228
G.-R. Xu
via the weak interaction such as Van der Waals forces and hydrogen bonding [61,
62]. Sonication [62], mechanical [63], intercalation [64], and chemical exfoliations
[65] are all belong to top-down methods. Zamora et al. firstly reported the synthesis
of MOF nanosheets ([Cu 2 Br(IN) 2 ] n ) by exfoliation [66]. This kind of nanosheets is
composed of one Cu dimer with ligands of bromine and isonicotinato. The layers
isonicotinato aromatic rings were stacked via π-π interactions, which could be deteriorated by mechanical force, and nanosheets with thickness of ~0.5 nm were fabricated. MOF-2 (ZnBDC, BDC = 1, 4-benzenedicarboxylate) nanosheet was synthesized by the similar strategy. The thickness and lateral dimensions were 1.5–6.0 nm
and 100 μm, respectively. In this work, the effect of various solvents was paid a
special attention [67].
Zn 2 (bim) 4 (bim = benzimidazole) was exfoliated by combination technique
of sonication and well ball-milling [62]. 2D layers with interlayer thickness of
0.988 nm were stacked with weak Van der Waals interactions. Exfoliation of
stacked crystals was realized with treatment by wet ball-milled and the consequent sonication, and thus fabricated nanosheets exhibited thickness of 1.12 nm.
Recently, Ni 8 (5-BBDC) 6 (m-OH) 4 (denoted as MAMS-1, BBDC = 5-tert-butyl- 1,3benzenedicarboxylic acid) nanosheets were fabricated by freeze–thaw process using
MAMS-1 as the raw materials [68]. Dispersion of MAMS-1 powder in hexane was
firstly carried out and then they were frozen in liquid nitrogen of 196 °C. Finally,
thawing in hot water of 80 °Cwas executed. At the solid/liquid interface, nanosheets
were formed by the shear force induced by volume change of hexane. Thus fabricated
nanosheets are with thickness and lateral size of ~4 nm and 10.7 mm, respectively.
A combined intercalation/chemical strategy to fabricate MOF nanosheets via
exfoliation using organic ligands as intercalants was reported by Zhou et al. [65].
Chemically labile dipyridyl ligand (i.e., 4,40- dipyridyl disulfide (DPDS)) was
inserted into layered MOF (Zn 2 (PdTCPP) (TCPP = tetrakis(4-carboxyphenyl)porphyrin)). The interactions among the MOFs interlayer could be weakened by
the coordination between DPDS and metal nodes. Single-layer MOF nanosheet with
yield of 57% was fabricated by exfoliation through the reduction of disulfide bonds
with trimethylphosphine.
Different from top-down method, in bottom-up process, MOFs nanosheets are
directly synthesized from organic precursors and metals. During bottom-up process,
lateral directional growth is not affected but the growth in vertical direction is
restricted. Interfacial synthesis [69], three-layer synthesis [70], surfactant-assisted
synthesis [71], modulated synthesis [72], and sonication synthesis [73] are all belong
to bottom-up fabrication methods.
Interfacial synthesis is the most widely used method among all the bottomup methods. The main interfacial synthesis methods include liquid/liquid,
liquid/air, and liquid/solid processes. Generally, two immiscible liquids such as
water/dichloromethane and water/ethyl acetate were used to dissolve organic ligands
and metal ions in liquid/liquid process [74, 75]. Synthesis of some MOF nanosheets
(e.g., Fe(py) 2 [Pt(CN) 4 ] (py = pyridine)) is an example of interface synthesis [76].
Particularly, liquid/air synthesis is the most widely used interfacial synthesis. In
liquid/air process, to control MOF nucleation and growth was controlled by reaction
G.-R. Xu
via the weak interaction such as Van der Waals forces and hydrogen bonding [61,
62]. Sonication [62], mechanical [63], intercalation [64], and chemical exfoliations
[65] are all belong to top-down methods. Zamora et al. firstly reported the synthesis
of MOF nanosheets ([Cu 2 Br(IN) 2 ] n ) by exfoliation [66]. This kind of nanosheets is
composed of one Cu dimer with ligands of bromine and isonicotinato. The layers
isonicotinato aromatic rings were stacked via π-π interactions, which could be deteriorated by mechanical force, and nanosheets with thickness of ~0.5 nm were fabricated. MOF-2 (ZnBDC, BDC = 1, 4-benzenedicarboxylate) nanosheet was synthesized by the similar strategy. The thickness and lateral dimensions were 1.5–6.0 nm
and 100 μm, respectively. In this work, the effect of various solvents was paid a
special attention [67].
Zn 2 (bim) 4 (bim = benzimidazole) was exfoliated by combination technique
of sonication and well ball-milling [62]. 2D layers with interlayer thickness of
0.988 nm were stacked with weak Van der Waals interactions. Exfoliation of
stacked crystals was realized with treatment by wet ball-milled and the consequent sonication, and thus fabricated nanosheets exhibited thickness of 1.12 nm.
Recently, Ni 8 (5-BBDC) 6 (m-OH) 4 (denoted as MAMS-1, BBDC = 5-tert-butyl- 1,3benzenedicarboxylic acid) nanosheets were fabricated by freeze–thaw process using
MAMS-1 as the raw materials [68]. Dispersion of MAMS-1 powder in hexane was
firstly carried out and then they were frozen in liquid nitrogen of 196 °C. Finally,
thawing in hot water of 80 °Cwas executed. At the solid/liquid interface, nanosheets
were formed by the shear force induced by volume change of hexane. Thus fabricated
nanosheets are with thickness and lateral size of ~4 nm and 10.7 mm, respectively.
A combined intercalation/chemical strategy to fabricate MOF nanosheets via
exfoliation using organic ligands as intercalants was reported by Zhou et al. [65].
Chemically labile dipyridyl ligand (i.e., 4,40- dipyridyl disulfide (DPDS)) was
inserted into layered MOF (Zn 2 (PdTCPP) (TCPP = tetrakis(4-carboxyphenyl)porphyrin)). The interactions among the MOFs interlayer could be weakened by
the coordination between DPDS and metal nodes. Single-layer MOF nanosheet with
yield of 57% was fabricated by exfoliation through the reduction of disulfide bonds
with trimethylphosphine.
Different from top-down method, in bottom-up process, MOFs nanosheets are
directly synthesized from organic precursors and metals. During bottom-up process,
lateral directional growth is not affected but the growth in vertical direction is
restricted. Interfacial synthesis [69], three-layer synthesis [70], surfactant-assisted
synthesis [71], modulated synthesis [72], and sonication synthesis [73] are all belong
to bottom-up fabrication methods.
Interfacial synthesis is the most widely used method among all the bottomup methods. The main interfacial synthesis methods include liquid/liquid,
liquid/air, and liquid/solid processes. Generally, two immiscible liquids such as
water/dichloromethane and water/ethyl acetate were used to dissolve organic ligands
and metal ions in liquid/liquid process [74, 75]. Synthesis of some MOF nanosheets
(e.g., Fe(py) 2 [Pt(CN) 4 ] (py = pyridine)) is an example of interface synthesis [76].
Particularly, liquid/air synthesis is the most widely used interfacial synthesis. In
liquid/air process, to control MOF nucleation and growth was controlled by reaction