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VO2-ZnO composite films with enhanced thermochromic properties for smart windows
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VO2-ZnO composite films with enhanced thermochromic properties for smart windows Bin Li, Jiahui Liu, Shouqin Tian∗, Baoshun Liu, Xinwei Yang, Zhao Yu, Xiujian Zhao∗∗ State Key Laboratory of Silicate Materials for Architectures, Wuhan University of Technology (WUT), No. 122, Luoshi Road, Wuhan, 430070, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Vanadium dioxide Ball milling method VO2-ZnO composite film Antireflection Optical properties
VO2 film is a promising thermochromic material in smart windows due to its reversible metal to insulator transition (MIT) accompanied with an abrupt change of transmittance in near-infrared region at around 68 °C (T > 68 °C, translucent; T < 68 °C, transparent), but which has not been widely applied because its low luminous transmittance (< 60%) and solar modulation efficiency (< 10%) are difficult to be improved simultaneously. In order to solve this problem, the ZnO-VO2 composite film was prepared by a facile method, in which commercial ZnO nanoparticles (NPs) and VO2 micro-particles were mixed by ball milling method to form the composites. By introducing ZnO NPs into the composite film, the luminous transmittance (Tlum) of the composite film was increased by 16.9% (from 54.9% to 63.9%) and the solar modulation efficiency (ΔTsol) was increased by 14.1% (from 9.9% to 11.3%) compared to the pure VO2 composite film. This was because ZnO NPs not only played the role of antireflection, but also prevented VO2 particles from agglomeration by dispersing around VO2 particles. Furthermore, the two-layered film based on ZnO-VO2 composites exhibited an astonishing ΔTsol of 18.8%, while maintaining excellent Tlum of 54.3%. This work could provide a simple and novel idea for us to improve the thermochromic properties of VO2 films and simultaneously to promote their practical application.
1. Introduction At present, the shortage of non-renewable energy and environmental pollution have been the biggest problem towards the world [1]. Therefore, the development and utilization of renewable energy such as solar energy have attracted more and more attention in recent years. And the thermochromic materials based ‘smart window’ can respond to environmental temperature and intelligently regulate the solar radiation entering the room and thus has been considered to greatly reduce the usage of air conditioner and building energy consumption [2,3]. Among all thermochromic materials, VO2 is undoubtedly the most promising one because of its reversible metal to insulator transition (MIT) accompanied with an abrupt change of transmittance in nearinfrared region at around 68 °C (T > 68 °C, translucent; T < 68 °C, transparent) [4,5]. Ideally, in cold winter, the near-infrared light can go through the VO2-based smart windows to keep the room warmth, but in hot summer, the near-infrared light is mostly reflected to the outside to prevent the room temperature from rising [6]. However, the inherent low luminous transmittance (Tlum < 60%), solar modulation efficiency (ΔTsol < 10%) and the mutual restriction between them greatly limited
∗
the practical application of pure VO2 films on smart windows [7,8]. According to the recent works, the fabrication of VO2-based nanocomposite film is considered to be one of the most effective way to address this issue [9,10]. And many works on VO2 nanocomposite films have been carried out [11–13]. Chen et al. successfully synthesized fine crystalline VO2 nanoparticles (NPs) with diameter of 25–45 nm by hydrothermal method [13], the best thermochromic properties (Tlum = 45.6%, ΔTsol = 22.3%) of the film obtained by dispersing nanoparticles in polyurethane (PU) evenly. Furthermore, doped VO2 NPs was also embedded into the matrix to improve thermochromic properties, and the Ti-doped VO2 nanoparticles-based nanocomposite film showed a 15% increase (from 46.1% to 53%) in Tlum and a 28% increase (from 13.4% to 17.2%) in ΔTsol compared to pure VO2 film [11]. It is believed that well crystalline VO2 NPs with a diameter under 20 nm uniformly dispersed in the medium will display the outstanding performance [9,14], but the hydrothermal synthesis of NPs often required severe conditions such as high temperature, high pressure and the long reaction times of over 24 h required [5]. Another effective strategy, the multilayered structure with antireflection coatings also has been designed to enhance the optical properties of VO2 film [15–18]. Above all,
Corresponding author. Corresponding author. E-mail addresses: [email protected] (S. Tian), [email protected] (X. Zhao).
∗∗
https://doi.org/10.1016/j.ceramint.2019.09.264 Received 22 August 2019; Received in revised form 6 September 2019; Accepted 26 September 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Bin Li, et al., Ceramics International, https://doi.org/10.1016/j.ceramint.2019.09.264
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ZnO has widely demonstrated to be a promising antireflection coating because of its high refractive index of 2.0 [17,19,20], the ZnO/VO2 bilayer film prepared by magnetron sputtering showed improved ΔTsol from 7.7% to 12.2% and appropriate Tlum of 50.3% [17]. However, it is too complex to be fabricated, most importantly, it is inferior in performance to the VO2 nanocomposite film. Therefore, it is still a great challenge to combine the two effective strategies and make the best of their advantages to improve the thermochromic properties of VO2 films to a crucial level. Inspired by the imagination, the VO2-ZnO composites have been fabricated by an economical and facile ball-milling method and then the composite films were fabricated by spin-coating the mixture on the float silica glass in this work. Hereby, ZnO NPs was creatively introduced into the composite to optimize the thermochromic properties of the VO2 composite films. With the increase of ZnO contents, the thermochromic properties of the film were constantly increased, the outstanding performance (Tlum = 64.2%, ΔTsol = 11.3%) was obtained for the sample with molar ratio of VO2/ZnO at 1:1. Furthermore, the film coated with two layers showed ultra-high ΔTsol of 18.8%, while still maintain excellent Tlum of 54.3%, which is superior to most VO2 nanocomposite films and multilayered VO2 films. This work could provide luciferous thought to enhance the performance of VO2 coatings and greatly promote the industrialization.
