Growth of Single-Crystalline KNbO3 Nanostructures

A. Magrez,*,† E. Vasco,‡ J. W. Seo,† C. Dieker,† N. Setter,‡ and L. Forro †

Laboratoire des Nanostructures et des NouVeaux Mate´riaux Electroniques, Institut de Physique de la Matie`reComplexe, and Laboratoire de Ce´ramique, Institut des Mate´riaux, Ecole Polytechnique Fe´derale de Lausanne,CH-1015 Lausanne-EPFL, Switzerland

ReceiVed: July 11, 2005; In Final Form: October 28, 2005

This communication reports on the growth of highly uniform KNbO3 nanowires exhibiting a narrow diameterdistribution around 60 nm and a length-to-width ratio up to 100. The nanowires were prepared by a hydrothermalroute, which enables simple, gram-scale production. A systematic study of the synthesized nanowires in termsof the morphological and chemical characteristics was carried out by varying the temperature-pressureconditions and the composition of the starting mixture. The results indicate that highly uniform single-crystallinenanowires form within a narrow window of the ternary phase diagram of KOH-Nb2O5-H2O.

Introduction

Nanoscale science based on one-dimensional structures hasfound a springboard in the discovery of carbon nanotubes. Sincethen, many different materials have been produced as one-dimensional (1D) nanostructures, such as, for example, nano-tubes and nanowires. Novel size- and shape-dependent crystalstructure1 (special polymorphisms induced by particle size havebeen reported recently for alkaline niobates2,3) and properties4

of nanoscale materials have been investigated intensively. Inparticular 1D oxide nanostructures have attracted attention butmainly in binary compounds such as, for example, TiO2, ZnO,VOx, etc. Despite very promising progress in the ability toprepare 1D oxide nanostructures,5 few reports concerning thesynthesis of nanowires of functional perovskite oxides areavailable so far.6,7 This paper presents the preparation of KNbO3

nanowires and the influence of growth conditions on the productcharacteristics, i.e., particle morphology, purity, and crystallinequality of the synthesized material. Perovskite KNbO3 (KNhereafter) is an attractive oxide for its acousto-optic, electro-optic, nonlinear optical, and piezoelectric properties. KN isemployed as a frequency doubling and mixing material and alsoas optical waveguides and a holographic storage medium. Inaddition, KN is a promising candidate as a lead-free andbiocompatible transducer with tunable piezoelectric response.8

The preparation of such a material as nanowires would drivethe development of a novel generation of nanoelectromechanicalsystems (NEMS) based on nanoscalable components.

Recently, KN nanostructured materials have been producedby hydrothermal synthesis9,10 and characterized by means ofatomic force microscopy assisted detection of induced piezo-electric vibrations.11 In this paper, we systematically study thehydrothermal route and explore the KOH-Nb2O5-H2O ternaryphase diagram. Our results indicate that the morphology of KNnanostructures strongly depend on the temperature-pressurecondition and the starting composition. Nanowires with a well-defined structure, a narrow diameter distribution, high aspect

ratio, and high density have been obtained in a narrow windowof the ternary phase diagram.

Experimental Section

For the synthesis of KN nanostructured materials, hydrother-mal treatment is applied, which is a suitable synthetic route formaterial preparation under mild conditions (low temperature).This method allows a reproducible shape control as well aslarge-scale synthesis, two aspects very important for theapplications mentioned above.

In a typical reaction, niobium pentoxide (Fluka) powder isadded to distilled water in which potassium hydroxide (Fluka)was dissolved. KOH acts as both the potassium source and themineralizer. The reactant mixture, which contains KOH-Nb2O5-H2O with different weight ratios, is subsequently stirredfor 2 h. The resulting slurry is poured into the Teflon vessel.Afterward, the autoclave is heated to a temperature ranging from100 to 225°C for 6 days, producing a white precipitate. Thesolid is filtered, washed with distilled water and ethanol, anddried at 120 °C overnight. For the transmission electronmicroscopy (TEM) study a Philips CM300 microscope was usedoperating at 300 kV. The TEM sample preparation involveddispersing the synthesized material in isopropyl alcohol bysonication, and a drop of suspension was put on a copper gridcovered with holey carbon. Scanning electron microscopy(SEM) micrographs were taken using a Philips XL 30 FEGoperated at 30 kV. X-ray powder diffraction experiments werecarried out using a Rigaku diffractometer in Bragg-Brentanogeometry with monochromatic Cu-KR radiation. The data werecollected in theθ-2θ mode.

