Nano Sized Ferrite Particles Biology Essay

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Nano-sized ferrite particles are extensively studied in the last several decades for its electrical and magnetic properties in wide range of technological applications 1,2. Nano-particles are defined as the material particles having characteristic dimensional length less than 100nm 3. The properties of nano-sized ferrite particles are quite unusual as compared with bulk materials. The surface to volume ratio of nano-sized particle is large which greatly alters the properties of materials i.e. coercivity (Hc), saturation magnetization (σs), Anisotropy energy (EA) 3 .

Ferrites or transition metal oxides are brittle and rigid ceramic material, available in different colors (silver gray, brown black etc.). Magnetite ferrite is one of the oldest known magnetic materials as lodestone. Magnetic ferrites are low conducting or electrically insulating magnetic materials. Neel in 1948 first time reported ferrimagnetic nature of ferrite material. In ferrimagnetic material the magnetic moments arising from different lattice atoms are unequal and oppositely aligned thus give a net magnetic moment 4. Ferrites do not generate any eddy current like other metal magnets. Thus they are more suitable material to use in high frequency alternating magnetic field.

1.2 Ferrites Structure

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Ferrites received enormous scientific attraction due to its magnetic and dielectric characteristics. These properties depend on the structure of ferrite. On the basis of their structure they are classified as follow:

1.2.1 Spinal Ferrites (SF)

Spinel ferrites are one of the earliest known magnetic materials. Magnetite (Fe3O4) was known many hundred years ago as lodestone. Its composition is FeOFe2O3. The general formula of spinal ferrite is XFe2O4, where X is a divalent metal such as iron, cobalt, nickel zinc, copper, manganese ion or mixture of divalent ions such that on average two valance electronic states are present 4,5. The accommodative possibility of number of divalent ions on same position in crystal has led ferrite to the subject of extensive research and technological applications.

Spinal crystal is a close packing of 32 oxygen ions arranged in face centered cubic (FCC) structure. This arrangement of anions gives two types interstices sites for cations, tetrahedral A-sites and octahedral B-sites. The metal cations are distributed among the tetrahedral A- site surrounded by four oxygen ions and octahedral B-sites surrounded by six oxygen ions as shown in figure1.

Figure : Unit cell of spinal ferrite structure 5

In a unit cell there are a total 64 tetrahedral sites (A) and 32 octahedral (B) sites available for cations. In case if all A-sites and B-sites are filled with divalent ions and trivalent ions respectively, there would be 224 positive charges due to 64 divalent and 32 trivalent ions in comparison of 64 negative charges due to 32 anions, and the structure would not be electrically neutral. However, only 8 tetrahedral and 16 octahedral sites are occupied by cation. The net positive and negative charges thus become equal and give an electrically insulating structure.

The structure shown in the fig.1 is an octant, which contains eight subcells, four tetrahedral and four octahedral structures respectively. Magnetic moments of ions are represented by arrows and clearly indicate that magnetic moment of ion at octahedral B-site is antiparallel to the magnetic moment of ion at tetrahedral A-site. Since ions at both sites have unequal magnetic moment, therefore it gives rise to the net magnetic moment and illustrates ferrimagnetic nature 4.

In spinal ferrites ratio of the cations at A-site and B-site greatly alter the structure and properties of spinal ferrites, and depend on size, electronic configuration and electronic energy of the metal ion. Spinal ferrites are further divided into three groups i.e. normal, inverse and mixed spinal ferrites 6.

In normal spinal structure eight tetrahedral A-sites are occupied by divalent atoms and the 16 octahedral B-sites are occupied by trivalent atoms. They are represented by chemical formula . A typical example of normal spinal ferrite is ZnFe2O4 7.

In inverse spinal ferrites, divalent atoms fill B-sites where as the trivalent atoms are equally distributed among A and B-sites. Their chemical formula is. Example of inverse spinal ferrite is cobalt ferrite (CoFe2O4) 6.

In mix spinal structure both tetrahedral (A) and octahedral (B) sites are partially filled by each kind of ions. Mixed spinal ferrites are represented by formula is, where is the inversion factor or inversion parameter. Value of depends on the method of preparation and the constituents of ferrites. For spinal structure and for inverse spinal structure x while for mixed spinal structure its value varies between 0 and1. MnFe2O4 is an example of mixed spinal ferrite 8.

1.2.2 Hexagonal Ferrite (HF)

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Hexagonal ferrites structure is very much complex in comparison to that of spinal structure. The general formula is where X is usually Strontium (Sr), Barium (Ba) or Lead (Pb). The X and O-2 ions form a close packed structure and give rise to three different interstices sites for the metal ions i.e. Tetrahedral, Octahedral and Trigonal bi-pyramid. The trigonal bi-pyramid interstices site is surrounded by 5 oxygen atoms. The lattice parameters of unit cell are and. Unit cell of hexaferrite contain 64 ions including 12 Fe+3 ions. These Fe+3 ions are distributed among the different interstices sites such that nine ions occupy octahedral B-Site, two on tetrahedral A-site and one occupy trigonal bi-pyramid site, surrounded by five coordinate sites. The arrangement of spins at different sites is such that we get four unpaired spins form a unit cell contributing to the magnetic moment of hexa ferrites 4.

1.2.3 Magnetic Rare Earth Garnets

Minerals like grossularite (3CaO. Al2O3.3SiO2), andradite (3CaO.Fe2O3.SiO2), spessarite (3MnO.Al2O3.3SiO2) have similar structure and are collectively called Garnet. Magnetic garnets crystallize in the same structure. The general formula of garnet is , where R is Yttrium or the rare earth ions (Gd, and Dy) in case of magnetic garnet. Garnets have dodecahedral or 12 sided crystal structure. Yttrium iron garnet (YIG) has unit cell containing 160 atoms and particularly important for the microwave applications. In the crystal structure of Garnets a new cation site dodecahedral site is available in addition to the tetrahedral and octahedral sites 4.

1.3 Properties

Ferrites with fascinating magnetic and electrical properties are very important due to their technological application.

1.3.1 Magnetic Properties

Ferrites are ferrimagnetic in nature as discussed earlier. On the bases of magnetic properties, ferrites can be divided into two groups: soft ferrites and hard ferrites.

Soft ferrites were produces by J. L. Snoek and his co-workers before 1950. Soft ferrites have high permeability, high electrical resistivity and saturation magnetization. They offer low conductivity, low magneto crystalline anisotropy and coercivity 9. Low coercivity results in easily magnetization and demagnetization of soft ferrites without dissipating much energy. Where the low conductivity prevent undesired eddy current.

