Bandgap Opening of Graphdiyne Monolayer via B, N‑Codoping for Photocatalytic Overall Water Splitting: Design Strategy from DFT Studies

While the hydrogen evolution reaction obtained during photocatalytic water splitting is easily facilitated by existing photocatalysts, the oxygen evolution reaction (OER) exhibits very sluggish reaction kinetics because it involves multi-proton-coupled electron-transfer steps. Therefore, the most important goal for developing efficient photocatalysts for overall water splitting is to design photocatalysts for which the valence band edge is sufficiently less than the oxidation potential of O₂/H₂O to meet the thermodynamic requirements of the OER. This is addressed in the present work by applying first-principles density functional theory calculations to systematically investigate the effect of B, N-codoping on the electronic band structure, thermal stability, dynamic stability, and optical properties of two-dimensional graphdiyne monolayers, which is a relatively new carbon allotrope consisting of sp- and sp²-hybridized carbon atoms that provides a bandgap of ∼1.0 eV. The results indicate that the bandgap energy increases with an increasing number of BN pair substitutions and that some of the B, N-codoped graphdiyne configurations representative of a BCN ternary structure provide direct bandgaps with energies of 2–3 eV appropriate for visible light absorption. Moreover, the valence and conduction band edges are appropriately matched with the oxidation and reduction potentials of water. We also demonstrate that the optimal B, N-codoped graphdiyne monolayers have excellent charge carrier mobilities and provide good separation between photogenerated electrons and holes.