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Introduction to Two-Dimensional Nanomaterials: Discovery, Types and Classifications, Structure, Unique Properties, and Applications

Ishrat Fatma1, Humira Assad1 and Ashish Kumar2*

1Department of Chemistry, Lovely Professional University, Phagwara, Punjab, India

2Nalanda College of Engineering, Bihar Engineering University, Department of Science, Technology and Technical Education, Government of Bihar, India

Abstract

In recent years, nanoscience and nanotechnology have advanced at a breakneck pace. The micro- to nanoscale reduction in particle diameter and tweaking of particle shape in materials result in unique characteristics and can be used in a variety of applications. Converting nanoscale components and entities into functionally distinct materials and technologies has newly gained popularity as a nanoarchitectonics method. Nanoarchitectonics in two dimensions has advanced significantly in recent years. Because of their large surface areas, 2D nanomaterials have become a popular topic for a diversity of surface-active applications. The increased demand for unconventional energy generation has spurred the coherent scheme and manufacture of 2D nanomaterials from the time when graphene was discovered. Hence, 2D nanomaterials have turned up to be a crucial class of nanomaterials for technology improvement because of their outstanding physiochemical properties. The multilayer architecture of these nanomaterials has robust in-plane bonding and moderate van der Waals within strata. These substances are being recommended for novel uses in energy, health, and the environment, all of which are critical societal sectors for long-term sustainability. Moreover, 2D nanomaterials counting graphene, hexagonal boron nitride, black phosphorus, and metal dichalcogenides have received huge consideration in present times because of their attractive characteristics and widespread usage in electronic goods, optoelectronic devices, photocatalysts, energy warehousing facilities, detectors, solar cells, lithium batteries, composite materials, and other fields. As a result, the objective of this book chapter is to present an overview of 2D nanomaterials, including their discovery, classification, structural modifications, and properties. Furthermore, based on the current knowledge, the uses of 2D nanomaterials are also discussed briefly.

Keywords: Nanotechnology, 2D nanomaterials, graphene, hexagonal boron nitride (h-BN), transition metal dichalcogenides (TMDCs)

1.1 Introduction

In recent times, nanotechnology has become a very popular area of research. It was the year 1959 when Nobel laureate Richard P. Feynman introduced nanotechnology in his very prominent lecture on “There’s Plenty of Room at the Bottom” [1]. Nanoscience is responsible for producing different types of materials at nanoscale. Nanoparticle is a very broad category of tiny materials possessing a size less than 100 nm [2, 3]. Nanomaterials are generally classified into four different types based on their dimensions, which include (i) Zero-dimensional (0D), (ii) One-dimensional (1D), (iii) Two-dimensional (2D), and (iv) Three-dimensional (3D). 0D nanomaterials include those whose all dimensions are estimated inside the nanoscale, i.e., <100 nm. These may be cubical, spherical, or polygonal in shape. Some common examples include metallic nanomaterials, quantum dots, nano-onions, etc. 1D nanomaterials are those whose one of its dimensions is greater than the nanoscale (minute). Some common examples include carbon nanotubes, carbon nanofibers, nanowires, etc. 2D nanomaterials are those whose two of its dimensions are greater than the nanoscale. Some common examples include graphene, nanolayers, nanocoatings, etc. 3D nanomaterials are those in which none of its dimensions is confined to the nanoscale. Some common examples include bulk powders, bundles of nanowires, etc. [4]. Out of several types of nanomaterials, 2D nanomaterials are recognized as the thinnest nanomaterial because of their dimensions and thickness according to the nanoscale as illustrated in Figure 1.1. 2D nanomaterials are amorphous as well as crystalline in nature. These are composed of many chemicals surrounded by a matrix-like substance such as metal or polymer. Nowadays, 2D nanomaterials have achieved great importance in terms of research as a result of their unique properties including electrical, mechanical, magnetic, optical, and piezoelectric properties, aside from their lubricant properties. A lot of work has been done so far and additional work is required in order to unveil various other properties, which would enable their use in various other fields of research.

Figure 1.1 2D nanomaterials.

