Cu₂O photocathodes for unassisted solar water-splitting devices enabled by noble-metal cocatalysts simultaneously as hydrogen evolution catalysts and protection layers

Here, we report a Cu₂O-based photocathode applied with pulsed-laser-deposited overlayers with the structure of Ga₂O₃/AZO/TiO₂ 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 Cu₂O, 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^-2 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 Cu₂O and Ga₂O₃ interface and surface stabilization by TiO₂ 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 BiVO₄ photoanode, which exhibits a stabilized peak photocurrent density of 0.1 mA cm^-2 under continuous illumination.