Formation of Co-O bonds and reversal of thermal annealing effects induced by X-ray irradiation in (Y, Co)-codoped CeO2 nanocrystals

We report an unconventional effect of synchrotron X-ray irradiation in which Co-O bonds in thermally annealed (Y, Co)-codoped CeO2 nanocrystal samples were formed due to, instead of broken by, X-ray irradiation. Our experimental data indicate that escaping oxygen atoms from X-ray-broken Ce-O bonds may be captured by Co dopant atoms to form additional Co-O bonds. Consequently, the Co dopant atoms were pumped by X-rays from the energetically-favored thermally-stable Co-O4 square-planar structure to the metastable octahedral Co-O6 environment, practically a reversal of thermal annealing effects in (Y, Co)-codoped CeO2 nanocrystals. The band gap of doped CeO2 with Co dopant in the Co-O6 structure was previously found to be 1.61 eV higher than that with Co in the Co-O4 environment. Therefore, X-ray irradiation can work with thermal annealing in opposing directions to fine tune and optimize the band gap of the material for specific technological applications.

Automated summary

In this study, an unconventional effect of synchrotron X-ray irradiation on thermally annealed (Y, Co)-codoped CeO2 nanocrystal samples was reported. The experimental data indicated that instead of breaking the Co-O bonds, the X-ray irradiation formed new Co-O bonds by capturing escaping oxygen atoms from the X-ray-broken Ce-O bonds. This effect caused a transition of Co dopant atoms from the energetically-favored Co-O4 structure to the metastable Co-O6 environment, resulting in a fine-tuning of the band gap of the material.