**2. Improvements of optical properties of VO2**

Luminous transmittances (*T*lum) and solar modulation ability (Δ*<sup>T</sup>*sol) are the most important indexes of thermochromic properties for VO2 smart coatings. The integral luminous transmittances (*T*lum) and solar transmittances (*T*sol) of the samples can be obtained by the following equations:

$$T\_{\rm lum,sol} = \int \mathbb{O}\_{\rm lum,sol}(\lambda) \mathrm{T}(\lambda) \mathrm{d}\lambda \Big/ \int \mathbb{O}\_{\rm lum,sol}(\lambda) \mathrm{d}\lambda \tag{1}$$

**2.1. Strategies for enhanced luminous transmittance and solar modulation ability**

[69–72], TiO2

films. However, the luminous transmittance is still not sufficient. TiO<sup>2</sup>

The optical calculation was performed upon a basic structure of a VO2

than the reported SiO2


Thermochromic smart coatings incorporating VO2

thin films and relative SEM images.

films [73] because TiO2

*n* value changes with the thickness of VO2

**Figure 3.** Schematic illustration of VO2

(d)–(f) corresponding to figures. (a)–(c) [12, 33, 72], respectively.

from 32 (without AR coating) to 55% for 50 nm VO2

reflection (AR) layer, such as SiO<sup>2</sup>

reflection material for VO<sup>2</sup>

An effective way to improve the luminous transmittance of VO<sup>2</sup>

tion ability of VO2

for VO2

that SiO2

VO2

/ZrO2

tor phase of VO2

layer for VO2

Many efforts have been made to improve the luminous transmittance and solar modula-

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

of multilayer structures is an effective way to improve the optical properties [11, 55, 68].

ricated 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

[73], ZrO2

antireflection layer successfully increased the luminous transmittance of the VO<sup>2</sup>

, and an improvement from 32.3 to 50.5% in *T*lum was confirmed for the semiconduc-

fabricated and demonstrated the highest *T*lum improvement among the reported at that time.

of refractive index *n* and thickness *d* [74]. Optimization was carried out on *n* and *d* for a maximum integrated luminous transmittance (*T*lum). The calculation demonstrates that the optimal

antireflection layer and buffer, respectively. Relative SEM images of three typical structures have been shown in figures.

, which was in good agreement with the calculations.

films fabricated by deposition, the design

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

7

films with additional layers have been fab-

coatings is to employ an anti-

was selected as AR

structure has been

layer with an AR layer

[74], etc. Lee et al. [70, 71] reported

/TiO2

. They deposited an optimized structure of

has a higher refractive index and is a more effective anti-

, and at *n* ≈ 2.2 it gives the highest *T*lum enhancement


. The optimized VO2

where *<sup>T</sup>* (*λ*) represents the transmittance at wavelength *<sup>λ</sup>*; *Φ*lum is the standard efficiency function for photopic vision; and *Φ*sol 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 (Δ*<sup>T</sup>*sol) of the films was calculated by <sup>Δ</sup> *<sup>T</sup>*sol <sup>=</sup> *<sup>T</sup>*sol, lt <sup>−</sup> *<sup>T</sup>*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 [63]. The luminous transmittance of VO<sup>2</sup> 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 (*T*lum) 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 [64–67]. 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%.

**Figure 2.** (a) Calculated luminous transmittance for single-layer VO<sup>2</sup> films with various thickness at semiconducting state (black line) and metallic state (red line) and (b) the solar spectrum and relative energy distribution.

#### **2.1. Strategies for enhanced luminous transmittance and solar modulation ability**

been discussed in Section 4. Recent progress for large-scale production of VO2

Luminous transmittances (*T*lum) and solar modulation ability (Δ*<sup>T</sup>*sol) are the most important

mittances (*T*lum) and solar transmittances (*T*sol) of the samples can be obtained by the following

*T*lum, sol = ∫Φlum, sol(*λ*)T(λ)dλ∕∫Φlum, sol(λ)dλ (1)

where *<sup>T</sup>* (*λ*) represents the transmittance at wavelength *<sup>λ</sup>*; *Φ*lum is the standard efficiency function for photopic vision; and *Φ*sol 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 (Δ*<sup>T</sup>*sol) of the films was calculated by <sup>Δ</sup> *<sup>T</sup>*sol <sup>=</sup> *<sup>T</sup>*sol, lt <sup>−</sup> *<sup>T</sup>*sol, ht, where *lt* and *ht* represent low temperature and

 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

grated luminous transmittance (*T*lum) 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 [64–67]. 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%.

state (black line) and metallic state (red line) and (b) the solar spectrum and relative energy distribution.

has been surveyed in Section 5. Finally, future development trends of VO2

**2. Improvements of optical properties of VO2**

indexes of thermochromic properties for VO2

high temperature, respectively.

