Date of Award
Doctor of Philosophy in Chemistry
As the environment of the earth continues to be a growing concern, it is important to use and develop techniques to study and improve sustainable materials. Complex materials with difficult matrices or multiple reaction pathways will always be a challenge for chemical analysis, but X-ray absorption spectroscopy is a valuable tool to bypass some of the obstacles.
Herein a promising photovoltaic material, FASnI3, was studied in situ by X-ray absorption spectroscopy to isolate the kinetic oxidation of the bulk material. FASnI3 photovoltaics can be damaged by both moisture and oxidation, but isolating oxidation information can give insight to future pursuits to improve the stability of the material. The technique is also non-damaging to the perovskite which is important when oxidative damage is the focus of the investigation. The element specificity of Sn K-edge X-ray absorption spectroscopy allows us to look at the oxidation of Sn(II) to Sn(IV) over time and isolate that decay. In three different environments, N2, dry air and ambient air, FASnI3 was found to oxidize by a contracting area kinetic model yet does not lose the octahedral lattice structure as no evidence of bulk SnO2 or SnI4 was observed. Maintaining the perovskite structure has implications for future salvage and repair after oxidative damage. Complimentary optical absorption, X-ray diffraction and 119Sn Mössbauer spectroscopy was included.
A promising set of water degradable polymers were also studied by X-ray absorption spectroscopy to quantify the speciation of sulfur within the bulk material. After oxidation in a bleach bath, polythionocaprolactone polymer has swelling ratio evidence of crosslinking, but the exact crosslink structures were previously quantitatively unidentified. X-ray photoelectron spectroscopy was useful in determining the presence of disulfide, but the technique benefits from further information gleaned from sulfur K-edge X-ray absorption spectroscopy. One other long chain polymer, polythionopentadecalactone-block-caprolactone, and two short chain polymers, polythionocaprolactone and polythionoheptalactone, were included to see differences in the chemical structure as it was previously known the shorter chain polymers were less porous. X-ray absorption electron spectroscopy’s element specificity makes it a good choice for studying the oxidative state of an atom in a complex matrix such as a polymer. Linear combination was used to quantitatively calculate the contributions of known references to identify the sulfur speciation. The short chain polymers were found to have larger contributions of disulfide and thioether or thiol within the bulk polymer whereas the longer chain samples had larger concentrations of sulfone. These differences can explain the variation in porosity between the short and long chain polymers. A gold M4,5-edge was also observed for Au3+ adsorbed polythionocaprolactone and gold was present which is promising for future gold extraction techniques. The same polymer type made in the same batch synthesis did exhibit a larger signal for sulfone than the gold adsorbed polymer indicating those sites are more solution accessible compared to the more densely packed pockets of disulfide and thioether; this highlights the importance of investigating the chemistry of the bulk material.
Smith, Melissa Ann, "X-ray absorption spectroscopy as a technique to study environmentally sustainable materials" (2023). Open Access Dissertations. Paper 1537.