Advanced Materials Research Vol. 584 (2012) pp 381-385 Online available since 2012/Oct/22 at www.scientific.net © (2012) Trans Tech Publications, Switzerland doi:10.4028/www.scientific.net/AMR.584.381
Photocatalytic Degradation of Organic Dyes Using ZnO/CeO2 Nanocomposite Material Under Visible Light R. Saravanan1,a, N. Karthikeyan1,b, S. Govindan1,c, V. Narayanan2,d and A. Stephen1,e 1
Department of Nuclear Physics, University of Madras, Guindy Campus, Chennai 600 025, India 2
Department of Inorganic Chemistry, University of Madras, Guindy Campus, Chennai 600 025, India
a
[email protected],
[email protected],
[email protected], d
[email protected],
[email protected] (corresponding author)
Keywords: Photocatalyst; Nanorods; Visible light.
Abstract. Nanorods of ZnO/CeO2 were synthesized by thermal decomposition method. The decomposition temperature and formation of composite material were confirmed by the thermogravimetric analysis (TGA) before the synthesis process. The prepared samples were characterized by different techniques. The structural and morphological properties of ZnO/CeO 2 nanorods were confirmed by X-ray diffraction (XRD) and high resolution scanning electron microscopy (HR-SEM). The chemical composition and specific surface area analysis were done by energy dispersive X-ray spectroscopy (EDX) and Brunauer–Emmett–Teller (BET) method. Further the catalyst was used for photocatalytic degradation of organic dye under visible light irradiation. Introduction In textile industries, azodyes such as methylene blue and methyl orange are used for dyeing purposes. These azodyes are found to have great hazardous effects to the human health and environment. One of the best ways to reduce contamination of water is by photocatalytic treatment. ZnO and TiO2 are mainly used for this application. Compared with TiO2, ZnO has higher catalytic activity and is also not expensive [1-3]. The major disadvantage of ZnO, is its large bandgap (3.2 eV), due to which it is sensitive only in UV light. The visible light photocatalytic activity of ZnO has been improved by various techniques like non-metal doping [4], metal doping [5] and composite materials [6]. The composite material prevents the electron-hole recombination, thus improving the photodegradation rate[7-8]. In this study ZnO and ZnO/CeO2 were prepared by thermal decomposition method. The prepared samples were characterized by various techniques and the results are discussed in detail. Experimental Zinc acetate dihydrate and cerium acetate used in the present study were of analytical reagent grade and used as received without further purification. Methylene blue (MB) was purchased from Aldrich chemicals. All aqueous solutions were prepared using double distilled water. The decomposition temperature and formation of composite material were confirmed by the TGA before the synthesis process and the ZnO was synthesized by thermal decomposition method as reported elsewhere [9]. The ZnO/CeO2 nanorod was also synthesized by the same method. Zinc acetate dihydrate and cerium acetate (90:10 weight ratio) were mixed, ground for one hour and annealed at 350 C for 3 hours. Measurment of photocatalytic activity The visible light photocatalytic activity procedure was followed as per our previous report [9]. The photocatalytic activity was estimated by measuring the decomposition rate of MB aqueous solution under visible light irradiation. The visible light irradiation was carried out using a projection lamp All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP, www.ttp.net. (ID: 14.139.186.162-22/11/12,12:15:29)
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(7748XHP 250 W, Philips, 532nm) in a photoreactor and an acetone jacket was used for the purpose of UV cut-off. Reaction suspensions were prepared by adding the required amount of photocatalyst into 500 ml of MB solution taken with an initial concentration of 3x10-5 moles/liter. The aqueous suspension containing MB and the photocatalyst was irradiated under constant stirring. The analytical samples from the suspension were collected at regular intervals of time, centrifuged and filtered. The concentration of MB in each sample was analyzed using UV-visible spectrophotometer at a wavelength of 664 nm. Result and discussion: The XRD peaks shown in figure 1 (a) indicate hexagonal wurtzite structure of ZnO and the lattice constant values are shown in table 1. These lattice constant matches well with the ICDD card no: 79-0208.
