- Email: [email protected]
TiO2 nanocomposite: Electrosynthesis and characterization
Synthetic Metals 165 (2013) 51–55
Contents lists available at SciVerse ScienceDirect
Synthetic Metals journal homepage: www.elsevier.com/locate/synmet
Poly ortho aminophenol/TiO2 nanocomposite: Electrosynthesis and characterization A. Ehsani a,∗ , M.G. Mahjani b , M. Bordbar a , R. Moshrefi b a b
Department of Chemistry, Faculty of Science, University of Qom, P.O. Box 37185-359, Qom, Iran Department of Chemistry, Faculty of Science, K. N. Toosi University of Technology, P.O. Box 15875-4416, Tehran, Iran
a r t i c l e
i n f o
Article history: Received 12 November 2012 Received in revised form 2 January 2013 Accepted 4 January 2013 Keywords: POAP Nanocomposite TiO2 Electrosynthesis
a b s t r a c t This paper aims to prepare the poly ortho aminophenol (POAP)/nanocrystalline TiO2 composite film. Polymerization of ortho aminophenol (OAP) proceeded after ultrasonic irradiation of nanocrystalline TiO2 . The aggregation of nano TiO2 can be reduced under ultrasonic irradiation, and the nanoparticles can be redispersed in the aqueous solution. The POAP deposits on the surface of the nanoparticle, which leads to a core–shell structure. Nanocomposite was characterized by FT-IR spectroscopy, UV–vis spectroscopy and transmission electron microscopy (TEM). The results indicate that a strong interaction exist at the interface of POAP and nano-TiO2 . The presence of nanocrystalline TiO2 strengthens the UV absorption of POAP and leads to a blue shift of the -polaron absorption of POAP. The electrochemical results indicate that a strong interaction exist at the interface of POAP and nano-TiO2 . Electrochemical properties of film were investigated by using cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). © 2013 Elsevier B.V. All rights reserved.
1. Introduction Conducting polymer/inorganic nanoparticle composites with different combinations of the two components have attracted more and more attention, since they have interesting physical properties and many potential applications [1]. However, conducting polymers are not molten in nature and generally are insoluble in common solvents, and the nanoparticles are easily aggregated due to their high surface energy, so it is difficult to prepare conducting polymer/inorganic nanoparticle composites by conventional blending or mixing in solution or melt form. Polyaniline (PANI) is one of the most important conducting polymers because of its unique electrical, optical, and optoelectrical properties, as well as its ease of preparation and its excellent environment stability [2,3]. Aminophenols are interesting members of the class of substituted anilines. The hydroxyl group in the phenyl ring can be oxidized to quinine and quinine can be reduced again. POAP gives a surface film of interesting electrochemical and electrochromic properties when it is electropolymerised in acidic solution. The POAP film coated electrode displays electrochromic character when the potential is driven from 0.1 to 0.7 V vs. SCE. This film is electroactive in aqueous and non-aqueous solutions containing protons but no response is observed at pH-value higher than pH 7. Peak
∗ Corresponding author. Tel.: +98 251 2103038; fax: +98 251 2854973. E-mail addresses: [email protected], [email protected] (A. Ehsani). 0379-6779/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.synthmet.2013.01.004
current increase and peak potential are displaced to more negative potentials as the pH decrease. The variety of results for conductivity of the POAP film reported in the literature [4–8] show that the electrochemical response of POAP is strongly influenced by the experimental procedure used to produce the polymer film, dopant anions and the purity starting monomer. Nanocrystalline TiO2 has unique physical and chemical properties and it can be used in advanced coating, cosmetic, sensor, solar cell and photocatalyst application [9–11]. Generally most applications are related to the crystalline structure of TiO2 to the crystalline structure of TiO2 , and amorphous TiO2 has limited applications [11]. Ultrasonic irradiation has been widely used in chemical synthesis. When an ultrasonic wave passes through a liquid medium, a large number of microbubbles form, grow, and collapse in a very short time of about a few microseconds, an effect that is called ultrasonic cavitation. Sonochemical theory calculations and the corresponding experiments suggested that ultrasonic cavitation can generate local temperatures as high as 5000 K and local pressures as high as 500 atm, with heating and cooling rates greater than 109 K/s, a very rigorous environment [11,12]. Therefore, ultrasound has been extensively applied in dispersion, emulsifying, crushing, and activation of particles. In the present work POAP/TiO2 composite was synthesized by in situ electropolymerization and ultrasonic irradiation technique was employed to prepare POAP/nanocrystalline TiO2 shell–core composite particles. Electrochemical properties of film were investigated by using electrochemical techniques, viz. CV and electrochemical impedance spectroscopy (EIS).
