Humidity responsive single-layered film fabricated by hydrophilic titanium oxide nanotubes

Abstract

We fabricated a humidity responsive film comprising a bundle of titanium oxide tubes, which changes its curvature corresponding to the relative humidity. The mechanism of changing the curvature of the film can be explained by adsorption, condensation, and evaporation of water molecules within the film. During adsorption, a liquid bridge is formed because of the growth of a water layer between tubes, thus contracting the tubes. When vapor pressure exceeds the equilibrium vapor pressure, condensation occurs at the meniscus of the liquid bridge, thus expanding the gap between tubes. We obtained the adsorption and desorption isotherms for the humidity responsive film by determining physical adsorption. Additionally, we demonstrated the varying motions of the humidity sensing film when a water droplet was applied on the surface. The sensor could distinguish between saturated and oversaturated humidity conditions, such as fog and rain, respectively, which is not possible using resistive humidity sensors. This humidity responsive film can be applied to environmental monitoring systems and possibly even to energy harvesting systems.