Atomic-scale visualization of single atom formation in metal-organic frameworks

Abstract

Recently, non-noble metal single atoms (SAs) have emerged as a groundbreaking class of materials, offering enhanced efficiency, reduced metal consumption, and widespread applicability. In the present study, zeolitic imidazolate framework-8 (ZIF-8) acts as a template for encapsulating Fe(acac)3 precursors and transforms into porous nitrogen-doped carbon (PNC) upon pyrolysis, effectively capturing Fe SAs at nitrogen defect sites. The evolution of Fe SA formation within the porous ZIF-8 structure was investigated using atomic-scale in situ high-resolution scanning transmission electron microscopy (HR-STEM) from 325 °C to 400 °C. The formation of SAs increases with temperature, e.g. the densities of SAs are 2.04 × 105 μm−2 and 4.21 × 105 μm−2 at 400 °C for 3 min and 30 min, respectively. More importantly, the formation rates of SAs are rapid in the early stage and then slow down with prolonged pyrolysis time. The two-stage formation process may be governed by the atomization of Fe atoms from dispersed and clustered Fe(acac)3 precursors, respectively. Theoretical calculations were performed to understand the formation mechanisms of the two stages. These insights, facilitated by atomic-scale in situ HR STEM observation, offer precise control over synthesis pathways, thereby advancing the design of tailored SA materials for catalytic applications and beyond.