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Polymerization and redox behavior of polypyrrole (PPy) films by in situ EQCM and PT techniques
Applied Surface Science 229 (2004) 372–376
Polymerization and redox behavior of polypyrrole (PPy) films by in situ EQCM and PT techniques$ Chongjun Zhaoa,b,*, Zhiyu Jiangb a
Photon Craft Project, Shanghai Institute of Optics & Fine Mechanics, Chinese Academy of Sciences and Japan Science and Agency, Shanghai 201800, China b Department of Chemistry, Fudan University, Shanghai 200433, China Received 30 December 2003; received in revised form 6 February 2004; accepted 6 February 2004 Available online 16 March 2004
Abstract Potentiodynamical polymerization and redox process of PPy films were examined in situ by both electrochemical quartz crystal microbalance (EQCM) and photothermal (PT) techniques. It was observed that the polymerization was favorable on the Au electrode covered by a thin as-grown polypyrrole (PPy) film rather than that on the bare Au electrode. It was confirmed that the PPy film was not fully oxidized during polymerization. The correlation of the doping/dedoping and the electrochromic properties of the PPy film was studied by combining in situ EQCM and PT techniques, and it is found that only the ion exchange in the potential of 0.33 to 0.17 V (versus Ag/AgCl) for anodic scan and in the potential range of 0.44 to 0.7 V (versus Ag/ AgCl) for cathodic scan was contributed to electrochromic behavior. # 2004 Elsevier B.V. All rights reserved. PACS: 82.35.þt; 82.45.þz; 81.15.Pq; 81.70.Cv Keywords: Electrochemical quartz crystal microbalance (EQCM); Photothermal (PT); Polypyrrole (PPy)
1. Introduction As one of the most important conducting polymers, PPy has been extensively studied for the uses in chemical sensors, electrocatalysis, rechargeable batteries and electrochromism [1]. It is necessary to understand the mechanism of the polymerization to prepare the PPy with good performances, and it is also important to study the correlation between the doping/dedopoing and the $
Shanghai Nanotech Promote Center (0259 nm 023). Corresponding author. Tel.: þ86-21-5991-1897; fax: þ86-21-5992-9373. E-mail address: [email protected] (C. Zhao). *
electrochemical properties such as the conductivity, structure and electrochromic properties of the PPy films. There is a distinct difference in absorption spectra (observed by color change) between the film in the reduced state and the oxidized state in the visible region [2,3], which can affect the local temperature of the electrode. If the dependence of both photothermal signal, DT, and frequency change, DF, on the potential were measured at the same time by combining the PT technique and the EQCM technique, the correlation between the doping/dedoping and the color change will be learned. EQCM is an in situ tool by measuring the mass change and able to measure nanogram range mass changes (108 to 109 g). The simplified Sauerbrey
0169-4332/$ – see front matter # 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2004.02.014
C. Zhao, Z. Jiang / Applied Surface Science 229 (2004) 372–376
373
equation for a uniform, thin film can be written as: DF ¼ 2:26 106 f02 Dm
(1)
So, Dm ¼
Df 2:26 106 f02
(2)
where DF is the observed frequency change (Hz); f0 is the fundamental oscillating frequency of the quartz (Hz); Dm is the attached mass on the quartz electrode surface (g cm2). In contrast with the EQCM, the photothermal technique belongs to photothermal spectroscopy (PTS) or photoacoustic spectroscopy (PAS). It is an in situ method for investigating the surface and structure change of an electrode by measuring the change of the local temperature of the electrode under irradiation of light at some wavelength bands. The light irradiates on the surface of the electrode, and the sensor at the backside of the electrode consisting of a polyvinylidene difluoride (PVDF) pyroelectric film that can translates the temperature difference into potential difference and is no less sensitive than 100 mV K1. The change of thickness or color of the film will affect the absorption, so they can be judged by measuring the local temperature of the electrode. The photothermal spectroscopy technique has been used to study the electrochemical deposition and redox process of nickel and polyaniline in our previous works [4–7]. In this paper, the PT technique is combined with the EQCM technique in situ to study the polymerization and redox process of PPy films.
2. Experimental The electrochemical cell for photothermal signal measurement is similar to that described elsewhere [5]. The construction is shown as Fig. 1. The working electrode used is a thin Pt foil with a thickness of 0.12 mm and an area of 0.75 cm2. The reaction area of the Pt foil electrode is controlled by epoxy resin. The counter electrode is a Pt wire electrode, and an Ag/AgCl electrode is used as reference electrode. The potentials reported in this paper are all versus this electrode. The photothermal signal measurements were carried out on two different laser instruments.
