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Morphological investigations of polypyrrole coatings on stainless steel
Synthetic Metals 156 (2006) 1280–1285
Morphological investigations of polypyrrole coatings on stainless steel A. Ashrafi a , M.A. Golozar a,∗ , S. Mallakpour b a
Department of Materials Engineering, Isfahan University of Technology, Isfahan 84156-83111, Iran b College of Chemistry, Isfahan University of Technology, Isfahan 84156-83111, Iran
Received 26 February 2006; received in revised form 6 September 2006; accepted 25 September 2006
Abstract In this paper, electrochemical polymerization of pyrrole on 316L stainless steel substrates was accomplished using a rotating disc electrode (RDE). By applying various current densities and disc rotation speeds, coatings were produced with the aid of galvanostatic technique. Experiments were performed in an aqueous solution containing 0.2 M pyrrole and 0.1 M oxalic acid. Current densities and disc rotation speed ranges were from 0.05 to 1.0 mA/cm2 and from 0 to 1500 rpm, respectively. Using a scanning electron microscope (SEM), the morphology of polypyrrole coatings was studied, and the morphology diagram was determined. The results obtained showed that various morphologies were obtained by changing the current density and/or disc rotation speed. These results also showed that apart from conventional morphologies of polypyrrole coatings reported in the literature, a new semicrystalline morphology was obtained under the conditions of very low current density (0.05 mA/cm2 ) and disc rotation speed (≤50 rpm). The degree of crystallinity of this morphology was estimated to be 68% by grazing-incidence small-angle X-ray scattering (GISAXS). The elemental analysis (CHN) revealed the ratio of semicrystalline polypyrrole to oxalic acid dopant to be 4:1. © 2006 Elsevier B.V. All rights reserved. Keywords: Polypyrrole; Electrochemical polymerization; Morphology
1. Introduction Most interesting and common properties of polymers, synthetic metals (electrically conductive polymers), and metals have led to development of several products including rechargeable batteries, corrosion resistant coatings, electrochromic displays, and bioactive devices [1–4]. Use of polypyrrole, polyaniline, polythiophene, and their derivatives as corrosion inhibitors as well as corrosion protection coatings on metallic substrates has been reported by several researchers [5–7]. Application of biocompatible and bio-functional (drug-eluting) uniform coatings of conductive polymers onto metallic medical devices, such as 316L stainless steel in cardiovascular stents and orthopedic implants has also been investigated [8]. Simultaneous formation and deposition of electrically conductive polymers are possible by electrochemical polymerization technique. Using this technique, controlling the thickness and uniformity of coatings is possible and practical. In addition,
∗
Corresponding author. Tel.: +98 311 3915736; fax: +98 311 2752. E-mail address: [email protected] (M.A. Golozar).
0379-6779/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.synthmet.2006.09.014
nontoxic and nonvolatile chemicals for polymer coating, allows this method an environmental friendly process. An aqueous electrolyte in electrochemical polymerization of pyrrole makes it more environmentally friendly process as well. Oxalic acid is widely used as dopant to produce corrosion resistant coatings on metallic substrates [9]. Some researchers have used neutral salts derived from a strong acid and a strong base to produce polypyrrole freestanding films with high electrical conductivity [10,11]. It has been shown that performance of polymer coatings depends on the nature, characteristics, and morphology. These properties are controllable with various parameters including current density, substrate rotation speed, and pH and temperature of the solution [12]. In this respect, the morphology could play an important role. It has also been reported that properties, such as adhesion, corrosion resistance and conductivity are affected by the morphology. Commercial production of electropolymerized conductive coatings and films often dictates a continuous process. For this reason, several researchers have recommended rotating cylinder electrode (RCE) or rotating disc electrode (RDE) [13,14]. Winand has reported morphological aspects of metallic coatings electrochemically deposited under various current densities and rotating disc speeds using RDE
A. Ashrafi et al. / Synthetic Metals 156 (2006) 1280–1285
and/or RCE [15]. Diagrams regarding morphology against current density and disc rotating speed provide useful and important information, and applications of the coatings and films produced [16]. In this paper electrochemical polymerization of pyrrole on 316L stainless steel substrates was performed using RDE and galvanostatic technique. The morphology diagram of the resulting coatings under various current densities and rotating disc speeds is reported. 2. Experimental 2.1. Materials and equipments Deposition of polypyrrole was carried out according to electrochemical polymerization of pyrrole using an EG&G Potentiostat/Galvanostat model A273. The working electrode (anode) was made of 316L stainless steel. Disc specimens with 15 mm diameter were prepared from 3 mm thick sheet. The rotating speed of the anode was controlled with an EG&G electrode rotator. A stainless steel sheet was used as the counter electrode (cathode). The reference electrode was Ag/AgCl. Pyrrole (98 wt.%) and oxalic acid (98 wt.%) were purchased from Merck Company, Germany. Pyrrole was distilled at ambient pressure before each experiment and kept under nitrogen gas. A 100 ml solution of 0.2 M pyrrole and 0.1 M oxalic acid in double distilled water was used for each experiment. The solution was degassed by nitrogen purging before addition of pyrrole and kept under nitrogen gas atmosphere during the experiment. Morphology of coatings was studied using scanning electron microscope (SEM). The polypyrrole coatings were investigated by grazing-incidence small-angle X-ray scattering (GISAXS) using a Philips X-ray diffractometer.
