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Chapter 1

1.1

 

1.1 Introduction to Nanomaterials

Introduction

Nanomaterials are cornerstones of nanoscience and nanotechnology. Nanostructure

science and technology is a broad and interdisciplinary area of research and

development activity that has been growing explosively worldwide in the past few

years. It has the potential for revolutionizing the ways in which materials and products

are created and the range and nature of functionalities that can be accessed. It is

already having a significant commercial impact, which will assuredly increase in the

future [1].

Figure 1.1.1: Evolution of science and technology and the future.

What are nanomaterials?

Nanoscale materials are defined as a set of substances where at least one dimension is

less than approximately 100 nanometers. A nanometer is one millionth of a millimeter

- approximately 100,000 times smaller than the diameter of a human hair.

Nanomaterials are of interest because at this scale unique optical, magnetic, electrical,

Chapter 1

1.2

 

and other properties emerge. These emergent properties have the potential for great

impacts in electronics, medicine, and other fields.

Figure 1.1.2: Nanomaterial (For example: Carbon nanotube).

Some nanomaterials occur naturally, but of particular interest are engineered

nanomaterials (EN), which are designed for, and already being used in many

commercial products and processes. They can be found in such things as sunscreens,

cosmetics, sporting goods, stain-resistant clothing, tires, electronics, as well as many

other everyday items, and are used in medicine for purposes of diagnosis, imaging

and drug delivery.

Engineered nanomaterials are resources designed at the molecular (nanometre)

level to take advantage of their small size and novel properties which are generally

not seen in their conventional, bulk counterparts. The two main reasons why materials

at the nano scale can have different properties are, increased relative surface area and

new quantum effects. Nanomaterials have a much greater surface area to volume ratio

than their conventional forms, which can lead to greater chemical reactivity and affect

their strength. Also at the nano scale, quantum effects can become much more

important in determining the materials properties and characteristics, leading to novel

optical, electrical and magnetic behaviours.

Nanomaterials are already in commercial use, with some having been

available for several years or decades. The range of commercial products available

today is very broad, including stain-resistant and wrinkle-free textiles, cosmetics,

sunscreens, electronics, paints and varnishes. Nanocoatings and nanocomposites are

finding uses in diverse consumer products, such as windows, sports equipment,

bicycles and automobiles. There are novel UV-blocking coatings on glass bottles

Chapter 1

1.3

 

which protect beverages from damage by sunlight, and longer-lasting tennis balls

using butyl rubber/ nano-clay composites. Nanoscale titanium dioxide, for instance, is

finding applications in cosmetics, sun-block creams and self-cleaning windows, and

nanoscale silica is being used as filler in a range of products, including cosmetics and

dental fillings.

Brief History of Nanomaterials

The history of nanomaterials began immediately after the big bang when

Nanostructures were formed in the early meteorites. Nature later evolved many other

Nanostructures like seashells, skeletons etc. Nanoscaled smoke particles were formed

during the use of fire by early humans. The scientific story of nanomaterials however

began much later. One of the first scientific reports is the colloidal gold particles

synthesized by Michael Faraday as early as 1857. Nanostructured catalysts have also

been investigated for over 70 years. By the early 1940’s, precipitated and fumed silica

nanoparticles were being manufactured and sold in USA and Germany as substitutes

for ultrafine carbon black for rubber reinforcements.

Nanosized amorphous silica particles have found large-scale applications in

many every-day consumer products, ranging from non-diary coffee creamer to

automobile tires, optical fibers and catalyst supports. In the 1960s and 1970’s metallic

nano powders for magnetic recording tapes were developed. In 1976, for the first

time, nanocrystals produced by the now popular inert- gas evaporation technique was

published by Granqvist and Buhrman [2]. Recently it has been found that the Maya

blue paint is a nanostructured hybrid material. The origin of its color and its resistance

to acids and bio corrosion are still not understood but studies of authentic samples

from Jaina Island show that the material is made of needle-shaped palygorskite (clay)

crystals that form a super lattice with a period of 1.4 nm, with intercalates of

amorphous silicate substrate containing inclusions of metal (Mg) nanoparticles. The

beautiful tone of the blue color is obtained only when both these nanoparticles and the

super lattice are present, as has been shown by the fabrication of synthetic samples.

Today nanophase engineering expands in a rapidly growing number of

structural and functional materials, inorganic and organic, allowing manipulating

mechanical, catalytic, electric, magnetic, optical and electronic functions. The

production of nanophase or cluster-assembled materials is usually based upon the

Chapter 1

1.4

 

creation of separated small clusters which then are fused into a bulk-like material or

on their embedding into compact liquid or solid matrix materials. e.g. nanophase

silicon, which differs from normal silicon in physical and electronic properties, could

be applied to macroscopic semiconductor processes to create new devices.

For instance, when ordinary glass is doped with quantized semiconductor ''colloids,'' it

becomes a high performance optical medium with potential applications in optical

computing.

