Supercritical temperature synthesis of fluorine-doped VO₂(M) nanoparticle with improved thermochromic property

Vanadium dioxide (VO₂) has been widely famous due to its excellent thermochromic activity. The bulk material VO₂ has a critical temperature of semiconductor state shift into the metallic (MST) at around 68 °C. MST of VO₂ is a reversible transformation in which the crystal structure of VO₂ changes from monoclinic below the critical temperature to rutile at above the critical temperature [1]. Smart windows application is the potential application for this material due to this transformation, among others such as dye sensitized solar cell [2], electronic materials [3], etc that were also reported. The practical implementation of this material, however, still facing many challenges due to its high critical temperature, although it has the closest critical temperature to room temperature compared to any other materials. Many ways have been performed to reduce the critical temperature of VO₂ such as by interfacial defect-mediated and size effect [4], non-stoichiometry [5], and so on. One of the methods to lower the critical temperature of VO₂ is by incorporation of doping using high valence cations as well as low valence anions such as halogen anions. Fluorine has been used as a dopant to improve the performance of VO₂ because it increases the heat ray shielding ability while maintaining good solar modulation ability and visible transmittance [6]. Addition of fluorine in the monoclinic structure of VO₂(M) reduces the structural differences to the rutile structure, leading to the decrease of the activation energy of the MST. Fluorine doping reduces the V–V interval distance of the monoclinic phase in which has two alternate intervals at the distance of 2.65 and 3.12 Å, to make it closer to the value of the V–V interval of the tetragonal rutile structure, with V–V distance of 2.87 Å [7, 8]. Another beneficial factor for using fluorine doping to enhance the color of the thin film has also been stated. The higher electronegativity of fluorine compared to oxygen make it possible for fluorine to substitute the oxygen in VO₂ to lower the O2p state. It will further act as a band gap widening to enhance the color of the resulting thin film to be more transparent [7]. The nanoparticle and thin film of VO₂ are usually synthesized using various methods such as chemical vapor deposition (CVD) [9, 10], physical vapor deposition [6, 11], magnetron sputtering [12, 13] radio frequency sputtering [14], thermal reduction followed by annealing [15] and chemically using sol-gel method [16]. Recent advances also include the use of organic solvents in a single polyol method [17], the use of tube furnace using SiO₂/Si substrate [18], cotton template method [19], controlled reactive high-power impulse magnetron sputtering [20], continuous hydrothermal flow synthesis [21], and microwave assisted solvothermal method [22]. The hydrothermal method offers several advantages such as the lower temperature synthesis and the availability to control morphology. The latest update on hydrothermal synthesis of VO₂ polymorph has been introduced recently [23]. In this work, fluorine doped VO₂ was synthesized to decrease the critical temperature as well as to achieve good thermochromic performance using the one-pot supercritical hydrothermal method at 490 °C. VO₂ with uniform size, high crystallinity nanoparticles with reduced critical temperature are the main goal.

