**4. Multifunctional design and construction**

Long et al. [66] proposed a novel sandwich structure of WO3

as a good protective layer for thermochromic VO2

.

/TiO2

**Figure 6.** Images of contact angle measurement of (a) the single-layer VO2

, and Cr2 O3 /VO2 /SiO2

, Cr2 O3 /VO2

Variation curves of Δ*T*sol for VO2

cycles (d).

single-layer VO2

/VO2

12 Emerging Solar Energy Materials

diffusion of W6+ to VO2

provided by WO3

ZnO, and VO2

/WO3

durability of thermochromic VO2

/SiO2

of WO3

functions as an AR layer to enhance the luminous transmittance (*T*lum) of VO2

in a constant-temperature humid environment with 90% relative humidity at 60°C. For the

environment. On the contrary, there shows almost no change in the optical transmittance

In the works above, the protective layers are usually single-layer films. To enhance the

protective coatings have demonstrated higher antioxidant activity under aging tests, which can be attributed to the different oxygen permeability through different inorganic films [83].

have been studied [82]. In this study, VO2

, the thermochromism nearly vanishes after 20 day's treatment in the tough

multilayer films with the same treatment. However, though protection is

, the solar modulation ability of the sample is weakly reduced due to the

films, bilayer coatings such as VO<sup>2</sup>

/VO2

/WO3

. The stability of samples was investigated

/TiO2

and (b) the proposed Cr2

O3 /VO2 /SiO2

with different duration time (c) and different fatigue

structure.

/ZnO, VO2

films with TiO<sup>2</sup>

, where WO3

not only

/SiO2 /

/ZnO

but also performs

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 TiO<sup>2</sup> : 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, TiO<sup>2</sup> (R) films are acted as buffer layer and growth template of VO<sup>2</sup> (M) films. However, TiO2 (R) films are less efficient photocatalysts than anatase TiO<sup>2</sup> (TiO2 (A)) films, which occupy an important position in the studies of photocatalytic active materials. Zheng et al. [11] constructed a TiO2 (R)/VO2 (M)/TiO<sup>2</sup> (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)/TiO<sup>2</sup> (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

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

Solar Modulation Utilizing VO2-Based Thermochromic Coatings for Energy-Saving Applications

**Figure 8.** (a) SEM cross-sectional profile of the sample with 210 nm period, (b) top-view SEM image of the sample with

**Figure 9.** (a) Proliferation viability of *Escherichia 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).

coatings on SiO2

with fluorooctyl triethoxysilane (FOS) overcoat.

, (d) planar VO2

, 210 nm

thermochromic smart coatings with

http://dx.doi.org/10.5772/intechopen.75584

films with moth-eye

15

layer (15 nm) deposited by CVD methods.

For self-cleaning function and improved stability, VO2

440 nm period, (c) TEM cross-sectional image to show the thickness of VO<sup>2</sup>

with 40 nm thickness, and 210 nm patterned VO<sup>2</sup>

patterned VO<sup>2</sup>

hydrophobic surface have been favored and studied by researchers. VO2

**Figure 7.** (a) FESEM image of a fractured cross section of the multilayer film (the insets are surface morphology of VO2 (M) (left) and TiO<sup>2</sup> (A) layers (right), respectively), (b) IR absorbance spectra of TiO2 (R)/VO2 (M)/TiO<sup>2</sup> (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 correspond to water droplet shapes on the surface), (d) variation of absorption spectra of RhB aqueous solution degraded by the multilayer film.

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 TiO<sup>2</sup> (R)/VO2 (M)/TiO<sup>2</sup> (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 [68]. 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

**Figure 8.** (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 VO<sup>2</sup> coatings on SiO2 , (d) planar VO2 , 210 nm patterned VO<sup>2</sup> with 40 nm thickness, and 210 nm patterned VO<sup>2</sup> with fluorooctyl triethoxysilane (FOS) overcoat.

vibrations of CH2

(M) (left) and TiO<sup>2</sup>

14 Emerging Solar Energy Materials

VO2

the SiO2

and CH3

absorption spectra of RhB aqueous solution degraded by the multilayer film.

groups (3000–2800 cm−1) can be distinctly detected. After UV

(R)/VO2

/SiO2

/TiO2

(R)/VO2

(M)/TiO<sup>2</sup>

(A) multilayer

(M)/TiO<sup>2</sup>

structure where

(A)

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

**Figure 7.** (a) FESEM image of a fractured cross section of the multilayer film (the insets are surface morphology of

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 correspond to water droplet shapes on the surface), (d) variation of

(A) layers (right), respectively), (b) IR absorbance spectra of TiO2

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

layer acts as the ion-barrier interlayer [68]. The proposed VST structure shows a

hydrophilicity of multilayer film. The photocatalytic activity of TiO<sup>2</sup>

with self-cleaning function have also been achieved by the VO2

**Figure 9.** (a) Proliferation viability of *Escherichia 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).

nanostructures have been fabricated to enhance the thermochromic properties, and the hydrophobic surface (contact angle 120°) can be achieved with additional overcoat [85]. 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 [86]. 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 **Figure 9**). 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 VO<sup>2</sup> and thus promote the VO2 biosafety.

#### **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)/TiO<sup>2</sup> (A) multilayer film was prepared on a glass with the area of 400 × 400 mm2 using magnetron sputtering method by Zheng et al. [11], where a combination of energy-saving, antifogging, and self-cleaning functions has been achieved (see **Figure 10(a)**). TiO2 (R)/VO2 (M)/TiO<sup>2</sup> (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)**).

**6. Conclusion and prospects**

**I.** Optical performances of VO2

**II.** Environmental stability of VO2

cal properties of VO2

tures. For practical applications, VO2

lives.

VO2

(M)/TiO<sup>2</sup>

As the most attractive thermochromic technology, VO<sup>2</sup>

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

**Figure 10.** (a) Photograph of large-scale (400 × 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

(a) multilayer film, (d) schematic diagram illustrating the basic components of a magnetron sputtering system.

Solar Modulation Utilizing VO2-Based Thermochromic Coatings for Energy-Saving Applications

ods, such as element doping, fabricating multilayer structures, and designing nanostruc-

transmittance and 15% solar modulation ability for sufficient energy-saving effect. Opti-

tions and simulations for better luminous transmittance and solar modulation ability.

layers for VO2 films can effectively improve the environmental stability of VO2


http://dx.doi.org/10.5772/intechopen.75584

coatings.

(R)/

17

thermochromic smart coatings can be improved by meth-

coatings is a great challenge for long-time use. Protective

smart coatings can be further improved by computational calcula-


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)**).

Solar Modulation Utilizing VO2-Based Thermochromic Coatings for Energy-Saving Applications http://dx.doi.org/10.5772/intechopen.75584 17

**Figure 10.** (a) Photograph of large-scale (400 × 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)/TiO<sup>2</sup> (a) multilayer film, (d) schematic diagram illustrating the basic components of a magnetron sputtering system.
