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

Visible-light-driven N 2 reduction into NH 4 + 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 N 2 photoreduction. In this work, a potential p–n heterojunction of Ru@Ni-MOF@H–C 3 N 4 was designed by valence control of the Ni 2+ / 3+ node in Ni-MOF to facilitate the separation of photogenerated carriers. Ni-MOFs containing the Ni 2+ / 3+ /sulfhydryl ligand as an amphoteric semiconductor and H–C 3 N 4 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–C 3 N 4 as the active sites of N 2 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–C 3 N 4 p–n heterojunction were explored by UPS results. Ru@Ni-MOF@H–C 3 N 4 possesses very high N 2 photoreduction activity to target NH 4 + (123.2 μmol g –1 h –1 ) and very high OER activity, indicating that the p–n heterojunction has both N 2 reduction and H 2 O oxidation active sites. This work highlights the construction of MOF-based p–n heterojunctions through valence control of the Ni 2+ / 3+ node and tunes charge separation and electron transmission through the built-in electron field for enhancing the N 2 photoreduction performance.