LETTER pubs.acs.org/NanoLett
Octopods versus Concave Nanocrystals: Control of Morphology by Manipulating the Kinetics of Seeded Growth via Co-Reduction Christopher J. DeSantis, Angela A. Peverly, Dennis G. Peters, and Sara E. Skrabalak* Department of Chemistry, 800 East Kirkwood Avenue, Indiana University, Bloomington, Indiana 47405, United States
bS Supporting Information ABSTRACT: Au/Pd octopods and concave core@shell Au@Pd nanocrystals have been prepared by coupling for the first time a seed-mediated synthetic method with co-reduction. The integration of these two methods is central to the formation of these binary Au/ Pd nanocrystals wherein the kinetics of seeded growth are manipulated via the co-reduction technique to control the final morphology of the nanocrystals. Significantly, the synthesis of these structures under similar reaction conditions illustrates that they are structurally related kinetic products. Detailed characterization by STEM-EDX analysis highlights the unique structural features of these nanocrystals and indicates that Pd localizes on the higher-energy features of the nanocrystals. Optical and electrocatalytic characterization also demonstrates their promise as a new class of multifunctional nanostructures. KEYWORDS: Kinetic control, octopodal, concave nanocrystals, branched, nanostars, multifunctional
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dvances in the synthesis of single metal nanostructures now allow for the precise assembly of atoms into particles of defined size and shape.1,2 This ability has provided a myriad of structures and enables control over their properties for optical,35 catalytic,6,7 and biomedical applications.811 However, it is often difficult to achieve bimetallic heterostructures with defined features because of different metal-to-capping agent interactions and rates of precursor reduction, galvanic replacement, and the thermodynamic miscibility of the two metal phases.12 Here, we report the synthesis and characterization of multifunctional Au/Pd octopods and concave core@shell Au@Pd nanocrystals, achieved by coupling for the first time a seed-mediated synthetic method with co-reduction. Significantly, the integration of these two methods provides a means of manipulating the kinetics of seeded growth to control the morphology of the final nanocrystals in favor of two distinct kinetic products. Octopodal1317 and concave1822 nanocrystals of even one metal are rare, with reports typically emphasizing the conditions which favor the formation of one of these structures in comparison to those which yield thermodynamically favored polyhedral nanocrystals. Yet, on account of their high surface energies, both octopods and concave nanocrystals should be considered kinetic products.23,24 Generally speaking, kinetic products are thought to form when reaction conditions favor high growth rates, wherein the rate of atomic addition to a surface exceeds that of adatom surface diffusion. As a result, adatoms are prohibited from migrating on the surface to adopt their lowest energy configurations and in turn the polyhedral structures favored by thermodynamics.23,24 As we have found, binary octopods and concave nanostructures can be independently accessed under r 2011 American Chemical Society
similar reaction conditions simply by controlling the kinetics of seeded growth. Seed-mediated synthetic methods separate particle nucleation from growth by providing preformed crystals as building blocks to construct the greater structure upon.25 Pioneered with the synthesis of Au nanorods,26,27 such syntheses have recently enabled heteroepitaxial deposition on the nanoscale, with a variety of shape-controlled core@shell structures reported.2833 Related to this Letter, Huang and co-workers19 recently reported the Au nanocube-directed synthesis of Au@Pd nanocrystals with tetrahexahedral and concave octahedral Pd shells while Lee and co-workers33 achieved heteroepitaxial deposition of Pd atop trisoctahedral Au nanocrystals. In both of these examples, a Pd precursor was reduced in the presence of Au cores, with the final structures achieved being primarily attributed to the AuPd epitaxial relationship and the structure-directing effects of capping agents or additives (i.e., a thermodynamic means of controlling crystal habit). As we have found, by coupling the co-reduction of Au and Pd precursors with a seed-mediated synthesis, the morphology of the final product can be controllably tuned to yield two distinct yet structurally related kinetic products. To synthesize Au/Pd octopods and concave Au@Pd nanocrystals, Au seeds (diameter
