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An important step in oxide photochemistry, the loading of electrons into shallow trap states, was studied using infrared (IR) spectroscopy on both, rutile TiO2 powders and single-crystal, r-TiO2 (110) oriented samples. After UV-irradiation or n-doping by exposure to H-atoms broad IR absorption lines are observed for the powders at around 940â€... cm-1. For the single crystal substrates, the IR absorption bands arising from an excitation of the trapped electrons into higher-lying final states show additional features not observed in previous work. On the basis of our new, high-resolution data and theoretical studies on the polaron binding energy in rutile we propose that the trap states correspond to polarons and are thus intrinsic in nature. We assign the final states probed by the IR-experiments to hydrogenic states within the polaron potential. Implications of these observations for photochemistry on oxides will be briefly discussed.

Boron-doped graphene was reported to be the best non-metal doped graphene electrocatalyst for the oxygen reduction reaction (ORR) working at an onset potential of 0.035 V (Jiao et al., 2014). In the present DFT study, intermediates and transition structures along the possible reaction pathways are determined. Both Langmuir-Hinschelwood and Eley-Rideal mechanisms are discussed. Molecular oxygen binds the positively charged B atom and forms an open shell end-on dioxygen intermediate. The associative path is favored with respect to the dissociative one. The free energy diagrams along the four-reduction steps are investigated with the methodology by Nørskov and co. (2004) in both acidic and alkaline conditions. The pH effect on the stability of the intermediates of reduction is analyzed in terms of the Pourbaix diagram. At pH = 14, we compute an onset potential value for the electrochemical ORR of U = 0.05 V, which compares very well with the experimental value in alkaline conditions.

Graphene (G) reactivity toward oxygen is very poor, which limits its use as electrode for the oxygen reduction reaction (ORR). Contrarily, boron-doped graphene was found to be an excellent catalyst for the ORR. Through a density functional study, comparing molecular and periodic approaches and different functionals (B3LYP vs PBE), we show how substitutional boron in the carbon sheet can boost the reactivity with oxygen leading to the formation of bulk borates covalently bound to graphene (BO3-G) in oxygen-rich conditions. These species are highly interesting intermediates for the Oî - O breaking step in the reduction process of O2 to form H2O as they are energetically stable. © 2013 American Chemical Society.

Very rarely do researchers use metal oxides in their pure and fully stoichiometric form. In most of the countless applications of these compounds, ranging from catalysis to electronic devices, metal oxides are either doped or defective because the most interesting chemical, electronic, optical, and magnetic properties arise when foreign components or defects are introduced in the lattice. Similarly, many metal oxides are diamagnetic materials and do not show a response to specific spectroscopies such as electron paramagnetic resonance (EPR) spectroscopy. However, doped or defective oxides may exhibit an interesting and informative paramagnetic behavior.Doped and defective metal oxides offer an expanding range of applications in contemporary condensed matter science; therefore researchers have devoted enormous effort to the understanding their physical and chemical properties. The interplay between experiment and computation is particularly useful in this field, and contemporary simulation techniques have achieved high accuracies with these materials.In this Account, we show how the direct comparison between spectroscopic experimental and computational data for some selected and relevant materials provides ways to understand and control these complex systems. We focus on the EPR properties and electronic transitions that arise from the presence of dopants and defects in bulk metal oxide materials. We analyze and compare the effect of nitrogen doping in TiO2 and ZnO (two semiconducting oxides) and MgO (a wide gap insulator) and examine the effect of oxygen deficiency in the semiconducting properties of TiO2-x, ZnO1-x, and WO3-x materials. We chose these systems because of their relevance in applications including photocatalysis, touch screens, electrodes in magnetic random access memories, and smart glasses.Density functional theory (DFT) provides the general computational framework used to illustrate the electronic structure of these systems. However, for a more accu

