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To testthis theory, ICP-AES was used to monitor the rate of Au0produc-tion. Samples were taken from the growth solution at specifictimeFigure 2. SEM images of reaction products from growth solutionscontaining 10 mM CTA-Br and (A) 0.5, (B) 2.0, and (C) 10.0 mMascorbic acid, resulting in the formation of {111}-faceted octahedra,{100}-faceted cubes, and high-index faceted trisoctahedra, respectively.Scale bars: 200 nm. points during the course of the reaction, and these samples werethen quenched using an excess of bis(p-sulfonatophenyl)-phenylphosphine dihydrate potassium salt (BSPP), which stronglycoordinates to gold ions in solution and to the gold particlesurface, thus slowing (inhibiting) the further reduction of goldions.55−57From each sample, the gold particles were isolated bycentrifugation, separating them from gold ions in solution, andsubsequently dissolved with aqua regia. ICP-AES was then used todetermine the total amount of gold in the sample. Note that theaddition of seeds to these reactions separates the nucleation fromthe growth event and thus the amount of Au0detected is due tothe growth of the seeds and not due to the nucleation of newseed particles. A comparison of the reactions containing0.0 and 5.0 mM NaBr, which produced trisoctahedra and{100}-faceted cubes, respectively, is shown in Figure 3C. Inboth sets of data, the amount of Au0increases with time due to theformation of gold nanoparticles. However, Au0is produced morerapidly for the reaction containing 0.0 mM NaBr than for thereaction containing 5.0 mM NaBr, indicating that the presence ofbromide is inhibiting the rate of gold particle formation and,accordingly, the rate of gold ion reduction. We note that {111}-faceted octahedra can also be produced at 5.0−10.0 mM NaBrby slightly lowering the concentration of reducing agent in thereaction solution, indicating that the kinetics of formation ofthese two products differ only slightly (Figure S1, SupportingInformation). This similarity in the rate of particle formation islikely due to {111} and {100} facets of gold being very close insurface energy.58−60A similar trend is observed when iodide is introduced toreactions run in the presence of CTA-Cl. However, iodide canbe added to the growth solutions in an amount 2 orders ofmagnitude lower than is required for bromide while achievingcomparable results. Growth solutions containing 0.0, 10.0, and75.0 μM NaI yield trisoctahedra, {111}-faceted octahedra, and{111}-faceted truncated bitetrahedra, respectively (Figure 4).
The ICP-AES data shown in Figure 4D indicate a decrease inthe rate of Au0generation with the addition of 75.0 μM NaI,confirming that, as with bromide, the addition of iodide tothe reaction inhibits the rate of gold ion reduction. Anotherdifference between the examples involving bromide and iodideis that the use of 75.0 μM iodide results in the formation of aplanar twinned product, truncated bitetrahedra, while highconcentrations of bromide produce cubes, which are singlecrystalline. Twin planes are a common defect observed inthe growth of noble metal nanostructures, and their presenceis indicative of slow, kinetically controlled growth condi-tions.61−67The growth of planar twinned truncated bitetrahe-dra with 75.0 μM NaI is therefore not surprising if iodide isacting to reduce the rate of gold ion reduction. We also do notobserve the growth of {100}-faceted cubes when iodide is used;however, the {111}-faceted octahedra do display some tiptruncations that expose {100} facets. Again, it is likely that thesmall difference in energy between {111} and {100} surfacefacets is the cause for these observations.As previously mentioned, the two effects of halides onparticle growth, namely, on the reduction rates of the gold ionspecies and on the binding of the halide to the gold particlesurface, are difficult to separate. For example, ICP-AES data forthe reactions containing 10.0 and 75.0 μM NaI cannot bedistinguished (data not shown), indicating that the introductionof iodide affects more than just the rate of reaction, since theformation of both {111}-faceted octahedra and {111}-facetedtruncated bitetrahedra appear to have the same rate of goldparticle generation yet produce different particle shapes. Thisindicates that the rate of growth of planar twinned seeds isfaster than the rate of growth of the single-crystalline counter-parts under the conditions that produce {111}-facetedtruncated bitetrahedra, but a comprehensive understandingof how all twinned particles fit into our model of kineticallycontrolled growth will still need to be determined.Effects of Silver Ions. In previous work,26we demonstratedthat particle shape can be controlled by varying the amount ofFigure 3. (A, B) SEM images of reaction products from growthsolutions containing 50 mM CTA-Cl and (A) 0.0 and (B) 5.0 mMNaBr, resulting in the formation of high-index faceted trisoctahedraand {100}-faceted cubes, respectively. Scale bars: 200 nm. (C) ICP-AES kinetics data of the reactions containing 0.0 mM (black squares)and 5.0 mM (red triangles) NaBr.Figure 4. (A−C) SEM images of reaction products from growthsolutions containing 50 mM CTA-Cl and (A) 0.0, (B) 10.0, and (C)75.0 μM NaI, resulting in the formation of high-index facetedtrisoctahedra, {111}-faceted octahedra, and {111}-faceted truncatedbitetrahedra, respectively. Scale bars: 200 nm. The truncatedbitetrahedra are the planar twinned analogue of the single crystallineoctahedra. (D) ICP-AES kinetics data of the reactions containing0.0 μM (black squares) and 75.0 μM (red triangles) NaI. silver ion additive in a growth solution containing CTA-Clsurfactant. Silver deposits epitaxially onto gold and prefers todeposit onto sites where it will have a high coordination numberwith respect to gold, such as kinks and step edges, and this leadsto the stabilization of high-index facets through silver actingas a capping layer. By incrementally raising the concentrationof silver ion in the growth solution, particles with increasinglyhigher index facets are formed due to the underpotentialdeposition of silver onto the surface of the growing goldparticle.26This corresponds to particles with an increasingnumber of exposed surface atoms per unit area: {110}-facetedrhombic dodecahedra, {310}-faceted truncated ditetragonalprisms, and {720}-faceted concave cubes, respectively. Notethat the {110}-faceted rhombic dodecahedra form concomitantlywith their planar twinned analogue, {110}-faceted bipyramids.34Using XPS and ICP-AES, we determined that the ratio of silverto gold on the surface of the particles (via XPS), as well as in theparticles as a whole (via ICP-AES), increases with greater con-centrations of silver ion in the growth solution