Design of Antireflection and Enhanced Thermochromic Properties of TiO2/VO2 Thin Films

VO2‐based thin films exhibit great potential applications in thermochromic smart windows. However, it is still a challenge to synergistically achieve the high luminous transmittance (Tlum) and large solar modulation (ΔTsol). In this paper, antireflective (AR) TiO2/VO2 thin films are designed through optical simulations, and the thin films with optimized film thickness are prepared by magnetron sputtering. The overall performances of the thin films are significantly improved. Compared with single‐layer VO2, the Tlum value of the TiO2/VO2 thin film is significantly increased from 29.03% to 46.29% by 59.5%, and the ΔTsol value is increased up to 16.03%. The Tlum and ΔTsol values of the TiO2/VO2/TiO2 sandwiched thin films can be further improved up to 50.49% and 20.11%, respectively, owing to the light interference at the interfaces between the multilayers. The results provide an effective strategy for improving the performance of smart windows.


Introduction
As a typical thermochromic material, vanadium dioxide (VO 2 ) can undergo reversible structural phase transition (SPT) from monoclinic phase (M 1 ) to rutile phase (R) at a critical temperature (T c ) of 68 °C. Meanwhile, metal-insulator phase transition (MIT) occurs, [1] resulting in sudden change of the electrical and optical properties. [2,3] At the temperature lower than T c , VO 2 is an insulator with high optical transmittance; but will be transformed into a metal with higher optical reflectance at the temperature above T c . The phase transition in VO 2 can also be triggered by force, light and electricity. [4][5][6] The phase transition endows VO 2 with potential applications in spacecraft thermal radiation device, [7] supercapacitor, [8] electrode material, [9,10] optical modulator, [11] optical switch, [12] microwave passive electronic device, [13] phase change memory, [14] field www.advmatinterfaces.de reaction magnetron sputtering. 200 nm TiO 2 / 100 nm VO 2 thin films show excellent overall thermochromic properties with the T lum value of 46.29% and the ΔT sol value of 16.03%, which is consistent with the simulation results. The T lum and ΔT sol values of TiO 2 /VO 2 /TiO 2 sandwiched thin films can be up to 50.49% and 20.11%, respectively. The design, preparation, and improvement mechanism of the films are discussed.

