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CHAPTER 1

INTRODUCTION

1.1 NANOMATERIALS

It is arguably Richard Feynman famed lecture in 1959 entitled “There's

Plenty of Room at the Bottom” suggesting that there was opportunity and benefits

from miniaturization and building atom by atom. His lecture is considered to be the

nourishment for innovative ideas towards the development of nanoscience and

technology.

Nanoscience and nanotechnology have gained much attention in

fundamental research and industrial applications. It mainly focuses on the evolution

and creation of materials at the nanoscale (between approximately 1-100 nanometers

or 10-9 - 10-7 meters) by mostly bottom up approach from quantum or atomic level

particles to nanoscale level or by top down approach through reduction process of

bulk materials [1]. Under this dimensions, the properties of nanomaterials differ

from the properties of their bulk counterparts in spite of the fact that they possess the

same chemical composition. These nanomaterials exhibit novel mechanical,

electrical, optical, chemical, magnetic and biological properties which are expected

to serve for various potential applications, such as their use in logic circuit, chemical

and biological sensors, optical devices, magnetic devices and so on.

1.2 SIGNIFICANCE OF NANOSCALE

The properties of materials can be different at the nanoscale for two main

reasons. First, nanomaterials have a relatively large surface area when compared to

the material in bulk form. This can make materials more chemically reactive

(chemically inert gold in bulk form can be tuned as a potent chemical catalyst at

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nanoscale) and affect their strength or electrical properties. Second, quantum effects

can begin to dominate the behavior of the materials affecting optical, electrical and

magnetic behavior at nanoscale. Materials can be produced that are nanoscale in one

dimension (wire) in two dimension (well) or in all three dimensions (nanoparticle).

Salient features of the nanomaterials are the properties related to its size effects

which significantly have the potential influence on both science and technology.

1.3 SURFACE EFFECTS

Surface area and quantum effects are the two important principle factors

of nanomaterials which cause the properties of nanomaterials to differ significantly

from other materials. When the particle size decreases, a greater portion of atoms are

found at the surface compared to those inside. Therefore, nanomaterials have greater

surface area and energy per unit mass compared with larger particles. Hence a given

mass of the material in nanoscale dimension will be much more reactive than the

same mass of the material made up of larger particles.

The specific surface area of a system and surface to volume ratio are

inversely proportional to particle size and both increases sharply for particles less

than 100 nm in diameter. Although, the total surface energy increases with the

overall surface area, it is strongly dependent on the dimension of the material. These

effects are different from the normal bulk structure or alternatively may induce a

simple relaxation (expansion or contraction) of the normal crystalline lattice, which

could in turn alter other material properties [2, 3].

However, the foremost challenges in synthesizing and processing of

nanomaterials are to overcome the surface energy and to prevent the nanostructures

or nanomaterials from growth in size, driven by the reduction of overall surface

energy [4]. The ability to control the materials properties by changing its surface

area is important in many technological applications.

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1.4 SIZE DEPENDENT PROPERTIES

1.4.1 Electrical Properties

The energy band structure and charge carrier density in the materials can

be modified from their bulk and in turn leads to changes in the associated electronic

properties of the materials. These effects are normally termed as quantum

confinement effect and relates to the structure and occupation of outermost

electronic energy levels of the material. When the size of the system becomes

comparable with the de - Broglie wavelength of electrons, the discrete nature of the

energy state becomes observable once again, although a fully discrete energy

spectrum is only observed in systems that are confined in all the three dimensions.

Discrete energy bands considerably changes the transport properties of the system.

In typical cases, the conducting materials become insulators below a critical length

scale, as the energy band cease to overlap.

Electrical transport properties for bulk system are determined by phonons

scattering, impurities and other carriers or scattering at rough surfaces. The electrical

transport is diffusive and the path of each electron represents a random walk. But in

the nanostructured systems, electrons can travel through the system without

randomization of the phase of their wave functions as the system dimensions are

smaller than the electron mean free path for inelastic scattering. This results in

additional localization phenomena which are basically related to phase interference.

In highly confined structures like quantum dots, conduction is mostly dependent on

the presence of other charge carriers and hence the charge state of the quantum dot.

In recent years, many advances are made in the field of molecular and

nano electronics. Single molecules are expected to be able to control electron

transport in molecular electronics. This offers the promise of exploring the vast

variety of molecular functions for electronic devices.

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1.4.2 Optical Properties

In nanostructured systems, the effect of reduced dimensionality on

electronic structure greatly affects the energies of the highest occupied molecular

orbital and the lowest unoccupied molecular orbital. Transition between these states

results in optical emission and absorption. Particularly, metals and semiconductors

exhibit large changes in optical properties as a function of particle size [5]. In

semiconductor micro crystallites, three - dimensional quantum confinement effects

can be observed when the particle size approaches to exciton Bohr radius. This

confinement results in novel optical properties. Due to these reasons, they have

gained much attention and are being utilized for various applications in

optoelectronic devices, such as optical data storage and high speed optical

communication. Optical properties such as fluorescence emission are also

specifically dependent on the size of the nanocrystals.

For example, it has been illustrated that with suitable size modifications,

CdSe nanocrystals can fluoresce throughout the visible range of the electromagnetic

spectrum [6]. Size - dependent optical properties are exhibited by gold and silver

nanoparticles [7]. The nanoparticles change its color with size due to surface

plasmon resonance and hence they are widely utilized for molecular sensing,

diagnostics and imaging [8].

1.4.3 Thermodynamic Properties

The surface atoms play a significant role in determining the

thermodynamic properties of nanostructured materials. The reduced coordination

number of the surface atoms considerably increases the surface energy resulting in

atomic diffusion at comparatively lower temperatures. The melting point of CdS

particles falls to as low as ~ 400 ºC with diameters less than 3 nm, much lower than

the bulk melting point ~ 1600 ºC for CdS [9]. Also, the gold nanoparticles with

diameter less than 3 nm, experiences a much lower melting point close to ~ 500 ºC

as compared to the bulk gold melting point ~ 1064 ºC [10]. This is due to the

increasing number of surface atoms with decreasing particle size. Moreover, the

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density of surface atoms varies considerably for different crystallographic planes,

possibly leading to different thermodynamic properties.

