Chapter VII NANO CHEMISTRY

INTRODUCTION The prefix (nano) in the word nanochemistry means a billionth (1 x 10-9 m). Atoms are very small and the diameter of a single atom can vary from 0.1 to 0.5 nm. It deals with various structures of matter having dimensions of the order of a billionth of meter.

BASICS OF NANOCHEMISTRY 1. Nanoparticles Nanoparticles are the particles, the size of which ranges from 1-50 nm. Generally they are obtained as colloids. The colloidal particles have a tendency to remain single crystal and hence are called as nanocrystals. A large percentage of atoms in nanocrystals are present on the surface Nanocrystals possess electronic, magnetic and opticalproperties. Since the nanoparticles exhibit an electronic behavior, governed by the quantum physics, they are also called as quantum dots.

2. Nanomaterials Nanomaterials are the materials having components with size less than 100 nm at least in one dimension. Nanomaterials, in one dimension, are layers such as a thin films or surface coatings. Nanomaterials, in two dimension, are tubes such as nanotubes and nanowires. Nanomaterials, in three dimension, are particles like precipitates, colloids and quantum dots.

3. Nanochemistry (or) Nanoscience Nanoscience is defined as the study of phenomena and manipulation of materials at atomic, molecular and macromolecular scales. 4. Nanotechnology Nanotechnology is defined as the design, characterization, production and applications of structures, systems and devices by controlling size and shape at 10-9 m scale or the single-atomic level.

DISTINCTION BETWEEN NANO PARTICLES, MOLECULES AND BULCK MATERIALS The size of nano particles are less than 100 nm in diameter, molecules are in the range of picometers, but bulk materials are larger in micron size. Molecule is a collection of atoms, nano particles are collection of few molecules that is less than 100 nm but bulk materials contains thousands of molecules. Surface area of nano particles is more than the bulk materials.

Strength of nano materials is 3 - 10 times higher than Nano particles possesses size dependent properties, but bulk materials possess constant physical properties. Corrosion resistance is more than the bulk materials, hence localised corrosion in nano materials is stopped. Behavior of bulk materials can be changed, but cannot enter inside the nano particles. Nano particles, due to its size, possess unexpected optical (visible) properties.

Gold nano particles appear deep red to black colour in solution compared to yellow colour with Gold. ZnO nano particles possesses superior UV blocking property compared to bulk material. Absorption of solar radiation in photovoltaic cell containing nano particles are higher than the film (bulk material).

10. Nano particles possesses lower melting point than the bulk materials. Gold nanoparticles melt at lower temperature (3000C) for 2.5 nm, but Gold slab melts at 1064OC.

11. Sinter ing of nano particles takes place at lower temper 11. Sinter ing of nano particles takes place at lower temper ature and in shor t time than the bulk mater ials. Electr ical proper ties, resistivity of nano par ticles ar eincreased by 3 times. Suspension of nano par ticles is possible, because nano particles possess high sur face area, but bulk mater ials cannot. The wear resistance of nano particles are 170 times higher than the bulk mater ials.

Table 7.1 Comparison of atom/molecule, nano particles/cluster, bulk materials

PROPERTIES OF NANO-MATERIALS 1. Melting Points Nano-materials have a significantly lower melting point and appreciable reduced lattice constants. This is due to huge fraction of surface atoms in the total amount of atoms. 2. Optical Properties Reduction of material dimensions has pronounced effects on the optical properties. Optical properties of nano-materials are different from bulk forms. The change in optical properties is caused by two factors

The quantum confinement of electrons within the nano-particles increases the energy level spacing. The optical absorption peak of a semiconductor nano-particles shifts to a short wavelength, due to an increased band gap. (ii) Surface plasma resonance, which is due to smaller size of nano-particles than the wavelength of incident radiation. The colour of metallic nano-particles may change with their sizes due to surface plasma resonance.

3. Magnetic Properties Magnetic properties of nano materials are different from that of bulk materials. Ferro-magnetic behaviour of bulk materials disappear, when the particle size is reduced and transfers to super-paramagnetics. This is due to the huge surface area.