52
A. Ehsani et al. / Synthetic Metals 165 (2013) 51–55
Fig. 1. The typical multi-sweep cyclic voltammograms during electropolymerization of POAP in the presence of TiO2 .
2. Experimental The chemicals used in this work were of Merck origin and used without further purification. Nanocrystalline TiO2 (anatase, 20–30 nm) was used in composite. All electrochemical measurements were carried out in a conventional three electrodes cell, powered by a potentiostat/galvanostat (EG&G273A) and a frequency response analyzer (EG&G, 1025). The system was run by a PC through M270 and M398 software using GPIB interface. The frequency ranges of 100 kHz to 100 mHz and modulation amplitude of 5 mV were employed for impedance studies. The glassy carbon (GC) with geometric area of 0.0314 cm2 was used as a working electrode. Saturated calomel electrode (SCE) and a platinum wire were used as reference and counter electrodes respectively. In order to create a rough surface for the polymer to attach, The GC electrode was polished with 0.05 m ␣-alumina powders on a polishing microcloth and then degreased with acetone in an ultrasonic bath for 10 min before rinsing with doubly distilled water. POAP/TiO2 composites were prepared by in a stirring solution containing 0.01 M monomer, 0.5 M HClO4 , 0.1 M LiCO4 , 5.0 × 10−3 M sodium dodecyl sulfate (SDS) and 10% of TiO2 in suspension. The aggregation of nano TiO2 reduced under ultrasonic irradiation and the nanoparticles redispersed in the aqueous solution. The coated electrode with POAP/TiO2 composite was thoroughly washed with distilled water and dried. The transmission electron microscopy (TEM) was performed using a CEM 902A ZEISS transmission electron microscope, with an accelerating voltage of 80 kV. Scanning electron microscopy (SEM) was performed by a Tescan Vega II HiVac instrument. 3. Results and discussion Cyclic voltammetry (CV) was used to deposit POAP coatings from an electrolyte containing 0.01 M monomer, 0.5 M HClO4 , 0.1 M LiCO4 , 5.0 × 10−3 M sodium dodecyl sulfate (SDS) and dispersed TiO2 nanoparticles at a linear potential sweep rate of 50 mV s−1 between −0.25 V and 0.9 V vs. SCE during 40 cycles on glassy carbon electrode. The solutions were stirred at 500 rpm with a magnetic stirred follower to disperse the TiO2 . Fig. 1 shows the typical multisweep cyclic voltammograms during electropolymerization in the presence TiO2 nanoparticles. As seen in the figure, OAP is oxidized irreversibly at around 600 mV without corresponding cathodic processes in the reverse scan. On the basis of the above analysis, a mechanism of formation of core–shell POAP/nanocrystalline TiO2 composite particles was
proposed, as shown in TEM images in Fig. 2. Under ultrasonic irradiation, the aggregates of nanocrystalline TiO2 are broken down and nanoparticles are redispersed in the aqueous solution at the nano scale, while at the same time, OAP monomer is polymerized and the synthesized POAP is absorbed on the surface of nanocrystalline TiO2 particles, which forms the shell of conductive POAP. The surfactant molecules are absorbed on the surface of the composite particles and have a stabilizing effect [13,14]. The incorporation of titanium oxide in the composite is confirmed by the SEM and EDX analysis, which shows the presence of titanium located at 4.50, 5.0 and 0.40 keV (Fig. 3). Also, the EDX spectrum of the electrochemically prepared composite material film shows a signal of carbon (C) at 0.25 keV and oxygen (O) at 0.53 keV characteristic of the POAP polymer. Because the gold is used as coating the mounted electrode in SEM analysis we observe also two peak at 2.07 and 9.8 keV of the gold. Thus, the content of TiO2 in the composite film is confirmed by SEM and EDX analyses. To elucidate the effect of TiO2 s on the property of POAP films, electrochemical performance of composite films was evaluated by carrying out CV measurements in 0.5 M HClO4 , as shown in Fig. 4. The cyclic voltammograms of the TiO2 /POAP films showed a couple of strong and broad oxidation and reduction waves. Their wave currents were stronger than that of pure POAP films electrodes. The voltammetric behavior of both films is similar and the CV curves show the voltammograms reveal the electrodes are stable in HClO4 solution within the sweeping potential range. Close comparisons of CV curves between TiO2 /POAP electrode and pure POAP electrode show that a TiO2 /POAP electrode not only displays a higher background current in the potential sweep but also there exist faradic currents, which are believed to arise from the contribution of the loaded TiO2 . This behavior occurs at all cycles and the higher current observed when TiO2 was in solution, indicates that the polymerization process is catalyzed by the presence of the TiO2 [15]. One possible explanation for the higher current is the increase in the concentration of OAP monomers near the electrode surface due to the influence of TiO2 . This is supported by the fact that the adsorption of positively charged ions on the surface of the nanoparticles is likely due to their negative zeta-potential value in aqueous solutions at pH 2 [16]. The z-potential values becomes slightly less negative when