Surfactant-Assisted Hydrothermal Growth of Single-Crystalline

Jul 18, 2007 - Latha Kumari , W. Z. Li , Shrinivas Kulkarni , K. H. Wu , Wei Chen , Chunlei Wang , Charles H. Vannoy , Roger M. Leblanc. Nanoscale Res...
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CRYSTAL GROWTH & DESIGN

Surfactant-Assisted Hydrothermal Growth of Single-Crystalline Ultrahigh-Aspect-Ratio Vanadium Oxide Nanobelts

2007 VOL. 7, NO. 9 1893-1897

Shufeng Shi, Minhua Cao,* Xiaoyan He, and Haiming Xie* Department of Chemistry, Northeast Normal UniVersity, Changchun, 130024, People’s Republic of China ReceiVed NoVember 27, 2006; ReVised Manuscript ReceiVed June 4, 2007

ABSTRACT: Single-crystalline vanadium oxide nanobelts were obtained through a surfactant-directed growth process under hydrothermal conditions using V2O5 as a precursor. The shape and size were observed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). X-ray powder diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), ultraviolet/ visible (UV/vis) spectroscopy, X-ray photoelecton spectroscopy (XPS), high-resolution TEM (HRTEM), and selected area electron diffraction (SAED) were used to characterize the composition and structure of the as-prepared nanobelts. The as-obtained vanadium oxide nanobelts are up to several hundreds of micrometers in length, 100-200 nm in diameter, and 20-30 nm in thickness. A possible mechanism was proposed to account for the formation of the nanobelts. The influence of the concentration of reactants, the reaction time, the concentration of the surfactant, and the reaction temperature on the morphology of the resulting products are discussed in detail. Furthermore, we tested the electrochemical intercalation properties with Li+ of the postannealing sample by calcining the obtained vanadium oxide nanobelts at 400 °C. It was found that the morphology and the structure of the synthesized product had an important influence on the electrochemical intercalation properties. Introduction In recent years, one-dimensional (1D) nanostructured materials, such as nanorods, nanowires, nanotubes, nanobelts, and nanoribbons, have received extensive attention because of their novel physical and chemical properties and the wide range of potential applications in nanodevices.1-4 Some of these materials have exhibited novel optical, electrical, magnetic, and mechanical properties, which are different from those of bulk or nanoparticle materials.5 It has been reported that some 1D nanostrutured materials have been employed as active components or interconnects for the fabrication of nanoscale electronic, optical, optoelectronic, electrochemical, and electromechanical devices. Nanobelts, which are a new group of 1D nanostrutures with a rectangular cross section, could be an ideal system for fully understanding dimensionally confined transport phenomena and show promising applications in building nanodevices.6,7 For many years, vanadium oxides and vanadium oxide-related compounds have received special attention because of their unique physical and chemical properties. Now, they have a wide range of practical applications on the basis of their structure and properties, such as electrochromic devices, cathodic electrodes for lithium batteries, catalyzers, gas sensors, and so on.8-12 These properties for applications are closely related to the layer structure of vanadium oxides. Because of the layer structure, different kinds of ions, molecules, and even polymers can be intercalated between the layers of vanadium oxide compounds. It may lead to the formation of the mixed valence vanadium (V5+ and V4+). Now various vanadium oxide nanostructures have been obtained. For example, Nesper et al. synthesized vanadium oxide nanotubes from vanadium(V) alkoxides and primary monoamines by a sol-gel reaction and subsequent hydrothermal treatment.13 Pinna et al. used a colloidal self-assembly made of sodium bis(2-ethyl-hexyl)sulfosuccinate Na(AOT)/isooctane/H2O to generate divanadium pentoxide nanorods and nanowires.14 Qian et al. prepared singlecrystal VOx‚nH2O nanoribbons relying on the presence of * To whom correspondence should be addressed. (M.C.) E-mail: [email protected].