Results and Discussion

Ternary Phase Diagram. The synthesis takes place insolution via a dissolution-precipitation process according tothe following reactions

* Author to whom correspondence should be addressed. Fax: 0041(21)693-4470. E-mail: arnaud.magrez@epfl.ch.

† Laboratoire des Nanostructures et des Nouveaux Mate´riaux Electron-iques, Institut de Physique de la Matie`re Complexe.

‡ Laboratoire de Ce´ramique, Institut des Mate´riaux.

3Nb2O5 + 8OH- f Nb6O198- + 4H2O (1)

Nb6O198- + 34OH- f 6NbO6

7- + 17H2O (2)

NbO67- + K+ + 3H2O f KNbO3 + 6OH- (3)

58 J. Phys. Chem. B2006,110,58-61

10.1021/jp053800a CCC: $33.50 © 2006 American Chemical SocietyPublished on Web 12/07/2005

As previously reported,12 niobium pentoxide first dissolvesinto Nb6O19

8- hexaniobate Lindqvist ion, in which NbO6octahedrons are sharing edges. This complex transforms after-ward, along reaction 2, into single octahedron NbO6

7- anions,which act further as elementary species for KN perosvkite, witha NbO3

- corner-sharing octahedron network. The final step(reaction 3) describes the KN precipitation. The structure ofthe Nb-containing species is presented in Figure 1.

Basically the exploration of the KOH(x)-Nb2O5(y)-H2O(z)ternary phase diagram was restricted to an area enclosed bycomposition lines corresponding to the limit of solubility of thesolid phases or lines deduced from the stoichiometry of reactions(1-3) occurring during the growth process.

On one hand, we consider the reaction completed when theproduct of the synthesis does not contain residual Nb2O5.Therefore for complete consumption of Nb2O5, the compositionof the starting mixture should match they/x e 0.34 [(3MNb2O5)/[(34 + 8)MKOH) ) 0.34]- 0 e z e 100% condition accordingto the stoichiometry of reactions 1 and 2. This condition alsoavoids the growth of undesirable KNb3O8 or K4Nb6O17 phases,which form layered structures.13 These niobates are formed viathe following reactions, in the liquid phase, depending on thesolution degree of alkalinity

Furthermore, the transformation of Nb2O5 into KNbO3 iscompleted if the conditiony/z e 2.46 - 0 e x e 100%,belonging to reaction 3 stoichiometry, is abided as well.

On the other hand, the explored area is limited by the limitsof solubility of solids. For KOH, the condition isx/z ) 1 - 0e y e 100%. The last condition corresponds to the niobiumpentoxide axis since its solubility is rather low (x ) z ) 0 - 0e y e 100%).These lines are plotted into the diagram (weightpercentage) of Figure 2, and compositions in agreement withall conditions are contained within the darkest area.

Representative SEM images of the products obtained at 200°C from different compositions are collected in Figure 3. Thesynthesized products are examined by X-ray powder diffraction(XRPD), and it is found that each sample corresponds to a KNsingle-phase system. The compositions used in the recentstudies9-11 are included to the phase diagram for comparison.

A strong dependence of the particle shape on the compositionof the starting mixture is observed. In particular, elongatedstructures are formed for points 3 and 4 in the phase diagram.For KOH content close to the saturation level in water (point1), the synthesized material reveals anisotropic structures.Nevertheless, these features are substandard nanowires sincethey exhibit nonuniform width. Their structures can be decribedas the stacking of cubic particles of different sizes that decreases

from the base toward the tip. Theoretically,14 it was found thatthis morphology is not an intermediate state of the growthtoward nanowires with uniform width but rather toward nanow-ires where the growth was interrupted by the presence of defects.Our study shows that either the KOH content or the thermalannealing time at 200°C affects the length and the width ofthe initial cubic particle as well as the number of the faces fromwhich the nanowires are formed.14 Furthermore, a decrease inthe population of defective nanowires compared to the numberof nanowires of uniform width was observed as the watercontent increased.