The conversion of anodic TiO2 films into TiOxCy in ultrahigh-vacuum (UHV) has been traced by photoemission spectroscopy in order to optimize the process parameters and study the different phase stabilities. In addition, density functional theory (DFT) calculations have been performed in order to elucidate the main questions about TiOxCy composition and stability. The experimental data indicate that the anodic TiO2 film is stable both in UHV and ethylene background up to ca. 600 K, and at this temperature, it starts to reduce leading to suboxide TiOx species. Above ca. 750 K, the formation of TiOxCy starts, since the oxygen vacancies begin to be replaced by carbon atoms. A surface enrichment in TiO2 and elemental carbon has been detected on the converted TiOxCy film at room temperature. Real-time measurements have shown that this phenomenon takes place during the cool down process and DFT calculations suggest a possible explanation: as the temperature decreases below ca. 750 K (temperature at which the formation of TiOxCy starts), the TiOxCy phase is not thermodynamically stable, and it decomposes into TiO2 and elemental carbon. The comparison of the experimental valence band data with DFT results has also allowed to establish that the film surface is not homogeneous and that segregation of TiO and TiC systems may take place. On the other hand, the local compositional study carried out by scanning photoelectron microscopy has shown that the conversion of the film is not homogeneous but depends on the grain orientation, in particular crystallites with an orientation close to <21 ̄1 ̄0> and <101 ̄0> planes show a higher grade of conversion. Both experimental and DFT data validate the use of TiOxCy as an innovative support for electrocatalysis.

We demonstrate that graphene oxide can be efficiently reduced by evaporating metal Titanium in high vacuum. A detailed description of this reaction is provided by combining in situ photoemission spectroscopy measurements and DFT calculations: the titanium atoms readily react with the oxygenated groups of graphene oxide, disrupting the C–O bonds, with the consequent formation of titania and the recovery of the sp2 hybridized carbon atoms. When all surface oxygen is consumed, titanium can react with the carbon substrate and form carbidic species. Resonant photoemission spectroscopy measurements allow identifying the presence and exact energy position in the valence band of the Ti–C and Ti–O–C states, which are supposed to control the electron and energy transfer across the TiO2/ graphene interface. Therefore with this study we provide a versatile method and the rationale for controlling, at the atomic level, the nature of the interface of graphene/metal oxide nanocomposites.

In this talk some examples on the use of constrained DFT and transition energy levels for the calculation of excitation and emission energies of pure and defective or doped metal oxides (e.g. TiO2 [1] or ZnO [2,3]) will be presented. Calculations are performed with the hybrid functional B3LYP, as implemented in the CRYSTAL09 code. Reference to the available experimental data will be made whenever possible. [1] C. Di Valentin, A. Selloni, Journal of Physical Chemistry Letters 2011, 2, 2223. [2] F. Gallino, G. Pacchioni, C. Di Valentin, Journal of Chemical Physics, 2010, 133, 144512. [3] H. Qui, F. Gallino, C. Di Valentin, Y. Wang, Physical Review Letters, 2011, 106, 066401.

Bulk excitons formed upon UV irradiation are found to become self-trapped, consistent with the observation of temperature dependent Urbach tails in the absorption spectrum and a large Stokes shift in the photoluminescence band of anatase. The electron and hole polaron trapping energy is considerably larger at the surface making energetically favorable for the polarons to travel from the bulk to the surface where the trapping sites correspond to undercoordinated Ti3+5c and O–2c surface atoms, or to isolated OH species in the case of a hydroxylated surface [1]. However, adsorbed molecular species can be better traps of photoinduced electrons and holes than surface undercoordinated ions or OH groups. In such cases a direct charge carrier transfer to the adsorbate takes place resulting in a net redox process. Here we explore the behaviour of a number of organic adsorbates as hole scavengers and of molecular oxygen as an electron one [2]. [1] C. Di Valentin and A. Selloni, J. Phys. Chem. Lett. 2, 2223 (2011). [2] C. Di Valentin and D. Fittipaldi, in preparation.

