Plasma-treatment induced H2O dissociation for the enhancement of photocatalytic CO2 reduction to CH4 over graphitic carbon nitride

Though graphitic carbon nitride (g-C3N4) is a star photocatalyst for CO2 reduction, its unsatisfactory efficiency and lower reduced-state product (primary product is CO) greatly limit the further application. Dissociation of H2O is known as key step to provide abundant protons for CO2 reduction. The sluggish kinetic of H2O dissociation on g-C3N4 restricts the generation of higher reduced-state hydrocarbon products. Herein, we designed holey g-C3N4 nanosheets with numerous surface defects by Ar plasma treatment. Density functional theoretical (DFT) calculations prove the Ar plasma-treated g-C3N4 (P-x-CN) exhibits better H2O adsorption and dissociation abilities than pure g-C3N4. The separation of photogenerated charge carriers in P-x-CN is also more efficient than pure g-C3N4, which offers higher density of surface photogenerated electrons. The probability of multiple electron reduction reactions to hydrocarbon products greatly increases. As a result, the optimal Ar plasma-treated g-C3N4 (P-80-CN) shows a 40 times higher efficiency of CO2 reduction to CH4 than the pure g-C3N4. This work demonstrates the important role of H2O adsorption and dissociation in tuning product selectivity of CO2 reduction reactions, and provides an effective plasma treatment to modify the surface structure of photocatalysts.