Synthesis, characterization and studies on optical properties of hierarchical ZnO–CdS nanocomposites

Materials Research Bulletin 47 (2012) 1755–1761

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Synthesis, characterization and studies on optical properties of hierarchical ZnO–CdS nanocomposites Manu Sharma, P. Jeevanandam * Department of Chemistry, Indian Institute of Technology Roorkee, Roorkee 247667, India

A R T I C L E I N F O

A B S T R A C T

Article history: Received 24 November 2011 Received in revised form 1 February 2012 Accepted 20 March 2012 Available online 29 March 2012

Cadmium sulfide coated zinc oxide hierarchical nanocomposites have been synthesised at room temperature by a simple solution based method. CdS nanoparticles were deposited on the surface of ZnO without using any surfactant, ligand or chelating agents. The nanocomposites were synthesised using different concentrations of thioacetamide, cadmium salts, and also by varying the reaction time. After characterization of the nanocomposites, optical properties were investigated by UV–visible diffuse reflectance and photoluminescence spectroscopy techniques. It was found that band gap of the ZnO–CdS nanocomposites is tunable between 2.42 and 3.17 eV. ß 2012 Elsevier Ltd. All rights reserved.

Keywords: A. Semiconductors B. Chemical synthesis D. Optical properties

1. Introduction Zinc oxide–cadmium sulfide nanocomposites have been extensively investigated due to their applications in the area of optics, electronics, and photo catalysis [1–4]. ZnO–CdS core–shell nanostructures have interesting applications in chemical engineering and biology [5–10]. One dimensional ZnO–CdS nanocomposites exhibit interesting optical properties such as tunable band gap, and the nanostructures are suitable for fast photon absorption, transportation, and collection [11]. In the ZnO–CdS nanocomposites, CdS acts as a visible sensitizer while ZnO, a semiconductor with a wide band gap (3.34 eV at 2 K), is responsible for charge separation which suppresses the recombination process [12]. ZnO–CdS nanocomposites also possess better physicochemical properties compared to the constituents. For example, the conductivity of ZnO nanorods–CdS nanoparticles composite is better than that of pure ZnO nanorods [13]. ZnO–CdS core–shell nanoparticles show improved sensing capabilities compared to pure ZnO and CdS [14]. Various methods, used for the synthesis of ZnO–CdS nanocomposites, have been briefly discussed below. Chemical bath deposition has been used to grow ZnO nanorods on glass substrate on which CdS nanoparticles were included by layer by layer assembly [15,16]. CdS quantum dots, grown on vertically aligned ZnO nanorods, have been synthesised by chemical bath deposition [17–20]. On ZnO nanorods synthesised by a vapor transport process, CdS shells have been epitaxially

* Corresponding author. Tel.: +91 1332 285444; fax: +91 1332 286202. E-mail address: [email protected] (P. Jeevanandam). 0025-5408/$ – see front matter ß 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.materresbull.2012.03.044

grown by metal organic chemical vapor deposition [21]. Pulsed laser deposition has been used for the fabrication of CdS–ZnO nanocomposites [22]. Vanalakar et al. have prepared CdS-sensitized ZnO nanorod electrodes by chemical bath deposition [23]. CdS quantum dots sensitized ZnO photoanode has been prepared by spray pyrolysis [24]. ZnO–CdS nanocomposites have been synthesized using ZnO particles with different shapes such as nanorods [25,26], nanotubes [27], irregular particles [28], nanowires [29], spheres [30], and plates [31]. In the present study, CdS nanoparticles have been deposited on the surface of ZnO hierarchical nanostructures. The main purpose of deposition of CdS nanoparticles on the surface of ZnO was to obtain improved optical properties for the ZnO–CdS nanocomposites compared to pure ZnO and CdS nanoparticles. The synthesis was carried out at room temperature without using any surfactant, chelating agent or ligand. After thorough characterization, optical properties of the CdS–ZnO hierarchical nanocomposites were investigated by UV–visible diffuse reflectance spectroscopy, and photoluminescence spectroscopy. 2. Experimental 2.1. Materials Zinc acetate dihydrate, and cadmium chloride dihydrate were received from Sisco Research Laboratories. Cadmium nitrate tetrahydrate was received from Rankem1, and cadmium sulfate, cadmium acetate dihydrate, and thioacetamide were received from Himedia1. Millipore water was used for the preparation of all the solutions. All the chemicals were used as received without further purification.

