Ground State of the Singly Ionized Oxygen Vacancy in Rutile TiO\u3csub\u3e2\u3c/sub\u3e

Results from electron paramagnetic resonance (EPR) and electron-nuclear double resonance (ENDOR) experiments are used to establish the model for the ground state of the singly ionized oxygen vacancy in the interior of bulk rutile TiO2 crystals. Hyperfine from 47Ti and 49Ti nuclei show that the unpaired electron in this S = 1/2 defect is localized on one titanium ion adjacent to the oxygen vacancy (i.e., the spin is not shared by two titanium ions). These defects are formed at low temperature (∼35 K) in as-grown oxidized crystals when sub-band-gap 442 nm laser light converts doubly ionized nonparamagnetic oxygen vacancies to the singly ionized paramagnetic charge state. The g matrix is obtained from EPR spectra and the 47Ti and 49Ti hyperfine and nuclear electric quadrupole matrices (A and Q) are obtained from ENDOR spectra. Principal values of the 47Ti and 49Ti hyperfine matrices are 64.54, 11.57, and 33.34 MHz. All the matrices have a principal axis along the [001] direction. In the basal plane, principal axes of the hyperfine and quadrupole matrices also coincide. The principal axes of the g matrix in the basal plane, however, deviate significantly from those of the A and Q matrices, thus indicating mixing of d orbitals due to the low symmetry at the Ti3+ ion site and participation of excited-state orbitals

Triplet Ground State of the Neutral Oxygen-vacancy Donor in Rutile TiO\u3csub\u3e2\u3c/sub\u3e

Electron paramagnetic resonance (EPR) is used to investigate the triplet (S = 1) ground state of the neutral oxygen vacancy in bulk rutile TiO2 crystals. This shallow donor consists of an oxygen vacancy with two nearest-neighbor, exchange-coupled 3+ ions located along the [001] direction and equidistant from the vacancy. The spins of the two trapped electrons, one at each 3+ ion, align parallel to give the S = 1 state. These neutral oxygen vacancies are formed near 25 K in as-grown oxidized TiO2 crystals by illuminating with sub-band-gap 442 nm laser light. The angular dependence of the EPR spectra provides the principal values and axes for the g and D matrices. Observations of the Ti and Ti hyperfine lines when the magnetic field is along high-symmetry directions show that the two 3+ ions are equivalent; i.e., they have equal hyperfine A matrices. The A matrix for each 3+ ion in the neutral S = 1 oxygen vacancy is approximately half of the A matrix reported earlier for the one 3+ ion in the singly ionized S = 1/2 oxygen vacancy [Brant et al., J. Appl. Phys. 114, 113702 (2013)]. The neutral oxygen vacancies are thermally unstable above 25 K. They release an electron to the conduction band with an activation energy near 63 meV and convert to singly ionized S = 1/2 oxygen vacancies. When undoped TiO2 is sufficiently oxygen deficient (i.e., reduced), this combination of conduction band electrons and singly ionized oxygen vacancies may result in carrier-mediated ferromagnetism at room temperature

Synthesis and Characterization of Rutile TiO2Nanopowders Doped with Iron Ions

Titanium dioxide nanopowders doped with different amounts of Fe ions were prepared by coprecipitation method. Obtained materials were characterized by structural (XRD), morphological (TEM and SEM), optical (UV/vis reflection and photoluminescence, and Raman), and analytical techniques (XPS and ICP-OES). XRD analysis revealed rutile crystalline phase for doped and undoped titanium dioxide obtained in the same manner. Diameter of the particles was 5–7 nm. The presence of iron ions was confirmed by XPS and ICP-OES. Doping process moved absorption threshold of TiO2into visible spectrum range. Photocatalytic activity was also checked. Doped nanopowders showed normal and up-converted photoluminescence

Radiationless deactivation of an intramolecular charge transfer excited state through hydrogen bonding: Effect of molecular structure and hard-soft anionic character in the excited state

Energy-gap dependency for radiationless deactivation from excited states of various molecules having strong intramolecular charge transfer (ICT) character has been investigated by observing fluorescence quenching on addition of alcohols. Molecules having strong ICT excited states were classified into three groups: (a) molecules that underwent considerable fluorescence quenching by ethanol (quenching constant, KSV > 20 M-1) and for which radiationless deactivation in protic solvents was much faster than anticipated from the ordinary energygap law observed in aprotic solvents, (b) molecules whose fluorescence exhibited substantial red shifts, and (c) molecules whose fluorescence were barely affected by the addition of ethanol (KSV < 1 M-1) and for which the energy-gap dependences on radiationless deactivation in protic solvents were not so different from those in aprotic solvents. Typical fluorophores for each case, i.e., a, b, and c, were aminoanthraquinone, aminophthalimide, and aminocoumarin, respectively. Differences in the fluorescence quenching phenomena are discussed in terms of the molecular structure and the hard-soft anionic character of the excited states, governed by changes in charge density on the carbonyl oxygen. An excited molecule having a hard anionic character on a specific site within the molecule, classified as group a, was concluded to undergo considerable fluorescence quenching through an intermolecular hydrogen bonding interaction with an alcohol having a hard cationic character. On the other hand, fluorescence of an excited molecule having a soft anionic character, classified as group c, cannot be quenched well by an alcohol because of the weak interaction on the carbonyl oxygen. The anomalous behavior of the excited aminophthalimides (group b), which are classified as hard anions but do not undergo fluorescence quenching, suggested the possibility that molecular rigidity is another factor controlling the radiationless deactivation process induced by hydrogen bonding