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Particle attachment and neck formation inside TiO2 nanoparticle networks determine materials performance in sensing, photo-electrochemistry, and catalysis. Nanoparticle necks can feature point defects with potential impact on the separation and recombination of photogenerated charges. Here, we investigated with electron paramagnetic resonance a point defect that traps electrons and predominantly forms in aggregated TiO2 nanoparticle systems. The associated paramagnetic center resonates in the g factor range between g = 2.0018 and 2.0028. Structure characterization and electron paramagnetic resonance data suggest that during materials processing, the paramagnetic electron center accumulates in the region of nanoparticle necks, where O2 adsorption and condensation can occur at cryogenic temperatures. Complementary density functional theory calculations reveal that residual carbon atoms, which potentially originate from synthesis, can substitute oxygen ions in the anionic sublattice, where they trap one or two electrons that mainly localize at the carbon. Their emergence upon particle neck formation is explained by the synthesis- and/or processing-induced particle attachment and aggregation facilitating carbon atom incorporation into the lattice. This study represents a substantial advance in linking dopants, point defects, and their spectroscopic fingerprints to microstructural features of oxide nanomaterials.

ConspectusEven in the gas phase single atoms possess catalytic properties, which can be crucially enhanced and modulated by the chemical interaction with a solid support. This effect, known as electronic metal-support interaction, encompasses charge transfer, orbital overlap, coordination structure, etc., in other words, all the crucial features of the chemical bond. These very features are the object of this Account, with specific reference to open-shell (paramagnetic) single metal atoms or ions on oxide supports. Such atomically dispersed species are part of the emerging class of heterogeneous catalysts known as single-atom catalysts (SACs). In these materials, atomic dispersion ensures maximum atom utilization and uniform active sites, whereby the nature of the chemical interaction between the metal and the oxide surface modulates the catalytic activity of the metal active site by tuning the energy of the frontier orbitals. A comprehensive set of examples includes fourth period metal atoms and ions in zeolites on insulating (e.g., MgO) or reducible (e.g., TiO

Site isolated vanadium ions supported on zeolites are central in many catalytic processes. However, an atomistic description of the species formed is often hampered by the heterogeneous nature of zeolites and the random distribution of active species. In this contribution we applied pulsed electron paramagnetic resonance spectroscopy on 17O exchanged ZSM-5 zeolite loaded with V4+ ions, via gas phase reaction with VCl4 vapors. From the magnetic parameters relative to 1H, 17O, 27Al and 51V isotopes we prove that a single, structurally well-defined VO(OH)+ species is formed at zeolite Al framework sites and derive a detailed atomistic model inclusive of bond lengths and angles. Comparison is set to vanadyl aquo complexes, highlighting analogies between surface and solution coordination chemistry. The results showcase the power of EPR spectroscopy and 17O isotopic labelling in the geometrical and electronic characterisation of inorganic complexes formed at the surface of oxide materials. Pulsed EPR on disorder systems can therefore compete in structural insight with more well-established X-ray techniques.

High surface area TiO2 was synthetized by means of a solvothermal method and it was then used to prepare two photocatalysts based on copper-modified TiO2. Two different preparation approaches have been adopted based on the use of the same amount of Cu2O (0.5% w/w) as a reactant. The spectroscopic characterization shows that the two preparation procedures lead to distinct distributions of Cu2O and other Cu-based species in the final composite materials and that the photoactivity of the solids is strictly related to the nature of the copper species. The joint application of CW-EPR and optical spectroscopy, both employed in catalytic conditions as a sort of “operando” mode, allowed monitoring the evolution of the photocatalytic systems occurring during the H2 evolution reaction (HER). The effect of water coordination on the Cu(II) species at the surface of titania has been revealed by EPR. The successive photoreduction of the system in the early steps of the photocatalytic reaction involves the reduction of Cu2+, the formation of Ti3+ centres in the oxide matrix and that of metallic copper particles that partially inactivate the system.

The influence of bromide ions on systems containing highly reactive radical species is of great interest for environmental remediation, atmospheric chemistry, and the synthesis of high-added-value ...

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.

