Enriching and Quantifying Porous Single Layer 2D Polymers by Exfoliation of Chemically Modified van der Waals Crystals

Abstract

2D polymer sheets with six positively charged pyrylium groups at each pore edge in a stacked single crystal can be transformed into a 2D polymer with six pyridines per pore by exposure to gaseous ammonia. This reaction furnishes still a crystalline material with tunable protonation degree at regular nano‐sized pores promising as separation membrane. The exfoliation is compared for both 2D polymers with the latter being superior. Its liquid phase exfoliation yields nanosheet dispersions, which can be size‐selected using centrifugation cascades. Monolayer contents of ≈30 % are achieved with ≈130 nm sized sheets in mg quantities, corresponding to tens of trillions of monolayers. Quantification of nanosheet sizes, layer number and mass shows that this exfoliation is comparable to graphite. Thus, we expect that recent advances in exfoliation of graphite or inorganic crystals (e.g. scale‐up, printing etc.) can be directly applied to this 2D polymer as well as to covalent organic frameworks.

The Current Understanding of how 2D Polymers Grow Photochemically

“…To obtain monolayers, which in a sense are the real 2D polymer macromolecules, [6] exfoliation is required. This is not always trivial and never quantitative, although progress has recently been achieved, both concerning the exfoliation technique itself [35] as well as an improved molecular design. [16,36] A polymerization mechanism has been proposed for monomer 1.…”

Two-Dimensional Polymers: Synthesis and Applications

“…Dynamic covalent bonds (e.g., imine and hydrazine bonds) were implemented to link the COF building blocks to further enhance the stability. In addition, the anthracene-based single-crystal approach including preorganization in a single crystal and photo-cross-linking steps has been extensively exploited to produce thin-layer 2D polymers with high stability via photoinduced dimerization of anthracene and solvent-assisted delamination . The quantification of nanosheet sizes, layer number, and mass after delamination can be readily achieved …”

“…8 The quantification of nanosheet sizes, layer number, and mass after delamination can be readily achieved. 8 Chemical Delamination. This method invokes the modification of the chemical structure of COFs to weaken the interlayer interactions and enlarge the interlayer spacing, leading to delamination of lamellar nanomaterials.…”

“…To date, many strategies have been developed to synthesize thin-layer 2D polymer via “top-down” and “bottom-up” routes. Despite the most widely used approach, the former route often involves the delamination of the layered hosts through exfoliation with polar solvents or mechanical forces to produce thin-layer polymer nanosheets, and the procedure is time-consuming with low yield. Furthermore, defects are inevitably introduced on nanosheets during delamination, and the size of the exfoliated polymer nanosheets is difficult to be controlled, yielding often extremely small nanosheets .…”

Impact of Pretreatment of the Bulk Starting Material on the Efficiency of Liquid Phase Exfoliation of WS2

“…One versatile method to obtain two-dimensional nanosheets is liquid phase exfoliation (LPE), which was demonstrated for graphene in 2008 [ 1 ]. LPE can deliver dispersions of nanosheets with high yield and is applicable to a wide range of materials, including h-BN, [ 2 , 3 ] transition metal dichalcogenides (TMDs) in their 2H-polytype (MoS 2 or WS 2 [ 2 , 3 , 4 ]) or 1T-polytype (TaS 2 [ 5 ] or ReS 2 [ 6 , 7 ]), III-VI semiconductors (GaS [ 8 ], InSe [ 9 ]), oxides (MnO 2 [ 10 ]), pnictogens (black phosphorus [ 11 , 12 , 13 ], Sb [ 14 , 15 ]), layered double hydroxides [ 16 ], naturally occurring minerals such as franckeite [ 17 ] or cylindrite [ 18 ], MXenes [ 19 ] or organic layered polymers [ 20 ] to name just a few. Liquid phase exfoliation is considered a two-step process involving mechanical delamination of the van der Waals crystals in liquid media through high energy processes such as sonication to overcome the interactions between the layers followed by subsequent colloidal stabilization in the liquid for example through the aid of suitable solvents or surfactants which prevent reaggregation.…”