Reactive sputter deposition of Cu-based photocathode thin films Katarina S. Flashar, Dominic Rapf, Ian D. Sharp, Verena Streibel Walter Schottky Institute, Technical University of Munich, D-85748 Garching, Germany Physics Department, TUM School of Natural Sciences, Technical University of Munich, D-85748 Garching, Germany In photoelectrochemical (PEC) water splitting, metal oxide-based semiconductor cathode materials remain comparatively underexplored, mainly because most transition metal oxides are native n-type semiconductors. Cu-based metal oxides are among the few within this class that exhibit native p-type conductivity and, thus, are essential for photocathode research. Among ternary Cu-oxides, copper bismuthate (CuBi 2 O 4 ) is an interesting photocathode candidate [1] . It has a suitable bandgap for visible light absorption and favorable photocurrent onset potentials [2] , but still suffers from poor charge transport characteristics [1] . To modulate its bandgap for more efficient light absorption and tune its charge transport properties, we have recently explored nitrogen incorporation into CuBi 2 O 4 (N:CuBi 2 O 4 ). As a starting point, we first developed a reactive co-sputtering and thermal annealing routine for the synthesis of high-quality copper bismuthate thin films. Second, we adjusted the nitrogen content in the gas mixture during reactive sputter deposition and post-deposition annealing treatments to tailor the extent of nitrogen incorporation. Using this N:CuBi 2 O 4 materials library, we investigate the impact of incorporated nitrogen on the optical and structural properties, as well as PEC activity, of these films. We find that PEC performance is noticeably enhanced with nitrogen incorporation. The exact origin of this enhancement is subject of ongoing studies. Alongside CuBi 2 O 4 , another promising Cu-based ternary transition metal oxide composition space comprises copper niobates (Cu x Nb y O z ). With band gaps in the visible range for several phases and suggested chemical stability, these materials are prospective photocathode candidates [3] . We are currently focusing on developing reactive co-sputtering protocols to grow well-defined thin films of the most promising phases, CuNb 3 O 8 and CuNb 13 O 33 . Given that the visible-light photocatalytic activity of copper niobates is still debated, we will elucidate this question using a combination of optical and PEC measurements. References 1. J. K. Cooper et al., Chem. Mater., 2021, 33, 3, 934–945. 2. N. T. Hahn et al., J. Phys. Chem. C, 2012, 116, 10, 6459–6466. 3. Weiss and Marschall, Appl. Catal., A, 2023, 661, 119234.
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