Electro-formation of self-assembled monolayers of thiol and sulfide on gold.

Abstract

The electro-deposition of thiol and sulfide monolayers has been studied with Electrochemical Quartz Crystal Microbalance (EQCM), chronoamperometry, FTIR, and cyclic voltammetry. The surface concentrations of alkanethiols/alkanethiolates calculated from the oxidative charge and from the EQCM signal using the Sauerbrey equation are in excellent agreement for depositions done in alkaline and pH neutral aqueous solutions. This allowed the measurement of the kinetics of the oxidative deposition of alkanethiols and alkanethiolates on gold. These experiments show that the oxidative depositions of alkanethiols and alkylthiolates occur in two steps. The oxidative deposition can be described by a combination of a potential dependent instantaneous nucleation-growth process and a potential independent Langmuir process. These results show that the rate of deposition of these compounds as well as their coverage can be controlled by an applied electric field. The effects of temperature and concentration on the oxidative deposition process are also reported. The electro-deposition of sulfide was found to be complex. In a pH neutral solution, sulfides go through a slow dissociative adsorption process and form a monolayer of thiolates. In alkaline solution, the oxidative adsorption of sulfide is faster. This observation is compatible with an SNZ reaction between physisorbed sulfides and hydroxyl ions.