Novel Visible-light-driven Bismuth-based Nanomaterials For The Photocatalytic Degradation Of Organic Pollutants

Since photocatalysis can degrade and mineralize organic pollutants and is high efficient,low cost,easily available and environmental friendly,it has been recognized as a promising method for water and wastewater treatment.It has been reported that various bismuth-based(Bi-based)semiconductor nanomaterials including bismuth oxyhalide,bismuth tungstate,bismuth vanadate possess photocatalytic activities.However,there are three limitations for their actual applications,i.e.,low response towards visible light,low mineralization efficiency and low stability.In this dissertation,various modification and optimization strategies,including phase control,morphology design,element doping,forming heterojunction and introducing lattice defects,were adopted for Bi-based photocatalysts.These strategies were found to partially solve the above three problems.Moreover,the physicochemical properties of the as-prepared photocatalysts were characterized and the photocatalytic mechanisms were elucidated in details.As a result,the rationality and feasibility of applying Bi-based photocatalysts for the photocatalytic degradation of organic pollutants were demonstrated.The main contents and achievements of this dissertation are as follows:1.In order to enhance the response of bismuth oxychloride(BiOCl)towards visible light,whose band gap is very broad,the oxygen-rich modification was adopted by tuning the ratio of Bi source and Cl source or adding NaOH in the solvothermal process.With the weakening of[Cl2]2-layers,Bi12O15Cl6 nanosheets and Bi12O17Cl2 nanobelts were obtained.Their band gaps were much narrower than that of BiOCl because their conduction band bottom positions were positive-shifted.The prepared Bi12O15Cl6 nanosheets and Bi12O17Cl2 nanobelts possessed strong responses towards visible light and were able to degrade bisphenol A efficiently under visible light irradiation,which is a typical non-dye organic pollutant.2.The efficiency of Bi-based photocatalysts for visible light absorption can be further enhanced by introducing a doping energy level between the valence band top and conduction band bottom of a semiconductor.Co-doped BiOCl nanosheets were fabricated and an additional Co-doping energy level was introduced into the band gap of BiOCl by the Co-doping modification.This strategy assisted the excitation of photogenerated electrons and made BiOCl possible to show photocatalytic activity under visible light irradiation.Moreover,the phase and the layered structure of BiOCl host crystal were retaied after Co-doping modification,and static electrostatic field between[Bi2O2]2+ and[Cl2]2-layers could induce the separation of charge carriers.As a result,the high-efficient photocatalytic degradation of bisphenol A under visible light irradiation was realized using the Co-BiOCl photocatalyst.3.Enhancing the oxidizability of photogenerated holes is a promising strategy to enhance the mineralization efficiency as the photocatalyst can directly oxidize H2O and generate ·OH.In order to tailor the band structure of bismuth oxybromide(BiOBr),Bi24O31Br10 nanobelts were synthesized with a higher oxidizability.The measured potential energy of its valence band top was more positive than the energy barrier of OH-/·OH.As a result,the photogenerated holes could oxidize H2O to produce ·OH,whose concentration in the photocatalytic reaction was substantially enhanced.As a result,the Bi24O31Br10 nanobelts possessed a high mineralization efficiency and could efficiently treat actual industrial wastewaters.4.A higher degradation and mineralization efficiency can be further improved by enhancing the charge separation efficiency of the photocatalyst.Bi24O31Br10 nanosheets with controllable thickness were synthesized.With a decrease in thickness,the oxygen vacancies on the surface or in the shallow lattice became dominant,which could efficiently trap the photogenerated electrons and enhance the charge separation efficiency.Tetracycline could be efficiently degraded uner visible light irradiation over the ultrathin Bi24O31Br10 nanosheets.On the other hand,by coupling ZnO with Bi24O3iBr10,the heterojunction of ZnO/Bi24O31Br10 were obtained.With the induction effect of the heterojunction interfaces,the photogenerated holes and electrons could be perfectly separared into the two crystal phases respectively,thus,the recombination of hole-electron pairs was substantially suppressed.In this way,the generation of reactive oxygen species over these photocatalysts was greatly enhanced,and free radicals became the main active species in the photocatalytic reaction.As a result,the ZnO/Bi24O31Br10 heterojunction exhibited an enhanced performance for the mineralization of bisphenol A.5.Since the[X2]2-layers in bismuth oxyhalide crystals were formed on the basis of van der Waals forces,halogen ions in bismuth oxyhalides tend to be released into the reaction system or exchanged with other anions,leading to the inactivation of the photocatalyst.In order to couple the high photocatalytic activity of bismuth oxyiodide(BiOI)with the good stability of bismuth tungstate(Bi2WO6),I-doped Bi2WO6 nanosheets was synthesized.The doped I element substantially enhanced the light absorption efficiency and charge separation efficiency of Bi2WO6.The stable crystal structure of Bi2WO6 was retained after I-doping modification.The fabricated I-doped Bi2WO6 nanosheets exhibited a high photocatalytic activity and stability under visible light irradiation,which would not be inactivated by the presence of S2-.