Le présent travail de thèse porte sur la synthèse et la caractérisation de nouvelles molécules et matériaux à base de polyoxométallates (POMs) pour des applications dans les domaines de l’énergie et l’environnement. Dans le domaine de l’environnement, notre choix s’est porté sur la transformation électro-catalytique et photo-catalytique des espèces polluantes comme les oxydes d’azote et les colorants azoïques toxiques comme l’Acide Orange 7. Pour ce qui concerne le domaine de l’énergie, nous nous sommes focalisés sur des systèmes électro- catalytiques pour la production de l’hydrogène (combustible à fort pouvoir calorifique) et pour la réduction du di-oxygène (intérêt dans le fonctionnement des piles à combustibles). Ces réactions nécessitent généralement plusieurs électrons pour se produire, c’est pour cette raison que notre choix s’est porté sur les catalyseurs à base de POMs. En effet, les POMs sont capables de stocker, puis de restituer, un grand nombre d’électrons sans changer de structure.Dans la première partie, divers POMs qui prennent en sandwich plusieurs métaux de transition sont synthétisés et caractérisés par des méthodes expérimentales (voltamétrie cyclique, coulométrie, microbalance) et théoriques (calculs DFT). Ces composés montrent une très bonne activité électro-catalytique pour la réduction des substrats tels que les oxydes d’azote, le di-oxygène et le peroxyde d’hydrogène.Dans la seconde partie, des matériaux aux propriétés améliorées sont synthétisés par incorporation de POMs dans des matrices tels que les réseaux moléculaires de type MOF (Metal Organic Framework) et dans les polymères liquides ioniques. L’association avec un semi-conducteur comme le TiO₂ est aussi considérée. Les matériaux obtenus sont caractérisés par diverses techniques : infrarouge à transformée de fourrier, électrochimie, spectroscopie en réflectance diffuse, analyse thermogravimétrique, diffraction de rayons X, spectroscopie de photo-électrons X, microscopie électronique à transmission. Les matériaux à base de POMs et de MOFs sont très performants pour la réduction électro-catalytique des protons, avec des potentiels de début effectif de la réaction, meilleurs que ceux des électrodes de platine. Les hybrides à base de POMs, de polymères liquides ioniques et du TiO₂ sont photo-sensibles sous lumière visible, contrairement à leurs composants. Ils démontrent une bonne activité vis-à-vis de la dégradation de l’Acide Orange 7. De plus, le composite obtenu par photo-déposition de nanoparticules d’argent sur ces matériaux présente de bonnes performances électro-catalytiques comme cathode pour la réduction du O₂ et du NO‾₃. / The present work concerns the synthesis and characterisation of new polyoxometalate (POMs) based molecules and materials for applications in energy and environment matters. As far as the environment is concerned, our efforts involved the electro-catalytic and photo-catalytic transformation of pollutants such as nitrogen oxides and recalcitrant azo dyes like Acid Orange 7. As for the domain of energy, we focused on electro-catalytic systems aimed at producing dihydrogen (a high calorific power fuel) and at reducing dioxygen (an important reagent in fuel cells). These reactions usually require several electrons in order to take place, which led us to choose POM-based catalysts. In fact, POMs are capable of stocking and returning an important number of electrons without changing their structure.In the first part, several POMs in which different transition metals are sandwiched in the equatorial plane of the molecular scaffold are synthesised and characterised by experimental (cyclic voltammetry, coulometry, microbalance) et theoretical (DFT calculations) methods. These compounds have shown a very good electro-catalytic activity towards the reduction of substrates such as nitrogen oxides, dioxygen and hydrogen peroxide.In the second part, some materials exhibiting improved properties are synthesised through the incorporation of POMs in matrices like Metal Organic Framework (MOF)-type molecular networks and ionic liquid polymers. The association with a semi-conductor such as TiO₂ has also been considered. The materials obtained were characterised by several techniques: Fourier transform infrared spectroscopy, electrochemistry, diffuse reflectance spectroscopy, thermogravimetric analysis, X ray diffraction, X ray photoelectron spectroscopy, transmission electron microscopy. The POM and MOFs based materials are very performing for the electro-catalytic reduction of protons, having onset potentials better than those exhibited by platinum electrodes. The hybrids consisting of POMs, ionic liquid polymers and TiO₂ are photo-sensitive under visible light, unlike each component taken individually. They have shown a good activity towards the degradation of Acid Orange 7. In addition, the composite obtained by photo-deposition of silver nanoparticles on these materials exhibits a good electro-catalytic performance as a cathode for the reduction of O₂ and NO‾₃.

