Many processes in nature and human-made settings rely on the unique properties of charged metal oxide:aqueous interfaces. Despite their ubiquity, these buried interfaces are challenging to study, since any analytical technique aiming to overcome the relatively small number density of interfacial versus bulk species must be highly sensitive and surface-selective. Nonlinear optical spectroscopies such as second harmonic generation (SHG) leverage the inherent anisotropy of metal oxide:aqueous interfaces to provide a direct optical probe of the E-field emanating from the charged solid, granting insight on properties such as interfacial charge density and surface coverage of adsorbed species. Moreover, recent advances in heterodyne detection (HD) have further empowered this technique to disentangle second- and third-order contributions to the measured SHG signal and facilitate the calculation of quantities such as interfacial potential and number of net aligned Stern layer water molecules without the use of traditional models describing charged surfaces. Although HD-SHG has developed extensively in its application to understanding dynamics at fused silica:aqueous interfaces, its application to strongly absorbing metal oxides remains nascent. In this thesis we demonstrate the application of HD-SHG analysis to the alpha-hematite:aqueous interface as a model absorbing metal oxide and one of the most ecologically and economically important ores of iron. More specifically, we highlight best practices in sample deposition, cleaning, and storage; the advantages and disadvantages of different sample geometries; the benefits of using an oscillator-only laser system for HD-SHG analysis; and the first steps toward developing an optical model to interpret phase-sensitive measurements performed at or near resonance. Major challenges remain for this system–including experimental design that fully accounts for the significant hysteresis of alpha-hematite–however, this thesis aims to serve as a record of the long and iterative journey toward expanding the scope of HD-SHG to a broader range of metal oxide:aqueous interfaces of environmental and technological relevance.

Creator

• Lozier, Emilie

DOI

• https://doi.org/10.21985/n2-gjqc-v747

Subject

• Physical chemistry
• Geochemistry
• Chemistry

Language

• en

Alternate Identifier

• http://dissertations.umi.com/northwestern:16725
• etdadmin_upload_1012560

Keyword

• solid aqueous interface
• second harmonic generation
• nonlinear optical spectroscopy
• metal oxide
• heterodyne detection
• hematite

Date created

• 2023-01-01

Resource type

• Dissertation

Rights statement

• In Copyright