13 CHAPTER 2 LITERATURE REVIEW 2.1 NANO TECHNOLOGY – AN INTRODUCTION Nano technology deals with small structures (or) small sized materials of nano meter scale. This chapter of research thesis is dedicated to deal with the published literature on multifunctional finishing of fabrics, various processes of functionalization and the concept and application of using TiO 2 and ZnO nano-particles for functional finishing and the concept and mechanism of photo catalytic oxidation. The methods of synthesis of TiO 2 and ZnO nano-particles and the required functional properties which can be added to the textiles by way of treatment with nano-particles are discussed. Textile finishing encompasses all the operation involved in (or) applied in treatment of fabrics from grey fabric stage to final finished fabric stage with the ultimate aim of improving the appearance and handle value of the fabrics. In this research thesis the term is employed to include all the processes that usually applied after coloration and improvise certain qualities of the fabrics. Such improved fabric characteristics may include appearance, fashion aspects and high performance properties for both personal needs and industrial applications. The fundamental difference between finishing and functional finishing lies in the area of improvement in functional requirement of fabrics. While finishing applies to general nature of added properties the functional finishing has a focused specific enhancement of the fabric properties and performance.
43
Embed
CHAPTER 2 LITERATURE REVIEW - Shodhgangashodhganga.inflibnet.ac.in/bitstream/10603/15500/7/07_chapter 2.pdf · 15 2.2 FUNCTIONAL FINISHES FOR TEXTILES The recent advances in functional
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
Transcript
13
CHAPTER 2
LITERATURE REVIEW
2.1 NANO TECHNOLOGY – AN INTRODUCTION
Nano technology deals with small structures (or) small sized materials
of nano meter scale. This chapter of research thesis is dedicated to deal with
the published literature on multifunctional finishing of fabrics, various
processes of functionalization and the concept and application of using TiO2
and ZnO nano-particles for functional finishing and the concept and
mechanism of photo catalytic oxidation. The methods of synthesis of TiO2
and ZnO nano-particles and the required functional properties which can be
added to the textiles by way of treatment with nano-particles are discussed.
Textile finishing encompasses all the operation involved in (or) applied
in treatment of fabrics from grey fabric stage to final finished fabric stage
with the ultimate aim of improving the appearance and handle value of the
fabrics. In this research thesis the term is employed to include all the
processes that usually applied after coloration and improvise certain qualities
of the fabrics. Such improved fabric characteristics may include appearance,
fashion aspects and high performance properties for both personal needs and
industrial applications.
The fundamental difference between finishing and functional finishing
lies in the area of improvement in functional requirement of fabrics. While
finishing applies to general nature of added properties the functional finishing
has a focused specific enhancement of the fabric properties and performance.
14
The nature of textile fabric substrate, whether woven, knitted (or) non –
woven (or) the type of yarn used in fabric like staple yarn (or) continuous
filament yarns plays a major role in deciding the means and methods of
applying functional finish to fabrics. Kathirvelu et al (2006). gave a detailed
account on functional finish of textiles by using traditional and conventional
type of chemicals.
Several research attempts were made to combine various finishes in
one bath. The economical advantages are that the main objectives are met in
one application and drying process. However the technological problem
encountered during such one bath finishing are many like similar nature of
effects help each other processes for example silicone elastomers impart water
repellency and softness enhance antistatic properties and finally the antistatic
finishes are made with softening. The finishes that are contradictory in nature
are counteracting each others, for example, hydrophobic finishes and
hydrophilic antistatic finishes (or) stiffening and elastomeric finishes (or)
stiffening and softening finishes.
The properties of final finished products are decided by the chemical
nature and the structure of natural, artificial and synthetic fibre. There are
several characteristics of fibres, both positive and negative properties, which
deserves due consideration in deciding the final applications for example,
some fibres easily burn (cellulose), some burn slowly and self – extinguish
(wool, silk) (or) some burn and melt (Synthetic fibres).
Cotton is known to possess the advantages of being imparted with a
wide variety of functional properties. Cellulosic fibres are chemically reactive
and similarly the natural protein fibres such as wool. Synthetic fibres are not
very reactive and many are inert. The cotton molecule has reactive groups
which permit permanent attachment of these functional compounds.
15
2.2 FUNCTIONAL FINISHES FOR TEXTILES
The recent advances in functional textiles have been in the direction of
adding features to the basic product and also to address the new emerging
problem that are associated with functions of textiles. The process of
imparting the desired functional properties to textile and clothing materials is
better known as functionalization (Burniston et al 2004). Three different
approaches are thought of in the area of developing functional textiles.
� New fibres / yarns
� New type of textile construction / production technology
� New types of textile finishing
In all cases the objective is to develop specific properties which enable
them to perform a particular function in the final product in a better way.
They are expected to be high – tech products with additional novel functions.
Many of them are used with the aim of improving wear, contour, safety and
health protection.
