Simple and large scale refluxing method for

Applied Surface Science 265 (2013) 591–596

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Simple and large scale refluxing method for preparation of Ce-doped ZnO nanostructures as highly efficient photocatalyst M. Rezaei, A. Habibi-Yangjeh ∗ Department of Chemistry, Faculty of Science, University of Mohaghegh Ardabili, P.O. Box 179, Ardabil, Iran

a r t i c l e

i n f o

Article history: Received 23 October 2012 Received in revised form 9 November 2012 Accepted 11 November 2012 Available online 23 November 2012 Keywords: Ce-doped ZnO Photocatalysis Nanostructures X-ray diffraction

a b s t r a c t A simple method was applied for preparation of Ce-doped ZnO nanostructures (mole fractions of Ce4+ ions are 0, 0.025, 0.05, 0.075 and 0.10) in water by refluxing for 3 h about at 90 ◦ C. This method is large scale, mild and involve no templates, surfactants or additives. The prepared nanostructures were investigated by powder X-ray diffraction (XRD), scanning electron microscopy (SEM), and UV–vis diffuse reflectance spectroscopy (DRS) techniques. The XRD patterns demonstrate that the nanostructures have the same crystal structure, and loading of Ce4+ ions does not change the structure of ZnO. The SEM images show that with increasing mole fraction of Ce4+ ions, morphology of the nanostructures changes from nanoplates to nanospheres. Photocatalytic activity of the nanostructures toward photodegradation of methylene blue (MB) was evaluated under UV irradiation. The results indicate that the nanostructures with 0.05 mole fraction of Ce4+ ions exhibit highest photocatalytic activity among the prepared samples. The influence of various operational parameters such as refluxing time, catalyst weight, calcinations temperature and pH of solution on the photodegradation reaction was studied. The optimum value for calcinations temperature was found to be at 400 ◦ C. Moreover, the nanostructures have highest photocatalytic activity at solutions with pH between 5.4 and 9. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Synthetic dyes that widely used in textiles, printing, dyeing and food industries are resistant to biological treatment and can produce harmful byproducts during hydrolysis, oxidation, or other chemical reactions taking place in the wastewater [1,2]. Traditional physical techniques (such as adsorption on various adsorbents, ultrafiltration and coagulation) only succeed in transferring the pollutants from water to another phase, thus creating secondary pollution [3]. Therefore, a low-cost complete mineralization process for the dyes would find extensive use for treatment of large volume of the wastewater. Heterogeneous photocatalysis using semiconductors is an effective method for environmental cleaning and remediation due to its potential to destroy a wide range of organic and inorganic pollutants at ambient temperatures and pressures [4–6]. This method is generally based on the generation of OH radicals which interact with organic pollutants leading to progressive degradation and subsequently complete mineralization [7,8]. This method is a clean, low temperature and non-energy intensive approach for treatment of the pollutants. High physical and chemical stability, inexpensive and nontoxicity were identified as the main reasons responsible for the

∗ Corresponding author. Tel.: +98 0451 5514702; fax: +98 0451 5514701. E-mail address: [email protected] (A. Habibi-Yangjeh). 0169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2012.11.053

wide acceptability of ZnO materials compared to other photocatalysts [9]. The initial step of heterogeneous photocatalysis processes is absorption of photons with a proper wavelength. The illumination induces electron promotion from valence to conduction bands for producing electron–hole pairs. Most of these electron–hole pairs recombine and only a small percentage of these pairs migrate to the surface of photocatalyst where they can be captured by adsorbed molecules to start the catalytic reactions [10,11]. It has been reported that some cations dopants can act as the trapping sites to decrease the electron–hole recombination rate and to increase photocatalytic activity [12]. Recently, much attention has been paid to rare-earth (especially Ce) doped ZnO for possible applications in various technologies [13–22]. Cyclohexanol conversion was investigated over CeO2 –ZnO materials prepared at 160 ◦ C for 2 h and subsequently calcined at 500 ◦ C [13]. Moreover, CeO2 –ZnO catalysts were prepared by calcinations of citrate precursor at 550 ◦ C for 3 h and applied them for cyclohexanone production [14]. Karunakaran et al. have prepared Ce-doped ZnO nanoparticles by sonochemical wet impregnation method and calcined them at 500 ◦ C and applied for detoxification of cyanide [15]. Fangli et al. studied infrared emissivity of Ce-doped ZnO films prepared in ethanol in presence of diethanol amine by spin coating method and subsequently annealed at 620 ◦ C for 2 h [16]. Ce-doped ZnO nanotubes were prepared in presence of a surfactant in aqueous solution of ethanol by hydrothermal method at 95 ◦ C for 4 h and photoluminescence of them was studied [17].

