Zinc oxide hollow spheres decorated with cerium dioxide. The role of morphology in the photoactivity of semiconducting oxides.

The photochemical activity of the recently proposed CeO2-ZnO photocatalytic material active under visible light has been improved by means of significant modifications of its morphology. A polymeric templating agent (Pluronic) has been used in the synthesis obtaining a particle morphology based on hollow spheres that is better defined in the case of high template concentration. The charge separation ability and the light-induced surface electron transfer under irradiation with visible polychromatic light in various ranges of wavelengths has been investigated by electron paramagnetic resonance. The reactivity of the photogenerated holes has been monitored by the spin trapping technique in the presence of DMPO. The hollow spheres morphology achieved through the synthesis here reported leads to systems with a higher photoactivity under visible irradiation than the same system displaying the classic platelets morphology. A parallel increase of the photocatalytic activity of this novel system in pollution remediation reactions is therefore predictable.

Nitrogen-doped semiconducting oxides. Implications on photochemical, photocatalytic and electronic properties derived from EPR spectroscopy.

Engineering defects in semiconducting metal oxides is a challenge that remains at the forefront of materials chemistry research. Nitrogen has emerged as one of the most attractive elements able to tune the photochemical and photocatalytic properties of semiconducting oxides, boosting visible-light harvesting and charge separation events, key elements in promoting solar driven chemical reactions. Doping with nitrogen is also a strategy suggested to obtain p-type conduction properties in oxides showing n-type features in their pristine state and to impart collective magnetic properties to the same systems. Here, we review the evolution in the understanding of the role of nitrogen doping in modifying the photochemical and electronic properties of the most common semiconducting oxides used in mentioned applications including: TiO2, ZnO, SnO2 and zirconium titanates. With an emphasis on polycrystalline materials, we highlight the unique role of Electron Paramagnetic Resonance (EPR) spectroscopy in the direct detection of open-shell N-based defects and in the definition of their structural and electronic properties. Synthetic strategies for the insertion of nitrogen defects in the various matrices are also discussed, along with the influence of the corresponding low-lying energy states on the general electronic properties of the doped solids.

Electron magnetic resonance in heterogeneous photocatalysis research.

The contribution of electron magnetic resonance techniques, and in particular of CW-EPR, to the experimental research on photocatalytic phenomena is illustrated in this paper with selected examples. In the first part of the paper the role of EPR in unravelling the nature and the features of extrinsic point defects in semiconducting oxides is epitomized using the important example of the photoactive nitrogen center in various semiconducting oxides. In the second part we describe how EPR can monitor the processes that follow the initial photoinduced charge separation in photocatalysis, namely the stabilisation, migration and surface reactivity of electrons and holes. Finally, we will discuss how the role of EPR in photocatalysis is not limited to monitor phenomena occurring in the solid or at its surface but it can be extended to the investigation of the liquid phase by employing the spin trapping techniques to monitor the nature and the concentration of the reactive free radicals formed along the photocatalytic process.

Copper-Modified TiO2 and ZrTiO4: Cu Oxidation State Evolution during Photocatalytic Hydrogen Production.

In the present work, two H2 evolution photocatalysts were prepared by employing two different oxides, TiO2 and zirconium titanate (ZrTiO4), as the support of various copper phases. For both the supports the same Cu loading (0.5% w/w) was adopted, but two different impregnation procedures have been followed, leading to different forms of Cu in the final composite material that are: (i) Cu(II) species dispersed on the oxide surface and (ii) Cu2O particles dispersed on the oxide surface. The present paper based on the parallel use of photocatalytic test and spectroscopic analysis performed in catalytic conditions illustrates the evolution of photocatalytic systems occurring during the H2 evolution reaction tests, pointing out that the as-prepared materials represent a pre-catalyst and they are modified during irradiation leading to the real working systems different from the starting ones. The herein presented spectroscopic analysis aims to contribute to the living debate on the oxidation state of copper in mixed Cu/oxide materials and on its role in hydrogen evolution under photocatalytic conditions.

The Existence of Nitrate Radicals in Irradiated TiO2 Aqueous Suspensions in the Presence of Nitrate Ions.

Evidence of the existence of nitrate radical in irradiated aqueous TiO2 suspensions in the presence of nitrate ions are reported for the first time. The joint use of UV/Vis and EPR spectroscopy showed that nitrate radicals are formed by hole induced oxidation of nitrate ions. Photocatalytic degradation of a model alkene compound allowed to highlight the presence of an intermediate organic nitrate deriving from nitrate radical attack to the double bond of the substrate. These results not only allow deeper understanding of photocatalytic processes, but open the route to new green photocatalytic syntheses initiated by nitrate radicals and to new insights in the field of atmospheric chemistry.

