chapter ii theoretical background -...

Chapter II Theoretical background

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2.1 Fundamentals of Magnetism

The magnetic properties arise mainly due to the electrons present in the

atom/materials, which have small magnetic moment by virtue of their motion.

Nucleus also has a small magnetic moment, but it is insignificant to that of the

electrons and it does not affect the gross magnetic properties. An electron can

contribute to the magnetic moment in two ways: the electron spin and the orbital

momentum [1-3]. The magnetic field resultant from electron spin is dependent on the magnetic quantum number ‘m’, whereas orbiting electrons create

magnetic fields around the atom. In general, the net magnetic field from orbital

momentum of the electrons is zero. Consequently, the net magnetic field from an

atom comes from the electronic spin. Spin is a universal property of electrons in

all states of matter at all temperatures. The electrons behave as if they were

spinning about its own axis, as well as moving in an orbit about the nucleus and

associated with this spin are definite amounts of magnetic moments and angular

momentum. The magnetic moment due to electron spin is equal to,

(2.1)

where e is the charge on the electron, h is Planck's constant, m is the mass of an

electron and c is the velocity of light. Substituting all the values in above

equation, the magnetic moment due to the spin and orbital motion of electrons

are found to be equal to 9.27 × 10-21 erg/Oe. Because it is such a fundamental

quantity, this amount of magnetic moment is given a special symbol µB and is

called as Bohr magneton.

It is well known that a bulk magnetic material consists of many magnetic

domains, and the magnetic properties are determined by the formation, structures

and movements of these magnetic domains under a variation of temperature or

magnetic field. In bulk material, the magnetic behavior is influenced by domains

and domain walls. Magnetic domains are regions in a crystal where the magnetic

moment orientation is different but aligns with the axis and each domain is

separated by a thin domain wall [4, 5]. Adjacent domains are separated by

domain boundaries or walls across which the direction of magnetization


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gradually changes. Normally domains are microscopic in size, and for a

polycrystalline specimen, each grain may consist of more than a single domain.

Thus, in a macroscopic piece of material, there will be a large number of

domains, and all may have a different magnetization orientation. The magnitude

of the magnetic field for the entire solid is the vector sum of magnetizations of

all the domains. The domains are formed in order to reduce the overall

magnetostatic energy of the system and are separated from one another by

domain or Bloch walls which are high energy areas defined as transition layer

that separates adjacent regions magnetized in different directions. The presence

of this domain walls and their mobility both reversibly and irreversibly are

directly responsible for magnetic hysteresis loop. Since the response of a

material to a magnetic field (H) is characteristic of the magnetic induction or the

flux density (B) and the effect that a material has upon the magnetic induction in

a magnetic field is represented by the magnetization (M). Thus, a universal

equation relating these three magnetic quantities, magnetic field, magnetic

induction and magnetization, can be established by

B = µo (H + M) (2.2)

where µo is a universal constant of magnetic permeability in a free space.

From equation (2.2), one can see that µoH is the magnetic induction generated

by the field alone and µoM is the additional magnetic induction contributed by a

material.

When a material is in the presence of a magnetic field, the permanent

magnetic dipoles may interact with the field, either contributing or reducing the

field within the material, causing a change in the overall inductance, which can

be shown as,

B = µ H (2.3)

Where µ is the permeability of a material in the applied field.


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2.2 Types of magnetism

Magnetic materials are classified in terms of their magnetic properties and

their uses. If a material is easily magnetized and demagnetized then it is referred

to as a soft magnetic material, whereas if it is difficult to demagnetize then it is

referred to as a hard (or permanent) magnetic material. The two most common

types of magnetism are diamagnetism and paramagnetism, which account for the

magnetic properties of most of the periodic table of elements at room

temperature. From a magnetic point of view the solids can also divided in two

categories (Fig. 2.1). The first includes the materials which do not exhibit any

spontaneous magnetization in the absence of an external field. The second group

is characterized by the spontaneous alignment of the magnetic moments. These

are the ferromagnetic and the antiferromagnetic materials. Finally, magnetic

materials can also be classified as ferrimagnetic although this is not observed in

any pure element but can only be found in compounds, such as the mixed oxides,

known as ferrites, from which ferrimagnetism derives its name.

