Creating hollow shell bimetallic oxide particles is of great interest for numerous technological applications, including electrocatalysis, lithium batteries, and supercapacitors. However, the oxidation mechanisms involved are not yet well understood. In-situ transmission electron microscopy (TEM) allows for direct observation of bimetallic nanoparticles during the oxidation process and offers understanding of these processes. Uniting electron energy loss spectroscopy (EELS) spectrum imaging with scanning TEM (STEM) delivers detailed data on the compositional heterogeneity of the resultant structures.
Materials and Methods
A Gatan imaging filter with a DENSsolutions Wildfire heating holder was utilized within an environmental TEM (ETEM) to attain an elevated temperature in a gas environment of 0.2 Torr pure O2. STEM-EELS spectrum imaging was performed using a GIF Tridiem® system to map the O k, Co L2,3, and Ni L2,3 edges over multiple particles, as seen in Figure 1.
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Figure 1. Series of in-situ EELS spectrum images showing particle oxidation. The top row shows a particle after initial oxidation at 450 °C, where a clear oxide shell is observed. The bottom row shows a different particle after further oxidation at 550 °C, where the entire particle was finally oxidized.
Han, L.; Meng, Q.; Wang, D.; Zhu, Y.; Wang, J.; Du, X.; Stach, E. A.; Xin, H. L., Interrogation of bimetallic particle oxidation in three dimensions at the nanoscale. Nat. Commun. 7 (2016) 13335. doi:10.1038/ncomms13335.
Summary
In-situ EELS spectrum imaging enabled the mapping of the composition of multiple bimetallic nanoparticles as they were oxidized. Research showed that oxide shells form initially, followed by diffusion of metal ions up to the surface where they are oxidized, forming voids in the structure. If the oxide cracks, permitting oxygen to infiltrate the outer shell, oxidation can also occur in the central region of the particle. The internal and external surfaces of the oxidized nanoparticles were found to be Co-rich.
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