Introduction

Vanadium dioxide (VO2) plays a crucial role in many fundamental research and practical applications. For instance, Mott field-effect transistor, light modulator and optical storage medium are potential products based on VO21,2,3. Moreover, VO2 with a metal-insulator phase transition (MIT) is a key material for applying to thermochromic smart windows because it exhibits a reversible structural transformation from an infrared-transparent monoclinic phase (VO2(M1)) at low temperature to an infrared-reflective rutile state (VO2(R)) at higher temperature than the transition, while maintaining certain visible transmittance4,5,6,7. Whereas, VO2 exhibits hysteresis in its phase transition properties and mechanical degradation on passing through the MIT because of stresses during the structural change8. In addition, the phase transition temperature (Tc) of MIT (68 °C) is always too high for the practical application of VO2-based materials4.

Nano-materials often exhibit extraordinary physical and chemical properties compared to their bulk counterparts9. One dimensional nanostructures, for example, nanorods represent particularly attractive because they present novel characteristics owing to their small radial dimension while retaining longitudinally connected substance10. In confined nanoscale system, more localized electronic states as well as narrower bands are usually supposed to increase the densities of states and lead to the superior phase transition behavior of VO211. Using the hydration-cleavage-exfoliation solvothermal process, Banerjee and co-workers have reduced the Tc of undoped VO2 by synthesizing various sized nanostructures12. In their research, the phase transition temperature during the cooling cycle is more significantly affected than the heating cycle by nanostructuring, therefore, the hysteresis width is observed to be much wider for all the nanostructures. Gao and co-workers have also regulated the hysteresis width through the nano-size effect13, which provides a key that nanoscale VO2(M1/R) possesses the probability of tuning hysteresis width for obtaining a sharper, more reproducible phase transition. Up to now, more than 20 compounds of vanadium oxide (VO, V2O3, VO2, V6O13, V8O15, V2O5 and so on14) and 10 polymorphs of VO2 (B, A, T, M1, M2, R phase and so on15) had been reported. Only the VO2(M/R) (the M1 phase is referred to as the M phase of VO2 in this study) experiences a fully reversible MIT at the vicinity of room temperature (RT). Moreover, low temperature synthetic method has usually generated VO2(B) nanobelts and subsequently can be transformed to VO2(M/R) by the post-heating treatment, but the nanostructure has been nearly destroyed16,17,18. So it should be a challenge to synthesize pure phase VO2(M/R) with a shape controlled nanostructure.

The ongoing debate associated to the fundamental origin of the phase transition behavior in VO2 involves electron-correlation-driven (Mott transition)19,20, structure-driven (Peierls transition)21,22, or the cooperation of both23. W doping is known as an effective route to regulate electron density in the conduction band for decreasing Tc by approx. 20–26 °C/at% W for the bulk and by 50–80 °C/at% W in nanostructures24,25,26,27. Synthesis of VO2(M/R) by controlling both the shape of nanostructures and the amount of W dopant could be a good strategy to narrow the hysteresis width while reducing Tc for obtaining an excellent phase transition property of VO2-based materials. Of note, systematically experimental investigation of nonstoichiometric effect in VO2 has been insufficient. The phase transition behavior has been demonstrated to be also sensitive to vanadium or oxygen related vacancies, even a deviation in the oxygen stoichiometry by a few percent can cause the lattice structure change and result in several orders of magnitude difference in the resistivity transition or the phase transition temperature shift28,29. Therefore, studying on oxygen nonstoichiometry induced reduction of Tc will contribute to the general understanding of the intrinsic MIT mechanism in VO2.

In this study, we successfully explored a one-pot hydrothermal method to prepare VO2(M/R) with desired morphology. It is inspiring to discover that the W dopant promotes the generation of pure phase VO2(M/R) nanorods with high aspect ratio. Moreover, the effect of oxygen nonstoichiometry on the structural phase transition and subsequently Tc of VO2 is discussed in detail.

Results

Shape-controlled synthesis and phase metamorphosis behavior upon W doping

Figure 1 shows the crystalline phase metamorphic behavior of WxV1−xO2 with x = 0, 0.5, 1.0 and 2.0 at% respectively where the temperature being kept at 280 °C but the different duration of the hydrothermal time being applied. For the undoped VO2, pure phase VO2(B) is obtained for the duration of the hydrothermal time for 6 h. By increasing the duration of the hydrothermal time from 12 to 72 h, the peak of {011} for VO2(M) (M {011} at around 27.8°) appears and becomes more significant. However, there always exists the secondary phase VO2(B) in the final product. Serial SEM images in Fig. 2 show the morphology transition behavior of the undoped VO2 upon increasing the duration of the hydrothermal time. Products of the metastable VO2(B) are the tangled nanobelts in the morphology for the 6 h-sample. By increasing the duration of the hydrothermal time from 12 to 72 h, VO2(B) nanobelts always exist as partial morphology except the block or snowflake VO2(M). In conclusion, we could not synthesize pure phase VO2(M) without W doping.

