Excellent Thermochromic Properties Research Articles

Facile and dynamic infrared modulation of durable VO2/CuI films for smart window applications

Vanadium dioxide (VO2) has been widely investigated as a smart window material due to its excellent thermochromic properties. However, with the increasing abundance of studies on VO2-based structures, its low solar modulation capability, environmental durability and high phase transition temperature have limited its commercial applications. Finding a simple way to solve the above problems simultaneously remains challenging. Here, we propose a bilayer structure combining VO2 and a transparent conductive material, copper iodide (CuI), for functional integration of thermochromic smart windows. The CuI layer is used as an anti-reflective layer and a protective layer to ensure high solar modulation capability (ΔTsol=15.41 %) and good expected service life (ESL=27 years), while realizing the dynamic modulation of infrared spectra at room temperature by Joule heating. This facile structure with overall performance offers a new possibility for the commercialization of VO2-based smart windows and room temperature dynamic modulation of infrared devices.

Stable and rapid thermochromic reversibility in acene alkylamine intercalated layered hybrid perovskites

Since the discovery of thermochromism in halide perovskites, new perovskite materials with excellent thermochromic properties have been developed. Among them, two-dimensional (2D) layered hybrid perovskites have been extensively explored because of their highly tunable structures and unique thermochromic responses. In this study, the thermochromism in acene alkylamine-spaced 2D perovskite single crystals (PEA)2PbI4 and (NEA)2PbI4 (PEA = 2-phenylethylammonium, NEA = 2-(2-naphthyl)-ethanammonium) is studied. These 2D perovskites undergo a distinct color change between orange and red during heating and cooling cycles, with the corresponding optical bandgap modulation monitored by variable-temperature absorption spectroscopy. The thermochromism in (PEA)2PbI4 and (NEA)2PbI4 is caused by lattice expansion, as confirmed by thermogravimetric–differential scanning calorimetry and variable-temperature powder X-ray diffraction. Compared with the thermochromism caused by structural phase transition, lattice-expansion-modulated thermochromism demonstrates rapid color change and luminescence recovery, and enhanced thermal cycling stability. This study presents a new strategy for developing thermochromic halide perovskites for multifunctional applications.

Air atmosphere available fast one-pot synthesis of VO2 nanoparticles with excellent thermochromic properties by a novel liquid-shielding method

As a typical strongly correlated transition oxide, vanadium dioxide (VO2) based nanomaterials have drawn many research attentions these years due to the giant metal-to-insulator phase transition (MIPT) at around 68 °C. However, due to the complexities of the V–O system and interplays between various vanadium oxide phases, the synthesis of high-quality VO2 nanopowders is still fraught with many challenges, especially in air atmosphere. In this paper, we report a novel air atmosphere available liquid-shielding synthesis method for thermochromic VO2 nanoparticles, by using low-eutectic molten-salt (LiCl–KCl) as the liquid-state air-insulation medium at elevated temperature and high-pressure pressed VOSO4–KCl pillars immersed in these liquid salts as the precursor. Small amounts of glucose are added to introduce a slight reductive environment, and well dispersed VO2 nanoparticles with excellent thermochromic properties can be directly synthesized at an ultra-low temperature of 375 °C. This feasible and atmosphere-available mass-production method is rarely reported in the related fields, which may provide a novel protocol strategy for the synthesis of high performance thermochromic VO2 and other functional oxide powders.

Synthesis of vanadium oxide nanostructures with excellent thermochromic properties via sol-gel chemistry in supercritical carbon dioxide

