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High Performance and Enhanced Durability of Thermochromic Films using VO2@ZnO Core-Shell Nanoparticles Yunxiang Chen, XianZhe Zeng, Jingting Zhu, Rong Li, Heliang Yao, Xun Cao, Shidong Ji, and Ping Jin ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b08889 • Publication Date (Web): 31 Jul 2017 Downloaded from http://pubs.acs.org on August 1, 2017

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ACS Applied Materials & Interfaces

High Performance and Enhanced Durability of Thermochromic Films using VO2@ZnO Core-Shell Nanoparticles Yunxiang Chen, †,‡,|| Xianzhe Zeng, †,‡,|| Jingting Zhu, †,‡ Rong Li, † Heliang Yao, † Xun Cao, *,† Shidong Ji, *,† and Ping Jin *,†,§ †

State Key Laboratory of High Performance Ceramics and Superﬁne Microstructure,

Shanghai Institute of Ceramics, Chinese Academy of Sciences, Dingxi 1295, Changning, Shanghai, 200050, China ‡ §

University of Chinese Academy of Sciences, Beijing 100049, China National Institute of Advanced Industrial Science and Technology (AIST),

Moriyama, Nagoya 463-8560, Japan

ABSTRACT: For VO2-based thermochromic smart window, high luminous transmittance (Tlum) and solar regulation efficiency (∆Tsol) are usually pursued as the most critical issues, which have been discussed in numerous researches. However, environmental durability, which has rarely been considered, is also so vital for practical application because it determines lifetime and cycle times of smart windows. In this paper, we report novel VO2@ZnO core-shell nanoparticles with ultrahigh durability as well as improved thermochromic performance. The VO2@ZnO nanoparticles-based thermochromic film exhibits a robust durability that the ∆Tsol keeps 77% (from 19.1% to 14.7%) after 103 hours in hyperthermal and humid environment, while relevant property of uncoated VO2 nanoparticles-based film badly deteriorates after 30 hours. Meanwhile, compared with the uncoated VO2-based film, the VO2@ZnO-based film demonstrates an 11.0% increase (from 17.2% to 19.1%) in ∆Tsol and a 31.1% increase (from 38.9% to 51.0%) in Tlum. Such integrated thermochromic performance expresses good potential for practical application of VO2-based smart windows. KEY WORDS: vanadium dioxide, VO2@ZnO, durability, excellent performance, thermochromic, smart window

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INTRODUCTION In recent years, “smart window” has received widespread attention for the purpose of reducing energy consumption of air conditioning in modern architectures.1-5 Smart window, designed to intelligently control the amount of transmitted light and heat, can respond to an external stimulus, such as light (photochromism),6-9 heat (thermochromism)10-13 or electricity (electrochromism).14-16 Vanadium dioxide (VO2) is one of the most ideal materials for smart window because of its fascinating thermochromic property accompanied by a reversible metal-insulator transition (MIT) at a near-room critical temperature (Tc). Attributed to the MIT, VO2(M) exhibits a dramatic change in near-infrared region which is transparent to NIR light at temperature below Tc and translucent to NIR light above Tc. Such property endows VO2(M) solar heat control ability in response to environmental temperature. Therefore, VO2(M) is a promising candidate for energy-efficient thermochromic smart window. Up to the present, most researches about VO2-based smart window such as film thickness optimization,17-21 doping,22-29 and composition30-33 have mostly concentrated on improving

thermochromic performance, including enhancing solar regulation

efficiency (∆Tsol),

increasing luminous transmittance (Tlum) and decreasing critical

temperature (Tc) closer to room temperature. Zhu et al have synthesized VxW1-xO2@SiO2 nanoparticles-based film with the ∆Tsol and Tlum of about 21.4% and 56.2% respectively which is extremely closed to the theoretical result (∆Tsol=25.27%, Tlum=46.03%).34 However, there remains another important challenge in practical application of VO2-based smart window: its environmental durability. The reason is that VO2 naturally transforms into the more stable V2O5 as a result of a progressive oxidation process.35 Tong et al have tried to protect the VO2 nanoparticles by using Al-O-based shell but this method only enhances the durability of VO2-based smart window for 4 times (from 5 days to 20 days),36 which is not enough for our daily life. Therefore, improving the durability of VO2-based smart window becomes an urgent assignment. Zinc oxide is an interesting material with high refractive index, which has been


