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ABSTRACT: The roles of silver ions and halides (chloride, bromide,and iodide) in the seed-mediated synthesis of gold nanostructureshave been investigated, and their influence on the growth of 10classes of nanoparticles that differ in shape has been determined.We systematically studied the effects that each chemical componenthas on the particle shape, on the rate of particle formation, and on thechemical composition of the particle surface. We demonstrate thathalides can be used to (1) adjust the reduction potential of the goldion species in solution and (2) passivate the gold nanoparticle surface,both of which control the reaction kinetics and thus enable theselective synthesis of a series of different particle shapes. We alsoshow that silver ions can be used as an underpotential depositionagent to access a different set of particle shapes by controlling growthof the resulting gold nanoparticles through surface passivation (moreso than kinetic effects). Importantly, we show that the density of silver coverage can be controlled by the amount and type ofhalide present in solution. This behavior arises from the decreasing stability of the underpotentially deposited silver layer in thepresence of larger halides due to the relative strengths of the Ag+/Ag0−halide and Au+/Au0−halide interactions, as well as thepassivation effects of the halides on the gold particle surface. We summarize this work by proposing a set of design considerationsfor controlling the growth and final shape of gold nanoparticles prepared by seed-mediated syntheses through the judicious use ofhalides and silver ions. 35822
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■ INTRODUCTIONAnisotropic noble metal nanoparticles have attracted a greatdeal of attention and research effort due to their shape- and size-dependent physical and chemical properties. These propertieshave led to the development of numerous applications in areassuch as spectroscopy,1−8catalysis,9,10energy,11and biology.12However, to take full advantage of the unique physical andchemical attributes of these nanostructures, it is necessary tohave synthetic methods for rationally controlling their size,shape, and composition.9,13Of these characteristics, shape hasproven to be one of the most challenging to deliberately control,yet it is arguably among the most useful parameters for tailoringthe properties of a nanoparticle, particularly for particlescomprised of gold. One of the most versatile methods forgenerating gold nanoparticles of a desired shape is the thermalseed-mediated synthesis,14−16which has led to the productionof a vast library of nanostructures, ranging from Platonic solidssuch as octahedra and cubes,15,17−19to plates and prisms,3,4,20,21to more exotic structures with high-index surface facets.22−25Itis acknowledged in the literature that additives, most commonlyhalides and silver ions, play a key role in directing the growth ofanisotropic nanostructures in this synthetic method.21,23,26−30For example, we have previously reported that low, but detect-able, concentrations of iodide impurities present in a bromide-containing surfactant significantly affect particle growth, withreactions yielding gold spheres, rods, or triangular prisms withincreasing concentrations of iodide ions due to the binding ofiodide to the gold nanoparticle surface.21In addition, silver ionsare a particularly interesting additive, because a number ofreports have observed the growth and stabilization of goldnanostructures bound by high-index facets due to theirpresence,22−24,30,31and these nanostructures are of potentialuse for catalytic applications due to the high number of low-coordinated surface atoms exposed on such facets.32,33We haverecently studied the role of silver ions in the seed-mediatedsynthesis for a series of reactions conducted with a chloride-containing surfactant.26It was determined that through theunderpotential deposition of a near-monolayer of silver ontothe surface of the gold nanoparticles, judicious control of thesilver ion concentration in the growth solution allowed for theselective preparation of a variety of gold nanostructures,including {110}-faceted rhombic dodecahedra and bipyramids,34{310}-faceted truncated ditetragonal prisms,26{720}-facetedconcave cubes,24and {111}-faceted octahedra with tailorablehollow features.35However, the role of chloride ions in thesesyntheses has, until now, not been addressed and a consistentexplanation for the role of the other commonly used halides,namely bromide and iodide, in the seed-mediated synthesis ineither the absence or presence of silver ions is still lacking. In theReceived: June 1, 2012Published: August 24, 2012 following article, we discuss the results of a detailed studydesigned to elucidate the independent and synergistic effects ofsilver ion and the halides (chloride, bromide, and iodide) on thegrowth of anisotropic nanostructures, with a focus on explaininghow these chemical components can act together to affectreaction kinetics and surface silver coverage to yield gold nano-structures of a particular morphology.