L'objectif de ce travail est de contribuer, par le biais d'approches expérimentales (XPS/chimisorption de sondes gazeuses) et théoriques (approches type DFT), à une meilleure compréhension fondamentale de la réactivité de surface de matériaux d'électrode positive et plus spécifiquement d'oxydes lamellaires lithiés LiMO2. La réactivité de surface du matériau LiCoO2 et l’influence d’une substitution de l’atome de cobalt par l’atome d’aluminium a été étudiée pour avancer dans la compréhension de l’effet bénéfique des « coatings » à base d’alumine. L’étude expérimentale a été centrée sur l’adsorption de SO2 et montre que la présence d’atomes d’aluminium induit une diminution de réactivité : elle génère des espèces de type sulfite, différentes des sulfates seuls identifiés pour LiCoO2. La modélisation de cette adsorption permet la mise en évidence de deux modes d’adsorption distincts : passage d’un contrôle rédox pour LiCoO2 à un contrôle acido-basique pour des matériaux contenant des atomes d’aluminium (LiAlO2), thermodynamiquement moins favorable. L’influence de la nature du métal de transition a été analysée sur la base d’études théoriques également menées sur LiMnO2 et LiNiO2. La modélisation de l’adsorption de SO2 sur les surfaces des matériaux LiMO2 révèle la présence des deux processus de chimisorption (formation de sulfites et sulfates), avec mise en évidence du rôle important des cations de sous-surface dans les processus de réduction conduisant aux sulfates. Cette étude théorique a permis d’interpréter les résultats expérimentaux obtenus pour Li(Ni1/3Mn1/3Co1/3)O2, l’identification de sulfites résulterait de la présence des ions Ni2+ alors que celle de sulfates serait liée à la présence des ions Mn4+ et Co3+. / This work is devoted to a better fundamental understanding of the surface reactivity of positive electrode materials, and specifically lamellar lithiated oxides LiMO2 through experimental (XPS/chemisorption of gaseous probes) and theoretical (DFT calculations) approaches. The beneficial effect of aluminum-based coatings on electrochemical performances is well known but the exact mechanisms are not totally understood. A detailed study of the surface reactivity of LiCoO2 and of the influence of Al/Co substitution is carried out. The experimental approach is focused on SO2 adsorption and shows that Al/Co substitution induces a decrease of the surface reactivity and a change in the nature of adsorbed species (identification of sulfite species whereas only sulfate species are characterized for LiCoO2). Theoretical calculations highlighs two different adsorption modes controlled by redox properties for LiCoO2 and by acid-base properties for -LiAlO2 (less energetically favorable). The theoretical study is extended to LiMnO2 and LiNiO2 in order to analyse the influence of the nature of the transition metal atom. The modelization of SO2 adsorption reveals two chemisorption processes (sulfite and sulfate formation), and highligths the key role of subsurface cations in the reduction process which leads to sulfates. Thus theoretical approach allows on interpretation of the experimental data obtained for Li(Ni1/3Mn1/3Co1/3)O2. The identification of sulfites may be explained by the presence of Ni2+ ions whereas sulfate species may result from the presence Mn4+ and Co3+ ions.

Le mécanisme de la réaction d’oxydation de l’hydrogène (HOR) dans Ni (111) est bien connu et se fait par étapes de Volmer-Heyrovsky, en milieu alcalin. Il a été proposé que la formation d'eau puisse jouer un rôle important. Dans cette thèse, j'ai étudié les surfaces de nickel et de nickel bimétallique en utilisant la théorie de la densité fonctionnelle (DFT). J'ai calculé des magnitudes thermodynamiques (comme les énergies libres d'adsorption de Gibbs) et des propriétés cinétiques (comme des barrières d'activation pour la formation d'eau). Plusieurs surfaces Ni / Cu ont été analysées. Celle qui contient 25% de Cu (sur la couche supérieure) a les meilleures performances: 1) l’énergie d’activation est de 0,2 eV, et 2) OH et H ne doivent pas être fortement adsorbés dans la plage de potentiel HOR. / The mechanism of hydrogen oxidation reaction (HOR) in Ni(111) is well-known and it happens through Volmer-Heyrovsky steps, in alkaline media. However it was proposed that water formation could play an important role. In this thesis, I have studied nickel and bimetallic nickel surfaces using density functional theory (DFT). I calculated thermodynamical magnitudes (like Gibbs energies of adsorption) and kinetic properties (like activation barriers for water formation). Several Ni/Cu surfaces were analyzed. The one with 25% of Cu (on top layer) has the best performance because: 1) the activation energy is 0.2 eV, and 2) OH and H are not to strongly adsorbed on the HOR potential range.