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2.4. Grazing-incidence small-angle X-ray scattering Grazing-incidence small-angle X-ray scattering was used in order to determine the degree of crystallinity of thin polypyrrole coatings without interference of substrates. For this purpose ˚ was selected. A an X-ray energy of 8.048 keV (λ = 1.54076 A) 0.5 mm × 0.5 mm incident beam slit was used in order to limit and concentrate the beam on the specimen surface at small incident angles. The incident beam had a very small angle (1◦ ) with respect to the sample surface. Therefore, the path length of the X-ray beam through the film was increased and thus any diffraction from the stainless steel substrate was omitted. The scattering measurements were conducted using a Huber fourcircle diffractometer. The sample was arranged horizontally and rotated from 10 to 90◦ (2θ). A 0.5 mm × 4.0 mm receiving slit was used because of the low scattering power and small scattering volume. 3. Results and discussion 3.1. Potential–time (E–t) diagrams Polypyrrole coatings were achieved according to electrochemical deposition of pyrrole on 316L stainless steel substrates by a galvanostatic method. Fig. 1 shows electrochemical polymerization diagrams (E–t) for various conditions. As is shown in Fig. 1a and b, application of current to the specimen, increases the potential in all cases after their respective induction periods.
2.2. Electrochemical polymerization Stainless steel specimens were mechanically polished with emery papers from #80 to #1200. After polishing, the specimens were degreased ultrasonically with isopropanol for 10 min. Electrochemical deposition was carried out using a galvanostatic method. Various current densities (0.05, 0.10, 0.25, 0.50, 0.75, and 1.0 mA/cm2 ) and disc rotation speeds (0, 50, 500, 1000, and 1500 rpm) were used. The specimen in a rotating disc electrode cell was set perpendicularly to the direction of rotation [17]. In all cases the charge (Q = It) was kept constant (0.5Q) at the polymerization potential. This means that for instance, the deposition time was 2000 s for coating of the specimen using current density of 0.1 A/cm2 . The polymerization experiments were conducted at 25 ◦ C under nitrogen atmosphere. 2.3. Elemental analysis Elemental analysis of C, H, and N was performed using an elemental analyzer Vario III (Elementar). Before the measurement, the specimens were rinsed with ethanol and kept at 50 ◦ C for 24 h to remove any absorbed water.
Fig. 1. Potential–time diagrams for various rotation speeds and current densities (a) without rotation (0 rpm) and various current densities, (b) with constant current density (0.25 mA/cm2 ) and various rotation speeds: (a) 0.05 mA/cm2 (—); 0.1 mA/cm2 (- - -); 0.25 mA/cm2 (· · ·); 0.5 mA/cm2 (- · -); 0.75 mA/cm2 ) and (b) 0 rpm (- · · -); 50 rpm (- · -); 500 rpm (· · ·); (- · · -); 1 mA/cm2 ( 1000 rpm (- - -); 1500 rpm (—).
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Fig. 2. Variation of peak potential against (a) current density and (b) disc rotation speed.