The use of specialized fibres that have required characteristics for
performing special functions, either by the characteristics of the polymer (or)
by additives before fibre spinning, can be a possible method for achieving
functional properties in textiles. The functional textiles are explained with
help of a schematic diagram given below:
16
Figure 2.1 Schematic diagrams on functional textiles
In alternate method, yarn (or) fabric engineering with different fibres (or)
different layers can also be employed.
2.3 OBJECTIVES OF FUNCTIONAL TEXTILE FINISHES
The versatility of textile fibres offers many avenues for functional
finishes that can add their own form of functionality, modify existing
properties of fibres (or) create new benefits that are unique. Textile industry
and researchers have developed increasingly many types of functional
finishes. The objective is to enhance comfort feeling to the consumers. There
are two broad divisions of functional finishes namely, physical based finishes
and chemical based finishes. At the present scenario of stricter regulations
being imposed on chemical substances, the phases of developments in
physical based finishes have gained momentum. A bird’s eye view of the
applications of nano technology in textile fabrics is give below:
17
Figure 2.2 Nano-technology in Fiber and Textile Manufacturing
In chemical based finishes, conventional methods of finishing can be
employed such as exhaustion, padding, low add-on processes, foam
application, printing, coating etc. Recently newer and alternatives methods of
finish applications like micro encapsulation etc have been developed. Certain
newer functional capabilities like slow and long term release of chemicals
during usage of fabric can be achieved from new methods. Nano technology
has emerged out to open new possibilities in functional finishes. Surface
modification by means of chemical modification, by the application of a
surface layer (or) by more environmental friendly treatment like enzymes (or)
physical modification with plasma technology are few techniques to mention
here.
Nanotechnology
in textiles
Nano fibers
and yarns
Nano finished
textiles
• Development of Single and
Multi - walled nano fibers,
such as Carbon Nano – Tube
(CNT) composite fibers.
• Production of nano fibers,
using electro-spinning
process.
• Nanotechnology can also
improve surface properties
and functionality of cotton
fabrics.
• A variety of chemical
finishes and coatings can be
developed.
18
Consumers demand newer and improved functional properties in
textiles. Globally the demands for technical textiles have increased as their
high performance properties are enhanced.
2.4 APPLICATION OF FUNCTIONAL FINISHES AND NANO
TECHNOLOGY
Nano textile finishing deals with application of ultra fine particles
produced using nano technology. In recent times, nanotechnology is
increasingly attracting global attention. It has a huge potential in a wide range
of end uses (Beringer et al 2004). The new and unique properties of nano
materials have attracted scientists, researches and business persons because of
their economical advantages. It is interdisciplinary and emerging field which
fundamentally manipulates structural materials to the nano level (or)
molecular level. Nanotechnology encompasses a wide range of technologies
concerned with structures on the nano meter scale. It has revolutionized
material science and led to the development and evolution of a range of new
improved materials.
By way of controlling atoms and molecule, one can generate functional
materials, devices and systems of nano meter scale by precisely placing the
individual atoms thereby creating excellent properties hither to unknown and
unexplored (Kathirvelu et al 2008e).
A glimpse through literature offers facts about many nanotechnology
based innovations that has great promise to the future. Nanotechnology has
already taken an important part of our daily life in one form (or) the other.
19
2.4.1 Effect of Particle Size
Nanotechnology is based on the fact that the properties of materials
can change drastically when the particle size falls below 100nm. The size of
nano-particles matters a lot with respect to the deciding of its importance nano
technology encompasses the structures whose thickness is smaller than 100
nm in one dimension. It exploits the characteristics of material that are in the
transitional zone between the atomic and microscopic level. The manufacture
and amputation of such nano level structures are also covered under this
umbrella. When the particle size is below 100nm there happens a drastic
change in the properties of materials. Surprisingly it may result in novel and
significantly improved physical, chemical and biological properties. They
exhibit fundamentally new behavior, when the size falls below a critical level.
The performance characteristics and functionalities that are previously
thought to be not possible are now made available for exploitation. It is a
convergent technology where the interdisciplinary interaction of sciences
mostly likes chemistry and physics play a vital role.
Nano-particles may either be natural (or) incidental (or) engineered and
may be amorphous (or) crystalline (or) polymeric (or) composites. They may
be of non metal metallic, semiconductor (or) a combination. The shape of
nano-particles may be spheres, tubes, rods, horns and placeless.
