EQCM AND SWITCH-FLOW CELL SET UP APPLIED TO ADSORPTION STUDIES

摘要

The electrochemical quartz crystal microbalance (EQCM) is a sensitive tool, capable of detecting mass changes in the submonolayer range. Its application to surface reaction studies greatly enhanced the comprehension of interfacial phenomena at the electrode surface and provides a unique in situ monitoring technique for adsorption studies. A switch-flow cell designed for EQCM measurements is presented where the inlet solution can be rapidly changed without interruption of the electrolyte flow allowing an easy introduction and removal of adsorbates from the electrochemical cell. In order to separate the resonance frequency shift due to the adsorption reaction (mass loading) from the contribution due to changes in solution viscosity and density during the switch-flow experiments (viscous loading correction), the quartz crystal resistance method is applied. The main advantage of this method consists of an in situ information sensitive to the local solution properties changes in proximity of the electrode interface. An accurate calibration with external measurements of viscosity and density has been carried out to validate the proportionality between the quartz crystal resistance variation and the viscous loading frequency shift as predicted by the theory of the electromechanical analogy for the oscillating quartz. As an example of application to adsorption studies, the adsorption of iodide anions on polycrystalline gold was investigated by this experimental set up. A negative frequency shift was detected when the iodide salt containing solution was allowed to flow in the cell at 0.2 VSSE, indicating a mass increase due to I-adsorption (Fig. 1). After the viscous loading correction the remaining frequency shift was attributed exclusively to mass loading from iodide adsorption and tests performed with different iodide salts showed the same quantitative contribution (Fig. 2). This was in a good agreement to theoretical expectations according to a I-monolayer formation in a closed packed hexagonal configuration. Interestingly the I- adsorption reaction was found to be irreversible in our experimental conditions. The simultaneous use of EQCM and switch-flow cell proved to be an interesting tool for adsorption studies on metal substrates.