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Optical and Photo-electrochemical characterization of manganese dioxide/polypyrrole nanocomposite
Leila Lamiri 1, 2, Assia Tounsi 2, Charif Dehchar1, Samiha Laidoudi1, Ouafia Belgherbi1, Belkacem Nessark2, Farid Habelhames2
1 Research Center in Industrial Technologies CRTI, P.O. Box 64, Cheraga 16014, Algiers. Algeria.
2 Laboratoire d’Electrochimie et Matériaux, Département de Génie des Procédés, Faculté de Technologie, Université Ferhat Abbas, Sétif 19000, Algeria.
Email: l.lamiri@crti.dz
Abstract. PPy-coated manganese dioxide (PPy-MnO2) synthesized by depositing PPy on the surface of γ-MnO2 particles in acetonitrile solution containing 10-1M lithium perchlorate (LiClO4) containing a monomer (pyrrole) and semiconductor (MnO2) nanoparticles. The composite materials (MnO2- PPy/ITO) were characterized by different methods including cyclic voltammetry, impedance spectroscopy, chronoamperometry, scanning electron microscopy (SEM). The cyclic voltammogram showed one redox couple characteristic of the oxidation and reduction composite material of composite material. The impedance spectroscopy study showed that the resistance of the film increases with the MnO2 incorporated in the polymer. The morphological analysis of the film surfaces showed that the MnO2 nanoparticle increased the roughness. These results give information on the use possibility of these materials for energy storage and as photovoltaic cells applications.
Keywords: Manganese dioxide, polypyrrole, electrodeposition, optical proprieties.
1. Introduction
Recently, the manganese dioxide (MnO2) has drawn much attention because of its unique electrochemical properties. The (MnO2) is featured by environmental friendliness, low cost and considerable catalytic activity toward electrochemistry and high chemical stability [1-5], but it has poor conductivity and deficient charge transfer [6-8]. Overcoming these disadvantages of MnO2, this metal-oxide can be incorporated with conducting polymers (CPs), which can improve the electrical conductivity of MnO2.
Among these conducting polymers polypyrrole (PPy) is deemed one of the most important conducting polymers due to its high electrical conductivity, good environmental stability, biocompatibility, and easy synthesis [9-11].
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Therefore, many investigations have been focus of PPy/MnO2 composite materials for applications in different fields such as catalysts [12–14], energy storage devices [15,16], supercapacitors [17, 18] and fuel cell [19-21].
Many researchers concerning MnO2-conducting polymer composites by using different methods. For instance, Khan et al [22] synthesized PPy/MnO2 composites by chemical method and studied their electrochemical, electrical properties, the synthesized materials showed semiconducting nature and their resistance was found to be dependent on temperature and MnO2 content in the composites. Wan et al [23] have been developed to prepare MnO2/PPy composites by in situ chemical oxidative polymerization; the results indicate that the combination of MnO2 and PPy enhances the capability of the composite and the largest specific capacitance, when assessed as electrode material for supercapacitor.
Researchers, [24] prepared manganese oxide (MnO2)/doped polypyrrole by chemical oxidative polymerization. It is observed that the dispersed MnO2 adhered to PPy chains increased the specific surface area of the nanocomposite. There are a few works on MnO2
based alloy electrodes development for fuel cell. Paul et al [25] developed MnO2 as electrocatalytic energy material for fuel cell electrode.
In the present investigations, we synthesized PPy-MnO2 composite material on an indium tin oxide (ITO) substrate by the electropolymerization of pyrrole in the presence of MnO2 nanoparticles. The composite films were characterized by SEM/EDS, UV–vis spectroscopy, cyclic voltammetry impedance spectroscopy and photocurrent tests.
2. Experimental
2.1. Materials and composite film preparation
Pyrrole (Py 98% Aldrich), they were stored at 4°C in the absence of light and used as received without further purification. The acetonitrile, pure product for analysis was obtained from Aldrich. The supporting electrolyte salt lithium perchlorate (LiClO4) was purchased from Fluka. The γ-MnO2 particles (99%, Aldrich).
Electrochemical polymerization and characterization were carried out with a three- electrode /one-compartment glass cell connected to a potentiostat/galvanostat (PGZ 401Voltalab) connected to a computer equipped with Voltamaster 4 software to select the electrochemical technique and set suitable parameters. The ITO substrate as working
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electrode, the reference electrode is a saturated calomel electrode with KCl (SCE), and the auxiliary electrode is a platinum wire. All experiments were performed at room temperature.
