First Principles Study On Photocatalytic Mechanisms Of C/Ti Doped SnS₂ Monomer And Heterostructure

As an excellent photocatalytic material,tin disulfide（SnS₂）material has attracted the attention of many researchers due to its minor band gap,stable structure,environmentally friendly properties.Its higher photogenerated electron hole recombination rate and lower solar energy utilization lead to its photocatalytic efficiency is difficult to reach the level required for practical applications.Previous research results have been achieved in the fields of SnS₂ doping for catalytic reduction of CO₂,catalytic oxidation of pollutants and semiconductor composite for water splitting,proving the effectiveness of doping and semiconductor composite to improve the photocatalytic performance of SnS₂ materials,so far the effects of doping and semiconductor composite on the photocatalytic mechanism of SnS₂ have not been well explained theoretically.Here in,we first investigated the reaction mechanism of C-doping to enhance the efficiency of SnS₂ for CO₂ reduction based on the first principles calculation.The C atoms doped into the body or surface occupying the interstitial position of C_i-2 produce a local shallow defect state above the valence band maximum（VBM）,which acts as a hole trap to extend the lifetime of photogenerated electrons and improve the photocatalytic efficiency.With further increase of C doping concentration,two adjacent C_i-2 positioned C atoms doped introduce two deep non-local defect states located below the conduction band minimum（CBM）,which can effectively extend the absorption spectra.Secondly,the reasons for the increasing and then decreasing efficiency of Ti-doped SnS₂ oxidative degradation of methyl orange were investigated.The Ti atom occupying the position of the Snatom in the body introduces two shallow localized unoccupied defect states below the CBM.This defect state acts as an electronic trap allowing the hole lifetime at the valence band to be prolonged,thus improving the degradation efficiency of the SnS₂-Ti nanoplate to methyl orange solution.With the increase of Ti doping concentration,the Ti atoms were mainly distributed on the surface,which introduced several new deep local defect states located near the center of the forbidden band as the complex center of the electron-hole pairs and suppressed the photocatalytic activity.Finally,the mechanism of photolytic water reaction of g-C₃N₄/SnS₂ heterostructure compounded by g-C₃N₄ and SnS₂ and the effect of In doping on it were investigated.The result shows that g-C₃N₄/SnS₂ belongs to Z-type heterostructure.In the g-C₃N₄/SnS₂,the electrons on the g-C₃N₄ are transferred to SnS₂,resulting in the formation of a built-in electric field from the g-C₃N₄ surface to the SnS₂ surface.Under visible light irradiation,the built-in electric field drives the recombination of excited electrons in the conduction band（CB）of SnS₂ and photogenerated holes in the valence band（VB）of g-C₃N₄,making it difficult for the valence band photogenerated holes of SnS₂ and the conduction band electrons of g-C₃N₄ to compound due to their existence in different spatial positions,increasing their chance of participating in photocatalytic reactions,which is the main reason for the significantly higher photocatalytic efficiency of the g-C₃N₄/SnS₂heterostructure.And the In doping substantially increases the number of electron transfers from the g-C₃N₄ surface to the SnS₂ surface,which boosts the intensity of the built-in electric field and enhances the separation efficiency of excited electrons and holes,thus improving photocatalytic efficiency of the heterostructure.