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Effect of Fenton reagent on the synthesis of polyaniline
Synthetic Metals 123 (2001) 293±297
Effect of Fenton reagent on the synthesis of polyaniline Hongping Zhu, Shaolin Mu* Department of Chemistry, School of Sciences, Yangzhou University, Shouxi Lake Campus, Yangzhou 225002, PR China Received 5 June 2000; received in revised form 27 November 2000; accepted 5 January 2001
Abstract Polyaniline has been prepared using Fenton reagent. The in situ visible spectra during the polymerization process show that the polymerization of aniline was carried out through an intermediate at 530 nm. The absorption band of the resulting product, polyaniline, appears at 700 nm. The conductivity of polyaniline prepared using Fenton reagent can reach 1:04 10 2 S cm 1, which is strongly dependent on the concentrations of FeSO4, H2SO4 and polymerization time. The cyclic voltammogram of polyaniline prepared using Fenton reagent is much different from that of polyaniline prepared normally, and has the good electrochemical reversibility and a fast charge transfer characteristic in the solution with pH 4.0. The FTIR spectrum indicates that no absorption band attributed to N±H stretching vibrations is present in polyaniline. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Polyaniline; Fenton reagent; In situ visible spectra; FTIR spectrum; Conductivity; Electrochemical properties
1. Introduction Polyaniline has been receiving signi®cant attention due to its high conductivity, good redox reversibility, swift change in ®lm color with potentials, photoelectrochemical conversion of light to electricity and high stability in air. It is hard to ®nd a polymer possessing so much valuable characteristics, which provide possible applications in battery electrodes [1,2], electrochromic devices [3,4], photoelectric cell [5,6], light-emitting diode [7] and biosensors [8,9]. It is anticipated that the preparation of polyaniline will get more and more attraction due to its wide applications. Polyaniline can be synthesized by the electrochemical polymerization or chemical polymerization of aniline. In the latter, various oxidants, such as ammonium peroxydisulfate [10±12], sodium peroxydisulfate [13], potassium bichromate [14] and hydrogen peroxide [15], were used for the oxidation of aniline monomer. Among the above oxidants, ammonium peroxydisulfate has been widely used for the preparation of polyaniline with high conductivity. From the point of view of oxidability and environmental protection, hydrogen peroxide could be a suitable oxidant for oxidation of aniline. This is because the oxidation±reduction potential of the couple of H2O2 is 1.77 V (versus NHE), aniline can be oxidized in such high potential, and hydrogen peroxide will be decomposed to water after reduction, it is possible to
* Corresponding author. Fax: 86-514-734-9819. E-mail address: [email protected] (S. Mu).
reduce environmental pollution. However, only a few reports described the synthesis of polyaniline through the chemical oxidation of aniline using hydrogen peroxide as an oxidant [15,16]. Inoue et al. reported the oxidation of aniline using hydrogen peroxide in the presence of Fe2, i.e. Fenton reagent, as oxidant to prepare polyaniline, its conductivity was 10 6±10 9 S cm 1 [15]. Such low conductivity may be caused by the deprotonation of polyaniline, since polyaniline was treated using a boiling aqueous solution of ammonium hydroxide before measurement of conductivity. Recently, Samuelson et al. reported the enzyme-catalyzed polymerization of aniline in the presence of hydrogen peroxide [16]. The above reports imply that the preparation of polyaniline can not be carried out using hydrogen peroxide alone as oxidant, because its oxidation rate is very slowly at room temperature. So actually it is impossible to prepare polyaniline through the oxidation of aniline using hydrogen peroxide as oxidant at room temperature. It is well known that the oxidation rate of a species is catalyzed by hydrogen peroxide in the presence of Fe2, or of other metallic ions with different oxidation states. Thus, we use Fenton reagent, i.e. the solution of H2O2 in the presence of FeSO4, to prepare polyaniline. The aim for this work is to gain a better understanding of the effect of Fenton reagent on the polymerization of aniline, and to approach the properties of polyaniline prepared by Fenton reagent. In this paper, the preparation of polyaniline using Fenton reagent, in situ visible spectra during polymerization of aniline, conductivity, electrochemical properties and FTIR spectrum of the polyaniline were reported.
