Nano-derived sensors for high-temperature sensing of H2, SO2 and H2S

The emission of sulfur compounds from coal-fired power plants remains a significant concern for air quality. This environmental challenge must be overcome by controlling the emission of sulfur dioxide (SO2) and hydrogen sulfide (H2S) throughout the entire coal combustion process. One of the processes which could specifically benefit from robust, low cost, and high temperature compatible gas sensors is the coal gasification process which converts coal and/or biomass into syngas. Hydrogen (H2), carbon monoxide (CO) and sulfur compounds make up 33%, 43% and 2% of syngas, respectively. Therefore, development of a high temperature (\u3e500°C) chemical sensor for in-situ monitoring H2, H2S and SO2 levels during coal gasification is strongly desired. The selective detection of SO2/H2S in the presence of H2, is a formidable task for a sensor designer. In order to ensure effective operation of these chemical sensors, they must inexpensively function within the gasifier\u27s harsh temperature and chemical environment. Currently available sensing approaches, which are based on gas chromatography, electrochemistry, and IR-spectroscopy, do not satisfy the required cost and performance targets.;There is also a substantial necessity for microsensors that can be applied inexpensively, have quick response time and low power consumption for sustained operation at high temperature. In order to develop a high temperature compatible microsensor, this work will discourse issues related to sensor stability, selectivity, and miniaturization. It has been shown that the integration of nanomaterials as the sensing material within resistive-type chemical sensor platforms increase sensitivity. Unfortunately, nanomaterials are not stable at high temperatures due to sintering and coarsening processes that are driven by their high surface to volume ratio. Therefore, new hydrogen and sulfur selective nanomaterial systems with potentially highly selective and stable properties in the proposed harsh environment were investigated. Different tungstates and molybdates (WO3, MoO3, MgMoO4, NiMoO4, NiWO4, Sr2MgWO6 (SMW), Sr2MgMoO6 (SMM), SrMoO4, and SrWO4) were investigated at the micro- and nano-scale, due to their well-known properties as the reversible absorbents of sulfur compounds. Different morphologies of aforementioned compounds as well as microstructural alterations were also the subject of the investigation. The fabrication of the microsensors consisted of the deposition of the selective nanomaterial systems over metal based interconnects on an inert substrate. This work utilized the chemi-resistive (resistive-type) microsensor architecture where the chemically and structurally stable, high temperature compatible electrodes were sputtered onto a ceramic substrate. The nanomaterial sensing systems were deposited over the electrodes using a lost mold method patterned by conventional optical lithography.;Development of metal based high temperature compatible electrodes was crucial to the development of the high temperature sensor due to the instability of typically used noble metal (platinum) based electrode material over ceramic substrates. Therefore, the thermal stability limitations of platinum films with various adhesion layers (titanium (Ti), tantalum (Ta), zirconium (Zr), and hafnium (Hf)) were characterized at 800 and 1200°C. Platinum (Pt)-zirconium (Zr)-hafnium (Hf) were investigated. The high-temperature stable composite thin film architecture was developed by sequential sputter deposition of Hf, Zr and Pt. In addition to this multilayer architecture, further investigation was carried out by using an alternative DC sputtering deposition process, which led to the fabrication of a functionally-gradient platinum and zirconium composite microstructure with very promising high temperature properties. The final process investigated reduced labor, time and material consumption compared to methods for forming multilayer architectures previously discussed in literature.;In addition to electrical resistivity characterization of the different thin film electrode architectures, the chemical composition, and nano- and micro-structure of the developed nanomaterial films, as well as sensing mechanism, were characterized by means of scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), X-ray and ultraviolet photoelectron spectroscopies (XPS and UPS), atomic absorption spectroscopy (AAS), X-ray diffraction (XRD), Raman spectroscopy, temperature programmed reduction (TPR) and transmission electron microscopy (TEM). The macro-configurations of the sensors were tested and analyzed for sensitivity and cross-sensitivity, response time and recovery time, as well as long term stability. The microsensor configuration with optimized nanomaterial system was tested and compared to a millimeter-size sensor platform in terms of sensitivity and accuracy. Electrochemical relaxation (ECR) technique was also utilized to quantify the surface diffusion kinetics of SO2 over the chosen sensor material surface. The outcomes of this research will contribute to the economical application of sensor arrays for simultaneous sensing of H2, H2S, and SO2

Transition metal oxides - Thermoelectric properties

Transition metal oxides (TMOs) are a fascinating class of materials due to their wide ranging electronic, chemical and mechanical properties. Additionally, they are gaining increasing attention for their thermoelectric (TE) properties due to their high temperature stability, tunable electronic and phonon transport properties and well established synthesis techniques. In this article, we review TE TMOs at cryogenic, ambient and high temperatures. An overview of strategies used for morphological, compositing and stoichiometric tuning of their key TE parameters is presented. This article also provides an outlook on the current and future prospects of implementing TMOs for a wide range of TE applications

