Solgel synthesis and luminescence of undoped and Mn-doped zincorthosilicate phosphor nanocomposites
J. El Ghoul a,n, L. El Mir a,b
a Laboratoire de Physique des Matriaux et des Nanomatriaux Applique lEnvironnement, Facult des Sciences de Gabs,Cit Erriadh Manara Zrig, 6072 Gabs, Tunisieb Al Imam Mohammad Ibn Saud Islamic University (IMSIU), College of Sciences, Departement of Physics, Riyadh 11623, Saudi Arabia
a r t i c l e i n f o
Article history:Received 19 July 2013Received in revised form20 October 2013Accepted 30 November 2013Available online 7 December 2013
a b s t r a c t
Zn2SiO4 and Zn2SiO4:Mn particles embedded in SiO2 host matrix prepared by sol gel method undersupercritical conditions of ethyl alcohol in two steps. Were prepared by a simple solid-phase reactionunder natural atmosphere at 1200 1C after the incorporation of ZnO and ZnO:Mn nanoparticles,respectively, in silica monolith. In the case of SiO2/Zn2SiO4 nanocomposite, the powder with an averageparticle size of 80 nm shows a strong luminescence band centred at around 760 nm in the visible range.In addition, the PL spectrum for the SiO2/Zn2SiO4:Mn nanocomposite showed that a dominant peak at525 nm appeared, which originated from the 4T16A1 transitions of Mn2 ions. The luminescenceproperties of nanocomposites were characterized by emission and excitation spectra as well theirdependencies of upon temperature and power excitation density.
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Over the last decade, the luminescent properties of silicate basedinorganic phosphors have been extensively investigated .Among the silicates, one of the promising candidates so far ismanganese doped zinc silicate (Zn2SiO4:Mn) phosphor , whichhas been used in cathode ray tubes, uorescent lamps, and plasmadisplay panels due to its high luminescence efciency and chemicalstability. Although researches about the effect of Mn2 ions onluminescent properties have been performed, most of the studiesconcerning the amount of Mn2 ions substituting for Zn2 ionshave chemically focused on the quantities of Mn2 ions in Zn2SiO4.Since the amount of Mn2 ions doped into the crystal lattice ofZn2SiO4 is closely related to the performance of Zn2SiO4:Mn2
phosphor such as decay time, color shift and concentration quench-ing, the determination of the quantities of Mn2 ions in the crystallattice is more essential. Furthermore, the site preference of Mn2
ions is indispensible to advance the luminescent properties of greenor yellow color-emitting phosphor.
It is well known that the luminescence properties of phosphordepend on its synthesis process. Usually, Zn2SiO4:Mn2 phosphorsare prepared by solid-state reactions . In solid-state reaction,
it requires high-temperature process with long period of time.Therefore, agglomerated particles with irregular shape and largesize are produced by the solid-state reaction method. Accordingly,grinding and milling process, which is harmful to the luminescentproperties, are necessary to reduce the particle size desired for theapplication . In recent years, many new preparation approacheshave been proposed, including spray pyrolysis route, solgel method,chemical vapor synthesis and hydrothermalmethod . Amongthese methods, the hydrothermal synthesis has great advantage,such as better distribution of metal ions, controllable morphologyand lower cost . Therefore, it is imperative to develop asimple synthesis route in order to potentially overcome most ofthese difculties in commercialization . It is in this context thatare situated the work of our team. It comes to new protocols ndnanoparticle synthesis by the solgel technique for various chemicaland physical applications [20,21]. Our objective in this work is, in arst step the in-situ synthesis of luminescent nanoparticles ofZn2SiO4:Mn in a silica matrix whose objective is to increase the lifeof components and prevent contamination caused by the externalenvironment. The solgel method has been conrmed to have moreadvantages in lowering the ring temperature, distributing theactivator ions homogeneously and improving the emission efciencyfor the powder phosphors [22,23].
In this study, the method is applied to prepare Zn2SiO4 andZn2SiO4:Mn particles embedded in silica monolith by the sameprotocol of solgel method combined with a furnace ring [3,9] butusing for the rst time, manganese doped zinc oxide nanoparticles
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Journal of Luminescence
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n Corresponding author. Tel.: 216 97 86 35 99; fax: 216 75 39 24 21.E-mail address: firstname.lastname@example.org (J. El Ghoul).
Journal of Luminescence 148 (2014) 8288
and studied the structural and optical properties of the obtainednanocomposites.
