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Chapter 1

INTRODUCTION

General Introduction to nanomaterials, their methods of preparation, properties and

applications are described in this Chapter. Aim of the present work and Chapter-wise

break-up of the thesis are also included in it.

1.1 Introduction to nanomaterialsLiterature reveals that the history of nanomaterials bEgan immediately after the big

bang when nanostructures were formed in the early meteorites [1]. Nature later evolved

many other nanostructures like seashells, skeletons etc. Nano scaled smoke materials were

formed during the use of fire by early humans. However, the scientific story of

nanomaterials bEgan much later. One of the first scientific reports is the colloidal gold

nanomaterials synthesized by Michael Faraday as early as 1857 [2]. 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. On Dec. 29,

1959 at the California Institute of Technology, Pasadena, Nobel Laureate Richard P.

Feynman gave a talk at the annual meeting of the American Physical Society that has

become one of the twentieth century’s classic science lectures, titled “There’s Plenty of

Room at the Bottom” [3]. He pointed out that if a bit of information requires only 100

atoms, then all the books ever written could be stored in a cube with sides 0.02 inch long

[4]. The history of nanomaterials is quite long; nevertheless, major developments with in

nanoscience have taken place during the last decade or so. Research in nanomaterials is a

multidisciplinary effort that involves interaction between researchers in the field of

physics, chemistry, mechanics, materials science and even biology and medicine.

Reportedly, the first nanoparticles based technology, which is a heterogeneous catalyst,

was developed in the early nineteenth century, followed by the use of silver halide

nanoparticles in photography [5].

The discovery of novel materials, processes and phenomena at the nanoscale as well

as the development of new experimental and theoretical techniques for research provide

fresh opportunities for the development of innovative nanosystems and nanostructured

materials. Nanosystems are expected to find various unique applications. Nanostructured

materials can be made with unique materials on the basis of their properties.

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The nanotechnology field in addition to the fabrication of nano-system provides the

platform to the development of better experimental and computational tools. Due to the

unique properties, the nanomaterials are used in industrial, commercial and defence

applications. Micro-and nanosystems are likely to be the next step in the “silicon

revolution".

1.2 The Word “nano”The word “Nano” means dwarf in Greek language. Normally nano is used as a prefix

for any unit like a second or a metre and it means a billionth part (10-9) of that unit. A

nanosecond is one billionth of a second and a nanometre is one billionth of a metre-about

the length of a few atoms lined up shoulder to shoulder. A world of things is built up from

the tiny scale of nanometres. In contemporary science, we consider atom as the smallest

particle and nanomaterials as the smallest building blocks of the nature. Nanometre is only

1 to 5 atoms wide while nanoparticles range from 1 to 100 nm. A hydrogen atom is 0.1

nm. We frequently quote hair as the thinnest. However nanometre is about 40,000 to

80,000 times smaller than the hair. While studying biology one talks of DNA, virus,

proteins, bacteria and cells as the smallest building blocks of a body. Width of DNA is

about 2 nm.; the size of proteins range from 5 to 50 nm; virus ranges from 75 to 100 nm;

bacteria is 1000 nm, red cells are approximately 7000 nm in diameter and 2000 nm in

height while white blood cells are approximately 10,000 nm in diameter. DNA, virus,

proteins, bacteria and cells are the natures tiniest and the most wonderful machines at

micro level or nano level. If one goes deep in to the functioning of proteins, DNA,

membranes and other nanoscale devices of the nature and copies these to build scientific

devices at nanoscale, one may find very encouraging results. Examples are fullerenes of

1nm, quantum dots of 8 nm and dendrimers of about 10 nm which have revolutionary

developments in the fields of materials science, electronics, biotechnology, medicine etc.

Cellular proteins and enzymes are one of the important constituents of human body. A few

nano structures are illustrated in figure 1.1.

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Figure: 1.1 (a) nanometre-Ten shoulder-to-shoulder hydrogen atoms (balls) span 1 nm,

DNA molecules are about 2.5 nm wide; (b) Thousands of nanometres- Biological cells,

like blood cells, have diameters in the range of thousands of nm. (c) Synthetic gold

nanoparticles of 50 nm.

Figure 1.2. Logarithmical length scale showing size of nanomaterials compared tobiological components

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1.3 Classification of nanostructured Materials

Figure 1.3 Classification of nanomaterials (a) Zero dimensional spheres and clusters,

(b) One dimensional nano-fiber, wires, and rods, (c) Two dimensional films, plates, and

networks, (d) Three dimensional materials (bulk materials).

Nanostructured materials can be created with various modulation dimensionalities

such as zero (e.g. atomic clusters, quantum dots and cluster assemblies), one (e.g.

multilayers), two (e.g. ultrafine-grained over layers or buried layers/ nanotube), and three

dimensional materials (e.g. bulk materials). Oswald was the first to realize that materials of

such dimensions should display novel and interesting properties, which have largely

dependent on their size and shape [6]. However, it is only from the last two decades that

significant interest has been devoted to inorganic materials consisting of a few hundred or

a few dozen atoms called clusters. The interest has been extended to a large variety of

metals and semiconductors because nanomaterials exhibited special properties, which

differ from corresponding macro-crystalline material. The bulk materials that are

constituted of atoms and molecules as such has been widely classified and satisfactorily

explained. On the basis of dimensionality , composition and shape nanomaterials are

classified in different categories which are described below.

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1.3.1 One Dimensional nanomaterialsMaterials with one dimension in the nanometer scale are typically thin films or

surface coatings, and include the circuitry of computer chips and the anti reflection and

hard coatings on eye glasses. Thin films have been developed and used for decades in

various fields such as electronics, chemistry and engineering. Thin films can be deposited

by various methods [6] and can be grown controllably to be only one atom thick[7] , so

called monolayer.

1.3.2 Two Dimensional NanomaterialsTwo-dimensional nanomaterials have two dimensions in the nanometer scale. These

include 2D nano structured films, with nanostructures firmly attached to a substrate, or

nanopore filters used for small particle separation and filtration. Free particles with a large

aspect ratio, with dimensions in the nanoscale range, are also considered 2D

nanomaterials. Asbestos fibers are an example of 2D nanoparticles.

