Water Photo-Oxidation Reaction on Clean and Doped Two-Dimensional Graphitic C₂N

In the search for new efficient photo-catalysts for hydrogen production through water splitting, the main attention has been paid to tuning the band gap width and its position with respect to vacuum level. However, actual electro-catalytic activity for the water oxidation reaction on a catalyst surface is no less important than those quantities. In this work we evaluate from first principles the thermodynamics of the reaction on relatively new candidates for water splitting: two-dimensional C₂N and that doped with phosphorus. We find that the 4-step reaction usually expected for water splitting will not proceed on these systems, resulting in oxygen atoms left strongly adsorbed to the surface. Another option, a 3-step reaction, is also found to be unfavorable. We also test an effect of higher oxygen coverage on the reaction thermodynamics, as suggested elsewhere. We find that, indeed, the doubled O-coverage makes the 4-step reaction feasible for the doped C₂N. However, an unacceptably high anode potential is required to make this reaction proceed. We thus conclude that the materials under consideration may not be efficient electro-catalysts for water splitting.