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OKWOIGWE, MODESTUS NDUBISI

PG/M.Sc./10/52477

SYNTHESIS AND CHARACTERIZATION OF TIN (IV) OXIDE NANOPARTICLES

BY SOL-GEL PROCESS

FACULTY OF PHYSICAL SCIENCES

DEPARTMENT OF PURE AND INDUSTRIAL CHEMISTRY

Chukwuma Ugwuoke

Digitally Signed by: Content manager’s Name

DN : CN = Webmaster’s name

O= University of Nigeria, Nsukka

OU = Innovation Centre

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DEPARTMENT OF PURE AND INDUSTRIAL CHEMISTRY

FACULTY OF PHYSICAL SCIENCES

UNIVERSITY OF NIGERIA, NSUKKA

SYNTHESIS AND CHARACTERIZATION OF TIN (IV) OXIDE

NANOPARTICLES BY SOL-GEL PROCESS

A RESEARCH PROJECT SUBMITTED IN THE PARTIAL FULFILLMENT OF

THE REQUIREMENT FOR THE AWARD OF MASTER OF SCIENCE

DEGREE IN INDUSTRIAL CHEMISTRY

BY

OKWOIGWE, MODESTUS NDUBISI

PG/M.SC./10/52477

SUPERVISOR: PROFESSOR U.C. OKORO

NOVEMBER, 2012

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CHAPTER ONE

INTRODUCTION

1.1 BACKGROUND OF THE RESEARCH

Miniaturization is a general aim of the technological development that is taking place to

produce smaller, faster, lighter and cheaper devices with greater functionality, while

using less raw materials and consuming less energy. Research on nanomaterials is a step

towards miniaturization of technology that will contribute significantly towards a suitable

usage of raw materials and energy1. When we bring constituents of materials down to the

nanoscale, the properties change. Some materials used for electrical insulations can

become conductive and other materials can become transparent or soluble.

Crystalline tin oxide is a wide-gap semiconductor (~ 3.6 eV), which, in its as-grown

state, is typically n-type. Because of its optical (transparent for visible light and reflective

for infra-red) and electrical properties, it is allied to good chemical and mechanical

stability. It has wide range of applications such as solid-state gas sensor, transparent

conducting electrodes, rechargeable Li batteries, liquid crystal displays, etc, 2-5

. Their

properties depend on microstructure, impurities and size effects of particles.

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1.2 STATEMENT OF THE PROBLEM

A country is said to be technologically advanced if she can produce smaller, faster,

lighter and cheaper devices with greater functionality while using less raw materials and

consuming less energy. Production of such devices can not be achieved using materials

with bulk particles because some of the properties of bulk particles are hidden.

SnO2 nanoparticles were synthesized to solve these technological problems, with

particular interest in incorporating them into the devices such as gas sensors, solar cells,

doped semiconductors, dye-sensitized solar cells, and transistors, electrodes for lithium

ion batteries, catalyst supports, and super capacitors. The routes to the synthesis of these

nanoparticles were always difficult because of high electrical energy requirements and

high cost of chemicals needed for the synthesis. Sol-gel synthesis of these nanoparticles

solves these problems of high energy requirement and high cost because the power

requirement is low, and the chemicals are cheap and readily available.

1.3 RESEARCH OBJECTIVES

The specific research objectives were to:

synthesize SnO2 nanoparticles at controlled crystal size using sol-gel

process;

investigate the effect of reaction temperature on the structure of the

synthesized SnO2 nanoparticles;

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investigate the effect of ammonia concentration on the structure of the

grown SnO2 nanoparticles; and

investigate the effect of annealing temperature on the structure and

morphology of the synthesized SnO2 nanoparticles.

1.4 JUSTIFICATION OF THE STUDY

Due to wide range applications of SnO2 nanoparticles to improve the overall efficiency

of such devices as gas sensors, solar cells, doped semiconductors, transistors, electrodes

for lithium ion batteries, catalyst supports, etc, it is worthwhile to be a partaker in their

synthesis.

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CHAPTER TWO

LITERATURE REVIEW

2.1 NANOTECHNOLOGY

“Nano” derives from the Greek word “nanos”, which means dwarf or extremely small6.

A nanometer is a billionth of a metre or 10-9

m. This means that a two- metre tall man is

two billion- nanometer tall. An ant is millions of nanometer across. Nanomaterials are

generally considered as the materials that have a characteristic dimension (e.g. grain size,

diameter of cylindrical cross-section, layer thickness) smaller than 100 nm. These

materials can be metallic, polymeric, ceramic, electronic, or composite 6.

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Figure 2.1 Sizes of Some Materials in Nanometers

2.2 CLASSIFICATIONS OF NANOMATERIALS

2.2.1. Classification Based on Their Pathway

The nanomaterials can also be classified into three types vis: natural, incidental, and

engineered nanomaterials depending on their pathway 6.

1 Natural nanomaterials, which are formed through natural processes, occur in the

environment (e.g. volcanic dust, lunar dust, magneto-tactic bacteria, minerals,

etc.).

2 Incidental nanomaterials occur as the result of man made industrial processes (e.g.

coal combustion, welding fumes, etc.).

3 Engineered nanomaterials are produced either by lithographically etching of a

large sample to obtained nanoparticles, or by assembling smaller subunits through

crystal growth or chemical synthesis to grow nanomaterials of the desired size and

configuration.

Engineered nanomaterials most often have regular shapes, such as tubes, spheres, rings,

etc. U.S. Environmental Protection Agency divides engineered nanomaterials into four

types. They are:

1. Carbon-based materials: these nanomaterials are composed mostly of carbon,

most commonly taking the form of a hollow sphere, ellipsodes or tubes. Spherical

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and ellipsoidal carbon nanomaterials are referred to as fullerenes, while cylindrical

ones are called nanotubes.

2. Metal-based materials: these nanomaterials include quantum dots, nanogold,

nanosilver and metal oxides, such as titanium dioxide. A quantum dot is a closely

packed semiconductor crystal comprised of hundreds or thousands of atoms, and

whole size is on the order of a few nanometers to a few hundred nanometers.

Changing the size of quantum dots changes their optical properties.

3. Dendrimers: these nanomaterials are nanosized polymers built from branched

units of unspecified chemistry. The surface of a dendrimer has numerous chain

ends, which can be tailored to perform specific chemical functions. This property

could also be useful for catalysis. Also, because 3-dimensional dendrimers contain

interior cavities into which other molecules could be placed, they may be useful

for drug delivery.

4. Composites: Nanocomposites are composite materials in which the matrix

material is reinforced by one or more separate nanomaterials in order to improve

performance properties. The most common materials used as matrix in nanocomposites

are polymers (e.g. epoxy, nylon, polyepoxide, polyetherimide), ceramics (e.g. alumina,

glass, porcelain), and metals (e.g. iron, titanium, magnesium). Comparing to the

conventional micro-composites, nanocomposites greatly improve the physical and

mechanical properties. It is well known that composite materials have advantages over

traditional materials.

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Nanocomposites, where nano-sized reinforcements (fillers) are dispersed in the base

material (matrix), offer a novel class of composites with superior properties and added

functionalities 7-10

. The nanoscale reinforcements over traditional fillers have the

following advantages 11

:

1. Low-percolation threshold (~0.1–2 vol.%).

2. Large number density of particles per particle volume (106–108 particles/μm3).

3. Extensive interfacial area per volume of particles (103–104 m2/ml).

4. Short distances between particles (10–50nm at ~1–8 vol.%).

Nanoparticles can substantially improve the mechanical properties of the host matrix

materials 12-16

. Even at very low filler volume content such as 1-5%, a considerable

improvement of the mechanical properties can be achieved 17-20

. It is observed that for

some nanocomposites, with the same filler volume fraction, the stiffness and strength

increases as the particle size decreases 21-26

. In general, the stiffness of nanocomposites

tends to increase as the filler volume fraction increases. This function may be nonlinear.

There may exist a critical volume fraction beyond which the stiffness starts to decrease 27

.

Nanocomposites can not only improve stiffness and strength, but also fracture toughness

28-38.

In general, the fracture toughness of nanocomposites increases as the volume fraction

increases, and increases as the nanofiller size decreases.

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Classification Examples

Dimension 3 dimensions < 100nm Particles, quantum dots, hollow

spheres, etc.

2 dimensions < 100nm Tubes, fibers, wires, platelets, etc.

1 dimension < 100nm Films, coatings, multilayer, etc.

Phase composition Single-phase solids

Crystalline, amorphous particles and

layers, etc.

Multi-phase solids

Matrix composites, coated particles,

etc.

Multi-phase systems Colloids, aerogels, ferrofluids, etc.

Manufacturing process Gas phase reaction

Flame synthesis, condensation, CVD,

etc.

Liquid phase reaction

Sol-gel, precipitation, hydrothermal

processing, etc.

Mechanical procedures Ball milling, plastic deformation, etc.

Table 2.1. Classification of Nanomaterials According to Different Parameters 39

.

2.2.2 Classification Based on Their Geometry

Nanotubes: When two dimensions are in the nanometer scale and the third is larger,

forming an elongated structure, they are generally referred as „nanotubes‟ or

nanofibers/whiskers/nanorods. Nanotubes are classified into two: inorganic nanotube and

carbon nanotube.

