Preparation of shape-controlling VO₂(M/R) nanoparticles via one-step hydrothermal synthesis

Abstract

In this study, we developed a facile one-step hydrothermal process that allows to synthesize high-purity VO₂(M/R) nanoparticles with various morphologies such as nanorods, nanogranules, nanoblocks, and nanospheres. W dopants are successfully implanted in VO₂(M/R) unit cells with high doping efficiency, which allows to regulate the size, morphology, and phase of obtained nanoparticles. The underlying regulation mechanism is presented in detail to reveal how hydrothermal products vary with W doping contents, which provides a synthetic strategy for the preparation of shape-controlling VO₂(M/R) nanoparticles with high purity to satisfy different specific demands for corresponding applications in the field of thermochromic smart windows.

Key words： one-step hydrothermal; W doping; shape-controlling; VO₂(M/R) nanoparticles

1 Introduction

Vanadium dioxide (VO₂) is a typical thermochromic material, which has attracted extensive attention all over the world owing to its promising prospects in the fields of energy saving and emission reduction [1–3]. Though various types of VO₂ polymorphs have been developed, only the revisable crystallographic transformation between monoclinic VO₂(M) and tetragonal rutile VO₂(R) phases shows a 4–5 orders of magnitude change in resistivity and infrared transmittance at the critical transition temperature T_c of 68°C [4]. This unique metal–insulator transition (MIT) performance allows the material to be used as a switch to automatically control infrared (IR) transmission to regulate room temperature depending on the outer temperature; therefore, VO₂(M/R) is an ideal material for developing thermochromic smart windows [5].

During past decades, various types of technologies have been used to prepare high-purity VO₂(M/R). Among them, most methods in previous reports have mainly focused on the direct preparation of VO₂(M) film [6–8]. However, the process of synthesizing VO₂(M) nanoparticles first followed by the preparation of film using the as-obtained VO₂(M) nanoparticles is an equally important approach, but it has received less attention. Despite the comparatively complicated technical process, it can considerably decrease stress, which originates from the phase transition, to enhance lifespan. In addition, VO₂(M) nanoparticles can be mixed with other appropriate flexible organics to prepare VO₂(M) films with better optical performance under some specific conditions such as substrates with large area and special shapes [5]. Hence, it is still urgent and desirable to develop technology for the preparation of VO₂(M) nanoparticles.

Owing to the considerable efforts by numerous researchers, VO₂(M/R) nanoparticles have been successfully prepared by different methods such as wet chemistry [9], molten salt synthesis [10], pyrolysis [11], confined-space combustion [12], and hydrothermal synthesis [13–15]. Among them, hydrothermal synthesis is mostly used owing to its simplicity, comparatively environmentally friendly reaction conditions, and ability to control the grain growth.

Usually, the preparation of VO₂(M/R) nanoparticles through the hydrothermal synthesis process is separated into two steps: hydrothermal reaction and annealing treatment. In general, during the hydrothermal process, VO₂(B) nanobelts are generated as the mesophase and are ultimately transformed into VO₂(M/R) during the subsequent annealing treatment. Then, the primitive nanostructures of VO₂(B) are nearly destroyed [16–18]. Nevertheless, one-step hydrothermal synthesis of VO₂(M/R), without annealing treatment, provides a possibility to prepare VO₂(M) nanoparticles by controlling morphology. Though various shapes for VO₂(M) nanoparticles have been presented in previous reports (e.g., nanorods [19], star-shaped nanoparticles [20], nanorings [21,22], nanoflowers [23], nanobeams [24], hollow spheres [25], and other shapes [26,27]), the shape-controlling mechanism still remains unclear, and there are only few reports that discuss it [28].

Herein, we reported a facile process through one-step hydrothermal synthesis to prepare high-purity VO₂(M/R) nanoparticles with various types of morphologies (e.g., nanorods, nanogranules, nanoblocks, and nanospheres) that can be well-regulated by different contents of W dopants implanted in VO₂ unit cells. In addition, we proposed a detailed possible explanation for the underlying shape regulation mechanism.

2 Experimental

2.1 Materials and characterization methods

All chemical agents (analytical grade purity) were obtained from J&K Scientific, Beijing and directly used as-received without further purification. Vanadium pentoxide (V₂O₅) served as the source of V atoms. Oxalic acid (C₂H₂O₄·2H₂O) was used as the reductant. Ammonium (meta) tungstate hydrate, (NH₄)₅H₅[H₂(WO₄)₆], was used as the source of W atoms. Ethyl alcohol (C₂H₅OH) and deionized water were used as solvents for the hydrothermal reaction.

