Article
Advanced Science,
Engineering and Medicine

Copyright © 2014 American Scientific Publishers
All rights reserved
Printed in the United States of America

Vol. 6, 965–973, 2014www.aspbs.com/asem

Synthesis and Characterization of Ultra-Fine
Colloidal Silica Nanoparticles
Mohd Qasim1 , J. Ananthaiah2 , S. Dhara2 , P. Paik1 , and D. Das1 ∗1School of Engineering Sciences and Technology, University of Hyderabad, Hyderabad 500046, India
2
School of Physics, University of Hyderabad, Hyderabad 500046, India

A facile synthesis of colloidal silica nanoparticles of various sizes, employing the hydrolysis and condensation of tetraethyl orthosilicate (TEOS) in absence of any surfactant, is reported. The surfactantfree colloids of SiO2 nanoparticles with diameter ∼ 5–6 nm are stable upto 50 days after synthesis.
The effects of different reaction parameters (e.g., concentration of TEOS, water, alcohol and ammonia) on particle sizes have been investigated. Particle sizes have been found to increase withincreasing TEOS and ammonia concentration but decrease with increasing water concentration.
Obtained products have been characterized through XRD, UV-Vis spectrophotometry, FTIR, HRTEM
and FE-SEM. The colloidal stability has been characterized by zeta potential and rheologicalmeasurements.

Keywords: Colloidal Silica, Rheology, Optical Properties, Silica Nanoparticles.
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1. INTRODUCTION
The colloidal silica nanoparticle is a stable dispersion ofsolid SiO2 nanoparticles in liquid media. Monodisperse
SiO2 nanoparticles are one of the most attractive materials in colloidal science because of their unique properties, such as surface structure and functionality (–OH),
optical properties, biocompatibility etc.1 Surface functionalized SiO2 finds application in electronics, sensors, catalysis, polishing, paints, fillers and pigments.2–5 Because of
the biocompatible nature and ease of surface modification,
amorphous SiO2 nanoparticles are a potential tool in medical biotechnology and used in several applications including target drug and gene delivery,6–8 molecular bioimaging,labeling of bio-marker of infected cancer and tumor cells
etc.7 8 However, SiO2 nanoparticles play different roles indifferent applications and their response strongly dependon the size and distribution, shape and surface properties.2
Among various methods reported for SiO2 nanoparticles
preparation Stober’s process is known to produce monodisperse particles.5 9 Stober process involves hydrolysis andcondensation of alkoxide of Si in aqueous alcohol solution, where liquid ammonia is used as a morphological catalyst. The size of SiO2 nanoparticles strongly
depends on the reaction parameters such as, solvent,∗

Author to whom correspondence should be addressed.

Adv. Sci. Eng. Med. 2014, Vol. 6, No. 9

catalyst, temperature, reaction time etc.10 SiO2 nanoparticles of size less than 5 nm to several micrometers can besynthesized by microemulsion method,11 organometallic
sphere endotemplate method,12 surfactant template method
etc. These techniques can yield smaller size particles withnarrow size distribution compared to those obtained from
Stober process, but large amount of surfactant and costabilizers are used in these processes.13 Therefore, the particles require through purification before being employed
for any specific application. Removing surfactant micelles
from SiO2 network structure is not an easy task since it
can entrap the micelles due to the electrostatic interactions.
Surfactant free SiO2 nanoparticles, therefore, have a great
promise in biological applications. Stober process generally produces larger size silica particles (0.05 to 2 m).9
A number of research groups modified Stober process toachieve silica nanoparticles of size <10 nm,14–21 but to thebest of our knowledge none of them have succeeded toachieve surfactant/metal ions free colloidal SiO2 of size
below 10 nm. Here, in this work, an effort has been madeto synthesize colloidal SiO2 nanoparticles of different sizes(5 to 30 nm) without the use of any surfactant or related
compounds. The colloidal SiO2 nanoparticles have been
prepared only by varying the concentration of TEOS, H2 O,
NH3 using sonochemical synthesis followed by the Stoberprocess. In the modified process very less TEOS has been

2164-6627/2014/6/965/009

doi:10.1166/asem.2014.1578

965

Synthesis and Characterization of Ultra-Fine Colloidal Silica Nanoparticles

Qasim et al.