Kathirvelu et al (2008d) reported the size and chemical composition
decides their physical properties. Their reactivity depends on the surface
chemistries including surface defect and impurities. The decreasing size of
nano-particles results in fundamental changes in properties of material. When
the particle size is reduced there is an enormous increase in the specific
surface area. The two different approaches generally thought of in nano-
particles production are top down and bottom up types. Schematic
representations of particle size and surface area at nano scale given below:
20
Particle size
10 cm 1 mm 1µm 1nm
Surface area
1 100 1, 00,000 10,000,000
Figure 2.3 Schematic representations of particle size and surface area
at nano scale
The top down approach is like precision engineering which starts from
the micro level structure and the components are gradually miniaturized. They
are primarily featured in physics. It involves enlarging the surface and
separation of the particles. It results in increase in free energy making the
system less stable. The second type of bottom – up approach is basically
assembling from atomic (or) molecular components which results in complex
structures. It is exploited well in chemistry and biology. In sol gel technique
colloidal particles are produced which ensures the particle size to be of nano
and no energy is required to enlarge the specific surface area.
21
2.4.2 Advantages of Two Different Approaches
In the “top down” approach of breaking down the bigger particles into
smaller nano-particles, the disadvantages are less precision, production of
waste and pollution and more energy consumption. In the “bottom up”
approach where the sub nano level particles are assembled to nano-particles,
they pose certain advantages like absolute precision, no wastage, positive
control on process, less energy required and more eco friendly nature.
2.4.3 Nanotechnology in Textiles
Nanotechnology has been playing are important role in recent
textile application (Kathirvelu 2003). The very first commercial application of
nanotechnology in textiles was in lifestyle applications. Surprisingly textiles
and cosmetics were among the first disciplines to use nano materials. It is
believed that textile fabrics gives one of the safest platform for the application
of nano-particles. There has been an underlying synergy between textiles
industry and nanotechnology which make it possible for large potential of
commercial exploitation. It is understood that the large amount of interfacial
area of both nano size particles and fibres in fabrics could be employed in a
more purposeful way that augers well for commercial textile potential
(sparkle news 2006). Due to their large surface area to volume ratio and high
surface energy, nano-particles can provide high durability of finish on fabrics.
This presents better affinity on fabrics and leading to better durability of the
function (Wong et al 2006). It is obvious that this coating of nano-particles
results in negligible and insignificant danger in physical and mechanical
properties like handle value, strength, air permeability and wetting properties
(Xin 2006). Very low consumption levels of chemicals and energy are the
advantages of using nano-particles in textile finishing.
22
Surface coating of textiles with nano sized particles of metal oxides
has gained more importance presently due to their specific advantages. Multi
functional properties like UV blocking, antibacterial, soil release and self
cleaning are achieved through nano coating. Both TiO2 and ZnO in nano-
particles form are capable of imparting self cleaning property to textiles. Nano
silver particles are very effective in imparting antibacterial property to
textiles. However its high cost is the major disadvantage. Nano-particles
coating influences the other fabric properties like Tensile strength, bursting
strength, Bending rigidity and air permeability. TiO2 is a wide band gap and
non toxic semiconductor. It has got a high degree of photo catalytic and self
cleaning properties. The catalytic activity of TiO2 is based on the electron /
hole pair formation due to photo excitation. Due to large surface area per unit
mass and volume, nano TiO2 particles show high degree of photo – catalytic
activities. The photo catalytic activity of TiO2 nano particle coatings depends
on the phase, the crystallite size and porosity of the coatings and the particle
size. Preparation of single phase aqueous solution of nano crystalline anatase
TiO2 at a low temperature of 38°C by using sol gel method was reported by
(Daoud et al 2004). By using chemical co precipitation – peptisation method
preparation of TiO2 solution at 72°C was reported by (Xing et al 2004).
Preparation of TiO2 sol with surface protective agent was reported. They also
reported characteristics of sol particles and evaluated the photo catalytic
activity. Both TiO2 and ZnO are bio – safe, bio compatible and can be used
for biomedical application without coating. Vigneshwaran et al (2006)
reported their work on ZnO soluble starch nano composites and synthesis of
nano ZnO using water as a solvent and soluble starch as a stabilizer. Yadav
et al (2006) studied and reported the performance of nano ZnO by wet
chemical method using Zinc nitrate and Sodium hydroxide as precursors and
soluble starch as stabilizing agent. Alessio Becheri et al (2007) reported their
work on synthesis and characterization of nano ZnO particles and their
application on cotton and wool fabric resulting in UV shielding. Kathirvelu et
23
al (2008a,b) reported their findings on nano ZnO using Zinc nitrate and
Sodium Hydroxide. Nanotechnology in textile applications general overview
are given below:
Figure 2.4 General overview of nanotechnology in textile applications
The present day applications of nano technology in textile industry are
in fibres, yarns, fabrics, nonwovens, finishing like dyeing and coating,
electronic textile and fibre modification.
2.4.4 Nanotechnology in Fibres, Fabrics And Dyeing
Production of nano-sealed synthetic fibres like Polyester, Polyamide
and Polypropylene is a promising application in fibres. The thermal,
electrical, mechanical and chemical properties of fibres can be improved by
24
using nanotechnology. New products like breathable textile laminates and
super absorbency fibres with open pore structure are possible in a variety of
polymers. High specific surface area of nano-particles makes it attractive for
usage in medical textiles. Polymeric nano fibres with nano-sealed diameters
are more suitable to such applications.
