Deliquescent Chromism of Nickel(II) Iodide Thin Films

Color-changing materials have a variety of applications, ranging from smart windows to sensors. Here, we report deliquescent chromism of thin, color neutral films of nickel(II) iodide (NiI₂) that are less than 10 μm thick. This behavior does not occur in the bulk material. Dark brown thin films of crystalline NiI₂ turn clear when exposed to humidity and can be switched back to the dark state when mildly heated (>35 °C). This optical transition between dark and clear states of an NiI₂ thin film is reversible with thermal cycling.