DOI: 10.1021/cg901348e
Facile Hydrothermal Synthesis of SrNb2O6 Nanotubes with Rhombic Cross Sections
2010, Vol. 10 2447–2450
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In-Sun Cho,† Sangwook Lee,† Jun Hong Noh,‡ Dong Wook Kim,‡ Hyun Suk Jung,§ Dong-Wan Kim,*, and Kug Sun Hong*,†,‡ †
Research Institute of Advanced Materials, Seoul National University, Seoul 151-742, Korea, Department of Materials Science & Engineering, Seoul National University, Seoul 151-744, Korea, §School of Advanced Materials Engineering, Kookmin University, Seoul 136-702, Korea, and Department of Materials Science & Engineering, Ajou University, Suwon 443-749, Korea
)
‡
Received October 28, 2009; Revised Manuscript Received April 23, 2010
ABSTRACT: Novel SrNb2O6 nanotubes with rhombic cross sections were produced via a facile hydrothermal route without any surfactants or templates by controlling the reaction conditions, such as the pH value and temperature. The prepared nanotubular powders were characterized using X-ray diffraction, field-emission electron microscopy (FESEM), and high-resolution transmission electron microscopy (HRTEM). From the FESEM and HRTEM analysis, it was found that the morphological evolution of the onedimensional tubular structure was originated from the stepwise bending growth in the [011] direction around the a-axis and its rhombic cross section was connected to the crystallographic nature.
*To whom correspondence should be addressed. E-mail: dwkim@ajou. ac.kr and
[email protected].
ethanol (99.9%), respectively, using constant magnetic stirring. After stirring for 20 min, the Nbcl5 solution was slowly added to the Sr(NO3)2 solution with constant magnetic stirring. Then, the pH of the solution was adjusted from 9.0 to 13.0 using 1 M NaOH (or KOH) solution. The mixture was then poured into an autoclave (300 mL), which was sealed, heated to the desired temperature (150-250 °C), and held at this temperature for 12 h without stirring, followed by natural cooling to room temperature. The white precipitate that formed was collected by centrifugation and washed with distilled water and absolute ethanol and finally dried at 90 °C in an oven. For the purpose of comparison, bulk SrNb2O6 powder was prepared by a solid-state reaction method. SrCO3 (High Purity Chemicals, 99.9%) and Nb2O5 (High Purity Chemicals, 99.9%) were used as the raw materials. Stoichiometric mixtures of the starting materials were homogenized by ball milling with zirconia media for 24 h. After drying and sieving, the powder was calcined at 1150 °C for 2 h in air. The crystal structures of the synthesized powders were determined using an X-ray powder diffractometer (Mac-science, M18XHF). The lattice parameters and atomic coordinates were obtained using GSAS Rietveld refinement software25 and a software package26 to refine the XRD patterns, which were measured by step scanning (0.02°) using Si (99.999%) as an external standard. The morphologies and microstructures were investigated using an FESEM (JEOL, JSM-6330F). The TEM images were recorded on a JEOL JEM-3000F microscope at an accelerating voltage of 300 kV. The cross-sectioned TEM specimens were prepared by the spin-coating method on a Si substrate and the focused ion beam (FIB)-based “lift-out” technique using a FIB-SEM hybrid system (SII NanoTechnology lnc., SMI3050SE). The specific surface area measurements were performed using a Brunauer-Emmett-Teller (BET) surface area analyzer (BELSORP-mini II, BEL Japan, Inc.). Panels a and b of Figure 1 show the typical FESEM images of the SrNb2O6 powders prepared by the hydrothermal method at 200 °C for 12 h at pH 11.0. It can be clearly seen that the powders are composed of uniform rhombic tubes with a diameter range of 200-300 nm, an average length of up to ∼3 μm, and a wall thickness of ∼50 nm. In the lattice image and corresponding fast Fourier transform (FFT) pattern at the side-edge of a single nanotube (Figure 1c), its single crystalline nature and tube axis
r 2010 American Chemical Society
Published on Web 04/29/2010
