Dual-Phase Transformation: Spontaneous Self ... - ACS Publications

Dec 23, 2016 - School of Materials Science and Engineering, Shanghai University, Shanghai 200444, China. §. School of Chemistry, University of New So...
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Dual-Phase Transformation: Spontaneous SelfTemplate Surface-Patterning Strategy for Ultra-transparent VO2 Solar Modulating Coatings Minsu Liu,† Bin Su,† Yusuf V. Kaneti,† Zhang Chen,‡ Yue Tang,† Yuan Yuan,§ Yanfeng Gao,‡ Lei Jiang,*,∥ Xuchuan Jiang,*,† and Aibing Yu† †

Department of Chemical Engineering, Monash University, Clayton, VIC 3800, Australia School of Materials Science and Engineering, Shanghai University, Shanghai 200444, China § School of Chemistry, University of New South Wales, Sydney, NSW 2052, Australia ∥ Laboratory of Bioinspired Smart Interfacial Science, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, China ‡

S Supporting Information *

ABSTRACT: Dual-phase transformation has been developed as a template-free surface patterning technique in this study. Ordered VO2 honeycomb structures with a complex hierarchy have been fabricated via this method, and the microstructures of the obtained VO2(M) coatings are tunable by tailoring the pertinent variables. The VO2(M) honeycomb-structured coatings have excellent visible light transmittance at 700 nm (Tvis) up to 95.4% with decent solar modulating ability (ΔTsol) of 5.5%, creating the potential as ultratransparent smart solar modulating coatings. Its excellent performance has been confirmed by a proof-of-principle demonstration. The dual-phase transformation technique has dramatically simplified the conventional colloidal lithography technique as a scalable surface patterning technique for achieving high-performance metal oxide coatings with diverse applications, such as catalysis, sensing, optics, electronics, and superwettable materials. KEYWORDS: surface patterning, solar-modulating coating, self-assembly, vanadium oxide, template-free design.16−19 Various techniques, such as magnetron sputtering deposition,20,21 chemical vapor deposition,22,23 and pulsed laser deposition,24,25 have been employed for controlling coating thickness precisely, since the reduced thickness led to higher Tvis. However, the ΔTsol was usually sacrificed simultaneously.26,27 In addition, multilayer VO2 strategies have been reported to enhance both ΔTsol and Tvis at the same time, but the improvement was still constrained by the performance from the VO2 single layer.20,28−32 Surface engineering is one of the most effective approaches to optimize both ΔTsol and Tvis.33 Gao et al.26 prepared random nanoporous VO2(M) films with an improved Tvis. Ordered surface patterns can further improve the performance. Xie et al.34 were pioneers in the fabrication of periodic microstructured VO2(M) films using a colloidal template, allowing further enhanced Tvis up to 81%. To obtain ordered 2D-

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olar modulating coating, a kind of glazing that can alter light transmission properties under certain stimuli, has attracted attention because of its high effectiveness in building energy savings and temperature management.1−3 Vanadium oxide (VO2) is one of the most ideal materials to fabricate such energy-efficient solar modulating coatings due to its passive thermochromic tunability in selective near-infrared regions.3−6 The light transmission properties can be reversibly switched between infrared-transparent as monoclinic VO2(M) and infrared-translucent as rutile VO2(R) above/below the controllable metal−insulator transition (MIT) temperature of VO2.7−11 Therefore, VO2 can be utilized as solar modulating coatings that can automatically block heat but not visible light for energy savings.12−14 Low visible light transparency, a critical problem, still impedes the practical applications of VO2-based solar modulating coatings.15 In addition, the visible light transmittance (Tvis) and solar-modulating ability (ΔTsol) of VO2 smart coatings are usually contradictive,16−18 and thus, the balance of ΔTsol and Tvis demands a proper fabrication © 2016 American Chemical Society

Received: September 12, 2016 Accepted: December 23, 2016 Published: December 23, 2016 407

DOI: 10.1021/acsnano.6b06152 ACS Nano 2017, 11, 407−415

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Figure 1. Highly ordered VO2 honeycomb microstructures can be fabricated through a facile dual-phase transformation process. (a) Digital photo of a piece of ordered honeycomb-structured VO2(M) film with the dimensions of 25 mm × 20 mm; (b) low magnification SEM image and a high magnification insert; (c) AFM image and the corresponding height profiles (i, ii) of the honeycomb structures; (d) TEM image of a cross-section of the honeycomb-structured film obtained using FIB; (e, f) HRTEM images with labeled lattice fringes and crystal planes of the selected areas shown in (d); (g) XRD patterns of the honeycomb-structured VO2(M) films with different thicknesses of 65, 95, and 195 nm, respectively, in which the characteristic (011) peak of VO2(M) was observed.

