A one-pot synthesis of a ternary nanocomposite based on mesoporous silica, polyaniline and silver

Article abstract of DOI:10.1039/c3ra44618b

The research on hybrid materials composed of inorganic and organic species in the nanometer range is motivated by the synergic combination of their native properties in a unique material. In this paper, we report a new one-pot synthesis of a ternary nanocomposite based on a conducting polymer (polyaniline-PANI) and silver, using mesoporous silica (MCM-41) as a hard template to promote the controlled growth of both polymer chains and silver nanoparticles. The presence of these reaction products was proved by the appearance of the characteristic diffraction peaks of face-centered cubic metallic silver phase in the X-ray diffractogram and by the presence of typical bands of polyaniline as emeraldine salt in the FTIR and UV-Vis spectra. The preservation of the ordered MCM-41 mesostructure after the polymerization reaction was also attested by XRD and TEM analysis and it was accompanied by a decreasing of the particle size as observed in the SEM images. N₂ adsorption-desorption isotherms support the statement that PANI chains and AgNP have been incorporated mainly into the channel of MCM-41, a fact also attested by TEM images. The electrical conductivity of this nanocomposite is on the order of 10^-3 S cm^-1, which is one million times higher than the value obtained for a MCM-41/polyaniline composite. Although some reports about the isolated incorporation of polyaniline or silver nanoparticles into the MCM-41 pores are found in the literature, to our knowledge this is the first time that such nanocomposite preparation is reported.

Full text of DOI:10.1039/c3ra44618b

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A one-pot synthesis of a ternary nanocomposite based
on mesoporous silica, polyaniline and silver†
Cite this: RSC Adv., 2013, 3, 26142
a
a
b
´
Ana Claudia De Abreu Rosa, Cintia Marques Correa, Roselena Faez,
a
a
*
Marcos Augusto Bizeto and Fernanda Ferraz Camilo
The research on hybrid materials composed of inorganic and organic species in the nanometer range is
motivated by the synergic combination of their native properties in a unique material. In this paper, we
report a new one-pot synthesis of a ternary nanocomposite based on a conducting polymer (polyaniline
– PANI) and silver, using mesoporous silica (MCM-41) as a hard template to promote the controlled
growth of both polymer chains and silver nanoparticles. The presence of these reaction products was
proved by the appearance of the characteristic diffraction peaks of face-centered cubic metallic silver
phase in the X-ray diffractogram and by the presence of typical bands of polyaniline as emeraldine salt
in the FTIR and UV-Vis spectra. The preservation of the ordered MCM-41 mesostructure after the
polymerization reaction was also attested by XRD and TEM analysis and it was accompanied by a
decreasing of the particle size as observed in the SEM images. N2 adsorption–desorption isotherms
support the statement that PANI chains and AgNP have been incorporated mainly into the channel of
MCM-41, a fact also attested by TEM images. The electrical conductivity of this nanocomposite is on the
order of 10ꢀ3 S cmꢀ1, which is one million times higher than the value obtained for a MCM-41/
polyaniline composite. Although some reports about the isolated incorporation of polyaniline or silver
nanoparticles into the MCM-41 pores are found in the literature, to our knowledge this is the first time
that such nanocomposite preparation is reported.
Received 23rd August 2013
Accepted 22nd October 2013
DOI: 10.1039/c3ra44618b
www.rsc.org/advances
electrical conductivity improvement. In this sense, it is already
published some studies focusing on the preparation of hybrid
1. Introduction
Nanomaterial science is an interesting research area due to the
distinct electronic, optical, magnetic and catalytic properties
that materials might show when are in the nanometric domain.
