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Chemical synthesis of hollow sea urchin like nanostructured polypyrrole
particles through a core-shell redox mechanism using a MnO2 powder as
oxidizing agent and sacrificial nanostructured template
L. Benhaddad a,b,c, M.C. Bernard b,c, C. Deslouis b,c, L. Makhloufi a, B. Messaoudi a,
A. Pailleret b,c,*, H. Takenouti b,c
Corresponding author:
E-mail:alain.pailleret@upmc.fr; Tel: +33 1 44 27 41 69; Fax: +33 1 44 27 40 74
aLaboratoire de Technologie des Matériaux et Génie des Procédés (LTMGP). Département de Génie des
Procédés. Université A. Mira, Route de Targa Ouzemmour, 06000 Béjaia, Algeria.
bCNRS, UPR 15, Laboratoire Interfaces et Systèmes Electrochimiques, (LISE, case courrier 133), 4 Place
Jussieu, F-75005, Paris, France
cUPMC Univ. Paris VI, UPR 15, LISE, 4 Place Jussieu, F-75005, Paris, France
Abstract
Hollow sea urchin shaped nanostructured polypyrrole powder was successfully synthesized
chemically in an acidic medium through a core-shell redox mechanism by using a
nanostructured MnO2 powder as oxidizing agent and sacrificial template simultaneously. The
morphology and the structure of MnO2 powder based reactant and produced polypyrrole
powder were characterized respectively by using FEG-SEM, TEM, EDX and XRD
techniques, which led us to demonstrate clearly the formation of hollow and open
microparticles of polypyrrole with the presence of nanotubes on their surface. Nanostructured
polypyrrole powder was found to be rather amorphous even though the shape of the
polypyrrole particles was induced by the crystalline and nanostructured sea urchin shaped
MnO2 powder on which they grew. In addition, neither MnO2 nor any manganese based
species were found within the produced polypyrrole powder, which ruled out the production
of composite materials. Moreover, Raman technique showed that the synthesized PPy powder
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was produced in the oxidized and thus conducting state. It actually possesses a 0.31 doping
level and a 0.05 S.cm-1 conductivity, as shown by XPS and impedance spectroscopy
measurements respectively. Cyclic voltammetry and UV-Visible spectroscopy studies
allowed us to identify the oxidation mechanism of pyrrole by our MnO2 powder through the
detection of soluble Mn2+ cations as reaction products isolated after filtration of the reaction
medium.
Keywords: polypyrrole powder; nanostructuration; core-shell redox mechanism; manganese
dioxide; sacrificial template; reactive template.
1. Introduction
The term “nanomaterials” encompasses a wide range of materials including nanocrystalline
materials, nanocomposites, nanoparticles, and nanotubes for example. Recently, the synthesis
of nanomaterials has attracted a great scientific interest due to their unique physical, chemical,
electrical, and magnetic properties, resulting mostly from their small size and their high
specific area [1-5], which makes them eligible for a vast range of application domains.
In particular, conducting polymers have been intensively studied for their one-dimensional
conjugated structures as well as their adjustable conductivity and surface reactivity [6–9].
Furthermore, nanostructured conducting polymers such as nanoparticles [10, 11], nanofibres
[12, 13], nanotubes [14] and nanowires [15-17] have received great attention, mainly for their
promising applications in batteries, conducting paints, chemical and electrochemical sensors,
and field emission applications [7,18–20]. Polypyrrole (PPy) is one of the most investigated
conducting polymers as a consequence of its high electrical conductivity, relatively good
stability in air, low toxicity and simple preparation [7, 8, 21–24]. Generally, two methods can
be used to prepare polypyrrole, namely electrochemical and chemical polymerizations. Both
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can involve a template during the polymerisation of pyrrole so as to obtain polypyrrole
nanostructures [25, 26].
