07) 2644–2647www.elsevier.com/locate/matlet

Materials Letters 61 (20

VO2(R) nanobelts resulting from the irreversibletransformation of VO2(B) nanobelts

Kai-Feng Zhang a, Xiang Liu b, Zhong-Xing Su a,⁎, Hu-Lin Li a

a College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou 730000, PR Chinab Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, PR China

Received 20 July 2006; accepted 9 October 2006Available online 7 November 2006

Abstract

VO2(R) nanobelts were prepared by the irreversible transformation of VO2(B) nanobelts at the elevated temperature. The morphology and sizeof the VO2(R) nanobelts were dependent on that of the precursor. VO2(B) nanobelts were synthesized by a hydrothermal route, and the process ofthe VO2(B) nanobelts' formation was also discussed. The product was characterized by a combination of techniques including XRD, TEM, FE-SEM, HRTEM, DTA and FT-IR. The as-obtained VO2(R) nanobelts have a monoclinic structure with a length of 350–600 nm, a wideness of100–150 nm and a thickness of 20–30 nm.© 2006 Elsevier B.V. All rights reserved.

Keywords: Nanomaterials; Crystal growth; X-ray diffraction; Vanadium dioxide

1. Introduction

Increasing attention has been focused on one-dimensional(1D) nanostructured materials (nanotubes, nanowires, nanobeltsand nanorods) due to their novel physical and chemical pro-perties, and their promising applications in numerous areas suchas nanoscale electronics and photonics [1–3]. Earlier effortswere devoted to the research of carbon nanotubes [4], metalsand II–VI semiconductors [5,6]. Recently, 1D nanostructures ofthe transition metal oxides such as ZnO, TiO2, Cu2O, V2O5, etchave attracted intense interest with their excellent performance[7–10]. Vanadium dioxide (VO2), one of the important func-tional materials, exists in four polymorphic forms, VO2(M),VO2(R) and two metastable phases VO2(A) and VO2(B) [11].Among these, VO2(R) with the rutile structure undergoes areversible metal–semiconductor phase transition at 68 °C,associated with drastic changes of the infrared transmission andthe electrical resistivity changes of an order of 104–105 [12,13],which makes it a candidate material for a wide variety ofpotential applications including thermochromic coating [14],

⁎ Corresponding author. Tel.: +86 931 891 2517; fax: +86 931 891 2552.E-mail address: zxsu@lzu.edu.cn (Z.-X. Su).

0167-577X/$ - see front matter © 2006 Elsevier B.V. All rights reserved.doi:10.1016/j.matlet.2006.10.039

optical and holographic storage [15], optical switching devices[16], and missile training systems [17].

So far, continuous work has been directed toward thepreparation and study of the properties of VO2(R). Zheng et al.reported that VO2(R) powder was fabricated by thermolysis of acomplex precursor (NH4)5[(VO)6(CO3)4(OH)9]·10H2O at450 °C [18]. Tsang and Manthiram demonstrated that VO2(R)powder resulted from the corresponding VO2(B) at around500 °C [19]. Until recently, one-dimensional VO2(R) nanorodswere presented by annealing the precursor VO2 hydrate nanorods[20]. Guiton et al. reported that VO2(R) nanowires withrectangular cross section were obtained by the bulk VO2

powders in vapor transport conditions [21].Herein, we report that VO2(R) nanobelts can be prepared by

thermally processing VO2(B) nanobelts at the elevated temper-ature. The corresponding precursor, VO2(B) nanobelts, weresynthesized by hydrothermal strategy using H2C2O4·2H2O asthe reducing and structure-directing reagent. To the best of ourknowledge, it is the first report about VO2(R) nanobelts.

2. Experimental section

All of the chemical reagents were of analytical grade andused as received. In a typical process, V2O5 (0.9094 g) and

Fig. 1. A) XRD pattern of the as-obtained VO2(R) nanobelts; B) XRD pattern ofthe precursor VO2(B).

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H2C2O4·2H2O (1.8912 g) solid powder were directly added to50 ml deionized water at room temperature. The resultant mix-ture was continuously stirred until a clear yellow-green solutionformed, and the system was then transferred into a Teflon-linedautoclave with a stainless shell. The autoclave was kept at

Fig. 2. Electron microscopic images of the VO2(R) nanobelts. A) TEM images of theC) FE-SEM images of the VO2(R) nanobelts; D) HRTEM images of the individual

160 °C for 48 h and then allowed to cool down to roomtemperature. The blue-black precipitate was collected andwashed several times with deionized water and anhydrousalcohol. The precursor VO2(B) nanobelts were obtained. Aportion of the as-obtained VO2(B) nanobelts was thermallytreated at 500 °C for 3 h in the flow of argon. The rutile type VO2

(R) nanobelts were subsequently obtained with the colorchanging from blue-black to black. The structure and morphol-ogy of the product were characterized by using a combination oftechniques including XRD (Cu Kα radiation, λ=1.54178 Å D/MAX-2400, Rigaku, Japan), TEM (Hitachi H-800, Japan), FE-SEM (Hitachi, S-4800 Japan) and HRTEM (JEOL-2010, Japan),and the transition properties were DTA (TG-DTA92A) and FT-IR (NIC-5DX).

