Les évolutions technologiques notamment dans la télécommunication ou le transport nécessitent des systèmes de stockage de l’énergie de plus en plus performants. Dans le cas des transports, l’utilisation d’énergie fossile est encore la plus rependu. Mais la raréfaction de ces ressources, et le réchauffement climatique en partie dû au gaz CO2 rejeté par la combustion de celle-ci a mené à une prise de conscience d’un besoin de transition énergétique. Le stockage de l’énergie sous forme électrochimique (batteries) permet de s’affranchir de ces défauts. L’un des matériaux d’électrode positive les plus utilisé pour les batteries Li-ion est le LiCoO2. Il est aujourd’hui remis en question, pour des raisons de performance, mais aussi de coût et de toxicité du cobalt. Une substitution du cobalt par d’autres métaux de transition a été testée avec succès. Le matériau LiNi1/3Co1/3Mn1/3O2 (NMC) a montré des performances électrochimiques supérieures au LiCoO2. Un autre matériau d’électrode positive en commercialisation, le matériau spinelle LiMn2O4, offre des capacités intéressantes.Les performances électrochimiques de ces matériaux peuvent être améliorées. La durée de vie de ces batteries est en partie limitée par des phénomènes aux interfaces électrode/électrolyte. Il est nécessaire d’étudier ces phénomènes encore mal compris. Lors de cette étude, nous nous intéresserons à la réactivité de surface d’un matériau modèle Li2MnO3 ; et des spinelles par une approche mêlant expérience et théorie. Notre stratégie opératoire est basée sur l’adsorption de sonde gazeuse suivit d’une étude multi-échelle sur des matériaux modèles couplant expérience et théorie.Une première étude porte sur la réactivité de surface des faces (001) des cristaux de Li2MnO3 par analyse XPS et Auger de l’adsorption de sonde SO2, ainsi que de de la modélisation de ces réactions d’adsorption. Puis une seconde étude s’intéresse au rôle du degré d’oxydation du manganèse dans la réactivité de surface avec une étude sur les spinelles. La dernière partie de la thèse porte sur l’impact des fautes d’empilement sur la réactivité de surface du matériau Li2MnO3 sous forme de poudre. / LiCoO2 from layered lithium oxide family, is the most widely used Li-ion batteries positive electrode material. To support the global demand for more efficient batteries material, much research has been performed to explore alternative materials as mixed transition metal oxides LiNi1/3Mn1/3Co1/3O2 (NMC), with a larger capacity (140 mAh.g-1 for LiCoO2 vs. 180 mAh.g-1 for NMC) and a good cyclability. Its main advantages are its lower cost and toxicity, due to the lower content in Co, as well as its higher thermal stability in the delithiated state, i.e. in the charge state of the battery. The large content in Mn at the tetravalent state, which is stable in temperature, allows to stabilize the layered structure and to postpone at higher temperatures all the reactions involved during the thermal degradation of NMC, i.e. the cationic migrations, the oxygen loss and the associated reduction of the transition metal ions. A new series of layered oxide compounds Li1+xM1-xO2, called the Li-rich, was recently shown to be very attractive as delivering exceptional reversible capacities (over 250 mAh.g-1).For these overlithiated manganese-rich compounds the redox processes were shown to involve reversibly (at least in part) the transition metal and the oxygen anion. For all these layered oxides, there is a partial dissolution of the transition metals in the electrolyte, whose rate strongly depends on the cycling conditions (temperature, potential window …) and especially on the aging and storage conditions.Within this framework, a strategy combining adsorption of gaseous probe molecules (SO2) monitored by XPS and periodic DFT calculations has been developed. At an experimental level, this methodology allows the identification of the adsorption mode and the determination of the active sites concentration. This strategy has been used with LiCoO2 and NMC material, and has shown the surface reactivity of these materials. The present work uses this strategy for studying the role of surface manganese on reactivity surface. We propose to study the material LiMn2O4 and Li2MnO3, as reference materials for respectively manganese +III/+IV and manganese +IV, to see the role of surface manganese oxidation state on layered lithium oxide surface for surface reactivity. We showed that the surface reactivity depends on the crystallographic face orientation of the material, in good agreement with experimental and theoretical studies.The surface reactivity depends on the crystallographic face in the case of LiMn2O4 crystals. The calculations of adsorption on the most stable surface show different an evolution of surface reactions. These calculations have been combining with X-ray photoelectron spectroscopy study of LiMn2O4 materials.

