Optimization of 1D ZnO@TiO₂ Core–Shell Nanostructures for Enhanced Photoelectrochemical Water Splitting under Solar Light Illumination

A fast and low-cost sol–gel synthesis used to deposit a shell of TiO₂ anatase onto an array of vertically aligned ZnO nanowires (NWs) is reported in this paper. The influence of the annealing atmosphere (air or N₂) and of the NWs preannealing process, before TiO₂ deposition, on both the physicochemical characteristics and photoelectrochemical (PEC) performance of the resulting heterostructure, was studied. The efficient application of the ZnO@TiO₂ core–shells for the PEC water-splitting reaction, under simulated solar light illumination (AM 1.5G) solar light illumination in basic media, is here reported for the first time. This application has had a dual function: to enhance the photoactivity of pristine ZnO NWs and to increase the photodegradation stability, because of the protective role of the TiO₂ shell. It was found that an air treatment induces a better charge separation and a lower carrier recombination, which in turn are responsible for an improvement in the PEC performance with respect to N₂-treated core–shell materials. Finally, a photocurrent of 0.40 mA/cm² at 1.23 V versus RHE (2.2 times with respect to the pristine ZnO NWs) was obtained. This achievement can be regarded as a valuable result, considering similar nanostructured electrodes reported in the literature for this application