Investigation of thermopower waves based energy sources

Miniaturisation of energy sources is critical for the development of the next generation electronic devices. However, reduction in dimensions of none of the commonly used energy generation technologies including batteries, fuel cells, heat engines and supercapacitors have resulted in efficient and reliable energy sources with high specific powers (power-to-mass ratio). Recently, the new concept of energy generation based on thermopower waves has shown promise for miniaturization. In such sources, exothermic chemical reactions of a reactive fuel are coupled to charge carriers of a thermoelectric (TE) material in its affinity, resulting in an intense thermal wave that self-propagates along the surface of the TE materials. This wave simultaneously entrains charge carriers, resulting in a large current. If the TE material also has a high Seebeck coefficient, a large output voltage and subsequently large specific power output are obtained. As the thermal wave results in a power output, it is called a thermopower wave. In the first stage of the PhD research, the author demonstrated thermopower wave systems based on thin films of Bi 2 Te 3 . Bi 2 Te 3 was implemented due to its high S (~ –200 μV/K) and σ (10 5 S/m). As Bi 2 Te 3 exhibits a low κ , the author devised a novel strategy by placing it on thermally conductive alumina (Al 2 O 3 ) substrate to compensate for this deficiency. The Bi 2 Te 3 based thermopower wave sources generated voltages and oscillations higher (at least 150 %) than the previously reported multi-walled carbon nanotube (MWNT) based thermopower wave sources, while maintaining a high specific power in the order of 1 kW/kg. In the second stage, the author implemented a novel combination of p-type Sb 2 Te 3 and n-type Bi 2 Te 3 as the core TE materials with complimentary semiconducting properties, to show the generation of voltage signals with alternating polarities. In the third stage, the author implemented zinc oxide (ZnO), which is a TE transition metal oxide (TMO), for the first time as the core material in thermopower wave sources. It was shown that both S (~ –500 μV/K at 300 °C) and σ (~ 4×10 3 S/m at 300 °C) of ZnO increased at elevated temperatures. By incorporating ZnO as the core TE material, the PhD candidate obtained voltages and oscillation amplitudes at least 200 % higher than any previously demonstrated thermopower wave systems (in the order of > 500mV), while maintaining a high specific power (~ 0.5 kW/kg). In the final stage, in order to exceed voltages larger than 1 V, the PhD candidate identified that manganese dioxide (MnO 2 ), which is another TE TMO, can exhibit exceptionally large S and moderate σ at elevated temperatures. As a result, the author implemented MnO 2 as the core TE material. It was shown that the S of MnO 2 increased dramatically with temperature, exhibiting a peak value of approximately –1900 μV/K at 350 °C. Consequently, voltages large enough (~1.8 V) to drive small electronic circuits were obtained, while maintaining high specific powers in the order of 1 kW/kg

Influences of Titanium Oxide Additions On The Electrochromic Properties Of WO3 Thin Films