used for all sets of experiments and has been added drop
wise after diluting with absolute ethanol. Liquid ammonia
has also been added drop wise in two steps to avoid sudden
increase in particles size in all the experiments. To the bestof our knowledge the present work is the first report on thesynthesis of SiO2 of particles sizes ∼ 5–6 nm in absence ofany surfactant. The stability of the SiO2 colloids has beenstudied using zeta potential () and rheological property
measurements. The changes in viscosity have been studied
as a function of shear rate and temperatures. The rheological behavior of all the colloidal silica nanoparticles has
been explained in terms of their morphology, zeta potential
and compositions and has been reported in the articles.

2. EXPERIMENTAL DETAILS
2.1. Materials
Tetraethyl orthosilicate (TEOS,99.99%, Sigma Aldrich),ammonia (25%, Qualigens), and ethanol (99.9%,
Hangzhou) were used in these experiments without any
purification. Deionized water was used in all experiments.

Figure 1. Photograph of colloidal SiO2 nanoparticles after eight weekof synthesis. All the samples are found to be stable with time.

Yield %= Observed weightg × 100/Theoretical weight (g)where the theoretical weight of the sample was calculated based on 100% conversion of TEOS into SiO2 .10
The yield (%) for each experiment is found to be ∼ 95%.

2.2. Synthesis of Colloidal Silica Nanoparticles
Under Different Conditions
2.3. Characterization Techniques
The experiments were performed using the precursor conParticle sizes, shape and morphology were measured using
centrations mentioned in Table I. All concentrations wereTransmission electron microscopy (FEI Tecnai T20G2
calculated based on the final concentration in the reacDeliveredby Publishing
to: unknown
S TWIN TEM)
and field emission scanning electrontion mixture. First, solution containing
appropriate
quan- Technology
IP: 117.215.217.75 On: Fri,microscopy
17 Oct 201405:08:56(Carl
Zeiss Ultra 55 FESEM). A drop of dilute
tities of absolute ethanol, ammonia anddeionized
water
Copyright: American Scientific Publishers
and well sonicated colloidal silica solution was placed on a
was sonicated in a 50 Hz TRANS-O-SONIC sonicator for
carbon coated copper grid, the solvent was evaporated and
10 minutes in 100 ml closed mouth beaker to ensure no
the grid was examined by TEM. For the FESEM, a drop of
evaporation of the solvent and complete mixing. Approsample was placed on carbon tape pasted on holder, solvent
priate amount of TEOS in absolute ethanol was added towas evaporated and the particles were coated with ∼ 10 Å
the above solution drop wise (0.03 ml/min) and after sonthin gold layers before examining in FESEM. The particle
icating for 100 minutes again ammonia was added drop
size was estimated using analysis image processing softwise (0.05 ml/min) to promote the condensation reaction.
ware. The compositional analysis of the sample was carriedContinuous sonication for about 70 minutes led to the forout by a Horiba Energy Dispersive X-ray Micro-analyzer
mation of white turbid suspension of silica nanoparticles.
attached to a Hitachi S-3400N SEM. Crystal structure of
Photograph of the as prepared colloidal silica nanopartithe samples was analyzed by SAED pattern obtained fromcles after eight weeks of synthesis have been shown in
TEM and from XRD data. The as-prepared and calcined silFigure 1. Powder of silica nanoparticles were obtained by
ica particles were analyzed by a Bruker D8 Advance X-ray
evaporating the solvent initially at 50  C for 2 hours andthen at 150  C for 2 hours in hot air oven. The conversion
Diffractometer with –2 geometry, using Cu K radiation,of the reactants into silica nanoparticles has been estimatedin the 2 range 5–50 . The optical absorption behavior wasusing the following formula,analyzed using a UV-Vis spectrophotometer (Perkin Elmer
Lambda 35). Fourier transformed infrared (FT-IR) analysis
Table I. Concentrations of the precursors used in different experiments.
was conducted on pellet made by mixing 1 mg of SiO2 in
100 mg KBr using Perkin-Elmer 2000 FT-IR spectromeName of sample
Solvent
TEOS (M)
H2 O (M)NH3 (M)ter. The Zeta potential and particle sizes of colloidal silS1
Ethanol
0.044
5.55
0.441
ica were measured by Zetasizer Ver. 6.20 (Malvern Serial
S2
Ethanol
0.134
5.55
0.441
Number: MAL1048519). Dynamic viscosity was measured
S3
Ethanol
0.268
5.55
0.441
using a standard rheometer (Anton Paar MCR 501) with
S4
Ethanol
0.134
2.77
0.441
controlled shear rate (CSR). The measuring system was a
S5Ethanol
0.134
8.33
0.441
S6
Ethanol
0.044
5.55
0.147
coaxial cylinder (CC-27). The temperature of the system
S7
Ethanol
0.044
5.55
0.735
was controlled using a Peltier device with an accuracy of
S8Isopropanol
0.044
5.55
0.735
± 0.1  C.
966

