With an urgent demand of energy efficient coatings for building fenestrations, vanadium dioxide (VO2)-based thermochromic smart coatings have been widely investigated due to the reversible phase transition of VO2 at a critical transition temperature of 68 °C, which is accompanied by the modulation of solar irradiation, especially in the near-infrared region. As for commercial applications in our daily life, there are still some obstacles for VO2-based smart coatings, such as the high phase transition temperature, optical properties (luminous transmittance and solar modulation ability), environmental stability in a long-time period, as well as mass production. In this review, recent progress of thermochromic smart coatings to solve above obstacles has been surveyed. Meanwhile, future development trends have also been given to promote the goal of commercial production of VO2 smart coatings.
1 Introduction
Due to environmental deterioration and energy shortage in human society, people pay more attention to finding effective energy efficient materials to reduce the energy consumption and greenhouse gas emission. According to the survey, buildings are responsible for about 40% of the energy consumption and almost 30% of the anthropogenic greenhouse gas emissions, which are owing to the use of lighting, air-conditioning, and heating [1‐5]. This has driven an urgent demand and research for energy efficient applications to reduce the building energy consumption. The heat exchange between the interior of the building and the outdoor environment through fenestrations leads to the largest energy consumption of buildings. Therefore, managing heat exchange through fenestrations is a feasible approach to reduce the building energy consumptions. In summers, solar radiation entering buildings should be controlled to reduce the air-conditioning energy consumption. On the contrary, thermal radiations from the buildings must be limited to consume lesser energy for heating in winters.
An effective way to achieve this goal would be using smart coatings on building fenestrations to control the solar radiation and thermal radiation. Therefore, smart coatings based on electrochromism [6‐10], thermochromism [11‐19], gasochromism [20‐22] and photochromism [23‐26] have been widely studied for energy efficient coatings. Thermochromic smart coatings can modulate near-infrared radiation from transmissive to opaque in response to the environmental temperature from low to high, which does not require extra stimuli and can save more energy consumption.
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Vanadium dioxide (VO2) is a typical thermochromic material that has been widely studied. VO2 exhibits an automatic reversible semiconductor-metal phase transition (SMT) at a critical transition temperature (Tc) of 68 °C [27], which has been widely investigated as smart coatings for buildings fenestrations [28‐32]. As shown in Fig. 1, for temperatures below the Tc, VO2 is monoclinic (P21/c, M1) phase with the transmittance of infrared (IR) radiation. On the contrary, the material is a tetragonal structure (P42/mnm,R), which is reflective for IR radiation [33, 34]. This feature makes VO2 an amazing material for thermochromic smart coatings [34‐42].
Fig. 1
a The crystallographic structure of VO2 (rutile) and VO2 (monoclinic) [53] and b schematic of thermochromic behavior before and after the phase transition [33]
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VO2 smart coatings are usually used in two forms including flexible foils based on VO2 nanoparticles [31, 43‐49] and VO2 based multilayer films [11, 12, 30, 50‐52]. However, for commercial application as smart coatings on energy efficient fenestrations, there are still several obstacles severely limiting the relative applicability of VO2 smart coatings. (i) The phase transition temperature (Tc) for pure bulk VO2 (68 °C) is too high to be applied on building fenestrations, while Tc around 40 °C is acceptable. (ii) For conventional VO2 coatings, relative modulation abilities are not efficient enough for energy saving. That can be explained by the fact that the modulation of VO2 for solar radiation is most attributed to the transmittance switch in the near-infrared region, which only accounts for 43% of solar energy in the solar spectrum [23]. (iii) The luminous transmittance \(T_{\text{lum}}\) for single layer VO2 with desirable (\(\Delta T_{\text{sol}}\)) is usually less than 40% (even 30%) due to the absorption in the short-wavelength range in both the semiconducting and metallic states of VO2, which should be larger than 50% at least for daily applications. (iv) For practical applications as smart coatings, VO2 must maintain excellent thermochromic performances during a long-time period—at least ten years. However, VO2 will finally transform into the V2O5 phase in the real environment, which is the most thermodynamically stable phase of vanadium oxide but does not possess the thermochromic property [54]. Therefore, environmental stability of VO2 is a great challenge for practical applications as smart coatings.
These obstacles must be overcome for practical applications and many efforts have been made to achieve this goal. Doping of proper ions can effectively reduce the phase transition temperatureof VO2: cations larger than V4+, such as W6+[55], Mo6+[56] and Nb5+[57], and anions smaller than O2−, such as F− [58], have been utilized to reduce the Tc. However, obstacles in (ii)–(iv) have not been solved. Although several reviews about VO2 coatings have been reported [32, 33, 59, 60], most of them are still in lab scale and few prospects of commercial applications are available.
In this review, we are going to view strategies of thermochromic VO2 smart coatings for improved thermochromic performance, environmental stability and large-scale production for commercial applications on building fenestrations. Firstly, strategies to enhance thermochromic performance (\(T_{\text{lum }}\) and \(\Delta T_{\text{sol}}\)) of VO2 coatings have been introduced as well as the balance between \(T_{\text{lum}}\) and \(\Delta T_{\text{sol }}\) (Section 2). Then, methods to improve the durability of VO2 coatings, including protective layers for multilayer films and core-shell structures for nanoparticles, will be summarized in Section 3. Meanwhile, multifunctional design of VO2 smart coatings such as photocatalysis and self-cleaning function has been discussed in Section 4. Recent progress for large-scale production of VO2 smart coatings has been surveyed in Section 5. Finally, future development trends of VO2 coatings have prospected for large-scale production as practical and commercial applications.
