Near-Ambient Nanocomposite Thermochromic Fenestration Elements from Post-Encapsulation-Annealed Tungsten-Alloyed Vanadium(IV) Oxide Nanocrystals

Heating, cooling, and lighting buildings consume an inordinate amount of energy, contributing greatly to the operating costs and carbon footprint of the built environment. The development of a thermochromic material capable of passively modulating the near-infrared (NIR) transmittance of fenestration elements, and thus the overall solar heat gain, has garnered intense interest owing to its potential to increase the energy efficiency of buildings. VO₂is a promising thermochromic material as a result of its characteristic metal-insulator transition (MIT), which engenders a discontinuous modulation of infrared transparency. Given its high thermodynamic transition, 67 °C, much effort has focused on decreasing the MIT of VO₂to near-ambient temperatures. However, dopant incorporation typically degrades crystallinity, which is reflected in a substantial decrease of NIR modulation. In this work, we demonstrate that the postsynthetic annealing of ultrasmall W_xV_1-xO₂nanocrystals encapsulated within SiO₂shells enables substantial improvements in crystallinity without sintering of the nanocrystals. The dispersion of SiO₂-encapsulated W-alloyed nanocrystals within a methacrylic acid/ethyl acrylate copolymer yields a smooth gradation of refractive indices. The nanocomposite films comprising VO₂nanocrystals alloyed with 2.3 at.% tungsten are cast onto glass and demonstrate a ΔT_NIRof 12.8% and a ΔT_Solof 10.6%, while maintaining a high degree of visible-light transmission (ca. 77% at 555 nm) and minimal modulation in the visible region (ΔT_Lum) at an operational temperature of 35 °C. The processing workflow from alloying of VO₂nanocrystals to their encapsulation within a SiO₂matrix for protected annealing and dispersion within a polymer further represents an entirely aqueous manufacturing route to thermochromic fenestration elements. More broadly, our results demonstrate the ability to access solid solutions of W_xV_1-xO₂with ultrasmall nanocrystalline dimensions while selectively tailoring the properties of the material for specific climates to achieve the desired combination of high visible-light transparency and NIR modulation.