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

Abstract

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.