Structure Control And Photocatalytic Water Splitting Properties Of Low Dimensional Sulfide Semiconductor

As a clean energy carrier with high energy density and large-scale utilization,hydrogen energy is regarded as an ideal fuel in the future.Photocatalytic water splitting,which mainly uses sustainable energy-solar energy to split water into hydrogen,is an energy saving,environmental protection and low cost hydrogen production method,and has become a hot research topic around the world.Among the various photocatalysts responsive to visible light,CdS and ZnCdS have been widely studied due to the advantages of narrow band gap,suitable band position and high optical absorption coefficient.However,the quantum dots and nanoparticles usually have large grain boundaries and surface defects,which provide a large number of composite centers for photogenic carriers and greatly limit the photocatalytic performance.On the other hand,the whole water splitting system still faces the problems of low efficiency and unstable performance of hydrogen production by solar energy.Therefore,in this thesis,ZnCdS-OLC and CdS-SnS₂composite nanosheet structures were constructed to reduce surface and interface defects,promote charge transfer and accelerate the kinetic process of hydrogen/oxygen evolution,and thereby improve the efficiency of hydrogen production by solar energy.Specific research contents are as follows:Zn_0.5Cd_0.5S and ZnCdS-OLC regular hexagonal nanosheets were synthesized by liquid phase method,and the hydrogen production rate of 10.8 mmol/g/h was obtained in the polysulfide electrolyte.The Zn_0.5Cd_0.5S nanosheets are composed of nanoparticles with plane size and thickness of 178 nm and 27 nm respectively.For the Zn_0.5Cd_0.5S-OLC composite,the Zn_0.5Cd_0.5S nanocrystals have been completely wrapped by carbon layers,forming a core-shell structure with the typical“type II”band alignment,which greatly benefits the transfer of electric charge in the paralleled direction and vertical direction.At the same time,compared with Zn_0.5Cd_0.5S quantum dots,the light absorption edges of the composite nanosheets and Zn_0.5Cd_0.5S nanosheets are redshift to 570 nm and 550 nm respectively,which broadens the spectrum absorption range and further increases the hydrogen production rate of the composite nanosheet to 10.8 mmol/g/h,higher than the pure Zn_0.5Cd_0.5S nanosheets（6.4mmol/g/h）and Zn_0.5Cd_0.5S quantum dots（3.4 mmol/g/h）.CdS-SnS₂composite nanosheets was synthesized by a two-step hydrothermal method.When the SnS₂/CdS mole ratio is 0.05/1,the hydrogen production rate is about 9 times that of CdS,but the hydrogen production rate is extremely unstable.With the increase of SnS₂content,the hydrogen production rate gradually decreased,while the stability gradually is improved.When the mole ratio of SnS₂/CdS is 0.2/1,the hydrogen production rate approaches CdS,while the stability is greatly improved.Further addition of SnS₂will weaken the hydrogen production performance.Further mechanism studies showed that CdS-SnS₂formed a Z-type energy band structure,and photogenerated electrons in the CdS conduction band help to produce hydrogen,while the holes left in the SnS₂valence band contribute to produce oxygen.Meanwhile,the holes in the valence band of CdS will be reduced by the electrons in the SnS₂conduction band,thus improving the stability of the photolysis water.In summary,ZnCdS-OLC and CdS-SnS₂composite nanosheet structures were constructed by liquid phase method and two-step hydrothermal method respectively,which significantly improved the photocatalytic hydrogen production efficiency and analyzed the corresponding hydrogen production mechanism,providing a certain foundation for further research and development of efficient photocatalysts.