Orientador: Luis Vicente de Andrade Scalvi / Resumo: Este trabalho visa apresentar o desenvolvimento e as principais conclusões referentes à combinação de heterojunções a base de GaAs com óxidos semicondutores, para aplicações em dispositivos optoeletrônicos. O texto foi dividido em duas partes principais, sendo a primeira parte voltada para a síntese e produção do SnO2, relacionada com a formação da heteroestrutura GaAs/SnO2; e a segunda parte é focada na produção dos ressonadores de onda acústicas de bulk,na ordem de GHz, a base de GaAs/ZnO. Na primeira parte, priorizou-se filmes de SnO2, que foram depositados por duas técnicas: sol-gel dip-coating e evaporação resistiva. Os filmes foram depositados sobre substratos de vidro soda-lime, e sobre substratos de GaAs, de quartzo e de a-SiO2. SnO2 foi também depositado sobre filme de GaAs depositado por sputtering. No caso da evaporação resistiva, a rota sol-gel é utilizada também para a preparação do pó que é utilizado como precursor para a evaporação resistiva de filmes de SnO2, combinando essas duas técnicas. Foram investigadas as propriedades ópticas e elétricas de filmes finos de SnO2 dopado com 1% de Er3+ e estruturas hibridas de GaAs/SnO2: Er3+. Entre os principais resultados, verificou-se: 1) espectros de luminescência diferentes do íon Er3+ ao se depositar SnO2 sobre substrato de vidro ou GaAs; 2) Microscopia eletrônica de varredura (MEV) e espectroscopia de energia dispersiva de raios-x (EDX) para filmes de SnO2, depositados por evaporação resistiva, atestaram uma relação... (Resumo completo, clicar acesso eletrônico abaixo) / Abstract: This work aims to present the development and the main conclusions obtained so far regarding the combination of GaAs-based heterojunctions with semiconductor oxides, for applications in optoelectronic devices. The text has been divided in two main parts, where the first one is related to the synthesis and production of SnO2, associated with the formation of the heterostructure GaAs/SnO2; and the second one if focused on the production of bulk acoustic wave resonators, with frequencies in GHz range, based on GaAs/ZnO. In the first part, attention was given to SnO2 films, deposited by two techniques: sol-gel dipcoating and resistive evaporation, on soda-lime glass substrates, and on GaAs, quartz and a-SiO2 substrates. SnO2 was also deposited on GaAs film deposited by sputtering. In the case of resistive evaporation, the sol-gel route is also used to prepare the powder which is used as a precursor for resistive evaporation of SnO2 films, then, by combining these two techniques. Optical and electrical properties of Er3+ -doped SnO2 thin films were investigated as well as the hybrid structure GaAs/SnO2 .Among the main results were: 1) different luminescence spectra of Er3+ ion when depositing SnO2 on glass or GaAs substrate; 2) scanning electron microscopy (SEM) and energy dispersive x-ray spectroscopy (EDX) for SnO2 films deposited by resistive evaporation show a relationship of the thermal annealing temperature with the concentration of Er ions in the surface layers; 3) this con... (Complete abstract click electronic access below) / Doutor

This project is aimed at semiconductive gas sensors based on tin dioxide. In the first part, gas sensors are divided depending on their principal of functionality. Next part is about functionality of tin dioxide gas sensors and the possibilities of active layer modification. Experimental describes gas sensor from its drawing until construction, testing and characterization. Several microelectronic technologies such as thin-film, thick-film, LTCC, spray-coating or wire-bonding were used for constructing the sensor. In the last part properties of gas sensors and differences in functionality between modified and unmodified gas sensor are summarized.

The ability to sense gas such as methane can provide an early warning system to protect human lives. High demand for the ability to sense the world around us has provided an extensive area of research for sensor technology. In particular, current sensor technology, specifically for methane, has provided sensors that require a heated environment to function. The majority of current methane sensors function at temperatures between 150°C and 450°C [1-3]. This thesis will explore an approach to produce a room temperature methane sensor.

