Synthesis and gas sensing properties of inorganic semiconducting, p-n heterojunction nanomaterials

En aquesta tesis utilitzant principalment Aerosol Assited Chemical Vapor Deposition, AACVD, com a metodologia de síntesis d'òxid de tungstè nanoestructurat s'han fabricat diferents sensors de gasos. Per tal d'estudiar la millora en la selectivitat i la sensibilitat dels sensors de gasos basats en òxid de tungstè aquest s'han decorat, via AACVD, amb nanopartícules d'altres òxids metàl·lics per a crear heterojuncions per tal d'obtenir un increment en la sensibilitat electrònica, les propietats químiques del material o bé ambdues. En particular, s'han treballat en diferents sensors de nanofils d'òxid de tungstè decorats amb nanopartícules d'òxid de níquel, òxid de cobalt i òxid d'iridi resultant en sensors amb un gran increment de resposta i selectivitat cap al sulfur d'hidrogen, per a l'amoníac i per a l'òxid de nitrogen respectivament a concentracions traça. A més a més, s'han estudiat els mecanismes de reacció que tenen lloc entre les espècies d'oxigen adsorbides a la superfície del sensor quan interactua amb un gas. I també s'ha treballat en intentar controlar el potencial de superfície de les capes nanoestructurades per tal de controlar la deriva en la senyal al llarg del temps, quan el sensor està operant, a través d'un control de temperatura.En esta tesis utilizando principalmente Aerosol Assited Chemical Vapor Deposition, AACVD, como metodología de síntesis de óxido de tungsteno nanoestructurado se han fabricado diferentes sensores de gases. Para estudiar la mejora en la selectividad y la sensibilidad de los sensores de gases basados en óxido de tungsteno estos se han decorado, vía AACVD, con nanopartículas de otros óxidos metálicos para crear heterouniones para obtener un incremento en la sensibilidad electrónica, las propiedades químicas del material o bien ambas. En particular, se han trabajado en diferentes sensores de nanohilos de óxido de tungsteno decorados con nanopartículas de óxido de níquel, óxido de cobalto y óxido de iridio resultante en sensores con un gran incremento de respuesta y selectividad hacia el sulfuro de hidrógeno, para el amoníaco y para el óxido de nitrógeno respectivamente a concentraciones traza. Además, se han estudiado los mecanismos de reacción que tienen lugar entre las especies de oxígeno adsorbidas en la superficie del sensor cuando interactúa con un gas. Y también se ha trabajado en intentar controlar el potencial de superficie de las capas nanoestructuradas para controlar la deriva en la señal a lo largo del tiempo, cuando el sensor está trabajando, a través de un control de temperatura.In this thesis, using mainly Aerosol Assited Chemical Vapor Deposition, AACVD, as a synthesis methodology for nanostructured tungsten oxide, different gas sensors have been manufactured. To study the improvement in the selectivity and sensitivity of gas sensors based on tungsten oxide, they have been decorated, via AACVD, with nanoparticles of other metal oxides to create heterojunctions to obtain an increase in electronic sensitivity, in the chemical properties of the material or at the same time in both. Particularly, we have worked on different tungsten oxide nanowire sensors decorated with nanoparticles of nickel oxide, cobalt oxide and iridium oxide resulting in sensors with a large increase in response and selectivity towards hydrogen sulfide, for ammonia. and for nitrogen oxide respectively at trace concentrations. In addition, the reaction mechanisms that take place between oxygen species adsorbed on the sensor surface when it interacts with a gas have been also studied. Furthermore, efforts have been put on trying to control the surface potential of the nanostructured layers to control the drift in the signal over time, when operating the sensors, through temperature control

