Ultrafast Elemental and Oxidation-State Mapping of Hematite by 4D Electron Microscopy
We describe a new methodology that sheds light on the fundamental electronic processes that occur at the subsurface regions of inorganic solid photocatalysts. Three distinct kinds of microscopic imaging are used that yield spatial, temporal, and energy-resolved information. We also carefully consider the effect of photon-induced near-field electron microscopy (PINEM), first reported by Zewail et al. in 2009. The value of this methodology is illustrated by studying afresh a popular and viable photocatalyst, hematite, α-Fe_2O_3 that exhibits most of the properties required in a practical application. By employing high-energy electron-loss signals (of several hundred eV), coupled to femtosecond temporal resolution as well as ultrafast energy-filtered transmission electron microscopy in 4D, we have, inter alia, identified Fe^(4+) ions that have a lifetime of a few picoseconds, as well as associated photoinduced electronic transitions and charge transfer processes.
Additional Information© 2017 American Chemical Society. Received: January 28, 2017; Published: March 8, 2017. This work was supported by the Air Force Office of Scientific Research (FA9550-11-1-0055) in the Gordon and Betty Moore Center for Physical Biology at the California Institute of Technology. The studies described in this paper were conceived by Ahmed Zewail, and most of the insights that we reveal were greatly influenced by his input. We thank Prof. Nathan S. Lewis at the California Institute of Technology for helpful comments. The authors declare no competing financial interest.
Supplemental Material - ja7b00906_si_001.pdf