Les électrocatalyseurs conventionnels utilisés dans les piles à combustibles à membrane échangeuse de protons (PEMFC) sont composés de nanoparticules de platine supportées sur des noirs de carbone de forte surface spécifique. A la cathode de la PEMFC, siège de la réaction de réduction de l’oxygène (ORR), le potentiel électrochimique peut atteindre des valeurs élevées - notamment lors de phases arrêt-démarrage - engendrant des dégradations irréversibles du support carboné. Une solution « matériaux » consiste à remplacer ce dernier par des supports à base d’oxydes métalliques. Ceux-ci doivent être résistants à la corrosion électrochimique, conducteurs électroniques et posséder une structure poreuse et nano-architecturée (permettant le transport des réactifs et produits et une distribution homogène de l’ionomère et des nanoparticules de platine). Dans ce travail, nous avons donc élaboré et caractérisé des électrocatalyseurs à base de nanoparticules de platine (Pt) déposées sur du dioxyde d’étain (SnO₂) et de titane (TiO₂) texturés (morphologies aérogel, nanofibres ou « loosetubes ») et conducteurs électroniques (dopés au niobium Nb ou à l’antimoine Sb). Le support permettant d’atteindre les meilleures propriétés électrocatalytiques est un aérogel de SnO₂ dopé à l’antimoine, noté ATO. En particulier, l’électrocatalyseur Pt/ATO présente une activité spécifique vis-à-vis de l’ORR supérieure à celle d’un électrocatalyseur Pt/carbone Vulcan® synthétisé dans les mêmes conditions, suggérant des interactions bénéfiques entre les nanoparticules de Pt et le support oxyde métallique (Strong Metal Support Interactions, SMSI).Des tests de durabilité simulant le fonctionnement d’une PEMFC en conditions automobile ont été effectués en électrolyte liquide à 80 °C sur ces deux électrocatalyseurs : cyclage entre 0,60 et 1,00 V vs l’électrode réversible à hydrogène (RHE) ou entre 1,00 et 1,50 V vs RHE. Le catalyseur Pt/ATO présente une durabilité accrue par rapport au catalyseur Pt/carbone Vulcan® de référence. Cependant, de nouveaux mécanismes de dégradation ont été mis en évidence dans cette étude : tout d’abord, l’élément dopant Sb est progressivement dissout au cours du vieillissement électrochimique, ce qui implique une perte de conductivité électronique. Cette perte est en partie liée à des incursions à bas potentiel, notamment durant les caractérisations électrochimiques. De plus, entre 5 000 et 10 000 cycles de vieillissement électrochimique (entre 0,60 et 1,00 V vs RHE ou entre 1,00 et 1,50 V vs RHE à 57 °C), le matériau support perd sa structure poreuse et forme un film amorphe peu conducteur. / Conventional electrocatalysts used in proton exchange membrane fuel cells (PEMFC) are composed of platinum nanoparticles supported on high specific surface area carbon blacks. At the cathode side of the PEMFC, where the oxygen reduction reaction (ORR) occurs, the electrochemical potential can reach high values - especially during startup-shutdown operating conditions - resulting in irreversible degradation of the carbon support. A “material” solution consists of replacing the carbon with supports based on metal oxides. The latter have to be resistant to electrochemical corrosion, be electronic conductor and have a porous and nano-architectural structure (for the transport of reagents and products and the homogeneous distribution of the ionomer and platinum nanoparticles).In this work, we have developed and characterized electrocatalysts composed of platinum (Pt) nanoparticles based on tin dioxide (SnO2) and titanium dioxide (TiO2) with optimized textural (aerogel, nanofibres or loosetubes morphologies) and electron-conduction properties (doped with niobium Nb or antimony Sb). The best electrocatalytic properties are reached for an antimony-doped SnO2 aerogel support, denoted ATO. The Pt/ATO electrocatalyst has especially a higher specific activity for the ORR than a Pt/carbon Vulcan® electrocatalyst, synthesized in the same conditions, suggesting beneficial interactions between the Pt nanoparticles and the metal oxide support (Strong Metal Support Interactions SMSI).Durability tests simulating automotive operating conditions of a PEMFC were carried out in liquid electrolyte at 57 °C on these two electrocatalysts by cycling between 0.60 and 1.00 V vs the reversible hydrogen electrode (RHE) or between 1.00 and 1.50 V vs RHE. The Pt/ATO electrocatalyst has an increased stability compared to the reference Pt/carbon Vulcan® electrocatalyst. However, new degradation mechanisms were highlighted in this study: first, the doping element (Sb) is progressively dissolved during electrochemical ageing, which implies a loss of electronic conductivity. This loss is partly due to incursions at low potential, including during electrochemical characterizations. Moreover, between 5,000 and 10,000 cycles of the accelerated stress tests (between 0.60 and 1.00 V vs RHE or between 1.00 and 1.50 V vs RHE at 57 °C), the support loses its porous structure and forms a poorly conductive amorphous film.

