Date of Degree

9-2018

Document Type

Dissertation

Degree Name

Ph.D.

Program

Chemistry

Advisor

Michael Mirkin

Committee Members

Mark Allen

Uri Samuni

Rein Ulijn

Subject Categories

Analytical Chemistry

Keywords

electrochemistry

Abstract

Electrochemistry at nanostructured interfaces is of significant current interest. Metal nanoparticles (NPs) and two-dimensional semiconductor nanomaterial are particularly important because of their applications in catalysis, energy conversion and storage, sensors, and medicine. Electrocatalytic processes at such nanostructured interfaces are essential for the generation of hydrogen and other types of fuels and photochemical energy conversion. Nanoscale electrochemical experiments require the fabrication and characterization of nanometer-size electrochemical probes. The main advantages of the nanoprobes include very fast mass-transfer rate, high signal-to-noise ratio and extremely fine spatial resolution of electrochemical imaging. During my Ph.D. research, nanoscale disk-type platinum electrochemical probes were developed for obtaining high resolution topographic image, including probing single catalytic nanoparticles and mapping out the catalytic activity of a semi-two-dimensional (semi-2D) nanosheet.

We first developed methodologies to fabricate and characterize disk-type platinum nanoelectrodes, including using air plasma to clean and activate nanoelectrode surfaces by removing most organic impurities, suitable for high resolution topographic imaging and investigating the electrochemical processes at single nanoparticles and two-dimensional nanomaterial. By employing the disk-type platinum nanoelectrodes as tips in the scanning electrochemical microscope (SECM), we can obtain the high-resolution topographic image in feedback mode and the catalytic activities image in the generation/collection mode of individual metal nanoparticle and two-dimensional nanomaterial. With a current-distance curve fitted to that expected from theory, we can evaluate the particle’s information of a nanoparticle. The direct probe of the electrocatalytic current at specified local sites with true nanoscopic resolution for two-dimensional nanomaterial can be used to analyze the catalytic activity of low-dimensional electrocatalysts which is highly dependent on their local atomic structures, particularly those less coordinated sites found at edges and corners.

A new mode of the SECM operation based on electron tunneling between the SECM tip and a NP immobilized on the insulating surface provides us a new tool to obtain high-resolution imaging of the NP topography. The obtained current vs. distance curves show the transition from the conventional feedback response to electron tunneling between the tip and the NP as the separation distances decrease to be ≤~3 nm. The tunneling mode of SECM operation also enables measurement of the heterogeneous kinetics at a single NP without attaching it to the electrode surface.

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