Among the large number of MOFs structures, 2D layered structures take a large proportion[5]. Recently, inspired by the favorable properties of single-layer-graphene and other functional inorganic nanolayers, 2D-MOFs tend to be fabricated into high aspect ratio metal organic frameworks nanosheets (MONs) isostructural to bulk crystals[51] (Figure 1-9). Accordingly, MONs can be defined as materials that possess the following: “1) organic ligands coordinated to metal ions or clusters with continuous connectivity in two-dimensions, but only non-covalent interactions in the third dimension; 2) highly anisotropic materials with one dimension approaching monolayer thickness and the others being at least an order of magnitude larger and approximately equal in size; 3) materials which can be isolated in a form with the dimensions outlined above as free standing sheets, not attached to a surface or other scaffold or as layers in a bulk material”[52].
Figure 1-9. Concept of the coordination nanosheets (CONASHs). Various combinations of metal ions and ligands produce a range of functionalized CONASHs[53].
However, synthesizing high surface-to-volume atom ratios MONs is still difficult since growing 2D frameworks to the microscale and at the same time maintaining crystal stacking in the few or mono-layer often leads to fragile nanosheets lacking long-range order. Moreover, in terms of characterization, owing to specific structure of MONs, sets
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of specific of proper techniques are required to identify its structures and explore properties. For instance, synchrotron grazing incidence X-ray diffraction (GIXRD), which is sensitive to lattice parameters parallel to horizon direction, can be used as an ideal technique for detecting the in-plane peaks of MONs[54–56]. Grazing incidence wide-angle X-ray scattering (GIWAX) and grazing incidence small-angle X-ray scattering (GISAX) are also powerful and versatile structural measurement techniques to probe nanoscale objects deposited on surfaces[57,58]. Solid state nuclear magnetic resonance spectroscopy (NMR), pair distribution function (PDF) data, X-ray photo spectroscopy (XPS), selected area electron diffraction (SAED) and Raman spectroscopy are also adopted to probe the structure and composition information of nanosheets. In addition to the structural analysis, the nanoscopic characteristics must also be investigated. Atomic force microscope (AFM) is used to determine the thickness of nanosheets. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) provide information about morphology with high resolution images.
Energy dispersive X-ray spectroscopy (EDX) and electron energy-loss spectroscopy (EELS) were utilized to analyze the elemental distribution of nanosheets.
1.2.1 Synthesis of MONs
Different from preparation of MOFs materials, the synthesis of MONs is not restricted to self-assembly of organic linkers and metal ions or clusters, but also include delicate exfoliation from stacked bulk crystals to provide highly anisotropy 2D freestanding nanosheets (Figure 1-10). However, it remains challenging to fabricate high quality MONs with high aspect-ratio in high yield because the breaking off 2D layers and structural collapse needs to be prevented during the process. Furthermore, it is very important to control the thickness of the lamellar to avoid aggregation. To address these challenges, researchers have developed two main approaches, top-down and bottom-up methods related to a variety of nanosheets which have yielded promising applications.
Top-down methods are mostly used for the 2D MOFs which are formed by layers stacking perpendicular to horizon direction with weak interactions, such as van der
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waals forces, hydrogen bonding, π-π interaction, etc. The non-covalent bonding between layers allows the exfoliation without disturbing the coordination within layers.
At present, the reported top-down approach includes liquid sonication exfoliation[59], Scotch tape method (mechanical exfoliation)[60], chemical exfoliation[61], Li-intercalation[62] and freeze–thaw method[63], to name a few. Although the top-down method is simple and efficient, the exfoliation rate and yield are generally too low for further application. Another important approach is bottom-up, which controls the growth of crystals within only two dimensions resulting in fewer layers or even monolayers without the need of further modification. Additionally, there are some researchers which successfully achieved the stacking of the layers via bottom-up method including interfacial synthesis[56], three-layer synthesis[64], modulated synthesis[65] and directly sonication synthesis[66]etc. In fact, the combination of solvents influenced the morphology and crystallinity of nanosheets via modulation of MOF growth kinetics. Although the research in MONs has developed with unexpectedly fast space, there are still some problems to solve such as chemical and mechanical stability or flexibility.
Figure 1-10. Schematic illustration of the bottom-up and top-down methodologies for producing MONs.[52]
1.2.2 Properties of MONs
In the past several years, MONs showed excellent performance in wide range of applications such as energy storage, biology, medicine and sensing[59,64,67–69]. This is
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due to its unique properties stemming from distinct ultrathin 2D structures. They do not only share the advantages of other bulk 2D MOFs, such as chemical tunability and dynamic behavior, but also overcome the shortcomings from stacked layers and buried active sites.
MONs, due to the large number of exposed active sites and short diffusion distance for the regents during reaction, have attracted more and more attention as electrochemistry catalysis. Oxygen evolution reaction (OER), a reaction related to metal–air batteries, and water splitting technologies—typically require expensive noble metal or metal oxides. MONs showed high potential to improve this situation. Recently, Huang et.
al.[70] used electrochemical exfoliation of a pillared-layer metal–organic framework to obtain a 2D ultrathin nanosheet which boost the oxygen evolution reaction. It is an efficient catalyst with overpotentials as low as 211 mV at 10 mA/cm2 and a turnover frequency as high as 30 s-1 at an overpotential of 300 mV. After exfoliation, the concentration of active sites increased 4 times than 3D bulk crystals which is important for the significant improvements of the OER performances (Figure 1-11a). In 2014, Gascon et. al. prepared high aspect ratio and high quality Cu(BDC) MOF nanosheets using a bottom-up strategy depending on diffusion-mediated modulation of the MOF growth kinetics. The two-dimensional nanosheets could be incorporated into polymer matrices to obtain ultrathin MOF–polymer composites with outstanding CO2 separation performance from CO2/CH4 gas mixtures, together with an unusual and highly desired increase in the separation selectivity with pressure. This is mainly due to superior occupation of membranes cross-section by nanosheets fillers distributing uniformly at different depth thus effectively eliminates the undesired plasticization effect and repeats gas discrimination to improve separation selectivity[64] (Figure 1-11b). Additionally, the suppressed fluorescence of materials could be turned on via solvent-mediated exfoliation which is beneficial for biology sensing platforms[71]. Zhang’s group adopted 2D M-TCPP M = Zn, Cu, Cd or Co, TCPP = tetrakis(4-carboxyphenyl)porphyrin) nanosheets as fluorescent sensing platforms for DNA detection. The 2D Cu-TCPP nanosheet exhibits the best performance for sensing DNA with detection limit of 20 ×
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10−12 M, which is much lower than the previously reported MOF particle-based fluorescent assays[72] (Figure 1-11c).
Figure 1-11. (a) Polarization curves of NiCo-UMOFNs, Ni-UMOFNs, Co-UMOFNs, RuO2 and bulk NiCo-MOFs in O2-saturated 1 M KOH solution at a scan rate of 5 mV/s. The dotted horizontal line is a guide to the eye showing a current density of 10 mA/cm2[73]; (b) FEB-SEM of nanosheets dispersed in a polymer membrane[64]; (c) Schematic illustration of 2D MOF nanosheet-based fluorescent DNA assay[72].