Freestanding Millimeter‐Scale Porphyrin‐Based Monoatomic Layers with 0.28 nm Thickness for CO 2 Electrocatalysis

Developing two‐dimensional (2D) and single atomic layered materials is a fascinating challenge. Here we successfully synthesize porphyrin‐based monoatomic layer (PML), a freestanding 2D porphyrin‐based material of monomer‐unit thickness (2.8 Å). The solvothermal method provides a bottom‐up approach for tailoring the monoatomic layer from the nanoscale to the milliscale. PMLs containing accurately tailorable M‐N 4 units (M=Cu and Au) were synthesized, which present metal center‐dependent performance for CO 2 electrocatalysis. PML with Cu‐N 4 centers performs high faradaic efficiencies of HCOO − and CH 4 (80.86 % and 11.51 % at −0.7 V, respectively) while PML with Au‐N 4 centers generates HCOO − and CO as major products (40.90 % and 34.40 % at −0.8 V, respectively). Irreversible restructuring behavior of Cu sites is also observed. Based on the graphene‐like properties and metal center‐selectivity relationships, we believe that PML will play a distinct role in various applications.