Facile and Low-Temperature Fabrication of Thermochromic Cr2O3

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Facile and Low-Temperature Fabrication of Thermochromic Cr2O3/ VO2 Smart Coatings: Enhanced Solar Modulation Ability, High Luminous Transmittance and UV-Shielding Function Tianci Chang,†,‡ Xun Cao,*,† Ning Li,†,‡ Shiwei Long,†,‡ Xiang Gao,§ Liv R. Dedon,∥ Guangyao Sun,†,‡ Hongjie Luo,# and Ping Jin*,†,⊥

ACS Appl. Mater. Interfaces 2017.9:26029-26037. Downloaded from pubs.acs.org by KAOHSIUNG MEDICAL UNIV on 08/13/18. For personal use only.

†

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 § Nanostructures Research Laboratory, Japan Fine Ceramics Center, Nagoya 456−8587, Japan ∥ Department of Materials Science and Engineering, University of California Berkeley, HMMB100, Berkeley, California 94720, United States # School of Materials Science and Engineering, Shanghai University, Shangda Road 99, Baoshan, Shanghai 200444, China ⊥ National Institute of Advanced Industrial Science and Technology (AIST), Moriyama, Nagoya 463-8560, Japan S Supporting Information *

ABSTRACT: In the pursuit of energy efficient materials, vanadium dioxide (VO2) based smart coatings have gained much attention in recent years. For smart window applications, VO2 thin films should be fabricated at low temperature to reduce the cost in commercial fabrication and solve compatibility problems. Meanwhile, thermochromic performance with high luminous transmittance and solar modulation ability, as well as effective UV shielding function has become the most important developing strategy for ideal smart windows. In this work, facile Cr2O3/VO2 bilayer coatings on quartz glasses were designed and fabricated by magnetron sputtering at low temperatures ranging from 250 to 350 °C as compared with typical high growth temperatures (>450 °C). The bottom Cr2O3 layer not only provides a structural template for the growth of VO2 (R), but also serves as an antireflection layer for improving the luminous transmittance. It was found that the deposition of Cr2O3 layer resulted in a dramatic enhancement of the solar modulation ability (56.4%) and improvement of luminous transmittance (26.4%) when compared to single-layer VO2 coating. According to optical measurements, the Cr2O3/VO2 bilayer structure exhibits excellent optical performances with an enhanced solar modulation ability (ΔTsol = 12.2%) and a high luminous transmittance (Tlum,lt = 46.0%), which makes a good balance between ΔTsol and Tlum for smart windows applications. As for UV-shielding properties, more than 95.8% UV radiation (250−400 nm) can be blocked out by the Cr2O3/VO2 structure. In addition, the visualized energy-efficient effect was modeled by heating a beaker of water using infrared imaging method with/without a Cr2O3/VO2 coating glass. KEYWORDS: smart coatings, Cr2O3/VO2 bilayer, low-temperature fabrication, high-thermochromic performance, UV-shielding

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INTRODUCTION The energy crisis has become one of the most severe problems of human society and has led to a high demand of energysaving materials. Because of excessive use of lighting, airconditioning, and heating, buildings are responsible for about 40% of total world energy consumption every year.1 An © 2017 American Chemical Society

effective way to reduce building energy consumption is using chromogenic coatings on building fenestration to control the Received: May 20, 2017 Accepted: July 19, 2017 Published: July 19, 2017 26029

