Research Article Cite This: ACS Appl. Mater. Interfaces 2019, 11, 22692−22702
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Self-Template Synthesis of Nanoporous VO2‑Based Films: Localized Surface Plasmon Resonance and Enhanced Optical Performance for Solar Glazing Application Shiwei Long,†,‡,§ Xun Cao,*,†,‡ Rong Huang,∥ Fang Xu,†,‡,§ Ning Li,⊥ Aibin Huang,†,‡ Guangyao Sun,†,‡ Shanhu Bao,†,‡ Hongjie Luo,# and Ping Jin†,‡,∇
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State Key Laboratory of High-Performance Ceramics and Superfine Microstructure and ‡Research Center for Industrial Ceramics, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, China § University of Chinese Academy of Sciences, Beijing 100049, China ∥ Key Laboratory of Polar Materials and Devices, Ministry of Education, East China Normal University, Shanghai 200241, China ⊥ Department of Materials Science and Engineering, College of Science, China University of Petroleum Beijing, No. 18 Fuxue Road, Beijing 102249, China # School of Materials Science and Engineering, Shanghai University, Shangda Road 99, Baoshan, Shanghai 200444, China ∇ Materials Research Institute for Sustainable Development, National Institute of Advanced Industrial Science and Technology, Nagoya 463-8560, Japan S Supporting Information *
ABSTRACT: Poly(tetrafluoroethylene) (Teflon) has been selected as the self-template structural material in the preparation of VO2 films using a reactive magnetron sputtering method and post-annealing process. VO2 films with spontaneous random nanoporous structures growing on quartz glasses have been deliberately established via bottom-up processing through this novel and facile approach. The nanoporous VO2 films exhibit an excellent optical performance based on the localized surface plasmon resonance, with ultrahigh luminous transmittance (Tlum‑L) up to 78.0% and the promoted solar modulation ability (ΔTsol) of 14.1%. Meanwhile, the ingenious microstructure of the film provides an antireflection function from multiple perspectives on visible light and indicates the potential of the windshield on vehicles for smart solar modulation. The nanoporous films expand the practical application of thermochromic VO2 to a fire-new field, breaking the optical performance envelope of the single-layer dense VO2 film away, and offering a universal method to prepare homogeneous nanoporous structures for thin films. KEYWORDS: sputtering, vanadium dioxides, PTFE, self-template, nanoporous, LSPR, windshield
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INTRODUCTION Intelligent windows or smart windows can automatically manage the heat by modulating the amount of solar radiation (especially the near-infrared (NIR) region) in response to environmental temperature variations and exhibit the potential application in building energy-saving and temperature control.1,2 Among the diverse thermochromic materials, vanadium dioxide (VO2) was usually regarded as the representative candidate due to the reversible phase change from an infrared-transmitted semiconductor state (VO2(M)) to an infrared-reflective metal state (VO2(R)) when below/ over the critical transition temperature (∼340 K), whereas the transmittance in the luminous range was nearly maintained.3,4 Therefore, VO2 films can be capable of impeding unnecessary heat at high temperature while allowing the incident solar radiation indoors automatically, displaying an ideal performance for energy savings.5 However, the relatively high phase transition temperature should be reduced to meet the requirement. © 2019 American Chemical Society
Fortunately, bringing in high valence dopants like tungsten can provide an efficient approach to reduce intrinsic transition temperature (∼68 °C) of VO2 by modifying the electronic density of states.6 Nevertheless, some other drawbacks, such as the low luminous transmittance (Tlum), unsatisfied solar modulation ability (ΔTsol), and the native dark brown color of films have still obstructed the actual use of VO2-based smart windows.7 Here, the low luminous transmittance and the native dark brown color of the films were probably caused by the small optical band gap of VO2, which gives rise to absorption in the short-wavelength range.8 Therefore, a large number of methods have been employed to enhance the optical performance (Tlum and ΔTsol) of VO2. Doping is also used here, dopants like Mg or F are able to increase Tlum but lead to the deterioration of ΔTsol.9,10 In addition, antireflection Received: February 26, 2019 Accepted: June 4, 2019 Published: June 4, 2019 22692
DOI: 10.1021/acsami.9b03586 ACS Appl. Mater. Interfaces 2019, 11, 22692−22702
Research Article
ACS Applied Materials & Interfaces
will further impel the development of the practical smart window and intelligent windshield for the promotion of energy savings.