Adv. Sci. Eng. Med. 6, 965–973, 2014

Qasim et al.

Synthesis and Characterization of Ultra-Fine Colloidal Silica Nanoparticles

3. RESULTS AND DISCUSSION

2 1/2 hours. XRD pattern shows that silica nanoparticles
are amorphous upto the calcination temperature of 900  C.
The effects of TEOS, H2 O and NH3 with concentrations
It is also found that with increasing calcination temperaon particle sizes and shape have been studied. The concenture the XRD peak intensity increases imploying crystaltrations of reactants and their effects on particle sizes arelization in silica phase. It is evident from the diffraction
shown in Tables I and II respectively. It was observed frompattern that at 1000  C amorphous silica transformed into
TEM (Fig. 2) and FESEM (Fig. 3) that the particle sizes
crystalline (Cristobalite) silica.23 The diffraction spectra ofof silica nanoparticles obtained from all the experiments
the samples calcined at 1100 and 1200  C are almost simito be very small. The largest particle size observed in thislar in nature. The observed diffration peaks at 2 = 2188
investigation is 31 ± 137 nm for as prepared sample S8.
[101], 28.48 [111], 31.39 [102], and 36.193 [200] corParticle size of 54 ± 061 nm is obtained for sample S6.
relates well with the cristobalite tetragonal phase (Ref.
About 2–3 micrographs of each sample were considered
JCPDF card number 89-3434). The crystallization temperto obtain the mean particle sizes and ∼ 150 particles were
ature of amorphous silica reported else where is 1300  C.24
measured for each sample. To the best of our knowledge
But, in this investigation the crystallization temperature is
this synthesis procedure with optimized condition (sample
observed at around 1000  C. The different synthesis routeS6) is the easiest way to produce ultrafine sized, highlyor different experimental conditions used in this study
pure and stable colloidal silica nanoparticles, without the
could lead to such difference in crystallization temperature.
use of any surfactant, template based technique, and metal
The crystallite size of all samples, calcinated at different
salt catalyst, which cause contamination to the particles.
temperatures, has been estimated from the XRD peak broaUltrafine silica nanoparticles are very prone to agglomeradining using Scherrer’s formula d = 09 / cos , wheretion mainly due to the hydrogen bonding present between
d is the crystallite size, is the wavelength of X-ray radithe surface –OH groups and minimize the high surface
ation (CuK,  is the Bragg angle and is the full widthenergy. TEM and FESEM analysis of the 50 days aged
at half maximum (FWHM) of the most intense diffraction
colloidal silica show the same mean sizes of the particles
peak.25 The crystallinity and crystallite size increases withas those of the freshly prepared samples.
increase in calcination temperatures. Around 80% increasein crystallite sze is observed in increasing the calcina3.1. Solid State Crystal Structuretion temperaturefrom 1000  C to 1100C (16.3 nm at
samples Technology
The solid state phase structure ofDelivered
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was checked by XRD with a CuKradiation (CuKOn:= Fri,1000
cination temperature
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AmericanPublishers caused less effect on crystallite size.
1540598 Å) at room temperature. Figure4(a) shows
the ScientificThe
crystallite
size of the sample calcined at 1200  C is
X-ray diffraction pattern of as prepared S1, S2, S3, S5,∼ 31.1 nm.
and S8 samples. XRD results show that all the SiO2 samples are amorphous in nature as expected and reported
3.2. FTIR Analysis
by others.22 The broad diffraction peak at 2 = 23 andTo confirm the chemical structure related to the functionalabsence of any diffraction peak from impurities confirmed
groups and purity of the prepared samples FT-IR analythe formation of pure amorphous silica nanoparticles.
sis was performed. The FTIR spectra of silica nanoparSame has also been confirmed from the SAED patterns
ticles (S8) is shown in Figure 5. The intense and broad(inset of TEM, Fig. 2).
band appearing at ∼ 1079–1167 cm−1 is assigned to the
The effect of calcination temperatures on the phaseasymmetric stretching vibrations of Si O Si bond.7 The
structure has also been studied. The XRD patterns ofabsorption band at ∼ 1079 cm−1 is arising from asymmetthe representative sample S8, calcined at different tem
ric vibration of Si O bond. The bands at ∼ 960 cm−1 andperatures varying from 150 C to 1200 C, are shown∼ 801 cm−1 can be assigned to the asymmetric vibration of
in Figure 4(b). All samples have been calcined forTable II.