Template synthesis is an elegant approach for the elaboration of nanostructures. For example,
it may involve the synthesis of a desired material within the pores of a nanoporous membrane
[27]. In fact, two types of templates have been reported in literature: hard templates such as
porous polycarbonate films [28], fibrillar V2O5 [29], and porous alumina [30], which usually
have to be removed after synthesis of nanostructured polypyrrole, with the risk to destroy the
synthesized polypyrrole, or some of its main properties, due to the use of strongly acidic/basic
aqueous or organic solutions or elevated temperatures [25], and soft templates such as reverse
microemulsions [31] and micelles [32], that may have the disadvantages of instability, low
efficiency and lack of versatility for a given monomer [25].
Among targeted structured materials, hollow spheres have received enormous interest in
recent years due to their unique properties and their numerous potential applications in
various domains as delivery vehicles for the removal of contaminated waste and the
controlled release of substances such as drugs, cosmetics, dyes and inks or as a catalyst carrier
[33]. In order to improve the processability of polypyrrole, great efforts have been made to
prepare polypyrrole hollow spheres. Recently, polymeric hollow spheres have been cited as
novel types of carriers and nanoreactors with designed properties because they exhibit
controllable permeability and surface functionality, which should enable many applications
[34-37]. As a result, many efforts are made up to now to find out a facile, efficient and
versatile approach to synthesize nanostructured polypyrrole based materials but it remains a
key research challenge for researchers.
To this end, we introduce in this contribution a simple and efficient chemical method to
synthesize pure nanostructured polypyrrole powder from an acidic aqueous medium by using
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nanostructured manganese dioxide. MnO2 was already mentioned in literature as a possible
oxidizing agent to produce conducting polymers from corresponding soluble monomers. It
was indeed observed for instance by N. Ballav et al. that MnO2, PbO2, and NH4VO3 particles
can act as oxidants for the polymerization of aniline to produce polyaniline without acting as
templates [38]. These oxidizing species are recoverable and recyclable, which is an advantage
over conventional oxidizing agent (NH4)2S2O8 [38]. The chemical synthesis of pure
nanostructured polyaniline powders from an adequate solution by using MnO2 powder as an
oxidizing agent and a template simultaneously was also reported [39]. However, if the
experimental conditions are not adequate, then a PANI/MnO2 composite material is formed
[40-42]. We also noticed from literature that MnO2 was claimed to act as an oxidizing agent
towards the pyrrole monomer to produce PPy/MnO2 composite materials [43-44].
Nevertheless, to the best of our knowledge, literature does not contain any report of the exact
and accurate mechanism (stoechiometry, other reaction products except PPy, properties of
PPy) of the chemical synthesis of polypyrrole powders using a MnO2 powder.
The objective of the present study was thus aimed at defining the mechanism of the chemical
polymerization of pyrrole into nanostructured polypyrrole materials from an acidic medium
with the help of a MnO2 powder synthesized by a hydrothermal method and used as oxidizing
agents and sacrificial nanostructured template simultaneously.
The structures and morphologies of all powders synthesized in this work were characterized
by various methods: Field Emission Gun-Scanning Electron Microscope (FEG-SEM),
Transmission Electron Microscopy (TEM), Energy Dispersive X-ray elemental analysis
(EDX), and X-ray Diffraction (XRD) techniques and Raman spectroscopy. UV-Visible
spectroscopy was used in the presence of a binding agent that is specific for the detection of
Mn2+ cations, as well as cyclic voltammetry. Our purpose was to accurately and undoubtedly
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identify the manganese based reaction product of the formation mechanism of our
nanostructured polypyrrole powders in the filtrated reaction medium.
2. Experimental Section
2.1. Chemicals
All the solutions were prepared using water deionised with double ion exchange columns.
Manganese sulphate (MnSO4, H2O), sulphuric acid (H2SO4), and sodium persulphate
(Na2S2O8) were purchased from Prolabo. The latter compound was used as oxidizing agent
towards Mn2+ cations for the synthesis of MnO2 powder. Pyrrole was purchased from Fluka
(97 % purity) and was purified by distillation. 25,26,27,28-tetrahydroxy-calix[4]arene-
5,11,17,23-tetrasulfonic acid (C28H24O16S4), noted Calix-S4, and sodium hydroxide (NaOH)
were purchased from Acros Organics and Prolabo respectively and used as received.
