Self-assembled monolayers on oxidized metals. 3. Alkylthiol and dialkyl disulfide assembly on gold under electrochemical conditions

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.