Enhanced Photocatalytic Water Splitting in a C₂ N Monolayer by C-Site Isoelectronic Substitution

Two-dimensional (2D) semiconductors have shown great promise as efficient photocatalysts for water splitting. Tailoring the band gap and band edge positions are the most crucial steps to further improve the photocatalytic activity of 2D materials. Here, we report an improved photocatalytic water splitting activity in a C₂ N monolayer by isoelectronic substitutions at the C-site, based on density functional calculations. Our optical calculations show that the isoelectronic substitutions significantly reduce the band gap of the C₂ N monolayer and thus strongly enhance the absorption of visible light, which is consistent with the observed redshift in the optical absorption spectra. Based on the HSE06 functional, the calculated band edge positions of C_2-x Si_x N and C_2-x Ge_x N monolayers are even more favorable than the pristine C₂ N monolayer for the overall photocatalytic activity. On the other hand, for the C_2-x Sn_x N monolayer, the conduction band minima is more positive than the oxygen reduction potential and, hence, Sn substitution in C₂ N is unfavorable for the water decomposition reaction. In addition, the isoelectronic substitutions improve the separation of e^- -h⁺ pairs, which, in turn, suppress the recombination rate, thereby leading to enhanced photocatalytic activity in this material. Our results imply that Si-, and Ge-substituted C₂ N monolayers will be a promising visible-light photocatalysts for water splitting.