Figure 1
figure 1

XRD patterns of WxV1−xO2 prepared at 280 °C with W doping levels ranging from 0.0 to 2.0 at% for different duration of hydrothermal time.

The filled dark blue diamond and olive star are characteristic peaks for B and A phase of VO2 respectively. The red and black columns belong to standard pattern in JCPDS card No. 65–2358 for VO2(M) and No. 76–0677 for VO2(R) respectively.

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Figure 2
figure 2

SEM images of undoped VO2 synthesized at 280 °C for different duration of hydrothermal time.

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By increasing the W-doped level to 0.5 at%, VO2(B) nanobelts always exist as partial morphology except the snowflake or rod-like VO2(M) for the duration of the hydrothermal time ≤48 h as shown in Figs 1 and 3. Inspiringly, the peaks of VO2(B) vanish and pure phase VO2(M) (Tc > RT) with uniformly rod-like morphology is successfully synthesized for the 72 h-sample. Meanwhile, for the hydrothermal samples prepared at 280 °C with W-doped of 1.0 and 2.0 at%, pure phase VO2(R) with uniformly rod-like morphology is obtained when the duration of the hydrothermal time ≥48 h and ≥12 h respectively as shown in Figs 1,4 and 5. The synchrotron radiation X-ray powder diffraction (SRXPD, λ = 0.50 Å) data confirm the pure phase VO2(M/R) is exactly free from the existing of the other V-O compounds and other VO2 phases, which was reported by our group in the recently study30. As an overall comparison, the schematic illustration of the morphology metamorphic behavior of VO2 is summarized in Fig. 6.

Figure 3
figure 3

SEM images of WxV1−xO2 (x = 0.5 at%) synthesized at 280 °C for different duration of hydrothermal time.

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Figure 4
figure 4

SEM images of WxV1−xO2 (x = 1.0 at%) synthesized at 280 °C for different duration of hydrothermal time.

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Figure 5
figure 5

SEM images of WxV1−xO2 (x = 2.0 at%) synthesized at 280 °C for different duration of hydrothermal time.

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Figure 6
figure 6

Schematic illustration of the morphology metamorphic behavior of VO2.

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WxV1−xO2 with x = 4.0, 6.0 and 10.0 at% were prepared at 280 °C for 72 h to investigate more W dopant on the crystalline phase metamorphic and morphology transition behavior of the as-obtained products as shown in Figs 7 and 8. Pure phase VO2(R) is still obtained for the 4.0 and 6.0 at% sample. When the W-doped level increases to 10.0 at%, VO2(B) nanobelts are grown again besides the main phase VO2(R). The results indicate a certain doping level of W could promote VO2(B) metamorphoses to pure phase VO2(M/R), which agrees with the previous reports14,18. Whereas, more excess W dopant would prevent the metamorphosis from VO2(B) into VO2(R) thoroughly. The intensity ratio between the XRD peaks of {110} and that of {101} of VO2(R) prepared at 280 °C for 72 h with different W-doped levels is listed in Table 1. It increases significantly from 2.1 to 5.7 with increasing the W-doped level from 1.0 to 2.0 at%. The strong intensity of the {110} reflections points to the strongly preferential growth direction of the structures, as has also been noted previously for VO2 nanowires prepared at high temperatures by vapor transport31,32,33. Simultaneously, the aspect ratio of the VO2(R) nanorods increases from nearly 5 to 10 with the increased dopant. Whereas, if the W-doped level increases from 4.0 to 10.0 at%, the intensity ratio {110}/{101} decreases from 2.5 to 1.1. Meanwhile, the aspect ratio of nanorods decreases with the increased W dopant as shown in Fig. 8. Finally, the bulk crystal of VO2(R) is grown for the 10.0 at% sample. The results indicate a certain doping level of W can promote the preferential growth of R {110} and the increased aspect ratio of VO2(R) nanorods, whereas the excess W would restrain.

Table 1 Intensity ratio between the XRD peaks of {110} and that of {101} of WxV1−xO2 prepared at 280 °C for 72 h with different W-doped levels.
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Figure 7
figure 7

XRD patterns of the WxV1−xO2 prepared at 280 °C for 72 h with W doping levels ranging from 4.0 to 10.0 at%.