Fig. 1. Pyroelectric transducer and photothermal spectroscopic electrochemical cell. (a) Pyroelectric transducer: (1) PVDF film; (2) stainless steel; (3) Teflon; (4) lead; (5) BNC cable. (b) Photothermal spectroscopic electrochemical cell: (1) auxiliary electrode; (2) Teflon; (3) pyroelectric transducer; (4) reference electrode; (5) quartz window; (6) silicon rubber ring; (7) working electrode.
One is a PDGL-3010F semiconductor diode laser with a power of 15 mW, and the wavelength of beams is 532 nm. The other is a He–Ne laser with wavelength of 632.8 nm and power of 30 mW. The working electrode foil is thick enough to interrupt the light completely. The temperature change at the working electrode was measured using a polyvinylidene difluoride (PVDF) pyroelectric film sensor placed at the electrode’s backside [5]. A model PARC 5210 lock-in amplifier and a model PAR 197 chopper at a frequency of 11 Hz were used to measure the photothermal signal. The oscillating frequency of EQCM and current density were recorded by a personal computer interfaced to a quartz crystal analyzer (Model CHI 400 time-resolved EQCM, f0 ¼ 8 MHz) and a potentiostat (Model 400, electrochemical working station). The oscillating frequency of EQCM, photothermal signals and currents were recorded simultaneously with the potential when the films were electrochemical polymerized by potentiodynamic technique. The oscillating frequency of EQCM and photothermal signals was recorded simultaneously with the potential when the redox process of the PPy films were carried out. All solutions were prepared from twice-distilled water. All reagents are analytical except that pyrrole is spectroscopically pure. The synthesis of PPy films was carried out in the mixed solution of 0.2 M pyrrole and 0.5 M LiClO4, while CV was carried out in 0.2 M KCl. The scan rate used was 10 mV s1.
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C. Zhao, Z. Jiang / Applied Surface Science 229 (2004) 372–376 0.30
3. Results and discussions
10
Fig. 2. Attached mass changes (dash line) and current density (solid line) response during the potentiodynamical polymerization of PPy films on the Au electrode covered by a as-grown thin PPy film; SR: 10 mV s1.
-2
i (mA cm )
8 6
0.15 3
0.10 2
0.05 1
0.00 0.0
0.1
0.2
0.3
0.4
0.5
4 2
-2
The PPy film was formed first on the bare Au electrode of EQCM by the potentiodynamical technique in the potential range of 0–0.4 V. It is observed that the frequency change does not decrease but increase, which can be attributed to the desorption of the small molecule such as pyrrole monomer from the Au electrode. This means that no PPy was formed in this potential range. However, when the Au electrode covered by a thin layer film prepared potentiodynamically in 0–0.5 V for two cycles was run in the potential range of 0–0.4 V again, the oscillating frequency of electrode decreases, i.e. attached mass increases, as shown in Fig. 2, suggesting that the PPy was formed on the Au electrode this time. The phenomena indicate that the as-grown film on the Au electrode was favorable to the subsequent polymerization of pyrrole monomer. This is consistent with the results reported by John and Wallace [8]. When the PPy films were prepared on the bare Au electrode of EQCM by the same method but in the potential range of 0–0.6 V, the current density and attached mass changes are shown in Fig. 3. In the figure, the attached mass change, Dm, increased in intensity with the cycle, which was somewhat different from PPy film doped by large molecule [9,10].
4
0.20
∆m (µg cm )
3.1. Potentiodynamical polymerization process of PPy films
0.25
0
0.6
E (V vs. AgCl/Ag) Fig. 3. Attached mass changes (dash line) and current density (solid line) response during the polymerizaiton of the PPy films on the bare Au electrode by potentio-dynamical technique; SR: 10 mV s1.
