Browsing by Subject "Metal oxide"
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Item Design of nanocomposites for electrocatalysis and energy storage : metal/metal oxide nanoparticles on carbon supports(2012-08) Slanac, Daniel Adam; Johnston, Keith P., 1955-; Stevenson, Keith J.; Mullins, Charles C.; Korgel, Brian A.; Ferreira, Paulo J.Controlling catalyst morphology and composition are required to make meaningful structure-activity/stability relationships for the design of future catalysts. Herein, we have employed strategies of presynthesis and infusion or electroless deposition to achieve exquisite control over catalyst composite morphology. The oxygen reduction (ORR) and the oxygen evolution reactions (OER) were chosen as model systems, as their slow kinetics is a major limiting factor preventing the commercialization of fuel cells and rechargeable metal air batteries. In acid, bimetallic (Pt-Cu, Pd-Pt) and monometallic (Pt) catalysts were presynthesized in the presence of capping ligands. Well alloyed Pt-Cu nanoparticles (3-5 nm) adsorbed on graphitic mesoporous carbon (GMC) displayed an ORR activity >4x that of commercial Pt. For both presynthesized Pt and Pt-Cu nanocrystals on GMC, no activity loss was also observed during degradation cycling due to strong metal-support interactions and the oxidation resistance of graphitic carbon. Similar strong metal-support interactions were achieved on non-graphitic carbon for Pd3Pt2 (<4 nm) nanoparticles due to disorder in the metal surface This led to enhanced mass activity 1.8x versus pure Pt, as well as improved stability. For basic electrolytes, we developed an electroless co-deposition scheme to deposit Ag (3 nm) next to MnOx nanodomains on carbon. We achieved a mass activity for Ag-MnOx/VC, 3x beyond the linear combination of pure component activities due to ensemble effects, where Ag and MnOx domains catalyze different ORR steps, and ligand effects from the unique electronic interaction at the Ag-MnOx interface. Activity synergy was also shown for Ag-Pd alloys (~5 nm), achieving up to 5x activity on a Pd basis, resulting from the unique alloy surface of single Pd atoms surrounded by Ag. Lastly, we combined arrested growth of amorphous nanoparticles with thin film freezing to create a high surface area, pure phase perovskite aggregate of nanoparticles after calcination. Sintering was mitigated during the high temperature calcination required to form the perovskite crystals. The high surface areas and phase purity led to OER mass activities ~2.5x higher than the benchmark IrO2 catalyst.Item Metal oxide photoelectrodes for solar water splitting(2015-08) Rettie, Alexander John; Mullins, C. B.; Bard, Allen J; Humphrey, Simon M; Hwang, Gyeong S; Korgel, Brian AEfficient solar water splitting – using sunlight to produce hydrogen from water – has been an ambitious goal of the scientific community for over 40 years. At its heart this is a materials problem, with the photoelectrodes used in a photoelectrochemical cell having to satisfy all the constraints of a photovoltaic material (light absorption, charge transport) as well as being stable in water and having appropriately positioned band edges. Of the metal oxide systems studied for this purpose, we identified iron oxide (hematite, α-Fe2O3), tungsten trioxide (WO3) and an emerging (at the time) material, bismuth vanadate (BiVO4) as the most promising. In this dissertation we sought to understand and address the shortcomings of these materials, namely, carrier transport in BiVO4 and α-Fe2O3 and light absorption in WO3. We synthesized high quality single crystals of undoped and Mo and W-doped BiVO4 using the floating zone technique and carried out fundamental transport measurements. Electrons were shown to form small polarons and the Hall effect mobility was low, ~10-1 cm2 V-1 s-1 at 300 K. Critically, the mobility measured by the Hall effect may be vastly different from the drift mobility. Small-polaron hopping was found to be in the adiabatic regime and anisotropic conductivity was related to the structural arrangement of vanadium ions. Electrons are also thought to form small polarons in α-Fe2O3, but a thorough analysis had not been performed. We grew single crystals of Ti:α-Fe2O3 and characterized their electron transport to evaluate this model and probe the large anisotropy thought to occur between the basal planes. These revealed that the adiabatic small-polaron model was appropriate. Interestingly, electron transport in Ti:α-Fe2O3 was shown to be near-isotropic, contradicting the common view in the literature. Finally, we studied the effects of sulfur or iodine incorporation in WO3 with the aim to improve its visible light harvesting ability. Both of these impurities did increase visible light absorption, but performance was degraded in all cases except for very low concentrations of sulfur doping. These impurities likely form inter-gap defect bands which allow the absorption of longer wavelength light, but also degrade transport properties if present in large amounts.Item Mixed metal oxide semiconductors and electrocatalyst materials for solar energy conversion(2013-12) Berglund, Sean Patrick; Mullins, C. B.The sun is a vast source of renewable energy, which can potentially be used to satisfy the world's increasing energy demand. Yet many material challenges need to be overcome before solar energy conversion can be implemented on a larger scale. This dissertation focuses on materials used for solar energy conversion through photo-electrochemical (PEC) processes. It discusses methods for improving PEC materials, namely mixed metal oxide semiconductors, by nanostructuring, incorporation of additional elements, and application surface electrocatalysts. In this dissertation several material synthesis techniques are detailed. A high vacuum synthesis process known as reactive ballistic deposition (RBD) is used to synthesize nanostructured bismuth vanadate (BiVO₄), which is studied for PEC water oxidation. Additionally, ballistic deposition (BD) is used to incorporate Mo and W into nanostructured BiVO₄ to improve the PEC activity. An array dispenser and scanner system is used to incorporate metals into copper oxide (CuO) and copper bismuth oxide (CuBi₂O₄) and over 3,000 unique material compositions are tested for cathodic photoactivity. The system is also used to test 35 elements as single component metal oxides, mixed metal oxides, and dopants for titanium dioxide (TiO₂) for use in dye-sensitized solar cells (DSCs). Lastly, RBD is used to deposit tungsten semicarbide (W₂C) onto p-type silicon (p-type) substrates as an electrocatalyst for PEC proton reduction. In many cases, the synthesis techniques and new material combinations presented in this dissertation result in improved PEC performance. The materials are thoroughly assessed and characterized to gain insights into their nanostructure, chemical composition, light absorption, charge transport properties, catalytic activity, and stability.