All magnetic materials may be grouped into five magnetic classes,

depending on the magnetic ordering and the sign of magnitude, temperature

dependence of the magnetic susceptibility and how they interact with fields

when they are placed in magnetic field. So there are five types of magnetism

exhibited by various materials as follows [6-9].

Fig. 2.1 Alignment of magnetic moments in different magnetic materials

a) ferromagnetism, b) and c) antiferromagnetism and d) ferrimagnetism.


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2.2.1 Diamagnetism

Diamagnetic materials have a weak, negative susceptibility to magnetic

fields and the relative permeability is slightly less than 1. Diamagnetic materials

are slightly repelled by a magnetic field and the material does not retain the

magnetic properties when the external field is removed. In diamagnetic materials

all the electrons are paired so there is no permanent net magnetic moment per

atom due to nullifying effect of orbital and spin angular moments. Diamagnetic

properties arise from the realignment of the electron orbits under the influence of

an external magnetic field. Consequently, when a diamagnetic material is placed

in a magnetic field, the induced magnetic moments oppose the applied field and

B < µo H. Ionic crystals and inert gas atoms are diamagnetic.

2.2.2 Paramagnetism

Paramagnetic materials have a small, positive susceptibility to magnetic

fields which varies inversely with temperature. There is a net magnetic moment

from electron spin. However, the individual atoms do not interact, and hence,

require large magnetic fields to orient the dipoles. When a paramagnetic material

is placed in a magnetic field, the magnetic moments experience a torque and

they tend to orient themselves in the direction of the magnetic field due to which

material gets slightly attracted by a magnetic field and does not retain the

magnetic properties when the external field is removed. Paramagnetic properties

are due to the presence of some unpaired electrons, and from the realignment of

the electron orbits caused by the external magnetic field. Alkali metals and

transition metals are examples of paramagnetic materials.

2.2.3 Ferromagnetism

Ferromagnetic materials have a large, positive susceptibility to an

external magnetic field. They have very large internal field. They exhibit a large

spontaneous magnetization and are able to retain their magnetic properties after

the external field has been removed. Ferromagnetic materials have some

unpaired electrons so their atoms have a net magnetic moment. They get their

strong magnetic properties due to the presence of magnetic domains. In these

domains, large numbers of atomic moments (1012 to 1015) are aligned parallel so


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that the magnetic force within the domain is strong. When a ferromagnetic

material is in the unmagnetized state, the domains are nearly randomly organized

and the net magnetic field for the part as a whole is zero. When a magnetizing

force is applied, the domains become aligned to produce a strong magnetic field

within the part. Ferromagnetic materials get their magnetic properties not only

because their atoms carry a magnetic moment but also because the material is

made up of small regions known as magnetic domains in which all the magnetic

moments are aligned. In each domain, all of the atomic dipoles are coupled

together in a preferential direction. Ferromagnetic materials become magnetized

when the magnetic domains within the material are aligned. This can be done by

placing the material in a strong external magnetic field or by passing electrical

current through the material. Some or all of the domains can become aligned or

some domains magnetized in one direction and some in another. The more

domains that are aligned, the stronger the magnetic field in the material. When

all of the domains are aligned, the material is said to be magnetically saturated.

When a material is magnetically saturated, no additional amount of external

magnetization force will cause an increase in its internal level of magnetization.

When the applied magnetic field is removed some of the domains lose

their orientation, but the material does not return all the way to a random

configuration. As a result it retains some magnetic properties; it has become a

permanent magnet. The magnetic properties of these materials can be described

by plotting a hysteresis loop for the magnetization, M, of the material as a

function of the applied magnetic field, B. The ferromagnetic susceptibility of a

material is quite temperature sensitive which decreases with increase in

temperature. But above a critical temperature known as the Curie temperature,

the material ceases to become ferromagnetic, and it becomes merely

paramagnetic. Iron, nickel, and cobalt are examples of ferromagnetic materials.

