Energy consumption has become an urgent issue not only for the global environment, but also for people’s lives. Among total energy consumption, buildings take nearly 40%. For buildings, energy exchange through windows accounts for over 50% by means of conduction, convection, and radiation. To reduce energy consumption, new structures should be developed for glass surfaces to enhance their thermal insulation properties. Vanadium dioxide (VO2) is the most well-known thermochromic material, which exhibits a notable optical change from transparent to reflecting in the infrared upon a semiconductor-to-metal phase-transition. In this chapter, we provide a comprehensive summary of advances on the VO2-based thermochromic coatings. Although the research on VO2 smart window has been carried on for several decades, the real commercial use of it has not yet been achieved. The hindrance factors against commercial use are conventionally known as the unsatisfactory intrinsic properties of VO2 material and have recently emerged as new challenges.
- solar modulation
- vanadium dioxide
- optical design
- multilayer structures
Nowadays, for environmental deterioration and energy shortage in modern human society, people are paying more attention to finding 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 owing to the use of lighting, air-conditioning, and heating [1, 2, 3, 4, 5]. Energy exchange through windows accounts for over 50% of energy consumed through a building’s envelope by means of conduction, convection and radiation, as shown in Figure 1(a). Therefore, energy saving of windows contributes the critical and important roles in building energy-efficient projects. Managing heat exchange through windows is a feasible approach to reduce the building energy consumptions. In summer, solar radiation entering buildings should be controlled to reduce the air-conditioning energy consumption. On the contrary, thermal radiation from the buildings must be limited to consume lesser energy for heating.
An effective route to achieve this goal would be using smart coatings on building windows to control the solar radiation. Therefore, smart coatings based on electrochromism [6, 7, 8, 9, 10], thermochromism [11, 12, 13, 14, 15, 16, 17, 18, 19], gasochromism [20, 21, 22] and photochromism [23, 24, 25, 26] have been widely investigated for energy-efficient coatings. Thermochromic-coated window can modulate near-infrared radiation (NIR) 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. It has two states: a transparent state with a higher solar transmittance and an opaque state with a lower solar transmittance. The thermochromic window [27, 28, 29], whose transition depends on the temperature, is widely investigated type of chromogenic window.
Vanadium dioxide (VO2) is one of the most promising thermochromic materials, which has been widely studied. VO2 exhibits an automatic reversible semiconductor–metal phase transition (SMT) at a critical transition temperature (
VO2 smart coatings are usually used in two forms including flexible foils based on VO2 nanoparticles [34, 46, 47, 48, 49, 50, 51, 52] and VO2-based multilayer films [11, 12, 33, 53, 54, 55]. However, for commercial application as smart coatings on windows, there are still many obstacles severely limiting the relative applicability of VO2 smart coatings. (I) The phase-transition temperature (
These obstacles have to 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 temperature of VO2: cations larger than V4+, such as W6+ , Mo6+  and Nb5+ , and anions larger than O2―, such as F― , have been utilized to reduce the
In this chapter, we will review 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 (and) of VO2 coatings have been introduced as well as the balance between and(Section 2). Then, methods to improve the durability of VO2 coatings, including protective layers for multilayer films, 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.
2. Improvements of optical properties of VO2
Luminous transmittances () and solar modulation ability () are the most important indexes of thermochromic properties for VO2 smart coatings. The integral luminous transmittances () and solar transmittances () of the samples can be obtained by the following equations:
where represents the transmittance at wavelength; is the standard efficiency function for photopic vision; and is the solar irradiance spectrum for an air mass of 1.5, which corresponds to the sun standing 37° above the horizon. The solar modulation ability () of the films was calculated by, where
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 . 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 luminous transmittance () of 30.2% and 25.1% for semiconducting and metallic VO2 (see Figure 2(a)). As for solar modulation ability, the majority of reported modulation abilities are less than 10%, which are not efficient enough for energy-saving function [64, 65, 66, 67]. For VO2 coatings before and after the phase-transition, the contrast of relative optical transmittance is mainly in the near-infrared region (780–2500 nm), which only account for 43%.
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, 55, 68]. 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 has been shown in Figure 3 with three typical structures for VO2 thin films and relative SEM images.
An effective way to improve the luminous transmittance of VO2 coatings is to employ an antireflection (AR) layer, such as SiO2 [69, 70, 71, 72], TiO2 , ZrO2 , etc. Lee et al. [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  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 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
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 SiN
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 . 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 Figure 4(a), (b)). Different crystallization of VO2 films can be obtained by introducing Cr2O3 layers with various thicknesses at a competitive temperature range from 250 to 350°C, where different thermochromic performance can be obtained (see Figure 4(c)). The Cr2O3/VO2 bilayer film deposited 350°C with optimal thickness shows an excellent = 12.2% with an enhanced = 46.0% (see Figure 4(d)), while the value of and 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 arose from the structural template effect and antireflection function of Cr2O3 to VO2.
