Oxygen Vacancy Regulation And Photocatalytic Properties Of Bismuth Oxychloride With Multistage Structure

With the rapid development of science and technology and industry,the emergence of many emerging industries leads to the discharge of a large number of industrial wastes into groundwater,which seriously harms human health and water resources ecological environment.Semiconductor photocatalysis,which uses solar energy as the main energy source,is considered as one of the most ideal technologies to deal with current water pollution problems due to its characteristics of energy saving,high efficiency and no secondary pollution.However,the application of photocatalysis in environmental remediation is limited due to the limited spectral response range and low photo-generated carrier separation efficiency of traditional photocatalysts represented by titanium dioxide（Ti O₂）.Bismuth oxychloride（Bi OCl）is considered as one of the most promising semiconductor photocatalysts owing to its unique open layered structure and reasonable energy band structure.However,the photocatalytic degradation efficiency of Bi OCl under visible light is not ideal due to the large band gap,its photocatalytic performance needs to be improved.The introduction of oxygen vacancy into Bi OCl materials is an effective method to enhance its photocatalytic activity.In addition,recent researches have found that the concentration of oxygen vacancies in the catalysts also has an significant effect on the photocatalytic performance.Based on the above analysis,in this paper,the coprecipitation method controlled by hydrogen ion,microwave-assisted solvothermal method and electrochemical method were used to introduce oxygen vacancies into Bi OCl materials for enhancing their photocatalytic performance.The oxygen vacancy content in Bi OCl was regulated by changing the process parameters,and the effect of oxygen vacancy concentration on the photocatalytic performance of Bi OCl was further explored.The main conclusions were reached in this paper through several aspects of the research:1.Bi OCl-X microspheres（X represents the content of oxygen vacancy in the sample,X=10.5%,13.6%,16.4%and 20.3%）with oxygen vacancies were synthesized by coprecipitation method at room temperature,and the oxygen vacancy concentration in Bi OCl-X was regulated by changing the content of H⁺in the reaction system.The as-prepared samples were further applied to the photocatalytic degradation of Rh B and CIP.The results indicated that the photocatalytic degradation rate increased at first and then decreased with the increase of oxygen vacancy concentration of Bi OCl-X from 10.5%to 20.3%.It is mainly because that the optimized increase of oxygen vacancy content could reduce the transfer resistance of photo-generated electrons and promote the separation of carriers,while excessive oxygen vacancies would act as photo-generated electron-hole recombination centers to improve their recombination efficiency,thus reducing the photocatalytic activity of the catalyst.Therefore,the Bi OCl-13.6%microspheres with optimized oxygen vacancy concentration exhibited the excellent photocatalytic activity.The degradation rate constants of Rh B and CIP were 0.834min^-1and 0.018min^-1,respectively,and the enhancement factors were about 1.7 times and 1.6 times,respectively.2.Bi OCl-X（X=100,120,140 and 160）photocatalysts with different oxygen vacancy concentrations were rapidly synthesized by microwave-assisted solvothermal method using ethylene glycol as solvent.With the increase of reaction temperature,the oxygen vacancy concentration of Bi OCl-X was increased,and its morphology structure was transformed from microflower to nanosheet.Benefiting from the introduction of oxygen vacancies,the Bi OCl-X showed superior photocatalytic performance than pure-Bi OCl nanosheets without oxygen vacancies.The characterization results revealed that the introduction of oxygen vacancy not only expanded the spectral absorption range of Bi OCl,but also improved the separation efficiency of photo-generated electron-holes.However,with the further increase of oxygen vacancy concentration,the transfer efficiency of photo-generated carriers in Bi OCl-X was decreased obviously,which indicated that excessive oxygen vacancies aggravated the rapid recombination of carriers.Therefore,the Bi OCl-100microflowers with the larger specific surface area and optimized oxygen vacancy concentration exhibited the most excellent photocatalytic activity.The degradation rates of Rh B,MB and CIP under visible light irradiation were 0.512,0.022 and0.022min^-1respectively,and the corresponding enhancement factors were as high as4.3,1.7 and 5.5.3.Using the Bi OCl microspheres in the first work as precursor materials,the Bi OCl-X（X=0,3,and 6）photocatalysts with higher surface oxygen vacancy concentration was obtained by electrochemical treatment of the above powders,and the oxygen vacancy was regulated by changing the treatment time.The obtained Bi OCl-X were used for the photocatalytic degradation of pollutants,the Bi OCl-6micropheres showed excellent photocatalytic degradation activity.The photocatalytic degradation rate of MB and CIP under visible light could reach 0.071min^-1and0.056min^-1,which is 7.0 and 4.3 times higher than that of the precursor Bi OCl-0,respectively.The improvement of photocatalytic performance could be attributed to the following two aspects:（1）The thickness of nanosheets in Bi OCl-6 microspheres was decreased obviously after electrochemical treatment,which led to the larger specific surface area,thus improving its adsorption capacity for pollutants;（2）The electrochemical surface treatment process increased the concentration of surface oxygen vacancies in Bi OCl,and the more surface oxygen vacancies could separate photogenerated carriers more effectively,which was more conducive to photocatalytic degradation reaction.In addition,after three repeated degradation experiments,Bi OCl-6 could still remove about 86%of MB which indicated the superior photocatalytic cycling performance.