RESULTS AND DISCUSSION VO2(M) films with highly ordered honeycomb microstructures have been prepared via a dual-phase transformation method. Figure 1a shows that the as-prepared VO2 film is transparent with a pale bronze color. The microstructure has been investigated using scanning electron microscopy (SEM), as shown in Figure 1b, revealing the highly ordered honeycomb patterns. Each domain is about 800 nm in diameter (size distribution is shown in Figure S1) and composed of numerous small nanoparticles with varying sizes of 10−80 nm. The dimensions of the honeycomb structures have been further verified by atomic force microscopy (AFM) measurements, as shown in Figure 1c. The diameter was measured to be ∼800 nm with a depth of 62 ± 5 nm from Figure 1c(i, ii), consistent with the SEM observations. Parts d and e of Figure 1 show the transmission electron microscopy (TEM) images of the crosssection of the highly ordered honeycomb VO2 structures, obtained by using a focused ion beam (FIB) technique. The film thickness was measured to be ∼65 nm, consistent with the AFM measurement. Further observations by optical microscopy (Figure S2) and SEM (Figure S3) show a high homogeneity of the honeycomb structures on the whole substrate. The as-prepared honeycomb microstructures are composed of monoclinic VO2. Selected areas of the cross-section of the film reveal the presence of a group of lattice fringes in 90° with interplanar spacings of 0.45 nm and indexed to (001) and (010) planes of VO2(M) (Figure 1e), respectively. Another distinct lattice fringe with a d-spacing of ∼0.24 nm corresponds to the VO2(M) (200) plane (Figure 1f). In addition, the lattice fringes indexed to the (011) and (101) planes of VO2(M) were observed from scratched nanoparticles of honeycomb-structured films (Figure S4). Figure 1g shows the X-ray diffraction

patterned microstructures, several techniques have been developed such as photolithography, electron beam lithography,35 and colloidal lithography.36 However, they usually suffer from low throughput and high costs or the use of templates (masks or colloids).37 Consequently, surface patterning has rarely been used in the large-scale fabrication, especially for solar modulating coatings. Therefore, a templatefree but controllable wet-chemical surface patterning technique is highly desirable. In this study, we present a dual-phase transformation strategy as a template-free surface-patterning method to prepare periodic microstructured monolayer films. This method only requires wet chemical precursors, which is highly effective for scalable manufacture and practical applications. The honeycomb structures resulted from the spontaneous self-template, which was realized by the dual-phase transformation between colloids and ionic states under the stimulus of moisture. Ordered VO2(M) honeycomb structures with a complex hierarchy were fabricated via this method, and the microstructures of the obtained VO2(M) coatings were tunable by tailoring the pertinent variables. To this end, we have realized the possibility of ultratransparent VO2 smart coatings. VO2(M) honeycomb-structured coatings have achieved excellent Tvis at 700 nm of 95.4% with decent ΔTsol of 5.5% from their broadband anti-reflection as a good balance between Tvis and ΔTsol. A further proof-of-principle demonstration confirmed the excellent performance of the as-prepared honeycombstructured VO2(M) films as solar modulating coatings, which efficiently prevented the melting of ice cubes. The proposed technique provides a path for fabricating 2D ordered structures, and it also presents insights to prepare ultratransparent VO2(M) solar modulating coatings. 408

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Figure 2. Formation mechanism of the highly ordered honeycomb-structured VO2(M) film was inspired by the spontaneous self-template and assembly (SSTA) during the dual-phase transformation process. (a) Homogeneous, fully solution-based precursor film was deposited at room condition (25 °C, 50% RH). (b) Precursor was spontaneously self-templated and assembled (SSTA) into hydrous sphere arrays after water evaporation in dry nitrogen (25 °C, ∼0 RH). (c) Hydrous spheres became hollow VO(OH)2 spheres after instant heating to 300 °C and (d) finally collapsed to honeycomb structures after being heated at a rate of 2 °C and maintained at 500 °C for 1 h. Microscopic photos of (e) the precursor film and (f) the film after SSTA process. SEM images of (g) captured hollow VO(OH)2 spheres and (h) final honeycomb structures.

