Heteroepitaxial TiO₂@W-doped VO₂ core/shell nanocrystal films: preparation, characterization, and application as bifunctional window coatings

Abstract

In this study, we have employed rutile TiO₂ nanoparticles (TiNPs) as cores for coating with vanadium sols of various concentrations and a tungsten doping of 2 at%. We grew the W-doped VO₂(M) nanocrystals (WVNCs) heteroepitaxially as shells onto the TiNP cores after a sintering process. Needle-like structures gradually appeared for the WVNCs on the TiNP surfaces upon increasing the concentration of the vanadium sol coating. These needle-like structures decreased the surface contact area sufficiently to result in superhydrophobicity. Falling water droplets rebounded completely from the TiNPs@WVNC films, thereby potentially preventing fouling of such materials. The superhydrophobicity of a TiNPs@WVNC film remained stable after 90 min of UV irradiation. The WVNC shells grew epitaxially on the TiNP seeds and enhanced the visible transmittance and near-infrared switching efficiency due to the needle-like structures; this was similar to the behavior of an anti-reflection coating. In addition, the TiNP cores in the TiNPs@WVNC films completely retained their photocatalytic properties. Such bifunctional (self-cleaning and thermochromic) nanomaterials might have applications in energy-saving smart windows.

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1. Introduction

Increasingly sophisticated intelligent window coatings respond to an external stimulus on an “as-needed” basis. Such materials include thermochromic coatings, which change their reflectance–transmission properties in response to temperature. Monoclinic-phase vanadium dioxide [VO₂(M)] is a promising material for use in thermochromic smart windows because its reversible insulator–metal phase transition (IMPT) between monoclinic and rutile phases [VO₂(M) ↔ VO₂(R)], at a phase transition temperature (T_p) of approximately 338 K, dramatically affects its electrical and optical properties.¹ VO₂(M) is a metallic material, which exhibits high infrared reflection when the temperature is above T_p, but becomes a semiconductor with a reasonable infrared transmission when the temperature is below T_p.^2,3 The modulation of T_p to room temperature is essential for many practical applications, and not just limited to smart windows. A number of approaches have been reported for decreasing T_p; for example, doping VO₂(M) with metal ions (e.g., W⁶⁺, Mo⁶⁺, Ta⁵⁺, Nb⁵⁺, and Ru⁴⁺) is a commonly used strategy, but it often decreases the thermochromic performance.⁴

Rutile TiO₂ has the same structure as VO₂(R) and considerably similar lattice parameters. Because the crystal structure of VO₂ is sensitive to its under- and/or over coating materials, rutile TiO₂ nanocrystals might be useful as seeds for assisting the nucleation and growth of nanostructured rutile VO₂. In the field of VO₂-based thermochromic smart windows, TiO₂ is also an important composite layer because of its good compatibility and crystalline structure similar to that of VO₂.⁵ It has been reported that TiO₂ films are effective underneath layers or cover layers for inducing the crystallization of VO₂(M/R),⁶ constructing heterostructures for photoemission,^7,8 enhancing the oxidation durability,^9,10 regulating the phase transition properties,¹¹ and improving the optical performance of VO₂.^1,12 Moreover, VO₂(M)/TiO₂ composites have their own advantages, not only for increasing visible transmittance but also for modifying the infrared modulation ability.^13,14 Compared to TiO₂–VO₂ multilayered structures, single-layered composite films are more attractive because of their ease of preparation, mixing homogeneity of the two components, and, most importantly, their display of near-infrared (NIR) modulation behavior at wavelengths that are significantly shorter than those for typical thin films of pure VO₂, thus providing advantages when applied in systems requiring efficient visible transmittance and NIR switching.¹⁵

Thin films of both TiO₂ and VO₂ can be prepared using a range of methods, primarily chemical vapor deposition (CVD),^16,17 sol–gel processing,^17–19 and physical vapor deposition (PVD).^20,21 CVD methods have the advantage of being more easily integrated into float-glass production lines. Although these methods can provide single-layered TiO₂(A)–VO₂ through the simultaneous deposition of TiO₂ and VO₂ at a critical temperature,^22,23 the diffusion of Ti atoms into VO₂ crystal lattices weakens the thermochromic performance of VO₂.²⁴ The reported methods for fabricating VO₂(M)/TiO₂ composites are limited to using PVD, which cannot meet the requirements for the production of large-area VO₂ thermochromic windows because of technical and cost problems. Because the crystallization temperature of TiO₂(A) is considerably lower than that of VO₂(R),²⁵ a “two-step method” method has been developed for producing TiO₂(A)/VO₂ single-layered composite films, in which VO₂ nanoparticles (NPs) are first prepared and dispersed in a titanium sol to form a composite film and then annealed for TiO₂ crystallization. This method largely restricts Ti diffusion and maintains the thermochromic performance of VO₂. Thus, the resulting materials become more cost-effective for large-scale coatings of glass. It has been demonstrated recently that combining TiO₂ and VO₂ within a phase composite produces a material that possesses both self-cleaning and thermochromic properties.^26,27 Furthermore, the films exhibit a decrease of 17 °C in the thermochromic switching temperature (T_p) to 51 °C. If this T_p value could be lowered further, such coatings would be strong candidates for application in self-cleaning solar control glazing. In this study, we report a “two-step method” for the preparation of TiO₂/VO₂ composites, in which rutile TiO₂ nanoparticles (TiNPs) were first dispersed in a W-doped vanadium sol to obtain a composite film, which was then annealed to ensure the crystallization of the W-doped VO₂. We found that the specific needle-like structures of the W-doped VO₂ nanocrystals (WVNCs) could be regulated by varying the ratio of TiO₂ cores to VO₂ shells. The films of the as-prepared composite particles on glass exhibited remarkable superhydrophobicity and hysteresis widths that were considerably narrower than that found for pure VO₂. Furthermore, this specific morphology increased the visible transmittance of the film and enhanced its regulation ability. Such films performed three functions: thermochromism (from the VO₂(M) layer) for solar energy modulation, photocatalysis (due to the TiNPs), and superhydrophobicity (for anti-fouling and self-cleaning effects). The as-obtained nanocomposite coatings appear to have applicability in smart windows.

