Andrea Vittadini

Role and effective treatment of dispersive forces in materials: Polyethylene and graphite crystals as test cases.

A semiempirical addition of dispersive forces to conventional density functionals (DFT‐D) has been implemented into a pseudopotential plane‐wave code. Test calculations on the benzene dimer reproduced the results obtained by using localized basis set, provided that the latter are corrected for the basis set superposition error. By applying the DFT‐D/plane‐wave approach a substantial agreement with experiments is found for the structure and energetics of polyethylene and graphite, two typical solids that are badly described by standard local and semilocal density functionals.

Theoretical Studies on Anatase and Less Common TiO2 Phases: Bulk, Surfaces, and Nanomaterials

Surfaces, and Nanomaterials Filippo De Angelis,† Cristiana Di Valentin,‡ Simona Fantacci,† Andrea Vittadini, and Annabella Selloni* †Computational Laboratory for Hybrid Organic Photovoltaics (CLHYO), Istituto CNR di Scienze e Tecnologie Molecolari, Via Elce di Sotto 8, I-06123 Perugia, Italy ‡Dipartimento di Scienza dei Materiali, Universita ̀ di Milano-Bicocca, I-20125 Milano, Italy Istituto CNR per l’Energetica e le Interfasi (IENI), c/o Dipartimento di Scienze Chimiche, Universita’ di Padova, I-35131 Padova, Italy Department of Chemistry, Princeton University, Princeton, New Jersey 08544, United States

Small gold clusters on stoichiometric and defected TiO2 anatase (101) and their interaction with CO: A density functional study

We have studied the interaction of small Aun (n=1–3) particles with the defect-free (stoichiometric) and defected (partially reduced) TiO2 anatase (101) surface using density functional calculations within a slab geometry. On the stoichiometric surface, gold particles prefer anion sites and “standing” geometries, in agreement with simple MO theory arguments. On the defected surface, Au strongly binds to the two cations close to the bridging oxygen vacancy. For both Au2 and Au3, “lying” adsorption geometries are now more likely, and starting from n=3 there is a tendency towards mixed binding, where cation and anion sites are simultaneously involved. Clustering of Au atoms is favored on both the stoichiometric and the reduced surfaces. CO strongly interacts with gold particles adsorbed on the stoichiometric surface. As found in previous gas-phase investigations, the CO-cluster bond is stronger when the cluster carries a positive charge. Accordingly, a weak interaction is computed for a gold atom supported o...

The adsorption of small molecules on the TiO2 anatase (101) surface by first-principles molecular dynamics

Abstract The (101) surface of the TiO 2 anatase polymorph and its interaction with small Bronsted acid molecules (H 2 O, H 2 S and HI) have been studied by means of first-principles molecular dynamics calculations. The structure and the density of states of the relaxed clean surface have been computed. Adsorption is predicted to be molecular for both H 2 O and H 2 S, because of favorable hydrogen bonding interactions with the surface. Only dissociation for HI is preferred, occurring with no sizeable barriers.

Tuning the catalytic activity of Ag(110)-supported Fe phthalocyanine in the oxygen reduction reaction.

A careful choice of the surface coverage of iron phthalocyanine (FePc) on Ag (110) around the single monolayer allows us to drive with high precision both the long-range supramolecular arrangement and the local adsorption geometry of FePc molecules on the given surface. We show that this opens up the possibility of sharply switching the catalytic activity of FePc in the oxygen reduction reaction and contextual surface oxidation in a reproducible way. A comprehensive and detailed picture built on diverse experimental evidence from scanning tunnelling microscopy, X-ray photoelectron spectroscopy and X-ray absorption spectroscopy, coupled with density functional theory calculations, sheds new light on the nature of the catalytically active molecule-surface coordination and on the boundary conditions for its occurrence. The results are of relevance for the improvement of the catalytic efficiency of metallo-macrocycles as viable substitutes for platinum in the cathodic compartment of low-temperature fuel cells.