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used in different types of coatings and composites as an antireflective layer. Above all, ZnO is a stable material in the environment. Previous researchers used ZnO thin film as an antireflective and protective layer. They deposited ZnO on the surface of VO2 film to improve the transmittance and stability of VO2 films.30, 37 The results show that the ZnO top layer can serve as an antireflective layer improving the solar regulation efficiency with high luminous transmittance and also a protective barrier to prevent VO2 film from oxidation and corrosion. As a consequence, we consider that ZnO could play a protective action as the shell for the VO2 nanoparticles. In this paper, we proposed the synthesis of VO2@ZnO core-shell nanoparticles which makes VO2-based films stable to practical architecture. VO2 nanoparticles were first synthesized by hydrothermal method, followed by coating with a thin layer of ZnO. The prepared VO2@ZnO nanoparticles-based smart window exhibits enhanced solar modulation efficiency, high luminous transmittance and excellent durability in heating, humid and oxygenic environment. This work sharply shortens the distance of VO2-based films towards VO2-based smart window for real application.

EXPERIMENTAL SECTION Preparation of VO2(M). VO2 nanoparticles were prepared following our previous works under a typical procedure.38-39 3.0 g commercial V2O5 (99.99%, Sinopharm Chemical Reagent Co., Ltd) powders and 4.15 g oxalic acid powders (H2C2O4.2H2O, Wako, Japan) were added to 50 mL deionized water at room temperature. The suspension was continuously stirred until a clear dark blue solution was formed. Then, the suspension was transferred into a 100 mL teflon-lined autoclave. The autoclave was maintained at 240 °C for 24 h and then air-cooled to room temperature. The resulting black-colored precipitate was collected by filtration, washed with deionized water and alcohol, and then dried at 80 °C in air for 8 h. Preparation of VO2@ZnO core-shell structure nanoparticles. To prepare the VO2@ZnO core-shell structure nanoparticles, 0.1 g of the as-prepared VO2 nanoparticle power was ultrasonically dispersed in 80 mL deionized water with 0.15 g hexadecyl trimethyl ammonium bromide (CTAB, 99%, Aladdin Reagent Co., Ltd.) to



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obtain a well-dispersed suspension. Then, 0.3 g zinc nitrate hexahydrate (Zn(NO3)·6H2O, Aladdin Reagent Co., Ltd) and 0.15 g Hexamethylenetetramine (HMT, Aladdin Reagent Co., Ltd) were rapidly added into the suspension under vigorous stirring until dissolved completely. The suspension was heated to 85 °C and maintain at this temperature for 8 h. Afterward, the suspension was centrifuged and the final VO2@ZnO nanoparticles were washed with deionized water and alcohol and then dried at 80 °C in air for 8 h. In order to obtain different thickness of ZnO shell, we attempt to add different amounts of Zn(NO3)·6H2O and HMT. The different parameters of our experiment are listed in Table 1. Preparation of VO2-based thermochromic films. First, 0.1 g as-prepared nanoparticle powders were dispersed ultrasonically into 10 mL ethanol evenly. Then 3 g polyvinyl butyral (PVB, M. W. 90000-120000, Aladdin Chemistry Co., Ltd.) was add into the suspension with constant stirring. Afterward, the mixture was uniformly cast onto a float glass substrate by spin-coating with the speed of 1000 r/min for 20 s. Removing the liquid by drying at 120 °C, finally the VO2-based thermochromic films were obtained. The whole process for the synthesis of VO2-based films is illustrated in Scheme 1. Characterization. The crystal phases of the nanocrystals were characterized by X-ray diffraction (XRD, Model D/Max 2550 V, Rigaku, Japan) with Cu Kα radiation (λ = 1.5418) at the voltage of 40 kV and the current of 40 mA, respectively. All the samples were measured at as canning rate of 5 °/min. The microscopic morphology and composition of the nanoparticle samples was obtained using a transmission electron microscope (TEM, JEOL,

