Research On SiO₂ Supported Core-shell Structure Photocatalysts And Hollow Structure Composite Photocatalysts

Photocatalytic is one kind of green and clean technologies.At present,it has been widely studied in degradation of organic pollutants,treatment of heavy metal wastewater,photolysis of water for hydrogen production,air purification,antibacterial,photocatalytic reduction and so on.The OH·,OOH·and hydrogen peroxide produced by TiO₂ under the light have high oxidation activity,low structural selectivity to degraded organic compounds,also can react with a variety of organic compounds,oxidize organic pollutants into CO₂,H₂O and simple inorganic compounds completely.Among many photocatalytic semiconductor materials,TiO₂is one of the most ideal semiconductor catalysts in the field of photocatalysis because of its stability,non-toxic and low cost.At present,the application and research of TiO₂ photocatalysis in the environment has been deepening and expanding at home and abroad,but it still has the following disadvantages:the nanoparticles are easy to agglomerate,the band gap is large,the quantum yield is low,and the utilization efficiency of sunlight is also low.So how to prevent agglomeration,improve quantum yield and the photocatalytic efficiency has become a key issue in the current research of TiO₂photocatalyst.In this paper,the core-shell structure and hollow structure photocatalysts were prepared based on TiO₂,and the photocatalytic activities were further improved by changing the morphology and structure of TiO₂.SiO₂ with uniform morphology,controllable structure and good thermal stability was used as the matrix core,and was coated with TiO₂ to prepare SiO₂@TiO₂ core-shell photocatalyst,SiO₂@TiO₂-Ag composite structure photocatalyst and hollow structure TiO₂ photocatalyst.Besides,the double-shell hollow TiO₂@ZIF-8 composite photocatalyst was synthesized by doping metal-organic framework(MOF)of ZIF-8 based on the preparation of hollow structure TiO₂ photocatalyst.The main research contents and conclusions are as follows:1.Preparation and research of SiO₂@TiO₂ core-shell structure photocatalyst.The simple and controllable SiO₂ microspheres were prepared by St(?)er method.The SiO₂@TiO₂core-shell structure photocatalysts were prepared by hydrothermal method with SiO₂microspheres as the core.The photocatalytic effect of SiO₂@TiO₂ core-shell structure photocatalysts with different Ti/Si ratios were prepared and studied.The morphology structure and the chemical properties of the composite photocatalysts were characterized by XRD,SEM,TEM,BET,FI-IR and Zeta point.The results showed as follows:The surface of the SiO₂ microspheres prepared by the St(?)er method was smooth and the diameter was about200-300nm.The XRD characterization results indicated that the crystal structure of TiO₂and SiO₂@TiO₂ core-shell structure photocatalysts prepared by hydrothermal method were belong to anatase.BET results showed that the SiO₂@TiO₂ core-shell structure photocatalysts with Ti/Si molar ratios of 1:1,2:1,5:1 and 8:1 had the type IV adsorption curve with H2 type hysteresis loop and belonged to mesoporous material,which were the same as TiO₂.The experimental results of photocatalytic degradation showed that the SiO₂@TiO₂ core-shell structure photocatalysts with different Ti/Si molar ratios had good degradation efficiency towards phenol and methylene blue,Ti/Si molar ratio of 5:1 showed the highest photocatalytic activity which had the photocatalytic degradation efficiency of 99.4%towards phenol and 99.2%towards methylene blue under UV light.2.Preparation and research of SiO₂@TiO₂-Ag composite structure photocatalyst.Based on the research of SiO₂@TiO₂ core-shell structure photocatalyst in the early stage,Ag was loaded on SiO₂@TiO₂and formed SiO₂@TiO₂-Ag composite structure photocatalyst,the effects of morphology and photocatalytic efficiency with different Ag content doping on SiO₂@TiO₂ core-shell structure photocatalyst were studied.The preparation conditions were optimized,and the optimal doping Ag content was determined.XRD,SEM,TEM,EDS,FI-IR,XPS were used to research the morphology structure and physicochemical properties.The results of photocatalytic activity showed that the SiO₂@TiO₂-Ag composite photocatalyst coated with Ag had higher photocatalytic activity than that of SiO₂@TiO₂ core-shell structure photocatalyst,and the highest photocatalytic degradation efficiency of 6 wt%Ag doped SiO₂@TiO₂-Ag composite photocatalyst towards phenol and methylene blue were 92.9%and83.5%under UV light,respectively.3.Preparation and research of hollow structure TiO₂ photocatalyst.In order to save the preparation cost,improve the synthesis efficiency of SiO₂@TiO₂ core-shell photocatalyst,the sol-gel method was used to synthesis SiO₂@TiO₂ core-shell photocatalyst.The SiO₂ core in the core-shell composite photocatalyst was removed by alkali