Physical properties of colloidal silica nanoparticles, S1–S8.

SamplenameMean particle
size (nm)by TEM
94
129
122
149
102
54
209
31

S1
S2
S3
S4
S5
S6
S7
S8

SD (nm)Mean particle
size (nm) by
FESEM

0.65
0.78
0.84
0.91
0.80
0.61
2.41
1.37

113
137
129
15
102
6
264
375

Adv. Sci. Eng. Med. 6, 965–973, 2014

SD (nm)Zeta
potential
(mV)

2.25
1.42
1.77
1.41
1.22
1.25
1.92
7.00

−20.1
−21.9
−21.7
−32.4
−25.4
−36.9
−32.7
−30.9

pH

Observed SiO2
concentration(mg/ml)Theoretical
SiO2 conc.
(mg/ml)Yield (%)

945
954
948
954
989
996
1023
1021

257
74
150
75
72
25
26
247

27
81
161
81
81
27
27
27

95.1
91.5
93.1
92.8
89.1
92.5
96.3
91.4

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Synthesis and Characterization of Ultra-Fine Colloidal Silica Nanoparticles

Figure 2.

Qasim et al.

TEM images and SAED pattern (inset) of all (S1–S8) SiO2 nanoparticles. The scale bar is 50 nm.

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concentration of primary particles at the induction period,
Si OH bond,18 and symmetric vibration of Si O bondrespectively. The band at ∼ 468 cm−1 corresponds to thei.e., [primary particles]  [TEOS]. Induction period is thebending mode of Si O Si bond. An intense characteristime period when generation of nuclei takes place fromthe supersaturated solution and induces formation of pritic absorption band around 3300–3500 cm−1 arises due tomary particles. Therefore, TEOS concentration determines
the O H stretching vibration mode of Si(OSi)n (OH)4−n(n = 2–4) linkage of the solid state network structure of silthe concentration of primary particles, but at the sametime the concentration of these primary particles deterica or due to the unbounded absorbed water.26–28 The bandat ∼ 1631 cm−1 is due to scissor bending vibration ofmines the subsequent growth of these particles. TEOS was
molecular water.29
diluted with ethanol and was added to the reaction system drop wise very slowly in order to promote the formation of smaller particles. Figure 6 shows the effects of
3.3. Effect of TEOS Concentration
TEOS concentration on size of silica nanoparticles. The
on the Size of Particles
particle size increases with increasing TEOS concentraThe effect of TEOS concentration on the size of SiO2
tion up to 0.134 M and after that it does not change
nanoparticles is controversial in some literature it is foundmuch. The particle size is seen to vary from 94 ± 06 tothat there is no significant effect of TEOS concentra9129±07 nm, measured by TEM, and 113 ±22 to 137 ±tion on particle sizes but, Bogush et al. and a group of3 4 10 15
14nm,
measured by FESEM. Both the TEM and FESEM
reported that increasing particles sizes
researchers
analysis show that the particles are almost monodispersed.
with increasing TEOS concentration. Helden et al.30 howThe particles sizes for all the samples are mentioned inever reported that high TEOS concentration can decrease
Table II.
the particle sizes also. In the present study, increased
size of the silica nanoparticles was observed with increas3.4. Effect of Water Concentration on
ing TEOS concentration (0.044–0.268 M), with 5.55 M
Size of Particles
water, and 0.441 M ammonia concentration (samples:
S1, S2, S3). Being the source of monomer, concentraIn the present study, decrease in size of the silica nanopartion of TEOS will determine the concentration of theticles was observed with increasing water concentration
nuclei/primary particles present in the system.4 Hence, thein the range 2.77–8.33 M, with 0.441 M ammonia,
increase in particle size is attributed to the increase in
and 0.134 M TEOS concentration (sample S4, S2, S5).
968

