Thin Film Ta-Oxynitride Semiconductor Architectures for Fundamental Investigations in Photoelectrocatalytic Water Splitting

Kyle J. Stephens, Emily Worobiej, Yanik Streit, Juliana Bruneli Falqueto, Christof Schneider, Arnold M. Müller, Christof Vockenhuber, Dengyao Yang, Motonori Watanabe, Daniele Pergolesi, Thomas Lippert;

Abstract:

Hydrogen and its applications are very important technologies for our society, and possessing various methods for its production is key to security and economic stability in these sectors. Photoelectrocatalytic water splitting is a unique route for hydrogen generation in that it utilizes resources of abundant water, unlimited sunlight, and readily available materials such as oxynitride semiconductors. Oxynitrides are popular candidates for photoactive material but suffer from degradation in corrosive electrolytes. In the endeavor to mitigate this, we employ crystalline thin film model systems fabricated by pulsed laser deposition to separate phenomena at interfaces from those in bulk. This work demonstrates novel fabrication, thorough characterization, and photoelectrochemical testing of two tantalum-containing oxynitride photoanodes, CaTaO_xN_y (CTON) and MgTa₂O_6-xN_x (MTON), in our specially designed multilayer architecture that involves TiN as a current collector and a NiO_x overlayer as the oxygen evolution catalyst. The two systems show stability for over 4 h at 1.5 V_RHE, and the CTON photoanode possesses larger photoresponse. Subsequently, CTON is used to examine the effect of NiO_x overlayer thickness where it is found 2.5 nm maximizes photocurrent density. These results provide the scientific community with new model systems and demonstrate their use to improve catalysts for water splitting technology.