Hydrothermal synthesis of single-crystal VO2(B) nanobelts

Kai-Feng Zhang a, Shu-Juan Bao a, Xiang Liu b, Jin Shi a,Zhong-Xing Su a,*, Hu-Lin Li a,*

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

Received 16 February 2006; received in revised form 8 May 2006; accepted 2 July 2006

Available online 8 August 2006

Abstract

Single crystalline VO2(B) nanobelts with a metastable structure were obtained through a simple hydrothermal synthetic method.

The VO2(B) nanobelts were characterized by means of X-ray diffraction, transmission electron microscopy, selected area electronic

diffraction, field-emission scanning electron microscopy and X-ray energy-dispersive spectroscopy techniques. The as-obtained

VO2(B) nanobelts are 400–600 nm long, typically 100–150 nm wide and 20–30 nm thick. The belt-like VO2(B) with a high surface

area may be beneficial to lithium insertion between the VO6 layers for application in batteries.

# 2006 Elsevier Ltd. All rights reserved.

Keywords: A. Oxides; A. Nanostructures; B. Chemical synthesis

1. Introduction

In recent years, with the exasperating of worldwide energy crisis and the development of portable electronic

devices, lithium-ion batteries have become more attractive than ever before, due to their higher cell voltage and energy

density compared to other rechargeable battery systems. Although the layered LiMO2 (M = Co, Ni and Mn) cathode

materials are presently used in the commercial lithium-ion cells, there are some common problems existing in

practical application such as high toxicity and limited capacity [1].

Vanadium forms a serious of binary oxides with general formula VO2+x including the well-known V2O5 and V2O3.

Some examples in the family of VO2+x (0 � x � 0.33) are VO2, V6O13, V4O9 and V3O7 with x values of 0, 0.17, 0.25

and 0.33. The VO2+x phase consists of distorted VO6 octahedra sharing both corners and edges can be described as

shear structures derived from a hypothetical VO3 with the ReO3 structure; the VO2 with this shear structure has been

designated as VO2(B) [2,3]. Among the VO2+x system, VO2(B) and V6O13 were found to show more promising

electrochemical performance compared to the well-known binary oxide V2O5 [4].

The discovery of carbon nanotubes in 1991 has greatly initiated intense experimental and theoretical interest in one-

dimensional (1D) nanostructured materials (nanotubes, nanobelts, nanowires and nanorods) [5], due to their distinctive

geometries, novel physical and chemical properties, and potential application in numerous areas such as nanoscale

electronics and photonics [6–12]. Huynh et al. reported that the operation properties of batteries depend not only on the

structure butalso on the morphology of the electrodematerial components [13]. Much effort hasbeen devoted to preparing

the bulk VO2(B) [2,14–17]. Tsang and Manthiram prepared VO2(B) nanopowder and tested the electrochemical

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Materials Research Bulletin 41 (2006) 1985–1989

* Corresponding authors. Tel.: +86 931 891 2585; fax: +86 931 891 2552.

E-mail addresses: zhangkf02@st.lzu.edu.cn (K.-F. Zhang), zxsu@lzu.edu.cn (Z.-X. Su), lihl@lzu.edu.cn (H.-L. Li).

0025-5408/$ – see front matter # 2006 Elsevier Ltd. All rights reserved.

doi:10.1016/j.materresbull.2006.07.001

properties [3]. Until recently, Li and coworkers reported the VO2(B) nanobelts tending to self-assemble the ‘‘paper form’’

[18]. Herein, we report a facile method for the large scale synthesis of VO2(B) nanobelts, which was based on the

hydrothermal strategy using V2O5 and H2C2O4�2H2O as vanadium source and morphology control reagent, respectively.

2. Experimental details

The VO2(B) nanobelts were synthesized under hydrothermal conditions. All of the chemical reagents used in the

experiment were analytical grade without further purification. In a typical procedure, V2O5 (0.9094 g) and

H2C2O4�2H2O (1.8912 g) solid powder were directly added to 50 ml deionized water at room temperature. The

mixture was continuously stirred until a clear yellow-green solution was formed, and the resultant system was then

transferred into a Teflon-lined autoclave with a stainless steel shell. The autoclave was kept at 160 8C for 48 h and then

allowed to cool down to room temperature. The blue-black precipitate was collected and washed several times with

distilled water and anhydrous alcohol. The final product was dried at 110 8C for 5 h in the flow of argon.

X-ray diffraction (XRD) pattern was obtained on a Rigaku (Japan) D/MAX-2400 X-ray diffractometer with Cu

Karadiation (l = 1.54178 A), employing a scanning rate of 0.028 S�1 in the 2u range of 10–608. Transmission electron

microscopy (TEM) images and selected area electronic diffraction (SAED) patterns were taken with Hitachi H-800

transmission electron microscopy, using an accelerating voltage of 100 KV. Field-emission scanning electron

microscopic (SEM) images were recorded with Hitachi S-4800 microscope, using an accelerating voltage of 10.0 kV.

HRTEM images were obtained on a JEOL-2010 electron microscope with an energy-dispersive X-ray analyzer, using

an accelerating voltage of 200 kV.

