Construction Of Zn_xCd_1-xS Heterojunction And Study On The Performance Of Photosplitting Water For Hydrogen Production

The development and utilization of hydrogen energy has become a new topic in the 21st century.The process of hydrogen production,storage,and using are the research focus and difficulty of the entire hydrogen industry.The development process of hydrogen has changed from traditional coal to hydrogen,methane steam reforming to natural gas hydrogen production,which will consume fossil energy and bring secondary pollution.After the traditional hydrogen production methods,water electrolysis,wind power,and solar photovoltaic hydrogen production appear.These new methods are of great significance because they consume less fossil energy,and have already been used in small-scale industrial applications.The most ideal is to use photocatalytic water splitting to produce hydrogen technology,which has the advantages of green,pollution-free,and no fossil energy consumption.The reserves of sunlight and ocean water are huge.The solar energy that irradiates the earth’s surface is equivalent to the energy generated by thousands of tons of coal and oil every year.If the photocatalytic hydrogen production technology can be industrialized（the solar hydrogen conversion efficiency exceeds 10%）in the future,human life will no longer need fossil energy.However,due to the shortcomings of ordinary photocatalysts such as insufficient utilization of sunlight,low separation efficiency of photogenerated carriers,low hydrogen production rate,and poor stability,it is particularly important to develop and research high-activity and low-cost composite catalysts.Zn_xCd_1-xS is a metal sulfide solid solution that can be used for solar energy conversion to hydrogen.It has the advantages of Cd S in response to visible light and the high potential and strong redox ability of Zn S,known as one of the most ideal sulfide catalysts.However,due to the low efficiency of sulfide carrier separation,its photo-splitting water hydrogen production activity is still not satisfactory for industrial use.Researchers need to modify it to improve the separation efficiency of photogenerated carriers.Commonly used methods include element doping,which can introduce impurity levels and vacancies into catalysts,narrow the band gap of semiconductor catalysts,and promote the transition and separation of photoelectrons.The generation of hollow structures and different crystal planes can promote light absorption and active sites.Constructing a heterojunction can form a built-in electric field to promote the separation of photogenerated carriers,etc.In terms of fully understanding the construction form of composite catalysts,this dissertation uses Zn_xCd_1-xS as the main catalyst,combined with reliable modification methods,adding vacancies,preparation of hollow structures,construction of heterojunctions,etc.The reaction mechanism（type I,type II,Z-Scheme,Schottky junction）is the main line of research,preparing the 0D/0D/3D Ni/Ni S/Zn_0.2Cd_0.8S,2D/3D Ni Fe-LDH/Zn_0.5Cd_0.5S,0D/2D Co₃S₄/Zn_0.5Cd_0.5S,0D/3D Zn_0.5Cd_0.5S/Zn Co Ni-LDH,2D/2D Zn_0.5Cd_0.5S-s/Mxene composite catalysts.The accuracy of the experimental results is verified by microstructure characterization,visible light response performance,separation of photogenerated carriers,and reasonable theoretical calculations.Chapter 1 is an introduction,which introduces in detail the development,principle of photocatalytic,Zn_xCd_1-xS catalysts,preparation methods of Zn_xCd_1-xS with different morphologies,the modification of Zn_xCd_1-xS,and the reaction mechanism of different heterojunctions.The significance of the selected topic and the main research contents of this paper are described.The second chapter is the experimental part,which introduces the experimental chemicals,characterization instruments,theoretical calculation methods,preparation of photocatalysts,calculation of quantum efficiency,preparation of photoelectrodes,and description of photocatalytic hydrogen production process.The third chapter introduces the photosplitting water hydrogen production performance and theoretical calculation of 2Ni/4Ni S/Zn_0.2Cd_0.8S composite containing Schottky junction and type I heterojunction.A flower-like structure of twin crystal 3D Zn_0.2Cd_0.8S was synthesized by a simple one-step hydrothermal method,the microstructural characterization demonstrated the successful preparation of the composite.There are three factors can separate carriers in 2Ni/4Ni S/Zn_0.2Cd_0.8S:The twin homogeneous junction of Zn_0.2Cd_0.8S will form a built-in electric fields by ZB and WZ.The Ni/Zn_0.2Cd_0.8S Schottky junction utilizes the high conductivity of metallic nickel particles to guide the photogenerated electrons to the surface active sites.The built-in electric field and potential difference can effectively separate photogenerated carriers between Ni S/Zn_0.2Cd_0.8S type I heterojunction.Under the synergistic effect of these three,the performance of the ternary 2Ni/4Ni S/ZCS composite was improved,including the response to visible light,transient photocurrent,impedance,and hydrogen production rate.Compared with Zn_0.2Cd_0.8S,the transient photocurrent of2Ni/4Ni S/Zn_0.2Cd_0.8S increased by 18 times,and the hydrogen production rate of photosplitting water increased by 83 times.The positron annihilation,electron spin resonance spectroscopy,and theoretical calculations were used in the characterization methods to verify that Ni and Ni S cocatalysts can prolong the photoelectron lifetime and narrow the energy band to promote electronic transitions,and achieve effective separation of photogenerated carriers.The fourth chapter introduces the photosplitting water hydrogen production performance and theoretical analysis of a space-structured 2D/3D direct Z-Scheme heterojunction Ni Fe-LDH/Zn_0.5Cd_0.5S.Using ZIF-8 as a precursor,a polyhedral hollow structure catalyst Zn_0.5Cd_0.5S was synthesized by Kirkendall effect and cation exchange.Layered double metal hydroxide（Ni