Classification of Nanomaterials

Nanomaterials have extremely small size which having at least one dimension 100 nm

or less. Nanomaterials can be nanoscale in one dimension (e.g. surface films), two

dimensions (e.g. strands or fibres), or three dimensions (e. g. particles). They can exist

in single, fused, aggregated or agglomerated forms with spherical, tubular, and

irregular shapes. Common types of nanomaterials include nanotubes, dendrimers,

quantum dots and fullerenes. Nanomaterials have applications in the field of nano

technology, and displays different physical chemical characteristics from normal

chemicals (i.e., silver nano, carbon nano tube, fullerene, photo catalyst, carbon nano,

silica).

According to Siegel [3], Nanostructured materials are classified as Zero

dimensional, one dimensional, two dimensional, three dimensional nanostructures.

Figure 1.1.3: Classification of Nanomaterials (a) 0D spheres and clusters (b) 1D

nanofibers, wires, and rods (c) 2D films, plates, and Networks

(d) 3D nanomaterials.

Nanomaterials are materials which are characterized by an ultra fine grain size (< 50

nm) or by a dimensionality limited to 50 nm. Nanomaterials can be created with

various modulation dimensionalities as defined by Richard W. Siegel [3], zero

(atomic clusters, filaments and cluster assemblies), one (multilayers), two (ultrafine-

Chapter 1

1.5

 

grained overlayers or buried layers), and three (nanophase materials consisting of

equiaxed nanometer sized grains) as shown in the above Figure 1.1.3.

Properties of Nanomaterials

Nanomaterials have the structural features in between of those of atoms and the bulk

materials. While most micro structured materials have similar properties to the

corresponding bulk materials, the properties of materials with nanometer dimensions

are significantly different from those of atoms and bulks materials. This is mainly due

to the nanometer size of the materials which render them: (i) large fraction of surface

atoms; (ii) high surface energy; (iii) spatial confinement; (iv) reduced imperfections,

which do not exist in the corresponding bulk materials.

Due to their small dimensions, nanomaterials have extremely large surface

area to volume ratio, which makes a large to be the surface or interfacial atoms,

resulting in more “surface” dependent material properties. Especially when the sizes

of nanomaterials are comparable to length, the entire material will be affected by the

surface properties of nanomaterials. This in turn may enhance or modify the

properties of the bulk materials. For example, metallic nanoparticles can be used as

very active catalysts. Chemical sensors from nanoparticles and nanowires enhanced

the sensitivity and sensor selectivity. The nanometer feature sizes of nanomaterials

also have spatial confinement effect on the materials, which bring the quantum

effects.

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

modified quite differently from their bulk and in turn will modify the electronic and

optical properties of the materials. For example, lasers and light emitting diodes

(LED) from both of the quantum dots and quantum wires are very promising in the

future optoelectronics. High density information storage using quantum dot devices is

also a fast developing area. Reduced imperfections are also an important factor in

determination of the properties of the nanomaterials. Nanostructures and

Nanomaterials favors of a self purification process in that the impurities and intrinsic

material defects will move to near the surface upon thermal annealing. This increased

materials perfection affects the properties of nanomaterials. For example, the

chemical stability for certain nanomaterials may be enhanced, the mechanical

properties of nanomaterials will be better than the bulk materials. The superior

Chapter 1

1.6

 

mechanical properties of carbon nanotubes are well known. Due to their nanometer

size, nanomaterials are already known to have many novel properties. Many novel

applications of the nanomaterials rose from these novel properties have also been

proposed [1].

Optical properties

One of the most fascinating and useful aspects of nanomaterials is their optical

properties. Applications based on optical properties of nanomaterials include optical

detector, laser, sensor, imaging, phosphor, display, solar cell, photocatalysis,

photoelectron chemistry and biomedicine.

The optical properties of nanomaterials depend on parameters such as feature

size, shape, surface characteristics, and other variables including doping and

interaction with the surrounding environment or other nanostructures. Likewise, shape

can have dramatic influence on optical properties of metal nanostructures. Figure1.1.4

exemplifies the difference in the optical properties of metal and semiconductor

nanoparticles. With the CdSe semiconductor nanoparticles, a simple change in size

alters the optical properties of the nanoparticles. When metal nanoparticles are

enlarged, their optical properties change only slightly as observed for the different

samples of gold nano spheres in Figure 1.1.4. However, when anisotropy is added to

the nanoparticle, such as growth of nano rods, the optical properties of the

nanoparticles change dramatically [4].

Figure 1.1.4: Fluorescence emission of (CdSe) ZnS quantum dots of various

sizes and absorption spectra of various sizes and shapes of gold

NPs.

Chapter 1

1.7

 

Electrical properties

Electrical Properties of Nanoparticles discuss about fundamentals of electrical

conductivity in nano tubes and nano rods, carbon nano tubes, photoconductivity of

nano rods, electrical conductivity of nano composites. One interesting method which

can be used to demonstrate the steps in conductance is the mechanical thinning of a

nano wire and measurement of the electrical current at a constant applied voltage. The

important point here is that, with decreasing diameter of the wire, the number of

electron wave modes contributing to the electrical conductivity is becoming

increasingly smaller by well-defined quantized steps.