The Liu et al.9,10 process, reproduced by Suyal et al.,11

explored several compositions with a constant Nb2O5 content(∼1 wt %) by modifying the KOH/H2O ratio (Figure 2). A SEMimage of point 2 powder confirms their observations. Thesample contains mainly isotropic sub-micrometer particles(average size of about 500 nm) with some elongated ones, whoseaspect ratios can reach up to 20. Our finding shows that theNb2O5 amount affects the particle shape as well: When theNb2O5 proportion is increased from 0.4 to 6.8 wt % (point 3f1 f C), the particle aspect ratio decreases down to the isotropicshape level for powders produced by Goh et al.12

In summary, point 3 is very likely the most favorablecomposition (38.7 wt % of KOH, 0.4 wt % of Nb2O5, and 60.9wt % of H2O) for growing regular nanowires since regularnanorods and nanowires, exhibiting well-defined edges parallelto low-indexed planes of KN, are produced mostly within theseconditions.

Temperature and Pressure.The applied temperature andpressure have a strong influence on the solubility of the differentspecies and reaction kinetics. Thus, SEM images of the productsobtained from the composition corresponding to point 3 are

Figure 1. Schematic illustration of the structural transformations of Nb-containing species along the chemical mechanism of KNbO3 synthesis byhydrothermal treatment. Nb atoms are located in the polyhedrons of oxygen atoms.

3Nb2O5 + 2OH- f 2Nb3O8- + H2O (4)

3Nb2O5 + 4OH- f Nb6O174- + 2H2O (5)

Figure 2. KOH-Nb2O5-H2O ternary phase diagram. The differentconditions to be respected by the starting compositions are presented.The exploration is restricted within the darkest area where all conditionsare abided. The dashed triangle represents the position of Figure 3 inthe full phase diagram.

Single-Crystalline KNbO3 Nanostructures J. Phys. Chem. B, Vol. 110, No. 1, 200659

presented in Figure 4 as a function of bath temperature andhydrostatic pressure in the autoclave.

First, for a temperature lower than 150°C, a second phasethat contains several micron-sized grains remains after thesynthesis. XRPD confirmed the presence of Nb2O5 impuritiesas unreacted material. Above 150°C, single-phase KN powderis synthesized. Lattice constants were estimated by refiningXRPD patterns assuming theAmm2 orthorhombic space groupfor all the prepared samples. The so-calculated values (a )0.3975(1) nm,b ) 0.5693(2) nm,c ) 0.5717(2) nm) are ingood agreement with those reported for distorted KN perovskitestructure.15 In addition the sample produced at 150°C almostexclusively contains nanowires with a regular size and a narrowdistribution of diameter around 60( 10 nm and an aspect ratioup to 100, as can be seen in Figure 3. These nanowires exhibitmorphological characteristics (length, width, aspect ratio, etc.)similar to those reported so far for BaTiO3 nanowires.4,5

Moreover, according to the authors’ knowledge, it is the firsttime that KN nanowires are produced in gram-scale quantitieswith such selectivity and such morphological characteristics.From the ideal composition found above, the increase oftemperature up to the orthorhombic-tetragonal phase transitiontemperature16 leads to the production of rough particles. Thiseffect could originate from the enrichment with niobium ionic

species in the liquid phase due to an increase of the solubilitylimit, tuned by the temperature and the pressure. Consequently,the control of the Nb2O5 dissolution via the bath temperatureand pressure plays a key role in modifying the morphology andcomposition of the final product.

Structure Characterization. To study the structural proper-ties of the wires, high-resolution transmission electron micros-copy (HRTEM) analysis was performed. Samples synthesizedat 150°C from the optimal mixture composition contain mainlyperfect nanowires with a diameter around 60 nm (Figure 4a).As can be seen in Figure 5b, the nanostructures produced aresingle-crystalline nanowires with edges parallel to a low-indexedplane of KN. The selected area electron diffraction (SAED)pattern, which was obtained from an entire nanowire, wasindexed by assuming the lattice constants refined from XRPDmeasurements. According to this, the nanowire axis runs alongthe [011] direction of KN referred to the orthorhombic unit cell,and the nanowire edges are parallel to the (010) and (001)planes.

Conclusion

In summary, regularly sized KN nanowires have beenprepared via a hydrothermal route at 150°C from a 38.7 wt %

Figure 3. Explored area of the KOH-Nb2O5-H2O ternary phase diagram. SEM images of some representative compositions are shown. (Thescale bar corresponds to 1µm.) Samples 1-5 were produced by hydrothermal synthesis at 200°C for 6 days. The KOH-Nb2O5-H2O ratios in wt% are as follows: (1) 38.1/1.9/60.0, (2) 40.6/0.9/58.5, (3) 38.7/0.4/60.9, (4) 44.4/0.4/55.2, (5) 30.4/0.4/69.2. The compositions of the A-D segmentare related to the syntheses in refs 9-11. Points B and C belong to ref 12 compositions. A part of they/x e 0.34 - 0 e z e 100% condition isalso presented as a full line.

