Electron trapping at point defects on hydroxylated silica surfaces

The origin of electron trapping and negative charging of hydroxylated silica surfaces is predicted based on accurate quantum-mechanical calculations. The calculated electron affinities of the two dominant neutral paramagnetic defects, the nonbridging oxygen center, equivalent to Si-O-center dot, and the silicon dangling bond, equivalent to Si-center dot, demonstrate that both defects are deep electron traps and can form the corresponding negatively charged defects. We predict the structure and optical absorption energies of these diamagnetic defects

La strada

Released in 1954, La strada was the third solo directorial work of Federico Fellini and followed the critical and commercial success of I vitelloni (1953). Together with the films that preceded La dolce vita (1960), La strada represented a more traditional narrative and sober visual style that contrasted with Fellini\u27s later experimental work. It is also an important film for understanding Fellini\u27s reputation and evolution as an auteur director, especially in the light of the debate that it triggered among the Italian and French critics of the time regarding the true \u27calling\u27 of Italian neorealism. The idea for La strada (which literally means the \u27road\u27 or \u27way,\u27 in reference to the lengthy journeys that the film\u27s characters take) came from Fellini and his screenwriter Tullio Pinelli, both of whom had observed traveling artists making their way across the Italian Apennines.1 One of the recurring characteristics of Fellini\u27s cinema was its focus on the forms of popular entertainment, a trait that is reflected in La strada\u27s poignant and careful portrait of the unique world of nomadic street entertainers in Italy.https://digitalcommons.chapman.edu/language_books/1003/thumbnail.jp

Anisotropic Effects of Oxygen Vacancies on Electrochromic Properties and Conductivity of γ\gamma-Monoclinic WO3_3

Tungsten trioxide (WO$_3$) is a paradigmatic electrochromic material, whose peculiar optical properties in the presence of oxygen vacancies or intercalated alkali atoms have been observed and investigated for a long time. In this paper we propose a rationalization of experiments based on first-principles calculations of optical and electrical properties of oxygen deficient (reduced) WO$_3$. Our approach is based on a parameter-free dielectric-dependent hybrid density functional methodology, used in combination with the charge transition levels formalism, for studying excitation mechanisms in the presence of defects. Our results indicate that oxygen vacancies lead to a different physics in $\gamma$-monoclinic WO$_3$, depending on the orientation of the W-O-W chain where the vacancy is created, thus evidencing strong anisotropic effects rooted in the peculiar structural properties of the original nondefective monoclinic cell. Different types of oxygen vacancies can hence be classified on the basis of the calculated ground state properties, electronic structure, and excitation/emission energies, giving a satisfactory explanation to a range of experimental observations made on oxygen deficient WO$_3$.Comment: Accepted for publication in J. Phys. Chem.

Steric and Orbital Effects Induced by Isovalent Dopants on the Surface Chemistry of ZrO2

Sometimes, dopants in oxide surfaces are referred to as single-atom catalysts, at least when these species are incorporated in the supporting lattice. Usually, single atom catalysts are transition metal atoms stabilized on an oxide surface, and the activity is due to the valence electrons of these species. However, the surface chemistry can be modified also by the presence of isovalent heteroatoms, where the total number of valence electrons of the active site is the same as for the regular surface. The effect of isovalent dopants on the chemical reactivity of tetragonal ZrO2 has been studied with first principles calculations. Zr ions in the bulk, subsurface, and surface sites have been replaced with Si, Ge, Sn, Pb, Ti, Hf, and Ce ions. Surface or subsurface sites are clearly preferred. The dopants modify the local structure of the surface and introduce new empty states in the band gap, thus affecting the Lewis acid properties of the surface. We studied the effect of the dopants on the decomposition of HCOOH. This can follow four paths with desorption of (a) H2, (b) CO, (c) H2O, or (d) CO2. On pure ZrO2 reaction (a) dehydrogenation is preferred followed by decarbonylation (b). Ti, Hf, and Ce have some effect on the decomposition but do not change the order of reactivity. On the contrary, in the presence of Si, decarbonylation becomes the preferred path. If Ge occupies surface sites, reaction (d) loss of CO2 is by far more favorable. With Sn, dehydrogenation remains energetically preferred but the ordering of the other reactions changes, while Pb makes CO2 desorption slightly preferred over release of H2. These effects virtually disappear when the dopants occupy subsurface sites. The study shows that "steric" and/or "orbital" effects of isovalent dopants on a catalyst surface are sufficient to change the reaction products compared to the undoped system

