NANO LETTERS
Radial Anisotropic Growth of Rhodium Nanoparticles
2005 Vol. 5, No. 3 435-438
James D. Hoefelmeyer, Krisztian Niesz, Gabor A. Somorjai,* and T. Don Tilley* Department of Chemistry, UniVersity of California, Berkeley, Berkeley, California 94720-1460, and the Chemical and Materials Science DiVisions, Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, California 94720 Received November 17, 2004
ABSTRACT In this contribution, we report the synthesis of rhodium multipods that result from a homogeneous seeded growth mechanism. Small Rh nanocrystal seeds were synthesized by the reduction of RhCl3 in ethylene glycol in the presence of PVP. These seed particles could be subsequently used, without isolation, to form larger rhodium nanoparticles. A reaction temperature of 190 °C led to isotropic cubic Rh particles. Lowering the reaction temperature resulted in more anisotropic growth, which gave Rh cubes with horns at 140 °C, and Rh multipods at 90 °C. The anisotropic growth occurred in the (111) direction, as determined by high-resolution TEM (HRTEM). Anisotropic growth proceeds via a seeded growth mechanism, and not by oriented attachment.
A current focus of research in nanochemistry is the development of fine control over the size and shape of particles. This has led to the empirical discovery of various methods for the control of nanoparticle dimensions. A widely adopted strategy for achieving shape control is based on manipulation of the kinetics of particle growth with a surfactant that preferentially binds to crystal faces of the growing particle. In this way, anisotropy is introduced into the particle geometry, such that the surface of the particle is predominantly composed of the growth-inhibited face. The generation of higher-order geometries, as in branched or dendritic structures, has remained a more difficult challenge. Rod, or branched, structures can proceed through two mechanisms: oriented attachment or seeded growth.1 Conceptually, the mechanism for the growth of branches would seem to involve controlled, epitaxial growth from the surface of a particle. Inorganic materials with branching structures on the nanometer scale include ZnO,2-5 SnO,6 CdS7/Se8/Te,9 TiO2,10 PbS,11 Au,12 as well as others. For branched cadmium chalcogenides, the interface of branching structures consists of a lattice match between (111) zinc blende and (0001) wurtzite, which results in highly uniform tetrapods. Wires of wurtzite ZnO with branching ribbons are formed as ZnO ribbons grow epitaxially on the (011h0) surfaces of the central wire, resulting in a single crystalline ‘liana’ structure.4 ZnO tetrapods with a wurtzite structure and arms that grow from the center in the (0001) direction have been reported.2,5 Metallic gold nanoparticles with branches result when seeded * Corresponding authors. E-mail:
[email protected]. 10.1021/nl048100g CCC: $30.25 Published on Web 02/01/2005
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growth is employed under mild temperatures.12 Nanocrystalline TiO2 nanorods and multipods were reported to form via oriented attachment of truncated diamond-shaped seeds.10 In this contribution, we report the synthesis of rhodium multipods that result from a homogeneous seeded growth mechanism, in which growth occurs radially from the Rh seed. The current evidence favors a seeded growth mechanism for the formation of the arms of the Rh multipods. A convenient route to noble metal colloids is reduction of the metal halide in alcohols in the presence of poly(vinylpyrrolidone) (PVP), as established for Os, Rh,13-15 Ir, Pd, Pt, and Au.16 Very recently, platonic Au nanocrystals17 and exquisite shape control of Pt nanostructures18-20 were obtained by reduction of anionic metal chloride salts with ethylene glycol in the presence of PVP. In addition, Teranishi et al. have demonstrated size tunability within a small size range for palladium21 and platinum22 nanoparticles using PVP as a stabilizer. A potentially useful method for controlling particle growth is based on the “seeded growth” method. Seeded growth with Au101 to produce monodisperse nanoparticles was clearly shown in the synthesis of Bi, Sn, and In particles.23 Homogeneous seeded growth of Pd21 and Pt22 nanoparticles has been used to produce larger particles from small seeds. This method takes advantage of an autocatalytic particle growth mechanism, which bypasses the nucleation step and leads to monodisperse particles.24 The Rh nanoparticles described here were obtained using homogeneous seeded growth. The addition of 10 mg (0.05 mmol) of RhCl3 in 1 mL of H2O (over a one minute period) to a stirred solution of 53 mg (0.48 mmol) of PVP in 10 mL
Figure 1. Rhodium-PVP seed particles from reduction of RhCl3 in ethylene glycol at 90 °C (left) and 190 °C (right), with average particle diameter of 3.6 and 5.5 nm, respectively. Bar ) 50 nm.