polyethylene glycol-400.15 Li et al. reported that vanadium dioxide single-crystalline nanobelts were synthesized by hydrothermal synthesis using formic acid as the reducing agent.16 All as-obtained vanadium oxide nanomaterials mentioned above have been synthesized by dissolving raw materials into a solvent (water or organic solvent) to form a clear solution or sol-gel. Recently, Zhang17a and Li17b reported V3O7‚H2O nanobelts by directly using V2O5 solid-state powder as precursors, respectively, and the as-obtained V3O7‚H2O nanobelts are an orthorhombic structure. However, for vanadium oxide nanostrutures, a well-ordered layered structure is generally desired because it has been found that such a structure exhibits an even larger capacity and good cycling performance than its orthorhombic structure.18 Hence, we have conducted a series of synthesis studies in an attempt to achieve the transformation from orthorhombic V2O5 solid-state powder to the corresponding layered phase. Herein, we describe a low-cost and simple approach for the preparation of vanadium oxide nanobelts by hydrothermal treatment of bulky V2O5 powder in aqueous solution with the aid of the surfactant sodium dodecylsulfate (SDS). In the experiment, we have achieved the transformation directly from solid-state powder to nanobelts without adding other reactants. That is the most important advancement than other techniques for synthesizing vanadium oxide nanomaterials. In addition, the raw material V2O5 powder is very cheap and easy to purchase. Controllable experiments were carried out to investigate the growth mechanism of the vanadium oxide nanobelts. We also studied the electrochemical performance of the as-obtained vanadium oxide nanobelts, which is useful to offer great opportunities for exploring the dependence of properties of a nanomaterial on its morphology and is promising for manufacturing potential electrochemical devices. Experimental Section Materials. All chemicals used were purchased without further purification. V2O5 analytical purity was purchased from Beijing Chemical Reagents Company. SDS was from Changchun chemical plant.

10.1021/cg060847s CCC: $37.00 © 2007 American Chemical Society Published on Web 07/18/2007

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Figure 1. FE-SEM images of the obtained vanadium oxide nanobelts with a molar ratio of SDS to V2O5 at 3. (a) Lower magnification SEM; (b) and (c) higher magnification; (d) TEM image of vanadium oxide nanobelts. Synthesis. The synthesis procedure was as follows: SDS (1.8 mmol) was dissolved in 60 mL of deionized water to form a colorless clear solution (0.03 M). Then vanadium pentoxide (V2O5) was added and stirred vigorously for about 20 min. A suspended orange solution was formed finally, which was transferred into an 80 mL Teflon-lined autoclave with a stainless shell. The autoclave was kept at 150 °C for 48 h and then cooled to room-temperature naturally. The precipitate was washed with deionized water and absolute alcohol several times to remove any possible residue and then dried at room temperature for 8 h. Electrochemical Measurements. The electrochemical properties of V2O5 composite were characterized by assembling experiment cells with lithium metal disks as a counter electrode. The working electrode was made by dispersing 84 wt % active materials (V2O5), 8 wt % carbon black, and 8 wt % polyvinylidene fluoride (PVDF) binder in Nmethylpyrrolidone (NMP) solvent to form a homogeneous slurry. The slurry was then spread on Al foil. The coated electrodes were dried in vacuum at 120 °C for 12 h. The V2O5 loading was 3-5 mg/cm2 in the experimental cells. The cells were assembled in an argon atmosphere filled glove box. The electrolyte was 1 M LiPF6 in a mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC) (1:1 volume). The cells were galvanostatically charged and discharged in a voltage range of 1.4-4.0 V. Characterization. The resulting products were characterized by X-ray powder diffraction (XRD) (Rigaku D-max-rA XRD with Cu-K radiation). The morphology and dimension of the products were observed by field emission scanning electron microscopy (FE-SEM, JEOL 7500B), the transmission electron microscopy (TEM, H-800), and high-resolution transmission electron microscopy (HRTEM). The Fourier transform infrared spectroscopy (FT-IR) pattern was recorded on a Perkin- Elmer model 580B spectrometer from 400 to 4000 cm-1. X-ray photoelecton spectroscopy (XPS, ESCALAB 250) and ultraviolet/ visible (UV/vis) spectroscopy were used to confirm the oxidation state of vanadium.

