Nanomaterials : Classification and Approach for Synthesis |ChemFam ...

Nanotechnology deals with the study of manipulating matter on an atomic scale. It refers to the constructing and engineering of the functional systems at very micro level or at atomic level. Generally nanotechnology deals with developing materials, devices or other structures that possess dimension of 1 to 100 nanometre. The emergence of nanotechnology began with the invention of Scanning Tunneling Microscope in the year 1981 and with the discovery of fullerene in 1985. Nanoparticles can display properties which are significantly different from the bulk material. For example, a foil of silver does not react with dilute HCl but silver nanoparticle rapidly reacts with dilute HCl.

Nanomaterials are cornerstones of nanoscience and nanotechnology. The usage of nanomaterials are enormous as energy storage devices such as fuel cells, detection of threats in defense, navy, drug delivery and water purification. Nanoparticles have a high surface to volume ratio. At this quantum levels the mechanical, electrical, optical, magnetic and catalytic properties of solids are significantly altered with the reduction in size in the range of nanometres.

Classification of Nanomaterials

According to Siegel, based on the number of dimensions nanomaterials are classified as Zero dimensional (0D),One dimensional (1D), Two dimensional (2D) and Three dimensional (3D) nanomaterials.

(i)Zero dimensional nanomaterials: In zero dimensional class of nanomaterials, all dimensions of space i.e x, y and z are at nanoscale. This means that no dimensions are greater than 100 nm range. This model includes nanoclusters and nanospheres.

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(ii)One-dimensional nanomaterials: In one dimensional class of nanomaterials, only the two dimensions x and y are at nanoscale and the third dimension is outside the nanoscale. It leads to needle shaped nanomaterials. It basically includes nanofibres, nanowires and nanotubes.

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(iii)Two dimensional nanomaterials: In this class of nanoparticles, only one dimension (such as x) is at nanoscale and the other two are outside the nanoscale. This includes nanofilms, nanolayers and nanocoatings with the thickness of nanometres.

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(iv)Three dimensional nanomaterials: These nanomaterials are not confined to the conventional nanoscale in any dimension. These materials have three arbitrary dimensions above 100 nm range. It includes dispersions of nanoparticles, bundles of nanowires and nanotubes in which all other dimensional structural elements are in close contact with each other and forms interfaces.

Class of Nanomaterials	Size	Examples
Zero dimension	< 100 nm	Nanospheres and nanoclusters
One dimension	< 100 nm	Nanofibres, nanotubes and nanowires
Two dimension	< 100 nm	Nanofilms, nanolayers and nanocoatings
Three dimension	> 100 nm	Quantum dots, bundles of nanowires and nanotubes

On further studies, nanomaterials are again divided into 4 more categories for easy understandings of their origins. These are —

(i)Carbon based materials: These are composed of carbon. They take the shape of hollow spheres or ellipsoids and are so referred as fullerenes or cylindrical forms are referred as nanotubes.

(ii)Metal based materials: These includes quantum dots, nanogold, nanosilver and metal oxides like TiO₂

(iii)Dendrimers: Dendrimers are repetitively branched molecules. Dendrimers are used in molecular recognition, nanosensing, light harvesting and drug delivery.

(iv)Composites: Composites are combination of nanoparticles with other nanoparticles or with larger, bulk-type materials.

Techniques for synthesis of nanomaterials

There are two general approaches for the synthesis of nanomaterials —
A) Top-down approach: The top down approach involves the breaking down of the bulk material into nanosized structures or particles. This technique is an extension of those that have been used for producing micron sized particles. Top down approaches are inherently simpler and depend either on removal or division of bulk material or on miniaturization of bulk fabrication processes to produce the desired structure with appropriate properties. The biggest problem that lies with the top down approach is the imperfection of surface structure. For example, nanowires made by lithography are not smooth and may contain a lot of impurities and structural defects on its surface. Examples of such techniques are high energy wet ball mining, electron beam lithography, aerosol spray etc.

B) Bottom-up approach: The alternating approach which has the potential of creating less waste and hence the more economical is the bottom up approach for synthesis of nanomaterials. Bottom-up approach refers to the build up of a material from the bottom: atom by atom, molecule by molecule or cluster by cluster. Many of these techniques are still under development or are just beginning to be used for commercial production of nanopowders. Organometallic chemical route, reverse micelle route, sol-gel synthesis, colloidal precipitation and electrodeposition are some of the well known bottom up techniques reported for the preparation of luminescent nanoparticles.

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Characterisation Techniques of Nanomaterials

Techniques	Information acquired
Scanning Electron Microscopy (SEM) with Energy-dispersive X-ray spectroscopy	Surface topography (up to 10nm) and composition
Transmission Electron Microscopy (TEM)	Surface morphology (up to 0.2nm)
FT-Raman Spectra	Distinguish single walled carbon nanotubes and multi walled carbon nanotubes
X-ray photoelectron spectroscopy	Electronic state of the element
Photoluminescence Spectra	CNT chirality or Asymmetry determination
Atomic Force Microscopy	Identification of individual surface atoms
Particle Size Analyzer	Particle size distribution

Synthesis of Nanoparticles by Chemical Reduction Method

Group VIB metal halides like CrCl₃, MoCl₃, WCl₄ can be reduced into their corresponding metals by using NaBEt₃H (sodium triethoxy boron hydride) with toluene as the solvent at room temperature.