In electrically conducting carbon nano tubes, only one electron wave mode is

observed which transport the electrical current. As the lengths and orientations of the

carbon nano tubes are different, they touch the surface of the mercury at different

times, which provides two sets of information: (i) the influence of carbon nano tube

length on the resistance; and (ii) the resistances of the different nano tubes. As the

nano tubes have different lengths, then with increasing protrusion of the fiber bundle

an increasing number of carbon nano tubes will touch the surface of the mercury

droplet and contribute to the electrical current transport [5].

Figure 1.1.5: Electrical behavior of nanotubes.

Mechanical properties

Mechanical Properties of Nanoparticles deals with bulk metallic and ceramic

materials, influence of porosity, influence of grain size, super plasticity, filled

polymer composites, particle-filled polymers, polymer-based nano composites filled

with platelets, carbon nano tube- based composites. The discussion of mechanical

properties of nanomaterials is, in to some extent, only of quite basic interest, the

reason being that it is problematic to produce macroscopic bodies with a high density

Chapter 1

1.8

 

and a grain size in the range of less than 100 nm. However, two materials, neither of

which is produced by pressing and sintering, have attracted much greater interest as

they will undoubtedly achieve industrial importance.

These materials are polymers which contain nanoparticles or nano tubes to

improve their mechanical behaviors, and severely plastic-deformed metals, which

exhibit astonishing properties. However, because of their larger grain size, the latter

are generally not accepted as nanomaterials. Experimental studies on the mechanical

properties of bulk nanomaterials are generally impaired by major experimental

problems in producing specimens with exactly defined grain sizes and porosities.

Therefore, model calculations and molecular dynamic studies are of major importance

for an understanding of the mechanical properties of these materials.

Filling polymers with nanoparticles or nano rods and nano tubes, respectively,

leads to significant improvements in their mechanical properties. Such improvements

depend heavily on the type of the filler and the way in which the filling is conducted.

The latter point is of special importance, as any specific advantages of a nano

particulate filler may be lost if the filler forms aggregates, thereby mimicking the

large particles. Particulate filled polymer-based nanocomposites exhibit a broad range

of failure strengths and strains. This depends on the shape of the filler, particles or

platelets, and on the degree of agglomeration. In this class of material, polymers filled

with silicate platelets exhibit the best mechanical properties and are of the greatest

economic relevance. The larger the particles of the filler or agglomerates, the poorer

are the properties obtained. Although, potentially, the best composites are those filled

with nano fibers or nano tubes, experience teaches that sometimes such composites

have the least ductility. On the other hand, by using carbon nano tubes it is possible to

produce composite fibers with extremely high strength and strain at rupture. Among

the most exciting nano composites are the polymer ceramic nano composites, where

the ceramic phase is platelet-shaped. This type of composite is preferred in nature,

and is found in the structure of bones, where it consists of crystallized mineral

platelets of a few nanometers thickness that are bound together with collagen as the

matrix. Composites consisting of a polymer matrix and defoliated phyllosilicates

exhibit excellent mechanical and thermal properties [1].

 

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Chapter 1

1.9

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Chapter 1

1.10

 

nanoparticles. This observation suggested that modification of the d band structure by

chemical bonding can induce ferromagnetic like character in metallic clusters [1].

Why nanoparticles exhibit such properties?

Quantum confinement

Quantum confinement is a very successful model for describing the size dependent

electronic structure of nanometer sized semiconductor structures [6]. Generally

speaking, it predicts increasing band gaps with decreasing particle sizes due to

shifting of the band edges. The majority of theoretical investigations on quantum

confinement effects were performed on isolated particles with idealized structures and

surface terminations. The quantum confinement effect can be observed once the

diameter of the particle is of the magnitude as the wavelength of electron wave

function [6]. When the materials are so small, their electronic and optical properties

deviate substantially from those of bulk materials. A particle behaves as if it were free

when the confining dimension is large compared to the wavelength of the particle.

During this state, band gap remains at its original energy due to continuous energy

state. However, as the confining dimension decreases and reaches a certain limit,

typically in nanoscale, the energy spectrum turns to discrete as it is shown in

Figure 1.1.7 [7]. As a result, band gap becomes size dependent. This ultimately results

a blue shift in optical illumination as the size of the particles decreases. Specifically,

the effect describes the phenomenon that results from electrons and electrons-holes

being squeezed into a dimension that approaches a critical quantum measurement,

called the excitons Bohr radius. In current application, a quantum dot confines in all

three dimensions such as a small sphere, a quantum wire confines in two dimensions,

and a quantum well confines in one dimension.

