Preparation, Modification And Photocatalytic Degradation Properties Of BiOX(X=Cl,Br,I) By Molten Salt Method

BiOX（X=Cl,Br,I）has been extensively utilized for photocatalytic degradation of water pollutants due to its stable chemical property,low cost,environment-friendly and controllable energy band structure.However,the practical application of BiOX is still restricted by its low solar energy utilization efficiency and easy recombination of photo-induced carriers.Besides,the most adopted method for the synthesis and modification of BiOX is hydrothermal/solvothermal method,which involves using organic solvents or surfactants in the reaction process,resulting in residual organic molecules on the surface of BiOX.The surface residual organic molecules not only inhibit the adsorption of the target reactant molecule,but also become the capture centers of photo-generated carriers,which is disadvantageous for improving its photocatalytic degradation activity.Molten salt method has been widely used for synthesis photocatalysts with unique microstructure due to its low cost,high efficiency and environmental friendliness.As a high temperature reaction medium,molten salts could generate strong polarizing force,which is beneficial to controllable preparation of ultrathin and well-defined two-dimensional materials.In addition,the as-prepared sample possesses high crystallinity and good activity.Therefore,molten salt method presents great potential in preparation and modification of BiOX.In this paper,BiOX was synthesized via molten salt method and characterized to investigate the intrinsic relationship between the micro-structure,band structure,photo-electronic properties and photocatalytic degradation performance.Step scheme heterostructure and bulk doping were constructed to broaden its light response range and improve photo-generated charge separation and transport efficiency,improving the photocatalytic activity of BiOX.The main research contents of this paper are as follows:（1）BiOX was synthesized by molten salt method and characterized by XRD,SEM,DRS,XPS and photoelectrochemistry tests to obtain microstructure,band structure and photo-electronic properties.The as-prepared BiOX presented typical 2D sheet-like morphology and the size of BiOX decreased in turn with the increase of X atom number.Among the prepared BiOX,BiOCl exhibited the most positive oxidation potential and highest photo-induced charges separation and transport efficiency.By normalizing the degradation rate constant to the specific surface area,BiOCl had the highest photocatalytic degradation rate for CIP under the Xenon lamp irradiation(0.024 g·min^-1·m^-2),which was 2.7 and 48 times higher than those of BiOBr(0.009 g·min^-1·m^-2)and BiOI(0.0005 g·min^-1·m^-2),respectively.（2）The binary In₂O₃-BiOCl step scheme heterojunction based photocatalyst was fabricated by molten salt method.The Mott-Schottky plots,XPS and EPR tests demonstrated that the photo-induced charges transfer between BiOCl and In₂O₃ interface followed step scheme path,which not only facilitated the effective separation of photo-induced carriers,but also maintained the original high redox ability of the BiOCl and In₂O₃.The In₂O₃-BiOCl composites possessed prominently enhanced photodegradation property toward CIP under the Xenon lamp irradiation.The optimized 20%In₂O₃-BiOCl showed the highest photodegradation efficiency,attaining 91%removal of CIP within 35 minutes.The calculated photocatalytic degradation rate constant for 20%In₂O₃-BiOCl(0.0713 min^-1)was about 39.6 and 3.2 times higher than those of pristine In₂O₃(0.0018 min^-1)and BiOCl(0.0223 min^-1),repectively.（3）Bulk tungsten(W⁶⁺)doped BiOCl was prepared by molten salt method.The introduced W⁶⁺restrained the growth of BiOCl crystal,which shortened the migration distance of photo-induced charges from bulk to the surface.The Bi³⁺in BiOCl lattice was partly substituted by W⁶⁺,forming a stable dopant level below lowest conduction band.The bulk W⁶⁺doping not only expanded its intrinsic optical absorption but also improved its charges separation efficiency,enhancing its visible-light photocatalytic activities.Especially,2%W-BiOCl exhibited superior photocatalytic degradation towards Rh B and the calculated photocatalytic degradation rate constant was 0.128 min^-1,which was approximately 10.7 times higher than that of pristine BiOCl(0.012 min^-1).（4）Bulk iodine（I^-）doped BiOCl was prepared by molten salt method.The crystal structure and photoelectric properties of samples were characterized and analyzed.It showed that the critical role of the doped iodine elements involving in the VB of BiOCl,broadening its intrinsic optical absorption and boosting the separation and transport of photo-generated carriers.In addition,bulk iodine doping process also created more oxygen vacancies in the crystal structure,further enhancing its optical absorption.The as-prepared bulk iodine doped BiOCl exhibited enhanced photodegradation property toward CIP under the visible light irradiation.The optimized bulk iodine doped BiOCl(BiOCl_1-zI_z)exhibited highest photocatalytic performance for CIP degradation and the calculated photocatalytic degradation rate constant was 0.06663 min^-1,which was approximately 12 times higher than that of pristine BiOCl(0.00591 min^-1).