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2. Experimental The chemicals used were all reagent grade. Aniline was distilled before use. The solid ferrous sulfate was gradually added into a stirring solution consisting of aniline, sulfuric acid and hydrogen peroxide. A platinum net was immersed into the solution. Aniline was polymerized on the platinum net during the polymerization process. Of course, most of polyaniline was precipitated into the reactor to be used in the measurements of conductivity and FTIR spectrum. The platinum net with polyaniline provides the test of cyclic voltammetry. The in situ visible spectra during the polymerization of aniline was measured using MPS-2000 spectrometer, which is a double-beam instrument. A model HPD-1A potentiostat±galvanostat was used for the cyclic voltammetry. The cyclic voltammograms were recorded using a YEW 3036 X±Y recorder. The scan rate was 50 mV s 1. FTIR spectrum of polyaniline was measured on a pressed pellet with KBr using a Nicolet 740 FTIR instrument. The conductivity of polyaniline was measured on a pellet using a four-probe technique. 3. Results and discussion 3.1. In situ visible spectra during the polymerization of aniline The measurements of the in situ visible spectra during the polymerization of aniline were carried out at an interval of 3 min between two measurements. Fig. 1 shows the in situ
visible spectra during the polymerization of aniline in the solution containing 0.2 mol dm 3 aniline, 0.2 mol dm 3 H2SO4, 0.4 mol dm 3 H2O2 and 1:0 10 3 mol dm 3 FeSO4. The reference solution for the measurement of in situ visible spectrum is the same as above solution, but in the absence of FeSO4. Fig. 1 shows that a band at 530 nm appears ®rst, and then the second band at 700 nm gradually appears. Finally, the band at 530 nm disappears. The change in visible spectra with time implies that the polymerization of aniline was carried out through an intermediate at 530 nm, since there are two bands on curve 8, but only one band appears on curves 7 and 10. The resulting band at 700 nm is attributed to the product, polyaniline. The measurements of the in situ visible spectra during the polymerization of aniline were carried out in the solution containing 0.2 mol dm 3 aniline, 0.4 mol dm 3 H2SO4, 0.4 mol dm 3 H2O2 and 1:0 10 3 mol dm 3 FeSO4. The curves of the visible spectra(omitted here) in this case are identical in shapes to those in Fig. 1, but the change in the absorbance with reaction time is slower than those in Fig. 1. This indicates that the polymerization rate of aniline in 0.4 mol dm 3 H2SO4 solution is slower than that in 0.2 mol dm 3 H2SO4 solution. This accounts for protonation of aniline. The protonation degree of aniline increases with increasing the concentration of acid. The stronger the protonation, the more dif®cult the oxidation of aniline. As a result, the starting polymerization rate of aniline in 0.4 mol dm 3 H2SO4 solution is lower than that in 0.2 mol dm 3 H2SO4 solution, but following polymerization rate of aniline in 0.4 mol dm 3 H2SO4 solution is faster than that in 0.2 mol dm 3 H2SO4 solution. The measurements of the in situ visible spectra during the polymerization of aniline were carried out in the solution containing 0.2 mol dm 3 aniline, 0.2 mol dm 3 H2SO4, 0.4 mol dm 3 H2O2 and 2:0 10 3 mol dm 3 FeSO4. Also, the curves of the visible spectra (omitted here) in this solution are identical in shapes to those in Fig. 1, but the absorbance in this solution increases more quickly than that in Fig. 1. This indicates that the polymerization rate of aniline increases with increasing concentration of FeSO4, i.e. the oxidability of hydrogen peroxide was enhanced due to increasing the concentration of ferrous sulfate, this is because the reaction between H2O2 and Fe2 forms a radical H2 O2 Fe2 ! Fe3 OH OH The radical concentration should increase with increasing concentration of Fe2, which accelerates the polymerization rate of aniline. 3.2. Conductivity of polyaniline
Fig. 1. In situ visible spectra at different times during the polymerization of aniline, solution consisted of 0.2 mol dm 3 aniline, 0.4 mol dm 3 H2O2, 0.2 mol dm 3 H2SO4 and 1:0 10 3 mol dm 3 FeSO4. Curves: (1) starting reaction; (2) 3 min; (7) 18 min; (8) 21 min; (9) 24 min; (10) 27 min.
Table 1 shows the effects of concentration of ferrous sulfate and polymerization time on the conductivity of polyaniline. The solution for polymerization consisted of 0.2 mol dm 3 aniline, 0.2 mol dm 3 H2SO4 and 0.4 mol dm 3 H2O2 with different concentrations of FeSO4. From
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Table 1 Effects of the concentration of FeSO4 and polymerization time on the conductivity of polyaniline Concentration of FeSO4 (mmol dm 3)
Conductivity (S cm 1)
1 2 3 4
1.04 5.01 1.31 4.01
Polymerization time (2 h)
10 10 10 10
2 4 4 5
Polymerization time (4 h) 6.94 10 3.34 10 <10 5 <10 5
4 5
Table 1, we can see that the conductivity of polyaniline prepared in 1:0 10 3 mol dm 3 FeSO4 is 1:04 10 2 S cm 1, which is the highest among concentrations of FeSO4 used and is also four orders of magnitude higher than that of polyaniline reported by Inoue et al. as mentioned above [15]. The conductivity of polyaniline decreases with increasing concentration of FeSO4 and polymerization time. Thus, the decrease of the conductivity in this case is caused by the over oxidation of polyaniline. Table 2 shows the effects of the concentration of sulfuric acid and polymerization time on the conductivity of polyaniline. The solution for polymerization consisted of 0.2 mol dm 3 aniline, 0.4 mol dm 3 H2O2 and 1:0 10 3 mol dm 3 with different concentrations of H2SO4. From Table 2, we can see that the conductivity of polyaniline prepared in 0.2 mol dm 3 H2SO4 is the largest among concentrations of sulfuric acid used. The conductivity of polyaniline decreased with increasing polymerization time and concentration of sulfuric acid. The latter accounts for the effect of acid concentration on the oxidability of hydrogen peroxide. From the above results we can draw a conclusion that the low conductivity of polyaniline is caused by the strong oxidability of Fenton reagent. It was found that polyaniline prepared in less than 0.2 mol dm 3 H2SO4 solution is yellow-brown. Its conductivity is very low. Therefore, in the following experiments polyaniline was prepared in the solution consisting of 0.2 mol dm 3 aniline, 0.2 mol dm 3 H2SO4, 0.4 mol dm 3 H2O2 and 1:0 10 3 mol dm 3 FeSO4. The color of polyaniline prepared in this solution is deeply blue, which is like that of the common polyaniline. The solubility of polyaniline is 172 mg in 100 ml of
Fig. 2. FTIR spectrum of polyaniline prepared using Fenton reagent.
