Photocatalytic activities of europium (III) and niobium (V) co-doped TiO₂ nanopowders synthesized in Ar/O₂ radio-frequency thermal plasmas

Europium (III) and niobium (V) co-doped TiO₂ nanopowders were synthesized in Ar/O₂ radio-frequency thermal plasmas by pyrolyzing aqueous precursors that contained various concentrations of Eu ³⁺/(Eu³⁺ + Nb⁵⁺ + Ti⁴⁺) = 0-0.5 at.% and Nb⁵⁺/(Eu³⁺ + Nb⁵⁺ + Ti⁴⁺) = 0-1.0 at.%. Phase identification was performed by X-ray diffraction (XRD), particle morphology was observed by transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM), and UV-vis absorption spectra were determined by UV-vis spectroscopy. The photocatalytic activities under UV and visible light irradiations were both evaluated by bleaching 20 μM of a methyl orange aqueous solution. All the resulting powders had a phase composition of anatase (30-66 nm crystalline size) (major phase: ∼80% mass proportion) and rutile (56-94 nm crystalline size) polymorphs, with a relatively wide particle size distribution from several nanometers (faceted crystallites) to ∼100 nm (spherical particles). The photocatalytic activities dominated by oxygen defects in the TiO₂ host lattice for the powders prepared with identical doping concentrations of Eu³⁺ and Nb⁵⁺ were both superior under UV and visible light irradiations over those obtained with different doping amounts. More interestingly, the co-doped powder exhibited an improved photocatalytic performance under 600-700 nm visible light illumination than the Eu³⁺-doped TiO₂ and P25 commercial powders.