What Limits the Performance of Ta3N5 for Solar Water Splitting?

Tantalum nitride (Ta3N5) is a promising photoelectrode for solar water splitting. Although near-theoretical-limit photocurrent has already been reported on Ta3N5, its low photovoltage and poor stability remain critical challenges. In this study, we used Ta3N5 nanotubes as a platform to understand the origins of these issues. Through a combination of photoelectrochemical and high-resolution electron microscope measurements, we found that the self-limiting surface oxidation of Ta3N5 resulted in a thin amorphous layer (ca. 3 nm), which proved to be effective in pinning the surface Fermi levels and thus fully suppressed the photoactivity of Ta3N5. X-ray core-level spectroscopy characterization not only confirmed the surface composition change resulting from the oxidation but also revealed a Fermi-level shift toward the positive direction by up to 0.5 V. The photoactivity degradation mechanism reported here is likely to find applications in other solar-to-chemical energy-conversion systems. [Display omitted] •The self-limiting photo-oxidation of Ta3N5 is the cause of its low photovoltages•X-ray core-level spectroscopy and photoelectrochemical studies support this claim•Complete protection of Ta3N5 from H2O is necessary for maximizing the performance Tantalum nitride (Ta3N5) promises high efficiency when used for artificial photosynthesis as a result of its superb optoelectronic properties and suitable band gap. Indeed, the photocurrent measured on Ta3N5 approaches its theoretical limit when tested for solar water splitting. The photovoltage and the stability, however, remain poor. Similar issues have been observed on other high-performance optoelectronic materials such as GaAs and GaP. Whereas photocorrosion in H2O has been understood as the key reason for the poor performance by these other materials, we show here that Ta3N5 is limited by a different mechanism. Our understanding provides a missing link in understanding what limits the performance of photoactive materials. It points to a new direction of improving the performance of solar water splitting. The performance of Ta3N5 as a photoelectrode for solar water splitting is compromised by the low photovoltage and poor stability. Wang and colleagues reveal that these issues are caused by the growth of a thin oxide layer on the surface. Although self-limiting in nature, this layer pins the Fermi level and leads to almost complete suppression of the photoactivity. The effect is quantitatively measured via X-ra