2.2. Characterization
The electrochemical impedance spectroscopy measurements were carried in the frequency range between 100 kHz and 100 mHz. The cell used during the impedance measurements was a traditional cell containing the CH3CN/ (LiClO4 0.1 M) solution,
The morphological characterization of composite film was examined by means of (SEM) model: JEOL JCM-5000 microscope operating.
The composite material deposited on transparent ITO electrode were characterised by UV-vis absorption spectroscopy, using a Shimadzu UV 1800 - PC spectrophotometer. The photocurrent measurements were recorded with a 500-W halogen lamp as polychromatic light source, and the illumination intensity is 100 mW/cm2.
3. Results and discussion 3.1. Cyclic voltammetry
The successive cyclic voltammograms relative to pyrrole (10-2M) in a solution of 10-1 M acetonitrile (CH3CN) in lithium perchlorate (LiClO4) on an ITO electrode recorded between the potential range -0.3 and 1.3 V vs. SCE, with a scan rate of 50 mV .s-1 are shown in Fig 1.
The cyclic voltammograms show during the positive scan potential a large anodic wave around 0.4V vs. SCE. Whereas in the reverse scan, a large cathodic wave is illustrated at 0 V vs. SCE. The anodic and cathodic waves correspond respectively to oxidation and reduction of the polypyrrole film [26]. It should be noted that the working electrode surface was covered by a polymer films upon the first cycle. The, the current intensity of the oxidation and reduction peaks progressively increased with the number of cycles, , indicating the formation and the growth of the conducting polymer film and suggesting a systematic increase in the electrode area as a result of the actual deposition of PBTh [27].
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Fig.1. Cyclic voltammograms relating to polypyrrole (PPy/ITO), in a CH3CN/LiClO4 0.1M) solution, recorded between 0.3 and 1.3 V vs. SCE at a scan rate of 50 mV s-1
Fig. 2 presents the cyclic voltammograms of the γ-MnO2 recorded in (CH3CN/ LiClO4 10-1 M) electrolyte solution and recorded in a potential range between -0,9 and 1,2 V vs.
SCE, with scan rate of 25 mV . s-1. The recording of the cyclic voltammograms is carried out under a low agitation (200 rpm). We observed a cathodic peak at -0.45 and a large peak at 0.36 V vs. SCE, corresponding to two reduction reactions, according to the following reactions:
MnO2 + H2O + e – MnOOH + OH –
MnOOH + H2O + e – Mn (OH) 2 + OH
During the scanning of the positive potentials we observe a not very marked peak at 0.25V / ECS corresponding to the reoxidation of the reduced species.
-0,4 -0,2 0,0 0,2 0,4 0,6 0,8 1,0 1,2 1,4
-1,0 -0,5 0,0 0,5 1,0 1,5 2,0
I (mA/cm2 )
E (V/ECS)
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Fig. 2. Cyclic voltammetry of γ-MnO2 in 0.1 M LiCLO4/CH3CN recorded at scanning rate 25 mV. s-1.
3.2. Incorporation of MnO2 into PPy films.
Fig. 3 shows the variation of the current density with time for the PPy-MnO2 composite material deposited by cycling on ITO in an aqueous solution containing 0.02g of MnO2 and 10-1 M of pyrrole by chronoamperometry for 60 s in in 0.1 M LiCLO4/CH3CN solution. The current density decreases very quickly, this is related to double-layer charging at the electrode surface.
Fig. 3. Current–time curve of the potentiostatic dispersion of MnO2 particles on PPy/ITO surface.
0 10 20 30 40 50 60
0,0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8
current density (mA/cm2)
t(s)
PPy-MnO2/ITO
-1,0 -0,5 0,0 0,5 1,0 1,5
-15 -10 -5 0 5 10 15 20
Courant (µA/cm2)
E (V/ECS)
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Fig. 4 represents the impedance diagram of ITO/PPy and ITO/PPy-MnO2 composite films, the curves were plotted over a frequency range between 100 kHz and 100 mHz. The Nyquist diagram shows a semi-circle at high frequencies and a straight line at low frequencies region.
The semi-circle part corresponds to the electron transfer limited process and the linear part to the diffusion process, the semicircle diameter increases this means by increasing the charge transfer resistance (Rct), as well as the capacitance. Also there is a decrease in the resistance of the electrolyte (Re), consequently the conductivity of ITO/PPy-MnO2 composite films decrease.
