Design,Synthesis And Application Of Cu_2-xS-based Nanoheterostructures In Photocatalytic Hydrogen Evolution

Water splitting to generate hydrogen in the presence of semiconductor photocatalysts has been considered an effective way to solve the problems of sustainable energy and environmental pollution.In recent years,many researchers have carried out lots of work in the field of semiconductor photocatalysis,and have made great progress in photocatalytic water splitting for hydrogen production.However,during the construction of semiconductor photocatalysts via traditional epitaxial growth or seeded growth.Due to the lattice mismatch between different components,it is inevitable to introduce more lattice defects and form extra carrier recombination centers at the heterointerface.Which inhibits the effective separation of charges and reduces the photocatalytic hydrogen evolution efficiency.The cation exchange method is suitable for the construction of heterostructured semiconductors with fine heterointerfaces.Cu_2-xS,as an excellent low-cost light-absorbing material,has a good development and application prospect in the field of photocatalysis.To break through the problem of insufficient photocatalytic efficiency caused by poor interface quality in semiconductor nanoheterostructures.Aiming at the above challenges,this paper focused on the precise synthesis of Cu_2-xS nanocrystals,and from the aspects of energy level matching,built-in electric field,fine heterogeneous interface to construct Cu_2-xS-based semiconductor nanoheterostructures(Cu_2-xS-MS（M=Zn/Cd）NHs).The relationship between the morphology,structure,composition and performance of photocatalytic hydrogen evolution was investigated.A new class of highly efficient and stable semiconductor photocatalysts without noble metal support has been prepared.Detailed conclusions are as follows:（1）High-quality Cu_2-xS nanocrystals were synthesized,and their morphology and size were precisely controlled.In the synthesis system of Cu_1.94S spherical nanocrystals prepared by thermal decomposition of Cu（acac）₂ and 1-DDT at high temperature,the particle size of the products was precisely controlled from 6 nm to 15 nm by adjusting the reaction system.Using ODE and TDE to replace part of1-DDT,spherical particles of 6 nm to 9 nm can be obtained;Adjusting the amount of Cu（acac）₂ and the reaction temperature,the size of spherical particles can be regulated from10 nm to 15 nm.In the system of synthesizing Cu_1.81S nanorods,the aspect ratio of Cu_1.81S nanorods was precisely controlled by adjusting the combination of sulfur source and the injection temperature.Using 1-DDT to replace part of tertiary t-DDT,the axial length of Cu_1.81S nanorods can be controlled in a wide range from 18 nm to 83 nm;Changing the injection temperature of t-DDT,the axis length of Cu_1.81S nanorods can be finely regulated from 68nm to 83 nm.In the synthesis system of Cu_1.81S nanosheets,the thickness of Cu_1.81S nanosheets could be accurately controlled from 12 nm to 32 nm by adjusting the Cu/S ratio and copper source concentration.In addition,using10 mmol DTBD as the sulfur source to replace t-DDT,precise regulation of Cu_1.81S nanocrystalline morphology and particle size was achieved by adjusting the composition ratio of ligand,which can stabilize the formation of regular morphology and uniform particle size nanoparticles,such as 57nm nanosheet,35 nm polyhedron and 18 nm spherical nanocrystalline.The above work provided abundant and stable matrix materials for subsequent research.（2）High-quality Cu_2-xS-based semiconductor nano-heterostructures were synthesized,and the precise control of morphology and particle size,heterogeneous interface,and domain composition was realized,which accelerated the directional separation of charge.The synthesis of semiconductor nano-heterostructure photocatalyst was innovatively carried out by the cation exchange method.Cu_2-xS nanocrystals with different morphologies and particle sizes were used as matrix materials,and Zn²⁺and Cd²⁺were used as hetero-cations respectively.By adjusting the dose of the hetero-cations,A series of Cu_2-xS-MS（M=Zn/Cd）NHs with wide spectral absorption,fine heterointerface,different structural components,built-in electric field,suitable edge potential and sufficient active sites were prepared.The lattice mismatch in the