Mild Synthesis of Size-Tunable CeO2 Octahedra for Band Gap Variation

CeO2 octahedral particles with average opposite corner lengths of 52, 67, 75, 85, 100, and 110 nm have been synthesized by heating a water–ethanol mixture of Ce(NO3)3 solution at 150 °C for 1 h. Simply varying the volume of Ce(NO3)3 solution used tunes the particle size. Experimental observations support direct formation of CeO2 from Ce3+ ions. Both light absorption and band-gap-related photoluminescence bands of these CeO2 octahedra red-shift continuously with increasing particle size. Optical band gap varies from 3.42 eV for 52 nm octahedra to 2.94 eV for 110 nm octahedra, so band gap tunability is possible over a very large size range. Mott–Schottky plots were obtained from electrochemical measurements to yield a band diagram of 52, 85, and 110 nm CeO2 octahedra with different valence band and conduction band energies, showing that particle size of semiconductor nanocrystals can significantly tune their band positions. The notable change in valence band positions for the 52 and 110 nm CeO2 octahedra may contribute to their potential difference in electrochemical oxygen evolution reaction activity.