For mass change in potential range of 0.4–0.6 V, a distinct increase is observed in reverse scan. The mass change in reverse scan is larger than that in forward scan, which means that the polymerization speed of pyrrole in reverse was larger than that in the forward scan. The ratio of the Dm to the polymerization charge, DQ for the PPy films in 0.4–0.6 V is listed in Table 1. In the table, the Dm/DQ in reverse scan in the former cycle was larger than that in the forward scan in the later cycle, indicating that a higher current efficiency occurs in the reverse scan. Some easily oxidized substance maybe involved in the reaction in the reverse scan. Scharifker et al. [11] proved that the oligomers were involved in the formation of polypyrrole on electrode surface, and the oligomers was easier to be oxidized than the pyrrole monomer, so this is consistent with the results in the present work. Polymerization of pyrrole on Pt electrode by potentiodynamic technique was also examined by PT technique, and the DT during the polymerization process was tested and presented in Fig. 4. It is interesting to find that the shape of the PT signal is just like that of the frequency change. The curves in each cycle of both frequency change, DF, and local temperature change, DT, are like the clock-wise rotating 908 the letter ‘‘U.’’ The change of the PT signal also increases with the cycle, and change of the PT signal in the reverse scan is also larger than that in the forward scan in the potential range of 0.4–0.6 V, which enhances the result obtained by EQCM technique.
C. Zhao, Z. Jiang / Applied Surface Science 229 (2004) 372–376
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Table 1 The ratio of the attached mass changes to the polymerization charge of the PPy films in the potential range of 0.4–0.6 V DE (V)
0.4–0.6 0.6–0.4 0.5–0.6 0.6–0.5 0.55–0.6 0.6–0.55
Dm/DQ First cycle (101 mg cm2 mC1)
Second cycle (101 mg cm2 mC1)
Third cycle (101 mg cm2 mC1)
Fourth cycle (101 mg cm2 mC1)
1.09 1.78 1.22 1.97 1.48 2.04
1.16 2.19 1.62 2.44 1.64 2.72
1.69 2.50 1.98 2.75 2.14 3.05
2.50 3.49 2.85 3.74 3.36 3.88
3.2. Electrochemical redox behaviors of PPy films A typical curve of the attached mass change, Dm, of PPy films in one cycle was shown in Fig. 5. In the anodic scan, when the potential increases in the potential range of 0.7 to 0.2 V, the mass decreases, which means that the expulsion of cation was involved. However, for the potential change beyond 0.2 V, the mass increases until 0.5 V, which implies that the insertion of anion was involved. In the cathodic scan, the mass changes are reversed, i.e. it decreases in the range of 0.5 to 0.4 V, while it increases in the range of 0.4 to 0.7 V. Although the very large molecules anion are not doped in the PPy films, both the motion of the cation and anion are involved, and Lee et al. [12] also observed the same phenomena in acetonitrile. Li and Qian [13,14] proved
that both anions and hydrated cations moved in neutral solutions. The change of PT signal for PPy films during the redox process is shown in Fig. 6. Compared to the PT signal during the redox process of PPy films, PT signal at 0.4 V in reverse scan during the polymerization process (corresponding to the maximum in polymerizaion) is smaller than that of the PPy film at 0.4 V and larger than that at 0.7 V during redox process. This implies that PPy film during polymerization process was not fully oxidized during polymerization, though the potential was kept anodic when the PPy film was prepared, and this was confirmed by Lee et al. [12]. When the potential scans in the positive direction, the change of the PT signal includes three parts: 0.7 to 0.33, 0.33 to 0.17, 0.17 to 0.5 V. In the potential range of 0.7 to 0.33 and 0.17 to 0.5 V, the PT signal changes little, which indicates that the color of the PPy films change little, though the ion exchange occurs in
-2
∆m (µg cm )
0.5
0.0
-0.5
-1.0 -0.8
-0.6
-0.4
-0.2
0.0
0.2
0.4
0.6
E (V vs. Ag/AgCl) Fig. 4. Photothermal signal of the polypyrrole films during the polymerization by potentiodynamic technique. Light source: He– Ne laser; l: 632.8 nm; P: 30 mW; chopping frequency: 11 Hz.
Fig. 5. Attached mass changes of the polypyrrole films during the redox process; SR: 10 mV s1.
