Study On Interfacial Charge Transfer In The Photoelectrochemical Reaction Of BiVO₄ Based Photoanode

Environmental pollution and energy crisis have been two important practical problems that human have to face.It is important to eliminate environmental pollutants and find clean new energy.Photoelectrochemical technology has been considered as a promising technology for environmental purification attributed to its great capacity to remove environmental pollutants and decompose water for hydrogen production under solar light illumination.Two kinds of BiVO4 based photoanodes were developed in this thesis for photoelectrochemical degradation of organic pollutants and water oxidation,respectively.Their fundamentals and influence factors were studied systematically.The interface charge transfer mechanism during the photoelectrochemical reaction was discussed mainly.For photoelectrochemical degradation of organic pollutants,a Fe00H/P:BiV04/rG0 composite photoelectrocatalyst was prepared by solvothermal method,photodeposition and electrostatic self-assembly,in that orde.Electrophoretic deposition was performed to prepare the composite film electrodes.The samples were systematically characterized and tested by photoelectrochemical method.The results showed that the optimum conditions for the preparation of BiV04 were as follows:the content of reaction substrate was 8 mmol,the reaction temperature was 120℃,the reaction time was 12 h,and pH=1.65.The obtained BiV04 was consistent with the monoclinic scheelite crystal structure with an average size of approximately 10 nm and a band gap of 2.36 eV.The remarkable improvement of photoelectrochemical proforemence was attributed to the combination of P doping,FeOOH loading and coupling with rGO which could effectively increase carrer concentration(Nd)and the interface charge transfer.The synergistic effect between FeOOH and rGO could suppress the recombination,and enhance the adsorption of organic pollutants on the surface of the electrode,thereby promote the charge transfer at the semiconductor/electrolyte interface(SEI).The effects of electrophoretic deposition time and the electrolyte conditions on the charge transfer at SEI of Fe00H/P:BiV04/rG0 photoanode for the photoelectrochemical degradation were studied by photoelectrochemical method.The photoelectrochemical degradation of clofibric acid was also studied.The results showed that the optimum deposition time was 6 min.And the optimum electrolyte conditions for the photoelectrochemical degradation of clofibric acid were as follows:the concentration of Na2SO4 was 0.2 mol L-1,the concentration of clofibric acid was 5 mg L-1,and pH was 7.Under these conditions,FeOOH/P:BiVO4/rGO photoanode showed a removal rate of 95.3%for degradation of clofibric acid in 6 h with an applied bias of 2 V vs Ag/AgCl and a vsible light illumination of 300 W.It also showed a good photoelectric stability.For the application of BiVO4 based photoelectrochemical technology in the field of water splitting for hydrogen production,the electrospinning technology was proformed to prepare the BiVO4 based photoanode.Its characterizations were carried out and analyzed,and the mechanism of interfacial charge transfer at SEI for water oxidation was studied by photoelectrochemical methods.It was found that the optimum electrospinning time was 30 min,and the presence of surface states(SS)on the surface of the BiVO4 photoanode was suggested.The SS acting as reaction sites(i-SS)in favor of the interfacial charge transfer at SEI,was suggested to be the adsorption of oxy/hydroxyl species on the surface of the electrode by oxygen vacancy.It was originated from the electron trapping process during the the reversible redox process of V5+/V4+in the bulk of BiV04 photoanode.On the other hand,the irreversible surface reductive reaction of VO2+/Vo2+ though the electron trapping process could raise the electron accumulation at SEI forming SS acting as recombination center(r-SS),thus decreasing the photoelectrochemical performance.W doped BiVO4 based photoanodes were prepared by electrospinning technology.The samples were further characterized and analyzed.The influence of W doping on the charge transfer at SEI for water oxidation was studied by photoelectrochemical method.The bulk and surface of BiVO4 electrodes were modified by W addition.The photoelectrochemical performancewas listed as follows:BiVO4<1%W-BiVO4<5%W-BiVO4<2%W-BiVO4.It had been found that a high PEC performance required not noly an optimum ratio of density of surface states(Nss)with respect to Nd,but also numerous Nss and Nd.Low concentration of W doping(0%and 1%)had a high concentration of r-SS;the middle concentration of W doping(2%)could passivate r-SS,while improve i-SS,thus increasing the charge transfer at SEI;the high concentration of W doping(5%)not only passivated r-SS,but also suppressed i-SS,which was not conducive to the interfacial charge transfer at SEI.