Adsorption and reaction of 4-nitrobenzoic acid on omega-functionalized alkanethiol monolayers on powdered silver: Infrared and Raman spectroscopy study

Abstract

We have investigated the stability of w-functionalized alkanethiol monolayers anchored on 2 mu m-sized silver particles by means of infrared and Raman spectroscopy. Although the alkyl chains of dodecanethiol (DDT) monolayers were deduced to have a trans-zigzag conformation by diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), those of 11-mercapto-1-undecanol (11-MUD) and 12-mercapto-1-dodecylamine (12-MDA) monolayers seemed to assume strongly disordered conformations on the powdered silver surface. We also found by DRIFTS that 4-nitrobenzoic acid (4-NBA) can be incorporated nearly equally into the hydrophilic(11-MUD and 12-MDA) and hydrophobic (DDT) group-terminated monolayers by forming Ag--COO- bond(s). This seemed to be best understood by presuming that the hydrophobic interchain interaction is less optimal on a microscopically rough surface than on a smooth surface when monolayers of 11-MUD and 12-MDA are assembled on Ag. It was also found by surface-enhanced Raman scattering (SERS) that photoreaction of 4-NBA more readily occurs at the 11-MUD-derivatized Ag particles than at the 12-MDA- and DDT- derivatized Ag particles. Invoking the role of water in such photoreactions, the hydroxyl-terminated monolayers appeared to be more labile to water than the amine-terminated monolayers. The present observations clearly illustrate that commercially available powdered silver is an efficient substrate for the characterization of self-assembled monolayers on silver substrates by the simultaneous use of DRIFTS and SERS.