Hydrothermal growth of VO2 nanoplate thermochromic films on glass with high visible transmittance

The preparation of thermochromic vanadium dioxide (VO2) films in an economical way is of interest to realizing the application of smart windows. Here, we reported a successful preparation of self-assembly VO2 nanoplate films on TiO2-buffered glass by a facile hydrothermal process. The VO2 films composed of triangle-shaped plates standing on substrates exhibit a self-generated porous structure, which favors the transmission of solar light. The porosity of films is easily controlled by changing the concentration of precursor solutions. Excellent thermochromic properties are observed with visible light transmittance as high as 70.3% and solar modulating efficiency up to 9.3% in a VO2 film with porosity of ~35.9%. This work demonstrates a promising technique to promote the commercial utilization of VO2 in smart windows.

crystallization treatment (500-550 °C for hours) were needed during those fabrications to limit their usability in industry.
Compared to those traditional solution-based deposition methods, hydrothermal method shows many advantages, such as easy implementation on the industrial scale, controllable porosity and crystal size, low-temperature processing, possibility to utilize a wide range of substrates, and being environmentally friendly. The hydrothermal technique has been used to grow ZnO films 13 , TiO 2 films 14 and other transition metal oxide functional thin films 15,16 on glass or conductive substrate with high quality. Crystal morphologies, especially tunable porosity of films can be controlled by synthesis processes, showing great impacts on functional performance 13 . In previous studies, hydrothermal technique and subsequent thermal treatment were used to synthesize various VO 2 (M) nanomaterials 17,18 , and VO 2 -based composite membrane were prepared by mixing VO 2 (M) nanopowders with transparent polymer (e.g.,VO 2 /SiO 2 core-shell 19 , VO 2 /ATO/polymer 20 and polymer-assisted deposition [21][22][23][24]. However, there is no report about using the hydrothermal method to prepare VO 2 (M) thin films on glass for smart windows.
To our knowledge, preparing a high quality metallic oxide thin film directly on glass by hydrothermal method is not easy 25 . The substrates with polarity and crystal orientations were usually used to grow fine organized thin films [26][27][28] . Our recent work has demonstrated that high quality epitaxial VO 2 thin films can be grown on sapphire substrates by hydrothermal method 29 . Compared to the costly single crystal substrate, the buffer layer prepared on glass is an economic way to grow fine films. For example, Podlogar et al. prepared ZnO buffer layers on glass to grow highly adhesive crystalline ZnO films 13 , and Masuda et al. grew super hydrophilic TiO 2 thin films on glass with SnO 2 :F layer (FTO) 30 .
Here, we successfully prepared VO 2 smart windows via a facile hydrothermal process followed by a short heat treatment. High quality and porosity of obtained VO 2 coatings make the films exhibit excellent thermochromic properties with good solar modulation ability and high visible light transmittance. To grow VO 2 thin films on glass, TiO 2 was selected as an buffer layer since TiO 2 film shows stable thermal properties, high transparency to visible light and easy preparation 9,31 . The porosity of VO 2 films was easily controlled by adjusting the concentration of the reaction solution. The possible growth mechanism was discussed based on the investigation into the effects of pH value and different precursor solutions on the growth process. The proposed simple process which is low cost and up-scalable would promote the application of VO 2 in smart windows.

Experimental
Experiment section. All reagents used in the experiment were analytically pure and purchased from Sinopharm Chemical Reagent Co., Ltd. Vanadyl oxalate aqueous solution was used to grow VO 2 thin films on glass substrates by the hydrothermal method. Before the growth of VO 2 films, TiO 2 buffers were firstly deposited on amorphous glass substrate by spin coating. A moderate-temperature treatment (400 °C) was carried out to achieve its crystallization and adhesion 32 . The detailed preparation process for TiO 2 buffers is as follows: firstly, tetrabutyltitanate (C 16 H 36 O 4 Ti, 10 ml) was added into the ethanol (5 ml) at room temperature and stirred for 30 min. Then the solution was transferred into a mixed solution of nitric acid (3 ml), deionized water (6 ml) and ethanol (80 ml) and stirred for 1 h. Finally a transparent and stable TiO 2 sol was obtained. The sol was spin coated at 3500 rpm for 30 s on a glass with diameter of 2 inches, which was ultrasonically cleaned for 10 min in a solution of acetone, 2-propanol and deionized water with volume ratios of 1:1:1. As-coated TiO 2 precursor layer was heated under 400 °C for 1 h to produce fine grained TiO 2 layer. The glass with TiO 2 buffers was used for the hydrothermal growth of VO 2 films. In the hydrothermal process, the vanadyl oxalate precursors were prepared by dissolving V 2 O 5 (0.182 g) in the aqueous solution (50 ml) containing oxalic acid (1.97 g) at 70 °C. The aqueous solution was diluted into 500 ml with deionized water, forming a 4 mmol/L solution with pH value ~2.4. The PH value was controlled by NH 4 OH. The vanadyl oxalate aqueous solution (60 ml) was transferred into a Teflon-lined autoclave (100 ml). The chemical reaction was carried out at 230 °C in an electric oven. After heating for 4 h, the autoclave was naturally cooled down in furnace. The side of TiO 2 layer was covered by a uniform film. The wafer samples were cleaned up with deionized water and alcohol, and dried by nitrogen. The thermochromic VO 2 windows were obtained through annealing the as-grown VO 2 films in a short annealing furnace at 400 °C for 60 s in 4 * 10 4 Pa of air. Unless specifically noted in the article, all samples used here are prepared as mentioned above.
