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

Abstract

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 (∼10⁴ cm² V⁻¹ s⁻¹) but very low hole mobility, which renders the C₆N₇ monolayer a promising candidate for water splitting under visible light.