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Thin Solid Films 534 (2013) 76–82

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Thin Solid Films

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Controlled synthesis of ZnO spheres using structure directing agents

Kurniawan Foe a, Gon Namkoong a,⁎, Tarek M. Abdel-Fattah b,e, Helmut Baumgart a,Mun Seok Jeong c,⁎⁎, Dong-Seon Lee d

a Department of Electrical and Computer Engineering, Old Dominion University, Applied Research Center, Norfolk, VA 23529, USAb Applied Research Center, Thomas Jefferson National Accelerator Facility, VA 23606, USAc Center for Integrated Nanostructure Physics (CINAP), Institute for Basic Science (IBS), Department of Energy Science, Sungkyunkwan University, South Koread School of Information and Communications, Gwangju Institute of Science and Technology, 261 Cheomdan-gwagiro (Oryong-dong), Buk-gu, Gwangju 500-712, South Koreae Department of Molecular Biology and Chemistry, Christopher Newport University, Newport News, VA 23606, USA

⁎ Corresponding author. Tel.: +1 757 269 5349.⁎⁎ Corresponding author.

E-mail addresses: [email protected] (G. Namkoon(M.S. Jeong).

0040-6090/$ – see front matter © 2013 Elsevier B.V. Allhttp://dx.doi.org/10.1016/j.tsf.2013.01.105

a b s t r a c t

Article history:Received 6 April 2012Received in revised form 29 January 2013Accepted 30 January 2013Available online 13 February 2013

Keywords:Zinc oxideSphereLiquid phase depositionThin filmMorphologyStructure directing agent

Controlled liquid phase deposition has been developed for fabricating zinc oxide (ZnO) nano/microspheresusing a mixture of precursor solution of zinc acetate dihydrate, ammonium hydroxide, and structuredirecting agents (SDAs) such as ethanol and urea. We found that ZnO spheres can be formed when theSDAs are optimized with the proper pH values. At pH values less than 12, an anisotropic growth of ZnOflowers and rod structures was produced with/without SDAs. On the contrary, at a pH value of 12 the direc-tional growth of ZnO was absolutely controlled and an isotropic growth of ZnO spheres was developed withthe presence of SDAs. We also found that the volume ratio of ethanol and urea in the solution was a key factorto modulate the uniform size distribution and diameter of the ZnO spheres from nanometer to micrometerrange.

© 2013 Elsevier B.V. All rights reserved.

1. Introduction

The II–VI compound zinc oxide (ZnO) semiconductor is a smartand versatile material because of its wide range applications in tech-nology. ZnO exhibits unique properties such as wide band gap energyof 3.37 eV [1] and a large exciton binding energy of 60 meV [2]. Due totheir hexagonal structure, ZnO crystals tend to grow anisotropically inthe form of nanorods, nanowires, and nanoflowers, where preferentialc-axis oriented growth is dominant [2,3]. Recently, ZnO spheres haveattracted intensive attention for their properties and potential appli-cations including photonic crystals [4], drug-delivery carriers [5],and sensors [6]. Thismeans that the formation of ZnO spheres requiresthe absolute control of directional crystal plane where c-plane growthshould be retarded and other growth planes should be enhanced toachieve symmetrical growth surfaces [7]. In addition, further controlof size and uniformdistribution of ZnO spheres is also critical. Current-ly, there exist many different syntheticmethods to achieve thin film ofZnO spheres, such as template based synthesis [8], vapor–liquid–solid[9], chemical vapor deposition [10], evaporation [11], irradiation [12],and hydrothermal synthesis [13]. Among various synthetic methods,

