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Vanadium Dioxide Nanoparticle-based Thermochromic Smart Coating: High Luminous Transmittance, Excellent Solar Regulation Efficiency and Near Room Temperature Phase Transition Jingting Zhu, Yijie Zhou, Bingbing Wang, Jianyun Zheng, Shidong Ji, Heliang Yao, Hongjie Luo, and Ping Jin ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b09011 • Publication Date (Web): 30 Nov 2015 Downloaded from http://pubs.acs.org on December 5, 2015

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ACS Applied Materials & Interfaces

Vanadium Dioxide Nanoparticle-based Thermochromic Smart Coating: High Luminous Transmittance, Excellent Solar Regulation Efficiency and Near Room Temperature Phase Transition

Jingting Zhu1, Yijie Zhou1*, Bingbing Wang1, Jianyun Zheng1, Shidong Ji1, Heliang Yao1, Hongjie Luo1,2, Ping Jin1,3*

1 State Key Laboratory of High Performance Ceramics and Superﬁne Microstructure, Shanghai institute of Ceramics, Chinese Academy of Sciences, Dingxi 1295, Changning, Shanghai, 200050, China

2 School of Materials Science and Engineering, Shanghai University, Shangda Rd. 99, Baoshan, Shanghai 200444, China

3 National Institute of Advanced Industrial Science and Technology (AIST), Moriyama, Nagoya 463-8560, Japan * Author for correspondence. Email: [email protected], Tel/Fax: +86-21-6990-6202; Email: [email protected], Tel/Fax: +86-21-6990-6213

Keywords: vanadium dioxide; thermochromic; smart coating; excellent performance; crystallinity; defect density

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Abstract An annealing-assisted preparation method of well-crystallized VxW1-xO2(M)@SiO2 core-shell nanoparticles for VO2-based thermochromic smart coatings (VTSC) is presented. The additional annealing process reduces the defect density of the initial hydrothermally prepared VxW1-xO2(M) nanoparticles and enhances their crystallinity so that the thermochromic film based on VxW1-xO2(M)@SiO2 nanoparticles can exhibit outstanding thermochromic performance with balanced solar regulation efficiency (∆Tsol) of 17.3%, luminous transmittance (Tlum) up to 52.2% and the critical phase transition temperature (Tc) around 40.4 oC, which is very promising for practical application. Furthermore, it makes great progress in reducing Tc of VTSC to near room temperature (25.2 oC) and simutaneously maintaining excellent optical properties (∆Tsol = 14.7% and Tlum = 50.6%). Such thermochromic performance is good enough to make VTSC applicable to practical architecture.

Introduction Smart materials, which switch reversibly between two different states in response to external stimuli, have been extensively pursued for fundamental scientific research and practical applications.1-6 Vanadium dioxide (VO2), as a typical smart material, can exhibit a well-defined reversible metal-to-insulator transition (MIT) under a variety of stimuli like temperature,6-8 stress,4, 5, 9 optical field,10, 11 electrical field 12, 13 and magnetic field,14 accompanied by a sharp change in electrical resistance spanning five orders of magnitude and optical change particularly in infrared region (IR) from a relative-transparent state to a more reflective state. However, the mechanism of the MIT in VO2 is still under debate, which is usually held to be initiated by the strong electron-electron correlations associated with the Mott transition 15 or electron-phonon interactions associated with the Peierls transition 16, 17. VO2 has attracted significant attention due to a variety of possible applications based on its ultrafast MIT such as intelligent window coatings,18, 19 bolometer,20 optical switches,21, 22 temperature sensors 23 and so on. Recently, the potential application is mainly focused on VO2-based thermochromic smart coatings (VTSC) which can respond to environmental temperature and regulate near infrared (NIR) irradiation by transforming from a transparent state at low temperature to a more reflective state at high temperature, and meanwhile maintaining visible transmittance.6, 18, 19 The smart solar regulation of VTSC can effectively reduce the building energy consumption which accounts for 30% - 40% of total social energy consumption in most countries.24 Although many efforts have been taken to apply VTSC in practical architecture and automobile, so far the performance is still not ideal. Because three issues related with VTSC cannot be well-balanced, that is, high luminous transmittance (Tlum), excellent solar regulation efficiency defined as the contrast of solar transmittance (∆Tsol) across MIT and adaptable critical temperature (Tc). Up to the present, most research work reported is concentrated on only one or two aspects of the VTSC. Some of them are focused on the optical performance regardless of Tc. For example , the optimal optical parameters are that the Tlum and ∆Tsol at about 50% and 7.1% respectively according to the optical computation for a single-layered VO2 film prepared by magnetic sputtering method.25 The optical performance could be optimized for ∆Tsol up to 12% and Tlum of 45% by depositing a TiO2/VO2/TiO2/VO2/TiO2 five-layered structure through physical vapor