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Table 1 The experimental details, and nomenclature of different ZnO–CdS nanocomposites. Experimental parameter

Nomenclature and details of sample preparation

A. Reaction time

A1. A2. A3. A4.

1 mM 1 mM 1 mM 1 mM

CdSO4, CdSO4, CdSO4, CdSO4,

10 mM 10 mM 10 mM 10 mM

B. Concentration of thioacetamide (reaction time = 6 h)

B1. B2. B3. B4.

1 mM 1 mM 1 mM 1 mM

CdSO4, CdSO4, CdSO4, CdSO4,

1 mM thioacetamide 2.5 mM thioacetamide 5 mM thioacetamide 10 mM thioacetamide

C. Cadmium salts (reaction time = 6 h)

C1. C2. C3. C4.

1 mM 1 mM 1 mM 1 mM

Cd(Ac)2, 10 mM thioacetamide CdCl2, 10 mM thioacetamide CdNO3, 10 mM thioacetamide CdSO4, 10 mM thioacetamide

D. Concentration of CdSO4 (reaction time = 6 h)

D1. D2. D3. D4.

0.1 mM CdSO4, 10 mM thioacetamide 0.25 mM CdSO4, 10 mM thioacetamide 0.50 mM CdSO4, 10 mM thioacetamide 1 mM CdSO4, 10 mM thioacetamide

E. Pure CdS

1 mM CdSO4, 10 mM thioacetamide, 75 8C (reaction time = 6 h)

F. Pure ZnO

Zinc acetate dihydrate, sodium citrate, ammonia solution, NaOH, 85 8C (reaction time = 12 h)

(reaction (reaction (reaction (reaction

time = 30 min) time = 1 h) time = 3 h) time = 6 h)

ethanol, and sonicated for 10 min. The emission spectra were then recorded for the suspensions at 350 nm excitation. 3. Results and discussion

(101)

Fig. 1 shows the XRD patterns of ZnO–CdS nanocomposites prepared at different reaction time periods (samples A1–A4). The XRD patterns show reflections due to ZnO (wurtzite; JCPDS File No. 80-0075), and CdS (cubic; JCPDS File No. 80-0019). After formation of the ZnO–CdS nanocomposites, the peak intensities due to ZnO decrease (see the inset in Fig. 1), and new peaks due to CdS are observed. This supports the fact that CdS particles are deposited on the surface of ZnO. The broadness of the diffraction peaks due to CdS indicates that the size of CdS crystallites is very small, as calculated by the Debye–Scherrer formula (2.2 nm). The calculated crystallite size of ZnO and CdS in the ZnO–CdS nanocomposites (samples A1–A4) is given in Table 2. The crystallite size of ZnO in the nanocomposites is about 28 nm while that of CdS is about

* ZnO # CdS

(100) (002)

Intensity (a.u)

*

Intensity (a.u)

* *

10

20

*

#

30

40

30 2θ

(103)

*

*

#

50

35

*

*

60

40

(200) (112) (201)

(311)

(102)

(220)

#

25

(110)

20

(111)

ZnO–CdS nanocomposites were prepared by a two step method. In the first step, ZnO hierarchical nanostructures were synthesised using zinc acetate dihydrate, trisodium citrate, and sodium hydroxide by a previously reported method [32]. About 2.1 g of zinc acetate dihydrate was dissolved in 200 ml of distilled water, and stirred for 10 min at room temperature. About 10 ml of 1.5 g trisodium citrate in water, and 4.2 ml of 25% ammonia solution were added. Then, about 20 ml of 2 M NaOH solution was added drop wise at room temperature with vigorous stirring. The temperature was then raised to 85 8C and kept for 12 h. The contents were centrifuged, washed with water, and dried at 60 8C. ZnO hierarchical nanostructures were obtained as powder. In the second step, CdS nanoparticles were deposited on the surface of ZnO hierarchical nanostructures using different cadmium salts (cadmium acetate, cadmium chloride, cadmium nitrate, and cadmium sulfate; 0.1–1 mmol (mM)), and thioacetamide (1– 10 mmol). About 250 mg of the zinc oxide powder was dispersed in 100 ml water. Appropriate concentration of cadmium salts and thioacetamide were then added, and the contents were stirred for different time periods (30 min to 6 h) at 25 8C. The yellow colored precipitates were centrifuged, washed with water, ethanol, and dried at 70 8C. The yield of the ZnO–CdS nanocomposites was about 250 mg. Pure CdS nanoparticles were also synthesized at room temperature but the yield was very low. Hence, the temperature of the synthesis was increased to 75 8C in order to get better yield (100 mg). The nomenclature of various ZnO–CdS nanocomposites, synthesised along with the synthesis details, are given in Table 1. Powder XRD patterns were recorded using a Brucker AXS D8 diffractometer operating with Cu-Ka radiation (l = 1.5418 A˚) with a scanning speed of 18/min. Thermal gravimetric analysis was carried out in air using a Perkin Elmer (Pyris Diamond) instrument with a heating rate of 5 8C/min from 25 8C to 800 8C. Field emission scanning electron microscope (FE-SEM) images were obtained using a FEI Quanta 200F microscope operating at an accelerating voltage of 20 kV. For recording optical absorption spectra of the ZnO–CdS nanocomposites, a Shimadzu UV-3600 UV–visible NIR spectrophotometer was used in the wavelength range 250–800 nm along with a diffuse reflectance accessory with BaSO4 as the reference. Photoluminescence spectroscopy measurements were carried out using a Varian Cary Eclipse fluorescence spectrophotometer. For the luminescence spectral studies, about 10 mg each of the ZnO–CdS nanocomposites was dispersed in about 5 ml

