Ultrathin two-dimensional oxide and oxysulphide nanomaterials and their potential applications

Over the past decade, two-dimensional (2D) materials have attracted a great deal of attention and have greatly improved. Metal oxysulphides (MOSs) and metal oxides (MOs) are the most suitable 2D nanomaterials among the wide range of 2D nanomaterials due to their stability and unique electronic and optoelectronic properties. Furthermore, the field of 2D materials has largely been devoted to stratified materials with layered crystal structures, as a result of prevailing synthesis methods that leave many nonlayered materials unexplored and potentially suitable for future applications in their 2D forms. However, despite many significant developments, the exfoliation of ultrathin 2D nanomaterials from their nonlayered metal counterparts remains a major technical challenge, thereby limiting their commercial application. Therefore, this PhD research explores the synthesis and fundamental properties of ultrathin 2D nonlayered titanium dioxide (TiO2), palladium sulphate (PdSO4), and silver sulphate (Ag2SO4) mainly via two facile methods: liquid metal approach for MOs and controlled liquid phase exfoliation (LPE) method for MOSs and investigates their properties and potential applications such as optoelectronic chemical sensing. The first objective of this thesis is to exfoliate ultrathin 2D TiO2 nanosheets from the gallium-titanium alloy in an aqueous solution. Gallium liquid metal and gallium-based alloys have low melting points near room temperature and undergo the Cabrera-Mott oxidation process in an ambient environment. The surface oxide composition is dominated by the metal oxide that has the lowest Gibbs free energy, which results in atomically thin 2D metal oxides on the surface. The author of this thesis uses gallium liquid metal as a reaction solvent and incorporates it with a small concentration of titanium metal powder (1 %wt) as the first step of the synthesis process. As a result of successful alloying in an N2 glove box, the surface of the Ga-Ti alloy becomes clear and glossy, indicating the absence of residual solid Ti metal powders. Compared to the base alloy, titanium dioxide has a lower Gibbs energy, thereby dominating the surface oxide composition. The second step is to delaminate the 2D oxide surface of the Ga-Ti alloy by utilizing the gas injection method. As part of this process, compressed air is injected into the liquid alloy and the 2D exfoliated nanosheets are collected in a solvent on top of the metal alloy. The synthesized ultrathin 2D TiO2 nanosheets (rutile phase) have a tetragonal crystal structure and feature nanometre thickness, sub-micro lateral dimensions, and high crystallinity. The 2D TiO2 nanosheets also exhibit a wide bandgap of 3.4 eV and a dielectric constant of 24. Following the successful delamination of ultrathin nonlayered 2D TiO2 nanosheets, the author of this thesis then explores the synthesis of nonlayered PdSO4 using LPE method. In this stage, palladium sulphide (PdS) bulk crystal is used as the initial material. PdS is cleaved into 2D structures by vigorous mechanical agitation under high forces of the probe sonication. Owing to the amount of oxygen dissolved in the liquid solvent (DMF) and utilizing the fact that PdS is unstable material in oxygen environments, sulphur atoms are continuously replaced by the active oxygen species in the solvent forming ultrathin PdSO4 nanosheets. A high-resolution transmission electron microscopy (HRTEM), X-ray powder diffraction (XRD), and X-ray photoelectron spectroscopy (XPS) are utilized to confirm the successful transformation of the obtained 2D nanomaterials. The comprehensive investigations of the material properties reveal that 2D PdSO4 nanosheets possess a narrow bandgap of 1.35 eV and a strong optical absorption across the visible light spectrum, making them a promising candidate for optoelectronic gas sensing applications. Moreover, it is expected that the abundant electron-hole pairs induced by light will be able to replace thermal excitation due to their high optical absorption in the visible spectrum. Gas sensing technologies are essential in leading decision-making and reducing pollution effects as pollution levels increase in many highly industrialized nations. Consequently, 2D PdSO4-based sensors operating at room temperature exhibit excellent performance and highly sensitive chemical sensing under visible light irradiation. Details of the sensor characteristics demonstrate an excellent selectivity and high sensitivity towards nitrogen dioxide (NO2) gas with an estimated limit of detection (LOD) of 1.84 ppb. In the final stage of this thesis, the author extends his investigation into nonlayered metal oxysulphides by exploring the synthesis of 2D Ag2SO4 nanosheets and their applications in optoelectronic gas sensing. The synthesis process is based upon two main strategies. Initially, silver sulphide (Ag2S) crystal powder is considered a stable material in ambient conditions, but it is converted into Ag2SO4 crystal powder through an oxidation process at 600°C for six hours. Following that, the Ag2SO4 is thinned down to ultrathin 2D nanosheets in a liquid medium utilizing LPE technique. The comprehensive studies of material properties indicate that 2D Ag2SO4 nanosheets exhibit an excellent excitation radioactive lifetime of ~2.06 ns and a narrow bandgap of 1.15 eV. The 2D Ag2SO4 nanosheets are thus realized in an optoelectronic gas sensing platform. In the presence of NO2, dipoles form on the surface of the 2D Ag2SO4 nanosheets due to the decomposition of NO2 and are enhanced under light illumination, which increases the sensor's performance. The synthesized 2D Ag2SO4 nanosheets are found to exhibit excellent sensitivity and selectivity towards NO2 gas molecules operating at room temperature and under blue light irradiation. Overall, the author has succeeded in establishing several important results in the course of this PhD research, including the synthesis of nonlayered 2D metal oxide (TiO2) and metal oxysulphides and revealing for the first time the extraordinary prospective of PdSO4 and Ag2SO4 nanosheets in optoelectronic chemical sensing. It is expected that the results of this PhD research will contribute to the development of scalable and low-cost synthetic pathways and will serve as a platform for further investigations into the synthesis and advanced applications of 2D materials in the near future.