Bioinspired Multifunctional Vanadium Dioxide: Improved

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Bioinspired Multifunctional Vanadium Dioxide: Improved Thermochromism and Hydrophobicity Xukun Qian,† Ning Wang,† Yunfeng Li,‡ Junhu Zhang,*,‡ Zhichuan Xu,† and Yi Long*,† †

School of Materials Science and Engineering, Nanyang Technological University, Singapore 639798, Singapore State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun 130012, PR China

‡

ABSTRACT: Vanadium dioxide (VO2) films with moth-eye nanostructures have been fabricated to enhance the thermochromic properties with different periodicity (d) to achieve antireflection (AR). It is revealed that the films with smaller d (210 and 440 nm) could increase both the luminous transmission (Tlum) and infrared transmission (TIR) at 25 and 90 oC, as the d is smaller than the given wavelength and the gradient refractive index produces antireflection. The average Tlum and TIR of VO2 increase with decreasing d. Compared with the planar film, the AR sample with periodicity of 210 nm and thickness of 140 nm can offer approximately 10% enhancement of Tlum and 24.5% increase in solar modulation (ΔTsol). With the addition of hydrophobic overcoat on the patterned VO2, ∼120° contact angle could be achieved. The present approach can tailor the optical transmittance in different wavelengths at high and low temperature together with self-cleaning, opening new avenues for producing hydrophobic VO2 with enhanced thermochromic properties for smart window applications.

1. INTRODUCTION

Biomimetic surfaces have been extensively explored for AR applications because they can dramatically suppress the reflection losses and increase light transmission over a wide range of wavelength and a large field of view.20−23 The biomimetic surface is composed of tapered arrays which are similar to the nipple arrays on moth eyes.21,22,24,25 The antireflection can be understood as the tapered structure offering a gradual refractive index (n) increase from the top of the taper with a refractive index of air (nair = 1) to the bottom with a higher n.24,26 The net reflectance of the AR surface can be regarded as a result of an infinite series of reflections at each incremental change in n.24 Since each reflectance comes from a different depth of the surface, each will have a different phase. If the transition occurs over an optical distance of half wavelength (λ/2) where all phases are present, there will be destructive interference and the reflectance will theoretically fall to zero.24 The reduction in reflection simultaneously gives rise to a corresponding increase in transmission. In order to yield the largest reduction in reflection over a broad range of wavelengths and incidence angles, three structural parameters are generally required: the height (h) of the AR arrays, the period (d), and the distance (s) between the arrays at the bottom. To avoid scattering from the optical interface, d has to be smaller than the wavelength of the incoming light, and by increasing h, the reflectivity will be furthered reduced.24 For AR

Vanadium dioxide (VO2) presents a first order phase transition from an insulating state to a metallic state at the critical temperature (Tc) around 68 °C. This transition is accompanied by a sharp change in electrical resistivity and optical transmittance in the infrared (IR) region. Specifically, it allows IR transmission at temperatures below Tc, but is highly reflective to IR light at temperatures above Tc. The transmission of visible light is nearly unaffected during the phase transition.1−3 VO2 thin films are thus excellent materials for technological applications such as IR uncooled bolometers,4 thermochromic coating on smart windows,5−7 field effect transistors,8 high damping materials,9 and smart radiator devices for spacecraft.10 In general, an ideal thermochromic coating on smart windows requires a sufficiently high luminous transmittance (Tlum), a large solar modulating ability (ΔTsol), and a low phase-transition temperature (Tc).11 In reality, there is no single material that can meet these requirements to date, and trade-offs are typically made. For the practical application of VO2-based smart windows, the low Tlum and ΔTsol are the two main drawbacks to be tackled.6,10 Numerous efforts, such as Mg- or Eu-doping,12,13 multilayer-stack design,14,15 nanocomposite film,7,16 porous film,17 and antireflective (AR) coatings,18,19 have been made to increase the luminous transmittance at the expense of solar modulation ability to different extents, except five-layer TiO2/VO2/TiO2/VO2/TO2 mutilayer stack films, which are complicated to construct.15 © 2014 American Chemical Society