1.4.4 Mechanical Properties

The densities of dislocations, surface area to volume ratio and grain size

greatly influence the mechanical properties of solid. The grain boundary sliding

results in an enhancement in damping capacity (ability to absorb energy by

converting mechanical energy into heat) of nanostructured materials. A decrease in

grain size strongly affects the strength and hardness. Single and multi - walled

carbon nanotubes exhibit high mechanical strengths and high elastic limits which

results in considerable mechanical flexibility and reversible deformation [11]. The

strength and toughness of both ceramics and metals can be greatly enhanced if they

are made out of nanoscaled crystallites instead of the usual micron - sized grains.

This effect is already being employed extensively to make superior ceramics and

tungsten carbide - cobalt composites [12] which are used for industrial machinery,

cutting tools, abrasives, bricks, pipes, floor tiles and jewelry etc.

1.4.5 Catalytic Properties

Nanomaterials have gained much attention for their varying physical and

chemical properties as a function of their dimension. Significant factors influencing

the catalytic activity and selectivity are surface structure, mobility of the active

species to restructure as well as the mobility of the adsorbates on these active

species. Nanomaterials have various advantages over its bulk counterpart like short

range ordering, enhanced interaction with environments due to the large number of

dangling bonds, great variety of the valence band electron structure and self

structuring for optimum performance in chemisorptions [13]. As a result of the size

reduction, large portion of their existing atoms are placed at the surface and higher

the surface area, the higher the surface atoms. Normally, increase in the surface area

gives more adsorption of reactant molecules on its surface, which results the higher

catalytic activity [14]. Internal surface area can be increased by introducing atomic

defects such as dislocations to enhance species diffusivity and chemical reactivity.

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This enhances their use in the field of oxidation - reduction chemistry with many

expected applications in fields like photocatalysis or photodegradation and

detoxification of chemical waste and environmental pollutants [15].

Nanoparticles can be employed to remove the contaminations in a

medium through chemical reaction to make it harmless. For example, ground water

can be purified by removing carbon tetrachloride from it using iron nanoparticles

[16]. Gold tipped carbon nanotubes can clean polluted water by trapping oil drops

from polluted water. Light activated nanoparticles like titanium dioxide are

continued to be studied for their ability to remove contaminants from various media.

Photoactive titanium dioxide (TiO2) nanoparticles are used for cleaning polluted

waters by removing various toxic metal ions like mercury, cadmium, arsenic,

chromium and copper through reduction. Nanoparticles can also be used in treating

water and contaminated air with various organic compounds, dyes and pesticides

[17].

1.4.6 Magnetic Properties

Large surface area to volume ratio in magnetic materials develops a

substantial proportion of atoms having a different magnetic coupling with

neighboring atoms, leading to differing magnetic properties [18].

Superparamagnetism is observed in magnetic nanoparticles by which the

magnetizations of the particles are randomly oriented and they are aligned only

under an applied magnetic field and the alignment disappears once the external field

is withdrawn. This is due to the presence of only one domain in magnetic

nanoparticles as compared with the multiple domains of bulk. Magnetic

nanoparticles of palladium (Pd), platinum (Pt) and surprising case of gold (Au) can

be obtained from non - magnetic bulk materials. Structural changes associated with

size effects develop ferromagnetism in Pt and Pd. However, gold nanoparticles

exhibit ferromagnetism when they are capped with appropriate molecules. The

charge localized at the particle surface gives rise to ferromagnetic like behavior [19].

This observation indicated that the modifications of the d - band structure by

chemical bonding can develop ferromagnetic like behavior in metallic clusters.

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Magnetic nanoparticles have variety of applications such as in

nanoelectronics, biomedical sensors, drug delivery, magnetic resonance imaging,

data storage, color imaging, bioprocessing, magnetic refrigeration and ferrofluids

[20, 21] etc. The need to increase storage space on magnetic storage devices such as

hard drives in computers resulted in the development of new field of study called

mesoscopic magnetism which involves the study of magnetic materials, specifically

for films of nanomagnets. The principle behind information storage mechanism

includes alignment of the magnetization in one direction of a very small region.

Magnetic storage devices such as hard drives are based on tiny crystals of cobalt

chromium alloys [22].

1.5 INTRODUCTION TO MAGNETIC MATERIALS

The orbital motion and spinning motion of electrons in an atom give rise

to the magnetic moment in a material. Therefore each atom represents a tiny

permanent magnet in its own domain. The revolving electron generates its own

orbital magnetic moment, measured in Bohr magnetons ( B) and also a related spin

magnetic moment due to the electron spinning by itself like the earth, on its own

axis. Basically every two electrons in an atom will form a pair such that they have

opposite spins with the resultant spin magnetic moment as zero. Unpaired electrons

are present in the 3d orbital of magnetic materials like iron, nickel, cobalt, etc.

A high spin magnetic moment can be observed in the magnetic materials due to the

interaction between the unpaired electron spin magnetic moment and the electrons

from the adjacent atom.

Magnetic behavior of materials are therefore dependent on these

unpaired electron spins. The spin magnetic moment of an electron is much larger

than its relative orbital magnetic moment. Every magnetic material can be defined in

terms of their magnetic behavior falling into one of five categories depending on

their bulk magnetic susceptibility. Magnetic susceptibility measures the degree of

magnetization of a material under the influence of an applied magnetic field. Its

value is positive for ferromagnetic and paramagnetic material and negative for a

diamagnetic material. Diamagnetism and paramagnetism are the most general form

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of magnetism, which relates the magnetic properties of most elements at room

temperature [23]. They are normally considered as nonmagnetic, whereas those

which are referred to as magnetic are typically classified as ferromagnetic.

Ferrimagnetism and it is the only form of magnetism that is not observed in any pure

elements and found only in compounds such as mixed oxides (ferrites). Pure

elements at room temperature exhibit a different type of magnetism known as

antiferromagnetism.

1.5.1 Classification of Magnetic Materials

Magnetic materials basically classified as diamagnetism, paramagnetism,

ferromagnetism, antiferromagnetism and ferrimagnetism.

1.5.1.1 Diamagnetism

When a material produces a magnetization effect (M) opposing the

direction of applied field magnetic field due to a change in motion of the spinning

electrons in the material then the material is classified as a diamagnetic material.

The measurement of the ability of a material with which it can be magnetized under

the influence of an external magnetic field is called its susceptibility. The

susceptibility value does not depend upon the temperature and is always negative.

1.5.1.2 Paramagnetism

In an atom, the unpaired electrons result in a net magnetic moment in

relation with electron spin. But the bulk material does not show any magnetic

property in absence of an external magnetic field because these magnetic moments

are randomly oriented. With the application of magnetic field these magnetic dipoles

can align in the field direction. Since these moments do not interact, a very large

magnetic field is needed to align all of them.

