Establishment Of Electrochemical In Situ Sum Frequency Generation Spectroscopy And Its Application In Studies Of Surfaces And Interface Processes

As a second order nonlinear technique,sum-frequency generation(SFG)spectroscopy is forbidden,under the electric-dipole approximation,in the bulk of a medium with inversion symmetry and then generate only at the surface of such media where the symmetry is broken and appear to be highly surface-specific for interfaces between the centrosymmetric media.So far,surface electronic properties and the vibrational spectroscopy of the electrochemical interface have been investigated by SFG.Molecular level insight like the molecular adsorption,orientation,packing,and dynamics has been obtained.However,the application of SFG spectroscopy in electrochemical interfaces has many limitations and obstacles,such as difficulties in studying complex reaction systems and the analysis of sophisticated spectra.We have developed broadband sum-frequency generation(BB-SFG)spectroscopy,which could enable us to obtain information on in situ electrochemical systems.Aiming at the difficulties in combining electrochemical studies with SFG spectroscopy,we show,through some selected examples,that comprehensive molecular information of electrochemical interface is brought by SFG techniques.(1)The regular variation of substrate SFG signal intensity as potential scans was found during the study of SCN-/Au system,which is different from the parabolic electrochemical SHG signal's behavior.Through systematic study of substrate SFG signal intensity variation of gold electrode during potential stepping in various solutions,we could deduce that a linear feature in substrate SFG intensity variation corresponds to a stable electric double layer state and a positive slope indicates the positive charge property on the side of solution at electrochemical interface.For the analysis of substrate SFG signal variation of cetyltrimethylammonium bromide(CTAB)on Gold electrode system,the adsorption structure model can be inferred,which was difficult to access through normal SFG analysis.(2)The properties of adsorbed CO on polycrystalline platinum electrode and different platinum nanostructures which were electrochemical deposited on glassy carbon substrates have been studied.We observe that the line shape of COad SFG spectra changed regularly as the aggregation degree of platinum nanoparticles increased and gradually approached the spectra on polycrystalline electrode.We use principal component generalized projections(PCGP)algorithm analysis for time domain spectra to reconstruct first-order polarization induced by the IR beams(P(1)).Different vibration decay dynamics of C-O are found.The inhomogeneous broadening phenomenon at lower aggregation Pt electrodes indicates a more apparent disordered CO adsorption structure.Combined with potential-dependent SFG results,we can derive that key factor for the observed SFG spectra changes is not the surface electronic states differences among different Pt-CO adsorption systems but adsorption structures of CO molecules caused by different surface nanoparticles arrangement,which may suggest the origin of abnormal infrared effects(AIREs).(3)CO2 reduction processes on polycrystalline copper electrode surface,polycrystalline gold electrode surface and polycrystalline gold electrode with underpotential deposition copper have been investigated.In C-H region two SFG peaks were observed on copper electrode surface,and no adsorbed CO intermediate was observed,which indicated that the hydrocarbon intermediate in ethanol pathway were detected on copper and CO molecule would immediately convert to other structure which is no SFG feature in both frequency intervals after approaching copper electrode.As for the process on gold electrode surface,the adsorbed CO was the main product detected and the spectra feature of CO from CO2 reduction was similar to direct absorbed CO on gold electrode.(4)Internal reflection SFG spectroscopy electrochemical cell and Li-ion battery electrochemical in situ cell were designed and built.Octadecanethiol(ODT)was used as probe molecule in order to assess the internal reflection geometry.For ODT adsorbed on 30-nm Au films,internal reflection geometry could achieve the enhancement of molecular resonant features compared to the standard external reflection geometry and avoid the interference from the liquid layer.With the Li-ion battery electrochemical in situ cell,we can study the surface processes at Li-ion battery electrodes.Preliminary experiment results at metal lithium electrode surface derived that the absorption of electrolyte molecules and solid electrolyte interphase(SEI)at lithium and other electrode material surfaces could be investigated by SFG using this cell.In summary,this dissertation focuses on the difficulties in current electrochemical SFG research.By selecting appropriate electrochemical systems,improving acquisition and analysis of SFG spectra,designing and developing electrochemical SFG in-situ cells to broaden the application of electrochemical SFG spectroscopy and achieve fundamental understanding of electrochemical interfaces.