Newly Emerging Metal–Organic Frameworks (MOF), MXenes and Zeolite …
229
occurs at solvent interface. The thickness could be controlled by organic ligands
dispersion. For example, Nano-a (nickel bis(dithiolene) nanosheets) nanosheet was
fabricated by coordination reaction that occurred at the interface of benzenehexathiol
(BHT) and nickel acetate [69]. Ethyl acetate thin layer containing BHT spread on the
surface of aqueous solution of Ni(OAc) 2 /NaBr. By this method, nano-1 nanosheet
with thickness of 0.6 nm was formed.
Although with the wide application, interfacial synthesis is faced with a challenge
in the low yield. In order to deal with this, a three-layer synthesis strategy was
explored [70]. In this process, DMF and acetonitrile were used as miscible solvents.
Because of the different densities, acetonitrile was on the top and DMF was at the
bottom, and middle layer composed of equal DMF and acetonitrile was formed.
Cu(NO 3 ) 2 was dissolved in the top layer and BDC ligand was in the bottom layer.
With the continuous diffusion of Cu
2+ and BDC ligands into the middle layer, MOF
nanosheets are formed. MOF nanosheets synthesis by this strategy had a very high
yield. CuBDS nanosheets fabricated in this process are characterized by the thickness
of 4–25 nm and lateral side of 0.5–4.0 μm, respectively. This method also displayed
great practicability in synthesis of many other MOF nanosheets such as ZnBDC,
CoBDC, Cu(1,4-NDC), and Cu(2,6-NDC) (NDC = naphthalenedicarboxylate).
Surfactant-assisted method has been used to fabricate MOF nanosheets with a
thickness below 10 nm [77]. The MOF products formation difference in the process
with and without surfactant was compared. Without surfactant, only the bulk crystals were formed. When the surfactant of (Polyvinylpyrrolidone, PVP) was introduced into the synthesis system, nanosheets with a thickness of ~8 nm were obtained
because of the restricted growth in the vertical direction. In addition to the surfactants,
many other small molecules (e.g., acetic acid and pyridine) could also play the same
role [78]. They can coordinate with specific planes of MOF crystals and induce the
anisotropic MOFs growth [79]. In addition to the methods mentioned above, many
other ones (e.g., chemical vapor deposition (CVD)) could also be used to synthesize
MOF nanosheets [80, 81].
Besides the intrinsic synthesis, functionalization of MOF nanosheets is also an
important aspect [82]. Recently, various functional materials such as noble metals
[83], metal sulfide [84], and carbon nanomaterials [85] were incorporated into MOF
nanosheets to obtain the maximized synergetic effect.
4.2 Zeolite Nanosheet
Zeolites nanosheets with various interlayer linkers have been synthesized [86–89]
with the purpose to tune various catalytic systems, for example, isomerization and
hydrocracking [90], selective formation of molecules [91], and epoxidation reaction [92]. Moreover, high-aspect-ratio zeolite nanosheets could further assemble
into zeolite films, which shows great potential in molecular sieving. The currently
used method to synthesize zeolite nanosheets was exfoliation. Nevertheless, because
of morphological damage (aggregation, curling, and fragmentation) and structural
229
occurs at solvent interface. The thickness could be controlled by organic ligands
dispersion. For example, Nano-a (nickel bis(dithiolene) nanosheets) nanosheet was
fabricated by coordination reaction that occurred at the interface of benzenehexathiol
(BHT) and nickel acetate [69]. Ethyl acetate thin layer containing BHT spread on the
surface of aqueous solution of Ni(OAc) 2 /NaBr. By this method, nano-1 nanosheet
with thickness of 0.6 nm was formed.
Although with the wide application, interfacial synthesis is faced with a challenge
in the low yield. In order to deal with this, a three-layer synthesis strategy was
explored [70]. In this process, DMF and acetonitrile were used as miscible solvents.
Because of the different densities, acetonitrile was on the top and DMF was at the
bottom, and middle layer composed of equal DMF and acetonitrile was formed.
Cu(NO 3 ) 2 was dissolved in the top layer and BDC ligand was in the bottom layer.
With the continuous diffusion of Cu
2+ and BDC ligands into the middle layer, MOF
nanosheets are formed. MOF nanosheets synthesis by this strategy had a very high
yield. CuBDS nanosheets fabricated in this process are characterized by the thickness
of 4–25 nm and lateral side of 0.5–4.0 μm, respectively. This method also displayed
great practicability in synthesis of many other MOF nanosheets such as ZnBDC,
CoBDC, Cu(1,4-NDC), and Cu(2,6-NDC) (NDC = naphthalenedicarboxylate).
Surfactant-assisted method has been used to fabricate MOF nanosheets with a
thickness below 10 nm [77]. The MOF products formation difference in the process
with and without surfactant was compared. Without surfactant, only the bulk crystals were formed. When the surfactant of (Polyvinylpyrrolidone, PVP) was introduced into the synthesis system, nanosheets with a thickness of ~8 nm were obtained
because of the restricted growth in the vertical direction. In addition to the surfactants,
many other small molecules (e.g., acetic acid and pyridine) could also play the same
role [78]. They can coordinate with specific planes of MOF crystals and induce the
anisotropic MOFs growth [79]. In addition to the methods mentioned above, many
other ones (e.g., chemical vapor deposition (CVD)) could also be used to synthesize
MOF nanosheets [80, 81].
Besides the intrinsic synthesis, functionalization of MOF nanosheets is also an
important aspect [82]. Recently, various functional materials such as noble metals
[83], metal sulfide [84], and carbon nanomaterials [85] were incorporated into MOF
nanosheets to obtain the maximized synergetic effect.
4.2 Zeolite Nanosheet
Zeolites nanosheets with various interlayer linkers have been synthesized [86–89]
with the purpose to tune various catalytic systems, for example, isomerization and
hydrocracking [90], selective formation of molecules [91], and epoxidation reaction [92]. Moreover, high-aspect-ratio zeolite nanosheets could further assemble
into zeolite films, which shows great potential in molecular sieving. The currently
used method to synthesize zeolite nanosheets was exfoliation. Nevertheless, because
of morphological damage (aggregation, curling, and fragmentation) and structural