Abstract

Semiconductor metal oxide photocatalysts are attractive for harnessing solar irradiation to drive useful chemical reactions due to their band gaps suitable to absorb a significant portion of the solar spectrum, band edge positions appropriate for relevant reactions, and stability relative to similar materials. Their photocatalytic performance can be further enhanced by the addition of a cocatalyst to improve efficiency and stability. However, metal oxides are subject to photoinduced changes during photocatalysis. Understanding the in-situ changes in the material is critical to designing optimally performing systems for energy conversion, production of commodity chemicals, and environmental remediation. The research detailed in this dissertation examines the effects of nanoscale features on charge-carrier extraction in metal oxide semiconductors. We use super-resolution, single-molecule fluorescence microscopy to map the spatio-temporal variations in charge-carrier extraction from individual bismuth titanium oxide nanoplates. We also apply cobalt phosphate as a cocatalyst to examine the mechanism of catalytic enhancement. First, we imaged the performance of bismuth titanium oxide nanoplates using in-situ single-molecule fluorescence imaging. We observe significant variations in the activity and stability of individual nanoplates with an average activity loss of 39% (± 30.1%, first standard deviation) over 20 minutes of imaging, which we attribute to the photoinduced generation of surface hydroxyl groups. Furthermore, we observed different nanoscale regions in the nanoplates that exhibit either higher activity or higher stability, with implications for the future design of nanoparticle photocatalysts. These results highlight the necessity of observing in-situ changes and understanding the influence of different nanoscale features on photocatalytic performance. We next examined the cocatalytic mechanisms of cobalt phosphate on bismuth titanium oxide nanoplates. We employed ensemble photocatalytic activity measurements with a variety of selective scavengers to identify the chemical mechanisms for reduction and oxidation. We observe that cobalt phosphate increases the rate of both reduction and oxidation reactions without changing the catalytic mechanism of the reactions. Comparison with photoinduced defects and examination of energy-level alignment imply that cobalt phosphate increases band bending in the nanoplates and thereby extracts holes more efficiently to increase charge separation. Further work is planned to support this mechanism. This work clarifies the role of cobalt phosphate in photocatalytic applications and emphasizes the significance of controlling charge-carrier dynamics to increase photocatalytic activity. We also applied super-resolution, single-molecule fluorescence imaging to cobalt phosphate deposited on bismuth titanium oxide nanoplates. However, we discovered that cobalt phosphate blocks the detection of the fluorescent product fluorescein in single-molecule fluorescence imaging. Our results indicate that cobalt phosphate introduces a non-radiative relaxation pathway for fluorescein bound to the cocatalyst surface, quenching the fluorescence that is necessary for single-molecule detection. This study demonstrates an additional limitation of single-molecule fluorescence imaging. The importance of correlating single-molecule and ensemble activity measurements to verify observations is highlighted for future studies, and methods of confirming compatibility with single-molecule fluorescence imaging are discussed. Overall, our results demonstrate that future photocatalyst design must consider the role of nanoscale features in controlling charge-carrier extraction to enhance the activity and stability of the system.

Committee Chair

Bryce Sadtler

Committee Members

Kelly Powderly; Matthew Lew; Richard Loomis; Yan Yu

Degree

Doctor of Philosophy (PhD)

Author's Department

Chemistry

Author's School

Graduate School of Arts and Sciences

Document Type

Dissertation

Date of Award

8-1-2026

Language

English (en)

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Available for download on Friday, July 30, 2027

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Chemistry Commons

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