- 上一篇：中药材实施GAP英文文献和中文翻译
- 下一篇：案例检索算法冲压模具英文文献和中文翻译
This indicates that, even at high silverion concentrations, the bromide is inhibiting the deposition ofsilver onto the particle surface and thus preventing the forma-tion of {111}-faceted octahedra. However, when the CTA-Brconcentration is reduced to 20 mM, thereby reducing the totalbromide concentration in the growth solution, the formation of{111}-faceted octahedra is observed (Figure S9). In the samemanner as the first experiment, reducing the bromide concentra-tion enables more silver to be deposited onto the gold surface ofeach particle.19Taken together, this set of experiments furtherconfirms our hypothesis that, at high concentrations of bromidein the growth solution, the deposition of silver onto the surfaceof the gold particles is inhibited.While we expect to observe the same trend in behavior withiodide, the insolubility of CTA-I prevents the study of iodide athigh concentrations. However, iodide can be added to solutionup to a concentration that is equal to that of the gold ion in thegrowth solution, and experiments conducted in this range ofiodide concentrations are in agreement with expected behavior.When the growth solution contains 10 μM iodide and 100 mMCTA-Cl, particles with extremely stellated features are pro-duced (Figure S10, Supporting Information). This particlemorphology most likely arises from poorly controlled growth asa result of iodide both binding to the particle surface andseverely destabilizing the AgUPD layer. It is possible that silverdeposited onto the gold particle surface is rapidly displaced ordestabilized by iodide, and this poorly controlled growth resultsin the generation of particles with stellated features and ill-defined surface facets.Concave versus Convex High-Index Faceted Nano-particles. It is now understood why the combination of silverions with chloride- or bromide-containing surfactants yieldsrelated high-index faceted nanostructures: concave cubes andtetrahexahedra, respectively. However, one important point notyet addressed is why the concave cubes have concave featureswhile the tetrahexahedra are convex nanostructures. Regardlessof the synthetic method used, concave nanostructures areunusual, but their growth is typically favored under kineticallycontrolled growth conditions.25,49,73−78We hypothesize thatthe convex tetrahexahedra are overall more thermodynamicallyfavorable structures than the concave cubes due to their moreisotropic shape. We believe that the concavity of the concavecubes is due to the relatively high stability of the AgUPD layer inthe presence of chloride and the rapid rate of gold ion reductionin CTA-Cl, which enables the formation of less thermodynami-cally favorable shapes. Effectively, the underpotential depositionof silver in the presence of a chloride-containing surfactant“locks in” a certain surface facet early in the reaction becauseof the stable AgUPD layer formed in chloride. For example, evenat the early stages of the concave cube reaction, transmis-sion electron microscopy (TEM) images show that the smallgrowing seed particles are in the shape of small concave cubes(Figure S11, Supporting Information). Over the course of thereaction, the small concave cubes continue to grow, maintainingtheir shape, until they reach their final size. We have foundthat smaller diameter (∼7 nm) seed particles are crucial forthe growth of concave cubes, presumably due to the higherreactivity of smaller seeds. This is demonstrated when seedswith a diameter of ∼40 nm are used and, rather than concavecubes, truncated ditetragonal prisms are formed even at con-centrations of silver ions that would normally produce concavecubes (100 μM silver ion). It is possible that the large seeds arenot reactive enough to support the growth of concave features.The energy barrier for growing concave features may be toohigh for these larger seeds to overcome. It then follows that,when a bromide-containing surfactant is used, the slowerreduction rate of [AuBr2]− in comparison to that of [AuCl2]−and the decreased stability of the AgUPD layer in bromide resultsFigure 9. Scheme illustrating how halides and silver ions can be used to direct the growth of gold seeds down different growth pathways to yielddifferent shaped products: (A) kinetically controlled products in the absence of silver ions; (B) Ag underpotential deposition-controlled productswhere the interactions of silver with the particle surface dictate product shape; (C) effect of varying the stability of the AgUPD layer with highconcentrations of chloride, bromide, or iodide in the growth solution, yielding concave cubes, tetrahexahedra, and stellated particles, respectively. in the development of convex, rather than concave, features dueto rearrangement of the surface atoms into a more thermo-dynamically favorable convex structure.■