Charge transfer processes involved in photocatalytic hydrogen production over CuO/ZrO₂–TiO₂ materials

Co-catalysts are widely employed to boost the photocatalytic hydrogen production; particularly, CuO has shown a remarkable improvement in the reaction rate. However, the impact of CuO on the charge transfer process during photocatalytic water reduction has been barely investigated. In this work, ZrO₂–TiO₂ (ZT) heterojunctions (5 mol% ZrO₂) have been obtained by means of the sol–gel method. Subsequently, copper nitrate impregnations are prepared by an incipient wetness impregnation method to get 0.5, 1, 3 and 5 wt% CuO loadings. A maximum in the photocatalytic activity is observed for the material containing 1 wt% CuO, followed by a drastic drop in the hydrogen generation rate. Electrochemical characterization shows that the charge-transfer resistance controls the photocatalytic experiments together with the additional interfacial resistance. The maximum photocatalytic activity is then given by a compromise between these two parameters. A further increase of the additional resistance, directly proportional to the CuO loading, reduces drastically the photocatalytic behavior most likely due to the electron trapping at the ZT–CuO interface.