Chapter 4
Synthesis and Characterization
of Silica Sol-Gel Material
Hypotheses are the scaffolds which are erected in front of a building and removed
when the building is completed. They are indispensable to the worker; but the worker
must not mistake the scaffolding for the building.
— Johann Wolfgang von Goethe
Chapter 4
4.1. Introduction
Sol-gel method has been used to formulate materials which can be used
as new laser materials, nanocomposites, biomimetic systems, and so on. Since,
potential in the creation of a wide variety of new materials, understanding of
the sol-gel method has become the center of interdisciplinary research
comprising physics, chemistry, biotechnology, biochemistry, electronics, and
related engineering branches. Sol-gel materials have a wide range of
applications, from environmental monitoring to biosensors for health care for
sensing clinically-important analytes.
Sol-gel derived silica thin films have gained prominence interest
because of the mild processing conditions and widespread understanding of the
sol-gel chemistry. The sol-gel method is receiving the worldwide attention of
scientists in the field of material science as in arrears to its versatility in
synthesizing inorganic nanomaterials. High purity, controlled porosity,
homogeneity, stable temperature, physical rigidity, nanoscale structuring, high
photochemical and thermal stability and excellent optical transparency are the
most remarkable features offered by this method for generating highly sensitive
and selective matrices to incorporate in several biomedical applications [1].
The
inorganic
nanomaterials
produced
by the
sol-gel
method
are
particularly remarkable for fabrication of biosensors since they possess a wide
range of physical attributes such as pH, temperature, water, viscosity and
solvent content. These experimental conditions are used to tailor different
property sol-gel materials for biosensor application.
In the synthesis of silica sol-gel materials, control over porosity is
important for maximum loading of enzyme and less likely to leach the
encapsulated enzymes. The porosity of sol-gel thin films allows small reactant
molecules to diffuse into the matrix while the large enzyme macromolecules
get entrapped in the pores. The reaction in the pores is studied by using the
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transparency of thin films which can be used for optical biosensor [2]. Thus,
the study of the synthesis conditions and the related resulting properties of the
xerogel are very important to understand the hybrid sol-gel systems for
application in biosensor.
4.2. Mechanism of Sol-Gel Reaction
Sol-gel reaction refers to the transition of definite compositions of
inorganic alkoxides precursors from liquid sol phase to solid gel phase. The
sol-gel term was known for more than 150 years, first coined in the late 1800s.
It generally mentions to a low-temperature method using inorganic precursors
that can produce ceramics and glasses with better purity and homogeneity.
Great concerted efforts of researchers from multidisciplinary fields have been
taken to transform sol-gel science to technology. Several products are already
commercially available for applications in optical coatings, biomedical, and
health care.
Sol-gel method involves the transformation of a sol from a liquid ‘sol’
into ‘gel’ phase and understanding of this summarized in various books and
reviews [1, 3]. The main reactions of sol-gel process are the hydrolysis and
polycondensation of metal alkoxides that result in a cross linked silica network.
A sol is first formed by mixing an alkoxide precursor like TEOS with water as
a co-solvent and an acid or base as catalyst at room temperature. The sol-gel
reaction starts when water is mixed with alkoxide precursor
.
Hydrolysis occurs by nucleophilic attack of the oxygen atom of water
molecules on a silicon atom of alkoxide. Subsequently, condensation reaction
takes place producing siloxane bonds and polymerization of monomers to
polymer concurred. Following three reactions describes a sol-gel mechanism,
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Considering that the water is a byproduct of the polycondensation
reaction, the H2O/alkoxide molar ratio (n=2) is theoretically sufficient to
promote a complete hydrolysis and condensation to yield anhydrous silica as
illustrated by equation [1].
where R is stands for methyl or ethyl group. An increase in
H2O/alkoxide
molar
ratio
(n)
promotes
hydrolysis
stoichiometric addition of water, when
condensation reaction is predominant; while at
reaction.
Under
the alcohol producing
water forming
condensation reaction is favored. Higher H2O/alkoxide molar ratio leads to
more complete hydrolysis of monomer before sufficient condensation occurs.
However, it was reported that variations in H2O/alkoxide molar ratios during
the synthesis of pure inorganic silica, results in change in the porosity and pore
size distribution of the obtained xerogel [4, 5].
The intrinsic characteristics of silica sol-gel matrix such as porosity,
polarity, surface area and rigidity mainly dependent on the progress of
hydrolysis and condensation reaction as illustrated in Equations 4.1-4.3.
