Materials Letters 109 (2013) 88–91

Contents lists available at ScienceDirect

Materials Letters

0167-57http://d

n CorrE-m

Zargar@

journal homepage: www.elsevier.com/locate/matlet

Synthesis of Al-doping ZnO nanoparticles via mechanochemicalmethod and investigation of their structural and optical properties

Ahmad Echresh n, Morteza Zargar ShoushtariPhysics Department, Shahid Chamran University, Ahvaz, Iran

a r t i c l e i n f o

Article history:Received 13 June 2013Accepted 15 July 2013Available online 20 July 2013

Keywords:NanoparticlesSemiconductorsStructuralDefects

7X/$ - see front matter & 2013 Elsevier B.V. Ax.doi.org/10.1016/j.matlet.2013.07.059

esponding author. Tel./fax: +98 611 333 1040.ail addresses: a-echresh@phdstu.scu.ac.ir (A. Escu.ac.ir (M. Zargar Shoushtari).

a b s t r a c t

In this study, pure and Al-doped ZnO nanoparticles were successfully synthesized by the mechan-ochemical method. The effect of doping on structural and optical properties was studied. The structuralinvestigations of products confirmed the hexagonal wurtzite structure for all products and also it wasobserved that doping caused a little change in lattice parameters. The results obtained by SEM revealedthat the average particle size of products increases by doping and nanoparticles have almost sphericalmorphology. UV–visible absorption and PL spectra showed that the maximum ultraviolet absorptionwavelength of products increases and the optical band gap shifts from 3.15 eV to 3.11 eV, with increasingthe Al concentration.

& 2013 Elsevier B.V. All rights reserved.

1. Introduction

In recent years, semiconductor nanostructures due to theirunique properties and applications have attracted much attention.Among many kinds of the metal oxide nanostructures that havebeen developed, ZnO is a material which is of great interest for avariety of applications [1,2]. Zinc oxide exhibits hexagonal wurt-zite structure and has a direct band gap semiconductor around3.37 eV at 300 K and the large exciton binding energy of 60 meV[3,4]. As an II–VI semiconductor, the ZnO is widely used in the gassensors [5], solar cells [6], and luminescent, electrical and chemicalsensors [7]. Doping affects the crystal growth when it is performedduring the nanoparticles formation process and also makes somechanges in its optical and electrical properties [8,9]. In order toimprove the properties of the n-type ZnO semiconductor, it can bedoped with metals such as Al, Ga and In [10,11]. The ZnO has somedefects which can either be native (oxygen vacancies, zinc inter-stitials) or incorporated ones (such as hydrogen atoms) that thesedefects could affect optical properties of the ZnO by creating adonor levels in its band gap [12]. Therefore, it is worthful toinvestigate the optical properties of Al-doped ZnO. Al-doped ZnO(AZO) nanostructures could be synthesized by various methodssuch as sol–gel [13], hydrothermal [14], arc plasma [15] andmagnetron sputtering [16].

As far as we know, producing the AZO nanoparticles by the simpleand economic mechanochemical method is reported for the first timein the present work. The structural and optical properties of thesamples were studied by X-ray diffraction (XRD), scanning electron

ll rights reserved.

chresh),

microscopy (SEM), thermogravimetry (TG), UV–visible absorptionspectroscopy (UV–vis) and photoluminescence (PL).

2. Experimental

The starting materials were ZnCl2, Na2CO3, NaCl and Al(No3)3 �9H2O with high purity that were purchased from Merck.The NaCl was used as a dilute additive to the starting powder. Thestoichiometric mixture of the starting powder was milled with theplanetary mill for 9 h at 250 rpm then powder was calcined in airat 600 1C for 30 min [17]. The reactions are described in thefollowing formula:

(1�x)ZnCl2+Na2CO3+8.6 NaCl+xAl (NO3)3 �9H2O-AlxZn1�xCO3+10.6 NaCl

AlxZn1�xCO3-AlxZn1�xOþ CO2 gð Þ þ⋯

The amount of x was equal to 0, 0.01, 0.03 and 0.09. Thediameter of the balls was 10 mm and ratio of the balls to thepowder mass was 10:1. In continue the samples were washed withdeionize water for a few times. Finally, AlxZn1�xO nanoparticleswere obtained from drying at 80 1C of washed powders.

3. Results and discussions

Thermogravimetry: Thermogravimetry analysis was carried outto determine the crystalline conditions. The obtained powder ofmilling was heated from the room temperature to 1000 1C with a

Fig. 1. TG plot of AZO nanoparticles (a) and X-ray pattern of AlxZn1�xO nanoparticles with x¼0, 0.01, 0.03 and 0.09 (b).

Table 1Lattice parameters, crystallites size, particle size, SGB, Maximum absorptionwavelength and optical band gap of AlxZn1�xO nanoparticles.

