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CONCLUSIONSCollectively, these data provide an understanding of the role ofsilver ions and halides in seed-mediated syntheses of goldnanoparticles, and they show how two different sets of particleshapes (Figure 9A,B) can be accessed via kinetic control, surfacepassivation, or a combination of both, in either the absence orpresence of silver ions. The mechanistic insight and designconsiderations put forth in this study have been used to explaina number of previously reported nanoparticle syntheses, such asthose involving the formation of concave cubes, tetrahexahedra,trisoctahedra, rods, and prisms. These design considerations willalso serve as guiding principles for further work in this area,including studies directed at understanding how twinnedstructures5,9fit within the framework of kinetic- and surface-controlled nanoparticle growth. In addition, similar interactionsmay be integral in the synthesis of nanoparticles of other metals,where metal−halide interactions and underpotentially depositedmetals are involved in controlling particle shape. For example,work by Xia and co-workers has shown that reaction kinetics arekey in directing the growth of nanoparticles composed of othermetals, such as silver, platinum, palladium, and rhodium.9,61,83Ultimately, the design considerations presented as a culminationof this study represent a step toward the rational synthesis ofgold nanoparticles of desired shapes, an advance required for theuse of these nanoparticles in many applications.■ EXPERIMENTAL SECTIONChemicals and Materials. Gold(III) chloride trihydrate(HAuCl4•3H2O, 99.9+%), silver nitrate (AgNO3, 99.9999%), sodiumborohydrate (NaBH4, 99.99%), sodium chloride (NaCl, 99.999%),sodium bromide (NaBr, 99.0+%), sodium iodide (NaI, 99.999%),L-ascorbic acid (AA, 99+%), cetyltrimethylammonium chloride(CTA-Cl, 25 wt % in H2O), and cetyltrimethylammonium bromide(CTA-Br, 99%) were purchased from Aldrich and used without furtherpurification. Hydrochloric acid (HCl, 1 mol/L volumetric solution)was purchased from Fluka and used without further purification.Preparation of Seed Particles. Au seeds with a diameter of 7 nmwere prepared by quickly injecting 0.60 mL of ice-cold, freshlyprepared NaBH4 (10 mM) into a rapidly stirred solution containing0.25 mL of HAuCl4 (10 mM) and 10.00 mL of CTA-Cl (100 mM) or CTA-Br (100 mM). The seed solution was stirred for 1 min and thenleft undisturbed for 2 h. Larger seeds were prepared by growing the7 nm diameter seeds into 40 nm diameter Au seeds by consecutivelyadding 200.0 μL HAuCl4 (10 mM) and 40.0 μL AA (100 mM) to asolution containing 8.0 mL of H2O and 1.6 mL of 100 mM CTA-Cl.The 7 nm diameter seed particles were diluted in 100 mM CTA-Cl(or CTA-Br) to generate a solution which was1/10 the concentrationof the original seed solution. Growth of 40 nm diameter seed particleswas initiated by adding 100.0 μL of the diluted 7 nm diameter seeds.The reaction mixture was swirled immediately after the addition of theseeds and then left undisturbed on the benchtop until the reaction wascomplete.Typical Seed-Mediated Synthesis. A typical growth solution wasprepared by consecutively adding reagents into 10.00 mL of CTA-Cl(or CTA-Br) in the following order, as necessary (Chart S1): NaCl,HCl to adjust pH, 0.50 mL of HAuCl4 (10 mM), NaBr or NaI, AgNO3(10 mM), and then 0.10 mL of AA (100 mM). The 7 nm diameterseeds were serially diluted in 0.1 M CTA-Cl to generate a solutionwhich was1/1000 the concentration of the original seed solution.Particle growth was initiated by adding 100.0 μL of the diluted 7 nmdiameter seeds or 0.5 mL of the 40 nm diameter seeds to the growthsolution, as required (Chart S1). The reaction mixture was swirledimmediately after the addition of the seeds and then left undisturbedon the benchtop until the reaction was complete.Instrumentation. Scanning electron microscopy (SEM) imageswere obtained using a Hitachi S-4800-II cFEG SEM. Transmissionelectron microscopy (TEM) images were obtained using a HitachiHD-2300 STEM. Inductively coupled plasma-atomic emission spectro-metry (ICP-AES) analysis was performed using a Varian Vista ICP-AES. Samples were prepared for ICP-AES by removing aliquots of thereactions at various time points, quenching the reactions with anamount of BSPP in excess of the gold concentration in the reaction,and then centrifuging the particles twice to isolate the particles fromthe reaction solution. The particles were then dissolved in fresh aquaregia and subsequently diluted with NANOpure H2O just priorto ICP-AES analysis. X-ray photoelectron spectroscopy (XPS) wasconducted using an Omicron ESCA Probe XPS spectrometer. Sampleswere prepared for XPS by centrifuging particle solutions to concentratethem, resuspending the particles in NANOpure H2O, and repeatedlydropcasting the particles onto a silicon substrate. XPS spectra weregathered using an Al Kα (1486.5 eV) anode with a power of 200 W(20 kV) and a hemispherical energy analyzer operated at a pass energyof 70.0 eV for survey scans and 24 eV for high-resolution scans.