Structure and stability of nickel/nickel oxide core-shell nanoparticles

Area of Science:

• Materials Science
• Nanotechnology
• Surface Chemistry

Background:

• Nickel nanoparticles (Ni NPs) are crucial in catalysis and materials science.
• Understanding their oxidation behavior is key to controlling their properties.
• Previous studies have explored Ni NP oxidation, but detailed structural analysis post-oxidation remains important.

Purpose of the Study:

• To investigate the structural and chemical changes in Ni nanoparticles during oxidation.
• To characterize the core-shell structure of oxidized Ni NPs.
• To determine the specific crystalline structure of the Ni NP core and the NiO phase.

Main Methods:

• Combined in situ X-ray Photoelectron Spectroscopy (XPS) and High-Resolution Transmission Electron Microscopy (HR-TEM).
• Preparation of pre-formed Ni NP films (4-8 nm diameter) within a controlled experimental setup.
• Controlled oxidation using O(2) gas and atmospheric exposure, with in situ XPS analysis.

Main Results:

• XPS revealed a composite Ni 2p spectrum indicating both metallic Ni core and NiO shell.
• The metallic Ni core signal persisted even after air exposure.
• HR-TEM and simulations confirmed a multitwinned icosahedral structure for the Ni NP core.
• Nickel oxide (NiO) was observed as distinct islands on the NP surface.

Conclusions:

• Nickel nanoparticles maintain a metallic core even after significant oxidation.
• The core exhibits a well-defined icosahedral structure.
• Oxidation results in the formation of NiO islands on the nanoparticle surface, suggesting a heterogeneous oxidation process.