Structural Design,Controllable Preparation And Photocatalytic Properties Of Bi_xO_yCl_z@TiO_2-n Fibers

With the rapid development of industrialization,environmental problems such as water pollution have drawn wide attention of all circles of the society.It has become one of the important tasks for researchers to seek an efficient and low consumption environment-friendly water treatment technology.In recent years,photocatalytic technology has shown great application potential because it is expected to directly use solar energy to solve the problem of water environment prevention.The oxygen-deficient TiO2(TiO2-x),as a photocatalyst with stable properties,low cost and visible light activity,is favored in water treatment,especially in the degradation of organic pollutants in water.However,its photocatalytic efficiency is poor,and there are some inherent defects,such as low light absorption efficiency,high recombination rate of photoinduced electrons and holes,weak adsorption and enrichment capacity,and difficulty in recovery and separation after water treatment.Therefore,the development of efficient and stable utility model TiO2-n based photocatalysts is of practical significance for the application of photocatalytic technology in practical water treatmentIn view of the above issues,in this paper,NaCl/TiO2-n fibers with graded mesoporous structure were prepared by a sol-gel method combined with centrifugal spinning technology and heat treatment in steam.The hydrochemical reaction of TiCl4 under the alkaline condition provided by NaOH aqueous solution can not only contribute to the controllable preparation of TiO2 products,but also play the role of"chlorine fixation" in the process,which means to convert chloride ions into NaCl salt crystals and realize resource recycling.In other words,the "self-manufactured and self-sacrificial" water-soluble pore forming agent is generated and utilized online.Based on the NaCl/TiO2-n fibers as the supporting material,three types of heterostructured BixOyCl2@TiO2-n fiber photocatalysts with graded porous structure,high visible light response range and photocatalytic activity were prepared by morphological regulation and heterostructure construction.They are p-n BiOCl@TiO2-n,direct Z-scheme Bi12O17Cl2@TiO2-n and all-solid-state Z-scheme Ag/Bi12O17Cl2@TiO2-n fibers,respectively.The basic research and exploration for their physical and chemical properties,structure-activity relationship and photocatalytic mechanism were launched by utilizing a series of instruments characterization means.The main research contents are as follows:(1)The BiOCl@TiO2-n fibers with p-n junction and grade porous structure were prepared by treating prefabricated NaCl/TiO2-n fibers through Bi(NO3)3 solution in a pot deposition process at room temperature and pressure.In this process,NaCl in the libers could be not only water-dissolved to form porous structure,but also used as the chlorine source to synchronously realize the in situ controllable preparation of BiOCl nanosheet crystal on the fiber surface.Flower-like BiOCl nanosheets and grade porous structure can provide more surface area and active sites for photocatalytic fibers,thereby enhancing the adsorption and enrichment ability of the products.The BiOCl@TiO2-n p-n heterojunction itself can effectively promote the spatial separation of photoinduced electrons and holes.In the photocatalytic degradation probe experiments of azo dye reactive brilliant red(X-3B)and colorless phenol in water,the as-prepared BiOCl@TiO2-n composite catalysts showed excellent photocatalytic activity and reusability stability,which provides the experimental basis for the establishment of correlation structure-activity relationship in water treatment.The dominant active species and their effects on photocatalytic reactions were confirmed by the scavenger experiments and electron spin resonance characterization.The possible photocatalytic mechanisms were discussed in detail.(2)BiOCl,as a wide band-gap semiconductor,is only excited in the ultraviolet region,resulting in low utilization rate of solar energy over BiOCl@TiO2-n fibers.Given this issue,the surface chemical modification of prefabricated NaCI/TiO2-n fibers was carried out in Bi(NO3)3 solution system via a hydrothermal method.Through regulating halogen content,a direct Z-scheme Bi12O17Cl2@TiO2-n photocatalytic fibers with broadband spectral response were prepared.In the preparation process,the NaCl in the fibers can synchronously realize the water-dissolution preparation of porous structure and act as the chlorine source to achieve the in situ modification of Bi12O17Cl2 nanosheets with high visible light response on the fiber surface.Combined with the UV-vis DRS analysis,the maximum of absorption wavelength of BiOCl is 354 nm and its band gap is 3.5 eV.The maximum of absorption wavelength over Bi12O17Cl2 red shifts to 510 nm and its band gap narrows to 2.41 eV,leading to the excellent visible light capture ability of the latter's compound Bi12O17Cl2@TiO2-n fibers.Nitrogen adsorption and desorption tests show that Bi12O17Cl2@TiO2-n fibers have ordered mesoporous structure.The pore sizes are mainly distributed in 3.3,4.5,11.8 and 20.1 nm.This graded mesoporous structure provides more surface area and active sites.On this basis,the selected photodeposition of Ag and scavenger experiments further revealed the design and construction of direct Z-scheme Bi12O17Cl2@TiO2-n heterojunction.Studies have shown that the formation of such Bi12O17Cl2/TiO2-n heterojunction can effectively promotes the separation of photoinduced electrons and holes.In the evaluation test of photocatalytic activity,Bi12O17Cl2@TiO2-n fibers showed excellent photocatalytic activity and stability,which provides strong evidence for above analysis.(3)In order to further improve the visible light capture ability and inhibit the recombination of photoinduced electrons and holes over photocatalytic fibers,the Ag/Bi12O17Cl2@TiO2-n fibers were prepared through the selective photoreduction deposition of Ag nanoparticles on the surface of Bi12O17Cl2@TiO2-n fibers.The FESEM and HRTEM characterization confirmed the selective deposition of Ag nanoparticles on the surface of Bi12O17Cl2@TiO2-n fibers contributes to the formation of Bi12O17Cl2-Ag-TiO2-n heterojunction interface.Further analysis shows that this new Bi12O17Cl2-Ag-TiO2-n interface can be attributed to the formation of all-solid-state Z-scheme heterojunction,and its construction can more effectively avoid the direct recombination of photoinduced electrons and holes.Moreover,because of the synergistic effect of the localized surface plasmon resonance(LSPR)induced by Ag nanoparticles,the optical light capture ability of the photocatalytic fiber product is significantly improved.Compared with the above Bi12O17Cl2@TiO2-n fibers,Ag/Bi12O17Cl2@TiO2-n fibers have wider spectral response range and stronger visible light absorption.Under the experimental conditions,Ag/Bi12O17Cl2@TiO2-n fibers exhibit the optimal photocatalytic activity and stability.Based on the experimental results and theoretical analysis of this preparation system,the possible photocatalytic mechanism was discussed and some new ideas were put forward.In conclusion,the research topic is clear and step-by-step,and the research results are expected to provide new ideas,methods and mechanisms for the preparation and application of new photocatalysts with broadband response in water treatment.