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 , and a maximum increase in by 86% (from 30.9 to 57.6%) has been obtained, which is better than the sample with single-layer antireflection (49.1%) .The same structure of TiO2/VO2/TiO2 has also been investigated by Zheng et al.  and Sun et al.  for improved thermochromic performance and skin comfort design. A novel sandwich structure of VO2/SiO2/TiO2 has been described by Powell et al. , where the SiO2 layers act as ion-barrier interlayers to prevent diffusion of Ti ions into the VO2 lattice. The best-performing multilayer film obtained in this work showed 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 around 18% for both semiconducting and metallic states.
A novel Cr2O3/VO2/SiO2 (CVS) sandwich structure has been proposed and fabricated based on optical design and calculations . 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 Figure 5(a) (three-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 is about 44.0% at approximately 40 and 90 nm of Cr2O3 and SiO2, respectively. In this work, the proposed CVS multilayer thermochromic film shows an ultrahigh = 16.1% with an excellent = 54.0%, which gives a commendable balance between and (see Figure 5(b), (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 Figure 5(d)).
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 . 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 SiN
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 (780–2500 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 . 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 of VO2 by mixing with specific materials, which shows a robust contrast in the visible light region in different temperatures [48, 78]. However, more investigations are required for a facile and low-cost 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 . 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.
To prevent VO2 films from degradation, introduction of protective layers above VO2 is an effective way that has been widely used. Chemically stable oxide films such as Al2O3 [56, 79], CeO2 [80, 81], WO3 , 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 , 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 , 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 1 week (168 h), which is far from the request for practical applications.
Long et al.  proposed a novel sandwich structure of WO3/VO2/WO3, where WO3 not only functions as an AR layer to enhance the luminous transmittance () 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’s 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 protection is provided by WO3, the solar modulation ability of the sample is weakly reduced due to the diffusion of W6+ to VO2.
In the works above, the protective layers are usually single-layer films. To enhance the durability of thermochromic VO2 films, bilayer coatings such as VO2/TiO2/ZnO, VO2/SiO2/ZnO, and VO2/SiO2 /TiO2 have been studied . In this study, VO2 films with TiO2/ZnO protective coatings have demonstrated higher antioxidant activity under aging tests, which can be attributed to the different oxygen permeability through different inorganic films . Zhan et al.  fabricated a complicated multilayer structure of SiN
The Cr2O3/VO2/SiO2 structure proposed by our lab shows robust environmental stability for long-time use . 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 Figure 6(a), (b)). Hydrophilicity of the single-layer VO2 indicates contact with water, which will accelerate the degradation process of relative thermochromic performance. On the contrary, the hydrophobicity 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 Figure 6(b)). 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 Figure 6(c), (d)).
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. However, TiO2 (R) films are less efficient photocatalysts than anatase TiO2 (TiO2 (A)) films, which occupy an important position in the studies of photocatalytic active materials. Zheng et al.  constructed a TiO2(R)/VO2(M)/TiO2(A) multilayer film, while the photocatalytic and photo-induced hydrophilic properties from the top TiO2(A) layer were studied for self-cleaning effects (see Figure 7(a)).
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 Figure 7(b). Before UV light irradiation, the characteristic alkyl C—H bond stretching vibrations of CH2 and CH3 groups (3000–2800 cm−1) can be distinctly detected. After UV light irradiation of 20 min, the absorbance of C—H bond stretching vibrations decreased drastically, which means 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 Figure 7(c)), 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 7(d) 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 acts as the ion-barrier interlayer . 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, which 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 . Fused silica substrates with AR patterns of different periods (0, 210, 440, 580, and 1000 nm) were prepared by reactive ion etching using 2D polystyrene colloidal crystals as a mask. Nipple arrays based on VO2/SiO2 have been realized and the additional fluorooctyl triethoxysilane (FOS) overcoat provides hydrophobicity of the surface (see Figure 8).
The biosafety of VO2 is also under consideration, while the ZnO layer has been used to provide the antibacterial property . 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
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 × 400 mm2 using magnetron sputtering method by Zheng et al. , where a combination of energy-saving, antifogging, and self-cleaning functions has been achieved (see Figure 10(a)). TiO2(R)/VO2 (M)/TiO2 (A) multilayer film was deposited using medium frequency reactive magnetron sputtering (MFRMS, see Figure 10(b)) 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 Figure 10(c)).
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 causes atoms from that target to be ejected with enough energy to travel to and bond with the substrate (see Figure 10(d)).
6. Conclusion 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. Method of multilayer structures has 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.
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-based 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.
Environmental stability of VO2 coatings is a great challenge for long-time use. Protective layers for VO2 films 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.
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.
We want to thank Tianci Chang for discussion about the VO2 materials. This chapter was financially supported by the National Natural Science Foundation of China (Grant No. 51572284) and the “Youth Innovation Promotion Association, Chinese Academy of Sciences” (Grant No.2018288).
Conflict of interest
There is no conflict of interest to declare.