Study On Carbon Quantum Dots-modified LDH-based Composite Photocatalysts And Their CO₂ Reduction Performance

The recycling of carbon dioxide（CO₂）is one of the most critical problems for humans to maintain a healthy environment.As a potential carbon resource from the perspective of green chemistry,the conversion of CO₂into high value-added hydrocarbon fuels（such as HCOOH,CO,and CH₄）has received increasing attention over the past years.Among existing feasible conversion technologies,photocatalysis provides unique advantages as follows:It is nontoxic and harmless to the environment;it uses inexhaustible energy supplied by solar;its reaction conditions are milder and more controllable than those of thermal catalysis and electrocatalytic conversion.Currently,layered double metal hydroxide（LDH）is an inexpensive and easily prepared inorganic functional materials materials.However,the photocatalytic effects of conventional LDH are not ideal due to defects such as less pore structure,slow electron mobility,and low light utilization.Due to the abundant hydroxide ions in the LDH structure,it has excellent performance in capturing CO₂molecules,and has a synergistic effect with a variety of inorganic materials（such as carbon nitride,indium oxide,cadmium selenide）,enhancing the efficiency of CO₂-sorption and charge separation efficiency,Therefore,a composite photocatalyst with the dual advantages of LDH and other inorganic materials can be prepared.However,the relatively poor surface redox potential and weak light harvest still limit the further improvement of CO₂reduction activity and hydrocarbon fuel selectivity.Therefore,it is urgent to find suitable auxiliary means to further improve the optoelectronic properties of binary composites.Based on this,this subject integrated carbon quantum dots（CQDs）with LDH-based binary composites semiconductor to solve the problems of low activity and poor dispersion,and to improve the electron separation and transmission efficiency and light respondence.We prepared carbon quantum dots-modified Ni Al-LDH/g-C₃N₄（LDH/CN/CQDs）and Ni Al-LDH/Cd S（LDH/Cd S/CQDs）composite photocatalysts,and their reaction mechanisms are explored through various characterization methods.The specific contents of the research are as follows:（1）A treble flower-like LDH/CN/CQDs photocatalyst was prepared via a hydrothermal method.Benefiting from the synergistic effect of LDH,CN and CQDs,LDH/CN/CQDs photocatalyst enhances not only the carrier separation efficiency,but also the CO₂photoreduction activity.Without any sacrificial agent or photosensitizer,the yield of CO reaches 5.2μmol·g^-1·h^-1.Among them,CQDs as an important"electronic bridge",transfer a large number of photogenerated electrons,improve the separation efficiency of electrons and holes,and reduce the carrier transport resistance;on the other hand,they can improve the light respondence,further enhancing the photocatalytic reduction performance.（2）Hierarchical LDH/Cd S/CQDs photocatalysts were prepared by hydrothermal method.Utilizing the multi-electron channels established by CQDs on the LDH/Cd S binary catalyst and the reflection-scattering effect of the hierarchical photocatalyst,the CO₂reduction activity of LDH/Cd S/CQDs into CO is greatly improved to 7.04μmol·g^-1·h^-1,and the shortcomings of full-spectrum light absorption ability is also ameliorated towards pure LDH.High-dispersed CQDs act as attachments to increase the active sites and reduce the impedance of the composites,thereby facilitating optoelectronic transmission.This work can provide a blueprint for developing and designing other efficient photocatalysts.