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CHAPTER 1
INTRODUCTION
1.1 Organic-Inorganic composites
Differing from the other bulk materials, the nanostructured materials
have evoked tremendous interest in regard to their attractive properties. This has
been mainly attributed to their size dependent effects which are related to the large
fraction of atoms at the surface and quantum size effects [1-4]. Recently, much
speculation has been shown on the fabrication and control of one-dimensional
materials such as nanowires, nanofibers, and nanorods owing to their abeyant
appositeness in electronics, optoelectronics, energy and other related fields.
According to the National Science Foundation (NSF), nanofibers have been
described as structures having a one dimension of 100 nanometers (nm) or less,
but generally considered having a diameter of less than one micron.
Actively, Organic-Inorganic composites system has reached great
heights of interest satisfying a broad range of applicability in several fields. In
many of the current applications, the unique properties of nanoparticle have been
delivered as fillers of composites or as coating materials. This polymer-
nanoparticle composite has stipulated the flexibility, stability and the
conformational ability for complicated structures while maintaining the
nanoparticle traits. The organic-inorganic composites not only possess the
characteristics of stability, excellent luminescence, electrical or magnetic
properties of inorganic particles but also the convenience to tailor-make the
structure, the facility to process it and also deplete the cost of the organic
molecules.
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Predicted to possess unique electronic and optical properties, these
composite nanofibers have been tuned by ways of optimizing the doping level of
inorganic materials. These fibers carry a whole package of applications in chemical
and biological sensors, light emitting diodes, rechargeable batteries nanoelectronic
devices, electromagnetic shielding and wearable electronics. Similarly, nanofibers
derived from ceramic materials such as zinc oxide (ZnO), titanium oxide and silicon
carbide possess optical characteristics (luminescence) that has been made use of in
light and field emitters.
Applications which make use of sub-micrometer diameter fibers have
many appealing benefits namely,
High surface area: Sensors, Protective clothing, drug delivery
Reduced pore size: Filters, Scaffolds for tissue engineering,
Adsorbents
Mechanical Strength: Reinforced composites
Higher packing densities: Energy storage systems
Small dimension: Micro and nano fabrications systems
Novel magnetic, electric and optical properties
ZnO has been considered a versatile material due to its direct band gap
(3.37eV), a large exciton binding-energy (60 MeV) at room temperature and a high
melting temperature (2248k). Due to its high exciton binding-energy, the excitons
have turned thermally stable at room temperature and thus ZnO has availed
numerous appliances in optoelectronic devices such as ultra-violet photo detectors,
photovoltaic devices etc. Apart from this, ZnO has been considered an eco-friendly
subsistence which makes it suitable for antimicrobial applications. Importantly, it
has also been listed as an eminent entity under GRAS (Generally Regarded As Safe)
by the US Food and Drug Administration and hence has been extensively used in
the formulation of personal and health care products. Most modern preparation
techniques have achieved the growth of morphology-controlled one-dimensional
ZnO structures. Unlike many of the materials with which it competes, ZnO has its
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own attributes of being inexpensive, relatively abundant, chemically stable, easy to
prepare and non-toxic [5].
Many methods have been developed to prepare the organic-inorganic
composites. One of the earliest and easiest methods has been the direct mixing of
the nanoparticles onto the polymer matrix. In this method, the nanoparticles would
be prepared before their incorporation into the polymer matrix. The expediencies of
this method have been facile technique, morphology as well as easy control over the
size of the nanoparticles. However, the nanoparticles have shown the liability to
aggregate due to their large specific surface energy. This has caused unavoidable
circumstances where the nanoparticles inhomogeneously distribute in the polymer
matrix, resulting in a loss of their function.
The second method to be discussed has been the layer-by-layer assembly
technique developed by Decher [6]. It has been proven to be one of the most
promising au courant methods of thin film deposition. Recently, this method has
also been successfully applied to thin films of nanoparticles and other inorganic
materials. Its simplicity and universality open a wide range of possible usances for
this technique, both in fundamental research and in advanced industrial
applications. The composites prepared using this method have found innumerable
applications in electroluminescence. However, difficulty has been seen when
applied to bulk materials.
Additionally, methods of electrochemistry, sol-gel and ultrasonic irradiation
have been used to prepare organic-inorganic composites. Among them, an
important and generally adopted technique has been the in-situ approach. The size
and distribution of the nanoparticles in the polymer matrix have been tuned by
designing and tailoring the structure of polymer matrix. In terms of their
composition, architecture, and arrangement of the organic and inorganic species, 1D
organic-inorganic hybrid nanomaterials exist mainly in the following groups (Figure
1).
