Self-Organisation of Copper Species at the Surface of Cu–TiO2 Systems During H2 Evolution Reaction: A Combined Investigation by EPR and Optical Spectroscopy

High surface area TiO2 was synthetized by means of a solvothermal method and it was then used to prepare two photocatalysts based on copper-modified TiO2. Two different preparation approaches have been adopted based on the use of the same amount of Cu2O (0.5% w/w) as a reactant. The spectroscopic characterization shows that the two preparation procedures lead to distinct distributions of Cu2O and other Cu-based species in the final composite materials and that the photoactivity of the solids is strictly related to the nature of the copper species. The joint application of CW-EPR and optical spectroscopy, both employed in catalytic conditions as a sort of “operando” mode, allowed monitoring the evolution of the photocatalytic systems occurring during the H2 evolution reaction (HER). The effect of water coordination on the Cu(II) species at the surface of titania has been revealed by EPR. The successive photoreduction of the system in the early steps of the photocatalytic reaction involves the reduction of Cu2+, the formation of Ti3+ centres in the oxide matrix and that of metallic copper particles that partially inactivate the system.