Journal of Biomaterials and Nanobiotechnology, 2011, 2, 390-399
doi:10.4236/jbnb.2011.24048 Published Online October 2011 (
Copyright © 2011 SciRes. JBNB
Silver Nanoparticles: Green Route, Stability and
Effect of Additives
Zaheer Khan1,2*, Javed Ijaz Hussain1, Sunil Kumar3, Athar Adil Hashmi1, Maqsood Ahmad Malik2
1Department of Chemistry, Jamia Millia Islamia (Central University), New Delhi, India; 2Department of Chemistry, Faculty of Sci-
ence, King Abdul Aziz University, Jeddah, Saudi Arabia; 3Department of Chemistry, University of Delhi, New Delhi, India.
Email: *
Received March 27th, 2011; revised June 29th, 2011; accepted July 26th, 2011.
Colloidal silver nanoparticles were prepared by reducing silver nitrate with oxalic acid in presence of cetyltrimethylammo-
nium bromide (CTAB). The synthesized silver particles show an intense surface plasmon band in the visible region. The
work reported in this paper describes the effect of concentration of various additives (NaCl, NaBr, NaNO3, Na2SO4 and
NaH2PO4) and ammonia on the growth and stability of Ag-nanoparticles. In all the cases the rate decreases as the [electro-
lytes] or [ammonia] increases. The nature, polarizability and coordinating ability of the anions play vital roles for nucleus
formation and the growth process, which subsequently form different size particles. Transmission electron microscopy, se-
lected areas electron diffraction, and UV-visible spectroscopy have been employed to characterize Ag-nanoparticles. The
effect of the following variables on the particle size and size distribution was investigated: the [oxalic acid], [CTAB] and
[Ag+]. The nanoparticles are stable in NaNO3 and NaH2PO4 solutions ; bu t NaCl, NaBr and Na2SO4 causes their aggrega-
Keywords: Silver Nanoparticles, Stability, Oxalic Acid, Additives, Ammonia
1. Introduction
The literature is replete with the investigations of the use
of noble metal nanoparticles(NP) (silver, gold and plati-
num) deposited on nanosized TiO2 and/or WO3 for car-
rying out the photocatalytic destruction of oxalic acid in
aqueous solution [1-4]; but the preparation and charac-
terization of Ag-NP involving oxalic acid (sacrificial
electron donors) has been neglected. Oxalic acid, an im-
portant reductant in many organic and bioorganic redox
reactions and its reduction process is eco-friendly and
has gained importance in green chemistry. It is found
naturally in varying concentrations, occurring in many
plants such as tea, rhubarb, spinach, cocoa, nuts, berries
and beans. Organic oxalic acid is essential for human
body and is completely harmless if consumed in organic
form. Inorganic oxalic acid causes trouble to human
body. When ingested, oxalic acid removes calcium from
the blood. Kidney damage can be expected as the cal-
cium is removed from the blood in the form of calcium
oxalate. Within the sub-group of ‘weak acids’, it is rela-
tively strong (pK1 = 1.22; pK2 = 4.28; E0 = +0.49 V for
C2O4H2/CO2 system in acid solution).
Advanced Ag-NP have been synthesized by thermal
decomposition and microwave irradiation methods by
the decomposition of silver oxalate in a glycol medium
using polyvinyl alcohol and polyvinyl pyrolidone as the
capping agents, respectively [5,6]. Itoh, et al prepared
silver nanoparticles via thermal decomposition of ox-
alate-bridging silver oleylamine complexes at 150˚C [7].
Even though many methods have been reported in the
literature, the interest in the field of genesis of Ag-NP
has not diminished. Among the various methods avail-
able, chemical reduction of metal salts is one of the pos-
sible ways of producing Ag-nanostructures as stable,
colloidal dispersions in water or organic solvents [8].
