NANO LETTERS
Icosahedral Face-Centered Cubic Fe Nanoparticles: Facile Synthesis and Characterization with Aberration-Corrected TEM
2009 Vol. 9, No. 4 1572-1576
Tao Ling,†,⊥ Lin Xie,†,⊥ Jing Zhu,*,† Huimin Yu,‡ Hengqiang Ye,§ Rong Yu,† Zhiying Cheng,† Li Liu,| Li Liu,| Guangwen Yang,| Zhida Cheng,† Yujia Wang,§ and Xiuliang Ma§ 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, Department of Chemical Engineering, Department of Computer Science and Technology, Tsinghua UniVersity, Beijing 100084, China, and Shenyang National Laboratory for Materials Science, Institute of Metal Research Chinese Academy of Science, Shenyang, 110016, China Received December 11, 2008; Revised Manuscript Received February 11, 2009
ABSTRACT Iron nanoparticles are highly desirable for their potential applications in magnetic and catalytic industry. However, their shape-controlled fabrication is still an important challenge. Here we successfully synthesized icosahedral face-centered cubic (fcc) Fe nanoparticles with size of 5-13 nm by a specifically designed thermodynamic governed synthetic route, which is facile but highly efficient and reproducible. With the aberration-corrected transmission electron microscopy (TEM), the unique icosahedral structure’s pseudo-2-fold, 3-fold, and pseudo-5-fold axes were directly observed for the first time and verified by computer simulation, which reveals that nanoparticles’ orientations have a large impact on HRTEM images at ultrahigh resolution. It is expected that as-synthesized Fe nanoparticles with sharp corners and edges would be beneficial for tailoring chemical and physical properties at the nanoscale.
Iron is the fourth most plentiful element on Earth1 and has been important as a material to mankind for more than 2000 years. In recent decades, nanosized iron particles have been extensively investigated and have found use in a wide range of applications including in magnetic recording media,2,3 biomedical applications,4 and catalysis.5 Recently, shapecontrolled synthesis of a variety of face-centered cubic (fcc) metal nanoparticles, such as Ag,6 Au,6,7 Pt,8,9 and Pd,10 has attracted much attention on account of the shape- and sizeenhanced optical, electronic, catalytic, and surface Raman scattering11 properties of these materials. As an ideal material for understanding the impact of surfaces on magnetism1 and as the most economical metal catalyst,1,12 the fabrication and characterization of Fe nanoparticles with well-defined shapes is an important challenge.13 * To whom correspondence should be addressed. E-mail: jzhu@ mail.tsinghua.edu.cn. † Beijing National Center for Electron Microscopy and Department of Materials Science and Engineering, Tsinghua University. ‡ Department of Chemical Engineering, Tsinghua University. § Institute of Metal Research Chinese Academy of Science. | Department of Computer Science and Technology, Tsinghua University. ⊥ These authors contributed equally to this work. 10.1021/nl8037294 CCC: $40.75 Published on Web 03/24/2009
2009 American Chemical Society
For bulk Fe, fcc structure is not the thermodynamically stable phase at ambient condition. Until now, various experimental methods for preparing Fe nanoparticles have been proposed, including thermal decomposition14 and chemical reduction.15 These solution-phase synthesis methods result only in the formation of spherical single crystalline products of body-centered cubic (bcc) phase.1 Nanostructures of fcc phase are only reported in the shape of decahedral and triangular faceted nanoparticles synthesized by gas-phase synthesis route16,17 and 5-fold-twinned nanorods using polythelene glycol (PEG) reduction coupled with annealing method.18 As predicted by thermodynamics, iron nanoparticles are expected to nucleate and grow into icosahedral morphology enclosed by {111} facets to minimize the total surface energy when the particle size falls down below ten nanometer scale.19,20 The possibility of icosahedron formation was suggested by mass spectrometer with 13 atoms.21 However, actual fabrication and direct observation of icosahedral Fe nanoparticles have not yet been reported. Herein, we report the specific synthesis of faceted icosahedral fccFe nanoparticles ranging from 5 to 13 nm, and the unique
Figure 1. Schematic diagram of the synthetic procedure of icosahedral fcc-Fe nanoparticles. PVP/Fe3+ fluid jet was sprayed out from the needle and uniformly dispersed onto an amorphous carbon or silicon monoxide film-coated Cu grid using standard electrospinning process. Afterward, the Cu grid was heated in vacuum at 400 °C for the fabrication of icosahedral fcc-Fe nanoparticles.