etching method to form the hollow structure TiO₂ photocatalyst with stable structure and good photocatalytic degradation efficiency.The results showed as follows:the SiO₂@TiO₂ core-shell photocatalyst synthesized by sol-gel method was simple and effective,and the core-shell structure morphology characteristics were obvious.The results indicated that the crystal structure of SiO₂@TiO₂ changed obviously as the calcination temperature increased,the crystal structure of SiO₂@TiO₂ core-shell photocatalyst was still anatase when the calcination temperature increased to 850?,which indicated SiO₂ can significantly inhibit the phase of TiO₂ transfer from anatase to rutile at high calcination temperature.The results of BET characterization showed that compared with TiO₂ and SiO₂@TiO₂ core-shell photocatalyst calcined at different calcination temperatures,the hollow structure TiO₂ calcined at different temperatures had the type IV adsorption curve with obvious H2 type hysteresis loop and belonged to mesoporous materials.The results of photocatalytic activity showed that the photocatalytic efficiency of hollow structure TiO₂ was higher than that of SiO₂@TiO₂core-shell photocatalyst calcined at different temperatures due to its hollow structure.The hollow structure TiO₂ calcined at 850?had the highest photocatalytic degradation efficiency of 97.5%towards phenol because of the highest intensity of diffraction peak and crystallinity of anatase crystal structure,which was conducive for photodegradation of phenol.However,when photodegradation of methylene blue,the hollow structure TiO₂ calcined at 650?had the highest photocatalytic degradation efficiency of 93.9%,this was because the hollow structure TiO₂ calcined at 650?had the highest specific surface area and anatase crystal structure to absorb and photodegradate methylene blue.4.Based on the preparation of hollow structure TiO₂ photocatalyst,the double-shell hollow TiO₂@ZIF-8 composite photocatalyst(HTZ)was synthesized by doping metal-organic framework(MOF)of ZIF-8 on hollow structure TiO₂.The effect of different molar ratios of ZIF-8 coating on hollow structure TiO₂ was studied;the photocatalytic degradation of methylene blue and phenol by TiO₂,SiO₂@TiO₂,hollow structure TiO₂ was compared.The results showed as follows:After hybrided ZIF-8 with hollow structure TiO₂,the crystal structure of double shell hollow structure photocatalyst was mainly anatase TiO₂and characteristic crystal structure of ZIF-8.According to BET analysis,the nitrogen adsorption and desorption curve of HTZs belonged to type IV adsorption curve with H2 type hysteresis loop,which was a typical mesoporou material,so it can increase the specific surface area of HTZs and promote the photocatalytic activity.The results of photocatalytic activity towards methylene blue showed that HTZ-2 had the highest photocatalytic efficiency of 99.1%due to the synergistic effect of ZIF-8 and hollow TiO₂,also because of the high specific surface area.However,the results of photocatalytic activity towards phenol showed that the degradation efficiency of HTZs was lower than that of hollow structure TiO₂.This was because the high specific surface area of HTZs had no obvious adsorption effect on phenol which belonged to one of the colorless organic pollutants.The hollow structure TiO₂coated with ZIF-8 did not increase the photocatalytic degradation efficiency of phenol,but hollow structure TiO₂ showed higher photocatalytic degradation efficiency because of the physical characteristics of hollow structure and stable anatase phase.The photocatalytic degradation of organic compounds by HTZs had selectivity,which was more suitable for the degradation of colored dye pollutants.In conclusion,in view of the shortcomings of TiO₂ in the research,we prepared SiO₂@TiO₂ core-shell photocatalyst,SiO₂@TiO₂-Ag composite structure photocatalyst,hollow structure TiO₂ photocatalyst and double-shell hollow TiO₂@ZIF-8 composite photocatalyst.The advantages of core-shell photocatalyst and hollow structure photocatalyst were systematically studied from the morphology,structure,physical properties and photocatalytic activity of photocatalysts.Firstly,the core-shell structure can increase the reaction interface area and improve the separation efficiency of photogenerated electrons and holes pairs.Secondly,the hollow structure photocatalyst prepared by core-shell structure photocatalyst can further improve the light absorption efficiency;provide more active sites and larger light receiving area.Thirdly,the introduction of MOF material of ZIF-8composited with hollow structure TiO₂ photocatalyst can improve the specific surface area,produce synergetic effect and increase the photocatalytic efficiency.This study provided a certain experimental basis for the preparation and application of core-shell photocatalyst and hollow structure photocatalyst based on TiO₂.