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the advantages of using an aqueous solution method clearly stemfrom its simple, efficient, low-cost, and large area deposition that in-volves low temperature and requires relatively little time. Depositionof thin films of metal oxide from aqueous solution requires a bettercontrol of the heterogeneous nucleation and growth processes. Liquidphase deposition (LPD) is one of the aqueous solution processes usedfor depositing thin films of metal oxides onto substrates [14–18].Since LPD process is in aqueous media, thin film of high quality metaloxides can be easily deposited onto different substrates [16]. ThereforeLPD is suitable for transferring heterogeneous precipitation of metaloxide nanostructures with different morphologies. Another advantagelies in that aqueous solution process provides flexible control of shapeand chemical composition of the crystalline ZnO precipitates by usingstructure directing agents (SDA) such as triethanolamine [19], ethanol[20], urea [21], polyethylene glycol [22], and citric acid [23]. For exam-ple, ethanol [20] and citric acid [23] were reported to be very effectivein modifying crystal growth planes of ZnO by absorbing crystal growthunits onto specific crystal planes. This limits the anisotropic growth ofZnO crystals and leads to the formation of spherical ZnOnanostructures.Even though synthesis of ZnO spheres was reported using hydrother-mal process [13], ZnO morphologies were not truly symmetrical andstill had many intrusions composed of nanostructures on the surface,inferring solid ZnO spheres were not achieved. In addition, a full controlof size and distribution of solid ZnO spheres was rarely reported.

In this paper, a systematic investigation of the synthesis of thin filmsof ZnO solid spheres using LPD process and SDAs, ethanol and urea, is

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reported. In particular, the morphological changes, size, shape, andcrystallinity of ZnO nanostructures are investigated. We found that byvarying the volume ratio of ethanol and urea in the LPD solution, wecould control both the size and distribution of ZnO spheres. In addition,the morphological development of ZnO structure as a function of reac-tion times (30–120 min) in LPD process is also studied to elucidatethe initial formation of the thin film of ZnO spheres.

2. Experimental details

All glasswarewerewell cleanedwith hydrochloric acid solution over-night, rinsed with deionized water (18.2 MΩ·cm), dried and kept insidean oven prior to use. Silicon substrates were cleaned thoroughly usingtrichloroethylene, acetone, methanol, and deionized water each for5 min in an ultrasonic bath. Subsequently, the substrates were driedand kept inside an oven at a temperature of 60 °C for 30 min. To studythe effect of structure directing agents (SDAs) on the morphology ofZnO crystals, a mixture of zinc acetate dihydrate (Zn(CH3COO)2·2H2O)solution and SDAs, ethanol (C2H5OH) and/or urea ((NH2)2CO),were pre-pared. In this case, zinc acetate dihydrate, ethanol, urea, and ammoniumhydroxide (NH4OH) were purchased from Sigma-Aldrich and BDH. Twodifferent concentrations (23 and 50 mM) of zinc acetate dihydrate

Fig. 1. SEM images of ZnO crystals synthesized at two different Zn precursor concen

(ZAD) were used to synthesize ZnO spheres and nanorods, respectively.Furthermore, an equivolume LPD solution of 23 mM of urea and ethanolwas also used as SDAwith a range of 10–12 on the pH scale. Particularly,a droplet of ammonium hydroxide was added to achieve desired pHvalues of LPD solution. The resultant mixture of LPD solution was stirredvigorously for 30 min. After stirring, the pHof the solutionwasmeasuredagain and if necessary, more ammonium hydroxide was added to adjustthe pH value. At a pH value of 10, a white color and clear LPD solutionwas obtained while the pH is >11, a clear solution has been obtained.

After the substrate was placed in a glass container with the LPD so-lution, the glass container was tightly sealed and kept inside an oven ata temperature of 100 °C for 2 h. Then the system was allowed to cooldown to room temperature for 90 min. Finally, the substrate was rinsedwith deionized water, dried, and kept in a clean environment.

All ZnOmorphologieswere analyzed using scanning electronmicros-copy (JEOL SEM JSM 6060 LV) with operating voltages of 20–25 kV andhigh resolution transmission electron microscopy (HR-TEM JEOL JEM-2100F)while the structural propertieswere determined byX-ray diffrac-tion (MiniFlexII, Rigaku) employing a monochromatized Cu-Kα radia-tion source (λ=0.15406 nm). For X-ray diffraction measurement, ω/2θscanswere usedwith a range from25 to 65°. In addition, Raman spectros-copy was performed for optical characterization. In the Raman scattering

trations (a–c) 23 mM and (d–f) 50 mM at different pH values (10, 11, and 12).