2.2.4 Antiferromagnetism

Antiferromagnetic materials have small positive susceptibilities at all

temperatures. Materials in which the atoms, ions or molecules have a permanent

dipole moment (resulting from unpaired electron spins), as in paramagnetic and


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ferromagnetic materials, but alternating ions within a domain have their

magnetic moments oriented in opposite directions, so the domain as a whole has

zero magnetization i.e. the interaction between neighbouring magnetic moments

may lead to an antiparallel alignment which results in vanishing the moments at

0K. Examples of an antiferromagnetic material are MnO, CoO, NiO, MnS, and

FeO etc. Such materials are generally antiferromagnetic at low temperatures. As

the temperature is increased, the domain structure breaks down and the material

becomes paramagnetic. A critical temperature in this case is called Neel

temperature. Below the Neel temperature the susceptibility generally decreases

with decreasing temperature. There is no spontaneous magnetization in

antiferromagnetic materials.

2.2.5 Ferrimagnetism

Those materials which exhibit spontaneous magnetization due to

antiparallel alignment between two magnetic sublattices but the resultant

magnetic moment do not vanish. Hence ferrimagnetic materials have non-zero

magnetization below the Curie temperature which is similar to ferromagnetic

materials. Hear alignment of spin is antiparallel but with in unequal numbers in

the two orientations and hence a net magnetic moment results. This

magnetization arises due to two main reasons,

i) The two sublattices are occupied by different types and different number

of magnetic ions and

ii) The two sublattices in two different crystallographic sites are occupied by

either same or different type of different number of magnetic ions.

Above a certain critical temperature, i.e. ferrimagnetic Curie temperature,

ferrimagnetic material becomes paramagnetic. As the magnetic properties

depends upon the interaction between the electrons associated with metal ions, in

these materials the neighbouring atomic magnetic moments becomes locked in

antiparallel with their neighbours. However, the magnetic moments in one

direction are weaker than the moments in the opposite direction leading to an

overall magnetic moment. Another difference between ferrimagnets and

ferromagnets is that in ferrimagnetic materials the saturation magnetization


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against temperature behaves in a more complicated way. For example, for some

ferrimagnets the magnetization can increase with increasing temperature and

then drops down. Fe3O4 is one of the most famous examples of ferrimagnetically

ordered solid.

2.3 Magnetic ordering in spinel ferrites

As far as the magnetic ordering in spinel ferrites is concerned, there are

three major superexchange interactions, jAB

, jAA

, and jBB

in spinel ferrites [10, 11]

Since the metal cations in spinel ferrites are mutually separated by larger oxygen

anions and hence the cation-cation distances are large in ferrites without net spin

in their crystal structure, direct exchange interactions are negligible. The

exchange forces between the metal ions in a ferrimagnetic material will act

through the oxygen ions by means of the indirect exchange mechanism, which is

known as the superexchange interaction [12], becomes strong enough to order

the magnetic moments. The major interaction that occurs in ferrites is the

superexchange interaction between octahedral and tetrahedral cations i.e. A-O-B

interactions [13-15]. The next acceptable interaction is B-O-B superexchange.

However A-O-A interaction is not coming into picture, as it is very weak. The

types of interactions in ferrite and the angle between them are shown in Fig. 2.2

schematically. The magnetic moments for all metal cations in A sites are

orientated parallel with respect to each other and the magnetic moments for all

cations in B sites are aligned parallel with one another as well. The magnetic

moment orientation of cations between A and B sites, however, is antiparallel to

each other in spinel ferrite. As there are twice as many of B sites as A sites, a net

magnetic moment results. Therefore, the magnetic structure of spinel ferrite is

ferrimagnetic ordering. Magnetization in ferrites occurs from the uncompensated

antiferromagnetism, so the magnitude of magnetization depends on composition,

cation distribution and the relative strength of the possible interactions. The

strength of exchange interactions controls the saturation magnetization and the

Curie temperature of the ferrites and this exchange interaction is controlled by

cation distribution. In addition, the superexchange interaction is also strongly


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dependent on the geometry of arrangement such as distance and angles of

cations in A and B sites.

