Designing Metal–Organic Framework-Based Ru@Ni-MOF@H–C ₃ N ₄ <i>p–n</i> Heterojunction through Valence Control of Metal Ions for Visible-Light Nitrogen Fixation

Visible-light-driven N2 reduction into NH4+ emerges as a promising approach for nitrogen fixation. However, the low efficiency of electron–hole separation and the multielectron transfer processes hamper the efficiency of N2 photoreduction. In this work, a potential p–n heterojunction of Ru@Ni-MOF@H–C3N4 was designed by valence control of the Ni2+/3+ node in Ni-MOF to facilitate the separation of photogenerated carriers. Ni-MOFs containing the Ni2+/3+/sulfhydryl ligand as an amphoteric semiconductor and H–C3N4 with a hollow sphere structure as an n-type semiconductor were successfully assembled as a p–n heterojunction, and Ru nanoparticles were introduced into the MOF shell on H–C3N4 as the active sites of N2 reduction. The microstructure was investigated by XAFS to understand the valence state and coordination environment of the p–n heterojunction. The built-in electric field and the Fermi level for the Ru@Ni-MOF@H–C3N4 p–n heterojunction were explored by UPS results. Ru@Ni-MOF@H–C3N4 possesses very high N2 photoreduction activity to target NH4+ (123.2 μmol g–1 h–1) and very high OER activity, indicating that the p–n heterojunction has both N2 reduction and H2O oxidation active sites. This work highlights the construction of MOF-based p–n heterojunctions through valence control of the Ni2+/3+ node and tunes charge separation and electron transmission through the built-in electron field for enhancing the N2 photoreduction performance.