dimethylformamide and 79 mg in 100 ml of dimethyl sulfoxide at 258C. As mentioned above, the conductivity of polyaniline reported here is much higher than that reported by Inoue et al. [15]. This difference is caused by many factors, such as the concentrations of FeSO4, H2SO4, and polymerization time. The concentrations of FeSO4 and H2SO4 used for the polymerization of aniline in the report of Inoue et al. are much higher than those in our work, and the polymerization time in their report is also much longer than that in our work. However, the main factor for affecting conductivity is that polyaniline prepared by Inoue et al. was treated using a boiling aqueous solution of ammonium hydroxide. As a result, polyaniline, emeraldine salt, was turned into emeraldine base that has a very low conductivity [17]. 3.3. FTIR spectrum of polyaniline Fig. 2 shows the FTIR spectrum of polyaniline prepared using Fenton reagent. In general, a band at about 3250 cm 1 appears in IR spectrum of polyaniline prepared electrochemically [18] or chemically using ammonium peroxydisulfate [19,20]. The band at about 3250 cm 1 is attributed to N±H stretching vibrations. However, no band arising from N±H stretching vibrations appears in Fig. 2. This is mainly different from the IR spectrum of polyaniline synthesized normally. Thus, the composition of polyaniline according to the result of FTIR spectrum is supposed as follows [21]:
Table 2 Effects of the concentration of H2SO4 and polymerization time on the conductivity of polyaniline Concentration of H2SO4 (mol dm 3)
0.2 0.4 0.6 0.8
Conductivity (S cm 1) Polymerization time (2 h) 1.04 1.02 3.03 4.37
10 10 10 10
2 2 3 4
Polymerization time (4 h) 6.94 6.74 2.61 3.04
10 10 10 10
4 4 4 5
In fact, the structure of polyaniline prepared in this work is very complicated, so the above structure is only an assumption. Polyaniline having this structure possesses a very low conductivity. The band at 1147 cm 1 is attributed to SO42 doped into polyaniline during the polymerization process based on a strong absorption band of SO42 appears at 1150±1050 cm 1 [22]. This result is very similar to the absorption bands of BF4 and NO3 ions doped into
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Fig. 3. Effect of pH value on the cyclic voltammograms of polyaniline in 0.5 mol dm 3 Na2SO4 solution. Curves: (1) pH 1.0; (2) pH 2.0; (3) pH 3.0; (4) pH 4.0.
polyaniline [18], which correspond to the absorption bands of BF4 and NO3 , respectively. 3.4. Cyclic voltammogram of polyaniline Fig. 3 shows the effect of pH value on the cyclic voltammogram of polyaniline in 0.5 mol dm 3 Na2SO4 solution from pH 1 to pH 4 at the scan potential range between 0.2 and 0.70 V. Only one pair of redox peaks occurs on the cyclic voltammogram, its anodic and cathodic peaks are at
0.25 and 0.05 V, respectively. This is much different from that of polyaniline prepared normally, in which there are two pairs of redox peaks at pH < 4 [23]. One pair of redox peaks occurs at about 0.1 V, which is almost independent of pH value and is caused by doping and dedoping of anions [24]. Another one occurs at about 0.74 V (at pH 1), their peak potentials shift toward negative potentials with increasing pH. Fig. 3 shows that anodic peak and cathodic peak currents decrease with increasing pH value, but the peak potentials change hardly as pH increases from 1 to 4. Therefore, polyaniline shown in Fig. 3 lost a pair of redox peaks at higher potentials. This means that the concentration of protons only affects the conductivity of polyaniline in this case. The difference of the cyclic voltammogram between the polyaniline prepared using Fenton reagent and the polyaniline synthesized normally may be caused by their different structures. Fig. 4(a) shows the effect of the scan rate on the cyclic voltammograms of polyaniline prepared using Fenton reagent. The solution for this experiment was 0.5 mol dm 3 Na2SO4 with pH 4. The scan rate was controlled between 25 and 600 mV s 1. The anodic peak and cathodic peak appear at 0.25 and 0.10 V, respectively. Their peak currents increase with increasing scan rate from 25 to 600 mV s 1, but their peak potentials are independent of the scan rate. This indicates that polyaniline prepared using Fenton reagent has a good electrochemical reversibility in the solution of pH 4.0. At the scan rate of 600 mV s 1, there is still a pronounced anodic peak and cathodic peak on the I±E curve. This means that the electrode reaction is controlled by
Fig. 4. Effect of scan rate on the cyclic voltammograms of polyaniline. (a) Polyaniline prepared using Fenton reagent; (b) polyaniline prepared electrochemically. Scan rate (mV s 1), curves: (1) 25; (2) 50; (3) 100; (4) 200; (5) 400; (6) 600.