Recently, nanostructured materials such as nanoplates, nanorods, and nanospheres have attracted considerable attention, because of their interesting size- and morphology-dependent properties and potential applications in various areas such as catalysis, optoelectronics, lithium-ion batteries, and drug delivery.1-5 In particular, tubular nanostructures have attracted increasing attention since the discovery of carbon nanotubes by Iijima in 1991.6 A variety of tubular nanostructures have been prepared including semiconductors,7,8 metals,9 sulfides,10 nitrides,11 metal oxides (niobates,12 phosphates,13,14 aluminates,15 tungstates,16 silicates17), etc. However, the successful synthesis of tubular nanostructures consisting of binary metal oxides or complicated composition materials is difficult and remains a great challenge. Strontium metaniobate (SrNb2O6), a constituent of Ba1-xSrxNb2O6, has attracted a great deal of attention recently and is currently being investigated as a potential material for pyroelectric, electro-optic, ferroelectric, and photorefractive devices.18,19 It has a monoclinic crystal structure with cell parameters of a = 7.722, b = 5.594, c = 10.986 A˚ and β = 90.370°.20 The photocatalytic activities of SrNb2O6 and its heterojunctions with WO3 for the degradation of organic dye molecules under UV irradiation were studied by An et al. and Huang et al., respectively.21,22 The photocatalytic production of H2 was also investigated by Kim et al.23 In all of these studies, however, the methods used for the preparation of the SrNb2O6 powders were limited to conventional solid-state reactions, which generally require repeated mechanical mixing and a high temperature process, which can lead to large agglomerations of particles and a low surface area. More recently, the high photocatalytic performance of one-dimensional (1-D) nanorods was reported for the first time by Chen et al.24 Herein, we report on a hydrothermal route for the synthesis of SrNb2O6 rhombic nanotubes. The detailed morphological evolution during the growth of these 1-D tubular nanostructures with rhombic cross sections was examined by controlling the reaction conditions, such as the synthetic temperature, time, and pH. In a typical synthesis of the SrNb2O6 nanotubes, 10 mmol of strontium nitrate (Sr(NO3)2, Aldrich Chemicals, 99.0%) and 20 mmol of niobium chloride NbCl5, Aldrich Chemicals, 99.0%) were dissolved in 100 mL of distilled water and 50 mL of absolute
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were clearly observed; that is, the interplanar spacing of the lattice planes along the length of the nanotube was 0.39 nm, which corresponds to the interplanar spacing of the (200) plane of monoclinic SrNb2O6. The XRD pattern taken from these powders was indexed as phase-pure monoclinic SrNb2O6 (JCPDS, No. 72-2088), as shown in Figure 1d.
Figure 1. FESEM (a, b), TEM images (c), and XRD pattern (d) of the synthesized SrNb2O6 rhombic nanotubes. The insets of part c are the HRTEM image and FFT pattern.
Cho et al. When using pH values below 11.0, both elongated tubular and octahedron-like morphologies with irregular size distributions were obtained (Figure S1 of the Supporting Information). On the basis of the XRD analysis (Figure S2), the octahedron-like morphologies observed at pH values below 11.0 were confirmed to be a H3.2Sr0.4Nb2O7 phase with a smaller Sr/Nb ratio, whose formation was attributed to the insufficient Sr content in the precipitates. The powder prepared at pH 13.0 was composed of broken nanotubes, agglomerated plates, and nanoparticles. The agglomerated plates and nanoparticles obtained at pH 13.0 were Sr(OH)2 and NaNbO3 phases, respectively. These phases were attributed to the phase separation of SrNb2O6 due to the reaction between SrNb2O6 nanotubes and Naþ ions in the solution under higher pH conditions (higher NaOH content). The relative amounts of impurity phases, such as H3.2Sr0.4Nb2O7, Sr(OH)2, and NaNbO3 phases observed at different pH values, were approximately calculated by a quantitative XRD analysis using the Rietveld method, and the results were summarized in Table S1. Although the amount of NaNbO3 was almost unchanged, the amount of the Sr(OH)2 phase was more increased when the pH value was increased from 12.0 to 13.0. As a result, it is considered that controling the pH value in the solution plays an important role in the formation of a uniform and tubular morphology without any impurity phases. TEM and HRTEM studies were conducted to further identify the morphology and crystallinity of the as-synthesized SrNb2O6 rhombic nanotubes. Figure 2a shows a cross-sectional TEM image of an individual nanotube. A rhombic tubular structure with an obtuse angle of 126° and an acute angle of 54° was observed. To further understand the crystallographic directions of the rhombic nanotube, HRTEM images and the corresponding FFT patterns in regions A and B were obtained (Figure 2b and c). As can be seen in these figures, the interplanar spacings of the lattice planes along the short and long diagonal directions of
Figure 2. (a) Cross-sectional TEM image of a single nanotube; (b and c) HRTEM images and corresponding FFT patterns in regions A and B, respectively; (d) overall crystallographic directions of the nanotube and the projected atomic structure of SrNb2O6 in the [100] direction. The angles shown in the cross-sectional TEM image were consistent with those obtained from the atomic structure.
