SYNTHESIS AND CHARACTERIZATION OF 2D MATERIALS FOR SENSING APPLICATION

Abstract

Rapid industrialization and urbanization are causing harm to the environment and human health owing to the emission of harmful gases. The fast detection of the emission of such harmful gases and the emitted concentration has become an important task that has brought high-performance gas sensors into huge demand. Most gas sensors work efficiently at higher operating temperatures, which also require higher power consumption and degrade the sensor quality after some time. However, the majority of the existing gas sensors suffer from poor selectivity and low portability. Advances in low-powered gas sensors, wireless sensors, and miniaturization of gas sensing technology have attracted the attention of the digital world. Over the last decade, researchers have been working on many new materials and have discovered several new materials for developing efficient gas sensors. Metal oxides have been the most widely used materials for commercial gas sensing, but high-temperature operation has been the major setback in their commercial applications. In addition, two dimensional (2D) materials have emerged as new and improved sensing materials owing to their inherited chemical, physical, and electronic properties. 2D materials especially transition metal dichalcogenides (TMDCs) based gas sensors are currently gaining considerable attention because of their longer environmental stability and ambient conditions; however, the low limit of detection and longer response and recovery times are still issues. To overcome these issues, when a 2D material is coupled with another 2D material such as a metal oxide or metal, the combined effects of these two materials appear. This combined effect demonstrates the possibility of improving the sensing performance of gas sensors by forming hybrid nanostructures. The present thesis involves the synthesis of nanocomposites based on 2D materials with metal oxides and other dimensional materials, and subsequent investigations have been performed on their utilization in gas detection. For this purpose, a hydrothermal method was primarily used to synthesize large-area, few layered nanostructures and to synthesize the nanocomposites thermal vapor deposited was also employed. These materials mainly belong to distinct families of 2D materials, namely transition metal dichalcogenide (MoS2), and transition metal oxides (MoO3 and SnO2). Active carbon black (CB) was synthesized using chemical vapor deposition (CVD). MoS2 with different nanocomposites was synthesized, and the effect of SnO2, MoO3, and MoS2- vii based composites, on the sensing performance of the fabricated gas sensors was investigated to achieve highly selective sensors at ambient conditions for NO2 and NH3 gases. To begin with, a mixed phase of 1T/2H-MoS2 has been synthesized with varying concentrations of Mo precursor in order of 1 M, 1.4 M, and 1.8 M using NH4+ intercalation by keeping the other experimental conditions the same. out of all the prepared nanostructures, 1.8M-1T/2H-MoS2 showed the maximum 1T character, indicating a higher amount of metallic character with a 1.5 eV bandgap. This enhancement in the 1T character and decrease in the bandgap value were achieved by increasing the concentration of NH4+ ions in the solution. The enhanced metallic character was also responsible for the enhanced optical absorption, indicating a direct excitonic transition from the valence band to the conduction band. Because 1T-MoS2 is not environmentally stable, the synthesis of mixed 1T/2H-MoS2 with a higher 1T character without any external aid is a good result. The variation in the morphology corresponding to the mixed phase also increased the surface-to-volume ratio and available active sites, which could be suitable for gas sensing. After studying the properties of the mixed phase of MoS2, the effect of SnO2 nanoparticles on the gas-sensing properties of MoS2 nanosheets was studied. Then to further improve the sensing performance, the active carbon black was also incorporated in SnO2/MoS2 and the output performance of the device was determined. To determine the optimum concentration of the SnO2, we have synthesized the different nanostructures with varying weight percentages of SnO2 (0.6 %, 0.8%, and 1%). Based on microstructural and electronic properties, MoS2/SnO2 with 0.8 % concentration was chosen as suitable for studying gas sensing performance. Further, different devices based on bare MoS2, SnO2/MoS2, and CB/SnO2/MoS2 were fabricated and their gas-sensing properties for NO2 around room temperature were evaluated. The gas sensing response of MoS2 for NO2 gas was found to be increased due to the incorporation of SnO2. Further, the ternary heterostructure interface of CB/SnO2/MoS2 also showed an increased and effective adsorption of NO2 gas molecules. The ternary hetero-interface of the CB/SnO2/MoS2 sensor showed a maximum sensing response of around 46 % for 100 ppm of NO2 gas, which is higher than the binary heterojunction of SnO2/MoS2 (43%) and bare MoS2 (42 %). Along with better sensor response, the response (26 s) and recovery times (73 s) are also faster than SnO2/MoS2 heterostructure and bare MoS2. This enhanced sensing performance of CB/SnO2/MoS2 is viii dedicated to the p-n heterojunctions and Schottky barriers generation at the interface, which has been justified by the electrical measurements. To further determine the efficiency of MoS2-based gas sensors with other oxides, MoS2/MoO3 heterostructures were synthesized with different reducing agents using the hydrothermal method and followed by thermal annealing in an Ar environment at 500 ˚C for 1h. The hydrothermal synthesis at 200 ˚C resulted in MoOxSy, which was confirmed by XRD and then thermal annealing was performed at 300˚ and 500 ˚C. The better crystallinity and identified phases were obtained at 500 ˚C and used further for gas sensing studies. In this work, the gas sensing properties of different MoS2:MoO3-based gas sensors have been studied and it turned out that MoS2:MoO3 based sensors showed dual detection for NO2 and NH3 gases. Here, three different heterostructures were prepared using hydrazine hydrate (HH-MoS2:MoO3), L-ascorbic acid (LA-MoS2:MoO3), and without reducing agent (MoS2:MoO3). The MoS2:MoO3-based sensor showed n-type sensing behavior dominated by MoO3 charge carriers for NH3 gas, whereas the same sensor showed p-type sensing behavior on exposure to NO2 gas. The HH-MoS2:MoO3 showed higher adsorption sites and more active sites resulting in a higher response for NH3 and NO2 gases but the incomplete recovery and longer recovery time proved to be the major drawbacks for this sensor. Besides that, MoS2:MoO3 showed a good sensing response for 5 ppm concentration of NO2 (36 %) and NH3 (52.3 %) gas with complete recovery and good response time. More importantly, one sensing surface has contributed to the selective detection of NO2 and NH3 at 50˚C with different sensing behavior. Interestingly, different conducting channels and adsorption sites play an important role in opposite sensing behavior, possibly due to the synergistic effects at the heterojunction interface. Here, in this work, the effect of different reducing agents on various MoS2:MoO3 based sensors has resulted in the availability of different adsorption sites and different morphology. It has been proved that HH-MoS2:MoO3 heterostructures are more prone to oxidation and clearly show higher response, but rapid oxidation may be the major cause of incomplete recovery. In contrast, MoS2:MoO3 contained the least sulfur vacancies on the surface, which played an important role in the complete recovery.