A maior parte da energia consumida no mundo provém da queima de combustíveis fósseis, responsáveis pelo desenvolvimento da sociedade moderna, mas, também, por diversos danos ao meio ambiente. Desse modo, a exploração de fontes limpas e alternativas de energia renovável, tais como a solar e a eólica, tem ganhado grande importância e atenção dos pesquisadores nos últimos anos. Entretanto, o armazenamento destas formas de energia precisa ser eficiente, para que possam ser utilizadas nos períodos de indisponibilidade da fonte. Neste sentido, as baterias são uma das mais eficazes formas de geração e armazenamento de energias renováveis. No entanto, seu uso ainda é limitado, principalmente para aplicação em veículos automotores, que são um dos responsáveis pela emissão de gases poluentes. O principal fator limitante está relacionado às baixas densidades energéticas nos materiais utilizados para compor os elementos das baterias modernas. O desenvolvimento experimental de novos materiais, para este fim, é um trabalho demorado e caro, de tal forma que métodos teóricos têm sido usados com sucesso para prever o comportamento de novos materiais, apropriados para utilização como componentes básicos das baterias. Neste projeto serão investigados, teoricamente, materiais nanoestruturados para aplicação como eletrodos de baterias de íons de lítio, uma das formas mais populares de baterias atualmente. Neste trabalho apresentamos as propriedades estruturais, eletrônicas e vibracionais da grafite e dos sistemas bidimensionais grafeno, SiC e CN utilizando cálculos ab initio baseados na teoria do funcional da densidade, onde se utilizou o método de pseudopotencial PAW (Projector Augmented Wave) e aproximação van der Waals (optB88-vdW) para o termo de troca-correlação. A grafite e o grafeno foram estudados para validar o método aplicado, o qual foi usado para estudar as estruturas de SiC (bicamadas) e CN (monocamada e bicamada) como possíveis nanomateriais para compor ânodos de baterias de íons de lítio. Estudamos a adsorção de Li nas bicamadas de SiC em diferentes sítios encontrando a posição estável e as possíveis vantagens e desvantagens de utilizá-las como ânodos de baterias de íons de lítio. As estruturas CN não se mostraram ser sistemas viáveis, uma vez que são dinamicamente instáveis. No entanto, estudamos as propriedades de camadas de CN hidrogenadas, que são dinamicamente estáveis e possíveis candidatas para aplicação em baterias de lítio. / Most of the energy consumed in the world comes from the burning of fossil fuels, responsible for the development of modern society, but also for various damages to the environment. Thus, the exploration of clean and alternative sources of renewable energy, such as solar and wind, has gained great importance and attention from researchers in recent years. However, the storage of these forms of energy must be efficient, so that they can be used during periods of unavailability of the source. In this sense, batteries are one of the most effective forms of generation and storage of renewable energies. However, its use is still limited, mainly for application in automotive vehicles, which are one of the responsible for the emission of polluting gases. The main limiting factor is related to the low energy densities in the materials used to compose the elements of modern batteries. The experimental development of new materials for this purpose is a time consuming and expensive work, so that theoretical methods have been used successfully to predict the behavior of new materials suitable for use as basic components of the batteries. In this project it will be investigated, theoretically, nanostructured materials for application as electrodes of lithium-ion batteries, currently, one of the most popular forms of batteries. In this work we present the structural, electronic and vibrational properties of graphite and two-dimensional graphene, SiC and CN systems using ab initio calculations based on the density functional theory, where the pseudopotential PAW (Projector Augmented Wave) method and van der Waals approximation (optB88-vdW) for the exchange-correlation were used. Graphite and graphene were studied to validate the applied method, which was used to study the SiC (bilayer) and CN (monolayer and bilayer) structures as possible nanomaterials to compose anodes of lithium-ion batteries. We studied the adsorption of Li in the SiC bilayers at different sites, finding the stable position and the possible advantages and disadvantages of using them as anodes of lithium-ion batteries. The CN structures have not been shown to be viable systems, since they are dynamically unstable. However, we have studied the properties of hydrogenated CN layers, which are dynamically stable and possible candidates for application in lithium batteries.

