30124 (1).pdf

NANOSCALE METALORGANIC FRAMEWORKS:

SYNTHESIS AND APPLICATION OF BIMODAL MICRO/MESO-STRUCTURE

AND NANOCRYSTALS WITH CONTROLLED SIZE AND SHAPE

Thèse

MINH-HAO PHAM

Doctorat en génie chimique

Philosophiae Doctor (Ph.D.)

Québec, Canada

© Minh-Hao Pham, 2013

iii

Résumé

Les composés à réseau moléculaire organométalliques (MOFs) ont émergé comme de

nouvelles classes de matériaux hybrides organo-inorganiques avec des potentialités

significatives en séparation, stockage de gaz, catalyse et support de médicaments. Ces

matériaux sont formés par un processus d‟assemblage dans lequel les ions métalliques sont

liés entre eux via un ligand organique, ce qui génère une surface de l‟ordre de 6500 m2g−1

et des volumes de pores supérieurs à 4.3 cm3g−1

. Dans cette thèse trois différentes

approches ont été développées pour la synthèse des nanocristaux MOFs à deux modes

micro-mésoporeux, ainsi que des nanocristaux MOFs à taille et forme contrôlable. En plus,

ces nanocristaux MOFs ont été utilisé comme un agent structurant pour la synthèse de

nanocomposite hybride platine-oxyde de titane (metal-oxideTiO2PtOx) qui ont été utilisé

comme photocatalyseurs pour la production d‟hydrogène à partir de l‟eau sous la lumière

visible.

Dans ce travail: (i) La première approche implique une méthode utilisant un surfactant,

suivi de traitement solvo-thermale en présence d‟acide acétique pour former des

nanocristaux MOFs micro-mésoporeux. L‟utilisation de surfactant non-ionique tell que

F127 (EO97PO69EO97) pour induire une structure mésoporeuse provoque labilité de la

cristallisation du mur des pores de la structure MOF. Tandis que la présence de l‟acide

acétique contrôle la vitesse de cristallisation du réseau MOFs pour former une

mésostructure bien définie à l‟intérieur des nanocristaux MOFs. En utilisant cette approche

des nanocristaux de [Cu3(BTC)2] et [Cu2(HBTB)2] de structure mésoporeuse avec des

diamètres de pores autour de 4.0 nm et des micropores intrinsèques ont été synthétisés. (ii)

La méthodologie de modulation de la coordination a été développée pour contrôler la forme

et la taille des nanocristaux MOFs. Des nanocubes et nanofeuilles de [Cu2(ndc)2(dabco)]n

de la structure MOFs ont été synthétisés en utilisant simultanément l‟acide acétique et la

pyridine ou la pyridine uniquement, respectivement comme modulateurs sélectifs. Ces

nanocristaux MOFs possèdent une cristallinité élevée et une grande capacité d‟adsorption.

La morphologie a été aussi étudiée en fonction de la capacité d‟adsorption de CO2. (iii) La

synthèse hydrothermale en contrôlant la taille de nanocristaux de carboxylates de structure

iv

MOFs, en utilisant simultanément des réactifs stabilisants et des réactifs contrôlant la

déprotonation a été démontrée. Dans le cas de FeMIL-88BNH2, la molécule triblock

copolymer a été utilisée comme un réactif stabilisant en coordonnant avec le métal et

contrôlant la croissance en formant des nanocristaux. L‟acide acétique joue le rôle comme

un agent déprotonant des liants carboxyliques en variant sa concentration dans le milieu

réactionnel, ainsi il régule la vitesse de nucléation, conduisant à aussi contrôler la taille

ainsi que le rapport longueur/largeur des nanocristaux. (iv) Finalement, des nanocomposites

hybrides Fe2O3TiO2PtOx de forme creuse possédant l‟activité photocatalytique

performante ont été développés en utilisant des nanocristaux FeMIL-88B composés de

centres Fe3(μ3O) liés par coordination insaturée comme template solide. Ce type de

nanocomposites non seulement absorbe la lumière visible mais aussi améliore la séparation

des électrons et des trous photo-générés, due à l‟épaisseur de paroi mince et les deux co-

catalyseurs (Fe2O3 and PtOx) localisés sur deux opposites surfaces du creux. En

conséquence, l'efficacité en photocatalyse de ce type de nanocomposites est élevée pour la

production d'H2 à partir de l'eau sous la lumière visible.

v

Abstract

Metalorganic frameworks (MOFs) have emerged as an important new class of porous

inorganicorganic hybrid solids with the potential for a significant impact on separation,

gas storage, catalysis and biomedicine. These materials are formed by assembly process in

which metal ions are linked together by rigid organic ligands, which creates enormous

surface areas (up to 6500 m2g−1

) and high pore volumes (up to 4.3 cm3g−1

). In this thesis,

three different synthetic approaches have been developed to achieve bimodal

micro/mesoporous MOF nanocrystals as well as nanosized MOFs with controlled size and

shape. In addition, using the synthesized MOF nanocrystals as templates, a new hollow

hybrid metal-oxideTiO2PtOx nanocomposite has also been prepared, and used as the

visible-light driven photocatalyst for the hydrogen production from water.

In this work, (i) the first approach involves nonionic surfactant-templated solvothermal

synthesis in the presence of acetic acid toward hierarchically micro-mesoporous MOF

nanocrystals. The use of a nonionic surfactant such as F127 (EO97PO69EO97) as

mesostructure template induces the ability to crystallize a MOF structure of pore wall,

while the presence of acetic acid allows control of the crystallization rate of the framework

to form well-defined mesostructures within the crystalline MOF nanocrystals. Using this

approach, [Cu3(BTC)2] and [Cu2(HBTB)2]-based MOF nanocrystals containing mesopores

with diameter around 4.0 nm and intrinsic micropores have been successfully synthesized.

(ii) Secondly, the coordination modulation methodology has been developed to control

shape and size of MOF crystals at the nanoscale. Nanocubes and nanosheets of

[Cu2(ndc)2(dabco)]n MOF have been rationally synthesized by using simultaneously acetic

acid and pyridine or only pyridine, respectively, as selective modulators. These MOF

nanocrystals exhibit high crystallinity and high CO2 sorption capacity. Their morphology-

dependent CO2 sorption property has also been demonstrated. (iii) Thirdly, the size-

controlled hydrothermal synthesis of uniform carboxylate-based MOF nanocrystals using

simultaneously stabilizing reagent and deprotonation-controlled reagent has been

demonstrated. In case of FeMIL-88BNH2, the molecular triblock copolymers as

stabilizing reagents coordinate with the metal ions and thus stabilize nuclei, which suppress

vi

the crystal growth to form nanocrystals. Acetic acid as deprotonation-controlled reagent

adjusts the deprotonation of the carboxylic linker via varying its concentration in the

reaction mixture, and thus regulates the rate of nucleation, leading to tailoring the size and

aspect ratio (length/width) of the nanocrystals. (iv) Finally, a new hollow hybrid metal-

oxideTiO2PtOx nanocomposite as an efficient photocatalyst has been developed by using

iron-based MIL-88B nanocrystals consisting of coordinatively unsaturated Fe3(μ3O)

clusters as template. The hollow nanocomposite not only absorbs visible light, but also

enhances the separation between photogenerated electrons and holes because of its thin

wall and the surface separation of two distinct functional cocatalysts (Fe2O3 and PtOx) on

two different surface sides of the hollow. As a result, the efficient photoactivity of the

nanocomposite photocatalysts has been found for the H2 production from water under

visible light irradiation.

vii

Table of Contents

Résumé .................................................................................................................................. iii

Abstract ................................................................................................................................... v

Abbreviations ......................................................................................................................... ix

Acknowledgements ................................................................................................................ xi

Preface ................................................................................................................................ xiii

Chapter 1 Introduction ....................................................................................................... 1

1.1 General introduction ............................................................................................... 1

1.2 Objectives of the thesis ........................................................................................... 2

1.3 References ............................................................................................................... 3

Chapter 2 Literature Review ............................................................................................. 5

2.1 Metalorganic frameworks ..................................................................................... 5

2.2 Mesoporous metal-organic frameworks ............................................................... 13

2.2.1 MOFs with mesocages .................................................................................. 13

2.2.2 MOFs with mesochannels ............................................................................. 21

2.2.3 Mesoporous MOFs from supramolecular templates ..................................... 26

2.2.4 Prospective applications of MOFs involving mesopores ............................. 30

2.3 Nanosized metal-organic frameworks .................................................................. 33

2.3.1 Coordination modulation .............................................................................. 34

2.3.2 Stabilizing reagent ........................................................................................ 36

2.3.3 Microemulsion .............................................................................................. 38

2.3.4 Synthetic parameters ..................................................................................... 41

2.3.5 Top-down approach ...................................................................................... 44

2.3.6 Potential application of MOFs involving nanosize ....................................... 45

2.4 Photocatalytic water splitting ................................................................................ 51

2.5 References ............................................................................................................. 54

Chapter 3 Characterizations ............................................................................................ 65

3.1 Introduction ........................................................................................................... 65

3.2 X-ray diffraction ................................................................................................... 65

3.3 Electron microscopy ............................................................................................. 67

3.4 X-ray photoelectron spectroscopy ........................................................................ 69

3.5 Fourier transform infrared spectroscopy ............................................................... 71

3.6 Ultraviolet-visible spectroscopy ........................................................................... 72

3.7 ζ-Potential analysis ............................................................................................... 73

3.8 Thermal analysis ................................................................................................... 74

3.9 Elemental analysis ................................................................................................ 75

3.10 Gas sorption .......................................................................................................... 76

3.10.1 Physisorption isotherm and surface area measurement. ............................... 76

viii

3.10.2 Micropore analysis ....................................................................................... 78

3.10.3 Size distribution of mesopores ..................................................................... 80

3.11 Gas chromatography analysis ............................................................................... 81

3.12 References ............................................................................................................ 81

Chapter 4 Route to Bimodal MicroMesoporous MOF Nanocrystals ........................ 83

4.1 Introduction .......................................................................................................... 89

4.2 Experimental ........................................................................................................ 90

4.3 Results and discussion .......................................................................................... 91

4.4 Conclusions ........................................................................................................ 100

4.5 Appendix ............................................................................................................ 101

4.6 References .......................................................................................................... 104

Chapter 5 Rational Synthesis of MOF Nanocubes and Nanosheets Using Selective

Modulators and Their Morphology-Dependent Gas-Sorption Properties ........ 107

5.1 Introduction ........................................................................................................ 113

5.2 Experimental ...................................................................................................... 115

5.3 Results and discussion ........................................................................................ 117

5.4 Conclusions ........................................................................................................ 123

5.5 Appendix ............................................................................................................ 123

5.6 References .......................................................................................................... 126

Chapter 6 Novel Route to Size-Controlled FeMIL-88BNH2 MOF Nanocrystals 129

6.1 Introduction ........................................................................................................ 135

6.2 Experimental ...................................................................................................... 137


6.4 Conclusions ........................................................................................................ 150

6.5 Appendix ............................................................................................................ 151

6.6 References .......................................................................................................... 152

Chapter 7 Hollow Fe2O3TiO2–PtOx Nanostructure with Two Distinct Cocatalysts

Embedded Separately on Two Surface Sides for Efficient Visible Light Water

Splitting to Hydrogen ............................................................................................. 157

7.1 Introduction ........................................................................................................ 163


7.3 Conclusions ........................................................................................................ 172

7.4 Experimental ...................................................................................................... 172

7.5 Appendix ............................................................................................................ 175

7.6 References .......................................................................................................... 179

Chapter 8 Conclusions and Prospects .......................................................................... 181

8.1 General conclusions ........................................................................................... 181

8.2 Prospects ............................................................................................................. 182

List of Publications ........................................................................................................... 185

ix

Abbreviations

AAS Atomic absorption spectroscopy

BDC 1,4-benzenedicarboxylic acid

BET BrunauerEmmettTeller

BJH BarrettJoynerHalenda

BPDC 4,4′-biphenyldicarboxylic acid

BPY 4,4′-bipyridine

BTB 1,3,5-tris[4-carboxyphenyl]benzene

BTC 1,3,5-benzenetricarboxylic acid

BTE 4,4′,4′′-(benzene-1,3,5-triyl-tris(ethyne-2,1-diyl))tribenzoate

BTTC Benzo-(1,2;3,4;5,6)-tris(thiophene-2′-carboxylate)

CB Conduction band

cbIM 5-chlorobenzimidazole

CTAB Cetyltrimethylammonium bromide

DABCO 1,4-diaza-bicyclo[2.2.2]octane

DBA 4-(dodecyloxy)benzoic acid

DMF N,N-dimethylformamide

DOT 2,5-dioxidoterephthalate

EDS Energy dispersive X-ray spectroscopy

FTIR Fourier transform infrared spectroscopy

GC Gas chromatography

H6L 1,3,5-tris (3′,5′-dicarboxy[1,1′-biphenyl]-4-yl)benzene

IUPAC International Union of Pure and Applied Chemistry

x

MOF Metalorganic framework

MTN Mobil thirty-nine

NDC 1,4-naphthalenedicarboxylic acid

2,6-NDC 2,6-naphthalenedicarboxylic acid

N-EtFOSA N-ethyl perfluorooctylsulfonamide

NHE Normal hydrogen electrode

PCP Porous coordination polymer

PTEI 5,5'-((5'-(4-((3,5-dicarboxyphenyl)ethynyl)phenyl)-[1,1':3',1''-terphenyl]-4,4''-

diyl)-bis(ethyne-2,1-diyl))diisophthalate

SAXS Small angle X-ray scattering

SBU Secondary building unit

SDC 4,4'-stilbenedicarboxylic acid

SEM Scanning electron microscopy

ST Super tetrahedron

TATAB 4,4′,4′′-s-triazine-1,3,5-triyltri-p-aminobenzoate

TATB 4,4′,4′′-s-trizaine-2,4,6-triyltribenzoic

TCPP Tetrakis(4-carboxyphenyl)porphyrin

TEM Transmission electron microscopy

T2DC Thieno[3,2-b]thiophene-2,5-dicarboxylate

TTEI 5,5',5''-(((benzene-1,3,5-triyltris(ethyne-2,1-diyl))tris(benzene-4,1-diyl))tris-

(ethyne-2,1-diyl))triisophthalate

UV-vis Ultraviolet-visible spectroscopy

VB Valence band

XPS X-ray photoelectron spectroscopy

XRD X-ray diffraction

xi

Acknowledgements

Firstly, I would like to thank Professor Trong-On Do for his excellent supervision and

guidance throughout the entirety of my PhD program. I am also sincerely grateful to

Professor Frédéric-Georges Fontaine at Université Laval for giving me the opportunity to

work with him. I gratefully acknowledge Professor Peter McBreen at Université Laval,

Professor James D. Wuest at Université de Montréal for their useful discussions.

I am very grateful for the generous and contentious help from the present and the former

members of the Do group and from my other colleagues at the Département de Génie

chimique of Université Laval. I wish to give thanks to all the professors and staff at the

Département de Génie chimique for their great assistance and cooperation. I am also

grateful to the technicians at Université Laval for providing advice and instructions to

operate equipments which I have used during my PhD studies.

I am most indebted to my family, especially my wife and my lovely daughter for their

unconditional love and invaluable support. I also thank my friends and acquaintances for

great and impressive time during my stay in Canada.

Finally, I would like to thank Vietnam Ministry of Education and Training for a

scholarship; the Natural Sciences and Engineering Research Council of Canada (NSERC),

the Centre en catalyse et chimie verte (C3V, Université Laval and the Centre in Green

Chemistry and Catalysis (CGCC, Québec for additional financial support.

Without all of you, it would not have been possible to realise this thesis.

Thank you.

xiii

Preface

This PhD thesis is built in the form of a collection of scientific papers whose first author is

the submitter of this thesis. The papers have been published or submitted for publication at

the time of the thesis submission.

The thesis is divided into eight chapters. The objectives of the thesis are elaborated in

chapter 1 after a brief general introduction. The literature review relating to the objectives

is thus presented in chapter 2, in which the background of metalorganic frameworks

(MOFs), the concepts and synthetic strategies toward mesoporous MOFs as well as the

prospective applications involving mesopore, the synthetic methodologies toward

nanosized MOFs and the potential applications involving nanosize, the photocatalytic water

splitting are reviewed. Chapter 3 outlines the background of characterizations that were

applied during the performing of this PhD work.

Chapter 4 reports the nonionic surfactant-templated solvothermal method in the presence of

acetic acid to achieve hierarchically porous MOF nanocrystals containing well-defined

mesopores with diameter around 4.0 nm and intrinsic micropores. The writing of this

chapter was supervised by Prof. Trong-On Do and Prof. Frédéric-Georges Fontaine who are

the co-authors. Part of the characterizations used in this research was conducted in

collaboration with Mr. Gia-Thanh Vuong who is also the co-author. This research is

published in Crystal Growth & Design 2012, 12, 1008–1013.

Chapter 5 presents the rational synthesis of nanocubes and nanosheets of

[Cu2(ndc)2(dabco)]n MOF by using simultaneously acetic acid and pyridine or only

pyridine, respectively, as selective modulators. The morphology-dependent CO2 sorption

property of these nanocrystals is also discussed in this chapter. The writing of this chapter

was supervised by Prof. Trong-On Do and Prof. Frédéric-Georges Fontaine who are the co-

authors. Part of the characterizations used in this chapter was conducted in collaboration

with Mr. Gia-Thanh Vuong who is also the co-author. This chapter is published in Crystal

Growth & Design 2012, 12, 3091–3095.

xiv

Chapter 6 reports the size-controlled hydrothermal approach toward uniform FeMIL-

88BNH2 nanocrystals with crystal size in the range of 30150 nm in width and of 50500

nm in length by using simultaneously F127 triblock copolymer as stabilizing reagent and

acetic acid as deprotonation-controlled reagent. The writing of this chapter was supervised

by Prof. Trong-On Do who is the co-author. Part of the characterizations used in this

research was conducted in collaboration with Mr. Gia-Thanh Vuong and Dr. Anh-Tuan Vu

who are also the co-authors. This chapter is published in Langmuir 2011, 17, 15261–15267.

Chapter 7 studies the application of the prepared FeMIL-88BNH2 nanocrystals as

template to constructing a new hollow hybrid metal-oxideTiO2PtOx nanocomposite as

the efficient photocatalyst for H2 production from water under visible light irradiation. The

writing of this chapter was supervised by Prof. Trong-On Do who is the co-author. Part of

the characterizations used in this research was conducted in collaboration with Mr. Cao-

Thang Dinh, Mr. Gia-Thanh Vuong and Prof. Ngoc-Don Ta who are also the co-authors.

This study has been submitted to a refereed journal.

Finally, Chapter 8 completes the thesis by summarizing general conclusions and prospects

for future work.

1

Chapter 1 Introduction

1.1 General introduction

Over the past two decades, there has been an enormous progress in research on porous

inorganicorganic hybrid materials that are known as metalorganic frameworks (MOFs)

or porous coordination polymers (PCPs).1-4

The phrase “metalorganic framework” that

was defined by Omar Yaghi et al. is now widely employed for porous crystalline structures

constructed from the coordinative bonding between metal ions and rigid organic linkers.5,6

MOFs have revealed various potential applications in separation,7,8

gas storage,9,10

catalysis,11,12

sensing,13

magnetic field,14

and biomedicine.15

The prospective applications

of MOFs are accounted for their stable framework,16

high porosity with enormous surface

area and pore volume,17,18

the ability to systematically vary and functionalize their pore

structure,19,20

and to rationally achieve pre-determined topologies with desired properties.21

Most of MOFs are microporous materials (pore size of < 2 nm) with large crystal sizes in

the micrometer scale. The small apertures of the micropores and the large crystal sizes of

MOFs significantly limit extensive applications of MOFs, especially in the participation of

large substances such as enzymes or in applications requiring nanoscale sizes such as drug

delivery.22,23

Moreover, the long micropores within MOF microcrystals also induce

diffusion resistance to adsorbed guest molecules. To upgrade the performance of MOFs,

several strategies to enlarge their pore size toward the mesopore regime (pore size of 250

nm)24,25

and to downsize their crystal size to the nanometer scale have thus emerged.26

The

mesopores and nanosizes promote the diffusion of guest molecules through MOFs and

increase accessible active sites.27

Furthermore, the mesopores also allow complex

substances to be encapsulated within MOF crystals. The MOF nanocrystals exhibit various

interesting features that are not observed in the bulk MOF materials such as unprecedented

adsorption, processability and guest inclusion.28

Several attempts to create mesopores within MOFs have been reported. The enlargement of

the organic linkers allows the expansion of the pore size of MOFs to the mesopore

2

regime.29

However, the obtained mesopore structures are usually collapsed upon the

removal of guest molecules. In many cases, the linker extension only forms catenated MOF

structures (i.e., interpenetration or interweaving of two or more identical and independent

frameworks) that have smaller pores rather than their non-catenated counterparts.30

The

surfactant-templated syntheses which employ the principle of the templated routes to

synthesis of conventional mesoporous materials have been adopted to generate mesopores

within MOF particles. However, the mesoporous MOFs have large particle size,31

or an

unidentifiable crystalline structure of pore wall.32

Similarly, various approaches have been used for synthesizing nanosized MOFs. The

microwave- and ultrasound-assisted syntheses produce MOF nanoparticles in a short period

of time.33

The MOF nanoparticles have also been synthesized by using microemulsion

systems.34

Stabilizing- and capping-reagents have been employed to suppress the crystal

growth of MOFs which lead to nanosized MOFs.35

But in general, it is difficult to control

the shape and size of MOF nanoparticles and the resulted nanoparticles usually appear as

aggregation rather than individual nanocrystals.36

Up to date, there is a great challenge to the preparation of mesoporous MOF nanocrystals

and uniform nanoscale MOFs with desired shape and size.

1.2 Objectives of the thesis

The aim of this thesis is to develop synthetic methods for preparing bimodal micro- and

meso-porous MOF nanocrystals, uniform nanosized MOFs with controlled-shape and size

and subsequently, the extensive application of the prepared nanosized MOFs.

The first objective is nonionic surfactant-templated solvothermal method in the presence of

acetic acid to achieve hierarchically porous MOF nanocrystals containing well-defined

mesopores and intrinsic micropores. The roles of the nonionic surfactant template and

acetic acid in forming well-defined mesostructures as well as influencing the size and

crystallinity of the hierarchically porous MOF nanoparticles have been demonstrated.

3

The second objective is coordination modulation methodology using simultaneously

distinct selective modulators or only one selective modulator for controlling rationally the

morphology of MOF nanocrystals. The effects of the nature and the concentration of

selective modulators on the shape and size of MOF nanocrystals have been demonstrated.

In addition, the morphology-dependent CO2 sorption property has also been illustrated.

The third objective is hydrothermal approach using simultaneously stabilizing reagent and

deprotonation-controlling reagent for the preparation of uniform nanosized carboxylate-

based MOFs with desired crystal sizes. The control of the stabilizing reagent and

deprotonation-controlling reagent over the size of the nanosized MOFs has been

demonstrated.

Finally, after the successful synthesis, the extensive application of the nanosized MOFs to

preparing a new hollow hybrid nanocomposite as an efficient photocatalyst for H2

production from water under visible light irradiation is the last objective of this dissertation.

1.3 References

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(4) Kitagawa, S.; Kitaura, R.; Noro, S.-i. Angew. Chem. Int. Ed. 2004, 43, 2334-2375.

(5) Yaghi, O. M., Li, G., Li, H. Nature 1995, 378, 703-706.

(6) Batten, S. R.; Champness, N. R.; Chen, X.-M.; Garcia-Martinez, J.; Kitagawa, S.;

Öhrström, L.; O'Keeffe, M.; Suh, M. P.; Reedijk, J. CrystEngComm 2012, 14, 3001-

3004.

(7) Li, J. R.; Sculley, J.; Zhou, H. C. Chem. Rev. 2012, 112, 869-932.

(8) Maes, M.; Schouteden, S.; Hirai, K.; Furukawa, S.; Kitagawa, S.; De Vos, D. E.

Langmuir 2011, 27, 9083-9087.

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(10) Wu, H.; Gong, Q.; Olson, D. H.; Li, J. Chem. Rev. 2012, 112, 836-868.

(11) Yoon, M.; Srirambalaji, R.; Kim, K. Chem. Rev. 2012, 112, 1196-1231.

(12) Corma, A.; García, H.; Llabrés i Xamena, F. X. Chem. Rev. 2010, 110, 4606-4655.

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4

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(30) Farha, O. K.; Malliakas, C. D.; Kanatzidis, M. G.; Hupp, J. T. J. Am. Chem. Soc.

2009, 132, 950-952.

(31) Ma, T.-Y.; Li, H.; Deng, Q.-F.; Liu, L.; Ren, T.-Z.; Yuan, Z.-Y. Chem. Mater. 2012,

24, 2253-2255.

(32) Zhao, Y.; Zhang, J.; Han, B.; Song, J.; Li, J.; Wang, Q. Angew. Chem. Int. Ed. 2011,

50, 636-639.

(33) Li, Z.-Q.; Qiu, L.-G.; Xu, T.; Wu, Y.; Wang, W.; Wu, Z.-Y.; Jiang, X. Mater. Lett.

2009, 63, 78-80.

(34) Taylor, K. M. L.; Rieter, W. J.; Lin, W. J. Am. Chem. Soc. 2008, 130, 14358-14359.

(35) Hermes, S.; Witte, T.; Hikov, T.; Zacher, D.; Bahnmüller, S.; Langstein, G.; Huber,

K.; Fischer, R. A. J. Am. Chem. Soc. 2007, 129, 5324-5325.

(36) Qiu, L. G.; Li, Z. Q.; Wu, Y.; Wang, W.; Xu, T.; Jiang, X. Chem. Commun. 2008,

3642-3644.

5

Chapter 2 Literature Review

2.1 Metalorganic frameworks

MOFs are porous crystalline materials with hybrid frameworks built from the linkages of

metal ions with organic linkers.1-3

The functional groups of the organic linkers for

coordinative bonding to the metal ions and the coordination geometry of the metal nodes

(metal ions or metal-containing clusters) play a decisive role in the formation of the

topologies of MOF structures. The appropriate groups are carboxylate, heterocyclic

nitrogen, amine, nitrile, phosphonate and sulfonate (Figure 2.1).4-8

Among them,

carboxylate group is the most commonly used functionality for designing and constructing

MOF structures.9,10

Multitopic linkers (i.e., containing two or more functional groups) with

rigid carbon backbones favor prominently in the quest for stable and robust frameworks

that retain MOF integrity in the absence of guest molecules. The rigid carbon backbones

are usually aromatic chains or alkenyl chains with double bonds. Metalloligands are also

used as linkers for the construction of MOFs.11-16

Furthermore, the linkers can be flexible

organic molecules, but in some cases they tend to give MOF structures in the form of dense

packing or interpenetration.17-19

Ditopic linkers

1,4-benzenedicarboxylic acid 1,4-diaza-bicyclo[2.2.2]octane 1,5-naphthalenedisulfonic acid

N,N-phenylenebis(salicylideneimine)dicarboxylic acid Bis[3-(4-pyridyl)pentane-2,4-dionato]copper(II)

6

Tritopic linkers

1,3,5-benzenetricarboxylic acid 2,4,6-tris(4-pyridyl)-1,3,5-triazine 1,3,5-benzenetriphosphonic acid

Tetratopic linkers

1,3,5,7-adamantanetetrabenzoic acid Tetrakis[4-(carboxyphenyl)oxamethyl]methane

1,3,5,7-tetrakis(4-phosphonophenyl)adamantine Tetrakis(4-cyanophenyl)methane

Figure 2.1 Multitopic organic linkers for MOF construction.4-16

The metal ions constructing MOF structures possess several coordination numbers. The

first row transition metal ions such as Cr3+

, Fe3+

, Cu2+

and Zn2+

are widely used for

constructing MOFs.10,20

The coordination of the functional groups of organic linkers to the

metal ions often generates polyatomic clusters containing two or more metal atoms, so-

7

called secondary building units (SBUs). The SBUs give points of extension where they

connect to the rigid carbon backbones of the organic linkers to form porous structures. The

geometry of the SBUs is determined by the coordination number, coordination geometry of

the metal ions and the nature of the functional groups. The variety of SBU geometries with

different numbers of points of extension has been observed in MOF structures.10,21

However, the metal-containing SBUs are not isolatable entities, and so it is essential to

establish exact chemical conditions that will yield a specific SBU in situ.8

Octahedron (6 points) Trigonal prism (6 point) Square paddle-wheel (4 points)

Triangle (3 points) Square pyramid (5 points) Cube (8 points)

Figure 2.2 Geometry and number of points of extension of representative SBUs. Metal:

polyhedral; C: black; O: red; Heteroatom: green, cyan. Reproduced with the permission of

The Royal Society of Chemistry.10

The most common SBUs are clusters of metal ions and carboxylate groups. The typical

examples are octahedral, trigonal prism and square paddle-wheel SBUs (Figure 2.2).10

The

octahedral SBU with the formula of M4O(COO)6 consists of a central bridging O atom

bonding to four tetrahedral metal atoms to form a tetrahedral M4O unit; and six carboxylate

groups connecting the edges of the tetrahedra. The six points of extension (i.e., six carbon

atoms of the carboxylate groups) are at the vertices of the octahedron. This cluster is found

for Zn, Co and Be. In case of trigonal prism SBU with the formula of M3O(COO)6, three

metal ions connect together via a central bridging O atom and each pair of the octahedral

8

metal atoms is bridged by two carboxylate groups. Each metal ion coordinates to a terminal

ligand such as solvent or anion. The six points of extension locate at the vertices of the

trigonal prism. This SBU is observed for several metals such as Cr, Fe and Al. In the square

paddle-wheel SBU, two metal ions are bridged by four carboxylate groups and each metal

ion is coordinated by a terminal ligand on the apical position. This structure is represented

by Cu2(COO)4 SBU.

The connection between the metal-containing SBUs via the carbon backbones of organic

linkers forms porous crystalline structures of MOFs.22

The judicious use of appropriately

shaped SBUs and rigid organic linkers results in the formation of pre-determined MOF

topologies.23,24

The well-known example is the design and preparation of MOF-5

framework.25

In this framework, the zinc-carboxylate SBUs with six points of extension in

octahedral geometries connect to phenyl rings of 1,4-benzenedicarboxylic (BDC) acids to

form porous cubic structure (Figure 2.3).

Figure 2.3 MOF-5 built from the connection of the octahedral SBUs with BDC acids. C:

gray; O: red; Zn: blue tetrahedral. All hydrogen atoms are omitted for clarity. The large

yellow sphere represents the largest van der Waals sphere that fits in the cavity of the

framework. Reproduced with the permission of Nature Publishing Group.25

In principle, an almost unlimited number of MOFs can be constructed due to a great

diversity of organic linkers and metal-containing SBUs. The connection of a specific SBU

with different organic linkers and vice versa leads to MOF structures with different

+

MOF-5

Zn4O(COO)6

Octahedral SBU

C6H4

Moiety

9

topologies. The illustrative example is shown in Figure 2.4. The connection of trigonal

prism SBUs with different organic linkers, either BDC acids as ditopic linker or 1,3,5-

benzenetricarboxylic (BTC) acids as tritopic linker, creates the different topologies of MIL-

88B and MIL-100 (MIL standing for Materials of the Institute Lavoisier).26,27

In the other

hand, the connection of the same tritopic BTC linkers with dicopper square paddle-wheel

SBUs instead of trigonal prism SBUs results in HKUST-1 topology (HKUST standing for

Hong Kong University of Science and Technology).28

As a result, the features of the

resulted frameworks such as pore structure, porosity, rigidity of the frameworks and

physicochemical properties are also different.