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Figure 1.1: Illustration of 1D organic–inorganic hybrid nanostructures.
The most elemental and primitive structure has been the homogeneous
organic-inorganic hybrid nanomaterial (Figure 1A), in which the inorganic phase
has been evenly dissolved in the organic part. A nanosized agglomeration of the
inorganic phase has been undetectable till now. Other types of 1D hybrid
nanomaterials exhibit the phase separation of the inorganic or organic part of the
nanoscale in their structures. Depending on the distribution of the nanosized
inorganic or organic domains, core-shell-type (Figure 1B), scattered type (Figure
1C, scattered phase in the entirety, core, or shell), and di-/ tri- multi block structured
1D hybrids (Figure 1D) have been reported. The spatial relationship of the inorganic
and organic phases not only determine the intrinsic properties and functions of the
corresponding hybrid, but also brings up a direct relation different synthetic
strategies.
1.2 Methods to prepare nanofibers
Many methods have so far been taken up for the preparation of fibers
namely:
i) Mechanical drawing [7]
ii) Self Assembly [8-9]
iii) Template synthesis [10]
iv) Phase separation [11]
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v) Electrospinning (ES)
In this research work, we have summarized the preparation of
ZnO/Polymer composites by combining in situ sol-gel and the method of
electrospinning to obtain different functional one-dimension composites for
application in the UV sensor, superhydrophobic and antibacterial field. Here, the
polymer matrix has allowed the nanometer scaled ZnO to homogeneously disperse
in the composite thus improving its stability, dispersion and mechanical strength. In
addition, the surface of the ZnO nanoparticles could be modified by the polymeric
matrix via the interactions between the two components. The most striking
character of our method has been the excellence in the compatibility between the
nanoparticles and the polymers obtained via modification of the surface of the
nanoparticles ensuring the homogeneous distribution of nanoparticles in the
polymer matrix. The existence of the polymer network not only serves as a template
medium but also stabilizes the nanoparticles. The structure of such an organic-
inorganic composite network has stabilized the nanoparticles in terms of a long
term, which has proved a landmark in the protection of their function.
1.2.1 Self assembly
Self-assembly is a process in which individual, pre-existing components
organize themselves through weak, non-covalent interactions (H-bonding,
electrostatic interactions) forces into desired patterns and structures. It is known as a
‘bottom-up’ method and it yields fibers with small diameters (less than 100 nm
thick and up to few micrometer lengths) and it offers novel properties and
functionalities, which cannot be achieved by conventional organic synthesis.
Figure 1.2 shows how small molecules are arranged in a concentric manner,
bonds form among the concentrically arranged molecules, and then nanofibers are
formed upon extension of these molecules normal to the plane. The main
disadvantage of the method is that it is a complex, long, and extremely elaborate
technique with low productivity.
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Figure 1.2: Schematic presentation of self-assembled nanofibers
1.2.2 Dry Spinning
Dry spinning is a method used to form polymeric fibers from solution.
However, instead of precipitating the polymer by dilution or chemical reaction as in
wet spinning, solidification is achieved by evaporating the solvent in a stream of air
or inert gas. In this method, the polymer is dissolved in a volatile solvent and the
solution is pumped through a spinneret composed of numerous holes. As the fibers
exit the spinneret, air is used to evaporate the solvent so that the fibers solidify and
can be collected on a take up wheel. Stretching of the fibers provides for orientation
of the polymer chains along the fiber axis. Dry spun fibers typically have lower void
content than wet spun fibers.
1.2.3 Drawing
Drawing is a process similar to dry spinning in fiber industry, which can
make one-by-one very long single nanofibers. However, only a viscoelastic material
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that can undergo strong deformations while being cohesive enough to support the
stresses developed during pulling, can be made into nanofibers through drawing. It
requires a minimum amount of equipments, and is a discontinuous process. As
shown in the Figure 1.3, a micropipette is dipped into a droplet near the solution-
solid surface contact line via a micromanipulator.