Determining structural models is pivotal to the rational understanding and development of heterogeneous catalytic systems. A paradigmatic case is represented by open-shell metals supported on oxides, where the catalytic properties crucially depend on the nature of the metal-oxygen bonds and the extent of charge and spin transfer. Through a combination of selective 17 O isotopic enrichment and the unique properties of open-shell s-state monovalent Group 12 cations, we derive a site-specific topological description of active sites in an MFI zeolite. We show that just a few selected sites out of all possible are populated and that the relative occupancies depend on the specific properties of the metal, and we provide maps of charge and spin transfer at the metal-oxygen interface. This approach is not restricted to zeotype materials, rather it is applicable to any catalysts supported on oxygen-containing materials.

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., Engineered nitrogen defects in semiconducting oxides imparts appealing photophysical and photochemical properties to the materials which can be selectively addressed through EPR spectroscopy.

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.

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.

Metal ion-doped titanium dioxide (TiO2) was synthesized by using the novel sol–gel method, where copper and manganese were used for dopant ions. The obtained Cu–TiO2 and Mn–TiO2 powders were investigated by electron paramagnetic resonance (EPR) spectroscopy before and after calcination treatment. EPR results indicate that the novel method can homogeneously spread out the metal ions into basic TiO2 structure. After calcination, the Cu2+ ion was in an axially distorted octahedral site, while Mn2+ and Mn4+ ions were created.

Lutetium-doped titanium dioxide (Lu-TiO2) was synthesized using a sol–gel method. The obtained compounds were characterized by X-ray diffraction and electron paramagnetic resonance. The photocatalytic decomposition of adenosine 5′-triphosphate (ATP) under UV irradiation was investigated to estimate the effect of the doped lutetium. Our results showed that the Lu-TiO2 nanoparticles have higher photoactivity than an undoped TiO2. Furthermore, the role of lutetium in Lu-TiO2, the structure of the photoreaction site on Lu-TiO2 and the reaction mechanism are also discussed.

Paramagnetic species are often involved in catalytic or photocatalytic reactions occurring at the solid-gas interface of heterogeneous catalysts. In this contribution we will provide an overview of the wealth and breadth of information that can be obtained from EPR in the characterization of paramagnetic species in such systems, illustrating the advantages that modern pulsed EPR methodologies can offer in monitoring the elementary processes occurring within the coordination sphere of surface transition-metal ions. To do so we selected three representative systems, where titanium ions in low oxidation states act as active catalytic sites, trying to outline the methodological approaches which characterize the application of EPR techniques and the questions that can be answered and addressed relative to the characterization of heterogeneous catalytic materials.

In the present study, we investigated the effect of cerium and erbium doping of the zirconium dioxide matrix. We synthesized doped samples using hydrothermal process. The amounts of dopant used were 0.5%, 1% and 5% molar (rare earth oxide over zirconium dioxide) respectively. The samples have been studied via X-ray Diffraction measurements for the structural characterization. UV visible diffuse reflectance was used for the optical analysis, Brunauer-Emmett-Teller (BET) model for the measurement of the surface area. Finally the samples have been analysed via electron paramagnetic resonance (EPR) for the electronic characterization. Then we tested the new synthetized materials to determine their photocatalytic activity in the reaction of degradation of methylene blue performed under irradiation by diodes (LEDs) emitting exclusively visible light.

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.

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.

© 2015 Elsevier B.V. A conserved cysteine located in the signature motif of the catalytic center (H-cluster) of [FeFe]-hydrogenases functions in proton transfer. This residue corresponds to C298 in Clostridium acetobutylicum CaHydA. Despite the chemical and structural difference, the mutant C298D retains fast catalytic activity, while replacement with any other amino acid causes significant activity loss. Given the proximity of C298 to the H-cluster, the effect of the C298D mutation on the catalytic center was studied by continuous wave (CW) and pulse electron paramagnetic resonance (EPR) and by Fourier transform infrared (FTIR) spectroscopies. Comparison of the C298D mutant with the wild type CaHydA by CW and pulse EPR showed that the electronic structure of the center is not altered. FTIR spectroscopy confirmed that absorption peak values observed in the mutant are virtually identical to those observed in the wild type, indicating that the H-cluster is not generally affected by the mutation. Significant differences were observed only in the inhibited state Hox-CO: the vibrational modes assigned to the COexo and Fed-CO in this state are shifted to lower values in C298D, suggesting different interaction of these ligands with the protein moiety when C298 is changed to D298. More relevant to the catalytic cycle, the redox equilibrium between the Hox and Hred states is modified by the mutation, causing a prevalence of the oxidized state. This work highlights how the interactions between the protein environment and the H-cluster, a dynamic closely interconnected system, can be engineered and studied in the perspective of designing bio-inspired catalysts and mimics.