Table 1 Content of ZnO NPs in different samples. Sample
VO2 (g)
ZnO (g)
Molar ratio of VO2:ZnO
VO2 S1 S2 S3
0.2074 0.2074 0.2074 0.2074
0 0.1017 0.2035 0.4070
– 1:0.5 1:1 1:2
2.3. Characterization The phase of the powders was characterized by X-ray diffraction (XRD, D8DISCOVER) at the power of 3 kW produced by the Cu Kα radiation, and scanning speed was set as 4°/min. The morphology and element distribution were investigated through a field emission scanning electron microscopy (FE-SEM, JSM-7500F, Japan JEOL) with an energy dispersive spectrometer (EDS). The transmittance and reflectance (300≤λ ≤ 2500 nm) of the films were obtained by UV–vis–NIR spectrophotometer (UV-3600, SHIMADZU) equipped with a heating apparatus at 30 °C and 90 °C, respectively. 3. Result and discussion 3.1. Structure and morphology of particles
2. Experimental section
Fig. 1a shows the XRD pattern of the commercial ZnO NPs, the crystallinity and purity of the particles is very good, so it can be used directly in the experiment. The XRD patterns of VO2 obtained through annealing treatment of V2O5 powders is shown in Fig. 1b. All the diffraction peaks matched well with the VO2(M) (PDF No: 82-661), and there were no peaks indexed to other vanadium oxides. The strong intensity of all peaks meant the good crystallinity of the particles, which is important to obtain splendid ΔTsol. Moreover, the fabrication process is highly reproducible and scalable. The above two powders were mixed in ethanol in different proportions, and the XRD pattern of the mixed sample obtained after evaporating ethanol is depicted in Fig. 1c. It can be seen that all the diffraction peaks of the three samples corresponded to a mixture of VO2(M) and ZnO respectively. According to previous work [21], Zn2V2O7 was easily formed by the reaction of VO2 and ZnO during heat treatment, but the result shows no diffraction peak of Zn2V2O7, which is beneficial for the ΔTsol positively. The appearance of V6O13 (corresponding to a weak diffraction peak at around 30°) in sample S2 and S3 due to part oxidation of the VO2(M) during evaporation. The morphology of powders was investigated by SEM measurement. Fig. 2a and b are the morphology of VO2(M). The powders exhibited large particle size (diameter of 300–1000 nm) because of the smaller ellipsoidal shaped particles adhere to each other and further growing to form large particles with irregular shape after annealing. However, the powder does not stack and bonding in large-scale but only partially agglomerate compared to that fabricated by pyrolysis method [22], which was contributed to uniform dispersion during ball-milling process. It can be seen in Fig. 2c, commercial ZnO powders with various shapes (including rods, granules, and flakes) show a small particle size of about 30 ± 10 nm. Fig. 2d, c, e present the SEM images of the
2.1. Preparation of VO2(M) particles The VO2(M) particles were obtained by annealing treatment with 5 g of commercial V2O5 powders (Xiya reagent co., Ltd., 99.5%) and 0.25 g of ammonium bicarbonate (Sinopharm Chemical Reagent Co., Ltd., AR) in atube furnace at 450 °C for 30 min, in which the vacuum was controlled under 200 Pa.
2.2. Preparation of VO2-ZnO composite films 0.2074 g of as-prepared VO2(M) particles and 0.2035 g of commercial ZnO NPs (Shanghai Macklin Biochemical Co., Ltd., 99.9%, Diameter ca. 30 ± 10 nm) were added into 50 mL of ethanol, the suspension was continuously stirring for 1.5 h with covering by plastic wrap. Then the ethanol was slowly evaporated at 60 °C for 5 h. After the ethanol being removed, the obtained mixed VO2-ZnO particles, 0.2055 g of polyvinylpyrrolidone (PVP, K30, Sinopharm Chemical Reagent Co., Ltd.,) and 8 mL of ethanol was milled in a 100 mL ball mill tank that containing 60 g of zirconia balls at 400 r/min for 8 h. The mixture obtained after ball-milling was centrifuged, then the suspension was coated on a common soda-lime-silica glass with the process of 500 r/ min for 20 s followed by 1000 r/min for 10 s. The detailed preparation process of VO2-ZnO composite films was presented in Scheme 1. All preparation methods were the same but the dosages of ZnO nanoparticles were different and listed in Table 1. And the content of PVP were half of the total mass of VO2-ZnO composites. All materials were used without further purification treatment in the experiment.
