Centimeter-scale perovskite SrTaO₂N single crystals with enhanced photoelectrochemical performance

Large-scale single crystals have potential applications in many fields, such as in ferroelectric and photoelectric energy conversion devices. Perovskite oxynitrides have also attracted attention in photoelectrochemical water splitting systems because of their high theoretical solar-to-hydrogen efficiencies. Nevertheless, the synthesis of perovskite oxynitride single crystals requires the coupling of cation exchange and ammonization processes, which is exceptionally challenging. The present study demonstrates an inorganic vapor method that provides, for the first time ever, high-quality epitaxial perovskite SrTaO₂N single crystals on the centimeter scale. Assessments using Raman spectroscopy, crystal structure analysis and density functional theory determined that the conversion mechanism followed a topotactic transition mode. Compared with conventional SrTaO₂N particle-assembled films, the SrTaO₂N single crystals made in this work were free of interparticle interfaces and grain boundaries, which exhibited extremely high performance during photoelectrochemical water oxidation. In particular, these SrTaO₂N single crystals showed the highest photocurrent density at 0.6 V vs. RHE (1.20 mA cm^-2) and the highest photocurrent filling factor (47.6%) reported to date, together with a low onset potential (0.35 V vs. RHE). This onset potential was 200 mV less than that of the reported in situ SrTaO₂N film, and the photocurrent fill factor was improved by 2 to 3 times.