2. Experimental section

2.1 Materials

Vanadium pentoxide (V₂O₅, Acros Organics, >99.6%) and tungsten(VI) oxide (WO₃, Hayashi Pure Chemical Industries, >99%) were employed as starting materials to prepare VO₂⁺ solutions. Rutile TiNPs (sizes: 11–20 nm; Acros Organics; CAS no. 13463-67-7) were used as received. Methylene blue (C₁₆H₁₈ClN₃S, MB, 98%) was purchased from Aldrich. All other chemicals and solvents were of reagent grade and purchased from Aldrich. All the reagents were used without further purification.

2.2 TiNPs@WVNCs films

TiNPs (ca. 0.02–0.05 g) were ultrasonically dispersed in anhydrous EtOH (50 mL) for 15 min. An aqueous solution of the Triton X-100 surfactant (Sigma Aldrich, 800 μL, 0.1 M) was added into the suspension, which was then stirred for 2 h. Another stock aqueous sol was produced using a previously reported method as follows:²⁸ V₂O₅ powder (15 g) was heated in a crucible until it was molten and then poured into distilled water at room temperature. After vigorous stirring, the sol was filtered, and solutions of various concentrations (ca. 1, 2, 3, and 4 at%) were obtained. Tungsten(VI) oxide powder was dissolved in a few drops of distilled water and added to the sol in quantities calculated to achieve a doping level (assuming 100% substitution) of 2 at%. The stock solution was subsequently added dropwise into the TiNPs suspension, followed by sonication for 30 min prior to use. Soda lime glass slides were ultrasonically cleaned for 10 min in 20% EtOH, 20% acetone, and deionized water. After drying the glass slides by purging with N₂, they were subjected to argon/oxygen plasma (AOP) treatment using a TCP 9400SE instrument (Lam Research).²⁹ The surface was chemically modified (strongly hydrophilic or polar) due to the AOP treatment. The introduction of these polar groups provided a more polar surface for the formation of the film. For each coating, the sol was spin-coated on the AOP-treated glass slides, and then dried in a clean air cabinet for 10–15 min. The TiNPs@WVNC films on the AOP-treated glass substrates were obtained after spin-coating at a speed of 3000 rpm. To obtain a crystalline coating, the as-obtained films were sintered in a vacuum quartz tube furnace under a low pressure of a reducing gas mixture (50% CO and 50% CO₂). Typically, at a pressure of approximately 3 Torr, the sample was heated from ambient temperature to 500 °C at a heating rate of approximately 20 °C min⁻¹ and then held at 500 °C for 2 h, before allowing it to cool naturally to room temperature. The annealing program was optimized to transform the amorphous shells into crystallized VO₂(M) while keeping the TiO₂ cores chemically intact. The samples of TiNPs that had been coated with the 1, 2, 3, and 4 at% vanadium sols and then sintered are denoted herein as TV1, TV2, TV3, and TV4, respectively.

2.3 Characterization of heteroepitaxial TiNPs@WVNCs films

The valence states of vanadium, tungsten, and titanium were studied using X-ray photoelectron spectroscopy (XPS; Scientific Theta Probe, UK). The structures of the samples were determined using X-ray diffraction (XRD; Rigaku D/max-RC). The phase transition behavior was analyzed through differential scanning calorimetry (DSC; Netzsch DSC-4000) at a temperature ramp rate of 10 °C min⁻¹ within the temperature range from −20 to +100 °C under a flowing N₂ atmosphere. The chemical compositions of the as-obtained samples were investigated using energy-dispersive X-ray spectrometry (EDX) combined with high-resolution scanning electron microscopy (HR-SEM; JEOL JSM-6500F, Japan). Transmission electron microscopy (TEM) images were obtained using a field emission transmission electron microscope (Philips Tecnai G2 F20) operated at an accelerating voltage of 200 kV. Static water contact angles (SWCAs) were measured using a FDSA MagicDroplet-100 contact angle goniometer. Each SWCA was determined, under normal laboratory ambient conditions at room temperature under 40% humidity, by fitting a Young–Laplace curve around the drop. Each mean value was calculated from at least 10 individual measurements; the measurement error was less than 3°. As-prepared superhydrophobic surfaces were mounted on a rotatable stage. The stage was tilted until a droplet on the stage slid, thus characterizing the roll-off angle of the surface. The tilt angle of the surface was fixed at a certain angle. The changes in the SWCAs after UV irradiation were monitored at specific time intervals.

