One-step ball milling synthesis of VO2 (M) nanoparticles with exemplary thermochromic performance

Vanadium dioxide (VO2) has demonstrated highly potential for smart windows because of its thermochromic property. This study represents the development of a facile but efficient method for the synthesis of VO2 (M) nanoparticles by ball milling method under ambient conditions, without release of waste liquid or gases. The key variables related to synthesis, including milling time and molar ratio of raw materials, have been investigated. It was found that the pure-phase VO2 (M) nanoparticles with the sizes of the particles ranged from 20 to 50 nm and relatively good dispersivity could be prepared by optimizing process parameters. For practice use to decrease the phase transition temperature, elemental W doping amount of 2 at.%, V1−xWxO2 (M) nanoparticles were also studied, and their glass coating exhibits high thermochromic performance with luminous transmittance (Tlum) of 44.18%, solar regulation efficiency (∆Tsol) of 9.64%, and the critical phase transition temperature (Tc) of ~ 42 °C. This work demonstrates a green and promising ball milling method to fabricate large scale VO2 (M) and V1−xWxO2 (M) nanoparticles for smart windows.


Introduction
Vanadium dioxide (VO 2 ) undergoes a reversible metal-toinsulator (MIT) transition at a critical temperature of 68 °C, accompanied with dramatic changes in optical and electrical properties [1]. Since the phase-transition property of VO 2 (M) was firstly reported by Morin in 1959, [2] it has been studied widely for application in thermochromic smart windows [3][4][5].
A variety of approaches have been explored to prepare VO 2 (M) particles or films. The main techniques employed to construct nanostructures of thermochromic VO 2 (M) are vapor deposition method, [6,7] combustion method, [8] hydrothermal method, [9][10][11] sol-gel method, [12,13] electrochemical method [14,15] and solution methods [16,17]. Among them, vapor deposition is an efficient method to fabricate high quality VO 2 (M)-film, but the equipment is usually complex and expensive. Combustion-assist fabrication is a cost effective and simple method to prepare VO 2 (M), but the violent reaction and uneven heat distribution during the combustion process result in the broad particle size distribution varying from 50 nm to 10 µm [18]. Fortunately, hydrothermal method can solve this problems mentioned above due to its controllability in particle size, morphology, and phase structure of VO 2 (M) nanoparticles. However, hydrothermal reaction is usually accompanied with high pressure (6.45-9.28 Mpa), [19] long reaction time (6-72 h) [9,20,21] and effluent disposal, which exists security risks and leads to the serious environmental pollution. Therefore, it is necessary to look for a novel method to synthesis the high quality VO 2 (M) nanoparticles with mild condition, simple process and environmental protection.
Recently, the facile ball milling method to synthesize VO 2 (M) has attracted much attention due to its own advantages, such as shorter preparation time, and less pollution than hydrothermal methods. Furthermore, this method has a good application prospect in large-scale production. Billik et al. [22] prepared the VO 2 (M) nanoparticles by ball milling V 2 O 5 and Na 2 SO 3 . However, the washing procedure partially leads to the reaction of VO 2 (M) with water and produces some other phases. Chika Takai et al. [23] obtained the pure VO 2 (M) particles by controlling the addition of paraffin wax and milling time, but the particles agglomerated severely. To sum up, the ball milling method is a green technique to prepare VO 2 (M) on a large scale, but the preparation of the high quality VO 2 (M) nanoparticles by this method remains a challenge, which need to be further optimized.
Herein, the pure VO 2 (M) nanoparticles with particle size ranging from 20 to 50 nm (average size of ~ 42 nm) and relatively good dispersivity are obtained by a facile onestep ball milling method with carbon as the reductant. Specifically, the variables, such as reaction time and molar ratio of reaction materials have been studied systematically. Furthermore, the low-temperature (300 °C) treatment can improve the ΔT sol of VO 2 (M) film from 6.38 to 9.12%, due to the enhanced crystallinity of VO 2 (M). Meanwhile, the W-doping can decrease the T c of V 1−x W x O 2 (M) greatly, and the excellent thermochromic performance with great ΔT sol (9.64%), high T lum (44.18%) and low T c (42 °C) could be obtained while W doping content is 2 at.%, which has good potential for practical application in smart window.

