Bifunctional Template-Induced VO2@SiO2 Dual-Shelled Hollow

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Research Article Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Bifunctional Template-Induced VO2@SiO2 Dual-Shelled Hollow Nanosphere-Based Coatings for Smart Windows Zhe Qu,†,‡ Lin Yao,*,† Jing Li,† Junhui He,*,† Jie Mi,‡ Shihui Ma,†,§ Siyao Tang,†,§ and Lili Feng§

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†

Functional Nanomaterials Laboratory, Center for Micro/Nanomaterials and Technology, and Key Laboratory of Photochemical Conversion and Optoelectronic Materials, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Zhongguancundonglu 29, Haidianqu, Beijing 100190, China ‡ The Affiliation Key Laboratory of Coal Science and Technology of Shanxi Province and Ministry of Education, Taiyuan University of Technology, Taiyuan 030024, Shanxi China § School of Chemical and Environmental Engineering China University of Mining and Technology (Beijing), Beijing 100083, China S Supporting Information *

ABSTRACT: Thermochromic vanadium dioxide (VO2) as one of the most promising candidates for smart windows has attracted widespread attention in recent years. Excellent optical performances (luminous transmittance, Tlum, and solar modulation efficiency, ΔTsol) of VO2-based coatings are usually pursued as crucial issues. In the current work, we report an ingenious approach for the synthesis of VO2@SiO2 dual-shell hollow nanospheres (DSHNs) and the preparation of DSHNs thermochromic coatings. A sequential bifunctional template-induced mechanism for the formation of DSHNs was proposed. Because of the unique hollow-core and dualshell structure, the as-prepared VO2@SiO2 DSHNs coatings exhibited appealing optical performances with enhanced luminous transmittance of 61.8% and solar modulation efficiency of 12.6%, compared with continuous and dense VO2 coatings. It has been proved that the improvement of visible transmittance could be ascribed to the effective reduction of refractive index (from 2.6 to 1.6 at 630 nm). In addition, its excellent thermochromic performance has been confirmed by the model cubes measurements, expressing a great potential as energy-efficient smart windows in high-rise buildings. The bifunctional template-induced synthetic strategy may inspire more facile, efficient and inexpensive processes for development of well-defined multishelled hollow nanostructures for varied applications. KEYWORDS: vanadium dioxide, double shell hollow nanosphere, bifunctional template, thermochromic, antireflection

1. INTRODUCTION

the most ideal inorganic candidate to construct smart windows and no substitution is known today. Theoretically, it was stimulated by effective medium theory (EMT) that systems composed of highly dispersed VO2 nanoparticles (NPs) in other dielectric hosts have advantages in improving the overall optical performances of thermochromic coatings over their unitary counterparts.10−12 Therefore, VO2-based core−shell structures have attracted much attention because of modified particle dispersion, refractive index, functionality, and stability.13−16 Gao’ s group and Jin’ s group have been active in constructing various VO2-based core−shell structures as building blocks for smart coatings in recent years. For example, coatings have been prepared using VO2@SiO2 core−shell nanoparticles, which have enhanced environmental stability as well as modulated optical constant.

Since the industrial revolution, a huge amount of nonrenewable energy has been consumed to promote the economy, which causes unprecedented influence on human society, including increasingly deteriorating energy crisis and global environment. As is well-known, energy consumption in buildings constitutes nearly half of total energy consumption around the world, and windows are one of the most energyinefficient components in buildings.1−3 For this reason, thermochromic smart windows, which can automatically control the solar energy entering indoor in response to ambient temperature, have been regarded as an appropriate candidate to reduce the energy consumption in architecture.4−6 It is well-known that vanadium dioxide (VO2) undergoes a reversible and ultrafast phase transition from semiconductor to metal at ∼68 °C, accompanied by the dramatic decrease of transmittance in the near-infrared region (NIR).7−9 Such a property enables VO2 to be considered as © XXXX American Chemical Society

