UNIT 5 NANOCHEMISTRY
5.1 INTRODUCTION
Nanochemistry is a relatively new branch of chemistryconcerned with the unique properties associated with assemblies of atoms or molecules of nanoscale (~1-100 nm), so the size of nanoparticles lies somewhere between individual atoms or molecules (the 'building blocks') and larger assemblies of bulk material which we are more familiar with.
Nanochemistry is the science of tools, technologies and methodologies for novel chemical synthesis e.g. employing synthetic chemistry to make nanoscale building blocks of desiredshape, size, composition and surface structure.
There are physical and chemical techniques in manipulating atoms to form molecules and nanoscale assemblies.
Physical techniques allow atoms to be manipulated and positioned to specific requirements for a prescribed use. Traditional chemical techniques arrange atoms in molecules using well characterized chemical reactions.
At this extremely small scale level, quantum effects can be significant, fascinating and potentially scientifically very rewarding innovative ways of carrying out chemical reactions are possible.
5.2 MOLECULE
It is the smallest particle in a chemical element or compound that has the chemical properties of that element or compound. Molecules are made up of atoms that are held together by chemical bonds. These bonds form as a result of the sharing or exchange of electrons among atoms.
The atoms of certain elements readily bond with other atoms to form molecules. Examples of such elements are oxygen and chlorine. The atoms of some elements do not easily bond with other atoms. Examples are neon and argon.
Molecules can vary greatly in size and complexity. The element helium is a one-atom molecule. Some molecules consist of two atoms of the same element.
For example, O2 is the oxygen molecule most commonly found in the earth's atmosphere; it has two atoms of oxygen.
However, under certain circumstances, oxygen atoms forming Ozonemolecule (O3) as triplets. Other familiar molecules include water, consisting of two hydrogen atoms and one oxygen atom (H2O), carbon dioxide, consisting of one carbon atom bonded to two oxygen atoms (CO2), and sulfuric acid, consisting of two hydrogen atoms, one sulfur atom, and four oxygen atoms (H2SO4).
5.3 NANOPARTICLE AND BULK MATERIAL
These are very tiny aggregations of atoms but bigger than most molecules. The small size of nanoparticles gives these particles 'unusual' structural and optical properties with applications in catalysis, electro optical devices etc.
Because nanoparticles can display properties significantly different from the bulk material and these properties can be exploited for many different uses. If you compare the size of nanoparticles to that of conventional industrially produced materials you find they have novel uses such as sunscreens and many present future applications.
There is no strict dividing line between nanoparticles and 'ordinary bulk' particles of a material such as baking powder or grains of sand, but particle size is considered as matters.
There are few naturally occurring nanoparticle assemblies e.g. phospholipid vesicles, polypeptide micelle of the iron storage protein, ferritin.
Nanoparticle research is currently an area of intense scientific research, due to a wide variety of potential applications in biomedical, optical, and electronic fields. Nanoparticles are of great scientific interest as they are effectively a bridge between bulk materials and atomic or molecular structures.
A bulk material should have constant physical properties regardless of its size, but at the nano-scale.
5.4 SIZE-DEPENDENT PROPERTIES
Size-dependent properties are observed such as quantum confinement in semiconductor particles, surface plasmon resonance in some metal particles and superparamagnetism in magnetic materials. The properties of materials change as their size approaches the nanoscale and as the percentage of atoms at the surface of a material becomes significant.
For bulk materials larger than one micrometer the percentage of atoms at the surface is minuscule relative to the total number of atoms of the material.
The interesting and sometimes unexpected properties of nanoparticles are not partly due to the aspects of the surface of the material dominating the properties in lieu of the bulk properties. Nanoparticles exhibit a number of special properties relative to bulk material.
For example, the bending of bulk copper (wire, ribbon, etc.) occurs with movement of copper atoms/clusters at about the 50 nm scale.
Copper nanoparticles smaller than 50 nm are considered super hard materials that do not exhibit the same malleability and ductility as bulk copper.
Ferroelectric materials smaller than 10 nm can switch their magnetization direction using room temperature thermal energy, thus making them useless for memory storage.
Suspensions of nanoparticles are possible because the interaction of the particle surface with the solvent is strong enough to overcome differences in density, which usually result in a material either sinking or floating in a liquid.
