Luminescence studies and stability of bismuth doped lanthanum oxide and oxysulphide

Different concentrations of bismuth doped lanthanum oxide (La2-xO3:Bix) phosphor powder (x = 0.001, 0.002, 0.003, 0.004, 0.006, 0.008 and 0.01) were synthesised by means of the sol-gel combustion method at 250 °C using citric acid as the fuel. The product was annealed at different temperatures and the luminescence properties were investigated. The maximum photoluminescence (PL) emission was obtained for the sample which was doped with x = 0.002 and annealed at 1200 °C in air. The X-ray diffraction (XRD) analysis confirmed that all samples crystallized in the La2O3 hexagonal phase. The scanning electron microscopy (SEM) data showed that the grain size increased with increasing annealing temperature and the shape of the grains changed from rectangular to more round, but faceted, after annealing at 1200 °C in air. Energy dispersive X-ray spectroscopy (EDS) confirmed the chemical composition, while diffuse reflectance spectroscopy was used to study the absorption of the La2-xO3:Bix samples. All the samples were absorbing in the ultraviolet range between 220 to 320 nm. The band gap of the La2O3 pure host sample was obtained from the reflectance data as 5.1 eV. Excitation at a wavelength of 308 nm resulted in a single broad blue luminescence emission band centred at 462 nm. The excitation and emission bands were attributed to transitions between the 1S0 ground state and the 3P1 excited state of Bi3+ ions, with a Stokes shift of 1.35 eV. It was found that the samples no longer exhibited PL after storage of several weeks. Further XRD measurements revealed that the La2O3 had changed to La(OH)3. This is consistent with reports that La2O3 can absorb moisture from the air and transform to La(OH)3, which was observed to occur completely in about a week. Unlike for La2O3:Eu and La2O3:Ho phosphors for which the transformation reduced the luminescence, but did not quench it completely, the luminescence of the degraded La2O3:Bi was negligible so that simply the presence of luminesce
African Laser Centre (ALC)