Switching adsorption and growth behavior of ultrathin [C₂C₁Im][OTf] films on Au(111) by Pd deposition

Abstract

Combining in vacuo deposition of ultrathin ionic liquid (UTIL) films with angle-resolved X-ray photoelectron spectroscopy (ARXPS), we demonstrate that by deposition of submonolayer amounts of Pd onto Au(111) the initial growth mode of the ionic liquid (IL) 1-ethyl-3-methylimidazolium trifluoromethanesulfonate ([CCIm][OTf]) can be switched from three-dimensional (3D) to two-dimensional (2D) growth, that is, from non-wetting to wetting. On clean Au(111), pronounced 3D growth occurs on top of an initially formed 2D wetting layer with cations and anions next to each other in a checkerboard arrangement. After pre- or postdeposition of only 0.7 ML Pd, two-dimensional layer-by-layer growth is found, which is attributed to strong attractive interactions between [CCIm][OTf] and surface Pd. For Pd post deposition onto the IL, the ARXPS data revealed particularly strong interactions between the dialkylimidazolium cation and Pd atoms, which considerably reduce the regular surface alloying of Pd with the Au substrate stabilizing Pd at the metal surface. In the context of heterogeneous catalysis using the SCILL (solid catalyst coated with ionic liquid layer) concept, these results directly provide a possible explanation on the molecular level for the beneficial influence of the IL layer in case of heterogeneous metal alloy catalysts.

Morphology, Structure, and Dynamics of Pentacene Thin Films and Their Nanocomposites with [C2C1im][NTf2] and [C2C1im][OTF] Ionic Liquids

“…Considering that PVD of ILs generates droplets, the thickness of the films is usually given in monolayers (ML), whereby 1 ML refers to one closed layer of ion pairs with cations and anions on top of each other. [36] The height (h) of 1 ML can be estimated from Equation (2). M is the molar mass of the IL, V m the bulk molecular volume, 1 the mass density, and N A the Avogadro constant.…”

Surface confinement induces the formation of solid-like insulating ionic liquid nanostructures

“…Several evidences have been reported of the surface-induced formation of ordered layered nanostructures of ILs extending up to several nm from the surface, at distances very large compared to the ion pair diameter. 27,[47][48][49][50][51][52][53] At odd with the case of the solvation layers observed at the bulk liquid/solid interface, which can be easily penetrated by a suitable probe, and typically reassemble reversibly upon the relief of the perturbation, in the case of solid-supported IL thin film the layers typically possess solid-like behavior, and cannot be displaced if not by the application of intense mechanical stresses. Solid supported IL nanostructures with extended layering have been produced in different conditions: pure bulk IL confined at solid interface, [54][55][56] IL diluted in solvent and dropcasted on solid surface, 48-49, 51, 57 and thin layers deposited by molecular beam deposition (electrospray or vacuum gradient).…”

“…Solid supported IL nanostructures with extended layering have been produced in different conditions: pure bulk IL confined at solid interface, [54][55][56] IL diluted in solvent and dropcasted on solid surface, 48-49, 51, 57 and thin layers deposited by molecular beam deposition (electrospray or vacuum gradient). 27,[52][53] . Different flat substrates have been investigated: insulator, 49, 57-58 carbonaceous materials 48 , and metals 56 .…”

The study of ionic liquid behavior at solid-liquid interfaces

“…A number of findings have been reported on triflate based ILs as well as salts as discussed before. [117][118][119][120][121] 81 These two results show frequency shifts in the OTf vibrational peaks as a function of film thickness. The first five rows in Table 3 the interfacial region.…”

“…Deposition of 0.7 ML of palladium before or after IL deposition switches the 3D growth to 2D (wetting) growth but with layer by layer configuration. 121 Additional studies compare multilayer and monolayer films of triflate based ILs, which show that the IR spectra for multilayer of IL resembles the bulk IR whereas the thin films look very different. This has been attributed to the arrangement of triflate at the surface with the SO3 group interacting with the solid surface while the CF3 group points outward.…”