Instrumentation characterization. The morphology of the reaction product was examined by using scanning electron micros-copy (SEM, Hitachi S-4800). The phase identification of the TiO 2 and VO 2 films was performed using X-ray diffraction (XRD, Bruker-AXS diffractometer, Model D8 ANVANCE) with Cu-Kα radiation source, Raman spectra (HR800, excitation wavelength: 633 nm, laser power: 1 mW) and Transmission Electron Microscope (TEM, FEI Tecnai G2 F20 S-TWIN). The chemical valence of vanadium ions was measured by XPS (PHI QUANTERA-II SXM) with Al-Kα radiation source (1486.6 eV). The porosity based on SEM images was calculated by using Image-Pro Plus (IPP) to compare the gray scale pixel of the area occupied by VO 2 nanoplates and exposed TiO 2 films. The optical transmittance spectra of samples at normal incidence from 300 to 3000 nm and were measured by using Shimadzu UV-3600 UV-VIS-NIR spectrophotometer with Heat Solid Transmission Accessory. Figure 1a shows the morphology images of polycrystalline TiO 2 buffers with grain size between 25 to 75 nm. The XPS full spectrum (Fig. 1b) of TiO 2 reveals a high purity component. The obtained VO 2 film is composed of nanoplates with an average thickness of ~40 nm, and a height of ~400 nm, which are regularly grown against substrates (Fig. 1c,d). There are smaller and more randomly oriented nanoplates close to the substrate, which is similar with the previous report for the growth of ZnO films 33 . As identified by XRD (Fig. 1e), the characteristic peaks agree with those of M-VO 2 in monoclinic structure (JCPDS No. 65-2358) and A-TiO 2 in anatase phase Scientific RepoRts | 6:27898 | DOI: 10.1038/srep27898 (JCPDS No. 21-1272) respectively. The remarkable (020) peak of VO 2 indicates that the growth of VO 2 films are preferentially oriented on substrates. For a randomly oriented VO 2 polycrystalline sample the intensity of (020) diffraction is only ~2.4% of the strongest peak (011). The preferred orientation of the VO 2 films supports the conclusion that the VO 2 nanoplates are regularly grown on substrates as shown in the cross-section structure of VO 2 films in Fig. 1d. The XRD pattern of TiO 2 buffers indicates the (004)-preferred orientation of anatase TiO 2 . It is known that the close-packed planes in anatase-TiO 2 (112) and rutile-VO 2 (200)/(020) are equivalent 34 , so we can infer that there is a lattice-matching relationship between anatase TiO 2 and rutile VO 2 with A-TiO 2 (112)//R-VO 2 (200)/(020). In this case, it is possible for VO 2 to grow in a preferred orientation manner guided by the A-TiO 2 buffer under hydrothermal growth temperature (230 °C). The M-VO 2 is a polymorphic phase transformed from R-VO 2 through a small distortion 35 . The R-VO 2 {200} planes correspond to the (020) and (002) planes in the M-VO 2 phase 36 . For the (004)-preferred orientation of anatase TiO 2 as determined by XRD, the preferred orientation of M-VO 2 should be (011)M considering the crystal distortion induced by the mismatch between TiO 2 and VO 2 . The angle between (112) and (004) in A-TiO 2 is ~61° and no good lattice-match relation exist along other directions, therefore, the inclined growth of plate-like VO 2 nanocrystals are observed in Fig. 1c,d. While the VO 2 nanoplates show the strong preferred orientation of (020)M, it should be related to other orientations of TiO 2 , i.e. (110) or (112) orientations of A-TiO 2 . For A-TiO 2 (110) or (112) orientations the VO 2 nanoplates would grow perpendicular or parallel to the substrate. The corresponding growth of VO 2 nanoplates can be observed in Fig. 1c,d. The existence of (110)-orientation TiO 2 is verified by TEM in Fig. 2. XPS measurements were performed to examine the oxidation states of V ions in VO 2 thin films (Fig. 1f) 37 . It is shown that the VO 2 thin films contain partial V 5+ ions together with V 4+ ions. The presence of V 5+ ions could be attributed to surface oxidization in the annealing process or storage in air and exist only on the surface as proved 5 . In order to understand more details about the oriented growth of VO 2 and TiO 2 layers, a cross-section sample of VO 2 /TiO 2 films was prepared and investigated by TEM. TEM images (Fig. 2a,b) show the well-connected 3-layer structure. The TiO 2 thin film has a thickness ~12.8 nm (Fig. 