JEM-2010,) with an energy-dispersive

spectrometer (EDS). The optical property of the thermochromic films were measured using UV-vis-near-IR spectrophotometer (HITACHI U-3010) with a temperature controlling unit. The phase transition properties of the powder samples were detected by differential scanning calorimetry (DSC200F3, NETZSCH) over the temperature range from 0 to 100 °C with a heating/cooling rate of 10 °C /min. For assessment of the visual and solar modulation performance of the films, the luminous transmittance (Tlum, 380-780 nm) and solar transmittance (Tsol, 260-2600 nm)


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were essential, which were calculated from the following equation ܶ୪୳୫ =

‫߮ ׬‬୪୳୫ ሺߣሻܶሺߣሻ݀ߣ ‫߮ ׬‬୪୳୫ ሺߣሻ݀ߣ

ܶୱ୭୪ =

‫߮ ׬‬ୱ୭୪ ሺߣሻܶሺߣሻ݀ߣ ‫߮ ׬‬ୱ୭୪ ሺߣሻ݀ߣ

where T(λ) denotes the transmittance at wavelength λ and φlum(λ) is the spectral sensitivity of the light-adapted eye. φsol is the solar irradiance spectrum for air mass 1.5 corresponding to the sun standing 37° above the horizon. In result, we can obtain the solar modulation efficiency (∆Tsol), which is usually used to characterize the thermochromic properties of VO2 films.

RESULTS AND DISCUSSION Figure 1a shows the XRD patterns of initial hydrothermally prepared VO2 nanoparticles and VO2 @ZnO nanoparticles. From these patterns, we could observe that all peaks of samples were corresponded to monoclinic VO2 (M) (JCPDS no. 043-1051) without any other impurity peaks. It means that the coating process of VO2(M) with ZnO shell will not destroy the crystals of the VO2 (M) particles. Moreover, no peaks in the XRD patterns are resulted from ZnO. Hence, we believe that ZnO was in a state of amorphous. To confirm the oxidation state of vanadium and zinc of the VO2@ZnO nanoparticle, X-ray photoelectron spectroscopy (XPS) was carried out and the results are exhibited in Figure 1b-d. Figure 1b is the full-scan spectrum of VO2@ZnO which demonstrates the existing of C, O, V and Zn. The signle C 1s peak (284.6 eV) in the survey is attributed to adventurous hydrocarbon contamination on the sample surface. The peak in 530.7 eV is assigned to O 1s owing to the O2- ions in the crystal lattice of the Zn and V ion array. The peak areas corresponding to V 2p and Zn 2p in XPS spectra are shown in Figure 1c and d. The main fitting peaks of V 2p3/2 and Zn 2p3/2 are centered at 516.59 eV and 1022.36 eV, which match well with the positions for V4+ in VO2 (516.30 eV) and Zn2+ in ZnO (1022.50 eV). These results confirm that the



