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The alloy phase behavior of nanoparticle (NP) interfaces has been used to tailor asymmetric growth. Using either Au-Pd core-shell or Au-Au(x)Pd(1-x) core-alloy NP starting materials, the deposition of Ag resulted in asymmetric and symmetric growth respectively. The phase segregation of the interface was confirmed by TEM and electrocatalytic activity.

The synthesis, processing, and galvanic exchange of three heterostructured nanoparticle systems is described. The surface accessibility and redox potential of a Au/Pd-Ag dumbbell nanoparticle, where a Au/Pd core/shell region, and a silver region make up the domains, was used to prepare the new nanostructures with controlled composition, morphology, and microstructure. Results indicate that the silver domain was particularly susceptible to galvanic displacement, and was exchanged to Au/Pd-M (M = Au, Pd, Pt). Interestingly, the dumbbell morphology remained after exchange, and the silver region was transformed to hollow, parachute, or concentric domains respectively. The morphology and microstructure change was visualized via TEM and HRTEM, and the composition changes were probed via STEM-EDS imaging and XPS. The electrocatalytic activity of the Au/Pd-M towards methanol oxidation was studied, with results indicating that the Au/Pd-Pt nanoparticles had high activity attributed to the porous nature of the platinum domains.

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The layer-by-layer formation of core/alloy nanoparticles is described. Using presynthesized gold nanoparticle cores, Au x Ag 1-x alloy shells were deposited and annealed with subnanometer precision using a microwave irradiation (MWI) mediated hydrothermal processing method. The alloy composition, thickness, and nanoparticle morphology governed the surface plasmon resonance characteristics of the particles, as well as growth characteristics. The mechanism for alloy deposition, annealing, and interdiffusion was explored using two gold precursors, [AuBr 4 ] − and [AuCl 4 ] − , and two hydrothermal temperatures (120, 160 °C). Findings indicate that use of [AuCl 4 ] − results in significant galvanic displacement, leading to nonuniform alloy formation and phase segregation at low annealing temperatures, which leads to loss of morphology control at intermediate compositions (x ≈ 0.25−0.75). In contrast, use of [AuBr 4 ] − results in alloy shells with low galvanic interactions, leading to optimum alloy distribution and high fidelity control of alloy-shell thickness that, in combination with higher hydrothermal processing temperatures, leads to uniform and monodisperse core/alloy microstructure across all compositions. The alloy deposition and core/alloy nanoparticle growth was followed in situ by monitoring the change in surface plasmon resonance (SPR) signatures by UV−vis, which were unique to alloy shell thickness, as well as composition, and morphology. The interfacial alloy composition was probed by modeling the SPR with discrete dipole approximation, the results of which suggest the final alloy shells are Au-rich compared to the feed ratios, owing in large part to both galvanic displacements as well as core-to-shell alloy interdiffusion.