Novel liquid-phase synthesis of two-dimensional transition metal compounds for electrocatalytic energy conversion applications
Creators: name: Mohiuddin, Md
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 401605
subject: Functional materials

description: Ultrathin two-dimensional (2D) nanomaterials offer enormous potential from fundamental studies to applications owing to their unique properties. Among the wide range of 2D nanomaterials, layered semiconducting transition metal dichalcogenides (TMDs) and non-layered transition metal compounds are most promising due to their thickness-dependent properties. This PhD thesis explores the novel liquid-phase synthesis and fundamental properties of several most promising 2D transition metal layered and non-layered compounds and their potentials in electrocatalysis application. Overall, this thesis consists of four main studies.<br><br>The first part of this thesis explores the liquid-phase exfoliation technique of layered MoS2 exploiting the piezoelectric nature, which leads to new opportunities for fast and efficient exfoliation processes. A surface acoustic wave (SAW) microcentrifugation device is utilised to apply concomitant electric field and mechanical shear force for the effective exfoliation of MoS2 nanosheets. Using the developed exfoliation method, at least an order of magnitude larger overall yield per unit of time can be achieved than previously reported liquid-phase exfoliation methods. Simultaneously, the higher monolayer yield can also be achieved. Moreover, the effect of the electric field in increasing the efficiency of liquid-phase exfoliation is also demonstrated.<br><br>Second, in extension to the first study, an enhancement in the efficiency of agitative liquid-phase exfoliation of stratified WS2 crystals under the influence of an electric field is studied. The exfoliation efficiency of WS2 nanoflakes has successfully been increased by using co-applied alternating electric field and mechanical wave. The loss of centrosymmetry at the facial layers of WS2, together with the matching between the dispersive component of this material and solvent that weakens the van der Waals forces, augment the exfoliation process. However, this is not seen for MoS2 and graphene. The outcomes provide the base for future investigations on the influence of electric field for the exfoliation of layered structures. As a proof-of-concept, the exfoliated 2D WS2 nanosheets are evaluated for potential electrocatalytic hydrogen evolution reaction (HER).<br><br>Third, facile and scalable synthesis of ultrathin hematite using inexpensive and water-soluble template through annealing has been explored. 2D nanosheets with the thickness down to 1 nm and lateral dimension up to 5 µm is realised. Interestingly, the morphology can easily be transformed into anisotropic porous planar nanostructure by introducing the aging process. The lattice match growth of nanosheets and shape transformation to anisotropic nanostructure due to small lattice mismatch on the template is thoroughly examined and explained. Further, nanostructures are tested for possible electrocatalytic hydrogen production.<br><br>Finally, ultrathin nanostructures such as nanosheets, nanonets of iron phosphide (FeP) have successfully been synthesised from ultrathin hematite through one step phosphorisation. The nanostructures are explored as free-standing electrocatalysts considering their high electrochemically active surface area up to 20.5 mF cm-2, which exhibit overpotential as low as 117 mV to achieve 10 mA cm-2 for hydrogen generation. The combination of ultrathin morphology and nanopores particularly in ultrathin FeP nanosheets are considered as key factors for high electrocatalytic activity.