Bio-inspired TiO2 nano-cone antireflection layer for the optical performance improvement of VO2 thermochromic smart windows

Abstract

Vanadium dioxide (VO 2) is a promising material for thermochromic glazing. However, VO 2 thermochromic smart windows suffer from several problems that prevent commercialization: low luminous transmittance (T lum) and low solar modulation ability (ΔT sol). The solution to these problems can be sought from nature where the evolution of various species has enabled them to survive. Investigations into the morphology of moths eyes has shown that their unique nanostructures provide an excellent antireflection optical layer that helps moths sharply capture the light in each wavelength from a wide angle. Inspired by this mechanism, a VO 2 thermochromic smart window coated with a TiO 2 antireflection layer with a novel nano-cone structure, is presented in this study to achieve high T lum and ΔT sol. Optimization for the key structure parameters is summarized based on the FDTD numerical simulations. The optimized structure exhibits a T lum of 55.4% with ΔT sol of 11.3%, an improvement of about 39% and 72% respectively compared to the VO 2 window without an antireflection layer. Furthermore, wide-angle antireflection and polarization independence are also demonstrated by this nano-cone coating. This work provides an alternative method to enhance the optical performance of VO 2 smart windows. Energy and environmental problems have become critical issues in modern society. It has been proven that buildings consume 20-40% of the total energy used. Most importantly, 60% of that energy is consumed to maintain thermal comfort by heating, ventilation and air conditioning (HVAC) systems 1. The huge consumption of energy by HVAC systems is mainly due to heat loss through the building envelope such as the roof, walls and windows. Among these building envelopes, approximately 50% of the energy is lost through windows, so more attention has been focused on energy efficient windows 2. Thermochromic smart windows are widely investigated because of their low cost and passive controllability of solar irradiance 3. Vanadium dioxide (VO 2) is one of the thermochromic materials to modulate transmittance of solar radiation because of its internal reversible phase change from metal (hot state) to insulator state (cold state) at a critical transition temperature of 68 °C 4. The phase change gives rise to an abrupt change of the near-infrared (NIR) transmittance, which renders VO 2 a promising candidate for thermochromic smart windows. To fulfill the demand of human vision and energy efficiency, improving luminous transmittance (T lum) and solar modulation ability (i.e. the difference of solar transmittance (T sol) between cold and hot states of materials or ΔT sol) simultaneously are key for the practical applications of VO 2 smart windows in buildings 5. Many efforts have been made to improve T lum and ΔT sol of thermochromic smart windows, such as chemical doping 6,7 , synthesizing VO 2 nanoparticles 8-11 and morphology modification 12-15. Among the different methods, depositing a planar antireflection layer is a commonly used solution to achi...

Self-Densified Optically Transparent VO2 Thermochromic Wood Film for Smart Windows

“…The transmittance spectra of VO 2 -TTWF under the cold and hot states are measured by a SHIMADZU UV3600 UV–vis–NIR spectrophotometer. There are two vital indices to quantify the optical performance of VO 2 -TTWF, namely, the luminous transmittance ( T lum ) and solar modulation ability ( ΔT sol ). , T lum is the amount of visible light transmitted by the VO 2 -TTWF that is useful for human vision under normal conditions; it is defined as where T (λ) is the transmittance of the VO 2 -TTWF at wavelength λ. y̅ (λ) is the photopic luminous efficiency of the human eye defined by the CIE (International Commission on Illumination) standard. The wavelength range of 380–780 nm corresponds to the limits of human vision.…”

“…They can replace conventional windows in buildings, vehicles, aircraft, and ships to save energy. Among different thermochromic materials, vanadium dioxide (VO 2 ) is widely investigated as it can modulate the transmittance of solar radiation because of its internal reversible phase change from a metal (hot state) to an insulator (cold state) at a critical-transition temperature of 68 °C. , The phase change gives rise to an abrupt change in the NIR transmittance so that VO 2 shows high transmittance in the NIR region at the cold state and becomes opaque to NIR light at the hot state. The solar energy in the NIR region accounts for around 49%, so the NIR regulation ability of VO 2 enables the window to save a lot of energy.…”

Element doping: a marvelous strategy for pioneering the smart applications of VO2

“…20a. 187 This structure is the ideal antireflective structure, which is equivalent to a dielectric layer with a continuously varying refractive index. FDTD simulation results show that the shape and distribution of VO 2 nano-cones have a significant impact on optical properties; films with a periodicity of 130 nm and aspect ratios between 1.9 and 8.8 exhibit the solar energy transmittance of 23.1% while maintaining T vis = 70.3% (Fig.…”

Chromogenic Technologies for Energy Saving

“…Several works have used SiO 2 , TiO 2 , and ZrO 2 antireflective layers [37]. A more sophisticated approach was proposed by Liu et al [50], who reported the design of nanocone antireflective layers made of TiO 2 , biomimicking moth-eye morphology and reaching a T lum of 55.4%, with ∆T sol of 11.3%. Moreover, the stability of VO 2 coatings is limited by chemical and physical deterioration issues.…”