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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)(-).

Defect states in reduced and n-type doped titania are of fundamental importance in several technologically important applications. Still, the exact nature of these states, often referred to as "Ti3+ centers", is largely unclear and a matter of debate. The problem is complicated by the fact that electronic structure calculations based on density functional theory (DFT) in the local density approximation (LDA) or semilocal generalized gradient approximation (GGA) provide results that do not account for many of the experimentally observed fingerprints of the formation of Ti 3+ centers in reduced TiO2. Here, we investigate the properties of at least four different types of Ti3+ centers in bulk anatase, (1) 6-fold-coordinated Ti6c3+ ions introduced by F- or Nb-doping, (2) Ti6c3+-OH species associated with H-doping, (3) undercoordinated Ti5c3+ species associated with oxygen vacancies, and (4) interstitial Ti5c3+ species. The characterization of these different kinds of Ti3+ centers is based on DFT+U and/or hybrid functional calculations, which are known to (partially) correct the self-interaction error of local and semilocal DFT functionals. We found that strongly localized solutions where an excess electron is on a single Ti3+ ion are very close in energy and sometimes degenerate with partly or highly delocalized solutions where the extra charge is distributed over several Ti ions. The defect states corresponding to these different solutions lie at different energies in the band gap of the material. This has important implications for the conductivity mechanism in reduced or n-type doped titania and suggests a significant role of temperature in determining the degree of localization of the trapped charge. © 2009 American Chemical Society.

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.

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.

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]}

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.

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.

The electronic structure and spectral properties of self-trapped holes (STH's) in SiO(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: STH(1), consisting of a hole trapped at the 2p nonbonding orbital of an O atom bridging two Si atoms, and 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, STH(2) can form only in correspondence of specific O-Si-O angles of about 80 degrees-90 degrees, which classifies the defect center as a molecular polaron.

The incorporation of nitrogen impurities in the anatase TiO2(101) surface is investigated by first principles density functional theory calculations. Several substitutional and interstitial configurations and different concentrations for the N impurities in the surface and subsurface layers are considered, as well as their interactions with oxygen vacancies in the TiO2 lattice. The stability of the various investigated systems is compared on the basis of their formation energy as a function of the oxygen chemical potential, which determines whether the system is in an oxidizing or reducing environment. Under oxygen-rich conditions, N bound to a surface O is preferred, whereas, under oxygen-poor conditions, substitutional N together with oxygen vacancies is favored, as previously found for bulk TiO2. The cost of formation of a surface oxygen vacancy is almost cancelled in the presence of N impurities in subsurface layers. The incorporation of nitrogen in the lattice modifies the electronic structure by introducing localized states in the band gap, consistent with the experimentally observed absorption of N-doped anatase samples in the visible region. Also, these N impurity states are excellent traps for the Ti3+ electrons deriving from surface oxygens vacancies. STM images for the various N-doped anatase (101) surface models have been computed to provide a reference to future experimental studies of this surface

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.

Charge traps at the surface of oxide materials play a fundamental role in various chemical processes, such as the activation of supported metal clusters. In this study, combining electron paramagnetic resonance with cluster model DFT calculations, we show that excess electrons at the surface of MgO, CaO, and SrO polycrystalline materials can be generated by preparing weakly hydroxylated surfaces followed by deposition of small amounts of alkali metals. The residual OH groups present on specific sites of the partially dehydroxylated surface act as stable traps for electrons donated by the alkali metal (Na in this case) which forms a Na+ ion distant from the trapped electron. This process results in the formation of thermally stable (H+)(e(-)) color centers at the surface of the oxide. The procedure could be of interest for the stabilization and activation of supported metal nanoparticles with potential use in catalysis

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.

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 formation of gold atoms and clusters from the reduction by hydrogen of mononuclear [Au(CH3)(2)(acac)] organometallic complexes deposited on the MgO surface has been studied by first principle density functional theory calculations. The comparison of measured and computed infrared data suggests the occurrence of a dissociative adsorption of the complex on the surface of the oxide, although a given fraction of undissociated molecules can cohexist. The reduction can follow various mechanisms, leading to the formation of neutral or positively charged mononuclear gold intermediates. We show that some of these intermediates can easily diffuse on the surface at room temperature and lead to nucleation of stable partially oxidized gold clusters

(H+)(e(-)) pairs generated at the surface of polycrystalline CaO are analyzed for the first time in terms of the interaction of the unpaired electron spin with the nuclear spin of the O-17 anions of the surface. CaO crystals enriched in the O-17 isotope are prepared and the corresponding hyperfine coupling constants are measured in electron paramagentic resonance (EPR) spectra. The results are analyzed on the basis of cluster model density functional theory calculations. The computed hyperfine coupling constants for (H+)(e(-)) pairs formed on the edge, corner, and reverse corner sites of the CoO surface allow a tentative assignment of two observed spectral features to specific morphological surface sites.

