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Title:Probing structure-function relationships at catalytic surfaces with emerging electroanalytical tools
Author(s):Simpson, Burton Herbert
Director of Research:Rodríguez-López, Joaquín
Doctoral Committee Chair(s):Rodríguez-López, Joaquín
Doctoral Committee Member(s):Gewirth, Andrew A; Murphy, Catherine J; Vura-Weis, Josh
Department / Program:Chemistry
Degree Granting Institution:University of Illinois at Urbana-Champaign
Scanning electrochemical microscopy (SECM)
Surface intermediates
Strontium titanate
Water splitting
Oxygen evolution reaction
Oxygen reduction reaction
Surface interrogation mode of scanning electrochemical microscopy (SI-SECM)
Redox titration
Abstract:In heterogeneous catalysis, adsorbed intermediates enable multi-step reaction pathways for otherwise energetically difficult chemical transformations. Understanding how catalyst surfaces impact the formation and reactivity of these adsorbed species is crucial to the rational design of new materials and structures. Many techniques based on spectroscopy, scanned probe microscopy, and electrochemistry have been developed to detect surface intermediates. These techniques have been used to chemically identify surface intermediates of many reactions and have played a role in understanding reactivity at well-defined interfaces. However, among these techniques, there is a need for a quantitative technique that can directly access reactivity in situ and can study the impact of local surface structure on adsorbate reactivity. Scanning electrochemical microscopy (SECM) is a powerful tool for probing local electrochemical reactivity at operating electrochemical interfaces. Recently, the surface interrogation mode of scanning electrochemical microscopy (SI-SECM) was introduced to allow the measurement of the coverage and reactivity of adsorbed reactive species formed on microelectrodes. Here, we detail work developing the numerical simulations framework that enables the application of this powerful technique to the spatially-resolved quantification of intermediates formed during photocatalytic reactions on semiconductor surfaces. This framework, allowed the measurement of the reaction kinetics and coverage of reactive oxygen species (ROS) formed on strontium titanate (SrTiO3) during photoassisted water oxidation. While developing the framework, it was discovered that simulations predicted that smaller SECM probes would enhance the surface interrogation signal while simultaneously improving the spatial resolution of SI-SECM measurements. This prediction was verified experimentally and leveraged to study the localized impact of substrate–adsorbate interactions on the reactivity of adsorbed ROS formed on pristine and defective sites on a (100) face of a SrTiO3 single crystal. Through some minor modifications, this framework even resolves the presence of multiple analytes of differing reactivities to produce a snapshot of the complex environment at the semiconductor–electrolyte interface. This capability was used to track the formation and decay of ROS formed on hematite during photocatalytic water oxidation. The development of these capabilities have significantly improved the utility of SECM for the studying mechanisms of photocatalytic transformations on semiconductor surfaces. More recently, we developed a novel platform to dynamically study the influence of surface structure on adsorbate reactivity. Strain engineering is an emerging concept in materials science where an engineered lattice deformation is used to tune the properties of functional materials. Our newly developed platform utilizes piezoelectric materials to induce strain in thin catalyst layers, which modulates the binding energy of molecular species to the surface. Because these supports are able to reversibly strain the supported catalysts, a larger array of surface configurations can be studied rapidly compared to typical approaches to strain engineering. While this technique is still in its infancy, refinements of device structure will enable detailed study of the relationship between surface structure and adsorbate reactivity. Many opportunities exist to improve the analytical performance of both SI-SECM and piezoelectric catalyst supports. For SI-SECM, it may be possible to develop methods to extract both surface coverage and reactivity from interrogation signals without the need for exhaustive simulation. This would allow the development of SI-SECM imaging modes to better understand the spatial distribution of adsorbate populations on operating semiconductor surfaces. Using surface interrogations to quantify changes in adsorbate reactivity on catalysts being dynamically strained by piezoelectric supports will allow discovery of an unprecedented level of detail regarding structure‒function relationships at catalyst surfaces. These new electroanalytical techniques provide a route to powerful in situ measurements of reactivity that complement existing knowledge to develop a mechanistic understanding of heterogeneous catalysis and inform catalyst design.
Issue Date:2017-07-09
Rights Information:Copyright 2017 Burton Simpson
Date Available in IDEALS:2017-09-29
Date Deposited:2017-08

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