H. Zhu, S. Mu / Synthetic Metals 123 (2001) 293±297
mass transfer, i.e. polyaniline has a fast charge transfer characteristic. Fig. 4(b) shows the effect of the scan rate on the cyclic voltammograms of polyaniline prepared electrochemically at 0.75 V. The electrolytic solution consisted of 0.2 mol dm 3 aniline and 0.2 mol dm 3 H2SO4. The solution for the cyclic voltammetry was 0.5 mol dm 3 Na2SO4 with pH 4.0. Fig. 4(b) shows that both anodic and cathodic peak currents increase with increasing scan rate, but the cathodic peak potential shifts towards negative potentials and the cathodic peak broadens with increasing scan rate. The shift of a peak potential with scan rate is characteristic of a quasi-reversible or an irreversible electrode reaction. The broadening of a peak leads to the occurrence of a current plateau on I±E curve. This implies that a diffusion-controlled reaction turns into a kinetics-controlled reaction due to the slow electron transfer reaction. Thus, comparison of results from Fig. 4(a) and Fig. 4(b) shows that the electrochemical reversibility and charge transfer rate of polyaniline prepared using Fenton reagent are better than those of polyaniline prepared electrochemically in the same electrolytic solution with 0.2 mol dm 3 H2SO4. 4. Conclusion The conductivity of polyaniline prepared using Fenton reagent is strongly dependent on the concentrations of FeSO4 and H2SO4, polymerization time, and is much lower than that of polyaniline prepared using ammonium peroxydisulfate. The latter is caused by the strong oxidability of Fenton reagent, which leads to over oxidation of polyaniline. This gives us a clue that seeking suitable oxidants and controlling polymerization time are very signi®cant for enhancing conductivity and improving electrochemical properties of polyaniline. The cyclic voltammograms at pH 4.0 show that the electrochemical reversibility and charge transfer characteristic of polyaniline prepared using Fenton reagent are better than those of polyaniline prepared electrochemically in the same electrolytic solution with 0.2 mol dm 3 H2SO4. Polyaniline prepared in this work has a lower conductivity, but has the good electrochemical reversibility and a fast charge transfer characteristic at higher pH values. So this material could be used for immobilization of enzymes to be fabricated biosensors. The electrical and optical properties of polyaniline prepared using Fenton reagent are different from those of
297
polyaniline synthesized normally. This can be attributed to their different structures. Thus further study for the structure and molecule weight of polyaniline prepared using Fenton reagent is required.
Acknowledgements This work was supported by the National Natural Science Foundation of China (20074027).
References [1] A.G. MacDiarmid, S.L. Mu, N.L.D. Somasiri, W. Wu, Mol. Cryst. Liq. Cryst. 121 (1985) 187. [2] P. Novak, K. MuÈller, K.S.V. Santhanam, O. Hass, Chem. Rev. 97 (1997) 207. [3] T. Kobayashi, N. Yonevama, H. Tamura, J. Electroanal. Chem. 177 (1984) 281. [4] C.D. Batich, H.A. Laitinen, H.C. Zhou, J. Electrochem. Soc. 137 (1990) 883. [5] J. Desilvestro, O. Hass, J. Chem. Soc., Chem. Commun. (1985) 346. [6] Y.H. Dong, S.L. Mu, Electrochim. Acta. 36 (1991) 2015. [7] S. Karg, J.C. Scott, J.R. Salem, M. Angelopoulos, Synth. Met. 80 (1996) 111. [8] P.N. Bartlett, R.G. Whitaker, Biosensor 3 (1987/1988) 359. [9] Y.F. Yang, S.L. Mu, J. Electroanal. Chem. 432 (1997) 71. [10] A.G. MacDiarmid, J.H. Chiang, M. Hmpern, W.S. Huang, S.L. Mu, N.L.D. Somasiri, W. Wu, S.I. Yaniger, Mol. Cryst. Liq. Cryst. 121 (1985) 173. [11] K. Tzou, R.V. Gregory, Synth. Met. 47 (1992) 267. [12] P.N. Adams, P.J. Laughlin, A.P. Monkman, Synth. Met. 76 (1996) 157. [13] H.S.O. Chan, P.K.H. Ho, K.L. Tan, B.T.G. Tan, Synth. Met. 35 (1990) 333. [14] E.M. GenieÂs, C. Tsintavis, A.A. Syed, Mol. Cryst. Liq. Cryst. 121 (1985) 181. [15] H. Inoue, Y. Kida, E. Imoto, Bull. Chem. Soc. Jpn. 39 (1966) 551. [16] W. Liu, J. Kumar, S. Tripathy, K.J. Senecal, L. Samuelson, J. Am. Chem. Soc. 121 (1999) 71. [17] H. Kim, C. Foster, J. Chiang, A.J. Heeger, Synth. Met. 26 (1988) 49. [18] S.L. Mu, J.Q. Kan, Synth. Met. 98 (1998) 51. [19] J.S. Tang, X.B. Jing, B.C. Wang, F.S. Wang, Synth. Met. 24 (1988) 231. [20] T. Hagiwara, M. Yamaura, K. Iwata, Synth. Met. 25 (1988) 243. [21] J.G. Masters, Y. Sun, A.G. MacDiarmid, A.J. Epstein, Synth. Met. 41±43 (1991) 715. [22] K. Nakanishi, P.H. Solomon, Infrared Absorption Spectroscopy, Holden-Day, Inc., San Francisco, 1997, p. 56. [23] W.S. Wang, B.D. Humphrey, A.G. MacDiarmid, J. Chem. Soc., Faraday Trans. I. 82 (1986) 2385. [24] D. Orata, D.A. Buttry, J. Am. Chem. Soc. 109 (1987) 3574.
