Effect of urea on the morphology and room temperature ferromagnetism of CeO₂ microstructures synthesized by hydrothermal method

CeO₂ microstructures with different morphologies were successfully synthesized via a facile hydrothermal method using Ce(NO₃)₃·6H₂O as the cerium source, urea as precipitator and carbon source, and cetyltrimethyl ammonium bromide as surfactant. X-ray diffraction (XRD) revealed the synthesized CeO₂ have the fluorite structure. Scanning electron microscope (SEM) indicated that the synthesized CeO₂ have cometary shape microstructures. The estimated direct band gaps (E _g ) of CeO₂ are 0.08–0.15 eV smaller than that of bulk CeO₂. X-ray photoelectron spectroscopy (XPS) showed that there are plenty of Ce³⁺ ions and oxygen vacancies at surface of CeO₂. The emission intensity decreases with the increase of urea from 10 to 40 mmol. All the synthesized CeO₂ exhibited the room temperature ferromagnetism (RTFM), and saturation magnetization (M _s ) increases with the increase of E _g up to 3.06 eV and then decreases with E _g increasing more. The RTFM mechanism of cometary shape CeO₂ has been proposed which is mainly associated with the oxygen vacancies and Ce³⁺ ions.