ZnO@TiO2 core–shell nanorod arrays with enhanced photoelectrochemical performance

Abstract

ZnO@TiO2 core–shell nanorod arrays (NRAs) were fabricated by radio frequency magnetron sputtering of a Ti layer on ZnO NRAs followed by post-oxidation treatment. Both the photoluminescence spectra and open circuit voltage decay measurement show that the thin TiO2 shell can effectively passivate the surface trap states of ZnO NRAs. Compared with the bare ZnO, ZnO@TiO2 core–shell nanostructure sensitized with CdS quantum dots (QDs) using as the photoelectrodes for photoelectrochemical (PEC) cells shows enhanced PEC performance. It is supposed that this enhanced PEC performance is attributed primarily to two effects. On one hand, the TiO2 shell layer suppresses the recombination of photogenerated carriers by passivating the surface recombination sites in ZnO NRAs and forming an energy barrier between the ZnO core and TiO2 shell. On the other hand, the rough surface of the TiO2 shell on ZnO NRAs facilitates the deposition of CdS QDs, which contributes to enhance light absorption in visible light region. However, the experiment results suggest that an optimal thickness of about 15nm for the TiO2 (or pre-deposited Ti) layer is highly desired, the further incremental thickness of the layer will restrain the PEC performance of photoelectrode due to several shortages, such as reduced light harvesting due to the shadow effect of the incomplete oxidized Ti layer, the introduction of additional leakage current, and the blocking effect for the penetration of the redox couples ions (S2−/Sn2−) into ZnO NRAs.