Litao Kang

Thermochromic VO2 Thin Films: Solution-Based Processing, Improved Optical Properties, and Lowered Phase Transformation Temperature

This paper describes a solution-phase synthesis of high-quality vanadium dioxide thermochromic thin films. The films obtained showed excellent visible transparency and a large change in transmittance at near-infrared (NIR) wavelengths before and after the metal-insulator phase transition (MIPT). For a 59 nm thick single-layer VO(2) thin film, the integral values of visible transmittance (T(int)) for metallic (M) and semiconductive (S) states were 54.1% and 49.1%, respectively, while the NIR switching efficiencies (DeltaT) were as high as 50% at 2000 nm. Thinner films can provide much higher transmittance of visible light, but they suffer from an attenuation of the switching efficiency in the near-infrared region. By varying the film thickness, ultrahigh T(int) values of 75.2% and 75.7% for the M and S states, respectively, were obtained, while the DeltaT at 2000 nm remained high. These results represent the best data for VO(2) to date. Thicker films in an optimized range can give enhanced NIR switching efficiencies and excellent NIR blocking abilities; in a particularly impressive experiment, one film provided near-zero NIR transmittance in the switched state. The thickness-dependent performance suggests that VO(2) will be of great use in the objective-specific applications. The reflectance and emissivity at the wavelength range of 2.5-25 microm before and after the MIPT were dependent on the film thickness; large contrasts were observed for relatively thick films. This work also showed that the MIPT temperature can be reduced simply by selecting the annealing temperature that induces local nonstoichiometry; a MIPT temperature as low as 42.7 degrees C was obtained by annealing the film at 440 degrees C. These properties (the high visible transmittance, the large change in infrared transmittance, and the near room-temperature MIPT) suggest that the current method is a landmark in the development of this interesting material toward applications in energy-saving smart windows.

Nanoporous Thermochromic VO2 Films with Low Optical Constants, Enhanced Luminous Transmittance and Thermochromic Properties

Nanoporous thermochromic VO(2) films with low optical constants and tunable thicknesses have been prepared by polymer-assisted deposition. The film porosity and thickness change the interference relationship of light reflected from the film-substrate and the air-film interfaces, strongly influencing the optical properties of these VO(2) films. Our optimized single-layered VO(2) films exhibit high integrated luminous transmittance (T(lum,l) = 43.3%, T(lum,h) = 39.9%) and solar modulation (ΔT(sol) = 14.1%, from T(sol,l) = 42.9% to T(sol,h) = 28.8%), which are comparable to those of five-layered TiO(2)/VO(2)/TiO(2)/VO(2)/TiO(2) films (T(lum,l) = 45%, T(lum,h) = 42% and ΔT(sol) = 12%, from T(sol,l) = 52% to T(sol,h) = 40%, from Phys. Status Solidi A2009, 206, 2155-2160.). Optical calculations suggest that the performance could be further improved by increasing the porosity.

VO2–Sb:SnO2 composite thermochromic smart glass foil

VO2/ATO/polymer thermochromic flexible foils for smart window application were prepared by casting VO2 and Sbxa0:xa0SnO2 (ATO) nanoparticles. The foils exhibit an increased solar-heat shielding ability, causing a 20% decrease in solar transmittance at an ATO content of 9%, while retaining an excellent solar modulation ability (6.9%). This development represents a breakthrough for applications to smart windows.

Solution-based fabrication of vanadium dioxide on F:SnO2 substrates with largely enhanced thermochromism and low-emissivity for energy-saving applications

Vanadium dioxide is a key material for thermochromic smart windows that can respond to environmental temperatures to modulate near infrared irradiation from a transparent state at low-temperature to an opaque state at high-temperature while maintaining the visible transmittance. This paper reports a novel VO2/FTO/glass multi-layered structure, which shows promising optical properties for application to energy-efficient smart windows. VO2 thin films are deposited on F-doped SnO2 (FTO) glasses by annealing a precursor film that is obtained via a solution-based process. The rutile-structured FTO substrate enhances the crystallinity of the VO2 films and lowers the synthesis temperature to ∼390 °C. The VO2/FTO/substrate double-layered films show both improved low-emissivity performance and distinct thermochromic properties. For a 65 nm thick VO2/FTO substrate double-layered film, low emissivities of 0.19 and 0.27 before and after the metal-insulator phase transition (MIPT) are obtained, while a solar transmittance modulation efficiency (η, in the wavelength range of 280–2600 nm) of 4.9% is achieved. A TiO2 anti-reflective coating (ARC) is incorporated to form a three-layered TiO2/VO2/FTO/substrate structure to boost the integrated visible transmittance (Tvis) while maintaining the low-emissivity performance. A 29.4% improvement for Tvis from 34.0% to 44.0% at room temperature is achieved for a 55 nm thick VO2 film coated with a TiO2 layer while emissivities of 0.13 and 0.24 before and after MIPT are maintained. Moreover, η is also increased significantly, from 4.3% for the VO2/FTO/substrate structure to 8.8% for the TiO2/VO2/FTO/substrate structure. Our results demonstrate a new approach of combining both thermochromism and low-emissivity performance for applications such as VO2-based energy-saving windows.

Self-assembly and synthesis mechanism of vanadium dioxide hollow microspheres

Rutile vanadium dioxide (VO2 (R)) hollow spheres with nanorods aggregating on their surface were successfully synthesized through a novel surfactant-assisted hydrothermal process in which polyvinylpyrrolidone (PVP) served as a soft template. The crystal structure and morphology were characterized by X-ray diffraction, scanning electron microscopy and transmission electron microscopy. Hollow spheres show an obvious Mott phase transition at 63.0 (heating semicycle) and 53.1 °C (cooling semicycle). Reaction conditions influencing the synthesis of these VO2 hollow spherical structures (i.e., reaction temperature, time and PVP addition amount) were investigated. Furthermore, a mechanism for the growth of the VO2 hollow spheres is proposed.