Chuanxiang Cao

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.

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.

VO2 thermochromic smart window for energy savings and generation

The ability to achieve energy saving in architectures and optimal solar energy utilisation affects the sustainable development of the human race. Traditional smart windows and solar cells cannot be combined into one device for energy saving and electricity generation. A VO2 film can respond to the environmental temperature to intelligently regulate infrared transmittance while maintaining visible transparency, and can be applied as a thermochromic smart window. Herein, we report for the first time a novel VO2-based smart window that partially utilises light scattering to solar cells around the glass panel for electricity generation. This smart window combines energy-saving and generation in one device, and offers potential to intelligently regulate and utilise solar radiation in an efficient manner.

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.

Fine crystalline VO2 nanoparticles: synthesis, abnormal phase transition temperatures and excellent optical properties of a derived VO2 nanocomposite foil

A simulation of the optical properties of nanocomposite coatings derived from VO2 nanoparticles (NPs) shows that the nanocomposite coatings have advantages over pure VO2 thin films in their solar energy modification ability (ΔTsol) and luminous transmittance (Tlum). These nanocoatings rely on fine quality VO2 NPs; methods to prepare NPs for this purpose are yet to be developed. By studying the formation mechanism of VO2 NPs, the NP preparation process was optimized, and fine crystal quality VO2 NPs with diameters from 25–45 nm were synthesized. The highest latent heat of these VO2 NPs is 43 J g−1, which is considerably higher than the 25 J g−1 reported previously and close to the 51 J g−1 of bulk VO2, which indicates that these VO2 NPs are highly crystalline. These NPs showed an asymmetrical phase transition and increased insulator–metal transition (IMT) temperatures. According to our results, the size of particles is not the only reason that should be responsible for the increased IMT temperatures. The high-quality NPs were dispersed in polyurethane (PU) and coated on polyethylene terephthalate (PET). The relationship between the solar energy modification ability (ΔTsol) and the luminous transmittance (Tlum) was studied by experiments and simulation. Although the best experimental values of ΔTsol = 22.3% and Tlum = 45.6% are still lower than the simulation results of ΔTsol = 23.7% and Tlum = 32.4%, these values represent the best for reported VO2 smart films or coatings.

The synthesis and performance of Zr-doped and W–Zr-codoped VO2 nanoparticles and derived flexible foils

Vanadium dioxide (VO2) is a key material for thermochromic smart window applications because of its Mott phase transition properties. However, the applications of VO2 have been restricted because of its drawbacks in performance, including high phase transition temperature (Tc), low luminous transmittance (Tlum), limited solar energy modulation ability (ΔTsol) and unpleasant color. Various studies have been undertaken to resolve these problems but the improvement in any one aspect is always accompanied by a deterioration of the others. This paper reports that Zr doping can simultaneously decrease Tc, increase Tlum, improve ΔTsol and modify the color of VO2 foils. The Tc value decreased from 68.6 °C to 64.3 °C at 9.8% Zr-doping; meanwhile, the composite foils prepared from Zr-doped VO2 nanoparticles exhibited excellent luminous transmittance (up to 60.4%) and solar energy modulation ability (up to 14.1%). The experimental optical band gap was 1.59 eV for the undoped VO2, which increased to 1.89 eV at 9.8% doping. As a result, the color of the Zr-doped foils was modified to increase their luminous transmittance and lighten the yellowish color of the VO2 foil. The first-principles calculation was in good agreement with the experiment results. The W–Zr-codoped VO2 nanoparticles were prepared to further decrease the transition temperature (28.6 °C), while simultaneously maintaining the luminous transmittance (48.6%) and solar energy modulation ability (4.9%) of the derived foils.

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.

Highly crystalline spindle-shaped mesoporous anatase titania particles: solution-phase synthesis, characterization, and photocatalytic properties.

Spindlelike mesoporous anatase titania particles were directly synthesized at a low temperature (95 degrees C) by using an aqueous peroxotitanium solution with polyacrylamide (PAM). The mesoporous titania had a BET-specific surface area of 89.6 m(2) g(-1) and showed high crystallinity, thermal stability, and good photocatalytic activity in the degradation of rhodamine B. PAM, as an additive, was confirmed to be crucial in the evolution of the specific structure, morphology, and crystalline phase, and a possible formation mechanism was suggested.

Crystallised mesoporous TiO2(A)–VO2(M/R) nanocomposite films with self-cleaning and excellent thermochromic properties

VO2(M/R) (Monoclinic/Rutile) exhibits reversible insulator–metal transition (IMT) near room temperature, and is a widely used film material for thermochromic smart windows. Recent studies have shown that VO2-based nanocomposite coatings show better optical performance (luminous transmittance, Tlum and solar energy modification ability, ΔTsol) than pure VO2 thin films. In previous studies, we succeeded in preparing VO2-based organic–inorganic (O–I) composite coatings and optimising their optical performance to a level higher than ever reported previously. Compared to O–I composite coatings, inorganic–inorganic (I–I) composite coatings show advantages in weatherability and multifunctions (for example, self-cleaning TiO2–VO2 composites). In this work, we succeeded in dispersing VO2 nanoparticles (NPs) in TiO2 sol by surface modification, and prepared high quality I–I nanocomposite films with comparable optical performance to that of the best pure VO2 thin film. With the help of thermal analysis, a two-step annealing process was developed, and crystallised TiO2(A)(anatase)–VO2(M/R) composite films were obtained. By introducing mesopores, the optical performance was improved to Tlum = 62.0% (20 °C), 60.5% (90 °C) and ΔTsol = 14.6%, which was better than that of any reported VO2-based pure films. In addition, due to the crystallised TiO2(A), these composite films showed self-cleaning properties (a low contact angle and photocatalytic decomposition of organic contaminants).

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.