M. Rezaei, A. Habibi-Yangjeh / Applied Surface Science 265 (2013) 591–596

Ma et al. have prepared a series of ZnO–CeO2 nanomaterials by a hydrothermal process in presence of a copolymer [18]. CeO2 –ZnO nanofibers were prepared in aqueous solutions of ethanol in presence of PVP by electrospinning method and calcined at 600 ◦ C for 3 h. Photocatalytic activity of the nanofibers was determined by degradation of Rhodamine B [19]. Ce-doped ZnO nanocrystals were synthesized in ethylene glycol in presence of citric acid at 150 ◦ C for 5 h and calcined at 400 ◦ C and their photoluminescence properties were studied [20]. Wang et al. have synthesized Ce-doped ZnO nanocrystalline by a gel-template combustion process in presence of gelatin [21]. Very recently, Ce1−x Znx Oy catalysts have been prepared in presence of sorbitol as fuel by combustion method at 500 ◦ C and applied them for hydrogen production by oxidative steam reforming of methanol [22]. As can be seen, little works have been reported about photocatalytic degradation of organic pollutants by Ce-doped ZnO nanomaterials. Moreover, the reported preparation methods mainly have high temperature or long reaction time and most of them involved environmentally malignant chemicals and organic solvents, which are toxic and not easily degraded in the environment. Therefore, searching new methodology with low-cost, green and mass-production method is of great importance. In the present paper, a simple and large scale refluxing method was applied for preparation of Ce-doped ZnO nanostructures in water as a green and template-free method. Moreover, influence of various operational parameters such as mole fraction of Ce4+ ions, refluxing time, catalyst weight, calcinations temperature and pH of solution on photodegradation reaction of methylene blue (MB) have been studied to achieve maximum degradation efficiency and the results were discussed. 2. Experimental

X = 0.100

X = 0.075

X = 0.050

Intensity

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X = 0.025

(002) (100)

(101)

(102)

(110)

(103) (112) (200) (201) (004) (202)

X = 0.000

10

20

30

40

50

60

70

80

2 Theta (deg.) Fig. 1. Powder XRD patterns for Ce-doped ZnO nanostructures with: (a) X = 0.000, (b) X = 0.025, (c) X = 0.050, (d) X = 0.075 and (e) X = 0.100.

double distilled water and ethanol, respectively to remove the unreacted reagents and dried in an oven at 60 ◦ C for 48 h. In order to investigate the effect of refluxing time on photocatalytic activity of the nanostructures, five more comparative samples were prepared, keeping the reaction parameters constant except that the products were prepared by refluxing for 1.5, 4.5, 6, 9 and 15 h.

