Physicochemical insights into semiconductor properties of a semitransparent tantalum nitride photoanode for solar water splitting

Abstract

The self-conductivity of tantalum nitride (Ta₃N₅) thin film-based semitransparent photoanodes was found to promote the current originating from the photoelectrochemical oxygen evolution reaction (PEC OER) without a conducting substrate. With surface modification by the NiFeO_x-electrocatalyst, an optimized Ta₃N₅ thin film fabricated directly on a transparent insulating quartz substrate generated a photocurrent density of ∼5.9 ± 0.1 mA cm⁻² at 1.23 V vs. the reversible hydrogen electrode under simulated AM 1.5G solar illumination. The correlation between the PEC OER performance of NiFeO_x-modified Ta₃N₅ photoanodes and the electrical properties of Ta₃N₅ thin films was investigated based on the Hall effect measurements. By changing the nitridation conditions, these properties can be tuned so that the higher the Hall mobility (0.2 to 1.7 cm² V⁻¹ s⁻¹) and the lower the carrier concentration (10²⁰ to 10¹⁹ cm⁻³). The surface chemical states of Ta₃N₅ thin films were investigated using X-ray photoelectron spectroscopy as a means of evaluating surface oxygen impurities and nitrogen vacancies, which may correlate with the PEC OER performance and the electrical properties of the material.