Droplet (millimetric size)
Micropipette is brougThe contact
Micropipedr
ht towardline
tte is touched the oplet surface
Nanofibeprod
Micropipette is withdrawn to Produce nanofibers
rbeing uced
Figure 1.3: Illustration of drawing method for preparing the fibers
Then the micropipette is withdrawn from the liquid at a certain speed,
yielding nanofibers. These steps are repeated many times on each droplet. The
solution viscosity, however, increases with solvent evaporation and some fiber
breaking occurs due to instabilities that occur during the process. Drawing process
is disadvantageous since the fiber size is dependent on the orifice size of the
extrusion mould; it is difficult to obtain fibers diameters less than 100 nm.
1.2.4 Template Synthesis
The template synthesis, as the name suggests, uses a nanoporous membrane
as a template to make nanofibers of solid (fibril) or hollow (tube) shape. The most
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important feature of this method lies in the fact that, nanometer tubes and fibrils of
different materials such as electronically conducting polymers, metals,
semiconductors, and carbons can be fabricated. On the other hand, the method
cannot make long continuous nanofibers. Extrusion of the polymer solution through
the porous membrane is achieved by water pressure. As soon as the polymer comes
into contact with the solidifying solution, fibers with diameter dependent on the
template pore size are produced as shown in Figure 1.4. The resultant fiber
diameters vary from a few to hundreds nanometers. On the other hand, this method
is limited in that nanofibers only a few micrometers long are obtained.
Water
Polymer Solu
Al2O3 Memb
Solidification Solution
tion
rane
Pressurized Water
Extruded fibers
STAGE 2
STAGE 1
Figure 1.4: Schematic representation of the template based method for preparation of fibers
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1.2.5 Phase Separation
The phase separation method of preparing fibers consists of dissolution,
gelation, and extraction using a different solvent, freezing, and drying, resulting in
nanoscale porous foam. The process takes relatively long periods of time to transfer
the solid polymer into the nano-porous foam. In this process, the polymer is
dissolved in an appropriate solvent at the desired concentration. The solution is then
stirred at a certain temperature for a period of time until a homogeneous solution is
obtained. This is followed by transferring the solution into a refrigerator set to the
gelation temperature of the polymer. The resultant gel is immersed in water several
times to allow solvent exchange. Finally, the gel is removed from water, transferred
to a freezer (-70oC), and then the frozen gel is lyophilized. A simple representation
of this process is given in figure 1.5, which shows how nanoporous polymer foam is
produced.
Figure 1.5: Schematic representation of the phase separation process
In this process, phase separation occurs due to physical incompatibility and
yields nanofibers; however, a long period is needed to transfer a solid polymer into
nano-porous foam. Fiber dimensions vary from 50 to 500 nm with a length of a few
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micrometers. Therefore, the limitation of this method is that no long continuous
fibers are produced and only the polymers that have gelation capability can be used
to produce the nanofibrous structure.
1.2.6 Electrospinning
Electrospinning is a process that creates nanofibers through an electrically
charged jet of polymer solution which is based on the uniaxial stretching of a
viscoelastic solution. The diameter of the fibers obtained through this method as
low in the range of 10µm to 10nm, which are typically 1 to 3 orders less than that
obtained by the conventional spinning process. To understand and appreciate the
process that enables the formation of various nanofiber assemblies, the principles of
electrospinning and the different parameters that affect the process have to be
considered. Unlike conventional fiber spinning methods like dry-spinning and melt-
spinning, electrospinning makes use of electrostatic forces to stretch the solution as
it solidifies. Similar to conventional fiber spinning methods, the drawing of the
solution to form the fiber will continue as long as there is enough solution to feed
the electrospinning jet. Thus without any disruption to the electrospinning jet the
formation of the fiber will be continuous.
Electrospun fibers show very high surface area to volume ratio, which makes
these fibers suitable for variety of applications such as sensors, adsorbents, filters
and energy storage materials. The main advantage of electrospinning method is,
relatively low cost and simplicity compared to that of most bottom-up methods. The
resulting nanofiber samples are often uniform in diameter and do not require
expensive further purification. Unlike submicron-diameter structures such as
whiskers, nanorods, carbon nanotubes, and nanowires, the electrospun nanofibers
are continuous. However, the use of conducting or semiconducting nanofibers for
electronic, opto-electronic, photonic or sensor devices is relatively new.
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Figure 1.6: Schematic diagram of the various electrospinning apparatus to obtain the different morphology of fibers.
