Effect of Bi3+co-doping on the up-converting and photocatalytic properties of SrGd2O4:Yb3+/Ho3+ phase

Abstract

In this investigation, samples of strontium-gadolinium-oxide (SrGd2O4) doped with different Yb3+ (2, 4 and 6 at%) and constant Ho3+ (1 at%) contents and co-doped with Bi3+ (2 at%) were prepared using the glycine-assisted sol–gel method. The pure orthorhombic SrGd2O4 phase, space group Pnma, was revealed using X-ray diffraction in all samples. Transmission electron microscopy discovered agglomerated clusters of spherical particles measuring approximately 150 nm in size. The inclusion of Bi3+ ions into the structure influenced the morphology since the formation of larger grains was observed, with sizes reaching up to ∼1.7 μm. The uniform distribution of all constitutive elements was confirmed by energy-dispersive X-ray spectroscopy. In all samples, up-conversion emission spectra revealed two dominant green (540 and 550 nm), red (671 nm), and infrared (758 nm) emission lines, due to the 5F4,5S2→5I8, 5F5→5I8, and 5F4,5S2 → 5I7 transitions, respectively. The sample co-doped with Bi3+ showed the most intense photoluminescent emissions, shorter luminescence decay, and the highest quantum yield. Additionally, a significant decrease in the energy band gap value was detected using diffuse reflectance spectroscopy for this sample. Methylene blue was used as a test pollutant to investigate its photocatalytic efficiency under simulated sunlight irradiation. The results show that Bi3+ co-doping deteriorates the photocatalytic efficiency of the SrGd2O4:Yb3+/Ho3+ phase by reducing hydroxyl radical formation.