Two-dimensional Intermediate Metal Oxysulfides for Room-temperature Gas Sensors

As low-dimensional material, two-dimensional (2D) semiconductor materials have garnered significant attention in scientific research and technological applications compared to other dimensionalities such as zero dimensional (0D), one dimensional (1D) and three dimensional (3D) nanostructures. Given high surface-to-volume ratio, 2D materials exhibit significantly enhanced surface reactivity, leading to substantial improvements in adsorption/desorption processes and other distinctive physiochemical properties. These characteristics are precisely tailored for gas sensing applications, making 2D materials a natural choice of various gas sensors. Metal oxides, such as SnO2, ZnO, TiO2 and WO3, as one of the most successful material groups of gas sensing, have been commonly used in chemiresistive sensors due to their outstanding long-term stability in ambient environments. However, most metal oxide-based sensors require a microheater to maintain an elevated operating temperature, stemming from their chemisorption mechanisms, which casts severe limitation on their industrial potential. Many explorations of 2D materials in room temperature gas sensors have been conducted in the last decade. For example, certain 2D metal chalcogenides exhibit room temperature gas sensing responses with the assistance of other external stimuli like ultraviolet (UV) lamp or bias voltage. Nevertheless, these external stimuli also introduce practical challenges to the sensors in terms of power-consumption and implementation complexity. On the other hand, metal sulfides, as a pivot subgroup of metal chalcogenides, are found to possess unique electrical and optical properties when it is downsized to 2D form. Given a low power visible light excitation, the metal sulfides generate substantial electron-hole pairs, leading to a dense re-distribution of free carrier on the material surfaces. With prolonged exciton radiative combination lifetime, the photo-induced free carriers are allowed to participate in physisorption-based gas-matter interactions forming electron dipoles between material surfaces and adsorbed analyte gas molecules, resulting in electrical resistance variances corresponding to the concentration changes of testing gas species at room temperature. Although metal sulfides are a promising material group for room temperature gas sensors, their sensing performance can rapidly degrade over time because of their poor stability in ambient air. Recognizing the strengths and weakness of both, an intermediate class of material between metal oxides and metal sulfides, metal oxysulfides emerges as an ideal material for a new generation of room temperature sensing materials. After a comprehensive literature review, the author realized that the emerging metal oxysulfides possess unique characteristics of both metal oxides and metal sulfides. Through a partial replacement of surface sulfur elements to oxygen elements under a controllable condition, the original sulfide phases of metal sulfide will transfer to oxysulfide crystal structures to obtain the ambient stability of metal oxides, with the intrinsically strong electro-optical properties retained. Such a material group exhibits a huge potential in the practical room temperature gas sensors based on physisorption mechanisms. However, the investigation of the metal oxysulfide is still in its early stages. An extensive understanding of the physicochemical and electro-optical properties of metal oxysulfides remains to be explored. Thus, deepening our knowledge of metal oxysulfides and their gas sensing properties presents both challenges and novel research opportunities in this emerging field. In this dissertation, zinc sulfide (ZnS) was initially chosen to showcase the potential of metal oxysulfides, transitioning from metal sulfides, as a substitute for the traditional high-temperature and energy-intensive UV lamps in gas sensing. Most existing research demonstrates that to achieve room-temperature gas sensing with ZnS, elaborate structural adjustments like doping, composites, and the addition of external stimuli (e.g., UV lamps) are needed. The transition to zinc oxysulfide presents a more straightforward alternative. This was accomplished through a hydrothermal method using ZnS, followed by calcination at 600°C, rendering it into a metastable state conducive for gas sensing. However, its gas sensing capability for nitrogen dioxide (NO2) did not return to the baseline without the assistance of external visible light. Notably, under blue light exposure, there was a significant increase in photo-induced free carriers, which became pivotal in the adsorption and desorption processes of NO2. The author then moved on to test the hypothesis of synthesizing metal oxysulfides using cobalt sulfide (CoS) as a representative material. The CoS was prepared through the chemical precipitation method and subsequently subjected to calcination. It was found that a calcination temperature of 600°C was optimal for inducing a transformation of CoS into its metastable state, producing cobalt oxysulfide. Material characterization highlighted that cobalt oxysulfide exhibited a unique crystal morphology characterized by a micro-cage structure which self-assembled from hexagonal sheets. Significantly, this newly achieved metastable state introduced novel room-temperature gas sensing capabilities not previously observed with pure CoS. The cobalt oxysulfide demonstrated superior performance in detecting hydrogen (H2) gas in room-temperature, with remarkable selectivity against various hazardous interfering gases. These findings validate the feasibility of synthesizing metal oxysulfides from metal dichalcogenides and underscore their potential as cutting-edge materials for high-performance gas sensing applications. Finally, the author extended the approach to nickel sulfide (NiS) to validate the initial method’s broader applicability. It was especially intriguing to uncover potential gas sensing properties in nickel oxysulfide, given that pure NiS exhibits no such capabilities. NiS was synthesized via the hydrothermal method and then calcined at an optimized temperature of 600°C, transforming it into its metastable nickel oxysulfide state. Material characterization revealed a distinctive hierarchical microstructure, notably the formation of self-assembled 2D micro-flower nanoflake structures in its crystal morphology. Further analysis validated the oxysulfide’s metastable state and confirmed its elemental composition after calcination. Remarkably, the synthesized nickel oxysulfide showed high sensitivity to H2 at room temperature, while it remained unresponsive to prevalent industrial gases. In summary, this research has pioneered the synthesis and application of 2D metal oxysulfides derived from metal sulfides, revealing their potential as superior materials for room-temperature gas sensing with external stimuli dependency. Through systematic exploration of zinc, cobalt, and nickel sulfides, the author underscored the transformative effects of calcination into their oxysulfide variant which introduces novel and enhanced gas sensing properties. The findings not only advance the understanding of metal oxysulfide materials but also pave the way for the development new class of cutting-edge, physisorption based room-temperature gas sensing materials suitable for a range of applications.