Moreover, properties of silica sol-gel material influenced by choice of
precursor, the molar ratio of H2O/alkoxide (n), type of solvent and co-solvent,
pressure, temperature, aging, drying and curing conditions. Physical properties
and internal microenvironment of sol-gel material is mainly dependent on
aging i.e. storage condition. As crosslinking of network increases with
expelling of internal solvent from pores of matrix results in change in internal
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polarity and viscosity. Furthermore, average pore size decreases that leads to
entrapped species inaccessible to the external analyte. This is the most
important concerns of silica sol matrices for biosensor application.
Reactions in acidic environments, the oxygen atom in Si-OH or Si-OR is
protonated and H-OH or H-OR are good leaving groups as demonstrated in
Figure 4.1. The electron density is shifted from the Si atom, making it more
accessible for reaction with water (hydrolysis)) or silanol (condensation).
Reactions in basic environments are based on nucleophilic attack by OH- or SiO- on the central Si atom. These species are formed by dissociation of water or
Si-OH. The reactions are of SN2 type where OH- replaces OR- (hydrolysis) or
silanolate replaces OH- or OR- (condensation). Acid catalyzed sol-gel exhibit
irregular chains due to the electrophilic reaction of H+. Whereas F- and OHanions directly attack Si(OR)4 groups by nucleophilic substitution which
involves the temporary expansion of the coordination number of silicone from
four to five or six which causes rapid hydrolysis and condensation reaction thus
affect the pore size, shape and distribution.
Figure 4.1: Acid and Base catalyzed reaction mechanism of sol-gel method
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Sol-gel method is a versatile process for material synthesis, which
exploits mild processing conditions such as ambient temperature and pressure
creating amorphous materials with high homogeneity and purity. Sols are
colloidal suspensions of solid particles in a liquid; prepared from metal
alkoxide or inorganic metal salts, water, ethanol and a catalyst. If the sol is
allowed to settle in a container, form a gel which is a porous three dimensional
network spans the entire volume. These gels are biphasic (both liquid and solid
phases) and can either collapse or maintain its structure depending on the
evaporation conditions. When the liquid is removed xerogels or aerogels are
produced under natural evaporation or supercritical conditions, respectively.
The physical properties of these materials can be tailored by manipulating
process conditions such as concentration and types of precursors used. The
aqueous sol-gel process is defined as the conversion of a precursor solution into
an inorganic solid through inorganic polymerization reactions induced by
water. In general, the precursor or starting compound is either an inorganic
metal salt (chloride, nitrate, sulfate, etc.) or a metal organic compound such as
an alkoxide. Metal alkoxides are the most widely used precursors, because they
react readily with water and are known for many metals [6].
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Fig. 4.2: Schematic representation of sol-gel method
In general, the sol-gel process consists of the following steps (Figure
4.2):
1. Preparation of a homogeneous solution either by dissolution of precursors
in an organic solvent or by dissolution of inorganic salts in water;
2. Conversion of the homogeneous solution into a sol by treatment with a
suitable reagent (generally water and with or without any acid/base catalyst)
3. Aging;
4. Shaping; and
5. Thermal treatment/Drying.
The first step in a sol-gel reaction is the formation of an inorganic
polymer by hydrolysis and condensation reactions, i.e., the transformation of
the molecular precursor into a highly cross-linked solid. Hydrolysis leads to a
sol, a dispersion of colloidal particles in a liquid, and further condensation
results in a gel, an interconnected, rigid and porous inorganic network
enclosing a continuous liquid phase. This transformation is called as the sol-gel
transition. There are two possibilities to dry the gels. Upon removal of the pore
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liquid under hypercritical conditions, the network does not collapse and
aerogels are produced. When the gel is dried under ambient conditions,
shrinkage of the pores occurs, yielding a xerogel. One of the highly attractive
features of the sol-gel process is the possibility to shape the material into any
desired form such as monoliths, films, fibers, and mono sized powders, and
subsequently to convert it into a ceramic material by heat treatment [3]. Some
important terms in sol-gel chemistry are described in following sections:
Aging
When a gel is preserved its pore liquid, its structure and properties
continue to change long after the gel point which is called aging. Four
processes occur, separately or simultaneously, during aging, comprising
polycondensation,
synerisis,
coarsening,
and
phase
transformation.
