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.

Fig. 1

Galvanic replacement of Ag nanocubes by Au at 23 and 90 °C. a Time series of in situ transmission electron microscopy (TEM) images (Supplementary Movie 1) and corresponding schematics showing the morphological evolution of an Ag nanocube during galvanic replacement (GR) reaction at 23 °C. Green arrows indicate pores that form in the deposited shell, and a cyan arrow points to a second GR reaction on the residual Ag core (formation of a rough shell) after it is again exposed to the Au solution through pores on the outer shell. b Time series of in situ TEM images (Supplementary Movie 2) and schematics showing the morphological evolution of an Ag nanocube during GR at 90 °C. Orange arrows indicate the inner shell with darker contrast, which should be the Au layer. Red arrows indicate observed void nucleation. Here, t ₀ indicates the start of the recording time

Fig. 2

Chemical and structural characterization of Ag/Au nanostructures synthesized ex situ. a Scanning TEM annular dark field image and corresponding energy dispersive X-ray spectroscopy (EDX) chemical maps of nanostructures synthesized ex situ at 23 °C. b Nanostructures synthesized at 100 °C after adding 0.3, 1.5, and 4.0 mL of 0.1 mM HAuCl₄. c EDX spectra for samples synthesized at 23 and 100 °C (with 1.5 mL of 0.1 mM HAuCl₄ added) showing the Cl K and Ag L lines. d Selected area electron diffraction from a single nanostructure in both samples (with 1.5 mL of 0.1 mM HAuCl₄ added), where diffraction spots for AgCl are frequently found in the 23 °C sample because of lower solubility of AgCl at 23 °C than at 100 °C

Fig. 3

Galvanic replacement of an Ag nanocube by Au at 70 °C. a Time series of in situ TEM images (Supplementary Movie 3) and corresponding schematic depicting how the morphological evolution can be explained via the propagation of two or more voids. b The projected area of the Ag nanocube cores vs. time at three different temperatures

Fig. 4

GR of an Ag nanocube using an AuCl precursor solution at 90 °C. Time series of in situ TEM images (Supplementary Movie 4) and the corresponding schematic depicting the creation of double-walled structures via void nucleation and propagation within the alloy shell. Red arrows indicate the nucleation of voids and their propagation within the shell