Enhancement of VO2 thermochromic properties by Si doping

Surface & Coatings Technology 276 (2015) 248–253

Contents lists available at ScienceDirect

Surface & Coatings Technology journal homepage: www.elsevier.com/locate/surfcoat

Enhancement of VO2 thermochromic properties by Si doping Xufei Wu a,⁎, Zhiming Wu a,⁎, Huafu Zhang b, Ruihua Niu c, Qiong He a, Chunhui Ji a, Jun Wang a, Yadong Jiang a a b c

School of Optoelectronic Information, State Key Laboratory of Electronic Thin Films and Integrated Devices, University of Electronic Science and Technology of China, Chengdu 610054, PR China School of Science, Shandong University of Technology, Zibo, Shandong 255049, PR China Southwest Institution of Technological Physics, Chengdu 610041, PR China

a r t i c l e

i n f o

Article history: Received 16 February 2015 Revised 4 June 2015 Accepted in revised form 6 July 2015 Available online 11 July 2015 Keywords: Vanadium dioxide Smart window Thermochromic property Metal-insulator transition

a b s t r a c t Silicon doped vanadium dioxide films were successfully prepared on indium tin oxide coated glass substrates at 255 °C annealing temperature by direct current magnetron sputtering. Maintaining spheroidal grains, the size of VO2 films nanoparticles decreased with silicon doping ratios increasing. Transmittance spectra indicated a 46 nm blue-shift of absorption edge and improved the samples' integrated luminous transmittances (from 28.4% at Si/V = 0 to 36.1% at Si/V = 0.17) with similar thickness of 90 nm. The samples presented excellent thermochromic properties that possessed higher than 40% near-infrared switching efficiency at 2000 nm and showed at least 9.2% solar modulation efficiency. Moreover, the metal–semiconductor phase transition temperature for heavily doped VO2 (Si/V = 0.17) decreased to 46.1 °C (22 °C lower than bulk). These features suggest the practical application of VO2 to smart windows. © 2015 Elsevier B.V. All rights reserved.

1. Introduction Vanadium dioxide (VO2) behaving excellent thermochromic property is the most promising material in solar energy conservation [1,2]. Undergoing a fully reversible first-order transition at a critical temperature (τc, 68 °C for bulk) [3,4], VO2 exhibits a structure distortion between a low temperature monoclinic phase (P21/c, M1) and a hightemperature rutile phase (P42/mnm, R), which leads to a corresponding change of near-infrared (NIR) spectrum at undercooling condition to a non-transparent NIR waveband at superheating ambient while keeping visible lucency [5]. In order to meet the utility of VO2, sol–gel method [6,7], sputtering deposition [8,9], chemical vapor deposition (CVD) [10,11], pulse laser deposition [12], and ion implantation [13] have been employed. Meanwhile great efforts have aimed to adjust suitable methods and apparatus for better performance of thermochromic VO2 films, such as polymer-assisted sol–gel deposition [14,15], atmospheric pressure or aerosol-assisted CVD [16,17]. However, VO2 film applied on practical thermochromic fenestration must satisfy the criteria that luminous transmittance (Tvis) and solar transmittance modulation (ΔTsol) should overpass 40% and 10% respectively [18]. Because of the discrepant trend between visible transmittance and solar modulation in vanadium dioxide films, thicker VO2 films present large solar modulation and a shortage of visible transmittance, while thinner films achieve visible criterion but a poor solar modulated ability. Additionally, the metal– semiconductor transition (MST) of vanadium dioxide should be decreased into vicinity of the comfort environment [18,19]. ⁎ Corresponding author. E-mail addresses: [email protected] (X. Wu), [email protected] (Z. Wu).

http://dx.doi.org/10.1016/j.surfcoat.2015.07.007 0257-8972/© 2015 Elsevier B.V. All rights reserved.