Polycondensation reactions continue to occur within the sol-gel network as
long as neighboring silanols are close enough to react. This increases the
connectivity of the network and its fractal dimension.
Syneresis
It is the spontaneous shrinkage of the gel and resulting expulsion of
liquid from the pores. Coarsening is the irreversible decrease in surface area
through dissolution and precipitation processes. Syneresis in alcoholic gel
systems is generally attributed to formation of new bonds through condensation
reactions, which increases the bridging bonds and causes contraction of the gel
network. In aqueous gel systems, or colloidal gels, the structure is controlled by
the balance between electrostatic repulsion and attractive van der Waals forces.
Therefore, the extent of shrinkage is controlled by additions of electrolyte. The
rate of contraction of silica gel during syneresis has a minimum at the
isoelectric point (IEP). For silica this point is at a pH of 2, at which the silicate
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species are uncharged. Since the condensation is the slowest at that point, this
suggests that the shrinkage is driven by the condensation reaction.
Drying
In drying, several things may happen when the liquid is removed from
the gel. When the liquid in the gel is replaced by air, the structure is maintained
and aerogel is formed. Xerogel is formed if the structure collapses. Normal
drying of the gel results in structural collapse due capillary forces drawing the
walls of the pores together and reducing the pore size. OH groups on opposite
sides may react and form new bonds by condensation. If the tension in the gel
is large that at which it cannot shrink any more cracking is occur.
The reaction kinetics of two similar tetraalkoxy silanes: TMOS and
TEOS can be distinctly different under identical reaction conditions. Under
acid catalyzed reaction conditions, a TMOS sol-gel undertakes both water and
alcohol producing condensation reactions whereas a TEOS sol-gel undergoes
only water producing condensation reaction. The early time hydrolysis and
condensation reactions of a TMOS sol-gel are statistical in nature and can be
quantitatively described by a few simple reaction rate constants while the
reaction behavior of a TEOS sol-gel is markedly no statistical. TEOS is an
effective precursor for synthesis of Silica sol-gel material; it has some
advantages such as convenience in controlling size distribution and pretty
compatibility with other organic additives. Also, it has more reactivity towards
condensation reaction and high affinity towards enzymes as compared to other
precursor. TEOS derived thin films are optically transparent and porous hence
suitable for the immobilization of enzymes.
Sol-gel method has several flexibilities and unique properties that are of
noteworthy in material science. The use of solutions allows the ready
construction of thin films and fibers with the high purity. It is used for creating
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a broad array of materials, especially oxides, in various forms including fibers,
composites and monoliths, thin films and coatings, porous membranes, and
powders. The formation of a gel requires the synthesis and the aggregation of
nano particles in the range 1-10 nm. The aggregation mechanism leads to the
synthesis of porous materials with small pore size, which is a nano-porous
material. Sol-gel products find application in numerous sectors including
abrasives,
aerospace,
agriculture,
analytical
chemistry,
architecture,
automotive, biomedical, chemical, construction, cosmetics, defense, food,
optics,
dentistry,
electronics,
electro-optics,
environmental,
beverages,
refrigeration and textiles [7].
The major problems of sol-gel methods are based on the hydrolysis and
condensation of molecular precursors is the control over the reaction rates. For
most transition metal oxide precursors, these reactions are too fast, resulting in
loss of morphological and also structural control over the final oxide material.
Furthermore, the different reactivity of metal alkoxides makes it difficult to
control the composition and the homogeneity of complex multi-metal oxides by
the sol-gel process.
One possibility to decrease and to adjust the reactivity of the precursors
is the use of organic additives like carboxylic acids, functional alcohols, which
act as chelating ligands and modify the reactivity of the precursors. An
alternative strategy involves the slow release of water by chemical or physical
processes, allowing control over the local water concentration and thus, over
the hydrolysis of the metal oxide precursors. In spite of all these efforts, the
strong sensitivity of aqueous sol-gel processes towards any slight changes in
the synthesis conditions and the simultaneous occurrence of hydrolysis and
condensation reactions make it still impossible to fully control the sol-gel
processing of metal oxides in aqueous medium.
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Chapter 4
Silica sol-gel materials have many advantages and some disadvantages
as displayed in Table 4.1.