2b). Two TiO 2 grains exist in the observation region, and they have different orientations as shown by the HRTEM images in Fig. S1 (supporting information). The VO 2 nanoplates show a triangle-like shape in Fig. 2a, which stand on the substrate. HRTEM images taken from two layers in Fig. 2c,e show clear lattice fringe, indicating good crystallinity of VO 2 and TiO 2 films. The interplanar spacing of 0.331 nm in Fig. 2c corresponds to the plane distance of (1-10) of monoclinic VO 2 (Fig. 2d). The interplanar spacings of 0.270 nm and 0.358 nm in Fig. 2e belong to the (− 110) plane and (011) plane of anatase TiO 2 (Fig. 2f), respectively. For the present orientations of A-TiO 2 and M-VO 2 as shown in Fig. 2(c-f), the equivalent planes, i.e. A-TiO 2 (112) and M-VO 2 (002)/(020) are not in the matching orientations. However, the right-hand grain of A-TiO 2 as shown in Fig. 2(b) and Fig. S1(c) exhibits an orientation that the left-hand grain rotates about 15° clockwise. In this case, the M-VO 2 (002) plane is parallel to the A-TiO 2 (112) plane of the right-hand grain, indicating the growth of VO 2 in Fig. 2 is guided by the left-hand TiO 2 . The To investigate the possible growth mechanism of VO 2 films, controllable hydrothermal processes were designed. Different precursor solutions and pH values were found to be key factors to affect the reaction process. The role of precursors in the hydrothermal process for preparing the VO 2 films were investigated, i.e. precursor solutions obtained from V(OH) 2 NH 2 dissolved in HNO 3 38 , hydrazine hydrate reacted with VOSO 4 39 , NH 4 VO 3 with 1,3-propylene glycol reduced in H 2 SO 4 40 , and V 2 O 5 dissolved in oxalate acid solution 41 . It is found that VO 2 films can be grown only in the vanadyl oxalate solution, which suggests that oxalate acid solution is a suitable solvent for the formation of VO 2 thin films.

Result and Discussion
The pH value of vanadyl oxalate solution was modulated by adding droplets of NH 4 OH. Figure 3(a-e) show the SEM images of VO 2 films prepared at different pH values. The morphology of VO 2 nanoplates greatly changes with increasing pH values. Obviously, the growth of VO 2 is greatly influenced by the pH value. At pH 3.46, the VO 2 nanoplates in Fig. 3a are twice thicker than those grown at pH 2.4 (Fig. 1c), making the nanoplates more like nanorods (length was ~300 nm). When the pH value rises up to 4.56, the nanorods become shorter (length is 250 nm) and wider (Fig. 3b). As the PH value equals to 6.21, nanorods disappear instead of rectangle-like grains distribute randomly on the film (Fig. 3c). At pH 7.45, irregularly shaped particles are loosely attached to substrates. At PH 8.12, more area of substrate is exposed. Furthermore, experiments revealed that nothing could be grown on the substrate while pH values ≥ 8.  46 . The IEP of TiO 2 is close to 6.2 as reported by Parks 47 . When PH is lower than 6.2, positive charge sites should dominate on the surface, whereas negative charge sites would be in majority. The adsorption affinity decreased rapidly as the pH value larger than IEP. Although the concentration of oxalate acids and the presence of metal cations in solution can influence the IEP, the pH dependence of adsorption does not change. It indicates that the protonated In the oxalic acid solution, the possible surface reaction would be like that: 1) the vanadyl oxalate species were adsorbed on the TiO 2 buffer. It is known that oxalate can form organic metallic cation complexes through the coordinating ability of the carboxyl group 48 . In that case, the negatively charged organic vanadium complexes ([(VO) x (C 2 O 4 ) y ] x−y ) should be adsorbed on the positive surface sites through the carboxylic group. 2) Undergoing the water shrinkage reaction between the adsorbed vanadyl oxalate and the neighboring hydrogen ions on the protonated surface, VO 2+ were adsorbed on the TiO 2 substrate, and then crystallized to VO 2 thin films. The schematic diagram of the growth process is shown in Fig. 3g. The different vanadic precursor solutions mentioned above have no carboxylic group, so there is no effective species to play the role of bridge between vanadium ions and positive charge-terminated surface of the TiO 2 thin films for achieving the growth of highly adhesive VO 2 films.