The filled dark blue diamond is characteristic peaks for B phase of VO2. The black column belongs to standard pattern in JCPDS card No. 76–0677 for VO2(R).

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Figure 8
figure 8

SEM images of the WxV1−xO2 prepared at 280 °C for 72 h with different W doping levels.

(a) 4.0 at.% (b) 6.0 at.% (c) 10.0 at.%.

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The shape-controlled mechanism revealed by TEM

The length of W-doped 4.0 at% VO2 nanorods (synthesized at 280 °C for 72 h) is about 2.5 μm with 600 nm in diameter as shown in the low magnification TEM image in Fig. 9A. The single-crystalline nanorods is confirmed by the lattice images of HRTEM and the inset SAED pattern as shown in Fig. 9. The lattice constants observed in Fig. 9B are 0.3236 and 0.2430 nm respectively, which can be indexed to the spacing of R {110} and R {101} and the angle between the two lattice images is 67.9° in arc and this corresponds to the angle between the designated crystal planes of R (110) and R (101). In addition, the (001) plane orientation is just perpendicular to the nanorod' growth direction R (110) and revealing the preferential growth direction of the VO2(R) nanorods is along [001]. The results demonstrate that the preferential growth of nanorod' growth direction R (110) is responsible for the increased aspect ratio of VO2(R) nanorods. It is generally known that the greater the d-spacing, the atom arrange more closely on the crystal plane. For the body-centered tetragonal VO2(R), (110) with the largest d-spacing contributes to the lowest surface energy for the preferential growth of VO2 grains. According to the SAED pattern shown in the inset of Fig. 9A, the bright diffraction spots reveal the good crystallinity of the sample. Based on the Bragg equation, the diffraction spots can be ascribed to different crystal planes of VO2(R). The three Bravais lattice points shown in the SAED of the inset of Fig. 9A correspond to crystal planes of R (110), R (101) and R (211) respectively as indexed therein. This definitely demonstrates the nanorods belong to VO2(R). Moreover, no fringe spacing belongs to tungsten oxides or their derivatives are detected by HRTEM, which confirms the capture of W atoms into the crystal lattice of VO2 as mother matrix and the formation of homogeneous solid-solution of WxV1−xO2.

Figure 9
figure 9

(A) TEM image of the single VO2 nanorod for the W-doped 4.0 at% sample and the corresponding SAED pattern (inset). (B) Lattice-resolved HRTEM image of the single nanorod.

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Influence of oxygen nonstoichiometry on the phase transition behavior

Figure 10A shows the DSC curve of the hydrothermally undoped sample treated at 280 °C for 72 h (HTh1). The endothermic and exothermal transition temperature is 62.3 and 49.3 °C during heating and cooling cycles respectively. Thus, the phase transition temperature (defined as Tc = (Tc,h + Tc,c)/2) of the undoped micron-sized block and snowflake-like sample is about 55.8 °C, which is much lower than the transition temperature of undoped bulk VO2 (about 68 °C) reported by Morin4 and undoped nanobelts VO2 (64 °C) reported by Whittaker24. The hysteresis width (ΔT = Tc,h−Tc,c) of the undoped sample is about 13.0 °C.

Figure 10
figure 10

(A) DSC curve of the hydrothermal sample treated at 280 °C for 72 h (HTh1) with W-doped at 0.0 at%. (B) DSC curves for the undoped samples synthesized by the (HTh1 + Annealing) (annealing at 500 °C for 1 h in furnace) method and the (HTh2 + Annealing) (hydrothermal treated at 160 °C for 72 h) process respectively. (C) XRD patterns of the undoped samples synthesized by the designated two fabrication processes. The red column belongs to standard pattern in JCPDS card No. 65–2358 for VO2(M). (D) SEM images of the undoped samples synthesized by the designated two fabrication processes.