thioacetamide, thioacetamide, thioacetamide, thioacetamide,

*

A4 A3 A2 A1 F

70

2θ Fig. 1. XRD patterns of ZnO–CdS nanocomposites prepared at different reaction times (30 min to 6 h) using 1 mM cadmium sulfate and 10 mM thioacetamide. The reflections due to ZnO are represented by ‘*’, and the reflections due to CdS are represented by ‘#’. The inset shows the effect of CdS deposition on the ZnO reflections.

M. Sharma, P. Jeevanandam / Materials Research Bulletin 47 (2012) 1755–1761 Table 2 Crystallite size of CdS and ZnO in ZnO–CdS nanocomposites (samples A1–A4) prepared using 1 mM cadmium sulfate and 10 mM thioacetamide. Sample

Reaction time

Crystallite size of ZnO (nm)

Crystallite size of CdS (nm)

A1 A2 A3 A4

30 min 1h 3h 6h

28.6 27.6 27.2 27.2

2.1 2.1 2.4 2.3

2 nm. There is no major change in the crystallite size of CdS in the nanocomposites when the reaction time is increased from 30 min to 6 h. XRD patterns of the ZnO–CdS nanocomposites prepared using different concentrations of thioacetamide (samples B1–B4), different cadmium salts (samples C1–C4), and different concentrations of cadmium sulfate (samples D1–D4) indicated that crystallinity of the nanocomposites increases with an increase in concentration of the reagents. Based on the XRD results, the best optimised conditions for the synthesis of ZnO–CdS nanocomposites were found to be 1 mM cadmium sulfate, 10 mM thioacetamide with a reaction time of 6 h at room temperature. Fig. 2 shows the typical TGA patterns of ZnO– CdS nanocomposites (samples A2–A4). The thermograms show the thermal stability of the nanocomposites up to about 500 8C. Only weight loss of about 4.5%, attributed to adsorbed moisture, is observed in this range. The weight gain at around 590 8C is due to the oxidation of CdS nanoparticles. The observed weight gain of the nanocomposites, A2, A3, and A4 are 3.3%, 3.6% and 4.5%, respectively. Fig. 3 shows the FE-SEM images of ZnO–CdS nanocomposites prepared at different reaction times using 1 mM CdSO4 and 10 mM thioacetamide (samples A1–A4) along with pure ZnO. At a reaction time of 30 min, no clear deposition of CdS nanoparticles on ZnO can be observed (Fig. 3, image A2). With increasing reaction time (30 min to 6 h), the quality of CdS coating is improved. The FE-SEM image of the nanocomposite prepared at 6 h (sample A4) shows the presence of spherical CdS nanoparticles on the ZnO hierarchical structures (Fig. 3, image A5). The observed mean size of the CdS nanoparticles on the ZnO nanostructures in sample A4 was 63  7.0 nm. The summary of EDXA results for the ZnO–CdS nanocomposites (samples A1–A4) prepared using 1 mM cadmium sulfate and 10 mM thioacetamide at different time intervals are given in Table 3. The EDXA results prove the formation of ZnO and CdS. FE-SEM images of the ZnO–CdS nanocomposites prepared using different

% Weight

100

99

1 h (A2) 3 h (A3)

98

6 h (A4)

97

96

95 100

200

300

400

500

600

700

800

o

Temperature ( C) Fig. 2. TGA patterns of ZnO–CdS nanocomposites prepared using 1 mM cadmium sulfate and 10 mM thioacetamide at different reaction times (samples A2–A4). The TGA patterns were recorded in air.