Scheme 1. The diagram for the fabrication process of VO2-ZnO composite film. 2
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Fig. 1. XRD patterns of (a) commercial ZnO, (b) VO2 and (c) different samples of VO2-ZnO composites with different molar ratios obtained after the evaporation of ethanol.
In which, T(λ) represents film transmittance of light at certain wavelength λ, ϕlum (λ) is the standard luminous efficiency function for the photopic vision of human eyes [23], ϕsol (λ) is the solar irradiance spectrum for air mass 1.5 corresponds to the sun standing 37° above the horizon [24]. In addition, the solar modulation efficiency (ΔTsol) was attained from ΔTsol = Tsol (30°C) − Tsol (90°C) , and the Tlum and ΔTsol are summarized in Table 2. It can be observed from Fig. 3a that the VO2-ZnO composite film exhibit splendid thermochromic performance with Tlum of 64.2% while ΔTsol is still maintained at 11.3%. Obviously, its optical properties satisfy the requirement of VO2 film applied on the smart windows [8]. Furthermore, compared with VO2 film, VO2-ZnO composite film significantly boost the optical properties with a 16.9% increased (from 54.9% to 64.2%) in Tlum and a 14.1% increased (from 9.9% to 11.3%) in ΔTsol. In order to investigate the effect of different ZnO contents on the optical properties of the composite films, the thermochromic properties of samples S1, S2, S3 were compared in Fig. 3b and d. Excitingly, Tlum increases (54.9% to 68.2%) with ZnO content increases (0 to 200%) monotonically (Fig. 3c and Table 2), and sample S3 shows a maximum Tlum of 68.2% (optimized by 24.2% compared with sample VO2), with regard to ΔTsol of samples VO2, S1, S2, the variation trend dependent on ZnO content is similar to that of Tlum. This phenomenon can easily explain by the reflectance spectrums depicted in Fig. 3c, it can be seen that the reflectance of different samples was decreased with increasing the content of ZnO NPs. This indicates that ZnO NPs dispersed uniformly in the VO2-ZnO composite film can also reduce the
samples after mixing particles in Fig. 2a and c with different molar ratios, and the similar morphology and particle size with that of powders in Fig. 2c revealed only the ZnO has been observed, which probably due to the higher magnification of the image, and VO2 particles are too large to been viewed in the image. The ZnO NPs acting as a dispersing medium were continuously filled in the voids between the VO2 powders during the ball milling process. 3.2. Thermochromic properties and morphology of VO2-ZnO composite films The phase transition temperature (Tc) of VO2-ZnO composites are all around 64 °C, and there is no change compared with the VO2 particles (as shown in Fig. S1 and Table S1). Subsequently, in order to characterize the optical properties of the VO2-ZnO composites, samples with different molar ratios of VO2:ZnO have been spin-coated on a normal float sodium-calcium-silicon glass to fabricate thin films. The transmittance of the films was measured from 300 to 2500 nm by UV–vis–NIR spectrophotometer at 30 °C and 90 °C respectively, the results are shown in Fig. 3. Meanwhile, the integral luminous transmittance (Tlum, 380-780 nm) and solar transmittance (Tsol, 300-2500 nm) were calculated from the transmittance spectrum of the film according to the equation below.
Tlum / sol =
∫ ϕlum/sol (λ) T (λ)dλ/∫ ϕlum/sol (λ)dλ
Fig. 2. SEM (a) and high-magnification SEM (b) images of VO2(M) powders, SEM image of (c) commercial ZnO and sample (d) S1, (e)S2, (f) S3 after the evaporation of ethanol. 3
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Fig. 3. Transmittance spectrum of (a) pure VO2 film and VO2-ZnO composite film and (b) samples S1, S2, S3, S22 at 30 °C and 90 °C respectively, (c) reflectance spectrum of samples VO2, S1, S2, S3 at 30 °C, (d) luminous transmittance (Tlum) and solar modulation efficiency (ΔTsol) dependent on molar ratio of ZnO to VO2.
