UMD Theses and Dissertations
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Item Magnetic and Toroidal Symmetry of Lithium Transition Metal Orthophosphates(2024) Gnewuch, Stephanie Kardia; Rodriguez, Efrain; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)LiCoPO4 is the foremost candidate material for a novel type of ferroic ordering calledferrotoroidicity. In this work, the synthesis of polycrystalline sample of LiCoPO4 is discussed, along with the structural analog LiMnPO4. Their magnetic susceptibility and magnetic structure were determined and analyzed and found to be consistent with previous reports on single crystal materials. This work also provides a thorough introduction to ferrotoroidicity, a history of its theoretical development, and a summary of the most studied candidate materials. The work then presents a detailed methodology for determining the toroidal structure which would result for the magnetic structure in candidate ferrotoroidal materials. The model provides a method for determining how many toroidal moments would be present, where they would be located within the unit cell, and along which crystallographic direction they would be oriented. Detailed examples for determining the magnetic structure are provided for LiCoPO4 and analogous structures with the olivine structure type, as well as several structures with the pyroxene structure type. The results demonstrate a method for understanding ferrotoroidal arrangements, anti-ferrotoroidal arrangements and non-toroidal structures.Item A Breath of Fresh Air: Study of Reactive Porous Metal Oxides for Chemical Warfare Agent and Simulant Defeat(2022) Leonard, Matthew; Rodriguez, Efrain E; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Organophosphorus molecules are a wide class of compounds that are used commerciallyas fire retardants, plasticizers, and pesticides. Organophosphorus pesticides were developed to replace environmentally persistent pesticides, such as dichlorodiphenyltrichloroethane, more commonly known as DDT. However, organophosphorus pesticides have been proven to be carcinogenic and to affect neurodevelopment negatively. A sub-class of organophosphorus molecules are highly toxic acetylcholinesterase (AChE) inhibitors, known as nerve agents. Although the Geneva convention banned the use of chemical warfare agents (CWA), the nerve agent sarin has been used as recently as 2013 on Syrian civilians. In 2018, a Novichok nerve agent was used in the attempted assassination of a former Russian spy and his daughter. Current CWA respiratory protection employs bituminous coal (BPL), carbon which is impregnated with a mixture known as ASZM-TEDA. BPL carbon impregnated with ASZM-TEDA has a wide range of reactivity, but has not changed significantly since its inception. The next-generation filtration material will need to have a large surface area to maximize reactive sites and be robust to withstand degradation. Mesoporous and nanoparticle metal oxides are highly active materials that show promise in nerve agent defeat. Within this dissertation, the goal is to develop and study reactive metal oxides to understand the factors that are important for the decomposition of CWAs and CWA simulants. In Chapter 1, I introduce the history of CWAs, the downfalls of current filtration technology, and the candidates for the next generation filter. In Chapter 2, the methods and characterization techniques used within this work are presented and discussed. In Chapter 3, to determine the effects of cation selection on methyl paraoxon decomposition, Ce4+ was isovalently doped into anatase type TiO2. Through UV/Vis spectroscopy, the degradation of methyl paraoxon was tracked and fit to pseudo-first-order kinetics, then normalized to the synthesized material’s surface area. The rate constant, normalized to the material’s surface area (kSA), reveals CeO2 is 3 to 4.6 times larger than that of TiO2 and the Ce-doped titanias. The Ce-doped titanias showed little to no change in methyl paraoxon decomposition compared to TiO2. The lack of change within the Ce-dopant titania revealed that crystal structure is a larger driving factor for methyl paraoxon decomposition than the cation identity (i.e. Ce4+ and Ti4+). Chapter 4 presents a study on the gas surface interaction between sarin and dry CuO nanoparticles (NP) through infrared (IR) spectroscopy. Sarin adsorbs to CuO through the P=O bond, and proceeds to decompose on the surface. Distinct red shifts in the delta(P-CH3) and rho(P-CH3) modes indicate the cleavage of the P-F bond, producing isopropyl methyl phosphonic acid (IMPA). Concurrently, a mode attributed to (O-P-O) begins to grow in, demonstrating that sarin forms a bridging species on the surface. Sarin continues to degrade on the dry CuO surface once the sarin feed is removed. Upon heating above 423 K, all modes associated with