CRYSTAL GROWTH & DESIGN 2008 VOL. 8, NO. 12 4340–4342
Communications Five-fold Twinned Nanorods of FCC Fe: Synthesis and Characterization Tao Ling,† Huimin Yu,‡ Xiaohua Liu,† Zhongyao Shen,‡ and Jing Zhu*,† Beijing National Center for Electron Microscopy, The State Key Laboratory of New Ceramics and Fine Processing, Laboratory of AdVanced Materials, Department of Materials Science and Engineering, and Department of Chemical Engineering, Tsinghua UniVersity, Beijing 100084, China ReceiVed January 29, 2008; ReVised Manuscript ReceiVed August 27, 2008
ABSTRACT: Face-centered cubic (FCC) Fe nanorods with a 5-fold twinning structure were successfully fabricated with a polyethylene glycol (PEG) reduction and annealing method. The 5-fold twinning structure of the synthesized nanorods was thoroughly investigated using scanning electron microscopy (SEM), selected area electron diffraction (SAED), and high resolution transmission electron microscopy (HRTEM). The characterization results suggest that the FCC Fe nanorod has a 5-fold twinning crystal structure with 〈110〉 as its growth direction, bounded by five {100} planes at side surfaces and capped by ten {111} planes at end faces. Five-fold twinning of nanoscaled particles1-3 is widespread in metals with a face-centered cubic (FCC) crystal lattice. These multitwinned nanoparticles can grow to a rodlike shape under strong kinetic control of the growth.4 In the past few decades, much effort has been directed to synthesize metal nanorods with 5-fold twinned structures, such as Ag,2,5-7 Cu,8 and Au.3 Previous research suggests that these 5-fold twinned metal nanorods have a truncated decahedron structure with a 〈110〉 growth direction, bounded by five {100} planes and capped by ten {111} planes. It is well-known that when the size of materials goes down into the nanoscale, due to the size effect, their structures may change and differ from their bulk phase. Previously, our group has reported the existence of 4H-Ag9,10 and HCP-Ni11 in nanowires. For bulk Fe, the FCC structure is not the thermodynamically stable phase at ambient conditions. Traditional synthetic routes for the production of Fe nanoparticles, that is, thermal decomposition12 and chemical reduction,13 yields body-centered cubic (BCC) structural Fe. Until now, there are only reports of FCC Fe nanoparticles encapsulated in and outside of carbon nanotubes with a single crystalline and nonfaceted structure.14,15 Moreover, metal nanoparticles with a faceted structure16-19 have attracted much attention due to their size and shape enhanced optical, electronic, catalytic, and surface Raman scattering properties. As an ideal material for studying the impact of surfaces on magnetism20 and as the most economical metal catalyst,20 the fabrication and characterization of Fe nanoparticles with faceted morphologies is an important challenge. To our knowledge, the fabrication of FCC Fe nanorods with a 5-fold twinning structure has not been reported yet. Herein, we report the * To whom correspondence should be addressed. E-mail:
[email protected]. † Beijing National Center for Electron Microscopy and Department of Materials Science and Engineering. ‡ Department of Chemical Engineering.
synthesis and characterization of the faceted FCC Fe nanorods with a unique 5-fold twinning structure. For the synthesis of FCC Fe nanorods, polyethylene glycol (PEG) was used as the moderate reduction agent to reduce Fe ions.21 The synthetic procedure was described as follows: 2 g of PEG (molecular weight ) 8000) was dissolved in 20 mL of deionized water, followed by magnetic stirring for 1 h to ensure the dissolution of PEG. Two grams of ferrous sulfate (FeSO4) was then added into the solution with stirring for 10 h. Afterward, a drop of FeSO4/ PEG solution was transferred onto an ultrathin carbon coated copper transmission electron microscopy (TEM) grid. The grid was put into a desiccator till dry, and then annealed at 400 °C for 30 min in a vacuum of 10-5 Pa. A typical TEM image shows randomly distributed nanorods and nanoparticles (Figure 1a). The energy dispersive X-ray spectroscopy (EDS) data demonstrated that these nanoparticles and nanorods were mainly composed of Fe, with Cr’s signal from the microscope and Cu’s signal from the TEM grid (Figure 1b). The selected-area electron diffraction (SAED) pattern (Figure 1a, inset) shows polycrystalline rings corresponding with the planes (111), (200), (220), and (311) of FCC Fe, calculated using a lattice constant of 0.362 nm. It confirmed that these nanorods and nanoparticles possess a FCC structure. Figure 1c is a TEM image of larger magnification, displaying a pentagonal faceted FCC Fe nanoparticle, from which the nanorod may evolve. Figure 1d clearly shows the pentagonal profiles of the nanorod. On the basis of the SAED evidence of the FCC structure (Figure 1a, inset) and the morphology of nanorods (Figure 1d), in combination with previous structural investigation on the metal nanorods,2,3,5-8 we presented the structural model of the 5-fold twinned nanorod of FCC Fe nanorods in Figure 2a-c. It consists of five identical subunits, named T1-T5, sharing one common edge
10.1021/cg800108n CCC: $40.75 2008 American Chemical Society Published on Web 10/29/2008
Communications
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Figure 1. (a) A TEM image of FCC Fe nanoparticles and nanorods, and the SAED pattern (inset) corresponding to FCC Fe. (b) EDS information obtained in (a). (c) A TEM image shows a facet FCC Fe nanoparticle, while (d) shows the clear pentagonal profiles of a FCC Fe nanorod.