than that of other samples, which greatly affected the ΔTsol and reflectance [4]. According to the previous work [25], as the thickness of the film was increased, the ΔTsol was remarkably improved with a part of scarification of Tlum. Therefore, then the S22 (coated sample S2 twice) was synthesized to further optimize the ΔTsol of VO2-ZnO composite films. As a result, sample S22 exhibited ultra-high ΔTsol of 18.8% as expected, and applicable Tlum of 54.3% (˃50%) is retained meantime. The optical performance is more excellent than that of most reported VO2 nanocomposite films and multilayered VO2 films as shown in Table 3. The thickness of sample S22 is about 950 nm in Fig. 4a, it means a high content of VO2 contained in the film, which is considered to improve the ΔTsol. Besides, the backscattered SEM image depicted in Fig. 4b shows that the VO2 particles (white spot) were distributed in the film evenly. This conclusion can also be demonstrated convincingly by the distribution of V element in Fig. 4c. A good dispersity of VO2 NPs in the VO2 nanocomposite films is essential to acquire outstanding Tlum, which has been testified by previous work [9,14,26]. Fig. 4d presents the EDS spectra on the surface of the film, and the ratio of Zn (15.49%)
Table 2 Thermochromic properties of different samples. Sample
VO2 S1 S2 S3 S22
Molar ratio of VO2 to ZnO
– 1:0.5 1:1 1:2 1:1
Tsol (%)
Tlum (%)
30 °C
90 °C
30 °C
90 °C
56.5 61.7 63.3 65.5 54.7
46.6 50.9 52.0 57.7 35.9
54.9 63.9 64.2 68.2 54.3
53.2 60.1 59.5 66.5 43.3
ΔTsol (%)
9.9 10.8 11.3 7.8 18.8
reflectance of VO2 films as same as the anti-reflection coatings (ARCs) which boosts both Tlum and ΔTsol simultaneously [15,17,19]. However, as the molar ratio of ZnO/VO2 reaching to 2:1, the ΔTsol of sample S3 was degraded from 11.3% to 7.8% significantly compared to sample S2, even lower than that of sample VO2 (9.9%). Analogously, the enormously reduction also emerges in the reflectance of sample S3. This is probably because of the high content of ZnO which is twice that of VO2, hence it is foreseeable that the content of VO2 in the film is much less
Table 3 Comparison of this work with some most reported VO2 nanocomposite films and multilayered VO2 films with outstanding optical properties. VO2 films
Synthesis method
Tlum (%)
ΔTsol (%)
Ref.
VO2 nanoparticles-based nanocomposite film VO2-Sb:SnO2 composite film TiO2(A)–VO2(M/R) nanocomposite films Ti-doped VO2 film VO2/Si-Al gel nanocomposite film VO2@ZnO nanoparticles-based thermochromic film VO2/ZnO bilayer film TEOS/VO2bilayer film VO2/ZnO composite film (our study)
Hydrothermal Hydrothermal Hydrothermal Hydrothermal Ball milling Hydrothermal Magnetron sputtering Sol-gel Ball milling
45.6 51.4 62.0 53.0 63.7 51.0 50.3 52.7 54.3
22.3 11.7 14.6 17.2 12.0 19.1 12.2 16.4 18.8
[13] [27] [28] [11] [8] [26] [17] [15]
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Fig. 4. (a) SEM image of the cross section, (b) backscattered SEM image of the film surface, (c) EDS spectrum of a region on the surface and (d) V (e) O (f) Zn element mapping of sample S22.
4. Conclusion
and V (13.25%) elements is approximately equal to 1:1, which is consistent with the original proportion of the powders. The element mapping images of O and Zn are shown in Fig. 4e and f. It can be seen clearly that the distribution of O and Zn elements is also very uniform and there is no partial aggregation in the film. These fully confirm that the ball milling method makes the powder disperse very uniformly in the matrix, which is beneficial to increase the Tlum of VO2-ZnO composite films. The explanation for the excellent thermochromic properties of VO2ZnO composite films is presented as follows. On the one hand, the mechanical ball milling can reduce the particle size of VO2 without distorting the crystal structure [8], which is beneficial for dispersing VO2 particles in the matrix uniformly, hence enhancing the Tlum but not reducing the ΔTsol. On the other hand, the transparent ZnO with appropriate refractive index (n = 2) play a role of antireflective effect in the VO2-ZnO composite films, which has been proved to make a great promotion on the optical properties [19,20,26]. Also, the size of VO2 particles is much larger than that of ZnO NPs. The suitable particle grading makes the ZnO NPs fill the gap between the VO2 particles during the ball milling process, the VO2 particles are sufficiently isolated without agglomeration. The model illustration of different films (VO2, S2 and S22) are shown in Fig. 5. In general, the excellent thermochromic properties were probably attributed to the well distribution and crystallinity of VO2 and the effect of antireflection of ZnO.
In summary, a simple and economical ball milling method has been utilized to prepare the VO2-ZnO composite films. Interestingly, the ZnO NPs is introduced to VO2 composite film to enhance the thermochromic properties obviously. And a two-layered film performed excellent optical properties (ΔTsol = 18.8%, Tlum = 54.3%). This is mostly ascribed to the antireflective effect of ZnO. In addition, the ZnO NPs was embedded into the composite to prevent VO2 particles from aggregation with the ball milling method which resulted in the ultra-high dispersity of VO2 particles in the films also play an important part of the improvement of thermochromic properties. Hereby, this work proposed a new thought to enhance the thermochromic properties, and the solution will have a great impetus to the commercial application of VO2-based smart windows.
Acknowledgments This work was supported by the National Natural Science Foundation of China (Grant No. 51772229), "111" project (No. B18038), National innovation and entrepreneurship training program for college students (No. 201910497034) and the Fundamental Research Funds for the Central Universities (No. 195201024). We also thank the Analytical and Testing Center of WUT for the help with carrying out XRD, TEM, and FESEM analyses.
Fig. 5. The schematic working mechanism of different films (VO2, S2 and S22). 5
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Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.ceramint.2019.09.264.