2.1. Materials Zinc acetate (Zn (CH3 COO)2 ·2H2 O extra pure), cerium sulfate (Ce(SO4 )2 ·4H2 O) extra pure, sodium hydroxide (NaOH) and absolute ethanol were obtained from Merck and employed without further purification. Double distilled water was used for the experiments. 2.2. Instruments The X-ray diffraction (XRD) patterns were recorded on a Philips Xpert X-ray diffractometer with Cu K␣ radiation ( = 0.15406 nm), employing scanning rate of 0.04 ◦ /s in the 2 range from 10◦ to 80◦ . Diffuse reflectance spectra (DRS) were recorded by a Scinco 4100 apparatus. Surface morphology and distribution of particles were studied via LEO 1430VP scanning electron microscopy (SEM), using an accelerating voltage of 15 kV. The samples used for SEM observations were prepared by transferring the particles, which at first were dispersed in the ethanol to glass substrate attached to the SEM stage. After allowing the evaporation of ethanol from the substrate, the particles on the stage were coated with a thin layer of gold and palladium. 2.3. Preparation of the nanostructures In a typical procedure for preparation of the nanostructures, zinc acetate dihydrate (4.691 g) and cerium sulfate (0.455 g) were dissolved in 450 ml of distilled water under stirring at room temperature. Then, aqueous solution of NaOH (5 M) was slowly added dropwise into the solution under stirring at room temperature until pH of the solution reached to 13. The solution subsequently was refluxed about at 90 ◦ C for 3 h. The formed suspension was centrifuged to get the precipitate out and washed three times with

2.4. Photocatalysis experiments Photocatalysis experiments were performed in a cylindrical Pyrex reactor with about 400 ml capacity. The reactor provided with water circulation arrangement to maintain the temperature at 25 ◦ C. The solution was magnetically stirred and continuously aerated by a pump to provide oxygen and complete mixing of the reaction solution. A UV Osram lamp with 125 W was used as UV source. The lamp was fitted on the top of the reactor. Prior to illumination, a suspension containing 0.1 g of the nanostructures and 250 ml of MB (2.75 × 10−5 M) was continuously stirred in the dark for 30 min, to attain adsorption equilibrium. Samples were taken from the reactor at regular intervals and centrifuged to remove the photocatalyst before analysis by spectrophotometer at 664 nm corresponding to maximum absorption wavelength of MB. The adsorption experiments were carried out in the dark to prevent photocatalytic degradation of the dye. 3. Results and discussion The XRD patterns for as-prepared Ce-doped ZnO nanostructures (mole fractions of Ce4+ ions are 0.000, 0.025, 0.050, 0.075 and 0.100) are shown in Fig. 1. For pure ZnO, the diffraction peaks are corresponding to (1 0 0), (0 0 2), (1 0 1), (1 0 2), (1 10 ), (1 0 3), (2 0 0), (1 1 2), (2 0 1), (0 0 4) and (2 0 2) planes in agreement with a wurtzite hexagonal crystalline phase. Moreover, no peaks attributable to possible impurities are observed. As can be seen, the XRD patterns for Ce-doped ZnO nanostructures are similar to pure ZnO and they do not display any peak corresponding to CeO2 system. Then, it can be concluded that Ce4+ ions would uniformly substitute into the Zn2+ sites or interstitial sites in ZnO lattice. The sharp


593

Fig. 2. SEM images for Ce-doped ZnO nanostructures with: (a) X = 0.000, (b) X = 0.025, (c) X = 0.050, (d) X = 0.075 and (e) X = 0.100.

diffraction peaks manifest that the as-prepared ZnO and Ce-doped ZnO nanostructures have high crystallinity. Scanning electron microscopy (SEM) was applied to study morphology of the nanostructures, which their records are shown in Fig. 2. It is evident that ZnO nanostructures are mainly nanoplates with different sizes and with increasing mole fraction of Ce4+ ions, the morphology is changing to nanospheres. Diffuse reflectance spectra (DRS) for the nanostructures with various mole fractions of Ce4+ ions were obtained and the results shown in Fig. 3. As can be seen, the spectra are similar to each other and they do not have any absorption in the visible region. The band gap of ZnO nanostructures (3.51 eV) is increased compared to that of bulk ZnO (3.37 eV). The enlargement of band gap (0.14 eV) or blue shift can be attributed to the quantum confinement effect [23]. The photocatalytic activity of the nanostructures was evaluated using MB as a model organic pollutant at 25 ◦ C. Photocatalytic degradation of MB generally yields carbon dioxide, nitrate,

sulfate and water [24]. The overall reaction can be written as follows: MB

Photocatalyst

−→

degradation products

(1)

Influence of various parameters on the photocatalytic degradation on Ce-doped ZnO nanostructures was carried out in order to obtain maximum degradation efficiency. The essential reaction parameters of (i) mole fraction for Ce4+ ions, (ii) refluxing time, (iii) calcinations temperature, (iv) catalyst weight, and (v) pH of solution were changed and the results discussed. Generally, photocatalytic activity dependent on doping amount of photocatalysts. The photocatalytic activities of the prepared Ce-doped ZnO nanostructures with different mole fractions of Ce4+ ions were evaluated by UV irradiation and the results demonstrated in Fig. 4. As can be seen, with increasing mole fraction of Ce4+ ions, firstly degradation of MB increases, and then with further doping, the degradation reaction decreases. In presence of the photocatalyst with 0.05 mole

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0.035 X=0.00

1.2

X=0.025

0.028

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1.0

kobs (min-1 )

Absorbance

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0.6

0.021

0.014

0.4

0.007 0.2

0.000

0.0 320

270

370

420

470

520

570

620

670

Fig. 3. UV-vis DRS for Ce-doped ZnO nanostructures with various mole fractions of Ce4+ ions.

fraction of Ce4+ ions, after 2.5 h irradiation, about all of MB was decomposed, whereas about 64% was decomposed on the pure ZnO. Dependence of photocatalytic reaction rate on concentration of pollutant is generally described by the following kinetic model [25]: rate = −

kK[MB] d[MB] = 1 + K[MB] dt

(2)

where k is first-order rate constant of the reaction and K is adsorption constant of the pollutant on the photocatalyst. Also, [MB] is concentration of MB (mol/L) at any time and t is the irradiation time. Eq. (2) can be simplified to a pseudo first-order equation [25]: ln

[MB] = −kKt = −kobs t [MB]◦

(3)

in which kobs is observed first-order rate constant of the photodegradation reaction on the nanostructures. Observed first-order rate constants for photocatalytic degradation of MB on the nanostructures are calculated using plots of ln A (logarithm of absorbance) versus irradiation time. In Fig. 5, dependence of observed first-order rate constant of the reaction on doping amount is shown. With increasing mole 1.8 X = 0.000 X = 0.025

1.5

X = 0.050 X = 0.075 X = 0.100

Absorbance

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0.9

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0

50

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0

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0.04

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Mole fraction of Ce

Wavelength (nm)

250

300

350

400

Irradiation time (min) Fig. 4. Photodegradation of MB by UV irradiation on Ce-doped ZnO nanostructures with various mole fractions of Ce4+ ions.

Fig. 5. Plot of observed-first order rate constant for the photodegradation reaction on the nanostructures versus mole fractions of Ce4+ ions.