fcc-Fe icosahedral structure was identified with sphericalaberration corrected transmission electron microscopy (TEM) combined with imaging simulation. Nanoparticle growth is a rather complicated process governed by many thermodynamic and kinetics factors, which control the structure, morphology, and size distribution of nanoparticles.22,23 Here we specifically developed a novel synthetic route governed by a thermodynamic growth regime to synthesize the icosahedral fcc-Fe nanoparticles. The concrete synthetic procedure was schematically depicted in Figure 1. First, a well-mixed polyvinylpyrrolidone (PVP, Mw ) 13 000) and ferric nitrate (Fe(NO3)3) solution was loaded into a hypodermic syringe in which PVP was functioned as a mild reducer.24 Then, upon applying a high voltage of 20 kV, a PVP/Fe3+ fluid jet was sprayed out from the needle and uniformly attached onto an amorphous carbon or silicon monoxide film coated Cu grid. Meanwhile, the excessive nanofibers were conveniently fished out by a stainless steel grid over the Cu grid. Finally, the Cu grid was heated at 400 °C for 1 h in vacuum, resulting in fabrication of novel icosahedral fcc-Fe nanoparticles. In this novel synthetic route, three key factors must be highlighted. At first, the uniform dispersing of nanoparticle precursors on the Cu grid is especially important, hence we designed the new method of high voltage spray to control the dispersive distribution of solution. Second, a mild reducing atmosphere is essential for the Fe3+ reduction. When using strong reduction agents, such as NaBH4, the fast reduction of Fe precursor usually results in the formation of spherical Fe nanoparticles in strings (Supporting Information, Figure S3a,b). As the reduction becomes extremely weak, the iron atoms will slowly aggregate into structures favored by thermodynamics. At last, growth in thermodynamic equilibrium condition is quite requisite for stable synthesis of the fcc-Fe nanoparticles, which were achieved by high temperature heating in vacuum (Supporting Information, Figure S3c,d). The as-synthesized fresh Fe nanoparticles were characterized using a JEOL JEM-2011 TEM operated at 200 kV. Bright field TEM images showed the nanoparticles were Nano Lett., Vol. 9, No. 4, 2009
Figure 2. Monodispersed fcc-Fe nanoparticles. (a) TEM image and high-magnification TEM image (inset) of as-synthesized nanoparticles. (b) Size distribution of the fcc-Fe nanoparticles according to HRTEM characterizations. (c) EDS spectra obtained for the region shown in (a) with Cu signal from the grid. (d) SAED pattern from the area in (a).
randomly distributed on the carbon film (Figure 2a). Figure 2b is the size distribution of as-synthesized icosahedral fccFe nanoparticles, which shows that the particle size distributed in the range of 5-13 nm, and the size of majority particles is about 9 nm. Energy dispersive X-ray spectroscopy (EDS) analysis demonstrated that these nanoparticles were mainly composed of Fe (Figue 2c). Selected-area electron diffraction (SAED) pattern (Figure 2d) exhibited polycrystalline rings corresponding to the fcc phase. After calibrating the camera length of the TEM using a gold film, the lattice constant of these Fe nanoparticles was calculated as 0.357 ( 0.005 nm. This value is in good agreement with the one for the fcc phase of Fe (0.356 nm),25 implying that the assynthesized nanoparticles are neither Fe oxides nor Fe/C compounds. High-resolution TEM (HRTEM) has been widely used in nanoparticles research to study their structure and morphology properties. However, due to the limited resolution and delocalization effects caused by spherical aberration (Cs) of the imaging lens, conventional TEM is not optimally suited for obtaining detailed information, such as size, shape, orientation and atomic arrangement of Ultrafine nanoparticles. Recently, Cs-corrected TEM has been successfully developed and shown its potential use in atomic-scale characterization of materials.26-31 Here, we report our HRTEM characterization of fcc-Fe nanoparticles using a FEI Titan 80-300 Cs-corrected microscope operated at 300 kV. In experiment, the Cs value was carefully adjusted to a range of -2∼3 µm. Justfocus was determined by minimizing phase contrast and all the images were taken at overfocus. Under these specific experimental conditions (Supporting Information, Figure S4), the {111} and {220} 1573
Figure 3. Structure diagrams, experimental, and simulated HRTEM images of icosahedral fcc-Fe nanoparticles situated in different axis orientations. (a-c) The 3-fold, 5-fold, and 2-fold structural models, where the 5-fold and 2-fold models are rotated with respect to the y-axis for 1.2° and x-axis for 1°, respectively; (d-f) the corresponding HRTEM images taken at overfocus; (g-i) the simulated HRTEM images of structures in panels a-c (spherical aberration coefficient, 2 µm; defocus, +6 nm for panels a and b, +4 nm for panel c; semiangle of beam convergence, 0.1 mrad; defocus spread, 2.3 nm).
