Thermochromic composite film of VO2 nanoparticles and [(C2H5)2NH2]2NiBr4@SiO2 nanospheres for smart window applications

Vanadium dioxide (VO2) has attracted consideration because of its thermochromic performance in smart windows. However, the practical applications of VO2-based smart windows are seriously hindered by their low luminous transmittance (Tlum), poor solar modulation efficiency (ΔTsol), and the monotonous "brown-yellowish" color. This study aims at developing a newly reported metal complex [(C2H5)2NH2]2NiBr4 and its core–shell structure of [(C2H5)2NH2]2NiBr4@SiO2, combined with the thermochromic VO2 to enhance optical performance (Tlum and ΔTsol). Density Functional Theory (DFT) calculations indicate that the as-obtained metal complex is thermodynamically stable, and the weaker NH…Br hydrogen bond makes the complex possesses a lower phase transition temperature (TC = 57.6 °C) than the reported [(C2H5)2NH2]2NiCl4 (TC = 75.4 °C). In addition, it is found that the SiO2 shell can effectively inhibit the deliquescence of the [(C2H5)2NH2]2NiBr4 complex, and DFT calculations reveal that the oxygen of SiO2 can bond with the hydrogen of ammonium, thus the water molecules in the air cannot continue to react with the complex. Interestingly, the combination of [(C2H5)2NH2]2NiBr4@SiO2/Polystyrene film with VO2/Polyvinylbutyral film demonstrates exemplary solar modulation abilities (Tlum,low = 52.9 %, Tlum,high = 37.3 %, ΔTsol = 25.7 %, Haze = 26.5 %), 2.1 times better than that of VO2/Polyvinylbutyral film (Tlum,low = 55.9 %, Tlum,high = 54.2 %, ΔTsol = 12.4 %, Haze = 17.2 %). Moreover, the color of VO2-based film changes from light-brown (at low temperature) to green (at high temperature).