CRYSTAL GROWTH & DESIGN
Vanadium Pentoxide Nanowires: Hydrothermal Synthesis, Formation Mechanism, and Phase Control Parameters Fu Zhou,*,† Xuemei Zhao,‡ Cunguang Yuan,† and Li Li† Department of Chemistry, and Center for Bioengineering and Biotechnology, China UniVersity of Petroleum, Qingdao, Shandong 266555, PR China
2008 VOL. 8, NO. 2 723–727
ReceiVed NoVember 18, 2006; ReVised Manuscript ReceiVed September 28, 2007
ABSTRACT: Large-scale orthorhombic V2O5 single-crystalline nanowires with diameters of 60–80 nm and lengths of up to hundreds of micrometers, which is by far the longest reported one-dimensional nanostructure of vanadium oxide fabricated, were synthesized by a template-free mild and direct hydrothermal reaction between VOSO4 · xH2O and KBrO3. The formation mechanism of the as-obtained orthorhombic V2O5 nanowires is briefly discussed. Several parameters, such as pH value in the solution, reaction temperature, and kinds of oxidants, are believed to play an important role in the phase of the final products.
1. Introduction Vanadium oxide and derived compounds have received significant attention recently because of their structural versatility combined with chemical and physical properties.1 While vanadium oxide phases containing mixed valence vanadium (V5+ and V4+) crystallize in three-dimensional (3-D) network structures, they can be regarded as layered structures in which VO5 square pyramids are connected by sharing corners and edges.2 The interactions between these layers are rather weak, as indicated by the exceptionally long V-O distance of 0.279 nm.3 This structural particularity permits the synthesis of a variety of binary vanadium oxides with the general formula VO2+x (0 e x e 0.33), such as V2O5, V2O3, V6O13, VO2, etc, which have attracted great attention because of their outstanding properties and potential applications as catalysts, chemistry sensors, high-energy density lithium ion batteries, and electrochemical and optical devices.4–9 In recent years, great attention has been focused on synthesis and applications of nanostructured materials, and one of the most dynamic research areas is the synthesis of one-dimensional (1D) nanostructures, such as nanowires, nanorods, nanobelts, and nanotubes.10–15 In the past decades, the fabrication of vanadium oxide 1-D nanostructures has been researched intensively. A variety of methods, such as thermal evaporation, surfactantassisted solution, and hydrothermal/solvothermal synthesis, have been developed to prepare vanadium oxides 1-D nanostructures.16–20 For instance, nanowires of V2O5 have been synthesized by polycondensation of vanadic acid, which involves ion exchange between Na+ and H+ ions in a resin from sodium metavanadate solutions.21,22 V2O5 nanorods and nanowires have been fabricated using a reverse-micelle technique by Pinna et al..23,24 Nesper and co-workers have developed a novel soft-chemistry synthesis method of a fundamentally new type of vanadium oxide nanotubes (VOx-NTs) involving an amine with long alkyl chains as a molecular, structure-directing template.25–29 Ivanovskii et al. have done a lot of research work on the electronic property of scroll-like divanadium pentoxide nanotubes.30–32 Cao et al. have made lots of efforts on the synthesis and electrochemical study of V2O5 nanorod/nanotube arrays.33–35 Li et al. have fabricated various vanadium oxide 1-D nanostructures and * Corresponding author. Tel: +86-532-86981562; fax: +86-532-86981318; e-mail:
[email protected]. † Department of Chemistry. ‡ Center for Bioengineering and Biotechnology.
studied their application as ethanol sensor materials and magnetic properties.36–38 However, all these synthetic methods of vanadium oxide 1-D nanostructures are either based on the transformation of V5+-contained precursors during reaction or through a complex and lengthy route. To the best of our knowledge, the simple direct hydrothermal synthesis of V2O5 1-D nanostructures from reagents with the oxidation state of vanadium at +4 is rarely reported before. Herein, we report the fabrication of orthorhombic V2O5 nanowires through a simple mild template-free hydrothermal synthetic method using VOSO4 · xH2O as precursors. The length of the as-obtained orthorhombic V2O5 nanowires reached up to hundreds of micrometers, which is by far the longest 1-D nanostructure of vanadium oxide that has been fabricated. The formation mechanism of the ultralong orthorhombic V2O5 nanowires is briefly discussed. Several parameters, such as reaction temperature, pH value in the solution system, and kinds of oxidant, were found to play an important role in controlling the phase of the final products.