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spectroscopy, Nd-YVO4 laser (λ=532 nm, 50 mW, VERDI) was used forexcitation and thermo-electrically cooled CCD attached triple monochro-mator (Acton TRIVISTA) was used for dispersion and detection.

3. Results and discussion

The morphology-controlled synthesis of ZnO micro/nanostructuresis of great interest for ZnO current and future applications. By adjustingthe precursor concentration and pH, different sizes andmorphologies ofZnOmicro/nanostructures have been prepared via an aqueous chemicalroute. Fig. 1 shows scanning electron microscopy (SEM) images of ZnOsynthesized as a function of concentrations of ZAD and pH values. Twodifferent ZAD concentrations of 23 mM and 50 mM and three differentpH values of LPD solution of 10, 11, and 12 were used. Fig. 1(a)–(c)shows different morphologies of ZnO crystals synthesized at a concen-tration of 23 mM. At a pH value of 10, in Fig. 1(a), ZnO flowers wereobtained with nanorods having diameters of 150–300 nm. By increas-ing pH value to 11, ZnO developed to microspheres with prism-likerods having diameters of 150–300 nm as shown in Fig. 1(b). At a pHvalue of 12, ZnO maintained a spherical shape with densely packednanorods in Fig. 1(c). In contrast at higher concentrations of 50 mM,

Fig. 2. SEM images of ZnO crystals synthesized under different SDAs in LPD pro

the size of nanorods increased, indicating that a higher concentrationof ZAD enhanced the overall growth as shown in Fig. 1(d)–(f). At a pHvalue of 10, ZnO hexagonal rods produced larger diameters of 300–900 nm and length of 8.5–10 μm compared with those produced withsmaller concentration (23 mM) of ZAD, as seen in Fig. 1(d). Further in-creasing the pH value to 11, prismatic ZnO rods with diameters of 250–500 nm and length of 8–10 μm were observed in Fig. 1(e). At thehighest pH of 12, a superstructure of ZnO rods with an overall diameterof 30 μmwas observed in Fig. 1(f). This superstructure consisted of nu-merous prismatic ZnO rods with individual diameters around 1 μm.Based on SEM images, it is clear that the anisotropic growth of ZnOwas dominant, leading to nanostructured shapes regardless of pHvalues and ZAD concentrations [24]. Therefore, it is critical to suppressthe directional growth of ZnO. In this case, we used structure directingagents (ethanol and/or urea) to control the morphology of ZnO struc-ture. Furthermore, ZnO crystals were synthesized at a concentration of23 mM instead of 50 mMsince higher ZAD concentration enhanced an-isotropic growth as shown in Fig. 1. The resulting ZnO revealed very dif-ferent morphologies for each sample depending on the pH value of theLPD solution. Fig. 2(a)–(c) shows SEM images of self-assembled ZnOmorphologies synthesized with urea as an SDA at different pH values.

cess: urea (a–c) and ethanol (d–f) at different pH values (10, 11, and 12).

Fig. 4. Typical Raman scattering spectra for the as-synthesized ZnO (a) nanorod synthe-sized at a pH value of 10 and (b) nanosphere synthesized at a pH value of 12 with urea.

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At a pH value of 10, we observed denser ZnO nanoflowers comparedwith one without urea. With further increased pH value to 11, a spher-ical ZnO crystal structure developed with bundles of nanorods and theoverall size of spherical ZnO is about 5.5 μm in length. At a pH valueof 12, solid and symmetrical ZnO spheres were observed with variousdiameters ranging from several hundred nanometers to a few microns.

When ethanol was used as an alternative SDA, a similar evolutionof ZnO morphology was observed, as shown in Fig. 2(d)–(f). At a pHvalue of 10, a spherical structure of ZnO crystals grew from the centerbut consisted of lots of hexagonal nanorods with a diameter around400 nm and length of 4.1–4.3 μm. With a pH value of 11, ZnOmicroflowers were observed with six-fold symmetry ZnO hexagonalpetals having a constant inter-arm angle of 60° in each direction inFig. 2(e). However, when the pH value was increased to 12, symmet-rical and solid ZnO crystal spheres were developed with larger diam-eters around 6–8 μm in Fig. 2(f). The diameter of solid ZnO spheresobtained with ethanol was much larger by more than 10 times com-pared with the case where urea was used as an SDA. In addition,when ethanol was used as an SDA, a more uniform size distributionof ZnO spheres was observed.