3. Results and discussion

Fig. 1A shows an XRD pattern of the as-obtained VO2(R)nanobelts, and all of the diffraction peaks could be perfectly indexedto the monoclinic crystalline phase of VO2(R). The lattice constantscalculated from XRD data were close to the literature values(a=5.751 Å, b=4.524 Å, and c=5.382 Å, β=122.64°, JCPDS FileNo. 43-1051). Also, the precursor was characterized by XRD analysis.

VO2(R) nanobelts (SAED patterns inset); B) TEM images of VO2(B) nanobelts;VO2(R) nanobelt.

Fig. 4. FT-IR spectra of VO2(R) nanobelts at 25 °C and 80 °C.

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As shown in Fig. 1B, the data of all the diffraction peaks were indexedto those of VO2(B) according to JCPDS card 31-1438.

As estimated from TEM and FE-SEM images (Fig. 2), the VO2(R)nanobelts are 350–600 nm long, 100–150 nm wide and 20–30 nmthick. High resolution transmission electron microscopy (HRTEM)images were taken from an individual VO2(R) nanobelt. As it isindicated in Fig. 2C, the nanobelts exhibit a good crystalline structureand clear lattice fringe. The interplanar spacing is calculated to be about0.43 nm, which is consistent with the distance between two (010)crystal planes of the monoclinic VO2(R) species.

According to the TEM and SEM images of the VO2(B) nanobelts,the morphology of the VO2(R) nanobelts is dependent on that of theVO2(B) nanobelts. SAED patterns (inset in Fig. 2A) suggest that the as-obtained VO2(R) nanobelt is single crystalline. Meanwhile, somefragments were found in VO2(R) samples. We think that the fragmentsmay result from the rupture of the belts during annealing. Therefore, itis significant to further explore the process of the VO2(B) nanobelts'formation. As for the synthesis of VO2(B) nanobelts, although Li et al.reported the VO2(B) nanobelts tending to self-assemble the “paperform” [22], we herein report a facile method for the large scale syn-thesis of VO2(B) nanobelts, which was based on the hydrothermalstrategy using V2O5 and H2C2O4·2H2O as vanadium source andmorphology control reagent, respectively. We found that the formationof VO2(B) nanobelts strongly depends on the chemical reactionconditions, especially the temperature and the molar ratio of the rawmaterials (H2C2O4·2H2O:V2O5). When the reaction temperature islower than 160 °C, no precipitation forms; VO2(B) nanobelts can beobtained in the range of 160–180 °C. Other vanadium oxides are foundin the product when the molar ratio of the raw materials is more than3:1. Furthermore, it proved that the quantity of the product is dependenton the reaction time; generally, the longer the reaction time, the morethe quantity of the product. However, the quantity keeps constant if thetime exceeds 48 h. Consequently, the optimum conditions for preparingVO2(B) nanobelts should be at 160 °C for 48 h with the molar ratio ofthe starting materials at 3:1. Also, we have prepared the NH4V4O10

nanobelts using H2C2O4·2H2O as the reducing and morphology controlreagent [23]. According to the previous reports [24–29], only VO2(B)powder could be obtained when SO2, H2 or NH3 are used as reducingreagents. So far, there are still no general mechanism for the formationof the nanobelts in the hydrothermal system, however, we suggest thatH2C2O4·2H2O not only acts as reducing reagent but also that itsbidentate coordination may play a crucial role in the control of one-dimensional belt-like morphology.

Fig. 3. DTA curves of the as-obtained VO2(R) nanobelts.

The transition properties of the as-obtained VO2(R) nanobelts werecharacterized by DTA and FT-IR. Fig. 3 is a typical DTA curve of thesample, and the endothermic peak implying the phase transition tem-perature of the nanobelts appears at 69 °C, which is a little bit higherthan the literature value of the powder species [19]. We think that itmay be due to the size effect of the nanostructured materials. FT-IRspectrum was used to test the optical properties of the VO2(R)nanobelts at room temperature (below the transition temperature) and80 °C (above the transition temperature). As shown in Fig. 4, theoptical switching properties were clearly observed according to thedifferences of the transmission at various temperatures. Additionally,changes in the vibrational bands from 400 to 1000 cm− 1 were alsoobserved while the structures of the VO2(R) nanobelts turned from themonoclinic to tetragonal phase during heating.

4. Conclusion

In summary, we have developed a new and facile hydro-thermal route to prepare VO2(R) nanobelts. This synthesisstrategy can be easily extended to other transition metal oxides.It is also possible to fabricate functional nanodevices usingthese 1D VO2(R) nanobelts as building blocks. Intensive re-search including electrical and optical properties is underway inour laboratory.

Acknowledgements

The authors would like to thank SEM (Hitachi S-4800)Laboratory of the Physical Department of Lanzhou Universityfor their great help in sample analysis. This work was finan-cially supported by the Natural Science Foundation of P.R.China (Grant no. 60471014).

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