Huang (2005) reported that the decreasing fibre diameter reduces the
contact angle between fibres resulting in excellent wetting behavior of final
product. Nanotechnology is applied in production of light weight fibres which
exhibit greater strength. (Nyati 2005) reported that nano fibre exhibit high
surface area, small fibre diameter, good filtration properties, thin layers and
high permeability.
The “die swell” effect is known as swelling of polymer when passing
through the capillary tube in electro spinning process. This could be
eliminated by addition of carbon nano tube to commercial polymer. In result,
the improvement strength of fibre against the high voltage between capillary
tube and collector. This helps in increasing the spinning speed in fibre
production. In an another interesting area of development, the hygroscopic
nature of polyester fibre can be increased by 30 times by coating it with a 50
nanometer film made up of 20 layers on the outside of fibre.
Wang et al (2004) reported a nylon filament yarn with the double
moisture uptake of conventional yarn in medical textile cardio vascular graft
made of woven and knitted structure are used to replace arteries in by-pass
surgery. Machine embroidered implants are used for connecting nerves during
reconstruction shoulder surgery. Wound healing dressings are another area of
developments in medical fields opening up new unexplored characteristics of
nano fibres.
25
Dyeing is another important area where nanotechnology finds
applications. Polypropylene is known to be a fibre most difficult to dye
because of absence of dye fixation sites. In polypropylene fibre dye site can
be created with nano die particles with modified quaternary ammonium salt.
The dyeing cost is also reduced .Since nano-particles have large surface area
they can be used as solvent for non-ionic, anionic, cationic dyes by using
nano clay and modified nano-clays. Such sorbent can be added to the polymer
matrix in polypropylene to improve the dyeability. Sparkle news (2006)
reported that nano clay textile composition have good dyeability, colour
fastness and less cost in dyeing and less waste water treatment problems. Also
report that they result in additional functional property improvement like
strength, UV absorbance and fire resistance. Mechanical blending of nano
clays can be added to the polypropylene matrix by using heat, in melting or
dissolving process
2.4.5 Nano Finishing Process
Perhaps the finishing of textiles is the most interesting area of
application for Russel (2002) reported the first work on nanotechnology in
textile called nano-tex. The exploitation of nanotechnology in textile finishing
was later taken up by an increased number of companies. Among the
techniques used to apply nano-particles onto textiles, coating is employed
majoritly. Cramer et al (2003) reported that the composition of coating
usually composed of nano-particles, a surfactant, ingredients and a carrier
medium for applying coating onto fabrics. Several methods like spraying,
transfer printing brushing, washing, rinsing and padding may be employed
Yen et al (2003) report that padding is the most widely used one.
A Padder is employed to apply nano-particles onto the fabric followed
by drying and curing. The functional properties that can be imparted to textile
by this method may include water repellences, soil resistance, wrinkle
26
resistance, anti-bacterial, anti-static, UV protection, flame retardation and
improvement of dyeability. It may also be used to impart new functional
properties like energy storage and communication.
Nano tex R was another commercial textile finish developed. Schulte
(2005) reported on the application of TiO2 nano-particles for textile finishing.
Nano-care R is reported to impart a new carefree wrinkle resistance finish
with minimum stains offering excellent liquid repellency and wrinkle
resistance. Another commercial finish nano-dry R was able to transport
perpiration away from the body and drying it quickly. Nano-fresh R is Known
to capture body odour giving the wearer an odurs free feeling. Schulenburg
et al (2008) reported about nano-pel R which makes the fabric breathable at
the same time being liquid and stain repellent.
2.4.6 Principles of Certain Commercial Types of Nano Textile Finishes
Nano-pel, a water and oil repellent treatment, can be applied to cotton,
wool, polyester, nylon, rayon and blended textiles. Nano-care, a wrinkle
resistant and oil repellency finish, is for pure cotton fabrics. Lennox Kerr
et al (2003) reported that both the above finishes have set a benchmark in
water and stain repellent performance of textiles.
Monomers containing methacrylate and per- fluoroalkyl group
producing a copolymer which exhibits water and oil repellency, where as a
fluorine- free monomer gives improved adhesivenes to fibre. Similarly Sahin
(1996) reported a monomer capable of improving durability through self-
crosslinking or reaction with reactive groups. According to Sello et al (1984)
these copolymer when treated with textiles liberate formaldehyde which is
detrimental to environmental safety.
27
The basic principles of nano-whiskers, according to Jain (2004),
consists of oligomeric or polymeric side branches attached to a flexible spine.
The branches stand outward from the surface to give a protection against
water or oil particles. Another variant of such a finish which does not produce
formaldehyde is called Nano-Tex Banks et al (1994) reports that the
formulation is capable of providing formaldehyde- free wrinkle resistance and
water and oil repellence when combined with a formaldehyde free resin such
as dimethyl urea glyoxal (DMUG) or butane tetra carboxylic acid (BTCA).
