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A reduction in the operation temperature of zirconia ceramic gas sensors is highly desirable for a number of practical reasons. This work seeks to investigate the factors that prevent a reduction in operation temperature and propose methods by which these may be resolved. A novel approach to sensor fabrication has been developed and employed with the advantage of reduced device complexity that should lead to subsequent cost and reliability benefits. Leakage rates in these devices have been shown to be small and electrochemical in origin. Leakage was greater than reported for gold seal devices, partly due to increased electrode activity. The flexibility of device configuration allows a variety of sensor geometries and functions to be realised. This flexibility led to the characterisation of sensors at the upper and lower ends of measurement range and the identification of deviations from theoretical performance. These deviations have been reconciled with theory extended to cover these limits. Such sensors are known to be sensitive to reducible gas species such as CO2 and H2O with a second limiting plateau allowing quantification of these gases. Such analysis capabilities have been found to be extended by incorporating a second pair of electrodes. These effects have not previously been reported. Sensors have been shown to be more sensitive to H2O than to CO2. To investigate the low temperature response of sensors, a variety of techniques and analyses have been developed and are employed with varying success. Impedance spectroscopy was by far the most useful and revealing tool but this is a function of the highly developed hardware and sophisticated control and analysis software bought as a complete system. Gas step changes and current / voltage sweeps were also useful as comparative techniques but could not separate out component mechanisms. Scanning electron microscopy proved to be a vital tool as it allowed vital information to be obtained concerning electrode and electrolyte microstructure. Again this is a function of a highly developed and sophisticated instrument. The techniques of pressure and concentration modulation were limited in terms of ease of use, measurement range and results interpretation. The main drawbacks were limited frequency ranges and laborious data collection and analysis. They do both however show large potential for improvement. Both amperometric and potentiometric sensors response rates were analysed with a variety of noble metal electrodes using each technique. Electrode material proved to have a marked effect on sensor performance with the best results obtained with silver and electro-deposited platinum. Scanning electron microscopy of silver showed that a finely divided and openly porous electrode was not required for high performance contrary to expectations. This is thought to be due to the solubility of oxygen in this metal. With platinum however, the improved microstructure is thought to be a signifîcant factor in electro-deposited and cermet electrode performance. Response rates in amperometric sensors did not show any significant temperature dependence although a restriction in measurement range was observed. Response rates were suspected to be mainly influenced by sensor geometry whilst measurement range was a function of sensor geometry, electrolyte conductivity and electrode activity. Improved electrolytes will provide improvements and may come in the form of attention to the YSZ system or by employing an alternative ion conductor such as ceria. Close attention to sensor dimensions provides possibility for enhancements. In amperometric devices for instance a long, thin diffusion barrier is required leading to a small internal cavity with a large electrode surface area and a thin electrolyte membrane.

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The use of InGaAsP semiconductor lasers as radiation sources in gas and liquid detection is described in this thesis. Single mode operation and tunability were studied in several schemes including diode lasers with a short external cavity (SXC), diode lasers with multiple short external cavities (MSXC), and a grating external cavity (GEC) diode laser. Comparisons of SXC, MSXC and GEC lasers are given in terms of tunability, side mode suppression ratio (SMSR), stability, and ease of construction and operation. In highly sensitive gas detection, the harmonic content of residual amplitude modulation (RAM) for current modulation of the diode lasers was studied based on the concept that the light intensity rather than the electric field is directly modulated by the injection current. Formulae for RAM and the absorption signals are given for injection