The kinetics of the oxidation of platinum metal have long been a topic of interest in the field of electrochemistry. Using a combination of cyclic voltammetry, potential step experiments, and sweep-hold experiments, this research studies the kinetics of the initial stages of oxide growth on Pt(100), Pt(111), and Pt(110) surfaces. By comparing the electrochemical results with surface X-ray diffraction (SXRD) experiments conducted at synchrotron facilities, it was found that the charge of the oxide peak is within 15 μC cm⁻² or about 0.1 ML for all three surfaces. This means that the amount of oxide formed on each surface is similar. It was also determined that the oxide formed on Pt(111) is a Pt(II) species, consistent with an oxide like PtO. From calculations from the potential step experiments, it was determined that on Pt(100) there are two distinct regions of current decay, but that double layer charging is not one of the two seen. Instead, it was determined that the oxidation is likely a two step process with the first step being an adsorption step and the second being a place exchange oxide formation step. It was also found that more charge is passed when conducting potential step experiments to the oxide region from potentials in the hydrogen underpotential deposition (HUPD) region than from potentials in the double layer region. Finally, the results of a sweep-hold experiment on Pt(100) show that the values for charge are similar when calculated via the data from a sweep-hold and potential step experiment from a potential in the double layer region. The results of this research help further the kinetic understanding of the platinum surface during its oxidation and reduction. / Graduate

Ce travail de thèse s’est intéressé au dépôt électrochimique de filmsde palladium ultra-minces sur Au(111), à leur caractérisation et àl’insertion d'hydrogène dans ceux-ci. La caractérisation des nanofilmsen milieu sulfurique montre des signatures bien définies, qui évoluentavec l’épaisseur des dépôts. Nous avons pu attribuer à chaque pic uneréaction spécifique, en accord avec les mécanismes de croissancerévélés par les mesures SXRD in situ. La croissance pseudomorphede la 1ère couche se fait avec une première étape d'adsorption, suiviepar un mécanisme de nucléation et croissance. La croissance 3D de latroisième couche démarre avant la fin de la deuxième couchepseudomorphe.L'absorption d'hydrogène dans les nanofilms a été étudiée en milieusulfurique. L’isotherme d’insertion présente un élargissement dudomaine de la solution solide, un plateau avec une pente dans ledomaine bi-phasique et une diminution du taux maximal d'insertion del’hydrogène par rapport au Pd massif. Ce taux diminue avecl’épaisseur mai approche celui de Pd massif au déla delà de 15 MC.Deux éléments ont été considérés pour expliquer le comportement desisothermes: les deux premières couches pseudomorphes sontcontraintes par le support et des « tours » tridimensionnelles relaxéesse forment au delà de la 2ème couche. / This thesis focused on electro-deposition, characterization andhydrogen strorage of ultrathin palladium film over Au(111). Theelectrochemical characterization of the nanofilms in sulphuric mediumshows well-defined features evolving with the deposit thickness. Wecould assign each peak to a specific reaction, in agreement with thegrowth mechanisms revealed by in situ SXRD measurements. Thepseudomorphic growth of the 1st layer firstly undergoes an adsorptionstep, followed by nucleation and growth mechanism, as shown bycurrent transient measurements. 3D growth of the 3rd layer beginsbefore the completion of the second pseudomorphic one.Hydrogen absorption in the nanofilms was studied in sulphuric mediumas well. Isotherms show an enlargement of the solid solution domain, asloppy plateau in the two-phase region, a decrease of maximuminsertion ratio (H/Pd)max compared to bulk Pd. This last valuedecreases with film thickness, approaching bulk Pd beyond about15 ML. Two contributions were considered to explain the isothermbehaviour: the two first Pd layers heavily constraint by the substrateand the 3D “towers like” relaxed structures growing on the secondpseudomorphic Pd layer.

