Preparation, characterization and photocatalytic activity of nano-sized ZnO/SnO2 coupled photocatalysts

doi:10.1016/S0926-3373(02)00115-7

Applied Catalysis B: Environmental

Volume 39, Issue 3, 6 December 2002, Pages 269-279

https://doi.org/10.1016/S0926-3373(02)00115-7 Get rights and content

Abstract

The nano-sized coupled oxides ZnO/SnO₂ in a molar ratio of 2:1 (Z₂S) and 1:1 (ZS) were prepared using the co-precipitation method and characterized with X-ray diffraction (XRD), UV–VIS diffuse reflectance spectroscopy and specific surface area (Brunauer–Emmett–Teller (BET)). Their photocatalytic activities were also evaluated using methyl orange (MO) as a model organic compound. The isothermal adsorption behavior of MO on Z₂S and the factors affecting the photocatalytic activity, such as the heat-treating temperature for the photocatalyst, the pH value of the reaction suspension and the addition of NaCl, KNO₃ and K₂SO₄ into the suspension, have been studied. It is found that the change in phase of Z₂S occurs with the calcination temperature. The band gap energy value observed by UV–VIS diffuse reflectance spectroscopy is 3.15 eV for Z₂S compared with those of 3.12 eV for ZS, 3.17 eV for ZnO and 2.53 eV for SnO₂. The isothermal adsorption behavior is a two-stage process. The photocatalytic degradation rate of MO on Z₂S is faster than that on ZS and ZnO by 40.2 and 66.1%, respectively. A schematic diagram of photocatalytic activity is also presented to explain the results.

Introduction

The degradation of organic pollutants in water and air by photocatalysis, using semiconductors, such as TiO₂ and ZnO, has attracted extensive attention during recent 20 years [1]. Previous studies have proved that such semiconductors can degrade most kinds of persistent organic pollutants, such as detergents, dyes, pesticides and volatile organic compounds, under UV-light irradiation [1], [2], [3], [4], [5]. However, the fast recombination rate of photogenerated electron/hole pairs hinders the commercialization of this technology [1]. It is of great interest to improve the photocatalytic activity of semiconductors for the degradation of organic compounds in water and air. In the past several years, there are a number of studies related to the photocatalytic activity of TiO₂ or ZnO coupled with metal oxide, like SnO₂ [6], [7], [8], [9], [10], [11], WO₃ [12], [13], [14], [15], [16], Fe₂O₃ [17], [18], ZrO₂ [19] and some rare earth oxides [20], [21], for the purpose of improving TiO₂ or ZnO photocatalytic activity. Coupled semiconductor photocatalysts may increase the photocatalytic efficiency by increasing the charge separation and extending the energy range of photoexcitation. At the same time, their physical and optical properties are greatly modified [16].

Recently, Tennakone and Bandara demonstrated that, when SnO₂ nano-crystallites (∼ 10–15 nm) are ground with ZnO powders (∼ 600 nm) in the ZnO content of 54 and 40% by weight, respectively, the coupled oxide catalysts ZnO/SnO₂ can generate hydrogen from water in the presence of a sensitizer (ruthenium bipyridyl complex) and a hole scavenger (i.e. ethanol) under visible light irradiation [6]. These results can be explained as originating from the transfer of energetic electrons (‘hot electrons’) injected to SnO₂ via dye-sensitization to ZnO, whose conduction band (CB) position is above that of SnO₂, leading to a wide separation of the electron and the hole.

The photocatalytic activity of coupled oxide photocatalyst is closely related to the ratio of the two oxides [15], [19]. The aim of the present study is to study the photocatalytic activity of Z₂S and ZS, which can be explained by a scheme illustrating the photocatalytic activity. For this purpose, the nano-sized coupled oxides Z₂S and ZS were prepared and their photocatalytic activities were evaluated using MO [22] as a model organic compound. The adsorption behavior of MO on Z₂S and the factors affecting the photocatalytic activity, such as the heat-treating temperature for Z₂S, the pH value of the reaction suspension as well as the concentration of NaCl, KNO₃ and K₂SO₄ in the suspension, have also been examined. It is found that the coupled oxide photocatalyst Z₂S has better photocatalytic activity to MO than ZS and ZnO.

