Review on nanomaterials: Synthesis and applications

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Abstract

The recent past in the technological development evidenced that evolution in Nanotechnology and nanoscience is the key factor. Nanotechnology is multidisciplinary science which deals with physics, chemistry, materials science and other engineering sciences. The applications of Nanotechnology are spreading in almost all the branches of science and technology. The present review article highlighted the types of nanoparticles and their synthesis methods, characterization techniques. There are many techniques and applications are reported in the last five years but here we strictly focused on the general synthetic approaches and applications of the nanomaterials which provide a general idea to the young researchers.

Introduction

Over the last century nanotechnology branch is flourishing to a great extent. And today many types of research are directly or indirectly related to the nanotechnology. Nanotechnology can be stated as the developing, synthesizing, characterizing and application of materials and devices by modifying their size and shape in nanoscale” In each and every stream the prefix “nano” is using as a keyword even in advertising the products also. Actually the word “nano” is derived from the Greek word nanos or Latin word nanus means which “dwarf”. It is the combination of physics, chemistry, material science, solid state, and biosciences. So profound knowledge in one field will not be sufficient, the combined knowledge of physics, chemistry, material science, solid state, and biosciences is required. The applications of Nanotechnology are spreading in almost all the branches of science and technology. The difference between the nanoscience and nanotechnology is the nanoscience gives the knowledge about the arrangement of atoms and their basic properties at nanoscale whereas the nanotechnology is the technology used in governing the matter at the atomic level for the synthesis of the novel nanomaterials with different characteristics [1]. The nanotechnology getting attention in almost all engineering branches but the common people didn’t get the knowledge about its existence in daily life but its vast usage in the medicine, engineering, environment, electronics, defense, and security is still increasing (Fig. 1). Even though so much work was done using this technology but still have space for developing the new novel nanomaterials in various fields for the progress of mankind. The researchers are fascinated and working for the progress of knowledge in terms of their size, capability, and expenditure. So special interest on the miniaturization of the device with economical is focusing mainly in the field of medicine, electronics. In upcoming days the nanotechnology controls mankind in living, working and communicating fields. So this gives interest in this subject and leads to the discussion of the basic and major topics of nanotechnology.

The basic and the key elements of nanotechnology are the “nanomaterials”. The nanomaterials are the materials with less than 100 nm size ones at least in one dimension. That means they have very less size than that of microscale. The nanomaterials are usually 10−9 m in size that means it is one billionth of a meter. The nanomaterials show different physicochemical properties than the bulk material which inherently depends on their size and shape. Surprisingly the nanomaterials produce a unique character with new characteristics and capabilities by modifying the shape and size at the nanoscale level. Nanomaterials may be of different shapes like nanorods, nanoparticles, nanosheets which can be characterized based on their dimensionality. Nanomaterials with zero-dimensional are nanoparticles, one dimensional is nanorods or nanotubes and two dimensional are generally films and layers type one. These are categorized mainly for the single isolated nanomaterials. By the interaction of two or more particles, their physical properties will alter. These particles of different constituents are called bulk or three-dimensional nanomaterials.

Based on the dimensions of nanoscale (<100 nm) they are classified as follows.

  • (1)

    Zero-dimensional nanomaterials (0-D): In this, all the three dimensions of the nanomaterials are in the nanoscale range. Nanoparticles will come into this classification.

  • (2)

    One dimensional nanomaterial (1-D): In this, in any one dimension it will be in nanoscale range and remaining two dimensions are out of the nanoscale range. Nanorods or nanotubes or nanowires are related to this class.

  • (3)

    Two-dimensional nanomaterials (2-D): Any two dimensions are in nanoscale range and remaining one dimension is out of it. These include nanofilms, nanolayers, and nanocoatings.

  • (4)

    Three dimensional or bulk nanomaterials (3-D): In any dimension, these nanomaterials are not in nanoscale range. That means in three arbitrarily dimensions they are >100 nm scale. These include nanocomposites, core shells, multi nanolayers, bundles of nanowires, bundles of nanotubes [2].

The nanomaterials are of different types based on their morphology, size, properties and the constituent in it. They are carbon-based nanomaterials, metal nanoparticles, semiconductor nanomaterials, polymeric nanomaterials, lipid-based nanomaterials.