0 10 20 30 40 50 60 70 80 90
0 5 10 15 20 25 30 35 40
-Z Im (Kohm.cm²)
Z Re (Kohm.cm²) PPy/ITO
PPy-MnO2/ITO
Fig.4. Nyquist plots of ITO/PPy, ITO/PPy-MnO2, in 0.1 M LiClO4/CH3CN
3.4. UV spectroscopy
The UV-vis absorption spectra of ITO/PPy and ITO/PPy-MnO2composite are shown in Fig. 5.As can be seen from the UV-visible absorption spectra, two absorption bands were observed in the absence of MnO2. The first at λmax = 440 nm specific to π–π* PPy transition and the second broad and badly defined at λmax=890 nm. In the case of ITO/PPy- MnO2composite, we noted an increase in the absorbance over all the wavelengths, which may
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be attributed to the interaction of between MnO2 nanoparticles and the PPy molecules;
resulting in a composite with new opticalproperties.
300 400 500 600 700 800 900 1000 1100
2,0 2,5 3,0 3,5 4,0
430 nm
910 nm
440 nm
Wavelength (nm)
Absorbance
PPy PPy+ MnO
2
890 nm
Fig .5.UV–vis absorption spectra of PPy and PPy-MnO2 /ITO
3.5. Morphology characterization by SEM and EDX
The SEM micrographs of the ITO/PPy and ITO/PPy-MnO2 composite films deposited on ITO electrodes are presented in Fig. 6. The SEM image of the ITO/PPy film (Fig. 6a) shows that the morphology of PPy has a porous and homogeneous. The image of the ITO/PPy-MnO2 composites (Fig. 6b) shows that the manganese dioxide particles were distributed into the PPy film with different sizes in the nanotubes [28,29]. The presence of MnO2 in the PPy was confirmed by EXD and the analyzed elements are shown in Figure 7b.
Intense rays of manganese were observed at 6.03 and 6.10 keV.
In addition, the EDX spectrum (Figure 6a) of the electrochemically prepared PPy film shows a signal of carbon (C) and nitrogen (N) at 0,31 keV and 0.4 keV, respectively, characteristic of the PPy polymer. The signals of chlorine (Cl) at 2,68 and 2,81 keV and and of the oxygen (O) at 0,57 keVindicates that the PPy film is doped by the perchlorate (ClO4-) ions (Fig. 6a).
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Fig.6. SEM images and EDX patterns of: (a) PPy/ITO, (b) ITO/PPy-MnO2 composite films
3.6. Photoelectrochemistry
PPy film deposited on ITO substrate (surface of 1 cm2) were used as working electrodes in a photoelectrochemical cell containing CH3CN/LiClO4 as electrolyte. Fig 7 shows the current density variation with time for PPy deposited by cycling on ITO in CH3CN/LiClO4 10−1 M,in a three-compartment photoelectrochemical cell, with an imposed potential of -1000 mV, excited by polychromatic light every 20 seconds. As shown, the polymer film presents a cathodic peak of photocurrent immediately after irradiation. This response indicates that the
(a)
(a) PPy/ITO
(b)
(b) MnO2+PPy/ITO
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recombination process was occurring in the film because of the presence of charge carriers in the bulk polymer, main due to the structural disorder. Thus, the prepared films have the conducting behavior of p-type polymers [30]. The incorporation of MnO2 in the polymer films increased the generated photocurrent.Thus, it can be suggested that the semiconducting MnO2 nanoparticles act as dissociation centers for the polymer excitons, which increase the number of charge carriers that get to (PPy-MnO2) composite interface.
10 20 30 40 50 60 70 80 90
-15 -10 -5 0 5
Current density µA/cm2
Time (s) PPy PPy-MnO2
Fig 7. Photocurrent density- time for PPy/ITO and PPy-MnO2, observed upon switching the light on and off.
4. Conclusion
The electrochemical and photophysical properties of alternating PPy and PPy-MnO2 were synthesized and its spectroelectrochemical properties were studied. The films of composite films was carried out electrochemically in a solution (CH3CN/LiClO4 10−1 M) by voltammetry cyclic method. The films of polymers and composite material obtained were characterized by SEM measurements and photocurrent. We observed that the incorporation of MnO2 particles modified the morphology and electrochemical properties of the PPy film.
The results obtained from the photo-electrochemistry measurements show that the photocurrent increases the composite material electrodes to be used in photovoltaic cells.
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