Chemical interaction and imaging of single Co3O4/graphene sheets studied by scanning transmission X-ray microscopy and X-ray absorption spectroscopy

Abstract

Scanning transmission X-ray microscopy (STXM) has been used to investigate the chemical, electronic and structural nature of Co₃O₄ nanocrystals grown on single nitrogen-doped graphene sheets through spatially resolved X-ray absorption near edge structure (XANES) spectroscopy and chemical imaging. It has been found that Co₃O₄ nanocrystals grown on N-doped graphene were partially reduced via Co³⁺(O_h) to Co²⁺(O_h), and the reduction varies spatially on and among individual Co₃O₄ nanocrystal-graphene sheets. Nitrogen sites on graphene have been shown to be major and important anchoring sites for Co₃O₄ nanocrystals in addition to the carbon and possibly oxygen sites. Macroscopic XANES of Co L-edge and K-edge were also measured to confirm the localized STXM result that Co³⁺ was partly reduced in the hybrid material. These insights should account for the superior performance of the covalently coupled Co₃O₄/graphene hybrid in energy related applications.