L’oxyde de zirconium est un matériau prometteur pour le stockage des actinides en formation géologique profonde. Pour la sureté de stockage, la stabilité de ces matrices dans les milieux géologiques doit être examinée. Une connaissance approfondie de la solubilité des phases solides de confinement des déchets radioactifs s’avère donc nécessaire. La solubilité de ZrO2 est très faible aux pH neutres (<10-8M), et augmente aux pH acides et alcalins. Cependant, les valeurs de solubilités publiées varient de plus de 6 ordres de grandeurs. L’objectif de ces travaux de thèse est de comprendre les processus ayant lieu, à l’équilibre, à l’interface ZrO2/solution et d’étudier la réactivité de surface de ce matériau en contact avec des solutions aqueuses. La solubilité de ZrO2 monoclinique et cubique a été explorée en approchant l’équilibre depuis les conditions de sous-saturation. Un protocole expérimental robuste a été développé permettant de mesurer, au plus précis possible, les très faibles concentrations de zirconium en solution (limite de quantification [Zr] ~10-11M). Les phases solides ont été caractérisées par BET, XRD, SAXS,HR-TEM and STEM-HAADF avant et après l’atteinte de l’équilibre afin d’étudier la réactivité de surface des matériaux mis en contact avec les solutions aqueuses. En vue de comprendre les mécanismes se déroulant à l’interface aussi bien que la faible solubilité dans les milieux aqueux, l’hydrolyse des liaisons Zr-O-Zr par les molécules d’eau a été étudiée par la méthode DFT en utilisant le code VASP. Cette étude nous a permis de proposer un mécanisme de dissolution dont l’étape limitante, aux pH neutres, est la difficulté de former un complexe activé à l’interface ZrO2/solution. / Zirconium dioxide is a promising ceramic for the specific immobilization of actinides in a geological disposal vault. An appropriate knowledge of the solubility controlling solid phases confining radioactive waste is necessary if one wants to assess the stability of ZrO2 in a disposal environmental conditions. The solubility of zirconium oxide is very low at neutral pH (<10-8M), but increases at highly acidic and alkaline pH. Solubility values may differ by more than 6 orders of magnitude at a given pH, indicating high uncertainties. The objective of this work is to understand processes governing the equilibrium between zirconium oxide and water, and to study the surface reactivity of the material in contact with aqueous solutions. The solubility of monoclinic and of cubic ZrO2 in aqueous solutions were investigated, approaching equilibrium from under-saturated conditions.The low solubility of zirconia makes its measurement a challenging task. A reliable experimental procedure was developed to measure low concentrations of zirconium (limit of quantitation[Zr] ~10-11M). In order to examine the surface reactivity, solid phases were further characterized by BET, XRD, SAXS, HR-TEM and STEM-HAADFbefore and after reaching equilibrium. In order to understand the reaction mechanism taking place at the zirconia/water interface as well as the low solubility in aqueous phase, hydrolysis of Zr-O-Zr bonds by a single and multiple water molecule were studied using the periodic DFT code, with Vienna ab initio simulation package (VASP). This study led to the proposal of a dissolution mechanism whose limiting step, in the neutral pH range, is the difficulty of formation of an activated complex.

