VО2-Based NC Coatings Modified with Other Light Functional Materials

Besides 7)_um and AT_S0, there are other optical performance parameters that should be emphasized in applications of V0₂-based smart windows, such as color, solar shading ability, and emissivity. Though Tium and AT_S0 have been greatly improved in V0₂-based NC coatings, they are still not ideal for practical applications. Since visible light occupies half of the energy of solar energy, researchers have started turning their attention to the visible waveband of solar energy. These works include composite V0₂ with visible-light- regulated materials or solar cells. All these works will be described in detail next.

Adjustment of Other Optical Performance of VO2-Based NC Coatings

VО2-based NC coatings with color modulation

V0₂ films/coatings are yellowish green, which is not a popular color for windows. Two methods are adopted to modulate the color of V0₂ films/coatings. The first is absorption edge regulation by doping [49, 55-57]. This method lightens/darkens the color of V0₂ films/coatings, improving/reducing their 7]_ит, but the adjustment range is limited and heavy doping would severely damage the lattice of the V0₂, reducing the AT_S0. The other method is composite V0₂ with materials of other colors. Although this will greatly reduce the 7]_um, changing the proportions and colors of the added materials would allow a wide range of color adjustment of the composite films/coatings.

Au is one kind of color material used to create a nanocomposite V0₂ coatings, where V0₂ is the matrix [6,58,59]. Au-V0₂ NC coatings were prepared by codepositing Au and V0₂ using magnetron sputtering [58, 59], sol-gel [60], pulsed laser deposition [61], or CVD [6]. By adjusting the concentration of the Au particles, the SPR of Au NPs would change and the composite coating can gradually change from brownish yellow to green and finally to blue (Fig. 3.12) [6]. As the concentration of Au particles increases, the reflection of

Figure 3.12 Examples of glass (3x5 cm² approximate size) with gold and V0₂ NC films. The films had a Au/V ratio determined by EDAX of (A) 0 (W- doped V0₂), (B) 0.09, (C) 0.15, (D) 0.30, (E) 0.36, and (F) (gold nanoparticle film) [6].

the coating in the infrared band is also enhanced, which helps to improve the thermal insulation performance of V0₂.

However, Au-V0₂ NC coatings are not economical enough, and the process is not suitable for large-scale preparation. Most of the Au- V0₂ NC coatings were used for studying the SPR of Au NPs on V0₂'s optical performance and the influence of carrier concentration on the phase transition temperature of V0₂. The simplest way to adjust color is by adding pigments into the V0₂ NPs dispersion used for organic NC coating preparation. According to CIELAB color system regulations, the color can be modulated using V0₂ NPs and two other different pigments.

Improvement of solar-heat shielding ability (Ts) for VО2 NPs

NIR light spans wavelengths of 760 to 2500 nm and contains about 47% of solar energy. T_s represents the ability to block this part of energy (1 - Tjnfr, where r_inf, refers to infrared energy transmittance). It is a pity that the T_s of V0₂ at high temperatures is not high enough, which leads to its low energy saving efficiency in a hot area.

To solve this problem, Gao et al. studied V0₂-based organic NC coatings with improved T_s by adding Sb:Sn0₂ (ATO) NPs (Fig. 3.13). Their results showed that both 7)_um and the visual appearance of the coatings remain almost unchanged while their T_inf_r and AT_S0 are monotonical as the ATO mass in the NC coatings increases, though the T_s of the NC coatings had visibly improved—about 20% at an ATO content of 9%. To test the applied properties, the NC

Figure 3.13 V0₂-based NC coatings with improved solar-heat shielding ability using ATO [50]. (а) ТЕМ image and (b) XRD of ATO NPs; (c, d) the transmittance curve (350-2500 nm) of an ATO-PU foil and V0₂-AT0- PU NC coating (with different solid contents of ATO], respectively, (e) The AT_S0] and the solar-heat shielding ability changing with the solid content of ATO; on increasing ATO, the T_s improved obviously, (f) Illustration of a testing system used to measure the solar thermal barrier effect of foils; 1 is a temperature monitor, 2 and 5 are test windows, 3 is a temperature probe, and 4 indicates infrared lamps, which are used to imitate solar light, (g) Temperature dependence on the irradiation time for blank float glass, V0₂-PU foils, and V0₂-AT0-PU foils (solid content of ATO is 30 wt.%). It was found that a V0₂-PU foil reduces the temperature by 17’C, compared with the blank float glass, while the V0₂-AT0-PU foil further reduces the temperature by 3.5" C.

coatings were applied to the blank float glass (4 mm in thickness) of a model house. The results showed that V0₂ NC coatings reduced the temperature by 17°C compared with the blank float glass, while adding ATO (30% by mass) further reduced the temperature by 3.5°C due to the enhancement of the infrared shielding ability.

VО2 NC coatings with low emission

Emission represents the ability to transfer heat. High values of emissivity imply strong energy exchange between the NC coatings and their surroundings, which leads to poor thermal insulation ability and high energy consumption requirement for buildings. Pure, dense V0₂ films can reach an emissivity value as low as

0.11-0.26 (metal-phase) and 0.63-0.67(insulator phase) [62], but the value would increase as the density of V0₂ films decreases. For V0₂-based NC coatings with dielectric materials (Si0₂, Ti0₂, PU, etc.) as the matrix, the emissivity is very high (usually >0.8) due to the low content of V0₂ NPs.

There are reports in which emissivity of V0₂ films could be reduced by coating low-emissivity materials on the V0₂ films, such as the noble metal Pt [62] or a transparent conductive oxide (AZO [63], FTO [64], etc.). These would reduce the emissivity of solution prepared V0₂ films from ~0.70 to ~0.2. Li et al. studied the V0₂- ATO composite coating. Their work showed that pure ATO had a emissivity of about 0.2 and the addition of V0₂ contents would increase the emissivity a little, but the composite films can serve as low-emittance coatings, especially for the lower V0₂ fractions. However, the preparation process in these methods is a little complicated and not suited for large-scale production.

Ag nanowires (Ag NWs) have been successfully prepared in recent years, which makes it possible to prepare low-emission coatings by the solution method. Luo et al. adopted this method [9], coated a Ag NW layer on top of a V0₂ NC coating, and obtained a multilayered V0₂ NC coating with low emissivity (Fig. 3.14). Compared to the results reported for V0₂/AZO, their V0₂/Pt structure samples showed obvious advantages in reducing the emissivity while maintaining the AT_S0. In the case of a single-layer V0₂ coating on quartz glass, the emissivity can be reduced to 0.21 by optimizing the coating thickness of Ag NWs, while the Д T_so remains 5.8%. This method has low operational difficulty and is suitable for large-scale production of low-emission V0₂ based NC coatings.