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Nitrogen doping represents an important strategy to modulate the optical, magnetic and photochemical properties of oxides for applications spanning from photocatalysis to optoelectronics and spintronics. In this work for the first time zirconium titanate, a material that exhibits many attractive properties, including an excellent dielectric constant, high corrosion resistance, high permittivity at microwave frequencies and excellent temperature stability, has been doped with nitrogen via a wet chemistry method. A detailed description of the geometrical and the electronic structure of the dopant centre has been obtained coupling Electron Paramagnetic Resonance (EPR) spectroscopy and density functional theory (DFT) calculations. Insertion of nitrogen impurities into the ZrTiO4 lattice modifies the optical properties causing an appreciable absorption in the visible range. The joint analysis of the EPR evidence and the DFT elaboration indicates that the nitrogen impurities preferentially occupy an interstitial position of the lattice generating intra-band gap states about 1 eV above the valence band edge that are responsible for the visible light absorption. The majority of the intra-band gap states are diamagnetic (N−) while a minor fraction is paramagnetic (N˙). These centres are photosensitive in that the ratio between N− and N˙ is modified, upon irradiation, because of electron excitation from the intra-band gap states to the conduction band.

Mixed solid system involving cerium and zirconium titanate (ZrTiO4) have been prepared using the sol-gel technique. Both X-ray diffraction and DFT calculations firmly indicate that, till a doping level of 10 mol%, cerium ions are dissolved in the titanate matrix (which has the scrutynite structure, analogous to those of the main TiO2 polymorphs) occupying the cationic sites and progressively altering its cell parameters. Cerium is hosted in the matrix both in the form of Ce4+ and Ce3+ ions (XPS results). The trivalent state seems to be favoured even though the state of the dopant depends on the treatment undergone by the material. DFT calculations describe the intra-band gap states formed in both cases and the strong localisation of the single electron in the case of Ce3+ (4f1). Differently from the case of Ce doped ZrO2, that shows photoactivity in the visible light because of the presence of cerium, the doped titanate is inactive in the same conditions. Under UV–vis illumination charge separation occurs (EPR results) and the low-loading doped systems (0.5%, 1%) form OH radicals, detected by spin trapping, more efficiently than the pristine matrix. The absence of photoactivity in the visible range is interpreted in terms of the detrimental role (charge recombination) played by both the occupied intra-band gap states associated to Ce3+ and the corresponding oxygen vacancies formed in the lattice by charge compensation.

Fluorine-doped titanium dioxide was prepared via sol-gel synthesis and subsequent calcination in air. The presence of fluorine in the lattice induces the formation of reduced Ti3+ centers that localize the extra electron needed for charge compensation and are observed by electron paramagnetic resonance. Density functional theory calculations using hybrid functionals are in full agreement with such description. The extra electron is highly localized in a 3d orbital of titanium and lies a few tenths of an electron volt below the bottom of the conduction band. The preparation via sol-gel synthesis using aqueous solutions of fluorides also causes the formation of surface F- ions that substitute surface hydroxyl groups (OH -) without generating reduced centers. © 2008 American Chemical Society.

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 parameters of various models with the 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 with the unpaired electron is concentrated in an O p-orbital with a modest delocalization toward the first neighboring ions.

Nitrogen boron co-doped TiO2 prepared via sol-gel synthesis and active under visible light, contains two types of paramagnetic extrinsic defects, both exhibiting a well resolved EPR spectrum. The first center is the well characterized [NiO](center dot) species (i = interstitial) also present in N-doped TiO2, while the second one involves both N and B. This latter center (labeled [NOB](center dot)) exhibits well resolved EPR spectra obtained using either 14 N or 15 N which show a high spin density in a N 2p orbital. The structure of the [NOB](center dot) species is different from that previously proposed in the literature and is actually based on the presence of interstitial N and B atoms both bound to the same lattice oxygen ion. The interstitial B is also linked to two other lattice oxygen ions reproducing the trigonal planar structure typical of boron compounds. The energy level of the [NOB](center dot) center lies near the edge of the valence band of TiO2 and, as such, does not contribute to the visible light absorption. However, [NOB](center dot) can easily trap one electron generating the [NOB](center dot) diamagnetic center which introduces a gap state at about 0.4 eV above the top of the valence band. This latter species can contribute to the visible light activity.

