VO₂(B) conversion to VO₂(A) and VO₂(M) and their oxidation resistance and optical switching properties

Vanadium pentoxide (V₂O₅), hydrogen peroxide (H₂O₂, 30 wt.%) and ethanol (CH₃CH₂OH) with analytical grade were purchased from Sinopharm Chemical Reagent Co., Ltd., and used without any further purification.

The synthesis of VO₂(B) nanobelts was based on our previous reports [22, 35] and slightly modified. In a typical synthesis, 0.455 g of commercial V₂O₅ was dispersed in 31 mL of redistilled water with magnetic stirring. Then 2 mL of H₂O₂ and 2 mL of ethanol were successively added into the solution, which was still stirred for 1 h at room temperature to obtain a brown liquid ([VO(O₂)₂]^- solution), as shown in equation 1: (1)V2O5+4H2O2→2[VO(O2)2]−+3H2O+2H+$$\rm V_2O_5+4H_2O_2\rightarrow 2[VO(O_2)2]^{-}+3H_2O+2H^+$$ After the solution achieved good homogeneity, the mixture was transferred into a 50 mL Teflon-lined stainless steel autoclave, then sealed and maintained at 180 °C for 48 h. When the reaction was finished, the blue-black precipitates were filtered off, washed with distilled water and anhydrous alcohol several times to remove any possible residues, and dried in vacuum at 75 °C for future applications.

In a typical synthetic route for transforming VO₂(B) to VO₂(A), 0.50 g of the VO₂(B) nanobelts was dispersed into 40 mL of deionized water with magnetic stirring. The mixed solution was transferred into a 60 mL stainless steel autoclave after the solution became suspension. The autoclave was sealed and maintained at 280 °C for 48 h and then cooled to room temperature naturally. The products were filtered off, washed with distilled water and absolute ethanol several times to remove any possible residues, and dried in vacuum at 75 °C.

In a typical transformation from VO₂(B) to VO₂(M), about 0.40 g of the VO₂(B) nanobelts were heated in a tube furnace with 5 °C/min heating rate under a high purity Ar (99.999 %) atmosphere at 700 °C for 2 h, and cooled to room temperature in the Ar flow to prevent the oxidation of VO₂(M).

X-ray powder diffraction (XRD) study was carried out on D8 X-ray diffractometer equipment with Cu Kα radiation, λ = 1.54060 Å. The data were collected between 5° and 70° with a scan speed of 4°/min. The morphology and dimensions of the products were observed by the transmission electron microscopy (TEM, JEM-2100). The sample was dispersed in absolute ethanol and was ultrasonicated before TEM. Thermogravimetric analysis and differential thermal analysis (TGA/DTA) were performed on SETSYS-1750 (AETARAM Instruments). About 10 mg of the sample was heated in an Al₂O₃ crucible in air atmosphere from ambient temperature to 700 °C at a constant rise of temperature (10 °C/min). The phase transition temperature (T_c) of VO₂(A) and VO₂(M) was measured by differential scanning calorimetry (DSC, DSC822^e, METTLER TOLEDO) with 5 °C/min heating rate. Optical properties of VO₂(A) and VO₂(M) were tested by variable-temperature Fourier transform infrared spectroscopy (IR, NICOLET 5700) with an adapted heating controlled cell. Variabletemperature IR patterns of the solid samples were measured using KBr pellet technique from 4000 to 400 cm^-1 with a resolution of 4 cm^-1. About 1 wt.% of the samples and 99 wt.% of KBr were mixed homogeneously, and then the mixture was pressed into a pellet.

Fig. 1 depicts the typical XRD patterns of the as-prepared VO₂(B). All the diffraction peaks from Fig. 1b can readily be indexed to the monoclinic crystalline phase (space group C2/m) of VO₂(B) in agreement with the literature values (JCPDS Card No. 65–7960) [17], whose plots are shown in Fig. 1a. No impurity phases, such as V₂O₅, V₃O₇, V₆O₁₃, VO₂(M), VO₂(A) and V₂O₃, were detected, indicating high purity of the as-obtained VO₂(B).

Fig. 1

After hydrothermal treatment of VO₂(B) at 280 °C for 48 h, the XRD of the product has been measured and is shown in Fig. 2. It can be observed from Fig. 2 that all the diffraction peaks can readily be indexed to the tetragonal crystalline phase (space group: P42/ncm, No. 138) of VO₂(A) (JCPDS Card No. 42–0876) [36], indicating that the VO₂(A) was successfully transformed from VO₂(B). The as-prepared VO₂(A) is of high purity what can be inferred by comparing Fig. 2a and Fig. 2b. After calcination, VO₂(B) was transformed into VO₂(M), as shown in Fig. 3. All the diffraction peaks in Fig. 3b can readily be indexed to the monoclinic crystalline phase (space group: P21/c, No. 14) of VO₂(M) (JCPDS Card No. 72-0514) [18] and the as-prepared VO₂(M) is of high purity what can be stated by comparing Fig. 3a and Fig. 3b. Therefore, it can be said that VO₂(B), VO₂(A) and VO₂(M) were successfully prepared.