A busca por materiais catalíticos fora dos metais de transição foi o objetivo dessa tese. Principalmente se considerarmos a escassez dos metais nobres como matéria prima para a catálise de moléculas diatômicas, em especial o oxigênio e o óxido de nitrogênio. A primeira molécula está diretamente relacionada com à obtenção de energia em células a combustível, especificamente, o ânodo das células a combustível. A segunda molécula está relacionada diretamente a eletroredução de íons nitrato, ou seja, na formação de uma molécula diatômica muito estável quimicamente e muito tóxica. Por isso foi utilizada a DFT (Teoria do Funcional da Densidade) para a realização de diversas simulações a 298,15K e 1 atm, com moléculas no estado gasoso de porfirinatos de silício (IV) reagindo com O2 e NO para a confirmação catalítica dos porfirinatos. Para isso foram utilizados os conjuntos de base 3-21G, 6-31G, 6-31G* e 6-31G(d), que mostraram tendências catalíticas dos porfirinatos estudados. Assim, por exemplo, pode-se evidenciar a tendência de hidrólise do porfirinato de silício (IV) através dos cálculos, já observado e demonstrado através de medidas experimentais. Porfirinatos de silício (IV) e octaetilporfirinatos de silício (IV) mostram a mesma tendência de formação de ligação química com O2 , forma de \"ponte\", diferenciando nos valores absolutos de DG e DH, e na ligação do O2 com o anel porfirínico. Com porfirinato, o oxigênio ligou-se com o silício e com o carbono meso; já com o octaetilporfirinato, o O2 ligou-se com o silício e com o nitrogênio do anel, promovendo uma ruptura de ligação entre o silício (presente no centro das porfirinas) e o nitrogênio do anel. A mesma tendência foi observada quando foram adicionadas hidroxilas nos carbonos meso do anel porfirínico, demonstrando que a presença de grupos doadores de elétrons nos carbonos meso não fortalece as ligações silício e nitrogênio presentes no sistema, e um átomo de cloro como contra-íon também não influencia no resultado final da reação. Quando reagimos o sistema com hidrogênio, observa-se a formação de duas moléculas de água fracamente ligadas ao sistema. Octaetilporfirinato de silício (IV) reagiu apenas com duas moléculas de NO, sendo observado a formação de uma estrutura pentagonal sobre o silício, rompendo-se duas ligações entre silício e nitrogênio do anel porfirínico. Quando colocado hidroxilas e cloro no sistema octaetilporfirinato de silício (IV), observou-se a reação apenas com uma molécula de NO, ligada ao nitrogênio do anel, sem a formação de NO+, comum em sistemas porfirínicos cujo átomo central é ferro, por exemplo. Também observou-se a tendência de formação do íon amônio e água, quando é colocado no sistema moléculas de hidrogênio para reagir com o NO preso ao anel porfirínico. Na prática, tanto os porfirinatos de silício (IV) quanto os octaetilporfirinatos de silício (IV) possuem contra-ions, sendo o mais comum o cloreto. Por isso, o estudo da labilidade do íon cloreto com as moléculas estudadas foi de fundamental importância na determinação da tendência desses sistemas. Assim pode-se inferir, por exemplo, que o cloreto é substituído pela molécula de oxigênio numa reação entre esses dois reagentes. Para finalizar, CO e H2 não reagiram com os porfirinatos de silício (IV), mostrando uma forte tendência à repulsão, devido à diferença de fase nos orbitais atômicos. / The search for catalytic materials out of the transition metals was the objective of this thesis. Mainly if we consider the scarcity of noble metals as raw materials for the catalysis of diatomic molecules, especially oxygen and nitrogen oxides. The first molecule is directly related to the getting of energy in fuel cells, specifically, the anode of the fuel cells. The second molecule is related directly to the electroreduction of nitrate ions, in other words, in the formation of a very stable diatomic molecule and also very poisonous. Therefore, DFT was used (Density Functional Theory) for the accomplishment of several simulations, at 298,15K and 1 atm, with molecules in the gaseous state of silicon porphyrinatos (IV) reacting with O2 and NO for their catalytic confirmation. For that, the bases 3-21G, 6-31G, 6-31G * and 6-31G(d) were used to study the catalytic tendencies of the porphyrinatos here studied. For instance, the tendency of hydrolysis of silicon porphyrinato (IV) can be evidenced through theoretical calculations, as already observed and demonstrated through experimental works. Silicon porphyrinatos (IV) and silicon octaethilporphyirinatos (IV) showed the same tendency of forming a chemical bond with O2. They form a \"bridge\" changing the absolute values for DG and DH, and form a O2 bond with the porphyrinic ring. With porphyrinatos, the oxygen binds to silicon and to meso carbon; but with octaethilporphyirinato, the O2 binds to silicon and to the nitrogen of the ring, promoting a rupture between the silicon (present in the center of the porphyrins) and the nitrogen of the ring. The same tendency was observed when hydroxyls were added in the meso carbons of the porphyrinic ring showing that the presence of donor groups of electrons in the meso carbons does not strengthen the silicon and nitrogen bonds presented in the system, and a chlorine counter-ion does not influence the final result of the reaction as well. When we reacted the system with hydrogen, the formation of two weak molecules of water bound to the system is observed. Silicon octaethilporphyrinato (IV) just reacted with two NO molecules, and was observed the formation of a pentagonal structure on silicon breaking two bonds between silicon and nitrogen of the porphyrinic ring. When one puts hydroxyls and chlorine in the system silicon octaethilporphyrinato (IV), the reaction was only observed with a molecule of NO bound to the nitrogen of the ring without the formation of NO+ common in porphyrinic systems where the central atom is iron, for example. Also, it was observed the tendency of formation of the ammonium ion and water when we put in the system molecules of hydrogen to react with the NO linked to the porphyrin ring. In practice, as the silicon porphyrinatos (IV) as the silicon octaethilporphyrinatos (IV) possess counter-ions, where the most common is with chloride. Therefore, the study of the lability of the ion chloride with the studied molecules was of fundamental importance in the determination of the tendency of these molecular systems. From there, for instance, we could observe that the chloride is substituted by the oxygen molecule in a reaction between the two reactants. To conclude, CO and H2 did not react with the silicon porphyrinatos (IV) showing a strong tendency to repulsion due to the phase difference in the atomic orbitals.

