Controlled Hydrothermal Synthesis and Photoluminescence of Nanocrystalline ZnGa(2)O(4):Cr(3+) Monospheres

The hydrothermal synthesis of nanocrystalline ZnGa(2)O(4):Cr(3+) (ZGC) red phosphor monospheres was accomplished in this work, and the effects of system pH, reactant content, reaction time, and citrate anions (Cit(3−)) on the phase and morphology evolution of the product were systematically studied. Under the optimized conditions of Cit(3−)/M = 1.0 molar ratio (M = total cations), pH = 5.0, and 0.2 mmol of Zn(2+), well-dispersed ZGC monospheres with an average diameter of ~454 ± 56 nm (average crystallite size ~15 nm) were successfully obtained via hydrothermal reaction at 180 °C for 18 h. Cit(3+) ions were demonstrated to be crucial to the formation of monospheres and substantially affect the pathway of phase formation. The ZGC monospheres calcined at 800 °C (average diameter ~353 ± 59 nm; average crystallite size ~30 nm) have an intensity ~6 times that of the original phosphor for the 700 nm red emission of Cr(3+) (the (2)E → (4)A(2) transition) under excitation with the O(2−) → Ga(3+) charge transfer band at 250 nm. Fluorescence decay analysis found that the 700 nm emission has lifetime values of ~5 ms for the ZGC phosphors.