(XRD) patterns of the VO2 honeycomb-structured films with thicknesses of 65, 95, and 195 nm, respectively. The characteristic (011) peak of VO2(M) located at 2θ = 27.8° is observed in the 65 nm thick sample. The intensity of this peak is enhanced when the film thickness is further raised to 95 nm. By increasing the film thickness to 195 nm, the (011) peak becomes more intense, and the presence of (1̅11), (200), (210), and (220) peaks can also be observed. All of these peaks are in good agreement with the diffraction peaks of monoclinic VO2 (JCPDS card no. 72-0514, space group: P21/c). Further characterization techniques including EDAX analysis (Figure S5) and Raman spectrum (Figure S6) also confirm that the honeycomb-structured VO2 films are composed of monoclinic VO2.34,38 The mechanism of the dual-phase transformation method and the formation process of the honeycomb microstructures have been carefully investigated. It has been proven that the VO2+ ions can spontaneously self-template and assemble

(SSTA) as arranged hydrous colloids arrays and deform to honeycomb microstructures after a heat treatment. The detailed processes can be found in the scheme, as shown in Figure 2. In the first step, an aqueous precursor, consisting of vanadyl dichloride (VOCl2), hydrochloric acid (HCl), hydrazine (N2H4), and polyvinylpyrrolidone (PVP), was spin-coated on a piece of quartz substrate (Figure 2a).39,40 Figure 2e shows the aqueous precursor film was highly homogeneous (magnified image in Figure S7a). In the second step, a SSTA process accompanied water evaporation from the precursor film. With evaporation of solvent, a large amount of hydrous colloids in microscale were spontaneously formed inside the deposited precursor film, as shown in Figure 2b. When the amount of hydrous colloids increased to a saturated level in such a homogeneous film, the interaction forces between these colloids in a confined space promoted their self-assembly to form an ordered array. The key to this SSTA process was the dual-phase transformation of the 409

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Figure 3. Microstructures of the obtained VO2(M) films are tunable by tailoring the pertinent variables. (a) Relationship between the pattern size and precursor concentration, and the insets are SEM images of three examples with the honeycomb size of 400, 800, and 1500 nm. (b) Relationship between the pattern distribution density and N2H4 concentration. Insets are SEM images of three examples with pattern distribution densities of 0, 0.1, and 2.6 units/μm2.

Figure 4. Obtained VO2(M) films have excellent visible light transmittance (Tvis) with excellent solar modulation ability (ΔTsol). (a) Temperature-dependent transmittance spectra of a 65 nm thick honeycomb-structured film and a GBR structured film prepared under the same conditions for comparison. Insets: visible light transmittance spectra at room temperature. (b) Comparison of this work with previously reported studies.

revealed honeycomb structures with nanocrystals forming inside only (Figure S8a,b). A small amount of air was infused, which can remove carbon from the decomposed PVP31 and promote the crystallization of the remaining VO2+ in the PVP framework to VO2 crystals (Figure S8c−f). The final product structure (Figure 2h) was observed as honeycomb structures with many microbowl domains. The diameter was measured to be 800 nm for each of the domains, consistent with the one of VO(OH)2 hollow spheres shown in Figure 2g. The microstructures (morphology, domain size, and distribution) of the obtained VO2(M) films were tunable by tailoring the pertinent variables in the reaction system. As shown in Figure 3a, the domain size of the honeycomb films could be controlled from 400 to 1500 nm (surface details in Figure S9), by adjusting the precursor concentrations (proportional to V4+ ions) from 0.18 to 0.44 mol/L. The domain size and the overall precursor concentration have shown a near-tolinear relationship. In addition, the concentration of N2H4 was found to play a critical role in the distribution density of the domains (Figure 3b). The film fabricated from the precursor with N2H4 concentration as low as 0.1 mol/L could not show obvious honeycomb microstructures. In this case, most of the N2H4 has been reacted to reduce V2O5 (V5+) to VO2+(V4+)

