Charge trapping in TiO2 polymorphs as seen by Electron Paramagnetic Resonance Spectroscopy

Electron Paramagnetic Resonance techniques (EPR) have been employed to investigate charge carriers trapping in the two main TiO2 polymorphs, anatase and rutile, with particular attention to the features of electron trapping sites (formally Ti3+ ions). The classic CW-EPR technique in this case provides signals based on the g tensor only. Nevertheless a systematic analysis of the signals obtained in the various cases (anatase and rutile, surface and bulk centers, regular and defective sites) have been performed providing useful guidelines in a field affected by some confusion. The problem of the localization of the electron spin density have been tackled by means of Pulse-EPR hyperfine techniques on samples appositely enriched with 17O. This approach has led to evidence a substantial difference, in terms of wavefunction localization between anatase (electrons trapped in regular lattice sites exhibiting delocalized electron density) and rutile (interstitial sites showing localized electron density).