Le but de ce travail a été de développer des cellules à base d’oxyde nanoporeux photosensibilisé sur support plastique. Dans ce contexte, une nouvelle voie d’élaboration de couches de nanoparticules d’oxyde à basse température a été développée par irradiation ultraviolet sous air. Tout d’abord, des couches de dioxyde de titane ont été préparées par cette méthode, les films obtenus étant constitués d’un réseau mésoporeux de nanoparticules interconnectées de dioxyde de titane de structure anatase d’après les caractérisations effectuées par microscopie électronique à balayage, diffraction des rayons X, porosimétrie d’adsorption d’azote et d’intrusion de mercure et analyse thermogravimétrique. Après sensibilisation des couches par un complexe polypyridyle de ruthénium, les cellules photovoltaïques élaborées avec des films en contact avec un électrolyte liquide présentent des rendements de conversion énergétique compris entre 1,6 et 2,5 % suivant la nature des particules utilisées. Cette voie a ensuite été élargie à d’autres oxydes tels que l’oxyde de zinc et le dioxyde d’étain ainsi qu’à des oxydes en configuration « cœur-écorce ». Les rendements obtenus avec le dioxyde d’étain, 1,5 à 1,8%, sont tout à fait remarquables par rapport à ceux décrits dans la bibliographie pour des couches traitées à haute température. Les rendements plus élevés avec les couches traitées aux UV étant liés à une amélioration des tensions de circuit ouvert et des facteurs de forme, les phénomènes physiques régissant les performances de ces dispositifs ont été étudiés par différentes techniques, notamment la spectroscopie d’impédance électrochimique et le déclin de tension de circuit ouvert. Enfin, les performances électrochromes des films de dioxyde de titane traités aux UV ont été caractérisées sur support verre et plastique en présence d’un électrolyte liquide ionique, les efficacités de coloration étant comparables aux systèmes élaborés à haute température. / This work aimed to develop dye-sensitized solar cells on plastic substrates. In this context, a new low-temperature method to make nanoporous oxide layers based on ultraviolet irradiation under air was studied. First of all, titanium dioxide layers were prepared with this method; the films obtained were composed of a mesoporous network of interconnected anatase titanium dioxide nanoparticles as evidenced by scanning electron microscopy, X-ray diffraction, nitrogen sorption and mercury porosimetries, and thermogravimetric analysis. After sensitizing the films with a ruthenium polypyridyl complex, the photovoltaic cells based on the films in contact with a liquid electrolyte gave conversion efficiencies between 1.6 and 2.5% depending on the nature of the particles used. This low-temperature method based on ultraviolet irradiation was then expanded to other oxides, such as zinc oxide and tin dioxide, as well as to core-shell structures. The conversion efficiencies obtained with tin dioxide were very high, i.e. 1.5 to 1.8%, compared to those usually reported in the literature for films sintered at high temperatures. The higher efficiencies obtained for the UV-treated films were related to higher open circuit potentials and higher fill factors. Therefore, the physical phenomena involved were investigated with various techniques; in particular, electrochemical impedance spectroscopy and open circuit voltage decay. Finally, the electrochromic performances of the low-temperature UV-processed nanoparticulate titanium dioxide films were studied on glass and plastic substrates with an ionic liquid. The coloration efficiencies were found to be comparable to those of high-temperature processed layers.

