Photoelectrochemical water oxidation at FTO|WO ₃ @CuWO ₄ and FTO|WO ₃ @CuWO ₄ |BiVO ₄ 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 CuWO ₄ -based photoelectrodes by chemical modification of a WO ₃ 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 WO ₃ either partially or completely to CuWO ₄ . The optical properties illustrate the transformation with the absorbance onset moving from the typical onset for WO ₃ (about 2.5 eV) to that of CuWO ₄ (about 2.1 eV). In pure CuWO ₄ photoelectrodes bulk recombination dominates the photoelectrochemical performance, while for FTO|WO ₃ @CuWO ₄ heterojunction photoelectrodes much larger charge separation and external quantum efficiencies are obtained. In addition, CuWO ₄ serves as a protective layer for the WO ₃ material that is not generally stable in neutral aqueous solutions. The FTO|WO ₃ @CuWO ₄ heterojunction material was further explored as an underlayer substrate, using a thin BiVO ₄ film as overlayer. The advantages of this configuration include improved light harvesting as BiVO ₄ is a direct semiconductor with a much larger absorption coefficient than CuWO ₄ , which is characterized by an indirect gap. It is found that the FTO|WO ₃ @CuWO ₄ underlayer efficiently extracts the photogenerated electrons from the BiVO ₄ 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.