4. Mechanical Properties The nano-materials have less defects compared to bulk materials, which increases the mechanical strength. (i) Mechanical properties of polymeric materials can be increased by the addition of nano-fillers. (ii) As nano-materials are stronger, harder and more wear resistant and corrosion resistant, they are used in spark plugs. Nano-crystalline carbides are much stronger, harder and wear resistant and are used in micro drills.

5. Electrical Properties (i) Electrical conductivity decreases with a reduced dimension due to increased surface scattering. However, it can be increased, due to better ordering in micro-structure. Polymeric fibres (ii) Nanocrystalline materials are used as very good separator plates in batteries, because they can hold more energy than the bulk materials. Nickel-metal hydride batteries made of nanocrystalline nickel and metal hydride, require far less frequent recharging and last much longer.

6. Chemical Properties Any heat treatment increases the diffusion of impurities, structural defects and dislocations and can be easily push them to the nearby surface. Increased perfection will have increased chemical properties.

SIZE DEPENDENT PROPERTIES Nearly all the properties as shown in following figure 7.1 like hardness, strength, ductility, melting point and density, change for nano materials. These behaviors vary so significantly by a mere reduction in grain size. Nanomaterials are composed of grains and grain boundaries. Nanometre sized grains contains only a few thousands of atoms with in each grain.

A large number of atoms reside at the grain boundaries. As the grain size decreases, there is a significant increase in the volume fraction of grain boundaries or interfaces. The properties of the materials are bound to be governed to a large extent by defect configurations. Hence the mechanical and chemical properties of nanomaterials are significantly altered due to defect dynamics. The elastic property of nanomaterials are different from that of bulk alloys due to the presence of increased fraction of defects

Fig 7.1 Shows how different properties change in the nano-materials

1. Nanocrystalline ceramics are tougher and 1. Nanocrystalline ceramics are tougher and stronger than those with coarse grains. 2. Nano-sized metals exhibit significant decrease in toughness and yield strength increases.

SYNTHESIS OF NANO - MATERIALS Nano-materials are synthesised in two methods. Top-down (or) Physical (or) Hard methods It involves conversion of larger particles into smaller particles of nano-scale structure. This methods is carried out by the following process.

1. Laser ablation 2. Chemical Vapour Deposition (CVD) 3. Electro-deposition

1. Laser Ablation In laser ablation, high-power laser pulse is used to evaporate the matter from the target. The stoichiometry of the material is preserved in the interaction. The total mass ablated from the target per laser pulse is referred to as the ablation rate. Reaction Setup A typical laser ablation setup in shown in the following figure.7.2 Fig 7.2 Laser ablation chamber equipped with a rotating target holder

When a beam of laser is allowed to irradiate the When a beam of laser is allowed to irradiate the target, a supersonic jet of particles is evaporated from the target surface. Simultaneously, an inert gas such as argon, helium is allowed into the reactor to sweep the evaporated particles from the furnace zone to the colder collector. The ablated species condense on the substrate placed opposite to the target. The ablation process takes place in vacuum chamber, either in vacuum or in the presence of some background gas.

2. Chemical Vapour Deposition (CVD) It is a process of chemically reacting a volatile compound of a material with other gases, to produce a non-volatile solid that deposits automatically on a suitably placed substrate. CVD reaction requires activation energy to proceed. This energy can be provided by several methods.

(a) Thermal CVD (b) Plasma CVD In thermal CVD, the reaction is activated by high temperature above 9000C. Typical apparatus comprises of gas supply system, deposition chamber and an exhaust system. (b) Plasma CVD In plasma CVD, the reaction is activated by plasma at temperature between 300 - 7000C.

(c) Laser CVD (d) Photo-laser CVD In laser CVD, pyrolysis occurs when laser thermal energy of laser heats falls on an absorbing substrate. (d) Photo-laser CVD In photo-laser CVD, the chemical reaction is induced by ultra violet radiation, which has sufficient photon energy, to break the chemical bond in the reactant molecules.