1.3.3 Three Dimensional nanomaterialsMaterials that are nano scaled in all three dimensions are considered 3D

nanomaterials. These include thin films deposited under conditions that generate atomic-

scale porosity, colloids, and free nanoparticles with various morphologies [7].

1.3.4 NanocompositesNanomaterials can be composed of a single constituent material or be a composite of

several materials. The nanocomposites found in nature are often agglomerations of

materials with various compositions, while pure single-composition materials can be easily

synthesized today by a variety of methods..

Today nanoparticles play key role in advance technology but nanoparticles having

limited applications so to increases functionality of materials the nanocomposites are come

into picture. The study of composite material, which are consisting of at least two phases

with different chemical composition. It has been of great interest from both fundamental

and practical point of view. The physical properties of nanocomposites can be combined to

produce material of desired response. Optical or magnetic characteristics changes when

particle sizes decreases to very small dimensions, which are in general of major interest in

the area of nanocomposites materials. Composites have excellent properties such as high

hardness, high melting point, low density, low coefficient of thermal expansion, high

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thermal conductivity, good chemical stability and improved mechanical properties such as

higher specific strength, better wear resistance and specific modulus and have good

potential for various industrial fields [8].

1.3.5 Nanoparticles uniformity and agglomerationBased on their chemistry and electro-magnetic properties, nanoparticles can exist as

dispersed aerosols, as suspensions/colloids, or in an agglomerate state (Figure 8). For

example, magnetic nanoparticles tend to cluster, forming an agglomerate state, unless their

surfaces are coated with a non-magnetic material. In an agglomerate state, nanoparticles

may behave as larger particles, depending on the size of the agglomerate. Hence, it is

evident that nanoparticle agglomeration, size and surface reactivity, along with shape and

size, must be taken into account when deciding considering health and environmental

regulation of new materials.

Figure 1.4. Classification of nanostructured materials from the point of view ofnanostructure dimensions, morphology, composition, uniformity and agglomeration

state.

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1.4 Methods of synthesis of nanomaterials: Bottom-Up and Top-Down

ApproachesLiterature survey reveals that nanomaterials are mainly fabricated by two methods:

top-down and bottom-up.

The top-down technique starts with a bulk material and then breaks it into smaller

pieces using mechanical, chemical or other form of energy. While in bottom-up approach

nanomaterials are synthesised from atomic or molecular species via chemical reaction etc.

Researchers, scientists and technologists are interested in nanoparticles (NPs) and

nanostructure materials (NMs) for different applications by controlling their size, shape,

distribution, composition, and dEgree of agglomeration.

Attrition or milling is a typical top-down method in making

nanoparticles /nanomaterials, where as the colloidal dispersion is a good example of

bottom-up approach for the synthesis of nanoparticles/nanomaterials. Bottom-up self-

assembly of colloidal sub-micron size spheres as well as top-down holographic laser

lithography in photo resists are reliable tools for the inexpensive, large-scale fabrication of

three-dimensional photonic crystals [12]. Lithography may be considered as a hybrid

approach, since the growth of thin films is bottom-up where as etching is top-down, while

nanolithography and nanomanipulation are commonly a bottom-up approach. These

approaches have few advantages and disadvantages.

The top-down approach is commonly used because it follows a traditional object-

oriented design, which advocates that complexity is best understood by starting with an

abstraction and decomposing it into smaller units. However, there are times when working

with an abstraction is problematic and it is more productive to start with the basics and

work upward. The major problem with top-down approach is the imperfection of the

surface structure. It is well known that the conventional top-down techniques such as ball

milling and lithography can cause significant crystallographic damage to the processed

patterns, and in case of lithography additional defects may be introduced even during the

etching steps [13]. For example, nanowires made by lithography are not smooth and may

contain a lot of impurities and structural defects on surface [14]. Such imperfections would

have a significant impact on physical properties and surface chemistry of nanostructures

and nanomaterials, since the surface to volume ratio in nanostructures and nanomaterials is

very large [15].

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The bottom-up approach refers to the build-up of a material from the bottom: atom-

by-atom, molecule-by-molecule, or cluster-by-cluster. In organic chemistry and/or

polymer science, we know polymers are synthesized by connecting individual monomers

together. In crystal growth, growth species, such as atoms, ions and molecules, after

impinging on to the growth surface, assemble into crystal structure one after another. The

bottom-up-approach has played significant role in the area of nanostructures and

nanomaterials research. There are several reasons for this. When structures fall into a

nanometre scale, there is little choice for a top-down approach. Bottom-up approach also

promises a better chance to obtain nano-structures with less defects, more homogeneous

chemical composition, and better short and long range ordering [16]. The main difficulty

of this approach is that an object may be segmented into multiple regions, some of which

may merge the object with its background.

The above mentioned approaches may be further classified in terms of physical and

chemical techniques. Broadly, physical technique is based on a process of transferring

growth species from a source or target. The process proceeds from atomic state and mostly

involves no chemical reactions. Various methods have been developed for the removal of

growth species from the source or target. However in case of chemical technique,

chemistry is very rich, and various types of chemical reactions are involved.

Chemical and Physical Methods

Chemical Methods: Chemical methods are widely used for the synthesis of

nanoparticles. Chemistry has played a major role in developing new materials with novel

and technologically important properties [17]. The advantage of chemical synthesis is its

versatility in designing and synthesizing new materials that can be refined into the final

product. The primary advantage that chemical processes offer over other methods is in

achieving good chemical homogeneity, as chemical synthesis offers mixing at the

molecular level. However, there are certain difficulties in chemical processing. In some

preparations, the chemistry is complex and hazardous. Contamination can also result from

the by-products being generated or side reactions in the chemical process. Agglomeration

can also be a major cause to the concern process at any stage in a synthetic process and it

can dramatically alter the properties of the materials. Finally, many chemical processes are

suitable for economical production and it is not always straight forward for all systems.

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Co-precipitation method is one such technique that has been used to produce

nanomaterials /nanocomposites for the present work.