fraction of Ce4+ ions from 0 to 0.05, the reaction rate constant is increased 2.5 times. It is well known that an important method for increasing photocatalytic activity is to suppress recombination of photogenerated electrons and holes to maximize the number of photogenerated electron–hole pairs which can participate in surface chemical reactions [26]. It is proposed that by doping ZnO nanoparticles with various cations, the trapping sites of charge carriers increase with increasing the amount of dopant concentration, which prolongs the lifetime of carriers [27]. For this reason, the photocatalytic activity increases with mole fraction of Ce4+ ions. But, when the amount of Ce4+ ions is higher than the optimum amount (x = 0.05), the high concentration of dopant ions act as recombination centers of electrons and holes and hence the photocatalytic activity decreases [28]. Moreover, at higher mole fractions of Ce4+ ions, the charge carriers can be trapped more than once on their way to the surface so they recombined before they can reach the surface of the photocatalyst [29]. For these reasons, photocatalytic activity of the nanostructures with 0.05 mole fraction of Ce4+ ions is higher than the other prepared samples. To examine the influence of refluxing time applied for preparation of the nanostructures with optimum mole fraction of Ce4+ ions, five more comparative samples were prepared, keeping the reaction parameters constant except that the nanostructures were prepared by refluxing for 1.5, 4.5, 6, 9 and 15 h (Fig. 6). The results demonstrated that the nanostructures prepared by refluxing for 9 h have higher photocatalytic activity relative to the other refluxing times. In order to study the effect of calcinations temperature on the photocatalytic activity, the nanostructures with 0.05 mole fraction of Ce4+ ions prepared by refluxing for 9 h were calcined at 200, 300, 400 and 500 ◦ C for 2 h and the results are depicted in Fig. 7. It is clear that photodegradation of MB increases with calcinations temperature and then decreases. Optimum calcination temperature for the prepared nanocrystallines is 400 ◦ C. It is accepted that increasing calcination temperature causes formation of photocatalysts with high crystallinity [30]. Therefore, it is expected that photocatalysts treated at higher temperatures might display better photocatalytic activity. The decrease in the photocatalytic activity for the catalyst treated at 500 ◦ C may be due to aggregation of the nanostructures. At higher calcinations temperature, the photocatalyst loses the active surface area because of sintering and crystalline growth, therefore the photocatalytic activity decreases. Similar results previously have been reported for various photocatalysts [31].

M. Rezaei, A. Habibi-Yangjeh / Applied Surface Science 265 (2013) 591–596 1.8

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0.20 Refl. for 1.5 h Refl. for 3.0 h

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kobs (min-1)

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0.1

Irradiation time (min) Fig. 6. Photodegradation of MB on the nanostructures with 0.05 mole fraction of Ce4+ ions prepared by refluxing at various times.

Photocatalytic activity of semiconductors depends on photocatalyst weight [32]. Hence a series of experiments were carried out to find the optimum photocatalyst amount by varying weight of the nanostructures between 0.05 and 0.20 g prepared by refluxing for 9 h that calcined at 400 ◦ C. In Fig. 8, the rate constant has been plotted versus weight of the photocatalyst. As can be seen, the photodegradation rate constant increases with increasing weight of the photocatalyst up to 0.15 g and then decreases. With increasing weight of the photocatalyst, the reaction rates generally should be increased, due to the fact that active sites of the photocatalyst were increased. However, more photocatalyst would also induce greater aggregation of the photocatalyst and the specific surface area decreased, leading to a reduction in the reaction rate. Moreover, this can be attributed to the scattering of light and reduction in light penetration through the solution [29]. As an important variable in aqueous phase mediated photocatalytic reactions, effect of pH on the photocatalytic degradation reaction was studied by keeping all other experimental conditions

0.15

0.2

0.25

Catalyst weight (g)

240

Fig. 8. Plot of observed-first order rate constant of the photodegradation reaction on the nanostructures calcined at 400 ◦ C versus weight of the photocatalyst.

constant and varying the initial pH of solution from 2 to 13 (Fig. 9). As can be seen, the degradation reaction suddenly decreases in solutions with lower pH. The zero point charge pH for the photocatalyst will be about at 9 [33]. At lower pH, the photocatalyst surface is positively charged and repulsive forces between the photocatalyst and the cationic dye will lead to a decrease in the dye adsorption and the photodegradation rate constant. Furthermore, the nanostructures have tendency to dissolve with decreasing the pH of solution [34]: Ce-doped ZnO + 2H+ = Cations of Zn2+ and Ce4+ + H2 O

(4)

Hence, in acidic solutions, the photocatalyst have low stability and the photocatalytic reaction rate is decreased suddenly with pH of solution. Above zero point charge pH, the photocatalyst surface is negatively charged by means of adsorbed OH− . Then, in the alkaline solutions, electrostatic attraction between surface of the photocatalyst with negative charge and the cationic dye will lead to increasing adsorption of MB (Fig. 9). Then, optimum values for pH of solutions are between 5.4 and 9.