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C. Zhao, Z. Jiang / Applied Surface Science 229 (2004) 372–376
4. Conclusion
Fig. 6. Photothermal signal of the polypyrrole films prepared during the redox process. Light source: He–Ne laser; l: 632.8 nm; P: 30 mW; chopping frequency: 11 Hz.
this potential range. However, in the potential range of 0.33 to 0.17 V, the PT signal increases significantly, which suggests that the color of the PPy film changes in this potential range. When the potential scans in the negative direction, the change of the PT signal is different from that in the positive direction. It consists of two regions: 0.5 to 0.44 and 0.44 to 0.7 V. When the potential changes in the range of 0.5 to 0.4 V, the PT remains constant though the insertion of anion is involved in this potential range, indicating that the ion exchange in this potential range affects the electrochromic properties of the film little. However, when the potential continues to decrease and changes in the potential range of 0.44 to 0.7 V, the PT signal decreases dramatically, suggesting that the color change of the PPy film occurs in this potential range. Combining the EQCM and CER technique, Syritski [15] found that the conductivity of PPy had no significant change despite ion exchange and they attributed to the motion of ion in the outer layer of the PPy films, and this provides us some support. So only the ion in the inner layer was contributed to electrochromic behavior. When the green laser with wavelength 532 nm and power 15 mW was used, the shapes of curves obtained by PT technique were similar to those with the red laser, except that the intensity was smaller.
Polymerization and redox process of PPy films were studied by combining the in situ EQCM and PT techniques. It is found that as-grown film was favorable to the subsequent deposition of the PPy film. It was confirmed that the PPy films during polymerization was not fully oxidized by aid of the PT technique. Both the cation and anion were involved in the doping–dedoping process of PPy films. Although the ionexchange occurs in the whole potential range, the electrochromic properties only occurred significantly in the potential range of 0.33 to 0.17 V in the positive direction and in the potential range of 0.44 to 0.7 V in the negative direction.
References [1] T.A. Skotheim, Handbook of Conducting Polymers, vol. 1, Marcel Dekker, New York, 1986. [2] K. Sato, M. Yamaura, T. Hagiwara, K. Murata, M. Tokumoto, Synth. Met. 40 (1991) 35. [3] H. Kim, W. Chang, Synth. Met. 101 (1999) 150. [4] Z. Jiang, X. Zhang, Y. Xiang, J. Electroanal. Chem. 351 (1993) 321. [5] Z. Jiang, Y. Xiang, J. Wang, J. Electroanal. Chem. 316 (1991) 199. [6] C. Zhao, Y. Gu, H. Chen, Z. Jiang, Electrochim. Acta 47 (2002) 1801. [7] Z. Jiang, Y. Xiang, B. Qian, C. Jin, J. Electroanal. Chem. 361 (1993) 257. [8] R. John, G.G. Wallace, J. Electroanal. Chem. 306 (1991) 157. [9] L.M. Abrantes, C.M. Cordas, E. Vieil, Electrochim. Acta 47 (2002) 1481. [10] D. Zhou, C.O. Too, G.G. Wallace, React. Funct. Polym. 39 (1999) 19. [11] B.R. Scharifker, E. Garcfa-Pastoriza, W.J. Marino, J. Electroanal. Chem. 300 (1991) 85. [12] H. Lee, H. Yang, J. Kwak, J. Electroanal. Chem. 468 (1999) 104. [13] Y. Li, R. Qian, J. Electroanal. Chem. 362 (1993) 267. [14] Y. Li, R. Qian, Synth. Met. 64 (1994) 241. ¨ pik, A. Talo, O. Forse´ n, Synth. Met. 119 [15] V. Syritski, A. O (2001) 309.