Received: July 15, 2014 Revised: August 19, 2014 Published: August 28, 2014 10766

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Figure 1. AFM images of AR samples with different periods: (a) 210 nm, (b) 440 nm, (c) 580 nm, (d) 1000 nm. (e) AFM cross-sectional profile. (f) SEM cross-sectional profile of 210 nm.

structures with fixed d and h, the distance s plays a negligible role in the AR performance and AR-structured surfaces are advantageous over AR-layer coatings because the reflection is reduced for omnidirectional incidence of light.27 Hitherto, AR surfaces have been extensively explored to increase transmission, eliminate ghost images, or veil glare caused by reflection from the optical surfaces and promote the performance of the devices.25,27 Recently, the Finite Difference Time Domain (FDTD) simulation has been used to analyze the moth eye mimic nanostructures and has revealed that the SiO2 nanoarrays (∼130 nm periodicity) coated with 5−50-nm-thick VO2 material could enhance the ΔTsol as high as 15.5% while maintaining the high visible transmission at ∼70%.28 In the present work, we experimentally demonstrate the fabrication and characterization of VO2 thermochromic film on AR surfaces for the first time. Compared with multi- or singlelayered antireflection stack,14,15 the biomimetic structuring eliminates the thermal stress induced by thermal cycling and coefficient of thermal expansion mismatch between different layers. Both Tlum and ΔTsol can be improved with AR surfaces and hydrophobicity could further be achieved with an overcoat on top of VO2.

2. METHODS Preparation of VO2 Film. Fused silica substrates with AR patterns of different periods (0, 210, 440, 580, and 1000 nm) were used to deposit VO2 film. The substrates were prepared by reactive ion etching using 2D polystyrene colloidal crystals as mask and the experimental details can be found elsewhere.25 Vanadium oxide precursor was prepared by dissolving vanadium pentoxide (300 mg, 99%, Alfa Aesar) with vigorous stirring in 30% hydrogen peroxide solution (30 mL, 30%, AnalaR NORMAPUR) at 70 °C.29 The substrates were successively cleaned with ammonia, acetone, and ethanol several times to make the surfaces hydrophilic followed by immersing in the precursor gel for 30 seconds and withdrawing at a constant speed of 10 mm/min (140 nm VO2) and 1 mm/min (∼40 nm VO2). After drying in air, the samples were annealed at 550 °C for 2 h in flowing argon with heating and cooling rates of 1 and 3 °C/min, respectively. The details of fluorooctyl triethoxysilane (FOS) sol−gel overcoat can be found in a previously published paper.19 Characterization. X-ray diffraction (XRD) was performed using a Bruker D8 Advance diffractometer (Cu Kα, λ = 0.15406 nm) with 40 kV and 30 mA. The surface morphologies of the films were observed by atomic force microscopy (AFM, DI-3100, Bruker, Germany) using tapping mode and scanning electron microscopy (SEM, 6340F, JEOL, Japan) connected with energy-dispersive X-ray spectroscopy (EDS, Oxford, UK). The film thickness was measured by surface profiler (Alpha-Step IQ, KLA-Tencor, USA). The AFM measurements were carried out in air at room temperature using commercial silicon probes. The high resolution transmission electron microscopy (TEM) was performed to characterize microstructure on JEOL 2010 -TEM 10767