In particular, the investigation of hybrid materials composed of
inorganic and organic species at the nanometer level is also
important due to the potential synergetic combination of their
properties in a unique material.1–3
Among these hybrid materials, those comprised by con-
ducting polymers as organic component have gained attention
due to their potential application in electronic nanodevices and
sensors.4,5 Porous inorganic matrixes with uniform pores
dimensions at nanoscopic domain are considered good hosts
for polymer formation. In the case of conducting polymers, the
conned ambient of the pores affords the formation of chains
with less branching, which is an important feature to guarantee
materials composed by inorganic hosts with pores in nanoscale,
such as mesoporous silica (SBA-15 and MCM-41) with different
conducing polymers (polyaniline, polypyrrole).6–8 Although, in
these papers, the reported electrical conductivity of the nano-
composite is lower than the pure polymer, the further inorganic
framework removal became an useful strategy to obtain con-
ducting polymers nanowires, which have application as nano-
electronics components.6,9 Studies involving hybrid materials
containing polyaniline (PANI), one of the most studied con-
ducting polymer, and MCM-41 have already been pub-
lished6,10–17 and although these composites are insulators
(electrical conductivity around 10ꢀ9 S cmꢀ1),14 they are still
attractive materials due to their notable photocatalytic activity
under visible light16 and to the electrorheological properties
that they shown when dispersed in uids.14 Additionally, the
embedding of PANI with metallic nanoparticles can also be an
attractive strategy to enhance some features of the bulk poly-
mer.18–23 For example, PANI/Ag composites24–34 show improved
electrical and thermal conductivities when compared to pure
PANI with additional antibacterial activity and optical proper-
ties. In this eld, several papers are focusing on the develop-
ment of new synthetic methods or adjusting old ones to prepare
homogenous nanocomposite of PANI/Ag from the direct
a
´
´
ˆ
´
Laboratorio de Materiais Hıbridos, Instituto de Ciencias Ambientais, Quımicas e
ˆ
~
~
Farmaceuticas, Universidade Federal de Sao Paulo, Rua Sao Nicolau, 210, CEP:
09913-030, Diadema, SP, Brazil. E-mail: ffcamilo@unifesp.br; Tel: +55 11 3319-3568
b
´
´
ˆ
Laboratorio de Materiais Polimericos e Biossorventes, Departamento de Ciencias da
´
~
~
~
Natureza, Matematica e Educaçao, Universidade Federal de Sao Carlos, Sao Carlos,
SP, Brazil
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c3ra44618b
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oxidation of aniline by silver cations in one-step reaction.24–28 In 1-butyl-3-methylimidazolium
bis(triuoromethanesulfonyl)-
2012, our group showed an innovative method to produce a amide (BMImTf2N) (Fig. 1). In this present paper, we tried a step
nanocomposite of polyaniline with metallic silver in a imida- further in promoting the controlled growth of both polyaniline
zolium-based liquid ionic, which was used as solvent and so chains and silver nanoparticles, using MCM-41 as a hard
template. For this reaction occur, we needed to overcome the template, which has a dened structural arrangement with
˚
low solubility of regular silver salts, by using AgN(SO2CF3)2 as cylindrical pores with 30 A. Our results indicate that AgN-
silver cations source, which is a soluble salt in ionic liquid. (SO2CF3)2 salt dissolved in the BMImTf2N ionic liquid is able to
Although the resulting material has been obtained with success, diffuse into the silica pores containing aniline monomer
the silver nanoparticles dispersed in the polymer matrix were previously adsorbed and to promote the polymerization reac-
not monodisperse and the polymeric chains were allowed to tion without provoke the hard template structure collapse or
growth without control.35 It is worth to mention that pure ionic reduction of structural ordering. The presence of metallic silver
liquids, especially containing imidazolium cations, have an particles in MCM-41/PANI should increase the electrical
ordered structures, which have already been extensively conductivity of this isolating binary composite.
reviewed in the literature.36,37 This supramolecular organization
is due to the coulomb interactions between the ions and in the
case of the imidazolium cations, the existence of an extended
network of cations and anions connected together by “pseudo”
2. Experimental section
2.1. Materials
Aniline (Aldrich, 97%) was distilled twice under reduced press-
ure before use. BMImTf2N was prepared according to procedure
already described in the literature.44 Other chemicals were used
as received.
hydrogen bonding and p–p stacking interactions. Because of
this organization, ILs have been used as so template for the
synthesis of materials, such as silica,38 metallic nanoparticles39
and polymer.40
In 2010, Park et al.41 reported the synthesis of nanosized
silica/Ag/PANI hybrid prepared using g-radiolysis and poly-
(vinylpyrrolidone) (PVP) as stabilizer, aiming its use as active
material in a hydrogen peroxide biosensor.42 At the same year,
Lee et al.43 described the synthesis of silver nanoparticles
distributed into PANI bridged silica network and its activity as
catalyst for the reduction of 4-nitrophenol, which is a common
water contaminant.
Although reports focusing on hybrid materials composed of
conducting polymers or metallic particles incorporate in MCM-
41 pores are found in the literature as above mentioned, those
concerning the combination of these three species are still
scarce.41–43 For our knowledge, there is no report in the litera-
ture concerning the preparation of PANI, mesoporous silica
(MCM-41) and silver.
2.2. Preparation of MCM-41
MCM-41 was prepared following procedure described in the
literature.45 Succinctly, tetraethylortosilicate (TEOS, Aldrich)
was hydrolyzed, under basic condition (NH4OH, Aldrich), using
micellar aggregates of cetyltrimethylammonium bromide
(CTAB, Aldrich) as template. The reactants molar ratio were
525(H2O) : 69(NH4OH) : 0.125(CTAB) : 1(TEOS). Aer 2 h, the
mesophase was isolated by ltration and the solid was calcined
under nitrogen atmosphere until the furnace temperature rea-
ches 550 ꢁC, remaining at this temperature for more 3 h under
air atmosphere.
2.3. Preparation of AgN(SO2CF3)2 (AgTf2N)
Into a 50 mL vacuum sublimator it was added 5.14 g
(18.0 mmol) of anhydrous LiN(SO2CF3)2 (Aldrich, 97%) followed
by 25 mL of H2SO4 (Aldrich, 98%). The solution was heated at
70 ꢁC with stirring, and HN(SO2CF3)2 was collected in the cooler
part of the sublimator as a white solid (90% yield). In the next
step, Ag2CO3 (12.0 mmol, newly synthesized) was added, under
stirring, to a solution of HN(SO2CF3)2 (5.62 g; 20.0 mmol) in
50 mL of distilled water under stirring. The reaction mixture
was heated at 65 ꢁC and then a white solid was isolated by
removing water under reduced pressure. To remove impurities,
such as silver oxide, the solid was dissolved in 50 mL of diethyl
ether and the remained solid was collected by ltration. Aer
solvent removal under reduced pressure, AgN(SO2CF3)2 was
obtained as white powder with 70% yield. The melting point
determined for this compound was 249–250 ꢁC, which is similar
to the commercial product (CAS: 189114-61-2).
Based on these facts, we developed an innovative method to
prepare
a
MCM-41/PANI/Ag nanocomposite by the
direct oxidation of aniline with Ag+ in an ionic liquid, called
2.4. Aniline adsorption
The monomer adsorption into the silica was performed in a
closed system containing two vials, one with liquid aniline and
another one with MCM-41 (previously dehydrated in an oven at
Fig. 1 Pictorial representation of the preparation of the MCM-41/PANI/Ag
composite.
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100 ꢁC).17 The system was vacuumed and heated to 40 ꢁC to
Nitrogen adsorption–desorption was measured at 77 K using
saturate the internal atmosphere with aniline vapor and nitrogen of 99.998% purity on a Quantachrome (NOVA 1200e)
promote the adsorption into the mesoporous silica. Hereaer in gas adsorption analyzer. Measurements were performed in the
the text, this material will be referred as MCM-41/Ani.
range of relative pressure from 0.05 to 0.99. Before the
measurement, the sample was out gassed at 60 ꢁC in the port of
the sorption analyzer for three hours. Specic surface area was
evaluated using the BET (Braunauer, Emmet and Teller)
2.5. Preparation of MCM-41/PANI/Ag
260 mg of MCM-41/Ani (containing 80 mg of aniline; method and pore size distribution (PSD) was calculated using
0.86 mmol) was added to 665 mg of AgN(SO2CF3)2 (1.72 mmol) the BJH algorithm.
dissolved in 1.7 mL of BMImTf2N at room temperature and
The electrical conductivity was measured by a four-point
under stirring. In a few minutes, the reaction mixture turned method using a Jandel Multi Height Probe with Jandel cylin-
green and aer 24 h, the solid was isolated by ltration, rinsed drical probe head (25.4 nm diameter ꢂ 48.5 nm high), tip
with acetone and dried under reduced pressure. Hereaer in spacing of 1.591 nm controlled by the RM3-AR Test Unit. The
the text, this material will be referred as MCM-41/PANI/Ag.
pressed pellets (15 mm in diameter and 0.50 mm in thickness,
The MCM-41/PANI/Ag composite (426 mg) was obtained as a respectively) used in this measurement were prepared using
green solid (yield ¼ 48%, considering 9% of organic content 100 mg of the sample. Electrical conductivity of the pressed
determined by thermogravimetry analysis and the initial mass pellet was an average of eight values measured from two sides
of the monomer).
and in different places and it was determined for two different
pellets.
2.6. Characterization techniques
MCM-41 was characterized by X-ray diffractometry (XRD),
thermogravimetry analysis (TGA), scanning (SEM) and trans-
mission (TEM) electron microscopy and by N2 adsorption–
desorption isotherm.
The ternary composite was characterized by XRD, TGA,
infrared (FTIR) and ultraviolet-visible (UV-Vis) spectroscopies,
SEM, TEM, N2 adsorption–desorption isotherm and by the
evaluation of the electrical conductivity using the four-probe
method.
3. Results and discussion
The choice for MCM-41 instead of other mesoporous silica was
due to its easy preparation and its ordered one-dimensional
parallel pores architecture at nanoscale range. The structural
order and pores regularity of synthesized MCM-41 was proved
respectively by the XRD and TEM (Fig. 1S and 2S – ESI†). The
presence of diffraction peaks attributed to the (100) (110) and
(200) planes are characteristic of a high ordered mesoporous
silica.46 The pore diameter and wall thickness estimated by
X-ray diffraction patterns (XRDP) of powdered samples were
recorded on a Rigaku diffractometer model Miniex using Cu-
Ka radiation.
˚
˚
TEM image are respectively 30 A and 11 A. The synthesized
MCM-41 shows a hexagonal elongated shape and sharp size
distribution of around 400 nm as observed in the SEM micro-
graph (Fig. 3S – ESI†).
Thermogravimetric analyses (TGA) was performed in a Shi-
madzu thermoanalyser TA-60, under synthetic air atmosphere
with a ux of 100 cm3 minꢀ1 and a heating rate of 10 ꢁC minꢀ1
.
The monomer adsorption in the MCM-41 pores was per-
formed by vapor diffusion method to guarantee that aniline
goes inside the pore and the polymer will be formed there
posteriorly, as conrmed by N2 adsorption isotherm experi-
ments already published.6 The monomer adsorption process
was monitored during 14 days, however since aer the seventh
day the mass kept constant, indicating the aniline saturation in
the MCM-41 internal and external surfaces. In order to
determine the amount of adsorbed aniline, the TGA curve was
Fourier transform infrared spectra (FTIR) were recorded on a
Bomem spectrophotometer, model MB-102 using the DRIFT
accessory. The samples were diluted in solid KBr before the
spectrum recording.
UV-Visible electronic absorption spectra (UV-Vis) of solid
samples were recorded in a Shimadzu spectrophotometer
model UV-2401PC equipped with an integration sphere appa-
ratus. The spectra were recorded on reectance mode and
transformed to absorbance using the equipment soware
(Kubelka–Munk method option). The samples were diluted in
BaSO4, which is also used as the reference substance.
Transmission electron microscopy (TEM) images were
recorded on a JEOL microscope model JEM 2100 operating at
200 kV coupled with a Thermo-Noran EDS. Samples were
prepared by dispersing the solid in isopropanol with the aid of
an ultrasonic bath followed by deposition onto a carbon-coated
Cu microgrid.
Scanning electron microscopy (FEG-SEM) micrographs of
uncoated samples were recorded on a JEOL eld emission gun
scanning electron microscope model JSM-7600F, using
secondary electrons detection mode.
Fig. 2 TGA curve of MCM-41/Ani.
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Scheme 1 Preparation of MCM-41/PANI/Ag composite in BMImTf2N.
registered (Fig. 2) and the aniline loss was estimated to be sample shows two mainly broad diffraction peaks in the region
ꢁ
around 33% (from room temperature until 200 C). The pres- of 20ꢁ to 25ꢁ attributed to the parallel arrangement periodicity
ence of aniline was also conrmed by FTIR (Fig. 4S – ESI†).
of the polymeric chains.48 These diffraction peaks were not
Aer the aniline impregnation in the MCM-41 pores, the observed in Fig. 3, suggesting that polymer chains were mostly
monomer was polymerized by AgTf2N in BMImTf2N, according generated inside the pores.
to the chemical reaction described in Scheme 1, which was
Analyzing the TGA curve of the MCM-41/PANI/Ag composite
studied in our previous work.35 This reaction is based on a it is possible to observe one main event (Fig. 4) from 200 C to
simple oxidation of aniline by silver cations, which acts as 650 ꢁC, which corresponds to 9% of weight loss and it was
oxidant agent and metal source. In ionic liquid, Ag+ was able to attributed to the polymer degradation. The onset temperature
oxidize aniline to PANI quickly, which is not usual in aqueous of the polymer decomposition is around 300 ꢁC, indicating good
media. One explanation for this fact is that the Ag+ reactivity thermal stability when compared to pure PANI. The residue
dissolved in the ionic liquid is not equal to the reactivity (around 89%) corresponds to the silica and metallic silver.
observed in common organic solvents. In addition, considering
ꢁ
that the oxidation of aniline to give the respective radical cation
is the slow step47 to obtain PANI, in our opinion the reaction
occurs faster in ionic liquid that in aqueous medium, because
the IL stabilizes this transition state more efficiently than water.
The isolated solid was green that is the characteristic color of
PANI in its most conductive state, the emeraldine salt form
(ES-PANI).
The X-ray diffraction pattern of MCM-41/PANI/Ag (Fig. 3)
shows the characteristic low angle peaks of MCM-41 indicating
that the in situ aniline polymerization did not cause the meso-
structure collapse. With an enlargement of high angle region it
is observed two diffraction peaks at 38ꢁ and 44ꢁ (inset in Fig. 3)
that are attributed to (111) and (200) planes of the face-centered
Fig. 4 TGA of the MCM-41/PANI/Ag composite.
cubic phase of silver (ICDD le no. 4-0783). A bulky ES-PANI
Fig.
5
FTIR spectra of the MCM-41/PANI/Ag composite (—) and pure
Fig. 3 X-ray diffraction pattern of the MCM-41/PANI/Ag composite.
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MCM-41 (---).
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Table 1 Physical properties of MCM-41 and MCM-41/PANI/Ag samples
Surface area
Pore diameter Pore volume
(BJH) (A)
(BET) (m2 gꢀ1
)
(BHJ) (cm3 gꢀ1
)
˚
Sample
MCM-41
MCM-41/PANI/Ag 410
1462
31.1
28.8
0.720
0.074
The N2 adsorption–desorption isotherms for MCM-41 and
MCM-41/PANI/Ag are shown in Fig. 7. MCM-41 shows a type
IV-isotherm according to IUPAC classication.53 The sharp
increase in the adsorbed volume of N2 at a relative pressure
range of P/Po ¼ 0.3–0.4 are due to capillary condensation of N2
into mesopores.54 The absence of hysteresis in the N2 adsorp-
tion isotherms is a characteristic feature of small pore MCM-
41.55 Hysteresis is only observed in MCM-41 with pores larger
Fig.
6
UV-Vis electronic absorption spectrum of the MCM-41/PANI/Ag
composite.
˚
than 40 A. There is not a conrmed explanation for this MCM-
In order to prove the presence of PANI in the composite, the
41 facet but it is suggested that it might be related to pore size
heterogeneity.56 Clearly in the MCM-41/PANI/Ag sample, the
inection point of the N2 adsorption curve shis to a lower
value of relative pressure and above this point it is observed a
long plateau that indicates less room to N2 adsorb due to the
presence of polymer and silver into the mesopores. The reduc-
tion of the surface area as well the pore volume for the
composite corroborates the lling of the pores with PANI and
silver, which partially blocks the adsorption of nitrogen mole-
cules. Table 1 shows the physical parameter of nitrogen
isotherms, such as BET surface area, pore volume and average
pore diameter calculated form N2 adsorption isotherms using
the BHJ model.
The morphology of the composite particles was analyzed by
SEM (Fig. 8) and TEM (Fig. 9). It is not possible to identify
segregated polymeric phases in the material or even silver
particles. The aniline polymerization changed the silica particle
size, since it is observed a considerable size decreasing when
compared to the MCM-41 particle (Fig. 3S – ESI†). Small elon-
gated cylindrical particles with diameters in the nanoscale
range (around 100 nm) distributed homogenously across the
entire area of the substrate are observed aer the polymeriza-
tion, while before the silica particle size is estimated to be of the
order of 400 nm.
FTIR spectrum was registered (Fig. 5). The bands centered at
3425 cmꢀ1 and 1630 cmꢀ1, observed also in the MCM-41 spec-
trum, are attributed to the O–H stretching and bending modes,
respectively. The Si–O–Si stretching and bending vibrations
occur at 1086 cmꢀ1 and 800 cmꢀ1, respectively. The most
notable differences comparing the MCM-41 and MCM-41/PANI/
Ag spectra are the appearance of the band at 1505 cmꢀ1
attributed to the benzenoid ring stretching vibration.49,50 The
enlargement in the 1250–1100 cmꢀ1 region comprises the band
at 1145 cmꢀ1 designated to the C–H in plane bending of the
quinoid rings (oxidized units) of PANI. The presence of Tf2N as
counter ions of the positive charge of these oxidized units is
conrmed by the appearance of the bands at 1200 cmꢀ1 and
1325 cmꢀ1, which are designated to CF3 and SO2 stretching
modes.51
In the UV-Vis spectrum (Fig. 6), registered in the solid state,
it is observed two bands. The rst centered at 435 nm is
attributed to the polaron-p* (A) transaction and another that
starts at 500 nm and extends until into the near infrared region
assigned to p-polaron (B) transition, which is the most evidence
of emeraldine salt formation.52 The surface plasmon resonance
absorption of the electrons in the conducting silver bands
usually observed around 400 nm can be overlapped by the
absorption A of PANI.
The representative TEM images of the composite (Fig. 9) still
exhibit elongated cylindrical particles (Fig. 9a) as observed in
pure MCM-41. Aer the polymerization, the hexagonal struc-
ture, characteristic of the MCM-41, is noted in the composite
(Fig. 9b and d), indicating that the present preparation method
had no effect on the uniform structure of the MCM-41. The
presence of silver was conrmed by energy dispersive X-ray
spectroscopy (EDX) (Fig. 9c). Only a very small amount of silver
nanoparticles (diameter around 5 nm), indicated by the arrows,
are observed outside the pore channels.
Finally, the electrical conductivity of the composite,
evaluated using a four probe method, was in the order of
10ꢀ3 S cmꢀ1, which is one million higher than the conductivity
of MCM-41/PANI composite (ca. 10ꢀ9 S cmꢀ1),14 since silicas are
an electrical isolating material in general. Considering that to
achieve good electrical conductivity, the polymeric chains of
Fig. 7 N2 adsorption–desorption isotherms of (a) MCM-41 and (b) MCM-41/
PANI/Ag composite.
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Fig. 8 SEM images of the MCM-41/PANI/Ag composite.
pores size and shape. The presence of silver nanoparticles in the
MCM-41/PANI composite increased the electrical conductivity
at about 106 times.
Acknowledgements
The authors acknowledge the Brazilian agency FAPESP (Grant
Number 07/50742-2) for nancial support and are thankful to
Prof. Vera R. L. Constantino (IQ-USP) for the X-ray diffraction
and UV-Vis experiments, to microscopy support of C2NANO –
Center for Nanoscience and Nanotechnology/MCT (TEM-MSC-
´
~
15017 proposal) and to Laboratorio de Simulaçao e Controle de
Processos (POLI-USP) for the TGA analysis.
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