Improvement of photocatalytic and photoelectrochemical activity of ZnO/TiO2 core/shell system through additional calcination: Insight into the mechanism

ZnO/TiO2 composites were prepared by sol-gel deposition of TiO2 on ZnO nanorods hydrothermally grown on electrically conductive indium tin oxide substrate (ITO). It has been shown that the ZnO/TiO2 interface plays a key role in enhancement of photodecomposition of methylene blue (MB) used as a model test pollutant, under monochromatic light irradiation (400 nm). The increase of photocatalytic activity was attributed to the shift of absorption edge of ZnO/TiO2 towards visible light in comparison with bare TiO2. Further enhancement of photocatalytic activity of ZnO/TiO2 was achieved through its additional calcination at 450 °C for 3 h. This treatment brings 40% increase in the rate of MB decomposition and a two-fold rise of the photocurrent in H2O oxidation. Measurements of open circuit potential (Voc) showed that the improved properties of additionally calcined ZnO/TiO2 composites stem from decrease of the electron-hole recombination rate. Scanning transmission electron microscopy (STEM) studies showed that the additional calcination resulted in formation of voids at the ZnO/TiO2 interface. Energy dispersive X-ray (EDX) and X-ray photoelectron (XPS) spectroscopies proved that formation of voids is accompanied by the outward diffusion of Zn ions into TiO2 layer and allowed to conclude about the existence of the Kirkendall effect at ZnO/TiO2 interface. Occurrence of this effect observed for the first time at such moderate temperature (450 °C) is attributed to a highly defective nature of the surface layer of the ZnO nanorods.