2.4 TiNPs@WVNC films for applications in bifunctional smart windows

The TiNPs@WVNC films underwent a reversible IMPT, which was accompanied by remarkable changes in their optical properties. Optical transmittances were monitored using a UV-vis-near-infrared spectrophotometer (Shimadzu UV3600) equipped with a thermo-regulated environmental cell. The reversible IMPT temperatures of the thin films were measured by recording the transmittance spectra as a function of temperature. For all the samples, the integral visible transmittance (T_int) was obtained based on the measured spectra, using the following equation:³⁰where X denotes the transmittance measured using a UV-vis-near-infrared spectrophotometer. The values of T_int were obtained using the formula ϕ_ρ = Φ_lum, where Φ_lum was the standard luminous efficiency function, and Φ_lum is 0 beyond this range.

The photocatalytic performance of the synthesized TiNPs@WVNC films was determined by evaluating the rate of degradation of MB. A glass plate (1 × 1 cm) presenting a thin TiNPs@WVNC film was immersed in an aqueous MB solution (20 ppm, 20 mL). The sample was placed in the dark to allow the complete absorption of MB onto the surface; it was then irradiated using a UV lamp. The concentration of MB in the solution was measured as a function of the irradiation time, monitoring the wavelength of maximum absorbance (λ_max) of MB at 664 nm. The photometric analysis of the TiNPs@WVNC films before and after irradiation was performed to measure the degradation efficiency of MB (D%), which is defined using the following expression:¹¹

where C₀ is the initial concentration of MB and C is the concentration of MB after irradiation of the samples for a certain period of time. The absorbance of the samples was measured using a UV-vis spectrophotometer with a resolution of 1 nm (Perkin-Elmer, lambda 25, 190–1100 nm). The decrease in the absorbance at λ_max of the samples after irradiation for a desired period of time allowed the determination of the rate of decolorization and, thus, the MB photodegradation efficiency, which also represents the activity of the TiNPs@WVNC films. The following equation was used to measure the rate of MB degradation (k) at a given time:

For comparison, thin films of a commercially available pure TiO₂ catalyst were also examined under similar conditions.

3. Results and discussion

3.1 Characterization and heteroepitaxy of TiNPs@WVNC films

The synthesis of the TiNPs@WVNC films in this study can be regarded as a process of generating “TiNP seeds” upon which the VO₂ crystals grew. We used sol–gel deposition to produce uniform precursor shells with various thicknesses on the TiO₂ cores. We used XPS to investigate the chemical states on the surfaces of the as-obtained TiNPs@WVNC films. Fig. 1a indicates that the sample consists of only six elements: carbon, vanadium, titanium, oxygen, silicon, and tungsten. The presence of signals for carbon and silicon atoms presumably reflected surface contamination and the glass slides, respectively. For the vanadium precursor film, we assigned the peaks centered at 517.15 and 524.2 eV to the V 2p3/2 and V 2p1/2 orbitals, respectively. The signal for the V 2p3/2 orbitals of the sample appeared at a binding energy of 517.8 eV, which can be attributed to V⁵⁺.³¹ In addition, two peaks were centered at 529.6 and 532.2 eV in the O 1s region (Fig. 1b). We assigned the major peak, positioned at the lower binding energy of 529.6 eV, to the O²⁻ ions in the V–O bonds.³² We assigned the second peak, located at 532.2 eV, to OH⁻ groups in the H₂O molecules. The V 2p core-level XPS spectrum recorded after sintering the sample revealed (Fig. 1c) that the V 2p doublet binding energies were 516.85 and 524.32 eV, corresponding to spin–orbital splitting of the V 2p3/2 and V 2p1/2 components, respectively.³³ The V 2p3/2 peak can be split into major and minor peaks at 516.85 and 515.7 eV, respectively. The major V 2p3/2 peak shifted from 517.15 to 516.85 eV after sintering, suggesting the generation of V⁴⁺ species. The binding energy for V 2p3/2 peak was slightly higher than that found for pure VO₂, but was in agreement with the value (516.3 eV) for W-doped VO₂,³⁴ suggesting that the V 2p3/2 binding energy increased slightly after W-doping and that the vanadium atoms in the doped sample resided in the +4 oxidation state. The minor V 2p3/2 peak at 515.7 eV could be assigned to the V–Ti bonds.³⁵ We attributed the binding energies at 458.15 and 464.25 eV to the Ti 2p3/2 and Ti 2p1/2 orbitals, respectively, of the rutile-phase TiNPs (Fig. 1d), corresponding to the position for Ti⁴⁺ species in TiNPs (the reported value for pure Ti is 458.0–458.5 eV (ref. 25)). Fig. 1e displays the signals for the W 4f species in the sample, with binding energies of W 4f5/2 and W 4f7/2 centered at 37.7 and 35.7 eV, respectively. According to the standard binding energies, there was only a small amount of tungsten in the sample, with the oxidation state of the tungsten ions in these films being solely W⁶⁺. Table 1 lists the ratios of titanium to vanadium (Ti/V) in the TV1, TV2, TV3, and TV4 samples. We observed that the Ti/V ratio decreased upon increasing the concentration of the vanadium sol coating on the TiNPs.

