Photo-Electrochemical And Photocatalytic Mechanism Of Binary BiOI-Based Composites

The development of industrialization and rapid population growth have led to serious environmental pollution and energy shortages,resulting in new demands for the improvement of the environment and the development of new energy sources.Photocatalytic technology has the advantages of high efficiency,environmental protection and economy,and is one of the effective means to control environmental pollution.And the core of this technology is photocatalyst.Among many new semiconductor photocatalysts,BiOI has attracted widespread attention due to its unique layered structure,narrow bandgap,and effective separation of photo-generated carrier.The current research focuses on overcoming the shortcomings of high recombination rate of photo-generated electron-hole pairs,poor energy band electric potential and difficulty recovery,in order to improve its photocatalytic performance and meet the treatment requirements of the actual water environment.In this dissertation,the morphology of the BiOI photocatalyst was adjusted,and three kinds of semiconductor heterojunction composites,BiOI/CoFe₂O₄,BiOI/g-C₃N₄ and BiOI/SnS₂,were constructed to improve the separation ability of photo-generated charges.The phase structure,microscopic morphology,and optical properties of the materials were characterized by X-ray diffraction（XRD）,scanning electron microscope（SEM）,high-resolution transmission electron microscope（HRTEM）,energy dispersive X-ray spectroscopy（EDS）,X-ray photoelectron spectroscopy（XPS）,ultraviolet-visible diffuse reflectance spectroscopy（UV-Vis DRS）,Fourier Infrared Spectroscopy（FT-IR）,and Photoluminescence Spectroscopy（PL）,and test the photo-electrochemical and photocatalytic properties,explore the photocatalytic mechanism of three kinds of composites.The specific research contents are as follows:1.Using the co-precipitation method,by changing the reaction solvent and adjusting the pH value,three-dimensional flower-like,two-dimensional flake-like,and micro-spherical tetragonal BiOI photocatalysts were prepared.The particle size distribution of flower-like BiOI is about 1μm,the average particle size of microspheres-like BiOI is about 0.51μm,and the thickness of flake BiOI is about 40 nm.Among them,the flower-like BiOI has the narrowest bandgap,which is 1.93 eV,showing a strong visible light response.On the other hand,BiOI microspheres degraded 92% of Rh B when irradiated with a xenon lamp for 90 min,showing the best photocatalytic activity.Crystallinity,microscopic morphology,and particle size synergistically affect the photocatalytic performance of BiOI semiconductor materials.2.Cubic spinel CoFe₂O₄ nanoparticles were prepared by hydrothermal method,and then the magnetically recovered flower-like BiOI/CoFe₂O₄ composite photocatalyst was synthesized by co-precipitation method.Spherical CoFe₂O₄ nanoparticles are uniformly distributed on the surface of the three-dimensional flower-shaped BiOI nanosheets,forming a direct Z-scheme heterojunction of BiOI/CoFe₂O₄.PL spectra and transient photocurrent response indicate that BiOI/CoFe₂O₄ semiconductor heterojunction reduces the recombination rate of photo-generated electron-hole pairs and promotes their migration and separation.The BiOI/CoFe₂O₄nanocomposite exhibits excellent visible light absorption capacity and excellent photocatalytic performance under simulated sunlight.Among them,the BiOI/CFO-3 photocatalyst has the smallest bandgap and the best photocatalytic activity.After 3 cycles of BiOI/CFO-3photocatalyst,the degradation efficiency still reaches 80.24%,showing excellent cycle stability.3.The g-C₃N₄ samples were prepared by the high-temperature calcination method,and the BiOI/g-C₃N₄ semiconductor composites were synthesized by the co-precipitation method.The composites contain flower-like tetragonal BiOI and lamellar g-C₃N₄,and Contain C,N,O,Bi,and I elements.The visible light absorption capacity of BiOI/g-C₃N₄ composite material is better than pure g-C₃N₄,and the photocatalytic activity is better.Among them,the first-order kinetic reaction rate constants of BiOI/g-C₃N₄（2:1）are 7.4 times and 1.6 times of pure g-C₃N₄and pure BiOI,respectively.On the other hand,O₂^-and h⁺play a major role in the photocatalytic degradation of Rh B dyes.The photo-generated carriers in BiOI/g-C₃N₄composites migrate by the type-I heterojunction mechanism.4.The flower-like SnS₂ was prepared by the hydrothermal method,and then the binary BiOI/SnS₂ composite sample was synthesized by the co-precipitation method.The composite sample contained tetragonal BiOI and hexagonal wurtzite SnS₂,indicating that BiOI and SnS₂were successfully composited.The specific surface area of BiOI/SnS₂-4 material is as high as111.1275 m2/g,which is conducive to the adsorption of Rh B and increases the number of reactive sites.BiOI/SnS₂-2 has the best photocatalytic activity to degrade 76.8%of Rh B dye under 90 min of light.O₂^-and OH play a major role in the photocatalytic degradation of Rh B in BiOI/SnS₂ composite samples.The improvement of the photocatalytic performance of BiOI/SnS₂ composites is due to the type-II heterojunction formed by p-BiOI and n-SnS₂,which enables rapid separation of photo-generated charges and generates more active free radicals during degradation.