Kathirvelu et al (2006) reported the commercial application called
Nano-Dry a three – dimensional molecular network is created surrounding a
fibre , that is nano-net architecture made with nano fibres. Synthetic fibres
like Nylon and Polyester exhibit hydrophobic nature.
In Nano- dry treatment, a hydrophilic network of nano net of durable
absorbent nano fibre is created on the hydrophobic substrate fibre of synthetic
fibre. This leads to a durable hydrophilic treatment. However the other
properties of synthetic material such as strength, colour fastness and hand
value are not affected. Jain (2004) reported that the Nano-Dry treatment
imparts durability by the combination of covalent attachment to the fibre
surface and the use of nano molecules. Approximately the percentage of
solids loading onto the fabric surface is 0.1-0.15% by weight of the fabrics.
The problem of build-up of static charges occurs in synthetic fibres
even at high RH levels. Nano-Tex is a treatment developed to impart the
positive qualities of cotton and synthetic.
28
Figure 2.5 The schematic representation of the 3-D molecular nano-net
of Nano-Dry
This finish is aiming to create a permanently attached carbohydrate
sheath around each synthetic fibre of the web. This gives the dual advantages
of utilizing the most desirable characteristics of both synthetic core and of the
natural sheath.
Figure 2.6 Schematic representation of a Nano-Touch treated fibre
Absorbent net of
Nano- material Manmade material
Manmade fibre
Nano-Touch sheath
29
In another development a permanent attachment of the carbohydrate
sheath is created around synthetic fibre, commercially known as Nano-
Touch. It consists of Nano- Wrap artchitecture which is hydrophilic in nature
with increased moisture regain. It also exhibits a durable antistatic nature,
according to (Jain 2004).
2.5 THE BASIC OF PHOTO-CATALYSIS
Fujishima et al (1972) describe photo-catalysis as a reaction which uses
light to activate a substance which modifies the rate of a chemical reaction
without being involved itself.
Figure 2.7 Schematic diagram of operation of a photo-chemically
excited TiO2 particle
Substances which promote reactions in the presence of light are
generally photo catalists. Deshpande et al (2006) reported that Zinc oxide,
Titanium di oxide, Zincsulphide, Tungsten oxide, Strontium titanate and
Hematite are photo catalysis materials. In a heterogenous photo-catalytic
system, semiconductor particles which are in close contact with a liquid or
gaseous reaction medium, when exposed to light gets to excited state and
initate subsequent reaction like redox and molecular transformations. TiO2 is
one such a material which exists in three crystalline forms; rutile, anatase and
30
brookite. Fujishma et al (1972) reported the photocatalystic splitting of water
on TiO2 electrodes.
Figure 2.8 Schematic diagrams showing the actions of a photo-catalyst
(NanoTiO2 -top) and the resultant benefits (bottom)
Teichner (2008) reported that TiO2 is a semi conductive material that
can be chemically activated by light, even though it absorbs only 5% of the
solar light reaching the earth. TiO2 produces pairs of electrons and holes when
UV radiation is absorbed by it. The electrons of valence band become excited
Photocatalyst
TiO2
CO
H2O
UV Lamp
Ultraviolet Ray
Organic Compound
+ H2O + O2
TiO2
Photocatalytic Reaction
VOC
and
Odour Bacteria Germs Fungus L
igh
t
Lig
ht
Odour
removal
Self-Cleaning
action
Air
Purification
Sterilization
31
when light falls on it with the excess energy the electron is moved to the
conduction band. Therefore thus the negative electron (e-) and the positive
holes (h+) pair is created. The energy difference between the valance and
conduction bands is called the Band Gap. The wave length of the light
necessary for photo excitation is given by the formula;
1240 (planeks constant, h) / 3.2 eV (band gap energy) = 388Am
The positive hole reacts with water molecule to form hydrogen gas and
hydroxyl radical. The negative electron reacts with oxygen molecule to form
oxide union. This reaction is fast at room temperature and atmospheric
pressure.
2.5.1 Nano TiO2 in Textiles
Ever since the introduction of nano-particles in textiles, efforts were on to
produce finished fabrics with multiple functional performances. Lee et al
(2003) reported the use of nano silver particles for imparting antibacterial
properties. Yadav et al (2006) published the research work on the use of ZnO
nano-particles for anti-bacterial and uv blocking properties. ZnO& TiO2 have
the advantages of being non-toxic and chemically stable. Colemana et al
(2005) reported that TiO2 has strong Oxidising power of its holes, high photo
stability and redox selectivity. Its commercial availability and ease of lab
preparation makes it more advantageous. It has three types of crystal structure
Viz, anatase, rutile and brookite. Anatase structure of TiO2 nano-particles are
the most active in photo catalysis. TiO2 nano-particles are generally produced
by using three methods Viz. wet synthesis, dry synthesis and milling. The
Bottom-Up approach is employed in both wet and dry synthesis where as the
Top-Down approach is used in the milling method (Kathirvelu et al 2008c).