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2 Improvements of optical properties
\(T_{\text{lum}}\) and \(\Delta T_{\text{sol}}\) are the most important indexes of thermochromic properties for VO2 smart coatings. The integral \(T_{\text{lum}}\) and \(T_{\text{sol}}\) of the samples can be obtained by the following equations
where \(T(\lambda )\) represents the transmittance at wavelength \(\lambda\); \(\varPhi_{\text{lum}}\) is the standard efficiency function for photopic vision; and \(\varPhi_{\text{sol}}\) is the solar irradiance spectrum for an air mass of 1.5, which corresponds to the sun standing 37° above the horizon. While the \(\Delta T_{\text{sol}}\) of the films was calculated by \(\Delta T_{\text{sol}} = T_{{{\text{sol,}}\,{\text{lt}}}} - T_{\text{sol,ht}}\), where lt and ht represent low temperature and high temperature, respectively.
VO2 smart coatings always suffer from the problem of low luminous transmittance due to the absorption in the short-wavelength range in both the semiconducting and the metallic states [61]. The luminous transmittance of VO2 coatings is largely dependent on relative thicknesses. Based on optical calculation, a single layer VO2 film (80 nm), for example, exhibits an integrated \(T_{\text{lum}}\) of 30.2% and 25.1% for semiconducting and metallic VO2 (see Fig. 2a). As for solar modulation ability, the majority of reported modulation abilities are less than 10%, which are not efficient enough for energy saving function [62‐65]. For VO2 coatings before and after the phase transition, the contrast of relative optical transmittance is mainly in the near-infrared region (780–2 500 nm), which only accounts for 43% of solar energy in the solar spectrum (see Fig. 2b).
Fig. 2
a Calculated luminous transmittance for single layer VO2 films with various thicknesses at semiconducting state (black line) and metallic state (red line) and b the solar spectrum and relative energy distribution [23]
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2.1 Strategies for enhanced luminous transmittance and solar modulation ability
Many efforts have been made to improve the luminous transmittance and solar modulation ability of VO2 based smart coatings. For VO2 films fabricated by deposition, the design of multilayer structures is an effective way to improve the optical properties [11, 52, 66]. As for VO2 nanoparticles prepared by solution methods, the formation of composite films is the most commonly used strategy [67, 68].
2.1.1 Multilayer design for VO2 thin films
Thermochromic smart coatings incorporating VO2 films with additional layers have been fabricated for improved thermochromic performances including desirable luminous transmittance and effective solar modulation ability. Schematic illustration of additional layers such as antireflection layers and buffer layers have been shown in Fig. 3 with three typical structures for VO2 thin films and relative SEM images.
Fig. 3
Schematic illustration of VO2 based films with a antireflection layer, b buffer layer, and c both of antireflection layer and buffer layers, respectively. Relative SEM images of three typical structures have been shown in Figs. d–f corresponding to Figs. a–c [12, 30, 69], respectively
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An effective way to improve the luminous transmittance of VO2 coatings is to introduce an antireflection (AR) layer, such as SiO2 [69‐72], TiO2 [73], ZrO2 [74], etc. Lee and Cho [70, 71] reported that SiO2 antireflection layer successfully increased the luminous transmittance of the VO2 films. However, the luminous transmittance is still not sufficient. TiO2 was selected as AR layer for VO2 films [73] because TiO2 has a higher refractive index and is a more effective antireflection material for VO2 than the reported SiO2. The optimized VO2/TiO2 structure has been fabricated and demonstrated the highest \(T_{\text{lum}}\) improvement among the reported at that time. The optical calculation was performed upon a basic structure of a VO2 layer with an AR layer of refractive index n and thickness d [74]. Optimization was carried out on n and d for a maximum integrated \(T_{\text{lum}}\). The calculation demonstrates that the optimal n value changes with the thickness of VO2, and at n ≈ 2.2 it gives the highest \(T_{\text{lum}}\) enhancement from 32% (without AR coating) to 55% for 50 nm VO2. They deposited an optimized structure of VO2/ZrO2 and an improvement from 32.3% to 50.5% in \(T_{\text{lum}}\) was confirmed for the semiconductor phase of VO2, which was in good agreement with the calculations.
Besides the antireflection layers on the top of VO2 films, buffer layers between the substrates and VO2 films also play important roles in the optical performances of integrated coatings. Some buffer layers as SiO2, TiO2, SnO2, ZnO, CeO2, and SiNx have been investigated in reported works [75‐78]. Nevertheless, thermochromic performances of VO2 coatings obtained based on above buffer layers are fair, which still can not match the requirements for practical applications.
