Nanostructured Metal Oxide Semiconductors towards Greenhouse Gas Detection

Climate change and global warming are two huge current threats due to continuous anthropogenic emissions of greenhouse gases (GHGs) in the Earth’s atmosphere. Accurate measurements and reliable quantifications of GHG emissions in air are thus of primary importance to the study of climate change and for taking mitigation actions. Therefore, the detection of GHGs should be the first step when trying to reduce their concentration in the environment. Throughout recent decades, nanostructured metal oxide semiconductors have been found to be reliable and accurate for the detection of many different toxic gases in air. Thus, the aim of this article is to present a comprehensive review of the development of various metal oxide semiconductors, as well as to discuss their strong and weak points for GHG detection

Wearable Nano-Based Gas Sensors for Environmental Monitoring and Encountered Challenges in Optimization

With a rising emphasis on public safety and quality of life, there is an urgent need to ensure optimal air quality, both indoors and outdoors. Detecting toxic gaseous compounds plays a pivotal role in shaping our sustainable future. This review aims to elucidate the advancements in smart wearable (nano)sensors for monitoring harmful gaseous pollutants, such as ammonia (NH3), nitric oxide (NO), nitrous oxide (N2O), nitrogen dioxide (NO2), carbon monoxide (CO), carbon dioxide (CO2), hydrogen sulfide (H2S), sulfur dioxide (SO2), ozone (O3), hydrocarbons (CxHy), and hydrogen fluoride (HF). Differentiating this review from its predecessors, we shed light on the challenges faced in enhancing sensor performance and offer a deep dive into the evolution of sensing materials, wearable substrates, electrodes, and types of sensors. Noteworthy materials for robust detection systems encompass 2D nanostructures, carbon nanomaterials, conducting polymers, nanohybrids, and metal oxide semiconductors. A dedicated section dissects the significance of circuit integration, miniaturization, real-time sensing, repeatability, reusability, power efficiency, gas-sensitive material deposition, selectivity, sensitivity, stability, and response/recovery time, pinpointing gaps in the current knowledge and offering avenues for further research. To conclude, we provide insights and suggestions for the prospective trajectory of smart wearable nanosensors in addressing the extant challenges

Innovative ozone sensors for environmental monitoring working at low temperature

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

Methods for Expanding the Diversity in the Response of Metal Oxide Based Gas Sensors

As all aspects of life become more automated and interconnected, sensors will be needed in various applications. In particular, gas sensors will find widespread use, in e.g. indoor air quality monitoring and breath analysis. Versus other detection methods, semiconducting metal oxide (SMOX) based sensors are more compact, sensitive, robust and inexpensive. Their major drawback is their inherent lack of selectivity. This limitation could be addressed by using arrays of SMOX materials with complementary sensing behavior. Today as a result of the historical development, despite the decades of research, most commercially available sensors are still based on SnO2. The work here examines three different options for creating complementary sensors: using a different n-type base metal oxide (WO3), noble surface loading and the creation of metal-oxide-metal-oxide mixtures. Based on a literature review, WO3 appeared promising and here its complementarity was verified. It was identified that the sensing behavior of WO3 is robust against changes in synthesis. Using operando diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy, it was possible to identify why the resistance of WO3 increases with humidity. From this finding it became apparent why the response to oxidizing gases strongly decreases in the presence of atmospheric humidity. In order to tune the sensing behavior, surface loading with metal oxides is commonly used. Although two mechanisms, chemical sensitization and Fermi level pinning, were already suggested in the 1980s, experimental evidence was limited. Here, the effect of rhodium, palladium and platinum loading on WO3 was examined. Using operando DRIFT spectroscopy, in situ transmission electron microscopy (TEM) and X-ray absorption spectroscopy (XAS) it was shown that the Fermi level pinning mechanism dominates. As a result, Rh-loading reduces the complementarity of WO3 and SnO2 based sensors. Finally, sensors based on SnO2 and Cr2O3 mixtures were examined. Reports of gas sensors based on combinations of metal oxides, in particular mixtures of n- and p-type materials, are common in literature. These mixed materials are usually created using sophisticated and expensive methods, like the electrospinning of nanofibers. Here sensors based on nanofibers were compared to those based on randomly dispersed particles. By breaking apart the nanofibers using soft mechanical grinding, it was possible, for the first time, to clearly separate the effects of the secondary structure from the coupling between the materials. It was identified that the junctions between the materials are largely responsible for the changed sensing. Furthermore, it was shown that by varying the ratio of the metal oxides, the sensor response can be tuned, i.e. shows a p- or n- type response, and in some cases no response. In total it has been shown that other n-type materials should be considered for integration into arrays with SnO2. It has been found that the applicability of noble metal oxide surface loadings to increase the complementarity of materials is limited. It has been shown that metal-oxide-metal-oxide mixtures can be used to tune the sensing behavior and that the mechanical mixing of materials is a sufficient preparation method to attain the desired results

