Elucidating the Crystallite Size Dependence of the Thermochromic Properties of Nanocomposite VO₂ Thin Films

Abstract

Fenestration elements that enable spectrally selective dynamic modulation of the near-infrared region of the electromagnetic spectrum are of great interest as a means of decreasing the energy consumption of buildings by adjusting solar heat gain in response to external temperature. The binary vanadium oxide VO 2 exhibits a near-room-temperature insulator–metal electronic transition accompanied by a dramatic modulation of the near-infrared transmittance. The low-temperature insulating phase is infrared transparent but blocks infrared transmission upon metallization. There is considerable interest in harnessing the thermochromic modulation afforded by VO 2 in nanocomposite thin films. However, to prepare a viable thermochromic film, the visible-light transmittance must be maintained as high as possible while maximizing thermochromic modulation in the near-infrared region of the electromagnetic spectrum, which necessitates the development of high-crystalline-quality VO 2 nanocrystals of the optimal particle size embedded within the appropriate host matrix and refractive index matched to the host medium. Here, we demonstrate the preparation of acrylate-based nanocomposite thin films with varying sizes of embedded VO 2 nanoparticles. The observed strong size dependence of visible-light transmittance and near-infrared modulation is explicable on the basis of optical simulations. In this article, we elucidate multiple scattering and absorption mechanisms, including Mie scattering, temperature-/phase-variant refractive-index mismatch between VO 2 nanocrystals and the encapsulating matrix, and the appearance of a surface plasmon resonance using temperature-variant absorptance and diffuse transmittance spectroscopy measurements performed as a function of particle loading for the different sizes of VO 2 nanocrystals. Nanocrystals with dimensions of 44 ± 30 nm show up to >32% near-infrared energy modulation across the near-infrared region of the electromagnetic spectrum while maintaining high visible-light transmission. The results presented here, providing mechanistic elucidation of the size dependence of the different scattering mechanisms, underscore the importance of nanocrystallite dimensions, refractive-index matching, and individualized dispersion of particles within the host matrix for the preparation of viable thermochromic thin films mitigating Mie scattering and differential refractive-index scattering.

Solution-Processed VO2 Nanoparticle/Polymer Composite Films for Thermochromic Applications

“…Agglomeration of NPs increases the absorption and scattering events, which in turn deteriorates the visible transmission of VO 2 films. 61 This lower IR switching ratio of our pure VO 2 films and other than optimum ratio of VO 2 / polymer composite films is believed to be related to NP aggregation and non-uniformity in films. With optimal ratio, VO 2 /PVA films had better IR switching and visible luminous transmission than VO 2 /PVP films.…”

“…This might need further investigation but is not within the scope of this article. Agglomeration of NPs increases the absorption and scattering events, which in turn deteriorates the visible transmission of VO 2 films . This lower IR switching ratio of our pure VO 2 films and other than optimum ratio of VO 2 /polymer composite films is believed to be related to NP aggregation and non-uniformity in films.…”

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

“…While this work focuses on achieving the selective modulation of NIR transmittance, additional thermochromic elements such as indenoquinacridone can be incorporated in order to obtain full-spectrum modulation. , In comparison to their electrochromic counterparts, thermochromic foils are not dependent on the electrical infrastructure of the building, and are thus particularly well suited for retrofitting applications. , Despite substantial progress over the last decade, a number of practical impediments have stymied the widespread adoption of VO 2 nanocomposite fenestration elements. − ,− As a result of the lattice changes accompanying underlying monoclinic–tetragonal structural phase transformations, substantial strain can be accumulated within VO 2 thin films. To address this challenge, ultrasmall nanocrystals of VO 2 have been embedded within flexible polymer nanocomposite thin films; such nanocomposites can accommodate strain-induced stresses without fracture or delamination . Ultrasmall nanocrystals further afford substantial benefits in terms of optical quality by mitigating particle-size-dependent Mie scattering mechanisms that contribute to optical haze. , In addition to the decrease in Mie scattering, the formation of a localized-surface plasmon resonance (LSPR) has been demonstrated to occur in particles below 100 nm in size, which affords an additional absorption mechanism that can contribute to the modulation of solar heat gain. ,− Ongoing challenges include the incorporation of dopants within VO 2 to enable near-ambient operation (bulk VO 2 has a transition temperature of ca.…”

“…To address this challenge, ultrasmall nanocrystals of VO 2 have been embedded within flexible polymer nanocomposite thin films; such nanocomposites can accommodate strain-induced stresses without fracture or delamination . Ultrasmall nanocrystals further afford substantial benefits in terms of optical quality by mitigating particle-size-dependent Mie scattering mechanisms that contribute to optical haze. , In addition to the decrease in Mie scattering, the formation of a localized-surface plasmon resonance (LSPR) has been demonstrated to occur in particles below 100 nm in size, which affords an additional absorption mechanism that can contribute to the modulation of solar heat gain. ,− Ongoing challenges include the incorporation of dopants within VO 2 to enable near-ambient operation (bulk VO 2 has a transition temperature of ca. 67 °C, which is too high to be of practical value), effective dispersion of embedded nanocrystals within the polymer matrix to prevent light scattering from clumped agglomerates, refractive-index matching of the active elements with the continuous polymeric matrix to avoid optical haze, and protection of the vanadium­(IV) oxide core from oxidation to V 2 O 5 (a thermodynamically favored reaction under ambient conditions). ,, Recent work by Zhou et al has highlighted the importance of protecting VO 2 active thermochromic elements from oxidation; these authors have emphasized the need for durability testing and have delineated empirical expressions based on the development of accelerated testing protocols .…”

Understanding the Semi-Switchable Thermochromic Behavior of Mixed Halide Hybrid Perovskite Nanorods

“…Therefore, we find that a study on the impact of the TC behavior of 1D perovskites on their glazing performance is still lacking and there is a need for analyzing their thermal performance and energy efficiency when subjected to varying climate conditions. Among TC coatings for windows, vanadium dioxide (VO 2 ) becomes the leading candidate due to its first-order phase transition from a monoclinic semiconducting state to a tetragonal metallic state at a transition temperature ( T t ) of 68 °C. , Besides, VO 2 can intelligently adjust the near-infrared (NIR) radiation according to the environmental temperature. − Moreover, as an organic hydrogel, cross-linked poly­( N -isopropyl acrylamide) or PNIPAm, a typical temperature-responsive hydrogel, combined with graphene oxide was employed for switchable glazing and adaptive solar control . However, despite the growing interest and potential advantages, state-of-the-art TCs suffer from limitations, such as narrow chromatic variation, high operating temperature, and UV-light instability, hindering their comprehensive utilization.…”