However the above condition prevails only in the presence of external

field. When the external field is removed the dipoles again return to their random

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orientation. This magnetic property is classified as the Paramagnetism and is found

in many materials like calcium, titanium and alloys of copper.

1.5.1.3 Ferromagnetism

The phenomenon by which the atoms are arranged in such a way that

their atomic magnetic moments align parallel to each other is known as

ferromagnetism. The presence of an internal field elucidates this phenomenon. The

internal field is considered to be strong enough to magnetize the material to

saturation. The Heisenberg model of ferromagnetism describes the parallel

alignment of magnetic moments in terms of an exchange interaction between

neighboring moments.

The presence of magnetic domains within the material, which are the

regions where the atomic magnetic moments are aligned, was postulated by Weiss.

How the material responds to an external magnetic field and as a consequence, why

the susceptibility becomes a function of applied magnetic field was explained by the

movement of these domains. Magnetization when all domains of a material are

aligned is referred to as saturation magnetization. The saturation magnetization

forms the basis of comparison between different ferromagnetic materials. In the

periodic table of elements only Fe, Co and Ni are ferromagnetic above room

temperature. The saturation magnetization decreases upon heating because the

thermal agitations of the atoms lead to the misalignment of the atomic moments.

With large thermal agitation the material becomes paramagnetic, and this transition

temperature is called Curie temperature, TC (Fe: TC = 770°C, Co: TC = 1131°C and

Ni: TC = 358°C). Ferromagnetic material has positive and high magnetic

susceptibility.

1.5.1.4 Antiferromagnetism

Antiferromagnetism is differing from ferromagnetism as in

antiferromagnetic material there is antiparallel alignment of the atomic magnetic

moment due to the exchange interaction between the neighboring atoms. These

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materials resemble the paramagnetic behavior as the antiparallel alignment of the

atomic magnetic moment cancels out the magnetic field. There exists a transition

temperature called Neel temperature (TN), above which these materials become

paramagnetic. The only exceptional element is Chromium (Cr: TN = 37 oC)

exhibiting antiferromagnetism at room temperature. Antiferromagnetic materials

have positive and very small susceptibility value.

1.5.1.5 Ferrimagnetism

Ferrimagnetism occurs in solids which have unequal numbers of both

parallel aligned (as in ferromagnetism) and anti-parallel aligned

(as in antiferromagnetism) magnetic moments. This is due to the presence of

different types of atomic composition of atoms or ions in the solids. These different

kinds of atoms or ions respond differently to an external magnetic field. Hence the

magnetic moment corresponding to a particular kind of ions may align in the field

direction while the moments of a different kind of ions can align in the opposite

direction. These different forms of alignment of magnetic moments result in a

spontaneous magnetization of the material. Ferrites are the ceramic material which

exhibits this type of magnetic behavior. Some ferrimagnetic materials are YIG

(yttrium iron garnet) and ferrites composed of oxides of iron, aluminum, cobalt,

nickel, manganese and zinc.

1.6 MAGNETISM IN ULTRAFINE NANOPARTICLES:

NANOMAGNETISM

Magnetic nanoparticles: Ultrafine magnetic particles with nanoscale

dimensions are found to exhibit novel properties compared with their conventional

coarse grained counterparts. Magnetic nanoparticles are influenced by unique

features like single domain nature and superparamagnetism. A large variation of

coercivity in magnetic nano particles compared to its bulk counterpart is also an

attractive phenomenon. The exceptional behaviour exhibited by nanoparticles is

basically due to two main reasons, finite size effects and surface effects. Brief

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descriptions on these special features of ultrafine nanoparticles are mentioned in the

following section.

1.6.1 Single Domain Particles

Normally, it is observed that when a particle is smaller than about

100 nm, a domain wall simply can't fit inside it, resulting in single domain particles.

The single domain particle has only one magnetic direction and it is uniformly

magnetized to its saturation magnetization. A single domain particle has high

magnetostatic energy (magnetic potential energy generated in the presence of a

magnetic field) but negligibly low domain wall energy compared to the multi

domain particle.

1.6.2 Variation of Coercivity with Particle Size

The variation in coercivity with particle diameter is illustrated in

Figure 1.1. From the figure it can be predicted that when the particle size is reduced,

the coercivity increases to a maximum value and then declines toward zero. In

multidomain particles, magnetization varies by domain wall motion. The size

dependence of coercivity is experimentally calculated by

abH aD

(1.1)

Where a and b are constants and D is the particle diameter [24].

The coercivity reaches a maximum when the particle size reduces below

a critical value Dc and become single domain. The particles with size less or equal to

Dc, change their magnetization by spin rotation. The coercivity decreases with the

decrease in particle size below Dc, because of thermal effects calculated by

3/2ahH g

D (1.2)

where g and h are constants.

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Figure 1.1 Variation of coercivity with particle size [24]

Below a critical diameter Dc, the thermal effects are strong enough to

spontaneously demagnetize a previously saturated assembly of particles. This results

in the reduction of coercivity to zero. Such particles are called superparamagnetic

and the phenomenon as superparamagnetism.

1.6.3 Superparamagnetism

The magnetic property exhibited by small ferromagnetic or ferrimagnetic

nanoparticles is referred to as superparamagnetism. This is observed in magnetic

nanoparticles with sizes close to few nanometers to couple of tenth of nanometers,

depending on the material. These nanoparticles are single domain particles and their

magnetization randomly flip direction under the influence of temperature. The time

between two flips of magnetization direction is called as Neel relaxation time.

Magnetic nanoparticles are considered to be in superparamagnetic state, when the

time used to measure the magnetization of the nanoparticles is much longer than the

Neel relaxation time and their magnetization appears to be in average zero in the

absence of an external magnetic field. Similar to the paramagnets they can be

magnetized under the influence of an external magnetic field but their magnetic

susceptibility is much higher than the paramagnets.

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One of the most crucial factors responsible for the paramagnetic behavior

of magnetic oxides like ferrites [25] is the finite size effect. Due to this effect the

thermal energy is sufficient to change the direction of magnetization of the entire

crystallite even when the temperature is below the Curie or Neel temperature. The

magnetic field is zero when the resulting fluctuations are in the direction of

magnetization.

In superparamagnetic material the magnetic moment of the entire

crystallite tends to align with the magnetic field, whereas in paramagnetic materials

each individual atom is independently influenced by an external magnetic field. The

crystalline anisotropy energy is the energy required to change the direction of

magnetization of a crystallite and depends both on the material properties and the

crystallite size.

