Nanoarchitectonics of MIL-101(Fe)/g-C3N4 S-Scheme heterojunction for photocatalytic nitrogen fixation: Mechanisms and performance

Recently, there has been a growing interest in the fundamental understanding of the mechanism of how MIL-101(Fe)/g-C₃N₄ heterojunctions facilitate the occurrence of photocatalytic nitrogen fixation reactions, especially the electron transfer mechanism has attained increasing attention in photocatalysis. Herein, we implemented a "triple win scenario" strategy to fabricate the S-scheme MIL-101(Fe)/g-C₃N₄ heterojunction through a straightforward solvothermal process. The MC-6 heterojunction minimizes the recombination of photogenerated carriers, enhances light utilization efficiency, and activates the N≡N bond, thus boosting photocatalytic nitrogen fixation efficiency. The transition metal iron activates the N≡N bond, while S-scheme heterojunctions reduce the photogenerated carrier recombination. In addition, the decrease in band gap (E_g) leads to an increase in visible light utilization efficiency. ISIXPS proved the mechanism of interelectron transfer of MIL-101(Fe)/g-C₃N₄ heterojunction under illumination. Upon the creation of the heterojunction, electrons migrate from g-C₃N₄ to MIL-101(Fe), establishing an inherent electric field due to the disparate Fermi levels between the two materials. The electrons (e^-) on the g-C₃N₄ CB with a more negative reduction potential and the holes (h⁺) on the MIL-101(Fe) VB are retained, which increased the redox capacity to a great extent required for the reduction of N₂ to NH₃. The ammonia production efficiency of MC-6 photocatalyst was 160 µmol g_cat^-1 h^-1, representing an 8-fold and 2.8-fold improvement over pristine g-C₃N₄ (20 µmol g_cat^-1 h^-1) and MIL-101(Fe) (57 µmol g_cat^-1 h^-1), respectively.