Designed electron transport path via Fe–O–Ni atomic bond for high CO2 reduction

The assembly of different Metal organic framework (MOFs) into hybrid heterostructures has proven to be a promising strategy that can effectively break through the limited regulatory capacity of single metal sites. Here, an S-scheme heterostructure (Fe₃Ni-MOF) based on homologous ligands (1,4-phthalic acid) of ultra-thin Ni-MOF and Fe-MOF nanoparticles with directional electron transport channels, was developed and used it for photoreduction of CO₂. Under the S-scheme electric field mechanism, the photogenerated carrier can achieve efficient directional separation through Fe–O–Ni atomic bond, which significantly reduces the energy barrier of the rate-determining step. Results show that the performance of Fe₃Ni-MOF (63.5 μmol g^–1) was 20 and 3.2 times higher than that of Ni-MOF and Fe-MOF, respectively, and exhibits excellent CO selectivity (96.4%) and stability. Transmission electron microscopy and atomic force microscopy revealed the two-molecular-layers structure of Ni-MOF and the micro-assembly structure of Fe₃Ni-MOF, which can shorten the electron transport distance and increase the molecular mass transfer rate. X-ray photoelectron spectroscopy, electron spin resonance and electron density difference calculations reveal that interfacial electric fields and atomic bonds work together to promote directional carrier separation, resulting in the accumulation of holes on Ni-MOF and electrons on Fe-MOF. The Gibbs free energy calculation and in-situ Fourier transformed infrared spectroscopy validate that the micro-assembled S-scheme heterostructures with directional electron transport channels can significantly reduce the activation energy barrier of the reaction. This study not only proves the feasibility of constructing MOFs S-scheme heterostructures using homologous ligands, but also provides a new way to overcome the limitations of monometallic MOFs. This strategy is expected to open up a new avenue to design efficient photocatalysts.