Temperature-controlled and shape-dependent ZnO/TiO2 heterojunction for photocathodic protection of nickel-coated magnesium alloys

Using nanomaterials to design and prepare photoanodes could enhance photoelectrochemical and photocathodic protection (PCP) performances. However, little is known about how the detailed nanostructure correlates with performance. Here, we report a morphologically tunable nano-ZnO semiconductor grown on a TiO2 nanotube array (ZnO/TiO2) for the PCP of nickel-coated magnesium alloy by manipulating the hydrothermal reaction temperature. While irregular ZnO nanosheet clusters were observed at relatively low and high temperatures, a smooth array of nanorods with a hexagonal cross-section structure was obtained at a moderate reaction temperature. Such nanostructure exhibited the optimum photoelectrochemical and PCP capacities, showing the highest photogenerated current density, which is 4.2 times higher than the single-component TiO2 photoanode. The apparent enhancement in the performance of the ZnO/TiO2 composite was related to the morphological regulation of the nano-ZnO and the formation of staggered gap heterojunction between ZnO and TiO2. This assessment is supported by the results from several characterizations, including a high-resolution transmission electron microscope, Mott-Schottky, and valence band X-ray photoelectron spectroscopy. These findings give a new insight into the relationship between the structure and photoelectrochemical performances of nano-ZnO, which has significant implications for designing and preparing high-efficiency photoanode in photocatalysis and corrosion protection.