Oxygen vacancies adjacent to Cu2+ ions in TiO2 (rutile) crystals

Abstract

Electron paramagnetic resonance (EPR) and electron-nuclear double resonance (ENDOR) are used to characterize Cu2+ ions substituting for Ti4+ ions in nominally undoped TiO2 crystals having the rutile structure. Illumination at 25 K with 442 nm laser light reduces the concentration of Cu2+ ions by more than a factor of 2. The laser light also reduces the EPR signals from Fe3+ and Cr3+ ions and introduces signals from Ti3+ ions. Warming in the dark to room temperature restores the crystal to its preilluminated state. Monitoring the recovery of the photoinduced changes in the Cu2+ ions and the other paramagnetic electron and hole traps as the temperature is raised from 25 K to room temperature provides evidence that the Cu2+ ions have an adjacent doubly ionized oxygen vacancy. These oxygen vacancies serve as charge compensators for the substitutional Cu2+ ions and lead to the formation of electrically neutral Cu2+-VO complexes during growth of the crystals. The Cu2+-VO complexes act as electron traps and convert to nonparamagnetic Cu+-VO complexes when the crystals are illuminated at low temperature. Complete sets of spin-Hamiltonian parameters describing the electron Zeeman, hyperfine, and nuclear electric quadrupole interactions for both the 63Cu and 65Cu nuclei are obtained from the EPR and ENDOR data. This study suggests that other divalent cation impurities in TiO2 such as Co2+ and Ni2+ may also have an adjacent oxygen vacancy for charge compensation.