**Acknowledgements**

**4. Characterization**

102 Porosity - Process, Technologies and Applications

the investigation.

while the VO2

Bg

VO2

phase will show the Ag

below and above the *τ*<sup>c</sup>

from the expression

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

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

the XRD and Raman scan, and then the solar modulation ability could be determined with tem-

the crystalline planes (011)/(−211)/(220)/(022)/(202) at the 2*θ* positions 28°/37°/55.5°/57.5°/65°,

 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

and the integrated solar modulating abilities (∆*T*sol, 250 nm < λ < 2500 nm) could be calculated

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

the effect of porosity on enhancing the thermochromic properties. Compared with the other property enhancement methods, such as ARC multilayers, biomimetic patterning, nanothermochromism and periodic patterning (**Figure 8**), the porous design shows the

trol, which could reduce the cost in the real applications. In the fabrication of nanoporous

 thin films, the PAD, freeze-drying as well as the dual-phase transformation are the three main methods for random nanoporous structures, while the colloidal lithography

the 2*θ* positions 28°/37°/55.5°/57.5°/65° [22, 70]. For the Raman scan [58, 71], the VO2

/mnm) will show the crystalline planes (110)/(101)/(211)/(220)/(002) at

peaks at the Raman shift positions 192/222/302/392/611 cm−1 and the

, and the integrated luminous transmission (*T*lum, 380 nm < λ < 780 nm)

*Tlum/sol <sup>=</sup> <sup>∫</sup>φlum/sol(λ)T(λ)dλ*/*∫φlum/sol(λ)d<sup>λ</sup>* (1)

phase could be firstly confirmed though

(M, *P*21

) − *T*sol(*τ* > *τ*<sup>c</sup>

materials as well as the thickness con-

).

nanomaterials and

/c) will show

(M)

on the surface although some artifacts always appear in the AFM images.

perature dependent UV-Vis-NIR characterization. As for the XRD, VO<sup>2</sup>

standing 37° above the horizon) [73]. ∆*T*sol is calculated from *T*sol(*τ* < *τ*<sup>c</sup>

In this chapter, we have elaborated the fabrication of nanoporous VO2

Regarding to the thermochromic properties, the VO<sup>2</sup>

(R, *P*42

**5. Concluding remarks and outlook**

advantages in easy-to-handling, low usage of VO2

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