Sample Latticeparameters(Å)

Averagecrystallitessize (nm)

Averageparticlesize (nm)

SGB(m2/m3)

λmax(nm) Eg(eV)

X¼0 a¼3.2418c/a¼1.6005

33.55 65 4.92�107 372.3120 3.15

X¼0.01 a¼3.2399c/a¼1.6001

30.15 75 5.47�107 373.9654 3.14

X¼0.03 a¼3.2433c/a¼1.6007

28.10 100 5.87�107 375.2454 3.13

X¼0.09 a¼3.2453c/a¼1.6010

24.80 114 6.65�107 377.3792 3.11

A. Echresh, M. Zargar Shoushtari / Materials Letters 109 (2013) 88–91 89

rate of 10 1C/min in the air. Fig. 1(a) shows a combined plot ofdifTG and TGA. Notably, the TGA data plots the weight loss of thenanoparticles which is found to take place till 550–600 1C and isdue to the release of CO2, H2O and N2 gases. Therefore, thecrystallization of AZO nanoparticles occurred at temperatures over600 1C.

Structural properties: Fig. 1(b) shows the X-ray patterns ofAlxZ1�xO with x¼0, 0.01, 0.03 and 0.09. A comparison with thestandard card demonstrates that the peaks can be indexed toknow the hexagonal wurtzite structure of ZnO with lattice con-stants of a¼3.250 Å, c¼5.207 Å (JCPDF card: 79-2205). On theother hand, the samples are quite pure and no impurities such asAl2O3 are observed. In addition, negligible changes of all diffractionpeak positions and lattice parameters of ZnO in all AZO samplescompared to that of pure ZnO suggest that Al+3 ions substitute theZn+2 ions and incorporate into the lattice of ZnO, that these resultsagree with the work reported by [18]. The lattice parameters andcrystallite size, measured using Xpowder software, are listed in theTable 1. The results reveal that the lattice parameters decreasewith doping, but with increasing the Al concentration, theseparameters increase. It can be assumed that the Al-doping resultsin a decrease of the lattice parameters due to the fact that the ionicradius of Al+3is smaller than ionic radius of Zn+2, but increasingthe Al concentration leads to increase in Coulomb repulsion forcesand expansion of the lattice parameters. As we know the grainboundary (GB) is the interface between two grains or crystallites.By increasing the Al doping, the average crystallites size wasreduced and specific GB area (SGB), which it introduces as theratio of GB area to volume, was increased. By approximately,SGB¼1.65/D formula [19], where D is the average crystallites size,was used to determine the specific GB area that is shown inTable 1. The SEM image of products is illustrated in Fig. 2. A carefulperusal of this image reveals that, nanoparticles have almostspherical morphology and the morphology of the nanoparticlesstay the same after doping. Also, The SEM images show an increasein the average particle size of products after doping that theirparticle size displays in Table 1. It seems that the increasing of SGBleads to the increment of average particle size.

Optical properties: Optical properties were investigated usingUV–visible and PL spectra. Fig. 3(a) displays the UV–vis absorptionspectra and the plot of (αE)2 versus E for pure ZnO andAl0.03Zn0.97O nanoparticles. Optical Band gap of nanoparticleswas measured using the following formula [18]:

ðαEÞ ¼ AðE � EgÞ1=2

Where α is an optical absorption coefficient, E is a photon energy,A is a constant coefficient and Eg is the optical band gap energy.The maximum absorption wavelength and the optical band gapvalues of products, obtained by extrapolation method, were listedin Table 1. The PL spectrum of the pure and Al-doped ZnOnanoparticles is presented in Fig. 3(b). UV emission peak, relatedto the recombination of electron–hole in near band edge levels isobserved at 389 nm for pure ZnO nanoparticles. One can see that,with increasing doping concentration, UV emission peak shifts tothe longer wavelength that might be a sign of the optical band gapdecreasing. It seems, Al-doping creates some energy donor levelsin the band gapthat leads to reduce the optical band gap of ZnOnanoparticles. These results are consistent with UV–visible results.The intensity of the UV emission peak decreases when the Alconcentration is rose from x¼0 to x¼0.09 which represented thenonradiative emission.

4. Conclusion

In this study, the pure and Al-doped ZnO nanoparticles weresynthesized via the mechanochemical method. According to theTG results, the appropriate temperature of calcinations is 600 1C.All products have the hexagonal wurtzite structure without anyimpurities such as Al2O3. The morphology of products was almostspherical and nochanges were observed with Al doping. Moreover,the average particle size incremented with increasing the Alconcentration. The optical studies showed a reduction in optical

Fig. 3. Absorption spectra (inset), variation of (αE)2 versus E (a) and PL (b) Of AlxZn1�xO nanoparticles.

