Where do photogenerated holes at the g-C3N4/water interface go for water splitting: H2O or OH−?

Abstract

Graphitic carbon nitride (g-C₃N₄), a metal-free two-dimensional photocatalyst, has drawn increasing attention due to its application in photocatalytic water splitting. However, its quantum efficiency is limited by the poor performance of the oxygen evolution reaction (OER). Therefore, it is important to clarify the behavior of photogenerated holes in the OER. In this work, we investigate the energy level alignment using the GW method and the exciton properties using the Bethe–Salpeter equation within the ab initio many-body Green's function theory at the g-C₃N₄/water interface. We found that the g-C₃N₄ substrate can elevate energy levels of OH⁻ and H₂O molecules at the interface by up to 0.6 eV. This effect can make the electronic levels of OH⁻ surpass the valence band maximum (VBM) of g-C₃N₄. However, orbital energies of H₂O molecules remain far below the VBM of g-C₃N₄. This indicates that a photogenerated hole after exciting g-C₃N₄ can relax to OH⁻ instead of neutral H₂O. Moreover, OH⁻ could be directly oxidized through electron transfer from OH⁻ to g-C₃N₄ by light near the optical absorption edge of g-C₃N₄, which is beneficial for efficient carrier separation at the interface.