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. (c) 2006 Elsevier B.V. All rights reserved.

The bonding, spin density related properties, and vibrational frequency of CO bound to single Au atom in the gas-phase or supported on MgO surfaces have been investigated with a variety of computational methods and models: periodic plane waves calculations have been compared with molecular approaches based on atomic orbital basis sets; pseudopotential methods with all electron fully relativistic calculations; various density functional theory (DFT) exchange-correlation functionals with the unrestricted coupled-cluster singles and doubles with perturbative connected triples {[CCSD(T)]. AuCO is a bent molecule but the potential for bending is very soft, and small changes in the bond angle result in large changes in the CO gas-phase vibrational frequency. At the equilibrium geometry the DFT calculated vibrational shift of CO with respect to the free molecule is about -150 cm(-1), whereas smaller values -60/-70 cm(-1) are predicted by the more accurate CCSD(T) method. These relatively large differences are due to the weak and nonclassic bonding in this complex. Upon adsorption on MgO, the CO vibrational shift becomes much larger, about -290 cm(-1), due to charge transfer from the basic surface oxide anion to AuCO. This large redshift is predicted by all methods, and is fully consistent with that measured for MgO/AuCO complexes. The strong influence of the support on the AuCO bonding is equally well described by all different approaches.}

Surface excess electrons are remarkable chemical entities that provide great opportunities for the design of new materials with precisely tuned electronic and magnetic properties. In this Account, we describe the structure and electronic properties of excess electron centers generated at the surface of insulating oxides. We also outline the elementary mechanisms that are at the basis of the generation of excess electrons at solid surfaces, setting a comparison to the general problem of excess electron localization in condensed media. Emphasis is given to morphological aspects relative to the surface-trapping sites as deduced from combined electron paramagnetic resonance and accurate quantum chemical calculations. The remarkable reactivity featured by the so formed "electron-rich" surfaces is illustrated, describing the reduction of simple diatomic molecules that form adsorbed radical anions via direct surface to adsorbate electron transfer

Innocence lost: The IR spectra of Au/CO complexes adsorbed on of a MgO(001) film differ from the corresponding neutral Au clusters. Calculations indicate this shift of 200-250 cm-1 can be associated with a negative charge on the Au/CO complex. For Au/CO complexes on so-called color centers a charge transfer is expected, however Au atoms on terrace sites are neutral. Thus, CO in fact induces the charge transfer instead of reporting the charge state of the adsorbed Au atoms. (Chemical Equation Presented) © 2006 Wiley-VCH Verlag GmbH & Co. KGaA.

Nitrogen-doped titanium dioxide (N-TiO2), a photocatalytic material active in visible light, has been investigated by a combined experimental and theoretical approach. The material contains singleatom nitrogen impurities that form either diamagnetic (Nb -) or paramagnetic (Nb ¥) bulk centers. Both types of Nb centers give rise to localized states in the band gap of the oxide. The relative abundance of these species depends on the oxidation state of the solid, as, upon reduction, electron transfer from Ti3+ ions to Nb ¥ results in the formation of Ti4+ and Nb -. EPR spectra measured under irradiation show that Nb centers are responsible for visible light absorption with promotion of electrons from the band gap localized states to the conduction band or to surface-adsorbed electron scavengers. These results provide a characterization of the electronic states associated with N impurities in TiO2 and, for the first time, a picture of the processes occurring in the solid under irradiation with visible light

In order to assign the defect responsible for the experimental electron paramagnetic resonance (EPR) signal with trigonal symmetry (T center), we have studied the properties of different paramagnetic centers in yttria-stabilized cubic zirconia by computing the EPR g -tensor from density functional perturbation theory. We have considered reduced vacancy-zirconium complexes and reduced Ti impurities. These first-principles calculations allow us to discard the experimental assignment of the T center to an extrinsic Ti3+ ion nearest neighbor to a single vacancy. Instead, the calculated EPR g tensors of both a Zr3+ or a Ti3+ ion at the center of a divacancy aligned along the 111 directions are compatible with the experimental EPR signal. However, since the EPR signal of the T center is correlated experimentally with an optical absorption band at 370 nm, calculated optical excitations allow us to decide in favor of the Ti3+ divacancy complex.