The photochemical and photophysical properties of a defective zinc oxide prepared by precipitation have been investigated using mainly electron paramagnetic resonance (EPR) and photoluminescence (PL) spectroscopy. As already reported in the literature the band gap of the oxide contains intra band gap states related to the presence of point defects (mainly cation vacancies) in the structure. The concentration and the energy levels of such defects, a fraction of them being paramagnetic, are the basis of a mechanisms of double and multiple excitations allowing visible photons with energy lower than the band gap value (hν Eg. Remarkably, some of the paramagnetic defects present in the bulk of the as prepared materials derive from excitation of VB electrons induced by ambient light illumination. The electron excitation from VB to CB has been observed upon irradiation under vacuum at 77 K with polychromatic light and applying four different filters to the emitted radiation with cut-off in the range between 400 nm and 495 nm. The process has been monitored by EPR as the photoexcitation ends up in the formation of trapped electron centers (signal at g = 1.96) and trapped hole centers (signal in the range 2.021 > g > 2.003) both clearly detected by this technique. A scheme of the energy state distribution in the band gap of the defective oxide, obtained coupling EPR and photoluminescence results, is proposed. Finally, a non-negligible fraction of the visible light generated carriers reach the surface of the nanocrystals, entailing the typical redox reactions of photocatalysis as indicated by the detection of hydroxyl radicals in solution performed by spin-trapping. For this reason, the wet chemistry prepared zinc oxide can be considered a visible light active (VLA) system.

In the present work, two H

Evidence of the existence of nitrate radical in irradiated aqueous TiO

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-C2H and OH groups. This is followed by subsequent additions of C2H2 molecules with formation of PAHs. During the dissociation of C2H2, 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 surface of things: Acetylene oligomerisation occurs on the surface of anatase TiO2 leading to the formation of polycyclic aromatic hydrocarbons and a reduced titania surface. The mechanism of the reaction has been elucidated by EPR measurements and DFT calculations (see figure).

The typical activation of a fourth generation ZieglerNatta catalyst TiCl4/MgCl2/phthalate with triethyl aluminum generates Ti3+ centers that are investigated by multi-frequency continuous wave and pulse EPR methods. Two families of isolated, molecule-like Ti3+ species have been identified. A comparison of the experimentally derived g tensors and 35,37Cl hyperfine and nuclear-quadrupole tensors with DFT-computed values suggests that the dominant EPR-active Ti3+ species is located on MgCl2(110) surfaces (or equivalent MgCl2 terminations with tetra-coordinated Mg). O2 reactivity tests show that a fraction of these Ti sites is chemically accessible, an important result in view of the search for the true catalyst active site in olefin polymerization.

Transition-metal ions with open-shell configurations hold promise in the development of novel coordination chemistry and potentially unprecedented redox catalysis. Framework-substituted Ti(3+) ions with tetrahedral coordination are generated by reductive activation of titanium silicalite-1 with triethylaluminum, an indispensable co-catalyst for heterogeneous Ziegler-Natta polymerization catalysts. Continuous-wave and pulse electron paramagnetic resonance methods are applied to unravel details on the local environment of the reduced transition metal-ions, which are shown to be part of the silica framework by detection of (29)Si hyperfine interactions. The chemical accessibility of the reduced sites is probed using ammonia as probe molecule. Evidence is found for the coordination of a single ammonia molecule. Comparison to similar systems, such as TiAlPO-5, reveals clear differences in the coordination chemistry of the reduced Ti sites in the two solids, which may be understood considering the different electronic properties of the solid frameworks.

