Investigation Of Several Inorganic Functional Materials With¹⁷O Solid State NMR Spectroscopy

The research presented in this thesis involves the use of a variety of solid-state NMR techniques, in combination with theoretical calculations to investigate the structure and properties of ceria nanoparticles and layered double hydroxides(LDHs) materials.Ceria and ceria-based materials have wide applications, such as industrial catalysis, solid oxide fuel cells (SOFCs) and photochemistry. Nanosized ceria materials show better performances than their corresponding bulk materials, which wasroughlyascribed to the increasing defectsites in nanomaterials. In recent years,XPS(X-ray Photoelectron Spectroscopy analysis), STM(Scanning Tunneling Microscope) and XANES (X-ray Absorption Near Edge Spectroscopy) to study the oxygen species. We used17O NMR spectroscopy to study the local structure of ceria nanoparticles with different particle size. In addition to signal due to the oxygen in the bulk (877ppm), two resonances at approx.1030and850ppm were observed. On the basis of the NMR data from probe molecule adsorption and theoretical calculations (CASTEP), the higher and lower frequency signals have been assigned to unsaturated oxygen ions on the surface and oxygen species connected to Ce3+ions.In order to further study the effects of Ce3+, we proformed17O SSNMR experiments to investigate a series of Ce3+doped La2O3materials. The intensities of the signals agree well with the model that the oxygen ions directly bound to Ce3+can be detected. Therefore,17O NMR can be developed as a sensitive method to probe the surface structure of nanosized materials and provide insight on rational design of nanomaterials with desired properties.Layered double hydroxides (LDHs), or "hydrotalcite-type"materials, possess the general formula M2+1-xM3+x(OH)2(An-x/n)·yH2O,where a fraction (x%, where x varies from17to33)of divalent metal cations M2+, e.g., Mg2+, located in a brucite-like environment are substituted by trivalent M3+, e.g., Al3+, cations.LDHs are important inorganic supramolecular compounds that can be designed to incorporate a variety of cations and anions and used as precursors for many functional mixed oxides. They have, therefore, received considerable attention in a wide range of applications including but not limited to, optical devices, catalysis, environmental managementand biological sciences. Recently, it was reported that solid state1H NMR spectroscopy coupled with very fast magic angle spinning (MAS) could be used to resolve and quantify the signals originating from both Mg30H and Mg2AlOH environments. However, due to large1H-1H homonuclear dipolar couplings are present in these systems, and ultrafast spinning rates (i.e.,>40kHz) were required. Herein, we present a highly efficient route for17O enrichment of LDHs using the "memory effect", and demonstrate how17O NMR could be used to investigate the cation ordering in LDHs and different species under moderate spinning rate(20kHz). Better resolution can be obtained with our approach than the results from1H ultrafast NMR spectroscopy and1Hâ†'17O cross polarization NMR results show efficient dipolar recoupling can be achieved.Therefore, the interactions among the interlayer anions, the cations and water can be investigated in a similar manner.