Morphology-controlled synthesis and luminescence properties of green-emitting NaGd(WO4)2:Tb3+ phosphors excited by n-UV excitation

Abstract

Tb3+-doped luminescent materials are widely used as green-emitting materials in the fields of lighting and display. A simple hydrothermal method was used to synthesize NaGd(WO4)2:Tb3+ phosphors without any surfactant or templates. The effects of pH value, reaction time and the doping concentration of Tb3+ on the phase structure, morphology and luminescent properties of the samples were investigated. X-ray powder diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive X-ray spectrum (EDS), photoluminescence spectra and decay curve were applied to characterize the obtained samples. The results indicate that well-defined uniform square plate-like NaGd(WO4)2:Tb3+ microcrystals can be synthesized at pH 8.0, 453.15 K, reaction time of 20 h. Time-dependent sampling and morphology characterization showed the probable formation mechanism of the square plate-like morphology is: nucleation, dissolution, recrystallization and oriented growth. The main excitation peak at 381 nm corresponding to 7F6→5D3 transition of Tb3+, located in the n-UV region (370–410 nm). Upon excitation with 381 nm, the emission spectrum of NaGd(WO4)2:Tb3+ phosphor showed a typical Tb3+ emission, originated from the 5D4→7FJ (J = 6, 5, 4, 3) transition. The strongest emission peak at 545 nm was attributed to 5D4→7F5 transition, which is located in the green light region. The luminescence intensity for the emissions from 5D4 level increased gradually with the increase of Tb3+ doping concentration and reached a maximum at the doping concentration of 80 mol%, then decreased, implying the occurrence of concentration quenching. The square plate-like NaGd(WO4)2:Tb3+ phosphor achieved the best properties, with the intensive green emission centered at 545 nm under n-UV excitation, indicating that it can be used as green component in n-UV-excited white light emitting diodes.