N-doped titanium dioxide has been prepared generating a nitrogen plasma in the atmosphere over a sample of pre-reduced TiO(2). This solid, investigated by EPR and XPS, is similar to materials prepared via sol-gel or different chemical methods. The cleaner preparation method, however, avoids complications in the assignments due to by-products of the chemical synthesis. The experiment unambiguously confirms previous theoretical forecast and indicates the formation of an interstitial paramagnetic nitrogen species already described in the case of sol-gel prepared materials. The energy of this species lies slightly over the valence band limit and well below the Ti(3+) states of reduced TiO(2). (C) 2009 Elsevier B. V. All rights reserved.

Recent experiments have indicated that titanium dioxide (TiO(2)) codoped with nitrogen and fluorine may show enhanced photocatalytic activity in the visible region with respect to TiO(2) doped only with nitrogen. Prompted by these findings, we have investigated N-F codoped TiO(2) through a combined theoretical and experimental study. Density functional theory (DFT) calculations have been carried out both within the generalized gradient approximation (GGA) and using hybrid functionals to accurately describe the electronic structure; substitutional as well as interstitial locations of nitrogen in the TiO(2) lattice were considered. From these calculations we infer that N-F codoping reduces the. energy cost of doping and also the amount of defects (number of oxygen vacancies) in the lattice, as a consequence of the charge compensation between the nitrogen (p-dopant) and the fluorine (n-dopant) impurities. The UV-visible spectra of the sol-gel prepared TiO(2) powders confirm the synergistic effect of N-F codoping: more impurities are introduced in the lattice with an increased optical absorption in the visible. EPR spectroscopy measurements on the codoped samples identify two paramagnetic species which are associated to bulk N impurities (N(b)(center dot)) and Ti(3+) ions. Preliminary photocatalytic tests also indicate an enhanced activity under vis-light irradiation toward degradation of methylene blue for the codoped system with respect to N-doped TiO(2).

Nitrogen doped titanium dioxide is attracting a continuously increasing attention because of its potential as material for environmental photocatalysis. In this paper we review experimental and theoretical work done on this system in our groups in recent years. The analysis is largely based on electron paramagnetic resonance (EPR) spectra and on their interpretation based on high-level ab initio calculations. N-doped anatase TiO2 contains thermally stable single N-atom impurities either as charged diamagnetic N-b(-) centers or as neutral paramagnetic N-b(.) centers (b stays for bulk). The N-atoms can occupy both interstitial or substitutional positions in the solid, with some evidence for a preference for interstitial sites. All types of N-b centers give rise to localized states in the band-gap of the oxide, thus accounting for the related reduction of absorption band edge. The relative abundance of these species depends on the oxidation state of the solid. In fact, upon reduction, oxygen vacancies form and transfer electrons from Ti3+ ions to the N-b(.) with formation of Ti4+ and N-b(.). EPR spectra measured under irradiation show that the Nb centers are responsible for visible light absorption with promotion of electrons from the localized N-impurity states to the conduction band or to electron scavengers like O-2 adsorbed on the surface. These results provide an unambiguous characterization of the electronic states associated with N-impurities in TiO2 and a realistic picture of the processes occurring in the solid under irradiation with visible light.

Electron paramagnetic resonance (EPR), X-ray photoelectron spectroscopy (XPS), and density functional theory (DFT) calculations are combined for the first time in an effort to characterize the paramagnetic species present in N-doped anatase TiO2 powders obtained by sol-gel synthesis. The experimental hyperfine coupling constants are well reproduced by two structurally different nitrogen impurities: substitutional and interstitial N atoms in the TiO2 anatase matrix. DFT calculations show that the nitrogen impurities induce the formation of localized states in the band gap. Substitutional nitrogen states lie just above the valence band, while interstitial nitrogen states lie higher in the gap. Excitations from these localized states to the conduction band may account for the absorption edge shift toward lower energies (visible region) observed in the case of N-doped TiO2 with respect to pure TiO2 (UV region). Calculations also show that nitrogen doping leads to a substantial reduction of the energy cost to form oxygen vacancies in bulk TiO2. This suggests that nitrogen doping is likely to be accompanied by oxygen vacancy formation. Finally, we propose that the relative abundance of the two observed nitrogen-doping species depends on the preparation conditions, such as the oxygen concentration in the atmosphere and the annealing temperature during synthesis.