Heptazine-based porous graphitic carbon nitride: a visible-light driven photocatalyst for water splitting

Graphitic carbon nitride (C3N4) based semiconductors are found to be potential metal-free photocatalysts for water splitting. However, due to the wide band gap, C3N4 has insufficient sunlight absorption which limits the energy conversion efficiency. Here, by means of density functional theory, we explore a heptazine-based porous graphitic carbon nitride, the C6N7 monolayer as a visible-light driven photocatalyst for water splitting. The C6N7 monolayer possesses a direct band gap of 1.90 eV and pronounced optical absorbance in the visible light region. More importantly, the band alignment of the C6N7 monolayer with respect to the water redox levels is found to satisfy the thermodynamic criteria for water splitting. By evaluating the free energy change in the oxidation/reduction reactions of the C6N7 monolayer, it is found that cocatalysts are required for water splitting. The C6N7 monolayer also favours separation of photoexcited electron-hole pairs, due to its high electron mobility (similar to 10(4) cm(2) V-1 s(-1)) but very low hole mobility, which renders the C6N7 monolayer a promising candidate for water splitting under visible light.