Journal of Crystal Growth 312 (2010) 2804–2813

Contents lists available at ScienceDirect

Journal of Crystal Growth

0022-02

doi:10.1

n Corr

E-m

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

Template free-solvothermaly synthesized copper selenide(CuSe, Cu2�xSe, b-Cu2Se and Cu2Se) hexagonal nanoplatesfrom different precursors at low temperature

Pushpendra Kumar, Kedar Singh n, O.N. Srivastava

Department of Physics, Faculty of Science, Banaras Hindu University, Varanasi 221005, India

a r t i c l e i n f o

Article history:

Received 8 January 2010

Received in revised form

2 June 2010

Accepted 14 June 2010

Communicated J.M. Redwingadopting the solvothermal method. Phase analysis, purity and morphology of the product have been

Available online 18 June 2010

Keywords:

A1. Crystal morphology

A1. Crystal Structure

A1. X-ray diffraction

A2. Growth from solutions

B1. Nanomaterials

B2. Semiconducting materials

48/$ - see front matter & 2010 Elsevier B.V. A

016/j.jcrysgro.2010.06.014

esponding author. Tel.: +91 542 2307308x21

ail address: kedarbhp@rediffmail.com (K. Sing

a b s t r a c t

Nonstoichiometric (Cu2�xSe) and stoichiometric (CuSe, b-Cu2Se and Cu2Se) copper selenide hexagonal

nanoplates have been synthesized using different general and convenient copper sources, e.g. copper

chloride, copper sulphate, copper nitrate, copper acetate, elemental copper with elemental selenium,

friendly ethylene glycol and hydrazine hydrate in a defined amount of water at 100 1C within 12 h

well studied by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM),

transmission electron microscopy (TEM), high resolution transmission electron microscopy (HRTEM)

and energy dispersive X-ray diffraction (EDAX) techniques. The structural and compositional analysis

revealed that the products were of pure phase with corresponding atomic ratios. SEM, TEM and HRTEM

analyses revealed that the nanoplates were in the range 200–450 nm and the as-prepared products

were uniform and highly crystallized. The nanoplates consisted of {0 0 1} facets of top–bottom surfaces

and {1 1 0} facets of the other six side surfaces. This new approach encompasses many advantages over

the conventional solvothermal method in terms of product quality (better morphology control with

high yield) and reaction conditions (lower temperatures). Copper selenide hexagonal nanoplates

obtained by the described method could be potential building blocks to construct functional devices

and solar cell. This work may open up a new rationale on designing the solution synthesis of

nanostructures for materials possessing similar intrinsic crystal symmetry. On the basis of the carefully

controlled experiments mentioned herein, a plausible formation mechanism of the hexagonal

nanoplates was suggested and discussed. To the best of our knowledge, this is the first report on

nonstoichiometric (Cu2�xSe) as well as stoichiometric (CuSe, b-Cu2Se and Cu2Se) copper selenide

hexagonal nanoplates with such full control of morphologies and phases by this method under mild

conditions.

& 2010 Elsevier B.V. All rights reserved.

1. Introduction

Consequences of nanostructured materials narrate to the factthat the characteristics of nanomaterials are poles apart fromthose of the bulk materials of the same composition due to sizedependent properties, flexible processing and easier syntheticprotocols. Compared with their bulk and thin films counterparts,nanostructured materials and their assembly are of deep-seatedsignificance due to their unique dimension dependent propertiesand potential applications as building blocks in electronics,optoelectronics, photovoltaic devices and solar cells. Recentlywide ranges of techniques have been developed to synthesizemetal chalcogenides with control of their shape and size. Copperchalcogenides have been studied to a lesser extent for its closest

ll rights reserved.

