Ultrasound assisted synthesis of a polyaniline hollow microsphere/Ag core/shell structure for sensing and catalytic applications

Article abstract of DOI:10.1039/c3ra23002c

Polyaniline hollow microspheres (PHM) have been synthesized by the ultrasound assisted emulsion polymerization technique using polystyrene microspheres as a template, followed by the formation of a polyaniline/Ag hollow microspheres (PHMAg) core/shell structure that was characterized by X-ray diffraction (XRD), Fourier transformation infrared spectra (FTIR) and scanning electron microscopy (SEM), transmittance electron microscopy (TEM). FTIR and SEM studies of the product confirmed the complete removal of the polystyrene. The presence of silver and its distribution on the polyaniline surface was confirmed by XRD and FESEM/TEM, respectively. The electrical conductivity of the PHMAg is found to be 4 times higher than the PHM. Finally, PHMAg has successfully been used as a sensor in the detection of pyrogallol oxidation and also as an effective catalyst in the reduction of 4-nitrophenol (4-Np) under ambient conditions.

Full text of DOI:10.1039/c3ra23002c

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PAPER
Ultrasound assisted synthesis of a polyaniline hollow
microsphere/Ag core/shell structure for sensing and
catalytic applications3
Cite this: RSC Advances, 2013, 3,
7
808
Ritwik Panigrahi and Suneel Kumar Srivastava*
Polyaniline hollow microspheres (PHM) have been synthesized by the ultrasound assisted emulsion
polymerization technique using polystyrene microspheres as a template, followed by the formation of a
polyaniline/Ag hollow microspheres (PHMAg) core/shell structure that was characterized by X-ray
diffraction (XRD), Fourier transformation infrared spectra (FTIR) and scanning electron microscopy (SEM),
transmittance electron microscopy (TEM). FTIR and SEM studies of the product confirmed the complete
removal of the polystyrene. The presence of silver and its distribution on the polyaniline surface was
confirmed by XRD and FESEM/TEM, respectively. The electrical conductivity of the PHMAg is found to be 4
times higher than the PHM. Finally, PHMAg has successfully been used as a sensor in the detection of
pyrogallol oxidation and also as an effective catalyst in the reduction of 4-nitrophenol (4-Np) under
ambient conditions.
Received 22nd November 2012,
Accepted 8th March 2013
DOI: 10.1039/c3ra23002c
www.rsc.org/advances
composites have been reported, where the ‘‘core’ may be a
conducting polymer or nonconducting materials i.e. styrene
1. Introduction
The preparation of hybrids as a new generation of functional
materials has attracted a lot of attention from materials
scientists in the last few years. The most important among
these are those involving wrapping of one component over
another by chemical or physical means to form core–shell
structures. It may be noted that the shell can alter the charge,
functionality and reactivity of the core in these core–shell
microsphere. The conducting polymers are characterized by
their conjugated p-electron system, which exhibits superior
13
electron conductivity in the oxidized state. The degree of
oxidation can also be controlled by chemical or electrochemi-
cal redox processes, which involve electron transfer and
exchange of counter ions for the compensation of charge on
the polymer chain. However, the preparation of hollow
microspheres acting as a shell for its application in the field
1
,2
nanocomposites. Hence, the various properties of the core
or final material), ranging from optical, magnetic, mechanical
(
14
of polymer nanocomposites is still in a developing stage. The
to catalytic, can be modified by selectively choosing the shell.
In addition, the core–shell breed of material possesses the
advantage of controlling the nature of the core as well as the
thickness of the shell for specific applications. Core–shell
materials can also be prepared by coating of polymer over
metal, semiconductor and ceramic cores. The core–shell
nanocomposites have been widely used in many fields due
to their enhanced optical, electrical, magnetic, dielectric
electrical, thermal, magnetic, catalytic properties are depen-
dent on the size, shape and morphology of spheres and their
coating with other polymers. Alternatively, the introduction of
the metal/semiconducting nanomaterials could result in the
15,16
strong electrostatic interaction between the polymer
and
nanoparticles and account for enhanced electrical and optical
17,18a–c
properties of the corresponding nanocomposites
their applications in the fields of drug delivery
and
and
19
3
,4,5
properties and increased stability.
so far for this purpose are layer by layer (LBL) deposition,
direct attachment of functionalized polymers to a surface,
Various techniques used
controlled release, optoelectronics and micro cavity reso-
6
20
nance.
7
8
Polyaniline remains one of the most important conducting
polymers, which has been extensively investigated in the last
few decades. This is mainly due to its ease of preparation, high
conductivity, good environmental stability under ambient
conditions, promising chemical, electrical and optical proper-
synthesizing materials in the presence of a polymeric ligand,
4
vapour deposition, etc. There exist a number of reports for
9
–12
polymer coated inorganic nanomaterials.
Very recently, the
synthesis of conducting polymer coated ‘‘core–shell’’ nano-
2
1–24
ties, as well as its unique redox tenability.
In addition, the
Department of Chemistry, Indian Institute of Technology, Kharagpur-721302, India.
E-mail: sunit@chem.iitkgp.ernet.in; Fax: +91-3222-255303; Tel: +91-3222-283334
doping level of polyaniline can easily be controlled through an
2
5,26
acid–base doping–dedoping process.
It finds multifaceted
3
Electronic supplementary information (ESI) available. See DOI: 10.1039/
c3ra23002c
applications in the field of electrochromic devices, secondary
7
808 | RSC Adv., 2013, 3, 7808–7815
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batteries, microelectronics, catalysis, electrostatic discharge
protection, electrorheological fluids, light emitting diodes,
was added to this and reaction was continued for 20 h, the
monodisperse latex type polystyrene colloidal particles were
obtained. Subsequently, 10 mL of the earlier prepared
polystyrene colloid after washing with distilled water were
slowly dispersed into (1 : 1) sulfuric acid and kept at 40 uC
under vigorous stirring conditions for 8 h. These resulting
sulfonated polystyrene particles were separated followed by
removal of excess sulphuric acid.
2
7–30
chemical sensors and corrosion protecting paint.
A variety
Au, Cu
3
1
32
33
34
35
of inorganic materials, such as SiO
2
,
Si, TiO
2
,
3
6
and Ag, are used to prepare the polyaniline composites.
Though a limited amount of work has recently been carried
out on the preparation of polyaniline/Au hollow microsphere
3
7–39
composites,
nanoparticles.
no such work has been reported using Ag
2.3 Preparation of polyaniline hollow microspheres (PHM)
In recent years, high intensity ultrasound has been used as a
novel technique in emulsion as well as radical polymeriza-
Sulfonated styrene particles were dispersed into 10 mL of
distilled water and 3 mmol of aniline monomer dissolved in 10
mL dil. HCl was added into this under stirring conditions,
which were continued for 6 h. Subsequently, the mixture was
subjected to ultrasonication for 90 min followed by the slow
addition of the 1.5 mmol APS solution, while keeping its molar
ratio at 2 : 1 with respect to aniline. As the polymerization of
aniline progressed, the resulting solution gradually turned
green, and was kept undisturbed for another 2 h. This was
subsequently centrifuged and washed several times with
distilled water followed by ethanol and dried under vacuum
at 40 uC for 24 h to obtain polyaniline (shell) coated
polystyrene (core) particles. Finally, the polystyrene core was
dissolved by adding THF to form hollow polyaniline micro-
spheres (PHM).
4
0–43
tions.
The presence of ultrasound induced polymerization
is found to be faster and can be carried out in the absence of the
initiator or co-stabilizers and at a low reaction tempera-
4
1,44,45
ture.
The observed effects could be attributed to the
formation and collapse of cavities/bubbles when the ultrasound
wave passes through the reacting medium. As a result, the ultra
high temperature and pressure conditions developed in the
4
6
system generate the radicals, and account for the enhanced
polymerization. Therefore, we have focused our work on the
preparation of uniform hollow polyaniline microspheres (PHM)
by ultrasound assisted polymerization of aniline monomers,
selecting polystyrene spheres as templates (or core) and use it in
developing hollow sphere polyaniline/Ag nanocomposites
(
PHMAg). The choice of Ag in our work is mainly guided by
2
(
.4 Preparation of polyaniline/Ag hollow microspheres
PHMAg)
Ag nanoparticles were prepared by chemical reduction
4
7,48
its highest electrical conductivity among all the metals
well as its enhanced catalytic activity, antibacterial properties
and good biocompatibility.
been characterized by XRD, FESEM, TEM, FTIR, UV-visible and
conductivity studies. Finally, the electrochemical activity and
catalytic efficiency of PHMAg has also been investigated in the
oxidation of pyrogallol and reduction of 4-Np, respectively.
These studies show that PHMAg acts as an efficient sensor as
well as catalyst for these environmentally polluting chemicals.
as
4
9–53
The synthesized PHMAg have
5
4a,b
according to the method already in the literature.
According to this, 25 mL of 0.16 mM AgNO was added drop
by drop to a 25 mL aqueous solution consisting of 5.8 mM
3
NaBH and 0.37 mM SDS. This results in the appearance of a
4
yellowish red colour due to the formation of Ag nanoparticles.
The reaction was carried out for 30 min maintained at 3–4 uC.
This results in the formation of a dark grey colloidal
dispersion due to Ag nanoparticles. In another experiment,
PHM were dispersed in 10 mL of water and added to the earlier
prepared colloidal dispersion of Ag nanoparticles at room
temperature and kept under stirring conditions for y5 h. The
resulting product was centrifuged and the surfactant was
removed by washing it with ethanol and water followed by
drying in vacuum under ambient conditions.
2
. Experimental
2
.1. Materials
Potassium peroxodisulfate, K S O ; ammonium peroxodisul-
fate, (NH ) S O (APS); 4-nitrophenol; pyrogallol; hydrochloric
2
2 8
4
2 2 8
acid, HCl; sulfuric acid, H SO , (Merck, Mumbai, India);
2
4
2
.5 Preparation of PHM and PHMAg-modified glassy carbon
sodium borohydride, NaBH ; sodium dodecyl sulfate, (SDS)
4
electrodes
(
(
Sisco Research Laboratory, Mumbai, India); tetrahydrofuran
THF) (Rankem); silver nitrate, AgNO (Finar Chemicals
OH (Hong Yang
2
1
3
A paste of PHMAg and distilled water (1.0 mg mL ) were
prepared followed by ultrasonic treatment for y30 min.
Subsequently, 1 drop of the above colloidal dispersion was
placed on a glassy carbon electrode and dried under ambient
conditions. It may be noted that prior to any electrochemical
measurements, the glassy carbon electrode was polished with
2 5
Limited, Ahmedabad, India); ethanol, C H
chemical Corporation); were of analytical grade and used as
received. Aniline (Merck, India) and styrene (Jyoti Lab., India),
were distilled twice under reduced pressure and stored with
cooling under nitrogen and used.
1
.0, 0.3, 0.05 mm alumina slurry followed by washing with
2.2 Preparation of sulfonated polystyrene core
distilled water and drying at room temperature.
Monodisperse polystyrene particles were prepared according
3
7
2.6 Catalytic activity of PHMAg
to the reported procedure with minor modifications. Styrene
monomer (0.1 moles) was added to 140 mL of distilled water
and stirred for about 20 min at 80 uC under argon atmosphere.
When 10 mL of 0.085 M solution of potassium peroxodisulfate
4-Aminophenol (4-Ap) can be prepared by the hydrogenation
of 4-Np at very high hydrogen pressure and high temperature.
It finds applications in pharmaceutical, photographic, and
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5
5
plastic industries, analgesic antipyretic drugs and so on.
However, 4-Np, according to the priority toxic pollutants list of
the United States Environmental Protection Agency, remains
5
6
as one of the major organic pollutants. Despite this, the
world production of 4-Np and its derivatives are increasing day
by day, as these are generally used in different commercial
5
7
industries, such as drug, fungicide, dye, leather etc. As a
result, the discharged wastes from such industries cause
environmental pollution and also harm life, causing irritation
to the eyes, skin, and respiratory tract and may cause
5
8
inflammation of these parts. Hence, a feasible way to reduce
-Np into 4-Ap is necessary and highly sought after. In this
regard, noble metals act as efficient reducing catalysts at
4
5
9
ambient condition. However, the use of pure noble metal
catalysts raises not only the production costs, but is also
associated with the difficulty in its recovery due to relatively
small dimensions. Therefore, metal decorated-hollow polymer
Fig. 1 XRD patterns of (a) PHM and (b) PHMAg.
platinum wire as the auxiliary electrode, a saturated calomel
electrode (SCE) as the reference electrode, and the PHMAg
electrode. Cyclic voltammograms of PHM and its correspond-
ing PHMAg have been recorded in the potential range of 0.8 to
(such as PHM) nanocomposites with a large surface area can
be a potential alternative as the cost of production decreases.
In addition, the recovery could also be easily facilitated when
noble metal catalyst particles are embedded on the hollow
polymer.
2
1
2
0.4 V and at a sweep rate of 50 mV s
at different
concentrations of pyrogallol.
In our work, the catalytic activity studies were carried out in
2
2
a quartz cuvette containing 2.75 mL H
aqueous solution of 4-Np. When 200 mL of NaBH
2
O and 30 mL (10 M)
2
1
4
(10 M) was
added to this, deep yellow colour 4-nitrophenolate ions were
formed. Subsequently, 6 mg of PHMAg catalyst was added into
this and the progress of the reaction was monitored by
observing the change of UV intensities during the reaction.
After the desired reaction time intervals, PHMAg catalyst was
separated by centrifugation of the reaction mixture and
3
. Results and discussion
3.1 Characterization of PHM and PHMAg; XRD, FTIR, SEM,
FESEM, TEM, EDS and UV-vis
X-ray diffraction (XRD) patterns of PHM and PHMAg are
presented in Fig. 1, which shows that the prepared PHM are
amorphous. On the contrary, XRD of its corresponding Ag
nanocomposite shows crystalline peaks, where sharp peaks
appear at 2h values of about 38.2u, 44.3u, 64.5u due to (111),
6
0
recycled.
2.7 Instruments
X-ray diffraction (XRD) analysis was carried out at room
temperature on a Phillip, Holland instrument with Cu-Ka
radiation (0.1541 nm) in the range 20u to 75u, with a scanning
rate of 5u per min. FTIR analysis was performed with a Perkin-
Elmer FTIR Spectrometer RXI using KBr discs and thin films
(200), (220) Bragg’s reflections of the cubic phase of silver.
[JCPDS Card Number–03-0931]
Fig. 2 shows the FTIR spectra of polystyrene; polyaniline
coated polystyrene and hollow polyaniline shell (PHM). The
presence of peaks at 1583 and 1494 cm
correspond to the CLC stretching of the quinoid and
benzenoid rings, whereas the peak at 1150 cm
21
in polystyrene
2
1
over the wave number range 400–4000 cm . Ultraviolet-visible
UV-vis) absorption spectra of PHMAg dispersed in distilled
(
21
could be
water through ultrasonic irradiation were recorded on a
Shimadzu UV-2450 UV-Visible spectrophotometer. The mor-
phology of the samples were studied by scanning electron
microscopy (SEM) on a JEOL, JSM-5800 scanning microscope
instrument operating at 20 kV and field emission scanning
electron microscopy (FESEM) on a Carl Zeiss at an accelerating
voltage of 20 kV. Transmission electron micrographs were
assigned to in plane bending of C–H bonds. A few additional
2
recorded on TECNAI G , SEI (Netherland) instrument operat-
ing at 200 kV. A Gatan multipole charge-coupled device (CCD)
camera was used to obtain digitally acquired images for the
samples placed on a carbon-coated Cu grid from dispersion in
ethanol. The conductivity was measured by a four-probe
method using a digital multimeter (Scientific Equipment
Roorkee, Model-LCS-02) at room temperature using the
powdered compressed pellets. The electrochemical experi-
ments were performed (HCH Instruments, Electrochemical
Analyzer) in a cell containing 20.0 mL of 0.1 M phosphate
buffer solution (PBS) at room temperature using a coiled
Fig. 2 FTIR spectra of (a) polystyrene particles, (b) polyaniline coated polystyrene
particles, and (c) polyaniline hollow microspheres (PHM).
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2
1
peaks are also observed at 1452, 750 and 698 cm
in the
spectra of polystyrene. In the case of poyaniline, the
2
1
characteristic 1575 and 1490 cm peaks correspond to the
6
1
CLC stretching of quinoid and benzenoid rings. In addition,
2
1
the peaks at 1298 and 1244 cm correspond to the C–N and
6
2
21
CLN stretching modes. Another peak appeared at 1132 cm
6
3a
due to the in-plane bending of C–H,
24 cm could be attributed to the out plane bending of C–
H. The bands observed at 686.5 cm
where as the peak at
2
1
8
6
4
21
in the spectrum
correspond to the ring out-of-plane deformation of the
6
3b
phenylene ring for polyaniline.
It is also noted that the
polystyrene (core)/polyaniline (shell) shows the presence of
peaks due to both polystyrene and polyaniline. Interestingly,
the peak intensity of polystyrene and polyaniline is signifi-
cantly reduced in the case of polystyrene coated with polyani-
line. When the polystyrene core is dissolved in THF, the peaks
corresponding to polyaniline remain virtually unaltered and
Fig. 4 (a) A SEM image and (b) a TEM image of polyaniline coated polystyrene
microspheres.
UV-vis absorption spectra of PHM and PHMAg are shown in
Fig. 7. The inset of this figure shows the UV absorption spectra
of silver nanoparticles. The characteristic peaks of polyaniline
appear at 260 nm and 360 nm, which are attributed to the p–
2
1
21
the peaks of polystyrene at 754 cm
and 697 cm
disappeared completely (Fig. 2c). This confirms the complete
removal of the polystyrene core and formation of hollow
polyaniline microspheres (PHM) (shell).
3
3
p*, polaron–p*, and p–polaron transitions, respectively.
Though the surface plasmon bands of silver nanoparticles
6
5
Fig. 3, 4a–b and 5a–b display the typical SEM and TEM
images of polystyrene spheres; polyaniline coated polystyrene
microspheres and PHM, respectively. It is noted that the
polystyrene particles exist as spheres with average diameter
y350 nm. The increment in diameter of the corresponding
polyaniline coated polystyrene microspheres is also visible.
These SEM images clearly show the formation of a hollow
polyaniline shell (PHM) when the polystyrene core is removed
by dissolving polyaniline-coated polystyrene in THF. These
images also show a small opening invariably generated in the
hollow polyaniline shell due to the removal of the polystyrene
core. It is anticipated that the mechanical properties of
polyaniline allow it to withstand the selective extraction of
the polystyrene core from the polystyrene (core)/polyaniline
are sensitive to their surrounding environment, the absorp-
tion spectra of PHMAg remained more or less unaltered. Fig. 7
clearly indicates that the typical absorption peak of silver
appears at 440 nm, corresponding to the surface plasmon
resonance (shown in the inset). Interestingly, this peak is not
observed in PHMAg, possibly due to the reduction in its
absorption intensity by surrounding polyaniline. Thus the
findings of UV-visible absorption studies further support the
(shell) and to exclusively form the hollow polyaniline shell.
The morphology of PHMAg has also been investigated by
FESEM and is shown in Fig. 6a. This further confirms the
formation of PHM with Ag particles (indicated as white spots)
dispersed on their surface. TEM images of nano-silver
assembled on the hollow polyaniline shell (PHM) are also
shown in Fig. 6b. It is evident that spherical Ag nanoparticles
with their diameter ranging from 30–50 nm are dispersed on
the surface of the hollow polyaniline shell.
Fig. 5 (a) A SEM image and (b) a TEM image of polyaniline hollow microspheres
(PHM).
Fig. 6 (a) A FESEM image and (b) a TEM image of polyaniline/Ag hollow
Fig. 3 A SEM image of polystyrene particles.
microspheres (PHMAg).
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Fig. 8 Cyclic voltammograms of (a) PHM and (b) PHMAg in presence of (I) 2
mM, (II) 6 mM and (III) 10 mM of pyrogallol.
Fig. 7 UV-vis spectra of (a) PHM (b) PHMAg (inset shows the UV-vis spectra of
selective and sensitive detection methods for pyrogallol is
more demanding. Therefore, keeping these points in our
the colloidal silver solution).
67
mind, we have tested the electrochemical activity of PHMAg in
detecting pyrogallol. Fig. 8 shows the cyclic voltammograms of
PHM and PHMAg in the potential range of 0.8 to 20.3 V and at
formation of PHMAg nanocomposites. Based on the above
results a schematic representation of the formation of PHMAg
is given in Scheme 1.
2
1
a sweep rate of 50 mV s recorded at different concentrations
of pyrogallol. It is evident that the oxidation peaks of
polyaniline appearing at y0.59 V and y0.42 V are due to
the oxidation of pyrogallol to its quinine form and is
independent of pyrogallol concentration. However, the current
density of the corresponding peaks increases with concentra-
tion of pyrogallol due to increasing concentration of electrons
in electrolyte solution confirming the catalytic effect of PHM.
When PHMAg is used as an electrode under identical
conditions, the oxidation peaks of polyaniline appeared more
or less in the same positions. Interestingly, the current
intensity of the oxidation peak (y0.61 V) increased gradually
with increasing concentration of pyrogallol. These findings
clearly demonstrate that the sensing of the electrocatalytic
oxidation of pyrogallol is significantly enhanced in the
presence of PHMAg. This is in all probability due to increased
charge transport of Ag nanoparticles due to the higher
conductivity through the PHM/Ag interface. In addition, silver
also facilitates the electrical contact of the pyrogallol with the
PHMAg electrode. Although Ag is a conductor and does not
make a continuous electron path alone, the presence of Ag
nanoparticles on the surface of conducting PHM creates many
active sites. This could ultimately lead to effective transport of
electrons through the PHMAg interface and account for its
efficient sensing in the oxidation of pyrogallol compared to the
polyaniline alone.
3.2 Conductivity measurement of PHM and PHMAg
The conductivity (D.C.) of the PHM and corresponding PHMAg
2
3
21
23
21
are found to be 2.05 6 10 S cm and 11.50 6 10 S cm
,
respectively. This suggests that the conductivity of polyaniline
is enhanced in presence of silver similar to that observed in
3
7
case of polyaniline/Au nanocomposite.
This could be
attributed to the high conductivity of Ag nanoparticles,
mediating the effective charge migration through the polyani-
line. It is also anticipated that a continuous electron phase is
formed through polyaniline, which is further benefited by the
5
2
presence of conducting Ag nanoparticles.
3.3 Pyrogallol sensing
The determination of phenol derivatives in pyrogallol (1,2,3-
trihydroxybenzene) has been extensively studied due to its
6
6
importance. It is commonly found in industrial effluents and
easily introduced into the living environment and is found to
be toxic due to its ability to generate free radicals. The
ingestion of pyrogallol in humans may cause gastrointestinal
tract irritation, haemolysis, methemoglobinemia, renal injury,
uraemia, and death. However, the development of the simple,
3.4 Catalytic properties – 4-Np reduction
Fig. 9 represents the absorbance versus wavelength plots in
presence of (a) with (I)/without NaBH4 (II), (b) PHM, (c) Ag
2
2
nanoparticles and (d) PHMAG for the reduction of 4-Np (10
8
M) carried out at different time intervals. It is observed that
-Np exhibits a strong absorption peak at 317 nm, which is
found to be red shifted to 400 nm in the presence of NaBH
4
4
.
This could be attributed to the formation of a 4-nitropheno-
lateate intermediate, which is found to be stable under
6
8
ambient conditions, even up to 14 days. Interestingly, the
peak intensity corresponding to 4-Np (400 nm) gradually
decreases in presence of PHMAg and Ag catalysts and the
Scheme 1 A scheme to demonstrate the probable mechanism of PHMAg
5
5
formation.
intensity of the 4-Ap (300 nm) peak increases.
The
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Scheme 2 A scheme to illustrate the probably mechanism of the reduction of
4
-Np to 4-Ap in the presence of (a) PHMAg and (b) Ag nanoparticles.
Fig. 9 (a) UV spectra of p-nitrophenol (I) and p-nitrophenol in presence NaBH
4
4
(
II), time dependent UV spectra for the catalytic reduction of 4-Np using NaBH
in the presence of (b) PHM, (c) Ag nanoparticles, (d) PHMAg.
5
9
microspheres. Alternatively, a synergistic effect between
polymer and metal nanoparticles cannot be ruled out. Such
synergistic effects could arise due to higher probability of
effective collision with Ag nanoparticles and 4-Np as PHMAg
adsorbs 4-Np on its surface via p–p stacking interactions
disappearance of the absorption peak of 4-Np confirms the
complete reduction of 4-Np to 4-Ap after 40 and 92 min,
corresponding to PHMAg and Ag, respectively. However, in the
presence of PHM, the peak intensity of 4-Np was found to
remain unchanged even after 4 h. The catalytic activity of the
Ag and PHMAg catalyzed reactions have also been correlated
5
5
between p-rich polyaniline and 4-Np.
As a result, the
concentration of 4-Np increases in and around silver nano-
particles embedded in PHMAg, making the reduction of 4-Np
much faster (Scheme 2). Here the nitrogens along the
polyaniline chain provide physical stabilization for the Ag
nanoparticles. On the contrary, when only Ag nanoparticles are
used as a catalyst, agglomeration occurs, resulting in the low
in terms of rate constants. As the concentration of NaBH
4
is
significantly higher compared to 4-Np, the above reaction can
6
9
be considered to follow pseudo first order kinetics. Fig. 10a
and 10b provides ln (A /A ) vs. time plots for the reduction of
-Np in presence of Ag nanoparticles and PHMAg as catalysts.
t
0
7
0–72
4
probability of effective collisions between Ag and 4-Np.
We
The linear plots in these figures further support our conten-
tion that the reaction is a pseudo first order reaction. The rate
constant of the reaction has also been calculated based on this
have carried out the experiments to study the recycled catalytic
activity of silver (please refer to supplementary information: S1
and S2 ). It is noted that y65% of catalytic activity is retained
3
2
2
21
and follows the order: PHMAg (9.39 6 10
min ) . Ag
even after the third recycle. However, a small decrease in
catalytic activity of the PHMAg catalyst in the third recycle is
probably due to the inevitable loss of the catalyst during the
washing process.
2
2
21
nanoparticles (3.2 6 10
min ). These studies clearly
demonstrate that the catalytic activity of PHMAg in our work
is about 2.6 times higher compared to the silver nanoparticles
alone. The value of the higher rate constant in the presence of
PHMAg could be attributed to the higher probability of
electron transfer from NaBH to 4-Np in the presence of Ag
nanoparticles embedded on the surface of polyaniline hollow
4
4. Conclusion
The formation of ultrasound assisted polymerized polyaniline/
Ag composite hollow spheres has been established by XRD,
FTIR, SEM, and TEM studies. Room temperature electrical
conductivity of the synthesized polyaniline/Ag composite
hollow spheres is found to be 4 times higher than the pure
polyaniline hollow spheres. Polyaniline/Ag composite hollow
spheres also exhibit better electrocatalytic activity in the
oxidation of pyrogallol and provide one of the most efficient
alternatives due to their rapid detection speed, remarkable
sensitivity, good selectivity, easy operation, low cost, ability to
be miniaturized and disposable nature. In addition, they are
also an effective catalyst in the reduction of the environmental
Fig. 10 Plots of ln(A
t 0
/A ) versus time for the catalytic reduction of p-nitrophenol
using NaBH in the presence of (a) Ag nanoparticles, (b) PHMAg
4
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