Surface-enhanced Raman spectroscopy at electrochemically roughened gold in vacuum and on photochemically activated substrates in aqueous solution

John Patrick Edmund Davies, Purdue University

Abstract

Electrochemically roughened gold electrodes were explored as SERS substrates for experiments in vacuum. Adsorbates studied in vacuum were benzene, toluene, benzonitrile, nitrobenzene, aniline, and pyridine. The gold surfaces were found to result in an enhancement of the Raman scattering from adsorbates in vacuum on the order of 10$\sp4$ over that observed on a smooth gold surface. With the exception of pyridine, the shifting of band frequencies for particular modes of an adsorbate, a phenomena typical in SERS, was absent in the spectra for adsorbate coverages $<$100 L. This lack of perturbation of the adsorbate vibrational modes was interpreted as indicating that very weak, if any, chemisorption was occurring on the gold surface. Increased adsorbate-surface interactions were observed by dosing thick layers of adsorbates ($\geq$1000 L) followed by warming of the substrate to temperatures close to but below the desorption temperature of the adsorbate. The increased interactions were characterized by the observation of band frequencies shifted from their values in the normal Raman spectra. In the case of nitrobenzene a reduction product, nitrosobenzene, was detected. Application of adsorbates on the electrochemically roughened surface in air prior to their insertion into vacuum (ex-situ application) resulted in the strongest adsorbate-surface interactions observed. Ex-situ application of nitrobenzene resulted in the formation of oxidized p-aminodiphenylamine (+2 oxidation state) and the benzidine dication. The reaction mechanism is discussed. A second study reported is on the preparation and SERS-activity of sputter deposited Ag-Cu and Ag-benzotriazole (BTA) thin films deposited on a polycrystalline Ag substrate. Both surfaces undergo surface restructuring under laser irradiation (514.5 and 647.1 nm) resulting in surface features characteristic of electrochemically pretreated metals (large metal clusters 500 to 1000 A in diameter). The mechanism behind the photoactivated restructuring is discussed. Spectra of thiocyanate and pyridine are reported for the Ag-Cu electrodes, and spectra of thiocyanate and BTA are reported for the Ag-BTA electrodes.

Degree

Ph.D.

Subject Area

Analytical chemistry

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