Various steps involved in synthesis of CVD The various steps involved in synthesis of CVD are summarized as follows. 1. Transport of gaseous reactants to the surface. 2. Adsorption of gaseous reactant on the surface. 3. Catalysed reaction occurs on the surface. 4. Product diffuses to the growth sites. 5. Nucleation and growth occurs on the growth site. 6. Desorption of reaction products away from the surface.

CVD Reactor The CVD reactors are of generally two types 1. Hot-wall CVD 2. Cold-wall CVD 1. Hot-wall CVD reactors are usually tubular in form, and heating is accomplished by surrounding the reactor with resistance elements. 2. But in cold-wall CVD reactors, substrates are directly heated inductively by graphite susceptors, while chamber walls are air (or) water-cooled

Fig 7.3 CVD Reactors

3. ELECTRO - DEPOSITION Template assisted electro-deposition is an important technique for synthesizing metallic nano-materials with controlled shape and size. Arrays of nano-structured materials with specific arrangements can be prepared by this method, using an active template as a cathode in an electrochemical cell.

Fig 7.4 Electro-deposition

The electro-deposition method consists of an. electrochemical cell The electro-deposition method consists of an electrochemical cell. The cell usually contains a reference electrode, a specially designed cathodes and an anode. The cathode, substrate on which electro-deposition of the nano-structure takes place, can be made of either non-metallic or metallic materials. By using the surface of the cathode, as a template, various desired nano-structures can be synthesized for specific applications

Bottom-up (or) Chemical (or) Soft methods (or) Small to Big methods It involves building-up of materials from the bottom by atom by atom (≈ 0.1 nm), molecule by molecule or cluster by cluster. This method is carried out by the following process

1. Precipitation 2. Thermolysis (a) Solvothermal method (b) Hydrothermal method

1. PRECIPITION Generally nano-particles are synthesised by the precipitation reaction between the reactants in presence of water soluble inorganic stabilizing agent. (i) Precipitation of BaSO4 Nano-particles

10 gm of sodium hexameta-phosphate (stabilizing agent) was dissolved in 80 ml of distilled water in 250 ml beaker with constant stirring. Then 10 ml of 1M sodium sulphate solution was added followed by 10 ml of 1M Ba(NO3)2 solution. The resulting solution was stirred for 1 hr. Precipitation occurs slowly. The resulting precipitate was then centrifuged, washed with distilled water and vacuum dried. In the absence of stabilizing agent, Bulk BaSO4 is obtained.

(ii) Precipitation by reduction Reduction of metal salt to the corresponding metal atoms. These atoms act as nucleation centres leading to formation of atomic clusters. These clusters are surrounded by stabilizing molecule that prevent the atoms agglomerating.

2. THERMOLYSIS Thermolysis is characterized by subjecting the metal precursors (usually organometallic compounds in oxidation state zero) at high temperatures together with a stabilizing agent. Nano-particles show an increase in size relating to the temperature rise. This is due to the elimination of stabilizing molecule, generating a greater aggregation of the particles.

(a) Hydrothermal synthesis It involves crystalisation of substances from high temperature aqueous solutions at high vapour pressure. Hydrothermal synthesis is usually performed below the super critical temperature of water (3740C). Method Hydrothermal synthesis is performed in an apparatus consisting of a steel pressure vessel called autoclave in which

Fig 7.5 Hydrothermal synthesis

(b) Solvothermal Synthesis a nutrient is supplied along with water. A gradient of temperature is maintained at the opposite ends of the growth chamber, so that the hotter end dissolves the nutrient and the cooler end causes seeds to take additional growth. (b) Solvothermal Synthesis Solvothermal synthesis involves the use of solvent under high temperature (between 1000C to 10000C) and moderate to high pressure (1 atm to 10,000 atm) that facilitate the interaction of precursors during synthesis.

Method A solvent is mixed with certain metal precursors and the solution mixture is placed in an autoclave kept at relatively high temperature and pressure in an oven to carry out the crystal growth. The pressure generated in the vessel, due to the solvent vapour, elevates the boiling point of the solvent.