2.2. Synthesis of powders
MnO2: This method has already been reported in our previous publications [4,5]. It consists
in mixing MnSO4, H2O (0.08 mol) with Na2S2O8 (0.08 mol) and 150 mL of deionised water at
room temperature. The mixture was stirred during 10 min so as to form a homogeneous pink
solution that was then kept at 90 °C for 24 h. The obtained powder was filtered, rinsed with
deionised water several times, and finally dried at 60 °C for 24 h.
Polypyrrole: The polypyrrole powders were chemically prepared by injecting liquid pyrrole
(Vpyrrole = 1.36 mL, 0.2 M) into a beaker containing MnO2 powder (2 g) suspended in an
aqueous solution of sulphuric acid (100 mL, 1 M) at room temperature. The polymerization of
pyrrole was found to be initiated immediately after addition of the monomer to the medium.
The mixture was stirred magnetically for 4 hours at room temperature. The black precipitate
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of polypyrrole was collected by filtration, rinsed repeatedly with deionised water, and finally
dried at 60 °C for 24 h.
2.3. Instruments
A Field Emission Gun-Scanning Electron Microscope (FEG-SEM Ultra 55 Zeiss), coupled
with Energy Dispersive X-ray elemental analysis (EDX), was used to visualise the
synthesized powders. In addition to these techniques, the X-ray diffraction (XRD) and TEM
techniques were also used. TEM images were obtained with a JEOL 2000 FX microscope
running at an accelerating voltage of 200 keV. Prior to analysis, the powders were crushed in
a mortar. A small amount was then added in a small volume of pure ethanol using ultrasonic
bath. A drop of this mixture was placed on a copper grid covered with a carbon coating and
allowed to dry in air before analysis.
A Jobin-Yvon (LABRAM model) Raman spectrometer was used with a He–Ne laser
(wavelength 632.81 nm). The 180◦ scattered Raman beam with respect to the incident beam
was focused on the spectrometer slit. The device using a Notch filter presents a very high
sensitivity which allows using weak laser intensity as low as 0.01 mW, avoiding therefore any
perturbation of the polymer powder.
Doping level of polypyrrole powder was determined from the measure of the sulphur to
nitrogen ratio that was carried out with the help of X-Ray Photoelectron Spectroscopy (XPS).
Conductivity of PPy powder was measured using impedance spectroscopy as well as a home
made device allowing a PPy powder based disk to be pressed between the flat faces of two
steel cylinders connected to a Bio-Logic SP300 potentiostat. The high frequency limit of the
obtained impedance spectroscopy spectrum provides a measurement of the electronic
resistance from which one can extract σ, the electronic conductivity (in S.cm-1) of the PPy
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powder by using the formula σ = 1/R.l/s, where R is the measured series resistance (in ohms),
l, the thickness of the PPy powder based disk (in cm) and s, its surface (in cm2).
2.4. Study of the polymerization of pyrrole in presence of MnO2
The study of pyrrole polymerization reaction was realized by using optical measurements. In
a preliminary series of experiments, UV-Visible spectra were recorded in the range of 350-
900 nm using a Hitachi U-4001 spectrophotometer in solutions containing different
concentrations of Mn2+ in the presence of Calix-S4 and prepared in NaOH aqueous medium at
pH 11.8, in a 1 cm standard cell. The purpose of this was to carry out a calibration procedure
aimed at allowing the identification of Mn2+ cations in solution. It is well known that this
binding agent gives a red coloration upon complexation of Mn2+ cations [45,46]. In order to
corroborate this quantitative analysis, the presence of Mn2+ was also detected by cyclic
voltammetry. Electrochemical measurements were carried out with a potentiostat (Gamry,
Femstat FAS1). The electrochemical experiments were carried out in a usual cell containing
three electrodes: a platinum electrode (area = 3.1 mm2) as working electrode, a SCE reference
electrode, and a large platinum grid as counter electrode.
