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Preparation of conducting polyaniline colloids under ultrasonication
Ultrasonics Sonochemistry 10 (2003) 77–80 www.elsevier.com/locate/ultsonch
Preparation of conducting polyaniline colloids under ultrasonication Mahito Atobe a
a,*
, Al-Nakib Chowdhury a, Toshio Fuchigami
a,*
, Tsutomu Nonaka
b
Department of Electronic Chemistry, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8502, Japan b Tsuruoka National College of Technology, 104 Aza-sawada, Oaza-ioka, Tsuruoka 997-8511, Japan Received 12 June 2002; received in revised form 13 August 2002; accepted 19 August 2002
Abstract The effects of ultrasonication on the chemical polymerization of aniline leading to the formation of conducting polyaniline colloids were examined. The formation rate of the colloids was significantly increased under ultrasonication. Furthermore, it was also observed that the morphological structure of the colloids thus prepared was greatly affected by the sonication. The polyaniline colloids were further characterized by a range of techniques including electric resistance meter, gel permeation chromatography, FTIR and cyclic voltammetry. It is noteworthy that the application of ultrasound to the polymerization resulted in a marked increase in the doping level, which reflected to the high electroconductivity of polyaniline colloids. Ó 2002 Elsevier Science B.V. All rights reserved. Keywords: Polyaniline; Conducting polymer colloid; Ultrasonication
1. Introduction Electroconducting polymers, such as polyaniline, exhibit not only electroconductivity but also unique optical and chemical properties. The diversity of properties exhibited by conducting polymers offers these materials to be used in numerous technological applications [1–4]. However, the poor processability owing to the low solubility of the polymers made it limited for practical applications. Over the last decade, an excellent progress in improving the processability of these polymers has been addressed [5,6]. The preparation of colloidal dispersion of conducting polymers is an attractive approach for improving their poor processability [7–10]. The use of colloids enables film casting from aqueous or non-aqueous solutions, either as stand-alone conducting polymers or as composite materials with other polymers. Moreover, the unique inherent properties of conducting polymers were also found to be essentially retained in their colloidal states as well [11]. In general, such
* Corresponding authors. Tel.: +81-45-924-5407; fax: +81-45-9245489 (M. Atobe), Tel.: +81-45-924-5406; fax: +81-45-924-5489 (T. Fuchigami). E-mail addresses: [email protected] (M. Atobe), fuchi@ echem.titech.ac.jp (T. Fuchigami).
properties of polymer colloids are originated from their morphological structures [12]. Therefore, it should practically be required that the structure of the colloid particles can be controlled purposively for their utilization. In our previous work, it was found that three dimensionally uniform and dense films of electropolymerized aniline, pyrrole and thiophene can be formed under ultrasonication [13–15]. This fact suggests that ultrasound can offer a useful method for controlling the structure and properties of conducting polymers. From the above point of view, we aimed to investigate the effect of ultrasonication on chemical polymerization of aniline leading to the formation of conducting polyaniline colloids, in the present work. 2. Experimental Polyaniline colloids were prepared using poly(ethylene oxide) (PEO) as a stabilizer and potassium iodate (KIO3 ) as an oxidant. A mixture of aniline (1.02 g), PEO (average Mv ; ca. 300,000, 0.80 g) and KIO3 (0.70 g) in an aqueous 1.25 M (M ¼ mol dm3 ) HCl solution (100 cm3 ) in a cylindrical glass cell was allowed to react for 12 h at 25 °C in the absence and presence of ultrasonication. The resultant colloidal dispersion was immediately
1350-4177/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 1 3 5 0 - 4 1 7 7 ( 0 2 ) 0 0 1 2 1 - 9
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M. Atobe et al. / Ultrasonics Sonochemistry 10 (2003) 77–80
ultracentrifuged at 40,000 rpm for 30 min and separated as deep green sediment. The sediment was rinsed several times with deionized water. Colloid production under ultrasonication employed a commercially available ultrasonic bath (38 kHz, 1.5 W cm2 (calorimetrically calibrated)). All the aqueous solutions used throughout experiments were prepared using deionized water. Dried down polyaniline colloids were subjected to a scanning electron microscope (SEM), electric resistance measurement (two-probe method) and FT-IR spectroscopy, while the solution of colloids in dimethyl formamide (DMF) was analyzed using gel permeation chromatography (GPC) with a polystyrene standard (detector: UV–vis). Polyaniline colloid (0.02 g) dispersed in aqueous 1.25 M HCl solution (100 ml) was utilized for cyclic voltammetry at 100 mV s1 on a glassy carbon electrode. A Pt foil and a saturated calomel electrode were used as counter and reference electrodes, respectively.
