NIRT: Active nanoparticles in nanostructured materials enabling advances in renewable energy and environmental remediation

Issued as final reportUniversity of Alabam

Synthesis and characterization of semiconductors via two routes: coordination frameworks and metal organic frameworks

The extension of the Creutz-Taube ion into an extended framework composed of mixed-valence ruthenium metal centers bridged by pyrazine organic spacers is described starting from pyrazine containing ruthenium precursors. The synthesis of a novel ruthenium(III) complex, [RuCl_2 (pyz)_4][BF_4] (where pyz = pyrazine), is described. The reaction of ruthenium precursors at high temperatures in a sealed reactor vessel yields a semiconducting, crystalline material, believed to be either a one- or two-dimensional network of ruthenium metal centers bridged by pyrazine, and is currently being studied under synchrotron x-ray diffraction experiments in order to determine the structure. The synthesis of cobalt precursors for an analogous experiment to form cobalt-based frameworks is reported. The synthesis of a novel anthracene-based protected tetrathiol is described, and reactions metal salts in various solvents to form extended frameworks has been attempted. Progress on the synthesis of alkyl-protected benzene selenols is reported, and initial reactions with lead salts are described. (Published By University of Alabama Libraries

Fundamental and applied studies of organic photovoltaic systems

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. (Published By University of Alabama Libraries

The nature of the metal-molecule interface revealed through tunneling spectroscopy

This dissertation describes the fabrication and characterization of molecular tunnel junctions made with a series of carboxylic acid derivatives chemisorbed to amorphous aluminum oxide (AlOx) electrodes with a variety of top metal electrodes. Para-substituted benzoic acids, terminal-substituted long alkyl chain (C16) alkanoic acids, and octadecylphosphonic acid were prepared on the native oxide of aluminum by a solution self-assembly technique. The self-assembled monolayers (SAMs) were characterized via x-ray photoelectron spectroscopy (XPS) and water contact angles. For the benzoic acid derivatives, several trends became apparent. The polarity of the solvent used during self-assembly was critical to monolayer formation. Solvents that were polar and protic in nature produced low coverage monolayers. In contrast, non-polar solvents produced much higher coverage monolayers. The thickness of the monolayers determined via XPS suggests that the plane of the aromatic ring is perpendicular to the AlOx surface with the carboxylate functional group chemisorbed in a bidendate chelating geometry. A similar saturation coverage was achieved for each benzoic acid derivative at approximately 2.7 x 1014 molecules cm-2. At saturation coverage, the benzoic acids and hydroxyls are present in an approximate ratio of 1:1. When Pb was vapor deposited on top of the benzoic acid derivatives and XPS measurements were obtained, only 4-SH benzoic acid showed a chemical reaction in the form of S-Pb. Au also showed a complete chemical reaction with 4-SH benzoic acid with a shift in the S(2s) peak 1.2 eV lower in binding energy indicative of a S-Au bond formation. Contact angle and XPS results of the long alkyl chain SAM derivatives reveal a higher surface coverage of molecules on AlOx when compared to the benzoic acids. Electrical measurements including: I vs. V, dI/dV [normalized as G(V)] vs. V, and IETS were also obtained. G(V) measurements obtained on tunnel junctions without SAMs showed large offsets in the G(V) minimum [G(V)min] conductance due to polar hydroxyl groups native to the AlOx surface introducing large asymmetry in the barrier. Upon addition of the carboxylic and phosphonic acid monolayers, the G(V)min shifted closer to zero bias. For tunnel junctions made with benzoic acid SAMs and Pb, several trends emerged. First, as the coverage of the SAM increased, the G(V)min decreased. Second, as the size of the para-substituent increased, the G(V)min decreased. Finally when a reactive para-substituent such as thiol is used, the barrier asymmetry is completely reversed due to a bond formation at the S-Pb interface. G(V) measurements obtained on various alkanoic acid SAMs showed no offset in the G(V)min. The data obtained was fit to a model to extract the tunnel barrier properties. The results showed that when a reactive terminal group such as thiol is used, the effective thickness of the tunnel barrier is larger than samples made with CH3 terminated SAMs. This suggests that less of the top metal can penetrate through monolayers when a chemical reaction occurs at the metal-molecule interface. Data obtained for tunnel junctions made with octadecylphosphonic acid SAMs showed very little penetration of the top metal likely due to higher saturation coverage and better packing. (Published By University of Alabama Libraries

Magnetic, electrical and magnetotransport properties of Cr O_2 and V O_2-based thin films and heterostructures