Fig 1. The XRD pattern of (a) ZnO, (b) CeO2 and (c) ZnO/CeO2 For the comparison purpose we prepared CeO2 by the same method at the same temperature and the XRD result is shown in figure 1 (b). The characteristic peaks of the prepared CeO2 were indexed to cubic structure and the lattice constant values are similar to JCPDS file no: 65-2975. Table 1: Lattice parameters and crystallite size values for all prepared samples ZnO CeO2 ZnO/CeO2 (Hexagonal) (Cubic) (Hexagonal & Cubic) JCPDS no: 79-0208 JCPDS no: 65-2975 Crystallite size – ZnO: 29 nm Crystallite size: 35 nm Crystallite size: 9 nm Crystallite size – CeO2: 6 nm Lattice parameter[Å] Lattice parameter[Å] Lattice parameter[Å] ZnO CeO2 a c a a c a 3. 26 5. 21 5.43 3.27 5.21 5.41 After ZnO is coupled with CeO2, two phases were detected (Figure 1 (c)). One of Hexagonal structure of ZnO and another of cubic structure of CeO2. The crystallite sizes (shown in table 1) of the samples were calculated using the scherrer equation. No other impurity peaks were detected in the XRD pattern. Hence the XRD result confirmed the formation ZnO/CeO2 mixture. The HR-SEM image of ZnO powder is shown in figure 2(a). The sample obtained by thermal decomposition method was found to have irregular nanorods with an average diameter of ~35 nm and lengths in few hundred nanometers. Thus, HR-SEM image evidently proved the formation of ZnO nanorods. Figure 2 (b) revealed agglomeration of spherical shaped CeO2 nanoparticles. The
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HR-SEM image of coupled material (ZnO/CeO2) is shown in figure 2(c), and the image indicates irregular rods along with spherical shape particles Thus, addition of CeO2 may influence the size and morphology of ZnO due to the synergetic growth.
Fig. 2. The HR-SEM image and the corresponding EDX result of (a) ZnO (b) CeO2 and (c) ZnO/CeO2 Energy dispersive X-ray analysis was carried out to determine the chemical compositions of the materials. It can be seen in the figure 2 a (left) that only the oxygen and the zinc are detected. Figure 2 b shows only the presence of cerium and oxygen. The peaks of Zn, Ce and O are clearly seen in figure 2 (c), which indicates that ZnO/CeO2 nanocomposites contain Zn, Ce and O without any impurities.
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The BET specific surface area of ZnO/CeO2 (13.23 m2/g) sample is higher than ZnO (8.67 m2/g) due to the lower particle size, which was confirmed through HR-SEM and XRD. The photocatalytic activity is dependent on the exposed surface area of the materials. The photocatalytic redox reaction takes place mainly on the surface of the photocatalysts, and therefore the surface properties significantly influence the efficiency of photocatalytic activity [10-11]. Hence the BET results suggest higher photocatalytic activity with ZnO/CeO2 nanocomposites than that of the ZnO nanorods. The Photocatalytic activity of ZnO and ZnO/CeO2 catalysts were measured interms of MB (methylene blue) decoloration. The solution of MB was irradiated by visible light for various time durations to evaluate the photocatalytic performance of ZnO and ZnO/CeO2 nanorods. Figure 3 (a) shows the decoloration of MB by ZnO/CeO2 catalyst under visible light irradiation. Degradation takes place only when ZnO/CeO2 is used and there is no decoloration for ZnO (as shown in figure 3 (b)) because ZnO has large bandgap and is insentive to visible light. These efficiency values are higher than the previous report[12-13] .The nanocomposite systems containing CeO2 nanoparticles, due to their lower band gap energy, show better photoactivity under visible light when compared to pure ZnO. Because of this relatively narrow bandgap energy, they can be excited by visible illumination; the enhancement was found due to the presence of CeO2 nanoparticles which inhibit the charge carrier recombination by electron capture resulting in more holes formation to produce hydroxyl radicals leading to increase in the rate of photodegradation reaction. The key factor for the enhancement of photocatalytic activity of the composite oxides is the dispersion of ceria as nanosize crystallites over the ZnO surface [9-15]. At the same time, the ZnO/CeO2 photocatalyst shows higher surface area compared with that of ZnO, which might also be beneficial for the improvement of the photocatalytic performance of the ZnO/CeO2 composite materials.