Recently, many approaches have been investigated into improving the visible and NIR performance [14,15,20–23]. It employs the strategies with a tunable thickness including the syntheses of nanoporous films, doping and deposition of antireflection layers. Nanoporous VO2 films with a suitable thickness attain a low optical constants prepared by polymer-assisted deposition, and as a consequence, less reflection and well visible transmittance (43.3% at 20 °C) make luminous properties of VO2 film better [23]. Doping is a feasible solution to achieve excellent thermochromic films. Gao et al. synthesizes quasi-spherical VO2 nanoparticles with highly crystallinity and uniform size by controlling phase and shape of VO2 nanostructures by antinomy doping [24]. To overcome the Tvis deficiency of VO2 film, Zr element has been introduced to prepare a composite VO2–ZrV2O7 film with a ratio of Zr/V = 0.12, and the doping method improves Tvis to 53.4% with 4.8% ΔTsol [14]. 1.1% Ti doping VO2-polyurethane thermochromic composited foils achieve a 15% enhancement in visible transmittance and a 28% improving in solar modulation [25]. Moreover, VO2 films doped high valence cations, such as W6 +, Mo6 +, Ta5 + and Ru4 +, decreased MST temperature [18]. Among them, tungsten doping has the largest MST temperature fall-off rate and has been decreased to ~25 °C [26]. Another solution to achieve well thermochromic performance is the addition of antireflective coating (ARC) layers. TiO2, CeO2 and SiO2 which possessed the suitable refractive index between air and VO2 films are the most efficiency ARC coating [27–29]. The Tvis of VO2 films increased with a nearly unchanged ΔTsol adding SiO2 antireflection layer. There still are several troubles to prevent VO2 from smart window applications. Ti doping VO2 films acquire both excellent visible and NIR performance, at the same time lead an increasing trend of MST temperature [15]. Zr doping vanadium dioxide films present a poor solar modulation (less than 6.4%) accompanying a highly MST temperature

X. Wu et al. / Surface & Coatings Technology 276 (2015) 248–253

2. Experimental details The samples of Si-doped VO2 films on ITO-coated glass substrates were prepared by DC magnetron sputtering technique. A V metallic disk target (99.99% purity) was attached on high resistance silicon chips (N 10000 Ω·cm, 5 mm × 2 mm, 0.5 mm thickness) to control Si doping ratios [31]. ITO-coated glasses were cleaned ultrasonically in detergent solution, acetone, ethanol, and deionized water. After drying in a high pressure N2 gas followed, the substrates were placed into the reactive chamber immediately. Then the chamber was evacuated to a base pressure of 1 × 10−3 Pa, high-purity Ar and O2 gases as working and reactive gases were introduced individually into chamber by using two mass flow controllers. The target was pre-sputtered for 15 min to avoid contaminations. The substrate temperature, deposition time, Ar/O2 flow ratio and chamber pressure were kept at 100 °C, 15 min, 98/1 and 1.8 Pa, respectively. After that, ultrapure oxygen ambient and a low substrate temperature of 255 °C had been employed to a 30 min in-situ annealing treatment under a pressure of 3.3 Pa. The crystal information was determined using an X-ray diffraction diffractometer (XRD, Shimadzu's XRD-7000). Raman shift spectra were performed using a 532.2 nm laser on a Raman microscope spectrometer (Horibar Corporation, LabRAM HR800). X-ray photoelectron spectroscopy (XPS, XSAM 800) was applied to characterize the chemical compositions of the deposited films. The morphology of the films was determined by field emission scanning electron microscopy (FE-SEM, FEI Inspect F). Energy dispersive spectrometer (EDS, Oxford instruments X-Max 51-XMX0019) was determined the ratio of silicon. The optical transmittance and reflectance were measured at wavelengths of 300–2500 nm at temperature of 25 °C and 90 °C by a spectrophotometer (Perkin Elmer, Lambda 75). The hysteresis loop in transmittance was measured by UV-visible spectrometer (Shimadzu Corporation, Pharma Spec-1700) at a fixed wavelength of 1100 nm with a traditional plate heating. 3. Results and discussion 3.1. Confirmation of structure Fig. 1 shows the XRD patterns of VO2 samples with different Si/V ratios. All the recorded XRD patterns have obvious backgrounds information of ITO-coated glass. Without silicon doping, a preferred (011) direction of monoclinic VO2 was formed despite of several indium tin oxide peaks. It indicates that the films possessed a polycrystalline structure with fine grains. For the sample with an additive mount of silicon to a ratio of Si/V = 0.03, the VO2 diffraction peaks at 2θ = 27.8° become weak and the full width at half maximum (FWHM) of peaks gradually widen from 0.23° to 0.40°, indicating a decrease of crystallite size. According to Scherrer's formula, average crystallite size along VO2 (011) peak is varied from 35 nm to 20 nm. Moreover, the diffraction peak of

1000

Intensity (a.u.)

