| The development of society has caused a sharp increase in human demand for energy.However,traditional non-renewable energy reserves are limited and will release harmful gases that pollute the environment when burned.In recent years,the conversion of solar energy to chemical energy by photocatalytic water splitting technology to produce hydrogen has been favored by researchers.Because the entire energy conversion process from raw materials of solar energy and water to products of oxygen and hydrogen is green,clean,low-cost,and pollution-free.Among the semiconductor photocatalytic materials,titanium dioxide(TiO2)has strong photochemical stability,strong oxidation reaction ability,and environmental friendliness.It was first studied by Japanese scholars Honda and Fujishima.Two-dimensional(2D)graphitic carbon nitrides(g-C3N4)has a suitable band gap and an easy-to-control interface,making it a popular material.Subsequently,bismuth vanadate(BiVO4)and other narrow band gap materials with strong optical response have also attracted attention.However,these single-component semiconductor photocatalysts inevitably suffer from the common problems of high carrier recombination rate,small specific surface area,and weak redox ability provided by band edge potentials,which restrict their visible light utilization and photocatalytic water splitting activity.Constructing a heterostructure is an effective strategy to improve the carrier separation rate,and it can also make the photocatalytic hydrogen evolution reaction(HER)and oxygen evolution reaction(OER)proceed on different components.In addition,metal and non-metal(co)doped heterostructures can further reduce the band gap and introduce impurity states to promote the transition of light-excited carriers.This paper aims to find efficient photocatalytic water-splitting materials,and calculated the electronic,optical and charge transfer properties of Cr,B doping and(Cr,B)codoping g-C3N4/BiVO4,and C,B doping and C&B codoping g-C3N4/TiO2,based on the hybrid density functional theory(HSE06)method.The specific research results are as follows:(1)The g-C3N4/BiVO4 heterostructure composed of g-C3N4 and BiVO4(010)surface can introduce a built-in electric field,which promotes interface charge transfer and prevents electron-hole pair recombination,resulting in g-C3N4 monolayer with negative charge and BiVO4(010)surface with positive charge.Under visible light irradiation,electrons are excited to the conduction band minimum(CBM)of the BiVO4(010)surface undergoing the HER,while the holes remain in the valence band maximum(VBM)of g-C3N4 monolayer aiding the OER.The band-edge potentials of BiVO4(010)surface are higher than that of g-C3N4 monolayer,which ensures a stronger redox reaction potential.Therefore,g-C3N4/BiVO4 is a typical Z-scheme heterostructure.Cr and B doping introduced Cr 3d states and B 2p states,respectively,which improved the visible light absorption capacity of the g-C3N4/BiVO4 heterostructure.In addition,the synergistic effect of the Cr 3d states and the B 2p states makes the optical absorption intensity of the(Cr,B)codoped g-C3N4/BiVO4 heterostructure higher than that of the pure,Cr,B doped g-C3N4/BiVO4 heterostructure.(2)The electric field are generated in the direction of the TiO2(101)surface to the g-C3N4 monolayer for the pristine,C and B doped g-C3N4/TiO2,and it was observed that the band-edge potential on the TiO2(101)surface was higher than that of the g-C3N4 monolayer.Thus,the pristine(2.591 eV),C doped(2.663 eV)and B doped(2.339 eV)g-C3N4/TiO2 are Z-scheme heterostructures,which promotes charge separation and retains a prominent redox ability.After C doping,the C 2p states are introduced,which promotes the transition of light-excited carriers.The band gap of B doped g-C3N4/TiO2 decreases,and the introduced mixed states of B 2p and N 2p makes the optical absorption edge red shift.The C&B codoped g-C3N4/TiO2 follows type-Ⅱ charge transfer mode because of their synergistic effect in C and B atoms,which changes the direction of the built-in electric field.It also has a narrow bandgap(1.309 eV)and effectively separate electron-hole pairs leading to strong optical absorption ability.The research results show that the band-edges matching of the semiconductor photocatalyst and the direction of the built-in electric field jointly determine whether the charges are selected to be Z-scheme or Ⅱ-type transfer mode. |
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