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Qasim et al.

Synthesis and Characterization of Ultra-Fine Colloidal Silica Nanoparticles

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Figure 3.

FESEM images of SiO2 nanoparticles(S1–S8).
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The obtained result is in good agreement with the results
reported by Rahman et al.4 Figure 6 illustrates the trend
in particle sizes with increasing water concentrations.
It is observed that as the R value (water/TEOS ratio)
increases the particles size also decreases. The R values for samples S4, S2 and S5 are 20.67, 41.41 and62.16, respectively. Generally, water promotes hydrolysis
and condensation reaction resulting in increase in particle sizes. At higher H2 O concentration the oligomers
in reaction solution get diluted, resulting in the formation of smaller particles. In the present investigation, thedecrease in size of the silica nanoparticles with increasingwater concentration may be associated with the dilutionof silicate concentration which slows down the hydrolysis and condensation reaction, resulting in restricted particle growth. Matsoukas and Gulari31 have also reported
that the increase in water concentration yields smaller particles. The estimated particle size is seen to vary from
149 ± 09–102 ± 08 nm. The particle size data obtained
from TEM (Fig. 2) correlates well with those obtained
from FESEM (Fig. 3). Although the average particle size
is seen to decrease with increasing water concentrationand their tendency to form a chain like structure (by joining many single particles) increases with increasing water
concentration.
Adv. Sci. Eng. Med. 6, 965–973, 2014

3.5. Effect of Ammonia Concentrationon the Size of Particles
Normally acid catalyzed hydrolysis is a very slow processcompared to the basic one and hence base catalyzed process is preferable to synthesize ultra-fine monodispersed
SiO2 nanoparticles. In this work NH3 is used as a catalyst
for hydrolysis and condensation of TEOS in ethanol. It is
worth mentioning that the higher NH3 and water content
in the reaction mixture increase the rate of hydrolysis.7 It
has been found in earlier work that slow drop wise addition of NH3 for a longer period controls the growth andlimits the size of particles, whereas faster rate of addition
reverse the effect and produce bigger size particles.9 On
addition of NH3 the pH of the solution increases and leads
to the formation of large number of SiO2 nuclei, which
subsequently grow into silica particles. From Figure 6,
it is clearly observed that as the NH3 is increased from
0.147 M to 5.55 M the particle size increased from
6 nm to ∼ 26.4 nm in presence of 0. 0.044 M TEOS
and 5.55M H2 O in the fixed volume of EtOH. (Samples
S6, S1, S7). The probable reason for this is that as the
NH3 increases the rate of hydrolysis of TEOS increases
to form [Si(OC2 H5 4−X (OH)X ], which subsequently condenses to produce large number of oligomers leading to
the formation of larger sized particles.2 4 At very low
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Synthesis and Characterization of Ultra-Fine Colloidal Silica Nanoparticles

Qasim et al.

(a)

(b)

Figure 6. Effect of precursors (TEOS, H2 O and NH3 ) concentration on
size of SiO2 nanoparticles. The filled squares represent estimated size
from TEM data and filled circles that from FESEM data.