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To study the unusual low Tc for the HTh1 synthesized undoped sample, we directly compare the DSC for this sample by the after annealing (HTh1 + Annealing) with that for the hydrothermal undoped one treated at 160 °C for 72 h and after annealing (HTh2 + Annealing) (annealing at 500 °C for 1 h in high-purity argon and this being also prepared by our group34) as shown in Fig. 10B. For the sake of comparison, we list Table 2 to show Tc and hysteresis width depending on the W-doped level and fabrication processes. When the undoped sample is synthesized by the (HTh1 + Annealing) process, Tc,h and Tc,c is about 59.7 and 47.2 °C respectively. Therefore, the Tc is about 53.5 °C, which is also lower than the (HTh2 + Annealing) fabricated undoped one (Tc being c.a 63.0 °C as shown in Table 2). Figure 10C shows the XRD patterns of the undoped samples synthesized by the designated two fabrication processes. The peaks of VO2(B) vanish and all of the peaks can be indexed to pure phase VO2(M) for the HTh1 synthesized undoped sample by the after annealing, which is similar to the (HTh2 + Annealing) fabricated one. The inset close-up shows that M (011) peak shifts to low angles when comparing the (HTh1 + Annealing) synthesized undoped sample with those by the (HTh2 + Annealing) synthesized one, which indicates the lattice spacing of M (011) increases. Both micron-sized snowflake and block-like morphologies are observed for the (HTh1 + Annealing) synthesized undoped sample as shown in Fig. 10D. Whereas, nanostructure is grown by the (HTh2 + Annealing) fabrication process. Thanks to the formation energies of oxygen vacancies in rutile oxides are very high, the high hydrothermal temperature (280 °C) and reductive hydrothermal atmosphere for the (HTh1 + Annealing) method may contribute to the generation of oxygen vacancies to form nonstoichiometric VO2-δ compared to the (HTh2 + Annealing) process (160 °C) and this would promote the lattice structural transition35,36.

Table 2 DSC parameters of the HTh1 synthesized sample with W-doped at 0.0 at% and of the undoped samples synthesized by the (HTh1 + Annealing) method and the (HTh2 + Annealing) process respectively.
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Discussion

To determine the oxygen stoichiometry, the thermogravimetric analysis of the samples was conducted as shown in Fig. 11. According to the TG curves, it can be found there exists one stage for the complete oxidization of the samples in the range of 300–600 °C. The weight gain (ΔTG) is about 10.4 %, 10.5 % and 9.6 % for the HTh1 synthesized undoped VOx, (HTh1 + annealing) undoped VOy and (HTh2 + annealing) undoped VOz respectively. The reaction equations for the oxidization of the samples can be given as follows (1):

Figure 11
figure 11

The thermogravimetric analysis of the samples.

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Where MO and MVOx represent molar mass of oxygen and VOx respectively. When combining the above formulas (2) and experimental results, we can work out x = 1.96, y = 1.95 and z = 2.00 respectively. The fact demonstrates that oxygen deficiency is formed in the HTh1 synthesized undoped VO1.96 and (HTh1 + annealing) undoped VO1.95 and the precisely stoichiometric VO2.00 is formed in the (HTh2 + annealing) undoped sample. Son and co-workers have synthesized monoclinic VO2 micro- and nanocrystals by optimizing the hydrothermal conditions37. In their research, the phase transition temperature of stoichiometric VO2.00 microrods is around 68 °C. Usually, the Tc of MIT for VO2 is affected by doping, nanoscaling, nonstoichiometry, strain and etc12,30,38,39. For the HTh1 synthesized undoped micron-sized VO1.96, the reason for the unusual low Tc may be due to the oxygen nonstoichiometry. The nano-size effect may be responsible for the relative lower Tc (63 °C) of the (HTh2 + Annealing) synthesized stoichiometric VO2.00 nanostructure.

Figure 12A,B shows the Raman spectra of the samples depending on dopant level and fabrication processes. The peaks in the Raman spectra are all identified as 144 (B1g), 191 (Ag), 223 (Ag), 260 (Ag), 308 (Ag), 338 (Ag), 388 (Ag), 437 (Ag), 442 (Eg), 499 (Ag), 617 (A1g) and 826 (B2g) cm−1 respectively and these Raman-active modes are the clear evidence of the existing of VO2(M) belonging to space group , which agrees with the identified Raman peaks by other researchers40,41,42,43. The intensity ratio between the peak of 191 and that of 223 cm−1 (191/223) of the HTh1 synthesized undoped VO1.96 is 1.6. When comparing the (HTh2 + Annealing) synthesized undoped VO2.00 with those by the (HTh1 + Annealing) synthesized undoped VO1.95, the intensity ratio decreases from 2.3 to 1.3. H. T. Kim and co-workers have studied Raman spectra for the MIT of the undoped VO2 in detail and deduced the conclusion that the Raman-active Ag modes at 191 and 223 cm−1 were explained by the pairing and the tilting of V cations respectively43. Hence, the decreased relative intensity of 191 cm−1 peak suggests the depairing of V cations and the occurring of the localized structural phase transition (SPT, induced possibly by oxygen nonstoichiometry for the HTh1 synthesized undoped VO1.96 and (HTh1 + annealing) undoped VO1.95) and this might cause the transformation from the intrinsic structure of the matrix of VO2(M) to the localized rutile structure. In addition, the local rutile structure is the structure-guided domain, which will act as the initial nucleation site for the whole SPT44. This process might promote MIT for the origin of the lowering Tc. However, this origin is still under the debate among the concerned experts as cited in the literature by Y. Xie et al. for an example, who pointed out that the atomic structure of isolated W dopant play a role in driving the nearby symmetric monoclinic VO2 lattice towards rutile phase, resulting in the depression of Tc45,46. Hence the exact mechanism for the observed unusual phenomena requires our further investigation.

