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Singly and multiply doped graphene oxide quantum dots have been synthesized by a simple electrochemical method using water as solvent. The obtained materials have been characterized by photoemission spectroscopy and scanning tunneling microscopy, in order to get a detailed picture of their chemical and structural properties. The electrochemical activity toward the oxygen reduction reaction of the doped graphene oxide quantum dots has been investigated by cyclic voltammetry and rotating disk electrode measurements, showing a clear decrease of the overpotential as a function of the dopant according to the sequence: N ∼ B > B,N. Moreover, assisted by density functional calculations of the Gibbs free energy associated with every electron transfer, we demonstrate that the selectivity of the reaction is controlled by the oxidation states of the dopants: as-prepared graphene oxide quantum dots follow a two-electron reduction path that leads to the formation of hydrogen peroxide, whereas after the reduction with NaBH4, the same materials favor a four-electron reduction of oxygen to water. (Chemical Equation Presented).

The nature of hole centers in a series of MeO2 (TiO2, ZrO2, HfO2) metal oxides doped with trivalent Al or Ga ions has been investigated coupling the classic continuous wave electron paramagnetic resonance (CW-EPR) technique with advanced density functional theory (DFT) calculations. The insertion of an aliovalent ion in the structure of the tetravalent oxides is compensated by the creation of oxygen vacancies leading to diamagnetic defective systems. The hole centers are observed by EPR after irradiation using ultraviolet (UV) frequencies (with consequent formation of an electron-hole pair) and trapping of the photogenerated electron. The distortion imparted by the presence of the dopant stabilizes these centers. This generates a rich superhyperfine structure, since the dopants employed in this investigation (Al and Ga) have a nonzero nuclear spin. The DFT calculations performed on a wide set of possible hole-trapping sites occurring in the solid, allow us to identify (comparing the calculated EPR parameters of various models with the experimental ones) the nature of the observed hole centers in all cases. These are always three-coordinated oxygen ions with one Al (or Ga) ion in the first coordinative sphere. As it has been observed in other cases of holes centers, the spin density associated with the unpaired electron is concentrated in an O p-orbital with a modest delocalization toward the first neighboring ions.

Oxides surfaces and thin films are finding continuous new technological applications and represent an important class of systems in materials science. Today we assist to a considerable effort to characterize the surfaces and the interfaces of oxide materials at an atomistic level. The intense experimental activity in this field has stimulated a parallel computational activity based on high-quality first principle calculations. In this review we focus our attention on the properties of oxide surfaces, and we describe the main factors that contribute to determine their behaviour: (1) nature of the bonding and electronic structure of the oxide; (2) surface morphology and defectivity; (3) doping and functionalization; (4) redox properties; (5) nano-dimensionality (e.g. in ultra-thin films). We also show how each of these parameters can affect the properties of supported metal atoms and nano-particles.

Using all electron fully relativistic DFT calculations we, have computed the EPR properties of Au atoms bound to various sites of the MgO surface. Changes in g-tensor and hyperfine coupling constants provide a way to identify the gold adsorption site and to map the surface morphology by comparison of measured and calculated EPR spectra. We found a strong reduction of the isotropic hyperfine coupling constant, a(iso)(Au), for adsorbed gold compared to the free atom; this reduction, which is about 45\% for terrace sites, is more pronounced when Au interacts with low-coordinated sites like steps, edges and corners where it is about 60\%. The reduction of a(iso)(Au) is accompanied by a corresponding increase of the superhyperfine interaction with the surface oxygen sites, as measured by a(iso)(O-17). Large anisotropies in the g-tensor are computed for all sites.

The formation of an oxygen vacancy and the simultaneous re-adsorption of an oxygen atom on regular and low-coordinated (LC) sites (edges and corners) of the surface of alkaline-earth oxides with cubic rock salt structure, MgO, CaO, SrO, and BaO, has been investigated using DFT cluster model calculations. The process corresponds to the formation of a surface Frenkel defect when the vacancy formation energy is partially compensated by the energy gained in the formation of a peroxo group. The structural and electronic properties of vacancies and peroxo groups along the series of alkaline-earth oxides have been analyzed. We found that the role of low-coordinated sites on the surface chemistry of alkaline-earth oxides is of crucial importance for MgO, but decreases for the heavier members. For instance, on BaO the formation of a peroxo group is practically site-insensitive. This is not the case of the vacancy formation, which is always favored on the low-coordinated sites. Fil: Di Valentin, Cristiana. Università degli Studi di Milano; Italia Fil: Ferullo, Ricardo. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Bahía Blanca. Instituto de Química del Sur. Universidad Nacional del Sur. Departamento de Química. Instituto de Química del Sur; Argentina. Università degli Studi di Milano; Italia Fil: Binda, Riccardo. Università degli Studi di Milano; Italia Fil: Pacchioni, Gianfranco. Università degli Studi di Milano; Italia

