Ultrathin Films of a Nitrile-Functionalized Ionic Liquid [C3CNC1Im][Tf2N] on Au(111) and Pt(111): Adsorption, Growth, and Thermal Behavior.

Abstract

We studied the adsorption and thermal behavior of the nitrile-functionalized ionic liquid (IL) [C₃CNC₁Im][Tf₂N] on Au(111) and Pt(111) between 150 and 600 K. Ultrathin films were prepared at 150 K by physical vapor deposition (PVD) and were characterized by angle resolved X-ray photoelectron spectroscopy (ARXPS). At 150 K, the IL adsorbs intact with a similar orientation on both surfaces: In the first layer, the so-called wetting layer, the cation lies flat on the surface and the anion is bound in cis-configuration with the SO₂ groups toward the surface and the CF₃ groups away from the surface. On Au(111), subsequent deposition of IL in the multilayer regime at 150 K shows 2D growth up until ∼0.75 ML and a transition to moderate 3D at higher coverages. Temperature-programmed XPS indicates a change in surface morphology toward more pronounced 3D islands for the multilayers on top of the wetting layer between 220 and 290 K. From 350 to 440 K, desorption of multilayers occurs, with IL decomposition starting at 375-400 K. On the more reactive Pt(111) surface, decomposition starts already above 280 K. Notably, this temperature is ∼80 K higher than the onset for decomposition of related nonfunctionalized imidazolium-based ILs, that is, [C₂C₁Im][OTf], [C₁C₁Im][Tf₂N], and [C₈C₁Im][Tf₂N] on Pt(111). This difference is attributed to the nitrile functionality. Our findings demonstrate that functionalizing ILs significantly modifies their thermal properties, which is of high relevance for SCILL (solid catalyst with an ionic liquid layer) systems.