Theses and Dissertations - Department of Chemistry & Biochemistry

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Characterization and catalytic applications of silver nanoparticles supported on hierarchically porous SiO_2 and Co_3O_4 monoliths
(University of Alabama Libraries, 2017) Yildirim, Yasemin; Bakker, Martin G.; University of Alabama Tuscaloosa
Hierarchically porous materials are of great interest in such applications as catalysis, drug delivery, chromatography, and electrochemical sensor arrays due to properties such as high surface area, large void volume, and tunable surface chemistry. In this study, we give the detailed report of the synthesis of supported Ag nanoparticles by nanocasting on to hierarchically porous SiO2 (silica) and Co3O4 (cobalt oxide) monoliths, as well as the use of these materials as heterogeneous catalysts for the hydrogenation and oxidation reactions. In the preliminary work, we investigated the synthesis of Ag supported on hierarchically porous SiO2 and Co3O4 monoliths, and the catalytic activity of these monoliths for the hydrogenation of MB (methylene blue) and MO (methyl orange) dyes in the presence of NaBH4 (sodium borohydride). The SiO2 monoliths were synthesized using a sol-gel technique. The Co3O4 monoliths were prepared by nanocasting, using the SiO2 monoliths as a template. The loading of Ag nanoparticles on the SiO2 and Co3O4 monoliths was done by a solution infiltration method using aqueous AgNO3 (silver nitrate) solution followed by reduction with ethylene glycol and hydrazine hydrate. Such monoliths also were used as continuous flow monolithic microreactors for the catalytic activity and stability studies for the hydrogenation of EO (eosin-Y) dye in the presence of NaBH4. Finally, the use of these monoliths for the oxidation of cyclohexene was investigated including the effects of temperature, oxidant, catalyst loading, and substrate to oxidant ratio. All these studies are presented in different sections depending on the different synthesis, procedures, and catalytic activity occurring in each heterogeneous catalyst.
Computational studies of atmospheric chemical processes, flexible catalysts, and of new materials for chemical hydrogen storage
(University of Alabama Libraries, 2014) Garner, Edward B.; Dixon, David A.; University of Alabama Tuscaloosa
Advanced electronic structure methods on high performance computers have been used to study new materials for technology applications and for atmospheric chemical processes of the halogens. Chapter 2 is focused on the thermodynamics of halogen oxides relevant to stratospheric ozone depletion chemistry. We calculated the thermodynamic properties of various key species to better understand what is happening in the atmosphere to help minimize our impact on the environment. This research is particularly important because of the lack of experimental data on these species. Chapter 3 is focused on the design of flexible catalysts for single electron transfer reactions using neutral Group 6B (Cr, Mo, W) pentacarbonyl complexes M(CO)5-L. It was found that various P-ligands such as phosphines, phosphalkenes, and phospha-quinomethanes can form radical cations and anions under redox conditions and that the radical site can be localized either on the metal or on the "non-innocent" ligand. More polar solutions will drive single electron transfer reactions to form the cationic and anionic metal based complexes with the appropriate oxidizing and reducing agents. Chapter 4 is the study of chemical hydrogen storage systems with a focus on borane amines. The goal was to develop economically viable and energy efficient processes to regenerate spent fuel formed by the release of hydrogen from ammonia borane. The thermodynamics for fuel regeneration processes of spent ammonia borane fuel, modeled as polyborazylene, were accurately predicted. A method using a modified Pictet-Trouton rule and calculated boiling points was used to estimate heats of formation of liquids for the prediction of the thermodynamics of reactions in the liquid phase. An effective tin catalyst with the potential to lower the cost for ammonia borane regeneration at an industrial scale was designed in collaboration with Los Alamos National Laboratory. However, it was found that at an industrial scale the process was limited due to the cost of transporting the tin catalyst around the spent fuel regeneration plant. Therefore, it was necessary to find a new method for regenerating spent ammonia borane fuel, and hydrazine was found to work very effectively in a one pot approach.
Computational studies of transition metal catalysts
(University of Alabama Libraries, 2010) Craciun, Raluca; Dixon, David A.; University of Alabama Tuscaloosa
High level electronic structure calculations were used to evaluate reliable, self-consistent thermochemical data sets for the second and third row transition metal hexafluorides, as well as for metal phosphines (M=Ni, Pd, Pt). For the transition metal hexafluorides, the electron affinities, heats of formation, first (MF₆ → MF₅ + F) and average M-F bond dissociation energies, and fluoride affinities of MF₆ (MF₆ + F⁻ → MF₇⁻) and MF₅ (MF₅ + F⁻ → MF₆⁻) were calculated. For the transition metal phosphines, the first metal-phosphine binding energy in MPH₃, M(PH₃)₂, MPH₃Cl₂ and M(PH₃)₂Cl₂ was calculated. The electron affinities, which are a direct measure for the oxidizer strength, increase monotonically in the second and third row series, from WF₆ to AuF₆, and from MoF₆ to AgF₆. The hexafluorides of the last two elements of each series, Pt, Au in the third row and Pd and Ag in the second, form extremely powerful oxidizers. The inclusion of spin orbit corrections is necessary to obtain the correct qualitative order for the electron affinities. The calculated electron affinities increase with increasing atomic number, are in good agreement with the available experimental values and, for the third row are: WF₆ (3.15 eV), ReF₆ (4.58 eV), OsF₆ (5.92 eV), IrF₆ (5.99 eV), PtF₆ (7.09 eV), and AuF₆ (8.20 eV). The electron affinities of the second row hexafluorides are even larger than for the second row: MoF₆ (4.23 eV), TcF₆ (5.89 eV), RuF₆ (7.01 eV), RhF₆ (6.80 eV), PdF₆ (7.95 eV), AgF₆ (8.89 eV). A wide range of density functional theory exchange-correlation functionals were also evaluated and only three gave satisfactory results as compared to the higher level electronic structure calculations. The corresponding pentafluorides are extremely strong Lewis acids. The optimized geometries of the corresponding MF₇⁻ anions show classical structures with M-F bonds for W through Ir and for Mo, Tc and Rh; however, for PtF₇⁻, AuF₇⁻, RuF₇⁻, PdF₇⁻, and AgF₇⁻ nonclassical anions were found with a very weak external F-F bond between an MF₆⁻ fragment and a fluorine atom. These anions are text book examples for "superhalogens" and can serve as F atom sources under very mild conditions.