Fig. 1 (a) Wide-range survey XPS spectra of TiNPs@WVNC films on AOP-treated glass slides. (b–d) High-resolution XPS profiles of the V 2p core level spectra of the (b) vanadium sol and (c) the TV3 film; (d) Ti 2p and (d) W 4f core level spectra of the TV3 film.

Table 1 Influence of TiNP seeds on thermochromic and photocatalytic properties

Sample	Ti/V^a (atom%)	Ti/V^b (atom%)	Tp^c (°C)	ΔTH^d (°C)	Roughness^e (nm)	Tint,20^f (%)	Tint,50^g (%)	Es^h (%)	K^i
a Obtained from XPS and EDX data, respectively.b Obtained from XPS and EDX data, respectively.c Obtained from DSC data.d Hysteresis between the heating and cooling cycles.e Obtained from AFM images.f Calculated by eqn (1) from 700 to 2500 nm at 20 and 50 °C, respectively.g Calculated by eqn (1) from 700 to 2500 nm at 20 and 50 °C, respectively.h Changes in the values of Tint at 20 and 50 °C.i Rate of MB degradation at 664 nm, as determined from the UV spectra.
TV1	1.61	1.54	43.1	5.6	279	43.3	37.5	13.4	0.0013
TV2	0.83	0.89	37.8	4.8	308	49.5	39.7	19.8	0.0015
TV3	0.32	0.39	31.4	3.4	632	56.4	39.3	30.3	0.0017
TV4	0.25	0.28	29.8	2.7	643	58.3	40.1	31.2	0.0018

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Fig. 2 displays the XRD patterns of the WVNC powders TV1, TV2, and TV4 and of the pure TiNPs. The observed diffraction peaks of the pure WVNC powders can be indexed to the monoclinic phase of VO₂(M) (JCPDS card no. 43-1051). No noticeable changes in the positions of the diffraction peaks occurred for samples TV1, TV2, and TV4, but they had shifted to a slightly lower angle when compared with that of the sample prepared without the TiO₂ cores; this shift could have resulted from the superposition of the diffraction peaks from rutile TiO₂ and VO₂(M), both near 27.8°. For TV1, TV2, and TV4, we observed the XRD peaks for both VO₂(M) and rutile TiO₂. The intensity of the VO₂(M) peak increased upon increasing the thickness of the coating on the TiNPs, which was consistent with the formation of VO₂(M) shells.

Fig. 2 XRD patterns for the pure TiNPs, TV1, TV2, TV3, TV4, and pure WVNC films. The diamonds and circles in the XRD patterns mark the main peaks of the M-phase VO₂ and rutile TiO₂, respectively.

Endothermic and exothermic peaks were evident in each DSC curve (Fig. 3), further confirming the formation of VO₂(M). Table 1 lists the values of T_p in the heating cycle and hysteresis (ΔT_H) between the heating and cooling cycles, recorded from the DSC curves for TV1, TV2, TV3, and TV4. The value of T_p increased upon increasing the TiNP seed content, despite having the same W doping content (e.g., T_p increased from 29.8 to 34.1 °C when the Ti/V molar ratio increased from 0.25 to 1.61). This result suggests that the addition of the TiNP seeds suppressed the W doping content in VO₂(M). The latent heats calculated from the heating cycles, as shown in Fig. 3, were 38.5 and 15.7 J g⁻¹ for the WVNCs prepared with and without TiNP seeds, respectively. The higher latent heat indicates that the TiNPs@WVNCs were highly crystalline and relatively perfect in their crystalline structure.³⁶ The hysteresis between the heating and cooling cycles increased upon increasing the content of TiO₂ seeds (from 2.7 to 5.6 °C), which is in contrast to the reported result that TiO₂ additives can remarkably narrow the hysteresis loop width;³⁷ we suspect that this was due to the effect of the morphology of the TiNPs@WVNCs.³⁸

Fig. 3 DSC curves of WVNCs in the (a) absence and (b) presence of TiNP seeds (Ti/V molar ratio: 0.32).

