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Designing semiconductors and catalysts for photoelectrochemical hydrogen production
Designing semiconductors and catalysts for photoelectrochemical hydrogen production
This thesis presents the results of a number of projects dealing with the challenge of hydrogen production from sunlight, promoting a medium to store and transport renewable energy. Different nanostructured organic and inorganic semiconductors as well as metallic co-catalysts were synthesized and combined to thin film devices, which can be envisioned to work like artificial leaves. These devices were extensively studied by various physical methods like X-ray diffraction, electron microscopy, X-ray photoelectron spectroscopy, spectrophotometry, nuclear magnetic resonance spectroscopy and density functional theory calculations. Finally, their abilities regarding the harvesting of sunlight and their catalytic properties for hydrogen evolution were investigated by photoelectrochemical methods. The first chapter describes the influence of tin doping on the performance of hematite photoanodes using model photoabsorber layers with different tin doping concentrations and concentration gradients prepared via atomic layer deposition. This study aims for the basic understanding of effects of dopants on fundamental rate-determining kinetic and recombination steps of metal oxide photoelectrodes. The second chapter elucidates the phenomenon of photocorrosion with the example of lithium doped copper oxide photocathodes. While this material appears to be an efficient material at first glance, it corrodes by copper reduction from its own photogenerated electrons in contact with water. This observation was studied in depth to reveal the underlying mechanism of photocorrosion. Additionally, a suitable protection approach for this material is discussed and the hydrogen evolution of those final devices is quantified. The third chapter presents the first study of covalent organic frameworks serving as photoelectrodes. By self-organization, this organic material grows conjugated two-dimensional sheets that stack in the third dimension, forming crystalline and porous polymers. The synthesized material called BDT-ETTA was grown as flat films with its one-dimensional pores oriented perpendicular to the surface of the underlying conductive substrate. Those devices were shown to exhibit a suitable band gap alignment for hydrogen evolution and were applied to reduce water by the use of sunlight. Finally, the combination with a platinum cocatalyst revealed the catalytic activity of the photoactive material itself as bottleneck for the targeted application, whereas the diversity of possible optical and electronic properties can be tuned by the selection of appropriate building blocks, offering an auspicious material system for the evolution of hydrogen from sunlight. The fourth chapter explores electrophoretic deposition as a well-working technique for the film deposition of covalent organic frameworks, also in combination with metallic platinum nanoparticles. With the example of the previously introduced BDT-ETTA, the influence of morphology and added cocatalyst on the photoelectrochemical performance is discussed. Devices exhibiting textural porosity, in addition to the intrinsic porosity of the covalent organic framework itself, showed an increased photoactivity compared to flat electrodes. Their combination with a nanosized platinum cocatalyst leads to strongly enhanced photocurrents, alleviating the catalytic bottleneck of the discussed material. Finally, the perspectives for the continuation of the above projects are discussed in the last chapter.
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Kampmann, Jonathan
2020
English
Universitätsbibliothek der Ludwig-Maximilians-Universität München
Kampmann, Jonathan (2020): Designing semiconductors and catalysts for photoelectrochemical hydrogen production. Dissertation, LMU München: Faculty of Chemistry and Pharmacy
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Abstract

This thesis presents the results of a number of projects dealing with the challenge of hydrogen production from sunlight, promoting a medium to store and transport renewable energy. Different nanostructured organic and inorganic semiconductors as well as metallic co-catalysts were synthesized and combined to thin film devices, which can be envisioned to work like artificial leaves. These devices were extensively studied by various physical methods like X-ray diffraction, electron microscopy, X-ray photoelectron spectroscopy, spectrophotometry, nuclear magnetic resonance spectroscopy and density functional theory calculations. Finally, their abilities regarding the harvesting of sunlight and their catalytic properties for hydrogen evolution were investigated by photoelectrochemical methods. The first chapter describes the influence of tin doping on the performance of hematite photoanodes using model photoabsorber layers with different tin doping concentrations and concentration gradients prepared via atomic layer deposition. This study aims for the basic understanding of effects of dopants on fundamental rate-determining kinetic and recombination steps of metal oxide photoelectrodes. The second chapter elucidates the phenomenon of photocorrosion with the example of lithium doped copper oxide photocathodes. While this material appears to be an efficient material at first glance, it corrodes by copper reduction from its own photogenerated electrons in contact with water. This observation was studied in depth to reveal the underlying mechanism of photocorrosion. Additionally, a suitable protection approach for this material is discussed and the hydrogen evolution of those final devices is quantified. The third chapter presents the first study of covalent organic frameworks serving as photoelectrodes. By self-organization, this organic material grows conjugated two-dimensional sheets that stack in the third dimension, forming crystalline and porous polymers. The synthesized material called BDT-ETTA was grown as flat films with its one-dimensional pores oriented perpendicular to the surface of the underlying conductive substrate. Those devices were shown to exhibit a suitable band gap alignment for hydrogen evolution and were applied to reduce water by the use of sunlight. Finally, the combination with a platinum cocatalyst revealed the catalytic activity of the photoactive material itself as bottleneck for the targeted application, whereas the diversity of possible optical and electronic properties can be tuned by the selection of appropriate building blocks, offering an auspicious material system for the evolution of hydrogen from sunlight. The fourth chapter explores electrophoretic deposition as a well-working technique for the film deposition of covalent organic frameworks, also in combination with metallic platinum nanoparticles. With the example of the previously introduced BDT-ETTA, the influence of morphology and added cocatalyst on the photoelectrochemical performance is discussed. Devices exhibiting textural porosity, in addition to the intrinsic porosity of the covalent organic framework itself, showed an increased photoactivity compared to flat electrodes. Their combination with a nanosized platinum cocatalyst leads to strongly enhanced photocurrents, alleviating the catalytic bottleneck of the discussed material. Finally, the perspectives for the continuation of the above projects are discussed in the last chapter.