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concentrations of thioacetamide (samples B1–B4) showed distorted hierarchical structures at lower concentrations of thioacetamide (up to 5 mM); spherical CdS particles could not be clearly observed on the surface of zinc oxide. However at 10 mM thioacetamide, good coating of CdS nanoparticles on ZnO could be noticed. The FE-SEM image of the composite prepared using cadmium acetate (sample C1) showed compact ball-like structure made up of small plates; no CdS spheres on ZnO surface could be observed. The ZnO–CdS nanocomposites prepared using cadmium chloride and cadmium nitrate (samples C2 and C3) showed distorted hierarchical structures. However, the nanocomposite prepared using cadmium sulfate (sample C4) showed good coating of CdS nanoparticles on the ZnO hierarchical structure. Fig. 4 shows the FE-SEM images of ZnO–CdS nanocomposites prepared using different concentrations of cadmium sulfate (samples D1–D4). The FE-SEM images show, with increasing cadmium sulfate concentration (up to 0.5 mM), distorted hierarchical, compact ball-like, and agglomerated hierarchical structures. However, at 1 mM concentration of CdSO4, good coating of CdS nanoparticles on ZnO can be noticed. Fig. 5 shows DRS spectra of the ZnO–CdS nanocomposites synthesised at different reaction times using 1 mM cadmium sulfate and 10 mM thioacetamide (samples A1–A4). The spectra of the nanocomposites show two band gap absorptions; one due to ZnO, and the other due to CdS. The band gap absorption of ZnO in the nanocomposites is closer to that of pure zinc oxide (380 nm). The band gap absorption of CdS in the nanocomposites show blue shift (45–65 nm) with respect to pure CdS nanoparticles (565 nm), depending on the reaction time. Fig. 6 shows the estimation of direct band gap of CdS in the nanocomposites calculated using the formula given below [33]. 2

ðahyÞ ¼ kðhy  Eg Þ

(1)

It can be seen that the band gap of CdS nanoparticles in the nanocomposites decreases from 2.49 eV to 2.42 eV when the reaction time is increased from 30 min to 6 h (see the inset in Fig. 6). Diffuse reflectance spectra of the nanocomposites prepared using different concentrations of thioacetamide (samples B1–B4), different cadmium salts (samples C1–C4), and different concentrations of cadmium sulfate (samples D1–D4) showed no change in the band gap absorption due to ZnO but showed blue shift of CdS band gap absorption with respect to pure CdS nanoparticles (565 nm). The summary of estimated band gap values for all the ZnO–CdS nanocomposites synthesized by varying different synthetic parameters is given in Table 4. The band gap of CdS nanoparticles in the composites B1 to B4 varied from 2.63 eV (1 mM thioacetamide) to 2.53 eV (10 mM thioacetamide). The band gap of CdS in the nanocomposites C1 to C4 varied from 2.58 eV (cadmium acetate) to 2.48 eV (cadmium sulfate). The nanocomposites D1 to D4 showed red shift (70–80 nm) of CdS band gap absorption when the CdSO4 concentration is increased from 0.1 mM to 0.5 mM. The band gap of CdS in the nanocomposites D1 to D4 varied from 3.17 eV (0.1 mM CdSO4) to 2.53 eV (1 mM CdSO4). It can be noticed that CdS nanoparticles in the nanocomposites possess higher band gap values compared to pure CdS nanoparticles. The observed optical properties of the ZnO–CdS nanocomposites are in accordance with those reported in the literature [34–36]. Fig. 7 shows photoluminescence spectra of the ZnO–CdS nanocomposites prepared at different reaction times (samples A1–A4). The photoluminescence spectrum of pure ZnO shows emission bands at 380 nm and 575 nm. The band at 380 nm is attributed to near band edge emission and that at 575 nm is attributed to oxygen vacancies [37]. Pure cadmium sulfide

M. Sharma, P. Jeevanandam / Materials Research Bulletin 47 (2012) 1755–1761

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Fig. 3. FE-SEM images of ZnO–CdS nanocomposites prepared using 1 mM cadmium sulfate and 10 mM thioacetamide at different reaction times (A1, pure ZnO; A2, 30 min; A3, 1 h; A4, 3 h; and A5, 6 h). Table 3 EDXA results of ZnO–CdS nanocomposites (samples A1–A4) prepared at different reaction times using 1 mM cadmium sulfate and 10 mM thioacetamide. Sample

A1. A2. A3. A4.

