Dual roles of [NCN]^2- on anatase TiO₂: A fully occupied molecular gap state for direct charge injection into the conduction band and an interfacial mediator for the covalent formation of heterostructured g-C₃N₄/a-TiO₂ nanocomposite

When low density of melamine is calcined with a-TiO₂ at 550 °C, two FT-IR peaks observed at 2048 and 2066 cm⁻¹ are assigned to NCN^2- covalently attached to a five-coordinated Ti atom and an oxygen vacancy, respectively, complemented by DFT computation. Admixture of NCN^2- HOMO(π_2p, 87.5 %) with the high energy Ti(3d, 12.5 %) destabilizes and upshifts the NCN^2- HOMO into the a-TiO₂ bandgap. A five-line EPR pattern derived from 71 % of atomic spin density localized on two equivalent ¹⁴N nuclei, observed under sub-bandgap excitation, verifies the presence of a molecular gap-state. The NCN^2--a-TiO₂ exhibits excellent H₂ production at a rate of 5101 μmol.h^-1.g^-1. Upon increasing melamine concentration, the rapid and simultaneous decreases of NCN^2- FT-IR and NCN^1- EPR signals accompanied by transitions of other spectroscopic data to those characteristic of tri-s-triazine demonstrate that the formation of heterostructured g-C₃N₄/a-TiO₂ proceeds via the intermediacy of NCN^2--a-TiO₂, thereby a mechanism is proposed.