Received: December 19, 2018 Accepted: April 9, 2019

A

DOI: 10.1021/acsami.8b22113 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces Unfortunately, solar modulation efficiency (ΔTsol) of thin films sharply decreased after coating VO2 NPs with a SiO2 shell.17 Furthermore, VO2@TiO2 core−shell structure-based smart coatings were demonstrated, which exhibited excellent thermochromic energy-saving effect, and photocatalytic selfcleaning performance.18 Interestingly, Gao’s group have also synthesized VO2@SiO2 nanorods with tunable localized surface plasmonic resonance in the metallic state, which was beneficial to improve ΔTsol without sacrificing the luminous transmittance (Tlum). What’s more, the inert oxide shell can also protect VO2 from oxidation to improve the life-span of smart coatings. Then, they attempted to protect the VO2 core by using an Al−O-based shell, which could enhance the durability of smart windows from 5 days to 20 days.19 Recently, zinc oxide (ZnO) shell20 and zinc sulfide (ZnS) shell21 were selected as protective barriers to improve the oxidation resistance of VO2-based smart coatings. In addition, the ZnS shell can modify the infrared emissivity of VO2 nanoparticles, making them more suitable for infrared camouflage application. It is not difficult to find that substantial efforts have confirmed that the balance between Tlum, ΔTsol, and stability of thermochromic coatings could be effectively tailored by manipulating the nanostructure and morphology of VO2-based NPs. In addition, the optical performance of nanoparticles composed of VO2 hollow nanospheres is superior to that of solid counterparts.22,23 However, to our best knowledge, most researches have been concentrated on simply coating VO2 NPs core with other transparent and dielectric thin shells. It is difficult to modulate the morphology and structure of composite nanoparticles, especially wellcontrolled multishelled hollow structures, which are expected to show outstanding thermochromic properties.24 Rational design and synthesis of VO2-based hollow nanospheres with high uniformity in a controlled manner still remain great challenges.25 Although Xu et al.26 prepared hollow VO2 spheres through a template-free hydrothermal method, in which the hollow spheres are formed as a result of outward Ostwald ripening by dissolution of the inner part and growth of the outer shell. The particle size (larger than 2.6 μm) of the as-prepared hollow spheres limited their further development as building block for smart windows. Under this consideration, it is necessary to develop a well-controlled strategy to synthesize nanoscaled VO2-based multishelled hollow structures with desirable morphologies and composition. In the current work, we reported a novel bifunctional template-induced method for the synthesis of VO2@SiO2 dualshell hollow nanosphere (DSHNs) via a well-controlled kinetic process. The amount of VO2+ loading in VO2@SiO2 DSHNs could be regulated by controlling the reaction time. The structural evolution of unique DSHNs was investigated and explained in terms of electrostatic interaction in detail. Then, DSHNs were utilized as building block for constructing thermochromic coatings with enhanced optical performance. By optimizing the nanoparticle size and nanostructure of DSHNs, substrate with the VO2@SiO2 DSNH coating attained enhanced luminous transmittance of 61.8% and solar modulation efficiency of 12.6%. It is important to note that this work realized the precise control over the morphology of VO2-based hollow NPs by delicate and rational design in the shell formation process instead of simply distributing or coating VO2 nanoparticles with other dielectric matrices. It may open a new path for preparation of well-defined core−

shell structures, especially multishelled hollow spheres, as building block for multifunctional coatings.

2. EXPERIMENTAL SECTION 2.1. Materials. Vanadium pentoxide (V2O5) was purchased from Kelong Chemical Reagent Company. Oxalic acid (H2C2O4·2H2O, 99.5+%) and poly(acrylic acid) (PAA, 30 wt % in water, Mw = 3000) were obtained from Aladdin Chemistry Co. Absolute ethanol (99.5%) and aqueous ammonia (25%) were purchased from Beihua Fine Chemicals. Tetraethyl orthosilicate (TEOS, 99+ %) was purchased from Alfa Aesar. All chemical regents were analytic grade and used without any further purification. Ultrapure water with a resistivity higher than 18.2 MΩ·cm was used in all experiments, and was obtained from a three-stage Millipore Mill-Q Plus 185 purification system (Academic). 2.2. Preparation of SiO2 Hollow Nanospheres (HNs). Typically, 0.2−0.3 g of PAA dissolved in 9 mL of aqueous ammonia was added dropwise into 180 mL of absolute ethanol, followed by the injection of 0.8−1.1 mL of TEOS under magnetic stirring at room temperature within 30 min. After 10 h, homogeneous SiO2 hollow nanospheres formed. Finally, ammonia was removed by stirring the sol in a ventilating cabinet. 2.3. Preparation of VO2@SiO2 Dual-Shell Hollow Nanospheres (DSHNs). VO2 precursor sol was prepared according to a previously reported method,27 4.6 g of V2O5 and 9.1 g of H2C2O4· 2H2O were added into a mixture of 10 mL of deionized water and 90 mL of ethanol, followed by refluxing at 120 °C for 10 h. The reaction occurred as shown in eq 1.