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VO2-ZnO composite films with enhanced thermochromic properties for smart windows Bin Li, Jiahui Liu, Shouqin Tian∗, Baoshun Liu, Xinwei Yang, Zhao Yu, Xiujian Zhao∗∗ State Key Laboratory of Silicate Materials for Architectures, Wuhan University of Technology (WUT), No. 122, Luoshi Road, Wuhan, 430070, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Vanadium dioxide Ball milling method VO2-ZnO composite film Antireflection Optical properties
VO2 film is a promising thermochromic material in smart windows due to its reversible metal to insulator transition (MIT) accompanied with an abrupt change of transmittance in near-infrared region at around 68 °C (T > 68 °C, translucent; T < 68 °C, transparent), but which has not been widely applied because its low luminous transmittance (< 60%) and solar modulation efficiency (< 10%) are difficult to be improved simultaneously. In order to solve this problem, the ZnO-VO2 composite film was prepared by a facile method, in which commercial ZnO nanoparticles (NPs) and VO2 micro-particles were mixed by ball milling method to form the composites. By introducing ZnO NPs into the composite film, the luminous transmittance (Tlum) of the composite film was increased by 16.9% (from 54.9% to 63.9%) and the solar modulation efficiency (ΔTsol) was increased by 14.1% (from 9.9% to 11.3%) compared to the pure VO2 composite film. This was because ZnO NPs not only played the role of antireflection, but also prevented VO2 particles from agglomeration by dispersing around VO2 particles. Furthermore, the two-layered film based on ZnO-VO2 composites exhibited an astonishing ΔTsol of 18.8%, while maintaining excellent Tlum of 54.3%. This work could provide a simple and novel idea for us to improve the thermochromic properties of VO2 films and simultaneously to promote their practical application.
1. Introduction At present, the shortage of non-renewable energy and environmental pollution have been the biggest problem towards the world [1]. Therefore, the development and utilization of renewable energy such as solar energy have attracted more and more attention in recent years. And the thermochromic materials based ‘smart window’ can respond to environmental temperature and intelligently regulate the solar radiation entering the room and thus has been considered to greatly reduce the usage of air conditioner and building energy consumption [2,3]. Among all thermochromic materials, VO2 is undoubtedly the most promising one because of its reversible metal to insulator transition (MIT) accompanied with an abrupt change of transmittance in nearinfrared region at around 68 °C (T > 68 °C, translucent; T < 68 °C, transparent) [4,5]. Ideally, in cold winter, the near-infrared light can go through the VO2-based smart windows to keep the room warmth, but in hot summer, the near-infrared light is mostly reflected to the outside to prevent the room temperature from rising [6]. However, the inherent low luminous transmittance (Tlum < 60%), solar modulation efficiency (ΔTsol < 10%) and the mutual restriction between them greatly limited
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the practical application of pure VO2 films on smart windows [7,8]. According to the recent works, the fabrication of VO2-based nanocomposite film is considered to be one of the most effective way to address this issue [9,10]. And many works on VO2 nanocomposite films have been carried out [11–13]. Chen et al. successfully synthesized fine crystalline VO2 nanoparticles (NPs) with diameter of 25–45 nm by hydrothermal method [13], the best thermochromic properties (Tlum = 45.6%, ΔTsol = 22.3%) of the film obtained by dispersing nanoparticles in polyurethane (PU) evenly. Furthermore, doped VO2 NPs was also embedded into the matrix to improve thermochromic properties, and the Ti-doped VO2 nanoparticles-based nanocomposite film showed a 15% increase (from 46.1% to 53%) in Tlum and a 28% increase (from 13.4% to 17.2%) in ΔTsol compared to pure VO2 film [11]. It is believed that well crystalline VO2 NPs with a diameter under 20 nm uniformly dispersed in the medium will display the outstanding performance [9,14], but the hydrothermal synthesis of NPs often required severe conditions such as high temperature, high pressure and the long reaction times of over 24 h required [5]. Another effective strategy, the multilayered structure with antireflection coatings also has been designed to enhance the optical properties of VO2 film [15–18]. Above all,
Corresponding author. Corresponding author. E-mail addresses: [email protected] (S. Tian), [email protected] (X. Zhao).
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https://doi.org/10.1016/j.ceramint.2019.09.264 Received 22 August 2019; Received in revised form 6 September 2019; Accepted 26 September 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Bin Li, et al., Ceramics International, https://doi.org/10.1016/j.ceramint.2019.09.264
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ZnO has widely demonstrated to be a promising antireflection coating because of its high refractive index of 2.0 [17,19,20], the ZnO/VO2 bilayer film prepared by magnetron sputtering showed improved ΔTsol from 7.7% to 12.2% and appropriate Tlum of 50.3% [17]. However, it is too complex to be fabricated, most importantly, it is inferior in performance to the VO2 nanocomposite film. Therefore, it is still a great challenge to combine the two effective strategies and make the best of their advantages to improve the thermochromic properties of VO2 films to a crucial level. Inspired by the imagination, the VO2-ZnO composites have been fabricated by an economical and facile ball-milling method and then the composite films were fabricated by spin-coating the mixture on the float silica glass in this work. Hereby, ZnO NPs was creatively introduced into the composite to optimize the thermochromic properties of the VO2 composite films. With the increase of ZnO contents, the thermochromic properties of the film were constantly increased, the outstanding performance (Tlum = 64.2%, ΔTsol = 11.3%) was obtained for the sample with molar ratio of VO2/ZnO at 1:1. Furthermore, the film coated with two layers showed ultra-high ΔTsol of 18.8%, while still maintain excellent Tlum of 54.3%, which is superior to most VO2 nanocomposite films and multilayered VO2 films. This work could provide luciferous thought to enhance the performance of VO2 coatings and greatly promote the industrialization.