Effect of Fenton reagent on the synthesis of polyaniline Hongping Zhu, Shaolin Mu* Department of Chemistry, School of Sciences, Yangzhou University, Shouxi Lake Campus, Yangzhou 225002, PR China Received 5 June 2000; received in revised form 27 November 2000; accepted 5 January 2001
Abstract Polyaniline has been prepared using Fenton reagent. The in situ visible spectra during the polymerization process show that the polymerization of aniline was carried out through an intermediate at 530 nm. The absorption band of the resulting product, polyaniline, appears at 700 nm. The conductivity of polyaniline prepared using Fenton reagent can reach 1:04 10 2 S cm 1, which is strongly dependent on the concentrations of FeSO4, H2SO4 and polymerization time. The cyclic voltammogram of polyaniline prepared using Fenton reagent is much different from that of polyaniline prepared normally, and has the good electrochemical reversibility and a fast charge transfer characteristic in the solution with pH 4.0. The FTIR spectrum indicates that no absorption band attributed to N±H stretching vibrations is present in polyaniline. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Polyaniline; Fenton reagent; In situ visible spectra; FTIR spectrum; Conductivity; Electrochemical properties
1. Introduction Polyaniline has been receiving signi®cant attention due to its high conductivity, good redox reversibility, swift change in ®lm color with potentials, photoelectrochemical conversion of light to electricity and high stability in air. It is hard to ®nd a polymer possessing so much valuable characteristics, which provide possible applications in battery electrodes [1,2], electrochromic devices [3,4], photoelectric cell [5,6], light-emitting diode [7] and biosensors [8,9]. It is anticipated that the preparation of polyaniline will get more and more attraction due to its wide applications. Polyaniline can be synthesized by the electrochemical polymerization or chemical polymerization of aniline. In the latter, various oxidants, such as ammonium peroxydisulfate [10±12], sodium peroxydisulfate [13], potassium bichromate [14] and hydrogen peroxide [15], were used for the oxidation of aniline monomer. Among the above oxidants, ammonium peroxydisulfate has been widely used for the preparation of polyaniline with high conductivity. From the point of view of oxidability and environmental protection, hydrogen peroxide could be a suitable oxidant for oxidation of aniline. This is because the oxidation±reduction potential of the couple of H2O2 is 1.77 V (versus NHE), aniline can be oxidized in such high potential, and hydrogen peroxide will be decomposed to water after reduction, it is possible to
* Corresponding author. Fax: 86-514-734-9819. E-mail address: [email protected] (S. Mu).
reduce environmental pollution. However, only a few reports described the synthesis of polyaniline through the chemical oxidation of aniline using hydrogen peroxide as an oxidant [15,16]. Inoue et al. reported the oxidation of aniline using hydrogen peroxide in the presence of Fe2, i.e. Fenton reagent, as oxidant to prepare polyaniline, its conductivity was 10 6±10 9 S cm 1 [15]. Such low conductivity may be caused by the deprotonation of polyaniline, since polyaniline was treated using a boiling aqueous solution of ammonium hydroxide before measurement of conductivity. Recently, Samuelson et al. reported the enzyme-catalyzed polymerization of aniline in the presence of hydrogen peroxide [16]. The above reports imply that the preparation of polyaniline can not be carried out using hydrogen peroxide alone as oxidant, because its oxidation rate is very slowly at room temperature. So actually it is impossible to prepare polyaniline through the oxidation of aniline using hydrogen peroxide as oxidant at room temperature. It is well known that the oxidation rate of a species is catalyzed by hydrogen peroxide in the presence of Fe2, or of other metallic ions with different oxidation states. Thus, we use Fenton reagent, i.e. the solution of H2O2 in the presence of FeSO4, to prepare polyaniline. The aim for this work is to gain a better understanding of the effect of Fenton reagent on the polymerization of aniline, and to approach the properties of polyaniline prepared by Fenton reagent. In this paper, the preparation of polyaniline using Fenton reagent, in situ visible spectra during polymerization of aniline, conductivity, electrochemical properties and FTIR spectrum of the polyaniline were reported.
