5 years ago

Solution Deposition of Phenylphosphinic Acid Leads to Highly Ordered, Covalently Bound Monolayers on TiO2 (110) Without Annealing

Solution Deposition of Phenylphosphinic Acid Leads to Highly Ordered, Covalently Bound Monolayers on TiO2 (110) Without Annealing
Erik S. Skibinski, William J. I. DeBenedetti, Melissa A. Hines
Solution-deposited phosphonic acids, O═P(OH)2R, 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 °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 ∼25% of the phosphinate monolayer to the corresponding phosphonate, producing a species that was somewhat more resistant to H2O rinsing.

Publisher URL: http://dx.doi.org/10.1021/acs.jpcc.7b04167

DOI: 10.1021/acs.jpcc.7b04167

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