CuO photocathodes for unassisted solar water-splitting devices enabled by noble-metal cocatalysts simultaneously as hydrogen evolution catalysts and protection layers.

Here, we report a CuO-based photocathode applied with pulsed-laser-deposited overlayers with the structure of GaO/AZO/TiO and 60 nm dense Pt simultaneously as a hydrogen evolution catalyst and protective layer under backside illumination from ITO substrate. And by depositing thin Au nanoparticles on the bottom of CuO, we observe the increase of light harvesting and the production of hot electrons from the surface plasmon resonance effect. The modified photocathode presents a photocurrent density of -4 mA cm at 0 V versus reversible hydrogen electrode and a thermodynamically-based energy conversion efficiency of 0.27% in a weak acidic electrolyte. The stability is tremendously improved compared with other structures without the dense Pt layer. This enhancement is attributed to the formation of a p-n junction at the CuO and GaO interface and surface stabilization by TiO and Pt. To demonstrate the impact on the practical application of the photocathode, we fabricate the bias-free solar water-splitting tandem device using a transparent BiVO photoanode, which exhibits a stabilized peak photocurrent density of 0.1 mA cm under continuous illumination.