Modification And Application Mechanism Of Bismuth-based Semiconductor Nanomaterials

Photocatalysis technology has been widely concerned in solving energy crisis and environmental pollution,and its research has made a lot of achievements.However,the discovery of efficient,environmentally friendly and stable catalysts has puzzled most researchers.According to the action mechanism of photocatalysts in different fields,most photocatalysts need to overcome the defects of wide band gap,easy recombination of photogenerated charge and low charge utilization rate.Due to the existence of Bi 6s and O 2p hybrid orbitals in valence bands of bismuth-based photocatalysts,most bismuth-based photocatalysts have relatively narrow visible light utilizable band gaps,so more and more attention has been paid to them.In this paper,bismuth-based semiconductor nanomaterials were modified by doping,oxygen vacancy introduction and heterostructure construction.Fe-doped Bi OCOOH,oxygen-vacancy Bi OCl/Bi₄O₅Br₂ and Co WO₄/Bi₂WO₆ Z-type heterojunctions were designed and synthesized to improve the photocatalytic performance of the three Bi-based materials.Firstly,with N,N-dimethylformamide as C source and organic solvent,Fe（NO₃）₃·9H₂O was added in the process of preparing Bi OCOOH by one-step solvothermal method,and Fe³⁺doping Bi OCOOH was realized under high temperature and pressure reaction conditions.Rhodamine B（Rh B）was used as the degraded pollutant to evaluate the performance of the synthesized catalyst under 300W xenon lamp light source.The results showed that after 50 min of light exposure,the degradation effect of pure Bi OCOOH on Rh B was not obvious,with a degradation rate of only 35%.However,the photocatalytic degradation efficiency of Rh B with 0.8%wt Fe doped Bi OCOOH was as high as 90%for the same light duration.It can be seen that the photocatalytic performance has been significantly improved.Scanning electron microscopy（SEM）analysis showed that Fe-doping made the Bi OCOOH sheets more porous and larger specific surface area,which was conducive to improving the adsorption of pollutant molecules by the catalyst.Fluorescence spectra（PL）,photocurrent and electrochemical impedance spectra show that the fluorescence intensity of doped materials decreases,the photocurrent intensity increases,the resistance of charge movement decreases,and the photogenerated charge recombination weakens.The ultraviolet-visible diffuse reflection spectra show that the doping energy level formed by Iron trivalent doping helps to reduce the band gap value of the material,so that the electrons can be excited by visible light.These are important reasons for the improvement of photocatalytic performance.The Bi OCOOH catalyst doped with 0.8%wt Iron trivalent was used for five cycles.Under the same experimental conditions,the degradation rate loss of Rh B remained at about10%after five cycles,which fully demonstrated the good stability of the prepared material.The active species tests confirmed that·O₂^-was the primary active substance,followed by h⁺,and finally·OH.Based on this,we propose a possible photocatalytic mechanism.Secondly,the compound heterojunction of Bi OCl and Bi₄O₅Br₂ was synthesized by using the reducibility of ethylene glycol in a two-step method.In this study,the antibiotic ciprofloxacin（CIP）was selected as the degradation target,and the performance of different catalysts was tested.After 90 min of illumination,the CIP degradation rates of bismuth oxychloride and Bi₄O₅Br₂ were about 33%and 43%,respectively.The CIP degradation rate of composites with 1 wt%of bismuth oxychloride was as high as 93%at the same time,and the photocatalytic degradation performance of composites was significantly improved.After five cycles,there was almost no obvious change in the morphology of the composites,and the degradation rate of CIP did not decrease significantly,which indicated that the composites had good stability.Finally,CoWO₄/Bi₂WO₆ Z-type heterostructure were successfully synthesized by one-step hydrothermal method by means of co-ionic system.The antibiotic ciprofloxacin（CIP）was also selected as the degradation target to test the catalytic performance of different samples.The experimental results showed that after 120 min of light exposure,The CIP degradation rates of Co WO₄ and Bi₂WO₆ were 11%and25%,respectively,at the same time,when the molar ratio of Co:Bi was 1:5（denoting as 1:5CBWO）.It can be seen that the heterogeneous interface was built and the photocatalytic performance of pure materials was improved.The operation steps of the three modification methods are simple.The modification principle is from the internal electronic structure of the material to the charge transfer between materials,and the research on the charge transfer mechanism is progressive layer by layer from inside to outside.