Template-free hydrothermal synthesis of Gd2O2SO4:Eu3+ hollow spheres based on urea-ammonium sulfate (UAS) system

Gd2O2SO4:Eu3+ hollow spheres were successfully synthesized through a template-free hydrothermal synthesis routine from commercially available Gd2O3, Eu2O3, HNO3, (NH4)2SO4 and CO(NH2)2 (urea) as the starting materials. The as-synthesized products were characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), thermogravimetric and differential scanning calorimetry (TG-DSC), scanning electron microscopy (SEM), field emission scanning electron microscopy (FE-SEM) and photoluminescence (PL) spectra. The XRD, FT-IR and TG-DSC results demonstrate that the precursor is composed of Gd2(OH)2CO3SO4·xH2O and could be converted into pure Gd2O2SO4 phase at 800 °C for 2 h in air. SEM and FESEM observations show that the obtained Gd2O2SO4 particles possess hollow sphere structure, which has a typical size of about 10 μm in diameter and about 1.5 μm in shell thickness. The formation mechanism of the Gd2O2SO4 hollow spheres is related to the Ostwald ripening process. PL spectroscopy reveals that the strongest red emission peak is located at 619 nm under 275 nm UV light excitation for the Gd2O2SO4:Eu3+ hollow spheres, which corresponds to the 5D0 → 7F2 transition of Eu3+ ions. Decay study demonstrates that the 5D0 → 7F2 transition of Eu3+ ions has a single exponential decay behavior and its corresponding fluorescence lifetime is 1.50 ms according to the linear fitting result.