Vanadium oxide VO2(M) synthesis was investigated using a simple supercritical CO2(scCO2) sol-gel process with vanadium(V) oxytriisopropoxide(VTIP) as the vanadium precursor and acetic acid as catalyst. scCO2 with different temperatures of 40, 60, 80 and 100 °C under 41.4 MPa was examined on the prepared sol-gels’ chemical constituents by FTIR and morphology by SEM, respectively. 80 °C and 41.4 MPa produced aerogel with smaller size of nanofiber. The calcination conditions were studied in regarding to different atmospheres, time durations and aerogel forms, by the XPS and XRD spectrum of the obtained vanadium oxides. Calcination in N2 at 500 °C with vanadium oxides aerogel casted on glass substrate for 15 min showed best calcination results, with V(IV) of 49.5%, V(V) of 47.1%, and V(III) of 3.4% in the obtained vanadium oxides. The smart glass was manufactured in scCO2 at 80 °C and 41.4 MPa, by direct synthesis of vanadium oxides aerogel on glass substrate, followed by calcination at 500 °C in N2 for 15 min. Phase transition of the obtained vanadium oxides was observed at 65 °C by DCS analysis. UV–vis–NIR spectrum at temperatures of 23 and 80 °C were measured. The prepared glass showed excellent regulation efficiency on IR transmittance (∆TIR=3.72%), and stable luminescence transmittance (∆Tlum=0.03%). This direct scCO2 sol-gel method shows promising application in manufacturing VO2 coated smart glass with excellent thermochromic switching properties.

One-pot Hydrothermal Synthesis of Vanadium Dioxide Nanoparticles

Vanadium dioxide (VO2) shows excellent thermochromic properties suitable for potential application in optical devices and smart windows coating. VO2 is interesting since actual coating on glass materials is possible along with large-scale production. This paper, VO2 nanoparticles were synthesized successfully using the sol-gel-assisted hydrothermal method. The as-prepared VO2 nanoparticles were annealed under vacuum, and the crystal phase formation, composition, morphology, and metal-insulator phase transition properties of the nanoparticles were studied. X-ray Diffraction (XRD) spectra revealed that a purely monoclinic phase of VO2 nanoparticles with an average crystallite size of 33.27 nm was synthesized. Scanning Electron Microscopy (SEM) micrographs also confirmed the formation of crystalline VO2 nanoparticles. Differential Scanning Calorimetry (DSC) curve shows that the change in temperature (∆T_c) between the exothermic and endothermic peaks is 13ºC. Moreover, Fourier Transform Infrared (FTIR) spectroscopy peaks ascertain the presence of VO2 functional groups in the synthesized nanoparticles.

Preparation and thermochromic properties of B-Mg co-doped nano-VO<sub>2</sub>

In order to reduce the phase transition temperature of vanadium dioxide (VO₂) materials and improve the optical properties of VO₂ materials, the boron-magnesium (B-Mg) co-doped vanadium dioxide (VO₂) powders were prepared through a sol-gel-assisted hydrothermal method in combination with the subsequent high-temperature annealing. The obtained powder was further prepared into a thin film. X-ray diffraction (XRD), X-ray electron spectroscopy (XPS), field emission scanning electron microscope (FESEM), differential scanning calorimeter (DSC), ultraviolet-visible-near-infrared spectrophotometer (UV-vis-NIR) were used to explore the microstructure of the material and effects of the doping concentrations of Mg on the phase transition temperature <italic>t</italic>_c and the optical properties of VO₂ materials. The B-Mg co-doped nano-VO₂ material that satisfies various application standards was obtained. The results show that the doped elements of B and Mg exist in the VO₂ structure as the form of B³⁺ and Mg²⁺. When the atomic percentages of B and Mg are 6.0% and 1.8%, the material shows a relatively excellent thermochromic properties, with the lowest phase transition temperature <italic>t</italic>_c = 30.8 ℃. The sunlight modulation amplitude Δ<italic>T</italic>_sol of the material is 11.8%, which is maintained above 10%. The average visible light transmittance <italic>T</italic>_lum of the material is 69.7%, which is 12.1% higher than that of the undoped VO₂ film. The properties above meet the requirements for the application in thermochromic smart windows. This work provides scientific basis for the preparation of VO₂ materials suitable for thermochromic smart windows.

