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Journal of Non-Crystalline Solids 352 (2006) 2565–2568

Sol–gel derived yttrium doped ZnO nanostructures

Ravinder Kaur a, A.V. Singh a, Kiran Sehrawat a, N.C. Mehra b, R.M. Mehra a,*

a Department of Electronic Science, University of Delhi South Campus, New Delhi 110021, Indiab USIC, University of Delhi, Delhi 110 007, India

Available online 22 May 2006

Abstract

Yttrium doped ZnO nanostructures were synthesized at room temperature by sol–gel technique. The sols were prepared using zincacetate di-hydrate and ethanol as the precursors with yttrium nitrate hexahydrate as the dopant. Lactic acid with water was used asthe acidic catalyst to control the hydrolysis reaction. Ammonia was added to vary the pH of the solution and the shape of the nanostruc-tures changed with the change in pH of the solution. The films were deposited on ultrasonically cleaned glass substrates by dip coatingtechnique. X-ray diffraction patterns indicated that the obtained nanostructures were polycrystalline in nature with (100), (002) and(101) reflections of hexagonal ZnO crystal structure. The ZnO films exhibited nanostructures with a rod/lathe like morphology onchanging the yttrium concentration. The diameters of the structures varied from 100 nm to 250 nm and the aspect ratio was found tobe in the range of 50–70.� 2006 Elsevier B.V. All rights reserved.

PACS: 73.22.�f; 73.50.Pz; 73.61.�r; 78.66.�w; 81.20.Fw

Keywords: Nanostructures; ZnO; Yttrium doping; Sol–gel

1. Introduction

Nanoscale one-dimensional structures have attractedgreat interest due to their extraordinary physical propertiesand potential applications [1–3]. Zinc oxide is an importantmaterial due to its exceptional physical properties such ashigh conductance, chemical and thermal stability, wideband gap (3.3 eV) and a large exciton binding energy of60 meV. Owing to these properties, it has been investigatedfor low voltage and short wavelength electro-opticaldevices such as light emitting diodes and UV lasers [4]. Dif-ferent nanostructures of ZnO including nanowires, nano-belts, nanoribbons, nanorods and nanonails have beenfabricated by thermal evaporation of oxide powders [5–7]. Sol–gel process, a simple and inexpensive techniquehas been employed to synthesize nanowires and nanorods[8–10]. The specific chemical, surface and nanostructuralproperties of zinc oxide make it a potential candidate espe-

0022-3093/$ - see front matter � 2006 Elsevier B.V. All rights reserved.

doi:10.1016/j.jnoncrysol.2006.01.090

* Corresponding author. Tel.: +91 11 24115849; fax: +91 11 24110876.E-mail address: rammehra2003@yahoo.com (R.M. Mehra).

cially for catalytic and gas sensing applications where theexposing surface area of the particles to target gas is veryimportant as the large surface atom/bulk atom ratio ofnanostructures enhances the sensing properties [11].

In the present work, yttrium doped ZnO (YZO) nano-structures were fabricated by changing the pH of the solu-tion. Further, a study on the shape of the resultingnanostructures and the aspect ratio has been performedas a function of the dopant concentration. The structuralproperties have been investigated using X-ray diffraction(XRD) patterns and scanning electron microscopy (SEM).

2. Experimental details

The precursor was prepared from zinc acetate(Zn(CH3COO)2 Æ 2H2O) and anhydrous ethanol, refluxedat 80 �C and stirred in a magnetic stirrer for about 4 h atroom temperature. The molarity of the solution was chosento be 0.2 M. Yttrium nitrate hexahydrate was used as thesource material for doping and its concentration was variedfrom 1 to 4 wt% in the present work. The solution was

2566 R. Kaur et al. / Journal of Non-Crystalline Solids 352 (2006) 2565–2568

hydrolysed with 2 mole of water per metal acetate by add-ing water dissolved in ethanol at 10 wt% concentrationdrop wise along with roughly 5 wt% lactic acid, in orderto remove turbidity and obtain a clear and stable solution.The pH of the obtained solution was 2.5. Ammonia wasdropped to obtain solutions of different pH values rangingfrom 2.5 to 5.

The precursor solution was always stored at 4 �C toavoid unwanted precipitation reactions. The reaction flask

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(101)

(002)

(100)

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nsity

(a.u

.)