layers such as TiO2, Cr2O3, and WO3, have been introduced to enhance Tlum (40−55%) and ΔTsol simultaneously based on the interference effect,11−14 but the improvement of Tlum cannot completely satisfy the requirement of architectural windows, which is at least 60% to avoid extra energy consumption on indoor lighting.15 Therefore, all aforementioned methods have hardly equilibrated the optical performance for both Tlum and ΔTsol. Surface modification seems one of the most feasible routes to take account of both Tlum and ΔTsol. Xie et al.16 masterly utilized monolayer polystyrene spheres as templates to fabricate periodic porous VO2 films, successfully enhanced the Tlum (70.2%), and obtained decent ΔTsol (7.9%), whereas Jiang et al.17 used a dual-phase transformation method to fabricate VO2 films with ordered honeycomb microstructures, achieving the excellent transmittance of 94.5% at 700 nm with ΔTsol of 5.5%. Besides, Long et al.18 fabricated periodic motheye nanostructures to improve both Tlum and ΔTsol for VO2 films, and added a hydrophobic overcoat on the VO2 films to realize the self-cleaning function. Furthermore, Long et al.19 also subtly prepared the Kirigami-inspired VO2 -based plasmonic elastomers and utilized mechanical stimulus to achieve adaptive, broadband, and highly efficient ΔTsol for thermochromic windows. In addition, except for ordered surface structures, some other previous works had also confirmed that random nanoporous structures on the VO2 thin film could significantly improve Tlum and maintain the ΔTsol, which may largely depend on the interference effect derived from reducing optical constants.20,21 Thus, nanoporous patterns play a positive role in optimizing the optical properties, especially in Tlum. For preparing ordered surface morphology, it is necessary to apply several advanced technologies, such as colloidal lithography,22 photolithography,23 or ion etching,24 which may be limited by the specific template or size and equipment. In consideration of the high costs and complex processes in the fabrication of ordered structures, spontaneous self-template seems a feasible method in actual use, which could form random pores. Besides, VO2 particles and films fabricated by the conventional chemical method exhibited a weak adhesion to a substrate, which is not suitable for long-term service and show inferior durability. Hence, self-template patterned VO2-based films fabricated by magnetron sputtering may be a wise choice in large-scale preparation. Inspired by these interesting design concepts, we first prepared a new type of spontaneous nanoporous VO2 thin films via the magnetron sputtering method. Herein, the poly(tetrafluoroethylene) (PTFE, Teflon) was deliberately introduced as the self-template on deposited composite films in consideration of its great chemical stability and nonadhesive properties,25,26 which could serve as the pore-forming material in the post-annealing process. As is expected, the obtained uniform porous VO2 exhibits ultrahigh luminous transmittance (Tlum‑L = 78.0% in maximum) and brilliant solar modulation ability (ΔTsol = 14.1%). This ultrahigh Tlum has satisfied the requirement of the windshield on vehicles, which need to be over 70% to guarantee the safety. In addition, it is exciting that both Tlum and ΔTsol had largely enhanced when designing the “double-layer” nanoporous VO2 film structure, which reached an excellent Tlum‑L of 63.3% and ΔTsol of 20.1%, respectively. We speculate that the located surface plasmon resonance (LSPR) mainly contribute to these optimizations for optical performance. We believe that the proposed universal approach
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EXPERIMENTAL SECTION