Symmetrical SnO2/W-doped VO2/SnO2 sandwich structures with high luminous transmittance, excellent solar modulation ability and low phase transition temperature

Until now, it is still a challenging task to obtain VO2-based smart windows with high luminous transmittance (Tlum), satisfactory solar modulation ability (ΔTsol) and low phase transition temperature (Tc) close to room temperature. In this work, symmetrical SnO2(d nm)/W doped VO2 (W-VO2, 60 nm)/SnO2(d nm) (d = 0, 90, 150, 230, 260, 330) multilayer films were proposed and deposited on quartz glass substrates by pulsed laser deposition. The bottom SnO2 layer acted as buffer to improve the crystallinity of VO2, while the top SnO2 layer served as antireflection layer to enhance the optical properties. When SnO2 layer reached a certain thickness, the crystallinity of W-VO2 interlayer was significantly improved, and therefore the optical properties of the obtained tri-layer films were greatly increased. In comparison with the W-VO2 monolayer film, Tlum and ΔTsol of the tri-layer film with d = 260 nm increased from 45.0% to 54.4% and from 2.6% to 10.7%, respectively, while maintaining a low Tc of 39.0 °C. Interestingly, the prepared SnO2/W-VO2/SnO2 tri-layer films showed lively golden yellow or iridescent colors. The results provide a possibility for the design of VO2-based smart windows with low phase transition temperature and excellent thermochromic properties.

Ultrahigh-sensitivity thermochromic smart fabrics and flexible temperature sensors based on intramolecular proton-coupled electron transfer

Ultrahigh-sensitivity including both ultralow-color-hysteresis and narrow temperature change is an important requirement of temperature sensors. However, it is an almost impossible challenge for the traditional ternary thermochromic mechanism. Herein, a new class of fluorane dyes was designed and synthesized which was used to fabricate the ultrahigh-sensitivity sensors and smart fabrics via microcapsule technology and screen printing. The new fluorane dyes (M2 and M3) performed excellent thermochromic properties with a narrow color-change temperature range (2.1 °C) and ultralow-color-hysteresis (<0.5 °C). The ultrahigh-sensitivity property is attributed to the intramolecular proton-coupled electron transfer (PCET) color-change mechanism. Moreover, the effect of particle size and core/shell ratio on the temperature sensitivity of microcapsule was investigated. The optimized microcapsules with 5 μm and 2/1 core/shell showed overall properties including the color-change temperature range as about 2 °C and the ultralow-color-hysteresis as about 0.3 °C. This unique design concept for ultrahigh-sensitivity sensors has great potential in human body care and other flexible sensors or smart fabrics fields.

Preparation and characterization of HfO2/VO2/HfO2 sandwich structures with low phase transition temperature, excellent thermochromic properties, and superior durability

In the last few decades, smart windows made from VO2-based thermochromic films have attracted extensive attention, but their actual commercial applications are limited by low luminous transmittance (Tlum), low solar modulation ability (ΔTsol), high phase transition temperature (Tc), and poor durability. In this study, glass/HfO2/VO2/HfO2 tri-layer films were designed and deposited on glass substrates by pulse laser deposition. Crystal structures, surface morphology, surface roughness, electrical properties, and optical properties of as-prepared sandwich structure films were analyzed. Results showed that both HfO2 buffer layer and antireflection layer (ARL) were monoclinic phase and grew along the (020) and (−111) crystal planes, respectively. HfO2 buffer layer not only reduced Tc of VO2 film by about 20 °C, but also played an important role in regulating crystal quality and surface morphology of VO2 films. More importantly, by covering films with HfO2 ARL, Tlum and ΔTsol of VO2 film were greatly improved. In particular, when the thicknesses of HfO2 buffer layer and ARL were 80 nm and 120 nm, the obtained HfO2/VO2/HfO2 tri-layer film reached a balance between high Tlum (∼47.2%), high ΔTsol (∼9.1%) and low Tc (∼49.1 °C). In addition, after 216 h of boiling water treatment, Tlum and ΔTsol of HfO2/VO2/HfO2 film covered with 120 nm thick ARL still remained at 49.3% and 7.0%, showing excellent durability. This research provides a new strategy for designing VO2-based smart windows with high performance and good durability.