The chemical reduction methods are probably the most
versatile, economical and easy to control the shape and
size of metal-NP. For stabilization of small particles, the
use of polymers, phospholipids, triblock polymers,
ligands, solid matrix and surfactants has also been sug-
gested [9-14]. Although a number of stabilizers are
available for the stabilization of nanosize particles in
solution, these are associated with some demerits [15].
The preparation of these materials in green solvents, such
as water [16] and other non-toxic solvents is becoming
popular. In this content, surfactant aggregates, especially
micelles, reverse micelles and macro emulsions, will get
Silver Nanoparticles: Green Route, Stability and Effect of Additives391
an edge over other stabilizers [17-20].
Henglein prepared the colloidal silver particles by ra-
diation method using 2-propanol, AgClO4 and poly-
phosphate as reductant, oxidant and stabilizer, respec-
tively, discussed the effects of adsorbed additives, or-
ganic solvent, and some metal cations (KI, NaSH,
C6H5SH, H2S, O2, CCl4, Na+, Ag+, Ba2+, Cd2+, Ni2+, Hg2+)
on the stability of resulting silver particles, and suggested
that Ag4
2+ species can be stabilized for along time in
presence of a polyanion even under air and growth stops
at the stage of this species [21-25]. Chemisorbed metal
cations effect was interpreted in terms of the donation of
electron density from the silver particles to the adsorbed
cations. Henglein also monitored the stepwise growth of
first silver clusters Ag+ ion reduction in aqueous solution
by spectroscopic methods [26]. Considerable spectro-
scopic data have accumulated on the synthesis of various
Ag-NP by silver-mirror reaction. It has been established
that ammonia concentrations and nature of reducing
agents play an important role in controlling the mor-
phologies of NP [27,28]. However, details of silver(I)
reduction (Ag-NP formation) involving oxalic acid as
reductant are not yet well-known in the absence and
presence of additives or ammonia. We have chosen ox-
alic acid to see the effects of different additives, because
its oxidation product, i.e., CO2, has no complex-formi-
ning tendency. A study of such a redox reaction should
provide information relevant to the hypothesis previously
advanced to the silver-mirror reaction [29,30]. We have
carried out the present study with the following aims: (1)
to determine the effects of different variables on the rates
of Ag-NP; (2) to establish the role of different additives
and ammonia on the growth of Ag-NP; and (3) to deter-
mine the effects of inorganic salts on the stability of sil-
ver sol. The observed results and the probable explana-
tions detailed in this paper.
2. Experimental Section
2.1. Chemicals
Oxalic acid (C2H2O4.2H2O, reductant, 99%), silver ni-
trate (AgNO3, oxidant, 99%), ammonia, cetyltrimethyl-
ammonium bromide (CTAB), ammonia, and inorganic
electrolytes (NaCl, NaBr, NaNO3, Na2SO4 and NaH2PO4)
were obtained from Merck India and used with out fur-
ther purification. Deionized water was used to prepare all
of the aqueous solutions. AgNO3 solutions were stored in
a dark glass bottle.
2.2. Preparation and Characterization of Ag-NP
CTAB solution (0.01 mol·dm–3, 4.0 ml) was mixed with
0.01 mol·dm–3 AgNO3 (10.0 ml). A 0.01 mol·dm–3 oxalic
acid (2.0 ml) was added to this solution. The color of the
reaction mixture gradually changed from colorless to
prefect transparent yellow, indicating the formation of
Ag-NP [5]. UV-260 Shimadzu, with 1cm quartz cuvettes
spectrophotometer was used to monitor the optical trans-
mission spectra of the silver sol under different experi-
mental conditions. The prepared NP was analyzed by
transmission electron microscopy (TEM) on a transmis-
sion electron microscope (JEOL, JEM-1011; Japan).
Samples were prepared by placing a drop of working solu-
tion on a carbon-coated standard copper grid (300 mesh)
operating at 80 kV. An Accumet, fisher scientific digital
pH meter 910 fitted with a combination electrode was
used for pH measurements.
