Single-atom nickel terminating sp² and sp³ nitride in polymeric carbon nitride for visible-light photocatalytic overall water splitting

Polymeric carbon nitride (PCN) has been widely used as a metal-free photocatalyst for solar hydrogen generation from water. However, rapid charge carrier recombination and sluggish water catalysis kinetics have greatly limited its photocatalytic performance for overall water splitting. Herein, a single-atom Ni terminating agent was introduced to coordinate with the heptazine units of PCN to create new hybrid orbitals. Both theoretical calculation and experimental evidence revealed that the new hybrid orbitals synergistically broadened visible light absorption <i>via</i> a metal-to-ligand charge transfer (MLCT) process, and accelerated the separation and transfer of photoexcited electrons and holes. The obtained single-atom Ni terminated PCN (PCNNi), without an additional cocatalyst loading, realized efficient photocatalytic overall water splitting into easily-separated gas-product H₂ and liquid-product H₂O₂ under visible light, with evolution rates reaching 26.6 and 24.0 μmol g^-1 h^-1, respectively. It was indicated that single-atom Ni and the neighboring C atom served as water oxidation and reduction active sites, respectively, for overall water splitting <i>via</i> a two-electron reaction pathway.