800

600

400

200

0

20

40

60

80

Diffraction Angle 2 /deg. Fig. 1. XRD pattern of VO2 films with different Si/V ratios deposited on ITO-coated glass substrate.

VO2 (200) appeared in Si doping films and no other crystal of vanadium oxide had been detected. As the additive amount of silicon to a ratio of Si/V = 0.11, the preferred orientation of (011) direction disappeared. The (011) direction was hardly distinguished with silicon doping increased to the ratio of Si/V = 0.17. Room temperature Raman spectra also have been used to characterize the existence of vanadium oxide. As shown in Fig. 2, a pure VO2 film showed Raman shifts at around 137, 193, 223, and 390 cm− 1, which verified that the dominant phase is VO2 (M) [17,32]. After Si doping with the ratio Si/V = 0.11, the other Raman shifts at 280, 300, 402, 521, and 993 cm−1 appeared. Although the shifts are characteristic for V2O5, the shift of VO2 (M) at 138 cm− 1 remains significantly strong. No Raman spectra shifts for SiO2 have been identified. XPS spectra were employed to analyze the detailed chemical compositions and valence state of vanadium. Fig. 3(a) shows a wide range XPS spectrum of undoped VO2 film. There are vanadium, oxygen, carbon, and indium, where indium and carbon are attributed to indium tin oxide substrate and surface contamination, respectively. Fig. 3(b) shows the high resolution scan of undoped VO2 film V2p3/2 core levels whose peak positions are fitted by Shirley function with software XPS peak 4.1. In Fig. 3(b), the vanadium element exists in two valence states, that are +4 valences with a binding energy 516.0 eV and +5 valences with a binding energy of 517.4 eV, and its +4 and +5 valences were evaluated with fractional percentages of 61.1% and 38.9%, respectively.

4500 138

4000

Intensity (a.u.)

[14]. For another solution, complicated manufacturing technique and precise thickness controlling are necessary for adding ARC coatings. However, inspired by the ARC coating of SiO2 and the structure of SiO2/VO2 core/shell structure [28,30], silicon introduced into vanadium dioxide films has a potential of thermochromic properties enhancing resulted from the transparency and refractive index of the SiO2 formation after doped VO2 films annealing at O2 ambient. With these considerations in mind, we investigated the samples of Si-doped VO2 film on indium tin oxide (ITO)-coated glass substrate deposited by direct current (DC) magnetron sputtering. A low temperature of 255 °C annealing process has been employed to prevent glasses from distortion and reducing the requirements of instrument. Compared with an undoped VO2 sample, 28.5% increases in Tvis maintaining 9.2% ΔTsol at the doping ratio of Si/V = 0.17, accompanied with a 10 °C decrease of MST temperature. Additionally, a conductive substrate enable to active controlling thermochromic variation.

249

Si/V=0 Si/V=0.11

3500 521 280

300

993

402

Si/V=0.11

3000 193 137

2500

223 193

390

200

400

Si/V=0

2000

600

800

1000 1200 1400

Raman Shift (cm-1) Fig. 2. Raman patterns of undoped and the ratio of Si/V = 0.11 VO2 films.

250

X. Wu et al. / Surface & Coatings Technology 276 (2015) 248–253 100k

15k

(b)

(a) V(LMM)

75k O(KLL)

10k

O1s

50k

V2p3/2(V5+)

V2p3/2(V4+)

V2s

Intensity (a.u.)

V2p C1s

25k

5k

In3d N1s

0k

1000

800

600

400

200

0

520

518

516

514

512

2500

(c) 100k

V3p V3s

(d)

Si2p 2000

V(LMM) O(KLL)

15k

1500

V2p3/2(V5+)

75k

V2p3/2(V4+)

O1s V2s

50k

108

104

100

96

10k

V2p C1s

25k In3dN1s

0k

1000

800

600

400

Si2s V3s Si2pV3p

200

0

5k

520

518

516

514

512

Binding Energy (eV) Fig. 3. XPS spectrum of undoped VO2 film (a) and the high-resolution profile V2p3/2 (b). (c) XPS spectrum of VO2 films with the ratio of Si/V = 0.11, (inset, Si 2p core level) and (d) its highresolution spectrum of the V2p3/2 core levels.

