HAXPES-Guided Buried Interface Engineering Enables Low-Onset-Potential Ta₃N₅ Photoanodes

Efficient electron-hole extraction is essential for achieving high performance in Ta₃N₅ photoanodes for solar water splitting. Prior studies on Ta₃N₅ emphasized surface interfaces, while the buried junction governing carrier extraction and photovoltage generation remains less explored. Here, we introduce a buried junction engineering strategy experimentally validated by angle-resolved hard X-ray photoelectron spectroscopy (AR-HAXPES), which directly resolves the energy band alignment at buried interfaces. A phase-pure NbN conductive interlayer forming an atomically coherent, lattice-matched junction with Mg:Ta₃N₅ is constructed. Depth-resolved AR-HAXPES reveals a 0.26 eV reduction in Schottky barrier height at the NbN/Mg:Ta₃N₅ interface compared with a conventional mixed-phase Nb_xN contact, confirming optimized Fermi level alignment and accelerated electron extraction. Consequently, the NbN/Mg:Ta₃N₅ photoanode achieves a low onset potential of 0.38 V vs RHE and a record applied bias photon-to-current efficiency of 4.38%. This work establishes HAXPES-guided buried interface engineering as a powerful strategy for advancing photoelectrodes for solar fuel production.