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2. Experimental The chemicals used were all reagent grade. Aniline was distilled before use. The solid ferrous sulfate was gradually added into a stirring solution consisting of aniline, sulfuric acid and hydrogen peroxide. A platinum net was immersed into the solution. Aniline was polymerized on the platinum net during the polymerization process. Of course, most of polyaniline was precipitated into the reactor to be used in the measurements of conductivity and FTIR spectrum. The platinum net with polyaniline provides the test of cyclic voltammetry. The in situ visible spectra during the polymerization of aniline was measured using MPS-2000 spectrometer, which is a double-beam instrument. A model HPD-1A potentiostat±galvanostat was used for the cyclic voltammetry. The cyclic voltammograms were recorded using a YEW 3036 X±Y recorder. The scan rate was 50 mV s 1. FTIR spectrum of polyaniline was measured on a pressed pellet with KBr using a Nicolet 740 FTIR instrument. The conductivity of polyaniline was measured on a pellet using a four-probe technique. 3. Results and discussion 3.1. In situ visible spectra during the polymerization of aniline The measurements of the in situ visible spectra during the polymerization of aniline were carried out at an interval of 3 min between two measurements. Fig. 1 shows the in situ
visible spectra during the polymerization of aniline in the solution containing 0.2 mol dm 3 aniline, 0.2 mol dm 3 H2SO4, 0.4 mol dm 3 H2O2 and 1:0 10 3 mol dm 3 FeSO4. The reference solution for the measurement of in situ visible spectrum is the same as above solution, but in the absence of FeSO4. Fig. 1 shows that a band at 530 nm appears ®rst, and then the second band at 700 nm gradually appears. Finally, the band at 530 nm disappears. The change in visible spectra with time implies that the polymerization of aniline was carried out through an intermediate at 530 nm, since there are two bands on curve 8, but only one band appears on curves 7 and 10. The resulting band at 700 nm is attributed to the product, polyaniline. The measurements of the in situ visible spectra during the polymerization of aniline were carried out in the solution containing 0.2 mol dm 3 aniline, 0.4 mol dm 3 H2SO4, 0.4 mol dm 3 H2O2 and 1:0 10 3 mol dm 3 FeSO4. The curves of the visible spectra(omitted here) in this case are identical in shapes to those in Fig. 1, but the change in the absorbance with reaction time is slower than those in Fig. 1. This indicates that the polymerization rate of aniline in 0.4 mol dm 3 H2SO4 solution is slower than that in 0.2 mol dm 3 H2SO4 solution. This accounts for protonation of aniline. The protonation degree of aniline increases with increasing the concentration of acid. The stronger the protonation, the more dif®cult the oxidation of aniline. As a result, the starting polymerization rate of aniline in 0.4 mol dm 3 H2SO4 solution is lower than that in 0.2 mol dm 3 H2SO4 solution, but following polymerization rate of aniline in 0.4 mol dm 3 H2SO4 solution is faster than that in 0.2 mol dm 3 H2SO4 solution. The measurements of the in situ visible spectra during the polymerization of aniline were carried out in the solution containing 0.2 mol dm 3 aniline, 0.2 mol dm 3 H2SO4, 0.4 mol dm 3 H2O2 and 2:0 10 3 mol dm 3 FeSO4. Also, the curves of the visible spectra (omitted here) in this solution are identical in shapes to those in Fig. 1, but the absorbance in this solution increases more quickly than that in Fig. 1. This indicates that the polymerization rate of aniline increases with increasing concentration of FeSO4, i.e. the oxidability of hydrogen peroxide was enhanced due to increasing the concentration of ferrous sulfate, this is because the reaction between H2O2 and Fe2 forms a radical H2 O2 Fe2 ! Fe3 OH OH The radical concentration should increase with increasing concentration of Fe2, which accelerates the polymerization rate of aniline. 3.2. Conductivity of polyaniline
Fig. 1. In situ visible spectra at different times during the polymerization of aniline, solution consisted of 0.2 mol dm 3 aniline, 0.4 mol dm 3 H2O2, 0.2 mol dm 3 H2SO4 and 1:0 10 3 mol dm 3 FeSO4. Curves: (1) starting reaction; (2) 3 min; (7) 18 min; (8) 21 min; (9) 24 min; (10) 27 min.