We used SEM to characterize the surface morphologies and nanostructures of our samples. Fig. 4 displays the SEM images and EDX analyses of the TiNPs coated using the 1, 2, 3, and 4 at% vanadium sols. The vanadium sols generated homogeneous thin films on the TiNPs prior to sintering, resulting in similar morphologies for the four samples (Fig. 4a–d). The thicker shells tended to assemble because of the lower number of heterogeneous nucleation sites.³⁹ Crystalline structures appeared for the films after sintering, due to the formation of VO₂ nanocrystals. Needle-like structures appeared initially on the TiNP surfaces of TV1 (Fig. 5a); their lengths ranged from 100 to 500 nm, indicating irregular thicknesses of the vanadium sol coating. For TV2, the needle-like structures of the WVNCs covered the entire surfaces of the TiNPs with more regular lengths ranging from 70 to 150 nm (Fig. 5b). Further increasing the concentration of the precursor sol to 3 at% caused the needle-like structures to exhibit high regularity (lengths of ca. 300 nm) on the surfaces of the TiNPs (Fig. 5c). An urchin-like structure appeared for the WVNCs of TV4, indicating that the aggregation of the TiNP cores had occurred on the TiNPs at a vanadium sol coating of 4 at% (Fig. 5d). In the absence of TiO₂ seeds, VO₂(M) exhibited an irregular shape.⁴⁰ The roughnesses measured using AFM, presented in Table 1, confirm that the WVNC coatings had significantly different morphologies and Ti/V ratios in the TV1, TV2, TV3, and TV4 samples. After annealing, the core/shell interface became obscure (Fig. 6a), indicating that the shell coating crystallized with a relatively rough morphology. The needle-like structures of the WVNCs on the surfaces of the TiNPs were clearly evident for TV3. The lattice-resolved HRTEM images of TV3 revealed distinguishable crystallographic lattice fringes, with the lattices coherently bound to the TiO₂ side faces (Fig. 6b). We could distinguish two different lattices for VO₂ and TiO₂, with nearly orthogonal and oblique crossing patterns (represented schematically in the rectangular region), respectively. The interplanar distances of 0.332 nm in the yellow region and 0.324 nm in the green region match well with the (−111) and (110) crystal planes of VO₂(M) and rutile TO₂, respectively, suggesting that the VO₂(M) crystals grew epitaxially on the rutile TiNPs; the appearance of a dislocation in the interface region, arising from the slightly different interplanar distances in VO₂(M) and rutile TiNPs, confirmed this epitaxial growth. Thus, a coupled interface existed between the VO₂ and TiO₂ phases, with the surfaces of the TiO₂ cores serving as propitious sites for heterogeneous nucleation of the VO₂ phase. We performed EDS analysis of the shell coating, applying a beam spot size of less than 1 nm. Signals were detected for both V and Ti co-existing on the shell, indicating some degree of Ti–V diffusion at the core/shell interface, presumably because of the infinite solubility of Ti in VO₂ lattices.³⁵ This finding also suggests that rutile TiO₂ seeds can induce the epitaxial growth of VO₂(M) nanocrystals.

Fig. 4 Top-view SEM images of the TiNPs coated with (a) 1, (b) 2, (c) 3, and (d) 4 at% vanadium sol without sintering.

Fig. 5 Top-view SEM images of the TiNPs@WVNC films TV1, TV2, TV3, and TV4.

Fig. 6 (a) TEM and (b) HRTEM images of a TiNPs@WVNC film prepared at a Ti/V molar ratio of 0.32.

When a liquid droplet was placed in contact with a superhydrophobic surface that was capable of self-cleaning, a high contact angle was formed with a small interfacial area between the liquid and the surface. In an ideal situation, the liquid would not wet the surface and would be free to roll off from it. This surface property, caused by the surface energy of the solid being lower than that of the liquid, results from either the chemistry of the material or its physical roughness, including both nanometer- and micrometer-scale features. Fig. 7a presents the SWCAs and roll-off angles of the surfaces of pure TiNPs, TV1, TV2, TV3, and TV4. Water droplets on the pure TiO₂ surface and TV1, TV2, TV3, and TV4 surfaces provided SWCAs of 85° ± 3°, 139° ± 3°, 151° ± 3°, 163° ± 3°, and 155° ± 3°, respectively. Thus, the SWCAs of the heteroepitaxial surfaces were larger than that found for the pure-TiNP surface. The SWCAs of the surfaces increased gradually upon increasing the fraction of needle-like WVNC structures up to that in TV3, but did not change significantly thereafter. Prior to UV irradiation, the water droplets on the pure TiNPs, TV1 and TV2 surfaces exhibited roll-off angles of 47° ± 3°, 18° ± 3°, and 11° ± 3°, respectively; increasing the density of needle-like structures even further, to obtain the TV3 and TV4 surfaces, caused the roll-off angles to decrease abruptly to approximately 1° ± 3°. The water droplets penetrated the TV1 and TV2 surfaces because of their lower densities of needle-like WVNC structures, resulting in relatively large roll-off angles. When water droplets resided on the TV3 and TV4 surfaces, with their greater fractional coverages of needle-like structures, air was presumably trapped in the cavities of these rough surfaces, resulting in composite solid–air–liquid interfaces and, therefore, relatively low roll-off angles. Fig. 7b and c display the SWCAs and roll-off angles, respectively, of the pure TiNPs surface and the TV1, TV2, TV3, and TV4 surfaces, plotted with respect to the UV irradiation time. VO₂(M) is a well-known semiconductor material with a band gap of 0.7 eV, and is seldom used as a photocatalyst. UV irradiation may not always change the VO₂(M), and VO₂(M) is not always oxidized under UV irradiation. The SWCA of the pure TiNPs surface was affected significantly, forming a completely hydrophilic surface (SWCA: <5°) upon increasing the UV irradiation time to 100 min (Fig. 7b). The SWCAs of the TV1 and TV2 surfaces decreased gradually to 113° and 133°, respectively, within 180 min because of their lower densities of needle-like structures. The SWCAs of TV3 and TV4 did not change significantly within 180 min of UV irradiation, due to their lower surface ratios—the fraction of the surface area over the total geometric area of a sample; this indicates the grooving on the surface, which allows air pockets to be formed between the solid surface and the overlaying liquid droplet. Fig. 7c plots the roll-off angles with respect to the UV irradiation time for the pure TiNPs surface and the TV1, TV2, TV3, and TV4 surfaces; data points marked with a roll-off angle of 90° represent droplets that were pinned to the substrates even when tilted vertically or flipped upside down. As expected, the roll-off angles for the pure TiNPs surface and the TV1 and TV2 substrates increased upon increasing the UV irradiation time, eventually reaching a state where the water droplet remained stuck to the surface even when it was turned by 90°. The roll-off angles of TV3 and TV4 did not change significantly within 180 min of UV irradiation. Such highly stable superhydrophobicity after UV irradiation may extend the lifetimes of smart windows. Impact experiments revealed that the needle-like structures exhibited superior slippage and robustness of their superhydrophobicity. We allowed water droplets to fall onto 35°-tilted pure TiNP and TV3 surfaces to observe their stickiness and rebound ability. We released droplets with a radius of 1 mm at rest from a height of 4.5 mm. As shown in Fig. 7d and e, the substrates were tilted at an angle of 35° to monitor their rebound trajectories. For the pure TiNPs surface, the droplets stuck to the surface (Fig. 7d). When needle-like WVNC structures were present on the TV3 surface, the droplets bounced completely from the substrate (Fig. 7e). Thus, needle-like structures improved the slippage of droplets on the pure TiNPs surfaces, resulting in effective rebounding.

