Mechanistic insights and performance benchmarking of doped semiconductor metal oxides for visible-light photocatalytic dye mineralization

Photocatalytic mineralization of synthetic dyes is a viable approach to wastewater treatment, but conventional semiconductor metal oxides such as TiO₂ (3.2 eV), ZnO (3.37 eV), and SnO₂ (3.6 eV) are constrained by wide band gaps that restrict photon absorption to the ultraviolet region, which accounts for less than 5% of the solar spectrum. This review demonstrates that rare-earth (RE) doping, governed by the 4f electronic configurations of lanthanide ions, addresses this limitation through multiple concurrent mechanisms. Across five host lattices (TiO₂, ZnO, SnO₂, CeO₂, and ZrO₂), RE incorporation narrows effective optical band gaps, extends photogenerated charge-carrier lifetimes through shallow f-state trapping, and enhances dye-surface interactions via Lewis acid-type RE3+ coordination. The review mechanistically distinguishes band-gap narrowing from mid-gap state formation as separate pathways to visible-light activity, a distinction routinely conflated in the literature, and provides a diagnostic framework based on Tauc analysis, photoluminescence spectroscopy, and photoelectrochemical measurements for their differentiation. Performance is evaluated through kinetic rate constants, quantum yields, and energy efficiency metrics rather than degradation percentage reporting, which obscures cross-study comparison.