Growth of CeO₂ nanocubes showing size-dependent optical and oxygen evolution reaction behaviors

CeO₂ nanocubes with average sizes of 9, 13, and 18 nm have been synthesized by preparing a slightly basic aqueous mixture of Ce(NO₃)₃, Na₂SO₄, and NH₄OH and heating the solution to 100 to 150 °C in 4 or 9 h. The nanocubes possess high crystalline quality. Their band gaps decrease gradually beyond the quantum confinement regime from 3.57 eV to 3.45 eV with increasing particle sizes. The 9 nm CeO₂ nanocubes have the most positive valence band energy and correspondingly they exhibit the best electrochemical oxygen evolution reaction activity. Since band gaps of semiconductor nanocrystals can be tuned substantially through particle size control to yield different band energies, this fact can be utilized to enhance the electrochemical and photocatalytic properties.