Download 2.1 Fundamentals of Magnetism The magnetic

Chapter II
Theoretical background
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=µH
(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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Theoretical background
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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Theoretical background
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, j , j , and j in spinel ferrites [10, 11]
AB
AA
BB
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 spinorbital 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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Theoretical background
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 semiconductors 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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Theoretical background
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 ntype 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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Theoretical background
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 oxidationreduction 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 hydrogencontaining 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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Theoretical background
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.
42
Chapter II
Theoretical background
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.
43
Chapter II
Theoretical background
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