[pt] Nanopartículas de dióxido de estanho foram obtidas a partir da reação de soluções aquosas de sais de estanho em sistemas de microemulsão e comparadas, em relação às suas propriedades texturais, estruturais e de condutividade elétrica, com nanopartículas do mesmo óxido obtidas pelo método convencional de precipitação. Foram realizadas alterações nos métodos utilizados a fim de se verificar a influência dos parâmetros de síntese, tais como: natureza da fase orgânica e do surfactante, tempo e temperatura de reação. Paralelamente, óxidos comerciais de titânio, alumínio e silício foram submetidos aos métodos de impregnação seca e de microemulsão. A influência desses métodos nas propriedades texturais, estruturais e no comportamento térmico desses óxidos foi estudada. As amostras obtidas foram caracterizadas por adsorção de N2, difração de raios X, espectroscopia de reflectância difusa na região do UV-visível e microscopia eletrônica de transmissão. Também foram realizadas análises de condutividade elétrica e de densidade aparente. As análises mostraram, no caso do óxido de estanho, que o método envolvendo microemulsões, apesar da grande dificuldade experimental, apresentou resultados semelhantes ao método convencional em relação ao tamanho de cristalito, à condutividade elétrica e à diferença de energia entre os orbitais HOMO-LUMO. Entretanto, o método de precipitação gerou o óxido de estanho com maior área superficial, 245 m(2)g(-1). Para os óxidos comerciais, diferenças significativas na textura das amostras foram encontradas. No caso do óxido de titânio, os resultados indicam que o método de impregnação afeta a estrutura desse óxido de forma a permitir a transformação da fase (anatásio/rutilo) em temperaturas mais baixas. / [en] Tin oxide nanoparticles have been obtained from the reaction of aqueous solutions of tin salts in microemulsion systems and compared with the same oxide nanoparticles obtained by the precipitation conventional method. Changes have been made in the methods used in order to verify the influence of synthesis parameters, such as: organic phase and surfactant nature, reaction time and temperature. In addition, titanium, aluminum and silicon commercial oxides have been subjected to dry impregnation and microemulsion methods. The influence of the method used in the textural and structural properties, and thermal behavior of these oxides have been studied. The samples have been characterized by adsorption of N2, X-ray diffraction, diffuse reflectance spectroscopy in the UVvisible range, and transmission electron microscopy. Furthermore, electrical conductivity and apparent density analysis have been performed. Despite difficult encountered in microemulsion systems experiments, tin oxide thus obtained is similar to the conventional method on crystallites size, electrical conductivity and the difference of energy between HOMO-LUMO orbitals results. However, precipitation method generated tin oxide with larger surface area, approximately 245 m(2)g(-1). For commercial oxides, significant differences on texture of the samples have been found. In the case of TiO2, the results also indicate that the dry impregnation method affects the structure so that phase transformation (anatase to rutile) occurs at lower temperatures.

Des matériaux hybrides de classe II ont été préparés à partir de précurseurs bis(tripropynylstannylés). Deux familles de précurseurs sol-gel incluant des espaceurs hydrocarbonés et thiophénique ont été obtenues et conduisent à des matériaux hybrides auto-organisés où les plans d’oxyde sont séparés par les espaceurs organiques. Ainsi l’espaceur rigide a donné lieu à une structure pseudo-lamellaire montrant une bande d’émission monomère avec un assez faible décalage vers le rouge par rapport à l'émission des précurseurs en solution. En revanche, alors que les xérogels thiényle plus désordonnés conduisent à une large émission caractéristique de la formation d’excimères ou de dimères. Par ailleurs, des films minces contenant les espaceurs alkylène et arylalkylène ont été préparés et ont montré une morphologie "pseudoparticulaire" poreuse et un ordre à courte distance contenant des réseaux SnOx. De façon inattendue, ces films minces hybrides détectent le dihydrogène dès une température de 50 °C dans la gamme 200-10000 ppm. A partir de ces films hybrides minces, le dioxyde d'étain cristallin (SnO2) a été préparé par un post-traitement thermique. Comme prévu, ces films SnO2 cassitérite détectent le dihydrogène et, dans une moindre mesure le monoxyde de carbone avec une température optimale de fonctionnement comprise entre 300 et 350 °C. / Class II hybrid materials were prepared from ditin hexaalkynides. Two families of precursors, including either hydrocarbon or oligothiophene-based spacers, were obtained and led by the sol-gel process to self-assembled organotin-based hybrid materials made of planes of oxide separated by organic bridges. Thus, the rigid thienyl spacer gave rise to a “pseudo-lamellar” structure that showed a monomer emission band with a rather small red-shift compared with to the emission of the precursor in solution. However more disordered thienyl xerogels led to broad emission features assigned to excimer or dimer formation. Moreover, thin films containing alkylene- and arylalkylene bridged have been prepared and showed a “pseudoparticulate” porous morphology and a short-range hierarchical order in the organic-inorganic SnOx pseudoparticles. Unexpectedly these hybrid thin films detect hydrogen gas at a temperature as low as 50 °C at the 200-10000 ppm level. From these hybrid thin films, crystalline tin dioxide (SnO2) were prepared by a thermal post-treatment. As expected, cassiterite SnO2 films detected H2 and to a less extent CO with a best operating temperature comprised between 300 and 350 °C.