Group III-nitrides: synthesis and sensor applications

Submitted in partial fulfilment of the requirements for the degree of Doctor of Philosophy in the Faculty of science Department of Chemistry University of the Witwatersrand. November 2016.An overview of the evolution of synthesis and applications of indium nitride and gallium nitride in modern science and technology is provided. The working principles and parameters of chemical vapour deposition (CVD) synthesis technique are explored in this study. In this study indium oxide, indium phosphate, indium nitride and gallium nitride materials are prepared by CVD. The versatility of CVD on the fabrication of one-dimensional (1D) structures is portrayed. Both change in dimensionality and change in size are achieved by a CVD technique. 1D indium oxide (In2O3) nanowires, nanonails and nanotrees are synthesised from vapour deposition of three-dimensional In2O3 microparticles. While 1D structures of the novel indium phosphate known as triindium bisphosphate In3(PO4)2 were obtained from reactions of In2O3 with ammonium phosphate. The effect of temperature, activated carbon and the type of indium precursor on dimensionality of the synthesized materials is studied. The inter-dependency between temperature and precursors is observed. The presence of activated carbon at high temperatures encouraged growth of secondary structures via production of excess indium droplets that act as catalysts. The combination of activated carbon and high temperature was found responsible for the novel necklace, nanonail, nanotree and nanocomb structures of In2O3. Indium nitride (InN) has for the first time been made by a combined thermal/UV photoassisted process. In2O3 was reacted with ammonia using two different procedures in which either the ammonia was photolysed or both In2O3 and ammonia were photolysed. A wide range of InN structures were made that was determined by the reaction conditions (time, temperature). Thus, the reaction of In2O3 with photolysed NH3 gave InN rod like structures that were made of cones (6 h/ 750 oC) or discs (6 h/ 800 oC) and that contained some In2O3 residue. Photolysis of In2O3 and NH3 by contrast gave InN nanobelts, InN tubes and pure InN tubes filled with In metal (> 60 %). The transformation of the 3D In2O3 particles to the tubular 1D InN was monitored as a function of time (1-6 h) and temperature (700-800 oC); the product formed was very sensitive to temperature. The band gap of the InN tubes was found to be 2.19 eV and of the In filled InN tubes to be 1.89 eV. Gallium nitride (GaN) and indium gallium nitride (InGaN) nanostructures were synthesized from thermal ammonification of gallium oxide (Ga2O3) as well ammonification of a mixture of In2O3 and Ga2O3 respectively. The effect of temperature on preparation of high purity GaN was studied. The GaN materials synthesized at 800 °C showed a mixture of the gallium oxide and the gallium nitride phases from the XRD analysis. However at temperatures ≥ 900 °C high quality GaN nanorods were obtained. The band-to-band ultraviolet optical emission value of 3.21 eV was observed from the GaN nanorods. However, the preparation of InGaN was complicated by the thermally stable In2O3. At lower temperatures inhomogeneous materials consisting of GaN nanorods and In2O3 were obtained. While at high temperatures (≥ 1050 °C) InGaN was obtained. However because indium has a high vapour pressure and a low melting point only a minute amount of it was incorporated in the crystal lattice. Hexagonally shaped nanoplates of In0.01Ga0.99N were successfully obtained. A shift in optical emission to longer wavelengths was observed for the InGaN alloy. A blue optical emission with the energy value of 2.86 eV was observed for the InGaN nanoplates. The two n-type group III-nitrides (InN, GaN) prepared in this study were used for the detection of CO, NH3, CH4 and NO2 gases in the temperature range between 250 and 350 °C. The InN sensor and GaN sensor responses were compared to the response of the wellestablished n-type SnO2 sensor under the same conditions. All the three sensors responded to all the four gases. However, InN and GaN were much more selective in comparison to SnO2. InN sensitivity to CO at 250 °C surpassed its sensitivity to any other gas at the studied temperature range. Its response towards CO at 250 °C was about five times more than that of SnO2 towards CO at the same temperature. While, GaN was the best CH4 sensor at 300 °C in comparison to InN and SnO2 sensors at all temperatures. Meanwhile SnO2 responded remarkably to both NH3 and CO across the studied temperature range with its performance improving with increasing temperature. The ability for InN to respond to both NH3 and NO2 at 250 °C opens up the possibility for an application of InN as an ammonia sensor in diesel engines. InN and SnO2 sensors were found susceptible to humidity interference in a real environmental situation. On the contrary, GaN sensor presented itself as an ideal candidate for indoor and outdoor environments as well as in bio-sensors because it showed robustness and inertness towards humidity. InN and GaN by showing activity at high temperatures only, presented themselves as good candidates for in-situ high temperate gas sensing applications. Response and recovery times for all sensors showed improvement with increasing temperature.MT201

