A high resolution study of surface reactivity by scanning electrochemical microscopy

The aim of this study was to investigate a number of different sample surfaces immersed in electrolyte solutions, using scanning electrochemical microscopy (SECM). The SECM was mainly used in the amperometric mode, whereby the faradaic current passing through the microelectrode probe was monitored. The response of this probe is sensitive to both the topography of the surface and its reactivity.The SECM was used to image changes in conductivity across a surface, where the samples were a platinum microdisc electrode and an interdigitated array electrode. Sites active to hydrogen peroxide were also detected.The etching of enamelled ceramic surfaces in acidic and alkaline media was studied. Etch rates (thickness etched with time) were obtained by monitoring the amount of hindered diffusion recorded at the probe. The decrease in etch rate as a function of time was attributed to the large area of enamel exposed in comparison to the volume of solution used. This caused the pH conditions to become less extreme as the reaction proceeded. It is also thought that the solution became saturated with sparingly soluble products of the surface reaction. In addition, etched holes were created in the enamel surface by generating protons electrochemically at the probe, both potentiostatically and galvanostatically. The evolution of the holes was monitored in situ. The release of heavy metals by the sample was also monitored, by combining the techniques of SECM and anodic stripping voltammetry. In this case, a mercury microelectrode probe was able to record high concentrations of lead close to the sample surface in comparison to the bulk solution.pH variations across a platinum electrode sample biased at various potentials were measured. Two different approaches were used. The reduction of oxide at the platinum probe was unsuccessful, due to uncertainties in the origin of the variations in tip response. The reduction of protons at the probe produced well resolved, readily interpretable images. These were analysed in terms of the reactions occurring at the platinum sample and edge effects, probably caused by a poor seal between the platinum and glass at the sample electrode.Diffusional transport of ions in solution through a polystyrene/divinylbenzene cation exchange membrane was investigated. The probe response was measured as it was moved in a direction perpendicular to the membrane and the approach curves obtained were analysed in terms of hindered diffusion to the probe, the concentration profile extending from the membrane surface and the scavenging effect of the probe. Attempts were made to image the local ion flux through individual ion channels in the membrane. It is thought that this failed due to the size of the probe used and the overlapping of diffusion fields from individual ion channels in close proximity