Time-Resolved Spectroscopy Of Surface Ligand Controlled Electron Transfer Of CdS Quantum Dots

Lignin is abundant and widely distributed in nature,which is an ideal raw for renewable carbon source.The monomers of lignin are basically connected by ?-O-4 bonds.The selective cleavage of ?-O-4 bonds by photocatalysis is an important breakthrough point in the production of industrial chemical raw materials,and one of the effective ways to solve energy and environmental problems.Quantum dot catalysts,with many special properties such as quantum confinement effect,have been widely used in photocatalysis to effectively improve solar energy conversion.To avoid agglomeration,it is often necessary to modify the surface of quantum dots with ligand molecules in the synthesis process.Thus,quantum dots can be better dispersed in the reaction solution and facilitate more sufficient contact with reactants.However,the surface of the quantum dots plays a vital role in photocatalysis,ligand modification of the QDs may change the catalytic properties.Therefore,in order to design highly efficient quantum dots photocatalysts,it is important to further understand the effects of ligands on the surface of quantum dots for better understanding the roles of ligands in the catalysis process.In view of the phenomena and problems mentioned,this thesis targeted on the photocatalytic dissociation of 2-phenoxy-l-benzene ethanol(PP-ol)by CdS QDs coated with mercaptocarboxylic acids(HS-(CH2)n-COOH,n=2,5,10).Time-resolved transient absorption spectroscopy(TA)was adopted to monitor the electron transfer in the catalytic reaction,and surface specific broadband sum frequency generation spectroscopy(BB-SFG)was used to investigate the adsorption and packing properties of the ligand molecules on the QDs surface.The research contents and conclusions are as follows:(1)Characterization of CdS QDs with capped mercaptocarboxylic acids ligands.The displacement of the surface ligands of CdS QDs has been detected by Fourier transform infrared spectroscopy(FTIR),and edge absorption of quantum dots has been detected by UV-VIS absorption spectrum(UV-VIS).The crystal shape,morphology and particle size of CdS QDs have been characterized by X-ray Diffraction(XRD),electron transmission microscope(TEM),dynamic light scattering(DLS).(2)The electron transfer kinetics between CdS QDs and electron acceptor(PP-ol)in thin film and solutions have been analyzed by TA.The main findings are as follows:i.In thin film,the electron transfer rate is negatively correlated with the alkyl length of the ligand.The electron transfer rate decreased exponentially with the alkyl length.The ligands act as the bridge bond in the electron transfer process,which is consistent with the tunneling model of the super exchange mechanism.ii.The electron transfer rate of quantum dots is clearly slowed down in solution,which indicates that the electron transfer process is hindered.(3)The arrangement of ligands on the surface of CdS QDs has been investigated by BB-SFG.The arrangement and orientation of the ligands on the surface of CdS QDs are closely related to the length of the linkers.Mercaptocarboxylic acids with shorter alkyl chains are arranged in a disordered manner on the surface,which greatly reduces the distance between QDs and the acceptors,and facilitating the electron transfer.However,mercaptocarboxylic acids with the longer alkyl chains form trans-extended ligand on the QDs,leading to the increase of electron transfer distance and hindering electron transfer process.