DOI: 10.1021/acsami.7b07137 ACS Appl. Mater. Interfaces 2017, 9, 26029−26037

Research Article

ACS Applied Materials & Interfaces

also be considered. Cr2O3 materials, which can be crystallized at very low temperature, provide a good candidate for structural template layers. The suitable refractive index (∼2.2−2.3) and crystal parameters are predicted to be beneficial for the optical performance of VO2 thin films. Refractive index of Cr2O3 is between the glass and the VO2, which is considered to enhance the luminous transmittance.24 Note that UV radiation is harmful to the health of human beings and results in aging of organic matters. Due to these risks, windows with good UVshielding function are essential for buildings. These motivations are not considered in much of the literature pertaining to VO2 based-smart coatings. In this paper, Cr2O3 has been selected to act as a structural template for the growth of VO2 films as well as the AR layer for improving the luminous transmittance. Thermochromic VO2 films can be successfully grown at a competitive lowtemperature range (250−350 °C) by using Cr2O3 template layers. The proposed Cr2O3/VO2 structure exhibits excellent thermochromic performance with enhanced luminous transmittance as well as solar modulation ability. The thermochromic performance of Cr2O3/VO2 structure is significantly improved compared to single layer VO2 films and even better than that reported in five-layered TiO2/VO2/TiO2/VO2/TiO2 films.25 As for the UV-shielding function, more than 95.8% UV (250−400 nm) can be blocked out by the Cr2O3/VO2 films, which can protect human skin from DNA damage induced by UV radiation exposure. These VO2 bilayer coatings exhibit good potential for industrial production in both scalability and process compatibility.

solar radiation entering or blackbody radiation leaving the buildings.2 As one of the typical thermochromic materials, vanadium dioxide (VO2) can initiate an automatic reversible semiconductor-metal transition (SMT) from the monoclinic (P21/c, M1 phase) to the tetragonal structure (P42/mnm) at a transition temperature (Tc) of 68 °C, giving rise to a dramatic modification of the optical properties from infrared (IR) transmitting to IR shielding in the near-infrared region, which is suitable for the application of energy-efficient windows. There have been many investigations and much progress in VO2based smart coatings in recent years.3−6 However, there are still some obstacles impeding practical applications of VO2-based smart coatings, including high-temperature fabrication, poor luminous transmittance, low solar modulation ability, and insufficient UV-protection ability. Among the common deposition technologies, magnetron sputtering plays a dominant role in VO2-based thin film manufacturing. Previous studies have shown that high-performance VO2 thin films are obtained at relatively high temperatures of over 450 °C due to the dependence of thermochromic on relevant crystallization.3,6−8 However, such high temperatures cause many problems in practical production of VO2 films. First, high temperature means high energy consumption as well as high product cost. Second, for conventional sodium glass, such a high process temperature leads to the diffusion of sodium ions from the substrate into the VO2 films, which will deteriorate the phase transition characteristics of the VO2 films.9 Furthermore, deposition at high temperature limits the choice of substrate to rigid materials such as glass and sapphire.10 For lowering the deposition temperature of VO2 films, many efforts have been made such as the postannealing and special excitation sources. Stoichiometric, crystalline VO2 films can be grown via sputtering at room temperature with post annealing at 450 °C.11 Unfortunately, additional annealing increases the nucleation probability of impurity vanadium oxides such as V2O3 or V2O5 (enthalpy of formation ΔHV2O5 = −1557 kJ/mol, ΔHV2O3 = −1226 kJ/mol, ΔHVO2 = −717 kJ/mol), as well as the mechanical stress in films derived from the difference in thermal expansion coefficient α between the film and substrate (αVO2 = 2.1 × 10−5/K, αglass = 3.3−8.5 × 10−6/K).12 High power deposition such as high power impulse magnetron sputtering (HiPIMS) and RF-superimposed DC magnetron sputtering were used to fabricate VO2 films at a relatively low temperature of around 300 °C.13,14 The requisite power, however, is too high to be applied on industrial production lines. Additionally, thermochromic properties of VO2 films fabricated by these methods were not desirable and suitable for applications in smart window technology.15−19 For VO2-based smart windows, luminous transmittance and solar modulation ability are the most critical performance parameters limiting the potential for the practical application. VO2 thin films always suffer from the problem of low luminous transmittance due to the absorption in the short-wavelength range in both the semiconducting and metallic states.20 An effective way to improve the luminous transmittance is to introduce an antireflection (AR) layer. Typical AR layers including TiO220 and SiO221 have been introduced to increase the luminous transmittance of VO2 thin films. Previous work has demonstrated that the refractive index (n) of AR layers of optimal VO2 films should range from 2.0 to 2.4.22,23 The corresponding solar modulation ability of the AR layer must