Neste estudo, investigaram-se mudanças nas propriedades coloidais de uma esmectita pilarizada com polihidroxicátion de alumínio. A solução pilarizante foi preparada mediante gotejamento de solução NaOH 0,4 mol L-1 em solução de AlCl3.6H2O 0,2 mol L-1, a qual foi adicionada à suspensão de argila de 1% m/m. Para avaliar mudanças nas propriedades das argilas, recorreu-se às técnicas de titulação potenciométrica descontínua, análise química total, DRX, FTIR, CTC e isotermas de adsorção/dessorção de N2. Além disso, foram realizados ensaios de adsorção de Cu2+ para avaliar a capacidade de remoção do metal de soluções aquosas pelas argilas pilarizadas e gerar informações relacionadas à interação entre adsorvente e adsorbato. Naturalmente, a argila apresentou espaçamento basal de 1,26 nm, ao passo que as pilarizadas apresentaram valores de 1,78 nm (500 oC) e 1,80 nm (350 oC). Dados da análise química total mostraram se tratar de uma montmorillonita com altos teores de Fe3+, e confirmou o aumento nos teores de Al3+ na estrutura da argila após pilarização. Os valores de área superficial específica e volume de microporos foram maiores para as argilas pilarizadas, enquanto que a CTC foi maior para a argila natural. A titulação potenciométrica mostrou modificação nas curvas de titulação com o processo de pilarização, em que nas argilas pilarizadas surgiram novos sítios de reação. Dentre as equações de adsorção aplicadas, Langmuir, Freundlich e Temkin, a primeira apresentou valores de r2 das equações linearizadas maior para todas as argilas e menor desvio médio (?g%) para argila natural, ao passo que a equação de Temkin mostrou valores de ?g(%) menores para as argilas pilarizadas. Parâmetros termodinâmicos confirmaram que a reação de adsorção de Cu2+ é espontânea para todas as argilas, principalmente para as argilas pilarizadas. Além disso, cálculos baseados em equações de Dubinin-Radushkevich evidenciam que a ligação é fraca entre metal e argila, caracterizando reações de fisissorção. / In this study, an Al-pillared smectite was synthesized and changes in its colloidal properties were investigated. The pillaring solution was prepared adding dropwise adequate volume of NaOH 0,4M to AlCl3.6H2O 0,2M solution. Then, the pillaring solution was added to a 1% w/w clay slurry with constant stirring. Potentiometric titration, chemical analysis, XRD, FTIR, CEC and N2 sorption/dessoption isotherms analysis were done to characterize the changes in clay properties. Moreover, adsorption experiments were carried out in order to evaluate the capacity of the pillared clays to remove Cu2+ from aqueous solution and to characterize the interaction between adsorbent and adsorbate. The natural clay has a basal spacing of 1.26 nm whereas the pillared clays reached 1.78 nm (500 oC) and 1.80 nm (350 oC) after calcination. Chemical analysis revealed that the montmorillonite used has high content of Fe3+ and the increase in the Al3+ amount in the structure of the pillared clays after pillaring process. The surface area and micropore volume were higher for the pillared clays and the CEC was higher for the natural clay. The pillaring process changed the potentiometric titration curves, wherein the pillared clays exhibited new reactive site. Experimental data were fit to Langmuir, Freundlich and Temkin adsorption equations, being the the first one the best (highest r2 value) for all the clays and lower standard deviation (?g%) for the natural clay. On the other hand, Temkin equation exhibited ?g% value lower for the pillared clays. Thermodynamics parameters demonstrate that the Cu2+ adsorption process is spontaneous for all the clays, but with higher values for the pillared materials. In addition, application of the Dubinin-Radushkevich equation revealed that the bond between the metal and the clay are weak, characterizing a physisorption.