2.3. Determination of Critical Micelle
Concentration (CMC)
The cmc values were determined from plots of the spe-
cific conductivity versus [CTAB] in the absence and
presence of AgNO3 and oxalic acid. The break point of
nearly two straight-line portions in the plot are taken as
an indication of micelle formation and this corresponds
to the cmc of CTAB [31] and found to be 10.1 × 10–4,
8.7 × 10–4, 8.8 × 10–4, 8.8 × 10–4 and 8.9 × 10–4 mol·dm–3
for water + CTAB, CTAB + oxalic acid (2.0 × 10–4
mol·dm–3), CTAB + AgNO3 (2.0 × 10–4 mol·dm–3),
CTAB + oxalic acid + AgNO3, respectively, at 30˚C.
3. Results and Discussion
3.1. Optimization of Reaction Conditions for
Transparent Ag-Sol
Generally, aqueous solutions of oxalic acid and AgNO3
were used to the preparation of silver oxalate (white
precipitate) [5-7]. Therefore, choice of the best condi-
tions for the preparation of Ag-NP is a crucial problem
that we address first. Under our experimental condi-
tions used ([oxalic acid (from 4.0 × 10–4 mol·dm–3 to 14.0
× 10–4 mol·dm–3), [Ag+] (from 4.0 × 10–4 mol·dm–3 to
20.0 × 10–4 mol·dm–3, [CTAB] (from 2.0 × 10–4 mol·dm–3
to 14.0 × 10–4 mol·dm–3 at 30˚C), we did not observe the
appearance of white precipitate, ruled out the possibility
of silver oxalate as the reaction product. It is well known
that colloidal aqueous solution of metal particles under-
goes acid hydrolysis or is unstable in acidic medium,
stability of metal nanoparticles depends strongly on the
pH of the working reaction mixture and its growth can be
stopped by adding the small amounts of minerals acids
[23,32,33]. Control of pH is not as straightforward in
micellar solutions as in ordinary solvents [34,35]. How-
ever, a series of experiments were performed in order to
see any change in the pH of the working solution. The
pH values were found to be nearly constant with in-
creasing [oxalic acid] (Table 1). It is not surprising from
Copyright © 2011 SciRes. JBNB
Silver Nanoparticles: Green Route, Stability and Effect of Additives
Copyright © 2011 SciRes. JBNB
Table 1. Values of kobs as a function of [oxalic acid], [CTAB] and [Ag+] for the Ag-NP formation at 30˚C.
104 [oxalic acid] (mol·dm–3) 104[CTAB] (mol·dm–3) 104[Ag+] (mol·dm–3) pH 104 kobs (s–1)
0.0 8.0 2.0 3.2 0.0
4.0 3.2 7.2
6.0 3.4 7.2
8.0 3.3 7.1
10.0 3.3 7.0
12.0 3.2 7.4
14.0 3.3 7.3
20.0 3.2 7.3
4.0 2.0 20 3.3 3.1
4.0 3.2 4.6
6.0 3.1 5.8
8.0 3.2 7.2
10.0 3.1 6.2
12.0 3.0 4.8
14.0 3.3 3.6
16.0 3.2 yellowish turbidity
20.0 3.1 yellowish turbidity
4.0 8.0 4.0 3.1 no yellow color
8.0 3.3 4.1
10.0 3.3 5.4
12.0 3.1 6.6
14.0 3.2 6.6
16.0 3.1 6.8
20.0 3.2 7.2
the fact that oxalic acid is a weak acid. Spectra of silver
sol solution possess a surface resonance plasmon (SPR)
band in the vicinity of 375 to 450 nm [36]. When AgNO3
(20.0 × 10–4 mol·dm–3) and CTAB (10.0 × 10–4 mol·dm–3)
reaction solution was allowed to react with oxalic acid
(4.0 × 10–4 mol·dm–3) in the absence and/or presence of
ammonia (from 0.0 to 60.0 × 10–4 mol·dm–3), the color-
less reaction solution became transparent yellow. UV-
visible spectral studies (Figure 1) showed that the
AgNO3-oxalic acid redox reaction resulted in SRP band
at 425 nm which was assigned to the Ag-NP [36]. It
should be emphasized here that before examining the
effect of electrolytes and ammonia on the stability and
growth of Ag-NP, the reaction was studied without add-
ing any agents too.
3.2. Effect of Reactant Concentrations and
Reduction of Ag+ ions by oxalic acid has been studied
kinetically as a function of [oxalic acid], [AgNO3] and
[CTAB]. The apparent first-order rate constant (kobs, s–1)
were determined from the slopes of ln[a/(1-a) versus time
plots [37,38]. The rate constant remained unchanged with
increase in [oxalic acid] (Table 1), showing first-order
dependence with respect to [oxalic acid]. Interestingly, as
the [CTAB] increased from 2.0 × 10–4 mol·dm–3 to 14.0
× 10–4 mol·dm–3, kobs increases, until it reaches a maxi-
mum, and then decreases monotonically due to the dilu-
tion effect. Kinetic determinations at higher [CTAB] (
16.0 × 10–4 mol·dm–3) were hampered due to the forma-
tion of yellowish turbidity (Table 1). We did not ob-
served the formation of yellow color at [AgNO3] = 2.0 ×
10–4 mol·dm–3. From these data, the over all mechanism
for the reaction can be represented in Scheme 1.
3.3. Salt Effect Results
Inert salts, especially the inorganic ones, act as catalysts
or inhibitors in the micelle mediated reactions [39,40].
Therefore, to see the effects of NaCl, NaBr, NaNO3,
Na2SO4 and NaH2PO 4 on the rate of Ag-NP, different
Silver Nanoparticles: Green Route, Stability and Effect of Additives393
300 400 500 600 700 800
Wav elength (nm)
Figure 1. UV-visible spectra of Ag-NP for [oxalic acid] = 4.0
× 10–4 mol·dm–3, [Ag+] = 20.0 × 10–4 mol·dm–3 and CTAB =
10.0 × 10–4 mol·dm–3 as a function of time 20 (), 40 () and
60 min () at 30˚C.
Scheme 1. Reduction of Ag+ by oxalic acid.
amounts of these salts were added to the reaction mixture
at 8.0 × 10–4 mol·dm–3 CTAB. Reaction solution became
turbid in presence of NaCl or NaBr ( 2.0 × 10–2 mol·dm–3),
which increases with these salts due to the strong Cl or
Br- ions affinity for Ag+ ions. The curves are explained
by taking into account two effects of anions, First, exclu-
sion of reactants from the reaction site (i.e., Stern layer,
as most of the ionic micelle mediated reactions are be-
lieved to occur in this region). As we increase the [elec-
trolytes], the 3, and 24
will try to get
incorporated into the reaction site through electrostatic
interactions, which are the normal behaviors found in the
literature [39-41]. Second, Fermi level of particles in-
creases with a rise in the concentrations of nucleoplilies
(3, and 24
) due to the adsorption of these
anions on to the surface of Ag-NP [23]. Hence, the kobs
decrease as the [electrolytes] increases. The difference in
the action of 3, and 24
ion is due to the
fact that 24
is specifically, i.e., high polarizability,
incorporated into the reaction site [42].
The adsorption of additives on colloidal silver parti-
cles in aqueous solution is accompanied by strong optical
changes. The SRP absorption band is damped and its
maximum is shifted to shorter or longer wavelengths,
depending on the nature of the adsorbed species and its
concentrations [21-25, 43, 44]. Stability of the Ag-NP in
presence of these salts was also investigated (Figure 2).
The SRP band remains unaffected in presence of NaNO3
or NaH2PO4 (from 25.0 × 10–3 to 75.0 × 10–3 mol· dm–3).
The intensity of SRP decreased sharply and the spectrum
became broader at low concentrations of Na2SO4. The peak
intensity was enhanced (hyperchromic-shift) by in- creasing
the [Na2SO4] from 25.0 × 10–3 to 75.0 × 10–3 mol·dm–3.