Physical Methods: Several physical methods are currently in use for the synthesis

and commercial production of nanostructured materials. In this section, various

experimental techniques, starting with physical techniques are discussed below.

1.4.1 Mechanical grinding

Mechanical attrition/milling is a typical example of ‘top-down’ approach of synthesis

of nanomaterials, where the material is prepared not by cluster assembly but by the

structural decomposition of coarser-grained structures as the result of severe plastic

deformation [18]. In this process small steel balls are allowed to rotate around the inside of

a drum and drop with gravity force on to a solid enclosed in the drum. This has become a

popular method to make nanocrystalline materials because of its simplicity, the relatively

inexpensive equipment (on the laboratory scale) needed, and the applicability to the

synthesis of essentially all classes of materials. The major advantage of this technique

often quoted is the possibility for easily scaling up to tonnage quantities of material for

various applications. The other significant advantage of this method is that it is

inexpensive, large scale and old well-established process, down to 2–20 nm possible. The

serious disadvantages of this technique are production of irregular nanoparticles,

introduction of defects, introduction of impurities from balls and milling additives.

Mechanical milling is typically achieved using high energy shaker planetary balls, or

tumbler mills. The energy transferred to the powder from refractory or steel balls depends

on the rotational (vibrational) speed, size and number of the balls, ratio of the ball to

powder mass, the time of milling and the milling atmosphere. Milling in cryogenic liquids

can greatly increase the brittleness of the powders influencing the fracture process. As with

any process that produces fine materials, adequate steps to prevent oxidation are necessary.

Hence this process is very restrictive for the production of non-oxide materials since then

it requires that the milling be allowed take place in an inert atmosphere and that the

powder materials be handled in an appropriate vacuum system or glove box. This method

of synthesis is suitable for producing amorphous or nanocrystalline alloy materials,

elemental or compound powders etc.

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1.4.2 Sputter deposition

Sputter deposition is a physical vapor depostion (PVD) method of depositing thin

films by sputtering, i.e. ejecting, material from a "target," i.e., source, which then deposits

onto a "substrate," e.g., a silicon wafer. Sputtered atoms ejected from the target have a

wide energy distribution. The sputtered ions can ballistically fly from the target in straight

lines and impact energetically on the substrates or vacuum chamber causing re-sputtering

(the re-sputtering is reemission of the deposited material during the deposition process by

ion or atom bombardment) or, at higher gas pressures, collide with the gas atoms that act

as a moderator and move diffusively, reaching the substrates or vacuum chamber wall and

condensing after undergoing a random walk. The sputtering gas is often an inert gas such

as argon. For suitable momentum transfer, the atomic weight of the sputtering gas should

be close to the atomic weight of the target, so for sputtering light elements neon is

preferable, while for heavy elements krypton or xenon is used. Reactive gases can also be

used to sputter compounds. The compound can be formed on the target surface, in-flight or

on the substrate depending on the process parameters [14]. The availability of many

parameters that control sputter deposition make it a complex process, but also allow

experts a large degree of control over the growth and microstructure of the film [15].

Sputtering is used extensively in the semiconductor industry to deposit thin films of

various materials in integrated circuit processing [16]. Because of the low substrate

temperatures used, sputtering is an ideal method to deposit contact metals for thin film

transistors [17]. An important advantage of sputter deposition is that even the highest

melting point materials are easily sputtered while evaporation of these materials in a

resistance evaporator is impossible. Sputtered films typically have a better adhesion on the

substrate than evaporated films [18]. The major disadvantage of the sputtering process is

that the process is more difficult to combine with a lift-off process for structuring the film

[19].

1.4.3 Laser ablation

In this method, vaporization of the material is affected using pulses of laser beam of

high power. Laser ablation [20] has been extensively used for the preparation of

nanoparticles and particulate films. In this process, a laser beam is used as the primary

excitation source of ablation for generating clusters directly from a solid sample for a wide

variety of applications [21]. The possibility for preparing nano-particulate web-like

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structures over large sample area is of particular interest in view of their novel properties

that can be applied to new technological applications [22]. Line-Spark Atomization (LSA)

is novel atomization system based on laser spark atomization of solids. Briefly, the LSA is

capable of evaporating material at a rate of about 20µg/s from a solid target under argon

atmosphere [23].

The small dimensions of the materials and the possibility to form thick films make the

LSA quite an efficient tool for the production of ceramic materials and coatings. In

addition, the laser spark atomizer can be used to produce highly mesoporous thick films

and the porosity can be modified by the carrier gas flow rate thus enabling for a control on

the microstructure of the coatings. The prepared nanomaterials by this technique are

suitable candidates for applications in membrane technology, catalysis and lithium-ion

batteries. Due to the above mentioned advantages; this technique has been mostly used for

the synthesis of single walled nano-tube (SWNT). However, it has also few limitations, in

complex system design; it is not always possible to find the desired wave length for

evaporation and the low energy conversion efficiency [24].

1.4.4 Ion Beam Deposition Techniques (Ion-Implantation)

Ion-implantation is a material engineering process in which ions of a material can be

implanted into another solid and therefore changes the physical properties of the solid. Ion-

implantation is used in semiconductor device fabrication and in metal finishing, as well as

for various applications in materials science research [25]. There are many examples in

which high energy (few KeV to hundreds of KeV) or low energy (< 200eV) ions are used

to obtain nanoparticles. Ions of interested materials are usually formed using an ion gun

specially designed to produce metal ions, which are accelerated to high or low energy

towards the substrate heated to few hundreds of 0C. Depending upon the energy of the

incident ions, various processes like sputtering and generation of electromagnetic radiation

may take place. This method can be used to obtain single element nanoparticles or

compounds and alloys of more than one element. Post calcination is also used for some

times to improve the crystallinity of the materials. In some experiments, it has been found

that for obtaining doped nanomaterials one can use ion-implantation method. There also a

possibility of making nanoparticles using swift heavy ions (few MeV energy) employing

ion accelerators like a pelletron accelerator [26].