CdS–ZnO, CdS–ZnO, CdS–ZnO, CdS–ZnO,

Zn

reaction reaction reaction reaction

time = 30 min time = 1 h time = 3 h time = 6 h

Cd

O

S

wt.%

at.%

wt.%

at.%

wt.%

at.%

wt.%

at.%

72.7 72.6 71.8 67.7

58.6 55.2 57.7 47.7

13.1 13.8 14.6 14.8

6.0 6.0 7.8 6.1

9.8 11.3 9.2 14.1

26.2 32.5 27.6 40.6

6.3 5.2 6.3 5.2

10.0 7.2 9.6 7.5

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Fig. 4. FE-SEM images of ZnO–CdS nanocomposites prepared using different concentrations of cadmium sulfate (D1, 0.1 mM; D2, 0.25 mM; D3, 0.5 mM; and D4, 1 mM). The concentration of thioacetamide was kept constant as 10 mM (reaction time = 6 h).

(αhν )

% R (a.u)

a

f

(a) 30 min. (b) 1h (c) 3h (d) 6h (e) CdS (f) ZnO

Band gap (eV)

(a) 30 min. (b) 1h (c) 3h (d) 6h (e) CdS (f) ZnO

to increase in the sulfur vacancies after the deposition of CdS on the ZnO surface. Gao et al. have reported an emission band in visible region around 550 nm in the ZnO–CdS nanocomposites [13]. This band has been attributed to emission from the defect states such as cadmium interstitials or sulfur vacancies in the CdS nanoparticles.

2

nanoparticles show a low intense emission band at about 575 nm. The emission spectra of ZnO–CdS nanocomposites also show two bands. The intensity of near band edge emission due to ZnO decreases in the nanocomposites where as intensity of the 575 nm band due to CdS increases in the nanocomposites. This is attributed

Reaction Time (minutes)

b

e e

b

d

c

d c

f a

2.00 300

400

500

600

700

800

2.25

2.50

2.75

3.00

3.25

3.50

Energy (eV)

Wavelength (nm) Fig. 5. DRS spectra of CdS–ZnO nanocomposites prepared using 1 mM cadmium sulfate and 10 mM thioacetamide at different time intervals (30 min to 6 h) (samples A1–A4).

Fig. 6. Estimation of band gap of ZnO–CdS nanocomposites prepared using 1 mM cadmium sulfate and 10 mM thioacetamide at different time intervals (30 min to 6 h). The inset shows the variation of band gap of CdS nanoparticles in the nanocomposites with reaction time.

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Table 4 Summary of estimated band gap values for the ZnO–CdS nanocomposites. For details on the samples, see Table 1. Experimental parameter

Sample

Band gap (eV)

A. Reaction time interval

A1 A2 A3 A4

2.49 2.46 2.43 2.42

B. Concentration of thioacetamide (reaction time = 6 h)

B1 B2 B3 B4

2.63 2.54 2.50 2.53

C. Cadmium salts (reaction time = 6 h)

C1 C2 C3 C4

2.58 2.55 2.52 2.48

D. Concentration of CdSO4 (reaction time = 6 h)

D1 D2 D3 D4

3.17 2.55 2.50 2.53

E

Pure CdS

2.20

F

Pure ZnO

3.25

25 6h 3h 1h 30 min. ZnO CdS

20

Intensity

15

10

5

0 400

450

500

Zn(CH3COO)2.2H2O + NaOH

650

nanostructures and deposition of CdS spheres on the ZnO hierarchical nanostructures is shown in Fig. 8. Cadmium sulfate on hydrolysis forms cadmium hydroxide, which then reacts with thioacetamide to give CdS nanoparticles [25]. An activated cadmium hydroxide complex is formed on the surface of ZnO and the surface bound cadmium hydroxide complex reacts with thioacetamide to form ZnO–CdS nanocomposite. The reactions involved are as follows. CdSO4 þ 2H2 O ¼ CdðOHÞ2 þ H2 SO4

(i)

ZnO þ CdðOHÞ2 ¼ ZnOCdðOHÞ2

(ii)