Computational thermodynamic studies of alkali and alkaline earth compounds, olefin metathesis catalysts, and borane -- azoles for chemical hydrogen storage
(University of Alabama Libraries, 2010) Vasiliu, Monica; Dixon, David A.; University of Alabama Tuscaloosa
Geometry parameters, frequencies, heats of formation and bond dissociation energies are predicted for the alkali (Li, Na and K) hydrides, chlorides, fluorides, hydroxides, and oxides and alkaline earth (Be, Mg and Ca) fluorides, chlorides, oxides and hydroxides at the coupled cluster theory [CCSD(T)] level extrapolated to the complete basis set (CBS) limit. The calculations including core-valence correlation corrections with the aug-cc-pwCVnZ basis sets (n = D, T, Q and 5) are mostly in excellent agreement with the available experimental measurements. Additional corrections (scalar relativistic effects, vibrational zero-point energies, and atomic spin-orbit effects) were necessary to accurately calculate the total atomization energies and heats of formation. The results resolve a number of issues in the literature. CCSD(T)/CBS level calculations with additional corrections are used to predict the heats of formation, adiabatic and diabatic bond dissociation energies (BDEs) and Bronsted acidities and fluoride affinities for the model Schrock-type metal complexes M(NH)(CRR')(OH)_2 (M = Cr, Mo, W; CRR' = CH_2, CHF, CF_2) and MO_2(OH)_2 transition metal complexes. The metallacyclobutane intermediates formed by addition of C_2H_4 to M(NH)(CH_2)(OH)_2 and MO_2(OH)_2 are investigated at the same level of calculation. The electronegative groups bonded to the carbene carbon lead to less stable Schrock-type complexes as compared to the complexes with a CH_2 substituent. The Schrock compounds with M = Cr are less stable than with M = W or Mo. The heats of formation and bond dissociation energies (BDEs) for the pyrrole, pyrazole, imidazole, triazole and tetrazole borane adducts were predicted using an isodesmic approach based on G3MP2 calculations. As potential hydrogen storage substrates, dehydrogenation energies for the elimination of one H_2 molecule were predicted as well as thermodynamic properties relative to their acid-base behavior. The H_3B-N bonds to an sp^2 nitrogen are much stronger than those to an sp^3 nitrogen for the 5-membered rings. The B-N BDEs for the azolylborate adducts are much larger than for the neutral azole borane adducts. The azole adducts with more number of nitrogens in the ring and with more BH_3 molecules to the azole nitrogens are more acidic.
Electrochemically assisted self assembly of mesoporous silica on conducting substrates as templates for electroactive metal oxides
(University of Alabama Libraries, 2012) Cordes, Nikolaus Lynn; Bakker, Martin G.; University of Alabama Tuscaloosa
Three dimensional, conductive, macroporous nickel foams and mesoporous carbon aerogels have been utilized as skeletal supports for an electrochemically assisted deposition of mesoporous silica particles and films. Substrate-supported mesoporous silica particles were synthesized directly onto the conducting supports using a combined sol-gel/electrochemical method, termed Electrochemically Assisted Self Assembly (EASA). The EASA method resulted in mesoporous silica/nickel foam composites exhibiting high specific surface areas (20-120 m^2 g^-1) and narrow silica mesopore size distributions (2.6-3.8 nm), as determined by nitrogen physisorption characterization. Increasing the EASA deposition time resulted in the increase of mesoporous silica particle diameters (from less than one micron for deposition times of 80 seconds to greater than one micron for deposition times of 3600 seconds), as well as an increase in the number of particles deposited and substrate surface coverage, as determined by scanning electron microscopy (SEM) and cyclic voltammetry, respectively. Transmission electron microscopy (TEM) analysis revealed that silica particles deposited by EASA possess a worm-like disordered morphology, due to poor surfactant ordering during the EASA process. Mesoporous silica particles on carbon aerogels resulted in an increase in electrochemical capacitance, from 7 F g^-1 for a bare carbon aerogel substrate to 20 F ^-1 for a mesoporous silica/carbon aerogel composite. Mesoporous silica/nickel foam composites were utilized as substrate-supported templates in a metal oxide nanocasting procedure. Aqueous and ethanolic cobalt nitrate infiltration and thermal decomposition produced Co3O4/mesoporous silica/nickel foam composites. Removal of the mesoporous silica particle template resulted in hierarchically porous Co_3 O_4 /nickel foam composite electrodes, as determined by X-ray diffractometry (XRD) and nitrogen physisorption. Co_3 O_4 /nickel foam composite electrodes fabricated from aqueous cobalt nitrate nanocasting exhibited relatively high surface areas (37-44 m^2 per gram of electrode), small micropore volumes, and broad mesopore size distributions. Co_3 O_4 /nickel foam composite electrodes fabricated from ethanolic cobalt nitrate nanocasting resulted in Co_3 O_4 /nickel foam electrodes with lower surface areas of 12-29 m^2 per gram of electrode. The electrochemical pseudocapacitance of Co_3 O_4 /nickel foam composite electrodes were investigated by galvanostatic constant current chronopotentiometry, with specific capacitances of 298-845 F per gram of deposited Co_3 O_4 at low current densities.
