Effect of W-Mo doping on the phase transition temperature of VO2 synthesized by hydrothermal microwave method

In this study, the combination of hydrothermal microwave technology and high-temperature method was used to efficiently control the formation of M-phase vanadium dioxide (VO2) nanoparticles, which were promising materials for optoelectronic switches and smart windows due to their excellent optoelectronic properties during the phase transition. The phase state and structure of VO2depended on its synthesis parameters, and the results showed that the optimal conditions for VO2(B) synthesis in a hydrothermal microwave were 120 °C for 2 h, which was a novel method for efficiently preparing VO2(B) at a low temperature. By vacuum annealing, VO2(B) could be transformed into monoclinic VO2(R), where VO2(R) converts into VO2(M) on cooling to room temperature. Furthermore, the phase transition temperature of W-Mo co-doped VO2(M) decreased by 14.8 °C, showing that the incorporation of W-Mo elements into the VO2-based structure affects the material’s phase transition temperature.


Introduction
Among the numerous metal oxides, vanadium oxide (VO 2 ) stood out for its excellent optical [1], electrical [2], and magnetic [3] properties. In recent decades, VO 2 has attracted the attention of academics due to its unique properties [4], resulting in its widespread application in numerous energy-saving industries. The richness of the VO 2 structure lies in its diverse crystalline phase composition [5]. Several common VO 2 crystalline phases have been identified to date [6], including the monoclinic VO 2 (B) [7], the rutile VO 2 (R) [8], and the monoclinic VO 2 (M) [9]. The electrochemical characteristics of VO 2 (B) have attracted the interest of numerous battery energy researchers [10]. In addition, VO 2 (B) was often used as a precursor crystal phase for other phase transitions [11]. VO 2 (B) can be converted to VO 2 (M) in a high-temperature environment [12]. At the critical temperature (T C = 68°C), VO 2 (M) and VO 2 (R) underwent a reversible structure (SPT) phase transition and a metal-insulator (MIT) phase transition [13,14]. The features after phase transformation caused drastic changes in both optical and electrical properties, VO 2 was therefore widely used in energy-efficient and comfortable intelligent windows [15], data storage [16], sensors [17], and so on. Although VO 2 (M) has favorable features, its somewhat higher phase transition temperature restricts its usage in certain applications. In recent years, researchers have studied the effect of elemental doping on the phase transition temperature of VO 2 (M). The results indicated that the doping atoms W [18], Mo [19], Eu [20], and F [21] could alter the phase transition temperature of VO 2 (M) [22]. Furthermore, the high-quality preparation of VO 2 (B) powders has a positive influence on the final properties tested, which is also necessary to obtain fine-sized VO 2 (M).
As VO 2 (B) is a synthetic precursor phase for VO 2 (M) [23], its preparation is particularly important. Until now, two different preparation techniques have been developed for obtaining VO 2 (B), which were distinguished by pressure: the traditional hydrothermal (HT) method [24] or the reduction treatment of vanadium oxide precursors under atmospheric pressure [25,26]. For example, Ji S et al [27] used the HT method to prepare VO 2 (B) at 180°C for 2-24 h. Dimitra Vernardou et al [28] successfully prepared VO 2 (B) by atmospheric pressure chemical vapor deposition (APCVD) at 500°C using VO(acac) 2 as a precursor. In conclusion, the Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. efficient synthesis of VO 2 (B) at low temperatures represents a novel obstacle. The hydrothermal microwave (HTMW) method has been reported as an effective way to accelerate the reaction rate of chemical synthesis [29,30]. To overcome the high temperature and lengthy time requirements of the HT method, we constructed a HTMW device that could use microwave frequencies to align polar molecules with the electric field produced by an alternating electric field, causing molecular friction [31], which could produce VO 2 (B) at 120°C and 0.5 h. Moreover, it has been demonstrated that the HTMW method exhibits a shorter synthesis time and a lower synthesis temperature to obtain fine VO 2 (B) powders compared to the HT.
Herein, we synthesized VO 2 (B) using a precursor solution consisting of vanadium pentoxide and citric acid. Citric acid served as a reducing agent and a stabilizing agent [32]. It adhered well to the surface of metal oxides and enhanced the particles' water solubility [33]. The chemical reaction was as shown in the following equation [34]: In the present work, the synthesis of M-phase VO 2 nanopowders was coordinated by combining the HTMW and the high-temperature calcination method. Systematically, the effects of microwave temperature and time on the production of VO 2 (B) powders were examined. The results showed VO 2 (B) was successfully synthesized under the HTMW condition at 120°C for 0.5 h, and it could be converted into VO 2 (M) by heat treatment. W-Mo co-doped VO 2 (M) was subsequently prepared by calcining the material at 560°C. The DSC results revealed that the T C of W-Mo co-doped VO 2 (M) decreased by 14 , ethanol (99.5%) were provided by Damao (Tianjin). The glass containers were cleansed with ultra-purified water and thoroughly dried. All reagents were analytical grade and used directly without further processing.