1.7 FERRITES

Ferrimagnetic materials or ferrite materials show lower permanent

magnetization because they have incomplete cancellation of the magnetic dipoles in

a domain. They are normally non - conductive ferrimagnetic ceramic compounds

derived from iron oxides such as hematite (Fe2O3) or magnetite (Fe3O4) as well as

oxides of other metals. Ferrites can be classified as spinel, garnet and hexaferrite

based on its crystal structure and compositions.

1.7.1 Spinel Ferrites

Magnetite (Fe3O4) is the main source for the production of spinel ferrites

through partial substitutions of the iron ions by other metallic cations. Its chemical

composition is explained by a simple formula AFe2O4, where ‘A’ represents a

divalent metal ion. (e.g: Zn2+, Fe2+, Ni2+, Mn2+, Mg2+, Co2+, Cd2+, etc…) and belong

to the space group Fd3m. Its lattice structure consists of a close packed oxygen

arrangement in which 32 oxygen ions form the unit cell. In spinel ferrites the anions

are packed in a face centered cubic (FCC) arrangement leaving two kinds of space

between anions: tetrahedrally coordinated sites (A), surrounded by four nearest

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oxygen atoms and octahedrally coordinated sites (B), surrounded by six nearest

neighbor of oxygen atoms.

Figure 1.2 Spinel ferrite structure [26]

This is shown in Figure 1.2. Its unit cell consists of 32 oxygen ions and

64 tetrahedral sites and 32 octahedral sites. There are only 8 tetrahedral sites and

16 octahedral sites occupied by metal ions, resulting in an electrically neutral

structure. Goldmann et.al [26] suggested that the crystal structure are best

understood by subdividing the unit cell into eight octant with edge ½ a, where ‘a’ is

the length of the unit cell. There exist an identical location of oxygen ions and

metals ions in all octant. Four oxygen ions on the body diagonals are accompanied

by each octant, lying at the corners of a tetrahedron. The location of each oxygen ion

is at a distance equal to one fourth of the length of the body diagonal from alternate

corners of the octant. The array of oxygen ions as a whole in the crystal constitute

fcc lattice with edge = a / 2. Hence, there are four such interpenetrating fcc oxygen

lattice.

1.7.1.1 Regular spinel ferrites

The regular spinel ferrites have 8 divalent metal ions occupying ‘A’ sites

and 16 trivalent Fe ions occupying ‘B’ sites. For example in Zn2+ [Fe23+] O4, Zn2+

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has strong affinity towards tetrahedral site (A); therefore they give rise to normal

ferrites as they enter the A - site of the lattice.

1.7.1.2 Inverse spinel ferrites

The 8 divalent metal ions are in the eight of sixteen octahedral sites. The

16 trivalent Fe ions are in eight octahedral and eight tetrahedral sites

Example; Nickel Ferrite Fe3+ [Fe3+Ni2+] O4

1.8 ZINC FERRITE (ZnFe2O4)

Some of the significant advantages of Zinc ferrites are their high

electromagnetic performance, excellent chemical stability, mechanical hardness, low

coercivity, moderate saturation magnetization, high magnetic permeability, non

toxicity, low eddy - current loss and high electronic conductivity [27]. Further, it is a

low cost, environmentally friendly and versatile material with n - type

semiconducting (band gap energy = 1.92 eV) and magnetic properties [28]. ZnFe2O4

nanoparticles have a regular spinel structure with a tetrahedral A - site occupied by

Zn2+ ions and an octahedral B - site occupied by Fe3+ ions. It exhibits anti

ferromagnetism at TN = 10 K [29].

1.8.1 Synthesis of Zinc Ferrite Nanoparticles

The major concerns in the synthesis of Zinc Ferrite nanoparticles are to

control the size, shape and properties to assemble the nanoparticles for a given

purpose. Normally two main methods are employed for the synthesis of Zinc ferrite

nanoparticles: bottom up method and top down method. In bottom - up method the

nanoparticles are built up from atom by atom, or molecule by molecule by

controlling the reaction parameters. Top down method is widely used in the

traditional nano particle manufacturing process by breaking down bulk materials

gradually into smaller sizes until they are nanosized. Mechanical breakdown such as

high energy ball milling is a common example of the top down method.

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Synthesis techniques can be scientifically classified into chemical and

physical methods according to the different synthesizing process involved. The

physical method consists of changes in the physical state, such as size, shape and

phase of the matter. For example, condensing gaseous metal vapors into

nanoparticles is a physical method. Most of the chemical methods [30 - 33] can be

considered as the bottom up method and physical methods like high energy ball

milling [34], lithography or pattering etc. are examples of top down method.

Numerous chemical methods have been employed recently to synthesis

ZnFe2O4 nanoparticles with varying sizes and shapes. Co - precipitation,

hydrothermal, thermal decomposition, gel-evaporation or polyol, sol - gel,

combustion and ball milling are the most commonly used methods for the synthesis

of high quality magnetic nanoparticles. In the present research work, the synthesis of

Zinc Ferrite nanoparticles was carried out using a low cost and simple hydrothermal

[35 - 37] and co - precipitation methods [38 - 40].

1.8.2 Hydrothermal Method

In the last few decades hydrothermal technique has gained much

attention from scientists and technologists of different disciplines for the synthesis

of various nanomaterials. The hydrothermal has geological origin with ‘hydro’

meaning water and ‘thermal’ meaning heat. The first person to use this word was a

British geologist, Sir Roderick Murchison (1792 - 1871). He used it to explain the

formation mechanism of variiour rocks and minerals in the earth’s crust through the

action of water at elevated temperature and pressure.

Hydrothermal technique can be defined as a method to synthesize

different chemical compounds and materials in a closed environment with water as a

solvent under high temperature (above 100 °C) and pressure (above 1 atm). In this

method small crystals will homogeneously nucleate and grow from solution when

subjected to a high temperature and pressure. During the nucleation and growth

process, water behaves both as a catalyst as well as solid state phase component.

Under the extreme conditions of the synthesis vessel (autoclave or bomb), water

often becomes supercritical, thereby increasing the dissolving power, diffusivity and

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mass transport of the liquid by limiting its viscosity. In contrast to other

methodologies, hydrothermal synthesis is environmentally favorable, inexpensive

and helps in the reduction of free energies for various equilibriums. Materials

synthesized hydrothermally are normally high - quality, single crystals with various

shapes and sizes. Although hydrothermal synthesis is an established synthesis

technique within the ceramics industry, it is developed recently within the scientific

community by the synthesis of one dimensional nano structures, such as carbon

nano tubes and oxide nanowires [41].

