Sol-Gel Strategies for Improved Thermochromic Properties

Considering the primary aim of building fenestration, a smart thermochromic window should meet three key criteria: (i) high transparency in the visible spectrum for indoor natural light conservation, (ii) stable and optimized ability to modulate the incident IR light, and (iii) appropriate transition temperature close to room temperature. Great efforts have been made to optimize the thermochromic properties of V0₂ coatings, including morphology [25, 42] and structure designing [11-13], doping [6, 43-45], composites [16,46,47], and surface engineering [48]. Recentstudies have demonstrated heteroatom doping to be an effective way to lower the phase transition temperature to room temperature [15, 44]. Moreover, the construction of a composite coating with a visible light antireflection effect has proved to boost the visible light transmittance without weakening the thermochromic properties [2-4].

Doping

The MIT phase transition temperature is closely related with the dimerization of the V-V chain V0₂ [49]. The element dopants can introduce electron or hole doping and internal strain/stress, which all affect the electronic structure and transition temperature of V0₂ [2,19]. In the frame of classical thermodynamics, the dependence of transition temperature r_c on the uniaxial stress о can be understood following the Clausius-Clapeyron equation [6]:

where s₀ is the initial strain, r_c° is the original phase transition temperature, and AH is the latent heat of the phase transition. Introduction of higher-valence-state cations (compared to V⁴⁺) into the crystal lattice of V0₂ leads to extra electrons, which can induce a decrease in ДН due to the W-induced dimerization of the V-V chain in the Rutile phase [5, 6], thereby reducing r_c according to the relation r_c = AH/AS. Besides, substituting V⁴⁺ by cations of larger ionic radii or inserting cations at interstitial sites of the V0₂ lattice can introduce compressive strain and induce structural deformation, which reduces r_c as well [5, 6, 50]. The sol-gel method has been demonstrated as a low-cost and feasible way to synthesize doped V0₂ coatings [1, 19]. V0₂ coatings doped by W⁶⁺ [44, 45], Mo⁶⁺ [15, 31, 51], Ti⁴⁺ [52], and Zr⁴⁺ [53] and codoped by W⁶⁺/Mo⁶⁺ [50] and Mg²⁺/W⁶⁺ [6] have been successfully synthesized by the sol-gel method.

W⁶⁺ is known to reduce the thermochromic switching temperature of V0₂ by the greatest extent per atomic percentage (20 C-26°C per at.%). Liang et al. [45] reported the synthesis of self-cleaning W-doped thermochromic V0₂ coatings on a fused quartz substrate via an organic sol-gel process with the addition of tungsten chloride as the tungsten source. The effects of doping with different levels (0-1.5 at.%) of tungsten were investigated on the thermochromic properties of V0₂ coatings. Figure 10.7 shows the optical transmittance spectra of V0₂ coatings doped with various

Figure 10.7 Thermochromic optical properties of W-doped V0₂ coatings with doping levels of 0-1.5 at.%. (a), (b) the optical transmittance spectra in the wavelength range of 200-2500 nm, and (c, d) the corresponding thermal hysteresis loops at the wavelength of 2000 nm. Cited from Ref. [45].

levels of W in the wavelength range of 250-2500 nm and the corresponding thermal hysteresis loops at the wavelength of 2000 nm. It was observed that W doping exhibited a significant effect on the thermochromic properties of V0₂ coatings, especially the transition temperature as the doping level increased.

Table 10.3 [45] summarizes the thermochromic optical properties of W-doped V0₂ coatings. The table shows that high values of visible light transmittance 7)_ит of 80.6% in the semiconductor state and 79.2% in the metallic state were realized for undoped V0₂ coatings. 7|_Um decreased as the doping level increased, and reached a minimum value of 71.6%/70.1% at W doping of 1 at.%. An increase in 7)_um for V0₂ coating with 1.5 at.% W doping was caused by a reduction in the thickness from 392 nm to 259 nm. At a wavelength of 2000 nm, both solar light modulation ДГ_5о1 and IR light modulation ДТ^о,,,,, were reduced as the doping level increased. As the doping level reached 1.5 at.%, the ДГ_5о1 became 6.1% and the ДТ^о,,,,, became 10%, compared with those of undoped V0₂ (9.1% and 26%, respectively). It is worth noting that the phase transition temperature r_c significantly dropped as the doping level increased, reducing almost 20°C per W at.% from the initial 56' C of the undoped V0₂. Phase transition temperatures of 42°C, 35°C, and 32°C were realized with W doping levels of 0.5 at.%, 1 at.%, and 1.5 at.%, respectively.

