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

The self-conductivity of tantalum nitride (Ta 3 N 5 ) 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 Ta 3 N 5 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 Ta 3 N 5 photoanodes and the electrical properties of Ta 3 N 5 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 cm 2 V -1 s -1 ) and the lower the carrier concentration (10 20 to 10 19 cm -3 ). The surface chemical states of Ta 3 N 5 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.