Construction Of Photocatalytic Heterojunction Of Bimetallic Sulfide Based On Electrospun Carbon Nanofibers

With the quickening of the industrial process,the energy crisis has become a major challenge to be solved.Hence,it is urgent to seek a new alternative clean energy.Solar energy can be used for energy conversion and environmental degradation,and has attracted the attention of scholars and researchers due to the advantage of easy availability,pollution-free and low cost.The hydrogen evolution based on the semiconductor photocatalytic is a clean and sustainable technology which directly converts solar energy into hydrogen energy.The advantages of simple operation and low cost endow it with practical application.The key of this technology is the preparation of catalysts with high H₂ performance and high photostability.Semiconductor sulfides have been reckoned as one of the most promising photocatalysts because of their narrow band gap,good visible-light response and excellent hydrogen production activity.However,bimetallic sulfide photocatalysts still face many problems:（a）easy recombination of photogenerated electrons and holes;（b）higher overpotential for H₂ evolution;（c）difficult recycling of photocatalysis,and these drawbacks greatly limit the improvement of catalytic efficiency of sulfide.Recently,the construction of photogenerated heterojunction is an effective strategy to solve the aforementioned problems.The heterojunction system can not only promote the separation efficiency of photoinduced carriers,but also help the introduction of cocatalysts to reduce the H₂ overpotential.Based on the regulation of energy band engineering,CdIn₂S₄ with in situ spinning CNFs/Co₄S₃and Cd_0.5Zn_0.5S combined with in situ spinning CNFs/CoNiS_xrespectively make up the ternary heterojunction systems.X-ray diffraction（XRD）,scanning electron microscope（SEM）,transmission electron microscopy（TEM）and X-ray photoelectron spectroscopy（XPS）are employed to explore the phase structure and morphology structure of heterojunctions;the photoelectric chemical system is used to study the photoelectric properties and catalytic performance.Besides,the interfacial band structure and electron transfer path of heterojunction are analyzed by ultraviolet photoelectron spectroscopy（UPS）.The main research contents and conclusions are as follows:1.In this work,carbon nanofibers containing Co₄S₃（CNFs/Co₄S₃）are in situ synthesized by chemically doped electrospinning and calcination method,and CdIn₂S₄nanosheets are grown on CNFs/Co₄S₃ by solvothermal method to prepare the nanofiber structure of CdIn₂S₄/CNFs/Co₄S₃ heterojunction.And by regulating the content of Co₄S₃ doped in CNFs,the influence of different content of Co₄S₃ on the H₂ activity is investigated.In the CdIn₂S₄/CNFs/Co₄S₃ Schottky heterojunction,CdIn₂S₄ as the main catalyst can provide abundant photogenerated electrons,and exhibits the morphology of nanosheets,which is beneficial to electron transport;CNFs can function as the electron-transfer bridges for promoting the separation efficiency of photogenerated carriers due to its high conductivity,and the introduction of black CNFs also greatly improves the response range of visible light;in situ introduced Co₄S₃ active sites can not only act as the acceptor of photoinduced electrons,but also significantly reduce the overpotential for H₂ generation.Meanwhile,a Schottky barrier is formed at the interface of CdIn₂S₄/CNFs,which can effectively promote the separation efficiency of photogenerated electrons.The synergistic effect among CdIn₂S₄ nanosheets,CNFs and Co₄S₃ active sites not only effectively solves the problem of photo-generated carrier recombination,but also greatly reduces the H₂overpotential,thus substantially improving the efficiency of photoconversion.In addition,the nanofiber structure of photocatalyst can effectively improve the recycle efficiency of the materials.Therefore,the ternary Schottky heterojunction CdIn₂S₄/CNFs/Co₄S₃ exhibited excellent visible light catalytic performance:the photocatalytic H₂ production rate reaches 25.87mmo L·g^-1·h^-1 after 2.5 h of light irradiation,and still shows high photostability after 15h of light irradiation.2.To investigate the effect of bimetallic transition sulfide as the active site,a facile electrospinning technique is utilized to synthesize CNFs/CoNiS_x with one-dimensional nanofiber structure by the precursor of polyvinylpyrrolidone,cobalt acetate and nickel acetate.Then,the heterostructure of Cd_0.5Zn_0.5S/CNFs/CoNiS_x（CZNS）is prepared by solvothermal method.Meanwhile,to compare the advantages of CoNiS_x as the active site,the Cd_0.5Zn_0.5S/CNFs/Co₄S₃（CZS-2）ternary heterojunction is prepared by the same process on the basis of the optimal cobalt content in the previous work.In the heterojunction Cd_0.5Zn_0.5S/CNFs/CoNiS_x,the main catalyst Cd_0.5Zn_0.5S is distributed in the morphology of nanoparticle on the CNFs,which fully contacts with the CNFs and provides plentiful photogenerated electrons;CoNiS_x active sites act together with CNFs to form the acceptor of photogenerated electrons.Compared with Cd_0.5Zn_0.5S/CNFs/Co₄S₃,the Cd_0.5Zn_0.5S/CNFs/CoNiS_x composite shows better results on the visible light absorption range,photocurrent,impedance and overpotential,thus improving the separation efficiency of photogenerated carriers more effectively.Accordingly,Cd_0.5Zn_0.5S/CNFs/CoNiS_x shows the best hydrogen production activity(11.2 mmol·g^-1)after 2.5 h visible light irradiation,which is 4.8 times than that of CZS-2.Besides,it still shows high hydrogen production activity after 15 h of visible light.