Ph.DDOCTOR OF PHILOSOPH

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

Inorganic and Organic Photovoltaic Materials for Powering Electrochromic Systems

abstract: ABSTRACT Autonomous smart windows may be integrated with a stack of active components, such as electrochromic devices, to modulate the opacity/transparency by an applied voltage. Here, we describe the processing and performance of two classes of visibly-transparent photovoltaic materials, namely inorganic (ZnO thin film) and fully organic (PCDTBT:PC70BM), for integration with electrochromic stacks. Sputtered ZnO (2% Mn) films on ITO, with transparency in the visible range, were used to fabricate metal-semiconductor (MS), metal-insulator-semiconductor (MIS), and p-i-n heterojunction devices, and their photovoltaic conversion under ultraviolet (UV) illumination was evaluated with and without oxygen plasma-treated surface electrodes (Au, Ag, Al, and Ti/Ag). The MS Schottky parameters were fitted against the generalized Bardeen model to obtain the density of interface states (Dit ≈ 8.0×1011 eV−1cm−2) and neutral level (Eo ≈ -5.2 eV). These devices exhibited photoconductive behavior at λ = 365 nm, and low-noise Ag-ZnO detectors exhibited responsivity (R) and photoconductive gain (G) of 1.93×10−4 A/W and 6.57×10−4, respectively. Confirmed via matched-pair analysis, post-metallization, oxygen plasma treatment of Ag and Ti/Ag electrodes resulted in increased Schottky barrier heights, which maximized with a 2 nm SiO2 electron blocking layer (EBL), coupled with the suppression of recombination at the metal/semiconductor interface and blocking of majority carriers. For interdigitated devices under monochromatic UV-C illumination, the open-circuit voltage (Voc) was 1.2 V and short circuit current density (Jsc), due to minority carrier tunneling, was 0.68 mA/cm2. A fully organic bulk heterojunction photovoltaic device, composed of poly[N-9’-heptadecanyl-2,7-carbazole-alt-5,5-(4’,7’-di-2-thienyli2’,1’,3’-benzothiadiazole)]:phenyl-C71-butyric-acidmethyl (PCDTBT:PC70BM), with corresponding electron and hole transport layers, i.e., LiF with Al contact and conducting/non-conducting (nc) PEDOT:PSS (with ITO/PET or Ag nanowire/PDMS contacts; the illuminating side), respectively, was developed. The PCDTBT/PC70BM/PEDOT:PSS(nc)/ITO/PET stack exhibited the highest performance: power conversion efficiency (PCE) ≈ 3%, Voc = 0.9V, and Jsc ≈ 10-15 mA/cm2. These stacks exhibited high visible range transparency, and provided the requisite power for a switchable electrochromic stack having an inkjet-printed, optically-active layer of tungsten trioxide (WO3), peroxo-tungstic acid dihydrate, and titania (TiO2) nano-particle-based blend. The electrochromic stacks (i.e., PET/ITO/LiClO4/WO3 on ITO/PET and Ag nanowire/PDMS substrates) exhibited optical switching under external bias from the PV stack (or an electrical outlet), with 7 s coloration time, 8 s bleaching time, and 0.36-0.75 optical modulation at λ = 525 nm. The devices were paired using an Internet of Things controller that enabled wireless switching.Dissertation/ThesisDoctoral Dissertation Electrical Engineering 201

Solution-processed metal oxide interlayers for hybrid organic:inorganic optoelectronic devices

Research on optoelectronic devices based on organic semiconductors has seen a steady rise in the last 20 years. Recently the incorporation of inorganic materials such as transition metal oxides as charge transport layers in these devices was found to be highly beneficial in terms of energy level alignment, stability and lifetime. Roll‐to‐roll processing of organic electronics and the use of flexible plastic substrates poses some limitations on the deposition of these oxides: they should be processed using solution‐based methods and not require high temperature post‐deposition treatments. In this work, thin films of four different transition metal oxides (MoO3‐x, V2O5‐x, WO3‐x, CoOx) were deposited using facile solution processes. The chemistry of the precursor solutions was carefully chosen to ensure the formation of the oxide of interest without the need for high temperature post‐deposition treatments (i.e. <150 °C). The oxides were incorporated in organic solar cells and light‐emitting diodes as hole transport layers and the effect of the solvo‐thermal processing conditions of the oxides on the behaviour of the devices was studied. All performance metrics were compared with those of poly(3,4‐ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS), a widely adopted hole transport material. Great improvements in both types of devices were recorded: in solar cells, the power conversion efficiency was up to 22% higher with a WO3‐x interlayer when compared with devices incorporating PEDOT:PSS; in light‐emitting diodes, the luminous efficacy was 24% higher using a MoO3‐x interlayer instead of a PEDOT:PSS one. In addition to this, the possibility of improving the characteristics and performances of PEDOT:PSS by blending it with MoO3‐x was explored. Different degrees of mixing were investigated, and the effect of increasing MoO3‐x percentage in the PEDOT:PSS/MoO3‐x hybrid on the behaviour of optoelectronic devices was studied. When compared to simple PEDOT:PSS, the hybrid produced an increase of 10% in the power conversion efficiency of solar cells and of 23% in the luminous efficacy of light‐emitting diodes. This thesis is divided into six chapters. Chapter 1 provides an insight into the underlying principles of device operation together with a review of the main characteristics of transition metal oxides and their incorporation in organic electronic devices. Moreover, a detailed analysis of different solution‐based methods that can be utilised for their deposition is given. Chapter 2 lists the different materials, recipes, and characterisation techniques used in this work. Chapters 3‐5 contain the results obtained in this research: Chapter 3 focuses on MoO3‐x and V2O5‐x oxides, Chapter 4 deals with the PEDOT:PSS/MoO3‐x hybrids and finally Chapter 5 contains results from WO3‐x and CoOx. Conclusions are laid out in Chapter 6, together with ideas and prompts for future expansions of this work.Open Acces

