Effects of Thermal Annealing and X-ray Exposure on Local Structures Surrounding Co in (Y. Co)-codoped CeO$_{2}$ Nanocrystals

Codoping of Y and Co in CeO$_{2}$ has been found to incur an unconventional bandgap narrowing effect in the host nanocrystal material. The dormant bandgap-tuning ability of Y appears to be turned on by the Co codopant in the oxygen-vacancy-rich CeO$_{2}$ host. The physical mechanism underlying such effect has also been revealed by DFT calculations. Synchrotron-Radiation-based X-ray absorption analysis has further demonstrated that the Co codopant atoms can be located in either of two different local bonding environments in the CeO$_{2}$ host. Thermal annealing is capable of driving Co atoms from one bonding environment to the other. On the other hand, when exposed to x-rays of sufficient photon energy, Co can resume the original local structure as in the as-made sample. A simple model is proposed to explain such intriguing interplay between the effects of annealing and x-ray exposure in these codoped nanoceria systems.