Applied electrochemistry for new class of Cerium-based functional materials

Abstract

The present work reports novel strategies for the controllable design of metal oxides and their heterojunction nanostructures, hence delivering functional materials of potential for energy and environmental applications. Numerous synthesis and characterisation methods were used: 1. Processing: Cyclic voltammetry, chronopotentiometry, chronoamperometry, hydrothermal synthesis, solvothermal synthesis, exfoliation, bottom-up, and precipitation. 2. Characterisation: XRD, ND, SEM, EDS, TEM, HAADF, STEM, EELS, EPR, Raman, FTIR, XPS, TOFSIMS, PL, TGA, UV-Vis, BET, Zeta Potential, AFM, KPFM, high-resolution NMR, MS, and linear sweep and cyclic voltammetry. A new class of Ce-based coordination polymer (Ce-CP) has been discovered, developed, and applied to a number of metal oxides: CeO2; TiO2; ZrO2; Fe2O3-, NiO-, and ZnO-decorated CeO2 heterostructures; MnO and CuO-decorated CeO2 heterostructures; C- and S-incorporated CeO2 heterostructures; and ZnO-derived MOF-5. The controllable fabrication techniques involve two principal structural and architectural approaches. The first aimed at controlling the density of surface and volumetric defects in ultrathin CeO2 films. This strategy is based on fundamental principles of aqueous chemistry for Ce-based materials using thermodynamics and electrochemistry. The second aimed at developing a controllable architectural design for materials through a simple, cost-effective, efficient, and reproducible method. A new class of Ce-CP was synthesised and used to fabricate materials with unprecedented microstructures, including cubes, ultrathin films, holey 2D nanostructures, hollow nanospheres, hollow pseudo-octahedra, 2D-3D mesoporous scaffolds, nanotubes, dumbbell-like shapes, rhombohedra, flower-like shapes, solid nanospheres, solid octahedra, and honeycomb scaffolds. The work reports several innovations: 1. New thermodynamic and kinetic data for Ce-based species in aqueous systems 2. Fabrication of ultrathin CeO2 with controllable thickness and defect concentrations 3. Room temperature synthesis of a new type of stratified Ce-based coordination polymer 4. Room temperature, template-free synthesis of polycrystalline holey 2D nanostructures with the extremely low thickness of two-unit cells 5. Engineering of the electronic structures of CeO2-based materials by band alignment 6. Synthesis of holey CeO2 nanosheets of controlled thicknesses using a bottom-up method 7. Room-temperature synthesis of hollow spheres and pseudo-octahedra through a directed nanoassembly technique 8. Template-free synthesis of mesoporous 2D-3D CeO2-x and CeO2-x-based transition-metal oxide nanostructures The development of new pathways to achieve controllable, precise, reproducible, and high-yield methods to fabricate materials with both superior and specific functionalities for targeted applications are reported. The versatility of the resultant fundamental and applied data can be utilised with other systems.