Hierarchical Structure Control And Photocatalytic Properties Of Metal Sulfide Semiconductor Materials

With the development of society,the problems of environmental pollution and energy shortage are becoming more and more serious.Photocatalysis,as a kind of advanced oxidation technology,has become the focus of research by researchers at home and abroad.Metal-organic frameworks(MOFs)and covalent organic frameworks(COFs),as new semiconductor materials,have many advantages,such as simple synthesis,adjustable structure,high specific surface area and crystallization,etc.,which have attracted extensive attention from researchers at home and abroad in the field of photocatalysis.At present,single component catalyst has the disadvantages of low visible light utilization rate,high photogenerated electron and hole recombination rate,and photocorrosion.To solve these problems,this paper will build heterojunctions with inorganic semiconductors and make full use of the conjugated skeleton of MOFs and the large?conjugated system of COFs to better separate electrons from holes.Through the formation of chemical bonds between heterojunctions,the separation of electrons and holes will be greatly promoted.In this paper,we designed,synthesized and characterized a series of multi-functional nanocomposites with metal sulfide@metal-organic framework or covalent organic framework@metal sulfide for the degradation of organic pollutants and the hydrolysis of hydrogen evolution.It has carried out three aspects of research,and achieved the following results:(1)Synthesis,photocatalytic properties and mechanism research of hollow CdS@Mn-ZIF-67Hollow CdS nanospheres were synthesized by template method.Mn-ZIF-67(doping Mn²⁺in Cobalt-2-methylimidazole)was assembled on the surface of hollow CdS nanospheres by coordination principle,and a new CdS@Mn-ZIF-67-x(Mn-ZIF-67 load rate:x=1:50%,x=2:82%,x=3:100%,x=4:120%)heterostructured photocatalyst was prepared.The sizes of semiconductor monomer materials and MOFs materials are precisely controlled.The structure and performance of the hollow CdS@Mn-ZIF-67 composite photocatalysts were studied by a series of testing and characterization methods,such as SEM,TEM,EDS,XRD,BET,UV-vis,TG,XPS,EIS,instantaneous photocurrent response and so on.The mechanism behind photocatalytic degradation of tetracycline and photocatalytic hydrogen evolution was deeply studied.The results showed that it can be used for photocatalytic degradation of tetracycline and photocatalytic hydrogen evolution.Under visible light,the degradation rate of tetracycline(TC)was up to 94.8%.The hydrogen evolution rate of the catalyst is10889.2?mol·h^-1·g^-1,which is about 74 times and 5 times of that of single Mn-ZIF-67and hollow CdS nanospheres,respectively.This kind of catalyst can effectively solve the photocorrosion problem of CdS,and the introduction of ZIF-67 also increased the active site of the catalyst.More importantly,the heterojunctions promote the migration of electrons and holes through coordination bond bridging and traditional double transfer mechanism,and the metal Mn doping greatly improves the photoelectric performance of the composite photocatalyst.In addition,the catalyst is very stable after repeated use,which has great potential for practical application.(2)Synthesis,photocatalytic properties and mechanism research of hollow/solid COFs-Ph@CdSThe stable structure of triazine COFs(COFs-Ph)was synthesized by using cheap and easily available raw materials(cyanuric chloride and p-phenylenediamine).The best mixed solvent was consisted of 1,4-dioxane,1,3,5-trimethylbenzene and HAc in the 5:5:1volume ratio by photocatalytic test selected.The hollow or solid structure of CdS was formed on the surface of COFs-Ph by the coordination of rich N atoms with metal Cd²⁺,and the heterojunction photocatalyst COFs-Ph@CdS-x(hollow/solid CdS load rate:x=1:53%,x=2:80%,x=3:106%,x=4:133%)heterostructured photocatalyst was obtained for the first time.The structure and performance of the hollow/solid COFs-Ph@CdS composite photocatalysts were studied by a series of testing and characterization methods.The mechanism behind photocatalytic degradation of tetracycline and photocatalytic hydrogen evolution was deeply studied.The degradation rate of tetracycline(TC)by hollow COFs-Ph@CdS-3 was as high as 87.2%under