Browsing by Author "Elliott, C. Michael, advisor"
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Item Open Access Bipyridyl cobalt complex mediators in dye-sensitized solar cells(Colorado State University. Libraries, 2008) Scott, Michael J., author; Elliott, C. Michael, advisorDye-sensitization of semiconductor substrates allows for efficient charge injection into the semiconductor conduction band. Dye-sensitized solar cells (DSSCs) exploit this for conversion of light into electrical energy. By employing mesoporous TiO2 a significant portion of visible light can be absorbed. The mesoporous TiO2, deposited on a transparent conducting oxide (TCO) medium, constitutes the photoanode of the DSSC. A wide range of materials may be used as a cathode. A redox electrolyte solution completes the cell. Typically, the I-/I3- redox couple has been employed in DSSCs. The use of bipyridyl cobalt complexes allows for tuning of the cell's electrochemistry, exploration of diverse cathode materials, and investigation of mediator solution additives. Cobalt complexes with alkyl, ester, and amide functionalities were considered throughout this body of work. The cobalt complexes were investigated on the basis of time dependence and electrode dependence. The cobalt complexes are stable for at least a period of one week when dissolved in γ-butyrolactone. Gold, carbon and modified TCO cathodes perform well in cells employing the alkyl substituted complex. Gold cathodes alone provide the best performance with cells employing the ester and amide substituted complex. An optically transparent cathode was developed for use in stacked DSSCs, allowing light that is not absorbed by the first DSSC in a stack to be absorbed by a second cell. A spectrally complementary dye in the second cell extends the light absorption to longer wavelengths. Spatial current images were obtained to investigate the local current behavior of cobalt mediated cells. Intentional electrode damage was visualized, and the effects of increased pressure on the cell were discussed. The use of phenothiazine (PTZ) moieties as co-mediators in cobalt mediated DSSCs was investigated. An anionic PTZ salt was most effective at reducing the photo-oxidized sensitizing dye. This PTZ salt enhanced the performance of DSSCs employing the alkyl substituted cobalt complex. Poor electronic coupling and decreased driving force prevents the PTZ salt from enhancing the performance of DSSCs employing the cobalt complexes with withdrawing functionalities.Item Open Access Energy storage and conversion materials: Part 1, Synthesis and characterization of ruthenium tris-bipyridine based fullerene charge transfer salts as a new class of tunable thermoelectric materials; Part 2, Synthesis and characterization of polymer thin films for use as a lithium ion battery separator(Colorado State University. Libraries, 2013) Bates, Daniel James, author; Elliott, C. Michael, advisor; Prieto, Amy L., advisor; Finke, Richard G., committee member; Van Orden, Alan, committee member; Crans, Debbie C., committee member; Dandy, David S., committee memberTo view the abstract, please see the full text of the document.Item Open Access I. Ground-state association between phenothiazine and tris(diimine)ruthenium(II) complexes: its role in highly efficient photoinduced charge separation. II. Ligand modifications of cobalt complexes to increase efficiency of electron-transfer mediators in dye-sensitized solar cells(Colorado State University. Libraries, 2012) Weber, John, author; Elliott, C. Michael, advisor; Rappe, Anthony, committee member; Levinger, Nancy, committee member; Woody, Robert, committee member; Van Orden, Alan, committee memberSupramolecular triad assemblies consisting of a central trisbipyridineruthenium(II) chromophore (C2+), with one or more appended phenothiazine electron donors (D) and a diquat-type electron acceptor (A2+) have been shown to form long-lived photoinduced charge separated states (CSS) with unusually high quantum efficiency. Up to now, there has been no explanation for why such large efficiencies (often close to unity) are achieved from these systems when other, seemingly similar, systems are often much less efficient. In the present study, using a bimolecular system consisting of chromophore-acceptor diad (C2+-A2+) and an N-methylphenothiazine donor we demonstrate that a ground-state association exists between the RuL32+ and the phenothiazine prior to photoexcitation. It is this association process that is responsible for the efficient CSS formation in the bimolecular system and, by inference, also must be an essential factor in the fully intramolecular process occurring with the D-C2+-A2+ triad analogs. Alkyl-substituted bipyridine ligands in cobalt II/III complexes were modified in order to serve as