Table 1 Content of ZnO NPs in different samples. Sample
VO2 (g)
ZnO (g)
Molar ratio of VO2:ZnO
VO2 S1 S2 S3
0.2074 0.2074 0.2074 0.2074
0 0.1017 0.2035 0.4070
– 1:0.5 1:1 1:2
2.3. Characterization The phase of the powders was characterized by X-ray diffraction (XRD, D8DISCOVER) at the power of 3 kW produced by the Cu Kα radiation, and scanning speed was set as 4°/min. The morphology and element distribution were investigated through a field emission scanning electron microscopy (FE-SEM, JSM-7500F, Japan JEOL) with an energy dispersive spectrometer (EDS). The transmittance and reflectance (300≤λ ≤ 2500 nm) of the films were obtained by UV–vis–NIR spectrophotometer (UV-3600, SHIMADZU) equipped with a heating apparatus at 30 °C and 90 °C, respectively. 3. Result and discussion 3.1. Structure and morphology of particles
2. Experimental section
Fig. 1a shows the XRD pattern of the commercial ZnO NPs, the crystallinity and purity of the particles is very good, so it can be used directly in the experiment. The XRD patterns of VO2 obtained through annealing treatment of V2O5 powders is shown in Fig. 1b. All the diffraction peaks matched well with the VO2(M) (PDF No: 82-661), and there were no peaks indexed to other vanadium oxides. The strong intensity of all peaks meant the good crystallinity of the particles, which is important to obtain splendid ΔTsol. Moreover, the fabrication process is highly reproducible and scalable. The above two powders were mixed in ethanol in different proportions, and the XRD pattern of the mixed sample obtained after evaporating ethanol is depicted in Fig. 1c. It can be seen that all the diffraction peaks of the three samples corresponded to a mixture of VO2(M) and ZnO respectively. According to previous work [21], Zn2V2O7 was easily formed by the reaction of VO2 and ZnO during heat treatment, but the result shows no diffraction peak of Zn2V2O7, which is beneficial for the ΔTsol positively. The appearance of V6O13 (corresponding to a weak diffraction peak at around 30°) in sample S2 and S3 due to part oxidation of the VO2(M) during evaporation. The morphology of powders was investigated by SEM measurement. Fig. 2a and b are the morphology of VO2(M). The powders exhibited large particle size (diameter of 300–1000 nm) because of the smaller ellipsoidal shaped particles adhere to each other and further growing to form large particles with irregular shape after annealing. However, the powder does not stack and bonding in large-scale but only partially agglomerate compared to that fabricated by pyrolysis method [22], which was contributed to uniform dispersion during ball-milling process. It can be seen in Fig. 2c, commercial ZnO powders with various shapes (including rods, granules, and flakes) show a small particle size of about 30 ± 10 nm. Fig. 2d, c, e present the SEM images of the
2.1. Preparation of VO2(M) particles The VO2(M) particles were obtained by annealing treatment with 5 g of commercial V2O5 powders (Xiya reagent co., Ltd., 99.5%) and 0.25 g of ammonium bicarbonate (Sinopharm Chemical Reagent Co., Ltd., AR) in atube furnace at 450 °C for 30 min, in which the vacuum was controlled under 200 Pa.
2.2. Preparation of VO2-ZnO composite films 0.2074 g of as-prepared VO2(M) particles and 0.2035 g of commercial ZnO NPs (Shanghai Macklin Biochemical Co., Ltd., 99.9%, Diameter ca. 30 ± 10 nm) were added into 50 mL of ethanol, the suspension was continuously stirring for 1.5 h with covering by plastic wrap. Then the ethanol was slowly evaporated at 60 °C for 5 h. After the ethanol being removed, the obtained mixed VO2-ZnO particles, 0.2055 g of polyvinylpyrrolidone (PVP, K30, Sinopharm Chemical Reagent Co., Ltd.,) and 8 mL of ethanol was milled in a 100 mL ball mill tank that containing 60 g of zirconia balls at 400 r/min for 8 h. The mixture obtained after ball-milling was centrifuged, then the suspension was coated on a common soda-lime-silica glass with the process of 500 r/ min for 20 s followed by 1000 r/min for 10 s. The detailed preparation process of VO2-ZnO composite films was presented in Scheme 1. All preparation methods were the same but the dosages of ZnO nanoparticles were different and listed in Table 1. And the content of PVP were half of the total mass of VO2-ZnO composites. All materials were used without further purification treatment in the experiment.