current modulation spectroscopy with diode lasers. Water vapour was detected by using InGaAsP SXC and DFB diode lasers, and an electronic subtracter was employed to reduce the detection noise. A sensitivity of ~ 1.6 x IO-6 in units of equivalent absorbance in an equivalent noise bandwidth of 1.25Hz was obtained. In liquid detection, InGaAsP laser diodes with multiple short external cavities (MSCX’s) were developed to provide a wide spectral coverage, up to 72nm spectral coverage was achieved. Liquid detection by MSXC diode lasers was studied in conjunction with multivariate calibration methods, i.e., principal component regression (PCR) and partial least squares (PLS). A sensitivity of 0.1% H2O in D2O was achieved and the limiting noise source was assessed. Three component mixtures of H2O, acetone and methanol were studied in terms of regression factors and outlier detection in the PCR and the PLS algorithms. To achieve even broader tunability by means of external cavity, work on making broad gain peak InGaAsP/InP lasers was initiated. / Thesis / Doctor of Philosophy (PhD)

Hoy en día, la necesidad de monitorizar y controlar el medio ambiente es a cada vezmás importante debido al creciente nivel de gases tóxicos que provienen de la expansiónde las actividades industriales, amenazando así el medio ambiente y la salud humana. Eldesarrollo de la nano-tecnología ha permitido fabricar sensores de gases portables,altamente sensibles, selectivos, de bajo coste y de bajo consumo de potencia.Los nanotubos de carbono (NTC) están ganando un interés a cada vez más considerablepor parte de la comunidad científica debido a su geometría y morfología únicas y susexcelentes propiedades electrónicas, mecánicas, térmicas i ópticas. Esto hace de ellosunos candidatos prometedores para un amplio rango de aplicaciones como por ejemplonuevos sensores de gases con propiedades mejoradas. En este contexto, mediante lapresente tesis, se ha realizado un profundo estudio para explorar las propiedades dediferentes sensores basados en nano-materiales híbridos constituidos por nanotubos decarbono junto a otros materiales con el fin de detectar gases tóxicos de manera eficiente.El trabajo realizado consistió en el diseño, la fabricación, la caracterización, y laoptimización de nanosensores híbridos.Esta tesis fue financiada en el marco del proyecto Europeo "Nano2hybrids", cuyoobjetivo era de diseñar la interfaz de las nano-partículas del metal con los nanotubos decarbono a través del control de los defectos estructurales y químicos producidos por ladescarga de un plasma de radiofrecuencia y aplicarlo a la detección de gases. Elbenceno fue elegido como gas principal, debido a sus graves efectos tóxicos a niveles depocas ppb y también debido a la no existencia en el mercado de un detector de bajocoste para benceno. De hecho, no hay en el estado de la técnica, un sensor de gas quepuede detectar de forma selectiva este gas a nivel operativo de ppb y trabajando atemperatura ambiente. Así, el reto de esta tesis era obtener un sensor altamente sensible,selectivo y estable, portátil y de bajo coste para la detección de benceno.En este sentido, se estudiaron exhaustivamente diferentes materiales basados ennanotubos de carbono funcionalizados, decorados con nanopartículas de metal o biendecorados o mezclados con óxidos metálicos, en términos de su adecuación para ladetección de gases (por ejemplo, sus sensibilidad, selectividad, estabilidad, y elmecanismo de detección, etc.). En particular se estudió la detección de diferentes gasescomo (benceno (C6H6 ), monóxido de carbono (CO), dióxido de nitrógeno (NO2), eletileno (C2H4), el sulfuro de hidrógeno (H2S), amoníaco (NH3) y agua (H2O)). Nuestrastareas consistieron en investigar experimentalmente y teóricamente el efecto de lascondiciones de preparación de los materiales (p.e. el tratamiento con plasma, lanaturaleza del precursor y tamaño de las nanoparticulas de metales), fabricación delsensor (p.e., técnica de deposición, el efecto del tipo de metal del los electrodos delsensor), y de las condiciones de caracterización del sensor (p.e., temperatura deoperación, flujo de gas,) sobre las propiedades sensoras de los mismos. Todo ello hapermitido adquirir conocimientos, explicar los mecanismos de funcionamiento en elsensado de gases de los diferentes materiales investigados y con ello desarrollar unsensor de gases adecuado para la detección de benceno.Hemos encontrado que los materiales híbridos que consisten en nanotubos tratados conplasma de oxígeno y decorados con diferentes nanopartículas de metal, muestran unamayor capacitad de detección a temperatura ambiente respecto a los nanotubos decarbono en bruto o los funcionalizados sólo con plasma. Las propiedades interfacialesde los