Protected ultrathin cuprous oxide film for photocatalysis: Excitation and relaxation dynamics

Abstract

In the search for cost-effective ways to produce green hydrogen, harnessing the most abundant renewable energy source---sunlight---directly through photoelectrochemical water splitting is highly desirable. Cuprous oxide (${\mathrm{Cu}}_{2}\mathrm{O}$) has proven itself as a material with great potential for the hydrogen evolution reaction (HER) and stands out by its Earth abundance and low processing cost. In this photoelectron spectroscopy study we investigated the electron dynamics in a heterostructure comprised of a ${\mathrm{Cu}}_{2}\mathrm{O}$ surface oxide grown on Cu(111) underneath a hexagonal boron nitride ($h$-BN) film, which replaces a conventional capping layer for corrosion protection. Our results show that it can be a viable cuprous oxide-based photoelectrode material. The $h$-BN film stays intact during the oxidation, and the oxide layer forms an ordered structure with defined valence and conduction bands. The conduction band is at a suitable energy to drive the HER for water splitting, but the photoexcited electrons display short lifetimes. Electrons are mainly excited in the copper substrate and are then captured in long-lived defect states in the ${\mathrm{Cu}}_{2}\mathrm{O}$ layer. The water splitting efficiency of this photocathode may still be improved by reducing the defect density.