precursor under certain stimuli. The formation of hydrous colloids was promoted by N2H4 but restrained by HCl. During drying, the role of HCl as the depressor weakened, and VO(OH)2(OH2)3 monomer41,42 or a similar complex with an −NH2 group formed in the precursor with the assistance of N2H4.43,44 After further olation and oxolation reactions,41 hydrous colloids were formed. The in situ microscopic observation in Figure 2f has verified the formation of colloids during drying (magnified image in Figure S7b), and this dualphase transformation process was fully reversible (movie S1). In the third step, the precursor film after SSTA was directly transferred to a preheated furnace at 300 °C, leading to a rapid film solidification and resulting in a periodic structured framework. The hydrous colloids quickly dehydrated to VO(OH)2 hollow spheres, with water and suspected NH3 released from the spheres as shown in Figure 2c. The captured hollow spheres as shown in Figure 2g indicate that the sphere diameter is around 800 nm. Further annealing to 500 °C resulted in the decomposition of VO(OH)2 to porous VO2 hollow spheres. In the final step, these porous hollow spheres collapsed to honeycomb structures due to the weakened support strength, as shown in Figure 2d. At the same time, the decomposition of PVP led to a thinner framework and thus 410

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The mechanism of why excellent performance could be obtained by honeycomb-structured VO2(M) films has been investigated both experimentally and theoretically. It has been proven that such honeycomb structures with complex structural hierarchy have higher visible light transmittance, but infrared radiation can still be effectively modulated (Figure 4a). The simulated transmittance spectra in Figure 6a show that a dense

ions. Therefore, only the excess N2H4 could promote the spontaneous formation of hydrous colloids in the films, and the number of colloids was dependent on the amount of excess N2H4. The honeycomb patterns could be formed at a higher N2H4 concentration and reach a saturated level at the N2H4 concentration of 0.4 mol/L (surface details as shown in Figure S10). Ordered honeycomb microstructures of VO2(M) films have led to exemplary optical transmittance (Tvis) and solarmodulating ability (ΔTsol) in our study. Figure 4a presents the temperature-dependent optical transmittance spectra of a 65 mm thick honeycomb-structured VO2(M) film and grainboundary-rich (GBR) VO2(M) film prepared under the same conditions for comparison. The GBR VO2(M) film has an excellent average Tvis of 87.1% at a wavelength of 700 nm and decent ΔTsol of 4.5%. In contrast, the honeycomb-structured VO2(M) film exhibits an ultrahigh average Tvis of 95.4% at 700 nm, which is nearly fully transparent, and delivers an even higher ΔTsol of 5.5%. Figure 4b shows a comparison of this work with recently reported VO2(M) films and shows that the honeycomb-structured VO2 film has excellent Tvis of 95.4% at 700 nm with a ΔTsol of 5.5%, superior to those recently reported VO2(M) films,18−21,26−29,31,33,34,45 including the periodic structures prepared by conventional colloidal lithography.34 The transition temperature (Tc) of the honeycombstructured VO2(M) film was also tunable by different amounts of tungsten (W) doping. As shown in Figure 5, the heating and

Figure 6. Optical properties of VO2(M) coatings are structure dependent. (a) Simulated temperature-dependent transmittance spectra of the unstructured and honeycomb-structured coatings. (b) Simulated reflectance spectra of the unstructured and honeycomb-structured coatings.

honeycomb structure has higher Tvis than an unstructured dense film. Anti-reflection was commonly observed in 2Dpatterned microstructures.50,51 Simulated results (Figure 6b) illustrate that the honeycomb structures can exhibit a strong anti-reflection between 350 and 1000 nm in the wavelength. This broadband anti-reflection was not common in many periodic structures fabricated through colloidal lithography52 but can be observed in the structures with improved hierarchy.53,54 The simulated optical path on an unstructured VO2(M) surface shows the surface is highly reflective toward visible light (Figure S12a), but the transmittance was improved on honeycomb structures (Figure S12b) with light accumulated on the thinner part of the film. Such a surface with improved structural hierarchy was studied to lengthen the optical path and enhance the light transmittance effectively.34,55,56 The rich grain boundaries can reduce the reflection of accumulated light effectively to further improve the light transmittance as shown in in Figure S12c. Figure S12d shows the simulated transmittance spectra account in the effect of grain boundaries, which agree with the experimental results in Figure 4a. In addition, the uniformities of grain boundaries and particle coverage are critical to the optical properties of VO2 coatings, whose effects have been extensively discussed in previous studies.27,57−59 Briefly, a loosely packed particulate structure