Hard ferrites are characterized by high coercivity, high permeability, high electrical resistivity and very high magneto crystalline anisotropy. High coercivity prevents the demagnetization of hard ferrites material and thus makes it suitable for the wide use as permanent magnet 10.

1.3.2 Electrical Properties

Ferrites are electrically insulator or semiconducting material. Conduction of charges in ferrites is quite different than semiconductor. Charge mobility is independent of the temperature variations in ferrites, and electron associated with any particular ion remains isolated. Presence of Iron ions in the ferrite structure with different valance state can affect the conductivity of the material; however the structure as a whole is non-conducting. Ferrites do not offer eddy current in alternating field. They are potentially good candidate in situations where electrical conductivity is undesired. They are particularly used as cores of induction coils operating at high frequencies because of their high permeability, saturation magnetization and low electrical conductivity 11.

1.3.3 Microwave Properties of Ferrites

Ferrites are extensively used in the complex microwaves communication, satellite Communication, navigation, radar technology and spectroscopy. Most common devices used in the microwave communication are isolators, circulators, resonators and phase shifter. These devices are made of ferrites having low coercivity and high remanence and moderate permeability. Permeability of ferrite depends on biasing magnetic field. Biasing field of ferrites is the sum of internal and external alternation field. In the absence of alternating field the magnetic moment of the electron is aligned along the easy axis where potential energy of the system is low. When ferrites are placed in the in external alternating field or microwave field, this external biasing field reinforces the internal magnetic field. This change will significantly affect the permeability of the ferrite. Magnetic moment of electron will be under the action of internal and external microwave field which results in precessional motion of magnetic moment about equilibrium position. This precessional motion of magnetic moment is opposed by the forces like resistance of the material, and the electron moment will precess in equilibrium precessional orbit under the action of external alternating field and internal damping. In this mechanism energy is transferred from the alternating field to the precessional motion of electron. This energy is dissipated by internal frication and results in heating of the material. This phenomenon is called resonance absorption. Microwave absorption as a result of resonance makes ferrites extremely valuable tool for their application in the stealth technology as microwave absorber. Stealth technology is the modern technique in which aircrafts and missiles are made invisible by coating them with microwave absorber ferrite paint 4,11.

1.4 Applications

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In early ages naturally occurring ferrites (Magnetite) were used as magnetized needles to find North and South. However the applications of magnetic ferrites were limited due to the poor knowledge of magnetic materials and their applications. In the last century researchers have discovered and produced ferrites with some fascinating properties.

Nowadays ferrites have found their applications in high density storage devices and magnetic inks12. As the particle size decreases, the number of magnetic domains in particles also decreases and the particle with size 5nm serve as the single domain particle. This 5nm single domain particle can be used to create storage density of 10Gbits/cm 3,13. Ferrites are non-conducting ceramic materials. Ferrites are extensively used in the situation where the electrical conductivity of material is undesired. They offer no eddy current in the A.C. field. Ferrites are particularly used in the high frequency applications. They are particularly used as cores of induction coils operating at high frequencies because of their high permeability, saturation magnetization and low electrical conductivity. Ferrites are also used for the catalysis of different organic compounds 14. Ferro fluid is currently the most promising material with numerous applications. Ferrites are dispersed in carrier liquid 15. They are used for heat exchange16. Ferro fluid can be used in a variety of applications. They are used in the diagnoses of the cancer in magnetic resonance imaging (MRI) device and magneto cytolysis for destruction of cancerous cells. Ferro fluid is also used as carrier for the targeted drug delivery 17.

Cobalt Ferrite

Cobalt Ferrite (CoFe2O4) is well known hard magnetic material with inverse spinal structure , in which Co+2 ions are localized at B-site where Fe+3 ions are present in both A-site and B-site 18. The unit cell is characterized by lattice parameters 8.38 Š and the interaction distances are b = 2.963 Å, u = 0.2714 Å, p = 2.963 Å, q = 0.3106Å, r = 38.336 Å and s = 3.127 Å 8,19. CoFe2O4 nanoparticles possess high chemical stability and mechanical hardness. It has a very high magnetostriction value, low electric loss, high coercivity 5.3kOe (Kilo Oersted), reasonable magnetization 80.8emu/g (Electromagnetic Unit per Gram ) 20 and very high uniaxial magnetocrystalline anisotropy along cube axis [1 0 0] 18,21,10. Cobalt ferrite show superparamagnetic behavior at room temperature however at low temperature it behaves ferrimagnetically.

These properties are temperature dependent and show variation with change in temperature. These properties make it a perfect material for use in stress sensor, precursors for making ferro-fluids, magnetic refrigeration 22. It exhibit superparamagnet 23 behavior at room temperature which is important for the enhanced contrast magnetic resonance imaging (MRI). The higher anisotropy and high coercivity make it a good candidate for the magnetic recording devices such as audio and video tape and high-density digital recording disks or computer memory 21,24. Cobalt ferrite is used in medical field for destruction of cancer cells by magnetic fluid hyperthermia 25, and targeted drug delivery 16. It is used as catalyst in the hydrolysis of different chemicals 26. CoFe2O4 has been used as microwaves absorber in different devices for stealth technology 27.

CHAPTER 2

LITERATURE REVIEW

2.1 Nano-Particles

Nano-particles are generally defined as the particles having size less than 100 nanometer (nm). In nano-particles surface to volume ratio is very large and the large number of atoms at the surface enhances the surface energy and wider band gape which plays a dominate role in improvement of many properties. The quantum confinement of electron for the nano-particles has effects on the electrical conductivity and magnetic susceptibility of the material. Higher surface energy enhances the chemical activity catalyst field.

2.2 Brief Background of Nanoparticles

Fabrication and application of nano-particles have attracted great interest in last several decayed. Nanostructured materials had its origin from the big-Bang. All matter is formed by the condensation of atoms into larger masses under the action of gravity. However the importance of the nano particles were first realized in 19th century. Nanoparticles of metal colloids were first reported by Faraday in 1857 while working with metal fine particles. He observed that the color of the glass is effected with doped particle size 28. In 1959 Richard Feynman gave a lecture "There's Plenty of Room at the Bottom" at the annual meeting of the American Physical Society (APS) at Caltech. In his lecture he discussed the possibility of having nano-sized utilities and argued that the behavior of the material at such a small size will be completely different from bulk materials. He said "As we go down and fiddle around with the atoms down there, we are working with different laws, and we can expect to do different things. We can manufacture in different ways. We can use, not just circuits, but some system involving the quantized energy levels, or the interactions of quantized spins. At the atomic level, we have new kinds of forces and new kinds of possibilities, new kinds of effects" 8. His article provides the guidance for work on the nano-materials and was regarded as the foundation of nanotechnology.

