Annabella Selloni

Crystal growth: Anatase shows its reactive side

Fluorine-containing species can cause titania to crystallize with an unusually large fraction of reactive {001} facets.

Reaction of O2 with Subsurface Oxygen Vacancies on TiO2 Anatase (101)

Oxide Chemistry Below the Surface Although metal oxides, such as titanium dioxide (TiO2), are used for catalytic oxidation reactions and photocatalysis, the O2 does not react directly with substrates. Vacancies in the surface region of the TiO2 rutile phase can transfer a negative charge to adsorbed O2 to create more reactive species. By contrast, in anatase—the phase associated with nanoscale TiO2 particles—subsurface vacancies form. Setvin et al. (p. 988) used a scanning tunneling microscopy tip to pull these vacancies to the surface in a niobiumdoped anatase crystal and followed the transformation of adsorbed O2− into a peroxo species and a bridging O2 dimer. Subsurface oxygen vacancies created at an anatase surface play a key role in forming a bridging oxygen (O2) dimer from adsorbed O2. Oxygen (O2) adsorbed on metal oxides is important in catalytic oxidation reactions, chemical sensing, and photocatalysis. Strong adsorption requires transfer of negative charge from oxygen vacancies (VOs) or dopants, for example. With scanning tunneling microscopy, we observed, transformed, and, in conjunction with theory, identified the nature of O2 molecules on the (101) surface of anatase (titanium oxide, TiO2) doped with niobium. VOs reside exclusively in the bulk, but we pull them to the surface with a strongly negatively charged scanning tunneling microscope tip. O2 adsorbed as superoxo (O2–) at fivefold-coordinated Ti sites was transformed to peroxo (O22–) and, via reaction with a VO, placed into an anion surface lattice site as an (O2)O species. This so-called bridging dimer also formed when O2 directly reacted with VOs at or below the surface.

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

A density functional study of carbon monoxide adsorption on small cationic, neutral, and anionic gold clusters

CO adsorption on small cationic, neutral, and anionic Aun (n=1–6) clusters has been investigated using density functional theory in the generalized gradient approximation. Among various possible CO adsorption sites, the on-top (one-fold coordinated) is found to be the most favorable one, irrespective of the charge state of the cluster. In addition, planar structures are preferred by both the bare and the CO-adsorbed clusters. The adsorption energies of CO on the cationic clusters are generally greater than those on the neutral and anionic complexes, and decrease with size. The adsorption energies on the anions, instead, increase with cluster size and reach a local maximum at Au5CO−, in agreement with recent experiment. The differences in adsorption energies for the different charge states decrease with increasing cluster size.

Titania-water interactions: a review of theoretical studies

The interaction between water (H2O) and titanium dioxide (TiO2) has a central role in many environment- and energy-related applications, such as the photodecomposition of organic pollutants, solar cells, and solar-hydrogen production. The importance of these applications has motivated strong interest and intensive experimental and theoretical studies of H2O adsorption on TiO2 surfaces for decades. This review attempts to summarize the major theoretical outcomes on this topic in the last twenty years, ranging from low coverages of adsorbed water molecules up to water multilayers on various TiO2 surfaces. Theoretical/computational methods as well as structural models are discussed and a detailed comparison of the results from various computational settings is presented. The interaction of water with photoexcited TiO2 surfaces is a challenging but very interesting subject for future studies.

Correlation between bonding geometry and band gap states at organic-inorganic interfaces: catechol on rutile TiO2(110).

Adsorbate-induced band gap states in semiconductors are of particular interest due to the potential of increased light absorption and photoreactivity. A combined theoretical and experimental (STM, photoemission) study of the molecular-scale factors involved in the formation of gap states in TiO(2) is presented. Using the organic catechol on rutile TiO(2)(110) as a model system, it is found that the bonding geometry strongly affects the molecular electronic structure. At saturation catechol forms an ordered 4 x 1 overlayer. This structure is attributed to catechol adsorbed on rows of surface Ti atoms with the molecular plane tilted from the surface normal in an alternating fashion. In the computed lowest-energy structure, one of the two terminal OH groups at each catechol dissociates and the O binds to a surface Ti atom in a monodentate configuration, whereas the other OH group forms an H-bond to the next catechol neighbor. Through proton exchange with the surface, this structure can easily transform into one where both OH groups dissociate and the catechol is bound to two surface Ti in a bidentate configuration. Only bidendate catechol introduces states in the band gap of TiO(2).

Adsorption modes of cysteine on Au(111): thiolate, amino-thiolate, disulfide.

The adsorption of cysteine on the (111) surface of gold has been studied by means of periodic supercell density-functional theory calculations. A number of different adsorption modes are examined, including adsorption through the thiol group in either thiolate or disulfide form, and adsorption through both the thiol and amino functional groups. We find that at intermediate coverage densities the latter mode of adsorption is favored, followed by thiolate adsorption at the bridge (slightly displace toward fcc) site. The N-Au and S-Au bond strengths in the amino-thiolate adsorption are estimated to be of the order of 6 and 47 kcal/mol, respectively. The electronic structure of the different systems is analyzed, with focus on the total and projected density of states, as well as on the detailed character of the electronic states at the interface. States near the Fermi energy are found to have a metal-molecule antibonding character, whereas metal-molecule bonding states mostly occur near the lower edge of the Au-d band.

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...

First principles study of adsorbed Cun (n=1–4) microclusters on MgO(100): Structural and electronic properties

We present a density functional study of the structural and electronic properties of small Cun (n=1,4) aggregates on defect-free MgO(100). The calculations employ a slab geometry with periodic boundary conditions, supercells with up to 76 atoms, and include full relaxation of the surface layer and of all adsorbed atoms. The preferred adsorption site for a single Cu adatom is on top of an oxygen atom. The adsorption energy and Cu–O distance are ES−A=0.99 eV and dS−A=2.04 A using the Perdew–Wang gradient corrected exchange correlation functional. The saddle point for surface diffusion is at the “hollow” site, with a diffusion barrier of around 0.45 eV. For the adsorbed copper dimer, two geometries, one parallel and one perpendicular to the surface, are very close in energy. For the adsorbed Cu3, a linear configuration is preferred to the triangular geometry. As for the tetramer, the most stable adsorbed geometry for Cu4 is a rhombus. The adsorption energy per Cu atom decreases with increasing the size of th...

Reaction pathway and free energy barrier for defect-induced water dissociation on the (101) surface of TiO2-anatase

The adsorption of a water molecule on a partially reduced TiO2 anatase (101) surface has been studied by first-principles molecular-dynamics simulations. At variance with the stoichiometric surface, dissociation of water close to the oxygen vacancy is energetically favored compared to molecular adsorption. However, no spontaneous dissociation was observed in a simulation of several picoseconds, indicating the presence of an energy barrier between the molecular and dissociated states. The free energy profile along a possible dissociation path has been determined through constrained molecular dynamics runs, from which a free energy barrier for dissociation of ∼0.1 eV is estimated. On the basis of these results, a mechanism for the dissociation of water at low coverage is proposed.