Journal of Alloys and Compounds 397 (2005) L1–L4

Letter

Sol–gel synthesis and characterization of nanocrystalline ZnO powders

M. Ristica,∗, S. Musica, M. Ivandaa, S. Popovicb

a Division of Materials Chemistry, Ru–der Boskovic Institute, P.O. Box 180, Bijeniˇcka Cesta 54, HR-10002 Zagreb, Croatiab Department of Physics, Faculty of Science, University of Zagreb, P.O. Box 331, HR-10002 Zagreb, Croatia

Received 8 January 2005; accepted 28 January 2005Available online 9 March 2005

Abstract

Nanocrystalline zinc oxide (ZnO) powders were prepared by fast hydrolysis of zinc 2-ethylhexanoate dissolved in 2-propanol, adding atetramethylammonium (TMAH) aqueous solution. XRD showed an average value of 25–35 nm for the basal diameter of supposed cylinder(prism)-shaped crystallites, whereas the height of these crystallites was 35–45 nm. TEM showed that the size of the majority of ZnO particlesvaried between 20 and 50 nm, thus indicating that particle and crystallite sizes in ZnO powders were approximately equal. The size of ZnOparticles did not change significantly for different amounts of zinc 2-ethylhexanoate in the precipitation systems investigated. Raman spectrao©

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eywords:Sol–gel; Nanocrystalline ZnO; XRD; FT-IR; Raman; TEM

. Introduction

Zinc oxide (ZnO) powders have found important applica-ions in the production of paints, ceramics, catalysts, varis-ors, etc. The size of ZnO particles in these powders is aery important factor for the specific application. Chemical,icrostructural and physical properties of ZnO powders areependent on the synthesis procedure. Very different methodsere used in the synthesis of ZnO powders.Music et al.[1,2] investigated the influence of the synthe-

is procedure on the formation of ZnO powders. ZnO didot crystallize upon hydrothermal treatment of Zn(NO3)2queous solution containing urea, up to 160◦C. HydrozinciteZn5(CO3)2(OH)6) was precipitated instead. Upon heating at60◦C in air, Zn5(CO3)2(OH)6 transformed to ZnO. The pre-ipitates obtained by abrupt adding of a concentrated NH4OHqueous solution to the Zn(NO3)2 solution yielded a com-lex compound, Zn5(OH)8(NO3)2(H2O)2− x(NH3)x, whichpon autoclaving transformed to ZnO. Matijevic and co-orkers[3,4] used urea and triethanolamine (TEA) process-

ing to prepare uniform ZnO particles. Urea processing[5]was also used to prepare ZnO powders, which werether utilized in the preparation of ZnO varistors. TaubertWegner[6] investigated the formation of monodisperse Zparticles by the hydrolysis of Zn2+ ions in a decomposinhexamethylenetetramine (HMTA) solution, into which auble starch was also added.

In recent years researchers have focused more osynthesis and properties of ZnO nanoparticles due toapplication in advanced technologies. ZnO nanopartwere prepared[7–11] by adding dry LiOH or its ethanolsolution to the ethanolic solution of Zn(ac)2·2H2O. Inubushet al. [12] prepared ZnO nanoparticles by the reactionZn(acac)2 with NaOH in ethanol. Mondelaers et al.[13]proposed the preparation of ZnO nanoparticles viaaqueous acetate–citrate gelation method. Microemumethod was also used[14,15]to produce ZnO nanoparticle

In the present work, we report about a novel laboraprocedure for the preparation of nanocrystalline ZnO pders. This procedure is based on the sol–gel method. Ipresent case, a tetramethylammonium hydroxide (TM

∗ Corresponding author. Tel.: +385 1 4680 107; fax: +385 1 4680 098.E-mail address:ristic@irb.hr (M. Ristic).

solution was added to the alcoholic solution of zinc 2-ethylhexanoate. TMAH is a strong organic alkali, which can

925-8388/$ – see front matter © 2005 Elsevier B.V. All rights reserved.oi:10.1016/j.jallcom.2005.01.045

L2 M. Ristic et al. / Journal of Alloys and Compounds 397 (2005) L1–L4

be used to easily adjust pH values near∼14. The advantageof TMAH in relation to a strong inorganic alkali (for exam-ple, NaOH) at very high pH is that it does not contaminatemetal oxides with alkali metal cations, which in the next stepwould affect the ohmic conductivity of the oxide material.In specific cases, the tetramethylammonium cation may alsoserve as a templating agent.