Surface to volume ratio

It plays a very important role in technology. As the surface area increases the

complete term increases but the volume remains same. Nanostructures and

nanomaterials possess a large fraction of surface atoms per unit volume. The ratio of

surface atom to interior atoms changes dramatically if one successively divides a

macroscopic object into smaller parts [6]. Such a drastic increase in the ratio of

surface atoms to interior atoms in nanostructures and nanomaterials might illustrate

Chapter 1

1.11

 

why changes in size range of nanometers are expected to lead to great changes in

physical and chemical properties of the material. As the scale decreases (keeping the

volume constant) the surface area of the material increases as shown in Figure 1.1.8

[8] and with increase in surface area the ability of the particles to interact with other

particle increases. Also the physical and chemical properties like interactive forces,

reactivity etc. increases. So the S/V ratio has a significant effect on the properties of

nanoparticles.

Figure 1.1.7: Representation of quantum confinement.

Figure 1.1.8: Representation of surface to volume ratio.

Chapter 1

1.12

 

Brownian motion

Brownian motion is characterized by the constant and erratic movement of minute

particles in a liquid or gas. The molecules that make up the fluid, in which the

particles are suspended, as a result of the inherently random nature of their motions,

colloid with the larger suspended particles at random, making them move, in turn,

also randomly. At macroscale we barely see the movement of particles or why they

move but at nanoscale the particles moves widely batted about by a smaller paticle.

Potential Applications of Nanomaterials

High – density magnetic data storage arrays provide a major technological driving

force for the exploration of magnetic nanomaterials. Surface and interface driven

properties play a role in all applications of magnetic nanoparticles. Various interesting

physical properties due to quantum size effect are currently of great fundamental and

application interest. Some of the recent developments associated with magnetic

nanomaterials are given here.

Information Storage

Since the 1940s with the sudden growth in popularity of magnetic recording,γ-Fe2O3

in the form of small single domain particles has dominated the magnetic storage

materials technology [9]. The data storage industry is driving towards higher densities

of stored ‘bits’. If a reliable data storage system based on a single 5 nm magnetic

nanoparticles acting as an individual bit of information could be created, storage

densities of 10 Gbit/cm2 would be possible [10]. The presence of silica was found to

enhance coercively of cobalt ferrite nanomaterials [11].

Electronic Devices

Today multifunctional laptops and palmtops are more users friendly, faster, handy and

have large memory capacities. Mobile phones, pocket sized memory storage devices

and the widely used MP3 players, iPods are perhaps the most convincing benefits of

nanomaterials. Most of them make use of magnetic materials such as ferrites.

Quantum Computers

With the ability to synthesize quantum dots on a commercial scale, it has become

possible to exploit the laws of quantum mechanics for novel quantum computers,

Chapter 1

1.13

 

using fast quantum algorithms. Quantum computers can perform several computations

at the same time and are much faster. Quantum dots were discovered by Louis E.

Brus, at Bell Labs in 1980 and he was awarded fist Kavli prize in nanoscience in 2008

for his pioneering efforts in this field [12]. Quantum dots are semiconductor materials

in the nano scale, normally in the size range of 1 – 20 nm. Most of the Q-dots are

composed of group II – VI and III – V elements of periodic table. Some examples of

Q – dots are CdSe, CdS, CdTe, GaAs, InAs etc. currently Q-dots are being used in

imaging, labeling, sensing [13] solar cells [14].

Sensors

Sensors made of nanocrystalline materials are extremely sensitive to change in the

environment. Some of the applications for the sensors made of nanocrystalline

materials are some detectors, ice detectors on aircraft wings and automobile engine

performance sensors. Ferrite nanoparticles can be used as LPG sensors [15].

Medical Applications

Nanomaterials have dimensions similar to those of biological molecules and hence

useful for biomedical applications. By attaching different biomolecules to

nanomaterials, they can be used in medical applications for specific functions.

Nanotechnology is being developed for both therapeutic (using nano drug delivery

system) and diagnostic (nano biosensors) applications. Nanoparticles have shown

potentials for detecting virus, pre-cancerous cells, treatment of hyperthermia etc.

targeted delivery systems using Cobalt ferrite nanoparticles can deliver drugs

effectively and conveniently with increased patience compliance, extended product

life and reduced healthcare cost [16, 17].

Filtration

Magnetic nanoparticles offer an effective and reliable method to remove heavy metal

contaminants from waste water by use of magnetic separation techniques. Nanoscale

particles increase the efficiency to absorb contaminants. Since nanomaterials posses

unique properties they can be used for variety of applications ranging from toothpaste

to satellites. Nanotechnology is finding applications in virtually all fields ranging

from science to engineering, influencing our life style in many ways.

Chapter 1

1.14

 

Catalysis

Catalysis from nanoparticles is beneficiary due to the extremely large surface to

volume ratio. The application potential of nanoparticles in catalysis ranges from fuel

cell to catalytic converters and photocatalytic devices. Catalysis is also important for

the production of chemicals. Platinum nanoparticles are now being considered in the

next generation of automotive catalytic converters because the very high surface area

of nanoparticles could reduce the amount of platinum required [18].