Preparation of Nanoporous VO2 Films. VO2 Films were fabricated on 10 × 10 mm2 quartz glass substrates via a reactive magnetron sputtering system and subsequent annealing. VO 2 (99.99%) and PTFE (99.95%) targets (diameter of 2 in.) were used for codeposition, whereas the deposition course was carried out using an integrated lock−load system. An initial pump-down process was executed to reach the original pressure of 5.0 × 10−4 Pa in a deposition chamber. Then, Ar (99.99% pure) gas as well as the Ar (97%) and O2 (3%) mixed gases (99.99% pure) with fixed (Ar + O2)/ Ar proportion were introduced into the atmosphere. Co-sputtering then took place at the direct current and radio frequency powers of 80 W and 60 W corresponding to VO2 and PTFE targets. The (Ar + O2)/Ar ratio was kept at 0.6 (O2 ∼ 1.1%) and the pressure of the chamber was kept at 0.6 Pa when the total gas flow is 40 sccm. Cosputtering was executed at room temperature through the entire deposition process for VOx/PTFE mixed films. Post-annealing was carried out after deposition in two steps. First, the composite film was annealed at 350 °C for 3 min, with a heating rate of ∼1.5 °C/s. Then, the annealing temperature increased to 450 °C and was maintained for 5 min. Finally, the samples decline to room temperature by furnace cooling. Moreover, when the continuous films were prepared, the steps were the same as in the fabricating nanoporous films, but the co-sputtered PTFE was not needed. Calculation of Optical Properties. The integral values of the solar transmittance (350−2600 nm) of the films and the luminous transmittance (Tlum, 380−780 nm) were obtained by the formulas Tsol =
∫ Φsol(λ)T(λ)dλ/∫ Φsol(λ)dλ
(1)
Tlum =
∫ Φlum(λ)T(λ)dλ/∫ Φlum(λ)dλ
(2)
where T(λ) means spectral transmittance at wavelength λ, Φsol(λ) is the solar irradiance spectrum for an air mass of 1.5 corresponding to the sun standing 37° above the horizon, and Φlum(λ) is the standard efficiency function for photopic vision. Herein, the solar modulation ability (ΔTsol) of films can be expressed by ΔTsol = Tsol ‐ L − Tsol ‐ H
(3)
where L and H stand for low and high temperatures, respectively. In consideration of room temperature, critical transition temperature (∼68 °C) as well as the pure phases of the VO2 films, we select 20 and 90 °C as the low/high temperatures to maintain the insulator/ metal states of the samples in the measurements, respectively. Characterization. X-ray diffraction (XRD) measurements were performed on a Rigaku Ultima IV diffractometer with a 2θ grazing angle mode using Cu Kα radiation (λ = 0.15418 nm), whereas the Raman shifts were performed using a Raman spectrometer (HORIBA, Lab RAM HR Evolution). Surface topography and root-mean-square (RMS) roughness of films were measured by an atomic force microscopy (AFM, SII NanoTechnology Ltd, Nanonavi Π) apparatus in the tapping mode. To confirm the thickness and determine the microstructure of the films, cross-sectional images and surface morphologies were observed using a field emission scanning electron microscope (FESEM, Hitachi SU8220 and FEI, Helios G4 UX). Transmittance spectra (350−2600 nm) and hysteresis loops (2000 nm) of the as-deposited films were obtained using a UV−vis−NIR spectrophotometer (Hitachi Corp., model UV-4100) equipped with an accessory heater. The temperature was measured precisely with a temperature sensor in contact with the surface of films, commanded by a temperature controlling unit. The angle-dependent transmittance in visible light was measured by an angle-resolved spectrum measurement system, as described in other literatures.27,28 22693
DOI: 10.1021/acsami.9b03586 ACS Appl. Mater. Interfaces 2019, 11, 22692−22702
Research Article
ACS Applied Materials & Interfaces
Figure 1. (a) Large-area FESEM images of the nanoporous VO2 film growing by sputtering and post-annealing process and the corresponding (b) amplified image of surface morphology. (c) Cross-sectional FESEM image of the nanoporous VO2 film. (d) AFM image of surficial morphology for the nanoporous VO2 film and the corresponding root-mean-square (RMS) roughness. (e) XRD pattern of the prepared nanoporous VO2 film. (f) Raman spectra of the nanoporous VO2 film. Simulation of Spectra. The optical constants (n, k) of dense VO2 (M/R) films were taken from our previous deposited films (Figure S12, Supporting Information) in the range of 350−2500 nm. The simulated spectra were calculated by the optical constants of void films and VO2 particle films. The transmittance for model 3 is assumed to be 100% overall wavelength range (350−2500 nm). The final model combines the former three models together by synthesizing their spectra with different percentages.
