Zheng, Chong (Professor of chemistry)
Ph.D. (Doctor of Philosophy)
Department of Chemistry and Biochemistry
Chemistry; Chemistry; Physical and theoretical; Chemistry--Computer simulation
Materials synthesis is a field at the intersections of chemistry and physics with wide-ranging applications. There is a rich diversity of techniques to develop novel materials, but very little fundamental understanding of the mechanisms that drive the formation of solids, leading to an inability to predict a synthesis for a material with targeted properties. Solvothermal synthesis has garnered much attention in the field due to its relative predictability by combining solution-phase dynamics with reactive inorganic precursors. By incorporating composition-guided organic chemistry, which often benefits from predictable properties, metal-organic frameworks (MOFs) synthesized solvothermally have emerged among the most rationally designed solids in modern science. MOFs are a class of crystalline materials composed of metal-centers linked by organic ligands, forming large, porous networks. Structures, thus some properties, can be predicted given motifs for previously determined metal-center geometries and ligand-bonding environs. Further, targeted properties can be chemically tuned via optimization of the ligand and/or metal. Early efforts in the field resulted in the intriguing materials that failed to be commercially viable due to stability issue. Metal-organic frameworks using lanthanide metal-centers (Ln-MOFs) are thought to increase thermodynamic stability of the material and present unique electronic properties such as photoluminescence. The projects presented herein focus on investigating the properties and stability of lanthanide metal-organic frameworks with a naphthalene-based ligand. To be a commercially viable material, among other things a MOF must be stable in addition to having practical properties. The increased complexity in both the accessible geometries and electronic properties of lanthanides relative to light transition metals makes this work largely exploratory. Novel, isostructural cerium, neodymium, and europium Ln-MOFs comprised of two-dimensional sheets of metal-carboxylate centers bridged by naphthalene were synthesized and photoluminescence properties analyzed. The series of Ln-MOFs studied show they have robust photoactivity that may be exploited in small molecule or ion sensing. Compound [Ce(NO₃)(NDC)][sub n] was found to be stable under basic and acidic aqueous conditions, but not thermally stable to 400°C. Small aromatic molecules were screened against [Nd(NO₃)(NDC)][sub n] and fluorescence quenching shown to be correlated to spectral overlap, with significant signal quenching of benzene, but no observed selective change in excitation or emission wavelengths. Further, the compound was found to be stable to 300°C in open air. In particular, compound [Eu(NO₃)(NDC)][sub n] was shown to be highly fluorescent in water and is readily quenched by trace concentrations of hazardous industrial by-product chromic acid. These investigations represent a broad effort to characterize Ln-MOFs in hopes of guiding the development of similar materials that exhibit robust chemical and thermal stability and relevant properties. Isoreticular synthesis is generally, but not exactly, an appropriate tool for replicating the synthesis with naphthalene-based ligands but different lanthanide metals. Procedurally altering the reported successful synthetic conditions with lanthanide metals is highly likely to produce isostructural and comparably stable compounds that exhibit unique electronic properties. Solvothermal synthesis using thiourea as a reactive solvent was also shown to produce unique lanthanide-metalloid complex germanium (II) sulfide doped lanthanum (III) hydroxide. The complex was found to photocatalytically degrade dye methylene blue in water under UV irradiation. While, not as efficient as known photocatalyst anatase titania, it represents a new class of lanthanum oxides doped with small band gap semiconductors that may be more easily optimized for photocatalytic processes than investigations on titanium dioxide have proven to be. Such intercalated lanthanum oxides may even have other relevant photo-driven applications, such as light harvesting or water-splitting.
Perkins, Timothy, "Synthesis and properties of lanthanide metal-organic frameworks and lanthanide-metalloid complexes : structural determination, photoluminescence, and applications" (2018). Graduate Research Theses & Dissertations. 5182.
Northern Illinois University
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