In our recent work, Cr2O3 has been selected to act as a structural template for the growth of VO2 films as well as the AR layer for improving the luminous transmittance [12]. The suitable refractive index (2.2–2.3) is predicted to be beneficial for the optical performance of VO2 thin films. Refractive index of Cr2O3 is between the glass and the VO2, which is considered to enhance the luminous transmittance. Meanwhile, Cr2O3 has similar lattice parameters with VO2(R), which can act as the structural template layer to lower the lattice mismatch between VO2 thin films and glass substrates and to reduce the deposition temperature of VO2 thin films (see Figs. 4a, b). Different crystallization of VO2 films can be obtained by introducing Cr2O3 layers with various thicknesses at a competitive temperature range from 250 °C to 350 °C, where different thermochromic performance can be obtained (see Fig. 4c). The Cr2O3/VO2 bilayer film deposited at 350 °C with optimal thickness shows an excellent \(\Delta T_{\text{sol}}\) = 12.2% with an enhanced \(T_{\text{lum,lt}}\) = 46.0% (see Fig. 4d), while the value of \(\Delta T_{\text{sol}}\) and \(T_{\text{lum,lt}}\) for the single layer VO2 film deposited high temperature at 450 °C is 7.8% and 36.4%, respectively. The Cr2O3 insertion layer dramatically increased the visible light transmission, as well as improved the solar modulation of the original films, which arised from the structural template effect and antireflection function of Cr2O3 to VO2.
Fig. 4
a Crystal structure of hexagonal Cr2O3, monoclinic VO2, and rutile VO2, respectively, b schematic illustration of Cr2O3/VO2 bilayer thermochromic film, c variation curve of \(T_{\text{lum,lt}}\), \(T_{\text{lum,ht}}\) and \(\Delta T_{\text{sol}}\) for VO2 films deposited with 40 nm Cr2O3 structural template layer at different temperatures, d transmittance spectra (250–2 600 nm) at 25 °C and 90 °C for VO2 films deposited with 40 nm Cr2O3 structural template layer at 350 °C and standard solar spectra [12]
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For better thermochromic performance, sandwich structures based on VO2 films have been fabricated. Double-layer antireflection incorporating TiO2 and VO2 (TiO2/VO2/TiO2) has been proposed [61] and a maximum increase in \(T_{\text{lum}}\) by 86% (from 30.9% to 57.6%) has been obtained, which is better than the sample with single-layer antireflection (49.1%) [73]. The same structure of TiO2/VO2/TiO2 has also been investigated by Zheng et al. [11] and Sun et al. [35] for improved thermochromic performance and skin comfort design. A novel sandwich structure of VO2/SiO2/TiO2 has been described by Powell et al. [66], where the SiO2 layers acts as ion-barrier interlayers to prevent diffusion of Ti ions into the VO2 lattice. The best performing multilayer film obtained in this work showed an excellent solar modulation ability (15.29%), which was very close to the maximum possible solar modulation for VO2 thin films. Unfortunately, the corresponding luminous transmittance is weak of around 18% for both semiconducting and metallic states.
A novel Cr2O3/VO2/SiO2 (CVS) sandwich structures have been proposed and fabricated based on optical design and calculations [30]. The bottom Cr2O3 layer provides a structural template for improving the crystallinity of VO2 and increasing the luminous transmittance of the structure. Then, the VO2 layer with a monoclinic (M) phase at low temperature undergoes a reversible phase-transition to rutile (R) phase at high temperature for solar modulation. The top SiO2 layer not only acts as an antireflection layer but also greatly enhances the environmental stability of the multilayer structures as well as providing a self-cleaning layer for the versatility of smart coatings. Optical simulation of luminous transmittances (semiconducting state) for the CVS structure has been shown in Fig. 5a (3-dimensional image). The thickness of the VO2 layer was fixed at 80 nm to demonstrate significant thermochromic performance while varying thicknesses of Cr2O3 and SiO2 were investigated for optimized optical properties. Four clear peaks are observed in the luminous transmittance simulations, which can be attributed to the interference effect of the multilayer structure. The highest value of \(T_{\text{lum,lt}}\) is about 44.0% at approximately 40 nm and 90 nm of Cr2O3 and SiO2, respectively. In this work, the proposed CVS multilayer thermochromic film shows an ultrahigh \(\Delta T_{\text{sol}}\) = 16.1% with an excellent \(T_{\text{lum,lt}}\) = 54.0%, which gives acommendable balance between \(\Delta T_{\text{sol}}\) and \(T_{\text{lum,lt}}\) (see Figs. 5b, c). The demonstrated structure shows the best optical performance in the reported structures grown by magnetron sputtering and even better than most of the structures fabricated by solution methods. To date, the proposed CVS structure exhibits the most recommendable balance between the solar modulation ability and the luminous transmittance to reported VO2 multilayer films (see Fig. 5d).
Fig. 5
a 3D surface image of the luminous transmittance (\(T_{{{\text{lum}},{\text{lt}}}}\)) calculation of the Cr2O3/VO2 (80 nm)/SiO2 multilayer structure on the thickness design of Cr2O3 (bottom layer) and SiO2 (top layer), b transmittance spectra (350–2 600 nm) at 25 °C (solid lines) and 90 °C (dashed lines) for the CVS structures with various thicknesses of SiO2 layers, c corresponding variation curves of \(T_{{{\text{lum}},{\text{lt}}}}\), \(T_{{{\text{lum}},{\text{ht}}}}\), and \(\Delta T_{\text{sol}}\) for b, d comparison of this work with recently reported VO2-based thermochromic films [30]
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There is some work focus on multilayer films with more layers for enhanced thermochromic performances. A five-layer thermochromic coating based on TiO2/VO2/TiO2/VO2/TiO2 has been studied [52]. A featured wave-like optical transmittance curve has been measured by the five-layer coating companying an improved luminous transmittance (45.0% at semiconducting state) and a competitive solar modulation ability (12.1%). Multilayer structure like SiNx/NiCrOx/SiNx/VOx/SiNx/NiCrOx/SiNx exhibits superior solar modulation ability of 18.0%, but the luminous transmittance (32.7%) and the complicated structure pose an enormous obstacle for practical application of this structure.