In 1962 Kubo 30 at Tokyo University gave theoretical formulation for the quantum confinement of electron. He suggested that the electron in the nano sized particles with size less than 10nm do not obey Fermi statistics. Kubo noted that the nano-sized particles had strong tendency to remain electrically neutral which effects specific heat, magnetization, and superconductivity. This effect is now called Kubo effect. Ohshima et al.31 synthesis fine particle of alumina, carbonate and studied its crystal structure and morphologies with the help of electron microscope and electron diffraction technique.

Field of nano materials has the potential for the theoretical as well as for experimental studies. In last several decades of 20th century, researchers have published large number of papers on synthesis, characterization and theoretical formulation of the properties of nanoparticles. Key factors that attracted scientists are the properties of nanoparticles which are enormously different than that of bulk materials.

Size of particle has effects on surface tension, vapor pressure and phase transition which is called thermodynamic size effect. Another sole property of the nanocrystals is defects in crystal. The density of crystal defects are so large that the spacing of two defect sites is of the order of the interatomic distance and it can drastically affect electrical, optical and mechanical properties of the materials. The size dependent sharp variation in the properties of nanoparticles is termed as quantum size effect32. Nanoparticles have large surface to volume ratio. Most of the atoms of particle lies on the surface and can sharply respond to variations of external electric and magnetic fields.

2.3 Magnetic Nano-Particles

The magnetic behavior of the nanoparticles is the unique character of the nano-particles. The origin of magnetism in atoms is the spin and orbital motion of electron. Magnetic dipole moments arising from spin and orbital motion are coupled to each other in different materials.

In diamagnetic materials the net magnetic dipole moment arising from the motion of electrons of different atoms cancels each other. In diamagnetic materials outermost shell of atom is completely filled, and when placed in magnetic field it magnetized in the direction opposite to the applied field. In paramagnetic materials outer most shell is partially filled, however the magnetic moments of all unpaired electrons are randomly oriented. Paramagnetic materials magnetized in the direction of applied field. Diamagnetic and paramagnetic magnetization is very weak and observed only in strong magnetic field. Other types of magnetic materials are ferromagnetic, ferrimagnetic and antiferromagnetic. In Ferromagnetic materials there is a strong interaction between the moments of unpaired electrons and result in a net magnetic moment and can be magnetized in weak magnetic field. In antiferromagnetic materials the magnetic moment arises from different lattice sites are equal and opposite. Therefore the net magnetic moment is zero. In 1940, Neel's discover Ferrimagnetism. In ferrimagnetic material the magnetic moments arising from different lattice atoms are unequal and oppositely aligned thus give a net magnetic moment 4.

Lodestone or magnetite (Fe3O4) is one of the oldest know ferrimagnetic material. It is naturally occurring mineral and known to the humans from several centuries. Elmore 33 in 1938 prepare magnetite for the first time by precipitation of ferrous chloride (FeCl2) and ferric chloride(FeCl3). Magnetite generally falls in group of magnetic spinal ferrites. Ferrites of different composition and structures have been the subject of major interest in all over the world in last century. There is always a need of ideal ferrite which is suitable for use in every set of required application.

The curiosity of researcher to have an ideal ferrite product with optimum performance, led them to the discovery of large number of different ferrites. Those with amended mechanical, chemical and physical properties have been achieved through different synthesis techniques.

2.4 Cobalt Ferrite nano-Particles

Cobalt ferrite (CoFe2O4) is one of the most important ferrite which has variety of technological applications. CoFe2O4 possess high chemical stability and mechanical hardness. Cobalt ferrite behave superparamagnetically at room temperature and frrimagnetically at temperature below 260oC 34,35. It has a very high magnetostriction. Magnetostriction is defined as the change in the shape or dimension of the lattice caused by the orientation of magnetic domains in the external magnetic field. Spin and orbital moment of electrons are strongly coupled in lattice. The orientation of the orbital magnetic moment is restricted. When direction of spin changes due to external magnetic field, it causes change in the orientation of the orbits. It produce a slight variation in the dimension of the lattice 5. In such materials magnetization is produced when subjected to external stress. CoFe2O4 has high DC resistivity and very small eddy losses. These properties make it a promising material for the stress sensor applications 36. It is used in the motors, transducers and actuators37. CoFe2O4 is used in microwaves communication, satellite Communication, navigation, and radar technology 27.

CoFe2O4 has very high uniaxial magnetocrystalline anisotropy.18,21. Some material preferred to magnetize in some particular direction. This property is called magnetocrystalline anisotropy of material. Magnetic moment of CoFe2O4 prefers to orient along the [1 0 0] axis of crystal. Magnetocrystalline anisotropy depend on temperature. Relation of anisotropy constant with temperature is 10:

Magnetocrystalline anisotropy energy is given by Wohlfarth theory:

Where K is the anisotropy constant, V is volume of grain and ÆŸ is the angle between direction of magnetization and easy axis. Magnetocrystalline anisotropy energy serves as a barrier for the superparamagnetic behavior of the CoFe2O4 nanoparticles. When become comparable to thermal activation energy , particles show superparamagnetic behavior 38. At room temperature becomes equal to and CoFe2O4 nanoparticles show superparamagnetic behavior.

Cobalt ferrite (CoFe2O4) has high coercivity 39. It requires large coercive force to demagnetize after saturation magnetization 40,41. The coercivity of CoFe2O4 varies with temperature. At room temperature its value is 5.3 kOe and increases to 25.2 kOe at temperature -268oC. Cobalt ferrite has reasonable magnetization because it contains Cobalt (Co) and Iron (Fe) which strongly respond to magnetic field because of their large magnetic moments. Saturation magnetization of CoFe2O4 has a value of 80.8emu/g at room temperature and it increases to 93.9emu/g at -268oC 10. This high anisotropy and coercivity are very important property for magnetic data recording. In magnetic data recording, information is stored by magnetizing materials. This magnetization can be reversed by thermal energy. In order to prevent data loss, either volume or anisotropy is increased to keep the magnetic energy of the grain above the thermal reversal energy 42. If volume of the grain is increased, its anisotropy and coercivity will decrease or vice versa. Therefore the high anisotropic and high coercive materials are particularly important for the magnetic data recording 18,24,43.