Development of Flexible Gas Sensors Based on Additive Fabrication Processes

Els sensors de gasos s’utilitzen per a monitoritzar ambients interiors i exteriors. Algunes aplicacions comuns són per a mesurar el nivell de contaminants als carrers, els gasos alliberats per les fuites industrials i d’automòbils, els gasos a la mineria, el contingut d’alcohol en sang a través de l’alè exhalat, etc. A mesura que creix el camp d’aplicació dels sensors de gasos, es fa necessari adaptar els sensors de gasos als nostres dispositius i pertinences diàries. Es requereixen materials mecànicament flexibles i resistents per a fabricar sensors de gasos flexibles. A banda de proves de detecció de gas, la resistència a la flexió dels sensors ha de provar-se per anomenar “flexible” a un sensor. L’objectiu principal d’aquesta tesi és fabricar sensors de gasos flexibles mitjançant processos additius emprant òxids metàl·lics com a materials sensibles. Els sensors de gasos flexibles es varen fabricar utilitzant un substrat polimèric flexible (Kapton). Els diferents processos emprats varen ser compatibles amb la temperatura de funcionament del substrat. Entre les tècniques emprades estan la plantilla, la serigrafia, la injecció de tinta, AA-CVD. A més a més, es varen realitzar processos superficials per a millorar l’adhesió dels òxids metàl·lics al substrat polimèric. La flexibilitat dels sensors es va provar realitzant una prova de flexió cíclica.Los sensores de gas se utilizan para monitorear ambientes interiores y exteriores. Algunas aplicaciones comunes son para medir: el nivel de contaminantes en las calles, los gases liberados por los escapes industriales y de automóviles, los gases en la minería, el contenido de alcohol en la sangre a través del aliento exhalado, etc. A medida que crece el campo de aplicación de los sensores de gas, se hace necesario adaptar los sensores de gas a nuestros dispositivos y pertenencias diarias. Se requieren materiales mecánicamente flexibles y resistentes para fabricar los sensores de gas flexibles. Además de las pruebas de detección de gas, la resistencia a la flexión de los sensores debe probarse para llamar “flexible” a un sensor. El objetivo principal de esta tesis es fabricar sensores de gas flexibles a través de procesos aditivos utilizando óxidos metálicos como materiales sensibles. Los sensores de gas flexibles se fabricaron utilizando un sustrato polimérico flexible (Kapton). Los diferentes procesos empleados fueron compatibles con la temperatura de la temperatura de funcionamiento del sustrato. Entre las técnicas empleadas están la plantilla, la serigrafía, la inyección de tinta, AA-CVD. Además, se realizaron procesos superficiales para mejorar la adhesión de los óxidos metálicos al sustrato polimérico. La flexibilidad de los sensores se probó realizando una prueba de flexión cíclica.Gas sensors are used to monitor indoor and outdoor environments. Some common applications are to measure: the level of pollutants in the streets, the gases liberated by industrial and car exhausts, gases in mining, blood alcohol content through the exhaled breath, etc. As the field of application for gas sensors is growing, it becomes necessary to adapt the gas sensors to our daily devices and belongings. This requires mechanically flexible and resistant materials to fabricate the flexible gas sensors. In addition to gas sensing tests, the resistance to bending of the sensors should be tested to call a sensor flexible. The main objective of this thesis is to fabricate flexible gas sensors through additive processes using metal oxides as sensitive materials. The flexible gas sensors were fabricated using a flexible polymeric substrate (Kapton). The different processes employed were compatible with the temperature of the operating temperature of the substrate. Among the techniques employed are stencil, screen-printing, inkjet-printing, AA-CVD. Also, surface processes were performed to improve the adhesion of the metal oxides to the polymeric substrate. The flexibility of the sensors was tested by performing a cyclical bending test

Innovative ozone sensors for environmental monitoring working at low temperature

L'abstract è presente nell'allegato / the abstract is in the attachmen

CO sensing characteristics of In-doped ZnO semiconductor nanoparticles

Abstract A study on the CO sensing characteristics of In-doped ZnO semiconductor nanoparticles (IZO NPs) prepared by a modified sol–gel technique is reported. The morphological and microstructural features of IZO NPs with various dopant concentrations (1 at.%, 2 at.%, 3 at.%, and 5 at.% In) were investigated by scanning electron microscopy (SEM) and X-ray powder diffraction (XRD). The influence of indium doping on defect characteristics of ZnO was also investigated by photoluminescence (PL). A thick film of IZO NPs was deposited by screen printing on an alumina substrate provided with a pair of Pt interdigitated electrodes to fabricate a simple conductometric sensor platform. The as fabricated In-doped ZnO sensors showed enhanced sensitivity to CO gas with respect to pure ZnO one. Sensors with low dopant loading (1 at.% and 2 at.% In) were found to be more sensitive with shorter response and recovery times than those with high dopant loading

Effect of Annealing on Metal-Oxide Nanocluster

Recently, the development of optoelectronic devices based on metal-oxide nanocluster has attracted intensive research interest. Nanoclusters are suitable for these because of their large surface-to-volume ratio and the presence of abundant oxygen vacancies or trap states. Metal–oxides such as ZnO, In2O3, and TiO2 synthesized using different technique produces high surface area films consisting of clusters and provides complete control over the film morphology. In this chapter, some of the metal oxides nanocluster film has investigated, and the effect of annealing on the structural, optical and electrical properties of the grown films when subjected to different annealing temperatures will be studied. Theoretically, these properties are presumed to improve after the heat treatment as the crystallinity, and the grain size of the film has increased due to the diminishing of oxygen vacancies. Thus, the greater surface-to-volume ratio, the better stoichiometry and higher level of crystallinity compared to bulk materials make nanocluster-based devices very promising for the mentioned application

