Wet ammonia Synthesis of Semiconducting N:Ta2O5, Ta3N5 and β-TaON Films for Photoanode Applications

Tantalum oxynitride (β-TaON) is a promising photoanode material for photoelectrochemical water splitting. However, the synthesis of phase-pure β-TaON films is a challenging task. This paper describes a novel approach to achieve this by the controlled nitridation of Ta2O5 films under a flow of ammonia gas (ammonolysis), which was obtained by bubbling argon through an aqueous ammonium hydroxide solution followed by a 5 Å molecular sieve to remove the water. Depending on the nitridation conditions, either nitrogen-doped Ta2O5 or Ta3N5 was obtained as a thermodynamically stable phase. The evolution of the nitridation process was monitored in-situ using optical transmission spectroscopy, which revealed that the transformation of Ta2O5 to Ta3N5 occurred within minutes, through a series of intermediate oxynitride phases. This suggested the possibility of synthesizing the desired β-TaON phase by quenching the sample at the right time during the nitridation process. By choosing the right ammonolysis conditions (750 °C, flow rate 50 ml/min.), we were able to slow down the nitridation process and terminate it before the oxide was fully converted to Ta3N5. A post-nitridation heat treatment at 800 °C indeed resulted in the formation of the desired β-TaON phase.