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deposition (PVD).26 The ∆Tsol can be further improved to 14.1% at Tlum about 43.3% by preparing nanoporous VO2 thin films via a solution-based method.27 Recently, the optical properties make a great progress for ∆Tsol up to 22% and Tlum up to 45% by making VO2-based thermochromic flexible foils with well-dispersed VO2(M) nanoparticles (NPs) in organic resin.28 In addition, other researchers pay much attention to lowering the Tc of VTSC while neglecting the optical performance. For instance, a 2 atom% W-doping could reduce Tc to room temperature (RT, ～25 o C) for VTSC prepared by PVD 29 or sol-gel method 30. Molybdenum doping could reduce Tc to 24 o C at a 7 atom% doping content.31 Magnesium is also shown to decrease the Tc to 45 oC at a 7 atom% doping amount.32 In all reported research work, reducing Tc by doping extra element always weakens ∆Tsol seriously, especially when the Tc drops to near RT.33-36 In a word, there is no study concerning all of the three issues (Tlum, ∆Tsol and Tc), that is, cutting Tc down to near RT while maintaining excellent optical properties. In this work, an annealing-assisted process was proposed to synthesize well-crystallized VxW1-xO2(M)@SiO2 core-shell nanoparticles which were applied for VTSC. VxW1-xO2(M) NPs were first synthesized by hydrothermal method, then followed by coating the NPs with a thin layer of silica which was efficient to protect VO2 NPs from aggregating and growing during annealing. Besides, VO2(M) NPs prepared simply by hydrothermal method were usually found to exhibit weak optical performance mainly originated from weak crystallinity and numerous defects.37 The annealing process allowed the prepared VxW1-xO2(M)@SiO2 NPs to decrease defect density and improve crystallinity so that they can exhibit outstanding thermochromic performance with ∆Tsol of 17.3% and Tlum up to 52.2%, and meanwhile with Tc around 40.4 oC. Furthermore, it is great progress to simutaneously reduce Tc to near room temperature (25.2 oC) and display splendid optical properties (∆Tsol = 14.7% and Tlum = 50.6%).

Experimental Section Synthesis of VxW1-xO2(M) Nanocrystals. In a typical reaction, VOSO4 (AR, Meryer Co., Ltd.) and the corresponding doping amount of (NH4)10W12O41•5H2O (Wako Pure Chemical Industries, Ltd.) were dissolved in 40 mL deionized (DI) water with continuous stirring to make a blue to dark green solution, 0.5 mL hydrazine monohydrate (N2H4 64-65%, Sigma-Aldrich Co., LLC.) was dropped into the solution slowly which was necessary for the final formation of VO2(M). The pH of the above mixed solution was then adjusted to 7 by adding diluted NaOH solution (0.1 mol/L) resulting in a brown precipitate forming. The precipitate was centrifuged and washed with DI water, and then dispersed in 40 mL DI water to form slurry. The slurry was stirred for 10 min and then transferred into a 100 mL Teflon-lined autoclave with a hydrothermal treatment at 240 oC for 36 h. The black product was collected under centrifugation, and washed with DI water and ethanol. Synthesis of VxW1-xO2(M)@SiO2 Nanocrystals. Coating VO2 with SiO2 is needed to separate VO2 NPs and prevent VO2 NPs from aggregating and growing during annealing. Using the approach suggested by Wei Li et al.,38 0.1 g as-synthesized VxW1-xO2(M) core particles was dispersed in 5 mL ethanol (AR, Shanghai Zhenxing No.1 Chemical Plant). Then the suspension was added to a big beaker charged with 280 mL absolute ethanol, 70 mL DI water and 5.0 mL concentrated ammonia solution (28 wt.%, Sinopharm Chemical Reagent Co., Ltd.) under stirring



