No description available.

No description available.

Made available in DSpace on 2014-10-09T12:40:47Z (GMT). No. of bitstreams: 0 / Made available in DSpace on 2014-10-09T14:08:00Z (GMT). No. of bitstreams: 1 02947.pdf: 6721410 bytes, checksum: 6b8fd88ede9d8533a129cf94a67a4313 (MD5) / Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) / Tese (Doutoramento) / IPEN/T / Instituto de Pesquisas Energeticas e Nucleares - IPEN/CNEN-SP / FAPESP:92/02990-0

Made available in DSpace on 2014-10-09T12:40:47Z (GMT). No. of bitstreams: 0 / Made available in DSpace on 2014-10-09T14:08:00Z (GMT). No. of bitstreams: 1 02947.pdf: 6721410 bytes, checksum: 6b8fd88ede9d8533a129cf94a67a4313 (MD5) / Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) / Tese (Doutoramento) / IPEN/T / Instituto de Pesquisas Energeticas e Nucleares - IPEN/CNEN-SP / FAPESP:92/02990-0

L’objectif de ce travail a été d’étudier des matériaux semiconducteurs de type p comme photocathode pour la réduction de protons dans le cadre de la photoélectrolyse de l’eau. Ainsi, deux types de matériaux ont été étudiés, des oxydes de cuivre, Cu2O et CuO, et des matériaux à structure pérovskite (ATiO3, A=Ca, Ba, Sr) dopées au fer et à l’azote. Les oxydes de cuivre ont été synthétisés par deux voies différentes afin d’obtenir des films : par voie sol-gel couplée au dip-coating et par électro-dépôt et anodisation du cuivre. La photocorrosion des oxydes de cuivre en milieu aqueux et sous illumination a été mise en évidence. La protection des oxydes de cuivre via une hétérojonction avec un semiconducteur de type n (TiO2 ou BaTiO3) a révélé une meilleure stabilité des électrodes au cours du temps ainsi que des photocourants élevés grâce à une composition et une architecture originales. Les pérovskites ont été synthétisées par voie sol-gel couplée au dip-coating. Ce sont des semiconducteurs de type n ; ainsi l’étude du dopage au fer, pour substituer le titane, et à l’azote, pour substituer l’oxygène, a mis en évidence un changement de nature de type n à type p, ainsi qu’une diminution de la largeur de bande interdite. Les propriétés physico-chimiques de toutes les électrodes synthétisées ont été caractérisées (structure cristalline, morphologie, propriétés optiques et électrochimiques) et discutées en fonction de leur composition et des paramètres de synthèse. Ces travaux ont permis d’élaborer des photocathodes originales, performantes et stables au cours du temps (oxydes de cuivre protégées), et de démontrer l’utilisation de pérovskites dopées pour cette application. / The aim of this work was to study p-type semiconducting materials as photocathodes for protons reduction into H2 for water splitting application. Two types of materials have been studied: copper oxides, Cu2O and CuO, and materials with a perovskite structure (ATiO3, A=Ca, Ba, Sr) doped by iron and nitrogen. Copper oxides have been synthetized by two different ways in order to obtain films: sol-gel process coupled with dip-coating and copper plating and anodization. Copper oxides photocorrosion has been highlighted in aqueous environment and under illumination. Their protection via a heterojunction with an n-type semiconductor (TiO2 and BaTiO3) improved electrodes stability over time and photocurrents, thanks to original composition and architecture. Perovskites have been synthetized by sol-gel process coupled with dip-coating. They are well-known as n-type semiconductors; so the study of doping with iron, to substitute titanium, and with nitrogen, to substitute oxygen, shows a change from n-type to p-type, and a reduction of the band gap. The physical and chemical properties of the synthetized electrodes were characterized (crystal structure, morphology, optical and photoelectrochemical properties) and discussed according to the composition and synthesis parameters. This work enables to obtain original, efficient, and stable over time, photocathodes (protected copper oxides) and to demonstrate the potential use of doped perovskites for this application.

