Structure Formation in the Wetting Layer of a Carbonyl-Functionalized Ionic Liquid on Au(111): How to Control the Functional Group?

Coating heterogeneous catalysts with ionic liquids (ILs), a strategy known as 'solid catalysts with ionic liquid layers', can fine-tune catalytic selectivity. Introducing functional groups into ILs enhances their interaction with reactants, but precise control over their positioning is crucial. The structural formation in the IL wetting layer of the carbonyl-functionalized IL [5-oxo-C₆C₁Im][NTf₂] on Au(111) is investigated using infrared reflection absorption spectroscopy and scanning tunneling microscopy under ultrahigh vacuum conditions, supported by density functional theory and molecular dynamics simulations. At low temperatures (<130 K), the IL forms disordered islands, which coalesce into ordered films near ambient temperature. At low coverage, the IL adopts flat, space-demanding adsorption geometries. Upon forming a closed film, adsorption shifts to more compact configurations, with the carbonyl group tilting toward the vacuum while the ring remains surface-bound. Deposition at 300 K forms crystalline structures in the sub-monolayer regime, where the cation side chain can either stand upright or lie flat depending on the coverage. The IL remains thermally stable and desorbs completely at 500 K without decomposition. These findings highlight how IL coverage and deposition conditions tune functional group orientation at the catalyst interface, optimizing SCILL performance.