Gold Speciation and Co-reduction Control the Morphology of AgAu Nanoshells in Formaldehyde-Assisted Galvanic Replacement

Abstract

Hollow AgAu nanostructures have a myriad of potential applications related to their strong and tunable localized surface plasmon resonances. Here, we describe how the hydrolysis of the Au precursor, AuCl 4 − , produces AuCl 4−x (OH) x − , where x is both time and pH-dependent, and how this can be used to control the morphology of hollow nanoshells in the co-reduction-assisted galvanic replacement of Ag by Au. Controlling the degree of hydrolysis is the key to obtain smooth shells: too small values of x (low hydrolysis) yield inhomogeneously replaced rough shells whereas too large values of x lead to the dominance of Au nucleation over galvanic replacement. Kinetic studies reveal two time constants for the galvanic replacement varying with temperature and composition; a short (<10 min) half-life component associated with the initial void creation and a long (>100 min) half-life component associated with the continuous reduction and replacement of Ag. By optimizing the reaction's pH and Au speciation, we obtained smooth alloy shells with fine control of composition, size, and shape over a broad range, thereby tuning the optical properties. This framework for understanding and controlling reaction kinetics and nanoshell morphology is applicable to other metallic systems and precursors, providing new ways to rationally design nanostructure syntheses.

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

“…b–c) Extinction spectra of 0.5 mM (q=0 to 4) measured: b) over time in nanopure water (pH 2.9, at t=30 min) and c) at higher pH at equilibrium, i. e., 24 hours after the preparation of the solution. Note that pH evolves during the hydrolysis process; here we report pH values measured at the time of extinction measurement …”

“…PVP10 and formaldehyde were added as stabilizing and reducing agents, respectively. Au is added in the proportion of 0.5 equivalent of Ag, see details in reference …”

“… TEM images of Ag−Au galvanic replacement product using pH‐optimized (pH 9) precursor solution showing the effect of pre‐adjusting the Ag nanoparticle's solution pH a) at pH 9.5 and b) at pH 11. See details in reference …”

“…The versatility offered by standard galvanic replacement methods can be expanded by introducing a secondary reducing agent in the reaction medium that will act upon both metals . More recently, altering the speciation of the oxidant metal to prevent the galvanic replacement reaction from occurring or to control the outcome of the galvanic replacement process was also reported . Manipulation of the metal‐complexing ligands as a control parameter has been little explored so far and will be discussed in this paper using the results from co‐reducer‐assisted galvanic replacement reactions carried out on silver nanoparticles using , and precursors for which we have manipulated the speciation in water with pH and pCl (by analogy to pH, the letter p in the expression pL is used to indicate the decimal cologarithm associated with the concentration of the ligand Cl).…”

Enhanced control of plasmonic properties of silver–gold hollow nanoparticles via a reduction-assisted galvanic replacement approach

“…Owing to their plasmonic properties, a commonly studied metal pair is Ag/Au; galvanic replacement operates because Au has a more positive redox potential and can gradually replace the Ag template. Taking advantage of the well-known shape control mechanisms for the Ag template, the Ag/Au pair has been used extensively as a synthetic approach to generate simple to exotic hollow structures, including spheres, 23,24 rods, 25 wires, 26 rings, 27 octahedrons, 28,29 multilayer spheres, 30 as well as boxes and cages. 31 By altering the resulting shell characteristics, including composition, the relative amount of oxidant metal added to the nanoparticle template suspension controls the optical properties of the nal solution.…”