Search

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.

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.

Nitrogen-doped titanium dioxide (N-TiO2), a photocatalytic material active in visible light, has been investigated by a combined experimental and theoretical approach. The material contains singleatom nitrogen impurities that form either diamagnetic (Nb -) or paramagnetic (Nb ¥) bulk centers. Both types of Nb centers give rise to localized states in the band gap of the oxide. The relative abundance of these species depends on the oxidation state of the solid, as, upon reduction, electron transfer from Ti3+ ions to Nb ¥ results in the formation of Ti4+ and Nb -. EPR spectra measured under irradiation show that Nb centers are responsible for visible light absorption with promotion of electrons from the band gap localized states to the conduction band or to surface-adsorbed electron scavengers. These results provide a characterization of the electronic states associated with N impurities in TiO2 and, for the first time, a picture of the processes occurring in the solid under irradiation with visible light