It has been experimentally observed that a bridging oxygen vacancy on the rutile TiO2(110) surface introduces localized Ti(3)3d(1) states about 1 eV below the conduction band which are not removed upon dissociation of a water molecule and formation of a pair of hydroxyl groups. Density functional calculations based on pure exchange-correlation functionals have not been able to satisfactorily reproduce and analyze these findings. Here we show that a correct description of the localized defect states on reduced and hydroxylated TiO2(110) is achieved only if proper geometry relaxation is accounted for using hybrid exchange functionals. We confirm the electron trapping nature of Ti(OH) groups but find no evidence that these defects should also act as hole traps by formation of Ti4(OH)(.) radicals.

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

Ab initio molecular dynamics (MD) is used to investigate NO reaction processes on the (001) surface of CaO. A novel path is proposed for the first steps of nitrogen oxides reactivity catalyzed by the CaO surface. The mechanism consists of the formation of anionic dimers, adsorbing on the surface cations, at the expense of oxidized NO species adsorbed on surface anions. The complete charge-transfer process takes place in two steps, producing, first monovalent anionic dimers (NO)(2)(-) and, later on, divalent anionic dimers (NO)(2)(-). These redox processes cause spin quenching and are observed in the short time scale of the ab initio MD simulation at 300 K. The results presented provide a rationalization of a recent electron spin resonance (ESR) investigation indicating that the spectroscopy is silent to most of the nitrogen oxide species adsorbed on CaO powders, despite deposition of paramagnetic NO molecules at room temperature

We report g tensors of surface color centers, so-called F-s(+) centers, of MgO calculated with two density-functional approaches using accurately embedded cluster models. In line with recent UHV measurements on single-crystalline MgO film, we determined only small g-tensor anisotropies and negative shifts Delta g equivalent to g-g(e) for all F-s(+) sites considered, namely, (001)-terrace, step, edge, and corner sites. The g values are very sensitive to the local structure of the defect: relaxation reverses the sign of Delta g. However, accounting for the spin-orbit interaction either self-consistently or perturbatively yields very similar results. In addition to the values of the tensor components, their direction with respect to the surface was determined. In contrast to edges, significant deviations from ideal C-2v symmetry were found for F-s(+) centers at steps. Recent data on single-crystalline thin films are reevaluated in the light of these results

Paramagnetic centers at the surface of ionic oxides in the form of trapped electrons can be generated by exposure of the material to alkali metal or hydrogen atoms or of molecular hydrogen under UV irradiation. For many years, it has been assumed that the resulting paramagnetic centers consist of oxygen vacancies filled by one electron. High-resolution electron spin resonance spectra and ab initio quantum chemical calculations show that the paramagnetic centers consist of (H+)(e(-)) electron pairs formed at morphological irregularities of the surface. At least three different kinds of (H+)(e(-)) centers, {[A], [B], and [C], have been identified with abundances of 80\%, 10\%, and 8\%, respectively. In this work, we compare a wide set of measured and computed g-factors and hyperfine coupling constants of the unpaired electron with the surrounding Mg-25, O-17, and H-1 nuclei and we propose a general assignment of the centers. (H+)(e(-)) pairs formed at Mg-4c, ions at steps and edges account for species [A], centers formed at Mg-4c ions at reverse corners correspond to species [B], and species [C] originates from (H+)(e-) pairs formed at Mg-3c ions at corners and kinks.}

Surface trapped hole centers have been generated by X-ray irradiation on an O-17 isotope enriched MgO sample. The resulting EPR spectrum allowed us to determine for the first time the complete O-17(-) hyperfine tensor (A = {[18.6, 18.6, -104.8] G). Comparison of the measured data with those obtained by embedded cluster density functional theory calculations allows to identify the hole trapping center with three-coordinated oxide anions localized at MgO corners. (C) 2005 Elsevier B.V. All rights reserved.}

The formation of Pd dimers on the surface of MgO has been studied by means of density functional theory (DFT) cluster model calculations. The following surface sites have been considered: regular five-coordinated anions at the (100) terraces, monoatomic steps, OH groups, and neutral vacancies (F centers). We discuss the energy balance of forming a dimer at a given site with respect to two isolated Pd atoms, one adsorbed at the defect and one on a regular terrace site. We found that all the defects considered lead to an energy gain when the dimer is formed, suggesting that they can be involved in nucleation and growth processes of metal clusters on the MgO surface. The dimerization energy is moderate for steps (approximate to0.8 eV), large for OH groups (approximate to1.3 eV) and rather small (<0.5 eV) for F centers. (C) 2004 Elsevier B.V. All rights reserved.