[ES] La presente Tesis Doctoral se centra en el estudio del comportamiento electroquímico de nuevos electrodos cerámicos basados en SnO2 dopado con Sb2O3. El estudio está orientado a la posterior aplicación de estos electrodos en procesos electroquímicos de oxidación avanzada. Inicialmente, se consideraron diferentes temperaturas de sinterización (entre 1050°C y 1250°C) y se observó que un aumento en la temperatura de sinterización favorecía la deposición del Sb2O3 en la superficie anódica, disminuyendo así la resistividad del electrodo. Posteriormente, se estudió el comportamiento electroquímico de estos nuevos electrodos obteniendo el potencial de descarga del oxígeno mediante la técnica de voltametría de barrido lineal. Los resultados obtenidos revelaron que el valor del potencial de descarga del oxígeno disminuye con el aumento de la temperatura de sinterización. Además, estos electrodos presentaron un comportamiento intermedio entre el ánodo de Pt y el ánodo de BDD. Este último electrodo se emplea habitualmente en los procesos electroquímicos de oxidación avanzada debido a su elevada capacidad para generar radicales hidroxilo, los cuales son capaces de oxidar los compuestos orgánicos a dióxido de carbono y agua. A continuación, se llevaron a cabo ensayos de oxidación electroquímica con estos electrodos cerámicos para seleccionar la temperatura de sinterización. Los resultados mostraron que con el electrodo cerámico sinterizado a 1250°C se alcanza una menor degradación del compuesto orgánico, mientras que los electrodos sinterizados a menores temperaturas presentan un comportamiento similar entre ellos. Por tanto, se seleccionó el electrodo cerámico sinterizado a 1200°C ya que presenta una baja resistividad y un buen comportamiento electroquímico para ser utilizado como ánodo en los procesos electroquímicos de oxidación avanzada. Seguidamente se analizó la vida útil de estos electrodos cerámicos, y se comprobó que la matriz cerámica incrementa en gran medida la estabilidad a la polarización anódica de los electrodos basados en SnO2. Posteriormente, se llevaron a cabo procesos de electro-oxidación en modo galvanostático de distintos contaminantes difíciles de eliminar por las técnicas convencionales utilizando como ánodo los nuevos electrodos cerámicos y el electrodo de BDD, para así poder comparar los resultados obtenidos. Se observó que, aunque con los electrodos cerámicos se consigue degradar el 100% de los distintos contaminantes, el electrodo de BDD es el más eficiente ya que genera mayor cantidad de especies oxidantes activas en su superficie. Para un mismo electrodo, se observó que un aumento en la densidad de corriente aplicada mejora la velocidad de degradación y mineralización de cada contaminante, mientras que la eficiencia eléctrica disminuye. Por otro lado, se estudió el efecto de la presencia de una membrana de intercambio catiónico entre ánodo y cátodo. Los resultados mostraron que el uso de la membrana beneficia la degradación y la mineralización del contaminante, ya que mejora la cinética de reacción anódica y evita la reducción de las especies oxidantes electrogeneradas. A continuación, se analizó la influencia de la concentración del Na2SO4 como electrolito soporte. En este estudio se observó que un aumento en la concentración del Na2SO4 mejora la mineralización del contaminante para el electrodo de BDD y, por el contrario, empeora para el electrodo cerámico. Esto se debe a la capacidad de cada electrodo para oxidar los sulfatos a persulfatos. Por último, se llevaron a cabo ensayos de ecotoxicidad de las muestras tratadas, y se demostró que, en presencia de membrana, a mayor densidad de corriente y con el electrodo de BDD la toxicidad de la muestra es mayor debido a la mayor presencia en disolución de iones persulfato. Por tanto, con el electrodo cerámico, además de alcanzar un elevado grado de degradación del contamina / [CAT] La present Tesi Doctoral es centra en l'estudi del comportament electroquímic de nous elèctrodes ceràmics basats en SnO2 dopat amb Sb2O3. L'estudi està orientat a la posterior aplicació d'aquests elèctrodes en processos electroquímics d'oxidació avançada. Inicialment, es van considerar diferents temperatures de sinterització (entre 1050°C i 1250°C) i es va observar que un augment en la temperatura de sinterització afavoria la deposició de Sb2O3 en la superfície anòdica, disminuint així la resistivitat de l'elèctrode. Posteriorment, es va