Exploring Cu oxidation state on TiO₂ and its transformation during photocatalytic hydrogen evolution

In-depth characteristic studies with H₂ activity and theoretical calculations were used to reveal the copper oxidation states most effective for photocatalytic hydrogen production when loaded on TiO₂. When the copper was originally present as CuO, photogenerated electrons initially reduced the Cu²⁺ to Cu⁺ in preference to proton reduction. The resulting Cu₂O then behaved as a secondary photocatalyst on the TiO₂ surface acting to improve the hydrogen production rate (1.4 times greater than neat TiO₂). When the copper was originally present as Cu⁰, an improved hydrogen generation rate was also evident (2.4 times greater than Cu₂O/TiO₂) and the metallic state was retained over the course of the reaction. In this case, the Cu⁰ deposits function as a co-catalyst for proton reduction. The findings reconcile past disagreements associated with this system, demonstrating both Cu⁺ (following reduction from Cu²⁺) and Cu⁰ are able to photocatalytically generate hydrogen, albeit by distinctly different mechanisms.