further confirm the structure, Raman shifts were executed, as displayed in Figure 1d. As expected, almost all known Raman vibration modes of monoclinic VO2 have been observed (144, 194, 226, 262, 310, 342, 389, 445, 500, and 615 cm−1).29,30 In addition, the Raman shift of the prepared dense films also matched well with monoclinic VO2 (Figure S3). Preparation and Formation Mechanism of Porous Films. To expound the fabrication process and formation mechanism of this nanoporous film, Figure 2a,b illustrate the preparation method and the speculation of the mechanism for pore formation at the post-annealing process in detail, respectively. First of all, the quartz glass substrate was rinsed
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RESULTS AND DISCUSSION Morphology and Structure. The field emission scanning electron micrograph (FESEM) image in Figure 1a shows the large-scale surface morphology and structure of the prepared VO2 film. One can see that random pores distribute on the film homogeneously. The amplified image of the red rectangle area in Figure 1a is illustrated in Figure 1b, which manifests that the random wormlike pores are in the nanoscale with the size of 50−200 nm (Figure S1b−d, Supporting Information), leading to the formation of plentiful continuous particles. Furthermore, Figures 1c and S1a provide the cross-sectional FESEM image of the film, demonstrating that the surface particles are teethlike and a portion of the pores are perforative with a depth of ∼210 nm (film thickness, Figure S1a), whereas some of the pores are not, which means that the teeth-like particles are probably connected to each other at the bottom, such as a nanonet. In addition, the atomic force micrograph (AFM) three-dimensional image (Figure 1d) presents a homogeneous surface morphology of the film. The root-mean-square (RMS) roughness is around 15.5 nm, implying that the smooth surface was obtained in the prepared film. Besides, some other AFM images (Figure S2b,c) also confirm the existence of nanopores. They indicate that the depth of the observed nanopores inside the film was about 30−100 nm, and the pitch of nanopores (periodicity) was about 100−200 nm, which agreed with the FESEM images above. Figure 1d shows the X-ray diffraction (XRD) pattern of the deposited film, and the main peaks can be indexed with (011), (2̅11), and (220) of VO2(M) (JCPDS card No. 72-0514, P21/c, a = 0.574 nm, b = 0.452 nm, c = 0.538 nm, and α = γ = 90°, β = 122.61°), while other peaks can also be indexed to pure VO2(M). Meanwhile, the XRD pattern is similar to other prepared dense VO2 films (Figure S2a). To
Figure 2. (a) Schematic of fabrication route for nanoporous VO2 films. (b) Mechanism in the formation of nanoporous films at the post-annealing process. 22694
DOI: 10.1021/acsami.9b03586 ACS Appl. Mater. Interfaces 2019, 11, 22692−22702
Research Article
ACS Applied Materials & Interfaces
Figure 3. (a) Transmittance spectra of the nanoporous VO2 films. (b) Temperature-dependent LSPR peak variation of the nanoporous VO2 films in the temperature range from 25 to 100 °C. Extinction (A) is calculated via A = −lg (transmittance). (c) Temperature-dependent hysteresis loops and the corresponding first-order differential curves can be seen in the inset at the top right corner. (d) Transmittance spectra of the double-layer nanoporous VO2 films. The colored areas indicate the normalized values of spectral irradiance corresponding to the visible (cyan) and NIR (red) ranges of solar spectra, and the yellow area indicates the values of eye sensitivity function. (e) CIE image of nanoporous VO2 films compared with continuous VO2 films. (f) Optical photograph of the nanoporous VO2 films on quartz.