In this dissertation, thin films of two promising rutile oxide materials (CrO2 and VO2) are studied. Additionally, magnetic tunnel junctions (MTJs) with these two materials as ferromagnetic (CrO2) and barrier layer(VO2) are fabricated and their properties are investigated. The CrO2 thin films are successfully grown on TiO2 (001) substrates by atmospheric pressure chemical vapor deposition (APCVD). Their structural and magnetic properties have been examined. The Stoner-Wohlfarth model is used to extract the distribution of the effective anisotropy field in the CrO2 (001) films for providing a better understanding of the out-of-plane magnetic behavior. The unexpected in-plane magnetic behavior is explained by the possible existence of stripe or vortex domain structures in the films. Besides CrO2, VO2 thin films and CrO2/VO2 heterostructures have been grown on TiO2 substrates of different orientations - (100), (110) and (001) - and their electrical and magnetic properties are studied. Finally, MTJs with CrO2 as the ferromagnetic electrode, heteroepitaxial VO2 as the barrier layer, and Co as the counter electrode are fabricated, and their transport and magnetic properties are investigated. The bias, temperature and barrier thickness dependence of the tunneling magnetoresistance (TMR) of these CrO2/VO2-based MTJs are presented. The Simmons and Brinkman models are used to estimate the barrier height of the tunneling device. In addition, the magnetic behavior of the MTJs at different temperatures is studied. (Published By University of Alabama Libraries

Nanostructured silver for applications in surface enhanced Raman spectroscopy and photoelectrochemical reactions

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. (Published By University of Alabama Libraries

The effect of fluoride on the crystallinity and photoactivity of titania

This dissertation describes the synthesis and characterization of fluorinated N-doped TiO_2 nanoparticles under ambient conditions. Samples were synthesized by sol-gel methods that utilized the controlled hydrolysis of titanium(IV) tetra-isopropoxide in acidic solutions. Nitrogen doping was achieved by two different methods. In one scheme triethylamine (TEA) was added post-synthesis to the nanoparticle formation. In the other scheme, ammonium chloride (NH_4 Cl) was added during the acid catalyzed hydrolysis reaction. A freeze-drying process of the sol-gel was used to prevent aggregation during dehydration and was found to retain the high surface area of the powder. Post-synthesis the hydroxyl groups on the surface were exchanged with fluoride by stirring the powders in acidic solution of NaF. Using this synthetic approach the amount of nitrogen and fluoride could be independently controlled. The nanoparticles were characterized by numerous spectroscopic techniques including DRS, FTIR, Raman, and XPS. Vibrational spectroscopy shows that the particles contain significant amounts of organic impurities after doping with TEA. In contrast, particles synthesized with NH_4 Cl showed much less contamination. XPS analysis revealed that a single nitrogen species with a binding energy of 400.6 eV when using TEA as the N precursor. In contrast, when NH_4 Cl was used as the nitrogen precursor two nitrogen species were observed with binding energies at 402.6 and 401.2 eV. These latter peaks are assigned to interstitial nitrogen in the N^(1+) and N^0 oxidation states. The as-synthesized nanoparticles also show a significant differences in their optical properties. In general, the particles doped from TEA and NH_4 Cl were yellow and white, respectively, despite containing approximately the same amount of nitrogen (~5% with respect to Ti). The difference is attributed to a high fraction of oxygen-vacancies in the TEA doped nanoparticles. XRD and Raman measurements determined that the as-synthesized samples were amorphous, but could be converted to the anatase phase by two different methods. Thermal annealing was shown to convert the amorphous particles to the anatase polymorph. The presence of surface fluoride was found to significantly lower the temperature to observe the amorphous to anatase transition. In the second method, stirring the powders in acidic solutions of NaF at room temperature for 12-168 hours produced the anatase phase with an average crystallite size of 4 nm. It was found that the phase transition only occurs when the pH is below the point of zero charge of the particles. The photoactivity of the nitrogen and nitrogen / fluoride-doped particles was tested for their ability to degrade methylene blue (MB) with visible light (> 400 nm). In general the particles with a surface fluoride were more photoactive that those without. In addition, particles with nitrogen were more photoactive than pure TiO_2 . By analyzing the decomposition products with electrospray ionization mass spectrometry and UV-Vis spectroscopy, it was possible to elucidate a different decomposition pathway for the nitrogen-doped samples. When TEA was the dopant precursor, MB primarily decomposed by a ring-cleavage pathway using superoxide. In contrast, when NH_4 Cl was the dopant precursor, MB decomposed through demethylation pathway induced by hydroxyl radicals. The as-synthesized particles were found to be more photoactive those thermally annealed. The loss of photoactivity could be ascribed to two main factors: (1) loss of nitrogen and fluoride and (2) loss of surface area by sintering. (Published By University of Alabama Libraries

