Anionic Dopants for Improved Optical Absorption and Enhanced Photocatalytic Hydrogen Production in Graphitic Carbon Nitride

Graphitic carbon nitride is an exemplar material for metal-free photocatalytic hydrogen production, essential to drive the change to a greener economy. However, its bandgap is too large, at 2.7 eV, for visible light harvesting, which hinders uptake in applications. From two sets of independent quantum mechanical simulations, we have determined the effect of two representative interstitial (hydrogen and fluorine) dopants on the electronic structure and optical properties of this material. From defect analysis, we have found that for a significant range of chemical potential the anionic fluorine dopant is favored. This dopant has significant effects on the optical absorption with the valence band edge shifted up by 0.55 eV, which extends light absorption into the visible. In contrast, hydrogen prefers to be cationic, with the conduction band edge shifted down by 0.45 eV, which strongly reduces hydrogen production as the thermodynamic driving force for proton reduction is significantly reduced. Fluorine is advantageous for improved H-2 production as band gap reduction is driven by raising of the valence band, with minimal effect on the thermodynamic driving force for hydrogen reduction. We propose that a design principle for improving carbon nitrides for hydrogen production is to use strongly electronegative dopants.