6; fax: +91 542 2368468.

h).

of kin chalcogenides nanoparticulates (especially CdS, CdSe, CdTe,ZnS, ZnSe, ZnTe, PbS, PbTe and PbSe) exhibiting size quantizationeffect that has potential use as building block in nanoelectronics,nano-optronics, nanosensors and actuators, optical signal proces-sing, switches and in biology [1–5]. Copper chalcogenides have alarge number of applications in various devices such as in solarcells, superionic conductors, photodetectors, photo-thermal con-version, electroconductive electrodes, microwave shielding, coat-ing, thermoelectric cooling, optical filter and as an opticalrecording material [6–15]. A few pioneering reports havedescribed the synthetic route for copper selenide nanostructurespreparation, focusing on their structural characterization andtheir photoluminescence properties [16–26]. Copper selenide maybe found in many phases and structural forms—differentstochiometrics such as CuSe, Cu2Se, Cu2Sex, CuSe2, a-Cu2Se,Cu3Se2, Cu5Se4, Cu7Se4, etc. as well as with non-stoichiometricform such as Cu2�xSe can be constructed into several crystal-lographic forms (monoclinic, cubic, tetragonal, hexagonal, etc.)

Fig. 1. Integrated characterization of CuSe nanoplates obtained at 100 1C for 12 h with copper chloride as copper source and elemental selenium powder by solvothermal

method (a–d) TEM images in different magnifications; inset of figures a and d, corresponding ED patterns carried on the plates (e) typical XRD pattern and (f) EDS spectrum

of as synthesized product.

P. Kumar et al. / Journal of Crystal Growth 312 (2010) 2804–2813 2805

Fig. 2. (a, b) SEM and (c–d) TEM images; (inset of c–d) ED pattern; (e) XRD pattern; and (f) EDS spectrum of the CuSe hexagonal nanoplates obtained at 100 1C for 12 h with

copper sulphate as copper source.

P. Kumar et al. / Journal of Crystal Growth 312 (2010) 2804–28132806

P. Kumar et al. / Journal of Crystal Growth 312 (2010) 2804–2813 2807

[27]. Special constitutions and properties of these compositionsmake copper selenide an ideal candidate for scientific research.Therefore, considerable progress on the study of copper selenidehas been made in recent years. It has been reported that thermalstability and band gaps of copper selenide vary with theirstochiometrics or phases [28,29]. The composition and the crystalstructure of the final products are usually dependent on thepreparation method [30–33]. However the preparation methodsand size or shape control are less flexible than other materialselenides such as ZnSe and CdSe and the studies relating to theiroptical properties are limited. Therefore, developing new methodsfor preparing high quality copper selenide nanomaterials andachieving control of their size or shape are very necessary.Obtaining novel nanomaterials with controllable size and shapeunder mild conditions with safe precursor at lower temperaturewith a relatively faster process is an issue that has engaged manyresearchers. Currently, the solvothermal process has proved to bea useful technique for generating novel materials with unusual

Fig. 3. Characterization of the nonstoichiometric Cu2�xSe nanoplates obtained at 100

images; (inset figure e) SAED pattern; (f) XRD pattern and (g) EDS spectrum.

properties. In the past years, sodium selenosulfate, as a promisingSe source, has been widely used to prepare metal selenides, whichare often nanocrystallites, such as CdSe and PbSe [34,35]. Graciaet al. [29] and Lakshmi et al. [30] have also reported thepreparation of chemically deposited copper selenide (Cu2�xSeand Cu3Se2) thin films, using sodium selenosulfate as theSe source, but in the present method we have used elementalSe powder as the selenium source. On the basis of previousresearch, our group has successfully expanded this method toprepare nanoscale metal chalcogenides [2,3]. Copper selenide is ap-type semiconductor having excellent electrical and opticalproperties and is suitable for photovoltaic application. Plate-likeCuSe nanostructures are of particular interest because thenanoplates are promising building blocks for nanodevices withcontrolled crystal orientation by a bottom-up route owing to theiranisotropic structures. Recently, much effort has been dedicatedto the synthesis of copper sulfide (CuS) nanoplates with differentmethods, e.g. solvothermal process, hydrothermal micro-emulsions

1C for 12 h with copper nitrate as copper source: (a) typical SEM and (b–e) TEM

Fig. 3. (Continued)

P. Kumar et al. / Journal of Crystal Growth 312 (2010) 2804–28132808

method, sono-chemical synthesis route, vacuum chemicalvapor reaction method, improved solvothermal method and bysolution-phase-arrested precipitation method in the range100–200 1C using different sources [36–43]. Despite all thesesuccesses, to the best of our knowledge, there is no report on thepreparation of hexagonal CuSe nanoplates up to now.