V2O5 + 3H 2C2O4 = 2VOC2 O4 + 2CO4 + 2CO2 + 2H 2O

(1)

Then 10 mL of VO2 precursor sol was injected to 20 mL of SiO2 hollow nanosphere sol at 0.5 mL/min. After injection, the temperature was subsequently increased to 60 °C, followed by stirring and refluxing for 1−4 h. The samples obtained at different hours were denoted as VS1, VS2, VS3, and VS4, respectively. Then the sol was allowed to cool to room temperature and sonicated for 30 min. 2.4. Preparation of VO2 @SiO2 DSHNs Coatings. The borosilicate glass was sonicated in a mixture of deionized water and absolute ethanol for 15 min followed by oxygen plasma (84 W, 5 min) treatment under an oxygen flow of 800 mL min−1. The cleaned glass substrate was immersed in the VO2@SiO2 sol for 60 s, then withdrawn from the sol at 150 mm/min, and dried at room temperature for 60 s. Each glass substrate was subjected the dipcoating procedure twice. The glass substrate coated by the VO2@SiO2 sol was annealed, with a ramp of 5 °C/min, for 1 h in a tube furnace at 550 °C under N2 at a flow velocity of 400 mL/min. For comparison, another 10 mL of VO2-containing precursor sol was diluted by 20 mL of ethanol, and a VO2 coating was also prepared by the same procedure as VO2@SiO2 DSHNs coatings. The preparation process is illustrated in Scheme 1. 2.5. Characterization. Transmittance and reflection spectra in the wavelength range from 300 to 2500 nm were measured at 15 and 90 °C on a Varian Cary 5000 UV−vis−NIR spectrophotometer with the assistance of a heating stage. The microscopic morphology was observed on a Hitachi S4800 field-emission scanning electron microscope (SEM) at 5 KV. Transmission electron microscopy (TEM) images were taken on a JEOL JEM-2100 at 200 kV. The refractive index and physical thickness of VO2@SiO2 DSHNs coatings were measured by Sentech spectroscopic ellipsometer at an incidence angle of 70°. The crystal structures of coatings were measured by Glancing Angle X-ray diffractometer (GAXRD, PANalytical X’pert Pro MPD) using Cu K radiation (λ = 0.154 nm). The roughness and surface morphology were investigated by atomic force microscopy (AFM) on an MM8-SYS scanning probe microscope (Bruker AXR). The energy-saving test was performed by a model cube experiment where a 50 W xenon bulb (NBet, HSX-F300) was employed as the light source. Blank glass and coated glass were embedded in the hole of the top of the model cubes. Temperature of the model cubes were B

DOI: 10.1021/acsami.8b22113 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces Scheme 1. Experimental Flow Chart for the Synthesis of VO2@SiO2 DSHNs and Corresponding Coatings

Figure 1. (a) TEM images of SiO2 hollow nanospheres. TEM images of VO2@SiO2 DSHNs: (b)VS1, (c) VS2, and (d) VS3, respectively.

monitored by temperature sensors inside the cubes. The characterizations were carried out at room temperature and 20% relative humidity. The two-dimensional infrared thermal images were obtained by an infrared thermal camera (FLIR SC620, FLIR Systems, Inc., OR, USA) and analyzed by accessory software in real time. The optical properties of all samples including luminous transmittance (380−780 nm) and solar transmittance (380−2500 nm) were calculated from the following equations:20