IMPA simultaneously decrease, indicating that IMPA desorbs from the surface. These observations were further corroborated through computational methods. Finally, in Chapter 5, I seek to enhance the reactivity of CuO by placing the cation Cu2+ within a Jahn-Teller active geometry. Mesoporous NiO and Cu-doped NiO were synthesized and exposed to diisopropyl fluorophosphate (DFP) in different environments and studied through diffuse reflectance IR Fourier transform spectroscopy (DRIFTS). Ordered mesoporous Cu-doped NiO was successfully synthesized through a hard templating method. Through X-ray diffraction (XRD), Cu2+ was incorporated into the NiO rock-salt lattice without phase separation for < 20%. The mesoporous metal oxides (MMO) maintained high surface areas (67.89-94.38 m2/g), with a main pore size of ~2.4 nm. Shifts in the Raman spectra indicate the dopant, Cu2+, reduces nickel vacancies resulting in a decrease in Ni3+ defect states. Upon DFP exposure, NiO was highly oxidative producing CO, CO2, carbonyls, and carbonates due to the active oxygen species formed by the Ni2+ vacancies. The mesoporous Cu-doped NiO samples were less reactive to DFP oxidation, due to the Cu2+ occupying the nickel vacancies, resulting in a reduction of active oxygen species.Item Tuning Crystallographic and Magnetic Symmetry in Lithium Transition Metal Phosphates and Thiophosphates(2022) Diethrich, Timothy; Rodriguez, Efrain E.; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Ferroic ordering needs no introduction; ferromagnetic, ferroelectric, and ferroelastic materials have had a significant impact on the materials science community for many years. While these three main types of ferroic ordering are well known, there is a fourth and final, lesser known ferroic ordering known as ferrotoroidicity. A ferrotoroidic material undergoes a spontaneous, physical alignment of toroidal moments under a critical temperature. This study is focused on broadening our current understanding of ferrotoroidics by studying two families of materials: LiMPO4, and Li2MP2S6, where M = Fe, Mn, and Co. While these two materials initially appear to be similar in some regards, many differences can be observed as a deeper dive is taken into their crystallography and magnetic structures. For a toroidal moment to exist, a specific orientation of magnetic moments is required, because of this, only certain magnetic point groups are allowed. For example, LiFePO4 has an “allowed” magnetic point group of m’mm, while it’s delithiated counterpart FePO4 has a “forbidden” magnetic point group of 222. This work has found that by using a new selective oxidation technique, lithium concentration can be controlled in the Li1−xFexMn1−xPO4 solid solution series. Neutron powder diffraction and representational analysis were used to find the magnetic point groups of each member of this series. In the end, each structure was solved and the largest transition temperature to date was reported for a potential ferrotoroidic material. The magnetic exchange interactions can be used to describe the magnetic phase changes that occuracross the Li1−xFexMn1−xPO4 series. The second group of materials in this study is the lithium transition metal thiophosphates of the formula Li2MP2S6, where M = Fe, Co. The structure of Li2FeP2S6 has been previously studied but no magnetic properties of this material have been reported. In addition, neither the structural nor magnetic properties have been reported for the cobalt analog. Single crystalXRD was used to confirm the previously reported crystal structure of Li2FeP2S6 and to find the novel crystal structure of Li2CoP2S6; both crystallize in a trigonal P31m space group. While isostructural in some regard, there are some crucial differences between these materials. The site occupancies are different, resulting in non-trivial charge balances and a unique thiophosphate distortion. Originally, these materials were chosen because their nuclear structure was predicted to host long-range antiferromagnetic order and potentially ferrotoroidic order. Contrary to expectations, magnetic susceptibility and field dependent measurements demonstrated paramagnetic behavior for both the iron and the cobalt sample down to 2 K. This result was further confirmed by a lack of magnetic reflections in the time-of-flight neutron powder diffraction data. While the phosphates and the thiophosphates demonstrated very different structural andmagnetic results, they both remain relevant materials for not only ferrotoroidics, but also magnetoelectrics, spintronics, quantum materials, and much more.Item NOVEL PLATINUM COMPLEXES SUPPORTED BY SULFONATED CNN PINCER LIGANDS RELEVANT TO AEROBIC METHANE FUNCTIONALIZATION CHEMISTRY(2021) Ruan, Jiaheng; Vedernikov, Andrei N; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Mild and selective aerobic methane functionalization