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Chapter-I: Introduction

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1.1 Introduction

Nanotechnology involves the creation and manipulation of materials at

the nanometre (nm) scale either by scaling up from single groups of atoms or

by refining or reducing bulk materials. A nanometre is 1 x 10-9 m or one

millionth of a millimetre. To give a sense of this scale, a human hair is of the

order of 10,000 to 50,000 nm, a single red blood cell has a diameter of around

5000 nm, viruses typically have a maximum dimension of 10 to 100 nm and

DNA molecule has a diameter of 2 to 12 nm. The use of the term

“Nanotechnology” can be misleading since it is not a single technology or

scientific discipline, but it is a multidisciplinary grouping of Physical,

Chemical, Biological, Engineering and Electronic.

Nanotechnology is a rapidly developing and expanding discipline and

has aroused growing media and public interest. New materials are being

discovered and astonishing claims are being made concerning their properties,

behaviours and applications. Development of new nanomaterials is a major

theme of all of these programmes. Ordinary materials such as carbon or silicon,

when reduced to the nanoscale, often exhibit novel and unpredictable

characteristics such as extraordinary strength, chemical reactivity, electrical

conductivity and other characteristics that the same material does not possess at

the micro or macro-scale. The nanomaterials have already been produced with

different morphologies such as nanotubes, nanowires, nanorods, cuboids etc,

which has special properties than spherical size materials.

Nanotechnologies are gaining more importance in commercial

applications. Nanoscale materials are currently being used in electronic,

magnetic, optoelectronic, biomedical, pharmaceutical, cosmetic, energy,

catalytic and material applications. Areas producing the greatest revenue for

nanoparticles are reportedly chemical-mechanical polishing, magnetic

recording tapes, sunscreens, automotive catalyst supports, bio-labelling,

electro-conductive coatings and optical fibres.

Despite the current interest nanoparticles are not a new phenomenon for

scientists being aware of colloids and sol particles, for more than 100 years.


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The scientific investigation of colloids and their properties were reported by

Faraday (1857) in his experiments with gold. He used the term “divided

metals” to describe the material which was produced. Zsigmondy (1905)

describes the formation of a red gold sol which is now understood to comprise

particles in the 10 nm size. Throughout the last century the field of colloidal

science has developed enormously and has been used to produce many

materials including metals, oxides, organic and pharmaceutical products.

Many other well known industrial processes produce materials which

have dimensions in the nanometre size range. One example is the synthesis of

carbon black by flame pyrolysis which produces carbon with a very high

surface to volume ratio. This is highly agglomerated but has a primary particle

size which can be in the order of 100 nm. Worldwide production of carbon

black was approximately six million tons in 1993 (IARC, 1996). Other

common materials produced by flame pyrolysis or similar thermal processes

include fumed SiO2, ultrafine TiO2 and Ni.

Nano sized particles are also found in the atmosphere where they

originate from combustion sources (traffic, forest fires), volcanic activity, and

atmospheric gas to particle conversion processes such as photochemically

driven nucleation. In fact, nanoparticles are the end product of a wide variety of

physical, chemical and biological processes, some of which are novel and

radically different.

The term "nano" comes from a Greek prefix, meaning dwarf. Nanoscale

objects have atleast one dimension that measures between 1-100 nm.

Nanophase materials of all types are being considered for advanced

applications in 21st Century. The synthesis, characterization and processing of

nanostructured materials are part of an emerging and rapidly growing fields [1].

Nanomaterials have a particular distribution of crystallites or domains at a

nanometer scale in a peculiar fashion. A strong macroscopic interaction occurs

between the basic units of crystallites. It determines the modified physical and

chemical properties of nanostructured materials. These nanostructures are

intensely interesting for many reasons [2].


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Fig.1.1 Examples of nanostructures in nature and nanotechnology

Nanotechnologists use similar principles to deliberately engineer at the

nanoscale to create products that make use of these unusual properties. Starting

with nanostructures, scientists rearrange them to assemble functional systems

that can be incorporated into products with unique properties. Fig. 1.1 shows

two examples. First, the propensity for carbon to form tubes at the nanoscale

can be used to generate arrays over micron sized conductors that illuminate flat

panel displays for mobile phone and secondly nanoparticles can be manipulated

to create effective, fully transparent UV blocking creams. These are two of

many examples of stronger, stickier, smoother and lighter products being

developed

In general, as per the human nature, one believes some information only

if one could see it. Science always believes on proof and repeatability of the

results, therefore nanotechnology has developed only after the invention of

electron microscope. The proverb ‘necessity is the mother of invention’ has

some meaning. In that way if one looks at the nanotechnology the applications

are countless. Properties of nanomaterials that are different from bulk are


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variation of the redox properties, band gap variation, enhancement in toughness

and strength, anomalous melting points and unusal crystal structures (in

metals). Nanotechnology is one of the frontier areas of science due to its

versatile applications in various fields.

1.2 History of Nanotechnology and Nanomaterials

The origins of nanotechnology received more attention after 1959, when

Richard Feynman, Physicist and Nobel Prize winner, presented a talk to the

American Physical Society in annual meeting entitled There’s Plenty of Room

at the Bottom [3]. In his talk, Feynman presented ideas for creating nanoscale

machines to manipulate, control and image matter at the atomic scale. In 1974,

Norio Taniguchi introduced the term ‘nanotechnology’ to represent extra-high

precision and ultra-fine dimension, and also predicted improvements in

integrated circuits, optoelectronic, mechanical devices and computer memory

devices [4]. This is the so called ‘top-down approach’ of carving small things

from large structures. In 1986, K. Eric Drexler in his book Engines of Creation

discussed the future of nanotechnology, particularly the creation of larger

objects from their atomic and molecular components, so called ‘bottom-up

approach’ [5]. He proposed ideas for ‘molecular nanotechnology’ which is the

self assembly of molecules into an ordered and functional structure. The

invention of the scanning tunneling microscope by Gerd Binnig and Heinrich

Rohrer in 1981 (IBM Zurich Laboratories), provided the real breakthrough and

the opportunity to manipulate and image structures at the nanoscale.

Subsequently, the atomic force microscope was invented in 1986, allowing

imaging of structures at the atomic scale. Another major breakthrough in the

field of nanotechnology occurred in 1985 when Harry Kroto, Robert Curl and

Richard Smalley invented a new form of carbon called fullerene (buckyballs), a

single molecule of 60 carbon atoms arranged in the shape of a soccer ball. This

led to a Nobel Prize in Chemistry in 1996. Since that time, nanotechnology has

evolved into one of the most promising fields of science, with multi-billion

dollar investment from the public and private sectors and the potential to create


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multi-trillion dollar industries in the coming decade. The nanomaterials began

immediately after the big bang theory when nanostructures were formed in the

early meteorites. Nature 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 [6]. Nanostructured catalysts

have also been investigated over 70 years. By the early 1940’s, precipitated and

fumed silica nanoparticles were being manufactured and sold in USA and

Germany as substitute for ultrafine carbon black for rubber reinforcement [7].

Nanosized amorphous silica particle has found large-scale applications in many

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

automobile tires, optical fibers and catalyst support. In the 1960s and 1970s

metallic nanopowders for magnetic recording tapes were developed [8].

Today nanophase engineering expands in a rapidly growing number of

structural and functional materials, inorganic and organic, allowing to

manipulating mechanical, catalytic, electric, magnetic, optical and electronic

properties. The production of nanophase or cluster-assembled materials is

usually based upon the creation of separated small clusters, which are fused

into a bulk material such as 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

[9].

1.3 Importance of Nanoparticles

The nanomaterials have created a high interest in recent years by virtue

of their unusual mechanical, electrical, optical and magnetic properties. The

production of nanophase materials is usually based upon the creation of


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separated small clusters, which are fused into a bulk-like material. Some

examples are given below:

(a) Nanophase ceramics are of particular interest because they are more ductile

at elevated temperatures as compared to the coarse-grained ceramics.

(b) Nanostructured semiconductors are known to show various non-linear

optical properties. Semiconductor quantum dot particles also show quantum

confinement effects which may lead to special properties, the luminescence in

silicon powder and silicon germanium quantum dots as infrared optoelectronic

devices.

(c) Nanosized metallic powders have been used for the production of gas tight

materials, dense parts and porous coatings. Cold welding properties combined

with the ductility make them suitable for metal-metal bonding especially in the

electronic industry.

(d) Single nanosized magnetic particles are mono-domains and one expects that

also in magnetic nanophase materials that grains correspond with domains,

while boundaries on the contrary to disordered walls. Very small particles have

special atomic structures with discrete electronic states, which give rise to

special properties in addition to the super-paramagnetism behaviour. Magnetic

nanocomposites have been used for mechanical force transfer (ferrofluids), for

high density information storage and magnetic refrigeration.

(e) Nanostructured metal clusters and colloids of mono or plurimetallic

composition have a special impact in catalytic applications. They may serve as

precursors for new type of heterogeneous catalysts which have been shown to

offer substantial advantages concerning to activity, selectivity and lifetime in

chemical transformations and electrocatalysis (fuel cells). Enantioselective

catalysis were also achieved using chiral modifiers on the surface of nanoscale

metal particles.

(f) Nanostructured metal-oxide thin films are receiving a growing attention for

the realization of gas sensors (NOx, CO, CO2, CH4 and aromatic hydrocarbons)

with enhanced sensitivity and selectivity. Nanostructured metal-oxide (MnO2)

finds application for rechargeable batteries for cars and consumer goods.


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Nanocrystalline silicon films for highly transparent contacts in thin film solar

cell and nano-structured titanium oxide porous films for its high transmission

and significant surface area enhancement leading to strong absorption in dye

sensitized solar cells.

