On the Nature of Reduced States in Titanium Dioxide As Monitored by Electron Paramagnetic Resonance. I: The Anatase Case

A systematic analysis of the reduced states in the titanium dioxide matrix (anatase polymorph) has been performed coupling the classic continuous wave electron paramagnetic resonance (CW-EPR) with advanced pulse-EPR techniques and introducing the O-17 magnetic isotope into the solid. Reduced states were originated in various ways including valence induction via aliovalent elements (F, Nb) and reducing treatments of the bare oxide including surface reaction with reducing agents (H, Na) and thermal annealing under vacuum with consequent oxygen depletion. Two main paramagnetic species were identified via EPR both amenable to Ti3+ ions. The former (EPR signal A: axial symmetry with g(parallel to) = 1.962 and g(perpendicular to) = 1.992) is observed in all case and has been conclusively assigned to reduced Ti3+ centers in regular lattice sites of the anatase matrix; the second (signal B: broad line centered at g = 1.93) is present only in reduced materials and is assigned, on the basis of the analysis of the hyperfine interaction of the centers with O-17 labeled ions in its environment, to a collection of slightly different Ti3+ centers located at the surface, or in the subsurface region. The hyperfine interaction of the lattice Ti3+ centers corresponding to signal A with O-17 was investigated by HYSCORE spectroscopy and resulted in a maximum hf coupling on the order of 2 MHz, which is nearly one order of magnitude less than that recently measured for reduced centers in rutile. This surprising result suggests that excess electrons corresponding to signal A are not localized on a single ion but are likely delocalized on several analogous titanium lattice ions. This result (compatible with recent theoretical calculations) has relevance with respect to the living debate about localization and delocalization of electrons in titania, which has been based, up to now, on conflicting evidence.