The preparation of colloid suspension particles in silicate hostmatrix has been done in three steps. In the rst one, nanocrystallineZnO and ZnO:Mn aerogels were prepared by a solgel methodunder supercritical conditions of ethyl alcohol (EtOH) based onOmri et al. protocol [9,24], where the water for hydrolysis wasslowly released by esterication reaction to control the size of theformed nanoparticles. In the second step, we have prepared ZnOand ZnO:Mn conned in silica aerogel according to the followingprocess: 0.5 ml of TEOS was rst dissolved in EtOH. Then, withconstant stirring of the mixture of TEOS and EtOH, 0.44 ml of waterand 30 mg of nanoparticles powder prepared in the rst step wereadded. The whole solution was stirred for about 30 min, resulting inthe formation of a uniform sol. The sols were transferred to tubes inultrasonic bath where 100 ml of uoride acid was added. The wetgel formed in few seconds. Monolithic and white aerogel wasobtained by supercritical drying in EtOH as described in the rststep. Finally, silica glasses containing Zn2SiO4 and Zn2SiO4:Mnparticles were obtained after ring aerogel at 1200 1C for 2 h.
The crystalline phases of annealed samples were identied byX-ray diffraction (XRD) using a Bruker D5005 powder X-raydiffractometer using a CuK source (1.5418 radiation). Crystallitesizes were estimated from the Scherrer's equation .
G 0:9B cos B
where is the X-ray wavelength (1.5418 ), B is the maximumof the Bragg diffraction peak (rad) and is the line width at halfmaximum.
Transmission electron microscopy (TEM, JEM-200CX) was used tostudy the morphology and particle size of the phosphor powders.The specimens for TEM were prepared by putting the as-grownproducts in EtOH and immersing them in an ultrasonic bath for15 min, then dropping a few drops of the resulting suspensioncontaining the synthesized materials onto TEM grid. The opticalabsorbance of the powders was determined using a Schimadzu UV-3101 PC spectrophotometer with integrating sphere in the wave-length range 2002000 nm. For photoluminescence (PL) measure-ments, the 450-W Xenon lamp was used as an excitation source. Theemitted light from the sample collected by an optical ber on thesame side as that of excitation was analyzed with a Jobin-YvonSpectrometer HR460 and a multichannel CCD detector (2000 pixel).The photoluminescence excitation (PLE) measurements were per-formed on a Jobin-Yvon Fluorolog 3-2 spectrometer. The lowtemperature experiments were carried out in a Janis VPF-600 Dewarwith variable temperature controlled between 78 and 300 K.
3. Results and discussion
3.1. Structural studies
The XRD spectra of undoped and Mn doped ZnO nanopwderhave been presented in Fig. 1. In the case of undoped ZnO, wenoticed the appearance of nine pronounced diffraction peaks at231.501, 34.191, 36.151, 47.391, 56.441, 62.791, 67.931, 68.911,
72.331, 76.971, 89.411 and 95.311 which can be attributed to the(1 0 0), (0 0 2), (1 0 1), (1 0 2), (1 1 0), (0 1 3), (1 1 2), (2 0 1),(0 2 2), (0 2 3) and (2 1 1) planes of ZnO, respectively . Theobtained XRD spectra matched well with the space group P63mc(1 8 6) (No. 36-1451) of wurtzite ZnO structure [24,26]. Afterdoping of ZnO with manganese, in addition to the peaks corre-sponding to ZnO, one secondary additional phase were detectedwhich can be attributed to ZnOMn3, (JCPDS Card 37-1485). Thelattice constants calculated from the XRD pattern, which are veryclose to ZnO ones, i.e., a 3:2498 and c 5:2066. These resultsare in a good agreement with those obtained by El Ghoul et al.. Due to the small size of the crystallites in the aerogel, thediffraction lines are broadened and are further found to depend onthe Miller indices of the corresponding sets of crystal planes. Forour samples, the (0 0 2) diffraction line is always narrower thanthe (1 0 1) line and the latter is narrower than the (1 0 0) line.After a correction for the instrumental broadening, an averagevalue of the basal diameter of the cylinder-shape crystallites wasfound to be 1422 nm, whereas the height of the crystallites was2330 nm.
Fig. 2 shows the X-ray diffraction patterns of SiO2/Zn2SiO4 andSiO2/Zn2SiO4:Mn nanocomposites treated at 1200 1C for 2 h in air.It is clear that all the diffraction peaks are in good agreement with
Fig. 2. X-ray diffraction pattern of the (a) SiO2/Zn2SiO4 and (b) SiO2/Zn2SiO4:Mnnanocomposite.
Fig. 1. X-ray diffraction pattern of the (a) undoped ZnO and (b) ZnO:Mn nanopowder.