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An inorganic nanotube is a cylindrical molecule often composed of metal oxides, and

morphologically similar to a carbon nanotube. Inorganic nanotubes have been observed

to occur naturally in some mineral deposits.39-43

Inorganic nanotubes are an alternative material to better-explored carbon nanotubes,

showing advantages such as easy synthetic access and high crystallinity,44

good

uniformity and dispersion, predefined electrical conductivity depending on the

composition of the starting material and needle-like morphology, good adhesion to a

number of polymers and high impact-resistance.45

They are therefore promising

candidates as fillers for polymer composites with enhanced thermal, mechanical, and

electrical properties. Target applications for this kind of composites are materials for heat

management, electrostatic dissipaters, wear protection materials, photovoltaic elements,

etc. Inorganic nanotubes are heavier than carbon nanotubes and not as strong under

tensile stress, but they are particularly strong under compression, leading to potential

applications in impact-resistant applications such as bulletproof vests.46-47

Carbon nanotubes (CNTs) are allotropes of carbon with a cylindrical nanostructure.

Nanotubes have been constructed with length-to-diameter ratio of up to 132,000,000:1,48

significantly larger than for any other material. These cylindrical carbon molecules have

unusual properties, which are valuable for nanotechnology, electronics, optics and other

fields of materials science and technology. In particular, owing to their extraordinary

thermal conductivity and mechanical and electrical properties, carbon nanotubes find

applications as additives to various structural materials, for instance, in (primarily carbon

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fiber) "baseball bats, car parts" and even "golf clubs" 48

, where nanotubes form only a

tiny portion of the material(s). Nanotubes are members of the fullerene structural family,

which also includes the spherical buckyballs, and the ends of a nanotube may be capped

with a hemisphere of the buckyball structure. Their name is derived from their long,

hollow structure with the walls formed by one-atom-thick sheets of carbon, called

graphene. These sheets are rolled at specific and discrete ("chiral") angles, and the

combination of the rolling angle and radius decides the nanotube properties; for example,

whether the individual nanotube shell is a metal or semiconductor. Nanotubes are

categorized as single-walled nanotubes (SWNTs) and mult-walled-nanotubes (MWNTs).

Figure 2.2 Different Shapes of Nanotubes 48

Nanolayers: The particulates which are characterized by only one dimension in

nanometer scale are nanolayers/nanoclays/nanosheets/nanoplatelets. These

particulate is present in the form of sheets of one to a few nanometer thick to

hundreds to thousands nanometers long.

Nanoparticles: Nanoparticles can be classified as particles less than 100nm in

diameter that exhibit new or enhanced size-dependent properties compared to

larger particles of the same material. When the three dimensions of particulates

are in the order of nanometers, they are referred as equi-axed (isodimensional)

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nanoparticles or nanogranules or nanocrystals. Nano-sized materials are naturally

present from forest fires to volcanoes and are also generated unintentionally from

anthropogenic sources as a by-product of combustion and deliberately as

fabricated nanomaterials. Nanoparticles have been used for centuries. The

coloured glass that we see in many old cathedrals from the middle age was made

of gold nanosized clusters that created different colour depending on the size of

the nanoparticles. The most prominent example of engineered nanoparticulate

material is carbon black which has been around us for decades in applications like

printing inks, toners, coatings, plastics, paper, tires and building products.

However, carbon black would for many reasons be excluded from the nanoparticle

category, as nanotechnology is about deliberately and knowingly exploiting the

nanoscale nature of materials49

.

Two of the major factors why nanoparticles have different properties (optical,

electrical, magnetic, chemical and mechanical) than bulk material are because the

size-range quantum effects start to predominate and the surface area to volume ratio is

increased7. The increase in the surface-area-to-volume ratio is a gradual progression

as the particles get smaller, which makes that atoms on the outside of the particle to

increasingly begin to dominate the ones inside the particle. This changes the

individual properties of the particle and how it interacts with other materials in the

surroundings. The increase in the relative surface area makes them very interesting for

the industry, as high surface area is a critical factor for instance in efficient catalysis

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and in structures like electrodes. This can not only improve the performance of

products like batteries, but also reduce resource usage in catalytic processes and hence

decrease the amount of waste. The large surface area also increases the mixing with

other materials in the surrounding and is especially beneficial in intermixed materials

like composites.

Figure 2.3 Various Types of Nanoscale Materials

50.

2.3 USES OF NANOPARTICLES

Nanotechnology often brings together different disciplines and this interdisciplinary

approach is expected to contribute to innovations that might solve many of today‟s

challenges in the society 51

.

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1 - Organic Light Emitting Diodes (OLEDs) for displays

2 - Photovoltaic film that converts light into electricity

3 - Scratch-proof coated windows that clean themselves with UV

4 - Fabrics coated to resist stains and control temperature

5 - Intelligent clothing measures pulse and respiration

6 - Bucky-tubeframe is light but very strong

7 - Hipjoint made from biocompatible materials

8 - Nano-particle paint to prevent corrosion

9 - Thermo-chromic glass to regulate light

10 - Magnetic layers for compact data memory

11 - Carbon nanotube fuel cells to power electronics and vehicles

12 - Nano-engineered cochlear implant

Figure 2.4 Potential Uses of Nanotechnologies51

Nanoparticles are of interest because of the new properties (such as reactivity and optical

behaviour) that they exhibit compared to larger particles of the same materials.

Manufactured nanoparticles are typically not products in their own right, but generally

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serve as raw materials, ingredients or additives in existing products. A selection of the

applications involving nanoparticles are presented here.

2.3.1 Nanoparticles in Medicine

Some nanoparticles contain active ingredient dissolved, encapsulated or adsorbed in

matrix material which are used as target delivery system52

. To see the effect of drug in

target tissue, to increase stability against degradation through enzymes and for

solubilization at intra-vascular route nanoparticles have been used 53

.

Drug delivery

In medical field nanoparticles are currently employed to deliver drugs 54

, heat, light or

other substances to specific types of cells (such as cancer cells)55-60

. Particles are

engineered so that they are attracted to diseased cells, which allow direct treatment of

those cells. This technique reduces damage to healthy cells in the body and allows for

earlier detection of disease. For the safe administration of nanoparticle through intravenous route

they are formulated in the form injection which consist spherical amorphous particle. Formulations

are less toxic in nature because in this co-solvent is not used to solubilize the drug. Ethyl glycol

molecules attached to nanoparticles stop white blood cells from recognizing the

nanoparticles as foreign materials, allowing them to circulate in the blood stream long

enough to attach to cancer tumours.

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Therapy Techniques

Nanoparticles, when activated by x-rays, can generate electrons that cause the destruction

of cancer cells to which they have attached themselves. Aluminosilicate nanopartcles can

quickly reduce bleeding in trauma patients by absorbing water, causing blood in a wound

to clot quickly 62

.

Nanoparticles may be used, when inhaled, to stimulate an immune response to fight

respiratory virus 63

.

Figure 2.5 Nanoparticles Carring and Releasing Drugs 61

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Diagnostic and Imaging Techniques

Nanoparticles have been studied for application for medical imaging.62-68

Quantum Dots

may be used in the future for locating cancer tumours in patients and in the near term for

performing diagnostic tests in samples. Concern about the toxicity of the material that

quantum dots are made from is one of the reasons restricting the use of quantum dots in

human patients. However, quantum dots composed of silicon is believed to be less

harmful than the cadmium contained in many quantum dots.

Iron oxide nanoparticles can be used to improve image from magnetic resonance

imagining (MRI) of cancer tumour. The nanoparticles are coated with a peptide that binds

to a cancer tumour. Once the nanoparticles are attached to the tumour, the magnetic

property of the iron oxide enhances the images from the MRI scan. Nanoparticles, such

as gold nanoparticles, can attach to proteins or other molecules, allowing detection of

disease indicators in a laboratory sample at a very early stage 69-70

. Gold nanoparticle,

such have antibodies attached can provide quick diagnosis for flu virus when light is

directed on a sample containing virus particles and the nanoparticles. The amount of light

reflected back increases because the nanoparticles cluster around virus particles, allowing

a much faster test than those currently used 70

.

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Anti-microbial Technique

One of the earliest nanomedicine applications was the use of nanocrystalline silver which

is an antimicrobial agent for the treatment of wounds. A new version of this technology

will be applied in burrn dressing coated with nanocapsules containing antibiotics. If an

infection starts, the harmful bacteria in the wound causes the nanocapsules to break open,

releasing the antibiotics which is quicker treatment of an infection and this reduces the

number of times a dressing has to be changed 70

2.3.2 Lithium-ion Batteries

The need to reduce hydrocarbon use and develop renewable energy sources has

been a hot topic in recent years. The energy crisis that people have experienced

in the last decade has indicated the need to develop and invest in renewable

energy. The Lithium-ion (Li-ion) battery is one of the possible solutions for

energy production and storage that will reduce the use of hydrocarbon fuel and

hence the amount of greenhouse gas generator 71

.