1.3 History of the electrospinning
The term “Electrospinning” derived from “electrostatic spinning”, was used
relatively recently (around 1994), but its fundamental idea dates back to more than
60 years. During 1934 to 1944, Formhals obtained a series of patents, describing an
experimental setup for the production of polymer filaments using an electrostatic
force [12-14]. A polymer solution, such as cellulose acetate was introduced into the
electric field. The polymer filaments were formed from the solution between two
electrodes bearing electrical charges of opposite polarity. One of the electrodes was
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placed into the solution and the other onto a collector. One ejected out of a metal
spinneret with a small hole, the charged solution jets evaporated to become fibers
which were collected on the collector. The potential differences in the fiber
characteristics depended on the properties of the spinning solution such as polymer
molecular weight and viscosity. When the distance between the spinneret and the
collecting device was short, spun fibers tended to stick to the collecting device as
well as to each other, due to incomplete solvent evaporation.
In 1952, Vonnegut and Neunauer were able to produce streams of highly
electrified uniform droplets of about 0.1mm in diameter. They invented a simple
apparatus for electrical atomization. A glass tube was drawn down to a capillary
having a diameter in the order of few tenths of millimeter. The tube was filled with
water or some other liquid and an electric wire connected with the source of
variable high voltage was introduced into the liquid [16].
In 1955, Drozin investigated the dispersion of a series of liquids into
aerosols under high electric potentials. He used a glass tube ending in a fine
capillary similar to the one employed by Vonnegut and Neubauer. He found that for
certain liquids and under proper condition, the liquid emerged from capillary as
highly dispersed aerosol consisting of droplets with relatively uniform in size. He
also captured different stages of dispersion [17].
In 1966, Simons patented an apparatus for the production of non-woven
fabrics of ultrathin and very light weight fibers forming different patterns using
electrical spinning [18]. The positive electrode was immersed into the polymer
solution and negative one was connected to a belt where the non-woven fabric was
collected. He found that fibers from low viscosity solutions tended to shorter and
finer whereas those from viscous solutions were relatively continuous.
In 1971, Baumgarten made an apparatus to electrospin acrylic fibers with
diameters in the range of 0.05-1.1µm. The spinning drop was suspended from
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stainless steel capillary tube and maintained constant in size by adjusting the feed
rate of an infusion pump [19]. A high voltage DC current was connected to the
capillary tube while the fibers were collected on a grounded metal screen. Since
1980’s and especially in the recent years, the Electrospinning process essentially
similar to that described by Baumgarten has regained more attention probably due
in part to surging interest in nanotechnology, as ultrafine fibers or fibrous structures
of various polymers with diameters down to submicron or nanometers can be easily
fabricated with this process [20-25].
Figure 1.7: The increasing number of publications on Electrospinning over the years is a clear indication of the importance of the area of research
Until 1993, this technique has been known as electrostatic spinning, are there
were only a few publications dealing with its use in the fabrication of thin fibers. In
early 1990s several research groups revived interest in this technique by
demonstrating the fabrication of thin fibers from broad range of organic polymers.
At this time the term Electrospinning is coined and is now widely used in literature.
This timely demonstration triggered a lot of experimental and theoretical studies
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related to Electrospinning. Number of publications in this field has been increasing
exponentially in the past few years, on account of remarkable simplicity, versatility
and potential uses of this technique. Many researchers began to explore the field.
Since then the number of publication in each year is increasing exponentially and
the figure 1.7 shows the exponential growth of the number of publications in this
area over the years.
1.4 Preparation of defect free fibers by controlling the process parameters
The morphology and diameter of the electrospun fibers are dependent on a
number of processing parameters that include: 1) the intrinsic properties of the
solution such as type of polymer, viscosity, etc., and 2) process parameters such as
applied electric field potential, flow rate, distance between the electrodes and
ambient parameters (temperature, humidity and air velocity in the chamber). The
presence of beads in electrospun fibers is a common problem [25-26]. Other shapes,
in particular ribbon like shapes with rectangular cross-section have also been
reported. A number of research articles discuss about the factors affecting the fiber
diameter. The major factors that affect the diameter of the electrospun fibers include
the concentration of the polymer, electrical conductivity of the polymer solution,
electric field strength and flow rate of the polymer solution etc.
Fridrikh et al. presented a simple analytical model for the forces that
determine jet diameter during electrospinning as a function of surface tension, flow
rate and electric current in the jet. Voltage can be considered the most essential
parameter in electrospinning, since it initiates the jetting and causes instabilities,
which stretch the jet [28].