Synthesis and gas sensing properties of single crystalline metal-oxide nanostructures

A la present tesis doctoral, s'han produït diferents tipus de nanoestructures basades en òxids metàl·lics, com per exemple nanofils de ZnO i octaedros d'In2O3, utilitzant el mètode de Deposició Química de Vapor (CVD) a altes temperatures. Per a la detecció d'òxid de nitrogen, s'ha descobert que la resposta dels nanofils d'ZnO està directament relacionada amb la quantitat de defectes presents en el material. Com més gran es el nombre de defectes, major és la resposta al diòxid de nitrogen. Tanmateix, per a la detecció d'etanol, la mostra que contenia un nombre mig de defectes, va ser la que dóna millors resultats. Pel que fa als octaedres de In2O3, podem dir que els octaedres d'In2O3 pur són excel·lents candidats per a la detecció de NO2, ja que tenen una excel·lent sensibilitat (0.43 ppb-1) a baixes temperatures (130ºC), mentres que la resposta a altres gasos com H2 és dos ordres de magnituds inferior en les mateixes condicions. A més a més, en presencia d'humitat, s'incrementa la sensibilitat a NO2 i, a la vegada, es redueix la sensibilitat a H2, pel que la selectivitat a NO2 també es veu incrementada. Finalment, utilitzant l'espectroscopia DRIFT, s'ha analitzat l'In2O3 expost a 1 ppm de NO2 a diferents temperatures i s'ha descobert que el mecanisme proposat per a descriure el procés de sensat es bastant més complicat del que s'ha reportat fins ara. Com a resultat de tots aquests experiments, s'ha donat un pas edanvant en l'explicació dels mecanismes de sensat dels nanofils d'ZnO i els octaedres de In2O3 a diferents temperatures.En la presente tesis doctoral, se han producido diferentes tipos de nanoestructuras basadas en oxidos metalicos, como por ejemplo nanohilos de ZnO y ocaedros de In2O3, utilizando el método de Deposición Química de Vapor (CVD) a altas temperaturas. Para la detección de dióxido de nitrógeno, se ha descubierto que la respuesta de los nanohilos de ZnO está directamente correlacionada con la cantidad total de defectos presentes en el material. Cuanto mayor es el número de defectos, mayor es la respuesta al dióxido de nitrógeno. Sin embargo, para la detección de etanol, la muestra que contenía un número medio de defectos fue la que dio mejores resultados. Por lo que respecta a los octaedros de In2O3, podemos decir que los octaedros de In2O3 puro son excelentes candidatos para la detección de NO2, ya que poseen una excelente sensibilidad (0.43 ppb-1) a bajas temperaturas (130ºC), mientras que la respuesta a otros gases como H2 es dos órdenes de magnitud inferior en las mismas condiciones. Además, en presencia de humedad, se incrementa la sensibilidad a NO2 y, a la vez, se reduce la sensibilidad a H2, por lo que la selectividad hacia NO2 tambien se ve incrementada. Finalmente, utilizando la espectroscopia DRIFT, se ha analizado el In2O3 expuesto a 1 ppm de NO2 a diferentes temperaturas y se ha descubierto que el mecanismo propuesto para describir el proceso de senado es bastante más complicado que lo que se ha publicado hasta ahora. Como resultado de todos estos experimentos, se ha arrojado luz nueva sobre los mecanismos de sensado de los nanohilos de ZnO y los octahedros de In2O3 a diferentes temperaturas.In the present doctoral thesis, several metal oxide nanostructures such as ZnO nanowires and In2O3 octahedra via a chemical vapour deposition (CVD) method at high temperatures. For the detection of nitrogen dioxide, it was found that the response of ZnO nanowires was directly correlated to the overall amount of defects of the material. The higher the number of defects is, the higher the response to nitrogen dioxide is. On the other hand, for the detection of ethanol, ZnO nanowires with an intermediate number of defects in which surface defects were dominant led to the best results. Additionally, regarding the In2O3 octahedra, we can say that pure In2O3 octahedra are excellent for detecting NO2 gas with an outstanding sensitivity (0.43 ppb-1) at low temperatures (130ºC), while the response to H2 remains two orders of magnitude lower under the same conditions. In addition, the presence of humidity increases the sensitivity to NO2 and, at the same time, reduces the response to H2, which results in an increased selectivity. Finally, by making use of the DRIFT spectroscopy, we have analyzed In2O3 material towards 1 ppm of NO2 at different temperatures and, it has been found that the mechanism proposed for the gas sensing is far more complicated than previously reported. As a result of these experiments, new light on the sensing mechanism of ZnO and In2O3 material towards NO2 gas at different temperatures has been shed

