R. Padmavathy and K. V. Rajendran
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AZojomo (ISSN 1833-122X) Volume 6 August
Result and Discussion
SnO2 nanocrystaline powders with an average diameter of 12-60 nm
have been successfully synthesized using different surfactants and a surface
modifier, cetyltrimethylammoniumbromide (CTAB), Polyethylene glycol (PEG) and
citric acid respectively at different pH via hydrothermal method. The
SnO2 nanopowders were characterized using X-Ray diffractometer (XRD),
UV-absorption spectroscopy, Scanning electron micrograph (SEM). XRD pattern
reveals that a high crystalline rutile SnO2 nanoparticles have been
synthesized. Significant morphological changes were observed in SEM due to
different surfactant, surface modifier and pH. The absorption edges exhibited a
blue shift, which can be ascribed to the quantum confinement effect. The
probable growth mechanisms of SnO2 are discussed.
SnO2, nanparticles, nanorods, nanopowders,
Semiconductor nanoparticles have been extensively studied from both
experimental and theoretical viewpoints, owing to their potential application in
solar energy conservation, Photocatalysis and in the field of optoelectronics
[1-4]. Metal oxide nanoparticles play an important role in the selective surface
modification of various substrates in the form of coating. Tin oxide is a direct
band gap n type semiconductor (Eg=3.6eV) and has been the most strategic
material used in applications, gas sensing, transparent electrodes and liquid
crystal displays etc. [5,6]. Low dimensional nanomaterials such as nanorods and
nanowires are expected to have properties notably different from those of bulk
material . Recent studies have shown that many fundamental physical or
chemical properties of semiconductor materials strongly depend on the size and
morphology of the materials. Several physical and chemical synthetic methods are
available for the fabrication of this material including solgel  CVD ,
annealing precursor powder , thermal evaporation and microwave heating .
Generally these preparation mentioned above usually involve high temperature,
complex procedures, sophisticated equipment or rigorous experimental conditions
Pang et.al  calcinated SnO2 nanocrystals at 600°C Li.et.al 
have prepared SnO2 box beam on quartz substrate via a simple vapour
deposition process at 1150°C.Mild method focus on micelle technique, electrical
deposition and hydrothermal method . Among these a single source
hydrothermal method is simple, cost-effective, nonpolluting and energy
economical method in the preparation of SnO2 nanoparticles. In this
study the focus is laid on the simple and efficient hydrothermal method for the
preparation of SnO2 nanoparticles and nanorods and the influence of
pH, surfactants and surface modifying agent on the size, morphology and optical
properties are discussed.
SnO2 nanopowders were successfully prepared by means of dissolving
0.002 mol of SnCl2. 2H2O (A.R) in 50ml of water containing
appropriate amount of NaOH. The above was stirred vigorously till a clear
solution was obtained, then 0.002 mol of a CTAB was added to the above solution.
After stirring the reactants were put into Teflon-lined stainless steel
autoclave of 100ml capacity. The sealed autoclave was then maintained at 130°C
for 24 h, and then cooled to room temperature naturally. A yellow precipitate
was then collected and was washed with deionized water and absolute alcohol
several times and then dried at 60°C for 3h.The obtained samples were then
calcinated at 400°C for 2h. The same procedure was followed for the preparation
of SnO2 nanoparticles using PEG and citric acid.
The prepared samples were subjected to different characterization including
X-Ray diffraction (XRD), UV-Vis absorption spectroscopy (UV) and scanning electron
micrograph (SEM). The crystalline structure of materials was analyzed by X-ray
diffraction (XPERT PRO with CuKα radiation λ=1.5406Å) at
scanning speed of 2°/min from 20° to 80°. The surface morphology was analyzed
using Scanning electron micrograph (JEOL, JSM-67001). The absorption spectra
were carried out in the range of 200 -2000nm by using SHIMDZU UV 310PC.
Result and Discussion
Fig.1 shows the XRD patterns of as synthesized SnO2 at 400°C,
exhibits varied intense peaks that are easily distinguishable. The peaks were
indexed as 110, 101, 200, 211, 220, 002, 310, 112, 301, 202, 321 and are in
agreement with the reported value of SnO2 (JCPDS 41-1445). No
characteristic peaks of impurities, such as tin metal and surfactants were
observed implying the formation of pure and single-phase tin oxide. From the
figure it can be noted that the peak intensity of the samples prepared using
CTAB was notably higher than samples prepared by citric acid and PEG.