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product was comprised of VO2 and ZnO. The morphology of our product was characterized by transmission electron microscopy (TEM). Figure 2a presents the TEM image of initial VO2(M). It reveals that the particle diameter of the prepared VO2(M) nanoparticles is ~40nm. According to Laaksonen’s computation for VO2 particle-based coating modeled by the four-flux theory, this size of VO2 nanoparticles are perfectly for VO2-based smart window.40 Figure 2b and Figure 2c are the TEM images of sample S2 (The TEM images of sample S1 and S3 are shown in Figure S1 of SI). It can be observed that almost every single VO2 nanoparticle was coated with ZnO shell separately and the thickness of the ZnO shell is about 15 nm according to Figure 2b (The thickness of sample S1 and S3 are about 5 nm and 25 nm respectively). In term of a single nanoparticle circled in Figure 2c, it can be clearly observed that VO2 nanoparticles were closely surrounded by ZnO shell. Figure 2d is the HRTEM image of the sample S2. It indicates that the VO2 core is a single crystal with a interplanar spacing of 3.2 Å corresponded to the (011) crystal plane. The result of selected area electron diffraction (SAED) also proved this conclusion (Figure S2). Besides, from Figure 2d we can find that there are no lattice fringes in ZnO shell, which means that ZnO is amorphous. These results are in agreement with the XRD results. Figure 2e and f present EDS line scan results of the single nanoparticle in Figure 2c. The compositional scanning curves of the single VO2@ZnO nanoparticle illustrate that the contents of Zn were slightly higher at both ends than that inside of the particle (Figure 2e), which further confirms that the VO2 particle was completely coated with ZnO shell. To investigate the thermochromic properties, we prepared films using these nanoparticles on float glass as model of VO2 smart windows. After coating the nanoparticles onto a glass substrate, The UV-vis--IR spectrophotometer was employed to measure the transmittance at a low (20 °C) and a high temperature (80 °C). The results were shown in Figure 3a and Figure 3b, and the calculated optical performance (∆Tsol and Tlum) is summarized in Table 2. Compare with uncoated VO2 film, ZnO coating VO2 films show greater effects on the properties. The ∆Tsol and Tlum are improved from 38.9% and 17.2% to 51.0% and 19.1% (sample S2). The back


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scattered SEM pictures of the film surface (Figure S3) show that the coated VO2 film has a better dispersity in the matrix than the uncoated VO2 film. Meantime, VO2@ZnO nanoparticle films with different amounts of zinc nitrate hexahydrate and HMT(sample S1, S2 and S3) were prepared by the same method. In the respect of solar regulation efficiency (∆Tsol), there is a little different between sample S1 (∆Tsol=18.9%) and S2 (∆Tsol=19.1%), but the luminous transmittance (Tlum) of sample S2 is increased from 47.7% (sample S1) to 51.0%. The optical properties of sample S3 degraded from ∆Tsol=18.9% and Tlum= 47.7% to ∆Tsol=16.6% and Tlum= 47.4% as compared with sample S1, which means that enthalpy of VO2 nanoparticle will decline due to coating with thicker ZnO shell. Therefore, an appropriate thickness of the ZnO shell for VO2 nanoparticles is the key of optical performance enhanced. Figure 3c shows the typical DSC curves of the uncoated VO2 and VO2@ZnO with various thickness of the ZnO shell. It can be seen from Figure 3c that the endothermic peak in the heating process of the uncoated VO2 at 67.7 °C, which is similar to the transition temperature of VO2 (Tc=68 °C). However, the phase transition temperature of the VO2@ZnO was decreased to 63.6 °C, which indicates that the phase transition is related to the thickness of the ZnO shell. The relationship is also clearly shown in Figure 3d, which shows that the Tc of VO2@ZnO slide from 66.9 °C to 63.6 °C with the increment of thickness of ZnO shell changing from 5 nm to 25 nm. The phenomena of enhanced optical properties and decreased phase transition temperature can be explained by the theory of the optical band gap. The band gap energy (Eg) can be determined as follows: ሺߙℎߥሻଶ = ‫ܣ‬ሺℎߥ − ‫ܧ‬୥ ሻ where α is the theoretical expression of the absorption coefficient, and A is a constant depending on the transition probability. The band gap energy can then be determined by extrapolating (αhν)2 to zero.41 The existence of ZnO widens the optical band gap from 2.27 eV in VO2 nanoparticles to 2.58 eV in VO2@ZnO nanoparticles, which is shown in Figure S4. The incorporation of Zn in the interface of VO2 and the



