Dissertations, Theses, and Capstone Projects

Date of Degree

6-2025

Document Type

Doctoral Dissertation

Degree Name

Doctor of Philosophy

Program

Chemistry

Advisor

Michael V. Mirkin

Committee Members

Daniel V. Esposito

Matthew Y. Sfeir

Chen Wang

Subject Categories

Analytical Chemistry | Materials Chemistry | Nanoscience and Nanotechnology | Other Chemistry

Keywords

Electrochemistry; nanotechnology; energy conversion; photocatalysis; scanning electrochemical microscopy; SECM; overal water splitting; nanoelectrodes

Abstract

Semiconducting materials play a crucial role in photocatalytic systems that harness solar energy, providing promising pathways toward sustainable energy solutions. Despite their promise, the search for semiconductors combining high efficiency, stability, and cost-effectiveness remains challenging. Local variations in semiconductor-liquid interfaces significantly influence the kinetics of photocatalytic reactions, while surface heterogeneities—stemming from native defects or catalytic modifications—further complicate structure-function analyses. Understanding these complexities requires advanced tools like scanning electrochemical microscopy (SECM), as they can directly reveal local variations in behavior and provide valuable insights into how the different structural motifs influence light absorption, carrier recombination, or reaction kinetics. Combining SECM with nanoelectrodes enables high-resolution imaging at the nanoscale, allowing in situ direct probing of active sites, a significant advantage distinguishing it from other characterization tools.

This thesis focuses on developing and applying high-resolution photo-SECM techniques to investigate overall water splitting (OWS) and other catalytic processes at the nanoscale, providing direct visualization and quantification of hydrogen and oxygen generation. The combination of high spatial resolution, quantitative measurements, and in situ capabilities positions photo-SECM as an indispensable tool for the future development of photocatalytic systems.

Chapter 1 introduces the foundational principles of SECM, emphasizing its versatility in probing electrochemical processes. Key advancements in nanoelectrode fabrication and technical challenges of nanoscale SECM are discussed.

Chapter 2 presents advancements in photo-SECM instrumentation, particularly a through-tip illumination approach coupled with a contactless optical system. This method allows localized illumination of a microscopic portion of the sample surface, with the tip simultaneously serving as a nanoelectrode for electrochemical measurements. An improved experimental setup was introduced to eliminate mechanical interactions between the optical fiber and the piezo-positioned tip. This innovation significantly enhances mechanical stability and allows high-resolution, artifact-free imaging and reliable nanoscale photoelectrochemical analysis, paving the way for more precise investigations of photoelectrochemical processes.

Chapter 3 presents the first direct observation of OWS at individual Al-doped SrTiO3 photocatalyst microparticles, achieved using high-resolution photo-SECM. This work confirms stoichiometric H2/O2 evolution on unbiased particles under illumination. Photoelectrochemical experiments on a single microparticle attached to a microelectrode tip further revealed particle-to-particle heterogeneity in activity and underscored the importance of single-particle studies to avoid masking individual contributions in bulk measurements.

Chapter 4 explores single phosphorus-doped BiVO4 microcrystals, revealing significant kinetic heterogeneity across the crystal surface, consistent with facet-dependent charge separation, where the {010} facet preferentially accumulates electrons, and the {110} facet favors holes. This study demonstrates photo-SECM's ability to probe sub-facet photocatalytic processes.

Chapter 5 introduces a theoretical framework for SECM measurements of electrocatalytic reaction rates involving kinetically controlled tip currents. The developed theory addresses challenges inherent to inner-sphere electron-transfer processes at nanometer-sized electrodes, enabling accurate quantification of hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) rates.

Chapter 6 investigates voltage-driven molecular catalysis and photoelectrocatalysis, demonstrating how the electrostatic potential drop across the double layer drives electron transfer between molecular catalysts immobilized directly on the electrode surface and dissolved reactants in a solution, enabling uphill reactions like water oxidation.

This PhD research contributes to the advancement of SECM as a powerful tool for studying photocatalytic and electrocatalytic processes at the nanoscale. Facilitating the illumination of a microscopic portion of the semiconductor while simultaneously quantitatively measuring the local fluxes of reaction products with high spatial resolution allowed a thorough investigation of the fundamental processes that govern photocatalysis. The methodologies developed in this research bridge fundamental science and practical applications, offering insights into the rational design of catalysts for energy conversion.

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