Electrochromic and photoelectrochromic switching devices based on aryl amine redox chemistry
(University of Alabama Libraries, 2014) Kelley, Melody; Blackstock, Silas C.; University of Alabama Tuscaloosa
Electrochromic Materials are of interest as fundamental components in smart windows, displays, and fabrics. A new class of organic arylamine molecular electrochromes (both ionic and neutral) has been developed to afford optimal optical and electrochemical switching properties in the absorptive/transmissive electrochromic device state. By controlling the number of redox active units, structuring their internal electronic coupling and alternating the charge state of the electrochrome, a range of optical states are afforded. N, N, N´, N´-Tetraanisyl-1,4- phenylenediame (TAPD) is optimized for use as multi-colored electrochromic molecular dye in the type I (solution) device state. Anionic polyarylamines are prepared and tested in transmissive singular type I electrochromic devices. The application of a small bias (2 V) leads to highly colored stable radical zwitterions. Lack of formal net charge in the oxidized form of the molecule influences its mass transport in the device to yield faster switching times. Devices with these anionic electrochromes also exhibit faster open circuit bleaching kinetics in contrast to neutral devices. In addition to their device studies, stable radical zwitterions have been synthesized (via chemical oxidation), and characterized using cyclic voltammetry, electron paramagnetic resonance (EPR) spectroscopy and optical spectroscopy. Information gained from these studies has led to an improved understanding of the rational design of isolable radical zwitterions. The phenomenon of electrochromism is further demonstrated in a novel redox auxiliary (RA) photoelectrochromic switch. An amino substituted norbornadiene (AA-N-Ts) undergoes photochemical rearrangement to its colorless quadricyclane isomer (AA-Q-Ts), which can be reversibly transformed back to the yellow AA-N-Ts upon oxidative catalysis at an electrode in solution. Similarly, amino substituted azobenzene derivatives isomerize between the contracted (cis) form and the expanded (trans) form as a result of photochemical and electrochemical stimuli, respectively. Using the technology of type I electrochromic devices, the photoelectrochromic switching of these systems is demonstrated with large duty cycles (~1000 cycles). The photoelectrochromic device (PECD) also provides an analytical platform for the study of redox auxiliary catalysis. A new photoelectrochromic switching media has been developed for cycling aromatic photoelectrochromic systems. The general utility of this media in the photoelectrochromic cycling redox auxiliary systems is also presented.
Electron spin-lattice relaxation of trityl radicals
(University of Alabama Libraries, 2017) Chen, Hanjiao; Bowman, Michael K.; University of Alabama Tuscaloosa
Tris(2,3,5,6-tetrathiaaryl)methyl radicals (trityls) belong to a family of persistent free radicals that have been widely used in a number of magnetic resonance applications because of their relatively narrow EPR line shape, long relaxation time and high stability. Trityls have been widely used as polarizing agents in a signal enhancing technique called dynamic nuclear polarization (DNP) for nuclear magnetic resonance (NMR). Under different DNP conditions, the enhancement result varies dramatically. The electron paramagnetic resonance (EPR) properties need to be taken into consideration. We examined the electron spin-lattice relaxation (T1e) of trityl radicals (Finland trityl and trityl OX063) frozen solutions at low temperatures in Chapter 3. The spin-lattice relaxation can be fitted with three classical relaxation processes: direct process, Raman process and Orbach-Aminov process. We also found fast-relaxing centers in high concentration Finland trityl and trityl OX063 frozen solutions. The spin diffusion through fast-relaxing centers plays an important role in spin-lattice relaxation. Chapter 4 shows two models that have been used to mimic the fast-relaxing centers, the trityl-micelle model and trityl-metal model. After studying the spin-lattice relaxation behavior, we found that the fast-relaxing centers could be divided into two different types, small clusters containing two or three mono-radicals for trityl-micelle model or large ‘crystal-like’ aggregations that involve a large number of radicals for trityl-metal model. Chapter 5 shows the spin-lattice relaxation results of trityl poly-radicals that contain more than one mono-radical core. In the poly-radical solution, there are two structural conformations of the poly-radical, extended and folded. The spin-lattice relaxation rate is mainly related to the concentration and the amount of extended or unfolded conformations. However, the spin diffusion is proportional to the number of fast-relaxing centers that correspond to compact, or folded, conformations of the poly-radicals. The folded formation of poly-radicals share similar relaxation properties with the fast-relaxing centers in mono-radicals that we found in Finland trityl and trityl OX063.
Electronic structure investigations of titanium oxide nanoclusters, boron-nitrogen heterocycles, and reaction products of lanthanides with oxygen difluoride and lanthanides with water
(University of Alabama Libraries, 2015) Mikulas, Tanya C.; Dixon, David A.; University of Alabama Tuscaloosa
Advanced electronic structure methods on high performance computers have been used to predict the reactions of lanthanides, properties of liquid chemical hydrogen storage systems, and Fe doped TiO2 nanoclusters. Chapter 2 describes a detailed experimental matrix isolation and computational study of the reactions of lanthanide atoms with F2O. The experimental data is analyzed in terms of the results of density functional theory and CCSD(T) calculations. The products OlnF and OLnF2 are observed, with most Ln in the +III oxidation state for both products. The bonding in these molecules is strongly dependent on the oxidation state of the lanthanide. The coupling of the spin on the O with that on the Ln is important in determining the Ln-O frequency. Chapter 3 describes the reactions of the lanthanides with H2O. The dominant products are LnO + H2 and HLnOH with the Ln in the +II oxidation state. The difference in the reactions of F2O and H2O are due to the differences in the reactant and product bond strengths. Chapter 4 describes combined experimental and computational studies of the liquid chemical hydrogen storage systems based on substituting a C-C with a B-N. Experimental structural analysis and high level electronic structure calculations suggest that the aromaticity of the 1,3-dihydro-1,3-azaborine heterocycle is intermediate between that of benzene and that of 1,2-dihydro-1,2-azaborine. The development of the first reported parental BN isostere of cyclohexane featuring two BN units is thermally stable up to 150 °C with a H2 storage capacity of 4.7 weight% is described. High level computations have been used to predict the reaction energetics of the formation of two cage compounds from the H2 desorption reactions. The photophysical properties resulting from BN/CC isosterism for 10 1,2-azaborine-based BN isosteres of stilbenes have been explained by using high level electronic structure calculations. Chapter 5 describes computational and experimental evidence for facile charge transfer from the transition metal ion Fe(II) to titanium sites in nanoscale TiO2 and its oxynitride, TiO2-xNx. The transfer has been characterized through core level and valance band photoelectron spectroscopies and detailed electronic structure calculations.