Table 4.1: Advantages and Disadvantages of Sol-gel material
There are two classes of hybrid silica sol-gel materials. Class (I)
materials are prepared by adding the dopant (biomolecules, dyes, etc.) material
to the starting sol whereas class (II) materials are prepared by co-condensation
of silicon alkoxides [Si(OR)4] one or more organosilanes [RʹSi(OR)3 (where Rʹ
could be an alkyl group identical to R, a different alkyl group or a completely
different functionality)].
In class (I) hybrid silica-based material the organic and inorganic
precursors are linked via weak interactions. The class (II) hybrid silica-based
material is composed of the organic and inorganic components are linked via
strong covalent bonds. These hybrid materials are attractive because they
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combine in a single solid both the properties of a rigid three-dimensional
network with the specific functionality of the organic component.
According to the IUPAC definition, the porous materials are classified
into three classes: microporous (< 2 nm), mesoporous (2-50 nm) and
macroporous (>50 nm). If the pore dimension is in the nanoscale range, such
materials denoted as nanoporous materials [8]. The nanoporous materials have
found growing interest in many applications, due to their large surface area,
tailored pore size distribution, manageable pore structure and versatile
composition.
Exclusively for biosensors, the use of nanoporous materials can
significantly improve the analytical performance of the biosensors since their
large surface area and versatile porous structure are beneficial for the loading
of large amounts of active catalysts and they have a fast diffusion rate [9]. The
porosity, pore size and its distribution can be controlled by catalyst and
experimental condition. The porous material has many advantages as an
immobilization matrix owing to its high chemical and physical properties, very
high chemical inertness and ageing behavior.
A porous structure of materials is beneficial in the development of
biosensor as it can enhance interfacial reaction and as a consequence improve
sensitivity. As well, tailored permeability for substrate enhances selectivity of
biosensor environment. The tailored porosity allows the small analyte molecule
to diffuse into the matrix while large protein macromolecule remains physically
entrapped in the pores. The transparent material is useful to characterize the
reaction that occurs in the pores of matrices by optical spectroscopy.
The porous Silica sol-gel matrices derived by HCl, HF, NaF, HNO3 and
NH4F catalysis is usually mesoporous with pore diameter in the range of 5-100
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nm. HCl catalyzed material has less pore size, on the other hand ammonia
catalysis tends to enlarge the pore size. Pore shapes are mainly controlled by
the type of catalyst. The pores of narrow necked and large abdomen shapes
with the free network structure of linear chains can be obtained by HCl
catalysis, while fine cylindrical pores can be achieved by fluorine anion (HF,
NaF and NH4F) and ammonia catalysis.
Generally, Silica particles are positively charged at low pH and
negatively charged at high pH. Basic to moderate acidic sols have a significant
amount of deprotonated silanol groups (SiO-) which increase the condensation
rate causing formation of highly branched silica species. Gelation of these
branched species results in the formation of a mesoporous region with pore size
2-50 nm. As the pH is lowered to the isoelectric point, the gelation time
increased and this leads to linear or randomly branched Silica gel having a
highly microporous structure with pore diameter ˂ 2 nm. At very high acid
concentrations (pH 1), dried xerogel become more mesoporous. This is due to
the protonation of silanols group to produce (SiOH2+) groups which are acting
to increase the rate of condensation.
4.3. Experimental
4.3.1. Materials
Tetraethyl orthosilicate (TEOS) was purchased from Sigma Aldrich and
was used without further purification. Methanol, iso-propanol, hydrochloric
acid and nitric acid were of analytical grade, procured from SD Fine chemicals
India. Microscopic slides (scientific plaza) were used as glass substrate.
4.3.2. Characterization of Silica sol-gel material
X-ray diffraction measurement was carried out with INXITU x-ray
diffractometer equipped with a crystal monochromator employing Cu-Kα
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radiation of wavelength 1.5406 Å in the 2θ range from 10 to 55º. The optical
activity of TEOS sol-gel thin films were monitored by using a UV-visible
spectrophotometer (Shimazdu model 1800). Fourier-transform infrared spectra
of TEOS sol-gel thin films were recorded using a Nicolet FTIR spectrometer
(Model 510 P).
