Subwavelength VO₂ Nanoparticle Films for Smart Window Applications

Abstract

High-quality VO2 nanoparticle (NP) films were prepared by a facile self-template synthesis method, including sputtering of V films and postannealing, and were identified as a parabolic nanocone array, namely, the moth-eye nanostructure. Compared to a VO2 flat thin film with a refractive index of ∼3 at a 550 nm wavelength, the fabricated subwavelength VO2 NP films with the graded and lower effective refractive index (less than 2) exhibited ultrahigh luminous transmittance (T lum) due to the decreased reflectance differences at air/VO2 and VO2/glass interfaces. Meanwhile, subwavelength rutile VO2 NPs provided the localized surface plasmon resonance excited at ∼1250 nm wavelength to enhance the solar modulation (ΔT sol). Using simulations, the optimal NP size and porosity of the NP film were designed to obtain the optimum balance between ΔT sol and T lum, which were confirmed by experimental results. The single-sided VO2 NP film with an ultrahigh T lum of 93.3% exhibited a transparent vision with a ΔT sol of 6.1%, and the double-sided VO2 NP film provided an excellent ΔT sol of 14%, while the T lum remained at ∼70%. The outstanding performances of the VO2 NP films exceeded the reported results for porous films, multilayers, and nanostructures. Thus, we believe that the VO2 NP film is a promising nanostructure to lead VO2-based coatings to an era for smart window applications, including the building glazing and windshields of vehicles.

Integrated Thermoelectric Design Inspired by Ionic Liquid Microemulsion-Based Gel with Regulatable Dual-Temperature Responsiveness

“…Thermochromic gel materials, especially hydrogels, play a crucial role in the thermoelectric dual-responsive smart windows benefiting from their high luminous transmittance, broadly tunable physical properties, ideal compatibility with color-changing materials, etc. − According to the color-changing mechanisms, thermochromic hydrogels usually contain poly­( N -isopropylacrylamide) (PNIPAM), , VO 2 , , block polymer, or nonionic surfactant, which can switch mode from transparency to opaqueness at high temperature by changing their aggregation morphology, crystal structure, microphase separation, or intrinsic cloud point, respectively. In addition, the copolymer that grafted thermosensitive block can also be used to construct thermochromic ion gel .…”

Fully Discrete VO2 Particulate Film with Ultra‐High Transmittance and Excellent Thermochromic Performance

“…At the same time, it is essential to endow some specific applications with visually transparent features to make full use of the sunlight spectrum. [3] The visually transparent VO 2 is essential for various devices, such as traditional smart windows that block near-infrared light; [13][14][15][16] radiation coolers for smart windows [3,4] and solar cells; [4] and microwave absorbing windows for communications, medical, and aerospace equipment. [10] The common demands for those applications are that the ultra-high T lum across the SMT transition needs to be maintained and the difference in NIR transmittance (ΔT sol ), mid-infrared emissivity (Δ𝜖) and microwave absorption (Δ𝛼) are as large as possible.…”

“…[10] The common demands for those applications are that the ultra-high T lum across the SMT transition needs to be maintained and the difference in NIR transmittance (ΔT sol ), mid-infrared emissivity (Δ𝜖) and microwave absorption (Δ𝛼) are as large as possible. However, the strong absorption of VO 2 (M) and reflectance of VO 2 (R) [15,17] in the visible light region give rise to the contradictory relationship between T lum and thermochromic performance (ΔT sol , Δ𝜖, and Δ𝛼), [13,16,18] considering the fact that thermochromic performance is positively related to the VO 2 content while T lum is negatively related to the VO 2 content. [3,4,16] For instance, increasing the VO 2 film thickness in the smart windows generally improves ΔT sol , but T lum is decreased.…”

“…However, the strong absorption of VO 2 (M) and reflectance of VO 2 (R) [15,17] in the visible light region give rise to the contradictory relationship between T lum and thermochromic performance (ΔT sol , Δ𝜖, and Δ𝛼), [13,16,18] considering the fact that thermochromic performance is positively related to the VO 2 content while T lum is negatively related to the VO 2 content. [3,4,16] For instance, increasing the VO 2 film thickness in the smart windows generally improves ΔT sol , but T lum is decreased. This causes tremendous difficulties in developing highly transparent VO 2 films with excellent ΔT sol .…”

“…This causes tremendous difficulties in developing highly transparent VO 2 films with excellent ΔT sol . [16,17,19] In addition, the concept of visually transparent VO 2 -based smart windows with passive radiative cooling regulation has recently been proposed. [3] A very low T lum of 27.8% and excellent Δ𝜖 of 0.4 were obtained by a VO 2 film composed of dispersed VO 2 nanoparticles (VO 2 NPs) and poly(methyl methacrylate).…”