Ultrathin Cage-based Covalent Organic Framework Nanosheets as Precursor for Pyrolysis-Free Oxygen Evolution Reaction Electrocatalyst

Abstract

Ultrathin covalent organic framework (COF) nanosheets are very appealing 2D materials, but the mass production of ultrathin COF nanosheets remains a great challenge. Here, by using cage-like bicyclocalix[2]arene[2]triazines tri-aldehyde (BCTAL) as the building block, 2D COF Cage-COF-1 was estimated to have a very weak interaction between adjacent layers (around 1/50 compared to that of graphite). As a result, 1.2 nm thick trilayer COF nanosheets were facilely exfoliated from the pristine COFs with large lateral size and high thickness homogeneity. The Cage-COF-1 nanosheet is featured by imine linkage, but it is catalytic inactive in oxygen evolution reaction (OER). After post-metalation with Co2+ under ambient conditions, remarkable catalysis activity and stability was observed for Cage-COF-1-Ns/Co, which has lower overpotential (330 mV) and Tafel slope (56 mV/dec) in catalytic OER compared to many other Co catalysts. This work has confirmed that weakening the interlaminar interaction is an effective strategy for the production of ultra-thin COFs nanosheets. Due to the fully exposed and accessible imine linkage, catalysis active metal site can be controllably produced via post-synthesis from nanosheets under very mild conditions.