Surface Chemistry on the Heterogeneous Metal Surface Investigated by the Sum Frequency Generation Microscope

Various surface phenomena, which are closely related to catalysis, corrosion, biology, etc., occur at a metal surface specifically due to unique nature of surface. Sum Frequency Generation (SFG) microscopy has been established as a powerful technique to investigate surface chemistry on heterogeneous metal surface. In this present work, the possibility about extending the application of SFG microscopy is described in detail. A new-generation SFG imaging setup with an improved spatial resolution and data acquisition speed has been developed and presented in the second chapter. In addition, the SFG theory and other experimental setups are introduced in the first two chapters. The main part of research presented in this thesis is about that the self-assembled monolayer of octadecanethiol (ODT) on the microcrystalline copper surface. The crystal grain and grain boundaries of the microcrystalline copper surface are mapped in the SFG image based on the strong brightness contrast of the SFG signal across the boundary. Local SFG spectra reveal significant difference with each other, as well as with the average SFG spectra, indicating the heterogeneity of the monolayer results from the copper grains with distinct crystallographic facets and orientations on copper surface. In addition, the statistical orientation analysis of amplitude ratio of CH3-sym/CH3-asym and corresponding contour maps imply that the orientation of ODT molecules is affected by the underlying copper. This work demonstrates that SFG imaging is a good approach to build the local spatial correlation of the optical non-resonant response of the metal surface to the conformation and orientation of the local adsorbed molecule. Besides that, the adsorption of near-atmospheric pressure methanol on the polycrystalline copper surface is investigated by SFG imaging. Methoxy is found as a stable intermediate which stably and abundantly exists on the metal surface. The pre-adsorbed oxygen promotes the dissociation of methanol with formation of a more ordered and higher density methoxy monolayer on copper surface. Another system studied by SFG imaging is the CO adsorption on a polycrystalline platinum surface. The stretching vibration mode from linearly bonded of CO adsorbed on the platinum surface is identified by the SFG spectra. The resonant frequency of CO stretching vibration varies on the different crystal domain areas.
The significant advantage of SFG microscopy over averaged SFG spectroscopy, as well as other surface techniques, is demonstrated in its ability to spatially correlate local molecular behaviors in response to the local properties of the metal surface. This work further demonstrates the powerful capability of SFG microscopy in characterizing molecular behaviors on the metal surfaces, as well as exploits the potential in analyzing surface chemistry on a heterogeneous metal surfaces.