Fundamental and applied studies of organic photovoltaic systems
(University of Alabama Libraries, 2014) Hill, Caleb M.; Pan, Shanlin; University of Alabama Tuscaloosa
Presented here are applied and fundamental studies of model organic photovoltaic (OPV) systems. Graphene oxide (GO) nanosheets were investigated as a potential electron acceptor in bulk heterojunction organic solar cells which employed poly[3-hexylthiophene] (P3HT) as an electron donor. GO nanosheets were transferred into organic solution through a surfactant-assisted phase transfer method. Electron transfer from P3HT to GO in solutions and thin films was established through fluorescence spectroscopy. Bulk heterojunction solar cells containing P3HT, P3HT-GO, and P3HT-phenyl-C₆₁-butyric acid methyl ester (PCBM, a prototypical elector acceptor employed in polymer solar cells) were constructed and evaluated. Single molecule fluorescence spectroscopy was employed to study charge transfer between conjugated polymers and TiO₂ at the single molecule level. The fluorescence of individual chains of the conjugated polymer poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) at TiO₂ surfaces was shown to exhibit increased intermittent (on/off "blinking") behavior compared to molecules on glass substrates. Single molecule fluorescence excitation anisotropy measurements showed the conformation of the polymer molecules did not differ appreciably between glass and TiO₂ substrates. The similarities in molecular conformation suggest that the observed differences in blinking activity are due to charge transfer between MEH-PPV and TiO₂, which provides additional pathways between states of high and low fluorescence quantum efficiency. The electrodeposition of individual Ag nanoparticles (NPs), which can be used to enhance light harvesting in organic photovoltaic systems, was studied in situ via dark field scattering (DFS) microscopy. The scattering at the surface of an indium tin oxide (ITO) working electrode was measured during a potential sweep. Utilizing Mie scattering theory and high resolution scanning electron microscopy (SEM), the scattering data were used to calculate current-potential curves depicting the electrodeposition of individual Ag NPs. The oxidation of individual presynthesized and electrodeposited Ag NPs was also investigated using fluorescence and DFS microscopies.
Gas phase ion energetic studies via photoelectron imaging and energy resolved mass spectrometry
(University of Alabama Libraries, 2015) Pruitt, Carrie Jo Michelle; Goebbert, Daniel J.; University of Alabama Tuscaloosa
Nitro containing compounds are well known energetic materials widely used in explosives. The shock sensitivity of nitro containing explosives has been linked to energetic properties such as bond dissociation energies, heats of formations, and molecular electronegativities. Measuring the energetic properties of nitro containing compounds can aide in the development and deployment of these materials, as well as influence practices in safe handling and use. The primary focus of this dissertation is investigating the energetic properties of nitro containing molecules using two experimental gas phase techniques. The first part of this dissertation describes experiments carried out on a homebuilt negative ion photoelectron imaging spectrometer. A detailed description of the construction of this instrument is described in this dissertation. Two studies included in this dissertation highlight the utility of photoelectron imaging. The first photoelectron imaging study was conducted on the CH- molecule. The results from this work allowed for the determination of fundamental properties such as the electron binding energy associated with two separate electronic transitions corresponding to electron detachment from the 1π and 3σ orbitals of CH-. In addition to this, analysis of the photoelectron angular distributions revealed evidence for a temporary excited state of CH- previously undetected. The second photoelectron imaging study focused on nitromethane anion and the dimer, trimer, and hydrated monomer cluster anions. In this work, vertical detachment energies, estimated electron affinities, and solvation energies for each cluster anion were identified from the photoelectron spectra. Theoretical calculations were used to predict cluster structures with calculated detachment energies in agreement with the experimental values. This work suggests the nitromethane clusters are formed by a single anion solvated by additional neutral molecules held together by O--C-H interactions. The later part of this dissertation describes collision induced dissociation and energy resolved mass spectrometry experiments using a commercial triple quadrupole mass spectrometer. Fragmentation pathways in copper nitrate cluster anions, nitrotoluene radical anions, and deprotonated nitrophenols anions as well as dissociation energies were determined from these studies. Dissociation energies were determined from threshold measurements where the apparent cross section of a fragment was measured as a function of the collision cell voltage. The values obtained from these works can be used in the negative ion thermochemical cycle to provide information on corresponding neutral species and are potential useful in the field of energetic materials.