Neste estudo, investigaram-se mudanças nas propriedades coloidais de uma esmectita pilarizada com polihidroxicátion de alumínio. A solução pilarizante foi preparada mediante gotejamento de solução NaOH 0,4 mol L-1 em solução de AlCl3.6H2O 0,2 mol L-1, a qual foi adicionada à suspensão de argila de 1% m/m. Para avaliar mudanças nas propriedades das argilas, recorreu-se às técnicas de titulação potenciométrica descontínua, análise química total, DRX, FTIR, CTC e isotermas de adsorção/dessorção de N2. Além disso, foram realizados ensaios de adsorção de Cu2+ para avaliar a capacidade de remoção do metal de soluções aquosas pelas argilas pilarizadas e gerar informações relacionadas à interação entre adsorvente e adsorbato. Naturalmente, a argila apresentou espaçamento basal de 1,26 nm, ao passo que as pilarizadas apresentaram valores de 1,78 nm (500 oC) e 1,80 nm (350 oC). Dados da análise química total mostraram se tratar de uma montmorillonita com altos teores de Fe3+, e confirmou o aumento nos teores de Al3+ na estrutura da argila após pilarização. Os valores de área superficial específica e volume de microporos foram maiores para as argilas pilarizadas, enquanto que a CTC foi maior para a argila natural. A titulação potenciométrica mostrou modificação nas curvas de titulação com o processo de pilarização, em que nas argilas pilarizadas surgiram novos sítios de reação. Dentre as equações de adsorção aplicadas, Langmuir, Freundlich e Temkin, a primeira apresentou valores de r2 das equações linearizadas maior para todas as argilas e menor desvio médio (?g%) para argila natural, ao passo que a equação de Temkin mostrou valores de ?g(%) menores para as argilas pilarizadas. Parâmetros termodinâmicos confirmaram que a reação de adsorção de Cu2+ é espontânea para todas as argilas, principalmente para as argilas pilarizadas. Além disso, cálculos baseados em equações de Dubinin-Radushkevich evidenciam que a ligação é fraca entre metal e argila, caracterizando reações de fisissorção. / In this study, an Al-pillared smectite was synthesized and changes in its colloidal properties were investigated. The pillaring solution was prepared adding dropwise adequate volume of NaOH 0,4M to AlCl3.6H2O 0,2M solution. Then, the pillaring solution was added to a 1% w/w clay slurry with constant stirring. Potentiometric titration, chemical analysis, XRD, FTIR, CEC and N2 sorption/dessoption isotherms analysis were done to characterize the changes in clay properties. Moreover, adsorption experiments were carried out in order to evaluate the capacity of the pillared clays to remove Cu2+ from aqueous solution and to characterize the interaction between adsorbent and adsorbate. The natural clay has a basal spacing of 1.26 nm whereas the pillared clays reached 1.78 nm (500 oC) and 1.80 nm (350 oC) after calcination. Chemical analysis revealed that the montmorillonite used has high content of Fe3+ and the increase in the Al3+ amount in the structure of the pillared clays after pillaring process. The surface area and micropore volume were higher for the pillared clays and the CEC was higher for the natural clay. The pillaring process changed the potentiometric titration curves, wherein the pillared clays exhibited new reactive site. Experimental data were fit to Langmuir, Freundlich and Temkin adsorption equations, being the the first one the best (highest r2 value) for all the clays and lower standard deviation (?g%) for the natural clay. On the other hand, Temkin equation exhibited ?g% value lower for the pillared clays. Thermodynamics parameters demonstrate that the Cu2+ adsorption process is spontaneous for all the clays, but with higher values for the pillared materials. In addition, application of the Dubinin-Radushkevich equation revealed that the bond between the metal and the clay are weak, characterizing a physisorption.