Here, we have successfully obtained the different purephases of copper selenides in ethylene glycol/hydrazine hydrate/water solutions via the solvothermal method under ambientconditions using copper chloride, copper sulphate, copper nitrate,copper acetate, elemental copper and elemental selenium powderas the raw materials. The diffusion of copper into selenium indispersed phase is reported by the solvothermal methodleaving behind contaminants. Surface energy and interparticleaffinity for size-dependent diffusion have been shown elegantlyleading to the evolution of copper selenide hexagonal nanoplates(in different phases like CuSe, b-Cu2Se, Cu2Se and Cu2–xSe,) in thenanometer size regime by an inexpensive, straightforwardmethod that is free from any capping agent, chelating agent,surfactant or template. This is the first report on copperselenide hexagonal nanoplates in stoichiometric as well asnonstoichiometric form by this method. This work mayprovide a new rationale pertaining to the design of the solutionsynthesis of nanoarchitectures for materials possessing similarintrinsic crystal symmetry. The formation mechanism is alsodiscussed.

2. Experimental details

2.1. Synthesis and characterizations of high quality copper selenide

(CuSe, Cu2�xSe, b-Cu2Se and Cu2Se) hexagonal nanoplates

In a typical synthesis of nanoplates analytical grade copperchloride, copper sulphate, copper nitrate, copper acetate, ele-mental copper and highly pure selenium (99.999%) powders,purchased from Sigma, were used without further purification.Ethylene glycol and hydrazine hydrate of analytical grade,purchased from Merck, Germany, were used as received. Forsynthesis copper chloride, copper sulphate, copper nitrate, copperacetate, elemental copper (2.0 g each) and selenium (0.5 g witheach copper source) were taken with deionized water, ethyleneglycol and hydrazine hydrate in the volume ratio of 7:2:1,respectively, in a 100 ml capacity Teflon lined stainless steelautoclavable bottle. Then the solution was sealed and kept underwater bath at 100 1C for 12 h. Finally, the precipitates werecollected and washed with anhydrous ethanol and hot distilledwater several times, then dried in vacuum at 50 1C for 6 h. TheX-ray diffraction patterns of as-synthesized freshly dried powdersor products were recorded by a Rigaku Rotoflux diffractometer(operating at 40 kV, 100 mA) with Cu-Ka radiation. Transmissionelectron microscopy (TEM) investigations were carried out using aTechni 20 G2-TEM, typical e-beam voltage employed being200 kV, and high resolution transmission electron microscopy

P. Kumar et al. / Journal of Crystal Growth 312 (2010) 2804–2813 2809

(HRTEM) was performed on a Philips CM300 FEG instrument withan acceleration voltage of 300 kV. One drop of the nanocrystalsdispersed in ethanol solution was placed on a 200 mesh carbon-coated copper grid and the grid was dried in vacuum beforeanalysis. Field-emission scanning electron microscopy wasapplied to investigate the size and morphology, which wascarried out with a field emission electron micro-analyzer usinga JEOL JSM6700 microscope, operating at 10 A and 15 kV.

3. Result and discussion

Typical synthesis of hexagonal platelets were carried out by asolvothermal system based on the modified reduction reactionamong copper chloride, copper sulphate, copper nitrate, copperacetate and elemental copper (as source materials) with hydra-zine hydrate and ethylene glycol (EG). The morphology of thecopper selenide (CuSe) nanoplates obtained using copper chloride(as copper source) at 100 1C for 12 h is shown in Fig. 1(a)–(c). Allthe platelets exhibit uniform hexagonal shapes on top–bottom

Fig. 4. b-Cu2Se hexagonal platelets obtained at 100 1C for 12 h with copper acetate (a) SE

magnifications; (e) HRTEM image; (f) ED pattern; (g) XRD pattern and (h) typical EDS

