Journal
JOURNAL OF PHYSICAL CHEMISTRY C
Volume 121, Issue 26, Pages 14213-14221Publisher
AMER CHEMICAL SOC
DOI: 10.1021/acs.jpcc.7b04167
Keywords
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Funding
- National Science Foundation (NSF) [CHE-1303998]
- NSF IGERT program [DGE-0903653]
- Office of Science of the U.S. Department of Energy [DE-AC02-05CH11231]
- Cornell Center for Materials Research Shared Facilities through the NSF MRSEC program [DMR-1120296]
- Division Of Chemistry
- Direct For Mathematical & Physical Scien [1303998] Funding Source: National Science Foundation
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Solution-deposited phosphonic acids, O=P(OH)(2)R, have been used to impart new functionality to a variety of metal oxide surfaces for applications ranging from organic field-effect transistors to biocompatible coatings on implants. Interestingly, the as-deposited monolayers are easily rinsed off, becoming robust and strongly adherent only after a long, low-temperature thermal anneal (e.g., 18 h at 120 degrees C). The need for this thermal treatment has raised questions about the nature of the bonding of the as-deposited monolayer. Is it merely physisorbed, requiring heat treatment for covalent bonding? To understand the first stages of monolayer formation, we have studied the reactivity and molecular bonding geometry of a prototypical, solution-deposited phosphinic acid, O=PH(OH)R, on the prototypical metal oxide surface rutile (110). We show that solution deposition produces near ideal, dense phenylphosphinate monolayers covalently bound in a bridged bidentate geometry. Three nearly orthogonal molecular vibrations-the P-H stretch vibration and the symmetric and antisymmetric OPO stretch-vibrations-provided an unambiguous signature of the three-dimensional structure and binding of the adsorbed monolayer; scanning tunneling microscopy provided information on long-range order and intermolecular conformation; and X-ray photoemission spectroscopy provided coverage quantification. Despite their covalent bidentate attachment and significantly higher binding energy than the corresponding carboxylic acid, a H2O rinse removed most of the phosphinate monolayer, demonstrating that hydrolytic stability does not result from covalent attachment alone. The H2O rinse also oxidized similar to 25% of the phosphinate monolayer to the corresponding phosphonate, producing a species that was somewhat more resistant to H2O rinsing.
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