2.1.2 Composite films based on VO2 nanoparticles
Composite films incorporating VO2 nanoparticles with inorganic or/and organic materials have many advantages. On the one hand, the structure of composite films may induce strains, which may have positive effects on the Tc and hysteresis-loop width of VO2 films [79]. On the other hand, according to the optical calculations performed by Li et al. [80], VO2 nanoparticles dispersed in suitable dielectric hosts show much higher luminous transmittance and solar energy transmittance modulation than pure VO2 films [80].
VO2-ZrV2O7 composite films have been successfully prepared by polymer-assisted deposition using V-Zr-O solution [79]. With similar thickness, the composite films exhibited significantly enhanced luminous transmittances with increasing Zr/V ratios (from 32.3% at Zr/V = 0 to 53.4% at Zr/V = 0.12), which can be attributed to the absorption-edge changes in the composite films. Nevertheless, the solar modulation ability of the samples showed slightly weakened increasing Zr/V ratios (from 6.0% at Zr/V = 0 to 4.8% at Zr/V = 0.12). Crystallized TiO2-VO2 composite films were prepared by dispersing VO2 nanoparticles in TiO2 sol and annealing by an optimized two-step annealing process [81]. The optical performance of these composite films could be improved by increasing their porosity by controlling annealing rate or by introducing mesopores.
Inorganic host BaSO4 has also been investigated due to its numerous advantages, such as stronger chemical inertness, acid and alkali resistance, relatively high density and whiteness, especially the high dispersion, good refractive index and transparency [82]. VO2-BaSO4 composite powders were prepared by a one-step hydrothermal process, and the existence of the BaSO4 could improve the optical properties of the VO2 by 43.0% in \(T_{\text{lum}}\) (from 30.4% to 43.5%) and 10.7% in \(\Delta T_{\text{sol}}\) (from 11.2% to 12.4%).
Besides the inorganic hosts as ZrV2O7, TiO2, and BaSO4, organic hosts also show great potentials in smart coatings. A temperature-responsive hydrogel based on poly(N-iso-propylacrylmide) (PNIPAm) has been reported [83]. The PNIPAm can undergo a hydrophilic to hydrophobic transition at the lower critical solution temperature (LCST). By tuning the thickness of the hydrogel and designing a suitable glass panel set-up, the PNIPAm exhibited an unprecedented good combination of the near-doubled average \(T_{\text{lum}}\) (70.7%), higher \(\Delta T_{\text{sol}}\) (25.5%) and lower transition temperature (32 °C). Further investigations have been carried out by dispersing VO2 nanoparticles into a PNIPAm hydrogel to form a hybrid thermochromic material [67]. The VO2/hydrogel hybrid nanothermochromic material makes a dramatically higher \(\Delta T_{\text{sol}}\) up to 35%, while still maintaining higher average \(T_{\text{lum}}\) (63%) (see Fig. 6).
Fig. 6
a Solar modulation mechanism of the VO2/hydrogel hybrid, b UV-Vis spectra of pure hydrogel (black), pure VO2 (red), and VO2/hydrogel hybrid (blue) at low and high temperature (the grey area indicates the normalized values of the visible (light) and IR(dark) spectra spectral irradiance. The light cyan area with sparse slashes is the luminous transmission difference, and the light yellow area is the IR transmission difference of the VO2/hydrogel hybrid) [67]
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Some transition metal complexes (TMCs) that exhibit thermochromism with little \(T_{\text{lum}}\) loss at high temperature are chosen to be hybridized with VO2 nanoparticles [45, 68, 84]. The used TMCs can partly absorb visible light and change color as a response to temperature change upon interaction with an appropriate donor solvent and effectively avoid severe damage to \(T_{\text{lum}}\) at high temperature because the absorption peaks are staggered with the extremum of the light-adapted eye sensitivity function. The thermochromic ionic liquid (IL) has been used by Zhu et al. [68]. Briefly, when heating from room temperature, the ionic liquid-nickel-chlorine (IL-Ni-Cl) complexes absorb increasing visible light around 656 nm and 705 nm, and gradually change color from colorless to blue. Compared with a pure VO2 film, the composite film of VO2 nanoparticles and IL-Ni-Cl complexes not only expresses more excellent optical performance (\(\Delta T_{\text{sol}}\) = 26.45% and \(T_{\text{lum,lt}}\) = 66.44%, \(T_{\text{lum,lt}}\) = 43.93%) but also adds a function of an obvious color change from brown to green as temperature rises, facilitating application and function exhibiting of smart windows (see Fig. 7). TMCs like IL-Ni-Br and IL-Co-Br have also been studied with robust improvements of thermochromic performances of VO2 nanoparticles [45, 84].
