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Even though the surface facets of the tetra-hexahedra and concave cubes are similar, the tetrahexahedra have asurface silver/gold ratio closer to that of the truncated ditetragonalprisms than to that of the concave cubes. to the 84% of a monolayer silver coverage on the concave cubes,the silver coverage on the tetrahexahedra is only 47% of a mono-layer, as determined by ICP-AES (Figure 8). Both character-ization techniques corroborate a lower coverage of silver on thetetrahexahedra than the concave cubes, reflecting a difference inthe behavior of silver ions in the presence of either bromide orchloride.To further probe the behavior of silver ions in CTA-Br, theconcentration of silver in the tetrahexahedra growth solution wasvaried from 40 to 100 μM. This is an important range of silverion concentration for reactions conducted in CTA-Cl, where at40 μMAg+, {310}-faceted truncated ditetragonal prisms areformed, while at 100 μMAg+, {720}-faceted concave cubesare formed. In addition to the dramatic difference in particlemorphology observed at these two different concentrations ofsilver in CTA-Cl, there is also a distinct change in surface silvercoverage due to the different number of exposed surface atomsassociated with the crystallographic facets which enclose eachparticle shape.26However, when CTA-Br is used rather thanCTA-Cl in the growth solution, a different behavior is observed.SEM characterization of the products formed from growthsolutions containing 100 mM CTA-Br and silver ion concentra-tions ranging from 40 to 100 μM reveals that tetrahexahedra areproduced at all concentrations of silver ions tested (Figure S6,Supporting Information). There is also little to no change in thedimensions of the tetrahexahedra at different concentrationsof silver ions. Characterization of the silver content by XPS(Figure 7) and ICP-AES (Figure 8) shows a relatively constantsilver coverage, regardless of the silver ion concentration inthe growth solution.
While there is a decrease in the amountof silver on the surface of the tetrahexahedra from the 47%monolayer coverage at 100 μM silver in the growth solutionto a 37% monolayer coverage of silver at 40 μM silver in thegrowth solution, this decrease does not affect the shape of theparticles.These results, obtained from growth solutions containinga high concentration (100 mM) of CTA-Br, are markedlydifferent from the case where a small concentration of bromide(<50 μM) and a high concentration (100 mM) of CTA-Cl areboth present in the growth solution. While the addition of smallamounts of bromide to CTA-Cl results in an increase in theamount of silver deposited onto the gold nanoparticles, a highconcentration of bromide in the form of CTA-Br inhibits thedeposition of silver in comparison to the use of CTA-Cl. Thisbehavior can be attributed to the same behavior of bromide thatwas previously discussed, namely (1) the decreased stability ofthe AgUPD layer in bromide as compared to chloride54,68−72and(2) bromide binding more strongly to the gold particle surfacethan chloride.54The ability of bromide to destabilize and createa more dynamic AgUPD layer facilitates the rearrangement ofsilver to more thermodynamically favorable surface sites, suchas atomic steps, and enables the stabilization the {730} facets ofthe tetrahexahedra even though the surface coverage of silver issignificantly lower than the {720}-faceted concave cubes.Additionally, a second contributing factor to the lower silvercoverage on the tetrahexahedra is the higher binding affinity ofbromide for the gold surface in comparison to that of chloride,which enables bromide to inhibit the deposition of silver byblocking the gold surface.We have experimentally demonstrated the enhanced bindingof bromide compared to chloride to the gold particle surfaceand the destabilization of the AgUPD layer by bromide in thefollowing experiments. First, growth solutions were preparedcontaining either 100 mM CTA-Cl and 100 mM NaBr or100 mM CTA-Br and 100 mM NaCl in addition to 0.5 mMHAuCl4, 1.0 mM ascorbic acid, 20 mM HCl, and 0.1 μLof7 nm diameter seeds (Chart S1). The products of bothreactions were tetrahexahedra (Figure S7, Supporting Informa-tion). In other words, if bromide is present in the reaction athigh concentrations (100 mM), in the form of either a salt(NaBr) or a surfactant (CTA-Br), then the effects of bromidedominate the effects of chloride (even if the chloride concentra-tion is also high, 100 mM) and tetrahexahedra are produced.Next, we demonstrate that more silver can be deposited onto thegold particle surface by reducing the bromide concentration inthe growth solution. In one experiment, a growth solution wasprepared with 10 μM silver ion (1/10th of the silver concentrationtypically used in the tetrahexahedra synthesis) and 100 mMCTA-Br (plus 0.5 mM HAuCl4, 1.0 mM ascorbic acid, 20 mMHCl, and 0.1 μL of 7 nm diameter seeds). The products of thisreaction display facets but are very polydisperse, indicative ofuncontrolled growth (Figure S8, Supporting Information). Notethat this is in contrast to the analogous experiment in CTA-Cl,where rhombic dodecahedra are produced. Importantly, wefound that by decreasing the CTA-Br concentration by a factorof 4, we were able to observe the growth of tetrahexahedra(Figure S8). This indicates that, when the concentration ofbromide was reduced, more silver was deposited onto the goldparticles, allowing the stabilization of the high-index facets of thetetrahexahedra. In another experiment, a very high concentrationof silver ions (>100 μM) was used in the growth solution of areaction seeded with concave cubes (Chart S1). When CTA-Clis used as a surfactant, since the concentration of silver ions isextremely high (500 μM), {111} facets can be stabilized, becausethose facets have the highest number of exposed surface atoms(i.e., they are the most densely packed).35The analogousexperiment can be conducted in CTA-Br; however, the pro-duct observed is simply overgrown concave cubes (Figure S9,Figure 8. ICP-AES data plotting the experimentally determined bulksilver/gold ratio (black squares) of tetrahexahedra prepared fromgrowth solutions with silver ion concentrations ranging from 40 to100 μM.Alsoplottedisacalculatedvalueforthesilver/goldratioexpected for a monolayer coverage of silver for a {730} facet (blue line).These data reveal that the tetrahexahedra have approximately a 47%monolayer coverage of silver that decreases to 37% with decreasingconcentrations of silver in the growth solution. Note that these silver/goldratios determined by ICP-AES are normalized for both the size andshape of the nanoparticles and thus represent a value reflecting thesilver coverage on the particle surface. See ref 26 for a more detaileddiscussion of this adjustment. Supporting Information).