Figure 3. Characterization of a nanorod with the “base” orientation. (a) Schematic diagram. (b) A low magnification TEM image of the nanorod. (c) The SAED pattern is indexed as superposition of 〈112〉 (marked by rectangle with real lines) and 〈001〉 (marked by rectangle with dashed lines) zone axes. (d) A HRTEM image of the rectangled part in (b), and the FFT spectrum obtained from the corresponding HRTEM image. M1 and M2 are the double diffraction spots. Figure 2. (a) Schematic illustration of the 5-fold twinned nanorod of FCC Fe bounded by five {100} planes and capped by ten {111} planes at both the tips of the Fe nanorods, whose growth direction is along 〈110〉. (b) and (c) Cross sections of the nanorod structure showing an arrangement of twins T1 to T5, and “base” (b) and “side” (c) orientations of domains with respect to the electron beam.
in the 〈110〉 direction as the 5-fold axis. And the FCC Fe nanorod was bounded by five {100} planes and capped by ten {111} planes. Panels b and c of Figure 2 are the cross sections of the nanorod structure displaying an arrangement of twins T1-T5 with respect to the electron beam. We further used SAED and HRTEM to test the 5-fold twinning structure of the FCC Fe nanorods. When the electron beam runs perpendicular to the (100) plane of one subunit, it is called the “base” orientation (Figures 2b and 3a). When the nanorod situated in this orientation, the base subunit T1 is in the 〈001〉 zone axis, while T3 and T4 are in the 〈112〉 zone axis, both of which are typical low index zone axes of the FCC structure. Figure 3b is a TEM image of a nanorod situated in this specific “base” orientation and Figure 3c is the corresponding SAED pattern, which consists of two specific crystallographic zones of FCC phase, that is, 〈112〉 and 〈001〉. The other diffraction spots are attributed to a double diffraction effect.5,8 Figure 3d is the HRTEM image of the rectangular part in Figure 3b. Here, {111} fringes of 〈112〉 zone axis, which was reflected by T3 and T4, can be visible across the whole nanorod. And in the central part of the nanorod, larger Moire fringes appear. As shown in the fast Fourier transform (FFT) spectrum (Figure 3d, inset), M1 corresponds to a
double diffraction arising from reflection of the{222} and {220} planes, while M2 results from reflection of the {111} and {220} planes. Similarly, when the electron beam is parallel to the (100) plane of one subunit, it is called the “side” orientation (Figures 2c and 4a). Here, T5 is situated in the 〈110〉 zone, while T2 and T3 are in the 〈111〉 zone axis orientation. Figure 4b is a TEM image of a nanorod situated in the “side” orientation, which was confirmed by SAED pattern (Figure 4c). It can be indexed as superposed patterns of the 〈110〉 and the 〈111〉 zone axes. Figure 4d is the HRTEM image of the rectangle part in Figure 4b. Due to the reflection of T5, one-half of the nanorod exhibits clearly two directional {111} planes of the 〈110〉 zone axis. For the other half of the nanorod, the {220} planes of 〈111〉 zone axis (T2 and T3) are not visible (Figure 4d) due to the resolution limit of TEM. We also did the sample tilting experiment to confirm the 5-fold twinning structure of FCC Fe nanorods. It is found that when tilting the nanorod around its long axis about 36°, the above-discussed SAED patterns of “base” or “side” orientation can be repeated. And, the SAED patterns of the two specific orientation can be translated by rotating the nanorod around its 〈110〉 axis about 18°. Collectively, the results of the sample tilting experiment agree well with our structural model of FCC Fe nanorod (Figure 2). In summary, we have successfully synthesized FCC Fe nanorods with a unique 5-fold twinning structure by reduction of FeSO4 in the presence of PEG, followed by annealing in a vacuum. The current synthetic procedure is very simple but highly reproducible and also provides a new route for non-noble metal faceted
4342 Crystal Growth & Design, Vol. 8, No. 12, 2008
Communications direction by five identical subcrystals, bounded by five {100} planes at side surfaces and capped by ten {111} planes at end faces. The magnetic and catalytic properties of these FCC Fe nanoparticles and nanorods will be carefully investigated to find attractive applications in nanotechnology.
Acknowledgment. This work was financially supported by National 973 Project of China, Chinese National Nature Science Foundation. We thank Mr. Morigen He for SEM characterization.
References
Figure 4. Characterization of a nanorod with the “side” orientation. (a) Schematic diagram. (b) A low magnification TEM image of the nanorod. (c) The SAED pattern is indexed as superposion of 〈110〉 (marked by rectangle with real lines) and 〈111〉 (marked by rectangle with dashed lines) zone axes. (d) A HRTEM image of the rectangled part in (b), and the FFT spectrum (inset) obtained from the corresponding HRTEM image.
nanoparticle synthesis. SAED and HRTEM characterizations revealed that the FCC Fe nanorod is arranged along the 〈110〉
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