(g) Polymer based composites with a high content of inorganic particles

leading to a high dielectric constant are interesting materials for photonic band

gap structure.

It has been realized that materials in nanodimensions exhibit properties

very different from their bulk counterparts. Increasing knowledge about the

unique properties of nanoparticles has lead to renewed interest in them for

potential applications. The applications of nanoparticles extend to wide-ranging

areas such as catalysis, biosensors, diagnostics, cell labeling, solar cells, fuel

cells, photonic band gap materials, single electron transistors, nonlinear-optical

devices and surface enhanced Raman spectroscopy. The realization of their

various potential applications is only limited by our imagination.

Our focus has been on the production processes rather than on those

which utilize nanoparticles to manufacture other products. Our view of a

production process includes synthesis, in which the material is formed, and

recovery in which the product is collected, modified, divided and packed for

dispatch. Since these are new materials, most of the research and industry effort

is on the development of processes and materials and scale-up of these

processes to industry scale. As understanding about the properties of a new

material increase, effort will tend to shift towards applications and more

exposures might be expected. Secondly, in the early stages of the life cycle of a

new material, many of the potential applications will be speculative and may

never come to fruition.

1.4 Morphology of Nanoparticles

As the particle size ranging from 1 to 100 nm shows unusal properties of

materials. From the literature survey it is seen that ZnO nanowires are used to

sense the ethanol in the range of 1 to 100 ppm, V2O5 nanofibres shows


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extremely high sensitivity for 1-butylamine and moderate sensitivity for

ammonia. The optical properties of metal nanoparticles have received attention

for making use of them in optical sensors, colour glasses and

photoelectrochemical cells. Metal nanowires and nanoparticles are used in the

nanocircuits. In medicine, gold nanoparticles are utilized for drug delivery and

identification of affected cells. It is important that the only triangular

nanoparticles can be easily identified the cancer cells. Since, they posses sharp

edges, they easily penetrate the cancer cells and the cancer cells can be

destroyed using the same. The most of triangular particles are bound to cell,

they will be killed using far IR radiation. It is also reported that in particular,

nanorods, nanowires, cuboids are showing enhanced photocatalytic activity

than the spherical size.

1.5 Methods for Synthesis of Metal Oxide Nanoparticles

Mainly two approaches have been used for the synthesis of

nanomaterials. Top–down and Bottom–up are the two approaches. Top–down

approach involves mainly physical methods where a bulk material is sliced into

pieces till the desired size is achieved. Lithographic techniques, laser induced

chemical etching and ball milling fall in to this category. However, these

methods are effective only down to the micrometer level. Reaching nanometer

scale makes these methods more expensive and technically difficult. The

bottom–up approaches mainly involve chemical and biological methods to

make nanoparticles. These involve controlled condensation of solute molecules

that is formed during a chemical reaction. The restriction of the condensation

or the growth leads to the formation of particles of desired size and shape [2].

However unlike the chemical synthesis of molecules of a desired structure, the

synthesis of nanomaterials with uniform size and shape is difficult. Thus, large

scale synthesis of nanomaterials remains a challenge.

The development of systematic studies for the synthesis of oxide

nanoparticles is a current challenge and, essentially, the corresponding

preparation methods may be grouped in two main streams based upon the


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liquid-solid and gas-solid nature of the transformations [10]. Liquid-solid

transformations are possibly the most broadly used in order to control

morphological characterstics with certain “chemical” versatility and usually

follow a “bottom-up” approach. A number of specific methods have been

developed, amongst them those which are broadly in use are given below.

I) Co-Precipitation Method

This involves dissolving a salt precursor in water (or other solvent) to

precipitate the hydroxide form with the help of a base. However, the use of

surfactants, sonochemical methods, and high-gravity reactive precipitation

appear as novel and viable alternatives to optimize the resulting solid

morphological characteristics [11-12].

II) Sol-Gel Method

The sol-gel method provides a highly useful means of preparing

inorganic oxides [13]. It is a wet chemical method and a multistep process

involving both chemical and physical processes such as hydrolysis,

polymerization, drying and densification. The name sol-gel is given to the

process because of the distinctive viscosity increase that occurs at a particular

point in the sequence of steps. Most of the sol-gel literature deals with

synthesis from alkoxides. The important features of the sol-gel techniques are

better homogeneity compared to the traditional methods, high purity, lower

processing temperature and more uniform phase distribution in

multicomponent systems, better size and morphological control, the possibility

of preparing new crystalline and nanocrystalline materials.

The various steps in the sol-gel technique may or may not be strictly

followed in practice. Thus, many complex metal oxides are prepared by a

modified sol-gel route without actually preparing metal alkoxides e.g. a

transition metal salt solution is converted into a gel by the addition of an

appropriate organic reagent.


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III) Microemulsion Technique

Microemulsions represent an approach based on the formation of

ternary mixture containing water, surfactant and oil. Metal precursors on water

will proceed precipitation as oxo-hydroxides within the aqueous droplets,

typically leading to monodispersed materials with size limited by the

surfactant-hydroxide contact [14].

IV) Solvothermal Method

In this case, metal complexes are decomposed thermally either by

boiling in an inert atmosphere or using an autoclave with the help of pressure.

A suitable surfactant is usually added to the reaction media to control particle

size growth and limit agglomeration [15]. The solvothermal method uses a

solvent under pressure and temperature above its critical point to increase the

solubility of solids and to speed up reactions of solids. Most materials can be

made soluble in proper solvent by heating and pressurising the system near to

its critical point. Lower supersaturation state can be achieved by this method.

VI) Chemical Vapour Deposition (CVD)

There are a number of CVD processes used for the formation of

nanoparticles, some of them are classical, metalorganic, plasma-assisted, and

photo CVD methodologies [17]. The advantages of this methodology consist of

producing uniform, pure and reproducible nanoparticles and films although

requires a careful initial setting up of the experimental parameters.

VII) Pulsed Laser Deposition (PLD)

Pulsed laser deposition heats a target sample (4000 K) and leads to

instantaneous evaporation, ionization, and decomposition with subsequent

mixing of desired atoms. The gaseous entities formed absorb radiation energy

from subsequent pulses and acquire kinetic energy perpendicularly to the target

to be deposited in a substrate generally heated to allow crystalline growth [18].

Irrespective of the preparation method used to obtain ultrafine nano-

oxides, the studies of nanoparticle preparation yielded compelling evidence


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concerning the fact that crystallization does not follow a traditional nucleation

and growth mechanism. Although subjected to further assessment, it appears

that the simple idea that a small primary size would be prime nucleation as the

key step of crystallization seems essentially correct and holds certain general

validity, at least in solid-solid crystallization mechanisms. When additional

liquid/gas phase crystallization steps are involved in the final formation of the

nanoparticle, other steps like Ostwald ripening may be also of prime

importance. In any case, a lot of novel insights have been recently uncover in

solid-solid transformations and two main theories describe crystallization to

proceed either by surface (single particle) or interface (two or multiple particle)

nucleation [19].

The primacy of one of them has been postulated to be a function of the

oxide chemical nature and temperature, being presumably surface effects

always predominant at higher temperatures. Both theories mostly received

support from kinetic approaches but very recent analyses sensitive to structural

order in the amorphous precursor materials have demonstrated the key role of

intraparticle local order (below 1 nm) in driving the nucleation temperature

onset in a broad interval of ca. 200 K, showing that the whole crystallization

mechanism of oxide nanoparticles appears only compatible with some kind of

intraparticle, dimensional-restricted (surface) mechanism [20].

VIII) Microwave Assisted Sol-Gel Method

In the last decades, microwave heating has been applied in several areas

as a very effective and non-polluting method of activation. Examples of this

technology are materials processing, industrial and commercial applications,

chemistry and biochemistry, among others. In the electromagnetic spectrum,

the microwave radiation is located between infrared radiation and radio waves.

The wavelengths are between 1mm and 1m with corresponding frequencies in

the range of 300 GHz to 300 MHz. Besides telecommunication and microwave,

radar equipments occupy many of the band frequencies in this region, most

routinely used, for industrial and scientific purposes are imposed by


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international convention be the 2.45GHz (wavelength of 12.2cm) [21]. The

microwave energy is lying between 1.24 meV to 1.24 µeV.

Microwave is a relatively new technology alternative for materials

processing that provides new approaches for enhancing the material properties

as well as economic advantages through energy saving and acceleration of

product development. Clearly, advantages in utilizing this technology include

penetration of radiation, controlled electric field distributions, rapid heating,

self-limiting reactions and selective and volumetric heating [22]. Depending on

the response to microwave heating, materials can be classified into three

principal groups with respect to their interaction with a microwave field:

transparent or low loss materials where microwaves pass through them without

any losses; conductors which reflect microwaves without any penetration;

absorbing or high loss materials, which absorb microwaves and dissipate the

electromagnetic energy as heat, depending on the value of the dielectric loss

factor. Microwave heating of dielectric materials lies in the ability of the

electric field to polarize the charge of the material where polarization cannot

follow the rapid change of the electric field [23].

Microwave heating offers several potential advantages over

conventional heating for inducing or enhancing chemical reactions. These

include direct microwave interaction with certain classes of molecules,

volumetric heating rather than heat flow through the vessel wall, less waste

heat, increased control over heating rates and the possibility of selective

enhancement of desired reactions. This is particularly attractive in the case of

catalytic reactions, where it may be possible to selectively promote desirable

reactions while suppressing undesirable ones. Also, as a fast, simple and energy

efficient method, microwave synthesis has been developed and is widely used

for the synthesis of zeolites and ceramic materials.

Compared with conventional methods, microwave synthesis has the

advantages of very short reaction times, production of small particles with a

narrow particle size distribution and high purity. These conveniences could be

attributed to fast homogeneous nucleation and ready dissolution of the gel.