Li-ion batteries have been extensively studied in recent decades due to their

high energy and power density, and low capacity fade. 71-78 They were first

introduced in the 1970s by M.S Whittinghan, and then commercialized in 1991

by Sony. They have been integrated into small portable devices such as laptops,

cameras, cell phones, camcorders add mp3 players. Moreover, their application

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in electric vehicle is growing. The Li-ion battery has attracted significant

attention from car manufacturers who are looking to improve fuel efficiency

and reduce greenhouse gas production. The main focus of the automobile

industry is to develop a Plug-in Hybrid Electric Vehicle (PHEV) using the Li-

battery as a power source and energy-storage system for the vehicle.79

The materials currently used for Li-ion batteries are LiCoO2 and graphite – the

first materials integrated by Sony into Li-ion batteries. The main disadvantages

of these batteries are that cobalt, which is one component of LiCoO2 is rare,

expensive and pollutes the environment. LiCoO2 occupies 70% of Li-ion

battery material cost. It is also a hazard that may harm or irritate eyes, skin or

respiratory tract and may cause harm if swallowed. Also, graphite used as

cathode has low volumetric capacity compared to Lithium metal. As a result,

newly developed materials are being sought out, including LiMn2O4, LiNiO2,

LiFPO4 and LiV2O4 for cathodes, 80-84

and SnO2, SiO2 and LiTiO12 for

anodes.85-92

Infact, SnO2 has been proposed as a possible anode material for

replacing the current carbon-based material (graphite) because it has higher

theoretical capacity (1491 mA h/g). The reaction Mechanism of SnO2 with

lithium can be summarized by two steps:

SnO2+ 4Li+ + 4e Sn + 2Li2O -------------------(1)

Li +Xe + Sn LixSn 0<x< 4.4 -------- (2)

In the first step, li-inserted SnO2 forms amorphous Li2O and metallic Sn.

Further reaction of the newly formed metallic Sn with lithium subsequently

leads to the formation of Li-Sn alloys with the composition, Li4.4Sn. However,

the alloying of lithuin into bulk metalic Sn causes internal damage due to the

large volume expansion, resulting in a loss of capacity and rechargeable. It has

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recently been reported that an electrode composed of nanostructured tin-based

materials can deliver a very high capacity (2 700 m A h/g at the 8c rate) and

still retain the ability to be discharged and charged for 800 cycle. This finding

has led to the renewed interest in the use of SnO2 nanoparticle as promising

anode for use in rechargeable lithium batteries 78

.

2.3.3 Fuel Cell

Nanoparticles minimize moving parts; reduce cost, size, and weight; increase power

density and cell voltage; improve efficiency of fuel cells by reducing resistive and mass

transfer losses;

Catalysts are used with fuels such as hydrogen or methanol to produce hydrogen ions.

Platinum, which is very expensive, is the catalyst typically used in the process.

Companies are either using nanoparticles of platinum to reduce the amount of platinum

needed, or using nanoparticles of other materials to replace platinum entirely and thereby

lower costs.

Fuel cells contain membranes that allow hydrogen ions to pass through the cell but do not

allow other atoms or ions, such as oxygen, to pass through. Nanotechnology creates more

efficient membranes that allow building lighter weight and longer lasting fuel cells.

Small fuel cells are being developed that can be used to replace batteries in hand held

devices such as PDAs or laptop computers. Most companies working on this type of fuel

cell are using methanol as a fuel and are calling them DMFC‟S, which stands for direct

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methanol fuel cell. DMFC‟S are designed to last longer than conventional batteries. In

addition, rather than plugging your device ready to an electrical outlet and waiting for the

battery to charge, with a DMFC‟S you can simply insert a new cartridge of methanol

into the device.

Fuel cells that can replace batteries in electric cars are also under development. Hydrogen

is the fuel most researchers propose for use in fuel cell powered cars. In addition to the

improvement of the catalyst and membrane discussed above, it is necessary to develop a

light weight and safe hydrogen fuel tank to hold the build network of refueling stations.

To build these tanks, it is better if the researchers develop lightweight materials that will

absorb the hydrogen and only release it when needed.

2.3.4. Solar Cell

The ability to create high efficiency solar cells is a key strategy to meeting growing

world energy needs. Nanotechnology is currently enabling the production of high-

efficiency organic Photovoltaics (OPVS) to help meet this challenges.Organic

photovoltaics are nanastructed thin films composed of layers of semi-conducting organic

materials (polymers or oligomers) that absorb photons from the solar spectrum . These

devices will revolutionize solar energy harvesting because they can be manufactured via

solution-based methods, such as ink-jet or screen printing; enabling rapid mass

production and driving down cost. Other benefit of using nanoparticles in solar cells is

reduced installation costs. This is achieved by producing flexible rolls instead of rigid

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crystalline panels. In a near future, nanoparticles can be incorporated in plastic film into a

case to form solar cells for such devices as mobile phones and laptop computers.

Currently, available nanotechnology solar cells are not as efficient as traditional ones;

however their lower cost offsets this. In the long term, nanotech version should both be

lower cost and, using quantum dots should be able to reach higher efficiency levels than

conventional ones.

2.3.5 Cosmetics and Skin Care

Eventually, nanotechnology may help us reverse aging at a cellular level. Until that day

comes, we‟ll have to be content with the ways that nanotechnology is being used in

cosmetics to keep our skin more youthful and provide protection from harmful sunlight.

Nanotechnology applications in skin care include: Sunscreen that uses zinc oxide

nanoparticles to block ultraviolet rays while minimizing the white coating on the skin;

Skin care lotions in which nutrients are encapsulated in nanoparticles suspended in an

liquid, making up a nanoemulsion. The small size of the nanoparticles, compared to

particles in conventional emulsions, allows the nanoparticles to penetrate deeper into the

skin, delivering the nutrients to more layers of skin cells. Lotions that use nanoparticles

called ethosomes to deliver nutrients that promote hair growth. Skin creams uses proteins

derived from stem cells to prevent aging of the skin. These proteins are encapsulated in

liposome nanoparticles which merge with the membranes of skin cells to allow delivery

of the proteins.

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2.3.6 Food

Storage bins are being produced with silver nanoparticles embedded in the plastic. The

silver nanoparticles kill bacteria from any material that was previously stored in the bins,

minimizing health risks from harmful bacteria. Silicate nanoparticles can provide a

barrier to glasses (for example oxygen), or moisture in a plastic film used for packaging.

This could reduce the possibility of food spoiling or drying out. Zinc oxide nanoparticles

can be incorporated into plastic packaging to block ultraviolent rays and provide anti

bacteria protection, while improving the strength and stability of the plastic film.

Nanosensor may be developed to detect bacteria and other contaminates, such as

salmonella, at a packaging plant. This will allow for frequent testing at a much lower

cost than sending samples to a laboratory for analysis. This point-of- packaging testing,

if conducted properly, has the potential to dramatically reduce the chance of

contaminated food reaching grocery store shelves. Pesticides can be encapsulated in

nanoparticles which can only release pesticides within an insect‟s stomach, minimizing

the contamination of plants themselves. Another development being pursued is a network

of nanosensors and dispensers used throughout a farm field. The sensors recognize

when a plant needs nutrients or water before there is any sign that the plant is deficient.

The dispensers then release fertilizer, nutrient, or water as needed, optimizing the growth

of each plant in the field one by one.

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2.3.7 Fabrics

Making composite fabrics with nano-sized particles or fibers allows improvement of

fabrics properties without a significant increase in weight, thickness, or stiffness as might

have been the case with previously used techniques. Nanowhiskers are incorporated into

fabrics to cause water to bead up, making fabrics water and stain resistant; nanopores can

provide superior insulation for shoe inserts in cold weather; Silver nanoparticles in

fabrics can kill bacterial, making clothing odour–resistant ; Nanoparticles can provide a

“Lotus plant” effect for fabrics used in awnings and other materials left out in the

weather, causing dirt to rinse off in the rain; fabric enhanced with nanopores can insulates

against heat or chill.

2.3.8 Coating

Nanopowders and manoparticles dispersions have seen increasing applications in

coatings. Due to their small size, very even coating can be achieved by painting

nanoparticle dispersions onto a Surface and baking of residual solvent. Nanoparticles

have been used for centuries. The coloured glass that we see in many old cathedrals from

the middle age was made of gold nanosized chesters that created different colour

depending on the size of the nanoparticles.

Optically transparent Conductive Coatings:Indium tin oxide (ITO) and antimony tin

oxide (ATO) are well known optically transparent, electrically conductive materials.

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Nanoparticles of these materials can be painted on surfaces such as interactive touch

screens to create conductive, transparent screens without relying on expensive Sputtering

techniques.

In addition,ITO and ATO can be used as an antistatic coating, utilizing their inherited

conductivity to dissipate static charge.

Optical Transparent Abrasion-Resistant Coating: Nanoscale aluminum oxide and

titanium oxide are optically transparent and greatly increase the abrasion resistance of

traditional coatings. Titanium oxide is of interest in many optical applications, since it is

highly reflective for most ultra-violent radiations. Zinc oxide and rare-earth oxides are

also ultra-violent reflective, but optically transparent and are therefore effective in

protecting surfaces from degradation brought about by exposure to ultra-violent radiation.

2.3.9 Building

An insulating material called aerogel , composed of silica nanoparticles separated by

nanopores ,use mostly air, making it an excellent insulator. For example, insulating the

walls of your house would only need about one-third the thickness if you use this

material instead of conventional insulation.

