Preparation And Modification Of TiO₂-BiVO₄ And Their Photoelectrocatalytic Performance On Degradation Of Typical Organic Pollutants

With the growth of population and the rapid development of economy,industry and agriculture,the total amount of pollutants discharged into aquatic environments increases year by year.Consequently,developing efficient and practical wastewater treatment technique has become a hotspot issue in the environmental field.Photoelectrocatalysis is an emerging advanced oxidation technique,which has the advantages of mild reaction conditions,utilization of solar energy,wide applicability,and no secondary pollution.It has broad application prospects in the field of wastewater treatment.The key to the efficient degradation of organic pollutants using photoelectrocatalysis is to design and fabricate high performance photoelectrocatalyst with wide visible light response,high conductivity,and large specific surface area.In this study,based on type ? heterojunction TiO₂-BiVO₄,polyimide(PI),glucose,and black phosphorus/red phosphorus(BP/RP)were employed to modify the composite material.The Z scheme was constructed to effectively separate photo-generated electron-hole pairs to improve the photoelectrocatalytic activity of the composite materials(TiO₂-BiVO₄-PI,TiO₂-C-BiVO₄,and TiO₂-BiVO₄-BP/RP).The prepared composite materials were well characterized by a variety of analysis and characterization methods.The photoelectrocatalytic properties and relevant mechanism were further studied,taking rhodamine B(Rh B),bisphenol compounds(BPs)and fluconazole(FLX)as target pollutants.The main findings are as follows.(1)Type ? heterojunction TiO₂-BiVO₄ composite material with fiddlehead-shaped morphology was designed via sol-gel method combined with solvothermal technology.The bionic structure effectively improved the performance of light absorption and adsorption of pollutants.At the same time,the external electric field can effectively inhibit the recombination of carriers by promoting the transfer of photogenerated electrons and holes in the type ? heterojunction.TiO₂-BiVO₄(5%)composite material showed the highest photoelectrocatalytic activity under the simulated sunlight and bias voltage(4 V).After 300 min,the degradation rate of Rh B(C0=10 mg/L;p H=5.3)achieved 97.2%,which was 17.8%higher than that of the photocatalytic system.In addition,the acute toxicity of degradation products was evaluated by Vibrio fischeri5269.Rh B was almost detoxified,because·O2^-produced by the system induced the fracture of C=C,C=N and C=O and ring opening in Rh B molecules,and thoroughly mineralization.(2)Using urea and pyromellitic dianhydride as raw materials,a PI material with excellent pore structure and light absorption characteristics was synthesized by thermal polymerization to modify TiO₂-BiVO₄.The doping of PI expanded the photoresponse range and improved the separation efficiency of electron-hole pair.Due to the existence of surface-surface contacted direct Z scheme between BiVO₄ and PI,holes and electrons were concentrated on the valence band of BiVO₄ and the conduction band of PI,respectively,resulting in a stronger redox activity.The photoelectrocatalytic activity of TiO₂-BiVO₄(1%)-PI(1%)composite material was the highest under the simulated sunlight and bias voltage(1 V).After 120 min,the degradation rates of the six BPs(C0=5 mg/L)were in the order as BPA(93.5%)>BPB(92.7%)>BPAP(85.6%)>BPF(75.9%)>BPAF(69.8%)>BPS(39.2%).After 360 min,the mineralization rate of BPs ranged from 31.7%to 66.8%.Under the attack of·O2^-and·OH,the intermediate products of BPs degradation were mainly formed via the substituent shedding or C-C bond breaking of phenol ring,hydroxylation,and ring opening of phenol ring,resulting in a lower mineralization rate.The ECOSAR program was used to analyze the changes in the toxicity of the intermediate products,and it was proved that the toxicity showed a decrease trend during the degradation process.(3)Using glucose as the carbon source,the TiO₂-C-BiVO₄ composite material was prepared by the sol-gel method combined with in-situ hydrothermal carbonization,in which the carbon existed in the graphite phase.Compared with TiO₂-BiVO₄,the doping of graphitic carbon improved the electron mobility of the composite material,broadened the light response range,enhanced the adsorption capacity of pollutants,and generated more active free radicals.As an electron conduction bridge,graphite carbon constructed a Z-type channel between TiO₂ and BiVO₄,adjusted the carrier transfer path,effectively inhibited the photogenerated electron hole recombination,and retained strong redox ability.The photoelectrocatalytic activity of TiO₂-C(1%)-BiVO₄(5%)composite material was the highest under the simulated sunlight and bias voltage(1 V).After 240 min,the degradation efficiency of BPS was 65.2%,and that of other BPs was over 95%.This was attributed to the fact that BPS molecule structure is stable,in which two hydroxyphenyl functional groups were connected by O=S=O.When the reaction time reached 360 min,BPs was almost completely mineralized.During the degradation process,the toxicity of BPB,BPAP and BPAF systems to Vibrio fischeri 5269 increased,but as the mineralization efficiency of BPs increased,the toxicity decreased significantly.(4)The BP/RP in-situ junction was fabricated under non-high temperature calcination using the ball milling-hydrothermal synergistic method.The multi-heterojunction TiO₂-BiVO₄-BP/RP composite material was successfully prepared by constructing a surface-to-surface Z-type heterojunction between BP/RP and BiVO₄two-dimensional material.Under the synergistic effect of the in-situ junction and the Z scheme,holes and electrons were concentrated on the valence band of BiVO₄ and the conduction band of BP/RP,respectively,which effectively improved the separation efficiency of photogenerated electron-hole pairs.Under the simulated sunlight and bias voltage(1 V),the degradation efficiency of TiO₂-BiVO₄-BP/RP on FLX(1 mg/L)was96.5%after 120 min.After ten cycles,the degradation efficiency of FLX was still over90.0%,and the morphology and chemical structure of TiO₂-BiVO₄-BP/RP did not change significantly,indicating that the multi-heterojunction composite material has excellent stability.Furthermore,TiO₂-BiVO₄-BP/RP exhibited high photoelectrocatalytic activity on the degradation of Rh B and BPA with degradation efficiency of 95.1%and 90.2%,respectively.