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interfacial stress which come from ZnO shell could distort the crystal of VO2 and increase the concentration of the defects, resulting in wide optical band and phase transition temperature decreased.42 The widened optical gap could lead to blue shift in the absorption edge from 546 to 480 nm, which result in a significant increase in the luminous transmittance.43 In addition, ZnO also be used as antireflection coating for VO2 film which has great effects on the luminous transmittance due to the higher refractive index.37 Therefore, the ZnO shell can act as an antireflection layer to increase the luminous transmittance of the films. VO2@ZnO core-shell structures effectively enhance the durability of VO2-based smart window. The coating shell serves as a barrier layer for oxygen and water diffusion so that it is expected to prevent the VO2 from being oxidized to V2O5. We designed an extreme environment for constant temperature of 60 °C and humidity of 90% which will accelerate VO2-based films losing the thermochromic performance in this condition of environment and evaluated the durability of VO2-based films by the degree of decrease in thermochromic performance. The transmittance spectra of uncoated VO2 film and VO2@ZnO film (sample 2) are show in Figure 4a and Figure 4b. For uncoated VO2 film, the transmittance contrast between the low-temperature semiconductor phase and high-temperature phase begins to decrease after 12 h. As time goes on, the transmittance measured in visible region and near-infrared region increase gradually, and finally the thermochromism vanishes completely after 30 h. The trend of the transmittance of VO2 at different time is in agreement with reported works.35-36, 44 Compared with the uncoated VO2 film, VO2@ZnO film shows striking durability. The transmittance curve at different temperature of VO2@ZnO film remains almost intact after 103 h testing which means that it still has good thermochromic performance. Figure 4c and Figure 4d show the tendency of transmittance variation at λ=1500 nm and ∆Tsol, which can evaluate the thermochromic performance of VO2-based films. It could be found that the transmittance contrast between 20 °C and 80 °C at λ=1500 nm (∆Tλ=1500) was intended to decrease with the time going on. For uncoated VO2 film, the ∆Tsol is 17.2% in initial state and then decreased to 0% after 30 h. The transmittance contrast at λ=1500


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nm decline to 0 at the same time. On the other hand, the ∆Tsol of VO2@ZnO film keeps 77% (from 19.1% to 14.7%) and the transmittance contrast at λ=1500 nm remained 80% (from 48.2% to 38.8%) of the initial sample after 103 h testing (Table 3). The phenomena demonstrate that the VO2@ZnO structures can not only enhance the optical performance of VO2 film but also effectively protect the thermochromic VO2 film from oxidation in hyperthermal and humid environment. According to the rigorous testing conditions which were employed to accelerate the degradation process of VO2-based films, we could speculate that the thermochromic properties of VO2@ZnO-based smart window can maintain more than 10 years in practical application.45-47 In summary, the core-shell of the as-prepared VO2@ZnO made great sense in improving the performance of VO2-based smart window. The excellent durability in this work is caused by the intact and compact ZnO shell which can provide well protection for VO2. Thermal treatment always be used for getting compact shell. Tong et al have reported VO2 nanoparticles coated with Al-O-based shell (V/AO) and heated the nanoparticle in vacuum at 600 °C for 3 h.36 The result shows that annealed V/AO nanoparticles have a better durability than unannealed V/AO nanoparticles, which means that thermal treatment can enhance the density of the shell and provide well protection for the shell. However, thermal treatment also can enhance the crystallinity of VO2 nanoparticle and make the nanoparticle grow. Thus large thermal stress would generate between the core and shell and the shell would crack. Li et al have reported the synthesis of VO2@TiO2 core/shell

nanoparticles

where

the

TiO2 shell

prepared

by

sol-gel

and

vacuum-annealing methods.48 It can be observed that the shell of TiO2 contain a considerable amount of cracks which was proved by SEM images. Huang et al have used diluted hydrochloric acid to etch V1-xWxO2@SiO2 in order to decrease the sizes of VO2.49 It also demonstrates that there are cracks in the SiO2 shell. In our work, intact and compact shell of ZnO can be formed without thermal treatment so that it can decrease the thermal stress and avoid producing cracks in the shell. Therefore, VO2@ZnO nanoparticles exhibit excellent durability for VO2-based films than previous reported works.



