**4. Characterization**

In order to fully characterize the structure and the thermochromic properties of VO2 nanoporous thin films, the advanced techniques including scanning electron microscopy (SEM), transmission electron microscopy (TEM), atomic force microscopy (AFM), X-ray diffraction (XRD), Raman spectroscopy as well as UV-Vis-NIR spectroscopy could be utilized in the investigation.

With respect to the nanoporous morphology, SEM is a powerful technique to observe the size, shape and the distributions of the nanopores on the surface in a large scale vision, while the details within the pore could be determined using the TEM in a cross-section view. Due to the non-destructive advantage, AFM is also an efficient way to scan the pore distribution on the surface although some artifacts always appear in the AFM images.

Regarding to the thermochromic properties, the VO<sup>2</sup> phase could be firstly confirmed though the XRD and Raman scan, and then the solar modulation ability could be determined with temperature dependent UV-Vis-NIR characterization. As for the XRD, VO<sup>2</sup> (M, *P*21 /c) will show the crystalline planes (011)/(−211)/(220)/(022)/(202) at the 2*θ* positions 28°/37°/55.5°/57.5°/65°, while the VO2 (R, *P*42 /mnm) will show the crystalline planes (110)/(101)/(211)/(220)/(002) at the 2*θ* positions 28°/37°/55.5°/57.5°/65° [22, 70]. For the Raman scan [58, 71], the VO2 (M) phase will show the Ag peaks at the Raman shift positions 192/222/302/392/611 cm−1 and the Bg peak at 258 cm−1. In the measurement of thermochromic performance, the transmittance of the normal incidence is recorded at the wavelength range 250–2500 nm at the temperature below and above the *τ*<sup>c</sup> , and the integrated luminous transmission (*T*lum, 380 nm < λ < 780 nm) and the integrated solar modulating abilities (∆*T*sol, 250 nm < λ < 2500 nm) could be calculated from the expression

$$T\_{\text{lum\'otol}} = \left\{ \rho\_{\text{lum\'otol}}(\lambda \lambda \text{T} \lambda \lambda \text{d}\lambda \| \| \rho\_{\text{lum\'otol}} \text{\'} \lambda \text{d}\lambda \text{}\lambda \text{\'} \right\} \tag{1}$$

with the MCC template is an effective approach for periodic nanoporous structures. The calculations reveal that the nanoporous structure could result in the decrease of optical constants and thus lead to the enhancement of visible transmission while maintain the

**Periodic**

*T*lum, exp. ∆*T*sol, exp. *T*lum, simu. ∆*T*sol, simu.

**Nanothermochromism**

**Biomimetic**

**Figure 8.** Methods proposed for enhancing the thermochromic performance of VO2

Although many efforts have been dedicated to optimize the effect of nanoporous structure

(<~80%) and the low solar modulating ability (<~30%) restrict the real applications in thermochromic smart windows. From the viewpoint of materials design, the periodic nanoporous

This research is supported by National Research Foundation, Prime Minister's Office, Singapore under its Campus for Research Excellence and Technological Enterprise (CREATE)

School of Materials Science and Engineering, Nanyang Technological University, Singapore

 thin films with the periodicity below 100 nm should give rise to the largely enhanced visible transmission as well as the highly reduced scattering, which could greatly improve the

thin films, the low visible transmission

nanomaterials [55].

**0**

**Porosity**

**patterning**

**20**

**40**

∆

*T*

**sol(%)**

**60**

**80**

**100**

Controlled Porosity in Thermochromic Coatings http://dx.doi.org/10.5772/intechopen.70890 103

decent solar modulating abilities.

**Acknowledgements**

programme.

**Author details**

Ning Wang, Yujie Ke and Yi Long\*

\*Address all correspondence to: longyi@ntu.edu.sg

VO2

on enhancing the thermochromic performance of VO2

**0**

**Multilayer**

**20**

**40**

*T***lu**

**ARC**

**m (%)**

**60**

**80**

**100**

thermochromic performance for smart window applications.

where *φ*lum is the standard luminous efficiency function for the photopic vision of human eyes [72], and the *φ*sol is the solar irradiance spectrum for air mass 1.5 (corresponding to the sun standing 37° above the horizon) [73]. ∆*T*sol is calculated from *T*sol(*τ* < *τ*<sup>c</sup> ) − *T*sol(*τ* > *τ*<sup>c</sup> ).
