Construction Of TiO₂-based Visible Light Photocatalysts And Photocatalytic Oxidation Performance Study

With the rapid economic development and continuous human activities,environmental pollution and energy shortage have become two major world problems,especially water pollution which is closely related to human health.Among various water pollutants,antibiotics are highly biotoxic,highly concentrated and difficult to degrade,which are difficult to be effectively degraded by traditional water treatment methods.Photocatalytic oxidation technology is a new advanced oxidation technology with high efficiency,safety,mild reaction conditions,energy saving,environment friendly and thorough purification,which shows great application prospects in the treatment of antibiotic pollution.The development of new,efficient and stable photocatalytic materials is an important direction for the development of photocatalytic technology.TiO₂,as the most classical photocatalyst,has the advantages of low cost,no pollution and good stability,and is recognized as the most economical,environmentally friendly and efficient photocatalytic material.However,the wide band gap of TiO₂leads to the problems that it can only absorb UV light and photogenerated carriers are easily compounded,which largely limit its practical application scope.To solve the above problems,this thesis adopts the strategies of ion doping,noble metal loading,defect modulation and construction of heterojunctions to modify TiO₂,based on which,the degradation performance and action mechanism of TiO₂-based composites on typical antibiotics in water are investigated.The work done in this paper and the main results achieved are:（1）N-TiO₂and Ag/AgCl/N-TiO₂photocatalysts were synthesized by sol-gel and photodeposition methods.the doping of N elements broadened the light absorption range of TiO₂and formed anatase TiO₂and rutile TiO₂heterojunctions;the surface loading of Ag further enhanced the light absorption capacity and improved the photogenerated charge transfer efficiency.Thanks to these advantages,Ag/AgCl/N-TiO₂showed 94%degradation of ciprofloxacin under visible light irradiation and still had good photocatalytic stability after four cycles.Kinetic studies showed that the photocatalytic degradation of ciprofloxacin followed quasi-primary reaction kinetics with a reaction rate constant of 0.015 min^-1for Ag/AgCl/N-TiO₂,which was about 2.5 and 1.7 times higher than that of TiO₂(0.006 min^-1)and N-TiO₂(0.009 min^-1).The improved photocatalytic activity was attributed to the enhanced visible light absorption and the increased efficiency of photogenerated electron-hole separation.The degradation of ciprofloxacin molecules into small molecules and finally into carbon dioxide and water was achieved by the synergistic effect of photogenerated holes and superoxide radicals.（2）Oxygen-rich vacancy-site TiO₂nanosheets and 2D/2D TiO₂/BiOBr heterojunction photocatalysts were synthesized by a simple hydrothermal and one-step in situ method,and the photocatalytic degradation performance of hygromycin hydrochloride was investigated.Among them,TOB-0.1（TiO₂to BiOBr molar ratio of 1:9）showed the best photocatalytic performance with 93.3%removal of hygromycin hydrochloride after 2 h of visible light irradiation,and still maintained more than 80%removal after 4 cycles.The reactive radical capture experiments showed that photogenerated vacancies were the main active species in the photocatalytic reaction process.It is shown that TOB-0.1 has the most excellent photocatalytic performance,which is mainly attributed to the introduction of oxygen vacancies to increase the visible light absorption and the 2D/2D interface formed between TiO₂and BiOBr to shorten the migration distance of photogenerated carriers and prolong the lifetime of photogenerated carriers.（3）TiO₂with Ti defects,BiOCl with Bi defects and TiO₂/BiOCl with bimetallic defects were synthesized by hydrothermal and post heat treatment methods,and the photocatalytic performance of the catalysts was evaluated by the degradation efficiency of tetracycline hydrochloride under visible light.Among them,the 90TB（TiO₂to BiOCl mass ratio of 9:1）composite showed the most excellent performance with 94.6%removal of tetracycline hydrochloride after 2 h of visible light irradiation.Cycling tests and comparison of surface morphology before and after the reaction showed that 90TB has excellent stability and reproducibility.Valence band spectroscopy and Mott Schottky curves confirmed the presence of Ti and Bi defects,photochemical tests showed that 90TB possessed the highest photogenerated carrier separation and migration efficiency,and active species capture experiments indicated that photogenerated holes played a major role in the photocatalytic reaction process.The synergistic effect of metal defects and heterojunctions optimizes the separation and migration of photogenerated carriers,resulting in the highest photocatalytic activity of 90TB.