The chemical adsorption and reaction of methyl fluoride, methyl chloride, methyl bromide, methyl iodide, methyl sulfide, dimethyl sulfide, dimethyl disulfide, and bis(trifluoromethyl) disulfide on clean, polycrystalline nickel surface (at 25°C) were investigated using an X-ray photoelectron spectrometer (XPS) and a thermal desorption system with mass spectrometric analysis (TD). The ion implantation of the methyl halide series on nickel was investigated by XPS. The nickel surface was cleaned by high temperature heating in vacuum or argon ion sputtering. An industrial lubricant, FC-43 (tri-hepta-fluorobutyl amine), was adsorbed on a gold-plated nickel surface at 25°C. Tne FC-43 adsorption system was investigated by TD. The adsorption of the methyl halide series on nickel was predominantly dissociative with several co-existing surface species being observed. The XPS results show that two carbon and two halogen species are formed. The possible structures for carbon are a CH₃-type and a CH₂-type or CH_nX (n = 0-3) adsorbed species. The halogen species appear to be neutral and ionic species. The TO results support the dissociative adsorption process for methyl halides on nickel. The adsorption of methyl fluoride and methyl chloride on nickel studied by TD resulted in the corresponding nickel halide being detected. The adsorption of methyl bromide and methyl iodide on nickel studied by TO resulted in the corresponding hydrogen halide, HX, being detected. The XPS results of the ion implantation of the methyl halides resulted in only one type of carbon that is similar to graphite. A halide type of halogen was also formed upon ion implantation. The XPS results for the adsorption of methyl sulfide, dimethyl sulfide, and dimethyl disulfide on nickel showed that only one carbon and one sulfur were observed. The possible structure for carbon is a CH₃-type of adsorbed specie. lhe sulfur species appears to be a sulfide S⁼-type adsorbed specie. The XPS results for the adsorption of bis(trifluoromethyl) disulfide on nickel showed that only sulfur was observed on the nickel surface. The TD results- for the methyl sulfide adsorption process was shown to be predominantly an associative type. The TO results for the dimethyl sulfide adsorption process were a mixture of dissociative and associative adsorption processes. The dimethyl disulfide adsorption process was determined by TD to be predominantly a dissociative type. The bis(trifluoromethyl} disulfide was determined by TD to be completely dissociative upon adsorption and reacted with nickel to form nickel sulfide. The TD results of FC-43 adsorption on gold-plated nickel showed that the predominant adsorption process was the associative type. / Ph. D.

In this study the applicability of Carbon-13 NMR and XPS to the detection of chemical changes in a solid composite rocket propellant was studied. Storage at elevated temperatures was used to simulate the propellant ageing process. In the XPS analysis, changes in the sources for the N(1s) and Cl(2p) photopeaks were investigated. The propellant loses "organic" nitrogen as it ages. Changes in the amount of Cl⁻ present are in doubt due to instrumental considerations. Carbon-13 NMR analysis showed that with increasing age of a sample there is a corresponding loss of vinylic groups from the binder. This loss of vinylic character is preferential in the order pendant>>cis>trans. Due to the long scan times involved this method is of limited utility for ageing analysis. / Master of Science

Bipolar electrochemistry is an interesting concept for high throughput screening techniques due to the ability to induce gradients in a range of materials and their properties, such as composition, particle size, or dopant levels, among many others. One of the key advantages of the method is the ability to test, create or modify materials without the need for a direct electrical connection. In this thesis, the viability of this method has been explored for a range of possible applications, such as metal recycling, nanoparticle modification and corrosion analysis. In the initial part of the work a process to electrodeposit gradients in metal composition was evaluated, with a view to applying the technique to the extraction and recycling of metals from fly ash. Compositional gradients in the metals under study could be readily obtained from controlled reference solutions, although the spatial resolution of the metals was not sufficient to perform separation. Only copper could be easily deposited from the fly ash solution. Bipolar electrodeposition was also successfully used to modify the particle size across substrates decorated with gold nanoparticles. The approach was demonstrated both for surfaces possessing either a uniform particle density or a gradient in particle density. In the latter case samples with simultaneous, orthogonal gradients in both particle size and density were obtained. A combination of the bipolar approach with rapid image analysis was also evaluated as a method for corrosion screening, using quantitative analysis of gradients in pitting corrosion damage on stainless steels in HCl as a model system. The factors affecting gradient formation and the initiation of corrosion were thoroughly investigated by the use of a scanning droplet cell (SDC) technique and hard x-ray photoelectron spectroscopy (HAXPES). The ability to screen arrays of different materials for corrosion properties was also investigated, and demonstrated for stainless steel and Ti-Al alloys with pre-formed compositional gradients. The technique shows much promise for further studies and for high throughput corrosion screening applications.

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This thesis is concerned with furthering our understanding of the basis of visible region photocatalytic activity exhibited by doped TiO2-based materials. A range of experimental techniques including high resolution X-ray photoemission spectroscopy and diffuse reflectance spectroscopy are used to investigate electronic structure and an attempt is made to link these results to the observed photocatalytic activity. Both anionic (N) and cationic (Rh and Sn) dopants are investigated. [See pdf file for full abstract].

<p>A fundamental understanding of heterogeneous catalysis requires analysis of model catalytic surfaces in tandem with complex technical catalysts. This work was divided in three areas, 1- preparation and characterization of model surfaces synthesized by vapor deposition techniques, 2- kinetic evaluation of model catalysts for formic acid decomposition and dry methane reforming, 3- characterization and kinetic evaluation of technical catalysts for the water gas shift reaction.</p> <p>In the first project, model PdZn intermetallic surfaces, a relevant catalyst for propane dehydrogenation, were prepared using an ALD approach. In this work, model surfaces were synthesized by exposing Pd(111) and Pd(100) surfaces to diethylzinc at ca. 10^-6mbar. Several different surface structures were identified by careful control of the deposition temperature of the substrate. Modifications in the adsorption properties of these surfaces towards carbon monoxide and propylene coincided with the structure of the PdZn surface layer. </p> <p>In the second project, formic acid decomposition kinetics were evaluated on model Pt catalysts. Formic acid decomposition was found to be structure-insensitive on Pt(111), Pt(100), and a polycrystalline foil under standard reaction conditions. CO selectivity remained < 1% for conversions <10%. Additionally, inverse Pd-Zr model catalysts were prepared by ALD of zirconium-t-butoxide (ZTB). Depending on treatment conditions, either ZrO_xH_y or ZrO₂ overlayers or Zr as sub-nanometer clusters could be obtained. The activity of the model catalyst surface towards dry reforming of methane if the initial state of the zirconium is metallic. </p> <p>In the third project, Au/Fe₃O₄ heterodimer catalysts were characterized for their thermal stability. In-situ TEM and XPS characterization demonstrates that the gold nanoparticles transform into gold thin films that wet the Fe₃O₄ surface as the reduction of the oxide proceeds. DFT calculations show that the adhesion energy between the Au film is increased on a partially reduced Fe₃O₄ surface. Additionally, Pt/Nb₂CT_x catalysts were characterized and kinetics evaluated for the water gas shift reaction. XPS and TEM characterization indicates that a Pt-Nb surface alloy is formed under moderate reduction temperatures, 350^OC. Water-gas shift reaction kinetics reveal that the alloy-MXene interface exhibit high H₂O activation ability compared to a non-reducible support or bulk niobium carbide. </p>