facets. The transmission electron microscopy (TEM) imagesshown in Fig. 1(a)–(d) indicate the high-yield growth feature ofthis simple solvothermal process. The edge length and thicknessof the nanoplates, estimated from Fig. 1(a), are 240–320 and30–40 nm, respectively. Fig. 1(b)–(d) gives the high-magnificationtransmission electron microscopy (TEM) images of the nanoplatesat different magnifications, which indicate the good uniformity ofthe sample. The selected area electron diffraction (SAED) patternperformed on the single hexagonal nanoplate shown in inset ofFig. 1(a and d) exhibits clear lattice fringes and hexagonallysymmetric spots pattern, indicating that the nanoplates are highlycrystallized and defect-free. The SAED pattern can be assigned to[0 0 0 1] zone axis projection of the hexagonal CuSe reciprocallattice. The phase composition and phase structure of theproducts were examined by XRD. The XRD pattern of theproduct obtained in the typical synthesis is shown in Fig. 1(e).All diffraction peaks could be perfectly indexed to a purehexagonal phase of CuSe with lattice constants a¼3.939 A andc¼17.25 A (JCPDS Card Number 340171, P63/mmc) and noother impurities were detected in the synthesized products.

M image; (b) typical TEM image (c–d) TEM images of single nanoplates in different

spectrum of as synthesized product.

Fig. 4. (Continued)

P. Kumar et al. / Journal of Crystal Growth 312 (2010) 2804–28132810

The intensity of (1 1 0) diffraction peak in the XRD pattern wasparticularly strong, indicating the presence of preferentialorientation in the sample, which has a higher number of {1 1 0}planes parallel to the normal sample than what would beexpected from a random powder. The energy-dispersive X-rayspectroscopy (EDS) spectrum (Fig. 1(f)) exhibits the Cu and Sesignals peaks of the elements presented with an approximateatomic ratio of 51.69:48.31, which is consistent with thestoichiometry of CuSe. The results indicate that the top–bottomsurfaces are {0 0 0 1} facets while the six side surfaces should be{1 1 2 0} facets, and the growth direction for developing thehexagon morphology. Using the general route in this work,uniform and impurity-free CuSe hexagonal plates using coppersulphate as copper source with average edge lengths of 500 nm,as shown in Fig. 2(a)–(d), can be obtained. The microstructureanalyses indicate that the as-prepared product is of pure phaseand single crystalline. Electron microscopy analyses show that theaverage edge lengths of the platelets are 400–500 nm and thethickness is estimated to be less than 100–150 nm. The X-raydiffraction (XRD) pattern, as shown in Fig. 2(e), can be steadilyindexed to hexagonal CuSe structure (JCPDS Card Number340171, P63/mmc). The energy-dispersive X-ray spectroscopy(EDS) spectrum (Fig. 2(f)) exhibits the Cu and Se signals peaksof the elements presented with an approximate atomic ratio of54.41:45.59, which is nearly consistent with the stoichiometry

of CuSe. The syntheses of hexagonal platelets through thegeneral route mentioned above were performed to demonstratethe feasibility of further extending this simple solvothermalapproach. Fig. 3 shows the morphology observation andmicrostructure analyses of Cu2�xSe nanoplates obtained usingcopper nitrate (as copper source) at 100 1C for 12 h. Both the SEMand TEM images, as shown in Fig. 3(a)–(e), demonstrate thesynthesis of deformed Cu2�xSe alloy hexagonal platelets. Edgelength and thickness of the deformed hexagonal platelets areabout 200–400 and 60 nm, respectively. The typical XRD pattern(Fig. 3(f)) of the as prepared Cu2�xSe platelets can be identified ascubic structures with space group of F43 m (JCPDS Card Number06-0680). Although there are more than 8 stoichiometries ofcopper selenides and some stoichiometries have different phases,each of them has its characteristic XRD pattern. Therefore, theXRD patterns give the most positive evidence of the formation ofCu2�xSe. Both the SAED pattern (inset of the Fig. 3(e)) and TEMimage (Fig. 3(b)–(e)) reveal the deformed feature of the product.In order to give further evidence to the composition informationof the product, EDS measurement was performed to detect theratio of the elements. The corresponding EDS spectrum shown inFig. 3(g) reveals the atomic ratio of the involving elements, Cu andSe, which is 64.44:35.56, presenting a slight deviation from thecorresponding nominal stoichiometry of Cu2Se (�2:1). Theweight percentages of Cu and Se were 60.50 and 39.50,

Fig. 5. Integrated characterization of Cu2Se nanoplates: (a–c) TEM images in different magnifications; (d) ED pattern of the nanoplate; (e) EDS spectrum and (f) typical XRD

pattern of as synthesized product obtained at 100 1C for 12 h with elemental copper and selenium powder by solvothermal method.

