Designed Synthsis And Photocatalytic Hydrogen Evolution Of Mesoporous Zn_xCd_1-xS Nanosphere

The construction of heterojunction,phase junction and Z-scheme structure in semiconductor photocatalyst can enhance the transfer and separation efficiency of photoinduced charge carriers by adjusting the energy band structure,which can improve the photocatalytic efficiency.Lattice matching is very important in the process of forming the junction while nano-twin structure possesses high lattice matcing and can form phase junction,therefore it’s of huge significance to design and synthesis of materials with nano-twin-induced phase junction.However,the existing methods to produce nano-twins,mostly with high-temperature synthesis procedure and tedious reaction steps,generally result in large grains with ill-defined morphology and dense structure with low surface area.Since photocatalytic reaction occurs on the surface of material,the design and synthesis of photocatalyst materials with mesostructure can effectively increase the active sites of photocatalysts and improve the diffusion capacity of reactant and product ions/molecules,thus facilitate the photocatalytic efficiency.Consequently,the potential photocatalyst material Zn_xCd_1-xS was chosen in this thesis for it poseesses both the hexagonal wurtzite crystal phase and cubic zinc blende crystal phase,and a facile and mild synthetic strategy was proposed to prepare mesoporous Zn_xCd_1-xS nanospheres with high surface area.The existence of mesoporous structure and nano-twins were demonstrated by various characterization in the thesis,and formation mechanisms were further discussed in detail.Moreover,the as-synthesized mesoporous Zn_x Cd_1-xS nanospheres were applied to the photocatalytic hydrogen evolution reaction,found that the nano-twin-induced phase junction indeed improves the photocatalytic hydrogen evolution reaction,and photoinduced charge carrier dynamics in phase junction were also studied.The major achievements are listed as following:1.Mesoporous Zn_xCd_1-xS nanosphere with nano-twin crystal structure was designed and synthesized by ligand-assisted coordinative self-assembly method.In this thesis,as-synthesized mesoporous Zn_xCd_1-x-x S nanospheres possess nanoscale spherical morphology with uniform particle size,and the particle size of the synthesized spheres can be controlled by simply adjusting the input metal proportion.The mesoporous Zn_xCd_1-xS nanospheres are composed of a large amount of nano-twins,with further analysis by high-resolution transmission electron microscopy（HRTEM）and X-ray powder diffraction（XRD）,we found that the nanoscale grains are wurtzite-zinc blende nano-twins and therefore form a large number of phase junctions.The samples possess high BET specific surface area(148-312 m² g^-1),and their pore size distributions are concentrated in 2-4 nm,indicating that the ligand-assisted coordinative self-assembly method was used to successfully construct the mesoporous structure in Zn_xCd_1-xS nanosphere.And the effect of organic coordinating ligands on the formation mechanism of mesoporous structure and nano-twin crystal structure was investigated in detail.2.Due to the photoelectric property of Zn_xCd_1-xS,we further explored its photocatalytic hydrogen production performance,and found that the as-synthesized mesoporous Zn_xCd_1-xS nanospheres had highly efficient and stable photocatalytic performance,and the best sample of it,mesoporous Zn_0.82Cd_0.18S nanospheres,possesses photocatalytic hydrogen production rate up to 13.46 mmol h^-1 g^-1（noble metal free）.By characterization of UV-vis diffused reflectance spectrum（UV-vis DRS）and surface photovoltage spectroscopy（SPS）,the light adsorption range and the band gap can be determined and calculated.By simply regulating the feed ratio of Zn/Cd,the light adsorption range of the sample can be extended from ultraviolet to the range of UV-visible light.We use transient photovoltage spectrum to further study the mechanism of photocarrier dynamic of mesoporous Zn_xCd_1-xS nanospheres,found that wurtzite-zinc blende nano-twin composed of type II staggered phase junction,which can effectively separate the photoinduced electron-hole pairs,make the photoinduced electrons flow into the cubic phase structure,and the photoinduced holes flow into the hexagonal phase structure.The nano-twin-induced phase junction can reduce the recombination of photogenerated electron-hole pairs,and slows down the photocorrosion on the sample,and plays an important role in improving the photocatalytic efficiency.