(Invited) Photoelectrochemical Investigation of Ta3N5 Films on TCO Substrates Synthesized Via Atomic Layer Deposition for Photocatalytic Water Splitting

Abstract

Solar driven photoelectrochemical (PEC) water splitting is a promising method to store solar energy in chemical bonds. Tantalum nitride (Ta3N5) has recently emerged as a promising candidate for PEC water splitting. Most of the studies on Ta3N5 begin with a similar synthetic route, consisting of an initial oxidation of Ta(0) metal to Ta(IV), followed by ammonolysis at elevated temperatures ( > 850 °C). Despite the simplicity of this method, there are also several drawbacks. Importantly, the high temperature ammonolysis limits the utilization of transparent conductive oxide (TCO) substrates which are necessary for fundamental photoelectrochemical measurements. In addition, lack of viable TCOs thwart use of Ta3N5 as the top electrode in a tandem PEC cell. As a solution to the above problems, we will present the synthesis of Ta-doped TiO2 (TTO) as a TCO substrate via atomic layer deposition (ALD) which we found to be stable in reducing atmospheres. ALD was also used to directly deposit thin films of Ta3N5 on TTO. While initial attempts produced as-deposited films are primarily amorphous TaOxNy, these films can be nitridized to Ta3N5 at far more moderate conditions compared to all previous reports and ALD–deposited tantalum oxide analogue films. The photoelectrochemical properties of the TTO/Ta3N5 films were investigated and the PEC water oxidation performance was analyzed. The excellent material control reported here allowed for a detailed material structure – function relationship to be determined and a path to improved performance elucidated. Initial results of a modified procedure, including ALD precursor and temperature variations, which allowed the direct synthesis of crystalline Ta3N5 on TTO substrates, will also be presented.