Functionalization of β-Ga2O3 Nanoribbons: A Combined Computational and Infrared Spectroscopic Study

The adsorption of H2O, alcohols (CH3OH and 1-octanol), and carboxylic acids (formic, acetic, and pentanoic) on beta-Ga2O3 nanoribbons has been studied using infrared reflection-absorption spectroscopy (IRRAS) and/or ab initio computational modeling. Adsorption energies and geometries are sensitive to surface structure, and hydrogen bonding plays a significant role in stabilizing adsorbed species. On the more stable (100)-B surface, computation shows that the physisorption of H2O or CH3OH is weakly exothermic whereas chemisorption via O-H bond dissociation is weakly endothermic. Experiment finds that a large fraction of a saturation coverage of adsorbed 1-octanol is displaced by exposure to acetic acid vapor. This is consistent with computational results showing that acids adsorb more strongly than methanol on this surface. The remaining alcohol, not displaced by acetic acid, suggests the presence of defects and/or (100)-A regions because computation shows that this less-stable surface adsorbs methanol more strongly than does the (100)-B. The v(C-H) modes of adsorbed 1-octanol are easily detected whereas no adsorbed H2O is observed even though H2O and CH3OH exhibit similar adsorption energies. It is inferred from this that the failure to detect H2O on the dominant (100)-B surface results from the orientation of the physisorbed H2O essentially parallel to the surface. Computation shows that this configuration is stabilized by H bonding. For chemisorbed formic acid, computation shows that a bridging carboxylate structure is favored over a bidentate or monodentate configuration. Computation also shows that chemisorption is favored on the (100)-A surface but physisorption is favored on the more stable (100)-B. Analysis of IRRAS data for acetic and pentanoic acids finds evidence for both types of adsorption. The carboxylate resists displacement by H2O vapor, which suggests that carboxylic acids may be useful for functionalizing beta-Ga2O3 surfaces. The results provide insight into the interplay between surface structure and reactivity on an oxide surface and about the importance of hydrogen bonding in determining adsorbate structure.