  Bimetallic nanoparticles are widely studied, for example in catalysis.
However, possible restructuring in the environment of use, such as segregation
or alloying, may occur. Taken individually, state-of-the-art analytical tools
fail to give an overall picture of these transformations. This study combines
an ensemble analysis (near-ambient-pressure X-ray photoelectron spectroscopy)
with a local analysis (environmental transmission electron microscopy) to
provide an in situ description of the restructuring of core--shell
nickel-cobalt nanoparticles exposed to cycles of reduction and oxidation. It
reveals a partial surface alloying accompanied by fragmentation of the shell
into smaller clusters, which is not reversible. Beyond this case study, the
methodology proposed here should be applicable in a broad range of studies
dealing with the reactivity of mono- or bi-metallic metal nanoparticles.