3. Results and Discussion
3.1. Characterization of MnO2 powder
The morphology of MnO2 powder obtained from the synthesis procedure reported in section
2.2, observed by FEG-SEM is presented in Figure 1a-b. The panoramic morphology of MnO2
powder synthesized by hydrothermal method reveals a sea urchin like morphology with 4 ~
10 µm in diameter. Elemental analysis by EDX (Figure 1c) confirms the presence of
manganese and oxygen, elemental components of MnO2. Indeed, the presence of manganese
is shown by two energy peaks appearing approximately at 5.5, and 6.3 keV whereas that of
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oxygen is shown by one energy peak situated approximately at 0.3 keV. The presence of
sulphur, observed as an impurity coming from manganese sulphate, is negligible.
Typical TEM image (Figure 1d) of MnO2 powder after grinding show that the nanosticks
observed previously by the FEG-SEM technique do consist of assemblies of straight sticks
with diameters within the 10-50 nm range, and up to several hundreds of nanometres in
length. In parallel, electron diffraction pattern showed that each stick is rather well
crystallized but its orientation changes from one to another. The observed sea urchin like
structure is thus a polycrystalline structure.
X-ray diffraction (XRD) pattern of the synthesized MnO2 powder is shown in Figure 1e. The
feature peaks recorded in the XRD spectrum were assigned by (hkl) values of 120, 031, 131,
230, 300, 002, 160, 401, 421, 003, 062 and 450. All of the reflections of the XRD pattern can
be readily indexed to the crystallographic variety γ-MnO2, which agrees with the values
recorded in the literature (JCPDS Card. N 14-0644).
3.2. Characterization of synthesized polypyrrole powders
The morphology of the resulting polypyrrole powder was observed by FEG-SEM (see Figs.
2a-b) and their corresponding EDX profile is presented in Figure 2c. The low magnification
FEG-SEM image of polypyrrole powder synthesized from γ-MnO2 illustrate that the
synthesized polypyrrole particles exhibit hollow sea urchin like morphologies dictated by
those observed for MnO2 powder with a rather uniform size and a more or less widely open
mouth. The high magnification FEG-SEM image of polypyrrole particles reveal that they are
composed of many nanometric nanotubes combined together on their surface (see Figure 2b).
Elemental analysis by EDX (Figure 2c) confirms the presence of carbon and nitrogen,
elemental components of polypyrrole, in the structure of synthesized powders, shown by
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energy peaks observed at 0.3 keV and 0.4 keV, respectively. The presence of sulphur and
oxygen is also obvious in these polypyrrole powders, which suggests, at this stage, that the
obtained polypyrrole powder is likely doped by SO42- anions present in the synthesis solution,
and as a consequence, that polypyrrole is in a doped, i.e. conducting state. It is worth noting
the complete absence of manganese in the polypyrrole powder in spite of the use of
manganese dioxide as an oxidizing agent towards pyrrole. From these first observations, it
can be concluded that hollow particles of pure nanostructured polypyrrole were successfully
synthesized by a chemical route.
The polypyrrole particles observed in Figs. 2b are in fact nanotubes according to TEM
images. Typical TEM images (see Figure 2d) show that the nanotubes of polypyrrole powder
synthesized with γ-MnO2 observed by the FEG-SEM technique actually possess a typical
outer diameter of 50 nm, a length up to 400 nm, and an inner diameter of about 10 nm which
vary along the nanotube. One can also distinguish, especially from Figure 2d that those
nanotubes are also closed at one extremity.
Figure 3 shows X-ray powder diffraction (XRD) pattern of the polypyrrole powder. As the
absence of manganese dioxide material has been established in this material, the XRD pattern
can be attributed only to polypyrrole. On Figure 3, XRD pattern exhibits a weak and broad
diffraction peak at 2 = 25°, which has already been encountered in literature and attributed to
amorphous PPy structures [47-51].
