Atomic-Scale Visualization of Single Atom Formation in Metal-Organic Frameworks

Recently, non-noble metal single atoms (SAs) emerge as a groundbreaking class of materials, offering enhanced efficiency, reduced metal consumption, and widespread applicability. In the present study, zeolite imidazole frameworks-8 (ZIF-8) acts as a template for encapsulating Fe precursors and transform into porous nitrogen-doped carbon (PNC) upon pyrolysis, effectively capturing Fe SAs at nitrogen defect sites. The evolutions of Fe SA formation within porous ZIF-8 structure using atomic-scale in situ high-resolution scanning transmission electron microscopy (HR-STEM) from 325 ℃ to 400 ℃ are investigated. The formation rates of SAs increase with temperature, most importantly, the formation rates of SAs are rapid in the early stage and then slow down with prolonged pyrolysis time, e.g. the densities of SAs are 2.04 × 105 /μm2 and 4.21 × 105 /μm2 at 400 oC for 3 min and 30 min, respectively. 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 are implemented to understand the formation mechanisms of the two stages. These insights, facilitated by atomic-scale in situ observation, offer precise control over synthesis pathways, thereby advancing the design of tailored SA materials for catalytic applications and beyond.