Nanoparticles often have unexpected visible properties because they are small enough to confine their electrons and produce quantum effects.
5.5 NANOPARTICLES
Nanoparticles have a high surface to volume ratio which has a dramatic effect on their properties compared to non-nanoscale forms of the same material. Examples: ZnO, CaO, CdS, BaTiO3
Examples:
Pieces of gold aregold-colored, but gold nanoparticles are deep red or even black when mixed with water.
Titanium dioxide is a white solid used in house paint where plainly it reflects visible light. However, titanium dioxide nanoparticles are so small that they do not reflect visible light, so they cannot be seen and are used in sun block creams because they block harmful ultraviolet light from the Sun without appearing white on the skin (as in TiO2 in paint).
Silver foil shows virtually no reaction with dilute hydrochloric acid but nanoparticles of silver rapidly react with hydrochloric acid because of the very large surface.
5.6 NANOCLUSTER
It denotes small, multiatom particles. Nanocluster sizes range from sub nanometer to about 10 nm in diameter. As a rule of thumb, any particle of somewhere between 3 and 3x107 atoms is considered a cluster. Two atom particles are sometimes considered as clusters. They have properties and structures which are very sensitive to their composition and size (ie., every atom counts).
Examples: ZnO, CdS, SiO2
5.7 NANORODS
Their dimensions range from 1 – 100 nm. They may be synthesized from metals or semiconducting materials by direct chemical synthesis. A combination of ligands acts as shape control agents and bond to different facets of the nanorod with different strengths. This allows different faces of the nanorod to grow at different rates producing an elongated object. They are used in display technologies, micro electro mechanical system and energy harvesting.
Examples: Zinc oxide nanorod/nanowire and Gold nanorods.
5.8 NANOTUBE
Carbon Nanotubesare an allotrope of carbon. They take the form of cylindrical carbon molecules and have novel properties that make them potentially useful in a wide variety of applications in nanotechnology, data storage, energy production, aerospace, automobile industry, drug delivery, doping, super capacitors, solar energy storage, electronics, optics and other fields of material science.
They exhibit extraordinary strength and unique electrical properties and are efficient conductors of heat. Nanotubes are members of the fullerene structural family which also includes buckyballs. Whereas buckyballs are spherical in shape, a nanotube is cylindrical with atleast one end typically capped with a hemisphere of the buckyball structure. There are two main types of nanotubes: single-walled nanotubes (SWNTs) and multi-walled nanotubes (MWNTs).
Examples: CNT, DNA, membrane, inorganic compounds.
5.9 NANOWIRE
A nanowire is a wire of dimensions of the order of nanometers (10-9 m). Molecular nanowires are composed of repeating molecular units either organic or inorganic. It can be defined as structures that have a lateral size constrained to tens of nanometers or less and an unconstrained longitudinal size. At these scales, quantum mechanical effects are important and hence such wires are also known as quantum wires.
Examples: Metallic (Ni, Pt, Au), semiconducting (lnP, Si, GaN, etc.) and insulating (SiO2, TiO2).
5.10 PROCESS OF SYNTHESIS
The nanomaterials are synthesized using a number of methods. These methods are generally classified into two categories, namely
- Top-down process and 2. Bottom-up process
5.10.1 Top-down process
To synthesize a nanomaterial, if a bulk material is used as a starting material then the method is known as top-down process.
In this process, a bulk material is crushed into fine particles using the process like mechanical alloying, laser ablation, sputtering, etc.
Advantages
1) The main advantage is that the covalent bonds holding together a single molecule which is far stronger.
2)This is mostly used by chemists to create structures by connecting molecules.
Fig. 1 Top-Down Process
Examples
1)Ball milling2) Laser Ablation
3) Sputtering4) Arc plasma
5) Electron beam evaporation6) Photolithography, etc.
5.10.2 Bottom-up Process
In some methods, the nanomaterials are prepared by arranging atom by atom. Due to the nucleation and growth, bigger size grains or a cluster of atoms having a size less than 100 nm are produced.
Advantages
1) Allows smaller geometries
2)CNT, silicon nanowires and organic semiconductors are produced by this process
3)It can make films and structures much easier
4)It is more economical than top-down process. It does not waste any material during production
Examples
1)Chemical vapour deposition2) Sol-gel method3) Electrodeposition, etc.