Results and Discussion Figure 1 shows SEM images of vanadium oxide nanobelts synthesized with a molar ratio of SDS to V2O5 of 3. A lower magnification FE-SEM image (Figure 1a) indicates that the product consists of a large number of uniform 1D nanobelts with lengths in the range of several tens to hundreds of

micrometers. We even can see the long belts using the naked eye. A higher magnification FE-SEM (Figure 1b) reveals that the nanobelts can bend and therefore exhibit perfect flexibility. The nanobelts have thicknesses ranging from 20 to 30 nm and widths of 100-200 nn. Furthermore, it can be clearly seen from Figure 1b,c that wider belts are formed by self-assembly of several individual nanobelts with uniform width. Some of them can also be bent as shown in Figure 1c. The high flexibility of the nanobelts may be attributed to their thin thickness and crystal structure of vanadium oxide. Figure 1d presents a typical TEM image of vanadium oxide nanobelts, which is consistent with the SEM image in width and length of nanobelts. The compositions of the raw material and the as-obtained vanadium oxide nanobelts are determined by X-ray powder diffraction (XRD), as shown in Figure S1 (Supporting Information). Figure S1a is the XRD pattern of the raw material, which can be indexed to the orthorhombic phase of V2O5 (JCPDS Card File No. 85-0601). It is worth noting that with the help of the surfactant-assisted hydrothermal treatment, a well-ordered layered structure of vanadium oxide was obtained as shown in Figure S1b. The first-, third-, fourth-, and fifth-order reflections could be clearly assigned at 2θ ) 8.18, 24.78, 33.24, and 41.95°, respectively, indicating that after hydrothermal treatment, the nonlamellar V2O5 precursor is transformed into a perfect layered structure. The microstructure of the layered vanadium oxide nanobelts was studied by the selected area electron diffraction (SAED) and the high-resolution transmission electron microscopy (HRTEM). The SAED pattern (Figure 2b) taken from a single nanobelt (Figure 2a) indicated that the nanobelts are single crystals. The HRTEM image (Figure 2c) recorded on the quadrate area shows the clearly resolved interplanar distances d ) 0.204 nm, which determines the (010) growth direction of the nanobelts. To further investigate the composition of vanadium oxide nanobelts, some necessary characteristics are carried out. Figure S2 (Supporting Information) shows the Fourier transform infrared (FT-IR) spectrum of the V2O5 raw material (Figure S2a)

Hydrothermal Growth of Vanadium Oxide Nanobelts

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Figure 2. (a) TEM image of a single nanobelt; (b) SAED image of nanobelt; (c) HRTEM image of nanobelt.

and the as-obtained nanobelts (Figure S2b). For the vanadium oxide nanobelts, as shown in Figure S2b, the bands at 2361.88, 1003.27, 764.74, and 528.48 cm-1 are attributed to the vibrational band characteristic of V-O. Compared with those for the V2O5 raw material, the bands corresponding to VdO stretching and V-O-V deformation modes are shifted to lower frequencies. Furthermore, a new band at 3442.09 cm-1 emerges, which can be attributed to the H-O-H stretching vibration. It might indicate that a certain amount of water molecules is embedded between the layers. Figure S3 (Supporting Information) presents UV/vis spectra of vanadium oxide nanobelts synthesized in the presence of the surfactant SDS. An adsorption peak centered about 790.00 nm can be attributed to the d-d electronic transition of V4+, suggesting the existence of V4+ in vanadium oxide nanobelts.19,20 Because this reaction was carried out without any other reducing agent, we think it is difficult to completely reduce V2O5 to VO2 under hydrothermal conditions. The oxidation state of vanadium oxide nanobelts can be determined by X-ray photoelecton spectroscopy (XPS) measurements. Figure S4 (Supporting Information) shows the spectra of the V2p core level. The V2p3/2 peak is centered at 517.3 ev. The binding energy is characteristic of vanadium in the +5 oxidation state. However, we also see the V2p3/2 peak around 516.0 ev, which represents the +4 oxidation state of vanadium. So, the XPS image further confirms that V5+ was partly reduced to V4+. Therefore, the chemical formula of the vanadium oxide nanobelts could be described as VOx‚nH2O according to above analysis. It was found from a series of experiments that the reaction time and the reaction temperature have significant effects on the morphology of vanadium oxide. First, keeping other parameters constant, only the reaction time was changed. After 12 h, single-crystalline vanadium oxide nanobelts had already been formed as shown in Figure 3a,b. The nanobelts have a diameter and length in the ranges of 100-150 nm and 10-15 µm, respectively, but we can occasionally see fragments. With an increase in the reaction time, the belts became longer, and the fragments became fewer. A TEM image of the product after a reaction time of 24 h, as shown in Figure 3c, reveals that the product mainly consists of belts with a length of about tens of micrometers. After heating for 36 h, the nanobelts continue to grow. Further extending the heating (48 h) leads to the formation of belts with an ultrahigh-aspect-ratio. Second, when the reaction was carried out at different temperatures, different morphologies of vanadium oxide were obtained. The short, broken nanobelts and lots of particles were formed at 120 °C (Figure 3d) and belts with different diameters and lengths at 180 °C (Figure 3e). The reason may be that at lower temperature, the precursor may not react completely; however, at 180 °C, the reaction may

