Hexagonal Boron Nitride (h-BN) in Solutes Separation
165
2.1 Mechanical Exfoliation
Mechanical exfoliation was the first effort in synthesizing atomic h-BN sheets, in
which the BN layer is mechanically cleaved or peeled [22]. The mechanical exfoliation method is low cost, which presents h-BN samples with great quality and
even can be performed in most physics or material science laboratories. Indeed, this
method has prepared a simple but potent technique to manufacture 2D materials
[23–27] which was initially applied to separate graphene in 2004 [25]. Then it has
been used for other substrate materials, like h-BN and molybdenum disulfide (MoS 2 )
exfoliation [23]. The usage of arranged sticky tape to peel off 2D nanosheets is one
of the popular mechanical exfoliation procedures [23, 28]. This procedure results in
about ten layers or 3.5-nm-thick h-BN nanosheets that could be owing to the strong
lip-to-lip interactions of h-BN basal planes [29]. The major disadvantage of mechanical exfoliation procedures is less yield of h-BN nanosheets [21], and the size of the
fabricated structures is typically restricted [20]. Other limitations include the lack of
control on atomically thin flakes production followed by transferred them at random
locations on the substrate. Additionally, thick flakes are transmitted along with thin
flakes which are necessary for further characterizations to identify mono- and fewlayer flakes [30–32]. Moreover, in the case of transparent 2D substances, like single
layer h-BN, the recognition of such thin flakes on the substrate is a challenge and
long-term due to its low optical absorption [33].
2.2 Chemical Exfoliation
The liquid solvents, like dichloromethane [34] and dimethylformamide [35] are used
to perform the chemical exfoliation techniques. The procedure of these techniques
is totally simple, which offers a great yield in comparison to the previous method,
mechanical exfoliation [21]. The process of exfoliation follows the below steps:
(I)
Self-curling of the sheets is obtained by adsorption of cations at the edges of
h-BN surface.
(II) Consecutive curling of BN layer is achieved by entering of anions and cations
into the interlayer region.
(III) The surface reaction with hydroxides produces layers with thickness ranging
from 2 to 4 nm, which is resulted in direct peeling off from the bulk material.
This technique offers considerable potential advantages including being facile
and low cost as well as simply transmittable to usual solvents, like ethanol and
water [36]. Du et al. [37] investigated a simple method for chemical exfoliation
of h-BN, which was derived from Hummers’ method of oxidation and exfoliation
of graphite to graphene [38]. The combination of h-BN powder with KMnO 4 and
H 2 SO 4 under heating and then H 2 O 2 addition resulted in the exfoliation of h-BN
to a small number of single-layer boron nitride nanosheets (BNNSs). Atomic force
165
2.1 Mechanical Exfoliation
Mechanical exfoliation was the first effort in synthesizing atomic h-BN sheets, in
which the BN layer is mechanically cleaved or peeled [22]. The mechanical exfoliation method is low cost, which presents h-BN samples with great quality and
even can be performed in most physics or material science laboratories. Indeed, this
method has prepared a simple but potent technique to manufacture 2D materials
[23–27] which was initially applied to separate graphene in 2004 [25]. Then it has
been used for other substrate materials, like h-BN and molybdenum disulfide (MoS 2 )
exfoliation [23]. The usage of arranged sticky tape to peel off 2D nanosheets is one
of the popular mechanical exfoliation procedures [23, 28]. This procedure results in
about ten layers or 3.5-nm-thick h-BN nanosheets that could be owing to the strong
lip-to-lip interactions of h-BN basal planes [29]. The major disadvantage of mechanical exfoliation procedures is less yield of h-BN nanosheets [21], and the size of the
fabricated structures is typically restricted [20]. Other limitations include the lack of
control on atomically thin flakes production followed by transferred them at random
locations on the substrate. Additionally, thick flakes are transmitted along with thin
flakes which are necessary for further characterizations to identify mono- and fewlayer flakes [30–32]. Moreover, in the case of transparent 2D substances, like single
layer h-BN, the recognition of such thin flakes on the substrate is a challenge and
long-term due to its low optical absorption [33].
2.2 Chemical Exfoliation
The liquid solvents, like dichloromethane [34] and dimethylformamide [35] are used
to perform the chemical exfoliation techniques. The procedure of these techniques
is totally simple, which offers a great yield in comparison to the previous method,
mechanical exfoliation [21]. The process of exfoliation follows the below steps:
(I)
Self-curling of the sheets is obtained by adsorption of cations at the edges of
h-BN surface.
(II) Consecutive curling of BN layer is achieved by entering of anions and cations
into the interlayer region.
(III) The surface reaction with hydroxides produces layers with thickness ranging
from 2 to 4 nm, which is resulted in direct peeling off from the bulk material.
This technique offers considerable potential advantages including being facile
and low cost as well as simply transmittable to usual solvents, like ethanol and
water [36]. Du et al. [37] investigated a simple method for chemical exfoliation
of h-BN, which was derived from Hummers’ method of oxidation and exfoliation
of graphite to graphene [38]. The combination of h-BN powder with KMnO 4 and
H 2 SO 4 under heating and then H 2 O 2 addition resulted in the exfoliation of h-BN
to a small number of single-layer boron nitride nanosheets (BNNSs). Atomic force