Due to chemical reactions, these methods produce the nanomaterials and hence these methods are called chemical methods.
Fig. 2 Bottom-up Process
5.11 SYNTHESIS METHODS
5.11.1 Precipitation method
The process by which a nanoparticle is separated out of solution as solid is called precipitation method. This occurs either by the action of gravity or through a chemical reaction that forms an insoluble nanoparticle out of two or more soluble compounds.
Example
1)Nanoparticles of molybdenum can be produced in toluene solution with NaB(C2H5)3H as reducing agent at room temperature. It provides a high yield of nanoparticles having dimensions of 1 to 5 nm.
MoCl3 + 6 NaB(C2H5)3H→2 Mo + 6 NaCl + 3 H2 + 6 B(C2H5)3
2)Nanoparticles of aluminium can be made by decomposing (CH3)2 C2H5NAl H3 in toluene and heating the solution to 105ºC for 2 hours.
5.11.2 Thermolysis method
Nanoparticles can be made by this method by decomposing solids at high temperature having metal cations and molecular anions or organometallic compounds.
Example
Lithium nanoparticles can be made by decomposing lithium azide, Li3N.
2 Li3N →6 Li + N2
Lithium azide is placed in an evacuated quartz tube and heated to 400ºC. At about 370ºC, Li3N decomposed releasing nitrogen gas. An increase in pressure is noticed in vacuum gauge. In a few minutes, the pressure drops back to the original value. This indicates the complete removal of nitrogen. The remaining lithium atoms form small colloidal metal particles of 5 nm size. Passivation is carried out using an appropriate gas.
Fig. 3 Thermolysis method
5.11.3 Hydrothermal method
This method can be defined as a method of synthesis of single crystals which depends on the solubility of minerals in hot water under high pressure. The crystal growth is performed in an apparatus consisting of a steel pressure vessel called autoclave, in which a nutrient is supplied along with water. It is used for oxide nanoparticle synthesis.
Example
BaTiO3nanoparticles are produced at about 4000ºC and alkaline solution is used to increase the solubility.
Ba(OH)2 + TiO2→BaTiO3 + H2O
CNT, MnO2, Fe2O3, TiO2, WO3, ZnO, BN, VO nanotubes can be prepared using this method.
5.11.4 Solvothermal method
It is the method used to prepare variety of materials such as metals, ceramics, polymers, semiconductors and nanoparticles of various forms. A solvent is mixed with certain metal originator and the solution mixture is placed in an autoclave kept at relatively high temperature (100ºC to 1000ºC) and pressure (1 to 104atm) in an oven to carry out the crystal growth
Example
ZnO quantum rod is prepared by dissolving zinc acetate in 2- propanol at 50ºC. It is then cooled to 0ºC. Sodium hydroxide is added to precipitate ZnO. The solution is then heated to 65ºC to allow ZnO growth. The rod shaped ZnOnanocrystals are obtained.
5.11.5 Electrodeposition method
The electrochemical reaction is the basic principle behind the electrodeposition process.
The electrodeposition set-up consists of a container, in which the electrolytes are taken in the required composition and pH. In electrodeposition process, three electrodes are used, namely working electrode (Pt), counter electrode and reference electrode (SCE). The Pt electrode consists of conducting materials such as indium tin oxide coated glass plates or metals such as Ti or Ni.
The required electrolytes, with suitable compositions and pH, are taken in the electrodeposition chamber. In order to deposit the nanocrystalline material, a grain refiner should be added into the electrolyte. The set-up is kept in a constant temperature. Suitable potential difference is applied between the counter and working electrode. The deposition can be made on cathode by maintaining a constant potential difference between the SCE and the working electrode. It is said to be potentiostatic method.
The deposition can also be made either in the cathode or anode. If the deposition is made in the cathode, then it is said to be cathodic deposition. If the deposition is made in the anode, then it is said to be anodic deposition. If the current through the circuit is made constant, then it is said to be galvanostatic method.
Fig. 4 Electrodeposition process
Advantages
1)It is one of the simplest and inexpensive methods.
2)The thickness of the film can be controlled by adjusting the deposition rate.
Drawbacks
1)Conducting electrodes are needed to produce electrodeposition.
