Nieuwe Methoden voor (Foto-) Elektrochemische Karakterisering van Dunne Film Nano-TiO2 en Fabricage van 3D Mesoporeuze TiO2 Nanoassemblages

The urgent need to mitigate atmospheric CO₂ levels and address the climate crisis has driven significant interest in electrochemical CO₂ reduction (eCO₂R), where renewable electricity is used to convert CO₂ into valuable chemicals and fuels. Semiconductor catalysts play a crucial role in CO₂ electrolyzer systems by leveraging their light absorption properties to facilitate charge transfer. A fundamental understanding of their band-edge positions is essential for gaining mechanistic insights into eCO₂R reactions at these semiconductor electrodes. However, conventional methods for determining band edges are inadequate for modern nanostructured semiconductor architectures. This thesis revisits core concepts in semiconductor electrochemistry and introduces a new (photo-)electrochemical strategy for reliable band-edge determination in nano-semiconductors. To enable precise characterization, the study employs an (electro)chemically stable thin-film n-type nano-TiO₂ system, ensuring that the current response originates solely from the semiconductor/electrolyte interface. The limitations of the conventional Mott-Schottky (MS) approach are first examined, highlighting its shortcomings when applied to nanostructured semiconductors. A novel methodology is then introduced, utilizing potentiodynamic measurements in redox probe solutions with known formal potentials to extract band-edge information. The onset of cathodic current in a high-work-function Schottky-type solution provides the flatband potential across a broad pH range, while the conduction band edge is inferred from the transition between rectifying and reversible current-potential behavior in solutions of decreasing pH using a low-work-function redox probe. Combining these measurements with an analysis of the potential drop at the nano-semiconductor/current-collector interface enables a comprehensive reconstruction of the semiconductor energy band diagram during potentiodynamic experiments. Expanding on these findings, the study investigates how photo-generated charge carriers modulate band-edge positions under illumination. Several key photoelectrochemical concepts are explored, including charge separation by the built-in electric field, quasi-Fermi level splitting, photovoltage generation, and valence band hole capture oxidation kinetics. The presence of photo-induced surface states is identified, and their role in band-edge unpinning is discussed. Additionally, the effects of concurrent heating at the nano-TiO₂/solution interface are systematically analyzed to distinguish thermal contributions from purely photoelectrochemical responses. Comparative studies between anatase and amorphous nano-TiO₂ thin films provide further insight into the influence of structural variations on electronic properties. While thin-film systems provide precise fundamental insights, their limited surface area restricts practical catalytic applications. To address this challenge, this thesis presents a new method for fabricating highly interconnected three-dimensional mesoporous TiO₂ nanoscaffolds. Initially discovered serendipitously during the electrochemical oxidation of TiN substrates, this process is systematically investigated to understand its underlying mechanism of formation. The resulting TiO₂ nanoscaffolds exhibit large electrochemically accessible surface areas (~300 m² per gram), serving as a natural extension of the thin-film system, where the current response remains solely attributable to the semiconductor/electrolyte interface. Their electronic conductivity is assessed via ion-insertion experiments, confirming a structurally integrated and electrically linked network. Furthermore, the ability to pattern TiO₂ nanoscaffolds on masked substrates demonstrates their compatibility with scalable device fabrication. By establishing new methodologies for characterizing nano-TiO₂ band energetics and introducing a scalable strategy to fabricate high-surface-area semiconductor architectures, this thesis advances semiconductor (photo-)electrochemistry and its applications in energy conversion. These findings lay a strong foundation for designing efficient semiconductor-based catalysts while providing a tutorial-style framework to promote the intelligent application of semiconductor (photo-)electrochemistry in addressing complex energy challenges.