Figure 2.4 Topologies of MOFs from the different connections between SBUs and organic

linkers. C: gray; O: red, Cu: cyan; metal such as Cr, Fe: green. Reproduced with the

permission of The Royal Society of Chemistry, Willey and Elsevier.26-28

Functional groups such as NH2, NO2, SO3H, CN, OH, OCxHy, CxHy, CF3,

COOH, azide (N3) and halogens have been attached on organic linkers, commonly

aromatic linkers, to generate functionalized MOFs.29-38

The functionalized MOFs are often

MIL-88B MIL-100 HKUST-1

10

monofunctional structures containing a single type of functional groups. The introduction

of functional groups creates isoreticular MOFs with the same network, such as IRMOF-n (n

= 2 5) possessing the network of IRMOF-1 (or MOF-5).33

The functional groups can

improve the stability and tailor the property of the parent structures for interesting

applications. Multifunctional MOFs containing two or more distinct chemical

functionalities have also been fabricated by integrating several organic linkers into a single

platform.39-41

The multifunctional MOFs provide integrated properties that their

monofunctional counterparts can not achieve.42

MOF structures can be built from two or more types of organic linkers instead of one type

of linker.43-46

In the example to [M2(dicarboxylate)2(N ligand)]n MOFs, the three dimension

structure of [Cu2(ndc)2(dabco)]n MOF is built from the connection of dicopper square

paddle-wheel SBUs with two distinct linkers, including 1,4-naphthalene dicarboxylic (ndc)

acid and 1,4-diaza-bicyclo[2.2.2]octane (dabco).47

In this structure, NDC molecules

connect with dicopper square paddle-wheel SBUs to form two dimension square lattices

that are connected together by dabco molecules at the lattice points to form the porous

structure (Figure 2.5).

Figure 2.5 Structure of [Cu2(ndc)2(dabco)]n. Cu: blue; N: green; O: red; and C: gray. All

hydrogen atoms are omitted for clarity. Reproduced with the permission of Elsevier.48

The crystalline structures of MOFs are normally quite rigid. However, the reversible

structural variation with retained topology can occur in various MOFs under external

stimuli. Such variation of the flexible frameworks is usually called “breathing effect”.49-51

11

The breathing effect is activated by the presence of inserted guests such as water, solvents

and gases. The breathing amplitude is governed by the host-guest interaction and the

flexibility of MOF skeleton. The structural variation results in a dynamic change in the pore

size and shape. During the breathing, the position of the Bragg peaks in the X-ray

diffraction powder pattern shifts as a result of the change of the unit cell parameters.

The large breathing effect has been found in some carboxylate-based MOFs such as MIL-

88 structures (MIL-88A constructed from fumaric acid, MIL-88B from BDC acid, MIL-

88C from 2,6-naphthalenedicarboxylic (2,6-NDC) acid and MIL-88D from 4,4′-

biphenyldicarboxylic (BPDC) acid),27,52,53

MIL-53,54-58

MIL-89,59

DUT-8,60

MCF-18,61

and

DMOF-1.62,63

The possible breathing of a carboxylate-based MOF occurs only if structural

rules are all satisfied.49

The rules include the existence of a mirror plane in the SBU with

the carboxylates in symmetrical positions toward them, for example in the square paddle-

wheel dimers of DMOF-1 and the trigonal prism trimers of MIL-88; ditopic linker; and the

absence of rigid odd cycles. Therefore, at variance to MIL-88, MIL-53 and DMOF-1; MIL-

101 owning pentagonal windows,64

MOF-5 consists of tetragonal SBUs without the mirror

planes,25

and HKUST-1 constructed from tritopic linkers28

do not breath. Moreover, it

seems that the ratio of number of carbons of the carboxylates surrounding the SBU to

number of metal atoms within the SBU must be greater than or equal to 2 for such

breathing effect.

In the illustrative example based on MIL-88B,65

the topology has large cages with a

trigonal bipyramid shape. The vertices of the bipyramid are occupied by trigonal prism

SBUs. The ditopic linkers are on the edges of the bipyramid and there is no link in the

equatorial plane of the bipyramind. The SBUs can rotate around the knee cap OO axes of

the carboxylates and the OOCCOO axes of the linkers (Figure 2.6, left). These rotations

accompanying with the absence of rigid linkages between the equatorial SBUs can induce a

change in the distance between the equatorial SBUs under external stimuli, leading to

breathing effect. Indeed, the as-synthesized form of MIL-88B contains a few solvent

molecules in the bipyramid cage. When the solvents are removed from the cage (dry form),

the length between the two non-equatorial SBUs increases whereas the distance between

12

the equatorial SBUs decreases. In contrast, the bipyramid flattens and its volume increases

if the solvents are exchanged into the cage (open form) (Figure 2.6, right). The expansion

of the cell volume between the dry and open forms of MIL-88B can be over 125 %.

Figure 2.6 (Left) One edge of the trigonal bipyramid and the possibility of rotation around

the knee cap OO axes (blue lines) with the senses of the rotations (blue arrows) of the

whole SBUs (green arrows) in MIL-88B. White arrow shows possible rotations around the

OOCCOO axis of the linker. (Right) Evolution of the bipyramid (top) and correlative

evolution of the structure along [001] (bottom) during breathing. Reproduced with the

permission of Science and The Royal Society of Chemistry.65

The organic linkers and metal-containing SBUs impart the characteristics of MOFs such as

high porosity with large surface area and pore volume,66,67

high stability,68

and the ability to

tailor the pore cavity environment.69,70

The extraordinary degree of variability for both

organic linkers and metal-containing SBUs leads to tunable properties that make MOFs of

interest for potential applications such as gas storage,71-76

separation,77-79

catalysis,80-82

sensing,83-86

biomedicine,87-89

and proton conductivity.90-94

Moreover, the post-synthetic

modifications also allow the manipulation of the physical and chemical properties of a

given MOF in a controllable manner.69,95-99

Therefore, the design and use of organic linkers

as well as the selection of metals focus on MOF structures suitable for their targeted

applications.6 For instance, the incorporation of electron-rich functional groups on the

9.6 Å 15.6 Å

19.1 Å 15.9 Å

Dry As-synthesised Open

V=3400 Å3

V=1500 Å3

13

organic linkers provides high affinity with CO2 gas which promotes preferential CO2

adsorption and separation,34,100,101

while the attachment of achiral functionalities offers

catalytic activity for enantioselective reactions.81

Similarly, coordinatively unsaturated

metal ions in square paddle-wheel or trigonal prism SBUs after the removal of axial

terminal ligands act as Lewis acid sites that have high affinity with various guests such as

adsorbates,102-104

catalytic organic molecules,105,106

porosity-customizing agents,107

and

reagents generating proton conductivity.108

A number of MOFs built from lanthanide

metals have luminescence that makes them ideal for sensing,109,110

while various MOFs

constructed from paramagnetic metals have magnetism suitable for magnetic

applications.111-113

2.2 Mesoporous metal-organic frameworks

One of the most important properties of MOFs is porosity with large surface area and high

pore volume. The IUPAC classifies porous solids into three categories according to their

pore size: microporous (d < 2 nm), mesoporous (2 nm d 50 nm) and macroporous (d

> 50 nm) materials. Therefore, most of the reported porous MOFs are microporous and

only a small fraction of MOFs are mesoporous (Table 2.1). The concept of the mesoporous

MOFs refers not only to structures possessing large cavities and/or channels with diameters

over 2 nm but also to MOF particles owning mesopore systems derived from

supramolecular templates which are similar to those of conventional mesoporous materials.

2.2.1 MOFs with mesocages

A number of MOFs having large cavities within the range of 2 50 nm (so-called

mesocages) have been prepared from one type of organic linker or the mixture of different

linkers in coordination with metal-containing SBUs that possess different geometric shapes.

One of these MOF structures is MIL-101.64

The structure of MIL-101 consists of trigonal

prism SBUs and ditopic linkers. The connection between the SBUs and the ditopic BDC

linkers constructs microporous super tetrahedron (ST) (Figure 2.7). The four vertices of the

ST are occupied by the SBUs while the ditopic linkers locate at the six edges of the ST. The

14

STs are computationally assembled through their vertices to produce an open framework

with the Mobil thirty-nine (MTN) zeotype topology. In this assembly, the STs serve as

tetrahedral TO4 units of the MTN zeotype. MIL-101 structure has two types of mesocages

with internal diameters of 29 Å and 34 Å. The small mesocage constructed by 20 STs has

12 pentagonal windows while the large mesocage defined by 28 STs has 12 pentagonal and

4 hexagonal windows. Although these cages are mesopores, the apertures of the windows

in MIL-101 are limited within the size range of micropore at 12 Å for the pentagonal

windows and 14.7 16 Å for the hexagonal windows.

The extension of the length of the ditopic linker creates larger mesocages within MIL-101

structure. Ferey et al. used 2,6-NDC acid that has a longer carbon backbone rather than that

of BDC acid for the preparation of MIL-101_NDC material.114

Similar to MIL-101 based

on BDC acid, MIL-101_NDC is also constructed from ST subunits and has the MTN

zeotype structure. However, the ST of MIL-101_NDC material (about 10.2 Å) is larger

than that of MIL-101 (about 8.6 Å) because of the longer carbon backbone of 2,6-NDC

acid. This results in two larger mesocages of MIL-101_NDC with internal diameters of 39

Å and 46 Å. The pentagonal and hexagonal windows have free diameters of approximately

13.5 Å and 18.2 20.3 Å, respectively.

15

Table 2.1 The structural features of representative mesoporous MOFs.

MOFs Linkers Mesoporous structure (Å)

Apertures (Å) Ref. Cages Channels Templates

MIL-101 BDC 29 and 34 12.0 and 14.7 64

MIL-101_NDC 2,6-NDC 39 and 46 13.5 and 18.2 114

MIL-100 BTC 25 and 29 4.8 and 8.6 26

Tb-mesoMOF TATB 39 and 47 13.0 and 17.0 115

UMCM-2 T2DC and BTB 26 32 116

DUT-6/MOF-205 NDC and BTB 2530 117,

66

MOF-210 BTE and BPDC 26.9 48.3 66

PCN-53 BTTC 22.2 16 118

PCN-68 PTEI 23.2 119

PCN-610/NU-100 TTEI 27 119, 120

NOTT-119 H6L 25 19 121

ZIF-95 cbIM 24 3.65 122

ZIF-100 cbIM 35.6 3.35 122

16

Table 2.1 (Continuous)

MOFs Linkers Mesoporous structure (Å)

Apertures (Å) Ref. Cages Channels Templates

Bio-MOF-1 Adenine and BPDC mesocage 28 (3-D) 67

IRMOF-16 TPDC 28.8 (3-D) 33

mesoMOF-1 TATAB 22.5 26.1 (3-D) 123

CYCU-3 SDC 28.3 31.1 (1-D) 124

JUC-48 BPDC 21.1 24.9 (1-D) 125

IRMOF-74 DOT 14 98 (1-D) 126

PCN-222 TCPP 37 (1-D) 127

UMCM-1 BTB and BDC 27 32 (1-D) 128

MMOF-n Disulfonate 61 75 F127 129

Zn-BDC BDC 30 N-EtFOSA 130

Cu3(BTC)2 BTC 25 N-EtFOSA 131

Cu3(BTC)2 BTC 38 310 CTAB/TMB 132

MOF-5 BDC > 100 DBA 133

17

Figure 2.7 Architecture of the mesocages and windows in MIL-101. In the scheme of the

MTN zeotype structure, the small and large mesocages are presented in green and red,

respectively. Metal octahedral: green; O: red; C: blue. Reproduced with the permission of

Science.64

By using tritopic BTC linker instead of ditopic BDC linker in coordination with the trigonal

prism SBU, mesoporous MIL-100 with the same MTN zeotype topology was previously

prepared by Ferey et al. in 2004.26

The structure of MIL-100 is also constructed from

tetrahedral ST subunits, in which four BTC linkers span four faces of the ST while the

vertices of the ST are still occupied by the trigonal prism SBUs (Figure 2.4). Each BTC

connects three trigonal prism SBUs via its three carboxylate groups and the phenyl ring of

BTC occupies the center of the triangle face obtained by joining the three SBUs. Therefore,

the ST in MIL-100 has smaller diameter rather than its counterpart in MIL-101, at 6.6 Å

18

and 8.6 Å, respectively. As a result, the mesocages of MIL-100 have smaller diameters at

25 Å for the small cage and 29 Å for the large cage. Similar to MIL-101 material, the

windows of MIL-100 have small diameters at 4.8 5.8 Å for the pentagonal windows and

8.6 8.6 Å for the hexagonal windows.

Using a longer tritopic linker, 4,4′,4′′-s-trizaine-2,4,6-triyltribenzoic acid (TATB), Kim et

al. prepared a mesoporous MOF from Tb3+

ions (namely Tb-mesoMOF) possessing the

same MTN zeotype topology as that of MIL-100 and MIL-101.115

In this case, the tritopic

TATB linkers connect trigonal-planar Tb4 clusters to produce STs (Figure 2.8). Each face

of the ST is spanned by the pair of TATB linkers that stack with each other. The outer

TATB of this pair joins three peripheral Tb3+

ions in a bidentate fashion while the inner

TATB links three central Tb3+

ions and other three peripheral Tb3+

ions in a bi-monodentate

fashion. Because the carbon backbone of TATB is elongated by three additional phenyl

rings when compared to that of BTC and BDC, the mesocages in Tb-mesoMOF are larger

than those of MIL-100 and MIL-101 as the result of the increase in the dimension of the

STs. The internal diameters of the mesocages are 39 Å and 47 Å. Although the mesocages

are larger, the apertures of the windows are also limited within micropore regime at 13 Å

for five-membered rings and 17 Å for six-membered rings.

Figure 2.8 The ST consisting of TATB acids and trigonal-planar Tb4 clusters and the

topology of Tb-mesoMOF. Each blue truncated tetrahedron on the topology represents one

ST. C: gray; O: red; N: blue; Tb: turquoise. Reproduced with the permission of Wiley

InterScience and American Chemical Society.115

TATB Super Tetrahedron Topology

47 Å 39 Å

19

Besides one type of linkers, some MOFs having mesocages are constructed from the

mixture of different types of linkers. Indeed, the coordination of ditopic thieno[3,2-

b]thiophene-2,5-dicarboxylate (T2DC) and tritopic 1,3,5-tris(4-carboxyphenyl)benzene

(BTB) with zinc in the form of octahedral Zn4O(COO)6 SBUs produces mesoporous

UMCM-2 MOF (UMCM standing for University of Michigan Crystalline Material).116

UMCM-2 structure has a mesocage with internal dimension of approximately 26 32 Å

and two different microporous cages (Figure 2.9).

Figure 2.9 Structure of UMCM-2. The cage II (in cyan) is mesoporous while the cages I (in

red) and case III (in yellow) are microporous. Reproduced with the permission of American

Chemical Society.116

DUT-6 (DUT standing for Dresden University of Technology) with mesocages is also built

from the mixture of ditopic 2,6-NDC linker and tritopic BTB linker which connect to

octahedral Zn4O(COO)6 SBUs (Figure 2.10).117

The carboxylate groups in the SBU come

from four BTB linkers in a square planar arrangement and two 2,6-NDC linkers. The MOF

structure provides dodecahedral mesocages with diameter of 25 – 30 Å which are built

from twelve Zn4O(COO)6 SBUs, six 2,6-NDC linkers and eight BTB linkers. The second

type of cage in DUT-6 is constructed from four Zn4O(COO)6 SBUs, two 2,6-NDC linkers

and four BTB linkers. These smaller cages are arranged to connect the mesocages to form a

periodic space filling. This MOF structure was independently reported as MOF-205 by

Yaghi et al. in 2010.66

T2DC BTB UMCM-2

II

III

I

Zn4O(COO)6 SBU

20

Figure 2.10 Structure of mesoporous DUT-6. Reproduced with the permission of Wiley

InterScience.117

Recently, MOF-210 with mesocages has been prepared from the mixture of ditopic BPDC

and tritopic 4,4′,4′′-(benzene-1,3,5-triyl-tris(ethyne-2,1-diyl))tribenzoate (BTE) linkers

(Figure 2.11).66

The largest mesocage with a dimension of 26.9 48.3 Å in MOF-210

consists of eighteen Zn4O(COO)6 SBUs, fourteen BTE and six BPDC linkers. The material

exhibits an enormous BET surface area of 6240 m2g

-1 and a total carbon dioxide storage

capacity of 2870 mg.g-1

.

Figure 2.11 Structure of MOF-210. The yellow and orange spheres indicate spaces in the

cages. Reproduced with the permission of Science.66

Although the MOFs possessing mesocages exhibit mesoporous features, their small

apertures of the windows within the micropore range below 20 Å limit the access of large

substances to the mesocages. The increase of the apertures to the mesopore range is still a

great challenge.

Mesocage Small cage Topology

BPDC BTE MOF-210

21

2.2.2 MOFs with mesochannels

2.2.2.1 Three dimension mesochannels

The common strategy to enlarge the channels of MOFs toward the mesopore range of 2

50 nm (so-called mesochannels) is the extension of organic linkers. The linker extension

takes the advantage of the increase of the channel size corresponding to the length of the

linker. However, the linker expansion can result in catenated structures with small

channels.134

Moreover, MOFs built from long linkers can be collapsed upon guest

removal.135

Therefore, the construction of a MOF with 3D mesochannels from a given

network topology via the extension of the linker should inhibit the catenation and

collapse.84,134

Figure 2.12 Structure of non-catenated IRMOF-16, interpenetrated IRMOF-15 isomer and

mesoMOF-1. The colored spheres represent the void inside the cavities. Reproduced with

the permission of Science, American Chemical Society and Elsevier.33, 123

TPDC Zn4O(COO)6

+

IRMOF-16

TATAB

+

Cu2(COO)4 mesoMOF-1

IRMOF-15

Octahedron

22

The typical non-catenated MOF with 3D mesochannels prepared via the extension of linker

is IRMOF-16.33

The MOF has the same topology of MOF-5 (IRMOF-1), in which

Zn4O(COO)6 octahedral SBUs connect to the aromatic backbones of ditopic carboxylate

linkers to form cubic network (Figure 2.12, top). In MOF-5, the carbon backbone has one

phenyl ring, giving micropores with the free- and fixed diameters of 11.2 Å and 18.5 Å,

respectively. In contrast, the carbon backbone in IRMOF-16 is elongated with two

additional phenyl rings, generating mesochannels with the free- and fixed diameters of 19.1

Å and 28.8 Å, respectively. IRMOF-16 should be synthesized under dilute conditions to

avoid catenation that results in the interpenetrated structure of IRMOF-15 isomer with

smaller micropores at 8.1 Å and 12.8 Å in free- and fixed diameters, respectively.

Zhou et al. prepared mesoMOF-1 with 3D mesochannels by the extention of tritopic

carboxylate linkers.123

MesoMOF-1 with the same topology of HKUST-1 is built from

dicopper square paddle wheel SBUs and tritopic 4,4′,4′′-s-triazine-1,3,5-triyltri-p-

aminobenzoate (TATAB) linkers (Figure 2.12, bottom). Although TATAB has a longer

carbon backbone rather than BTC in HKUST-1, mesoMOF-1 has non-interpenetrated

structure because of the nonplanarity of TATAB linkers. In mesoMOF-1 structure, each

Cu2(COO)4 SBU connects four TATAB linkers, and each TATAB links three SBUs to

form an octahedron, in which all six vertices are occupied by the SBUs, and four of the

eight faces are spanned by TATAB linkers. Eight such octahedra occupy the eight vertices

of a cube to form a cuboctahedron through corner sharing. These cuboctahedra propagate to

an open framework with 3D mesochannels of 22.5 26.1 Å in diameter.

Recently, An et al. have reported an alternative strategy for constructing mesoporous MOFs

which addresses the size of the vertex rather than the length of organic linkers. They used

large zinc-adeninate building units (ZABUs) as vertices to construct bio-MOF-100 (Figure

2.13).67

In bio-MOF-100, each ZABU consists of eight Zn2+

cations interconnected by four

adeninates and two µoxo groups. Monodentate BPDCs occupy the remaining coordination

sites on each tetrahedral Zn2+

cation. The ZABU is connected to four adjacent ZABUs via

twelve BPDC linkers to generate open structure with mesocages and 3D mesochannels with

23

a diameter of 28 Å along the axes. This MOF structure exhibits the largest pore volume

reported to date, up to 4.3 cm3g−1

.

Figure 2.13 Structure of bio-MOF-100: ZABU can be treated as a distorted truncated

tetrahedron. Each truncated tetrahedron connects four adjacent tetrahedra to generate open

framework. Zn: dark blue tetrahedra; C: grey spheres; O: red spheres; N: blue spheres.

Reproduced with the permission of Nature Publishing Group.67

2.2.2.2 One dimension channels

The construction of infinite rod-shaped SBUs has been employed to generate MOFs with

1D mesochannels such as CYCU-3 (CYCU standing for Chung-Yuan Christian

University),124

and JUC-48 (JUC standing for Jilin University China) materials (Figure

2.14).125

In CYCU-3 structure, each octahedral Al3+

ion binds to four oxygen atoms from

four carboxylate groups of 4,4'-stilbenedicarboxylic acids (SDC) and two oxygen atoms

from the bridged hydroxide groups. The octahedral AlO6 units form corner-shared 1D

SBUs that are linked together by the SDC linkers to generate two types of 1D channels,

including hexagonal mesochannels with the cross-section size of approximately 28.3 31.1

Å and triangular micropores with the pore size of 14.4 Å. Similarly, JUC-48 is constructed

from rod-shaped Cd2+

carboxylate SBUs. These SBUs are interconnected through the

biphenyl chains of BPDC linkers to generate a non-interpenetrated network with 1D

hexagonal mesochannels of 24.5 27.9 Å.

Large cage Topology

ZABU

24

Figure 2.14 Structures of CYCU-3 and JUC-48. Reproduced with the permission of The

Royal Society of Chemistry and Wiley InterScience.124, 125

Figure 2.15 Structure of the series of nine IRMOF-74-I to XI. C: gray; O: red; metal: blue.

Reproduced with the permission of Science.126

The construction of infinite rod-shaped SBUs in combination with the extension of organic

linkers has been used for enlarging 1D channels of MOFs. The strategy has been employed

for the systematic expansion of MOF-74 from its original link of one phenyl ring (I) (2,5-

dioxidoterephthalate, DOT) to eleven (XI), which affords an isoreticular series of nine

MOF-74 structures (termed IRMOF-74-I to XI) (Figure 2.15).126

The topology of IRMOF-

74 is built from infinite rod-shaped SBUs. The SBU is in a hexagonal arrangement joined

CYCU-3 JUC-48

SBU MOF-74

25

along the short and long axes by metal-oxygen bonds and organic DOT links, respectively.

The formation of the short axis is ensured by the coordination of 5-coordinate metal atoms

to the carboxylic and the ortho-positioned hydroxyl functionalities of DOT links. DOT

links (long axes) join the infinite rod-shaped SBUs to make IRMOF-74 structure with 1D

hexagonal channels. Upon the elongation of DOT links, the nine members of this series

with non-interpenetrated robust structures have hexagonal channels in the range of 14 98

Å in size.

In a different assembly, the structure of PCN-222 (PCN standing for porous coordination

network) containing 1D mesochannels is built from Zr6 clusters instead of infinite rod-

shaped SBUs (Figure 2.16).127

In this structure, only eight edges of the Zr6 octahedron are

bridged by the carboxylates from square planar tetrakis(4-carboxyphenyl)porphyrin (TCPP)

linkers, while the remaining positions are occupied by terminal hydroxy groups. Therefore,

the framework of PCN-222 can be viewed as zirconium-carboxylate layers in the ab plane

which are pillared by TCPP linkers along the c axis. PCN-222 provides 1D hexagonal

mesochannels with a diameter of 37 Å. Moreover, the isomers of PCN-222 with

uncoordinated porphyrin or metalloporphyrins including Fe, Mn, Co, Ni, Cu, and Zn can be

produced, which affords a new series of PCN-222.

Figure 2.16 Structure of PCN-222. Zr: black; C: gray; O: red; N: blue; metal in porphyrin:

orange. Reproduced with the permission of Wiley InterScience.127

Zr6 octahedron

TCPP Topology Mesochannel

26

The combination of different types of organic linkers has been employed for the

construction of MOFs with 1D mesochannels. Koh et al. have synthesized mesoporous

UMCM-1 by combining ditopic BDC with tritopic BTB in the presence of zinc cations

(Figure 2.17).128

Each octahedral Zn4O SBU in UMCM-1 is coordinated by two BDC

linkers and four BTB linkers. This structure generates micropous cages and 1D hexagonal

mesochannels with a diameter of 27 32 Å.

Figure 2.17 Structure of UMCM-1. Reproduced with the permission of Wiley

InterScience.128

2.2.3 Mesoporous MOFs from supramolecular templates

The template-assisted synthetic approach has been adopted to prepare mesoporous MOFs.

Similar to traditional mesoporous materials, the co-assembly approach uses structure-

directing agents (supramolecular templates) for generating mesostructures (Figure 2.18).

For the co-assembly into mesostructures and the growth of MOF directed by the templates,

the interaction between MOF precursors and the supramolecular templates with

considerable strength is essential. Otherwise, macroscopic phase segregation may take

place, and the MOF will crystallize by itself despite the templates.136

27

Figure 2.18 Schematic illustration of the synthesis of mesoporous MOFs using

supramolecular templates. Reproduced with the permission of Wiley InterScience.132

2.2.3.1 Non-ionic templates

Nonionic triblock copolymers have been used as supramolecular templates for fabricating

mesoporous MOFs. Ma et al. used the surfactant F127-induced co-assembly for producing

MOFs with order mesopore structures from two disulfonic linkers (1,5-

naphthalenedisulfonic and ethanedisulfonic acids) and different metal ions (Cd2+

, La3+

,

Cu2+

and Sr2+

).129

The MOF walls of the mesopores are built from the crystalline

frameworks of metal-disulfonate. Since the easy coordination of the sulfonate groups with

the metal ions facilitating the formation of large MOF crystals rather than the co-assembly

between the surfactant and the MOF precursors into mesostructures, the crown ether 1,10-

diaza-18-crown-6 was added to control the release of the metal ions, which slowed down

the coordination rate between the metal centers and the sulfonate claws.

In such acidic synthesis system, the PEO segments of the triblock copolymer F127

(PEOPPOPEO) were protonated, producing positively charged head groups. The

positively charged surfactants (SoH

+), the disulfonate anions (X

−) and the metal cations (I

+)

assembled together via (SoH

+)X

−I+ mechanism to form ordered hexagonal mesostructures.

The propagation of the metal-disulfonate coordination (X−I+) formed the crystalline MOF

walls of the mesostructures. After the removal of the F127 surfactant by acidic ethanol

28

extraction, the obtained MOF particles contained mesopores with the pore diameters in the

range from 6.1 to 7.5 nm.

Nonionic N-ethyl perfluorooctylsulfonamide (N-EtFOSA) has also been used as

supramolecular templates for synthesizing hierarchically micro- and mesoporous MOF

particles. N-EtFOSA molecules self-assemble into cylindrical micelles, in which the

fluorocarbon tails direct toward the inside of the micelles. By using N-EtFOSA, Zhao et al.

synthesized MOF nanospheres with well-ordered hexagonal mesopores from Zn2+

ions and

BDC acids in an IL/SCCO2/surfactant emulsion system (IL = 1,1,3,3-

tetramethylguanidinium acetate ionic liquid and SCCO2 = supercritical CO2).130

Because of

the strong interaction of the CO2 molecules with the fluorocarbon tails, the CO2 molecules

existed as the core of the micelles. The coordination of Zn2+

ions with BDC linkers in the

IL generated the microporous MOF walls. After the removal of the IL, CO2, and the

surfactant, uniform MOF nanospheres with well-ordered mesopores and microporous walls

were achieved. The sizes of micropores, mesopores and the wall thickness were about 0.7,

3.0 and 2.5 nm, respectively.

Recently, Peng et al. have used N-EtFOSA as nonionic template for synthesizing

mesoporous MOF nanoplates based on HKUST-1 structure in the IL solution.131

In this

case, the surfactant has a dual role in the formation of the mesoporous MOF nanoplates. On

the one hand, the surfactant molecules play the role of a template in the mesopore

formation. On the other hand, the surfactant can selectively adsorb onto the crystallographic

planes of the MOF, thus serving as a directing agent and kinetically controlling the

anisotropic growth of the MOF. In the process, Cu2+

ions in the IL first react with the

deprotonated BTC to form nanosized framework building blocks. The nanosized building

blocks then assemble with N-EtFOSA molecules to form mesostructured MOF particles.

The removal of the surfactant from the mesostructured particles gives mesopores with the

diameter around 2.5 nm.

29

2.2.3.2 Cationic templates

In addition to nonionic surfactants, cationic surfactants have been used for synthesizing

mesoporous MOFs. In 2008, Qiu et al. used cetyltrimethylammonium bromide (CTAB) as

cationic template for preparing MOFs with disordered mesostructures from Cu2+

cations

and BTC linkers.132

Under the similar reaction conditions of the synthesis of HKUST-1

from Cu2+

cations and BTC linkers, the CTAB molecules self-assembled into micelles that

directed the formation of the mesostructures. The walls of the mesostructures were

constructed from nanosized HKUST-1 domains. After the removal of the template by

solvent extraction, the mesopores with a diameter up to 5.6 nm were fabricated, resulting in

hierarchically micro- and mesoporous MOFs, in which the mesopores were interconnected

by the intrinsic micropores of HKUST-1 with the diameter of 8.6 Å. Furthermore, the

hydrophobic swelling agent 1,3,5-trimethylbenzene (TMB) was added to elongate the

mesopores via swelling CTAB micelles. The diameter of the mesopores can increase to 31

nm in the presence of TMB.

In the same synthetic system, Sun et al. used citric acid as chelating agent for establishing a

bridging interaction between copper ions and CTAB templates, which ensured the co-

assembly of the MOF precursors and the templates into mesostructures.136

The chelating

agents interacted simultaneously with the copper ions and the CTAB molecules through

Coulombic attraction and coordination.

2.2.3.1 Anionic templates

Long-chain carboxylic acid 4-(dodecyloxy)benzoic (DBA) was used for the preparation of

sponge and pomegranate MOF-5 with mesopores and macropores in the range of 10 100

nm (Figure 2.19).133

In a different mechanism from other surfactant templates, DBA served

a dual purpose of having a carboxylate group for binding to Zn2+

ions of the SBUs, and a

long alkyl chain for space filling. In the nucleation and crystal growth of MOF-5, DBA

molecules attached to the growing crystal using their carboxylate functionalities and

30

hampered the local crystal growth to make mesopores and macropores using the long alkyl

chains. DBA molecules were removed from the crystals by solvent exchange.