These properties make cobalt ferrite a perfect material for use in stress sensor, precursors for making ferro-fluids, magnetic refrigeration 22. It exhibits superparamagnetic 23 behavior which is important for the enhanced contrast magnetic resonance imaging (MRI). The higher anisotropy makes it a good candidate for the magnetic recording devices such as audio and video tape and high-density digital recording disks or computer memory 21,24. Cobalt ferrite is used in medical field for destruction of cancer cells by magnetic fluid hyperthermia25, and targeted drug delivery16. It is used as catalyst in the hydrolysis of different chemicals26. CoFe2O4 has been used as microwaves absorber in different devices for stealth technology27

All these properties were found to be strongly size dependent. It is observed that with reduction in size of particle below a finite value, CoFe2O4 exhibits superparamagnetic behavior. Anisotropy, coercivity ( Hc) of the CoFe2O4 increases and saturation magnetization (σs) decreases with decrease in size 32,44. Grigorova et al. (1998) reported that size of the particle depends on annealing temperature and showed that particle size increases with increase of annealing temperature 10.Maaz et al. (2007) also verified the same behavior 21. However the literature suggests that the magnetic properties of cobalt ferrite are not only size dependent but also depend on the method of preparation.

2.4.1 Synthesis techniques of Cobalt Ferrite Nano-Particles

To achieve the smallest size of particles, various techniques were developed for the synthesis of nano-sized cobalt ferrite. They include sonochemical reaction8, mechanical alloying 45, spray-drying, freeze-drying, hot-spraying, evaporation-condensation, matrix isolation, laser-induced vapor phase reactions and aerosols 25,precipitation 10,44, Coparticipation21, normal micelle and reverse micelle methods 46, micro emulsion method22, sol-gel, and glass crystallization47.

These synthesis techniques of the cobalt ferrites can be broadly divided into two main categories namely chemical and physical methods.

Chemical methods are very commonly used for the synthesis of the ferrites. In this method chemicals are dissolved in proper solvents like water, ethanol, propanol and acetone etc. to make precursors. Precursors are mixed in definite molar ratios and converted to solid nano phase by precipitation at room temperature or higher temperature. Other methods are chemical vapor deposition or chemical vapors condensation. In these methods precursors are converted into gaseous state at high temperature and low pressure. This gas is allowed to deposit on substrate and condensed to form nanoparticles 48,49,50. Sol-gel 20 is also one of the wet chemical rout for the synthesis of cobalt nanoparticles.

In chemical methods properties of nano-crystal such as crystal structure, particle size, particle size distribution, and electrical, magnetic and mechanical properties are varied with different parameters of chemical reaction involved in synthesis. Temperature, pH and reactants concentration are those parameters.

Other group of synthesis techniques is Physical method of synthesis. There are several physical methods which are utilized to prepare nano-crystalline materials. Among them most widely used is mechanical deformation or mechanical alloying. In this method precursors are subjected to high energy ball milling or high energy shear stress. Precursors are then annealed at high temperature to obtain nano-crystals23,45,51. Other physical methods like microwave decomposition 52 and laser ablation 5 etc. are also in practice for the synthesis.

2.5 Literature Review

These preparation techniques greatly alter the size and properties of the particles. For the first time it was reported by Schuele 53 et al. (1961) that the size and properties of the particle are greatly dependent upon the method of preparation. Schuele and his co-workers synthesized nanoparticles of CoFe2O4 with size 16nm by hydrothermal process. The saturation magnetization (σs) of the CoFe2O4 particles was observed to be 70emu/g (electromagnetic units/grams) while coercivity (Hc) about 0.75kOe (kilo oersted). Ding 45 et al. (1995) prepared CoFe2O4 with 30nm particles size by mechanical alloying method. Sample was annealed above 700oC. The saturation magnetization (σs) and coercivity (Hc) values were reported to be 77emu/g and 2-2.7kOe respectively. In 1995 Moumen 54 and co-workers published their work regarding the preparation of CoFe2O4 nanoparticles. They prepared 2-5nm particle by chemical micelle method with σs 50emu/g and Hc 8.8kOe and also reported that the particle size varies with reactant and surfactant concentration. Blaskov 34 et al. (1996) reported the size of particle as 5nm by chemical coprecipitation method followed by low temperature calcination. They observed σs value 15emu/g and Hc value 13kOe. Muller 47 et al. (1996) showed that annealing temperature has effect on size and magnetic properties of particles. They prepared particle of size 26-210nm by glass crystallization method with various annealing temperatures and observed saturation magnetization (σs) 74-88emu/g and coercivity (Hc) 0.51-0.89kOe. Pillai and Shah 24 (1996) prepared nano-particles having size about 50nm using water in oil micro emulsion technique. They observed coercivity 1.44kOe and saturation magnetization 65emu/g. Grigorova 10 et al. in 1998 achieved 4.7-59nm particle size by coprecipitation method followed by annealing at various temperatures. They measured values of σs 5.4-47.5emu/g and Hc 10-14kOe. In year 1998 shafi 8 et al. reported synthesis of cobalt ferrite nano-particles of size less than 5nm by sonochemical method and reported the observed value of magnetization about 45emu/g. They also reported that surface area of sample reduces with crystallization and its value decreases to 52m2/g for sample heated at 700 oC. Yan 55 et al. (1999) prepared cobalt ferrite nanoparticles by combustion method using glycine as fuel. The magnetic properties were observed to be strongly dependent on glycine ratio. The surface area they observed was 29m2/g which is much larger then observed for bulk sample. Shi and Ding 51 used combination of coprecipitation and mechanical alloying to produce fine particles of size about 10nm. They observed variation in saturation magnetization and coercivity value with increase of annealing temperature. Giri 56 et al. (2000) reported first time photomagnetism in cobalt ferrite prepared by coprecipitation method. . Ahn 26 et al. in 2000 prepared nano-particles of size 4.9nm by micro emulsion method. The observed coercivity and magnetization values were 15.1kOe and 15.2emu/g respectively at 5K. Chinnasamy 57 et al. develop a modified coprecipitation rout called seed mediated growth dominated coprecipitation. They produced 40nm particles and achieved a high coercivity value 4.3kOe close to the theoretical value 5.3kOe. Further they also showed that particle size in the conventional co-precipitation method varies with molar ratio and flow rate of sodium hydroxide (NaOH). Kim 2 et al. in year 2003 reported synthesis of CoFe2O4 particle at various temperatures by coprecipitation method, and also reported that size and magnetic properties depend on temperature of the precipitates. They synthesized CoFe2O4 nano-particle with size ranging from 2-15nm. Kim et al. observed magnetization and coerecivity value 2-58.3emu/g and 0-193Oe. Cao and Gu 35 in 2005 utilize coprecipitation method to prepare CoFe2O4 nanoparticles of particle size 22nm. They observed that cobalt ferrite behave superparamagnetically at room temperature . In year 2005, Cannas 58 et al. prepared nanoparticle of size ranging from 15-58nm by thermal decomposition method and show that size of particles varies with pH value. They also observed that the coercivity and saturation magnetization value decreased with increase of pH. Maaz 21 et al. (2007) synthesized 15-48nm particles by coprecipitation method and observed that Hc value to be10.5kOe at low temperature of -198oC. Kumar 59 et al. in 2008 studied temperature dependence on particle size, surface area coercivity and magnetization. They prepared cobalt ferrite by precipitation method and annealed sample at various temperature from 100-900oC. They observed that surface area of the sample decreases where particle size, magnetization and coercivity of the sample increases with increase of annealing temperature. In year 2012 Zalite 50 et al. prepared CoFe2O4 nano particles by sol-gel self-combustion method and by high frequency plasma technique and observed particle size of 10-20nm and 38-40nm and surface area of 37-43m2/g and 28-30m2/g respectively. They also studied magnetic properties of the material and observed saturation magnetization value53.4emu/g and 75.4emu/g and coercivity value 1.17kOe and 780Oe respectively for the sample prepared by sol-gel self-combustion and high frequency plasma method.