Sol-gel and precipitation methods are known as wet approaches. Dry
32
synthesis includes combustion, furnace and plasma synthesis according to
(Daoud et al 2004).
However the matter of great difficulty is achieving the uniformity of
size distribution of the nano-particles in all these methods. To less extent the
degree of agglomeration is another factor of concern. Certain degree of spread
in nano particle size is inevitable. The process parameter employed in
synthesis has influence over the size distribution. The particle size distribution
uniformity can be improved by adopting additional separation steps. However
the drawback in this case is that the process yield parcentage is affected.
Burniston et al (2004) revealed that nano-particles agglomerate together even
at lower temperature than their bulk size counter parts. High energy inputs or
temperature during synthesis may lead to more agglomeration by coaleseing
together of particles.
The band gap value is 3.0 eV for the rutile type TiO2 and 3.2 eV for
anatase type. Both of this structure absorbs only UV rays. Daoud et al (2004)
reported that the rutile type can absorb rays that are closer to the visible light
rays. According to Bozzi et al (2005), the anatase type of TiO2 exhibits higher
photo catalytic activity than the rutile type.
2.6 SYNTHESIS OF TiO2 NANOPARTICLES
Synthesis of Nano-particles in general involves the manipulation of
inorganic, organic and biological systems. Many new methods and devices
are being constantly brought into the field by various researchers. Over the
past decade, nano-particles have evolved as a separate class of materials.
Surprisingly various science disciplines have come together in the making and
synthesis of nano-particles. Nano-particles of ZnO in one end, find
application in hygiene products like diapers and on the other end used to alter
the characteristics of solid rocket propellents by addition.
33
The techniques of synthesis of nano-particles are divided into three
types Viz, UV Vapour-Phase, Solution Precipitation and the Solid-state
process. Sometimes two or more of these types are combined for synthesis.
During the early stages of development the Vapour-phase method was in use.
Presently the solid state processer is most widely used for micron sized
particles. According to the published literature, Ferro corporation, one of the
leading nano powder manufacturer, uses the solid state synthesis method.
Hundreds of form of lithium cobalt oxide nano powder is being produced by
using solid- state synthesis.
Lee et al (2006) reported that TiO2 nano-particles should fulfil a wide
range of requirements such as particle size, size distribution, morphology,
crystallinity and phase etc. so as to become useful and suitable for application.
Manufacture or synthesis of TiO2 nano-particles with well defined physical
and chemical properties has been a challenging task. Many researches have
published this works related to a variety of methods used for synthesis of
TiO2 nano-particles. Trunga et al (2004) reported the sol-gel process, Wu
et al (2006) about hydrothermal method and Kim etal (2003) reported on
solvothermal and emulsion precpitation methods.
According to Lee et al (2003) photo assisted sol-gel method was yet
another variant of the sol-gel technique. The other newer techniques reported
in the literature include oxidation of titanium tetrachloride in a modified
diffusion flame reactor, crystallization in reverse micelles or in the
supercritical CO2 and polymer templating method. Nano-particle synthesized
through different methods exhibit distinct physiochemical properties. Certain
wet chemical methods have been developed to synthesis powder with
spherical shape and uniform size. Arami et al (2007) reported that sol-gel
process, emulsion and pyrolysis have been used to prepare mono dispersed
34
spherical TiO2 nano-particle. It is important that the nano-particle exhibits
crystallinity and narrower size distribution. Sol-gel synthesized TiO2 nano-
particle are amorphous and require further calcinations steps to improve
crystallization such heat treatment lead to particle agglomentation and change
in size, sol-gel method for TiO2 nano-particle has another disadvantage of the
use of costly organic solvents. Similarly methods like forced hydrolysis and
homogeneous precipitation have the disadvantages of very low reaction time
due to very long concentration of the reacting formulations. Anatase TiO2
nano-particle can be synthesized hydrothermal methods using amorphous
TiO2. Chang et al (1995) reported a technique by using TiCl4. Deshpande
et al (2006) reported a method by using TiOCl2 aqueous solution Riv et al
(2006) reported sol-gel method using titanium alkoxides. Fang et al (2003)
reported hydrothermal methods amorphous TiO2, TiCl4 or TiOCl2 or TiO2
aqueous solution and sol-gel method using titanium alkoxides.
In order to regulate the physiochemical properties of TiO2 nano-
particle, organic modifiers are used in synthesis. Such modifiers include
hydroxyl group containing, carboxylate group and amine group containing
organics. As discussed earlier a variety of methodologies for synthesizing
TiO2 nano-particle have been invertigated by researchers. The commonly
used starting materials (Precusors) in the above methods are Titanium Iso-
propoxide, titanium tetra chloride, titanium (IV) sulphate and amorphous
titanium di oxide.