Nanoparticles-based paint can reduce the chance of mold and mildew growing in moist

areas of buildings such as bathrooms or on the exterior walls. The paint contains

nanoparticles of silver that inhabits the growth of mildew and bacteria. Solar cells built

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with nanoparticles can be installed as a coating on windows or other building materials.

This is always referred to as “Building integrated photovoltaics”

2.3.10 Sensor

Semiconductor metal oxides used as detecting elements in nanotechnogy-based

sensors have captured greater portion of sensor market 93-96

. These detecting elements

change their electrical characteristics, such as resistance or capacitance, when they

absorb a gas molecule 97-99

.

Because of the small size of nanotubes, or nanoparticles, a few gas molecules are

sufficient to change the electrical properties of the sensing elements. This allows the

detection of a very low concentration of chemical vapours. The goal of using

nanoparticle-based sensor is to have small, inexpensive sensors 100-105

. These highly

sensitive sensors can sniff out chemicals just as dogs are used in airports to smell the

vapours given off by explosives or drugs. Other obvious applications of nanomerial-

based sensors are monitoring methane in coal mines, 106

toxic gas detection, 107

monitoring of fruit freshness, 108

detecting the release of chemical weapons, 109,110

monitoring of pollution from automotive and power generation plants, 111-116

chemical

process monitoring, 117

bio-technological processing, 118

monitoring of gases that

contribute to smog in urban areas, 119-122

food quality, 123-126

and medical diagnosis 127-

134. Among these metal oxide nanomaterial-based sensors, SnO2 nanomaterials is the

most extensively researched. 135-150

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2.4 HEALTH EFFECTS OF NANOPARTICLES

Despite the widespread development of nanotechnology and nanomaterials through the

last 10-20 years, it is only recently that focus has been turned onto the potential

toxicological effects on humans, animals, and the environment through the exposure of

fabricated nanomaterials. 151

With that being said, it is a new development that potential negative health and

environmental impacts of a technology or a material is given attention at the developing

stage and not after years of application. 152

The term “nano(eco-)toxicology” has been developed on the request of a number of

scientists and is now seen as a separate scientific discipline with the purpose of

generating data and knowledge about nanomaterial effects on humans and the

environment. 153,154

Toxicological information and data on nanomaterials is limited and ecotoxicological data

is even more limited. Some toxicological studies have been done on biological systems

with nanoparticles in the form of metals, metal oxides, selenium and carbon 155

, however

the majority of toxicological studies have been done with carbon fullerenes 156.

The European Scientific Committee on Emerging and Newly Identified Health Risks

results from human toxicological studies on the cellular level can be assumed to be

applicable for organisms in the environment, even though this of cause needs further

verification. 157

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2.4.1 How Nanoparticles Enter the Body

Exposure of nanomaterials to workers, consumers, and the environment seems inevitable

with the increasing production volumes and the increasing number of commercially

available products containing nanomaterials or based on nanotechnology 158.

Exposure is a key element in risk assessment of nanomaterials since it is a precondition

for the potential toxicological and ecotoxicological effects to take place. If there is no

exposure – there is no risk. Nanoparticles are already being used in various products and

the exposure can happen through multiple routes.

Human routes of exposure are dermal (for instance through the use of cosmetics

containing nanoparticles); inhalation (of nanoparticles for instance in the workplace);

ingestion (of for instance food products containing nanoparticles); and injection (of for

instance medicine based on nanotechnology).

Although there are many different kinds of nanomaterials, concerns have mainly been

raised about free nanoparticles. 159,160 Free nanoparticles could either get into the

environment through direct outlet to the environment or through the degradation of

nanomaterials (such as surface bound nanoparticles or nanosized coatings).

30

2.4.1.1 Environmental Routes of Exposure

Environmental routes of exposure are multiple. One route is via the wastewater system.

At the moment research laboratories and manufacturing companies must be assumed to

be the main contributor of carbon-based nanoparticles to the wastewater outlet.

For other kinds of nanoparticles for instance titanium dioxide and silver, consumer

products such as cosmetics, crèmes and detergents, is a key source already and discharges

must be assumed to increase with the development of nanotechnology.

However, as development and applications of these materials increase this exposure

pattern must be assumed to change dramatically. Traces of drugs and medicine based on

nanoparticles can also be disposed of through the wastewater system into the

environment.

Drugs are often coated, and studies have shown that these coatings can be degraded

through either metabolism inside the human body or transformation in environment due

to UV-light. 161

This emphasizes the need to study the many possible processes that will

alter the properties of nanoparticles once they are released in nature.

Another route of exposure to the environment is from wastewater overflow or if there is

an outlet from the wastewater treatment plant where nanoparticles are not effectively held

back or degraded.

Additional routes of environmental exposure are spills from production, transport, and

disposal of nanomaterials or products. 162

31

While many of the potential routes of exposure are uncertain scenarios, which need

confirmation, the direct application of nanoparticles, such as for instance nano zero valent

iron for remediation of polluted areas or groundwater is one route of exposure that will

certainly lead to environmental exposure. Although, remediation with the help of free

nanoparticles is one of the most promising environmental nanotechnologies, it might also

be the one raising the most concerns. The Royal Society and The Royal Academy of

Engineering actually recommend that the use of free nanoparticles in environmental

applications such as remediation should be prohibited until it has been shown that the

benefits outweigh the risks. 163

The presence of manufactured nanomaterials in the environment is not widespread yet, it

is important to remember that the concentration of xenobiotic organic chemicals in the

environment in the past has increased proportionally with the application of these

meaning that it is only a question of time before we will find nanomaterials such as

nanoparticles in the environment – if we have the means to detect them. 164

The size of nanoparticles and our current lack of metrological methods to detect them is a

huge potential problem in relation to identification and remediation both in relation to

their fate in the human body and in the environment. 165

Once there is a widespread environmental exposure to human, exposure through the

environment seems almost inevitable since water, sediment, and living organisms can

take up nanoparticles from water. They may take up nanoparticles by ingestion from the

vegetation or sediment. In this way, nanoparticles can be transported to food chain.166

32

2.4.2 How Nanoparticles Interact with Living Organisms

Nanoparticles, can have the same dimensions as biological molecules such as proteins.

In living systems, they may immediately adsorb onto the surface of some of the large

molecules they encounter as they enter the tissues and fluids of the body.

This ability of nanoparticles to have molecules “sticking” to their surface depends on the

surface characteristics of the particles and can be relevant for drug delivery uses. Indeed,

it is possible to deliver a drug directly to a specific cell in the body by designing the

surface of a nanoparticle so that it adsorbs specifically onto the surface of the target cell.

But the interaction with living systems is also affected by the dimensions of the

nanoparticles. For instance, nanoparticles smaller than a few nanometres may penetrate

inside biomolecules, which is not possible for larger nanoparticles. Nanoparticles may

cross cell membranes. It has been reported that inhaled nanoparticles can reach the blood

and may reach other target sites such as the liver, heart or blood cells.

Key factors in the interaction with living structures include nanoparticle dose, the ability

of nanoparticles to spread within the body, as well as their solubility. Some nanoparticles

dissolve easily and their effects on living organisms are the same as the effects of the

chemical they are made of. However, other nanoparticles do not degrade or dissolve

readily. Instead, they may accumulate in biological systems and persist for a long time,

which makes such nanoparticles of particular concern.

33

There remain many unknown details about the interaction of nanoparticles and biological

systems and more information on the response of living organisms to the presence of

nanoparticles of varying size, shape, chemical composition and surface characteristics is

needed to understand and categorize the toxicity of nanoparticles.

2.4.3 Characteristics of Nanoparticles Relevant for Health Effects

The characteristics of nanoparticles that are relevant for health effects are:

Size –Size is the general reason why nanoparticles have become a matter of discussion

and concern. The very small dimensions of nanoparticles increases the specific surface

area in relation to mass, which again means that even small amounts of nanoparticles

have a great surface area on which reactions could happen. If a reaction with chemical or

biological components of an organism leads to a toxic response, this response would be

enhanced for nanoparticles. This enhancement of the inherent toxicity is seen as the main

reason why smaller particles are generally more biologically active and toxic that larger

particles of the same material. 167

Size can cause specific toxic response if for instance nanoparticles will bind to proteins

and thereby change their form and activity, leading to inhibition or change in one or more

specific reactions in the body. 168

Besides the increased reactivity, the small size of the

nanoparticles also means that they can easier be taken up by cells and that they are taken

up and distributed faster in organism compared to their larger counterparts. 169, 170

34

Due to physical and chemical surface properties all nanoparticles are expected to absorb

to larger molecules after uptake in an organism via a given route of uptake. 171

Some nanoparticles such as fullerene derivates are developed specifically with the

intention of pharmacological applications because of their ability of being taken up and

distributed fast in the human body, even in areas which are normally hard to reach – such

as the brain tissue. 172

Fast uptake and distribution can also be interpreted as a warning

about possible toxicity, however this need not always be the case. 173

Some nanoparticles

are developed with the intension of being toxic for instance with the purpose of killing

bacteria or cancer cells, 174

and in such cases toxicity can unintentionally lead to adverse

effects on humans or the environment.