Fig. 3(c) shows the XPS spectrum of VO2 film with a ratio of Si/V = 0.11 and the inset is the high resolution scan of Si2p core level. The vanadium states were 40.6% for +4 valence and 59.4% for +5 valence in Fig. 3(d). This result was in accordance with the V5 + appearance of Raman shift, and the Si2p core suggested that silicon exists in the films with a formation of substitution solid solution.

3.2. Morphology Fig. 4 shows the SEM surface morphology for the deposited films on ITO-coated glass substrate. Keeping compact and spheroidal particles, particle size obviously decreased with increasing Si/V ratios. Randomly measuring the diameter of 600 grains consecutively starting from a

Fig. 4. SEM images of films with different Si/V ratios. (a) Si/V = 0, (b) Si/V = 0.03, (c) Si/V = 0.11, and (d) Si/V = 0.17.

X. Wu et al. / Surface & Coatings Technology 276 (2015) 248–253

251

corner of the SEM image, mean vanadium oxide particle size went down from 47 nm (Si/V = 0), 44 nm (Si/V = 0.03), 32 nm (Si/V = 0.11) to 28 nm (Si/V = 0.17). These changes suggested that Si doping has a great effection in the film particles formation process. Crystals with less defection could more easily reach a critical balance to grow into larger grains. However, the films' critical size of stable nuclei was well-known determined by both of crystalline growth speed and nucleation velocity. Introduced Si as new defection, the smaller grain size attained with a fixed condition because of the same growth speed which depended on deposition conditions and substrate. At Si/V = 0.17, the vanadium oxide film became more smoothly and their grain cannot recognize due to the fuzzy boundaries, most of particles were so small that it was hardly distinguish by SEM, and only a few of grains could separate out. It is good agreement with other reported results that doping can reduce the grain size [15]. 3.3. Optical properties 90% of total solar energy distributed between 250 nm and 1500 nm in the solar irradiance [33]. It indicated that the solar modulation ability of VO2 was determined by the difference of transmittance from insulator phase of undercooling to superheating after transition to metal phase. Especially in near-IR region (NIR), vanadium dioxide exhibited large variance transmittance across MST, while keeping an unchanged visible transmittance, i.e., a nearly fixed luminous transmittance. Therefore, the integrated optical properties of vanadium dioxide films were formulated by luminous (vis, 380–780 nm), solar (sol, 240–2500 nm) radiation and NIR switching efficiency. It can be obtained by the following equation that: Z Ti ¼

Z φi ðλÞT ðλÞdλ= φi ðλÞdλ

ð1Þ

ΔT i ¼ T i ðT b T c Þ−T ðT N T c Þ

ð2Þ

where T is the transmittance, i denotes vis or sol for the calculation, φvis is the light-adapted human eye's spectral sensitivity, i.e., standard luminous efficiency function, and φsol is the solar irradiance spectrum for air mass 1.5 (corresponding to the sun standing 37° above the horizon). The optical and MST properties of silicon-doped vanadium dioxide films were summarized in Table 1. Fig. 5 shows the transmittance spectra of composite films with different Si/V ratios. The samples' films thickness was measured to be around 90 nm. With Si/V ratios increasing, Tvis significantly increased from undoped sample's 28.4% to the ratio of Si/V = 0.17 sample's 36.1% (films Tvis with a 41.8%), gaining a 27% improvement. Room temperature reflectance spectra were presented in Fig. 6, and it decreased with increasing Si/V ratios, at the same time a reflectance valley shifted from 548 nm to 510 nm. Due to all samples with a similar thickness, the transmittance improving was attributed to silicon doping. Silicon was oxidized at the process of reactive sputtering and in-situ annealing in pure oxygen ambient and generated silicon-oxygen bond. First, the substitution of Si broke the structure of VO2 and replaced V–O bond to Si–O bond. Thus absorption efficiency of VO2 was cut down by the decreasing intensity of vanadium oxygen bond. It led to an improvement of visible