ZnOCdðOHÞ2 þ CH3 CSNH2 ¼ ZnOCdS þ H2 O þ CH3 CONH2 (iii)

85oC, 12 h

+ Na3C6H5O7 + aq. NH3 ZnO Hierarchial Nanostructures

aq. CdSO4

CdS

o o o o oo CdS CdS ooo o o o o ooooo o o o O o o o o o o o ooo o ooo o o oooo ooo ooooooo oo ooo H3C C NH2 o o o oo ooooooo o o o o o o oooooo o + o o oo oo o + H 2O CdS o ooo oooo o o o o CdS o o oo o oooo o o

Cd(OH)2

CdS

CdS

600

Fig. 7. Photoluminescence spectra of the ZnO–CdS nanocomposites (samples A1– A4) prepared using 1 mM cadmium sulfate and 10 mM thioacetamide at different reaction time intervals (30 min to 6 h).

The change in emission peak intensity has been attributed to the interaction between the two semiconductors (ZnO and CdS). Kundu et al. have reported that the ZnO–CdS nanocomposite shows two emission bands; at 325 nm and 500 nm [36]. The decrease in emission intensity with an increase in the loading of CdS nanoparticles on ZnO has been attributed to decrease in the number of hydroxyl species on the surface of zinc oxide due to an increase in the content of CdS in the nanocomposites. The mechanism of formation of ZnO–CdS nanocomposites has been discussed below. During the synthesis of ZnO–CdS nanocomposites, when cadmium salts were reacted with thioacetamide in the absence of ZnO, no yellow colored product was formed. This indicates that ZnO plays an important role in the deposition of CdS nanoparticles. A schematic of formation of hierarchical ZnO

CdS

550

Wavelength (nm)

CdS

ZnO - CdS Nanocomposite

C

Cd(OH)2 Cd(OH)2

Cd(OH)2

S H3C

Cd(OH)2

NH2

Cd(OH)2

Cd(OH)2

Cd(OH)2

Cd(OH)2

Cd(OH)2

Cd(OH)2

ZnO - Cd(OH)2

Fig. 8. Schematic indicating the formation of ZnO–CdS nanocomposites.

M. Sharma, P. Jeevanandam / Materials Research Bulletin 47 (2012) 1755–1761

4. Conclusions ZnO–CdS hierarchical nanocomposites were successfully synthesized using a simple solution based method at room temperature. The method is an easy way to prepare ZnO–CdS nanocomposites without using any ligand or chelating agent. The nanocomposites show better optical properties compared to the individual constituents. The band gap of CdS–ZnO nanocomposites depend on various experimental parameters such as reaction time, concentrations of thioacetamide and cadmium salt, and also chemical nature of the cadmium salt. The band gap of CdS in the ZnO–CdS nanocomposites is tunable from 2.42 eV to 3.17 eV and these composites are expected to be useful in applications such as sensing, optoelectronics, and photocatalysis. Acknowledgments Generous funding from the Department of Science and Technology, Government of India in the form of a research grant (Project No. SR/S1/PC-06/2007) is gratefully acknowledged. The authors would like to thank Ms. Geetu Sharma for her help. References [1] S.K. Panda, S. Chakrabarti, B. Satpati, P.V. Satyam, S. Chaudhuri, J. Phys. D: Appl. Phys. 37 (2004) 628. [2] F. Fang, D.X. Zhao, B.H. Li, Z.Z. Zhang, J.Y. Zhang, D.Z. Shen, Appl. Phys. Lett. 93 (2008) 233115/1. [3] P. Vasa, P. Taneja, P. Ayyub, B.P. Singh, R. Banerjee, J. Phys.: Condens. Matter 14 (2002) 281. [4] X. Wang, G. Liu, G.Q. Lu, H.M. Cheng, Int. J. Hydrogen Energy 35 (2010) 8199. [5] X.Q. Meng, D.X. Zhao, J.Y. Zhang, D.Z. Shen, Y.M. Lu, X.W. Fan, X.H. Wang, Mater. Lett. 61 (2007) 3535. [6] K. Rajeshwar, N.R. de Tacconi, C.R. Chenthamarakshan, Chem. Mater. 13 (2001) 2765. [7] A.P. Alivisatos, Science 271 (1996) 933. [8] M.A. Anderson, S. Gorer, R.M. Penner, J. Phys. Chem. B 101 (1997) 5895.

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