Inhibition of H-2 and O-2 Recombination: The Key to a Most Efficient Single-Atom Co-Catalyst for Photocatalytic H-2 Evolution from Plain Water

In the present work, it is shown that anodic TiO2 nanotubes (NTs) can be decorated with Pt, Pd, Rh, and Au single atoms (SAs) by a simple "dark deposition" approach. Such TiO2 NTs with surface trapped noble metal SAs provide a high activity for photocatalytic H-2 generation from pure water, i.e., in absence of a sacrificial agent. However, noble metals also act as active centers in the undesired hydrogen back-oxidation (H-2 + O-2 -> H2O), leading to a decrease in the overall photocatalytic H-2 production efficiency. Here it is reported that the use of noble metal co-catalysts, in the form of single atoms, can inhibit this recombination. From the different noble-metal SAs investigated, Pd SAs yield the highest H-2 production rate of 0.381 mu mol h(-1) cm(-1) at a density of 0.41 x 10(5) Pd atoms mu m(-2). Overall, the results provide a path to a highly efficient photocatalytic performance for water splitting by the suppression of the H-2/O-2 recombination, which can be effectively achieved using Pd in the form of SAs as photocatalytic co-catalysts.