This review reveals that the properties of CoFe2O4 nanoparticles show reasonable variation with preparation techniques. Each technique has its own advantages and disadvantages. Some problems are also associated with these techniques. If one technique enhances a property it may also affect other properties. The quantity of CoFe2O4 nanoparticle powder produced by these techniques is limited. No single method can be adapted to achieve large scale production with enhanced magnetic properties, smallest particle size and minimum size dispersion.

There is always a need for the development of a method, which can overcome these problems. Comparative studies of different preparation techniques can play a vital role in discovering a preparation technique to overcome the above stated problems. However in the past no attention was paid to such investigation of the synthesis techniques. Grigorova et al. (1997) gave some data about the properties of CoFe2O4 synthesized through different routes. That data was collected from the work of different peoples. In last few decades no single project, based on the comparative studies of these techniques is reported. The purpose of present project is to use different methods for the production of CoFe2O4 nanoparticle powder, and to select out of these methods, the most suitable one to overcome the above stated short comings.

In past cobalt ferrite has been investigated extensively; however a very little attention has been given to the large scale production of cobalt ferrite and comparative studies of the different synthesis techniques. Keeping in view the industrial importance of the large scale production of the Cobalt ferrite nanoparticles, the major aim of the present project is the comparative study of different synthesis techniques, like co-precipitation and sol-gel. The dependence of structure and morphology of CoFe2O4 nanoparticles on the method of preparation will be studied. The effect of heat treatment on the properties of CoFe2O4 prepared by different methods will also be investigated. This comparative study will help us to identify the most suitable method for the large scale production of CoFe2O4, and also to minimize the production cost.

CHAPTER 3

Synthesis and Characterization Techniques

3.1 Materials

The large scale synthesis of mano-dispersed nano particles of is one of our basic aims of this project. Selection of the proper chemicals is very important for this purpose. Reagent grade chemicals of Ferric Chloride Hexahydrate (FeCl36H2O), Cobalt Chloride Hexahydrate (CoCl26H2O) Sodium Hydroxide (NaOH), Ethyl Alcohol (C2H6O), Oleic Acid and Propylene Oxide were used for the synthesis. Oleic Acid and Propylene Oxides were used as surfactant for the particle coating. Chemicals were used without further purification. All these chemicals were supplied by Scharlu (Germany) which are listed in table 3.1 along with their specification.

Table 3.1:

S.NO

Name of Chemicals

Chemical Formula

Molecular weight

Grade

Name of Company

1

Ferric Chloride Hexahydrate

FeCl36H2O

270.32

Extra pure

Scharlu

2

Cobalt Chloride Hexahydrate

CoCl26H2O

237.93

Extra pure

Scharlu

3

Sodium Hydroxide

NaOH

39.99

Extra pure

Scharlu

4

Ethanol

C2H6O

46.07 g

Extra pure

Scharlu

5

Oleic Acid

C18H34O2

282.4614 g

Extra pure

Scharlu

6

Propylene Oxide

C3H6O

58.08 g

Extra pure

Scharlu

Our main objectives were to prepare the same required sample by two different routes from the same raw chemicals. The materials preparation is a bit different, so we will discuss them separately.

Solid chemicals are weighed on sartorious G.M.B.H analytical balance and transfer to clean and dry flasks. Liquid samples were measured with grade B volumetric flasks, burettes and measuring/transferring pipettes. All the glassware used for the synthesis is best quality borosilicate glass.

3.2 Washing of glassware

The most important task in synthesis of any sample is washing of the glassware. To reduce the undesired impurities glassware should be properly washed. It can be done by using concentrated Nitric acid (NH3) or Hydrochloric acid (HCl). Glassware is first washed with acid and then it is rinsed with water for several times so that all acid is removed. Glassware is then rinsed with double distill water for few times to remove all the undesired impurities and then dried in oven.

3.3 Material Preparation for Coprecipitation

For the coprecipitation method precursors are made by dissolving proper amount of chemicals in double distilled water. A specific ratio of these precursor solutions are mixed in a clean flask or beaker and are intensively stirred to get a homogenous solution of the precursors.

3.3.1 Preparation of Ferric Chloride Hexahydrate (FeCl36H2O) solution

To prepare 0.4M solution of ferric chloride hexahydrate (FeCl36H2O), 108.12g of FeCl36H2O is dissolved in one liter of double distilled water. The solution was shaken for some time so that all the FeCl36H2O are dissolved homogenously in double distilled water.

3.3.2 Preparation of Cobalt Chloride Hexahydrate (CoCl26H2O) solution

0.2M solution of Cobalt chloride hexahydrate (CoCl26H2O) is prepared by dissolving 47.6g of CoCl26H2O in one liter of double distilled water. The solution was shaken for some time to obtain homogenous solution.

3.3.3 Preparation of Sodium Hydroxide (NaOH) solution

3M solution of sodium hydroxide (NaOH) was prepared by adding 120g of NaOH in a clean liter volumetric flask. Doubly distilled water is added till the solution is up to the mark. The flask is vigorously shaken to obtain homogenously dissolved solution of NaOH.

