**3. Introduction of hexagonal boron nitride**

hBN is structurally similar to graphite and has hardness comparable to graphite. Since hBN is the isoelectric analog of graphite structure and shares very similar structural characteristics and many physical properties, is so-called white graphite. It is not present in nature and is synthesized.

Due to its unique properties, including high resistance to oxidation, high thermal conductivity, good thermal insulation, chemical inertness, excellent lubrication, non-toxicity, and environmental friendliness, hBN has diverse industrial applications in surface coatings, composites, lubricants, and insulators. Due to the impressive properties of nanoscale materials and the development of the application of nanomaterials in the industry, ongoing research is carried to develop new methods for synthesis of nanomaterials. However, until now, there is no ensured large-scale and high yield method to achieve a significant amount of boron nitride nanosheets (BNNSs).

Although researches on 2D nanomaterials have been began several decades ago, the wave of interest and attention to these materials get started in 2004 when Novoselov discovered single-layer graphene with superb electronic properties [1]. Many efforts have been made to achieve 2D materials including graphene, boron nitride, and several dichalcogenides. Boron nitride (BN) is one of the most promising systems ever to be the lightest compound of the three and four groups in the periodic table. BN is composed of equal numbers of N and B atoms, which are configured in hexagonal arrange, similar to carbon atoms in graphene. For naming, the term "single-layer BN" is used for monolayer of BN, and in the case of multilayers, is called BNNSs.

#### *Nanostructures*

As shown in **Figure 4**, single-layer BN has honeycomb structure consisting of isoelectric borazine rings and benzene structures. B▬N bonds have a covalent nature, but due to the electronegativity difference, these bonds have ionic properties with a length of 1.45 Å. The distance between the two centers of the borazine rings is 2.54 Å (compared with 2.46 Å for graphene). The edges of the plates could be zigzagged (boron or nitrogen on the edges) or armchair (nitrogen-boron on the edge) [37].

The lateral dimension of BNNSs is in range from several hundred nanometers to several 10 micrometers [39, 40]. The dimensions of the nanosheets are different depending on the synthesis method. **Figure 5** shows a transmission electron microscopy (TEM) micrograph possessing boron nitride plates, with a lateral dimension of 600 nm.

The single layers of BN can be placed on each other to form few-layer BNNSs. The vdW interaction between layers holds BN layers together, so that the distance between these sheets is 0.333 nm, while the layer's distance in carbon structures is about 0.337 nm [37].

The inorganic analog of graphene, sometimes assigned white graphene, is isoelectronic similar to graphene. However, due to electronegativity differences between the boron and the nitrogen atoms, π electrons are shifted into nitrogen atomic centers, forming the insulating materials [42, 43].

The arrangement of atomic layers in BN and its nanosheets differs with graphite and graphene. The arrangement in the graphene is called AB stacking mode, so that each carbon atom is located at the top of the center of the neighboring layer benzene ring. While in the layers of BN, the stacking mode is AA, and each atom at the upper and lower layers has nitrogen atoms due to polar-polar or electrostatic interactions [6]. Although the AA stacking mode is always observed in nanosheets obtained from the top-down approaches, this order is not always seen in bottomup synthetic techniques [39, 40]. In addition, the calculations show that the B–N layers have relative displacement from AA to AB stacking mode, along the favorable energy [44].

Duo to the difference in electronegativity, B▬N bonds have ionic characteristic which is compared to covalent C▬C bonds in graphene. This can lead to lip-lip interactions between the layers, i.e., chemical interactions as bridges or spotwelds. This phenomenon helps to reduce energy and then stabilize the formation

**27**

*Two-Dimensional Nanomaterials*

**3.1 The preparation of BNNSs**

**Figure 5.**

*middle [41].*

the exfoliation process.

[58–61] are based on the later approach.

**3.2 The properties and applications of BNNSs**

also have special properties due to high surface area.

*DOI: http://dx.doi.org/10.5772/intechopen.85263*

of few-layered BNNSs by reducing the number of dangling bonds at the edges, as well as reduction of frustration effect (forming B▬B and N▬N bonds instead of favorable B▬N bonds) [45, 46]. Interestingly, such a strong interaction has only a negligible effect on the distance between the BN layers compared to graphene [6].