The paramagnetic defects present in pristine zirconium dioxide (ZrO 2) and those formed upon reductive treatments (either annealing or UV irradiation in H2) are described and rationalized by the joint use of electron paramagnetic resonance (EPR) and DFT supercell calculations. Three types of Zr3+ reduced sites have been examined both in the bulk of the solid (one center) and at the surface (two centers). Trapping electron centers different from reduced Zr ions are also present, whose concentration increases upon annealing. A fraction of these sites are paramagnetic showing a symmetric signal at g = 2.0023, but the majority of them are EPR silent and are revealed by analysis of electron transfer from the reduced solid to oxygen. The presence of classic F-type centers (electrons in bulk oxygen vacancies) is disregarded on the basis of the g-tensor symmetry. This is expected, on the basis of theoretical calculations, to be anisotropic and thus incompatible with the observed signal. In general terms, ZrO2 has some properties similar to typical reducible oxides such as TiO2 and CeO2 (excess electrons stabilized at cationic sites), but it is much more resistant to reduction than this class of materials. While point defects in doped (Y 3+, Ca2+) ZrO2 materials have been widely investigated for their role as ionic conductors, the defectivity of pristine ZrO2 is much less known; this paper presents a thorough analysis of this phenomenon. © 2013 American Chemical Society

Understanding the interfacial hole transfer between photoirradiated anatase TiO2 nanoparticles and organic adsorbates at the surface is a crucial step toward the full comprehension and control of the photooxidation processes, which are fundamental in photocatalytic applications for energy production, environmental care, and medical cancer phototherapy. In this density functional study, we use the hybrid functional B3LYP in order to investigate how the hole transfer process takes place at the anatase (101) surface when a series of organic alcohols and acids are adsorbed. The results allow us to propose a mechanism of photoinduced proton dissociation and a scale of scavenging power of the different organic adsorbates (glycerol > tert-butanol > iso-propanol > methanol > formic acid). We observe that the surface dipole originated by the molecular adsorption causes a reduction in the cost to form a hole at a surface oxygen. This can then be trapped by the organic adsorbate only if proton dissociation takes place. In the case of glycerol, the hole transfer triggers further photoinduced chemical reactivity than the simple proton dissociation. Only in the case of cathecol, which is an excellent hole scavenger, the proton dissociation is not a required step

Despite the importance of tungsten oxide in various areas of materials science including catalysis and photocatalysis, relatively few systematic theoretical studies have been devoted to this system. In this review we report the results of first principle density functional theory calculations based on a hybrid functional that properly reproduces the band gap and other fundamental properties of WO3. We briefly describe the dependence of the band gap on the crystalline phase of WO3. Then, we address the nature of defects and dopants in bulk WO3. As WO3 can be easily reduced to WO3−x , we first discuss the nature of isolated O vacancies showing that three different situations arise from the removal of one O atom along each of the three crystallographic directions of RT monoclinic WO3. The data provide insight into the origin of electrochromism of this material. Then we discuss the role doping of WO3 with substitutional atoms in order to increase the activity for water splitting and we show that Hf is a promising dopant. The redox properties of WO3 are discussed also in relation to H2 adsorption on the WO3(001) surface. Finally, the role of nanostructuring is analyzed by studying the properties of (WO3)3 cyclic clusters deposited on the rutile TiO2(110) surface. Charge transfers at the (WO3)3/TiO2 interface and their role on the activity of this heterogeneous catalyst are discussed

Anatase TiO2 doping with boron, carbon, nitrogen, and fluorine atoms has been considered in a systematic study by performing periodic DFT calculations with the hybrid B3LYP functional and large supercells. The effect on the electronic structure of replacing lattice O atoms with B, C, N, or F dopants, or to include the same atoms in interstitial positions has been considered. Clear trends emerge as a function of the atomic number of the doping element. B, C, and N atoms in substitutional positions result in magnetic impurities whose energy levels fall in the energy gap of the material. The position of these gap states closely follows the effective nuclear charge of the dopant, with B that gives states high in the gap and N which introduces states just above the top of the valence band. Fluorine, being very electronegative, has filled states below the O 2p valence band and leads to the formation of Ti3+ ions due to charge compensation. Interstitial impurities have a quite different electronic structure, which again depends on the nuclear effective charge. B acts as a net electron donor with formation of B 3+ and three Ti3+ ions; C donates only two electrons to the lattice with formation of a C2+ ion; N forms a direct bond with a lattice O, and does not donate electrons to the host lattice. We also discuss possible internal charge transfers between high-lying electronic states in the gap (Ti3+ 3d1 states) and low-lying acceptor states of the dopant. The interplay between substitutional and interstitial dopants, between dopants and oxygen vacancies or titanium interstitials, and between co-dopants (B,N; N,F) are discussed in view of their beneficial effects for photocatalytic processes. © 2011 Elsevier B.V. All rights reserved