Facile fabrication of VO2/SiO2 aerogel composite films with excellent thermochromic properties for smart windows

In order to improve the solar modulation ability and near-infrared shielding performance of VO2-based films, in this work, VO2/SiO2 aerogel composite films have been fabricated by a facile mechanical ball-milling method. It has been clarified that the solar modulation efficiency (ΔTsol) of the composite film can be increased by 187.5% (from 6.4% to 18.4%) by introducing SiO2 aerogel into the composite film when the visible light transmittance (Tlum) is about 40%, which can be ascribed to the synergistic effect of Rayleigh scattering effect of SiO2 aerogel and the good dispersion of the particles. The VO2/SiO2 aerogel composite particles not only have excellent dispersion, but also the SiO2 aerogel can effectively slow down the oxidation of VO2 and improve the oxidation resistance of VO2. In particular, the universality of the Rayleigh scattering effect of SiO2 aerogel is demonstrated, which can act together with the doping effect of Mg2+ ions to further improve the thermochromic properties of VO2 films. This study provides a new idea for improving the thermochromic properties of VO2 films and meanwhile is of great significance in promoting the practical application of VO2 films in the field of smart windows.

Mist CVD of vanadium dioxide thin films with excellent thermochromic properties using a water-based precursor solution

Mist chemical vapor deposition (mist CVD), which is capable of producing oxide films over large areas at a high productivity and low cost, has been proposed as a fabrication method for VO2 thin films for smart windows. However, the thermochromic properties of the VO2 films previously prepared by mist CVD are not sufficient for application in smart windows. In this study, to obtain a high-quality VO2 film, we investigated the effects of the solvent of the precursor solution on the resulting film in mist CVD. Films consisting of a single phase of VO2 were obtained when a water-based precursor solution was used. In contrast, V2O3 films are formed when a methanol-based precursor solution is used. The VO2 film deposited from water solution exhibited high visible transmittance along with a large change in the infrared transmittance with temperature change. The high quality of the VO2 film indicates that mist CVD is an effective method for the fabrication of VO2-based smart windows.

Size-Controllable M-Phase VO2 Nanocrystals for Flexible Thermochromic Energy-Saving Windows

Ultrafine M-phase VO2 nanocrystals with good crystallinity are desirable for highly efficient thermochromic flexible films. Herein, we report an oxygen-controlled hydrothermal method to achieve the one-step preparation of stoichiometric and size-controllable VO2(M) nanoparticles for scaled-up production. The specific ratio of oxygen intake to V ions was first determined to guarantee the acquisition of stoichiometric VO2(M). Then, the hydrothermal conditions were optimized to perform size-controllable growth of VO2(M) nanoparticles. VO2(M) nanoparticles with average particle sizes of 25, 32, 41, and 52 nm were prepared by changing the precursor solution concentration under the determined oxygen intake/V ion ratio. Smaller VO2(M) nanoparticles exhibit better thermochromic performance because a smaller size favors light transmission and promotes blue-shifting of the local surface plasmon resonance. The flexible film fabricated with 25 nm VO2(M) nanoparticles shows an excellent thermochromic performance with a solar modulation efficiency (ΔTsol) of 19.2% and a luminous transmittance (Tlum) up to 66.42%. This work proposes a simple and effective hydrothermal process producing size-controllable VO2(M) nanoparticles with low cost and scalable production and demonstrates that the size reduction of VO2(M) nanoparticles is favorable for flexible films to exhibit excellent thermochromic properties.

Bioinspired thermochromic transparent hydrogel wood with advanced optical regulation abilities and mechanical properties for windows

The huge heat loss/gain through windows is the cause of great energy consumption in buildings. In addition, the traditional fabrication method for glass causes many environmental problems. Recently, transparent wood has emerged as a promising alternative to traditional glass because of its high transmittance, strong mechanical properties, excellent thermal insulation ability and sustainability. In this study, inspired by jellyfish, a thermochromic transparent hydrogel wood that can smartly regulate solar irradiation is proposed as a smart window material by impregnating Poly(N-isopropylacrylamide)-polyacrylamide hydrogel into delignified wood. The novel thermochromic transparent hydrogel wood inherits the excellent thermochromic properties of PNIPAM and strong mechanical properties of wood, showing advanced optical regulation ability (i.e. Tlum = 82.7% and 39.8% at the cold and hot states &ΔTsol = 38.1%), low transition temperature (i.e. Tc = 22.9 °C), mechanically robust (i.e. σ = 11.6 MPa along the axial direction) and low thermal conductivity (i.e. K = 0.37 W m−1 K−1 along the perpendicular direction of the wood growth). A field test conducted in October in Hong Kong shows that thermochromic transparent hydrogel wood can reduce the indoor air temperature by 4.3 °C. Furthermore, a computational simulation for an office building proves that 2.6–10.2% energy could be saved by thermochromic transparent hydrogel wood in four different climate-zone cities. Besides, thanks to the flexibility, thermochromic transparent hydrogel wood can be easily fitted on existing windows, demonstrating the great potential for use in energy-efficient buildings.

