  Enhanced visible light photocatalytic activity of transition metal-doped
ceria (CeO2) nanomaterials have experimentally been demonstrated, whereas there
are very few reports mentioning the mechanism of this behavior. Here we use
first-principles calculations to explore the origin of enhanced photocatalytic
performance of CeO2 doped with transition metal impurities (Fe, Cr and Co).
When a transition metal atom substitutes a Ce atom into CeO2, t2g and eg levels
of 3d orbits appear in the middle of band gap owing to the effect of cubic
ligand field, and the former is higher than latter. Interestingly, t2g subset
of FeCe (CoCe and CrCe)-Vo-CeO2 is split into two parts: one merges into the
conduction band, the other as well as eg will remain in the gap, because O
vacancy defect adjacent to transition metal atom will break the symmetry of
cubic ligand field. These eg and t2g levels in the band gap are beneficial for
absorbing visible light and enhancing quantum efficiency because of forbidden
transition, which is one key factor for enhanced visible light photocatalytic
activity. The band gap narrowing also leads to a redshift of optical absorbance
and high photoactivity. These findings can rationalize the available
experimental results and provide some new insights for designing CeO2-based
photocatalysts with high photocatalytic performance.