2. Experimental

Zinc 2-ethylhexanoate containing 1% of ethylene glycolmonomethylether supplied by Alfa Aeser® was used. TMAH[(CH3)4NOH, 25%, w/w aqueous solution, electronic grade,99.99% (metal basis)] was also supplied by Alfa Aeser®. Wa-ter free 2-propanol and absolute ethanol supplied by Kemikawere used. Twice distilled water used for washing of the pre-cipitates was prepared in our laboratory.

Chemical conditions for the precipitation of samplesS1–S6 are given inTable 1. Samples S1–S6 were precipitatedat room temperature (RT) as follows: onto a proper weightof zinc 2-ethylhexanoate (starting chemical), 90 ml of 2-propanol was added. After strong mixing a clear (transparent)solution was obtained. Then, 10 ml of 25% TMAH was addedto this solution under strong mixing. The formed colloidalsuspensions (S1–S4) were aged for 30 min, washed threet a-t er-f ax-i d S6w witho be-f . Thea

Ra-m tion( ure-m rdeda TheF uterl ro-c ed ints Ra-m argon

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laser withλ = 514.5 nm, as the excitation source. The scat-tered light was analyzed by a Dilor Z-24 Raman spectrome-ter. TEM observation of the samples was performed using aPhilips transmission electron microscope (model Morgagni268).

3. Results and discussion

In the present work, we have found that very fine ZnOparticles can be produced by a fast hydrolysis of zinc 2-ethylhexanoate dissolved in 2-propanol. A stable colloidal(milky) dispersion was obtained by adding aqueous solutionof TMAH to the alcoholic solution of zinc 2-ethylhexanoate.A gradual coagulation of colloidal particles was observedupon complete addition of the TMAH solution. For that rea-son the suspensions S1–S4 were aged for 30 min, whereasthe suspensions S5 and S6 were aged for 24 h. The aim ofprolonged ageing was to check for a possible influence of thecoagulation process on the properties of ZnO particles.

All samples were identified by XRD as a single phasecontaining ZnO (PDF card nos.: 89-1397; 89-0511; 36-1451)[16]. Diffraction lines of ZnO were broadened, and diffrac-tion broadening was found dependent on Miller indices ofthe corresponding sets of crystal planes. For most samplesthe diffraction line 0 0 2 was narrower than the line 1 0 1,a ateda thatc theh ald so ctionl n fort asald nm,ws e S1( nOp ivenf

S wed avs eda bandc eo cedb ag-g rph.H teds orp-t ency( -t per-m andc the

imes with ethanol and two times with twice distilled wer, then dried at 60◦C. Washing of the precipitates was pormed using a Sorvall RC2-B ultraspeed centrifuge (mmum operational range, 20,000 rpm). Samples S5 anere precipitated in the same way as samples S1–S4nly difference in the ageing of the colloidal suspension

ore the separation of a solid phase from the liquid phasegeing time for samples S5 and S6 was 24 h.

All samples were characterized by XRD, FT-IR andan spectroscopies and TEM. X-ray powder diffrac

Philips counter diffractometer, model MPD 1880) measents were performed at RT. FT-IR spectra were recot RT using a Perkin-Elmer spectrometer (model 2000).T-IR spectrometer was coupled with a personal comp

oaded with the IRDM (IR data manager) program to pess the recorded spectra. The specimens were pressmall discs using a spectroscopically pure KBr matrix.an spectra were recorded using a coherent Innova-100

able 1hemical conditions for the preparation of nanocrystalline ZnO powd

ample ZEH (g) 2-Propanol (ml) TMAH (m

1 0.982 90 102 1.496 90 103 1.995 90 104 5.002 90 105 1.009 90 106 5.010 90 10

EH: zinc 2-ethylhexanoate containing 1% ethylene glycol monomether, TMAH: tetramethylammonium hydroxide, 25% (w/w) aqueolution.

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nd that one was narrower than the line 1 0 0. This indicn asymmetry in the crystallite shape. It was supposedrystallites were in the form of cylinder (prism), havingeight (direction of the crystalc-axis) bigger than the basiameter (crystal axes,a1 anda2). The crystallite size wabtained by measurements of the broadening of diffra

ines and applying the Scherrer equation, after a correctiohe instrumental broadening. An average value of the biameter of the cylinder-shaped crystallites was 25–30hereas the height of the crystallites was 35–45 nm.Fig. 1hows a characteristic part of the XRD pattern of samplnanocrystalline ZnO). The XRD pattern of commercial Zowder, exibiting only instrumental broadening, is also g

or comparison.FT-IR spectra of samples S1–S6 are shown inFig. 2.