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / A espectroscopia de fotoelétrons excitados por radiação ultravioleta (UPS Ultraviolet Photoelectron Spectroscopy) é uma técnica de análise tipicamente de superfícies, particularmente relevante para estudos onde se deseja obter informações sobre os níveis de valência de átomos situados na superfície de um material. Esta técnica tem importantes aplicações em áreas em que reações de superfícies são fundamentais, como, por exemplo, estudos de corrosão, oxidação e de catálise. Neste trabalho, descreve-se a implantação de UPS no Laboratório de Física Aplicada do Centro de Desenvolvimento da Tecnologia Nuclear LFA/CDTN. Para a produção de fótons na região do ultravioleta é utilizada uma lâmpada de He, que emite fótons com energias de 40,8 e 21,2 eV. Essa instrumentação, construída em nosso laboratório, foi instalada em uma câmara de ultra-alto vácuo (UHV) e testada em medidas da banda de valência de monocristais. Para comprovar a funcionalidade da técnica, testes foram conduzidos em monocristais de Cu (100), Cu90Au10 (100) e Cu3Au (100), sendo que os resultados aqui apresentados, mostram o bom desempenho da instrumentação desenvolvida. A oxidação in-situ, em UHV, de filmes finos de Ni foi também investigada por XPS (XRay Photoelectron Spectroscopy), na tentativa de se observar a influência da pressão parcial de oxigênio e da temperatura na preparação de bicamadas Ni/NiO por oxidação in-situ. Nossos resultados mostram que para exposição do Ni a 103 L de O2 a temperaturas de até 523 K ocorre a formação de no máximo 10 Å de óxidos de níquel. / The Ultraviolet Photoelectron Spectroscopy (UPS) is an important surface analysis technique used in surface valence band studies. This technique has been applied to fundamental research on areas like corrosion, oxidation, and catalysis. In this work, a He lamp for UPS was constructed, installed and tested in the Laboratório de Física Aplicada at Centro de Desenvolvimento da Tecnologia Nuclear LFA/CDTN. The Helium discharge lamp has photon emission of 40,8 eV and 21,2 eV energy. The instrumentation built in our laboratory was installed in an Ultra High Vacuum (UHV) chamber and the obtained UPS results confirm the good perfomace of the instrument. The surface analysis tests were carried out on Cu (100), Cu90Au10 (100) and Cu3Au (100) surfaces. Furthermore, the preparation of NiO/Ni bilayer by in-situ oxidation of nickel films was also investigated by X-Ray Photoelectron Spectroscopy (XPS). The influence of oxygen partial pressure and the temperature changes in the growth of NiO during the in-situ oxidation of Ni were studied. For Ni exposure to 103 L O2 from room temperature up to 523 K, we observe the formation of, at most, 10 Å Ni oxides.

Novel photovoltaic cells receive considerable attention from researchers as evidenced by high numbers of published articles. Different types of materials are currently being examined in order to reduce the cost and improve the efficiency of solar cells. Essentially, solar cells are constructed by placing layers of light absorber between electron and hole transport materials. Electricity generation by solar cells involves multiple processes. These processes require an understanding of the physical properties of the surfaces and interfaces of the materials. In this thesis, materials for novel photovoltaic cells are studied by X ray photoelectron spectroscopy (XPS), a surface and interface characterisation technique. The materials studied in this thesis are colloidal quantum dots (CQDs) of the core/shell systems CdTe/CdSe and PbS/CdS, and CQDs that have been surface passivated using Cl- (CdTe/Cl) and CdS (CdTe/CdSe/CdS and PbS/CdS). Moreover, CsSnI3, a perovskite material, is also studied in both bulk and thin film form. CQDs can be used as light absorbers in solar cells while CsSnI3 can be employed as the hole transport material. The role of the core shell structure and surface passivation treatment is to improve or maintain charge transport as well as acting as a protective layer to the CQDs. Depth profiling synchrotron radiation XPS is used to determine these structures. In the CdTe/CdSe samples, the elemental ratio between Se (shell) and Te (core) increases with decreasing sampling depth, demonstrating the presence of a CdSe shell located at the surface of the CQDs. The shell thicknesses of the core-shell systems are estimated from XPS and show that the addition of the third thin shell (of CdS) protects the CQD during ligand exchange. Cl- passivation is shown to reduce the energy the valence band maximum and the energy gap of CdTe CQDs. This is associated with the passivation of midgap trap states due to the removal of dangling bonds at the surface of CQDs. Surface passivation is shown to improve the stability of CQDs to air exposure. This is indicated by a significant reduction of the surface oxide species in the passivated PbS/CdS samples. In the unpassivated core-only PbS samples, however, oxidation rapidly occurs which affects the electronic states required for charge transport in solar cells. XPS studies of CsSnI3 show that this material is reactive to air exposure. Surface preparation techniques are performed to remove the contamination layer and reveal the physical properties of the perovskite itself. This is confirmed by the elemental ratios from XPS. The metallic character of CsSnI3 is also observed in the valence band spectra as evidenced by the appearance of the Fermi edge.