Microwave synthesis of VO 2 (B)
0.4732 g V 2 O 5 and 0.8795 g C 6 H 8 O 7 ·H 2 O were stirred into 40 ml distilled water until the mixture became a yellow suspension. The suspension was transferred to a 100 ml microwave reactor, which was sealed and maintained at 120°C for 0.3-2 h before being cooled to room temperature. Following filtration, washing with water and ethanol to remove unreacted compounds, and vacuum drying at 60°C, 0.3124 g monoclinic structure VO 2 (B) was obtained.

Preparation of VO 2 (M)
and element-doped VO 2 0.1562 g VO 2 (B) was treated in a vacuum and a vacuum-less atmosphere at 560°C-600°C for 2 h. After the heated samples naturally cooled to room temperature, the 0.1468 g powder obtained in a vacuum was VO 2 (M).

Characterization
The crystal structures of VO 2 were studied by x-ray diffraction (XRD, D8 FOCUS, Bruker Germany) using a Cu-Kα radiation source at 40 kV and 20 mA, with a 2θ range from 10°to 80°. The size and morphology were studied through the use of scanning electron microscopy (SEM, JSM-7610F Plus) and transmission electron microscopy (TEM, TESCAN MIRA4). Element abundance was calculated using energy dispersive spectroscopy (EDS, JSM-7610F Plus). The phase transition parameters of the resulting compounds were obtained using differential scanning calorimetry (DSC Q100) with a heating rate of 10°C min −1 in nitrogen and a temperature range of 10°C to 100°C. X-ray photoelectron spectroscopy (XPS) data were collected by a Thermo Fisher Scientific K-Alpha (USA). An inductively coupled plasma optical emission spectrometer (ICP-OES, Agilent 7700 ) was used to measure the metal contents in the samples.   figure 1(g) demonstrated that the HTMW-prepared sample formed a V 2 O 5 peak. As the reaction time was prolonged to 0.5-2 h (figures 1(b)-(e)), the V 2 O 5 XRD peak disappeared, leaving only the VO 2 (B) peaks. This trend indicated that in the HTMW method, it was necessary to maintain proper reaction time to reduce V 2 O 5 , so as to successfully form VO 2 (B). Table 1 summarized the effects of temperature and time on the synthesis of VO 2 (B) by the HTMW and HT methods. Consistent with the preceding XRD characterization data, the synthesis of VO 2 (B) using the HTMW method required substantially lower temperatures and shorter reaction times than the HT method. In a word, compared to the HT method, the HTMW method has the benefits of low energy consumption and high efficiency. VO 2 (B) was a sub-stable phase that could be irreversibly transformed into VO 2 (R) under high temperature annealing conditions, where VO 2 (R) transforms into VO 2 (M) after cooling to room temperature [11]. Figures 2(a)-(b) represent the XRD patterns of VO 2 (M) obtained in a vacuum atmosphere, revealing the effective synthesis of VO 2 (M). It could be observed that all diffraction peaks were easily assigned to the monoclinic crystalline phase (space group: P 21/c ) of VO 2 (M) (JCPDS card No. 82-0661) [38]. However, changing the reaction atmosphere to a vacuum-less atmosphere resulted in the conversion of VO 2 (B) to V 2 O 5 , despite the other reaction conditions remaining unchanged (figures 2(c)-(d)), indicating that VO 2 (M) can only be successfully prepared by annealing at 560°C for 2 h in a vacuum atmosphere. Figure 3 illustrated the XRD patterns of VO 2 (M) with and without the elements. Pure VO 2 (M) (JCPDS card No. 820661) was produced without the additive, and the diffraction peak at 27.8°corresponds to the (011) crystal plane of VO 2 (M) ( figure 3(c)). However, as shown in figures 3(a)-(b), the addition of W and Mo elements results in a reduction of VO 2 (M) due to the appearance of peaks associated with W-VO 2 and Mo-VO 2 , and the (011) crystal plane exhibits a reasonable deviation [39]. No impurity phases VO 2 (B), V 2 O 5 , VO 2 (A), or V 2 O 3 were detected, indicating that W-VO 2 (M) and W-Mo-VO 2 (M) possess high purity and well crystallization.  The crystal grain size of the sample in figure 3 could be calculated using the Scherrer formula:

Results and discussion
where D is the average size of the VO 2 particles, λ is the wavelength of x-ray radiation (λ = 0.154056), K is a constant, typically 0.943, β is the width of the characteristic peak (011) at full-width at half maximum (FWHM), and θ is the Bragg angle (011). According to the date in table 2, the grain size of the pre-doped and post-doped samples fell from 35 nm to 21 nm, this conclusion was also verified by the later SEM results. The morphology of VO 2 prepared before and after annealing was characterized by SEM, as shown in figures 4(a)-(b). VO 2 (B) prepared by the HTMW method exhibited a layered sheet structure with a width of approximately 500 nm and a typical length of several microns, whereas VO 2 (M) prepared for the annealing treatment of VO 2 (B) displayed a clearly layered sheet structure of nanoparticles stacked together. The small size   of the particles in the layered sheet structure was propitious to weaken the light scattering, as they are used in VO 2 -based thermochromic smart windows [15]. Figures 4(c)-(d) showed the TEM images of the obtained monoclinic VO 2 (M). It was evident that the image remained laminated, further validating the reported layered structure of VO 2 by SEM. The TEM image corresponding to the (011) plane of VO 2 (M) revealed a lattice spacing of 0.32 nm and a clean crystal lattice, indicating that VO 2 (M) possessed excellent crystallinity [40]. As shown in figure 5(a)'s SEM image, VO 2 (M) size decreased to a nanoflower shape after doping with W and Mo, revealing that doping with W and Mo also affects the morphology of VO 2 (M). And such an ultra-small size enables more efficient doping of W-MO components into VO 2 (M), making it more responsive in devices such as smart windows and sensors [17]. Figure 5 showed the EDS with elemental mapping images analysis of W-Mo-VO 2 (M) and W-VO 2 (M) samples. As displayed in figure 5(a), the W-Mo-VO 2 (M) sample contained only four elemental peaks, namely O,  V, Mo, and W, and no other impurity peaks, indicating the product was pure. The distribution ranges of O, V, Mo, and W elements matched their respective SEM images, showing that the elements were uniformly dispersed in VO 2 (M). The near proximity of the Mo and W distribution ranges revealed that Mo and W ions were successfully doped into the VO 2 (M) lattice [40]. Figure 5(b) depicted a typical EDS spectrum of a W-VO 2 (M) sample, indicating that the sample consists of just V, O, and W elements. In addition, the contents of Mo and W in doped-VO 2 (M) were measured by ICP-OES characterization. The results were shown in table 3. In the W-Mo-VO 2 (M) sample, the ratio of W:Mo was 1:1.18, and the percentage content of W element in the W-VO 2 (M) sample was 0.998%, and the result was approximately consistent with the actual doping ratio. Therefore, the present study can obtain homogeneous W-Mo-VO 2 (M) composite powders.