Due to the lack of knowledge and lack of studies, the toxicity of nanoparticles is often

discussed on the basis of ultra fine particles (UFPs), asbestos, and quartz, which due to

their size could in theory fall under the definition of nanotechnology. 174, 176

An estimation of the toxicity of nanoparticles could also be made on the basis of the

chemical composition, which is done for instance in the USA, where safety data sheets

for the most nanomaterials report the properties and precautions related to the bulk

material. 177

Within such an approach lies the assumption that it is either the chemical composition or

the size that is determining for the toxicity. However, many scientific experts agree that

35

the toxicity of nanoparticles cannot and should not be predicted on the basis of the

toxicity of the bulk material alone. 178, 179

The increased surface area-to-mass ratio means that nanoparticles could potentially be

more toxic per mass than larger particles (assuming that we are talking about bulk

material and not suspensions), which means that the dose-response relationship will be

different for nanoparticles compared to their larger counterparts for the same material.

This aspect is especially problematic in connection with toxicological and

ecotoxicological experiments, since conventional toxicology correlates effects with the

given mass of a substance. 180, 181

Inhalation studies on rodents have found that ultrafine particles of titanium dioxide

causes larger lung damage in rodents compared to larger fine particles for the same

amount of the substance. However, it turned out that ultra fine- and fine particles cause

the same response, if the dose was estimated as surface area instead of as mass. 182

This indicates that surface area might be a better parameter for estimating toxicity than

concentration, when comparing different sizes of nanoparticles with the same chemical

composition. Besides surface area, the number of particles has been pointed out as a key

parameter that should be used instead of concentration. 183

Although comparison of ultrafine particles, fine particles, and even nanoparticles of the

same substance in a laboratory setting might be relevant, it is questionable whether or not

general analogies can be made between the toxicity of ultrafine particles from

36

anthropogenic sources (such as cooking, combustion, wood-burning stoves, etc.) and

nanoparticles, since the chemical composition and structure of ultrafine particles is very

heterogeneous when compared to nanoparticles which will often consists of specific

homogeneous particles. 184

From a chemical viewpoint nanoparticles can consist of transition metals, metal oxides,

carbon structures and in principle any other material, and hence the toxicity is bound to

vary as a results of that, which again makes it impossible to classify nanoparticles

according to their toxicity based on size alone. 185

Chemical composition and surface characteristics – The toxicity of nanoparticles

depends on their chemical composition, but also on the composition of any chemicals

adsorbed onto their surfaces. However, the surfaces of nanoparticles can be modified to

make them less harmful to health.

In addition to the physical and chemical composition of the nanoparticles, it is important

to consider any coatings or modifications of a given nanoparticles. 186

A study by Sayes et al. 188

found that the cytotoxicity of different kinds of some

derivatives varied by seven orders of magnitude, and that the toxicity decreased with

increasing number of hydroxyl- and carbonyl groups attached to the surface. According

to Gharbi et al. 189

, it is in contradiction to previous studies, which is supported by Bottini

et al who found an increased toxicity of oxidized carbon nanotubes in immune cells

when compared to pristine carbon nanotubes 190

.

37

The chemical composition of the surface of a given nanoparticle influences both the

bioavailability and the surface charge of the particle, both of which are important factors

for toxicology and ecotoxicology. 191

The chemical composition also influences properties such as lipophilicity, which is

important in relation to uptake through cells membranes in addition to distribution and

transport to tissue and organs in the organisms. Coatings can furthermore be designed so

that they are transported to specific organs or cells,which has great importance for

toxicity. 192

It is unknown, however, for how long nanoparticles stay coated especially inside the

human body and/or in the environment, since the surface can be affected by for instance

light if they get into the environment. Experiments with non-toxic coated nanoparticles,

turned out to be very cell toxic after 30 min. exposure to UV-light or oxygen in air. 192

Shape – Although there is little definitive evidence, the health effects of nanoparticles are

likely to depend also on their shape. A significant example is nanotubes, which may be of

a few nanometres in diameter but with a length that could be several micrometres. A

recent study showed a high toxicity of carbon nanotubes which seemed to produce

harmful effects by an entirely new mechanism, different from the normal model of toxic

dusts.

38

2.4.4 How Inhaled Nanoparticles Affect Health

Particulate matter present in air pollution, especially from traffic emissions, is known to

affect human health, although it is not clear exactly how. Epidemiological studies on

ambient air pollution have not proved conclusively that nanoparticles are more harmful

than larger particles, but these studies may not be well suited to demonstrate such

differences.

Inhaled particulate matter can be deposited throughout the human respiratory tract, and

an important fraction of inhaled nanoparticles deposit in the lungs. Nanoparticles can

potentially move from the lungs to other organs such as the brain, the liver, the spleen

and possibly the foetus in pregnant women. Data on these pathways is extremely limited

but the actual number of particles that move from one organ to another can be

considerable, depending on exposure time. Even within the nanoscale, size is important

and small nanoparticles have been shown to be more able to reach secondary organs than

larger ones.

Another potential route of inhaled nanoparticles within the body is the olfactory nerve;

nanoparticles may cross the mucous membrane inside the nose and then reach the brain

through the olfactory nerve. Out of three human studies, only one showed a passage of

inhaled nanoparticles into the bloodstream.

Materials which by themselves are not very harmful could be toxic if they are inhaled in

the form of nanoparticles.

39

The effects of inhaled nanoparticles in the body may include lung inflammation and heart

problems. Studies in humans show that breathing in diesel soot causes a general

inflammatory response and alters the system that regulates the involuntary functions in

the cardiovascular system, such as control of heart rate.

The pulmonary injury and inflammation resulting from the inhalation of nanosize urban

particulate matter appears to be due to the oxidative stress that these particles cause in the

cells.

2.4.5 The Health Implications of Nanoparticles Used As Drug Carriers

Nanoparticles can be used for drug delivery purposes, either as the drug itself or as the

drug carrier. The product can be administered orally, applied onto the skin, or injected.

The objective of drug delivery with nanoparticles is either to get more of the drug to the

target cells or to reduce the harmful effects of the free drug on other organs, or both.

Nanoparticles used in this way have to circulate long distances evading the protection

mechanisms of the body. To achieve this, nanoparticles are conceived to stick to cell

membranes, get inside specific cells in the body or in tumours, and pass through cells.

The surfaces of nanoparticles are sometimes also modified to avoid being recognized and

eliminated by the immune system.

With dermal administration, it was found that particle size was less important than the

total charge in terms of permeation through the skin. For instance, only negatively

40

charged particles were found to overcome the skin barrier and only when concentration

of charge was high enough.

Nanoparticles may be used effectively to deliver genes to cells, to treat cancer, as well as

in vaccination.

The use of nanoparticles as drug carriers may reduce the toxicity of the incorporated drug

but it is sometimes difficult to distinguish the toxicity of the drug from that of the

nanoparticle. Toxicity of gold nanoparticles, for instance, has been shown at high

concentrations. In addition, nanoparticles trapped in the liver can affect the function of

this organ.

Nanoparticles have the potential to cross the blood brain barrier, which makes them

extremely useful as a way to deliver drugs directly to the brain. On the other hand, this is

also a major drawback because nanoparticles used to carry drugs may be toxic to the

brain.

2.4.6 Assessments of Harmful Effects of Nanoparticles

Traditionally, doses are measured in terms of mass because the harmful effects of any

substance depend on the mass of the substance to which the individual is exposed.

However, for nanoparticles it is more reasonable to measure doses also in terms of

number of particles and their surface area because these parameters further determine the

interactions of nanoparticles with biological systems.

41

Several hypotheses were proposed for the adverse health effects of nanoparticles as part

of ambient air pollution. These hypotheses address nanoparticle characteristics, their

distribution, and their effects on organ systems, including effects on immune and

inflammatory systems.

However, some of these hypotheses may be of limited or no relevance for engineered

nanoparticles. For instance, the adhesion of toxic substances onto the surface of

nanoparticles may be of less relevance for production and handling facilities of large

volumes of engineered nanoparticles compared to the particles in ambient air.

In addition, drawing conclusions from tests on healthy animal models may be unsuitable

as some of the effects of nanoparticles may only be a risk for susceptible organisms and

predisposed individuals, but not to healthy people. For instance, age, respiratory tract

problems and other pollutants can modify the pulmonary inflammation and oxidative

stress induced by nanoparticles.

Because of the specific characteristics of nanoparticles, conventional toxicity tests may

not be enough to detect all their possible harmful effects. Therefore, a series of specific

tests was proposed to assess the toxicity of nanoparticles used in drug delivery systems.

One mechanism of toxicity of nanoparticles is likely to be the induction of oxidative

stress in cells and organs. Testing for interaction of nanoparticles with proteins and

various cell types should be considered as part of the toxicological evaluation.

42

With the exception of airborne particles delivered to the lung, information on the

behaviour of nanoparticles in the body including distribution, accumulation, metabolism,

and organ specific toxicity is still minimal.

2.4.7 Effects of Nanoparticles on the Environment

A number of studies have examined the uptake and effects of nanoparticles at a cellular

level to evaluate their impact on humans; it can reasonably be assumed that the

conclusions of these studies may be extrapolated to other species, but more research is

needed to confirm this assumption. Moreover, careful examination and interpretation of

existing data and careful planning of new research is required to establish the true impact

of nanoparticles on the environment, and the differences with larger, conventional forms

of the substances.

Persistent insoluble nanoparticles may cause problems in the environment that are much

greater than those revealed by human health assessments.