3. Results and discussion Polymerization of aniline under ultrasonication was completed within 12 h, since no colloidal sediments in the supernatant decanted from the reaction mixtures used for the colloid preparation was detected even after standing for seven days. In contrast, the supernatant from the reaction mixture in the absence of ultrasonication still gave a larger amount of colloidal sediment. Therefore, it can be stated that the rate of colloid formation in the presence of ultrasonication is much higher than that in the absence. In this point, an ultrasonic effect on the chemical polymerization seems to be quite different from that on the electrochemical polymerization, which is known to be deceleratingly polymerized under ultrasonic irradiation [13–15]. It is well known that PEO is depolymerized by ultrasound and its real molar mass is thus decreased [16].
In order to confirm whether the molar mass of PEO used as a stabilizer is decreased under ultrasonic irradiation with 1.5 W cm2 at 12 h, the molar mass of PEO was measured by GPC (detector: RI) before and after irradiation. As a result, the molar mass was little affected under the present conditions. This fact indicates that increase in the rate of chemical polymerization is essentially attributed to an effect associated with ultrasound. Therefore, the difference in the polymerization rate as mentioned above seems to lie in the different mechanism as follows. The electrochemical (electrooxidative) polymerization process involves the formation of radical cations generated by electrooxidation of monomer molecules, radical coupling reactions for chain extension, further oxidation of oligomers, the insertion of counterions and eventual polymer deposition, and all of them take place only at the electrode interfaces. Therefore, it can be considered that the radical–radical coupling, further oxidation of oligomers and deposition are adversely affected as the reaction products are transported away from the electrode surface under hydrodynamic condition such as ultrasonic agitation, and consequently the polymerization rate is decelerated. In contrast, the chemical polymerization process can take place at anywhere in the bulk solution. Under this situation, the radical coupling reactions for chain extension may be accelerated by the ultrasonic agitation, and eventually the polymerization is enhanced. Fig. 1 shows SEM images of dried-down polyaniline colloids. It can be observed that the morphological structure of polyaniline colloids prepared in the absence of ultrasonication Fig. 1(a) is needle-shaped and finely net-worked. On the other hand, the morphology of colloids under the sonication Fig. 1(b) is quite different from the above one, i.e. grains are deposited and they are not net-worked. Additionally, electroconductivity of polyaniline colloids prepared in the presence of ultra-
Fig. 1. SEM photographs of the dried-down polyaniline colloids formed (a) without and (b) with ultrasonication.
M. Atobe et al. / Ultrasonics Sonochemistry 10 (2003) 77–80
sonication (3:0 102 S cm1 ) was found to be ca. 17 times as high as that in the absence. In general, it is known that needle-shaped particles have a conductivity superior to that of grain-shaped ones [17]. However, polyaniline colloids prepared under ultrasonication exhibit relatively high conductivity regardless of its morphological structure. In addition, the grain structure colloids obtained by the sonication may be used as chromatographic beads. GPC elution patterns of polyaniline colloids dissolved in DMF are shown in Fig. 2. The elution patterns sketched in Fig. 2(a) and (b) stand for the polyaniline colloids prepared in the absence and presence of ultrasonication, respectively. In both the cases, the elution patterns seem to be fairly different from those of electrochemically prepared polyaniline [18] and to be multimodal in a range of 103 –106 . Fig. 2 suggests that the polyaniline colloids mainly consist of the corresponding high polymer and oligomer. Although an influence of ultrasonication on the molecular weight distribution could be observed at 104 –105 , this was not so drastic. Therefore, an increase of conductivity by ultrasonication is not rationalized as due to the sonication effect on the molecular weight. The FT-IR spectrum of the vacuum dried powder of the colloids prepared with and without sonication were almost similar to each other in most of the wavelength region (see Fig. 3(1)), but the band at 1140 cm1 is seem to be very intense and broad in the spectra of the sample with sonication compared to that without sonication, as shown in Fig. 3(2). This band is a vibrational mode of þ
79
between the benzoid and quinoid rings [19]. Therefore, it could be considered that the polyaniline colloid prepared with sonication has a higher doping level than that without sonication. This may result in an increase in the electroconductivity of the colloids. A dispersion of polyaniline colloids in 1.25 M HCl was electrochemically active and exhibited a clear redox property in cyclic voltammograms (CVÕs), as shown in Fig. 4. CVÕs for the colloids prepared in the absence and presence of ultrasonication seem to be almost equivalent to each other. However, it is noticeable that the first oxidation peak in the former is considerably small and broad, while that in the later is large and sharp. This fact indicates that the insertion of counterions (dopants) into the colloids prepared with sonication is facilitated compared with that without sonication, which also reflected to the high doping level of polyaniline colloids. Therefore, the higher structure of polyaniline colloids may be also modified by ultrasonication.
