A slightly longer and refined version of this was originally written for a college assignment.
Nanoscience concerns a basic understanding of physical, chemical, and biological properties on atomic and near-atomic scales. It employs controlled manipulation of these properties to create materials and functional systems with unique capabilities.
The term “nano” comes from ancient Greek and means “dwarf” (nános = dwarf). The “nanoscale” is typically measured in nanometres, which is one billionth of a metre i.e 1 nm = 10^-9 m
First given by renowned physicist Richard Feynman in 1959 in his talk, “There’s Plenty of Room at the Bottom” and later popularised by Eric Drexler, nanotechnology is a highly interdisciplinary field, involving physics, chemistry, biology, materials science, and other engineering disciplines.
Nanomaterials are one of the main products of nanotechnology as nanoparticles, nanotubes, nanorods etc. They are materials that have at least one dimension in the nanometer scale, typically between 1 and 100 nanometers.
In ISO/TS 80004, a nanomaterial is defined as the “material with any external dimension in the nanoscale or having an internal structure or surface structure in the nanoscale”, with nanoscale defined as the “length range approximately from 1 nm to 100 nm”.
This includes both
- nano-objects, which are discrete pieces of material, and nanostructured materials, which have internal or surface structures on the nanoscale;
- a nanomaterial may be a member of both these categories.
They exhibit unique physical and chemical properties that are different from their bulk counterparts due to their small size and large surface area and due to quantum mechanical effects.
Nanomaterials can be made from a variety of different materials, such as metals, ceramics, polymers, and biomaterials, and can be used in a wide range of applications, including electronics, medicine, energy, and environmental remediation.
Some examples of nanomaterials include carbon nanotubes, quantum dots, and nanoparticles of gold, silver, and titanium dioxide.
When the size or dimension of a material is continuously reduced from a macroscopic size (meter or centimetre) to a micro size, the properties remain the same at first and slowly begin to change as the particle size is reduced further.
The size at which the properties of materials change entirely from those in the bulk material is said to be in the nanoscale range, generally observed at below 100nm.
To summarise – the mechanical, electrical, optical, electronic, catalytic, and magnetic properties of solids are significantly altered with a great reduction in particle size.
Thus, the properties of nanomaterials are significantly different from those of atoms and bulk materials due to their nanometer dimensions which cause
- a large fraction of surface atoms (explained by the inverse relationship between the particle size and surface area)
- high surface energy
- spatial confinement
In the case of materials, spatial confinement can also refer to the confinement of atoms, molecules, or electrons to a small region of space, such as in a nanoparticle or a quantum dot. This confinement can have a significant impact on the physical and chemical properties of the material, as the behavior of the atoms, molecules, or electrons is strongly influenced by their surroundings.
- reduced imperfections
These differences are related to the spatial arrangement of molecules or structure, electronic structure, energetics, chemical reactivity, phase change or catalytic activity.
The two principal factors which cause the properties of the nanomaterials to differ significantly from other materials:
- increased relative surface area, and
- quantum effects.
These factors can change or enhance properties such as reactivity, strength and electrical characteristics such as conductivity. Some of these are explained below:
- High surface area: Nanomaterials have a much greater surface area per unit mass compared with larger particles due to their small size.
- Enhanced mechanical properties: Nanomaterials can have increased strength and toughness compared to bulk materials due to the strengthening effect of the surfaces and interfaces.
- Increased thermal stability: Nanoparticles can have increased thermal stability compared to bulk materials, due to their smaller size and increased surface area-to-volume ratio. The inter-atomic spacing decreases with size and this is due to long-range electrostatic forces and short-range core-core repulsion. The melting point of nanoparticles decreases with size.
Additionally, the small size of nanomaterials can also result in quantum confinement effects, where the properties of the material are determined by the size and shape of the nanoparticles rather than their composition.
It changes the optical, electronic and magnetic properties of the material. The band gap increases as the size of the material are reduced to the nanometer range.