43

2.5 METAL OXIDE NANOPARTICLE SYNTHESIS

Increasing recent interests have been found in chemical synthesis & processing of

nanostructure materials. The recent popularity of “Nanoscience” not only revitalized the

use of many “old” method of synthesis of nanostructured materials, but also motivated

many “new” and modified ones. There are basic approaches to the synthesis of materials.

The first is the “top-down” approach, which involves breaking down the bulk material

into particles with nanometer-sized grains. The other approach is the “bottom-up”

approach, in which individual atoms or molecules are put together to form the required

nanoparticles.

Numerous techniques are available to synthesize oxides, and specifically oxide nanoma-

terials. All classes of nanomaterials including 1, 2 and 3-dimensional materials, have

unique advantages; however, this work focuses on 3-dimensional nanoparticles. A brief

literature review presented in this section specifically discusses synthesis methods carried

out in a liquid medium. From the numerous liquid phase synthetic methodologies sol-gel

nanoparticle synthesis was selected for this research.

Oxide nanoparticles have been reported in the literature to provide an increase in the

sensor response as high as two orders of magnitude compared to larger particles used in

the same application 193

. Advances in both chemical synthesis and characterization tech-

niques in the past two decades have made it possible to reproducibly create nanomateri-

als. The breadth of materials that can be made, high degree of reaction control for size,

44

shape, and chemical composition made liquid-based synthesis particularly attractive

when compared to and more costly vapor phase techniques.

2.5.1 Non-Aqueous Liquid Phase Reactions

Reactions that are carried out in water-free solvents are defined as non-aqueous. There

are two areas where this method is most often utilized. The first is in the synthesis of

silicon-based ceramics including SiC, Si3N4, SiBxOy and SiOxCy from silane polymers 194,

195. The second is in the synthesis of oxide powder using a combination of a metal salt

and a metal alkoxide precursor to react and create the oxide material.196

Particles formed

by these reactions are typically on the upper end of the size limit that would be

considered a nanoparticle (100 nm) and most fall within the submicron size range of 200

– 500 nm 197, 198

An additional disadvantage of this technique is that chlorides will be

formed that are difficult to remove completely from the final material. Non-aqueous

synthesis is similar to solvothermal synthesis

2.5.2 Coprecipitation Techniques

Coprecipitation is used mainly to prepare multi-component oxides by precipitating inter-

mediate compounds such as hydroxide and oxalate compounds 199. Dopants can also be

added to the mixed oxides if the reaction is controlled carefully. In the literature this

technique has proven useful for synthesis of nanomaterials for advanced energy

materials such as cathodes in rechargeable lithium based batteries 200 and particularly in

45

making nanocrystalline oxides (yttria-stabilized zirconia and cerium oxide) for solid oxide

fuel cells 201, 202. Difficulty in obtaining nanoparticles via coprecipitation arises during the

washing and drying of the hydroxide or oxalate intermediate compounds. Significant

aggregation of these compounds prior to the firing causes an increase in particle size. It

has been reported and is possible to obtain high quality nanomaterials via

coprecipitation when proper washing, drying and firing procedures are developed 203.

2.5.3 Emulsion-Based Synthesis

Microemulsion processing is a particularly important synthetic method as it is capable of

producing oxide nanoparticles whereas large scale emulsion processing typically leads to

particles on the orders of micrometers. Microemulsions are thermodynamically stable,

optically transparent dispersions of two immiscible liquids, water and oil for example 204

.

Mixtures of surfactants are used to reduce the interfacial tension to values approaching

zero (less than 0.001 Nm-1

in some cases) which enables spontaneous dis-persion of the

two phases through thermal motion. Equilibrium water (assuming water-in-oil

microemulsion) domain sizes range from 10 – 100 nm depending on the type and

concentration of the surfactant 205

. The process of colloidal and nanoparticle formation is

complex and includes relationships between nucleation, nanoparticle formation,

intermediate growth (for colloids) and eventual coagulation and flocculation. All of these

factors depend on the specific interactions between molecular species in the microemul-

sion. Successful microemulsion synthesis of oxide materials is enabled due to the fact

46

that particle nucleation is initiated simultaneously in a large number of micelles (spheri-

cal water droplets stabilized by surfactants in the oil medium) with well isolated nuclea-

tion sites due to the presence of surfactant films acting as stabilizers for the oxide parti

cles. Surfactant stabilized water micelles act as nano-sized reactors for performing syn-

thetic reactions, and the sizes of the colloids or nanoparticles formed are directly deter-

mined by the sizes of these emulsified droplets 206

. Monodisperse particles are formed

only when the nucleation and growth stages are strictly separated (one of the advanta-

geous properties of microemulsion synthesis. It is possible to adjust the size of emulsi-

fied water droplets by controlling the molar ratio of water to surfactant(s) and thus it be-

comes feasible to control the size of the oxide nanoparticles synthesized using this tech-

nique. The outstanding dispersion, small particle size distribution and shape control im-

parted by the microemulsion synthesis technique make it a very attractive method for ox-

ide nanoparticle synthesis. One aspect of microemulsion preparation that must be con-

sidered, as described by Shi and Verweij 207

, is the purification of particles after syn-

thesis. Their results showed that non-agglomerated nanoparticles with diameter less than

5 nm could be obtained and after careful cleaning procedures homogeneous coatings

could be made. SnO2 nanoparticles have been synthesized in the size range of 3–5 na-

nometers using microemulsion processing, yet no sensing data with materials made using

this technique are available 208

.

47

2.5.4 Hydrothermal and Solvothermal Synthesis

The term hydrothermal is geologic in origin and many of the mechanism were originally

elucidated for mineral systems by Morey 209

. Hydrothermal synthesis, applied to ceramic

nanomaterial processing, is defined as an aqueous chemical reaction in a sealed container

at a temperature that autogenously generates at elevated pressure 210

. Processing in this

manner allows low temperature, between 100 - 374°C which are the boiling and critical

point for water, respectively, synthesis of fully crystalline oxide nanoparticles to be

realized 211, 212

.

There are several advantages to hydrothermal processing including high purity (> 99.5%)

and chemical homogeneity, small particle size (< 5 nm possible), narrow particle size

distribution, single step processing, low energy usage, fast reaction times, low cost

equipment, the ability to generate met stable compounds, and, importantly, no

calcinations is required for many materials since they are fully crystallized by the

reaction 213

.

48

fact that nanomaterials including one, two and three dimensional structures can be syn-

thesized to the fully crystalline state is one of the reasons oxide nanostructures made via

hydrothermal synthesis have grown in popularity during the past decade. Continued

expansion of this field is expected as microwave, ultrasonic, electrochemical and

mechanical systems are being combined with the hydrothermal processing to improve

reaction kinetics and reduce processing time 214

. An example of how reaction time was

reduced dramatically was demonstrated by Jouhannauad et al215

. who used microwave

hydrothermal synthesis to make 5nm SnO2 nanoparticles with process times as short as

60 seconds. This is a huge improvement as most synthesis techniques require at minimum

6 hours of reaction time 216

. Driving reaction times down to minutes may enable

combinatorial style nanomaterials research where literally hundreds of variants could be

examined in a short period of time to help further understanding of the reaction

mechanisms involved.

Solvothermal synthesis refers to reactions done in non-aqueous solvents, using similar

processing schemes as hydrothermal 217

. Many of the same advantages are realized with

solvothermal synthesis including high purity, nanoparticles < 5nm with a small size

distribution and fully crystalline materials 218

.The limitation of this technique when

compared to hydrothermal processing is that many of the reactants and solvents used

must be stored, mixed and sealed in an oxygen and moisture free environment prior to the

reaction 219

. Some materials are difficult to synthesize via hydrothermal reactions, NiO

for example. It is possible to synthesize NiO nanoparticles solvothermally due to the

49

large number of organic precursors, solvents and the combination of reaction pathways

available 220

. As progress is made in the understanding of the organic chemistry involved

in solvothermal synthesis it may ultimately surpass hydrothermal synthesis in popularity

providing the ability to create uniquely tailored materials for a specific application.

Numerous solvothermal synthesis techniques for created oxide nanoparticles useful in gas

sensing have been published 219-223

.

2.5.5 Sol-Gel Processing

Sol-gel process is a chemical route to synthesize glassy or ceramic materials at relatively

low temperatures, based on wet chemistry processing, which involves preparation of a

sol, its gelation and the removal of the liquid within the porous gel, which can be

consolidated by heat treatment.

The precursor

In a sol-gel process, ceramic materials are prepared from a chemical solution of precursor

molecules. Precursors for sol-gel materials are typically metal alkoxides, but in principle,

any precursor of a suitable compound which can be hydrolysed could be used. An alkoxy

is a ligand formed by removing a proton from the hydroxyl on an alcohol, such as

methoxy (CH3O−) or ethoxy (CH3CH2O

−), where the hyphen indicates an electron that is

capable of forming a bond. Metal alkoxides are metal atoms which have one or more

alkoxide side groups.224

Precursors with different side groups have different reaction

50

kinetics which influence the gel structure and hence the properties of the final material

225, and using mixtures of different precursors, specific properties can be obtained.

