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

Gd 2 O 2 SO 4 :Eu 3+ hollow spheres were successfully synthesized through a template-free hydrothermal synthesis routine from commercially available Gd 2 O 3 , Eu 2 O 3 , HNO 3 , (NH 4 ) 2 SO 4 and CO(NH 2 ) 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 Gd 2 (OH) 2 CO 3 SO 4 · x H 2 O and could be converted into pure Gd 2 O 2 SO 4 phase at 800 °C for 2 h in air. SEM and FESEM observations show that the obtained Gd 2 O 2 SO 4 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 Gd 2 O 2 SO 4 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 Gd 2 O 2 SO 4 :Eu 3+ hollow spheres, which corresponds to the 5 D 0  →  7 F 2 transition of Eu 3+ ions. Decay study demonstrates that the 5 D 0  →  7 F 2 transition of Eu 3+ ions has a single exponential decay behavior and its corresponding fluorescence lifetime is 1.50 ms according to the linear fitting result.