The reactivity of low-coordinated anions of MgO towards CO has been studied by means of ab initio cluster model DFT calculations. Differently from MgO terraces, where the O2- anions are totally unreactive, oxide anions at step, edge and corner sites exhibit a very high basicity and reactivity towards CO. The reaction: [O-LC(CO)(n-1)](2-) (s) + CO (g) -> [O-LC(CO)(n)](2-) (s) leads to the formation of polymeric (CnOn+1)(2-) species, where n goes from 1 to 5. The reaction is non-activated and exothermic; only the formation of the tetramer appears to be nearly thermoneutral. The formation of the hexamer (n = 6) is thermodynamically unfavorable. For each value of n, several isomers have been considered. The vibrational properties of the [OLC(CO)n] 2- surface complexes have been computed and used to interpret the low-temperature infrared spectra obtained in the group of Zecchina [G. Spoto, E.N. Gribov, G. Ricchiardi, A. Damin, D. Scarano, S. Bordiga, C. Lamberti, A. Zecchina, Prog. Surf. Sci. 76 (2004) 71]. (c) 2005 Elsevier B.V. All rights reserved.

This paper deals with the microscopic mechanism of nanolithography of self-assembled monolayers (SAM) of alkanethiol molecules on Au(111) induced by the exposure of the film to a beam of "cold" Cs atoms. Density functional theory calculations have been carried out to elucidate the mechanism of interaction of the Cs atoms with the SAM. We found that the film damage occurs in two steps: the Cs atom penetrates the SAM and at a distance of 10-12 Angstrom from the surface donates one electron to Au, forming a Cs+ cation which binds strongly to the surface and interacts with the polar head of the SR molecule. The thermal energy released in this process largely exceeds the energy required to stimulate the desorption of RS-SR disulfide molecules from the Au surface with consequent damage of the film. No chemical interaction occurs between Cs or Cs+ and the hydrocarbon chain of the thiol molecule.

Recent experimental studies have determined that carbon doping dramatically improves the photocatalytic activity of TiO2 in the visible-light region. Using density functional theory (DFT) calculations within the generalized gradient corrected approximation, we investigate various structural models of carbon impurities in both the anatase and rutile polymorphs of TiO2 and analyze the associated modifications of the electronic band structure. We compare the stability of all these diverse species on the basis of their energy of formation as a function of the oxygen chemical potential, which determines whether the system is in an oxidizing or reducing environment. At low carbon concentrations, we find that, under oxygen-poor conditions, substitutional (to oxygen) carbon and oxygen vacancies are favored, whereas, under oxygenrich conditions, interstitial and substitutional (to Ti) C atoms are preferred. Higher carbon concentrations undergo an unexpected stabilization caused by multidoping effects, interpreted as inter-species redox processes. Carbon impurities result in modest variations of the band gap but induce several localized occupied states in the gap, which may account for the experimentally observed red shift of the absorption edge toward the visible. Our results also indicate that carbon doping may favor the formation of oxygen vacancies in bulk TiO2

Electron paramagnetic resonance (EPR), X-ray photoelectron spectroscopy (XPS), and density functional theory (DFT) calculations are combined for the first time in an effort to characterize the paramagnetic species present in N-doped anatase TiO2 powders obtained by sol-gel synthesis. The experimental hyperfine coupling constants are well reproduced by two structurally different nitrogen impurities: substitutional and interstitial N atoms in the TiO2 anatase matrix. DFT calculations show that the nitrogen impurities induce the formation of localized states in the band gap. Substitutional nitrogen states lie just above the valence band, while interstitial nitrogen states lie higher in the gap. Excitations from these localized states to the conduction band may account for the absorption edge shift toward lower energies (visible region) observed in the case of N-doped TiO2 with respect to pure TiO2 (UV region). Calculations also show that nitrogen doping leads to a substantial reduction of the energy cost to form oxygen vacancies in bulk TiO2. This suggests that nitrogen doping is likely to be accompanied by oxygen vacancy formation. Finally, we propose that the relative abundance of the two observed nitrogen-doping species depends on the preparation conditions, such as the oxygen concentration in the atmosphere and the annealing temperature during synthesis.