Bonding Properties of Isolated Metal Atoms on Two-Dimensional Oxides

We report a systematic comparative dispersion-corrected DFT study of single (K, Au, and Pt) atom adsorption over a wide range of metal-supported oxide ultrathin films (MgO on Ag and Mo, ZnO on Cu, Ag, and Au, SiO2 on Pt and Ru, TiO2 on Ag and Pt, and ZrO2 on Pt and ZrPt). These films include reducible and non-reducible oxides and have been prepared and characterized experimentally, showing very unusual and interesting behavior toward metal atom adsorption. The interaction of K and Au with the metal/oxide substrates is dominated by charge-transfer aspects, where K tends to assume positive charge and Au negative charge. This fact reflects into a general trend where metal-supported oxide films displaying a large work function (i.e., deep empty states) tend to bind K cations strongly, while supports with small work function (i.e., shallow donor states) strongly stabilize Au in anionic form. The correlation between adsorption energy and work function is not strong enough to neglect several other aspects related to chemical and morphological properties of the specific oxide/metal interface. The case of Pt is completely different: here, covalent contributions to the bonding prevail, and the bond strength depends on factors such as the surface morphology and local atomic coordination, rather than the support's work function

Defect calculations in semiconductors through a dielectric-dependent hybrid DFT functional: the case of oxygen vacancies in metal oxides

We investigate the behavior of oxygen vacancies in three different metal-oxide semiconductors (rutile and anatase TiO2, monoclinic WO3, and tetragonal ZrO2) using a recently proposed hybrid density-functional method in which the fraction of exact exchange is material-dependent but obtained ab initio in a self-consistent scheme. In particular, we calculate charge-transition levels relative to the oxygen-vacancy defect and compare computed optical and thermal excitation/emission energies with the available experimental results, shedding light on the underlying excitation mechanisms and related materials properties. We find that this novel approach is able to reproduce not only ground-state properties and band structures of perfect bulk oxide materials, but also provides results consistent with the optical and electrical behavior observed in the corresponding substoichiometric defective systems.Comment: Accepted for publication in J. Chem. Phy

Recyclable and reusable nano-CuFe2O4 catalyzed C-O cross-coupling

An efficient protocol was developed and validated for the synthesis of biaryl/aryl alkyl ethers using CuFe2O4 nano powder as a recyclable catalyst via the reaction between aryl halides and phenols/alcohols. Variety of aryl ethers were synthesized efficiently in the presence of catalytic amount of CuFe2O4, KOH as base, under ligand free conditions in nitrogen atmosphere with DMSO as solvent at 120 oC. The catalyst is air-stable, inexpensive, magnetically separable and recyclable up to four cycles

Electronic structure and phase stability of oxide semiconductors: Performance of dielectric-dependent hybrid functional DFT, benchmarked against GWGW band structure calculations and experiments

We investigate band gaps, equilibrium structures, and phase stabilities of several bulk polymorphs of wide-gap oxide semiconductors ZnO, TiO2,ZrO2, and WO3. We are particularly concerned with assessing the performance of hybrid functionals built with the fraction of Hartree-Fock exact exchange obtained from the computed electronic dielectric constant of the material. We provide comparison with more standard density-functional theory and GW methods. We finally analyze the chemical reduction of TiO2 into Ti2O3, involving a change in oxide stoichiometry. We show that the dielectric-dependent hybrid functional is generally good at reproducing both ground-state (lattice constants, phase stability sequences, and reaction energies) and excited-state (photoemission gaps) properties within a single, fully ab initio framework.Comment: Minor changes in the final published versio