3.3.4 Sample preparation by Coprecipitation method

For the sample preparation by coprecipitation method, first equal amount of cobalt chloride hexahydrate (CoCl26H2O) 0.2M solution and ferric chloride hexahydrate (FeCl36H2O) 0.4M solution was put in a large flat bottom flask placed at hot plate and vigorously stirred for one hour. At the same time sodium hydroxide (NaOH) solution was added drop wise at rate of 4ml/min into the solution till pH value raise to12.11.

At this stage a small amount of Oleic acid (C2H6O) was added to the solution. Oleic acid acts as surfactant for coating of nanoparticles and to optimize the aggregation of the nano particles at the time of synthesis. That solution was heated at reaction temperature 80oC and vigorously stirred for another 2 hrs. Solution turned into dark brown or black precipitates of cobalt ferrite (CoFe2O4). The product was then cooled at room temperature. To remove impurities like chlorine and sodium, precipitates were washed several times with double distilled water and then with ethanol. After washing precipitates were dried in oven at 100oC and left for 24hrs. Sample was then grinded into fine powder of CoFe2O4. The XRD of the sample show that it is amorphous phase of CoFe2O4 which contain 10% water.

3.4 Material preparation for Sol-gel method

Sol-gel method is the most widely used technique for the synthesis of cobalt ferrite. This technique is less complicated than any other synthesis method. For this a proper proportion of chemicals are dissolved in the ethanol and stirred for some tim so that gel is obtained and then this gel is dried according the requirements of final product.

3.4.1 Ferric chloride hexahydrate (FeCl36H2O) and ethanol solution

To prepare 0.4M solution 72.08g of ferric chloride hexahydrate (FeCl36H2O) of was added in 600ml of ethanol. Flask is well shaken so to that FeCl36H2O is dissolved in ethanol.

3.4.2 Cobalt chloride hexahydrate (CoCl26H2O) and ethanol solution

Molar solution of 0.2M is obtained by dissolving 31.7g of CoCl26H2O into 600ml of ethanol. A well dissolved and homogenous solution is obtained.

3.4.3 Sample preparation by sol-gel method

Both 0.4M solution of FeCl36H2O and 0.2M solution of CoCl26H2O are mixed in a large beaker under intensive stirring so that a homogenous mixture is obtained. Then I added 280ml of propylene oxide into the mixture and left for the stirring for 6hrs. However the mixture turned into dark gel in 30minutes only. The mixture was then boiled at 80oC for 12hrs and black or dark brown powder of CoFe2O4 was obtained. The sample obtained was grinned into fine powder with help of motor and piston. The XRD of the sample show that structure was amorphous and no peak was obtained in result.

3.5 Heat treatment

The process of heat treatment can be used to modify mechanical strength and physical properties of the materials. Sample heating and cooling rates can also affect the properties of the materials. Materials are annealed at higher temperature to relive internal stress, which improve the microstructure and mechanical properties like tensile strength, hardness, ductility and toughness etc.

The microstructure of the materials is greatly affected by the heat treatment. Majority of materials transform from amorphous to crystalline materials at temperature higher than room temperature. Cobalt ferrite shows crystallization at temperature higher than 200oC. The crystallinity of CoFe2O4 increases with increase in temperature and highly crystalline CoFe2O4 material is achieved at temperature higher than 600oC 60.

Samples prepared by coprecipitation method and sol-gel method were amorphous. They were filled in separate crucibles and heated to 800oC with heating rate of 10oC per minute. Samples were annealed for 8 hours at 800oC and then they were cooled at cooling rate of 10oC per minute. At this high temperature sample turn from amorphous to crystalline form and its color changes from brown to dark black. Sample was removed from the furnace and grinned to fine powder and filled in separate bottles.

Heating and annealing of sample was carried with help of furnace (model no. LHT04/18) made by Naberthem Germany, and installed in Material Research Laboratory (MRL), University of Peshawar. Furnace heating and cooling rates can be automatically adjusted. Maximum heating range of the furnace is from 30oC to 1800oC.

3.6 Characterization techniques

Following techniques were used to characterize the samples:

Differential Thermal Analysis (DTA) was used to determine phase transformation temperature.

Thermogravenmatric Analysis (TGA) was used to find out changes in mass with temperature.

X-ray Dirractometer (XRD) was used for phase analysis and crystallite size determination

Scanning Electron Microscope (SEM) was used for the shape, size and morphology of the particle.

Energy Dispersive X-ray Spectrometer (EDX) was used to determine the chemical composition.

Atomic Force Microscope was used to study the surface of the particles

Surface Area Analyzer was used to determine surface area and pour size distribution

Fourier Transform Infrared Spectroscopy was used to determine group of materials.

3.6.1 Differential Thermal Analysis (DTA)

Differential thermal analysis (DTA) is a well-established tool to study the phase transformation temperature of different materials. In this technique we study variation in a property as function of externally applied and programed temperature. The term also stands for variety of techniques that used the study of temperature dependent reactions which cause variation in physical and chemical properties of substance. These dependent reactions are either endothermic or exothermic. Endothermic reactions are boiling, melting, vaporization, sublimation, desolvation, chemical degradation, solid-solid phase transition, etc. and exothermic are crystallization, oxidative decomposition and etc. These reactions either raise or lower the temperature of the specimen. The differential changes in temperature of the specimen are plotted against the temperature. The curve obtained is called thermogram 61,62.

In this technique temperature variation of the material specimen and an appropriate reference material is measured under controlled condition as a function of temperature. Different reaction i.e. exothermic and endothermic reaction will cause variation in the temperature of the material where the temperature of the specimen remains the same. These temperature variation cause change in the enthalpy (ΔH) which is positive for endothermic reaction and negative for exothermic reaction 62.

Material specimen and reference material are placed in a furnace, whose temperature is controlled with computer software. Temperature variation are detected with thermocouples and displayed in the computer software which record the data electronically. If the temperature of the specimen leads the temperature of the reference material it will be an exothermic reaction. On other hand if the sample temperature lags behind the reference temperature it will be an endothermic reaction.

DTA analysis was carried at Centralized Research Laboratory (CRL) university of Peshawar. Both sample and reference material were placed in the aluminum pan and heated at heating in Diamond TG/DTA made by Perkin Elmer, USA. The heating rate of the furnace was 10 degree centigrade per minute (10oC/min) in temperature ranging from 30-1300oC.