[63]. The luminous transmittance of VO<sup>2</sup>

Based on optical calculation, a single layer VO2

**Figure 2.** (a) Calculated luminous transmittance for single-layer VO<sup>2</sup>

equations:

6 Emerging Solar Energy Materials

VO2

pected for large-scale production as practical and commercial applications.

smart coatings

coatings have pros-

smart coatings. The integral luminous trans-

coatings is largely dependent on relative thicknesses.

film (80 nm), for example, exhibits an inte-

films with various thickness at semiconducting

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 VO<sup>2</sup> coatings is to employ an antireflection (AR) layer, such as SiO<sup>2</sup> [69–72], TiO2 [73], ZrO2 [74], etc. Lee et al. [70, 71] reported that SiO2 antireflection layer successfully increased the luminous transmittance of the VO<sup>2</sup> films. However, the luminous transmittance is still not sufficient. TiO<sup>2</sup> was selected as AR layer for VO2 films [73] because TiO2 has a higher refractive index and is a more effective antireflection material for VO<sup>2</sup> than the reported SiO2 . The optimized VO2 /TiO2 structure has been fabricated and demonstrated the highest *T*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 luminous transmittance (*T*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*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*lum was confirmed for the semiconductor phase of VO2 , which was in good agreement with the calculations.

**Figure 3.** Schematic illustration of VO2 -based films with (a) antireflection layer, (b) buffer layer, and (c) both of antireflection layer and buffer, respectively. Relative SEM images of three typical structures have been shown in figures. (d)–(f) corresponding to figures. (a)–(c) [12, 33, 72], respectively.

Besides the antireflection layers on the top of VO<sup>2</sup> films, buffer layers between the substrates and VO2 films also play important roles in the optical performances of integrated coatings. Some buffer layers as SiO<sup>2</sup> , TiO2 , SnO2 , ZnO, CeO2 , and SiN*<sup>x</sup>* have been investigated in reported work [64, 75–77]. Nevertheless, thermochromic performances of VO2 coatings obtained based on above buffer layers were fair, which still cannot match the requirements for practical applications.

In our recent work, Cr2 O3 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 VO<sup>2</sup> thin films. Refractive index of Cr<sup>2</sup> O3 is between the glass and the VO2 , which is considered to enhance the luminous transmittance. Meanwhile, Cr<sup>2</sup> O3 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 VO<sup>2</sup> thin films (see **Figure 4(a)**, **(b)**). Different crystallization of VO<sup>2</sup> films can be obtained by introducing Cr2 O3 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 Cr2 O3 /VO2 bilayer film deposited 350°C with optimal thickness shows an excellent Δ*T*sol = 12.2% with an enhanced *T*lum, lt = 46.0% (see **Figure 4(d)**), while the value of Δ*<sup>T</sup>*sol and *<sup>T</sup>*lum, lt for the single-layer VO2 film deposited high temperature at 450°C is 7.8 and 36.4%, respectively. The Cr<sup>2</sup> O3 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 Cr<sup>2</sup> O3 to VO2 .

For better thermochromic performance, sandwich structures based on VO<sup>2</sup> films have been fabricated. Double-layer antireflection incorporating TiO<sup>2</sup> and VO2 (TiO2 /VO2 /TiO2 ) has been proposed [63], and a maximum increase in *T*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. [38] for improved thermochromic performance and skin comfort design. A novel sandwich structure of VO2 /SiO2 /TiO2 has been described by Powell et al. [68], where the SiO2 layers act as ion-barrier interlayers to prevent diffusion of Ti ions into the VO<sup>2</sup> 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.

