Al- and Ga-Doped TiO2, ZrO2, and HfO2: The Nature of O 2p Trapped Holes from a Combined Electron Paramagnetic Resonance (EPR) and Density Functional Theory (DFT) Study

Abstract

The nature of hole centers in a series of MeO2 (TiO2, ZrO2, HfO2) metal oxides doped with trivalent Al or Ga ions has been investigated coupling the classic continuous wave electron paramagnetic resonance (CW-EPR) technique with advanced density functional theory (DFT) calculations. The insertion of an aliovalent ion in the structure of the tetravalent oxides is compensated by the creation of oxygen vacancies leading to diamagnetic defective systems. The hole centers are observed by EPR after irradiation using ultraviolet (UV) frequencies (with consequent formation of an electron–hole pair) and trapping of the photogenerated electron. The distortion imparted by the presence of the dopant stabilizes these centers. This generates a rich superhyperfine structure, since the dopants employed in this investigation (Al and Ga) have a nonzero nuclear spin. The DFT calculations performed on a wide set of possible hole-trapping sites occurring in the solid, allow us to identify (comparing the calculated EPR paramet...