Photoreactivity improvement of TiO₂/Sr₄Al_l4O₂₅:Eu,Dy heterojunction photocatalyst by Fe doping and annealing in N₂–H₂ mixed gas

Sr₄Al_l4O₂₅:Eu,Dy phosphor beads were impregnated with Fe, followed by TiO₂ sol–gel coating. The heat treatment atmospheres after TiO₂ coating were N₂, N₂–H₂ mixed gas, and air. X-ray diffraction, scanning electron microscopy, and energy dispersive analysis confirmed the crystallinity, spherical nano-TiO₂ morphology, and elemental composition of the Fe-doped TiO₂/Sr₄Al_l4O₂₅:Eu,Dy phosphor beads. X-ray photoelectron spectroscopy analysis verified the existence of oxygen-deficient Ti^δ+ due to the Fe doping and reducing atmosphere. Additionally, the thermal treatment of Sr₄Al₁₄O₂₅:Eu,Dy/TiO₂ beads in the N₂–H₂ reducing atmosphere enhanced N doping in the TiO₂ matrix. The red shift of the diffuse absorption spectra increased with the amount of Fe doped on phosphor, indicating heterojunction narrowing in Sr₄Al₁₄O₂₅:Eu,Dy/Fe-TiO₂. Photoluminescence emission deteriorated with increasing Fe doping and N₂ heat treatment. However, the N₂–H₂ mixed gas preserved up to 90% of the photoluminescence intensities regardless of the amount of Fe dopant on the phosphor/TiO₂ bead surface. The photoreactivity performance was evaluated using a toluene volatile organic compound as pollutant under 30 lx ultraviolet (UV) and 1200 lx visible light sources. The 0.06 M Fe-impregnated Sr₄Al₁₄O₂₅:Eu,Dy/TiO₂ beads heat treated in a N₂–H₂ reducing atmosphere exhibited outstanding photocatalytic activity of up to above 95% for 60 min in both UV and visible light activated reactions. Moreover, the 0.06 M Fe-doped Sr₄Al₁₄O₂₅:Eu,Dy/TiO₂ phosphor beads demonstrated plausible reusability with photocatalytic efficiencies above 90% in five repeated test cycles.