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

, monoclinic VO2

bilayer thermochromic film, (c) variation curve of *T*lum, lt, *T*lum, ht and Δ*T*sol for VO2

peaks are observed in the luminous transmittance simulations, which can be attributed to the interference effect of the multilayer structure. The highest value of *<sup>T</sup>* lum, lt is about 44.0% at approx-

thermochromic film shows an ultrahigh Δ*<sup>T</sup>* sol = 16.1% with an excellent *<sup>T</sup>* lum, lt = 54.0%, which gives a commendable balance between Δ*<sup>T</sup>* sol and *<sup>T</sup>* lum, lt (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 modu-

There is some work focus on multilayer films with more layers for enhanced thermochromic

studied [52]. A featured wave-like optical transmittance curve has been measured by the fivelayer coating companying an improved luminous transmittance (45.0% at semiconducting

were investigated for optimized optical properties. Four clear

, and rutile VO2

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

structural template layer at 350°C and standard solar spectra [12].

structural template layer at different temperatures, (d) transmittance spectra (250–2600 nm) at 25 and 90°C

, respectively. In this work, the proposed CVS multilayer

/VO2

/TiO2

multilayer films (see **Figure 5(d)**).

, respectively, (b) schematic illustration

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

9

films deposited with

/VO2

/TiO2

has been

thicknesses of Cr2

O3

of Cr2 O3 /VO2

40 nm Cr2

for VO2

imately 40 and 90 nm of Cr2

O3

**Figure 4.** (a) Crystal structure of hexagonal Cr2

films deposited with 40 nm Cr<sup>2</sup>

and SiO2

O3

lation ability and the luminous transmittance to reported VO<sup>2</sup>

performances. A five-layer thermochromic coating based on TiO<sup>2</sup>

and SiO2

O3

O3

A novel Cr2 O3 /VO2 /SiO2 (CVS) sandwich structure has been proposed and fabricated based on optical design and calculations [33]. The bottom Cr<sup>2</sup> O3 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 Solar Modulation Utilizing VO2-Based Thermochromic Coatings for Energy-Saving Applications http://dx.doi.org/10.5772/intechopen.75584 9

Besides the antireflection layers on the top of VO<sup>2</sup>

O3

enhance the luminous transmittance. Meanwhile, Cr<sup>2</sup>

(see **Figure 4(a)**, **(b)**). Different crystallization of VO<sup>2</sup>

O3

 to VO2 .

fabricated. Double-layer antireflection incorporating TiO<sup>2</sup>

rier interlayers to prevent diffusion of Ti ions into the VO<sup>2</sup>

was very close to the maximum possible solar modulation for VO2

, TiO2

O3

, SnO2

in reported work [64, 75–77]. Nevertheless, thermochromic performances of VO2

obtained based on above buffer layers were fair, which still cannot match the requirements

 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 VO<sup>2</sup>

(R), which can act as the structural template layer to lower the lattice mismatch between

layers with various thicknesses at a competitive temperature range from 250 to 350°C,

bilayer film deposited 350°C with optimal thickness shows an excellent Δ*T*sol = 12.2% with an enhanced *T*lum, lt = 46.0% (see **Figure 4(d)**), while the value of Δ*<sup>T</sup>*sol and *<sup>T</sup>*lum, lt for the single-layer

tion 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 antire-

proposed [63], and a maximum increase in *T*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 struc-

improved thermochromic performance and skin comfort design. A novel sandwich structure

tilayer film obtained in this work showed excellent solar modulation ability (15.29%), which

the corresponding luminous transmittance is weak around 18% for both semiconducting and

phase transition to rutile (R) phase at high temperature for solar modulation. The top SiO2

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

has been described by Powell et al. [68], where the SiO2

thin films and glass substrates and to reduce the deposition temperature of VO<sup>2</sup>

where different thermochromic performance can be obtained (see **Figure 4(c)**). The Cr2

film deposited high temperature at 450°C is 7.8 and 36.4%, respectively. The Cr<sup>2</sup>

For better thermochromic performance, sandwich structures based on VO<sup>2</sup>

is between the glass and the VO2

O3

ings. Some buffer layers as SiO<sup>2</sup>

thin films. Refractive index of Cr<sup>2</sup>

for practical applications.

In our recent work, Cr2

8 Emerging Solar Energy Materials

flection function of Cr<sup>2</sup>

/VO2

/TiO2

O3 /VO2

improving the crystallinity of VO2

/TiO2

/SiO2

optical design and calculations [33]. The bottom Cr<sup>2</sup>

ture of TiO2

/SiO2

metallic states.

