Construction Of Ultrathin Cd_0.5Zn_0.5S Nanosheets Based Photocatalysis Systems And Study On Their Surface Reaction Kinetics Of H₂ Evolution

Hydrogen energy entitles pollution-free and high calorific value and is considered to be an ideal substitute for fossil fuels,and,photocatalysis represents a green strategy to acquire hydrogen by solar energy conversion.At present,developing efficient and stable photocatalytic H₂evolution system is still a core issue in this field.Zinc cadmium sulfide solid solution(Cd_xZn_1-xS,0<x<1)is a class of H₂evolution catalyst with superior activity,however,the sever charge recombination,sluggish surface reaction kinetic and hole-induced photocorrosion are the main obstacles that limiting its application.Recent years,many works have been done by researchers to suppress the bulk or surface charge recombination of Cd_xZn_1-xS,whose works mainly focus on the modification of physical process of photocatalysis and lose sight of the surface catalysis of H₂evolution.In this dissertation,the basic properties of Cd_xZn_1-xS were revealed first.After that,two-dimensional ultrathin Cd_xZn_1-xS nanosheets was prepared,and,reaction circumstances modulation was employed to promote charge consumption for enhancing the surface reaction kinetic of Cd_xZn_1-xS.Eventually,a highly efficient photocatalytic H₂production system with stable activity was obtained.This dissertation paid mainly attention to reveal the formation mechanism of 2D Cd_xZn_1-xS nanosheets and its surface reaction kinetics enhancement mechanism in different solvent systems,based on above,some universal principle of photocatalytic H₂production process has been clarified.The specific works are as follows:（1）Cd_xZn_1-xS（0<x<1）solid solution nanoparticles are fabricated by hydrothermal method.X-ray diffraction（XRD）,scanning electron microscopy（SEM）,ultraviolet-visible absorption spectrum（UV-vis）,photoluminescence spectrum（PL）,etc.are employed to clarify the basic physical and chemical properties of Cd_xZn_1-xS solid solution,as well its properties dependence on Zn/Cd moles ratio.The results show that the band gaps of Cd_xZn_1-xS increases gradually with increasing in Zn content,accompanying positively shifted valence potential and negatively shifted conduction potential,as well blue-shifted UV-Vis absorption spectrum and PL emission peak.These regular change in band gaps makes it possible to balance the two contradictory properties of light absorption capacity and redox potential of charge carriers of the catalyst.As a result,Cd_0.5Zn_0.5S exhibits the best H₂evolution activity in 0.35 M Na₂S/0.25 M Na₂SO₃,which is not only much higher than that of Cd S and Zn S,but exceeding those with other Zn/Cd moles ratios.（2）2D mesoporous ultrathin Cd_0.5Zn_0.5S nanosheets were fabricated following a cation exchange strategy using inorganic-organic hybrid Zn S-ethylenediamine[denoted as Zn S（en）_0.5]as hard template.The finally Cd_0.5Zn_0.5S nanosheets displays a thickness of～1.5 nm with abundant pore structure,which is mainly due to the difference of the internal and external diffusion rates of metal ions,the phase transition of Cd_0.5Zn_0.5S（en）_xintermediate and lattice distortion caused by the escape of ethylenediamine molecules.Such unique ultrathin mesoporous structure has shorten the migration distance of charge carriers and greatly enriched the surface charge of the catalyst,which effectively promotes the surface reaction and mass transfer process of H₂evolution,endowing the nanosheets prominent activity of 19.1 mmol?g^-1?h^-1in the presence of TEOA（Wavelength range:350nm-780 nm）,nearly two times higher than that nanoparticles(8.5 mmol?g^-1?h^-1).The nanosheets even exhibit superior activity of 1395μmol?g^-1?h^-1in pure water without any co-catalyst for its unique structure,contrasting with the negligible activity of nanoparticles counterpart.In addition,for taking full advantage of Cd_0.5Zn_0.5S nanosheets as a platform catalyst,Ni Co₂S₄/Cd_0.5Zn_0.5S binary composite was prepared by mechanical mixing,the Schottky junction formed by the two further promoted the surface separation of charge carriers and kinetic of H₂evolution,leading to H₂production rate reaches 62.2 mmol?g^-1?h^-1and 2436μmol?g^-1?h^-1in TEOA and pure water,respectively.However,the severe photocorrosion of Cd_0.5Zn_0.5S in pure water is still a burning question.（3）The surface catalytic process of water splitting over Cd_0.5Zn_0.5S nanosheets was strengthen by introducing Na OH into the reaction system,achieving H₂evolution activity remarkably improved without adding any co-catalyst or electron donor in water.The surface reaction kinetic of the system was studied by using kinetics theory,and a"photocatalyst-electrolyte effect"was proposed.It is found that the H₂production rate of Cd_0.5Zn_0.5S is positively correlated with the OH^-concentration and reaction temperature,reaching a high level of 12.44 mmol?g^-1?h^-1and～20 mmol?g^-1?h^-1at 30 ^oC and 40 ^oC（6 M Na OH,0.5 h）,respectively,which are two orders of magnitude higher than in pure water.The enhanced surface reaction significantly also inhibits the photocorrosion of catalyst efficiently.The adsorption of OH^-on Cd_0.5Zn_0.5S is the rate-limiting step of the whole reaction process,and increased reaction temperature and Na OH concentration can both accelerate the adsorption of OH^-,which directly promote the dissociation of water molecules.The calculation shows that the apparent activation energy of the reaction is about 24 k J?mol^-1,it is also proposed that the pre-exponential factor can be used as a descriptor for the surface reaction efficiency of photocatalysis.Moreover,the driving force of the water oxidation half reaction increases with increasing in OH^-concentration,leading to?OH becomes the main product of the water oxidation.The proposed surface reaction strengthening strategy is also applicable to other chalcogenide photocatalysts.（4）Metal Co decorated Cd_0.5Zn_0.5S nanosheets was prepared,and then,?OH scavengers were further introduce into the Na OH solution,such as reductive inorganic salts,alcohols,biomass etc.to design a general applicable"photocatalytic cascade reaction"for H₂evolution.Aqueous methanol reforming was employed as a model reaction,it is found that this"photocatalytic cascade reaction"not only greatly enhances the surface reaction process of the Co/Cd_0.5Zn_0.5S,but also overcomes the diffusion limitation of the hole sacrificial agent in the conventional system,and,the stability issues of sulfide photocatalysts was solved.When the concentration of Na OH is 4 M,the H₂production kinetic of the system fits zero-order with the concentration of methanol,and its apparent rate depended on the adsorption and oxidation kinetic of OH^-on the catalyst.The addition of OH^-has changed the reaction process of methanol oxidation from a hole-dependent direct path to a indirect oxidation process mediated by?OH,and also reduced the apparent activation energy of methanol decomposition.The H₂production activity of the system can reach 74.7±3.7 mmol?g^-1?h^-1at 30 ^oC in 4 M Na OH,and can be further enhanced to178.4±5.9 mmol?g^-1?h^-1at 60 ^oC.This surface reaction enhancement strategy by"photocatalytic cascade reaction"is also applicable to other catalytic systems,which provides some reference for the future practical application of photocatalytic H₂production.