The optical modulation properties of the prepared VO 2 films were investigated to evaluate its potential for the smart windows. For realizing the application of VO 2 in smart window a technological challenge is to improve the maximum visible transmittance (T-vis) to an acceptable value (> 60%), while maintain the high solar modulating efficiency (∆T sol ) of VO 2 49 . To improve T-vis, one way is to fabricate porous films 6,12,49 , and another way is to deposit an antireflection film or reduce the thickness of the continuous films of VO 2 to less than 80 nm 12,24 . In this work, the standing nanoplate structure facilitates the penetration of solar light, namely apt to achieve high T-vis. The obtained VO 2 films are in fact self-generated porous films, which would produce excellent combination thermochromic property. Cao et al. reported a nanoporous VO 2 film exhibiting good thermochromic properties, the highest value of T-vis and ∆T sol were 75% and 7.9% respectively 11 . In our work, the T-vis can be easily adjusted by changing the porosity of VO 2 films through diluting the concentration of vanadyl oxalate in solution (Fig. S3). The porosity of VO 2 films on glass increases with decreasing the concentration of vanadyl oxalate. By comparing the area occupied by VO 2 nanoplates and exposed TiO 2 films, the calculated porosities for the VO 2 films grown in different concentration vanadyl oxalate solutions are shown in Fig. 4a. The samples are marked as: #1, 0.73 mmol/L; #2, 1.1 mmol/L; #3, 1.5 mmol/L; #4, 2.2 mmol/L; #5, 4.0 mmol/L (as in Fig. 1c), respectively. The sample #1 has the largest porosity of ~54.9%, it suggests that higher T-vis could be achieved.
Such self-generated porous nanostructures exhibit a good combination property of thermochromism (combining visible light transmittance and solar modulating efficiency). Figure 4b shows temperature-dependent transmittance of the porous VO 2 nanoplates thin films. The right insets are the corresponding coating photos. The hysteresis loops of transmittance at 2000 nm for different VO 2 thin films are shown in Fig. 4c, the T c and hysteresis loop width (Δ T) of #5 is 70.1 °C and 12.9 °C respectively. Both of T c and Δ T are increased as the porosity of thin films increasing, which is considered that the discontinuity of grain in thin films causes a loose grain boundaries limit propagation of MIT, and results higher T c and wider Δ T 50 . The T-vis, ∆T sol , and near-infrared (NIR) switching efficiency (∆T nm 2000 ) are shown in Fig. 4d, T-vis monotonously increases with the porosity of thin films as predicted. While the ∆T sol shows a plateau for samples #2-#4. Pleasurable thermochromic properties are observed in the sample #2 with 35.9% porosity, the T-vis value is as high as ~70.3% with the ∆T sol up to 9.3%. The results are even better than those of periodic and aperiodic porous VO 2 (M) films fabricated by complicated chemical and physical processes 6,24,49 , the multilayered TiO 2 (or SiO 2 )/VO 2 /substrate films 9 , and the VO 2 -based composite thin films 20,51 . The excellent thermochromic properties of our VO 2 films benefit from the special nanoplates structure which provides pores to solve the issue of low visible transmittance, meanwhile keep the thickness of films up to ~400 nm.
The integrated solar transmittance (T sol , 300-2500 nm) and the ∆T sol values are obtained from the following equation: ∫ ∫ where T λ denotes transmittance at wavelength λ, ϕ sol is the solar irradiance spectrum for air mass 1.5 (corresponding to the sun standing 37° above the horizon) 52 .

Conclusion
In summary, we have successfully fabricated nanoplates VO 2 films on glass substrates with TiO 2 -buffers, for the first time, by a facile hydrothermal method. The obtained VO 2 films show unique self-assembly porous structure with the porosity controllable by the concentration of the precursor solution. Excellent thermochromic properties are achieved with a visible light transmittance of 70.3% and a solar modulating efficiency of 9.3%. The investigation of growth process reveals that the appropriate adsorbent media, such as oxalate groups adsorbing on TiO 2 buffers, are necessary for the preparation of VO 2 thin films on glass by the hydrothermal technique. The preparation process of thermochromic VO 2 films adopted in this work is facile, low-cost and up-scalable. The experiments proved its potential in promoting the practical application of VO 2 in smart windows.