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EXPERIMENTAL SECTION

Sample Preparation. Cr2O3/VO2 bilayer structures were grown on 10 × 10 mm2 and 75 × 75 mm2 quartz substrates by magnetron sputtering. Growth conditions for Cr2O3 and VO2 layers were optimized for the purpose of maximizing the optical properties. The deposition of Cr2O3 structural template layers was carried out using a Cr2O3 ceramic target (purity 99.9%, 4-in. diameter) of 150 W RF power with 40 sccm argon flow (purity 99.99%). Deposition times of Cr2O3 were deliberately set on changeable time to determine the effect of the thickness of Cr2O3 on the fabrication and optical properties of VO2 thin films. Then the VO2 thermochromic layer was deposited via sputtering using a V2O3 ceramic target (purity 99.9%, 4-in. diameter) under the condition of 100 W DC power with Ar and O2 flows of 39 and 1 sccm, respectively. It has been demonstrated that threshold thickness for VO2 films to show significant thermochromic performance is ∼50 nm,26 thus the thickness of the VO2 film was fixed at ∼80 nm for optimal results. The pressure of the deposition chamber was maintained at 6.00 mTorr, and the substrate temperature was kept at the range from 250 to 350 °C with a temperature interval of 25 °C. The substrate temperatures were systematically calibrated by a surface thermometer before the deposition. Instrumentations and Characterizations. Following growth, the Cr2O3/VO2 bilayer films were subjected to extensive structural characterization and property studies. Thin film X-ray diffraction (XRD) analysis was conducted on a Rigaku Ultima IV diffractometer with Cu Kα radiation (λ = 1.5418 Å) using 2θ scanning model. The morphology and thickness of the films was measured by scanning electron microscope (SEM, Hitachi SU8220) and an atomic force microscope (AFM, SII Nano Technology Ltd., Nanonavi P). X-ray photoemission spectroscopy (XPS) analysis was performed on Thermo Fisher Scientific ESCAlab250 to investigate the valence state of elements in the films. Transmittances spectra of the bilayer structures in the wavelength range of 250 to 2600 nm were measured using a UV−vis spectrophotometer (Hitachi U-3100). Raman analysis was carried out using a HR Evolution Raman spectrometer. 26030


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Figure 1. VO2 films can be successfully deposited at low temperatures (250−350 °C) by introducing Cr2O3 as structural template. (a) Crystal structure of hexagonal Cr2O3, monoclinic VO2, and rutile VO2, respectively; (b) Schematic illustration of Cr2O3/VO2 bilayer thermochromic film; (c) Digital photo of the large scale Cr2O3/VO2 thermochromic film on 75 × 75 mm2 glass substrate; (d) AFM image of the (40 nm) Cr2O3/VO2 film on quartz glass substrate; (e) Cross-section SEM image of the (40 nm) Cr2O3/VO2 film on quartz glass substrate; (f) Raman spectra of VO2 film deposited with 40 nm Cr2O3 structural template layer.

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Calculation of Optical Properties. VO2 (M) thin films have been widely studied to be utilized as smart windows. For the investigation of the optical properties, vis-near-infrared transmittances spectra of Cr2O3/VO2 bilayer films and single layer VO2 films deposited at different temperatures were measured. The integral luminous transmittances (380−780 nm) and solar transmittances (350−2600 nm) of the films were obtained by the equations Tlum =

∫ Φlum(λ)T(λ) dλ/∫ Φlum(λ) dλ

(1)

Tsol =

∫ Φsol(λ)T(λ) dλ/∫ Φsol(λ) dλ

(2)

RESULTS AND DISCUSSION In this work, we propose a facile preparation method to deposit VO2 thin film using Cr2O3 as a structural template at a lowtemperature range from 250 to 350 °C. The Cr2O3 thin film serves as a structural template to reduce the deposition temperature of VO2 films as well as an antireflection layer to enhance luminous transmittance of Cr2O3/VO2 bilayer structures. In previously reported work,27−29 Al 2O3 is frequently used as the substrate for epitaxial growth of VO2 thin films because of its similar lattice parameters (Al2O3, hexagonal, a = 0.475 nm, c = 1.297 nm; VO2(R), tetragonal, a = 0.455 nm, c = 0.286 nm). However, Al2O3 films need high deposition temperatures over 700 °C in magnetron sputtering. Cr2O3 thin films can be readily deposited across a large temperature range from 30 to 900 °C due to the low formation enthalpy of the material (ΔHCr2O3 = −1129 kJ/mol).30,31 Meanwhile, Cr2O3 has the same lattice structure and very similar lattice parameters (hexagonal, a = 0.496 nm, c = 1.359 nm) to Al2O3. Jin et al. reported low-temperature growth of Al2O3 films by embedding Cr2O3 films as structural templates, which largely reduced the deposition temperature of Al2O3 films from 1000 to 400 °C.32 In this article, Cr2O3 was chosen to replace of Al2O3 as the structural template layer to lower the lattice mismatch between VO2 thin films and quartz glass substrates and to reduce the deposition temperature of VO2 thin films. Figure 1a shows the crystal structure of hexagonal Cr2O3, VO2 (M), and VO2 (R). The refractive index of Cr2O3 is in accordance with the requirement for the AR layer of VO2