Figure 4 presents the Raman spectrum corresponding to polypyrrole powder synthesized by
using γ-MnO2. The most important peak is the one at ca. 1599 cm-1, which represents the
backbone stretching mode of C=C bonds and is considered to be an overlap of the
contribution of the reduced and oxidized forms of polypyrrole [52-57]. The peak positions of
C=C backbone stretching bonds are in favour of dominating oxidized species. It was reported
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by J. Duchet et al. [53] that the peak at 1503 cm-1 represents a skeletal band. Moreover, a
broad feature comprised of several overlapping bands and centered at 1348 cm-1 is assigned to
a hybrid mode of intra-ring C-C bond stretching and mainly inter-ring C-C bond stretching
[52], antisymmetrical C-N stretching [53] or ring stretching [55,56]. The peak at 1258 cm-1
has been assigned to the antisymmetrical C-H in-plane bending mode [53] or N-H in-plane
deformation [55]. Further evidence for the presence of oxidized species is given by the double
peaks which appear at ca. 1084 cm-1 and 1049 cm-1 and assigned to the symmetrical C-H in
plane bending mode associated with the bipolarons (dication) and polaron (radical cation)
structures respectively [52,53]. Moreover, the bands at ca. 983 cm-1 and 941 cm-1 which are
assigned to the ring deformation associated with polarons and bipolarons respectively
[52,53,56] are also the signature of oxidized PPy. All these interpretations tend to demonstrate
that the PPy powder is in its doped (oxidized) state, in good agreement with the presence of
sulphate anions revealed by our EDX analysis.
3.3. Study of the polymerization mechanism of pyrrole
3.3.1. Optical measurements
As reported previously, MnO2 powder, used as oxidizing agent for the chemical
polymerization of pyrrole, totally disappears as such at the end of the synthesis step, as shown
by the absence of manganese in the structure of the polypyrrole powders revealed by EDX
and XRD. This means that manganese produced from MnO2 reduction should be found in the
supernatant solution as an ionic and therefore soluble species. These results incited us to
identify the reduction product of manganese dioxide and to study the kinetics of polypyrrole
polymerization reaction. In this purpose, we have used UV-Visible spectroscopy and cyclic
voltammetry. In order to define the nature of these ions (Mn2+, Mn3+), it was postulated that
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MnO2 was reduced into Mn2+ ions and these ions were analyzed by the method already used
by Nishida et al. [45].
In order to optimize the complexation conditions, Mn2+ aqueous solutions were prepared from
different concentrations of MnSO4, H2O. The influence of Mn2+ concentration on the intensity
of absorbance band was studied directly after mixing 1 mL of Calix-S4 solution and 3 mL of
Mn2+ solution, at different concentrations, at pH 11.8. It is obvious that the concentration of
the testing solution affects the intensity of the absorbance band of the complex. The higher the
concentration of manganese (II) cations is, the higher is the band intensity of complex Calix-
S4 / Mn (II), as presented in Figure 5, but this increase is limited at an absorbance of 3.5
because above this absorbance value, marked fluctuations appeared. The plot of this variation
of absorbance with Mn2+ concentration shows that the absorbance of these solutions varies
linearly as a function of Mn2+ ions concentrations, which obeys the Lambert-Beer Law (curve
not shown) and constitutes thus a calibration curve.
In order to verify our hypothesis, polypyrrole powders were prepared as explained in section
2.2. After polymerization and filtration, the retrieved solution was diluted at an adequate
concentration (1.16 10-4 M), taking as a hypothesis that the whole initial amount of MnO2 was
reduced to Mn2+. It was observed that the added volume of Calix-S4 (1 mL) to 3 mL of
retrieved diluted polymerization solution changed immediately the coloration of initial
solution from colourless to red. Also, the spectrum related to the characterization of this
mixture by UV-Vis, shown in Figure 6, illustrates the presence of an absorption band
perfectly overlapped with that presented in Figure 5c and relative to Mn2+ ions at a
concentration of 1.16 10-4 M.