When large amounts of DBA were available, the mesoporous and macroporous system

permeated from the center to the surface of the crystals, producing spongeous MOF-5

(spng-MOF-5). With lesser amounts of DBA, the crystals were „starved‟ of DBA at the

midpoint during the crystal growth, giving pomegranate MOF-5 (pmg-MOF-5) with

spongeous core and solid outer shell.

Figure 2.19 Schematic mesopore and macropore structures of spng-MOF-5 and pmg-MOF-

5. Reproduced with the permission of American Chemical Society.133

2.2.4 Prospective applications of MOFs involving mesopores

In addition to the physicochemical properties involving organic linkers and metal-

containing SBUs similar to those of microporous MOFs, mesoporous MOFs offer extensive

features involving mesopores which are not exhibited by microporous MOFs. Combining

the advantages of MOFs with the benefits of the mesopore networks, mesoporous MOFs

can afford further applications such as in pharmaceutical field, enzymatic catalysis and the

synthesis of nanoparticles.

31

2.2.4.1 Large molecule encapsulation

Mesoporous MOFs with wide pore apertures allow large and complex substances such as

inorganic clusters, organic and biological molecules to access inside their pore systems.

Yaghi et al. demonstrated that the pore apertures of IRMOF-74-IV to -IX were large

enough for natural proteins and inorganic clusters to enter the mesopores, such as vitamin

B12 with the largest size dimension of 27 Å in IRMOF-74-IV, myoglobin (globular protein)

with spherical dimension of 21 35 44 Å in IRMOF-74-VII, GFP (barrel structure) with

diameter of 34 Å and length of 45 Å in IRMOF-74-IX, and MOP-18 (inorganic spherical

cluster) with diameter of 34 Å in IRMOF-74-V.126

In another research, Yaghi et al. also

indicated that the molecular dye rhodamine was preferably incorporated in the mesopores

and macropores of spng- and pmg-MOF-5 rather than the micropores of MOF-5 because

the diameter of rhodamine was much smaller than that of the meso- and macropores.133

Mesoporous MOFs have been employed as host matrix materials for heterogeneous

biocatalysis.137

Hierarchically porous MOFs contain mesopores as nanospaces to

accommodate biocatalysts and micropores for the selective diffusion of reactants and

products, resulting in shape or size-selective biocatalysis. Ma et al. reported the

encapsulation of proteins, such as microperoxidase-11 with molecular dimension of about

3.3 1.7 1.1 nm,138

cytochrome c with molecular dimension of about 2.6 × 3.2 × 3.3

nm,139

and myoglobin with molecular dimension of about 2.1 × 3.5 × 4.4 nm,140

in

hierarchically microporous and mesoporous Tb-mesoMOF. Since the pore apertures of Tb-

mesoMOF with diameters of 1.3 nm and 1.7 nm are smaller than the protein molecules, the

proteins must undergo a change in conformation that is initiated by the surface contacts

between the proteins and Tb-mesoMOF crystals to migrate through these small apertures

into the mesocages (3.9 nm and 4.7 nm in diameter).139

While microperoxidase-11

encapsulated in Tb-mesoMOF exhibited superior enzymatic catalysis toward the oxidation

of 3,5-di-t-butyl-catechol to o-quinone by hydrogen peroxide, myoglobin squeezed into Tb-

mesoMOF showed superior catalytic activities toward the size-selective oxidation of 2,2′-

azinobis(3-ethyl-benzthiazoline)-6-sulfonate and pyrogallol by H2O2.

32

The functionalized walls of mesoporous MOFs can generate specific connections with

complex organic molecules via post-synthetic modifications, leading to the possibility of

tethering desired molecules onto the pore walls. Lui et al. employed strain-promoted

“click” modification based on cyclooctyne derivatives for the incorporation of various

functional groups onto the pore walls of azide-functionalized mesoporous bio-MOF-100

(i.e., bio-MOF-100 built from 2-azidobiphenyldicarboxylic acids instead of BPDC

acids).141

The succinimidyl ester moieties of the functional groups allowed subsequent

bioconjugation of biomolecules onto the pore walls. Therefore, this bioconjugation strategy

can be used for tethering peptides, proteins (including enzymes), nucleic acids, polymers,

dyes and nanoparticles onto the internal surface of mesoporous MOFs.

2.2.4.2 Confined nanospace synthesis

The mesocages of MOFs can act as nanoreactors where various chemical reactions can

perform. A number of inorganic nanoparticles have been prepared in mesoporous MOFs

without the collapse of the MOF structures. The size of the nanoparticles is restricted by the

dimension of the mesocages. Because of the order arrangement of the mesocages, this

restriction produces calibrated monodispersed nanoparticles on the MOF matrices. These

impregnations can lead to significant changes in the textural properties of the MOFs such as

adsorption capacity and catalytic activity.

In 2010, Ferey et al. prepared fluorinated inorganic clusters, [(n-C4H9)4N]2[Mo6Br8F6], in

the mesocages of MIL-101 via a post-synthesis.142

The inclusion of the fluorinated clusters

in the mesocages improved hydrogen sorption of MIL-101 due to the interaction of

terminal fluorine atoms with hydrogen molecules. Aluminium-based MIL-100 was also

used as a host for synthesizing Pd nanoparticles with a size around 2.0 nm embedded within

the mesocages without degradation of the porous host.143

The H2 uptake in the composite

MIL-100/Pd was almost twice that of the pristine MIL-100 at room temperature.

Recently, ultrafine metallic Pt and bimetallic Au-Pd nanoparticles have been prepared in

the mesocages of MIL-101 materials.144

While the metallic Pt-immobilized MIL-101

33

nanocatalyst showed high catalytic activities in all three phases, including liquid-phase

ammonia borane hydrolysis, solid-phase ammonia borane thermal dehydrogenation and

gas-phase CO oxidation, the bimetallic Au-Pd-immobilized MOF nanocatalyst showed high

catalytic activity for the generation of hydrogen from formic acid. Ni nanoparticles with a

diameter up to 1.9 nm were also embedded in the mesocages of mesoMOF-1 by gas-phase

loading of nickelocene and subsequent reduction.145

These Ni-embedding mesoMOF-1

materials acted as catalysts for the hydrogenolysis of nitrobenzene to aniline or the

hydrogenation of styrene to ethylbenzene.

In addition to metallic and bimetallic nanocatalysts, polyoxometalates such as

polyoxotungstates [PW4O24]3-

and [PW12O40]3-

were fabricated into the mesocages of the

MIL-101 for catalyses.146,147

The hybrid polyoxotungstates/MIL-101 materials behaved as

true heterogeneous catalysts for H2O2-based alkene epoxidation.

2.3 Nanosized metal-organic frameworks

Various strategies have been developed to miniature the size of MOF crystals to the

nanometer scale, which offers a host of intriguing properties distinguishing from those of

the bulk counterparts. A number of MOF morphologies with zero-dimensional (e.g.,

nanocubes), one-dimensional (e.g., nanowires, nanorods), two-dimensional (e.g.,

nanoplates, nanosheets) and anisotopic nanostructures have been fabricated. In general,

bottom-up technique using atomic or molecular precursors has been used extensively

because the morphology of MOF nanocrystals can be precisely controlled. In this

technique, capping reagents, stabilizing reagents, microemulsions and the variations in

synthetic parameters have been employed for the productions of nanosized MOFs.

Recently, top-down technique using bulk MOF precursors has been adopted to prepare

nanosized MOFs.

34

2.3.1 Coordination modulation

Coordination modulation approach uses capping reagents (termed modulators) for

inhibiting the growth of MOF nanoparticles by coordinatively bonding to the surface-

exposed metal sites (Figure 2.20).148

The modulator has a single functional group similar to

that of the multitopic organic linkers. During the nucleation and crystal growth of MOFs,

the modulators compete with the organic linkers in order to coordinate with the metal sites.

The competition impedes the coordination interaction between the metal ions and the

linkers which favors the formation of MOF nanocrystals. Furthermore, the selective

coordination modulation can change the rate of framework extensions in different

directions, resulting in controllable morphologies of MOF nanocrystals.

Figure 2.20 Coordination modulation approach toward MOF nanocrystals with different

morphologies. Reproduced with the permission of Wiley InterScience.148

Monocarboxylic acids and their salts are commonly used as modulators for fabricating

carboxylate-based MOF nanocrystals. The possibility to terminate the crystal growth of

MOF nanoparticles by coordination modulation was first demonstrated by Fischer et al., in

which p-perfluoroethylbenzoic acid (pfmbc) was used as modulator for yielding MOF-5

nanoparticles.149

In the growth solution of MOF-5, pfmbc modulator competed with BDC

linker for coordination to the vacant edges of surface-exposed Zn4O sites of MOF-5

35

colloids, yielding stable MOF-5 nanocrystals. The size of MOF-5 nanocrystals was

changed in the range of 100 200 nm by adjusting the ratio of BDC and pfmbc.

Kitagawa et al. showed that the selective coordination modulation in one of coordination

modes constructing MOF structure led to anisotropic growth of the MOF nanocrystals.148

In

three-dimensional pillared-layered [Cu2(ndc)2(dabco)n] structure as a model, the ndc linkers

bind to dicopper square paddle wheel SBUs to form two-dimensional square lattices that

are connected by pillar dabco linkers at the lattice points in the [001] direction. This

assembly yields anisotropic framework dominated by two coordination modes, including

copper-carboxylate and copper-amine. This allows the selective modulation of the copper-

carboxylate modes by monocarboxylic acid. In the presence of acetic acid as modulator, the

coordination interaction between copper and ndc was impeded because both ndc and acetic

acid had the same carboxylic functionality, while the coordination of copper with dabco

was not affected. Therefore, the selective modulation enhanced the relative crystal growth

in the [001] direction, leading to the formation of the square nanorods. The length and

thickness of the nanorods were adjusted by acetic acid concentration, in which the length

corresponding to the copper-amine coordination directions increased whereas the width

corresponding to the copper-carboxylate coordination directions decreased with an increase

in the concentration of acetic acid.

Recently, Kitagawa et al. have also used acetic acid as modulator for preparing the

nanoplates of twofold interpenetrated [Cu2(bdc)2(bpy)]n MOF.150

The interpenetrated

structure is composed of two identical but distinct frameworks that base on dicopper square

paddle wheel SBUs interconnected with bdc in the equatorial plane and bpy in the apical

positions. Each distinct framework is similar to [Cu2(ndc)2(dabco)n] framework in which

bdc and bpy linkers are used in lieu of ndc and dabco, respectively. [Cu2(bdc)2(bpy)]n

microcrystals were synthesized by first forming 2D square grids of [Cu2(bdc)2(solvent)2]n

from the reaction of copper ions and bdc acids. The coordination of bpy linkers to the

dicopper SBUs of the square grids via the exchange reaction with the solvent molecules

formed the bulk 3D framework. By adding acetic acid to the mixture of copper acetate and

bdc in the first step of the synthesis, the square-like nanoplates were yielded. The width and

36

thickness of the nanoplates increased with an increase in the modulator concentration.

Compared with the synthesis of [Cu2(ndc)2(dabco)n] nanorods, the modulator here had the

opposite effect on the width corresponding to the copper-carboxylate coordination

directions.

Monocarboxylate salts were used as modulators for synthesizing nanosized MOFs built

from lanthanide cations (Ln = Dy3+

, Eu3+

, or Tb3+

) and BTC acid.151

In the absence of the

modulators, [Ln(BTC)(H2O)] MOFs had rod-shaped microcrystals with a length of 60 μm.

While the addition of sodium formate as modulator resulted in fairly uniform bean-shaped

nanocrystals with a length of 125 nm and a width of 100 nm, the presence of sodium

acetate yielded smaller nanocrystals with a length of 90 nm and a width of 75 nm.

2.3.2 Stabilizing reagent

Stabilizing reagents suppress the crystal growth of MOF nanoparticles by adsorbing on the

growing facets. The adsorption slows down the crystal growth rate of all the growing

facets, which facilitates systematically the formation of the nanocrystals. The deceleration

of the crystal growth rate depends on the adsorbance of the growing facets. Therefore, the

relative crystal growth rates can be manipulated by using variable amounts of stabilizing

reagents, resulting in the different morphologies of MOF nanocrystals. The appropriate

stabilizing reagents are usually surfactants. In contrast to modulators, the stabilizing

reagents can be removed from the surface of the resulted MOF nanocrystals.

Ma et al. synthesized MOF-5 nanocrystals by using a multi-step solvothemal process in the

presence of CTAB surfactant as stabilizing reagent.152

The surfactant was added to stabilize

the incubated mother solution. The presence of CTAB thus slowed significantly the

subsequent nucleation. After the nucleation, amines were added to accelerate the

deprotonation of the linkers, resulting in the rapid growth of MOF-5 nanocrystals. In the

incubated solution without CTAB, the fast uncontrolled-nucleation and crystal growth

resulted in the microcrystals rather than the nanocrystals. Furthermore, the combination of

CTAB with different amines (triethylamine, piperidine and decylamine) effected the

37

growth along different crystallographic axes, thereby allowing the formation of anisotropic

nanoparticles. While decylamine with a long carbon chain induced the nanorods,

triethylamine and piperidine induced the octahedral nanocrystals. After the synthesis, the

CTAB surfactant was removed cleanly from the surface of MOF-5 nanocrystals by

washing.

Xu et al. also demonstrated that the addition of different amounts of CTAB controlled the

morphology of Eu2(FMA)2(OX)(H2O)44H2O nanocrystals (FMA = fumarate, OX =

oxalate).153

They used the Bravais–Friedel–Donnay–Harker method for simulating the

crystal growth and morphology control. In the absence of CTAB, the large rhombus

truncated bipyramid microcrystals were fabricated. The gradual increase in CTAB amount

in microemulsion system prepared the elongated hexagonal nanoplates and hexagonal

nanoplates.

Figure 2.21 The morphologies of Eu2(FMA)2(OX)(H2O)44H2O MOF simulated by

modulating the growth rates of different growing facets. Reproduced with the permission of

The Royal Society of Chemistry.153

38

The crystal structure of Eu2(FMA)2(OX)(H2O)44H2O MOF has three possible growing

facets {022}, {004} and {111} in which the growth rate Rhkl is in the order R{022} > R{004} >

R{111} (Figure 2.21). In the absence of CTAB, the faster growing facet {022} tends to

disappear, so the crystal morphology is dominated by {004} and {111} facets, leading to

the formation of the large rhombus truncated bipyramid crystals. The addition of CTAB

makes high surface energy facets {022} and {004} more thermodynamically favorable by

preferential adsorption. Therefore, the crystal growth rate of {111} facet is faster than those

of {022} and {004} facets, resulting in the elongated hexagonal nanoplates. When more

CTAB is present, the crystal growth rate of {111} facets is also significantly prohibited, so

R{111} R{022} > R{004}, leading to the formation of the hexagonal nanoplates.

The similar effect of CTAB surfactant on the morphology of nanoscale MOFs was

observed for HKUST-1.154

Since the growth rates of the growing facets were determined by

the amount of CTAB, the morphological evolution of HKUST-1 nanoparticles from cube,

to truncated cube, cuboctahedron, truncated octahedron, and to octahedron was occurred

with an increase in the CTAB concentration. Furthermore, the presence of CTAB that

slowed the nucleation rate led to an increase in the size of HKUST-1 crystals.

2.3.3 Microemulsion

Microemulsion is thermodynamically stable mixture of hydrophobic liquid, water and

surfactant, frequently in the presence of co-surfactant such as short chain alcohols. In case

water disperses in hydrophobic liquid, the mixture is called reverse microemulsion. In the

reverse microemulsion, surfactant-stabilized aqueous nanodroplets are dispersed in

hydrophobic organic phase. Such aqueous nanodroplets can act as nanoreactors for

synthesizing MOF nanoparticles. In these synthetic processes, the precursors of MOFs are

dissolved in the nanodroplets and the subsequent crystallization of MOF nanoparticles is

induced by the collision between the nanodroplets or external stimuli. The size of the

resulted MOF nanoparticles can be controlled via the control over the size of the

nanodroplets by varying the molar ratio of water to surfactant (denoted as W).

39

The first example of the syntheses of nanosized MOFs in reverse microemulsions was

reported in 2006.155

The crystalline Gd(BDC)1.5(H2O)2 nanorods and Gd(1,2,4-BTC)(H2O)3

nanoplates were prepared at room temperature in CTAB/1-hexanol/isooctane/water reverse

microemulsions. The research demonstrated that the morphologies and sizes of the MOF

nanoparticles were affected by the W value. Indeed, Gd(BDC)1.5(H2O)2 nanorods of 100

125 nm in length by 40 nm in width were obtained with W = 5 while the larger rods of 1

2 m in length and around 100 nm in width were obtained with W = 10.

The reverse microemulsion synthesis of nanosized MOFs at room temperature can generate

amorphous particles due to the rapid coordination between metal ions and organic linkers.

In these cases, the elevated temperatures favour the crystal growth of the crystalline MOF

nanoparticles. Liu et al. indicated that the synthesis of nanosized MOFs from Gd and BHC

at room temperature using reverse microemulsion only produced amorphous materials.156

Therefore, the reverse microemulsion synthesis at high temperature was used for preparing

crystalline Gd2(BHC)(H2O)6 nanoparticles. The block-like nanocrystals with dimensions of

25 50 100 nm were prepared in CTAB/1-hexanol/n-heptane/water microemulsion (W =

10) after heating at 120 °C for 18 h.

By using the same CTAB/1-hexanol/n-heptane/water microemulsion, the rod-like

Mn(BDC)(H2O)2 nanoparticles with diameter of 50 100 nm and length of 750 nm to

several µm were obtained either at room temperature or under microware heating at 120

°C.157

The crystalline Mn3(BTC)2(H2O)6 nanoparticles were also fabricated in the similar

reverse microemulsion by using isooctane as continuous hydrophobic phase instead of n-

heptane. The results indicated that the synthetic temperature impacted on the morphology

of the Mn3(BTC)2(H2O)6 nanocrystals. While the spiral rod Mn3(BTC)2(H2O)6

nanoparticles with diameter of 50 100 nm and length of 1 2 µm were obtained at room

temperature, the block-like nanocrystals with much lower aspect ratio and the length in the

range of 50 300 nm were fabricated at 120 °C under microware heating.

40

Figure 2.22 Model for the formation and growth of [Zn(ip)(bpy)]n nanorods in AOT/n-

heptane/DMF reverse microemulsion. Reproduced with the permission of Nature

Publishing Group.158

Because some organic linkers do not dissolve in aqueous phase, non-aqueous reverse

microemulsions in which water is replaced by a polar organic solvent such as N,N-

dimethylformamide (DMF) have been employed to synthesize MOF nanoparticles.

Kitagawa et al. used non-aqueous AOT/n-heptane/DMF reverse microemulsion for the

preparation of [Zn(ip)(bpy)]n nanocrystals at ambient temperature (AOT = dioctyl

sulphosuccinate sodium).158

Ultrasonic irradiation was required to induce the

thermodynamically controlled crystallization of the MOF nanoparticles. The formation of

the nanocrystals was accomplished within a few minutes. The crystalline nanorods with

dimensions of 300 50 15 nm were obtained at W = 1. The increase in W value led to the

larger nanorods. In contrast, the gel-like amorphous particles precipitated immediately in

the AOT/n-heptane/DMF microemulsion system without sonication. The crystal growth of

the nanorods was explained by the reversible formation of metallinker bonds under high-

energy irradiation in combination with constant merging of the nanodroplets (Figure 2.22).

During the crystal growth, the particle size extended rapidly the nanodroplet size of the

initial microemulsion, leading to an aggregation of the growing nanocrystals and the

41

surface coordination of AOT. Furthermore, the surface coordination of AOT also limited

the diffusion of the metal ions and organic linkers to the crystal surface which limited the

particle growth.

2.3.4 Synthetic parameters

The synthetic parameters including compositional parameters (metal source, reactant

concentration, pH, solvent and molar ratio of reactants) and process parameters (time,

temperature and heating source) can be adjusted to achieve MOF nanocrystals. However,

there isn‟t general trend in variation of the synthetic parameters to generate MOF

nanocrystals except microwave- and ultrasound-assisted syntheses. Therefore, the syntheses

of nanosized MOFs by varying the synthetic parameters have to be fine-tuned for each

system in a trial-and-error approach.

The syntheses under high-energy irradiation of microwave or ultrasound allow the

fabrication of MOF nanocrystals in a short period of time.159

In microwave-assisted

synthesis, the high dielectric absorptivity of polar solvents results in a thermal conversion

of high-energy microwave and thus efficient heating of reaction solution. The rapid and

local heating leads to a fast and homogeneous nucleation. In case of ultrasonic-assisted

synthesis, the cavitational collapse of the vacuum bubbles produced in the sonochemical

reaction leads to very fast and intense local heating and high pressure, which allow both a

rapid synthesis and a tuning of the kinetics of the reaction. The approaches are facile, rapid

and environmentally friendly, but it is not easy to control the morphology and the size of

the resulted nanocrystals.

Several MOF nanocrystals have been prepared by using microwave-assisted synthesis.

MIL-101_NH2 nanocrystals were prepared under microwave irradiation from an aqueous

solution of 2-aminoterephthalic acid and FeCl36H2O at 60 °C for 5 min.160

The resulted

nanoparticles had an octahedral shape with diameter of 173 ± 60 nm. MIL-100 nanocrystals

were similarly prepared from an aqueous mixture of iron powder, BTC and HF acid at 200

°C for 30 min. The resulted MIL-100 particles appeared as an aggregation of the smaller

42

spherical nanoparticles of 59 ± 46 nm rather than the individual nanocrystals. Through the

systematic investigation into reaction conditions toward MIL-88A nanoparticles, Chalati et

al. showed that hydrothermal microwave-assisted synthesis was a convenient and rapid

route to obtain monodispersed MIL-88A nanoparticles with controllable size smaller than

100 nm.159

Ultrasonic-assisted method was applied to the synthesis of various nanosized MOFs such as

Zn3(BTC)212H2O,86

MOF-2,85

[Tb(btc)(H2O)6]n,83

and HKUST-1.161

The size and shape of

the Zn3(BTC)212H2O nanocrystals were strongly influenced by the irradiation time. The

short period of reaction time in a few minutes produced the sphere-like nanoparticles of 50

– 100 nm in size. The irradiation up to 30 min prepared the wire-like nanoparticles with

diameter of 100 – 200 nm and length of up to 100 mm. Further increase in reaction time

increased the diameter of the wire-like crystals to 700 – 900 nm. The nanoparticles had a

trend of aggregation rather than isolated nanocrystals. Nanobelts and nanosheets of MOF-2

were prepared under ultrasonic irradiation at ambient temperature. The nanobelts with a

width of 150 – 300 nm and a length of 2 – 5 m were obtained after 10 min irradiation. The

nanosheets were formed with the reaction time up to 20 min. Further increase of the

reaction time led to an increase in dimension of the nanosheets. Nanowires of

[Tb(btc)(H2O)6]n with an average diameter of 50 nm and length of up to a few micrometers

were produced under ultrasound irradiation at 70 °C and atmospheric pressure. HKUST-1

nanocrystals were fabricated by ultrasonic-assisted synthesis at ambient temperature and

atmospheric pressure for short reaction times. The resulted HKUST-1 nanocrystals had a

very wide range of particle size distribution.

The reaction temperature affects on the rate of nucleation and crystal growth, resulting in

an impact on the shape and size of MOF crystals. MIL-88A with several crystal sizes was

synthesized under hydrothermal conditions by varying the reaction temperature in the range

of 65 150 °C.159

The bar-like nanocrystals with a length of 250 ± 55 nm were obtained at

65 °C. The increase in the reaction temperature led to an increase in the crystal size. The

synthesis at 150 °C produced the submicro-crystals of MIL-88A with a length of above 1.2

m.

43

The synthetic processess using temperature programs were applied to obtain MOF

nanoparticles from supersaturated crystallization solutions. The temperature program

usually has three steps including the incubation of mother solution, the nucleation from the

incubated solution at high temperature and the crystal growth at lower temperature. The

nanocrystals of MOF-5, IRMOF-3, and HKUST-1 were synthesized by using temperature

programs. The homogeneous MOF-5, IRMOF-3 nanocrystals were generated by the

controlled nucleations and crystal growths of these MOFs in the presence of CTAB.152

In

contrast, the HKUST-1 particles up to 200 nm in size accompanying the forming nuclei

were observed within a 10 min period of the crystal growth because the nucleation of

HKUST-1 overlapped with their crystal growth.162

Metal source affects significantly on the formation of MOF nanoparticles. Horcajada et al.

investigated three iron salts including iron(III) nitrate, iron(III) chloride and iron(III)

acetate for the synthesis of iron-based MIL-89 nanoparticle in ethanol solution.163

While

the precipitate of iron muconate MIL-89 appeared immediately in case of the nitrate and

chloride salts, the use of iron acetate led to the formation of MIL-89 nanoparticles. It was

assumed that iron acetate clusters contained metal-coordinated acetate moieties that acted

as modulators suppress the extended crystallization for the formation of MIL-89

nanocrystals.

The concentration of reactants has significant effect on the size of MOF crystals. Khan et

al. showed that the size of MIL-101 crystals decreased from submicro- to nano-scale with a

dilution of the aqueous reaction mixture by water.164

The decrease in MIL-101 crystal size

in the diluted reaction mixtures was explained by a great reduction of the crystal growth

rate compared with small reduction of the nucleation rate. Similarly, the addition of water

in the reaction mixture of Zr-based MOFs led to a decrease in the nanocrystal size.165

The

dilution of the stable mother liquor of HKUST-1 with a counter solvent induced the

formation of the nanocrystals.166

The stable mother liquor of HKUST-1 was first obtained

by mixing copper acetate, BTC and a large amount of dodecanoic acid as modulator in

butanol. The cubic HKUST-1 submicro-crystals with a size of 614 11 nm were obtained

from the concentrated mother liquor (concentration of BTC, c = 31.2 mM, and dilution

44

factor, ethanol/mother liquor, d = 1). In contrast, the highly diluted solution (c = 7.9 mM,

and d = 8) produced the smaller nanocrystals of 138 1 nm in size. The formation of the

smaller HKUST-1 nanocrystals in the diluted solution was attributed to the fast nucleation

that contrasted the slow nucleation in the concentrated solution.

The investigation of Khan et al. illustrated that high pH values were helpful to decrease the

size of MIL-101 nanocrystals.164

It was assumed that the high pH accelerated the

deprotonation of BDC acid into benzenedicarboxylate, resulting in high nuclei

concentration that led to small MIL-101 nanocrystals. Similarly, the addition of ammonium

hydroxide to vary pH value was used for the control of the crystal size of zirconium-based

UiO-66 nanoparticles (UiO standing for University of Oslo).167

The increase in amount of

NH4OH reduced the crystal size of the Zr-MOFs. The formation of zirconium hydroxide or

oxide because of the addition of NH4OH was also attributed to the formation of the small

UiO-66 nanocrystals. However, the change of the pH value of reaction medium can lead to

the formation of different MOF structures from identical precursors.156

Bataille et al. recently demonstrated the effect of solvents on the size of crystalline 1D

tubular MOF built from Cu2+

ions, 1,2-bis(4-pyridyl)ethane and p,p-diphenyl-

diphosphinate.168

Under mild conditions, the elongated nanorods with a well-defined range

of lengths and cross sections were formed in ethanol, whereas the needle microcrystals

were crystallized in water. In another research, the use of water or methanol for the

synthesis of MIL-88A nanocrystals led to the larger nanoparticles when compared to the

use of DMF.159

This was related to the higher solubility of fumaric acid (the organic linker

of MIL-88A) in DMF rather than in methanol and water as well as the higher dipolar

moment of DMF.

2.3.5 Top-down approach

Li et al. used a top-down delamination for fabricating crystalline MOF nanosheets from the

bulk crystals of layered MOF-2 [Zn2(BDC)4(H2O)22DMF]n.169

The structure of MOF-2 is

constructed from zinc square paddle-wheel SBUs and BDC acids. The connection between

45

the zinc square paddle-wheel SBUs and BDC acids creates 2D layers that are held together

by hydrogen-bonding interactions to form MOF-2 framework. The breakage of these

hydrogen bonds via an exfoliation resulted in the crystalline MOF nanosheets with

thickness ranging from a single layer to multi-layers. Under ultrasonic treatment in acetone

at room temperature, the resulted nanosheets had a thickness of 1.5 6.0 nm,

corresponding to the thickness of two layer to several layers of MOF-2.

2.3.6 Potential application of MOFs involving nanosize

2.3.6.1 Adsorption and heterogeneous catalysis

The reduction of MOF crystal size to the nanometer scale results in a dramatic decrease in

diffusion length and increase in accessible active sites as compared to the bulk counterparts.

These lead to improving adsorption as well as more efficient catalytic performance of MOF

nanoparticles.

Figure 2.23 Schematic illustration of the structural transformation of flexible

[Cu2(bdc)2(bpy)]n PCP. Top: Nonporous closed and guest-included open phases. Bottom:

Novel open empty phase was observed with the crystal downsizing. Reproduced with the

permission of Science.150

46

Groll et al. demonstrated that the methanol adsorption kinetics of flexible [Zn(ip)(bpy)]n

MOF accelerated dramatically with the crystal downsizing.158

The shape of the sorption

isotherm of [Zn(ip)(bpy)]n MOF varied considerably from the bulk to the nanocrystal

though the overall adsorption capacities were almost identical. It is known that flexible

PCPs change their structures in response to molecular incorporations but recover their

original configurations after the incorporated guests are removed.150

Therefore, the crystal

downsizing can suppress the structural mobility of the flexible PCPs. Kitagawa et al.

illustrated that the crystal downsizing of twofold interpenetrated structure of

[Cu2(bdc)2(bpy)]n MOF induced a novel structural flexibility (Figure 2.23).150

In addition to

two intrinsic phases that were generated by sorption process (i.e., a nonporous closed phase

and a guest-included open phase), a new open empty phase that was not observed in case of

the bulk counterpart was isolated when downsizing the crystals to the nanoscale. The

induction of such molecular-scale shape-memory effect could be exploited as intelligent

functional materials that respond to the microscopic environmental changes.

Table 2.2 Catalytic activity of the bulk microcrystals and the nanocrystals of HKUST-1 in

the oxidative dehydrogenation of dibenzylamine to dibenzylimine.170

Catalyst Conversion (%) Yield (%) (c)

TOF (h−1

) (d)

bulk 17 15 0.8

nanocrystal-A (a)

27 24 1.3

nanocrystal-B (b)

53 41 2.5

a Prepared in the presence of polymer poly(acrylic acid sodium salt).

b Prepared in the presence of both polymer poly(acrylic acid sodium salt) and CTAB.

c The other product is benzaldehyde.

d TOF was calculated by dividing the molar amounts of dibenzylamine converted in 3 h by the molar amounts

of Cu used.

47

The catalytic activity of porous MOFs depends on the particle size that determines the

concentration of accessible catalytic sites and the diffusion distance in the pore system.

Jiang et al. demonstrated that the downsizing of HKUST-1 crystals from the micrometer to

nanometer scale enhanced greatly the catalytic performance in the oxidation of

dibenzylamine to dibenzylimine.170

HKUST-1 catalysts with a crystal size less than 200 nm

had higher oxidation activity up to three times rather than the bulk of 10 – 20 μm in the

crystal size (Table 2.2).