Effects of the parameters of the electric field on electrospinning process and
forming fibers are, once again, diversified. An increase in voltage and, thus, in
electric field, was found mainly to decrease, but also increase the fibers diameter.
Similarly, an increase in distance (decrease in field) has found to both increase and
decrease the fibers diameter. These findings indicate that by appropriately varying
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one or more of the above parameters, nanofibers can be successfully electrospun
from a rich variety of materials that include polymers, biopolymers, DNA, protein,
composites, and ceramics and even relatively small macromolecules such as
phospholipids.
The fibers obtained in the Electrospinning processes are randomly oriented.
For several useful applications one need well aligned and long fibers. Several
research groups attempted to obtain well-aligned fibers as illustrated in figure 1.6
[29-44]. More recently Lin and coworkers developed near-field Electrospinning
process to deposit solid nanofibers in a direct, continuous and controllable manner.
Sundaray et al fabricated the well aligned electrospun fibers using a modified
rotating drum with sharp pin inside [40].
Intrigued by their potential application as scaffolds in cell biology and tissue
engineering, a large number of biodegradable that include poly(caprolactone),
poly(L-lactide) and poly(glycolide) have been directly electrospun into nanofibers
[45-48]. In addition to these synthetic organic polymers, natural biopolymers such
as DNA, silk fibroin, human or bovine fibrinogens, dextran, collagens and even
viruses have also been successfully used for Electrospinning. In general, the
condition controlling the Electrospinning using these biomacromolecules are
identical to those observed for the case of conventional synthetic polymers.
Lrsen et al. were the first to combine Electrospinning with sol-gel methods
to design vesicles and nanofibers made from inorganic oxides [49]. A variety of
functional components can be directly added to the solution for Electrospinning to
obtain nanofibers within diversified range of compositions and well defined
functionalities. To this end, incorporation of nanoparticles such as Zinc Oxide,
Carbon nanotube, silver and iron oxides have all been demonstrated. Various
ceramic meal oxides fibers were obtained by high temperature calcinations of the
precursor organic-inorganic composite fibers assembled by Electrospinning. It was
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generally observed that the calcinations temperature has a great influence on both
the crystalline phase and the surface morphology of the fibers.
These provide a wide range of properties such as strength, weight, elasticity,
porosity and charged surface areas. Moreover electrospinning also provides the
capacity to lace together a variety of nanoparticles or nanofillers types that can be
encapsulated into a nanofibers matrix. Functional micro/nano particles may be
dispersed in polymer solutions, which are then electrospun to form composites in
the form of continuous nanofibers and nanofibrous assemblies. All these endow
electrospinning with outstanding manufacturing capabilities but utilizing an easy
process and capable of excellent flexibility. Additionally, electrospinning seems to
be the only method that can be further developed for mass production of one-by-one
continuous nanofibers from various polymers. A number of processing techniques
such as drawing, template synthesis, phase separation and self-assembly have been
used to prepare polymer nanofibers in recent years. However these methods have
disadvantages such as: material limitation, they are time-consuming and they
require complicated processing systems. As far as electrospinning is concerned it is
not only a simple one-step top-down process for fabricating nanofibers, but also the
co-processing of polymer mixtures, chemical cross- linking can be carried out that
provide a variety of path-ways for controlling the chemical composition of the
nanofibers.
1.5 Characteristics of electrospun fibers
Electrospun nanofibers possess unique traits, such as: extraordinary high
surface area, coupled with remarkable porosity, excellent structural and mechanical
properties, flexibility, low basis weight, and cost effectiveness among others.
Another interesting aspect of using nanofibers has been its feasibility to modify not
only their morphology and their content but also the surface structure to carry
various functionalities. Nanofibers have been post-synthetically functionalized in
an. Furthermore, its feasibility to control secondary structures of nanofibers has
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been successful in preparing nanofibers with core/sheath structures, nanofibers with
hollow interiors and nanofibers with porous structures.
Economically, the electrospinning process has been relatively cheaper when
compared to that of the bottom-up nano-fiber fabricating methods. The resulting
nanofibers have been prone to be uniform, continuous and require no expensive
purification protocols. These nanofibers have been found relatively apparent to be
scaled up for productivity due to the top-down process and the design of multiple
jets for synchronous electrospinning. Moreover, the nanofibers have one dimension
on the microscopic scale but another dimension macroscopically. This unique
characteristic endows nano-fiber mats with appealing benefits possessed by
functional materials on the nanometer scale, gaining advantage over conventional
solid membrane in regard to ease in processing, ease of packaging and shipping.