þ
BANH ¼ Q or BANHAB (B: benzoid unit, Q: quinoid unit) and may be attributed to the existence of the positive charge and the distribution of the dihedral angle
Fig. 2. GPC elusion patterns for DMF solutions of polyaniline colloids prepared (a) without and (b) with ultrasonication.
Fig. 3. FT-IR spectrum of the polyaniline colloids prepared (a) without and (b) with ultrasonication (1) in the region of 4600–400 cm1 and (2) in the region of 1200–1000 cm1 .
80
M. Atobe et al. / Ultrasonics Sonochemistry 10 (2003) 77–80
debted to the JSPS for a postdoctoral fellowship (1999– 2001).
References
Fig. 4. CV of dispersions of polyaniline colloids prepared (a) without and (b) with ultrasonication. (c) CV stands for the background.
4. Conclusions The rate of polymerization during the preparation of conducting polyaniline colloids was greatly increased by ultrasonic irradiation. The morphological structure, doping level and the electrochemical properties of the colloid were also significantly affected by the irradiation. The colloid preparation under ultrasonic irradiation could be an effective means for controlling the structure and properties of processable conducting polymers. Acknowledgements This study was financially supported by grant-in-aid for Scientific Research (contract no. 416316) from the Japanese Ministry of Education, Culture, Sports, Science and Technology of Japan. A.-N.C. is greatly in-
[1] H. Koezuka, K. Hyodo, A.G. McDiarmid, J. Appl. Phys. 58 (1985) 1279. [2] M. Orita, K. Yoshino, Chem. Exp. (1986) 679. [3] J. Alper, Science 246 (1989) 208. [4] K. Nishio, M. Fujimoto, T. Suzuki, J. Power Sources 34 (1991) 153. [5] J. Barisci, J. Mansouri, G. Spinks, G.G. Wallace, Synth. Met. 84 (1997) 361. [6] S. Maeda, D.B. Cairns, S.P. Armes, Eur. Polym. J. 33 (1997) 245. [7] R.B. Bjorklund, B. Liedberg, J. Chem. Soc. Chem. Commun. (1986) 1293. [8] A. Ohtani, T. Shimidzu, Bull. Chem. Soc. Jpn. 62 (1989) 234. [9] S. Maeda, S.P. Armes, Synth. Met. 69 (1995) 499. [10] M.R. Simmons, P.A. Chaloner, S.P. Armes, S.J. Greaves, J.F. Watts, Langmuir 14 (1998) 611. [11] M. Aldissi, S.P. Armes, Progr. Org. Coat. 19 (1991) 21. [12] B. Vincent, J. Waterson, J. Chem. Soc. Chem. Commun. (1990) 683. [13] M. Atobe, S. Fuwa, N. Sato, T. Nonaka, Denki Kagaku (presently Electrochemistry) 65 (1997) 495. [14] M. Atobe, T. Kaburagi, T. Nonaka, Electrochemistry 67 (1999) 1. [15] M. Atobe, T. Nonaka, Trans. MRS-J 25 (2000) 81. [16] T.J. Mason, J.P. Lorimer, Applied Sonochemistry, Wiley-VCH, Weinheim, 2002 (Chapter 5). [17] E.C. Cooper, B. Vincent, J. Phys. D 22 (1989) 1580. [18] A. Watanabe, K. Mori, Y. Iwasaki, Y. Nakamura, J. Chem. Soc. Chem. Commun. (1987) 3. [19] J. Tang, X. Jing, B. Wang, F. Wang, Synth. Met. 24 (1988) 231.
