The Preparation Of Bi_xO_yX_z Material And Its Photocatalytic Reduction Of CO₂

Semiconductor photocatalysis technology is a new technology to solve the environmental pollution and energy crisis.It utilizes the inexhaustible sunlight to successfully convert CO₂ into useful hydrocarbons such as CO,CH₄,CH₃OH,HCOOH or other high value-added compounds.However,traditional photocatalysis is faced with problems such as low electron-hole separation rate and poor light absorption capacity.Therefore,how to effectively improve the utilization rate of solar energy and catalytic efficiency is the primary task of current photocatalytic technology research.Bi OX（X=Cl,Br,I）is one of the new high efficiency photocatalysts.We prepare bismuth-rich catalyst Bi_xO_yX_z by increasing the content of Bi in the synthesis process,which not only retains the structural advantages of Bi OX,but also has a higher conduction band position for efficient CO₂ conversion.However,pure Bi_xO_yX_z is often limited by wide band gap structure and low quantum conversion rate,which makes its catalytic activity is not high in the visible light region.In order to solve the above problems,this paper chose Bi_xO_yX_z as the research object,from the construction of van der Waals（vd W）heterojunction and the doping of single atom to regulate the charge separation efficiency and transfer path of Bi_xO_yX_z,and studied the influence of different electron transfer mechanism on the photocatalytic performance in detail.The catalyst Bi₃O₄Cl is synthesized with tetracycline hydrochloride as the source of Cl.Bi₃O₄Cl and g-C₃N₄ are mixed uniformly in ethanol solution by ultrasound to generate Bi₃O₄Cl/g-C₃N₄ 2D/2D vd W heterojunction,and the influence of vd W heterojunction formation on electron transport mechanism is explored.The two catalysts shows typical two-dimensional nanosheet structure,and they are closely attached to form a vd W heterojunction.It has higher photocurrent response intensity and longer fluorescence lifetime.At the same time,vd W heterojunction has more excellent CO₂ adsorption capacity,which provides a guarantee for the efficient adsorption and activation of CO₂.Finally,the conversion process of Bi₃O₄Cl/g-C₃N₄ photocatalytic reduction of CO₂ is studied,the formation of intermediate products is analyzed,and the corresponding reaction mechanism is proposed.The fundamental reason for the improved catalytic performance of Bi₃O₄Cl/g-C₃N₄ 2D/2D vd W heterojunction is that the vd W force accelerates the photoinduced electron migration from g-C₃N₄ to Bi₃O₄Cl surface,and improves the photoexcited electron-hole migration and separation efficiency.Under the condition of20%g-C₃N₄ loading,CO₂ conversion efficiency is the highest,and the yields of CO and CH₄are 6.6μmol g^-1 h^-1 and 1.9μmol g^-1 h^-1,respectively.The loading of monatom Fe into the lattice of Bi₄O₅I₂ by a simple solvothermal method not only provides more reactive active sites,but also effectively avoids the problem that the traditional monatom with high specific surface energy is easy to aggregate into clusters.Hence,by increasing the solvothermal temperature,Fe³⁺is substituted for Bi³⁺to precipitate elemental Bi in situ,and the synergistic effect of Fe³⁺and Bi promoted the reduction ability of the catalyst.The catalyst has ultrathin nanosheet structure,which greatly reduces the diffusion distance of the carrier from the inside to the surface,and significantly reduces the possibility of carrier recombination.X-ray near-edge absorption spectroscopy（XANES）and extended X-ray absorption spectroscopy（EXAFS）confirm that Fe exists in the crystal lattice of Bi₄O₅I₂ in the form of monatomic Fe³⁺.High angle annular dark field scanning electron microscopy（HAADF-STEM）can clearly observe the presence of Fe single atoms in the crystal lattice.The effect of single atom Fe on the electronic structure of Bi₄O₅I₂ has been studied by DFT calculation.Fe atoms regulate the band gap structure of the catalyst and act as a springboard for electron transition to promote carrier separation.Finally,by comparing the CO₂ adsorption model and reaction pathway of Bi₄O₅I₂ and Bi₄O₅I₂-Fe30,the catalyst doped with Fe single atom shows a stronger local charge density around the catalyst,and the energy of intermediate product formation is reduced,indicating that the CO₂ activation process is easier.Therefore,Bi₄O₅I₂-Fe30 has higher CO and CH₄ yields of 23.77 and 4.98μmol g^-1 h^-1,respectively（approximately 5 times increased）.This paper provides a new research idea for the synthesis and development of high-performance photocatalytic materials.