Hongjie Luo

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.

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 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.

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.

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.

Superhydrophobic silica aerogel microspheres from methyltrimethoxysilane: rapid synthesis via ambient pressure drying and excellent absorption properties

Silica aerogels are nanostructured porous solids with an open pore structure, high surface area, high porosity, low bulk density, and low thermal/electrical conductivity, which have drawn great interest in both science and technology. Generally, silica aerogels are prepared via a sol–gel process with a subsequent drying step, in which a supercritical condition is usually employed to dry wet silica gels. However, the supercritical drying is a limiting factor for synthesis efficiency and cost. Ambient pressure drying (APD) is considered an alternative and more practical approach. However, the tedious surface modification and solvent exchange steps involved in conventional APD for the synthesis of silica aerogels (bead- or bulk-type) are still challenges. In this study, a novel APD method was successfully developed for the drying of precursor silica microspheres that were obtained via a water-in-oil (W/O) emulsion method using methyltrimethoxysilane (MTMS). The resultant silica aerogel microspheres were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), and the Brunauer–Emmett–Teller (BET) method. The results indicate that silica aerogel microspheres possess similar characteristics to those aerogels prepared by supercritical drying. The silica aerogel microspheres have a mean diameter of approximately 300 μm, a bulk density as low as 0.08 g cm−3, a specific surface area as high as 853 m2 g−1, a pore size distribution between 2 and 45 nm, and an average pore size of 16 nm. In addition, the aerogels are superhydrophobic with a contact angle as high as 172°, and they exhibit excellent absorption capability and recyclability for organic liquids and oils. A new production route for silica aerogel microspheres will greatly expand the commercial utilization of these materials.

One-step preparation of nitrogen-doped and surface-passivated carbon quantum dots with high quantum yield and excellent optical properties

Nitrogen-doped carbon quantum dots (N-CQDs) are prepared via one-step hydrothermal processing using citric acid and linear-structured polyethyleneimine (LPEI) polymer as raw materials. The LPEI serves not only as a nitrogen source for N-doping but also as a surface-passivation agent for surface modification. The as-obtained N-CQDs show a mean size of 1.67 nm and a high fluorescence quantum yield (QY) of 37.4%. In addition, with the remaining LPEI on the surface, the N-CQDs also show good water-solubility and reducibility for noble metal ions to construct CQDs-containing composites. After a set of careful examinations of the as-obtained N-CQDs, the so-called up-conversion photoluminescence (UCPL) of CQDs under an excitation of a xenon lamp, which are observed in previous reports, is found to be a false appearance caused by the second-order diffraction light from the monochromators of the spectrofluorimeter.