Fig. 5. Transmittance spectra for VO2 samples with different Si/V ratios. The solid and dashed lines are measured at 25 °C and 90 °C, respectively.

transmittance. Second, XPS and Raman result shows that the addition of silicon promoted the component of amorphous + 5 states vanadium. Compared with vanadium dioxide, vanadium oxide with +5 states possessed higher transmittance in visible spectrum. Both of them resulted in the improvement of the samples luminous properties. In addition, from Fig. 5, by simply calculating the wavelength of maximum value of the first derivative of transmittance spectra, the silicon doped vanadium oxide films absorption edge (λk) shifted 46 nm from 510 nm (Si/V = 0) to 464 nm (Si/V = 0.17). The blue shift resulted in another enhancement of visible luminous. As reported, dopants are sometimes induced by a blue-shift spectrum. Mg doping of VO2 films enhances optical transmittance with a blue-shift in the absorption edge from 490 nm to 440 nm [34]. Doping with F at 2.0% also generates a blue-shift in the absorption edge of vanadium dioxide from 445 nm to 415 nm, leading to an increased Tvis [35]. The absorption edge of vanadium oxide films was ruled by scattering effects and absorption. Scatter effects dominated in large particles while absorption dominated for small ones [36]. Characterized the morphology of vanadium dioxide films, its grain sizes were hugely changed. With similar ingredient films, the absorption maintained a constant. Long wavelength radiation was easier scattered by larger grain size so that undoped VO2 film had an absorption edge at the side of long wavelength. Then silicon doping increased vanadium dioxide films' defect-nucleation site density, causing a smaller crystal particle agreed with our previous reported zirconium doping [31]. Deposited in same ambient, small size particles formed with similar driving force and increasing defect-nucleation site density, leading to a blue shift of absorption edge. However, increasing luminous transmittance has a side-effect of NIR-light switching efficiency and solar modulation. The NIR-light switching efficiency was defined as the difference in the NIR-light transmittance at 2000 nm before and after the MST. A pure VO2 film, improving Tvis to 41% to satisfy the criterion of luminous, was inevitable deteriorating ΔTsol to 6.7% [22]. V1 − xTixO2 films NIR-light switching efficiency were changed from 50% to 34% at a doping ratio of x = 0.091 [15]. Zr doped VO2 films elevated Tvis from 35.4% to 53.4% while it

Table 1 Optical and MIT properties of silicon-doped vanadium dioxide films. Sample

Doping ratio (Si/V)

I II III IV

0 0.03 0.11 0.17

Tvis (%)

Tsol (%)

25 °C

90 °C

25 °C

90 °C

28.4 29.6 32.1 36.1

24.5 25.6 28.4 33.3

34.4 34.9 37.3 39.7

21.9 22.7 26.0 30.5

ΔTsol (%)

Transmittance at 2000 nm (%) 25 °C

90 °C

12.5 12.2 11.3 9.2

57.3 57.4 58.1 55.9

7.0 6.4 10.2 15.0

Transition temperature ± half hysteresis width (°C)

Absorption edge (λk/nm) 25 °C

90 °C

57 ± 6.25 53.4 ± 6.1 47.2 ± 6.45 46.1 ± 7.2

510 500 488 464

512 504 502 486

252

X. Wu et al. / Surface & Coatings Technology 276 (2015) 248–253

increased transmittance. Meanwhile, IR switching efficiency of Si doping VO2 films merely dropped from 64.9% with pure to 59.7% with Si/V = 0.11 at 2500 nm. It is stable compared with W-doped VO2 film IR switching efficiency as ~ 35% (1% [38] and others W-doped [16,37, 39]) or even 23% (2 wt.% W-doped) [40]. In general, Si doping VO2 films offered an efficient way to acquire excellent thermochromic VO2 films. 4. Conclusion

Fig. 6. Reflectance spectra for VO2 samples with different Si/V ratios.

possessed a poor ΔTsol decreasing from 6.0% to 4.8% [14]. In the present work, combining the ITO-coated glass transmittance, our samples' NIRlight switching efficiency varied in the range 40%–50%. In lightly doped VO2 (Si/V = 0.03) sample, the NIR switching ability was little changed. Moreover, the sample doping at a ratio of Si/V = 0.17, still remained 40% switching ability. The NIR switching efficiency suggested a high performance of solar modulation (ΔTsol). With silicon doping ratios in the films, the solar modulation efficiencies of samples exceeded 9.2% (Si/V = 0.17), comparing with pure VO2 films' 12.5%. It indicated that VO2 with silicon doping enhanced luminous properties and maintained NIR-light efficiency and solar modulation. For application of vanadium dioxide films to smart window, the SMT temperature should be adjusted. In the present work, the undoped vanadium oxide film on ITO-coated glass exhibited a hysteresis loop centered at 57 °C with a width of 12.5 °C. Moreover, adding silicon to the ratios of Si/V = 0.17, the MST temperature decreased to 46.1 °C with a width of 14.4 °C, implying a decrease in the phase transition temperature of about 11 °C, which is 22 °C lower than of single crystal (68 °C), as shown in Fig. 7. Maintaining the spheroidal grain particles, the hysteresis loop nearly unchanged. The phenomenon is resulted from the imperfection of crystal structure and interfacial energy changed. Compared with tungsten (W) doping vanadium dioxide films, silicon doping films indicated same trend of decreasing SMT temperature [37]. As tungsten content increased, VO2 films decreased the transmittance of room temperature (~ 25 °C), whereas Si doping VO2 films

Fig. 7. Hysteresis loops for VO2 samples of different Si/V ratios. Curves with solid symbols represent the transmittance variation at heating branch while hollow symbols at cooling branch.

Silicon doped vanadium dioxide films with different Si/V ratios have been successfully prepared at a low annealing temperature of 255 °C on ITO-coated glass by DC magnetron sputtering and characterized using various techniques. The XRD, Raman and XPS analyses indicate that silicon doping existed in the formation of a substitution solid solution. The addition of silicon greatly influenced the film microstructure. With similar thickness, the luminous transmittance of samples increased 27% due to silicon doping and the blue-shift of absorption-edge induced by grain sizes decreasing. Additionally, VO2 films remain excellent solar modulation efficiency and decrease MST temperature to 46.1 °C, which is suitable for the requirements of practical thermochromic material. The results indicate that Si doping is an efficient way to satisfy the application of VO2 films to smart windows. Acknowledgment This work was supported by National Natural Science Foundation of China (Grant nos. 61235006 and 61421002). References [1] M. Kamalisarvestani, R. Saidur, S. Mekhilef, F.S. Javadi, Performance, materials and coating technologies of thermochromic thin films on smart windows, Renew. Sust. Energ. Rev. 26 (2013) 353–364. [2] E.S. Lee, X. Pang, S. Hoffmann, H. Goudey, A. Thanachareonkit, An empirical study of a full-scale polymer thermochromic window and its implications on material science development objectives, Sol. Energy Mater. Sol. Cells 116 (2013) 14–26. [3] M. Maaza, K. Bouziane, J. Maritz, D.S. McLachlan, R. Swanepool, J.M. Frigerio, M. Every, Direct production of thermochromic VO2 thin film coatings by pulsed laser ablation, Opt. Mater. 15 (2000) 41–45. [4] F.J. Morin, Oxides which show a metal-to-insulator transition at the neel temperature, Phys. Rev. Lett. 3 (1959) 34–36. [5] Y. Gao, H. Luo, Z. Zhang, L. Kang, Z. Chen, J. Du, M. Kanehira, C. Cao, Nanoceramic VO2 thermochromic smart glass: a review on progress in solution processing, Nano Energy 1 (2012) 221–246. [6] N. Wang, S. Magdassi, D. Mandler, Y. Long, Simple sol–gel process and one-step annealing of vanadium dioxide thin films: synthesis and thermochromic properties, Thin Solid Films 534 (2013) 594–598. [7] L. Kang, Y. Gao, H. Luo, A novel solution process for the synthesis of VO2 thin films with excellent thermochromic properties, ACS Appl. Mater. Interfaces 1 (2009) 2211–2218. [8] H. Zhang, Z. Wu, Q. He, Y. Jiang, Preparation and investigation of sputtered vanadium dioxide films with large phase-transition hysteresis loops, Appl. Surf. Sci. 277 (2013) 218–222. [9] S. Chen, H. Ma, J. Dai, X. Yi, Nanostructured vanadium dioxide thin films with low phase transition temperature, Appl. Phys. Lett. 90 (2007) 101117. [10] M. Saeli, C. Piccirillo, I.P. Parkin, I. Ridley, R. Binions, Nano-composite thermochromic thin films and their application in energy-efficient glazing, Sol. Energy Mater. Sol. Cells 94 (2010) 141–151. [11] M.E.A. Warwick, R. Binions, Chemical vapour deposition of thermochromic vanadium dioxide thin films for energy efficient glazing, J. Solid State Chem. 214 (2014) 53–66. [12] Y.L. Wang, X.K. Chen, M.C. Li, R. Wang, G. Wu, J.P. Yang, W.H. Han, S.Z. Cao, L.C. Zhao, Phase composition and valence of pulsed laser deposited vanadium oxide thin films at different oxygen pressures, Surf. Coat. Technol. 201 (2007) 5344–5347. [13] R. Lopez, L.A. Boatner, T.E. Haynes, R.F. Haglund, L.C. Feldman, Enhanced hysteresis in the semiconductor-to-metal phase transition of VO2 precipitates formed in SiO2 by ion implantation, Appl. Phys. Lett. 79 (2001) 3161. [14] J. Du, Y. Gao, H. Luo, Z. Zhang, L. Kang, Z. Chen, Formation and metal-to-insulator transition properties of VO2–ZrV2O7 composite films by polymer-assisted deposition, Sol. Energy Mater. Sol. Cells 95 (2011) 1604–1609. [15] J. Du, Y. Gao, H. Luo, L. Kang, Z. Zhang, Z. Chen, C. Cao, Significant changes in phasetransition hysteresis for Ti-doped VO2 films prepared by polymer-assisted deposition, Sol. Energy Mater. Sol. Cells 95 (2011) 469–475. [16] C.S. Blackman, C. Piccirillo, R. Binions, I.P. Parkin, Atmospheric pressure chemical vapour deposition of thermochromic tungsten doped vanadium dioxide thin films for use in architectural glazing, Thin Solid Films 517 (2009) 4565–4570.

