Galvanic Transformation Dynamics in Heterostructured Nanoparticles

Abstract

Corrosion is a significant problem for the stability of structural metals and potentially for functional nanomaterials in operating environments. When two metals with different electrochemical potentials form a junction, galvanic corrosion occurs, resulting in the sacrificial dissolution of the metal with a higher oxidation potential (lower electrode potential). Here, it is shown that bimetallic hetero-nanostructures composed of phase-segregated metals undergo galvanic corrosion in aqueous environments. Such selective etching of the sacrificial metal in heterojunction particles leads to the formation of unusual and kinetically stabilized half-spheroid particles. By using a fluid cell and in situ scanning transmission electron microscopy, a two-stage corrosion process can be observed where the Cu experiences a fractal breakdown before the Ag corrodes due to the lack of a protective oxide layer. However, when treated with a mild Ar plasma, the stability of these structures against corrosion is enhanced due to the conversion of the amorphous native oxide to a denser, thin layer of CuO on the Cu surface. Taken together, this work highlights the importance of considering the effects of galvanic corrosion on the stability of multicomponent nanoparticles, and it shows how mass transport in a nanoscale system is influenced by redox processes.

Electron Microscopy Studies of Soft Nanomaterials

“…Studies of etching kinetics of NPs on one hand provide mechanistic understanding of the reactivity of different facets of NPs which can be relevant to understanding of corrosion and catalytic stability of NPs, and on the other hand can serve as a means of harvesting nonequilibrium shapes of NPs (e.g., concave, cages, and branched) with desired properties. Different etching mechanisms have been reported so far, including the conventional oxidative etching where metal atoms are oxidized back to ions by oxidative species, the galvanic corrosion where two metals are in contact and one is preferentially etched due to its higher oxidation potential to protect another, and the galvanic replacement where one metal is replaced by another with a higher reduction potential . Different from the role of a reducing agent when imaging the growth of NPs, an electron beam can also generate oxidative species from radiolysis of solvents, which either are involved in redox reaction networks of the system or serve as the real etchant.…”

“…Compared with core–shell structures, galvanic corrosion can be more obvious in Janus NPs, where both components are exposed to the etchant. In a recent paper, a set of Janus NPs were synthesized and their etching under the electron beam was observed . For a Ag–Cu nanocrystal, due to the higher oxidation potential of Cu, the breakdown of Cu occurred first, followed by the dissolution of Ag (Figure a).…”

“…Green arrows indicate pores that form in the deposited Au shell, and a cyan arrow points to a second galvanic replacement reaction on the residual Ag core after it is again exposed to HAuCl 4 through pores on the outer shell. (a, b) Reprinted with permission from ref . Copyright 2021 Wiley-VCH.…”

“…In a recent paper, a set of Janus NPs were synthesized and their etching under the electron beam was observed. 482 For a Ag−Cu nanocrystal, due to the higher oxidation potential of Cu, the breakdown of Cu occurred first, followed by the dissolution of Ag (Figure 34a). Instead of dissolving gradually, the Cu portion was broken into pieces and flowed away from the particle, which could be attributed to the presence of CuO that has a crystal symmetry distinct from Cu and forms a nonconformal protective layer on metal surface.…”

Effects of the Encapsulation Membrane in Operando Scanning Transmission Electron Microscopy

“…Thanks to the implementation of a localized sealed reaction environment, in situ gas electron microscopy has gradually become an essential technique for visualizing nanoscopic reactions in real time (and often in real-space). For example, the combination of gas supply and high temperature heating capability allows for growth of nanomaterials in micron-sized reactors inside the TEM, inducing preferential oxidation in alloys, , and examining structural robustness and stability of catalyst materials under various corrosive environments. , Additionally, the electrochemical signal can be also monitored during reactions introduced by external gaseous species, providing the possibility to characterize structure–functional relationships for catalysis, metal-air battery, and gas sensor development based on a similar environmental system …”