Tlum,sol =

∫ φlum,sol(λ)T(λ) dλ ∫ φlum,sol(λ) dλ

increase in the reaction time to 3 h leaded to most of the vanadium precursor getting into the cavity and being stuck to the internal surface of SiO2 HNs, thus forming a VO2+ inner shell (the layers between two red lines in Figure 1c and d). The TEM of sample VS1 after calcination are presented in the Figure S3a and c, it can be seen that the VO2 nanoparticle appears inside and outside the SiO2 HNs, which could be confirmed by the HRTEM images. The clear crystal fringes in HRTEM images (Figure S3b and d) indicate that the VO2 nanoparticles existed as well-crystallized single-phase particles. The interplanar spacing of crystal planes is consistent with the (011) plane of the monoclinic VO2(M) phase, which is in accordance with the GAXRD analysis. Thus, both of TEM and HRTEM confirm the existence of VO2(M) nanoparticles in the DSHNs coating. Interestingly, it was found that after prolonging the reaction time to 4 h, some of the obtained particles gradually became yolk−shell structured nanoparticles (shown in Figure S4) instead of DSHNs. The unexpected phenomenon has aroused our greatest interest in exploring the formation mechanism of VO2@SiO2 DSHNs. We here propose an elaborate mechanism for the formation process of VO2@SiO2 DSHNs, which is presented in Scheme 2. First, SiO2 HNs were prepared by a modified Stöber method. PAA reacted with aqueous ammonia to generate ammonium polyacrylate (APA). TEOS partially hydrolyzed and then deposited on the surface of APA polyanionic spheres to form SiO2 shells. Typically, the shells of SiO2 HNs prepared by this method were the porous. As a result, such opening pores enabled VO2+ ions to freely diffuse through the silica shell and entered into the interior of hollow nanospheres under vigorous stirring. What should be noted here is that APA not only acted as a core template for the synthesis of SiO2 shells in the first step but also as a scaffold for the immobilization of dispersed vanadium precursor on the internal surface of SiO2 shells because of electrostatic repulsion. VO2+ ions were attracted by the OH− groups on SiO2 HNs but restrained by the COONH4+ groups on APA chains after getting into the voids, which prompted them to concentrate on the internal

(2)

where T(λ) represents the transmittance value at wavelength λ, the φlum (λ) denotes standard efficiency function for photopic vision, and φsol (λ) is the solar irradiance spectrum for an air mass of 1.5. The solar modulation efficiency was calculated by ΔTsol = Tsol,c − Tsol,h

(3)

where Tsol,c and Tsol,h represent the Tsol before and after phase transition, respectively.

3. RESULTS AND DISCUSSION 3.1. Morphology and Structure of SiO2 HNs, VO2@ SiO2 DSHNs, and VO2@SiO2 DSHN Coatings. In this work, we employed a flexible one-step template-induced method to produce SiO2 HNs.28,29 The microstructure of SiO2 HNs and visible transmittance of SiO2 HNs-based coatings were optimized as shown in Figures S1 and S2 and Table S1. Here, we focus on the VO2@SiO2 DSHNs. To precisely explore and control the kinetic formation process of VO2@ SiO2 DSHNs, time-dependent experiments were carried out. The change of nanoparticle microstructure over the refluxing time were recorded by TEM. At the initial stage (t = 1 h), there were many clusters adrift around SiO2 HNs, and a small part of them attached to the outside surface of SiO 2 nanospheres (as denoted by red arrow and circle in Figure 1b). According to eq 1 in the Experimental Section, it is reasonable to believe that these clusters were mainly composed of the vanadium precursor (VO2+ ions).30 With the refluxing time increasing (t = 2), clusters in the free state gradually disappeared, and some of them coated on the outer surface of SiO2 shell while others diffused into the nanospheres through micropores on the SiO2 shell under vigorous stirring. A further C

DOI: 10.1021/acsami.8b22113 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces Scheme 2. Schematic Illustration of the Formation Mechanism of VO2@SiO2 DSHNs

surface of SiO2 shell. When the amount of VO2+ ions in the cavity reached saturation, the number of COONH4+ groups would not be enough to prevent VO2+ locating into the center of nanospheres, thus forming the yolk−shell structure. In the final step, the calcination process promoted the removal of the template APA and the crystallization of VO2+ to VO2 (M). On the basis of the above discussion, it is easy to adjust the ratio of VO 2 inside and outside the as-prepared hollow SiO 2 nanospheres by changing the reaction time. From the optical measurements in the following content, there could be an optimal ratio between the amount of VO2 inside and outside of the SiO2 HNs, which enabled the Tlum and ΔTsol of asprepared DSHNs coatings to achieve the best balance. To our best knowledge, we realized the precise control for the VO2based multishell hollow nanospheres for the first time. The rational and delicate design of complex multishell or yolk− shell nanoparticles with desirable shape, composition, and physicochemical properties are worthwhile to be further researched by utilizing appropriate templates, certain shell materials, various driving forces, etc. Borosilicate glasses were dip-coated with the SiO2 HNs and VO2@SiO2 DSHNs sol and annealed at 550 °C for 1 h under N2 to crystallize the VO2 (M) with good thermochromic property. As shown in Figure 2a, the glass substrate was homogeneously covered by SiO2 HNs with ∼0−25 nm interspace between them. With respect to the VO2@SiO2 DSHNs coatings, it can be observed that there were a large quantity of continuous VO2 nanoparticles (as indicated by red arrows in Figure 2b) aggregating around the SiO2 HNs in the coating composed of VS1. The morphology of VO2 nanoparticles outside the SiO2 HNs were similar to that of the reference VO2 sample (Figure S4). Meanwhile, the SiO2 HNs were connected with each other, and the interspace among them became small or even disappeared. As a result, the surface of the whole coating was rather coarse. With the reaction going on (t = 3 h), there were less and less VO2 nanoparticles exposed outside the SiO2 HNs. It is clear that the glass substrate was uniformly covered by spherical nanoparticles (as denoted by red arrow) in Figure 2d, indicating most of VO2 NPs were encapsulated in the silica nanospheres. Therefore,

