Comparative Molecular Resolution STM Imaging of Thiourea, Ethylthiourea, and Sulfur Self-Assembled Adlayer Structures on Silver (111)

Abstract

Self-assembled adlayer structures of thiourea (TU), ethylthiourea (ETU), and sulfur (S) adsorbed on Ag(111), from neutral and alkaline aqueous solutions, are investigated with a scanning tunneling microscope operating under potential control. Voltammograms obtained under various different potential routines are very similar for the three substances. At potentials close to −1.2 V (versus saturated calomel electrode), a (√7×√7)R19.1° adlayer structure with superlattice and nearest neighbor distances of ds = 0.76 nm and dn = 0.44 nm, respectively, is imaged, irrespective of the adsorbate molecule. In this potential region, the first pair of conjugated current peaks related to electroadsorption/desorption processes is recorded. As either the electroadsorption time is increased or the applied potential is shifted positively, self-assembled TU and S monolayers evolve into more compact structures. TU adlayers compress into hexagonal arrangements with dn = 0.33 nm, and S forms adsorbed S-trimers with the initial ordered array superstructure and coverage and shorter dn. On the other hand, ETU only presents the hexagonal arrangement with dn = 0.44 nm common to all three adsorbates at the early ordered adsorption stages. The dynamics of adsorbate patterns can be explained by the interplay of adsorbate−substrate and adsorbate−adsorbate interactions, principally hydrogen bonding for TU, steric size effects due to ethyl substituents for ETU, and a clustering process assisted by polymerization for S. As TU on Ag(111) and TU on Au(111) exhibit similar adlayer structures, the electroadsorption free energy difference was estimated from the electrodesorption voltammetric peaks. The resulting value is similar to the desorption energy differences calculated for other sulfur-containing molecules.