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Gianfranco Pacchioni

Publications and source records attributed to Gianfranco Pacchioni.

At least 19 recordsLinked to original sources
PubMed2006

Origin of photoactivity of nitrogen-doped titanium dioxide under visible light.

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

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PubMed2006

Electronic structure of defect states in hydroxylated and reduced rutile TiO2(110) surfaces.

It has been experimentally observed that a bridging oxygen vacancy on the rutile TiO2(110) surface introduces localized Ti3+ 3d1 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.

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PubMed2006

Nature of point defects on SiO2/Mo(112) thin films and their interaction with Au atoms.

We have studied by means of periodic DFT calculations the structure and properties of point defects at the surface of ultrathin silica films epitaxially grown on Mo(112) and their interaction with adsorbed Au atoms. For comparison, the same defects have been generated on an unsupported silica film with the same structure. Four defects have been considered: nonbridging oxygen (NBO, [triple bond]Si-O(*)), Si dangling bond (E' center, [triple bond]Si(*)), oxygen vacancy (V(O), [triple bond]Si-Si[triple bond]), and peroxo group ([triple bond]Si-O-O-Si[triple bond]), but only the NBO and the V(O) centers are likely to form on the SiO(2)/Mo(112) films under normal experimental conditions. The [triple bond]Si-O(*) center captures one electron from Mo forming a silanolate group, [triple bond]Si-O(-), sign of a direct interaction with the metal substrate. Apart from the peroxo group, which is unreactive, the other defects bind strongly the Au atom forming stable surface complexes, but their behavior may differ from that of the same centers generated on an unsupported silica film. This is true in particular for the two most likely defects considered, the nonbridging oxygen, [triple bond]Si-O(*), and the oxygen vacancy, [triple bond]Si-Si[triple bond].

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PubMed2006

Bonding trends and dimensionality crossover of gold nanoclusters on metal-supported MgO thin films.

Bonding of gold clusters, , 16, and 20, on MgO(100) and on thin MgO films supported on Mo(100) is investigated using first-principles density-functional theory. Enhanced adhesive bonding is found for clusters deposited on metal-supported MgO films of thickness up to about 1 nm, or 4 to 5 MgO layers, originating from electrostatic interaction between the underlying metal and metal-induced excess electronic charge accumulated at the cluster interface with the oxide film. The increased wetting propensity is accompanied by a dimensionality crossover from three-dimensional optimal cluster geometries on MgO(100) to energetically favored two-dimensional structures on the metal-supported films.

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PubMed2006

Charge transfers at metal/oxide interfaces: a DFT study of formation of K delta+ and Au delta- species on MgO/Ag(100) ultra-thin films from deposition of neutral atoms.

Ultra-thin oxide films grown on a metal substrate and of thickness smaller than 1 nm may exhibit unusual properties with respect to thicker films or single crystal oxide surfaces. In a previous study [G. Pacchioni, L. Giordano and M. Baistrocchi, Phys. Rev. Lett., 2005, 94, 226104] we have suggested that a Au atom adsorbed on a MgO/Mo(100) thin film becomes negatively charged by direct electron tunneling from the Mo metal and that this is related to the low MgO/Mo(100) work function. Here we show, based on periodic DFT supercell calculations, that charge transfer can occur also in the opposite direction by adsorption of electropositive K atoms on MgO/Ag(100) films. We predict the occurrence of a charge transfer also for Au on MgO/Ag(100) films despite the fact that here the work function is 1 eV larger than in MgO/Mo(100). The formation of a layer of adsorbed negative (Au delta-/MgO/Ag) or positive (K delta+/MgO/Ag) adsorbates results in an increase or decrease, respectively, of the MgO/Ag(100) work function as predicted by the classical Gurney model for ionic adsorbates on metal surfaces.

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PubMed2006

Vibrational and electron paramagnetic resonance properties of free and MgO supported AuCO complexes.

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 MgOAuCO complexes. The strong influence of the support on the AuCO bonding is equally well described by all different approaches.

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PubMed2006

Ab initio molecular dynamics simulation of NO reactivity on the CaO(001) surface.

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(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.

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PubMed2006

Binding of single gold atoms on thin MgO(001) films.

In the present Letter the first electron paramagnetic resonance spectra of single metal atoms on a single crystalline oxide surface are presented. For Au atoms on a MgO(001) film investigated here an analysis of the angular dependent resonance positions and the hyperfine coupling to (17)O shows that the atoms are bound on top of oxygen ions on the terrace of the film. This result is in perfect agreement with scanning tunneling microscopy measurements at 5 K presented here. The measured hyperfine matrix components allow an experimental verification of the theoretically proposed binding mechanism of Au atoms on MgO. In particular, the large reduction of the isotropic hyperfine coupling constant of supported Au as compared to free atoms is not due to a charge transfer at the interface but a hybridization of orbitals and a resulting polarization of the unpaired electron.