Synthesis of Metal Oxides with Controllable Morphology and Surface Charge and Their Composites for Gas Sensor Application

Tohoku University殷澍課

Synthesis and Gas Sensing Properties of Transition Metal Dichalcogenides materials (TMDs)

En el procés de monitorització industrial, el control d'emissions dels cotxes, la seguretat de la qualitat de l'aire interior i exterior i la protecció del medi ambient, la detecció contínua i fiable de diversos gasos és fonamental. Els òxids metàl·lics semiconductors, els materials més utilitzats en aplicacions de detecció de gasos, tenen limitacions substancials com ara un alt consum d'energia, una mala estabilitat a llarg termini, una selectivitat limitada i, sobretot, una alta sensibilitat creuada a la humitat. Els materials nous que permeten un funcionament a baixa temperatura poden resoldre problemes relacionats amb l'energia, donant lloc a xarxes de sensors millors i més fiables. Com a resultat, materials 2D com els dicalcogenurs de metalls de transició (TMD) han sorgit com a opcions viables per a la detecció de gasos. Aquests materials de nova generació tenen el potencial de millorar les propietats de detecció dels materials sensibles als gasos, com ara la sensibilitat, la selectivitat, l'estabilitat i la velocitat (temps de resposta-recuperació). Això es deu a les seves propietats úniques inherents, que inclouen el gruix a nanoescala, una gran superfície específica, abundants llocs de vora actiu i una alta sensibilitat a les molècules de gas a temperatures més baixes i fins i tot a temperatura ambient. La tesi actual intenta augmentar la fabricació d'aquests materials en capes 2D de nova generació i utilitzar-los per a aplicacions de detecció de gasos en aquest camp d'estudi. A més, els materials de detecció de gasos investigats en aquesta tesi tenen el potencial d'abordar l'esmentat anteriorment en la seva forma prístina o després d'alguna funcionalització. En aquest sentit, aquesta tesi proposa sensors de gas quimioresistius basats en diversos materials TMD.En el proceso de monitoreo industrial, el control de emisiones de automóviles, la seguridad de la calidad del aire interior y exterior y la protección del medio ambiente, la detección continua y confiable de varios gases es fundamental. Los óxidos de metales semiconductores, los materiales más utilizados en aplicaciones de detección de gases, tienen limitaciones sustanciales, como un alto consumo de energía, poca estabilidad a largo plazo, selectividad limitada y, sobre todo, alta sensibilidad cruzada a la humedad. Los nuevos materiales que permiten el funcionamiento a baja temperatura podrían resolver los problemas relacionados con la energía, lo que daría como resultado redes de sensores mejores y más fiables. Como resultado, los materiales 2D como los dicalcogenuros de metales de transición (TMD) han surgido como opciones viables para la detección de gases. Estos materiales de próxima generación tienen el potencial de mejorar las propiedades de detección de los materiales sensibles al gas, como la sensibilidad, la selectividad, la estabilidad y la velocidad (tiempo de respuesta-recuperación). Esto se debe a sus propiedades únicas inherentes, que incluyen espesor a nanoescala, gran área de superficie específica, abundantes sitios de borde activos y alta sensibilidad a las moléculas de gas a temperaturas más bajas e incluso a temperatura ambiente. La tesis actual intenta ampliar la fabricación de estos materiales en capas 2D de próxima generación y utilizarlos para aplicaciones de detección de gases en este campo de estudio. Además, los materiales de detección de gases investigados en esta tesis tienen el potencial de abordar lo mencionado anteriormente, ya sea en su forma original o después de alguna funcionalización. En este sentido, esta tesis propone sensores de gas quimiorresistivos basados en varios materiales TMDs.In the industrial monitoring process, car emission control, indoor and outdoor air quality safety, and environmental protection, continuous and reliable detection of various gases is critical. Semiconducting metal oxides, the most extensively used materials in gas sensing applications, have substantial limitations such as high power consumption, poor long-term stability, limited selectivity, and, most notably, high humidity cross-sensitivity. Novel materials that allow for low-temperature operation might solve power-related issues, resulting in better and more reliable sensor networks. As a result, 2D materials like transition-metal dichalcogenides (TMDs) have emerged as viable options for gas sensing. These next-generation materials have the potential to improve gas-sensitive materials' sensing properties such as sensitivity, selectivity, stability, and speed (response-recovery time).This is owing to their inherent unique properties, which include nanoscale thickness, large specific surface area, abundant active edge sites, and high sensitivity to gas molecules at lower temperatures and even at room temperature. The current thesis attempts to scale up the fabrication of these next-generation 2D layered materials and utilise them for gas sensing applications in this field of study. Furthermore, the gas sensing materials investigated in this thesis have the potential to address the aforementioned either in their pristine form or after some functionalization. In this regard, this thesis proposes chemoresistive gas sensors based on several TMDs materials