Another important technical aspect in TiO2 nano-particle is the
arrangements of nano-particle into well define structures or assemblies or
porous aggregates. Such nano structure exhibits unique properties like pore
size, thermal stability and their reproducibility. Yang et al (2006) reported
35
that the interaction between the primary building block is controlled in such a
way that at ambient conditions the nanostructure with useful properties is
formed. Sumio (2005) reported a large no .of such nano structure, their
synthesis and their unique properties.
2.6.1 Sol-Gel Method for Synthesis Of Tio2 Nano-Particles
One of the most widely used and successful technique for preparing
TiO2 nano-particle is sol-gel technique. By careful control of the process
parameter and the chemical structure of the starting precursor, the nano-
particle properties can be altered. Bessekhouad et al (2003) reported the sol-
gel based method by hydrolysis and polycondensation of titanium alkoxide ,
Ti(OR) n to form Oxopolymers, which are then transformed into an oxide
network. Ristic et al (2005) reported that the microstructure of TiO2 nano-
particle (anatase & rutile) decide its photo catalytic activity.
Dhage et al (2004) reported a method consisting of using titanium (iv)
n-butoxide (Tico-bu) 4) in isoprophl alcohol (i-PrOH) Acetylaceton (acac)
was used as a chemical addition to moderate the reaction speed. Deionised
water was used for hydrolysis in solution with i-PrOH under mechanical
stirring. White precipitate of titanium oxy hydroxide was obtained and
washed with water for several times. The molar ratio of these reactants
Ti(O-BU)4: H2O: I- Prop. OH: acac=1:100: 2: 0.01. Then HNO3 was added to
the final solution and refluxing at 850C for 8 hrs was found to influence the
size of TiO2 nano-particle. Then the sol was gelled by drying at 1000C for
3hours and then calcined in vaccum (400-7000C) to give TiO2 nano-particle.
36
2.6.2 Flame Spray Pyrolysis
Processes that are yielding high production rate, continuous production
and relatively low cost like flame spray pyrolysis are of more commercial
importance. This method yields fine, pure and single-phase particles. Chang
et al (2007) reported the properties like phase composition and particle sizes
of nano-particle obtained by flame spray pyrolysis. The process is basically
evaporating through pyrolysis the liquid droplet of precursor to obtain TiO2
nano-particle. The precursor solution was obtained by dissolving titanium
tetra isopropoxide (TTIP) in solvent. These prepared nano-particle exhibit
high purity and clear crystallinity.
The experimental set up consists of an ultrasonic atomizer, a diffusion
flame burner and a thermo phoretic sampler. A liquid droplet of precursor was
generated by ultrasonic atomizer, at 1.7MHz Frequency. The argon gas burner
evaporates solvent and precipitates the nano-particle. Hydrogen gas was used
as a fuel while oxygen and air were used as oxidants. Droplets of precursor
were supplied to the flame zone through the central tube. The argon gas flow
rate was 2lit/min. in the central tube. Through the second tube the gas was
passed at 1lit/min to keep the straight path way of the liquid droplets in the
flame region. Through the outer tube, hydrogen, oxygen and air were passed
at certain flow rate. A cold glass tube cooled at 12oC by cooling water flow is
used to collect nano-particle finally. The concentration of precursor liquid and
flame temperature influence particle size.
A new method called one-pot synthesis used trichloroethylene as
reaction medium. Trunga et al (2004) discussed that the method has
advantage over multicomponent solution system when TiO2 is used as a
reinforcement for polymer dissolved in trichloroethylene.
37
Trichloroethylene 60ml was mixed with 2ml of Titanium Isopropoxide
with water and stirred rigorously at room temperature. Hydrolysis and
condensation reaction occured. The prepared powder was washed by ethanol
and acetone for several times, dried at 1000C in vaccum for 3days and heated
at 4000C for 10 hrs in air.
Kim et al (2003) reported a method by hydrolysis of TEOT (tetra
ethylor thotitanate) in two stage mixing process. They attempted further to
make a continuous process by small reactor which controlled the rate of
nucleation. An ageing tube was employed for controlling the growth of nano-
particle. The hydrolysis rate is controlled by water vapourization and feed rate
of TEOT solution. The apparatus has a small reactor for two feed reagents
water and TEOT, a syringe pump a heating tape for vapouring H2O and a long
silicon tube for ageing the particles. The small reactor is continuously fed
with the two feeds.
TiO2 Primary nuclear is generated by the polymerization of the
hydrolysis product. According to Hong et al (2003), a part of them is grown to
fine nano-particle while passing through the ageing tube. In this part of
literature survey a review has been made over the various methods developed
for synthesis of nano-particles. Each method has it own merits and demerits.
The choice of method and precursor depend on the end use
requirement of nano-particle. According to Bessekhouad et al (2003) in
future, these methods could be suitably changed to meet the specific
functional needs of nano-particle.
