Theoretical Study On Photocatalytic Mechanism And Its Regulation Of G-C₃N₄/TiO₂-B Heterostructures

TiO₂,as a typical wide band gap semiconductor,is widely used in the fields of photocatalysis,biomedicine,gas sensor and solar cell due to its abundant reserves,non-toxic and corrosion resistance.In addition,the g-C₃N₄/TiO₂-B heterostructure was constructed to improve the visible light absorption of TiO₂-B.We reveal its microscopic photocatalytic mechanism and propose site-selective doping and metal-nonmetallic co-doping means to improve its photocatalytic ability.The main research results of this thesis are as follows:Firstly,we used hybrid density functional theory to explore the possibility of g-C₃N₄/TiO₂-B heterojunction as a photocatalyst for water splitting,and clarified the photocatalytic mechanism of g-C₃N₄/TiO₂-B heterostructure.Our studies show that the band gap of g-C₃N₄/TiO₂-B{100}heterostructures is significantly reduced compared with the TiO₂-B surface and the utilization rate of visible light is increased.The band edges straddles the redox potential of water and over all water splitting can be realized.We found that there is a built-in electric field from g-C₃N₄ toward TiO₂-B{100}at the interface of the g-C₃N₄/TiO₂-B{100}heterostructure,which provides a driving force for the migration of photoinduced electrons on valence band of g-C₃N₄ and photoinduced holes on conduction band of TiO₂-B{100}.Meanwhile,the photoinduced electrons are retained in the conduction band of g-C₃N₄ and the photoinduced holes are retained in the valence band of TiO₂-B{100}.Therefore,g-C₃N₄/TiO₂-B{100}heterostructures follow a direct Z-scheme photocatalytic mechanism.Secondly,we propose a strategy of selective substitution of F-doping to regulate the photocatalytic mechanism.The results show that F_N2@g-C₃N₄/TiO₂-B（001）heterostructure still follows a direct Z-scheme photocatalytic mechanism.The synergistic effect of the built-in electric field and band bending accelerates the recombination of photoinduced electrons in conduction band of TiO₂-B（001）with the photoinduced holes in conduction band of F_N2@g-C₃N₄.The interfacial charge transfer at the interface of F_C1@g-C₃N₄/TiO₂-B（001）heterostructure is reversed,resulting the reversed built-in electric field,which follows type-II photocatalytic mechanism,in which the photoinduced electrons and holes accumulate continuously in the conduction band of TiO₂-B（001）and the valence band of g-C₃N₄,respectively.Therefore,our study shows that F@g-C₃N₄/TiO₂-B（001）heterostructure is a photocatalyst with full solar spectral response.Finally,we propose a scheme of Li-F co-doping to enhance the interfacial charge transfer and improve the separation efficiency of photoinduced carriers.The results show that the Li-F co-doping can significantly enhance the interfacial interaction,promote the interfacial charge transfer,and enhance the intensity of the built-in electric field.Our calculation and analysis demonstrate that the co-doped Li@g-C₃N₄/F@TiO₂-B（001）heterostructure has a staggered band alignment,which realized the spatial separation of the photoinduced electron-hole pairs.The built-in electric field,band bending and band offset provide a strong driving force for the directional migration of the photoinduced electrons and holes.Thermodynamic analysis of HER showed that the Gibbs free energy decreased under the drive of the photoinduced external potential.Li-F co-doping significantly reduced the HER overpotential,which confirms that Li-F co-doping can effectively improves the photocatalytic hydrogen evolution ability.In conclusion,this thesis verifies that g-C₃N₄/TiO₂-B heterostructure can be used as a high quality photocatalyst for hydrogen evolution from water splitting firstly.In addition,the photocatalytic mechanism and hydrogen evolution sites of g-C₃N₄/TiO₂-B（001）heterostructure have been regulated by selective substitution of F atom.Finally,the interfacial interaction of g-C₃N₄/TiO₂-B（001）heterostructure is enhanced by metal-nonmetallic co-doping,which improved the photocatalytic ability of the heterostructure.This thesis provides a new idea and theoretical basis for the research of photocatalytic heterostructures modification experimentally.