The Effect And Mechanism Of Doping Behaviour On The Photo-electric Properties Of Bi₂WO₆

Semiconductor photocatalyst Bi2WO6 has demonstrated excellent application prospect in the degradation of organic contaminantes.But its band gap is relatively large and the recombination rate of the photon-generated carrier is high,which prohibited its rapid development and applications.Recently,ions modification of Bi2WO6 has attracted extensive attentions to researchers,many transition metal elements are tried to incorporate into Bi2WO6 and many delightful achievements have been gained.But these studies seldom investigated the ion-doping improvement mechanism deeply,as a consequence,it remains unclear that whether the ions are effectively doped.It is also not very certain that the doped ions substituted Bi or W positions.To solve the abovementioned problems,first principles density functional theory(DFT)calculation is initially used in this thesis to study the substituted position,bond length,charge population,band structure,density of state and defects formation energy of Bi2WO6.Subsequently,different typical kinds of ions doped Bi2WO6 are synthesized through hydrothermal or solvothermal methods based on the theoretical calculation results.Effects of the doping ions on crystal structure,chemical composition,microstructure and photocatalytic properties are investigated and the improvement mechanism of the photocatalytic properties are explored.Finally,the humidity sensitive characteristics of Bi2WO6 film and the photocatalytic property of Bi2WO6/Bi2O3 heterojunction complex are further explored on the basis of the above researches.For the oxygen vacancy-containing BixWO6,the DFT results indicate that oxygen vacancy could significantly reduce the band gap of Bi2WO6,increase the internal electric field and charge density,promote the generation of photo-induced electrons and effectively inhibit electron-hole recombination.In addition,oxygen vacancy-containing BixWO6(x= 1.81,1.87,1.89,1.92,2.01)are synthesized by solvothermal method.After W self-doping process,the crystal structure and micro-structure of Bi2WO6 change slightly.However,the contents and photo absorption performance of the Bi2WO6 could be affected by the W self-doping.The Bi1.89WO6 displays the best photocatalytic property,where the RhB is degraded by 98%after irradiated 180 minutes.For the Mg2+ ions doped Bi2WO6,DFT calculations show that the crystal structure of Bi2WO6 deforms slightly and the band gap decreased.Mg impurity energy level has not formed,but its Fermi level decreases,energy levels split and locality decreases,which in favor of the electron excitation.The Mg2+ ions doped Bi2WO6 are synthesized by hydrothermal method.After Upon doping with Mg2+ ions,the crystal structure of Bi2WO6 deforms,surface area increases and the light absorption edges also red-shift.The 1%Mg2+ ions doped Bi2WO6 shows the best photocatalytic property,the RhB is degraded by 94%after irradiated 45 minutes.For the Co2+ ions doped Bi2WO6,DFT results show that Co impurity energy level forms in the band structure of Bi2WO6,but the band gap changed little.Thecrystal structure of Bi2WO6 is deformed and more oxygen vacancies form.The Co2+ions doped Bi2WO6 are synthesized through solvothermal method.Upon doping with Co2+ ions,the surface area increases and more oxygen vacancies form,but the band gap decreases.The 1.78%Co2+ ions doped Bi2WO6 shows the best photocatalytic property,the RhB is degraded by 99.5%after irradiated 20 minutes.For the In3+ ions doped Bi2WO6,DFT results show that no impurity energy level is formed in the band structure of Bi2WO6 and the band gap changes little.In3+impurity density of state emerges in valence band and conduction band.In3+ ions doped Bi2WO6 are synthesized by solvothermal method.In3+ ions have little effect on the crystal structure of Bi2WO6 when the doping amount is less than 15%,but when the doping amount is 20%,the crystal structure of Bi2WO6 is deformed.The band gap decreased,the surface area increased and the light absorption edges red-shifted upon doping with In3+ ions.The 8%In3+ions doped Bi2WO6 shows the best photocatalytic property,the RhB is degraded 93%after irradiated 120 minutes.For the Sn2+ ions doped Bi2WO6,DFT results show that Sn impurity energy level formed in the band structure of Bi2WO6 and the band gap decreased.Sn2+ ions doped Bi2WO6 are synthesized by solvothermal method.Sn2+ ions have little effect on the basic phase and microstructure of Bi2WO6.But the band gap decreased,the surface area increased and the photocatalytic property increased.The 10%Sn2+ ions doped Bi2WO6 shows the best photocatalytic property,the RhB is degraded by 84.6%after irradiated 120 minutes.For the Er3+ ions doped Bi2WO6,DFT results show that the crystal structure of Bi2WO6 is slightly deformed and Er impurity energy level forms in the band structure of Bi2WO6 and the band gap decreased after Er3+ substituted Bi position.Er3+ ions doped Bi2WO6 are synthesized by solvothermal method.Er3+ ions have little effect on the basic phase but the particles changed more compacted.The band gap decreased a little and the photocatalytic property increased dramatically.The 4%Er3+ ions doped Bi2WO6 shows the best photocatalytic property,the RhB is degraded by 91.5%after irradiated 150 minutes,which is 83%higher than that of undoped one.To further understand the impact mechanism of ion-doping and oxygen vacancies on electronic structure and crystal structure of Bi2WO6,DFT calculation are applied to study effects of different typical doping ions substituted Bi,O and W positions on band structure,density of states,electronic structure of Bi2WO6.Results show that the band gap decreases when Na,Zn,Sb substituted Bi position and Zn impurity energy level is formed in the band structure of Bi2WO6;The band gap also decreased when N,S,F,Cl and Br substitute oxygen position;When V,Nb,Cr and Mo substituted W position,the band gap decreased and the impurity energy levels form in conduction band,but the valence band structure changed little.The DFT calculation results of Na,Mg,Cr,Co,In,Sn or Br doping Bi2W06 show that oxygen vacancies formed more easily than that of the undoped one.Bi2.1WO6 films with different Co2+ ions doping amount are prepared through precursor-spin coating method and their humidity sensitive characteristics are studied.The Bi2.1WO6 film is composed of nano-rods and its fraction of coverage is the highest when the pH value is 4 and the heat-treatment temperature is 300?.The particle size and band gap of Bi2.1WO6 film are decreased upon doping with Co2+ions.Co2+ ions doped Bi2.1WO6 film has higher sensitivity between 10%-60%humidity and has lower sensitivity between 60%-100%humidity.The oxygen ion conduction mechanism and the existence of oxygen vacancy could be proved through AC impedance test of doped Bi2WO6,and then established the relationships between ion doping,oxygen vacancy and photocatalytic property.Bi2+xWO6+1.5x(x=-0.25,0,0.25,0.5,1.0)complex photocatalysts are synthesized by solid phase method.Their photocatalytic characteristics are deeply explored.The absorbtion in visible light area of the complexes are higher than that of the undoped one.When x?0.25,the photocatalytic performance of the complex are much better than that of the undoped one.Among these complexes,the Bi2.5WO6.75 shows the best photocatalytic property,the RhB is degraded nearly 100%after irradiated 25 minutes.