The article describes a new concept of an optical hybrid thin-film material that exhibits reversible humidity-driven light transmittance (scattering) properties. The film consists of a dispersive porous structure, with embedded hygroscopic and deliquescent compounds, that is able to scavenge water molecules from humid air to fill up the pores and become transparent to the incident light. Upon exposure to dry air, water is released from the structure and the material recovers its original light scattering properties. The developed thin films can change their transparency when exposed to air with different relative humidity (RH), adjusting the light throughput. This material can, therefore, be used to design new optical switching systems, having the advantage that they do not require liquid crystal, transparent conductive glass substrates, or complex layer-by-layer architectures for operation. In this work, a thin film was prepared by the sol–gel technique using calcium chloride as deliquescent substance embedded in a hybrid silica–titania matrix. At dry air, the thin films show a highly dispersive state, with a light transmittance T = 0.06, while at 51% RH the film exhibits a transparent state with a transmittance T =0.67 (measured at 633 nm).
Adv. Func. Mater. 2018
Conceptual working principle of a humidity-driven light transmittance switching material. a) Scheme of the dry/scattering and humid/transmitting states. b) Scheme of the dry and wet pores, representing the different refractive indices n in dry air, and the n matching between matrix and pores in humid air.