Collections > Electronic Theses and Dissertations > Characterization of Water Oxidation Catalysts for Implementation in a Dye-Sensitized Photoelectrosynthesis Cell

Chapter One is an introduction to solar energy conversion and the Dye-Sensitized Photoelectrosynthesis Cell (DSPEC). It goes into detail about the development of the individual parts of the DSPEC, including the semiconductor electrodes, photosensitizers and catalysts that have been implemented in similar devices. It then gives a brief history of the use of iridium oxide nanoparticles as water oxidation catalysts. Chapter Two describes the film formation process for iridium oxide nanoparticles, focusing particularly on the electroflocculation technique. Studies were performed which determined the mechanism of electroflocculation, focusing on the short electroflocculation time periods. Three different electroflocculation methods – constant potential, potential pulsing and potential cycling – were implemented and the resulting iridium oxide nanoparticle films were compared electrochemically and microscopically. Electroflocculation was also compared to chemical flocculation and a direct pH change method of film formation. Chapter Three is a general characterization chapter on iridium oxide nanoparticles, using a variety of analytical techniques to probe the structure, surface chemistry and reactivity of these nanoparticles. It probes the pH effect on electroflocculated NPs electrochemically and relates it to its reactivity as a water oxidation catalyst. UV-Vis spectroelectrochemistry of the iridium oxide nanoparticles is discussed and the individual spectra of each Ir oxidation state is presented. Raman spectroscopy of the freely diffusing iridium oxide nanoparticles and iv electroflocculated nanoparticles demonstrates the increase in crystallinity via electroflocculation. SEM demonstrates the morphology of the electroflocculated films. Change in surface charge of the iridium oxide nanoparticles with respect to pH is depicted with zeta potential measurements. Finally, XPS of various forms of the iridium oxide nanoparticles shows the different forms of iridium within the nanoparticles, likely distinguished by surface and core iridium sites. Chapter Four details the behavior of iridium oxide nanoparticles in organic media through a few different methods. First, the behavior of an electroflocculated iridium oxide nanoparticle film is examined in an aprotic solvent, as well as the changes in the electrochemical behavior when a proton source is added. A hot injection thermal degradation synthesis of iridium oxide nanoparticles is also explored. Exchange into organic media via valeric acid capping ligands is discussed. This leads to the ferrocenation of the iridium oxide nanoparticles using a Click reaction with a phosphate terminated ligand. Electrochemical tagging of the iridium oxide nanoparticles elaborates on the surface chemistry and diffusion coefficient in aprotic media. Chapter Five explores a dip-coated layer-by-layer synthesis of a chromophore-catalyst photoanode assembly consisting of a Ru(II) polypyridal dye and iridium oxide nanoparticle catalyst. Preparation of Ru(II) polypyridyl-iridium oxide nanoparticle (IrOx NP) chromophore-catalyst assemblies on a FTO|nanoITO|TiO2 core/shell by a layer-by-layer procedure is described for application in Dye Sensitized Photoelectrosynthesis Cells (DSPEC). Significantly enhanced, bias-dependent photocurrents with Lumencor 455 nm 14.5 mW/cm2 irradiation are observed for core/shell structures compared to TiO2 after derivatization with [Ru(4,4’-PO3H2bpy)2(bpy)]2+ (RuP2) and uncapped IrOX NPs at pH 1 and pH 5.8 in HClO4 and NaSiF6 buffers, respectively, with a Pt cathode. Photocurrents arising from photolysis of the resulting photoanodes, v FTO|nanoITO|TiO2|-RuP2,IrO2, are dependent on TiO2 shell thickness and applied bias, reaching 0.2 mA/cm2 at 0.5 V vs AgCl/Ag with a shell thickness of 6.6 nm. Long term photolysis in the NaSiF6 buffer results in a marked decrease in photocurrent over time due to surface hydrolysis and loss of the chromophore from the surface. Long term stability, with sustained photocurrents, has been obtained by Atomic Layer Deposition (ALD) of overlayers of TiO2 to stabilize surface binding of -RuP2 prior to the addition of the IrOX NPs. Chapter Six focuses on the electrochemical characterization of three novel Ru(II) quaterpyridine complexes and the assessment of their potential for benzyl alcohol oxidation catalysis. The terminal ligands for these complexes are varied between CH3CN, Cl and vinyl pyridine. Their electrochemical behavior in acetonitrile and aqueous media are reported. When dissolved in aqueous media, an exchange of the CH3CN ligands with H2O allows the complexes to reach higher oxidation states, suggesting potential for water oxidation catalysis. Benzyl oxidation catalysis is also explored; two of the three complexes are catalysts for this reaction.