2θ (deg.)θ

Fig. 1. A typical XRD pattern of the ZnO nanostructures.

Fig. 2. SEM pictures of undoped ZnO films prepared from

was then placed in a heated water bath for speeding up thehydrolysis reaction.

Corning (7059) glass substrates, after cleaning with ace-tone and methanol in an ultrasonic bath for 20 min each,were dipped in the solution, withdrawn at a rate of�10 cm/min and then dried at 250 �C for 20 min. Thiscycle was repeated 10–15 times. The thickness of the filmswas in the range of 200–250 nm. The deposited films wereannealed in air at 450 �C for 1 h.

The structural properties of the films were investigatedby Phillips–Holland X-ray diffractometer (Model PW1830/00), whereas the thickness was measured by DEK-TECK3-ST surface profilometer. The surface morphologyof the films was analyzed by scanning electron microscopy(SEM, JEOL JSM-6300). The composition wt% of Y toZn, of YZO films was found to be equal to that added inthe solution, as was confirmed by the elemental dispersionanalysis of X-rays (EDAX) measurements performed onthe films.

3. Results and discussion

A typical X-ray diffractogram of the nanostructureobtained from an undoped ZnO film prepared with a solu-tion of pH 2.5 is shown in Fig. 1. It depicts the hexagonalwurtzite crystal structure of the films having planes (100),

solutions with pH values of (a) 2.5, (b) 3.5 and (c) 4.

Fig. 3. SEM pictures of yttrium doped ZnO films prepared from solutions with pH value of 4 and yttrium concentration of (a) 1 wt%, (b) 2 wt%, (c) 3 wt%and (d) 4 wt%.

R. Kaur et al. / Journal of Non-Crystalline Solids 352 (2006) 2565–2568 2567

(002) and (101) at angles 2h = 31.84�, 34.43� and 36.35�,respectively. As for the other samples prepared from solu-tions of different pH, the XRD patterns were almost iden-tical, except for a slight increase in sharpness of the peakswith increasing pH.

SEM studies of the films revealed presence of differenttypes of ZnO nanostructures with change in pH value ofthe solution and also by introducing known amounts ofthe dopant.

Fig. 2 shows the SEM micrographs of undoped ZnOfilms obtained from solutions with increasing pH values.As can be seen from Fig. 2(a), a pH value of 2.5 resultsin a highly homogenous distribution of fine nanoparticles.With the increase in pH to 3.5, the nanostructure showed atransition to dendritic growth (Fig. 2(b)) which resulted inthe formation of nanowires as the pH was further increasedto 4 (Fig. 2(c)). It was observed that the size of particlesdepends on the rate of hydrolysis. Increasing the pH byadding ammonia inhibits the isotropic agglomeration ofparticles, so particles do not grow further. Instead, ananisotropic agglomeration occurs which leads to the for-mation of dendrites and wires.

Fig. 3 shows the change in the shape of nanostructuresin yttrium doped ZnO films prepared from solution witha pH 4 with different dopant concentrations. The films

having 1 wt% of yttrium exhibited nanofibres with an aver-age diameter in the range of 200–250 nm as is evident fromFig. 3(a). As the yttrium concentration was increased,nanorods (Fig. 3(b)) were formed which converted to Tand Y shaped thicker rods for a concentration of 4 wt%(Fig. 3(c) and (d)). The diameters of the structures variedfrom 100 to 250 nm and the aspect ratio was found to bein the range of 50–70.

4. Conclusion

The nanostructured morphology of the sol–gel derivedYZO films can be tailored by using appropriate pH ofthe solutions and the dopant concentration. The transitionfrom nanoparticles to nanorods can be attributed to achange in pH of the solution affecting the rate of hydroly-sis, which affects the initial nuclei of ZnO.

It was also observed that for pH remaining same, thenanostructures showed different morphologies as a func-tion of yttrium concentration. It is expected that Y inZnO lattice acquires substitutional position at low concen-tration while at higher concentrations, it takes the intersti-tial sites (due to reduction in zinc sites) which results in theformation of nanorods. This is similar to the formation ofZnO nanostructures by controlling Zn vapor as has been

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reported by Leung et al. [12]. A comprehensive study is inprogress to elucidate the exact growth mechanisms at dif-ferent stages.

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