2.3.6.2 Hierarchical structure assembly

The fabrications of MOF films by assembly of preformed MOF nanocrystals have been

reported. The resulted films have a hierarchically porous structure consisting of micropores

of MOF nanocrystals and mesopores formed by inter-nanoparticles voids. Such

hierarchically porous structure facilitates the diffusion and permeation of guest molecules

through MOF films. Therefore, the MOF films can be employed as smart separation

membranes, chemical sensors or nanodevices.

Ferey et al. prepared MOF thin films from preformed MIL-101 nanoparticles by using a

dip-coating method.171

The homogenous colloidal MIL-101 nanocrystals with an average

diameter of 22 nm were first produced by using a microwave-assisted synthesis. The

nanoparticles in a colloidal solution were then deposited on bare silicon wafer. The

thickness of the film depended on the concentration of the suspension and was controlled

by the number of dip-coating cycles. The thin films could be used as sensors for vapors due

to the selective adsorption property of MIL-101.

The flexible MIL-89 nanoparticles were also used for the assembly of MOF thin film by

using the similar dip-coating method.163

The homogeneous colloidal MIL-89 nanoparticles

with a size varied from 20 to 40 nm were synthesized in ethanol media with the presence of

acetate moieties as modulator. The thin films were then prepared via the deposition of the

nanoparticles on silicon wafers. Each dip-coating process deposited a single layer of the

48

colloid on the substrate. Therefore, the film thickness could be increased and controlled

through repetition of the dip-coating process.

Recently, the crack-free films of amine-functionalized MIL-101 have been fabricated by

dip-coating of the nanoparticles in the presence of polyethylenimine (PEI).172

The

suspensions of MIL-101NH2 nanoparticles were first fabricated in ethanol containing PEI.

The films were then prepared by dip-coating Anodiscs (anopore alumina) in the suspension

a number of times. The presence of PEI in the suspension enhanced the interaction of MIL-

101NH2 nanoparticles with the surface of Anodisc through hydrogen bonding, which

helped the nanoparticles to be homogeneously adsorbed on the surface during the dip-

coating. In addition, the PEI molecules on the MIL-101NH2 nanoparticles reduced and

homogenized the stress forces between the nanoparticles, which contributed to the

formation of the crack-free films. The films exhibited high selectivity for CO2 over N2 and

high CO2 capacity, which offered an efficient separation of CO2 from N2.

Glass-supported Eu1-xTbxMOF films were fabricated from the suspension of the pre-

synthesized MOF nanocrystals by using a spin-coating method.151

Eu1-xTbxMOF

nanocrystals were synthesized by coordination modulation approach using

monocarboxylate salts as modulators. The film thickness was controlled by the nanocrystal

concentration in the suspension and the spin-coating rate. The films had strong

luminescence property and efficient Tb3+toEu

3+ energy transferability. Therefore, they

were potential candidates for applications in the field of color displays, luminescence

sensors and structural probes.

The preparation of porous MOF thin films on the inner walls of capillary columns for GC

separations has been reported. Gu and Yan fabricated MIL-101 coated capillary silica

column for the high-resolution GC separation of xylene isomers and ethylbenzene by using

a dynamic coating method (Figure 2.24).173

The suspension of MIL-101 nanocrystals was

first filled into the capillary column under gas pressure, and then pushed through the

column to leave a wet coating layer on the inner wall. The capillary column was further

treated using a temperature program before the GC separation experiment. By using the

49

same dynamic coating process, Yan et al. have recently fabricated IRMOF coated-capillary

columns from IRMOF-1, IRMOF-3 nanocrystals for the high-resolution GC separation of

persistent organic pollutants.174

Figure 2.24 The thin film of MIL-101 on the inner wall of capillary silica column and the

GC separation of xylene isomers and ethylbenzene of the coated capillary column.

Reproduced with the permission of Wiley InterScience.173

2.3.6.3 Biomedical application

MOFs suitable for biomedical applications must have biologically friendly compositions

and be stable under biological conditions.87

The biocompatible metal cations are Ca, Mg,

Zn, Fe, Ti, or Zr whose oral lethal doses 50 (LD50) range from a few μg/kg up to more than

1 g/kg (calcium). Meanwhile, the common linkers are exogenous compounds such as

polycarboxylates which do not intervene in the body cycles. The incorporation of

functional groups on these linkers can modulate the host-guest interactions, allowing a

better control of drug release. The other linkers are endogenous compounds such as gallic,

fumaric and muconic acids. The endogenous linkers might be used in the body, which

would strongly decrease the risk of adverse effects. The imaging property of MOFs as

contrast agents in magnetic resonance imaging, optical imaging or X-ray computed

tomography allows following both detection of the drug-loaded nanoparticles and

efficiency of a given therapy. The prerequisite for medical applications is the preparation of

homogeneous and monodispersed MOF nanoparticles because some administration routes

such as systemic circulation require precise nanoscale sizes.

c)

50

Variety of nanoscale MOFs have been tested as drug-delivery nanocarriers and contrast

agents.175

Among them, the non-toxic iron-carboxylate MOFs such as MIL-53, MIL-88A,

MIL-88B, MIL-89, MIL-100 and MIL-101NH2 are shown as potential candidates for

these purposes.160

In addition to the advantage of high pore volume and large surface area

for high drug loading capacity and efficient delivery of drugs in the body, these MOF

nanoparticles are synthesized in biologically favourable aqueous or ethanol media with

controlled particle sizes. The surface of the nanocrystals can be engineered to achieve

suitable stability, bio-distribution and targeting abilities (Figure 2.25). Furthermore, the

paramagnetic iron atoms, free and coordinated water molecules in the networks of these

MOF nanoparticles allow them be effective contrast agents.

Figure 2.25 Scheme of engineered core–corona iron carboxylate MOFs for drug delivery

and imaging. Reproduced with the permission of Nature Publishing Group.160

51

Nanosized Gd- and Mn-based MOFs have also been reported as potential contrast agents in

magnetic resonance imaging.176

The paramagnetic metal ions enhance image contrast by

increasing the rate of water proton relaxation when a magnetic field is applied. Moreover,

the nanosized MOFs can be doped with emissive lanthanide ions to afford optical property

for imaging applications.

2.3.6.4 Templates

The use of nanosized MOFs as templates for the synthesis of core-shell nanostructures was

reported. The metal oxide shells such as silica and titania were coated on several nanosized

MOFs. Lin et al. prepared silica shells with variable thickness on Ln(BDC)1.5(H2O)2

nanocrystals (Ln = Eu3+

, Gd3+

, Tb3+

) by using sol-gel procedure.176

The nanocrystals were

first functionalized with polyvinylpyrollidone to reduce particle aggregation in solution,

followed by a treatment with tetraethylorthosilicate (TEOS) in an ammonia/ethanol

mixture. The shell thickness was tuned by varying the reaction time or the amount of

TEOS. The MOF core and silica shell nanostructures were functionalized with medical

agents for biomedical applications.177

The MOF core could be removed by dissolving at

low pH to fabricate hollow silica nanoparticles. Recently, amorphous titania shell has been

deposited on MIL-101 nanocrystals by using acid-catalyzed hydrolysis and condensation of

titanium(IV) bis(ammonium lactato)dihydroxide in water at room temperature.178

The

thickness of the titania shell could be varied by variation of the concentration of HCl acid

as catalyst for the hydrolysis and condensation or by the reaction time. The subsequent

calcination of this structure generated a composite of mixed metal oxides as a catalyst.

2.4 Photocatalytic water splitting

Water splitting to generate H2 and O2 using solar energy in the presence of semiconductor

photocatalysts has revealed a potential means of clean, renewable fuel production.179-181

Photocatalytic semiconductor has a band structure in which the conduction band (CB) is

separated from the valence band (VB) by a band gap. When a semiconductor photocatalyst

is illuminated by light with energy equal to or larger than that of the band gap, the electrons

52

are excited from the VB into the CB, leaving positive holes in the VB. The photogenerated

electrons and holes can participate in redox reactions.

Figure 2.26 Principle of water splitting using semiconductor photocatalysts

Figure 2.27 Relationship between the band structure of semiconductors and the redox

potential of water splitting.182,183

In water splitting, water molecules are reduced by photogenerated electrons to generate H2

and are oxidized by photogenerated holes to produce O2 (Figure 2.26). To induce water

splitting, the bottom level of the CB must be more negative than the redox potential of

H+/H2 (0.0 V vs. NHE at pH = 0) while the top level of the VB must be more positive than

h

+

CB

VB

0.0 V

+1.23 V

H2O

H2

H2O

O2

Band gap

Po

ten

tia

l v

s. N

HE

(p

H 0

)0

g-C3N42.6

6eV

53

the redox potential of O2/H2O (+1.23 V vs. NHE at pH = 0).182

In principle, various

semiconductor materials have the levels of CB and VB suitable for water splitting (Figure

2.27). However, the band gap should be narrower than 3.0 eV for the visible light-driven

photocatalysis ( > 415 nm).

The surface separation of the photogenerated electrons and holes is required to achieve

efficient water splitting. The downsizing of the particle size of the photocatalysts shortens

migration of the electrons and holes toward surface active sites, resulting in a significant

decrease in the recombination probability. However, the small nanoparticle size can

generate a quantum size effect that widens the band gap.184

The recombination between

photogenerated electrons and holes is dominant if the photocatalytic surface does not have

active sites for the redox reactions. Therefore, nanoparticulate cocatalysts such as Pt, Rh,

NiOx and RuO2 for H2 evolution and Co3O4, Mn3O4 for O2 evolution are loaded to form

active sites on the surface of photocatalysts.179,185

In addition, the physical separation

between the distinct active sites for the reduction and oxidation reactions enhances the

water splitting activity.186

The backward reaction forming water from evolved H2 and O2 is one of the reasons leading

to inefficient overall water splitting. Therefore, sacrificial reagents (hole or electron

scavengers) are usually used for half reactions of water splitting to produce either H2 or

O2.187

In the half reaction for H2 evolution, photogenerated holes in the VB oxidize hole

scavenger such as alcohols, S2-

and 2-

3SO ions instead of water. This enriches

photogenerated electrons in the CB, and thus H2 evolution reaction is enhanced.182

In

contrast, photogenerated electrons in the CB are consumed by electron scavenger such as

Ag+ and Fe

3+ ions in the half reaction for O2 evolution, resulting in an enhancement of O2

evolution reaction.

Metal oxide, sulfide, nitride and graphitic carbon nitride (g-C3N4) photocatalysts have been

developed for water splitting.182,183,188,189

For example, CdS with suitable band structure is

an excellent photocatalyst for H2 evolution under visible light in the presence of hole

scavengers.190-192

However, the photocorrosion in which S2-

in CdS rather than H2O is

54

oxidized by photogenerated holes makes CdS inactive for overall water splitting and

decreases the stability of the photocatalyst.193

On the other hand, WO3 catalyzes efficiently

the half reaction for O2 evolution under visible light in the presence of electron scavengers,

but is inactive for H2 evolution due to the CB level more positive rather than the redox

potential of H+/H2.

194,195 g-C3N4 and its modified derivatives also exhibit photocatalytic

activity for water reduction into H2 or water oxidation into O2 in the presence of a proper

hole or electron scavenger under visible light irradiation.183,196,197

Among the semiconductor photocatalysts, titania material has revealed as the most

promising candidate because of inexpensive, non-toxic and robust photocatalyst under

photochemical conditions. However, the wide band gap (3.0 eV for the rutile phase and 3.2

eV for the anatase phase) makes pristine TiO2 only active under UV light that is only a

small fraction of the solar light spectrum. Therefore, the modifications are required to

induce the photon absorbance within the visible light region.184,198,199

These processes

include modifications of chemical composition such as doping with transition metal ions

(e.g., V, Cr, Mn, Fe, Ni) or with nonmetallic elements (e.g., N, S, C, B, F, Cl, Br); and

surface chemical modifications such as sensitizing with organic dyes,200

metal

nanoparticles,201,202

or narrow band gap semiconductors.203,204

In general, the substitution of

transition metals for the Ti sites (metal-doping) seems to be easy while the replacement of

nonmetallic elements for the O sites (nonmetal-doping) is more difficult due to differences

in charge states and ionic radii. The small size of the nanoparticle is beneficial for the

doping. Moreover, the activity of TiO2 materials depends on their primary particle size and

shape.205

Therefore, the progress of TiO2-based photocatalysts still attracts enormous

researches.

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65

Chapter 3 Characterizations

3.1 Introduction

The aim of this thesis is to develop synthetic methods for preparing bimodal micro-/meso-

porous MOF nanocrystals, uniform nanosized MOFs with controlled shape and size and

subsequently, the extensive application of the prepared nanosized MOFs. To accomplish

this work, X-ray diffraction (XRD); electron microscopy techniques including scanning

electron microscopy (SEM), transmission electron microscopy (TEM) and the

complementary energy dispersive X-ray spectroscopy (EDS); X-ray photoelectron

spectroscopy (XPS); Fourier transform infrared spectroscopy (FTIR); ultraviolet-visible

spectroscopy (Uv-vis); ζ-potential analysis; thermal analysis including thermogravimetric

analysis (TGA) and differential thermal analysis (DTA); atomic absorption spectroscopy

(AAS); gas adsorption and gas chromatography (GC) have been used for characterizations.

In this chapter, the principles of the characterizations are outlined. The details on the

experiments are presented in the following chapters.

3.2 X-ray diffraction

X-ray diffraction is a powerful technique in the analysis of crystalline solids and

mesoporous materials. This techique provides valuable information on the phase structure

of crystals and the pore array of mesostructures.

The wavelength of X-ray lies in the Ångstrøm-sized range which is similar to the distance

between atoms in solids.1 When X-rays enter a crystal, the lattice planes of the crystal can

cause constructive and destructive interferences of the scattered X-rays. According to

Bragg‟s law, the constructive interference that generates diffraction peaks is obtained when

the path difference of the scattered X-rays is a multiple integer of the X-ray wavelength:

66

nλ = 2dsinθ (3-1)

Where n is the order of interference

is the wavelength of X-ray

d is lattice plane distance

is the angle of incidence

When is known, the distance between the lattice planes can be easily deduced.

Figure 3.1 Schematic illustration of the Bragg‟s law.

The Debye-Scherrer equation can be applied to calculate the mean size of crystals from

XRD pattern:

cos

KD

(3-2)

Where D is crystal size

K is Scherrer constant, in general set to 0.9

is the wavelength of X-ray

is full width at half maximum in radians

is the angle of incidence

d2

Incident beam Diffracted beam

67

Although the mesoporous materials are usually not crystalline at atomic level, the periodic

pore structure in a long range can cause the interferences of the scattered X-rays in the

same way of crystalline structure (Figure 3.1 and Figure 3.2). However, due to their long

range regularity in the nanometer scale, the diffraction takes place at low range of the 2θ

angle, generating small angle X-ray scattering (SAXS) pattern. Since the mesopore

diameter (dpore) can be determined from the pore size distribution by nitrogen adsorption-

desorption isotherm analysis, the wall thickness (tw) can be calculated by following

equation:

w o poret = a d (3-3)

Where ao is the distance between the centers of two adjacent mesopores, and can be

deduced from d-spacings.

Figure 3.2 Scheme of a long-range hexagonal order in mesoporous materials (left) and

SAXS pattern of the representative mesoporous material MCM-41 (righ).2

3.3 Electron microscopy

Electron can be considered as a wave with a short wavelength. The wavelength of electrons

accelerated by high voltage is much smaller than that of visible light. Therefore, the

accelerated electrons allow imaging on small scales. The SEM and TEM are commonly

used microscopy techniques.3,4

These techniques use an electron optical system to produce

2-Theta (deg.)

Inte

nsi

ty

tw

dpore

d

68

a fine electron beam with high energy. The high-energy electron beam is then focused onto

solid specimen.

Figure 3.3 Emissions from solid specimen under the irradiation of high-energy electron

beam.5

There are several emissions from the solid specimen when the high-energy electron beam

irradiates this specimen (Figure 3.3).5 The incident electrons dislodge secondary electrons

from the specimen by transferring their energy to the specimen. Because of owning low

energy, the secondary electrons generated at the surface of the specimen are emitted

whereas the others generated at deeper regions are quickly absorbed. Therefore, the emitted

secondary electrons are very sensitive to the surface of the specimen. The SEM technique

detects these emitted secondary electrons for study of the relief of the surface.

Under the irradiation of the high-energy electron beam, the electrons in the inner shells of

the atoms in the specimen are also emitted which leaves vacant orbits. The occupancy of

these vacant orbits by the outer shell electrons results in the emission of X-ray photons

whose energy corresponds to the energy difference between the outer shell electrons and

the inner shell electrons. Therefore, the energy of the X-ray photons characterizes

Specimen

Transmitted electrons

Incident electron beam

Secondary electrons

Auger electrons

Backscattered electronsX-rays

Cathodoluminescence

Absorbed electrons

69

individual elements, which allows the analysis of the composition of the specimen via EDS

spectrum.

In TEM technique, the specimen must be thin enough to transmit the incident electrons.

The interaction of the electrons with the specimen when it passes through allows projecting

a real image of the specimen onto the viewing device.

3.4 X-ray photoelectron spectroscopy

X-ray photoelectron spectroscopy is the most widely used surface analysis technique.6,7

The XPS measures the energy of electrons emitted from core levels (Figure 3.4). The

binding energy of the electrons (EB) can be calculated by following equation:

B KE = h E (3-4)

Where hν is the photon energy

EK is the kinetic energy of the electron

ϕ is the spectrometer work function

Because the three quantities on the right-hand side the equation are measurable, it is easy to

calculate the binding energy of the electrons.

The emitted electrons contribute to characteristic peaks in the XPS spectrum. The peaks of

the electrons from p, d and f orbitals are split into two peaks due to the interaction of the

electron angular momentum with the orbital angular momentum (Figure 3.5). Each electron

has a quantum number associated with its spin angular momentum (s2). The value of s can

be either +1/2 or 1/2. The two angular momenta are added vectorially to produce the

quantity j in the expression nlj with j = |l + s|. The area ratio of the split peaks is equivalent

to the ratio of the values (2j + 1). For example, an electron from p orbital (l = 1) has two j

values of 1/2 and 3/2, and thus the area ratio of the p1/2 and p3/2 peaks is 1:2 (Table 3.1).

70

Figure 3.4 Schematic diagram of XPS process with the emission of an electron in 2p

orbital.

Table 3.1 Summary for the splitting of core levels.

Orbital j values Area ratio (2j1 + 1) : (2j2 + 1)

s 1/2

p 1/2, 3/2 1:2

d 3/2, 5/2 2:3

f 5/2, 7/2 3:4

K

Kinetic

Energy

L

M

1s

2s

2p

3s

3p3d

Ef

Ev

Binding

Energy

0

Photoelectron

Photon

h

71

Figure 3.5 Example for the splitting of Ti 2p XPS peak into Ti 2p1/2 and Ti 2p3/2 peaks in

TiO2.

The binding energy identifies an electron specifically, both in terms of its parent element

and atomic energy level. The change in the oxidation state and in the surroundings of an

atom shifts the binding energy of the core electrons and this binding energy change is

named chemical shift. In general, the removal of valence electrons increases the binding

energy whereas the addition of valence electrons decreases the binding energy. Moreover,

the binding enegy of the core levels increases when electrons are withdrawn from the atom

by chemical bonding. Therefore, the chemical shift provides valuable information on the

oxidation state, chemical environment and functional group.

3.5 Fourier transform infrared spectroscopy

The FTIR spectroscopy is used for investigation of the chemical bonds in materials by the

excitation of the vibrations by the infrared radiation with frequencies in the range of 400

4000 cm-1

.8 The vibrations are infrared active if the molecule has a permanent dipole

moment or the dipole moment of the molecule changes during the oscillations (Table 3.2).9

Each chemical bond with several vibrational modes can absorb several frequencies with

different intensities. The stretching vibrations cause stronger absorptions of IR radiation

rather than the bending vibrations (Figure 3.6). The absorbed frequencies are the

characteristics of chemical bonds. Therefore, FTIR technique is suitable for identification

of functional groups.

Binding Energy (eV)

454456458460462464466468

CP

S

Ti 2p1/2

Ti 2p3/2

72

Table 3.2 Selection rules for Raman and infrared activity of vibrations.9

PD for the dipole moment, for the polarizability and Q for the normal coordinate of the

molecule

Figure 3.6 Different vibrational modes of an asymmetric triatomic molecule.

3.6 Ultraviolet-visible spectroscopy

The solid state UV-vis spectroscopy is employed to characterize the absorbance of light of

semiconductor photocatalysts.10

The energy of photons in the visible region and the near

ultraviolet region with the wavelength from 200 nm to 800 nm can excite electrons from

the VB into the CB of a semiconductor. When a semiconductor is illuminated by light with

Bend Symmetric stretchAsymmetric stretch

73

energy matching or exceeding that of the band gap of the semiconductor, the light induces

the photoexcitation (Figure 3.7). The wavelength at which the light absorption occurs,

together with the intensity of the absorption are recorded by an optical spectrometer and

plotted in the form of UV-vis spectrum.

Figure 3.7 Schematic photoexcitation in semiconductor.

The band gap energy (E) of semiconductor can be calculated from the absorption edge on

the UV-vis spectrum by following equation:

hcE =

λ (3-5)

Where h is Plank constant (6.626 x 10-34

J.s)

c is the speed of light (3.0 x 108 m/s)

λ is wavelength at absorption edge (nm)

The conversion factor: 1eV = 1.6 x 10-19

J

3.7 ζ-Potential analysis

The electrical state of a charged surface is determined by the spatial distribution of ions

around it. The ionic distribution is called electrical double layer (EDL).11

In a physical

model, one layer of the EDL (Stern layer) is firmly bound to the particle as a fixed charge,

while the other layer (diffuse layer) is loosely associated with the particle. The stern layer

contains ions opposite in sign to the surface charge. The diffuse layer contains an excess of

Photon

h

+

CB

VB

Band gap

74

counter-ions opposite in sign to the fixed charge, and a deficit of co-ions of the same sign

as the fixed charge. As a result, the electrical potential is a function of the distance from the

charged surface of the particle suspended in a medium (Figure 3.8). The potential at the slip

plane is identified as ζ-potential.

Figure 3.8 Scheme for the ionic distribution and electrical potential of a negatively charged

particle.12

ζ-potential analysis is widely employed to identify the electrical charge of particles. The

absolute value of ζ-potential implies the stability of a colloidal dispersion. Colloids with

high ζ-potentials are electrically stable due to the strong repulsion of adjacent particles,

whereas others with low ζ-potentials tend to an aggregation since the attraction between

adjacent particles exceeds the repulsion.

3.8 Thermal analysis

Thermogravimetric analysis (TGA) quantifies the loss weight of sample as a function of

increasing temperature.13,14

The mass change with an increase in temperature gives valuable

information about the removal of guest molecules as well as the stability of the material at

certain temperature. The profile of the TGA thermogram can be used for calculating the

amount of the components present in the sample.

+ ++++

++

++

+++

+

+

+++++

+ +

++

++

+

+

+

+

+

+

++

+

+ + +

+ +

+

+

+

+

+

+

+

+

+ +

++

+

+

+

+

+

Distance

Pote

nti

al

Negatively charged surface

Sterm layer

Diffuse layer

Slip plane

0

+

+

+

75

The mass change of the materials is typically evoked by evaporation or decomposition of

components, by oxidation or reduction. These processes are usually exothermic or

endothermic. The complementary differential thermal analysis (DTA) allows identifying a

process either exothermic or endothermic by recording temperature difference between the

sample and an inert reference while both are subjected to the same heating program.

Therefore, the thermal analysis techniques provide information relating to certain physical

and chemical phenomena that occur during heating.

3.9 Elemental analysis

Elemental analysis for carbon is based on the conversion of all of the carbon in materials to

CO2 gas.15

This analysis is carried out by a multistep process including the mineralization

of the material in the presence of excess oxygen at high temperature about 1000 °C, the

subsequent complete oxidation of the resulted gases mixture and the quantification of CO2

gas. The amount of CO2 gas after the process is determined by gas chromatography.

Atomic absorption spectroscopy is used for the qualitative and quantitative analysis of

metal elements present in materials.16

The technique is based on the absorption of light of

free atoms in gaseous state. Electrons of the free atoms can be promoted to excited states

for a short period of time by absorbing radiations. The atoms of each chemical element

absorb a characteristic wavelength. This enables the qualitative analysis of a sample. The

absorbance is proportional to the concentration of the element in the sample. Thus, the

concentration can be calculated based on the Beer-Lambert law:

oIA = log

I (3-6)

Where A is absorbance

Io is the intensity of the light passing through the reference

I is the intensity of the light passing through the sample

76

3.10 Gas sorption

3.10.1 Physisorption isotherm and surface area measurement.

One of the most important properties of MOFs is porosity. Adsorption-desorption isotherm

analysis is widely used for determining their surface area, pore volume and pore size

distribution.17

The isotherm is the plot of adsorbed volume at the standard pressure against

the corresponding equilibrium pressure (P/Po) at a constant temperature.

There are six types of physisorption isotherm (Figure 3.9).18,19

The reversible Type I

isotherm is given by microporous materials in which the rapid rise at low pressures

corresponds to monolayer adsorption, followed by a plateau because of the filling of the

micropores. The reversible Type II and III isotherms are the normal forms obtained with

non-porous or macroporous adsorbents. Type IV and V isotherms are characteristic of

multilayer adsorption consisting of capillary condensation onto mesoporous materials. The

Type V is uncommon isotherm relating to the Type III isotherm in that the adsorbent-

adsorbate interaction is weak. Type VI isotherm is typical of stepwise multilayer adsorption

on a uniform nonporous surface.

Figure 3.9 Types of physisorption isotherms.18, 19

P/Po

Am

ou

nt

adso

rbed

I II III

IV V VI

77

Figure 3.10 Types of hysteresis loops.18, 19

The characteristic of the Type IV isotherm is its hysteresis loop, i.e. the adsorption and

desorption branches do not coincide in the multilayer range, which is associated with

capillary condensation in mesopores. The hysteresis loops exhibit a wide variety of shapes

(Figure 3.10). Type H1 hysteresis is a symmetrical loop consisting of almost vertical and

nearly parallel adsorption/desorption branches. The type H1 is attributed to uniform

cylindrical pores with a narrow pore size distribution. Type H2 is an asymmetrical loop

consisting of a desorption branch much steeper than the adsorption branch. The type H2 is

assigned to “ink-bottle” pores with narrow necks and wide bodies. Type H3 is associated

with slit-shaped pores formed by the aggregate of plate-like particles. Type H4 with nearly

horizontal and parallel branches over a wide range of P/Po is often associated with narrow

slit-like pores.

There are several theories to determine the surface area of porous materials. The most

common theory is the Brunauer-Emmett-Teller (BET) model. The surface area is

determined by using the BET equation:20

o

o m m o

P/P 1 c 1 P

n(1 P/P ) n c n c P

(3-7)

Where n is the amount adsorbed at the relative pressure P/Po

nm is the monolayer capacity

c is a constant related exponentially to the heat of adsorption in the first

adsorbed layer.

P/Po

Am

ou

nt ad

sorb

ed

H1 H3 H4H2

78

In practice, the c value is nearly taken as:

1( ) /c Lq q RTe (3-8)

Where q1 is the heat of adsorption of the first layer of gas molecules

qL is the heat of the gas liquefaction

R and T are the gas constant and the absolute temperature, respectively.

The BET equation gives a linear relationship between o

o

P/P

n(1 P/P ) and P/Po. The values of

nm and c are calculated from the intercept m

1

n c and slope

m

c 1

n c

. The surface area can thus

be calculated from the monolayer capacity on the assumption of close packing as:

m mA = n α L (3-9)

Where m is the molecular cross-sectional area

nm is the monolayer capacity

L is the Avogadro constant

Generally, nitrogen is considered to be the most suitable gas for surface area determination

and it is usually assumed that the BET monolayer is close-packed, giving m = 0.162 nm2

at 77 K.

3.10.2 Micropore analysis

Several methods have been established for micropore analysis. The common methods are t-

plots, αs-plots, Dubinin-Radushkevich (DR), Dubinin-Astakhov (DA), Horvath-Kawazoe

(HK), Saito-Foley and the methods based on the density functional theory (DFT).

79

In this thesis, t-plot method has been used for the analysis of the volume and the surface

area of micropores.18

The method is based on t-curve that is the plot of the standard

adsorbed amount with the statistical thickness of the adsorbed multilayer. The statistical

thickness is calculated as the function of P/Po in the range 0.45 < t < 1.0 nm by using the de

Boer equation:

1/ 2

o

13.99t =

P0.034 log

P

(3-10)

The t-plot is a straight line that passes through the origin in the case of non-porous

materials (Figure 3.11). If the material is microporous, the t-plot exhibits a positive

intercept and the micropore volume can be calculated from this intercept. The slope, s, of

the linear branch is proportional to the external surface area, i.e. the total area of the outside

and pores that are not micropores, as following equation:

tS s 15.47 (3-11)

The constant 15.47 here represents the conversion of the gas volume to liquid volume.

The micropore surface area can be calculated as the differential between the total surface

area SBET and the external surface area St.

If the material contains mesopores, the capillary condensation will occur when the relative

pressure reaches a value relating to the radius of the pore by the Kelvin equation.

Therefore, the t-plot will show an upward deviation commencing at the relative pressure at

which the finest pores are just being filled.

80

Figure 3.11 t-plots of non-porous (curve A), microporous (curve B), mesoporous (curve C)

and bimodal micro-mesoporous (curve D) materials.18

The Horvath-Kawazoe method is used for the characterization of the size distribution of

micropores.21

The HK model yields a relation between pore filling pressure and the pore

width from the free energy change of the adsorption because the adsorbate molecules

transfer from the bulk gas phase to the adsorbed phase, through following equation:

c

o

P (H)ln

P RT

(3-12)

Where Po is the bulk gas saturation pressure

Pc is the pore filling pressure

ϕ is the heat of adsorption

H is the pore width

R is the gas constant

T is temperature.

3.10.3 Size distribution of mesopores

The size distribution of mesopores is estimated by using the Barrett-Joyner-Halenda (BJH)

method on the desorption branch data based on the Kelvin equation:22

t (Å) t (Å)

A

B

C

D

Am

ou

nt ad

sorb

ed

81

m

o K

2 VP 1ln

P RT r

(3-13)

Where P/Po is the relative pressure of vapor in equilibrium with a meniscus having a

radius rK (Kelvin radius)

γ and Vm are the surface tension and molar volume of liquid nitrogen at its

boiling point

R and T are the universal gas constant and boiling point of nitrogen.

3.11 Gas chromatography analysis

Gas chromatography (GC) is an analytical separation technique used for identification of

compounds that can be vaporized without decomposition from a complex mixture. The

separation of gas phase molecules in GC system is a dynamic process that is based on the

difference in the partitioning coefficients of the molecules over a moving mobile gas phase

and a stationary phase. The higher the affinity with the stationary phase, the more a

compound partitions into that phase and the longer it takes before it passes through the GC

system.23

As a result, the compounds are separated along GC column and reach the end of

the column at different periods of time (so-called retention times). The compounds exiting

the end of the column are identified and quantified by an appropriate electronic detector.

The signal from the detector is related to their concentrations.

3.12 References

(1) Toney, M. F. In Encyclopedia of materials characterization, Brundle, C. R.; Evans,

C. A.; Wilson, S. Eds., Manning Publications, Greenwich, 1992.

(2) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S. Nature 1992,

359, 710-712.

