Ionic liquids confined in carbon nanotubes: multiphase evolution driven by electron beam and temperature variations

Abstract

Ionic liquids, composed solely of cations and anions with high thermal stability, have attracted significant attention for various applications. In advanced energy and nanotechnology applications such as supercapacitors, ionic thermoelectric conversion systems, and nanolubrication technologies, ionic liquids are often confined within nanoscale spaces and exposed to external stimuli, including electrical and thermal inputs. While molecular dynamics simulations have predicted unique properties and phase transitions of ionic liquids under nanoconfinement, direct experimental observation of their multiphase evolution under external stimuli remains limited. In this study, we used a nano-manipulator to fill 1-butyl-3-methylimidazolium hexafluorophosphate ([Bmim][PF₆]) into individual multi-walled carbon nanotubes and employed in-situ transmission electron microscopy (TEM) to observe the evolution of complex multiphase structures under electron beam irradiation and temperature changes. Under intense electron beam irradiation, we observed the slow growth of irregularly shaped nanobubbles (∼20 nm in size) caused by electrolysis reactions, as well as liquid film thinning with increasing irradiation time. In contrast, heating alone caused only slight structural changes below approximately 400°C, indicating that the thermal decomposition is effectively suppressed at temperatures below 400°C due to the nanoconfinement effect. Above this temperature, we observed both abrupt and gradual transformations in nanobubble size and liquid film thickness, with the liquid film between the nanobubbles and carbon walls thinning to as little as 1.5 nm, forming an ultra-thin layer of soft matter adhering to the solid surface. The multiphase structures exhibited little change upon cooling from 1100°C to room temperature, indicating that thermal decomposition dominates over evaporation at high temperatures.