The main constituent in this type of nanomaterials is the carbon. Carbon nanotubes and fullerenes are related to this type. Basically, the CNTs are embedded with graphene sheets which are rolled into a tube. These are much stronger than steel and can be useful for structural enhancement. The CNTs are of a single-walled type and multi-walled type.

Fullerenes are the hollow cage structure particles with sixty or more carbon atoms. These are allotropes of carbon. Its structure similar to hollow football with pentagonal and hexagonal carbon units is organized in a regular pattern. They show good electrical conductivity, electron affinity, and high strength [1], [2].

The starting materials of the metal nanomaterials are divalent and trivalent metal ions. There are different methods for the preparation of metal nanoparticles like chemical or photochemical methods. By using reducing agents the metal ions are reduced to the metal nanoparticles. These have a high surface area and have the good adsorption ability of small molecules. They are widely used in different research areas, environmental and bioimaging studies. Not only a single nanoparticle but also the mixing of two or more nanoparticles with the size control can also be achieved. By doping different metals even the rare earth metals can change the main element characteristics. By doping different elements in different constitutions their properties also get vary [3], [4], [5], [6].

Semiconductor nanomaterials have metallic and non-metallic properties. They exhibit wide band gaps by modifying it shows different properties. These are widely used in photocatalysis, electronic devices. For instance, ZnS [7], ZnO [8], CdS [9], CdSe [10], CdTe [11] are related to group II-VI semiconductor materials. GaN [12], GaP [13], InP [14], InAs [14] are from group III-V. In recent times, semiconductor grapheme nanocomposites attracted the researches. The graphene can improve the physical and chemical properties of the semiconductor. For gas sensing sensitivity [15], [16], [17], piezoelectric properties [18], [19] graphene composites materials can be utilized.

The nanocomposite is a polyphase solid material where one of the phases has one, two or three dimensions of less than 100  nm. Nanocomposites have a high surface to volume ratio which differs from typical composites. Based on the size and the shape the physicochemical properties may differ as follows [2], [20].There are different types of nanocomposites like that of nanomaterials. The different types are Ceramic Matrix Nanocomposites (CMNC), Metal Matrix Nanocomposites (MMNC) and Polymer Matrix Nanocomposites (PMNC).

In them, the polymer composite of graphene-based composites is developing in recent days to a large extent. Graphene is composed of carbon moiety. The single layer carbon atoms arranged in hexagonal matrix [21], [22]. It exists with zero band gap and the electrons are almost as the massless particles which consist of the good electrical medium in 2D [23]. The precursor of graphene is graphene oxide (GO) whose electronic conductivity [24] is very less. So the conversion of GO to reduced graphene oxide rGO gives better results with good conductivity. There are different methods like Exfoliation method [25], [26], [27], CVD method [28], [29], thermal reduction [30], [31], chemical reduction [32], [33], [34], multistep reduction method [35], [36], [37], [38] to convert the GO to rGO. The different types of semiconductor graphene family nanocomposites are a metal oxide/grapheme nanocomposites, metal chalcogenide/grapheme nanocomposites. The metal oxides possess a variety of applications. In them ZnO [39], [40], In2O3 [41], TiO2 [42], MnO2 [43], Fe2O3 [44] shows the photocatalytic, photovoltaic, drug delivery [45], gas sensors [46], [47], batteries [48], [49] and cytotoxicity activities [50]. The basic key role for these applications is the interaction of matter with surroundings.

The nanomaterials with in the critical size (<100 nm) show unique and fascinating properties led to an interest in them. These properties of nanomaterials are different from the bulk material. While the macro-structured properties are identical to their bulk. On decreasing the particle size the number of particles on the surface increases. The coordination number for surface atoms is less than the inner atoms and so they mobile easily. Based on the surface property we can apply the nanomaterials in various fields like catalysis, functional coatings, adsorbents, nanoelectronics, sensors.

The excitation, emission, chemical reactivity, and stability properties are also size-dependent properties at nano-regime. Not only had the size the shape also played an important role in judging the nanomaterial property. When the nanoparticle size is nearer to the de Broglie wavelength limit and diameter is less than the quasiparticle interaction then they exhibit quantum size effects. The controlling and manipulation of nanomaterials properties is possible by controlling their size during their synthesis by using different methods. Due to having more surface area they can be applied as catalysts. Mostly the metal nanoparticles can be utilized as good catalysts. By using the surface phenomenon the nanomaterials can be applied in the following fields [1].