The interaction of oxygen with the surface of CeO2–TiO2 mixed oxides prepared via sol–gel was investigated by means of electron paramagnetic resonance (EPR). Upon admission of molecular oxygen onto the surface of the as prepared materials (which underwent final oxidative calcination) the formation of superoxide O2− ions is observed without the need for preliminary annealing in a vacuum and consequent oxygen depletion. The superoxide species is symmetrically adsorbed (“side-on” structure) on the top of a Ce4+ ion. Surprisingly the electron transfer is fully reversible at room temperature having the typical behavior shown by molecular oxygen carriers, which, however, link to oxygen in a completely different manner (“end-on” structure). We suggest that the active sites are Ce3+ ions present in the stoichiometric cerium titanate which forms during the synthesis. The features of these Ce3+ ions must be different from those of the same ions formed in CeO2 by reductive treatments, which show a different reactivity to O2. The observation reported here opens up innovative perspectives in the field of heterogeneous catalysis and in that of sensors as the total reversibility of the electron transfer is observed in a significant range of oxygen pressure.

We report direct evidence for quintuplet spin states in a particular kind of reduced TiO

Aromatase catalyses the conversion of androgens into estrogens and is a well-known target for breast cancer therapy. As it has been suggested that its activity is affected by inhibitors of phosphodiesterase-5, this work investigates the potential interaction of sildenafil with aromatase. This is carried out both at molecular level through structural and kinetics assays applied to the purified enzyme, and at cellular level using neuronal and breast cancer cell lines. Sildenafil is found to bind to aromatase with a KD of 0.58 ± 0.05 μM acting as a partial and mixed inhibitor with a maximal inhibition of 35 ± 2%. Hyperfine sublevel correlation spectroscopy and docking studies show that sildenafil binds to the heme iron via its 6th axial water ligand. These results also provide information on the starting molecular scaffold for the development of new generations of drugs designed to inhibit aromatase as well as phosphodiesterase-5, a new emerging target for breast cancer therapy.

Undoped TiO 2 and Fe 3+ doped (0.1, 0.3, 0.6 and 1 wt.%) TiO 2 nanoparticles have been synthesized by the acid-catalyzed sol–gel method. Iron cations are introduced in the initial solution, before gelification, what promotes their lattice localization. The Fe 3+ doped TiO 2 films have been fabricated using a dip-coating technique. The effect of iron content on the crystalline structure, phase transformation and grain growth were determined by X-ray diffraction (XRD), Raman spectroscopy, UV–visible diffused reflectance spectroscopy (DRS) and Electron paramagnetic resonance (EPR) spectroscopy. It has demonstrated that all catalysts are composed of mixed-phase crystals of anatase and brookite with anatase as dominant phase. The crystallinity of the brookite and anatase phases decreased with increasing the iron content. The analysis of EPR result further confirms that Fe 3+ ion are successfully doped in the TiO 2 lattice by substituting Ti 4+ . It was demonstrated that Fe 3+ ion in the TiO 2 films plays a role as the intermediate for the efficient separation of photogenerated hole–electron pairs and increases the photocurrent response of the film under UV light irradiation. The maximum photocurrent is obtained on the Fe 3+ doped TiO 2 film with 0.1% Fe, which is 1.46 times that achieved on undoped TiO 2 film.

A systematic analysis of the reduced states in the titanium dioxide matrix (anatase polymorph) has been performed coupling the classic continuous wave electron paramagnetic resonance (CW-EPR) with advanced pulse-EPR techniques and introducing the 17O magnetic isotope into the solid. Reduced states were originated in various ways including valence induction via aliovalent elements (F, Nb) and reducing treatments of the bare oxide including surface reaction with reducing agents (H, Na) and thermal annealing under vacuum with consequent oxygen depletion. Two main paramagnetic species were identified via EPR both amenable to Ti3+ ions. The former (EPR signal A: axial symmetry with g∥ = 1.962 and g⊥ = 1.992) is observed in all case and has been conclusively assigned to reduced Ti3+ centers in regular lattice sites of the anatase matrix; the second (signal B: broad line centered at g = 1.93) is present only in reduced materials and is assigned, on the basis of the analysis of the hyperfine interaction of the ce...