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for 30 min. Afterwards, 0.5 mL tetraethylorthosilicate (TEOS, AR, Aladdin Chemistry Co., Ltd.) was slowly added dropwise into the above mixed solution within 10 min, and the reaction was continued for 4 h at room temperature under vigorous mechanical stirring. The final product was collected under centrifugation, followed by washing with DI water and ethanol, and finally dried in an oven at 80 oC for 2 h. Additional annealing treatment was essential to obtain well-crystallized VxW1-xO2(M)@SiO2 powders at 600 oC for 20 min at nitrogen atmosphere. To sum up, the whole process for the synthesis of the NPs is illustrated in Scheme 1.

Scheme 1 The experimental flow chart for the synthesis of well-crystallized VxW1-xO2(M)@SiO2 powders.

Synthesis of VxW1-xO2(M)@SiO2 Nanoparticle-based Coating. 0.3 g as-prepared powders were dispersed ultrasonically in 15 mL ethyl alcohol for 30 min with an appropriate amount of ANTI-TERRA-U (dispersant, BYK-Chemie GmbH) and 5 g polyvinyl butyral (PVB, M. W. 90000-120000, Aladdin Chemistry Co., Ltd.), and then the mixture was uniformly cast onto a float glass substrate by spin-coating with the speed of 600 r/min for 20 s and then 1000 r/min for 20 s. After removing the liquid by drying in an oven at 80 oC for 60 min, the VO2 nanoparticle-based thermochromic film was obtained. Characterization. X-ray diffraction(XRD, Model D/Max 2550 V, Rigaku, Japan) was used to identify the crystal phases of the nanocrystals with Cu Kα radiation (λ = 1.5418 Å) at the voltage of 40 kV and the current of 40 mA, respectively. The W doping concentration was detected by inductively coupled plasma measurements (ICP, Thermoelectric Corporation, IRIS Intrepid). The morphology of the NPs was characterized by field-emission scanning electron microscopy (FE-SEM, JEOL, JSM-6700F, Tokyo, Japan) and transmission electron microscopy (TEM, JEOL, JEM-2010, Tokyo, Japan). The phase transition properties of the resultant products were detected by differential scanning calorimetry (DSC200F3, NETZSCH) at a heating/cooling rate of 10 oC•min-1 with temperature ranging from 0 to 100 oC. Optical transmittance characteristics were evaluated at wavelengths ranging from 350 to 2600 nm at 15 or 20 and 100 oC on a UV-vis– near-IR spectrophotometer (HITACHI U-3010) equipped with a temperature controlling unit.


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In order to assess the visual and energy saving performance of all the samples, the integrated luminous transmittance (Tlum, 380–780 nm) and solar transmittance (Tsol, 240–2600 nm) were essential, which could be obtained from the following equation / where denotes the transmittance at wavelength λ, is the spectral sensitivity of the light-adapted eye and is the solar irradiance spectrum for air mass 1.5 corresponding to the sun standing 37° above the horizon.6, 39, 40 As a result, the solar modulation efficiency (∆Tsol) could be calculated, which is usually used to characterize the thermochromic properties of VTSC.