Figure 3.1: Schematic of Differential Thermal Analyzer 62

3.6.2 Thermal gravimetric Analysis (TGA)

Thermal gravimetric analysis (TG) is the technique which records the change in the weight or mass of materials as a function of time, either at a finite and fixed temperature or over a range of temperature at fixed heating rate. The thermal gravimetric analysis is the process in which material specimen is heated at high enough temperature so that a component of the material decomposed into gas and evaporates. It is commonly used to determine characteristics of the materials like degradation temperature, absorbed moisture contents, oxidation temperature etc 61,62. The curve also indicate the temperature at which sample should be treated to obtain heat resistive and stable material.

In TG analysis specimen is placed on an alumina or platinum pan. The pan mass is measured by a sensitive balance which detect very small variation in the mass. The Cahn balance design is shown in Figure 3.2

Figure 3.2: Example of TG design 63

The balance is designed such that it can control the atmosphere which is accomplished by gases released by the specimen. For this purpose a thermocouple mounted very close to the pan. The chamber containing the balance is often compressed air or inert gas, which help to protect the balance chamber and its associated electronic circuitry from gas diffusion 61-63.

TG analysis was carried out using Diamond TG/DTA made by Parkin Elmer, USA. Sample were heated in alumina pan at heating rate of 10 degree centigrade per minute (10oC/min) in temperature ranging from 30-1300oC.

3.6.3 X-ray Diffraction and Crystal Structure Analysis

X-rays are generally defined as the electromagnetic radiation with wavelength in range of 0.1-1Å. These radiations are more energetic than ultraviolet (UV) radiations. Its energy ranges from 10-100keV. These radiations were accidently discovered by Wilhelm Conrad Rontgen in Germany in 1895. This discovery brings revolution in modern physics. X-rays are of two types, Continuous and characteristic x-rays. The later one is used for the characterization of the materials. In year 1912 Von Laue discovered the phenomenon of x-rays diffraction by crystal.

When a beam of X-ray encroaches a crystalline solid material and interact with electrons or ions of the materials, the rays of the beam are scattered in all directions. Figure 3.1 shows direction of x-rays by different atomic planes of crystal as shown.

C:\All Data\Mphil thesis\Thesis(ch+ref)\Thesis\Braggs.png

Figure 3.3: Diffraction of X-rays by atomic Planes of crystal 64

Two rays of incident beam are shown in the figure, which are scattered from two different planes. Lawrence Bragg provides a simple mathematical form of diffraction condition. This is known as Bragg's Law. According to Bragg's Law these rays will produce Bragg's peak if they interfere constructively. For the constructive interference their path difference must be integral multiple of wavelength of the radiation 65.

1

In this equation is the wavelength of the incident ray, n is the order of reflection and an integer, is the angle between incident and scattered beams where is inter planer spacing.

Bragg's law allows a precise measurement of inter-planer distance which can be used to determine the lattice parameters. For cubic crystal cell lattice parameters are equal and lattice angle are 90o.

and

2

Most of solid materials have crystalline structure. When they are subject to x-rays, at some particular angles x-rays are reflected with maximum intensities. This gives a pattern in which intensities of the reflected x-rays are plotted against the 2θ value. The peaks observed in this pattern correspond to various crystal planes. According to Bragg's law, peaks for large d-spacing is observed at small angles where for small d-values peaks are observed at large scattering angles 65.

3.6.3.1 Phase Identification or Qualitative Analysis by XRD

The data recorded from the x-rays diffraction can be used to identify different phases of the materials. A phase of material can be defined as the region in which chemical composition and physical properties are uniform. Most of crystalline materials have single or multi phases. Each phase have unique diffraction pattern which is used to identify it.

Since early decades of 20th century, scientists had recorded large number of standard diffraction patterns. That database includes more than 550,000 patterns of different materials. International Center for Diffraction Data (ICDD) founded Joint Committee for powder Diffraction Studies (JSPDS) in 1941. It gives a comprehensive set of database with name of Powder Diffraction File (PDF) which is updated annually and includes information about d-spacing, relative intensities, lattice perimeter etc 66.

For the phase identification of materials, its x-rays diffraction data sample is compared with the JSPDS (Joint Committee for Powder Diffraction studies) data. The values of d-spacing and intensities obtained are compared with d-value and intensities given in JCPDS card. Thus different phases of materials are identified on the basis of well-matched values of powder pattern and cards.

3.6.3.2 Crystallite Size Determination

The data obtained from the x-rays diffraction of the material can also be used to determine the size of crystallite. For the particle having crystallite size less than 10Å give broaden diffraction peaks are observed. The term particle size is usually used for the crystallite size in this range. The crystallite size is given by scherrer equation 65,67:

3

where λ is the wavelength of x-rays, β is the full width at half maximum (FWHM) of the peak in radians, θ is the Bragg angle, C is a constant with value ranging from 0.9-1.0 depending on crystallite shape and d is the crystallite size.

X-ray analysis was carried by JEOL-JDX-3532 Diffractometer with Cu Kα-radiations (λ=1.54Å) source fitted with Ni-filtered. X-ray tube was operated at 30kV with tube current 30mA. Sample was scanned in the range of 10o-70o at step angle of 0.02o and scanning speed of 1o per minute. The time taken by each step was 1.2sec. X-ray was supported with computer program loaded with ICDD x-rays diffraction files PDF-2. Phases were identified using full data search match software.

3.6.4 Scanning Electron Microscope (SEM)

Scanning electron microscope is one of the most sophisticated instruments available for the microstructural characterization and analysis of the materials. It uses high energy electrons rather than ordinary light to form an image. When high energy electron interacts with specimen it generates variety of different signals. Such as x-rays, Auger electron, secondary electrons, back scattered electrons and cathode luminescence. These signals are used to study morphology, topography (Surface studies) and compositional analysis. SEM has high resolution, large depth of focus field and provides three dimensional imaging. It has high magnification which makes the instrument useful for the material research 68.

The fundamental principles of Scanning Electron Microscope (SEM) were developed in the 1930. In 1938, Von Ardenne built first instrument which was later developed by Zwerykinetal in 1942 with resolution of 500Å. The modern type of the instrument has been constructed in Cambridge by Otale and Mc Muller (England 1948). Nixon and his colleague, extanded and work of Otale in 1959 and developed electron microscope with better contrast. In 1966, Cambrige Science Instrument Co. announced the word's first commercial SEM 69.

Scanning Electron Microscope has many sophisticated components. All these components should be in proper function for accurate analysis of the sample under investigation. Control console and electron optical column are the two main parts of the SEM.

Control console is the main unit of SEM to control functions of various components e.g. magnification, resolution, electron gun, electromagnetic lenses and image contrast. In modern SEM most of the functions of components are controlled through computer software.