Design of Thin Films by Theoretical Simulations
Using the optical characteristic matrix and the optical constants, the T lum and ΔT sol values of VO 2 thin films with different thicknesses are calculated according to the Equations (1) and (2), and Figure 1a shows the results. The T lum value of VO 2 thin films is reduced with increasing film thickness. However, the ΔT sol value increases firstly and then decreases with increasing film thickness, owing to the enhanced absorption and scattering of sunlight. [37] Therefore, the thickness of VO 2 thin films should be optimized to achieve both high T lum and ΔT sol . Herein, the optimized thickness of VO 2 thin films is 100 nm, for which the T lum and ΔT sol can be well balanced with the values of 33.53% and 14.81%, respectively. Figure 1b shows the optical transmittance of the VO 2 thin film with a thickness of 100 nm. A large gap exists in the transmittance spectra of infrared band at 90 °C and 20 °C. As shown in Figure S1 (Supporting Information), the suitable refractive index (n) of the AR film for VO 2 film is 2.0-2.5. Because the refractive index of the as-prepared TiO 2 films is 2.1-2.2, and the preparation of TiO 2 is simple and economical, TiO 2 is selected as the AR film. Figure 1c illustrates the optical properties of TiO 2 /VO 2 thin films. As the thickness of TiO 2 is increased from 0 to 500 nm, the ΔT sol value increases firstly, and then remains at a constant value, that is, it is feasible to improve the solar modulation property of VO 2 thin films through adjusting the thickness of TiO 2 . As displayed in Figure S2 (Supporting Information), the T lum value changes periodically with increasing thickness of TiO 2 due to the interference effect. Referencing to Figure 1c, the highest T lum can be obtained at the TiO 2 film thickness of around 50 nm. However, if both T lum and ΔT sol are taken into account, the optimized thickness of TiO 2 should be at about 200 nm. So TiO 2 antireflective films with the thickness of 50-200 nm are involved in the following to study the effects of AR film thickness. Figure 1d shows the calculated transmittance curves of the TiO 2 /VO 2 thin films with the TiO 2 thickness of 0, 50, 100, 150, and 200 nm, at 20 °C. The transmittance curves in visible light are consistent with the above analysis. For the TiO 2 film thickness of 50 nm and 200 nm, the transmittance curves have a peak at around 550 nm, accordingly these two film systems have higher T lum . Moreover, a transmittance peak is evidenced in the near-infrared light band, and it will be shifted to higher band with increasing TiO 2 thickness. The transmittance to infrared radiation is increased at 20 °C, but not at 90 °C ( Figure S3, Supporting Information), due to the temperature www.advmatinterfaces.de dependent refractive index of VO 2 . Hence, the TiO 2 /VO 2 thin films can be designed to play an AR role at 20 °C, but not at 90 °C. So, the modulation ability of VO 2 to infrared radiation is improved by the thicker TiO 2 antireflection layer. The T lum and ΔT sol values of the TiO 2 /VO 2 thin films with the TiO 2 film thickness of 200 nm can be up to 52.5% and 20.2% respectively, which is higher than that of VO 2 films by 56.58% and 36.39%. Figure 2a shows the X-ray diffraction (XRD) patterns of the V thin films after thermal annealing at 550 °C but at different oxygen partial pressure for 2 hours. When the thermal annealing is done at the oxygen partial pressure of 10 Pa, the XRD peaks can be ascribed to V 2 O 3 (JCPDS no.71-0345) and VO 2 (M) (JCPDS no.75-0514). [23] The XRD peaks of VO 2 (M) are significantly enhanced at the oxygen partial pressure of 15 Pa. However, at the oxygen partial pressure of 20 Pa, the XRD peaks at 30.94° and 34.16° are ascribed to V 2 O 5 (JCPDS no.89-0611), owing to excessive oxygen. Based on this, the oxygen partial pressure of 15 Pa is adopted in the following experiments, and the annealing time is elongated appropriately. Figure 2b shows the XRD pattern of the film annealed at 550 °C for 4 h, the XRD peaks of VO 2 (M) are substantially enhanced, indicating the formation of highly crystalline VO 2 (M). Figure 2c,d shows the survey X-ray photoelectron spectroscopy (XPS) spectra and the XPS spectra of V 2p and O 1s. O and V are from the VO 2 films, but C is from the surface contamination. Two splitting energy levels of V2p are evidenced, and the binding energy difference between V 2p 3/2 and O 1s is 13.87 eV, confirming the formation of VO 2 (M). [38,39] Figure S4 (Supporting Information) shows the XPS spectra along the depth. The close position of V indicates the uniform composition in the films. Figure 3a displays the scanning electron microscopy (SEM) images of the thin films, and Figure 3b shows the particle size distribution. The grain size in the films increases with the oxygen partial pressure and annealing time gradually. As displayed in the inset of Figure 3a, the transmittance of the films is improved with increasing oxygen partial pressure. When the V film is annealed under 20 Pa oxygen partial pressure, the film becomes light gray rather than the yellowish brown of VO 2 film. Figure 3c shows the element distribution in the annealed thin films, and V and O are uniformly distributed. Figure 3d shows the SEM image of the cross-section of the thin film, confirming that the thickness of the VO 2 thin film is about 100 nm. Figure 4a,b displays the survey XPS spectra and the XPS spectra of Ti 2p. The peaks of Ti 2p, Ti 2s, and O 1s orbitals  Figure 4c shows the thickness of TiO 2 thin films as a function of sputtering time, and the deposition rate is calculated to be 90 nm h −1 . Figure 4d,e shows the SEM image and the elemental distribution in the TiO 2 thin films. The film surface is very dense, and Ti and O are uniformly distributed, indicating the uniform composition in the film.

