Preparation And Photocatalytic Performance Of Composite Photocatalyst Based On Bi₂WO₆

Photocatalytic technology has great application potential in the field of environmental pollution control because it can use solar energy to achieve the purposes of organic matter degradation,heavy metal and gaseous pollutants reduction.In addition,it has the advantages of a low energy consumption and thorough treatment.Bismuth tungstate(Bi₂WO_6,BW)is a ternary metal oxide and n-type semiconductor.It has visible light activity and shows the visible light absorption edge at about470 nm,with moderate band gap.Therefore,it is a photocatalyst with good application prospect.However,its rapid electron-hole pair recombination rate and insufficient light response range make its photocatalytic efficiency low,so it is difficult to meet the actual industrial demand.In view of its shortcomings,this paper uses quantum dot compositing,biochar compositing and heterojunction structure to modify Bi₂WO₆ in order to facilitate its carrier migration,delay its electron-hole pair recombination,expand its optical response range,and ultimately improve its photocatalytic efficiency.According to the characteristics of different composite materials,the performance and degradation mechanism of removing pollutants such as rhodamine B（Rh B）,tetracycline hydrochloride（TC）and hexavalent chromium（Cr（Ⅵ））under visible light were investigated.Combined with characterization analysis,the composition and structure of the prepared materials were explored,the electron transfer paths were speculated,and the improvement of its photocatalytic performance was comprehensively analyzed.The specific research contents and results are as follows:（1）Preparation and photocatalytic properties of quantum dot doped Bi₂WO₆ photocatalystN,S organic dots were synthesized by thermal polycondensation and were ultrasonically compounded with Bi₂WO₆ to prepare N,S organic dots/Bi₂WO₆ composites（NSD/BW）.The degradation efficiency of modified samples to Rh B and TOC of aquaculture wastewater under visible light irradiation was studied.The experimental results manifested that NSD/BW-0.2 sample showed the best photocatalytic activity,and the degradation efficiency to Rh B could reach 91%in70 min,while BW was only 36%.Besides,the TOC removal rate of aquaculture wastewater could reach 2.15 times that of BW.Through relevant characterizations,it was found that NSD endowed composite materials with narrower band gap,inhibited the recombination of photogenerated electron-hole pairs,and expands the light absorption range of NSD/BW to the near-infrared region,greatly improving the utilization of light and ultimately improving its photocatalytic degradation performance.（2）Preparation and photocatalytic properties of biocarbon-doped Bi₂WO₆ photocatalystModified orange peel biochar was prepared by high temperature calcination and KOH modification,and was introduced into the hydrothermal synthesis process of Bi₂WO₆ to prepare biocarbon/Bi₂WO₆ composites（BW/BC）.The photocatalytic degradation experiment was carried out with Rh B and aquaculture wastewater.The experimental results manifested that BW/BC-5sample showed the best photocatalytic activity,and the degradation efficiency to Rh B could reach94.90%in 140 min,which was 47.10%higher than BW.Moreover,the TOC removal rate of aquaculture wastewater is 1.72 times that of BW.By characterization analysis,it was found that BW maintained its original flower ball morphology on the surface of BC,and the introduction of BC regulated the band gap of BW,enhanced its visible light adsorption capacity and inhibited the recombination of photogenerated electron-hole pairs as an electron transfer channel.In addition,the VB position of BW is low,which is not enough to oxidize to form·OH.But the free radical capture experiment shows that·OH is an important active substance in the system,which indicates that the oxygen-containing functional group of BC is the key factor controlling the formation of·OH in the system.（3）Preparation and photocatalytic performance of Bi₂WO₆/Zn In₂S₄ heterojunction photocatalystBi₂WO₆/Zn In₂S₄Ⅱheterostructure photocatalyst（BW/ZIS）was prepared by introducing Bi₂WO₆ into the synthesis of Zn In₂S₄.The photocatalytic experiments were carried out with Rh B,Cr（VI）and aquaculture wastewater.The experimental results manifested that BW/ZIS-0.2 sample showed the best activity,and the degradation efficiency of Rh B in 25 min can reach 97%,which was30%higher than ZIS（BW has almost no degradability）.BW/ZIS-0.2 sample also has a photoreduction efficiency of 97.36%for Cr（VI）.Besides,the TOC removal rate of aquaculture wastewater is 9.02 times that of BW.The excellent photocatalytic performance of BW/ZIS-0.2sample can be attributed to the expansion of the light response range and accelerated migration of carriers in the type II heterojunction structure,which inhibits the recombination of photogenerated electron-hole pairs and ultimately significantly improves the photocatalytic degradation performance.（4）Preparation and photocatalytic properties of MIL-101/Bi₂WO₆ heterojunction photocatalystIron based MIL-101 was prepared by hydrothermal method and was introduced into the synthesis process of Bi₂WO₆ to prepare MIL-101/Bi₂WO₆（MIL/BW）Z-type heterojunction photocatalyst.Photocatalytic experiments were carried out with Rh B,TC and TOC of aquaculture wastewater as target pollutants.The experimental results manifested that MIL/BW-3%sample showed the best photocatalytic activity,and the degradation efficiency of Rh B in 15 min could reach96%,which was 61%higher than BW.MIL/BW-3%sample also has a photodegradation efficiency of 81%for TC in 120 min,which was 29%higher than BW.Furthermore,the TOC removal rate of aquaculture wastewater is 2.01 times that of BW.By characterization analysis,it was found that BW was grown on the surface of MIL-101 in the form of stacked nanosheets and the light response range of MIL/BW-3%sample was extended.Meanwhile,Z-type heterojunction greatly facilitated carrier migration and ultimately improved the photocatalytic degradation performance.Through the free radical capture experiment,it was found that singlet oxygen（¹O₂）was the main active substance in the reaction system.This is mainly due to the Fenton effect between MIL-101 and hydrogen peroxide（H₂O₂）,which generates the hydroperoxyl（·O₂H）and further increases the content of superoxide radical（·O₂^-）and hydroxyl radical（·OH）in the system,which jointly promote the formation of ¹O_2.Finally,the photocatalytic degradation performance was significantly improved by strong·O₂^-system.