Fig 7.6 Solvothermal synthesis

Solvothermal synthesis of zinc oxide Example for solvent Ethanol, methanol, toluene, cyclohexane, etc., Solvothermal synthesis of zinc oxide Zinc acetate dihydrate is dissolved in 2-propanol at 500C. Subsequently, the solution is cooled to 00C and NaOH is added to precipitate ZnO. The solution is then heated to 650C to allow ZnO growth for some period of time before a capping agent (1-dodecanethiol) is injected into the suspension to arrest the growth. The rod shaped ZnO nano-crystal is obtained.

Uses 1. Many geometries including thin film, bulk powder, single crystals can be prepared. 2. Thermodynamically stable novel materials can also be prepared easily.

NANO-WIRES Nano-wire is a material having an aspect ratio ie., length to width ratio greater than 20. Nano-wires are also referred to as “quantum wires”. 1. Nano-wires of metals : Au, Ni, Pt. 2. Nano-wires of semiconductors : InP, Si, GaN 3. Nano-wires of Insulators : SiO2, TiO2 4. Molecular nanowires : DNA

Characteristics of Nano-wires 1. Nano-wires are one-dimensional material. 2. Conductivity of a nano-wire is less than that of the corresponding bulk materials. 3. It exhibits distinct optical, chemical, thermal and electrical properties due to this large surface area. 4. Silicon nano-wires show strong photo luminescence characteristics.

Synthesis of Nano-wires Nanowires can be synthesised by any one of the following methods. 1. Template-assisted synthesis Template assisted synthesis of nanowires is simple way to fabricate nanostructures. These templates contain very small cylindrical pores or voids within the host material and the empty spaces are filled with the chosen material to form nanowires. Examples for templates Alumina (Al2O3), nano-channel glass, mica films, ion track-edged polymers.

2. VLS method It involves the absorption of the source material from the gas phase into a liquid droplet of catalyst. Upon supersaturation of the liquid alloy, a nucleation event generates a solid precipitate of the source material. This seed serves as a preferred site for further deposition of material at the interface of the liquid droplet, promoting the elongation of the seed into a nanowire.

Applications of Nano-wires Nanowires are used for enhancing mechanical properties of composites. It is also used to prepare active electronic components such as pn junction and logic gates. Semiconductor nanowire crossings are expected to play a important role in future of digital computing. Nanowires find applications in high-density data storage either as magnetic read heads or aspatterned storage media.

NANO-RODS Characteristics of Nano-rods Nano-rod is a material having an aspect ratio in the range 1 to 20 with short dimension of the material being 10-100 nm. Characteristics of Nano-rods 1. Nano-rods are one-dimensional materials. 2. It also exhibits optical and electrical properties

Synthesis Applications Nano-rods are produced by direct chemical synthesis. A combination of ligands act as shape control agents and bond to different facets of the nano-rods with different strength. Applications It finds applications in display technologies and micro mechanical switches

NANO CLUSTER Nano clusters constitute an intermediate state of matter between molecules and bulk materials. These are fine aggregates of atoms or molecules. They are bound by forces, which may be metallic, covalent, ionic, hydrogen bond or vander waals force in character. The size of nanocluster ranges from sub-nanometer to 10 nm in diameter. It has been found that clusters of certain critical size (clusters with a certain number of atoms in the group) are more stable than others. Nanoclusters consisting of upto a couple of hundred atoms, but larger aggregates containing 103 or more atoms are called nanoparticles.

Magic number It is the number of atoms in the clusters of criticle sizes with higher stability. Different types of clusters can be distinguished by the nature of the force between the atoms. Clusters containing a transition metal atoms have unique chemical, electronic and magnetic properties, which vary with the number of constituent atoms, the type of element and the charge on the cluster.

Production of Nano Cluster Fig 7.7 Production of nano clusters from atoms or molecules or from bulk materials

Clusters can be produced from atomic or molecular constituents or from the bulk materials as shown in the figure. Atomic clusters or molecular clusters are formed by nucleation of atoms or molecules respectively. Clusters of the same type may be obtained by top down process also. Sources of Clusters There are many kinds of cluster sources. Two of them are 1. Supersonic nozzle source 2. Gas-aggregation source

1. Supersonic Nozzle Source Here metal is vapourized in an oven and the vapour is mixed with an inert carrier gas (seeded) at a pressure of several atmosphere at a temperature of 75 - 1500 K. The metal/carrier gas mixture is then allowed through a nozzle in to high vacuum, which creates supersonic beam. Seeding produces large clusters while in the absence of a carrier gas smaller clusters are formed.

