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Müller, Alexander (2015): Transmission electron microscopic investigation of several nanostructured photoelectrodes for photoelectrochemical water splitting. Dissertation, LMU München: Fakultät für Chemie und Pharmazie
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Abstract

Most renewable energy sources suffer from intermittency and have to be coupled with sophisticated energy conversion and storage technologies. An elegant solution is offered by photoelectrochemical water splitting, where solar energy is directly converted into chemical energy by splitting water into oxygen and the energy carrier hydrogen. Photoelectrochemical water splitting requires two photoelectrodes which are immersed in an aqueous electrolyte. These photoelectrodes are semiconductors with valence and conduction bands straddling the redox potential of water. Upon illumination, electrons and holes are produced, separated and transferred to the electrolyte, leading to the evolution of oxygen at the photoanode and the evolution of hydrogen at the photocathode. The resulting hydrogen can be stored, transported and then either burnt in fuel cells to regain electrical energy or used for industrial applications like the Haber-Bosch process. The photoelectrodes are often nanostructured to increase the surface area, at which the reaction takes place. This strategy has been realized with several morphologies such as nanotubes, inverse opals, etc. and has often lead to performance increases of several hundred percent. Therefore, detailed knowledge of the morphology is important and can be obtained by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). TEM is a powerful technique that allows imaging samples with a resolution down to the sub-Ångstrom scale. In addition, TEM can be combined with spectroscopic methods such as electron energyloss spectroscopy (EELS) and energy-dispersive X-ray spectroscopy (EDX) to quantify the chemical composition. In this thesis, three different materials systems were studied by TEM: noble metal nanoparticles on TiO2 for hydrogen evolution with the sacrificial agent MeOH, Fe2O3/WO3 dual absorber photoanodes and photocathodes out of the novel material FeCrAl oxide. Titania is one of the most researched photoanode materials. However, it only absorbs UV light. Au and Au/Ag core-shell nanoparticles were deposited by the project partners Michael Karnahl and Sandra Peglow of the LIKAT and the INP Greifswald, respectively, on anatase thin films by photodeposition and radio frequency magnetron sputtering. These noble metal nanoparticles absorb visible light by surface plasmon resonance and also act as co-catalysts for electrons excited in the titania and injected into them. Cross-section were prepared for a detailed TEM investigation of the microstructure. The distribution of the nanoparticles varied greatly with the synthesis method, as photodeposited particles grew in and on top of the titania, whereas the plasma-deposited nanoparticles only grew on top. Different growth and coarsening mechanisms could be identified and correlated to the synthesis conditions by careful particle size distribution determination. In addition to defect-free nanoparticles, several defects such as five-fold twinning, grain boundaries and stacking faults were found. The TEM analysis was complemented by optical absorption and photocatalysis measurements, and the synthesis as well as the properties could be correlated to microstructural features. Due to its narrow band gap, hematite is a popular photoanode material. However, it also has several disadvantages, which were addressed by several studies. Tin-doping increased the transfer efficiency and therefore the photocurrent, with the tin being enriched at the surface of the hematite nanoparticles and hinting at a structure-function relationship. Deposition of a Co3O4 co-catalyst and the introduction of a conductive scaffold all further increased the photocurrent. Another performance-increasing approach, combining multiple photocatalytically active materials, was tested with Fe2O3/WO3 dual absorbers prepared by Ilina Kondofersky of the group of Prof. Thomas Bein. WO3 was systematically applied as a scaffold and/or as a surface treatment. The arrangement of the different materials and the interfaces between them was studied in detail by TEM. Both the host-guest approach and the surface treatment strongly increased the performance compared to the pure materials and several beneficial interactions could be identified. For example, WO3 strongly scatters visible light, resulting in increased absorption by Fe2O3 and higher current densities. We also determined a cathodic shift in the onset potential to 0.8 V and, compared to pure Fe2O3, increased transfer rates of up to 88 %, and can therefore conclude that the Fe2O3/WO3 dual absorbers are a very promising system. In spite of all the different performance-enhancing strategies developed so far, it is becoming apparent that all currently available materials, regardless of how heavily they are improved, will not reach sufficient performances. This has led to the search for novel materials and in this thesis, meso- and macroporous photocathodes with the overall stoichiometry Fe0.84Cr1.0Al0.16O3 were investigated in close cooperation with Ilina Kondofersky. Using TEM cross-sections, a phase separation into Fe- and Cr-rich phases was observed for both morphologies and could be correlated to the precursor stabilities. In comparison to the mesoporous layer, the macroporous photocathode had a significantly increased charge collection efficiency and therefore performance, proving the benefits of tuning the morphology. In all studies, performance-increasing strategies were successfully applied and we found the performance to depend heavily on the morphologies. By combining the results of all techniques, insight into the complex interplay between synthesis conditions, morphology and properties could be achieved and the gained knowledge is expected to benefit future work.