Size-selective Preparation Of Ⅰ-Ⅲ-Ⅵ Quantum Dot-based Photocatalysts And Their Application In Hydrogen Production

In recent years,due to the growing energy and environmental crisis,the use of photocatalytic technology to convert solar energy into chemical energy has received widespread attention.Photocatalytic water splitting is considered to be the most promising method for converting solar energy into hydrogen.An efficient and stable photocatalyst is the key to the development of clean energy.Among them,theⅠ-Ⅲ-Ⅵquantum dot materials have attracted great interest in the field of photocatalysis because of their unique quantum confinement effect,adjustable band gap,wide light absorption range,and many active sites.High-quality quantum dots still rely on the oil-phase high-temperature hot injection method.TheⅠ-Ⅲ-Ⅵquantum dot materials prepared in the water phase are not only water-soluble and the preparation method is green and environmentally friendly,but the scaled water phase preparation of quantum dots has the problem of uneven size,and the interaction of multiple metal components will exacerbate the growth of quantum dot process non-uniformity.Therefore,takingⅠ-Ⅲ-Ⅵgroup sulfide quantum dots as the research object,with the purpose of solving the problem of uneven preparation of quantum dots in the aqueous phase,high-qualityⅠ-Ⅲ-ⅥAg-In-Zn-S and Cu-In-Zn-S.For the preparation of quantum dot photocatalysts,size-selective separation preparation and supported promoters were carried out,the structural properties and photocatalytic activity before and after size-selective separation were studied,and feasible alternative preparation routes were explored.The research content is as follows:（1）In order to solve the problem of non-uniform size of water-phase quantum dots,two kinds of Ag-In-Zn-S quantum dots with different Ag contents were prepared by hydrothermal method.Firstly,the quantum dots are agglomerated to the state where precipitation occurs by adding the destructive solvent isopropanol quantitatively.Then the quantum dots are centrifuged step by step through centrifugation.The size distribution of the separated quantum dots is more uniform than that of the original solution,and the quantum dot photocatalyst with excellent hydrogen production performance is selected by electrochemiluminescence.AIZS-#4 is the quantum dot photocatalyst with the highest hydrogen production activity,and the hydrogen production rate is 1.68 mmol·g^-1h^-1,under the condition of S^2-/SO₃^2-as the sacrificial agent.Only simple separation can achieve the hydrogen production rate of the original quantum dot photocatalyst 6 times.This work provides a simple alternative method for obtaining high-quality quantum dot photocatalyst,and has a certain guiding role in the large-scale preparation of high-quality quantum dot.（2）Using the Ag-In-Zn-S quantum dots separated by size selection as the metamaterial,the catalytic performance of the quantum dot photocatalyst was improved by adding cocatalysis,and cheap NiS₂ was used as a cocatalyst to obtain Ag-In-Zn-S quantum dots/NiS₂ composite photocatalyst.The effects of NiS₂ loading and size before and after loading on the cocatalyst on the performance of photocatalytic hydrogen production were explored.The reaction mechanism of the composite photocatalytic reaction system with NiS₂ as cocatalyst was proved by in situ transient photovoltage.When the loading is 0.5%,the photocatalytic activity of s-AIZS/NiS₂-0.5is the highest.Under the condition of L-ascorbic acid as sacrificial agent,the photocatalytic activity reaches 5.96 mmol·g^-1h^-1,which is 4.4 times higher than that of s-AIZS quantum dots after separation,and 5 times higher than that of AIZS quantum dots in the original solution.This work proves that the quantum dot-loaded cocatalyst improves the photocatalytic performance of the original liquid quantum dot after separation to a large extent,and the first experiment proves that NiS₂ can enhance the effective separation mechanism of the electron-hole pair of the composite photocatalyst.It has a certain guiding role in the study of the mechanism of the semiconductor/NiS₂ composite photocatalytic system,and the development of active metal sulfides as a co-catalyst provides new ideas.（3）Cu-In-Zn-S quantum dots are combined with carbon dots in situ water to solve the problem of uneven structure of Cu-In-Zn-S/carbon dot composite system.Separation is carried out by means of size selection.After separation,the composite quantum dots have different optical properties and hydrogen production activities,among which Cu-In-Zn-S/C-#2 has the highest photocatalytic activity,and the total hydrogen production reaches 12.09 mmol·g^-1h^-1（L-ascorbic acid as sacrificial agent）,which is 2.4 times of that of Cu-In-Zn-S/C before separation,and 4.2 times of the original Cu-In-Zn-S.This work provides a new way for the preparation and screening of high-quality I-III-VI quantum dots/carbon dots composite photocatalyst,and also provides guidance for the large-scale production of composite quantum dots photocatalyst.