Electronic property of a metal is based on their electronic band structure. The band structure depends upon the particle size. The delocalized bands are seen in the molecular states. The band structure of nanocrystal is in between the discrete states of atoms (and molecules) and continuous bands of crystals. The adjacent lines energy separation depends on the size of the particle. On decreasing the size the separation of energy levels increases. The metallic character decreases and gradually changes to the semiconductor nature.

There are different types of magnetic materials like dia, para, Ferro, antiferro, ferromagnetic materials. The soft and hard magnetic materials are different types of magnetic materials based on their coercivity. The soft materials have low coercivity with small hysteresis area and these materials can be magnetized by a low magnetic field. The hard magnets are contrasted to this. The coercivity will change based on the particle size. Generally on increasing the surface area and decrease in grain size the coercivity and saturation magnetization increases. So the nanoparticles have the more surface area with less grain size and show high magnetic strength. If further, the particle size decreased then the ferromagnetic particles changes to paramagnetic materials due to instability. These paramagnetic materials behave in a different way from the bulk material and so they called as superparamagnetic. The magnetic materials can be applicable in data storage capacity, electronic circuits, actuators, transformers, power generators and many more.

The nanomaterials show novel optical properties. The interaction of the material with light is known as the optical property which depends on the size, shape, doping, surface characteristics, and also on the surrounding environment interaction. This property is due to the due to their surface plasmon and quantum confinement of electrons. Free electron density and the dielectric medium of the nanomaterial will judge the surface plasmon resonance energy. Gold particle exhibits the surface plasmon resonance band at 520 nm, for silver, it is at 410 nm. The size-dependent Optical properties depend upon the size. For instance, a 2.3 nm size CdSe emits blue light whereas the 5.5 nm size CdSe emits red light. Based on the optical properties the nanomaterials can use in polymers to increase their refractive index. In LCD’s and LED’s, better resolution images can develop. They can applicable for optoelectronic materials. All the properties mainly depend on the size and shape of nanomaterials. So the controlling and manipulation of nanomaterials properties is possible by controlling their size during their synthesis by using different methods [51].

Section snippets

Synthesis of nanomaterials

The synthesis of nanoparticles can be done by three different approaches. They are as follows.

  • (1)

    Biological methods

  • (2)

    Physical methods

  • (3)

    Chemical methods

  • (i)

    The biological method is simple and easy, generally with a single step, eco-friendly. In this context, we can use the microorganisms and also the different plant parts for the preparation of the nanomaterial’s [52].

Characterization of nanomaterials

The nanoparticles exhibit different physicochemical properties. On varying their size and even a small dimension in nanoscale they will exhibit different properties. To examine their properties characterization of nanoparticles wants to be done with different instruments. They are UV Spectrophotometer, Fourier Transform Infrared (FT-IR) Spectroscopy, Atomic Force Microscopy (AFM), Transmission Electron Microscopy (TEM), Scanning Electron Microscopy (SEM), Vibrating Sample Magnetometer (VSM),

Applications of nanoparticles

Ferrite nanoparticles are almost widely used in each and every field due to their magnetic, electrical, optical and chemical properties. Their applications range from medical to modern industries. They are applied in the area of biomedical [124], wastewater treatment, catalyst, information technologies. They are used as sensors and biosensors [125] in which electrochemical, optical, piezoelectric and magnetic field are applicable. In energy storage devices [126] they are applicable in the form

Conclusions

Day to day the synthesis of novel nanomaterials are increasing. The nanomaterials with mixed compositions are also synthesizing to apply in different fields. The facile synthesis methods will produce the nanoparticles of desired size, shape and property one which can withstand the external conditions but still, they need some improvement. Nowadays wide research in going into the fields of biomedicine, electronic storage devices, and sensors but still there is a scope for the development of

Acknowledgements

This work was supported by DST (Department of Science and Technology) – India in the form of INSPIRE Faculty award (IFA-14/ENG-70) and Indo – Korea (INT/Korea/P-27) joint project. The authors LAK and IVK are thankful to Koneru Lakshmaiah Educational Foundation (KLEF) for their financial support and encouragement to write this review article.

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