Ferromagnetic Interactions in Highly Stable, Partially Reduced TiO2 : The S=2 State in Anatase.

We report direct evidence for quintuplet spin states in a particular kind of reduced TiO2 anatase obtained by the mild oxidation of TiB2 under hydrothermal conditions. Continuous-wave and pulse EPR spectroscopy at X and Q band frequencies provide compelling evidence for the presence of S=2 states, stable in a wide range of temperatures up to room temperature. A tentative model, corroborated by spin-polarized DFT calculations, is proposed, which consists of four ferromagnetically interacting Ti3+ ions with distances ranging from 0.5 nm to 0.8 nm and tetrahedral arrangement.

Mechanism of the Cyclo-Oligomerisation of C2 H2 on Anatase TiO2 (101) and (001) Surfaces and Their Reduction: An Electron Paramagnetic Resonance and Density Functional Theory Study.

Dehydroxylated, hydroxylated and hydrated anatase TiO2 samples have been exposed to acetylene at room temperature. The interaction leads to the formation of polycyclic aromatic hydrocarbons (PAHs) and is accompanied by the appearance of Ti3+ ions, as shown by electron paramagnetic resonance (EPR) spectra. Fully or partly dehydroxylated samples show higher reactivity, whereas the hydrated samples are chemically inert. The experimental results point towards a crucial role of the more reactive (001) facets of anatase nanoparticles. Density functional theory calculations show that acetylene physisorbs on the anatase (101) surface without activation of the C-H bond. The reduced (101) surface (O vacancies) leads to acetylene activation but not to dissociative adsorption. In contrast, the dehydroxylated (001) anatase surface is very active and leads to the spontaneous splitting of the C-H bond with formation of Ti-C2 H and OH groups. This is followed by subsequent additions of C2 H2 molecules with formation of PAHs. During the dissociation of C2 H2 , radical species do not form and electrons are not transferred to the surface because direct Ti-C covalent bonds form on the surface. However, the ring closure in the formation of the aromatic compounds leaves behind hydrogen atoms that donate their valence electrons to the oxide. This results in the appearance of EPR-active Ti3+ centres.

On the redox mechanism operating along C2H2 self-assembly at the surface of TiO2.

The interaction of acetylene with the TiO2 surface at room temperature entails a complex set of self-assembly reactions with the formation of products having relatively high molecular weight. In a previous paper by some of us (Jain, S. M.; et al. J. Mater. Chem. A 2014, 2, 12247-12254), the C2H2-TiO2 reaction has been monitored, essentially by Fourier transform infrared spectroscopy, at the surface of P25 (a mixture of anatase and rutile, typical benchmark material in the field of photocatalysis) in order to elucidate the nature of the products of this surface reaction. In the present paper, the same process was followed, for the first time, using electron paramagnetic resonance (EPR) and monitoring by the thermogravimetric analysis the weight loss of the material upon heating in order to further investigate the complex mechanism of the surface reaction. This was done using pure anatase and comparing the EPR results with those concerning both rutile and P25. The self-assembly mechanism occurring at the interface is accompanied by the formation of EPR visible Ti(3+) centers due to electrons injection in the TiO2 substrate. This finding clarifies that at least one of the reaction channels of this complex process (namely, the formation of polycyclic aromatic hydrocarbons) is based on the heterolytic dissociative chemisorption of acetylene, followed by a redox interaction between the adsorbate and the solid, which allows the creation of the building blocks necessary to assemble polyaromatic molecules.

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).

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.

Hydration structure of the Ti(III) cation as revealed by pulse EPR and DFT studies: new insights into a textbook case.

The (17)O and (1)H hyperfine interactions of water ligands in the Ti(III) aquo complex in a frozen solution were determined using Hyperfine Sublevel Correlation (HYSCORE) and Pulse Electron Nuclear Double Resonance (ENDOR) spectroscopies at 9.5 GHz. The isotropic hyperfine interaction (hfi) constant of the water ligand (17)O was found to be about 7.5 MHz. (1)H Single Matched Resonance Transfer (SMART) HYSCORE spectra allowed resolution of the hfi interactions of the two inequivalent water ligand protons and the relative orientations of their hfi tensors. The magnetic and geometrical parameters extracted from the experiments were compared with the results of DFT computations for different geometrical arrangements of the water ligands around the cation. The theoretical observable properties (g tensor (1)H and (17)O hfi tensors and their orientations) of the [Ti(H(2)O)(6)](3+) complex are in quantitative agreement with the experiments for two slightly different geometrical arrangements associated with D(3d) and C(i) symmetries.