Multilayer Thin Films

This book, "Multilayer Thin Films-Versatile Applications for Materials Engineering", includes thirteen chapters related to the preparations, characterizations, and applications in the modern research of materials engineering. The evaluation of nanomaterials in the form of different shapes, sizes, and volumes needed for utilization in different kinds of gadgets and devices. Since the recently developed two-dimensional carbon materials are proving to be immensely important for new configurations in the miniature scale in the modern technology, it is imperative to innovate various atomic and molecular arrangements for the modifications of structural properties. Of late, graphene and graphene-related derivatives have been proven as the most versatile two-dimensional nanomaterials with superb mechanical, electrical, electronic, optical, and magnetic properties. To understand the in-depth technology, an effort has been made to explain the basics of nano dimensional materials. The importance of nano particles in various aspects of nano technology is clearly indicated. There is more than one chapter describing the use of nanomaterials as sensors. In this volume, an effort has been made to clarify the use of such materials from non-conductor to highly conducting species. It is expected that this book will be useful to the postgraduate and research students as this is a multidisciplinary subject

Sensitivity Enhancement of Metal-Oxide Chemical Sensors for Detection of Volatile Organic Compounds

Metal-oxide chemical sensor technology has been praised as a cheap and efficient method of detecting both reducing and oxidizing gases depending on the metal-oxide’s carrier type. The research conducted in this thesis explored methods of enhancing the sensitivity of an n-type metal-oxide material (indium tin oxide, ITO) to a volatile organic compound (VOC) through changes in both device and testing characteristics. Two methods of testing prototype sensors were developed which consisted of short and long-term exposure to ethanol at different temperatures and concentrations. Maximum sensitivity at 2000 ppm was achieved in devices with thin, annealed metal-oxide layers with a high temperature of operation; this sensitivity measurement was achieved using a prolonged exposure test with 100-nm of annealed ITO at an operating temperature of 360°C and yielded a sensitivity of 32.5%. A fabrication process consisting of two lift-off processes for the metal-oxide and contact metal was developed to create the prototype devices. Preliminary characterization on the metal-oxide materials confirmed its thickness, crystallinity / crystal structure, and grain size. In addition to the electrical tests, a future work chemical sensor was thermally and electrically simulated using SolidWorks and Silvaco Atlas, respectively; a proposed fabrication process of the device is also presented, along with a basic outline of future work experiments to further study sensitivity enhancements through other metal-oxide materials, noble catalytic metals, device architecture, and signal processing of proposed electrical testing