1,720,967 research outputs found
Hole diffusion across leaky amorphous TiO2 coating layers for catalytic water splitting at photoanodes
No full textWe investigate the mechanism of hole diffusion across leaky amorphous TiO2 (am-TiO2) layers. Through ab initio molecular dynamics simulations, we construct an atomistic model of am-TiO2 consistent with the experimental characterization. We first demonstrate that the oxygen vacancies do not occur in am-TiO2, as they can be assimilated by the amorphous structure upon structural rearrangement. Hence, their role in hole diffusion is ruled out. In contrast, O-O peroxy linkages are formed in pristine am-TiO2 upon injection of excess holes, with an associated defect level lying at 1.25 +/- 0.15 eV above the valence band of the material. We show that such linkages can provide a viable mechanism for hole diffusion in am-TiO2, as illustrated by a diffusion path of 1.2 nm with energy barriers lower than 0.5 eV in our atomistic model of am-TiO2
Absolute Energy Levels of Liquid Water
No full textWe determine the ionization potential (IP) and the electron affinity (EA) of liquid water together with the absolute redox level of the standard hydrogen electrode (SHE) by combining advanced electronic-structure calculations, ab initio molecular dynamics simulations, thermodynamic integration, and potential alignment at the water vacuum interface. The calculated SHE level lies at 4.56 eV below the vacuum level, close to the experimental reference of 4.44 eV inferred by Trasatti. The band edges are determined through a hybrid functional designed to reproduce the band gap achieved with highly accurate GW calculations. Our analysis yields IP = 9.7 eV and EA = 0.8 eV, consistent with both photoemission spectra of liquid water and thermodynamical data for the hydrated electron
Extrinsic Defects in Amorphous Oxides: Hydrogen, Carbon, and Nitrogen Impurities in Alumina
No full textWe present a procedure for addressing extrinsic defects in amorphous oxides, in which the most stable defect configurations are identified through ab initio molecular dynamics in various charge states and studied through hybrid functional calculations. The protocol is further complemented with an electron counting scheme based on maximally localized Wannier functions, which allows one to identify the nominal charge state and the composition of the defect core unit. Here, we apply this approach to the study of hydrogen, carbon and nitrogen impurities in amorphous alumina (alpha-Al2O3), a highly relevant material for technological applications. Hydrogen is found to be amphoteric with a thermodynamic +1/-1 charge transition level lying at similar to 4.6 eV above the valence band maximum, in qualitative agreement with results obtained with crystalline models, but at a substantially different energy level. Hydroxyl groups are further shown to lead to the same defect states as observed for hydrogen. Application of our procedure to carbon and nitrogen impurities leads to structural configurations that are found to depend on the total charge set in the simulation cell. Through the adopted electron counting rule, we assess that carbon and nitrogen impurities are only found in neutral and in singly positive charge states, respectively. This indicates that neither carbon nor nitrogen give charge transition levels in the band gap, in strong contrast with results achieved for crystalline models. In addition, the defect core units are shown to incorporate a varying number of oxygen atoms, by which their formation energy depends on the oxygen chemical potential mu O. In oxygen-poor conditions, both the carbon and the nitrogen impurities favor bonding to Al atoms, while they tend to form single or double bonds with oxygen atoms as mu(0) increases. Based on the band alignment of alpha-Al2O3 with three technologically relevant semiconductors (GaAs, GaN and alpha-Fe2O3), we discuss the possible role of point defects in degrading the performance of electronic devices and in favoring hole transport across the oxide in water-splitting setups
Oxygen defects in amorphous Al2O3: A hybrid functional study
No full textThe electronic properties of the oxygen vacancy and interstitial in amorphous Al2O3 are studied via ab initio molecular dynamics simulations and hybrid functional calculations. Our results indicate that these defects do not occur in amorphous Al2O3, due to structural rearrangements which assimilate the defect structure and cause a delocalization of the associated defect levels. The imbalance of oxygen leads to a nonstoichiometric compound in which the oxygen occurs in the form of O2- ions. Intrinsic oxygen defects are found to be unable to trap excess electrons. For low Fermi energies, the formation of peroxy linkages is found to be favored leading to the capture of holes. The relative +2/0 defect levels occur at 2.5 eV from the valence band. Published by AIP Publishing
Evaluation of Photocatalysts for Water Splitting through Combined Analysis of Surface Coverage and Energy-Level Alignment
No full textTo examine whether suitable conditions occur for the water splitting reaction at their interfaces with liquid water, we determine the pH-dependent surface coverage for a series of semiconductors, including GaAs, GaP, GaN, CdS, ZnO, SnO2, and rutile and anatase TiO2. For this, we calculate acidity constants at surface sites through ab initio molecular dynamics simulations and a Grand Canonical formulation of adsorbates. The resulting pH values at the point of zero charge show excellent agreement with experiment and thereby support the validity of our approach. By combining information concerning the surface coverage with the alignment of the band edges with respect to the relevant redox levels, we scrutinize the potential of the considered semiconductors as photocatalysts and identify the corresponding optimal pH ranges for hydrogen and oxygen evolution. More specifically, our results indicate that GaN stands out among these semiconductors as the most promising candidate for the overall water splitting, with the potential of further improvement through alloying. With the surface coverage, our computational analysis brings a new descriptor that is currently beyond experimental reach
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Alignment of Redox Levels at Semiconductor-Water Interfaces
No full textWe determine the band alignment between various semiconductors and liquid water by combining molecular dynamics (MD) simulations of atomistic interface models, electronic-structure calculations at the hybrid-functional and GW level, and a computational standard hydrogen electrode. Our study comprises GaAs, GaP, GaN, CdS, ZnO, SnO2, rutile TiO2, and anatase TiO2. For each semiconductor, we generate atomistic interface models with liquid water at the pH corresponding to the point of zero charge. The molecular dynamics are started from two kinds of initial configurations, in which the water molecules are either molecularly (m) or dissociatively (d) adsorbed on the semiconductor surface. The calculated band offsets are found to be strongly influenced by the adsorption mode at the semiconductor-water interface, leading to differences larger than 1 eV between m and d models of the same semiconductor. We first assess the accuracy of various ab initio electronic-structure schemes. The use of a standard hybrid functional leads to large errors for the conduction band edge but nevertheless accounts accurately for the position of the valence band edge. One-shot GW calculations with a starting point at the semilocal density functional level do not yield any improvement. It is necessary to turn to one-shot GW calculations based on a hybrid-functional starting point to achieve a noticeable improvement in the determination of the band edges, with mean average errors ranging between 0.23 and 0.27 eV. The use of state-of-the-art quasiparticle self-consistent GW schemes does not lead to any further improvement for the set of semiconductors under investigation. Further improvement with mean average errors of 0.20 eV is obtained when turning to hybrid-functional and GW methods, in which the experimental band gap of the semiconductor is enforced by construction. The present work sets a benchmark for the accuracy by which band edges at semiconductor-water interfaces can be obtained with current advanced electronic-structure methods. In particular, the importance of providing an atomistic description of the semiconductor-water interface is emphasized
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
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