In this work, bare and O-doped g-C3N4 nanosheets were synthesized. The results show that the structure of bare and O-doped g-C3N4 (OCN) nanosheets are layered and the absorption of visible light increases after O-doping. Density function theory calculations demonstrate that the oxygen atom is replaced at the edge site of g-C3N4. The reaction pathway of CO2 reduction to fuel/chemicals on the OCN surface is as follows: CO2→HCOO→ HCOOH→ HCO + H2O→ H2CO→ H2COH→ H3COH→ CH3 + H2O→ CH4. The main product on the OCN and CN surfaces are methane and methanol, respectively. The Gibbs activation energy of CH3OH formation on the CN and OCN surfaces are 1.72 and 0.14 eV, respectively. The rate-determining step on the OCN surface is HCOOH→ HCO + H2O, whereas on the CN surface is CH3OH formation from the H3CO intermediate. Therefore, the Gibbs activation energy on the O-doped g-C3N4 decreases for the methanol production in comparison with the bare g-C3N4. In general, the O-doping of g-C3N4 improves the photocatalytic activity through the decrement in the band gap and enhances the reaction rate of CO2 conversion to fuels/chemicals.

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