simulated sunlight conditions,which was 1.1,1.2,1.1 and 10 times higher than that of solid COFs-Ph@CdS-3 photocatalyst,solid CdS,hollow CdS and COFs-Ph,respectively.After five photocatalytic cycles,the photocatalytic degradation rate of tetracycline was high.On the other hand,the hydrogen evolution rate of the catalyst hollow COFs-Ph@CdS-4 was 726.0?mol·h^-1·g^-1,which was 2.1,17.7,6.3 and48.4 times higher than that of solid COFs-Ph@CdS-1 photocatalyst,solid CdS,hollow CdS and COFs-Ph,respectively.Through a large number of photocatalytic experimental data,it is proved that there is a great difference in photocatalytic activity between hollow composite photocatalyst and solid composite photocatalyst,and the hollow structure is more conducive to photocatalytic degradation and hydrogen evolution.This kind of catalyst can effectively solve the photocorrosion problem of CdS.Triazine COFs-Ph have large conjugated structures and coordination bonds between heterojunc-tions,which can greatly promote the transfer of photogenerated electrons and holes.Through the mechanism study,the matching band structure forms the Z-Scheme heterojunction,which provides favorable conditions for the separation of electrons and holes.Have an in-depth understanding of the basic research of triazine COFs-Ph materials,and this study provides a new idea for the design of novel COFs-like heterojunctions.(3)Synthesis,photocatalytic properties and mechanism research of LZU1@P-MoS₂and LZU1@Co-MoS₂Co²⁺-doped composite photocatalyst LZU1@Co-MoS₂ and P-doped composite photocatalyst LZU1@P-MoS₂ were prepared by hydrothermal method.Using Schiff-base COFs(COFs-LZU1,condensation from p-phenylenediamine and benzene-1,3,5-tricarboxaldehyde),through the coordination of N atoms in Schiff-base with metal ions,MoS₂ doped Co²⁺/P was uniformly loaded on the surface of spherical COFs-LZU1 to obtain LZU1@Co-MoS₂ and LZU1@P-MoS₂ heterojunction.The structure and performance of the LZU1@Co-MoS₂ and LZU1@P-MoS₂composite photocatalysts were studied by a series of testing and characterization methods.The mechanism behind photocatalytic degradation of tetracycline and photocatalytic hydrogen evolution was deeply studied.In-depth comparative analysis with undoped composite photocatalyst LZU1@MoS₂,undoped MoS₂,Co-MoS₂ and P-MoS₂.Under visible light irradiated for 60 min,the degradation rate of tetracycline(TC)for 50 mg/L,40 mg/L,30 mg/L and 20 mg/L,LZU1@P-MoS₂ composite photocatalyst could reach 77.6%,88.6%,89.5%and 99.2%,respectively.And the structure was stable after five cycles.Besides,the hydrogen evolution rate of LZU1@P-MoS₂ and LZU1@Co-MoS₂was 16875.2?mol·h^-1·g^-1 and 14125.5?mol·h^-1·g^-1.The hydrogen evolution rate of LZU1@P-MoS₂ was 3.8,69.9,462.3 times higher than that of P-MoS₂,MoS₂,COFs-LZU1,respectively.This kind of catalyst can effectively inhibit the recombination of electrons and holes,mainly because the coordination bonds between heterojunctions provide a path for the migration of electrons and holes;The doping of non-metallic P in MoS₂ makes the S vacancy become an effective electron trap center,which hinders the recombination of photogenerated electrons and holes.The photocatalytic mechanism shows that this kind of catalyst is a double channel heterojunction,which further promotes the separation of electrons and holes.The above studies indicate that the heterojunction based on metal sulfide@MOFs or COFs@metal sulfide has excellent activity in photocatalytic degradation of organic pollutants and high hydrogen evolution performance.The separation of photogenerated carriers can effectively prevent photocorrosion and improve photocatalytic efficiency.The methods adopted in this paper are as follows:from the composition of catalyst structure,the large conjugated system with COFs or MOFs and the coordination bonds constructed between heterojunctions can provide migration path for photogenerated carriers.Froming the energy band structure of the catalyst,the impurity level or electron trap formed by metal or non-metal doping,the double channel mechanism and the Z-type mechanism formed by matched energy band can further promote the efficient separation of photogenerated carriers.Therefore,the research in this paper will provide a new reference method for the design of photocatalyst for practical organic wastewater treatment.