Polymerization and redox behavior of polypyrrole (PPy) films by in situ EQCM and PT techniques$ Chongjun Zhaoa,b,*, Zhiyu Jiangb a
Photon Craft Project, Shanghai Institute of Optics & Fine Mechanics, Chinese Academy of Sciences and Japan Science and Agency, Shanghai 201800, China b Department of Chemistry, Fudan University, Shanghai 200433, China Received 30 December 2003; received in revised form 6 February 2004; accepted 6 February 2004 Available online 16 March 2004
Abstract Potentiodynamical polymerization and redox process of PPy films were examined in situ by both electrochemical quartz crystal microbalance (EQCM) and photothermal (PT) techniques. It was observed that the polymerization was favorable on the Au electrode covered by a thin as-grown polypyrrole (PPy) film rather than that on the bare Au electrode. It was confirmed that the PPy film was not fully oxidized during polymerization. The correlation of the doping/dedoping and the electrochromic properties of the PPy film was studied by combining in situ EQCM and PT techniques, and it is found that only the ion exchange in the potential of 0.33 to 0.17 V (versus Ag/AgCl) for anodic scan and in the potential range of 0.44 to 0.7 V (versus Ag/ AgCl) for cathodic scan was contributed to electrochromic behavior. # 2004 Elsevier B.V. All rights reserved. PACS: 82.35.þt; 82.45.þz; 81.15.Pq; 81.70.Cv Keywords: Electrochemical quartz crystal microbalance (EQCM); Photothermal (PT); Polypyrrole (PPy)
1. Introduction As one of the most important conducting polymers, PPy has been extensively studied for the uses in chemical sensors, electrocatalysis, rechargeable batteries and electrochromism [1]. It is necessary to understand the mechanism of the polymerization to prepare the PPy with good performances, and it is also important to study the correlation between the doping/dedopoing and the $
Shanghai Nanotech Promote Center (0259 nm 023). Corresponding author. Tel.: þ86-21-5991-1897; fax: þ86-21-5992-9373. E-mail address: [email protected] (C. Zhao). *
electrochemical properties such as the conductivity, structure and electrochromic properties of the PPy films. There is a distinct difference in absorption spectra (observed by color change) between the film in the reduced state and the oxidized state in the visible region [2,3], which can affect the local temperature of the electrode. If the dependence of both photothermal signal, DT, and frequency change, DF, on the potential were measured at the same time by combining the PT technique and the EQCM technique, the correlation between the doping/dedoping and the color change will be learned. EQCM is an in situ tool by measuring the mass change and able to measure nanogram range mass changes (108 to 109 g). The simplified Sauerbrey
0169-4332/$ – see front matter # 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2004.02.014
C. Zhao, Z. Jiang / Applied Surface Science 229 (2004) 372–376
373
equation for a uniform, thin film can be written as: DF ¼ 2:26 106 f02 Dm
(1)
So, Dm ¼
Df 2:26 106 f02
(2)
where DF is the observed frequency change (Hz); f0 is the fundamental oscillating frequency of the quartz (Hz); Dm is the attached mass on the quartz electrode surface (g cm2). In contrast with the EQCM, the photothermal technique belongs to photothermal spectroscopy (PTS) or photoacoustic spectroscopy (PAS). It is an in situ method for investigating the surface and structure change of an electrode by measuring the change of the local temperature of the electrode under irradiation of light at some wavelength bands. The light irradiates on the surface of the electrode, and the sensor at the backside of the electrode consisting of a polyvinylidene difluoride (PVDF) pyroelectric film that can translates the temperature difference into potential difference and is no less sensitive than 100 mV K1. The change of thickness or color of the film will affect the absorption, so they can be judged by measuring the local temperature of the electrode. The photothermal spectroscopy technique has been used to study the electrochemical deposition and redox process of nickel and polyaniline in our previous works [4–7]. In this paper, the PT technique is combined with the EQCM technique in situ to study the polymerization and redox process of PPy films.
2. Experimental The electrochemical cell for photothermal signal measurement is similar to that described elsewhere [5]. The construction is shown as Fig. 1. The working electrode used is a thin Pt foil with a thickness of 0.12 mm and an area of 0.75 cm2. The reaction area of the Pt foil electrode is controlled by epoxy resin. The counter electrode is a Pt wire electrode, and an Ag/AgCl electrode is used as reference electrode. The potentials reported in this paper are all versus this electrode. The photothermal signal measurements were carried out on two different laser instruments.
Fig. 1. Pyroelectric transducer and photothermal spectroscopic electrochemical cell. (a) Pyroelectric transducer: (1) PVDF film; (2) stainless steel; (3) Teflon; (4) lead; (5) BNC cable. (b) Photothermal spectroscopic electrochemical cell: (1) auxiliary electrode; (2) Teflon; (3) pyroelectric transducer; (4) reference electrode; (5) quartz window; (6) silicon rubber ring; (7) working electrode.
One is a PDGL-3010F semiconductor diode laser with a power of 15 mW, and the wavelength of beams is 532 nm. The other is a He–Ne laser with wavelength of 632.8 nm and power of 30 mW. The working electrode foil is thick enough to interrupt the light completely. The temperature change at the working electrode was measured using a polyvinylidene difluoride (PVDF) pyroelectric film sensor placed at the electrode’s backside [5]. A model PARC 5210 lock-in amplifier and a model PAR 197 chopper at a frequency of 11 Hz were used to measure the photothermal signal. The oscillating frequency of EQCM and current density were recorded by a personal computer interfaced to a quartz crystal analyzer (Model CHI 400 time-resolved EQCM, f0 ¼ 8 MHz) and a potentiostat (Model 400, electrochemical working station). The oscillating frequency of EQCM, photothermal signals and currents were recorded simultaneously with the potential when the films were electrochemical polymerized by potentiodynamic technique. The oscillating frequency of EQCM and photothermal signals was recorded simultaneously with the potential when the redox process of the PPy films were carried out. All solutions were prepared from twice-distilled water. All reagents are analytical except that pyrrole is spectroscopically pure. The synthesis of PPy films was carried out in the mixed solution of 0.2 M pyrrole and 0.5 M LiClO4, while CV was carried out in 0.2 M KCl. The scan rate used was 10 mV s1.
