Studying (Electro)Chemical Catalysis Of Au Nanoparticles And Carbon Nanostructures Using Single Molecule Fluorescence Microscopy

Abstract

This dissertation presents the results on studies of chemical and electrochemical catalysis using single molecule fluorescence microscopy. With its real-time, single-turnover observations at millisecond, sub-diffraction limit resolutions, the single molecule fluorescence microscopy is a powerful technique in interrogating nanocatalysts in catalyzing chemical and electrochemical transformations. In this work, three major topics are covered: (i) revealing a hidden surface reaction intermediate in a catalytic process on a single nanoparticle, (ii) visualizing monolayer graphene oxide sheets in electrocatalysis and monitoring their activity changes upon electrochemical reduction, and (iii) studying electrochemical catalysis of arrays of aligned single-walled carbon nanotubes. Detecting and characterizing reaction intermediates is important and powerful for elucidating reaction mechanisms, but challenging in general because of the low populations of intermediates in a reaction mixture. Studying surface reaction intermediates in heterogeneous catalysis presents additional challenges, especially the ubiquitous structural heterogeneity among the catalyst particles and the accompanying polydispersion in reaction kinetics. The single molecule fluorescence spectroscopy was used in this work to study the oxidative N-deacetylation of amplex red catalyzed by Au@mSiO2 nanorods and 5.3 nm Au nanoparticles. The distribution of microscopic reaction times for the catalysts followed the initial-rise-and-then-decay behaviors, indicating a kinetic intermediate straddled by two rate determining steps. The rate constant for each step was quantified, and a working mechanism was formulated for the catalytic kinetics. Graphene oxide sheets are a novel material that can be potentially used for the electrochemical, optoelectronic and mechanical related applications. Challenges in the research of graphene oxide sheets include imaging the single-sheet and understanding the electrochemical activity. By using a fluorogenic reaction, the electrochemical catalysis of graphene oxide sheets was studied in real time. Morphologies of individual sheets were revealed by this fluorescence microscopy approach, including the boundaries, wrinkles and folded regions. A graphene oxide sheet might be divided into different regions whose electrochemical activities were studied and compared. A constant negative potential was applied to the graphene oxide sheets for their partial reduction while monitoring the activity changes. The fundamental understanding of single-walled carbon nanotubes in electrochemical processes is important for tailoring their structures for optimal performances. Single molecule fluorescence microscope was applied to study large arrays of aligned single-walled carbon nanotubes. Super-resolution imaging technique was used to precisely locate the active sites at the sub-diffraction limit resolution. After the single molecule study, the same piece of sample was imaged by AFM and SEM to overlay the active sites on top of nanotubes. Multiple active sites were observed on the same nanotube at different locations, and compared in terms of the dependence of substrate concentrations and applied potentials.