In situ measurements of water adsorption on a platinum electrode by an electrochemical quartz crystal microbalance

Abstract

Adsorption of water at solid electrode-aqueous electrolyte solution interfaces is well known. According to the double-layer model proposed by Bockris and Reddy [l], the first layer closest to the electrode surface is composed only of water molecules when no specific adsorption takes place. In the case where a specifically adsorptive species is introduced into the solution, this species replaces the first-layer water molecules. Some of the adsorbed water molecules remain in the first layer and can interact laterally with specifically adsorbed species. In order to understand the adsorption and electron transfer processes at the molecular level, therefore, it is essential to examine the details of water adsorption, particularly to quantify the number of adsorbed water molecules and to determine the orientation and structure of the adsorbed water layer. Besides electrochemical measurements, adsorption of water at solid electrodeaqueous solution interfaces has been examined by in situ techniques such as the radiotracer method [2-61, surface-enhanced Raman scattering (SERSI [7-lo], UV reflectance spectroscopy [l l] and electrochemically modulated IR reflectance spectroscopy (EMIRS) [12,13]. The recently developed electrochemical quartz crystal microbalance (EQCM) [14] is a useful instrument for examining adsorption at the monolayer level because of its high sensitivity. However, EQCM studies of phenomena at the monolayer level are rather limited, probably owing to the difficulty of constructing a stable EQCM in solution. So far the EQCM has been used for studying the underpotential deposition of metals [15-181, the electrosorption of anions [19], adsorbed oxygen monolayers [20] and self-assembled monolayers of viologen derivatives [21] and ferrocene derivatives [22-251. Among these