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Abstract:
Metal oxides are a promising class of photoelectrode materials for photoelectrochemical (PEC) water splitting because they are in general cheaper and more stable in aqueous solutions than conventional III-V semiconductors. Since only few—if any—of the simple binary oxides show the desired properties, recent efforts in the field have shifted towards investigating the more complex multinary oxides. To study the fundamental properties and performance limitations of such novel photoelectrode materials, one needs to be able to deposit thin and compact films of high electronic quality. Pulsed laser deposition (PLD) is a versatile physical vapor deposition technique that meets these demands. Therefore, in this thesis, this powerful tool is used to (i) deposit dense and high-quality BiVO4 photoelectrodes and (ii) to comprehensively evaluate the new and promising material α-SnWO4. In the first part, the complex PLD process of BiVO4 films by ablating a BiVO4 target is systematically elucidated with a special focus on the deviations from an ideal stoichiometric target-to-substrate transfer. By correlating the V:Bi ratio of the films with their charge carrier transport properties and PEC performance, remarkable AM1.5 sulfite oxidation photocurrents of 2.4 ± 0.2 mAcm-2 at E = 1.23 V vs. RHE with stoichiometric films are achieved without any doping or nanostructuring. BiVO4 photoelectrodes with similar PEC performance are additionally prepared for the first time by the alternating ablation of Bi2O3 and V2O5 targets. This approach is shown to be an attractive alternative route to control the cation stoichiometry and lays the foundation for the future growth of epitaxial BiVO4 films. The second part of this thesis contains a comprehensive evaluation of α-SnWO4 as a novel photoelectrode material. α-SnWO4 has recently attracted attention in the field due to the combination of a nearly ideal bandgap (~1.9 eV) and a very early photocurrent onset potential (~0 V vs. RHE). Using phase-pure pulsed laser deposited films, the close-to-optimum band alignment and bandgap is confirmed, and other important parameters such as the charge carrier mobility, lifetime, diffusion length, and the PEC stability are reported for the first time. A high-temperature treatment is shown to enhance the charge carrier mobility of α-SnWO4 films by more than two orders of magnitude, as measured with time-resolved microwave conductivity (TRMC). This results in one of the highest effective charge carrier diffusion lengths ever measured in a metal oxide photoelectrode (~200 nm). A complimentary analysis by time-resolved terahertz spectroscopy (TRTS) shows that this improvement can be attributed to larger grain/domain sizes with increasing heat-treatment temperature. In other words, grain boundaries significantly limit the charge carrier transport in α-SnWO4. In addition, a hole-conductive NiOx protection layer is introduced to prevent self-passivation of the surface of the α-SnWO4 films (formation of a thin SnO2 layer), which drastically increases the sulfite oxidation photocurrent by a factor of ~100 setting a new benchmark AM1.5 photocurrent density (~0.75 mA cm-2 at E = 1.23 V vs. RHE) and IPCE (~38% at λ = 355 nm) for α-SnWO4. These findings provide important insights into the key PEC properties and performance limitations of α-SnWO4, and allow the identification of strategies to further improve the performance of this promising photoanode material.