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Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Highly Enhanced Thermochromic Performance of VO2 Film Using “Movable” Antireflective Coatings Fang Xu,†,‡,§ Xun Cao,*,†,‡ Zewei Shao,†,‡,§ Guangyao Sun,†,‡,§ Shiwei Long,†,‡,§ Tianci Chang,†,‡,§ Hongjie Luo,*,∥ and Ping Jin†,‡,⊥
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†
State Key Laboratory of High-Performance Ceramics and Superfine Microstructure, and ‡Research Center for Industrial Ceramics, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, China § University of Chinese Academy of Sciences, Beijing 100049, China ∥ School of Materials Science and Engineering, Shanghai University, Shanghai 200444, China ⊥ National Institute of Advanced Industrial Science and Technology (AIST), Moriyama, Nagoya 463-8560, Japan S Supporting Information *
ABSTRACT: The VO2 film with “movable” antireflective (AR) coating was initially designed and successfully prepared to deal with the challenges that VO2 faced. The “movable” AR layer, including water and other organic solvents, not only endowed VO2 with active-passive regulation mode but also dramatically enhanced the thermochromic performance. Furthermore, we combined solid and movable antireflection layer for further structural optimization, and the result turned out to be superior to any multilayer structure reported previously. We believe that this revolutionary concept of AR coating will open up a new avenue for low-cost smart window applications. KEYWORDS: smart window, vanadium dioxide, movable antireflection layer, thermochromic performance, active-passive control
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absorption in the short-wavelength range for both semiconductor (S-) and metallic (M-) phases of VO2. During the past few decades, researchers have made great efforts to solve the above problems toward the realization of VO2 thermochromic windows. Facing the challenges, introducing an antireflection (AR) layer has been proved to be an valid method to increase the Tlum and/or ΔTsol.7,8 The AR layer can greatly reduce the light loss in multielement lenses by making use of phase changes and the dependence of the reflectivity on index of refraction. And the roughness is necessary to reduce surfaces reflection because smooth surfaces shine more than rough ones.9 In previous works, typical AR layers such as SiO2, TiO2, ZrO2, and Cr2O3 have been introduced to improve optical properties of VO2 films. Babulanam et al. studied the antireflection (AR) layer on VO2, and found that the luminous transmittance (Tlum) was increased while the phase transition of VO2 was fixed by a SiO2 AR layer.10 However, AR layer prepared by conventional magnetron sputtering method is restricted by expensive equipment. And more investigations are required for a low-cost and facile approach to reach the balance between luminous transmittance and solar modulation ability of VO2-based smart window.
large quantity of energy is consumed to maintain the indoor comfort in buildings, a large proportion of which is lost through windows. Managing heat loads in building infrastructure through central heating and air-conditioning has been estimated to produce ∼30% of all anthropogenic greenhouse gas emissions.1−3 To cope with the rapid growth of energy demand and severe environmental conditions, the development of energy-saving materials and renewable resources has already received extensive attention. Among these, thermochromic material has become the most studied materials for its significant ability to regulate solar energy in response to changes of external environment.4 The thermochromic smart window, typically based on a vanadium dioxide (VO2) functional layer, has attracted particular interest due to its unique transition feature. VO2 is well-known by exhibiting a reversible SMT (semiconductor-metal transition) at Tc6, accompanied by a dramatic change of the infrared transmittance. Although investigations on VO2 smart windows have been carried on since the 1980s on a worldwide scale,5 there are still some difficulties hindering VO2-based smart windows in practical applications: (a) the single passive regulation mode of VO2, which changes phase only in response to external temperature variations; (b) low efficient solar modulation ability(ΔTsol) in practical applications, usually less than 10%;6 (c) poor luminous transmittance(Tlum) on account of © XXXX American Chemical Society
Received: November 27, 2018 Accepted: January 23, 2019 Published: January 23, 2019 A
DOI: 10.1021/acsami.8b20794 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Letter
ACS Applied Materials & Interfaces
Figure 1. Schematic illustration of VO2/movable AR structure used in thermochromic smart windows.