Figure 2. Top-view SEM images of (a) SiO2 HNs coating and VO2@ SiO2 DSHNs coatings of (b) VS1, (c) VS2, and (d) VS3, respectively.

the top-view SEM images of coatings subject to refluxing for varied periods of time also confirm the fabrication of welldefined VO2@SiO2 DSHNs. The cross-sectional SEM images of samples VS1, VS2, and VS3 are shown in Figure S6. It can be observed that the average thickness of VS1, VS2, and VS3 were about 85, 70, and 83 nm, respectively. These films with thickness of 40−80 nm could show comparatively balanced combination of visible transmittance and NIR switching efficiency. Figure S7a and b shows AFM images of the bare VO2 and VO2@SiO2 DSHNs composite coatings, and there is significant difference in their surface morphology. The surface of the bare VO2 coating was comprised of nonoriented nanoparticles and was very smooth. As for the sample of VS3, the glass substrate was fully covered by nanospheres with height difference within hundred nanometers scale, which D

DOI: 10.1021/acsami.8b22113 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 3. (a) Temperature-dependent transmittance spectra of VO2 and VO2@SiO2 DSHNs coatings of VS1, VS2, VS3, and VS4, respectively. (b) Tlum and ΔTsol of samples subject to refluxing for varied periods of time.

VS4 coating reduced to ΔTsol = 8.1% and Tlum = 57.1%, which was the lowest one among all the samples. Therefore, the optimal result was realized when the refluxing time was 3 h. Particularly, the increase of ΔTsol was achieved in this work without sacrificing the visible transmittance, which is different from the results in some other literatures.35,36 The novel VO2@SiO2 DSHNs showed an enhancement in optical performance compared with other core−shell structures in previous reports (Table 1). It could be partly attributed to the

agrees well with the SEM observations. The root mean-square (rms) roughness of the coating was estimated to be 22.7 nm in an area of 2 μm × 2 μm, as compared to that (2.5 nm) of the bare VO2 coating, indicating the coating became coarsen. The increase of surface roughness may contribute to the optical performance improvement of VS3 coatings.31 GAXRD was used to verify the particle size and crystallinity of as-prepared samples. Figure S8 shows the GAXRD patterns of prepared pristine VO2 and VO2@SiO2 dual DSHNs coatings (VS3). All peaks of samples are associated with the (011), (220), (111), (022), and (130) lattice planes of VO2 (M) without any impurity detected (JCPDS no. 043-1051, space group = P21/c). Moreover, the diffraction peak of bare VO2 is relatively stronger and narrower than that of VO2@ SiO2 DSHNs. The average crystallite size of VO2 NPs in the bare VO2 coating and the DSHNs coating were estimated to be ∼26.5 and ∼13.8 nm, respectively, by Scherrer’s formula (see Supporting Information) from the diffraction peak (011). The results indicated that the introduction of SiO2 HNs could effectively prevent the VO2 from aggregation and growth. 3.2. Optical Performance and Analysis. Core−shell structures, especially single or multishelled hollow ones, have attracted considerable attention and have been widely applied in many areas, including energy storage, drug delivery, and catalysis.32−34 However, the research of “smart strategy” for VO2-based thermochromic core−shell nanoparticles with wellcontrolled morphology and performance is still in its infancy. It is highly possible to get well-performed thermochromic coatings with excellent optical performance by accurately regulating the pertinent variables in the formation process of complex multishell nanoparticles. To investigate the thermochromic and optical performances of VO2@SiO2 DSHNs coatings, the UV−vis−IR spectrophotometer with the assistance of heating stage was employed to measure the temperature-dependent transmittance. The experimental results and the calculated optical performances (Tlum and ΔTsol) are presented in Figure 3a and b and Table S2. Comparing with the continuous and dense VO2 coatings, the VO2@SiO2 DSHNs coatings exhibited much better optical performances. With increase of the refluxing time, the Tlum increased from 42.5% to 66.3% (VS1) first and, then, decreased to 61.7% (VS2) and 61.8% (VS3), respectively. Meanwhile, ΔTsol increased from 7.6% to 10.7% (VS1), 10.5% (VS2), and 12.6% (VS3), respectively. However, with extending the refluxing time to 4 h, the optical property of

