Surface-enhanced Raman scattering study on temperature-induced adsorption structure change of acrolein on coldly evaporated silver films

Abstract

Raman scattering and surface-enhanced Raman scattering (SERS) spectra were measured for acrolein adsorbed on a coldly evaporated silver film under the base pressure of 10 −8 Torr in the temperature range of 15–240 K. The SERS spectrum measured at 120 K indicates that the adsorbate exists mainly in a physisorption state. The physisorbed adsorbate desorbs from the silver surface around 150 K. When the substrate temperature was increased further to 180 K, there appeared SERS features which were different from those observed at 120 K. The main adsorbate at 180 K (type A) has the CO bond attached to the silver surface. On increasing the temperature to 240 K the type A species is converted to another adsorption state (type B), in which acrolein lies flat on the silver surface with both the CO and CC bonds coordinated to the silver. When the temperature is reduced to 15 K, the type B species persists as a main adsorbate. During the successive temperature change, the SERS spectra always exhibit a set of bands ascribable to a third adsorption state (type C), which is concluded to be silver(I) acrylate. Thus, under a base pressure of 10 −8 Torr a part of acrolein is oxidized to acrylate. The SERS spectrum was measured also for acetaldehyde adsorbed on the silver film at 180 K. The spectral feature can clearly be ascribed to silver(I) acetate, indicating that an oxidation reaction takes place at the surface. The adsorbate assumes an end-on structure with the symmetry axis of the carboxyl group perpendicular to the surface. When the temperature is increased from 180 to 240 K, the adsorbed acetate ion gives rise to a broad SERS band centered around 1544 cm −1 ascribable to an asymmetric CO 2 −stretching vibration. The result indicates that the temperature increase causes tilting of the symmetry axis. This temperature-induced tilting process is reversible because, when the temperature was reduced again to 15 K, the 1544 cm −1 band disappeared. This kind of reversible orientation change was also observed for silver acrylate (type C) adsorbed on the coldly evaporated silver film.