Improving and Evaluating the Stability of Photovoltaics

Advancements in photovoltaic technologies are hindered by stability and degradation. In silicon photovoltaics, these degradation mechanisms include potential induced degradation (PID) and current-induced degradation (CID), among others. In this thesis, impedance spectroscopy is used to examine passivated emitter and rear cell (PERC) silicon modules with PID and CID. A comparison between control and degraded modules is done to identify key differences in the impedance spectra and determine the extent of the degradation. PID was observed at the module level as a dramatic reduction in shunt resistance, with a small amount of spatial inhomogeneity present in the degradation. It was found that accurate characterization of CID via measurement of the minority carrier lifetime requires a high bias voltage at the module level that exceeds the capabilities of a standard impedance spectrometer. Because of this, CID was also examined at the cell level, where reductions in minority carrier lifetimes could be accurately measured. A correlation between the reduction in minority carrier lifetime due to CID and a reduction in the power conversion efficiency was observed. Thus, the PID and CID mechanisms studied here induce unique changes in the impedance spectroscopy results, making them distinguishable and quantifiable. Finally, the ability to mitigate CID through the use of different silicon wafers and a current induced regeneration process was characterized by impedance spectroscopy. The stability of emerging photovoltaics, such as those based on a metal halide perovskite light-absorbing film, is also tackled. Atmospheric pressure spatial atomic layered deposition (AP-SALD) is a scalable technique to produce dense, uniform, pinhole free thin films. This is ideal for improving the stability of perovskite devices by preventing the ingress of moisture, which causes degradation of the perovskite film. Tungsten oxide is a transparent semiconductor that is attractive for use as a charge-transporting layer in perovskite photovoltaics. By including a nebulizer and ozone generator in our custom-built AP-SALD system, it was possible to produce tungsten oxide films for the first time by this method. Optimized deposition parameters resulted in hexagonal tungsten oxide, which was characterized then annealed into monoclinic tungsten oxide. The growth rate of this procedure suggests layer-by-layer growth, which is consistent with an ALD growth mode. Due to high deposition temperatures, an inverted p-i-n architecture is required to implement tungsten oxide into a perovskite structure. Preliminary trials at incorporating tungsten oxide address the crystal structure and wettability of the WO3 film. Further tests are necessary to create a device using WO3 films fabricated by AP-SALD

Chemical Vapour Deposition of p-type First Row Transition Metal Oxides for Use in Sensing Volatile Organic Carbon Species

There has been an increased attention towards gas detection due to levels of toxic and hazardous gases to which humans are exposed to everyday. Semiconductor gas sensors, mainly metal oxide sensors have good sensitivity due to difference in conductivity even at low concentrations. Metal oxide are desirable materials for sensors due to their low cost, robust structure, relatively low energy consumption and good response to a variety of gases and vapours. Several metal oxides have been used for gas sensor devices, including but not limited to SnO2, WO3 and TiO2. Since the discovery for its use as gas sensors, SnO2, a n-type material is one of the most widely used gas sensor material. However, there are drawbacks such as poor baseline stability, interference from humid conditions and low selectivity.1,2 Recently Cr2-xTixO3, a n-type doped p-type material has improved properties com- pared to SnO2, a pure n-type MOS. It displays better stability towards relative humidity, a low baseline drift, higher selectivity and better stability.3 Since its discovery as a gas sensing material by Moseley and Williams, it has been widely used in research and com- mercially with the ability to detect various gases and vapours such as NH3, CO, H2S and different alcohols.4,5 Ceramic substrate are still commonly used as the based for MOS sensors due to its low cost, ease of fabrication, chemical stability and robustness of the substrate (sensor can be made via screen-printing). However, there are certain limitation, which is why Micro-electro-mechanical system (MEMS) technologys have been extensively researched upon over recent years. Some of the reasons for such drive is because MEMS miniaturised ii heating element (micro-hotplates) can operate at high temperatures quickly and preserve power consumption (mW ranges) due to its low thermal mass.6 However, due the MEMS substrate brittleness, certain synthetic techniques are not viable such as screen printing. Therefore, one aspect of the project is to directly and successfully deposit CTO sensing ma- terial onto the MEMS substrate via AACVD. Once deposited, its sensitivity and selectivity towards VOCs will be investigated and compared to ceramic screen printed sensors. This project is carried out in collaboration with Alphasense Ltd (http://www.alphasense.com/), a leading UK gas sensor manufacturer. The mechanism of gas sensing has long been debated and described. Here two models are described, band bending model and surface trap limited model. The latter was used to explain the mechanism and interaction of the sensors tested in this project. Another aspect of this project is to find optimum parameters to generate n-type doped CWO and p-type doped CNO thin films via AACVD. Detailed analysis (elemental compo- sition, oxidation state, structural information and morphology) of the deposited thin films were studied using XRD, EDX and XPS. These materials sensing abilities were tested towards various different VOCs. This project was carried out in collaboration with Alphasense Ltd, a leading UK gas sensor company. The project was divided into the following main sections: • Synthesis of CTO thin films onto glass, alumina and gas sensing substrates • Characterisation of CTO thin films • Synthesis of CTO on MEMS platforms • Gas sensor tests at Alphasens