Investigation of Ni@CoO core-shell nanoparticle films synthesized by sequential layer deposition

M.C Spadaro,P Luches,F Benedetti,S Valeri,S Turchini,G Bertoni,A.M Ferretti,E Capetti,A Ponti,S D’Addato

doi:10.1016/j.apsusc.2016.05.177

Abstract

Films of Ni@CoO core-shell nanoparticles (NP Ni core size d≈11nm) have been grown on Si/SiOx and lacey carbon supports, by a sequential layer deposition method: a first layer of CoO was evaporated on the substrate, followed by the deposition of a layer of pre-formed, mass-selected Ni NPs, and finally an overlayer of CoO was added. The Ni NPs were formed by a magnetron gas aggregation source, and mass selected with a quadrupole mass filter. The morphology of the films was investigated with Scanning Electron Microscopy and Scanning Transmission Electron Microscopy. The Ni NP cores have a shape compatible with McKay icosahedron, caused by multitwinning occurring during their growth in the source, and the Ni NP layer shows the typical random paving growth mode. After the deposition of the CoO overlayer, CoO islands are observed, gradually extending and tending to merge with each other, with the formation of shells that enclose the Ni NP cores. In situ X-ray Photoelectron Spectroscopy showed that a few Ni atomic layers localized at the core-shell interface are oxidized, hinting at the possibility of creating an intermediate NiO shell between Ni and CoO, depending on the deposition conditions. Finally, X-ray Magnetic Circular Dichroism at the Ni L2,3 absorption edge showed the presence of magnetization at room temperature even at remanence, revealing the possibility of magnetic stabilization of the NP film.

Highlights

Research on metal/oxide core-shell nanoparticles (NPs) is a major area of interest in material science because of the fundamental role of these materials in magnetic recording, catalysis, nanomedicine, and photovoltaics [1, 2]
The Ni@CoO films have been investigated with Scanning Electron Microscopy (SEM) and Scanning Transmission Electron Microscopy (STEM), in order to understand their growth morphology
Films of Ni@CoO core-shell NPs grown on Si/SiOx were successfully obtained with sequential layer deposition, by depositing a first layer of CoO, a layer of preformed, mass-selected Ni NPs obtained by a magnetron gas-aggregation source, and an overlayer of CoO

Summary

Introduction

Research on metal/oxide core-shell nanoparticles (NPs) is a major area of interest in material science because of the fundamental role of these materials in magnetic recording, catalysis, nanomedicine, and photovoltaics [1, 2]. In situ X-ray Photoelectron Spectroscopy showed that a few Ni atomic layers localized at the core-shell interface are oxidized, hinting at the possibility of creating an intermediate NiO shell between Ni and CoO, depending on the deposition conditions.

Results

Conclusion