We used the Electron Paramagnetic Resonance (EPR) technique to study the interaction of SO 2 molecules on bare and partially hydroxylated alkali-earth oxide (AEO) surface. When SO 2 is adsorbed on the surfaces of higher basicity (CaO and SrO) disproportionation reaction occurs with the formation of SO 2 − radical. We show also that, when the same reaction is present on partially hydroxylated surfaces a fraction of the electrons exchanged during the redox process are trapped on the oxide surface forming colour centres ((H + )(e − ) centres).

H2 absorption and desorption reactions in MgH2 promoted by ball-milling with 1 mol% MgNb2O6, Mg4Nb2O9 and Mg3Nb6O11 have been investigated. MgH2 was milled with the bare oxides for 12 h under a high purity Ar atmosphere. Absorption and desorption reactions in the ball-milled samples were studied by in situ X-ray diffraction (XRD) in isothermal conditions with Anton Paar XRK 900 reaction chamber. XRD patterns for absorption were recorded at 573 K under hydrogen pressure of 0.9 MPa and for desorption at 623 K in vacuum. Experimental data were analysed according to the Rietveld method. Ball-milled samples showed the presence of a mixture of β and γ allotropes of MgH2, with significantly broadened diffraction peaks due to reduced crystallite size and strain, together with bare additives. The presence of Mg–Nb oxides significantly accelerates the hydrogen absorption and desorption processes. The amount of hydrogen absorbed in the presence of Mg–Nb–O phases is lower than the maximum stoichiometric capacity, because of the presence of a non-reactive MgO layer on the surface of the powders or at the grain boundaries. Experimental results are discussed on the basis of thermodynamic and kinetic arguments.

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.

Continuous wave (CW) and pulse electron paramagnetic resonance (EPR) experiments, in conjunction with density functional theory (DFT) calculations, provide a detailed description of defective centres produced upon nitrogen doping of polycrystalline ZnO. Two distinct paramagnetic species are formed upon annealing of ZnO nanoparticles in an NH3 atmosphere, which are characterized by the interaction of the unpaired electron with one and two N nuclei. HYSCORE experiments provide the full hyperfine and quadrupole interaction tensors for the monomeric defect, which, on the basis of quantum chemical calculations, is assigned to a nitrogen ion substituting a lattice oxygen ion. © 2010 The Royal Society of Chemistry.

Mo{sub 1-x}W{sub x}O{sub 3} oxides with different cationic fraction (x=0.2, 0.5 and 0.8) and, for comparison purposes, pure MoO{sub 3} and WO{sub 3} were prepared. Along with textural and structural characterizations, absorbance FT-IR, diffuse reflectance UV-vis-NIR and EPR spectroscopies were employed to study the changes in the electronic properties of these materials passing from Mo{sub 1-x}W{sub x}O{sub 3} in oxidizing atmosphere to Mo{sub 1-x}W{sub x}O{sub 3-{delta}} in reducing conditions. XRD analysis showed that the Mo-W mixed oxides are constituted by two or three crystalline phases, whose abundance and composition are well characterized by structural refinement with the Rietveld method. Only the sample with the highest Mo content (x=0.2) shows a predominant mixed phase and also a superior ability to lose oxygen with respect to the other mixed oxides. The oxygen loss in the reduced oxides induces the formation of defects with electronic levels in the band gap of the material, in particular, electrons trapped in oxygen vacancies and/or at cationic sites (polarons). While the nature of defect sites induced in the mixed and in the pure oxides is similar, the photo-ionization energies, the ratio between surface and bulk defects and the stability of the defects in oxygen at increasing temperaturemore » are peculiar of each mixed oxide. - Graphical abstract: The combined use of different techniques provides new insight into structural, optical and electronic properties of semiconductor Mo{sub 1-x}W{sub x}O{sub 3-{delta}} mixed oxides.« less

The reaction of excess electrons at the surface of MgO with ammonia leads to surface ammoniated electrons analogous to those formed when alkali metals are dissolved in anhydrous ammonia. Surface excess electrons are found to be solvated by up to three ammonia molecules, and well-resolved CW and pulsed EPR spectra allow for a precise description of the unpaired electron spin density distribution over the solvent molecules. The large majority of the electron spin density resides in the first-shell nitrogen fragments. HYSCORE spectra allow obtaining for the first time the full hyperfine interaction of the solvated electron with the ammonia protons, which is consistent with a small and negative spin density in the 1H 1s orbital. Furthermore, the hyperfine and nuclear quadrupole tenors of the second-shell nitrogens could be unravelled.