We have studied the interaction of K atoms with the surface of polycrystalline alkaline-earth metal oxides (MgO, CaC, SrO) by means of CW- and Pulsed-EPR, UV-Vis-NIR spectroscopies and DFT cluster model calculations. The K adsorption site is proposed to be an anionic reverse corner formed at the intersection of two steps, where K binds by more than 1 eV, resulting in thermally stable species up to about 100 K. The bonding has small covalent and large polarization contributions, and the K atom remains neutral, With one unpaired electron in the valence shell. The interaction results in strong modifications of the K electronic wave function which are directly reflected by the hyperfine coupling constant, (K)a(iso). This is found to be a very efficient "probe" to measure the degree of metal-oxide interaction which directly depends on the Substrate basicity. These results provide an original and general model of the early stages of the metal-support interaction in the case of ionic oxides

Point defects on oxide surfaces are presumed to be preferential nucleation sites for supported metal clusters. Under typical growth conditions, dimers constitute the first step in island nucleation. Firstprinciples calculations on the formation of Pd dimers on regular and defect sites of the MgO(100) surface show that nucleation occurs with large dimer binding energies at divacancies and charged oxygen vacancies (F+ centers), while it is less favorable on steps and neutral F centers. The extensive database of defect trapping/attachment properties gives a firm basis to rationalize recent atomic-force microscopy findings and provides guidelines valid, in general, for ionic substrates

The existence of alkali metal anions in interstitial cavities of crystalline silica and microporous materials (e.g., zeolites) has been suggested based on NMR and EPR measurements but has never been studied theoretically. In this work we report results of density functional theory calculations on the interaction of a Li- anion with cluster models of alpha quartz containing up to approximate to100 Si and O atoms. We also consider the interaction of Li- with orthosilicic acid, Si(OH)(4). The properties of Li- incorporated in a silica structure have been compared with those of the hydride anion, H-. We found that, in agreement with previous theoretical studies, H- forms stable structures where a Si atom becomes penta-coordinated with a bipyramidal trigonal structure. Li- behaves completely differently. Because of its larger size, Li- does not come close enough to the tetra-coordinated Si atom to induce the structural distortion observed for H-. On the contrary, the Li atom becomes incorporated into the silica structure, with formation of a =Si-Odelta--Lidelta+ linkage. The negative charge is transferred to the host silica structure where becomes trapped at pre-existing defects or induces additional structural changes in the network.

We have investigated the origin of the experimentally observed change in photoactivity of anatase and rutile TiO2 induced by substitutional N-doping using state-of-the-art density functional theory calculations. Our results show that in both polymorphs N 2p localized states just above the top of the O 2p valence are present. In anatase these states cause a redshift of the absorption band edge towards the visible region. In rutile, instead, this effect is offset by the concomitant N-induced contraction of the O 2p band, resulting in an overall increase of the optical transition energy. Experimental trends are well described by these results.

Adsorption of NO and NO, molecules on the surface of CaO has been investigated by means of density functional theory (DFT) cluster model calculations. Different embedding schemes (shell models and point charges) have been compared. Beside adsorption sites at flat terraces, defect sites like steps and oxygen vacancies (F and F+ centers) have been studied. The spin properties of the adsorbed molecules have been monitored by computing hyperfine coupling constants. At variance with MgO, NO is strongly bound to the oxide anions at terraces (approximate to 0.8 eV) and steps (approximate to1.4 eV). The spin is localized on the NO molecule and the bonding has covalent character. NO2 adsorption is more complex. On the terrace sites, for the N-down adsorption we found two minima with similar energies but different surface-NO2 distances. In one case (short distance) the spin is entirely on NO2 in the second one (long distance), the bonding has partial charge-transfer character, with localization of 50\% of the spin density on the surface oxide anions. The relative stability of the two structures depends on the level of treatment. In another isomer the molecule is oriented O-down and the bonding has a net charge transfer character (formation of NO2-). On step sites the N-down mode is definitely preferred, the O-N distance is short and the spin is on NO2. Finally, F and F+ centers on CaO(1 0 0) have similar characteristics as on MgO(1 0 0) and exhibit a strong reactivity towards NO and NO2