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1.4.5 Electric Arc Deposition

This is one of the simplest and useful methods, which leads to mass scale

production of fullerenes and carbon nano tubes using strong arc discharge between

electrodes. In order to achieve strong arc, the electrodes are kept in water cooled vacuum

chamber. The positive electrode itself acts as the source of material. Inert gas or reactive

gas introduction is necessary for discharge action. Usually the gap between the electrodes

is ~1mm and high current ~50 to 100 amperes is passed from a low voltage power supply

(12-15 volts). When an arc is set up, anode material evaporates. This is possible as long as

the discharge is maintained. The adjustment of the electrode gap without breaking the

vacuum becomes essential, as one of the electrodes burns and consequently the gap

increases. The large fullerens quantity can be produced by the arc between the two

graphite electrodes and due to which the temperature rises to a value as high as ~35000C.

In case of fullerens, the formation occurs at low helium pressure as compared to that used

for nanotubes formations [27]. Also, fullerens are obtained from purification of soot

collected from inner walls of vacuum chamber, where as nanotubes are found to be formed

only at high He gas pressure and in the central portion of the cathode. No carbon nanotube

is found on the chamber walls. Some nanoparticles of carbon are also usually found

around the region where nanotubes are formed. In principle, formation of other

nanocrystals or tubes of other materials should also be possible by this method. However,

this method is mostly found to be suitable for fullerens or carbon nanotube deposition.

1.4.6 Molecular Beam Epitaxy (MBE)

This technique of deposition can be used to deposit elemental or compound quantum

dots, quantum wells, quantum wires, etc in a very controlled manner. High dEgree of

purity in materials is achievable using ultra high vacuum of typically~10-10 torr [28].

Besides the ultrahigh vacuum system, MBE mostly consists of real time structural and

chemical characterization capability, including reflection high energy electron diffraction

(RHEED), X-ray photoelectric spectroscopy (XPS) and Auger electron spectroscopy

(AES). Other analytic instruments may also be attached to the deposition chamber or to a

separate analytic chamber, from which the growth of films can be transferred to the

chamber without exposing to the ambient condition. In the MBE, the evaporated atoms or

molecules from one or more sources do not interact with each other in the vapour phase

under such a low pressure. Special sources of deposition known as effusion cells and are

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employed to obtain molecular beams of the constituent elements. The rate of deposition is

kept very low and substrate temperature is rather high in order to achieve sufficient

mobility of the elements on the substrate and layer by layer growth to obtain

nanostructures. Ultra high vacuum environment ensures absence of impurity or

contamination, and thus a highly pure film can be readily obtained. Individually controlled

evaporation of sources permits the precise control of chemical composition of the deposit

at any given time. The disadvantage of this method is that it is complex in nature and

expensive. The main advantage of MBE is that it offers growth monitoring by RHEED.

1.4.7 Electrodeposition

This technique has been used for a long time to make electroplated materials. By

carefully controlling the number of electrons transferred, the weight of material transferred

can be determined in accordance with Farday’s laws of electrolysis. This states that the

number of moles of product formed by the electric current is directly proportional to

number of moles of electrons supplied. In nanoscience/nanotechnology, the main aim is to

place only a single layer on a surface by electro-deposition in a very controlled way. For

accuracy of results the current and time must be measured carefully and any other factors

involved in consuming currents such as impurities must be known in great detail. Hence

there is the necessity of super clean rooms. Nanostructured film of platinum can be

produced by the electro-deposition from liquid crystalline mixtures. The films obtained are

remarkably flat, uniform and shiny in appearance. The concept of electroplating from

liquid crystalline mixtures can be used for other metals including palladium (Pd), Ni and

Au oxides and semiconductors. The unique nature of nanostructured films from liquid

crystals makes them of considerable interest for a wide range of applications; these include

batteries, fuel cells, and solar cells, windows that can disperse heat and change properties

depending on the environment. The major advantage of this is highly influenced by the

surface characteristics of the electrode substrate, and the shape and size of the deposits

depend on the substrate. However, it has also some limitations, typically restricted to

electrically conductive substrate materials, difficulties in the preparation of desired

templates and additional high temperature calcination steps are expensive and unsuitable

for polymer substrates.

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1.4.8 Gas-Phase Synthesis of nanomaterials

The gas-phase synthesis methods are of increasing interest because they allow elEgant

way to control process parameters in order to be able to produce size, shape and chemical

composition controlled nanostructures. Before discussing a few selected pathways for gas-

phase formation of nanomaterials, some general aspects of gas-phase synthesis need to be

discussed. Gas-phase processes have inherent advantages, some of which are noted below.

1) An excellent control over size, shape, crystallinity and chemical composition of

synthesized nanomaterials.

2) Highly pure materials can be obtained.

3) Multi-component systems are relatively easy to form.

4) Easy control of the reaction mechanisms.

Most of the synthesis routes are based on the production of small clusters that can

aggregate to form nanoparticles (condensation). Condensation occurs only when the

vapour is supersaturated and in these processes homogeneous nucleation in the gas-phase

is utilized to form materials.

1.4.8.1 Gas Condensation Processing

In this technique, a metallic or inorganic material is vaporized using thermal

evaporation sources such as Joule heated refractory crucibles, electron beam evaporation

devices or sputtering sources in an atmosphere of 1-50 mbar He (or any other inert gas like

Ar, Ne, Kr) [29]. A rotating cylindrical device cooled with liquid nitrogen was employed

for the particle collection. Subsequently, the nanoparticles are removed from the surface of

the cylinder by means of a scraper in the form of a metallic plate. In addition to this cold

finger device, other techniques, which are used frequently in aerosol science, have now

been implemented for the use in gas condensation systems such as corona discharge, etc.

These methods allow for the continuous operation of the collection device and are better

suited for larger scale synthesis of nanomaterials. In this technique, clusters form due to

homogenous nucleation in the gas phase and grow by coalescence and incorporation of

atoms from the gas phase. The cluster or particle size depends critically on the residence

time of the materials in the growth rEgime and can be influenced by the gas pressure, the

kind of inert gas, i.e. He, Ar or Kr, and on the evaporation rate/vapour pressure of the

evaporating material [29]. The average particle size of the nanoparticles increases due to

increasing gas pressure, vapour pressure and mass of the inert gas. A major advantage over

convectional gas flow is the improved control of the particle sizes due to above mentioned

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mechanism/process. In this technique, excellent intermixing on the scale of the particle

size can be obtained. However, control of the composition of the elements has been

difficult and reproducibility is poor. In the gas condensation method, quantities have so

far been limited to a laboratory scale. These quantities are sufficient for materials testing

but not for industrial production.

