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 MeO 2 (TiO 2 anatase, ZrO 2 , HfO 2 ) metal oxides doped with three-valent 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 UV frequencies (with consequent formation of an electron-hole pair) and trapping of the photogenerated electron. The hole centers formed in this way are, in the majority of cases, characterized by the presence of one dopant atom (Al or Ga) in the first coordination sphere of the oxygen ion that traps the hole. 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 non-zero 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 parameters of various models with experimental ones) the nature of the observed hole centers in all cases. These are always three-coordinated oxygen ions with one Al (or Ga) ion in the first coordinative sphere. As it has been observed in other cases of holes centers, the spin density associated to the unpaired electron is concentrated in an oxygen p-orbital with a modest delocalisation towards the first neighbouring ions.3

Origin and Electronic Features of Reactive Oxygen Species at Hybrid Zirconia-Acetylacetonate Interfaces

“…Specifically, spectra similar to that observed for HSGZ are reported in the literature only for crystalline zirconia samples subjected, in order, to (1) activation by a thermal pretreatment in vacuo and (2) contact with oxygen at high temperature or under UV irradiation. 7,8,10,11 In contrast, HSGZ samples analyzed in this work were simply produced at room temperature in the presence of air and kept in the dark for at least 12 h before EPR measurements.…”

Three‐dimensional mapping of crystalline ceramic waste form materials

“…A slight contrast change exhibited by the TiO 2 indicates that Ga incorporated into both phases. Aliovalent Ga doping, Ga +3 on Ti +4 sites is well known in these materials and low Ga levels in TiO 2 were expected to result from the diffusion of Ga during high‐temperature sintering . The very bright region labeled “Platinum” is a thin layer of Pt, the material used to secure the samples onto the tips of pins, which were mounted into the TXM.…”

Communication: Hole localization in Al-doped quartz SiO2 within ab initio hybrid-functional DFT

“…In fact, in order to measure the O hyperfine constants, 17 O enriched samples have to be prepared with complex and costly procedures. 50…”