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Figure 5 displays the ∆Tsol, Tlum and durability in constant temperature (60 °C) and humidity (90%) for different VO2-based smart films which have been reported. For VO2@SiO2 core-shell structure, the ∆Tsol is 21.4% at Tlum=56.2%.34 However, the durability of VO2@SiO2 is only 72 h. WO3/VO2/WO3 composite film could increase durability to 480 h and maintain high Tlum at 55.4%, but its ∆Tsol is less than 4.5%.50 In contrast, VO2/Al-O core-shell structure was promised to be an effect way to enhance the durability and keep the optical performance from declining. The durability of VO2@Al (OH)3 film extends to 120 h when its Tlum keeps at 53.1% and ∆Tsol at 14.8%, and the durability of VO2@Al2O3 film further extended to 480 h when Tlum keeps 46.7% and ∆Tsol keeps 13.2%.36 Compared with the above results, the VO2@ZnO core-shell nanoparticles in this work present much more excellent durability in hot and humid environment (>103 h) and enhance optical performance (Tlum=51.0%, ∆Tsol=19.1%) at the same time, which is superior to any other previous works.

CONCLUSION In summary, we report a facile method to prepare a novel thermochromic VO2@ZnO core-shell structure, which displays an excellent integrated performance. The VO2@ZnO-based films exhibit ultrahigh durability (>103 h using accelerated experiment), accompanied by enhanced thermochromic performance (∆Tsol of 19.1% and Tlum of 51.0%). This outstanding durability could be ascribed to the good protection of ZnO shell for VO2 nanoparticles. And the enhanced optical properties were attributed to the widened optical gap band and the antireflective effect of ZnO. This study provides a new solution for optimizing the optical properties and promoting the durability, which can be very promising for the practical application of VO2-based thermochromic smart windows.


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ASSOCIATED CONTENT Supporting Information TEM images for various VO2@ZnO nanoparticles, SAED patterns for VO2@ZnO nanoparticles, SEM pictures for surface of VO2 and VO2@ZnO films, the spectra for UV- vis absorption and (αhv)2-hv relationship of VO2, VO2@ZnO and ZnO, reflectance spectra of VO2 and VO2@ZnO films at 20 °C.

AUTHOR INFORMATION Corresponding Author *

E-mail, [email protected].

*


*


Author Contributions ||

Y.C. and X.Z. contributed equally to this work.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This study was financially supported by the National Natural Science Foundation of China (NSFC, No. 51372264, 51572284) and the Shanghai Sailing Program (No. 17YF1429800).



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Figure Captions

Scheme 1. Experimental flow chart for the synthesis of VO2@ZnO core-shell structure nanoparticle and VO2@ZnO film.

Figure 1. (a) XRD patterns of uncoated VO2 and VO2@ZnO with different amounts of Zn(NO3)·6H2O and HMT. (b) XPS full-scan spectrum of VO2@ZnO. High-resolution scan of (c) V 2p and (d) Zn 2p.

Figure 2. (a) TEM image of uncoated VO2 nanoparticles. (b) and (c) TEM images of VO2@ZnO core-shell structure nanoparticle. (d) HRTEM image of the VO2@ZnO core-shell structure nanoparticle. (e) EDS line-scan of the nanoparticle in panel c. (f) EDS spectrum of VO2@ZnO nanoparticles.

Figure 3. Optical transmittance spectra at 20 °C and 80 °C of uncoated VO2 film and VO2@ZnO film (a), Sample S1, S2, S3 for films (b). (c) DSC curves of the uncoated VO2 and Sample S1, S2, S3. (d) The dependence of critical temperature (Tc) on the thickness of ZnO shell.

Figure 4. Optical transmittance spectra of uncoated VO2 (a) and VO2@ZnO (b) in constant temperature (60 °C) and humidity (90%). Curves of transmittance at λ=1500 nm and solar regulation efficiency (∆Tsol) for different time: (c) uncoated VO2 and (d) VO2@ZnO.