The crystalline structure was characterized by X-ray diffraction (XRD) performed on a D8 discover (Bruker, USA) instrument with the Cu Kα radiation at a 40-kV accelerating voltage and 40-mA current. The size and morphology of nanoparticles were evaluated by field emission scanning microscopy (FESEM, JEOL, Japan). The elemental composition of samples was determined by an energy-dispersive X-ray spectroscopy (EDS) instrument coupled to FESEM. The chemical bonds of the samples were analyzed to confirm the oxidation state of vanadium using X-ray photoelectron spectroscopy (XPS, ESCALab250Xi, Thermo Fisher Scientific, UK).

2.2 One-step hydrothermal synthesis of VO₂(M/R) nanoparticles

Fig.1 Schematic diagram of the one-step hydrothermal synthesis of VO₂ nanoparticles

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Figure 1 shows the schematic diagram of the one-step hydrothermal synthesis of VO₂ nanoparticles. In our experiment, 0.9455 g of H₂C₂O₄ was first dispersed into 37.5 mL of deionized water under magnetic stirring in a constant-temperature bath. After complete dissolution in 5 min, 0.6825 g of V₂O₅ powder was gradually added into the reaction solution to form a brick-red suspension, which could be uniformly dispersed in another 5 min. Subsequently, we used various contents of the (NH₄)₅H₅[H₂(WO₄)₆] powder for each group (see Table 1) to add into the mixed solution under vigorous stirring conditions. In the next 30 min, the suspended particles in the mixed solution were completely dissolved. Meanwhile, the suspension color became dark blue, which was the final precursor solution. Then, the precursor solution was carefully transferred into a 50-mL Teflon-lined reaction kettle with a stainless-steel shell. Then, the reaction kettle was heated at the temperature of 280°C for 24 h with a heating rate of 8°C/min, followed by natural furnace cooling until room temperature. Next, a dark blue precipitate was observed at the bottom of the kettle and separated from the solution by vacuum filtration. Subsequently, the precipitate was sequentially washed three times with deionized water, anhydrous alcohol, and deionized water. Finally, dark blue nanoparticles were obtained after centrifuging at 7000 r/min for 5 min and drying under vacuum at 60°C for 8 h.

Tab.1 Contents of (NH₄)₅H₅[H₂(WO₄)₆] powder for different groups

W doping ratio/at%*	weight/g
0	0
1.0	0.03758
2.0	0.07589
3.0	0.11274
4.0	0.15178

Note: * W doping ratio refers to the atomic percent of Watoms in the total of Wand V atoms in VO2 crystals

3 Discussion

3.1 Doping efficiency of W atoms

In this section, we list the samples prepared with the W doping ratio of 1.0 at% as the value with the minimum W doping content set in our experiments. If W dopants are successfully demonstrated to be implanted into the end products for this group, it can also be applicable to other groups with a larger ratio because higher concentration indicates a stronger reaction rate.

Fig.2 Element mapping and surface spectrogram of VO₂ nanoparticles prepared with the W doping ratio of 1.0 at%. (a) SEM images of nanoparticles. (b) Overall diagram of the key elements: yellow color for W element, red color for O element, and green color for V element. (c) W element. (d) O element. (e) V element. (f) Surface spectrogram of different elements in the range of 0–8 keV. Here, cps is counts per second

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Figures 2(a)–2(e) show EDS elemental mapping for the distribution of key elements in the prepared nanoparticles. It is determined that W dopants are uniformly distributed on the evaluated surface. Figure 2(f) shows the surface spectrogram in the range of 0–8 keV for VO₂ nanoparticles with the W doping ratio of 1.0 at% obtained by EDS coupled to FESEM. In addition to V and O elements, peaks for the W element are also detected at approximately 1.9, 2.1, and 8.4 keV, which confirm that W element has been successfully implanted into the final hydrothermal products.

Tab.2 Atomic percent of key elements in VO₂ nanoparticles determined by EDS

nominal W doping ratio/at%*	actual W doping ratio/at%*	elements	atom%	doping efficiency/%
0	0	O V W	64.33 35.67 0	0
1.0	0.91	O V W	66.74 32.96 0.30	91.0
2.0	1.47	O V W	63.82 35.65 0.53	73.5
3.0	2.16	O V W	65.36 33.89 0.75	72.0
4.0	2.72	O V W	64.74 34.30 0.96	68.0

Note: * Nominal W doping ratio refers to the atomic ratio of W atoms in the total of W and V atoms added to the mixture solution; actual W doping ratio refers to the atomic ratio of W atoms in the total of W and V atoms in the final products.