Then, the VO2

A novel Cr2

of VO2

and VO2

VO2

VO2

VO2

Cr2 O3

VO2

films, buffer layers between the substrates

have been investigated

, which is considered to

has similar lattice parameters with

films can be obtained by introducing

coatings

thin films

O3 /VO2

O3 inser-

films have been

) has been

layer

, and SiN*<sup>x</sup>*

has been selected to act as a structural template for the growth of

and VO2

has also been investigated by Zheng et al. [11] and Sun et al. [38] for

(CVS) sandwich structure has been proposed and fabricated based on

and increasing the luminous transmittance of the structure.

O3

layer with a monoclinic (M) phase at low temperature undergoes a reversible

(TiO2

/VO2

lattice. The best-performing mul-

layer provides a structural template for

/TiO2

layers act as ion-bar-

thin films. Unfortunately,

films also play important roles in the optical performances of integrated coat-

, ZnO, CeO2

**Figure 4.** (a) Crystal structure of hexagonal Cr2 O3 , monoclinic VO2 , and rutile VO2 , respectively, (b) schematic illustration of Cr2 O3 /VO2 bilayer thermochromic film, (c) variation curve of *T*lum, lt, *T*lum, ht and Δ*T*sol for VO2 films deposited with 40 nm Cr2 O3 structural template layer at different temperatures, (d) transmittance spectra (250–2600 nm) at 25 and 90°C for VO2 films deposited with 40 nm Cr<sup>2</sup> O3 structural template layer at 350°C and standard solar spectra [12].

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 Cr2 O3 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 *<sup>T</sup>* lum, lt is about 44.0% at approximately 40 and 90 nm of Cr2 O3 and SiO2 , respectively. In this work, the proposed CVS multilayer thermochromic film shows an ultrahigh Δ*<sup>T</sup>* sol = 16.1% with an excellent *<sup>T</sup>* lum, lt = 54.0%, which gives a commendable balance between Δ*<sup>T</sup>* sol and *<sup>T</sup>* lum, lt (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 VO<sup>2</sup> 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 TiO<sup>2</sup> /VO2 /TiO2 /VO2 /TiO2 has been studied [52]. A featured wave-like optical transmittance curve has been measured by the fivelayer coating companying an improved luminous transmittance (45.0% at semiconducting

(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

solar modulation ability can be robustly enhanced due to the contribution from the visible

ing 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 Δ*T*sol

light region. That means that the transmittance in the visible light region for VO<sup>2</sup>


In previous work, researchers usually focus on the thermochromic properties of VO2

tivalent element and there are several kinds of vanadium oxide, such as VO, V2

O5

effective way that has been widely used. Chemically stable oxide films such as Al<sup>2</sup>

Al oxide is a typical material that has been investigated as a protection layer for VO2

in the damp environment, which can be attributed to the corrosion of water to Al<sup>2</sup>

ings. In work reported by Ji [56], different thicknesses of Al oxide protective layers have been

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

thy to mention that in above cases, the test period of the samples is less than 1 week (168 h),

[66], etc. have been studied to keep VO2

the luminous transmittances and solar modulation ability. However, environmental stability is

near the room temperature. Therefore, how to maintain the thermochromic performance of

in air. It should be noted that the selected materials to be used as protective layers might

films from degradation, introduction of protective layers above VO<sup>2</sup>

by DC magnetron sputtering. The durability of the samples was evaluated

in dry air at 300°C and humid environment. The similar structure was also

coatings during a long-time period is an inevitable problem that must be overcome.

**3. Methods to improve the stability of VO2**

. Among them, V2

will gradually transform into the intermediate phases of V6

O3

films can protect the VO<sup>2</sup>

which is far from the request for practical applications.

, V2 O5


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

 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

 **for long-time use**

is the most thermodynamically stable phase and VO2

O7

O13 and V3

does not possess thermochromic optical change properties

, where dual enhancement in the optical properties and the

protective layers were fabricated by atomic layer deposition

from oxidation in the heating test but not sufficient

coatings from lab to industrial production. Vanadium is a mul-

smart coat-

11

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

to improve

O5

is an

coat-

[56, 79],

O3 , VO2 , V6 O13,

and finally into V<sup>2</sup>

O3

O3

. It is wor-

away from oxidant like water and

for VO2

of VO2

V4 O9 , V3 O7

VO2

CeO2

O2

To prevent VO2

modulation ability of VO2

another great challenge for VO2

, and V2

[57]. However, unlike VO2

[80, 81], WO3

stability is preferred.