2. Gas-aggregation source The source utilizes the property of aggregation of atoms in an inert media. The vapours generated by any method are introduced in to a cold inert gas at a higher pressure. The species at high temperature are thermalized. The gas phase is super saturated with the species and they aggregate. These sources produce continuous beams of clusters of low-to-medium boiling metals

NANO TUBES Nano-tubes are one of the most widespread studied and used materials, consists of tiny cylinders of carbon and other materials like boron nitride. Nano-tubes of carbon and inorganic compounds with structures comparable to the layered structure of graphite have been prepared. Studies on carbon nano-tubes are quite extensive.

Carbon Nanotubes (CNT) Carbon nanotubes are allotropes of carbon with a nanostructure having a length-to-diameter ratio greater than 1,000,000. When graphite sheets are rolled into a cylinder, their Single walled carbon nanotubes Fig 7.8 Single walled carbon nano tubes

edges joined and form carbon nanotubes i. e edges joined and form carbon nanotubes i.e., carbon nanotubes are extended tubes of rolled graphite sheets. Nanotubes naturally align themselves into “ropes” and held together by vanderwaals forces. But each carbon atoms in the carbon nanotubes are linked by the covalent bond.

STRUCTURE (OR) TYPES OF CARBON NANOTUBES Carbon nanotubes are lattice of carbon atoms, in which each carbon is covalently bonded to three other carbon atoms. Depending upon the way in which graphite sheets are rolled, two types of CNTs are formed. 1. Single - walled nanotubes (SWNTs). 2.Multi – Walled nanotubes (MWNTs)

Fig 7.9 Structure of Single walled carbon nanotubes

1. Single - walled nanotubes (SWNTs) SWNTs consist of one tube of graphite. It is one-atom thick having a diameter of 2 nm and a length of 100 m. SWNTs are very important, because they exhibit important electrical properties. It is an excellent conductor. Three kinds of nanotubes are resulted, based on the orientation of the hexagon lattice.

(a) Arm-chair structures: The lines of hexagons are (a) Arm-chair structures: The lines of hexagons are parallel to the axis of the nanotube. (b) Zig-zag structures: The lines of carbon bonds are down the centre. (c) Chiral nanotubes: It exhibits twist or spiral around the nanotubes. It has been confirmed that arm-chair carbon nanotubes are metallic while zig-zag and chiral nanotubes are semiconducting.

2. Multi - walled nanotubes (MWNTs) MWNTs (nested nanotubes) consist of multiple layers of graphite rolled in on themselves to form a tube shape. It exhibits both metallic and semiconducting properties. It is used for storing fuels such as hydrogen and methane.

Fig 7.10 Multiwalled Carbon Nanotubes

SYNTHESIS OF CARBON NANOTUBES Carbon nanotubes can be synthesized by any one of the following methods. 1. Pyrolysis of hydrocarbons. 2. Laser evaporation. 3. Carbon arc method. 4. Chemical vapour deposition.

1. Pyrolysis 2. Laser evaporation Carbon nanotubes are synthesized by the pyrolysis of hydrocarbons such as acetylene at about 7000C in the presence of Fe-silica or Fe-graphite catalyst under inert conditions. 2. Laser evaporation It involves vapourization of graphite target, containing small amount of cobalt and nickel, by exposing it to an intense pulsed laser beam at higher temperature 12000C in a quartz tube reactor. An inert gas such as argon is simultaneously allowed to pass into the reactor to sweep the evaporated carbon atoms from the furnace to the colder copper collector, on which they condense as carbon nanotubes.

