OPTICAL IMAGING AND MECHANISTIC STUDIES OF ELECTROCHEMICAL PHENOMENA AT THE NANOSCALE

Abstract

In this work, we use optical methods to study electrochemical reactions and processes occurring on the nanometer length scale. Optical methods are advantageous over traditional electrochemical methods because of their high spatial resolution and sensitivity at both the single nanoparticle and single molecule level. This dissertation describes a series of studies in which super-localization and dark-field optical imaging is used to provide insight into spatial and temporal heterogeneity in nanoscale electrochemical systems with <25 nm spatial resolution. In the first set of experiments, three-dimensional (3-D) super-resolution imaging is used to determine the tip-substrate distance in nanoscale scanning electrochemical microscopy (SECM) with precision better than 25 nm. Correlating the tip-substrate distance using both optical and electrochemical techniques showed excellent agreement. Second, single nanoparticles (NP) were delivered through a nanopipette, and their resistive-pulse signals were correlated with a fluorescence optical signal. The diffusion trajectories of individual NP delivered to the external solution and to an electrified interface were obtained by 3-D super-resolution imaging, and showed signatures of both sub-diffusive and super-diffusive behavior, depending on the balance of forces between the flow from the pipette and the applied potential at the electrified substrate. Next, we studied the influence of surface oxide layers on single silver NP electrodissolution by tracking the intensity and spatial variation of scattering from single nanoparticles over time. We discovered that silver NPs can undergo electrodissolution in either a spatially symmetric or asymmetric manner, based on the nature of the surface oxide layer. Moreover, we also reported the simultaneous electrodeposition of silver oxide at the electrode surface during the electrodissolution of silver NPs, which enabled us to study the effect of multiple simultaneous redox reactions and their effects on one another. Overall, these experiments reveal local heterogeneity in nanoscale electrochemical processes and allow for many single nanoparticles to be measured in parallel, revealing relationships that are hidden using traditional electrochemical measurements.