2. Experimental Procedures All the reagents were of analytical grade and were used without further purification. In a typical synthetic procedure of orthorhombic V2O5 nanowires, 10 mmol of VOSO4 · xH2O and 5 mmol of KBrO3 were dissolved in 40 mL of distilled water and stirred magnetically for 30 min to get good homogeneity. Nitric acid was added dropwise under stirring until the final pH of the solution was about 1–2. The resulting clear solution was then transferred into a 50 mL Teflon-lined stainless steel autoclave. The autoclave was maintained at 160 °C for 24 h. After the sample was cooled to room temperature naturally, the yellow precipitates were filtered off, washed with distilled water and anhydrous alcohol several times, and dried in a vacuum at 80 °C for 12 h. X-ray diffraction (XRD) patterns of the final products were recorded by using a Philips X’Pert Super diffractometer with graphite monochromatized Cu KR radiation (λ ) 1.54178 Å) in the 2θ range of 5–80°. The morphology of the products was examined by a field emission scanning electron microscope (FESEM, JEOL JSM - 6300F, 15 kV). The transmission electron microscopy (TEM) images were performed with a Hitachi 800 TEM with an accelerating voltage of 200 kV. The electron diffraction (ED) patterns and high resolution transmission electron microscopy (HRTEM) images were obtained using a JEOL2010 TEM at an acceleration voltage of 200 kV. The samples used for characterization were dispersed in absolute ethanol and were ultrasonicated before TEM, FESEM, and HRTEM tests.
10.1021/cg060816x CCC: $40.75 2008 American Chemical Society Published on Web 12/20/2007
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Figure 1. XRD pattern of the typical product. Figure 3. (a, b) TEM images of the typical product. Inset: SAED pattern.
Figure 4. (a, b) HRTEM images of the typical product. Inset: SAED pattern.
Figure 2. (a-d) FESEM images of the typical product.
3. Results and Discussion 3.1. XRD Patterns of the Products. XRD analysis was carried out to investigate the phases of the as-obtained products. The XRD pattern of the typical product is given in Figure 1. All the diffraction peaks could be indexed as the orthorhombic phase of V2O5 in agreement with literature values (JCPDS card 85-0601) with calculated lattice constants a ) 3.563 Å, b ) 11.516 Å, c ) 4.374 Å. The (001) peak was extraordinarily strong compared with other peaks, which differed greatly from the XRD data for the powder sample (JCPDS card 85-0601), indicating that the as-obtained orthorhombic V2O5 might have special morphologies. 3.2. The Morphology and Structure of As-Obtained Orthorhombic V2O5 Nanowires. The FE-SEM images of the typical product are shown in Figure 2. The microstructures of the product were long uniform nanowires, and the proportion of the nanowires in the sample was almost 100% (Figure 2a). The length of as-obtained orthorhombic V2O5 nanowires reached hundreds of micrometers as calculated from the panoramic image shown in Figure 2a, which was by far the longest length reported for the fabrication of 1-D vanadium oxide nanostructures, indicating the efficiency of our synthetic method. The diameters of the nanowires ranged from 60 to 80 nm (Figure 2b-d), and the as-obtained nanowires had a strong tendency to self-assemble into nanowire arrays. Figure 3 gives the TEM images of the typical product, which clearly showed that
nanowire arrays of orthorhombic V2O5 with diameters around 60 nm and lengths of up to hundreds of micrometers were obtained. The inset SAED image in Figure 3b indicated that the as-obtained V2O5 nanowires are single crystallined. The HRTEM image of a typical orthorhombic V2O5 nanowire is shown in Figure 4a. The distance between the neighboring planes is about 0.287 nm (Figure 4b), consistent with that of the (040) plane of orthorhombic V2O5, indicating the individual nanowire growth direction of [010]. The inset SAED pattern in Figure 4a, taken from an individual nanowire, reveals its good single-crystal nature. 3.3. Formation Mechanism of As-Obtained Orthorhombic V2O5 Nanowires. To study the formation mechanism of as-obtained orthorhombic V2O5 nanowires, the evolution process of the typical product was examined thoroughly by detailed XRD and TEM tests. The synthetic process was ceased at definite reaction periods of 1, 6, and 12 h, and the as-obtained intermediate products were separated for XRD and TEM studies. Although we knew that the reaction did not stop immediately after the autoclave was removed from the heater owing to heat transfer reasons, we did believe that the intermediate precipitates obtained represented certain stages in the formation process. The XRD pattern and TEM images of the intermediate product were shown respectively in Figures 5 and 6. As shown in Figure 5a, when the reaction was carried out for 1 h, the diffraction peaks of the main intermediate products could be indexed as a set of peaks characteristic of the 00l reflections for the layered phase V2O5 · xH2O, which was consistent with the reported data.39–41 The layer spacing of the intermediate phase was determined to be 12.85 Å from the 001 reflection, which agreed well with literature data.42 When the
Vanadium Pentoxide Nanowires
Figure 5. XRD patterns of the intermediate products at the reaction time of (a) 1 h, (b) 6 h, (c) 12 h.