4. Chemical vapour deposition 3. Carbon arc method It is carried out by applying direct current (60 - 100 A and 20 - 25 V) arc between graphite electrodes of 10 - 20 m diameter. 4. Chemical vapour deposition It involves decomposition of vapour of hydrocarbons such as methane, acetylene, ethylene, etc., at high temperatures 11000C in presence of metal nanoparticle catalysts like nickel, cobalt, iron supported on MgO or Al2O3. Carbon atoms produced by the decomposition condense on a cooler surface of the catalyst.

Properties of CNTs 1. CNTs are very strong, withstand extreme strain in tension and posses elastic flexibility. 2. The atoms in a nano-tube are continuously vibrating back and forth. 3. It is highly conducting and behaves like metallic or semiconducting materials. 4. It has very high thermal conductivity and kinetic properties.

Uses of CNTs 1. It is used in battery technology and in industries as catalyst. 2. It is also used as light weight shielding materials for protecting electronic equipments. 3. CNTs are used effectively inside the body for drug delivery. 4. It is used in composites, ICs.

APPLICAITONS OF NANO MATERIALS (OR) NANO PARTICLES Nano-technology finds significant impact on all most all the industries and all areas of society. Since nano-materials possess unique beneficial chemical, physical and mechanical properties, they can be used for a wide variety of applications

I.Medicine 1. Nano drugs Nano materials are used as nano drugs for the cancer and TB therapy, 2. Laboratories on a chip Nano technology is used in the production of laboratories on a chip.

3. Nano-medibots Nano particles function as nano-medibots that release anti-cancer drug and treat cancer. 4. Gold-coated nanoshells It converts light into heat, enabling the destruction of tumours. 5. Gold nano particles as sensors Gold nano particles undergo colour change during the transition of nano particles.

6. Protein analysis Protein analysis can also be done using nanomaterials. 7. Gold nanoshells for blood immuno assay Gold nano shells are used for blood immuno assay. 8. Gold nano shells in imaging Optical properties of the gold nano shells are utilized for both imaging and therapy.

9. Targeted drug delivery using gold nano particles It involves slow and selective release of drugs to the targeted organs. 10. Repairing work Nano technology is used to partially repair neurological damage.

II.Industries 1. As Catalyst It depends on the surface area of the material. As nano-particles have an appreciable fraction of their atom at the surface, its catalytic activity is good. Bulk gold is chemically inert, where as gold nano-particles have excellent catalytic property.

2. In water purification Nano-filtration makes use of nano-porous membranes having pores smaller than 10 nm. Dissolved solids and colour producing organic compounds can be filtered very easily from water. Magnetic nano-particles are effective in removing heavy metal contamination from waste water. 3. In fabric industry The production of smart-clothing is possible by putting a nano-coating on the fabric. (i) Embedding of nano-particles on fabric makes them stain repellent. (ii) Socks with embedded silver nano-particles fills all the bacteria and makes it odour free.

4. In Automobiles (i) Incorporation of small amount of nano- particles in car bumpers can make them stronger than steel. (ii) Specially designed nano-particles are used as fuel additive to lower consumption in vehicles. 5. In food industry The inclusion of nano-particles in food contact materials can be used to generate novel type of packing materials and containers.

6. In energy sector In solar power, nano-technology reduces the cost of photovoltaic cells by 10 to 100 times.

III.Electronics Quantum wires are found to have high electrical conductivity. The integrated memory circuits have been found to be effective devices. A transistor, called NOMFET, (Nano particle organic memory field effect transistor) is created by combining gold nano particles with organic molecules. Nano wires are used to build transistors without p - n junctions. Nano radios are the other important devices, using carbon nanotubes. MOSFET (Metal Oxide Semi conductor Field Effect Transistor), performs both as switches and as amplifiers.

IV.Bio-materials (Biology) Nano materials are used as bone cement and bone plates in hospitals. It is also used as a material for joint replacements. Nano technology is being used to develop miniature video camera attached to a blind person’s glasses. Nano materials are also used in the manufacture of some components like heart valves and contact lenses. Nano materials are also used in dental implants and breast implants. CNTs are used as light weight shielding materials for protecting electronic equipments against electromagnetic radiation.