These outstanding properties have shown up polymer nanofibers and composite
fibers as good candidates for several applications.
1.6 Modifications of electrospinning
Polymer fibers obtained by using conventional electrospinning apparatus
have been randomly oriented. Researchers have attempted diverse approaches to
achieve alignment in electrospun fibers. During a course of electrospinning,
production of fibers in a continuous and controllable manner has been reported.
With this technique, known as near-field electrospinning, the production of
nanofibers with desired pattern has been made possible. Recently, Atomic force
microscope (AFM) based voltage-assisted electrospinning technique has been
reported to achieve aligned fibers.
Single fiber of polyethylene oxide (PEO) polymer with nanometer scale
diameters have been formed by this method. Ceramic hollow nanofibers have been
developed by Li and Xia using coaxial electrospinning. They have been able to
control the wall thickness by acclimatizing the experimental parameters. They have
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also proposed the use of these hollow ceramic fibers in nano fluidic devices or
optical waveguides [50-52].
Figure 1.8: (a) Flat polymer nanofibers, (b) Beaded polymer nanofibers, (c) Non-porous smooth polymer nanofibers, (d) porous submicron polymer fibers, (e) Randomly oriented ceramic nanofibers and (f) Aligned ceramic nanofibers 1.7 Applications of Electrospinning
The simplicity of the fabrication scheme, the usage of commensurable
materials, as well as the unique and profiting features associated with electrospun
nanofibers, have together made this technique attractive in a number of applications
[53-54]. Major areas of research on nanofibers include biomedical applications,
energy storage such as solar cells and fuel cells, sensors, and filtration. The
potential applications of electrospun nanofibers have been summarized in figure
1.9.
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Figure1.9: Application of electrospun fibers in different field Modification of electrospinning 1.8 Motivation and Statement of the problem
In particular, one-dimensional structures such as nanofibers have been
found more attractive due to their large specific surface area, uniform diameter
distribution and well-defined charge carriers transporting path. In the recent times,
many researchers among the academic and industrial communities have shown keen
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interest towards the preparation of polymer nanofibers. Our interest has also been
on making composite fibers through electrospinning for varied applications. In
addition to a wide variety of polymer and composite nanofibers, inorganic
nanofibers such as ZnO have also been fabricated using this simple technique,
through calcination of polymer composite nanofibers containing inorganic
precursors. However, difficulty has been observed in obtaining direct ultra fine
fibers of inorganic materials having lengths in the order of millimeter as they tend
to break during formation pertaining to their thermal and other mechanical stresses.
Importance has been stressed on the understanding of the causes behind the
formation of defects in the fibers and the breakage of fibers during thermal
treatment encouraging development methods to control their formation. Here, in our
work, we have investigated the effect of thermal treatment on fiber morphology and
the possible mechanism behind such structural changes in fibers.
Dispersing nanoparticles in a polymer matrix has been one of the best
methods to stabilize the nanoparticles and to achieve properties that combine the
excellent thermal and mechanical properties of polymer matrix with the attractive
functional properties of the stable inorganic filler materials. Diverse approaches
such as spin coating, film casting etc have been employed to achieve this. However,
major difficulties associated with these methods are random distribution and
aggregation of nanoparticles in polymer matrix. In this work, we have focused on
the dispersion of Nanocrystalline ZnO in fibrous polymer matrix through two
different approaches for comparison namely the Insitu sol-gel method and the Ex-
situ method (Direct dispersion of ZnO nanoparticles in polymer solution). Detailed
studies of the fibrous membrane prepared through different routes have been
systematically studied and the results have been compared.
For application in ultraviolet photo detectors, requirement of high
quality ZnO nanostructures have been essential since the defects in the material
introduce additional transition states leading to unwanted emissions in the visible
region. In order to achieve this high quality, the importance of understanding the
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mechanism behind the defect-formation and means of eliminating them during the
preparation of ZnO nanomaterials has been compulsive. Also, conspicuousness in
maintaining low cost of synthesis and easy affordability has to be kept in mind.
There have not been many investigations in the scientific literature on the
fabrication of UV photo-detectors-based on polymer/ZnO fibrous composites.