Fig. 3. (a) Time depentdent optical absorption spectra of MB( ZnO/CeO2) used as a catalyst and (b) The photocatalytic degradation of MB under visible light illumination Conclusion ZnO and ZnO/CeO2 were prepared by thermal decomposition method. Compared to other methods thermal decomposition is simple, fast and a low cost method. The HR-SEM image confirms the formation of nanorods and the structure of the composite materials were identified by XRD. The BET results suggested that the ZnO/CeO2 nanocomposites have higher surface area compared with ZnO. The composite material (ZnO/CeO2) exhibited higher efficiency in the degradation of methylene blue under visible light. Acknowledgment: One of the author (RS) is grateful to the University of Madras for the financial assistance in the form of University Research Fellowship.
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References: [1] [2] [3] [4] [5] [6]
[7] [8] [9] [10] [11]
[12]
[13] [14] [15]
H. C. Akyol, M. Yatmaz, Bayramoglu, Photocatalytic decolorization of Remazol Red RR in aqueous ZnO suspensions, Appl. Catal. B: Environ. 54 (2004) 19-24. J. Lizama, J. Freer, H.D. Baeza, Mansilla, Optimized photodegradation of Reactive Blue 19 on TiO2 and ZnO suspensions, Catal. Today. 76 (2002) 235-246. M.R. Hoffmann, S.T. Martin, W.Y. Choi, D.W. Bahnemann, Environmental applications of semiconductor photocatalysis, Chem. Rev. 95 (1995) 69-96. C. Shifu, Z. Wei, Z. Sujuan, L. Wei, Preparation, characterization and photocatalytic activity of N-containing ZnO powder, Chem. Eng. J. 148 (2009) 263-269. K.G. Kanade, et. al. Self-assembled aligned Cu doped ZnO nanoparticles for photocatalytic hydrogen production under visible light irradiation, Mater. Chem. Phys. 102 (2007) 98-104. K.R. Gopidas, M. Bohorquez, P.V. Kamat, Photophysical and photochemical aspects of coupled semiconductors: charge-transfer processes in colloidal cadmium sulfide-titania and cadmium sulfide-silver(I) iodide systems, J. Phys. Chem. 94 (1990) 6435-6440. L. Zheng, et. al. Network Structured SnO2/ZnO Heterojunction Nanocatalyst with High Photocatalytic Activity. Inorg. Chem. 48 (2009) 1819-1825. G. Li, et. al.Article Role of Surface/Interfacial Cu2+ Sites in the Photocatalytic Activity of Coupled CuO−TiO2 Nanocomposites. J. Phys. Chem. C 112(2008) 19040-19044. R. Saravanan, et. al. ZnO/CdO composite nanorods for photocatalytic degradation of methylene blue under visible light , Mater. Chem. Phys. 125 (2011) 277-280. R. Georgekutty, M. K. Seery, S. C. Pillai, A highly efficient Ag-ZnO photocatalyst: synthesis, properties and mechanism, J. Phys. Chem. C. 112 (2008) 13563-13570. X. Yang, Y. Wang, L. Xu, X. Yu, Y. Guo, Silver and indium oxide codoped TiO2 nanocomposites with enhanced photocatalytic activity, J. Phys. Chem. C .112 (2008) 1148111489. B. G. Mishra, G. Ranga Rao, Promoting effect of ceria on the physicochemical and catalytic properties of CeO2–ZnO composite oxide catalysts, Journal of Molecular Catalysis A: Chemical 243 (2006) 204–213. M. Faisal, et. al. Role of ZnO-CeO2 Nanostructures as a Photo-catalyst and Chemi-sensor, J. Mater. Sci. Technol., 27(7) (2011) 594-600. J.M. Hermann, J. Disdier, P. J. Pichat, Photoassisted platinum deposition on TiO2 powder using various platinum complexes, Phys. Chem. 90 (1986) 6028-6034. A. Sclafani,J. M. Hermann, Influence of metallic silver and of platinum-silver bimetallic deposits on the photocatalytic activity of titania (anatase and rutile) in organic and aqueous media, J. Photochem. Photobiol. A. 113(1998) 181-188.
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Photocatalytic Degradation of Organic Dyes Using ZnO/CeO2 Nanocomposite Material under Visible Light 10.4028/www.scientific.net/AMR.584.381