Ball milling preparation of VO 2 (M)
VO 2 (M) nanoparticles were prepared by high energy mill (model 8000D Mixer) under air condition with milling rotational speed of 1425 rpm. The optimum value of the ballto-powder weight ratio was 30:1.
The mole ratio of reaction and the milling time were investigated. Firstly, the mixture of different molar ratios (2:0.5-2:8) V 2 O 5 (98%, Wuxi Zhan Wang chemical reagent, Ltd.): activated carbon (AR, Tianjin Da Mao chemical reagent, Ltd.) was milled for 1 h. Then, the raw materials were milled with the optimum molar ratio of 2:1 for different time (0 min, 10 min, 30 min, 1 h, 2 h, 4 h). Subsequently, the samples were separated from the milling vessel and washed 3 times by DI water and alcohol, then dried in an oven at 80 °C for 6 h.
We also prepared tungsten (W) doped VO 2 (M) by adding tungsten acid (99%, Shanghai Macklin Biochemical Co, Ltd.) (1%, 2% and 3% W/V molar ratio of W) and milled with V 2 O 5 : activated carbon of 2:1 for 2 h. After milling, the samples were separated from the milling vessel and washed 3 times by DI water and alcohol (AR, Sinopharm Chemical Reagent Co, Ltd.), and dried in an oven at 80 °C for 6 h. In the end, all of them were treated at 300 °C for 4 h in a vacuum atmosphere.

Synthesis of VO 2 (M) thermochromic films
Firstly, 0.05 g as-prepared VO 2 (M) powder was dispersed ultrasonically in 10 mL ethyl alcohol for 30 min. Then 0.6 g polyvinyl butyral (PVB, Shanghai Macklin Biochemical Co, Ltd.) was added into the VO 2 (M) dispersion with constant stirring at room temperature until complete dissolution. The mixture was uniformly cast onto the microscope slide substrate by 41 μm roller bar with the speed of 50 m/min for three times. Then, the films were dried at 80 °C for 6 h to remove the ethanol. Finally, thermochromic films were obtained.

Characterization
The crystal structures of the as-prepared samples were determined by X-ray diffraction (XRD, D8 Focus, Bruker AXS Gmbh, Germany ) performed using a Cu−Kα radiation source (λ = 1.54056 Å), with a speed of 4°/min in a 2θ range from 10° to 80° at room temperature. The morphology of the nanoparticles and film were examined by scanning electron microscopy (SEM, MIRA3, TESCAN, Czech Republic) and transmission electron microscopy (TEM, JEM-2100F, JEOL, Tokyo, Japan). Thermal properties were detected by differential scanning calorimetry (Discovery DSC 2500, TA, New Castle, America) at a heating/cooling rate of 10 °C min −1 under nitrogen flow with temperature ranging from 0 to 100 °C.
Thermochromic switching parameters were monitored on an UV-vis-NIR spectrophotometer (SHIMADZU UV-3600 Plus) equipped with a temperature controlling unit.
The integrated luminous transmittance (T lum , 380-780 nm) and solar transmittance (T sol, 250-2500 nm) were essential, which could be obtained from the following equation.
where T(λ) denotes the transmittance at wavelength λ, φ lum (λ) is the spectral sensitivity of the light-adapted eye, and φ sol (λ) is the solar irradiance spectrum for air mass 1.5 corresponding to the sun standing 37° above the horizon [24,25]. As a result, the ΔT sol could be calculated. Figure 1a shows the XRD patterns of samples milled with different molar ratios of V 2 O 5 to activated carbon (2:0.5-2:8) for 1 h. The small diffraction peaks of VO 2 (M) (JCPDS no. 043-1051) appear when the molar ratio is 2:0.5, which suggests that V 2 O 5 can be effectively reduced to VO 2 by activated carbon during ball milling process. With further increase of the carbon content, all the diffraction peaks are corresponding to VO 2 (M) when the molar ratio is 2:1. However, the peak intensity decreases when further decreases the molar ratio, indicating that the excess carbon can reduce the crystallinity, which may be due to that the impurity atoms penetrate interstitial sites in the milling process [26].

