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 degrees C, two FT-IR peaks observed at 2048 and 2066 cm(-1) are assigned to NCN2- covalently attached to a five-coordinated Ti atom and an oxygen vacancy, respectively, complemented by DFT computation. Admixture of NCN2- HOMO(pi(2p), 87.5 %) with the high energy Ti(3d, 12.5 %) destabilizes and upshifts the NCN2- HOMO into the a-TiO2 bandgap. A five-line EPR pattern derived from 71 % of atomic spin density localized on two equivalent N-14 nuclei, observed under sub-bandgap excitation, verifies the presence of a molecular gap-state. The NCN2--a-TiO2 exhibits excellent H-2 production at a rate of 5101 mu mol.h(-1).g(-1). Upon increasing melamine concentration, the rapid and simultaneous decreases of NCN2- FT-IR and NCN1- 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 NCN2--a-TiO2, thereby a mechanism is proposed.