**1. Introduction**

In recent decades, the usage of traditional energy materials, including the oil and the coal meets more and more challenges due to the increase of air pollution, energy shortage and the global warming. Therefore, the concepts of sustainable and environment-friendly production were raised by scientists for energy-saving, and various clean energy technologies have been proposed for industries, for example, fuel cell [1–4], solar cell [5–7] and wind turbines [8–11].

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On the other hand, the alternative energy-saving approach is to develop green-energy buildings equipped with state-of-the-art smart windows, for example, electrochromic/thermochromic smart windows [12–17].

nanoparticles in the nanocomposite. Long's group investigated the micropatterning [55] and

thin films, which both benefited the VO<sup>2</sup>

showed the high *T*lum (~43%) and the large ∆*T*sol (~12%). Across the strategies, the nanoporous

As is well known, the porous structure could effectively increase the specific area of materials and thus supply large active areas under low loading. On the other hand, the porous design could also reduce the optical constants (refractive index 'n' and the extinction coefficient 'k'), which could benefit the materials with enhanced visible transmittance. The optical calcula-

method, based on the assumption that the optical constants should be linearly dependent on the volume fraction or the 'n' and 'k' is linearly decreased with the porosity. As shown in

structure gave rise to an obvious decrease of optical constants (n, k) compared with the normal thin film, and the optical calculations revealed the largely enhanced *T*lum and ∆*T*sol with

tributed and the periodic porous structures. In the random case, as reported by Gao's group [53] and Long's group [57], the thermochromic properties could be enhanced to *T*lum > 40% and ∆*T*sol > 14%. In contrast, for the periodic porous structure, as reported by Xie's group [58] and further developed by Long's group [59, 60], the visible transmittance could be above 46% while maintaining the ∆*T*sol above 13%. Actually, the periodic nanoporous design is more

experimental/reference *T*lum (versus film thickness). Reference data is from Jin et al. [56]. (c) Optical calculations of

/VO2

thin films could be performed with an optical-admittance recursive

thin film. (b) Experimental (solid) and reference (dash) n, k (versus wavelength) and

thin films based on an optical-admittance recursive method, where dotted lines and solid lines

/TiO2

/VO2

/TiO2

Controlled Porosity in Thermochromic Coatings http://dx.doi.org/10.5772/intechopen.70890

 **with porous structure**

thin films (**Figure 1a**), the porous

thin films, there are normally the random dis-

thin films with improved

materials as well as the

structure, which

95

nanopatterning [51] of VO2

tions of nanoporous VO2

**Figure 1.** (a) Nanoporous VO2

represent the *T*lum at insulating state and the ∆*T*sol, respectively [53].

the nanoporous VO2

thickness control.

*T*lum and ∆*T*sol. Mlyuka et al. reported the five-layer TiO<sup>2</sup>

**2. Enhanced thermochromic properties of VO2**

**Figure 1**, as for the random distributed nanoporous VO2

increasing the porosity of the thin films.

With respect to the porous structure of VO2

design showed the advantages in easy-to-handling, low usage of VO2

Vanadium dioxide (VO2 ), as a promising coating material for thermochromic smart windows have been investigated for half a century, since Morin found the intrinsic metal-toinsulator transition (MIT) of VO2 in 1959 [18]. Below a critical temperature (*τ*<sup>c</sup> ) ~ 68°C, VO2 shows the monoclinic insulating phase (VO2 (M)) with zig-zag V-V chains along the *c*-axis (*P*21 /c, V-V separation is 0.262, 0.316 nm) [19]. Above the *τ*<sup>c</sup> , VO2 is transformed to rutile metallic phase (VO2 (R)) with linear V-V chains along the *c*-axis (*P*42 /mnnm, V-V separation is 0.288 nm) [19]. The increase of the electrical resistance across the MIT is always in 3–5 orders of magnitude, and the first-order transition could occur simultaneously with the time less than 500 fs [20]. Along with the MIT, the IR transmittance of VO<sup>2</sup> could also be modulated by a large magnitude owing to the change of the optical parameters (refractive 'n' and extinction coefficient 'k') [21]. As a coating material, VO2 shows the high IR transmittance at the cold state while exhibits the large absorption as well as the strong reflection at the hot state, which gives rise to large IR modulating ability [22–25]. Due to the little difference of optical parameters in the visible region, VO2 shows the little transmittance difference in the visible region [26–28]. The solar modulating ability especially in the IR region makes VO<sup>2</sup> a promising coating material for thermochromic smart windows.