Table 1. Summary of the Optical Performance of VO2-Based Core−Shell-Structured Coatings sample VO2@SiO2 core− shell structure VO2@TiO2 core− shell nanorods VO2@SiO2 core− shell nanorods platelike VO2@ SiO2 structure VO2@SiO2 core− shell 2D SiO2/VO2 photonic crystals VO2(M)−ZnO dandelions VO2 @ ZnO core− shell VO2@SiO2 dualshell nanoparticle

Tlum‑L/H (%) (380−780 nm)

ΔTsol (%) (380−2500 nm)

preparation

55.3/58.7

7.5

hydrothermal

17

68.2/41.6

10.27

sol−gel

18

46.5/48.1

11.98

hydrothermal

47

36.0/40.4

8.4

hydrothermal

48

59.7/56.5

11.3

hydrothermal

38

41.5/46.9

3.1

sol−gel

14

52.2/n.a.

9.3

hydrothermal

49

ref

51.0/46.5

19

hydrothermal

20

61.8/60.2

12.6

sol−gel

this work

following reasons: (1) the protection of SiO2 shell for VO2 (M) nanoparticles from aggregating and growing during the annealing process, which has been proved by SEM observations and GAXRD measurements. Annealing VO2 NPs without any prevention would lead to severe aggregation and degrade optical performance, especially luminous transmittance, as reported by previous work.37 (2) Well-dispersed VO2 nanoparticles with rather small sizes (sub-40 nm) could effectively avoid light scattering, thus improving the thermochromic properties of coatings, which was calculated from effective medium theory and four-flux theory by Laaksonenh et al. and Li et al.10,38 As a result, despite the fact that VO2@SiO2 DSHNs prepared in our work contained E

DOI: 10.1021/acsami.8b22113 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 4. (a) Wavelength dependent refractive index (n) and extinction coefficient (k) of VO2@SiO2 DSHNs coating at room temperature. (b) Comparison between measured and simulated transmittance spectra of VO2@SiO2 DSHNs coatings of different thickness over the solar spectrum. The solid line and dash line represent the experimental and calculated results, respectively.

illustrated in Figure 5a. The model cubes were employed to simulate the real environment in high-rise buildings. Two

less VO2 as compared with other solid core−shell nanoparticles, the fabricated coatings could still maintain a reasonable performance, which is superior to other porous films and many other VO2−SiO2 composite systems.39−42 Furthermore, the AR tech, especially via surface engineering (e.g., layer-by-layer assembly, etching, and lithography techniques), is one of the most flexible and cost-effective methods to achieve high transmittance. But the in-depth and ingenious research is rather rare on the integration of the AR tech with thermochromic coatings. Theoretically, the Fresnel equation provides an essential mathematical model of the reflection of thin films.43 For an ideal single-layered thin film, the refractive index (RI = nc) should satisfy two requirements: (1) nc = (na*ns)1/2 and (2) d = λ/4nc, where na and nc represent the RI of the air and the substrate, respectively, d denotes the thickness of the single-layer thin films, and λ denotes the wavelength of incident light. It can be calculated that an ideal single layer AR coating should have a RI of 1.23, when na and ns are 1 and 1.52, respectively.44 To obtain a desirable refractive index, it is necessary to adjust the void size, thickness of SiO2 shell and the amount VO2 in VO2@SiO2 DSHNs. The refractive index (nv) and extinction coefficient (kv) were measured over the solar spectra, as shown in Figure 4a. Unfortunately, it was difficult to measure the optical constants of bare VO2 coating by ellipsometry probably because of the sever aggregation of disorder-arrayed nanoparticles.45 Comparing with the bare VO2 (RI ≈ 2.5 at 600 nm) reported by previous literatures,28,46 the RI of VO2@SiO2 DSHNs coatings obtained in the current work show dramatic reduction, and reached 1.63 at the wavelength of 630 nm, which is closer to the ideal one, imbuing the coating with a better AR effect. Furthermore, we fitted the transmittance spectra at low temperature by finite-different time-domain (FDTD) using the measured optical constants (Figure 4b). It is found that the calculated results are in impressive accordance with the experimental spectra. Therefore, the dual-shell hollow nanoparticles of VO2@SiO2 make a great sense in enhancing the optical performance of thermochromic coatings. 3.3. Energy-Saving Tests. As one of the most promising candidate materials for smart windows, the ability to control the IR transmittance is crucial for VO2-based coatings.50−52 To further demonstrate the application potential of VO2@SiO2 DSHNs coatings, experimental devices were established as