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different periods in a 2 μm × 2 μm scanning area as shown in Figure 1a−d. The periodical structures can be observed with uniform tapered arrays oriented perpendicular to the substrate. There is no hint of VO2 agglomeration between arrays and the substrate array profile is well preserved. As calculated in the cross-section profile (Figure 1e), the periods of different VO2 cone-like arrays are 210, 440, 580, and 1000 nm and the tapered nanopillar structures could be observed. The SEM cross-sectional image of 210 nm sample (Figure 1f) showed that the distance between the pillars varied from ∼200 to 240 nm and the pillar height was ∼180 nm. To further examine the coverage of VO2 film, scanning electron microscopy (SEM) observation of the topography and EDS were performed. The SEM picture for the sample with 440 nm period as shown in Figure 2a is in good agreement with AFM observation and there is no VO2 agglomeration observed. As the gradual change of the refractive index from air to VO2 is essential to antireflection, continuous film coverage is required. The O Kα1, Si Kα1, and V Kα1 profiles (Figure 2b) indicate that the concentrations along the line for the three elements are distributed evenly, which indicates that VO2 film has been covered continuously on arrays. Figure 3 shows the transmittance spectrum as a function of wavelength for the planar and AR samples measured at 25 and 90 °C, respectively. A large IR-transmittance contrast with temperature change has been observed in all samples, indicating the metal-insulator transition of VO2 film.3,12,32 Two regimes of the spectrum have been analyzed separately, visible (380−780 nm) and IR (780−2500 nm) wavelength ranges. The Tlum (25 °C) of the AR film with 210 nm period reached a maximum value of 60.1% at wavelength of around 700 nm. The spectra shows a number of distinct features at 25 °C and the transmittance curves especially in the visible region varied greatly with surface periods. Compared with the planar film, the enhancement of transmittance for AR film with 210 nm period is obvious over the spectral range from around 560 to 2500 nm. One rarely observed feature worth noting here is the shift of transmittance peaks. For the AR film with 440 nm period, two transmittance peaks emerge and the edge shifts to the blue end of the spectrum, which is beneficial to the luminous transmission enhancement and the reasons remained unkown. For the 1000-nm-period sample, both transmittances at high and low temperature peaks have a large red-shift compared with the planar surface. This phenomenon could be utilized to tailor the optical response in different selected wavelength ranges. At this moment, the mechanisms for such a phenomenon remain unknown. Tsol, Tlum, TIR, and ΔTsol were calculated and listed in Table 1. It is found that the planar VO2 film shows a relatively low Tlum, i.e., 39.2% at 25 °C and 41.7% at 90 °C. With AR structures except the 1000 nm period, the average Tlum at both

with an electron beam energy of 200 keV. The TEM sample prepration was done using the lift-out technique in a focused ion beam (FIB) system. The spectrophotometer is equipped with a heating and cooling stage (PE120, Linkam, UK). The reported data is averaged from five different sampling points. The integral Tlum (380−780 nm), IR transmittance TIR (780−2500 nm), and solar transmittance (Tsol, 280−2500 nm) were calculated from eq 1: Tlum/IR/sol =

∫ ϕlum/IR/sol(λ)T(λ) dλ/∫ ϕlum/IR/sol(λ) dλ

(1)

where T(λ) denotes spectral transmittance, φlum (λ) is the standard luminous efficiency function of photopic vision in the wavelength range of 380−780 nm,30 φIR (λ) and φsol (λ) are the IR/solar irradiance spectrum for air mass 1.5 (corresponding to the sun standing 37° above the horizon).31 ΔTsol is obtained by ΔTsol = Tsol,25 °C − Tsol,90 °C.

3. RESULTS AND DISCUSSION Atomic force microscopy (AFM) is employed to examine the three-dimensional topography of pattered VO2 films with

Figure 2. (a) Top-view SEM image of AR surface with 440 nm period and (b) EDS profile taken along the red line in (a).

Figure 3. Transmittance spectra in 250−2500 nm range at 25/90 °C for planar and AR samples with 140 nm thickness, respectively.