Preparation of W/Zr co-doped VO2 with improved microstructural and thermochromic properties

W/Zr co-doped VO2 nanoparticles were successfully synthesized via hydrothermal method with the aim of improving thermochromic properties. The phase transition temperature (Tc) is significantly reduced by an appropriate amount of W doping. The values of luminous transmittance (Tlum) and solar modulation ability (ΔTsol) of VO2 films were significantly improved by Zr doping. The addition of W and Zr has a remarkable influence on the microstructural properties of VO2 nanoparticles, and a dandelion and petals like morphology can be observed for V0.95W0.01Zr0.04O2 and V0.94W0.02Zr0.04O2 samples, respectively. The dandelion-like structure in V0.95W0.01Zr0.04O2 is attributed to the contacting and fusing of the villous structure, and the petals-like structure in V0.94W0.02Zr0.04O2 sample is ascribed to the shrinkage and crimping of the layered structure. V0.95W0.01Zr0.04O2 film has shown an excellent thermochromic properties with Tc, Tlum, and ΔTsol of 46.9 °C, 60.7%, and 10.6%, respectively. Further increase in W concentration has led to degradation in Tlum and ΔTsol values and Tc reduces to near room temperature. This work provides a feasible approach to design W and Zr doped VO2 films for primary application in smart windows.

Stabilizing vanadyl acetylacetonate using imidazolium Ionic liquids for VO2 thermochromic thin films

This study examines the role of different imidazolium ionic liquids (ILs) on the stabilization of vanadyl acetylacetonate, VO (acac)2, as a vanadium precursor by using UV-Vis spectroscopy. The possible stabilization mechanisms of VO (acac)2 in IL solution were investigated. Furthermore, a fabrication method is proposed to prepare monoclinic vanadium oxide VO2 (M) thin films using a mixture of ionic liquid and poly-ionic liquid (PIL). The ILs 1-ethyl 3 allyl imidazolium bis(trifluoromethansulfonylimide) ( [EAIM] [TFSI]), 1-ethyl 3-methyl imidazolium trifluoromethanesulfonate ( [EMIM] [TFO]) and 1-ethyl 3-methyl imidazolium acetate ( [EMIM] [AcO]) were investigated. It is shown that ( [EAIM] [TFSI]) has the best coordinating ability to VO (acac)2 for protecting the central vanadium atom from oxidation. For thermochromic film formation, the effect of annealing temperature (250°C – 400°C) and time (10 minutes - 120 minutes) on crystallization was studied using X-ray diffraction. The possible role of PIL decomposition on the formation and crystallization of VO2 (M) is discussed. The prepared films showed excellent thermochromic properties with 20.3 % solar modulation efficiency.

Thermochromic VO2–SiO2 composite coating from ammonium citrato-oxovanadate(IV)

Vanadium dioxide (VO2) coating plays an important role in energy saving and environmental protection due to its unique reversible phase transition. To solve the daylighting issue of VO2 coating, a VO2 (M)–silicon dioxide (SiO2) composite coating is fabricated from ammonium citrato-oxovanadate(IV) by a SiO2-assisted coating method. The VO2 (M)–SiO2 composite coating possesses excellent thermochromic properties that have produced varying results, i.e. 49.2% of visible transmittance, 52.3% of transmittance reduction at 2000 nm wavelength, 12% of solar energy modulation (ΔT sol ) and a phase transition temperature of 56.0 °C. Our findings may pave the way to extending the large-scale application of smart windows based on thermochromic VO2.