Figure 5. Characterization of energy-saving effect of VO2@SiO2 DSHNs coating. (a) Schematic illustration of model cube measurements; The infrared thermal images of (b) cube 2 with VO2@SiO2 DSHN-coated glass and (c) cube 1 with blank glass at 5 s infrared irradiation. (d) Relationships between radiation time and temperature within cube 1 (red line) and 2 (black line), respectively.

cubes were made of some board wrapped by tinfoil (4 mm in thickness) for thermal isolation. Internal space of the cubes was about 216 cm3 (6 cm × 6 cm × 6 cm). Meanwhile, a hole in the top of cubes was covered by blank (cube 1) or the VO2@ SiO2 DSHNs-coated glass (cube 2) as windows. Besides, a temperature detector was placed in the center of each cube to monitor their actual temperature changes. The measurements were carried out at 23.1 °C initially without irradiation of infrared lamp. After irradiation for ∼5 s, the temperature of coated glass in Figure 5b sharply climbed (∼106 °C) and exceeded the phase transition temperature of VO2 (68 °C), while the temperature of the upper surface of blank glass was much lower (∼45 °C, Figure 5c). It is reasonable to believe that the DSHNs coated substrate underwent an ultrafast phase transition during the heating process. Then, the rate of inner F

DOI: 10.1021/acsami.8b22113 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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and Chemistry, the Chinese Academy of Sciences (CAS) (2017-YL), the National Natural Science Foundation of China (21571182), the National Key Research and Development Program of China (2017YFA0207102), Youth Innovation Promotion Association of CAS (2016023), the Science and Technology Commission of Beijing Municipality (Z151100003315018), and the National Natural Science Foundation of China (61307065).

temperature increase of cube 2 became slower than that of cube 1 after the VO2 completed the phase transition. Consequently, after irradiation for 180 s, while the inner temperature of cube 1 increased to 68 °C, the inner temperature of cube 2 only increased to 55 °C. The temperature difference between cube 1 and cube 2 reached as high as 13 °C (Figure 5d). From the above experiment, it can be found that the as-prepared VO2@SiO2 DSHNs coating was highly effective to resist the infrared radiation, with great potential in practical applications.

