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

Aanchal Alagh

• español

  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.

• English

  Objective 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. The ability to manipulate the electrical structure of TMD semiconductors is a fundamental key aspect of their practical use in electronics and semiconductors. Furthermore, 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. Furthermore, the 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.

  Methodology and Conclusions While the literature describes several methodologies for fabricating these 2D layered materials, achieving an edge-enriched morphology is crucial for gas sensing applications. Therefore, the present doctoral thesis is dedicated to the development of these 2D layered materials to obtain such a morphology to implant them as chemoresistive gas sensors. In this regard, a facile synthesis route is developed via a combination of the aerosol-assisted chemical vapour deposition (AA-CVD) method with/without H2-free atmospheric pressure chemical vapour deposition (APCVD) to develop 3D assemblies of TMDs materials, such as WS2, MoS2, WSe2, WS2/Cu2O, WS2/PtO and WS2/PdO. Moreover, the developed CVD growth process demonstrates a simple and efficient route to synthesize large-area multi-layered TMDs materials, which have strong prospects for scalable production. Furthermore, all these grown materials indicate their potential for next-generation gas sensors. In particular, our results indicate that multi-layered transition metal dichalcogenides have high sensitivity towards NO2 molecule detection due to the enhanced charge transfer.

  In the line of research, the research conducted also discusses the impact of two different morphologies of WS2 (nanotriangles and nanoflakes) on the gas sensing properties, achieved by adopting a combination of AACVD and APCVD techniques. The results demonstrate that the final morphology of WS2 films depends mainly on that of pre-deposited WO3 layers. From the gas sensing point of view, the highest sensitivity was recorded for the WS2 NT sensor, with an unprecedented ultra-low detection limit below 5 ppb at 150°C. Additionally, this material has demonstrated its ability to detect 800 ppb of NO2 even when operated at room temperature (25 °C). The high sensitivity and the unprecedentedly low limit of detection achieved (in the ppt range), were attributed to the porous surface and the increased number of sulfur edges in WS2 nano triangles (NTs), which were created by the random 3D assembly of WS2 nanosheets on WS2 nanoneedles. Furthermore, WS2 NTs have shown excellent response stability during long-term stability tests conducted over 8 months. It is worth noting that the assembly of WS2 nanotriangles on 1D nanoneedles or nanorods shows a highly increased porosity and increased number of edges for gas interaction in comparison to the more closely packed nanoflake assembly. Hence, these results shed light on the important role played by the morphology in enhancing sensor performance.

  Moreover, numerous reports in the literature suggest enhancing the gas sensing characteristics of TMDs by modifying their surface chemistry, either by noble metal decoration or functionalization with other materials, for instance, decoration with metal oxides. In this regard, one aspect of this thesis focuses on improving the selectivity of WS2 NTs sensors for the detection of hydrogen sulphide gas. This gas is of particular interest as it poses serious threats to humans. Hence, a chemoresistive sensor was fabricated by using a hybrid material consisting of copper-oxide nanoparticles-decorated multi-layered tungsten disulfide nanostructures (Cu2O/WS2). The gas-sensing studies performed show that this hybrid nanomaterial has an excellent sensitivity towards hydrogen sulphide (11-times increase in response compared to that of pristine WS2 sensor from 54% to 610%.) at moderate temperature (150 °C). Additionally, functionalization of pristine WS2 sensor with Cu2O nanoparticles further enhances the gas sensing performance towards the targeted gas even at room temperature (13-times increase in response compared with that of pristine WS2sensor). Moreover, results obtained from humidity cross-sensitivity of Cu2O/WS2 sensor indicate superior gas sensing response (with a negligible decrease in response) as compared to pristine WS2 sensor, when the ambient humidity is increased to 50%, which is rarely found in metal oxide-based sensors. Moreover, the fabricated sensor shows excellent stability over 7 months with only a 16% fall-off in its sensing response from 610% to 510%.

  To further enhance the sensitivity of 2D layered TMDs materials at room temperature, a unique and efficient route to add functionality to the metal sulphides is also explored. Pristine WS2 was loaded with different metal oxides and the gas sensing properties of metal oxides (such as with palladium oxide and platinum oxide nanoparticles) loaded WS2 have been explored in the following study. The sensing results revealed that loading of WS2 nanosheets with either PtO or PdO ions significantly increases their gas response towards NO2 at room temperature (i.e., double the response as compared to pristine WS2 sensor from 10% to 26.5%, thereby lowering the detection limit of the sensor (lower than 25 ppb after loading, which is far lower than that of pristine WS2 gas sensor), at room temperature. Moreover, this study takes advantage of an appealing method for adding functionality to the transition metal dichalcogenide host matrix. However, this method is not limited to only these materials and other materials can be incorporated into the host matrix by adopting this 2-step growth process which takes advantage of a combination of AACVD with the CVD technique.

  To elucidate the gas sensing properties of other TMDs material, tungsten diselenide (WSe2), a tungsten-based dichalcogenide with close similarities to WS2 was investigated and the corresponding results are presented in this thesis. This material was grown in the Namur Institute of Structured Matter (NISM), University of Namur, in the lab of Prof. Jean François Colomer (collaborator). It was found that for growing selenium based TMDs, it is necessary to incorporate H2 in the CVD chamber for the reaction to take place. This is because, unlike Sulfur, Selenium is not a very strong reductant. Therefore, WSe2 nanomaterial was grown via selenization of WO3 nanomaterial in the presence of H2 and Ar gas during the growth process. It was found that using this technique vertically aligned 2H-WSe2 nanosheets assembled on nanowires were obtained. The integration of as-synthesized, 3D WSe2 with unique structural arrangements resulted in exceptional gas sensing characteristics with dual selectivity towards NH3 and NO2 gas. Additionally, the selectivity of the sensor can be tuned by selecting its operating temperature (150°C for NH3 and 100°C for NO2). The cross-sensitivity test revealed that NO2, H2, C6H6, and CO have a negligi

• català

  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.