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On the basis of this study,we also provide a series of generally applicable design param-eters for the directed synthesis of gold nanoparticles in the seed-mediated method through the controlled use of halides andsilver ions.A number of perse and, at times, conflicting theories exist toexplain the role of halides in the seed-mediated synthesis, inboth the absence and presence of silver ions. The mostwidespread explanation for the role of bromide anions is thatthey bind to {100}-faceted surfaces, stabilizing those surfacesand slowing gold deposition at that site, thus leading to theformation of {100}-faceted nanoparticles, such as cubes.15,25,36,37However, other proposed explanations include the selectivebinding of bromide to {110} or {111} surfaces16,27,29or forma-tion of cetyltrimethylammonium bromide (CTA-Br) micelles,bilayers, or CTA-Au-Br complexes that physically templatenanorod formation or slow the rate of gold deposition.16,29,37,38In the presence of silver ions, bromide has been proposed tolead to the formation of silver bromide (or CTA-Ag-Br), whichdeposits selectively onto certain surfaces, through eitheradsorption14,27,29,37,39or underpotential deposition.23,40,41Foriodide, which has only been investigated more recently, the caseis even more complex. Iodide is commonly thought to bindto and stabilize {111} facets,21,42−45but some researchers haveproposed that it acts as a reducing agent to increase the rate of[AuCl4]− reduction.28,43However, in other reports iodide is claimedto slow the growth of gold nanoparticles42results which arecontradictory. In the presence of silver ions, the effects ofiodide on particle growth are proposed to dominate the effectsof silver ions.46Taken together, iodide has been proposed to(1) bind to the surface with silver ion and form a layer of silveriodide, (2) act as a catalyst for the reduction of gold ions onto{111} surfaces,44,46and (3) seed gold nanoparticle growth byacting as a nucleation site.43Consequently, there is little con-vergence and self-consistency among the proposed theories forthe role of silver ion and the halides in nanocluster growth.Assuming that bromide and iodide act via similar mechanismsin each variation of the seed-mediated synthesis (the reactionconditions are very similar), it would be useful to determine therole of each additive.
Moreover, it would be worthwhile to developa series of design considerations by which the principles of actionof these two anions in the presence and absence of silver ions canbe better understood and applied to the syntheses of gold nano-particles of a desired shape.One of the most illustrative examples of the use of halides incontrolling gold nanoparticle shape is the case of the silver-assisted syntheses of concave cubes24and tetrahexahedra,22bothof which are bound by similar high-index facets. The syntheticconditions for producing these two particles are nearly identical,except that the {720}-faceted concave cubes24are synthesizedin cetyltrimethylammonium chloride (CTA-Cl), while the samesynthesis in the analogous bromide-containing surfactant,cetyltrimethylammonium bromide (CTA-BR), results in convex{730}-faceted tetrahexahedra (Figure 1).22These are closelyrelated shapes that are best described as a cube having eithersquare-pyramidal depressions (concave cubes) or square pyramids(tetrahexahedra) on each of the six faces. The reason for thisdifference in morphology has, up to this point, not beenelucidated. To understand why the growth of concave cubesand tetrahexahedra is favored in chloride- and bromide-containingsurfactants, respectively, we have studied the role of halides in theabsence of silver ions and the role of silver ions in the presence oftwo different ranges of halide concentrations. However, ourfindings are of interest to all particles, regardless of shape, preparedby this synthetic method.In the following article, we first clarify the role of each of thehalides in the absence of silver ions and show that the introduc-tion of bromide or iodide to a growth solution containing achloride-containing surfactant slows the rate of gold ion reduc-tion and results in particles bound by lower-energy surfacefacets. We also show that iodide decreases the rate of gold ionreduction more significantly than bromide does.