Deprotonating a base: High surface coverage is found to be capable of inducing proton transfer from adsorbed ammonia to surface oxygen atoms on ZnO of both single crystals and nanoparticles by means of HREELS and UHV-FTIRS vibrational spectroscopies and confirmed by density functional theory calculations

We report on a DFT study of the adsorption of nbutylphosphonic acid [CH3(CH2)3PO(OH)2] on the anatase (101) TiO2 model surface. The pure GGA (PBE and rPBE) approach is compared to a hybrid functional (B3LYP). Dispersion forces are taken into account in the Grimme framework correcting both energies and gradients. The surface coverage is varied from 0.25 ML (one phosphonic unit every four surface Ti5c) to 1 ML (one phosphonic unit/Ti5c). The overall picture resulting from these three approaches is qualitatively the same, although quantitative details are different. At low coverage, the alkylphosphonic acid adsorbs in a monodissociated bidentate mode, forming two PO−Ti5c bonds with one additional OH−Os bond, where Ti5c and Os refer to surface titanium and oxygen atoms. At full coverage, the molecules adsorb in a monodentate mode and reorient in order to minimize lateral repulsion and maximize H-bonds. The contribution of dispersion forces to the adsorption energy increases with the coverage. The 0.75 and 1 ML coverages, which are the most favored, correspond nicely to the reported experimental coverages. The results suggest that for entropic reasons the surface could be rather disordered, with coexisting domains of different coverages 0.75 and 1 ML. No additional states in the band gap of the semiconductors are formed for this hybrid organic−inorganic system. The workfunction decreases by 0.7 eV when a full self-assembled monolayer is supported on the surface.

(WO3)3 clusters deposited on the (110) rutile TiO2 surface are excellent catalysts for the formaldehyde (CH2O) polymerization reaction (J. Phys. Chem. C 2010, 114, 17017). The present B3LYP study unravels the possible paths of this catalyzed reaction. According to the stoichiometry of the r-TiO2 surface, the (WO3)3 clusters can be neutral, singly charged, or doubly charged. We find that only neutral (WO3)3 and anionic (WO3)3- clusters are reactive toward CH2O molecules. In both cases it is possible to determine more than one mechanism on the basis of a nucleophilic attack of the formaldehyde O atom to the W ions of the cluster. The reaction proceeds through successive attacks of other CH2O molecules and the formation of acetal and polyacetal intermediates, which inhibits the chain propagation. Only in the case of the anionic (WO3)3− catalyst is a totally different reaction path possible at low temperatures. This path involves the formation of radical species where the unpaired electron is localized on the organic moiety bound to the cluster. The polymer chain propagation follows a radical mechanism with low activation barriers. Thus, a cluster’s electron charging speeds up the formaldehyde polymerization at low temperatures. On the basis of these unexpected results, we conclude that electron-rich supports and low working temperatures are the keys to kinetic control of the reaction favoring a fast radical chain propagation mechanism.

The electronic properties of doped tungsten oxide (WO3) have been studied using DFT calculations with hybrid functionals. While the position of the top of the valence band (VB) in WO3 is good for O-2 evolution in water splitting, the conduction band (CB) is too low for H-2 production. Furthermore, the band gap should be reduced to improve activity with visible light. Doping can be used to alter the position of the energy levels, thus resulting in a more efficient photocatalyst. Replacing W in the lattice by isovalent Mo or Cr ions narrows the band gap but shifts the CB edge further down. Replacing O by S has the effect to narrow the energy gap by introducing localized occupied states above the VB and shifts the CB minimum upward-two effects that go in the right direction. Substitution of W with low-valent Ti, Zr, or Hf ions widens the band gap and shifts the CB edge to higher energies. However, a low-valent ion replacing W induces the formation of compensating defects: in Hf-doped WO3 oxygen vacancies (V-O) have negative formation energies. The simultaneous presence of substitutional Hf and of an O vacancy results in a shift of both VB and CB to higher energies and a reduction of the band gap, with potential benefit for photocatalytic hydrogen production. Codoping with Hf + 2F or Hf + V-O + S has also been investigated.