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Figure 3. Schematic illustration of the morphological evolution process and the corresponding intermediate morphologies of the rhombic SrNb2O6 nanotubes.
the cross-sectional rhombic nanotube were 0.56 and 1.10 nm, which correspond to those of the (010) and (001) planes of monoclinic SrNb2O6, respectively. Moreover, the interplanar spacing of the lattice planes parallel to the surface normal of a cross-sectioned rhombic nanotube was 0.50 nm, which corresponds to that of the (011) plane of monoclinic SrNb2O6; that is, the parallel direction of the rhombic nanotube edges was the [011] direction. A possible atomic model corresponding to the HRTEM images in Figure 2b and c is shown in Figure 2d. From the projected atomic structure of monoclinic SrNb2O6 along the a-axis, the obtuse and acute angles of the nanotube were consistent with those of the atomic structure shown in Figure 2d. Generally, the cross sections of 1-D tubular nanostructures tend to be circular, due to their relatively low surface energy, but the SrNb2O6 in this study was processed into rhombic nanotubes with well-defined hollow interiors. In order to understand the possible morphological evolution mechanisms of the SrNb2O6 nanotubes with rhombic cross sections, controlled experiments were conducted at 200 °C while varying the hydrothermal time (Figure S3). The detailed FESEM images of the intermediate morphologies are shown in Figure 3, together with a schematic illustration of the whole evolution process of the rhombic nanotubes. During the initial stage of the reaction, amorphous nanoparticles were formed (after 1 h) which subsequently selfassembled to form elongated plates and bent plates by oriented aggregation (after 3 h) (Figure 3a). After a prolonged reaction time, these bent plates grew laterally, and then further bending occurred at the intermediate stage (Figure 3b). The final stage of evolution was from incomplete nanotubes to enclosed rhombic nanotubes by the fusion of the edges (after 12 h) (Figure 3c). The results of experiments conducted at different temperatures demonstrate similar morphological evolution (Figure S4). In general, the preferential growth direction of nanostructures is determined by the minimization of the free energy, which consists of the surface energy and the strain energy.27,28 Therefore, the evolution of the nanoplates in the [100] direction at the initial stage should minimize their free energy. Although the energetic favorability of SrNb2O6 has not yet been clarified, the stacking sequence of the a-axis consists of alternating layers of “ABAB...” (where A is the SrO8 polyhedra and B is the NbO6 octahedra), whereas that of the c-axis is “ABBABB...” (Figure S7) and, thus, the distance between the (100) planes is smaller than that between the (001) planes, which may cause the initial anisotropic growth of the nanoplates to proceed in the [100] direction by the self-assembly of the nanoparticles, as in other systems.29 Then, based on the diffusion-limited aggregation model, the protruding parts of the nanoplates can easily grasp the nanoparticles and grow quickly to build a new wall from the nanoplate edge, leaving the hollow interior.30 A similar
hypothesis was made for the growth mechanism of IrO2 nanotubes.31 It is believed that the stepwise bending growth leading to the formation of a rhombic cross section is attributed to the crystallographic nature of SrNb2O6, as shown in Figure 2d. After the rhombic nanotubes are formed, further increases in the time and temperature (Figures S3e and S4d, respectively) provide the nanoparticles with sufficient energy to arrange themselves and diffuse into the center of the vacant structure, making the tube walls thicken and finally forming the rhombic nanorods for the sake of their morphological stability. In contrast to the other tubular morphologies reported previously,16,32 the formation of the rhombic SrNb2O6 nanotubes involved the stepwise bending process of nanoplates, which is also affected by the internal pressure, because nanorods were observed when the filling factor (the amount of solution in the hydrothermal reactor) was reduced to 40% (Figure S5). This highly anisotropic crystal habit along the a-axis has been observed in the case of other SrNb2O6 micro/ nanorods.24,33 From the BET surface area measurements, the SrNb2O6 rhombic nanotubes exhibited increased surface area (8.1 m2/g) compared to their bulk counterpart (1.1 m2/g). This increased surface area of SrNb2O6 nanotubes originates from their unique morphological features. In summary, a simple hydrothermal method was used to produce SrNb2O6 rhombic nanotubes with a diameter range of 200-300 nm, an average length of ∼3 μm, and a wall thickness of 50 nm by controlling the reaction conditions. This rhombic tubular morphology originated from the stepwise bending process of the elongated nanoplates formed in the early growth stage around the a-axis. The nanotubes exhibited a higher BET surface area than the bulk counterpart due to their unique morphology. Acknowledgment. This work was cosupported by a Korea Science and Engineering Foundation (KOSEF) grant funded by the Korea government (MOST) (R01-2007-000-11075-0) and the Nano R&D program through the National Research Foundation of Korea funded by MEST (2009-0082659) and research program 2009 of Kookmin University in Korea. Supporting Information Available: Additional experimental results (XRD, FESEM, TEM) for the effects of experimental conditions (pH, hydrothermal time, temperature, and filling factor) on the phase and morphology. This material is available free of charge via the Internet at http://pubs.acs.org.
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