This effect is caused by the phenomenon resulting from electrons and electric holes being constricted into a dimension which approaches the critical quantum measurement known as the exciton Bohr radius.
- Unique electronic properties: Nanomaterials can have unique electronic properties, such as increased conductivity and optical properties, due to quantum confinement effects.
- Increased optical properties: Nanoparticles can have increased optical properties like absorption and scattering, due to their small size and increased surface area-to-volume ratio.
- Increased reactivity: A large fraction of the atoms are located at the surface of the nanomaterial which increases its reactivity and catalytic activity. As growth and catalytic chemical reactions occur at surfaces, a given mass of material in the nanoparticulate form will be much more reactive than the same mass of material made up of larger particles.
This can be beneficial for certain applications such as catalysts.
The very high surface area to volume ratio of nanoparticles also provides a tremendous driving force for diffusion, especially at elevated temperatures.
The properties of a nanomaterial can also depend on factors such as shape, composition, and the presence of defects, and some properties can also be associated with specific risks.
Nanomaterials can be classified based on their size, shape, chemical composition, and physical properties. Some common classification methods include:
Size-based classification: Nanomaterials can be classified based on their sizes, such as nanoparticles (1-100 nm), nanocomposites (100-1000 nm), and nanostructures (>1000 nm).
Shape-based classification: Nanomaterials can also be classified based on their shape, such as spherical, rod-like, and sheet-like.
Composition-based classification: Nanomaterials can be classified based on their chemical composition, such as metal nanomaterials, carbon nanomaterials, and semiconductor nanomaterials.
Property-based classification: Nanomaterials can be classified based on their physical properties, such as magnetic, optical, and electronic properties.
Synthesis-based classification: Nanomaterials can be classified based on how they are synthesized, such as top-down and bottom-up methods.
A single material can belong to multiple classifications. The classification method used will depend on the specific context and purpose of the research.
Three main classifications are listed below:
I. CLASSIFICATION BASED ON MATERIALS
Broadly speaking, nanomaterials can be categorized into four main types based on material such as:
(1) Inorganic-based nanomaterials:
Inorganic nanomaterials include materials such as metals, metal oxides, and semiconductors and are typically synthesized through physical or chemical methods (precipitation, gas condensation, or chemical vapour deposition).
Examples of inorganic nanomaterials include gold nanoparticles, titanium dioxide, and silicon dioxide. These materials have a wide range of applications, including catalysis, electronics, and energy storage.
(2) Metal-based materials:
These include quantum dots, nanogold, nano silver and metal oxides like TiO2. A quantum dot is a closely packed semiconductor crystal comprised of hundreds of thousands of atoms whose size is on the order of a few nanometers to a few hundred nanometers.
(3) Carbon-based nanomaterials;
These are composed of carbon, taking the form of hollow spheres, ellipsoids or tubes. The spherical and ellipsoidal forms are referred to as fullerenes while cylindrical forms are called nanotubes.
(4) Organic-based nanomaterials;
Organic nanomaterials are typically synthesized through chemical methods, such as self-assembly or polymerization and include dendrimers.
Dendrimers are nanosized, repetitively branched molecules. The name comes from the Greek word dendron. These nanomaterials are nanosized polymers built from branched units and defined cavities. They can be functionalised at the surface and can hide molecules in their cavities.
The surface of a dendrimer has numerous chain ends, which can perform specific chemical functions. Dendrimers are used in molecular recognition, nanosensing, light harvesting, and opto-elelectrochemical devices. They may be useful for drug delivery.
(5) Composite-based nanomaterials.
Composites are a combination of nanoparticles with other nanoparticles or with larger, bulk-type materials. Nanoparticles like nanosized clays are added to products (auto parts, packaging materials, etc) to enhance mechanical, thermal and flame-retardant properties.