Mass spectrometry studies of peptides cationized by trivalent metal ions
(University of Alabama Libraries, 2018) Commodore-Botoklo, Juliette Joan; Cassady, Carolyn J.; University of Alabama Tuscaloosa
The field of proteomics is dedicated to understanding how a protein’s structure and function relates to human health and disease. Peptide sequencing by mass spectrometry is important to the proteomics movement. Unfortunately, sequencing of many peptides and proteins, such as those with residues containing acidic and neutral side chains, can be difficult. Acidic side chains undergo facile deprotonation that make analysis challenging and can hinder formation of positive mode ions. New methods of sample preparation and dissociation techniques are needed to increase sequence information. This dissertation includes an extensive study of the effects on electron transfer dissociation (ETD) mass spectrometry of biological and model acidic non-phosphorylated and phosphorylated peptides adducted to trivalent lanthanide metal cations. Mass spectra contained herein provide abundant information about the primary structure of peptides. The ETD process requires multiply positively charged ions that can be difficult to obtain with acidic peptides. This work demonstrates that addition of trivalent lanthanide metal cations allows highly acidic peptides to be analyzed by ETD by forming multiply positively charged precursor ions by electrospray ionization (ESI). Using trivalent lanthanide cations as ionizing agents yields extensive sequence information for highly acidic peptides including definitive identification of phosphorylation sites. Peptides forming [M + Met + H]4+ and [M + Met]3+ generate full sequence coverage in many cases, but [M + Pr – H]2+ generates less sequence coverage. (Met is the trivalent metal cation.) The spectra contain primarily a mix of non-metallated and metal adducted c- and z- ions. All metallated product ions incorporate at least two acidic sites or a highly acidic phosphoresidue, which strongly suggests that the trivalent metal cation coordinates with residues that contain highly acidic side chains. All trivalent lanthanide cations are suitable for sequencing highly acidic peptides except europium and radioactive promethium. ETD spectra contain high signal-to-noise ratios making identification of product ions straightforward. Sequence coverage generally improves with increasing peptide chain length. Trivalent chromium enhances protonation of neutral peptides, which is important to ETD analysis. ESI conditions, particularly drying and nebulizing gas pressures are critical to formation of [M + 2H]2+ by neutral peptides.
Metal oxide thin films by chemical vapor deposition for photocatalytic water splitting
(University of Alabama Libraries, 2016) Panikar, Archana Sathyaseelan; Gupta, Arunava; University of Alabama Tuscaloosa
Photoelectrochemical cells (PEC) are devices which convert solar energy into consumable chemical energy by splitting water into oxygen and hydrogen. Photocatalytic activity at a semiconductor oxide surface forms the backbone of the PEC and thus the quest for high activity oxide materials and improving the cells efficiency is a widely explored field of research. Metal oxide semiconductors with band gaps in the visible spectrum are actively sought as photocatalytic electrode materials. The major advantages are that oxides are nontoxic, stable, and inexpensive. However, their overall efficiency is usually limited by short carrier diffusion length due to structural defects, limited light absorptivity and sluggish kinetics at the interface. To overcome these limitations crystalline semiconductor oxides synthesized by high temperature techniques are desired. A direct liquid injection chemical vapor deposition technique has been employed to synthesize films of Fe2O3 (hematite) and BiVO4 (bismuth vanadate) for use as photocatalysts. The high temperature synthesis technique is optimized to obtain good quality crystalline smooth films on fluorine doped tin oxide substrates and their photoelectrochemical characteristics have been studied. It is observed that the interlayer oxide material used for growth of the Fe2O3 and BiVO4 has a significant role in their photoactivity.The interlayer oxide serves as an efficient electron transport layer and also influences the grain characteristics of the film. For hematite it is observed that a n-type metal oxide interlayer (e.g. Nb2O5 or TiO2) helps improve the photoactivity as compared to a p-type oxide (NiO). BiVO4 has a poor electron diffusion length, and a WO3 interlayer improves the photocurrent in BiVO4 films by improving the charge collection efficiency. The low absorption coefficient of hematite requires a dense electrode for greater light absorption; however, the electrode thickness is limited by the poor hole diffusion length (~4 nm). Plasmonic metal nanostructures of gold (Au), silver (Ag), and copper (Cu), which are known to concentrate and scatter broad range wavelengths of incident light, are promising for enhancing the light absorption cross-section of a semiconducting material. Gold nanoparticles embedded in hematite films have been synthesized. About three times higher light absorption and photocurrent enhancement are obtained. A thickness-dependent study of photoactivity indicates a greater enhancement of gold-embedded hematite thin films compared to thicker films due to reduced charge transport distance and optimal local field enhancement effect. The embedded structure also has the advantage of consistent performance and protection of plasmonic nanostructures from electrochemical corrosion, resulting in long cycles of operation.
Nanocrystals and thin films of oxide and chalcogenide spinels for spintronic applications
(University of Alabama Libraries, 2010) Wang, Yu-Hsiang; Gupta, Arunava; University of Alabama Tuscaloosa
Spin-based transport in semiconductor systems has been proposed as the foundation of a new class of spintronic devices. For the practical realization of such devices it is important to identify magnetic materials with diverse electronic transport properties (metallic, semiconducting, insulating) and sufficiently high Curie temperature (T_C ) that can be readily integrated with standard semiconductors. Promising classes of materials for this purpose are the magnetic spinel oxides and chalcogenides. Some of these spinel-based materials are also attractive for biomedical applications. The facile solution-based synthesis of monodisperse nanocrystals of a wide variety of magnetic ferrites and nanocrystals of the chalcospinel CuCr_2 Se_4 , along with their structural and magnetic properties, is presented in the first section of the dissertation. The following section presents a theoretical investigation of the electronic band structure of two quaternary chalcospinel systems, Cd_x Cu_1-x Cr_2 Se_4 and Cd_x Cu_1-x Cr_2 S_4 , and also a number of anion-substituted Cr-based chalcospinels. A wide range of half-metal compositions are predicted both for the cation and anion substituted chalcospinels. The synthesis of spinels has been expanded to the growth of ferrites films using the direct liquid injection chemical vapor deposition (DLI-CVD) technique, which is detailed in the last section of the dissertation. High quality epitaxial NiFe_2 O_4 films have been grown using this technique with the magnetic properties of the films being comparable to those observed in the bulk, even for films grown at a high deposition rate. The growth of other thin film ferrites, such as lithium ferrite and barium hexaferrite, which are useful for higher frequency microwave applications are being investigated. The eventual goal is to use extend the DLI-CVD technique for the synthesis of chalcospinels films - in particular those predicted to be half-metallic - which have the potential for a variety of applications in spintronic devices.
