Bimetallic nanoparticles display unique optical and catalytic properties that depend on crystallite size and shape, composition, and overall architecture. They may serve as multifunctional platforms as well. Unfortunately, many routes toward shape and architecturally controlled bimetallic nanocrystals yield polydisperse samples on account of the challenges associated with homogeneously nucleating a defined bimetallic phase by co-reduction methods. Developed by the Skrabalak laboratory, seed-mediated co-reduction (SMCR) involves the simultaneous co-reduction of two metal precursors to deposit metal onto shape-controlled metal nanocrystalline seeds. The central premise is that seeds will serve as preferential and structurally defined platforms for bimetallic deposition, where the shape of the seeds can be transferred to the shells. With Au-Pd as a model system, a set of design principles has been established for the bottom-up synthesis of shape-controlled bimetallic nanocrystals by SMCR. This strategy is successful at synthesizing symmetrically stellated Au-Pd nanocrystals with a variety of symmetries and core@shell Au@Au-Pd nanocrystals. Achieving nanocrystals with high morphological control via SMCR is governed by the following parameters: seed size, shape, and composition as well as the kinetics of seeded growth (through manipulation of synthetic parameters such as pH and metal precursor ratios). For example, larger seeds yield larger nanocrystals as does increasing the amount of metal deposited relative to the number of seeds. This increase in nanocrystal size leads to red-shifts in their localized surface plasmon resonance. Additionally, seed shape directs the overgrowth process during SMCR so the resultant nanocrystals adopt related symmetries. The ability to tune structure is important due to the size-, shape- and composition-dependent optical properties of bimetallic nanocrystals. Using this toolkit, the light scattering and absorption properties of Au-Pd octopods, 8-branched nanocrystals, could be tuned and were shown to be highly sensitive to changes in refractive index. The refractive index sensitivity displayed a linear correlation to the localized surface plasmon resonance initial position, where the sensitivity is greater than that of monometallic Au structures. Due to their bimetallic composition and unique architecture enabled by SMCR, Au-Pd octopods are promising refractive index based sensors. This Account summarizes the underlying principles for synthesis of bimetallic nanocrystals by SMCR, which have been established by systematic manipulation of synthetic parameters in a model Au-Pd system. These principles are anticipated to be general to other bimetallic systems, allowing for the design and synthesis of new nanocrystals with fascinating optical and catalytic properties.

Branched nanocrystals display interesting optical and catalytic properties on account of their high surface areas and tips with small radii of curvatures. However, many synthetic routes toward branched nanocrystals result in inhomogeneous samples on account of asymmetric branching. Seed-mediated coreduction is a recently developed route to symmetrically branched nanocrystals where the symmetry of the seeds is transferred to the final stellated morphologies. Here, general guidelines to stellated nanocrystals are outlined by surveying coreduction of Au and Pd precursors in the presence of a variety of shape-controlled Au seeds to achieve Au/Pd nanostructures. Single-crystalline, twinned, and anisotropic seeds were analyzed to expand the classes of stellated nanostructures synthetically accessible. Significantly, single-crystalline Au seeds adopt {100}-terminated intermediates prior to branching, regardless of initial seed shape. We compared these results with those obtained with shape-controlled Pd seeds, and seed composition was identified as an important synthetic parameter, with Pd seeds being more resistant to shape changes during overgrowth. This difference is attributed to the greater diffusion rate of Au atoms on Au seeds compared to Au atoms on Pd seeds. These results provide guidelines for the seeded synthesis of symmetrically branched nanocrystals and architecturally defined bimetallic nanostructures in general.

Colloidal plasmonics is a rapidly developing field, with applications of metal nanoparticles that range from biosensing to photocatalysis. Historically, Au- or Ag-only nanoparticles have been investigated for these uses. However, there is emerging interest in multifunctional plasmonic colloids as well as a desire to expand the light scattering and absorption properties of Au or Ag nanoparticles. Both of these goals can be achieved by incorporating a second metal. In this Perspective, recent advances in the synthesis and application of Au–Ag and Au–Pd nanostructures are discussed. The highlighted nanostructures were synthesized through the seed-mediated method, an approach that provides for structurally complex nanoarchitectures while maintaining sample homogeneity. Overall, these architecturally controlled bimetallic nanostructures are representative of a new class of plasmonic colloids in which composition and structure can be manipulated to achieve unique optical properties and functionality.

Stellated metal nanostructures are a class of plasmonic colloids in which large electric field enhancements can occur at their sharp tips, making them excellent candidates for surface-enhanced Raman spectroscopy (SERS) and surface-enhanced infrared spectroscopy (SE-IRS) platforms. Shape-dependent rules for convex polyhedra such as cubes, nanorods, or octahedra exist, which describe far-field scattering and near-field enhancements. However, such rules are lacking for their concave counterparts, which include stellated structures. Here, the optical response of stellated Au nanocrystals with O h , D 4h , D 3h , C 2v , and T d symmetry were modeled to systematically investigate the role of symmetry, branching, and particle orientation with respect to excitation source using finite difference time domain (FDTD) calculations. Furthermore, these results are compared to experimentally obtained localized surface plasmon bands determined by UV−visible (UV−vis) spectroscopy for ensembles of similarly shaped metallic nanostructures. These UV−vis spectra include that of the first reported all-Au octopodal structure achieved from seed-mediated methods. From these studies, symmetry-dependent rules governing the far-field and near-field optical response are outlined. These rules can be applied to a variety of concave structures to yield the optimized far-field and near-field responses for SERS and SE-IRS platforms.

The predictable synthesis of bimetallic nanostructures via co-reduction of two metal precursors is challenging due to our limited understanding of precursor ligand effects. Here, the influence of different metal-ligand environments is systematically examined in the synthesis of Pd-Pt nanostructures as a model bimetallic system. Nanodendrites with different spatially defined Pd-Pt compositions are achieved, where the local ligand environments of metal precursors dictate if temporally separated co-reduction dominates to achieve core-shell nanostructures or whether electroless co-deposition proceeds to facilitate alloyed nanostructure formation. As the properties of bimetallic nanomaterials depend on crystal ordering and composition, chemical routes to structurally defined bimetallic nanomaterials are critically needed. The approaches reported here should be applicable to other bimetallic compositions given the established reactivity of coordination complexes available for use as precursors.