Preparation and Photocatalytic Activity of ZnO-SnO2, ZnO-ZnWO4 and SnO2-ZnO-ZnWO4 Nanocomposites

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.