Density functional theory (DFT) has become the most widely used first-principles computational method to simulate different atomic, molecular, and solid phase systems based on electron density assumptions. The complexity of describing a many-body system has been significantly reduced in DFT. However, it also brings in potential error when dealing with a system that involves the interactions between metallic and non-metallic species. DFT tends to overly-delocalize the electrons in metallic species and sometimes results in the overestimation of reaction energy, metallic properties in insulators, and predicts relative surface stabilities incorrectly in some instances. There are two approaches to overcoming the failure of DFT using standard exchange-correlation functionals: One can either use a higher level of theory (and thus incur a greater computational cost) or one can apply an efficient correction scheme. However, inaccurate corrections and improper calculation models can also lead to more errors. In the beginning of this dissertation, we introduce the correction methods we developed to accurately model the structure and electron density in material surfaces; then we apply the new methods in surface reactivity studies under experimental conditions to rationalize and solve real life problems. We first investigate the post-DFT correction method in predicting the chemisorption energy (Echem) of a NO molecule on transition metal surfaces. We show that DFT systematically enhances back-donation in NO/metal chemistorption from the metal d-band to NO 2π* orbital, and relate the back-donation charge transfer to the promotion of an electron from the 5σ orbital to the 2π* orbital in the gas-phase NO G2Σ-←X2Π excitation. We establish linear relationships between Echem and ΔEG←X and formulate an Echem correction scheme to the (111) surfaces of Pt, Pd, Rh and Ir. As a precursor to further optimization of DFT corrections on transition metal oxide surfaces, we systematically compare the alumina (α-Al2O3) and hematite (α-Fe2O3) (0001) surfaces to study how the atomic positions treatment during geometry optimizations would affect the electronic structure and modeled reactivity, since they are often reported to have a minimal effect. Our results suggest that both can vary significantly in quantitative and qualitative ways between partially constrained or fully relaxed slab models. We continue to use the α-Fe2O3 (0001) surfaces to optimize the Hubbard U method implemented in DFT that determines the Coulomb repulsion correction (Ud) to localize Fe d-electrons. It successfully restores the insulating properties of bulk hematite, but underestimates the stability of the oxygen-terminated surface. It is mainly due to the fact that all the chemically distinct surface Fe atoms were treated the same way. Here we develop a linear response technique to derive specific Ud values for all Fe atoms in several slab geometries. We also find that in a strongly correlated system, the O p-orbitals also need the Hubbard correction (Up) to accurately predict the structural and electronic properties of bulk hematite. Our results show that the site-specific Ud, combined with Up as Ud+p, is crucial in obtaining theoretical results for surface stability that are congruent with the experimental literature results of α-Fe2O3 (0001) surface structure. Besides methodology development, we continue to apply our specific Ud+p method in the engineered application of the Chemical Looping Combustion (CLC) process in which transition metal oxides play the role of oxidizing fuel molecules for full CO2 capture. Current molecular dynamic studies use partially constrained surface models to simulate the CH4 reaction on hematite surfaces without the detailed comparison of the early stage adsorption products. Here we use hematite (α-Fe2O3) and magnetite (Fe3O4) surfaces as analogous to systematically study the early adsorption products of CH4. Our results show that the reaction favors the homolytic pathway on O-terminated surface, and that as a reduced form of hematite, the magnetite surface also shows excellent reactivity on CH4 dissociation. Knowing how to simulate DFT surface model properly we continue to enrich our theoretical methods for more complicated systems under aqueous conditions. We focus on various structures of the lithium-ion battery material, LiCoO2 (LCO) (001) surface, involving hydroxyl groups. We assess the relative stabilities of different surface configurations using a thermodynamic framework, and a second approach using a surface-solvent ion exchange model. We find that for both models the –CoO–H1/2 surface is the most stable structure near the O-rich limit, which corresponds to ambient conditions. We also found that this surface has nonequivalent surface geometry with the stoichiometric –CoO–Li1/2 surface, leading to distinct band structures and surface charge distributions. We go on to probe how those differences affect the surface reactivity in phosphate anion adsorption. All of the work presented in this dissertation reveals the importance of accurately modeled material structures in theoretical studies to achieve correct physical properties and surface reactivity predictions. We hope our DFT correction schemes can continue to contribute to future surface studies and experimental measurements, and to enlighten new ideas in future DFT methodology improvements.