the positively charged OAP monomers are adsorbed on the surface of the TiO2 [16,17] and they became carriers of OAP increasing their concentration near the electrode surface and therefore the polymerization rate. Other reasons for the current increase could be that the increase of available area due to the incorporation of the TiO2 . To further investigate the UV–vis spectra for POAP and POAP/TiO2 were obtained. UV- and are shown in Fig. 5. Clearly, the prepared POAP/nanocrystalline TiO2 composites not only can strongly absorb the UV light but also can absorb the visible and near-IR light. The characteristic peaks of POAP appear at 225, 380 and 500 nm, which were attributed to –*, polaron–*, and –polaron transitions, respectively. It is interesting to note that, presence of TiO2 nanoparticles, the UV absorption 380 nm is strengthened and the absorption at 500 nm due to the ð-polaron transition is shifted. This result indicates there is strong interaction between POAP and TiO2 nanoparticles. Fig. 6 displays the FT-IR spectra of electropolymerised POAP in the presence (a) and absence of TiO2 nanoparticles (b) under ultrasonic irradiation. The spectrum of POAP shows two peaks at 3380 cm−1 and 1602 cm−1 due to the characteristic bands of the N H stretching vibrations and the axial stretching of the C O groups in the POAP structure. The peaks in the region of 1400–1600 cm−1 are attributed to the stretching of C H and C C groups. The peak at 1384 cm−1 is clearly seen that could be assigned to the C N stretching vibration of a secondary aromatic amine. The band at 1121 cm−1 is ascribed to the stretching of the C O C
A. Ehsani et al. / Synthetic Metals 165 (2013) 51–55
53
Fig. 2. The transmission electron microscopy images of POAP/TiO2 in different magnification.
linkages and further support that the o-aminophenol changed into POAP. It can be noted that the hydrogen-bond absorption at 3380 cm−1 is strengthened in the presence of nano TiO2 . The spectrums of POAP are similar in the main peaks although with different intensity. In addition, the incorporation of nano TiO2 leads to the shift of some FTIR bands of POAP. The results also suggest that there is strong interaction between the POAP and TiO2 nanoparticles. To illustrate the performance of the resulting electrodes in supercapacitors, constant current discharge was performed to
measure the capacitance in a 0.5 M HClO4 solution. Typical galvanostatic discharge curves at a current of 0.1 mA between 0 and 0.8 V are shown in Fig. 7. It can be clearly seen that the TiO2 /POAP electrode cell presents higher capacitance than the pure POAP and this has been considered as the contribution of faradic pseudo capacitance of TiO2 /POAP supercapacitor. Electrochemical impedance spectroscopy (EIS) is one of the best techniques for analyzing the properties of conducting polymer electrodes and it has been broadly discussed in the literature using a variety of theoretical models [18–22]. In the case of electrochemical
Fig. 3. SEM image and EDX analysis of POAP/TiO2 nanocomposite.
54
A. Ehsani et al. / Synthetic Metals 165 (2013) 51–55
Fig. 4. Comparative cyclic voltammograms for POAP and POAP/TiO2 electrodes with a potential sweep rate of 100 mV s−1 between −0.2 and 0.85 V in acidic solution.
Fig. 7. Galvanostatic discharge curves of the POAP (1) and POAP/TiO2 (2).
Fig. 5. UV–vis absorption spectra of POAP (1) and POAP/TiO2 (2).
system, EIS can reveal information regarding processes occurring in the polymer matrix when it is doped. This may include kinetic values of the doping process and parameters of the diffusion of ions into the polymers. EIS was analyzed for POAP films in two different synthesis conditions in acidic solution of HClO4 . Fig. 8 shows the Nyquist diagrams of electrodes in OCP. The plot in Fig. 8 depicts a single semi-circle in the high frequency region and a straight line
Fig. 6. FT-IR spectra of electropolymerised POAP in the presence (a) and absence of TiO2 nanoparticles (b) under ultrasonic irradiation.
in the low-frequency region for all spectra. The high-frequency arc is the overall contact impedance generated from the electrical connection between TiO2 /POAP composites and the backing plate as well as the charge transfer at the contact interface between the electrode and the electrolyte solution. In spite of the similar shape of the impedance spectra, there is an obvious difference between the diameters of the four semi-circles. That is, the diameters of the semicircles decline with presence of TiO2 s in POAP. In other words, the bulk-film transport of electrons and the charge transfer resistance (Rct ) of TiO2 /POAP core–shell structure films are lower than that of the pure POAP films. This means that the polymer deposited on the TiO2 surfaces have faster electron transport in the bulk-film and charge transfer in the parallel POAP film/solution interface and TiO2 s/solution interface, compared to that in the originally single POAP film/solution interface. This fact may suggest that the TiO2 has an obvious improvement effect, which makes the composites
Fig. 8. Nyquist diagrams of POAP (1) and POAP/TiO2 (2) in OCP in acidic solution. Inset shows the equivalent circuit.