efficient electron-transfer mediators in dye-sensitized solar cells. Attempts at halogen substitution reactions are described. Ultimately isopropyl groups appended to bipyridine ligands were modified by introducing a hydroxyl group at the benzylic position. The electrochemical behavior of the modified ligand is described, as well as its performance as part of a cobalt complex electron-transfer mediator in dye-sensitized solar cells.Item Open Access Indirect electrochemical detection of DNA hybridization based on catalytic oxidation of cobalt(II) and concentration gradient formation in redox conducting polymers(Colorado State University. Libraries, 2008) Xue, Di, author; Elliott, C. Michael, advisorSince the new concept was introduced back in 1993, efforts to develop electrochemical methods for detecting nucleic acid hybridization (e.g., DNA) have mushroomed. Compared with nearly all other analytical techniques, electrochemical instrumentation is inexpensive, robust, and relatively simple to operate. The first part of the dissertation (Chapter 1 to Chapter 4) describes the development of a novel electrochemical DNA sensor based on catalytic oxidation of a cobalt bipyridine "mediator molecule" on an ITO electrode. Interaction of the surface bound DNA probe with the DNA target results in formation of hybrid duplex, which subsequently brings redox catalyst molecules from solution to the electrode surface. The mode of selective catalyst binding is intercalation between base pairs of ds-DNA. This surface-bound catalyst "turns on" the redox chemistry of the mediator molecule which is otherwise kinetically inert to oxidation on ITO. With this approach, we demonstrate detection of a 20-mer DNA target oligonucleotide at picomolar concentrations with outstanding signal-to-noise. The second part of our research (Chapter 5) mainly concerns redox polymer films containing permanently locked concentration gradients. Upon redox gradient formation, the conducting polymer displays interesting properties, such as solid diode behavior and electroluminescence. Previous methods explored drying and/or cooling the film to physically immobilize its gradient. Unfortunately, this preservation was temporary, and underwent degradation over time. Our work is aimed to overcome this limitation by covalently attaching counterions to the polymer backbone and thus permanently locking the redox gradients. Both parts of this dissertation utilize heteroleptic metal complexes possessing redox potentials close to zero (vs SSCE). Compounds with highly negative potentials are strongly reducing and highly positive potentials means strong oxidizing capabilities, which exerts strict requirements on supporting electrolytes and solvents, including high impurity, broad potential window as well as exclusion of environmental interference. Thus, the closer the potential to zero (vs SSCE), the more stable (electrochemically) the complex and the easier the electrochemical measurements.Item Open Access Non-conjugate potential-stepping phenothiazine and phenoxazine based polymer hole-transport material for dye-sensitized solar cells & increasing void space in porous TiO2: to study diffusion properties of a cobalt mediator(Colorado State University. Libraries, 2012) Persson, Kristoffer, author; Elliott, C. Michael, advisor; Rappé, Anthony K., committee member; Kipper, Matthew J., committee memberAs energy demands increase so has the search for alternative sources of energy. Although, fossil fuels have proven useful in energy production, they are also detrimental due to the negative impact on our environment. Considering the current alternative energy sources, such as wind, hydroelectric, biofuels, etc, one source of alternative energy shines above the rest, solar energy. Solar energy provides a possible solution to the energy demands of our modern world with little effect on the environment. The only waste produced from the solar cell industry is from producing and recycling the cells. After production, solar cells require no resources to function other than solar radiation, and no waste is produced. The sun has been powering life on this planet for billions of years, and bombards the earth with 3x1024 J of energy per year. Only 0.02% of this energy is currently needed to power the world, thus making the sun a viable solution to energy demands, while decreasing current pollution issues. This thesis focuses on dye sensitized solar cell (DSSCs), in particular, the Grätzel cell, which incorporates thin films of TiO2 as the semiconductor, DSSC's work very similarly to a battery, but instead of using chemical energy to drive electrons through the circuit, it uses photons. Several issues have arisen with these types of solar cells and their use in the modern world. One particular problem is