*The transmission electron microscope image of two relatively large BNNSs, which have been overlapped in the* 

2D nanosheets can be synthesized with two bottom-up and top-down approaches that relate to the synthesis of sheets from boron and nitrogen precursor [47, 48] and also the separation of layers. Chemical reaction [49] and CVD [39, 40, 50]

are based on the former and micromechanical cleavage [51, 52], high-energy

are always disadvantages in any way. In synthesis processes, a great amount of effort was put into preventing the formation of a strong chemical bond between the substrate and nanosheets. The crystallization process time, the nucleation on the substrate and the low density of critical nuclei are the important factors in synthetic methods. On the other hand, in the top-down view, exfoliation of layers is used. Nanosheets obtained from exfoliation usually have a higher crystallinity, but their lateral dimensions are limited by the material used. Also, due to lip-lip interactions between sheets, exfoliation of layers to isolate them is difficult. Therefore, the production of single layer is associated with a lot of problems. But in the bottomup approach, there is a lot of control over the supply of thin nanosheets with high lateral dimensions. However, the crystallinity of obtained nanosheets is less than

electron beam [53, 54], ball milling [55], and chemical [49, 56, 57]/liquid exfoliation

Each technique has unique advantages for a specific application; however, there

hBN has attracted many attentions due to its low density, high thermal conductivity, electrical insulation, high resistance to oxidation, low chemical efficiency, and low refractive index. BNNSs also inherit these properties, and in addition, they

**Figure 4.** *Structural view of 2D BNNSs [38].*

#### **Figure 5.**

*Nanostructures*

dimension of 600 nm.

about 0.337 nm [37].

able energy [44].

As shown in **Figure 4**, single-layer BN has honeycomb structure consisting of isoelectric borazine rings and benzene structures. B▬N bonds have a covalent nature, but due to the electronegativity difference, these bonds have ionic properties with a length of 1.45 Å. The distance between the two centers of the borazine rings is 2.54 Å (compared with 2.46 Å for graphene). The edges of the plates could be zigzagged (boron or nitrogen on the edges) or armchair (nitrogen-boron on the edge) [37]. The lateral dimension of BNNSs is in range from several hundred nanometers to several 10 micrometers [39, 40]. The dimensions of the nanosheets are different depending on the synthesis method. **Figure 5** shows a transmission electron microscopy (TEM) micrograph possessing boron nitride plates, with a lateral

The single layers of BN can be placed on each other to form few-layer BNNSs. The vdW interaction between layers holds BN layers together, so that the distance between these sheets is 0.333 nm, while the layer's distance in carbon structures is

The inorganic analog of graphene, sometimes assigned white graphene, is isoelectronic similar to graphene. However, due to electronegativity differences between the boron and the nitrogen atoms, π electrons are shifted into nitrogen

The arrangement of atomic layers in BN and its nanosheets differs with graphite and graphene. The arrangement in the graphene is called AB stacking mode, so that each carbon atom is located at the top of the center of the neighboring layer benzene ring. While in the layers of BN, the stacking mode is AA, and each atom at the upper and lower layers has nitrogen atoms due to polar-polar or electrostatic interactions [6]. Although the AA stacking mode is always observed in nanosheets obtained from the top-down approaches, this order is not always seen in bottomup synthetic techniques [39, 40]. In addition, the calculations show that the B–N layers have relative displacement from AA to AB stacking mode, along the favor-

Duo to the difference in electronegativity, B▬N bonds have ionic characteristic which is compared to covalent C▬C bonds in graphene. This can lead to lip-lip interactions between the layers, i.e., chemical interactions as bridges or spotwelds. This phenomenon helps to reduce energy and then stabilize the formation

atomic centers, forming the insulating materials [42, 43].

**26**

**Figure 4.**

*Structural view of 2D BNNSs [38].*

*The transmission electron microscope image of two relatively large BNNSs, which have been overlapped in the middle [41].*

of few-layered BNNSs by reducing the number of dangling bonds at the edges, as well as reduction of frustration effect (forming B▬B and N▬N bonds instead of favorable B▬N bonds) [45, 46]. Interestingly, such a strong interaction has only a negligible effect on the distance between the BN layers compared to graphene [6].