Several years back, scientists supposed that 2D nanomaterials could not subsist in natural surroundings because of their ecological instability. However, currently, about 700 2D materials have been predicted theoretically, even though many of them are not created yet. The preliminary studies on germanene as well as silicene were done in 1994, 10 years earlier than graphene, which was discovered in 2004. For this great achievement, Andre Geim and Konstantin Novoselov received the Nobel Prize in 2010 [5]. In 1962, Boehm gave the suffix “ene” to the foils of carbon, which contain only one layer. In 1991, the outcomes associated to nanotubes of carbon were firstly informed. These outcomes drove enormous numbers of research reassuring the researchers to create 1D nanoribbon or isolate the 2D graphite from the 2D crystals. The detection of graphene made investigators to shift to examine and produce some non-carbon-based 2D nanomaterials. After that, various other nanomaterials were identified, out of which the researchers were firstly attracted towards TMDs because of their exceptional electronic features. They possess excellent flexibility as well as high transparency. Since TMDs behave just like semiconductors, they are mostly used in numerous electrical appliances like transistors, Li-ion batteries, etc. Thereafter, MXene was discovered in 2011 and was called Ti3C2Tx. Later, an allotrope of carbon known as silicene was discovered with opposite properties to that of graphene. Their properties mainly depend on the type of the substrate used. It was the year 2014 when graphene was discovered by Guy Le Lay. Work is still ongoing to discover various other 2D nanomaterials. Recently, different types of (2D) nanomaterials have been produced such as nano-plates, nano-walls, nano-prisms, etc. These nanomaterials (2D) possess a layered configuration along with strong in-plane bonds as well as van der Waals forces in between these layers. At present, 2D nanomaterials such as graphene, h-BN etc., have received plenty of recognition because of their unique properties as well as extensive applications in electronics, lithium batteries, catalysts, solar cells, composites, etc.

1.2 Types of Two-Dimensional (2D) Nanomaterials or Particles

2D nanoparticles are of different types, which include carbon-based nanomaterials like graphene, black phosphorus, transition metal oxides, hexagonal boron nitride, etc. In order to determine their various properties/features and applications, 2D nanomaterials are generally classified into three different classes: (i) layered van der Waals solids, (ii) layered ionic solids, and (iii) surface-assisted non-layered solids [6] as illustrated in Figure 1.2.

1.2.1 Layered van der Waals Solids

These are considered as one of the most important types of 2D nanomaterials, which possess strong in-plane bonding (covalent/ionic), as well as weak out-of-plane van der Waals bonding among the layers. It is this weak van der Waals bonding along with surface tension that is responsible for the exploitation of graphite nanosheets by using micromechanical and liquid exfoliation [7, 8]. The length and thickness of these nanosheets are always in μm and <1 nm, respectively. Exfoliation of these nanosheets causes an increase in surface tension as well as in aspect ratio. Transition metal dichalcogenides also known as TMDs are commonly used complexes for studying layered van der Waals solids like MoS2. About 40 TMDs have been explored so far bearing layered structure. Because of their unique exotic characteristics, these materials are used in semi-conducting and thermoelectrical devices [9, 10].

Figure 1.2 Types of 2D nanomaterials.

1.2.2 Layered Ionic Solids

These are also an important type of 2D nanomaterials, which consist of polyhedral layers (charged) inserted in the middle of hydroxide or halide layers through various electrostatic interactions. The exfoliation of these materials is done through various exchange methods including ion exchange and ion exchange liquid exfoliation mode. Moreover, adsorption of heavy material in addition to interchange of native ions with tedious ions is responsible for increasing the line gaps between the basal planes, due to which the bonding strength of these basal planes decreases. Europium hydroxide is considered as one of the best-layered ionic solids. Other examples include perovskite oxides such as K1.5Eu0.5Ta3O10 and metal oxides like Eu(OH)2.5(DS)0.5 that are exfoliated by using the ion exchange method [11, 12].

1.2.3 Surface-Assisted Non-Layered Solids

These kinds of two-dimensional nanomaterials are manufactured effectively either by using epitaxial growth or by chemical vapor deposition (CVD) methods. One of the best examples of such type is...