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Unfortunately the exact nature of the interaction of the microwaves with the

reactants during the synthesis of materials is somewhat uncertain and

speculative. However, it is well known that the interaction between dielectric

materials and the microwaves leads to dielectric heating in which electric

dipoles in such materials respond to the applied electric field. In liquids, this

constant reorientation leads to a friction between the molecules, which

subsequently generate heat. Many microwave phenomena are poorly

understood such as non-thermal effects and the superheating effect [21-23].

1.6 Properties of ZnO

1.6.1 Physical Properties

The unique and fascinating properties of II-VI compound

semiconductors have triggered tremendous motivation among the scientists to

explore the possibilities of using them in industrial applications, zinc oxide

(ZnO) is a piezoelectric, dielectric, transparent, semiconducting oxide with a

direct band gap of 3.37 eV at room temperature and a large exciton binding

energy (60 MeV), which is 2.4 times the effective thermal energy (KBT = 25

MeV) at room temperature. This is one of the key parameters that ZnO exhibits

near-UV emission, transparency, conductivity and resistance to high

temperature electronic degradation. In addition, ZnO is the hardest of the II-VI

semiconductors due to the higher melting point (1975°C) and large cohesive

energy (1.89 eV). Physical properties of ZnO are given in Table 1.1.


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Table 1.1: Physical Properties of ZnO

Sr. No. Properties Value

1 Structure Hexagonal

2 Lattice parameter ‘a’ 3.2495 Å

3 Lattice parameter ‘c’ 5.2069 Å

4 c/a 1.602

5 Density 5.606 g cm-3

6 Stable phase at 300 K Wurtzite

7 Melting point 1975°C

8 Thermal conductivity 0.6 , 1-1.2 µSiemen

9 Static dielectric constant 8.656

10 Refractive index , 2.008, 2.029

11 Energy gap 3.37 eV, direct

12 Exciton binding energy 60 MeV

13 Ionicity 62%

1.6.2 Crystal Structure

Zinc oxide crystallizes in three forms: hexagonal wurtzite, cubic zinc

blende and the rarely observed cubic rock salt. The wurtzite structure is most

stable and thus most common at ambient conditions, shown in Fig.1.2 (a). The

zinc blende form can be stabilized by growing ZnO on substrates with cubic

lattice structure, shown in Fig.1.2 (b). The rock salt NaCl-type structure is only

observed at relatively high pressures ~10 GPa. The hexagonal and zinc blende

ZnO lattices have no inversion symmetry (reflection of a crystal relatively any

given point does not transform it into itself). This and other lattice symmetry

properties result in piezoelectricity of the hexagonal and zinc blende ZnO, and

in pyroelectricity of hexagonal ZnO. The hexagonal structure has a point group

6 mm (Hermann-Mauguin notation) and the space group is P63mc or C6v. The

lattice constants are a = 3.25 Å and c = 5.2 Å, their ratio c/a ~ 1.60 is close to


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the ideal value for hexagonal cell c/a = 1.633. As in most II-VI materials, the

bonding in ZnO is largely ionic, which explains its strong piezoelectricity. Due

to this ionicity, zinc and oxygen planes bear electric charge (positive and

negative, respectively). Therefore, to maintain electrical neutrality, those planes

reconstruct at atomic level in most relative materials, but not in ZnO because

its surfaces are atomically flat, stable and exhibit no reconstruction.

Fig.1.2 (a) Wurtzite structure of ZnO

Fig.1.2 (b) Zinc blende structure of ZnO


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1.6.3 Lattice Parameters

Lattice parameters are considered important, when one has to develop

semiconductor devices. There are mainly four factors which determine the

lattice parameters of the semiconductors. (i) Free-electron concentration which

affects the potential of the bottom of conduction band normally occupied by

electrons. (ii) Concentration of impurities, defects and the difference in ionic

radii between these defects and impurities with respect to substituted matrix

ions. (iii) External strains. On the other hand, the strict periodicity of the lattice

is disturbed by many imperfections. These imperfections have a considerable,

controlling influence on mechanical, thermal, electrical and optical properties

of semiconductors. They determine the plasticity, hardness, thermal and

electrical conductivities. Commonly the lattice parameters of any crystalline

material are measured accurately by high resolution X-ray diffraction

(HRXRD). Table 1.2 shows a comparison of measured and calculated lattice

parameters of ZnO, c/a ratio reported by several groups [24-25].

Table 1.2: Measured and calculated lattice constants

Name of the method

Lattice parameter a (Å)

Lattice parameter c (Å)

Ratio of c/a

X-ray diffraction

method. 3.2496 5.2042 1.6018

Powder X-ray

diffraction

method.

3.2501 5.2071 1.6021

Linear

Combination of

Atomic Orbital

(LCAO) method.

3.286 5.241 1.595


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1.6.4 Band Structure

ZnO has wurtzite structure with direct energy band gap of 3.37 eV at

room temperature. The lowest conduction band of ZnO is predominantly n-type

and the valance band is p-type (six fold degenerate). The valance band splits

into three subbands A, B and C by spin orbit and crystal-field interactions, as

illustrated in Fig.1.3 [26]. The free excitation binding energies associated with

the A, B and C valance bands are 63, 50 and 49 meV, respectively [27]. The

high excitation binding energy leads to excitonic recombination even at room

temperature, in contrast to conventional electron-hole-plasma(EHP) transition,

which is promising for high-efficiency, low threshold photonic devices. Optical

anisotropy exists near the band edge due to different selection rules for light

polarization perpendicular and parallel to c-axis [28]. Transition for the A and

B excitation is mainly allowed for light polarization perpendicular to C-axis,

where as transition for the C excitation is mainly allowed for light polarization

parallel to C-axis.

Fig.1.3. Valance band splitting in hexagonal ZnO by crystal-field and spin-

orbit coupling


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1.6.5 Technology Development

In the field of semiconductor, ZnO is not new, its lattice parameter and

optical properties were studied by Bunn in 1935 and Mallow in 1954. Damen

studied the vibrational properties of ZnO with Raman scattering in 1966 and its

growth though the chemical vapour transport reported by Gali and Coker. ZnO

has been used as LED (1968) and it’s green emission (1969), yellow emission

(1970) are reported by Drapak. The main obstacle for the development of ZnO

has been the lack of reproducibility and low resistivity of p-type ZnO as

recently discussed by Look and Claflin in 2004. The attempt is made to prepare

various morphologies in presence of various oxides mixed with ZnO and to

improve its photo catalytic activity.

1.6.6 Applications of ZnO

As an important low-cost basic II-VI functional semiconductor material,

ZnO has many remarkable applications in functional devices, due to its large

exciton binding energy (60 meV) at room temperature. A great effort has been

paid to the synthesis of ZnO nanostructures, such as nanorods, nanowires [29-

30], nanobelts [31] and nano-tubes [32], which have been attracted intense

attention due to their potential applications in a variety of novel nano-devices,

such as field-effect transistors, single-electron transistors, photodiodes [33] and

chemical sensors [34-36].

The applications of zinc oxide powder are numerous and the principle

ones are summarized below. Most applications exploit the reactivity of the

oxide as a precursor to other zinc compounds. For material science

applications, zinc oxide has high refractive index, high thermal conductivity,

antibacterial and UV-protection properties. Consequently, it is added into

various materials and products, including plastics, ceramics, glass, cement,

rubber, lubricants [37], paints, ointment, adhesive, sealants, pigments, foods,

batteries, ferrites, fire retardants, etc. Zinc oxide is a constituent of cigarette

filters for removal of selected components from tobacco smoke. A filter

consisting of charcoal impregnated with zinc oxide and iron oxide removes


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significant amounts of HCN and H2S from tobacco smoke without affecting its

flavor. Zinc oxide along with stearic acid is used in the vulcanization of rubber.

Zinc oxide is widely used for concrete manufacturing. Addition of ZnO

improves the processing time and the resistance of concrete against water.

Many sunscreens use nanosized zinc oxide (along with nano titanium dioxide)

because such small particles do not scatter light and therefore do not appear

white. Zinc oxide is added to many food products, for eg. breakfast cereals as a

source of zinc, a necessary nutrient. Zinc white is used as a pigment, in paints

which is more opaque than lithopone, but less opaque than titanium dioxide.

Chinese white is a special grade of zinc white used in artists' pigments. It is

also a main ingredient of mineral makeup. Various plastics, such as

polyethylene naphthalate (PEN), can be protected by applying zinc oxide

coating, the coating reduces the diffusion of oxygen with PEN [38]. Zinc oxide

layers can also be used on polycarbonate (PC) in outdoor applications. The

coating protects PC from solar radiation and decreases the oxidation rate and

photo-yellowing of PC [39]. ZnO has wide direct band gap (3.37 eV).

Therefore, it’s most common potential applications are in laser diodes and light

emitting diodes (LEDs) [40]. Some optoelectronic applications of ZnO overlap

with that of GaN, which has a similar bandgap (~3.4 eV at room temperature).

Compared to GaN, ZnO has a larger exciton binding energy (~60 meV, 2.4

times of the room-temperature thermal energy), which results in bright room-

temperature emission from ZnO. It can be combined with GaN for LED-

applications. For instance as TCO layer and ZnO nanostructures provide better

light outcoupling [41]. Other properties of ZnO favorable for electronic

applications include its stability to high-energy radiation and to wet chemical

etching [42] radiation resistance [43] makes ZnO a suitable candidate for space

applications. ZnO is currently the most promising candidate in the field of

random lasers to produce an electronically pumped UV laser source. The

pointed tips of ZnO nanorods result in a strong enhancement of an electric

field. Therefore, they can be used as field emitters [44]. Aluminium-doped ZnO

layers are used as transparent electrodes. The constituents Zn and Al are much


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cheaper and less toxic compared to the generally used indium tin oxide (ITO).