1.8.3 Co-Precipitation Method

Co-precipitation synthesis involves dissolution of compound salt

precursor in aqueous media and subsequent precipitation from the solution by pH

adjustment. It is a useful method for the preparation of ceramics and metal oxide

powders. Apart from its simplicity, atomic mixing of the constituents by chemical

co - precipitation yields a final product of near perfect stoichiometry without high

temperature treatment.

1.9 BACKGROUND ON PHOTOCATALYSIS

1.9.1 Definition of Photocatalysis

The phenomenon connecting photochemistry and catalysis is referred as

photo catalysis. It implies that light and a catalyst are necessary to bring about or to

accelerate chemical transformations [42].

1.9.2 Photocatalytic Processes

Wastewater pollutants consisting of organic chemicals coming from

industrial or domestic sources must be removed or destroyed before discharging to

the environment. These pollutants may also be found in ground and surface water,

which also need to be treated to achieve acceptable drinking water quality [43]. The

increasing public concern with these environmental pollutants has necessitated

developing novel treatment methods, in which photo catalysis is gaining

considerable attention in the field of pollutants degradation [44].

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Natural purification of aqueous systems such as lagoons, ponds, streams,

rivers and lakes are caused by sunlight initiating the decomposition of organic

molecules to carbon dioxide and other mineral products. The process is accelerated

by the presence of various natural sensitizers. The utilization of colloidal

semiconductors and the introduction of catalysts to promote specific redox processes

on semiconductor surfaces were developed in the last two decades [45].

Photocatalytic detoxification of wastewater is a process that combines

heterogeneous catalyst and solar energy [46]. Semiconductor photo catalysis, with a

main focus on TiO2, has been utilized to a variety of problems of environmental

interest in addition to water and air purification. The application of illuminated

semiconductors for degrading undesirable organics dissolved in air or water is well

documented and has been successful for a wide variety of compounds [17]. Organic

compounds such as alcohols, carboxylic acids, amines, herbicides and aldehydes

have been photo catalytically disintegrated in laboratory and field studies. The

photocatalytic process can mineralize the hazardous organic chemicals to carbon

dioxide, water and simple mineral acids [47]. Thus one of the primary advantages of

photocatalytic process over existing technologies is that there is no further

requirement for secondary disposal methods.

Another advantage of this process is that when compared to other

advance oxidation technologies especially those using oxidants such as hydrogen

peroxide and ozone, expensive oxidizing chemicals are not needed as ambient

oxygen is the oxidant [48]. Photo catalysts are also self - regenerated and can be

used again or recycled. Finally, the solar photocatalytic process can also be utilized

to annihilate nuisance odor compounds and naturally occurring organic matter which

contain precursors for trihalomethanes formed during chlorine disinfection step in

drinking water treatment [49].

1.9.3 Mechanism of Photocatalysis

During the photocatalytic process, the illumination of a semiconductor

photo catalyst with ultraviolet (UV) radiation activates the catalyst, establishing a

redox environment in the aqueous solution [46]. Semiconductors act as sensitizers

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for light induced redox processes due to their electronic structure, which is

characterized by a filled valence band and an empty conduction band [17]. Band

gap is the gap between energy levels of the valence and conduction band where no

electron states exists.

The photons absorbed by the semiconductor photo catalyst have energies

equal to or higher than its band gap or threshold energy. Each photon of required

energy (i.e. wavelength) that hits an electron in the occupied valence band of the

semiconductor atom can elevate that electron to the unoccupied conduction band

leading to excited state conduction band electrons and positive valence band holes

[50].

A positive hole created in the valence band oxidizes the water molecules

(Fig. 1.3) and breaks it apart to form hydrogen gas and hydroxyl radical. And the

excited electron in the conduction band reduces oxygen molecule to form oxide

anion, which tends to degrade the methylene blue dye present in the solution. This

cycle continues till the reaction is exposed to light source. The degradation of

methylene blue dye can be enhanced by the increased number of photo - excited

electrons in the conduction band.

Figure 1.3 Mechanisms of Photocatalysis

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1.10 REVIEW OF LITERATURE

1.10.1 Ferrite Nanomaterials

Ferrite nanoparticles have attracted great attention because of their broad

applications in many fields such as magnetic materials, biomedicine and diverse

catalytic processes. Ferrite nanoparticles comprising of nano sized grains have a

significant fraction of grain boundries with a high degree of atomic disorder along

the grain boundries. Superparamagnetisim, spin canting, core / shell structure,

metastable cation distribution etc., are some of the properties observed in different

nano structures of ferrites. These properties depend on number of factors such as

composition, grain size, surface morphology, anisotropy and inter particle

interactions [51 - 53]. The magnetic and the electrical properties of ferrites are found

to be dependent on the synthesis and sintering conditions of the material and are

observed to be highly sensitive to the cation distribution. Numerous synthesis

techniques are being employed for synthesis of ferrite nanoparticles, which exhibit

novel properties when compared to their properties in bulk. Nanostructures obtained

through physical methods like mechanical milling contains high amount of defects.

Goya et al., [54] reported that during the synthesis of ZnFe2O4 oxygen ions escape

from the spinel structure, thereby generating anion vacancies during milling. Some

non - conventional methods which are used extensively for the synthesis of materials

are co - precipitation, thermal decomposition, sol gel and hydrothermal methods.

Co - precipitation and hydrothermal method are the preferred techniques for

synthesizing ferrite nanostructures because of increased homogeneity, purity and

reactivity observed in the as synthesized ferrites. The important advantages of these

techniques are low cost, simplicity and control over particle size.

Photosynthesis is an essential part of life on earth which is a

photochemical process. The general perception is that research on photocatalysis

began only after the invention of photo electrochemical water splitting by Fujishima

and Honda in 1972 [55]. The photocatalysis process was depicted in the early work

of Plotnikov [56] in 1920’s, in his book entitled, “Allaemeine photochemie”. In

1953, Markham and Laidler [57] minutely observed photo oxidation on the surface

21

of zinc oxide in aqueous solutions. Extending this work, Markham and Laidler

comprehensively studied the photocatalytic properties of zinc oxide. Further

advancement in photocatalysis began by 1970’s, by analyzing the surface properties

of different photocatalysts.