Hu et al. [44] investigated the thermochromic properties of porous W-doped V0₂ coatings prepared from an aqueous vanadium solution/polyvinylpyrrolidone (PVP) sol. Ammonium tungstate hydrate (H₄oN₁₀04₁ W₁₂ • xH₂0) was selected as the W doping source. The transmittance spectra at 20°C (solid lines) and 90 C (dashed lines) in the wavelength of 250-2500 nm and the corresponding inset SEM images and d(Tr)/d(T)-T plots at the wavelength of 2000 nm for porous W-doped V0₂ coatings on silica substrates with different doping levels are exhibited in Fig. 10.8. All the W-doped V0₂ coatings showed similar porous structures with favorable holes for visible light transmittance. It was found that all films exhibited significant optical modulation in the IR region. The IR transmittance change decreased with an increase in the W doping level. However, the visible light transmittance was hardly affected by the increased W doping level, remaining at ~60%. According to the

Table 10.3 Thermochromic properties of VO2 coatings with different levels ofW doping

0	226	80.6/79.2	-1.5	81.4/72.3	9.1	26	22	56
0.5	234	78.9/77.8	-1.0	79.5/71.2	8.3	20	15	42
1.0	392	71.6/70.1	-1.5	71.7/63.2	8.6	22.7	16	35
1.5	259	74.5/74.3	-0.2	76.7/70.6	6.1	10	12	32

Table 10.4 Thermochromic optical parameters of W-doped V0₂ coatings with increased doping levels from 0 at.% to 2 at.%

0	57.3/58.8	44.4/45	-0.6	50.9/41.5	9.4	52.9	20	55
0.5	63.1/64.2	49.3/51.6	-2.3	55.7/48.5	7.2	45.9	15	45
1	57.5/58.9	44.8/47.4	-2.6	50.2/44.2	6	38.5	12	35
2	58.4/59.2	44.6/45.7	-1.1	49.4/42.5	6.9	37	15	28

Figure 10.8 Transmittance spectra at 20°C (solid lines) and 90 C (dashed lines) in the wavelength of 250-2500 nm and the corresponding inset SEM images and d(Tr)/d(T)-T plots at the wavelength of 2000 nm for porous W-doped VO2 coatings on silica substrates with different doping levels: (a) 0 at.%, (b) 0.5 at.%, (с) 1 at.%, and (d) 2 at.%, respectively. Cited from Ref. [44].

transmission spectra, the IR modulation ДГ₂ооопт at the wavelength of 2000 nm and solar spectrum modulation ДT_so are given in Table 10.4 [44]. Both decreased with increasing W doping levels. A significant effect of W doping was observed on the phase transition temperature. Compared to the undoped porous V0₂ coatings with a transition temperature of 55°C, the doped ones, with W doping levels of 0.5 at.%, 1 at.%, and 2 at.%, exhibited respective transition temperatures of 45^CC, 35°C, and 28°C, which was close to room temperature (20 C-30°C).

Although a significant decrease in the critical transition temperature has been achieved even close to room temperature by doping with high-valence cations with large ionic radii, such as W⁶⁺ and Mo⁶⁺, a deterioration in visible transmittance and solar modulation is observed [44, 45]. Therefore, a dopant that can reduce the transition temperature without sacrificing the luminous transmittance is eagerly awaited. According to the Clausius- Clapeyron equation (Eq. 10.1), Zr⁴⁺, with a larger ion radius (r =

0.71 A and coordination number [CN] = 6), rather than V⁴⁺ (r = 0.58 A and CN = 6) [54], has the probability to lower the critical transition temperature by causing lattice distortion in VO2. In addition, Zr dopants could widen the optical gaps of V0₂, resulting in enhanced visible transmittance [5]. The positive effect of Zr⁴⁺ doping has been demonstrated on the thermochromic optimization of V0₂ coatings. Lu et al. [53] prepared Zr-doped V0₂ coatings on a glass substrate through the sol-gel method by using VO(acac)2 as the vanadium precursor and Zr(N0₃)₄ as the Zr⁴⁺ source. The effect of Zr doping on the metal-semiconductor transition and thermochromic transmittance switching was investigated. It was found that as the doping level of Zr⁴⁺ increased up to 2 wt.%, the critical temperature of phase transition decreased to 50°C, with little variation in the visible light transmittance [53].

