Reversible Shape and Plasmon Tuning in Hollow AgAu Nanorods

Abstract

The internal structure of hollow AgAu nanorods created by partial galvanic replacement was manipulated reversibly, and its effect on optical properties was mapped with nanometer resolution. Using the electron beam in a scanning transmission electron microscope to create solvated electrons and reactive radicals in an encapsulated solution-filled cavity in the nanorods, Ag ions were reduced nearby the electron beam, reshaping the core of the nanoparticles without affecting the external shape. The changes in plasmon-induced near-field properties were then mapped with electron energy-loss spectroscopy without disturbing the internal structure, and the results are supported by finite-difference time-domain calculations. This reversible shape and near-field control in a hollow nanoparticle actuated by an external stimulus introduces possibilities for applications in reprogrammable sensors, responsive materials, and optical memory units. Moreover, the liquid-filled nanorod cavity offers new opportunities for in situ microscopy of chemical reactions.

Controllably Hollow AgAu Nanoparticles via Nonaqueous, Reduction Agent‐Assisted Galvanic Replacement

“…For the reactions performed with 0.2Au:Ag, we obtained the expected hollow nanocage in the absence of ascorbic acid (Figure S2, Supporting Information). When present, ascorbic acid slowly reduces the removed Ag + as well as a portion of the added Au 3+ onto the particle surface during replacement such that instead of hollow nanocages, NPs synthesized at 65 °C with a reducing agent have a closed, smooth surface with solvent trapped inside the void of the particle (Figure ) . At higher Au:Ag ratio, such as 0.5Au:Ag, the coreduction agent prevents the NPs from dealloying to small fragments (Figure S2, Supporting Information).…”

“…Nanoshells and other hollow architectures can indeed perform better in applications such as SERS, imaging, and photothermal therapy . Furthermore, hollow NPs are of interest owing to their high surface area to mass ratio, their ability to carry a cargo, and their potential to enable studies of chemical reactions in nanoscale volumes …”

“…Further galvanic replacement removes Ag by a dealloying process, generating hollow Au nanocages . Dealloying can be overcome by adding a reducing agent to the reaction (a process called coreduction), leading to alloy structures even for stoichiometries larger than 0.33Au:Ag . This approach also often reduces the roughness and occurrence of pinholes if the coreduction step is slow; such structures can be completely pinhole‐free and liquid‐filled, enabling aqueous, electron‐beam driven electrochemical reactions …”

“…Galvanic replacement reactions involving Ag and Au are usually performed in aqueous media . Of the few reducing agent‐assisted reactions, to the best of our knowledge all have been performed in aqueous media: one for Pd and Pt, and two other, for Ag and Au, with only one work reporting closed NP structures . Replacement in aqueous media limits the synthesis, functionalization, and application of NPs: nonaqueous syntheses could, for instance, allow for the encapsulation of water‐insoluble molecules inside the NP cavity, as well as enable the galvanic replacement of, or with, metals unstable in water.…”

Enhancing Galvanic Replacement in Plasmonic Hollow Nanoparticles: Understanding the Role of the Speciation of Metal Ion Precursors

“…The oxidation layer that may form on templates made of more active metals like Zn or Al has been reported to slow down reaction kinetics and change the structural outcome, and even prevent the galvanic exchange from occurring . Finally, coupling the standard galvanic replacement process described above with a secondary reducing agent acting upon both metallic species (often termed “co‐reducer” in reference to the oxidant metal) provides the ability to tune the alloying process and to manipulate shell morphology and composition beyond what is achievable with simple galvanic replacement . The secondary reducing agent in the solution introduces new reaction pathways and products owing to the competition for oxidant ions between the metal template and the co‐reducer, which reduces the oxidant metal's hunger for template electrons.…”