Cu_1.94S-Zn S spherical nanoheterostructure was1.70%,and the lattice mismatch in rod-like and sheet-like Cu_1.81S-Cd S was only about 0.16%.The construction of a continuous anion framework,built-in electric field and fine heterointerface was conducive to rapid charge transfer,and the time scale of photogenerated charge transfer from Cu_1.94S domain to Zn S domain was only about 91 fs.Further,MPA was used as a surfactant to perform surface ligand exchange on the heterostructure,and the stable dispersion of Cu_2-xS-MS（M=Zn/Cd）NHs in the aqueous solvent was achieved.The above work laid a technical foundation for the subsequent application research of photocatalysis.（3）A highly efficient and stable Cu_2-xS-based semiconductor nanoheterostructure photocatalyst without noble metal loading was developed.The structure-activity relationship and the mechanism of hydrogen evolution in the heterostructure were clarified.The photocatalytic experimental results showed that the photocatalytic hydrogen evolution efficiency of Cu_2-xS-MS（M=Zn/Cd）NHs was significantly improved compared with that of pristine Cu_2-xS,Zn S and Cd S nanoparticles.The photocatalytic hydrogen evolution efficiency of spherical Cu_1.94S-Zn S NHs was 0.918 mmol h^-1·g^-1,which was 38-fold and 17-fold more than those of pristine Cu_1.94S and Zn S NCs,respectively.The optimal H₂ production efficiency of the Cd S-Cu_1.81S HNRs was 2.714mmol h^-1·g^-1,representing 150-fold and 108-fold improvements compared with pristine Cu_1.81S and Cd S NRs,respectively.The photocatalytic hydrogen evolution efficiency of sheet-like Cu_1.81S-Cd S HNPs reached up to 1.867 mmol h^-1·g^-1,which was 98 and 18 times higher than that of sole Cu_1.81S and Cd S NPs,respectively.After the photocatalytic hydrogen evolution experiment,the TEM and XRD results showed that the morphology and structure of each heterostructured photocatalyst did not change significantly,indicating that this type of photocatalyst possessed excellent photostability.In rod-like and sheet-like Cu_1.81S-Cd S NHs photocatalyst systems,there existed a similar structure-activity relationship,and the kinetic equilibrium between the reduction reaction of H⁺and the oxidative consumption of holes jointly controlled the hydrogen evolution rate of the photocatalyst.H⁺was reduced by photogenerated electrons on the surface of the Cd S domains to generate H₂,while the holes on the surface of the Cu_1.81S domains were removed by the coupling of Na₂S and Na₂SO₃.In a certain range,increasing the content of the Cd S domain on a single nanoparticle could provide more H⁺reduction sites on its surface,thus improving the efficiency of H₂ production.When oxidation sites were scarcely or insufficiently,the kinetics of hole consumption in the photocatalytic reaction was too slow,which conversely promoted the electron-hole pair recombination,thus inhibiting the photocatalytic hydrogen evolution process and reducing the H₂ production rate.In addition,the photocatalytic efficiency of the heterostructures prepared by the cation exchange method and the in-situ photoreduction deposition of noble metals was synergistically investigated.The photocatalytic hydrogen evolution efficiency of sheet-like Cd S-Pt NHs prepared by the in-situ photoreduction noble metal deposition method was0.263 mmol h^-1·g^-1,which was only 2.5 times higher than that of single Cd S,and far lower than that of Cu_1.81S-Cd S HNPs prepared by cation exchange method.The above results showed that the cation exchange method possessed great advantages and potential in the field of preparing high-efficiency photocatalysts.The achieved photocatalytic hydrogen evolution efficiency of heterostructured semiconductors prepared by cation exchange reaction could compare to that of noble metal-loaded catalysts.In this study,the photocatalytic properties of Cu_2-xS-based semiconductor nanoheterostructures were studied and explored from three aspects:the synthesis of Cu_2-xS matrix materials,the fine construction of heterostructures,and the effect of photocatalytic water splitting for hydrogen production.The design and construction of nanoheterostructured photocatalysts provided new ideas and promoted the development of the cation exchange method in the field of efficient photocatalyst preparation.This dissertation contains 86 figures,29 tables and 299 references.