Performances of VO 2 and TiO 2 /VO 2 Thin Films
As shown in Figure 5a, the transmittance curves of the thin films at 20 °C and 90 °C show great difference in the infrared band, owing to the phase transition. The T lum value of the VO 2 thin film with thickness of 100 nm is 29.03%, and the ΔT sol is 15.8%, which is in good agreement with the calculated results. Figure 5b shows the transmittance curves of TiO 2 /VO 2 thin films with the TiO 2 thickness of 50, 60, and 70 nm and Figure 5c displays the magnified curves in the visible band. The transmittance curves of both TiO 2 /VO 2 thin films and VO 2 thin films show a peak in the visible band, while there is little difference in the infrared band. However, the transmittance of TiO 2 / VO 2 thin films is significantly increased in the visible band due to the strongly suppressed reflectance, and the peak value of the visible transmittance becomes nearer to the peak position of the standard spectral sensitivity of the light-adapted eye at 555 nm ( Figure S5, Supporting Information), contributing to the T lum enhancement effect. If the thickness of TiO 2 AR layer is increased up to 60 nm, the T lum of TiO 2 /VO 2 thin films is up to 47.06%, which is higher than that of VO 2 thin film by 62.11%, with the ΔT sol value of 14.36%. If the 200 nm thick AR TiO 2 layer is utilized, the transmittance of the TiO 2 /VO 2 thin films changes greatly. As shown in Figure 5d, the transmittance of TiO 2 /VO 2 thin films is significantly increased in the visible band, and a peak emerges in the near infrared band at 20 °C, but changes little at 90 °C. Figure 5e depicts the enlarged transmittance curve in the visible band. For the 200 nm thick TiO 2 AR layer, the transmittance peak appears at about 600 nm in visible range. The T lum and ΔT sol values of TiO 2 /VO 2 (200/100 nm) thin films can be up to 46.29% and 16.03%, respectively, which are substantially improved, as compared with VO 2 thin films. As shown in Figure 5f, the measured T lum and ΔT sol values of the TiO 2 /VO 2 thin films change almost with the same trend as that of the simulation results.

Optical Properties of TiO 2 /VO 2 /TiO 2 Sandwiched Thin Films
Based on the above results, if the AR TiO 2 films are deposited on both sides of VO 2 thin films, forming TiO 2 /VO 2 /TiO 2

