Photocatalytic water splitting has gained increasing attention,since it utilizes renewable resources,such as water and solar energy,to produce hydrogen.Using the first-principles density functional theory,we investigate the properties of the single layer C2N which was successfully synthesized.We reveal that monolayer C2N has a substantial direct band gap of 2.45 eV.To regulate its band gap,four different nonmetal elements (B,O,P,and S) on the cation and anion sites are considered.Among them,B-doped N site is the most effective one,with the lowest formation energy and a band gap of 2.01 eV.P-doped N site is the next,with a band gap of 2.08 eV,though its formation energy is higher.The band alignments with respect to the water redox levels show that,for these two dopings,the thermodynamic criterion for the overall water splitting is satisfied.We therefore predict that B-or P-doped C2N,with an appropriate band gap and an optimal band-edge position,would be a promising photocatalyst for visible-light water splitting.