SnO nanolayer-protected CuO nanowires as an efficient and stable photocathode for solar water splitting.

The poor stability of CuO is a major obstacle to its widespread use as a photocathode for the hydrogen evolution reaction (HER), highlighting the urgent need for a facile and reproducible protection strategy. In this work, we present a simple colloid-based method to deposit a uniform SnO overlayer onto CuO nanowires (NWs) grown on a porous copper foam (CF), forming the CF/CuO@SnO composite photocathode. The SnO nanolayer composed of densely packed, single-crystalline nanoparticles exhibits an ultrathin thickness of 5-10 nm, along with excellent transparency, conductivity, and chemical stability. Following further decoration with Pt nanoparticles, the resulting CF/CuO@SnO/Pt photocathode delivers an impressive photocurrent density of 3.64 mA cm at 0 V vs. RHE and retains 74.1 % of its initial activity after 90 min of continuous illumination in a neutral electrolyte. The Faradaic efficiency for HER reaches 67.4 %, nearly five times higher than that of pristine CuO, underscoring the enhanced photostability and high solar-to-hydrogen conversion efficiency of the composite photocathode. This superior photocatalytic performance can be attributed to two key functions of the SnO overlayer: (1) acting as a transparent, conductive protection layer to inhibit CuO photocorrosion, and (2) forming a p-n heterojunction that improves charge-carrier transport. To the best of our knowledge, this is the first study to demonstrate a simple colloid-based fabrication of an SnO layer to suppress CuO photocorrosion, offering broad applicability for mitigating the photostability challenges of other unstable photoelectrodes.