Freestanding millimeter-scale porphyrin-based monoatomic layers with 0.28 nm thickness for CO2 electrocatalysis.

Developing novel 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 A). The solvothermal method provides a bottom-up approach for tailoring the monoatomic layer from the nanoscale to the milliscale. PMLs containing accurately tailorable M-N4 units (M=Cu and Au) were synthesized, which present metal center-dependent performance for CO2 electrocatalysis. PML with Cu-N4 centers performs high faradaic efficiencies of HCOO- and CH4 (80.9% and 11.5% at -0.7 V, respectively) while PML with Au-N4 centers generates HCOO- and CO as major products (40.9% and 34.4% at -0.8 V, respectively). The 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.