1.4.8.2 Chemical Vapour Deposition (CVD)

Chemical vapour deposition, a hybrid method using different chemicals in vapour

phase and are conventionally used to obtain coatings of variety of materials viz. inorganic

and organic materials. It is widely used in industry because of relatively simple

instrumentation, ease of processing, possibility of depositing different types of materials

and economically avaibility. Under certain deposition conditions, nanocrystalline films or

single crystalline films are possible. The chemical processes used in the CVD of thin films

can be classified in to following types of reactions: thermal decomposition, oxidation,

reduction and hydrolysis. In general, CVD process generates active gaseous species and

these species are transported in to the reaction chamber and the gaseous precursor

undergoes gas-phase reaction, forming an intermediate phase. The intermediate species are

then absorbed onto the heated substrate, and heterogeneous reactions occur at the gas-solid

interface that produces the deposit and byproducts. The deposit is defused along the heated

substrate surface, forming the crystallization centre for subsequent growth of the films.

The main CVD process parameters include deposition temperature, gas vapours/ pressure,

input gas ratio, and flow rate. The deposition temperature is the dominant parameter.

Although CVD is complex system but it has distinctive advantage i.e. easy to control the

layer thickness, good layer homogeneity and universal process. However, the drawbacks

of CVD method are to include the chemical and safety hazards caused by the toxicity,

corrosive, flammable and explosive precursor gases.

1.4.9. Sol-Gel Process

Sol and Gel have are used scientific interests for a long time and may be defined

as follows: State of distribution of colloid (solid materials with size 1-1000 nm,

include103-109 atoms) materials in liquid state is called sol and the colloid materials,

through van der Waals attraction, eventually connect to form a gel, a 3-dimensional solid

network having high porosity and high specific surface area and this process is known

from long times. The first silica gels were made in 1845 by M. Ebelmen in France and the

oldest sols prepared in a laboratory were synthesized with gold [30]. In the 1950s and

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1960s, Roy and co-workers [31] used sol-gel method to synthesize a variety of novel metal

oxide compositions with very high levels of chemical homogeneity, including Si, Al, Zr,

Nd etc, which could not be made using traditional oxide powder methods [32]. The sol-gel

method is a wet-chemical synthesis technique for preparation of glasses, ceramics and

nanoparticles at low temperature. It is based on control of hydrolysis and condensation of

alkoxide precursors. It is possible to fabricate metal oxide in a variety of forms, such as

ultra-fine powders, fibres, thin films, porous aerogel materials or monolithic bulky glasses,

ceramics and metal oxides. In addition, thin films have been synthesized which are used in

forming monolithic optical lens and optoelectronics and photonic components.

An overview of sol-gel process steps: The sol-gel process, as the name implies,

involves transition from a liquid ‘sol’ (colloidal solution) into a ‘gel’ phase [33]. Usually

inorganic metal salts or metal organic compounds such Poly vinyl alcohol (PVA) is used

as precursors. A colloidal suspension or a ‘sol’ is formed after a series of hydrolysis and

condensation reaction of the precursors. Then the sol materials condense into a continuous

liquid phase (gel). With further drying and heat treatment, the ‘gel’ is converted into, in

general, dense ceramic or glass materials. Generally three reactions are used to describe

the sol-gel process: hydrolysis, alcohol condensation and water condensation. Because

water and alkoxides are immiscible, alcohol is commonly used as co-solvent. Due to the

presence of the solvent, the sol-gel precursor, alkoxide, mixes well with water to facilitate

the hydrolysis. The following example describes various processes.

Hydrolysis:

(1.1)

Here R (alkyl group) is hydrolyzed, during the hydrolysis reaction, the alkoxide

groups (OR) is replaced with hydroxyl group (OH) through the addition of water.

Condensation:

(1.2)

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Linkage of additional =Si-OH tetrahedral occurs as a polycondensation reaction as

shown in Eq. (1.3). After ageing and drying process the water and alcohol are expelled

from –O-Si-O-Si-O- network and eventually resulting in a SiO2 network found.

…….. (1.3)

The hydrolysis and poly-condensation reactions initiate at numerous sites within

the solution (metal alkoxide, water and alcohol) as mixing proceeds. When sufficient

interconnected Si-O-Si bonds are formed in a rEgion, they respond cooperatively as

colloidal (sub micrometre) materials or a sol. The size of the sol materials and the cross-

linking within the materials depend upon the pH among other variables i.e. size of

materials increases with increases the pH of the reaction [34]. Since the sol is a low-

viscosity liquid, it can be cast into a mold. The mold must be selected to avoid adhesion of

the gel.

Gelation: With time the colloidal materials and condensed silica species link together

to become a three-dimensional network. The physical characteristics of the gel network

depend greatly upon the size of materials and extent of cross-linking prior to gelation. At

gelation, the viscosity increases sharply, and a solid object results in the shape of the mold.

With appropriate control of the time-dependent change of viscosity of the sol, fibres can be

pulled or spun as gelation occurs. As mentioned earlier, it is possible to fabricate metal

oxide in a variety of forms, such as ultra-fine powders and thin films etc.

Ageing: Ageing of a gel involves maintaining the cast object for a period of time,

hours to days, completely immersed in liquid. During ageing, polycondensation continues

and these occurs re-precipitation of the gel network, which increases the thickness of

interparticle necks and decreases the porosity. The strength of the gel thereby increases

with ageing. An aged gel must develop sufficient strength to resist cracking during drying.

Drying: During drying, the liquid is removed from the interconnected pore network.

Large capillary stresses can develop during drying when the pores are small (< 20 nm).

28

These stresses will cause the gels to crack catastrophically unless the drying process is

controlled by decreasing the liquid surface energy by addition of surfactants or elimination

of very small pores, by hypercritical evaporation, which avoids the solid-liquid interface,

or by obtaining monodisperse pore sizes by controlling the rates of hydrolysis and

condensation. After hypercritical drying, the aerogel has a very low density and is a very

good thermal insulation when sandwiched between glass plates and evacuated [35].

Densification: Although there are many applications of sol-gel silica prepared and

dried at or near room temperature (especially those involving trapping functional organic

or biological molecules with the gel pores), heat treatment of the porous gel at high

temperature is necessary for the production of dense glass or ceramics from the gel silica.

After the high temperature annealing, the pores are eliminated and the density of the sol-

gel materials ultimately becomes equivalent to that of the fused glass. The densification

temperature depends considerably on the dimension of the pores, the dEgree of connection

of the pores, and the surface areas in the structure [36].

1.4.10 Co-precipitation Method

This method has been used in the present work and is discussed in details in Chapter

second.

1.5 Properties of nanomaterialsIt has been well established that all the materials (like metals, semiconductors or

insulators) have size dependent physio-chemical properties e.g. mechanical, structural,

thermal, electrical and magnetic properties, below a certain critical size [37]. The critical

size depends upon the details of the materials viz. its composition and structure. However,

for most of the materials, the critical size is below~100 nm [38]. Interestingly at such a

small size even the shape of the material and interactions between nanomaterials decide

the properties of the materials. This opens up a huge possibility of tailor-making the

materials, which has different properties just due to their size, shape and/or assembly.

Mechanical properties of the materials depend upon the composition and nature of

bonds between the atoms viz. covalent, ionic, metallic etc. As a result, purest materials

may be inherently weak or strong or brittle. Presence of impurities also affects all these

properties. Most of the materials have various impurities like C, O, N, P, S etc. present in

them as well as there are point defects, grain boundaries, dislocations etc., which are

29

responsible for the deviations of the properties, expected from high purity and ordered

materials. When the size of materials is reduced to nanoscale, materials tend to be single

crystals. Indeed it is possible to determine various mechanical properties like elastic

properties, hardness, ductility etc. of different nanostructures.

In case of thermal properties, variety of nanoparticles like Au, Ag, CdS etc. have been

investigated for their thermal stability and melting. Melting begins at the surface. As the

particle size decreases, surface to bulk atoms ratio increases dramatically. In small

materials or clusters the central atom may be considered as surrounded by first, second,

third …up to n compact shells of atoms. Number of atoms in shells is given as 10n2+2.

The first shell would have 12 atoms; second shell would have 42 atoms and so on. It can

be easily seen that number of surface atoms is quite large in nanoparticles and surface to

bulk atoms ratio goes on increasing with decreasing materials size. Large surface is related

to large surface energy [39]. This energy can be lowered by melting. Melting of

nanoparticles is usually determined either by X-ray diffraction or electron diffraction.

Heating temperature and increase duration of heating is increases the lattice parameter

[40].

In case of nanosized grains, resistivity is in general larger than that in polycrystalline

materials [41]. The electrons get scattered at grain boundaries resulting in increase of

resistance. Therefore electrical resistance of polycrystalline materials is larger than that of

corresponding single crystal materials. In materials having nanocrystalline grains larger

number of boundaries exist, compared to polycrystalline materials having micrometre

sized grains. Therefore resistivity of materials having nanosized grain is generally quite

large.

Among various other interesting properties, magnetism is one of the most important

properties of nano sized materials because it has diverse applications like in information

storage, electronic circuits, transformers, motors, sensors and medical field [42]. With

progress in miniaturization of semiconductor devices, a need is felt to miniaturize the

magnetic components too to achieve larger data storage capacity and faster devices in

general. This has led to achievements of some new and interesting devices as well as

understanding of some basic phenomena. With this background of magnetic materials, one

may proceed to understand properties of some special types of nano magnetic materials in

which materials are reduced to nanometre scale at least in one of the dimensions. Magnetic

nanoparticles, assemblies of nanoparticles, magnetic nanowires, magnetic thin films or

multilayer thin films and some metal oxide films, doped semiconductor materials or thin

30

films have become the focus of attention due to their interesting magneto-resistive or

magneto optical properties [43].

1.6 Some Important Applications of nanomaterialsAbility of materials to dramatically change their properties at nanoscale has opened

the possibility of making new devices, instruments, consumer goods etc: which are multi-

functional and can be used in a much better way in future. Rapid progress in synthesis and

to understanding the properties of nanomaterials in just a few years had led them to enter

in the world market in a big way. So in this section, some important applications of

nanomaterials are briefly discussed.

The electronic devices with typical dimensions of few nanometers i.e. three directions

display, Single Electron Transistor (SET), Spin Valves and Magnetic Tunnel Junctions

(MTJ) are conceptually new devices based on nanomaterials [44]. Nanoscale crystal

influences the chemical, electronic and optical behavior of materials [45]. Nanomaterials

also play an important role in the fields of sports equipments and toys [46]. Tennis balls

using nano clay are able to fill pores in better way and trap the air pressure inside. This

increases the life of balls. Toys industry also has been well geared to embrace

nanomaterials.

Textile industry is also quite excited about use of nanomaterials [47]. Nanoparticles

are also important in cosmetics [48]. Zinc oxide and titanium oxide nanoparticles of fairly

uniform size are able to absorb ultraviolet light and protect the skin from these radiations

[49]. The nano-based cosmetics are becoming quite popular for their known applications

[50]. Silver nanoparticles are used in refrigerators, air purifiers or air conditioners, water

purifiers and photography for their known antibacterial properties [51].

Space and defence scientists are also trying to adopt nanomaterials as alternative

materials and to replace the conventional materials by nanomaterials due to their

functional properties i.e. very low density materials, known as aerogels, which is very

good for used of various applications in spacecrafts and defense for reducing the weight of

application instrument [52-53]. Even some special lightweight suits, jackets etc. can be

made using aerogels as they can be made of such materials that are poor conductors of heat

[54]. A great revolution occurs in the field of biotechnology and medical science due to

use of nanomaterials [55]. Initial testing reports show that nanomaterials are successfully

used in various drug delivery systems, cancer tumor therapies or their detection [56]. Such

materials were show the great impact in solving over health related problems also. There is

31

considerable nano-based research going on to help diabetic and HIV affected patients [57].

