Fabrication of Thin-Film Ta₃N₅ on Ta-Doped TiO₂ as a TCO for Photocatalytic Water Oxidation

Hamed Hajibabaei, Thomas W. Hamann

Abstract. The solar-driven photoelectrochemical (PEC) water splitting is a promising route to directly store solar energy into chemical bonds. Recently, a new class of semiconductors, e.g. tantalum nitride (Ta₃N₅), emerged as a promising candidate for PEC water splitting. Most of the studies on tantalum nitrides (Ta₃N₅), basically, share a similar synthesis recipe. The synthesis procedures, generally, begin with the oxidation of Ta(0) to Ta(IV), followed by ammonolysis at elevated temperatures ( > 850 °C). Despite of the simplicity that comes with this method, however, this method puts multiple negative impacts on the PEC performance of the Ta₃N₅. First of all, high-temperature analysis, limits the selection of TCO, therefore, Ta₃N₅ is commonly prepared on Ta foil which exclude the applicability of the Ta₃N₅ as a photoanode in tandem PEC cells. Secondly, it has been well documented that upon oxidation/ammonolysis some resistive nitrogen-poor interfaces, between Ta₃N₅ and Ta, are formed which suppress the electron collection efficiency at Ta₃N₅/Ta junction. Moreover, high-temperature ammonolysis, practically, makes it hard to control the morphology, interfaces and inherent properties of semiconductors.

Addressing these issues, we systematically developed Ta-doped TiO₂ (TTO) as a stable TCO in reducing atmosphere. Moreover, to avoid high-temperature ammonolysis, ALD was used to directly deposit thin films of Ta₃N₅. Preliminary analysis, however, shows that the as-deposited films are amorphous TaO_xN_y. It was found that ALD-deposited TaO_xN_y films can be nitridized to Ta₃N₅ at moderate conditions in comparison to analogs ALD–deposited tantalum oxide. Afterward, thin film of Ta₃N₅ was fabricated on TTO and the PEC water oxidation performances were analyzed.