NANOCOMPOSITES OF 2D NANOMATERIALS AND METAL OXIDES FOR MULTIFUNCTIONAL APPLICATIONS

Abstract

With the progression of science and technology, air pollution has emerged as a significant concern for society. The emission of hazardous gases and volatile organic compounds (VOCs) into the environment not only poses a threat to atmospheric conditions but also poses risks to human health. VOCs are organic substances capable of vaporizing rapidly and dispersing into the air, even at room temperature, due to their low boiling points. According to the World Health Organization (WHO), air pollution stands as the primary cause of premature death and various illnesses. Consequently, it becomes imperative to detect and regulate the release of these pollutants to mitigate their adverse effects on both the environment and human health. Ethanol a common VOC, is deeply ingrained in our daily lives. However, it's important to acknowledge that prolonged exposure to ethanol can pose serious risks to human health, including irritation of the nose and throat, vomiting, kidney failure, nausea, headaches, and damage to the central nervous system. In extreme cases, it has even been linked to cancer. Additionally, as a flammable gas with an explosion range of 3.3 – 19%, ethanol contributes to numerous traffic accidents. Its vapor can form explosive mixtures when combined with other gases, further exacerbating safety concerns. Given these risks, addressing the immediate and potential dangers associated with ethanol exposure has become a paramount concern. Hence, it is crucial to implement timely monitoring and detection systems for ethanol gas, particularly at room temperature, as an integral component of safety measures. With this aim, a room temperature chemiresistive ethanol gas sensor based on hydrothermally synthesized zinc oxide (ZnO) incorporated-molybdenum diselenide (MoSe2) nanosheets has been investigated. The sensing properties of the MoSe2/ZnO nanocomposite sensor have been investigated systematically by exposing the sensor to various ethanol gas concentrations (10- 500 ppm) in dry N2 and dry air. The synergistic Ph.D. Thesis (Nikita Jain) vii effect due to the incorporation of ZnO nanorods in MoSe2 nanosheets has been found to enhance the sensor response to ethanol gas (when operated in dry N2) with improved response and recovery time of 8.4 and 14.7 seconds respectively, high selectivity, stability, and reproducibility. The nanocomposite-based sensor has shown a high gas sensing response (Rg/Ra) of 37.8 to 500 ppm of ethanol gas in dry N2. While the response of the nanocomposite-based sensor has decreased to 15.3 to 500 ppm of ethanol gas in dry air which suggests that the sensor has performed better when operated in dry N2 than in dry air. The sensor has demonstrated a p-type characteristic response. Importantly, the sensor has operated at RT and has been able to detect ethanol down to 10 ppm. Besides, the sensor has also established prolific long-term stability of 4 weeks. The sensor has exhibited improved response (8.4 s) and recovery (14.7 s) time to 500 ppm ethanol gas compared to previously reported values. The enhancement in performance of the sensor has been due to the formation of a p-n junction at the interface of the MoSe2/ZnO nanocomposite sensor. Furthermore, potential barrier modulation at the interface has provided a positive effect on sensitivity performance. After studying the enhanced performance of the MoSe2/ZnO nanocomposite sensor due to the formation of p-n heterojunction at the interface of MoSe2 nanosheets and ZnO nanorods, we have explored a ternary nanocomposite of MoSe2-ZnO heterojunctions decorated rGO (MoSe2/ZnO/rGO). In this work a highly stable, exceptionally selective, and reliably repeatable ethanol gas sensing device has been successfully developed using the ternary MoSe2/ZnO/rGO nanocomposite, promising long-term stability. Importantly, the ternary nanocomposite sensing device has exhibited a fantabulous sensing response of 50.2 to 500 ppm ethanol gas. The ternary nanocomposite sensing device has been able to detect ethanol down to 1 ppm at room temperature. The developed ternary nanocomposite sensing device has shown a considerably fast response time (6.2 s) and recovery time (12.9 s) to 500 ppm ethanol gas. Besides, the sensing device also establishes prolific long-term stability of 6 weeks. The superior performance of the developed ternary nanocomposite sensing device has been owed to the formation of a p-n heterojunction between MoSe2 nanosheets and ZnO nanorods with the aid of rGO nanosheets. The attachment of the Ph.D. Thesis (Nikita Jain) viii MoSe2 nanosheets and ZnO nanorods onto rGO nanosheets has not only provided various p-n heterojunctions but has also offered more active sites for the adsorption and desorption of ethanol molecules. This has improved the gas-sensing response of the sensing device toward ethanol significantly. Additionally, the conductive network of rGO nanosheets has facilitated fast electron transfer between MoSe2 nanosheets and ZnO nanorods, endowing the ternary nanocomposite sensing device with quick response and recovery time. The widespread adoption of Internet of Things (IoT) technology has created an increasing demand for reliable gas sensor networks across a variety of applications, including air pollution monitoring, industrial safety, smart cities, and personal healthcare. In this context, we have investigated self-powered devices that seamlessly integrate gas sensing capabilities with energy generation. This exploration aims to enhance the efficiency and sustainability of sensor networks, ensuring they meet the growing demands of modern applications. In this study, a novel self-powered ethanol gas sensor, exhibiting excellent selectivity, sensitivity, and stability, has been developed based on n-type SnS nanoflakes at room temperature. The n-type SnS- based HEC serves as the power source for self-powered ethanol gas sensors, enabling the detection of various concentrations of ethanol gas at RT. Consequently, the power supply and gas sensor have been effectively combined into a single device, demonstrating a successful integration of both functionalities. X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), high-resolution transmission electron microscopy (HRTEM), Energy dispersive X-ray (EDX) and Brunauer-Emmett-Teller (BET) analysis have confirmed the formation of orthorhombic SnS nanoflakes with a high specific surface area (6.15 m2 g-1). The observed voltage-current (V-I) characteristic curves of the HEC at RT have shown a maximum current (Imax) of 40 μA and voltage of 1.03 V. The sensing performance of the self-powered ethanol gas sensor has been analysed for various concentrations of ethanol gas (10 - 100 ppm). The sensor has exhibited a response value (Ra/Rg) of 41.3 to 100 ppm ethanol gas concentration, with quick response/recovery times of 27.3 s/31.4 s respectively at RT. The sensor has shown promising potential for prolonged ethanol gas detection (30 days). The experimental results have demonstrated that the Ph.D. Thesis (Nikita Jain) ix n-type SnS-based self-powered ethanol gas sensor represents a promising platform for integration into future large-scale IoT systems. This breakthrough paves the way for more versatile and scalable IoT solutions, enabling continuous environmental monitoring and data collection across various settings.