Water Co-Adsorption in Ultrathin Films of Ionic Liquids on Pt(111)

The influence of pre-adsorbed water (D₂O) on the growth, adsorption, orientation, and thermal behavior of two ionic liquids (ILs) on Pt(111) was studied within the SCILL (solid catalyst with ionic liquid layer) concept. Nonfunctionalized [C₁C₁Im][Tf₂N] and nitrile-functionalized [C₃CNC₁Im][Tf₂N] were deposited at ∼100 K onto clean Pt(111) or onto single-layer and multilayer crystalline (CI) or amorphous (ASW) D₂O films and analyzed at various temperatures by angle-resolved X-ray photoelectron spectroscopy (ARXPS). On both clean and D₂O-covered Pt(111), the ILs initially grow in a 2D layer-by-layer mode, forming a closed wetting layer at ≈ 0.5 ML IL coverage, followed by moderate 3D growth above 0.8-1.0 ML. At 100 K, the ILs partially displace D₂O from the Pt surface, yielding co-adsorption structures where IL contacts Pt(111) directly and is also located in a second layer. [Tf₂N]^- anions adsorb in cis-configuration with oxygen atoms toward the surface, while the cations adopt mixed parallel and tilted orientations. Heating to 130 K induces rearrangement, increasing direct IL-Pt(111) contact by favoring parallel cation alignment. Co-adsorbed D₂O remains but desorbs at ≈10 K lower temperature, indicating weaker binding to Pt. The similar interfacial behavior of both ILs shows that the nitrile group does not significantly influence adsorption geometry or thermal stability.