**1. Introduction**

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 [1–5]. Energy

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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 [6–10], thermochromism [11–19], gasochromism [20–22] and photochromism [23–26] 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 [27–29], 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 (*T*<sup>c</sup> ) at 68°C [30], which has been widely investigated as smart coatings for building fenestrations [31–35]. As shown in **Figure 1(b)**, for temperatures below the *T*<sup>c</sup> , VO2 is monoclinic (P21 /*c*, M1) phase with the transmittance of NIR. On the contrary, the material is a tetragonal structure (P42 /*mnm*, R), which is reflective for NIR [36, 37]. This feature makes VO2 an amazing material for thermochromic smart coatings [37–45]. Based on VO2 -thermochromic coatings, smart windows can let the solar energy (mainly caused by NIR) in and out during the cold and hot days, respectively, which are shown in **Figure 1(c)**.

which is the most thermodynamically stable phase of vanadium oxide but does not possess

**Figure 1.** (a) Schematic of energy exchange in winter and summer days. (b) Typical optical properties of thermochromic

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

in (II)–(IV) mentioned above have not yet been solved. Although several reviews about VO2 coatings have been reported [35, 36, 61, 62], most of them are still in lab scale and few pros-

thermochromic performance, environmental stability, and large-scale production for commercial applications on building fenestrations. Firstly, strategies to enhance thermochromic per-

protective layers for multilayer films, will be summarized in Section 3. Meanwhile, multi-

: cations larger than V4+, such as W6+ [57], Mo6+ [58] and Nb5+ [59], and

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

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

5

coatings have been introduced as well as the balance between

smart coatings such as photocatalysis and self-cleaning function has

is a great chal-

(monoclinic phase)

. However, obstacles

coatings, including

smart coatings for improved

the thermochromic property [56]. Therefore, environmental stability of VO2

coatings before and after phase-transition temperature. Inset is the crystallographic structure of VO2

(rutile phase). (c) Schematic of energy-efficiency based on thermochromic smart coatings.

anions larger than O2―, such as F― [60], have been utilized to reduce the *T*<sup>c</sup>

*<sup>T</sup>*lumand <sup>Δ</sup> *<sup>T</sup>*sol (Section 2). Then, methods to improve the durability of VO2

In this chapter, we will review strategies of thermochromic VO2

lenge for practical applications as smart coatings.

pects of commercial applications are available.

formance (*T*lum and <sup>Δ</sup>*<sup>T</sup>*sol) of VO2

functional design of VO2

temperature of VO2

and VO2

VO2 smart coatings are usually used in two forms including flexible foils based on VO<sup>2</sup> nanoparticles [34, 46–52] and VO2 -based multilayer films [11, 12, 33, 53–55]. 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 (*T*<sup>c</sup> ) for pure bulk VO2 (68°C) is too high to be applied on building windows, while *T*<sup>c</sup> 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 (*<sup>T</sup>* lum) for single layer VO2 with desirable solar modulation (Δ*<sup>T</sup>* 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 10 years. However, VO2 can easily transform into the V2 O5 phase in the real environment, Solar Modulation Utilizing VO2-Based Thermochromic Coatings for Energy-Saving Applications http://dx.doi.org/10.5772/intechopen.75584 5

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 [6–10], thermochromism [11–19], gasochromism [20–22] and photochromism [23–26] 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 [27–29], whose transition depends on the temperature, is widely investigated type

investigated as smart coatings for building fenestrations [31–35]. As shown in **Figure 1(b)**,

(mainly caused by NIR) in and out during the cold and hot days, respectively, which are

for commercial application as smart coatings on windows, there are still many obstacles

abilities are not efficient enough for energy saving. That can be explained by the fact that the

near-infrared region, which only accounts for 43% of solar energy in the solar spectrum [23].

(Δ*<sup>T</sup>* sol) is usually less than 40% (even 30%) due to the absorption in the short-wavelength

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

smart coatings are usually used in two forms including flexible foils based on VO<sup>2</sup>

is monoclinic (P21

) is one of the most promising thermochromic materials, which


exhibits an automatic reversible semiconductor–metal phase

an amazing material for thermochromic smart coat-


(68°C) is too high to be applied on building windows,

for solar radiation is most attributed to the transmittance switch in the

O5

) at 68°C [30], which has been widely

/*c*, M1) phase with the transmittance of

smart coatings. (I) The phase-transition

coatings, relative modulation

with desirable solar modulation

, which should be larger than

phase in the real environment,

/*mnm*, R), which is reflective

of chromogenic window.

4 Emerging Solar Energy Materials

Vanadium dioxide (VO2

has been widely studied. VO2

for temperatures below the *T*<sup>c</sup>

ings [37–45]. Based on VO2

shown in **Figure 1(c)**.

temperature (*T*<sup>c</sup>

modulation of VO2

while *T*<sup>c</sup>

VO2

for NIR [36, 37]. This feature makes VO2

nanoparticles [34, 46–52] and VO2

severely limiting the relative applicability of VO2

) for pure bulk VO2

(III) The luminous transmittance (*<sup>T</sup>* lum) for single layer VO2

10 years. However, VO2 can easily transform into the V2

range in both the semiconducting and metallic states of VO2

transition (SMT) at a critical transition temperature (*T*<sup>c</sup>

, VO2

NIR. On the contrary, the material is a tetragonal structure (P42

around 40°C is acceptable. (II) For conventional VO2

**Figure 1.** (a) Schematic of energy exchange in winter and summer days. (b) Typical optical properties of thermochromic coatings before and after phase-transition temperature. Inset is the crystallographic structure of VO2 (monoclinic phase) and VO2 (rutile phase). (c) Schematic of energy-efficiency based on thermochromic smart coatings.

which is the most thermodynamically stable phase of vanadium oxide but does not possess the thermochromic property [56]. Therefore, environmental stability of VO2 is a great challenge for practical applications as smart coatings.

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+ [57], Mo6+ [58] and Nb5+ [59], and anions larger than O2―, such as F― [60], have been utilized to reduce the *T*<sup>c</sup> . However, obstacles in (II)–(IV) mentioned above have not yet been solved. Although several reviews about VO2 coatings have been reported [35, 36, 61, 62], most of them are still in lab scale and few prospects of commercial applications are available.

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 (*T*lum and <sup>Δ</sup>*<sup>T</sup>*sol) of VO2 coatings have been introduced as well as the balance between *<sup>T</sup>*lumand <sup>Δ</sup> *<sup>T</sup>*sol (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.