P. Kumar et al. / Journal of Crystal Growth 312 (2010) 2804–2813 2811

respectively. The as-prepared (b-Cu2Se) product exhibits thehigh-yield growth and good crystallization, as shown inFig. 4(a)–(d) using copper acetate as the source of copper. Theedge length and thickness are estimated to be 180–210 nm andabout 40 nm, respectively. Fig. 4(e) presents HRTEM image with

interplanar separation d¼0.208 nm along the [2 2 0] axis ofb-Cu2Se nanoplatelets. The SAED pattern (Fig. 4(f)) analogous toothers indicates its single crystalline nature. The SAED pattern(Fig. 4(f)) and TEM and HRTEM images (Fig. 4(b)–(e)) reveal thehigh crystalline feature of the product. The typical XRD pattern

P. Kumar et al. / Journal of Crystal Growth 312 (2010) 2804–28132812

(Fig. 4(g)) of the as prepared b-Cu2Se nanoplatelets can beidentified as cubic structures with space group of F (JCPDS CardNumber 04-0839). The EDS spectrum shown in Fig. 4(h) indicatesthat the as-prepared product is of pure phase with anapproximate stoichiometry of 67.32:32.68, which is close to 2:1of Cu2Se. The weight percentages of Cu and Se were 62.50 and37.50, respectively. This result further supports the 2:1stoichiometry of Cu2Se as shown by EDAX analysis. In order tocomprehensively understand this general method, the formationprocess of the hexagonal platelet structure is studied more indetail. Fig. 5(a–c) gives the typical TEM images of the product(Cu2Se) obtained at 100 1C for 12 h using elemental copper as thesource material. The edge length and thickness of the thussynthesized nanoplates are estimated to be 300–700 nm andabout 80 nm, respectively. Both the SAED pattern (Fig. 5(d)) andTEM image (Fig. 5(a–c)) reveal the high crystalline feature of theproduct. The EDS spectrum shown in Fig. 5(e) indicates that theas-prepared product is of pure phase with an approximate 2:1stoichiometry of Cu2Se. The typical XRD pattern (Fig. 5(f)) of theas prepared Cu2Se nanoplatelets can be identified as hexagonalstructures with space group of P42/n (JCPDS Card Number29-0575). It can be concluded that using this facile and unifiedsolvothermal method hexagonal micro/nanoplatelets ofcopper selenide can be successfully synthesized using differentcopper sources. The results also demonstrate that the atomicratios of the involving elements are tunable, which allows thepossibility of searching for materials possessing optimizedthermoelectric performance. Comparing the images of all theproducts indicates that the size of the plates varies while theshape is unchanged using different copper source materials. All ofthe SAED patterns show hexagonal and cubic structures. Theintensity and highly ordered diffraction spots mean that theparticles are well crystallized, which is important for their use inhigh mobility-life solar cells.

4. Reaction mechanism

Based on the above observations, a growth mechanism ofnanoplates is proposed. Se source was derived from the reductionof Se by N2H4; this highly reactive Se can be easily converted intoSe2� , which results in a high monomer concentration. In theinitial step, hydrazine hydrate complexes with Cu2 + and forms atransparent soluble complexes solution, which effectivelydecreases the concentration of Cu2 + and avoids the precipitationof CuSeO3, thus providing a more homogenous solution environ-ment for the reaction. The chemical reaction involved in the entiresynthesis of nanoplates can be formulated as follows:

2Cu2þþ4OH�-2CuOkþ2H2O

4CuOþ4Se2�þ4H2O-4CuSeþ8OH� ð1Þ

2Cu2þþ6OH�-2CuðOHÞ�3

4CuðOHÞ�3 þ4Se2�-4CuSekþ12OH� ð2Þ

Se2� is released slowly and interacts with surplus N2H4 toform the molecular precursor immediately. So the decompositionof precursor can proceed thoroughly under the present condition.The application of N2H4 as the coordination agent is determinablefor the phase of the products. So it can be drawn that thecomplexing ability of groups containing atom N (such as NH2 orNH3) can effectively determine the final phase of the products.Compared with the CuO deposit (Eq. (1)), it is easier for theCu(OH)3

� (Eq. (2)) to release Cu2 +, which can facilitate the growth

of nanoparticles under non-equilibrium kinetic growth conditionswith a high monomer concentration. A similar phenomenon wasfound during the preparation of Cu2Te and PbSe using N2H4 �H2Oas the complexing agent and the exact mechanism was not fullyunderstood [2,44]. All the above results reveal that N2H4 �H2O, asa solvent, favors the formation of copper selenide nanostructure.The exact mechanism for the formation of nanoplates is stillunclear, but it is reasonably concluded that the appropriate ratioof solvents volume plays a critical role in the formation ofnanoplates.

5. Conclusions

In summary, a facile and general solvothermal process wasdeveloped to synthesize hexagonal platelets in different phases ofcopper selenides (CuSe, Cu2�xSe, b-Cu2Se and Cu2Se) usingethylene glycol and hydrazine hydrate as both solvent andreducing agent. This powerful route demonstrates that it ispossible to obtain well-defined hexagonal plates ofcopper selenide with tunable atomic ratios in stoichiometric aswell as in nonstoichiometric form by a reduction reaction usingsimple and convenient copper source as precursors, which couldbe further extended to design the morphologies of other inorganiccrystals.

Acknowledgements

Pushpendra Kumar is grateful for support from the UniversityGrant Commission, New Delhi, for providing financial assistanceunder the Rajeev Gandhi National Fellowship Scheme as SRF(RGNFS-SRF). We are thankful to Mr. Upendra Kumar Parashar,Mr. Jai Singh, Vijay Ji, Vimal Ji (Department of Physics, B.H.U.),Prof. K Tripathi, Mr. Manish Mishra, Mr. Hemant Kumar (Depart-ment of Medicine, I.M.S, B.H.U) and to Prof. N. Dalal and Prof. G. F.Strouse (Department of Chemistry and Biochemistry, Florida StateUniversity, Tallahassee, USA.) for their kind suggestions and helpthroughout the work.

References

[1] Y.D. Li, J.W. Wang, Z.X. Deng, Y.Y. Wu, X.M. Sun, D.P. Yu, P.D. Yang, J. Am.Chem. Soc. 123 (2001) 9904–9905.

[2] P. Kumar, K. Singh, Cryst. Growth Des. 9 (2009) 3089–3094.[3] P. Kumar, K. Singh, Curr. Nanosci. 6 (2010) 89–93.[4] W.Z. Wang, Y. Geng, P. Yan, F.Y. Liu, Y. Xie, Y.T. Qian, Inorg. Chem. Commun. 2

(1992) 83–85.[5] Y.D. Li, Y. Ding, Z.Y. Wang, Adv. Mater. 11 (1999) 847–850.[6] C. Kaito, A. Nonaka, S. Kimura, N. Suzuki, Y. Saito, J. Cryst. Growth 186 (1998)

386–392.[7] M.G. Bawendi, D.J. Carroll, W.L. Wilson, L.E. Brus, J. Chem. Phys. 96 (1992)

946.[8] B.P. Zhang, T. Yasuda, Y. Segawa, H. Yaguchi, K. Onabe, E. Edamatsu, T. Itoh,

Appl. Phys. Lett. 70 (1997) 2413–2415.[9] W. Wang, Y. Geng, P. Yan, F.Y. Liu, Y. Xie, Y.T. Qian, J. Am. Chem. Soc. 121

(1999) 4062–4063.[10] Y. Li, Y. Ding, Y.T. Qian, Y. Zhang, L. Yang, Inorg. Chem. 37 (1998) 2844–2845.[11] T. Chivers, Dalton Trans. (1996) 1185–1194.[12] J.J. Ritter, M. Pichai, Inorg. Chem. 34 (1995) 4278–4280.[13] H. Toyoji, H. Yao, Jpn. Kokai Tokkyo Koho Jp02 173 622.[14] F. Mongellaz, A. Fillot, J. De. Lallee, Proc. SPIE-Int. Soc. Opt. Eng. 156 (1994)

2227.[15] S.T. Lakshmikvmar, A.C. Rastogi, Sol. Energy Mater. Sol. Cell 32 (1994) 7–19.[16] A.A. Korzhuev, Fiz. Khim. Obrab., Mater. 3 (1993) 131.[17] Z.P. Qiao, Y. Xie, J.G. Xu, X.M. Liu, Y.J. Zhu, Y.T. Qian, Can. J. Chem. 78 (2000)