Figure 5. Hysteresis loops of the obtained honeycomb-structured VO2(M) films with tungsten doping of 0, 0.47, 0.95, and 1.42% at a wavelength of 1200 nm.

cooling hysteresis loops at a wavelength of 1200 nm indicate the Tc can be reduced by W doping. The Tc of an undoped honeycomb-structured VO2 film is ∼70 °C, which is similar to the previous studies.46,47 The Tc of 1.42 atom % W doped honeycomb-structured VO2 film is ∼45 °C. The effectivity of W to reduce Tc is 17.6 °C/%, which agrees well with the previous reports.48,49 The broad hysteresis indicates the honeycomb-structured VO2(M) film has a comparatively rich grain boundary. The width of the hysteresis can also be effectively reduced from 22 °C (undoped) to 6.7 °C (1.42% W doped). W doping has limited influences on the optical properties (Figure S11), resulting in 2−3% lowered Tvis and 0.2−0.5% lowered ΔTsol (Table S1). 411

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Figure 7. Proof-of-principle demonstration shows that the as-prepared VO2(M) films can serve as energy-efficient smart windows to efficiently prevent ice melting, compared with the blank glass counterpart. (a) Schematic illustration of the set-ups to melt 10 g ice cubes by heat through honeycomb-structured VO2 coated glass and (c) blank glass; (b, d) digital photos of the corresponding ice cubes after radiation; (e) relationships between ice residue weight and heat radiation time through honeycomb-structured VO2 coated glass, GBR VO2 coated glass, and blank glass. The color changes of the ice are due to the diffusion of dye in the melting process.

sacrifices ΔTsol to enhance Tvis,57 and a uniform surface coverage is essential to achieve balanced Tvis and ΔTsol.27 A further proof-of-principle demonstration has been conducted where the as-prepared VO2(M) film can serve as an energy-efficient solar modulating coating to prevent the melting of ice, compared with the blank glass counterpart. Parts a and c of Figure 7 are schematic presentations of set-ups to melt ice cubes. Briefly, one 25 × 20 mm glass window coated by the honeycomb-structured VO2 (65 nm in thickness, spectra in Figure 4a) was embedded inside 10 mm thick insulation foam and placed under a 150 W heating resource at a distance of 30 cm. The heat would only pass through the window and melt ice cubes (10 g in weight) below at a distance of 10 cm. As control experiments, a blank glass and a GBR VO2(M) coated glass (spectra in Figure S13) with similar size were used, and other conditions were the same. The GBR VO2(M) coating was fabricated with a similar Tvis (94.7%) at 700 nm to honeycombstructured VO2(M) coating. It was found that the honeycombstructured VO2(M)-coated glass could reject a certain amount of heat to slow the melting. Figure 7b shows that almost no melting happened after 10 min of heating, and only a minimal amount of ice melted after 20 min with a small water stain on the tissue paper. However, the control sample shows different behaviors. Figure S14 shows the ice under the GBR VO2(M) coating was melting quicker than under the honeycombstructured VO2. Figure 7d shows that the ice under the blank glass was melting quickly, and a large water stain can be observed on the tissue paper after 10 min of heating, with more than half of the ice cube melted after 20 min. Figure 7e is the quantitative presentation of the ice-melting process as demonstrated above. It shows that 28.6% of ice in weight melted after 10 min heat radiation through blank glass, but only 11.2% and 8.7% melted, respectively, under same conditions through GBR VO2(M) and honeycomb-structured VO2(M)coated glasses. After 20 min of heat radiation for the blank, GBR VO2(M)-coated, and honeycomb-structured VO2(M)-

coated glasses, the weight losses were 63.8%, 34.9%, and 23.1%, respectively. The ice was fully melted within 27 min under the blank glass but survived over 35 min under GBR VO2(M) coated glass and 40 min under honeycomb-structured VO2 coated glass. Therefore, the as-prepared honeycomb-structured VO2(M) films are highly effective to block infrared radiations, with great potential as highly transparent but energy-efficient solar modulating coatings in residential and commercial buildings.