Nitrogen doped TiO{sub 2} represents one of the most promising material for photocatalitic degradation of environmental pollutants with visible light. However, at present, a great deal of activity is devoted to the anatase polymorph while few data about rutile are available. In the present paper we report an experimental characterization of N doped polycrystalline rutile TiO{sub 2} prepared via sol-gel synthesis. Nitrogen doping does not affect the valence band to conduction band separation but, generates intra band gap localized states which are responsible of the on set of visible light absorption. The intra band gap states correspond to a nitrogen containing defect similar but not coincident with that recently reported for N doped anatase. - Graphical abstract: Nitrogen doped TiO{sub 2} represents one of the most promising material for photocatalitic degradation of environmental pollutants with visible light. However, at present, a great deal of activity is devoted to the anatase polymorph while few data about rutile are available. In the present paper we report an experimental characterization of N doped polycrystalline rutile TiO{sub 2} prepared via sol-gel synthesis.

Nitrogen-doped TiO2 materials were successfully prepared following three different preparation routes (sol−gel, mechanochemistry, and oxidation of TiN) and characterized by X-ray diffraction, electron microscopy, and various spectroscopic techniques. All samples absorb visible light, and the one obtained via sol−gel, showing the anatase structure, is the most active in the decomposition of organic compounds under visible light. Various nitrogen-containing species have been observed in the materials, whose presence and abundances depends on the preparative route. Ammonium NH4+ ions are residual of the synthesis using ammonium salts (sol−gel, mechanochemistry) and are quite easily eliminated, as shown by the parallel behavior of both NMR and XPS spectra. Cyanide CN− ions form at high temperature in parallel with the phase transition of the solid to rutile. Molecular nitric oxide forms in the case of materials exhibiting close porosity. The already reported bulk radical species, Nb•, is the only paramagnetic...

On the basis of experimental evidence and DFT calculations, we propose a simple yet viable way to stabilize and chemically activate gold nanoclusters on MgO. First the MgO surface is functionalized by creation of trapped electrons, (H+)(e-) centers (exposure to atomic H or to H2 under UV light, deposition of low amounts of alkali metals on partially hydroxylated surfaces, etc.); the second step consists in the self-aggregation of gold clusters deposited from the gas phase. The calculations show that the (H+)(e-) centers act both as nucleation and activation sites. The process can lead to thermally stable gold cluster anions whose catalytic activity is enhanced by the presence of an excess electron. © 2008 American Chemical Society.

Various Mg/Nb/O mixed phases have been prepared by solid state reaction in order to mimic similar compounds formed in the ball milled system composed of MgH2 and Nb2O5. The latter oxide has a role as a promoter of the hydrogen uptake and desorption in the Mg/MgH2 system for hydrogen storage. One mixed phase in particular (Mg3Nb6O11) have shown a reversible uptake of molecular hydrogen, confirmed by thermal desorption experiments followed by mass spectroscopy. Comparison of the behaviour of this phase with NbO shows how a reversible interaction with hydrogen may be linked to the presence of octahedral niobium clusters in both oxides. © 2008 International Association for Hydrogen Energy.