Fig. 7 (a) SWCAs and roll-off angles of pure TiNPs, TV1, TV2, TV3, and TV4 surfaces. (b) SWCAs and (c) roll-off angles of pure TiNPs, TV1, TV2, TV3 and TV4 surfaces, plotted with respect to the UV irradiation time. Real-time images of the water droplets impacting on (d) pure TiNPs and (e) TV3 surfaces inclined at an angle of 35°.

3.2 TiNP@WVNC films for applications in bifunctional smart windows

The values of T_int and the NIR switching efficiency (E_s) are two main parameters used to characterize smart windows. Although these two parameters are strongly related, their changes can be conflicting.⁴¹ A large visible transmittance is usually accompanied by a small E_s value, and vice versa. Developing methods to improve the E_s value while simultaneously maintaining a comparatively high visible transmittance has been challenging. We studied the thermochromic properties of the TiNPs@WVNC films by measuring the optical transmittance spectra of TV1, TV2, TV3, and TV4. Fig. 8a reveals that all these films displayed excellent visible transmittance at 20 and 50 °C as well as excellent E_s values. For TV1, TV2, TV3 and TV4 films that were coated with a thickness of approximately 87 nm, the values of T_int for the semi-conductive states reached 31%, 44%, 62%, and 64%, respectively, with E_s values in the range of 20–45% (Table 1). The values of T_int for TV3 and TV4 are higher than those for single-layer VO₂ coatings with similar film thicknesses (reported values for 50 nm-thick films include 40% and 45%).⁴² Thus, the needle-like structured surfaces can be regarded as anti-reflecting coatings (ARCs) with enhanced transmittance.⁴³ Single-layer VO₂ crystals rarely display optical performance that includes high T_int values as well as good E_s values. Thus, our current solution-based method appears to have many advantages over other preparation methods.

Fig. 8 (a) Transmittance spectra of the TiNPs@WVNCs of TV3 at a thickness of approximately 87 nm. The solid and dashed lines represent the spectra obtained at 50 and 20 °C, respectively.

To test the applicability of the photocatalytic effects in smart windows, we immersed our TiNPs@WVNC films into a reaction solution containing MB for various lengths of time. Fig. 8b displays the absorption of a solution of MB after UV irradiation in the presence of the TV3 film (0 min: time at which the container was placed in the dark). The TiNP cores coated with the WVNC shells retained their photocatalytic properties, and electron/hole pairs were excited under irradiation of UV light. The rate of MB degradation (k) over thin films of commercial TiNPs (particle size: 20 nm; P25) was approximately 0.0013. Apart from TV1, the rates of MB degradation over the thin films of TiNPs@WVNC were all higher than that found over the commercial TiNPs (Table 1). We suspect that the heteroepitaxial crystallinity induced the photocatalytic ability of VO₂, and thus enhanced the rate of MB degradation. Therefore, these heteroepitaxial TiNPs@WVNC systems exhibited bifunctional properties—superior thermochromicity and photoactivity—under UV irradiation, due to their large surface areas compared to those of the thin films of pure TiNPs.

4. Conclusion

TiNPs can be heteroepitaxially coated with WVNCs, resulting in needle-like structured nanocrystals on their surfaces. The morphology changed from the particle shape of rutile TiNPs in the absence of the vanadium sol coating to needle-like shapes for the TiNPs@WVNCs. Films of the needle-like structured TiNPs@WVNCs exhibit superhydrophobicity (SWCA: >150°) and can completely rebound water droplets. The needle-like structures of the heteroepitaxial nanocrystals substantially enhanced the modulation of the visible transmittance. In addition, the TiNP cores within the W-doped VO₂ shells completely retained their photocatalytic properties. This study not only provides a simple method for synthesizing core/shell TiNPs@WVNC films for bi-functional (photocatalytic and thermochromic) window coatings but also demonstrates their stable superhydrophobicity after UV irradiation.

Acknowledgements

We thank the Ministry of Science and Technology of the Republic of China for supporting this research financially.