Poly-aryl, also known as polyphenylene compounds are a class of dendrimer, which contain a large number of aromatic rings. They are of interest because they display restricted rotation of their stearically congested aromatic rings. These extended structures have the potential to act as precursors for even larger aromatic systems and have many applications including electronic devices, drug delivery and catalysis. A total of 23 novel poly-aryl compounds have been examined using single crystal X-ray diffraction and a number of structural patterns have emerged. Six of the compounds contain alkynes and it has been observed that their conformation is governed by a combination of conjugation between the alkyne and aryl groups and inter-molecular interactions. In the more extended poly-aryl compounds steric congestion rules out any possibility of conjugation between the rings and their conformation is governed by intra-molecular non-bonded interactions in the core of the molecules and by inter-molecular interactions in their periphery. Where possible, solution NMR measurements were carried out on the poly-aryl compounds and confirmed that the solution structures are in agreement with those obtained from individual crystal.

Ce travail de thèse s'est intéressé au dépôt électrochimique de filmsde palladium ultra-minces sur Au(111), à leur caractérisation et àl'insertion d'hydrogène dans ceux-ci. La caractérisation des nanofilmsen milieu sulfurique montre des signatures bien définies, qui évoluentavec l'épaisseur des dépôts. Nous avons pu attribuer à chaque pic uneréaction spécifique, en accord avec les mécanismes de croissancerévélés par les mesures SXRD in situ. La croissance pseudomorphede la 1ère couche se fait avec une première étape d'adsorption, suiviepar un mécanisme de nucléation et croissance. La croissance 3D de latroisième couche démarre avant la fin de la deuxième couchepseudomorphe.L'absorption d'hydrogène dans les nanofilms a été étudiée en milieusulfurique. L'isotherme d'insertion présente un élargissement dudomaine de la solution solide, un plateau avec une pente dans ledomaine bi-phasique et une diminution du taux maximal d'insertion del'hydrogène par rapport au Pd massif. Ce taux diminue avecl'épaisseur mai approche celui de Pd massif au déla delà de 15 MC.Deux éléments ont été considérés pour expliquer le comportement desisothermes: les deux premières couches pseudomorphes sontcontraintes par le support et des " tours " tridimensionnelles relaxéesse forment au delà de la 2ème couche.

In this thesis the morphology and the atomic structure of quasi-one-dimensional structures grown on Si were determined by means of diffraction experiments in combination with kinematic diffraction theory calculations. In the first and the second study a formerly unknown superstructure of Dy/Tb on Si(111) was characterized by means of STM, DFT, SPA-LEED experiments and kinematic diffraction calculations. Here, a structure model could be proposed which contains half as many subsurface Si vacancies as compared to the well-known superstructure of Dy/Tb on Si(111) it was derived from. Due to the decreased number of subsurface Si vacancies the reconstruction is subject to an uni-axial strain which is mitigated by the formation of domains separated by anti-phase domain boundaries. It could be shown that two different types of domains alternate across the surface forming quasi-one-dimensional domains. Additionally, the distribution of the domains could be derived by comparison with kinematic diffraction calculations. In the third study a deeper insight into the complex system of bundled rare-earth silicide nanowires on Si(001) was given. Here, the distributions of the NW width, the bundle width and the bundle distance were deduced from the diffraction patterns collected by SPA-LEED and the subsequent comparison to kinematic diffraction theory calculations. Additionally, it was shown that the (2 x 1) reconstruction sometimes observed on top of the NWs by STM cannot exist over larger parts of the sample and instead a (1 x 1) reconstruction needs to be assumed to explain the experimentally observed diffraction data. In the fourth study the atomic structure of the gold induced atomic wires of the Si(111)-(5 x 2)-Au system was analyzed. The Patterson function of the in-plane SXRD data was compared to the Patterson functions derived from the atomic structure models proposed in literature (AN, EBH, KK) ruling out the AN-model. By comparison of the experimental out-of-plane SXRD data to the corresponding (calculated) SXRD data for the EBH- and the KK-model the KK-model could be identified as the most probable model. Additionally, a refined atomic structure model was derived for the KK-model. In conclusion, the results presented in this thesis clearly display the power of diffraction experiments especially in conjunction with the comparison to kinematic diffraction theory calculations and prove that they are applicable even to low dimensional (e.g., quasi-one-dimensional) structures. Furthermore, it was shown that diffraction experiments can deliver complementary information (e.g., information on deeper atomic layers) as compared to local probing methods (e.g. STM or Atomic Force Microscopy) and especially the combination of local probing methods, DFT calculations and diffraction experiments allows for the explanation of even very complicated material systems.