Figure 2. Rh nanoparticles grown on Rh seed particles at 90 °C (left), 140 °C (middle), and 190 °C (right). Bar ) 50 nm.
of ethylene glycol at 90-190 °C resulted in reduction of the metal salt to Rh(0)n. The reaction was stirred for 1 h at the designated temperature, and the product was isolated by precipitation with an 2-propanol/hexane mixture. The precipitate can be redispersed in water and purified by additional precipitation/redispersion steps. Transmission electron microscopy (TEM) was used to characterize the products of the reaction, which resulted in nanoparticles (xj ) 3.6 nm, σ ) 0.7 nm, 90 °C; xj ) 5.5 nm, σ ) 0.7 nm, 190 °C) as shown in Figure 1. The seed particles could be used subsequently, without isolation, to form larger rhodium nanoparticles. A second addition of RhCl3 (100 mg, 0.48 mmol) in 1 mL H2O (over 10 minutes) to the Rh seed sol led to the formation of larger particles. The nature of the product Rh particles was found to be highly dependent on temperature (Figure 2). A reaction temperature of 190 °C led to isotropic cubic Rh particles (xj ) 12.7 nm, σ ) 1.5 nm). Lowering the reaction temperature resulted in more anisotropic growth, which gave Rh cubes with horns at 140 °C and Rh multipods at 90 °C. Below 90 °C, reduction did not occur. The anisotropic growth occurred in the (111) direction, as determined by highresolution TEM (HRTEM) (Figures 3, 4). This finding is consistent with the observation that Pt nanorods are elongated in the (111) direction.19 The Rh cubes are single crystals and are terminated at the surface by six (100) faces. A smaller fraction of the sample consists of truncated cubes and 436
twinned nanocrystals. The Rh cubes with horns, as well as the Rh multipods, are also single crystalline. The Rh multipods have a central region that is ∼3 nm in diameter with two to seven arms having a thickness of ∼3 nm. The lengths of the arms range from 3 to 9 nm. Interestingly, when a 5.5 nm Rh seed was used to grow a particle at 90 °C, a Rh multipod was formed, having arms approximately 5 nm in diameter. The nature of the seed clearly has an influence on the anisotropic particle geometry. The powder X-ray diffraction (XRD) pattern of Rh metal exhibits three reflections in the range 40-100° 2θ, which correspond to the 111, 200, and 220 reflections. Since the reflection intensity is proportional to the X-ray coherence length of the crystal, the relative intensities of these reflections vary with nanoparticle shape. The order of intensities for a Rh crystal having the shape of a cube should be 200 < 220 < 111, since the distance from the crystal centroid to the crystal face center (100 direction) is shorter than to the edge midpoint (110 direction), which are both shorter than to the corner (111 direction) for a cube. The Rh multipod reflection intensity should be diminished for the 200 and 220 reflections (vs 111), since these directions are not parallel to the length of the arms. In Figure 5, the XRD patterns of Rh multipods, horned cubes, and cubes are compared. The 111/200 and 111/220 intensity ratios are highest for the Rh multipods, and decrease for the Rh horned cubes and Rh Nano Lett., Vol. 5, No. 3, 2005
Figure 3. HRTEM of Rh nanocube viewed through the 100 axis (left), FFT (top right), and lattice simulation (bottom right). Bar ) 4 nm.
Figure 4. HRTEM of Rh multipod viewed through the 110 axis (left), FFT (top right), and lattice simulation (bottom right). Bar ) 4 nm.
cubes. The increased intensity of the 111 reflection confirms the anisotropic nature of the Rh nanocrystals grown at lower temperature. The anisotropic geometry of the particles can arise from two different mechanisms, seeded growth or oriented attachment.1 In the seeded growth mechanism, anisotropic growth is favored at lower temperatures. The oriented attachment process occurs by attachment of high surface energy facets of seeds, while more stable ones are conserved. The established ordering of surface energies, Rh(111) < Rh(100) < Rh(110),25 suggests that oriented attachment would favor the conservation of 111 surfaces at the expense Nano Lett., Vol. 5, No. 3, 2005
of the others. Assuming cuboctahedron seeds26 and oriented attachment, there is no way to propagate in the 111 direction without sacrificing 111 surfaces. Thus, this mechanism seems to be disfavored. The observation that growth at 90 °C on a larger seed resulted in multipods with thicker arms is consistent with the seeded growth mechanism. Finally, the Rh seeds are isolable after reaction for 1 h, which means oriented attachment does not occur at elevated temperature over time. We therefore conclude that the multipods result from seeded growth, which is anisotropic in the 111 direction. In conclusion, small Rh nanocrystal seeds were synthesized by the reduction of RhCl3 in ethylene glycol in the presence 437
References
Figure 5. XRD patterns and intensity ratios of (a) Rh multipods (b) Rh horns (c) Rh cubes.
of PVP. The seeded growth of Rh nanocrystals resulted in monodisperse, well-defined samples. The nanocrystal shape was temperature dependent, due to anisotropic growth at lower temperatures, which resulted in Rh cubes at 190 °C and multipods at 90 °C. Future work in this area will focus on the use of well-defined seeds for further tuning the growth conditions. Acknowledgment. This work was supported by the Director, Office of Energy Research, Office of Basic Energy Sciences, Chemical Sciences Division, of the U.S. Department of Energy under Contract No. DE-AC03-76SF00098. We thank the National Center for Electron Microscopy, and Chris Nelson, for the use of HRTEM. We thank the UC Berkeley Electron Microscope Lab for the use of TEM. The authors thank Peidong Yang and A. Paul Alivisatos for helpful discussions and use of the powder X-ray diffractometer (A.P.A.). Lattice simulations were adopted from a visualization program: http://home3.netcarrier.com/∼chan/ SOLIDSTATE/CRYSTAL/fcc.html produced by Winston Chan.
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NL048100G
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