The catalytic activity of pure and Ni-doped MgO surfaces in N2O decomposition has been investigated theoretically with DFT B3LYP cluster model calculations. The barrier to abstraction of O from N2O to form a surface peroxo group, O-2(2-), is found to be rate limiting on both pure and Ni-doped MgO. In the presence of Ni impurities, however, the barrier is reduced from 1.37 eV to 1.19 eV, thus accounting for the experimentally observed increase in catalytic activity as function of the Ni content. Dissociation reaction was found to take place also by direct interaction with surface Ni ion with a competing activation barrier of 1.22 eV. However, this latter process is less exothermic (1.45 eV vs 2.19 eV). The 0 abstraction by a Surface oxygen is followed by 0 diffusion and recombination with final desorption of an O-2 molecule from the surface. This process occurs preferentially on pure MgO while requires higher barriers in the presence of Ni. This can explain the observed decrease of catalyst activity when the Ni concentration becomes higher than 50%. (c) 2005 Elsevier B.V. All rights reserved

The nature of electron traps at the surface of polycrystalline MgO is analyzed for the first time in terms of interaction of the electron spin with the nuclear spin of the O-17 anions of the surface. MgO crystals enriched in the O-17 isotope have been prepared and the corresponding hyperfine coupling constants have been measured in EPR spectra. The trapped electrons are produced by exposure of the samples to H-2 followed by UV irradiation, with consequent production of paramagnetic (H+)(e(-)) centers. The EPR spectrum shows a multiplet structure with two main sextets separated by 51 and 10 G, respectively, due to the interaction of the unpaired electron with two nonequivalent O-17 nuclei. The results are interpreted based on accurate quantum chemical calculations. It is suggested that the paramagnetic centers observed correspond to (H+)(e(-)) pairs formed at step sites of the MgO surface

The basicity of regular and low-coordinate (LC) sites (steps, edges and corners) at the surface of alkaline earths with NaCl structure (MgO, CaO, SrO, and BaO) has been investigated by using BF3 as a probe molecule. B-O and B-F distances; O-B-F bond angles; B-F asymmetric stretching frequencies; O, B and F 1s core-level binding energies; and the interaction energy of adsorbed BF3 were determined by means of DFT calculations on cluster models. These adsorption properties were compared with those of complexes of BF3 with molecules with various basicities (water, ammonia, phosphine, etc.). We show that many properties of adsorbed BF3, and in particular the experimentally accessible shifts in vibrational frequency, in B and F 1s core levels, and in BF3 desorption temperature, exhibit a linear correlation with the surface basicity as measured by the vertical ionization potential of the oxide anions. On the other hand, shifts of the O 1s core level binding energy do not provide a simple way to detect surface basicity. On a given oxide surface, the differing basicities of various sites result in measurable differences in adsorption properties. This suggests the potential use of BF3 as a probe molecule for titrating LC sites on the surface of ionic oxides.

The NO/NiO(100) system represents an excellent test case for the theory of surface chemical bond since accurate information about geometry, adsorption strength, and spin properties is available from experiments performed on NiO and Ni-doped MgO powders, single crystals, and thin films. We used cluster models to describe the NO/NiO interaction in combination with density functional theory (DFT) and wave function-based methods. We have identified four major aspects of the interaction: (1) the bonding cannot be described by a single determinant; (2) a spin-polarized DF-B3LYP approach gives reasonable adsorption properties at the price of a physically incorrect spin distribution; (3) a key ingredient of the interaction is the Coulomb repulsion within the Ni 3d shell; since this term is described very differently depending on the exchange-correlation functional it can result in overbound generalized gradient approach or Becke, Lee, Yang, and Parr or in strongly unbound (HFLYP) systems depending on the DFT approach; (4) the proper inclusion of the dynamical correlation is essential to treat the on-site Coulomb repulsion within the Ni 3d shell and to provide an accurate bond strength. In fact, the explicitly correlated complete-active-space second-order perturbation theory method gives results in overall agreement with the experiment. This shows the importance of treating on the same footing spin and electron correlation as well as the multiconfiguration character of the wave function. (C) 2002 American Institute of Physics