Information). The optical design and simulation of VO2-based thin film have been carried out for the sake of improving the overall optical properties. According to the results of simulation, we find that the introduction of smart movable antireflection layer to VO2 thin film can significantly improve the solar modulation ability and visible light transmittance. Herein, we provide an interesting approach to design a new type of intelligent window controlled by dual modes. A schematic diagram of how VO2/Movable AR bilayer structure will work as an intelligent window coating is shown in Figure 1. The VO2 single layer with a certain amount of solvent are sealed in insulating glass. As shown in the left half of the Figure 1, when under the large external temperature difference, the solvent vapor in the insulating glass is uniformly attached to the surface of the glass on the cold side, just like the atomization of window surface in winter, forming the antireflection layer to VO2 film. Conversely, the AR layer will disappear when the temperature difference decreases between inner and outer face of glass, such as in summer. As a result, the luminous transmittance changes depending on the external temperature difference, which provide a new method of visualization for VO2 films. More importantly, there are two approaches to realize the effect of intelligent antireflection. The first one is automatic regulation, that is, according to the change of temperature difference indoor and outdoor, a certain amount of solvent which is preplaced in the hollow glass, changes through the transformation of gaseous state and liquid state. The other is active regulation, which controls the amount of solvent by detecting the temperature difference and the humidity between the insulating glass to adjust the thickness of the antireflection layer, and can obtain the dual-mode regulation of smart thermochromic window. A piece of 400 × 400 mm2 flat glass coated with VO2 film is used as smart window for a model house (Figure 2a), which exhibits VO2 films applying to building smart windows. Figure
Meanwhile, the passive regulation mode has also blocked the practical application of VO2 smart coatings: the phase transition of VO2 depends on external temperature stimulation. In previous works, few researches had focused on the initiative and diversity of VO2 regulation. Recently, a study of Mo-doped VO2 thin films with largely enhanced optical performance exceeding the normal range for VO2 single layer films has greatly attracted our attentions.11 According to the discussion and analysis, it was found that extra water or ice may be attached to the sample’s surface owing to the low temperature, which will affect the experimental result.12 In terms of this interesting phenomenon, we put forward a new concept of movable antireflection (AR) layer which could largely enhance the thermochromic performance of VO2 film. In this paper, VO2 thin films with movable antireflection (AR) coatings were proposed for the first time. The inexpensive and eco-friendly H2O selected as the movable AR layer has dramatically improved the optical performances of VO2 films and realize the two-phase intelligent regulation through the conversion of liquid and gaseous. Other solvents like glycerin can also serve as the movable AR layer to achieve dual mode regulations. The overall optical properties of VO2/H2O structure are significantly improved in comparison to single layer VO2 film and the ΔTsol is higher than the reported multilayer films so far. Furthermore, this novel structure increases the contrast of optical transmittance in visible range, which not only make mainly contribution to solar modulation ability of VO2, but also offers a novel promising method for visualization of VO2based materials. The single VO2 film is grown on 20 × 20 mm2 quartz substrate by magnetron sputtering. After that, the movable H2O layer is introduced into insulating glass with VO2 film inside (Supporting Information). All characterizations include AFM, XRD, SEM and UV−vis spectrophotometer (Supporting B
DOI: 10.1021/acsami.8b20794 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Letter
ACS Applied Materials & Interfaces
Figure 2. (a) VO2 thermochromic film deposited on 400 × 400 mm2 glass substrate; (b) AFM image of the VO2 film. (c) XRD pattern of VO2 films. (d) Cross-section SEM image of VO2 film.