Photocatalytic activity of S- and F-doped TiO(2) in formic acid mineralization.

Two series of doped titanium dioxide samples (S-TiO(2) and F-TiO(2)) were prepared by the sol-gel method in the presence of different amounts of dopant source (thiourea and NH(4)F, respectively), followed by calcination at 500, 600 or 700 °C, and characterised by BET, UV-vis absorption, XPS, HRTEM, XRD and EPR analyses. Reference undoped materials were prepared by the same synthetic procedure. Their photocatalytic activity under visible light was investigated employing the photocatalytic degradation of formic acid in aqueous suspension as test reaction. S-doped TiO(2) showed a photocatalytic activity quite similar to that of undoped materials. In this regard, the insertion of S, characterised by a relatively large ionic radius, into the TiO(2) crystalline structure appears rather difficult, as confirmed by XPS analysis. On the contrary, moderate F doping was beneficial in increasing the rate of formic acid photocatalytic degradation, especially for photocatalysts calcined at high temperature, consisting of highly crystalline pure anatase, in which the rate of detrimental charge carrier recombination was reduced. For both series of doped materials, high doping levels appear to limit the semiconductor photoactivity, probably due to the formation of a progressively increasing number of charge recombination centres. The EPR characterisation of the investigated doped TiO(2) samples evidenced the presence of nitrogen containing species (nitric oxide radical encapsulated in micro-void, with no photoactivity, and N(b)˙ species, active in visible light sensitisation) and of titanium reduced centres Ti(3+), due to charge imbalance consequent to dopant introduction in the TiO(2) lattice either in anionic (F(-)) or in cationic form (S(6+)).

Decreasing the oxidative potential of TiO(2) nanoparticles through modification of the surface with carbon: a new strategy for the production of safe UV filters.

A safe UV filter may be obtained by inhibiting the photo-generation of free radicals through modification of the surface of TiO(2) nanoparticles with carbon.

Non-UV-induced radical reactions at the surface of TiO2 nanoparticles that may trigger toxic responses.

Kept in the dark: The non-photocatalytic generation of free radicals from fine and ultrafine TiO(2) particles has been studied by means of a spin-trapping/ESR spectroscopy technique (see figure). The amount and kind of free radicals generated depends on the crystalline structure, but not on the particle dimensions.Titania is generally considered to be an inert and safe material. Several studies, however, have reported that nanosized TiO(2) may elicit toxic effects. In some cases the observed adverse effects have been related to free radicals. Although new studies mainly concern irradiated titania, the role and the mechanisms of the generation of free radicals by TiO(2) in the absence of UV irradiation are not well known. The purpose of the present study is to investigate the free-radical-generation mechanisms by nano- and micronsized anatase or rutile powders under normal laboratory illumination or in the dark by means of a spin-trapping/ESR spectroscopy technique. This technique is used to identify the nature and the amount of free radicals released in solution, and in the solid-state to characterise the paramagnetic centres at the surface of particles that may participate in the reactions. The following radical-generating mechanisms have been considered: 1) the generation of oxygenated free radicals (HO(2) (.), O(2) (.-), HO(.)) following the reaction of TiO(2) with oxygen, water or H(2)O(2) and 2) the generation of carbon-centred radicals following the cleavage of the C--H bond in a model molecule. Although no free radicals were detected in a simply buffered solution, anatase and rutile generated O(2) (.-) and HO(.), respectively, in the presence of H(2)O(2). Both polymorphs were also active in the cleavage of the C--H bond. Although the formation of O(2) (.-) appears to be related to exposure to sunlight, the generation of HO(.) and carbon-centred free radicals also occurs in the dark. When samples of equal surface area were tested, micron- and nanosized anatase was found to react in the same way indicating that a reduction in diameter does not generate new kinds of reactive sites. The data presented herein may have implications in the assessment of the health risk associated with the exposure to TiO(2) nanoparticles and in the ecotoxicological impact following their possible leakage into the environment.

Origin of photoactivity of nitrogen-doped titanium dioxide under 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 single-atom 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.

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.

Characterization of paramagnetic species in N-doped TiO2 powders by EPR spectroscopy and DFT calculations.

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.