estudiar el comportament electroquímic d'aquests nous elèctrodes obtenint el potencial de descàrrega de l'oxigen mitjançant la tècnica de voltametria de passada lineal. Els resultats obtinguts van revelar que el valor del potencial de descàrrega de l'oxigen disminueix amb l'augment de la temperatura de sinterització. A més, aquests elèctrodes ceràmics van presentar un comportament intermedi entre l'ànode de Pt y l'ànode de BDD. Aquest últim elèctrode s'utilitza habitualment en els processos electroquímics d'oxidació avançada a causa de la seua elevada capacitat per a generar radicals hidroxil, els quals són capaços d'oxidar els compostos orgànics a diòxid de carboni i aigua. A continuació, es van dur a terme assajos d'oxidació electroquímica amb aquests elèctrodes ceràmics per a seleccionar la temperatura de sinterització. Els resultats van mostrar que amb l'elèctrode ceràmic sinteritzat a 1250°C s'aconsegueix una menor degradació del compost orgànic, mentre que els elèctrodes sinteritzats a menors temperatures presenten un comportament similar entre ells. Per tant, es va seleccionar l'elèctrode ceràmic sinteritzat a 1200 °C ja que presenta una baixa resistivitat i un bon comportament electroquímic per a ser utilitzat com a ànode en els processos electroquímics d'oxidació avançada. Seguidament, es va analitzar la vida útil d'aquests elèctrodes ceràmics, i es va comprovar que la matriu ceràmica incrementa en gran mesura l'estabilitat a la polarització anòdica dels elèctrodes basats en SnO2. Posteriorment es van dur a terme processos d'electro-oxidació en mode galvanostàtic de diferents contaminants difícils d'eliminar per les tècniques convencionals utilitzant com a ànode els nous elèctrodes ceràmics i l'elèctrode de BDD, per a així poder comparar els resultats obtinguts. Es va observar que, encara que amb els elèctrodes ceràmics s'aconsegueix degradar el 100% dels diferents contaminants, l'elèctrode de BDD és el més eficient ja que genera una major quantitat d'espècies oxidants actives en la seua superfície. Per a un mateix elèctrode, es va observar que un augment en la densitat de corrent aplicada millora la velocitat de degradació i mineralització de cada contaminant, mentre que l'eficiència elèctrica disminueix. D'altra banda, es va estudiar l'efecte de la presència d'una membrana d'intercanvi catiònic entre ànode i càtode. Els resultats van mostrar que l'ús de la membrana beneficia la degradació i la mineralització del contaminant, ja que millora la cinètica de reacció anòdica i evita la reducció de les espècies oxidants electrogenerades. A continuació, es va analitzar la influència de la concentració del Na2SO4 com a electròlit suport. En aquest estudi es va observar que un augment en la concentració del Na2SO4 millora la mineralització del contaminant per a l'elèctrode de BDD i, per contra, empitjora per a l'elèctrode ceràmic. Això es deu a la capacitat de cada elèctrode per a oxidar els sulfats a persulfats. Finalment, es van dur a terme assajos d'ecotoxicitat de les mostres tractades, i es va demostrar que, en presència de membrana, a major densitat de corrent i amb l'elèctrode de BDD la toxicitat de la mostra és major a causa de la major presència en dissolució d'ions persulfat. Per tant, amb l'elèctrode ceràmic, a més d'aconseguir un elevat grau de degradació del con / [EN] The present Doctoral Thesis focuses on the study of the electrochemical behaviour of new ceramic electrodes based on SnO2 doped with Sb2O3. The study is oriented at the subsequent application of these electrodes in electrochemical advanced oxidation processes. Initially, different sintering temperatures were considered (between 1050°C and 1250°C) and it was observed that an increase in the sintering temperature favoured the deposition of Sb2O3 on the anodic surface, thus decreasing the resistivity of the electrode. Later, the electrochemical behaviour of these new electrodes was studied by means of the oxygen discharge potential using the linear sweep voltammetry technique. The results obtained revealed that the value of the oxygen discharge potential decreases with increasing the sintering temperature. Furthermore, these electrodes showed an intermediate behaviour between the Pt anode and the BDD one. This last electrode is commonly used in advanced electrochemical oxidation processes due to its high capacity to generate hydroxyl radicals, which are capable of oxidizing the organic compounds to carbon