2b shows the microstructures of VO2 films measured by AFM as well as the surface morphology observed by SEM. The VO2 film with thicknesses of ∼90 nm shows a smooth surface and the RMS roughness of the surface is 4.50 nm, indicating high quality of VO2 films. Figure 2c presents XRD pattern of VO2 films in detail. All observed diffraction peaks are corresponded to the monoclinic VO2 (JCPDS: 043−1051) which clearly indicates the high purity of the VO2 films. The cross-section SEM image of VO2 film with approximately 90 nm thickness is shown in Figure 2d. Figure 3a shows the schematic diagram of H2O/VO2 bilayer film. At high temperatures, the water layer disappears, and VO2 can reflect an influx of infrared radiation due to its metallic state above Tc. The water layer will cover the surface of VO2 when the temperatures drops and has significant effect on the enhancement of overall transmittance of the bilayer film. From the surface of the VO2 layer to the air, the H2O layer gradually transforms from liquid to vapor, and its density continuously decreases to the same level as air. As a result, there would be few reflections if there is not an obvious interface between the air and gas−water transition area. What’s more, this graded-
index coating on substrate has been proved to have excellent antireflection characteristics.13 Figure 3b shows the simulated transmittance spectra of VO2(90 nm) attached with H2O(∼100 μm) by Essential Macleod software based on continuous and density mediums.14−16 As we can see from the simulated transmittance spectra, compared to single VO2 layer, the overall transmittance of visible and infrared regions has been greatly improved after adding the water layer at room temperature while the transmittance remains the same at high temperatures. The distinct difference between high- and low-temperature transmission curve result in an enhancement in optical performance, including luminous transmission and solar regulation efficiency. And the calculation of optical properties is shown in Supporting Information. Figure 3c shows the actual experimental results which are consistent with the simulation. The thickness of water layer can be controlled by the gap thickness of insulating glass. On account of antireflection effect, one can see that the introduction of water layer can dramatically improve the luminous transmittance of VO2 in the semiconducting state at room temperature. It is obvious that a C
DOI: 10.1021/acsami.8b20794 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Letter
ACS Applied Materials & Interfaces
Figure 3. (a) Schematic illustration of H2O/VO2 film; (b) optical simulation of VO2/H2O double-layer; (c) transmittance spectra of VO2 films with and without H2O layer; (d) Photographs of different thickness of VO2 with H2O layer on 20 × 20 mm2 quartz at 20 °C (left) and 80 °C (right).
Tlum,lt of 42.5%, much larger than that in the metallic state. In the meantime, ΔTsol dramatically improves to 18.2%. The digital photos of different thickness of VO2 with H2O layer at 20 °C (left) and 80 °C (right) prepared on 20 × 20 mm2 quartz are presented in Figure 3d, which also exhibit a
giant contrast in the visible region can make great contribution to the solar regulation efficiency. Combined with the data in the Table 1, for single layer VO2 film, we can see that Tlum,lt is 35.3%, whereas ΔTsol is only 10.1%. When the water layer is introduced to the surface of VO2, the sample shows enhanced D
DOI: 10.1021/acsami.8b20794 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Letter
ACS Applied Materials & Interfaces
and the letters at the bottom are more transparent to the naked eyes (optical properties in detail have been presented in Table 2). Covering protective layer for VO2 films is an effective way to prevent films from degradation. Meanwhile, to obtain the VO2based films with ultrahigh optical performance, we optimize the double-layer structure by introducing other layers such as HfO2 and SiO2. Further investigations of VO2/HfO2/H2O and VO2/SiO2/H2O multilayer structures have been carried out. Thicknesses variation of HfO2 and SiO2 were investigated to optimize thermochromic performance with VO2 layer of 90 nm. As shown in Figure S1a, different thicknesses of HfO2 layers are deposited on 90 nm thick VO2 films to obtain the optimum performance of multilayer film. After analyzing experimental results, it is found that the HfO2 sample with a sputtering time of 20 min has the best optical matching with VO2 film of 90 nm. The optimum HfO2 thickness is determined by cross-section SEM to be 130 nm (Figure S2a). Figure 4a shows the transmittance spectra of VO2/HfO2/ H2O multilayer films. To our knowledge, the visible light contributes about half of solar energy. Note that compared with single layer VO2, an increase of 63.6% (35.3−40.4%) for luminous transmittances and ultrahigh solar modulation ability (20.8%) have been achieved by the VO2/HfO2/H2O multilayer structure (at 130 nm HfO2). Similarly, for the SiO2 layer, the optimum thickness is determined to be 150 nm through