A. Ehsani et al. / Synthetic Metals 165 (2013) 51–55
have more active sites for faradic reactions and a larger specific capacitance than pure POAP. Similar behavior has been reported concerning polyaniline/TiO2 composite film performance [22]. 4. Conclusions POAP was deposited on the surface of TiO2 forming a core–shell structure. Polymerization of OAP preceded under ultrasonic irradiation in the presence of nanocrystalline TiO2 particles. The aggregation of nano TiO2 in the aqueous solution can be broken down under ultrasonic irradiation, and the formed POAP deposits on the surface of the nanoparticle. SEM, FT-IR, CV and EIS were used for characterization of composite. The introduction of nanocrystalline TiO2 leads to a blue shift of the ð-polaron absorption. The results of FTIR, UV–vis, analyses show that the interaction between POAP and nanocrystalline is strong. TiO2 has an obvious improvement effect, which makes the composites have more active sites for faradic reactions and a larger specific capacitance than pure POAP. Acknowledgment The authors would like to express their deep gratitude to the Iranian Nano Council for supporting this work. References [1] R. Gangopadhyay, A. De, Chemistry of Materials 12 (2000) 608–622.
55
[2] G. Gustafsson, Y. Cao, G.M. Treacy, F. Klavetter, N. Colaneri, A.J. Heeger, Nature 357 (1992) 477–479. [3] M.J. Sailor, E.J. Ginsburg, C.B. Gorman, A. Kumar, R.H. Grubbs, N.S. Lewis, Science 249 (1990) 1146–1149. [4] M.G. Mahjani, A. Ehsani, M. Jafarian, Synthetic Metals 160 (2010) 1252–1259. [5] A. Ehsani, M.G. Mahjani, M. Jafarian, Synthetic Metals 161 (2011) 1760–1765. [6] A. Ehsani, M.G. Mahjani, M. Jafarian, A. Naeemy, Progress in Organic Coatings 69 (2010) 510–516. [7] A. Ehsani, M.G. Mahjani, M. Jafarian, Synthetic Metals 162 (2012) 199–204. [8] F. Gobal, K. Malek, M.G. Mahjani, M. Jafarian, V. Safarnavadeh, Synthetic Metals 108 (2000) 15–19. [9] B. O’Regan, M. Gratzel, Nature 353 (1991) 737–740. [10] M. Machida, K. Norimoto, T. Watanabe, K. Hashimoto, Fujishima, Journal of Materials Science 34 (1999) 2569–2574. [11] H. Xia, Q. Wang, Composites 14 (2002) 2158–2165. [12] K.S. Suslick, Science 3 (1990) 1439–1445. [13] N. Sakmeche, S. Aeiyach, J.J. Aaron, M. Jouini, J.C. Lacroix, P.C. Lacaze, Langmuir 15 (1999) 2566–2574. [14] X. Zhang, J. Zhang, Z. Liu, Carbon 43 (2005) 2186–2191. [15] D.V. Bavykin, F.C. Walsh, Titanate and Titania Nanotubes: Synthesis, Properties and Applications, Royal Society of Chemistry, Cambridge, 2010. [16] D.V. Bavykin, F.C. Walsh, Journal of Inorganic Chemistry 8 (2009) 977–997. [17] P. Herrastia, A.N. Kulakb, D.V. Bavykinb, C. Ponce de Léonb, J. Zekonytec, F.C. Walshb, Electrochimica Acta 56 (2011) 1323–1328. [18] S.A.M. Refaey, Synthetic Metals 140 (2004) 87–94. [19] H. Heli, H. Yadegari, Electrochimica Acta 55 (2010) 2139–2148. [20] A. Ehsani, M.G. Mahjani, M. Jafarian, Turkish Journal of Chemistry 35 (2011) 1–9. [21] A. Ehsani, M.G. Mahjani, M. Jafarian, A. Naeemy, Electrochimica Acta 71 (2012) 128–133. [22] S. Abaci, B. Nessark, R. Boukherroub, K. Lmimouni, Thin solid films 519 (2011) 3596–3602.
