Self-assembled monolayers of aromatic thiol and selenol on silver: Comparative study of adsorptivity and stability

We have investigated comparative adsorptivities and relative stabilities of self-assembled monolayers of thiol (benzenethiol, BT) vs selenol (benzeneselenol, BSe) on a silver surface by diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy and surface-enhanced Raman spectroscopy (SERS). BT and BSe are chemisorbed on silver as benzenethiolate and benzeneselenolate, respectively, after deprotonation with a tilted orientation with respect to the substrate surface; the benzene rings of BT and BSe are tilted by 25 degrees and 37 degrees, respectively, from the substrate normal. Competitive adsorption experiments show that adsorption of BT is more favorable by 0.3 kcal/mol. Temperature-dependent DRIFT spectra indicated that the monolayer of benzenethiolate on silver is thermally more stable than that of benzeneselenolate. This could be evidenced from the fact that the vibrational peaks of the benzenethiolate species were observed up to 458 K, while those of benzeneselenolate became substantially weakened around 418 K. The more negative desorption potential of the BT monolayer clearly indicates that the benzenethiolate binds more strongly to the silver surface than the benzeneselenolate does.