1143.[18] J.J. Zhu, O. Palchik, S.G. Chen, A. Gedanken, J. Phys. Chem. B 104 (2000)

7344–7347.[19] Y. Xie, X.W. Zheng, X.C. Jiang, J. Lu, L.Y. Zhu, Inorg. Chem. 41 (2002) 387–392.[20] S. Xu, H. Wang, J.J. Zhu, H.Y. Chen, J. Cryst. Growth 234 (2002) 263–266.[21] M. Kemmler, M. Lazell, P. O’Brien, D.J. Otway, J.H. Park, J.R. Walsh, J. Mater.

Sci. Mater. Electron 13 (2002) 531–535.

P. Kumar et al. / Journal of Crystal Growth 312 (2010) 2804–2813 2813

[22] M. Dhanam, P.K. Manoj, R. Prabhu-Rajeev, J. Cryst. Growth 280 (2005) 425–435.[23] Z. Zulkarnain, N. Saravanan, C.L. Tan, Mater. Lett. 59 (2005) 1391–1394.[24] R.P. Rupa, T.J. Teny, M. Lakshmi, K.P. Vijayakumar, C.Sudha Kartha, Thin Solid

Films 473 (2005) 208–212.[25] R.S. Mane, S.P. Kajve, C.D. Lokhande, H. Sung-Hwan, Vacuum 80 (2006)

631–635.[26] Y.J. Hsu, C.M. Hung, Y.F. Lin, B.J. Liaw, T.S. Lobana, S.Y. Lu, C.W. Liu, Chem.

Mater. 18 (2006) 3323–3329.[27] K.B. Shafizade, I.V. Ivanova, M.M. Kaizinets, Thin Solid Films 55 (1978)

211–220.[28] S.K. Haram, K.S.V. Santhanam, M. Numann-Spallar, C. Levy-Clement, Mater.

Res. Soc. Bull. 27 (1992) 1185–1192.[29] V.M. Gracia, P.K. Nair, M.T.S. Nair, J. Cryst. Growth 203 (1999) 113–124.[30] M. Lakshmi, K. Bindu, S. Bini, K.P. Vijayakumar, C.Sudha Kartha, T. Abe

Y. Kashiwaba, Thin Solid Films 370 (2000) 89–95.[31] M. Lakshmi, K. Bindu, S. Bini, K.P. Vijayakumar, C.Sudha Kartha, T. Abe

Y. Kashiwaba, Thin Solid Films 386 (2001) 127–132.

[32] R.D. Heyding, R.M. Murray, Can. J. Chem. 54 (1976) 841–848.[33] R.D. Heyding, Can. J. Chem. 44 (1966) 1233–1236.[34] S.S. Kale, C.D. Lokhande, Mater. Chem. Phys. 62 (2000) 103–108.[35] G.A. Kitaev, A.Z. Khvorenkova, Russ. J. Appl. Chem. 72 (1999) 55.[36] W.J. Lou, M. Chen, X.B. Wang, W.M. Liu, J. Phys. Chem. C 111 (2007)

9658–9663.[37] P. Zhang, L. Gao, J. Mater. Chem. 13 (2003) 2007–2010.[38] H.L. Xu, W.Z. Wang, W. Zhu, Mater. Lett. 60 (2006) 2203–2206.[39] K.J. Wang, G.D. Li, J.X. Li, Q. Wang, J.S. Chen, Cryst. Growth Des. 7 (2007)

2265–2267.[40] W.M. Du, X.F. Qian, X.D. Ma, Q. Gong, H.L. Cao, J. Yin, Chem. Eur. J. 13 (2007)

3241–3247.[41] Y.S. Liu, D.H. Qin, L. Wang, Y. Cao, Mater. Chem. Phys. 102 (2007)

201–206.[42] A. Ghezelbash, B.A. Korgel, Langmuir 21 (2005) 9451–9456.[43] Y.C. Zhang, T. Qiao, X.Y. Hu, J. Cryst. Growth 268 (2004) 64–70.[44] X. Wang, G. Xi, Y. Liu, Y. Qian, Cryst. Growth Des. 8 (2008) 1406–1411.