1,721,056 research outputs found
Molecular and hybrid systems for photocatalytic dihydrogen production in water by combining rhodium or cobalt molecular catalysts with a ruthenium photosensitizer or cadmium-free quantum dots
No full textCe mémoire de thèse est consacré au développement de systèmes photocatalytiques moléculaires et hybrides (moléculaires/inorganiques) pour la réduction photo-induite des protons en dihydrogène (H2) en solution purement aqueuse. Les objectifs étaient d’identifier de nouveaux catalyseurs efficaces et robustes pour la production de H2, d’augmenter la stabilité et l’efficacité des phostosensibilisateurs existants et enfin de tester de nouvelles combinaisons photosensibilisateurs/catalyseurs.Un nouveau complexe de rhodium à ligand terpyridine ([Rh(tpy)(CH3CN)Cl2]+) a tout d’abord été isolé et caractérisé. Ce complexe a montré des performances intéressantes en tant que catalyseur pour la production de H2 sous irradiation de lumière visible en présence du photosensibilisateur de référence, le complexe de ruthénium tris-bipyridine ([Ru(bpy)3]2+) et d’ascorbate comme donneur d’électron sacrificiel. Afin d’augmenter la stabilité des systèmes catalytiques utilisant le photosensibilisateur [Ru(bpy)3]2+, ce dernier a été substitué par le complexe de ruthénium tris-dibenzènesulfonate-phenanthroline hydrosoluble ([Ru((SO3Ph)2phen)3]4-), qui présente un coefficient d'absorption deux fois plus élevé et un état excité à plus longue durée de vie tout en conservant des potentiels assez similaires. Combiné à un catalyseur de cobalt tétraazamacrocyclique ([Co(CR14)Cl2]+) et à l’ascorbate, ce photosensibilisateur s’est montré plus stable conduisant à une production de H2 plus élevée lors de la photocatalyse par rapport à [Ru(bpy)3]2+, avec près de 5000 cycles catalytiques réalisés par molécule de catalyseur en 24 h d’irradiation. La dernière partie de ce manuscrit décrit des systèmes hybrides associant des photosensibilisateurs inorganiques à base de nanocristaux semi-conducteurs de quantum dots sans cadmium de type CuInS2/ZnS à un catalyseur moléculaire. Afin d’accroitre le rendement quantique d’émission des quantum dots de CuInS2/ZnS, le cœur CuInS2 de ces derniers a été dopé au zinc. Il a également été montré que ces quantum dots sont capables d’activer, en plus du catalyseur de cobalt tétraazamacrocyclique, d’autres familles de catalyseurs pour la production de H2 comme des complexes de rhodium à ligands polypyridinyle et de nickel à ligand phosphine.This thesis is devoted to the development of molecular or hybrid (molecular/inorganic) photocatalytic systems for the photo-induced reduction of protons to dihydrogen (H2) in purely aqueous solution. The aims were: to identify new efficient and robust catalysts for H2 evolution, to increase the stability and efficiency of existing photosensitizers and finally to test new photosensitizer/catalyst combinations.A new rhodium complex with a terpyridine ligand ([Rh(tpy)(CH3CN)Cl2]+) was first isolated and characterized. This complex showed interesting performance as a catalyst for H2 production under visible light irradiation in the presence of the benchmark photosensitizer, the ruthenium tris-bipyridine complex ([Ru(bpy)3]2+) and ascorbate as a sacrificial electron donor. In order to increase the stability of the catalytic systems using the photosensitizer [Ru(bpy)3]2+, the latter was substituted by the water-soluble ruthenium tris-dibenzenesulfonate-phenanthroline complex ([Ru((SO3Ph)2phen)3]4-) which exhibits a twofold higher absorption coefficient and a longer lifetime of its excited state while maintaining quite similar potentials. Combined with a tetraazamacrocyclic cobalt catalyst ([Co(CR14)Cl2]+) and ascorbate, this photosensitizer was shown to be more stable leading to a higher H2 production during photocatalysis compared to [Ru(bpy)3]2+, with nearly 5000 catalytic cycles performed per catalyst molecule in 24 h of irradiation. The last part of this manuscript describes hybrid systems combining inorganic photosensitizers based on semiconductor nanocrystals of cadmium-free CuInS2/ZnS quantum dots with a molecular catalyst. In order to increase the emission quantum yield of the CuInS2/ZnS quantum dots, the CuInS2 core of the latter has been doped with zinc. It was also shown that these quantum dots are capable of activating, in addition to the tetraazamacrocyclic cobalt catalyst, other families of catalysts for H2 production such as rhodium with polypyridinyl ligands and nickel complexes with phosphine ligands
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