One application which has begun to be commercially available is the use of

ZnO as the front contact for solar cells [45]. Transparent thin-film transistors

(TTFT) can be produced with ZnO. As field-effect transistors, they even may

not need a p-n junction [46] thus avoiding the p-type doping problem of ZnO.

Some of the field-effect transistors even use ZnO nanorods as conducting

channels [47]. The piezoelectricity in textile fibers coated in ZnO have been

shown capable of fabricating "self-powered nanosystems" with everyday

mechanical stress from wind or body movements [48-49]. Zinc oxide nanorods

are devices detecting changes in electrical current passing through zinc oxide

nanowires due to adsorption of gas molecules. Selectivity to hydrogen gas was

achieved by sputtering Pd clusters on the nanorod surface. The addition of Pd

appears to be effective in the catalytic dissociation of hydrogen molecules into

atomic hydrogen, increasing the sensitivity of the sensor device. The sensor

detects hydrogen concentrations down to 10 parts per million at room

temperature [50].

1.7 Properties of Cerium Oxide

1.7.1 Physical Properties

Ceria (CeO2) is an oxide with important applications in areas of

catalysis, electrochemistry, photochemistry and materials science [51-55]. Also

it is highly efficient ultraviolet (UV) absorber to protect light-sensitive

materials, as a coating material for protection of corrosion of metals, as an

oxidation catalyst and as a counter electrode for electrochemical devices [56-

59]. The physical properties of CeO2 are represented in Table 1.3. Cerium

oxide has outstanding physical and chemical properties. Recently Zhang et al.

has reported the CeO2 nanocrystal microspheres as a novel adsorbent for the

removal of Cr (VI) from waste water [60]. In its most stable phase, bulk CeO2

adopts a fluorite-type crystal structure in which each metal cation is surrounded

by eight oxygen atoms. The band gap of pure ceria is ~3.2 eV, but crystal


21

defects or impurities can transform the material in a good n-type

semiconductor.

Table 1.3 Physical Properties of CeO2

Sr. No. Properties Value

1) Molar mass 172.115 g/mol

2) Appearance White or pale yellow solid, slightly hygroscopic

3) Density 7.65 g/cm3, solid 7.215 g/cm3, fluorite phase

4) Melting point 2400°C

5) Solubility in water Insoluble

6) Band gap

~3.2 eV

In the area of catalysis, nanoparticles of ceria have been studied since

the early 1970s, but they were poorly characterized. In recent years, substantial

progress has been made and the use of better synthetic methods.

1.7.2 Crystal Structure of CeO2

The fluorite structure is most stable and thus most common at ambient

conditions shown in Fig.1.4. Many researchers were reported the fluorite

structure of CeO2. Small nanoparticles exhibited a nearly amorphous structure

[58, 61-65]. Due to its fluorite structure, the oxygen atoms in a ceria crystal are

all in a plane with one another, allowing for rapid diffusion as a function of the

number of oxygen vacancies. As the number of vacancies increases, the ease at

which oxygen can move around in the crystal increases, allowing the ceria to

reduce and oxidize molecules or co-catalysts on its surface. It has been shown

that the catalytic activity of ceria is directly related to the number of oxygen


22

vacancies in the crystal, frequently measured by using X-ray Photoelectron

Spectroscopy to compare the ratios of Ce3+ to Ce4+ in the crystal.

Fig. 1.4 Fluorite structure of CeO2

1.7.3 Applications of CeO2

Nanostructured metal oxides have attracted considerable attention from

many researchers due to their diversity of applications in various technological

fields of science and technology. Ceria has been used in catalytic converters in

automotive applications. CeO2 possesses many attractive properties that make

it highly promising for a wide range of applications such as solid electrolytes in

solid oxide fuel cells [66], automotive three-way catalyst [67], ultraviolet

absorbers [68-69] and oxygen sensors [70-71]. It is also used as a catalyst for

large-scale fluid cracking in refineries and dehydrogenation of ethyl benzene to

styrene [72] as a sulphur adsorbent for the removal of H2S from hot fuel gas

streams [73]. The reports on the synthesis and applications of coupled

bicomponents ZnO-CeO2 materials are important. The mixed oxide of ZnO

reported that it has high photo catalytic activity due to tuning in the band gap,

which is one of the key concepts for photocatalysis. Therefore we have carried

out this study on microwave assisted process for preparation of ZnO, ZnO-

CeO2 and ZnWO4 nanoparticles and properties of these nanoparticles.


23

1.8 Metal Oxide Nanomaterials

Metal oxides play a very important role in many areas of chemistry,

physics and materials science [74]. The metal elements are able to form a large

diversity of oxide compounds. These can adopt a vast number of structural

geometries with an electronic structure that can exhibit metallic, semiconductor

character [75]. In technological applications, oxides are used in the fabrication

of microelectronic circuits, gas sensors, piezoelectric devices, fuel cells, dye

sensitized solar cells, coatings for the passivation of surfaces against corrosion.

In the emerging field of nanotechnology, a goal is to make nanostructures with

special properties with respect to those of bulk [76-77].

Metal oxide nanoparticles can exhibit unique physical and chemical

properties due to their limited size and a high density of corner surface sites.

Particle size is expected to influence three important groups of basic properties

in any material. The first one comprises the structural characteristics, namely

the lattice symmetry and cell parameters [78]. Bulk oxides are usually robust

and stable systems with well-defined crystallographic structures. However, the

growing importance of surface free energy and stress with decreasing particle

size must be considered. Changes in thermodynamic stability associated with

size can induce modification of cell parameters [79] and in extreme cases the

nanoparticle can disappear due to interactions with its surrounding environment

and a high surface free energy [80].

In order to display mechanical and structural stability, a nanoparticle

must have a low surface free energy. As a consequence of this requirement,

phases that have a low stability in bulk materials can become very stable in

nanostructures. This structural phenomenon has been detected in TiO2, VOx ,

Al2O3 and MoOx oxides [81]. Size-induced structural distortions associated

with changes in cell parameters have been observed in nanoparticles of Al2O3,

NiO, Fe2O3, WO3, MoO3, CeO2, Y2O3 and ZnO [82-84]. As the particle size

decreases, the increasing number of surface and interface atoms generates

stress/strain and concomitant structural perturbations [85-87]. Beyond this


24

“intrinsic” strain, there may be also “extrinsic” strain associated with a

particular synthesis method which may be partially relieved by annealing [88].

Also, non-stoichiometry is a common phenomenon. On the other hand,

interactions with the substrate on which the nanoparticles are supported can

complicate the situation and induce structural perturbations or phases not seen

for the bulk state of the oxide [89].

1.9 Nanomaterials in Semiconductor

The important terms involved in a photoactive semiconductor are

conduction band (CB), valence band (VB), band gap, traps sites and Fermi

level. The bands are the allowed energy states that an electron can occupy in a

material. The highest energy band occupied by an electron is called the valence

band while the next available lowest empty energy level, is called the

conduction band. The bands are clearly differentiated in a semiconductor than a

metal. In case of semiconductor nanoparticles, as the size of the nanoparticles

increases the band gap of the semiconductor decreases, which is shown in

Fig.1.5


25

Fig.1.5 The variation of particle size with band gap energy

The Fermi level is a probability distribution curve that represents a 50%

possibility of locating an electron at a given energy level. For an n-type

semiconductor such as ZnO, TiO2 the Fermi level is close to the conduction

band. Light energy greater than the band gap of the semiconductor excite the


26

electrons from the valence band to the conduction band leaving behind a hole

in the valence band. For example, TiO2 is a large band gap semiconductor and

hence produces e-h pairs on illumination with UV light. The electrons and

holes are available for carrying out redox activities at the semiconductor

surface. Photogenerated e-h pairs are also delocalized in the semiconductor.

These locations are called trap sites (et and ht). The e-h pairs undergo

recombination which results in decreasing the efficiency of the semiconductor.

The numbers of photogenerated electrons in TiO2 are dictated by the ability of

the surroundings to scavenge electrons and holes and the recombination

between the photo generated e-h pairs.

An increase in the band gap of the semiconductor with decrease in the

particle size is defined as quantization effect. Nanoparticles of semiconductors

display unique size-dependent properties (quantization effect) that alter its

photochemical, photophysical, photochromic, optical and electrochemical

responses. Both the large (e.g. ZnO, TiO2, SnO2 and WO3) as well as small

band gap semiconductor (e.g. CdSe and CdS) display this property. Charge

generation, separation, retention and transfer across a semiconductor and its

surroundings is greatly affected due to the quantization effect [90-94]. The

presence of surface bound species which include surrounding electrolytes,

sensitizers like other semiconductors, metals and dyes play an active role in

determining the mechanism of charge transfer taking place at the

semiconductor and surrounding species interface. Photo-induced excitation

leads to charge separation in the semiconductor followed by electron or hole

transfer to the surroundings dictated by the energetics of the system.

Furthermore, defects created in the semiconductor largely depend upon method

and doping. These defects play a vital role in controlling the

photoelectrochemical and photocatalytic behavior of the semiconductor. Two

major drawbacks of any individual large band gap semiconductor have been

identified as, recombination of photogenerated charges (electron and holes) and

limited light harvesting ability. These factors are limiting in the economical

usage of the semiconductor. The former can be minimized by using metal on a


27

semiconductor because metal acts as sink for the electron and decreases

recombination. Sensitization with dyes is one of the most commonly used

methods to overcome the limited light harvesting ability of a large band gap

semiconductor [95]. A semiconductor can be sensitized using another

semiconductor (e.g. TiO2-CdSe) or a dye (Azo dyes) while charge

recombination can be minimized using metal deposits (Ag, Pt). When a metal

is used with a semiconductor, the metal facilitates in decreasing recombination

of the photogenerated charges. Thus both drawbacks of the semiconductor can

be effectively overcome depending upon the material used to create composite

with a semiconductor.