Figure 6. TEM images of the intermediate products at the reaction time of (a) 1 h, (b) 6 h, (c) 12 h.
reaction time was prolonged to 6 h, the XRD pattern of the intermediate product (Figure 5b) still displayed the characteristic peaks of the 00l reflections for the layered phase V2O5 · wH2O (y < x). However, the layer spacing of the intermediate phase was reduced to 10.12 Å as determined from the 001 reflection. The smaller interplanar spacing was caused by the removal of water from the interplanar region of the layered phase V2O5 · xH2O. Thermogravimetric (TG) tests on the intermediate products (the results not shown here) also proved that the crystalline water intercalated in the middle of vanadium oxide layers decreased with the prolonged reaction time. After 12 h of reaction, all the diffraction peaks could be indexed as the orthorhombic phase of V2O5 in agreement with literature values (JCPDS card 89-0612), indicating that nearly all the intercalated water was driven out and pure phase of orthorhombic V2O5 was obtained (Figure 5c). Figure 6 displayed the morphological evolution of the intermediate products during our synthetic route. The TEM image of the 1 h intermediate product was given in Figure 6a. It could be clearly observed that the intermediate product at 1 h mainly consisted of small nanoparticles and short nanofiber arrays. Figure 6b showed the TEM image of the intermediate product at 6 h. There were nearly no nanoparticles left, and the product was composed of nanowire arrays in the length range of several micrometers. After 12 h of hydrothermal reaction, the product showed well-developed nanowire morphology as shown in Figure 6c. The diameter of the nanowires was in the range of 30–50 nm, and the length reached tens of micrometers. Prolonged reaction time was necessary for the further development of the nanowires. After 24 h of reaction, large-scale uniform orthorhombic V2O5 nanowires with diameters of 60–80
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nm, and lengths of up to hundreds of micrometers were obtained as the final product, as shown in Figure 2. On the basis of the above experimental results, the possible formation mechanism might be concluded as follows. It is wellknown that V2O5 can be regarded as a layered structure in which VO5 square pyramids are connected by sharing corners and edges.2 The interactions between these layers are rather weak, as indicated by the exceptionally long V-O distance of 0.279 nm.3 In particular, this structure permits H2O molecules to be embedded between the layers without a far-reaching restructuring, which leads to the formation of the V2O5 · xH2O phase. In our synthetic route, at the initial phase of reaction, the strong oxidant KBrO3 oxidized the VO2+ cations in the solution into small V2O5 nanoparticles. Since the layered structure of V2O5 was favorable for the intercalation of H2O molecules, the initially formed V2O5 nanoparticles had a strong inclination to have H2O molecules embedded between their layers and transformed into the V2O5 · xH2O phase. Meanwhile, the layered structure of vanadium oxides and their derivatives was favorable for the development of 1-D nanostructures, as proven by the works of other chemists.36–38 As were reported by other research groups, these newly formed nanoparticles had a very strong tendency to aggregate into ribbon-like particles,43–46 similar to the intermediate products observed at a reaction time of 1 h. Therefore, the initially formed ribbon-like particles served as the precursor to grow into 1-D nanostructures in the prolonged reaction, thus leading to the formation of short nanowire array intermediate products as shown in Figure 6a. With the reaction proceeding, the embedded H2O molecules between the layers of V2O5 · xH2O were gradually removed, as proven by the reduction of the d value of the 001 reflection (Figure 5b). Furthermore, the initially formed short and immature nanowire arrays grew further into a longer and better-developed 1-D nanostructures base on the well-known Ostwald ripening process. In the prolonged hydrothermal treatment, the H2O molecules were all removed from the interplanar regions of the layers in V2O5 · xH2O, leading to the formation of the orthorhombic V2O5 phase (Figures 5c and 1). Meanwhile, the final product developed into a full-developed nanowire morphology with diameters in the range of 60–80 nm and lengths up to hundreds of micrometers (as shown in Figure 2), which was by far the longest 1-D nanostructures reported for vanadium oxides. 