Preparation of conducting polyaniline colloids under ultrasonication Mahito Atobe a
a,*
, Al-Nakib Chowdhury a, Toshio Fuchigami
a,*
, Tsutomu Nonaka
b
Department of Electronic Chemistry, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8502, Japan b Tsuruoka National College of Technology, 104 Aza-sawada, Oaza-ioka, Tsuruoka 997-8511, Japan Received 12 June 2002; received in revised form 13 August 2002; accepted 19 August 2002
Abstract The effects of ultrasonication on the chemical polymerization of aniline leading to the formation of conducting polyaniline colloids were examined. The formation rate of the colloids was significantly increased under ultrasonication. Furthermore, it was also observed that the morphological structure of the colloids thus prepared was greatly affected by the sonication. The polyaniline colloids were further characterized by a range of techniques including electric resistance meter, gel permeation chromatography, FTIR and cyclic voltammetry. It is noteworthy that the application of ultrasound to the polymerization resulted in a marked increase in the doping level, which reflected to the high electroconductivity of polyaniline colloids. Ó 2002 Elsevier Science B.V. All rights reserved. Keywords: Polyaniline; Conducting polymer colloid; Ultrasonication
1. Introduction Electroconducting polymers, such as polyaniline, exhibit not only electroconductivity but also unique optical and chemical properties. The diversity of properties exhibited by conducting polymers offers these materials to be used in numerous technological applications [1–4]. However, the poor processability owing to the low solubility of the polymers made it limited for practical applications. Over the last decade, an excellent progress in improving the processability of these polymers has been addressed [5,6]. The preparation of colloidal dispersion of conducting polymers is an attractive approach for improving their poor processability [7–10]. The use of colloids enables film casting from aqueous or non-aqueous solutions, either as stand-alone conducting polymers or as composite materials with other polymers. Moreover, the unique inherent properties of conducting polymers were also found to be essentially retained in their colloidal states as well [11]. In general, such
* Corresponding authors. Tel.: +81-45-924-5407; fax: +81-45-9245489 (M. Atobe), Tel.: +81-45-924-5406; fax: +81-45-924-5489 (T. Fuchigami). E-mail addresses: [email protected] (M. Atobe), fuchi@ echem.titech.ac.jp (T. Fuchigami).
properties of polymer colloids are originated from their morphological structures [12]. Therefore, it should practically be required that the structure of the colloid particles can be controlled purposively for their utilization. In our previous work, it was found that three dimensionally uniform and dense films of electropolymerized aniline, pyrrole and thiophene can be formed under ultrasonication [13–15]. This fact suggests that ultrasound can offer a useful method for controlling the structure and properties of conducting polymers. From the above point of view, we aimed to investigate the effect of ultrasonication on chemical polymerization of aniline leading to the formation of conducting polyaniline colloids, in the present work. 2. Experimental Polyaniline colloids were prepared using poly(ethylene oxide) (PEO) as a stabilizer and potassium iodate (KIO3 ) as an oxidant. A mixture of aniline (1.02 g), PEO (average Mv ; ca. 300,000, 0.80 g) and KIO3 (0.70 g) in an aqueous 1.25 M (M ¼ mol dm3 ) HCl solution (100 cm3 ) in a cylindrical glass cell was allowed to react for 12 h at 25 °C in the absence and presence of ultrasonication. The resultant colloidal dispersion was immediately
1350-4177/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 1 3 5 0 - 4 1 7 7 ( 0 2 ) 0 0 1 2 1 - 9
78
M. Atobe et al. / Ultrasonics Sonochemistry 10 (2003) 77–80
ultracentrifuged at 40,000 rpm for 30 min and separated as deep green sediment. The sediment was rinsed several times with deionized water. Colloid production under ultrasonication employed a commercially available ultrasonic bath (38 kHz, 1.5 W cm2 (calorimetrically calibrated)). All the aqueous solutions used throughout experiments were prepared using deionized water. Dried down polyaniline colloids were subjected to a scanning electron microscope (SEM), electric resistance measurement (two-probe method) and FT-IR spectroscopy, while the solution of colloids in dimethyl formamide (DMF) was analyzed using gel permeation chromatography (GPC) with a polystyrene standard (detector: UV–vis). Polyaniline colloid (0.02 g) dispersed in aqueous 1.25 M HCl solution (100 ml) was utilized for cyclic voltammetry at 100 mV s1 on a glassy carbon electrode. A Pt foil and a saturated calomel electrode were used as counter and reference electrodes, respectively.