where T (λ) represents the transmittance at wavelength λ, Φlum is the standard efficiency function for photopic vision, and Φsol is the solar irradiance spectrum for an air mass of 1.5, which corresponds to the sun standing 37° above the horizon. While the solar modulation ability (ΔTsol) of the films was calculated by ΔTsol = Tsol,lt − Tsol,ht where lt and ht represent 25 and 90 °C, respectively. Infrared Thermal Images Characterization. Infrared thermal imaging characterization was carried out using a 250 W infrared lamp (Philips BR125 IR) as the source of infrared radiation. A 75 × 75 mm2 glass substrate coated with Cr2O3/VO2 film and a blank glass of the same size as a counterpart were set in heat-insulating foam. A beaker with 10 mL deionized water was placed at a distance of 10 cm under the glass, while the infrared lamp was set at a distance of 20 cm above the glass. The characterizations were carried out with room conditions of 20 °C and 40% relative humidity, and the infrared thermal images were recorded by an infrared thermal camera (Avio, InfRec) and analyzed by accessory software in real time. 26031


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Figure 2. XRD patterns of VO2 films (a) deposited with different thicknesses of the Cr2O3 template layer 275 °C and (b) deposited with 40 nm Cr2O3 structural template layer at different temperatures. B and M refer to VO2 (B) and VO2 (M), respectively.

thin films. Cr2O3/VO2 bilayer films are expected to exhibit better luminous transmittances than single layer VO2 thin films. A schematic illustration of Cr2O3/VO2 thermochromic films is shown in Figure 1b. Almost all of UV radiation can be blocked out by the thermochromic layer, the visible transmittance of the bilayer film will be enhanced due to the antireflection function of Cr2O3, and a large amount of infrared radiation can be reflected as VO2 is opaque for wavelengths in near-infrared region above Tc. Thus, the proposed lowtemperature fabricated Cr2O3/VO2 bilayer films can realize a combination of excellent solar modulation ability, enhanced luminous transmittance and UV-shielding function. A digital photo of prepared Cr2O3/VO2 film on a large scale 75 × 75 mm2 glass is presented in Figure 1c. The large-scale sample exhibits high luminous transmittance and the background letters can be clearly seen with the naked eyes. The microstructures of Cr2O3/VO2 bilayer films have been measured by AFM, as shown in Figure 1d. The sample with 40 nm Cr2O3 structural template shows a smooth surface and uniform grain size with the root-mean-square (RMS) roughness of 4.50 nm, which confirms fine crystallization of the VO2 (M) phase. Figure 1e shows a scanning electron microscopy (SEM) image of the cross-section of (40 nm) Cr2O3/VO2 film, where a bilayer structure can be observed. VO2 layers have a uniform thickness of about 80 nm for all samples with various thicknesses of the Cr2O3 structural template layer (Figure S5). Raman features for VO2 grown on 40 nm Cr2O3 structural templates at 350 °C have been displayed in Figure 1f. Almost all known Raman vibration modes of monoclinic VO2 have been presented (136, 193, 222, 305, 335, 384, 433, 496, 615 cm−1).33 No impurity peak can be observed, proving that pure VO2 (M) films have been fabricated with the Cr2O3 structural template at low temperatures. Figure 2a shows the XRD patterns of VO2 films with varying Cr2O3 thickness grown at the deposition temperature of 275 °C. The single layer VO2 film deposited without Cr2O3 template layer shows oriented peaks including (001), (002) and (003) of VO2(B) (JCPDS:81-2392). VO2 (B) is a metastable phase of vanadium dioxide, defined as an intermediate phase between V2O5 and V2O3.34 The dominant peaks of VO2 (B) in the XRD pattern of the single layer VO2 film without a Cr2O3 structural template may result from the low deposition temperature, which is in agreement with other.34,35 When a 6 nm Cr2O3 layer is introduced, the VO2 XRD pattern shows a mixed phase including VO2 (B) and monoclinic vanadium dioxide (VO2 (M), JCPDS: 72-0514) and