Adsorption of NO on MgO ultrathin films grown on a Mo(100) substrate results in the formation of N 2 O at low temperature (120 K). In addition, small amounts of N 2 O are produced at 280 K. Isotope exchange experiments showthat the reaction does not involve lattice oxygens. Ab initio DFT/B3LYP cluster model calculations have been performed to understand the mechanism of the reaction. Various defect sites on the MgO surface have been considered. It is found that the reaction occurs preferentially at neutral oxygen vacancies (F centers) through the adsorption of a first NO molecule followed by the addition of a second NO; from the adsorbed dimer, N 2 O 2 , N 2 O forms with an activation barrier of about 0.1 eV, leaving a regular site instead of the original F center. This result is consistent with the experimental observation that subsequent exposure of the surface to NO results in a reduced production of N 2 O.

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.

The dispersion of small amounts of Ce4+ ions in the bulk of ZrO2 leads to a photoactive material sensitive to visible light. This is shown by monitoring with EPR the formation and the reactivity of photogenerated (λ > 420 nm) charge carriers. The effect, as confirmed by DFT calculations, is due to the presence in the solid of empty 4f Ce states at the mid gap, which act as intermediate levels in a double excitation mechanism. This solid can be considered an example of a third-generation photoactive material. © 2014 American Chemical Society.

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 se 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. (C) 2013 Elsevier Ltd. All rights reserved.

CONSPECTUS: 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 accurate description of the oxide band gap and of the electron localization of the impurity states associated with dopants or defects, we resorted to the use of hybrid functionals (B3LYP), where a portion of exact exchange in the exchange-correlation functional partly corrects for the self-interaction error inherent in DFT. In many cases, the self-interaction correction is very important, and these results can lead to a completely different physical picture than that obtained using local or semilocal functionals. We analyzed the electronic transitions in terms of their transition energy levels, which provided a more accurate comparison with experimental spectroscopic data than Kohn-Sham eigenvalues. The effects of N-doping were similar among the three oxides that we considered. The nature of the impurity state is always localized at the dopant site, which may limit their application in photocatalytic processes. Photocatalytic systems require highly delocalized photoexcited carriers within the material to effectively trigger redox processes at the surface. The nature of the electronic states associated with the oxygen deficiency differed widely in the three investigated oxides. In ZnO1-x and WO3-x the electronic states resemble the typical F-centers in insulating oxides or halides, with the excess electron density localized at the vacancy site. However, TiO2 acts as a reducible oxide, and the removal of neutral oxygen atoms reduced Ti(4+) to Ti(3+).

No abstract available]

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.

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.

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

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.

Graphene inertness towards chemical reactivity can be considered as an accepted postulate by the research community. This limit has been recently overcome by chemically and physically modifying graphene through non-metal doping or interfacing with acceptor/donor materials (metals or semiconductors). As a result, outstanding performances as catalytic, electrocatalytic, and photocatalytic material have been observed. In this critical Review we report computational work performed, by our group, on the reactivity of free-standing, metal- and semiconductor-supported B-doped graphene towards oxygen, which is at the basis of extremely important energy-related chemical processes, such as the oxygen reduction reaction. It appears that a combination of doping and interfacing approaches for the activation of graphene can open unconventional and unprecedented reaction paths, thus boosting the potential of modified graphene in many chemical applications.

(WO(3))(3) clusters deposited on the (110) rutile TiO(2) surface are excellent catalysts for the formaldehyde (CH(2)O) 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-TiO(2) surface, the (WO(3))(3) clusters can be neutral, singly charged, or doubly charged. We find that only neutral (WO(3))(3) and anionic (WO(3))(3)(-) clusters are reactive toward CH(2)O 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 CH(2)O molecules and the formation of acetal and polyacetal intermediates, which inhibits the chain propagation. Only in the case of the anionic (WO(3))(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.

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.

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.

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.

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(s), and Cu interstitial, Cu(i), 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.

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.

