Study On The Enhanced Photocatalytic Properties Of Semiconductor Materials By Excitons Modulation

Semiconductor photocatalysts have attracted much attention due to their broad prospects in alleviating energy shortage and solving environmental pollution problems.In the past,we used the traditional energy band theory to explain the photocatalytic mechanism.Although the solving process is simplified,the interaction between electron and hole generated by semiconductor optical excitation is neglected.Once the interaction between them is taken into account,a new photobiological species,exciton,emerges in the photocatalytic reaction.The exciton effect of two-dimensional structural materials with low electron shielding effect and high dielectric property is particularly significant.Photogenerated electrons and holes are easy to interact strongly in local two-dimensional structures,and a large number of excitons are generated through the combination of coulombic forces.Excitons are not conducive to the degradation of organic pollutants by semiconductor photocatalysts.In order to accelerate the dissociation of excitons in 2D structure-semiconductors g-C3N4 and BiOCl,nitrogen vacancy is introduced in g-C3N4,precious metal Ag is deposited on BiOCl surface and a heterojunction is constructed among Ag,AgIO3 and BiOCl.The exciton dissociation is promoted and the photocatalytic properties of g-C3N4 and BiOCl are enhanced.The main conclusions are as follows:(1)Nitrogen vacancy g-C3N4 is synthesized by thermal polymerization.Nitrigen vacancy causes energy disorder of the system.Under light,nitrogen vacancy preferentially captures excitons in g-C3N4,and then prompts excitons to separate into free electrons and holes in the energy disorder regions.The accumulation of free electrons raises the conduction band position of g-C3N4,and the quantum limiting effect causes the valence band position to be more positive.The nitrogen vacancy g-C3N4 has a strong redox ability.(2)The Ag-OVs-(001)BiOCl photocatalyst is prepared by photo-reduction method.A lot of oxygen vacancies are generated in BiOCl under continuous light,and the "hot electrons" generated by electrons in the oxygen vacancy in the high-energy state make Ag+adsorb on the oxygen vacancy through electrostatic attraction to reduce Ag+ to Ag,and form Ag-OVs-(001)BiOCl photocatalyst.Under light,the electrons generated by Ag spread to BiOCl through TSPR and competed for the holes in the primary excitons of BiOCl to form the secondary excitons.The electrons in the primary exciton are released to the BiOCl conduction band,and the secondary excitons are captured by oxygen vacancy and released holes to the BiOCl valence band.The degradation rate of RhB by photocatalyst is 2.64 times that of BiOCl under simulated sunlight irradiation for 2h.(3)The direct Z-scheme Ag/BiOCl/AgIO3 photocatalyst is synthesized by photo-reduction.The strong exciton effect of BiOCl makes Ag+and IO3-preferentially generate AgIO3,and further attracts Ag+to form AgIO3·Ag+colloidal particles.The colloidal particles are attracted by the electrons to the(001)facet of BiOCl,and Ag+is reduced by a small amount of electrons to Ag and dispersed in the(001)facet of BiOCl,forming the Ag/BiOCl/AgIO3 photocatalyst.The transition from type-I Ag/BiOCl/AgIO3 to the direct Z-scheme Ag/BiOCl/AgIO3 is achieved by the electrons flowing towards BiOCl.Ag-electrons capture the holes of the primary excitons in BiOCl to generate secondary excitons,which accelerates the dissociation of excitons in BiOCl and makes the direct Z-scheme Ag/BiOCl/AgIO3 heterojunction photocatalyst degrade RhB under simulated sunlight and near-infrared light.In the process of cyclic degradation,the electrons released from the primary exciton in BiOCl react with AgIO3 to form AgI,which forms the traditional type-Ⅱ Ag/BiOCl/AgI photocatalyst.It has excellent degradation stability under the simulate sunlight and near-infrared light.