Non-metallic nanostructures for light-driven chemistry and catalysis

Zhang, Wenxin

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https://hdl.handle.net/2142/132765 Copy

Description

Title

Non-metallic nanostructures for light-driven chemistry and catalysis

Author(s)

Zhang, Wenxin

Issue Date

2025-12-01

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

Jain , Prashant

Doctoral Committee Chair(s)

Shim, Moonsub

Committee Member(s)

• Chen, Qian
• Schleife, Andre

Department of Study

Materials Science & Engineerng

Discipline

Materials Science & Engr

Degree Granting Institution

University of Illinois Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

• Plasmonic semiconductor quantum dots
• Photocatalysis
• Nanomaterials
• Localized surface plasmon resoance.

Abstract

Since ancient times, nanoparticles have been part of human technology, even if people did not consciously recognize their existence. For example, the Lycurgus Cup and stained-glass windows in churches demonstrate early uses of nanomaterials to reflect and manipulate light for aesthetic purposes. With the advancement of science, we now possess a much deeper understanding of nanotechnology. Through deliberate synthesis, nanoparticles can be engineered to exhibit unique optical and catalytic properties. As one of the mostly commonly studied nanomaterials materials, coinage metal (Au, Ag, Cu) nanostructures display a remarkable phenomenon: under excitation by visible wavelengths of light, their free electrons undergo collective oscillations known as localized surface plasmon resonance (LSPR) oscillations. This effect has been harnessed both for harvesting visible light and for catalyzing thermodynamically uphill reactions. However, the scarcity and high cost of coinage metals limit their widespread application as plasmonic catalysts. To address this challenge, my research focuses on developing light-harvesting materials and catalysts from earth-abundant alternatives that exhibit LSPR, strong light absorption, and photocatalytic activity when engineered at the nanoscale to fulfill the requirement of large enough free carrier concentration. Within this context, semiconductor nanocrystals, namely two forms of copper-deficient cuprous sulfide (Cu₂₋ₓS), are studied in two separate investigations to gain new insights into their light harvesting abilities. In one case, the influence of defects and associated free carriers on the nature of light absorption was studied, and in the other the use of light excitation to control defect and associated free carrier concentration was demonstrated. The first study, presented as Chapter 2, focuses on modulating the free carrier concentration in copper sulfide nanocrystals using laser irradiation and such modulation is activated via band-gap excitation. Copper sulfide nanocrystals undergo spontaneous oxidation in air, leading to copper deficiencies and the generation of free holes that give rise to LSPR in the NIR region. However, we demonstrated that the deficiency level and free hole concentration can be decreased by photoassisted reduction in an oxygen-free environment. This method enables the use of visible light excitation not only to tune the LSPR frequency but to completely turn it off. The mechanism of this process involves visible light activation of a chemical reaction wherein copper oxide present on the surface of oxidized copper sulfide nanocrystals undergoes dissociation. The copper thus formed is injected back into the lattice of Cu2-xS nanocrystals and O2 is liberated into the oxygen-free environment. Kinetic studies and activation barrier measurements reveal the photoactivation is primarily due to a photothermal effect with a small contribution from photoexcited carriers catalyzing the dissociation reaction. These findings highlight opportunities for optical control of redox states and potential applications as optical switch. More importantly, this also serves as a lesson for the semiconductor photocatalysis community that photocatalytic effects may not necessarily originate from photoexcited carriers; photoinduced heating can be responsible. It is necessary to resolve and control for such a photothermal effect. Exploring new materials for plasmonic properties is also critical for advancing the field. The second study, presented in Chapter 3, investigates potential LSPR behavior in another form of copper sulfide, CuS, known as covellite. CuS has long presented a puzzle in terms of the oxidation state of copper, the structure of the lattice, and whether or not the material has copper vacancies and associated free carriers. This also raises questions about the near-infrared (NIR) absorption of nanocrystals of copper sulfide: is this absorption due to a free-carrier-derived LSPR or due to band-to-band transitions. In this work, I synthesized CuS nanocrystals and conducted a refractive index test of whether the NIR absorption has an LSPR nature. The test showed that the NIR absorption band is an LSPR and therefore the material has a large free carrier concentration, which was estimated from the LSPR frequency. I investigated the oxidation states of Cu and S in CuS nanocrystals using X-ray photoelectron spectroscopy (XPS) and Auger spectroscopy. We found both Cu⁺ and Cu²⁺ in near equal proportions. A copper-vacancy-rich structure based on this composition is consistent with the free hole concentration estimated from the LSPR frequency. In essence, we have addressed the puzzle about the structure of CuS and established the presence of copper vacancies and LSPRs. In conclusion, semiconductor nanoparticles like copper sulfide nanocrystals hold promise as catalysts for light-induced reactions. Their dual mechanisms—band-gap absorption and LSPR—offer flexibility and tunability across different wavelength regimes. For example, it exhibits a band-gap absorption extending across the visible region, while its LSPR lies in the NIR with fine-tuning possible via free-carrier-concentration control. Nonetheless, robust examination of photocatalytic activity remains a significant challenge. Experimental outcomes are influenced by numerous factors including colloidal nanoparticle stability, surface ligand density, degree of oxidation, and chemical impurities. Further systematic investigation is necessary to rigorously establish their photocatalytic attributes and unlock their potential in photocatalysis.

Graduation Semester

2025-12

Type of Resource

Thesis

Handle URL

https://hdl.handle.net/2142/132765

Copyright and License Information

Copyright 2025 Wenxin Zhang

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