1,720,959 research outputs found
A high-resolution core level spectroscopy study of Ir: From flat to reconstructed and stepped surfaces
No full textThe ability to distinguish surface atoms with different coordination numbers is of fundamental importance in many fields of materials science, ranging from magnetism to heterogeneous catalysis. In this study, we exploited the capability of high-resolution core-level photoelectron spectroscopy in combination with density functional theory-based calculations to investigate this kind of atomic configurations. The measurement of the 4f7/2 core level of the (111), (100), (110), (311), and (510) surfaces allowed us to highlight the differences between surface atoms with different coordination, also related to surface reconstruction processes, atomic diffusion, and morphological changes. The shifts in core levels were correlated with the modification of the effective coordi- nation number, which takes into account the role of the variability in the Ir-Ir interatomic distances, and with the variations in the centroids of the projected d-band ΔBd, a quantity in turn related to chemical reactivity. Besides solid surfaces, the results may aid in understanding the properties of Ir nanoparticles whose active sites on nanofacets, such as edges, corners, and steps, are of paramount relevance in heterogeneous catalysis, especially in the electrocatalytic oxygen evolution reaction
The highest oxidation state observed in graphene-supported sub-nanometer iron oxide clusters
Full text linkIron oxide nanoclusters are of interest for a broad range of applications, but limited experimental information on their oxidation mechanism is available outside of the gas phase. Here, the oxidation of graphene-supported size-selected Fe-n clusters is studied using high-resolution X-ray Photoelectron Spectroscopy.Size-selected iron oxide nanoclusters are outstanding candidates for technological-oriented applications due to their high efficiency-to-cost ratio. However, despite many theoretical studies, experimental works on their oxidation mechanism are still limited to gas-phase clusters. Herein we investigate the oxidation of graphene-supported size-selected Fe-n clusters by means of high-resolution X-ray Photoelectron Spectroscopy. We show a dependency of the core electron Fe 2p(3/2) binding energy of metallic and oxidized clusters on the cluster size. Binding energies are also linked to chemical reactivity through the asymmetry parameter which is related to electron density of states at the Fermi energy. Upon oxidation, iron atoms in clusters reach the oxidation state Fe(II) and the absence of other oxidation states indicates a Fe-to-O ratio close to 1:1, in agreement with previous theoretical calculations and gas-phase experiments. Such knowledge can provide a basis for a better understanding of the behavior of iron oxide nanoclusters as supported catalysts
Unveiling Inequality of Atoms in Ultrasmall Pt Clusters: Oxygen Adsorption Limited to the Uppermost Atomic Layer
Full text linkThe concept of preferential atomic and molecular adsorption site is of primary
relevance in heterogeneous catalysis. In the case of ultrasmall size-selected
clusters, distinguishing the role played by each atom in a reaction is extremely
challenging due to their reduced size and peculiar structures. Herein, it is
revealed how the inequivalent atoms composing graphene-supported Pt12 and
Pt13 clusters behave differently in the photoinduced dissociation of O2, with only
those in the uppermost layer of the clusters being involved in the reaction. In this
process, the epitaxial graphene support plays a fundamental active role: its
corrugation and pinning induced by the presence of the clusters are crucial for
defining the preferential adsorption site on the Pt atomic agglomerates, facilitating
the lateral diffusion of physisorbed oxygen at a distance that induces its
selective adsorption in the topmost layer of the clusters, and inducing an
inhomogeneous charge distribution within the clusters which directly affects the
O2 adsorption. The inhomogeneous oxidation of the clusters is resolved by
means of synchrotron-based X-ray photoelectron spectroscopy and supported by
ab initio density functional theory calculations. The possibility to identify the
active sites on Pt clusters induced by cluster–support interactions has the
potential to enhance the experimentally supported design of nanocatalysts
Ultra-Low Atomic Diffusion Barrier on Two-Dimensional Materials: The Case of Pt on Epitaxial Graphene
No full textUnderstanding the energetics of atomic diffusion on graphene and two-dimensional (2D) materials is critical for advancing ultraminiaturized nanodevices, where even single-atom dynamics can significantly impact their functionality and performance, and for designing next-generation catalysts with superior activity and selectivity. In this work, we demonstrate that the combination of fast, high-resolution X-ray photoelectron spectroscopy (HR-XPS) and density functional theory (DFT) simulations provides a powerful approach to probe Pt atoms diffusion on epitaxial graphene. HR-XPS with its high chemical sensitivity and temporal resolution allows in situ tracking of Pt 4f7/2spectral components associated with monomers, dimers, and larger clusters at low temperature. This capability enabled us to monitor the rapid decay of monomer coverage and the subsequent aggregation into larger clusters. By fitting the time evolution of the different Pt species using a kinetic model, we extracted a diffusion barrier of 128 ± 6 meV, in excellent agreement with the 130 meV value obtained by nudged elastic band (NEB) calculations. These findings establish fast HR-XPS as a noninvasive, high surface-sensitive, and chemically specific technique for quantifying ultralow diffusion barriers of atoms on weakly interacting two-dimensional materials. This approach provides a practical framework for exploring surface dynamics and for guiding the controlled assembly of small atomic clusters or ordered superlattices on 2D templates
Anisotropy-driven double corrugation: Coexistence of one- and two-dimensional wave patterns in epitaxial graphene on iridium
No full textTailoring the electronic properties of graphene is crucial for a variety of applications. In this study, we investigate the graphene growth on the high Miller-index, anisotropic Ir(311) surface, where it self-organizes into one-dimensional ripples accompanied by a short-wavelength, two-dimensional wave pattern that is spatially confined between them. By employing a combination of spectroscopy- and microscopy-based techniques, we show that carbon atoms on the ripples interact weakly with the substrate, whereas those in the flatter region between two ripples experience stronger interaction with iridium atoms, leading to a partial rehybridization of carbon orbitals towards sp3 character. Complementary density functional theory calculations identify at least three distinct families of non-equivalent carbon atoms within the graphene layer and reveal that atoms on ripples are subjected to compressive strain. Since both compressive strain and corrugation are key factors in influencing graphene's chemical reactivity, the coexistence of two different wave pattern on graphene/Ir(311) system points to a region-specific reactivity. This spatial modulation of properties offers exciting potential for the design of bifunctional catalysts, particularly for hydrogen storage applications and for advanced materials in spintronic
Limitations in Determining Oxidation States in Condensed Matter at the Subnanometric Scale
Full text linkThe oxidation state is a fundamental chemical concept commonly employed to rationalize, classify, and predict the chemical reactivity of a variety of compounds. Understanding and defining the elemental oxidation state of solid materials at the atomic level becomes increasingly complex as their physical dimensions are reduced from tens of nanometers─where properties are still dominated by bulk or outer atomic crystal plane characteristics, to the subnanometric limit. In this work, we highlight the significant limitations in determining even a basic quantity, such as the oxidation state, when oxidized low-nuclearity mass-selected clusters are investigated by means of X-ray photoelectron spectroscopy (XPS), widely recognized as the elective approach to resolve different oxidations states. The lack of crystalline order in these nanoclusters, unlike that in periodic bulk systems and in solid surfaces, leads to a broad distribution of measured core levels, as shown in the case study of W nanoclusters. These cannot be unambiguously assigned to a valence state based solely on the knowledge of bulk matter behavior but need close comparison with specific theoretical modeling. Our results emphasize the substantial challenges inherent in understanding the unique properties of nanoscale materials, particularly in making a rigorous and quantitative determination of a fundamental property that takes a relevant role in many chemical processes, and represent crucial knowledge for advancing technologies that rely on the miniaturization of matter in various processes
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
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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