Fig. 2

Fig. 3

Fig. 4 shows the typical TEM images of the asprepared VO₂(B), VO₂(A) and VO₂(M) nanobelts, which reveal that the as-obtained VO₂(B), VO₂(A) and VO₂(M) have similar morphology. The TEM images indicate that all of the as-obtained VO₂(B), VO₂(A) and VO₂(M) consist of a large number of 1D nanobelts with the length in the range of several to tens of micrometers and width ranging from 80 to 200 nm.

Fig. 4

In the past decades, VO₂(B), VO₂(A) and VO₂(M) nanobelts have been extensively studied, however, their oxidation resistance properties have not been reported in the literature. Therefore, in this contribution, the oxidation resistance properties of VO₂(B), VO₂(A) and VO₂(M) nanobelts were investigated by TGA/DTA with the flowing air, as shown in Fig. 5 and Fig. 6. Fig. 5 shows the TG curves of the as-obtained VO₂(B), VO₂(A) and VO₂(M) nanobelts in air atmosphere, which reveal that their oxidation process by O₂ starts at 341, 408 and 465 °C, respectively. The oxidation process of VO₂(B), VO₂(A) and VO₂(M) is finished at 472, 594 and 675 °C, respectively. The above results suggest that the VO₂(M) nanobelts have the best thermal stability and oxidation resistance. After the TGA/DTA test, a yellow powder was obtained, whose color is the same as that of V₂O₅, indicating that VO₂ is oxidized to V₂O₅.

Fig. 5

Fig. 6

As shown in Fig. 5, before beginning of VO₂ oxidation, the weight loss was not zero, which was caused by the loss of water absorbed on its surface. The weight loss of VO₂(B) is the largest while that of VO₂(M) is the least. The reason could be that VO₂(B) was directly prepared by a hydrothermal route and VO₂(M) was obtained by heating VO₂(B) at 700 °C, which could reduce the absorbed water. The weight loss of VO₂(B), VO₂(A) and VO₂(M) nanobelts in their oxidative process is 8.29, 9.01 and 9.54 %, respectively. The weight loss values of VO₂(B), VO₂(A) and VO₂(M) nanobelts correspond to the oxidation of the bulk VO₂ to V₂O₅ (9.64 %).

Fig. 6 shows the heat flow curves of the asobtained VO₂(B), VO₂(A) and VO₂(M) nanobelts in air atmosphere, which suggests that the fierce oxidation of VO₂(B), VO₂(A) and VO₂(M) nanobelts occurs at 426, 507 and 645 °C, respectively. The sharp endothermic peak at about 685 °C is the melting point of V₂O₅, which further confirms that the oxidative product is V₂O₅. By the way, for VO₂(M) curve, a peak at 68 °C appears, which is the T_c of VO₂(M). Based on these results, it can be stated that the VO₂(B), VO₂(A) and VO₂(M) nanobelts have good thermal stability and oxidation resistance properties below 341, 408 and 465 °C in air atmosphere, respectively while the VO₂(M) nanobelts have the best thermal stability and oxidation resistance, which is beneficial for the application of VO₂(M) in air.

When the phase transition of VO₂(A) or VO₂(M) occurs, they exhibit a noticeable endothermal profile in the heating DSC curve, which corresponds to the phase transition of VO₂(A) or VO₂(M). Fig. 7 shows the typical DSC curves of VO₂(A) and VO₂(M). The T_c of VO₂(A) is about 162 °C, while for VO₂(M), the T_c is about 67 °C.

Fig. 7

According to the DSC results of the as-obtained VO₂(A) and VO₂(M), they reveal a noticeable endothermic peak in the heating cycle. It was reported that the optical properties have drastically changed when the reversible phase transition of VO₂(A) or VO₂(M) occurred. Therefore, we further developed the as-obtained VO₂(A) and VO₂(M) as the optical switching devices. The optical switching properties of VO₂(A) and VO₂(M) were investigated by variable-temperature infrared spectra, as shown in Fig. 8 and Fig. 9. It can be clearly seen from Fig. 8 that VO₂(A) has the optical switching properties at different vibratory absorption bands, revealing that it is a potential candidate for optical switching devices at the vibratory absorption bands from 700 to 650 cm^-1 and from 600 to 550 cm^-1. In case of VO₂(M), as shown in Fig. 9, it can be clearly observed that VO₂(M) has the optical switching properties. The optical transmission below T_c is higher than that above T_c, suggesting that it has good thermochromic properties. Besides, it also reveals that VO₂(M) has potential applications in optical switching devices at the variety of vibratory absorption bands owing to its large transmission changes. These optical properties of VO₂(A) and VO₂(M) verify that they are beneficial for the development and application of optical switching materials.

Fig. 8

Fig. 9

It has been reported [1, 25, 34, 37–40] that the optical spectra, usually explained as the observed bands, are related to the electromagnetic resonance between incident photons (with a specific wavenumber) and variation of chemical bond polarization associated with a specific vibration mode. At low temperature (T < T_c) the electrons involved in the V⁴⁺–V⁴⁺ bonds between VO₆ octahedra are localized. However, these electrons are delocalized at high temperature (T > T_c). In the metal state (T > T_c), this delocalization involves a screening effect for the incident photons, which occurs at the surface of the sample [37]. As a result, no vibrational absorption bands can be observed. In this circumstance, the transmittance drastically decreases. The variable-temperature infrared spectra confirm the strong reversible metal-insulator phase transition at around T_c.