Development of electrochemical probe microscopy and related techniques
Edwards, Martin Andrew
This thesis presents work on the development of a number of scanned electrochemical probe microscopies. Such techniques have widespread applications, from materials science to the life sciences. Advances in flexible instrumentation, coupled with the theoretical description of electrochemical systems, are central themes which allowed for the fruitful investigation of a variety of experimental systems. Theoretical descriptions of scanning ion conductance microscopy (SICM) were developed, particularly to investigate the effect of tip-geometry on imaging resolution. This technique has already found a number of applications in the life sciences, but image resolution has not previously been addressed adequately. Images were recorded showing tip-convolution that were in agreement with theoretical predictions.\ud The scanning microcapillary contact method (SMCM) was developed, as a method of assessing spatial heterogeneities in electrode activity on the submicron length-scale. An electrolyte filled microcapillary containing a reference/auxiliary electrode was approached to a substrate (working) electrode surface, via piezoelectric positioners. Contact of the electrolyte meniscus with the substrate electrode was sensed by a current flowing. Electrochemical measurements were performed before the microcapillary was retracted and another point on the sample was characterised. Spatial heterogeneities in electrode activity were imaged on a sub-micron length-scale and the activity of basal plane highly oriented pyrolytic graphite (HOPG) was demonstrated.\ud Tip position modulation scanning electrochemical microscopy (SECM-TPM), where an ultramicroelectrode (UME) is oscillated perpendicularly to a surface and an amperometric current is recorded, was investigated experimentally and theoretically. A model including convective mass-transport was developed that gave an accurate description of the experimental situation. It was demonstrated that SECM-TPM is a potentially powerful approach for the measurement of the permeability of a sample.\ud SECM experiments were performed investigating the growth of Ag particles at a liquid/liquid interface, which was caused through the electrodissolution of a Ag UME in an aqueous phase, and the reduction of the Ag+ ion by an electron donor in the organic phase. A model was created that allowed for the interpretation of data.\ud Cyclic voltammetry investigations of HOPG covered with a Nafion film containing a redox mediator confirmed the activity of basal plane HOPG, as demonstrated by SMCM measurements. Nafion slowed diffusion sufficiently to allow the spatial-decoupling of surface sites with different activity.
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