deposited for VO2

process of VO2

(ALD). The Al2

affect the optical properties of VO<sup>2</sup>

investigated [79], while the Al2

O3

O5

**Figure 5.** (a) 3D surface image of the luminous transmittance (*T*lum, lt) calculation of the Cr2 O3 /VO2 (80 nm)/SiO2 multilayer structure on the thickness design of Cr2 O3 (bottom layer) and SiO<sup>2</sup> (top layer), (b) transmittance spectra (350–2600 nm) at 25 (solid lines) and 90°C (dashed lines) for the CVS structures with various thickness of SiO2 layers, (c) corresponding variation curves of *T*lum, lt, *T*lum, ht, and Δ*T*sol for (b), (d) comparison of this work with recently reported VO2 -based thermochromic films.

state) and a competitive solar modulation ability (12.1%). Multilayer structure like SiN*<sup>x</sup>* / NiCrO*<sup>x</sup>* /SiN*<sup>x</sup>* /VO*<sup>x</sup>* /SiN*<sup>x</sup>* /NiCrO*<sup>x</sup>* /SiN*<sup>x</sup>* 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.

#### **2.2. Balance between luminous transmittances and solar modulation ability**

Regarding practical application of VO2 -based thermochromic smart coatings, high solar modulation ability (Δ*T*sol) accompanied by high luminous transmittance (*T*lum) is required. Nevertheless, we can find that it is tough to make a good balance between luminous transmittance (*T*lum) and solar modulation ability (Δ*<sup>T</sup>*sol). A unilateral pursuit of distinguished solar modulation ability or ultrahigh luminous transmittances is meaningless.

Most work on VO<sup>2</sup> -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 [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 VO<sup>2</sup> 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 Δ*T*sol 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, V2 O3 , VO2 , V6 O13, V4 O9 , V3 O7 , and V2 O5 . Among them, V2 O5 is the most thermodynamically stable phase and VO2 will gradually transform into the intermediate phases of V6 O13 and V3 O7 and finally into V<sup>2</sup> O5 [57]. However, unlike VO2 , V2 O5 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 VO<sup>2</sup> is an effective way that has been widely used. Chemically stable oxide films such as Al<sup>2</sup> O3 [56, 79], CeO2 [80, 81], WO3 [66], 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 VO<sup>2</sup> , where dual enhancement in the optical properties and the stability is preferred.

state) and a competitive solar modulation ability (12.1%). Multilayer structure like SiN*<sup>x</sup>*

(bottom layer) and SiO<sup>2</sup>

variation curves of *T*lum, lt, *T*lum, ht, and Δ*T*sol for (b), (d) comparison of this work with recently reported VO2

the luminous transmittance (32.7%) and the complicated structure pose an enormous obstacle

modulation ability (Δ*T*sol) accompanied by high luminous transmittance (*T*lum) is required. Nevertheless, we can find that it is tough to make a good balance between luminous transmittance (*T*lum) and solar modulation ability (Δ*<sup>T</sup>*sol). A unilateral pursuit of distinguished solar

near-infrared region (780–2500 nm), while inconspicuous contrast in the visible light region


exhibits superior solar modulation ability of 18.0%, but

O3 /VO2

(top layer), (b) transmittance spectra (350–2600 nm)

(80 nm)/SiO2

layers, (c) corresponding


NiCrO*<sup>x</sup>*

/SiN*<sup>x</sup>*

thermochromic films.

10 Emerging Solar Energy Materials

Most work on VO<sup>2</sup>

/VO*<sup>x</sup>*

structure on the thickness design of Cr2

/SiN*<sup>x</sup>*

for practical application of this structure.

Regarding practical application of VO2

/NiCrO*<sup>x</sup>*

/SiN*<sup>x</sup>*

**Figure 5.** (a) 3D surface image of the luminous transmittance (*T*lum, lt) calculation of the Cr2

at 25 (solid lines) and 90°C (dashed lines) for the CVS structures with various thickness of SiO2

O3

**2.2. Balance between luminous transmittances and solar modulation ability**

modulation ability or ultrahigh luminous transmittances is meaningless.

/

multilayer


Al oxide is a typical material that has been investigated as a protection layer for VO2 coatings. In work reported by Ji [56], 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 [79], while the Al2 O3 protective layers were fabricated by atomic layer deposition (ALD). The Al2 O3 films can protect the VO<sup>2</sup> from oxidation in the heating test but not sufficient in the damp environment, which can be attributed to the corrosion of water to Al<sup>2</sup> O3 . 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. [66] proposed a novel sandwich structure of WO3 /VO2 /WO3 , where WO3 not only functions as an AR layer to enhance the luminous transmittance (*T*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'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 .