K.-F. Zhang et al. / Materials Research Bulletin 41 (2006) 1985–19891986

Fig. 1. XRD pattern of the as-obtained nanobelts.

Fig. 2. EDS analysis of a single VO2(B) nanobelt.

3. Results and discussion

3.1. XRD and EDS results

Fig. 1 shows an X-ray diffraction (XRD) pattern of the VO2(B) nanobelts. All of diffraction peaks can be readily

indexed to a pure monoclinic crystalline phase (space group c2/m) of VO2(B) with lattice constants a = 12.09 A,

b = 3.702 A, c = 6.433 A, and b = 106.68 (JCPDS, No. 31-1438). Chemical analysis on the nanobelts was taken by X-ray

K.-F. Zhang et al. / Materials Research Bulletin 41 (2006) 1985–1989 1987

Fig. 3. (a) TEM images of the VO2(B) nanobelts; (b) field-emission SEM images of the VO2(B) nanobelts; (c) HRTEM images of the individual

VO2(B) nanobelt.

energy dispersion spectrum (EDS) (Fig. 2), which indicates that, with the exception of Cu raised from TEM grid, Vand O

are present without any impurities. However, the atom ratio of V to O cannot be calculated because the peak of the

element V overlaps one of the element O.

3.2. TEM and SEM analysis

The morphology and size of the resulting products are shown in Fig. 3. As estimated from TEM and SEM images,

the as-obtained VO2(B) nanobelts are 400–600 nm long, typically 100–150 nm wide and 20–30 nm thick. Especially,

the yield of the belt-like morphology is statistically counted to be higher than 95%, and the quantity of the products can

reach gram grade. The selected area electronic diffraction (SAED) patterns (inset in Fig. 3a) taken from a single

nanobelt and recorded from the [1 0 0] zone axis indicates that the nanobelts are single crystal with a preferential

growth direction along the [0 1 0] direction. Fig. 3c is a representative high-resolution transmission electron

microscopy (HRTEM) image of a individual VO2(B) nanobelt, which shows the clear lattice fringes with spacing of

0.64 nm between the adjacent lattice planes corresponding to the distance between two (0 0 1) crystal planes of the

monoclinic VO2(B) species. The result further confirms the nanobelts grow along the [0 1 0] direction. Fig. 4 shows

the projection of the VO2(B) structure along [0 1 0]. Additionally, as shown in Fig. 3, the layered structure was found at

the edges of the single nanobelt. VO2(B) can be regarded as a layered structure in which the distorted VO6 octahedra

share both corner and edges [19]. We suggested that VO2(B) may grow along the [0 1 0] direction layer by layer in (a,

c) planes. At the edges of the single nanobelts, the imperfection of crystal growth may lead to this layered structured

observed. Further studies on the crystal structure will make for explaining the results.

3.3. Mechanism analysis of synthesis

We found that the formation of VO2(B) nanobelts strongly depends on the chemical reaction conditions, especially

the temperature and the molar ratio of the raw materials (H2C2O4�2H2O:V2O5). When the reaction temperature is

lower than 160 8C, no precipitation forms; VO2(B) nanobelts can be obtained in the range of 160–180 8C. Other

vanadium oxides are found in the product when the molar ratio of the raw materials is more than 3:1. Furthermore, it

proved that the quantity of the product is dependent on the reaction time; generally, the longer the reaction time is, the

more the quantity of the product is. However, the quantity keeps constant if the time exceeds 48 h. Consequently, the

optimum conditions for preparing VO2(B) nanobelts should be at 160 8C for 48 h with the molar ratio of the starting

materials of 3:1. Compared with previous report, we absolutely excluded the impurity ion NH4+ by using V2O5 as

vanadium source. In our earlier research, NH4V4O10 species were found in lower temperature when NH4VO3 was

employed as the starting material [20]. Furthermore, VO2(B) nanobelts could be obtained at lower temperature, using

H2C2O4�2H2O as the reducing and morphology control reagent. According to the previous reports [2,3,8–11], only

VO2(B) powder could be obtained when SO2, H2 or NH3 are used as reducing reagents. So far, there are still no general

K.-F. Zhang et al. / Materials Research Bulletin 41 (2006) 1985–19891988

Fig. 4. Projection of the VO2(B) structure along [0 1 0]. There are packing of edge sharing octahedral that are only linked by corners in (a, c) planes

[19].

mechanism for the formation of the nanobelts in hydrothermal system, however, we suggest that H2C2O4�2H2O not

only acts as reducing reagent but also that its bidentate coordination may play a crucial role in the control of 1D belt-

like morphology.

4. Conclusions

In summary, we have successfully synthesized VO2(B) nanobelts by a facile hydrothermal route. It has apparent

advantages over the traditional approaches for the preparation of belt-like nanomaterials. The belt-like VO2(B) with a

high surface area may be applied in rechargeable lithium-ion batteries. It is believed that this synthetic strategy will be

helpful to explore and control 1D nanostructured materials, and it may also be potentially extendable to prepare other

1D nanomaterials

Acknowledgments

The authors would like to thank SEM (Hitachi S-4800) laboratory of Physical Department of Lanzhou University

for their great help in sample analysis. This work was financially supported by the Natural Science Foundation of the

PR China (Grant No. 60471014).

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