steps are the crucial points in the formation of nanopores and teeth-like nanoparticles. LSPR Effect and Optical Performance. As expected, this kind of porous structure serves as a positive role for optical performance. Figure 3a exhibits the transmittance spectra at 20 and 90 °C for the prepared optimal nanoporous VO2 films (labeled as VP), whereas the prepared films with other thickness and PTFE volume fraction are shown in Figure S6 as the references. Observably, the optimal VP film has an ultrahigh transmittance at a visible range, whether in the semiconductor state or metallic state. The film has reached an ultrahigh luminous transmittance (Tlum) of 78.0% (Tlum‑L) and 72.8% (Tlum‑H) at 20 and 90 °C, respectively, and delivers an even higher solar modulation ability (ΔTsol) of 14.1%, exhibiting a splendid optical performance compared with dense VO2 films (Figure S7a,b). Moreover, the reflectance spectrum of nanoporous VO2 films (Figure S7d) does not exhibit an obvious reflective peak near ∼1200 nm when VP is in high-temperature conditions, implying that there is a strong absorption in this wavelength range according to Kirchhoff’s law. Here, we speculate that the formation of the localized surface plasmon resonance (LSPR) absorption peak in metallic states might contribute more to the enhancement of solar modulation efficiency, which we will discuss below. Furthermore, double-layer nanoporous VO2 films (labeled as 2VP, Figure S7c) were also designed. The spectra (Figure 3d) of 2VP display a larger difference in low/high temperature, especially in the near-infrared range, with an obvious LSPR absorption peak. In addition, the colored areas represent the normalized values of spectral irradiance corresponding to the visible (cyan) and NIR (red) ranges of solar spectra, as well as the eye sensitivity function (yellow), respectively.7 Under such circumstances, the double-layer nanoporous film shows a Tlum‑L of 63.3% with a ΔTsol of 20.1% and also exhibits a large enhancement of optical performance in equilibrating luminous transmittance and solar modulation ability. The optical
to make it pollution-free and dry. Then the co-sputtering was carried out using VO2 ceramic and PTFE targets, resulting in depositing a continuous VOx/PTFE mixed film on quartz glass. Finally, the deliberate post-annealing was executed on extraordinary parameters, leading to the formation of VO2 films with random nanopores and nanoparticles on the surface, and even the perforative pores. Notably, the post-annealing route is one of the most vital steps. To our knowledge, the PTFE has the lowest adhesive energy in solid states.31,32 Thus, when a continuous VOx/PTFE mixed film was formed, PTFE may form as nearly nanoscale spherical particles and uniformly distribute in the amorphous VOx film on the co-sputtering step, so as to guarantee the lowest surface energy on the interface with VOx,33 not joint VOx, and form a dispersive system in the mixed dense film. Herein, the post-annealing process was intentionally executed in two steps. Primarily, the annealing temperature was slowly increased to ∼350 °C with the heating rate of ∼1.5 °C/s and maintained for 3 min. Above the melting point of ∼327 °C,34 PTFE would fuse and undergo an endothermic process, and we speculate that PTFE particles may start to melt and combine together adequately to aggregate the lager fusing particles, because this approach would decline the interfacial area and reduce the surface energy to make PTFE reach a metastable state. Then, the annealing temperature was elevated to 450 °C and maintained for 5 min, which is above the sintering point of ∼380 °C and a boiling point of ∼400 °C for PTFE.35,36 In this case, PTFE particles may volatilize and ablate in the furnace, forming the pores and particles from bottom to the surface of films. Further annealing is aimed at making the rest of the VOx to transform into crystalline VO2 in the end. In comparison, when we take the primarily annealing step out, the VO2 particles of the film collapsed (Figure S4), forming a broken surface of the film that is full of cracks and heterogeneous particles. Herein, we speculate that improper annealing temperature may be the main cause (Figure S5). Therefore, annealing temperature and 22695
DOI: 10.1021/acsami.9b03586 ACS Appl. Mater. Interfaces 2019, 11, 22692−22702
Research Article
ACS Applied Materials & Interfaces performance of all of the prepared films was calculated and are collected in Table 1, and the nanoporous VO2 film is