1.10 Impact of Nanomaterials in Catalysis

Nanomaterials are believed to exhibit unique science at nanoscale

regime. The interest in nanomaterials arises from the fact that their physico-

chemical properties are a function of their dimensions. Some of the important

features that influence the catalytic and selectivity are surface structure,

mobility of the active species to restructure as well as the mobility of the

adsorbates on these active species, selective pore size in case of porous

catalysis and metal-support interface sites. In comparison to bulk materials in

nano regime possesses several properties like short range ordering, enhanced

interaction with environments due to high number of dangling bonds, great

variety of the valence band electron structure and self-structuring for optimum

performance in chemisorption and catalysis.

As a consequence of the size reduction, larger portion of their

constituent atoms are located at the surface (surface to volume ratio) and higher

the surface area, higher will be the surface atoms. In general, increase in the

surface area provides more adsorption of reactant molecules on its surface,

which results the higher catalytic activity. Similarly increase in the number of

surface atoms creates more number of active sites, which ultimately result in

higher catalytic activity. In case of fuel cell applications, the current generated

at an electrode is proportional to the active surface of catalysts on the electrode


28

surface, so higher power density fuel cells can be formed from nanomaterials,

because nanomaterials have a higher surface to volume ratio. Researchers have

also shown that the electrolytic properties of the materials are sensitive to

particle size, so increased catalytic activity can be observed for nanoparticles

and nanomaterials. It is important to understand that smaller the particles, the

larger the portion of their constituent at the surface. A surface atom plays an

important role in catalysis, as the reaction takes place at the surface of the

particle. Furthermore, increasing surface area increases the relative contribution

of the surface energy and therefore the thermodynamic stability is decreased

with decreasing particle size [96-97].

Nanomaterials are characterized by a very high number of low co-

ordination number species at edge and corner sites which can provide a large

number of catalytically active sites. Such materials exhibit chemical and

physical properties characteristic of neither that of isolated atoms nor of the

bulk material. The excess surface energy makes the particles more reactive and

structurally sensitive towards their environment. Modern surface science

studies indicate that, during chemisorptions and catalytic reaction, these

particles restructure, and the adsorbed molecules also possesses a high degree

of surface mobility. The adsorbate-induced restructuring facilitates the

breaking of surface chemical bonds and the rearrangement of adsorbates as the

cluster assumes a thermodynamically most stable configuration. As any

materials interact with their environment through solid/gas, solid/liquid, and

solid/solid interfaces, the nanometer scale surface created can be modified to

perform certain functions [98].

Nanosize particles play a crucial role in surface reactivity,

chemisorption and catalysis. Owing to their size, the electronic structure of the

particles significantly differs from that of bulk. The electronic structure of

catalysts has been shown to be important in adsorption and since adsorption is

a necessary step in heterogeneous catalytic reactions, it would be expected that

the changes in the electronic structure would influence the rate of reaction. The

small size also significantly affects the electrochemical behaviour of the


29

nanocrystal materials. Electrochemical model and theories on nanoparticles and

charge-transfer mechanisms for some metal oxide revealed that the properties

of semiconductor gradually transmit to molecular properties as the size of a

crystal is successively decreased in the nanometer range [99]. While going

from bulk to nano size, the semiconductor energy levels become more

separated from each other and the effective band gap increases. As a result of

increase in band gap, the conductivity as well as the density of states in the

conduction and valence band will be reduced. Because of this unusual

behaviour of metals and semiconductors while going from bulk to nano, the

Fermi level and density of states at the interface will alter. Since the Fermi

level is associated with the redox ability of the system, there will be a

considerable change in the rate of redox processes. It would therefore be

possible to control the electrocatalytic and photocatalytic activity of metals and

semiconductors by changing their size [96,100].

1.11 Classification of Nanomaterials Based on its Shape

The main categories of nanoparticle according to shape and the type of

application in which they may be used [101-102] are given in Table 1.4.

Table 1.4 Different shapes of nanoparticles with examples

Morphology of the Nanostructure

Name of the Material

Nanotubes Carbon, (fullerenes)

Nanowires Semiconductors, metals, oxides, sulfides, nitrides

Nanocrystals,

quantum dots

Insulators, semiconductors, metals, magnetic

materials

Other nanoparticles Ceramic oxides

a) Nanotubes

Nanotubes are a particularly novel form of nanoparticle about which

there is great interest and excitement. Carbon nanotubes (CNT) were first


30

discovered by Iijima (1991), and are a new form of carbon molecule. They are

similar in structure to the spherical molecule C60 (buckminsterfullerene)

discovered in the 1980s, but are elongated to form tubular structures 1-2 nm in

diameter [103]. In their simplest form, nanotubes comprise a single layer of

carbon atoms arranged in a cylinder. These are known as single-wall carbon

nanotubes. They can also be formed as multi-wall carbon nanotubes having

diameters significantly greater up to 20 nm and length greater than 1 mm.

CNTs have great tensile strength and are considered to be 100 times stronger

than steel while being only one sixth of its weight thus making them potentially

the strongest. They also exhibit high conductivity, high surface area, unique

electronic properties and potentially high molecular adsorption capacity [104].

Applications which are currently being investigated include; polymer

composites (conductive and structural filler), electromagnetic shielding,

electron field emitters, super capacitors, batteries, hydrogen storage and

structural composites [105]. Hoffman et al. identified four main synthesis

methods for CNTs. These are:

• Laser ablation,

• Arc discharge,

• Chemical vapour deposition (CVD) and

• Plasma-enhanced CVD.

Many of the applications envisaged for CNTs will require bulk

quantities of the material to be produced. For applications where large

quantities of CNTs are required, CVD offers the most suitable approach for

scaling up due to the relatively low growth temperature, high yields and high

purities that can be achieved. Production of relatively large volumes of

unaligned MWCNTs, have been achieved by this and other methods and these

materials are beginning to be used for commercial applications [106].

A recent advance in the development of CNTs is the production of

controlled architectures of aligned CNTs using an injection CVD process. The

CNTs are produced by spraying a solution of ferrocene dissolved in xylene into

a two-stage furnace. The carrier gas is a mixture of argon and hydrogen. The


31

ferrocene decomposes to provide an iron catalyst, which nucleates the nanotube

growth. The xylene acts as the carbon feedstock. The vapour passes into the

second furnace where the nanotubes grow on quartz substrates [107].

One major focus of current research on nanotubes is on scaling-up of

production rates to kilogram quantities. Nanotubes have also been produced

from other materials including silicon and germanium but the development of

various forms and applications for CNTs remain the main focus of activity

[108]. From an occupational hygiene perspective, the large aspect ratios of

CNTs, their durability and the desire to produce bulk quantities make them of

particular interest [109].

b) Nanowires

Nanowires are small conducting or semi-conducting nanoparticles with

a single crystal structure and a typical diameter of a few nanometers and a large

aspect ratio [110]. They are used as interconnectors for the transport of

electrons in nanoelectronic devices. Various metals have been used to fabricate

nanowires including cobalt, gold and copper. Silicon nanowires have also been

produced.

Most approaches to the fabrication of nanowires are derived from

methods currently used in the semi-conductor industry for the fabrication of

microchips. Van Zant (2000) provides a comprehensive review of microchip

fabrication, which makes useful background reading. Typically they involve

the manufacture of a template followed by the deposition of a vapour to fill the

template and grow the nanowire. Deposition processes include electrochemical

deposition and CVD. The template may be formed by various processes

including etching, or the use of other nanoparticles, in particular, the nanotubes

[96,111].

c) Quantum Dots

Quantum dots of semiconductors, metals and metal oxides has the

particle size in all dimension at the forefront of research for the last five years

due to their novel electronic, optical, magnetic and catalytic properties


32

[96,112]. The number of atoms in a quantum dot, which ranges from 1000 to

100,000, makes it neither an extended solid structure nor a single molecular

entity. This has led to various names being attributed to such materials

including nanocrystals and artificial atoms. To date, chemistry, physics and

material science have provided methods for the production of quantum dots

and allow tighter control of affecting factors, for example, particle growth and

size, solubility and emission properties [113].

The majority of research has centred around semiconductor quantum

dots, as they exhibit distinct 'quantum size effects'. The light emitted can be

tuned to the desired wavelength by altering the particle size through careful

control of the growth steps. Various methods can be employed to make

quantum dots however the most common is by wet chemical colloidal

processes [114]. This method gives a better results and the morphology of the

materials.

d) Other Nanoparticles

This catch all category includes a wide range of primarily spherical or

aggregated dendritic forms of nanoparticles. Dendritic forms are where

spherical or other compact forms of primary particles aggregate together to

form chain like or branching structures. Welding fume is the best known

example of this. This “other nanoparticle” category includes existing common

nanoparticles such as ultrafine carbon black and fumed silica which are

synthesized in bulk form through flame pyrolysis methods. Nanoparticles of

this type may be formed from many materials including metals, oxides,

ceramics, semiconductors and organic materials. The particles may be

composites having, for example, a metal core with an oxide shell or alloys in

which mixtures of metals are present. Many of the production processes

involve the direct generation of aerosols through gas phase synthesis, similar to

flame pyrolysis but other production processes including wet chemistry

methods [115]. This group of particles may be categorized as being less well

defined in terms of size and shape, generally larger, and likely to be produced


33

in larger bulk quantities than other forms of nanoparticles. From an

occupational hygiene perspective, the likelihood of aerosol generation and their

availability in bulk quantities makes these nanoparticles of particular interest

[96,116].

