How Ag Nanospheres Are Transformed into AgAu Nanocages

Abstract

Bimetallic hollow, porous noble metal nanoparticles are of broad interest for biomedical, optical and catalytic applications. The most straightforward method for preparing such structures involves the reaction between HAuCl and well-formed Ag particles, typically spheres, cubes, or triangular prisms, yet the mechanism underlying their formation is poorly understood at the atomic scale. By combining in situ nanoscopic and atomic-scale characterization techniques (XAFS, SAXS, XRF, and electron microscopy) to follow the process, we elucidate a plausible reaction pathway for the conversion of citrate-capped Ag nanospheres to AgAu nanocages; importantly, the hollowing event cannot be explained by the nanoscale Kirkendall effect, nor by Galvanic exchange alone, two processes that have been previously proposed. We propose a modification of the bulk Galvanic exchange process that takes into account considerations that can only occur with nanoscale particles. This nanoscale Galvanic exchange process explains the novel morphological and chemical changes associated with the typically observed hollowing process.

Self‐Templating Approaches to Hollow Nanostructures

“…However, neither mechanism can explain the hollowing process alone because the exchange ratio of Ag:Au does not agree with the expected 3:1 ratio for galvanic exchange and the unchanged size of the product is inconsistent with the Kirkendall effect. More recently, Moreau et al studied the galvanic replacement between HAuCl 4 and citrate‐capped Ag nanoparticle and attributed the hollowing process to a “nanoscale Galvanic exchange” process, which followed the vacancy coalescence hypothesis for the formation of voids . In their understanding, a monolayer of Au is first deposited on the surface of Ag via citrate reduction.…”

Ag–Au bimetallic nanocomposites stabilized with organic–inorganic hybrid microgels: synthesis and their regulated optical and catalytic properties

“…17,18 Compared to the co-reduction approach, the galvanic replacement (GR) reaction strategy had received more attention due to offering facile reaction conditions and allowing the fabrication of bimetallic NPs with tunable structures that can be controlled by adjusting the concentration ratio of metal precursor salts. [19][20][21] The unique porous structure of Ag-Au bimetallic NPs is characterized by an increased number of surface active sites and a large void volume, which not only promotes electron transfer on the nanocatalyst surface, but also accelerates the reaction of the substrate in contact with the catalyst and thus improves catalytic activity. [22][23][24] The surface of bimetallic NPs is commonly coated with chemically inert protective layers (SiO 2 , dendrimers, polymers, etc.)…”

In Situ Techniques for Probing Kinetics and Mechanism of Hollowing Nanostructures through Direct Chemical Transformations

“…If the calculated SAXS patterns agree well with the experimental patterns, the evolution of the nanoparticles more likely involves the proposed intermediate structures . With in situ SAXS, Moreau et al studied the galvanic replacement reaction of Ag nanoparticles with AuCl 4 − ions, revealing the absence of the Kirkendall effect responsible for the formation hollow nanoshells . The conclusion is derived from the unchanging size of the nanoparticles in the course of the reaction.…”

“…Besides the size and geometry of nanoparticles, the variation of the local coordination and oxidation states of metal atoms during galvanic replacement reactions and oxidation reactions can be probed by in situ extended X‐ray absorption fine structure (EXAFS) spectroscopy . Due to the acquisition time of a decent EXAFS spectrum being much longer than that of a scattering pattern, in situ EXAFS spectroscopy is usually applicable for slow reactions that transform solid nanoparticles to hollow ones.…”