The Construction Of BiOIO₃ Based Composites And Their Photocatalytic Mechanisms For Enhanced Visible Light Photocatalytic Removal Of NO_x

Recently, air quality has surged to worldwide attention. Of these pollutants, excess NO_x（NO and NO₂） could lead to serious environmental and health problems including acid rain, photochemical smog, and even pulmonary edema. For NO_x purification, semiconductor photocatalysis as a green technology that could use sunlight to purify air pollutants, provides an attractive alternative to conventional approaches. Most recently, an novel Bi-based photocatalyst, BiOIO₃, composed of（Bi₂O₂）²⁺ and（IO₃）^- layers, shows excellent UV light photocatalysis. However, the large band gap endows BiOIO₃ with only UV light activity. In order to enhance its visible photocatalysis, we try to modify BiOIO₃ using semiconductors coupling. In this study, RGO/BiOIO₃ were fabricated by a one-pot hydrothermal method. BiOI/BiOIO₃ and Ag/AgCl/BiOIO₃ were prepared by a combination of hydrothermal method and chemical precipitation method. The as-prepared samples were systematically characterized by XRD, XPS, SEM, TEM, N2 adsorption-desorption isotherms, UV-vis DRS, photocurrent generation, FT-IR, Raman and ESR. Their photocatalytic activities were also evaluated towards removal of NO_x at ppb-level under visible light irradiation. The results are as follows:RGO/BiOIO₃ photocatalysts: RGO/BiOIO₃ were synthesized by a simple hydrothermal method. During which BiOIO₃ nanoplates were formed in situ on RGO sheets resulting from partial reduction of GO. The two components of the composites displayed intimate interfacial contact. In contrast to pure BiOIO₃, The as-prepared RGO/BiOIO₃ exhibited slightly increased UV photocatalytic activity toward removal of NO from air, as BiOIO₃ would be expected to compete with RGO with regard to absorption and utilization of UV light. However, RGO/BiOIO₃ showed highly enhanced visible photocatalytic activity, relative to that of pure BiOIO₃. Evidence showed that RGO acting as a semiconductor could be excited under visible light to yield charge carriers to induce photocatalytic reaction. In addition, photo-generated holes were found to be the main active species inducing the photo-oxidation of NO_x under visible light, whereas holes and ·OH were considered to be responsible for UV light photo-activity.BiOI/BiOIO₃ photocatalysts: BiOIO₃ was prepared by a simple hydrothermal method, then BiOI were in situ grown on the surface of BiOIO₃ by a chemical precipitation method. Herein, the concepts of 2D-2D heterostructures and exposed active facets were integrated into BiOI/BiOIO₃ for enhanced photocatalysis. The BiOIO₃ nanosheets with exposed {010} facets were first synthesized. As the arrangement of atoms of（010） facets of BiOIO₃ was the same as that of the（001） facets of BiOI, the pristine BiOIO₃ nanosheets then functioned as substrates to induce the preferential growth of BiOI along the（001） plane via providing interfacial oxygen atoms. As the oxygen atoms at the interface were shared by BiOIO₃ and BiOI nanosheets, large contact areas and an intimate interface could be achieved. Benefiting from ehanced visible absoption and efficient charge transfer, along with the active sites provided by expsosed facets, the as-obtained BiOI/BiOIO₃ showed increased visible light photocatalytic efficiency and stability for the removal of ppb-level NO_x.Ag/AgCl/BiOIO₃ photocatalysts: BiOIO₃ was first prepared by a simple hydrothermal method, then Ag/AgCl particles were in situ introduced into BiOIO₃ system by a chemical precipitation method, and thus obtaining Ag/AgCl/BiOIO₃ composites. Enhanced visible-light absorption and charge carrier separation were achieved after the introductionof Ag/AgCl particles into BiOIO₃ systems. The ternary Ag/AgCl/BiOIO₃ photocatalysts were applied to the visible light photocatalytic oxidization of NO_x in air and exhibited enhanced activities in comparison with Ag/AgCl and pure BiOIO₃, which was related to the surface plasmon resonance effects of Ag metal and the effective carrier separation ability of BiOIO₃.This study provides new insights into the preparation, modification of Bi based photocatalysts and their photocatalytic mechanism and also establishes technological base for their future application.