Contents lists available at SciVerse ScienceDirect
Synthetic Metals journal homepage: www.elsevier.com/locate/synmet
Poly ortho aminophenol/TiO2 nanocomposite: Electrosynthesis and characterization A. Ehsani a,∗ , M.G. Mahjani b , M. Bordbar a , R. Moshrefi b a b
Department of Chemistry, Faculty of Science, University of Qom, P.O. Box 37185-359, Qom, Iran Department of Chemistry, Faculty of Science, K. N. Toosi University of Technology, P.O. Box 15875-4416, Tehran, Iran
a r t i c l e
i n f o
Article history: Received 12 November 2012 Received in revised form 2 January 2013 Accepted 4 January 2013 Keywords: POAP Nanocomposite TiO2 Electrosynthesis
a b s t r a c t This paper aims to prepare the poly ortho aminophenol (POAP)/nanocrystalline TiO2 composite film. Polymerization of ortho aminophenol (OAP) proceeded after ultrasonic irradiation of nanocrystalline TiO2 . The aggregation of nano TiO2 can be reduced under ultrasonic irradiation, and the nanoparticles can be redispersed in the aqueous solution. The POAP deposits on the surface of the nanoparticle, which leads to a core–shell structure. Nanocomposite was characterized by FT-IR spectroscopy, UV–vis spectroscopy and transmission electron microscopy (TEM). The results indicate that a strong interaction exist at the interface of POAP and nano-TiO2 . The presence of nanocrystalline TiO2 strengthens the UV absorption of POAP and leads to a blue shift of the -polaron absorption of POAP. The electrochemical results indicate that a strong interaction exist at the interface of POAP and nano-TiO2 . Electrochemical properties of film were investigated by using cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). © 2013 Elsevier B.V. All rights reserved.
1. Introduction Conducting polymer/inorganic nanoparticle composites with different combinations of the two components have attracted more and more attention, since they have interesting physical properties and many potential applications [1]. However, conducting polymers are not molten in nature and generally are insoluble in common solvents, and the nanoparticles are easily aggregated due to their high surface energy, so it is difficult to prepare conducting polymer/inorganic nanoparticle composites by conventional blending or mixing in solution or melt form. Polyaniline (PANI) is one of the most important conducting polymers because of its unique electrical, optical, and optoelectrical properties, as well as its ease of preparation and its excellent environment stability [2,3]. Aminophenols are interesting members of the class of substituted anilines. The hydroxyl group in the phenyl ring can be oxidized to quinine and quinine can be reduced again. POAP gives a surface film of interesting electrochemical and electrochromic properties when it is electropolymerised in acidic solution. The POAP film coated electrode displays electrochromic character when the potential is driven from 0.1 to 0.7 V vs. SCE. This film is electroactive in aqueous and non-aqueous solutions containing protons but no response is observed at pH-value higher than pH 7. Peak
∗ Corresponding author. Tel.: +98 251 2103038; fax: +98 251 2854973. E-mail addresses: [email protected], [email protected] (A. Ehsani). 0379-6779/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.synthmet.2013.01.004
current increase and peak potential are displaced to more negative potentials as the pH decrease. The variety of results for conductivity of the POAP film reported in the literature [4–8] show that the electrochemical response of POAP is strongly influenced by the experimental procedure used to produce the polymer film, dopant anions and the purity starting monomer. Nanocrystalline TiO2 has unique physical and chemical properties and it can be used in advanced coating, cosmetic, sensor, solar cell and photocatalyst application [9–11]. Generally most applications are related to the crystalline structure of TiO2 to the crystalline structure of TiO2 , and amorphous TiO2 has limited applications [11]. Ultrasonic irradiation has been widely used in chemical synthesis. When an ultrasonic wave passes through a liquid medium, a large number of microbubbles form, grow, and collapse in a very short time of about a few microseconds, an effect that is called ultrasonic cavitation. Sonochemical theory calculations and the corresponding experiments suggested that ultrasonic cavitation can generate local temperatures as high as 5000 K and local pressures as high as 500 atm, with heating and cooling rates greater than 109 K/s, a very rigorous environment [11,12]. Therefore, ultrasound has been extensively applied in dispersion, emulsifying, crushing, and activation of particles. In the present work POAP/TiO2 composite was synthesized by in situ electropolymerization and ultrasonic irradiation technique was employed to prepare POAP/nanocrystalline TiO2 shell–core composite particles. Electrochemical properties of film were investigated by using electrochemical techniques, viz. CV and electrochemical impedance spectroscopy (EIS).