3.4. Phase Control Parameters of As-Obtained Orthorhombic V2O5 nanowires. 3.4.1. pH Value. The successful fabrication of orthorhombic V2O5 nanowires can only be realized at a narrow pH value range of 1–2. Comparative experiments were made by adjusting the pH value to the range of 2–3 while keeping other synthetic parameters stable. The XRD pattern of as-synthesized product (Figure 7a) showed typical reflection peaks of the layered phase V2O5 · xH2O. A similar phenomenon had also been discovered in the previous studies, suggesting that vanadium oxide 1-D nanostructures could only be produced in a narrow range of pH values.38,47 3.4.2. Reaction Temperature. Reaction temperature in our synthetic route was also an important factor impacting the phase of the final product. Comparative experiments were made by varying the reaction temperature in the range of 140–180 °C while keeping other synthetic parameters unchanged. The results indicated that the temperature range of 160–180 °C was favorable for the fabrication of orthorhombic V2O5 nanowires. When the reaction temperature dropped to below 160 °C, such as 150 °C, the XRD pattern of the product displayed a set of peaks characteristic of 00l reflections for the layered phase of V2O5 · xH2O, as shown in Figure 7b. The possible reason for the influence of reaction temperature on the phase of the final
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value in the solution system, reaction time, and kinds of oxidant, were found to play an important role in controlling the phase of the products. These ultralong V2O5 nanowires, which have large specific areas and short diffusion lengths, may be a superior cathode material for application in lithium ion batteries. Further work is under way in our laboratory to study the electrochemical properties of these ultralong orthorhombic V2O5 nanowires, which will be reported later.
References
Figure 7. XRD patterns of the products fabricated under various synthetic parameters: (a) pH value: 2–3; (b) reaction temperature: 150 °C; (c) oxidant: KClO3; (d) oxidant: K2S2O8.
product might be that with the reduction of reaction temperature, the driving force for the removal of H2O molecules from the interplanar regions of V2O5 · xH2O was deficient, and thus the transformation from layered phase V2O5 · xH2O into orthorhombic V2O5 was impossible. 3.4.3. Kinds of Oxidants. The kinds of the oxidants in our synthetic route also played an important role in the successful synthesis of orthorhombic V2O5 nanowires. Only strong oxidants such as KBrO3 could be applied in our synthetic method. Other oxidants, which were weaker than KBrO3 in their oxidizing ability, such as KClO3 and K2S2O8 were applied in our comparative experiments while keeping other synthetic parameters stable. Only layered phase V2O5 · xH2O was obtained as the final products, as shown in Figure 7c,d. It was believed that only oxidants with strong oxidizing abilities such as KBrO3 could supply the necessary driving force for the removal of H2O molecules from the interplanar regions of the layered structures of V2O5 · xH2O. The morphologies of the products prepared under the above various synthetic parameters were studied by TEM technique. Only small nanoparticles and arrays of very short nanorods with a length of only several micrometers were obtained (TEM images not shown), indicating the lack of the driving force for the formation of fully developed high-quality ultralong orthorhombic V2O5 nanowires under these conditions. On the basis of the above comparative experiments results, the optimal synthetic parameters for the fabrication of ultralong well-developed orthorhombic V2O5 nanowires could be concluded as follows. The pH value in the solution should be in the range of 1–2, the reaction temperature should not be lower than 160 °C, and strong oxidants such as KBrO3 were indispensable for the successful synthesis of orthorhombic V2O5 nanowires.
4. Conclusion In conclusion, we have developed a novel facile hydrothermal method to synthesize uniform orthorhombic V2O5 nanowires with the lengths up to hundreds of micrometers, which was by far the longest reported 1-D nanostructure fabricated for vanadium oxides. The synthetic method was based on the mild and direct reaction between VOSO4 · xH2O and KBrO3 and needed no templates or complex treatments, which was very economical and environmentally friendly. The formation mechanism of the ultralong orthorhombic V2O5 nanowires was briefly discussed. Several parameters, such as reaction temperature, pH
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