peaks of both phases are inconspicuous. Compared to the single layer VO2 film grown at the same temperature, the appearance of VO2 (M) means that the VO2 (B) phase has begun to transform to the VO2 (M) phase. With increasing thickness of the Cr2O3 template layer, peaks of the VO2 (B) phase are gradually reduced, while VO2 (M) phase peaks are gradually enhanced. With 60 and 80 nm Cr2O3 template layers, the XRD patterns show strong peaks of VO2(M) phase and peaks corresponding to the (104) and (116) surfaces of Cr2O3; in contrast, no obvious peaks from VO2 (B) phase can be observed. That is to say that the Cr2O3 template layer is helpful promoting the crystallization of VO2 (M) and the transformation of VO2 (B) to VO2 (M). The phase transformation is possible due to the similar lattice parameters of Cr2O3 and VO2 (R). During the deposition process, VO2(R) can be easily fabricated and then transformed to VO2 (M). The phase transformation of B phase to M phase of VO2 often takes place during high-temperature annealing process.35,36 Thus, the introduction of the Cr2O3 structural template has the additional advantage of lowering the temperature for VO2 (M) crystallization. Further investigations of Cr2O3/VO2 bilayer films were carried out with fixed thickness of VO2 and Cr2O3 of 80 and 40 nm, respectively, while the substrate temperature was changed from 250 to 350 °C. Corresponding XRD patterns are provided in Figure 2b. At a deposition temperature of 250 °C, the XRD pattern of the single layer VO2 film shows no obvious peak indicating that amorphous VO2 was formed at that temperature (Figure S3). On the contrary, the sample deposited at the same temperature with 40 nm Cr2O3 as template layer still shows a primary phase of monoclinic vanadium dioxide. Weak peaks belonging to VO2 (B) can be detected at deposition temperatures of 250 and 275 °C. At 300 °C and above, XRD patterns unequivocally show monoclinic VO2 with no trace of VO2 (B) or other impurity phases. Meanwhile, with increasing deposition temperature, XRD patterns of corresponding VO2 (M) show increasing intensity due to the increased crystallization. According to XRD results, the single layer VO2 film deposited at 275 °C is VO2 (B) and thus does not possess optical change properties near room temperature, resulting in no obvious thermochromic performance from corresponding optical spectra (Figure S8b). However, when the deposition temperature was maintained at 275 °C, a ΔTsol of 5.1% can be achieved by introducing 6 nm Cr2O3 structural template layer (Figure 3a). The thermochromic performance is carried out by 26032


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Figure 3. Transmittance spectra (350−2600 nm) at 25 and 90 °C for VO2 films (a) deposited with different thicknesses of the Cr2O3 template layer at 275 °C and (c) deposited with 40 nm Cr2O3 structural template layer at different temperatures; corresponding variation curves (b and d) of Tlum,lt, Tlum,ht, and ΔTsol for panels a and c, respectively.

