EELS study of Fe- or Co-doped titania nanosheets.

Ti0.6Fe0.4O2 and Ti0.8Co0.2O2 nanosheets are Fe- and Co-doped titanium oxides, respectively, and they are synthesized by the exfoliation of lepidocrocite-type layered titanates. We have investigated these nanosheets by electron energy-loss spectroscopy (EELS) using a monochromated transmission electron microscope. The energy-loss near-edge structures (ELNESs) of Fe-L and Co-L indicate that Fe(3+) and Co(2+) ions are substituted in the octahedral sites in each nanosheet. The Ti-L edges of Ti0.6Fe0.4O2 and Ti0.8Co0.2O2 nanosheets correspond to the octahedral coordination of Ti(4+) and oxygen atoms as well as an undoped titania nanosheet (Ti0.87O2). On the other hand, the electron transitions from 2p3/2 to 3d eg in Ti-L3 regions are different in each nanosheet. We have also investigated the electron-beam-induced damage of Ti0.6Fe0.4O2 and Ti0.8Co0.2O2 nanosheets. The results indicated that Fe(3+) ions in the Ti0.6Fe0.4O2 nanosheets were selectively reduced to Fe(2+) ions in the reduction process by electron irradiation. In contrast, the chemical shift of the Ti-L edge of the Ti0.8Co0.2O2 nanosheets indicated that Ti(4+) ions were reduced. These results suggest that the substitution of 3d metals in titania nanosheets affects their crystal and electronic structures and material properties such as their long-range atomic configuration and reduction mechanism.