Mechanistic insight into the water photooxidation on pure and sulfur-doped g-C3N4 photocatalysts from DFT calculations with dispersion corrections

• Possible mechanisms of OER on g-C3N4 are investigated by DFT calculations.
• High overpotential is caused by the high energy cost associated with the first proton removal step.
• Sulfur doping into g-C3N4 reduces the high overpotential for OER.
• This work provides insights for designing better anode to achieve high OER activity on g-C3N4.

In this work, first-principle methods are employed to build thermodynamic models for both the pure and sulfur atom modified g-C3N4 photocatalysts. Three possible mechanisms of oxygen evolution reaction (OER) following four one-electron pathway are investigated. The hydroxyl (OH) species as a key intermediate is found to strongly interact with the catalyst and its newly observed stability indeed significantly affects the overpotential of OER. On pure g-C3N4, the first removal of proton from water, the rate-determining step, can not become surmountable at room temperature until an overpotential of 0.88 V (2.11 V vs SHE) is appended, in accord with the experimental observation that water photooxidaton hardly occurs on g-C3N4 without any modification. Interestingly, the sulfur doping not only leads to a different reaction mechanism but also lowers the overpotential, consistent with the experimental finding that the reaction rate for OER could be further enhanced by sulfur-modified g-C3N4. Our theoretical results provide useful insights for designing better anodes to achieve high OER activity on graphitic carbon nitride based photocatalysts.

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