Naseri et al., [27] synthesized crystalline ZnFe2O4 by the thermal

treatment method followed by calcinations at various temperatures from 723 K to

873 K. Poly (vinyl pyrrolidon) (PVP) was used for stabilizing the particles and

avoiding agglomeration. TEM images showed the average particle sizes in the range

of 17 nm - 31 nm. Vibrating sample magnetometer (VSM) demonstrated the

magnetic properties which displayed super paramagnetic behaviors for the calcined

samples. Also suggests that, the saturation magnetization and coercivity field are

primarily dependent on the methods of preparation of the ferrites. Electron

paramagnetic resonance (EPR) spectroscopy confirmed the presence of unpaired

electrons and calculated the peak - to - peak line width ( Hpp), the resonant

magnetic field (Hr) and the g - factor values.

Mendonca et al., [58] reported the structural and magnetic measurements

of ZnFe2O4 nanoparticles obtained through co - precipitation chemical method. The

rietveld analysis of X - ray patterns showed that (i) the samples were single phase

(ii) the average particle size increases with synthesis temperature and (iii) the

cationic disorder increases with decrease in of the mean particle size. The

Zero - Field - Cooled (ZFC) and Field - Cooled (FC) magnetization measurements

indicate that the blocking temperature increases with increase in of the particle size.

Neel temperature to larger particles and blocking effects to smaller particles were

observed in the sample grown at T = 850 °C. It was observed that the coercive field

does not decrease with the temperature following the Neel relaxation and

Bean - Livingston approaches.

Yu et al., [59] studied well crystallized ZnFe2O4 ultrafine particles with

octahedral shape and a size of 300 nm prepared by the hydrothermal reaction using

metal Zn sheet and FeCl2 as reactants in ammonia solutions at 180 oC. The

synthesized ferrite particles exhibit high magnetization with maximum saturation

22

magnetization ( max) 61.2 emu / g at 80 K and 54.6 emu / g at 300 K respectively.

The results obtained suggest that hydrothermal process is an effective pathway for

preparing high quality ferrite ultrafine particles.

Surinwong et al., [60] reported the synthesis of cubic zinc ferrite

nanoparticles by ultrasonic cavitation - assisted solvothermal technique using ethyl

alcohol - water mixed solvent. They also presented the effects of the ultrasonic

cavitation and the use of C2H5OH - H2O mixed solvents in diminishing average

particle size and in improving uniform distribution of particle size. The average size

of nanoparticles obtained was 20 nm as observed from SEM images and the

crystallite size was approximately 10 nm as measured from XRD results.

Field - dependent magnetization of the nanoparticles observed at room temperature,

with magnetization of 24.32 emu / g at 1 T.

Fan et al., [61] used colloid mill and hydrothermal technique to

synthesize nanocrystalline ZnFe2O4 photocatalysts. This technique included a very

rapid mixing of Fe3+ cations with reducing agent. The reduction process was carried

out in a colloid mill reactor, followed by a slow oxidation of iron nuclei and

structural transformation in a separate hydrothermal process. The results showed

that ZnFe2O4 nano crystals with the uniform crystallite sizes were obtained in - situ

forming iron nuclei as the source of Fe. These ZnFe2O4 nano crystals indicated

better abilities to photo decompose acid orange II azodye molecule under UV

irradiation due to quantum confinement effect and high surface area structure, as

compared to bulk ZnFe2O4 sample synthesized by the conventional solid state

method. Final results suggested that the ZnFe2O4 nano crystals have excellent

chemical and thermal stabilities and exhibit good photo catalytic activities. These

properties make it a potential candidate for application in the field of industrial

photo - degradation of organic azodye pollutants.

Li et al., [62] reported spinel zinc ferrite nanospheres with diameters of

about 212 nm synthesized in high yield via a general, one - step and template - free

solvothermal route. The synthesized nano spheres had cubic spinel structure having

well size uniformity and regularity. The absorption edge of ZnFe2O4 nano spheres

23

rose to a higher energy in the UV - Vis absorption. The ZnFe2O4 nanospheres

exhibited remarkably high surface photovoltage response in the UV and visible

region, ensuring the enhanced separation of photogenerated electrons and holes. The

typically improved photo catalytic activity of the ZnFe2O4 nano spheres was

determined in the decomposition of rhodamine B under Xe lamp irradiation.

Hydroxyl radicals on the surface of photo illuminated ZnFe2O4 nanospheres were

evaluated by the photoluminescence technique, revealed hydroxyl radicals played an

important role in the photocatalytic reaction. The synthesized nanostructure showed

an enhanced ability to remove organic pollutants in wastewater.

Fan et al., [63] reported the synthesis of visible light induced

cobalt - doped zinc ferrite (Zn(1 x)CoxFe2O4) photocatalysts via a facile

reduction - oxidation route, which involved rapid reduction of Fe3+ and Co2+ cations

in colloid mill reactor, followed by oxidation of iron and cobalt nuclei and structural

transformation under hydrothermal conditions. The results suggested that metallic

Co and Fe nuclei could be synthesized by reduction in the colloid mill.

Zn(1-x)CoxFe2O4 nano crystals with uniform size were successfully obtained with

improved photo catalytic activity in the degradation of methylene blue under visible

light irradiation. The intrinsic chemical stability and enhanced magnetic property

observed in Zn - Co ferrites nano crystals make it a potential candidate for its

application in the field of industrial photo degradation of organic pollutants.

Sun et al., [64] prepared magnetic ZnFe2O4 nanoparticles of 10 nm size

using sodium oleate as morphology - controller. The synthesized ZnFe2O4 showed

wide absorption ability in visible light range and high photocatalytic properties with

the degradation of rhodamine B (RhB). The enhanced photo catalytic efficiency

observed in the nano crystals was attributed to the nano size effect and the

octahedral system with active (1 1 1) surfaces. The nano crystals showed

superparamagnetism and were easily separable by magnetic field.

Wang et al., [65] reported nanocrystalline ZnFe2O4 “timber - like”

superstructures through the thermal decomposition of zinc ferrioxalate precursor

from metal sulfates and sodium oxalate without adding any additives. They observed

24

that the porous nanocrystalline ZnFe2O4 superstructures showed superparamagnetic

properties at room temperature and wound find applications in magnetic devices.

Xue et al., [66] reported the synthesis of macro porous nano crystalline

zinc ferrite with single spinel - phase by a facile self - propagating combustion

method using zinc nitrate, iron nitrate and glycine. They observed that the crystallite

size of nanocrystalline ZnFe2O4 was maintained to a certain extent by simply

modifying the ratio of metal nitrate and glycine. The final results showed that the

samples had small nanoparticle sizes, large surface areas and macro pores. The

magnetic measurements revealed that the synthesized nanocrystalline ZnFe2O4 could

behave as super paramagnet at 300 K.

