(A) Schematic illustration of the fabrication of single-shell (SSHM), double-shell (DSHM), and triple-shell (TSHM) hollow MIL-101 superstructures by layer-by-layer self-assembly growth and etching processes. (B₁)–(B₈) TEM ((B₁), (B₃), (B₅), (B₇)) and HRTEM ((B₂), (B₄), (B₆), (B₈)) images of solid MIL-101 ((B₁), (B₂)), SSHM ((B₃), (B₄)), DSHM ((B₅), (B₆)), and TSHM ((B₇), (B₈)). Scale bars: (B₁)–(B₄), (B₆), (B₈), 50 nm; (B₅), (B₇), 200 nm. Insets of (B₂), (B₄), (B₆), (B₈): the corresponding FFT patterns (Reproduced from Ref. [131]. Copyright©2017, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim). (C) Schematic illustration of the synthesis of single-shelled (SSHMs), double-shelled (DSHMs), and tripled-shelled (TSHMs) hollow MOFs through shell-by-shell soft-templating method, respectively. (D₁)–(D₃) SEM images of SSHMs (D₁), DSHMs (D₂), and TSHMs (D₃). (E₁)–(E₃) TEM images of SSHMs (E₁), DSHMs (E₂), and TSHMs (E₃) (Reproduced from Ref. [134]. Copyright©2017, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim).