MXene‐Derived Highly Oriented TiN Enables Ta ₃ N ₅ Photoanodes for Transmission‐Mode Bias‐Free Solar Water Splitting

ABSTRACT

The incompatibility between optical transparency and photocarrier extraction in Ta ₃ N ₅ photoanodes, arising from the harsh ammonolysis process, has constrained their potential for bias‐free water splitting, despite intrinsically ideal band structures. Here, we report a transparent and nitridation‐compatible bottom electrode based on 2D MXene‐derived TiN thin films. The (111)‐preferred orientation of MXene‐derived TiN facilitates efficient electron extraction from Ta ₃ N ₅ , while maintaining high optical transmittance at an ultrathin thickness. By integrating with n ‐GaN, we design a heterostructured electron collector that further improves charge separation, corroborating the directional electron extraction in Ta ₃ N ₅ /TiN/ n ‐GaN via photoelectron spectroscopy and carrier dynamics studies. The transparent photoanode generates a photocurrent density of 9.2 mA cm ⁻² at 1.23 V versus a reversible hydrogen electrode for water oxidation, also enabling transmission‐mode bias‐free solar water splitting. A triple‐tandem photoelectrochemical device with optimized band gap matching achieves a solar‐to‐hydrogen conversion efficiency of 13.2%, approaching 83% of the theoretical limit of Ta ₃ N ₅ . This study opens up an unexplored interface engineering for carrier and light utilization in metal nitrides toward efficient solar hydrogen production.