1.2 Magnetic Nanoparticles

Introduction

Magnetic nanoparticles (MNPs) have been the focus of an increasing amount of the

recent literature, which has chronicled research into both the fabrication and

applications of MNPs. The explosion of research in this area is driven by the

extensive technological applications of MNPs which includes single – bit elements in

high – density magnetic data storage arrays, magneto – optical switches, and novel

photo – luminescent materials. In biomedicine, MNPs serve as contrast enhancement

agents for Magnetic Resonance Imaging, selective probes for bio – molecular

interactions and cell sorters. Nanoparticles of magnetic materials are also find

applications as catalyst, nucleators for the growth of high-aspect-ratio nano-materials,

and toxic waste remediation. Methodologies for the synthesis of MNPs are being

developed by scientists working in fields such as spanning Biology, Chemistry and

Materials Science. In the last decade, these efforts have provided access to nanoscale

magnetic materials ranging from inorganic metal clusters to custom – built Single

molecular Magnets [19].

The properties associated with the bulk magnetic materials are the same as that

in magnetic nanoparticles but due to the smaller size, magnetic nanoparticles show

interesting properties as compared to bulk particles. In magnetic nanoparticles some

of the new phenomenon like superparamagnetism and spin canting can be realized

which may not be seen in bulk magnetic particles. The saturation magnetization of the

magnetic nanoparticles is found to have less value when compared to bulk magnetic

particles. However, the coercivity of the magnetic nanoparticles is more value than

that of bulk magnetic particles.

Chapter 1

1.15

 

Magnetic Materials and Superparamagnetism

Materials which can be magnetized are called magnetic materials. The response of

materials is different at electronic, atomic, molecular and microscopic level to an

applied magnetic field. In different materials, the magnetic effect varies from weak to

strong. Application wise, magnetic materials are classified as soft and hard magnetic

materials and on account of their behavior, they are classified as diamagnetic,

paramagnetic, ferromagnetic, antiferromagnetic, ferrimagnetic and

superparamagnetic. The diamagnetic materials are weakly repelled by an external

magnetic field and show negative susceptibility, paramagnetic materials show small

and positive susceptibility and are weakly attracted by external magnetic field.

Ferromagnetic materials exhibit a large and positive susceptibility and are strongly

attracted towards magnetic field. In case of diamagnetic and paramagnetic materials,

the magnetic properties do not persist on the removal of external magnetic field.

In case of ferromagnetic materials, these properties are stable even after the external

magnetic field is removed.

Antiferromagnetic substances have small positive susceptibility at all

temperature but their susceptibility vary in a particular way with temperature. When

the spins are antiparallel due to negative interaction, it results in anti –

ferromagnetism.

Ferrimagnetic materials have magnetic moments of adjacent atoms as aligned

in opposite directions, but their magnitudes are not equal and opposite. So, there

exists a net magnetic moment which may have a value less than that in ferromagnetic

materials. The moment disappears above Curie temperature.

Superparamagnetism is a phenomenon in which magnetic materials may

exhibit the behavior similar to paramagnetism even when at temperatures below the

Curie or the Neel temperature [20,21]. The whole particle represents as single domain

having all atomic moments ordered. The magnetization of the particle is no longer

fixed in the direction dictated by the particle shape or crystal anisotropy, as the

frequently jump between different stable orientations of the magnetization. When a

system is in metastable equilibrium, the probability ‘p’ per unit time will switch out

the metastable state and will get into the more stable demagnetized state, given by:

Chapter 1

1.16

 

⎟⎟⎠

⎞⎜⎜⎝

⎛ Δ−=

bTkfVvP exp0 (1)

In equation (1), v0 is an attempt frequency factor equal to approximately 10-9s-1, V is

the particle volume and ΔfV is the free energy barrier that the particle moment must

surmount to leave the metastable state. In a particle with strong crystalline anisotropy,

Δf is equal to magneto crystalline anisotropy Kb. (The magnetization appears to be

zero in a time scale large as compared to the typical time flipping).

Application of an external magnetic field to an ensemble of such thermally

demagnetized particle results in a much larger magnetic response than that would be

the case of paramagnets, because now each local moments have magnitude N’μm

instead of μm, where N’ is the number of magnetic atoms in the particle. Thus, the

susceptibility of a super – paramagnetic is increased (N’) 2 fold.

For KuV << KT, the magnetic moments exhibit superparamagnetic relaxation

and the relaxation time can be given in a simplified way as:

⎟⎟⎠

⎞⎜⎜⎝

⎛=

TkKV

B

exp0ττ (2)

In equation (2), τ0 is the characteristic time (10-10s), K is the anisotropy energy

(20,000 J/m3 for iron oxide) and V is the volume of the particle, kB is the Boltzmann

constant and T is the temperature.

When the superparamagnetic relaxation time is short compared to the time

scale of the experimental method used for the magnetic properties, the sample

resembles a paramagnetic. The relaxation time depends on temperature and so by

decreasing the temperature, τ0 can be made larger than the time scale of measurement

and then the magnetic moment of the particle becomes measurable.