Fig. 7
a Color change mechanism for IL-Ni-Cl complexes (upper) and color change of VO2/IL-Ni-Cl composites film, b UV-Vis-NIR transmittance spectra of pure IL-Ni-Cl complexes film, pure VO2 nanoparticles film, and VO2/IL-Ni-Cl composite film at 20 °C and 80 °C (the yellow-orange area indicates the normalized values of the visible (yellow) and NIR (orange) spectra spectral irradiance, and the cyan area indicates the values of eyes ensitivity function) [68]
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2.2 Balance between luminous transmittances and solar modulation ability
Regarding practical application of VO2 based thermochromic smart coatings, high solar modulation ability accompanied by high luminous transmittance is required. Nevertheless, we can find that it is tough to make a good balance between luminous transmittance and solar modulation ability. A unilateral pursuit of distinguished solar modulation ability or ultrahigh luminous transmittances is meaningless.
Most work on VO2 based smart coatings pursue large contrast of optical transmittance in the near infrared region (780–2 500 nm), while inconspicuous contrast in the visible light region (380–780 nm) is desirable for both semiconducting and metallic states. In the solar spectrum, ultraviolet light, visible light and infrared light is responsible for about 7%, 50%, 43% of solar energy, respectively [23]. Therefore, if there is an increased contrast in the visible light region for VO2 based smart coatings between the semiconducting and the metallic state, relative solar modulation ability can be robustly enhanced due to the contribution from the visible light region. That means that the transmittance in the visible light region for VO2 smart coating of metallic state should be maintained at least 50%, while the coating shows higher luminous transmittance of semiconducting state. Some works have been reported to increase \(\Delta T_{\text{sol}}\) of VO2 by mixing with specific materials shows a robust contrast in the visible light region in different temperatures [45, 68]. However, more investigations are required for a facile and cheap method to achieve the balance between luminous transmittances and solar modulation ability of VO2 based smart coatings.
3 Methods to improve the stability of VO2 for long-time use
In previous work, researchers usually focus on the thermochromic properties of VO2 to improve the luminous transmittances and solar modulation ability. However, environmental stability is another great challenge for VO2 coatings from lab to industrial production. Vanadium is a multivalent element and there are several kinds of vanadium oxide, such as VO, V2O3, VO2, V6O13, V4O9, V3O7, and V2O5. Among them, V2O5 is the most thermodynamically stable phase and VO2 will gradually transform into the intermediate phases of V6O13 and V3O7 and finally into V2O5 [54]. However, unlike VO2, V2O5 does not possess thermochromic optical change properties near the room temperature. Therefore, how to maintain the thermochromic performance of VO2 coatings during a long-time period is an inevitable problem that must be overcome.
3.1 Protective layers for VO2 films
To prevent VO2 films from degradation, introducing of protective layers above VO2 is an effective way that has been widely used. Chemically stable oxide films such as Al2O3 [54, 85], CeO2 [86, 87], WO3 [64], etc., have been studied to keep VO2 away from oxidant like water and O2 in air. It should be noted that the selected materials to be used as protective layers might affect the optical properties of VO2, where dual enhancement in the optical properties and the stability is preferred.
Al oxide is a typical material that has been investigated as a protection layer for VO2 coatings. In work reported by Ji et al. [54], different thicknesses of Al oxide protective layers have been deposited for VO2 by DC magnetron sputtering. The durability of the samples was evaluated at a high temperature around 300 °C in dry air and highly humid environment. They found that the Al oxide protective layers provided good protection and delayed the degradation process of VO2 in dry air at 300 °C and humid environment. The similar structure was also investigated [85], while the Al2O3 protective layers were fabricated by atomic layer deposition (ALD). The Al2O3 films can protect the VO2 from oxidation in the heating test but not sufficient in the damp environment, which can be attributed to the corrosion of water to Al2O3. It is worthy to mention that in above cases, the test period of the samples is less than one week (168 h), which is far from the request for practical applications.
Long et al. [64] proposed a novel sandwich structure of WO3/VO2/WO3, where WO3 not only functions as an AR layer to enhance the \(T_{\text{lum}}\) of VO2 but also performs as a good protective layer for thermochromic VO2. The stability of samples was investigated in a constant-temperature humid environment with 90% relative humidity at 60 °C. For the single layer VO2, the thermochromism nearly vanishes after 20 day treatment in the tough environment. On the contrary, there shows almost no change in the optical transmittance of WO3/VO2/WO3 multilayer films with the same treatment. However, though the protection provided by the WO3, the solar modulation ability of the sample is weakly reduced due to the diffusion of W6+ to VO2.
In works above, the protective layers are usually single layer films. To enhance the durability of thermochromic VO2 films, bilayer coatings as VO2/TiO2/ZnO, VO2/SiO2/ZnO, and VO2/SiO2 /TiO2 have been studied [88]. In this study, VO2 films with TiO2/ZnO protective coatings have been demonstrated higher antioxidant activity under aging tests, which can be attributed to the different oxygen permeability through different inorganic films [89]. Zhan et al. [90] fabricated a complicated multilayer structure of SiNx/NiCrOx/SiNx/VOx/SiNx/NiCrOx/SiNx, which exhibits enhanced thermal stability up to 375 °C. However, aging test in a humid environment is not applied to the samples.
