Surface properties of small metal particles

Open Access
- Author:
- Mawby, Lillian Mackenzie
- Graduate Program:
- Chemistry
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- November 05, 2021
- Committee Members:
- Robert Hickey, Outside Unit & Field Member
Kenneth Knappenberger, Major Field Member
Raymond Schaak, Major Field Member
Philip Bevilacqua, Program Head/Chair
Benjamin Lear, Co-Chair & Dissertation Advisor
Bellamarie Ludwig, Co-Chair of Committee - Keywords:
- surfaces
nanoparticles
metal powders
NMR
gold
aluminum
silane
ligands
ssNMR - Abstract:
- Small metal particles behave differently from their bulk material counterparts. The major cause of this deviation is the large surface area to volume ratio which leads to a greater fraction of high energy atoms at the surface. This induces large changes in both the physical and chemical properties of the particles. I have studied some of these surface properties to provide fundamental insight to further the ability of scientists and engineers to tune these materials in a precise and informed manner. This will advance the utility of these systems for use in future real-world applications such as catalysis, optoelectronics, fuel alternatives, drug delivery, and energy storage. This dissertation will explore how surface molecules can be used to modify the physical and chemical properties of two classes of small metal particles. 1. While it is well known that thiol ligands have an affinity to bind to the surface of gold nanomaterials, the influence these molecules undoubtedly have on the core is still under speculation. Dipolar ligand effects show a correlation with the surface potential in planar and bulk metal systems, indicating the strong effect of ligands on the surface’s underlying electronic properties. We use this as our hypothetical basis in deciphering the effects of ligand dipole moments on the magnetic susceptibility of colloidal gold nanoparticles. By using azobenzene-containing thiols, we can alter the dipole moment in situ using external stimuli. To measure the magnetic susceptibility of the particles coated in these photoisomers, we used the Evans method of NMR. We determined that for small gold nanoparticles, the change in the dipole moment of the ligand is not the dominant factor contributing to the differing electronic properties of the core. In comparing the importance of through-bond to through-space interactions at the metal-molecule interface, the former has greater influence over the core electronics. 2. Aluminum (Al) powder undergoes a reaction with steam to produce heat and hydrogen, both of which can be used for power and energy applications. One possible use of these powders would be to generate power in oxygen-deficient environments. However, to use the powder in this capacity it must be fed into an oxidizer flow in a precise and controlled manner, meaning its rheological properties need to be tailored to produce an ideal flow behavior. Hydrophobic alkoxysilane coatings on Al powders can limit cohesion by restricting strong intermolecular forces at the surfaces of the particles thus improving the flow properties. While these coatings show viability in tailoring the rheology, mechanistic studies of the synthetic aspect of the coating process are sparse. Examination of the synthetic parameters that are part of the wet chemical coating procedure may lead to a better flowing powder. We test three different reaction temperatures, two humidity settings, two stir rates, and two lengths of cure time to ascertain which would be the preferred parameter setting for a better flowing powder with fewer agglomerates. Results show that the reaction temperature and the cure time both influence the powder’s apparent density, indicating that there are likely fewer agglomerates in those with a higher density. However, flow tests showed little variability between the powders coated using different settings. This may benefit future applications that require the coating process to be substantially scaled up. Magic angle spinning solid-state NMR is useful for observing the molecular coordination states of the surface aluminum species and furthering our understanding of the silane coating reaction. Upon comparison between the untreated powder and coated powder, it was evident that there is a surface restructuring of the Al. Control experiments were run to extract the variable causing this rearrangement. The results led us to the conclusion that the hydrochloric acid used to catalyze the hydrolysis of the silane to silanol is the cause. While the predominant species of Al for the untreated powder is pentahedral, powder exposed to HCl shows a greater majority of octahedral sites. We then pretreated the Al powder with HCl to use in the coating process and found that these pretreated powders can drive the silane hydrolysis without any catalyst. This outcome and the surface rearrangement evidence suggest the occurrence of surface etching which may create catalytic sites on the powder surfaces that drive the silane hydrolysis.