Crucial roles of triazinic-NO and CO groups in photocatalytic water splitting on graphitic carbon nitride

Abstract

As a typical metal-free photocatalyst, water splitting on graphitic carbon nitride (g-C₃N₄) has drawn increasing attention. Uncovering its reaction mechanism, which still remains a mystery despite extensive studies, would be of great help for the design of efficient metal-free photocatalysts. Through first-principles calculations we discovered that overall water splitting on g-C₃N₄ relies heavily on two nonadiabatic transitions and yielding of the vital active site CO. The two nonadiabatic transitions, one between two excited states while another between the excited and the ground states, trigger the reaction and split an H₂O molecule into a triazinic-N–H and triazinic-N–OH pair. Formation of triazinic-N–OH stabilizes electrons in the π* orbitals of g-C₃N₄ and thereby facilitates the reduction reaction. CO is developed from triazinic-N–OH. The significance of CO lies in that it provides a site at which another H₂O molecule could split spontaneously into a C–OOH and N–H pair without the aid of photons so that the oxidation evolution reaction could occur thereafter. The local electric field imposed upon the H₂O molecule that is hydrogen-bonded to the triazinic N is a key factor affecting the nonadiabatic transitions and thus splitting of this H₂O. To increase the water splitting efficiency, the stability of CO should be improved as it tends to evolve into triazinic-NO which is inactive for H₂O splitting.