Hydrogen adsorption on the WO3 (001) surface has been studied by performing DFT calculations with the B3LYP functional and periodic slab models. We show that in the case of WO3 (001) surface the common practice to fix the bottom layers of the oxide slab results in spurious surface states and in the incorrect description of the band structure. The problem can be removed by full relaxation of all of the slab layers, resulting in converged values of the surface energy (0.33 J/m(2)) and of the band gap (2.75 eV vs computed bulk value of 3.10 eV). When H is adsorbed on the surface, we find that the under-coordinated O or the surface in-plane O-1,O-p,O-y sites have about the same binding energy, 2.5 eV. We find comparable adsorption energies also for one subsurface in-plane O site (O-2,O-p,O-y), which suggests an easy migration of the H atom on the surface or into the bulk. In all cases, H binds to the O anions as a proton and donates the valence electron to the 5d level of one or more W ions (formal change of oxidation state from W6+ to W5+). The corresponding occupied W 5d states lie high in the gap and sometimes even merge with the conduction band. The more or less localized nature of the donated electron depends on the site where the proton is bound, suggesting a high mobility of the excess electrons. Dissociative adsorption of H-2 is a weakly exothermic process at low H coverage but becomes weakly endothermic at high coverage. The partial modification and occupation of W 5d states, by hydrogen adsorption on the surface or introduction in the lattice of WO3, can explain the experimentally observed change in optical and electric properties of this material when exposed to H-2 gas, also known as the "chemichromic" effect because of the analogies with the well-known "electrochromic" effect.

A combined experimental and first principles study of Cu defects in bulk ZnO is presented. Cu particles are epitaxially deposited on the polar O-ZnOð000 1Þ surface at room temperature. Upon heating, a broadening of the quasielastic peak in high resolution electron energy loss spectra is observed, corresponding to an electronic doping effect of Cu atoms in bulk ZnO with an ionization energy of 88 meV. Cu impurities in ZnO, although commonly acting as acceptors, are presently observed to induce shallow donor states. We assign these to interstitial Cu species on the basis of a hybrid density functional study.

Transition metal doping of ZnO is considered as a promising way to obtain a diluted magnetic semiconducting oxide. In this work we investigate copper doping of ZnO by means of density functional theory, using a hybrid exchange-correlation functional and a periodic approach with localized atomic basis functions. Isolated copper species, such as copper substitutional to zinc, Cu, and Cu interstitial, Cu is., are analyzed in terms of transition energy levels and hyperfine coupling constants with reference to available spectroscopic data. We also examine the potential magnetic interaction between copper species, their interaction with oxygen vacancies, and the possibility of copper clustering. The relative stability of the various copper impurities considered in this study is finally compared on the basis of their formation energy at different oxygen chemical potentials and Fermi level values.

Using hybrid functional electronic structure calculations, we have investigated the structure and energetics of photogenerated electrons and holes in the bulk and at the (101) surface of anatase TiO2. Excitons formed upon UV irradiation are found to become self-trapped, consistent with the observation of temperature-dependent Urbach tails in the absorption spectrum and a large Stokes shift in the photoluminescence band of anatase. Electron and hole polarons are localized at Ti and O lattice sites, respectively. At the surface, the trapping sites generally correspond to undercoordinated Ti3+5c and O surface atoms or to isolated OH species in the case of a hydroxylated surface. The polaron trapping energy is considerably larger at the surface than in the bulk, indicating that it is energetically favorable for the polarons to travel from the bulk to the surface. Computed oneelectron energy levels in the gap and hyperfine coupling constants compare favorably with oxidation potential and EPR measurements.

Hybrid functional density functional theory calculations of the oxygen vacancy (VO) in monoclinic bulk WO(3) provide a coherent rationalization for the strong dependence of WO(3-x) electronic properties on the VO concentration and of the semiconductor-to-metal transition, a phenomenon intimately connected to the electrochromic effect. Different VO centers containing W(4+), W(5+), and W(6+) species are expected to coexist. Optical transition levels are 0.7-1.0 eV below the conduction band minimum, in agreement with experiments. The complex nature of V(O) in WO(3) can only be detected with methods that properly describe band gaps and polaronic distortions at defect sites.

