Surface-enhanced Raman spectroscopy as a probe of adsorbate structure and bonding at platinum-group and noble metal surfaces
Abstract
Surface-enhanced Raman spectroscopy (SERS) has been employed as an in-situ probe of adsorbate structure and bonding at platinum-group and coinage metal surfaces, the former surfaces being prepared as ultrathin overlayer films on SERS-active gold. In addition to the constant-current electrodeposition of such overlayers, a new deposition method was developed, involving the spontaneous redox replacement of an underpotential-deposited copper or lead monolayer with a Pt-group metal cation solute. This method yields remarkably uniform transition metal films, displaying intense SERS for adsorbates bound to the overlayer and free from substrate interferences. Armed with these overlayer strategies, initial research was aimed at exploring chemistry at these surfaces, including chemisorption under equilibrium, as well as reactive conditions. The vibrational properties of several archetypical chemisorbates were examined, including monoatomic adsorbates, benzonitrile, and ethylene. Periodic trends in chemisorbate-surface bonding were revealed, reflecting the extent of electron donation and/or back-donation interactions between the chemisorbate and the metal. Such trends are found to correlate with the electronic properties of the metal, such as the electron filling and energy of the metal d-band. SERS and voltammetric measurements were also utilized to examine electrocatalytic processes, including the electrooxidation of formic acid and ammonia at Pt-group metal surfaces. Such studies yield information on the nature of the adsorbed species present under reaction conditions and their roles in the reaction mechanisms. Finally, the potential of SERS for the analysis of biologically-relevant systems was demonstrated with the detection and identification of aqueous saccharides. The sensitivity achieved with SERS enables high-quality Raman spectra to be obtained from small amounts of aqueous saccharides using low laser power and short signal acquisition times. The spectral fingerprints obtained furnish an effective means of distinguishing structurally similar saccharides, as well as determining the components of saccharide mixtures and oligosaccharides.
Degree
Ph.D.
Advisors
Weaver, Purdue University.
Subject Area
Analytical chemistry
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