We perform first principles calculations to investigate the catalytic behavior of C9N4 nanosheet for water splitting.For the pristine C9N4,we find that,at different hydrogen coverages,two H atoms adsorbed on the 12-membered ring and one H atom adsorbed on the 9-membered ring show excellent performance of hydrogen evolution reaction (HER).Tensile strain could improve the catalytic ability of C9N4 and strain can be practically introduced by building C9N4/BiN,and C9N4/GaAs heterojunctions.We demonstrate that the HER performance of heterojunctions is indeed improved compared with that of C9N4 nanosheet.Anchoring transition metal atoms on C9N4 is another strategy to apply strain.It shows that Rh@C9N4 exhibits superior HER property with very low Gibbs free energy change of-30 meV.Under tensile strain within ～ 2％,Rh@C9N4 could catalyze HER readily.Moreover,the catalyst Rh@C9N4 works well for oxygen evolution reaction (OER)with an overpotential of 0.58 V.Our results suggest that Rh@C9N4 is favorable for both HER and OER because of its metallic conductivity,close-zero Gibbs free energy change,and low oneset overpotential.The outstanding performance of C9N4 nanosheet could be attributed to the tunable porous structure and electronic structure compatibility.