lattice plane distances of fcc-Fe which are 0.206 and 0.126 nm, respectively, can be clearly observed and atom sites appear bright on a dark contrast. Image simulations were carried out using software package MacTempasX. Careful HRTEM experiments and simulations indicated that the assynthesized fcc-Fe nanoparticles were faceted with icosahedral morphology. An icosahedral structure consists of 20 tetrahedra and has 3-fold, 5-fold, and 2-fold symmetrical axes, whose schematic diagrams are given in Figure 3a-c, respectively. Figure 3d shows a typical experimental HRTEM image of a fcc-Fe nanoparticle situated in the 3-fold orientation. The {111} fringes from the tetrahedrons in the upper and lower parts of the Fe nanoparticle, as marked in Figure 3a, intersect with each other and form angles of 60°. Also, the atom structures of {111} zone axis could be clearly seen in the 1574
central part. Simulated image (Figure 3g, Cs ) 2 µm, ∆f ) 6 nm) is in good agreement with the experimental image (Figure 3d). HRTEM images of exact 5-fold axial direction is hard to obtain in experiment since it is unstable for an icosahedron to lie on a carbon film against one of its corners. Figure 3e is an experiment image of the pseudo-5-fold axis. It is worth noting that the image shows very unique ring structures with 2-fold symmetry, but not the distinct 5-fold rotational symmetry as the simulation result of the accurate 5-fold orientation (Supporting Information, Figure S5b). For an atomic model rotated with respect to the y-axis for 1.2° (depicted in Figure 3b), the simulated HRTEM image (Figure 3h, calculated with Cs ) 2 µm, ∆f ) 6 nm) shows a very good agreement with the observed one in details of the ring structures (Figure 3e). Nano Lett., Vol. 9, No. 4, 2009
Similarly, an icosahedron of the exact 2-fold orientation could hardly remain steady on the substrate against one of its edges. A typical HRTEM image of the pseudo-2-fold axis is shown in Figure 3f. Atomic structures of 〈110〉 and 〈111〉 zone axes can be clearly resolved with good contrast and surface facets that are sharply terminated. Additionally, the white arrows (Figure 3f) indicate that surface atoms were peeled off the nanoparticle due to high-energy electron bombardment. However, the structure of central 〈112〉 zone axis could hardly be discerned and does not conform to the simulation result of the exact 2-fold orientation (Supporting Information, Figure S5d). Thereby, the atomic model for simulation is intentionally rotated with respect to the x-axis. For a rotation angle of 1°, as illustrated in Figure 3c, the simulated HRTEM image (Figure 3i, calculated with Cs ) 2 µm, ∆f ) 4 nm) agrees well with the experiment image in detail. To date, HRTEM has established itself a most powerful tool in nanoparticle characterization. Despite its complexity in image interpretation, HRTEM is most suited for structure analysis at subnanometer scale. Recently, quantitative works, such as atom displacement and local strain measurement in thin film and multiple twinned structures were achieved by Cs-corrected TEM technique.28,30 However, for clusters of several nanometers in diameter, misorientation should be included in the quantitative analysis since the particle’s shape functions in reciprocal space is rather large and will still be interesected by the Ewald sphere for some tilts away from the nearest zone axis. Our results give an example that, although the particle is tilted away from its exact orientation, the image still shows fine lattice structures and the structure might be erroneously analyzed without careful comparison with simulation. Hence, sample orientation, as well as structure models and imaging conditions, has to be considered thoroughly for reliable quantitative analysis of the HRTEM image, which would benefit high-precision measurements to characterize the structural, physical, and chemical properties of nanoparticles. The morphological stability and size distribution of nanomaterials have been successfully investigated by thermodynamic calculation.19,20,32 For fcc structural metal nanoparticles, theoretical and experimental investigations have suggested that small sizes favor icosahedral nanoparticles, decahedral ones are favored at intermediate sizes, and single crystalline morphologies at large sizes. Here, we estimate the critical size distribution of icosahedral fcc-Fe nanoparticles with reference to decahedral nanoparticles. The total Gibbs free energy of Fe nanoparticles, for icosahedron (Gi) and decahedron (Gd) can express as Gi ) Uc + Us + Ue + Ut
(1)
Gd ) Uc + Us + Ue + Ut
(2)
where Uc, Us, Ue, and Ut are the cohesive energy, the surface energy, the elastic strain energy, and the twin boundary energy, respectively. On the assumption that particles with different shapes contain the same number of atoms have the Nano Lett., Vol. 9, No. 4, 2009
Figure 4. Critical size calculation for icosahedral fcc-Fe nanoparticles. (a) Schematic diagrams of icosahedron and decasahedron with defined particle size d. (b) The energy difference ∆Gid between the icosahedron and the decahedron versus the particle size of an icosahedron (di). In the diameter range for ∆Gid < 0, icosahedral fcc-Fe nanoparticle is more energetically favorable. di* is the critical particle size for an icosahedron.