Figure 4. Evolution of the morphology and composition of the synthesized products with the bath temperature (T) and hydrostatic pressure (P) inthe hydrothermal autoclave. Representative XRPD spectra are inserted (vertical arrows indicate Nb2O5 phase). SEM images are given for each pairof (T,P) values (scale bar corresponds to 2µm). The orthorhombic-tetragonal phase transition temperature of the KNbO3 bulk phase is includedas a reference.

60 J. Phys. Chem. B, Vol. 110, No. 1, 2006 Magrez et al.

KOH, 0.4 wt % Nb2O5, 60.9 wt % H2O mixture. Theso-synthesized powder is composed of single-crystalline KNnanowires with an aspect ratio and average diameter of 100and 60 nm, respectively. These morphological characteristicsare suitable and promising for NEMS applications. The influenceof the composition of the starting mixture as well as of thetemperature-pressure within the hydrothermal autoclave on theproduct purity and particle shape have clearly been identified.Future works are oriented to the understanding of the growthkinetics of KN nanowires, aiming at a further characterizationof the growth mechanism. Nevertheless, these results will allowus to perform similar growth studies on comparable perovskitematerials to synthesize other functional perovskite materials asone-dimensional nanostructures.

Acknowledgment. This work was carried out within theNCCR Nanoscience program of the Swiss National ScienceFoundation. The financial support is gratefully acknowledged.Authors thank the Centre Interdisciplinaire de MicroscopieElectronique at the EPFL for access to electron microscopes.

References and Notes

(1) Fernandez-Garcia, M.; Martinez-Arias, A.; Hanson, J. C.; Rod-riguez, J. A.Chem. ReV. 2004, 104, 4063.

(2) Shiratori, Y.; Magrez, A.; Pithan, C.Chem. Phys. Lett.2004, 391,288.

(3) Shiratori, Y.; Magrez, A.; Dornseiffer, J.; Haegel, F.-H.; Pithan,C.; Waser, R.J. Phys. Chem. B2005, 109, 20122.

(4) Hu, J.; Odom, T.; Lieber, C. M.Acc. Chem. Res.1999, 32, 435.(5) Patzke, G. R.; Krumeich, F.; Nesper, R.Angew. Chem., Int. Ed.

2002, 41, 2446.(6) Urban, J. J.; Yun, W. S.; Gu, Q.; Park, H.J. Am. Chem. Soc.2002,

124, 1186.(7) Mao, Y.; Banerjee, S.; Wong, S. S.J. Am. Chem. Soc.2003, 125,

15718.(8) Saito, Y.; Takao, H.; Tani, T.; Nonoyama, T.; Takatori, K.; Homma,

T.; Nagaya, T.; Nakamura, M.Nature2004, 432, 84.(9) Liu, J. F.; Li, X. L.; Li, Y. D. J. Nanosci. Nanotechnol.2002, 2,

617.(10) Liu, J. F.; Li, X. L.; Li, Y. D. J. Cryst. Growth2003, 247, 419.(11) Suyal, G.; Colla, E.; Gysel, R.; Cantoni, M.; Setter, N.Nano Lett.

2004, 4, 1339.(12) Goh, G. K. L.; Lange, F. F.; Haile, S. M.; Levi, C. G.J. Mater.

Res.2003, 18, 338.(13) Uchida, S.; Inoue, Y.; Fujishiro, Y.; Sato, T.J. Mater. Sci.1998,

33, 5125.(14) Vasco, E.; Magrez, A.; Forro´, L.; Setter, N.J. Phys. Chem. B2005,

109, 14331.(15) Katz, L.; Megaw, H. D.Acta Crystallogr.1966, 22, 639.(16) Pruzan, P.; Gourdain, D.; Chervin, J. C.High-Pressure Res.2002,

22, 243.

Figure 5. (a) TEM image of KN nanowires synthesized at 150°C from the optimal mixture composition. (b) HRTEM image and (c) SAED patternof a KN nanowire. On the basis of the SAED, the zone axis was determined to [100].

Single-Crystalline KNbO3 Nanostructures J. Phys. Chem. B, Vol. 110, No. 1, 200661