dioxide and water. Then, electrochemical oxidation tests were carried out with these ceramic electrodes to select the sintering temperature. The results showed that with the ceramic electrode sintered at 1250°C, a lower degradation and mineralization of the organic compound is achieved, while the electrodes sintered at lower temperatures showed a similar behaviour. Therefore, the ceramic electrode sintered at 1200°C was selected as it presents low resistivity good electrochemical behaviour to be used as anode in electrochemical advanced oxidation processes. Next, the service life of these ceramic electrodes was analyzed, and it was found that the ceramic matrix greatly increases the anodic polarization stability of the electrodes based on SnO2. Subsequently, electro-oxidation processes were carried out in galvanostatic mode for different contaminants that are difficult to remove by conventional techniques, using the new ceramic electrodes and the BDD electrode as anodes, in order to compare the results obtained. It was observed that ceramic electrodes can be used as anodes for electrochemical oxidation, since for high current densities it is possible to degrade a 100% of the different contaminants. However, the BDD electrode is the most efficient one since it generates more active oxidant species on its surface. For a given electrode, an increase in the current density improves the degradation and mineralization of each contaminant, while the electrical efficiency decreases. On the other hand, the effect of the presence of a cation-exchange membrane between anode and cathode was studied. The results showed that its use benefits the degradation and mineralization of the contaminants, since it improves the kinetics of the anodic reaction and avoids the reduction of the electrogenated oxidant species. Next, the influence of the concentration of Na2SO4 as supporting electrolyte was analyzed. In this study it was observed that an increase in the Na2SO4 concentration improves the mineralization of the contaminant for the BDD electrode and, on the contrary, worsens for the ceramic electrode. This is due to the ability of each electrode to oxidize sulfates to persulfates. Finally, ecotoxicity tests on the treated samples were carried out, and it was shown that, in the presence of the membrane, at higher current density and with the BDD electrode, the toxicity of the sample is greater due to the higher presence of persulfate ions. Therefore, with the ceramic electrode, in addition to achieving a high degree of contaminant degradation, the treated samples are less toxic than the samples treated with the BDD electrode. / Agradezco al Ministerio de Economía y Competitividad por la financiación recibida mediante los proyectos CTQ2015-65202-C2-1-R y RTI2018-101341-B-C21, y a la cofinanciación con los fondos FEDER, que han permitido llevar a cabo la investigación en la Universitat Politècnica de València para la realización de la presente Tesis. También agradecer a Sergio Mestre Beltrán por proporcionarnos los nuevos electrodos cerámicos en los que se ha basado este trabajo. / Mora Gómez, J. (2020). Eliminación de contaminantes orgánicos emergentes mediante procesos electroquímicos de oxidación avanzada [Tesis doctoral]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/158751 / TESIS

In this work the influence of solid electrolyte additives on the gas sensing properties of tin oxide layers was investigated systematically for the first time. NASICON (NAtrium, Super Ionic CONductor, Na(1+x)Zr2SixP(3-x)O12; 0 <= x <= 3) was used as a model for solid electrolyte additives. The structure of that material is ideally suitable for studies of the correlation between material parameters and the gas sensitivity of the layers. In the NASICON structure the content of mobile Na+-ions can be varied by a factor of four resulting in a simultaneous change of the ionic conductivity sigma(Na+) by approximately three orders of magnitude without considerable structural alterations. Powders of SnO2 and NASICON (x = 0; 2.2; 3) were prepared separately by means of sol-gel routes and mixed in a volume ratio of 80/20. Pastes were prepared from these powders with different compositions and screen printed on alumina substrates with a fourfold structure of thin film gold electrode combs. Four different compositions were characterised simultaneously at elevated temperatures in various gas atmospheres. The conductivity of the