Table 1 shows the effects of concentration of ferrous sulfate and polymerization time on the conductivity of polyaniline. The solution for polymerization consisted of 0.2 mol dm 3 aniline, 0.2 mol dm 3 H2SO4 and 0.4 mol dm 3 H2O2 with different concentrations of FeSO4. From
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Table 1 Effects of the concentration of FeSO4 and polymerization time on the conductivity of polyaniline Concentration of FeSO4 (mmol dm 3)
Conductivity (S cm 1)
1 2 3 4
1.04 5.01 1.31 4.01
Polymerization time (2 h)
10 10 10 10
2 4 4 5
Polymerization time (4 h) 6.94 10 3.34 10 <10 5 <10 5
4 5
Table 1, we can see that the conductivity of polyaniline prepared in 1:0 10 3 mol dm 3 FeSO4 is 1:04 10 2 S cm 1, which is the highest among concentrations of FeSO4 used and is also four orders of magnitude higher than that of polyaniline reported by Inoue et al. as mentioned above [15]. The conductivity of polyaniline decreases with increasing concentration of FeSO4 and polymerization time. Thus, the decrease of the conductivity in this case is caused by the over oxidation of polyaniline. Table 2 shows the effects of the concentration of sulfuric acid and polymerization time on the conductivity of polyaniline. The solution for polymerization consisted of 0.2 mol dm 3 aniline, 0.4 mol dm 3 H2O2 and 1:0 10 3 mol dm 3 with different concentrations of H2SO4. From Table 2, we can see that the conductivity of polyaniline prepared in 0.2 mol dm 3 H2SO4 is the largest among concentrations of sulfuric acid used. The conductivity of polyaniline decreased with increasing polymerization time and concentration of sulfuric acid. The latter accounts for the effect of acid concentration on the oxidability of hydrogen peroxide. From the above results we can draw a conclusion that the low conductivity of polyaniline is caused by the strong oxidability of Fenton reagent. It was found that polyaniline prepared in less than 0.2 mol dm 3 H2SO4 solution is yellow-brown. Its conductivity is very low. Therefore, in the following experiments polyaniline was prepared in the solution consisting of 0.2 mol dm 3 aniline, 0.2 mol dm 3 H2SO4, 0.4 mol dm 3 H2O2 and 1:0 10 3 mol dm 3 FeSO4. The color of polyaniline prepared in this solution is deeply blue, which is like that of the common polyaniline. The solubility of polyaniline is 172 mg in 100 ml of
Fig. 2. FTIR spectrum of polyaniline prepared using Fenton reagent.
dimethylformamide and 79 mg in 100 ml of dimethyl sulfoxide at 258C. As mentioned above, the conductivity of polyaniline reported here is much higher than that reported by Inoue et al. [15]. This difference is caused by many factors, such as the concentrations of FeSO4, H2SO4, and polymerization time. The concentrations of FeSO4 and H2SO4 used for the polymerization of aniline in the report of Inoue et al. are much higher than those in our work, and the polymerization time in their report is also much longer than that in our work. However, the main factor for affecting conductivity is that polyaniline prepared by Inoue et al. was treated using a boiling aqueous solution of ammonium hydroxide. As a result, polyaniline, emeraldine salt, was turned into emeraldine base that has a very low conductivity [17]. 3.3. FTIR spectrum of polyaniline Fig. 2 shows the FTIR spectrum of polyaniline prepared using Fenton reagent. In general, a band at about 3250 cm 1 appears in IR spectrum of polyaniline prepared electrochemically [18] or chemically using ammonium peroxydisulfate [19,20]. The band at about 3250 cm 1 is attributed to N±H stretching vibrations. However, no band arising from N±H stretching vibrations appears in Fig. 2. This is mainly different from the IR spectrum of polyaniline synthesized normally. Thus, the composition of polyaniline according to the result of FTIR spectrum is supposed as follows [21]:
Table 2 Effects of the concentration of H2SO4 and polymerization time on the conductivity of polyaniline Concentration of H2SO4 (mol dm 3)
0.2 0.4 0.6 0.8
Conductivity (S cm 1) Polymerization time (2 h) 1.04 1.02 3.03 4.37
10 10 10 10
2 2 3 4
Polymerization time (4 h) 6.94 6.74 2.61 3.04
10 10 10 10
4 4 4 5
In fact, the structure of polyaniline prepared in this work is very complicated, so the above structure is only an assumption. Polyaniline having this structure possesses a very low conductivity. The band at 1147 cm 1 is attributed to SO42 doped into polyaniline during the polymerization process based on a strong absorption band of SO42 appears at 1150±1050 cm 1 [22]. This result is very similar to the absorption bands of BF4 and NO3 ions doped into
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Fig. 3. Effect of pH value on the cyclic voltammograms of polyaniline in 0.5 mol dm 3 Na2SO4 solution. Curves: (1) pH 1.0; (2) pH 2.0; (3) pH 3.0; (4) pH 4.0.