To understand the effect of SDAs on the directional growth of ZnO,X-ray diffraction (XRD) wasmeasured as shown in Fig. 3. All diffractionpeaks in the pattern matched well with the standard hexagonal struc-ture (P63mc, PDF Card 36-1451) for bulk ZnO [25]. Particularly, theZnO nanorods at low pH values of 10 or 11 (Fig. 3(a)) exhibit the strongpeak of crystal growth in the [002] direction. In contrast, for the ZnOnanospheres obtained by using SDAs at high pH value of 12 (Fig. 3(b))the [002] peak is substantially suppressed and other growth directionssuch as [100], [101], [110], and [103] are significantly enhanced, leadingto the isotrophic growth of the ZnO nanospheres. Raman-scatteringspectra are also consistent with X-ray diffraction measurement on thegrowth mode of ZnO structures. Fig. 4 demonstrates Raman-scatteringspectra of ZnO nanorods and spheres with wave-numbers in therange of 300–800 cm−1 at room temperature. In Fig. 4(a), a sharp andstrong peak at 439 cm−1 was observed for ZnO nanorods, which wereattributed to the optical phonon E2 (high) mode of the ZnO [26,27].This corresponds to the characteristic Raman active peak for the hexag-onal phase of ZnO [26–28]. Further analysis reveals that the full widthat half maximum of the E2 (high) mode is 7.66 cm−1. Besides, threevery small peaks at 335, 381, and 413 cm−1 were also observed in the

Fig. 3. XRD patterns of ZnO (a) nanorods synthesized at a pH value of 10 (see Fig. 2(d))and (b) nanospheres synthesized at a pH value of 12 with urea (see Fig. 2(c)). Indexedpeaks correspond to the hexagonal phase.

spectra which are assigned to the E2 (high)–E2 (low), A1 (TO) and E1(TO) modes [29–31], respectively. Additionally, a suppressed peak at583 cm−1 is seen in the spectrum and attributed to E1 (LO). For ZnOspheres, two relatively weak peaks of E1 and E2 were found at 420 and441 cm−1, as shown in Fig. 4(b). Other relatively weak peaks locatedat 636, 679, 711, and 765 cm−1 could not be explained within theframework of the bulk phonon modes, and may be attributed to

Fig. 5. Phase stability diagram of the ZnO(s)–NH3(l) system at 25 °C as a function ofprecursor concentration and pH, where the colored lines denote the thermodynamicequilibrium between the Zn2+ soluble species and the corresponding solid phases.

Fig. 6. SEM images delineating the temporal evolution of ZnO nanostructure morphologies (pH=12) at different reaction times (30, 60, 90, and 120 min) of LPD processes.(a–d) ZnO crystals were synthesized without the presence of both ethanol and urea as SDAs, while (e–h) with ethanol and urea.

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multi-phonon processes [32]. One of the distinct differences betweenRaman spectra between ZnOnanorods and spheres is the relative inten-sity of E2 (high) mode. The relatively weak E2 (high) mode of ZnOspheres compared to that of ZnO nanorod in Fig. 4 indicates that thehexagonal phase was suppressed and instead isotropic growth tookplace for ZnO spheres (Fig. 4(b)) [26–28].

Based on the above-described results, the growth mechanismsof self-assembled solid ZnO nano/microspheres can be discussedin terms of concentration of ZAD, pH values, and SDAs. The main

reactions involved in the growth are illustrated in the followingequations:

Zn2þ þ 2OH

−←→ZnðOHÞ2 ð1Þ

ZnðOHÞ2 þ 2OH−←→½ZnðOHÞ4�2− ð2Þ

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½ZnðOHÞ4�2−←→ZnO2−2 þ 2H2O ð3Þ