1.12 Properties of Metal Oxide Nanoparticles

a) Optical Properties

The optical property is one of the fundamental properties of metal

oxides and can be experimentally obtained from absorption measurements.

While reflectivity is clearly size-dependent as scattering can display drastic

changes when the oxide characteristic size is in the range of photon

wavelength, absorption features typically common and main absorption

behavior of solids [117]. Due to quantum-size confinement, absorption of light

becomes both discrete like and size dependent. For nano crystalline

semiconductors, both linear and non-linear optical properties arise as a result of

transitions between electron and hole discrete or quantized electronic levels

[118]. The effective mass theory (EMA) is the most elegant and general theory

to explain the size dependence of the optical properties of nano-meter

semiconductors, although other theories such as the free exciton collision

model (FECM) or those based on the bond length-strength correlation have

been developed to account for several deficiencies of the EMA theory [119].

Other optical excitations, which showed quantum-size confinement

effects, concern the excitation of optical phonons of oxides. The effects of size

on the phonon spectra of oxide materials have been well established by using

Raman scattering experiments on nanocrystals, in combination with the

theoretical phonon confinement models [120]. Essentially, the phonon

confinement model provides the theoretical background for the study of

nanocrystalline materials and this is the main factor responsible for the changes

observed in the Raman spectrum, which are caused by the size effect. Some

examples of application of the confinement model for qualitative interpretation


34

of Raman results in series of nanostructured oxides like anatase ZnO, TiO2,

CuO, Cr2O3, ZrTiO4, CeO2 or manganese oxides are reported [11-123].

In all cases, “non-stoichiometry” size-dependent defect affects optical

absorption features of nanosized oxides. Typical point defects in

nanostructured oxides concern oxygen or cation vacancies or the presence of

alien’s species, like Cu2+ and Ce3+. Vacancy defects introduce gap states in

proportion to the defect number; in fact, a random distribution of vacancy

defects introduce a Gaussian like density of states which may produce mid-gap

states and be localized near the valence and conduction bands depending on the

electronic nature (donor/acceptor) of the defect and giving characteristic

“localized” features in the UV-Visible spectrum. Such point defects mainly

contribute to the Raman spectra by producing a broadening of the peaks [124].

Alien cations display specific features, like the localized d-d or f-f transitions

of Cu/Ce. Besides electronic modifications, point defects, the alien ions, like

Cu2+ and Ce3+, induce strain and concomitant structural differences in atomic

positions with respect to bulk positions. Strain effects are inherent to

nanostructured materials and may be comprised in the general, ambiguous term

of “surface” effects usually claimed to account for significant deviations in the

confinement theories [125]. Surface effects and, particularly, nonstoichiometry

related to the preparation method are critically important for very low particle

size and to produce characteristic features in the UV-Visible spectrum for

certain oxides such as SnO2 or ZrO2 [96,126].

b) Electrical Properties

Oxide materials can present ionic or mixed ionic/electronic conductivity

and it is experimentally well established that both can be influenced by the

nanostructure of the solid. The number of electronic charge carriers in a metal

oxide is a function of the band gap energy according to the Boltzmann

statistics. The electronic conduction is referred to as n or p hopping type

depending on whether the principal charge carriers are electrons or holes. The

number of “free” electron/holes of an oxide can be enhanced by introducing


35

non-stoichiometry and in such case are balanced by the much less mobile

oxygen/cation vacancies [127]. In an analogous manner to hopping type

conduction, ionic conduction takes place when ions can hop from site to site

within a crystal lattice as a result of thermal activation, and is typically

interpreted on the basis of a modified Fick´s second law. Four mechanism

types have been observed for ionic conduction: direct interstitial, interstitialcy,

vacancy and Grotthus. As charge species (defects; impurities) in polycrystalline

oxides typically segregate to particle boundaries to minimize strain and

electrostatic potential contributions to the total energy, there is a contribution to

the conductivity parallel to the surface which becomes important at the

nanoscale regime [128]. The charge carrier (defect) distribution also suffers

strong modification from bulk materials, as there is presence of charge carriers

through the whole material as a consequence of the shielded electrostatic

potential depletion at surface layers of nanosized materials [129]. As a result of

these nanoscale derived effects, it is well known that CeO2 exhibits an

improved n-type conductivity which may be four order of magnitude greater

than the corresponding to bulk/micro-crystalline ceria and is ascribed to a

significant enhancement of the electronic contribution [130]. Alteration of the

transport properties is also observed in ZrO2 but the physical ground is still far

from being understood [131]. The strong size dependence observed for the

electrical conductance in the context of gas-sensing devices has been recently

reviewed for the SnO2, WO3, and In2O3 oxides. In proton conductors, like

SrCe0.95Yb0.05O3-d enhanced conduction and faster kinetics under H-

atmospheres are observed in nanosized samples as these phenomena are largely

determined by boundary/interfacial effects [132]. Interesting to stress here is

that some of the most dramatic effects of the nanostructure on ionic transport in

oxides are observed in the field of Li+ ion batteries. An outstanding

enhancement in conductivity due to Li+ ion vacancy has been achieved using

Li-infiltrated nanoporous Al2O3 [133].


36

c) Mechanical Properties

Main mechanical properties concern low yield (stress and hardness) and

high (superplasticity) temperature observables. Information on oxide

nanomaterials is scarce and mainly devoted to analyze sinterability, ductility

and superplasticity. These mechanical properties are also found to be strain-rate

dependent; an enhanced strain rate sensitivity at room temperature is observed

for TiO2 and ZrO2 with decreasing primary particle grain size. In spite of such

facts, it is clear that oxide materials (like ZnO, Al2O3, ZrO2, CeO2 and TiO2)

sintered under vacuum or using the spark plasma technique display enhanced

field strength and hardness with respect to conventional or bulk ceramic

materials and have the additional properties of being transparent (films), being

potential materials for the aerospatial industry [134]. Superplasticity refers to

the capacity of oxide materials. Essentially, polycrystalline tetragonal ZrO2

appears as the most celebrated example of a superplasticity ceramic and

together with TiO2, the ZrO2 are the only nano-oxides subjected to studies. At

room temperature, nanocrystalline oxides may have a small amount of ductility

beyond that exhibited by bulk materials but they are not superplastics. At high

temperatures, they seem to exhibit significant compressive ductility and strain

rate sensitivities that indicate superplasticity [135].

d) Chemical Properties

Metal oxides are used for both their redox and acid/base properties in

the context of absorption and catalysis. The three key features essential for

their application as absorbents or catalysts are (i) the coordination environment

of surface atoms, (ii) the redox properties and (iii) the oxidation state at surface

layers. Both redox and acid/base properties are interrelated and attempts can be

found in the literature to establish correlations of both properties. In a simple

classification, oxides having only s or p electrons in their valence orbitals tend

to be more effective for acid/base catalysis, while those having d or f outer

electrons find a wider range of uses [136].


37

The solid in a given reaction conditions that undergoes reduction and

reoxidation simultaneously by giving out surface lattice oxygen anions and

taking oxygen from the gas phase is called a redox catalyst. This process

necessarily demands microscopy reversibility and implies dynamic operation.

The commonly accepted mechanism was developed by Mars van Krevelen and

essentially implies that redox systems require high electronic conduction

cations to manage electrons and high oxygen-lattice mobility. Based on modern

isotopic exchange experiments, the redox mechanism of chemical reactions can

be more specifically divided in (i) extra facial oxygen in which adsorbed

(oxygen) species react (electrophilic reaction) and (ii) interfacial oxygen where

lattice oxygen vacancies are created (nucleophilic reaction) [137]. There are

enormous evidences which exhibit that nucleophilic oxygen is capable of

carrying out selective oxidations while it seems that electrophilic species seems

to work exclusively on non-selective ones. Latter, it was shown that

hydrocarbon selective oxidation starts with H-abstraction steps and that the

filling of oxygen vacancies requires the cooperation of a significant number of

cations. So, typically, an oxidation reaction demands to optimize three

important steps: the activation of the C-H bond and molecular oxygen and the

desorption of products (to limit over-oxidation). The effect of size on these key

steps is unknown but can be speculated to be related to the oxidation state of

surface cations and their ability to manage electrons and the influence of non-

stoichiometry on the gas-phase oxygen species handling and activation [138].

Many oxides also display acid/base properties. Oxide materials can

contain Bronsted and Lewis acid/base sites. Bronsted acid (A) and base (B)

interactions consist of the exchange of protons as HA + B = A- + HB+. In any

solid, two independent variables, the acid/base strength and amount (density

per surface unit) need to be addressed to give a complete picture of its

acid/base characteristics. Such characteristics are basically linked to the nature

(valence/cation size) of the element present in the oxide and general views of

the behavior of Bronsted/Lewis acidity as a function of solid state variables

have been published. Essentially, Lewis acidity is characeristic of ionic oxides


38

and practically absent in covalent oxides. The strongest Lewis acid oxides are

Al2O3 and Ga2O3. As a general rule, for stronger Lewis acid, the few sites are

available (amount) due to the higher level of surface hydroxylation. As

mentioned, Lewis acidity is mostly associated to oxides with ionic character;

Lewis basicity is mostly associated with them. This means that the stronger the

Lewis acid sites, the weaker the basic sites and vice versa. On the contrary,

most of the ionic metal oxides do not carry sufficiently strong Bronsted acidity

to protonate pyridine or ammonia at room temperature although the more acids

of them can do it at higher temperatures. In spite of this, the surface OH groups

of most ionic oxides have a more basic character than acid character. Finally,

strong Bronsted acidity appears in oxides of elements with formal valency five

or higher (WO3, MoO3, N2O5, V2O5 and S-containing oxides) [139].

