Sputtering Flexible VO2 Films for Effective Thermal Modulation.

Flexible vanadium dioxide (VO2) thermochromic films show great potential for large-scale fabrication and possess broader applications compared with VO2 coatings on rigid substrates. However, the fabrication of flexible VO2 films remains a challenge so far, leading to the scarcity of research on flexible VO2 films for smart windows. With the aim to obtain a flexible VO2-based films with excellent optical properties and a long service life, we designed and successfully fabricated a flexible ITO/VO2/ITO (IVI) film on the colorless transparent polyimide substrate, which could be directly attached to glasses for indoor temperature modulation. This flexible IVI film effectively enhances the luminous transmittance (Tlum) and solar modulation ability (ΔTsol) (15 and 68% increase relative to a VO2 single layer), reduces the thermal emissivity (εT) (50.7% decrease relative to a VO2 single layer), and exhibits better durability than previously reported structures. Such excellent comprehensive performance offers it great potential in practical applications on smart windows. This work is supposed to provide a new strategy for facile direct fabrication of flexible VO2 films and broaden the applications of flexible VO2 in more coatings and devices.

Tuning Phase Transition and Thermochromic Properties of Vanadium Dioxide Thin Films via Cobalt Doping.

Vanadium dioxide (VO2) featuring a distinct thermally triggered phase transition is regarded as the most attractive thermochromic material for smart window applications. However, the high transition temperature (∼67 °C) and moderate luminous transmittance (<50%) of the pristine VO2 circumvent room temperature applications. In this work, epitaxial cobalt-doped VO2 thin films were fabricated to tailor the electric and optical properties on a c-plane sapphire substrate. At the highest doping concentration of 10%, the transition temperature of VO2 is reduced to 44 °C, accompanied by a high luminous transmittance of 79% for single-element Co-doped VO2. The roles of cobalt doping and detailed band variation are fully explained experimentally and by modeling (DFT calculation), respectively. Furthermore, the dramatically increased carrier concentration in cobalt-doped VO2 underscores the promising future of cobalt-doped VO2 unveiled by temperature-dependent Hall effect measurement.

Morphology-controlled WO3-x homojunction: hydrothermal synthesis, adsorption properties, and visible-light-driven photocatalytic and chromic properties.

In this paper, a facile one-step hydrothermal method for the synthesis of crystalline-amorphous WO3-x core-shell nanopowders (nanoparticles, nanorods, and nanowires) is reported. The core-shell structure, the size and the morphology of the core, and the shell ratio can be controlled by pH, the content of oxalic acid and the content of ammonium tungstate. A possible formation and growth mechanism is proposed. It was found that oxalic acid determines the crystalline-amorphous core-shell structure and [Formula: see text]/OH- plays a key role in retarding the crystal growth along the [001] axis. The synthesis method may introduce a new way to monitor and tune the stoichiometric and substoichiometric composition of semiconductor oxide nanostructure. The homojunction exhibits an enhanced adsorption ability, obvious visible photocatalytic efficiency, and good photochromic and chemochromic properties.

Selective photochromism in a self-coated WO3/WO3-x homojunction: enhanced solar modulation efficiency, high luminous transmittance and fast self-bleaching rate.

Pursuing excellent photochromic (PC) properties is still a longstanding challenge for energy-efficient smart coatings. Herein, we have prepared a novel series of self-coated crystalline WO3-amorphous WO3-x homojunctions by a one-step hydrothermal process, in which oxalic acid was used as a capping agent to prevent polycondensation and the crystallization of [WO6]. The as-prepared nanopowders with tight interfaces have a large specific surface area and rich surface electrons, which leads to a dramatic PC property reaching up to ΔT sol = 64.74% at the near-infrared (NIR) range of 1000-2600 nm with high luminous transmittance (T lum = 94.6% in the virgin state and T lum = 32.47% in the colored state). Meanwhile, the as-prepared crystalline WO3-amorphous WO3-x homojunction reveals fast reversible PC circulation; most particularly, the self-bleaching rate increases at least three times-the self-bleaching time is less than 8 h. Moreover, as the microstructure of the homojunction is tuned, the solar modulation range can be selective and tuned, so that the solar modulation efficiency is up to the best energy-saving state by controlling the main absorption according to the solar energy distribution.

