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

•The strategy of low melamine/TiO2 ratio isolates a NCN2−—TiO2 transient species.•NCN2−—TiO2 exhibits excellent activities in H2 production, a novel photocatalyst.•Admixture of NCN2− HOMO π2p with Ti 3d upshifts the state into the bandgap of TiO2.•Five-line EPR pattern from a hole at NCN1−—TiO2, authentication of a gap state.•Heterostructured g-C3N4/TiO2 formation proceeds via the intermediacy of NCN2−—TiO2.