Two-Dimensional Ultrathin Chalcogenide:Applications And Reaction Mechanism Research In Photocatalytic CO₂ Conversion

The ever-increasing socio-economic development and the continuous improvement of human living standards have brought about serious environmental pollution and energy shortages.How to effectively solve energy and environmental problems has become a worldwide issue.Excessive use of fossil fuels emits a large amount of carbon dioxide(CO2)that causes global warming.The concentration of CO2 in the atmosphere has increased from 280 ppm before industrialization to the current 400 ppm,an increase of nearly 30%,and continues to rise.By the 22nd century,it is expected to rise to 500 ppm,resulting in a global temperature increase of 1.9 ?.In order to alleviate the greenhouse effect and energy dilemma,the development of innovative energy technologies is imminent.In this regard,natural photosynthesis provides us with inspiration:Artificial photosynthesis can be used to convert carbon dioxide and water into useful carbon-based fuels and oxygen at normal temperature and pressure,which not only reduces the concentration of CO2 in the environment,but also produces renewable green energy.In this thesis,two-dimensional ultra-thin atomic layer materials are studied and combined with first-principles calculations to reveal the structure-activity relationship between the macroscopic properties of two-dimensional materials and their microstructures,deepening the understanding of the mechanism of photocatalytic CO2 reduction in 2D materials,providing new ideas and new material systems for more effective CO2 photocatalytic reduction and solar energy conversion.In this paper,we designed three kinds of cheap materials and developed the controllable preparation method as well as characterization method of its two-dimensional shape.Meanwhile,we explored the macroscopic function and exposed the essential mechanism for the two-dimensional ultra-thin atomic layer material from the atomic scale to realize the light-driven CO2 reduction.The main research contents of this thesis are as follows:1.Intermediate energy band WO3 ultra-thin nanosheets to achieve infrared light CO2 overall splitting.At present,in the field of CO2 photocatalysis,most of the research is concentrated in the high-energy visible and ultraviolet light regions,while the use of infrared light,which accounts for about 50%of the total solar energy,has rarely been reported.On the one hand,low-energy infrared light is difficult to be absorbed by conventional semiconductors,and on the other hand,the band edge position of the catalyst for CO2 photocatalytic reduction need to meet strict potential matching conditions.In order to effectively use infrared light to realize CO2 photocatalytic reduction reaction,we designed an intermediate band semiconductor system.Taking the oxygen-defective WO3 ultra-thin atomic layer nanosheet as an example,the theoretical calculations show that when the oxygen defect reaches a certain concentration,a new intermediate energy band appears in the middle of the band gap,so that the WO3 nanosheet can strongly absorb the infrared light to realize the valence band-intermediate band-conduction band transition of photogenerated carrier.The experiments have confirmed that the two-dimensional structure of WO3 atomic layer nanosheets not only provides abundant surface active sites,but also inhibits carrier recombination;on the other hand,due to the existence of intermediate bands,WO3 atomic layer nanosheets have more strong infrared absorption capability for efficient catalytic CO2 reduction.Furthermore,we applied it to the infrared photocatalytic CO2 reduction test,and the oxygen-deficient WO3 atomic layers display IR-driven CO2 overall splitting to CO and O2,while their catalytic activity proceeds without deactivation even after 3 days,showing high photoncatalytic stability.This work opens new avenues for designing IR-driven photocatalytic reactions such as water splitting and N2 reduction,thus largely promoting solar energy utilization efficiency.2.New mechanism of metallic CuS high efficiency infrared light driven CO2 reduction.Although the ultra-thin intermediate band semiconductor atomic layer can realize infrared photocatalytic CO2 reduction,its preparation process is complicated,carrier mobility is low,and it is also limited to a conventional semiconductor system,which limits its wide application.In view of this,we extend the research goal to a high electron density conductor system,and for the first time,metallic ultra-thin CuS atomic layer nanosheets are prepared,and high-efficiency infrared light-driven CO2 reduction performance is realized.The properties of the metal were confirmed by variable temperature conductivity test and theoretical calculation.Synchrotron radiation ultraviolet photoelectron spectroscopy fully proves that metallic CuS nanosheets can realize a new interband-intraband coupling electron transfer under infrared radiation,so that photogenerated electrons and holes can participate in redox reaction efficiently.In addition,combined with first-principles calculations and in-situ infrared spectroscopy,we speculated on its possible photocatalytic reaction mechanism.Infrared photocatalytic reduction of CO2 performance tests showed that metallic CuS nanosheets were able to produce carbon monoxide by 100%reduction of carbon dioxide under infrared illumination,and the yield was as high as 14.5?mol g-1 h-1.This work provides a new direction for the study of infrared catalysts.3.Bimetallic site catalysts enable high selectivity visible light to drive CO2 reduction to produce CH4.The ultimate goal of artificial photocatalysis is to achieve 100%selectivity of the desired reaction product in an ideally high yield.However,since the dissociation energy of the C=O bond is as high as?750 kJ mol-1,the photoreduction process of the extremely stable CO2 molecule is extremely difficult and complicated,and multiple proton-coupled electron transfer is involved in the reduction process,resulting in various reduction products,such as carbon monoxide(CO),methanol(CH3OH),methane(CH4)and even higher hydrocarbons.The distribution of the final product is usually determined by the kinetic and thermodynamic parameters of the CO2 reduction pathway.Taking the simplest hydrocarbons(CH4)and carbon oxides(CO)as an example,the formation of CH4 is thermodynamically superior to the formation of CO;but from a kinetic point of view,compared to the two-electron reduction process of CO2 to CO,the need for 8 electrons in the CH4 formation process is more diff-icult to achieve.At present,there have been reports on the conversion of CO2 photocatalysis to CH4,but the selectivity of CH4 production is generally low.In this paper,an atomic layer thickness bimetallic site nanosheet was designed,benefited from the formation of a new highly stable M-C-O-M intermediate(M stands for metal site),which can regulate the selectivity of the product by regulating the reaction pathway.We prepared the CuIn5S8 single crystal cell layer for the first time,and the introduction of S vacancies caused the localization of the charge of Cu and In atoms around it.In situ FTIR spectra,Gibbs free energy calculations and synchrotron-radiation vacuum ultraviolet photoionization mass spectra unveil these Cu-In dual sites simultaneously bond with the C and O atoms of adsorbed CO2 to form a stable Cu-C-O-In configuration,which not only lowers the overall activation energy barrier but also converts the endoergic protonation step to an exoergic reaction process,thus changing the reaction pathways to form CH4 instead of CO.As a result,the sulfur-deficient CuIn5S8 single-unit-cell layers achieve near 100%selectivity for visible-light-driven CO2 reduction to CH4 with a rate of 8.7?mol g-1 h-1(over CO evolution),where the selectivity and activity are the highest level among all other reported single-component photocatalysts without any sacrificial agents evaluated under the similar conditions.This work provides a theoretical basis and a new research direction for further exploration of high-efficiency and high-selectivity photocatalytic systems.