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

• Published in: Journal of physical chemistry. C Vol. 121; no. 26; pp. 14213 - 14221
• Main Authors: Skibinski, Erik S, DeBenedetti, William J. I, Hines, Melissa A
• Format: Journal Article
• Language: English
• Published: American Chemical Society 06.07.2017
• ISSN: 1932-7447; 1932-7455
• DOI: 10.1021/acs.jpcc.7b04167

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.