Figure 1. XRD pattern of prepared SnO2 pattern
using (a) citric acid, (b) CTAB, (c) PEG at pH=2.
The crystalline sizes obtained using different surfactants, surface modifier
at different PH were calculated using Debye Scherer's equation  and listed
in Table 1.
Table1. Grain Size and Morphology of SnO2 nanoparticles
synthesized using surface modifier, cationic and non-ionic surfactant at different
Fig. 2 represent the single XRD peak of the sample using citric acid to
obtain the value of full wave half maxima â which was found to be 1.26
Figure 2. Single XRD peak of prepared SnO2 pattern
using citric acid.
The absorption spectra of the samples obtained from different surfactant and
surface modifying agent were shown in Fig. 3 exhibits a blue shift in the
absorption band edge which could be attributed to well known quantum size effect
Figure 3. Absorption spectra of synthesized SnO2
using (a) citric acid, (b) CTAB, (c) PEG at pH=2.
One of the key factors associated with the nanomaterial is the size
dependence of their physical and chemical properties. As the small size of
nanoparticles, result in spatial confinement of the charge carrier wave
function, which is termed as quantum size effect. The quantum size effects not
only include blue shift of the absorption edge and exciton energy but it also
cover the increase in exciton association strength and biding energy .
Considering the blue shift of the absorption position from the bulk
SnO2, the absorption onsets of the present samples can be assigned to
the direct transition of electron in SnO2 nanocrystals . In order
to obtain the band gap value the slope was drawn to the higher wavelength region
as reported. The sample synthesized using surface modifier exhibited a
absorption edge at 310 nm which is blue shifted considerably compared with the
samples prepared using cationic (316 nm) and non-ionic surfactants (332 nm).
From the graph (Fig. 4) it can be observed that the grain size and band gap
varies linearly, implying that as the grain size increases the absorption edge
is shifted towards the higher wavelength region. As pH varies from 2 to 7 the
absorption spectra is shifted towards red implying larger particle size at
higher value of pH.
Figure 4. Relation between grain size and band gap.
Fig. 5 depicts the SEM images of SnO2 nanoparticles synthesized by
using two different surfactants, and a surface modifier (CTAB, PEG, Citric acid)
at three different pH, which have a great influence in determining the
morphology and size of SnO2 nanoparticles. In the case of surface
modifying agent citric acid no distinct morphological changes (spherical shape)
were observed but exhibited variations in the grain size at different pH values.
In the case of cationic surfactant CTAB, the electrostatic interaction takes
place between CTA+ cations and Sn(OH)62- the
cation CTA+ condense into aggregates in which counterions
Sn(OH)62- anions are interrelated in the interfaces
between the head group to form CTA+- Sn(OH)62-
pair. Morphological changes were observed by increasing the value of pH.
Spherical shape nanoparticles were observed at pH=2 whereas both particles and
rods at pH=7 and the formation of rod could be due to the oriented growth
aggregation of nanoparticles. PEG being a non-ionic surfactant SnO2
formation is not possible by electrostatic interaction, and the formation can be
attributed to weak Vanderwall's interaction. Spherical shape was observed at
pH=2 and then modified into cauliflower like shape at pH=5 latter exhibited a
triangular shape for pH=7. The change in the morphology with different
surfactants, surface modifying agent and pH are listed in table1.
Figure 5. SEM images of SnO2 using (a) citric
acid, (b) CTAB, (c) PEG at different pH values.
SnO2 nanoparticles having particle sizes from 12 - 60 nm were
productively synthesized by a simple hydrothermal method. XRD results showed
that the nanoparticles and rods were single crystalline SnO2 with
rutile structure. SEM photographs exhibits different morphologies for different
pH and surfactants. The absorption edge showed a prominent blue shift. From the
above discussion it can be concluded that SnO2 nanoparticles prepared
using citric acid a surface modifier showed a lesser particle size compared to
the samples prepared from CTAB a cationic surfactant and PEG a nonionic
surfactant. This convenient synthesis strategy can be applied as general
approach for the preparation of other metal oxides nanoparticles and
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R. Padmavathy and K. V. Rajendran
E-mail: [email protected]
This paper was also published in print form
in "Advances in Technology of Materials and Materials Processing", 11 (2009)