Interfacial Molecular Adsorption Geometry And Electrochemical Reaction Investigated By Surface-enhanced Raman Spectroscopy And Single-molecule Break Junction Technique

Molecular adsorption at interfaces plays a key role in charge transfer and mass transfer processes in the fields of sensors,catalysis,energy storage and conversion,biochemistry,and molecular electronics.Therefore,with the help of advanced characterization techniques,it is of great significance to probe the effect of electrode surface and interface electronic structure on molecular adsorption at the molecular level,but it is still full of challenges.Since the development of shell-isolated nanoparticle-enhanced Raman spectroscopy（SHINERS）in 2010,the use of an ultrathin silica layer wrapped with a gold core with SERS activity as a signal amplifier,for the interface Species’vibrational signatures can produce electromagnetic field enhancements of 7 to 8 orders of magnitude.In addition,Scanning tunneling microscope break junction（STM-BJ）has been developed as a novel technique to monitor the properties of single-molecule adsorption on interfaces,which can reveal the role of molecules and interfaces by analyzing single-molecule characteristic conductance peaks and their changes and the adsorption geometry of the molecules.In summary,this thesis combines SHINERS and STM-BJ techniques to monitor molecular adsorption and its electrochemical reactions at the interface at the molecular level.The main contents and conclusions are as follows:1.Using STM-BJ and SHINERS techniques,on the atomically flat Au（111）model surface,systematically study the adsorption structure of pyridine molecules by the interfacial electronic effects induced by imidazolium ionic liquids,water,and organic solvents at the molecular level.Among them,the results of STM-BJ show that the interfacial electronic effects induced by air,water,and organic solvents lead to multiple sets of conductance values in the pyridine-like molecular junction.The interfacial electronic effect induced by BMI⁺modified Au（111）can hinder the flat adsorption geometry of the pyridine ring of 4,4’-BPY withπ-bonding interaction,resulting in the main vertical adsorption geometry of the molecule through the nitrogen atom withσ-bonding interaction,and only a single conductance value appears.In addition,in situ Raman spectroscopy directly shows that the CC stretching vibration peaks on the adsorbed pyridine ring appear and gradually increase in intensity with the increase of the volume ratio of water in the mixed solution.At the same time,the adsorbed BMI⁺on the interface also decreased,resulting in the transformation of 4,4’-BPY from the inclined adsorption geometry to the flat adsorption geometry.Finally,the DFT calculation results further show that the interface electronic effect induced by the solvent-modified interface leads to the molecular adsorption geometry transition.This work not only provides a simple and feasible strategy for effectively regulating the contact geometry of molecules and electrodes in pyridine-like molecular junctions,but also reveals the mechanism of interfacial electronic effects regulating molecular adsorption geometry and electron transport at the molecular level.2.Using STM-BJ and SHINERS technology combined with electrochemical methods,systematically study the electrochemical reaction of pyridine molecules at the Au（111）/ionic liquid interface at the molecular level.In situ electrochemical Raman spectroscopy confirmed for the first time that electrochemically induced pyridine molecules were transformed into radical cations in ionic liquids,and the new Raman peaks at 1644 and 1632 cm^-1 were assigned to positively charged pyridine rings CC parallel stretching vibration mode.At the same time,the in situ electrochemical STM-BJ monitors the conductance information of the conversion of pyridine-like molecules to radical cations from the single-molecule level,and exhibits a binary switch close to400%at the redox potential.This work not only provides a feasible method for designing novel redox-like molecular switches,but also provides molecular-level insights into the reaction process and electron transfer of pyridine-like molecules in ionic liquids.3.Surface-enhanced Raman spectroscopy（SERS）combined with electrochemical methods was used to monitor the oxygen evolution reaction process catalyzed by copper-bipyridine complexes at the molecular level.The UV-vis and EPR spectra showed that the copper-bipyridine complex was deprotonated in an alkaline solution at p H 12.5 to form（2,2’-BPY）Cu（OH）₂ with a tetragonal planar structure.In addition,in situ Raman spectroscopy found that the new Raman peaks at 557,1033 and 1324 cm^-1were assigned to the stretching vibration mode of Au-O,the ring breathing vibration mode of bipyridine molecule and the CC stretching vibration mode in the ring.Comparing the blank experiment and the heavy water experiment,it can be seen that the deprotonation of Cu-OH on（2,2’-BPY）Cu（OH）₂ to form Cu-O leads to the ring breathing vibration and the CC stretching vibration mode of the bipyridine molecule.Further combine with gold atoms or Au-O to form Au-O-Cu or Au-OO-Cu,and finally release oxygen.This work not only reveals the mechanism of（2,2’-BPY）Cu（OH）₂-catalyzed oxygen evolution reaction at the molecular level,but also provides theoretical guidance for the design of copper-bipyridine complexes to catalyze the oxygen evolution reaction.