Solid-like Ordering of Imidazolium-Based Ionic Liquids at Rough Nanostructured Oxidized Silicon Surfaces

The investigation of ionic liquids (ILs) confined in a solid porous matrix is of particular interest considering that these substances are increasingly used as electrolyte in devices employing nanostructured nanoporous materials for the electrodes. Furthermore, the confinement of the ILs into a porous matrix would allow overcoming the difficulties of its packaging, leakage and portability. In order to support applications, a deeper understanding of the interaction of ILs with the nanoporous solid material and its increased interface is required.
In this work, we report on the modification of morphological and mechanical properties of the imidazolium-based [Bmim][NTf2] ionic liquid upon surface confinement on a cluster-assembled nanostructured rough oxidized silicon (ns-SiOx) surface. An atomic force microscopy investigation revealed that upon the interaction with the ns-SiOx film, [Bmim][NTf2] locally rearranges into ordered, layered, stiff and poorly conducting solid-like domains, coexisting with, and embedded into, the liquid IL film. The observed interfacial layering of [Bmim][NTf2] deposited on ns-SiOx suggests that the behavior of the IL-electrode interface in photoelectrochemical devices employing nanostructured nanoporous materials can be far more complex than expected under the hypothesis of an IL-based electrolyte in the stable liquid phase. The observed effects reported in this work could in principle take place also inside the bulk nanoporous matrix, where they could be further amplified by the extreme spatial confinement.