CONCLUSIONS This study has demonstrated a template-free surface-patterning technique to fabricate honeycomb-structured VO2(M) films. The major findings can be summarized as follows: (1) This template-free structure engineering technique can generate ordered 2D microstructures without the use of any “soft/hard” templates. It has dramatically simplified the conventional colloidal lithography technique as a scalable surface-patterning technique. (2) The VO2(M) honeycomb structures fabricated via this technique have a complex structural hierarchy. The honeycomb structures, including the domain size and distribution, were also tunable by tailoring the pertinent variables. (3) The resulting honeycomb-structured VO2(M) films presented extremely high visible light transmittance (Tvis) up to 95.4% with decent solar modulating ability (ΔTsol) of 5.5%. (4) These honeycomb-structured VO2(M) films have been investigated, experimentally and theoretically, to show an exemplary effect in broadband anti-reflection and transmission enhancement. These findings are very important for preparing ultratransparent but energy efficient smart windows. The proposed dual-phase transformation strategy has great potential to be extended as a scalable surface-patterning technique for achieving high-performance metal oxide coatings with diverse applications, such as catalysis, sensing, optics, electronic, and superwettable materials 412

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comparison were subtracted by our premeasured background spectrum (Figure S15). Ice-Melting Demonstration. The demonstration experiment was performed using a 150 W infrared bulb (Philips BR125 IR). A piece of 25 × 20 mm glass was embedded in 10 mm thick heat insulating polyethylene foam (Abelflex, Ormonoid). Ice cubes were prepared from 10 g of methylene blue solution (0.05 wt % in H2O) and frozen in a covered mold at −18 °C for 24 h. The infrared bulb was placed at a distance of 30 cm above glass embedded foam, and the ice cubes were place at a distance of 10 cm under the foam during the test. The ice-melting demonstration was conducted under room conditions of 18 °C and 43% RH. The GBR coating was deposited at the 500 rpm for 10 s and 4000 rpm for 60s; all other conditions remained the same as for 47 nm thick GBR films. Numerical Simulations. The 3D finite difference time domain simulations were performed. The optical constants (n, k) of VO2(M/ R) were taken in the from 250 to 2500 nm.21 A uniform mesh size of 10 nm was employed in all directions of the simulation region. The boundary conditions of the simulation region were set to PML (perfectly matched layer) at the z direction and periodic at the x and y directions. The plane wave model was used as an incident beam source and traveled along the z-axis. The numerical models are constructed based on experimental data, and all models have a constant mass or volume.