Results and Discussion Fig. 1(a) shows the XRD patterns of the initial hydrothermally prepared VO2(M) NPs, the silica-coated VO2(M) NPs, the direct annealed uncoated VO2(M) and the final annealed VO2@SiO2 NPs. All peaks of the four kinds of particles correspond to monoclinic VO2(M) (JCPDS no.043-1051) without any other impurity peaks detected. There is no obvious difference in peak intensity between the uncoated and coated VO2(M), but the peak intensity is relatively stronger after annealing. It indicates that annealing process can obviously enhance the crystallinity of VO2(M). Moreover, the diffraction peak of the annealed VO2(M) without coating is even stronger and narrower than that of the annealed VO2@SiO2, which means that the crystals of the direct annealed uncoated VO2(M) have grown. Under careful control of the molar ratio of (NH4)10W12O41•5H2O to VOSO4, VO2(M) with various W doping contents could be obtained. Fig. 1(b) shows the XRD patterns for samples with different doping amounts of W element which are coated with SiO2 and annealed at 600 oC for 20 min at nitrogen atmosphere. All the diffraction peaks can also be indexed to well-crystallized monoclinic VO2(M). The annealed VO2(M)@SiO2 samples with different W doping contents (0 at.%, 1 at.%, 2 at.% and 3 at.%) are further denoted by W0%, W1%, W2%, and W3%, respectively. From the slow scanning (0.2 degree per min, step size 0.02 degree) XRD curves between 26.5° and 29.0° ((011) peaks) shown in Fig. 1(c), the peaks are found to monotonically shift toward smaller angle as the increase of W doping amount. According to the Bragg’s Law (2d sin ), the peak shift indicates the interplanar lattice along (011) expanding with the incorporation of W6+ ions. Table 1 shows the d-spacing of (011) plane for the W-doped VO2(M) and the precise W doping amount in the VO2(M) detected by ICP.

Fig. 1 XRD patterns of (a) uncoated VO2(M), VO2(M)@SiO2, annealed uncoated VO2(M) and annealed



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VO2(M)@SiO2 sample powders, and (b) the annealed VxW1-xO2@SiO2 with various W-doped contents; (c) Magnified patterns after intensity normalization processing for (b) of the (011) peak at slow scanning speed of 0.2 degree per minute from 26.5° to 29.0° (W0%, W1%, W2%, and W3% indicate the W : V molar ratio of W-doped VO2 coated by SiO2 is 0%, 1%, 2% and 3%, respectively).

Table 1 The d-spacing of (011) plane for VO2(M) with different W doping amounts

Sample

W-doped amount (at.%)

ICP (at.%)

d-spacing(Å)

W0% W1% W2% W3%

0 1 2 3

0 0.85 1.77 2.89

3.199 3.203 3.207 3.218

The morphology and the size of the initial VO2(M) NPs, the annealed VO2(M) particles without coating, the silica-coated VO2(M) NPs and the final annealed VxW1-xO2(M)@SiO2 NPs (W0%, W1%, W2% and W3%) are presented in Fig. 2 and Fig. S1. It reveals that the particle diameter of the prepared NPs except the annealed uncoated ones in this experiment is almost smaller than 40 nm. Based on Laaksonen’s computation 41 for VO2 particle-based coatings modeled by the four-flux theory (detailed expounded in the SI), the transmittance of VTSC at short wavelengths will decrease obviously due to the light scattering if the particle size is over 40 nm. In this work, the VO2 NPs as-prepared except the annealed uncoated ones are small enough to avoid light scattering in visible region if the NPs could be well-dispersed in some matrix. Fig. 2(a) inset shows that the average size of the original VO2 NPs is around 18.9 nm and the particle size distribution is relatively narrow according to the statistics from 300 NPs in the SEM picture. But from the high-resolution TEM (HRTEM) image in Fig. 2(b), it could be observed that the initial prepared VO2 NPs have a poor crystallinity with indistinct and disorder lattice fringes as well as some point defects. While annealing the original VO2 NPs to enhance their crystallinity without coating with a layer of silica beforehand, the particles of VO2 aggregate together and grow severely which is shown in Fig. 2(c). The TEM images of the VO2(M)@SiO2 and the annealed VO2(M)@SiO2 are exhibited in Fig. 2(d) and (e), respectively. It can be observed from Fig. 2(d) that almost every single VO2 nanoparticle is coated with SiO2 shell separately and the thickness of the SiO2 shell is about 3 nm, which means that the VO2(M) particles are well-protected by silica shell. Comparing Fig. 2(d) with (e), it demonstrates that heat treatment has little influence on the morphology and the size of VO2(M) attributed to the protection of SiO2 shell. Besides, Fig. 2(f) illustrates the HRTEM image of a single nanoparticle circled in Fig. 2(e), and it indicates that the nanoparticle is a single crystal with interplanar spacing about 3.2 Å corresponding to the (011) crystal plane, and the distinct lattice fringes verify that the crystallinity of the annealed VO2(M)@SiO2 samples is obviously enhanced, which is in accordance with the previous XRD analysis. From the TEM images for the annealed VO2(M)@SiO2 NPs with different doping contents of tungsten (0 at.%, 1 at.%, 2 at.% and 3 at.%) in Fig. S1, it could be seen that the W-doped amount has no obvious impact on the morphology of the NPs, but the particle size has a little increase upon doping with tungsten.


