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These effectsarise from larger, less electronegative halides both forminghalide−gold ion complexes with decreasing reduction poten-tials and having an increasing binding affinity for the goldnanoparticle surface. We summarize previous results involvingthe control of particle shape by silver underpotential deposi-tion26and present new data showing that silver ions also reducethe rate of particle growth by physically and chemicallyinhibiting the deposition of gold onto the particle surface. X-rayphotoelectron spectroscopy (XPS) characterization of thesurface of each particle type and inductively coupled plasmaatomic emission spectroscopy (ICP-AES) kinetic studies areused to show that, in the presence of a silver ion additive, theaddition of a larger halide to a surfactant containing a smallerhalide causes a decrease in the stability of the AgUPD layer (it ismore susceptible to surface rearrangement) relative to itsstability in the absence of the larger halide. Destabilization ofthe AgUPD layer is due to the decreasing binding affinity oflarger halides for the AgUPD layer and is dependent on the con-centration of the halide. Consequently, judicious control overthe type and amount of halide in solution allows for the relativestrengths of the Ag+/Ag0−halide and Au+/Au0−halide inter-actions to be tailored, enabling control over the silver coverageand shape of the gold nanoparticles produced. We discuss theimplications of these results for understanding the formation ofthe concave cubes and tetrahexahedra and provide a series ofdesign considerations for the shape-controlled growth of goldnanoparticles prepared by the seed-mediated syntheticapproach.Figure 1. In a seed-mediated synthesis of gold nanoparticles, the use ofa silver ion additive in the presence of a chloride-containing surfactant(CTA-Cl) results in the growth of {720}-faceted concave cubes (uppergrowth pathway), while the use of a bromide-containing surfactant(CTA-Br) under otherwise identical conditions results in the growthof {730}-faceted tetrahexahedra (lower growth pathway). ■ RESULTSIn a typical seed-mediated synthesis of gold nanoparticles, smallseeds (∼7 nm diameter) were first prepared by the rapidreduction of HAuCl4 by sodium borohydride in the presence ofCTA-Cl, a chloride-containing surfactant. In some instances,larger seeds were desired, and these were obtained by regrowingthe ∼7 nm diameter seeds in a growth solution containingHAuCl4,CTA-Cl,andascorbicacidtoyieldsphericalparticleswith an average diameter of ∼40 nm. An aqueous growth solutionwas then prepared which contained at least the followingreagents: HAuCl4, surfactant (either CTA-Cl or CTA-Br), andascorbic acid. In these reactions, the ascorbic acid reduces Au3+toAu+to yield a colorless solution, and the addition of seed particlescatalyzes the reduction of Au+to Au0to generate larger goldparticles.14−16Au+is stabilized against disproportionation bybinding to CTA-X to form a stable CTA-X-AuX2− complex.47,48Experiments were conducted to elucidate the roles of silver ionsand halides in controlling the growth and final shape of theresulting nanoparticles. These investigations were carried out bysystematically varying the constituents of the growth solutions.Hydrochloric acid was used to control the solution pH, NaBr andNaIwereusedtointroduceBr− and I− species, respectively, andAgNO3 wasusedasthesourceofAg+. The particle size can becontrolled by varying the amount of seeds added to the growthsolution, with larger amounts of seeds generating many smallparticles, while smaller amounts of seeds lead to fewer, but larger,particles. However, in the experiments that follow, the volume ofseed solution added was held constant across each set of reactionsto limit the number of variables. In addition to the experimentaldetails provided in each subsection of the Results and Discussion,a full list of experimental conditions can be found in theSupporting Information (Chart S1).Effects of Reducing Agent Concentration.
