Construction Of CdS-based Heterostructure And Its Photocatalytic Hydrogen Evolution Performance

Among the visible light responsive photocatalysts,CdS is favored by scientific researchers because of its low cost,suitable adjustable band gap and excellent electronic affinity.However,the photocatalytic activity of CdS is limited to some extent because the photogenerated electron-hole pair generated by CdS excitation is easy to compound,S^2-is easy to be corroded by photogenerated hole or dissolved oxygen in solution.Many strategies have been proposed by researchers for these two problems,and commonly used strategies include co-catalyst loading,defect engineering,heteroatom doping,and construction of heterojunctions.In this paper,the problems of electron-hole recombination and photocorrosion in CdS semiconductors are studied through the surface loading of Mo-based semiconductor materials,the preparation of core-shell structures and the establishment of different types of charge transfer mechanisms.The mechanism of activity enhancement of CdS photocatalyst and the change of its corrosion resistance to light were studied through photoelectrochemistry,optical simulation and other characterization,providing a new guiding idea for the design and synthesis of CdS-based photocatalyst system with high activity and excellent stability.Aiming at the problem of electron-hole reciprocal recombination caused by CdS excitation.The composite material is prepared by compounding with molybdenum base semiconductor.Molybdenum-based semiconductor nanomaterials（such as MoS₂and MoP₄）have been widely studied in the field of photocatalysis due to their unique advantages.It has excellent electron mobility and narrow band gap,and can be used as an excellent cocatalyst or composite semiconductor.Therefore,MoS₂ nanoparticles were loaded onto CdS nanoparticles by hydrothermal method to form porous spherical MoS₂/CdS composite,which showed enhanced photocatalytic hydrogen evolution activity and excellent cycle stability.The spherical structure is formed by irregular stacking of two different sizes of MoS₂ and CdS nanoparticles,which contain a large number of stacking holes.The formation of stacking holes can enhance the absorption and utilization of light,shorten the distance between photogenerated carriers and the surface of catalyst.The research shows that the introduction of MoS₂can significantly enhance the electric field intensity around CdS,and the strong electric field intensity indicates that there is a large amount of charge accumulation at the interface.Therefore,MoS₂/CdS composite exhibits high photocatalytic hydrogen evolution activity(9.5 mmol·g^-1·h^-1)and excellent cycle stability.MoO₃ was converted into MoP₄ in situ in the atmosphere of PH₃ by gas-solid synthesis and loaded onto CdS nanorods to form MoP₄/CdS composites.After MoO₃is converted into MoP₄ in situ in PH₃ atmosphere,the Moatom in the obtained MoP₄forms a Mo-S bond with S^2-in CdS as a charge transfer channel.Moreover,the formation of Mo-S bond can enhance the stability of S^2-on CdS surface and slow down the impact of photoetching.The photoelectrochemical test results show that the enhanced photocatalytic activity may be attributed to the effective inhibition of the electron-hole pair and the effective spatial separation of the electron-hole pair caused by the Z-Scheme charge transfer mechanism.Therefore,during the photocatalytic process,the rate of MoP₄/CdS reached 12.36 mmol·g^-1·h^-1,which was much higher than CdS nanorods（22.6 times）,and even higher than that of CdS loaded with 2.0wt%noble metal Pt.On this basis,the design and preparation of special structure composite materials and the establishment of Z-Scheme charge transfer mechanism can not only achieve the separation of electron-hole pairs in space,but also further prevent S^2-from being oxidized and slow down photoetching.Based on this,the composite of Zn In₂S₄ nano-sheets containing S vacancies wrapped on the surface of CdS nanorods was prepared by hydrothermal method.Zn In₂S₄ nanosheets grown on the surface of CdS nanorods help to enhance the absorption and utilization of visible light,and the shell can effectively protect the core,making it have better structural stability.The abundant S vacancies on the surface of Zn In₂S₄ can attract electrons as reactive sites.In addition,the establishment of Z-Scheme charge transfer mechanism is conducive to the spatial separation of electron-hole pairs,preventing S^2-from being oxidized,thus effectively slowing down photoetching.Therefore,the CdS@ZIS-V_S photocatalytic hydrogen evolution rate reached 18.06 mmol g^-1 h^-1,which was 16.9 times and 19.6 times higher than CdS(1.16 mmol·g^-1·h^-1)and ZIS(0.92 mmol·g^-1·h^-1),respectively.The preparation of core-shell structure and the establishment of Z-Scheme charge transfer mechanism can promote the separation of electron-hole pairs and effectively slow down photoetching.On this basis,a more effective strategy was proposed to further improve the photocatalytic activity and structural stability.Using hydrothermal method,self-assembly polymerization and chemical bath deposition method to prepare a double-layer core-shell structure CdS@Polydopamine@SnO_2-xcomplex shows enhanced photocatalytic hydrogen evolution activity and excellent cycle stability.The conducting polymer PDA as the intermediate layer can be used as a bridge to accelerate the charge transfer.The preparation of core-shell structure can enhance the absorption and utilization of visible light,and the shell can protect the core and enhance the structural stability.The establishment of Z-Scheme charge transfer mechanism is conducive to the spatial separation of electron-hole pairs,preventing S^2-from being oxidized and thus more effectively slowing down the photoetching.Therefore,in the process of photocatalysis,the photocatalytic hydrogen evolution rate of CdS@PDA@SnO_2-x(82.27 mmol·g^-1·h^-1)is 34.3 times than CdS(2.4 mmol·g^-1·h^-1),and the catalytic activity remains stable after 10 cycles.