Figure 3. TEM images of the obtained vanadium oxide nanobelts at different reaction times and temperatures: (a) 12 h, (b) 24 h, (c) 36 h; (d) 120 °C, (e) 180 °C.

Figure 4. TEM images of vanadium oxide nanobelts obtained with different molar ratios of SDS to V2O5 (a) 1:1, (b) 1:2, (c) 1:3, (d) 0.

be so fast due to higher temperature so that it does not have enough time to form the well-proportioned belts. Furthermore, the concentration of the surfactant SDS also has effects on the morphology of the product. To study this effect, a series experiments were carried out with different molar ratios of [SDS]/[V2O5] but using constant temperature and reaction time. When the molar ratio [SDS]/[V2O5] is 1:1, the as-synthesized belts are wider (Figure 4a) than those obtained at a 2:1 ratio of [SDS]/[V2O5] (Figure 4b). On increasing the molar ratio, the belts became narrower and longer. However, when the molar ratio of [SDS]/[V2O5] was increased to 4:1 (Figure 4c), the belts became shorter and some particles emerged. So a molar ratio [SDS]/[V2O5] of 3:1 is the optimal condition for getting the narrow and long vanadium oxide nanobelts (Figure 1). If no SDS was used, a large number of particles and a few nanobelts were formed (Figure 4d).

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In our experiment, we hypothesized that the surfactant which controls the morphologies of vanadium oxide nanobelts synthesized embeds in and wraps each layer of the V2O5 layer precursor. When using a lower concentration of SDS, the film which coats the V2O5 precursor is thin, the reaction rate is rapid, and the growth of the nucleus is relatively random, so the belts are wide. With a higher SDS concentration, the film which coats the V2O5 precursor becomes thick, the surrounding space of the surfactant is more and more narrow, and the reaction rate is slower and slower, so when it reaches the extreme, we obtain the best nanobelts. The nanobelts are obviously narrower and longer than previously obtained vanadium nanobelts. However, once the concentration of SDS is over the limit, the best belts cannot be obtained. Through the above experimental results, we know that the surfactant is crucial for the formation of vanadium oxide nanobelts. Although the mechanism is not clear between the V2O5 raw material and the surfactant under hydrothermal conditions, we presume it may be attributed to the template function of the surfactant. Recently, much work has been focused on V2O5 nanostructures by the hydrothermal process, and various V2O5 nanomaterials have been prepared, such as nanorods, nanowires, nanosheets, and nanobelts. However, their growth process differs from ours. Those hydrothermal methods are all based on homogeneous nucleation and solution processes.21,22 In our paper, a special study shows that insoluble V2O5 solid-state powder could be directly used as a raw material for the synthesis of vanadium oxide nanobelts under hydrothermal conditions without adding any other nucleation. The formation mechanism of vanadium oxide nanobelts is the same as that for the formation process of LiMn2O4.23 It follows two steps, dissolution and surfactant-directed recrystallization. First, the bulky V2O5 powder partially dissolves in water under hydrothermal conditions, so it still includes V2O5 particles in the solution. The undissolved V2O5 particles can serve as seeds for the nucleation of the nanobelts. As we know, the surfactant can selectively adsorb different crystallographic planes of the nucleus, resulting in the formation of the anisotropic nanostructures. Therefore, the formation of the vanadium oxide nanobelts can be regarded as a seed-mediated, surfactant-directed growth process. It is most important that insoluble oxide raw material was required in our experiments; the seeds and nutrition source for our vanadium oxide nanobelts are from the same bulky particles. Because of the layered structure of vanadium oxide, it has been well studied for various applications. As an intercalation compound, vanadium oxide has attracted a lot of attention as an electrode for electrochemical applications. The electrochemical properties are influenced by many factors such as crystallinity, surface area, and preparation methods. In our experiment, we investigated the charge-discharge capability of the postannealing sample obtained by calcining the as-prepared vanadium oxide nanobelts at 400 °C. Some previous reports revealed that the influence of a significant amount of H2O in the materials on the electrochemical properties is vital, and it may react with lithium to limit the cycling life and capacity of the batteries.24 So, it has been demonstrated that the capacity and cycling performance will be remarkably improved after the water content in electrode materials is reduced by thermal treatment.24 This explains why we did not directly measure the electrochemical behavior of the vanadium oxide nanobelts. The XRD image of the postannealing sample is shown in Figure S5 (Supporting Information). It clearly reveals that after calcining, not only was the crystallinity of the sample greatly improved, but also the