Table 1. Summary of Optical Properties for VO2 Films with Various Movable AR Layers sample
Tlum,lt(%)
Tlum,ht(%)
Tsol,lt (%)
Tsol,ht(%)
ΔTsol(%)
VO2(90 nm) VO2(90 nm)/H2O VO2(90 nm)/ glycerin VO2(90 nm)/ PhMe
35.3 42.5 42.9
32.5 34.2 40.0
36.3 44.9 45.4
26.2 26.7 31.9
10.1 18.2 13.5
43.5
36.0
46.2
29.0
17.2
Table 2. Summary of Optical Properties for Different Thickness of the VO2 Films sample
Tlum,lt(%)
Tlum,ht(%)
Tsol,lt (%)
Tsol,ht(%)
ΔTsol(%)
VO2(30 nm) VO2(30 nm)/H2O VO2(130 nm) VO2(130 nm)/ H2O
52.6 62.1 21.0 26.2
57.4 58.6 20.7 21.0
62.0 70.6 28.5 34.6
56.0 56.9 15.8 16.1
6.0 13.7 12.7 20.2
similar effect of enhanced luminous transmittance. Under the same light source, it can be seen from the picture that the color of VO2 films becomes darker as the thickness of the VO2 film gradually increased (90 to 130 nm). But when the water layer is introduced as the top layer, the samples show a lighter color
Figure 4. UV−vis−NIR spectra at 25 and 90 °C for (a) VO2/HfO2/H2O multilayer films and (b) VO2/SiO2/H2O multilayer films and (c) VO2 films with different kinds of movable AR. (d) Contrast of this work with previously published data about VO2-based films. E
DOI: 10.1021/acsami.8b20794 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Letter
ACS Applied Materials & Interfaces ORCID
Figures S1b and S2b. Moreover, VO2/SiO2/H2O multilayer structure also processed high optical properties (As shown in Figure 4b, the summary of optical properties was shown in Table S1). The introduction of these two materials isolates VO2 from the water layer, improving both optical performance and longevity of VO2-based multilayers. In addition to the water layer, other organic solvents, such as glycerin, toluene, etc., could also work as antireflection layers (As shown in Figure 4c and Table 1). What’s more, the multiple choices of solvents may expand the diversity of movable AR layer for VO2. For VO2 films fabricated by deposition, multilayer structure design has been demonstrated an effective way and a contrast of this work with published articles about multilayer films has been integrated in Figure 4d (the summary of solar and luminous transmittance is shown in Table S2).17−27 We can see that the improvement of overall optical performances is a gradual trend of VO2 thin films from the data. Furthermore, the expensive equipment and complex preparation technology also limit its practical applications. Although other methods such as composite materials, biomimetic nanostructuring have been investigated to enhance thermochromic performance in VO2,28 few satisfactory performances can be attained by sputtering method. Above all, the proposed VO2/HfO2/H2O multilayer thermochromic film namely achieves ultrahigh ΔTsol = 20.8% and excellent Tlum,lt = 40.4% simultaneously. Note that ΔTsol was promoted beyond most of reported studies using multilayer and even better than most of structures fabricated by chemical methods. And the introduction of inexpensive and eco-friendly water not only endowed VO2 films with outstanding thermochromic performance but also the dual mode regulations. In conclusion, we prepare a novel VO2/H2O bilayer structure that gives dramatically high ΔTsol (18.2%) and Tlum (42.5%). The movable AR layer provides a facile and flexible approach to significantly improve thermochromic performance of VO2 films. Through further structural optimization, we obtain the VO2/HfO2/H2O multilayer structure with ultrahigh solar modulation (20.8%), which is superior to any multilayer structure reported previously, as optical performance is dramatically enhanced via the combined effects of solid and movable antireflection layer. This is the first time that the concept of movable antireflection layer has been proposed and demonstrated to be feasibile. This innovative approach may broaden our traditional thinking about optimizing smart windows.