(6) Biological nanomaterials
These nanomaterials are of biological origin and are used for nanotechnological applications. The important features of these particles are:
- self-assembly properties and
- specific molecular recognition
eg DNA nanoparticles, and nanostructured peptides.
Various self-assembled peptide structures can be designed to release compounds under specific conditions and are used in drug delivery systems.
II. CLASSIFICATION BASED ON DIMENSION
According to Siegel, nanostructured materials are classified as:
1) Zero-dimensional nanomaterials
A nanoparticle is defined as a nano-object with all three external dimensions in the nanoscale i.e x,y and z are at the nanoscale and no dimensions are greater than 100nm, whose longest and shortest axes do not differ significantly.
Here two dimensions x, y are nanoscale and the other is outside the nanoscale. This leads to needle-shaped nanomaterials. It includes nanofibres, nanotubes, nanorods and nanowires. Eg A nanofiber has two external dimensions in the nanoscale, with nanotubes being hollow nanofibers and nanorods being solid nanofibers.
Here one dimension x is at the nanoscale and two are outside the nanoscale. The 2D materials exhibit plate-like shapes. It includes nanofilms, nanolayers, and nanocoatings with nanometre thickness.
Eg : A nanoplate/nanosheet has one external dimension in the nanoscale, and if the two larger dimensions are significantly different it is called a nanoribbon.
These are the nanomaterials that are confined to the nanoscale in any dimension. These materials have three arbitrary dimensions above 100nm. The bulk 3D nanomaterials are composed of multiple arrangements of nanosized crystals in different orientations.
It includes dispersions of nanoparticles, bundles of nanowires and nanotubes as well as multi-nanolayers(polycrystals) in which the 0d, 1d, and 2d structural elements are in close contact with each other and form interfaces.
III. CLASSIFICATION BASED ON METHODS OF SYNTHESIS
Nanomaterials can be classified on the basis of synthesis methods in several ways, including:
Bottom-up: Bottom-up synthesis methods involve building up the material from smaller building blocks, such as molecules or atoms, through processes such as self-assembly, chemical vapor deposition, and molecular beam epitaxy.
Top-down: Top-down synthesis methods involve breaking down larger materials into smaller particles through techniques such as mechanical grinding, milling, and laser ablation.
Physical vapor deposition: Physical vapour deposition method involves the condensation of a material from a vapor phase onto a substrate, examples of these methods include, evaporation, sputtering, and laser ablation
Chemical vapor deposition: Chemical vapor deposition method is a chemical process where a material is deposited on a substrate from a gaseous phase, it can be used to synthesize a wide range of materials including metals, semiconductors and ceramics.
Hydrothermal and Solvothermal: Hydrothermal and Solvothermal methods are aqueous-6 processes that involve the use of high pressure and high temperature to create nanomaterials.
Each method has its own advantages and disadvantages, and the appropriate method will depend on the desired properties of the final material and the desired application.
IV. CLASSIFICATION OF NANOSTRUCTURED MATERIALS
While nanomaterials refer to materials that have at least one dimension on the nanometer scale – it also includes nanostructured materials which are materials that have a specific structural organization on the nanometer scale. They are categorized by the phases of matter they contain.
1) A nanocomposite is a solid containing at least one physically or chemically distinct region or collection of regions, having at least one dimension in the nanoscale.
2) A nanofoam has a liquid or solid matrix, filled with a gaseous phase, where one of the two phases has dimensions on the nanoscale.
3) A nanoporous material is a solid material containing nanopores, and voids in the form of open or closed pores of sub-micron length scales.
4)A nanocrystalline material has a significant fraction of crystal grains in the nanoscale.
In conclusion, nanomaterials, a major product of nanotechnology are materials with at least one dimension on the nanometer scale. They exhibit unique physical and chemical properties due to their high surface-to-volume ratio, and quantum confinement effects caused by their extremely small size. They can be broadly classified into inorganic, organic and biological categories and have an extremely versatile range of applications in fields such as electronics, energy, medicine, and environmental science.
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