Probing surface chemistry of halide perovskite nanocrystals using NMR and Dynamic Nuclear Polarization
No full textLa découverte des nanocristaux (NCs) de CsPbBr3 en 2015 et leurs propriétés uniques d’émission lumineuse ont suscité un vif intérêt pour leur synthèse colloïdale. Les NCs de CsPbBr3, dotés d’un cœur ionique, se distinguent des autres semi-conducteurs mais sont plus sujets à la dégradation due à des facteurs environnementaux tels que la lumière, la chaleur et l’humidité, en comparaison avec leurs homologues plus covalents (par ex. CdSe, ZnO). Pour améliorer la stabilité des NCs, les chercheurs ont utilisé divers ligands (agents stabilisants) qui jouent un rôle crucial dans le contrôle de la croissance, de la forme et de la stabilité colloïdale. Dans cette étude, des ligands natifs tels que des alkylammoniums (par ex. dodécylammonium) et des carboxylates (par ex. oléate) ont été employés pour neutraliser les charges de surface et améliorer la stabilité en formant une couche protectrice autour des NCs.Les investigations sur la chimie de surface des NCs, utilisant la Résonance Magnétique Nucléaire (RMN) en phase liquide et solide (ssNMR), ont fourni des informations précieuses sur la thermodynamique de liaison des ligands et la composition potentielle de la surface. Ces études visent à améliorer la reproductibilité des synthèses et à optimiser la stabilité, notamment via des échanges de ligands. Cependant, la compréhension de la terminaison de surface et de la stabilisation par ligands des NCs est clairement limitée par la sensibilité insuffisante de la ssNMR.Quelques études ont utilisé la Polarisation Nucléaire Dynamique (DNP) pour surmonter cette limitation de sensibilité, mais avec une efficacité réduite par rapport à des systèmes modèles (par ex. solution congelée). Cette thèse explore les principaux défis rencontrés lors de l’utilisation de la DNP pour polariser des NCs de pérovskites à 100 K et montre qu’une amélioration de la sensibilité d’un facteur x10-20 peut être atteinte grâce à des agents de polarisation de pointe et un système de rotation cryogénique à hélium en cycle fermé permettant des expériences DNP jusqu’à 30 K.Ce travail a été mené sur des NCs de CsPbBr3 synthétisés en utilisant une version légèrement modifiée de la méthode classique d’injection à chaud, visant à améliorer la qualité et la stabilité des structures cuboïdes monodisperses. Plus précisément, nous discutons d’abord du protocole de préparation des échantillons pour les expériences DNP, en prenant en compte la labilité des ligands et les modifications potentielles de surface. Nous abordons également l’effet de divers paramètres expérimentaux tels que l’oxygène dissous (dans le solvant TCE), le stockage des échantillons, l’absorption des micro-ondes, le chauffage des échantillons, ainsi que la qualité du verre et la relaxation spin-réseau (1H T1) afin d’expliquer pourquoi de tels systèmes sont si difficiles à polariser à 100 K. Pour remédier à cette limitation, nous discutons de trois stratégies reposant sur l’utilisation de KBr pour diluer l’échantillon, d’agents de polarisation DNP de pointe et de l’accès à des températures ultra-basses (~30 K).Malgré ces avancées, la corrélation des noyaux faiblement abondants (par ex. 13C et 15N) reste un défi. Ce travail démontre le potentiel de combiner la DNP avec des NCs portant des ligands marqués isotopiquement (15N pour les ammoniums et 13C pour les carboxylates) et propose une méthode pour récupérer ces ligands coûteux. De plus, une caractérisation complète des NCs à l’aide de techniques telles que les spectroscopies UV-Vis et photoluminescence, XRD, SEM et TEM confirme leur qualité structurelle et optique. Les images TEM complètent les analyses atomiques de terminaison de surface fournies par la RMN, offrant une compréhension plus approfondie de la chimie de surface des NCs de CsPbBr3 et mettant en lumière les voies d’optimisation de leur stabilité.The discovery of CsPbBr3 perovskite nanocrystals (NCs) in 2015 and their unique light-emitting properties sparked interest in their colloidal synthesis. CsPbBr3 NCs with an ionic core differ from other semiconductors and are more prone to degradation caused by environmental factors such as light, heat, and humidity compared to their more covalent counterparts (e.g., CdSe, ZnO). To enhance the stability of NCs, researchers have employed a variety of ligands (stabilizing agents) that play a crucial role in controlling growth, shape, and ensuring colloidal stability. In this study, native ligands such as alkylammonium (e.g., dodecylammonium) and carboxylates (e.g., oleate), were used to neutralize surface charges and