Um estudo teórico detalhado das estruturas e energias do ligante 1, 7, 11, 17-tetraoxa-2, 6, 12, 16-trazaocicloocsano ([20]aneN4O4) coordenado com íons metálicos de transição Fe2+, Co2+, Ni2+, Ru2+, Rh2+ e Pd2+ foi realizado em nível de teoria B3LYP/Lanl2DZ. As geometrias dos complexos foram totalmente otimizados em simetria Cs com os íons metálicos coordenados com quatro átomos de nitrogênio (complexos 1a e 1aq) ou quatro átomos de oxigênios (complexos 1b e 1bq) e duas moléculas de água. Os arranjos octaédricos (1a e 1b) e quadrado-planares (1aq e 1bq) foram consideremos neste trabalho. A estrutura teórica está em excelente acordo com a estrutura de difração de raio-x experimental determinada para o complexo octaédrico de Ni2+ de [20]AneN4O4. Os cátions M2+ ligam-se preferencialmente aos átomos de nitrogênios com energia de ligação que aumenta na ordem Fe2+ < Ru2+ < Co2+ < Ni2+ < Rh2+ < Pd2+. Para os metais de transição da primeira fila, os complexos de spin alto são mais estáveis que os complexos de spin baixo. Em contraste, para os metais de transição da segunda fila, os estados de spin baixo mostraram-se mais estáveis que os estados de spin alto. As ligações metal-ligante nos complexos foram analisadas em termo das interações covalentes e iônicas e ajudaram a entender porque os complexos (1a e 1aq) são mais estáveis que os complexos (1b e 1bq). Os complexos poliaminas [20]aneN4 e poliéteres [20]aneO4 foram obtidos substituindo os átomos de nitrogênio e oxigênio da posição alfa dos macrociclos [20]aneN4O4 e [20]aneO4N4, respectivamente. O macrociclo [20]aneO4 tem preferência em complexar íons metálicos da primeira fila, enquanto o macrociclo [20]aneN4 prefere complexar os íons metálicos da segunda fila. / A detailed theoretical study of structures and energies of the 1,7,1l,17-tetraoxa-2,6,12,16-tetraaza-cycloeicosane ligand ([20]AneN4O4) coordinated to Fe2+, Co2+, Ni2+, Ru2+, Rh2+ and Pd2+ transition metals ions was carried out with the B3LYP/Lanl2DZ method. The geometries of the complexes were fully optimized in Cs symmetry with the metal ions coordinated either to four atoms nitrogen (complexes 1a e 1b) or to the four atoms oxygen (complexes 1aq e 1bq). The octahedral and square planar arrangements were considered in this work. The theoretical structure is in excellent agreement with the experimental X-ray diffraction structure determination for the [20]AneN4O4 octahedral Ni2+ complex. The M2+ cations bind preferentially to the nitrogen atoms with binding energies that increase in the order Fe2+ < Ru2+ < Co2+ < Ni2+ < Rh2+ < Pd2+. For the first-row transition metals, the highspin complexes are more stable than the low-spin complexes. In contrast, for the second-row of transition metals, the low-spin states were found more stable than the high spin states. The metal-ligand bonds in the complexes were analyzed in terms of the covalent and ionic interactions and helped to understand why complexes (1a e 1aq) are more stable than complexes (1b e 1bq). The polyamines [20]aneN4 and polyethers [20]aneO4 complexes were obtained substituting the atoms N or O of the alfa position of the macrocycles [20]aneN4O4 and [20]aneO4N4, respectively. The macrocycle [20]aneO4 prefers to complex first-row transition metals; however, the macrocycle [20]aneN4 prefers to complex second-row transition metals.

