Doctor of Philosophy, The Ohio State University, 2012, Chemistry

This dissertation involves the study of the interaction of carbon monoxide (CO) and nitric oxide (NO) on derivatives of low temperature conducting metal oxides, ruthenium and vanadium oxides. The interactions of these gases with the metal oxides lead to changes in conductivity which show promise for possible applications as a new class of resistive based ambient gas sensors that alleviate the current limitations of CO and NO sensors that operate at elevated temperatures. These sensors are based on hydrated ruthenium oxide (RuOx(OH)y) and vanadium pentoxide (V2O5) . RuOx(OH)y was prepared a wet precipitation reaction involving ruthenium chloride with a base. This material was amorphous, made up of 20–50nm particles and contains Ru(III) and Ru(IV), as determined by XPS. Thick films were made of air and supercritical dried particles of RuOx(OH)y. The conductivity of these films decreased in the presence of CO in air and this change was reversible. Infrared spectroscopy showed the formation of carbonates and water in the presence of CO, which disappeared upon replacement of CO with air. Upon thermal treatment of RuOx(OH)y above 200°C, a decrease in the conductivity change in the presence of CO at room temperature is observed. These changes were accompanied by a conversion of the amorphous RuOx(OH)y to a crystalline RuO2 and consequently a conversion of Ru(III) to Ru(IV). This dissertation proposes the oxidation of CO on RuOx(OH)y leads to reduction of the ruthenium and subsequently a decrease in conductivity of the thick films. With the conversion to crystalline RuO2, the material becomes metallic and conductivity changes are diminished. Changes in RuOx(OH)y conductivity with CO provides an opportune platform for an ambient CO sensor. The interferences from ambient concentrations of hydrocarbons, ammonia, CO2, NO and NO2, were shown to have no effect on the conductivity . This dissertation also discusses the study of the interaction of NO with vanadium oxides. The V (open full item for complete abstract)

Committee: Prabir Dutta (Advisor); Sheikh Akbar (Committee Member); Susan Olesik (Committee Member) Subjects: Chemistry

Doctor of Philosophy, University of Toledo, 2023, Physics

Low-cost thin film absorber layer materials with high absorption coefficients (> 105 cm-1 in visible spectral range) and bandgap close to the ideal value for efficient photovoltaic conversion efficiency are leading candidates for thin film photovoltaic (PV) applications. This dissertation discusses the fabrication and optical and microstructural properties of magnetron-sputtered glancing angle deposited CdTe thin film absorber layer material and its application as an interlayer in CdS/CdTe solar cells. In addition, optoelectronic properties of non-toxic and earth-abundant absorber layer material, antimony selenide (Sb2Se3), and optimization of polycrystalline VO2 fabrication from amorphous vanadium oxide (VOx) film along with its optical properties have been discussed. Sb2Se3 is a promising candidate as an absorber layer material in PV applications. I have performed optical property characterization of thin film Sb2Se3 and identified electronic losses when used in a PV device. The indirect bandgap, direct bandgap, and Urbach energy have been determined to be 1.12 eV, 1.17 eV, and 21.1 meV, respectively using photothermal deflection spectroscopy. Optical properties of Sb2Se3 in the form of complex dielectric function (ε = ε1 + iε2) spectra in 0.75 to 4 eV spectral range is determined using spectroscopic ellipsometry. The line shape of ε is obtained using a parametric model which incorporates an Urbach tail, a band edge function, and five critical point oscillators. The optical property spectra in ε and structural parameters in terms of the thickness of solar cell layer components are used as input parameters for external quantum efficiency (EQE) simulation to investigate the electronic and optical losses in Sb2Se3-based solar cells. A carrier collection length of ~ 400 nm and a ~97 % carrier collection probability near the heterojunction in the Sb2Se3 solar cell are identified by comparing experimental and simulated EQE. Next, I describe deposition and characterizati (open full item for complete abstract)

