Ce-induced built-in electric field engineering in Bi7O9I3/TiO2 S-scheme heterojunction for enhanced photocatalytic nitrogen fixation

Photocatalytic nitrogen fixation offers a sustainable route for ammonia synthesis under ambient conditions, but its practical application is severely limited by inefficient charge separation and sluggish interfacial charge-transfer kinetics. Herein, we develop a Ce-doped Bi 7 O 9 I 3 /TiO 2 S-scheme heterojunction through dopant-mediated Fermi level engineering to strengthen the interfacial built-in electric field (BIEF) and enhance photocatalytic nitrogen fixation. The charge transfer behavior and N 2 reduction mechanism was investigated using photoelectrochemical measurements, Kelvin probe force microscopy (KPFM), ultraviolet photoelectron spectroscopy (UPS), theoretical calculations, and in situ FT-IR spectroscopy. Ce doping shifts the Fermi level of Bi 7 O 9 I 3 upward, thereby enlarging the Fermi level difference between Bi 7 O 9 I 3 and TiO 2 and strengthening the interfacial BIEF. The enhanced BIEF promotes directional interfacial charge transfer and suppresses photogenerated carrier recombination, leading to improved charge separation. Consequently, the optimized photocatalyst achieves an NH 3 production rate of 53.70 μmol g −1 h −1 in pure water, which is 8.21- and 3.76-fold higher than those of pristine Bi 7 O 9 I 3 and the undoped Bi 7 O 9 I 3 /TiO 2 heterojunction, respectively. In situ FT-IR spectroscopy and theoretical calculations indicate that the photocatalytic nitrogen reduction proceeds through an associative distal pathway. This work demonstrates that dopant-induced Fermi level engineering can effectively regulate interfacial electric fields and charge transfer behavior in S-scheme heterojunctions, providing a rational strategy for BIEF engineering in efficient photocatalytic nitrogen fixation.