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sandwiched thin films, the optical properties might be further improved. The eigenmatrix method is adopted to predict the performances of the film systems. Figure 6a,b shows the calculated T lum and ΔT sol values of the films, respectively. The optical properties of the TiO 2 /VO 2 /TiO 2 sandwiched thin films depend on the TiO 2 thickness, which is similar to that of TiO 2 /VO 2 . For thinner TiO 2 thin films, the TiO 2 /VO 2 /TiO 2 sandwiched thin films show better antireflection but degraded modulation. As shown in Figure 6c, the AR effect of the TiO 2 /VO 2 /TiO 2 sandwiched thin films with both the upper and lower TiO 2 layers of 50 nm is the best, with T lum up to 69.49%, which is higher than that of VO 2 by 107.25%, but the ΔT sol is only 11.49%. The modulation can be improved if thicker TiO 2 films are utilized. Figure 6d shows the transmittance curves of the TiO 2 /VO 2 /TiO 2 sandwiched thin films with the TiO 2 film thickness of 100, 150, and 200 nm. Two peaks appear in the visible and near-infrared band. For the film system with the TiO 2 film thickness of 200 nm, the T lum and ΔT sol can be up to 60.20% and 18.72%, respectively, which are higher than that of VO 2 by 79.54% and 26.40%.
The TiO 2 /VO 2 /TiO 2 sandwiched thin films are prepared by magnetron sputtering. Figure 7a,b displays the SEM images of TiO 2 /VO 2 /TiO 2 thin films. Figure 7c shows the SEM images of the cross-section of TiO 2 (200 nm), VO 2 /TiO 2 (100/200 nm), and TiO 2 /VO 2 /TiO 2 (200/100/200 nm) thin films, and the interfaces between thin films are clear. Figure 7d shows the optical photographs of the naked glass (d1), VO 2 thin film (d2), TiO 2 /VO 2 films (d3-d4), and TiO 2 /VO 2 /TiO 2 sandwiched films (d5-d6). Compared with VO 2 thin film, the visible light transmittance of TiO 2 /VO 2 thin films is improved substantially, particularly, the TiO 2 /VO 2 /TiO 2 (50/100/50 nm) thin film sample. Figure 8a shows the XRD patterns of TiO 2 , VO 2 , and TiO 2 / VO 2 /TiO 2 sandwiched thin films for comparison. The peaks of TiO 2 (A) and VO 2 (M) are evidenced in the TiO 2 /VO 2 /TiO 2 thin films. The TiO 2 thin film under VO 2 is in anatase structure, but the TiO 2 thin film on VO 2 is amorphous if no annealing is done. The refractive indices of both polycrystalline and amorphous TiO 2 films in this work are in the range of 2.1-2.2, and thus affect the antireflection little. The infrared (IR) transmittance at 2500 nm against temperature is measured for the VO 2 and TiO 2 /VO 2 /TiO 2 thin films, and the results are shown in Figure 8b. The phase transition temperatures of the VO 2 and TiO 2 /VO 2 /TiO 2 thin films are 68.5 °C and 72 °C, respectively. As compared with VO 2 (M), the phase transition temperature of TiO 2 /VO 2 /TiO 2 thin films is slightly elevated with widened hysteresis, which might be ascribed to two reasons. Firstly, since the thermal expansion coefficient of VO 2 is larger than that of TiO 2 , the thermal compressive stress is generated in the  www.advmatinterfaces.de VO 2 thin films during heating process, and hinders the phase transition in VO 2 . [21] Secondly, the interdiffusion at the interface between VO 2 and TiO 2 affects the phase transition temperature. Since the radius of Ti 4+ is smaller than that of V 4+ , substitution doping will occur when Ti 4+ ions are diffused into VO 2 , resulting in slightly shortened V-O bonds in VO 2 and more difficult phase transition. This leads to slightly elevated phase transition temperature and widened hysteresis of the TiO 2 / VO 2 /TiO 2 thin films. [40] Figure 8c displays the transmittance curves of the TiO 2 /VO 2 /TiO 2 thin films, and Figure 8d summarizes the optical properties for comparison. The T lum and ΔT sol values of TiO 2 /VO 2 /TiO 2 (50/100/50 nm) sandwiched thin films are 50.49% and 17.49%, while those of TiO 2 /VO 2 / TiO 2 (200/100/200 nm) sandwiched thin films are 47.65% and 20.11%, which are better than the reported results of TiO 2 /VO 2 / TiO 2 (100/150/190 nm) thin film with T lum of 30.1% and ΔT sol of 10.2%. [41] Figure 9 summarizes the calculated and experimental results, as well as those reported in references, for comparison. Generally, the thin films reported previously could not have high T lum and large ΔT sol simultaneously. The thin films designed and prepared in this work can overcome this issue. The T lum and ΔT sol values of the TiO 2 /VO 2 (200/100 nm) thin films can be up to 46.29% and 16.03%, and can be further improved up to 47.65% and 20.11%, respectively, for TiO 2 /VO 2 /TiO 2 (200/100/200 nm) sandwiched thin films, which are far beyond the available data. Therefore, TiO 2 /VO 2 /TiO 2 sandwiched thin films exhibit potential applications in smart windows.

Conclusion
In this work, the optical properties of TiO 2 /VO 2 thin films are studied by optical simulation and experimental measurements. It is found that TiO 2 thin films could effectively modulate the optical properties of VO 2 owing to the enhanced antireflection. The T lum value of the TiO 2 /VO 2 thin films with the TiO 2 thickness of 200 nm can be up to 46.29%, which is increased by 59.4%, as compared with that of VO 2 , the ΔT sol value is also improved up to 16.03%. The optical properties of TiO 2 /VO 2 / TiO 2 sandwiched thin films can be further improved. The T lum and ΔT sol of the TiO 2 /VO 2 /TiO 2 (200/100/200 nm) sandwiched thin films can be up to 47.65% and 20.11%, respectively, owing to the enhanced reflection of light at the multiple interfaces. The results provide an idea for design of high-performance thermochromic thin films.

