Fabrication of ultrathin two-dimensional MOF nanosheets with cage-like cavities showing excellent adsorption for lead(ii)

Abstract

In this work, we present a top-down method for the preparation of 2D MOF nanosheets with cavity structures. The pro-ligand 25,26,27,28-tetrakis[(carboxyl)methoxy]calix[4]arene was elaborately selected, and a layered MOF with cavity structures was constructed. The large molecular skeleton and cup-shaped feature of the calix[4]arene caused large layer separations and weak interlayer interactions among the 2D layers, which enabled the layered MOF to be readily delaminated into ultrathin 2D MOF nanosheets. Owing to the cup-shaped feature of the calix[4]arene, there are permanent cage-like cavities loaded on the as-prepared MOF nanosheets. By decorating oxygen-containing functional groups (carboxyl and ether groups) in the cage-like cavities, the resultant Cu-MOF nanosheets showed excellent adsorption performance for Pb²⁺. The intimate contact and sufficient interactions on the exposed surface areas of Cu-MOF nanosheet resulted in ultrahigh adsorption selectivity and anti-interference ability for Pb²⁺, together with an outstanding Pb²⁺ uptake capacity of 738.65 mg g⁻¹, which were obviously better than those of its 3D precursor. The possible adsorption mechanism was systematically investigated by the investigations of zeta potential, FT-IR, XPS, and DFT calculations. This study opens the door to achieving ultrathin MOF nanosheets with cavity structures, which would well expand the applications of MOF nanosheets.