Nanostructured silver for applications in surface enhanced Raman spectroscopy and photoelectrochemical reactions
(University of Alabama Libraries, 2014) Clayton, Daniel Adam; Pan, Shanlin; University of Alabama Tuscaloosa
Initial work focused on characterizing silver and its surface enhanced Raman spectroscopy (SERS) capabilities. Silver nanowires were chosen as an ideal material and scanning confocal microscopy studies were performed to identify hot spots. The silver nanowires were found to exhibit fluorescence blinking that was attributed to small silver clusters undergoing rapid interchange from Ag0 to Ag₂O. Control of this blinking was accomplished through the removal of oxygen and through electrochemical control of the system. SERS was also recorded from these nanowires. Deconvolution of the SERS signal from the fluorescence was accomplished either by increasing the SERS analyte concentration or increasing the total number of "hot spots" in the focus volume. Silver applications were studied by performing a SERS study of Rhodamine 6G (R6G) and Poly(3-hexylthiophene-2,5-diyl) (P3HT). A Tollens' silver substrate was utilized as the SERS substrate and similar blinking effects were found to arise. P3HT was cast from 4 different solvents:dichloromethane, chlorobenzene, THF, and toluene. The solvent effects were studied, with kinking of the polymer noted in the non-chlorinated solvents. Single molecule studies in conjunction with polarization control indicated that the P3HT formed in an overlapping manner with only partial charge transfer within the molecule. Finally silvers interactions with TiO₂ were studied. Micron scale single crystal anatase TiO₂ was synthesized by using HF in a hydrothermal process forming a truncated bipyramidal structure consisting of [101] and [001] faces. Fluorine was present in small amounts on the surface of the TiO₂ as confirmed by x-ray photoelectron spectroscopy (XPS). An annealing process was used to remove the fluorine. Nitrogen doping was attempted, but was not found to occur in significant amounts. Visible light sensitivity was noted in annealed samples but did not occur in the bulk as demonstrated through photoelectrochemical measurements. Silver photoreduction directly on the surface of the TiO₂ crystals revealed visible light sensitivity at surface defects. No facial preference was noted for the silver growth through energy-dispersive X-ray spectroscopy (EDX) images. A secondary method of silver attachment through a linker molecule showed that on-resonance silver structures provided greater SERS enhancement dependent upon the direction of the linker molecule.
New materials for optical sensing of explosives copolymers containing 2-vinyl-4,6-diamino-1,3,5-triazine and co-crystals of electron rich aromatic molecules and 1,3-dinitrobenzene
(University of Alabama Libraries, 2013) McNeil, Steven Keith; Nikles, David E.; University of Alabama Tuscaloosa
This dissertation focuses on the development of electron rich polymers with an affinity for nitroaromatics. Thin polymer films of the electron rich polymers could be applied in an optical waveguide sensor to detect nitroaromatics by changes in the optical properties of the polymer thin films. Charge transfer complexes between electron rich aromatic reagents and electron deficient nitroaromatics were produced providing an understanding of the intermolecular interactions between the electron donor and electron acceptor. Electron rich copolymers were synthesized with 2-vinyl-4,6-diamino-1,3,5-triazine (VDAT) using a published literature procedure. The polymerization procedure was extended to a variety of electron rich monomers, resulting in the production of a number of electron rich copolymers. Thin films of the copolymers were spin coated and their optical properties were characterized by spectroscopic ellipsometry before and after exposure to a nitroaromatic vapor. The exposure to the nitroaromatic vapor allowed the formation of complexes with the electron rich copolymers and the nitroaromatic molecules, creating a change in the optical properties of the polymer films. This refractive index change after exposure to a nitroaromatic demonstrated the possibility of these films to be applied in an optical waveguide sensor for explosive detection. Co-crystals were grown between electron rich donors and the electron deficient 1,3-dinitrobenzene by the slow evaporation method. When the electron donor solution and electron acceptor solution were combined in a crystallization dish, significant color changes were observed. The interaction between the electron donor and electron acceptor were characterized using analytical techniques.
Part A: arenepolythiols synthesis and doping study of semiconducting metal-organic frameworks part b: mechanistic study of the bis(trineopentylphosphine)palladium catalyzed Buchwald-Hartwig amination
(University of Alabama Libraries, 2016) Hu, Huaiyuan; Shaughnessy, Kevin H.; University of Alabama Tuscaloosa
Part A A synthesis is reported for 9,10-dimethyl-2,3,6,7-anthracenetetra(thioacetate), an anthracenetetrathiol derivative that may find use in molecular electronics and conducting metal-organic frameworks (MOFs). The easily accessible 2,3,6,7-tetrahydroxy-9,10-dimethylanthracene was treated with 2-(4-pyridyl)ethanethiol in molten p-toluenesulfonic acid to yield the tetra(thioether), which was methylated and then deprotected with triethylamine in the presence of acetic anhydride to yield 9,10-dimethyl-2,3,6,7-anthracenetetra(thioacetate), a protected form of the tetrathiol. Separately, the first synthesis of benzenepentathiol and streamlined syntheses of 1,2,4,5-benzenetetra(thioacetate) and benzenehexathiol are reported. By using ethylenediamine as the solvent, MOFs of benzenepolythiols are successfully synthesized. Semiconducting [Pb(C6S6)3]n, with conductivity of 7.8 x 10-8Ω-1cm-1, was selected as the doping substrate. The conductivity of p-type doped Pb3-xTlx(C6S6) increased to 2.4 × 10-6 Ω-1cm-1, which is strong evidence that at least some of the Tl has been incorporated substitutionally on Pb sites and is acting as an electrical dopant in Pb3-xTlx(C6S6). However, analogous methods using n-type dopants, like benzenenpentathiol, Bi and Sb, didn't form products with increased conductivity. Part B The catalyst derived from Pd2(dba)3 and trineopentylphosphine displays high catalytic efficiency as well as functional group tolerance for the coupling of a wide scope of sterically demanding aryl bromides and chlorides with sterically hindered aniline derivatives. More interestingly, the Pd2(dba)3/ PNp3 showed limited sensitivity to the steric demand of the substrates based on both isolated yields and time required for reaction completion. To simplify the mechanistic study of this catalyst system, Pd(PNp3)2 was synthesized and characterized to be used as the pre-catalyst. In the Pd(PNp3)2 catalyzed catalytic cycle, ligand dissociation is rate limiting for aryl bromides, and oxidative addition is rate limiting for aryl chloride. Since oxidative addition occurs after the rate limiting step for aryl bromides, the identity of aryl bromide should not affect the overall catalytic rate. With hindered aryl amines, the coupling rate is independent of the steric demand of aryl bromides. With aniline, the coupling rate is slower and less hindered aryl bromides react faster than more hindered examples. The aniline inhibition is due to competitive conversion of the aryl-palladium intermediate into a catalytically inactive palladacycle, which occurs faster with more hindered aryl-palladium complexes.
Polymer-mediated metal and metal oxide nanoparticles: synthesis, characterization and controlled assembly
(University of Alabama Libraries, 2009) Bai, Litao; Street, Shane C.; University of Alabama Tuscaloosa
Functionalized nanoparticles and their assembly have been the subject of intense recent research activity. The stability and assembly of functionalized nanoparticles are still challenges for practical applications. Polymers are excellent stabilizing agents which can be used in nanoparticles synthesis. This dissertation describes polymer mediated nanoparticles which synthesized by both chemical and photo reduction techniques. The synthesis methods and assembly of polymer mediated nanoparticles were systematically studied. Two kinds of polymers were used in this research: PAMAM dendrimer and hyperbranched polyethylenimine. PAMAM dendrimer was used as the host for the synthesis of FePt nanoparticles with narrow size distribution by chemical reduction. Magnetic and structural properties of the as-made and annealed samples were studied. The as-made FePt nanoparticles have the chemically disordered fcc structure and can be transformed into the chemically ordered fct structure after annealing. But the annealing process caused both the size and size distribution to increase. To prevent size increase during annealing, dendrimer mediated alloys were encapsulated in silica microspheres. Their magnetic properties can be manipulated by synthesis conditions. Polyethylenimine is a kind of hyperbranched polymer, which was used to encapsulate nanoparticles. With polyethylenimine, Pt and Co₃O₄ nanoparticles were synthesized by photochemical processes. Their formation mechanisms were discussed. The chemical and electrochemical catalytic activity of polyethylenimine mediated Pt nanoparticles were studied. The results indicate that polyethylenimine is a good capping and reducing agent, and a carrier which improve samples stability and allows patterning. Ordered discrete magnetic nanoparticle arrays were also fabricated. Polyethylenimine was used as a stabilizer to synthesize magnetite nanoparticles. Polyethylenimine coating of magnetite nanoparticles can effectively prevent the aggregation of the nanoparticles. By combining the capillary filling technique with a magnetic field, polyethylenimine mediated magnetite nanoparticles form ordered assembly, in which the resolution of the pattern is doubled. The formation process of ordered structures was systematically investigated. This method is technologically feasible and scalable to make ordered magnetic nanoparticles over a large area. The result of this dissertation demonstrates that polymer stabilizers play an essential role in the synthesis of nanoparticles as well as their assembly.
Preparation of perfluorinated ionomers for fuel cell applications
(University of Alabama Libraries, 2012) Sayler, Todd Stephen; Thrasher, Joseph S.; University of Alabama Tuscaloosa
One of the major issues with the current membrane technology for polymer electrolyte membrane fuel cells is the low conductivity seen at low relative humidity. This dissertation discloses the preparation of perfluorinated polymers with higher densities of acid sites and higher conductivities to overcome this issue. These materials are prepared using a system designed to safely synthesize and polymerize tetrafluoroethylene (TFE) on a hundred gram scale. The copolymerization of TFE and perfluoro-2-(2-fluorosulfonylethoxy) propyl vinyl ether (PSEPVE) to prepare materials with varying ratios of the two monomers was carried out by solution, bulk, and emulsion polymerization techniques. Additionally, the homopolymer of PSEPVE has been prepared and characterized by MALDI-TOF mass spectrometry, which shows the low molecular weight distribution seen in other similar materials in the literature is due to a high rate of β-scission termination. Spectroscopic measurements and thermal analysis were carried out on these samples to obtain better characterization than was currently available. Producing polymers with a higher amount of PSEPVE, and thus higher density of acid sites, leads to the materials becoming water soluble after hydrolysis. However, addition of a curable ter-monomer allows the polymer chains to be crosslinked to regain water insolubility. Using this approach, water insoluble membranes with high densities of acid sites and conductivities up to 5.5 times higher than Nafion® 115, the standard benchmark for fuel cell membranes, have been produced. Preparation of high molecular weight, low EW copolymers of TFE and PSEPVE is difficult due to the reactivity ratios of the two monomers. Literature reactivity ratios for VDF and PSEPVE are more favorable for preparation of high molecular weight, low EW copolymers. Here, alternating copolymers of VDF and PSEPVE are prepared for the first time; where high molecular weight samples have been shown to possess low swelling characteristics in water. It has also been found the lower molecular weight samples that are soluble in perfluorohexane can be converted to perfluorinated polymers by direct fluorination with 20% elemental fluorine in nitrogen with 254 nm UV irradiation.