References

1. J. H. Park, J. M. Coy, S. Kasirga, C. Huang, Z. Fei, S. Hunter and D. H. Cobden, Measurement of a solid-state triple point at the metal–insulator transition in VO₂, Nature, 2013, 500, 431–434.
2. F. C. Case, Improved VO₂ thin films for infrared switching, Appl. Opt., 1991, 30, 4119–4123.
3. F. Beteille and J. Livage, Optical switching in VO₂ thin films, J. Sol-Gel Sci. Technol., 1998, 13, 915–921.
4. J. B. Goodenough, The two components of the crystallographic transition in VO₂, J. Solid State Chem., 1971, 3, 490–500.
5. M. Hervieu, The surface science of metal oxides, Adv. Mater., 1995, 7, 91–92.
6. Y. Muraoka and Z. Hiroi, Metal–insulator transition of VO₂ thin films grown on TiO₂ (001) and (110) substrates, Appl. Phys. Lett., 2002, 80, 583–585.
7. K. Okazaki, H. Wadati, A. Fujimori, M. Onoda, Y. Muraoka and Z. Hiroi, Photoemission study of the metal–insulator transition in VO₂/TiO₂ (001): Evidence for strong electron–electron and electron–phonon interaction, Phys. Rev. B: Condens. Matter Mater. Phys., 2004, 69, 165104.
8. R. Eguchi, S. Shin, A. Fukushima, T. Kiss, T. Shimojima, Y. Muraoka and Z. Hiroi, Thermoelectric properties and figure of merit of a Te-doped InSb bulk single crystal, Appl. Phys. Lett., 2005, 87, 201902.
9. Z. T. Zhang, Y. F. Gao, L. T. Kang, J. Du and H. J. Luo, Effects of a TiO₂ buffer layer on solution-deposited VO₂ films: Enhanced oxidization durability, J. Phys. Chem. C, 2010, 114, 22214–22220.
10. G. Fu, A. Polity, N. Volbers and B. K. Meyer, Annealing effects on VO₂ thin films deposited by reactive sputtering, Thin Solid Films, 2006, 515, 2519–2522.
11. J. Chen, Y. He and C. Li, Synthesis of doubly functional nanowhiskers from a colloid solution including a V–Ti complex precursor, J. Mater. Sci., 2012, 47, 5287–5297.
12. L. Kang, Y. Gao, H. Luo, Z. Chen, J. Du and Z. Zhang, Nanoporous thermochromic VO(2) films with low optical constants, enhanced luminous transmittance and thermochromic properties, ACS Appl. Mater. Interfaces, 2011, 3, 135–138.
13. M. Wilkinson, A. Kafizas, S. M. Bawaked, A. Y. Obaid, S. A. Al-Thaiti, S. N. Basahel, C. J. Carmalt and I. P. Parkin, Combinatorial atmospheric pressure chemical vapor deposition of graded TiO₂–VO₂ mixed-phase composites and their dual functional property as self-cleaning and photochromic window coating, ACS Comb. Sci., 2013, 15, 309–319.
14. N. R. Mlyuka, G. A. Niklasson and C. G. Granqvist, Thermochromic multilayer films of VO₂ and TiO₂ with enhanced transmittance, Sol. Energy Mater. Sol. Cells, 2009, 93, 1685–1687.
15. S. Y. Li, G. A. Niklasson and C. G. Granqvist, Nanothermochromics: Calculations for VO₂ nanoparticles in dielectric hosts show much improved luminous transmittance and solar energy transmittance modulation, J. Appl. Phys., 2010, 108, 063525.
16. T. D. Manning, I. P. Parkin, C. S. Blackman and U. Qureshi, APCVD of thermochromic vanadium dioxide thin films–solid solutions V_2−xM_xO₂ (M = Mo, Nb) or composites VO₂–SnO₂, J. Mater. Chem., 2005, 15, 4560–4566.
17. S. A. O'Neill, R. J. H. Clark, I. P. Parkin, N. Elliott and A. Mills, Anatase thin films on glass from the chemical vapor deposition of titanium(IV) chloride and ethyl acetate, Chem. Mater., 2003, 15, 46–50.
18. A. Kafizas, C. W. Dunnill and I. P. Parkin, The relationship between photocatalytic activity and photochromic state of nanoparticulate silver surface loaded titanium dioxide thin-films, Phys. Chem. Chem. Phys., 2011, 13, 13827–13838.
19. D. P. Partlow, S. R. Gurkovich, K. C. Radford and L. J. Denes, Switchable vanadium oxide films by a sol–gel process, J. Appl. Phys., 1991, 70, 443.
20. N. Martin, C. Rousselot, D. Rondot, F. Palmino and R. Mercier, Microstructure modification of amorphous titanium oxide thin films during annealing treatment, Thin Solid Films, 1997, 300, 113–121.
21. M.-H. Lee and M.-G. Kim, RTA and stoichiometry effect on the thermochromism of VO₂ thin films, Thin Solid Films, 1996, 286, 219–222.
22. H. Kakiuchida, P. Jin and M. Tazawa, Optical characterization of vanadium titanium oxide films, Thin Solid Films, 2008, 516, 4563–4567.
23. J. Du, Y. F. Gao, H. J. Luo, L. T. Kang, Z. T. Zhang, Z. Chen and C. X. Cao, Significant changes in phase-transition hysteresis for Ti-doped VO₂ films prepared by polymer-assisted deposition, Sol. Energy Mater. Sol. Cells, 2011, 95, 469–475.