38
2.7 SYNTHESIS OF ZnO NANO-PARTICLES
Zinc oxide is a semiconductor of n type and with direct band gap. Chen
et al (2003) reported various techniques used to synthesize ZnO nano-particle
both chemical and physical methods. The chemical method include thermal
hydrolysis technique, hydrothermal processing and sol-gel method. Physical
methods are spray pyrolysis, vapour condensation and thermo chemical
decomposition of precursor. Presently the vapour phase and sol-gel methods
are widely used. In case of vapour phase method the drawback is that the
powder is in the agglomerate form due to difficulty in control of reaction
condition. Also it is slow process and more energy consuming.
The sol-gel method though producing uniform ZnO nano-particles,
requires stricter control on reaction condition. Due to high costs it is not
commercialized but suitable for research only. Discussed the problem
associated with the poor yield efficiency of ZnO nano-particles and the
difficulity associated with particle size control. New and other methods were
tried in recent years like thermal decomposition, supercritical precipitation
and colloidal synthesis. Ma et al (2008) discussed a method by thermolysis of
Zinc oxide propionate as a precursor and trioctyl phosphine oxide at a high
temp of 160oc. Guo et al (2000) reported various methods of ZnO nano-
particle synthesis under the broad section namely physical , thermal and
chemical methods. Um et al (2007) reported a method called the levitational
gas condensation (LGC) method. Um et al (2007) reported a modified method
of LGC and phase evolution.
The main parts of such an apparatus are high frequency induction
generator, levitation and evaporation chamber and oxygen concentration
39
control unit. Wang et al (2004) reported a new combustion method, in which
the mixture of Zinc nitrate and fuel powder is ignited to form ceramic oxides.
Rapid heat conduction takes place resulting in quick reduction of temperature.
Fine ZnO nano-particles can be produced without agglomeration. Glycine was
used as fuel for this purpose due its inexpensiveness and its heat of
combustion being more on the negative side when compared with urea or
citric acid.
In another study reported, Zinc nitrate was used as the precursor. It has
the dual function of being a precursor and an oxidant. Zinc Nitrate of
analytical grade and glycine were mixed at a desired molar ratio. The mixture
became like transparent slurry matter. Vigourous stirring was carried out to
make a homogenous mixture. Then it was heated to 100oC using a hotplate to
dry it. The dired up precursor is now having combustibility. A mini gas
burner was employed for combustion of precursor. A large volume of gases
evolved during combustion. This method has the advantages of simplicity,
less cost and speed.
Another method was reported by Ko et al (2006) for synthesis of ZnO
nano powder. This rapid method is based on DC thermal plasma synthesis
yielding a high production rate.
By changing the plasma gas combination and flow rate it was possible
to control the shape of ZnO nano-particle .The new type of DC plasma reactor
operated at 70 Kw and atmospheric pressure. Zinc powder with impurities
less than 50 ppm was used in this process. The Zinc powders were passed
through nitrogen gas and made to fall on plasma flame. Then it was
vapourised, oxidized and finally quenched.
40
2.8 STRUCTURAL CHARACTERIZATION OF NANO -PARTICLES
According to Wang (2000), there are several methods used for
characterization of the nano-particle.
X-ray powder Diffraction method (XRD) for studying the crystal
structure of nano particles, Fourier Transform Infrared Spectroscopy (FTIR)
for identifying the name of chemical groups and bonds, Transmission
Electron Microscope (TEM) for identifying the nano-particle diameter,
Particle Size Analyser (PSA) for measuring the range of size of nano-particles
and Scanning Electron Microscopy (SEM) for capturing the images capturing
the images containing topological information of fabrics.
2.8.1 X-Ray Powder Diffraction Method (XRD)
It is a very useful experimental technique to study the crystal structure
of solids. Structural parameters like lattice constant and geometry, orientation
of single crystals, preferred orientation of polycrystals, defects and stresses
can be studied using this technique (Jenkins et al 1996). Identification of
unknown materials are also a possibility by using this technique. In this
method a collimated x-ray beam is made incident on a specimen and
diffracted by the crystalline phases is the specimen. The wave length ranges
from 0.7 to 2Å. According to Bragg’s law.
2 sinA d θ= (2.1)
where;
d = the spacing between atomic planes in the crystalline phase and
A = the X-ray wavelength.
41
The intensity of the diffracted x ray is measured. This is a function of
the diffraction angle 2θ and the specimen’s orientation.
According to Jenkins and Snydes the specimen’s crystalline phases and
its structural properties are identified by using the diffraction pattern.
According to scherer’s equation the crystalline domain diameter D, is given
by the following equation;
0.89
cosD
W
λ
θ
×=∆ ×
(2.2)
where;
λ = the wavelength of the incident X-ray beam, (1.54 A˚ for the Cu Kα)
θ = the Bragg’s diffraction angle
∆W = the full width of the X-ray pattern line at half peak-height in radians