Preparation And Photocatalytic Properties Of Bismuth Based Photocatalysts

Semiconductor photo-catalysis,a kind of green and environmental pollution control technique,uses sunlight to generate electrons and holes,and then the organic pollutants absorbed on the surface of catalyst are degraded into CO₂ and H₂O through a series of oxidation-reduction reactions.Among the bismuth-based photo-catalysis,CuBi₂O₄ has attracted widely attention due to the unique electronic structure,higher cathode photovoltage,and narrower band gap?1.5-1.8 eV?.However,CuBi₂O₄ also has some obvious disadvantages,such as long and complicated preparation process for preparation,weak oxidation ability,easy recombination of the photogenerated electron-hole pairs,low carrier transmission efficiency and etc.To overcome these problems,a short-time hydrothermal method was developed to prepare CuBi₂O₄ and several new CuBi₂O₄ composite photocatalysts was fabricated in this dissertation.Firstly,the influencing factors for hydrothermal method of CuBi₂O₄ was investigated,aiming to optimize the process conditions and shorten the preparation period.Secondly,the tetragonal CuBi₂O₄/amorphous BiFeO₃ composite photocatalysts and g-C₃N₄/CuBi₂O₄ composite photocatalysts were prepared,to suppress the photo-generated electrons-hole pairs recombination and improve the carrier transport efficiency of CuBi₂O₄.Finally,the physicochemical properties of the prepared photocatalyst such as structure,morphology and band gap are analyzed,and the photocatalytic activity of organic pollutants is evaluated to simulate the photocatalytic degradation effect of organic pollutants.The main contents and achievements of this dissertation are as follows:In order to solve the problem of long preparation period of traditional hydrothermal method,a short-time hydrothermal method at mild reaction conditions was developed to prepare CuBi₂O₄ nanocolumn arrays,by using Bi?NO₃?·5H₂O in acetic acid and Cu?NO₃?·3H₂O in ethanol as precursor solutions.The influences for the crystal structure,morphology,and band gap of CuBi₂O₄ by the experimental conditions including hydrothermal time,hydrothermal temperature,and NaOH concentration were investigated by X-ray diffractometer?XRD?,scanning electron microscope?SEM?,and UV-vis spectrophotometer.In addition,the removal rate of methylene blue was used to evaluate the photocatalytic performance of the obtained CuBi₂O₄ photocatalyst.The results showed that a tetragonal CuBi₂O₄ nanocolumn arrays with a band gap of 1.75 eV was fabricated within 30 min at 120?and the NaOH concentration of 6.5 M.Its show good visible light absorption?700 nm?and photocatalytic property.In the presence of a small amount of H₂O₂?50?L?,using a xenon lamp as the light source???420 nm?for irradiation of 30 min,the removal rate of methylene blue?50 mL,0.02mM?was 91%,which is comparable to the CuBi₂O₄ prepared by traditional methods.After 5photocatalytic cycle experiments,the removal rate of methylene blue by the CuBi₂O₄ was decreased 6.6%,and the CuBi₂O₄ maintain the tetragonal structure.Therefore,we proposes a novel strategy to prepare CuBi₂O₄ within 30 min at a moderate temperature of 120?,which provides a facile strategy for the fast synthesis of metal-oxide-based photocatalysts at mild reaction conditions.In order to reduce the recombination of photo-generated electrons-hole pairs and improve the photocatalytic activity,a tetragonal CuBi₂O₄/amorphous BiFeO₃ composite photocatalyst?T-CBO/A-BFO?was prepared by a one-step hydrothermal method.The influences for the crystal structure,morphology,and band gap of T-CBO/A-BFO by different ratios of tetragonal CuBi₂O₄ and amorphous BiFeO₃ were investigated via XRD,SEM,and UV-vis spectrophotometer.In addition,the removal rates of methylene blue and methyl orange were utilized to evaluate the photocatalytic performance of T-CBO/A-BFO.The results demonstrated when the composite ratio of T-CBO and A-BFO is 1:1,the morphology of the prepared T-CBO/A-BFO is irregular particles,with a visible light absorption of 670 nm,which broadened the application ranges in visible light.The T-CBO/A-BFO was irradiated within 30 min,and the removal rate of methylene blue?50 mL,0.02 mM?was increased from 56%?T-CBO?and 19%?A-BFO?to 97%,increasing of 41%and 78%,respectively.Within 120 min of illumination,the removal rate of methyl orange?50 mL,10 mg/L?increased from 28%?T-CBO?and 63.6%?A-BFO?to 80.4%,increasing of 52.4%and 16.8%,respectively.Thus,the photocatalytic activity was significantly improved.After 5 photocatalytic cycle experiments,the removal rate of methyl orange by T-CBO/A-BFO was decreased 8.4%.But the characteristic peaks appeared in XPS spectrum of Bi₂O_2.75 after cyclic experiment.Thus,the stability of the T-CBO/A-BFO also needs to be improved.Nevertheless,T-CBO/A-BFO has the advantages of simple preparation method,narrow band gap,and high utilization rate of visible light and high photocatalytic activity.To further improve the carrier transfer efficiency during photocatalytic reaction,a g-C₃N₄/CuBi₂O₄ composite photocatalyst was prepared by NaBH₄ reduction method.The influence for the crystal structure,morphology,and band gap of the g-C₃N₄/CuBi₂O₄ by different composite ratios of g-C₃N₄/CuBi₂O₄ and different mass ratios of NaBH₄/Bi?NO₃?₃were studied via XRD,SEM and UV-vis spectrophotometer.In addition,the removal rate of methyl orange was used as an index to evaluate the photocatalytic performance of g-C₃N₄/CuBi₂O₄ composite photocatalyst.The result showed when a mass ratio of g-C₃N₄ to CuBi₂O₄ is 2:1 and NaBH₄ to Bi?NO₃?₃ is 0.06,a g-C₃N₄/CuBi₂O₄-R_0.06 with a band gap of 2.35eV and an absorption threshold of 528 nm was obtained.The removal rate of the prepared g-C₃N₄/CuBi₂O₄-R_0.06 for methyl orange?50 mL,10 mg/L?within 30 min was increased from 11%?g-C₃N₄?,14%?CuBi₂O₄?,and 57%?g-C₃N₄/CuBi₂O₄?2:1??to 99.3%,increasing of 88.3%,85.3%and 42.3%,respectively.After 5 photocatalytic cycle experiments,the removal rate for methyl orange of g-C₃N₄/CuBi₂O₄-R_0.06 was decreased 12.3%,and the crystal structure did not change,which demonstrated that the g-C₃N₄/CuBi₂O₄-R_0.06 is stable and reusable.