The nature of Ag(111)/TiO2 rutile and anatase interfaces, of interest for the design of memristors, has been studied by means of density functional theory calculations using various computational approaches. We have considered interfaces where the lattice mismatch of the oxide crystalline phase and the metal electrode does not result in excessive strain. The bonding at the interface is very weak, and the charge transfer is negligible for stoichiometric oxides. The formation of O vacancies has a lower cost at the interface with Ag than on the bare titiania surface and results in stronger adhesion between the Ag electrode and the reduced TiO2-x oxide. The diffusion of Ag and O atoms or ions across the interface is a thermodynamically unfavorable process which can occur only at high temperatures or under the effect of an external electric field. Once Ag atoms are incorporated in the bulk of TiO2 they can be stabilized in an interstitial site (more favorable) or a position substitutional to Ti. In both cases Ag is ionized and transfers the valence electron to the host crystal with formation of Ti3+ states. The Ag atoms remain positively charged, even when extended Ag chains are formed (nanofilaments). For an Ag filament inside TiO2 to exhibit conductive behavior, a higher density of Ag atoms is required, but this is hardly possible in the regular bulk crystalline lattice of TiO2 without inducing a structural breakdown. © 2011 American Physical Society.

Various hybrid functionals combined with both plane wave and localized basis sets have been used for a systematic study of the structural and electronic properties of all phases of WO3. It is found that hybrid functionals work at least as well as the standard DFT/GGA functional in predicting lattice constants and equilibrium volumes. However, the adoption of hybrid functionals has the advantage to considerably improve the Kohn-Sham band gap which is always severely underestimated by the standard DFT calculations. The HSE06 functional in combination with a plane wave basis set describes well the band gap ofWO(3), while the B3LYP functional associated with a localized basis set slightly overestimates it. The band gap can be made fully consistent with experiment by fixing the amount of Hartree-Fock exchange in the hybrid functional to 15\%.

The O-17 and H-1 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 O-17 was found to be about 7.5 MHz. H-1 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 H-1 and O-17 hfi tensors and their orientations) of the [Ti(H2O)(6)](3+) omplex are in quantitative agreement with the experiments for two slightly different geometrical arrangements associated with D-3d and C-i symmetries.

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.

Distortion is the key: In situ EPR spectroscopy provides the first experimental confirmation that the adsorption of O2 molecules on a stoichiometric ultrathin MgO(001) film on Mo(001) leads to the spontaneous formation of O2.- radicals. The results show that polaronic distortion of the MgO lattice (see picture; Mg yellow, O blue) stabilizes the radical, and this distortion is only possible in very thin films. Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

GaN:ZnO solid solutions have been identified as a promising system for photoatalytic or photoelectrohemical water splitting under visible-light irradiation. However, the origin of their activity at longer wavelength with respect to the parent materials (GaN and ZnO) absorbing in the UV spectrum is still matter of debate. Previous theoretical studies were based on standard GGA or GGA+U calculations which largely underestimated the band gap values of the two semiconductors and are thus not best suited methods for the required analysis. The present is a hybrid density functional study (B3LYP) which provides more accurate description of the electronic structure of the parent semiconductors and is thus also more reliable for the evaluation of the mixed GaN:ZnO system. For small concentrations of ZnO in GaN, loal inhomogeneity of Zn or O concentration must be invoked to observe a red-shift of the absorption edge. For larger concentrations, some random alloy distributions, enthalpically more expensive but entropically more favorable, are found to present reduced band gap values because of a positive shift of the N 2p states from the GaN component interfacing the ZnO fragments as a consequence of the repulsive interaction with the Zn 3d states. © 2010 American Chemical Society.

A hybrid density functional study based on a periodic approach with localized atomic orbital basis functions has been performed in order to compute the optical and thermodynamic transition levels between different charge states of defect impurities in bulk ZnO. The theoretical approach presented allows the accurate computation of transition levels starting from single particle Kohn-Sham eigenvalues. The results are compared to previous theoretical findings and with available experimental data for a variety of defects ranging from oxygen vacancies, zinc interstitials, and hydrogen and nitrogen impurities. We find that H and Zn impurities give rise to shallow levels; the oxygen vacancy is stable only in the neutral V-O and doubly charged V-O(2+) variants, while N-dopants act as deep acceptor levels.