can be attributed to the interference effect of light from the interface between VO2 and Cr2O3 layers. An optical calculation and optimization were executed for the VO2 (80 nm)/Cr2O3/glass structure with a variable film thickness of Cr2O3 using a self-made optimization program. As shown in Figure S9a, the luminous transmittance of VO2 films improves with increasing Cr2O3 film thickness and demonstrates a wave-like feature because of the interference effect of the multilayer for both semiconducting and metallic VO2. 80nm semiconducting VO2 exhibits a maximum luminous transmittance of 41.8% with a 45 nm Cr2O3 template layer. According to the calculation, an increase in luminous transmittance from 33.4% to 41.8% can be expected with the use of Cr2O3 template in addition to the low-temperature effect. Simulated transmittance spectra of the (80 nm) VO2/ (40 nm) Cr2O3/glass structure are shown in Figure S9b. According to the experimental data, the prominent luminous transmittance of the (80 nm) VO2/(40 nm) Cr2O3/glass structure is 47.8%, which is higher than the simulated value. That is due to the columnar structure of the VO2 films, which is observed in the SEM images and leads to the increase of the luminous transmittance for the proposed Cr2O3/VO2 films. For optimizing the optical performance, the deposition temperature of Cr2O3/VO2 films was varied between 250 to 350 °C with a temperature interval of 25 °C, while the thickness of Cr2O3 and VO2 was fixed at 40 and 80 nm, respectively. The transmittance spectra of the samples are shown in Figure 3c. A solar modulation ability of 5.8% can be realized from an ultralow deposition temperature of 250 °C with a 40 nm Cr2O3 structural template. This gives a significant improvement over amorphous single layer VO2 films deposited

the VO2 (M) derived from VO2 (B) when the Cr2O3 structural template is introduced. However, the solar modulation ability is low due to the residual VO2 (B) phase. As the Cr2O3 thickness is increased from 6 to 40 nm, the solar modulation ability (ΔTsol) of corresponding samples increases gradually from 5.1% to 8.3%. This is consistent with the XRD results indicating that better crystallization of VO2 (M) can be realized with a thicker Cr2O3 structural template. It is worthy to note that with a 40 nm Cr2O3 template layer, an excellent ΔTsol of 8.3% has been obtained from the bilayer Cr 2O3/VO2 film. For comparison, an ideal single layer VO2 sample has been fabricated by a traditional high-temperature process of 450 °C and the corresponding spectrum is shown in Figure S8c. The calculated ΔTsol from the transmittance spectra is 7.8%. Better thermochromic characteristics of VO2 films deposited at a low temperature of 275 °C can be obtained by introducing Cr2O3 film as template layer as compared with the single layer VO2 film grown using a traditional high-temperature process (>450 °C). As mentioned above, Cr2O3 can act as the antireflection layer (AR) for VO2 films. With Cr2O3 layer thickness increasing from 6 to 40 nm, the Tlum,lt increases from 39.5% to 47.8% (Figure 3b), while Tlum,lt of the single layer VO2 film is only 36.4% (Figure S8c). An improvement about 31% can be achieved due to the antireflection properties of the Cr2O3 layer. However, when the thickness of the Cr2O3 layer increases continuously, the Tlum,lt decreases because of the additional absorption and intrinsic yellow emission of the Cr2O3 layer. At the same time, a deviation in the luminous region appears in the spectral transmittance of the bilayer Cr2O3/VO2 composite films, which 26033


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For successful smart window applications to be realized, vanadium dioxide thermochromic films should block UV radiation as much as possible in addition to possessing excellent solar modulation ability and high luminous transmittances. UV radiation in the solar spectrum can be absorbed by the skin, leading to erythema, burns, and even skin cancer.37 To further confirm UV absorbance of VO2 thin films, transmittance spectra including the UV region were measured and are shown in Figure 4a. Extremely low transmittance in the UV region (250−400 nm) can be observed in both semiconductor state and metallic state of Cr2O3/VO2 film. Furthermore, in order to evaluate the UV-shielding function of the film quantitatively, an integral transmittance of UV radiation (denoted as TUV) can be adapted as TUV = ∫ ΦUV(λ)T(λ) dλ/∫ ΦUV(λ) dλ, where λ is ranged from 250 to 400 nm. On the basis of the calculation, TUV,lt and TUV,ht for the proposed Cr2O3/VO2 film are only 4.2% and 3.8%, respectively, which indicates the realization of remarkable UVshielding function. In comparison, high values of TUV have been exhibited by different glass, where TUV for quartz glass, BK7 glass, and soda-lime glass are 91.9%, 88.2%, and 80.6% (Figure S10), respectively, underscoring the capability Cr2O3/VO2 smart windows to protect human skin from harmful UV damage. Figure 4b shows a comparison of our work with recently reported VO2 thin films prepared by magnetron sputtering (Detailed optical properties have been summarized in Table S1). A development trend can be drawn from data among the optical properties of VO2 thin films, where high solar modulation ability (ΔTsol) accompanied by high luminous transmittance (Tlum) is required for practical application. A unilateral pursuit of distinguished solar modulation ability or ultrahigh luminous transmittances is meaningless. Although VO2 with excellent thermochromic performance has been reported by chemical methods,4,38−40 few satisfied performance can be achieved by sputtering method. In our work, Cr2O3/ VO2 bilayer film shows an excellent ΔTsol = 12.2% with an enhanced Tlum,lt = 46.0%, superior to these works reported recently and makes a commendable balance between ΔTsol and Tlum, which is desirable for application of VO2 films as smart windows.