In another series of experiments, the above-mentioned spectrophotometric procedure was
used to follow the production of Mn2+ in the course of the polymerization procedure, and
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therefore the reaction kinetics of the production of our polypyrrole powders. For that purpose,
an adequate volume of the reaction mixture was extracted, filtered so as to remove the
polypyrrole and manganese dioxide powders, and then the Mn2+ concentration was
determined and plotted as a function of time (Figure not shown). It was found that this latter is
constant with time beyond a 25 minutes reaction time, which therefore provides a first
estimation of the reaction duration.
3.3.2. Electrochemical measurements
Another method allowing the identification of the reaction product resulting from MnO2
reduction is the cyclic voltammetry technique. The aqueous solution resulting from filtration
of the reaction medium was used as an electrolytic solution. The potential scan rate was 10
mV.s-1 and the potential scan started from 0.7 V vs. SCE towards the positive direction up to
1.3 V vs. SCE, followed by a cathodic scan down to 0.7 V vs. SCE. The resulting cyclic
voltammogram shown on Figure 7 is typical for the electrochemical behaviour of Mn2+
cations in acidic aqueous solutions. The reaction product was therefore undoubtedly identified
as Mn2+. The anodic peak indeed reveals the oxidation of dissolved Mn2+ cations into
manganese dioxide, produced as an insoluble thin film on the working electrode surface,
whereas the cathodic peak reveals its reduction in the reverse reaction [58]. This important
observation demonstrates that the Mn2+ ions that were not detected in the structure of the
produced polypyrrole powders are present in the retrieved polymerization solution of pyrrole
and consequently confirms the results obtained by the UV-Visible technique.
3.4 Discussion
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From these results, it can be concluded that the manganese dioxide sea urchins have oxidized
pyrrole to produce hollow sea urchin shaped polypyrrole particles (see Eq. 1 with the
hypothesis that the doping rate in PPy is 0.33 per monomer [59]) and were reduced into Mn2+
cations (see Eq. 2), which were produced in solution as soluble species.
nenHPynPy n33.22)( 33.0 (Eq. 1)
OHMneHMnO 2
2
2224 (Eq. 2)
This redox mechanism can be represented by the following redox reaction:
n
PyOnHnMnnPynHnMnO )(61476167 33.0
2
2
2
(Eq.3)
In our experiments, oxidation of pyrrole occurred at the manganese dioxide/aqueous solution
interface, leading to the progressive dissolution of MnO2 into soluble Mn2+ cations. As this
interface therefore steps back towards the heart of the initial MnO2 particles, the polypyrrole
particles are likely to grow towards the interior of the native particles, so as to keep in touch
with the manganese dioxide surface, although this is not incompatible with a simultaneous
growth on the external side of the growing PPy particles. Such mechanism may suggest that
pyrrole monomers and sulphate anions may diffuse through the growing and hollow
polypyrrole particles envelope from the outside to the inside so as to feed the reaction site
with reactants. Simultaneously, Mn2+ cations have to diffuse away through the walls of these
polypyrrole structures so as to leave the reaction site where they were produced. Another
possibility, possibly more realistic, is that reactants and reaction products respectively reach
and leave the reaction site by using the mouth frequently observed on the resulting hollow sea
urchin shaped polypyrrole particles. In any case, it is obvious from our structural
characterization experiments that these manganese dioxide nanostructured particles did not
act only as oxidizing agents in the course of this mechanism but also as sacrificial templates
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imposing their shape to the resulting polypyrrole particles during a so-called core-shell redox
mechanism.
The resulting polypyrrole powder possesses a 0.31 doping level as shown by XPS
characterisation (see Survey XPS spectrum in Figure SI 1 of Supplementary Information).