amples S1–S4, aged as suspension for 30 min, shoery broad IR band with the peak centered at 430 cm−1 andhoulders at 535 and 400 cm−1, whereas S5 and S6 (ags suspension for 24 h) showed a very broad and strongentered at 430 cm−1 with a shoulder at 535 cm−1. The shapf the IR spectrum of ZnO particles is generally influeny particle size and morphology, the degree of particlesregation, or the crystal structure of the ZnO polymoayashi et al.[17] compared the recorded and calculapectra of ZnO. ZnO particles showed three distinct absion peaks located between the bulk TO-phonon frequωT⊥) and the LO-phonon frequency (ωL||). These absorpion peaks shifted towards lower frequencies when theitivity of the surrounding medium was increased. Serna

o-workers[18,19] considered the relationship between

M. Ristic et al. / Journal of Alloys and Compounds 397 (2005) L1–L4 L3

Fig. 1. Characteristic parts of X-ray powder diffraction patterns: (a) sampleS1 (nanocrystalline ZnO) and (b) commercial ZnO powder. Measurementstaken at room temperature.

Fig. 2. FT-IR spectra of samples S1–S6. The spectra recorded at room tem-perature.

Fig. 3. Raman spectra of samples S1–S6. The spectra recorded at roomtemperature.

shape of the IR spectrum on one side, and the physical shapeand aggregation of ZnO particles on the other. Tanigaki et al.[20] prepared ZnO particles by the high-temperature oxida-tion of zinc powder. They observed different shapes in thespectra of ZnO particles as a result of the used preparationroute.

Samples S1–S6 were also investigated by Raman spec-troscopy. The corresponding spectra are given inFig. 3. Thesespectra are characterized by a very strong band at 438 cm−1.The band at 332 cm−1 is well pronounced; the bands at 392and 418 cm−1, however, are visible as shoulders. A broadband at 578–580 cm−1 with a shoulder at 536–542 cm−1 anda very weak band at 652–656 cm−1 are also observed in thesespectra.

Damen et al.[21] investigated the Raman effect on ZnOcrystals of the size of several millimeter, and measured: twoE2 vibrations at 101 and 437 cm−1, one transverseA1 at381 cm−1 and one transverseE1 at 407 cm−1, one longitu-dinalA1 at 574 cm−1 and one longitudinalE1 at 583 cm−1.Calleja and Cardona[22] investigated the resonance effect ofRaman scattering on a ZnO single crystal byE2,A1T,E1L andE1T phonons including several second-order features withphoton energies between 1.6 and 3 eV. Exarhos and Sharma[23] applied Raman spectroscopy in the investigation of ZnOfilms (wurtzite phase) and concluded that these films exhib-ited a certain degree of residual stress inferred from theER ode.T d byd

2aman shift relative to the single crystal position of this mhe shape of the Raman spectrum of ZnO was influenceoping of ZnO particles with antimony ions[24]. With an

L4 M. Ristic et al. / Journal of Alloys and Compounds 397 (2005) L1–L4

Fig. 4. TEM photographs of ZnO powders: (a) S1, (b) S2, (c) S5.

increase in the Sb-doping content up to 2.15% the band at528 cm−1 appeared, whereas the band at 580 cm−1 shiftedto 568 cm−1. Heating of Sb-doped ZnO at high temperaturescaused a gradual decrease in the relative intensities of thebands at 528 cm−1 and 568 cm−1 in relation to the band at433 cm−1.

Raman spectra of samples S1–S6, shown inFig. 3, indi-cated distinct changes in relation to the Raman spectrum ofbig ZnO crystals. The Raman band at 392 cm−1, observedfor samples S1–S4, was located at 381 cm−1 for big ZnOcrystals. Also, the band at 407 cm−1, observed for big ZnOcrystals, shifted to 418 cm−1 for nanocrystalline ZnO. Theseeffects, as well as the increase in relative intensities of theband at 580 cm−1 and the shoulder at 542 cm−1 and theirbroadening, can be assigned to the effect of the very smallsize of ZnO particles produced in the present work. The effectof very small size SnO2 particles, as well as the aggregationeffect of these particles were considered in the interpretationof the Raman spectrum of SnO2 in our previous work[25].

TEM measurements have shown that very small ZnO par-ticles are present in all powders.Fig. 4 shows TEM pho-tographs of selected ZnO powders S1, S2 and S5. The size ofthe majority of ZnO particles in these powders varied between20 and 50 nm. Taking into account the results of crystallitesize measurements by XRD, it can be concluded that the crys-tallite size is approximately equal to the particle size in ZnOp

Acknowledgement

The authors wish to thank Professor Nikola Ljubesic forhis assistance with electron microscopy.

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