Facile Synthesis of Island-like ZrO₂-VO₂ Composite Films with Enhanced Thermochromic Performance for Smart Windows

Abstract

VO₂-based film, as a very promising thermochromic material for smart windows, has attracted extensive attention but has not been widely applied because it is difficult to simultaneously improve in terms of both solar-modulation efficiency (ΔT_sol) and visible transmittance (T_lum) when made using the magnetron-sputtering method, and it has poor durability when made using the wet chemical method. Herein, island-like ZrO₂-VO₂ composite films with improved thermochromic performance (ΔT_sol: 12.6%, T_lum: 45.0%) were created using a simple approach combining a dual magnetron-sputtering and acid-solution procedure. The film’s ΔT_sol and T_lum values were increased initially and subsequently declined as the sputtering power of the ZrO₂ target was raised from 30 W to 120 W. ΔT_sol achieved its maximum of 12.6% at 60 W, and T_lum reached its maximum of 51.1% at 90 W. This is likely the result of the interaction of two opposing effects: Some VO₂ nanocrystals in the composite film were isolated by a few ZrO₂ grains, and some pores could utilize their surface-plasmon-resonance effect at high temperature to absorb some near-infrared light for an enhanced ΔT_sol and T_lum. More ZrO₂ grains means fewer VO₂ grains in the composite film and increased film thickness, which also results in a decrease in ΔT_sol and T_lum. As a result, this work may offer a facile strategy to prepare VO₂-based films with high thermochromic performance and promote their application in smart windows.

1. Introduction

Thermochromic windows are considered a promising way to reduce building energy consumption significantly due to their simple structure, good solar modulation, and zero-energy input characteristics [1]. Among these thermochromic films, VO₂ films have attracted much attention because they can modulate near-infrared (NIR) transmittance via their reversible and ultra-fast transition between monoclinic phase (M phase) and rutile phase (R phase) at about 68 °C [1]. In this sense, VO₂ thermochromic smart windows can respond to changes in the surrounding temperature and then automatically adjust the amount of solar radiation entering indoors. The smart windows can block NIR from entering the room when the temperature is high, thus reducing energy consumption caused by air conditioning, and they can allow NIR to enter indoors when the temperature is low in winter, raising the indoor temperature and thus reducing energy consumption caused by warming. Therefore, VO₂ films have emerged as a promising material for the upcoming generation of smart-window coatings due to the above advantages. In fact, VO₂ films are not widely applied in buildings, mainly because of undesirable modulation efficiency (ΔT_sol), the inherent low visible luminous transmittance T_lum, high phase-transition temperature (T_c), and lackluster durability [2,3,4]. In particular, ΔT_sol and T_lum are especially difficult to improve simultaneously.

To solve the above issues, numerous techniques have been employed, such as doping, a multilayer-film structure, a core–shell structure, interfacial tension, etc. Many metal oxides such as TiO₂, ZnO, WO₃, and ZrO₂ have been introduced into VO₂-based films as layers to form multilayer VO₂-based films for enhanced thermochromic performance. ZrO₂ is one of the metal oxides that may be utilized as a protective layer and anti-reflection layer for VO₂ films since it is relatively stable and has a good refractive index. ZrO₂/VO₂/ZrO₂ three-layer films were prepared by pulsed-laser deposition and showed enhanced visible transmittance while ΔT_sol remained at only 8% [5]. Additionally, Zr doping has a significant impact on the thermochromic performance of VO₂-based films [6,7,8]. For instance, ZrW₂O₈/VO₂/ZrW₂O₈ thin films exhibited good light transmittances of 71.9% compared to 39.8% of intrinsic VO₂ thin films [6]. W/Zr co-doped VO₂ nanoparticles synthesized by a hydrothermal method showed excellent thermochromic properties with T_c, T_lum, and ΔT_sol of 46.9 °C, 60.7%, and 10.6%, respectively [8]. In this regard, Zr doping or the addition of ZrO₂ in VO₂-based films would result in an evident improvement in the thermochromic performance. However, these VO₂-based films do not exhibit good stability [9,10,11,12,13]. Hence, dual magnetron sputtering is employed to prepare ZrO₂-VO₂ composite films because the films obtained by sputtering usually have strong adhesion and thus exhibit good stability.