Figure 5. Solar regulation efficiency (∆Tsol), luminous transmittance (Tlum) and durability in constant temperature (60 °C) and humidity (90%) of different VO2-based smart window coatings.


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Table 1. Amounts of Zn(NO3)·6H2O and HMT for VO2@ZnO. Sample

Zn(NO3)·6H2O (g)

HMT (g)

S1

0.15

0.7

S2

0.3

0.15

S3

0.5

0.25

Table 2. Summary of the luminous transmittance, solar transmittance and solar regulation efficiency for VO2 and VO2@ZnO films Tlum (%)

Tsol(%) ∆Tsol

Sample 20°C

80°C

20°C

80°C

VO2

38.9

36.2

47.0

29.8

17.2

S1

47.7

43.6

53.9

35.0

18.9

S2

51.0

46.5

57.1

38.0

19.1

S3

47.4

43.1

50.8

34.2

16.6

Table 3. Summary of the transmittance at λ=1500 nm, solar regulation efficiency (∆Tsol) in different time for VO2@ZnO films. Tλ=1500 (%) ∆Tλ=1500 (%)

∆Tsol (%)

31.1

48.2

19.1

79.3

31.4

47.9

18.9

102

79.4

32.2

47.2

18.4

2x102

79.9

33.7

46.2

17.7

5x102

80.8

37.5

43.3

16.5

103

81.7

42.9

38.8

14.7

Time (hours) 20 °C

80 °C

0

79.3

10



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Ultrathin Nanostructures with High Optical Performance and Optimization for Smart Windows by Etching. J. Mater. Chem. A 2013, 1 (40), 12545-12552. 50. Long, S. W.; Zhou, H. J.; Bao, S. H.; Xin, Y. C.; Cao, X.; Jin, P. Thermochromic Multilayer Films of WO3/VO2/WO3 Sandwich Structure with Enhanced Luminous Transmittance and Durability. RSC Adv. 2016, 6 (108), 106435-106442.



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Scheme 1. Experimental flow chart for the synthesis of VO2@ZnO core-shell structure nanoparticle and VO2@ZnO film. 404x266mm (150 x 150 DPI)


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Figure 1. (a) XRD patterns of uncoated VO2 and VO2@ZnO with different amounts of Zn(NO3)•6H2O and HMT. (b) XPS full-scan spectrum of VO2@ZnO. High-resolution scan of (c) V 2p and (d) Zn 2p. 222x174mm (150 x 150 DPI)



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Figure 2. (a) TEM image of uncoated VO2 nanoparticles. (b) and (c) TEM images of VO2@ZnO core-shell structure nanoparticle. (d) HRTEM image of the VO2@ZnO core-shell structure nanoparticle. (e) EDS linescan of the nanoparticle in panel c. (f) EDS spectrum of VO2@ZnO nanoparticles. 318x200mm (150 x 150 DPI)


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Figure 3. Optical transmittance spectra at 20 °C and 80 °C of uncoated VO2 film and VO2@ZnO film (a), Sample S1, S2, S3 for films (b). (c) DSC curves of the uncoated VO2 and Sample S1, S2, S3. (d) The dependence of critical temperature (Tc) on the thickness of ZnO shell. 214x169mm (150 x 150 DPI)



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Figure 4. Optical transmittance spectra of uncoated VO2 (a) and VO2@ZnO (b) in constant temperature (60 °C) and humidity (90%). Curves of transmittance at λ=1500 nm and solar regulation efficiency (∆Tsol) for different time: (c) uncoated VO2 and (d) VO2@ZnO. 249x183mm (150 x 150 DPI)


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Figure 5. Solar regulation efficiency (∆Tsol), luminous transmittance (Tlum) and durability in constant temperature (60 °C) and humidity (90%) of different VO2-based smart window coatings. 198x168mm (300 x 300 DPI)



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Table Of Contents (TOC) graphic 214x73mm (150 x 150 DPI)


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