2)After the deposition is made, the films should be annealed at high temperature. High temperature annealing is expensive.
5.11.6 Chemical vapour deposition method (CVD)
It is a method of chemically reacting a volatile compound of a material to be deposited, with other gases, to produce a non-volatile solid that deposits atomistically on a suitably placed substrate. The CVD apparatus consists of a quartz tube container which is heated by a tubular furnace. A silicon substrate is kept inside the quartz container in a tungsten/graphite boat. The chamber is evacuated and an inert gas is passed into the chamber. The reactants are admitted into the chamber and the chamber is maintained at a suitable temperature. The coating is formed on the silicon substrate because of the chemical reaction. The unused gases flow through the outlet.
The chemical reaction, the hydrogen reduction of silicon tetrachloride, is used to produce the epitaxial growth of pure silicon. This reaction takes place at 1200ºC.
SiCl4 + 2 H2→Si + 4 HCl
Fig. 5 Chemical Vapour Deposition process
Advantages
1)CVD is a low cost and high yield method.
2)Compared to other methods, the temperature of the deposition is low.
3)High purity (more than 99 % pure) nanomaterials are produced using this method.
4)Both single wall and multiwall nanotubes can be produced.
5)The diameter of the nanotube can be controlled by controlling the thickness of the catalytic film.
5.11.7 Laser ablation method
It consists of a high intense laser beam, a cylindrical furnace, an air or water cooled metallic trap with filter, a vacuum pump and an inlet for the gas. A CO2 laser with an output wavelength of 10.6 micrometer and power output of 1 kW can be focused into small spot in the order of an mm using a lens produces a very high energy, and it is made to incident on a target material. If the temperature produced by the laser beam on the target material is greater than its sublimation point, the explosion of the target material may occur and hence the target material gets efface from the surface. The nanomaterials are collected on the air or water cooled traps. A continuous flow of nitrogen gas with a pressure of 1 torr is maintained during the synthesis of the nanomaterials. A tube furnace is an additional energy source used to maintain the temperature of the target.
Fig. 6 Laser Ablation process
Advantages
1)No solvent is used, hence is ecofriendly.
2)The nanomaterials with perfect structures can be produced
3)Both SWNT and MWNT can be produced.
4)It is easy to operate and running cost is low.
5)Heating of the target is minimum.
6)The process is gentler than other methods.
Drawbacks
1)High temperature should be maintained.
2)Laser source with high output energy should be needed.
3)Only a small quantity of nanomaterials can be produced.
5.12 PROPERTIES OF NANOMATERIAL
The physical, mechanical, optical and electronic properties of the nanomaterials change from their bulk material. Some of the properties of the nanomaterials are given below:
5.12.1 Physical Properties
Melting point
The melting temperature of gold in bulk form is around 1300 K. The melting temperature significantly decreases up to around 7000 K, when the particle size is decreased around 10 to 20 nm. The melting temperature of CdS is slightly higher than 1700 K in the bulk form. The melting temperature of CdS reduces to 600 K, when its particle size is around 10 to 15 nm.
Interatomic distance
The interatomic distance gets reduced when there is a reduction in particle size.
Magnetic moment
The magnetic moment of a nanomaterial increases with the reduction in coordination number. This shows that the smaller size particles are more magnetized. The magnetic moment of iron has greater by up to 30 % when it is nanoscale than when it is in the bulk form. Clusters with small size particles are spontaneously magnetized.
5.12.2 Mechanical properties
The hardness of a nanomaterial increases with the reduction in particle size. The hardness of Cu gets increased to two times when its particle size is 50 nm and becomes 5 times hardner than the bulk material at 6 nm grain size.
The nanophase materials have high strength and super hardness. It is due to the fact that nanophase materials are free from dislocation. Super elasticity has also been observed in nanophase materials.
5.12.3 Optical properties
Nanoclusters of different particle size exhibit different absorption spectra and they lie in the visible region. Due to this, the nanoscale gold solution with different particle size appears as orange, purple, red or greenish.
5.12.4 Electrical properties
Resistivity is the inverse of conductivity. In general, the resistivity of nanomaterials is greater than that in polycrystalline materials. This is because, the electrons get scattered at grain boundaries resulting in the increase of resistance.