Zhan et al. [84] fabricated a complicated multilayer structure of SiN*<sup>x</sup>*

humid environment is not applied to the samples.

**Figure 6(a)**, **(b)**). Hydrophilicity of the single-layer VO2

environmental stability due to the dual protection from the Cr2

fatigue cycles, while VO2

pollutants. There are three different polymorphs of crystalline TiO<sup>2</sup>

lytic and photo-induced hydrophilic properties from the top TiO2

stable phase at all temperatures and the most common natural form of TiO2

(R)/VO2

(R)/VO2

tase (tetragonal) and brookite (orthorhombic). Rutile TiO2

**4. Multifunctional design and construction**

Semiconductor photocatalysts like TiO2

Zheng et al. [11] constructed a TiO2

self-cleaning effects (see **Figure 7(a)**).

Self-cleaning property of the TiO2

lattice parameters, TiO<sup>2</sup>

films. However, TiO2

/SiO2

long-time use [33]. The top SiO2

, which exhibits enhanced thermal stability up to 375°C. However, aging test in a

angle of the films change abruptly from 24.1° (hydrophilicity) to 115.0° (hydrophobicity) (see

will accelerate the degradation process of relative thermochromic performance. On the con-

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,

roughness for hydrophobic surfaces (see **Figure 6(b)**). The double protection from Cr2

ings, which is desirable for long-time use. The proposed CVS structure shows remarkable

shows negligible deterioration even after accelerated aging (60°C and 90% relative humidity)

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.

films, which occupy an important position in the studies of photocatalytic active materials.

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

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

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

trary, the hydrophobicity exhibited by the CVS structure is helpful to keep the VO2

structure proposed by our lab shows robust environmental stability for

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

top coatings in this work has been deliberately optimized with enhanced

makes an excellent promotion for the environmental stability of the CVS coat-

layer is chemically stable and makes the static water contact

O3

single-layer samples almost become invalid (see

are widely and frequently employed to decompose

(TiO2

(R) films are acted as buffer layer and growth template of VO<sup>2</sup>

(R) films are less efficient photocatalysts than anatase TiO<sup>2</sup>

and the SiO2

NiCrO*<sup>x</sup>*

The Cr2

/SiN*<sup>x</sup>*

O3 /VO2

fabrication of SiO2

of 103 h and 4 × 103

**Figure 6(c)**, **(d)**).

and SiO2

/NiCrO*<sup>x</sup>*

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

indicates contact with water, which

/SiN*<sup>x</sup>*

/VOx

/SiN*<sup>x</sup>* / 13

isolated

O3

layer, which

: rutile (tetragonal), ana-

(A) layer were studied for

. Due to similar

(TiO2

(M)

(A))

(R)) is a thermodynamically

(A) multilayer film, while the photocata-

(A) multilayer film was evaluated by

In the works above, the protective layers are usually single-layer films. To enhance the durability of thermochromic VO2 films, bilayer coatings such as VO<sup>2</sup> /TiO2 /ZnO, VO2 /SiO2 / ZnO, and VO2 /SiO2 /TiO2 have been studied [82]. In this study, VO2 films with TiO<sup>2</sup> /ZnO protective coatings have demonstrated higher antioxidant activity under aging tests, which can be attributed to the different oxygen permeability through different inorganic films [83].

**Figure 6.** Images of contact angle measurement of (a) the single-layer VO2 and (b) the proposed Cr2 O3 /VO2 /SiO2 structure. Variation curves of Δ*T*sol for VO2 , Cr2 O3 /VO2 , and Cr2 O3 /VO2 /SiO2 with different duration time (c) and different fatigue cycles (d).

Zhan et al. [84] fabricated a complicated multilayer structure of SiN*<sup>x</sup>* /NiCrO*<sup>x</sup>* /SiN*<sup>x</sup>* /VOx /SiN*<sup>x</sup>* / NiCrO*<sup>x</sup>* /SiN*<sup>x</sup>* , which exhibits enhanced thermal stability up to 375°C. However, aging test in a humid environment is not applied to the samples.

The Cr2 O3 /VO2 /SiO2 structure proposed by our lab shows robust environmental stability for long-time use [33]. 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 Cr2 O3 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 Cr2 O3 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)**).