Such value is in very good agreement with that of 0.33 usually encountered in literature for
polypyrrole [59]. Moreover impedance spectroscopy measurements allowed us to determine a
series resistance of 282.4 ± 4.4 Ω (average value calculated over ten impedance spectroscopy
measurements, see impedance spectroscopy spectrum in Figure SI 2 of Supplementary
Information) and thus a 0.05 S.cm-1 conductivity. By comparison with the doping level value,
this conductivity value may reveal conductivity limitations due to chain length and
nanostructured character of polypyrrole powders. The nanotubes observed on the surface of
the sea-urchin shaped polypyrrole nanostructures are about two hundred fifty nanometers long
and their walls are about twenty-five nanometers thick (see Figure 2). As a consequence, one
can expect a majority of short polypyrrole chain lengths that is likely to lead to a higher
number of interchain electron transfer events and therefore to a lower conductivity, especially
in an amorphous material (see Figure 3) like the polypyrrole powder synthesised in this work.
Conclusion
In this contribution, hollow sea-urchin shaped nanostructured polypyrrole particles were
successfully synthesised by using sea urchin shaped MnO2 nanostructured powders as
oxidizing agents and sacrificial templates simultaneously. The reaction kinetics of pyrrole
polymerization was studied by UV-Visible spectroscopy and cyclic voltammetry
measurements. The reaction was found to occur within about 25 minutes. We demonstrated
that MnO2 particles oxidize pyrrole monomers into oxidised polypyrrole and were reduced to
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soluble Mn2+ cations identified in the retrieved polymerization solution, which explains the
absence of manganese in the produced polypyrrole powders. The reaction mechanism is
clearly redox and of the core-shell type. The synthesized amorphous polypyrrole powders
were found to be doped with sulphate anions and thus in the conducting state. They possess a
0.31 doping level as well as a 0.05 S.cm-1 conductivity in our experimental conditions. From
all these observations, it can be mentioned that we have accurately and undoubtedly defined
an easy and direct method to synthesize large amounts of pure, amorphous, conducting and
nanostructured polypyrrole particles that are likely to possess in addition a high specific area.
We also believe that our investigations bring a necessary light on the redox reactivity between
MnO2 and pyrrole monomer that was rather badly defined in literature so far. As such, they
may affect strongly and beyond expectations i) the understanding of chemical synthesis of
PPy in the presence of MnO2 and another oxidizing agent, ii) the electrochemical deposition
of composite polypyrrole/MnO2 thin films, and iii) the expectable surface properties
(oxidation power) of MnO2 particles in the resulting materials, keeping in mind that the
oxidation potential of polypyrrole is lower than the oxidation potential of pyrrole. It is likely
that such conclusions will be of large interest for numerous investigations frequently reported
in literature lately concerning the synthesis, properties and applications of PPy/MnO2 powders
(mixtures or core/shell) and PPy/MnO2 composite thin films in view of applications as
electrode materials in batteries, supercapacitors and electrocatalysis for instance.
Acknowledgements
The present work was carried out in the frame of French – Algerian cooperation project
CMEP- PHC Tassili N°06 MDU 686. The authors are grateful to EGIDE for financial
support. They thank Mrs Pillier and Mr. Borensztajn for their efficient assistance in TEM,
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FEG-SEM and EDX experiments, as well as Mr C. Bazin and Mrs. H. Benidiri for the XRD
measurements. Mr C. Calers (LRS, UMR 7197) is warmly acknowledged for his
collaboration during XPS measurements.
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FIGURES CAPTIONS
Figure 1: (a-b) FEG-SEM images, (c) EDX profile, (d) TEM image and (e) XRD pattern of
MnO2 powder.
Figure 2: (a) Low magnification and (b) high magnification FEG-SEM images, (c) EDX
profile, and (d) TEM image of polypyrrole powder synthesized with γ-MnO2.
Figure 3: XRD pattern of polypyrrole powder synthesized with γ-MnO2.
Figure 4: Raman spectrum of polypyrrole powder synthesized with γ-MnO2.