When mono-valent anions (e.g., 3 or 24
) are
added into the Ag-NP solution, they adsorb onto the sur-
face of silver cluster, 2
, without affecting the nano-
particle dispersivity and no change is observed in SRP
band. The bivalent anions such as , causes the in-
teraction of one anion with several Ag-NP at low con-
centrations, such type of multi-interaction causes the
aggregation of free Ag-NP, which in turn, decreases the
intensity of SRP band. When the anion concentration is
increased, enough anions adsorb onto the surface of
Ag-NP and a charge reversal of the shell occurs. As a
result, redispersal of the Ag-NP occurs. Consequently,
intensity of SRP band increases. Interestingly, no turbid
solution of AgCl or AgBr was detected in presence of
NaCl or NaBr. This result indicated that essentially all
Ag+ ions were transformed to the Ag0 during the redox
process and/or adsorbed on the surface of silver cluster,
. Thus, we may safely conclude that Cl or Br-
ions could not detach the adsorbed Ag+ from the 2
[37]. In case of NaCl, hyperchromic- and blue-shift of
the intensity and SRP band were observed with increas-
ing the [NaCl], respectively. Immediately after addition
of NaCl the stronger SRP band centered at ca. 425 nm
changed into a broad absorption shoulder at about 400
nm. On the other hand, different behavior was observed
for NaBr. No absorption at 425 nm of colloidal silver
appeared. It thus seems that the Ag-NP is rather unstable
in presence of NaBr under the same [NaCl] and is rap-
idly converted into larger particles. This red-shift is at-
tributed to the strong interactions between the adsorbed
Ag+ and Br ions on the surface of Ag2
, the nu-
cleoplilicity of the Br ion is better than that of Cl ion
[45,46]. The surface neutralization or adsorption slightly
induces the particle aggregation, as reflected by the shift
of SPB. It should be emphasized that the solutions were
non-opalescent before as well after the addition of Br as
viewed by the naked eye.
3.4. Ammonia Effect Results
Ammonia is a mandatory reagent for silver-mirror reac-
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Silver Nanoparticles: Green Route, Stability and Effect of Additives
Copyright © 2011 SciRes. JBNB
300 400 500 600700
Wa v e len gth (nm)
[NaCl](mol dm-3)
300 400 500 600 700
W a v e le n g th (n m)
[NaNO3] (mol dm-3)
300 400 500 600 700
W a vele n g th (n m )
[Na2SO4] (mol dm-3)
300 400 500 600 700
Wav elength (nm)
[NaH2PO4] (mol dm-3)
300 400 500 600 700
0.5 104[NaBr] (mol dm-3)
Wav elength (nm)
Figure 2. Effect of [electrolytes] on the surface plasma resonance band of Ag-NP. The concentration of Ag-NP was diluted
from the original solution by 4 times.
tion and their concentration play a major role in control-
ling the Ag-NP size and shape [27,47-49]. In order to
gain insight into the role of ammonia, the oxalic acid-
silver(I) reaction was also studied in presence of [ammo-
nia]. The evolution of the absorption spectra with time is
presented in Figure 3 in an ammonia solution containing
4.0 × 10–4 mol·dm–3 oxalic acid, 20.0 × 10–4 mol·dm–3
AgNO3 and 10.0 × 10–4 mol·dm–3 CTAB. As shown in
Figure 1 the SRP band centered at about 425 nm de-
creased in intensity and shifted to shorter wavelengths
with increasing time (blue shift of about 15 nm). It
seemed possible that the decrease in the intensity was
related to the complexation of ammonia with Ag+ ions
during the course of reaction. To conform this hypothesis,
Silver Nanoparticles: Green Route, Stability and Effect of Additives395
250 350450 550 650 750
Wave length (n m)
Figure 3. UV-visible spectra of Ag-NP for [oxalic acid] = 4.0
× 10–4 mol·dm–3, [Ag+] = 20.0 × 10–4 mol·dm–3 and CTAB =
10.0 × 10–4 mol·dm–3 in presence of [ammonia] = 20.0 × 10–4
mol·dm–3 as a function of time 20 (), 40 (), 60 (), 80 ()
and 100 min (*) at 30˚C.