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Fig. 2 (a) SEM picture of the original VO2 nanoparticles (the insert is the particle size statistics from 300 NPs) and (b) HRTEM picture of one original VO2 NP, (c) TEM image of the direct annealed uncoated VO2(M) particles, (d and e) TEM images of the VO2@SiO2 and the annealed VO2@SiO2 respectively, (f) HRTEM image of the nanoparticle circled in (e).

Fig. 3(a and b) presents the transmittance spectra of VTSC based on the prepared particles: the uncoated VO2(M), the annealed VO2 without coating, the VO2(M)@SiO2 and the annealed VxW1-xO2(M)@SiO2 NPs (W0%, W1%, W2% and W3%), and the cross-sections of the samples are displayed (Fig. S2) showing the similar thickness of different VO2 nanoparticle-based films. The calculated optical performance (∆Tsol and Tlum) is summarized in Table 2. Both the silica coating and annealing show great effects on the properties. The Tlum and ∆Tsol are improved from 56.6% and 8.7% to 59.7% and 11.3% respectively after the original VO2(M) NPs are coated with a layer of silica (Fig. 3(a)), due to the better dispersity in matrix of the coated VO2 NPs than that of the uncoated ones (also be verified by the back scattered SEM pictures of the films in Fig. S3). Meantime, SiO2 shell also protects VO2(M) from weakening the optical properties from Tlum = 56.2% and ∆Tsol = 21.4% to Tlum = 40.0% and ∆Tsol = 8.7% during annealing, which is mainly attributed to its protection for particles that prevents VO2 NPs from aggregating and growing. Annealing VO2 without coating initially will degrade optical performance, which was also certified by experiments of Paik42 et al. Besides, annealing process improves the ∆Tsol of the VO2(M)@SiO2 from 11.3% to 21.4% on account of enhancing the crystallinity of the VO2(M). The overall performance of the annealed VO2(M)@SiO2 with ∆Tsol of 21.4% and Tlum up to 56.2% is encouraging, because it is much higher than ∆Tsol = 7.0% at Tlum = 45.3% for a VO2 film with all of nanoparticles aggregating together, 27 ∆Tsol = 7.5% at Tlum = 55.3% for VO2(M)@SiO2 nanostructures with weak crystallinity, 6 and ∆Tsol = 13.3% at Tlum = 53.6% for VO2(M)@TiO2 nanorods with a length over 100 nm.40 The reason for the great property enhancement in this work is owing to the good dispersion, the excellent crystallinity and the rather small size of the VO2(M) NPs. When VO2(M) NPs meet all the conditions of being well-crystallized, well-dispersed and less than 40 nm in size, VTSC will present simultaneous improvement of ∆Tsol and Tlum, which has been simulated from four-flux theory by Li 43 and Laaksonen 41 et al detailed expounded in the SI. Among them, good crystallinity of VO2 nanoparticles is essential to the excellent



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thermochromic performance of VTSC.28, 44 Moreover, if VO2 nanoparticles are well-dispersed in matrix, the spectral transmittance for coatings can be calculated shown in Fig. 4 (a) and the exact ∆Tsol & Tlum for simulated samples are listed in Table S1. The growing contribution of scattering for larger particles can be clearly seen as a lowering of the transmittance as the particle size is increased. This effect is unobvious when particle size below 40 nm and starts to be pronounced for size in the range from 100 to 200 nm, and it is more prominent for semiconducting VO2. Besides, the most remarkable feature for metallic VO2 nanoparticles is the pronounced minimum at λ≈ 1200 nm. The minimum is clearly caused by the localized surface plasmon resonance (LSPR) absorption which leads to a significant modulation of solar energy transmittance around Tc. 39 However, if VO2 nanoparticles have a poor dispersion in the film, they cannot exhibit LSPR and will act as the film produced by physical methods in Fig. 4(b).