An electron optical column for the SEM consists of electron gun and electromagnetic lenses and scan coils. Two common types of the electron guns used in SEM to generate electrons are thermionic Emission Gun and Field Emission Gun. In thermionic emission electron are generated by passing high curnt through filament. These electrons are accelerated from cathode to the specimen by application of high voltage ranging from 1kV to 30kV 68. The filament is surrounded by a grid cap, or wehnelt cylinder with circular aperature centered at the filament apex. The grid cap is biased negatively between 0-500 volts with respect to the cathode. The electric field produced in such arrangement converge the emitted electrons. While in field emission gun, electrons are generated by applying high negative voltage to cathode about 3000V across the filament.

Figure 3.4: Scanning Electron Microscope 70

Electron beam is converged by passing through electromagnetic lenses (made of two electromagnetic pole pieces) similar to optical lenses used to converge beam of light. This converged electron beam is then allowed to interact with material specimen placed at the bottom of the column. Air of the column is removed by a powerful vacuum pump.

The electron beam scans across the specimen surface and generate different signals as discussed earlier. These signals are detected by different detectors and converted to electrical signals. These electrical signals are amplified and electronically recorded as image of the specimen. The electron bombardment can cause heating and excess negative charge on the specimen which affects the image quality particularly in case of nonconductive materials. In case of nonconductive material, excess negative charges and heat is removed by conductive coating of specimen. Coating also increase mechanical strength of specimen and increase back scattered electrons 68.

Scanning Electron Microscope (SEM) JSM5910 made by JEOL was used for SEM images. Its operating voltage is 30kV and maximum magnification is 300000X and resolution of 2.3nm.

3.6.5 Chemical Analysis of Surface Composition by EDX

The most important part of material characterization is the chemical analysis. Micro-constituent of the material are identified through different characterization techniques. Such as by X-ray scanning of the materials, atomic absorption spectroscopy, optical emission spectroscopy, infrared spectroscopy, Reman spectroscopy, Electron spin resonance spectroscopy, fluorescence spectroscopy and Rutherford back scattering, photoelectron spectroscopy, Auger electron spectroscopy and secondary ion mass spectroscopy. The above mentioned techniques are widely used in the fields of solid state material and surface sciences.

Energy dispersive x-rays spectroscopy (EDX) is the most commonly used technique for the elemental analysis or qualitative analysis of the materials. Materials are subjected to high energy charge particles like electron, proton or high energy x-rays. These incident radiations knockouts the inner shell electrons or excite it to the higher energy levels, leaving a hole in the inner shell. The electron transition to fill this hole from higher level gives an x-ray photon of definite energy. These x-rays photon or characteristic x-rays are used to identify the element present in material for the qualitative analysis. This technique is also used to carry the quantitative analysis of the material specimen. The number of photon of particular wavelength crossponds to the number of atoms of a particular element present in specimen 68,69.

The microstructure of samples was studied with help of X-rays Energy Dispersive Spectrometer (EDX-INCA 200, Oxford Instrument, U.K) installed in Centralized Resource Laboratory (CRL), Department of Physics, University of Peshawar. The EDX is capable to analyze elements ranging from Boron to Uranium on periodic table. In terms of Energy, resolution of EDX is 133eV.

3.6.6 Atomic Force Microscope (AFM)

Atomic force microscopy is the topographical studies of the surface of the material specimen. In this technique a sharp tip of cantilever is brought near to the surface of specimen so that it touches it and can senses the interatomic forces acting near the surface. Tip of AFM observe repulsive force it touches the sample but at small distance it senses attractive force due to the Van-der-Walls attractive forces.

It was first designed to measure the roughness of the sample. A beam of laser is focused on the reflective cantilever, which deflect the beam into a position sensitive array of photodiodes detector. The cantilever tip motion on the surface of the sample effect the reflected angle and position of the laser beam. This assembly is used to scan the surface of sample and a computer software sketch the image of the specimen.

Figure 3.5: Atomic Force Microscope 69

AFM has advantage over the SEM that is does not require any conductive coating of the sample. It has high resolution and low cost in comparison of SEM69.

3.6.7 Surface Area Analyzer

Surface area of the solid is very important to understand the behavior of the solids. It can affect catalyst activity, adsorption capacity and processing of most powder and porous materials. Surface area mainly increases with decrease in particle size. Some physical and chemical changes such as dissolution or decomposition create pores in the particle. It can also increase the surface area 71.

When solid particles is exposed to a gas, the gas molecules adsorbed on the solid particles. The adsorbed amount depends on the nature of absorbent and pressure. This adsorption can be physical or chemical. In physical adsorption gas molecules connect their selves due to some physical phenomena while in chemical adsorption; molecule attached their self with particle through chemical reaction. The physical adsorption in which gas molecules made a layer around the particle is more important to determine surface area of the particle.

The graph obtained by plotting the gas adsorbed at constant temperature against the pressure is called adsorption isotherm and depends only on shape of the particle. A number of theories are developed to explain the adsorption process and isotherms. The first attempt was made by Langmuir for monolayer adsorption71.

Two methods are developed for calculating surface area and pore size. These methods are Brunauer-Emmett-Teller (BET) and Barret-Jouner-Halenda (BJH).

3.6.7.1 Brunauer-Emmett-Teller (BET)

Langmuir theory was further developed by Brunauer, Emmett and Teller by consideration of multilayer adsorption. The multilayer adsorption theory is known as Multilayer BET theory. According to BET first layer attached itself with the particle by dipole interaction. The orientation of the first layer dipoles provides a base for the second layer adsorption. In this way multilayers adsorption take place. It is most widely used technique for the determination of the surface area and pore size71,72.

3.6.7.2 Barret-Jouner-Halenda (BJH)

This model was developed by Barret, Joyner and Halenda to calculate pore volume and radius. They assume that thickness of the residual layer on the plane surface and the pores is the same. Thickness of the layer can be change with pressure of the specimen which help to determine the volume of the pore 71,72.

Surface area analyzer (Model No: NOVA2200e) installed in CRL, UOP, made by Quantachrome, USA, was used to determine the surface area and pore size of the specimen under vacuum heating and nitrogen adsorption 73.

3.6.8 Fourier Transform Infrared Spectroscopy (FTIR)

Fourier transform infrared spectroscopy (FTIR) is a powerful technique to see inside the structure of the material molecules and atoms. It can be used to identify molecules and atoms by the absorbed resonant frequencies corresponding to different rotational and vibrational energy levels. The bend of frequencies absorbed by the material are clearly indicated as absent or in the spectrum. For the fine nanoparticles broadening is observed in the missing band 63,74.