J. El Ghoul, L. El Mir / Journal of Luminescence 148 (2014) 8288 83
those of the standard pattern reported by the Joint Committee onPower Diffraction Standard (JCPDS No. 08-492 and 37-1485)[21,28]. The peak signatures of hexagonal wurtzite ZnO were alsoobserved. Therefore, the hexagonal ZnO and willemite Zn2SiO4may coexist in the composite . After incorporation of thesenanoparticles ZnO:Mn in SiO2 and thermal treatment, a new zincic(-Zn2SiO4) compound was formed (Fig. 2b). The willemite(-Zn2SiO4) crystals were well developed. This analysis showsthat the peaks of these samples are indexed to the -Zn2SiO4(JCPDS Card 37-1485) . The difference in the peak intensityand width shows that the seeded sample has a higher degree ofcrystallinity. In fact the presence of manganese or vanadium inthese materials improves their crystallinity and this was observedin our previous works [9,20,21]. The lattice constants calculatedfrom the XRD pattern are a 13:944 and c 9:314, which arevery close to willemite -Zn2SiO4 ones, i.e., . This resultindicates that -Zn2SiO4 has a rhombohedral structure . It isclear that the crystalline phase is the most dominant one corre-sponding to the -phase Zn2SiO4, in parallel we note the appear-ance two other phases of silica that correspond to cristobalite andquartz . Average grain size (G) of the crystallites Zn2SiO4:Mnvaries from 70 to 90 nm , which has been estimated usingScherrer's formula (1). However, Jiang et al.  predicted thatthe optimum particle size for the Zn2SiO4:Mn nanopowder was100 nm.
We investigated the size and morphology of our samples byTEM. The TEM images of undoped and Mn-doped ZnO nanopar-ticles (Fig. 3) show that very small ZnO particles are present in theas-prepared aerogel powder. The size of the majority of ZnOparticles in this powder varied between 15 and 30 nm . Takinginto account the results of crystallite size measurements by XRD, it
can be concluded that the crystallite size is approximately equal tothe particle size in ZnO and ZnO:Mn powder prepared in thepresent work. Fig. 4 shows the TEM micrographs of the samplesZn2SiO4 and Zn2SiO4:Mn indicating that the well-crystallizedZn2SiO4 particles. At high temperature at 1200 1C, Zn and Sispecies, move and diffuse inside the porous body to form Zn2SiO4phase with a particle size greater than 80 nm (Fig. 4a). In the caseof Zn2SiO4:Mn colloid suspension is formed in silica host matrixwith a particle size of about 70 nm (Fig. 4b). Energy dispersivespectroscopy (EDX) analysis during the TEM observation, areshown in Fig. 4c and d. The concentrations of Zn, Si, and O areshown in the corresponding tables, which correspond to thestoichiometric composition of Zn2SiO4. The excess Zn ions overthe stoichiometric ratio of Zn2SiO4 crystals exist at the near-surface side due to low Si concentrations at a distance far fromthe quartz surface, and they are also texturized along with somepreferred orientations of Zn2SiO4:Mn2 crystals as seen in theXRD results. The other peaks such as Cu and C come from coppergrid and contamination.
3.2. Optical studies
Study of the optical absorption spectra provides a very usefultool for the investigation of optically induced transition andinsight in the energy gap and band structure of crystalline andnon crystalline materials. However, the absorption spectrum of-Zn2SiO4:Mn nanoparticles in the silica matrix at room tempera-ture are presented in Fig. 5. For the purpose of comparison, Zn2SiO4nanoparticles without Mn doping are presented in this gure aswell. A strong absorption band with a maximum at 215 nm isobserved in both -Zn2SiO4:Mn and Zn2SiO4 nanoparticles,
Fig. 3. Typical TEM photograph showing the general morphology (a) ZnO and (b) ZnO:Mn nanopowder and its EDX analysis.
J. El Ghoul, L. El Mir / Journal of Luminescence 148 (2014) 828884
indicating that this band originates from the absorption of Zn2SiO4host lattice. A typical value of absorption edge at 220 nm(5.63 eV), which was the absorption edge of Zn2SiO4 .Furthermore, by a comparison of the absorption curve of Zn2SiO4nanoparticles with that of Zn2SiO4:Mn nanoparticles, it can beclearly seen that there is more absorption in the latter than in theformer in the range 220280 nm. Obviously, this is attributed to theabsorption of Mn2 charge transfer transition in Zn2SiO4 host lattice.Due to the low concentration of Mn2 , the Mn2 absorption is notstrong . The values obtained for the band gap energies of theZn2SiO4 pur is about 5.493 eV, whereas the -Zn2SiO4:Mn is around5.347 eV (Fig. 5, inset) . We suggest that this decrease is due tothe reduction of the bandwidth of ZnO after doping with Mn. In fact,it was shown that the band gap energies of the ZnO materials show
a decrease with the increase of doping concentration [24,33]. For thelow Mn concentration the reduction in the band gap has beentheoretically explained as a consequence of exchange interactionbetween d electrons of the transition metal ions (Mn) and the s andp electron of the host band.
3.3. Photoluminescence property
In the present investigation, the emission spectra of SiO2/Zn2SiO4 nanocomposite at different temperatures, recorded inthe spectral range 3651000 nm are shown in Fig. 6. The samplehas broad PL bands centered at different positions. The strikingfeature is the absence of almost any of the usually reported visibleemission bands in the range 400650 nm (2.42.7 eV), and theappearance of a strong and wide near infrared (NI...