1,720,959 research outputs found
Raman spectroscopy of charge transfer interactions between single wall carbon nanotubes and [FeFe] hydrogenase
No full textWe report a Raman spectroscopy study of charge transfer interactions in complexes formed by single-walled carbon nanotubes (SWNTs) and [FeFe] hydrogenase I (CaHydl) from clostridium acetobutylicum. The choice of Raman excitation wavelength and sample preparation conditions allows differences to be observed for complexes involving metallic (m) and semiconducting (s) species. Adsorbed CaHydl can reversibly inject electronic charge into the LUMOs of s-SWNTs, while charge can be injected and removed from m-SWNTs at lower potentials just above the Fermi energy. Time-dependent enzymatic assays demonstrated that the reduced and oxidized forms of CaHydl are deactivated by oxygen, but at rates that varied by an order of magnitude. The time evolution of the oxidative decay of the CaHydl activity reveals different time constants when complexed with m-SWNTs and s-SWNTs. The correlation of enzymatic assays with time-dependent Raman spectroscopy provides a novel method by which the charge transfer interactions may be investigated in the various SwNT. CaHydl complexes. Surprisingly, an oxidized form of CaHydl is apparently more resistant to oxygen deactivation when complexed to m-SWNTs rather than s-SWNTs
Opening space for H-2 storage: Cointercalation of graphite with lithium and small organic molecules
No full textCointercalation of graphite with lithium and organic molecules, such as benzene and tetrahydrofuran (THF), is studied using first-principles calculations. The molecules play an important role in expanding the interlayer graphene distance to similar to 7.7 A. The increased space permits multiple H-2 species to be bound to Li cations with a binding energy of 10-22 kJ/mol. Furthermore, in the interstitial area free of Li cations, the negative charge in the graphene sheets enhances the H-2 binding energy to similar to 9 kJ/mol through electrostatic attraction. In order to restrain nucleation of lithium hydrides, the densest Li array is determined to be a Li-4(THF)C-72 structure, which absorbs 3.4 wt % hydrogen molecules reversibly. Cointercalation offers an experimentally accessible approach to designing optimized hydrogen storage materials that have not been investigated previously
Wiring-up hydrogenase with single-walled carbon nanotubes
No full textMany envision a future where hydrogen is the centerpiece of a sustainable, carbon-free energy supply. For example, the energy in sunlight may be stored by splitting water into H(2) and O(2) Using inorganic semiconductors and photoelectrochemical approaches(1) or with artificial photosynthetic systems that seek to mimic the light absorption, energy transfer, electron transfer, and redox catalysis that occurs in green plants.(2) Unfortunately, large scale deployment of artificial water-splitting technologies may be impeded by the need for the large amounts of precious metals required to catalyze the multielectron water-splitting reactions. Nature provides a,variety of microbes that can activate the dihydrogen bond through the catalytic activity of [NiFe] and [FeFe] hydrogenases,(3,4) and photobiological approaches to water splitting have been advanced.(5) One may also consider a biohybrid approach; however, it is difficult to interface these sensitive(6,7) metalloenzymes to other materials and systems. Here we show that surfactant-suspended carbon single-walled nanotubes (SWNTs) spontaneously self-assemble with [FeFe] hydrogenases in solution to form catalytically active biohybrids. Photoluminescence excitation and Raman spectroscopy studies show that SWNTs act as molecular wires to make electrical contact to the biocatalytic region of hydrogenase. Hydrogenase mediates electron injection into nanotubes having appropriately positioned lowest occupied molecular orbital levels when the H2 partial pressure is varied. The hydrogenase is strongly attached to the SWNTs, so mass transport effects are eliminated and the absolute potential of the electronic levels of the nanotubes can be unambiguously measured. Our findings reveal new nanotube physics and represent the first example of "wiring-up" an hydrogenase with another nanoscale material. This latter advance offers a nonprecious metal route to the design of new biohybrid architectures and building blocks for hydrogen-related technologies
Transparent conductive single-walled carbon nanotube networks with precisely tunable ratios of semiconducting and metallic nanotubes
No full textWe present a comprehensive study of the optical and electrical properties of transparent conductive films made from precisely tuned ratios of metallic and semiconducting single-wall carbon nanotubes. The conductivity and transparency of the SWNT films are controlled by an interplay between localized and delocalized carriers, as determined by the SWNT electronic structure, tube-tube junctions, and intentional and unintentional redox dopants. The results suggest that the main resistance in the SWNT thin films is the resistance associated with tube-tube junctions. Redox dopants are found to increase the delocalized carrier density and transmission probability through intertube junctions more effectively for semiconductor-enriched films than for metal-enriched films. As a result, redox-doped semiconductor-enriched films are more conductive than either intrinsic or redox-doped metal-enriched films
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe 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
Get PDF“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
Get PDFWe 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
Novel organometallic fullerene complexes for vehicular hydrogen storage
No full textTheoretical studies have predicted that scandium can bind to the twelve five-membered rings in C-60. It is then possible to stabilize four dihydrogen ligands (H-2) on each Sc atom with a binding energy of similar to 30 kJ/mol, ideal for vehicular hydrogen storage. The resulting C-60[ScH2(H-2)(4)](12) complex is predicted to be a minimum energy structure with similar to 7.0 wt% reversible hydrogen capacity. However, wet chemical synthesis of the calculated eta(5)-coordinated fullerene complex is unprecedented. The chemistry of C,, is generally olefinic (i.e., eta(2)-coordination, in which the metal is coordinated to two carbon atoms contributing two electrons to the bonding). Furthermore, stabilization of multiple dihydrogen ligands on a single transition metal has not been demonstrated. Recently we have probed new synthesis techniques in order to coordinate C,, with either Fe, Sc, Cr, Co or Li. The new compounds were characterized with solid-state nuclear magnetic resonance, and structures have been proposed. All of the structures were found to have unique binding sites for hydrogen employing the technique of temperature programmed desorption. Furthermore, some of the structures were shown to have significant hydrogen capacities with volumetric measurements. (c) 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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