(3) Bindell, J. B. In Encyclopedia of materials characterization, Brundle, C. R.; Evans,


(4) Sickafus, K. E. In Encyclopedia of materials characterization, Brundle, C. R.; Evans,


82

(5) Delannay, F. In Characterization of Heterogeneous Catalysts, Delannay, F. Eds.,

Marcel Dekker Inc., New York, 1984.

(6) Defosse, C. In Characterization of Heterogeneous Catalysts, Delannay, F. Eds.,

Marcel Dekker Inc., New York, 1984.

(7) Heide, P. V. D. X-ray Photoelectron Spectroscopy: An introduction to Principles and

Practices, John Wiley & Sons, Inc., Hoboken, New Jersey, 2011.

(8) Grifftiths, P. R.; Haseth, J. A. D. Fourier Transform Infrared Spectrometry, John

Wiley & Sons Inc., Hoboken, New Jersey, 2007.

(9) Kuzmany, H. Solid-State Spectroscopy: An Introduction, Second Ed., Springer, New

York, 2009.

(10) Kisch, H. Angew. Chem. Int. Ed. 2013, 52, 812-847.

(11) Delgado, A. V.; González-Caballero, F.; Hunter, R. J.; Koopal, L. K.; Lyklema, J.

Pure & Appl. Chem. 2005, 77, 1753-1805.

(12) Liese, A.; Hilterhaus, L. Chem. Soc. Rev. 2013, 42, 6236-6249.

(13) Coats, A. W.; Redfern, J. P. Analyst 1963, 88, 906-924.

(14) Brown, E. Introduction to Thermal Analysis: Techniques and Applications, Springer,

2001.

(15) Greenfield, S.; Edwards, D. J. H.; Barnard, M.; Burgess, C.; Hill, S. J.; Jarvis, K. E.;

Lord, G.; West, M.; M. Sargent; Potts, P. J.; Price, J.; Newman, E. J. Accred. Qual.

Assur. 2006, 11, 569-576.

(16) Welz, B.; Sperling, M. Atomic Absorption Spectrometry, Third Ed., Wiley-VCH

Verlag GmbH, Weinheim, Germany, 2007.

(17) Walton, K. S.; Snurr, R. Q. J. Am. Chem. Soc. 2007, 129, 8552-8556.

(18) Gregg, S. J.; Sing, K. S. W. Adsorption, Surface Area and Porosity, Second Ed.,

Academic Press, London, 1982.

(19) Sing, K. S. W.; Everett, D. H.; Haul, R. A. W.; Moscou, L.; Pierotti, R. A.;

Rouquerol, J.; Siemieniewska, T. Pure & Appl. Chem. 1985, 57, 603-619.

(20) Brunauer, S.; Emmett, P. H.; Teller, E. J. Am. Chem. Soc. 1938, 60, 309-319.

(21) Horvath, G.; Kawazoe, K. J. Chem. Eng. Jpn. 1983, 16, 470-475.

(22) Barrett, E. P.; Joyner, L. G.; Halenda, P. P. J. Am. Chem. Soc. 1951, 73, 373-380.

(23) Visser, T. In Handbook of Vibrational Spectroscopy, Chalmers, J. M.; Griffiths, P. R.

Eds., John Wiley & Sons Ltd, Chichester, 2002, Vol. 2.

83

Chapter 4 Route to Bimodal MicroMesoporous MOF

Nanocrystals

Minh-Hao Pham,ý Gia-Thanh Vuong,

ý Frédéric-Georges Fontaine,

ffi and Trong-On Do

*,ý

ýDepartment of Chemical Engineering, and

‡Department of Chemistry

Centre en catalyse et chimie verte (C3V)

Laval University, Quebec, G1V 0A6, Canada

Published in Cryst. Growth Des. 2012, 12, 1008−1013.

85

Résumé

Afin de surpasser les contraintes imposées par la taille des micropores (taille des pores < 2

nm) des composés moléculaires à réseau organométallique (MOFs), on peut soit réduire la

taille des cristaux ou bien créer des mésopores dans les canaux des cristaux de structure

MOFs. Notre approche est de les réaliser en même temps, en utilisant une méthode basée

sur l‟utilisation de surfactant nonionique (des microblocs comme template en présence

d‟acide acétique, qui non seulement génère des canaux mésoporeux bien définis à

l‟intérieur des nanocristaux microporeux de structure MOFs, mais en plus donne une

meilleure cristallinité. Deux exemples de ce type de MOFs ont été sélectionnés pour

illustrer notre approche en utilisant [Cu3(BTC)2] et [Cu2(HBTB)2] basés sur les structures

MOFs (H3BTC = 1,3,5-benzenetricarboxylic acide; H3BTB = 1,3,5-Tris[4-

carboxyphenyl]benzene).

87

Abstract

To overcome the pore size constraints (pore size < 2 nm) of crystalline microporous metal-

organic frameworks (MOFs), one can either reduce crystal size or create mesopore

channels in the MOF crystals. Our approach does both, using a solvothermal nonionic

surfactant-templated assembly of microblocks in the presence of acetic acid, which not only

generates well-defined mesopore channels within the microporous MOF nanocrystals but

also gives high crystallinity. Two examples were selected to illustrate our approach using

[Cu3(BTC)2] and [Cu2(HBTB)2]-based MOFs, (H3BTC = 1,3,5-benzenetricarboxylic acid;

H3BTB = 1,3,5-Tris[4-carboxyphenyl]benzene).

89

4.1 Introduction

Crystalline MOFs, constructed from the assembly of organic linkers with metal ions or

metal clusters, are scaffoldings with all or most atoms on internal surfaces, which leads to

exceptionally high surface areas (up to 6500 m2g

-1) and pore volumes (up to 3.6 cm

3g

-1).

1-3

They have emerged as an important new class of materials with a significant impact in

molecular separation, gas storage, catalysis, sensing, and drug delivery.1-4

Although MOFs

exhibit very large surface areas, the microporosity of these materials (pore size < 2 nm)

restrains the accessibility of a significant portion of the surface area to large guests, which

limits their potential applications. Therefore, to upgrade the performance of these materials,

attempts have been made to synthesize porous materials with large pore diameters in the 2

to 50 nm range.2,3,5

For example, MIL-101,2 UMCM-2

3 and PCN-105

5b exhibit mesoporous

behavior owing to mesoporous cages found throughout the structure; however, the

mesopores are often restricted by small apertures that prohibit large molecules from

accessing the space inside. Increasing the organic linker length is another option but, with

few exceptions,1,6

the MOFs built up from long-linkers tend to collapse upon guest

removal7 or form catenated structures.

8 As in the case of microporous zeolites,

9,10 materials

with bimodal micro-/mesoporosity are of considerable interest for many potential

applications, such as drug release, gas storage, separation and catalysis because they offer

the benefits arising from the combined two regimes.10, 11

However, no rational synthetic

procedure to incorporate mesopores in MOFs that can be applied to a large array of

carboxylate-based MOFs has yet been reported.12

We report here a novel methodology

toward the synthesis of a new family of bimodal micro-mesoporous MOF nanocrystals via

the coassembly of nanosized MOF building blocks with a non-ionic triblock copolymer

surfactant in the presence of acetic acid. The use of a large triblock copolymer surfactant

such as F127 (EO97PO69EO97) as a mesostructured template induces the ability to

crystallize a MOF structure of pore walls while the presence of acetic acid allows

controlling the rate of crystallization of the framework to form well-defined mesostructures

within the crystalline MOF nanocrystals.

90

4.2 Experimental

Chemicals: CH3COOH (99.7%, Fisher), Pluronic F127 (EO97PO69EO97, average Mn

12,600, Aldrich), 1,3,5-Benzenetricarboxylic acid (H3BTC, 95%, Adrich), 1,3,5-Tris(4-

carboxyphenyl)benzene (H3BTB, 98%, Aldrich), Cu(NO3)23H2O (99%, Aldrich), and

anhydrous ethanol as solvent were purchased from commercial sources and used without

further purification.

Synthesis: Bimodal micro-mesoporous MOFs nanocrystals were prepared by using a

solvothermal synthesis in the presence of triblock copolymer F127 as the template and

acetic acid with various molar ratios of acetic acid/Cu2+

in ethanol. In a typical synthesis of

bimodal micro-mesoporous [Cu3(BTC)2] based MOFs, namely, UL1MOF-x, 0.618 mL

(10.8 mmol) of acetic acid was added to a 6 mL ethanol solution containing 0.435 g (1.8

mmol) of Cu(NO3)23H2O. To this solution, 0.216 g of F127 surfactant pre-dissolved in 6

mL of ethanol was added. After stirring for 2 h, 0.221 g (1.0 mmol) of H3BTC was added.

After stirring for an additional 2 h, the reaction mixtures were heated at 120 °C for 16 h.

The product was washed several times with ethanol, recovered by centrifugal separation,

and then dried at 70 °C for 4 h. To remove the surfactant, the products were treated with

tetrahydrofuran (THF) in the presence of acetic acid as solvent (molar ratio acetic acid/THF

= 0.15/1.0) with the ratio of 1 g of the material for 400 mL of the solvent at 100 °C for 24

h. The same procedure was used for the synthesis of bimodal [Cu2(HBTB)2] based MOFs,

namely, UL2MOF-x, except that H3BTB was used as linkers instead of H3BTC, with the

molar ratio of Cu(NO3)23H2O/0.28 H3BTB/33 CH3COOH/0.05 F127/620 C2H5OH, at 110

°C for 16 h.

Characterizations: Powder XRD patterns in the 2θ range of 0.5 - 5° for SAXS and 5 - 50°

for WAXD were collected on a Bruker SMART APEX II X-ray diffractometer equipped

with Cu K radiation (40 kV, 30 mA). SEM images were taken with a JEOL 6360

instrument with an accelerating voltage of 3 kV. TEM images were obtained with a 120 kV

JEOL JEM 1230 electron microscope. HRTEM investigations were carried out on a JEOL

JEM-2100F electron microscope operating at 200 kV (point resolution 0.1 nm). N2

91

adsorption-desorption isotherms were measured at the temperature of liquid nitrogen with a

Quantachrome Autosorb-1 system. The BET surface areas were calculated from the

experimental pressure ranges identified by established consistency criteria reported by

Walton and Snurr for metal-organic frameworks,13

herein the 0.05 0.15 P/P0 range of

linearity of the adsorption isotherm. The pore size distributions were obtained from the

analysis of the desorption branch of the isotherms using BJH model. TGA/DTA analysis

was carried out with a Netzsch STA 449C thermal analysis instrument from room

temperature to 500 °C with a heating rate of 5 °C min-1

under an air flow of 20 mL min-1

using the Netzsch Proteus data analysis package. The elemental analysis of carbon and

copper was performed using a Perkin-Elmer 240C elemental analyzer and a Perkin-Elmer

Analyst 200 Atomic Absorption Spectrometer, respectively.

4.3 Results and discussion

A typical mixture for the synthesis of the bimodal micro-mesoporous nanomaterials

(namely, ULMOF-x; x = acetic acid/Cu2+

molar ratio) is composed of acetic acid (AA),

copper(II) nitrate trihydrate, and non-ionic triblock copolymer surfactant (Pluronic F127).

The solution was stirred for 2 hours before the carboxylate linker was added (H3BTC for

UL1MOF-x or H3BTB for UL2MOF-x) and was followed by a solvothermal treatment. The

crystals obtained were extracted with tetrahydrofuran in the presence of acetic acid to

remove the surfactant.

Figure 4.1 shows the SAXS and WAXD patterns for the as-made UL1MOF-x samples

obtained from different acetic acid to Cu(II) molar ratios (AA/Cu2+

= 0, 4, and 6) in the

presence of Pluronic F127 and H3BTC, and for pristine Cu3(BTC)2 (HKUST-1)14

as well,

as prepared by Hartmann et al. using the same copper starting material.15

The presence of a

single d100 peak in the SAXS spectra for the materials prepared in the presence of acetic

acid and F127 suggests a disordered wormhole-like mesostructure.10,16

This peak grows in

intensity and shifts from 1.4° to 1.1° when the AA/Cu2+

molar ratio is increased from 0 to

6, suggesting a more uniform mesostructure and larger d100 for these materials. It is

important to note that using an AA/Cu2+

ratio of 12, under the same synthetic conditions,

92

does not alter significantly the intensity and the position of the d100 peak, indicating that a

similar mesostructure is obtained for this sample (Table S4.1). As expected, no d100 peak is

present for the pristine HKUST-1 material since it possesses no mesopores. A comparison

of the WAXD of UL1MOF-6 samples with pristine HKUST-1 sample confirms that both

materials have the same microcrystalline network.15

However, the broad diffraction peaks

indicate that UL1MOF-6 is composed of small nanocrystals. By considering the pristine

HKUST-1 to be 100% crystalline, the crystallinity of the samples UL1MOF-0, 2, 4, 6, and

12 was determined to be 58, 85, 89, 94 and 92% crystalline, respectively.17

All these results

suggest that higher crystallinity and more uniform wormhole-like mesostructure are

obtained for UL1MOF-x by increasing the AA/Cu2+

molar ratio. It emphasizes the key role

of acetic acid in the formation of both the micropore and well-defined mesopore structures

within the MOF nanocrystals.

Figure 4.1 Powder XRD patterns for UL1MOF-x. SAXS (left) and WAXD (right) patterns

for the as-made UL1MOF-x obtained from various molar ratios of acetic acid/Cu2+

: (a) x =

6, (b) x = 4, (c) x = 0, and (d) the pristine Cu3(BTC)2 MOF. UL1MOF-x were synthesized

with a molar ratio of Cu2+

/H3BTC/CH3COOH/C2H5OH/F127 = 1 : 0.56 : x : 115 : 0.01 at

120 °C for 16 h.

10 20 30 40 501 2 3 4 5

SAXS WAXD

a

b

c

d

2 d(nm)

1.1 8.8

1.5 5.9

1.4 6.3

0.9 9.8

2-Theta (degree)

Inte

nsit

y (

a.u

.)

93

Figure 4.2 N2 adsorption-desorption isotherms: (a) the solvent-extracted UL1MOF-6

displays a type I isotherm at relatively low P/P0 pressures for a micropore structure and a

typical type IV isotherm with a H4 hysteresis loop associated with mesopore channels

(inset: the mesopore diameter distribution for the UL1MOF-6), (b) the pristine Cu3(BTC)2

exhibits a type I isotherm characterising a micropore structure with a smaller pore volume.

The nitrogen adsorption/desorption isotherm at 77 K of the solvent-extracted UL1MOF-6 is

presented in Figure 4.2. Data for the pristine HKUST-1 sample are included for

comparison. The latter material displays a type I isotherm at relatively low P/Po pressures,

which is characteristic of the micropore structure, with BET surface area and pore volume

of 1420 m2g

-1 and 0.60 cm

3g

-1, respectively. However, for the UL1MOF-6 sample, in

addition to the type I isotherm indicative of a microporous structure with a diameter of 8.5

Å, as in pristine HKUST-1 (Figure S4.1), a typical type IV isotherm with a H4 hysteresis

loop at relatively high P/Po (P/Po = 0.4-0.9) was observed. This is related to the capillary

condensation associated with mesopore channels, indicating the presence of an additional

mesoporous structure in this sample. The adsorption/desorption isotherms in Figure 4.2 are

nearly horizontal and parallel over a wide range of P/Po. This loop type is associated with

uniform mesopores of the extracted UL1MOF-6.18

The total surface area (BET surface area

= 1630 m2g

-1) and pore volume (0.79 cm

3g

-1) of UL1MOF-6 are comprised of both types of

P/Po

0.0 0.2 0.4 0.6 0.8 1.0

Vo

lum

e (

cm

3/g

)

0

200

400

600

Pore diameter (nm)2 4 6 8 10

Volu

me (

cm

3/Å/g)

0.00

0.02

0.04

4.0 nm

a

b

94

pores, and are significantly higher than those obtained for the HKUST-1 sample, due to the

presence of the mesostructure. The mesopore diameter distribution was calculated using the

Barrett-Joyner-Halenda (BJH) method (inset of Figure 4.2), indicating a narrow mesopore

diameter distribution with an average value of 4.0 nm. This mesopore diameter is also

comparable to the known value for the mesoporous SBA-16 silica synthesized using the

same F127 surfactant.19

This also indicates that the ordered mesostructure is still preserved

after the removal of surfactant molecules from this UL1MOF-6 sample.

Figure 4.3 Electron microscopy images of UL1MOF-6. (a) SEM image showing the

isolated octahedral mesoporous MOF crystals with the size in a range of 100-200 nm, (b, d,

f) Bright-field HRTEM images with different magnifications indicating a three-dimensional

wormhole-like mesostructure, (c, e, g) Dark-field HRTEM images obtained on the same

area of the sample. The bright spots in the dark-field images correspond to MOF

nanocrystal building blocks.

The SEM image of the UL1MOF-6 sample shows the overall octahedral morphology of

[Cu3(BTC)2]-based MOF produced by our procedure (Figure 4.3a). The discrete octahedral

nanocrystals do not aggregate and exhibit a uniform crystal size ranging from 100 to 200

nm. Representative TEM images of the UL1MOF-6 show similar morphology (Figure 4.3b,

c and Figure S4.2) to that of the micrometer-sized HKUST-1 single crystals obtained by the

original procedure (Figure S4.3).14,15

Indeed, the bipyramidal hollow octahedrons are

95

clearly observed using TEM images with different views (Figure 4.4). For the

characterization of mesopore-ordered framework consisting of crystalline nanoparticle

domains, bright/dark-field high resolution-TEM images were recorded on the same area to

substantiate claims of high crystallinity and mesoporosity on this material.20

Shaped

octahedral crystals were clearly observed (Figure 4.3d, e) and Figure 4.3f, g reveal a quite

uniform pore size with a disordered mesopore structure, which is consistent with the SAXS

results and reminiscent of MSU-116

and UL-zeolite10

which have wormhole-like mesopore

structures. The bright spots in the dark-field and the dark spots in the bright-field HRTEM

image (Figure 4.3g-f, respectively) correspond to continuous small MOF nanocrystals

within the well-defined octahedron. All these results suggest that the octahedral

mesocrystals are constructed from nanometric crystallites which are embedded along the

organic hydrophilic block from the surfactant chains to form crystalline mesopore walls,

while preserving the mesostructural integrity. Note that these MOF samples are quite

sensitive to the electron beam compared to other mesostructured metal oxides obtained with

F127 as a structure-directing agent and makes it challenging to obtain selected-area electron

diffraction patterns.21

Figure 4.4 (a-d) TEM images recorded along different directions: (a) along the [111]

direction, (b) along the [110] direction, (c) along the [001] direction and (d) along the [112]

direction. (e) Schematic representations of the TEM images, and the octahedral

reconstructed three-dimensional shape of a UL1MOF-6 nanocrystal.

96

TGA-DTA measurements for the as-synthesized and extracted-UL1MOF-6 materials were

carried out under air flow (Figure S4.4). For the extracted-UL1MOF-6 sample, the TGA-

DTA profile shows a weight loss of ~10% at below 180 °C that can be attributed to the

solvent desorption within the pore channels of the extracted sample. However no

significant weight loss (˂ 5% was found for the UL1MOF-6 sample before extraction

suggesting a F127 template inside the mesopore structure. Furthermore, no essential

difference in the thermal stability between materials was observed; their structure is

thermally stable up to 280 ºC. In the temperature range of 250 ºC350 ºC, because of a

simultaneous decomposition of the F127 template22

and the MOF structure of UL1MOF-6,

it is difficult to identify one from the other. However, the weight loss of the as-synthesized

UL1MOF-6 (59%) in the range of 280-330 °C is greater than that of extracted-UL1MOF-6

(51%), implying the presence of the F127 copolymer in the as-made sample. The FTIR

spectra of these samples are also identical (Figure S4.5). In addition, C and Cu elemental

analysis was also performed. The atomic ratio of C to Cu of the UL1MOF-6 before

surfactant extraction is 9.5 which is higher than those of the UL1MOF-6 after surfactant

extraction (C/Cu = 7.8) and as compared to those of the pristine HKUST-1 (C/Cu = 7.6).

This clearly indicates the presence of surfactant F127 inside mesopore channels of the

UL1MOF-6 nanocrystals before extraction.

The same procedure was also applied successfully for the synthesis of [Cu2(HBTB)2]-based

MOF mesocrystals (namely, UL2MOF-x) using H3BTB as linker instead of H3BTC. Figure

4.5 shows the SAXS and WAXD patterns of the [Cu2(HBTB)2] based MOF prepared with

the same surfactant and acetic acid concentrations in the synthetic solution (AA/Cu2+

= 6).

For comparison, the pristine microporous [Cu2(HBTB)2] MOF was also prepared by the

method of Mu et al.23

The SAXS patterns of all samples exhibit a diffraction peak at the 2θ

value of 3.1o that is present in the pristine MOF crystal structure. For the UL2MOF-6

sample, a single d100 peak was observed at lower 2θ value indicating the presence of

wormhole-like mesopores in this sample which is not present for the pristine sample. In

addition, the nitrogen adsorption/desorption isotherm at 77 K of the UL2MOF-6 sample

shows a type IV isotherm with a H4 hysteresis loop corresponding to mesopores having a

diameter of 3.9 nm (Table S4.1).

97

Figure 4.5 Powder XRD patterns for UL2MOF-6. SAXS (left) and WAXD (right) patterns

for the as-made UL2MOF-6 (a) and the pristine Cu2(HBTB)2 MOF (b). UL2MOF-6 was

synthesized using H3BTB as the organic linker and a molar ratio

Cu2+

/H3BTB/CH3COOH/C2H5OH/F127 = 1 : 0.28 : 33 : 620 : 0.05 at 110 °C for 16 h.

Interestingly, this material contains numerous flower-like crystals, and almost all of them

show the same morphology (Figure 4.6a-c). The flower-like crystals were obtained as

multilamellar nanosheets with three-dimensional intergrowth. The overall thickness of the

lamellar stacking is 40 60 nm. Bright-field and dark-field HRTEM images recorded on

this UL2MOF-6 sample indicate that small nanocrystals are also uniformly embedded in a

continuous matrix within the crystals (Figure 4.6d, e). The nanocrystallite sizes are

consistent with those from X-ray diffraction peak broadening. Furthermore, its crystallinity

is 85% by considering the microporous [Cu2(HBTB)2] MOF to be 100% crystalline.

2-Theta (degree)

2 d(nm)

1.2 7.4

3.1 2.9

10 20 30 40 50

SAXS WAXD

a

b

Inte

nsit

y (

a.u

.)

1 2 3 4 5

98

Figure 4.6 Electron microscopy images of UL2MOF-6 sample. (a-c) SEM images with

different magnifications indicating a flower-like morphology that is composed of

nanosheets, (d, e) HRTEM images of the edge of a nanosheet: (e) bright-field image

showing a 3D disordered wormhole-like mesostructure, (d) dark-field image obtained on

the same area of the sample shows the bright spots corresponding to MOF nanocrystal

building blocks.

99

Scheme 4.1 Possible mechanism for the formation of the bimodal micro-mesoporous MOF

nanocrystals.

On the basic of the results of this study and literature precedents, we propose a possible

mechanism for the formation of bimodal MOF nanocrystals illustrated in Scheme 4.1.

Because of the presence of acetic acid in the reaction solution containing copper(II) ions,

acetate coordination species,24

probably a derivative of the metal-acetate bidentate bridging

[Cu2(CH3COO)4], form. Upon the addition of the linkers, nanosized building blocks

including MOF components are yielded. As recently reported by Kitagawa et al. in MOF

chemistry using copper(II) salt, the similar MOF building blocks with a size of a few

nanometers were formed from organic linkers and acetic acid in the early stage of the

reaction.25

The nanosized building blocks here will coassemble with the surfactant micelles

into mesostructures, as previously reported in the synthesis of mesoporous inorganic

materials.20

At high temperature (120 °C), the building blocks will fuse to form MOF

crystals, while maintaining the mesostructure.25-27

It is noted that the key factor to obtain well-defined mesoporous materials is that the growth

of the framework must be slow enough for the coassembly of the nanosized building blocks

and the surfactant micelles into a mesostructure to occur while avoiding phase separation.24

Since acetic acid competes with the carboxylate linkers,25

the growth of the framework to

generate large crystals in the early stage of the reaction which can induce a phase

100

segregation is limited. Moreover, acetic acid can also impact the deprotonation of the

linkers. A higher acetic acid concentration thus leads to a lower growth rate of the MOF.28

Therefore, by tuning the acetic acid concentration in this synthetic process, it is possible to

form the well-defined mesostructures within MOF nanocrystals. The use of large triblock

copolymer surfactant such as F127 in the current method allows yielding thick pore walls

that induce the ability to crystallize the MOF structure to obtain highly crystalline bimodal

MOF nanocrystals.24

4.4 Conclusions

To the best of our knowledge, this is the first synthesis of such bimodal MOF nanocrystals

using the co-assembly of nanosized MOF building blocks with a nonionic triblock

copolymer surfactant in the presence of acetic acid. Bimodal ULMOF nanocrystals are

considered to be of substantial scientific and technological importance, owing to easier

transport of guest molecules through the mesopores, short diffusion pathways, and exposed

active sites within the MOF crystals. In addition to diffusional effects, the grafting of novel

functionalities within the mesopore surface by post-synthesis will also be studied, since a

larger pore diameter should ease the functionalization processes.29

We are currently looking

at the synthesis of other carboxylate MOFs to establish the generality of this approach.

101

4.5 Appendix

Table S4.1 Synthetic parameters and physicochemical properties of bimodal ULMOF-x nanocrystals

Sample AA/Cu2+(*)

F127/Cu2+(*)

SBET

m2/g

Vpore

cm3/g

Smeso

m2/g

Smeso/SBET

(%)

Vmeso

cm3/g

Smicro

m2/g

Vmicro

cm3/g

dmeso

nm

Crystallinity

%

UL1MOF-12 12 0.01 1570 0.77 300 19.1 0.29 1270 0.48 4.0 92

UL1MOF-6 6 0.01 1630 0.79 280 17.2 0.29 1350 0.50 4.0 94

UL1MOF-4 4 0.01 1105 0.64 240 21.7 0.31 865 0.33 4.0 89

UL1MOF-2 2 0.01 860 0.51 200 23.3 0.25 660 0.26 4.0 85

UL1MOF-0 0 0.01 468 0.31 148 31.6 0.18 320 0.13 4.0 58

Cu3(BTC)2 0 0.00 1420 0.60 - - - - - - 100

UL2MOF-6 33 0.05 770 0.38 205 26.6 0.16 565 0.22 3.9 85

Cu2(HBTB)2 0 0.00 586 0.25 - - - - - - 100

(*)molar ratio. SBET is the BET surface area. Vpore is the total pore volume determined by using the desorption branch of the N2 isotherm at P/Po =

0.95. Smeso is the mesopore surface area estimated by subtracting Smicro from SBET. Smicro and Vmicro are the micropore surface area and micropore

volume, respectively, calculated by using t-plot method. Vmeso is the mesopore volume calculated by subtracting Vmicro from Vpore. dmeso is the

mesopore diameter determined from the local maximum of the BJH distribution of pore diameters in the desorption branch of the N2 isotherm. The

crystallinity of the samples was determined based on the areas of the peaks in the 2 range of 5o to 14

o by considering their respective pristine

MOFs to be 100 % crystalline.

102

Figure S4.1 Micropore diameter distribution for the UL1MOF-6, using the HK method.

Figure S4.2 TEM images of UL1MOF-6 sample showing octahedral shape nanocrystals

with the size in a range of 100 nm to 200 nm. These MOF nanocrystals contain a uniform

three-dimension wormhole-like mesotructure.

Figure S4.3 SEM image of the pristine microporous Cu3(BTC)2 MOF crystal prepared

using the method in ref 15. The crystal has octahedral shape with the size of 15 µm.

103

Figure S4.4 TGA/DTA profiles of the as-synthesized UL1MOF-6 (a) and extracted

UL1MOF-6 samples (b).

Figure S4.5 FTIR spectra of the as-synthesized UL1MOF-6, extracted UL1MOFs-6 and

pristine HKUST-1.

TGA (%)

40

60

80

100

Temperature/oC

50 150 250 350 450

DTA (uV/mg) exo

0

1

2

3

4

5

6

7

TGA: solid lines

DTA: dashed lines

(a)

(b)

Wavenumber (cm-1

)

500 1000 1500 2000 2500 3000 3500 4000

Extracted-UL1MOF-6

As-synthesized UL1MOF-6

Pristine HKUST-1

Tra

ns

mit

tan

ce

104

4.6 References

(1) (a) Yaghi, O. M.; O'Keeffe, M.; Ockwig, N. W.; Chae, H. K.; Eddaoudi M.; Kim, J.

Nature 2003, 423, 705-714. (b) Furukawa, H.; Ko, N.; Go, Y. B.; Aratani, N.; Choi,

S. B.; Choi, E.; Yazaydin, A. Ö.; Snurr, R. Q.; O‟Keeffe, M.; Kim, J.; Yaghi, O. M.

Science 2010, 329, 424-428.


Margiolaki, I. Science 2005, 309, 2040-2042.


(4) (a) Horcajada, P.; Chalati, T.; Serre, C.; Gillet, B.; Sebrie, C.; Baati, T.; Eubank, J.

F.; Heurtaux, D.; Clayette, P.; Kreuz, C.; Chang, J. S.; Hwang, Y. K.; Marsaud, V.;

Bories, P. N.; Cynober, L.; Gil, S.; Férey, G.; Couvreur P.; Gref. R. Nat. Mater. 2010,

9, 172-178. (b) Yanai, N.; Kitayama, K.; Hijikata, Y.; Sato, H.; Matsuda, R.; Kubota,

Y.; Takata, M.; Mizuno, M.; Uemura, T.; Kitagawa, S. Nat. Mater. 2011, 10,

787‒793. (c Satoru, S.; Higuchi M.; Matsuda, R.; Yoneda, K.; Hijikata, Y.; Kubota,

Y.; Mita, Y.; Kim, J.; Takata, M.; Kitagawa, S. Nat. Chem. 2010, 2, 633‒637.

(5) (a) Koh, K.; Wong-Foy, A. G.; Matzger, A. J. Angew. Chem. Int. Ed. 2008, 47, 677-

680. (b) Fang, Q. R.; Yuan, D.Q.; Sculley, J.; Lu W. G.; Zhou, H. C. Chem. Commun.

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(6) Klein, N.; Semkovska, I.; Gedrich, K.; Stoeck, U.; Henschel, A.; Mueller, U.; Kaskel,

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(7) (a) Eddaoudi, M.; Kim, J.; Rosi, N.; Vodak, D.; Wachter, J.; O'Keeffe, M.; Yaghi, O.

M. Science 2002, 295, 469-472. (b) Jiang, H.-L.; Tatsu, Y.; Lu, Z.-H.; Xu, Q. J. Am.

Chem. Soc. 2010, 132, 5586-5587.

(8) Farha, O. K.; Malliakas C. D.; Kanatzidis M. G.; Hupp J. T. J. Am. Chem. Soc. 2010,

132, 950-952.

(9) Vuong, G. T.; Do, T. O. J. Am. Chem. Soc. 2007, 129, 3810-3811.

(10) Do, T. O.; Kaliaguine, S. Angew. Chem. Int. Ed. 2001, 40, 3248-3251.

