Temperature-controlled evolution of microstructures that promote charge separation in a TaON photoanode for enhanced solar energy conversion

Abstract

The microstructure of a semiconductor is strongly associated with the semiconductor liquid junction (SCLJ) electric field, which drives charge separation and transfer. Here, we have explored the water-saturated ammonia nitriding of a Ta₂O₅ precursor, a thermodynamically favorable reaction, to synthesize and tune the microstructure of a TaON material in a wide temperature range of 850–1050 °C. We found that, with increasing temperature, the microstructure evolved from porous particles with profiles of the parent precursor, to dispersed sintering particles with large sizes and smooth surfaces, due to grain growth. Furthermore, bulk charge separation and transfer was shown to be dependent on the temperature-controlled microstructure evolution. As a result, when coupled with a loading of Co(OH)_x as a cocatalyst, the TaON particles grew at 1000 °C with high crystallinity, smooth surfaces and less grain boundaries, yielding a highest water splitting photocurrent of 2.3 mA cm⁻² at 1.23 V_RHE in 1 M NaOH under AM 1.5G simulated sunlight (100 mW cm⁻²). Therefore, we believe that owing to the temperature-sensitive microstructure evolution, water-saturated ammonia nitriding may be a potential avenue for synthesizing high-quality oxynitride photocatalysts for effectively separating and transporting charges.