Nanostructured CeO₂ photocatalysts: optimizing surface chemistry, morphology, and visible-light absorption

Emerging photocatalytic applications of cerium dioxide (CeO₂) include green hydrogen production, CO₂ conversion to fuels, and environmental remediation of various toxic molecules. These applications leverage the oxygen storage capacity and tunable surface chemistry of CeO₂ to photocatalyze the chosen reaction, but many open questions remain regarding the fundamental physics of photocatalysis over CeO₂. The commonly ascribed 'bandgap' of CeO₂ (∼3.1 eV) differs fundamentally from other photocatalytic oxides such as TiO₂; UV light excites an electron from the CeO₂ valence band into a 4f state, generating a polaron as the lattice distorts around the localized charge. Researchers often disregard the distinction between the 4f state and a traditional, delocalized conduction band, resulting in ambiguity regarding mechanisms of charge transfer and visible-light absorption. This review summarizes modern literature regarding CeO₂ photocatalysis and discusses commonly reported photocatalytic reactions and visible light-sensitization strategies. We detail the often misunderstood fundamental physics of CeO₂ photocatalysis and supplement previous work with original computational insights. The exceptional progress and remaining challenges of CeO₂-based photocatalysts are highlighted, along with suggestions for further research directions based on the observed gaps in current understanding.