<p>Diamond and cubic boron nitride surfaces have extreme properties that can be exploited in novel tribological, electrochemical and electronic applications. Normally insulating diamond surfaces can exhibit high surface conductivities due to hydrogen termination and the nature of the surrounding atmosphere. Successful growth of cubic boron nitride thin films is hindered when harsh synthesis methods are used.</p><p>Three significant surface-related properties are addressed in this thesis using computational methods: (1) the structure, energy stability and reactivity of clean and differently terminated diamond surfaces, (2) the high surface conductivity of diamond, and (3) the adsorption-induced stability, reactivity and reconstruction of the cubic boron nitride (100) surface. Density Functional Theory (DFT) has been used at the GGA level under periodic boundary conditions to simulate the diamond and cubic boron nitride surfaces. </p><p>The diamond surface structures are shown to be insensitive to hydrogen desorption. Oxygen atoms bind in different positions and with different bond strengths. Hydroxyl groups experience both attractive hydrogen bonding and steric repulsions within the adsorbed species. The reconstruction of diamond (111)-1x1 is strongly dependent on the species adsorbed onto the surface. Electron transfer was observed from a diamond surface into a water-based adlayer, yielding a p-type doped surface, depending on the nature of the surface and the adlayer. The cubic boron nitride (100)-1x1 surface was shown to reconstruct into a 2x1 configuration on both the boron- and nitrogen-rich side through the formation of B-B bonds, as well as N–N dimer-induced surface relaxation. Hydrogen stabilized the (100)-1x1 surface, but the partial removal of hydrogen yielded non-reactive dimer formation on the surface. </p>

Diamond and cubic boron nitride surfaces have extreme properties that can be exploited in novel tribological, electrochemical and electronic applications. Normally insulating diamond surfaces can exhibit high surface conductivities due to hydrogen termination and the nature of the surrounding atmosphere. Successful growth of cubic boron nitride thin films is hindered when harsh synthesis methods are used. Three significant surface-related properties are addressed in this thesis using computational methods: (1) the structure, energy stability and reactivity of clean and differently terminated diamond surfaces, (2) the high surface conductivity of diamond, and (3) the adsorption-induced stability, reactivity and reconstruction of the cubic boron nitride (100) surface. Density Functional Theory (DFT) has been used at the GGA level under periodic boundary conditions to simulate the diamond and cubic boron nitride surfaces. The diamond surface structures are shown to be insensitive to hydrogen desorption. Oxygen atoms bind in different positions and with different bond strengths. Hydroxyl groups experience both attractive hydrogen bonding and steric repulsions within the adsorbed species. The reconstruction of diamond (111)-1x1 is strongly dependent on the species adsorbed onto the surface. Electron transfer was observed from a diamond surface into a water-based adlayer, yielding a p-type doped surface, depending on the nature of the surface and the adlayer. The cubic boron nitride (100)-1x1 surface was shown to reconstruct into a 2x1 configuration on both the boron- and nitrogen-rich side through the formation of B-B bonds, as well as N–N dimer-induced surface relaxation. Hydrogen stabilized the (100)-1x1 surface, but the partial removal of hydrogen yielded non-reactive dimer formation on the surface.

Several recent studies that have proposed MUSIC models for gibbsite surfaces have purported to achieve a very good fit with potentiometric titration data. However, in order to achieve such results, several significant parameters, such as the number of surface sites, site densities, and pKa values were sometimes re-introduced in the model as fitted parameters, and physically unrealistic modeling assumptions were sometimes used. In addition, recent evidence supports the idea that some of the gibbsite potentiometric titration data from these studies may be unreliable. In order to re-interpret the potentiometric titration data, we used several recently published methods. In order to detect possible problems with estimates of gibbsite basal and edge surface area, we synthesized two gibbsite samples with different aspect ratios and characterized their surface areas using BET, AFM, and computerized image analysis routines. We also estimated pKa values for acid/base reactions at gibbsite surfaces by applying a new bond-valence method to gibbsite (001)-type and (100)-type surface structures based on ab initio calculations. The resulting pKa estimates are not to be taken as precise values due to difficulties and assumptions associated with calculating reasonable ab initio surface structures. Instead, we believe they represent a more reasonable range than has been previously estimated. Using these estimates, we propose an improved MUSIC model for gibbsite, which seems to predict surface adsorption, not perfectly, but within a reasonable range for a number of titration data sets without re-introducing any of our estimated parameters as adjustable parameters. Discrepancies that exist between model predictions and various potentiometric titration data sets are likely due to error associated with potentiometric titrations and pKa predictions.