Figure 5: UV-Visible spectra of Calix-S4/Mn2+ complex in aqueous solutions containing
Calix-S4 (1 mL) and Mn2+ (3 mL) at different concentrations: (a) 0,29 10-4 M, (b) 0,58 10-4
M, (c) 1,16 10-4 M, (d) 1,74 10-4 M, (e) 2,32 10-4 M, (f) 2,9 10-4 M, (g) 3,48 10-4 M, (h) 4,06
10-4 M, (i) 4,64 10-4 M, (j) 5,22 10-4 M, (k) 5,8 10-4 M.
Figure 6: Overlapped UV-Visible spectra showing (a) that presented in Figure 6 (c) and (b)
that recorded in an aqueous solution containing Calix-S4 (1 mL) and 3 mL of retrieved
polymerization solution diluted so as to get the same Mn2+ concentration (1,16 10-4 M), taking
as an hypothesis that the whole initial amount of MnO2 was reduced to Mn2+.
Figure 7: Cyclic voltammogramm of platinum electrode in retrieved polymerization solution
diluted so as to get the same Mn2+ concentration (1,16 10-4 M), taking as an hypothesis that
the whole initial amount of MnO2 was reduced to Mn2+. Potential scan rate: 10 mV.s-1.
24
Figure 1: (a-b) FEG-SEM images, (c) EDX profile, (d) TEM image and (e) XRD pattern of
MnO2 powder.
0 1 2 3 4 5 6 7 8 9 10 11 12 13
(c)
Energy (keV)
Mn
O
S Mn
Mn
20 30 40 50 60 70 80
(e)
2
/ (°)
002
003
421
401
160
300
230
031
131
120
450
062
(
d
)
25
01234567
(c)
O
N
S
C
Energy (keV)
Figure 2: (a) Low magnification and (b) high magnification FEG-SEM images, (c) EDX
profile, and (d) TEM image of polypyrrole powder synthesized with γ-MnO2.
26
5 10152025303540
2 / (°)
Figure 3: XRD pattern of polypyrrole powder synthesized with γ-MnO2.
600 800 1000 1200 1400 1600 1800 2000
Arbitrary Unit / (a.u.)
941
1049
1084
983
1258
1348
1503
1599
Wavenumbers / cm
-1
Figure 4: Raman spectrum of polypyrrole powder synthesized with γ-MnO2.
27
400 500 600 700 800 900
0,0
0,5
1,0
1,5
2,0
2,5
3,0
3,5
4,0
(k)
(a)
Absorbance
Wavelength / nm
Figure 5: UV-Visible spectra of Calix-S4/Mn2+ complex in aqueous solutions containing
Calix-S4 (1 mL) and Mn2+ (3 mL) at different concentrations: (a) 0,29 10-4 M, (b) 0,58 10-4
M, (c) 1,16 10-4 M, (d) 1,74 10-4 M, (e) 2,32 10-4 M, (f) 2,9 10-4 M, (g) 3,48 10-4 M, (h) 4,06
10-4 M, (i) 4,64 10-4 M, (j) 5,22 10-4 M, (k) 5,8 10-4 M.
28
400 500 600 700 800 900
0,0
0,5
1,0
1,5
2,0
(b)
(a)
Absorbance
Wavelength / nm
Figure 6: Overlapped UV-Visible spectra showing (a) that presented in Fig. 6 (c) and (b) that
recorded in an aqueous solution containing Calix-S4 (1 mL) and 3 mL of retrieved
polymerization solution diluted so as to get the same Mn2+ concentration (1,16 10-4 M), taking
as an hypothesis that the whole initial amount of MnO2 was reduced to Mn2+.
0,7 0,8 0,9 1,0 1,1 1,2 1,3 1,4
-8
-6
-4
-2
0
2
4
6
8
Current / µA
Potential / V
ECS
Figure 7: Cyclic voltammogramm of platinum electrode in retrieved polymerization solution
diluted so as to get the same Mn2+ concentration (1,16 10-4 M), taking as an hypothesis that
the whole initial amount of MnO2 was reduced to Mn2+. Potential scan rate: 10 mV.s-1.
