Stabilized Solar Hydrogen Production with CuO/CdS Heterojunction Thin Film Photocathodes

Cupric oxide (CuO) is a promising material for large-scale, economic solar energy conversion due to the abundance of copper, suitable band gap, and ease of fabrication. For application as a photocathode for water splitting, the main challenge is prevention of the inherent photocorrosion in aqueous media. Photoelectrochemical measurements of bare CuO thin films prepared by oxidation of electroplated Cu indicated that the vast majority of the photocurrent in 1 M phosphate buffer solution (pH 7) comes from photocorrosion of the CuO into metallic Cu, with a faradaic efficiency for hydrogen evolution of ∼0.01%. We found that deposition of an n-type CdS buffer layer underneath a protective TiO<sub>2</sub> layer yielded a stable and efficient photoelectrode, with the champion electrode giving 1.68 mA cm<sup>–2</sup> at 0 V<sub>RHE</sub> and an onset potential of ca. 0.45 V<sub>RHE</sub>. We attribute a favorable band alignment of CuO/CdS for the record photovoltage obtained with this material and a high conformality of the TiO<sub>2</sub> layer on the sulfide surface for the high stability of hydrogen-producing photocurrents (faradaic efficiency ∼100%).