Journal
CHEMISTRY OF MATERIALS
Volume 27, Issue 20, Pages 6960-6968Publisher
AMER CHEMICAL SOC
DOI: 10.1021/acs.chemmater.5b01862
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Funding
- DOE BES [DE-FG02-03ER15476]
- Office of Basic Energy Sciences of the US Department of Energy [DE-AC02-05CH11231]
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In situ techniques of transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS) were used to investigate the thermal stability of Ni-Co core-shell nanoparticles (NPs). The morphological, structural, and chemical changes involved in the core-shell reconfiguration were studied during in situ annealing through simultaneous imaging and acquisition of elemental maps in the TEM, and acquisition of 0 Is, Ni 3p, and Co 3p XP spectra. The core-shell reconfiguration occurred in a stepwise process of surface oxide removal and metal segregation. Reduction of the stabilizing surface oxide occurred from 320 to 440 degrees C, initiating the core-shell reconfiguration. Above 440 degrees C, the core-shell structure was disrupted through Ni migration from the core to the shell. This resulted in the formation of a homogeneous Ni Co mixed alloy at 600 degrees C. This study provides a mechanistic description of the alteration in the core-shell structures of NPs under vacuum conditions and increasing annealing temperature, which is crucial for understanding these technologically important materials.
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