The optical properties of metal nanomaterials are determined by a set of parameters that include composition, particle size and shape, overall architecture, and local environment. This Tutorial Review examines the influence of each of these factors on the localized surface plasmon resonance of colloidal metal nanoparticles. This examination is paralleled with a discussion of the advances which have enabled the synthesis of structurally defined metal nanomaterials, as these samples serve as the best platforms for elucidating the fundamental properties of plasmonic colloids. Based on the analysis of such samples, five guidelines are presented to aid the rational design and synthesis of new metal nanostructures for advanced applications in nanomedicine, energy, chemical sensing, and colloidal plasmonics in general.

Gold-palladium octopods and new concave and shape-controlled alloy nanostructures are synthesized by seed-mediated co-reduction, wherein two metal precursors are reduced in the presence of seeds that serve as preferential sites for the growth of the larger nanostructures. Here, the first comprehensive study of this technique is presented in a model Au-Pd system and provides insight into the mechanism of formation for these architecturally distinct nanocrystals. A systematic evaluation of synthesis conditions decoupled the roles of (i) Au:Pd precursor ratio, (ii) reaction pH, and (iii) capping agent concentration in morphology development. These factors provide control of growth kinetics and ultimately the morphology and composition of the final nanostructures. Significantly, elucidating the overgrowth processes during seed-mediated co-reduction will lead to the synthesis of other architecturally controlled bimetallic nanocrystals.

Au/Pd octopods and concave core@shell Au@Pd nanocrystals have been prepared by coupling for the first time a seed-mediated synthetic method with co-reduction. The integration of these two methods is central to the formation of these binary Au/Pd nanocrystals wherein the kinetics of seeded growth are manipulated via the co-reduction technique to control the final morphology of the nanocrystals. Significantly, the synthesis of these structures under similar reaction conditions illustrates that they are structurally related kinetic products. Detailed characterization by STEM-EDX analysis highlights the unique structural features of these nanocrystals and indicates that Pd localizes on the higher-energy features of the nanocrystals. Optical and electrocatalytic characterization also demonstrates their promise as a new class of multifunctional nanostructures.

There is currently a worldwide need to develop efficient photocatalytic materials that can reduce the high-energy cost of common industrial chemical processes. One possible solution focuses on metallic nanoparticles (NPs) that can act as efficient absorbers of light due to their surface plasmon resonance. Recent work indicates that small NPs, when photoexcited, may allow for efficient electron or hole transfer necessary for photocatalysis. Here we investigate the mechanisms behind hot hole carrier dynamics by studying the photodriven oxidation of citrate ions on Au@SiO@Au core-shell NPs. We find that charge transfer to adsorbed molecules is most efficient at higher photon energies but still present with lower plasmon energy. On the basis of these experimental results, we develop a simple theoretical model for the probability of hot carrier-adsorbate interactions across the NP surface. These results provide a foundation for understanding charge transfer in plasmonic photocatalytic materials, which could allow for further design and optimization of photocatalytic processes.

Branched metal nanoparticles often display unique physicochemical properties on account of their structures; however, most examples are asymmetric, with branches randomly distributed from the cores of the nanoparticles. This asymmetry can give rise to variable properties between samples. Here, we report the synthesis of symmetrically branched Au/Pd nanocrystals including five-branched pentapods with D(3h) symmetry, 24-branched nanocrystals with O(h) symmetry, 12-branched nanocrystals with T(d) symmetry, and eight-branched octopods and bowties with O(h) and D(4h) symmetry, respectively. These structures are achieved by seed-mediated co-reduction wherein the shapes of the seeds direct the number and symmetry patterns of the branches. Compositional boundaries exist at the interfaces between the seed and overgrowth metals to provide visualization via advanced electron microscopy of the relationships between seed structure and the symmetry of branched nanocrystals. Significantly, seed structure plays a definitive role in determining the final shape of convex metal nanocrystals, and the results presented here illustrate a similar relationship for branched nanocrystals and will enable the design of new architecturally distinct nanostructures.