Karsten Harbauer

With the rising anthropogenic CO₂ concentrations in the atmosphere and the growing interest in converting solar energy into value-added products, interest in (photo)electrochemical reduction of CO₂ is increasing. A key component for this process is an optically transparent and electrically conductive substrate, usually in the form of a transparent conducting oxide (TCO) film deposited on glass. However, most traditional TCO materials, such as fluorine-doped SnO₂ (FTO) or tin-doped In₂O₃ (ITO), are not stable under cathodic conditions. TiO₂ has been widely reported as an excellent protection layer material in photo-electrochemical (PEC) research, and niobium is one prospective dopant to improve its conductivity. In this work, we demonstrate that commercial FTO glass can be protected against (photo)corrosion under cathodic reaction conditions in pH-neutral electrolytes by direct-current sputtering of thin Nb-doped TiO₂ (Nb:TiO₂) overlayers. Combined with a pre-deposition FTO etching procedure, the Nb:TiO₂-covered FTO electrode offers excellent electrochemical stability while maintaining good transparency and conductivity.

The ever-increasing energy demand and environmental concerns associated with fossil-based fuels have driven the urgent need for sustainable methods to produce chemical fuels. One approach is to perform solar water splitting to produce hydrogen and oxygen. For this purpose, metal oxide-based semiconductor are promising photoelectrode materials due to their relatively high abundance and stability in aqueous solutions. However, metal oxides typically have only slow surface reaction kinetics for water oxidation [1]. To overcome this, water oxidation co-catalysts, such as CoP_i, FeOOH, NiFeO_x, and MnO_x, have been deposited on the surface of numerous metal oxide semiconductors resulting in an improved performance [2-6]. However, the processes at the semiconductor/co-catalyst interface are not yet well understood. Here, we investigate the role of MnO_x and Ni:MnO_x co-catalysts (thickness < 10 nm) deposited by atomic layer deposition (ALD) on Ta-O-N thin films as a model system. By systematically controlling the partial pressure of NH₃, H₂ and H₂O during post-annealing of Ta thin films, different phases (e.g. Ta₂O₅, TaO_xN_y, Ta₃N₅) with varying valence band maximum positions were successfully prepared. The valence band position of the co-catalyst was also varied by modifying the Ni content. Photoelectrochemical studies with and without hole scavenger reveal that MnO_x and Ni:MnO_x enhances the photocurrent of Ta-O-N films by a factor of ~ 4, but they do not affect the charge injection efficiency. Instead, the photocurrent increase is likely a result of higher charge separation efficiency. We attribute this to the formation of a band bending at the Ta-O-N/MnO_x and Ta-O-N/Ni:MnO_x interface, as suggested by the open circuit potential (OCP) and in-line X-ray photoelectron spectroscopy (XPS) data. This is further supported by analyzing Ta-O-N films with varying thickness of the co-catalysts; thicker co-catalysts result in increasing band bending, which correlates with the photocurrent trend as well as the effective carrier diffusion length measured by time-resolved microwave conductivity (TRMC). Finally, the influence of the band offsets between the different photoanode in the Ta-O-N systems and the co-catalysts is discussed. References [1] M. De Respinis, Chem. Mater. 2015, 27, 7091. [2] F. F. Abdi, ChemCatChem 2012, 5, 490. [3] W. D. Chemelewski, J. Am. Chem. Soc. 2014, 136, 2843. [4] J.-W. Jang, Nat. Commun. 2015, 6, 7447. [5] D. Xu, Electrochim. Acta. 265, 2018, 10. [6] A. Ramirez, J. Phys. Chem. C. 2014, 118, 14073.

Our recently reported multifunctional plasma and deposition sensor [Welzel et al., Appl. Phys. Lett. 102, 211605 (2013)] was used for the characterization of two different plasma sources: a magnetron sputtering deposition source and an ion beam source. The multifunctional sensor, based on a conventional quartz crystal monitor (microbalance) for mass increase/decrease measurements, can measure quasi-simultaneously the deposition/etching flux, the energy flux, and the charged particle flux. By moving the sensor or the plasma source stepwise against each other, the lateral (radial) flux profiles of the different sources can be measured with a lateral resolution of about 8 mm, the diameter of aperture in front of the quartz crystal. It is demonstrated that this compact and simple multifunctional sensor is a versatile tool for the characterization of different kinds of plasma sources for deposition and etching purposes. By combining the different measured quantities, the ion-to-neutral ratio and the mean energy per deposited atom can be calculated, parameters that are essential for the characterization of plasma deposition and etch processes.