52
A. Ehsani et al. / Synthetic Metals 165 (2013) 51–55
Fig. 1. The typical multi-sweep cyclic voltammograms during electropolymerization of POAP in the presence of TiO2 .
2. Experimental The chemicals used in this work were of Merck origin and used without further purification. Nanocrystalline TiO2 (anatase, 20–30 nm) was used in composite. All electrochemical measurements were carried out in a conventional three electrodes cell, powered by a potentiostat/galvanostat (EG&G273A) and a frequency response analyzer (EG&G, 1025). The system was run by a PC through M270 and M398 software using GPIB interface. The frequency ranges of 100 kHz to 100 mHz and modulation amplitude of 5 mV were employed for impedance studies. The glassy carbon (GC) with geometric area of 0.0314 cm2 was used as a working electrode. Saturated calomel electrode (SCE) and a platinum wire were used as reference and counter electrodes respectively. In order to create a rough surface for the polymer to attach, The GC electrode was polished with 0.05 m ␣-alumina powders on a polishing microcloth and then degreased with acetone in an ultrasonic bath for 10 min before rinsing with doubly distilled water. POAP/TiO2 composites were prepared by in a stirring solution containing 0.01 M monomer, 0.5 M HClO4 , 0.1 M LiCO4 , 5.0 × 10−3 M sodium dodecyl sulfate (SDS) and 10% of TiO2 in suspension. The aggregation of nano TiO2 reduced under ultrasonic irradiation and the nanoparticles redispersed in the aqueous solution. The coated electrode with POAP/TiO2 composite was thoroughly washed with distilled water and dried. The transmission electron microscopy (TEM) was performed using a CEM 902A ZEISS transmission electron microscope, with an accelerating voltage of 80 kV. Scanning electron microscopy (SEM) was performed by a Tescan Vega II HiVac instrument. 3. Results and discussion Cyclic voltammetry (CV) was used to deposit POAP coatings from an electrolyte containing 0.01 M monomer, 0.5 M HClO4 , 0.1 M LiCO4 , 5.0 × 10−3 M sodium dodecyl sulfate (SDS) and dispersed TiO2 nanoparticles at a linear potential sweep rate of 50 mV s−1 between −0.25 V and 0.9 V vs. SCE during 40 cycles on glassy carbon electrode. The solutions were stirred at 500 rpm with a magnetic stirred follower to disperse the TiO2 . Fig. 1 shows the typical multisweep cyclic voltammograms during electropolymerization in the presence TiO2 nanoparticles. As seen in the figure, OAP is oxidized irreversibly at around 600 mV without corresponding cathodic processes in the reverse scan. On the basis of the above analysis, a mechanism of formation of core–shell POAP/nanocrystalline TiO2 composite particles was
proposed, as shown in TEM images in Fig. 2. Under ultrasonic irradiation, the aggregates of nanocrystalline TiO2 are broken down and nanoparticles are redispersed in the aqueous solution at the nano scale, while at the same time, OAP monomer is polymerized and the synthesized POAP is absorbed on the surface of nanocrystalline TiO2 particles, which forms the shell of conductive POAP. The surfactant molecules are absorbed on the surface of the composite particles and have a stabilizing effect [13,14]. The incorporation of titanium oxide in the composite is confirmed by the SEM and EDX analysis, which shows the presence of titanium located at 4.50, 5.0 and 0.40 keV (Fig. 3). Also, the EDX spectrum of the electrochemically prepared composite material film shows a signal of carbon (C) at 0.25 keV and oxygen (O) at 0.53 keV characteristic of the POAP polymer. Because the gold is used as coating the mounted electrode in SEM analysis we observe also two peak at 2.07 and 9.8 keV of the gold. Thus, the content of TiO2 in the composite film is confirmed by SEM and EDX analyses. To elucidate the effect of TiO2 s on the property of POAP films, electrochemical performance of composite films was evaluated by carrying out CV measurements in 0.5 M HClO4 , as shown in Fig. 4. The cyclic voltammograms of the TiO2 /POAP films showed a couple of strong and broad oxidation and reduction waves. Their wave currents were stronger than that of pure POAP films electrodes. The voltammetric behavior of both films is similar and the CV curves show the voltammograms reveal the electrodes are stable in HClO4 solution within the sweeping potential range. Close comparisons of CV curves between TiO2 /POAP electrode and pure POAP electrode show that a TiO2 /POAP electrode not only displays a higher background current in the potential sweep but also there exist faradic currents, which are believed to arise from the contribution of the loaded TiO2 . This behavior occurs at all cycles and the higher current observed when TiO2 was in solution, indicates that the polymerization process is catalyzed by the presence of the TiO2 [15]. One possible explanation for the higher current is the increase in the concentration of OAP monomers near the electrode surface due to the influence of TiO2 . This is supported by the fact that the adsorption of positively charged ions on the surface of the nanoparticles is likely due to their negative zeta-potential value in aqueous solutions at pH 2 [16]. The z-potential values becomes slightly less negative when the positively charged OAP monomers are adsorbed on the surface of the TiO2 [16,17] and they became carriers of OAP increasing their concentration near the electrode surface and therefore the polymerization rate. Other reasons for the current increase could be that the increase of available area due to the incorporation of the TiO2 . To further investigate the UV–vis spectra for POAP and POAP/TiO2 were obtained. UV- and are shown in Fig. 5. Clearly, the prepared POAP/nanocrystalline TiO2 composites not only can strongly absorb the UV light but also can absorb the visible and near-IR light. The characteristic peaks of POAP appear at 225, 380 and 500 nm, which were attributed to –*, polaron–*, and –polaron transitions, respectively. It is interesting to note that, presence of TiO2 nanoparticles, the UV absorption 380 nm is strengthened and the absorption at 500 nm due to the ð-polaron transition is shifted. This result indicates there is strong interaction between POAP and TiO2 nanoparticles. Fig. 6 displays the FT-IR spectra of electropolymerised POAP in the presence (a) and absence of TiO2 nanoparticles (b) under ultrasonic irradiation. The spectrum of POAP shows two peaks at 3380 cm−1 and 1602 cm−1 due to the characteristic bands of the N H stretching vibrations and the axial stretching of the C O groups in the POAP structure. The peaks in the region of 1400–1600 cm−1 are attributed to the stretching of C H and C C groups. The peak at 1384 cm−1 is clearly seen that could be assigned to the C N stretching vibration of a secondary aromatic amine. The band at 1121 cm−1 is ascribed to the stretching of the C O C
A. Ehsani et al. / Synthetic Metals 165 (2013) 51–55
53
Fig. 2. The transmission electron microscopy images of POAP/TiO2 in different magnification.
linkages and further support that the o-aminophenol changed into POAP. It can be noted that the hydrogen-bond absorption at 3380 cm−1 is strengthened in the presence of nano TiO2 . The spectrums of POAP are similar in the main peaks although with different intensity. In addition, the incorporation of nano TiO2 leads to the shift of some FTIR bands of POAP. The results also suggest that there is strong interaction between the POAP and TiO2 nanoparticles. To illustrate the performance of the resulting electrodes in supercapacitors, constant current discharge was performed to
measure the capacitance in a 0.5 M HClO4 solution. Typical galvanostatic discharge curves at a current of 0.1 mA between 0 and 0.8 V are shown in Fig. 7. It can be clearly seen that the TiO2 /POAP electrode cell presents higher capacitance than the pure POAP and this has been considered as the contribution of faradic pseudo capacitance of TiO2 /POAP supercapacitor. Electrochemical impedance spectroscopy (EIS) is one of the best techniques for analyzing the properties of conducting polymer electrodes and it has been broadly discussed in the literature using a variety of theoretical models [18–22]. In the case of electrochemical
Fig. 3. SEM image and EDX analysis of POAP/TiO2 nanocomposite.
54
A. Ehsani et al. / Synthetic Metals 165 (2013) 51–55
Fig. 4. Comparative cyclic voltammograms for POAP and POAP/TiO2 electrodes with a potential sweep rate of 100 mV s−1 between −0.2 and 0.85 V in acidic solution.
Fig. 7. Galvanostatic discharge curves of the POAP (1) and POAP/TiO2 (2).
Fig. 5. UV–vis absorption spectra of POAP (1) and POAP/TiO2 (2).
system, EIS can reveal information regarding processes occurring in the polymer matrix when it is doped. This may include kinetic values of the doping process and parameters of the diffusion of ions into the polymers. EIS was analyzed for POAP films in two different synthesis conditions in acidic solution of HClO4 . Fig. 8 shows the Nyquist diagrams of electrodes in OCP. The plot in Fig. 8 depicts a single semi-circle in the high frequency region and a straight line
Fig. 6. FT-IR spectra of electropolymerised POAP in the presence (a) and absence of TiO2 nanoparticles (b) under ultrasonic irradiation.
in the low-frequency region for all spectra. The high-frequency arc is the overall contact impedance generated from the electrical connection between TiO2 /POAP composites and the backing plate as well as the charge transfer at the contact interface between the electrode and the electrolyte solution. In spite of the similar shape of the impedance spectra, there is an obvious difference between the diameters of the four semi-circles. That is, the diameters of the semicircles decline with presence of TiO2 s in POAP. In other words, the bulk-film transport of electrons and the charge transfer resistance (Rct ) of TiO2 /POAP core–shell structure films are lower than that of the pure POAP films. This means that the polymer deposited on the TiO2 surfaces have faster electron transport in the bulk-film and charge transfer in the parallel POAP film/solution interface and TiO2 s/solution interface, compared to that in the originally single POAP film/solution interface. This fact may suggest that the TiO2 has an obvious improvement effect, which makes the composites
Fig. 8. Nyquist diagrams of POAP (1) and POAP/TiO2 (2) in OCP in acidic solution. Inset shows the equivalent circuit.
