Direct observation of the nanoscale Kirkendall effect during galvanic replacement reactions

Abstract

Galvanic replacement (GR) is a simple and widely used approach to synthesize hollow nanostructures for applications in catalysis, plasmonics, and biomedical research. The reaction is driven by the difference in electrochemical potential between two metals in a solution. However, transient stages of this reaction are not fully understood. Here, we show using liquid cell transmission electron microscopy that silver (Ag) nanocubes become hollow via the nucleation, growth, and coalescence of voids inside the nanocubes, as they undergo GR with gold (Au) ions at different temperatures. These direct in situ observations indicate that void formation due to the nanoscale Kirkendall effect occurs in conjunction with GR. Although this mechanism has been suggested before, it has not been verified experimentally until now. These experiments can inform future strategies for deriving such nanostructures by providing insights into the structural transformations as a function of Au ion concentration, oxidation state of Au, and temperature.

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

“…The major void continuously enlarges in size and the Ag core gradually shrinks (Figure b,c), which is similar to the hollowing mechanism dominated by Kirkendall surface diffusion presented in Figure and Figure . Depleting the Ag core results a hollow nanobox (Figure d) . The involvement of the Kirkendall effect observed from the in situ characterization is different from the hollowing mechanism associated the conventional galvanic replacement reactions shown in Figure .…”

“…Therefore, in situ TEM with specialized liquid cells becomesnecessary to help understand the detailed hollowing mechanism. Chee et al studied the galvanic replacement reaction of Ag nanoparticles with oxidative AuCl 4 − ions using in situ liquid‐phase TEM . The contact of AuCl 4 − ions and the Ag nanoparticles spontaneously triggers the galvanic replacement reaction to deposit a Au layer on the outer surface of the Ag nanoparticles.…”

“…The contact of AuCl 4 − ions and the Ag nanoparticles spontaneously triggers the galvanic replacement reaction to deposit a Au layer on the outer surface of the Ag nanoparticles. In situ TEM observation revealed that the following hollowing process is driven by the combination of the a continuous galvanic replacement reaction and the excessive outward diffusion of Ag (i.e., the Kirkendall effect) . At room temperature (i.e., 23 °C), following the deposition of a Au layer on a Ag nanocube, voids appear simultaneously at the corners of the Ag nanocube.…”

“…At room temperature (i.e., 23 °C), following the deposition of a Au layer on a Ag nanocube, voids appear simultaneously at the corners of the Ag nanocube. Uniform enlargement of the voids and their coalescence forms a symmetric gap between the Ag core and Au shell, resulting in the formation of a core–void–shell nanorattle at 36 s of reaction . However, increasing the reaction temperature to 90 °C favors the anisotropic coalescence of voids to form an asymmetric geometry with one major void ( Figure a, highlighted by arrows).…”

“…a–d) TEM images of the nanoparticle formed after the galvanic replacement reaction starts for different times: (a) 5.6 s, (b) 11.2 s, (c) 13.6 s, and (d) 17.6 s. The scale bar in (a) also applies to (b–d). Adapted with permission . Copyright 2017, Springer Nature.…”