First-principle Studies Of The Mechanism Of Water Splitting On Graphitic Carbon Nitride

Recently,graphitic carbon nitride(g-C3N4)as a semiconductor polymer has been attracting much interests from the scientists in the field of photocatalysis due to its advantages,such as its ability to decompose water under the light radiation,easiness to synthesis and low cost.In this paper,by using first principle density functional theory,the configurations and the electronic structures of g-C3N4 were firstly studied as well as the halogen doping effect towards the performance of photocatalytic water splitting.Secondly,we also explored the mechanism of g-C3N4 photocatalytic water oxidation by determining the reaction active center,the reaction path and the overpotential on g-C3N4.Finally,the photocatalytic water splitting activity of S doped g-C3N4 was studied.The calculation results revealed that the pure g-C3N4 exhibits a plane configuration with a direct energy band gap.The top of valence band is mainly composed of the orbitals of edge N atoms,indicating that the electrons will excitate from the N atoms which possess higher electronegativity.For the doped g-C3N4 system,halogen atoms(X)tend to substitute edge N atoms.The material still maintains a plane configuration while the bond lengths of X-C and X-N in the doped g-C3N4 are longer than those of the C-N bond in pure g-C3N4.The new energy band will facilitate the mitigration of photo-generated electrons.The calculated optical adsorption spectrum indicates the increased adsorption of visible-light,which is in accordance with the results of experiments.In addition,the calculated electrode potential of halogen doped g-C3N4 results revealed that the doped semiconductor systems have larger driving force for water splitting.To explore the mechanism of oxygen evolution reaction on the surface of g-C3N4,all possible reaction paths were considered.The calculation results show that the reaction proceeding through an OH*intermediate is most possible.Based on this reaction pathway,the calculation results also indicate a large overpotential for the oxygen evolution reaction,consistent with the experimental observation that the reaction rate is very low.After an sulfur atom was doped into g-C3N4,the reaction path was changed,the production of H2O*and O*from HOO*changed to HO*+ O*.Due to the significantly enhanced binding energy of OH*,the overpotential is correspondingly reduced,facilitating the reaction process.The decreasement of overpotential on sulfur doped g-C3N4 well interpretes the experimental observation that the reaction rate of oxygen evolution is improved after doping an sulfur atom into g-C3N4.