Two-dimensional Metal Sulfide and Metal Oxysulfide for Physisorption-driven Micro-gas Sensor

Given the unique physiochemical properties of two-dimensional (2D) materials such as large surface-to-volume ratios, high surface activities, and facile surface functionalization, emerging micro-gas sensors based on ultra-thin functional materials have been developed in recent years. However, to achieve complete reversibility and reasonable sensitivity, these sensors mostly require external excitation sources such as heating and ultraviolet light illumination. The reason for this can be attributed to the fact that a significant amount of research has been centered around commonly observed 2D metal oxides and metal sulfides, which have served as the key sensitive materials for most commercially available gas sensors. These materials come with limitations stemming from their sensing mechanisms. For example, most metal oxides require elevated temperatures for sensor activation due to their inherent chemisorption characteristics. Additionally, sensors based on metal sulfides encounter issues related to the degradation of sensing performance over the long term due to oxidation effects. Therefore, the investigation of new types of sensing materials based on the physisorption mechanism is promising for the next generation of gas sensors. The physisorption-based sensor has been receiving increasing attention in recent years, because of its unique selectivity and more importantly the capability to operate at low temperatures or room temperature. Unlike chemisorption, materials based on physisorption exhibit characteristics such as weak bond attraction to the target gas, a unique charge transfer mechanism, and reversibility of the materials without chemical structure changes. This implies the potential to achieve sensing responses at low temperatures. Furthermore, the study of enhancing the sensing substrate has not been a central focus within this field. However, as research into low-dimension nanomaterials has progressed, there is an obvious demand for improving the sensor substrate to enhance its compatibility with these materials. The improvement in this direction holds potential benefits for the exploration of future sensing applications. After a comprehensive literature review, the PhD candidate realized that metal oxysulfides have a huge potential for room temperature gas sensing purposes due to the benefit of oxygen replacement in metal sulfide that not only retains sulfur element but also presents the physisorption property. The study of metal oxysulfide-based gas sensors is still in the early stage. Therefore, an understanding of the mechanism, synthesis method, sensing performance, and the application of these types of materials can be a critical challenge and a research gap. Furthermore, by studying the improvement of the sensing substrate, it becomes feasible to improve practical applications by enhancing material compatibility with the sensing substrate. This, in turn, paves the way for further investigation spanning sensor fabrication and material synthesis.The PhD candidate of this thesis will study the methodology of the metal oxysulfides-based gas sensor as well as its synthesis method, characterization, and optical and gaseous properties in comparison with metal sulfides. Moreover, the PhD candidate also investigated the fabrication and improvement of the interdigital transducer (IDT) type sensor as the application sensing substrate for the selecting materials. The research began from the fabrication of IDT with undercut generation of negative photoresist Az nlof 2020 for fence effect reduction, which allows the sensor substrate compatible with the deposited material for higher temperature physical vapor deposition (PVD) temperature and narrow fingers of the sensor electrodes. Besides that, the study of modified indium sulfide (In2S3) for room temperature gas sensing was investigated by nitrogen dioxide (NO2) dynamic test by using the drop casting method to apply the material on the sensing substrate. The PhD candidate also employed Raman spectroscopy and Ultraviolet–visible spectroscopy (UV-vis) for the materials characterization of its vibration analysis, absorption property analysis, and bandgap analysis. In addition, X-ray photoelectron spectroscopy (XPS) and X-Ray diffraction analysis (XRD) are also applied for the proof of the material. According to the unique optical property of the In2S3, its modified material has the potential for room-temperature gas sensing. An aging and argon (Ar) annealing method is applied, both providing the sensing performance improvement under visible light illumination at room temperature. This leads to the study of the metal oxysulfides in the following chapters.After investigating the aging effect of In2S3, the PhD candidate hypothesized that the 2D metal oxysulfides, induced by the partial sulfur replacement with oxygen in metal sulfide, can be considered suitable sensitive materials owing to their excellent long-term stability and complete reversibility. However, the correlation of oxygen-to-sulfur content with gas sensing performances has yet to be known, hindering the optimization process of such an emerging NO2 gas sensing approach. In this work, we realize 2D ultrathin indium oxysulfide by combing annealing of bulk In2S3 crystals and subsequently liquid phase exfoliation (LPE), while the degree of sulfur replacement with oxygen is controlled by the initial annealing temperature. At an optimum annealing temperature of 800 °C, the sample exhibits an optical bandgap of 2.19 eV and a photoexcited carrier lifetime of 272.82 ns. As a result, it demonstrates a response factor of 7.26 for 126 ppb NO2 under blue light illumination at room temperature, while complete reversibility, high selectivity, and excellent long-term stability are also achieved. The sensing mechanism is based on physisorption and presents an extremely low limit-of-detection (LOD) of 0.0342 ppb, far beyond reported room-temperature optoelectronic NO2 sensors in this field.In the final stage of this thesis, the PhD candidate investigates the potential of Ar annealing for metal sulfide materials. With the argon annealing for the flower shape copper sulfide (CuS), the material presents a form shift to self-assembly porous by nanostructure stack, due to a low-level oxygen involved in the annealing process, which is characterized by X-ray photoelectron spectroscopy (XPS), X-Ray diffraction analysis (XRD) and energy dispersive X-ray spectroscopy (EDS). Besides, high resolution transmission electron microscopy (HRTEM) and atomic force microscopy (AFM) show that the material presents nano structure around 10-15 nm. Given that, this hole-type metal oxysulfides structure allows the gas molecule to perform physisorption behavior for the better adsorbed on the material surface, and the material also presents p-type and turns into copper oxysulfide. By applying it to the sensor substrate, the sensor presents a high selectivity to NH3, which also performs long-term stability and excellent room temperature without any external source for sensor excitation. This room-temperature copper oxysulfide gas sensing presents a viable solution for efficient and low-cost NH3 gas monitoring.In summary, the PhD candidate successfully demonstrates several achievements in the course of this PhD research, including breakthroughs in the studies of gas-matter interactions for metal oxysulfide materials from transducing substrate fabrication, sensing materials synthesis, and high-performance practical applications. It is expected that the outcomes of this PhD research will contribute to the development of low-cost, efficient, and high-performance room-temperature gas sensing devices.