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Figure 5. The electrochemical performance of anhydrous vanadium oxide nanobelts obtained, (a) charge-discharge curves in the initial cycle; (b) the cycle performance of capacitances.

layered structure was maintained; both are very important for keeping an even larger capacity and good cycling performance. Figure 5a shows the profile of the postannealing sample in the initial charge-discharge cycle in a range between 3.486 and 1.399 V vs Li+/Li and under a constant current density of 0.2 mA‚cm-2. The postannealing sample exhibits an initial discharge capacity of as high as 323 mAh/g, which is markedly higher than those in the references reported previously.13,17,25 It is even larger than that (307.5 mAh/g) recently reported by Xie.26 That may be attributed to its layered structure and lower water content of the calcined sample. In addition, the discharge curve exhibits a stepwise shape, which is due to the phase change of LixV2O5 during Li+ intercalation. Figure 5b summaries cycling performance of the vanadium oxide nanobelts treated at 400 °C. It can be clearly seen that the postannealing sample exhibited a high initial capacity. However, with the increase of the cycling numbers a drastic electrochemical degradation was observed. It retains 61% of the initial discharge capacity after 11 cycles and reaches a stabilized capacity of about 150 mAh/g. The decreasing of discharge capacity of the sample is likely due to partial fracture of its shape and structural degradation after the redox cycle. That has been studied during the charge-discharge process of other vanadium nanostructures.27 All the results above obtained from the electrochemical investigation indicate that the electrochemical properties are related to the crystal structure and morphology of the electrode material. Conclusion In conclusion, uniform vanadium oxide nanobelts have been synthesized on a large scale under hydrothermal conditions in the presence of a surfactant. In addition, these nanobelts are transformed directly from the V2O5 solid-state powder without adding any other nucleus. The surfactant not only provides a

Hydrothermal Growth of Vanadium Oxide Nanobelts

microenvironment for the growth of vanadium oxide nanobelts but also plays a template role for directing the formation of nanobelts. The postannealing vanadium oxide nanobelts with a high aspect ratio are beneficial for lithium ion insertion between the V2O5 layers for application in batteries. Acknowledgment. The authors thank the National Natural Science Foundation of China (NSFC, 20401005), the Jilin Distinguished Young Scholars Program Foundation, and the Huo Yingdong Foundation for financial support. This work also was supported by analysis and testing foundation of Northeast Normal University. Supporting Information Available: XRD patterns; FTIR, UV/vis, and XPS core level spectra. This material is available free of charge via the Internet at http://pubs.acs.org.

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