Fig. 2.2 Different types of interactions for different types of

lattice sites in ferrite

2.4 Factors influencing magnetic properties

2.4.1 Microstructure

Microstructure refers to the microscopic structure of solid materials. This

is an important parameter for ferrites. For the better performance parameters and

properties, uniform microstructure is an essential condition. It means all the

grains should be of same size and minimum porosity. Microstructural aspects of

ferrites have always some special interest, such as to attain proper saturation, to

minimize anisotropy, to minimize magnetostriction and to avoid foreign ions

that can strain the lattice. There are several conditions maintained to get proper

microstructure for better properties, some of them are variation of sintering

conditions, additives, etc. In 1977, Igarashi et al. put forward the following

relationship from his experimental findings [16].

µ α D1/3 (2.4)

Where, D is the diameter of a grain.

2.4.2 Composition and cation distribution

The magnetic properties of spinel ferrites are greatly influenced by

composition and cation distribution. Variation of the cation distribution between

the cationic sites leads to different magnetic properties even if the composition


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of the spinel is the same. For example, a blocking temperature change of as

much as 50°C has been observed in MnFe2O4 nanoparticles with a 29%

inversion in distribution of cations [17]. When comparing similar systems with

different composition such as CoFe2O4 and MgFe2O4, there is always a large

difference in the blocking temperature (~150K) that can be attributed to the spin-

orbital coupling of the cations as well as superparamagnetic properties. While

there are three unpaired d electrons present in Co2+, all of the electrons are paired

in Mg2+. So while Co2+ cations have a large spin-orbital coupling, the paired

electrons of Mg2+ do not provide any contribution to the electron spin. Magneto

crystalline anisotropy arises from spin-orbit coupling [18-20]. If we relate the

spin-orbit coupling factor to the Stoner-Wohlfarth theory, there would be an

increased energy barrier. As a result, a larger blocking temperature is required to

overcome this large anisotropy energy barrier. Hence, the influence of cation

distribution and chemical composition can greatly influence the tunability of the

magnetic properties of spinel ferrites.

2.5 Electrical properties

2.5.1 Basic science for conductivity

The materials are classified into three types viz. conductor, semiconductor

and insulator. If a partially filled energy band lying just above completely filled

band in a material, it will show metallic conduction. If the band is completely

full, the applied field cannot impart any change in electron movement as no

empty state is there and therefore cannot accommodate the electron with

changed energy. The next empty band lies far above and cannot be reached by

the electrons of filled band. Such material will be insulators. The bands are

separated by an energy gap, called band gap (or forbidden band i.e. Eg)

Eg = Ev−Ec (2.5)

Where, Ev is valence band, Ec is conduction band. If the forbidden energy

gap is narrow, at temperature T > 0K, it may be possible for some electrons from

valence band to have sufficient thermal energy to jump into higher empty band.

As a result, movement of charge carriers is possible because of availability of


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empty states. This type of material is called as semiconductor [21, 22]. Partially

filled band can also result from the overlap of completely filled band with an

empty band. The formation of energy bands in metal, insulator and

semiconductor are shown in Fig. 2.3

Fig. 2.3: The band structure and Fermi level of a) a conductor,

b) an insulator and c) a semiconductor.

