The performance of Ta 3 N 5 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-ray spectroscopy and photoelectrochemical studies. The information sheds light on strategies for improving photoelectrode performance. SUMMARYTantalum nitride (Ta 3 N 5 ) is a promising photoelectrode for solar water splitting. Although near-theoretical-limit photocurrent has already been reported on Ta 3 N 5 , its low photovoltage and poor stability remain critical challenges. In this study, we used Ta 3 N 5 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 Ta 3 N 5 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 Ta 3 N 5 . 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.

Solar rechargeable battery combines the advantages of photoelectrochemical devices and batteries and has emerged as an attractive alternative to artificial photosynthesis for large-scale solar energy harvesting and storage. Due to the low photovoltages by the photoelectrodes, however, most previous demonstrations of unassisted photocharge have been realized on systems with low open circuit potentials (<0.8 V). In response to this critical challenge, here it is shown that the combined photovoltages exceeding 1.4 V can be obtained using a Ta N nanotube photoanode and a GaN nanowire/Si photocathode with high photocurrents (>5 mA cm ). The photoelectrode system makes it possible to operate a 1.2 V alkaline anthraquinone/ferrocyanide redox battery with a high ideal solar-to-chemical conversion efficiency of 3.0% without externally applied potentials. Importantly, the photocharged battery is successfully discharged with a high voltage output.

Detailed understanding of the photocatalyst/electrolyte interface is critical to further development of photocatalysts. The authors observed a unique phenomenon whereby interface reactions between Ta 3 N 5 and Co(OH) 2 under photo-electrochemical conditions improved the performance of Ta 3 N 5 for solar water oxidation. The photo-induced interface not only improved the surface kinetics but also inhibited severe Fermi-level pinning for bare Ta 3 N 5 . The strategy developed here sheds light on new future routes to modifying photocatalysts for better performance.