same cohesive energy Uc for each case. Therefore, the change in total Gibbs free energy of an individual fcc-Fe nanoparticle for a transformation from icosahedron to decahedron can be expressed as ∆Gid ) Gi - Gd
(3)
∆Gid versus the particle size of icosahedron di is plotted in Figure 4. By setting ∆Gid ) 0, the critical particle size of the icosahedron di* is easily obtained as di* ) 11 mm. It means that icosahedra are energetically favorable when the particle size goes down below 10 nm. From this point of view, if we want to obtain fcc-Fe icosahedral nanoparticles, we must specially design and develop a novel method that can synthesize ultrafine nanoparticles with ∼10 nm size under thermodynamic growth conditions. This is exactly why we presented the new methodology that emphasizes three key factors including high voltage-controlled well dispersion of precursor solution, mild reduction, and growth at high temperature in vacuum. Moreover, the size distribution of our synthesized particles is in the range of 5-13 nm, which is in good agreement with thermodynamic calculation. Hence, our methodology is successful to fabricate icosahedral fccFe nanoparticles with well-controlled shape and size. In summary, icosahedral fcc-Fe nanoparticles of sizes 5-13 nm were successfully prepared by a thermodynamic governed synthetic route, which is facile but highly efficient and reproducible. We believe that this strategy would provide a new route for synthesis of other diverse non-noble metal faceted nanoparticles. Additionally, with the help of aberration-corrected TEM, the unique icosahedral structure of these fcc-Fe nanoparicles was identified by their typical HRTEM images of pseudo-2-fold, 3-fold, and pseudo-5-fold axes. Careful comparison between experiment and simulation reveals that nanoparticles’ orientations have a large impact on the HRTEM image at ultrahigh resolution. These icosahedral fcc-Fe nanoparticles may present an ideal opportunity for attempting to understand the impact of surfaces on magnetism. Moreover, iron is by far the least expensive metal catalyst, and such well-shaped icosahedral particles with 1575
sharp corners and edges would be beneficial for tailoring catalytic properties at the nanoscale. Acknowledgment. This work was financially supported by National 973 Project of China, Chinese National Nature Science Foundation. We would like to thank Professor Andy Godfrey for helpful paper revising. Supporting Information Available: Methods, thermodynamic calculation for critical size of icosahedral fcc-Fe nanoparticle, Figures S1-S5, and Table S1. This material is available free of charge via the Internet at http:// pubs.acs.org. References (1) Huber, D. L. Small 2005, 1, 482–501. (2) Jorgensen, F. The Complete Handbook of Magnetic Recording; McGraw-Hill, New York, 1995. (3) Lin, X.; Chen, L.; Zhu, J. Metall. Trans. A 1991, 22A, 2709–2711. (4) Molday, R. S.; Mackenzie, D. J. Immunol. Methods. 1982, 52, 353– 367. (5) Li, F.; Vipulanandan, C.; Mohanty, K. K. Colloids Surf., A 2003, 223, 103–112. (6) Sun, Y. G.; Xia, Y. N. Science 2002, 298, 2176–2179. (7) Kim, F.; Connor, S.; Song, H.; Kuykendall, T.; Yang, P. D. Angew. Chem., Int. Ed. 2004, 43, 3673–3677. (8) Wang, Z. L. J. Phys. Chem. B 2000, 104, 1153–1175. (9) Tian, N.; Zhou, Z. Y.; Sun, S. G.; Ding, Y.; Wang, Z. L. Science 2007, 316, 732–735.
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NL8037294
Nano Lett., Vol. 9, No. 4, 2009
