Thermochromic behavior of chloro-nickel(II) in deep eutectic solvents and their application in thermochromic composite films

Abstract

Novel non-aqueous thermochromic solutions were facilely prepared by dissolving transition metal chlorides, such as NiCl₂·6H₂O, CrCl₃·6H₂O, FeCl₃, and CoCl₂·6H₂O, in an inexpensive, widely available ionic liquid analogous deep eutectic solvent (DES). It was found that only the NiCl₂·6H₂O dissolved choline chloride-based DES exhibited a stable and significant thermochromic behavior over a wide working temperature ranging from room temperature to about 150 °C. Typically, the colour of the 0.1 M NiCl₂·6H₂O dissolved DES solution changed in this order from pale green (room temperature), spring green (∼70 °C), to blue (∼120 °C). An effective strategy was proposed to produce more thermochromic solutions by mixing Ni(II) with other metal ions in the choline chloride-based DES. The thermochromic PVDF composite film was demonstrated by incorporating DES-based Ni(II) solution into the microporous PVDF film. The preliminary results are expected to stimulate interest of the development of high performance thermochromic materials for the facile fabrications from DESs.