Oxygen-Deficient Metal Oxides and Thiol-Functionalized Gold Nanoparticles as Model Ice Nucleating Particles

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Restricted (Penn State Only)
Author:
Mitch, Ryan
Graduate Program:
Chemistry
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
April 24, 2026
Committee Members:
Benjamin Lear, Major Field Member
Jerry Harrington, Outside Unit & Field Member
James Hodges, Major Field Member
Miriam Freedman, Chair & Dissertation Advisor
Kenneth Knappenberger, Program Head/Chair
Keywords:
ice nucleation
functional groups
gold nanoparticles
aerosol particles
clouds
climate
mineral dust
metal oxides
adsorbate
spinels
defect engineering
Abstract:
Aerosol particles perturb Earth’s radiation budget directly through interactions with shortwave and longwave radiation, and indirectly by facilitating phase transformations of water to form clouds. A small subset of aerosol particles induces ice nucleation, or the birth of an ice embryo, leading to subsequent cloud glaciation and, thus, altering the cloud-climate feedback loop. However, due to their complex composition and dynamic chemical properties, their specific radiative forcing mechanisms in this context are uncertain. To investigate the impact of individual chemical characteristics on the ice nucleation activity of aerosol particles, two model systems are selected. Active sites along the surface of aerosol particles exist as topographic defects, whose geometry and size may be amenable to ice nucleation. Metal oxides in mineral dust aerosol may possess not only these features, but also point defects in their lattice structure, such as oxygen vacancies. Yet the influence of chemical point defects on their ice nucleation activity has not been considered. To elucidate this relationship, ternary spinel metal oxides are synthesized and immersed in different calcination atmospheres as part of a defect engineering procedure. Zinc aluminate induces ice nucleation at higher temperatures compared to magnesium aluminate, particularly under the influence of a N2-rich, reducing environment, suggesting that oxygen vacancies primarily catalyze ice nucleation. This work sheds light on the relationship between chemical defects on the surface of mineral dust aerosol particles and their ice nucleation activity, highlighting a potentially important ice activation mechanism. Molecules positioned at the air-particle interface impact ice nucleation through the synergy of hydrophilicity and rigidity. While both properties are essential to dictating water-particle interactions, the degree to which intermolecular interactions along the particle surface dictate ice nucleation activity is not clear. Gold nanoparticles functionalized with para- substituted aromatic thiol ligands and monosubstituted octanethiol ligand analogs are explored. Both the backbone and non-thiol functional group influence ice nucleation activity via different mechanisms, with the octane chain facilitating ice nucleation at higher temperatures compared to the phenyl ring due to the reduced degree of π-stacking. Likewise, ligands exhibiting strong, two-way hydrogen bonding, particularly those containing carboxylic acid and ester groups, are not as effective as ligands exhibiting one-way hydrogen bonding or an intermediate degree of two-way hydrogen bonding. This work highlights the complex interplay between backbone and non-thiol functional group structure in the context of the ice nucleation activity of AuNPs and aerosol particles containing molecular adsorbates.