Adsorption of docosanethiol from solution on polycrystalline silver surfaces: an XPS and NEXAFS study

Adsorption and molecular ordering of docosanethiolate (CH3(CH2)(21)SH) self-assembled monolayers prepared by immersing polycrystalline silver substates in a 3 mu mol ethanol solution were investigated as a function of immersion time by X-ray Photoelectron Spectroscopy (XPS) and Near Edge X-ray Absorption Fine Structure Spectroscopy (NEXAFS). The changes in the adsorbate and substrate core level spectra show that initial physisorption of docosanethiol on the air oxidized surface is followed by reduction of the surface oxide by the thiol, resulting in chemisorbed silver alkanethiolates. The NEXAFS data reveal, contrary to previously reported observations for the adsorption of docosanethiol on gold surfaces, that on Ag substrates even after the shortest immersion times a preferential orientation of the hydrocarbon backbones exists in the film. These results indicate a domain growth adsorption mechanism, where alkanethiolate molecules nucleate around the initial alkanethiolate islands. A high oxygen precoverage promotes alkanethiolate film formation. A two-step adsorption mechanism explains the experimental results and, using simple thermodynamic arguments, the catalytic effect of surface oxygen on film formation. (C) 1998 Elsevier Science B.V. All rights reserved.