Design, fabrication and characterisation of gas sensors based on nanohybrid materials

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

Nanostructured TiO2 Layers for Photovoltaic and Gas Sensing Applications

Titanium dioxide (TiO2) has been an important material for decades, combining numerous attractive properties in terms of economy (low price, large availability) or ecology (non-toxic), as well as broad physical and chemical possibilities. In the last few years, the development of nanotechnologies offered new opportunities, not only in an academic perspective but also with a view to many applications with particular reference to the environment. This chapter focuses on the many ways that allow to tailor and organize TiO2 crystallites at the nanometre scale to make the most of this amazing material in the field of photovoltaics and gas sensing

Doctor of Philosophy

dissertationCurrently, all over the world, a lot of money is being pumped into the healthcare domain to facilitate development of rapid, point-of-care disease diagnostic platforms, which are relatively cheap with enhanced ease-of-use capabilities that can be deployed in low-resource settings which have higher prevalence of disease infected cases. Volatile organic compounds (VOCs) represent one class of biomarkers that has been less explored but possesses immense potential from a disease diagnostic standpoint. Tuberculosis (TB) has been a cause of significant health concern affecting a large population of people in Africa and Asia and recently, researchers have identified four specific TB VOCs from the breath of infected patients, through GC-MS analysis techniques. Rapid, accurate diagnosis is critical for timely initiation of treatment and, ultimately, control of the disease. Lack of access to appropriate diagnostic tools is caused, in part, by shortcomings as currently available diagnostics are often ill-adapted to resource-limited settings or specific patient needs, or may be priced out of reach. Although many countries still rely on basic tools such as smear microscopy, new diagnostics are changing the TB diagnostics landscape. Some groups have previously attempted to develop breath-based TB detection techniques utilizing evanescent wave technology and colorimetry-based pattern detection techniques, but no sensors exist for detection of the four methyl ester-based VOCs. In the research presented in this dissertation, we have attempted to develop a low-cost, metal functionalized titania nanotubular array-based sensor platform for electrochemical detection of the four major TB volatile organic biomarkers (VOBs). TiO2 or titania nanotubes is an easy-to-synthesize, robust, wide bandgap (~3.2 eV) semiconductor material with excellent vectorial charge transport properties. In addition, the nanotubular morphology presents a large surface-area-to-volume ratio with sufficient metal bound active sites which facilitates efficient binding with the VOBs of interest. Titania nanotubes with an optimized morphology and stoichiometry and functionalized with cobalt through the incipient wetting impregnation, and an in-situ lattice functionalization method for electrochemical detection of the four TB VOBs and their subsequent integration into a sensor hardware, has been investigated. The potential light assisted, plasmonic-based sensing capabilities of gold nanoparticle functionalized TiO2 nanotubes have been illustrated as well. In the end, a similar but slightly tweaked sensing platform has been tested for the detection of nonpulmonary colorectal cancer as well, extending the detection capabilities of the fabricated sensor substrate and leaving room for further research for screening of other life-threatening diseases. Improved access to better TB screening and diagnostics may present potential opportunities that may include efforts to accelerate market entry and/or scale-up of the innovative sensing platform that addresses unmet needs

A Review on Metal Oxide-Graphene Derivative Nano-Composite Thin Film Gas Sensors

Most of the available commercial solid-state gas/vapor sensors are based on metal oxide semiconductors. Metal oxides (MOs) change their conductivity while exposed to gas or vapors ambient can be utilized as gas or vapor sensing materials. In recent days, graphene has attracted tremendous attention owing to its two-dimensional structure with an extremely high surface to volume ratio, electron mobility, and thermal conductivity. However, intrinsic graphene is relatively inefficient for the adsorption of gas/vapor molecules. In this regard, graphene oxide (GO) and reduced graphene oxide (rGO), which are graphene species functionalized with different oxygen groups that offer a higher amount of adsorption sites improving the sensitivity of the film. Up to now, many research groups across the globe have reported the promising performance towards gas detection using various GO/rGO-metal oxide nanocomposites. This chapter reviews the composites of graphene oxide or reduced graphene oxide and metal oxides in nanoscale dimensions (0-D, 1-D, 2-D, and 3-D) for gas sensing applications considering two specific focus areas, that is, synthesis of nanocomposites and performance assessment for gas/vapor sensing

One-Dimensional Oxide Nanostructures as Gas-Sensing Materials: Review and Issues

In this article, we review gas sensor application of one-dimensional (1D) metal-oxide nanostructures with major emphases on the types of device structure and issues for realizing practical sensors. One of the most important steps in fabricating 1D-nanostructure devices is manipulation and making electrical contacts of the nanostructures. Gas sensors based on individual 1D nanostructure, which were usually fabricated using electron-beam lithography, have been a platform technology for fundamental research. Recently, gas sensors with practical applicability were proposed, which were fabricated with an array of 1D nanostructures using scalable micro-fabrication tools. In the second part of the paper, some critical issues are pointed out including long-term stability, gas selectivity, and room-temperature operation of 1D-nanostructure-based metal-oxide gas sensors

