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

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.