The Cr2O3/VO2/SiO2 structure proposed by our lab shows robust environmental stability for long-time use [30]. The top SiO2 layer is chemically stable and makes the static water contact angle of the films change abruptly from 24.1° (hydrophilicity) to 115.0° (hydrophobicity) (see Figs. 8a, b). Hydrophilicity of the single layer VO2 indicates well contact with water, which will accelerate the degradation process of relative thermochromic performance. On the contrary, the hydrophobicity that exhibited by the CVS structure is helpful to keep the VO2 isolated from the water, which can protect the coatings against oxidation. Wettability is dependent on the chemical composition and structure of the surface. The surface of silicon is normally hydrophilic without additional treatments, but previous studies have demonstrated that the wettability of the silicon surface can be significantly changed by structuring the surfaces. So, fabrication of SiO2 top coatings in this work has been deliberately optimized with enhanced roughness for hydrophobic surfaces (see Fig. 8b). The double-protection from Cr2O3 and SiO2 makes an excellent promotion for the environmental stability of the CVS coatings, which is desirable for long-time use. The proposed CVS structure shows remarkable environmental stability due to the dual-protection from the Cr2O3 and the SiO2 layer, which shows negligible deterioration even after accelerated aging (60 °C and 90% relative humidity) of 103 h and 4 × 103 fatigue cycles, while VO2 single layer samples almost become invalid (see Figs. 8c, d).
Fig. 8
Images of contact angle measurement of a the single layer VO2 and b the proposed Cr2O3/VO2/SiO2 structure. Variation curves of \(\Delta T_{\text{sol}}\) for VO2, Cr2O3/VO2 and Cr2O3/VO2/SiO2 with different duration time c and different fatigue cycles d [30]
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3.2 Core-shell structures
Unlike VO2 films prepared PVD methods, thermochromic coatings based on VO2 nanoparticles have different surface morphologies and larger specific area. Therefore, protective layers may not be suitable for VO2 nanoparticles. Core-shell structures for VO2 nanoparticles have been demonstrated to be an effective way to improve performances of the VO2 core by using selected materials as shells [29, 55, 91‐93].
Silica (SiO2) is the most studied material utilized as shell to improve the durability of VO2 nanoparticles [29, 55, 94, 95]. Firstly, the SiO2 layer is optically transparent and chemically stable. Secondly, the utilization of SiO2 layer can prevent nanoparticles from agglomeration [95]. Thirdly, the SiO2 layer is helpful to improve the chemical stability and mechanical stability of VO2 nanoparticles. Gao et al. [29] synthesized VO2@SiO2 core-shell structure via solution method with PVP pretreatment. The proposed VO2@SiO2 structures can effectively enhance the anti-oxidation and acid-resisting properties of VO2. After annealing at 300 °C in air for 2 h, most VO2 nanoparticles without SiO2 shell have transformed into V2O5, while no trace of V2O5 can be observed in the VO2@SiO2 sample. The acid-corrosion experiment of the samples in a hydrochloric acid solution (pH = 1) also confirmed the chemical stability of VO2@SiO2.
Al oxide is a promoted material as the shell to protect nanoparticles from corrosion, which has been demonstrated to protect perovskite solar cells from corrosion [96]. VO2/Al-O core-shell structures have been fabricated, where different duration tests have been carried out [97]. For the uncoated sample, the VO2 nanoparticles were oxidized into V2O5 when heated at 300 °C, while the coated VO2/Al-O remains stable even heated up to 350 °C. While treated in the damp heating environment, the uncoated sample loses thermochromic properties only after 48 h, while the VO2 nanoparticles coated with Al-O shell remains stable even after 20 days.
In the study by Chen et al. [43], ZnO was selected as the shell to prevent VO2 nanoparticles from being oxidized. From the TEM images of VO2@ZnO shown in Figs. 9a, b, it can be observed that VO2 nanoparticles were closely surrounded by ZnO shells. Compared with uncoated VO2 film, ZnO coating VO2 films show greater effects on the properties (see Fig. 9c). The \(\Delta T_{\text{sol}}\) and \(T_{\text{lum}}\) are improved from 38.9% and 17.2% to 51.0% and 19.1% (see Fig. 9d). An extreme environment for a constant temperature of 60 °C and humidity of 90% which will accelerate samples losing the thermochromic performance in this condition of the environment and evaluated relative durability. For the uncoated VO2 film, the thermochromism vanishes completely after 30 h treatment. Compared with the uncoated VO2, the VO2@ZnO shows striking durability. The transmittance curve at different temperatures of VO2@ZnO film remains almost intact after 103 h testing, which means that it still has good thermochromic performance (see Fig. 9e).
