The characterisation of thin insulating films on metal electrodes immersed in aqueous electrolytes using the AC admittance technique
The full potential of Langmuir Blodgett (LB) Technology will only be realised in the development of sensors, such as coated metal electrodes, if the problems related to film stability in an aqueous environment can be overcome. To this end, the ac admittance technique has been applied to characterise LB films of Barium Stearate and Poly-ButylMethacrylate, deposited onto platinum, copper and aluminium electrodes immersed in a range of electrolytes. Cyclic voltammetry was used as a complementary characterisation technique for the coatings on platinum. Equations were derived to relate the measured cell admittance to the equivalent circuit parameters. Approximations to these equations, valid for limited frequency ranges, enable parameter estimates to be made from the double log plots of capacitance and loss against frequency. The admittance data was analysed in terms of an equivalent circuit model adapted from the work of Macdonald (1952,1971,1973,1974,1976,1985). This treatment enables the electrical measurements to be related to the electrical double layer, electrolyte and deposited film and represents the first attempt to characterise LB films in this way. The equivalent circuit model was rigorously applied to the characterisation of i) PCB electrodes spin coated with PVC and ii) sputtered copper electrodes coated with Barium Stearate using the LB Technique. Several processes are found to contribute to the molarity, electrolyte and time dependent changes in coating admittance. For example, ionic uptake leads to an increase in coating admittance which can introduce a marked frequency dependent dispersion. For less stable coatings, film reorientation and desorption and changes in the oxide of the metal electrode can also affect the measured admittance. LB films of PBMA deposited onto platinum were sufficiently stable for immobilising electroactive species. Valinomycin incorporated in an LB film of PBMA and deposited onto platinum exhibited a reversible tt [K+] dependent conductance.