Two-dimensional doped molybdenum oxides and their catalytic performances
Creators: name: Haque, Farjana
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 401605
subject: Functional materials

description: This PhD research explores the crystal and band structure tuning of molybdenum oxides and investigates their feasibilities in applications such as catalysis and biology. To extract the full potential of molybdenum oxides this PhD mainly scrutinised two dimensional (2D) and low dimensional nanosheets of molybdenum oxides. This approach facilitated the controlled tuning of surface chemistry to realise various exciting properties that were unexplored in their bulk counterparts. With the aim of future practical applications, high throughput synthesis methods (e.g. hydrothermal) and post-treatments were designed and implemented for the targeted applications.

In this PhD, hexagonal molybdenum oxide was synthesised to explore their electrocatalytic properties in hydrogen evolution reaction (HER). Molybdenum based compounds are an emerging class of non-metallic catalytic materials for the HER in acidic media. However, most of them lose considerable catalytic performance and exhibit poor long-term stability in alkaline media. Here, planar molybdenum oxide, with high alkaline stability and ordered intracrystalline pores, is developed as the HER candidate. The pores with diameters in the order of ~5-7 Å are HER-active and appear after an NH4+ doping-driven phase transition from the orthorhombic to hexagonal phase. Such a unique structure facilitates the diffusion of ionic entities and water molecules to the HER sites and helps in the removal of gaseous products, therefore improving the surface-active area and reaction kinetics. These intracrystalline pores also reduce the long-term stress on electrodes. The corresponding HER activity is extremely stable for &gt; 40 h in an alkaline medium at an overpotential of 138 mV with a Tafel slope of 50 mV dec-1. Such properties offer a superior combination compared to those of other reported molybdenum-based nanostructures, hence providing a great opportunity for developing high-performance alkaline non-metal HER catalysts.

The promising catalytic performance of molybdenum oxides achieved through electrocatalytic study led to exploring, doping options as the layered crystal structure of these compounds allows for a broad range of doping options, which may give rise to plasmon resonance making them ideal for various other catalysis such as photocatalytic reactions. Photochemical reactions enabled by plasmon-induced hot electrons offer unique advantages in solar-light-driven catalysis and energy conversion. Noble metal nanostructures have been the most studied category of plasmonic materials so far but the lifetime of produced hot electrons is extremely short. Therefore, forming plasmonic metal-semiconductor heterojunctions has been the most conventional approach to extend the hot electron lifetime for many practical applications. Here, extraordinary long-lived hot electrons in ultra-thin degenerately doped molybdenum oxides with surface plasmon resonance in the visible and near-infrared region were discovered. The lifetime is at the nanosecond scale, which is at least 4 orders enhanced compared to noble metal counterparts. Such a peculiar property is ascribed to the quasi-metallic feature of molybdenum oxides driven by hydrogen dopants-induced bandgap trap states, in which the electron-phonon scattering dominates over the ultrafast electron-electron scattering in the decay dynamics of plasmon-induced hot electrons. The plasmonic dye oxidation was carried out without the coupling of semiconductors, possibly providing a viable way towards expanding the candidates for direct plasmonic photocatalysis from the domain of degenerately doped semiconductors.

The significance of dopants in molybdenum oxides and resultant ultrathin low-dimensional compounds realised through this PhD indicate unrestricted features of such materials, which fostered further investigation of cytotoxicity properties using the developed compounds. In humans and mammals, molybdenum acts as a cofactor for various enzymes, such as aldehyde oxidase, xanthine oxidase, and sulphite oxidase that proofs to be essential trace elements. As such low dimensional molybdenum oxides are an excellent candidate for various biological applications due to their selectivity, broad-ranged doping option, non-toxic nature, human body compatibility, and low-cost features. Additionally, enhanced surface area and reactivity allow them to easily translocate cell membranes, efficiently bind molecular species, and catalyse chemical reactions. Although many applications of MoO3 nanostructures in various forms have been studied in biological systems in the past decade, thus, a detailed study of the toxicity and selectivity of MoO3 and doped MoO3 nanomaterials in cancer cells would be of immense interest. Herein, 2D molybdenum oxide has been doped with hydrogen and nitrogen, where the moderately N and H co-doped MoO3 revealed the potential utility for treating metastatic cancer cells as well as exhibited excellent capability of catalytic ultrafast dye degradation in dark. The doped molybdenum oxide has been tested as a catalyst for dye degradation over methylene blue (MB) in the dark and to determine the reactive oxygen species (ROS) causing the ultrafast degradation of MB three different scavenger test has been employed. To investigate the cytotoxicity properties and the mechanism of the cell death of the nanoparticles several assays such as 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl-2H-tetrazolium bromide (MTT) assay, ROS induction, mitochondrial membrane potential, Nuclear morphological changes have been performed on both cervical cancer cell lines, Henrietta Lacks (HeLa), and human embryonic kidney 293T (HEK293T). Additionally, an annexin assay had been utilised to confirm cell death due to exposure to molybdenum oxides. Thus, this research presented the cytotoxicity of MoO3 nanoplates toward invasive cancer cells and investigated the mechanism of cell death.
 
Overall, the influence of doping and structural modification in 2D molybdenum oxides and resultant upgrades in catalysis and biomedical applications are demonstrated in this thesis, which is expected to provide useful guidelines in developing high-performance catalytic and biomedical devices.