polyaniline [18], which correspond to the absorption bands of BF4 and NO3 , respectively. 3.4. Cyclic voltammogram of polyaniline Fig. 3 shows the effect of pH value on the cyclic voltammogram of polyaniline in 0.5 mol dm 3 Na2SO4 solution from pH 1 to pH 4 at the scan potential range between 0.2 and 0.70 V. Only one pair of redox peaks occurs on the cyclic voltammogram, its anodic and cathodic peaks are at
0.25 and 0.05 V, respectively. This is much different from that of polyaniline prepared normally, in which there are two pairs of redox peaks at pH < 4 [23]. One pair of redox peaks occurs at about 0.1 V, which is almost independent of pH value and is caused by doping and dedoping of anions [24]. Another one occurs at about 0.74 V (at pH 1), their peak potentials shift toward negative potentials with increasing pH. Fig. 3 shows that anodic peak and cathodic peak currents decrease with increasing pH value, but the peak potentials change hardly as pH increases from 1 to 4. Therefore, polyaniline shown in Fig. 3 lost a pair of redox peaks at higher potentials. This means that the concentration of protons only affects the conductivity of polyaniline in this case. The difference of the cyclic voltammogram between the polyaniline prepared using Fenton reagent and the polyaniline synthesized normally may be caused by their different structures. Fig. 4(a) shows the effect of the scan rate on the cyclic voltammograms of polyaniline prepared using Fenton reagent. The solution for this experiment was 0.5 mol dm 3 Na2SO4 with pH 4. The scan rate was controlled between 25 and 600 mV s 1. The anodic peak and cathodic peak appear at 0.25 and 0.10 V, respectively. Their peak currents increase with increasing scan rate from 25 to 600 mV s 1, but their peak potentials are independent of the scan rate. This indicates that polyaniline prepared using Fenton reagent has a good electrochemical reversibility in the solution of pH 4.0. At the scan rate of 600 mV s 1, there is still a pronounced anodic peak and cathodic peak on the I±E curve. This means that the electrode reaction is controlled by
Fig. 4. Effect of scan rate on the cyclic voltammograms of polyaniline. (a) Polyaniline prepared using Fenton reagent; (b) polyaniline prepared electrochemically. Scan rate (mV s 1), curves: (1) 25; (2) 50; (3) 100; (4) 200; (5) 400; (6) 600.
H. Zhu, S. Mu / Synthetic Metals 123 (2001) 293±297
mass transfer, i.e. polyaniline has a fast charge transfer characteristic. Fig. 4(b) shows the effect of the scan rate on the cyclic voltammograms of polyaniline prepared electrochemically at 0.75 V. The electrolytic solution consisted of 0.2 mol dm 3 aniline and 0.2 mol dm 3 H2SO4. The solution for the cyclic voltammetry was 0.5 mol dm 3 Na2SO4 with pH 4.0. Fig. 4(b) shows that both anodic and cathodic peak currents increase with increasing scan rate, but the cathodic peak potential shifts towards negative potentials and the cathodic peak broadens with increasing scan rate. The shift of a peak potential with scan rate is characteristic of a quasi-reversible or an irreversible electrode reaction. The broadening of a peak leads to the occurrence of a current plateau on I±E curve. This implies that a diffusion-controlled reaction turns into a kinetics-controlled reaction due to the slow electron transfer reaction. Thus, comparison of results from Fig. 4(a) and Fig. 4(b) shows that the electrochemical reversibility and charge transfer rate of polyaniline prepared using Fenton reagent are better than those of polyaniline prepared electrochemically in the same electrolytic solution with 0.2 mol dm 3 H2SO4. 4. Conclusion The conductivity of polyaniline prepared using Fenton reagent is strongly dependent on the concentrations of FeSO4 and H2SO4, polymerization time, and is much lower than that of polyaniline prepared using ammonium peroxydisulfate. The latter is caused by the strong oxidability of Fenton reagent, which leads to over oxidation of polyaniline. This gives us a clue that seeking suitable oxidants and controlling polymerization time are very signi®cant for enhancing conductivity and improving electrochemical properties of polyaniline. The cyclic voltammograms at pH 4.0 show that the electrochemical reversibility and charge transfer characteristic of polyaniline prepared using Fenton reagent are better than those of polyaniline prepared electrochemically in the same electrolytic solution with 0.2 mol dm 3 H2SO4. Polyaniline prepared in this work has a lower conductivity, but has the good electrochemical reversibility and a fast charge transfer characteristic at higher pH values. So this material could be used for immobilization of enzymes to be fabricated biosensors. The electrical and optical properties of polyaniline prepared using Fenton reagent are different from those of
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polyaniline synthesized normally. This can be attributed to their different structures. Thus further study for the structure and molecule weight of polyaniline prepared using Fenton reagent is required.
Acknowledgements This work was supported by the National Natural Science Foundation of China (20074027).
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