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 adsorption of small amounts of alkali metal atoms (Li, Na, K, Rb, and Cs) on the surface of MgO powders and thin films has been studied by means of EPR spectroscopy and DFT calculations. From a comparison of the measured and computed g values and hyperfine coupling constants (hfccs), a tentative assignment of the preferred adsorption sites is proposed. All atoms bind preferentially to surface oxide anions, but the location of these anions differs as a function of the deposition temperature and alkali metal. Lithium forms relatively strong bonds with MgO and can be stabilized at low temperatures on terrace sites. Potassium interacts very weakly with MgO and is stabilized only at specific sites, such as at reverse corners where it can interact simultaneously with three surface oxygen atoms (rubidium and cesium presumably behave in the same way). Sodium forms bonds of intermediate strength and could, in principle, populate more than a single site when deposited at room temperature. In all cases, large deviations of the hfccs from the gas-phase values are observed. These reductions in the hfccs are due to polarization effects and are not connected to ionization of the alkali metal, which would lead to the formation of an adsorbed cation and a trapped electron. In this respect, hydrogen atoms behave completely differently. Under similar conditions, they form (H(+))(e(-)) pairs. The reasons for this different behavior are discussed.

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.

Nb2O5 is known as good promoter for adsorption/desorption kinetics of hydrogen in magnesium hydride. In this article the interaction with hydrogen of bare Nb2O5, the oxidic component of the mixed MgH2/Nb2O5 system, is investigated in various conditions (i.e. employing atomic, molecular and nascent hydrogen). The state of the hydrogen-Nb2O5 system was monitored using various techniques including: X-ray Diffraction, Electron Paramagnetic Resonance, Diffuse Reflectance UV-Vis Spectroscopy, Thermal Desorption Spectroscopy -Mass Spectrometry, Differential Scanning Calorimetry and Thermal Programmed Desorption. Niobium (V) oxide is not at all inert while interacting with hydrogen. This oxide is partially reduced by hydrogen, which is incorporated in the solid and released as both molecular hydrogen and water. This peculiar behaviour, reminiscent of some properties of the bronze family, suggests an active chemical role played by Nb2O5 in the mixed MgH2/Nb2O5 system.

Dehydroxylated, hydroxylated and hydrated anatase TiO

Electron Paramagnetic Resonance (EPR) plays a paramount role in understanding the nature and the main features of point defects in the bulk of both ionic and covalent solids. In the present chapter, after a brief introduction to the technique, the application of EPR to unravel the surface counterparts of bulk point defects in the case of metal oxides will be illustrated with particular attention to the defects capable of charge carriers trapping.

Sulfur contamination of alkaline earth oxide surfaces has important consequences in many fields of chemistry, such as surface science and catalysis. We used the Electron Paramagnetic Resonance (EPR) technique to study the interaction of CS2 molecules under different pressure and temperature conditions, with the bare and electron-enriched surface of MgO. CS2 reacts on the oxide surface through two distinct reaction paths, one leading to diamagnetic species, the other to paramagnetic surface entities. Both these reaction paths lead to the fragmentation of the CS2 molecule and, in some cases to oligomerization processes. EPR experiments allowed to follow the formation of the unstable CS 2 − radical intermediate and its evolution through formation of several surface radicals like S−, S 3 − and S n − (n ⩾ 3), depending on the reaction conditions.

Photosensitive, germanium-doped multicomponent silicate glasses with different amount of boron oxide and sodium oxide were prepared and investigated by means of electron paramagnetic resonance (EPR) and ultraviolet–visible (UV–Vis) spectroscopy in order to investigate the structural changes occurring in the glasses under UV laser irradiation at 248 nm (KrF excimer UV laser). Spectral data were recorded before and after UV laser irradiation: all the glasses showed EPR signals only after UV laser irradiation, independently from the presence of a UV–Vis absorption band on the as prepared glasses. The main results showed that the presence or the absence of boron and sodium and their amount play a role in the type and the number of defects, their time and thermal decay, under the same UV laser exposure conditions. A clear correlation between the EPR signal and the UV–Vis absorption is observed only for the 240 nm band. It is also demonstrated that the sodium ions decrease the number of paramagnetic defects in these glasses and deeply change the germano-silicate glass structure. The presence, the absence and the co-presence of sodium and boron ions has a role in the structure, the number, the nature and the stability of the paramagnetic defects. It is important to understand the role of sodium and boron in order to increase the photosensitivity of germano-silicate glasses, for their interest in the field of ion-exchangeable glasses for photonic waveguides and Bragg-grating based devices.