Structure Formation in an Ionic Liquid Wetting Layer: A Combined STM, IRAS, DFT and MD Study of [C₂C₁Im][OTf] on Au(111)

Abstract

In a solid catalyst with ionic liquid layer (SCILL), ionic liquid (IL) coatings are used to improve the selectivity of noble metal catalysts. To understand the origins of this selectivity control, we performed model studies by surface science methods in ultrahigh vacuum (UHV). We investigated the growth and thermal stability of ultrathin IL films by infrared reflection absorption spectroscopy (IRAS). We combined these experiments with scanning tunneling microscopy (STM) to obtain information on the orientation of the ions, the interactions with the surface, the intermolecular interactions, and the structure formation. Additionally, we performed DFT calculations and molecular dynamics (MD) simulations to interpret the experimental data. We studied the IL 1‐ethyl‐3‐methylimidazolium trifluoromethanesulfonate [C2C1Im][OTf] on Au(111) surfaces. We observe a weakly bound multilayer of [C2C1Im][OTf], which is stable up to 390 K, while the monolayer desorbs at ∼450 K. [C2C1Im][OTf] preferentially adsorbs at the step edges and elbows of the herringbone reconstruction of Au(111). The anion adsorbs via the SO3 group with the molecular axis perpendicular to the surface. At low coverage, the [C2C1Im][OTf] crystallizes in a glass‐like 2D phase with short‐range order. At higher coverage, we observe a phase transition to a 6‐membered ring structure with long‐range order.

Temperature-Dependent Structure Formation in the Wetting Layer of the Ionic Liquid [C2C1Im][OTf] on Au(111)

“…The calculated IP density is 1.33 IP nm 2 , which is in good agreement with our measurements. In contrast to the previous work by Buḧlmeyer et al, 43 we did measure not only the reported hexagonal structure of [C 2 C 1 Im][OTf] on Au(111) but also oblique structures at temperatures ≤190 K.…”

“…In the previous study by Buḧlmeyer et al, the 2D glass structure was directly imaged for smaller IL islands and they achieved long-range order by increasing the IL film coverage. 43 In contrast to the work by Buḧlmeyer et al, we measured the change from a 2D glass-like structure to a long-range order as we increased the sample temperature while keeping the coverage constant.…”

“…The initial placements of [C 2 C 1 Im][OTf] molecules on the different surface slabs were done with the new in-house Python script build_adsorbates.py, with initial guesses based on relative energies of adsorbed dimers shown in our recent study of hexagonal adsorption patterns. 43 Geometries of the patterns at 100 K were optimized until all Cartesian force components were below 0.030 eV/Å, whereas the patterns at 190 and 320 K were optimized until all Cartesian force components were below 0.040 eV/Å. For some of the latter, the gradient component criterion could not be reached at all, then, convergence was signaled if the change of total energy was below 1 × 10 −5 eV.…”

“…Recently, some of the authors studied some fundamental aspects of the WL of [C 2 C 1 Im][OTf] on Au(111) using infrared reflection adsorption spectroscopy (IRAS), STM, density functional theory (DFT), and molecular dynamics (MD) simulations. 43 It was shown that the [OTf] − anion adsorbs perpendicular to the surface, with the SO 3 group facing the Au(111). The imidazolium ring of the [C 2 C 1 Im] + cation lies flat on the surface.…”

“…This growth behavior has already been reported for ILs on various metal substrates. 42,43,55 We also investigated the film formation behavior of the IL on Au (111) > and forms the basis for multilayer growth (Figure 5c,f,i). This structural change is most evident when the associated FT of the surface pattern is considered (Figure 5g−i).…”