DEVELOPMENT OF 2D MOLYBDENUM DISULFIDE BASED NO2 GAS SENSOR

Abstract

The rapid increase in environmental pollution and industrial activities has led to a growing demand for efficient, reliable, and cost-effective gas sensing technologies. Toxic gases such as nitrogen dioxide (NO₂) and hydrogen (H₂) pose significant risks to human health, environmental safety, and industrial operations. In this context, the present thesis focuses on the development and investigation of nanostructured molybdenum disulfide (MoS₂) thin films for gas sensing applications, with particular emphasis on understanding their structural properties and sensing behavior toward NO₂ and H₂ gases. The research begins with the synthesis of MoS₂ thin films using a combination of electron beam evaporation and chemical vapor deposition (CVD) techniques. Molybdenum (Mo) thin films of varying thicknesses were initially deposited on suitable substrates and subsequently sulfurized under controlled conditions to obtain high-quality MoS₂ layers. Systematic optimization of synthesis parameters such as film thickness, annealing temperature, and sulfurization temperature was carried out to achieve uniform, crystalline, and reproducible thin films. Among the different samples studied, the 20 nm thick MoS₂ films exhibited superior crystallinity, structural stability, and surface uniformity, making them suitable for gas sensing applications. Comprehensive structural and morphological characterization was performed using X ray diffraction (XRD), Raman spectroscopy, field emission scanning electron microscopy (FESEM), and atomic force microscopy (AFM). The XRD analysis confirmed the formation of polycrystalline hexagonal-phase MoS₂, while Raman studies validated the presence of characteristic vibrational modes corresponding to in plane and out-of-plane lattice vibrations. Surface morphology analysis revealed the formation of nanosheet and flower-like structures, providing a high surface-to-volume ratio and abundant active sites for gas adsorption. A significant contribution of this work is the investigation of MoS₂–hydrogen interaction using in-situ X-ray diffraction. This approach enabled real-time monitoring of structural changes in MoS₂ under controlled hydrogen gas environments, eliminating sample-to-sample variations. The study revealed that exposure to hydrogen gas leads to noticeable changes in diffraction peak intensity, indicating structural modifications and possible lattice strain effects. These findings provide deeper insights into the interaction mechanisms between hydrogen molecules and layered MoS₂ structures. Furthermore, the gas sensing performance of MoS₂ thin films toward NO₂ gas was systematically investigated as a function of operating temperature and gas concentration. The fabricated sensor exhibited n-type semiconducting behavior, with an increase in electrical resistance upon exposure to the oxidizing NO₂ gas due to electron withdrawal. The sensing response improved significantly with increasing temperature, achieving optimal performance at 150 °C. The sensor demonstrated a RAMESH KUMAR ix maximum response of approximately 14.2% at 20 ppm concentration, along with a response time of about 102 seconds and a recovery time of 94 seconds. Additionally, the sensor response increased with increasing NO₂ concentration, indicating strong adsorption and efficient charge transfer processes. Despite the promising performance, certain limitations such as reduced recovery at higher gas concentrations and potential structural defects were observed. These findings highlight the need for further improvements in selectivity, stability, and environmental adaptability of MoS₂-based sensors. Overall, this thesis establishes a strong correlation between synthesis conditions, structural properties, and gas sensing performance of MoS₂ thin films. The integration of advanced synthesis techniques with in-situ characterization provides a comprehensive understanding of gas–material interactions. The results demonstrate that nanostructured MoS₂ is a highly promising material for next-generation gas sensors, with potential applications in environmental monitoring, industrial safety, and clean energy systems.