SnO2/VO2/SnO2 tri-layer thermochromic films with high luminous transmittance, remarkable solar modulation ability and excellent hydrophobicity grown on glass substrates

The SnO2/VO2/SnO2 (SVS) multilayer films were prepared using pulsed laser deposition and were characterized by X-ray diffraction, Raman spectroscopy, scanning electron microscopy, Hall Effect Measurement System, double beam spectrophotometer and contact angle measurements. The results showed that SVS multilayer films with excellent thermochromic properties were obtained by adjusting the preparation temperature of the SnO2 buffer layer and antireflection layer. When the preparation temperature of the SnO2 layer was 320 °C, the phase transition amplitude (ΔR) of the VO2 film reached 354 times and both the phase transition temperature (Tc) and hysteresis loop width (ΔH) could be effectively reduced. Meanwhile, the SVS multilayer film had satisfactory luminous transmittance (50.2%) and solar modulation ability (15.3%), achieving the balance of optical properties. In addition, all the SVS multilayer films exhibited excellent hydrophobicity. Boiling water experiments show that the service life of SVS tri-layer film is twice that of SV bi-layer film. This work suggests that the SVS multilayer films are a good candidate for VO2-based smart windows.

Comparative study of thermochromic properties on fresh and aged vanadium dioxide films

Vanadium dioxide (VO2) thin films were fabricated by means of DC reactive magnetron sputtering at low temperature, and then were aged in air for 5 years. The thermochromic properties of the fresh VO2 film and aged one were investigated by UV–visible spectrophotometer. The experimental results show that the fresh VO2 film shows excellent thermochromic properties with a high visible transmittance of 40.8% and a large switching efficiency of 73% at 2500 nm. For the film aged in ambient air for 5 years, the visible transmittance is increased to 54.0% due to a blue-shift of absorption edge and a decrease in luminous absorbance. More interestingly, the aged VO2 film still keeps a relatively large switching efficiency of 51% at 2500 nm, suggesting the deposited VO2 film has a good thermochromic stability in ambient air. These characteristics indicate the practical applications of VO2 film to smart optical devices.

Citrate-assisted hydrothermal synthesis of vanadium dioxide textured films with metal-insulator transition and infrared thermochromic properties

The present study reports the fast synthesis of VO2 (М1) oriented films on the r-Al2O3(012) substrates by the hydrothermal method using citric acid and vanadium (V) oxide as precursors with post deposition annealing in inert atmosphere. The effects of synthesis parameters (solution concentration and autoclave filling factor) on the films composition, morphology and electric properties are considered using XRD, Raman spectroscopy, AFM and SEM. The obtaining VO2(M1)/r-Al2O3 films show a metal-insulator transition with resistivity change of ~3.5 orders of magnitude and excellent thermochromic properties in the long-wavelength IR region. This work demonstrates a promising technique to promote the commercial implementation of VO2 as material for design of infrared (IR) switch devices.

Sn dopants improve the visible transmittance of VO2 films achieving excellent thermochromic performance for smart window

How to achieve high visible-light transmission meanwhile maintain satisfying solar modulation (ΔTsol) remains a challenge for VO2-based thermochromic smart windows. In this study, Sn dopants exhibit the ability of enhancing the visible transmittance (Tlum) of VO2 films, as a result, endowing the films with excellent thermochromic properties. Sn-doped VO2 films with different Sn contents (0at%, 0.43at%, 0.74at%, 1.73at%, 1.86at%) are prepared via hydrothermal method. The 0.74at% Sn-doped film has a Tlum value about twice that of the undoped film. The improvement of the visible transmittance is rationalized by ellipsometry and optical absorption measurements, in which it is revealed that the Sn dopants reduce the refractive index (n) and extinction coefficient (k) of VO2 in the VIS-NIR wavelength range, and widen the band gap (Eg). The 0.74at% Sn-doped VO2 film realizes excellent thermochromic performance with the ΔTsol up to ~15.4% and the Tlum reaching ~51.8%. This work demonstrates the unique characteristic of Sn dopants to improve the visible transmittance of VO2 films, suggesting a rational way in optimizing the thermochromic property of VO2.