226, 227

Hydrolysis

Metal alkoxides are readily hydrolysed. This reaction exchanges the alkoxy groups (–OR)

of the precursor with hydroxyl groups (–OH), where, for generality, R represents an alkyl

so that –OR is an alkoxy, and releases alcohol. Depending on the amount of water and

catalyst present, hydrolysis may continue to completion, so that all alkoxy groups are

exchanged by hydroxyl groups, but the reaction may go backwards in a reesterification

process as well. Mineral acids or bases are used to catalyze the hydrolysis reaction, and

the rates of the two reactions depend on the catalyst concentration. Water and

alkoxysilanes are immiscible, but vigorous stirring improves mixing, and the alcohol

released as by-product of the hydrolysis helps homogenizing the initially phase separated

system. After hydrolysis of the precursors, a single phase system is obtained.

Condensation

The hydrolyzed precursor forms bond with other precursors in a condensation process to

build polymeric molecules (Si-O-Si) and water or alcohol is released. The chemical

reactions can be described schematically as follows:

Si–OR + H2O ↔ Si–OH + ROH (1)

Si –OR + HO–Si ↔Si –O–Si + ROH (2)

51

Si–OH + HO–Si ↔ Si –O–Si + H2O (3)

The silanol group (Si–OH) produced by the hydrolysis reaction, (1), is converted into

siloxane (Si–O–Si) crosslinks by either reaction (2) or reaction (3). Some silanol groups

may remain after the chemical reactions have taken place because of incomplete

condensation.228

Whether the water-producing (3) or the alcohol-producing (2) reaction

takes place depends on the side groups taking part in the reaction. The number of

hydroxyl groups increases with the degree of hydrolysation, which in turn depends on the

water concentration and the pH. The probability of side group encounters depends on the

mobility of the molecules, and hence temperature and steric hindrance. For precursor

molecules that can form at least two bonds, these condensation reactions can continue to

build larger and larger molecules in a polymerization process. The structure of the

polymeric network depends on the functionality of the precursors, i.e., the number of

bonds that each monomer can form.

Gelation

During formation of a gel, polymer clusters grow by condensation. These clusters collide

and link together, until, eventually, a single giant cluster extending throughout the

solution is produced. The time it takes for this macroscopic polymer to be made is called

the „gel-time‟, and it can be measured as the time of an abrupt rise of the viscosity.229

At

a very low pH, gel formation is very low, and the sol contains separate polymetric

52

clusters. The gel may be dried without shrinkage of the solid skeleton to form an aerogel,

which has extremely low density and refractive index, 230, 231

or dried with shrinkage of

the solid skeleton to form a xerogel, a much denser, but still porous, material. Xerogels

are used, e.g., as permeable matrix for immobilization of reagent dyes in optical sensing

systems, 232

or catalysts 233

in reactors. Furthermore, thin film coatings of xerogel are

used, e.g., as spin coatable dielectrics (spin-on glass) in microelectronics 236

, optical

coatings 237

, self-cleaning coatings 238

, or scratch protective coatings 239, 240

. When the gel

is annealed above 450℃ in oxygen atmosphere, the organic compounds of the material

are calcinated, and purely inorganic glass materials are obtained. At high temperature >

1000℃, sintering will close the porosity of the material, and dense materials can be

obtained. The bottom-up formation of solid material, which is provided by the sol-gel

process, enables control of the chemical composition and porosity of the material. This

makes it possible to produce materials of, e.g., special mechanical, optical or thermal

properties, which can not be made in any other manner. The low viscosity of the sol at

early stages of gelation enables thin film coatings to be made by spin-coating 241

, spray-

coating 242

, or dip-coating 243

, and the gel films are easy structured by casting or imprint

244-246. This makes sol-gel derived materials very versatile.

53

Figure 2.6 Sketch of the Chemical Structure at Different Stages of a Sol-gel Process.

EtO denotes an ethoxy group (CH3CH2O−). The alkoxysilane precursor (a) undergoes a

hydrolysation process (1) during which the alkoxy side groups are exchanged by

hydroxyl groups. The hydrolyzed precursor (b) forms bonds to other precursors in a

condensation process (2) to build polymeric molecules (c). As the polymerization process

(3) proceeds, a gel (d) is eventually formed. By thermal annealing of the gel, the organic

compounds are calcinated (4) and a purely inorganic material (e) is produced.

The processes described above based on the use alkoxides can also be applied in using

inorganic precursors such as tin (IV) chloride pentahydrate (SnCl2.5H2O).

54

Figure 2.7 Simplified Chart of Sol-gel Processes 247

2.6 CHARACTERIZATION TECHNIQUES

2.6.1 Scanning Electron Microscope

Scanning Electron Microscope (SEM) is a type of electron microscope that images a

sample by scanning it with a beam of electrons in a raster scan pattern. The electrons

interact with the atoms that make up the sample producing signals that contain

information about external morphology (texture), chemical composition, and crystalline

structure and orientation of the materials making up the sample.

55

Figure 2.8 SEM (JEOL-JSM 5800)

2.6.2 X-ray Diffraction

X-rays are electromagnetic radiation of wavelength about the same size as an atom. They

occur in that portion of the electromagnetic spectrum between gamma-rays and the

ultraviolet. The discovery of X-rays in 1895 enabled scientists to probe crystalline

structure at the atomic level. X-ray diffraction has been in use in two main areas, for the

fingerprint characterization of crystalline materials and the determination of their

structure. Each crystalline solid has its unique characteristic X-ray powder pattern which

may be used for its identification. Once the material has been identified, X-ray

crystallography may be used to determine its structure, i.e. how the atoms pack together

in the crystalline state and what the interatomic distance and angle are etc. X-ray

diffraction is one of the most important characterization tools used in solid state

chemistry and materials science.

56

Figure 2.9 PANalytical System Diffractometer (Model: DY-1656)

57

CHAPTER THREE

MATERIALS AND METHODS

3.0 Equipment

Beakers

Test-tubes

Magnetic stirrer

Electronic weighing machine

P h meter

Mortar

Pestle

Spatula

Sringe

Flask

Oven

3.1 Chemicals

The chemicals used were tin (IV) chloride pentahydrate (SnCl4.5H2O, Fluka), ammonium

hydroxide (NH4OH 25%, Merck), absolute ethanol (C2H5OH 98%, R & M Chemicals)

and distilled water from the Electro-chemical laboratory of National Centre for Energy

58

Research and Development, University of Nigeria, Nsukka. All the chemicals are

analytical grade and are used as received without further purification.

3.2 Synthesis of SnO2 Nanoparticles

Ethanol (100 cm3) was added to the mixture of tin (IV) chloride pentahydrate (3.0 g) in a

beaker (100 cm3), and the mixture was stirred for 25 minutes to form a homogeneous

solution. Ammonia solution (12 cm3) was added to the solution at constant reaction

temperature of 40 oC under constant stirring, to form a gel. The resulting gel was filtered

and washed with ethanol to remove any impurities that resulted from side reactions. After

air drying at room temperature for 48 hours, the obtained powder was ground using

mortar and pestle, and finally annealed at a temperature of 180 oC.

3.3 Synthesis of SnO2 Nanoparticles at Various Reaction Temperatures

Ethanol (100 cm3) was added to the mixture of tin (IV) chloride pentahydrate (3.0 g) in a

beaker (100 cm3), and the mixture was stirred for 25 minutes to form a homogeneous

solution. Ammonia solution (12 cm3) was added to the solution at constant reaction

temperature of 40 oC under constant stirring, to form a gel. The resulting gel was filtered

and washed with ethanol to remove any impurities that resulted from side reactions. After

air drying at room temperature for 48 hours, the obtained powder was ground using

mortar and pestle, and finally annealed at a temperature of 180 oC. The experiment was

repeated by varying the reaction temperature at 60, 80, and 100 oC, while keeping all

other conditions constant.

59

3.4 Synthesis of SnO2 Nanoparticles at Various Ammonia Concentrations

Ethanol (100 cm3) was added to the mixture of tin (IV) chloride pentahydrate (3.0 g) in a

beaker (100 cm3), and the mixture was stirred for 25 minutes to form a homogeneous

solution. Ammonia solution (12 cm3) was added to the solution at constant reaction

temperature of 40 oC under constant stirring, to form a gel. The resulting gel was filtered

and washed with ethanol to remove any impurities that resulted from side reactions. After

air drying at room temperature for 48 hours, the obtained powder was ground using

mortar and pestle, and finally annealed at a temperature of 180 oC. The experiment was

repeated by using 1.07, 8.7, 9.7, and 9.9 mole per dm3 of ammonia in each case while the

reaction and annealing temperatures were kept constant at 40 and 180 oC respectively.

3.5 Synthesis of SnO2 Nanoparticles at Various Annealing Temperatures

Ethanol (100 cm3) was added to the mixture of tin (IV) chloride pentahydrate (3.0 g) in a

beaker (100 cm3), and the mixture was stirred for 25 minutes to form a homogeneous

solution. Ammonia solution (12 cm3) was added to the solution at constant reaction

temperature of 40 oC under constant stirring, to form a gel. The resulting gel was filtered

and washed with ethanol to remove any impurities that resulted from side reactions. After

air drying at room temperature for 48 hours, the obtained powder was ground using

mortar and pestle, and finally annealed at different temperatures of 140, 160, 180, 200,

220 and 240 oC.

