Matching Items (3)

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Electrocatalytic and photoelectrosynthetic hydrogen production using metalloporphyrins and molecular-modified gallium phosphide photocathodes

Description

Metalloporphyrins represent a class of molecular electrocatalysts for driving energy relevant half-reactions, including hydrogen evolution and carbon dioxide reduction. As electrocatalysts, they provide a strategy, and potential structural component, for

Metalloporphyrins represent a class of molecular electrocatalysts for driving energy relevant half-reactions, including hydrogen evolution and carbon dioxide reduction. As electrocatalysts, they provide a strategy, and potential structural component, for linking renewable energy sources with the production of fuels and other value-added chemicals. In this work, porphyrins are used as structural motifs for exploring structure-function relationships in electrocatalysis and as molecular building blocks for assembling photoelectrochemical assemblies leveraging the light capture and conversion properties of a gallium phosphide (GaP) semiconductor. These concepts are further covered in Chapter 1. A direct one-step method to chemically graft metalloporphyrins to GaP surfaces is described in Chapter 2. Structural characterization of the hybrid assemblies is achieved using surface-sensitive spectroscopic methods, and functional performance for photoinduced hydrogen production is demonstrated via three-electrode electrochemical measurement combined with product analysis using gas chromatography. In Chapter 3, preparation of a novel cobalt porphyrin modified with 3-fluorophenyl groups at all four meso-positions of the porphyrin ring and a single 4-vinylphenyl surface attachment group at one of the β-positions is described. Electrochemical measurements show the 3-fluorophenyl groups perturb the reduction potentials of the complex to more positive values as compared to non-fluorinated analogs, illustrating synthetic control over the redox properties of the catalysts. The use of grazing angle attenuated total reflectance Fourier transform infrared spectroscopy to characterize chemically modified GaP surfaces containing grafted cobalt fluoro-porphyrins is presented in Chapter 4. In these hybrid constructs, porphyrin surface attachment is achieved using either a two-step method involving coordination of cobalt fluoro-porphyrin metal centers to nitrogen sites on an initially applied thin-film polypyridyl surface coating, or via a direct modification strategy using a cobalt fluoro-porphyrin precursor bearing a covalently bonded 4- vinylphenyl surface attachment group. Finally, Chapter 5 describes binuclear copper porphyrins in which two copper porphyrin macrocycles are doubly fused at the meso-β positions are shown to be active electrocatalysts for the hydrogen evolution reaction. The enhancement in catalytic performance over analogous non-fused copper porphyrins indicates extended macrocycles provide an advantageous structural motif and design element for preparing electrocatalysts that activate small molecules of consequence to renewable energy.

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Created

Date Created
  • 2019

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Synthesis and characterization of nanocrystalline nickel-zinc spinel ferrite thin films using the spin-spray deposition method

Description

The overall objective of this project is to optimize the development of magnetic ferrite thin films targeted for enabling low-loss broadband communication devices, miniaturized low-microwave inductors and electromagnetic noise suppressors.

The overall objective of this project is to optimize the development of magnetic ferrite thin films targeted for enabling low-loss broadband communication devices, miniaturized low-microwave inductors and electromagnetic noise suppressors. The focus of this objective is to design and build a reactor and improve the spin-spray process. Each film is then characterized and optimized to have a high permeability and high frequency in the range of 500 MHz - 3 GHz. Films produced by the µ-droplet deposition regime yields a higher Snoek's product than the continuous liquid layer regime. The highest Snoek's product occurs when it is deposited at an oxidant pH of 8.28. The Ni-Zn-Co ferrite magnetic domains were imaged using the Lorentz TEM in which multi-grain domains are experimentally observed for the first time.

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Agent

Created

Date Created
  • 2013

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Studies of epitaxial silicon nanowire growth at low temperature

Description

Silicon nanowires were grown epitaxially on Si (100) and (111) surfaces using the Vapor-Liquid-Solid (VLS) mechanism under both thermal and plasma enhanced growth conditions. Nanowire morphology was investigated as a

Silicon nanowires were grown epitaxially on Si (100) and (111) surfaces using the Vapor-Liquid-Solid (VLS) mechanism under both thermal and plasma enhanced growth conditions. Nanowire morphology was investigated as a function of temperature, time, disilane partial pressure and substrate preparation. Silicon nanowires synthesized in low temperature plasma typically curved compared to the linear nanowires grown under simple thermal conditions. The nanowires tended bend more with increasing disilane partial gas pressure up to 25 x10-3 mTorr. The nanowire curvature measured geometrically is correlated with the shift of the main silicon peak obtained in Raman spectroscopy. A mechanistic hypothesis was proposed to explain the bending during plasma activated growth. Additional driving forces related to electrostatic and Van der Waals forces were also discussed. Deduced from a systematic variation of a three-step experimental protocol, the mechanism for bending was associated with asymmetric deposition rate along the outer and inner wall of nanowire. The conditions leading to nanowire branching were also examined using a two-step growth process. Branching morphologies were examined as a function of plasma powers between 1.5 W and 3.5 W. Post-annealing thermal and plasma-assisted treatments in hydrogen were compared to understand the influences in the absence of an external silicon source (otherwise supplied by disilane). Longer and thicker nanowires were associated with longer annealing times due to an Ostwald-like ripening effect. The roles of surface diffusion, gas diffusion, etching and deposition rates were examined.

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Agent

Created

Date Created
  • 2011