Semiconductors are mainly classified into two groups

1) Intrinsic semiconductor and

2) Extrinsic semiconductor

Intrinsic Semiconductor

When the energy band gap is sufficiently narrow, some of the electrons

occupying states at the top of the valence band may gain sufficient thermal

energy to transfer to empty states in conduction band. Such electrons can

contribute to conductivity of the material, because of the temperature at which

conductivity become appreciable depends on the crystal structure; such crystals

are properly called intrinsic semiconductors. The density of conduction electrons

in a semiconductor increases with temperature so that its conductivity also

increases.

Extrinsic semiconductor

Crystals are never perfect and usually contain some foreign atoms, which

may be present in substitutional or interstitial solid solution. These atoms have

valence electrons, which are bound to their nucleus by force differing from those

binding such electrons in the other atoms. In terms of band model, this means


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that there are energy levels present in the crystal which differ in energy. If the

electrons occupying this energy level can contribute to conductivity in a crystal,

then such a crystal is called an extrinsic semiconductor. If the substitutional

impurity atoms have five or more valence electrons, they are said to ‘donor’

(having excess of electrons) of the crystal. It increases the concentration of

electrons in the conduction band without generating any extra holes in the

valence band. Since the electron concentration is greater than hole concentration,

the former become the majority carrier. The energy of these levels is usually

somewhat less than the energy level at the bottom of the conduction band,

electrons being the majority carrier, it is called as n-type semiconductor.

On the other hand, if the impurity atoms have three or less valence

electrons, they are said to be ‘accepter’. The energy level so called acceptor level

is usually slightly higher than the level at the top of valence band. An electron

from the valence band can be easily excited to this localized level leaving a hole

in the valence band. This itself does not generate any electron in the conduction

band. The majority carriers are holes. These types of extrinsic semiconductors

are known as p-type semiconductors. Fig. 2.4 shows the band diagram of the

different types of semiconductors. When a semiconductor is doped with donor or

acceptor impurities, impurity energy levels are introduced. The conductivity of

doped semiconductors is then much higher than that observed for intrinsic

semiconductor.

Fig. 2.4: Band diagram for (a) n-type semiconductor and

(b) p-type semiconductor


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2.5.2 Conductivity in spinel ferrite

The conductivity mechanism in spinel ferrites is quite different from that

in semiconductors. In ferrites, the temperature dependence of mobility affects

the conductivity and the carrier concentration is almost unaffected by

temperature variation. Spinel ferrites in general are semiconductors with their

conductivity values varying between 102 and 10-11 Ω-1·cm-1. The low

conductivity is associated with the simultaneous presence of Fe2+ and Fe3+ ions

on equivalent lattice sites i.e. usually the octahedral sites. The presence of Fe2+

results in n-type behaviour. The conductivity arises due to the mobility of extra

electrons on ferrous ion which requires little energy to move to a similarly

situated adjacent ferric ion through the crystal lattice. The valence states of the

two ions are interchanged. The movement is described by hopping mechanism,

in which the charge carriers jump from one ionic site to the next site under the

influence of an electric field [23-25]. The hopping probability depends upon the

activation energy, which is associated with the electrical energy barrier

experienced by the electrons during hopping.

In many cases the slope of log·ρ vs 1/T plots changes at certain

temperature i.e. at Curie point. According to Verwey et al. [26], the conductivity

of high resistivity oxides is due to hopping effect which can be increased by the

addition of small amount of constituents to the structure. The presence of Fe2+

ions is sometimes desirable as it reduces magnetostriction effect and resistivity.

Most of the spinel ferrites are semiconductor and their resistivity ρ decreases

with increase in temperature according to the Arrhenius relation,

ρ = ρo exp (2.6)

Systematic experimental investigation of the electrical properties of

oxidic spinels allows one to place them among controlled valence semi-

conductors as described by Verwey. Further works by Morin, [27] Johnston and

Heikes [28], Jonkar [29], Holestein [30] have helped to elucidate conduction.

The mechanism of electrical condition in these oxides involves an electron

transfer process in which the charge carriers hop from one site to other site.