4.3.3. Synthesis of Silica sol-gel material
A. Preparation of silica stock solution
The preparation of silica samples was carried out by the previously
reported method [10] as illustrated in Figure 4.3. The sols were prepared by
hydrolysis and condensation of TEOS in the presence of water and
hydrochloric acid (0.1 M) with different TEOS concentrations. The samples
were prepared for the various concentrations and thin films were prepared. Out
of which, thin films prepared from samples A1 and A2 are crack free and
uniform in nature. Hence, these are chosen for further study and explained in
detail. The sample A1 was prepared by mixing 4.5 mL of TEOS and 0.25 mL of
0.1 M HCL in 1.4 mL of double distilled water. Sample A2 was prepared by
adding 2 mL of TEOS and 0.25 mL of 0.1 M HCL in 1.5 mL of double distilled
water. These reaction mixtures were placed in 25 mL stopper glass container
and stirred for 5 h at 300 rpm until visible homogeneity was obtained. The
samples A1 and A2 were further kept in a polystyrene container for aging for
about 24 h and used as stock solutions for preparation of thin films.
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Figure 4.3: Schematic illustration for synthesis of silica sol-gel material
It was investigated that as the TEOS concentration decreases, the
gelation times become larger as shown in Table 4.2.
Table 4.2: Effect of silane content on viscosity and gelation time
Silica
TEOS:H2O:HCl Stirring Ageing Viscosity
Gelation
Time
Samples
Molar ratio
(h)
(h)
(Pa s)
A1
18:5.6:1
5
24
14
5
A2
8:5.6:1
5
24
16
3
tgel (day)
B. Preparation of thin films
I) Pre-treatment for glass plates
Prior to casting, glass plates were sonicated in distilled water for 30
minutes to remove impurities. These glass plates were etched by treating with
concentrated HNO3 for 2 h. Further, the plates were washed many times with
double distilled water. These pre-treated glass plates wetted with isopropanol
for even spreading of stock solution.
II) Deposition Method
Various physical and chemical deposition methods have been reported
to prepare thin films, including chemical bath deposition, spray pyrolysis
method, dip coating and spin coating. However, spin coating method is widely
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Chapter 4
employed for the highly reproducible fabrication of thin film over large areas
with high structural uniformity and homogeneity at ambient temperature.
Spin coating
Preparation of silica sol-gel thin films by spin coating method is
represented in Figure 4.4. The clear stock solutions of samples A1 and A2
further diluted in methanol (1:3). Methanol was used to decrease viscosity to
enhance spreading of silica sol on the substrate. About 100 µl of the diluted
stock solution was placed on glass plate (area about 4 cm2) and spun at 2800
rpm for 15 min by using spin coater. These films were then dried at room
temperature.
Figure 4.4: Preparation of silica thin films by spin coating method
4.4. Results and Discussion
4.4.1. X - Ray Diffraction method
The phase formation process in silica matrix triggered by sol-gel method
was monitored using an x-ray powder diffraction method as shown in Figure
4.5. For the as-dried gel, a much broadened peak at the 2θ of 23° is observed,
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corresponding to the amorphous silica matrix conforming to JCPDS file (791711) [11].
The figure shows that the silica film is in crystalline form. This
indicates that no crystalline phase was formed during the initial drying of silica
sol-gel material prepared with TEOS at room temperature. Therefore, FTIR
spectroscopy was used in order to reconnaissance the thin film material.
Figure 4.5: The X-ray diffraction pattern of silica sol-gel material
4.4.2. Fourier - transform infrared spectroscopy
FTIR study is carried out to confirm the presence of the silica network
in samples synthesized of two different concentrations of TEOS (Sample A1
and A2) as represented in Figure 4.6. The silica gel spectra illustrate several
frequency regions as given below:
A] 4000 cm-1- 3000 cm-1: In this spectral range, the bands are mainly due to
overtones and combination of vibration of Si-OH or H2O. The band at 34303445 cm-1corresponds to molecular water hydrogen bonded to Si-OH group
[12].
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B] 3000 cm-1 - 1350 cm-1: In this spectral range, the bands are due to the
overtones and combination of vibration of organic residue, molecular water and
SiO2 network. The band at 1638cm-1 corresponds to vibration of molecular
water. In the spectra, water bands observed at around 1640 cm−1 corresponding
to bending vibrations indicate hygroscopic character of the powdered samples
[13].
C] 1300 cm-1 - 400 cm-1: This spectral region is associated with combinations
of vibration of silica network. The band at 1082 cm-1 assign to asymmetric
stretching vibrations of Si-O-Si bridging sequences. The band at 937-944 cm-1
attributed to stretching vibrations of the free silanol group on the surface of
silica network. The band at 791-795 cm-1 corresponds to bending vibration of
C-H of CH3-Si group. The band at 470-474 cm-1 associated with out of plane
deformation of Si-O bonds.
