Self-Assembled Monolayers on Oxidized Metals. 3. Alkylthiol and Dialkyl Disulfide Assembly on Gold under Electrochemical Conditions

Abstract

Alkylthiol monolayers were assembled in ethanol solutions onto gold surfaces held at positive potentials. The developing monolayer introduces a barrier to electron transfer; hence, measurement of the current corresponding to ethanol oxidation at the applied potential provides a convenient means for real-time monitoring of the self-assembly (SA) process and its completion. Monolayers produced by the new method are formed considerably faster than similar monolayers prepared by the common procedure (no applied voltage). Two other processes which occur under the same applied potential include gold surface oxidation and oxidative desorption of the monolayer, both related to the presence of small amounts of water in the ethanol solution. It is shown that the interplay between the combined processes allows considerable control over the SA process before, during, and after monolayer formation, such as the possibility to perform multiple adsorption−desorption cycles for wettability and surface control. An important consequence of understanding the mechanism of alkylthiol SA onto oxidized gold concerns alkylthiol vs dialkyl disulfide adsorption. While the two types of molecules produce similar monolayers on reduced gold surfaces, a totally different result is obtained with oxidized gold, namely, alkylthiols form compact monolayers whereas dialkyl disulfides do not. This, together with the possibility to determine the extent of gold surface preoxidation, opens the way to rational design of mixed monolayers.

Quantifying thiol–gold interactions towards the efficient strength control

“…As proposed by Ron et al 37 , thiols may directly react with the oxidative gold surface to form Au-S bonds via the oxidation-reduction reaction involving the reduction of the gold oxide and the direct adsorption of thiols onto the reduced gold surface. The formed species on both the oxidized and reduced gold surfaces are gold (I) thiolates.…”

“…Proper cleaning of the gold surface is necessary to obtain high-quality thiol-gold-based SAM. For this purpose, rational methods for preparing highly reproducible gold surfaces, including the oxidative [35][36][37][38] and reductive 36,37 pretreatments, have been proposed. Briefly, gold substrates could be oxidized to a positive charge state via conventional methods, such as ultraviolet/ozone, oxygen plasma, electrochemical oxidation and piranha solution oxidation.…”

Studies on Self‐Assembled Alkanethiol Monolayers Formed at Applied Potential on Polycrystalline Gold Electrodes

“…On adding 1-dodecanethiol, peaks very similar to those obtained at lower potentials appeared, but falling nearly to zero instead of to their initial constant levels. This can be explained by the fact that, upon coating, the redox processes taking place continue only where there are pinholes and defects in the covering layer [19].…”

“…Examining the curve it can be seen that after restoring the initial potential of 0.0 V the frequency is close to its initial value, suggesting that removal of gold from the crystal has not occurred. It is known, however, that at potentials such as 0.9 Voxidation of ethanol to acetaldehyde starts to take place, and that a redox reaction between gold oxide and ethanol can occur [19,24], also leading to production of acetaldehyde. Similar effects have been observed with oxides on passivated nickel and iron electrodes [33].…”

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

“…The oxidative adsorption of alkanethiolates on a gold surface can be used to form the thiol SAM and has been also studied electrochemically [37,38] as well as with EQCM [31,32,39] but much less compared with the reductive desorption.…”