Minoru Kanehira

Enhanced chemical stability of VO2 nanoparticles by the formation of SiO2/VO2 core/shell structures and the application to transparent and flexible VO2-based composite foils with excellent thermochromic properties for solar heat control

Vanadium dioxide is a key material for thermochromic smart windows that can respond to environmental temperature and modulate near infrared irradiation by changing from a transparent state at low temperature to a more reflective state at high temperature, while maintaining visible transmittance. VO2 thermochromism is commonly used in films on glass that function as smart windows. Flexible VO2 nanocomposite foils are able to combine the intrinsic properties of VO2 nanoparticles with the added functionalities contributed by nanoscale and interface effects, such as increased visible transparency and infrared modulation ability. These foils are promising for applications in construction and automotive glasses to increase energy efficiency. However, VO2 nanoparticles may be unstable, and they are difficult to prepare in stable dispersive suspensions. In this paper, we report a novel all-solution process that can be used to prepare transparent, stable and flexible VO2-based composite films. These films exhibit UV-shielding properties and an excellent temperature-responsive thermochromism in the near infrared region. A typical film has a solar modulation efficiency of 13.6%, which is the highest value for VO2 thermochromic films with comparable visible transmittance. Coating the VO2 nanoparticles with a thin SiO2 shell significantly improved their anti-oxidation and anti-acid abilities. This result represents an important breakthrough in VO2 thermochromism, and it may have applications for near infrared modulation of glass used in construction or cars.

Phase and shape controlled VO2 nanostructures by antimony doping

Quasi-spherical VO2 nanoparticles with uniform size and high crystallinity are ideal functional materials for applications in field-effect transistors, smart window coatings and switches. However, the synthesis of these VO2 nanoparticles has long been a challenge. This article presents a novel doping strategy for the simultaneous control of the size, morphology and polymorphology of VO2 nanoparticles. Doping can induce the change in crystal structure and exhibits a significant promoting effect on the formation of doped monoclinic VO2 (VO2 (M)). Specifically, by antimony (Sb3+) doping, hexagonal-shaped, well crystalline monoclinic VO2 nanoparticles with tunable sizes (8–30 nm) and controllable polymorphs were synthesized via a one-pot, hydrothermal method. Sb3+ dopants, which are larger in radius and lower in valence than V4+ ions, can introduce extra oxygen vacancies during the nucleation and growth of VO2 nanoparticles. These positively charged nuclei may suppress the adsorption of VO2+ aqua ions, and therefore inhibit the growth of the VO2 (M) nanoparticles. Comparably, Sb5+ dopants that possess higher valence counts than V4+ ions can induce the growth of VO2 (M) particles to 200–300 nm width and above 500 nm length. The Sb3+-doped VO2 (M) nanoparticles exhibit excellent properties in metal–semiconductor transformation at transition temperatures ranging from 55–68 °C. Films obtained by casting these nanoparticles show excellent optical properties (both visible transmittance and infrared regulation), compared with those prepared from gas phases, such as sputtering. This synthetic strategy that involves the doping of an element with a different valence count than the matrix cation may be useful for controlling the solution growth of some technologically significant nanomaterials. In addition, the formation mechanism of solid and crystalline transformation was also studied by designing a specific reaction autoclave.

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.

Core-shell VO2@TiO2 nanorods that combine thermochromic and photocatalytic properties for application as energy-saving smart coatings

Vanadium dioxide (VO2) is a Mott phase transition compound that can be applied as a thermochromic smart material for energy saving and comfort, and titanium dioxide (TiO2) is a well-known photocatalyst for self-cleaning coatings. In this paper, we report a VO2@TiO2 core-shell structure, in which the VO2 nanorod core exhibits a remarkable modulation ability for solar infrared light, and the TiO2 anatase shell exhibits significant photocatalytic degradation of organic dye. In addition, the TiO2 overcoating not only increased the luminous transmittance of VO2 based on an antireflection effect, but also modified the intrinsic colour of VO2 films from yellow to light blue. The TiO2 also enhanced the chemical stability of VO2 against oxidation. This is the first report of such a single nanoparticle structure with both thermochromic and photocatalytic properties that offer significant potential for creating a multifunctional smart coating.

VO2 (A) nanostructures with controllable feature sizes and giant aspect ratios: one-step hydrothermal synthesis and lithium-ion battery performance

In this paper, uniform 1D VO2 (A) nanostructures with controllable morphologies were prepared via an easy one-step hydrothermal method for the first time. Aspect ratios are tunable from 10:1 to 1000:1 by changing the synthesis parameters such as the concentration of vanadium source and the reaction time. The mechanism of morphology evolution was investigated and discussed based on the nucleation and growth process of VO2 (A) particles. Electrochemical analyses of VO2 (A) nanostructures were performed, and the results showed a capacity of 277 mAh g−1 for a Li-ion battery using these nanostructures as cathode materials. This shows a significant improvement compared with other vanadium oxides such as VO2 (B) (approximately 180 mAh g−1) and V2O5 (approximately 230 mAh g−1). Unfortunately, the VO2 (A) nanostructures exhibit high initial irreversible loss due to the formation of an intermediate layer after electrochemical reactions.