## 3 Results and Discussion  
The synthetic conditions took a significant part in the formation of VO\({}_{2}\)(B) nanobelts. Figure 1A demonstrates the XRD pattern of the samples at various ratio of V:C. When the ratio of V:C is 1:1.25, \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Phase** & **Transition Temperature (K)** & **a(Å)** & **b(Å)** & **c(Å)** & **Crystal System** \\ \hline VO\({}_{2}\)(M) & 341 & 5.74 & 4.16 & 5.38 & Monoclinic \\ VO\({}_{2}\)(R) & 341 & 4.53 & 4.53 & 2.87 & Monoclinic \\ VO\({}_{2}\)(A) & 435 & 8.44 & 8.44 & 7.67 & Tetragonal \\ VO\({}_{2}\)(B) & — & 12.03 & 3.69 & 6.42 & Tetragonal \\ \hline \hline \end{tabular}
\end{table} 
Table 1: The crystal data of VO\({}_{2}\) polymorphs. an unknown solid was obtained based on our knowledge. With the increase of V:C, the characteristic peaks of VO\({}_{2}\)(B) (JCPDS, No. 81-2392) were obtained and pure phase of VO\({}_{2}\)(B) was synthesized with V:C and increased to 1:1.75. However, when V:C continued to increase (V:C = 1:2), VO\({}_{2}\)(B) was not obtained and other vanadium oxides were prepared. Thus, V:C = 1:1.75 is suitable for the synthesis of VO\({}_{2}\)(B) with pure phase. Figure 1B describes the XRD patterns of VO\({}_{2}\)(B) prepared with various hydrothermal times, which discloses the evolution process of VO\({}_{2}\)(B)'s formation. As depicted in Figure 1B, VO\({}_{2}\)(B) was obtained at 6-96 h indicating that VO\({}_{2}\)(B) could be synthesized in a short time. Figure 2 shows the morphology of the samples obtained by SEM. When V:C = 1:1.25, it can be seen that nanosheets are obtained (Figure 2a). With V:C increasing to 1:1.75, VO\({}_{2}\)(B) nanobelts are synthesized, as shown in Figure 2b. Thus, V:C = 1:1.75 is favorable for preparation of VO\({}_{2}\)(B) with belt-like morphology. The SEM images of the samples obtained at 6 and 12 h are shown in Figure 2c,d respectively. They both reveal that VO\({}_{2}\)(B) nanobelts are formed. This result indicates that VO\({}_{2}\)(B) nanobelts can be synthesized at a short time. Figure 3 shows the TEM image of the samples. The TEM images are consistent with SEM images, which also reveals that VO\({}_{2}\)(B) with belt-like morphology could be synthesized. Figure 1: XRD patterns of the samples with various conditions: (**A**) various ratio of V:C (180 \({}^{\circ}\)C, 48 h); (**B**) reaction time (V:C = 1:1.75, 180 \({}^{\circ}\)C). Figure 2: SEM images of the products under various conditions: (**a**) V:C = 1:1.25, 48 h; (**b**) V:C = 1:1.75, 48 h; (**c**) V:C = 1:1.75, 6 h; (**d**) V:C = 1:1.75, 12 h. (at 180 \({}^{\circ}\)C). Further composition and structure information about the as-obtained VO2(B) was provided by EDS, FTIR, and Raman. The SEM image and its corresponding EDS spectrum of the as-obtained sample is shown in Figure 4, which indicates that the sample composes of vanadium and oxygen elements in consonance with the observation of XRD patterns. The IR and Raman spectra of the as-obtained VO2(B) is shown in Figure 5a. The absorption peaks among 1100 and 400 cm\({}^{-1}\) are assigned to V-O vibration band in the IR spectrum [27; 28]. The peak at 535 cm\({}^{-1}\) is ascribed to the stretching modes of V-O-V, and that at 920 cm\({}^{-1}\) is ascribed to V-O bonds. Meanwhile, the characteristic absorption peak of the VO2(B) nanobelts with layered structure is observed at 998 cm\({}^{-1}\), which is assigned to the symmetric stretching vibration in VO2(B) (V\({}^{4+}\) = O). Figure 4: SEM image and EDS spectrum of the VO2(B) nanobelts. Figure 3: TEM images of the VO2(B) nanobelts at different scales: (**a**) 500 nm; (**b**) 200 nm. Raman spectrum of the VO\({}_{2}\)(B) nanobelts is shown in Figure 5a, which demonstrates a series of bands located at 100-1100 cm\({}^{-1}\) are the vibrations of V-O type of VO\({}_{2}\)(B). Specially, the intense peak at 134 cm\({}^{-1}\) corresponds with B\({}_{38}\) and B\({}_{28}\) symmetry, which is indexed to the stretching vibration pattern of (V\({}_{2}\)O\({}_{2}\))\({}_{n}\) in agreement with the chain conversion [29, 30].