Fig. 2

Synthetic strategy and structural characterisation. a Schematic overview of the etching transformation of four-coordinated Cu atoms to three-, two- and one-coordinated Cu atoms with organic ligands (w represents oxygen atom from the water, and h represents oxygen atom from the hydroxide anion, OH⁻). b–e Model structures and real morphologies of the four different MOF crystal states after etching. b Pristine HKUST-1 MOFs, c stacked-nanosheet MOFs, d random-etching MOFs, e collapsed MOFs. f Transmission electron microscopy (TEM) top-view image of a five-layered CASFZU-1 thin nanosheet. g Selected area electron diffraction (SAED) pattern (taken from the white-circled region in f) at the top shows diffraction within a few-layered nanosheet. A simulated SAED pattern of the HKUST-1 crystal down the \([\bar 111]\) axis is also shown. h High-resolution TEM (HR-TEM) image of the white-framed region at the edge of the nanosheet shown in f. i The crystal structure of HKUST-1 along the \([1\bar 1\bar 1]\) direction. Scale bars represent 500 nm for b, d, and e, 200 nm for d, 100 nm for f and 2 nm for h. Colour scheme for chemical representation: cyan, Cu; red, O; grey, C; white, H