materiales híbridos resultantes pueden ser adaptadas, lo que ofrece una enormeflexibilidad para el ajuste de sus propiedades sensoras. Cuando se combinaron en unamatriz de micro-sensores que opera a temperatura ambiente, nanotubos decorados condiferentes metales, de forma que unos resulten sensibles al benceno y otros insensibles,esto permitió por primera vez la realización de un prototipo de bajo coste capaz dedetectar selectivamente y a temperatura ambiente el benceno presente a nivel de trazas(por debajo de 50 ppbs) en una mezcla de gases. El prototipo realizado presenta unostiempos de respuesta y de recuperación de 60 s y 10 minutos respectivamente además deuna buena estabilidad y reproducibilidad. Este prototipo se encuentra protegido por unapatente que ha sido licenciada a una compañía que se encargará de la comercializaciónindustrial del producto.In the last few years, there has been a growing demand for monitoring the environment,especially with the increasing concern by the release of toxic gases emitted by manmadeactivities. The development of nanotechnology has created a huge potential for buildinghighly sensitive, selective, low cost, and portable gas sensors with low powerconsumption.Nowadays, carbon nanotubes are receiving an intense interest from the scientificcommunity, due to their unique geometry, morphology, electronic, mechanical, thermaland optical properties, which make them a promising candidate for many industrialapplications including new gas sensors for the detection of toxic species. In this context,in this thesis a deep study is devoted to explore the sensing properties of differenthybrid nanomaterials based on carbon nanotubes for an efficient detection of toxicgases. The design, fabrication, characterization, and optimization of gas sensors usinghybrid materials have been carried out.This thesis was financially supported by the European project "Nano2hybrids", whichexploits the interface design of metal nanocluster-carbon nanotube hybrids via controlof structural and chemical defects in a plasma discharge, for designing gas sensors withsuperior performance. Benzene was chosen as the principal target gas due to its serioustoxic effects at low ppb levels and the fact that there are no reliable, low cost andselective benzene detectors in the market. In fact, no gas sensor able to selectivelydetect this gas at ppb levels and operating at ambient temperature has been reported upto now in the literature. So, the challenge of the project was to fabricate sensitive,highly selective, stable, portable, and low cost benzene gas sensor employing hybridnanomaterials.Herein, functionalized MWCNTs, metal decorated MWCNTs, and metal oxidedecorated MWCNTs or metal oxide and MWCNT mixtures were deeply investigated interms of their gas sensing performances (e.g, sensitivity, selectivity, stability, detectionmechanism,. etc) towards the detection of different gases (benzene (C6H6), carbonmonoxide (CO), nitrogen dioxide (NO2), ethylene (C2H4), hydrogen sulfide (H2S),ammonia (NH3), and water (H2O)). Our tasks were to investigate experimentally andtheoretically the effects of material preparation conditions (e.g., plasma treatment,nanocluster precursor and size), sensor fabrication (e.g., deposition technique,electrodes sensor metal), and sensor characterization conditions (e.g., operatingtemperature, gas flow) on the gas sensing properties of our devices, and to acquireknowledge in order to develop a selective benzene detector. Based on experimental andtheoretical results, different mechanisms for the interaction between gases and thehybrid materials tested have been proposed.We found that hybrid materials consisting of oxygen plasma treated multiwalled carbonnanotubes decorated with different metal nanoparticles showed room temperaturesensing capability. Responsiveness to gases of these hybrid materials was higher thanthat of pristine or plasma functionalized carbon nanotubes. Metal decoated CNTs can betailored for the recognition of different gases and vapors with different reactivities,which offers enormous flexibility for tuning the interfacial properties of the resultinghybrid materials and thus, of their sensing properties. When combined in a microsensorarray operating at room temperature, the use of benzene-sensitive and benzeneinsensitivemetal-decorated multiwalled carbon nanotubes, allowed for the first time theimplementation of a low cost detector prototype, which can selectively detect benzenewhen present at trace levels (below 50 ppb) in a gas mixture. Sensors present responseand recovery times of 60 s and 10 min respectively, good stability and reproducibility.This type of sensors are protected by a patent, and licensed to a company for industrialcommercialization.