Eliminating Sub‐Surface Transport Barrier in Ta ₃ N ₅ Photoanodes via Sulfur‐Assisted Nitrogen Incorporation

ABSTRACT The conversion of tantalum oxide to tantalum nitride (Ta 3 N 5 ) is fundamentally constrained by limited nitrogen diffusion and strong Ta─O bonds, resulting in defect gradients that impair charge transport and reduce solar‐to‐hydrogen (STH) efficiency in photoelectrochemical water splitting. Using angle‐resolved hard X‐ray photoelectron spectroscopy, we uncover a previously unrecognized sub‐surface charge transport barrier in Ta 3 N 5 , originating from non‐uniform defect distributions due to diffusion‐limited nitridation. To overcome this, we introduce a sulfur‐assisted bond‐weakening strategy via thiourea (TU) pretreatment, which forms S─Ta─O linkages that destabilize Ta─O bonds and promote uniform nitrogen incorporation. This eliminates defect gradients and the associated interfacial barrier, enhancing surface charge injection efficiency by over 37%. When integrated with gradient Mg doping, the approach further improves bulk carrier separation efficiency to 94% and yields a record half‐cell STH efficiency of 4.12% in TU‐Mg:Ta 3 N 5 photoanodes. These findings establish sulfur‐mediated bond weakening as a general route for improving nitridation and photocarrier transport in nitride photoelectrodes.