X. Wu et al. / Surface & Coatings Technology 276 (2015) 248–253 [17] C. Piccirillo, R. Binions, I.P. Parkin, Synthesis and functional properties of vanadium oxides: V2O3, VO2, and V2O5 deposited on glass by aerosol-assisted CVD, Chem. Vap. Depos. 13 (2007) 145–151. [18] S.Y. Li, G.A. Niklasson, C.G. Granqvist, Thermochromic fenestration with VO2-based materials: three challenges and how they can be met, Thin Solid Films 520 (2012) 3823–3828. [19] I.P. Parkin, T.D. Manning, Intelligent thermochromic windows, J. Chem. Educ. 83 (2006) 393–400. [20] M.E.A. Warwick, R. Binions, Advances in thermochromic vanadium dioxide films, J. Mater. Chem. A 2 (2014) 3275–3292. [21] N.R. Mlyuka, G.A. Niklasson, C.G. Granqvist, Mg doping of thermochromic VO2 films enhances the optical transmittance and decreases the metal-insulator transition temperature, Appl. Phys. Lett. 95 (2009) 171909. [22] N.R. Mlyuka, G.A. Niklasson, C.G. Granqvist, Thermochromic multilayer films of VO2 and TiO2 with enhanced transmittance, Sol. Energy Mater. Sol. Cells 93 (2009) 1685–1687. [23] L. Kang, Y. Gao, H. Luo, Z. Chen, J. Du, Z. Zhang, Nanoporous thermochromic VO2 films with low optical constants, enhanced luminous transmittance and thermochromic properties, ACS Appl. Mater. Interfaces 3 (2011) 135–138. [24] Y. Gao, C. Cao, L. Dai, H. Luo, M. Kanehira, Y. Ding, Z.L. Wang, Phase and shape controlled VO2 nanostructures by antimony doping, Energy Environ. Sci. 5 (2012) 8708–8715. [25] S. Chen, L. Dai, J. Liu, Y. Gao, X. Liu, Z. Chen, J. Zhou, C. Cao, P. Han, H. Luo, M. Kanahira, The visible transmittance and solar modulation ability of VO2 flexible foils simultaneously improved by Ti doping: an optimization and first principle study, Phys. Chem. Chem. Phys. 15 (2013) 17537–17543. [26] Z.F. Peng, W. Jiang, H. Liu, Synthesis and electrical properties of tungsten-doped vanadium dioxide nanopowders by thermolysis, J. Phys. Chem. C 111 (2007) 1119–1122. [27] H. Koo, D. Shin, S.-H. Bae, K.-E. Ko, S.-H. Chang, C. Park, The effect of CeO2 antireflection layer on the optical properties of thermochromic VO2 film for smart window system, J. Mater. Eng. Perform. 23 (2013) 402–407. [28] D. Li, Y. Shan, F. Huang, S. Ding, Sol–gel preparation and characterization of SiO2 coated VO2 films with enhanced transmittance and high thermochromic performance, Appl. Surf. Sci. 317 (2014) 160–166.