1.8

1.8 Room Temp. 200 ºC

1.5

300 ºC

1.5

400 ºC 500 ºC

1.2

Absorbance

Absorbance

1.2

0.9

0.9 pH = 2.0 pH = 3.5 pH=5.4

0.6

0.6

pH = 7.0 pH = 9.0 pH = 11

0.3

0 -30

-10

pH = 13

0.3

10

30

50

70

90

Irradiation time (min) Fig. 7. Photodegradation of MB on the nanostructures with 0.05 mole fraction of Ce4+ ions prepared by refluxing for 9 h calcined at various temperatures.

-30

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Irradiation time (min) Fig. 9. Photodegradation of MB on the nanostructures with 0.15 g versus solution pH.

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Acknowledgement

100

The authors wish to acknowledge University of Mohaghegh Ardabili, for financial support of this work.

80

% Degradation

References 60

40

20

0

1

2

3

4

Number of runs Fig. 10. Plot of degradation percent for photodegradation of MB on the nanostructures at optimized conditions versus number of runs.

In order to investigate reusability and stability of the photocatalyst, the photodegradation experiments were carried out on the photocatalyst in optimized conditions (mole fraction of Ce4+ ions = 0.05, calcinations temperature = 400 ◦ C, catalyst weight = 0.15 g, and pH = 5.4) and the results were demonstrated in Fig. 10. In the experiments, the photocatalysts were recycled after washing and drying at 60 ◦ C for 24 h. As can be seen, the degradation percent decreases to 85% after five runs, indicating that the photocatalytic activity has a better repeatability. Decrease of the degradation percent can be attributed to adsorption of organic intermediates and by-products of the photodegradarion reaction in the cavities and on surface of the photocatalyst that influences the surface activity of the photocatalyst. 4. Conclusion Ce-doped ZnO nanostructures (0 ≤ mole fraction of Ce4+ ≤ 0.1) were successfully prepared by a simple and green method in water at 90 ◦ C. The sharp diffraction peaks manifest that the as-prepared nanostructures have high crystallinity. With increasing mole fraction of Ce4+ ions, morphology of the nanostructures is changing from nanoplates to nanospheres. Among the prepared photocatalysts, the nanostructures with 0.05 mole fraction of Ce4+ ions have highest activity towards photodegradation of methylene blue under UV irradiation. Moreover, the photocatalyst prepared by refluxing for 9 h have higher activity relative to the other nanostructures. The photocatalytic activity increases with calcinations temperature up to 400 ◦ C and then decreases. The photocatalyst at pH between 5.4 and 9 have better activity relative to acidic and alkali solutions. The prepared photocatalyst is reusable for five numbers of runs with at least 85% of activity.

