The characterization of Co-nanoparticles supported on graphene

TL;DR

This study combines density functional theory and X-ray photoelectron spectroscopy to analyze the oxidation states and magnetic properties of cobalt nanoparticles supported on graphene, revealing thickness-dependent oxidation behavior and protective oxide formation.

Contribution

It provides new insights into the oxidation states and protective oxide formation of Co nanoparticles on graphene, combining theoretical and experimental approaches.

Findings

Low Co thickness leads to non-homogeneous island formation and oxidation.

Thicker Co layers form a protective oxide while remaining metallic at the interface.

Co exists predominantly in the Co2+ oxidation state without Co3+ evidence.

Abstract

The results of density functional theory calculations and measurements using X-ray photoelectron spectroscopy of Co-nanoparticles dispersed on graphene/Cu are presented. It is found that for low cobalt thickness (0.02 nm - 0.06 nm) the Co forms islands distributed non-homogeneously which are strongly oxidized under exposure to air to form cobalt oxides. At greater thicknesses up to 2 nm the upper Co-layers are similarly oxidized whereas the lower layers contacting the graphene remain metallic. The measurements indicate a Co2+ oxidation state with no evidence of a 3+ state appearing at any Co thickness, consistent with CoO and Co[OH]2. The results show that thicker Co (2nm) coverage induces the formation of a protective oxide layer while providing the magnetic properties of Co nanoparticles.