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

Abstract

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. VO2 is 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 VO2 to 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 x V1–x O2 nanocrystals encapsulated within SiO2 shells 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 VO2 nanocrystals alloyed with 2.3 at.% tungsten are cast onto glass and demonstrate a ΔT NIR of 12.8% and a ΔT Sol of 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 VO2 nanocrystals to their encapsulation within a SiO2 matrix 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 x V1–x O2 with 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.

Thermochromic Fenestration Elements Based on the Dispersion of Functionalized VO2 Nanocrystals within a Polyvinyl Butyral Laminate

“…The distinctive exothermic features correspond to variations in defect density within the ultrasmall VO 2 nanocrystals. 16,22,25,45 Ultrasmall dimensions of active fillers, below the wavelength of visible light, are a necessary but not sufficient criterion for mitigating light scattering in thermochromic fenestration elements. 10,25 Agglomerated ultrasmall particles can scatter light much the same way as larger particles, and thus, a key imperative is to ensure the effective dispersion of the particles within laminate matrices.…”

“…Encapsulation of VO 2 Nanocrystals with a SiO 2 Shell and Surface Functionalization of VO 2 @SiO 2 Core�Shell Nanoparticles VO 2 nanocrystals with dimensions of 26 ± 8 nm, crystallized in the monoclinic phase, were prepared by Dimien, Inc., using a hydrothermal process. 41 In a typical method to constitute SiO 2 shells, 0.0726 g of VO 2 nanocrystals were dispersed in 128 mL of ethanol and 32 mL of DI water (ρ = 18.2 MΩ•cm, Barnstead International NANOpure Diamond system) by ultrasonication (Branson 5510) for 1 h. Ammonium hydroxide (1600 μL) and tetraethyl orthosilicate (300 μL) were added to the dispersion and allowed to stir at 25 °C for 24 h. 10,16 On the basis of our previous work, the selected concentration of tetraethyl orthosilicate and reaction time of 24 h yields a shell thickness of 19 ± 1 nm (vide infra). 16 Residual unreacted tetraethyl orthosilicate was removed from solution by centrifugation (8500 rpm, 10 min) (Heraeus Megafuge 8), followed by three washes with ethanol and one with acetone.…”

“…41 In a typical method to constitute SiO 2 shells, 0.0726 g of VO 2 nanocrystals were dispersed in 128 mL of ethanol and 32 mL of DI water (ρ = 18.2 MΩ•cm, Barnstead International NANOpure Diamond system) by ultrasonication (Branson 5510) for 1 h. Ammonium hydroxide (1600 μL) and tetraethyl orthosilicate (300 μL) were added to the dispersion and allowed to stir at 25 °C for 24 h. 10,16 On the basis of our previous work, the selected concentration of tetraethyl orthosilicate and reaction time of 24 h yields a shell thickness of 19 ± 1 nm (vide infra). 16 Residual unreacted tetraethyl orthosilicate was removed from solution by centrifugation (8500 rpm, 10 min) (Heraeus Megafuge 8), followed by three washes with ethanol and one with acetone. The encapsulated nanocrystals were then dried under a flow of nitrogen on a Schlenk line at room temperature prior to functionalization with 3methacryloxypropyltrimethoxysilane.…”

“…25 Utilizing the thermally triggered metallization of VO 2 nanocrystals in thermochromic fenestration elements requires not just control of particle composition and crystallinity (which govern the onset transition temperature, hysteresis, and sharpness) but also the particle dimensions, dispersion, interphase with the polymer matrix, and particle loading to alleviate common degradation mechanisms and mitigate light scattering. 7,8,10,16,18,25 Selection of the polymer matrix and functionalization of active fillers are both of critical importance to the design of high-optical-quality thermochromic fenestration elements. Films comprising large particles or agglomerations of smaller particles are prone to Mie scattering, which is a primary origin of optical haze.…”

“…10,27,29,31,38,39 The SiO 2 shell further imbues protection from sintering and provides a handle for chemical functionalization based on the tethering of organofunctional silanes. 10,16,40 An important imperative is thus to design appropriate functionalization schemes to enable the dispersion and embedding of VO 2 @SiO 2 nanocrystals in PVB. We report, here, the use of a coupling agent that is tethered to the SiO 2 shells encapsulating VO 2 nanocrystals and forms siloxane linkages with the PVB matrix.…”