The XPS spectra of W-Mo co-doped VO 2 (M) and W-doped VO 2 (M) presented in figures 6-7 both clearly showed intense peaks of V, O, C, and W, but only the W-Mo-VO 2 (M) sample showed peaks of Mo elements, which indicated that W and Mo were successfully incorporated into the lattice of VO 2 (M); During the doping process, the V atoms were replaced by W and Mo atoms, which was further confirmed by the presence of W-O and V-O bonds in the high-resolution O1s XPS spectra (figures 6(b) and 7(b)), along with the demonstration that the doped W atoms and Mo atoms were placed in the positions of V atoms in [VO 6 ] octahedra. The O1s and V2p spectra of the two samples were shown in figures 6(c) and 7(c), respectively. The phase of vanadium oxide could typically be determined based on the location of the distinctive V2p3/2 peak, as this is the position most sensitive to the phase transition [41]. As indicated in figures 6(c) and 7(c), the V 2p nuclear peaks in both samples were well-matched to the V 4+ state, indicating that the chemical valence was well-controlled [42]. The peaks of 524.17 eV and 524.27 eV were attributed to V2p1/2, while the binding constant of V2p3/2 was approximately 516 eV. This peak represented the oxidation state of V 4+ , and the absence of a V 5+ peak indicates that VO 2 has been fully reduced. In the W-Mo-VO 2 (M) sample (figure 6(d)), Mo is present as Mo 6+ with binding energies of 232.18 eV and 235.5 eV for the Mo3d5/2 and Mo3d3/2 peaks, respectively. Figure 6(e) showed the highresolution spectra of W4f, which exhibited two distinct peaks at binding energies of 35.23 eV and 37.36 eV, belonging to the energy levels of hexavalent tungsten W4f7/2 and W4f5/2, respectively [39,43]. Figure 7(d) showed that W is also present in W-VO 2 (M) as W 6+ , with binding energies of 34.93 eV for W4f7/2 and 37.12 eV for W4f5/2.
The phase transition between VO 2 (M) and VO 2 (R) was accompanied by a massive change in properties [44]. Combined with the above characterization analysis indicated the successful conversion of VO 2 (B)→ VO 2 (R)→ VO 2 (M). However, the high phase transition temperature (T C = 68°C) of VO 2 (M) limited many applications [45]. To confirm the influence of the as-synthesized samples on the T C , the phase transition property of W-doped and W-Mo co-doped VO 2 (M) was studied by DSC. As shown in figure 8, the endothermic peaks occurred at 68°C throughout the heating process, which was consistent with the stated phase transition temperature of VO 2 (M) [46]. Furthermore, the appearance of endothermic peaks also indicated the reversible

Conclusions
In this work, we successfully synthesized VO 2 (B) by the reduction of V 2 O 5 and citric acid through the HTMW method. Using microwave frequency to cause molecular friction between polar molecules and an electric field, the HTMW method was an effective method for accelerating the reaction rate of chemical synthesis, allowing VO 2 (B) to be synthesized at a lower temperature and in less time while retaining its crystalline structure. This work demonstrated that the HTMW method has the advantage of being easier to prepare VO 2 (B) at low temperatures than the HT method, which was a promising choice for the low-temperature production of nanoparticle particles. Meanwhile, the XRD indicated that VO 2 (B) could be converted into VO 2 (M) at 560°C in a vacuum. In addition, the W-Mo double doping elements reduced the critical temperature (T C = 68°C) of metal-insulator phase transition (VO 2 (M) ⇌ VO 2 (R)). By observing endothermic peaks of W-Mo co-doped VO 2 (M) at 52.6°C using DSC, it was determined that W-Mo doping may be exploited to tune its T C . The DSC demonstrates that the W-Mo co-doped VO 2 (M) produced has thermogenic mutation characteristics.