<p>In dye-sensitized solar cells, the functions of light absorption and charge transport are separated. A photosensitive ruthenium-polypyridine dye in the cell absorbs light, injects an electron to a semiconductor and is then regenerated by a redox couple, typically iodide/triiodide. Quantum chemical calculations of the electronic structure of triiodide have been carried out with the restricted active space SCF method, including spin-orbit coupling, and with density functional theory. It was shown that the difference in charge density between the terminal and central atoms results in a splitting of the core levels. The calculations gave a value of the splitting of 0.8 - 1.0 eV for the <em>3d</em> and <em>4d</em> levels. Experimentally, the electronic structure has been investigated with photoelectronspectroscopy. The measured terminal/center splitting is 1.1 eV.The spin-orbit interaction of the <em>4d </em>levels of triiodide has also been calculated. The splitting was determined to be 1.6 eV. The experimental value is 1.7 eV. An assignment of the peaks in the computed spectrum of triiodide was made and the features of the experimental spectrum have beenidentied.The theoretical valence spectrum of triiodide has been computed and assigned. The results can be used in the analysis of photoelectron spectra of the molecule. Information about the electronic structure of the redox couple can help in the understanding of the electron transfer processes and forfurther development of the solar cells.  Furthermore, the solvation structure of the prototype dye, the tris(bipyridine)ruthenium(II) complex, in water and its interaction with iodide and chloride has been studied by means of molecular dynamics simulations. The trajectory analysis showed that the water molecules in the first solvation shell form a chain in between the bipyridine ligands. It was found that the iodide ions are more likely than chloride to enter between the ligands, which can be important for the electron transfer processin the solar cell.</p>

<p>Recently, materials such as hole conductors (CuI, CuSCN) and light absorbers (Ru-complexes, CuInS₂) have been actively investigated for application in nanocrystalline solar cells. In this thesis combinations of these materials have been studied.</p><p>In the first part of the thesis, various methods were applied to characterize the electronic structure and photoconversion mechanism of the dye molecule Ru(dcbpyH₂)₂(NCS)₂ when combined with materials for the use in photovoltaic devices. Specifically, the adsorption and electronic structure of the dye molecules adsorbed to semiconductors were investigated by means of photoelectron spectroscopy. The results indicate a chemical bond between the dye molecules and the hole conductors (CuI, CuSCN) via the NCS- groups. In addition, preparation of a TiO₂/Ru-dye/CuI solid state model system was studied <i>in situ</i>. These experiments showed a partial breaking of the TiO₂-dye bond caused by CuI evaporation. Photovoltage measurements were also performed. These investigations showed a shift in the light absorption threshold of the dye molecules adsorbed onto the hole conductors (CuI and CuSCN), indicating new defect states at the dye/CuSCN interface. Also, charge accumulation and transport in solar cells with CuSCN were compared to liquid electrolyte cells. Measurements showed that the lifetime and transport time of electrons in solar cells with CuSCN are much shorter than in electrolyte cells.</p><p>In the second part of the thesis, the deposition of CuInS₂ onto various metal oxides by spray pyrolysis has been studied with x-ray diffraction and photoelectron spectroscopy. The measurements showed that the morphologies of the substrates play a significant role in the formation of CuInS₂ layers. Also, the presence of CdS at TiO₂ has a positive influence on the formation of CuInS₂.</p>