1,720,979 research outputs found
Macroscopic and microscopic electron transfer kinetics of HOPG and graphite intercalated compound investigated by cyclic voltammetry and SECM
Full text linkHighly oriented pyrolytic graphite (HOPG) is one of the most used host materials for obtaining and investigating graphite intercalated compounds, because of the high degree structural order of this polycrystal. Experiments on electrochemically intercalated HOPG in sulphuric acid have a model character, as the results obtained can be usefully generalised, not only with respect to other graphite compounds but also for the intercalation of other layered host lattices. In addition, the HOPG/H2SO4 system has an attractive potential for the possibility of electrochemically producing graphite oxide, ideally, by reversible oxidation/reduction cycles, which is of interest for energy storage and graphene production on an industrial scale. However, the oxidation/reduction cycles in such electrochemical intercalation process are not reversible and topotactic, so that the HOPG structure is considerably altered. This alteration may affect, for instance, the quality of the electrochemically produced graphene. In particular, the impact the electrochemical intercalation has on the conductivity of basal planes of HOPG, and so on graphene sheets, is still debated. In this work, we investigated both the macroscopic and microscopic electron transfer (ET) kinetics of the HOPG surface, before and after the intercalation of 1 M H2SO4 to obtain graphite intercalated compound, by using cyclic voltammetry (CV) and scanning electrochemical microscopy (SECM), respectively. The heterogeneous kinetic constant (k0) of the HOPG was evaluated quantitatively by using the redox systems [Fe(CN)6]3–/4– and [Ru(NH3)6]3+/2+. The morphology of the samples was also investigated by atomic force microscopy (AFM), which revealed a widespread formation of blisters and precipitates during the HOPG intercalation process. The CV and SECM results indicate that, upon intercalation, the electrochemical behaviour of the HOPG changes sensibly and the ET decreases sensibly. However, this effect depends on the redox mediators employed and it results more dramatic for the [Fe(CN)6]3–/4– system, for which a decrease of k0 by orders of magnitude was obtained. The decrease of ET can be correlated to the blisters and precipitates, which occur during the HOPG intercalation, as observed by AFM
Uniaxial Alignment of a Monolayer of Flat-on Free-Base Porphyrins on an Exfoliable Insulating Substrate
No full textPorphyrins are an extremely valuable class of molecules engaged in a variety of roles spanning from biology to optoelectronics. Manipulation of the chemical and physical properties of the inner cavity of porphyrins has been recognized as crucial for the exploitation of these systems in organic devices, particularly when porphyrins self-organize at the interface with a flat-on orientation of the macrocycle. Such an orientation has been mostly observed on metallic surfaces. Unfortunately, the physical-chemical properties of the molecules result in being largely perturbed due to the molecule-metal interaction. In addition, conducting substrates are unsuited to exploit electrically driven devices based on organic layers. To overcome these issues, we performed a topology-based analysis of insulating organic single crystal structures to identify a surface which (i) ensures easy exfoliation through mechanical methods, (ii) ensures epitaxial match with an overlayer of close-packed flat-on porphyrin molecules, and (iii) displays chirality. The outcome of this work is represented by a unique crystal of mixed 2,5-diketopiperazine and fumaric acid in a 1:1 ratio. We demonstrate that the (110) surface of this crystal fulfills the aforementioned requirements and, thanks to its peculiar subnanometric corrugations, allows one to grow uniaxially aligned monolayers of flat-on porphyrin molecules assembled through van der Waals interactions
Detecting the nir fingerprint of colors: The characteristic response of modern blue pigments
Full text linkReflectance spectroscopy in the ultraviolet (UV), visible (Vis), and near infrared (NIR) range is widely applied to art studies for the characterization of paints and pigments, with the advantages of non-invasive techniques. Isolating and detecting the fingerprint of pigments, especially in the NIR range, is quite challenging, since the presence of vibrational transitions of the most common organic functional groups prevents to relate the optical spectrum of a composite sample, as an artwork is, to each one of its elements (i.e., support, binder, and specific pigment). In this work, a method is presented to obtain the UV-Vis-NIR optical response of the single components of a model composite sample reproducing an artwork, i.e., the support, the binder, and the pigment or dye, by using diffuse reflectance spectroscopy. This allowed us to obtain the NIR spectral fingerprint of blue pigments and to identify specific features possibly applicable for detecting cobalt and phthalocyanine blue colors in artwork analysis
Customised porphyrin coating films for graphite electrode protection: An investigation on the role of peripheral groups by coupled AFM and cyclic voltammetry techniques
No full textControlling the molecular arrangement of properly designed materials is the ultimate target of studies in surface protection by organic thin films. The properties of the films are even more critical when the protection ability must be match with the function of the underneath active surface as in the case of electrodes covered by suitable protective as well conductive materials. In this context, we demonstrate that the electrode protection against anion intercalation, exerted by a vacuum deposited thin film of meso-tetraphenylporphyrin (H2TPP) on a highly-oriented pyrolytic graphite (HOPG) electrode, can be maximized by tailoring the peripheral groups of the porphyrin skeleton. From atomic force microscopy and cyclic voltammetry investigations, our results highlight the important role played by the molecular/electronic structure of the porphyrins to enhance the molecule-substrate interaction and the molecular assembly in the case of a perfluorinated derivative, thus resulting in effective protection ability in acid environment outperforming theH2TPP film
Disclosing the Graphite Surface Chemistry in Acid Solutions for Anion Intercalation
No full textGraphitic electrodes in ion batteries and graphite electrochemical exfoliation for graphene production are technological processes based on the anion intercalation mechanism in the stratified crystal structure upon immersion in acid solutions. To the detriment of the graphite mother-crystal, decomposition of the electrolyte solution or production of unexpected graphite intercalation compounds (GICs) is a side effect related to the anion intercalation mechanism. In this work, we studied the surface of highly oriented pyrolytic graphite (HOPG) samples treated with electrolyte solutions of 2 M HClO4 and 1 M H2SO4 in order to identify elemental and molecular species involved in the intercalation processes and, at the same time, find local surface defects through which solvated ions easily reach the HOPG subsurface. Results from cyclic voltammetry, X-ray photoelectron spectroscopy (XPS), and time-of-flight secondary ion mass spectrometry (ToF-SIMS) are described and discussed. Large blister formation and more general HOPG surface damage induced by treatment with acid solutions were revealed and mapped. In both HOPG samples, oxidized in 2 M HClO4 and 1 M H2SO4, the surface distribution of perchlorate and hydrogen sulfate ions gives direct evidence that the anion intercalation happens via surface defects among HOPG crystallites. These results pave the way for a complex intercalation process where more than one molecular species could be responsible for blister formation and carbon detriment
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
Porphycene Protonation: A Fast and Reversible Reaction Enabling Optical Transduction for Acid Sensing
No full textNano-sensor materials, especially if they target biological purposes, require fine control and tuning of the properties at the molecular scale when intramolecular properties have to be exploited, for example, in optical sensors. By means of optical spectroscopies (namely, spectrophotometry, spectrofluorimetry, and surface differential reflectivity), we demonstrate that thin films of porphycene show a spectacular chromatic change when exposed to acid vapors (specifically to hydrochloric acid). After exposure, the original optical properties of the porphycene films are recovered within a few seconds and without any thermal annealing. In addition, since clear spectroscopic signatures are observed both in absorption and in reflectivity, the parent porphycene is shown to be suitable even for deposition of films onto opaque substrates. These findings are of significant interest in view of potential engineering of the molecules and implementation in devices
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
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