The mostdirect way of controlling the reaction kinetics, and thus particleshape, in these syntheses is by changing the concentration ofthe reducing agent, ascorbic acid. Increasing the amount ofreducing agent in the growth solution increases the rate ofgold ion reduction, which should create a preference for thegrowth of more kinetically favorable (or less thermodynamicallyfavorable) particle morphologies. To understand the role ofother additives, such as silver ions and halides, it is important tofirst fully explain and isolate the effect of ascorbic acid, becauseit is present in all of the reaction solutions. To establish the roleof the reducing agent, growth solutions were prepared whichcontained 10 mM CTA-Br, 0.125 mM HAuCl4, 500 μLof40 nm diameter seeds, and either 0.5, 2.0, or 10.0 mM ascorbicacid (Chart S1). Scanning electron microscopy (SEM) imagesof the products of these reactions reveal that the concentrationof reducing agent does indeed have a significant impact on theshape of the product (Figure 2). At 0.5 and 2.0 mM ascorbicacid, the reactions generate well-formed {111}-faceted octahedra(Figure 2A) and {100}-faceted cubes (Figure 2B), respectively.When the concentration of ascorbic acid is increased to 10.0 mM,the products resemble ill-formed trisoctahedra (Figure 2C).These particles are likely bound by high-index facets, as sug-gested by characterization studies of well-formed trisocta-hedra;49however, we suspect that these reaction conditions arenot ideal for generating such structures and that this particularparticle morphology is indicative of poorly controlled growthdue to a very rapid rate of gold ion reduction. Nonetheless, theseobservations indicate that, as the concentration of reducing agentis increased, particles bound by higher-energy surface facets areproduced. These results are consistent with many other studies ofthe use of ascorbic acid in the seed-mediated synthesis.25,38,50,51Effects of Halides in the Absence of Silver Ions. Wehave determined that, in the absence of silver ions, halides affectparticle growth through two separate but cooperative pathways.First, the halides present in solution will complex gold ionspecies, thereby affecting the reduction potential and solubility ofthe gold ions in the CTA-X-AuX2− complexes and thus changingtheir reduction rate.48,51Indeed, the reduction potentials forAu+-halide complexes decrease in the order [AuCl2]− >[AuBr2]− >[AuI2]− (1.154 > 0.960 > 0.578 V), meaning that the additionof bromide or iodide to a chloride-containing surfactant willdecrease the ability of Au+to be reduced by ascorbic acid.52The solubilities of the [AuX2]− complexes decrease in the sameorder,53and this decreasing solubility relative to the highsolubility of CTA-X-AuX2− will also slow the rate of goldreduction. Second, the halides can bind to the surface of the goldnanoparticles, inhibiting the subsequent growth of the particles.The binding strength of the halides to an Au particle surfaceincreases in the order Cl− <Br− <I−.54Thus, the introduction ofbromide or iodide will slow the rate of gold nanoparticleformation as compared to the rate in the presence of chloridealone both by lowering the reduction potential and solubility ofthe gold ion species in solution and by binding strongly to thesurface of the gold particles. Note that the relative importance ofthese separate pathways is difficult to distinguish, but both followsimilar trends with respect to affecting the rate of gold deposi-tion, and thus we are able to control particle shape by simplyvarying the concentrations of different halides in the growthsolution. Indeed, when the previous reactions are conducted withthe chloride-containing surfactant CTA-Cl rather than thebromide-containing CTA-Br, we can produce trisoctahedra evenat low concentrations of ascorbic acid (1.0 mM, Figure 3A). Thisindicates that the rate of gold ion reduction is more rapid in thepresence of CTA-Cl than in CTA-Br, since high-index facets areformed at a comparatively low concentration of reducing agent.To investigate the consequences of bromide anions on particleformation, a growth solution containing 50 mM CTA-Cl and5.0 mM NaBr was prepared (in addition to 0.5 mM HAuCl4,1.0 mM ascorbic acid, and 0.1 μL of 7 nm diameter seeds;Chart S1). We note that the concentration of bromide in thissolution is much greater than the concentration of gold ion inthe reaction. This reaction produced{100}-facetedcubes(Figure3B),which are the same structures observed when the amount ofreducing agent is decreased with a bromide-containing surfactant,consistent with the conclusion that the introduction of bromideinto CTA-Cl is reducing the rate of gold ion reduction.