It has been experimentally observed that Ti doping of bulk ZrO(2) induces a large red-shift of the optical absorption edge of the material from 5.3 to 4.0 eV [Livraghi et al., J. Phys. Chem. C, 2010, 114, 18553-18558]. In this work, density functional calculations based on the hybrid functional B3LYP show that Ti dopants in the substitutional position to Zr in the tetragonal lattice cause the formation of an empty Ti 3d band about 0.5 eV below the bottom of the conduction band. The optical transition level ε(opt)(0/-1) from the topmost valence state to the lowest empty Ti impurity state is found at 4.9 eV in a direct band gap of 5.7 eV. The calculated shift is consistent with the experimental observation. The presence of Ti(3+) species in Ti-doped ZrO(2), probed by means of electron paramagnetic resonance (EPR), is rationalized as the result of electron transfers from intrinsic defect states, such as oxygen vacancies, to substitutional Ti(4+) centers.

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.

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 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) (-) radicals. Hyperfine sublevel correlation (HYSCORE) spectroscopy allows the superhyperfine interaction tensor of O(2) (-)Na(+) species to be determined, demonstrating the direct coordination of the O(2) (-) adsorbate to 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.

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 report the results of density functional theory calculations on the electronic structure of boron-doped anatase TiO2. For the calculations we used both standard and hybrid exchange-correlation functionals. On the basis of a close comparison of computed and measured observable properties, in particular hyperfine coupling constants from electron paramagnetic resonance experiments and core level binding energies from X-ray photoelectron spectroscopy spectra, we propose the existence of both substitutional to oxygen and interstitial boron atoms in the bulk of anatase. Boron substitional to oxygen results in a paramagnetic defect, [BTi 3], which introduces new states in the midgap of the material. The analysis of the stability of this center, however, shows that it should not be the dominant species and should convert into interstitial boron after annealing at high temperature. Various possible sites for interstitial boron have been considered where the dopant is coordinated to three, [BO3], or to four, [BO4], oxygen atoms. The electronic characteristics of interstitial boron are rather independent of the site where the atom is incorporated. Boron in interstitial positions behaves as a three-electron donor with formation of Bh3+ and reduction of Ti4+ to Ti 3+. The possible copresence of substitutional and interstitial boron could also result in internal charge transfer leading to more complex situations. © 2009 American Chemical Society.

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.

In this Letter the interaction of Li atoms with terrace sites of MgO(001) surfaces is characterized by combining EPR spectroscopy with theoretical calculations. The Li atoms adsorbed to MgO terrace sites show a reduction of the isotropic hyperfine coupling constant by approximately 50% with respect to Li atoms in the gas phase. In combination with theoretical calculations it can be shown that this reduction of the hyperfine coupling constant is not due to a charge transfer between Li and MgO but can be understood by a polarization of the Li atoms which are essentially neutral. (C) 2007 Elsevier B.V. All rights reserved.

Recent experiments have indicated that titanium dioxide (TiO(2)) codoped with nitrogen and fluorine may show enhanced photocatalytic activity in the visible region with respect to TiO(2) doped only with nitrogen. Prompted by these findings, we have investigated N-F codoped TiO(2) through a combined theoretical and experimental study. Density functional theory (DFT) calculations have been carried out both within the generalized gradient approximation (GGA) and using hybrid functionals to accurately describe the electronic structure; substitutional as well as interstitial locations of nitrogen in the TiO(2) lattice were considered. From these calculations we infer that N-F codoping reduces the. energy cost of doping and also the amount of defects (number of oxygen vacancies) in the lattice, as a consequence of the charge compensation between the nitrogen (p-dopant) and the fluorine (n-dopant) impurities. The UV-visible spectra of the sol-gel prepared TiO(2) powders confirm the synergistic effect of N-F codoping: more impurities are introduced in the lattice with an increased optical absorption in the visible. EPR spectroscopy measurements on the codoped samples identify two paramagnetic species which are associated to bulk N impurities (N(b)(center dot)) and Ti(3+) ions. Preliminary photocatalytic tests also indicate an enhanced activity under vis-light irradiation toward degradation of methylene blue for the codoped system with respect to N-doped TiO(2).