The VO2 thermochromic smart windows have various advantages in energy saving. To begin with, the phase transition temperature (*τ*<sup>c</sup> ) of VO2 is close to the room temperature, which cannot be found in other phase transition materials (*τ*<sup>c</sup> (V2 O3 ) = −123°C, *τ*<sup>c</sup> (V2 O5 ) = 257°C, *τ*c (V6 O13) = −123°C, *τ*<sup>c</sup> (TinO2n+1) = 127–377°C) [27]. Secondly, the *τ*<sup>c</sup> of VO2 could be further reduced to ambient temperature through doping with other high valence metal cations, for example, W6+ [22, 29–33], Mo6+ [34–36]. Finally, several synthetic methods, for example, atmospheric pressure CVD [36–40], magnetron sputtering [41–45], sol-gel [35, 46, 47] and hydrothermal assembly [48–50], have been developed to fabricate VO2 nanostructures for applications. However, for thermochromic applications, VO2 still meets several challenges. Firstly, it is hard to achieve the high visible transmittance (*T*lum) and the large solar modulating abilities (∆*T*sol) simultaneously, since there is always a tradeoff between the *T*lum and ∆*T*sol [51]. Secondly, the thermochromic property is hard to maintain when reducing the *τ*<sup>c</sup> to room temperatures via doping [31]. Finally, the VO2 coating is not stable in the air [52].

In order to improve the thermochromic performance of VO2 coating, several interesting strategies, including nanoporosity, nanothermochromism, patterning as well as multilayer structures have been investigated by the scientists. Gao's group reported the enhanced luminous transmittance (*T*lum = ~40%) and improved thermochromic properties (∆*T*sol = ~14%) of nanoporous VO2 thin films with low optical constants, and the optical calculations suggested that the further improved performance could be expected by increasing the thin film porosity [53]. Li et al. [54] calculated the nanothermochromics of VO2 nanocomposite by dispersing VO2 nanoparticles in the dielectric host, which revealed that the thermochromic performance could be largely enhanced (*T*lum = ~65%, ∆*T*sol = ~20%) with spherical morphologies of the VO2

nanoparticles in the nanocomposite. Long's group investigated the micropatterning [55] and nanopatterning [51] of VO2 thin films, which both benefited the VO<sup>2</sup> thin films with improved *T*lum and ∆*T*sol. Mlyuka et al. reported the five-layer TiO<sup>2</sup> /VO2 /TiO2 /VO2 /TiO2 structure, which showed the high *T*lum (~43%) and the large ∆*T*sol (~12%). Across the strategies, the nanoporous design showed the advantages in easy-to-handling, low usage of VO2 materials as well as the thickness control.

#### **2. Enhanced thermochromic properties of VO2 with porous structure**

On the other hand, the alternative energy-saving approach is to develop green-energy buildings equipped with state-of-the-art smart windows, for example, electrochromic/thermochro-

dows have been investigated for half a century, since Morin found the intrinsic metal-to-

(R)) with linear V-V chains along the *c*-axis (*P*42

is 0.288 nm) [19]. The increase of the electrical resistance across the MIT is always in 3–5 orders of magnitude, and the first-order transition could occur simultaneously with the time

lated by a large magnitude owing to the change of the optical parameters (refractive 'n' and

the cold state while exhibits the large absorption as well as the strong reflection at the hot state, which gives rise to large IR modulating ability [22–25]. Due to the little difference of

visible region [26–28]. The solar modulating ability especially in the IR region makes VO<sup>2</sup>

) of VO2

reduced to ambient temperature through doping with other high valence metal cations, for example, W6+ [22, 29–33], Mo6+ [34–36]. Finally, several synthetic methods, for example, atmospheric pressure CVD [36–40], magnetron sputtering [41–45], sol-gel [35, 46, 47] and