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(1) Chang, T.; Cao, X.; Li, N.; Long, S.; Gao, X.; Dedon, L. R.; Sun, G.; Luo, H.; Jin, P. Facile and Low-Temperature Fabrication of Thermochromic Cr2O3/VO2 Smart Coatings: Enhanced Solar Modulation Ability, High Luminous Transmittance and UV-Shielding Function. ACS Appl. Mater. Interfaces 2017, 9, 26029−26037. (2) Xu, F.; Cao, X.; Luo, H.; Jin, P. Recent Advances in VO2-based Thermochromic Composites for Smart Windows. J. Mater. Chem. C 2018, 6, 1903−1919. (3) Lee, H. Y.; Cai, Y.; Velioglu, S.; Mu, C.; Chang, C. J.; Chen, Y. L.; Song, Y.; Chew, J. W.; Hu, X. M. Thermochromic Ionogel: A New Class of Stimuli Responsive Materials with Super Cyclic Stability for Solar Modulation. Chem. Mater. 2017, 29, 6947−6955. (4) Gao, Y.; Luo, H.; Zhang, Z.; Kang, L.; Chen, Z.; Du, J.; Kanehira, M.; Cao, C. Nanoceramic VO2 Thermochromic Smart Glass: A Review on Progress in Solution Processing. Nano Energy 2012, 1, 221−246. (5) Li, M.; Magdassi, S.; Gao, Y.; Long, Y. Hydrothermal Synthesis of VO2 Polymorphs: Advantages, Challenges and Prospects for the Application of Energy Efficient Smart Windows. Small 2017, 13, 11701147. (6) Powell, M. J.; Quesada-Cabrera, 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, 1369−1376. (7) Morin, F. J. Oxides Which Show a Metal-to-Insulator Transition at the Neel Temperature. Phys. Rev. Lett. 1959, 3, 34−36. (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) Son, J.-H.; Wei, J.; Cobden, D.; Cao, G.; Xia, Y. Hydrothermal Synthesis of Monoclinic VO2 Micro- and Nanocrystals in One Step and Their Use in Fabricating Inverse Opals. Chem. Mater. 2010, 22, 3043−3050. (10) Li, S. Y.; Niklasson, G. A.; Granqvist, C. G. Nanothermochromics: Calculations for VO2 Nanoparticles in Dielectric Hosts Show much Improved Luminous Transmittance and Solar Energy Transmittance Modulation. J. Appl. Phys. 2010, 108, 063525. (11) 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, 1707. (12) Granqvist, C. G. Electrochromics and Thermochromics: Towards a New Paradigm for Energy Efficient Buildings. Materials Today: Proceedings 2016, 3, S2−S11. (13) Ghosh Chaudhuri, R.; Paria, S. Core/shell Nanoparticles: Classes, Properties, Synthesis Mechanisms, Sharacterization, and Applications. Chem. Rev. 2012, 112, 2373−2433. (14) Ke, Y.; Balin, I.; Wang, N.; Lu, Q.; Tok, A. I.; White, T. J.; Magdassi, S.; Abdulhalim, I.; Long, Y. Two-Dimensional SiO2/VO2 Photonic Crystals with Statically Visible and Dynamically Infrared Modulated for Smart Window Deployment. ACS Appl. Mater. Interfaces 2016, 8, 33112−33120. (15) Fleer, N. A.; Pelcher, K. E.; Zou, J.; Nieto, K.; Douglas, L. D.; Sellers, D. G.; Banerjee, S. Hybrid Nanocomposite Films Comprising Dispersed VO2 Nanocrystals: A Scalable Aqueous-Phase Route to Thermochromic Fenestration. ACS Appl. Mater. Interfaces 2017, 9, 38887−38900.

4. CONCLUSION In summary, a facile bifunctional template-induced approach has been designed to synthesize novel VO2@SiO2 dual-shell hollow nanoparticles as building block for thermochromic smart coatings. In the synthetic process, vanadium precursor gradually entered into prepared SiO2 hollow nanospheres to form a dual-shell hollow structure even yolk−shell nanoparticle. Taking advantage of electrostatic interaction, APA acted not only as template for the formation of SiO2 shell but also as a scaffold for immobilization of vanadium precursor on the internal surface of the SiO2 shell to form a dual-shell structure. The distinct DSHNs provided thermochromic coatings with reduced RI (from 2.6 to 1.6 at 630 nm), as well as outstanding optical performances (Tlum = 61.8%, ΔTsol = 12.6%). In addition, the energy-saving tests indicated that the VO2@SiO2 DSHNs coatings effectively blocked infrared radiation, which is of great significance for the practical application of smart windows to maintain a pleasant indoor environment. Furthermore, the facile bifunctional templateinduced approach has a potential to be developed as a universal strategy for the preparation of well-defined core− shell structures, especially multishelled hollow nanospheres.

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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.8b22113. TEM images of SiO2 HNs prepared by different amount of TEOS; transmittance spectra and optical performances of varied SiO2 HNs thin films; TEM and HRTEM images for VS1 and VS3 after calcination; TEM image for VS4; SEM image for bare VO2; cross-sectional SEM images for samples VS1, VS2, and VS3; GAXRD pattern and AFM image for VO2 and VO2@SiO2 coatings; summary of Tlum and ΔTsol of the different samples (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Junhui He: 0000-0002-3309-9049 Jie Mi: 0000-0002-9374-2307 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was financially supported by the Innovative Talents Cultivation Project of Technical Institute of Physics G

DOI: 10.1021/acsami.8b22113 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

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DOI: 10.1021/acsami.8b22113 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces transparent VO2 Solar Modulating Coatings. ACS Nano 2017, 11, 407−415.

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DOI: 10.1021/acsami.8b22113 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX