Colloids and Surfaces A: Physicochem. Eng.

Aspects 399 (2012) 11–17

Contents lists available at SciVerse ScienceDirect

Colloids and Surfaces A: Physicochemical and

Engineering Aspects
journal homepage: www.elsevier.com/locate/colsurfa

Reduction of aromatic nitro compounds on colloidal hollow silver nanospheres

Raji Vadakkekara, Mousumi Chakraborty ∗ , Parimal A. Parikh ∗∗
Department of Chemical Engineering, S. V. National Institute of Technology, Surat – 395 007, Gujarat, India

a r t i c l e i n f o a b s t r a c t

Article history: Hollow silver nanospheres colloids were successfully prepared by a simple reaction of silver nitrate
Received 21 December 2011 (AgNO3 ), sodium hydroxide (NaOH) and hydroxylammonium hydrosulfate ((NH2 OH)2 ·H2 SO4 ) in the
Received in revised form 13 February 2012 presence of gelatin. Formations of colloidal hollow silver nanospheres were observed by UV–vis
Accepted 17 February 2012
absorption spectroscopy, particle size analyzer and transmission electron microscopy (TEM) and X-ray
Available online 25 February 2012
diffraction (XRD). Dispersion destabilization of nanospheres colloids was detected by Turbiscan. Supe-
rior catalytic performance was observed in a test reaction of 4-nitrophenol (4-NP) and 4-nitroaniline
Keywords:
(4-NA) reduction in the presence of freshly prepared ice cold aqueous solution of sodium borohydride
Colloidal hollow silver nanospheres
Stability
(NaBH4 ) at room temperature. The reduction process was monitored by UV–vis absorption spectroscopy.
Nitro compounds The reaction followed pseudo-first order kinetics with respect to nitro compound and the rate constant
Reduction was found to be larger compared to other silver catalyst used for the reduction reaction.
Kinetics © 2012 Published by Elsevier B.V.

1. Introduction approaches have been used for the synthesis of hollow metal-
lic spheres such as heterophase polymerization combined with a
Nanostructures with hollow interiors have attracted much sol–gel process, emulsion/interfacial polymerization, laser pyroly-
attention and have been intensely investigated in recent years sis, ultrasonication, UV radiation method, hydrothermal reactions
because of their low density, low toxicity, large specific surface at high temperature photoreduction and layer-by-layer (LBL) self-
area, high chemical and thermal stability, and surface perme- assembly techniques [9,15–18]. The templates, such as silica beads,
ability properties which are substantially different from those of lipsome, polypyrrole, Sephadex G-100 beads, polyvinylpyrroli-
solid nanoparticles [1–3]. Nanoshell is a new class of nanoparticles done (PVP), polystyrene spheres and poly-(styrene-methyl acrylic
with tunable plasmon resonance, consists of a spherical dielec- acid) (PSA), have been used to synthesize hollow platinum, pal-
tric nanoparticles surrounded by an ultrathin, conductive, metallic ladium, gold and silver spheres [5,19–22]. Compared with other
layer allowing materials to be specifically designed to match the templates, polystyrene spheres are commonly used as templates
wavelength required for a particular application [4,5]. Nanoshell because they possess better dispersibility and tunable sizes ranging
can be designed and fabricated by varying the composition and from nanometers. However, the surface properties of polystyrene
dimensions of the layers of nanoparticles. Nanostructures with hol- spheres significantly affected the formation of silver shells.
low interiors have been widely studied in many areas including Hollow silver spheres were successfully prepared by Wang et al.
catalysis, drug delivery, as the stationary phase for selective sepa- [13] by reducing AgNO3 with ascorbic acid and using poly-(styrene-
ration, artificial cells, light fillers, photonic crystals, low-dielectric methyl acrylic acid) (PSA) spheres as templates in the presence
prosthetic materials and protection of environmentally sensitive of sodium polyacrylate as a stabilizer. Ag/polypyrrole composite
biological species, medical imaging, sensors, photothermal therapy, nanoparticles and hollow composite capsules had been prepared
anode materials for limiting batteries, confined-space chemical by Shi et al. [19] via a one-step redox reaction between silver
reactors, etc. [6–11]. nitrate and pyrrole monomer at room temperature. Ferrer et al.
Nanostructures with hollow interiors are commonly prepared [22] reported about the internal structure of Ag–Au bimetallic
by coating the surface of colloidal particles with thin layers of nanoparticles with hollow interiors and alloyed shells, synthesized
its desired materials, followed by selective removal of colloidal by chemical reduction of metallic precursor and subsequent gal-
templates by calcination or wet chemical etching [12–14]. Many vanic replacement reaction. Lee et al. [5] synthesized silica/silver
heterogeneous particles with hollow structure by water-in-oil
(W/O) emulsion and polyol process. Yang et al. [23] developed a
novel route to wrap inorganic nanoparticles in polymer hollow
∗ Corresponding author. Tel.: +91 261 2201641; fax: +91 261 2227334.
∗∗ Corresponding author. spheres (polystyrene-encapsulated silver hollow sphere), which
E-mail addresses: mch@ched.svnit.ac.in (M. Chakraborty), pap@ched.svnit.ac.in included self-assembly polystyrene (PS) latex particles at the aque-
(P.A. Parikh). ous/oil interface, sintering and ␥-ray radiation. Hollow spheres and

0927-7757/$ – see front matter © 2012 Published by Elsevier B.V.

doi:10.1016/j.colsurfa.2012.02.016
12 R. Vadakkekara et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 399 (2012) 11–17

nanospheres of Au had been prepared by Yang et al. [24] in the pres- agents, gelatin as stabilizing agents and NaOH as accelerator
ence of gelatin and found that the temperature of the reaction is the reagent. 1 × 10−2 mol L−1 AgNO3 solution and 3 × 10−2 mol L−1
key factor for the production of the Au hollow and nanospheres. (NH2 OH)2 ·H2 SO4 solution were mixed separately with 2% (mol%)
Darroudi et al. [25,26] synthesized Ag-NPs using glucose–gelatin gelatin solution. Then, the pH of AgNO3 solution was adjusted
mixture and only gelatin as a reducing and stabilizing agent. They to 9 with NaOH. Equal volume of (100 mL) of AgNO3 and
found that the particle size of Ag-NPs obtained in gelatin solution (NH2 OH)2 ·H2 SO4 solution were mixed and continuously stirred for
was smaller than in glucose–gelatin solution. To the best of our 6 h. The temperature was kept maintained at 5 ◦ C.
knowledge, till date nobody reported to synthesize colloidal hollow
silver nanospheres using gelatin as protective agent or inhibitor. 2.3. Procedure for the reduction of nitro compounds using
In the present work, attempts have been made to synthesize hol- colloidal hollow silver nanospheres as catalysts
low Ag nanospheres colloids using gelatin as both inhibitor and
stabilizer. Gelatin is more eco-friendly, easily available and thus Catalytic efficiency of the colloidal hollow silver nanospheres
offers cheaper alternative than other stabilizer. Gelatin is a natural for the reduction of 4-NP and 4-NA was analyzed. For the reduc-
biopolymer and it has been widely used as a protective medium for tion reaction, in a standard cuvette having 1-cm path length, 1 mL
synthesizing nanospheres, hollow spheres, porous microspheres; of water, 1 mL of nitro compound (5 × 10−4 M), 1 mL of ice cold
wound dressings, drug carriers, and tissue scaffolds. As a biopoly- solution of NaBH4 (8 × 10−2 M) were taken. Next 100 ␮L of col-
mer, gelatin appears the most suitable as it forms homogeneous, loidal hollow Ag nanospheres (0.85 × 10−4 g Ag) was added to the
optically transparent and thermoreversible physical gel in aqueous above solution mixture and time dependent absorption spectra
solvent [27–30]. were measured. All reactions were carried out at room temper-
Catalysis is the most important chemical application of metal ature and atmospheric pressure. The detailed kinetic aspects of
nanoparticles. The reduction of nitro compounds to amino com- the reduction were studied under varied experimental condition.
pounds with an excess amount of NaBH4 has often been used The catalytic efficiency of colloidal hollow Ag nanospheres towards
as model reaction to examine the catalytic performance of metal reduction of these nitro compounds was compared.
nanoparticles [31–36]. Aromatic amines are generally prepared by
catalytic hydrogenation and stoichiometric reduction of aromatic
2.4. Characterization techniques
nitro compounds. Aromatic amines are widely used as the inter-
mediates for the synthesis of dyes, pharmaceuticals, agrochemical
The absorption spectra of hollow Ag nanospheres and rate of the
and wide range of biological activity [30,37,38]. Kundu et al. [31]
reduction reaction were recorded on a UV–vis spectrophotometer
synthesized silver nanoparticle catalysts in silica matrix (SNSM)
(HACH, Germany). Size of the nanospheres was obtained using par-
which was found to be active for the reduction of aromatic nitro
ticle size analyzer (Malvern Zetasizer, Nano ZS 90, U.K.). Surface
compounds to the corresponding amino derivatives. Hwang et al.
morphology was investigated with transmission electron micro-
[33] studied the reduction of nitro compounds on Pd colloids pre-
scope (TEM), Philips Tecnai-20 operated at 200 kV provides 0.27 nm
pared by ␥-irradiation. The silver nanoparticles with about 10 nm
point resolution. TEM was prepared by one droplet of nanoparti-
diameter were immobilized onto the halloysite nanotubes and used
cles sample on carbon film of copper grid having 300 mesh size and
for the catalyzed reduction of 4-NP with NaBH4 in alkaline aque-
3 nm diameter. XRD pattern of the sample was examined by X-ray
ous solutions [35]. Leelavathi et al. [36] used reduction of nitro
diffraction (XRD) on a Philips, X’Pert-MPD equipped with a Ni fil-
compounds as a model reaction to study the catalytic activity of
tered CuK␣ radiation source ( = 1.542 Å) of 40 kV and 30 mA. Mean
quantum cluster of silver. They suggested that the reaction followed
crystallite size of the hollow silver nanosphere was calculated using
pseudo-first order kinetics. They found that the rate of reduction
the Scherrer equation by broadening of X-ray line using integral
of 4-NP increased with increasing of the amount of NaBH4 which
breadth ˇ [39]. Stability of hollow Ag nanospheres was analyzed
ultimately reduced the induction time (IT, time required to observe
using transmission and back scattering profiles, scanning the col-
visual change, i.e., the start of reduction of nitro compounds).
loidal sample by light rays of 880 nm wavelength using Turbiscan
In light of the above literature, stable hollow silver nanospheres
classic MA 2000 (Formulaction, France).
colloids have been synthesized using AgNO3 and (NH2 OH)2 ·H2 SO4
in the presence of gelatin as stabilizer. Turbiscan has been used to
monitor the stability of colloidal hollow silver nanospheres. The 3. Results and discussions
reduction of nitro compounds (4-nitrophenol and 4-nitroaniline)
is carried out as a model reaction to study the catalytic activity of 3.1. Preparation of the colloidal hollow Ag nanospheres
hollow silver nanospheres colloids.
The possible reaction mechanism for the preparation of colloidal
hollow Ag nanospheres may be summarized as below:
2. Experimental
AgNO3 + NaOH → AgOH + NaNO3 (1)
2.1. Materials

Silver nitrate (AgNO3 ), gelatin, hydroxylammonium hydro- 2AgOH + (NH2 OH)2 ·H2 SO4 + 2NaOH → 2Ag + N2 + Na2 SO4
sulfate ((NH2 OH)2 ·H2 SO4 ), sodium borohydride (NaBH4 ), 4-
nitrophenol (4-NP), and 4-nitroaniline (4-NA) were purchased from + 6H2 O (2)
Finar Chemicals, India and used as received without further purifi-
Gelatin inhibited the direct reaction of NH2 OH with AgNO3 and
cation. Millipore water (Millipore, Elix, India) was used throughout
coagulation of the produced colloidal Ag hollow nanospheres. Tem-
the experiments for preparing the aqueous solutions.
perature of the reaction was the key factor for the production of
hollow silver nanospheres colloids. At lower temperature, the vis-
2.2. Synthesis of hollow silver nanospheres colloids cosity of the solution increased which reduced the diffusion rate
of Ag+ ions and NH2 OH and the overall rate of reaction too. At
Hollow silver nanospheres colloids were prepared by the reduc- low temperature (5 ◦ C), more nitrogen gas was produced and hol-
tion of silver nitrate in water, using (NH2 OH)2 ·H2 SO4 as reducing low Ag nanospheres were formed on the surface of nitrogen gas
 R. Vadakkekara et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 399 (2012) 11–17 13

Fig. 2. DLS histogram of hollow Ag nanospheres.

3.3. Particle size analysis

The size distribution and average particle size of hollow Ag

nanospheres were obtained using particle size analyzer (Fig. 2).
Fig. 1. UV–vis absorption spectra of the hollow Ag nanospheres. From Dynamic Light Scattering (DLS) histogram, it could be seen
that hydrodynamic diameters of hollow Ag nanospheres were
within the range of 11–40 nm with average diameter 22.56 nm.
bubbles. This may be the reason for the formation of controlled
shape of hollow Ag nanospheres [24].
3.4. TEM

3.2. UV–visible spectroscopy The morphology of the prepared hollow Ag nanospheres was
studied by TEM. From TEM image (Fig. 3a and b), it could be seen
The absorption in the visible region with max = 420 nm corre- that Ag nanospheres had a hollow spherical structures with the
sponds to the Surface Plasmon Resonance (SPR) absorption region average dimensions of about 10–48 nm and the central parts of the
for silver particles 10–40 nm in size (Fig. 1). The strong SPR is due to spheres were light whereas the edges were dark and the spherical
the high electronic polarizability of small particles, which yields a surfaces were covered by densely packed Ag nanoparticles. The size
very high extinction coefficient. The resonance frequency is depen- distributions of the particles were shown in Fig. 3(c).
dent on the size, shape, material properties, surrounding medium
and the state of aggregation of the nanoparticles. The light green 3.5. XRD
color of the colloidal sample was indicative of the silver nanopar-
ticles, providing another piece of evidence for the formation of The peaks of XRD spectrum (Fig. 4) of the hollow silver
smaller hollow sphere. nanospheres were clearly distinguishable and could be perfectly

Fig. 3. (a) TEM photograph of hollow Ag nanospheres; (b) a single hollow sphere; (c) particle size distribution for hollow Ag nanospheres.
14 R. Vadakkekara et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 399 (2012) 11–17

Fig. 4. XRD pattern of hollow Ag nanospheres. Fig. 6. UV–vis spectra of hollow Ag nanospheres colloids at different time.

indexed to crystalline silver, not only in peak position, but also in time intervals were superimposing. This indicated that shape and
their intensity. The peaks at 2 of 36.3◦ , 42.7◦ , 63.5◦ , 76.1◦ and 83.8◦ particle sizes of the silver nanoparticles were not changing with
would be attributed to the (1 1 1), (2 0 0), (2 2 0), (3 1 1) and (2 2 2) time.
crystallographic planes of face-centered cubic (fcc) silver crystals To monitor the stability, the absorbance of the colloids was mea-
[25]. The mean silver crystallite size, calculated from the Scherrer sured at different intervals of time, shown in Fig. 6. There was no
equation was 36 nm. change in the peak position (420 nm) and intensity of absorbance
up to 24 h. As the particles increase in size, the absorption peak
3.6. Stability of the colloidal hollow silver nanospheres usually shifts toward the red wavelength. The stable position of
absorbance peak indicated that nanoparticles did not aggregate.
In the system under study, several pathways of nanodisper-
sion breakdown are possible, such as oxidation of metal particles 3.7. Reduction of 4-NP and 4-NA with colloidal hollow silver
accompanied by their dissolution, aggregation of particles, and also nanospheres
sedimentation. Gelatin as a good stabilizer can cover them and
prevent their agglomeration and growth. The process of colloidal Till date, though reduction of nitro compounds using vari-
aggregation is affected by the particle surfaces charge, the ionic ous transition metal catalysts like Pd, Au, Ag, etc. was reported
strength of the solution, the redox potential of the system and the [32–35] nobody had used hollow Ag nanospheres colloids as cata-
reaction rate [27]. lyst for the reduction of nitro compounds. The catalytic reduction
As prepared hollow Ag nanospheres colloid solution were of 4-nitrophenol and 4-nitroaniline with excess amount of NaBH4
scanned from bottom (0 mm) to the top of the vial (∼70 mm) for as hydrogen source was spectrophotometrically studied using
a period of 20 min (Fig. 5). Scanning was performed at different colloidal hollow Ag nanospheres solution. The absorbance was
time intervals up to 24 h. It was observed that, Backscattering (BS) monitored as a function of time. Aqueous solution of 4-NP showed
profiles of gelatin protected hollow silver nanospheres at different absorption maximum at 317 nm and 4-NA at 380 nm due to the
n → * transition (Figs. 7 and 8). It was observed that, after addi-
tion of freshly prepared ice cold solution of NaBH4 , the peak of
4-NP was red shifted from 317 to 400 nm and color of the reaction
mixture was changed from pale yellow to deep yellow due to the
formation of 4-nitrophenolate ion in alkaline solution [31]. Though
no shifting of peak of 4-NA at 380 nm was observed intensity of
peak increased after addition of NaBH4 (Fig. 8) and the color of the
solution (yellow) unaltered for a couple of days without the addi-
tion of hollow Ag nanospheres. In the absence of the catalyst, the

Fig. 7. UV–vis spectra for 4-NP and successive reduction of 4-NP catalyzed by col-
loidal hollow Ag nanospheres at intervals of 2 min. Conditions: [4-NP] = 5 × 10−4 M;
Fig. 5. BS profile of colloidal hollow Ag nanospheres. [NaBH4 ] = 8 × 10−2 M; hollow Ag nanospheres = 0.85 × 10−4 g; temperature = 303 K.
 R. Vadakkekara et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 399 (2012) 11–17 15

Fig. 9. Plot of ln [A] versus time for the reduction of 4-NP using colloidal hol-
Fig. 8. UV–vis spectra for 4-NA and successive reduction of 4-NA catalyzed by col- low Ag nanospheres as a catalyst in the presence of variable concentrations of
loidal hollow Ag nanospheres at intervals of 2 min. Conditions: [4-NA] = 5 × 10−4 M; NaBH4 . Conditions: [4-NP] = 5 × 10−4 M; hollow Ag nanospheres = 0.85 × 10−4 g;
[NaBH4 ] = 8 × 10−2 M; hollow Ag nanospheres = 0.85 × 10−4 g; temperature = 303 K. [NaBH4 ] = 2 × 10−2 to 12 × 10−2 M.

of NaBH4 , k values were increased and after a certain concentration

thermodynamically favorable reduction of the nitro compounds (E0
of NaBH4 the lines were parallel indicating comparable k values.
for 4-NP/4-NA = −0.76 V and H3 BO3 /BH4 − = 1.33 V, versus NHE) was
not observed under the used experimental condition [31,32].
3.7.2. Effect of concentration of colloidal hollow silver
With the addition and proper mixing of hollow Ag nanospheres
nanospheres on reduction of 4-NP and 4-NA
colloids, the peak at 400 nm (due to 4-nitrophenolate ion) grad-
Spiro [41] observed that in heterogeneous/microheterogeneous
ually decreased, while new peak at 295 nm observed after 2 min
catalysis, the rate of the reaction generally increases linearly with
and color of the solution turned to brown, which substantiated the
the amount of the catalysts. The effect of catalyst amount on reduc-
formation of 4-aminophenol (4-AP). The peak at 400 nm fully dis-
tion of nitro compound was studied by varying the concentration
appeared at 6 min, this indicated the completion of the reduction
of hollow silver nanospheres colloids (0.17 × 10−4 to 0.85 × 10−4 g
reaction, whereas Leelavathi et al. [36] found that the reduction
Ag) in the reaction mixture, keeping other parameters constant. A
of 4-NP completed after 12 min using 50 mg of 10% Ag/Al2 O3 . In
plot of rate constant (k) versus amount of hollow silver nanospheres
the case of 4-NA the peak at 380 nm gradually decreased and new
was shown in Fig. 11. At lower concentration of Ag (0.17 × 10−4 ),
peak started to appear at 300 nm after 2 min, due to the formation
rate of reduction of 4-NP was found to be 8.33 × 10−2 min−1 and
of 4-phenylenediamine [31,36]. The peak at 380 nm completely
IT was high (8 min). For 4-NA reduction, rate was 4.2 × 10−2 min−1
disappeared at 8 min.
for 0.17 × 10−4 g of Ag and IT was 10 min. With increasing the Ag
With excess amount of NaBH4 (>100 times of reactants, 4-NP
concentration, rate of reduction of both the nitro compounds was
and 4-NA), the reaction could be considered as pseudo-first order
increased (Fig. 11).
reaction with respect to nitro compounds.

Pseudo-firstorderrateequation, [At ] = [A0 ]·e−kt (3) 3.7.3. Effect of temperature on the reduction of 4-NP and 4-NA
The effect of temperature was studied in the range of 10–50 ◦ C
where At , absorbance at time t represents corresponding concen- for the reduction reaction of nitro compounds the results are given
tration of the reactant; A0 , absorbance at the initial stage, i.e. initial in Table 1. It was observed that at 10 ◦ C, the time required to com-
concentration of the reactant and k, pseudo-first order rate con- plete reduction was high and rate was 5.98 × 10−2 min−1 for 4-NP
stant. and 3.78 × 10−2 min−1 for 4-NA. At 50 ◦ C, rate of 4-NP reduction
The use of large amount of NaBH4 increased the pH of the was found to 39.50 × 10−2 min−1 and 29.36 × 10−2 min−1 for 4-NA
medium and it slowed down the degradation of BH4 − . The hydro- reduction. With increasing temperature, IT was decreased and rate
gen gas evolved from the NaBH4 helped in proper mixing of the
solution, therefore the catalyst nanospheres were well distributed
in the reaction medium and offered a favorable condition for the
reaction and also prevented aerial oxidation of the products [32].
The diffusion and adsorption of nitro compound on hollow silver
sphere and followed by electron transfer mediated by Ag surface
from BH4 − to nitro compound [40]. The rate of reduction of 4-
NP is significantly higher than rate of reduction of 4-NA and this
was ascertained by comparing the absorbance changes at specified
intervals of time (Figs. 7 and 8).

3.7.1. Effect of NaBH4 concentration for reduction of 4-NP and

4-NA
In all experiments, concentration of NaBH4 was excess (>100
times) than nitro compounds to get the reaction independent of
NaBH4 concentration. A plot of ln [A] versus time was obtained at
Fig. 10. Plot of ln [A] versus time for the reduction of 4-NA using colloidal hollow Ag
different concentration of NaBH4 , keeping concentration of hol-
nanospheres as a catalyst in the presence of variable concentrations of NaBH4 . Con-
low silver nanospheres colloids and nitro compounds constant ditions: [4-NA] = 5 × 10−4 M; Ag hollow sphere = 0.85 × 10−4 g; [NaBH4 ] = 2 × 10−2 to
(Figs. 9 and 10). It was observed that with increasing concentration 12 × 10−2 M.
16 R. Vadakkekara et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 399 (2012) 11–17

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