Efeitos termoelétricos descrevem o surgimento de campos elétricos em função de gradientes de temperatura e vice-versa. Neste trabalho investigamos as propriedades termoelétricas de materiais de baixa dimensionalidade e nanoestruturas através de cálculos de primeiros princípios das propriedades de transporte destes sistemas, usando o código TRANSAMPA, que é baseado em funções de Green fora do equilíbrio e do código SIESTA, baseado em teoria do funcional da densidade. Inicialmente estudamos nanofitas de grafeno e como estas são alteradas pela presença de impurezas substitucionais de Boro e Nitrogênio. Entre os principais resultados, mostramos que fitas na configuração ferromagnética apresentam efeito Seebeck dependente do spin, que pode ser ajustado por efeito de campo. A seguir, vemos que o coeficiente Seebeck (S) em bicamadas de grafeno pode ser ajustado por potenciais de gate, de forma a escolher os portadores de carga, atingindo S =_250 _V/K. Também estudamos a dependência de S com a temperatura (T) e o tamanho do gate, calculamos a condutividade térmica por dinâmica molecular e a eficiência termoelétrica (ZT). Na seqüencia, mostramos que grafeno dopado com Mn mostra caloritrônica de spin ajustável via gate e como a termocorrente varia com T e _T. Finalmente, calculamos as propriedades termoelétrica de uma junção molecular Au-BDT-Au e como elas variam em função do alongamento da junção. Também propomos um esquema geral para maximizar ZT de junções moleculares em geral. / Thermoelectric effects describe how electric fields arise in response to temperature gradients and vice versa. In this thesis we investigate the thermoelectric properties of low-dimensional materials and nanostructures theoretically. We perform ab initio calculations of the electronic transport properties using the TRANSAMPA code, based in nonequilibrium Greens functions, and the SIESTA code, based in density functional theory. First, we study graphene nanoribbons and how their properties are altered by substitutional impurities. Among our main results for this system, we show that ribbons in the ferromagnetic configuration present spin-dependent Seebeck effect, which can be tuned by a field effect. We show that the Seebeck coefficient (S) of bilayer graphene is highly tunable by a gate potential, with ambipolar behavior, reaching S = _250 _V/K. We also study how S varies with temperature (T) and gate length. We calculate its thermal conductivity by molecular dynamics, and its thermoelectric efficiency (ZT ). Then, we show that Mn doped graphene features a gate-tunable spin-dependent S, which is robust under changes in T and _T, rendering this material suitable for spin caloritronics. Finally, we calculate how the thermoelectric properties of an Au-BDT-Au molecular junction vary with mechanical stretching, and propose a general recipe to improve ZT in molecular junctions in general.

Devido a estabilidade e capacidade de armazenar oxigênio (oxygen storage capacity) óxidos de cério (CeO2, CeO2 - x, 0 < x < 1/2, Ce2O3) possuem muitas aplicações, tais como catalisadores e suporte para metais de transição (Ni, Pt, Pd, Rh, etc.). A capacidade do átomo de cério alterar reversivelmente seu estado de oxidação entre Ce4+ e Ce3+ é crucial em suas aplicações e pode ser induzida por metais de transição ou outra espécie químicam (átomos, moléculas) depositados sobre suas superfícies. Porém, não se tem conhecimento de estudos da adsorção de metais transição sobre clusters neutros de óxido de cério, nem mesmo estudos de clusters de composição fixa e carga variável. Portanto, neste projeto são estudadas as propriedades físico-químicas dos clusters de (CeO2)6- , (CeO2)6 e (CeO2)6+ e da adsorção dos ametais F e Na e dos metais de transição Ni, Pd e Pt sobre (CeO2)6. Os resultados foram obtidos mediante cálculos ab initio da teoria do funcional da densidade (do inglês - Density Functional Theory, DFT), onde os funcionais PBE e HSE06 foram utilizados para descrição do funcional de troca-correlação. Os resultados apontam divergências dos funcionais PBE e HSE06 na sequência de energia total relativa das 22 configurações representativas de (CeO2)6. As energias de ionização e afinidade eletrônica indicam que as configurações (CeO2)6 de maior energia perdem um elétron mais facilmente que as de menor energia, que acomodam melhor o elétron. O pGMC (do inglês -putavive Global Minimum Configuration) aniônico é derivado do pGMC neutro pela localização do elétron no orbital Ce fz3 e consequnete mudança do estado de oxidação Ce4+ para Ce3+. O pGMC catiônico não é derivado do pGMC neutro, esse cluster não possui ligações Ce-O terminais e o elétron é removido de um orbital com maior contribuição do estado O p. Nas configurações de adsorção apenas ligações iônicas Ce-X ou O-X com maior diferença de eletronegatividade são formadas, nas quais ocorre transferência de carga para o elemento mais eletronegativo. Análises das configurações de menor energia mostram que, ao contrário da adsorção de flúor, os átomos de Na, Ni, Pd e Pt transferem carga para o cluster. Contudo, as configurações Pd/(CeO2)6 e Pt/(CeO2)6 não possuem momento magnético resultante. Apenas nas configurações Na/(CeO2)6 e Ni/(CeO2)6 o elétron se localiza no orbital Ce fz3 com a concomitante