Ti⁴⁺ is another kind of dopant for thermochromic V0₂ coatings, which can increase the critical phase transition temperature but does not change the transmission/absorption characteristics of the V0₂ coatings [52, 55]. Hu et al. [52] used the hybrid of an inorganic V2O5 sol and an organic titanium sol to synthesize Ti-doped V0₂ coatings with different Ti concentrations. Figure 10.9a shows the IR transmittance spectra of an undoped V0₂ film and a V0₂ film doped with 2.8 at.% Ti at 40°C and 90°C. Little difference was found in the transmittance spectra of the doped and undoped V0₂ coatings, which indicates the zero effect of Ti doping on the IR transmittance switching of V0₂ coatings. Besides, as shown in Fig. 10.9b, the critical transition temperature increases and the hysteresis width decreases as the Ti doping concentration increases. The special functions of Ti doping provide a way to adjust the critical phase transition temperature of V0₂ in the range above the original 68°C without influencing the IR transmittance switching properties.

Considering the different effects of dopants, codoping may provide a way to satisfy both decreasing the r_c and maintaining an acceptable Ti_um, favoring access to V0₂ coatings with sufficient

Figure 10.9 (a) IR transmittance spectra of undoped V0₂ coating and V0₂

coating doped with 2.8 at.% Ti at 40“C and 90' C. (b) Variation in the transition temperature and hysteresis width of V0₂ films as a function of Ti doping concentration. Cited from Ref. [52].

luminous transmittance and solar spectrum modulation at rather low transition temperatures.

Wang et al. [6] prepared Mg/W-codoped V0₂ coatings on a silica substrate through a sol-gel method with PVP as a cross- linking reagent. In this study, the large r_c-reducing rate of W and the superior T’lum -enhancing ability of Mg were simultaneously demonstrated on the Mg/W-codoped V0₂ coatings, resulting in a good combination of low r_c of ~35°C and a high 7)_um of 81.3%. The UV-vis-NIR transmittance spectra of Mg/W-codoped V0₂ coatings were recorded at 15³C and 90°C (Fig. 10.10a). A decrease in the transmission was observed for all of the samples above the wavelength of 1000 nm upon heating from 15°C to 90³C, in accordance with the intrinsic MIT characteristics of V0₂. Among all the samples, the single Mg-doped V0.95Mg0.05O2 exhibited the highest T_so in the entire spectrum (250-2500 nm) at both temperatures, especially in the visible range, indicating the ability of Mg to enhance 7)_um. On the other hand, the single W-doped V0.9sW0.02O2 showed the lowest phase transition temperature (27.05^CC) and the lowest T_so (250-2500 nm) at the same time, revealing the weakness of W as a dopant. On doping with 2 at.% W, the 7)_um increased with an increase the Mg doping concentration, and the optimal 7)_um was realized on V0.94W0.02Mg0.04O2, with the lowest visible-light consumption by reflection (%R) and absorption (%A) (Fig. 10.10b). Figure 10.10c show hysteresis loops for the temperature-dependent transmittance

Figure 10.10 (a) Transmittance spectra and (b) recorded reflection and

absorption spectra of pristine and Mg/W-codoped V0₂ coatings in the UV- vis-NIR range at 15°C (solid lines) and 90 C (dashed lines), (c) Hysteresis loop for the temperature-dependent transmittance of the pristine and Mg/W-codoped V0₂ coatings at a wavelength of 2500 nm. (d) Dependence of r_c on Mg doping concentration on the basis of doping with 2 at.% W with a fitted linear relationship. Cited from Ref. [6].

of pristine and Mg/W-codoped V0₂ coatings at a wavelength of 2500 nm, and the corresponding dependence of r_c on Mg doping concentration was investigated. As shown in Fig. lO.lOd, the r_c exhibited a tendency to increase, where continuous Mg doping in the presence of a 2 at.% W dopant resulted in a slight increase in r_c from 27.05°C (V0.98W0.02O2) to 35.08 °C (V0.93W0.02Mg0.0sO2) at a rate of 1.6°C/at.%.

In addition, Mo/W [50, 56] and Ti/W [57] codoping has been carried out to tune the r_c of V0₂. Mo/W-codoped V0₂ thin films exhibited an interesting nonlinear effect between со- and single- doped materials [56]. The co-doped V0₂ films showed higher r_c- lowering efficiency than those single-element-doped films [56].

Ti/W codoping induced a higher r_c in the material than single W doping because of the decrease in the carrier density arising from the electron transfer from the donor W to the acceptor Ti, while the transmission was largely compromised compared with that of pure V0₂ [57]. Therefore, codoping with proper metal ions can simultaneously realize low r_c and high luminous transmittance through a synergetic effect.