Table 1. Thermochromic Properties of Five Samples with Periodicity of 0, 210, 440, 580, and 1000 nm with VO2 Film Thickness of 140 nm

sample

Tlum (25 °C) (%)

Tlum (90 °C) (%)

Tlum (average) (%)

ΔTlum (%)

TIR (25 °C) (%)

TIR (90 °C) (%)

TIR (average) (%)

ΔTIR (%)

Tsol (25 °C) (%)

Tsol (90 °C) (%)

ΔTsol (%)

Planar surface d = 210 nm d = 440 nm d = 580 nm d = 1000 nm

39.2 43.6 41.5 40.1 14.5

41.7 45.3 48.1 44.2 11.7

40.5 44.5 44.8 42.2 13.1

−2.5 −1.7 −6.6 −4.1 −2.8

62.0 67.5 63.2 61.7 58.5

44.1 48.1 50.9 51.9 46.9

53.1 57.8 57.1 56.8 52.7

17.9 19.4 12.3 9.8 11.6

46.8 52.1 49.3 48.3 34.5

41.1 45.0 48.9 46.1 27.9

5.7 7.1 0.32 2.17 6.6

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Figure 4. Effects of periodicity on (a) Tlum and TIR and (b) ΔTlum, ΔTIR, and ΔTsol.

25 and 90 °C increases compared with planar surface as shown in Figure 4a. This is in accordance with the theoretical prediction that the antireflection only happens when d is less than λ.33,34 Compared with the planar film, both 210 and 440 nm AR samples provide approximately 10% relative increase of average Tlum compared with planar surface (44.5% vs 40.5%). The average Tlum of the sample decreases with the increase of periodicity from 44.5% to 13.1% (Figure 4a) which is consistent with the theory, as smaller periodicity offers better antireflection.33,34 The large reduction of Tlum for a 1000 nm sample is due to the large red shift as observed in Figure 3 which arises from the fact that d (1000 nm) is larger than the visible wavelength.28 However, the transmission in IR range (TIR) of 1000 nm period is not affected as much as Tlum by the patterning due to the larger wavelength of IR (Figure 4a) . The average TIR for 1000 nm period is 52.7%, close to that of planar sample (53%). Similar to the visible range, the antireflection also happens in the IR range as the average TIR for 210, 440, and 580 nm is larger than that for the planar surface. In addition, the TIR decreases from 57.8%, 57.1%, 56.8%, to 52.7% with increasing periodicity (Figure 4a) and the ΔTIR decreases with increasing periodicity (Figure 4b). ΔTlum and ΔTsol does not have an apparent relationship with periodicity (Figure 4b) as the biomimetic surface could enhance antireflection with increased transmission in both visible and IR ranges at 20 and 90 °C, but the difference between low and high temperautre is more complicated. More importantly, compared with plannar VO2, the ΔTsol of 210 nm sample has increased from 5.7% to 7.1% together with relative ∼10% enhancement of Tlum, and this was attributed by the larger surface area of VO2 exposed to solar radiation due to the biomimetric surface construction.28 Furthermore, the ΔTsol of 440 and 580 nm samples has dropped dramatically and the cause may partially be the reduction of ΔTlum and ΔTIR compared with the planar surface (Figure 4b). The averaged Tlum (44.5%) and ΔTsol (7.1%) of 210 nm sample as highlighted in Table 1 are less than the theoretical calculation which predicted that with the VO2 thickness of ∼10 nm, the ΔTsol could reach as high as 15.5% while maintaining the high Tlum of ∼70% with the pitch size of 180 nm,28 and the main discrepancies are due to the coating thickness difference and pitch difference, as it is difficult to

Figure 5. (a) Transmittance spectra in 250−2500 nm range at 25/90 °C for planar VO2, 210 nm patterned VO2 with ∼40 nm thickness, and 210 nm patterned VO2 with fluorooctyl triethoxysilane (FOS) overcoat. Inset: the contact angles. (b) TEM cross-sectional image to show the thickness of VO2 coatings on SiO2.