There are several reports on the fabrication of UV detectors using different
ZnO structures such as particles, films, wires, etc. Still, extensive research needs to
be carried out on this prominent arena. In particular, improvements have been taken
up to enhance the quality of the ZnO nanostructures in order to achieve esteemed
performance and reliability. Solution processed optoelectronic devices have some
advantages in terms of ease of fabrication, low cost, etc. However, difficulty in
realizing the feasible UV photodetectors has been observed since complications
pertaining to the uniform dispersion between the two electrodes. Further, UV
detectors based on wet chemical synthesized semiconductors have shown slow
photo response because of the high density of defects.
Basically, photodiodes have been found to be light-sensitive devices
used to detect optical signals through electronic processes, and generally work
under photoconductive mode. The operation involves three steps: (1) Carrier
generation by absorption of the incident light photon of energy higher than the band
gap of the device (2) Carrier transport and (3) Current flow in the external circuit to
provide the output signal. Based on the requirement of the device and earlier
reports, we have noticed tremendous scope for improvement namely: (1) To
eliminate intrinsic defects in ZnO materials during fabrication (2) To enhance the
pathway for carrier transport in the active material (3) To improve the interaction of
ZnO nanostructures with electrodes.
Our research on fabrication of ZnO nanoparticles in fibrous polymer
matrix has proven the high optical quality of ZnO obtained, which has further
elevated the interest regarding the UV sensor applications of ZnO. Electrospinning
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process allows the direct fabrication of high quality ZnO nanofibers and their
composites without introducing the intrinsic defects. Compared with the device
based on ZnO nanowires, particles and films fabricated by other techniques, the UV
sensor based on electrospun fibers has been simple to fabricate.
The main advantages of the scheme proposed here been the ease of
preparation under controlled conditions, enhanced interfacial interaction between
organic and inorganic materials and an overall stabilizing effect during preparation.
Defined material structure i.e. controlling the morphology and particle size has led
to higher efficiency of exciton generation under UV irradiation and charge
separation. Therefore, it might be possible to enhance the UV sensitivity of the
device if we replace the Nano ZnO particles, films with fibers and their composites.
Increased surface area of the materials has shown to improve the performance of the
device. In this respect, nanofibers might fulfill the requirements because they have
high surface area to volume ratio, defined structure, and high aspect ratio.
Super hydrophobic surfaces, with a water contact angle (WCA) greater than
120°, have proved very useful in applications such as self cleaning, antisnow/fog
and contamination prevention. The wettability of the solid surface has turned out to
be a characteristic property of materials and has been strongly dependent on both
the surface energy and surface roughness. Conventionally, super-hydrophobic
surfaces have been produced mainly in two ways by creating a rough surface or by
modifying the surface with materials having low surface energy, such as fluorinated
or silicon compounds. Of late, a variety of techniques have been proposed for
constructing superhydrophobic materials, such as etching (chemical etching and
plasma etching), chemical/physical vapor deposition CVD/PVD), densely packed
aligned carbon, sol–gel processing, etc. These tend to modify surface topography
and enhance hydrophobicity by coating a hydrophobic thin layer or monolayer. A
large number of materials both inorganic (such as ZnO, CNTs, Fluorinated
materials, SiO2, etc.) and hydrophobic polymers have been used to prepare
superhydrophobic surfaces.
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In some cases, the reported approaches employ either expensive
materials or complicated procedures, thereby limiting the applications of
superhydrophobic surfaces. In most practical cases, it has not been possible to
modify the surface roughness without simultaneously affecting the chemical nature
of the surface. In addition, common acknowledgement has been made on the
difficulty in obtaining super hydrophobic surfaces from water soluble polymers.
Primarily, our objective has been to attempt a scheme to prepare superhydrophobic
surface using initially water soluble polymers with the addition of ZnO
nanoparticles by using electrospinning method. Here, the wettability of fibrous
composite membranes has been studied in different ways namely: (1) Effect of
concentration of precursor (2) Effect of thermal treatment (3) various routes to
prepare fibrous membrane, etc.