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2.5.3 Factors affecting the conductivity in ferrites

The electrical properties of ferrites are affected by the distribution of

cations in the sites, type and amount of dopant, by the amount of Fe2+ present,

sintering condition, grain size and grain growth parameters [31-33]. The

resistivity of the ferrites shows an exponential dependence on temperature and in

many cases the slope of the ln σ vs. 1/T plots changes at certain temperature

called Curie temperature. Hear activation energy is changing from ferrimagnetic

to paramagnetic region. This anomaly strongly supports the influence of

magnetic ordering upon the conductivity process in ferrites. Also the

temperature dependence of the conductivity arises only due to the mobility and

not due to the number of charge carriers in the sample. When these impurities

are added to the ferrite in small amounts they do not form a solid solution at all,

or otherwise form a solid solution which is not homogeneous. They tend to

collect in the grain boundaries and form a highly resistive substance in it. [34]

The grain size, grain boundaries and porosity are important factors in the

microstructure, which influence the electrical properties of ferrites. In addition to

the above considerations, the activation energy is also influenced by the grain

size. Bigger grain size implies increased grain-to-grain contact area for the

electron to flow, and therefore, a lower barrier height. Since the grain size is

known to increase with sintering temperature [35, 36.], the activation energy is

expected to decrease. At higher sintering temperature, it is obvious that there is

more densification or less porosity. Due to the reduced porosity, the individual

grains come closer and an effective area of grain-to-grain contact increases [37] In a number of ferrites tetragonal distortions from cubic spinel structure exists

due to the presence of Jahn-Teller ions (such as Mn3+ and Cu2+) especially at the

octahedral site. This distortion in spinel structure affects the distance between

the neighbouring Fe2+ and Fe3+ ions and hence the conduction process of the

hopping electrons is also affected. Mazen et al. [38] have found that the

activation energy changes at the transition of tetragonal to cubic phase in copper

ferrite.


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2.6 Transport properties

Seebeck (1821) found that an electromotive voltage is established in a

circuit consisting of two conductors made up of different materials, if the

junctions of these conductors are kept at different temperature T1 and T2. This

voltage is termed as thermal emf. The emf difference depends upon the nature of

the solid under consideration, the temperature difference between the two ends

and the ambient temperature at which the solid is maintained. Experiments show

that in a narrow temperature interval, it is proportional to the difference in the

temperature of the junctions A and B is called differential or specific

thermoelectric power.

VT = α (T2 – T1) (2.6)

The sign of (α) depends upon the nature of the majority carriers (α) is

positive for holes and negative for electrons.

There are three sources of the thermal emf

i) The directional current of the carriers in the conductors, due to the

presence of a temperature gradient (the volumetric component),

ii) The change in the position of Fermi level (the junction component) and

iii) The drag of the electrons by the phonons (phonons drag effect).

Suppose that a temperature difference (T2-T1) is maintained at the

terminals of a uniform conductor so that there is a temperature gradient dT/dX.

Electrons in the hot region are more energetic and therefore have greater

velocities than those in the cold region. Consequently there is a net diffusion of

electrons from the hot end toward the cold end which leaves behind exposed

positive metal ions in the hot region and accumulates electrons in the cold

region. This situation prevails until the electric field developed between the

positive ions in the hot region and the excess electrons in the cold region prevents

further electron motion from the hot to cold end. A voltage is therefore developed

between the hot and cold ends with the hot end at positive potential. The potential

difference dV across a piece of material due to a temperature difference dT is

called Seebeck effect.


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The differential thermoelectric power corresponding to this component is

expressed as

α = dV /dT (2.7)

As a rule, in n type conductor α is directed from the hot end to the cold

end.

A temperature difference between two points in a conductor or

semiconductor results in a voltage difference between these two points. Stated

differently, a temperature gradient in a conductor or a semiconductor gives rise to

a built-in electric field. This phenomenon is called the Seebeck effect or the

thermoelectric effect. The Seebeck coefficient gauges the magnitude of this

effect. The thermoelectric developed per unit temperature difference in a

conductor is called the Seebeck coefficient. Only the net Seebeck voltage

difference between different metals can be measured. The principle of the

thermocouple is based on the Seebeck effect.