Fig. 2. SEM image of AlxZn1�xO nanoparticles with X¼0 (a), X¼0.01 (b), X¼0.03 (c) and X¼0.09 (d)

A. Echresh, M. Zargar Shoushtari / Materials Letters 109 (2013) 88–9190

band gap of Al0.09Zn0.91O nanoparticles in comparison with theZnO nanoparticles.

Acknowledgment

The authors acknowledge Shahid Chamran university of ahvazfor financial support of this work.

References

[1] Rashidi H, Ahmadpour A, Bamoharram F, Heravi M, Ayati A. The novel, onestep and facile synthesis of ZnO nanoparticles using heteropolyoxo metalatesand their photoluminescence behavior. Adv Powder Technol 2013;24:549–53.

[2] Kazeminezhad I, Sadollahkhani A, Farbod M. Synthesis of ZnO nanoparticlesand flower-like nanostructures using nonsono- and sono-electrooxidationmethods. Mater Lett 2013;92:29–32.

[3] Nair MG, Nirmala M, Rekha K, Anukaliani A. Structural, optical, photo catalyticand antibacterial activity of ZnO and Co doped ZnO nanoparticles. Mater Lett2011;65:1797–800.

[4] Chieng BW, Loo YY. Synthesis of ZnO nanoparticles by modified polyolmethods. Mater Lett 2012;73:78–82.

[5] Khun K, Ibupoto ZH, Ali SU, Chey CO, Nur O, Willander M. Iron ion sensorbased on functionalized ZnO nanorods. Electroanalysis 2012;24:521–8.

[6] Zhifeng L, Chengcheng L, Jing Y, Lei E. Preparation of ZnO nanoparticles andcharacteristics of dye-sensitized solar cells based on nanoparticles film. SolidState Sci 2010;12:111–4.

[7] Wadesa A, Nour O, Willander M. The effect of the interlayer design on theelectroluminescence and electrical properties of n-ZnOnanorod/p-type blendedpolymer hybrid light emitting diodes. Nanotechnology 2009;20:065710.

[8] He HY, Liang Q. Enhancement in the optical transmittance of ZnO:Al powdersby Mo co-doping. Curr Appl Phys 2012;12:865–9.

[9] Liu C, Meng D, Pang H, Wu X, Xie J, Yu X, et al. Influence of Fe-doping on thestructural, optical and magnetic properties of ZnO nanoparticles. J Magn MagnMater 2012;324:3356–60.

[10] Fang T, Watson JL, Goodman J, Dimkpa CO, Martineau N, Das S. Does dopingwith aluminum alter the effects ofZnO nanoparticles on the metabolism of soilpseudomonads. Microbiol Res 2013;168:91–8.

[11] Morales AE, Pal U. Effect of In, Sb and Ga doping on the structure andvibrational modes of hydrothermally grown ZnO nanostructures. Curr ApplPhys 2011;11:525–31.

[12] Tuomisto F, Look DC, Farlow GC. Defect studies in electron-irradiated ZnO andGaN. Physica B 2007;401–402:604–8.

A. Echresh, M. Zargar Shoushtari / Materials Letters 109 (2013) 88–91 91

[13] Zhang J, Que W. Preparation and characterization of sol–gel Al-doped ZnO thinfilms and ZnO nanowire arrays grown on Al-doped ZnO seed layer byhydrothermal method. Sol Energy Mater Sol Cells 2010;94:2181–6.

[14] Yun S, Lee J, Yang J, Lim S. Hydrothermal synthesis of Al-doped ZnO nanorodarrays on Si substrate. Physica B 2010;405:413–9.

[15] Farbod M, Zargar Shoushtari M, Parhoodeh S. Fabrication and characterizationof Zn1�xAlxO nanoparticles by DC arcplasma. Physica B 2011;406:205–10.

[16] Nomoto JI, Hirano T, Miyata T, Minami T. Preparation of Al-doped ZnOtransparent electrodes suitable for thin-film solar cell applications by varioustypes of magnetron sputtering depositions. Thin Solid Films 2011;520:1400–6.

[17] Moballegh A, Shahverdi HR, Aghababazadeh R, Mirhabibi AR. ZnO nanoparti-cles obtained by mechanochemical technique and the optical properties. SurfSci 2007;601:2850–4.

[18] Kumar RS, Sathyamoorthy R, Sudhagar P, Matheswaran P. Effect of aluminumdoping on the structural and luminescent properties of ZnO nanoparticlessynthesis by wet chemical method. Physica E 2011;43:1166–70.

[19] Straumal BB, Mazilkin AA, Protasova SG, Straumal PB, Myatiev AA, Schutz G,et al. Grain boundaries as the controlling factor for the ferromagneticbehavior of Co-doped ZnO. Philoso Mag 2013;93:1371–83.