24. I. Balberg, B. Abeles and Y. Arie, Phase transition in reactively co-sputtered films of VO₂ and TiO₂, Thin Solid Films, 1974, 24, 307–310.
25. R. Li, S. D. Ji, Y. M. Li, Y. F. Gao, H. J. Luo and P. Jin, Synthesis and characterization of plate-like VO₂(M)@SiO₂ nanoparticles and their application to smart window, Mater. Lett., 2013, 110, 241–244.
26. U. Qureshi, T. D. Manning, C. Blackman and I. P. Parkin, Composite thermochromic thin films: (TiO₂)–(VO₂) prepared from titanium isopropoxide, VOCl₃ and water, Polyhedron, 2006, 25, 334–338.
27. U. Qureshi, T. D. Manning and I. P. Parkin, Atmospheric pressure chemical vapour deposition of VO₂ and VO₂/TiO₂ films from the reaction of VOCl₃, TiCl₄ and water, J. Mater. Chem., 2004, 14, 1190–1194.
28. T. J. Hanlon, J. A. Coath and M. A. Richardson, Molybdenum-doped vanadium dioxide coatings on glass produced by the aqueous sol–gel method, Thin Solid Films, 2003, 436, 269–272.
29. J. K. Chen, C. Y. Hsieh, C. F. Huang, P. M. Li, S. W. Kuo and F. C. Chang, Macromolecules, 2008, 41, 8729–8736.
30. C. X. Cao, Y. F. Gao and H. J. Luo, Pure single-crystal rutile vanadium dioxide powders: Synthesis, mechanism and phase-transformation property, J. Phys. Chem. C, 2008, 112, 18810–18814.
31. Y. Wang and Z. Zhang, Synthesis and transport properties of nanostructured VO₂ by mechanochemical processing, Phys. E, 2009, 41, 548–551.
32. C. J. Cui, G. M. Wu, J. Shen, B. Zhou, Z. H. Zhang, H. Y. Yang and S. F. She, Synthesis and electrochemical performance of lithium vanadium oxide nanotubes as cathodes for rechargeable lithium-ion batteries, Electrochim. Acta, 2010, 55, 2536–2541.
33. Y. Zhang, X. Liu, G. Xie, L. Yu, S. Yi, M. Hu and C. Huang, Hydrothermal synthesis, characterization, formation mechanism and electrochemical property of V₃O₇·H₂O single-crystal nanobelts, Mater. Sci. Eng., B, 2010, 175, 164–171.
34. F. Li, X. H. Wang, C. L. Shao, R. X. Tan and Y. C. Liu, Biodegradable polyester hybrid nanocomposites containing titanium dioxide network and poly(ε-caprolactone): Synthesis and characterization, Mater. Lett., 2007, 61, 1328–1332.
35. Y. Li, S. Ji, Y. Gao, H. Luo and P. Jin, Modification of Mott phase transition characteristics in VO₂@TiO₂ core/shell nanostructures by misfit-strained heteroepitaxy, ACS Appl. Mater. Interfaces, 2013, 5, 6603–6614.
36. Z. Chen, Y. F. Gao, L. T. Kang, C. X. Cao, S. Chen and H. Luo, Fine crystalline VO₂ nanoparticles: Synthesis, abnormal phase transition temperatures and excellent optical properties of a derived VO₂ nanocomposite foil, J. Mater. Chem. A, 2014, 2, 2718–2727.
37. Y. Li, S. Ji, Y. Gao, H. Luo and M. Kanehira, Core/shell VO₂@TiO₂ nanorods that combine thermochromic and photocatalytic properties for application as energy-saving smart coatings, Sci. Rep., 2013, 3, 1370–1373.
38. J. Y. Suh, R. Lopez, L. C. Feldman and R. F. Haglund, Semiconductor to metal phase transition in the nucleation and growth of VO₂ nanoparticles and thin films, J. Appl. Phys., 2004, 96, 1209–1212.
39. A. F. Demirörs, A. van Blaaderen and A. Imhof, A general method to coat colloidal particles with titania, Langmuir, 2010, 26, 9297–9303.
40. O. Monfort, T. Rochb, L. Satrapinskyy, M. Gregor, T. Plecenik, A. Plecenik and G. Plesch, Reduction of V₂O₅ thin films deposited by aqueous sol–gel method toVO₂(B) and investigation of its photocatalytic activity, Appl. Surf. Sci., 2014, 322, 21–27.
41. Z. T. Zhang, Y. F. Gao, Z. Chen, J. Du, C. X. Cao, L. T. Kang and H. J. Luo, Thermochromic VO₂ thin films: Solution-based processing, improved optical properties, and lowered phase transformation temperature, Langmuir, 2010, 26, 10738–10744.
42. Z. Chen, Y. Gao, L. Kang, C. Cao, S. Chen and H. Luo, Fine crystalline VO₂ nanoparticles: Synthesis, abnormal phase transition temperatures and excellent optical properties of a derived VO₂, J. Mater. Chem. A, 2014, 2, 2718–2727.
43. G. Xu, P. Jin, M. Tazawa and K. Yoshimura, Optimization of antireflection coating for VO₂-based energy efficient window, Sol. Energy Mater. Sol. Cells, 2004, 83, 29–37.

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