Even in the 21st century, infectious diseases continue to pose a dominant
public health threat in many developing countries. The World Health Organization
has reported that the developing countries contribute to 25% of the world’s death
caused by microbes. Development of resistance to antibiotics has been a major
drawback in the treatment of many infectious diseases. Therefore, the need for new
strategies has been in need to identify and develop alternative antimicrobial agents
to control bacterial infections. Since the survival of microorganisms on surfaces in
the environment could also be the result of the increased spread of diseases,
antimicrobial coatings on surfaces have been of great interest. In addition, nano
structured coatings have been expected to provide better safety and stability to the
surfaces below. In recent years, numerous antibacterial materials including metals,
semiconducting oxides and polymeric/composite materials have been extensively
investigated. Among the many inorganic antibacterial agents, ZnO has been
extensively used because of its ability to withstand harsh processing conditions, UV
blocking property and superior durability apart from being less toxic and cost
effective when compared to the organic antibacterial materials. It has been observed
from the available literatures that several mechanisms have been proposed to
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interpret the antibacterial behavior of ZnO, be it nanopowder or film. Though
polymer/ZnO composite has received considerable attention, its application in the
field of microbial protection has not been explored in detail. The defined
morphology and large surface to volume ratio of fibers may provide better
interaction with microorganism. Therefore it has been anticipated that these Fibrous
Composite Membranes (FCM) may exhibit much stronger antibacterial activity.
Hence bactericidal tests of Free standing-FCM (FS-FCM) have been surveyed here.
Our interest has been to prepare the “ZnO particle enriched fibers”. To
prepare the pure ZnO fibers, the component polymer inside the composite fibers has
to be removed under high temperature. Although pure inorganic oxide nanofibers
such as TiO2, ZnO could easily be synthesized in this manner, the removal of the
polymer reduces the elasticity and mechanical strength of composite nanofibers.
Improved properties might result if the inorganic precursors inside the composite
nanofibers could be converted into inorganic oxides while the component polymer
inside the composite nanofibers is being retained. Very little attention has been
directed towards the preparation of ZnO nanostructures on the fiber surface to form
functional nanostructures. Methods of assembling inorganic nanoparticles into
polymer matrixes include a mixture of preformed nanoparticles and polymers,
plasma deposition, and in situ growth. A new interest has been incited in the latter
mode of synthesis inside polymer matrixes. However, the in situ synthesis of ZnO
nanocrystals in solid polymer matrixes has still remained a highly sophisticated
challenge.
Thus the objectives of the present studies have been:
1. To Prepare and characterize the fibrous membrane (Polymer/ZnO and ZnO
nanofibers using combination of sol-gel and electrospinning methods.
2. To investigate the effect of process parameter (e.g. Electric field, viscosity,
electrode distance, flow rate, etc.) on fibers morphology
3. To obtain the ultra long ZnO fibers by controlled thermal treatment (Heat
flow rate, Different calcinations temperature etc) of composite fibers.
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4. To study the optical quality of the composite fibers.
5. To fabricate the UV sensor
6. To study the UV shielding property of the composite fibrous membrane.
7. To investigate the bactericidal property of fibrous membrane.
8. To study the wettability nature of composites membrane.
9. To develop a scheme for preparation of “ZnO particle enriched” fibers
10. To compare the efficiency of FCM prepared by different approaches and
to prove the role of electrospinning to enhance the properties of the
prepared material.
1.9 Organization of the thesis
The overview of the research work has been summarized into six
chapters, the contents of which have been briefly outlined:
In chapter 1, a brief outline of nanofibers and their application in various
fields, together with the current research has been presented. In addition, the
objectives and the scope of the present research work have been given.
In chapter 2, an overview of Electrospinning technique along with the
details of the experimental facilities employed for characterizing the prepared
samples has been detailed.
In chapter 3, the details of the preparation and characterization of fibrous
composite membrane and pure ZnO fibrous membrane have been explained. The
influence of various process parameters (Viscosity, strength of electric field,
distance between the electric field, flow rate, etc) on the morphology of electrospun
fibers have been discussed. This study has been extended to prepare an ultra long
and unbroken ZnO nano fibers and a thorough investigation on the effects of
thermal treatment on fibers morphology has been performed.
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In chapter 4, the optical properties of the composite membranes have
been investigated and the results described. The fabrication of UV sensor has been
given. UV shielding property of the fibrous composite membrane has been studied.
In chapter 5, the preparation of superhydrophobic surface without
additional chemical modification has been described. The wettability of the
composite fibrous membrane has been studied using water contact angle
measurements and the results have been presented. The bactericidal properties of
the membrane have also been studied for antibacterial applications. A method of the
preparation of “ZnO particles enriched fibers” has been developed and the results
discussed.
In chapter 6, Summary of all the results obtained and a brief scope for the
future work has also been given. A list of references and publications based on this
work has been confined.