All chemical reagents used were of commercially available analytical grade and were used without further purification. In a typical hydrothermal synthesis, 1 g, 0.05 moles of V₂O₅ powder was added to 28 ml of H₂O. The requisite quantity of NH₄F as fluorine dopant source was added, and then the mixture was reacted with 5 ml H₂O₂ 30%. The slurry was stirred for 24 h before reacted with 2.75 mmol of N₂H₄·H₂O. The mixture was further stirred for 20 min. The homogenous mixture was further heated hydrothermally in a supercritical fluid reactor at 490 °C for 30 min. The final product was collected via centrifugation, washed three times with deionized water and ethanol and further dried in a vacuum drying oven at 60 °C overnight. The thermochromic thin films were prepared by dispersion of 100 mg of VO₂ nanoparticle and the various concentration of fluorine-doped VO₂, each in a mixture of water and isopropanol (10:9 volume ratio). The chosen polymer was polyvinylpyrrolidone K30, and trimethoxymethylsilane was used as the dispersion agent. The mixture was stirred overnight at room temperature with the addition of zirconia beads with 1 mm diameter size. The film was cast using doctor blade method on quartz glass. The morphology and phase structure of the nanoparticle was examined by scanning electron microscopy (Hitachi SU-6600), transmission electron microscopy (TEM, Jeol JEM-2000 EXII) and x-ray diffraction with Cu Kα line (XRD, Bruker D2 Phaser). The binding energy of the contained elements, as well as the actual fluorine doping content, were confirmed via x-ray photoelectron spectroscopy (XPS, ULVAC PHI 5600, ULVAC PHI Co., Ltd) with sputtering of argon ion at 3 kV at 3 × 3 wide area for 64 times of scans for 2 min. The XPS data analysis was continued by subtracting the background using Shiley method and curve-fitting the obtained signal using Gauss–Lorentz method to get the parameter that indicated the character in percentage for each contained element. Solid-state NMR experiments were conducted on a home-build spectrometer operating at 188.344 MHz with a Varian 4 mm T3 probe in a magnetic field of 2.7 T at room temperature. High-resolution solid-state ¹⁹F NMR spectra of fluorine doped VO₂ samples spun at 15 kHz were obtained with a DEPTH background suppression sequence with a recycle delay of 120 s. The chemical shift are relative to CFCl₃ (also known as Freon-11) referenced externally to the CF₂ signal of PTFE at −122 ppm.

The reduced critical temperatures of the products were measured using thermogravimetric differential thermal analyzer (TG/DTA RIGAKU Thermoplus TG8120). The thermochromic performances of the VO₂ thin films were analyzed in the wavelength range of 190–2500 nm using UV Visible-near-infrared spectrophotometer (Jasco V-600, JASCO) equipped with film heating attachment. The thin film was cast using Doctor Blade frame with 12.5 μm thickness.

Hydrothermal synthesis was performed using hydrazine monohydrate as the reducing agent. The reaction proceeds at a supercritical temperature at 490 °C. Below the supercritical temperature, impurities of mixed valence species of V₆O₁₃ and vanadium (V)·nH₂O complex was formed [24]. An increase in temperature enhances the chance to produce pure phase of VO₂(M) as well as reducing the reaction times [25]. Another report noticed that the size of the nanoparticle product is not affected by increasing temperature [26]. The reaction took only 30 min to finish. The role of the hydrogen peroxide is to dissolve the vanadium (V) source. The molar concentration of hydrazine monohydrate is crucial to reduce the V⁵⁺ to V⁴⁺ species. Vanadium possesses several valence states; therefore, an excessive amount of hydrazine monohydrate concentration will reduce the V⁵⁺ further to V³⁺ in the form of V₂O₃. The unfinished reaction with less amount of reductant will give a mixture product of various metastable mixed valences Magneli phases V_nO_2n−1 species in between as well as the metastable phases of VO₂(A) and VO₂(B) [27]. These metastable species need an annealing process at high temperature to convert them to monoclinic structure. Therefore, a two-steps hydrothermal synthesis is sometimes needed [28, 29]. Fluorine doping is relatively easy to be incorporated that can promote the formation of VO₂(M) along with Ti, Cr, F, Mo, Sn, Sb and W [30]. Therefore, the fluorine doping of VO₂ can be synthesized the same way as the pristine VO₂.

The XPS pattern of the fluorine-doped VO₂(M) is presented in figure 1(a). The XPS pattern shows the existence of vanadium, oxygen, and fluorine. The argon residue peaks arise from the use of the sputtering process. Figure 1(b) shows that The XPS can also determine the valence state of the vanadium. By referencing the O1s to 530 eV. The V⁴⁺ can be determined from the binding energy of the V2p_3/2 peaks around 516 eV and for V⁵⁺ at 517 eV [13, 31]. The actual concentration of fluorine doping is 0.1%, 0.13% and 0.62% for the as-prepared 5, 10 and 15 moles%, respectively. The fluorine peak was very weak, plotted at the 684 eV. The fluorine peak in figure 1(c) was not observable without etching process. After 64 times of scans using ion argon sputtering 3 kV for two minutes, the peak for F1s is visible, might be related to its limited doping amount. The existence of fluorine doping is also confirmed using ¹⁹FMAS NMR, as presented in figure 1(d). A single peak was observed at −124 ppm with a background suppression pulse sequence repeated 96 times. The T₁ of the peak at −124 ppm was obtained as ca. 1.6 s by combining a saturation recovery method and a background suppression method. Estimation of amount of the fluorine dopant is not obtained due to the low signal-to-noise ratio.