Much work is going on to understanding: how the nature does it all and mimic it. Thus

nanomaterials have quite useful applications and out of these two main applications i.e.

Fire-retardancy and UV-Protection of nanomaterials treated fabrics were studied in present

work.

1.7 UV protection textilesNanomaterials treared fabric are used to block UV-radiation by intEgrating fibres with

metal oxide nanomaterials, dyes or pigments. Generally there are conventional finishing

method of dip pad-dry-cure (e.g. MgO, ZnO) or to use sol-gel techniques for TiO2 treated

fabrics. In present time the there are number of inorganic nanomaterials like some

semiconductor oxides i.e.TiO2, ZnO, SiO2, Al2O3 are good UV-blockers than organic and

conventional size blockers and having superior properties such as no-toxicity, superior

chemical stability at high temperature exposure, better durability, an exceptional UV-

protective factor (UPF) rating for UV-A and UV-B radiations and upper ability to absorb

and scatter UV radiation [58]. In fact, these are very small materials , like metal oxide

nanorods (10-50 nm in length) have an higher blocking efficiency because of Rayleigh’s

scattering theory, the intensity of the scattered light varies as the sixth power of the particle

size and varies inversely with the fourth power of wavelength . The sol-gel method is the

UV-blocking most used treatment for cotton, assuring to maintain the effect even after 50

home launderings. Fabrics coated by methyl red dye with nano TiO are good antibacterial,

antiflammable properties and also be used for air freshening, removing bad odours from

confined space [59-60].

1.8 Fire RetardanceIn present time to study the application of nanomaterials treated fabrics some

investigations were made towards to study fire-retardancy or flame retardancy (FR) of a

fabric, its burn time, its strength, anti-bacterial, sterilizing efficiency, UV-light and

chemical protection etc. For applications these fabrics are composed by different types of

hydrous aluminasilicates, or other nano-materials such as diamine (diaminodiphenyl

methane), boroxosiloxanes, Sb2O3 or metal oxide nanocomposites [61]. With these

nanomaterials treated fabrics the heat distortion temperature (HDT)( i.e. temperature

needed to burn the fabrics) increases from 650C to 1520C. One of the major obstacles of

these processes is to set up the dispersion of the nanosized fillers into the polymer matrix

32

compounds [62]. Now researchers are trying to avoid this problem and also avoid their

natural interaction tendency that heavily obstacle for the successive aggrEgation process

with yarns. MgO, Al2O3 nanomaterials can also apply on fabrics to increase the

mechanical strength, abrasion resistance and the fire retardancy of textile [63]. Other

nanomaterials can also be used as additives in extrusion, coating and finishing textile

applications (composite fibres) to enhance fire-retardancy (FR) of fabrics [64].

1.9 Commercialization of nanomaterialsIn future there is fundamental issues to commercialization of nanotechnology

for the development of nanoscale electronics and computing applications and also be used

to create the molecular machines and manufacturing capabilities at the nanometre scale. At

present, most of the people have limited production capabilities at research scale only and

no development for large scale quantities. However, researchors were working with co-

workers or alone to develop new applications for nanomaterials that will undoubtedly have

an immense impact on our future life. These applications have importance in, to improve

the quality of polymers, life of batteries, electronics goods, cosmetics, sensors, fuel cells,

and catalysis to coatings on metals, computer screens and other displays. Other people

were study nanoparticles for biological applications such as drug delivery, screening,

diagnostics and even cure. The most effective thing in nanotechnology is the cost and

availability of nanomaterials and in present time few of nanotechnological applications are

close to commercialization. It means these are commercially available within the next few

years. Hence in the next decade we shall be able to harness the fruits of the tremendous

progress that have been achieved in material properties upon the utilization of

nanomaterials in practical scales.

1.10 Fundamental Issues in nanomaterialsThe fundamental issues in the domain of nanomaterials are ability to control

the size of the system, ability to obtain the required composition, not just the average

composition but also in the details such as defects, concentration gradients, etc., ability to

control the modulation dimensionality during the assembly of the nano-sized building

blocks, one should be able to control the extent of the interaction between the building

blocks as well as the architecture of the material itself. More specifically the following

issues have to be considered for the future development of nanomaterials.

33

Development of new synthesis methods for bulk materials as well as for their

nanostructured materials is major issues for researchors. Although there should be better

understanding of the influence of the size of building blocks in nanostructured materials as

well as the influence of size on the physical, chemical and mechanical properties of the

material.

1.11 Aim of the Present WorkNanoparticles can be synthesized by either chemical or physical methods. it

can be made physically by ball milling, a process in which course materials are ground to

nanoscale materials. Disadvantages of ball-milling include the high mechanical energy

needed and contamination by the material of which the ball mill is made. In chemical

methods, by-products are formed during reactions but they can generally be removed.

Chemical techniques provide the power to mix at the molecular level, allowing for the

production of highly homogeneous materials. Nanoparticles can be synthesized chemically

through co-precipitation method sol-gel processing, atomic layer deposition methods,

hydrothermal techniques, and chemical vapour deposition [65]. The co-precipitation

process actually offers unique opportunity for the synthesis of optical materials with

composition controlled and this method has been used for the synthesis of nanocomposites

in the present work. The homogeneous mixture of several components in liquid state

makes it possible to vary the materials over a wide range of compositions at molecular

level; therefore, the optical properties of the materials can be tailored. Metal/ rare earth

metal dopants were create grain in the host materials [66]. In summary, the co-

precipitation method provides an efficient and cost-effective platform to explore the

effects of different dopants.