Magnetic Properties and Vapochromic Reversible Guest-Induced Transformation in a Bispyrazolato Copper(II) Polymer: an Experimental and Dispersion-Corrected Density Functional Theory Study

Dispersion-corrected density functional theory (DFT-D) calculations, Electron Spin Resonance spectroscopy (EPR), and variable temperature magnetic moment measurements were used to investigate the structure and the electronic/magnetic properties of bispyrazolato-copper(II) coordination polymer and of its hydration product. The Cu(II) ions are antiferromagnetically coupled through the sigma system of the pyrazolate rings in both compounds. Theoretical electron density maps reveal that water molecules interact simultaneously and to a comparable extent with two Cu(II) centers (through the electronegative O end) and two pyrazolate rings (through the partly positively charged H atoms), which is compatible with the observed internuclear distances. DFT-D calculations indicate that low kinetic barriers are involved in the rearrangement of the host structure.

A theoretical study of the H2O and H2S chemisorption on Cu2O(111)

Abstract Density functional theory coupled to the molecular cluster approach has been used to study the bonding of two Bronsted acids (H 2 X, X=O and S) to the Cu 2 O(111) non-polar surface. Both molecular and dissociative chemisorption have been considered. The interaction between surface Cu(I) Lewis acid sites and the nucleophilic X end of the undissociated H 2 X has been investigated for different molecular orientations, i.e., with the molecular plane either perpendicular (atop ⊥ ) or parallel (atop ‖ ) to the surface. As far as the dissociative chemisorption is concerned, both partial and total deprotonation of H 2 X have been considered. For both acids, the atop ‖ chemisorption corresponds to the absolute minimum, even if the partial deprotonation of H 2 S is found isoenergetic to H 2 S atop ‖ .

An LCAO-LDF study of the chemisorption of H2O and H2S on ZnO(0001) and ZnO(101̄0)

Local density functional theory coupled to the molecular-cluster approach has been used to study the molecular and dissociative chemisorption of H2O and H2S on the relaxed ZnO(0001) and ZnO(1010) surfaces. The geometrical parameters and vibrational frequencies of the adsorbate have been computed. The agreement with experiment is good. Theoretical outcomes indicate that the basicity of the Lewis-base site available on the (0001) surface (the oxygen ions of the second layer partly exposed at the surface) is negligible, and that both H2O and H2S exist even on relaxed ZnO(0001) only as undissociated molecules. At variance to this, the dissociative chemisorption of H 2 S on relaxed ZnO(1010) is highly favoured, while as far H2O is concerned, dissociative and molecular chemisorption energies are very close to each other.

Energetics of atomic hydrogen diffusion on Si(100)

Abstract We present first principles calculations of the potential energy surface for the diffusion of a single hydrogen atom on Si(100)2 × 1. Surface relaxation is found to be very important for the energetics of diffusion. A strong anisotropy is predicted for hydrogen motion: H should diffuse mainly along dimer rows, where activation energies are ~ 1.3 eV, while the barrier for row-to-row hopping is ~ 0.5 eV higher. Our results indicate that diffusion can be considered a fast process compared to H 2 recombinative desorption.

Surface carboxylate species on Cu(100) studied by angle-scanned photoelectron diffraction and LCAO-LDF calculations

Chemisorption site geometries of formate and acetate species on Cu(100) have been studied by means of angle-scanned X-ray photoelectron diffraction (XPD) and first-principle quantum-mechanical calculations on a Cu60-formate cluster in the framework of local density functional theory (LDF). According to our LDF calculations, the short bridge site results to be more stable than the cross bridge one by about 20kcalmol. The reasons for the more effective interaction of the adsorbate in the short bridge site have been clearly outlined on the basis of the analysis of the LDP wavefunctions. LDF-optimized structural parameters have been used as a starting input in a series of single-scattering cluster spherical wave (SSC-SW) simulations of the O 1s XPD curves, which have experimentally confirmed the occupation of the short bridge site proposed by LDF calculations. The SSC-SW simulations have also outlined the role of the low-frequency-hindered rotational modes of the adsorbate (librations) and have furnished an estimate of the frequency (∼ 70 cm−1) of the hindered rotation on the axis perpendicular to the surface. Furthermore, as regards the formate species, the experimental data give evidence of a Cu-O distance of 1.95 A, in agreement with the LDF calculations and with angle-resolved photoemission fine structure (ARPEFS) literature results. An estimate of the OCO angle (129° ± 5°) has been obtained from polar scans for both formate and acetate species.