at the same temperature that show none of the thermochromic performance in accordance with the XRD results (Figure S3). With increased deposition temperatures between 300 and 350 °C, there is no obvious change in Tlum,lt compared to the 275 °C sample; however, Tlum,ht decreased gradually from 43.2% to 36.9% with a temperature increase from 275 to 350 °C (Figure 3d). A relatively higher temperature of 350 °Cstill lower than traditional deposition temperatures of VO2 (M)led to an exemplary optical transmittance (Tlum,lt = 46.0%) and solar modulation ability (ΔTsol = 12.2%). Conversely, Tlum,lt and ΔTsol of the ideal single layer VO2 film are 36.4% and 7.8%, respectively. Excellent thermochromic properties with enhanced luminous transmittance can be realized by introducing Cr2O3 as an AR layer; simultaneously, the deposition temperature of the VO2 film has been greatly reduced due to structural template effect of Cr2O3. Optical properties of all samples were calculated from the transmittance spectra and summarized in Tables 1 and 2. Table 1. Optical Properties of Samples with Different Thicknesses of the Cr2O3 Structural Template sample

Tlum,lt (%)

Tlum,ht (%)

Tsol,lt (%)

Tsol,ht (%)

ΔTsol (%)

VO2-450 °C VO2-275 °C 6 nm Cr2O3/VO2-275 °C 20 nm Cr2O3/VO2-275 °C 40 nm Cr2O3/VO2-275 °C 60 nm Cr2O3/VO2-275 °C 80 nm Cr2O3/VO2-275 °C

36.4 42.4 39.5 43.9 47.8 43.7 37.2

37.3 42.6 38.8 42.3 43.2 41.5 36.0

42.1 36.3 36.2 39.1 42.1 41.6 41.5

34.3 35.9 31.1 32.9 33.8 34.8 33.5

7.8 0.4 5.1 6.2 8.3 6.7 8.0

Table 2. Optical Properties of Samples Deposited at Different Temperatures sample 40 40 40 40 40

nm nm nm nm nm

Cr2O3/VO2-250 Cr2O3/VO2-275 Cr2O3/VO2-300 Cr2O3/VO2-325 Cr2O3/VO2-350

°C °C °C °C °C

Tlum,lt (%)

Tlum,ht (%)

Tsol,lt (%)

Tsol,ht (%)

ΔTsol (%)

52.2 47.8 45.7 46.3 46.0

49.7 43.2 41.2 40.2 36.9

46.7 42.1 41.1 42.0 41.6

41.0 33.8 31.8 31.2 29.4

5.8 8.3 9.3 10.7 12.2

Figure 4. (a) Transmittance spectra (250−2600 nm) at 25 and 90 °C for VO2 films deposited with 40 nm Cr2O3 structural template layer at 350 °C and standard solar spectra. (b) Comparison of this work with recently reported VO2-based thermochromic films prepared by magnetron sputtering.3,14,16,23,41−45 26034


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Figure 5. Infrared thermal imaging characterizations demonstrated that the Cr2O3/VO2 bilayer films can effectively prevent water heating, compared to the blank glass counterpart. (a) Schematic presentation of infrared thermal imaging characterizations for glass coated with Cr2O3/VO2 and blank glass. Infrared thermal images of the beaker with 10 mL of deionized water recorded by infrared camera with continuous irradiance comes from the infrared lamp for blank glass (b) and glass coated with Cr2O3/VO2 (c). (d) Maximum temperature curve comes from corresponding infrared thermal images for glass coated with Cr2O3/VO2 (black line) and blank glass (red line).