has always been a challenge. Shilov developed a PtII based system that has a potential to solve the problem, but the non-practical PtIV oxidant needs to be substituted by more accessible oxidants, such as O2. On pursuing this goal, several series of PtII and PtIV methyl complexes derived from two pre-ligands, Ph-dpms [(6-phenylpyridin-2-yl)(pyridin-2-yl)methanesulfonate, HL1-] and Ph-pcpps [7-(6-phenylpyridin-2-yl)-6,7-dihydro-5H-cyclopenta[b]pyridine-7-sulfonate, HL2-] were synthesized. Their reactivity in O2 and X-H (X = C, Si) bond activation (PtIIMe), and in CH3-X (X = O, N, C) reductive elimination (PtIVMe), was studied. The three steps of the proposed catalytic cycle suitable for aerobic methane functionalization were investigated.The reverse of the first step, methane C-H activation, was probed by reacting K[(L1)PtIIMe] with acid H(Et2O)2BArF4 {BArF4 = tetrakis[3,5- bis(trifluoromethyl)phenyl]borate} in CH2Cl2 at -78 °C. Although methane and no stable PtIV(Me)H species were detected at both -78 °C and room temperature, the resulting solutions were shown to activate Si-H bonds of Et3SiH and Me3SiH to form Pt hydrido complexes. The second step, aerobic oxidation of PtIIMe complexes, was investigated using K[(L1)PtIIMe] and K[(L2)PtIIMe]. K[(L1)PtIIMe] reacts with O2 in MeOH solutions to form diastereomeric (L1)PtIVMe2 complexes, which are barely reactive in CH3-X reductive elimination. Notably, K[(L2)PtIIMe] reacted with O2 in MeOH or acetone / TFE to selectively form three out of four possible diastereomeric (L2)PtIV(Me)OH complexes with (L2)PtIVMe2 as a minor by-product. The remaining fourth diastereomer of (L2)PtIV(Me)OH was prepared using H2O2 as oxidant. The third step, CH3-X reductive elimination, was studied using a series of PtIVMe species supported by L1 and L2. The PtIVMe(Y) species (Y = Cl, I, OCH2CF3, OH) having methyl trans- to sulfonate and one (L2)PtIV(Me)OH complex having pyridyl trans- to methyl demonstrated facile CH3-X reductive elimination (X = Me2SO+, OH, O2CCF3, and Me2NPh+) using the corresponding nucleophiles with yields of the CH3-X products up to 99%. Two other (L2)PtIV(Me)OH complexes having methyl ligand trans- to pyridyl formed predominantly C-C coupling products in aqueous DMSO solutions of CF3CO2H. Overall, this work demonstrates the potential of our novel sulfonated pincer ligands to support aerobic functionalization of methane at a Pt center.Item Magnetism and superconductivity in topotactically modified transition metal chalcogenides(2020) Wilfong, Brandon Cody; Rodriguez, Efrain E; Paglione, Johnpierre; Chemical Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Inspired by the structure of the simplest iron-based chalcogenide superconductor, FeSe, the class of tetrahedral transition metal chalcogenides (TTMCs) exhibit interesting chemical and physical properties due to its structure. This structure consists of tetrahedrally coordinated transition metal chalcogenides stacked to form two dimensional layers held together by van der Waals forces. This structure and its associated tetrahedral coordination of transition metal to chalcogenide, square transition metal sublattice, van der Waals layered structure, and d-electron filling at the Fermi level yields interesting properties from superconductivity to frustrated itinerant magnetism. In this dissertation work, we demonstrate that the anti-PbO type FeCh (Ch = S, Se, Te) structure offers a perfect platform for the study of superconductivity in the iron-based system as well as new physics as the class is expanded to different transition metals. Prior to this work, the binaries of the TTMC family was limited to iron, but has been expanded to cobalt. In the cobalt compound, CoSe, superconductivity in the FeSe binary is suppressed and a frustrated spin glass like magnetic state emerges. Beyond the binaries, we have shown that topotactic hydrothermal synthetic routes on the iron chalcogenide system can lead to novel intercalated phases where long range magnetic order can co-exist with superconductivity in the (LiOH)FeSe system. This synthetic scheme also allows the intercalation of organic molecules, specifically ethylenediamine, to form organic-inorganic hybrids which can offer a new avenue for designing heterolayer compounds with complex interlayer interactions and bonding.Item SYNTHESIS OF MAIN GROUP ELEMENT CLUSTERS OF GROUPS 13 AND 15 FROM DISPROPORTIONATION PATHWAYS AND ZINTL POLYANIONS(2019) Stevens, Lauren Marie; Eichhorn, Bryan W; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This thesis details the synthesis and characterization of main group element clusters of Groups 13 (aluminum) and 15 (phosphorus, arsenic). Aluminum clusters were synthesized from metastable Al(I)X (X = Cl, Br) solutions, which have proven adept at fostering the growth of metalloid clusters of the form AlnRm (where n > m). Group 15 – transition metal coordination complexes and ligand-free binary anions are synthesized through the use of Zintl ion precursors, originating from phases K3Pn7 (Pn = P, As, Sb). The novel cluster [(Bu2O)3Li][Li4Al5Ph12] has been synthesized and characterized, reported here in seven different crystallographic modifications. In addition to being the first low-valent phenyl aluminum cluster, this anion exhibits an unusual metastability in both the solid-state and solution, as indicated through ESI-MS, LDI-MS, and solid-state NMR analyses. The Zintl-derived coordination complexes [(η4-As7)Co(η3-As3)]3-, [(η4-As7)Rh(COD)]2-, and [(η4-As7)Ir(COD)]2- are reported as the first As / Group 9 clusters, and are isoelectronic to known coordination species of Zintl anions and transition metal carbonyl fragments. Additionally, the product [(η4-As7)Co(η3-As3)]3-is the first known carbon-free binary anion of As / Co. These complexes have been characterized via LDI-MS, NMR, single crystal XRD, and quantum chemical calculations. The doubly substituted coordination complex [(en)(CO)3Mo(η4-P7)Mo(CO)3]3- completes a series of previously reported Group 6 polyphosphide complexes, and is compared to its W congener, [(en)(CO)3W(η4-P7)W(CO)3]3-. Unlike coordination complexes, which retain the nuclearity of the seven-atom precursors [Pn7]3-, binary intermetalloids [Mo2P16]4- and [Rh3As16]3- show extensive reorganization of the original polypnictide cages. These anions feature cyclo-[P10]2- and cyclo-[As5]1- subunits, which are the first to be isolated and described in products of Zintl anions. Additionally, these are the first binary systems to be reported for Mo/P and Rh/As, and could potentially be used for the formation of binary phases (i.e. RhAs2) upon oxidationItem TOWARDS ENERGY EFFICIENT AND ATOM ECONOMICAL CHEMICAL CYCLES FOR NITROGEN FIXATION(2018) Duman, Leila Margaret; Sita, Lawrence R; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Converting the abundant, but largely inert, dinitrogen (N2) into the N-containing products upon which society relies (including proteins, explosives, and fertilizers) is a vital synthetic target as the worldwide population, and therefore demand for such commodities, continues to grow. The development of energetically and atomically efficient methods of cleaving, functionalizing, and releasing N2 as other valuable N-containing products (a process known as fixation) is of the utmost importance to sustainably meet the global demand. Transition-metal-mediated examples of N2 coordination, activation, cleavage, and N-atom product release are an attractive and active field of study in developing N2 fixation cycle that operate under mild conditions and consume few resources. Herein, the effects of a sterically reduced pentamethylcyclopentadienyl, amidinate (CPAM) ligand framework around group 6 metal centers (M = Mo, W) are investigated as a means of lowering barriers to reaction in order to complete a novel N2 fixation cycle. The design and implementation of this set of reactions are reported, including synthesis and characterization of new dinitrogen and dinitrogen-derived organometallic compounds, as are the groundbreaking thermally mediated N≡N cleavage from dinuclear M(II) (μ-N2) to M(V) (μ-N)2 dinuclear complexes, N atom functionalization via silylation to form terminal Mo(IV) trimethylsilyl imidos, and a controlled, “dry hydrolysis” with Me3SiCl and a readily available silanol or alcohol proton source (XOH) to releases hexamethyldisilazane (HN(SiMe3)2) and a silyl ether (XOSiMe3) while regenerating the M(IV) dichloride starting material. Reactivities within and modifications to this cycle are explored, including the introduction of innovative reagents, and the versatility of the CPAM system is demonstrated while making strides towards more targeted and sustainable means of synthetic N2 fixation.Item MULTI-LAYERED, VARIABLE POROSITY SOLID- STATE LITHIUM-ION ELECTROLYTES: RELATIONSHIP BETWEEN MICROSTRUCTURE AND LITHIUM-ION BATTERY PERFORMANCE(2019) Hamann, Tanner; Wachsman, Eric; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The global drive to create safer, higher capacity energy storage devices is increasingly focused on the relationship between the microstructures of electrochemically- active materials and overall battery performance. The advent of solid-state electrolytes with multi-layered, variable porosity microstructures opens new avenues to creating the next generation of rechargeable batteries, while creating new challenges for device integration and operation. In this dissertation, microstructures of solid-state Li-ion conducting electrolytes were characterized to identify the primary limiting factors on electrolyte performance and identify structural changes to improve porous electrolyte performance in dense-porous bilayer systems. LLZO-based garnet