Investigating nanoscale dynamics in light absorbing nanostructures using transmission electron microscopy

Alcorn IV, Francis Marion

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https://hdl.handle.net/2142/121238 Copy

Description

Title

Investigating nanoscale dynamics in light absorbing nanostructures using transmission electron microscopy

Author(s)

Alcorn IV, Francis Marion

Issue Date

2023-07-12

Director of Research (if dissertation) or Advisor (if thesis)

Jain, Prashant K

Doctoral Committee Chair(s)

• Jain, Prashant K
• van der Veen, Renske M

Committee Member(s)

• Gewirth, Andrew A
• Zuo, Jian-Min

Department of Study

Chemistry

Discipline

Chemistry

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

• TEM
• Plasmons
• Catalysis
• in operando studies
• Photochemistry

Abstract

Plasmonic nanoparticles—those displaying a prominent light-absorption feature known as a localized surface plasmon resonance (LSPR) arising from the collective oscillation of charge carriers within the nanostructure—are promising candidates for applications including light harvesting for improved photovoltaics, single-molecule sensing, biomedical applications, and, most importantly for this dissertation, direct photocatalysis. Light excitation of these plasmonic nanostructures results in several processes that can be applied to catalyze reactions on the surface of the nanostructure. Firstly, large electric fields can distort chemical bonds of surface adsorbates to facilitate their reactions. Additionally, the LSPR can decay to produce energetic charge carriers that can drive oxidative or reductive reactions or form reactive radical ions. Lastly, eventual thermalization of excited-state carriers leads to heating of the nanostructure surface; the elevated temperatures can be used to enhance mass transport and reaction rates. Examples of reactions that have been successfully catalyzed by plasmonic excitation include hydrogen dissociation, alkene epoxidation, ammonia synthesis, and reduction of carbon dioxide into valuable chemicals. Another striking example of plasmonic catalysis is discussed in Chapter 3 in this dissertation. Specifically, we studied the effect of plasmonic excitation of Au–Cu alloy nanoparticles on electrocatalytic carbon dioxide reduction in an aqueous environment. In these studies, it was found that plasmonic excitation enhances rates of Faradaic reactions on the surface of the nanoparticles, as evidenced by higher current densities of the reaction under plasmonic excitation than in dark conditions. However, the primary product of electrocatalysis, which is carbon monoxide (CO) in the absence of plasmonic excitation, changes to hydrogen (H2). In other words, the reaction selectivity is overturned due to plasmonic excitation. In these experiments, the electrochemical reactions carried out without plasmonic excitation were performed at the same electrode surface temperature as those measured under plasmonic excitation, thus the effect of photothermal heating is fully accounted for, which is often a pitfall of studies in the field of plasmonic catalysis. The measured effects of plasmonic excitation are photochemical and non-thermal. While these studies are promising for developing novel, effective plasmonic technologies, there is a large knowledge gap on if and how these materials may evolve under the action of plasmonic excitation, and how such evolution may impact catalytic activity. Nanomaterials are known to dynamically evolve in a myriad of ways impacting catalytic behavior in both advantageous (activating) and deleterious (disabling) manners. Such dynamics can now be probed using in-situ transmission electron microscopy (TEM) techniques, discussed in detail in Chapter 1; but a lack of environmental TEM instruments equipped with light irradiation do not allow in-situ studies into light-induced structural dynamics in plasmonic nanomaterials. This technical gap is addressed by the development of the dynamic environmental TEM (DETEM) at the University of Illinois Urbana-Champaign. This instrument is an upgraded TEM equipped with modifications including a gas handling system, an electron spectrometer, and several lasers for performing pump-probe studies and in-situ light irradiation of the specimen. Using this microscope, dynamics of plasmonic nanomaterials under light excitation were investigated, which led to new findings about photoinduced restructuring, as described in Chapters 2 and 4 in this dissertation. Chapter 2 focuses on structural dynamics in copper–copper oxide (Cu2O) core–shell nanoparticles, a promising material for applications including plasmon-induced alkene epoxidation and carbon dioxide reduction. In these DETEM studies, hollowing of these nanoparticles into hollow nanoshells via the nanoscale Kirkendall effect was observed under simultaneous plasmonic excitation and electron-beam irradiation. This process was investigated with remarkable spatiotemporal resolution, capturing the nucleation of nanoscopic voids, which eventually grow to result in the hollow nanostructure. The number of void nuclei and the crystallographic direction along which they grow were measured. The process was found to be induced by electron beam irradiation, but plasmonic excitation was found to kinetically accelerate the hollowing rate by increasing the rate of void nucleation. These results point to a structural transformation that can potentially occur when utilizing these nanoparticles in plasmonic applications, but more importantly establish the capabilities of the DETEM for real-time studies of nanoscale dynamics in plasmonic materials under light. In Chapter 4, I describe the investigation of plasmon-induced dynamics of Au–Cu alloy nanoparticles—the plasmon-modulated electrocatalysis capabilities of which were described in Chapter 2—using the DETEM. Plasmonic excitation of these nanoparticles was found to induce coalescence of closely spaced nanoparticles in a manner kinetically distinct from the equivalent process induced by electron- beam irradiation. Specifically, coalescence occurred under plasmonic excitation in a switch-like manner in which the reaction was rapid after a waiting time that was much longer than the time required for the chemical transformation. This result reflects a potential process that can occur when these nanoparticles are used in photocatalytic applications and one likely to impact catalytic activities. Altogether, while plasmonic materials are promising technologies, there are still fundamental questions to be addressed about how these materials respond to energetic stimuli, particularly light irradiation. Results herein reflect several such stimuli-induced dynamics, highlighting the capabilities of advanced TEM techniques for undertaking real-time, nanometer-resolution studies and drawing crucial insights. Continued development of TEM technologies, including advances in electron sources, detectors, and data storage and processing will not only benefit the field of plasmonic, but other applied materials as well, as briefly discussed in Chapter 5.

Graduation Semester

2023-08

Type of Resource

Thesis

Copyright and License Information

Copyright 2023 Francis Alcorn IV

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