improve stability by forming a protective shell around the NCs.Investigations into surface chemistry of the NCs, using liquid-state and solid-state Nuclear Magnetic Resonance (ssNMR), have provided valuable insights into ligand binding thermodynamics and potential surface composition. These studies aim to enhance synthesis reproducibility and optimize stability for example via ligand exchange. Nevertheless, progress in understanding the surface termination and ligand stabilization of the NCs using ssNMR is clearly impeded by the current lack of sensitivity of this technique.A few studies made use of Dynamic Nuclear Polarization (DNP) to improve this sensitivity limitation but with limited efficiency compared to model systems (e.g. frozen solution). This thesis explores the main challenges encountered when using DNP to polarize perovskite NCs at 100 K and shows that x10-20 improvement in sensitivity can be demonstrated using state-of-the-art polarizing agents and a home-built closed-cycle cryogenic helium spinning system enabling DNP experiments down to 30 K.The work was conducted on CsPbBr3 NCs synthesized using a slightly modified version of the classical hot-injection method, aiming to improve the quality and stability of the monodisperse cuboidal structures. More precisely, we first discuss the sample preparation protocol used to conduct DNP experiments on NCs, considering ligand lability and potential surface modifications. We also discuss the effect of various experimental parameters such as dissolved oxygen (in the TCE solvent), sample storage, microwave absorption, sample heating, as well as glass quality and 1H T1 to explain why such systems are so difficult to polarize at 100 K. In order to improve this limitation, we discuss three strategies relying on the use of KBr to dilute the sample, state-of-the-art DNP polarizing agents and access to ultra-low temperature ~30 K.Despite these advances, correlating nuclei with low natural abundance (e.g., 13C and 15N) remains challenging. This work demonstrates the potential of combining DNP with NCs with isotopically labeled ligands (15N for ammonium and 13C for carboxylates) and outlines a method for recovering these costly ligands. Moreover, comprehensive characterization of the NCs using techniques such as UV-Vis and photoluminescence spectroscopies, XRD, SEM, and TEM confirms their structural and optical quality. TEM imaging complements atomic-level analyses of surface termination provided by NMR, offering a deeper understanding of the surface chemistry of CsPbBr3 NCs and highlighting the stability optimization pathways
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
Dispelling the Myths Behind First-author Citation Counts
Get PDFWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
koamabayili/VECTRON-author-checklist: VECTRON author checklist
No full textWe have done our best to complete the author checklist relating to the use of animals in the hut study. Note that the objective for the hut study was to evaluate the IRS treatment applications for residual efficacy against Anopheles mosquitoes, including the local An. coluzzii mosquito population. Cows were only used to attract mosquitoes into the huts and no tests were carried out directly on the cows. The author checklist is intended for use with studies where experiments are carried out on animals, which is why we have had such difficulty in completing this for the hut study, as many of the questions do not relate to how the cows were used
Safer-by-Design Fluorescent Nanocrystals: Metal Halide Perovskites vs Semiconductor Quantum Dots
No full textInternational audienceDespite the young age of the research field, substantial progress has been made in the study of metal halide perovskite nanocrystals (HPNCs). Just as their thin-film counterparts are used for light absorption in solar cells, they are on the way to revolutionizing research on novel chromophores for light emission applications. Exciting physics arising from their peculiar structural, electronic, and excitonic properties are being discovered with breathtaking speed. Many things we have learned from the study of conventional semiconductor quantum dots (CSQDs) of II−VI (e.g., CdSe), IV−VI (e.g., PbS), and III−V (e.g., InP) compounds have to be thought over, as HPNCs behave differently. This Feature Article compares both families of nanocrystals and then focuses on approaches for substituting toxic heavy metals without sacrificing the unique optical properties as well as on surface coating strategies for enhancing the long-term stability
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