Hall effect and Thermoelectric properties are widely used in the

interpretation of the conduction mechanism in semiconductors. However in case

of low mobility ferrites, it is sometimes difficult to measure the Hall effect as the

ferrites are not a band type semiconductors and the conduction takes place due to

hopping of electrons or holes. In such a case the measurements of the

thermoelectric power is the only alternative. The sign of the thermo-emf gives

the vital information about the type of conduction in the ferrite i.e. whether it is

p-type or n-type. The substitution of cations of the low valence state gives rise to

p-type of conduction while the substitution of cations of high valence state to n-

type of conduction [39].

2.7 Catalysis

In metal oxides the cations and the anions are surrounded by each other

leading to an ordered long range bulk structure, which is largely determined by

the stoichiometry. Metal oxides are widely used as catalysts as well as catalyst

supports. The surfaces are more complex in their structure and are highly

heterogeneous. Metal oxide surfaces exhibit both basic and acidic characters,


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based on their composition, which is important for some reactions in catalysis.

They also exhibit a wide range of activities and selectivity for a variety of

chemical reactions, partly due to the rich variety of surface sites and the ability

of their surface cations to assume different valence states [40].

Transition metal oxides are used widely for large number of chemical

reactions. The cations in transition metal oxides often exist in more than one

oxidation state that makes them especially active for reactions of the oxidation-

reduction class [41]. It was demonstrated that a mixture of metal oxides brings

out combined effect or a synergistic behavior, which was well known among the

transition metal oxides that enhance the catalytic activity [43, 43] for several

reactions. In addition to being used as catalysts, transition metals are also

important as supports and promoters.

Oxides containing two or more different kinds of metal cations are known

as mixed metal oxides. Oxides can be binary, ternary and quaternary and so on

with respect to the presence of number of different metal cations. Among the

mixed metal oxides, spinel type oxides remain prominent due to their

applications in catalysis. Spinels show interesting catalytic properties, in which

the properties are controlled by the nature of ions, their charge and site

distribution between tetrahedral (Td) and octahedral (Oh) sites. Among the

spinel compounds ferrospinels have been used as effective catalysts because of

the ease with which iron can exchange its oxidation state between +2 and +3.

Another important feature attributed with these materials, from the commercial

standpoint, is that spinel structure provides high stability so that these materials

can withstand reducing conditions to a reasonable extent. Even if reduction of

Fe3+ to Fe2+ occurs, spinel structure remains unaltered and upon reoxidation the

original state can be retained [44].

In general, cations on the surface possess Lewis acidity, i.e. they behave

as electron acceptors. The oxygen ions behave as proton acceptors and are thus

Bronsted bases. According to the Bronsted acid concept, an acid is a hydrogen-

containing species able to release a proton and a base is any species capable of

combining with a proton. Lewis concept is that an acid accepts an electron pair;


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conversely a base is any species that can donate an electron pair [45]. The

surface composition of metal oxides is determined by the surface anion to cation

ratio and oxidation states of surface cations as it depends on the stoichiometry of

the oxide and the orientation of the exposed crystal planes. Non-stoichiometry

often arises from preferential removal of surface oxide leading to reduction of

the surface by pretreatment of the samples. For mixed metal oxides, in addition

to surface anion to cation ratio, the ratio of the different cations is also of

interest. In this case, the cation ratio at the surface and the bulk depends on the

surface tension of the individual oxides and the bulk strain of the solid solution

due to mismatch of the ionic sizes or coordination symmetry. Sometimes, the

chemisorbed species may lower the surface energy of solid inducing surface

aggregation of the component that binds more strongly with the adsorbate.

Formation of surface compound that is different from the bulk is also possible in

presence of adsorbate that has different oxidation states [46].