ZnO2−2 þ H2O←→ZnO þ 2OH

−: ð4Þ

As shown in Fig. 5, phase stability diagram of the ZnO(s)–NH3(l)system indicates that [Zn(OH)4]2− could be formed from many dif-ferent species such as Zn(OH)+, Zn(OH)2, Zn(OH)3−, and others,depending on the parameters including the concentration of Zn2+

and the pH value, and not limited to Eq. (2) [33,34]. And all of theseintermediate species are actually in equilibrium, with the majorforms being different under different reaction conditions. The growthprocess of ZnO structures starts with Zn2+ and OH− ions coordinatewith each other, as in Eq. (1), and followed by dehydration by protontransfer, forming Zn2+⋯O2−⋯Zn2+ bonds, and leading to an agglom-erate of the form of [Znx(OH)y](2x−y)+, which has an octahedral ge-ometry. Water molecules formed by dehydration migrate into thesolution. In the above equations, the O2− in ZnO comes from ammo-nia, not from the solvent H2O. Also, it could be from ethanol.

Particularly, the initial pH of solution determines the role of zinccations [Zn(NH3)4]2+ as the growth unit of ZnO [35] that can be cor-related to the shape and morphology of the ZnO nanostructures,according to following reactions:

NHþ4 þ OH

−←→NH3 þ H2O ð5Þ

Zn2þ þ 4NH3←→½ZnðNH3Þ4�2þ ð6Þ

when pH≤10

½ZnðNH3Þ4�2þ þ 2OH−;→ZnO þ 4NH3 þ H2O ð7Þ

Zn2þ þ 2OH

−→ZnðOHÞ2 ð8Þ

when pH>10

½ZnðNH3Þ4�2þ þ 2OH−→ZnO þ 4NH3 þ H2O ð9Þ

½ZnðOHÞ4�2−→ZnO þ H2O þ 2OH−: ð10Þ

At pH≤10 zinc cations [Zn(NH3)4]2+ form ZnO and zinc hydroxide[Zn(OH)2], as in Eqs. (7) and (8), while at pH>10 the addition of moreammonia dissolves most of the zinc hydroxide, yielding the clear solu-tion according to Eqs. (2) and (3). The precursor, Zn(OH)42− in the

Fig. 7. (a) Low and (b) high magnifications of HR-TEM cross-sectional images of a ZnO nanowith its corresponding selected area electron diffraction (SAED) image in the inset.

solution at high pH values also acts as the growth unit to facilitateZnO crystallites [35]. However, the addition of more ammonia to theaqueous solution to adjust the higher pH values results in the largeamount of NH3 which subsequently prevents the amalgamation of thenucleus, thereby leading to dispersed and dense morphologies of ZnO[36]. Indeed, we found that the increase in the pH value to 12 yieldeddensely packed ZnO morphologies that were shown in SEM images inFig. 1(c).

The temporal and morphological evolution of the ZnO structurescomparedwith/without SDAs at pH value of 12 provided further insightinto the initial nucleation of ZnO. When SDAs are not used, the initialnucleation (up to 30 min) forms highly dense nucleation (Fig. 6(a)and (b)) that led to highly packing density of nanostructures at a pHvalue of 12 (Fig. 6(c) and (d)). When SDAs were added, we found theinitial nucleation of ZnO morphologies (Fig. 6(e) and (f)) was similarto that of ZnO synthesizedwithout SDAs (Fig. 6(a) and (b)). However, ata reaction of time of 90 min, the adjacent nanorods of ZnOmicroflowersagglomerated into solidmicrospheres [19] with a few voids observed inFig. 6(g). When the reaction time is further increased to 120 min, sym-metrical and solid ZnO sphereswere synthesizedwith SDAs as shown inFig. 6(f). This result can be linked to the fact that the addition of SDAs inthe solution at pH value of 12 resulted in enhanced bridging of theneighboring ZnO nanoparticles. Indeed, analysis of a high resolutionTEM (HR-TEM) revealed a very high density of nanocrystallites andgrains with diameters around 2 nm as shown in Fig. 7(a). We thinkthat these nucleated nanocrystallites act as the building blocks to formZnO nanospheres. Furthermore, SDAs play a critical role in suppressingthe directional growth of preferred (0002) plane of ZnO. As shown inFig. 7(b), lattice fringes with 0.20–0.24 nm of lattice space show differ-ent crystal growth directions but do not show preferential directionalgrowth. These results are consistent with Raman spectra and X-raydiffraction measurement which indicated SDAs (urea and ethanol)acted as modification of growth planes of ZnO, particularly suppressing(0002) plane while enhancing lateral growth planes of ZnO. There-fore, based on these SEM, TEM, and X-ray diffraction measurement,SDAs such as ethanol and urea effectively suppress the growth ofc-plane while enhancing other crystal growth directions of ZnO[20,23] and bridge adjacent nanorods to form agglomerated spherestructures by modifying chemical reaction of ZnO surface. As a conse-quence, the clusters of ZnO nano-particles aggregate together to aspherical aggregation.