the experiments were performed under different condi-
tions: (i) addition of oxalic acid followed by ammonia
and vice versa and (ii) allowing ammonia and Ag+ ions
reaction to proceed for 10 min, followed by addition of
4.0 × 10–4 mol·dm–3 oxalic acid. Observations of these
experiments are summarized in Figure 4, as absorbance-
and kobs-[ammonia] profiles. At 425 nm, the absorbance
first decreased then increased continuously and de-
creased with [ammonia], respectively, for the certain
reaction time i.e., 60 and 20 min, to the experimental
condition (i) (Figure 4, and ) whereas the absorbance
first decreased until it reached a maximum then de-
creased with [ammonia] to the addition of ammonia fol-
lowed oxalic acid (Figure 4, ). For the reaction condi-
tions (ii), absorbance decreased progressively with [am-
monia] (Figure 4, ). The reduction of Ag+ ions by ox-
alic acid results in a fast formation of Ag-NP. Spectro-
scopic and kinetic results (Figures 1, 3, 4) show that in
the presence of ammonia the reduction rate is slower and
that significant changes in the reactivity of Ag+ ions oc-
cur in solutions containing ammonia. It is clear that am-
monia play a crucial role in the reduction of Ag+ ions by
oxalic acid. In the presence of ammonia the Ag+ ions are
expected to form [Ag(NH3)2]+ [50]. The intensity and
rate of the Ag-NP decreased after the addition of ammo-
nia, indicating that the formation of [Ag(NH3)2]+ species,
which ultimately decreases the reduction potential of Ag+
ions as well as the generation of metallic silver at the
early stages of the reaction (separates the Ag+ ions from
the oxalic acid). Instant and/or after 10 min addition of
oxalic acid into the reaction solution (AgNO3 and am-
0 102030405060
Absorbance at 425 nm
104[amm onia] (mol dm-3)
Figure 4. Effect of [ammonia] on the surface plasmon res
rmation ( and ). At higher [ammonia] 30.0 × 10-4
sence of ammonia, Scheme 1 mechanism can be
oxalic acid by [Ag(NH3)2] is
TEM images of silver sol (or ange-
nance absorbance. Reaction conditions: [oxalic acid] = 4.0 ×
10–4 mol·dm–3, [Ag+] = 20.0 × 10–4 mol·dm–3 and CTAB = 8.0
× 10–4 mol·dm–3 under different order of ammonia mixing:
(1) Ag+, CTAB, oxalic acid and ammonia (time = 20 ()
and 60 min(), (2) Ag+, CTAB, ammonia and oxalic acid
(time = 20 () and 60 min ( ) and (3) Ag+, CTAB, ammonia
and oxalic acid after 10 min ().
mol·dm–3, there is no significant changes in the reaction
rate. On the basis of these observations, we may safely
concluded that oxalic acid is capable to reduce Ag+ to
Ag0. In presence of ammonia there is a competition be-
tween oxalic acid and ammonia to react with Ag+. Obvi-
ously, ammonia has strong affinity towards Ag+ and
small time is enough to form [Ag(NH3)2]+. Complexation
lowers the reduction potentials to such an extent that the
reactive site (electrons gaining tendency) of Ag+ is par-
tially blocked by the presence ammonia molecules in
[Ag(NH3)2]+ [50] but not totally prevent the Ag+ reduc-
tion. These results are also in agreement with the idea
that once a metal atom which acts as a nucleation center
is formed; it acts as a catalyst for the reduction of re-
maining metal ions present in solution via autocatalysis
In pre
odified as Scheme 2.