Thus the surface acid-base properties of metal oxides can influence the

substrate and reactant adsorption followed by reaction. Metal oxides have unique

catalytic properties towards alkylation reactions that are mainly influenced by

their acid-base properties. The acid-base properties of the metal oxides can be

tuned by choosing the different metal cations and also by varying their

compositions. Also from the electronic structure point of view, the mixing of

two or more different metal oxides influences the overlap between metals orbital

to different extents. The catalytic conversion, desired product selectivity or yield

depends upon the above factors. It is well known that with decrease in the size of

particles for a given volume of material, the number of atoms at the surface

(surface area) increases tremendously. Thus, the reduction in the size of the

particles renders them excellent catalysts [47, 48].

2.8 Photocatalysis

Environmental pollution is a matter of worldwide concern in our present

day world. Dyes are extensively used in the textile industry. Textile processing

industries in particular contribute significantly to this problem since they use a


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substantial volume of water along with a high percentage of reactive dye stuffs.

They are the copious source of coloured organics emanating as a waste from the

textile dyeing process. Hence, the waste water released by these industries is

characterized by a significant amount of suspended solids and un-reacted

dyestuff, broadly fluctuating pH and high temperature. Due to the high

concentration of organics in the effluents and the higher stability of modern

synthetic dyes, the conventional biological treatment methods are ineffective for

the complete colour removal and degradation of organics and dyes [49, 50].

Other conventional methods of colour removal from an aqueous medium include

techniques like coagulation, filtration, adsorption by activated carbon and

treatment with ozone [51]. However, the disposal of toxic sludge is a severe

drawback in all the above methods. Each method has its own advantages and

disadvantages. For example, the use of charcoal is technically easy but has a

high waste disposal cost. While in filtration, low-molar-mass dyes can pass

through the filter system. Hence, the necessity of investigating new alternatives

for the adequate treatment of the dye present in waste water is inevitable.

The efficient photocatalytic degradation of hazardous wastes is one of the

most desirable and challenging goals in the research of the development of

environment friendly catalysts. Use of inorganic photocatalyst such as the metal

oxides is cheaper way of removing organic matters and pollutant gases.

Recently, a number of researchers have shown the photocatalytic decomposition

of different dyes in presence of UV light or Visible light [51-53]. Several earlier

studies reported that, the photocatalytic degradation of dyes follows first order

kinetics [54, 55].

Photocatalysis is a process by which the irradiation of a metal oxide

semiconductor produces photo-excited electrons (e−) and positively charged

holes (h+). A photocatalytic reaction is initiated when a photoexcited electron is

promoted from the filled valence band of a semiconductor photocatalyst (SC) to

the empty conduction band as the absorbed photon energy hυ, equals or exceeds

the band gap of the semiconductor photocatalyst, leaving behind a hole in the

valence band. In concert, electron and hole pair (e−–h+) is generated.


43

Photoexcitation: Photocatalyst + hυ → e− + h+

Oxygen ionosorption: (O2)ads + e− → O2• −

Ionization of water: H2O → OH− + H+

Protonation of superoxides: O2• − + H+ → HOO•

Thus the hydroperoxyl radical formed in has also scavenging properties similar

to O2 thus doubly prolonging the lifetime of photohole:

HOO• + e− → HO2−

HOO− + H+ → H2O2

Both the oxidation and reduction can take place at the surface of the

photoexcited semiconductor photocatalyst. Recombination between electron and

hole occurs unless oxygen is available to scavenge the electrons to form

superoxides (O2•−), its protonated form the hydroperoxyl radical (HO2•) and

subsequently H2O2 [56, 57]. The selection of a suitable photocatalyst is thus

challenging. Most of the investigations have focused on mixed-metal oxides

which show relatively high reactivity and chemical stability under ultraviolet

(UV) light photocatalytic degradation of organic contaminants using solar

radiation is highly economical compared with the processes using artificial UV

radiation.


44

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chapter ii theoretical background -...

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