Fig. 3 Optical transmittance spectra at low and high temperature of (a) the uncoated VO2, the direct annealed uncoated VO2, the VO2@SiO2 and the annealed VO2@SiO2, and (b) the annealed VxW1-xO2@SiO2. (c) DSC curves of the annealed VxW1-xO2@SiO2. (d) The dependence of ∆Tc on the doping levels. (W0%, W1%, W2% and W3% indicate that the adding molar ratio of W : V is 0%, 1%, 2% and 3%, respectively.) Table 2 Summary of the phase transition temperature, thermal hysteresis width and optical properties for as-prepared samples: the uncoated VO2, the annealed VO2 without coating, the VO2@SiO2 and the annealed VxW1-xO2@SiO2 with different W-doped amounts.

Sample Uncoated VO2 Annealed VO2 VO2@SiO2 W0% W1% W2% W3%

Phase transition temperature Tc(oC) 47.0 77.0 47.9 84.7 55.9 40.4 25.2

Thermal hysteresis width ∆Tc(oC) 19.7 17.5 20.6 50.5 36.0 22.3 8.5

Luminous transmittance Tlum (%) 15 oC 100 oC 56.6 56.0 40.0 39.3 59.7 56.5 56.2 49.7 55.5 50.4 52.2 48.3 50.6 47.5

Solar transmittance Tsol (%) 15 oC 100 oC 58.6 49.9 47.3 38.6 57.9 46.6 60.5 39.1 59.2 39.8 56.7 39.4 55.9 41.2


Solar regulation efficiency ∆Tsol (%) 8.7 8.7 11.3 21.4 19.4 17.3 14.7

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Fig. 4 Ideal film models and their simulated spectral transmittance for (a) VO2 nanoparticles well-dispersed and (b) VO2 nanoparticles agglomerated in a dielectric medium with nm=1.5 calculated with the four-flux method. For calculation, the following layer thickness d and volume fraction of nanoparticles f are used: d=10 µm and f=0.01. (According to Li 43 and Laaksonen’s41 theory)

It is well known that tungsten element is the most effective dopant for reducing the Tc of VTSC,45 which is also used in our experiment. Fig. 3(c) shows the typical DSC curves of the annealed VxW1-xO2(M)@SiO2 NPs with various W-doped contents, and the curves of the uncoated VO2(M), the annealed VO2(M) without coating and the VO2(M)@SiO2 samples are shown in Fig. S6. It can be seen from Fig. S6(a) that the phase transition temperatures of the uncoated VO2 and the VO2@SiO2 particles are same and both around 47 oC lower than the normal transition temperature 68 oC which might be caused by the nano-size effect 46. However, in Fig. 3(c), the endothermic peak of the undoped annealed VO2@SiO2 at approximately 85 oC is much higher than 68 oC, which is similar to the reports in previous literatures.28, 35, 47 This phenomenon also exists similarly in the annealed uncoated VO2(M) sample with a Tc of 77 oC shown in Fig. S6(b). The result of the increased Tc could be explained by the Lopez’s analysis 48 that nucleation first occurs at special sites such as extrinsic defects, and a stronger driving force is required for the phase transition if the defects density is reduced with heat treatment.44 Furthermore, a higher Tc for the annealed VO2@SiO2 than that of the annealed uncoated one is mainly due to the thermal insulation of the silica shell. There exist some additional small peaks in the DSC curves of Fig. 3(c), which may be caused by various types of coexisting nanoparticles,28 but other exact factors should be explored in future. Further analysis of the DSC curves reveals that thermal hysteresis width (∆Tc) sharply decreases with W doping amount increasing, also clearly shown in Fig. 3(d) which shows that the ∆Tc linearly decreases with a rate of 14.5 oC per at.% W in the range from 50.5 oC in undoped annealed VO2@SiO2 to 8.5 oC at a doping level of 2.89 at.%. Although heat change of phase transition indicates thermal hysteresis width of VO2 can be adjusted by doping, inner mechanism is not clear so far. 49 In Fig. 3(c) and Table 2, it can also be discovered that Tc is declined with the increase of W-doped amount, which will weaken the optical performance seriously due to weaker crystallinity and more