3. Results and discussion Polymerization of aniline under ultrasonication was completed within 12 h, since no colloidal sediments in the supernatant decanted from the reaction mixtures used for the colloid preparation was detected even after standing for seven days. In contrast, the supernatant from the reaction mixture in the absence of ultrasonication still gave a larger amount of colloidal sediment. Therefore, it can be stated that the rate of colloid formation in the presence of ultrasonication is much higher than that in the absence. In this point, an ultrasonic effect on the chemical polymerization seems to be quite different from that on the electrochemical polymerization, which is known to be deceleratingly polymerized under ultrasonic irradiation [13–15]. It is well known that PEO is depolymerized by ultrasound and its real molar mass is thus decreased [16].
In order to confirm whether the molar mass of PEO used as a stabilizer is decreased under ultrasonic irradiation with 1.5 W cm2 at 12 h, the molar mass of PEO was measured by GPC (detector: RI) before and after irradiation. As a result, the molar mass was little affected under the present conditions. This fact indicates that increase in the rate of chemical polymerization is essentially attributed to an effect associated with ultrasound. Therefore, the difference in the polymerization rate as mentioned above seems to lie in the different mechanism as follows. The electrochemical (electrooxidative) polymerization process involves the formation of radical cations generated by electrooxidation of monomer molecules, radical coupling reactions for chain extension, further oxidation of oligomers, the insertion of counterions and eventual polymer deposition, and all of them take place only at the electrode interfaces. Therefore, it can be considered that the radical–radical coupling, further oxidation of oligomers and deposition are adversely affected as the reaction products are transported away from the electrode surface under hydrodynamic condition such as ultrasonic agitation, and consequently the polymerization rate is decelerated. In contrast, the chemical polymerization process can take place at anywhere in the bulk solution. Under this situation, the radical coupling reactions for chain extension may be accelerated by the ultrasonic agitation, and eventually the polymerization is enhanced. Fig. 1 shows SEM images of dried-down polyaniline colloids. It can be observed that the morphological structure of polyaniline colloids prepared in the absence of ultrasonication Fig. 1(a) is needle-shaped and finely net-worked. On the other hand, the morphology of colloids under the sonication Fig. 1(b) is quite different from the above one, i.e. grains are deposited and they are not net-worked. Additionally, electroconductivity of polyaniline colloids prepared in the presence of ultra-
Fig. 1. SEM photographs of the dried-down polyaniline colloids formed (a) without and (b) with ultrasonication.
M. Atobe et al. / Ultrasonics Sonochemistry 10 (2003) 77–80
sonication (3:0 102 S cm1 ) was found to be ca. 17 times as high as that in the absence. In general, it is known that needle-shaped particles have a conductivity superior to that of grain-shaped ones [17]. However, polyaniline colloids prepared under ultrasonication exhibit relatively high conductivity regardless of its morphological structure. In addition, the grain structure colloids obtained by the sonication may be used as chromatographic beads. GPC elution patterns of polyaniline colloids dissolved in DMF are shown in Fig. 2. The elution patterns sketched in Fig. 2(a) and (b) stand for the polyaniline colloids prepared in the absence and presence of ultrasonication, respectively. In both the cases, the elution patterns seem to be fairly different from those of electrochemically prepared polyaniline [18] and to be multimodal in a range of 103 –106 . Fig. 2 suggests that the polyaniline colloids mainly consist of the corresponding high polymer and oligomer. Although an influence of ultrasonication on the molecular weight distribution could be observed at 104 –105 , this was not so drastic. Therefore, an increase of conductivity by ultrasonication is not rationalized as due to the sonication effect on the molecular weight. The FT-IR spectrum of the vacuum dried powder of the colloids prepared with and without sonication were almost similar to each other in most of the wavelength region (see Fig. 3(1)), but the band at 1140 cm1 is seem to be very intense and broad in the spectra of the sample with sonication compared to that without sonication, as shown in Fig. 3(2). This band is a vibrational mode of þ
79
between the benzoid and quinoid rings [19]. Therefore, it could be considered that the polyaniline colloid prepared with sonication has a higher doping level than that without sonication. This may result in an increase in the electroconductivity of the colloids. A dispersion of polyaniline colloids in 1.25 M HCl was electrochemically active and exhibited a clear redox property in cyclic voltammograms (CVÕs), as shown in Fig. 4. CVÕs for the colloids prepared in the absence and presence of ultrasonication seem to be almost equivalent to each other. However, it is noticeable that the first oxidation peak in the former is considerably small and broad, while that in the later is large and sharp. This fact indicates that the insertion of counterions (dopants) into the colloids prepared with sonication is facilitated compared with that without sonication, which also reflected to the high doping level of polyaniline colloids. Therefore, the higher structure of polyaniline colloids may be also modified by ultrasonication.