electrolytes were fabricated with varied porosity and characterized using 3D Focused Ion Beam (FIB) Tomography, enabling digital reconstructions of the underlying 3D microstructures. Ion transport through the microstructures was analyzed using M-factors, which identified garnet volume fraction and bottlenecks as primary limiters on effective conductivity, followed by geometric tortuosity. Notably, a template-based porous microstructure displayed a low tortuosity plane and a high tortuosity direction, as opposed to the more homogenous tape-cast porous microstructures. To evaluate the performance of these microstructures in Li symmetric cells, dense-porous bilayers were digitally constructed using the FIB Tomography microstructures as porous layers with fully infiltrated Li-metal electrodes, and equilibrium electric potentials were simulated. The bilayers had area-specific resistance (ASR) values similar to the ASR value of the dense layer alone. The bilayer ASR also decreased as porous layer porosity increased, due to ion transport occurring primarily through the dense layer-electrode interface and higher porosity creating higher interfacial area. Artificial bilayers were created with porous layers composed of columns for a range of column diameters/particle sizes, porous layer porosities, and porous layer thicknesses. The bilayer ASR decreased with increasing porosity and decreasing column diameter, similar to the FIB Tomography bilayers. However, bilayer ASR dramatically increased when only partially infiltrated with electrodes, and instead increased with increasing porosity and decreasing column diameter. The simulation results showed that fabricating solid-state bilayer symmetric cells with low ASR required high porosity porous microstructures with small particle sizes, and electrodes completely infiltrated to the dense layer.Item Solid electrolyte interphases on graphite anodes and sulfur cathodes(2018) Wang, Luning; Eichhorn, Bryan W; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This dissertation describes chemical compositions and fundamental mechanisms of formation of solid electrolyte interphase (SEI) layers on a graphite anode and a microporous carbon/sulfur cathode. Direct analysis on the authentic SEIs by solution NMR spectroscopy is combined with investigations on model chemical systems. We uncovered the presence of highly similar SEI components (i.e., lithium ethylene mono-carbonate, LEMC and lithium methyl carbonate, LMC) on the two types of electrodes, but the components are generated from completely different chemical routes. The solution and solid-state properties of the compounds are fully characterized. Single crystalline X-ray diffraction (XRD) studies on the two compounds reveal layered packings and structural disorders in the lattice. Electrochemical impedance spectroscopy (EIS) measurements indicate high Li+ ionic conductivity (> 10-6 S/cm) of LEMC. Remarkable difference is observed regarding the chemical compositions and formation routes of the SEIs on the identical sulfur cathodes in Li-S vs. Na-S batteries. The formation of SEIs is found closely associated with fundamental issues such as ion solvation structures, cation coordination, etc. Operando studies on solid-liquid interface in an electrochemical cell carried out by ambient-pressure X-ray photoelectron spectroscopy (AP-XPS) visualize ionic structures in an electrical double layer (EDL) in a Au/sulfide aqueous system. We observe monolayer adsorption of bisulfide (HS-) on Au and spatial distributions of HS- in liquid/gas interface.Item ELECTRONIC MODIFICATION WITHIN THE WELL-ESTABLISHED CPAM FRAMEWORK AS A MEANS TOWARD INCREASED REACTIVITY(2017) Thompson, Richard; Sita, Lawrence R.; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Early transition metals (group IV-VI) supported by the pentamethylcyclopentadienyl-amidinate mixed ligand set (CPAM) have been found to enable a number of important chemical transformations including (living) coordinative polymerization of alpha-olefins, fixation of dinitrogen and group transfer chemistry involving oxo, imido and sulfido ligands to unsaturated organic substrates, including carbon dioxide. A great deal of the allure and success associated with these complexes is their modularity, particularly as it concerns the amidinate component which is tunable at both the N-bound substituents as well as the distal position. Accordingly, a great deal of work has established that by reducing the sterics in all three positions engendered higher reactivity. There exists, however, a practical “steric wall” such that the size of substituents can only be contracted so much. Tuning of the electronic character of these well-established systems could prove to be a novel and potent method for affecting reactivity of these complexes within an already well understood steric environment.