1.13 Advanced Oxidation Processes

Advanced oxidation processes (AOP’s), uniting together ozone and high

output ultraviolet technologies, in conjunction with hydrogen peroxide and

catalyst are successfully used to decompose many toxic and bio-resistant

organic pollutants in aqueous solution to acceptable levels, without producing

additional hazardous by-products or sludge which require further handling.

Advanced oxidation processes involve the generation of hydroxyl (•OH)

radicals which oxidize the pollutants. Advanced oxidation processes can be

broadly classified into the following groups:

i) Homogeneous photocatalysis,

ii) Heterogeneous photocatalysis

i) Homogeneous photocatalysis

The applications of homogeneous photodegradation (single-phase

system) to treat contaminated water, involves the use of an oxidant to generate

radicals, which attack the organic pollutants to initiate oxidation. The major

oxidants used are: Hydrogen peroxide (UV /H2O2), Ozone (UV /O3),

Hydrogen peroxide and Ozone (UV /O3/ H2O2),


39

Photo-Fenton system (Fe+3 / H2O2).

ii) Heterogeneous photocatalysis

Heterogeneous photocatalytic process consists of utilizing the near UV

radiation to photo-excite a semiconductor catalyst in the presence of oxygen.

Under these circumstances oxidizing species, either bound hydroxyl radicals or

free holes, are generated as shown in Fig.1.6. Using photocatalysis, organic

pollutants can be completely mineralized reacting with the oxidizers to form

CO2, water and dilute concentration of simple mineral acids. The process is

heterogeneous because there are two active phases, solid and liquid. This

process can also be carried out utilizing the near part of solar spectrum (λ <

380nm) which transforms into a good option to be used [140].

Fig.1.6 Scheme showing some of the events that might be taking place on an irradiated semiconductor particle


40

The semiconductor may be in the form of a powder suspended in the

water or fixed on a support. The most active photocatalyst for this application

is the anatase form of TiO2/ZnO because of its high stability, good performance

and low cost [141].

1.14 History and Current Status of Photocatalysis

Environmental pollution and energy shortage, being regarded as the top

two challenges facing mankind in the next 50 years, have attracted much

attention from both government agencies and scientific communities.

Semiconductor based heterogeneous photocatalytic technology was developed

as a promising solution to the two challenges. The utilization of

semiconductors as a photocatalyst started from the first report by Fujishima and

Honda in 1972 [142]. At that time, the energy crisis was spreading around the

world, the water splitting into hydrogen and oxygen under ultraviolet (UV)

irradiation by using TiO2 photoanode inspired extensive research concerning

solar energy conversion and created a new era in the field of photocatalyst. In

1977, Bard reported that CN- was photocatalytically oxidized into OCN- on

TiO2 [143] and the applications of photocatalysis were extended into the field

of waste water treatment and environmental remediation. Since the beginning

of the 1990s, environmental pollutants have aroused increased public concern.

Heterogeneous photocatalytic reactions on ZnO nanoparticles (n-type

semiconductors) are now recognized as an ideal technology for removal of all

kinds of pollutants both in water and air [144-148]. Most of the investigations

have focused on mixed-metal oxides which show relatively high reactivity and

chemical stability under ultraviolet (UV) light.

Over the last 10 years the scientific and engineering interest in the

application of semiconductor photocatalysis has grown exponentially. In the

areas of water, air and waste water treatment the photocatalysis by metal oxides

has many potential applications as an environmental control technology [149].

Many background information reviews on photocatalysis have been appeared

in the literature, note worthy recent reviews are provided by Ollis and Al-


41

Akabi, [150] Blake, [151] Mills et al., [152] Bahnemann et al. [153] Pichat,

[154] Aithal et al., [155] and Lewis [156].

Semiconductor photocatalysis with a primary focus on TiO2 as a durable

photocatalyst has been applied to a variety of problems of environmental

interest in addition to water and air purification. It has been shown to be useful

for the destruction of microorganisms such as bacteria [157] and viruses [158].

Semiconductors (e.g., TiO2, ZnO, Fe2O3, CdS and ZnS) can act as sensitizers

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

characterized by a filled valence band and an empty conduction band [159].

The approaches also include the incorporation of transition metals doping [160-

163]. Spinel zinc ferrite (ZnFe2O4) is a narrow band gap semiconductor that has

a potential application in the conversion of sunlight, because of its sensitivity to

ultra-violet light and no photochemical corrosion [164]. In contrast, on bulk

semiconductor electrodes only one species, either the hole or electron, is

available for reaction due to band bending [165]. However, in very small

semiconductor particles and suspensions both species are present on the

surface. Therefore, careful consideration of both the oxidative and the reductive

paths is required.

1.15 Literature Survey

The fine powders of oxides have been prepared by many methods,

including forced hydrolysis, sol-gel, hydrothermal, co-precipitation, surfactant

templating and spray pyrolysis [166]. However, microwave-assisted synthesis

is very beneficial to find a fast, simple and energy efficient approach to

produce fine metal oxide nanoparticles [167]. Microwave assisted synthesis is

relatively new technology to produce inorganic compounds since 1986 for

materials processing to enhance the material properties as well as economic

advantages [168]. Microwave heating offers several potential advantages over

the conventional heating for inducing or enhancing chemical reactions [169].

These includes direct microwave interaction with certain class of molecules,

volumetric heating rather than heat flow through the vessel wall, less waste


42

heat, increased control over heating rates and the possibility of selective

enhancement of desired reactions [170]. Microwave synthesis has been

developed and widely used for the synthesis of zeolites and ceramic materials.

Compared with conventional methods, microwave synthesis has advantages: 1)

Production of small particles with a narrow size distribution with high purity

and 2) within very short reaction time. These conveniences could be attributed

to fast homogeneous nucleation and ready dissolution of the gel [171].

Microwave heating is in situ mode of energy conversion therefore it is very

attractive for chemist due to its simplicity and cost effective technique.

A number of methods have been used for synthesizing ZnO

nanopowders [172-178] and in recent years, a new method has been reported as

microwave-assisted synthesis. Due to its unique features such as short reaction

time, enhanced reaction selectivity, energy saving, and high reaction rate [179],

the application of microwave-assisted synthesis of ZnO nanoparticles has been

rapidly growing [179-183]. Many methods have been reported for the synthesis

of ZnO nanoparticles, including the sol-gel [184], microwave method [185],

evaporative decomposition of solution [186], template-assisted growth [187],

wet chemical synthesis [188] and gas-phase reaction [189]. Some researchers

have been highlighted the performance of ZnO for degradation of some organic

compounds [190-191]. In addition, ZnO has more functions than TiO2 [192]

and recent researches have pointed out that ZnO can also be used in the acidic

or alkaline conditions through the proper treatment [193-194].

Hong et al., [195] reported the synthesis of Zinc oxide (ZnO)

nanoparticles using zinc nitrate and NaOH by ultrasonic irradiation technique

at room temperature. The ZnO nanoparticles were heated individually in an

electric furnace for two hours at 700°C. The morphology and optical

properties of the C60 and ZnO nanoparticles were characterized by X-ray

diffraction, SEM, transmission electron microscopy and UV-Visible

spectroscopy. The photocatalytic activity of the heated and unheated C60 and

ZnO nanoparticles were studied for the decomposition of Methylene blue,

Methyl orange and Rhodamine B.


43

Comparison of dye degradation efficiency using ZnO powders with

various sizes reported by Wang et al., [196]. ZnO powders with various size

scales (mean diam.size: 10, 50, 200, and 1000 nm) were prepared by two

different preparation methods, thermal evaporation method and chemical

deposition method. The size 50 nm prepared by thermal evaporation method

shows the highest photocatalytic activity. In addition the tetrapod ZnO

nanopowders had the higher efficiency than irregular ZnO nanoparticles.

However, the smallest 10 nm ZnO nanoparticles prepared by chemical

deposition method and the degradation efficiency was discussed through the

photocatalytic experiments using 50 nm ZnO nanoparticles as photocatalyst.

Giraldi et al., [197] reported annealing effect on photocatalytic efficiency of

ZnO nanoparticles prepared by chemical technique. The samples were annealed

and effects on the photocatalytic properties, surface decontamination and the

consequent particle change, in terms of crystallinity are studied. The as-

prepared samples correspond to a metastable phase (oxy or hydroxy zinc

acetate) and post annealing leads to ZnO crystals. The XRD patterns showing

only the ZnO phase for heat treatment at 100°C, FTIR data shows that

carboxylate group remains attached to the ZnO surface up to 300 °C. Up to

300 °C the presence of these carboxylate groups, provided by the synthesis

method, showed to be more relevant to photoactivity than the specific surface

area. At higher temperatures, crystallinity becomes the dominant factor and an

increasing of crystallinity favors the photoactivity.

The study of photoconductivity and photoluminescence of ZnO

nanoparticles is reported by Kripal et al., [198]. They studied the

photoconductvity and photoluminescence properties of ZnO nanoparticles

(NPs) prepared by using co-precipitation method and thioglycerol as a caping

agent and annealed at 300°C. The TEM and XRD pattern confirm the

hexagonal wurtzite structure of ZnO nanoparticles. The UV-Visible absorption

spectrum of ZnO NPs shows blue shift of absorption peak as compared to bulk

ZnO. The photoluminescence (PL) spectra of as-synthesized ZnO NPs show

band edge emission as well as blue-green emission.


44

Growth and gas sensing properties of p and n-type ZnO nanostructures

are reported by Ramgir et al.,[199]. ZnO nanoparticles (NPs) of 5 to15 nm size

and nanowires (NWs) of 50 to100 nm diameter were prepared using simple

chemical method, exhibiting p and n-type characteristics, respectively. ZnO

NW-films show good sensitivity and selectivity towards H2S in ppm range with

fast response and recovery times. Interestingly, ZnO NP-films showed p-type

conductor.