253

[29] Z. Chen, Y. Gao, L. Kang, J. Du, Z. Zhang, H. Luo, H. Miao, G. Tan, VO2-based doublelayered films for smart windows: optical design, all-solution preparation and improved properties, Sol. Energy Mater. Sol. Cells 95 (2011) 2677–2684. [30] Y. Gao, S. Wang, H. Luo, L. Dai, C. Cao, Y. Liu, Z. Chen, M. Kanehira, Enhanced chemical stability of VO2 nanoparticles by the formation of SiO2/VO2 core/shell structures and the application to transparent and flexible VO2-based composite foils with excellent thermochromic properties for solar heat control, Energy Environ. Sci. 5 (2012) 6104–6110. [31] H. Zhang, Z. Wu, X. Wu, X. Wei, Y. Jiang, Preparation and phase transition properties of nanostructured zirconium-doped vanadium oxide films by reactive magnetron sputtering, Thin Solid Films 568 (2014) 63–69. [32] G.I. Petrov, V.V. Yakovlev, J. Squier, Raman microscopy analysis of phase transformation mechanisms in vanadium dioxide, Appl. Phys. Lett. 81 (2002) 1023. [33] C.A. Gueymard, D. Myers, K. Emery, Proposed reference irradiance spectra for solar energy systems testing, Sol. Energy 73 (2002) 443–467. [34] J. Zhou, Y. Gao, X. Liu, Z. Chen, L. Dai, C. Cao, H. Luo, M. Kanahira, C. Sun, L. Yan, Mgdoped VO2 nanoparticles: hydrothermal synthesis, enhanced visible transmittance and decreased metal-insulator transition temperature, Phys. Chem. Chem. Phys. 15 (2013) 7505–7511. [35] W. Burkhardt, T. Christmann, S. Franke, W. Kriegseis, D. Meister, B.K. Meyer, W. Niessner, D. Schalch, A. Scharmann, Tungsten and fluorine co-doping of VO2 films, Thin Solid Films 402 (2002) 226–231. [36] R. Lopez, L. Feldman, R.F. Haglund, Size-dependent optical properties of VO2 nanoparticle arrays, Phys. Rev. Lett. 93 (2004) 177403. [37] T.D. Manning, I.P. Parkin, M.E. Pemble, D. Sheel, D. Vernardou, Intelligent window coatings: atmospheric pressure chemical vapor deposition of tungsten-doped vanadium dioxide, Chem. Mater. 16 (2004) 744–749. [38] R. Binions, C. Piccirillo, I.P. Parkin, Tungsten doped vanadium dioxide thin films prepared by atmospheric pressure chemical vapour deposition from vanadyl acetylacetonate and tungsten hexachloride, Surf. Coat. Technol. 201 (2007) 9369–9372. [39] Z. Huang, C. Chen, C. Lv, S. Chen, Tungsten-doped vanadium dioxide thin films on borosilicate glass for smart window application, J. Alloys Compd. 564 (2013) 158–161. [40] J.Q. Shi, S.X. Zhou, B. You, L.M. Wu, Preparation and thermochromic property of tungsten-doped vanadium dioxide particles, Sol. Energy Mater. Sol. Cells 91 (2007) 1856–1862.