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serious lattice distortion induced by additional point defects according to previous reports.33-37, 50, 51 Amazingly, the optical property of the VTSC prepared in our experiment with a small W-doped amount does not weaken obviously, even when the W doping content is at 2 at.%, the value of ∆Tsol and Tlum are still comparable with W0% sample (seen from Fig. 3(b) and Table 2). Maintaining the luminous transmittance above 50%, the solar regulation efficiency is 17.3% with Tc around 40 oC and 14.7% with Tc around 25 oC, which are both very promising for the practical application. These results are attributed to the annealing process for VxW1-xO2(M) which mostly reduces the defects and enhances the crystallinity.44, 52 Point defects in crystal structure will cause lattice distortion which is a main reason to damage ∆Tsol of VO2(M),53-55 meanwhile there are numerous point defects in the initial hydrothermally prepared VO2 and doping VO2 with tungsten will introduce more additional point defects. However, annealing treatment could eliminate most point defects to relieve lattice distortion,56,57 which will make a great contribution to increasing the ∆Tsol. Fig. 5 displays the ∆Tsol, Tlum and Tc for different types of VTSC that have been reported. For F-doped VO2, ∆Tsol is only 10.7% at Tlum = 48.7% and Tc =35 oC. When ∆Tsol is improved to 13.1% at Tlum = 48.1%, the Tc returns to 66 oC as a result.33 Doping VO2 with Mg increases Tlum from 45.3% to 54.2% and reduces Tc from 67.4 oC to 61.1 oC, but ∆Tsol is less than 11% and the Tc is too high to be applied in application.34 W-Zr codoping seems to maintain high Tlum around 50% and decreases Tc to 28.6 oC, however, ∆Tsol is only about 4.9% at the same time.36 In addition, W-doped VO2 prepared by microemulsion method keeps ∆Tsol at around 13% and reduces Tc from 81.3 oC to 43.1 oC with a low content of dopant, while Tlum decreases from 52.2% to 39%, and the situation worsens sharply with higher concentration dopants.35 Among these methods, VTSC shows great thermochromic performance, but there is still a long distance to practical architecture. The thermochromic properties (∆Tsol = 14.7%, Tlum = 50.6% and Tc = 25.2 oC) in this work are far more superior to any other previous works reported.

Fig. 5 (a) Solar energy modulation ability (∆Tsol) and phase transition temperature (Tc) and (b) ∆Tsol and luminous transmittance (Tlum) of the different VO2 coatings: F-doped VO2(M),33 Mg-doped VO2(M),34 W-doped VO2(M) by microemulsion,35 W-Zr-codoped VO2(M) 36 and VxW1-xO2@SiO2 in this work. The results of this work are better than any previously reported data.

Conclusion In summary, our work reported VO2-based thermochromic coatings with simutaneously balanced near room temperature Tc (25.2 oC) and splendid optical properties (∆Tsol = 14.7% and Tlum = 50.6%) for great progress. Doping VO2 nanoparticles with tungsten element, coating VO2(M) with


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silica and annealing in inert atmosphere were all essential to obtain the excellent VTSC for practical application. This study provided a new solution for optimizing the optical properties of VTSC which can be very promising for the real application. Supporting Information Available The TEM images for various W-doped VO2, the SEM pictures for cross-sections of different VO2 NP-based films, the SEM pictures for the uncoated VO2 and VO2@SiO2 films, details on the simulation, summary of the ∆Tsol & Tlum for simulated samples and DSC curves of the uncoated VO2, VO2@SiO2 and uncoated but annealed VO2 are listed in Supporting Information. Acknowledgments The authors are grateful to the high-tech project of MOST (2014AA032802), the national sci-tech support plan the National Natural Science Foundation of China (NSFC, No.: 51372264), and the Science and Technology Commission of Shanghai Municipality (STCSM, No.: 14DZ2261200).

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