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Fang Xu: 0000-0001-7596-761X Xun Cao: 0000-0003-4417-6350 Shiwei Long: 0000-0002-5233-389X Tianci Chang: 0000-0001-7521-9339 Ping Jin: 0000-0003-4129-049X Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was financially supported by the Key Research and Development Plan of Anhui Province (1804a09020061), the Youth Innovation Promotion Association, Chinese Academy of Sciences (No.2018288) and the Shanghai Pujiang Program (18PJD051).
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b20794.
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REFERENCES
(1) Kamalisarvestani, M.; Saidur, R.; Mekhilef, S.; Javadi, F. S. Performance, materials and coating technologies of thermochromic thin films on smart windows. Renewable Sustainable Energy Rev. 2013, 26 (10), 353−364. (2) Huovila, P. Buildings and Climate Change: Status, Challenges, and Opportunities; United Nations Environment Programme, 2007. (3) Omer, A. M. Energy use and environmental impacts: A general review. J. Renewable Sustainable Energy 2009, 1 (5), 053101. (4) Zhou, Y.; Cai, Y.; Hu, X.; Long, Y. Temperature-responsive hydrogel with ultra-large solar modulation and high luminous transmission for ″smart window″ applications. J. Mater. Chem. A 2014, 2 (33), 13550−13555. (5) Babulanam, S. M.; Eriksson, T. S.; Niklasson, G. A.; Granqvist, C. G. THERMOCHROMIC VO2 FILMS FOR ENERGY-EFFICIENT WINDOWS. Sol. Energy Mater. 1987, 16 (5), 347−363. (6) Li, S. Y.; Niklasson, G. A.; Granqvist, C. G. Thermochromic fenestration with VO 2 -based materials: Three challenges and how they can be met. Thin Solid Films 2012, 520 (10), 3823−3828. (7) Parkin, I. P.; Manning, T. D. Intelligent Thermochromic Windows. J. Chem. Educ. 2006, 83 (3), 393−400. (8) Wang, S.; Liu, M.; Kong, L.; Long, Y.; Jiang, X.; Yu, A. Recent progress in VO2 smart coatings: Strategies to improve the thermochromic properties. Prog. Mater. Sci. 2016, 81, 1−54. (9) Yao, L.; He, J. Recent progress in antireflection and self-cleaning technology − From surface engineering to functional surfaces. Prog. Mater. Sci. 2014, 61 (8), 94−143. (10) Babulanam, S. M.; Eriksson, T. S.; Niklasson, G. A.; Granqvist, C. G. Thermochromic VO 2 films for energy-efficient windows. Sol. Energy Mater. 1987, 16 (5), 347−363. (11) Lv, X.; Cao, Y.; Lu, Y.; Ying, L.; Zhang, Y.; Song, L. Atomic Layer Deposition of V1−xMoxO2 Thin Films, Largely Enhanced Luminous Transmittance, Solar Modulation. ACS Appl. Mater. Interfaces 2018, 10 (7), 6601−6607. (12) Chang, T.; Cao, X.; Jin, P. Comment on ″Atomic Layer Deposition of V1-xMoxO2 Thin Films, Largely Enhanced Luminous Transmittance, Solar Modulation″. ACS Appl. Mater. Interfaces 2018, 10, 26814−−26817. (13) Xi, J. Q.; Schubert, M. F.; Kim, J. K.; Schubert, E. F.; Chen, M.; Lin, S.-Y.; Liu, W.; Smart, J. A. Optical thin-film materials with low refractive index for broadband elimination of Fresnel reflection. Nat. Photonics 2007, 1 (3), 176−179. (14) Loka, C.; Moon, S. W.; Choi, Y. S.; Lee, K. S. High Transparent and Conductive TiO 2 /Ag/TiO 2 Multilayer Electrode Films Deposited on Sapphire Substrate. Electron. Mater. Lett. 2018, 14 (2), 125−132. (15) Kim, Y. T.; Cho, J. Y.; Heo, J. Formation of antireflection structures for silicon in near-infrared region using AlO x /TiO x bilayer and SiN x single-layer. J. Non-Cryst. Solids 2018, 489, 22−26.