Spin canting

A spin canting is one of the parasitic phenomenons, it negatively reflects in a

capability of nano objects to be easily controlled by an external magnetic field. It is

governed by finite size effects and surface effects.

In case of nanoparticles, the surface plays very important role in order to

decide the magnetic properties. Normal coordination, in the interior, breaks down at

the surface to have very different forms [22]. Spin structures of such nanoparticles is

Chapter 1

1.17

 

very complicated as compared to their bulk counterpart [23]. Spin canting can be

investigated by using Mossbauer spectroscopy in high magnetic field [24].

The main effect of finite size on magnetic nanoparticles is that, the broken

exchange bonds for the surface atoms become dominant. This can have a particularly

strong effect on ionic materials. Since the exchange interactions are mainly anti –

ferromagnetic and which happens only through super – exchange interactions. It

crucially depends on the bond length and the bonds are moved from the surface, there

can be frustration and canted spin order in case of these magnetic nanoparticles.

Some important uses of magnetic nanoparticles

High density magnetic data storage arrays provide a major technological driving force

for the exploration of MNPs. If a reliable data storage system based on a single 5 nm

MNP acting as an individual bit of information could be created, storage densities of

10 Gbit/cm2 would be possible [25]. MNPs have also demonstrated to be functional

elements in magneto – optical switches [26], sensors based on Giant Magneto

resistance [27], and magnetically controllable Single Electron Transistor devices

[28], or photonic crystals [29]. One of the first stages in the development of these

MNP – based materials is the creation of ordered 2 – and 3 – dimensional arrays of

MNPs [30]. Two dimensional arrays are typically fabricated by the slow evaporation

of highly monodispersed MNP solutions onto a substrate [31]. Structural control can

be achieved by the application of a magnetic field [32] or patterning using dip – pen

nanolithography [33]. Three dimensional nanoparticles assembles with complex

structures can also be fabricated by the slow evaporation technique in the presence of

an applied magnetic field [34]. These technological applications are all in the addition

to the numerous known and developed applications of aqueous suspensions of MNPs

(ferro fluids) [35].

1.3 Introduction to Ferrites

Among the numerous classes and types of magnetic materials, the ferrite occupies a

special class. Ferrites are widely used magnetic materials as they posses excellent

electrical as well as magnetic properties. The interesting physical and chemical

properties of ferro spinels arise from their ability to distribute the cations among the

available sites. The knowledge of cation distribution is essential to understand the

Chapter 1

1.18

 

physical properties. On account of their combined properties, they can be used in

many applications which include fabrication of magnetic cores of read/write heads for

high speed digital tapes or disc recording [36, 37].

Ferrites are the ferrimagnetic materials consisting of iron oxide and metal

oxides. On the basis of their crystal structure, they can be grouped into four main

types, namely spinel ferrite (MFe2O4, M = Co, Ni, Cu, Mn etc.) hexagonal ferrite

(MeFe12O19, Me = Ba, Sr or Pb, Ca), Ortho ferrite (RFeO3, e.g. BaTiO3, PbTiO3),

Garnet (R3Fe5O12, R = Y or rare earth ions like La, Gd, Dy etc). In the family of

ferrites, over many years, spinel ferrites have been subject of concern due to their

wide possibilities of applications in electronics and radio engineering. The advantages

of spinel ferrite as magnetic materials are due to their high values of electrical

resistivity, high saturation magnetization, high permeability, low eddy current and

dielectric losses [38].

Why Ferrites in 21st Century?

Figure 1.3.1: Number of publications on Magnetic materials (Year wise).

Though, the field of ferrite materials is well cultivated, but due to its potential

applications in various field, even after 60-65 years, after first artificial synthesis and

characterization of spinel ferrite system, scientists, researchers, technologists and

engineers are still interested in various types of ferrite systems, substituted with

different cations, prepared by different synthesis techniques, in bulk, thin film and

Chapter 1

1.19

 

nanocrystalline form and its various properties such as, structural, magnetic,

electrical, dielectric, optical, infrared spectral, mechanical etc.

From Figure 1.3.1 it is quite clear that since the year 2000 number of

publications on magnetic materials, particularly in the field of nanocrystalline ferrites

increases so rapidly [39], which supports the fact that research in the field of ferrites

in 21st century is still the subject of large interest and possibilities.

Nanocrystalline spinel ferrites

Recently, nano – magnetic materials have attained great interest because of their

potential applications in many fields like high density data storage, ferro – fluid,

magneto – optical recording, magneto – caloric refrigeration, magnetic resonance

imaging, drug delivery etc [40], the synthesis of magnetic materials on the nano scale

has been a field of intense study, due to the novel macroscopic properties shown by

particles of quantum dimensions located in the transition region between atoms and

bulk solids [41]. Quantum size effect and the large surface area of magnetic

nanoparticles dramatically change some of the magnetic properties and exhibit

superparamagnetic phenomena and quantum tunneling of magnetization, because

each particle can be considered as single magnetic domain.

