Atomic Hydrogen Reactions of Alkanethiols on Au(111): Phase Transitions at Elevated Temperatures

Abstract

Scanning tunneling microscopy (STM) was used to observe the chemical transformations of an octanethiolate monolayer on Au(111) exposed to gas-phase atomic hydrogen at 27 °C. Reaction begins with the steady conversion of the closepacked ϕ phase to the high-density, liquid-like ϵ phase. Following this step is a rapid event in which both ϕ and ϵ disappear from the surface and are replaced with low-density striped-phase structures. The ϵ phase, which does not form at room temperature, leads to this more complex reaction behavior. Experimental observations are reproduced by a simple set of kinetic equations describing the transformations between phases and the relative reactivity of molecules in each phase. Reaction kinetics had previously been attributed to enhanced reactivity near defect sites; with the phase transformation model, no such enhancement is necessary. The reaction rate of monolayers formed from 8-mercapto-1-octanol confirms this proposal, as the much higher density of defects in these monolayers does not cause significant changes in the reaction.

Influence of Structural Dynamics on the Kinetics of Atomic Hydrogen Reactivity with Low-Temperature Alkanethiolate Self-Assembled Monolayers

“…This observation is also true for SAMs exposed to atomic H at T S = 270 K. However, at T S = 250 K, the gold islands appear gradually throughout the reaction, even while close-packed thiols are still present on the Au(111) surface. We propose that the slow growth of gold islands is partially due to the reduced mobility of thiols on the Au(111) surface at low temperature, although it is interesting to note that the Kandel group has also recently reported on similar reaction-induced surface evolution at higher temperatures than our results . This suggests that other factors likely impact the rate of gold adatom growth, such as atomic H flux and overall reaction speed.…”

“…We propose that the slow growth of gold islands is partially due to the reduced mobility of thiols on the Au(111) surface at low temperature, although it is interesting to note that the Kandel group has also recently reported on similar reaction-induced surface evolution at higher temperatures than our results. 44 This suggests that other factors likely impact the rate of gold adatom growth, such as atomic H flux and overall reaction speed.…”

Molecular Imaging of Viologen Adlayers and In Situ Monitoring Structural Transformations at Electrode–Electrolyte Interfaces

“…Compared with noncovalent interactions (hydrogen-bonding and van der Waals interactions), the strong Au–S interaction provides a unique opportunity to construct molecular adlayers using thiolated molecules. As a model system, self-assembled monolayers (SAM) of alkanethiols have been thoroughly studied over the past few decades using scanning tunneling microscopy (STM) techniques. − It was found that alkanethiols first formed a flat-oriented adlayer when the coverage was low, but when the concentration of molecules on the surface increased, the molecular orientation changed from flat to tilted or vertical, with respect to the substrate surface . Small thiolated aromatic molecules have also been reported to form ordered structures on a gold substrate. , …”