Sun et al.,[200] reported on bar-like nano ZnO prepared by

hydrothermal method at 230°C using ZnSO4 and CO(NH2)2 as the materials

and CTAB as the surface active agent. The XRD shows that the nanoparticles

of ZnO have hexagonal wurtzite structure. The observed results by SEM

shows that the products showed rod-like morphology with average diameter of

about 60-80 nm and length of about 260-580 nm. Methylene blue in aqueous

solution could decompose rapidly under photocatalysis of bar-like nano ZnO

when pH value of the solution was 8.0 and the degradation rate of Methylene

Blue reached 100% after 90 minutes.

Dalai et al., reported [201], hollow ZnO microspheres with interior

diameter of ∼10 µm were synthesized by a facile solution method using

polymethyl methacrylate (PMMA) microspheres as template. The hollow

microspheres are built by layers of nanoparticles with high crystallinity. The

shell thicknesses are ∼100 nm. The XRD, SEM, TEM and XPS measurements,

a possible formation mechanism for the hollow ZnO microspheres is discussed.

As-synthesized hollow ZnO microspheres exhibit novel absorption band in the

region of UV. Band gap of the hollow ZnO microsphere is 3.53 based on its

UV-Visible absorption spectrum.

The study of photochemical properties of CeO2-coated ZnO nanorods

were carried out by Lau et al., [202]. Well-aligned ZnO nanorods were

deposited by a mild hydrothermal process and coated with nanosized CeO2

particles (∼5 nm) by an oxidative-soak-coating method at 45° C. The low

growth temperature proved useful in avoiding interfacial reaction between the

two phases. Correlation of photoluminescence results indicated that the defects


45

responsible for the deep level emission (DLE) from ZnO were largely located

at the surface. The CeO2 coating reduced the DLE but also the photocatalytic

activities as surface hydroxyl groups were involved in the nucleation of the

CeO2 phase. Wang et al., [203] reported the hydrothermal process for CeO2-

ZnO composite nanostructure, by using Zn (NO3)2·6H2O, Ce (NO3)3·6H2O and

hexamethylenetetramine on Si substrate at 90°C. The structures, morphologies

and the composite of CeO2/ZnO samples have been investigated by XRD,

TEM, SEM and EDX at room and it shows that the obtained CeO2/ZnO

composite nanostructures have different morphology for composite. The result

shows that the uniform and dense CeO2 nanoparticles cover the whole surface

of single crystal ZnO rods when the Ce3+/Zn2+ molar ratio of reaction solution

is 1/10. This kind of the CeO2/ZnO composite nanostructure may find potential

application in the selective detection of CO.

Wu et al., [204] developed a layer-by-layer (LBL) assembly approach to

synthesize ZnO nanorod-based hybrid nanomaterials such as ZnO/CeO2,

ZnO/CdS and ZnO/Ag on ZnO nanorod templates at room temperature. The

ZnO nanorod-based hybrid nanomaterials have been characterized by TEM,

HRTEM, XRD and XPS. It is indicated that a uniform coating layer consisting

of homogeneous nanoparticles is deposited on the surface of ZnO nanorods due

to the strong electrostatic attraction between metal ions and polyelectrolyte

modified ZnO nanorods. The general approach presented here can be extended

to the synthesis of other one-dimensional (1D) nanostructure-based hybrid

Nanomaterials.

CeO2/ZnO nanostructured microspheres as catalysts for the oxidative

coupling of CH4 with CO2, the conversion of CH4 corresponded with that using

the CeO2/ZnO nanoparticles observed by Yongjun et al., [205]. CeO2/ZnO

nanostructured microspheres with an average diameter of ≈3.8 µm were

sysnthesized by a solid-stabilized emulsion technique. The CeO2/ZnO

nanostructured microspheres were characterized with SEM, XRD, CO2-TPD,

BET measurement. Based on the oxidative coupling reaction of CH4 with CO2

as an oxidant, the catalytic performance of the CeO2/ZnO nanostructured


46

microspheres was evaluated and compared with that of the CeO2/ZnO

nanoparticles. The surface of the CeO2/ZnO nanostructured microspheres

consisted of petal-like structures with a petal thickness of ≈90 nm and a petal

depth of 0.4-0.9 µm. Using CeO2/ZnO nanostructured microspheres as

catalysts for the oxidative coupling of CH4 with CO2, the conversion of CH4

corresponded with that using the CeO2/ZnO nanoparticles, while the CeO2/ZnO

nanostructured microspheres had much longer operating life.

ZnWO4 as a kind of tungstate has received considerable attention due to

its applications as an X-ray and γγγγ-scintillator, photoanodes and solid-state laser

host, as well as for acoustic and optical fibers reported by Montini et al. [206].

Tungstates of divalent transition metals (MIIWO4, M=CoII, NiII, CuII, ZnII) were

prepared by reaction of transition metal nitrates with sodium tungstate. The

precipitates were then calcined at 500ºC. The materials were characterized by

means of ICP-AES, UV, XRD, SEM and surface area analysis. The higher

activity for the decolorization of Methylene Blue (MB) and Methyl Orange

(MO) of ZnWO4 compared to that of the other investigated tungstates was

correlated with its strong tendency of excitons self-trapping.

The enhanced photocatalytic performance was observed by Huang et al.,

[207] ZnWO4 nanoparticles prepared by hydrothermal technique with different

annealing temperature and time. The high photocatalytic activity confirmed by

degradation of formaldehyde and RhB in gaseous and water respectively. The

highest photocatalytic activity both appeared at 450°C for 1 h. The

photocatalytic activity of ZnWO4 was a little higher than that of P-25

(Degussa) in gaseous phase and lower in aqueous.

Jia et al., [208] reported the structural and optical properties of ZnWO4

and CdWO4 nanofilms by a novel route through the combination of reverse

micelle system with dip-coating technology. Here collodion is used as a

dispersant and film-forming agent to obtain nanofilm with a good quality films.

The SEM and XRD results indicate ZnWO4 and CdWO4 nanoparticles with

monoclinic system and wolframite structure. The nanofilm's PL bands shows

blue shift compared with bulk materials where as red shift compared with


47

nanoparticles. FTIR absorption bands between 400 and 900 cm-1 prove the

presence of ZnWO4.

Wang et al., [209] used a microwave assisted sol-gel method. The

results indicate the cubic fluorite structure. TEM reveals that the size of

crystalline particles of 4 to 6 nm. Preparation of monodisperse cerium oxide

nanoparticles were carried out by Soucek et al. [210]. The products were

characterized by Raman spectroscopy, XRD, UV-Visible spectroscopy and

TEM. The TEM images and UV-Visible spectra show that the CeO2 particles

prepared are uniform nanosized and absorb UV light in the range of 250 t0 400

nm. The sizes of the nanoparticles were obtained about 5 to 20 nm.

Zhang et al. [211] synthesized hierarchical CeO2 nanocrystal

microspheres by sol-gel method at of 120°C temperature. The product was

characterized by XRD, TEM and clearly revealed the microstructure of the

CeO2 nanospheres. These nanospheres are of porous structure and 50 to 100 nm

in diameter. Photocatalytic degradation of various dyes with different

chromophores such as Methyl Violet, Methylene Blue and Rhodamine B by

using titanium oxide was reported by Hosseinnia et al. [58]. They have been

synthesized single phase anatase titania powders by a simple precipitation

method. The powders were characterized by XRD, TEM and surface area, after

calcinations at 550°C for 3 hrs. TEM results show the particle size about 20 nm

and surface area of 75 m2/g. From the detailed literature survey it is seen that

very few scientists have been given attention for the semiconductors like ZnO,

ZnWO4, CeO2 and ZnO-CeO2 by using microwave assisted method. Therefore

it has an advantage of very short reaction time, production of small particles

with a narrow size distribution and high purity. These conveniences could be

attributed to fast homogeneous nucleation and ready dissolution of the gel.

Microwave heating is in situ mode of energy efficient method.

1.16 Plan of the Research Work

From the industrial point of view, it is of prime importance to use new

materials as a photocatalyst. Metal oxide nanomaterials are the benign catalyst.


48

The metal oxide nanoparticles as a photocatalyst are supposed to modify the

route of chemical reaction thereby reducing the cost of production.

Photodegradation of hazardous organic compounds like dye and pesticides

from industrial effluents is of prime importance and at ease can be done by

metal oxide nanoparticles within shorter duration due to its large surface to

volume ratio.

It is observed from the literature survey that mixed oxides are more

favourable than single oxides due to its increase in surface area, tuning of the

band gap of energy, which is required for photocatalyst.

The important task of the present work is to prepare ZnO and its mixed

oxides such as ZnO-CeO2 and ZnWO4 by using very simple microwave

assisted sol-gel method. The properties such as crystal structure, catalytic and

spectral properties are greatly influenced by the metal ions present at two

different sites. Therefore, to find out the effect of substitution of different metal

ions on the properties of metal oxide, various techniques such as scanning

electron microscopy (SEM), transmission electron microscopy (TEM), electron

dispersive X–ray analysis (EDS), X–ray diffraction analysis (XRD), thermo-

gravimetric analysis (TGA), Differential thermal analysis (DTA), UV-Visible

spectroscopy and IR spectroscopy are used. The microwave assisted method is

employed for the synthesis of zinc oxide, ZnO-CeO2, and ZnWO4. The ZnO

and mixed ZnO with CeO2 and WO3 are tested for the photodegradation of dyes

such as Methyl Orange, Methylene Blue and Rhodamine B. The effect of

catalyst loading, pH and initial concentration of dyes on the photocatalytic

degradation efficiency using ZnO and mixed ZnO nanorods as a photocatalyst

was tested.


49

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