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(Figure 5).26Thisresult matches well with what would be expected for a sub-monolayer coverage of silver on each of the different facet types.Thus, the data provide quantitative evidence that silver ionscontrol gold nanoparticle growth in the seed-mediated synthesisin the presence of CTA-Cl through underpotential depositionof up to a monolayer of silver onto the facet with the highestnumber of exposed surface atoms for which there is sufficientsilver to cover.26Once silver deposits onto a particular facet, itslows further growth of that facet by inhibiting gold deposition,and therefore that facet is retained in the final nanoparticlemorphology. To further validate this claim, we showed that byintroducing more silver ion than that required to form concavecubes, we were able to selectively stabilize {111} facets togenerate octahedra with hollow features, which correlates wellwith {111} being the most densely packed facet for face-centeredcubic gold.35In the current study, we are also interested in theeffects that the silver ion additive has on the reaction rate, andthrough an ICP-AES study of the rate of gold nanoparticleformation, we show that increasing amounts of silver ion additiveslow the rate of nanoparticle growth (Figure 5). This is inagreement with what we would expect, since silver inhibits golddeposition onto the particles, higher concentrations of silver ionsshould inhibit gold deposition more effectively. However, wenote that the slowed rates of gold particle growth are very similarfor substantially different concentrations of silver ions in thegrowth solution. Note that 1 and 100 μM represent the entire[Ag+] range studied in our shape-controlled syntheses. Thissuggests that rather than reaction kinetics being the dominantfactor affecting particle growth and shape, as is the case in theabsence of silver ions, surface effects are more influential in thecase of the Ag0underpotential deposition-controlled particlesyntheses.Effects of Trace Halides in the Presence of Silver Ions.The effects of halides on particle growth are more complicatedwhen silver ions are introduced into the reaction because thehalides interact not only with the gold ions in solution and thegold nanoparticle surface but also with the silver ions in solutionand the AgUPD layer on the surface of the particles. Fortunately,the effects of halides on the underpotential deposition of silveronto bulk gold surfaces have been studied by others in greatdetail,68−72and while these studies are typically conducted onplanar gold surfaces in electrochemical cells, we show herehow the same effects explain the behavior of these chemicalcomponents in the context of the growth of gold nanoparticlesin the seed-mediated synthesis. It has been observed that thepresence of a chloride adlayer over an AgUPD layer results in theformation of the most stable AgUPD layer in comparison to eitherbromide or iodide and that adlayers of bromide or iodide leadto AgUPD layers with decreasing stability.68−72This behavior isexemplified in a series of experiments conducted by Michalitschand Laibinis,68where an AgUPD layer was first electrochemicallyprepared on a gold surface and then subsequently strippedeither in the presence of chloride, bromide, or iodide or in theabsence of halides (presence of sulfate ions). The stripping peakin the absence of a halide was found to be at 534 mV, while thepeak shifted to 615, 574, or 415 mV in the presence of chloride,bromide, or iodide, respectively.68These data indicate that thestability of an AgUPD layer decreases in the order Cl− >Br− >absence of halide > I−. These results would be expectedwhen considering either enthalpic or solubility arguments. In asimplified view, the formation of an AgUPD layer can bedescribed as the breaking (or dissociation) of an Au−halidebond accompanied by the formation (or association) of an Ag−halide bond. Whether considering enthalpies of formation orsolubility constants, the formation of an AgUPD layer is morefavorable in the presence of halides in the order Cl− >Br− >I−Figure 5. (A) Chart of the silver/gold ratios for {110}-faceted rhombicdodecahedra, {310}-faceted truncated ditetragonal prisms, and {720}-faceted concave cubes as determined by ICP-AES, a bulk character-ization technique, and XPS, a surface characterization technique. Thesilver/gold ratio on the surface of the particles increases in the orderrhombic dodecahedra < truncated ditetragonal prisms < concave cubesdue to higher-index facets having more open surfaces with moreexposed surface sites. Note that the bulk silver/gold ratios determinedby ICP-AES are normalized for both the size and shape of thenanoparticles and thus represent a value reflecting the silver coverageon the particle surface.