[1] O. Ozdemir, B. Armagan, M. Turan, M.S. Celik, Dyes and Pigments 62 (2004) 49. [2] S. Wang, H. Li, S. Xie, S. Liu, L. Xu, Chemosphere 65 (2006) 82. [3] P.P. Selvam, S. Preethi, P. Basakaralingam, N. Thinakaran, A. Sivasamy, S. Sivanesan, Journal of Hazardous Materials 155 (2008) 39. [4] N. Sobana, M. Muruganandam, M. Swaminathan, Catalysis Communications 9 (2008) 262. [5] R. Liu, P. Hu, S. Chen, Applied Surface Science 258 (2012) 9805. [6] W. Lv, B. Wei, L. Xu, Y. Zhao, H. Gao, J. Liu, Applied Surface Science 259 (2012) 557. [7] W. Su, J. Chen, L. Wu, X. Wang, X. Wang, X. Fu, Applied Catalysis B: Environmental 77 (2008) 264. [8] H. Xia, H. Zhuang, T. Zhang, D. Xiao, Materials Letters 62 (2008) 1126. [9] R. Kitture, S.L. Koppikar, R. Kaul-Ghanekar, S.N. Kale, Journal of Physics and Chemistry of Solids 72 (2011) 60. [10] P.V. Kamat, Journal of Physical Chemistry C 111 (2007) 2834. [11] Y. Zhang, Q. Wang, J. Xu, S. Ma, Applied Surface Science 258 (2012) 10104. [12] W. Zhang, Z. Zhong, Y. Wang, R. Xu, Journal of Physical Chemistry C 112 (2008) 17635. [13] B.G. Mishra, G.R. Rao, Bulletin of Materials Science 25 (2002) 155. [14] B.G. Mishra, G.R. Rao, Journal of Molecular Catalysis A: Chemical 243 (2006) 204. [15] C. Karunakaran, P. Gomathisankar, G. Manikandan, Materials Chemistry and Physics 123 (2010) 585. [16] D. Fangli, W. Ning, Z. Dongmei, S. Yingzhong, Journal of Rare Earths 28 (2010) 391. [17] L. Yan, L. Xiulin, L. Jiangang, Journal of Rare Earths 28 (2010) 571. [18] T.-Y. Ma, Z.-Y. Yuan, J.-L. Cao, European Journal of Inorganic Chemistry 5 (2010) 716. [19] C. Li, R. Chen, X. Zhang, S. Shu, J. Xiong, Y. Zheng, W. Dong, Materials Letters 65 (2011) 1327. [20] A. George, S.K. Sharma, S. Chawla, M.M. Malik, M.S. Qureshi, Journal of Alloys and Compounds 509 (2011) 5942. [21] Y.-L. Wang, Y.-H. Xie, Z.-K. Xue, G.-G. Liu, Journal of Synthetic Crystals 40 (2011) 917. [22] L.-A. Ying, J. Liu, L. Mo, H. Lou, X. Zheng, International Journal of Hydrogen Energy 37 (2012) 1002. [23] L.I. Berger, Semiconductor Materials, CRC, Boca Raton, 1997. [24] H. Lachheb, E. Puzenat, A. Houas, M. Ksibi, E. Elaloui, C. Guillard, J.M. Herrmann, Applied Catalysis B 39 (2002) 75. [25] M.A. Behnajady, N. Modirshahla, R. Hamzavi, Journal of Hazardous Materials B 133 (2006) 226. [26] L.G. Devi, B.N. Murthy, S.G. Kumar, Materials Science and Engineering B 166 (2010) 1. [27] R. Mishra, R.S. Yadav, S.S. Sanjay, A.C. Pandey, C. Dar, Journal of Experimental Nanoscience 5 (2010) 17. [28] S.-M. Chiu, Z.-S. Chen, K.-Y. Yang, Y.-L. Hsuc, D. Gan, Journal of Materials Processing Technology 60 (2007) 192. [29] S. Sakthivel, B. Neppolian, M.V. Shankar, B. Arabindoo, M. Palanichamy, V. Murugesan, Solar Energy Materials and Solar Cells 77 (2003) 65; N. Daneshvar, D. Salari, A.R. Khataee, Journal of Photochemistry and Photobiology A: Chemistry 157 (2003) 111. [30] M.V. Shankar, S. Anandan, N. Venkatachalam, B. Arabindoo, V. Murugesan, Journal of Chemical Technology and Biotechnology 79 (2004) 1279. [31] E. Sanatgar-Delshade, A. Habibi-Yangjeh, M. Khodadadi-Moghaddam, Monatshefte Fur Chemie 142 (2011) 119. [32] S. Sakthivel, B. Neppolian, M.V. Shankar, B. Arabindoo, M. Palanichamy, V. Murugesan, Solar Energy Materials and Solar Cells 77 (2003) 65. [33] N. Sobana, M. Swaminathan, Separation and Purification Technology 56 (2007) 101. [34] N. Daneshvar, D. Salari, A.R. Khataee, Journal of Photochemistry and Photobiology A: Chemistry 162 (2004) 317.