Studies of the Electrochemical Removal and Efficient Re-formation of a Monolayer of Hexadecanethiol Self-Assembled at an Au(111) Single Crystal in Aqueous Solutions

Abstract

The electrochemistry of a hexadecanethiol monolayer deposited on a Au(111) single crystal electrode has been examined in electrolyte solutions of different pH values, and it has been found that a significant fraction of the hexadecanethiol monolayer can be electrochemically removed and redeposited repetitively from the Au(111) surface. We suggest that this behavior is caused by the low solubility of the reduced molecules which prevents their diffusion into the bulk of the solution. The solubility of the thiol is confirmed to be a most important factor in the oxidative redeposition process by identical experiments performed on butanethiol and nonanethiol layers which show a decrease in the oxidative redeposition with the increasing solubility of the thiol. Preliminary work suggests that surface roughness may also play a part in influencing the extent of oxidative redeposition, with smoother surfaces giving more redeposition. The reductive desorption/oxidative redeposition of hexadecanethiols in an aqueous solution of high pH (0.5 M KOH) consists of two distinct processes. This is suggested to arise from the presence of domains of thiols of different ionic permeabilities.

Electrochemical oxidative adsorption and reductive desorption of a self-assembled monolayer of decanethiol on the Au(111) surface in KOH+ethanol solution

“…They are 54.9, 57.5, 58.9, 63.0 and 65.1 µC cm -2 in 10 µM, 50 µM, 100 µM, 500 µM and 1 mM thiol solutions, respectively, and are smaller than 73.3 µC cm -2 , which is the expected value for the saturated coverage, i.e., (√3 x √3)R30° structure [44][45][46][47][48]. The value of 73.3 µC cm -2 has been reported for the reductive desorption of the SAM formed externally in an ethanol solution containing only the thiol [23,24].…”

“…Au -SR + e -=> Au + RS - (1) many electrochemical studies of the reaction (1) have been carried out because the peak area and the shape of the cathodic peak for the reductive desorption and its peak potential provide useful information of the SAM such as the adsorbed amount, stability, adsorption energy, orientation, and substrate morphology [17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34][35][36]. For example, it was found that the longer the alkylchain, the more negative the reductive peak potential, reflecting the stronger van der Waals attractive interaction among alkylchains [17].…”

“…For example, it was found that the longer the alkylchain, the more negative the reductive peak potential, reflecting the stronger van der Waals attractive interaction among alkylchains [17]. The reductive desorption process has been investigated by many techniques including electrochemistry [17][18][19][20][21][22][23][24][25][26], FT-IR [27][28][29], electrochemical QCM [30][31][32], and in situ STM [33][34][35][36]. Several groups suggested the formation of an "aggregate" or "micelle" near or on the electrode surface after the reductive desorption of the SAM [28,33,35].…”

Copper Electrodeposition on Alkanethiolate Covered Gold Electrodes*

“…For this annealing procedure, the vacancies and step edges move by diffusion of single substrate atom vacancies [26,45]. This [74,75], demonstrating that they bind strongly; below -500 mV sulphide desorbs from Au(lll) [7]; below -300 mV iodide desorbs from Pt(110), causing surface mass transport of the substrate [72]; -200 mV suffice to desorb the weaker bonded sulphate ion from platinum surfaces [76]). In the positive potential range, the situation can be much more complex; often an adsórbate is first oxidized and the product can desorb, e.g.…”

A Catalytic 2-Aminoethanethiol Reduction on a Gold Electrode through a Sulfur-Gold Bond Formation

“…4 In other studies by Morin and coworkers, the amount oxidatively re-deposited was affected by the pH of the solution. 5,6 It was suggested that the amount of the redeposition by the oxidation decreases with the increasing solubility of the thiols because of the diffusion into the bulk of the solution.For this study, 2-aminoethanethiol (HSCH 2 CH 2 NH 3 + ) in a pH 6 aqueous solution, which has not been studied for the reductive desorption reaction, was used as a thiol compound. It has a short chain length with an ionic amino group and is easily dissolved in an aqueous solution.…”