The removal of lattice O atoms, as well as the addition of interstitial H atoms, in TiO(2) is known to cause the reduction in the material and the formation of ``Ti(3+){'' ions. By means of electronic structure calculations we have studied the nature of such oxygen vacancy and hydrogen impurity states in the bulk of the anatase polymorph of TiO(2).The spin polarized nature of these centers, the localized or delocalized character of the extra electrons, the presence of defect-induced states in the gap, and the polaronic distortion around the defect have been investigated with different theoretical methods: standard density functional theory (DFT) in the generalized-gradient approximation (GGA), GGA+U methods as a function of the U parameter, and two hybrid functionals with different admixtures of Hartree-Fock exchange. Theresults are found to be strongly dependent on the method used. Only GGA+U or hybrid functionals are able to reproduce the presence of states at about 1 eV below the conduction band, which are experimentally observed in reduced titania. The corresponding electronic states are localized on Ti 3d levels, but partly delocalized solutions are very close in energy. These findings show the limited predictive power of these theoretical methods to describe the electronic structure of reduced titania in the absence of accurate experimental data. (C) 2008 American Institute of Physics. [DOI: 10.1063/1.2996362]}

In this work we present a simplified procedure to use hybrid functionals and localized atomic basis sets to simulate scanning tunneling microscopy (STM) images of stoichiometric, reduced and hydroxylated rutile (110) TiO2 surface. For the two defective systems it is necessary to introduce some exact Hartree-Fock exchange in the exchange functional in order to correctly describe the details of the electronic structure. Results are compared to the standard density functional theory and planewave basis set approach. Both methods have advantages and drawbacks that are analyzed in detail. In particular, for the localized basis set approach, it is necessary to introduce a number of Gaussian function in the vacuum region above the surface in order to correctly describe the exponential decay of the integrated local density of states from the surface. In the planewave periodic approach, a thick vacuum region is required to achieve correct results. Simulated STM images are obtained for both the reduced and hydroxylated surface which nicely compare with experimental findings. A direct comparison of the two defects as displayed in the simulated STM images indicates that the OH groups should appear brighter than oxygen vacancies in perfect agreement with the experimental STM data. (C) 2007 American Institute of Physics

The electronic structure and spectral properties of self-trapped holes (STH's) in $\mathrm{Si}{\mathrm{O}}_{2}$ are revisited using pure and hybrid density functional theory (DFT) approaches and cluster models. The structure and electron spin resonance (ESR) parameters [hyperfine coupling constants (hfcc's) and $g$ factors] have been determined for the two classical variants of STH's: ${\mathrm{STH}}_{1}$, consisting of a hole trapped at the $2p$ nonbonding orbital of an O atom bridging two Si atoms, and ${\mathrm{STH}}_{2}$, a metastable defect where the hole is delocalized over the $2p$ orbitals of two bridging O atoms. The computed ESR parameters fully support the experimental assignments based on the analysis of the ESR spectra. However, a nearly quantitative agreement between measured and computed hfcc's and $g$ factors is found only with a hybrid functional where 50% of the Hartree-Fock exchange is mixed in with the DFT exchange. Standard hybrid functionals, such as B3LYP, or pure DFT functionals, such as BLYP, fail completely in describing the two defects. According to these results, ${\mathrm{STH}}_{2}$ can form only in correspondence of specific O-Si-O angles of about 80\ifmmode^\circ\else\textdegree\fi{}--90\ifmmode^\circ\else\textdegree\fi{}, which classifies the defect center as a molecular polaron.

Nitrogen doped titanium dioxide is attracting a continuously increasing attention because of its potential as material for environmental photocatalysis. In this paper we review experimental and theoretical work done on this system in our groups in recent years. The analysis is largely based on electron paramagnetic resonance (EPR) spectra and on their interpretation based on high-level ab initio calculations. N-doped anatase TiO2 contains thermally stable single N-atom impurities either as charged diamagnetic N-b(-) centers or as neutral paramagnetic N-b(.) centers (b stays for bulk). The N-atoms can occupy both interstitial or substitutional positions in the solid, with some evidence for a preference for interstitial sites. All types of N-b centers give rise to localized states in the band-gap of the oxide, thus accounting for the related reduction of absorption band edge. The relative abundance of these species depends on the oxidation state of the solid. In fact, upon reduction, oxygen vacancies form and transfer electrons from Ti3+ ions to the N-b(.) with formation of Ti4+ and N-b(.). EPR spectra measured under irradiation show that the Nb centers are responsible for visible light absorption with promotion of electrons from the localized N-impurity states to the conduction band or to electron scavengers like O-2 adsorbed on the surface. These results provide an unambiguous characterization of the electronic states associated with N-impurities in TiO2 and a realistic picture of the processes occurring in the solid under irradiation with visible light.