Calibration of the Electrochemical Quartz Crystal Microbalance

TLDR: In this article, the average sensitivity of the quartz crystal microbalance was calculated for the case of localized mass perturbation, where the mass change is continuously but nonuniformly distributed across the surface.

Abstract: The quartz crystal microbalance seems to be a very usefui tool in electrochemical studies, but, up to now, no attempts have been made to calibrate the microbalance under electrochemical conditions. The aim of this paper is to determine not only the average sensitivity for different active areas but also the differential sensitivity as a function of the radial position of a localized change of mass. In this way, for homogeneous mass perturbation, it is possible to calculate the change of mass (Am) from the frequency shift (AJ) for different active areas by taking into account the average sensitivity value. In the case of localized Am, the differential sensitivity value allows one to calculate hm from hfif the position of the event is known. The calibration procedure was made for an AT-6 MHz quartz crystal. Quartz crystals are important sensing devices in liquid phase. They can be used to monitor changes in electrode mass (1-3) or changes in the liquid properties (4, 5). In an electrochemical process, a shift of the value of the quartz crystal resonance frequency (fo) can be attributed to a change in mass (Am) of the electrode, provided that the true relationship between hf and Am is known. The problem of the sensitivity of the quartz crystal microbalance (QCM) has been already dealt with in the air (6-8). The differential sensitivity of the QCM has been calculated from the local damage produced in a thin polymer film by an ion beam sputtering. In the liquid phase the distribution of the vibration amplitude of the quartz crystal has been investigated by putting a tungsten wire probe in contact with the quartz crystal to provoke a change in frequency (9, 10). The results obtained in both liquid and gas phases are rather similar, in spite of the properties of liquid phase which extend the vibration of the quartz crystal even beyond the active region defined by the metal deposit. These works show the complexity of the system and that the boundary conditions used for the derivation of Sauerbrey's equation (11, 12) are not always fulfilled. The problem of the QCM calibration has been extensively studied in air. on the contrary, despite the wide use of the QCM in electrochemistry, the calibration procedure of this technique in electrochemical conditions is not depicted in the literature. The aim of this work is to calibrate the electrochemical quartz crystal microbalance (EQCM), not only in the case where hm is uniformly spread on the active electrode surface, but also for localized mass changes. The last case is very relevant to all electrochemical phenomena which produce a change of mass on a small area, e.g., in localized corrosion or gaseous bubble evolution. The general case where the mass change is continuously but nonuniformly distributed across the surface is not addressed in the paper.