Experimental Section
Simulation Model and Method: The optical properties of VO 2 thin film and TiO 2 /VO 2 multilayer thin films were simulated using the transfer matrix method. [63] The optical constants (n and k) of VO 2 , TiO 2 , and the soda-lime glass were from the experimental results. [23] The thicknesses of VO 2 and TiO 2 thin films were changed in the ranges of 0-300 nm and 0-500 nm, respectively. In the calculations, wavelength, film thickness, and refractive index were input variables. For the multilayer thin films and for given optical characteristic matrix of each layer, the equivalent thin film characteristic matrix could be obtained by multiplying the matrices. Based on the calculated transmittance spectra, the luminous (lum, 380-780 nm) and solar (sol, 300-2500 nm) properties of the thin films were evaluated as: in which ϕ lum (λ) denotes the standard spectral sensitivity of the lightadapted eye, as shown in Figure S5 (Supporting Information); ϕ sol (λ) represents the solar irradiance spectrum for air mass 1.5 corresponding to the sun standing 37° above the horizon, as shown in Figure S6 (Supporting Information); [64] and T(λ) represents the transmittance of the thin film at wavelength λ. Accordingly, ΔT sol could be calculated from the transmittance at 20 °C and 90 °C, as: Preparation and Characterizations of the Thin Films: The TiO 2 /VO 2 multilayer thin films were prepared on the ordinary soda-lime glass by magnetron sputtering. Prior to the film deposition, the deposition chamber was evacuated to 2 × 10 −4 Pa, and 30 sccm pure Ar (99.9995%) was introduced to a pressure of 0.75 Pa. Firstly, V thin films were prepared by radio frequency (RF) sputtering of a vanadium target (d = 50 mm, 99.99% purity) with a power of 100 W for half an hour, and then the V thin films were thermally annealed in O 2 atmosphere, resulting in the formation of VO 2 thin films. Finally, TiO 2 thin films were deposited on the VO 2 thin films by reactive sputtering of Ti target (d = 50 mm, 99.99%) in Ar-O 2 mixture with 20% O 2 partial pressure. The preparation process of TiO 2 /VO 2 films is presented in Scheme 1, and the preparation conditions for VO 2 and TiO 2 are listed in Table S1 (Supporting Information). When preparing TiO 2 /VO 2 /TiO 2 sandwiched films, TiO 2 thin film was deposited by magnetron sputtering firstly and then V film. The TiO 2 /V thin films were annealed in oxygen, and then another TiO 2 thin film was deposited. The samples were not taken out from the magnetron sputtering cavity, ensuring the cleanliness between films.
During the sputtering process, a quartz oscillator was used to monitor the film thickness, and an ellipsometer was used to accurately determine the film thickness. The cross-section morphology of the thin films was analyzed by the field emission scanning electron microscopy (FESEM, JSM-7000F). The elemental composition and phase of the thin   [25,37, www.advmatinterfaces.de films were characterized by XPS, equipped with a monochromatic Al Kα source, operating at 12.5 kV/16 mA) and XRD (Bruker D8 Advance X-ray diffractometer with Cu Kα radiation at 1.542 Å). The surface morphology of the thin films was observed by FESEM, and the element distribution in the thin films was detected by the energy dispersive X-ray spectroscopy. The absolute spectral transmittance was measured by a Hitachi U-4100 spectrometer, in the wavelength range of 300-2500 nm, at 20 °C and 90 °C, corresponding to VO 2 (M) and VO 2 (R), respectively. The optical transition behavior was studied by measuring the IR transmittance at 2500 nm against the temperature with a heating-cooling rate of 2 °C min −1 , and the phase transition temperature (T c ) is defined as the average of the half-maximum temperature on the heating and cooling curves, as described previously. [63]

Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.