In situ electron energy loss spectroscopy study of metallic Co and Co oxides

Determining the Co valence, particularly in Co-based nanocatalysts is a longstanding experimental challenge. In this paper, we utilize in situ electron energy-loss spectroscopy and first-principles density functional theory calculations to distinguish between metallic Co, $Co3O4$⁠, as well as CoO. More specifically, differences in the $OK$- and $CoL$-edges are utilized to determine the Co valence in different Co-oxide particles. We will further demonstrate that while the metallic $CoL3/L2$-ratio equals that of partially reduced $Co3O4$⁠, the near-edge fine-structure of the metallic $CoL$-edge exhibits additional features not present in any Co-oxide. The origin of these features will be discussed. Based on our experimental and theoretical results, we will propose a fitting method to distinguish metallic Co from Co-oxides.