Synthesis, structural transformation and surface properties of two dimensional metal oxides and their compounds

This PhD project explores the synthesis and fundamental properties of two dimensional (2D) metal oxides, and investigates their feasibilities in applications such as catalysis and electronics. The field of 2D materials research is mostly focused on layered crystals, out of which ultra-thin planes (few monolayers) can be readily extracted. The high surface area of 2D nanosheets allows for efficient interaction with the molecular entities in the vicinity of the oxide, which can be bound to or electrostatically adsorb onto the basal surface. In the first part of this PhD, layered α-MoO3 is exfoliated into 2D nanosheets forming defect-rich 2D α-MoO3-x. The synthesized compound is found to display superior activity for the electrocatalytic hydrogen evolution reaction (HER) with a low overpotential and fast electron transfer for hydrogen production. The combination of oxygen deficient structure and large surface area of the 2D nanosheets with structural defects and steps played as key factors to achieve high HER activity. Thus far, the field of 2D materials research is predominantly focused on naturally stratified or layered materials due to the ease of their exfoliation. However, there are many important types of metal oxide compounds, which do not occur in the form of natural layered crystals, and hence cannot be readily transformed into 2D nanosheets. If these crystals are synthesized in 2D morphologies, they can offer the required large surface-area for various catalytic reactions. In the second part of this PhD, a two-step synthesis approach is explored to synthesize 2D nanosheets of non-layered crystals. First, layered α-MoO3 is exfoliated into 2D α-MoO3-x. Subsequently, these defect rich 2D α-MoO3-x nanosheets are transformed into stable 2D PbMoO4 nanosheets using a solution phase topotactic reaction. The transformed 2D PbMoO4 nanosheets display trap states within their bandgap, enabling their efficient performance as a photocatalyst under visible light irradiation. The transformed 2D nanosheets act as an excellent photocatalyst to degrade the organic pollutants in water. The presented method in this work can be likely extended to establish a variety of highly stable defect rich 2D metal molybdates for potential applications in visible light based photocatalysis, which are otherwise challenging to synthesize. Finally, synthesis of a non-layered transparent conductive oxide (TCOs) as 2D nanosheets is also explored using a liquid metal reaction approach. This highly scalable technique produces ultra-large 2D indium tin oxide (ITO) nanosheets, which can potentially be utilized in the future industrial production of TCOs. ITO is a widely used transparent conductor which finds application in every-day electronics such as touch screens and flat panel displays. One key limitation of ITO is the brittle nature of this ceramic, prohibiting its use in flexible electronics. Additionally, the commercial deposition of this material mostly relies on vacuum based techniques. Herein, the wafer scale printing synthesis of highly flexible 2D ITO nanosheets with a typical thickness of 1 nm is presented. A low temperature liquid metal printing approach is utilized to directly deposit 2D ITO onto a variety of substrates including polymers. The final nanosheets feature two orders of magnitude lower light absorption when compared with graphene, while maintaining suitable electrical conductance for device fabrication. The developed technique will enable low cost, printable and flexible optoelectronics, while also providing an alternative to graphene with superior transparency for the creation of van der Waals heterostructures.