Elucidation of the Deposition Processes and Spatial Structures of Alkanethiol and Arylthiol Molecules Adsorbed on Pt(111) Electrodes with in Situ Scanning Tunneling Microscopy

Abstract

In situ scanning tunneling microscopy (STM) was used to examine the spatial structures of n-alkane thiols (1-hexanethiol, 1-nonanethiol, and 1-octahexanethiol) and arylthiols (benzenethiol and 4-hydroxybenzenethiol) adsorbed on well-ordered Pt111 electrodes in 0.1 M HClO4. The electrochemical potential and molecular flux were found to be the dominant factors in determining the growth mechanisms, final coverages, and spatial structures of these organic adlayers. Depending on the concentrations of the thiols, deposition of self-assembled monolayers (SAMs) followed either the nucleation-and-growth mechanism or the random fill-in mechanism. Low and high thiol concentrations respectively produced two ordered structures, (2 x 2) and (square root of 3 x square root of 3)R30 degrees , between 0.05 and 0.3 V. On average, an ordered domain spanned 500 A when the SAMs were made at 0.15 V, but this dimension shrank substantially once the potential was raised above 0.3 V. This potential-induced order-to-disorder phase transition resulted from a continuous deposition of thiols, preferentially at domain boundaries of (square root of 3 x square root of 3 x )R30 degrees arrays. All molecular adlayers were completely disordered by 0.6 V, and this restructuring event was irreversible with potential modulation. Since all thiols were arranged in a manner similar to that adopted by sulfur adatoms (Sung et al. J. Am. Chem. Soc. 1997, 119, 194), it is likely that they were adsorbed mainly through their sulfur headgroups in a tilted configuration, irrespective of the coverage. Both the sulfur and phenyl groups of benzenethiol admolecules gave rise to features with different corrugation heights in the molecular-resolution STM images. All thiols were adsorbed strongly enough that they remained intact at a potential as negative as -1.0 V in 0.1 M KOH.

Electrochemical scanning tunneling microscopy examination of the structures of benzenethiol molecules adsorbed on Au(1 0 0) and Pt(1 0 0) electrodes

“…This was the only ordered structure seen on Pt(1 0 0) at h 6 0.5, as nucleation seeds observed at the initial stage of adsorption also contained this ( p 2 · p 2)R45°structure. This result is similar to the observation of benzenethiol admolecules arranged in p(2 · 2) on nucleation seeds on Pt(1 1 1) [20].…”

“…We recall our previous STM study of benzenethiol molecules adsorbed on Pt(1 1 1), where p(2 · 2) and ( p 3 · p 3)R30°structures are also identified, as those found for sulfur adatoms [20]. Two nearest neighbor molecules are 0.56 and 0.48 nm apart in these two structures, which are less than the van der Waals diameter of a benzene molecule.…”

Alkanethiols on Platinum: Multicomponent Self-Assembled Monolayers

“…8 A recent scanning tunneling microscopy (STM) study reported a high-coverage phase for alkanethiols on Pt(111) with a ( 3 × 3)R30°structure that corresponds to a 5.1 × 10 14 cm -2 coverage. 49 These literature coverage values are thus somewhere between the XPS-determined coverages for SAMs on p-Pt and TS-Pt.…”

“…75,76 Similarly, a recent STM study provided evidence of local ( 3 × 3)R30°ordering for alkanethiols on single-crystal Pt(111), but also reported difficulties with imaging any long-range ordering. 49 The presence of a disordered component such as S2 could naturally account for these observations.…”

The coordination chemistry of gold surfaces: Formation and far-infrared spectra of alkanethiolate-capped gold nanoparticles

“…(1)) Au nþm þ m RSH ! Au n ðAu-S-RÞ m þ mH þ þ me À ð1Þ has been invoked for spontaneous self-assembly [8,9] and for the electrochemically assisted formation of thiol SAMs on Au [10,11] and Pt [12]. (For the former case dioxygen or adventitious impurities serve as oxidant; for the latter, the oxidation is electrochemical).…”