Section snippets

Preparation for nano-sized photocatalysts

Nano-sized coupled oxide photocatalyst Z₂S was prepared with the co-precipitation method. SnCl₄·5H₂O and ZnSO₄·7H₂O (analytic reagent grade) were used as the starting materials and NaOH as the co-precipitant without further purification. ZnSO₄·7H₂O and SnCl₄·5H₂O in a molar ratio of 2:1 were dissolved in a minimum amount of deionized water. Then the 4 mol/l of NaOH solution was added to the above solution to adjust pH to about 7 and a white amorphous precipitate was formed. The precipitate was

Phase and mean size of the photocatalysts

The XRD patterns of the Z₂S powders calcined at different temperatures and for different times are as shown in Fig. 1(1) and (2), respectively. The phases of the photocatalysts calcined at 500 and 600 °C are the mixture of ZnO and SnO₂, but the grain sizes get larger with the increasing calcination temperature. The Zn₂SnO₄ phase emerges at 700 °C and ZnO and SnO₂ almost disappear at 900 °C. For the coupled oxide calcined at a fixed temperature of 600 °C, Fig. 1(2) shows that there are no changes in

Conclusions

1.
The nano-sized photocatalysts Z₂S and ZS can be synthesized by using the co-precipitation method. With the increase in calcination temperature, not only the mean grain sizes of Z₂S grow, but also the change in phase of the photocatalyst occurs.
2.
Z₂S exhibits better photocatalytic activity to MO than ZS and ZnO. The photocatalytic reaction by Z₂S is 40.2 and 66.1% faster than those by ZS and ZnO, respectively. A mechanistic scheme for photocatalytic activity is presented to explain the

Acknowledgements

The authors wish to thank Profs. H. Wang, Y.S. Shen and D.K. Peng of Department of Materials Science & Engineering of University of Science & Technology of China and Dr. S. Wen, Profs. G.X. Wang, J.Q. Lei, F.Y. Wang and G.X. Chen of Guangzhou Institute of Geochemistry for helps in this study. Support from the Natural Science Foundation of Guangdong province, China (teamwork project “Environmental fate and control technology of the chemicals with adverse health effects in the Pearl River Delta”)

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Preparation, characterization and photocatalytic activity of nano-sized ZnO/SnO2 coupled photocatalysts

Abstract

Introduction

Section snippets

Preparation for nano-sized photocatalysts

Phase and mean size of the photocatalysts

Conclusions

Acknowledgements

J. Mol. Catal. Part A. Chem.

Chemosphere

Appl. Catal. Part A. Gen.

J. Catal.

Mater. Chem. Phys.

J. Photochem. Photobiol. Part A. Chem.

J. Solid State Chem.

J. Photochem. Photobiol. Part A. Chem.

Mater. Chem. Phys.

J. Mol. Catal. Part A. Chem.

J. Mol. Catal. Part A. Chem.

J. Photochem. Photobiol. Part A. Chem.

Mater. Chem. Phys.

Sens. Actuators B

Mater. Chem. Phys.

Sens. Actuators B

J. Photochem. Photobiol. Part A. Chem.

J. Mater. Sci.

NanoStruct. Mater.

Chemosphere

Appl. Catal. Part A. Gen.

J. Photochem. Photobiol. Part A. Chem.

J. Photochem. Photobiol. Part A. Chem.

Catal. Today

Solar Energy

Water Res.

Appl. Catal. Part B. Environ.

Appl. Catal. Part B. Environ.

Appl. Catal. Part B. Environ.

Preparation, characterization and photocatalytic activity of nano-sized ZnO/SnO₂ coupled photocatalysts