(11) (a) Holland, B. T.; Abrams, L.; Stein, A. J. Am. Chem. Soc. 1999, 121, 4308-4309.

(b) Qiu, L. G.; Xu, T.; Li, Z. Q.; Wang, W.; Wu, Y.; Jiang, X.; Tian, X. Y.; Zhang, L.

D. Angew. Chem. Int. Ed. 2008, 47, 9487-9491. (c) Zhao, Y; Zhang, J.; Han, B.;

Song, J.; Li, J.; Qian Wang, Q, Angew. Chem. Int. Ed. 2011, 50, 636-639. (d) Li, Y.;

Zhang, D., Guo, Y.-N.; Guan, B., Tang, D.; Liu, Y.; Huo Q. Chem. Comm., 2011, 47,

7809-7811.

(12) Roy, X.; MacLachlan, M. J. Chem. Eur. J. 2009, 15, 6552-6559.


(14) Chui, S. S.-Y.; Lo, S. M.-F.; Charmant, J. P. H.; Orpen, A. G.; Williams, I. D.

Science 1999, 283, 1148-1150.

(15) Hartmann, M.; Kunz, S.; Himsl, D.; Tangermann, O. Langmuir 2008, 24, 8634-8642.

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(16) Bagshaw, S. A.; Prouzet, E.; Pinnavaia, T. J. Science 1995, 269, 1242-1244.

(17) Based on the areas of the peaks in the 2 range of 5° to 14°.

(18) (a) Sing, K. S. W.; Everett, D. H.; Haul, R. A. W.; Moscou, L.; Pierotti, R. A.;

Rouquerol, J.; Siemieniewska, T. Pure & Appl. Chem. 1985, Vol. 57, No. 4, 603-619.

(b) Gregg, S. J.; Sing, K. S. W. Adsorption, Surface Area and Porosity; Academic

Press, London, 1997.

(19) Zhao, D.; Huo, Q.; Feng, J.; Chmelka, B. F.; Stucky, G. D. J. Am. Chem. Soc. 1998,

120, 6024-6036.

(20) Yang, P.; Zhao, D.; Margolese, D. I.; Chmelka, B. F.; Stucky, G. D. Nature 1998,

396, 152-155.

(21) Lebedev, O. I.; Millange, F.; Serre, C.; Van Tendeloo, G.; Férey. G. Chem. Mater.

2005, 17, 6525-6527.

(22) Jiua, J.; Kurumada, K.; Peib, L.; Tanigaki, M. Colloids and Surfaces B:

Biointerfaces, 2004, 38, 121.

(23) Mu, B.; Li, F.; Walton, K. S. Chem. Commun. 2009, 2493-2495.

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40, 784-792


Angew. Chem. Int. Ed. 2009, 48, 4739-4743.

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(28) Hermes, S.; Witte, T.; Hikov, T.; Zacher, D.; Bahnmuller, S.; Langstein, G.; Huber,


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107

Chapter 5 Rational Synthesis of MOF Nanocubes and

Nanosheets Using Selective Modulators and Their Morphology-

Dependent Gas-Sorption Properties

Minh-Hao Pham,ý,§

Gia-Thanh Vuong,ý,§ Frédéric-Georges Fontaine,

ffi,§ and Trong-On Do

*,ý,§

†Department of Chemical Engineering,

‡Department of Chemistry, and

§Centre en catalyse

et chimie verte (C3V), Laval University, Quebec City, G1V 0A6

Center in Green Chemistry and Catalysis (CGCC), Quebec, Canada

Published in Cryst. Growth Des. 2012, 12, 3091−3095 (Most read article).

109

Résumé

Des nanocubes et des nanofeuilles de [Cu2(ndc)2(dabco)]n de structure MOFs (ndc = 1,4-

naphthalene dicarboxylate; dabco = 1,4-diazabicyclo[2.2.2]-octane) ont été synthétisés en

utilisant simultanément l‟acide acétique et la pyridine ou la pyridine uniquement,

respectivement comme les modulateurs sélectifs. Cette approche permet de contrôler la

croissance de nanocristaux MOFs sur les différentes directions pour la synthèse de taille et

de forme contrôlée de MOFs dont la structure est composée de deux ou plus de molécules

liantes en utilisant des modulateurs sélectionnés. Ces nanocristaux MOFs possèdent une

cristallinité élevée et une grande capacité d‟adsorption de CO2 comparée aux matériaux

MOFs de [Cu2(ndc)2(dabco)]n de forme de nano-barres générées en utilisant seulement

l‟acide acétique comme le modulateur sélectif, qui est dû à l‟impact de l‟effet de

morphologies sur leur capacité d‟adsorption.

111

Abstract

Nanocubes and nanosheets of [Cu2(ndc)2(dabco)]n metal-organic framework (ndc = 1,4-

naphthalene dicarboxylate; dabco = 1,4-diazabicyclo[2.2.2]octane) were synthesized by

using simultaneously acetic acid and pyridine or only pyridine, respectively, as selective

modulators. This approach can tailor crystal growing on different directions for the size-

and shape-controlled synthesis of MOF nanocrystals whose structure is composed of two or

more types of linkers using selective modulators. These MOF nanocrystals exhibit high

crystallinity and higher CO2 uptakes compared to that of the bulk MOF material or of the

[Cu2(ndc)2(dabco)]n nanorods generated by using only acetic acid as the selective

modulator, which may be due to the morphology effect on their gas sorption properties.

113

5.1 Introduction

Porous metal-organic framework (MOF) crystals have emerged as potential candidates in a

number of practical applications such as drug delivery,1 biomedical imaging,

2 sensing

3 and

gas adsorption.4 In comparison with bulk materials, nanostructured MOFs exhibit unique

shape- and size-dependent properties such as more accessible active sites, short diffusion

pathways within the porous nanocrystals, and the ability to functionalize the external

surfaces of MOF nanocrystals.3b,5,6

A number of synthetic methods have been reported for the preparation of MOF

nanoparticles including microemulsions,4b,7

surfactant-mediated methods,8 microwave-

assisted routes,1a

and sonochemistry.9 However, the precise control over the shape and size

of MOF nanoparticles still remains challenging. Recently, selective coordination

modulation approaches have been reported to synthesize MOF nanocrystals using a single

capping reagent (so-called modulator).10,11

For example, nanosized MOF-5 has been

synthesized using p-perfluoromethylbenzenecarboxylate (pfmbc) as a modulator.10a

Because MOF-5 is constructed from one type of linker (i.e., 1,4-benzenedicarboxylic acid)

and Zn4O clusters, it has an isotropic framework with only one coordination mode.12

In the

presence of the pfmbc, the carboxylate functionality of the modulator can impede the

coordination between the zinc and the linkers to regulate the rate of the framework

extension. Since the pfmbc also binds isotropically to the particle surface, such process can

yield spherical/cubic MOF-5 nanocrystals. Tsuruoka et al. reported the synthesis of

[Cu2(ndc)2(dabco)]n nanorods (ndc = 1,4-naphthalene dicarboxylate; dabco = 1,4-

diazabicyclo[2.2.2]octane) using acetic acid as the modulator.10b

Because the acetic acid

selectively impedes the Cu-ndc coordination, nanorods are obtained solely.

The selective modulation methodology has been also used to control the shape and size of

MOF crystals at the micrometer scale. Umemura et al. have recently reported the

morphology-controlled synthesis of [Cu3(btc)2]n octahedron, cuboctahedron and cubic

microcrystals (btc = benzene-1,3,5-tricarboxylate) with a mean size of about 2 m by

changing the concentration of modulator (n-dodecanoic acid or lauric acid).13

The control

114

over the shape and size of MOF microcrystals was also achieved with additives that allow

the protonation or deprotonation of the organic linkers. For example, the introduction of

bases such as diethylamine, triethylamine and acids such as acetic acid or HNO3 allows

adjusting the particle formation rate and the size and shape of microcrystals.14

Recently, we reported a new method for the synthesis of uniform MOF nanocrystals with

controllable size and aspect ratio (length/width) using nonionic triblock copolymer F127

and acetic acid as stabilizing and deprotonation-controlling agents, respectively.15

The

alkylene oxide segments of the F127 copolymer can coordinate metal ions and stabilize the

MOF nuclei in the early stage of the formation of MOF nanocrystals, and acetic acid can

control the deprotonation of carboxylic linkers during the synthesis, thus enabling the

control of the nucleation rate, leading to tailored size and aspect ratio of nanocrystals.

Scheme 5.1 Coordination modulations toward [M2(dicarboxylate)2(N ligand)]n MOF

nanocrystals with controlled morphologies.

Route 1: Simultaneous modulation of metal-carboxylate and metal-nitrogen coordination

modes using both monocarboxylic acid and amine to prepare nanocubes.

Route 2: Selective modulation of metal-nitrogen mode using only amine toward

nanosheets.

Route 3: Selective modulation of metal-carboxylate mode using only monocarboxylic acid

toward nanorods.

Metal Ion Dicarboxylate N Ligand

1

Modulation

Monocarboxylic acid

Amine

Nanocube

Nanorod

+

Sele

cti

ve

Sim

ult

an

eo

us

(001)

(001)

a

c b

(010)(100)

2 Nanosheet

3

115

Inspired by the methods for the synthesis of semiconductor nanocrystals,16

crystalline

MOFs constructed from two or more types of linkers (i.e., having different coordination

modes), such as [M2(dicarboxylate)2(N-ligand)]n,17

could have controllable nanoscale size

and shape using simultaneously different selective modulators. Herein, to demonstrate our

approach we have chosen the three-dimensional [Cu2(ndc)2(dabco)]n MOF, composed of

two distinct linkers,17a

as a member of [M2(dicarboxylate)2(N ligand)]n MOF series. This

MOF crystallizes in a tetragonal space group (P4/mmm), in which the dicarboxylate ligands

(ndc) link the metal ions in the square paddle-wheel dicopper clusters to form two-

dimensional square lattices, which are connected by the bidentate nitrogen pillar ligands

(dabco) at the lattice points. Hence, the structure is dominated by two coordination modes:

copper-ndc on the four (h00) and (0k0) facets and copper-dabco on two remaining (00l)

facets. This allows modulating simultaneously the coordination modes of copper-ndc and

copper-dabco by monocarboxylic acids and amines, respectively. Monocarboxylic acid,

such as acetic acid, having the same carboxylate functionality as the ndc linker impedes the

coordination interactions between copper and ndc in the [100] direction, whereas amine

containing a nitrogen atom with a lone pair similar to that of dabco impedes the

coordination between copper and dabco in the [001] direction. As a result, both the [100]

and [001] directions of the crystal growth can be regulated to form nanocubes using both

modulators (Scheme 5.1, Route 1), instead of using a single modulator: pyridine for

nanosheets (Route 2) and acetic acid for nanorods (Route 3).

5.2 Experimental

Chemicals: Copper(II) acetate monohydrate (99%), 1,4-naphthalenedicarboxylic acid

(94%), 1,4-diazabicyclo[2.2.2]octane (99%), pyridine (99%), and N,N-

dimethylformamide (DMF, 99.8%) were purchased from Sigma-Aldrich. Acetic acid

(99.7%) was purchased from Fisher Scientific. All chemicals were used as received without


Synthesis: For a typical synthesis of [Cu2(ndc)2(dabco)]n nanocubes, a solution of 70 mg of

1,4-naphthalenedicarboxylic acid (0.06 M), 18 mg of 1,4-diazabicyclo[2.2.2]octane (0.03

116

M) and 0.38 mL of pyridine (0.9 M) in 5 mL of DMF was poured into a solution of 90 mg

of Cu(CH3COO)2H2O (0.06 M) and 0.26 mL of acetic acid (0.6 M) in 7.5 mL of DMF in a

glass vial under vigorous stirring. The synthetic mixture was stirred for 30 min before

transferred into an autoclave and subsequently heated at 100 °C for 24 h. The resulted

product was filtered, washed several times with anhydrous ethanol, and dried at 75 °C

overnight.

The same process was applied to synthesizing [Cu2(ndc)2(dabco)]n nanosheets. In this

synthesis, a solution of 70 mg of 1,4-naphthalenedicarboxylic acid (0.06 M), 18 mg of 1,4-

diazabicyclo[2.2.2]octane (0.03 M) and 0.70 mL of pyridine (1.5 M) in 5 mL of DMF was

first poured into a solution of 90 mg of Cu(CH3COO)2H2O (0.06 M) in 7.5 mL of DMF.

The subsequent process was exactly the same as that for the nanocubes.

For the benchmark, the bulk [Cu2(ndc)2(dabco)]n material was also prepared. First, a

solution of 70 mg 1,4-naphthalenedicarboxylic acid (0.06 M) and 18 mg 1,4-

diazabicyclo[2.2.2]octane (0.03 M) in 5 mL DMF was poured into a solution of 90 mg

Cu(CH3COO)2H2O (0.06 M) in 7.5 mL DMF. The subsequent steps were performed the

same as that for the nanocubes and nanosheets.

Characterization. Transmission electron microscopy (TEM) images were obtained using a

JEOL JEM 1230 microscope operating at 120 kV. Samples for TEM measurements were

prepared by depositing each drop of the dispersions of the products in anhydrous ethanol

onto a carbon-coated copper grid (200 mesh). The excess of the solvent was wicked away

with a filter paper, and the grids were then dried in air. Powder X-ray diffraction (XRD)

patterns were collected on a Bruker SMART APEX II X-ray diffractometer with Cu K

radiation ( = 1.5406 Å in the 2θ range 5 30° at a scan rate of 1.0o min

-1.

Thermogravimetric analysis (TGA) was carried out with a Q5000 thermal analysis

instrument (TGA Q5000) from room temperature to 600 °C with a heating rate of 5 °C

min‒1

under an air flow of 25 mL min‒1

. The adsorption-desorption of N2 at 77 K and of

CO2 at 273 K were performed with a Quantachrome Autosorb-1 system. The products were

outgassed under vacuum at 373 K overnight before the gas sorptions. The

117

Brunauer‒Emmett‒Teller (BET) surface area was calculated from the experimental

pressure range identified by established consistency criteria reported by Walton and Snurr

for MOFs,18

herein the 0.05‒0.2 P/Po range of linearity of the N2 adsorption isotherm. The

pore volume was calculated as the N2 uptake at the relative pressure of 0.9.


Nanocubes of [Cu2(ndc)2(dabco)]n MOF were achieved by using simultaneously acetic acid

and pyridine as selective modulators. Figure 5.1 shows TEM images of a series of samples

prepared with different pyridine concentrations, while keeping the same acetic acid

concentration at 0.6 M. At pyridine concentrations less than 0.6 M, nanocubes along with

nanobars and nanorods were observed (Figure 5.1a and Figure S5.1). However, at higher

pyridine concentrations (≥0.6 M) only nanocubes were produced (Figure 5.1b-d). The

three-dimensional shape of the MOF nanocrystals was determined from TEM images

oriented differently toward the incident electron beam.19

The size of nanocubes decreases

slightly with increasing the pyridine concentration, from 100 35 nm at 0.6 M to 80 20

nm at 1.2 M pyridine. However, uniform size and well-defined nanocubes were observed at

0.9 M pyridine (Figure 5.1c).

In the presence of 1.5 M pyridine without acetic acid, the formation of nanosheets was

observed (Figure 5.2 and Figure S5.2). This is attributed to the selective modulation of

pyridine for the copper-dabco mode. On the contrary, in the absence of pyridine at the same

acetic acid concentration (0.6 M), only nanorods were yielded (Figure S5.3) as previously

reported by Kitagawa et al., due to the selective modulation of acetic acid for copper-ndc

mode.10b

Therefore, the formation of the nanocubes in our approach suggests that pyridine has a role

in modulating the copper-dabco coordination on the two (00l) facets and impedes the

crystal growth in the [001] direction, whereas acetic acid plays a role in modulating the

copper-ndc mode on the four remaining (h00) and (0k0) facets and thus suppresses the

118

crystal growth in the [100] direction. As a result, the crystal growth in both the [100] and

[001] directions is regulated simultaneously to yield nanocubes.

Figure 5.1 Morphology evolution of [Cu2(ndc)2(dabco)]n nanocrystals prepared at 0.6 M

acetic acid with various concentrations of pyridine: (a) 0.3 M; (b) 0.6 M; (c) 0.9 M and (d)

1.2 M. The scale bar is 200 nm.

Figure 5.2 TEM images of [Cu2(ndc)2(dabco)]n nanosheets prepared in the presence of 1.5

M pyridine without acetic acid.

119

When the pyridine concentration was kept unchanged (e.g., 0.6 M) at acetic acid

concentrations less than 0.6 M, nanocubes with non-uniform sizes along with nanobars

were found (Figure S5.4a). However, at higher acetic acid concentrations ( 0.6 M), only

nanocubes were yielded (Figure 5.1b and Figure S5.4b, c). The size of the nanocubes

decreases slightly with an increasing concentration of acetic acid. It has been reported that a

larger amount of the modulators enables a larger external surface area of the nanocrystals to

be capped,10b

leading to smaller nanocubes when prepared at higher pyridine and acetic

acid concentrations.

Figure 5.3 XRD patterns for [Cu2(ndc)2(dabco)]n nanocubes prepared by keeping the acetic

acid concentration (0.6 M) unchanged with various concentrations of pyridine: (a) 0.6 M;

b) 0.9 M; (c) 1.2 M; (d) the nanosheets prepared at 1.5 M pyridine without acetic acid; (e)

the nanorods prepared at 0.6 M acetic acid without pyridine and (f) the simulated pattern

created from CIF file.17a

2 Theta (degree)

5 10 15 20 25 30

Inte

nsit

y

Simulation

Nanorods

Nanocubes: 1.2 M

Nanocubes: 0.9 M

Nanocubes: 0.6 Ma)

b)

c)

d)

e)

(100)

(200)

(110)

(001)

(101)

(210)

(201)

(102)

(300)

(310)

(103)

(311)

(310)

Nanosheets

f)

120

The XRD patterns for a series of the nanocubes prepared by keeping unchanged the 0.6 M

acetic acid concentration but varying the concentration of pyridine, along with the

nanosheets fabricated with 1.5 M solution of pyridine in the absence of acetic acid, exhibit

intense diffraction peaks that match those of the simulation, indicating highly crystalline

nanomaterials (Figure 5.3). It is also noted that the relative ratios of peak area for the (001)

reflection of the nanocubes and of the nanosheets are smaller than that of the nanorods.

This is attributed to the suppression of the crystal growth along the [001] direction of the

nanocubes and the nanosheets in the presence of pyridine.

Figure 5.4 Gas adsorption (solid symbols)-desorption (open symbols) isotherms of N2 at 77

K (a) and CO2 at 273 K (b) for [Cu2(ndc)2(dabco)]n nanocubes prepared at 0.6 M acetic

acid and 0.6 M pyridine (circles); for the nanosheets prepared at 1.5 M pyridine without

acetic acid (triangles); for the nanorods prepared at 0.6 M acetic acid without pyridine

(diamonds); and for the bulk MOF material (squares).

The adsorption and desorption of N2 at 77 K and CO2 at 273 K for the nanocubes prepared

at 0.6 M acetic acid and 0.6 M pyridine, the nanosheets prepared at 1.5 M pyridine without

acetic acid and the nanorods prepared at 0.6 M acetic acid without pyridine are shown in

Figure 5.4 and Table 5.1. The obtained results indicate that the BET surface area and pore

volume of the nanomaterials are greater than those of the bulk MOF material. As seen in

Figure 5.4b, the nanomaterials have a high CO2 uptake at 273 K and 1 bar.20

Interestingly,

a)

P/Po

0.0 0.2 0.4 0.6 0.8 1.0

N2 a

dsorb

ed /cm

3g-1

0

100

200

300

400

500

Pressure (bar)

0.0 0.2 0.4 0.6 0.8 1.0

CO

2 a

dsorb

ed /cm

3g-1

0

40

80

120 b)

Bulk material

Nanorods

Nanosheets

Nanocubes

121

the nanocube and nanosheet samples show slightly higher CO2 uptakes than that of the bulk

and the nanorod materials at 273 K and 1 bar (Table 5.1); even if the BET surface area and

pore volume of the nanocube sample are lower than those of the nanorod one. Lee et al.

have reported that the gas sorption property of MOF microcrystals can vary according to

crystal morphology or exposed surfaces.21

Therefore, in the case of [Cu2(ndc)2(dabco)]n

nanocrystals, it is expected that the nature of the exposed crystal facets terminated by the

different modulators and the orientation of the two porous channels of the MOF structure

relative to these exposed facets will lead to a differentiation in N2 and CO2 uptakes. TGA

profiles indicate the same thermal stability of the nanocubes, nanosheets and nanorods, up

to 270 °C (Figure S5.5).

Table 5.1 Porosity and CO2 uptake of the nanocrytals and the bulk material.

Sample

Modulator Concentration

(M) BET surface

area (m2g

-1)

Pore volume

(cm3g

-1)

CO2 uptake at 1 bar,

273 K (mmolg-1

) Acetic acid Pyridine

nanocubes 0.6 0.6 1040 0.50 5.0

nanosheets 0 1.5 1175 0.54 4.8

nanorods 0.6 0 1180 0.56 4.3

bulk material 0 0 916 0.47 3.5

To investigate the capability to modulate of other amines, including tertiary and primary

amines, for the control of the morphology of the nanocrystals, we employed triethylamine,

n-dodecylamine, n-octylamine, and methylamine while keeping the same 0.6 M acetic acid

concentration. However, the introduction of these amines in the synthetic mixture did not

completely yield cubic nanocrystals (Figure 5.5). This can be due to their weak competition

with the dabco linker for interaction with copper dimers, these amines being worse ligands

than pyridine. Moreover, these aliphatic amines could be protonated and rendered inactive

by acetic acid.

122

For pyridine, the lone pair on its nitrogen atom is in the same plane and in the same

direction with the dipole moment and conjugates with the electrons of the aromatic ring;

these may induce the nitrogen atom to compete with that of dabco linker for the

coordination to the metal ions.22

As seen in Figure 5.5, in the case of triethylamine with

concentrations less than or equal to 0.6 M, a precipitate containing nanocubes along with

nanobars was formed. However, higher concentrations of triethylamine only yielded a

reddish orange solid probably caused by the reduction of copper ions. With the other

amines such reduction occurred at a much lower amine concentration (at 0.1 M) and the

coordination modulation for copper-dabco mode eventually did not occur. As a

consequence, only nanorods were produced.

Figure 5.5 TEM images of [Cu2(ndc)2(dabco)]n nanocrystals fabricated in the presence of

amines with 0.6 M acetic acid. (a-c) The different concentrations of triethylamine: (a) 0.1

M, (b) 0.3 M, (c) 0.6 M, (d) 0.1 M n-dodecylamine, (e) 0.1 M n-octylamine, and (f) 0.1 M

methylamine. The scale bar is 500 nm.

123

5.4 Conclusions

We demonstrated the simple coordination modulation approach for the rational synthesis of

nanocubes and nanosheets of [Cu2(ndc)2(dabco)]n MOF whose structures are composed of

two coordination modes (metal-carboxylate and metal-amine modes) using simultaneously

selective modulators (acetic acid and pyridine) or only one selective modulator (pyridine),

respectively. The introduction of selective capping reagents that can cap different facets of

growing MOF crystals allows us to prepare nanocrystals with controlled morphologies. In

MOF chemistry, a large number of structures have crystal facets with intrinsically distinct

chemical features that allow different functionalities to adhere selectively onto the facets.23

Such methodology allows to tailor relative ratio of exposed crystal faces.

5.5 Appendix

Figure S5.1 TEM images of [Cu2(ndc)2(dabco)]n nanocrystals prepared at 0.6 M acetic acid

with the concentration of pyridine at 0.1 M (a) and 0.45 M (b). The scale bar is 200 nm.

124

Figure S5.2 TEM images of [Cu2(ndc)2(dabco)]n nanosheets prepared in the absence of

acetic acid with the concentration of pyridine at 1.5 M. The scale bar is 200 nm.

Figure S5.3 TEM image of [Cu2(ndc)2(dabco)]n square nanorods prepared at 0.6 M acetic

acid in the absence of pyridine.

125

Figure S5.4 TEM images of [Cu2(ndc)2(dabco)]n nanocrystals prepared at 0.6 M pyridine

with the concentration of acetic acid at 0.3 M (a); 0.9 M (b) and 1.2 M (c). The scale bar is

200 nm.

Figure S5.5 TGA curves for [Cu2(ndc)2(dabco)]n nanocubes prepared at 0.6 M acetic acid

with 0.6 M pyridine (a), for the nanosheets fabricated at 1.5 M pyridine in the absence of

acetic acid (b) and for the nanorods prepared by 0.6 M acetic acid without pyridine (c).

Temperature (oC)

100 200 300 400 500 600

Weig

ht (%

)

20

40

60

80

100

(a)

(b)

(c)

126

5.6 References

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Furukawa, S.; Takashima, Y.; Yoshida, K.; Isoda S.; Kitagawa, S. Angew. Chem. Int.

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129

Chapter 6 Novel Route to Size-Controlled FeMIL-88BNH2

MOF Nanocrystals

Minh-Hao Pham,ý Gia-Thanh Vuong,

ý Anh-Tuan Vu,

ffi and Trong-On Do

*,ý

†Department of Chemical Engineering, Laval University, Quebec City, Quebec G1V 0A6,

Canada

‡Institute of Chemistry, Vietnamese Academy of Science and Technology, Building A18, 18

Hoang Quoc Viet, Cau Giay, Hanoi, Vietnam

Published in Langmuir 2011, 27, 15261–15267

131

Résumé

Une nouvelle approche pour la synthèse des monocristaux de composés organiques

moléculaires à réseau métallique (MOF) avec la taille et la morphologie contrôlées a été

développée. Cette approche utilise simultanément des copolymères non ioniques (tribloc

F127 comme agent stabilisant et l‟acide acétique comme agent de déprotonation. Le

segment allylène du copolymère tribloc F127 coordonne avec l‟ion métallique et stabilise

les nucléides du MOF au premier stage de la formation du monocristal du MOF. L‟acide

acétique contrôle la déprotonation de la molécule liante carboxylique durant la synthèse, ce

qui lui confère le contrôle de la vitesse de nucléation, conduisant à la formation

architecturale du monocristal (la taille et le rapport de longueur sur largeur). Le FeMIL-

88BNH2 est un cristal de fer de structure MOF, ce type de matériaux a été sélectionné

pour illustrer cette approche. Le résultat montre que cette approche n‟est pas seulement

pour synthétiser des nanocristaux uniformes, mais aussi contrôler la taille et la morphologie

du matériel. La taille et le rapport d‟aspect (longueur sur largeur augmentent avec

l‟augmentation de la concentration d‟acide acétique dans le mélange réactionnel. Le

copolymère non ionique tribloc F127 et l‟acide acétique peuvent être éliminés facilement

du monocristal FeMIL-88BNH2 produit par lavage avec l‟éthanol, ainsi les groupes

amines dans Fe-MIL-88B-NH2 sont disponibles comme fonction basique pour des

applications pratiques. Cette approche est destinée pour la synthèse d‟une variété de

membres de MOFs d‟origine carboxylique, tels que: MIL-53, MIL-89, MIL-100, et MIL-

101.

133

Abstract

A new approach for the synthesis of uniform metal-organic framework (MOF) nanocrystals

with controlled- sizes and aspect ratios has been developed using simultaneously nonionic

triblock copolymer F127 and acetic acid as stabilizing and deprotonating agents,

respectively. The alkylene oxide segments of the triblock copolymer can coordinate with

metal ions and stabilize MOF nuclei in the early stage of the formation of MOF

nanocrystals. Acetic acid can control the deprotonation of carboxylic linkers during the

synthesis and, thus enables the control of the rate of nucleation, leading to the tailoring of

the size and aspect ratio (length/width) of nanocrystals. Fe-MIL-88B-NH2, as an iron-based

MOF crystal was selected as a typical example to illustrate our approach. The results reveal

that this approach is used for not only the synthesis of uniform nanocrystals but also the

control of the size and aspect ratio of the materials. The size and aspect ratio of

nanocrystals increase with an increase in the concentration of acetic acid in the synthetic

mixture. Nonionic triblock copolymers F127 and acetic acid can be easily removed from

the Fe-MIL-88B-NH2 nanocrystals products by washing with ethanol, and thus, their amine

groups are available for practical applications. The approach is expected to synthesize

various nanosized carboxylate-based MOF members such as MIL-53, MIL-89, MIL-100,

and MIL-101.

135

6.1 Introduction

Metal-organic frameworks (MOFs) are crystalline porous materials whose structure is

composed of metal-oxide units joined by organic linkers through strong covalent bonds.

These materials exhibit high surface areas (up to 6500 m2g

-1) and high pore volumes (1-2

cm3g

-1), and are of considerable interest for many potential applications.

1,2 However,

because of their microporosity (generally, pore size of < 2 nm), the potential applications of

these materials are limited. To overcome this limitation, as in the case of microporous

zeolites,3,4

one can either create mesopore channels within MOF crystals or reduce the

crystal to nanosize. Attempts have been made to synthesize porous MOFs with large pore

diameters in the range of 2 to 50 nm.5-7

For example, MIL-1015 and UMCM-2

6 exhibit

mesoporous behavior owing to mesoporous cages found throughout the structures;

however, the mesopores are often restricted by small apertures that prohibit large molecules

from accessing the space inside. Increasing the length of the organic linker is another

option, but with few exceptions,8 MOFs built up from long linkers tend to collapse upon

guest removal9 or form catenated structures.

10 Recently, the synthesis of hierarchically

micro- and mesoporous MOF materials in an ionic liquid/supercritical CO2/surfactant

emulsion system was reported.11

These materials combine advantages of both meso- and

micropores and have potential applications in gas separation and catalysis.

Recently, nanoscale MOF crystals have emerged as important candidates with a significant

impact for drug delivery,12

biomedical imaging applications,13,14

chemical sensing,15

and

gas separation,16

owing to easier transport of guest molecules through nanosize, short

diffusion pathways and exposed active sites within MOF nanocrystals.17,18

Furthermore, the

external surface of MOF nanocrystals also offers the ability to functionalize core-shell

nanostructures for biomedical sensing and imaging.19

The adsorbate-induced structural

flexibility of some MOFs, in which the unit cell parameters can vary significantly when the

guests are adsorbed within their pores enables the induction of stress at the interfaces

between MOF thin film and second material. This behavior yields a signal transduction for

chemical detectors.15

The tunable and amenable nature of pores within MOFs by the

136

judicious choice of metal clusters and organic linkers offers a potential application for

selective gas separation.20

Even though different methods have been developed for the syntheses of crystalline

nanosized MOFs, including water-in-oil microemulsions,21,22

surfactant-mediated

hydrothermal syntheses,23

sonochemistry,24

microwave-assisted routes,12,25

and

coordination modulation,26,27

the precise control over the size and shape still remains a

challenge. For example, the microwave- and ultrasound-assisted approach allows for

fabrication of MOF nanocrystals for a short period by heating reaction mixtures under

microwave irradiation.12,24

Although this method is simple and environmentally friendly, it

is difficult to tune the size and shape of the resulted nanocrystal. In the presence of a

surfactant under microwave heating, the synthetic procedure can also alter serendipitously

the morphologies of MOF nanocrystals.25

Using the surfactant-mediated synthetic method,

Uemura et al. indicate that organic polymers can be used as inhibitors for the synthesis of

coordination polymer nanoparticles,23

because the protecting polymers can coordinate

weakly with metal ions to provide steric stabilization that allows for the formation of

nanoparticles. In the case of the reverse-phase microemulsion-based methodology,

surfactant molecules can be used for the formation of microemulsion nanodroplets

containing metal ions and linkers.21

The nanodroplets act as microreactors for the formation

of MOF nanopaticles. Although this method enables the tuning of the size and shape of

nanocrystals by adjusting reaction conditions, it usually leads to the aggregation of the

nanocrystals. In some cases, using the reverse-phase microemulsion at room temperature,

only amorphous material is produced.14

The coordination modulation using capping

reagents (modulators) with the same functionality of organic linkers enables the impeding

of coordination interactions between metal ions and organic linkers to fabricate MOF

nanocrystals.26,27

This modulation allows for the control of the rate of framework extension

and crystal growth. Carboxylate-based MOF nanocrystals, such as nanosized MOF-5

colloids26a

and [Cu2(ndc)2(dabco)n] nanorods (ndc = 1,4-naphthalenedicarboxylate, and

dabco = 1,4-diazabicyclo[2.2.2]octane)27

are prepared by using monocarboxylic acids (p-

perfluoromethylbenzenecarboxylate) and acetic acid as modulators, respectively.

