Defect Engineering and Phase Modulation in Fe-Doped TiO2 Nanocrystals: Unraveling the Origin of Enhanced Visible-Light Absorption

Engineering lattice distortion and electronic structure is a promising strategy for tailoring the functional properties of semiconductor nanocrystals. Herein, Fe-doped TiO2 nanocrystals containing 0–1.0 wt % Fe were synthesized via a wet-chemical route to investigate the interplay between Fe incorporation, phase evolution, and optical properties. X-ray diffraction and Rietveld refinement revealed anatase/brookite heterophase nanocrystals without detectable secondary iron oxide phases, indicating no discernible phase segregation. Fe incorporation promoted brookite stabilization at low dopant concentrations and systematically modified crystallite size, crystallinity, and lattice microstrain. Raman and FTIR analyses revealed increasing local disorder and distortion of the TiO6 coordination environment. These structural changes were accompanied by enhanced visible-light absorption and a reduction in the optical band gap from 3.00 to 2.60 eV. Crystal-field analysis yielded a splitting energy of ∼1.92 eV, supporting Fe3+ ions in distorted octahedral oxygen environments. Notably, the correlation between lattice microstrain and band gap energy demonstrates the role of dopant-induced structural distortion in modulating the electronic structure of TiO2. These findings establish controlled Fe incorporation as an effective approach for tuning the structural and optical properties of TiO2 nanocrystals for visible-light-driven photocatalysis, optoelectronics, and solar energy applications.