60

CHAPTER FOUR

RESULTS AND DISCUSSION

CHAPTER FOUR

RESULTS AND DISCUSSION

The tin oxide nanoparticles were prepared by sol-gel method by adding absolute ethanol

to a mixture of tin (iv) chloride pentahydrate precursor and water to homogenize the

initially separated system. The mixture was vigorously stirred to improve mixing.

Ammonia solution was added to the resulting mixture to form a gel. The process involved

were hydrolysis and polymerization. In contact with water, tin (IV) chloride precursor

was hydrolysed during which the chloride groups were exchanged by hydroxyl groups.

Cl

SnCl

Cl

Cl

OH

SnOH

OH

OH(A)

(1) (2)

The hydrolysed precursor formed bonds with other precursors in a condensation process

to build polymeric molecules. As the polymerization process proceeded, a gel was

eventually formed. By thermal annealing of the gel, pure tin (iv) oxide nanoparticles were

obtained.

61

OH

SnOH

OH

O

SnOH

OH

OH

O

SnOH

O

SnOH

OH

OH

OH

SnOH

OH

O

SnOH

OH

O

SnOH

OH

OH

O

SnO O

SnO

O

O

O

Sn

O

O

O

SnO

O

O

SnOH

O

O

+H2O

(C)

+H2O +H

2O

(3) (4) (5)

(B)

The influence of the sol-gel processing conditions on the particle size was the main

subject of interest here. Three independent techniques were used to characterize the size

of the nanoparticles: XRD, SEM and UV-Vis. Crystallite sizes of nanocrystalline

materials can be estimated from the peak broadening in an XRD pattern according to the

Scherrer equation 249

:

D =

Where D is crystallite size

λ is X-ray wavelength (0.1541 for Cu-Kα)

is the full width at the half maximum of the diffraction peak (FWHM)

is the Bragg‟s diffraction angle . The assumptions made when using Scherrer

equation are that the particles are spherical in shape, are less than 1000 Å diameter, strain

free, and are uniform in size. 249, 250

62

4.1 EFFECT OF REACTION TEMPERATURE ON THE SYNTHESIS OF

SnO2 NANOPARTICLES

The X-ray diffraction patterns were recorded for crystallographic identification of the

synthesized samples as shown in Figure 4.1.

Figure 4.1 The XRD patterns of SnO2 Samples Prepared at Different Reaction

Temperatures.

The main diffraction peaks were observed at 25.5, 33.60 and 52.0 degrees at

corresponding plane of (110), (101) and (211) respectively. The results complied with

100 oC

60 oC

oC

40 oC

80 oC

63

the standard SnO2 XRD pattern (ICDD file number 41-1445). All diffraction peaks can be

indexed to pure tetragonal SnO2 crystalline phase. The patterns with the broadest peaks

correspond to the samples synthesized at the lowest temperatures. The samples made at

the lowest temperatures also show the fewest peaks, especially beyond 50° (2θ). This

may indicate that these samples contain some residual amorphous materials in the final

product. The particle sizes calculated using the Scherrer equation are also indicated in

table 4.1. It is observed that the peaks become broader as the reaction temperature

increases from 40 oC to 100

oC, indicating decreasing particles size.

Table 4.1 The Crystallite Sizes of SnO2 Samples Prepared at Different Reaction

Temperatures.

The higher temperature favours a fast hydrolysis reaction and results in high super

saturation which also can be attributed to high nucleation rate.251

This will lead to the

formation of a large number of small nuclei and eventually lead to the formation of small

particles.252

Reaction Temperature (oC) crystallite size (nm)

40 5.3

60 5.2

80 5.1

100 5.1

64

4.2 EFFECT OF AMMONIA CONCENTRATION ON THE SYNTHESIS

OF SnO2 NANOPARTICLES

The study on the effect of ammonia concentration on the synthesis of tin oxide

nanoparticles was carried out by using 1.18, 3.30, 5.36, and 7.44 mole per dm3 of

ammonia in each case while the reaction and annealing temperatures were kept constant

at 40 and 180 oC respectively in experiment 3.4

Table 4.2 The Crystallite Sizes of SnO2 Samples Prepared at Different Ammonia

Concentrations

Table 4.2 shows the crystallite sizes of SnO2 particles using different ammonia

concentration. As the concentration of ammonia varies from 1.18 to 7.44 mole per dm3,

which result in the increase of the pH from1 to 10.1, the crystallite size increases

marginally from 5.4 to 6.2 nm. The concentration of ammonia fed into the reaction

mixture was found to have significant effect on SnO2 particle distribution.

[NH3] (mol dm

-3) pH crystallite size (nm)

1.18 1.0 5.4

3.30 8.6 5.7

5.36 9.4 6.0

7.44 10.1 6.2

65

Figure 4.2 The XRD patterns of SnO2 Samples Prepared Using different Ammonia

Concentrations

This behavior is similar with the result reported by Park et al. on silica nanoparticles.42

At low ammonia concentration, 1.18 mole per dm3, well-dispersed SnO2 nanoparticles

with soft aggregation were obtained. The particles showed better distribution when the

concentration of ammonia is increased up to 5.4 mole per dm3. However, a further

increased in the concentration of ammonia results in high particle agglomeration believed

to be due to excessive generation of primary particles at super saturation state.

5.34 M

7.48 M

3.20 M

1.07 M

66

4.3 EFFECTS OF ANNEALING TEMPERATURE ON THE SYNTHESIS

OF SnO2 NANOPARTICLES

The synthesis of SnO2 nanoparticles has been described in experiment 3.5

Figure 4.3 The XRD patterns of SnO2 Samples Annealed at Different Temperatures.

It is clear from figure 4.3 and table 4.3 that the peak width increases with the decreasing

annealing temperature, which is an indication of particle size decrease with the decrease

in annealing temperature. The effect of particle size with increase in annealing

temperature is explained by this fact: In heating process when the particles are formed,

they collide and either coalesces with one another to form a large particle, or they

coagulate. The process which occurs depends on the temperature and available energy.

140 oC

160 oC

180 oC

200 oC

220 oC

240 oC

67

Table 4.3 The crystallite sizes of SnO2 samples prepared at different annealing temperatures.

4.4 OPTICAL STUDIES OF SnO2 NANOPARTICLES

Figure 4.4 shows the UV-Vis diffuse reflectance spectra of SnO2 nanoparticles annealed

at 160, 200, and 240 oC, where the scale labeled “absorbance” are the negative logarithm

values of the experimentally determined diffuse reflectance of the samples.

Table 4.4 The Band gap Energies of SnO2 Annealed at Different Temperatures

Temperature (oC) Absorption edge wavelength (nm) Band gap energy (eV)

160 312 3.98

200 315 3.94

240 318 3.90

Annealing temperature (0C) crystallite size (nm)

140 2.8

160 4.1

180 5.3

200 6.3

220 7.9

240 9.2

68

The sharp rise of the spectra at the absorption edge demonstrates high crystalline

nanocrystals with less surface defects. As the size of the nanoparticles decrease, the

higher surface area to volume ratio incorporates various surface related defects in the

nanoctystals. This causes a broadening of the absorption spectra at the absorption edge.

Therefore, the sharp rise in the spectra can help us to conclude that in the nanocrystals,

the surface related defects are less and the crystals are homogeneously distributed.

Figure 4.4 UV-Vis Diffuse Reflectance Spectra of SnO2 Nanoparticles Annealed at

Different Temperatures

69

The values of the absorption edge wavelength of

Band Gap Energy (E) =

Where h is plank‟s constant = 6.626 x 10-34

Joule second

C is speed of light = 3.0 x 108 metres per second

λ is absorption edge wavelength in metres

1 eV = 1.6 x 10-19

Joules (conversion factor)

The band gap energies at annealing temperatures of 160, 200 and 240oC are larger than

the bulk SnO2 (3.6 eV). The band gap is found to be crystal size dependent and increases

with decreasing crystal size.

4.5 MORPHOLOGICAL OBSERVATIONS

Figure 4.5 shows the morphologies of samples annealed at different temperatures, 160oC,

200oC, 240

oC. Spherical morphology with a highly porous, foam-like structure can be

observed. The particle sizes of the nanoparticles increased with the increasing annealing

temperature

70

Figure 4.5 The SEM Morphology of the SnO2 Nanoparticles at Different Annealing

Temperatures.

.

200 oC

160 oC

240 oC

71

CHAPTER FIVE

CONCLUSION 5.1 CONCLUSION

Tin Oxide nanoparticles of uniform size were synthesized successfully by using sol-gel

method. The XRD analysis shows that the synthesized particles are crystalline in nature

and of tetragonal phase. The sol-gel method is very efficient for synthesizing tin oxide

nanoparticles of uniform size in economic and quick method. By varying the processing

methodology, time, temperature and concentration, the nanocrystals of different sizes and

morphologies can be easily synthesized to an industrial scale at low cost.

5.2 SUGGESTION FOR FURTHER WORK

The results of the experiments in this research work lead to the possibilities of future

work. Parts of the synthesized SnO2 nanoparticles should be deposited on a substrate and

the result compared with the particles synthesized without deposition on the substrate to

find out whether the substrate has any effect on the size of the crystals. Also, the effect of

tin (IV) chloride pentahydrate concentration on the size of the SnO2 nanoparticles should

be investigated.

72

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