Unfortunately, the VO₂-based films prepared by magnetron sputtering are dense and absorb more visible light, leading to a low visible transmittance. In order to improve the visible transmittance, a solution process is usually employed to fabricate porous VO₂ films [14,15]. The porous VO₂ films exhibit enhanced visible transmittance due to the increased porosity and good solar-modulation efficiency because of strong localized surface-plasmon-resonance (LSPR) effects at high temperatures caused by small isolated VO₂ grains [16,17,18]. Therefore, the method combining magnetron sputtering and a solution process is a promising way to prepare VO₂-based film with enhanced thermochromic performance. However, the method was not employed to synthesize ZrO₂-VO₂ composite films because ZrO₂ grains showed better solution resistance than VO₂. In this work, island-like ZrO₂-VO₂ composite films on glass were created using a simple technique combining dual magnetron sputtering and an acid-solution process. The obtained ZrO₂-VO₂ composite films showed enhanced thermochromic performance compared with pure VO₂ films. The effect of ZrO₂ content on the thermochromic properties was investigated in detail to account for the enhancement in thermochromic performance.

2. Materials and Methods

Without additional purification, all chemicals were utilized after being acquired from Sinopharm Chemical Reagent Co., Ltd. In this experiment, Nanchang National Materials Technology Co., Ltd.’s V target (99.95% purity) and ZrO₂ target (99.99% purity) were employed. The magnetron-sputtering equipment used in the experiment was JPD-500. The size of the vacuum chamber was Φ500 × H420 mm and the size of rotating substrate table was Φ150 mm. Figure 1a displays magnetron-sputtering diagram. Before sputtering, cooling water was turned on. It was ensured that the vacuum chamber was evacuated to 3.0 × 10⁻³ Pa. At this time, argon with a purity of 99.99% was introduced into the reaction chamber and the flow of argon was adjusted to 200 sccm. First, ZrO₂-V composite films were deposited through direct-current magnetron sputtering of V targets at a power of 90 W and radio-frequency magnetron sputtering of ZrO₂ targets simultaneously at various power. The total continuous sputtering duration of the V target was 15 min, and the zirconia target was intermittently sputtered 9 times for 30 s each time. The sputtering pressure was 0.4 Pa. Figure 1b depicts the comprehensive manufacturing procedure of ZrO₂-V composite films. Finally, ZrO₂-VO₂ composite films were obtained through post-annealing the ZrO₂-V composite films in a tube furnace. Specifically, the films were specifically placed in a tube furnace with an air pressure of 1000 Pa. the temperature was ramped up to 450 °C and held for 1 h at a rate of 5 °C/min, followed by a natural cooling process. For comparison, ZrO₂-VO₂ composite films with different sputtering power of ZrO₂ were denoted as sample ZrO₂-30 W, ZrO₂-60 W, ZrO₂-90 W, and ZrO₂-120 W.

ZrO₂-30 W, ZrO₂-60 W, ZrO₂-90 W, and ZrO₂-120 W were placed in a PTFE etching flower basket and each was firstly subjected to a 5 s treatment in hydrochloric acid at a molar concentration of 5.6 mol/L, noting that there was no obvious change in optical properties after the acid-solution process. The acid-solution-process time was therefore increased to 20 s. Immediately after corrosion, the films were taken out of the acid solution and ultrasonically cleaned for one minute in deionized water and anhydrous ethanol. The obtained VO₂-based films were dried with N₂ and labelled as ZrO₂-30 W-acid, ZrO₂-60 W-acid, ZrO₂-90 W-acid, and ZrO₂-120 W-acid. The preparation conditions of pure VO₂ film and ZrO₂-doped VO₂ film are shown in

Table 1. Preparatory conditions of pure VO₂ film and ZrO₂-doped VO₂ film.

Sample	Sputtering Pressure	Preparation of VO2 Thin Films		Preparation of ZrO2 Thin Films		Acid Treatment
		Sputtering Power	Sputtering Time	Sputtering Power	Sputtering Time
Pure VO2 film	1 × 103 Pa	90 W	15 min		9 times for 30 s each time
ZrO2-30 W				30 W
ZrO2-60 W				60 W
ZrO2-90 W				90 W
ZrO2-120 W				120 W
ZrO2-30 W-acid				30 W		20-s
ZrO2-60 W-acid				60 W
ZrO2-90 W-acid				90 W
ZrO2-120 W-acid				120 W

.