137

In this study, we report a new approach for the size-controlled synthesis of uniform

carboxylate-based MOF nanocrystals with high crystallinity using simultaneouslly nonionic

triblock copolymer and acetic acid. In this approach, alkylene oxide segments of triblock

copolymer PEO-PPO-PEO [poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene

oxide)] that can coordinate with metal ions28

play a crucial role in yielding MOF

nanocrystals. Acetic acid owning carboxylic functionality that enables the tuning of the rate

of the deprotonation of the carboxylic linkers allows us to not only control rationally the

size of the MOF nanocrystals but also obtain high crystallinity. Fe-MIL-88B-NH2

Fe3O(H2N-BDC)3 with H2N-BDC = 2-aminoterephtalic acid] as an iron-based MOF

nanocystal is selected to illustrate our approach, because this iron-based MOF material is

recognized as having nontoxic nature, highly flexible framework and drug adsorption

capacity, and the controlled delivery of drugs in the human body.12

Fe-MIL-88B-NH2

nanocystals are obtained from iron salts [FeCl3 or Fe(NO3)3], 2-aminoterephtalic acid,

acetic acid, Pluronic F127, and water as synthetic medium. This approach is also expected

to achieve other members of MOFs.

6.2 Experimental

Chemicals: FeCl36H2O (Aldrich, 97%), Fe(NO3)39H2O (Aldrich, 98%), H2N-BDC

(Aldrich, 99%), Pluronic F127 (EO97PO69EO97, with an average Mn = 12600, Aldrich), and

CH3COOH (Fisher, 99.7%) were obtained. All chemicals were used as received without


Synthesis: The size-controlled synthesis of Fe-MIL-88B-NH2 nanocrystals was

accomplished using a hydrothermal route with FeIII

salt and 2-aminoterephthalic acid as the

metal source and organic linker, respectively, with Pluronic F127 and acetic acid. The

reaction mixtures with molar ratios Fe3+

/H2N-BDC/H2O/F127/CH3COOH of

1:0.5:1255:x:y were crystallized for 24 h at 110 °C. The x value (F127/Fe3+

molar ratio)

and y value (CH3COOH/Fe3+

molar ratio) were altered to control the size of nanocrystals.

In a typical synthesis, 0.16 g of F127 (x = 0.02) was dissolved in 13.34 mL of deionized

water and a volume of 1.66 mL of 0.4 M aqueous solution of FeCl36H2O (0.66 mmol) was

138

poured into this surfactant solution. The resulted solution was stirred for 1 h before 0.3 mL

of acetic acid (y = 8) was injected. After stirring for an additional 1 h, 60 mg (0.33 mmol)

of H2N-BDC was added. The reaction mixture was stirred for 2 h before transferring into an

autoclave for the crystallization. The dark brown solid product was recovered and washed

several times with ethanol by centrifugation to remove the surfactant and excess reactants.

To investigate the effect of the F127/Fe3+

molar ratio (x value) and CH3COOH/Fe3+

molar

ratio (y value) on the crystals size, the x and y values were tuned from 0.01 to 0.16 and

from 0 to 16, respectively. The yield of the reactions was approximately 61% based on

H2N-BDC. For comparison, the Fe-MIL-88B-NH2 microcrystals were also prepared under

the same synthetic conditions, except that both F127 and acetic acid were not added.29

Characterizations: Transmission electron microscopy (TEM) images were obtained using

a JEOL JEM 1230 microscope operating at 120 kV. Samples for TEM measurements were

prepared by depositing a drop of the dispersions of the products in absolute ethanol onto

carbon-coated copper grids (200 mesh). The excess of the solvent was wicked away with

filter paper, and the grids were dried in air. The mean sizes of nanocrystals were

determined from statistic distributions evaluated according to TEM images on 100

particles. Scanning electron microscopy (SEM) images were taken on a JEOL 6360

instrument at an accelerating voltage of 3 kV. Powder X-ray diffraction (XRD) patterns

were collected on a Bruker SMART APEX II X-ray diffractometer with Cu K radiation (

= 1.5406 Å in the 2θ range 5 20° at a scan rate of 1.0o min

-1. All samples were dried at

100 °C overnight to remove guest solvent molecules within the pores before XRD scan.

Fourier transform infrared (FTIR) spectra were recorded on a FTS 45 spectrophotometer in

the spectral range 4000 400 cm-1

using the KBr disk method. Thermogravimetric analysis

(TGA) was carried out with TGA/SDTA 851e thermogravimetric analyzer from room

temperature to 600oC with a heating rate of 5 °C min

-1 under an air flow of 50 mL min

-1. -

potential measurements were performed with a Zetasizer Nano ZS in water at 25 °C.

139


Different stages for the synthesis of Fe-MIL-88B-NH2 (i) without and (ii) with triblock

copolymer F127 and (iii) with the simultaneous presence of Pluronic F127 and acetic acid

in the synthetic mixture were studied to illustrate the role of F127 and acetic acid in the

control over the size of Fe-MIL-88B-NH2. Figure 6.1 shows electron microscopy images of

Fe-MIL-88B-NH2 samples prepared without and with triblock copolymer F127. In the

synthesis without Pluronic F127, using the method described in ref 29, Fe-MIL-88B-NH2

microsized crystals were produced from an aqueous reaction mixture of FeCl36H2O and

H2N-BDC with the morphology of bipyramidal hexagonal prism, and size of 3.5 µm in

length and 1.2 µm in width (Figure 6.1a). Under the same synthetic conditions, however, in

the presence of triblock copolymer F127, Fe-MIL-88B-NH2 nanocrystals were obtained.

Even though the particle size decreases from micro- to nanometers in the presence of F127,

the morphology of the bipyramidal hexagonal prism still remains unchanged (Figure 6.1a,

b).12,30

Furthermore, when the F127/Fe3+

molar ratio varies from 0.01 to 0.04, no significant

change in size of nanocrystals was found (Figure 6.1b-d); the average size of the

nanocrystals is 50 5 nm in length and 30 5 nm in width. However, when the molar ratio

of F127/Fe3+

in the synthetic mixture was above 0.04 (i.e., F127/Fe3+

= 0.08 and 0.16),

TEM images of these samples exhibit non-uniform nanoparticles. Some big nanocrystals

besides small nanocrystals were observed (Figure 6.1e, f). These results indicate that

triblock copolymer F127 surfactant plays some role in the preparation of Fe-MIL-88B-NH2

nanocrystals. However, the size of nanocrystals could not be controlled by adjusting the

amount of F127 in the synthetic mixture.

140

Figure 6.1 Representative electron microscopy images of different Fe-MIL-88B-NH2

samples: (a) SEM image of Fe-MIL-88B-NH2 micro-crystals (Inset: schematic morphology

of the crystals); (b-f) TEM images of Fe-MIL-88B-NH2 nanocrystals prepared from

different F127/Fe3+

molar ratios: (b) x = 0.01, (c) x = 0.02, (d) x = 0.04, (e) x = 0.08 and (f)

x = 0.16.

To control the size of Fe-MIL-88B-NH2 nanocrystals, both Pluronic F127 and acetic acid

(CH3COOH) were introduced into the reaction mixtures. The simultaneous presence of

acetic acid and the F127 surfactant in the synthetic solution strongly influences the size and

aspect ratio of the bipyramidal hexagonal prism of Fe-MIL-88B-NH2 nanocrystals. In this

study, the F127/Fe3+

molar ratio was kept unchanged (x = 0.02); however, the

CH3COOH/Fe3+

molar ratio (y value) varies from 4 to 16. Figure 6.2 shows representative

TEM images of Fe-MIL-88B-NH2 samples obtained from different CH3COOH/Fe3+

molar

ratios with the same F127/Fe3+

molar ratio of 0.02. Interestingly, the size and aspect ratio of

the resulted nanocrystals increase with an increase of the amount of CH3COOH added in

the synthetic mixture. At a low CH3COOH/Fe3+

molar ratio (y 4), no significant

difference in the size and shape of the nanocrystals was observed (Figure 6.2a), as

compared to those of the nanocrystals prepared without acetic acid (Figure 6.1c). However,

with an increase of the CH3COOH/Fe3+

molar ratio from 4 to 8 and to 16, the size of

nanocrystal products increases rapidly from 50 5 nm to 150 10 nm and to 500 10 nm

141

in length and from 30 5 nm to 60 10 nm and to 150 10 nm in width, respectively; the

aspect ratio (length/width) increases from 1.5 to 3.5. The same trend was also observed

when Fe(NO3)39H2O was used as a metal source instead of FeCl36H2O (Figure S6.1,

Figure S6.2). On the contrary, when the CH3COOH/Fe3+

molar ratio was kept unchanged (y

= 0.04) but the F127/Fe3+

molar ratio (x value) varied from 0.01 to 0.04, the size and shape

of the nanocrystals are almost unchanged (Figure S6.3). This behavior is similar to that for

the case without acetic acid.

Figure 6.2 TEM images of Fe-MIL-88B-NH2 nanocrystal samples prepared with the same

F127/Fe3+

molar ratio of 0.02 (x = 0.02) for all experiments at different CH3COOH/Fe3+

molar ratios (y = CH3COOH/Fe3+

): (a) y = 4, (b) y = 8, (c) y = 12 and (d) y = 16. (Inset:

distributions of the width and the aspect ratio of nanocrystals, respectively).

Therefore, adjusting the molar ratio of CH3COOH/Fe3+

allows for the control of the size of

the nanocrystals. It is obvious that, with a high CH3COOH/Fe3+

molar ratio (y 8) in the

synthesis mixture, highly well-defined morphologies and isolated Fe-MIL-88B-NH2

Crystal width (nm)

100 140 180 220

%

0

10

20

30

40

Aspect Ratio

2 3 4

%

0

10

20

30

40

Crystal width (nm)

100 140 180 220

%

0

10

20

30

40

Aspect Ratio

2.5 3.5 4.5

%

0

10

20

30

40

50

Crystal width (nm)

30 50 70

%

0

10

20

30

40

50

Aspect Ratio

1.5 2.5 3.5

%

0

10

20

30

40

50

Crystal width (nm)

20 30 40 50

%

0

10

20

30

40

Aspect Ratio

1.0 1.5 2.0

%

0

10

20

30

40

142

nanocrystals were obtained. This also suggests that a high concentration of acetic acid

favors the formation of well-defined crystalline nanocrystals.

In the previous research on the synthesis of carboxylate-based MOF nanocrystals via

coordination modulation using acetic acid as the capping reagent (in the absence of the

F127 surfactant),27

the presence of acetic acid allows the formation of [Cu2(ndc)2(dabco)n]

nanorods and the higher concentration of acetic acid leads to the smaller width of the

nanorods. In that case, acetic acid competes with the dicarboxylic linker (1,4-naphthalene

dicarboxylic acid) to coordinate with dicopper clusters to create nanocrystals. A larger

amount of acetic acid can passivate a larger fraction of external surface area, resulting in

smaller nanocrystals. On the contrary, in this work, for the synthesis of Fe-MIL-88B-NH2

crystals, the presence of acetic acid (without the F127 surfactant) in the synthetic mixture

led to the formation of Fe-MIL-88B-NH2 microsized crystals (Figure S6.4). This could be

mainly due to the difference in the synthesis medium: the aqueous medium for Fe-MIL-

88B-NH2 instead of the organic solvent for [Cu2(ndc)2(dabco)n].

Furthermore, in the simultaneous presence of F127 and acetic acid, nanosized Fe-MIL-

88B-NH2 crystals were obtained. The nanosize of Fe-MIL-88B-NH2 increases with an

increase of the concentration of acetic acid in the synthetic mixture. These results suggest a

different behavior of acetic acid in the formation of the Fe-MIL-88B-NH2 crystals in the

presence of the F127 surfactant. In this case, triblock copolymer F127 plays an important

role in the formation of MOF nanocrystals because of its ability to coordinate with metal

ions,28

while acetic acid tailors the size of nanocrystals by controlling the deprotonation of

carboxylic linkers during synthesis.

Figure 6.3 shows the powder XRD patterns for the Fe-MIL-88B-NH2 nanocrystal samples

prepared with various molar ratios of CH3COOH/Fe3+

(y = 0 16) in the presence of

surfactant F127 (F127/Fe3+

= 0.02), and the microcrystal sample prepared in the absence of

both triblock copolymer F127 and acetic acid. As a reference, the simulated XRD pattern

for the chromium(III)-based MIL-88B structure (denoted as Cr-MIL-88B) created from the

crystallographic information file (CIF) is also present.31

As seen in Figure 6.3, the XRD

143

reflections of the nanocrystal samples match those of the simulated XRD pattern of Cr-

MIL-88B, readily indexing hexagonal space group P63/mmc of the MIL-88B structure of

the synthesized nanocrystals.29,31,32

On the basic of the Bragg peak positions, the calculated

unit cell parameters of the dried forms of all nanocrystals are a 9.7 Å and c 19.0 Å,

which are similar to those of the anhydrous form of Cr-MIL-88B (a 9.6 Å, c 19.1

Å).31,33

The intensity of the diffraction peaks for the nanocrystals increases with an

increasing CH3COOH/Fe3+

molar ratio, suggesting that, with a higher amount of acetic acid

in the synthetic mixture, a higher crystallinity of the samples can be obtained. This is

consistent with the morphologies observed by TEM images. This also suggests the role of

acetic acid in the improvement of the crystallinity of the nanocrystals.

The XRD pattern for the Fe-MIL-88B-NH2 microcrystals exhibits the diffraction peak with

the highest intensity at 2 of 12.2. It is noted that the slight discrepancies in intensity and

the 2 position of the reflections are due to the structural flexibility of MIL-88B.29,30,33

Because of the structural flexibility, the guest species (organic and/or inorganic species) in

the pore channels can lead to the motions of the skeleton accompanied by the position shift

as well as the change in intensity of the reflections. The magnitude of the motion is strongly

affected by the degree of pore-filling and the nature of the guests.33-35

Generally, it is

difficult to remove completely guest molecules within the pore channels of large MOF

crystals (microsize) compared to those in small MOF crystals (nanosize), because of longer

diffusion pathways. Furthermore, the shifted position of the reflections, as seen in Figure

6.3, is presumably attributed to the effect of the crystal size of Fe-MIL-88B-NH2.

144

Figure 6.3 Powder XRD patterns for Fe-MIL-88B-NH2 nanocrystal samples prepared at

different molar ratios of CH3COOH/Fe3+

(y = CH3COOH/Fe3+

) in the presence of

surfactant F127 (F127/Fe3+

= 0.02), and the micro crystals prepared in the absence of both

surfactant F127 and acetic acid. The simulated X-ray diffraction pattern for chromium (III)-

based MIL-88B structure created from CIF file.31

To identify the presence of amino groups in the resulted nanocrystals, FTIR spectra for

different Fe-MIL-88B-NH2 samples were recorded. Curves a and b of Figure 6.4 shows

FTIR spectra for the nanocrystal samples prepared in the presence of both Pluronic F127

(F127/Fe3+

= 0.02) and acetic acid (CH3COOH/Fe3+

= 8) and in the presence Pluronic

F127, however, in the absence of acetic acid, respectively. For comparison, FTIR spectra

Inte

nsit

y

002101

102 103200

201

2-Theta (degree)

6 8 10 12 14 16 18 20

Simulation

y = 0

y = 4

Micro-crystals

y = 8

y = 12

y = 16

145

for the Fe-MIL-88B-NH2 microcrystals (Figure 6.4c) and free 2-aminoterephthalic acid

linker (Figure 6.4d) are also presented. In general, the samples of Fe-MIL-88B-NH2 nano-

and microcrystals exhibit similar FTIR spectra (curves a-c of Figure 6.4). Two bands at

around 3490 cm-1

and 3370 cm-1

, which are attributed to the symmetric and asymmetric

stretching absorptions of primary amine groups, were observed. These FTIR bands

correspond to those of free 2-aminoterephthalic acid (Figure 6.4d), indicating the amino

groups in the nanoscale MOF crystals.36

No band at around 1700 cm-1

that is characteristic

of protonated carboxylic groups was observed in curves a and b of Figure 6.4, suggesting

the absence of protonated carboxylic groups (because of acetic acid) in the nanocrystals.37

This also indicates that most acetic acid was removed from the nanocrystals upon washing.

It can be concluded that the introduction of acetic acid during the synthesis did not affect

amine groups in the nanocrystal products.

Figure 6.4 FTIR spectra of Fe-MIL-88B-NH2 nanocrystals prepared (a) in the presence of

both Pluronic F127 and acetic acid with the F127/Fe3+

molar ratio of x = 0.02 and

CH3COOH/Fe3+

of y = 8; (b) in the presence of Pluronic F127 (x = 0.02) and absence of

acetic acid; (c) Micro-crystals; and (d) Free 2-aminoterephthatic acid.

Wavenumber (cm-1

)

500 1000 1500 2000 2500 3000 3500 4000

a

b

Tra

ns

mit

tan

ce

-COOH

-NH2

c

d

146

Figure 6.5 -potential distributions in aqueous solution in at pH ~7 of a series of Fe-MIL-

88B-NH2 nanocrystals prepared from FeCl3.6H2O with different CH3COOH/Fe3+

molar

ratios (y = CH3COOH/Fe3+

) at the same F127/Fe3+

molar ratio of 0.02 (x = 0.02), before

(A) and after washing (B).

To identify the presence of the surfactant on the surface of the nanocrystal products after

the synthesis, -potential measurements were carried out in the neutral aqueous solution

(pH ~7) for a series of Fe-MIL-88B-NH2 nanocrystals prepared with different

CH3COOH/Fe3+

molar ratios in the presence of the F127 surfactant before washing (i.e., the

Zeta Potential (mV)

-80 -60 -40 -20 0 20 40 60 80

To

tal

Co

un

ts

y = 0

y = 4

y = 8

y = 12

y = 16

A

Zeta Potential (mV)

-80 -60 -40 -20 0 20 40 60 80

To

tal

Co

un

ts

y = 0

y = 4

y = 8

y = 12

y = 16

B

147

product obtained after removing the mother liquors without further purification) and after

washing with ethanol. The -potential curves of these samples are shown in Figure 6.5.

Before washing, in aqueous solution (at pH ~7), for all samples, no charge essential on the

surface of nanoparticles was observed. The -potential of these samples is close to the zero

point of charge (ZPC) (Figure 6.5A). No charge before washing could be attributed to

nonionic copolymers F127 capped on the nanocrystal surface. However, upon washing

several times with ethanol, in the aqueous solution at the same pH ~7, these samples exhibit

negative charges, which vary in the range of – 41 and – 51 mV (Figure 6.5B). The

Fe−MIL-88B−NH2 structure [Fe3O(solvent)2Cl(NH2-BDC)3msolvent] is constructed from

the trimers of µ3-O-bridged FeIII

octahedra [Fe3O(COO)6], which are connected by 2-

aminoterephtalic acids. Each FeIII

trimer needs a Cl- counteranion to maintain electric

neutrality. Chlorine anions are attributed to such a negative potential. This also indicates

that most of the surfactant was removed from the particle surface upon washing.

Figure 6.6 Thermogravimetric analysis (TGA) curves (solid) and their first derivative

thermogravimetric (DTG) curves (dashed) for Fe-MIL-88B-NH2 nanocrystals prepared

with CH3COOH/Fe3+

molar ratio of y = 8 and F127/Fe3+

molar ratio of x= 0.02 before (red)

and after washing (blue).

Before washing

After washing

Deri

v. W

eig

ht

(%/o

C)

0.0

0.2

0.4

0.6

0.8298

oC

254 oC

Temperature (oC)

100 200 300 400 500 600

Weig

ht

(%)

20

40

60

80

100

TGA DTG

148

TGA and derivative thermogravimetric (DTG) curves of the Fe-MIL-88B-NH2 nanocrystal

sample prepared with the CH3COOH/Fe3+

molar ratio of 8 and F127/Fe3+

molar ratio of

0.02 before and after washing are shown in Figure 6.6. Both samples exhibit similar TGA-

DTG profiles, but some results show the presence of the surfactant in the material before

washing. The TGA curves exhibit a major weight loss in the range of 200400 °C with two

maximum peaks at 254 °C and 298 °C (see DTG curves). The former is consistent with the

decomposition temperature of Pluronic F127,38

implying the presence of the F127

surfactant in the materials. The latter is attributed mainly to the MOF structure

decomposition.39

It is noted that the intensity of the peak at 254 °C and the loss weight in

the temperature range of 200265 °C for the Fe-MIL-88B-NH2 nanocrystals before

washing are much higher than those for this nanocrystal after washing, suggesting the most

of the F127 surfactant was removed. The difference in the weight loss of this sample before

and after washing was about 10 wt %.

The size-controlled formation of Fe-MIL-88B-NH2 nanocrystals in the presence of Pluronic

F127 and acetic acid can be explained by the fact that the alkylene oxide segments of the

triblock copolymer enable the weak coordination with metal ions.28

Therefore, the presence

of the triblock copolymer leads the stabilization of MOF nuclei at the early stage of the

synthesis. The subsequent crystal growth may involve the Ostwald ripening at the expense

of metal ions and linkers as well as smaller nanoparticles to yield MOF nanocrystals. The

formation of large nanoparticles at the cost of small nanoparticles is presumably due to the

energy difference between large and small nanoparticles.40

Besides, the nucleation and

growth of MOF nanocrystals could occur within the mesophases of F127 that act as

nanoreactors.41

Furthermore, the nucleation and crystal growth can be affected by the

presence of acetic acid. There is indeed a competitive interaction of dicarboxylic linkers

and acetic acid with iron ions during the nucleation and crystal growth processes. It is

shown that the formation of carboxylate-based MOFs is primarily dependent upon the

degree of deprotonation of carboxylic linkers.26a

The amount of acetic acid could control

the degree of deprotonation of carboxylic linkers. A higher CH3COOH/Fe3+

molar ratio

(i.e., a higher concentration of acetic acid) leads to a lower degree of deprotonation of the

H2N-BDC linker, and thus, the rate of nucleation and crystal growth could be slower,

149

resulting in bigger MOF nanocrystals.22, 42

The size-controlled fabrication of Fe-MIL-88B-

NH2 nanocrystals is illustrated by Scheme 6.1.

Upon adding H2N-BDC (linker molecules) into the aqueous solution containing the F127

surfactant and iron(III) salt, even at room temperature, dark precipitates immediately

appeared, indicating a rapid formation of Fe-MIL-88B-NH2 framework units, whereas this

phenomenon was not observed in the reaction mixture without F127 under the same

synthesis conditions. In the presence of acetic acid, a slower change in color from yellow

for the linker molecules to dark for MOF species in the early stage was also observed

because of the slow cost of linker molecules for the MOF formation.

Scheme 6.1 Schematic representation for the size-controlled fabrication of Fe-MIL-88B-

NH2 nanocrystals

The aspect ratio of nanocrystals is also controlled by the acetic acid concentration. The

aspect ratio increases with an increase of the molar ratio of CH3COOH/Fe3+

(Figure 6.2).

At a molar ratio of CH3COOH/Fe3+

equal or higher than 8, the growth of nanocrystals

along the [001] direction corresponding to the length of nanocrystals is preferable.30

Because all 2-aminoterephthalic acid molecules are oriented along the [001] direction,30

the

interaction between metal ions and the carboxylic groups of linker molecules along the

[001] direction is more favorable than others, resulting in faster growth. However, at low

concentrations of acetic acid or without acetic acid in the presence of the triblock

150

copolymer F127 surfactant, no significant change in the size and aspect ratio of the

nanocrystals is observed even with high F127 concentrations (Figure 6.1), indicating the

same effect of the F127 surfactant in all directions during the crystal growth.

6.4 Conclusions

We have demonstrated a new route to the synthesis of Fe-MIL-88B-NH2 nanocrystals with

controlled sizes and aspect ratios using simultaneously triblock copolymer and acetic acid

in the synthetic mixture. The alkylene oxide segments of block copolymer enable the

coordination with metal ions and the stabilization of MOF nuclei in the early stage and

have a key role in yielding uniform MOF nanocrystals. The size and aspect ratio of

nanocrystals can be adjusted by altering the amount of acetic acid in the synthetic mixture.

Acetic acid can control the degree of deprotonation of carboxylic linkers. A higher

concentration of acetic acid leads to a lower degree of deprotonation of the H2N-BDC

linker and, thus, a lower rate of nucleation and crystal growth, resulting in larger MOF

nanocrystals. This allows the tailoring of the crystal size from a few tenths to a few

hundredths of nanometers and the aspect ratio. We believe that this approach can be

extended to other carboxylate-based MOF nanocrystals. These nanocrystal materials have

potential applications in different fields, such as catalysis, chemical sensors, and related

advanced nanodevices, because of nanoscale size, short diffusion pathways, and more

exposed active sites.

151

6.5 Appendix

Figure S6.1 TEM images of Fe-MIL-88B-NH2 nanocrystals prepared from Fe(NO3)39H2O

at different molar ratios of CH3COOH/Fe3+

(y = CH3COOH/Fe3+

), while keeping the same

F127/Fe3+

molar ratio of 0.02 (x = 0.02): (a) y = 0, (b) y = 8, (c) y = 12 and (d) y = 16.

Figure S6.2 Powder XRD pattern for Fe-MIL-88B-NH2 nanocrystals prepared from

Fe(NO3)39H2O at CH3COOH/Fe3+

= 12 and F127/Fe3+

= 0.02 (red line), and the simulated

XRD pattern for chromium (III)-based MIL-88B structure (blue line).

2-theta (degree)

6 8 10 12 14 16 18 20

Inte

nsity

(a.u

)

002

103200

101

Simulation

152

Figure S6.3 TEM images of Fe-MIL-88B-NH2 nanocrystals prepared at the same

CH3COOH/Fe3+

molar ratio of 4 (y = 4) with different F127/Fe3+

molar ratios: (a) x = 0.01,

(b) x = 0.02 and (c) x = 0.04. The scale bar is 100 nm.

Figure S6.4 SEM image of Fe-MIL-88B-NH2 micro-crystals prepared from the synthetic

mixture containing FeCl36H2O, H2N-BDC and acetic acid (CH3COOH/Fe3+

= 8) without

F127 surfactant.

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157

Chapter 7 Hollow Fe2O3TiO2–PtOx Nanostructure with Two

Distinct Cocatalysts Embedded Separately on Two Surface Sides

for Efficient Visible Light Water Splitting to Hydrogen

Minh-Hao Pham,ý Cao-Thang Dinh,

ý Gia-Thanh Vuong,

ý Ngoc-Don Ta,

ffi and Trong-On Do*

,ý

†Department of Chemical Engineering, Laval University, Quebec City, Quebec G1V 0A6,

Canada

‡ School of Chemical Engineering, Hanoi University of Science and Technology, Hanoi,

Vietnam

Submitted to a refereed journal

159

Résumé

Un nouveau nanocomposite oxyde métalliqueTiO2PtOx de forme creuse a été synthétisé

en utilisant des nanocristaux de MIL-88B comme un tensioactif solide dans lequel, la

structure est composée de centres Fe3(μ3O) liés par coordination insaturée. Les sites acides

de Lewis sur la surface des nanocristaux du MIL-88B, qui sont formés par la

déshydratation des centres trimériques métalliques Fe3(μ3O), sont greffés en premier par

le précurseur du titane contenant le groupe amine via l‟attachement de leur atome d‟azote

par des paires d‟électrons avec les sites acides de Lewis. Ceci conduit à la déposition d‟une

couche mince de TiO2 sur la MIL-88B surface. Après calcination, le nanocomposite

composé d‟un hybride Fe2O3TiO2 creux avec l‟épaisseur de mur contrôlable (15 30 nm)

est préparé. De plus, deux fonctions catalytiques distinctes (oxydation et réduction) qui

sont séparément localisées localisés sur deux opposites surfaces du creux: une comme

Fe2O3 issue à partir de la structure MOF par calcination embarquée à l'intérieur de la

surface du creux, et l‟autre par le dopage du métal tel que PtOx sur le côté extérieur de la

surface du creux. Ce nanocomposite creux non seulement absorbe la lumière visible, mais

aussi améliore la séparation des électrons et trous photogénérés, due à l‟épaisseur de paroi

mince et les deux co-catalyseurs (Fe2O3 and PtOx) localisés sur deux opposites surfaces du

creux. En conséquence, ce type de nano-composites est très performant pour la production

d'H2 à partir de l'eau sous la lumière visible.

161

Abstract

A new hollow hybrid metal-oxideTiO2PtOx nanocomposite was synthesized by using

MIL-88B nanocrystals consisting of coordinatively unsaturated metal centers Fe3(μ3O) as

template. Lewis acid sites on the surface of the MIL-88B nanocrystals, which were formed

from the dehydrated trimeric metal centers, were first grafted with titanium precursor

containing amine group via attaching their nitrogen atoms with lone pairs of electrons. This

led to the successful deposition of TiO2 thin shell on the MIL-88B. The calcined hollow

nanocomposite is composed of hybrid metal oxideTiO2 with controllable wall thickness

(15 30 nm) and two distinct functional (oxidative and reductive) cocatalysts that are

separately located on two different surface sides of the hollow: one as Fe2O3 from the iron-

based MOF nanocrystal via calcination embedded inside of hollow surface, and the other

by metal-doping such as PtOx on the outside of hollow surface. This hollow nanocomposite

not only absorbs visible light, but also enhances the separation between photogenerated

electrons and holes due to the thin wall thickness and the surface separation of two distinct

functional cocatalysts. As a result, an efficient visible-light photo-activity of this

nanocomposite is found for the H2 production from water.

163

7.1 Introduction

Because of the depletion of fossil fuels and the serious environmental problems

accompanying their combustion, a new form of energy that is clean, renewable, low-cost

and a viable alternative to fossil fuels is urgently needed.1 The solar-driven splitting of

water to produce H2 with semiconductor photocatalysts represents an attractive pathway

toward solving important energy and environmental problems.2 However, the significant

challenge of using sunlight for H2 generation lies in designing an efficient system using

visible rather than UV light.

