Maria Cristina Paganini

Generation of superoxide ions at oxide surfaces

The superoxide radical anion O2- is both an important intermediate in heterogeneous catalytic oxidation and a useful probe for positive charges in ionic solids, such as metal oxides and zeolites. The paper illustrates the main circumstances under which stable superoxide anions are formed at surfaces: (i) direct surface–oxygen electron transfer; (ii) photoinduced electron transfer; (iii) surface intermolecular electron transfer; (iv) decomposition of hydrogen peroxide.

Charge trapping in TiO2 polymorphs as seen by Electron Paramagnetic Resonance spectroscopy

Electron Paramagnetic Resonance (EPR) techniques have been employed to investigate charge carrier trapping in the two main TiO2 polymorphs, anatase and rutile, with particular attention to the features of electron trapping sites (formally Ti(3+) ions). The classic CW-EPR technique in this case provides signals based on the g tensor only. Nevertheless a systematic analysis of the signals obtained in the various cases (anatase and rutile, surface and bulk centers, regular and defective sites) has been performed providing useful guidelines on a field affected by some confusion. The problem of the localization of the electron spin density has been tackled by means of Pulse-EPR hyperfine techniques on samples appositely enriched with (17)O. This approach has led to evidence of a substantial difference, in terms of wavefunction localization between anatase (electrons trapped in regular lattice sites exhibiting delocalized electron density) and rutile (interstitial sites showing localized electron density).

Colour centres at the surface of alkali-earth oxides. A new hypothesis on the location of surface electron traps

Abstract Irradiation of highly dehydrated MgO by UV light in the presence of surface adsorbed hydrogen leads to the formation of particular types of surface colour centres indicated with F + S (H) (one-electron, paramagnetic) and F S (H) (two electrons, diamagnetic). F S centres are the surface counterparts of the well-known F colour centres formed in the bulk of ionic solids by high-energy irradiation or metal addition. In the particular case of F + S (H), the unpaired electron is in magnetic interaction with a nearby proton belonging to a hydroxyl group deriving from H 2 heterolytic dissociative chemisorption (H 2 →H + +H − ) and consequent H + stabilization on a surface oxide ion. The joint use of EPR, FT-IR and DR-UV–vis spectroscopies has allowed clarification of the mechanism of formation of these centres, which is based on the ionization of adsorbed hydryde groups by UV light and stabilization of the ionized electron into suitable positively charged surface electron traps. A fraction of these traps coincides with the site capable of stabilizing the hydride ion (in the form of bridged Mg 3 H), which is built up by an array of three Mg 2+ ions reproducing a (111) facelet of the oxide. The same site can be also seen as a O 2− 3c vacancy (3-coordinated surface anionic vacancy). Ab-initio quantum chemical calculations confirm the proposed assignment, which goes beyond the original model of 5-coordinated surface anion vacancies at the flat (100) MgO plane, which is thus left in favour of a new model that describes the surface electron traps as being localized in less coordinated regions of the surface.

The nature of paramagnetic species in nitrogen doped TiO2 active in visible light photocatalysis.

Nitrogen doped TiO2, a novel photocatalyst active in the decomposition of organic pollutants using visible light, contains two different types of paramagnetic centres (neutral NO radicals and NO2(2-) type radical ions respectively) which are likely related to specific properties of the solid.

The nitrogen–boron paramagnetic center in visible light sensitized N–B co-doped TiO2. Experimental and theoretical characterization

Nitrogen boron co-doped TiO(2) 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 [N(i)O]˙ species (i = interstitial) also present in N-doped TiO(2), while the second one involves both N and B. This latter center (labeled [NOB]˙) 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]˙ 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 lies near the edge of the valence band of TiO(2) and, as such, does not contribute to the visible light absorption. However, [NOB]˙ can easily trap one electron generating the [NOB](-) 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.

TiO2@CeOx core-shell nanoparticles as artificial enzymes with peroxidase-like activity.

The Ce4+↔Ce3+ redox switch is at the basis of an all-inorganic catalytic cycle that is capable of mimicking the activity of several natural redox enzymes. The efficiency of these artificial enzymes (nanozymes) strongly depends on the Ce4+/Ce3+ ratio. By capitalizing on the results obtained on oxide/oxide model systems, we implemented a simple and effective procedure to obtain conformal TiO2@CeOx core-shell nanoparticles whose thickness is controlled with single-layer precision. Since the Ce3+ species are stabilized only at the interface by the electronic hybridization with the TiO2 states, the modulation of the shell thickness offers a simple method to tailor the Ce4+/Ce3+ ratio and therefore the catalytic properties. The activity of these nanoparticles as artificial peroxidase-like enzymes was tested, showing exceptional performances, even better than natural horseradish peroxidase enzyme. The main advantage with respect to other oxide/oxide nanozymes is that our nanoparticles, having a tunable Ce4+/Ce3+ ratio, are efficient already at low H2O2 concentrations.

Probing the Local Environment of Ti3+ Ions in TiO2 (Rutile) by 17O HYSCORE

Reduced states in TiO(2) : (17)O hyperfine sublevel correlation spectroscopy was used to monitor the local environment of stable Ti(3+) ions generated in a (17)O-enriched polycrystalline TiO(2) (rutile) sample. A hyperfine interaction of about 8 MHz is found, which is analogous to that observed for molecular Ti(3+) aqua complex cations and suggests a localized nature of the unpaired electron wave function for these centers at 4 K.

Cerium-Doped Zirconium Dioxide, a Visible-Light-Sensitive Photoactive Material of Third Generation

The dispersion of small amounts of Ce(4+) ions in the bulk of ZrO2 leads to a photoactive material sensitive to visible light. This is shown by monitoring with EPR the formation and the reactivity of photogenerated (λ > 420 nm) charge carriers. The effect, as confirmed by DFT calculations, is due to the presence in the solid of empty 4f Ce states at the mid gap, which act as intermediate levels in a double excitation mechanism. This solid can be considered an example of a third-generation photoactive material.

Trapped molecular species in N-doped TiO2

Nitrogen-doped TiO2, a novel photocatalyst active in the decomposition of organic pollutants using visible light, contains several different types of paramagnetic centers. These are molecular species, such as NO and NO2 radicals and other species, deeply interacting with the TiO2 structure. All or part of these species is related to specific properties of the solid. Electron paramagnetic resonance has been employed to characterize the N-containing paramagnetic species present in N-doped anatase TiO2 powders obtained via sol-gel synthesis. In the present work attention is focused on molecular species generated during the synthesis process and segregated in cavities of the TiO2 structure.

Structural and spectroscopic characterization of CeO2–TiO2 mixed oxides

Mixed CeO2–TiO2 systems have been synthesized using the sol–gel technique in a symmetric range of nominal compositions (10, 50 and 90 mol% CeO2). The solid materials were characterised using a variety of diffractometric and spectroscopic techniques. The intimate contact between the two components during the synthesis leads to heterogeneous systems which are based on the presence, besides the two individual oxide phases, of a mixed phase of a cerium titanate (Ce2Ti2O7, pyrochlore structure) which contains Ce3+ ions that impart particular optical properties to the solid (a pronounced red shift with respect to the band gap transition of the two oxides). Ce3+ ions are present at the surface of the systems together with tetravalent Ce and Ti ions. The mixed solids can be reduced by annealing under vacuum and, upon reoxidation under mild conditions with O2, form superoxide species adsorbed on Ce4+ which have, as already reported for low loading CeO2–TiO2 systems, peculiar spectroscopic properties with respect to superoxide adsorbed on bare cerium dioxide.