The crystal structure of the film was characterized on an Empyrean diffractometer using grazing angle X-ray diffraction (GAXRD) measurement (Cu Kα, λ = 0.154178 nm, generated at 4 kW output power). The morphology and element distribution of the film were characterized by a field-emission scanning-electron microscope (SEM, JSM-5610LV, Tokyo, Japan). The valence and composition of elements of the film were determined by X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi/ESCALAB 250Xi. Thermo Fisher, Waltham, MA, USA). To evaluate the optical properties of composite films, the solar transmittance of the film in the range of 300–2500 nm at 20 °C and 90 °C was analyzed by an ultraviolet-visible near-infrared spectrophotometer (UV-3600, Shimadzu Corporation, Kyoto, Japan). The integrals T_lum and ΔT_sol were calculated with the following Formulas (1) and (2) [1,2,3,19,20,21,22,23,24,25].

$$T_{\mathrm{lum}}={{\displaystyle \int }}_{380}^{780}{\nu }_{\mathrm{m}}\left(\mathsf{\lambda }\right)T\left(\mathsf{\lambda }\right)d\mathsf{\lambda }/{{\displaystyle \int }}_{380}^{780}{\nu }_{\mathrm{m}}\left(\mathsf{\lambda }\right)d\mathsf{\lambda }$$

(1)

$$T_{\mathrm{sol}}={{\displaystyle \int }}_{300}^{2500}{\phi }_{\mathrm{sol}}\left(\mathsf{\lambda }\right)T\left(\mathsf{\lambda }\right)d\mathsf{\lambda }/{{\displaystyle \int }}_{300}^{2500}{\phi }_{\mathrm{sol}}\left(\mathsf{\lambda }\right)d\mathsf{\lambda }$$

(2)

The ΔT_sol was calculated with Formula (3).

$$\mathsf{\Delta }{T}_{sol}=T_{sol}\left(20°C\right)-T_{sol}\left(90°C\right)$$

(3)

where T(λ) is the transmittance, λ is the wavelength of the incident light, ν_m(λ) denotes the spectral sensitivity of the light to the human eye, and φ_sol(λ) represents the irradiance spectrum of the sunlight at an atmospheric mass of 1.5 (corresponding to the sun from the horizon 37°) [1,2,3,26,27,28,29].

3. Results and Discussion

3.1. Structures of the Films before and after Acid-Solution Treatment

The XRD patterns of the samples before (ZrO₂-30 W, ZrO₂-60 W, ZrO₂-90 W, ZrO₂-120 W) and after (ZrO₂-30 W-acid, ZrO₂-60 W-acid, ZrO₂-90 W-acid, ZrO₂-120 W-acid) the acid-solution procedure are shown in Figure 2. It can be seen that every sample had strong diffraction peaks at 2θ = 27.9°, 37.1°, 42.4°, and 55.6°, which could be matched to VO₂ (M) (JCPDS No.44-252, a = 5.753Å, b = 4.526Å, c = 5.383Å, space group: P2₁/c, α = 90°, β = 122.6°). No further noticeable peaks were observed, with the exception of the amorphous peak of the glass substrate at 2θ = 20°.This indicates that VO₂ in the film only exists in the form of M phase. In the XRD diagram, we did not find an obvious diffraction peak of ZrO₂, which may have been due to the short sputtering time of ZrO₂, and the content of ZrO₂ prepared was lower than the XRD measurement limit. According to Goedicke’s experimental analysis, ZrO₂ films prepared by magnetron sputtering are crystalline without annealing. There were obvious diffraction peaks at ZrO₂ (111), ZrO₂ (220), ZrO₂ (211), and ZrO₂ (220), and they grew preferentially with the increase in sputtering power. The successful preparation of ZrO₂ was proven in the subsequent XPS test analysis [30,31,32,33].

From Figure 2a, it is clear that the VO₂ film after the introduction of ZrO₂ grains grew preferentially along the (020) crystal plane corresponding to 2θ = 37.1°. Additionally, the intensity of the peak at 2θ = 37.1° was progressively declined as sputtering power increased, showing the benefit of a proper ZrO₂ concentration for the preferred growth of VO₂ grains along the (020) crystal plane. Moreover, the film’s diffraction-peak intensity was decreased when the sputtering power reached 120 W. This is probably because the excess ZrO₂ affected the crystallization of VO₂. The intensity of the peak at 2θ = 37.1° was gradually decreased as the ZrO₂ power increased after the acid-solution process, whereas the intensity of the peak at 2θ = 27.9° gradually increased, as shown in Figure 2b. This indicates that hydrochloric acid preferentially corrodes (020) facets of VO₂ grains during the acid process [32,33,34,35].

