Cooperative Adaptive Junctions Govern Overall Photoelectrochemical Water Splitting

The solar-to-hydrogen conversion efficiencies for overall water splitting on particulate systems is low, below 3%. This limitation is in significant part due to the poor understanding, and thus inability to engineer, carrier-selective electron and hole contacts to the hydrogen- and oxygen-evolving electrocatalysts on the light-absorbing semiconductor particle. With dual-working-electrode and element-specific electric-potential measurements on SrTiO₃ model semiconductors by operando ambient-pressure X-ray photoelectron spectroscopy, we show that selective carrier collection emerges from cooperative adaptive junctions. Under illumination, hole collection by metal-oxide electrocatalysts drives metal cation oxidation that increases the effective interface electron barrier and improves hole selectivity. Simultaneously, electrons accumulate on metal hydrogen catalysts like Pt, forming hydridic species that lower the electron barrier. These findings challenge the idea that differences in crystal-facet work function govern charge separation and establish the new conceptual framework of cooperative adaptive junctions for improving particulate photocatalysts across materials systems.