Nanosheets doped with isolated single-atom reaction sites for hydrogen evolution are a potential means of suppressing recombination during photocatalytic water splitting. Oxide nanosheets show higher activity during photocatalytic hydrogen evolution from CH 3 OH/water after doping with singleatom reaction sites in the lattice. However, water splitting into H 2 and O 2 from pure water has not yet been achieved on such materials because backward reactions, such as oxygen reduction, are also accelerated at these same reaction sites. In the present study, calcium niobate nanosheets incorporating Rh single-atom reaction sites covered with ultrathin NiO x atomic layers are fabricated. The nanosheets having a Ni/Rh atomic ratio of 7/1 are found to promote evolution of H 2 and O 2 from water. In this system, the ultrathin NiO x atomic layers prevent oxygen penetration but allow H + to access the Rh reaction sites, thus promoting photocatalytic water splitting for H 2 and O 2 production. This is the first demonstration of perfect water splitting using a nanosheet photocatalyst with single-atom reaction sites.

The bandgap energy of such metal oxides can be reduced by replacing oxygen with nitrogen. In the cases of oxynitrides and nitrogen doped (N−doped) oxide, the p−orbital of the N 3− anion hybridizes with the p−orbital of the O 2− anion, contributing to the formation of the valence band maximum. N−doped TiO 2 [7] and N−doped NbO x [8] nanosheets were reported in the 2000s. In recent years, N−doped perovskite nanosheets (LaTa 2 O 6.