The direct oxidation of +
t possible. Thus, the reaction may generally be ex-
pressed by Equation (6) (Kos is the rapid equilibrium for
the encounter complex formation between the redox
couple [52]). By analogy with previous results [52], we
assume that complex decomposes in a rate-determining
one-step one-electron oxidation-reduction mechanism to
give Ag0 and corresponding radical due to the sacrificial
electron donor’s property of oxalic acid.
3.5. TEM Images
Figures 5 and 6 show
monia) also decreases the intensity and rate of Ag-NP
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Silver Nanoparticles: Green Route, Stability and Effect of Additives
Copyright © 2011 SciRes. JBNB
Scheme 2. Oxidation of oxalic acid by Ag-ammonia complex.
Figure 5. TEM images of Ag-NP. Reaction conditions: [oxalic acid] = 4.0 × 10–4 mol·dm–3, [Ag+] = 20.0 × 10–4 mol·dm–3 and
CTAB = 10.0 × 10–4 mol·dm–3.
Figure 6. TEM images of Ag-NP. Reaction conditions: [oxalic acid] = 4.0 × 10–4 mol·dm–3, [Ag+] = 20.0 × 10–4 mol·dm–3 and
CTAB = 10.0 × 10–4 mol·dm–3 in presence of [ammonia] = 20.0 × 10-4 mol·dm–3.
Silver Nanoparticles: Green Route, Stability and Effect of Additives397
icles were prepared based on the re-
[1] V. Iliev, D. T Eliyas and L. Pet-
ental, Vol. 63, No. 3-4, 2006, pp. 266-271.
red color prepared by the oxalic acid-Ag+ reaction in Environm
absence and presence of ammonia). One can see
well-dispersed spherical Ag-NP, with size ranging from
3.5 to 9 nm and a monodisperse size distribution can be
easily evidenced from the TEM image of Figure 5(a).
The sol is quite polydisperse. The monodispersity is
manifested also in the 2D hexagonal colloidal assemblies
observed in most of the micrographs we scanned [53].
As can be seen in Figure 6(a) and (b) (typical example),
show that the particles are spherical, polydispersed, and
small-sized of diameter ca. 16 nm in presence of ammo-
nia. Interestingly, the CTAB stabilized Ag-NP were
formed faster than in the CTAB + ammonia containing
silver sols. TEM image of Figure 6(a) also reveal that
these Ag-NP aggregate and/or deposited onto the surface
of particles in an unsymmetric manner to form neck-
lace-like structure. Such type of aggregation we did not
observed without ammonia. Presence of ammonia, de-
creased the nucleation rate, because the ammonia present
in the reaction mixture reduce the reduction potential of
Ag+ ions which inhibit the particle nucleation and growth
(Table 1). As a result, the size of the particles is higher
in presence of ammonia. Comparison of spectroscopic,
kinetic and TEM data clearly indicates that the shape, the
size distribution, nature of reaction-time curves, mecha-
nism, aggregation, and polydispersity strongly depends
on the presence of [ammonia]. The crystalline nature of
the Ag-NP was revealed by the electron diffraction pat-
terns. The typical selected-area diffraction pattern are
shown in Figures 5(c) and 6(c) and it clearly exhibited
diffraction rings with interplanar spacing The rings pat-
terns are consistent with the plane families{110}, {111},
{200}, {220}, {311}, {331}and {422}, of pure
face-centred cubic silver structure in absence and pres-
ence of ammonia [54, 55].
4. Conclusions
The silver nanopart
duction reaction of silver nitrate and oxalic acid using
CTAB as a stabilizing agent. Additives and ammonia
plays a major role in this method and their concentrations
are an important parameter to determine the growth rate.
TEM and selected area electron diffraction confirmed
that formation of spherical, aggregated and face-cen-
tered-cubic Ag-NP, respectively. The AG-NP is stable in
NaNO3 and NaH2PO4 solutions but the presence of NaCl,
NaBr and Na2SO4 causes their aggregation.
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