Gas Sensors Based on Conducting Polymers

Since the discovery of conducting polymers (CPs), their unique properties and tailor-made structures on-demand have shown in the last decade a renaissance and have been widely used in fields of chemistry and materials science. The chemical and thermal stability of CPs under ambient conditions greatly enhances their utilizations as active sensitive layers deposited either by in situ chemical or by electrochemical methodologies over electrodes and electrode arrays for fabricating gas sensor devices, to respond and/or detect particular toxic gases, volatile organic compounds (VOCs), and ions trapping at ambient temperature for environmental remediation and industrial quality control of production. Due to the extent of the literature on CPs, this chapter, after a concise introduction about the development of methods and techniques in fabricating CP nanomaterials, is focused exclusively on the recent advancements in gas sensor devices employing CPs and their nanocomposites. The key issues on nanostructured CPs in the development of state-of-the-art miniaturized sensor devices are carefully discussed. A perspective on next-generation sensor technology from a material point of view is demonstrated, as well. This chapter is expected to be comprehensive and useful to the chemical community interested in CPs-based gas sensor applications

Photocatalytic degradation of methyl violet in water using TiO2/Cellulose-N-MWCNTs

ABSTRACT TiO2-carbon based composites are of great significance in a wide range of applications including photocatalytic degradation. This is attributed to the high photodecomposition efficiency of the composites as compared to independent TiO2. Carbon materials such as cellulose polymer and multiwalled carbon nanotubes (MWCNTs) are considered as good supports for TiO2 owing to their unique properties such as lightweight, large surface area and high aspect ratio. Lately, the study of cellulose-MWCNTs composite has been an area of academic interest due to its large mass fraction, and prowess to facilitate toughening mechanisms in fiber bridging. However, a cost-effective method that can improve the dispersion and interfacial adhesion of the MWCNTs in the polymer is still required. Thus different modification methods of MWCNTs have been explored to increase the binding sites of the material. In this study, it was hypothesized that the cellulose’s potential as a TiO2 support can be improved by hybridizing it with MWCNTs resulting in high TiO2-C photocatalytic activity through synergistic effect. A catalytic decomposition of Fe-Co/CaCO3 was used over C2H2 to fabricate the MWCNTs. Thereafter, the MWCNTs were functionalized by (1) acid-treatment (referred to as fMWCNTs), (2) nitrogen doping by in situ and ex situ methods (referred to as in situ N-MWCNTs and ex situ N-MWCNTs, respectively) and (3) both acid treatment and nitrogen doping (referred to as in situ fN-MWCNTs and ex situ fN-MWCNTs). Moreover, cellulose-N-MWCNTs (C@fN-MWCNTs) hybrid was prepared by electrospinning a solution of cellulose acetate/in situ fN-MWCNTs (11/0.115) in DMAc at 25 kv and 1 mL/h. The prepared MWCNTs and cellulosic materials were further used as support materials of TiO2 in the photodegradation of methyl violet (MV 6B). The supported TiO2 catalysts were prepared by a sol-gel method and then analyzed using various techniques, such as transmission electron microscopy (TEM), X-ray diffraction (XRD), thermogravimetric analysis (TGA), and Raman spectroscopy. iii TGA results revealed that in situ N-MWCNTs contained high impurities inclusive of as Fe, Co, Ca, and amorphous carbon which were identified by XRD analysis. Nevertheless, TGA, BET and TEM, showed that acid treatment of MWCNTs improves their purity, surface area and anchoring sites for the TiO2, respectively. Furthermore, SEM results showed that C@fNMWCNTs hybrid interacts with TiO2 better than cellulose fibers. This was in accord with the PL results which showed a reduction in the electron/hole recombination. However, the surface area of C@fN-MWCNTs was very low compared to cellulose fibers which resulted in low dye adsorption capacity by C@fN-MWCNTs. The photocatalytic degradation activity commercial TiO2 was enhanced by 3.7% and 5.6% after being supported on cellulose and C@fN-MWCNTs, respectively. Thus, incorporating in situ fNMWCNTs with cellulose did improve the cellulose’s potential as a TiO2 support. However, the overall photocatalytic degradation performance of TiO2/C@fN-MWCNTs was less than that of in situ TiO2/fN-MWCNTs. This may be due to the reduction in the surface area, which resulted in reduced adsorption and thus lowers degradation efficiency.EM201