Table 2. Thermochromic Properties and Contact Angle of Planar VO2 (∼40 nm thickness), 210 nm Patterned VO2 and 210 nm Patterned VO2 with FOS Overcoat sample

Tlum (25 °C)/%

Tlum (90 °C)/%

ΔTsol/%

contact angle/°

Planar VO2 210 nm patterned VO2 210 nm patterned VO2/FOS

63.8 66.8 65.9

60.1 68.4 68.0

3.0 3.2 3.1

26 58 118

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Notes

experimentally reduce the pitch size to 180 nm and the coating thickness to 10 nm as calculated. However, the enhanced thermochromic properties compared with the planar surface did prove the efficacy of biomimetic surface patterning. To further protect film from oxidation and provide hydrophobicity, an overcoat with 2 vol % FOS mixed with methanol/H2O/tetraethyl orthosilicate (TEOS)/ethanol19 was coated on the ∼40 nm thick VO2 film with 210 nm patterening. The UV−vis and contact angle of the planar VO2, 210 nm patterned VO2 and FOS overcoated 210 nm VO2 is shown in Figure 5a. The contact angle of the patterned VO2 is larger than the plannar VO2, as it is well-known that nanopatterning could offer enhanced hydrophobicity.26 With FOS coated Si gel overcoat, the contact angle was largely increased to 116° as shown in the inset in Figure 5a and Table 2, as FOS-Si gel coating could provide low surface energy coating which enhance hydrophobicity.19 The coating thickness variation of VO2 was further examined from TEM observation. Figure 5b is a bright-field image of the prepared VO2@SiO2 nanopillar cross-section. The thickness of VO2 coating in the valley was ∼40 nm while the thickness of coating on the top was ∼40 nm. VO2 nanopillar were filled with Pt during TEM sample preparation. Compared with 210 nm samples with 140 nm thickness as shown in Table 1, the Tlum is increased and ΔTsol is decreased due to the reduced thickness. Similar to 140 nm VO2 film, nanopatterning does provide ∼40 nm VO2 films antireflection as there is relative ∼10% enhanced Tlum with nearly unchanged ΔTsol. FOS overcoated does not influence the Tlum and ΔTsol, but it does offer largely enhanced contact angle from 58° to 118° which will provide self-cleaning and antioxidation.19

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research is supported by the Singapore National Research Foundation under CREATE programme: Nanomaterials for Energy and Water Management and Singapore minster of education (MOE) Academic Research Fund Tier 1 RG101/13. The electron microscopy and XRD work were performed at the Facility for Analysis, Characterization, Testing and Simulation (FACTS) in Nanyang Technological University, Singapore.

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4. CONCLUSIONS In summary, biomimetic patterned AR VO2 surfaces with different d have been fabricated to enhance the thermochromic performance of VO2 coating. It was found that smaller d (210, 440, and 580 nm) could increase both the Tlum and TIR at 25 and 90 °C and the 1000 nm sample has a drastic reduction in Tlum due to the large redshift of the peaks which is attributed by larger d than the visible wavelength. The average Tlum and TIR together with ΔTIR increases with decreasing d as smaller d would favor the antireflection. It is of great interest that 210 nm patterned samples of different thicknesses could offer both enhancement of Tlum and ΔTsol. As far as our knowledge is concerned, it is the first time investigating the enhancing thermochromic performance of VO2 thin film with moth eye mimic nanostructures experimentally. Patterning enhances the contact angle of pure VO2 and the contact angle could be further increased to ∼120° by applying FOS sol−gel overcoat which is of great practical signifiance to promote self-cleaning. It could be predicted that with further reducing the d (< 200 nm), Tlum and ΔTsol could continue to increase, which offers a completly new direction of solving the critical and intrisinc problems of low Tlum and ΔTsol in thermochromic VO2 coatings.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Author Contributions

Xukun Qiana and Ning Wang contributed equally to this work. 10770

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