Magnetic nanoparticles of spinel ferrites are of great interest in fundamental

science, especially for addressing the fundamental relationships between magnetic

properties and their crystal chemistry and structure. Crystal chemistry shows how the

chemical composition (chemical formula), internal structure and physical properties

of minerals are linked together. Spinel ferrites have been investigated in recent years

for their useful electrical and magnetic properties and applications in information

storage systems, magnetic bulk cores, magnetic fluids, microwave absorbers and

medical diagnostics. The synthesis and magnetic structure characterization of spinel

metastable nano ferrites have been investigated with much interest [42] and a lot of

attention has been focused on the preparation and characterization of

superparamagnetic metal oxide nanoparticles of spinel ferrites [42]. The rich crystal

chemistry in spinel ferrite systems offers excellent opportunities for understanding

and fine tuning the superparamagnetic properties of nanoparticles by chemical

manipulations.

Chapter 1

1.20

 

A wide variety of techniques are being used to synthesize nanostructured

materials including gas condensation, rapid solidification, electrodeposition,

sputtering, crystallization of amorphous phases, wet-chemical methods like co

precipitation, sonochemical reactions, sol–gel method, combustion, reverse micelle

technique, hydrothermal route and mechanical attrition-ball milling [42].

The magnetic, electric and dielectric behavior of spinel ferrites decisively

depends upon the structural properties. Therefore, the non-destructive methods of

characterization such as X-ray diffraction and infrared spectroscopy especially suited

for such investigations. The wavelength of X-ray, electrons and neutrons are

comparable to the interplanner distances in solids. Because of high penetrating power,

X-ray can provide important information regarding structural properties of matter.

The angle of diffraction and intensity of diffracted beam together are characteristics of

a particular crystal structure since no two atoms have exactly the same size and X-ray

scattering ability; the intensities of diffracted beam will be unique for every material.

This uniqueness helps to identify the structure and determine the structural parameters

of the material [43].

The use of nanoparticles as catalysts in organic transformation has attracted

considerable interest in recent years [44]. Spinel ferrites offer more interesting

catalytic activities compared to the corresponding single component metal oxides

[45]. Spinel ferrite nanocrystals are regarded as one of the most important inorganic

nanomaterials because of their electronic, optical, electrical, magnetic and catalytic

properties, all of which are different from their bulk counterparts. These properties are

dependent on chemical composition and microstructrual characteristics, where the

particle size and shape might be controlled in the fabrication processes [44].

Irradiation of ferrites is a powerful tool to enhance crystallographic defects

and changes in properties of ferrites [45]. Various kinds of radiation such as fast

neutron, energetic ions and γ rays were used to study the effect of radiation on the

properties of ferrites with different compositions. The obtained data are important for

evaluation of the ferrites performance in electronic components used in highly

radioactive environment such as nuclear facilities. Even the satellites and spacecrafts

can be exposed to cosmic radiations of rather high accumulated dose. The changes in

hysteresis loop parameters and magnetization in bulk spinel ferrites due to

Chapter 1

1.21

 

γ irradiation and heavy ions were investigated for several types of ferrites. However,

if the change in electrical or magnetic properties of ferrites is linearly dependant on

absorbed dose, the ferrites can be used to develop sensitive dosimeter [45].

Defect distribution plays a dominant role in tuning the properties of ferrites.

The positron annihilation lifetime (PAL) spectroscopy is one of the powerful

techniques for measuring the change in properties of structural defects in a wide

variety of solids. The PAL technique was used to study the electron density and to

determine the structure, nature, and concentration of point and extended defects in

ferrites. Also, the induced defects and changes in electron density and grain size by

irradiation can be accurately analyzed using this technique. The injected positron,

emitted from a 22Na source, into solids is thermalized and diffused to a depth of a few

hundred μm. The positron is either delocalized or trapped in defects, which lead to an

increase of its lifetime. Also, the positron may extract an electron from the

surrounding material to form a positronium atom (Ps) with two states; a singlet state

(para-Ps, p-Ps) and a triplet state (ortho-Ps, o-Ps). In both cases, the positron will

eventually annihilate with an electron and will have a corresponding lifetime with a

corresponding intensity for each state at annihilation site. Each lifetime component

has a corresponding intensity (I1, I2) relating to the number of annihilations occurring

at a particular lifetime (τ1, τ2). Therefore, the detection of annihilation g rays due to

trapping of positron in tetrahedral and octahedral sites is an indication for lifetime of

the impeded positrons in ferrite sample which reflect the size of defects and the ionic

distribution. The PAL technique was therefore employed successfully in studying

several types of ferrites [45].