Firstly, it is hard to achieve the high visible transmittance (*T*lum) and the large solar modulating abilities (∆*T*sol) simultaneously, since there is always a tradeoff between the *T*lum and ∆*T*sol [51]. Secondly, the thermochromic property is hard to maintain when reducing the *τ*<sup>c</sup>

egies, including nanoporosity, nanothermochromism, patterning as well as multilayer structures have been investigated by the scientists. Gao's group reported the enhanced luminous transmittance (*T*lum = ~40%) and improved thermochromic properties (∆*T*sol = ~14%) of nano-

the further improved performance could be expected by increasing the thin film porosity

 nanoparticles in the dielectric host, which revealed that the thermochromic performance could be largely enhanced (*T*lum = ~65%, ∆*T*sol = ~20%) with spherical morphologies of the VO2

thin films with low optical constants, and the optical calculations suggested that

(TinO2n+1) = 127–377°C) [27]. Secondly, the *τ*<sup>c</sup>

thermochromic smart windows have various advantages in energy saving. To begin

(V2 O3

), as a promising coating material for thermochromic smart win-

(M)) with zig-zag V-V chains along the *c*-axis

shows the little transmittance difference in the

is close to the room temperature, which

(V2 O5

still meets several challenges.

coating is not stable in the air [52].

coating, several interesting strat-

nanocomposite by dispersing

) = −123°C, *τ*<sup>c</sup>

of VO2

, VO2

) ~ 68°C, VO2

a

) = 257°C,

could be further

nanostructures for

is transformed to rutile

/mnnm, V-V separation

could also be modu-

shows the high IR transmittance at

in 1959 [18]. Below a critical temperature (*τ*<sup>c</sup>

mic smart windows [12–17].

insulator transition (MIT) of VO2

94 Porosity - Process, Technologies and Applications

shows the monoclinic insulating phase (VO2

optical parameters in the visible region, VO2

with, the phase transition temperature (*τ*<sup>c</sup>

O13) = −123°C, *τ*<sup>c</sup>

/c, V-V separation is 0.262, 0.316 nm) [19]. Above the *τ*<sup>c</sup>

extinction coefficient 'k') [21]. As a coating material, VO2

cannot be found in other phase transition materials (*τ*<sup>c</sup>

promising coating material for thermochromic smart windows.

hydrothermal assembly [48–50], have been developed to fabricate VO2

applications. However, for thermochromic applications, VO2

to room temperatures via doping [31]. Finally, the VO2

In order to improve the thermochromic performance of VO2

[53]. Li et al. [54] calculated the nanothermochromics of VO2

less than 500 fs [20]. Along with the MIT, the IR transmittance of VO<sup>2</sup>

Vanadium dioxide (VO2

metallic phase (VO2

(*P*21

The VO2

porous VO2

VO2

*τ*c (V6 As is well known, the porous structure could effectively increase the specific area of materials and thus supply large active areas under low loading. On the other hand, the porous design could also reduce the optical constants (refractive index 'n' and the extinction coefficient 'k'), which could benefit the materials with enhanced visible transmittance. The optical calculations of nanoporous VO2 thin films could be performed with an optical-admittance recursive method, based on the assumption that the optical constants should be linearly dependent on the volume fraction or the 'n' and 'k' is linearly decreased with the porosity. As shown in **Figure 1**, as for the random distributed nanoporous VO2 thin films (**Figure 1a**), the porous structure gave rise to an obvious decrease of optical constants (n, k) compared with the normal thin film, and the optical calculations revealed the largely enhanced *T*lum and ∆*T*sol with increasing the porosity of the thin films.

With respect to the porous structure of VO2 thin films, there are normally the random distributed and the periodic porous structures. In the random case, as reported by Gao's group [53] and Long's group [57], the thermochromic properties could be enhanced to *T*lum > 40% and ∆*T*sol > 14%. In contrast, for the periodic porous structure, as reported by Xie's group [58] and further developed by Long's group [59, 60], the visible transmittance could be above 46% while maintaining the ∆*T*sol above 13%. Actually, the periodic nanoporous design is more

**Figure 1.** (a) Nanoporous VO2 thin film. (b) Experimental (solid) and reference (dash) n, k (versus wavelength) and experimental/reference *T*lum (versus film thickness). Reference data is from Jin et al. [56]. (c) Optical calculations of the nanoporous VO2 thin films based on an optical-admittance recursive method, where dotted lines and solid lines represent the *T*lum at insulating state and the ∆*T*sol, respectively [53].

efficient in controlling the porosity and optimizing the thermochromic properties than the random counterpart, since the porosity could be easily estimated from the structure design.