Thereafter the potential increases beyond the polymerization potential, i.e. beyond the horizontal line of potential. At this time, nucleation begins and then the potential decreases. This decrease is assumed to be due to change of surface potential accompanied with formation of the first polypyrrole nuclei. Following this stage, a very slight and gradual increase in potential is seen. This would be due to decreasing concentration of the monomer and oligomers near the surface of the specimen. Thus, the polymerization needs higher potential to proceed. This indicates that the electrochemical polymerization is based on a diffusion controlled process. By increasing the current density, the peak due to nucleation potential increases (Fig. 2a). It is believed that as the current density is increased, more monomer in the solution converts to oligomers and the chain length of the oligomers in the solution increases as well. Therefore, the potential needed to initiate coating is assumed to increase. It is also seen that the peak potential increases with increasing the disc rotation speed (Fig. 2b). It is also observed that increasing current density decreases the time needed to reach the polymerization potential (induction period) (Fig. 3a). It is believed that increasing the current density increases the possibility of the substrate to reach the polymerization potential. Therefore, the induction period would decrease. As mentioned before, rotating the specimen and/or increasing the rotation speed decrease the polymerization potential. It should be mentioned that during polymerization, hydrogen gas evolution occurred on the substrate surface. By rotating the specimen, removal of hydrogen gas from the surface is assumed to be promoted. Therefore, polymerization is assumed to take place at potentials lower than those needed in the case with no rotation. It is also observed that increasing the disc rotation speed gives rise to substantially no measurable change in induction period (Fig. 3b).
Fig. 3. Induction period as a function of (a) current density and (b) disc rotation speed.
3.2. Morphology Using SEM, morphology of various polypyrrole coatings produced on 316L stainless steel specimens was investigated. Examples are shown in Fig. 4. As shown in this figure, various morphologies, such as semicrystalline, planar, semiplanar, island-type, semispheroid, spheroid (cauli-flower), needle-like, and trunk shape were obtained. At very low current densities and disc rotation speeds, such as 0.05 mA/cm2 and 50 rpm, respectively, a semicrystalline morphology is formed. This morphology looks like a hexagonal type microstructure. The nucleation and growth of these hexagonal crystals are assumed to continue until the entire substrate surface is covered with polypyrrole coating. It must be mentioned that these crystals are different from ferrous oxalate crystals which are formed during initial stages of polymerization. This difference is shown in their GISAXS diffraction peaks in Fig. 6. The ferrous oxalate crystals have been reported to consist of monoclinic and/or orthorhombic structure [18]. The semicrystalline polypyrrole coating shows peaks at 2θ = 13, 22, 27, 29, 47, and 48◦ . The ferrous oxalate dihydrate, on the other hand, shows peaks at 2θ = 18.5, 29.1, 44.6, and 44.7◦ and the amorphous polypyrrole only a broad around 2θ = 25◦ . These observations clearly ruled out the possibility. It is also worth mentioning that the polypyrrole coating is black while the ferrous oxalate dihydrate deposit is grey. By increasing the current density or disc rotation speed, the morphology of coatings is changes from semicrystalline to needle-like through planar and spheroid. At a very high current density, such as more than 1 mA/cm2 , the rate of polymerization would be higher than the rate of deposition. Therefore,
A. Ashrafi et al. / Synthetic Metals 156 (2006) 1280–1285
some inhomogeneities, such as trunk-shape morphologies are assumed to form. These inhomogeneities are assumed to be due to the depolarization of substrate as well as hydrogen gas evolution on the surface of the specimen. As mentioned previously, the hydrogen gas is a by-product of electropolymerization of pyrrole. Fig. 5 shows a morphology diagram as a function of current density as well as disc rotation speed. With increasing current density at a constant rotation disc speed, the mor-
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phology changes in the order of planar, semiplanar, semispheroid, needle-like, and finally trunk-shape. The SEM observations of the coatings show that the smoothness decreases in the order of, semicrystalline, planar, semiplanar, island type, semispheroid, spheroid, needle-like, and trunk-shape. Thus, it could be concluded that smoothness of coatings decreases with increasing current density. It is worth mentioning that the tendency is almost the same in the metallic electrochemically deposited coatings. In the metal electrochem-
Fig. 4. Morphology of polypyrrole coatings on 316L stainless steel substrates (a) semicrystalline, (b) planar, (c) semiplanar, (d) island type, (e) semispheroid, (f) spheroid, (g) needle-like, (h) trunk-shape.