In order to further investigate the impact of the acid-solution process of the morphology evolution of the samples, SEM characterization was performed on samples ZrO₂-60 W, ZrO₂-60 W-acid, ZrO₂-90 W, and ZrO₂-90 W-acid, and the results are shown in Figure 3. It can be seen that the film’s grain size was comparatively homogeneous and became small after the acid-solution process. As the power increased, the film became rougher and gradually thicker before the acid-solution process. After the acid-solution process, voids gradually developed in the films to form numerous islands on the surface that resulted in a strong surface-plasmon-resonance (LSPR) effect and a higher ΔT_sol, and the thickness of the films gradually decreased, which was extremely encouraging for achieving an increase in the films’ T_lum [36,37,38].

3.2. Thermochromic Properties of VO₂-Based Films

Figure 4a,b exhibit the solar transmittances of samples ZrO₂-30 W, ZrO₂-60 W, ZrO₂-90 W, ZrO₂-120 W, and samples ZrO₂-30 W-acid, ZrO₂-60 W-acid, ZrO₂-90 W-acid, and ZrO₂-120 W-acid, respectively, at 20 °C and 90 °C.

Table 2. The solar-modulation efficiency (ΔT_sol) and visible transmittance (T_lum) of the obtained films.

Sputtering Power of ZrO2 (W)	ZrO2-VO2 Composite Film before Acid-Solution Process		ZrO2-VO2 Composite Film after Acid-Solution Process
	Tlum (%)	ΔTsol (%)	Tlum (%)	ΔTsol (%)
0	28.2	12.4	-	-
30	46.9	13.3	47.1	12.2
60	40.2	14.3	45.0	12.6
90	38.2	11.9	51.1	9.4
120	31.2	9.2	41.9	7.7

lists the films’ solar-modulation efficiency (ΔT_sol) and luminous transmittance (T_lum). When the sputtering power of the V target was 90 W, the pure VO₂ film obtained by magnetron sputtering showed a ΔT_sol of 12.4% and T_lum of 28.2%. The VO₂-based films’ T_lum increased dramatically when ZrO₂ was introduced, reaching 46.9% when the ZrO₂ target’s sputtering power was 30 W (ZrO₂-30 W). This is because the introduction of ZrO₂ decreased the refractive index of VO₂-based films. Furthermore, the introduction of ZrO₂ improved the crystallinity of the film, resulting in an increase in ΔT_sol to 13.3% at 30 W (ZrO₂-30 W-acid) and 14.3% at 60 W. (ZrO₂-60 W-acid). However, when the power exceeded 60 W (ZrO₂-90 W and ZrO₂-120 W), too much ZrO₂ affected the oxidation and crystallization process of the V film, resulting in a decrease in T_lum and ΔT_sol. After a 20 s treatment in hydrochloric acid at a molar concentration of 5.6 mol/L, the obtained films showed very good acid resistance. From Figure 5 and Table 2, the T_lum was increased after the acid-solution process, probably attributable to the reduction in film thickness and the generation of few pores in the film. The reduction of VO₂ content in the film probably led to a decrease in ΔT_sol. Consequently, it is unrealistic to enhance T_lum by extending acid-solution-processing time, which is in good agreement with previous works [32]. In particular, the ZrO₂-VO₂ obtained after the acid-solution process (ZrO₂-90 W-acid) exhibited the highest T_lum of 51.1% while keeping a good ΔT_sol of 9.4%.

In order to investigate the effect of the acid-solution process on the composition of island-like ZrO₂-VO₂ composite film, XPS characterization was performed on samples ZrO₂-90 W and ZrO₂-90 W-acid, and the results are shown in Figure 6. It is evident that both films contained C, V, Zr, and O elements. The binding energies for O 1s, V 2p, Zr 3d, and C 1s, where the C contents came from adventitious carbon, were 530 eV, 515 eV, 180 eV, and 284.8 eV, respectively.