þ
BANH ¼ Q or BANHAB (B: benzoid unit, Q: quinoid unit) and may be attributed to the existence of the positive charge and the distribution of the dihedral angle
Fig. 2. GPC elusion patterns for DMF solutions of polyaniline colloids prepared (a) without and (b) with ultrasonication.
Fig. 3. FT-IR spectrum of the polyaniline colloids prepared (a) without and (b) with ultrasonication (1) in the region of 4600–400 cm1 and (2) in the region of 1200–1000 cm1 .
80
M. Atobe et al. / Ultrasonics Sonochemistry 10 (2003) 77–80
debted to the JSPS for a postdoctoral fellowship (1999– 2001).
References
Fig. 4. CV of dispersions of polyaniline colloids prepared (a) without and (b) with ultrasonication. (c) CV stands for the background.
4. Conclusions The rate of polymerization during the preparation of conducting polyaniline colloids was greatly increased by ultrasonic irradiation. The morphological structure, doping level and the electrochemical properties of the colloid were also significantly affected by the irradiation. The colloid preparation under ultrasonic irradiation could be an effective means for controlling the structure and properties of processable conducting polymers. Acknowledgements This study was financially supported by grant-in-aid for Scientific Research (contract no. 416316) from the Japanese Ministry of Education, Culture, Sports, Science and Technology of Japan. A.-N.C. is greatly in-
[1] H. Koezuka, K. Hyodo, A.G. McDiarmid, J. Appl. Phys. 58 (1985) 1279. [2] M. Orita, K. Yoshino, Chem. Exp. (1986) 679. [3] J. Alper, Science 246 (1989) 208. [4] K. Nishio, M. Fujimoto, T. Suzuki, J. Power Sources 34 (1991) 153. [5] J. Barisci, J. Mansouri, G. Spinks, G.G. Wallace, Synth. Met. 84 (1997) 361. [6] S. Maeda, D.B. Cairns, S.P. Armes, Eur. Polym. J. 33 (1997) 245. [7] R.B. Bjorklund, B. Liedberg, J. Chem. Soc. Chem. Commun. (1986) 1293. [8] A. Ohtani, T. Shimidzu, Bull. Chem. Soc. Jpn. 62 (1989) 234. [9] S. Maeda, S.P. Armes, Synth. Met. 69 (1995) 499. [10] M.R. Simmons, P.A. Chaloner, S.P. Armes, S.J. Greaves, J.F. Watts, Langmuir 14 (1998) 611. [11] M. Aldissi, S.P. Armes, Progr. Org. Coat. 19 (1991) 21. [12] B. Vincent, J. Waterson, J. Chem. Soc. Chem. Commun. (1990) 683. [13] M. Atobe, S. Fuwa, N. Sato, T. Nonaka, Denki Kagaku (presently Electrochemistry) 65 (1997) 495. [14] M. Atobe, T. Kaburagi, T. Nonaka, Electrochemistry 67 (1999) 1. [15] M. Atobe, T. Nonaka, Trans. MRS-J 25 (2000) 81. [16] T.J. Mason, J.P. Lorimer, Applied Sonochemistry, Wiley-VCH, Weinheim, 2002 (Chapter 5). [17] E.C. Cooper, B. Vincent, J. Phys. D 22 (1989) 1580. [18] A. Watanabe, K. Mori, Y. Iwasaki, Y. Nakamura, J. Chem. Soc. Chem. Commun. (1987) 3. [19] J. Tang, X. Jing, B. Wang, F. Wang, Synth. Met. 24 (1988) 231.