that the iodide/triiodide (I-/I3-) mediator, which currently produces the most efficient DSSCs, is corrosive and volatile. To address this and other issues, a conductive phenothiazine (PTZ) and phenoxazine (POZ) based polymer is hypothesized to be a suitable replacement for the mediator and solvent by acting as a charge separator and hole transport material, without any volatile or corrosive problems. This polymer would hypothetically function similarly to proposed electron transport in DNA. When charges are injected into a DNA strand they are transferred through π-stacking interactions at the center of the helix, which allows electrons to tunnel through the DNA strand. A potential-stepping block co-polymer incorporating phenothiazine (PTZ) and phenoxazine (POZ) groups attached to the polymer backbone can π-stack like the base pairs in DNA. By creating a two-block co-polymer, with one composed of PTZ monomers and the other of POZ monomers, charge separation can be achieved by trapping the hole on the POZ groups due to their more negative oxidation potentials. This potential-stepping polymer charge separator is the focus of the first part of this thesis. The second section of this thesis is centered on diffusion issues in DSSCs where the I-/ I3- mediator is replaced with tris((2,2'-bipyridyl-4,4'-di-tert-butyl)cobalt(III) perchlorate, CoDTB+3 (ClO4-)3 (DTB = 4,4'-di-tert-butyl-2,2'-bipyridine). The cobalt mediator has many advantages: it is non-corrosive, non-volatile, and it is able to be tuned to optimize the electron transfer process to the dye by simple structural modifications of the ligand. However, cobalt mediators have small diffusion coefficients on the order of 1x10-7 cm2 s-1 or less in TiO2 mesoporous thin films. It has been hypothesized that changing the structure of the TiO2 layer or increasing the void space in the films in the correct manner may dramatically increase the effective rate of diffusion. The introduction of appropriate void space will hypothetically create channels and allow faster diffusion of the mediator. The second half of this thesis explores the effects of introducing void space into TiO2 thin films using various nanoparticles.Item Open Access Photo-induced electron transfer in cu(i) bis-phenanthroline based assemblies. Part I: Chromophore-acceptor diads. Part II: Donor-chromophore-acceptor triads(Colorado State University. Libraries, 2013) Lazorski, Megan, author; Elliott, C. Michael, advisor; Shores, Matthew P., committee member; Chen, Eugene, committee member; Bailey, Travis S., committee member; Sites, James R., committee memberThe photophysical behavior of [Cu(I)P2] (P=2,9-disubstituted-1,10-phenanthroline ligands) in donor-chromophore-acceptor (D-C-A) triads and chromophore-acceptor (C-A) diads is a complex and fascinating area of under developed, yet fundamental, electron transfer chemistry. In metal polypyridyl D-C-A and C-A triads/diads, metal polypyridyl chromophores (C) in which the polypyridyl ligands are covalently linked to acceptor (A) and/or donor (D) moieties, photo-excitation of the chromophore initiates a series of electron transfer events that result in the formation of a charge separated (CS)/charge transfer (CT) state, respectively. The majority of high-performing metal polypyridyl D-C-A/C-A complexes, on which [Cu(I)P2] D-C-A/C-A research is based, incorporate ruthenium (as [Ru(II)L3] where L=polypyridyl ligand) or other rare, expensive, and sometimes toxic metals such as osmium, rhenium and platinum. Although [Ru(II)L3] D-C-A/C-A's have historically set the benchmark for metal polypyridyl D-C-A/C-A performance, it is clear that these complexes are not a practical choice if D-C-A's or C-A's were incorporated into a device for large scale production. However, bisphenanthroline complexes of copper, a much more earth abundant, cheaper and less toxic metal, exhibit very similar photophysical properties to [Ru(II)L3] and have thus gained recognition as promising new materials for D-C-A/C-A triad/diad construction. In order to understand the electron transfer (ET) events occurring in [Cu(I)P2] D-C-A/C-A triads/diads, a complex must be synthesized that is capable of forming a CS with high quantum efficiency (Φcs/ct) and a long CS/CT lifetime (τcs/ct). Therefore, the intent of the research reported herein is to synthesize novel, yet functional heteroleptic [Cu(I)P2] D-C-A/C-A triads/diads and study their fundamental, photo-initiated electron transfer chemistry, specifically the formation of a CS/CT state. Many challenges, which are not present for [Ru(II)L3], make the design and synthesis of [Cu(I)P2] D-C-A/C-A assemblies an art in itself. Therefore, a significant amount of effort was spent on fabricating ligand architectures that (1) are appended with acceptor and/or donor moieties