From TEM analysis it is observed that the size of SiO2 particles increases from 21 nm, obtained in EtOH, to 31 nm
obtained in isopropanol. Similar results have also beenreported by Wang et al.3 where it was observed that asthe chain length of the alkyl group increases the dielectric
constant of the solvent decreases, which led to the increasein particle sizes. It is worth mentioning that the dielectric
constants of ethanol and isopropanol are ∼ 24.3 and 19.9,
respectively. The dominance of van der Waals attractive
DeliveredFigure 4. (a) X-ray diffraction patterns ofas preparedby
S1,Publishing
S2, S3, S5, Technology to: unknown
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SiO2 nanoparticles (S8)and S8 SiO2 nanoparticles; (b) XRD patterns
mary particles
depends on the dielectric constant of the
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calcined at different temperatures.
solvent. In a solvent with low dielectric constant the attractive force between the nuclei dominates over the static
NH3 concentration ultrafine silica nanoparticles of dimenrepulsive forces and hence promotes the reaction kinetics
sions 5–6 nm are obtained. This finding will help otherwith subsequent particle growth.319
researchers to prepare ultra fine silica nanoparticles without using any surfactant.
3.7. Zeta Potential
Zeta potential measurement was carried out to check the
3.6. Effect of Other Alcohols on the Size of Particles
stability of colloidal silica nanoparticles and is seen to
To observe the effect of solvent on the particle sizes, twovary from −20.1 to −36.9 mV depending of the partidifferent types of alcohols, such as ethanol (sample S7),
cle size and morphology and is mentioned in Table II.
and isopropanol (sample S8), were chosen keeping all
the other experimental conditions constant (see Table I).

Figure 5.

970

FT-IR spectra of as prepared SiO2 nanoparticles (S8).

Figure 7. Variation of zeta potential of all colloidal SiO2 with reactant
concentrations (TEOS, H2 O and NH3 ) for colloidal SiO2 nanoparticles.

Adv. Sci. Eng. Med. 6, 965–973, 2014

Qasim et al.

Synthesis and Characterization of Ultra-Fine Colloidal Silica Nanoparticles

The zeta potential values of the samples suggest that SiO2
nanoparticles are moderately stable in suspension. Samples S1, S2, S3, and S5 have lower zeta potentials thansamples S4, S6, S7, and S8. It is observed (in Tables I
and II) that a changes in particles sizes (9–12 nm) andzeta potentials (Fig. 7) are weakly dependent on changes
in TEOS concentration. Change in water concentrationfrom 2.77 M (S4) to 5.55 M (S2) changes the zeta potential value from −32.4 to −21.9 mV. Further increase inwater concentration does not affect the zeta potential of
colloidal SiO2 (S5). With increasing ammonia concentration, zeta potential decreases at beginning (−36.9 mV for
S6 to −20.1 mv for S1) followed by a rapid increase to−32.7 mV for S7 with further increase in the ammonia
concentration. NH4 OH produces hydroxyl ions, which getadhered to the silica particle surfaces and helps in stabilizing the colloids by static repulsion force against vander Waals forces of attraction. However, the change inzeta potential depends on the particle sizes and the surface
charge density, which is strongly associated with the pHof the solution.

