| Monoclinic phase bismuth vanadate(m-BiVO4)is a narrow band gap semiconductor photocatalyst for hydrogen production and degradation of organic pollutants.However,ideal m-BiVO4 has a lower separation rate of photo-excited electron-hole pairs and thus has low photocatalytic performance.Studies have reported that vacancy,doping and other defects can improve the photocatalytic activity of m-BiVO4.In this paper,based on the density functional theory(DFT),the Cambridge Sequential Total Energy Package(CASTEP)module from Materials Studio(MS)computational simulation software is used to systematically study the structural stability,electronic and optical properties of defective m-BiVO4 via first-principle calculations.Firstly,the influences of Bi,V and O vacancies on the structural,electronic and optical properties of BiVO4 are investigated.The results indicate that despite the lattice parameters of BiVO4 change with vacancy types slightly,the structure can still exist stably.For simplicity,BiVO4 with Bi,V and O vacancy located at different site are labeled as V1Bi,V2Bi,V1V,V2V,V1O,V2O,V3O and V4O.Besides,ideal BiV04 is a direct band gap semiconductor while VBi,VV and VO BiVO4 are indirect band gap semiconductor.Taking V1Bi,V1V and V1O BiVO4 as an example,the calculated optical properties show that the static dielectric constants ?1(O),refractive index n(O)and reflectivity(R0)of defective BiVO4 are larger than those of ideal crystal.The optical absorption spectra ?(?)indicate that the optical band gaps of defective BiVO4 are reduced and will effectively enhance the optical absorption in visible light range.Secondly,the effects of Nb,N and Nb/N atoms on the material's performance of m-BiVO4 are investigated by considering their elastic properties,Debye temperature,electronic band structures,band edges and optical properties.Geometrical optimizations show that despite the lattice parameters of Nb-BiVO4,N-BiVO4,Nb/N-BiVO4 change slightly,structural stability is not affected.Results of elastic properties indicate that ideal BiVO4 is the stiffest,Nb-BiVO4 is the most dutile and most easily formed,N-BiVO4 is the hardest.The values of Poisson's ratio v show the ionic bonding is dominated in ideal and defective BiVO4.Debye temperature G?D of ideal BiVO4 is larger than those of defective BiVO4.Ideal and Nb-BiVO4 are indirect band gap semiconductor while N-BiVO4 and Nb/N-BiVO4 are dorect band gap semiconductor.Band edge positions show that the reduction and oxidation capabilities are in the order of Nb/N-BiV04>N-BiV04>Nb-BiVO4>ideal BiV04 and Nb-BiVO4>ideal BiVO4>N-BiVO4>Nb/N-BiVO4,respectively.The calculated optical properties indicate that red shifts occur in N-BiVO4 and Nb/N-BiVO4 which will make visible light absorption more effective.Finally,the influences of scandium(Sc)doping and oxygen vacancy(VO)on the structural stabilities,electronic structures,elastic properties,optical properties of monoclinic BiVO4(m-BiVO4)are investigated.It is found that the indirect band gap is maintained in BiVO4 even with V replaced by Sc(sub Sc-V).However,it becomes direct band gaps for BiVO4 with Bi replaced by Sc(sub Sc-Bi)and that with V replaced by Sc and compensated by Vo(sub sc-V+Vo).The reduction and oxidation capabilities are in the sequence of sub sc-v+Vo>sub Sc-Bi>ideal>sub sC-V and ideal>sub sc-v+Vo>sub Sc-Bi>sub sC-V,respectively.Elastic properties show that ideal BiVO4 is the stiffest,sub Sc-Bi BiVO4 is the most ductile and easily formed,sub sC-V+VO BiVO4 is the hardest.Optical properties reveal that red shift takes place both in BiVO4with sub sC-Bi and sub Sc-V typed defects,resulting in enhanced absorption in visible light range. |
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