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Fig. 4. (Continued ).
ical deposition, however, the morphologies are compact crystalline, sharp crystalline, crystalline/nodular, and open nodular [11,12]. It is also observed that the area of smooth morphology increases with increasing the disc rotation speed (Fig. 5). At low current densities (such as 0.05 mA/cm2 ), the area of planar morphology increases as the disc rotation speed is increased and finally extend to other morphology zones, such as semiplanar, spheroid type, and so on.
3.3. Grazing-incidence small-angle X-ray scattering The GISAXS pattern of amorphous polypyrrole shows a single broad peak, which indicates that the elecropolymerized polypyrrole is amorphous. This amorphous peak has been reported to be observed at 2θ = 25◦ [19]. In the case of semicrystalline polypyrrole, on the other hand, three sharp peaks are observed at 2θ = 15.9, 22.5, 27.2, 29.1, 35.7, 39.3, 42.9, 47.1, and 48.3◦ . The peaks strongly indicate that the electropolymer-
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nique using rotating disc electrode in oxalic acid aqueous solution. The following conclusions were derived from the results obtained:
Fig. 5. Morphology diagram of polypyrrole coatings on 316L stainless steel substrates (Scr: semicrystalline; Pr: planar; Spr: semiplanar; I: island; Ssph: semispheroid; Sph: spheroid; N: needle-like; T: trunk-shape).
- Increasing the current density at constant rotation disc speed increases the potential peak and decreases induction period. - Increasing the rotation disc speed at constant current density decreases both the potential peak and induction period. - Various morphologies, such as semicrystalline, planar, semiplanar, island-type, semispheroid, spheroid, needle-like, and trunk-shapes were observed depending upon the conditions of the deposition. - Apart from morphologies reported previously, a new semicrystalline morphology was observed in the coating prepared at a very low current density (0.05 mA/cm2 ) and rotation disc speed lower than 50 rpm. Its crystallinity was 68%. - Increasing the rotation disc speed increases the area of smoother morphology zone. - At low rotation disc speed, increasing the current density promoted rougher morphologies, whereas at high speed, smoother morphologies. Acknowledgements The authors would like to thank the Vice Chancellor for Research and Center of Graduate Studies, Isfahan University of Technology for the financial support received. References
Fig. 6. GISAXS pattern of amorphous polypyrrole, semicrystalline polypyrrole, and iron oxalate hydrate.
ization conditions induce crystals. The degree of crystallinity of this coating is defined as follows [20]: Total area under crystalline peaks × 100% Total area under crystalline and amorphous peaks Using the fitted results obtained from GISAXS patterns (Fig. 6), the degree of crystallinity of semicrystalline polypyrrole coatings produced was estimated to be 68%. 3.4. Elemental analysis The elemental analysis result of semicrystalline sample is as follows: C = 52.35%, H = 3.79% and N = 16.33%. These results are in good agreement with calculated percentage for carbon (61.35%), hydrogen (3.79%), and nitrogen (15.90%) content in the polymer repeating unit. The results show that the ratio of pyrrole to dopant (oxalate) molecules is 4:1. 4. Summary and conclusion Electrochemical deposition of pyrrole on 316L stainless steel substrates was successfully carried out by galvanostatic tech-