a coaxial cylinder (CC-27). The temperature of the system
was controlled using a Peltier device with an accuracy of± 0.1  C. In Figure 8(a) the variation of dynamic viscosity
is shown as a function of shear rate. In the low shear
rate range (< 200 s−1 ) the dynamic viscosity varies from
1.4 to 2 mPa s depending on the sample and does notchange with shear rate. Thus, in the low shear rate range
all the samples show behavior of a typical Newtonian fluid.
It is observed that S5 and S3 exhibit larger viscosity thanother samples. This is because of higher concentration ofsilica (S3) and water (S5) in these samples as both lead toextensive networking or formation of chain like structure
as discussed later. It was observed that the samples S4,
S2, and S5, because of their increasing water contents,
show increasing trend in viscosity. Sample S5 shows larger
viscosity than S4 and S2. It was confirmed by TEM that as
the amount of water content increases the particles tend toform longer chain or networks by joining with each other.
This is due to increasing tendency of hydrogen bonding
between particles and formation of chain like structure thatgives rise to larger viscosity. Sample S1, S6, and S7 show
almost similar viscosity because they have same amount
3.8. Rheological Measurementsof water and silica content.
Dynamic viscosity of the colloidal silica was measured
In the high shear rate range i.e., beyond 200 s−1 , theusing a standard rheometer (Anton Paar MCR 501) withviscosity abruptly changes slope and increases with thecontrolled shear stress (CSS). The measuring system wasincreasing shear rate showing shear thickening behavior. Similarto:
typeof shear thickening behavior is reported
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05:08:56of silica nanoparticles.32 33 It wasdispersion
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that in polar liquid silica nanoparticles could
interact via hydrogen bonding and hence they have strong
tendency to form microstructures. In the present samples
the solvent contains ethanol and water and both of themare polar liquids. Hence there is a strong tendency to form
nanostructures or nanoclusters due to hydrogen bonding
as discussed earlier. In the low shear rate range the equilibrium nanostructure remains undisturbed and the samples behave as a stable non-flocculated sol with constantviscosity. As the shear rate is increased the short-range
hydrodynamic lubrication forces can bring the nanoclusters closer and facilitate the microstructure formation and(b)hence can form a gel like structure. This can lead tohigher rates of energy dissipation and abrupt increase in
the dynamic viscosity. Further investigation is needed toshed more light on the shear thickening behavior of thesamples. Since the viscosity of colloids strongly depends
on temperature, temperature dependence of viscosity has
been measured for two samples, namely S1 and S4 anda typical variation is shown in Figure 8(b). The viscosity
decreases for both the sample rapidly with the increasing
temperature and shows an Arrhenius type of behavior i.e.,= o exp [E/RT] where E is the activation energy ando is the pre-exponential factor. The logarithm of the visFigure 8. (a) Variation of shear viscosity of colloidal SiO2 nanoparticositywith inverse of the temperature for the two samples
cles as function of shear rate at 27 C. (b) Temperature variation of shear
isshown
in Figure 8(b) (inset). The activation energies forviscosity for two colloidal SiO2 samples namely S1, S4 at shear rate 5
the two samples, S1 and S4 are 4031 and 3776 cal/mol
s−1 and inset shows the Arrhenius behavior of viscosity.
Adv. Sci. Eng. Med. 6, 965–973, 2014971

Synthesis and Characterization of Ultra-Fine Colloidal Silica Nanoparticles

Qasim et al.

4. CONCLUSIONS

Figure 9. UV-vis absorption spectra of colloidal SiO2 nanoparticles
sample S1, S3, S4 and S6 and variation of max with particles size (inset).

respectively. These values are comparable to the activationenergy reported in the binary mixture of pure water andethanol with similar concentration.34

Ultra-fine colloidal silica nanoparticles of different sizes
and morphology have successfully been synthesized by amodified Stober process using a low frequency ultrasonicbath. Obtained sizes varied from 5–31 nm. Colloidal silica
nanoparticles with such tight distribution in sizes, whichconveniently fall into primary size range, have been prepared without the use of any surfactant or metal salt as catalyst. The effect of reactants concentration (TEOS, water,
ammonia and alcohol) on particle sizes has been investigated. The particle sizes are seen to increase with increasing TEOS and ammonia concentration but decrease withincreasing water concentration. Alcohol of larger molecular weight enhances the particle growth. The maximum in
UV-Vis absorption spectra is seen to shift at higher wavelengths with decreasing size of the silica nanoparticles.
At low shear rate (<200 s−1 ), a typical Newtonian behavior is observed for all the colloidal silica nanoparticles, but
at high shear rate (>200 s−1 ), a shear thickening behavior
is observed.

Acknowledgment: Mohd Qasim greatly acknowledges
the financial support obtained from UGC in the form of
3.9. UV-Vis Spectroscopy
MANF fellowship in carrying out this research work. The
In silica nanoparticles the optical phenomena is related totechnical support received from the Centre of Excellence
the presence of different defects due to incomplete forin Materials Science (Nanomaterials) Aligarh Muslim Unimation of Si O Si tetrahedral network at the silica surversity, School of Engineering Sciences and technology
35 Publishing Technology to: unknown
byface, such as oxygen and siliconDelivered
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the effect of particles size on electronic
absorption behavat the Universityof Hyderabad is greatly appreciated.
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Publishers
ior, UV-Vis spectra of powder samples S6 (5.4 nm), S1(9.4 nm), S3 (12 nm), S4 (15 nm) have been taken after
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Received: 26 March 2014. Accepted: 29 April 2014.

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