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PubMed2006

Electron traps on oxide surfaces: (H+)(e-) pairs stabilized on the surface of 17O enriched CaO.

(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 17O anions of the surface. CaO crystals enriched in the 17O 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 CaO surface allow a tentative assignment of two observed spectral features to specific morphological surface sites.

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PubMed2006

Density-functional model cluster studies of EPR g tensors of Fs+ centers on the surface of MgO.

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 Deltag identical with 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 Deltag. 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.

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PubMed2006

Au and Pd atoms adsorbed on pure and Ti-doped SiO2/Mo(112) films.

The interaction of Pd and Au atoms with a silica surface and SiO2Mo(112) ultrathin films has been studied with periodic density-functional theory-generalized gradient approximation calculations. On both unsupported and supported silica, Pd and Au are weakly bound. No charge transfer occurs to the empty Pd and Au orbitals. Differently from Au, Pd can easily penetrate with virtually no barrier into the hexagonal rings of the supported silica film and binds strongly at the SiO2-Mo interface. The same process for Au implies overcoming a barrier of 0.9 eV. Completely different is the behavior of Ti-doped silica films. Au forms a direct covalent bond with substitutional Ti at the expense of the Ti...O-Mo interface bond which breaks. The global process is exothermic by 1 eV and nonactivated, showing that Ti doping results in solid anchoring points for the adsorbed Au atoms and for nucleation and growth of small gold particles. The effect of Ti doping is less pronounced for Pd but still visible with substantial enhancement of the Pd adsorption strength.

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PubMed2006

Identification of color centers on MgO(001) thin films with scanning tunneling microscopy.

Localized electronic defects on the surface of a 4 monolayer (ML) thin MgO(001) film deposited on Ag(001) have been investigated by low-temperature scanning tunneling microscopy and spectroscopy. Depending on the location of the defect, we observe for the first time different defect energy levels in the band gap of MgO. The charge state of defects can be manipulated by interactions with the scanning tunneling microscope tip. Comparison with ground state energy levels of color centers on the MgO surface obtained from embedded cluster calculations corroborates the assignment of the defects to singly and doubly charged color centers.

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PubMed2006

Excess electrons stabilized on ionic oxide surfaces.

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.

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PubMed2005

Nature of the chemical bond between metal atoms and oxide surfaces: new evidences from spin density studies of K atoms on alkaline earth oxides.

We have studied the interaction of K atoms with the surface of polycrystalline alkaline-earth metal oxides (MgO, CaO, 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 400 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.

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PubMed2005

Gold atoms and dimers on amorphous SiO(2): calculation of optical properties and cavity ringdown spectroscopy measurements.

We report on the optical absorption spectra of gold atoms and dimers deposited on amorphous silica in size-selected fashion. Experimental spectra were obtained by cavity ringdown spectroscopy. Issues on soft-landing, fragmentation, and thermal diffusion are discussed on the basis of the experimental results. In parallel, cluster and periodic supercell density functional theory (DFT) calculations were performed to model atoms and dimers trapped on various defect sites of amorphous silica. Optically allowed electronic transitions were calculated, and comparisons with the experimental spectra show that silicon dangling bonds [[triple bond]Si(.-)], nonbridging oxygen [[triple bond]Si-O(.-)], and the silanolate group [[triple bond]Si-O(-)] act as trapping centers for the gold particles. The results are not only important for understanding the chemical bonding of atoms and clusters on oxide surfaces, but they will also be of fundamental interest for photochemical studies of size-selected clusters on surfaces.

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PubMed2005

Charging of Au atoms on TiO2 thin films from CO vibrational spectroscopy and DFT calculations.

Au atoms have been deposited on oxidized and reduced TiO2 thin films grown on Mo(110). The gold binding sites and the occurrence of Au-TiO2 charge transfer were identified by measuring infrared spectra as a function of temperature and substrate preparation. The results have been interpreted by slab model DFT calculations. Au binds weakly to regular TiO2 sites (De < 0.5 eV) where it remains neutral, and diffuses easily even at low temperature until it gets trapped at strong binding sites such as oxygen vacancies (De = 1.7 eV). Here, a charge transfer from TiO2 to Au occurs. Au(delta-)CO complexes formed on oxygen vacancies easily lose CO (De = 0.4 eV), and the CO stretching frequency is red-shifted. On nondefective surfaces, CO adsorption induces a charge transfer from Au to TiO2 with formation of strongly bound Audelta+CO complexes (De = 2.4 eV); the corresponding CO frequency is blue-shifted with respect to free CO. We propose possible mechanisms to reconcile the observed CO desorption around 380 K with the unusually high stability of Au-CO complexes formed on regular sites predicted by the calculations. This implies: (a) diffusion of AuCO complexes above 150 K; (b) formation of gold dimers when the diffusing AuCO complex encounters a Au atom bound to an oxygen vacancy (reduced TiO2) or a second AuCO unit (oxidized TiO2); and (c) CO desorption from the resulting dimer, occurring around 350-400 K.

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