The nature of nitrogen-doped MgO, N-MgO, is investigated by density functional theory (DFT) calculations using both periodic supercells and emebedded Cluster models. The electronic structure and the spin properties, in particular, the hyperfine coupling constants, of substitutional for O and interstitial N species are determined by using a hybrid exchange-correlation functional in order to account for the self-interaction problem in DFT. Both substitutional and interstitial N introduce magnetic impurities in bulk MgO and generate new energy levels at about 0.5-1.7 eV above the top of the MgO valence band. We have also considered the Simultaneous presence of neutral magnesium vacancies (V-Mg) and neutral oxygen vacancies (V-O) in the material, and we found that a charge transfer can occur between the N-impurity states and these intrinsic defects. In particular, a substitutional nitrogen behaves as all electron donor in the presence of V-Mg, and as an electron acceptor in the presence of V-O centers. The Occurrence of the internal charge transfer may alter the magnetic properties of the material.

The formation of O-2(-) radical anions by contact of O-2 molecules with a Na pre-covered MgO surface is studied by a combined EPR and quantum chemical approach. Na atoms deposited on polycrystalline MgO samples are brought into contact with O-2. The typical EPR signal of isolated Na atoms disappears when the reaction with O-2 takes place and new paramagnetic species are observed, which are attributed to different surface-stabilised O-2(-) raclicals. Hyperfine sublevel correlation (HYSCORE) spectroscopy allows the superhyperfine interaction tensor of O2-Na+ species to be demonstrating the direct of the O-2(-) adsorbate to determined, coordination surface Na+ cations. DFT calculations enable the structural details of the formed species to be determined. Matrix-isolated alkali superoxides are used as a standard to enable comparison of the formed species, revealing important and unexpected contributions of the MgO matrix in determining the electronic structure of the surface-stabilised Na+-O-2(-) complexes.

Oxidation of magnesium nitride, Mg3N2, leads to segregated nitrogen impurities within the bulk of MgO. Electron paramagnetic resonance experiments in conjunction with quantum chemical calculations allow us to identify these paramagnetic centers as N2- radical anions trapped in anion vacancies in the bulk of MgO. The characteristics of the defect center, derived from the spin Hamiltonian parameters, exhibit similarities but also some interesting differences with respect to the classical N2- species in KCl, a sort of prototype of molecular anions trapped in ionic crystals. © 2010 American Chemical Society.

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.

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.

The preferred charge and spin state of an interstitial (Ti(int)) atom in reduced bulk rutile or anatase Ti(1+x)O(2) has been investigated with spin-polarized hybrid density functional theory (DFT) calculations. A neutral Ti(int) placed in an interstitial cavity of titania spontaneously transforms into a Ti(3+) ion with nearly one electron localized on the 3d shell; the remaining three electrons are donated to the lattice Ti ions. Spin polarization is essential to obtain a correct description of the electronic structure of the defect.

Co-doping of TiO(2) with Cr and Sb was recently found to have a beneficial effect on the photocatalytic activity under visible-light irradiation. With the present comparative standard and hybrid density functional study we get new insight into the electronic structure of the system and provide a theoretical support to the experimental findings. Hybrid DFT better describes (i) the Cr d states splitting and thus the semiconducting properties of the Cr-doped system, and (ii) the localized nature of the Ti(3+) states produced by Sb doping. An electron transfer from the Sb-induced states to the Cr 3d levels is considered to be the reason for the enhanced photostability of the co-doped Cr/Sb system. (C) 2008 Elsevier B. V. All rights reserved.

We have studied the electron paramagnetic resonance (EPR) properties of a classical group of paramagnetic centers in solid KCl, the H center (a trapped Cl(2)(-) species) and the analogous O(2)(-) and N(2)(-) centers. The calculations have been performed at the density functional theory level using pure and hybrid exchange-correlation functionals in combination with cluster models embedded in point charges. The attention has been focused on the reproduction of key EPR parameters such as the g-and the A-tensors. The A-tensor is satisfactorily reproduced for all three species. The g-tensor is well described in the case of the H center, while large deviations are found for the O(2)(-) and N(2)(-) trapped radicals. The different behavior is rationalized in terms of different symmetries of the singly occupied molecular orbitals, s in Cl(2)(-) and p in O(2)(-) and N(2)(-).