One-step ball milling synthesis of VO 2 (M) nanoparticles
In order to explore the formation of VO 2 (M) in the ball milling process, the mixture of V 2 O 5 and C with the molar ratio of 2:1 has been milled for different time. As shown in Fig. 1b, the diffraction peaks of VO 2 (M) gradually generate with prolongation of the milling time to 1 h, and the diffraction peak intensity of VO 2 (M) significantly decreases with further increase of the milling time. This phenomenon indicates that the excessive milling process can reduce the crystallinity of VO 2 (M), which may be because of the accumulated lattice defects in VO 2 (M) [27].
The morphology and size of the mixture milled for different time could be observed from the SEM picture in Fig. 2. It reveals that the raw V 2 O 5 contains large particles, and the particle size decreased sharply after milled for 10 min, as shown in Fig. 2b. With further increase of milling time, the particle size gradually decreased, and the smallest particle with size from 20 to 50 nm (average size of 42 nm) can be obtained when the milling time is 2 h. However, as shown in Fig. 2f, the particles aggregated severely when further increase the milling time to 4 h, which can severely degrade the thermochromic performance.
In order to confirm the reaction mechanism, the clear Ca(OH) 2 aqueous and the 0.1 wt.% PdCl 2 solution are used to detect the gaseous product obtained during ball milling process. The gas is collected by polythene plastic bag, and then inject into the airtight vial containing indicator. As can be seen in Fig. S1, the indicator of pellucid Ca(OH) 2 solution markedly become turbid after injecting the gas, while the indicator of PdCl 2 solution still remained clarification, demonstrating that the gaseous product is CO 2 rather than CO. Thus, the reaction in the milling process can be proposed as the following: As shown in Fig. 3, in the ball milling collision process, the particle size decreased rapidly, which can increase the surface energy of raw material. In addition, it is reported that the temperature of the particle surface (1 μm 2 ) can reach up to 1000 K with the duration of 10 −4 -10 −3 s in the collision process [28][29][30]. Benefited by the increased surface energy of raw materials and instantaneous high temperature during collision process, V 2 O 5 has been reduced to VO 2 (M) by carbon.
The phase transition temperature of VO 2 (M) can be investigated by DSC analysis, due to its first-order reversible phase transition companied with energy absorbed and released during heating and cooling processes. Figure 4a shows the DSC curves of the VO 2 (M) obtained by ball milling for 2 h. A typical metal-insulator transition characteristic could be detected in the sample, which the DSC curves show endothermic peaks at about 72.1 °C during the heating process and exothermic peaks at about 55.4 °C during the cooling process. Besides that, vis-near-infrared transmittance spectra of the composite film of VO 2 (M) is characterized at 15 °C (before phase transition) and 100 °C (after phase transition) for detecting its optical modulation  Fig. 4b. The transmittance spectrum of VO 2 (M) film exhibits high T lum (380-780 nm) of 52.78% and great ∆T sol (250-2500 nm) of 6.38% (Fig. 4b), indicating the VO 2 (M) nanoparticles prepared by ball milling method have great potential for practical application in smart windows.

Low-temperature treatment
The thermochromic performance is not only determined by particle size, but also by crystallinity [4]. In order to further improve the thermochromic performance of the VO 2 (M) nanoparticles synthesized by one-step ball milling,  Optical transmittance spectra at low and high temperature of the VO 2 (M) the powder is treated at different temperature from 300 to 800 °C. As shown in Fig. 5a, the peak intensity is relatively stronger after low-temperature (300 °C for 4 h) treatment, indicating that the crystallinity of VO 2 (M) is obviously enhanced. Fortunately, the particle size remains unchanged after low-temperature treatment. However, the nanoparticles grow into elongated nanorods when the temperature is increased to 400-800 °C, as clearly shown in Figs. S2(b-d). Figure 5c and d show the TEM images of the original VO 2 (M) and the low-temperature treated VO 2 (M) particles. Figure 5e illustrates the HRTEM image of the nanoparticle circled in Fig. 5c. Compared with the original VO 2 (M) particles (Fig. 5c), the low-temperature treated VO 2 (M) particles exhibit much more clear lattice fringes, and the interplanar distance of d = 3.2 Å can still be indexed to the (011) planes of monoclinic VO 2 (Fig. 5f ). Meanwhile, the distinct lattice fringes verify that the crystallinity of the low-temperature treated VO 2 (M) particles has been enhanced compared with the original VO 2 (M) particles, which is in consistence with XRD results. It indicates that the low temperature treatment is beneficial to maintain original morphology of VO 2 (M), as well as improve crystallinity of VO 2 (M) and further promote the thermochromic property.
In order to clarify the positive effect of low-temperature treatment on the thermochromic performance of VO 2 (M), the transmittance spectra of films based on original VO 2 (M) and low-temperature treated VO 2 (M) have been detected. As shown in Fig. 6, it can be seen that the thermochromic performance of the low-temperature treated VO 2 (M) film (T lum , ~ 50.45%; ∆T sol , ~ 9.12%) is improved when compared with the original VO 2 (M) film (T lum , ~ 52.78%; ∆T sol , ~ 6.38%). It indicates that the low-temperature treatment can obviously enhance the thermochromic performance of VO 2 (M).