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The XRD pattern of pristine and all the fluorine doped VO₂ were matched to PDF-44-0252 VO₂(M) (P21/c), presented in figure 2(a). The lattice constants are a = 5.75 Å, b = 4.52 Å, c = 5.38 Å. There is no impurity detected for all peaks. There is also no sign of ammonium fluoride peak which indicates that all the fluorine reacted and was homogeneously incorporated in the VO₂ crystal lattice. The Miller indices marked in the graph, and peak intensity dominated by the (011) planes of the low-symmetry-insulating monoclinic phase. The FHWM calculation for pristine VO₂(M) gives the value of 0.2578. The crystallite size measured using Scherrer's equation (λ = 0.154 Å) is ranged within 18–33 nm. With the increasing of the concentration of fluorine doping, there was a slight shifting of the peaks as can be seen in figure 2(b). The intensity of the peaks was also reduced especially for the concentration of the 0.62% fluorine doping. The blue shifting peaks show the reduced crystallinity. The peaks for fluorine-doped VO₂ shifted to higher 2θ value. The ionic radius of fluorine is smaller than that of oxygen and therefore when the fluorine atom substitutes the oxygen in the VO₂ lattice, the interplanar distance d_(hkl) shortens resulting to higher 2θ angle shifting [32]. The chosen temperature reaction of 490 °C can be rationalized by the purity of the VO₂ phase obtained at various temperature synthesis, as presented in figure 2(c). At 250 °C up to 350 °C reaction temperature, no VO₂ phase was obtained. The samples synthesized at these temperature heating consists of V₂O₅·nH₂O species. Above the critical point of water, the VO₂ phases was started to form. At 450 °C there is still an impurity peak detected at 15°. The peak at 450 °C showed a mixture of VO₂(M) and V₆O₁₃ phases. At 490 °C a pure phase of VO₂(M) can be obtained consistently. The reaction only took 30 min to proceed without any further heating or annealing process necessary. This is comparable to some other reported hydrothermal methods that need a lower temperature of 250° but the reaction time is longer at 24 h [7].

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The TEM images are presented in figure 3. The morphology of the pristine VO₂ morphology is an irregular cubic like nanopowder, as similar to the reported by other groups [33, 34]. Fluorine doping does not significantly alter the morphology and the size of the particle. The particle size is within 50–100 nm. The difference of the reducing temperature is related to crystallites size. It is accepted that doping reduces the crystallite size [6]. Calculation of single crystallite size based on Scherrer method showed the single crystallite size to be within the range of 15–19 nm for both 0.1% and 0.13% of fluorine doping. For 0.62% of fluorine doping, the crystallite size is smaller by up to 2–12 nm. Although the value is different, The TEM result supported the result of XRD patterns analysis that the higher concentration of fluorine doping, the smaller size was obtained. The small size of fluorine did not affect the particle growth. The morphology is homogeneous, and doping did not significantly change the uniformity. Compared to other published result, these nanoparticles do not show the tendency of VO₂(M) particle to grow along the preferential [110] direction of one nano-dimensional structure such as nanowires or nanorods in which may in some case reduce the transparency [35].