It is clear that the effect of water content in sol is pronounced for gel.
The samples A1 and A2 show slight differences in intensity of peaks. The broad
peak in 1400- 1600 regions confirm the presence of SiO4. The intensity of
absorption decreases on the low frequency side of 1082-1032 cm-1 and 944-937
cm-1 bands and increases at high frequency peak 3430-3445 cm-1, 1635-1638
cm-1, 791-795 cm-1, 470-474 cm-1.
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Chapter 4
Figure 4.6: FTIR spectra of silica sol-gel thin films for a) sample A1 and b) A2
As shown in Table 4.3, the FTIR spectra show characteristic vibrational
bands at 1082, 799 and 470 cm-1; corresponding to the stretching, bending and
out of plane deformation of Si-O bonds, respectively. The position and shape of
the main Si-O-Si vibrational band at 1082 cm-1, and the well pronounced
shoulder suggests a stoichiometric silicon dioxide structure.
Table 4.3: Assignments of band in the FTIR spectra in silica thin films
Wavenumber
Mode
Comment
ν Si-OH or
OH stretching
(cm−1)
3440
H2 O
1082
νa Si-O-Si
Network Si-O-Si stretching
1035
νa Si-O-Si
Silicon sub-oxide, Si-O-Si angle
<144o
944
νs Si-O-R
R: H;C2H5
799
ν Si-C, ρa CH3
Si-O bending
470
Si-O out of plane deformation
ν: stretching; δ: bending; ρ: rocking; a: asymmetric; and s: symmetric
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4.4.3. UV- Vis spectroscopy
The optical properties of silica thin films were studied by recording the
spectra over 400 and 1100 nm using a UV-VIS spectrophotometer as illustrated
in Figure 4.7. The optical properties revealed the formation of highly
transparent silica thin films. The optical properties of silica thin films are
changing with a decrease in silane content. Both films show the similar
behavior up to the wavelength 600 nm, after that films show a variation in
transmittance.
Thin film prepared by using sample A2 shows slight increase in the
transmittance about 95% in the range of wavelength 600 nm to 1100 nm as
compared to thin film prepared by using sample A1 (88 %). This behavior may
be due to the less silane content in sample A2 as compared to sample A1.
However, this increase in transmittance of thin films of sample A2 is favorable
for fabrication of biosensor due to its greater optical transparency.
Figure 4.7: Transmittance spectra of silica thin film of samples A1 and A2
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4.4.4. Scanning Electron Microscopy and Elemental X-ray mapping
In order to confirm the formation of the silica matrix during the sol-gel
reaction, EDS mapping was used to characterize the silica distribution. Figure
4.8 represents the Si-mapping analysis of the silica sol-gel thin film. The
electron beam penetrates 2-3 microns below the surface of sample to reveal the
embedded silica distribution. The bright spots represent that the Si element
spreads uniformly throughout the matrix. It suggests that the sol-gel reaction of
TEOS and water take place throughout the channel networks of the Silica
matrix [14].
Figure 4.8: (a) SEM image, and (b) Elemental X-ray mapping for the
element Si of Silica sol-gel thin film
4.4.5. EDS
The purity of the Silica matrix was ascertained from the EDS studies
(Figure 4.9). It was indicated the presence of silicon, oxygen and carbon as the
elemental composition. The carbon present in the system originated from the
alcohol evolved in reaction.
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Figure 4.9: EDS spectra of Silica sol-gel thin film
4.4.5. Field Emission Scanning Electron Microscopy
FESEM micrographs of surface of silica sol-gel thin film are shown in
Figure 4.10. The silica matrix is in the form of porous membrane (a), and their
uniform surface is clearly visible at high magnification. At low magnification
(b), cracked surface is observed, which is phenomenon of the silica sol-gel
material.
Figure 4.10: FESEM micrographs of Silica sol-gel thin films from (a) High
magnification, and (b) Low magnification
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4.5. Conclusion
This chapter addresses the influence of silane content on properties of
silica thin films. These thin films were synthesized using the sol - gel method
and spin coating method was employed for the synthesis of uniform and crack
free thin films. The results showed that the optical properties of silica thin films
were affected by decrease in silane (TEOS) concentration. The greater optical
transparency of the silica thin film prepared by using the sample A2 makes it
valuable in the application of enzyme nanobiosensor.
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