Controlling Photocatalytic Activity via a “Dual Role” Dopant: A Systematic Study of Fe-Doped TiO2 Microspheres with Exposed {001} Facets

This work presents a facile hydrothermal method for synthesizing iron-doped anatase TiO2 hierarchical microspheres composed of embedded single crystals with exposed {001} facets. In this approach, ferric fluoride (FeF3) uniquely serves as both the iron source for doping and a morphology control agent. The influence of a wide range of Fe doping concentrations (from 2.23% to 20.13% atomic ratio) on the phase structure, morphology, optical properties, and photocatalytic activity was systematically investigated. The results show that at low doping levels (<15%), single-phase anatase microspheres are formed, with the constituent nanosheets becoming progressively thinner as the dopant amount increases. At higher concentrations, a phase transition occurs, yielding mixed phases of anatase and rutile (17.46% Fe) or rutile and an unidentified phase (20.13% Fe). Photocatalytic activity tests revealed that low Fe content (2.23% and 5.38%) enhanced the generation of hydroxyl radicals compared to undoped TiO2, while higher concentrations led to decreased activity. This non-monotonic trend is attributed to the “dual role” of the iron dopant: (1) morphology control by fluoride ions, which increases the exposure of active {001} facets; (2) Fe-induced charge separation at low concentrations, which enhances carrier lifetime, and Fe-induced recombination centers at high concentrations, which diminishes photocatalytic efficiency. This work provides a novel strategy for precisely tuning the morphology and electronic structure of TiO2 and offers insights into the complex role of dopants in photocatalysis.