mudança do estado de oxidação Ce4+ para Ce3+. Nas configurações de menor energia a adsorção de sódio é favorecida em 0,05 eV em relação a adsorção de flúor. Por sua vez, o níquel é o metal de transição que se adsorve mais fortemente ao cluster, ao passo que o paládio apresenta a menor energia de adsorção. / Due to the stability and oxygen storage capacity, cerium oxides (CeO2, CeO2 - x, 0 < x < 1/2 , Ce2O3) have many applications, such as catalysts and support for transition metals (Ni, Pt, Pd, Rh, etc.). The ability of the cerium atom to reversibly change its oxidation state between Ce4+ and Ce3+ is crucial in its applications and can be induced by transition metals or another chemistry species (atoms, molecules) deposited on their surfaces. However, no studies of the transition metals adsorption on cerium oxide neutral clusters, nor studies of clusters of fixed composition and variable charge are known. In this project, the physicochemical properties of the (CeO2)6- , (CeO2)6 and CeO2)6+ clusters and the adsorption of the F and Na ametals and Ni, Pd and Pt transition metals on (CeO2)6.The results were obtained by ab initio calculations of the density functional theory (DFT), where the PBE and HSE06 functionals were employed for the exchange-correlation functional. The results point out divergences of the PBE and HSE06 functional in the 22 representative (CeO2)6 configurations relative total energy sequence. Ionization and electron affinity energies indicate that the higher energy (CeO2)6 configurations lose an electron more easily than the lower energy ones, which better accommodate the electron. The anionic pGMC (putavive Global Minimum Configuration) is derived from the neutral pGMC by the location of the electron in the Ce fz3 orbital and consequent oxidation state change from Ce4+ to Ce3+. The cationic pGMC is not derived from the neutral pGMC, this cluster has no terminal Ce-O bonds and the electron is removed from an orbital with the highest contribution of the Op state. In the adsorption configurations only ionic bonds Ce-X or O-X with the greatest difference in electronegativity are formed, in which charge transfer occurs to the most electronegative element. Unlike fluorine adsorption, lower energy configurations analyses show that Na, Ni, Pd e Pt atoms transfer charge to the cluster. However, the Pd/(CeO2)6 and Pt/(CeO2)6 configurations have no resulting magnetic moment. Only in the Na/(CeO2)6 and Ni/(CeO2)6 configurations the electron is located in the Ce fz3 orbital with the concomitant oxidation state change from Ce4+ to Ce3+. In the lower energy configurations the sodium adsorption is favored in 0,05 eV in relation to the fluorine adsorption. In turn, nickel is the transition metal that most strongly adsorbs on cluster, while palladium has the lowest adsorption energy.

En rupture avec les approches classiques de métallurgie consistant à allier un ou deux éléments majoritaires avec d'autres éléments en proportions minoritaires, un nouveau concept de matériaux est né : des alliages multi-composants formant une solution solide et pour lesquels tous les composants sont fortement concentrés. Ces nouveaux alliages, appelés alliages à haute entropie, présentent des propriétés mécaniques prometteuses, telles qu'une résistance mécanique élevée combiné à une grande ductilité. Par définition, ce nouveau concept de matériau rend possible l'exploration d'un champ quasi-infini de compositions chimiques. Toutefois, la stabilité thermodynamique de ces systèmes est mal connue, limitant fortement le choix des compositions. Dans ce contexte, le premier objectif de cette étude était de déterminer le domaine d'existence de la solution solide cubique à faces centrées (cfc) pour le système Co-Cr-Fe-Mn-Ni. Pour cela, la stabilité de phase cfc a été étudiée théoriquement et expérimentalement. En utilisant l'approche Calphad (Calculation of PHAse Diagram) et une nouvelle base de données (TCHEA-1), les phases stables de 10 626 compositions ont pu être calculées, à plusieurs températures. La comparaison entre calculs et résultats expérimentaux indique que la solution solide cfc est correctement décrite par cette base de données. Ainsi, il a été montré que la phase cfc est stable sur une large gamme de composition, décrite intégralement. Il est désormais possible de choisir une composition formant une solution solide stable à haute température pour ce système. Des calculs DFT (Density Functional Theory) ont ensuite permis d'analyser l'évolution de l'enthalpie de mélange en fonction de la composition mais aussi du nombre d'éléments. Des différences notables ont pu être constatées avec les prédictions faites par la base de données TCHEA-1. De plus, ces calculs ont mis en avant l'absence d'interaction ternaire et quaternaire pour le système d'étude. Ensuite, l'influence de la composition sur le durcissement par solution solide a été étudiée, pour permettre l'optimisation des propriétés mécaniques. Ainsi, l'évolution des propriétés structurelles et mécaniques des alliages multi-composants a été étudiée expérimentalement. Vingt-cinq alliages du système Co-Cr-Fe-Mn-Ni formant une solution solide cfc ont été traités. Le paramètre de maille a été mesuré par rayons X tandis que la dureté et le module d'élasticité