W-doped VO 2 (M) nanoparticles
In order to decrease the T c of VO 2 (M), tungsten element as the dopant is used in our experiment. Figure 7 shows the XRD patterns of the low-temperature treated VO 2 (M) particles with different doping amounts of W element. Obviously, the diffraction peaks (011) are found to monotonously shift toward a smaller angle with the increase of the W doping amount, and the d-spacing of VO 2 (011) plane calculated by the Bragg's Law (Table 1) increases with the enhancement of the W-doping concentration. This phenomenon can be attributed that the W 6+ ions with larger radius has successfully substituted V 4+ ions, [31] indicating that the W element has been effectively doped in the VO 2 (M). The morphology of the V 1−x W x O 2 (M) with various W-doped contents is characterized by SEM, as shown in Fig. S2. The morphology of the V 1−x W x O 2 (M) nanoparticles basically remain and the average particle size is ~ 40 nm. It demonstrates that the W-doping has little influence on the morphology and the size of VO 2 (M).
As shown in Fig. 8a, the DSC curves of the V 1−x W x O 2 (M) with various W-doped contents confirm the effect of W doping on the thermal properties of V 1−x W x O 2 (M). As expected, the pure VO 2 (M) after low-temperature treatment exhibited a T c value of 67.7 °C. For W-doped VO 2 (M), the T c value of the W-doped VO 2 (M) reduced from 67.7 to 27.1 °C with W-doped concentration increased from 0 to 3 at.% W doping, respectively. And the T c of the V 1−x W x O 2 (M) decreases with a rate of 13.5 °C per at.% W, which agrees well with the reported results [25]. Further analysis of the DSC curves (Fig. 8b) reveals that thermal hysteresis width (∆T c ) sharply decreases with increasing W doping amount, indicating that the phase transition of vanadium dioxide becomes more sensitive with the increase of doping amount. Figure 8c exhibits the transmittance spectra of the films based on V 1-x W x O 2 (M) with the uniform thickness (W0%, ~ 1.6 μm, W1%, ~ 1.7 μm, W2%, ~ 1.4 μm, W3%, ~ 1.5 μm, Fig. S4) and the calculated optical performance (ΔT sol and T lum ) is summarized in Table 2. Clearly, as the W doping amount increases from 0 to 3 at.%, the T lum and ΔT sol of V 1−x W x O 2 (M) decreases from 50.45% and 9.12% to 46.56% and 2.45%, respectively. This is due to the poor crystallinity and serious lattice distortion induced by additional point defects [11,25,32,33]. Table 3 summarizes the thermochromic properties of VO 2 (M) films obtained by different methods in recent years. Compared to other methods, VO 2 (M) obtained by the hydrothermal methods exhibit better thermochromic properties. Guo et al. [34] reported that the undoped VO 2 (M) nanoparticles with average size of ∼ 30 nm synthesized by hydrothermal method exhibit excellent thermochromic performance with the solar modulation efficiency of 12.34% and luminous transmittance of 54.26%. Dai et al. [11] reported F-doped VO 2 nanoparticles, which had lower phase transition temperature of 35 °C at 2.93 at.% F and    However, for the hydrothermal method, the high pressure is a potential security liability, the high reaction temperature (240 °C) and long reaction time (24 h) result in high cost and inefficiency, which hinder its industrialization. Compared to the hydrothermal method, ball milling method to prepare V 1−x W x O 2 in our work is low-cost and high efficiency, which is suitable for mass production.

Conclusion
In this study, VO 2 (M) nanoparticles with particle size ranging from 20 to 50 nm (average size of ~ 42 nm) and relatively good dispersivity have been successfully synthesized by a facile ball milling method. The ball milling parameters are systematically studied and the optimum parameters are obtained. After low-temperature treatment, the ΔT sol is improved from 6.38 to 9.12% due to the enhanced crystallinity of VO 2 (M). When W doping content is 2 at.%, the thermochromic film based on V 1−x W x O 2 (M) nanoparticles can exhibit balanced ΔT sol of 9.64%, T lum of 44.18%, critical phase transition temperature of around 42.0 °C. This work may provide a simple, efficient, economic and environment-friendly approach for practical application in smart window.
Funding This work is supported by University of Jinan (XKY2069) and Shandong Shenna Smart Advanced Materials Co., Ltd.

Compliance with ethical standards
Conflict of interest The authors declare that they have no conflict of interest.
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