Yao et al., [67] prepared ferromagnetic zinc ferrite nanocrystals at

ambient temperature via the thermal decomposition of metal - surfactant complexes.

The prepared zinc ferrite nano crystals were super paramagnetic at room temperature

with a blocking temperature TB = 68 ± 2 K. A saturation magnetization

MS = 65.4 emu / g at T = 10 K occurred due to the variation in inversion degree of

the spinel structure. A coercive field of HC = 102 ± 5 Oe in the blocked state shows

the anisotropic of nanoparticles. Magnetization data in the prepared ZnFe2O4 nano

crystals confirms the surface spin canting in the nano crystals. Their results

demonstrate that magnetic nano properties of magnetic particles can be improved by

just varying the particle size, which might be a significant way to design novel

magnetic materials.

Jeong et al., [68] prepared highly crystalline ZnFe2O4 nanocrystals of two

different particle sizes via the nonaqueous nanoemulsion synthesis. The results

showed the explicit marginal size distribution and precise magnetic properties of the

nano crystals. The experimental results suggest that the ZnFe2O4 nano crystals can

be used for biomedical applications as MRI imaging and DNA transfection.

Zhang et al., [69] reported the successful synthesis of magnetic

composite of ZnFe2O4 / BiVO4 by the one - step chemical co - precipitation

method. The photocatalytic activity of BiVO4 was enhanced by composite it with

25

ZnFe2O4 that contained a narrow band gap. This favored the ZnFe2O4 / BiVO4

composite to display enhanced visible - light absorption and to produce additional

photo - generated electrons. The photo catalytic ability of the composite was

observed better than that of pure BiVO4 under visible light irradiation. The n - type

ZnFe2O4 nanoparticles were uniformly deposited on the surface of n - type BiVO4.

This helped in the formation of n - n type heterogeneous structures in the

composite system. The photo - generated electrons and holes could move to a

position at a comparatively positive and negative potential, respectively, due to

different potentials of Conduction band (CB) and Valance band (VB) observed in

ZnFe2O4 and BiVO4, which increased the separation efficiency between the

photo - generated electrons and holes and this improves the photo catalytic activity.

Zhu et al., [70] prepared porous ZnFe2O4 nanorods with a diameter

around 50 nm and a length of several micrometers by a micro emulsion - based

method in conjugation with calcinations at 500 oC. The porous ZnFe2O4 nano rods

displayed much better sensing performance than ZnFe2O4 nanoparticles when used

as the ethanol sensor at room temperature.

Jia et al., [28] prepared porous ZnFe2O4 nano rods by the thermal

decomposition of ZnFe2(C2O4)3 precursor. The precursor was synthesized by

template surfactant - free solvothermal method. The final results demonstrated that

the prepared ZnFe2O4 conserved the precursor morphology of 1D nanorods with

diameters of 100 - 200 nm and lengths of different micrometers having

interconnected to each other to form porous nanorods. The synthesized ZnFe2O4

nano rods in the form of subsequently light - driven photo catalyst showed improved

photo catalytic decomposition activity for methylene blue.

Konicki et al., [71] reported on the synthesis of magnetic ZnFe2O4

nanoparticles by microwave assisted hydrothermal method and the nanoparticles

were used as an adsorbent for the removal of acid dye Acid Red 88 (AR88) from

aqueous solution. The influence of various parameters such as AR88 concentration

(10 - 56 mg L-1), pH of the solution (3.2 - 10.7), and temperature (20 - 60 oC) were

studied. The Langmuir and Freundlich models of adsorption were used for the

26

analysis of the observed data. The Langmuir model showed a well similarity with

the equilibrium data. The adsorption kinetic data were examined by the pseudo -

first - order and pseudo - second - order kinetic models and intraparticle diffusion

model. The adsorption kinetics was observed to follow the pseudo second - order

kinetic model.

Zhang et al., [72] investigated the synthesis of Zn(1 x)FexO (x = 0.02,

0.04, 0.06, 0.08, 0.10) powders by sol - gel approach through the reduction of Zn

nitrate and Fe nitrate by citrate. XRD studies revealed that Zn(1 x)FexO samples were

in single phase with the ZnO wurtzite structure as the Fe content is less than 2 %.

But a secondary phase ZnFe2O4 was observed when Fe concentration increased to

4 %. Raman results showed that the crystalline quality decreases with the increase of

Fe concentration. The oxygen vacancy (Vo) defects present in the material were

shown by the PL spectra.

Sun et al., [73] reported the synthesis of magnetically recyclable ZnFe2O4

/ ZnO nanocomposites immobilized on different content of graphene using an

ultrasound aided solution method. These nanocomposites showed favorable

photocatalytic activity under solar light irradiation. The content of graphene and the

molar ratio of ZnFe2O4 to ZnO could be modified by adjusting the amount of zinc

salts and graphene oxide dispersions. When the molar ratio of ZnFe2O4 to ZnO was

0.1 and the weight ratio of graphene to ZnFe2O4 / ZnO was 0.04, an excellent

photocatalytic activity under solar light irradiation was observed. The recycling

process of photocatalyst nanoparticles can be facilitated by the presence of magnetic

ZnFe2O4.

Hou et al., [74] reported the synthesis of graphene - supported ZnFe2O4

multi - porous microbricks hybrid via a facile deposition - precipitation reaction,

followed by a hydrothermal treatment. The morphology, structure and optical

properties of the hybrid were well characterized and the results indicated formation

of an intimate contact between ZnFe2O4 microbricks and graphene sheets. A much

higher photocatalytic activity was observed in the grapheme - supported ZnFe2O4

multi - porous microbricks hybrid than the pure ZnFe2O4 multi - porous microbricks

27

and ZnFe2O4 nanoparticles under the visible light irradiation ( > 420 nm) from the

photocatalytic degradation of p - chlorophenol experiments. Enhancement of

photocatalytic performance was attributed to the fast photogenerated charge

separation and transfer due to the high electron mobility of graphene sheets,

improved light absorption, high specific surface area as well as multi - porous

structure of the hybrid. The main active oxygen species in the photocatalytic

reaction were found to be the hydroxyl radicals by the photoluminescence and

radicals trapping studies. The work suggested some insights into the synthesis of

grapheme - based hybrid photocatalysts with high photocatalytic activity.