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; 0, B and F Is 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 8 and F is 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 0 Is core level binding energy do not provide a simple way to detect surface basicity. On a given oxide surface, the differeing 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

Recent experimental results (Hoeft et al 2001 Phys. Rev. Lett. 87 086101) have questioned the capability of current theoretical methods for describing the bonding of NH3, CO, and NO with the NiO(100) surface. We show that these systems do indeed represent a challenge to theory. For different reasons, density functional theory (DFT) fails in describing the bonding of these molecules to the NiO surface. The gradient-corrected functionals which work better for the properties of NH3/NiO and CO/NiO (energies, geometries, vibrations) provide wrong answers for NO/NiO and vice versa. This is not due to the well-known difficulty as regards DFT describing the insulating character of NiO. In fact, exactly the same problem is found for isolated Ni2+ impurities in MgO. A correct description of the bonding of both closed-shell (NH3 and CO) and open-shell (NO) molecules to NixMg1-xO is obtained only after inclusion of dynamical correlation and dispersion forces via wavefunction-based methods. However, even with correlated calculations some uncertainties exist regarding the predicted value of the energy of adsorption of NO on NiO. While CASPT2 calculations reach reasonable agreement with experiment, the results of approximate coupled-cluster calculations (the multi-configuration coupled-electron-pair approach) substantially underestimate the adsorption energy.

The adsorption of NO and NO2 molecules at the BaO surface has been investigated by means of density functional theory (DFT) cluster model calculations. Regular adsorption sites at flat terraces have been compared with those on steps and corners. The properties of the adsorbed molecules have been monitored by computing various observable properties, such as core level binding energies, hyperfine coupling constants, and vibrational frequencies. NO binds strongly at the oxide anions at terraces, steps, and corners of the BaO surface. The bonding has a substantial polarity due to delocalization of charge from the surface to the adsorbate, but cannot be described as a full charge transfer interaction. The spin is almost entirely localized on the NO adsorbed molecule. NO2 binds to the BaO surface in two different ways, N-down and O-down. In both orientations the oxide anion on the surface is oxidized with formation of an NO2- species which interacts electrostatically with the neighboring Ba cations. The spin is localized on the surface anions by the effect of the creation of a hole in the O(2p) valence shell. The O-down mode is more stable on terrace sites, while on low-coordinated sites the two orientations have similar stabilities. For both NO and NO 2: the low-coordinated sites exhibit a much larger reactivity than the flat terraces. The formation of O- ions in the case of NO 2 adsorption can be very important for the further reactivity of the surface.

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 N2O at low temperature (120 K). In addition, small amounts of N2O are produced at 280 K. Isotope exchange experiments show that 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, N2O2, N2O 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 N2O

Methanol adsorption on MgO thin films has been studied by Fourier transform infrared (FTIR) and thermal desorption spectroscopies (TDS), and by ab initio cluster model calculations. Depending on the preparation conditions, films with various concentrations of defects have been obtained. These films exhibit different reactivity toward adsorbed methanol. In particular, at temperatures of 580 K, H-2 is released from defect-rich films while no production of hydrogen is observed on defect-poor films. The calculations show that methanol can interact in three different ways with the surface giving rise to physisorption, chemisorption, or heterolytic dissociation into CH3O- and H fragments depending on the adsorption site. Chemisorption and dissociation occur only at defect sites, like low-coordinated ions at steps. Oxygen vacancies (F centers) are proposed as the sites present on the defect-rich films which are responsible for hydrogen release at high temperature

Cluster-assembled materials open fascinating new routes for tuning physical and chemical properties by changing cluster size and often behave completely differently than their bulk analogues. By depositing gas phase Pd clusters on MgO thin films, model catalysts are fabricated which exhibit remarkable catalytic activity, In contrast to the high selectivity of Pd (111) surfaces for the cyclotrimerization of acetylene to benzene, small supported Pd-n clusters reveal a strongly size-dependent selectivity and catalyse the formation of benzene as well as of other hydrocarbons. The understanding at the atomistic level of the observed processes has been obtained by means of first-principles quantum-mechanical simulations, The theoretical studies have shown the importance of the surface defects of the oxide substrate in stabilizing the supported clusters but also in promoting their catalytic activity. For instance, Pd atoms bound at the regular sites of the MgO (100) terraces do not promote the acetylene to benzene conversion while they become active catalysts when bound at oxygen vacancies (F centers). (C) 2001 Elsevier Science B.V. All rights reserved.