Figure 12
figure 12

(A) Raman spectra of the HTh1 synthesized sample with W-doped at 0.0 at% in the Raman shift range 100–1000 cm−1. (B) Raman curves for the undoped samples synthesized by the (HTh1 + Annealing) method and the (HTh2 + Annealing) process respectively.

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Conclusions

In this study, pure phase VO2(M/R) with controlled morphology were successfully prepared via one-step hydrothermal method. The addition of a certain level of W (0.5–2.0 at%) is vital to synthesize the pure phase VO2(M/R) nanorods. The assured level of W doping can promote the preferential growth of {110} to form VO2(M/R) nanorods with high aspect ratio. It must be emphasized that the unusual low Tc equals to 55.8 and 53.5 °C is observed for the nonstoichiometric VO1.96 and VO1.95 in the bulk respectively and the Tc is 63.0 °C for the precisely stoichiometric VO2.00 nanostructure. The present study demonstrates an improvement of the phase transition behavior and reduces the hindrances for the advanced applications of VO2-based materials.

Methods

Materials

Oxalic acid (H2C2O4·2H2O, AR) and vanadium pentoxide (V2O5, AR) were used as source material to prepare the vanadium precursor solution. Deionized water (ρ = 18.2 MΩ.cm) was used to prepare all aqueous solutions. Ammonium tungstate hydrate ((NH4)5H5[H2(WO4)6]·H2O, AR) was chosen as the W dopant. All of these reagents were used without further purification.

The preparation process

The detail of this part has been described in previous report30. Briefly, V2O5 and oxalic acid (1: (1–3) in molar ratio) were directly added to 75 ml deionized water at RT. Then, a certain amount of W dopant was dispersed into the above solution with magnetic stirring. After mixing for 1 h, the resulting precursor was transferred into a 100 mL stainless steel autoclave with polyphenylene cup, then being sealed and maintained at 280 °C for 6–72 h. After the autoclave cooling to RT, a dark blue precipitate was obtained. The product was washed with deionized water and acetone for several times, then centrifuged at 8000 rpm for 8 min and dried in vacuum at 60 °C for 6 h.

In this study, (NH4)5H5[H2(WO4)6]·H2O was used as the W dopant and the reported W-doped content here is based on the quantity of W atoms added in the feed. The sample synthesized by the duration of the hydrothermal time for 6 or 72 h is simplified to the 6 or 72 h-sample.

Characterization techniques

The phase purity of the products was examined by an X-ray diffractometer (XRD, PANalytical X'pert Pro MPD) in the 2θ range of 5–80° with the step of 0.0083° using Cu-Kα radiation (λ = 1.54178 Å). The operating voltage and current were kept at 40 kV and 40 mA, respectively. The morphology and dimensions of the products were investigated using a field emission scanning electron microscope (FESEM, S-4800, Hitachi Japan) under the operating voltage of 2 kV. A JEOL-2100F instrument operated at 200 kV was used to acquire high-resolution transmission-electron-microscopy (HRTEM) images and selected area electron diffraction (SAED) patterns. Raman scattering spectra of the samples were recorded on a LabRAM HR800 micro-Raman spectrometer using a 532 nm wavelength YAG laser. The phase transition properties depending on the surrounding temperature of the as-prepared VO2 were studied by differential scanning calorimetry (HDSC, PT500LT/1600) under the temperature range from 25 to 100 °C under the circulatory heating/cooling cycles. The thermogravimetric analysis (TG) of the samples was conducted on a Nicolet 6700-Q50 thermal analyzer under dry air flow in the range of 50–650 °C with a heating rate of 5 °C min−1.

Additional Information

How to cite this article: Chen, R. et al. Shape-controlled synthesis and influence of W doping and oxygen nonstoichiometry on the phase transition of VO2. Sci. Rep. 5, 14087; doi: 10.1038/srep14087 (2015).