As seen in Figure 7a,b, the peak of Zr 3d could be fitted into Zr 3d_5/2 and Zr 3d_3/2, located at 182.1 eV and 184.5 eV, respectively, corresponding to Zr⁴⁺. After the acid-solution process, the peaks of Zr 3d_5/2 and Zr 3d_3/2 were located at 182.2 eV and 184.6 eV, respectively, which also corresponded to Zr⁴⁺ in ZrO₂, and the intensity of the peak was almost unchanged, indicating that the Zr content was nearly unchanged after the acid-solution process. This is probably due to the excellent chemical stability ZrO₂ displayed in acidic solutions. After the acid-solution process, only a small amount of Zr element was probably reacted. This is confirmed by the XPS result in Figure 6. The results shows that the molar ratio of Zr/V increased from 0.02 to 0.05 during the acid-solution process, indicating that V was lost from the film at a faster rate than Zr. The 20 s of the acid-solution process did not entirely remove Zr [39]. If extending the processing time, the solar-modulation rate of the films would decrease significantly. The characteristic peak of V in the sample before the acid-solution process had two peaks corresponding to V 2p_1/2 and V 2p_3/2, as shown in Figure 7c. For V 2p_3/2, there were two binding energy peaks located at 516.2 eV and 517.5 eV that could be assigned to +4 and +5 valences of V, respectively [5,40,41]. The primary source of V’s +5 valence was V₂O₅, indicating that the film surface was partly oxidized. O 1s spectra could be fitted into two peaks. The peak at ~530 eV was attributed to the V-O bond in the crystal lattice, whereas the peak at ~532 eV belonged to the surface-chemisorbed oxygen species [30,31]. After the acid-solution process, the film’s peak position was barely altered, whereas the content of the chemisorbed oxygen species was decreased, indicating that the island-like ZrO₂-VO₂ composite film exhibited good oxygen resistance, different from previous works [38]. The resistance was recorded at gradient temperatures ranging from 20 °C to 90 °C in order to determine the phase-transition temperature (T_c) of samples ZrO₂-90 W and ZrO₂-90 W-acid. Figure 8 depicts the electrical hysteresis loop of the film. After the introduction of ZrO₂ to the film, it can be seen that the T_c of the film increased to 52 °C, which may be attributed to the interfacial stresses caused by the ZrO₂ and VO₂ grains. Additionally, the electrical hysteresis loop’s breadth was about 8 °C. After the acid-solution process, some channels or voids were generated in the film and the stress between the crystal grains was released; hence, the T_c of the film rose to 58 °C and the width of the electrical hysteresis loop was expanded to 12 °C, which can be mainly attributed to the hysteresis effect in the thermal transmission process caused by the channels or voids between the grains [37,38,39].

Above all, the island-like ZrO₂-VO₂ composite films (ZrO₂-90 W-acid), especially those prepared by magnetron sputtering, exhibited improved visible transmittance and superior solar modulation compared with pure VO₂ films. As demonstrated in

Table 3. Comparison of thermochromic performance of this work with previous works.

	Thermochromic Properties		Reference
System	Tlum (%)	ΔTsol (%)
VO2	39.93	9.95	Sang et al. [7]
VO2	55.0	18.0	Kim et al. [42]
W/Zr-doped VO2	60.7	10.6	Guo et al. [8]
Zn-doped VO2	41.3	15.3	Kang et al. [43]
Terbium-doped VO2 film	54.0	8.3	Wang et al. [44]
Two-dimensional nanostructure VO2 film	61.3	11.9	Long et al. [45]
W-doped VO2 film	61.7	11.7	Zhang et al. [46]
Island-like ZrO2-VO2 composite films	45.0 51.1	12.6 9.4	This work

, the thermochromic performance attained was superior to that in the majority of earlier works.

4. Conclusions

In this work, ZrO₂-VO₂ composite films with enhanced thermochromic properties were prepared by a combination of magnetron sputtering and an acid-solution process. After the acid-solution treatment, the grains with poor crystallinity in the film were etched, and some channels or voids were generated at the same time, leading to an increase in the T_lum of the film. Without affecting the oxidation of the V film, the ΔT_sol of the film was increased with the increment of the ZrO₂ content because its introduction could improve the crystallinity of the film. Excessive ZrO₂ probably affected the crystallization and oxidation process of V in the film, leading to a decrease in the ΔT_sol and T_lum. When the sputtering power of ZrO₂ was 30 W and 60 W, the composite films prepared by the acid-solution process exhibited better thermochromic properties. Therefore, this work can provide a very facile and effective method to prepare VO₂-based films with good thermochromic performance for smart windows.