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.