particles, suitable particle size, and appropriate dielectric constant of the surrounding medium. To investigate the position variation of LSPR peaks, temperature-dependent vis−NIR spectra were measured from 25 to 100 °C (Figure S10a) and the corresponding switched temperature-dependent extinction was calculated based on the formula A = −lg (transmittance), which is illustrated in Figure 3b. Interestingly, it is evident that the intensity of LSPR appears at high temperature and shows gradually attenuation with the reduced temperature, especially in the range of 60−80 °C (Figure S9c,f), which is near the transition temperature of VO2. The high-temperature strengthened LSPR phenomenon may ascribe to the emergence of intermediate states during the structural and electronic phase transition in VO2 thin films,43 which presents a temperature-dependent fraction of metallic phase, offering an increased carrier concentration of the surficial VO2 nanoparticles, giving rise to the gradual enhancement of relevant LSPR intensity. Notably, a gradual red-shift (1070−1260 nm) of the LSPR absorption peak is observed when the temperature is over 70 °C during the heating process (Figures 3b and S9c), this phenomenon may be interpreted through the modified formula based on Maxwell-Garnett effective medium theory.44 Here, to simplify, we consider the metal state surficial VO2 as the ideal spherical nanoparticles, and the wavelength of the LSPR peak could be given by45
Table 1. Optical Properties for Prepared Films sample
ΔTsol
Tlum‑L
Tlum‑H
50 nm VO2 140 nm VO2 210 nm VO2 nanoporous VO2 film (VP) 2VP
10.0 15.8 14.8 14.1 20.1
39.3 17.2 9.8 78.0 63.3
35.2 12.2 6.4 72.8 57.1
undisputedly the most excellent film in overall performance. Moreover, the optical energy band gaps (Eg) based on transmittance spectra have also demonstrated that the Eg values of the nanoporous film are little higher (1.67−2.0 eV) than dense VO2 films (Figure S9),37 which could lead to a significant decrease in the absorption of a short-wavelength range, including visible range, resulting in an enhancement of luminous transmittance (Tlum). The blue shift of the absorption edge in the transmission spectra may contribute to the main cause in broadening the band gap.38 The phenomenon of localized surface plasmon resonance (LSPR) on VO2 has drawn attention due to its temperaturedependent behavior, which can also affirm the existence of itself and was not observed on pure noble metals.39,40 The transmittance spectra of the nanoporous VO2 film (Figure 3a) suggest that the LSPR absorption effect in the NIR range (∼1200 nm) can be observed at 90 °C but not at 20 °C, which may be due to the metallic phase formation above the transition temperature of VO2. Furthermore, the existence of sub-100 nm scale surficial isolated VO2 nanoparticles mentioned above is another key to the generation of LSPR, which agrees with our previous work.41 According to the modified formulas proposed by Bohren42 based on Mie’s electrostatic field theory, the relationship of the dielectric function of spherical particles (ε = ε1 + iε2), the dielectric constant of the surrounding medium (εm), as well as the extinction cross section (σext), scattering cross section (σsca), and absorption cross section (σabs) are given by σext =
ε2(λ) 18πVεm3/2 λ [ε1(λ) + 2εm]2 + [ε2(λ)]2
(4)
σsca =
32π 4V 2εm2 [ε1(λ) − εm]2 + [ε2(λ)]2 λ4 [ε1(λ) + 2εm]2 + [ε2(λ)]2
(5)
σabs = σext − σsca
λSPR = λnp
2+f εm + 1 1−f
(7)
where λnp = 2πc/ωnp for the VO2(R) nanoparticles, f is the particle “filling factor”, and εm is also the dielectric constant of the surrounding medium for the VO2(R) nanoparticles. Obviously, from formula (7), one can see that λSPR is determined by f and εm. Thus, an enlarged f or εm cloud induces a red-shift of λSPR. In this work, εm is composed of the surrounding glass substrate and air. In other words, it changed little when the phase transition of VO2 occurs and can be treated as a constant. When the temperature exceeds Tc, the semiconductor to metal transition is triggered. In this case, the particle size of surficial metallic VO2(R) gradually increases, which results in an enlargement of filling factor f in the nanoporous VO2 film. Therefore, the λSPR exhibits a red-shift in the heating process. Moreover, it is noteworthy that the LSPR intensity increases when exceeding the Tc and leads to a strengthened absorption at lower transmittance near ∼1200 nm. In addition, the 780− 1350 nm wavelengths range occupied most of the solar energy in the NIR range, as is shown in the red area in Figure 3d. Meanwhile, the LSPR effect almost disappears during the low temperature (