<p>This thesis addresses the electronic structure of vanadium and copper oxides using soft X-ray absorption (SXA) spectroscopy and resonant inelastic X-ray scattering (RIXS) at high brightness synchrotron radiation sources. In RIXS incident photons, tuned to the energy of specific absorption resonances, are inelastically scattered leaving behind a low energy valence excitation in the system studied. Effects of electron localization are reflected by the occurrence of low-energy excitations in form of dd- and charge-transfer excitations that are modelled by cluster calculations. Band-like states are dominating when the intermediate core excited state is delocalized.</p><p>RIXS at V 2p and O 1s resonances has been used to study the electronic structure of the monovalent vanadium oxides VO₂ and V₂O₃, and of the mixed valence compounds, NaV₂O₅ and V₆O₁₃. For NaV₂O₅ and V₆O₁₃ significant contributions from localized low-energy excitations reflect the partly localized character of their valence band electronic structure, whereas VO₂ and V₂O₃ appear mostly as band-like. Effects of carrier doping are addressed for the case of Mo doping into VO₂ and reveal a quasi-rigid band behavior. In the cases of VO₂ and V₆O₁₃ the temperature dependent metal-insulator transition could be monitored by following the spectral evolution of bands originating from V 3d and V 3d - O2p hybridized states. For Na₂V₃O₇ nanotubes it was possible to selectively probe states from the apical and the basal oxygen sites of VO₅ pyramids that constitute these nanotubes. Furthermore, the RIXS technique has been demonstrated to be highly valuable in characterizing the charge transfer processes that accompany lithium insertion into vanadium oxide battery cathodes. Finally, for insulating cuprates RIXS at O 1s, Cu 3p and Cu 3s resonances has been recorded at high-resolution for the detailed investigation of crystal field excitations.</p>

This thesis addresses the electronic structure of vanadium and copper oxides using soft X-ray absorption (SXA) spectroscopy and resonant inelastic X-ray scattering (RIXS) at high brightness synchrotron radiation sources. In RIXS incident photons, tuned to the energy of specific absorption resonances, are inelastically scattered leaving behind a low energy valence excitation in the system studied. Effects of electron localization are reflected by the occurrence of low-energy excitations in form of dd- and charge-transfer excitations that are modelled by cluster calculations. Band-like states are dominating when the intermediate core excited state is delocalized. RIXS at V 2p and O 1s resonances has been used to study the electronic structure of the monovalent vanadium oxides VO2 and V2O3, and of the mixed valence compounds, NaV2O5 and V6O13. For NaV2O5 and V6O13 significant contributions from localized low-energy excitations reflect the partly localized character of their valence band electronic structure, whereas VO2 and V2O3 appear mostly as band-like. Effects of carrier doping are addressed for the case of Mo doping into VO2 and reveal a quasi-rigid band behavior. In the cases of VO2 and V6O13 the temperature dependent metal-insulator transition could be monitored by following the spectral evolution of bands originating from V 3d and V 3d - O2p hybridized states. For Na2V3O7 nanotubes it was possible to selectively probe states from the apical and the basal oxygen sites of VO5 pyramids that constitute these nanotubes. Furthermore, the RIXS technique has been demonstrated to be highly valuable in characterizing the charge transfer processes that accompany lithium insertion into vanadium oxide battery cathodes. Finally, for insulating cuprates RIXS at O 1s, Cu 3p and Cu 3s resonances has been recorded at high-resolution for the detailed investigation of crystal field excitations.

No description available.

Thesis (Ph.D.); Submitted to the Univ. of New Mexico, Albuquerque, NM (US); 20 Dec 2004. / Published through the Information Bridge: DOE Scientific and Technical Information. "LA-14197-T" A. Ayala. 12/20/2004. Report is also available in paper and microfiche from NTIS.