ont été étudiés par nano-indentation. Le rôle de chaque élément sur le comportement mécanique fut ainsi explicité. Finalement, un modèle permettant d'estimer le durcissement de solution solide pour ce système est étudié / For a long time, development of alloys was restricted to one principal element, or two, with minor elements added for performance optimization. In 2004, a new concept of materials was born: multi-component alloys forming a solid-solution and in which all components are very concentrated. These new alloys, named high entropy alloys, can combine high mechanical resistance and large ductility. By definition this new material concept should make it possible to explore an almost infinite field of chemical compositions. But in the meantime, the thermodynamic stability of these systems was poorly known and severely limits the choice of alloy compositions. In this context, the first objective of this study was to fully determine the composition range of existence of a unique fcc solid solution within the multi-component Co-Cr-Fe-Mn-Ni system. To address this problem, the phase stability was theoretically and experimentally investigated. Using the Calphad approach and a new database (TCHEA-1), the stable phases of 10 626 compositions could be calculated, at several temperatures. The comparison between calculation and experimental results indicates that the fcc solid solution is accurately described by this database. Finally, it was shown that the fcc phase is stable over a wide range of composition, which was completely described. Now, it is possible to choose a priori a composition which will form a solid solution within this system. The heat of mixing of the fcc phase were compared between density functional theory (DFT) and Calphad calculations for binaries, ternaries, quaternaries and quinary systems. Significant differences were found with the predictions made by the TCHEA-1 database. In addition, these calculations have highlighted the absence of ternary and quaternary interaction for the Co-Cr-Fe-Mn-Ni system. However, the influence of the composition on the fcc solid solution strengthening was not fully understood, which limits mechanical optimization. So, the evolution of structural and mechanical properties of multi-components alloys was experimentally investigated. Several alloys from the Co-Cr-Fe-Mn-Ni system forming a unique fcc solid solution were processed. The lattice parameter was measured by XRD while the hardness and elastic modulus were measured by nano-indentation. The role of each element on the mechanical behaviour is presented. Finally, a model to assess the solid solution strengthening for this system is studied

Neste trabalho estudamos propriedades estruturais, magnéticas e eletrônicas de complexos de cobre em duas classes de ligantes semelhantes com potenciais aplicações nas áreas de nanotecnologia e farmacologia. A primeira classe de ligantes é do tipo bases de Schiff, conhecidas por sua vasta gama de aplicações em química e biologia, aqui estudadas como miméticos de sítios de proteínas e também como potenciais metalo-fármacos. Procuramos investigar as características estruturais e eletrônicas buscando correlacionar as informações obtidas experimentalmente com as obtidas através de cálculos de estrutura eletrônica. Em especial, no estudo da competição pelo íon Cu entre as bases de Schiff e a proteína albumina, mostramos a influência de diversos fatores como, por exemplo, a geometria dos ligantes e a sua estrutura eletrônica. A segunda classe estudada é constituída de bases modificadas de DNA com a habilidade de complexar metais (espécie [M-DNA]) e formar estruturas com acoplamento ferromagnético. Observamos para as espécies [M-DNA] que o acoplamento ferromagnético é estabilizado através de dois efeitos: a) diferentes estados de carga que podem gerar distorções na coordenação quadrado planar; b) a adição do backbone do DNA. Utilizamos neste estudo o método Projector Augmented Wave (PAW) e também bases locais dentro da Teoria do Funcional da Densidade, atrav´es dos c´odigos computacionais CP-PAW e Gaussian03. / We studied structural, electronic and magnetic properties of copper complexes with two similar classes of ligands with potential applications in nanotechnology or pharmacology. The first class of compounds consists of Schiff bases and corresponding copper complexes. We investigated structural and electronic properties of these complexes searching for correlations among the information obtained experimentally and by electronic structure calculations. In special, in the study of the competition for the Cu ion between Schiff bases and the albumin protein, we show the influence of different factors such as the geometry of the ligands and their electronic structure. The second class of ligands focused in our studies are modified DNA bases coordinated to copper, where EPR studies have shown ferromagnetic interactions among the metal centers. We studied in this case the influence of the charge state and of the backbone in the magnetization of the Cu chain. In all cases presented here, we used ab-initio electronic structure calculations in the framework of the Density Functional Theory (DFT). This has been done using the CP-PAW code and the Gaussian03 code.