Subwavelength VO₂ Nanoparticle Films for Smart Window Applications

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 (Tlum) 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 (ΔTsol). Using simulations, the optimal NP size and porosity of the NP film were designed to obtain the optimum balance between ΔTsol and Tlum, which were confirmed by experimental results. The single-sided VO2 NP film with an ultrahigh Tlum of 93.3% exhibited a transparent vision with a ΔTsol of 6.1%, and the double-sided VO2 NP film provided an excellent ΔTsol of 14%, while the Tlum 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.