Structure, heats of formation, and bond dissociation energies of group IIIa-group IVA-group VA molecules for chemical hydrogen storage systems
(University of Alabama Libraries, 2010) Grant, Daniel Justin; Dixon, David A.; University of Alabama Tuscaloosa
The potential of Group IIIA—IVA—VA compounds for chemical hydrogen storage have been evaluated from thermodynamic properties, heats of formation and bond dissociation energies (BDEs), from CCSD(T) calculations in conjunction with correlation consistent basis sets extrapolated to the complete basis set limit, including additional core-valence, scalar-relativistic, and atomic spin-orbit corrections. Geometry optimizations and frequencies were computed at the CCSD(T)/MP2 levels. Diatomic distances, frequencies, and anharmonic constants were obtained from a potential energy curve fit at the CCSD(T) level. Calculations show that AlH_3NH_3(g), AlH_3PH_3(g), [AlH_4^-][NH_4^+](s), [AlH_4^-][PH_4^+](s), and [BH_4^-][PH_4^+](s) can potentially serve as hydrogen storage systems, in addition to BH_3NH_3 and [BH_4^-][NH_4^+](s). Dehydrogenation of methyl-substituted ammonia boranes is most favorable across B-N where methylation at N reduces the reaction exothermocity, becoming more thermoneutral. The adiabatic π-bond energy is defined as the rotational barrier between the ground state and C_s transition state structures, the intrinsic π-bond energy as the adiabatic rotational barrier corrected for inversion, and σ-bond energy, as the adiabatic dissociation energy minus the adiabatic π-bond energy. Within the substituted boranes H_(3-n)BX_n (X = F, Cl, Br, I, NH_2, OH, and SH), fluorines have the largest BDEs while the second and third largest are for hydroxyl and amino. Hydride and fluoride affinities have been predicted to judge the Lewis acidities with the highest affinities found for BI_3, lowest for B(NH_2)_3, and within the boron trihalides, the acidity increases down the periodic table. Although the sequential dehydrogenation of diammoniosilane is exothermic, further dehydrogenation is largely endothermic, requiring an effective coupling process to remove three hydrogen molecules thermoneutrally. Except for methyliodosilane, methyl and halide substitution increases the Si-X and Si-C BDEs compared to the halosilanes and methylsilane, respectively. The differences in the adiabatic and diabatic BDEs in the PF_xO and SF_xO compounds are employed to explain trends in their stepwise BDEs. The adiabatic BDE for removal of fluorine from stable closed-shell SF_6 to give the unstable SF_5 radical is 2.8 times the BDE for removal of fluorine from the unstable SF_5 radical to give stable closed-shell SF_4. Simlar principles govern the BDEs of the phosphorous fluorides and the phosphoro and sulfur oxofluorides.
Surface enhanced nanostructured electrode materials for solar energy harvesting and conversion
(University of Alabama Libraries, 2016) Shan, Zhichao; Pan, Shanlin; University of Alabama Tuscaloosa
This dissertation presents surface enhanced photoanodes and oxygen evolution reaction (OER) catalysts for solar water splitting to produce hydrogen. The enhancement could be achieved by either introduced surface plasmon enhanced metallic nanostructures, such as Au or Ag nanoparticles, or adjusted surface structure, chemical composition and band structure of TiO2. This dissertation also presents various electrochemical and spectroscopic techniques used to characterize nanostructured materials for solar water splitting reactions. Firstly, a model photoanode comprised of Ag@Ag2S core-shell nanoparticles (NPs) on a nanostructured TiO2 substrate is presented for visible light sensitive photoelectrochemical properties. The nanostructured electrode is coated with TiO2 nanowires (NW) on Ti plate to provide a high surface area for efficient light absorption and efficient charge collection from Ag@Ag2S NPs. Pronounced photoelectrochemical responses of Ag@Ag2S NPs under visible light responses were obtained. These responses were attributed to collective contributions of local surface plasmon enhancement, enhanced charge collection by Ti@TiO2 NWs, and high surface area of the nanostructured electrode system. The shell thickness and core size of the Ag@Ag2S core-shell structure can be controlled and the optimal photoelectrochemical performance with a core size of 17 nm (in diameter) and shell thickness of 8 nm was formed. Secondly, a Au@CdS/Ti@TiO2 nanostructured photoanode was prepared by decorating a CdS thin film layer onto a Au/Ti@TiO2 NWs substrate. Compared to CdS/Ti@TiO2 NWs photoanode, Au@CdS/Ti@TiO2 exhibits a significant enhancement to water splitting efficiency. iii The enhanced photoelectrochemical catalytic activity is attributed to the surface plasmon enhancement of Au nanoparticles. XPS, XRD, SEM, EDS, high resolution TEM, AC impedance and other electrochemical methods were applied to resolve the structure-function relationship of the nanostructures of Ag@Ag2S/Ti@TiO2 NWs and Au@CdS/Ti@TiO2 NWs electrodes. The studies of the photocatalytic activity of the core-shell structure, as well as a core-shell structure predictive model can further improve the understanding of the interplay between the shell thickness and core size and guide the design of highly efficient core-shell materials. Lastly, chapter 5 of this dissertation presents a high efficiency, durable, and low-cost oxygen evolution reaction (OER) catalyst based on earth-abundant elements, carbon, oxygen, and titanium for renewable energy conversion and storage devices. In this study, we report a highly active nanostructured electrode NanoCOT (C, O and Ti) for an efficient OER in alkaline solution. The NanoCOT electrode is synthesized from the carbon transformation of nanostructured TiO2 in an atmosphere of methane, hydrogen and nitrogen by a CVD process. The NanoCOT exhibits highly enhanced OER catalytic activity in alkaline solution, providing a current density of 1.33 mA/cm2 at an overpotential of 0.42 V, which is about 4 times higher than an IrO2 electrode and 15 times higher than a Pt electrode because of its nanostructured high surface area and favorable OER kinetics. The enhanced OER catalytic activity of NanoCOT is attributed to the presence of a continuous energy band of the titanium oxide electrode with predominantly reduced defect states of Ti (e.g., Ti1+, Ti2+ and Ti3+) formed by chemical reduction with hydrogen and carbon. OER performance of NanoCOT can also be further enhanced by decreasing its overpotential 150 mV at a current density of 1.0 mA/cm2 after coating its surface electrophoretically with 2.0 nm IrOx nanoparticles (NPs).

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Browsing Theses and Dissertations - Department of Chemistry & Biochemistry by Author "Bakker, Martin G."

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