capable of being reduced/oxidized resulting in the formation of a CS/CT, (2) are able to be easily modified so the amount of energy stored in the CS/CT can be tuned, (3) favor the self-assembly of [Cu(I)P2] complexes, (4) are able to facilitate processes that maximize the Φcs/ct. Once the ligands were obtained, the complexation equilibria behavior of these [Cu(I)P2] triads and diads were studied. Despite efforts to design ligand architectures that favor heteroleptic formation, the thermodynamic driving force for heteroleptic D-C-A triad formation is less favor-able than expected. Thus, mixing stoichiometric quantities of D, C and A results in a statistical mixture of C-A, C-D and D-C-A products. Furthermore, since the ligands are labile and will re-arrange to the most thermodynamically stable configuration of products when these complexes are dissolved, isolation of the D-C-A product is impossible. However, recent advances in ligand design have shown promise for resolving this on-going issue. Despite having a mixture of products with the D-C-A, the electron transfer processes of the [Cu(I)P2] D-C-A triads and C-A diads were investigated. Using Transient Absorption (TA) laser spectroscopy, the CT state in the constructed C-A diads and the CS state in the D-C-A triads were detected and the lifetimes were determined. However, it was found that those lifetimes could be modulated to a small degree by solvent in the C-A diads (c.a. 6x longer in polar solvents), and drastically via the application of a magnetic field in D-C-A triads (c.a. 60x longer). The ability to modulate the lifetimes enabled the deconvolution of the effects due to the C-A diad vs D-C-A triad in the statistical product mixtures. Although the response in a magnetic field was a somewhat expected result, as similar effects occur in the [Ru(II)L3 D-C-A/C-A's, the magnitude of change in the lifetime and the quantum efficiency offers new insight into the electron transfer events that occur in the CS/CT formation process for [Cu(I)P2] D-C-A/C-A complexes.Item Open Access Synthesis and characterization of transitional metal polypyridine complex fullerene salts(Colorado State University. Libraries, 2010) Hong, Jie, author; Elliott, C. Michael, advisor; Prieto, Amy Lucia, committee member; Van Orden, Alan K., committee member; Ladanyi, Branka M., committee member; Hochheimer, Hans D., committee memberIn recent years, ionic fullerene (C60) salts have attracted much attention due to their interesting chemical and physical properties. Transition metal polypyridine complex, most notably, tris (2, 2'-bipyridine)ruthenium-based compounds, [RuL3]m+, have a number of photochemical and electrochemical properties which make them of interest for both fundamental and applied studies. The similarity of electrochemical potentials of [RuL3] and C60"’ gives rise to the possibility of a new charge-transfer ionic salts [Ru L3m+]n(C6on-)m- In chapter I, the background of ionic C60 salts as well as the motivation of preparing transition metal polypyridine complex C60 salts is described in detail. A chart of electrochemical data is generated and used as a guideline to predict the possible stoichiometries of ionic salts throughout the whole research work. In Chapter II, the synthesis and characterization of three ionic salts using [Ru(bpy)3]m+ (m = 1, 2) as cations are fully described. All three salts are semiconducting with the highest conductivity at ~ 10 S • m-1. Interesting paramagnetism is reported as well. The detailed discussions based on single crystal and powder X-ray diffraction studies are useful in better understanding the electronic conduction and magnetism. The physical properties of ionic C60 salts can be rationalized based on the crystal packing. In the presence of a bulky cation Ru(bpy)3m+, an expanded crystal lattice is found with weak site-site interactions. In Chapter III, the ligand substitution effect of [RuL3 m+]n(C60n-)m is examined. Four ligands: 4, 4'-dimethyl-2, 2'-bipyridine (4DMB), 5, 5'-dimethyl-2, 2'-bipyridine (5DMB), 4, 4'-di-tert-butyl-2, 2'-bipyridine (TBB) and 4, 4', 5, 5'-tetramethyl-2, 2'- bipyridine (TMB) are chosen as the targets. The ligand substitution not only changes the redox potentials of cation Ru(bpy)3m1 but also alters its size. This provides a route for tuning the properties of [RuL3 ’”^]n(C6on-)m • Electrical and magnetic properties of all compounds as prepared are investigated. The highest conductivity found is also close to 10 S • m-1. In Chapter IV, the effect of substituting ruthenium by chromium in metal complex ionic C60 salts is studied. Cr(bpy)3 m+ and Ru(bpy)3 m+ are very similar in size but of quite different redox potentials. The electrical conductivity of their corresponding ionic salts shows large dependence on the redox potentials.