Fig. 9
a Experimental flow chart for the synthesis of VO2@ZnO core-shell structure nanoparticles and VO2@ZnO film, b–c TEM images of VO2@ZnO core-shell structure nanoparticle, d optical transmittance spectra at 20 °C and 80 °C of uncoated VO2 film and VO2@ZnO film, e optical transmittance spectra of VO2@ZnO in a constant temperature (60 °C) and humidity (90%) [43]
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4 Multifunctional design and construction
Nowadays, multifunctional fenestrations of the buildings are favored by customers. As is known to all, the fenestrations of the buildings and vehicles always need to be cleaned, which would lead to additional pollutants from the use of detergents and wasting a mass of labors. Semiconductor photocatalysts like TiO2 are widely and frequently employed to decompose pollutants. There are three different polymorphs of crystalline TiO2: rutile(tetragonal), anatase (tetragonal) and brookite (orthorhombic). Rutile TiO2 (TiO2 (R)) is a thermodynamically stable phase at all temperatures and the most common natural form of TiO2. Due to similar lattice parameters, TiO2 (R) films are acted as buffer layer and growth template of VO2 (M) films. Nevertheless, TiO2 (R) films are less efficient photocatalysts than anatase TiO2 (TiO2 (A)) films, which occupy an important position in the studies of photocatalytic active. Zheng et al. [11] constructed a TiO2(R)/VO2(M)/TiO2(A) multilayer film, while the photocatalytic and photoinduced hydrophilic properties from the top TiO2(A) layer for self-cleaning effects (see Fig. 10a).
Fig. 10
a FESEM image of a fractured cross-section of the multilayer film (the insets are surface morphology of VO2(M) (left) and TiO2(A) layers (right), respectively), b IR absorbance spectra of TiO2(R)/VO2(M)/TiO2(A) multilayer film with stearic acid overlayer at various irradiation time under UV light, c CAs of the multilayer film with stearic acid overlayer dependence on irradiation time (the insets are corresponding water droplet shapes on the surface), d variation of absorption spectra of RhB aqueous solution degraded by the multilayer film [11]
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Self-cleaning property of the TiO2(R)/VO2(M)/TiO2(A) multilayer film was evaluated by the decomposition of stearic acid under UV radiation. The degradation of stearic acid was related to the decrease in IR absorption of the C—H stretches, which has been summarized in Fig. 10b. Before UV light irradiation, the characteristic alkyl C—H bond stretching vibrations of CH2 and CH3 groups (3 000–2 800 cm−1) can be distinctly detected. After UV light irradiation of 20 min, the absorbance of C—H bond stretching vibrations decreased drastically, which meant that a considerable proportion of stearic acid was decomposed. The IR absorbance slowly became weak with the increase of irradiation time, and finally almost faded away after 180 min irradiation time. In addition, the degradation of stearic acid also can be confirmed by the changes of the contact angle of the multilayer film. The contact angles of the surface transform from 99.5° (hydrophobic) to 11.5° (hydrophilic) (see Fig. 10c), which can be ascribed to the degradation of stearic acid and the photoinduced hydrophilicity of multilayer film. The photocatalytic activity of TiO2(R)/VO2(M)/TiO2(A) multilayer film also has been demonstrated by the decomposition rate of RhB under UV light irradiation. Figure 10d shows that the absorption spectra of RhB aqueous solution degraded by the multilayer film under UV light irradiation. Thermochromic smart coatings with self-cleaning function have also been achieved by the VO2/SiO2/TiO2 structure where the SiO2 layer act as the ion-barrier interlayer [66]. The proposed VST structure shows a significant degradation rate of stearic acid and is comparable to that of a standard Pilkington Activ glass, which is a commercially available self-cleaning glass that contains a thin TiO2 layer (15 nm) deposited by CVD methods.
For self-cleaning function and improved stability, VO2 thermochromic smart coatings with hydrophobic surface have been favored and studied by researchers. VO2 films with moth-eye nanostructures have been fabricated to enhance the thermochromic properties and the hydrophobic surface (contact angle 120°) can be achieved with additional overcoat [98]. Fused silica substrates with AR patterns of different periods (0, 210, 440, 580 and 1 000 nm) were prepared by reactive ion etching using 2D polystyrene colloidal crystals as a mask. A nipple arrays based on VO2/SiO2 have been realized and the additional fluorooctyltriethoxysilane (FOS) overcoat provides hydrophobicity of the surface (see Fig. 11).
Fig. 11
a SEM cross-sectional profile of the sample with 210 nm period, b top-view SEM image of the sample with 440 nm period, c TEM cross-sectional image to show the thickness of VO2 coatings on SiO2, d planar VO2, 210 nm patterned VO2 with 40 nm thickness, and 210 nm patterned VO2 with FOS overcoat [98]
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The biosafety of VO2 is also under consideration, while the ZnO layer has been used to provide the antibacterial property [99]. ZnO coated VO2 thin films exhibited excellent antibacterial property proved by SEM observation results that ZnO coated samples cause the membrane disruption and cytoplasm leakage of E. coli cells and fluorescence staining results that the amounts of viable bacteria are evidently lower on the surface of ZnO coated films than that of uncoated films (see Fig. 12). The sterilization mechanism of ZnO films is believed to be attributed to the synergistic effect of released zinc ions and ZnO nanoparticles by elaborately designing a verification experiment. More importantly, the ZnO layer with an appropriate thickness can significantly reduce the cytotoxicity of VO2 and thus promote the VO2 biosafety.