In spite of the drudgery efforts by researchers for the synthesis of highly

efficient ZnFe2O4 photocatalyst; its visible absorption, dispersibility of catalyst in

polluted water and reusability still remain ambiguous, which prohibit ZnFe2O4 for

commercial applications. In the current work, ZnFe2O4 nanoparticles were

synthesized using different surfactants. The structure, morphology, magnetic

properties were studied and the photocatalytic activity of band gap tunable ZnFe2O4

nanoparticles prepared using PVA for the degradation of methylene blue was

investigated.

1.11 OBJECTIVES OF THE THESIS

The synthesis of nanoparticles with a uniform size and morphology is

one of the most significant challenges in nanotechnology. Thus, a control over the

particle size is very important for magnetic semiconductor nanoparticles systems. It

is very difficult to achieve the confinement of particles without surface capping

agent. The main objectives of the research work as follows:

1. To synthesize ZnFe2O4 nanoparticles using different surfactant such

as CTAB, TEA, DEA, EA and PVA by simple hydrothermal and

co - precipitation method.

2. To study structural, morphological and magnetic properties of the

synthesized nanoparticles.

28

3. To characterize the synthesized materials by Powder X - ray

diffraction (XRD), Fourier transforms infrared spectroscopy (FTIR),

Thermo gravimetric analysis (TGA), Differential thermal analysis

(DTA), Field emission Scanning electron microscopy (FESEM),

Transmission electron microscopy (TEM), Vibrating sample

magnetometer (VSM) and diffuse reflectance spectroscopy (DRS).

4. To investigate the Photocatalytic activities of band gap tunable zinc

ferrite nanoparticles prepared using PVA as surfactant for the

degradation of methylene blue dye.

Based on the above objectives, this research work was performed and

then results are accounted in the following chapters.

1.12 ORGANIZATION OF THE THESIS

The thesis deals with the synthesis and characterization of ZnFe2O4

nanoparticles with different surfactant such as Cetyltrimethylammoniumbromide

(CTAB), Triethylamine (TEA), Diethylamine (DEA), Ethylamine (EA) and

Polyvinyl alcohol (PVA). The ZnFe2O4 nanoparticles using surfactant CTAB, TEA,

DEA, EA have been synthesized through hydrothermal method. Co precipitation

method was employed for the preparation of ZnFe2O4 nanoparticles using PVA

surfactant with different molar concentration of Fe3+ ions and these nanoparticles

were utilized for photocatalytic activity. The present thesis consists of five chapters.

Chapter 1 gives a brief introduction to nanoparticles, features of

nanoparticle and properties and significance of nanoscale materials. It also deals

with magnetic materials, magnetism, different types of magnetism, ferrites,

zinc ferrites, photocatalytic mechanism and photocatalytic applications. Different

synthesis techniques of nanoparticles are presented in this chapter. Elaborate

literature survey of the recent work on the chosen ZnFe2O4 materials has also been

described. The main objective of the present work is given at end of this chapter.

29

Chapter 2 briefly describes the different advanced techniques used to

characterize the prepared nano scale materials. In the present research work, the

synthesized nanostructures were characterized by various characterization

techniques such as X - ray diffraction (XRD), Fourier transform infrared

spectroscopy (FTIR), Thermo gravimetric analysis (TGA), Differential thermal

analysis (DTA), Scanning electron microscopy (SEM), Transmission electron

microscopy (TEM), Energy dispersive X - ray analysis (EDX), Vibrating sample

magnetometer (VSM), UV - Visible absorption spectroscopy (UV - Vis) and diffuse

reflectance spectroscopy (DRS). In addition to this, it also presents the principles,

description of the instrumentation used for the photocatalytic studies.

Chapter 3 discusses the synthesis of zinc ferrite nanoparticles by

hydrothermal method using different surfactants and their properties. Section 3.1

presents the synthesis of super paramagnetic Zinc Ferrites nanoparticles via

hydrothermal method, using CTAB as a surfactant. The effect of adding CTAB in

various concentrations, namely, 0 g, 0.5 g, 1.0 g, and 1.5 g are investigated with

respect to the phase formation, densification, morphology, particles size and

magnetic properties of synthesized ZnFe2O4 nanoparticles. Section 3.2 includes

effect of TEA on the structural and magnetic properties of Ferromagnetic ZnFe2O4

nanoparticles by hydrothermal method. The concentration of Triethylamine was

varied to 2 ml, 4 ml, 6 ml, 8 ml and 10 ml respectively as mentioned above. The

formation mechanism for the single phase ZnFe2O4 was discussed with respect to

amount of TEA. The hematite phase was observed by the lower amount of

Triethylamine. The weak ferromagnetism was confirmed by VSM measurements.

Section 3.3 summarizes the use of facile hydrothermal method for the synthesis of

the super paramagnetic ZnFe2O4 nanoparticles using different amounts of DEA. The

lower amount 2 ml of DEA in the reaction medium produced mixed phase - Fe2O3

and ZnFe2O4. The formation mechanism is also discussed. Magnetic property of

nanoparticles was investigated by VSM measurements. Superparamagnetic property

was observed 4 ml, 6 ml, 8 ml and 10 ml of DEA. Section 3.4 gives ZnFe2O4

nanoparticles synthesized by a surfactant assisted hydrothermal method using

different concentration of ethylamine namely, 2 ml, 4 ml, 6 ml, 8 ml and 10 ml. The

30

amount of 2 ml and 4 ml of ethylamine yielded mixed phase (hematite and Zinc

ferrite). The phase evolution studies regarding the formation mechanism of such

nanoparticles are also reported. High resolution TEM image shows the single

crystalline structure of the ZnFe2O4 with higher amount of DEA. Vibrational and

magnetic properties of all synthesized nanoparticles have also been discussed.

Chapter 4 describes the synthesis of ZnFe2O4 / ZnO nanoparticles by

co - precipitation method using polyvinyl alcohol (PVA) as the surfactant. In the

work we have kept molar concentration of Zn2+ as 0.1 M and the weight of

surfactant polyvinyl alcohol (PVA) as 2 g for all the experiment and ferric nitrate

with different molar concentrations varying from 0 - 0.2 M (in steps of 0.02 M) were

added in the reaction medium to obtain the final products. The effects of Fe3+ molar

concentration, the phase formation, chemical analysis, morphology, optical and

magnetic properties were investigated. Furthermore, the photocatalytic activity of

ZnFe2O4 / ZnO nanoparticles for the degradation of methylene blue (MB) under

visible light irradiation is discussed.

Chapter 5 gives the summary and conclusions obtained from the present

investigation. Also it seeks out suggestions for future development of the work.