Abstract Recent advances in materials synthesis resulted in a rediscovery of the low cost copper in its one and two-dimensional forms and project newer applications of this metal in fields not considered before. In this thesis, one-dimensional copper, i.e. nanowires are synthesized by a hydrothermal route and explored for their chemical, electrical, catalytic and plasmonic properties with highlighted advantages, benefited from their size and shape compared to thin film and bulk copper. Characterization of copper nanowires and their native oxides were performed using a number of analytical techniques such as X-ray photoelectron and Auger spectroscopy, Raman spectroscopy, X-ray diffraction as well as scanning probe and electron microscopy techniques to elucidate the oxidation mechanism and to assess the feasibility of the oxidized materials for various applications. A few atomic layers of cuprous oxide seem to form on the surface of the nanowires instantly, maybe already during synthesis, which then slowly grows further when exposing the nanowires to ambient air leading to the appearance of cupric oxide as well. Because of the surface oxides, individual nanowires and their bundled networks exhibit semiconducting behavior, which complicates the direct use of such materials for interconnections in electronics. However, even with the presence of native oxides, copper nanowires hold promise in many other applications such as the ones explored here for plasmonics and heterogeneous catalysis. As demonstrated in this work, surface plasmon absorption properties of the nanowires can be exploited for chemical sensing of surface adsorbed molecules (model compound Rhodamine 6G) by efficiently amplifying its Raman spectrum without using any lithographically defined sensor template. Further, it is shown that phenol contamination in water may be efficiently eliminated by converting it to nontoxic polyphenol as well as to CO2 owing to the highly efficient catalytic property of the mixed oxide phases on the surface of the nanowires. The results published in this thesis contribute to the understanding of the chemical and physical behavior of copper nanowires and other low dimensional copper nanostructures that undergo rapid surface oxidation. / Tiivistelmä Jatkuva elektronisten laitteiden ja anturien pienentäminen on hyvin linjassa teknologian kehittymisen kanssa. Pyrkimys monitoimisiin ja tehokkaisiin materiaaleihin on muuttanut tavanomaisten materiaalien käsitystä. Viimeisimmät edistysaskeleet materiaalisynteesissä ovat johtaneet edullisen kuparin uudelleenlöytämiseen sen yksi- ja kaksidimensionaalisissa muodoissa ennustaen metallille uusia sovellutuksia alueilla, joissa sitä ei ole aiemmin hyödynnetty. Tässä väitöstyössä on tutkittu hydrotermisesti syntetisoitujen yksiulotteisten kuparinanojohtimien kemiallisia, sähköisiä, katalyyttisiä ja plasmonisia ominaisuuksia sekä näiden pieneen kokoon ja muotoon perustuvia etuoja ohutkalvo- ja bulkkikupariin verrattuna. Kuparinanojohtimia ja niiden luonnollisia oksideja karakterisoitiin useilla analyysitekniikoilla kuten röntgenelektroni- ja Auger-eletronispektroskopialla, Raman-spektroskopialla, röntgendiffraktiolla sekä pyyhkäisykärki- ja elektronimikroskopialla selvittäen hapettumismekanismeja ja oksidien soveltuvuutta eri käyttötarkoituksiin. Muutaman atomikerroksen paksuinen kupari(I)oksidikerros havaittiin muodostuvan välittömästi, luultavasti jo materiaalisynteesin aikana nanojohtimien pinnalle. Nanojohtimien altistuessa ympäröivälle ilmalle oksidikerros kehittyi hitaasti johtaen kupari(II)oksidin muodostumiseen. Pintaoksidien johdosta yksittäiset nanojohtimet ja niistä yhteenkasautuneet verkostot käyttäytyvät puolijohdemaisesti mikä monimutkaistaa näiden materiaalien käyttöä sellaisenaan elektroniikan johtimissa. Luonnollisista oksideista huolimatta kuparinanojohtimet ovat lupaavia monissa muissa sovelluksissa, kuten tässä työssä tutkituissa plasmonisessa ja heterogeenisessä katalyysissä. Väitöstyössä osoitetaan, että nanojohtimen pintaplasmonisia absorptio-ominaisuuksia voidaan hyödyntää pintaan absorboituneiden molekyylien kemiallisessa havainnoinnissa (mallinnettu yhdiste rodamiini 6G) vahvistamalla Raman–spektriä käyttämättä lainkaan litografiapohjaista anturisapluunaa. Myöskin vesien fenolikontaminaatio voidaan tehokkaasti muuntaa myrkyttömiksi polyfenoleiksi ja hiiidioksidiksi hyödyntämällä nanojohtimien pinnalla olevia oksideja tehokkaana katalyyttinä (jopa parempi kuin kaupallisten kupariin pohjautuvat katalyytit). Tässä väitöstyössä julkaistut tulokset edistävät kuparinanojohtimien sekä muiden pienikokoisten ja nopeasti hapettuvien kuparinanorakenteiden kemiallisen ja fyysisen käytöksen ymmärtämistä. Tieteellisten kehitysaskeleiden lisäksi tämä väitöstyö voi myös toimia lähteenä pienirakenteisten yleisten metallien sovelluksille.