Fe³⁺-doped Anatase TiO₂ with d–d Transition, Oxygen Vacancies and Ti³⁺ Centers: Synthesis, Characterization, UV–vis Photocatalytic and Mechanistic Studies

The current work emphasizes the preparation, characterization, recyclability, stability, and mechanistic study of nanosized Fe3+-doped TiO2 photocatalyst. The structural, optical, and photocatalytic properties of undoped and doped TiO2 were studied by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), N2 absorption–desorption, UV–vis diffuse reflectance spectroscopy (DRS), photoluminescence (PL), electron spin resonance (ESR), X-ray photoelectron spectroscopy (XPS), Raman and UV–visible spectroscopy. XRD analysis showed that prepared powders with different iron content (200, 100, 50, and 25 molar ratios) consisted of only anatase phase. FTIR study confirmed the chelation of acetate with titanium precursor through the bidentate bridge mode; as a result, the condensation pathways are effectively altered by the acetate ligands favoring the formation of the anatase phase, this result gives further confirmation to the XRD analysis. A decrease in charge carrier recombination rate and the presence of oxygen vacancies and related Ti3+ centers in the prepared photocatalysts were confirmed through PL and EPR spectroscopic studies. XPS results have indicated the presence of dopant electronic states (Fe3+, Fe2+ and Fe4+), which could be due to substitution of Fe3+ ions in-place of Ti4+ in the crystal lattice. UV–vis DRS spectrum showed that undoped TiO2 exhibits an absorption edge in the UV region, the position of which was shifted toward the visible region on incorporation of Fe3+ into it. This red shift of the optical absorption in doped TiO2 was the outcome of d–d transition of Fe3+ (2T2g → 2A2g, 2T1g) and the charge transfer transition between interacting iron ions (Fe3+ + Fe3+ → Fe4+ + Fe2+). These Fe3+ 3d states in addition to oxygen vacancies and Ti3+ centers create band states, thereby favoring the electronic transition to these levels and resulting in narrowing of TiO2 band gap. A direct confirmation is the increase in the magnitude of Urbach energy with the lowering in the band gap of Fe3+-TiO2. The production of hydroxyl radicals (OH– + h+ → OH•) which are the main scavengers for the photogenerated holes (h+) was monitored by a PL technique using terephthalic acid (TA). The observed trend was TFe50 > TFe100 > TFe25 > TFe200 > TiO2, implying that the TFe50 powder produced an enhanced amount of OH• radicals under light irradiation, which helps in its highest photocatalytic activity against the degradation of methylene blue and 4-chlorophenol under UV and visible light irradiation.