Photoelectrochemical water oxidation at FTO|WO3@CuWO4 and FTO|WO3@CuWO4|BiVO4 heterojunction systems: An IMPS analysis

Small perturbation techniques such as intensity-modulated photocurrent spectroscopy (IMPS) have become an essential tool to unravel the complex, interrelated processes that govern the charge carrier dynamics in photoelectrochemically active materials for solar water splitting. We have fabricated CuWO4-based photoelectrodes by chemical modification of a WO3 nanorod array as a partially sacrificial template. The electrodes have been characterized by photoelectrochemical techniques including IMPS as a function of the annealing temperature, transforming WO3 either partially or completely to CuWO4. The optical properties illustrate the transformation with the absorbance onset moving from the typical onset for WO3 (about 2.5 eV) to that of CuWO4 (about 2.1 eV). In pure CuWO4 photoelectrodes bulk recombination dominates the photoelectrochemical performance, while for FTO|WO3@CuWO4 heterojunction photoelectrodes much larger charge separation and external quantum efficiencies are obtained. In addition, CuWO4 serves as a protective layer for the WO3 material that is not generally stable in neutral aqueous solutions. The FTO|WO3@CuWO4 heterojunction material was further explored as an underlayer substrate, using a thin BiVO4 film as overlayer. The advantages of this configuration include improved light harvesting as BiVO4 is a direct semiconductor with a much larger absorption coefficient than CuWO4, which is characterized by an indirect gap. It is found that the FTO|WO3@CuWO4 underlayer efficiently extracts the photogenerated electrons from the BiVO4 overlayer, hence, the inclusion of a second heterojunction plays an essential role in improving the charge separation and internal quantum efficiency, minimizing both bulk and surface recombination.