Scheme 1. The diagram for the fabrication process of VO2-ZnO composite film. 2
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Fig. 1. XRD patterns of (a) commercial ZnO, (b) VO2 and (c) different samples of VO2-ZnO composites with different molar ratios obtained after the evaporation of ethanol.
In which, T(λ) represents film transmittance of light at certain wavelength λ, ϕlum (λ) is the standard luminous efficiency function for the photopic vision of human eyes [23], ϕsol (λ) is the solar irradiance spectrum for air mass 1.5 corresponds to the sun standing 37° above the horizon [24]. In addition, the solar modulation efficiency (ΔTsol) was attained from ΔTsol = Tsol (30°C) − Tsol (90°C) , and the Tlum and ΔTsol are summarized in Table 2. It can be observed from Fig. 3a that the VO2-ZnO composite film exhibit splendid thermochromic performance with Tlum of 64.2% while ΔTsol is still maintained at 11.3%. Obviously, its optical properties satisfy the requirement of VO2 film applied on the smart windows [8]. Furthermore, compared with VO2 film, VO2-ZnO composite film significantly boost the optical properties with a 16.9% increased (from 54.9% to 64.2%) in Tlum and a 14.1% increased (from 9.9% to 11.3%) in ΔTsol. In order to investigate the effect of different ZnO contents on the optical properties of the composite films, the thermochromic properties of samples S1, S2, S3 were compared in Fig. 3b and d. Excitingly, Tlum increases (54.9% to 68.2%) with ZnO content increases (0 to 200%) monotonically (Fig. 3c and Table 2), and sample S3 shows a maximum Tlum of 68.2% (optimized by 24.2% compared with sample VO2), with regard to ΔTsol of samples VO2, S1, S2, the variation trend dependent on ZnO content is similar to that of Tlum. This phenomenon can easily explain by the reflectance spectrums depicted in Fig. 3c, it can be seen that the reflectance of different samples was decreased with increasing the content of ZnO NPs. This indicates that ZnO NPs dispersed uniformly in the VO2-ZnO composite film can also reduce the
samples after mixing particles in Fig. 2a and c with different molar ratios, and the similar morphology and particle size with that of powders in Fig. 2c revealed only the ZnO has been observed, which probably due to the higher magnification of the image, and VO2 particles are too large to been viewed in the image. The ZnO NPs acting as a dispersing medium were continuously filled in the voids between the VO2 powders during the ball milling process. 3.2. Thermochromic properties and morphology of VO2-ZnO composite films The phase transition temperature (Tc) of VO2-ZnO composites are all around 64 °C, and there is no change compared with the VO2 particles (as shown in Fig. S1 and Table S1). Subsequently, in order to characterize the optical properties of the VO2-ZnO composites, samples with different molar ratios of VO2:ZnO have been spin-coated on a normal float sodium-calcium-silicon glass to fabricate thin films. The transmittance of the films was measured from 300 to 2500 nm by UV–vis–NIR spectrophotometer at 30 °C and 90 °C respectively, the results are shown in Fig. 3. Meanwhile, the integral luminous transmittance (Tlum, 380-780 nm) and solar transmittance (Tsol, 300-2500 nm) were calculated from the transmittance spectrum of the film according to the equation below.
Tlum / sol =
∫ ϕlum/sol (λ) T (λ)dλ/∫ ϕlum/sol (λ)dλ
Fig. 2. SEM (a) and high-magnification SEM (b) images of VO2(M) powders, SEM image of (c) commercial ZnO and sample (d) S1, (e)S2, (f) S3 after the evaporation of ethanol. 3
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Fig. 3. Transmittance spectrum of (a) pure VO2 film and VO2-ZnO composite film and (b) samples S1, S2, S3, S22 at 30 °C and 90 °C respectively, (c) reflectance spectrum of samples VO2, S1, S2, S3 at 30 °C, (d) luminous transmittance (Tlum) and solar modulation efficiency (ΔTsol) dependent on molar ratio of ZnO to VO2.