Fig. 12
a Proliferation viability of E. coli after culture of 24 h on samples VZ-0, VZ-1, VZ-2 and VZ-3, accompanied by the SEM morphology of E. coli after culture of 24 h on surfaces of b VZ-0, c VZ-1, d VZ-2 and e VZ-3 (the scale bar is 20 μm. The insets show the corresponding partially enlarged SEM images and the scale bar is 1 μm) [99]
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5 Large-scale production of VO2 smart coatings
For commercial applications on building fenestrations in our daily life, large-scale production of VO2 based smart coatings is a great challenge that must be developed. For VO2 based films, magnetron sputtering is the most commonly used method and several works about large-scale production of VO2 based films by magnetron sputtering have been reported. A large-scale TiO2(R)/VO2 (M)/TiO2 (A) multilayer film was prepared on a glass with the area of 400 mm × 400 mm using magnetron sputtering method by Zheng et al. [11], where a combination of energy-saving, antifogging and self-cleaning functions has been achieved (see Fig. 13a). TiO2(R)/VO2 (M)/TiO2 (A) multilayer film was deposited using medium frequency reactive magnetron sputtering (MFRMS, see Fig. 13b) system to sputter planar rectangular metal targets in a suitable atmosphere. The proposed structure shows excellent ability to block out infrared irradiation, which causes a temperature reduction of 12 °C compared with the blank glass (see Fig. 13c).
Fig. 13
a Photograph of large-scale (400 mm × 400 mm) multilayer film at room temperature (the inset is corresponding structure diagram of the multilayer film), b photograph of the magnetron sputtering system, c photographic illustration of the testing system, 1: temperature monitor, 2: temperature probe, 3: infrared lamps, 4: blank glass, 5: glass with TiO2(R)/VO2(M)/TiO2(A) multilayer film, d schematic diagram illustrating the basic components of a magnetron sputtering system [11, 100]
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The magnetron sputtering coating system could be applied in architecture commercial glasses and the designed large area sputtering cathode can make the coating on large area glass substrates. The optimized design and precise manufacturing can guarantee to get a higher vacuum and a shorter cycle time by using a smaller pumping system. Sputtering is a vacuum process used to deposit thin films on substrates. It is performed by applying a high voltage across a low-pressure gas (usually argon) to create a “plasma”, which consists of electrons and gas ions in a high-energy state. During sputtering, energized plasma ions strike the target, which is composed of the desired coating material, and caused atoms from that target to be ejected with enough energy to travel to and bond with the substrate (see Fig. 13d).
In terms of VO2 nanoparticles, which are usually synthesized by solution methods, the first requirement for commercial application is the preparation of VO2 nanoparticles with uniform particle sizes. Then, VO2 nanoparticles would be dissolved in suitable resins and coated on substrates such as glasses and PET. Calculations based on effective medium theory made by Li et al. [80] shows that dilute composites with VO2 nanoparticles embedded in hosts with properties mimicking glass or polymer can yield significantly decreased luminous absorption jointly with much enhanced transmittance modulation of solar energy. These results demonstrate that VO2-based nanothermochromics open new avenues toward energy-efficient fenestration. Polymer hosts such as polyurethane (PU), poly(vinylpyrrolidone) (PVP) and polyamide (PA) have been used. Surface modulation of VO2 nanoparticles will be carried out and the treated nanoparticles will be dispersed in the polymer hosts to form the suspension. The VO2 based coatings are prepared by casting the suspension on the substrates. Preparation procedure for VO2@SiO2 nanoparticles and flexible composite films has been shown in Fig. 14a [29]. Thermal stable PET film covered with VO2 has been stuck on the glass on which has a size of 1.65 m × 1.65 m of a model house (see Fig. 14b) [101‐104]. The demonstration of the VO2 glazing’s application indicated that compared with the use of the ordinary glazing, the use of VO2 glazing could save 10.2%–19.9% cumulative cooling load (see Fig. 14c).
Fig. 14
a Preparation procedure for VO2@SiO2 nanoparticles and flexible composite films, b photograph of a model room with VO2 glazing (1.65 m × 1.65 m), c cooling load comparison during the demonstration of VO2 glazing [29, 101]
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6 Conclusions and prospects
As the most attractive thermochromic technology, VO2-based smart coatings have gained great attention by researchers and many efforts have been made to promote the real commercialization. Methods such as multilayer stacks, composite films, core-shell structures have been carried out to improve thermochromic performance with enhanced luminous transmittance, solar modulation ability and environmental stability. However, more efforts are still needed to make this technology into our daily lives.
(i)
Optical performances of VO2 thermochromic smart coatings can be improved by methods, such as element doping, fabricating multilayer structures, and designing nanostructures. For practical applications, VO2 smart coatings should have 50% luminous transmittance and 15% solar modulation ability for sufficient energy-saving effect. Optical properties of VO2 smart coatings can be further improved by computational calculations and simulations for better luminous transmittance and solar modulation ability.
(ii)
Environmental stability of VO2 coatings is a great challenge for long-time use. Protective layers for VO2 films as well as core-shell structures for VO2 nanoparticles can effectively improve the environmental stability of VO2 coatings. Future work can be carried out by choosing materials with versatility for protective, antireflection and self-cleaning functions.
(iii)
Large-scale production of VO2 smart coatings is necessary to turn this technology from the lab into the industrial and commercial application. Traditional methods, such as hydrothermal synthesis, spray pyrolysis and sol-gel, etc., are limited due to their low production and complicated process. An effective way to solve this problem is fabricating VO2 based smart coatings during the production of glasses, just like the deposition of low-emissivity (low-E) coatings on the glass production lines.
Acknowledgements
This study was financially supported by the National Natural Science Foundation of China (Grant No. 51572284) and the “Youth Innovation Promotion Association, Chinese Academy of Sciences”.
Open AccessThis article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.
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