Fe-LDH）was sterically grown on Zn_0.5Cd_0.5S surface,the structure can maximize the contact with water to expose more active sites.The successful preparation of the space-structured Ni Fe-LDH/Zn_0.5Cd_0.5S was confirmed by microstructural analysis.The XPS,ESR spectra,theoretical calculations（DFT）and TRPL spectra proved the existence of electrons between Zn_0.5Cd_0.5S and Ni Fe-LDH.The electrons in the CB of Zn_0.5Cd_0.5S can combine with the Ni Fe-LDH VB holes to form an electron transfer channel,which proves that a direct Z-Scheme heterojunction is formed,the efficient separation of photogenerated electrons and holes can be achieved.The space-structured surface of the composite is 2.5times larger than pure Zn_0.5Cd_0.5S,and the red shift of the UV-vis diffuse reflectance spectrum proves that the refractive effect increases the light absorption of this structure.The Z-Scheme heterojunction 6LDH/Zn_0.5Cd_0.5S exhibits larger transient photocurrent and smaller impedance,and the rate of photosplitting water hydrogen production is increased by 11.6times.The fifth chapter introduces the photosplitting water hydrogen production performance and theoretical analysis of the layered 0D/2D Co₃S₄/Zn_0.5Cd_0.5S type II heterojunction.Combined with cheap Co-based cocatalysts,smooth layered Co precursors with different thicknesses were prepared through changing the reaction time,the wrinkled and layered Co₃S₄ surface was formed after sulfuration.This morphology makes it easier to load Zn_0.5Cd_0.5S nanoparticles on the surface of Co₃S₄,and the microstructure characterization proved the successful preparation of the 0D/2D composite Co₃S₄/Zn_0.5Cd_0.5S.The movement of elements binding energy in XPS indicate that Zn_0.5Cd_0.5S and Co₃S₄ are in the electron-gaining and electron-lossing states,respectively,this is due to the generation of a built-in electric field.The experimental results show that the transient photocurrent and hydrogen production rate of the composite are increased by 70 times and 14.6 times,respectively.The type II mechanism of the photosplitting process of Co₃S₄/Zn_0.5Cd_0.5S is proposed.The layered structure of the composite makes visible light refraction,and the absorption edge of UV diffuse reflection indicates red-shift.The layered structure improves the response of the sample to visible light.Using theoretical calculations to prove the transfer of electrons,Zn_0.5Cd_0.5S will accept electrons from Co₃S₄,making the Zn 4s and S 2p orbitals in the Zn_0.5Cd_0.5S CB closer to the Fermi level at the interface,The S 2p orbital traverses the entire conduction and VB at the interface plays the role of electron transport.The experimental results,characterizations and theoretical calculations demonstrate that the composite form type II heterojunction.The sixth chapter introduces the photocatalytic hydrogen production performance of the novel hollow flower-like 0D/3D Zn_0.5Cd_0.5S/Zn Co Ni-LDH type II heterojunction.Based on the principle that hollow structure can increase active sites and light refraction during sample modification,composite were prepared by precipitation and cation exchange methods using Zn Co-MOF as the precursor.The microstructure characterization indicated that the samples were successfully prepared.Through the experimental results and the generation of superoxide radicals and hydroxyl radicals in the characterization of electron spin resonance spectroscopy,it is proved that the composite Zn_0.5Cd_0.5S/LDH forms a type II n-n heterojunction,which can effectively realize the separation of photogenerated carriers and improve the photosplitting water hydrogen production efficiency.The 0D/3D Zn_0.5Cd_0.5S/Zn Co Ni-LDH composite has stronger visible light response performance,red-shifted UV-Vis diffuse reflection absorption spectrum,transient photocurrent and hydrogen production rate increased 3 times and 7.3 times,respectively.The seventh chapter presents the performance and theoretical analysis of 2D/2D Zn_0.5Cd_0.5S-s/Mxene composite containing S vacancies and Schottky junction.According to the principle that vacancies and heterojunction can effectively improve the separation of photogenerated carriers,two-dimensional sheet Zn_0.5Cd_0.5S was synthesized by solvothermal method,and the introduction of S vacancies in Zn_0.5Cd_0.5S was realized by simple treatment with hydrogen peroxide,a composite was prepared by combining Mxene（Ti₃C₂）.The microstructure characterization and electron spin resonance spectroscopy demonstrated the successful preparation of the sample.The experimental results show that the composite has excellent visible light response performance,the formation of Schottky junctions and vacancies can narrow the band gap,and the UV-Vis diffuse reflectance spectrum is red-shifted.Compared with Zn_0.5Cd_0.5S-s,the transient photocurrent of the composite increased by 11.5times.The photosplitting water hydrogen production rate of the composite increased by 23.2times.The Zn 4s and S 2p orbitals of Zn_0.5Cd_0.5S-s/Mxene were proved to have electron transport ability in theoretical calculations,the density of states of Zn 4s and S 2p near the Fermi level in Zn_0.5Cd_0.5S-s,indicating that there is a built-in electric field between Zn_0.5Cd_0.5S-s and Mxene.The experimental results,characterization and theoretical calculations prove that the Zn_0.5Cd_0.5S-s/Mxene Schottky is formed.The Schottky junction and S vacancies can effectively separate the carriers and improve the hydrogen production efficiency.The eighth chapter is the comparison of different heterojunctions in the full dissertation in terms of materials,preparation methods,morphological characteristics,visible light response performance,photoelectrical performance,hydrogen production performance and reaction mechanism,and summarizes the advantages and disadvantages of different heterojunctions and their industrial applications.Chapter nine is the conclusion.The scientific research achievements,conference exchanges,awards and acknowledgements related to this paper are introduced.