A Theorical Study Of Water Adsorption And Splitting On Graphitic Carbon Nitrides Surface

Photocatalytic water splitting is one of the most important method forconverting solar energy into hydrogen energy with strong attraction to scentists allover the world. The catalytic process of water splitting had been widely studied inthe past decades. Graphitic carbon nitride, namely g-C₃N₄, is a new photocatalyticmaterial with environmental-friendliness and low cost, which had attracted a lot ofattention recent years. However, limited by the accuracy of experiment can reach sofar, the detail structure of this material is still unclear, and so do the water splittingmechanism on the g-C₃N₄surface. Therefore, some theoretical work is necessary forthe improvement of this material. In this paper, we used g-C₃N₄cluster model as abasic model to do some simulation of this material by density functional theory.Structure optimization, IRC computation, NBO charge analysis and frontier orbitalanalysis were adopted in this work. We also discussed how the doping of Pt make adifference in the catalytic process.Firstly, we did some calculation for comparsion of different calculating method,and finally determined to use the B3LYP method for all calculation in considerationof both computational performance and efficiency. At the same time, we designed ag-C₃N₄cluster model consist three tri-s-triazine as the most suitable surface model.The results showed that the corrugated g-C₃N₄is more stable than the planarstructure, and the corrugated g-C₃N₄was selected as a basic model for the afterwork.Secondly, we simulated the water splitting process on the pure g-C₃N₄surfaceand proposed three single water molecule reaction pathways and three double watermolecules reaction pathways. The computation result showed that the energybarriers of these reaction pathways are relatively high, and the correspondingwavelengths of the energy values are out the range of visible light, which make itdifficult to react. Though the energy barriers of these reaction pathways are high, thedifference in value indicated that the g-C₃N₄functioned as catalyst in these reaction.Lastly, we simulated the water splitting process on the Pt-doped g-C₃N₄surfaceand proposed two single water molecule reaction pathways and two double watermolecules reaction pathways. Compared to the energy barriers of Pt-excludedreaction pathways mentioned before, the energy barriers of Pt-contained reactionpathways decreased by more than half. More importantly, the correspondingwavelengths of these energy values are in the range of visible light, which indicatedthat the reaction is easier to happen. These computation results is in good agreementof experiments. NBO charge analysis and frontier orbital analysis showed that the doping of Pt atom increased the reaction activity of system and make thereformation of H2molecule easier. In conclusion, this work successfully explainedthe function of the doping of Pt as a cocatalyst.