Even though both urea and ethanol can produce isotropic ZnOspheres, we also found that characteristics of the size and distributionof ZnO spheres are quite different. ZnO spheres obtained using ureaproduced random size distribution having diameters from hundredsof nanometers to a few micrometers as shown in Fig. 2(c). On the

sphere. HR-TEM image of ZnO nanosphere from the area is marked by black rectangle

Fig. 8. SEM images of ZnO spheres controlled using (a) the volume ratio of ethanol to Zn precursor to urea of 1:1:1 and (b) of 1.25:1:1.

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other hand, when ethanol is used as an SDA, uniform size distributionof ZnO spheres (Fig. 2(f)) was obtained while the diameters of ZnOspheres were about 6–8 μm which is more than 10 times largerthan those of ZnO spheres obtained by urea. Therefore, it is inferredthat the size and distribution of ZnO spheres can be controlled bycombining both urea and ethanol as SDAs. To further control thesize and distribution of ZnO spheres, two different volume ratios ofethanol to ZAD to urea, 1:1:1 and 1.25:1:1 in the LPD solution wereused. When volume ratio of ethanol of 1:1:1 was used, uniform distri-bution of ZnO spheres was indeed found as shown in Fig. 8(a). The av-erage of the diameters of the ZnO crystal nanospheres was determinedto be at 694±64 nm. When volume ratio of ethanol was modified to1.25:1:1, the average diameters of ZnO were increased to 1420±90 nm, as shown in Fig. 8(b). Therefore, the controlled volume ratio ofethanol and urea allowed for the control over the size and distributionof ZnO spheres.

4. Conclusions

In summary, controlled synthesis of self-assembled solid ZnOspheres was performed using liquid phase deposition (LPD) pro-cess. We found that urea and ethanol as SDAs in the LPD solutionplayed a significant role in the formation of ZnO spheres, particu-larly at higher pH values of 12. XRD and Raman spectroscopy clear-ly indicated that the presence of ethanol and urea efficientlysuppressed the dominant hexagonal phase of ZnO. We also foundthat the volume ratio of ethanol and urea in the LPD solution wasthe crucial parameter that controlled the distribution and diameterof the solid ZnO crystal spheres from nanometer to micrometerrange.

Acknowledgment

This project is supported by the National Science Foundationunder grant no. BRIGE-0824311 and also by the Research Center Pro-gram of IBS (Institute for Basic Science) in Korea.

References

[1] P. Gao, Y. Ding, Z.L. Wang, Nano Lett. 3 (2003) 1315.[2] M. Huang, S. Mao, H. Feick, Q. Yan, Y. Wu, H. Kind, E. Weber, R. Russo, P. Yang,

Science 292 (2001) 1897.[3] L. Vayssieres, Adv. Mater. 15 (2003) 464.[4] G.R. Yi, J.H. Moon, V.N. Manoharan, D.J. Pine, S.M. Yang, J. Am. Chem. Soc. 124

(2002) 13354.[5] C.E. Fowler, E. Khushalan, S.J. Mann, J. Mater. Chem. 11 (2001) 1968.[6] J.H. Holtz, S.A. Asher, Nature 389 (1997) 829.[7] J.S. Song, F. Tronc, M.A. Winnik, J. Am. Chem. Soc. 126 (2004) 6562.[8] H. Qian, G. Lin, Y. Zhang, P. Gunawan, R. Xu, Nanotechnology 18 (2007) 355602.[9] G. Deng, A. Ding, X. Zheng, W. Cheng, P. Qiu, Mater. Res. Bull. 40 (2005) 903.

