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Description
The molecular modification of semiconductors has applications in energy

conversion and storage, including artificial photosynthesis. In nature, the active sites of

enzymes are typically earth-abundant metal centers and the protein provides a unique

three-dimensional environment for effecting catalytic transformations. Inspired by this

biological architecture, a synthetic methodology using surface-grafted polymers with

discrete chemical recognition sites

The molecular modification of semiconductors has applications in energy

conversion and storage, including artificial photosynthesis. In nature, the active sites of

enzymes are typically earth-abundant metal centers and the protein provides a unique

three-dimensional environment for effecting catalytic transformations. Inspired by this

biological architecture, a synthetic methodology using surface-grafted polymers with

discrete chemical recognition sites for assembling human-engineered catalysts in three-dimensional

environments is presented. The use of polymeric coatings to interface cobalt-containing

catalysts with semiconductors for solar fuel production is introduced in

Chapter 1. The following three chapters demonstrate the versatility of this modular

approach to interface cobalt-containing catalysts with semiconductors for solar fuel

production. The catalyst-containing coatings are characterized through a suite of

spectroscopic techniques, including ellipsometry, grazing angle attenuated total reflection

Fourier transform infrared spectroscopy (GATR-FTIR) and x-ray photoelectron (XP)

spectroscopy. It is demonstrated that the polymeric interface can be varied to control the

surface chemistry and photoelectrochemical response of gallium phosphide (GaP) (100)

electrodes by using thin-film coatings comprising surface-immobilized pyridyl or

imidazole ligands to coordinate cobaloximes, known catalysts for hydrogen evolution.

The polymer grafting chemistry and subsequent cobaloxime attachment is applicable to

both the (111)A and (111)B crystal face of the gallium phosphide (GaP) semiconductor,

providing insights into the surface connectivity of the hard/soft matter interface and

demonstrating the applicability of the UV-induced immobilization of vinyl monomers to

a range of GaP crystal indices. Finally, thin-film polypyridine surface coatings provide a

molecular interface to assemble cobalt porphyrin catalysts for hydrogen evolution onto

GaP. In all constructs, photoelectrochemical measurements confirm the hybrid

photocathode uses solar energy to power reductive fuel-forming transformations in

aqueous solutions without the use of organic acids, sacrificial chemical reductants, or

electrochemical forward biasing.
ContributorsBeiler, Anna Mary (Author) / Moore, Gary F. (Thesis advisor) / Moore, Thomas A. (Thesis advisor) / Redding, Kevin E. (Committee member) / Allen, James P. (Committee member) / Arizona State University (Publisher)
Created2018
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Description
Chemical modification of (semi)conducting surfaces with soft-material coatings containing electrocatalysts provides a strategy for developing integrated constructs that capture, convert, and store solar energy as fuels. However, a lack of effective strategies for interfacing electrocatalysts with solid-state materials, and an incomplete understanding of performance limiting factors, inhibit further development. In

Chemical modification of (semi)conducting surfaces with soft-material coatings containing electrocatalysts provides a strategy for developing integrated constructs that capture, convert, and store solar energy as fuels. However, a lack of effective strategies for interfacing electrocatalysts with solid-state materials, and an incomplete understanding of performance limiting factors, inhibit further development. In this work, chemical modification of a nanostructured transparent conductive oxide, and the III-V semiconductor, gallium phosphide, is achieved by applying a thin-film polymer coating containing appropriate functional groups to direct, template, and assemble molecular cobalt catalysts for activating fuel-forming reactions. The heterogeneous-homogeneous conducting assemblies enable comparisons of the structural and electrochemical properties of these materials with their homogeneous electrocatalytic counterparts. For these hybrid constructs, rational design of the local soft-material environment yields a nearly one-volt span in the redox chemistry of the cobalt metal centers. Further, assessment of the interplay between light absorption, charge transfer, and catalytic activity in studies involving molecular-catalyst-modified semiconductors affords models to describe the rates of photoelectrosynthetic fuel production as a function of the steady-state concentration of catalysts present in their activated form. These models provide a conceptual framework for extracting kinetic and thermodynamic benchmarking parameters. Finally, investigation of molecular ‘proton wires’ inspired by the Tyrosine Z-Histidine 190 redox pair in Photosystem II, provides insight into fundamental principles governing proton-coupled electron transfer, a process essential to all fuel-forming reactions relevant to solar fuel generation.
ContributorsWadsworth, Brian Lawrence (Author) / Moore, Gary F (Thesis advisor) / Moore, Thomas A. (Committee member) / Trovitch, Ryan J (Committee member) / Arizona State University (Publisher)
Created2020