Diborane (B2H6) is a hydride gas often employed in high-purity industrial surface processes such as chemical vapor deposition or epitaxial layer growth. The use of diborane at industrial scales is complicated by the formation of higher-order borane contaminants in pure diborane gas via a complex series of gas-phase reactions. An advanced, rationally designed sorbent could stabilize diborane through interfacial interactions, dramatically reducing the decomposition rate without permanently trapping the molecule. However, the design of such a sorbent would require a nuanced understanding of diborane's fundamental surface chemistry, about which little is known. In the work presented in this thesis, a novel ultra-high vacuum (UHV) system was designed and employed to characterize the fundamental interactions of diborane with a variety of surfaces. In situ Fourier-transform infrared (FTIR) spectroscopy and temperature-programmed desorption (TPD) experiments were used in conjunction with density-functional theory (DFT) calculations to elucidate binding geometries and interaction mechanisms. On non-functionalized model surfaces such as CaF2 or amorphous carbon, diborane adsorbed only at cryogenic temperatures. Hydroxylated surfaces such as amorphous silica (SiO2) adsorbed significantly more diborane, which remained at slightly higher temperatures. FTIR spectra indicated the presence of hydrogen bonding between diborane and surface hydroxyl groups. DFT calculations revealed that the interaction takes the form of a novel bifurcated dihydrogen bond. In contrast with previous reports, diborane exhibited only weak interactions with the surface hydroxyl groups of silica. DFT calculations further elucidated that the irreversible reaction of diborane with surface hydroxyls is only possible in the presence of a second nucleophile (such as adventitious water). On the metal-organic framework (MOF) UiO-66 NH2, unique chemistry was observed in which diborane reacted with the –NH2 groups of the MOF linkers, yielding stable surface-bound products. DFT calculations determined the reaction mechanism to be dissociative adsorption of diborane, resulting in two amine-bound –BH3 moieties. Importantly, it was found that these fragments persisted at room temperature and could only leave the surface via the reverse reaction. The discovery that diborane can be stored as separate fragments that re-combine to yield the parent molecule has important implications for the development of new diborane sorbents. We hypothesize that surfaces designed with fixed, precisely spaced nucleophiles could enable the reversible storage of diborane. / Doctor of Philosophy / Diborane (B2H6) is a useful but hazardous gas employed in both academia and industry, often in processes that require ultra-high-purity source gases. However, diborane reacts with itself at room temperature, making the contamination of pure diborane very difficult to avoid. This problem could potentially be solved with a specially designed solid material that would sequester diborane without destroying it, but the design of such a material would require a much better understanding of diborane's chemistry with surfaces than currently exists. In this work, we employed ultra-high vacuum (UHV) methods to study the interactions between diborane and a variety of surfaces, with the ultimate goal of determining guiding principles for the design of diborane-stabilizing sorbents. Among the materials we studied were inorganic carbon, silica (SiO2), and a class of advanced microporous materials known as metal-organic frameworks (MOFs). Inorganic materials were found not to interact meaningfully with diborane. A novel hydrogen bond was discovered between diborane and the surface of silica, but the interaction was found to be too weak to provide significant stabilization. Most MOFs behaved similarly to silica. The MOF UiO-66-NH2, however, was found to react with diborane. Through a combination of computer simulations and UHV experiments, the precise nature of the reaction was determined. On the surface of UiO 66 NH2, diborane splits into two surface-bound BH3 molecules, where it is trapped until the reaction reverses. Importantly, it was found that BH3 can only leave the surface by recombining into diborane—effectively storing diborane on the surface to be released later. We hypothesize that this useful chemistry is due to the fixed distance between chemical groups on the MOF surface. This discovery suggests a promising strategy for the design of advanced diborane sorbents.
| Identifer | oai:union.ndltd.org:VTETD/oai:vtechworks.lib.vt.edu:10919/111586 |
| Date | 22 August 2022 |
| Creators | Jones, Nathan B. |
| Contributors | Chemistry, Morris, John R., Morris, Amanda, Deck, Paul A., Esker, Alan R. |
| Publisher | Virginia Tech |
| Source Sets | Virginia Tech Theses and Dissertation |
| Language | English |
| Detected Language | English |
| Type | Dissertation |
| Format | ETD, application/pdf, application/pdf |
| Rights | Creative Commons Attribution-NonCommercial 4.0 International, http://creativecommons.org/licenses/by-nc/4.0/ |
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