Microwave hydrothermal synthesis, annealing and luminescence properties of BaWO₄:3%Eu³⁺ microcrystals

The uniform BaWO₄ microcrystals with scheelite-type tetragonal structure were fabricated via a microwave hydrothermal method. The morphologies and sizes of the products can be tuned by adjusting synthetic conditions including pH value, the amount of cetyltrimethylammonium bromide (CTAB) and the reaction time. The increasing amount of CTAB surfactant (from 0 to 0.05 mmol) causes the morphological transformation from flower-like to octahedron-like. Particularly, in the time-dependent experiments, the nanoparticles prefer to adsorb or nucleate on (111) crystal planes in the growth process, while the (101) planes become narrow or even partly disappear. Doping of Eu³⁺ (3 mol%) has no effect on the morphology and the phase structure. The emission spectrum (λ_ex = 394 nm) of the BaWO₄:3%Eu³⁺ exhibits the maximum peak at 613 nm, corresponding to ⁵D₀ → ⁷F₂ transition of Eu³⁺. Moreover, uniform and well-defined octahedron-like crystals have better fluorescence properties than irregular flower-like crystals. Annealing treatment, aiming at removing the residual surfactant on the surface, can effectively improve the crystallization and reduce the defects in the crystals. BaWO₄:3%Eu³⁺ microcrystals with octahedron-like annealed at 850 °C for 2 h has the strongest luminescence properties, which can be considered as a red phosphor for white light-emitting diodes (WLEDs).