Fig.3 Comparison of the nominal W and actual W doping content

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Table 2 shows the difference between the nominal and W doping ratios. Table 2 shows that the real content of W in the final product is always lower than their corresponding theoretical values. In addition, the doping efficiency (ratio of the actual to nominal W doping ratios) is always less than 1, which also decreases with an increase in the W doping ratio (see Fig. 3). This occurs because the starting materials in hydrothermal reaction have their own different diffusion speeds. Consequently, there is always a portion of W element that is not capable of participating in the reaction. Even if the doping efficiency in our experiment is slightly lower than that in the previous research [29], VO₂ nanorods exhibit superior doping efficiency. However, such doping efficiency in our experiment is acceptable because our reaction time is only third compared to that in the abovementioned study. In the following section, to be convenient for the readers, we still use the data of nominal W doping ratio as the W doping ratio in this study.

3.2 Mechanism of phase evolution

During the hydrothermal synthesis of VO₂(M/R) with W dopants, the possible reaction can be characterized as follows. First, V₂O₅ particles are dispersed in oxidic acid, and V⁵⁺ is reduced to V⁴⁺. The formula is shown in Eq. (1).

(1){{\displaystyle \mathrm{V}}}_{\mathrm{2}}{{\displaystyle \mathrm{O}}}_{\mathrm{5}}+{{\displaystyle \mathrm{H}}}_{\mathrm{2}}{{\displaystyle \mathrm{C}}}_{\mathrm{2}}{{\displaystyle \mathrm{O}}}_{\mathrm{4}}\to {{\displaystyle \mathrm{V}\mathrm{O}\mathrm{C}}}_{\mathrm{2}}{{\displaystyle \mathrm{O}}}_{\mathrm{4}}+{{\displaystyle \mathrm{H}}}_{\mathrm{2}}\mathrm{O}

In the precursor solution, {{\displaystyle \mathrm{V}\mathrm{O}\mathrm{C}}}_{\mathrm{2}}{{\displaystyle \mathrm{O}}}_{\mathrm{4}} is a labile substance, which can be easily resolved into V⁴⁺. Then, V⁴⁺ ions can further form the stabilized octahedral structure of the growth unit of [VO₆]. Then, [VO₆] continues to develop into the new crystal nucleus at high temperature, which can grow into one-dimensional VO₂(B) for the following step. However, as an intermediate phase in the hydrothermal process, VO₂(B) is easily transformed into VO₂(M/R) at high temperature. Based on Leroux’s research [30], the octahedral structure of [VO₆] will be destroyed at first, and the connection between adjacent units of [VO₆] will be also broken. Then, the obtained micro molecules regrow by elongating in two directions. Thus, the metaphase of VO₂(B) is transformed into VO₂(M). During the entire process, high temperature (generally, above 280°C) is necessary for the entire phase transition to obtain pure VO₂(M) [29].

W dopants are usually added to control the phase transition temperature between VO₂(M) and VO₂(R). In recent years, numerous studies have demonstrated that W dopants can not only regulate the morphology and size of as-obtained VO₂ nanoparticles but also facilitate the phase transition from VO₂(B) to VO₂(M). A possible explanation for the mechanism is that W⁶⁺ can replace V⁴⁺ in the unit cells of VO₂(B), which leads to lattice distortion owing to the larger ionic radius of W⁶⁺ (60 pm) compared to that of V⁴⁺ (58 pm). As a result, the connection between the adjacent units of [VO₆] will break faster, giving rise to the rapid formation of VO₂(M/R). This observation provides a feasible method for the regulation of morphology, size, and purity of VO₂(M) by changing the content of W dopants added in the hydrothermal reaction. In our experiments, the source of W⁶⁺ is provided by ammonium metatungstate, which can be easily dissolved in water to generate W⁶⁺, as shown in Eq. (2).