A. Ehsani et al. / Synthetic Metals 165 (2013) 51–55
have more active sites for faradic reactions and a larger specific capacitance than pure POAP. Similar behavior has been reported concerning polyaniline/TiO2 composite film performance [22]. 4. Conclusions POAP was deposited on the surface of TiO2 forming a core–shell structure. Polymerization of OAP preceded under ultrasonic irradiation in the presence of nanocrystalline TiO2 particles. The aggregation of nano TiO2 in the aqueous solution can be broken down under ultrasonic irradiation, and the formed POAP deposits on the surface of the nanoparticle. SEM, FT-IR, CV and EIS were used for characterization of composite. The introduction of nanocrystalline TiO2 leads to a blue shift of the ð-polaron absorption. The results of FTIR, UV–vis, analyses show that the interaction between POAP and nanocrystalline is strong. TiO2 has an obvious improvement effect, which makes the composites have more active sites for faradic reactions and a larger specific capacitance than pure POAP. Acknowledgment The authors would like to express their deep gratitude to the Iranian Nano Council for supporting this work. References [1] R. Gangopadhyay, A. De, Chemistry of Materials 12 (2000) 608–622.
55
[2] G. Gustafsson, Y. Cao, G.M. Treacy, F. Klavetter, N. Colaneri, A.J. Heeger, Nature 357 (1992) 477–479. [3] M.J. Sailor, E.J. Ginsburg, C.B. Gorman, A. Kumar, R.H. Grubbs, N.S. Lewis, Science 249 (1990) 1146–1149. [4] M.G. Mahjani, A. Ehsani, M. Jafarian, Synthetic Metals 160 (2010) 1252–1259. [5] A. Ehsani, M.G. Mahjani, M. Jafarian, Synthetic Metals 161 (2011) 1760–1765. [6] A. Ehsani, M.G. Mahjani, M. Jafarian, A. Naeemy, Progress in Organic Coatings 69 (2010) 510–516. [7] A. Ehsani, M.G. Mahjani, M. Jafarian, Synthetic Metals 162 (2012) 199–204. [8] F. Gobal, K. Malek, M.G. Mahjani, M. Jafarian, V. Safarnavadeh, Synthetic Metals 108 (2000) 15–19. [9] B. O’Regan, M. Gratzel, Nature 353 (1991) 737–740. [10] M. Machida, K. Norimoto, T. Watanabe, K. Hashimoto, Fujishima, Journal of Materials Science 34 (1999) 2569–2574. [11] H. Xia, Q. Wang, Composites 14 (2002) 2158–2165. [12] K.S. Suslick, Science 3 (1990) 1439–1445. [13] N. Sakmeche, S. Aeiyach, J.J. Aaron, M. Jouini, J.C. Lacroix, P.C. Lacaze, Langmuir 15 (1999) 2566–2574. [14] X. Zhang, J. Zhang, Z. Liu, Carbon 43 (2005) 2186–2191. [15] D.V. Bavykin, F.C. Walsh, Titanate and Titania Nanotubes: Synthesis, Properties and Applications, Royal Society of Chemistry, Cambridge, 2010. [16] D.V. Bavykin, F.C. Walsh, Journal of Inorganic Chemistry 8 (2009) 977–997. [17] P. Herrastia, A.N. Kulakb, D.V. Bavykinb, C. Ponce de Léonb, J. Zekonytec, F.C. Walshb, Electrochimica Acta 56 (2011) 1323–1328. [18] S.A.M. Refaey, Synthetic Metals 140 (2004) 87–94. [19] H. Heli, H. Yadegari, Electrochimica Acta 55 (2010) 2139–2148. [20] A. Ehsani, M.G. Mahjani, M. Jafarian, Turkish Journal of Chemistry 35 (2011) 1–9. [21] A. Ehsani, M.G. Mahjani, M. Jafarian, A. Naeemy, Electrochimica Acta 71 (2012) 128–133. [22] S. Abaci, B. Nessark, R. Boukherroub, K. Lmimouni, Thin solid films 519 (2011) 3596–3602.