[10] Z. Gu, M.P. Paranthaman, J. Xu, Z.W. Pan, ACS Nano 3 (2009) 273.[11] Z.L.S. Seow, A.S.W. Wong, V. Thavasi, R. Jose, S. Ramakrishna, G.W. Ho, Nanotech-

nology 20 (2009) 045604.[12] J. Geng, B. Liu, L. Xu, F.-N. Hu, J.-J. Zhu, Langmuir 23 (2007) 10286.[13] W.-W. Wang, Y.-J. Zhu, L.-X. Yang, Adv. Funct. Mater. 17 (2007) 59.[14] U. Bach, D. Corr, D. Lupo, F. Pichot, M. Ryan, Adv. Mater. 14 (2002) 845.[15] Y. Aoi, H. Kambayashi, T. Deguchi, K. Yato, S. Deki, Electrochim. Acta 53 (2007)

175.[16] A. Nakata, M. Mizuhata, S. Deki, Electrochim. Acta 53 (2007) 179.[17] B. McDonald, T. Cui, J. Colloid Interface Sci. 354 (2011) 1.[18] T. Ichikawa, S. Shiratori, Inorg. Chem. 50 (2011) 999.[19] H. Jiang, J. Hu, F. Gu, C. Li, J. Phys. Chem. C 112 (2008) 12138.[20] Z. Yan, Y. Ma, D. Wang, J. Wang, Z. Gao, T. Song, J. Phys. Chem. C 112 (2008) 9219.[21] S.-N. Bai, J.-S. Shieh, T.-Y. Tseng, Mater. Chem. Phys. 41 (1995) 104.[22] J. Liu, X. Huang, Y. Li, Q. Zhong, L. Ren, Mater. Lett. 60 (2005) 1354.[23] H. Zhang, D. Yang, S. Li, X. Ma, Y. Ji, J. Xu, D. Que, Mater. Lett. 59 (2005) 1696.[24] S. Hirano, N. Takeuchi, S. Shimada, K. Masuya, K. Masuya, K. Ibe, H. Tsunakawa,

M. Kuwabara, J. Appl. Phys. 98 (2005) 094305.[25] International Center for Diffraction Data, PDF-2 card 36-1451.[26] J.J. Wu, S.C. Wu, J. Phys. Chem. B 106 (2002) 9546.[27] H. Gao, F. Yan, J. Li, Y. Zeng, J. Wang, J. Phys. D: Appl. Phys. 40 (2007) 3654.[28] Y.J. Xing, H. Xi, Z.Q. Xue, X.D. Zhang, J.H. Song, R.M. Wang, J. Xu, Y. Song, S.L. Zhang,

D.P. Yu, Appl. Phys. Lett. 83 (2003) 1689.[29] K.A. Alim, V.A. Fonoberov, M. Shamsa, A.A. Baladin, J. Appl. Phys. 97 (2005)

124313.[30] J. Marquina, Ch. Power, J. Gonzalez, Rev. Mex. Fis. 53 (2007) 170, (Suppl.).[31] Z. Li, Z. Hu, F. Liu, J. Sun, H. Huang, X. Zhang, Y. Wang, Mater. Lett. 65 (2011) 809.[32] T.C. Danmen, S.P.S. Porto, B. Tell, Phys. Rev. 142 (1966) 570.[33] O.H. Elsayed-Ali, H.E. Elsayed-Ali, T.M. Abdel-Fattah, J. Hazard. Mater. 185 (2011)

1550.[34] C. Hsueh, H. Chen, J.K. Gimzewski, J. Reed, T.M. Abdel-Fattah, ACS Appl. Mater.

Interfaces 2 (2010) 3249.[35] J.M. Jang, S.D. Kim, H.M. Choi, J.Y. Kim, W.G. Jung, Mater. Chem. Phys. 113 (2009)

389.[36] J. Zhang, L. Sun, J. Yin, H. Su, C. Liao, C. Yan, Chem. Mater. 14 (2002) 4172.