Developments and applications of electrochemical microscopy

Abstract

This thesis is concerned with the use of electrochemical microscopy, in particular the development and application of the scanning electrochemical microscope (SECM). The concept of intermittent contact (IC), i.e. detecting the oscillation amplitude damping of an SECM imaging probe as it makes physical contact with a surface, is presented as a non-electrochemical means to provide tip-substrate distance feedback, in IC-SECM. This is briefly demonstrated for localised etch pit formation on a calcite crystal. A new imaging mode that incorporates a hopping imaging mode with the principles of intermittent contact is demonstrated for a range of samples, in hopping (H)IC-SECM. HIC-SECM uses an oscillating probe, so alternating current data are also obtained, and this type of scanning mode allows three-dimensional visualisation of the flux around an interface, which greatly enhances the information content compared to other types of electrochemical imaging. The resolution achievable by constant-distance imaging using IC-SECM is greater than constant-height imaging, although the dimensions of the electrode used in imaging will limit the resolution. Pt disk nanoelectrodes were also prepared, that were milled using focussed ion beam-scanning electron microscopy (FIB-SEM). These probes were used to image a model substrate, a gold band on glass, using IC-SECM in a low-force soft-tapping setup. The extension of this work to electrochemical systems focuses on studies of electrodeposition of silver nanoparticles on basal plane highly oriented pyrolytic graphite (HOPG). A nucleation-aggregative growth-detachment mechanism is proposed as an important feature of the process, through both macroscopic and microscopic scanning electrochemical cell microscopy (SECCM). The active sites for electron transfer on HOPG, a topic of recent debate, were also probed in macroscopic and microscopic experiments for silver electrodeposition. The difficulties of working on the electrochemistry of quinones in non-aqueous media are highlighted, with particular focus on methods to overcome the issue of electrode fouling. Different degrees of surface fouling were observed depending both on the electrode material, the compound studied and its concentration. IC-SECM was introduced as a means of establishing a close tip-substrate gap without needing to do electrochemistry for positioning and thus minimising fouling effects, from which high rate constants could be measured and the effect of electrode material on electron transfer kinetics investigated