To achieve high efficiency, a sunlight driven photocatalyst is usually composed of

semiconductor that harvests visible light and cocatalysts that are loaded and highly

dispersed on the photocatalyst surface to create active sites and reduce the activation energy

for gas formation. The cocatalysts are typically a noble metal such as Pt and Rh as

reductive sites for H2 evolution and metal oxide such as iron oxide and manganese oxide as

oxidative sites.2,3

Upon band gap excitation in the photocatalyst, the photogenerated

electrons and holes migrate toward the reductive and oxidative surface sites, respectively,

thereby facilitating efficient electrons-holes separation. This photocatalytic system thus

improves the photocatalytic activity for water splitting, as compared to those of the

photocatalyst loading either reduction or oxidation cocatalyst.4 Furthermore, the two

distinct cocatalysts, which are highly dispersed and isolated from each other onto the same

photocatalyst, are highly desired to promote water splitting in harmony under visible light

irradiation. However, to date, the demonstration of this concept still remains a major

challenge.5

Among the semiconductor-based photocatalysts (e.g., titanium dioxide,6 oxide solid

solutions,7 and tantalum-nitrogen compounds

8 etc.), TiO2 is considered to be the most

possible candidate for commercial scale-up due to inexpensive, nontoxic and robust

photocatalyst under photochemical conditions. Due to its large band gap (3.0 eV and 3.2

eV for rutile and anatase, respectively), TiO2 only absorbs UV light. Several approaches,

including doping with transition metals such as Fe, Cr, Cu or non-metals, N, S, C or

164

hydrogenation have been taken to narrow the band gap of TiO2 and thus extends the

photoactivity in visible region.9 Alternatively, the coupling of TiO2 with narrow band gap

semiconductors could also result in visible light-induced photoactivities.10

On the other hand, metal organicframeworks (MOFs), which are formed by polymeric

connections of metal centers as “clusters” in coordination with organic linkers, have

emerged as an interesting class for diverse applications such as gas storage, gas separation,

catalysis, chemical sensing and drug delivery,11

owing to their incredibly tunable physical

and chemical properties as well as rationally tailored crystal morphologies.12

Among them,

various MOF structures consist of coordinatively unsaturated metal centers (namely, MOF-

UMCs) such as trimeric Cr3(μ3O) or Fe3(μ3O) clusters of MIL-88 and MIL-101. Each

trimeric metal(III) center in the MOF frameworks possesses terminal water molecules that

can be removed by vacuum and temperature treatements to provide Lewis acid sites. These

dehydrated UMCs are very beneficial to grafting organic molecules or trapping gases,

because of their strong interactions with electron-rich functional groups. Among them,

amine functional groups are often used as grafting units.13

Herein, we report a novel approach to develop a new type of hollow metal oxideTiO2

nanostructure using nanocrystals of MOF-UMCs as hard template. This type of

nanocomposite is composed of hollow hybrid metal oxideTiO2 with controllable hollow

wall thickness (15-30 nm) and two distinct functional (i.e. oxidative and reductive)

cocatalysts that are separately located on two hollow surface sides: one of which, i.e. metal

oxide from MOF template via calcination, embedded inside of hollow surface and the other

by metal doping such as PtOx outside of hollow surface. This hollow nanocomposite not

only absorbs visible light, but also greatly enhances photogenerated electron/hole

separation because of the hollow thin wall and the isolation of the two distinct functional

cocatalysts. As a result, a highly efficient visible-light photo-activity is found for the H2

production from water.

To illustrate our approach, we have selected iron-based MIL-88B nanocrystal consisting of

unsaturated Fe3(μ3O) clusters as a hard template, titanium (IV) (triethanolaminato)

165

isopropoxide C3H7OTi(OC2H4)3N with amine group (namely, TEAI) as a grafting reagent

and H2PtCl6 as a Pt source for the synthesis of a hollow α-Fe2O3TiO2PtOx nanostructure.

Lewis acid sites are created from the UMCs of the MIL-88B framework after removal of

terminal water molecules by vacuum/temperature treatment, to which TEAI with amine

group can be grafted via the lone electron pair of nitrogen atom (Scheme 7.1). Even though

numerous Lewis acid sites are available in the dehydrated framework, because of the dense

state of MIL-88B,14

TEAI is grafted on its outer surface. To increase the shell titania

thickness, TEAI-grafted MIL-88B nanocrystals were treated with a dilute solution of TEAI

in ethanol with 5 vol % H2O. During this treatment, the hydrolysis and condensation of

TEAI produce MIL-88B core/TiO2 shell nanostructure. The thickness of the titania shell

can be also controlled by the TEAI concentration.

Scheme 7.1 Schematic illustration of the procedure for the design of hollow α-

Fe2O3TiO2PtOx photocatalysts with two cocatalysts separate from each other using MIL-

88B nanocrystals containing UCMs as the hard template for visible light-induced water

splitting into hydrogen.

166


In a typical procedure, MIL-88B nanocrystals with desired size were prepared by following

the method described in our previous work, by using FeCl36H2O, 2-amino-1,4-benzene

dicarboxylic acid, Pluronic F127 and acetic acid.15

The resulted bipyramidal hexagonal

prism crystals with an average length of 500 nm and width of 150 nm are shown in

Figure 7.1a, b and Figure S7.1a. The MIL-88B structure with the formula of

Fe3O(H2N‒BDC 3Cl(H2O)2 was also confirmed by the powder X-ray diffraction (Figure

S7.1b).

Figure 7.1 TEM and HRTEM images of MIL-88B (a, b); MIL-88B@TiO2 (c, d) and

hollow α-Fe2O3-TiO2-PtOx (e-h).

In the MIL-88B framework, each Fe3(μ3O) cluster contains two terminal water molecules,

after removal of these water molecules by a dehydration treatment, Lewis acid sites are

created and available for amine grafting.16

TEAI-grafted MIL-88B nanocrystals were

achieved by treating the dehydrated MIL-88B with solution of TEAI in 2-propanol. TEM

images indicate no essential change in morphology of the TEAI-grafted nanocrystals as

compared to that of the pristine MIL-88B sample (Figure S7.2a, b). However, the presence

a)

f)

e)

d)

c)

b)

100 nm 100 nm

50 nm50 nm

100 nm

50 nm

h)

5 nm

5 nm

g)

167

of Ti was found as indicated by X-ray photoelectron spectroscopy (XPS) because of the

TEAI molecules anchoring on the MIL-88B outer surface (Figure S7.2c). Subsequently, the

TEAI-grafted MIL-88B nanocrystals were treated in refluxing dilute solution of TEAI in

ethanol (5 vol % H2O). Because of the grafting, hydrolysis and condensation of TEAI,

MIL-88B core/TiO2 shell nanostructure with controllable titania shell thickness can be

obtained. As seen by TEM images, the as-made obtained MIL-88B core/titania shell

nanomaterial exhibits the same morphology of the pristine MIL-88B (Figure 7.1c, d and

Figure S7.3a). However, this material shows a rough surface instead of the smooth surfaces

of the pristine sample, which indicates the formation of titania shell on MIL-88B. The XPS

spectrum also indicates a large amount of Ti on this sample (Figure S7.3b). The powder

XRD pattern of the as-made MIL-88B core/titania shell nanoparticles shows only

diffraction peaks which are characteristic of the MIL-88B structure (Figure 7.2a); no

diffraction peak that is characteristic of TiO2 was observed, indicating the amorphous TiO2

shell. This is also confirmed by the HRTEM observation (Figure S7.3a – inset).

Figure 7.2 Powder XRD patterns of MIL-88B@TiO2 (a), α-Fe2O3TiO2PtOx (b) and

simulated patterns for α-Fe2O3 (c), anatase TiO2 (d), MIL-88B (e) for references.

2-Theta (deg.)

10 20 30 40 50 60 70

Inte

nsity *

* * *

(a)

(b)

(c)

(d)

(e)

168

In order to generate the hollow photocatalyst with two cocatalysts located on two distinct

sides, the as-made MIL-88B core/titania shell nanomaterial was loaded with H2PtCl6 by

wet impregnation, followed by calcination in air. After the calcination, the color of this

sample changed from dark brown to dark red, because of the decomposition of Fe-based

MIL-88B core into iron oxide. Importantly, no essential change in morphology of this

sample after the calcination was observed, as showed by TEM (Figure 7.1e) and SEM

(Figure S7.4) images; however, shrinkage in size was found. The TEM image exhibits

clearly a hollow nanostructure with a mean wall thickness of 15 nm. Note that under the

same calcination conditions, the pristine MIL-88B nanocrystals (i.e., without titania shell)

were destroyed, forming aggregated irregular iron oxide particles (Figure S7.5). The TEM

and HRTEM images show PtOx nanoparticles with the size of 3 4 nm dispersed on the

hollow surface (Figure 7.1f, g). Because of solid MIL-88B@TiO2 core-shells, H2PtCl6 only

embedded on their outer surface, and thus PtOx cocatalyst located on the outer side of the

hollow. HRTEM image also indicates that each individual hollow nanoparticle is composed

of metal oxides that are well crystallized with d-spacing of 0.35 nm matching the d101 of

anatase TiO2, and of 0.25 nm matching the d110 of α-Fe2O3 (Figure 7.1h).

XRD pattern of the hollow α-Fe2O3TiO2PtOx sample shows diffraction peaks that match

those of anatase TiO2 and α-Fe2O3 phases (Figure 7.2b). Four additional peaks (asterisked

in Figure 7.2b) of this sample are the characteristic of Fe-titanate mixed oxide (Fe2TiO5),

which could be formed at the interface of TiO2 and α-Fe2O3.

The formation of the hollow structure and the shrinkage of the MIL-88B core/titania shell

structure after the calcination are attributed to the decomposition of the MIL-88B core into

iron oxide nanoparticles that occupy much less volume, and the transformation of

amorphous phase to crystalline phase of TiO2 shell under the thermal treatment in air, as

demonstrated by the thermogravimetric analysis (Figure S7.6). N2 sorption isotherm of the

hollow α-Fe2O3TiO2PtOx sample indicates the BET surface area of 40 m2/g.

The thickness of Fe2O3TiO2 hybrid wall can be controlled by adjusting the concentration

of TEAI in the refluxing solution. When the TEAI concentration increases from 6 mM to

169

50 mM the thickness increases from 15 nm to 30 nm (Figure 7.1f and Figure S7.8).

However, at high concentrations of TEAI, isolated TiO2 nanoparticles were formed.

XPS was also recorded to investigate the chemical states of Ti and Fe in the hollow α-

Fe2O3TiO2PtOx material (Figure 7.3). The Ti2p spectrum fitted with Gaussian-

Lorentzian function reveals obviously a dominant Ti2p3/2 peak with binding energy (BE) at

458.0 eV, which is characteristic of Ti4+

state in TiO2 lattice,17

along with two shoulders.

The first shoulder at 456.5 eV corresponds to Ti3+

state because of oxygen deficiency in

TiO218

whilst the second shoulder at 458.6 eV arises from Ti4+

state in TiOFe structure.19

In the TiOFe bond, the Pauling electro-negativity differential between Fe3+

(1.83) and

Ti4+

(1.54) ions induces the possibility of electron transfer from Ti4+

to Fe3+

, which makes

Ti4+

ion less electron and Fe3+

more electron rich and thus results in an increase of core

electron BE of Ti4+

and a decrease of core electron BE of Fe3+

. For the XPS spectrum of

Fe2p, three dominant peaks: Fe 2p3/2 at 712.1 eV, Fe 2p1/2 at 725.5 eV and a satellite peak

at 719.1 eV are in accord with the presence of α-Fe2O3.20

Moreover, the fitted Fe 2p3/2 peak

showed an additional peak at 710.3 eV, which is assigned to Fe3+

state in the TiOFe

bond. The absence of Fe 2p3/2 peak at 709.3 eV suggests that no Fe2+

is present in the

sample.

Figure 7.3 XPS spectra of Ti 2p and Fe 2p core levels of the hollow α-Fe2O3TiO2PtOx.

Binding Energy (eV)

454456458460462464466468

CP

S

Ti 2p1/2

Ti 2p3/2

Ti3+

Ti4+

Ti4+

(Ti-O-Fe)

2

4

6

8

10

12

x103

Binding Energy (eV)

705710715720725730

CP

S

Fe 2p1/2

Fe 2p3/2

Satellite

Fe3+

(Fe-O-Ti)

Fe3+

20

25

x103

30

15

170

The presence of Ti4+

and Fe3+

states in the TiOFe bond suggests Fe-doping in the TiO2

lattice. It is inferred that during the calcination, the MIL-88B core is decomposed. In

parallel, the amorphous shell is conversed to crystalline titania one and iron ions from the

MIL-88B enter into the TiO2 lattice, producing Fe-doped TiO2. Because of high iron

content in MIL-88B@TiO2, iron oxide nanoparticles are also formed inside of the Fe-doped

TiO2 shell. The XPS analysis, which probes only a few nanometres deep of the surface,

indicates the surface Ti/Fe atomic ratio of 1:1.9, whereas the bulk Ti/Fe atomic ratio

calculated from EDS is 1:4.0, corresponding to 20 wt % TiO2 and 80 wt % Fe2O3. These

results indicate that the hollow particle is rich in Fe and some α-Fe2O3 segments inside,

however, is rich in Ti outside of hollow Fe2O3TiO2.

UV-vis diffuse reflectance spectrum of the hollow α-Fe2O3TiO2PtOx nanostructure

exhibits the absorbance in visible region with a band edge around 610 nm (Figure 7.4, left).

The absorption onset is similar to that of α-Fe2O3 with a band gap at 2.0 eV.21

Therefore,

the visible light absorption of the hollow hybrid could be attributed to α-Fe2O3, Fe-doped

TiO2 and Fe2TiO5, in which the visible light absorption of Fe-doped TiO2 overlaps with that

of α-Fe2O3 and Fe2TiO5.

Figure 7.4 UV-vis diffuse reflectance spectrum (left) and the photoactivity for water

splitting under visible light irradiation (right) of hollow α-Fe2O3TiO2PtOx photocatalyst.

Wavelength (nm)300 400 500 600 700

F(R

)

0.0

0.1

0.2

TiO2 (P25)

hollow

Reaction time (h)

0 5 10 15 20 25

Am

ou

nt

of

evo

lve

d H

2 (

mo

l.g

-1)

0

1000

2000

3000

4000

5000

6000

171

The photocatalytic activity of the hollow α-Fe2O3TiO2PtOx nanocomposite for hydrogen

generation was carried out under visible light irradiation ( > 420 nm) using lactic acid as

sacrificial reagent. The results showed that 1.1 mmolh1g

1 of H2 gas was evolved in this

catalytic system. To investigate the stability of α-Fe2O3TiO2PtOx photocatalyst, an

experiment of 5 cycles with intermittent N2 bubbling after every 5 h was carried out

without catalyst regeneration. The obtained result indicated an excellent stability for the

photocatalyst during the course of the 25 h irradiation without noticeable catalytic

deactivity (Figure 7.4, right). The amount of H2 gas was evolved linearly during the entire

period of each cycle, with a total of 5.5 mmolg1

H2 produced after 5 h.

For comparison, the composite of α-Fe2O3, TiO2 and PtOx with the same chemical

composition of the hollow α-Fe2O3TiO2PtOx sample was also prepared using the same

procedure of synthesis and starting MIL-88B nanocrystals, except that titanium (IV)

butoxide (i.e., without amine group) as grafting reagent instead of TEAI. For this calcined

sample, under the same photocatalytic conditions, only small quantity of H2 was produced

after the 5 h irradiation. It is obvious that the absence of amine group in the titanium

precursor and the rapid hydrolysis of titanium butoxide in ethanol (5 vol % H2O) led to a

physical mixture of TiO2 particles and MIL-88B nanocrystals rather than MIL-88B@TiO2

core-shell nanostructure (Figure S7.9). Consequently, no essential hydrogen was formed

under visible light for this material. Furthermore, when PtOx-loaded α-Fe2O3 powder

prepared from MIL-88B, PtOx-loaded TiO2 and PtOx-loaded Fe2TiO5 were used under the

same photocatalytic conditions, only trace of H2 was found in these cases.

Although α-Fe2O3 derived from the calcined MIL-88B absorbs visible light, it cannot drive

the proton reduction of water because its conduction band is more positive than the

potential of 2H+/H2 couple.

22 Similarly, the Fe2TiO5 mixed oxide with the CB below the

potential of 2H+/H2 cannot reduce the proton. TiO2 only absorbs UV light and thus does not

catalyze the visible light-induced H2 evolution. The above results demonstrate that Fe-

doped TiO2 thin wall is responsible for the visible light-induced activity of α-

Fe2O3TiO2PtOx and the high isolation of α-Fe2O3 and PtOx cocatalysts embedding on

172

two different sides of the hollow hybrid enhances their photocatalytic activity for water

splitting through the efficient separation of photogenerated electrons-holes.

7.3 Conclusions

We have developed an approach toward a new hollow hybrid metal-oxideTiO2PtOx

photocatalyst for visible light-induced water splitting into H2 by using nanosized MOFs

consisting of coordinatively unsaturated metal centers as template. This hollow

photocatalyst contains two distinct functional cocatalysts on two different sides of the thin

wall. Because of the surface separation of the cocatalysts, the separation between

photogenerated electrons and holes is enhanced, resulting in the efficient photocatalysis.

This approach can be extended to achieve distinct photocatalysts by varying the chemical

composition and morphology of the MOF templates.

7.4 Experimental

Chemicals: FeCl36H2O (Aldrich, 97 %), 2-amino-1,4-benzenedicarboxylic acid (Aldrich,

99 %), Pluronic F127 (EO97PO69EO97, average Mn = 12600, Aldrich), CH3COOH (Fisher,

99.7 %), titanium(IV) (triethanolaminato)isopropoxide (80 wt% in 2-propanol, Aldrich),

titanium (IV) butoxide (Aldrich, 97 %), 2-propanol (Aldrich, 99.5 %), ethanol (95 %),

H2PtCl66H2O (Aldrich, ≥ 37.5 % Pt basis and lactic acid (Aldrich, 85 90 % in water).

All chemicals were used as received without further purification.

Synthesis of MIL-88B nanocrystals: MIL-88B nanocrystals were prepared by following

our previously reported approach.15

Briefly, the aqueous dispersion (15 mL) of 2-

aminoterephtalic acid (60 mg, 0.33 mmol), FeCl36H2O (180 mg, 0.66 mmol), Pluronic

F127 (160 mg) and acetic acid (0.6 mL, 10.5 mmol) was heated at 110 oC for 24 h. The

resulted solid was isolated and washed several times with ethanol by centrifugation to

remove the surfactant and excess reactants.

173

Titanium precursor grafting on MIL-88B nanocrystals: The as-made MIL-88B (100

mg) was dried overnight at 150 oC under vacuum to remove terminal water molecules. To

this dehydrated form, 2 M TEAI solution in 2-propanol (4 mL) was poured and stirred at

room temperature for 2 h. The TEAI-grafted MIL-88B was then collected, washed with 2-

propanol by centrifugation and dried overnight at 100 oC.

TiO2 coating on MIL-88B nanocrystals: TEAI-grafted MIL-88B (100 mg) was dispersed

in 80 mL of 6 mM TEAI solution in ethanol (5 vol. % H2O) and then refluxed for 24 h

under vigrous stirring. The MIL-88B@TiO2 core-shell was collected and washed several

times with ethanol by centrifugation.

Preparation of α-Fe2O3TiO2PtOx: 0.57 mL of H2PtCl6 aqueous solution (10 mM) was

added to the suspension of MIL-88B@TiO2 (100 mg) in deionized water (12 mL) and

stirred for 1 h at room temperature, followed by a rotary evaporation with a water bath at 50

oC to remove the water. The powder was calcined in air with heating rate at 5

oC min

-1 at

500 oC for 3 h to yield a dark red PtOx-loaded composite.

Photocatalytic H2 evolution: Visible light-induced H2 evolutions were carried out in 300

mL septum-sealed glass reactors. In each run, 20 mg of α-Fe2O3TiO2PtOx was well

dispersed with magnetic stirring in a 50 mL of aqueous solution containing lactic acid (10

wt.%). The reactor was deoxygenated by bubbling nitrogen to remove oxygen and then

placed in front of a 300 W Xe-lamp with a 420 nm cut-off filter (FSQ-GG420, Newport)

under constant stirring. 0.5 mL of the gas in the headspace of the reactor was analyzed by

GC to determine the amount of evolved H2.

Characterizations: Transmission electron microscopy (TEM) images were obtained on a

JEOL JEM 1230 microscope operating at 120 kV. High resolution TEM images were

performed on a Tecnai G2 20 instrument operated at 200 kV. Scanning electron microscopy

(SEM) images were taken on a JEOL 6360 instrument at an accelerating voltage of 3 kV.

Powder X-ray diffraction (XRD) patterns were collected on a Bruker SMART APEX II X-

ray diffractometer with Cu Kα radiation (λ = 1.5406 Å) at a scan rate of 1.0o min

1. All

174

samples were dried at 100 oC overnight to remove guest solvent molecules within the pores

before the XRD scan. XPS measurements were carried out in an ion-pumped chamber

(evacuated to 109

Torr) of a photoelectron spectrometer (Kratos Axis-Ultra) equipped with

a focused X-ray source (Al Kα, hν = 1486.6 eV). The UV-vis spectra were recorded on a

Cary 300 Bio UV-visible spectrophotometer. N2 adsorption-desorption isotherms were

measured at the temperature of liquid nitrogen with a Quantachrome Autosorb-1 system.

The BET surface areas were calculated in the range of 0.05−0.3 P/Po. Thermogravimetric

analysis (TGA) was carried out with a TGA/SDTA 851e thermogravimetric analyzer from

room temperature to 600 oC with a heating rate of 5

oC min

1 under an air flow of 50 mL

min1

.

Operation conditions for GC analysis:

Carrier gas: nitrogen at 15 Psi

Column: Carboxen-1010 Plot Capillary 30 m x 0.53 mm.

Detecter: TCD

Column temperature: 30 oC

Retention time of H2: 0.9 min

175

7.5 Appendix

Figure S7.1 SEM image (a) and powder XRD pattern (b) of MIL-88B nanocrystals

Figure S7.2 TEM images (a, b), XPS survey and high-resolution Ti 2p (inset) spectra (c) of

TEAI-grafted MIL-88B nanocrystals.

Figure S7.3 TEM and HRTEM (inset) images (a), XPS survey and high-resolution Ti 2p

(inset) spectra (b) of MIL-88B@TiO2 nanoparticles prepared with the concentration of

TEAI at 6 mM TEAI.

2-Theta

5 10 15 20 25 30

Inte

nsity

(002)

(101)

(102)(103) (200)

(201)

(202)

(311) (204) (302)

Simulation

Experimetal

b)

_

Binding Energy (eV)456460464468

Binding Energy (eV)

02004006008001000

CP

S

Ti 3p

C 1s

N 1s

O 1s

Fe 2p

Ti 2p

O KLLAuger

CP

S

Ti 2p1/2

Ti 2p3/2c)

Binding Energy (eV)456460464468

Binding Energy (eV)

02004006008001000

CP

S

Ti 3p

C 1s

N 1s

O 1s

Fe 2p

Ti 2pO KLLAuger

CP

S

Ti 2p1/2

Ti 2p3/2b)

176

Figure S7.4 SEM image of -Fe2O3TiO2PtOx sample.

Figure S7.5 SEM (a) and TEM (b) images of iron oxide sample obtained by the calcination

of the pristine MIL-88B nanocrystals. HRTEM image (inset) indicates the fusion of iron

oxide nanoparticles. The scale bar for the inset is 5 nm.

177

Figure S7.6 The thermogravimetric analysis of MIL-88B and MIL-88B@TiO2[PtCl6]2-

.

The weight retention differential at 500 oC of 8.0 % is attributed to the coated TiO2 and

loaded PtOx.

Figure S7.7 N2 sorption isotherms at 77 K of -Fe2O3TiO2PtOx.

Temperature (oC)

100 200 300 400 500 600

We

igh

t (%

)

20

40

60

80

100

MIL-88B@TiO2-[PtCl6]2-

MIL-88B

TGA

P/Po

0.0 0.2 0.4 0.6 0.8 1.0

N2 u

pta

ke

(cm

3/g

)

0

50

100

150

200

250

Desorption

Adsorption

SBET

= 40 m2/g

178

Figure S7.8 The hollow nanoparticles with various thicknesses of Fe2O3TiO2 hybrid wall

prepared with the concentration of TEAI at 25 mM (a) and 50 mM (b) while maintaining

other reaction parameters. The scale bar is 50 nm.

Figure S7.9 TEM images of the sample prepared by using titanium (IV) butoxide without

amine group as grafting reagent instead of TEAI before (a) and after the calcination (b).

The simple mixtures of TiO2 nanoparticles and MIL-88B nanocrystals (a) or iron oxide

particles (b) were observed instead of MIL-88B core/TiO2 shell or hollow hybrid

nanostructure.

b)a)

500 nm 500 nm

b)a)

179

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Chapter 8 Conclusions and Prospects

8.1 General conclusions

This thesis has developed three different synthetic methods for preparing bimodal micro-

mesoporous MOF nanocrystals and uniform nanosized MOFs with controlled shape and

size. The use of the prepared MOF nanocrystals as template for fabricating the efficient

photocatalyst for water splitting into H2 under visible light irradiation has also been

demonstrated.

As the first target of this thesis, the nonionic surfactant-templated solvothermal method in

the presence of acetic acid has been developed to achieve bimodal micro-mesoporous MOF

nanocrystals. The result in chapter 4 has illustrated the successful application of this

method to preparing [Cu3(BTC)2]- and [Cu2(HBTB)2]-based MOF nanocrystals containing

mesopores and intrinsic micropores by using nonionic F127 copolymer as micellar

template, acetic acid to control the crystallization of MOF pore wall and ethanol as solvent.

These bimodal porous MOF nanocrystals exhibit well-defined mesostructure with diameter

of 3.9 4.0 nm and high crystallinity.

In chapter 5, the coordination modulation methodology using simultaneously distinct

selective modulators or only one selective modulator to control the shape and size of MOF

nanocrystals has been demonstrated. The nature and the concentration of selective

modulators play crucial roles in forming the morphology of [Cu2(ndc)2(dabco)]n

nanocrystals as well as in controlling the crystal size. By using simultaneously acetic acid

and pyridine or only pyridine as selective modulators, nanocubes and nanosheets of

[Cu2(ndc)2(dabco)]n MOF have been prepared, respectively. The MOF nanocrystals exhibit

high crystallinity, high adsorption capacity and morphology-dependent sorption property

for CO2.

In chapter 6, the hydrothermal approach in the simultaneous presence of stabilizing reagent

and deprotonation-controlled reagent toward uniform carboxylate-based MOF nanocrystals

182

with controlled-size has been discussed. Uniform nanoscale FeMIL-88BNH2 with

desired crystal sizes in the range of 30 150 nm in width and of 50 500 nm in length has

been produced by using the triblock copolymer as stabilizing reagent and acetic acid as

deprotonation-controlling reagent in aqueous synthetic medium. The size and aspect ratio

(length/width) of the nanocrystals were controlled by acetic acid concentration. The

nanocrystals appeared as individual crystals after removing the surfactant.

After the successful synthesis, the application of the prepared FeMIL-88BNH2

nanocrystals to fabricating an efficient photocatalyst for H2 production from water under

visible light has been illustrated in chapter 7. The nanocrystals have a dual function,

including scaffolding to create hollow hybrid nanostructure and precursor to generate

surface oxidation sites inside of the hollow as well as to modify the chemical composition

of deposited TiO2 thin shell toward visible light absorbance. By loading surface reduction

sites such as PtOx on the outer of the hollow, the resulted nanocomposite containing two

cocatalytic sites (i.e., oxidation and reduction) on two different surface sides induces the

efficient separation between photogenerated electrons and holes and visible light-driven

catalytic activity for water splitting into H2 in the presence of lactic acid as sacrificial

reagent.

8.2 Prospects

While the first synthetic method gives advantages of both mesopore and nanosize which

should overcome the obstacles of the small pore and large crystal of microporous MOFs,

the second approach allows designing MOF morphology at the nanometer scale and the

third method allows controlling exactly the size of MOF nanocrystals. The last objective of

this thesis draws further applications of MOF nanocrystals.

There are several directions that can be further pursued in the future:

1. Investigation on other MOFs that are synthesized in organic solvents to see if the

first methodology can work in different organic solvents.

183

2. Study on other [M2(dicarboxylate)2(N ligand)]n MOF structures to see if the second

methodology can be universally applied.

3. Test on different MOFs synthesized in aqueous media to see if the third

methodology can be extended.

4. Indicating the correlations between the interaction of MOF precursors with

surfactant in different reaction media (organic solvent and water), the nature of

MOF structures and either the ability to form of mesostructure that leads to bimodal

porous MOFs or macroscopic phase segregation toward MOF nanocrystals without

mesopores.

5. Using nanosized MOFs with different chemical compositions as templates for

fabricating photocatalyts for efficient conversion of solar energy to fuel.

185

List of Publications

1. Minh-Hao Pham, Gia-Thanh Vuong, Anh-Tuan Vu, and Trong-On Do. “Novel

Route to Size-Controlled FeMIL-88BNH2 MetalOrganic Framework

Nanocrystals” Langmuir 2011, 27, 15261–15267.

2. Minh-Hao Pham, Gia-Thanh Vuong, Frédéric-Georges Fontaine, and Trong-On Do.

“A Route to Bimodal Micro-Mesoporous Metal−Organic Frameworks Nanocrystals”

Cryst. Growth Des. 2012, 12, 1008−1013.

3. Minh-Hao Pham, Gia-Thanh Vuong, Frédéric-Georges Fontaine, and Trong-On Do.

“Rational Synthesis of Metal−Organic Framework Nanocubes and Nanosheets Using

Selective Modulators and Their Morphology Dependent Gas-Sorption Properties”.

Cryst. Growth Des. 2012, 12, 3091−3095 (Most read article).

4. Minh-Hao Pham, Cao-Thang Dinh, Gia-Thanh Vuong, Ngoc-Don Ta, and Trong-On

Do. “Hollow Fe2O3TiO2–PtOx Nanostructure with Two Distinct Cocatalysts

Embedded Separately on Two Surface Sides for Efficient Visible Light Water Splitting

to Hydrogen”. Submitted to a refereed journal, 2013.

5. Gia-Thanh Vuong, Minh-Hao Pham, and Trong-On Do. “Synthesis and engineering

porosity of a mixed metal Fe2Ni MIL-88B metal–organic framework”. Dalton Trans.

2013, 42, 550–557.

6. Cao-Thang Dinh, Minh-Hao Pham, Freddy Kleitz, and Trong-On Do. “Water

Soluble CdS-Titanate-Nickel Nanocomposites for Visible Light Photocatalytic

Hydrogen Production”. Submitted to a refereed journal, 2013.

7. Cristina Mottillo, Yuneng Lu, Minh-Hao Pham, Matthew Cliffe, Trong-On Do, and

Tomislav Friščić. “Mineral neogenesis as an inspiration for mild, solvent-free

synthesis of bulk microporous metal–organic frameworks from metal (Zn, Co)

oxides”. Green Chemistry 2013, 15, 2121-2131.

8. Gia-Thanh Vuong, Minh-Hao Pham, and Trong-On Do. “Study on the

Crystallization Mechnism of MIL-88B Metal-Organic Frameworks”. Submitted to a

refereed journal, 2013.

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