(2){{\displaystyle \left({{\displaystyle \mathrm{N}\mathrm{H}}}_{\mathrm{4}}\right)}}_{\mathrm{5}}{{\displaystyle \mathrm{H}}}_{\mathrm{5}}\left[{{\displaystyle \mathrm{H}}}_{\mathrm{2}}{{\displaystyle \left({{\displaystyle \mathrm{W}\mathrm{O}}}_{\mathrm{4}}\right)}}_{\mathrm{6}}\right]\to {{\displaystyle \mathrm{N}\mathrm{H}}}_{\mathrm{3}}+{{\displaystyle \mathrm{H}}}_{\mathrm{2}}\mathrm{O}+{{\displaystyle \mathrm{H}}}_{\mathrm{2}}{{\displaystyle \mathrm{W}\mathrm{O}}}_{\mathrm{4}}

3.3 Phase behavior

Figure 4 shows XRD patterns of the hydrothermal reaction products prepared at 280°C for 24 h with different W doping ratios. For undoped VO₂, the phase of the end product is comprised of VO₂(B) and VO₂(M), which indicates that pure VO₂(M) cannot be obtained without W dopants in our experiments. However, when the W doping ratio increases to 1.0 at%, the end products are VO₂(M) nanoparticles with high purity because nearly all diffraction peaks are consistent with those of a standard VO₂(M) (JCPDS#43-1051) phase. When the W doping ratio is 2.0 at%, the end products are still pure VO₂(M) nanoparticles. Moreover, the intensity of peaks is also intensified compared to that of 1.0 at% doping ratio, which demonstrates a much higher crystallinity for the VO₂(M) crystal structure. Whereas, when the W doping ratio reaches 3.0 at%, the M(130) peak between 64° and 66° is split into R(310) and R(002) peaks (see Fig. 4), which is regarded as a diagnostic criterion for the phase transition from VO₂(M) to VO₂(R). It can be confirmed that the end product is a mixture of VO₂(M/R). However, minor impure peaks still appear at 29° despite of high purity of VO₂(M/R) nanoparticles. After the W doping ratio reaches 4.0 at%, except for the more obvious impure peaks, new impure peaks also appear at approximately 55°, which indicates that the purity of prepared VO₂(M/R) decreases. Overall, in our experiment, a certain amount of W dopants can facilitate the formation of VO₂(M) compared to nanoparticles prepared with the W doping ratio of 0, 1.0, and 2.0 at%. It can also promote the formation of VO₂(R) at the W doping ratio of 3.0 at%. However, excess amount of W dopants (4.0 at%) will lead to the generation of undesirable peaks during the hydrothermal reaction.

Fig.4 XRD patterns of prepared nanoparticles with different W doping ratios (left) and a magnified view (right) of the XRD data in the range of 64°<2θ<66°

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3.4 Morphological behavior

For undoped samples (see Fig. 5), the morphology of end products comprises hexagon nanoparticles with an average side length of 1.5 mm, aggregation bulk of multiple nanorods with an average length of 4 mm, and some snowflake nanoparticles (see Fig. 5(b)), which are mainly caused by the preferable growth of the M(011) crystal facet. Combined with the analysis of XRD patterns in Fig. 4, there still exists a portion of VO₂(B) nanobelts, which has not been transformed into VO₂(M) nanoparticles without W dopants at 280°C for 24 h.

Fig.5 SEM images of the prepared nanoparticles without W dopants. (a) Hexagon nanoparticles and nanorods. (b) Snowflake nanoparticles

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Figure 6 shows the morphology of nanoparticles prepared with the W doping ratio of 1.0 at%. The end products are a mixture of VO₂(M) nanogranules with an average diameter of 200 nm and VO₂(M) nanorods with an average length of 1 mm. Compared to undoped nanoparticles, the change in both phase and morphology is attributed to the replacement of a portion of V⁴⁺ in the VO₂ unit cell by W⁶⁺. This change accelerates the decomposition of VO₂(B) owing to aggravated lattice distortion. Furthermore, the addition of W⁶⁺ brings extra electrons, which exist in d_II orbitals and can be transferred to adjacent V⁴⁺. Thus, new chemical pairs of V³⁺–W⁶⁺ and V³⁺–V⁴⁺ are generated, which results in the formation of new oxygen vacancies of the unit cells. Consequently, the ability to incorporate surrounding elements is intensified, and the preferable growth of the M(011) peak is accelerated to generate nanorods with small size, which is confirmed by Fig. 4, where the M(011) peak becomes sharper when the W doping ratio changes from 0 to 3.0 at%.

