Photocatalytic activity of ZnO thin films prepared by two-step thermal oxidation of Zn films

ZnO thin films were prepared by two-step thermal oxidation of dc-magnetron sputtered Zn films on glass substrates. The structural and optical properties of ZnO films were investigated by X-ray diffraction (XRD), Scanning electron microscopy (SEM) and optical transmittance. XRD reveals that Zn films completely convert to ZnO films after annealing in oxygen at 550 degrees C for 2 h. SEM shows that floccule-like ZnO nano-wires form on the surface of the films when annealing temperatures exceed 500 degrees C. The average diameter of the nano-wire is about 20 nm. The optical transmittance measurements show that ZnO films are highly transparent in the visible region and the absorption edge blueshifts with increasing oxidation temperature from 450-550 degrees C. The optical bandgap at 550 degrees C were calculated to be 3.27 eV. Photocatalytic decomposition results show that the decomposition rate of phenol aqueous solution on ZnO films increases with increasing oxidation temperature. Maximum photodecomposition of 72.6% is obtained with 10 mg/l phenol concentration for 6 hours.