Experimental Section, characterization instrument and calculation of optical properties; transmittance spectra and SEM image of VO2/HfO2 films and VO2/SiO2 films; optical properties of VO2 films with different thickness; summary of recently reported VO2-based films (PDF)
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DOI: 10.1021/acsami.8b20794 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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ACS Applied Materials & Interfaces (16) Kovacs, A.; Mescheder, U. Optical characterization of pore filling in mesoporous multilayers by ultrathin atomic layer deposited hafnium dioxide. J. Vac. Sci. Technol., A 2018, 36 (3), 031508. (17) Zhu, B.; Tao, H.; Zhao, X. Effect of buffer layer on thermochromic performances of VO 2 films fabricated by magnetron sputtering. Infrared Phys. Technol. 2016, 75, 22−25. (18) Powell, M. J.; Quesadacabrera, R.; Taylor, A.; Teixeira, D.; Papakonstantinou, I.; Palgrave, R. G.; Sankar, G.; Parkin, I. P. Intelligent Multifunctional VO2/SiO2/TiO2 Coatings for SelfCleaning, Energy-Saving Window Panels. Chem. Mater. 2016, 28 (5), 1369−1376. (19) Zheng, J.; Bao, S.; Jin, P. TiO2(R)/VO2(M)/TiO2(A) multilayer film as smart window: Combination of energy-saving, antifogging and self-cleaning functions. Nano Energy 2015, 11, 136− 145. (20) Mlyuka, N. R.; Niklasson, G. A.; Granqvist, C. G. Thermochromic multilayer films of VO2 and TiO2 with enhanced transmittance. Sol. Energy Mater. Sol. Cells 2009, 93 (9), 1685−1687. (21) Long, S.; Zhou, H.; Bao, S.; Xin, Y.; Cao, X.; Jin, P. Thermochromic multilayer films of WO3/VO2/WO3sandwich structure with enhanced luminous transmittance and durability. RSC Adv. 2016, 6 (108), 106435−106442. (22) Panagopoulou, M.; Gagaoudakis, E.; Boukos, N.; Aperathitis, E.; Kiriakidis, G.; Tsoukalas, D.; Raptis, Y. S. Thermochromic performance of Mg-doped VO2 thin films on functional substrates for glazing applications. Sol. Energy Mater. Sol. Cells 2016, 157, 1004− 1010. (23) Li, R.; Ji, S.; Li, Y.; Gao, Y.; Luo, H.; Jin, P. Synthesis and characterization of plate-like VO2(M)@SiO2 nanoparticles and their application to smart window. Mater. Lett. 2013, 110, 241−244. (24) Sun, G.; Cao, X.; Zhou, H.; Bao, S.; Jin, P. A novel multifunctional thermochromic structure with skin comfort design for smart window application. Sol. Energy Mater. Sol. Cells 2017, 159, 553−559. (25) Gagaoudakis, E.; Kortidis, I.; Michail, G.; Tsagaraki, K.; Binas, V.; Kiriakidis, G.; Aperathitis, E. Study of low temperature rfsputtered Mg-doped vanadium dioxide thermochromic films deposited on low-emissivity substrates. Thin Solid Films 2016, 601, 99−105. (26) Choi, Y.; Jung, Y.; Kim, H. Low-temperature deposition of thermochromic VO 2 thin films on glass substrates. Thin Solid Films 2016, 615, 437−445. (27) Qian, X.; Ning, W.; Li, Y.; Zhang, J.; Xu, Z.; Yi, L. Bioinspired Multifunctional Vanadium Dioxide: Improved Thermochromism and Hydrophobicity. Langmuir 2014, 30 (35), 10766−10771. (28) Cui, Y.; Ke, Y.; Liu, C.; Chen, Z.; Wang, N.; Zhang, L.; Zhou, Y.; Wang, S.; Gao, Y.; Long, Y. Thermochromic VO2 for EnergyEfficient Smart Windows. Joule 2018, 2 (9), 1707−1746.
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DOI: 10.1021/acsami.8b20794 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
