Theoretical Research And Design Of High Efficiency Photocatalytic Material BiOI

Semiconductor photocatalysis technology has promising potential prospects in addressing energy shortage and environmental pollution.However,due to the defects of traditional photocatalysts(the wide band gap and low quantum conversion efficiency),the further development of photocatalysis technology is facing difficulties.Different from traditional photocatalysts,novel photocatalysts usually have unique structures or other features,which is significant for the enhancement of photocatalytic activity.As a novel photocatalyst,BiOX(X = F,Cl,Br,?)has special layered crystal structure,suitable band gap,and high photocatalytic activity.In BiOX,the band gap of BiOI is the smallest(1.9 eV),which corresponds to a threshold wavelength of 650 nm,so it has higher photocatalytic performance under visible-light irradiation.At present,most of the researches about BiOI are focused on the experiment,so the microscopic mechanism of the excellent photocatalytic activity of BiOI has not been deeply understood.And more importantly,BiOI also exits some defects(the high recombination rate of photo-generated electron-hole pairs and inability to produce hydrogen),making it need to be further improved.Therefore,it is necessary to study the structure and properties of BiOI,and make some feasible modification based on it.Hence,this paper systematically study on the structural,electronic and relative properties of perfect BiOI,defect physics of BiOI,low-index stoichiometric BiOI surfaces,graphene-based materials/BiOI heterostructures,and BiOI/BiOIO3 heterostructure based on the density functional theory calculations.The obtained results could provided the theoretical guidance for the future research of BiOI and similar materials.The main research contents and results are as follows:(1)The crystal structure and electronic structure of BiOI were studied by different methods,and the most suitable method for BiOI was determined.The properties of BiOI were calculated,and the relationship between the microstructure,the composition of electronic structure and the photocatalytic properties were clarified.The calculated results showed that the results obtained by the three DFT methods have no obvious difference,and all the methods can reproduce the crystal structure of BiOI well.Bi atoms and O atoms form covalent bonds,while Bi atoms and I atoms form ionic bonds(in the lower energy regions).BiOI has anisotropic characteristics,and visible-light absorption by BiOI is mainly because of photon absorption along the direction of the a/b axis.Considering the accuracy of the calculated results,utilization of computational resources,and computational time consumption,the GGA+U method is suitable for BiOI.(2)The physical properties and effects forphotocatalytic performances of three types point defects(including impurity defect,antisite defect,and vacancy defect)in BiOI have been systematically investigated.And the relatively suitable methods of point defects modification were found.The calculated results showed that the lattice distortions induced by these point defects are very gentle.Owing to the existence of point defects,the distribution of electrons and the direction of electron transfer exhibit some different features,but the change is very localized.In the cases of BiOI:I@Bi and BiOI:Vac@O,there appears an isolated defect band within the band gap,which has smaller distance from the top of VB.Thus,this type of defect band could act as photogenerated holes trap,and enhance the photocatalytic performance.In the case of BiOI:Vac@Bi,there is an overlapping defect band at the top of VB,which is also favorable for the enhancement of photocatalytic performance.Importantly,the inducing of point defects could vary the band edge position of BiOI.In the cases of BiOI:I@Bi and BiOI:Vac@O,the CB and VB edge positions are straddle the redox potential of water,resulting in as the promising photocatalyst candidates for overall water splitting driven by visible light.(3)The crystal structure,electronic structure,and related properties of(001)/(101)/(100)/(110)surfaces were calculated and analyzed.The relationship between the surface properties and photocatalytic properties was clarified.The calculated results showed that the surface relaxations of these four low-index stoichiometric BiOI surfaces are relatively small,especially that of(001)surface could be ignored.Besides,due to the non-fully-coordinated Bi atoms and O atoms on the surface,it appears a few surface states below the bottom of conduction band and above the top of valence band in the first layer except the(001)surface.The dangling bond density of Bi atoms on the(001)surface is zero nm2,leading to the lowest surface energy,thus it process the predominant surface area(51.4%)in the equilibrium morphology of BiOI.Considering the relationship between surface structure,electronic structure and photocatalytic activity,the BiOI(101)surface maybe has higher photocatalytic activity,because the facile transfer behave from BiOI to outside and the capuche center for photo-generated carriers.(4)The interfacial microstructure and properties of graphene-based materials/BiOI heterostructures(including graphene/BiOI,GO/BiOI,and g-C3N4/BiOI)were calculated and analyzed.And the intrinsic mechanism of enhanced photocatalytic activity of these heterostructures was clarified.The calculated results showed that the variation of lay spacing for the several layers at the interface is not very important and the positive interface formation energy also can prove that all heterostructures are stable.There is a slight charge transfer along the interfacial normal direction from BiOI to graphene\GO\g-C3N4.Importantly,the charge depletion and accumulation mainly occur in the area near the interface,which means dipole moment is generated at the interface.And the introduced internal electric field can promote the carrier separation and restrain the recombination of electron-hole pairs.The band positions have some changes after these heterostructures formed.The GO/BiOI and g-C3N4/BiOI heterostructures are all standard type-II heterostructures,which can promote the directional separation of carriers,and it is satisfactory for photocatalytic reaction.(5)The interfacial microstructure and properties of vertical and in-plane BiOI/BiOIO3 heterostructures were investigated.The effects of different bonding methods on the interfacial properties and photocatalytic properties were analyzed.And the intrinsic mechanism of enhanced photocatalytic activity of these heterostructures was clarified.The calculated results showed that the most suitable interface spacing of vertical-A\vertical-B\in-plane BiOI/BiOIO3 is 3.0 A,1.3 A,and 2.3 A,respectively.And all the interface energy is all positive,which indicated that the heterostructures are stable.The charge depletion and accumulation mainly occur in the area near the interface,which means dipole moment is generated at the interface.And the introduced internal electric field can promote the carrier separation and restrain the recombination of electron-hole pairs.The band positions have some changes after these heterostructures formed.The in-plane heterostructure is the type ? heterostructure,which is not satisfactory.While the two vertical heterostructures are all type ? heterostructures,which can promote the directional separation of carriers,and it is favorable for photocatalytic reaction.