|dc.description.abstract||The inescapable exhaustion of fossil fuel reserves has led to a deep interest in alternative and renewable energy resources. Hydrogen, as a clean, affordable and environmentally friendly energy carrier, is a potential replacement for fossil fuels. Consequently, the related detection technologies are rapidly growing, and this trend is likely to continue in the future. The superior hydrogen sensing materials combined with the micro/nanofabrication technologies offer new capabilities to develop hydrogen sensors. Palladium and its alloys are proved as one of the most significant hydrogen sensing materials utilized in hydrogen sensors to detect or monitor hydrogen leakage and concentrations. Furthermore, the use of low-dimensional palladium-based hydrogen sensors enables the miniaturization devices to be suitable for hydrogen-based applications, while possessing the advantages of high detection performance, low power consumption and sample measurement circuitries.
The main purpose of this thesis is to investigate the low-dimensional palladium-based hydrogen sensors with superior sensing performance which can be fabricated by a simple, low-cost, and faster fabrication process. First, the detection mechanism of the palladium-based hydrogen sensors and current research interests in palladium and its low-dimensional structures are presented in the literature review. Next, theoretical models were utilized to analyse the hydrogen atom diffusion process in palladium and the equilibrium state of the palladium-hydrogen system. The important parameters were calculated to determine the fundamental relations between thickness, surface area to volume ratio, temperature, and hydrogen partial pressure in the palladium-based hydrogen sensors. Palladium-based hydrogen sensors with different dimensional structures were fabricated and measured to verify the theoretical relation of the parameters. For the two-dimensional palladium-based thin film, a 19.3 nm patterned palladium-yttrium nanosheet showed a superior linear gas response due to the high permeability of the sensing element. The one-dimensional palladium nanowires were fabricated by electrospinning and evaporation systems to simplify the fabrication process of nanowires and reduce the cost of production. The Palladium/ Poly Vinyl Alcohol (Pd/PVA) nanowires were regarded as hydrogen-actuated switches with high sensitivity due to the broken palladium nanowires which appeared during the detection process. Additionally, a novel concept of dimension, called quasi-one-dimensional (1.5D) was generated by a combination of two-dimensional and one-dimensional structures. Owing to the unique surface texture of the paper, the 1.5D palladium microfiber networks were formed on the paper substrate. The larger surface area to volume ratio of the palladium microfiber networks contributed to the faster response time and superior sensitivity whether at room temperature or high-temperature. Moreover, the sensors were suitable for flexible sensing applications without performance degradation. The fabrication of the thinner palladium microfiber networks was presented to determine the thickness effect in performance. A graphite layer was used as a buffer layer to enhance the adhesion of the substrate surface, which in turn formed the continuous palladium microfibers. The excellent linearity, faster response, superior sensitivity, simple fabrication process, low cost, good repeatability and stability of these hydrogen sensors are promising for wide-ranging hydrogen applications.||