Fabrication of BaTaO 2 N Thin Films by Interfacial Reactions of BaCO 3 /Ta 3 N 5 Layers on a Ta Substrate and Resulting High Photoanode Efficiencies During Water Splitting

Thin films of barium tantalum oxynitride (BaTaO 2 N) with thicknesses of 150–680 nm are grown on TaN/Ta substrates via the interfacial reaction of BaCO 3 /Ta 3 N 5 layers in a N 2 atmosphere. A TaN thin film is first deposited on a Ta substrate by the Ar/N 2 ion sputtering of a Ta target, and then covered with a TaO x layer by Ar/O 2 sputtering. The TaO x layer is subsequently nitrided in an NH 3 flow to a Ta 3 N 5 layer. Finally, Ba metal is deposited on the Ta 3 N 5 /TaN/Ta substrate under vacuum, oxidized, and carbonized to produce BaCO 3 . The interfacial reaction between the BaCO 3 and Ta 3 N 5 layers is conducted in a N 2 atmosphere at high temperatures. X‐ray diffraction patterns confirm BaTaO 2 N and Ta 2 N phases on the Ta substrates, while cross‐sectional scanning electron microscopy images show a three‐layer BaTaO 2 N/Ta 2 N/Ta structure. Cobalt oxide (CoO x )‐deposited BaTaO 2 N/Ta 2 N/Ta photoanodes for water oxidation provide a high photocurrent of 4.6 mA cm −2 at 1.23 V (vs RHE) and an incident photon‐to‐current conversion efficiency of 9% at 600 nm under simulated AM1.5G irradiation. The efficiency of this photoanode is remarkably improved compared with previously reported photoanode thin films. Interestingly, a BaTaO 2 N/Ta 2 N/Ta electrode with similar photoanode performance is also obtained via the interfacial reaction between TaN and BaCO 3 layers.