Fig.6 SEM images of prepared nanoparticles with the W doping ratio of 1.0 at% under different magnification. (a) ×5k. (b) ×20k

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Fig.7 XPS spectrum of nanoparticles prepared with the W doping ratio of 1.0 at%. (a) General patterns. (b) V element. (c) W element. (d) O element

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To further confirm our explanation, we conducted XPS analysis of nanoparticles prepared with the W doping ratio of 1.0 at%, as shown in Fig. 7. Figure 7(a) shows the general patterns of the XPS spectrum, in which the peaks at 515–525 eV are attributed to V_2p, peaks at 35–42 eV are associated with W_4f, peak at 529.6 eV is associated with O_1s, peak at 631.5 eV is associated with V_2s, peak at 71.5 eV is associated with V_3s, and peaks at 43.2 eV and 700.0–850.5 eV are associated with V_3p and V (Auger), respectively. In addition, the peak at 516.5 eV is associated with V_2p3/2 (see Fig. 7(b)), which is below the reference value (517 eV) given for pure polycrystalline fully oxidized V₂O₅ [31] and demonstrates the existence of vanadium in low oxidation states (mainly V⁴⁺). However, this value is higher than that of pure VO₂ in previous reports [32–36], which indicates that part of V⁴⁺ is reduced into V³⁺ during the exchange reaction [37]. In Fig. 7(c), the peaks at 35.6 eV and 37.6 eV are associated with W_4f7/2 and W_4f5/2, respectively, which demonstrates the existence of W⁶⁺ in VO₂ crystal cells. Therefore, our explanation for the regulation mechanism of W⁶⁺ is further confirmed. When the W doping ratio is 2.0 at%, nanoparticles completely disappear (see Fig. 8). Instead, only nanorods are present with an average length of 3 mm. More W⁶⁺ are implanted into VO₂ unit cells when more W⁶⁺ are added into the hydrothermal solution, which induces the generation of larger crystalline grains and accelerates the growth of VO₂(M).

Fig.8 SEM images of nanoparticles prepared with the W doping ratio of 2.0 at% under different magnification. (a) ×5k. (b) ×10k

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Fig.9 SEM images of nanoparticles prepared with the W doping ratio of 3.0 at% under different magnification. (a) ×5k. (b) ×30k

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Fig.10 SEM images of nanoparticles prepared with the W doping ratio of 4.0 at% under different magnification. (a) ×5k. (b) ×15k

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Figure 9 shows nanoparticles prepared with the W doping ratio of 3.0 at%. Except for a portion of nanorods, most of them grew into massive nanoblocks with an average length of 700 nm. This result indicates that excess W⁶⁺, which was implanted into VO₂ crystal cells, promotes lattice distortion and restrains the preferable growth of the M(001) crystal facet. This conclusion is further confirmed by the wider M(001) peaks in Fig. 4. Thus, massive nanoparticles with square and spherical shapes are generated. When the W doping ratio is increased to 4.0 at%, nanospheres are formed with an average diameter of 2 mm; these spheres are made of many nanoparticles with rough surfaces, as shown in Fig. 10. The lattice distortion of VO₂ crystal cells owing to the implantation of W⁶⁺ is further promoted, and the preferable growth of M(001) crystal facet is further restrained. Impure crystals are also observed in the end products.

Fig.11 Overall schematic diagram of evolution of VO₂ nanoparticles and regulation scheme

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Finally, the overall schematic diagram of the evolution of VO₂ nanoparticles and the regulation scheme based on different W doping contents is shown in Fig. 11. The morphologies of nanoparticles prepared in our experiment varied from nanogranules, nanorods, nanoblocks to nanospheres with an increase in the W doping content from 0 to 4.0 at%. In addition, we confirmed that W dopants can promote the phase transformation from VO₂(B) to VO₂(M), and the average size of as-obtained nanoparticles is increased from 200 nm to 2 mm with an increase in the W doping content.

4 Conclusions

In summary, we prepared high-purity VO₂(M/R) nanoparticles with various types of morphologies (e.g., nanogranules, nanorods, nanoblocks, and nanospheres) through one-step hydrothermal synthesis that is based on different W doping content. In our experiments, W dopants are essential in regulating the morphology of VO₂(M/R) nanoparticles through restraining the preferable growth of VO₂(M/R). In addition, we confirmed that a certain number of W dopants can promote the phase transformation from VO₂(B) to VO₂(M/R); however, excess content can lead to the generation of undesirable crystals and lower purity of nanoparticles. In general, our study provides a facile process by combining hydrothermal synthesis with W doping strategies to prepare high-purity shape-controlled VO₂(M/R) nanoparticles. This process is expected to be used in industrial mass production in the field of thermochromic smart windows because the process is simple, environmentally friendly, and allows to control the crystal growth. In addition, the approach used in this study to control the process of crystal growth with inorganic dopants to obtain shape-controlled nanoparticles can be also applied to the preparation of other metallic crystals to obtain desired morphologies.

Acknowledgements

The authors acknowledge the financial support from the Shenzhen Science and Technology Innovation Council (No. JCYJ20170413141208098), the National Natural Science Foundation of China (Grant No. 51601101)

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