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The phase transition behavior is presented by DTA pattern in figure 4. The critical temperature of undoped VO₂(M) is 64 °C. This critical temperature is lower and comparable to the reported critical temperature of bulk VO₂(M) [27], nanosheets and petaloid clusters by Luo's group [36], and VO₂ synthesized using thermolysis [37]. The lower value of the critical temperature of the VO₂(M) is because of the small size VO₂(M) particle. The critical temperature can be observed in the heat-flow plot, by looking at the sudden changes in the DTA graph. The critical temperature of the VO₂ reduced to 58 °C and 48 °C for fluorine dopant of 0.10% and 0.13%, respectively. Therefore, the best concentration that reduces the critical temperature the most is the 0.13%. This result is comparable to fluorine doped VO₂ synthesized using CVD at 60 °C [9]. Another group was reported to be able to VO₂ is to lower the critical temperature up to 35 °C using fluorine doping [7]. The mechanism for MST has been studied by many researchers [8, 38, 39]. Although there is some debate existing about the MST mechanism, most researchers tend to convince that the MST is following Mott–Hubbard mechanism [40, 41]. The mechanism for the reducing temperature for fluorine doping has been explained. It is related to the shortening the distance of V–V interval in the monoclinic system, and therefore the MST can happen at lower temperature [7]. The critical temperature for the 0.62% fluorine doping can hardly be observed. Although the minimum value of the DTA is at 37.15 °C, this concentration of 0.62% dopant showed less pronounce thermochromic performance as to be explained below. The reason for the less obvious observed critical temperature for the 0.62% of fluorine doping could be the changing the structure due to the increasing amount of fluorine anion which means there are possibly V³⁺ existed in the system that hindrance the MST process. Another reason could be due to a big proportion of fluorine that trapped at grain boundaries because of the reduced crystallite size [6]. For the higher concentration of fluorine, the density of states distribution of the V3d valance band crossed the Fermi level. This condition is resulting in poor switching due to the high of metallic character [11]. A similar phenomenon of the retarded phase transition in increasing fluorine concentration has been reported by other group [7]. There is a possibility that the critical temperature of the 0.62% dopant fall below room temperature, since although not obvious there is still a thermochromic performance observed for 0.62% dopant.

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The thin films of the fluorine-doped VO₂(M) were produced using doctor blades method. The thickness of the film is 12.5 μm, and the film is homogeneous with light brown-yellow color. The reduced temperatures of the fluorine doping were further tested using a UV–vis NIR spectrophotometer. The percent transmittance was measured at a low temperature of 25 °C and a higher temperature at 95 °C. The thermochromic properties are showed in figure 5. It might be observed that all pristine and fluorine doped VO₂(M) all giving thermochromic property, although not obvious for the 0.62% of fluorine concentration. The effect of fluorine doped can be seen from the higher transmittance. The higher concentration of fluorine shows the higher transmittance at 0.62%. The thermochromic properties are the widest at low concentration of fluorine doping and getting narrow the higher the concentration of the fluorine doping. At 1500 nm, the 0.10% of fluorine doping gives the widest infrared reduction percentage. When measured at 2500 nm the pristine VO₂(M) gives the widest infrared reduction percentage followed by the increasing concentration of fluorine doping progressively. The infrared reductions at 2500 nm are 63.45%, 55.37%, 23.32% and 4.48% for pristine VO₂(M), 0.1%, 0.13% and 0.62% of fluorine doping, respectively.

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In summary, we successfully synthesized fluorine-doped VO₂(M) using the hydrothermal method at a supercritical temperature of 490 °C. The obtained pristine and fluorine doped adopted the structure of VO₂(M). The supercritical temperature helps the growth of pure VO₂(M) nanoparticle as well as maintaining its homogeneity and morphology. Nanoparticle VO₂(M) possess more reduced critical temperature compared to the bulk VO₂(M). Fluorine doping induces the smaller particle size. The higher the concentration of the fluorine doping, the smaller the particles size obtained. The pristine VO₂(M) has the critical temperature of 64 °C. The best reduction of critical temperature was observed by fluorine doping of 0.13% up to 48 °C. The thin films of the fluorine-doped VO₂(M) showed pronounced a thermochromic property that is suitable for smart window applications.

The authors acknowledge the support from the JSPS KAKENHI Grant Number JP16H06439, JP16H06440 (Grant-in-Aid for Scientific Research on Innovative Areas), and the Dynamic Alliance for Open Innovations Bridging Human, Environment and Materials, the Cooperative Research Program of 'Network Joint Research Center for Materials and Devices'.