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When more iodide ispresent in the growth solution, the rhombic dodecahedrabecome larger in size and are the dominant product overbipyramids until truncated ditetragonal prisms form at 0.10 μMiodide (Figure S5, Supporting Information). XPS data confirmthat, as more iodide is added to the growth solution, more silveris deposited onto the surface of each particle (Figure S5).Furthermore, ICP-AES shows that, with increasing amounts ofiodide, the initial rate of particle formation increases (Figure S5).Taken together, these results concerning the behavior of iodide inthepresenceofsilverionsareanalogoustothoseobservedforthebehavior of bromide in the presence of silver ions, yet the iodideconcentration necessary to observe these differences in the shapeof the nanoparticles is 2 orders of magnitude lower than whatis necessary for bromide, consistent with observations in theabsence of silver ions.The overall trend observed in these experiments is that, whentrace amounts of bromide or iodide are added to growthsolutions containing a silver ionadditiveinCTA-Cl,moresilveris deposited onto the surface of the gold nanoparticles than isdeposited in the absence of the bromide or iodide ions.Concurrently, when more silver is deposited onto the particlesurface, a more open and higher-index surface is stabilized,consistent with an underpotential deposition-controlled growthmechanism.26Moreover, the addition of either bromide or iodideto these reactions leads to an increased rate of particle growth.Initially, these observations may seem counterintuitive, sincebromide and iodide bind more strongly to the gold particlesurface than does chloride and thus could inhibit the depositionof silver onto the particle surface. However, these results areconsistent with the influence of halides on the stability of AgUPDlayers on a gold surface. The addition of either bromide or iodideto these reactions induces a destabilization of the AgUPD layer,with the destabilizing effect being more significant for iodide thanfor bromide. We hypothesize that when bromide or iodidedestabilizes the AgUPD layer, more opportunities are created forsilver to rearrange on the particle surface, either through a cycle ofoxidation and redeposition or through local surface mobility.68−72This mobility facilitates the relocation of silver to moreenergetically favorable surface sites, such as corners or atomicsteps, thus allowing facets with more exposed surface atoms to bestabilized at lower concentrations of silver ions than would berequired in the presence of chloride only.
The higher mobilityimparted to the AgUPD layer by bromide or iodide as comparedto chloride also results in a faster rate of gold ion reduction ontothe particles because the more dynamic surface is more easilyaccessible for gold deposition. This behavior explains both theXPS and ICP-AES data, which show that increasing concen-trations of either bromide or iodide in the growth solution lead toboth greater amounts of silver on the surface of each particle andfaster rates of gold ion reduction. This relatively high stability ofan AgUPD layer in the presence of chloride also explains why theuse of silver underpotential deposition is a very effective meansfor controlling particle shape in reactions conducted in CTA-Cl,since the overall strength of the Au−AgUPD−Cl interaction makesthe reactions very sensitive to the concentration of silver ions inthe growth solution, thereby enabling the synthesis of a widevariety of particle shapes. Therefore, if one wants to work underkinetically controlled growth conditions, it is more optimal towork in a chloride-containing surfactant than one comprised ofbromide or iodide.Effects of High Concentrations of Halides in thePresence of Silver Ions. Thus far, the effects of adding smallamounts of bromide or iodide to growth solutions containing achloride-containing surfactant and silver ions have been explored.In this section, we demonstrate that high concentrations of eitherbromide or iodide in the presence of silver ions will actuallyinhibit the underpotential deposition of silver onto the goldparticle surface because of the strong binding of these halides togold surfaces. However, due to the destabilizing effects that thesehalides have on the AgUPD layer, high-index facets can still bestabilized despite the lower coverage of silver. These results arerelevant when, for example, a bromide-containing surfactantis used rather than a chloride-containing surfactant, and thisdistinction is the difference between producing concave cubes24and tetrahexahedra,22which are prepared from growth solutionscontaining 100 μM silver and either 100 mM CTA-Cl orCTA-Br, respectively (in addition to 0.5 mM HAuCl4,1.0mMascorbic acid, 20 mM HCl, and 0.1 μL of 7 nm diameter seeds;Chart S1).First, a combination of XPS and ICP-AES was used tocharacterize and compare the metallic compositions of thetetrahexahedra and concave cubes. Following the same protocolas described previously, XPS characterization of the sur-face silver/gold ratio of the tetrahexahedra reveals that thetetrahexahedra have a significantly lower silver coverage (Ag/Autetrahexahedra = 0.14) than the concave cubes (Ag/Auconcave cubes =0.26) (Figure 7). The XPS results were confirmed by ICP-AES, acomplementary technique. Because the ICP-AES analysis is abulk characterization technique, the silver coverage can be ex-pressed in terms of a percentage of a monolayer.26In comparisonFigure 7. XPS data on the silver/gold ratio for particles generated in100 mM CTA-Br with different concentrations of AgNO3 in thegrowth solution. In the range of 40−100 μM silver ions, all of theproducts formed in CTA-Br are tetrahexahedra and have similar silver/gold ratios. In the case of CTA-Cl, 40 μM silver ions yields truncatedditetragonal prisms that have a silver/gold ratio of 0.14, while 100 μMsilver ions yield concave cubes that have a silver/gold ratio of 0.26, asshown in the figure.