Studies of the relations between structure and the electromagnetic response of

ferromagnetic semiconductors are useful in understanding their properties. Infrared

absorption spectroscopy has been used to study the occurrence of various absorption

bands in the spectra and analyzed on the basis of different cations present on A- and

B-sites of spinel lattice. It is used to determine the local symmetry in crystalline/non-

crystalline solids, ordering phenomenon in spinels, presence/absence of Fe2+- ions,

also to determine force constants, elastic moduli, Debye temperature, and heat

capacity of ferrite materials [46].

Chapter 1

1.22

 

In case of nanoparticles, the calcinations temperature determines the size of

the particle and crystallinity. Crystallization temperature is different for different

materials. In the thermogravimetric analysis, the temperature region which does not

contain any peaks represents the crystallization region. So this is an efficient tool for

the determination of temperature at which crystallization or recrystallization occur

[47]. Study of phase transition in nanoparticles in comparison with that in the bulk

materials has wide spread appreciation [48].

1.4 Aim and Outline of the present work According to the existing literature, cobalt ferrite, CoFe2O4, is an inverse spinel taken

to be collinear ferrimagnet [49], with magnetic moment of 3 μB at 80 K and Neel

temperature of 825 K [50]. Nanocrystalline CoFe2O4 with fcc spinel structure has

potential applications in the field of medicine, high density data storage, catalysis,

sensors, magnetic recording [51]. Cobalt ferrite exhibits high dc resistivity, positive

anisotropy constant, large magnetostriction, high permittivity and low losses made

them suitable for microwave device applications.

Transition metal chromites with the general chemical formula MCr2O4, are of

interest because of their technological applications as catalysts (ZnCr2O4), refractories

(CaCr2O4 and MgCr2O4), and pigments and glazes (CoCr2O4) [52]. Spinel cobalt

chromite, CoCr2O4, is a new spiral magnet that has been recently found to exhibit

strong magnetoelectric coupling [53]. Thus, expected to be useful in tailoring electric

polarization. In addition, they often serve as model systems for studying low

temperature magnetic phenomena [54].

Cobalt ferrite and cobalt-ferrite based composites have received recent

attention, as promising materials for magnetomechanical strain sensing and actuating

applications, and multiferroic materials [55]. CoCr2O4 is a normal spinel with canted

ferromagnetic structure and its Curie temperature is 97 K [56]. Cobalt ferrite,

CoFe2O4, and cobalt chromite, CoCr2O4, and their solid solutions CoCrxFe2-xO4 (x =

0.0 – 2.0) have attracted large number of chemists, physicists and metallurgists to

study their different aspects [51 – 57].

Manganese ferrite, MnFe2O4, is considered as a most controversial and

complex spinel ferrite [58]. The cation distribution, degree of inversion, oxidation

state of Fe3+ and Mn2+ and particularly its magnetic properties are still a subject of

Chapter 1

1.23

 

interest. On the other hand, MgFe2O4, is a partially inverse spinel and it can be

considered as a collinear ferrimagnet whose degree of inversion depends on the

thermal history of the sample.

Apart from the spinel composition and cation distribution, it is known that

physical properties can also tailored both by controlling the size of the nanocrystalline

domains and by the morphology of the material itself.

The purpose of the work is double sided. Initially we have presented the

important observations which were obtained from more conventional experimental

techniques such as XRD, TEM, EDAX, TGA, DSC etc. and highlight the changes

occurring at the different stages of substitution. The emphasis is thereafter diverted to

demonstrate the ability of nanoparticles to degrade pollutants and positron

annihilation techniques to sense such changes. The latter two aspects are of

importance since such works are scarcely available in the literature so far, and

secondly it offers a viable investigative probe for such studies which are highly

essential in the current scenario of novel material and arising challenges in their

understanding of macrostructure and preparative parameters [59]. The system under

investigation MgxMn1-xFe2O4 (x = 0.0 – 1.0) belongs to a large class of compounds

having general chemical formula A2+B23+O4

2- and crystallize in the spinel structure.

Nanoparticles of spinel ferrite systems with general chemical formulae,

CoCrxFe2-xO4 (x = 0.0 – 2.0) and MgxMn1-xFe2O4 (x = 0.0 – 1.0) have been

synthesized by the co-precipitation route. The synthesized samples have been

characterized by employing various experimental tools.

(i) X-ray powder diffractometry at 300 K.

(ii) Energy dispersive analysis of X-rays at 300 K.

(iii) Particle size distribution study at 300 K.

(iv) Transmission electron microscopic study at 300 K.

(v) Thermo gravimetric and differential scanning calorimetric analysis at

30 – 700 °C.

(vi) Mossbauer spectroscopic study at 300 K.

(vii) Photocatalytic degradation of organic pollutant like nitrobenzene and

herbicide like 2, 4 – Dichlorophenoxyacetic Acid (2, 4–D), studies have been

carried out under UV light irradiation.

Chapter 1

1.24

 

(viii) Crystal defects and cation redistribution study on nanocrystalline cobalt-ferri-

chromites by positron annihilation spectroscopy at 300 K.

Chapter 1

1.25

 

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Chapter 1

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