Surface Interactions of Diborane

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.

Name thesis:degree_name

Doctor of Philosophy

Level thesis:degree_level

doctoral

Discipline thesis:degree_discipline

Chemistry

Department dc:contributor.department

Chemistry

Grantor dc:publisher

Virginia Tech

Year dc:date.issued

2022

Author dc:creator

• Jones, Nathan B.

Chair dc:contributor.committeechair

• Morris, John R.

Committee members dc:contributor.committeemember

• Morris, Amanda
• Deck, Paul A.
• Esker, Alan R.

• Ultra-High Vacuum
• Diborane
• Silica
• Metal-Organic Frameworks
• Adsorption
• Desorption
• Infrared Spectroscopy
• Temperature-Programmed Desorption

Statement dc:rights

• Creative Commons Attribution-NonCommercial 4.0 International

Licence dc:rights.uri

• http://creativecommons.org/licenses/by-nc/4.0/

Language dc:language.iso

en

Dc Identifier Other

vt_gsexam:35381

OAI identifier oai:identifier

oai:vtechworks.lib.vt.edu:10919/111586