The term rare earths was propounded by Johann Gadolin in 1794 [67]. In fact such

elements are neither rare nor earths. Rare earth elements were called “rare” because just

after their discovery they were thought to be present in the earth’s crust only in a small

amounts, and “earths” because their oxides have an earthy appearance [68]. Rare earth

elements include lanthanum, cerium, neodymium, europium, holmium, erbium, and

lutetium etc. Despite their names, rare earth elements are not rare in nature except

promethium which is a radioactive element. Among rare-earths which are all more

abundant than lead [69] the most common is Cerium. The present work explores the

synthesis of bulk nanocrystalline of metal oxides such as MgO NPs and its

nanocomposites were synthesized through co-precipitation method using their metallic

34

salt. Bazzi and Kepniski have described the nanocrystalline materials synthesis by various

methods: Co-precipitation in high-boiling polyalcohol solutions, inverse microemulsion

,solvothermal method and hydrothermal solgel auto-combustion [70] etc.

The work presented in the thesis deals with the synthesis and

characterization of nanoparticles/ nanocomposites of MgO NPs/NCs (metal oxide). In

addition, the thesis also incorporates the applications of as-synthesized nanomaterials as

fire-retardant and for UV-protection in different fabrics. Co-precipitation method was

adopted for the preparation of the samples. Metal-doped materials have been the subject of

increasing interest due to their prospective utilization for various field applications.

Special attention has been given to metal oxide nanoparticles (NPs) [71-76] due to their

remarkable electrical and optical properties owing to the surface plasmon resonance

(SPR), the nanometre scale feature which is the origin of the observed colours in metal

colloids [77]. The surface plasmon resonance (SPR) is regarded as the collective

excitation of the conduction band electrons. The electric field of an incident light beam

polarizes these electrons with respect to the heavier ionic cores. A restoring force develops

at the NP surface with the resulting excitation of the free electrons oscillations. This

surface plasmon absorption shows distinctive features (e.g. frequency, shape, width) that

are directly connected to the composition and morphology of the NPs [78-79].

Potential applications of nanocomposites materials, which are consisting of

spherical metal NPs /NCs allowing the controlled precipitation of metallic materials and

which are synthesized by relative simple method of preparation. In addition, the optical

properties of small nanostructured materials which are strongly dependent on the structural

and geometric parameters of the materials (e.g. size, shape, composition) are ultimately

determined by the preparation process [80-81]. Nanocomposites are characterized by the

dispersion of nanosized metals or semiconductors [82]. Some of the examples are Tin,

Magnesium, Silver, Copper, Nickel, or Lead [83]. The dispersoids have a large surface-to

volume ratio. This makes these materials potentially attractive for sensing purposes

because, in principle, these were provide innumerable surface sites for physical adsorption

of water molecules [84]. Humidity sensors have assumed importance in recent years

because of the need to monitor and control environmental humidity in various industrial

processes [85]. For automated control of humidity, sensors using changes of electrical

conductivity are necessary. Research has been carried out on various systems to explore

the possibility of utilizing them as humidity sensors [86-92]. Commercial humidity sensors

are mostly comprised of either polymer films [93-94] or porous ceramics [95]. These

35

sensors are based on the principle of electrical resistance change as a function of relative

humidity. Adsorption of water molecules on the surface of these systems causes their

electrical properties to change.

The structural evolutions have been investigated by X-ray Diffraction (XRD),

Scanning Electron Microscopy (SEM), Fourier Transform Infrared (FTIR) Spectroscopy,

Transmission electron microscopy (TEM), Ultra Violet-Visible-Near Infrared (UV-VIS-

NIR) Spectroscopy and Least oxygen index (LOI).

The main objective of this thesis is to demonstrate the synthesis and characterization

of the binary oxide nanocomposites such as MgO-X nanocomposites (X= NiO, CuO,

Co3O4, Fe2O3, CeO2, Al2O3) . The results obtained and the interpretation theirof have been

compiled in the form of this thesis. The Chapter wise breakup of the thesis is as follows.

The Chapter-1 deals with the general introduction and classification of nanostructured

materials. Interesting properties, their methods of preparation and applications of

nanomaterials are also described. Aim of the present work and Chapter wise breakup of

the thesis are also incorporated in this Chapter.

The Chapter-2 contains a brief description of the experimental techniques employed

and of parameters investigated in the present work. This Chapter of the thesis is focused on

the use of co-precipitation method for the preparation of metal oxides nanoparticles and its

various nanocomposites.

In Chapter-3, description of preparation of MgO nanoparticles by using the co-

precipitation technique were discussed. For this, the aim was to explore the possibilities of

this method with regard to materials preparation at large scale and control of the chemical

composition and particle size. Interesting aspects of nanoparticles for different durations

of calcination at fixed temperature and for fixed durations of calcination at different

calcination temperature of samples were investigated by various characterization

techniques such as X-ray Diffraction, Scanning Electron Microscopy, IR-Spectroscopy

and UV-VIS Spectroscopy, Transmission Electron Microscopy.

In Chapter-4, preparation of Magnesium Oxide-Transition metal oxide( NiO, CuO,

Fe2O3,Co3O4) nanocomposites by using the co-precipitation method and calcined samples

were characterized by using various characterization techniques such as X-ray

Diffraction, Scanning Electron Microscopy, IR-Spectroscopy and UV-VIS Spectroscopy,

Transmission Electron Microscopy.

In Chapter-5, the Magnesium Oxide-Rare Earth metal oxide(CeO2) binary

nanocomposites were synthesized by using the co-precipitation method and calcined

36

samples of different concentrationswere characterized by using various characterization

techniques such as X-ray Diffraction, Scanning Electron Microscopy, IR-Spectroscopy

and UV-VIS Spectroscopy, Transmission Electron Microscopy.

In Chapter-6, the MgO-Al2O3 nanocomposites with the different dopant

concentration of Al2O3 were synthesized by using the co-precipitation methode and

calcined samples were then characterized by using various characterization techniques

such as X-ray Diffraction, Scanning Electron Microscopy, IR-Spectroscopy and UV-VIS

Spectroscopy, Transmission Electron Microscopy.

The Chapter-7 deal with the demonstration of preparation of nanocomposites treated

fabrics (cotton and polyester-blend fabrics ) with different nanocomposites concentrations

and its application as fire retardancy and UV protection.

In Chapter-8, conclusions drawn from various results ,which were drawn in

various Chapters were discussed and the future scope of the present work is also described

in this Chapter.

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