EXPERIMENTAL SECTION Materials. Vanadium pentoxide (V2O5, 99.6%, Sigma-Aldrich), polyvinylpyrrolidone (PVP K-30, MW = 58000, Alfa Aesar), hydrochloric acid (HCl, 32%), nitrogen gas (N2, 99.9995%, Air Liquide), ammonium tungstate ((NH4)10H2(W2O7)6, 99.9%, Alfa Aesar), and hydrazine hydrate (N2H4·H2O, 99%) were used as received without any further purification. Preparation of Precursor Solutions. Typically, 11 mmol of V2O5 powder was added to 60 mL of deionized water under vigorous stirring and heating at 65 °C. Then, 8 mL of HCl was slowly added into the solution. After being stirred for 5 min, the solution became yellowish in appearance. In the next step, 1 mL of N2H4·H2O was added dropwise into the solution. After another 10 min of stirring, a clear blue solution was obtained. Next, another 1 mL of N2H4·H2O was added dropwise into the blue solution for another 10 min under stirring. Once the solution cooled, 6 g of PVP K-30 was added into the solution under vigorous stirring for 2 h. Finally, distilled water was added into the solution to form a total volume of 100 mL. For the fabrication of grain-boundary-rich VO2(M) film, the total amount of 0.5 mL of N2H4·H2O instead of 2 mL was added in the precursor, and all other procedures were the same as the above-mentioned procedures. For tungsten doping, a certain amount of (NH4)10H2(W2O7)6 was slowly added into water on a hot-plate under vigorous stirring and heated to 65 °C to dissolve, and an appropriate amount of (NH4)10H2(W2O7)6 solution was added to the V4+ solution. Fabrication of Honeycomb-Structured VO2(M) Film. First, a quartz substrate was carefully treated with deionized water, ethanol, and piranha solution, respectively. A 200 μL portion of the V4+ precursor solution was dropped onto the quartz substrate and spincoated at 500 rpm for 10 s and 3000 rpm for 60 s. The coated quartz wafer was carefully placed in one cleaned quartz tube and inserted into a horizontal tube furnace. The furnace was preheated to 300 °C, and nitrogen (N2) was injected at a flow rate of 350 sccm for 1 h. Next, the N2 flow was reduced to 100 sccm, and the sample was moved into the heating zone of 300 °C. The film was annealed at a heating rate of 2 °C/min. Then, 100 mL of air was injected into the N2 flow at a rate of 1 sccm through a syringe pump at a temperature region of 300−400 °C. The final annealing temperature was maintained at 500 °C for 1 h. The annealed film was collected after cooling overnight to room temperature. Grain-boundary-rich VO2(M) films and W-doped honeycomb-structured VO2(M) films were prepared under the same conditions with the deposition of corresponding precursors. The film thickness was measured to be 65 nm for honeycomb structures and 47 nm for GBR films. Characterization. XRD measurements were performed using an Empyrean thin-film XRD Xpert materials research diffractometer system at 2θ = 10−80° through a Cu Kα radiation and grazing analysis program with a 12° incident angle. The surface morphology of the films was investigated using a field-emission scanning electron microscope (FEI Nova NanoSEM 230) and an AFM system (Bruker Dimension Icon SPM). The cross-sectional chips were milled and cut using a FIB system (XT Nova Nanolab 200) and analyzed using a TEM system (FEI Tecnai G2 20) and a high-resolution transmission electron microscopy (HRTEM) system (Phillips CM200) with an energy dispersive X-ray spectroscopy (EDAX) system. The hollow VO(OH)2 spheres were captured by drying the precursor solution on liquid bridge masks to reduce the negative influence from moisture and oxygen in ambient environment.60,61 Further composition analyses were performed using a Raman microscope (Renishaw inVia) with 514 nm laser source. Optical characterizations were performed using a Cary 5 UV−vis−NIR spectrophotometer equipped with a heating chamber for thermochromic transmittances. All transmittance data were obtained after subtracting the background including air and quartz substrate. The background spectrum (Figure S15) was first measured, and the coating transmittance was recorded based on the background, which is a function of WinUV software (Version 4.3, Agilent Technology). Some reported data for

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.6b06152. Additional materials and characterizations (PDF) Reversible colloidal formation and decomposition (AVI)

AUTHOR INFORMATION Corresponding Authors

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

Minsu Liu: 0000-0003-0787-1725 Bin Su: 0000-0002-7122-6694 Lei Jiang: 0000-0003-4579-728X Author Contributions

M.L. conducted the experimental work with assistance from B.S., Y.T., Y.V.K., Z.C., Y.G., and Y.Y., while M.L. also proposed the research; X.J., L.J., and A.Y. provided supervision for the experimental work. Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS We gratefully acknowledge the financial support of the Australian Research Council (ARC) (Project Nos. DP160104456 and FT0990942) and access to Melbourne Centre of Nanofabrication (MCN), Monash Centre for Electronic Microscopy (MCEM), and the UNSW node of the Australian Microscopy and Microanalysis Research Facilities (AMMRF). We also thank Prof D. Zhao (Fudan University) and Dr. A. Gentle (University of Technology Sydney) for valuable discussions. REFERENCES (1) Baetens, R.; Jelle, B. P.; Gustavsen, A. Properties, Requirements and Possibilities of Smart Windows for Dynamic Daylight and Solar Energy Control in Buildings: A State-of-the-Art Review. Sol. Energy Mater. Sol. Cells 2010, 94, 87−105. 413

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DOI: 10.1021/acsnano.6b06152 ACS Nano 2017, 11, 407−415