Committee: Nikolas J. Podraza (Committee Chair); Robert W. Collins (Committee Member); Yanfa Yan (Committee Member); Song Cheng (Committee Member); Terry Bigioni (Committee Member) Subjects: Physics

Doctor of Philosophy, The Ohio State University, 2022, Chemical Engineering

Catalysts play an important role in the chemical industry since they help increase the speed of chemical reactions and reduce waste generation. Catalysts are typically functionalized onto the surface of solid supports such as mesoporous silica materials to ensure easy separation from the product and make the manufacturing process more economically viable. Mesoporous silica materials (pore size > 2 nm) such as SBA-15 and MCM-41 are generally preferred for supporting catalysts because of their thermal stability, robustness, and tunable pore architecture. Whereas such silica supported catalytic materials hold promise, key synthesis-structure-activity relationships remain to be discovered to enable commercial implementation of these laboratory-based catalysts. Such synthesis-structure-activity relationship studies are important to make supported catalysts more industrially viable and better performing than their homogeneous counterparts. The current work focuses on obtaining synthesis-structure-activity relationships by varying design parameters for the mesoporous silica support and catalyst analogue. This is done using two catalytic systems: (i) Knoevenagel condensation catalyzed by tertiary amines functionalized on SBA-15, and (ii) Oxidative dehydrogenation of propane catalyzed by vanadium oxides functionalized on SBA-15. Whereas the SBA-15 support is predominantly mesoporous, it also has secondary micropores (pore size < 2 nm) in its structure. Our results reveal that reducing the micropore volume in the SBA-15 support (NMP SBA-15) enhances the catalytic performance of tertiary amines for Knoevenagel condensation in comparison to their regular micropore counterparts (REG SBA-15). Such performance enhancement is observed for a wide range of tertiary amine densities on the SBA-15 surface. This indicates the generality of the inhibiting effect of micropores on performance of tertiary amines for this reaction. The micropore volume of SBA-15 is observed to significantly d (open full item for complete abstract)

Committee: Nicholas Brunelli (Advisor); Aravind Asthagiri (Committee Member); Jessica Winter (Committee Member) Subjects: Chemical Engineering

PhD, University of Cincinnati, 2001, Arts and Sciences : Physics

The transition metal oxide Vanadium Oxide evokes wide ranging research interest due to diverse and complex physical phenomenon it exhibits. The material has a rich phase diagram with four phases that are characterized by remarkably different properties. In recent years, the unique properties and phenomenon observed in the insulating regime of Vanadium Oxide have received a lot of attention. In addition to the long standing issue of the unusual magnetic ordering in the magnetically ordered insulating phase, recent experimental studies including neutron and resonant x-ray scattering, and x-ray absorption studies have raised new, interesting questions of the complex physics of the disordered and magnetically ordered insulating phases and the transition between these phases. In this work we have tried to gain a comprehensive physical understanding of insulating Vanadium Oxide by developing a microscopic S=2 bond model that is based on spin and orbital degrees of freedom, and is consistent with the parameters and phenomenology of this system. We have used this model to study insulating Vanadium Oxide in different temperature and parameter regimes, and thereby tried to elucidate the role played by spin and orbital physics in governing the interesting behavior observed in this system.We find that using the S=2 bond model with spin and orbital degrees of freedom, we can satisfactorily explain not only the anomalous magnetic ordering, but also all the other properties of the disordered and magnetically ordered insulating phases observed by experimental studies. The model also shows a phase transition between these phases which is completely consistent with the phenomenology of the magnetic transition in insulating Vanadium Oxide. In addition, the S=2 model predicts changes in the phase transition phenomenology for the AFI transition in the presence of magnetic field, and the possibility of an additional phase transition at lower temperatures. In this work, we have explored a (open full item for complete abstract)

Committee: Fu-Chun Zhang (Advisor) Subjects: Physics, Condensed Matter