Paper NS+AS-WeA3
Oxidation State Sensitive Imaging of Ceria Nanoparticles

Wednesday, November 12, 2014, 3:00 pm, Room 304

Scanning transmission electron microscopy (STEM) coupled with electron energy loss spectroscopy (EELS) has been successfully applied to track changes to composition as well as bonding environment with atomic resolution. These measurements impose exacting experimental and instrumentation requirements that include aberration-corrected optics, electron sources with narrow energy spreads, and extremely stable room environments. Therefore it seems prudent when possible that other techniques with less demanding experimental requirements supplement EELS measurements. Imaging or diffraction techniques greatly relax these aforementioned requirements as the need for corrective optics and narrow energy spreads can be eliminated. Moreover the possibility of beam induced artifacts is reduced because the total electron dose needed to form an image or diffraction pattern is less than an EELS spectrum image.

Low angle annular dark field (LAADF) STEM is presented as an alternative to EELS measurements to identify changes to local changes of oxidation state in ceria (CeO₂) nanoparticles. This relationship was established through the use of EELS, in-situ measurements, and image simulations. Ceria has numerous energy related applications due to the ability of ceria readily store and release oxygen. The formal charge of the cerium ions transition from 4+ to 3+ as oxygen vacancies are formed. These oxygen vacancies cause local distortions to the crystal and subsequently produces additional diffuse scattering to low angles. The LAADF STEM signal is sensitive to this change in scattering and contrast variations in the image become resolved. Additionally, preliminary experiments on other metal oxide nanoparticles suggest that this approach may be applied to other material systems and processes as well.