Electron transfer dissociation mass spectromerty studies of peptides

Electron transfer dissociation (ETD) is an important tandem mass spectrometry technique in peptide and protein sequencing. In the past, ETD experiments have primarily involved basic peptides. A limitation of ETD is the requirement that analytes be at least doubly cationized by electrospray ionization (ESI). In this research, a method has been developed for enhancing protonation of acidic and neutral peptides. This has allowed doubly protonated ions, [M+2H]2+, to be produced from peptides without basic residues and has enabled their study by ETD. This dissertation includes the first extensive study of non-basic peptides by ETD. The effects of a basic residue on ETD were investigated using a series of heptapeptides with one lysine, histidine, or arginine residue. The spectra contain primarily c"- and z'-ions, which result from cleavage of N-C_α bonds along the backbone. Almost all of product ions include the basic residue. Enhanced fragmentation occurs on the C-terminal side of the basic residue. Also, cn-1 formation is enhanced, where n is the number of residues in the peptide. Addition of Cr(III) nitrate to a solution of the neutral peptide heptaalanine yields abundant [M+2H]2+ formation by ESI. Eleven metal ions were tested and Cr(III) gave by far the most intense supercharging of peptides. In contrast, Cr(III) does not increase protonation of proteins. Experiments were performed to explore the supercharging mechanism. Addition of Cr(III) to the sample solution was used to produce [M+2H]2+ in the remainder of this research. Neutral peptides with alkyl side chains were studied by ETD and found to produce b- and c-ions. Two mechanisms are proposed for b-ion formation, which involves cleavage of backbone amide (O=C)-N bonds. The length of peptide chain affects ETD fragmentation, but the identity of the alkyl residue has minimal effect. Acidic peptides with one or two aspartic or glutamic acid residues produce b-, c- and zOe-ions. The mechanism of b-ion formation is probably the same as that for neutral peptides, while c- and zOe-ions result from a radical mechanism involving oxygen atoms on the acidic side chains. For highly acidic heptapeptides, c- and zOe-ions are the major products, which supports a radical mechanism. (Published By University of Alabama Libraries

Applications of polyamidoamine dendrimers in polymer electrolyte membrane fuel cells

Dendrimers are highly branched macromolecules with well-ordered three- dimensional architectures. Polyamidoamine (PAMAM), the most common class of dendrimers, have been widely studied due primarily to the following three features: 1) the interior amine and amide groups that can interact with ionic metal precursors through ligand exchange reactions; 2) the presence of an interior void space in the higher generation dendrimers; and 3) the exterior primary amine groups that permit further functionaliztion. These unique structural features have inspired many potential applications. This dissertation describes two applications of PAMAM dendrimers in polymer electrolyte membrane fuel cells (PEMFCs). First, in an effort to improve the utility of Pt in PEMFCs, PAMAM G4 was used as both a template and a stabilizer to synthesize dendrimer encapsulated Pt nanoparticles (Pt DENs) by photoreduction. These nanoparticles are highly monodisperse, exhibit high specific activity for the oxygen reduction reaction, and are inert to methanol oxidation, showing great potential for application in PEMFCs. Then, a simplified membrane electrode assembly (MEA) has been fabricated by the electrostatic self-assembly between Nafion® and Pt DENs and characterized. Two methods were proposed to increase Pt loading: layer-by-layer self-assembly and immobilization of Pt DENs and carbon powder on carbon fibers. Approximately 80 layers were proposed to reach the required loading using a dipping machine. Immobilization of Pt DENs and carbon powder simultaneously on carbon fibers can easily be achieved by electrochemical coupling, which is promising for replacing the conventional method of electrode fabrication. Secondly, in order to reduce the methanol crossover in direct methanol fuel cells (DMFCs), PAMAM G0 doped Nafion® membranes were prepared. Direct TEM imaging of the Naifon® embedded with nanoparticles demonstrates that PAMAM G0 can penetrate into the bulk of Nafion® through cluster channels to re-organize the distribution of sulfonate clusters by interacting with the sulfonic acid groups in different clusters. The presence of PAMAM G0 in the Nafion® membrane causes reduction of both methanol permeability and proton conductivity, but a very beneficial trade off can be reached when a doping concentration of 10⁻⁴ M PAMAM G0 is used. The fuel cell performance is much improved When Nafion® was treated with 10⁻⁴ M PAMAM G0. (Published By University of Alabama Libraries

Nanocrystals and thin films of oxide and chalcogenide spinels for spintronic applications

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. (Published By University of Alabama Libraries