Highly Enhanced Thermochromic Performance of VO2 Film Using "Movable" Antireflective Coatings.

The VO2 film with "movable" antireflective (AR) coating was initially designed and successfully prepared to deal with the challenges that VO2 faced. The "movable" AR layer, including water and other organic solvents, not only endowed VO2 with active-passive regulation mode but also dramatically enhanced the thermochromic performance. Furthermore, we combined solid and movable antireflection layer for further structural optimization, and the result turned out to be superior to any multilayer structure reported previously. We believe that this revolutionary concept of AR coating will open up a new avenue for low-cost smart window applications.

A plasmonic non-stoichiometric WO3-x homojunction with stabilizing surface plasmonic resonance for selective photochromic modulation.

Here, we fabricated an oxygen-deficient WO3-x crystalline/amorphous homojunction with a strong and tunable localized surface plasmon resonance (LSPR) absorption and realized selective solar modulation over infrared light by controlling the experimental parameters. The homojunction shows good LSPR stability in photochromic cycles, which paves the way to control LSPR applications.

Facile and Low-Temperature Fabrication of Thermochromic Cr2O3/VO2 Smart Coatings: Enhanced Solar Modulation Ability, High Luminous Transmittance and UV-Shielding Function.

In the pursuit of energy efficient materials, vanadium dioxide (VO2) based smart coatings have gained much attention in recent years. For smart window applications, VO2 thin films should be fabricated at low temperature to reduce the cost in commercial fabrication and solve compatibility problems. Meanwhile, thermochromic performance with high luminous transmittance and solar modulation ability, as well as effective UV shielding function has become the most important developing strategy for ideal smart windows. In this work, facile Cr2O3/VO2 bilayer coatings on quartz glasses were designed and fabricated by magnetron sputtering at low temperatures ranging from 250 to 350 °C as compared with typical high growth temperatures (>450 °C). The bottom Cr2O3 layer not only provides a structural template for the growth of VO2 (R), but also serves as an antireflection layer for improving the luminous transmittance. It was found that the deposition of Cr2O3 layer resulted in a dramatic enhancement of the solar modulation ability (56.4%) and improvement of luminous transmittance (26.4%) when compared to single-layer VO2 coating. According to optical measurements, the Cr2O3/VO2 bilayer structure exhibits excellent optical performances with an enhanced solar modulation ability (ΔTsol = 12.2%) and a high luminous transmittance (Tlum,lt = 46.0%), which makes a good balance between ΔTsol and Tlum for smart windows applications. As for UV-shielding properties, more than 95.8% UV radiation (250-400 nm) can be blocked out by the Cr2O3/VO2 structure. In addition, the visualized energy-efficient effect was modeled by heating a beaker of water using infrared imaging method with/without a Cr2O3/VO2 coating glass.

Selective and Tunable Near-Infrared and Visible Light Transmittance of MoO3-x Nanocomposites with Different Crystallinity.

In this Communication, we report MoO3-x nanocomposites in which the near-infrared and visible light transmittance can be selectively modulated through the crystallinity. The MoO3-x nanocomposites were fabricated by a hydrothermal method, and their optical properties were characterized by UV-Vis spectrometer. The obtained results proved the possibility to tune the nanocomposite's optical properties in the UV/Visible spectral region: crystalline MoO3 mainly regulates the near-infrared range (800-2600 nm), and amorphous MoO3-x mainly changes the visible range from 350 nm to 800 nm and MoO3-x , with semi-crystalline structures mainly modulating around 800-1000 nm. These kinds of optical modulations could be attributed to small polar absorption, free electron absorption and plasmon absorption according to different crystallinity. Our work may create new possibilities for future applications such as photochromism, photocatalysis, and electrochromism.