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C. Zhao, Z. Jiang / Applied Surface Science 229 (2004) 372–376 0.30
3. Results and discussions
10
Fig. 2. Attached mass changes (dash line) and current density (solid line) response during the potentiodynamical polymerization of PPy films on the Au electrode covered by a as-grown thin PPy film; SR: 10 mV s1.
-2
i (mA cm )
8 6
0.15 3
0.10 2
0.05 1
0.00 0.0
0.1
0.2
0.3
0.4
0.5
4 2
-2
The PPy film was formed first on the bare Au electrode of EQCM by the potentiodynamical technique in the potential range of 0–0.4 V. It is observed that the frequency change does not decrease but increase, which can be attributed to the desorption of the small molecule such as pyrrole monomer from the Au electrode. This means that no PPy was formed in this potential range. However, when the Au electrode covered by a thin layer film prepared potentiodynamically in 0–0.5 V for two cycles was run in the potential range of 0–0.4 V again, the oscillating frequency of electrode decreases, i.e. attached mass increases, as shown in Fig. 2, suggesting that the PPy was formed on the Au electrode this time. The phenomena indicate that the as-grown film on the Au electrode was favorable to the subsequent polymerization of pyrrole monomer. This is consistent with the results reported by John and Wallace [8]. When the PPy films were prepared on the bare Au electrode of EQCM by the same method but in the potential range of 0–0.6 V, the current density and attached mass changes are shown in Fig. 3. In the figure, the attached mass change, Dm, increased in intensity with the cycle, which was somewhat different from PPy film doped by large molecule [9,10].
4
0.20
∆m (µg cm )
3.1. Potentiodynamical polymerization process of PPy films
0.25
0
0.6
E (V vs. AgCl/Ag) Fig. 3. Attached mass changes (dash line) and current density (solid line) response during the polymerizaiton of the PPy films on the bare Au electrode by potentio-dynamical technique; SR: 10 mV s1.
For mass change in potential range of 0.4–0.6 V, a distinct increase is observed in reverse scan. The mass change in reverse scan is larger than that in forward scan, which means that the polymerization speed of pyrrole in reverse was larger than that in the forward scan. The ratio of the Dm to the polymerization charge, DQ for the PPy films in 0.4–0.6 V is listed in Table 1. In the table, the Dm/DQ in reverse scan in the former cycle was larger than that in the forward scan in the later cycle, indicating that a higher current efficiency occurs in the reverse scan. Some easily oxidized substance maybe involved in the reaction in the reverse scan. Scharifker et al. [11] proved that the oligomers were involved in the formation of polypyrrole on electrode surface, and the oligomers was easier to be oxidized than the pyrrole monomer, so this is consistent with the results in the present work. Polymerization of pyrrole on Pt electrode by potentiodynamic technique was also examined by PT technique, and the DT during the polymerization process was tested and presented in Fig. 4. It is interesting to find that the shape of the PT signal is just like that of the frequency change. The curves in each cycle of both frequency change, DF, and local temperature change, DT, are like the clock-wise rotating 908 the letter ‘‘U.’’ The change of the PT signal also increases with the cycle, and change of the PT signal in the reverse scan is also larger than that in the forward scan in the potential range of 0.4–0.6 V, which enhances the result obtained by EQCM technique.
