Abstract CeO 2 nanorod photocatalysts with systematically tuned oxygen vacancy concentrations were synthesized via a one-step NaBH 4 -assisted hydrothermal method to elucidate the role of defect engineering in photocatalytic water splitting. A series of reduced samples (Ce1–Ce5) and pristine CeO 2 were thoroughly characterized. Increasing NaBH 4 dosage induced XRD peak broadening with crystallite size reduction (5.37–4.44 nm) and a UV–Vis DRS red shift with bandgap narrowing (2.89–2.72 eV). Urba