High-Temperature Ammonolysis of Thin Film Ta2O5 Photoanodes: Evolution of Structural, Optical, and Photoelectrochemical Properties

Tantalum nitride (Ta3N5) and oxynitride (TaON) are promising materials for photoelectrochemical (PEC) water splitting due to near-ideal band gaps and band edge positions. However, the high-temperature ammonolysis process that is usually used to make these materials depends sensitively on the process parameters and specific design of the annealing system, and reproducing highly efficient (oxy)nitride photoanodes from recipes reported by other laboratories has proven to be challenging. To understand and monitor the nitridation process in more detail, we employ an optical absorption spectroscopy technique that allows us to follow the transformation of thin Ta2O5 films in situ at temperatures up to 800 °C. Our results show that the incorporation of nitrogen in a dry ammonia atmosphere starts at 575 °C and is accompanied by a gradual red-shift of the Ta2O5 absorption edge and an expansion of the lattice due to the larger ionic radius of N3– relative to O2–. Although coloration of the material due to an N-2p → ...

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