C. Zhao, Z. Jiang / Applied Surface Science 229 (2004) 372–376
375
Table 1 The ratio of the attached mass changes to the polymerization charge of the PPy films in the potential range of 0.4–0.6 V DE (V)
0.4–0.6 0.6–0.4 0.5–0.6 0.6–0.5 0.55–0.6 0.6–0.55
Dm/DQ First cycle (101 mg cm2 mC1)
Second cycle (101 mg cm2 mC1)
Third cycle (101 mg cm2 mC1)
Fourth cycle (101 mg cm2 mC1)
1.09 1.78 1.22 1.97 1.48 2.04
1.16 2.19 1.62 2.44 1.64 2.72
1.69 2.50 1.98 2.75 2.14 3.05
2.50 3.49 2.85 3.74 3.36 3.88
3.2. Electrochemical redox behaviors of PPy films A typical curve of the attached mass change, Dm, of PPy films in one cycle was shown in Fig. 5. In the anodic scan, when the potential increases in the potential range of 0.7 to 0.2 V, the mass decreases, which means that the expulsion of cation was involved. However, for the potential change beyond 0.2 V, the mass increases until 0.5 V, which implies that the insertion of anion was involved. In the cathodic scan, the mass changes are reversed, i.e. it decreases in the range of 0.5 to 0.4 V, while it increases in the range of 0.4 to 0.7 V. Although the very large molecules anion are not doped in the PPy films, both the motion of the cation and anion are involved, and Lee et al. [12] also observed the same phenomena in acetonitrile. Li and Qian [13,14] proved
that both anions and hydrated cations moved in neutral solutions. The change of PT signal for PPy films during the redox process is shown in Fig. 6. Compared to the PT signal during the redox process of PPy films, PT signal at 0.4 V in reverse scan during the polymerization process (corresponding to the maximum in polymerizaion) is smaller than that of the PPy film at 0.4 V and larger than that at 0.7 V during redox process. This implies that PPy film during polymerization process was not fully oxidized during polymerization, though the potential was kept anodic when the PPy film was prepared, and this was confirmed by Lee et al. [12]. When the potential scans in the positive direction, the change of the PT signal includes three parts: 0.7 to 0.33, 0.33 to 0.17, 0.17 to 0.5 V. In the potential range of 0.7 to 0.33 and 0.17 to 0.5 V, the PT signal changes little, which indicates that the color of the PPy films change little, though the ion exchange occurs in
-2
∆m (µg cm )
0.5
0.0
-0.5
-1.0 -0.8
-0.6
-0.4
-0.2
0.0
0.2
0.4
0.6
E (V vs. Ag/AgCl) Fig. 4. Photothermal signal of the polypyrrole films during the polymerization by potentiodynamic technique. Light source: He– Ne laser; l: 632.8 nm; P: 30 mW; chopping frequency: 11 Hz.
Fig. 5. Attached mass changes of the polypyrrole films during the redox process; SR: 10 mV s1.
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C. Zhao, Z. Jiang / Applied Surface Science 229 (2004) 372–376
4. Conclusion
Fig. 6. Photothermal signal of the polypyrrole films prepared during the redox process. Light source: He–Ne laser; l: 632.8 nm; P: 30 mW; chopping frequency: 11 Hz.
this potential range. However, in the potential range of 0.33 to 0.17 V, the PT signal increases significantly, which suggests that the color of the PPy film changes in this potential range. When the potential scans in the negative direction, the change of the PT signal is different from that in the positive direction. It consists of two regions: 0.5 to 0.44 and 0.44 to 0.7 V. When the potential changes in the range of 0.5 to 0.4 V, the PT remains constant though the insertion of anion is involved in this potential range, indicating that the ion exchange in this potential range affects the electrochromic properties of the film little. However, when the potential continues to decrease and changes in the potential range of 0.44 to 0.7 V, the PT signal decreases dramatically, suggesting that the color change of the PPy film occurs in this potential range. Combining the EQCM and CER technique, Syritski [15] found that the conductivity of PPy had no significant change despite ion exchange and they attributed to the motion of ion in the outer layer of the PPy films, and this provides us some support. So only the ion in the inner layer was contributed to electrochromic behavior. When the green laser with wavelength 532 nm and power 15 mW was used, the shapes of curves obtained by PT technique were similar to those with the red laser, except that the intensity was smaller.
Polymerization and redox process of PPy films were studied by combining the in situ EQCM and PT techniques. It is found that as-grown film was favorable to the subsequent deposition of the PPy film. It was confirmed that the PPy films during polymerization was not fully oxidized by aid of the PT technique. Both the cation and anion were involved in the doping–dedoping process of PPy films. Although the ionexchange occurs in the whole potential range, the electrochromic properties only occurred significantly in the potential range of 0.33 to 0.17 V in the positive direction and in the potential range of 0.44 to 0.7 V in the negative direction.
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