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DISCUSSIONDesign Considerations for Controlling ParticleGrowth. Considering the results from all of the previouslydiscussed experiments, we now can explain the roles of silverions and halides in seed-mediated syntheses and how they canbe used to control particle shape and surface facet structure(Figure 9). Essentially, in the absence of silver ions, the rate ofreaction is the dominant factor that controls product growth.Slower rates of reaction yield lower-energy surface facets andcan be achieved by using a lower amount of ascorbic acid or byintroducing a larger halide (bromide or iodide). In the presenceof silver ions as an underpotential deposition agent, surfaceeffects are more influential than kinetic effects in controllingparticle growth, with higher amounts of silver on the particlesurface yielding higher-index surfaces. More silver can bedeposited onto the particle surface by increasing the amount ofsilver ion in the growth solution or by introducing a traceamount of bromide or iodide. However, at high concentrationsof halides, the AgUPD layer is destabilized and the binding ofthe halide to the particle surface inhibits the deposition ofsilver, although the very dynamic particle surface due to thedestabilized AgUPD layer can still yield high-index facetedtetrahexahedra with a bromide-containing surfactant. However,these effects are not suitable for achieving well-faceted particlesin the presence of iodide. In contrast, the very stable AgUPDlayer in the presence of chloride can “lock-in” the concavefeatures of the concave cubes early in growth while the seedsare small and highly reactive. Although it would be ideal tomeasure the amount of halides on the surface of the nano-particles to further confirm our results, this is not possibleto do by XPS due to some of the silver halide peaks overlappingwith peaks for silver without a bound halide.Here we provide six design considerations that expand uponthe above discussion. These design considerations can befollowed to rationally control product shape in the seed-mediated synthesis of gold nanoparticles.(1) Increasing the concentration of ascorbic acid speeds the rateof gold ion reduction, and thus particle growth, leading tomore kinetically favored particles with higher-energy surfacefacets. This is based on the observation that {111}-facetedoctahedra, {100}-faceted cubes, and {221}-facetedtrisoctahedra form at 0.5, 2.0, and 10.0 mM ascorbicacid, respectively.(2) In the absence of other shape-directing additives, and underotherwise identical conditions, the addition of a larger halidewill slow particle growth, facilitating the synthesis of particleswith lower-energy surface facets. This is based on theobservation of a trend in particle shapes opposite to thatobserved with increased ascorbic acid and is supportedby ICP-AES kinetics data. The slowed growth is due to adecrease in the reduction potential and solubility of theAu+-halide complexes formed in solution in the order[AuCl2]− > [AuBr2]− > [AuI2]−, as well as an increasingbinding affinity of the larger halides for the gold surfacein the order Cl− <Br− <I−. Thus, more thermodynami-cally favorable products form with increasing concen-trations of bromide and iodide, with the effects of iodidebeing stronger than those of bromide.(3) In the absence of larger halides (bromide and iodide),increasing concentrations of silver ions stabilize particles witha greater number of exposed surface atoms per unit surfacearea, enabling the formation of high-index nanostructures.This design consideration is consistent with the observedformation of octahedra, rhombic dodecahedra, truncatedditetragonal prisms, and concave cubes at increasinglylarger concentrations of silver ion in solution and isconfirmed by XPS and ICP-AES surface characterizationas well as kinetics studies.26The underpotential deposi-tion of up to a monolayer of silver onto a particular surfacefacet on a growing gold particle inhibits further golddeposition on that surface, and the facet is maintained inthe final particle shape. A given concentration of silver ionswill stabilize the most densely packed facet for which thereis sufficient silver to deposit a near-monolayer onto.(4) In the presence of silver ion as a shape-directing additive, theaddition of a trace amount of a larger halide decreases thestability of the AgUPD layer, and particles with more openfacets can be synthesized without adding as much silver ion.We have observed that, in the presence of trace bromide oriodide, particles with more open facets, such as truncatedditetragonal prisms, are stabilized at a concentration ofsilver ion in solution that, in the presence of chloride only,would lead to the growth of a particle bound by lower-energy facets, such as rhombic dodecahedra. Thisobservation is further supported by XPS surface character-ization and ICP-AES kinetics experiments. Chloride actsto stabilize the AgUPD layer, while bromide and iodidedestabilize the AgUPD layer, with iodide having greaterdestabilizing effects than bromide.