Ce travail de thèse porte sur l’effet de la fluoration sur la réactivité du dioxyde de titane. Dans ce travail, trois familles de TiO2, dont l’anatase pure, le rutile pure et le TiO2 de la phase mixte anatase/rutile ont été fluorées par la méthode dite du choc thermique à différentes températures, de 400 – 950°C. Les influences de la fluoration sur les propriétés et la composition élémentaire de leur surface ont été étudiées par spectroscopie photoélectronique à rayonnements X (XPS). L’évolution de la structure cristalline, la morphologie et les propriétés optiques de ces catalyseurs en fonction de la fluoration a été également étudiée en détail par diffraction des rayons X, nanosonde Auger et spectroscopie de réflectance diffuse UV - Visible. Leur activité photocatalytique a été évaluée par la dégradation du bleu de méthylène en solution. Jusqu’à 500°C, la fluoration est uniquement surfacique et ne modifie ni la structure ni la morphologie des particules TiO2. Cependant la fluoration augmente la teneur en groupement OH de surface, ce qui contribue à l’augmentation de l’activité photocatalytique. Par contre, au-delà de 500°C, la méthode de fluoration forme une phase parasite anisotrope, K2Ti6O13 qui réduit les performances photocatalytiques. La réactivité de surface des catalyseurs fluorés a été également évaluée par l’adsorption de sondes gazeuses acide SO2 et basique NH3, couplée à l’analyse XPS. Les résultats montrent que tous les catalyseurs possèdent des surfaces amphotères dont l’acidité et la basicité sont significativement influencées par la fluoration. / The thesis aimed to investigate the influences of fluorination on the reactivity of titanium dioxide. In this work, three crystallographic families of TiO2: pure anatase, pure rutile and TiO2 P25 (mixed phase anatase/rutile), were fluorinated by thermal shock method at different temperatures, from 400 to 950°C. The influence of fluorination on the properties and elementary composition of their surface was studied by X ray photoelectron spectroscopy (XPS). The evolution of crystal structure, morphology and optic properties of these catalysts versus the fluorination was also studied in detail by X ray diffraction, Auger nanoprobe and diffuse reflectance UV – Visible spectroscopy. Their photocatalytic activity was evaluated by the degradation of methylene blue in solution. According to the results, the fluorination below 600°C only occurs on the surface and does not modify the structure and the particle size of TiO2. However, the fluorination enhances the surface hydroxyl groups, which are assigned to the improvement of photocatalytic activity. For the fluorination over 600°C, the parasite anisotropic K2Ti6O13 phase is formed, resulting in the reduction of photocatalytic performances.The surface reactivity of our catalysts was also evaluated by the adsorption of probe molecules acid SO2 and basic NH3, coupled with XPS analysis. The results show that the surface of all catalysts is amphoteric with the acidity and basicity significantly affected by fluorination.

Extending electronic devices beyond the limitations of current micro-electronics manufacturing will require detailed knowledge of how to make contacts to semiconductor surfaces. In this work, we investigated several methods by which such connections to silicon surfaces could be achieved. Scanning tunneling microscopy (STM) was our main experimental tool, allowing direct imaging of the surfaces at the atomic level. First, the growth of self-forming linear nanostructures of organic molecules on silicon surfaces offers a possibility of creating devices with hybrid organic-silicon functionality. We have studied the growth of many different molecules on a variety of hydrogen-terminated silicon surfaces: H-Si(100)-2x1, H-Si(100)-3x1, and H-Si(111)-1x1. We found molecular growth patterns affected by steric crowding, by sample doping level, or by exposure to ion-pump created radicals. We formed the first contiguous "L-shaped" molecular lines, and used an external electric field to direct molecular growth. We attempted to study a novel method for nanoscale information transfer along molecular lines based on excitation energy transfer. The second part of the work focuses on the development and use of a new multiple-probe STM instrument. The design and the custom STM control software written for it are described. Connections to Si surfaces were achieved with a combination of lithographically defined metal contacts and STM tips. Two-dimensional surface conductivity of the Si(111)-7x7 surface was measured, and the effect of modifying the surface with organic molecules was investigated. A novel method, scanning tunneling fractional current imaging (STFCI), was developed to further study surface conductance. This method allowed us to determine, for the first time, that the resistance of steps on the Si(111)-7x7 surface is significantly higher than that of the surface alone.