Ultrafast Water Transport in Two-dimensional Vermiculite–Ionic Liquid Composite Membrane to Enhance Rapid Humidity Sensing

Abstract Recent advances in humidity sensors have emphasized their critical roles in various fields, from food processing to healthcare. Vermiculite (V), as a two-dimensional (2D) material, can be exploited in humidity sensors with numerous advantages such as low cost, thermal stability, and ease of functionalization for large-scale manufacturing. Here we demonstrated that the 2D characteristics of V, combined with ultrafast transport of confined water in its nanocapillaries, significantly enhance the rapid adsorption and desorption of water, thereby endowing the humidity sensor with rapid sensing capabilities. Furthermore, we employed the ionic liquid (IL), [EMIM][BF4] as an intercalating agent to modify V utilizing the electrostatic and hydrogen bonding interactions between them. The ultrafast transport of water in the V-[EMIM][BF4] membrane was not only improved, but the confined water in nanocapillaries was also transformed from a ''constrained'' to a comparatively "relaxed'' state. This "relaxed" water allowed it to rotate into suitable orientation for efficient proton transfer. Consequently, the V-[EMIM][BF4] membrane-based sensor exhibited the improved transient response of 5 s and 34 s in the range of 30%-80% relative humidity. This study leveraged the benefits of IL-modified V membranes to pave the way for cost-effective humidity sensing devices with rapid responses.