The maximum temperature curves derived from the infrared thermal images are presented in Figure 5d. Initial temperatures for the two curves are very close at around 20 °C (19.67 and 20.12 °C, respectively). After 10 min of irradiation, the temperature for the beaker with Cr2O3/VO2-coated glass increased to 25.48 °C, while the control beaker with blank glass increased to 30.50 °C. The temperature of the beaker under Cr2O3/VO2-coated glass increased slowly after 15 min and stayed at 28.70 °C after infrared irradiation of 30 min. Conversely, the temperature of the control with blank glass sharply increased and came to 36.25 °C after 30 min. Compared to initial temperatures, the temperature of the beaker with Cr2O3/VO2 coated glass increased by 46.0% (19.67−28.70 °C), while the temperature of the control with blank glass increased by 80.2% (20.12−36.25 °C). Therefore, the Cr2O3/VO2 thermochromic films grown at low temperatures can highly effectively block infrared irradiation, showing great potential to be used as solar energy modulation coating.

Infrared thermal imaging characterization has been carried out for visually displaying the solar modulation ability of our Cr2O3/VO2 layer, as shown in Figure 5a. In short, a 75 × 75 mm2 glass coated with Cr2O3/VO2 bilayer with a deposition temperature of 350 °C (spectra in Figure 3a) was set in insulation foam and placed 20 cm below a 250 W infrared lamp. The heat from the infrared lamp pass through the glass and heat deionized water in a beaker 10 cm below the glass. A blank glass substrate of the same size was used as a contrast experiment (experimental conditions were duplicated as above). Figure 5c shows the infrared thermal images of the beaker at different irradiation times with glass coated by Cr2O3/ VO2. At the beginning, the beaker is close in temperature with the ambient environment and the infrared image shows blue color indicating low thermal emissivity of the beaker. With increasing irradiation time, the top part of the beaker changes its color to green, indicating a medium thermal emissivity. After 20 min of irradiation, all parts of the beaker are green in the infrared thermal image and remain a stable green hue up to 30 min of irradiation. On the contrary, Figure 5b presents the infrared thermal images of the beaker with the blank glass substrate. The deionized water was easily heated by the infrared lamp, and the infrared thermal shows a high thermal emissivity red color. The obvious differences between the infrared thermal images in Figure 5b and 5c indicate that the use of the Cr2O3/ VO2 film can effectively prevent the water heating by infrared radiation.

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CONCLUSION In summary, Cr2O3/VO2 bilayer coatings with high thermochromic performance have been demonstrated at a relative low deposition temperature (250−350 °C) using magnetic sputtering methods. A scaled-up 75 × 75 mm2 sample was successfully fabricated proving the strong potential of our method for industrial application. On the basis of the developing strategy of smart coatings, these Cr2O3/VO2 bilayer coatings exhibit an excellent balance of optical performance 26035


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between solar modulation ability (ΔTsol = 12.2%) and luminous transmittance (Tlum = 46.0%). The Cr2O3 insertion layer dramatically increased the visible light transmission, as well as improved the solar modulation of the original films, which arises from the structural template effect and antireflection function of Cr2O3 to VO2. Meanwhile, the proposed Cr2O3/ VO2 films have a remarkable UV-shielding function (>95.8%) at 250−400 nm for protecting human beings and items from damage and premature aging. Infrared thermal imaging characterization has shown that these bilayer films can dramatically reduce temperature change due to irradiation, providing valid evidence of the Cr2O3/VO2 bilayer structure’s potential as energy-saving smart window coatings.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b07137. XRD and XPS results of as-prepared Cr2O3 films; XRD patterns, XPS analysis, SEM and AFM images, reflectance spectra, and thermal hysteresis loops of Cr2O3/VO2 films; transmittance spectra of single layer VO2 films deposited at different temperatures; optical simulation of Cr2O3/VO2 bilayer films; and summary of optical properties of recently reported VO2 thin films prepared by magnetron sputtering (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (X. Cao). *E-mail: [email protected] (P. Jin). ORCID

Tianci Chang: 0000-0001-7521-9339 Xun Cao: 0000-0003-4417-6350 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was financially supported by the National Natural Science Foundation of China (NSFC No. 51572284).

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REFERENCES

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Facile and Low-Temperature Fabrication of Thermochromic Cr2O3

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