layers, when measured in air, decreases considerably with increasing Na+-content in the NASICON additive. This is correlated with enhanced activation energy of the electronic conductivity. The sensitivity of the layers to polar organic molecules like R-OH (alcohols), R-HO (aldehydes) and ROOH (carboxylic acids) is highly enhanced by the NASICON additive. This is observed especially on the admixtures with NASICON of high Na+-content (x = 2.2 and x = 3). On the other hand, the sensitivity to substances with mid-standing functional groups like 2-propanol or propanone can not be enhanced by NASICON additives. Furthermore the sensitivity of these composite layers to CO, H2, NH3, methane, propane, propene and toluene (all exposed as admixtures with air) is lower than the sensitivity of pure SnO2-layers. These observations are well correlated with the results of gas consumption measurements on SnO2/NASICON powders by means of FTIR spectroscopy. In spite of the lack of surface analytical data, a model of surface chemical gas reactions based on a triple phase boundary (SnO2/NASICON/gas atmosphere) was developed, which explains the experimental observations qualitatively. It is assumed that the decrease of the electronic conductivity as observed in the presence of NASICON additives with increasing Na+-content is due to an enhanced electron depletion layer. This is formed in the SnO2 grains by Na+/e- interactions across the SnO2/NASICON-interface. The enormous enhancement of the sensitivity to polar organic molecules may be due to specific nucleophilic interactions with the Na+-ions and coupled Na+/e--interactions at the triple phase reaction sites.

In this work the influence of solid electrolyte additives on the gas sensing properties of tin oxide layers was investigated systematically for the first time. NASICON (NAtrium, Super Ionic CONductor, Na(1+x)Zr2SixP(3-x)O12; 0 <= x <= 3) was used as a model for solid electrolyte additives. The structure of that material is ideally suitable for studies of the correlation between material parameters and the gas sensitivity of the layers. In the NASICON structure the content of mobile Na+-ions can be varied by a factor of four resulting in a simultaneous change of the ionic conductivity sigma(Na+) by approximately three orders of magnitude without considerable structural alterations. Powders of SnO2 and NASICON (x = 0; 2.2; 3) were prepared separately by means of sol-gel routes and mixed in a volume ratio of 80/20. Pastes were prepared from these powders with different compositions and screen printed on alumina substrates with a fourfold structure of thin film gold electrode combs. Four different compositions were characterised simultaneously at elevated temperatures in various gas atmospheres. The conductivity of the layers, when measured in air, decreases considerably with increasing Na+-content in the NASICON additive. This is correlated with enhanced activation energy of the electronic conductivity. The sensitivity of the layers to polar organic molecules like R-OH (alcohols), R-HO (aldehydes) and ROOH (carboxylic acids) is highly enhanced by the NASICON additive. This is observed especially on the admixtures with NASICON of high Na+-content (x = 2.2 and x = 3). On the other hand, the sensitivity to substances with mid-standing functional groups like 2-propanol or propanone can not be enhanced by NASICON additives. Furthermore the sensitivity of these composite layers to CO, H2, NH3, methane, propane, propene and toluene (all exposed as admixtures with air) is lower than the sensitivity of pure SnO2-layers. These observations are well correlated with the results of gas consumption measurements on SnO2/NASICON powders by means of FTIR spectroscopy. In spite of the lack of surface analytical data, a model of surface chemical gas reactions based on a triple phase boundary (SnO2/NASICON/gas atmosphere) was developed, which explains the experimental observations qualitatively. It is assumed that the decrease of the electronic conductivity as observed in the presence of NASICON additives with increasing Na+-content is due to an enhanced electron depletion layer. This is formed in the SnO2 grains by Na+/e- interactions across the SnO2/NASICON-interface. The enormous enhancement of the sensitivity to polar organic molecules may be due to specific nucleophilic interactions with the Na+-ions and coupled Na+/e--interactions at the triple phase reaction sites.