than that of other samples, which greatly affected the ΔTsol and reflectance [4]. According to the previous work [25], as the thickness of the film was increased, the ΔTsol was remarkably improved with a part of scarification of Tlum. Therefore, then the S22 (coated sample S2 twice) was synthesized to further optimize the ΔTsol of VO2-ZnO composite films. As a result, sample S22 exhibited ultra-high ΔTsol of 18.8% as expected, and applicable Tlum of 54.3% (˃50%) is retained meantime. The optical performance is more excellent than that of most reported VO2 nanocomposite films and multilayered VO2 films as shown in Table 3. The thickness of sample S22 is about 950 nm in Fig. 4a, it means a high content of VO2 contained in the film, which is considered to improve the ΔTsol. Besides, the backscattered SEM image depicted in Fig. 4b shows that the VO2 particles (white spot) were distributed in the film evenly. This conclusion can also be demonstrated convincingly by the distribution of V element in Fig. 4c. A good dispersity of VO2 NPs in the VO2 nanocomposite films is essential to acquire outstanding Tlum, which has been testified by previous work [9,14,26]. Fig. 4d presents the EDS spectra on the surface of the film, and the ratio of Zn (15.49%)
Table 2 Thermochromic properties of different samples. Sample
VO2 S1 S2 S3 S22
Molar ratio of VO2 to ZnO
– 1:0.5 1:1 1:2 1:1
Tsol (%)
Tlum (%)
30 °C
90 °C
30 °C
90 °C
56.5 61.7 63.3 65.5 54.7
46.6 50.9 52.0 57.7 35.9
54.9 63.9 64.2 68.2 54.3
53.2 60.1 59.5 66.5 43.3
ΔTsol (%)
9.9 10.8 11.3 7.8 18.8
reflectance of VO2 films as same as the anti-reflection coatings (ARCs) which boosts both Tlum and ΔTsol simultaneously [15,17,19]. However, as the molar ratio of ZnO/VO2 reaching to 2:1, the ΔTsol of sample S3 was degraded from 11.3% to 7.8% significantly compared to sample S2, even lower than that of sample VO2 (9.9%). Analogously, the enormously reduction also emerges in the reflectance of sample S3. This is probably because of the high content of ZnO which is twice that of VO2, hence it is foreseeable that the content of VO2 in the film is much less
Table 3 Comparison of this work with some most reported VO2 nanocomposite films and multilayered VO2 films with outstanding optical properties. VO2 films
Synthesis method
Tlum (%)
ΔTsol (%)
Ref.
VO2 nanoparticles-based nanocomposite film VO2-Sb:SnO2 composite film TiO2(A)–VO2(M/R) nanocomposite films Ti-doped VO2 film VO2/Si-Al gel nanocomposite film VO2@ZnO nanoparticles-based thermochromic film VO2/ZnO bilayer film TEOS/VO2bilayer film VO2/ZnO composite film (our study)
Hydrothermal Hydrothermal Hydrothermal Hydrothermal Ball milling Hydrothermal Magnetron sputtering Sol-gel Ball milling
45.6 51.4 62.0 53.0 63.7 51.0 50.3 52.7 54.3
22.3 11.7 14.6 17.2 12.0 19.1 12.2 16.4 18.8
[13] [27] [28] [11] [8] [26] [17] [15]
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Fig. 4. (a) SEM image of the cross section, (b) backscattered SEM image of the film surface, (c) EDS spectrum of a region on the surface and (d) V (e) O (f) Zn element mapping of sample S22.
4. Conclusion
and V (13.25%) elements is approximately equal to 1:1, which is consistent with the original proportion of the powders. The element mapping images of O and Zn are shown in Fig. 4e and f. It can be seen clearly that the distribution of O and Zn elements is also very uniform and there is no partial aggregation in the film. These fully confirm that the ball milling method makes the powder disperse very uniformly in the matrix, which is beneficial to increase the Tlum of VO2-ZnO composite films. The explanation for the excellent thermochromic properties of VO2ZnO composite films is presented as follows. On the one hand, the mechanical ball milling can reduce the particle size of VO2 without distorting the crystal structure [8], which is beneficial for dispersing VO2 particles in the matrix uniformly, hence enhancing the Tlum but not reducing the ΔTsol. On the other hand, the transparent ZnO with appropriate refractive index (n = 2) play a role of antireflective effect in the VO2-ZnO composite films, which has been proved to make a great promotion on the optical properties [19,20,26]. Also, the size of VO2 particles is much larger than that of ZnO NPs. The suitable particle grading makes the ZnO NPs fill the gap between the VO2 particles during the ball milling process, the VO2 particles are sufficiently isolated without agglomeration. The model illustration of different films (VO2, S2 and S22) are shown in Fig. 5. In general, the excellent thermochromic properties were probably attributed to the well distribution and crystallinity of VO2 and the effect of antireflection of ZnO.
In summary, a simple and economical ball milling method has been utilized to prepare the VO2-ZnO composite films. Interestingly, the ZnO NPs is introduced to VO2 composite film to enhance the thermochromic properties obviously. And a two-layered film performed excellent optical properties (ΔTsol = 18.8%, Tlum = 54.3%). This is mostly ascribed to the antireflective effect of ZnO. In addition, the ZnO NPs was embedded into the composite to prevent VO2 particles from aggregation with the ball milling method which resulted in the ultra-high dispersity of VO2 particles in the films also play an important part of the improvement of thermochromic properties. Hereby, this work proposed a new thought to enhance the thermochromic properties, and the solution will have a great impetus to the commercial application of VO2-based smart windows.
Acknowledgments This work was supported by the National Natural Science Foundation of China (Grant No. 51772229), "111" project (No. B18038), National innovation and entrepreneurship training program for college students (No. 201910497034) and the Fundamental Research Funds for the Central Universities (No. 195201024). We also thank the Analytical and Testing Center of WUT for the help with carrying out XRD, TEM, and FESEM analyses.
Fig. 5. The schematic working mechanism of different films (VO2, S2 and S22). 5
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Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.ceramint.2019.09.264.
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