Nanomaterials for Biological Applications: Drug Delivery and Bio-sensing

The idea of utilizing nanomaterials in bio-related applications has been extensively practiced during the recent decades. Magnetic nanoparticles (MPs), especially superparamagnetic iron oxide nanoparticles have been demonstrated as promising candidates for biomedicine. A protective coating process with biocompatible materials is commonly performed on MPs to further enhance their colloidal and chemical stability in the physiological environment. Mesoporous hollow silica is another class of important nanomaterials that are extensively studied in drug delivery area for their ability to carry significant amount of guest molecules and release in a controlled manner. In this study, different synthetic approaches that are able to produce hybrid nanomaterials, constituting both mesoporous hollow silica and magnetite nanoparticles, are described. In a two-step approach, pre-synthesized magnetite nanoparticles are either covalently conjugated to the surface of polystyrene beads and coated with silica or embedded/enclosed in the porous shell during a nanosized CaCO3 templated condensation of silica precursors, followed by acid dissolution to generate the hollow structure. It was demonstrated that the hollow interior is able to load large amount of hydrophobic drugs such as ibuprofen while the mesoporous shell is capable of prolonged drug. In order to simplify the fabrication procedure, a novel in-situ method is developed to coat silica surface with magnetite nanoparticles. By refluxing the iron precursor with mesoporous hollow silica nanospheres in polyamine/polyalcohol mixed media, one is able to directly form a high density layer of magnetite nanoparticles on silica surface during the synthesis, leaving reactive amine groups for further surface functionalization such as fluorescence conjugation. This approach provides a convenient synthesis for silica nanostructures with promising potential for drug delivery and multimodal imaging. In addition to nanoparticles, nanowires also benefit the research and development of instruments in clinical diagnosis. Semiconductive nanowires have demonstrated their advantage in the fabrication of lab-on-a-chip devices to detect many charge carrying molecules such as antibody and DNA. In our study, In2O3 and silicon nanowire based field effect transistors were fabricated through bottom-up and top-down approaches, respectively, for ultrasensitive bio- detection of toxins such as ricin. The specific binding and non-specific interaction of nanowires with antibodies were also investigated

Nanomaterials for Biological Applications: Drug Delivery and Bio-sensing

The idea of utilizing nanomaterials in bio-related applications has been extensively practiced during the recent decades. Magnetic nanoparticles (MPs), especially superparamagnetic iron oxide nanoparticles have been demonstrated as promising candidates for biomedicine. A protective coating process with biocompatible materials is commonly performed on MPs to further enhance their colloidal and chemical stability in the physiological environment. Mesoporous hollow silica is another class of important nanomaterials that are extensively studied in drug delivery area for their ability to carry significant amount of guest molecules and release in a controlled manner. In this study, different synthetic approaches that are able to produce hybrid nanomaterials, constituting both mesoporous hollow silica and magnetite nanoparticles, are described. In a two-step approach, pre-synthesized magnetite nanoparticles are either covalently conjugated to the surface of polystyrene beads and coated with silica or embedded/enclosed in the porous shell during a nanosized CaCO3 templated condensation of silica precursors, followed by acid dissolution to generate the hollow structure. It was demonstrated that the hollow interior is able to load large amount of hydrophobic drugs such as ibuprofen while the mesoporous shell is capable of prolonged drug. In order to simplify the fabrication procedure, a novel in-situ method is developed to coat silica surface with magnetite nanoparticles. By refluxing the iron precursor with mesoporous hollow silica nanospheres in polyamine/polyalcohol mixed media, one is able to directly form a high density layer of magnetite nanoparticles on silica surface during the synthesis, leaving reactive amine groups for further surface functionalization such as fluorescence conjugation. This approach provides a convenient synthesis for silica nanostructures with promising potential for drug delivery and multimodal imaging. In addition to nanoparticles, nanowires also benefit the research and development of instruments in clinical diagnosis. Semiconductive nanowires have demonstrated their advantage in the fabrication of lab-on-a-chip devices to detect many charge carrying molecules such as antibody and DNA. In our study, In2O3 and silicon nanowire based field effect transistors were fabricated through bottom-up and top-down approaches, respectively, for ultrasensitive bio- detection of toxins such as ricin. The specific binding and non-specific interaction of nanowires with antibodies were also investigated