[1] B. Lakard, G. Herlem, S. Lakard, R. Guyetant, B. Fahys, Polymer 46 (2005) 12233. [2] A.A. Hermas, M. Nakayama, K. Ogura, Electrochim. Acta 50 (2005) 3640. [3] H.K. Hans, A.D. Child, W.C. Kimbrell, Synth. Met. 71 (1995) 2139. [4] S. Kuwabata, S. Masui, H. Tomiyori, H. Yoneyama, Electrochim. Acta 46 (2000) 91. [5] M.A. Malik, R. Wlodarczyk, P.J. Kulesza, H. Bala, K. Miecznikowski, Corros. Sci. 47 (2005) 771. [6] J.O. Iroh, W. Su, Electrochim. Acta 46 (2000) 15. [7] A. Ashrafi, M.A. Golozar, S. Mallakpour, Iran Polym. J. 12 (2003) 485. [8] Z. Weiss, D. Mandler, G. Shustak, A.J. Domb, J. Polym. Sci. A 42 (2004) 1658. [9] J.O. Iroh, W. Su, Electrochim. Acta 46 (2000) 15. [10] M. Ogasawara, K. Funahashi, T. Demura, T. Hagiwara, K. Iwata, Synth. Met. 14 (1986) 61. [11] M. Yamaura, K. Sato, T. Hagiwara, K. Iwata, Synth. Met. 48 (1992) 337. [12] W. Su, J.O. Iroh, Synth. Met. 95 (1998) 159. [13] P. Kathirgamanathan, M.M.B. Qayyum, J. Electrochem. Soc. 141 (1994) 147. [14] P. Kathirgamanathan, A.M. Souter, D. Baluch, J. Appl. Electrochem. 24 (1994) 283. [15] R. Winand, Electrochim. Acta 39 (1994) 1091. [16] D.R. Gabe, G.D. Wilcox, J.C. Garcia, F.C. Walsh, J. Appl. Electrochem. 28 (1998) 759. [17] V.S. Kolosnitsyn, O.A. Yapryntseva, Russ. J. Appl. Chem. 77 (2004) 222. [18] W. Su, J.O. Iroh, Electrochim. Acta 44 (1999) 4655. [19] Y. Nogami, J.P. Pouget, T. Ishiguro, Synth. Met. 62 (1994) 257. [20] L. Mandelkern, Crystallization of Polymers, second ed., Cambridge University Press, 2002.
Morphological investigations of polypyrrole coatings on stainless steel A. Ashrafi a , M.A. Golozar a,∗ , S. Mallakpour b a
Department of Materials Engineering, Isfahan University of Technology, Isfahan 84156-83111, Iran b College of Chemistry, Isfahan University of Technology, Isfahan 84156-83111, Iran
Received 26 February 2006; received in revised form 6 September 2006; accepted 25 September 2006
Abstract In this paper, electrochemical polymerization of pyrrole on 316L stainless steel substrates was accomplished using a rotating disc electrode (RDE). By applying various current densities and disc rotation speeds, coatings were produced with the aid of galvanostatic technique. Experiments were performed in an aqueous solution containing 0.2 M pyrrole and 0.1 M oxalic acid. Current densities and disc rotation speed ranges were from 0.05 to 1.0 mA/cm2 and from 0 to 1500 rpm, respectively. Using a scanning electron microscope (SEM), the morphology of polypyrrole coatings was studied, and the morphology diagram was determined. The results obtained showed that various morphologies were obtained by changing the current density and/or disc rotation speed. These results also showed that apart from conventional morphologies of polypyrrole coatings reported in the literature, a new semicrystalline morphology was obtained under the conditions of very low current density (0.05 mA/cm2 ) and disc rotation speed (≤50 rpm). The degree of crystallinity of this morphology was estimated to be 68% by grazing-incidence small-angle X-ray scattering (GISAXS). The elemental analysis (CHN) revealed the ratio of semicrystalline polypyrrole to oxalic acid dopant to be 4:1. © 2006 Elsevier B.V. All rights reserved. Keywords: Polypyrrole; Electrochemical polymerization; Morphology
1. Introduction Most interesting and common properties of polymers, synthetic metals (electrically conductive polymers), and metals have led to development of several products including rechargeable batteries, corrosion resistant coatings, electrochromic displays, and bioactive devices [1–4]. Use of polypyrrole, polyaniline, polythiophene, and their derivatives as corrosion inhibitors as well as corrosion protection coatings on metallic substrates has been reported by several researchers [5–7]. Application of biocompatible and bio-functional (drug-eluting) uniform coatings of conductive polymers onto metallic medical devices, such as 316L stainless steel in cardiovascular stents and orthopedic implants has also been investigated [8]. Simultaneous formation and deposition of electrically conductive polymers are possible by electrochemical polymerization technique. Using this technique, controlling the thickness and uniformity of coatings is possible and practical. In addition,
∗
Corresponding author. Tel.: +98 311 3915736; fax: +98 311 2752. E-mail address: [email protected] (M.A. Golozar).
0379-6779/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.synthmet.2006.09.014
nontoxic and nonvolatile chemicals for polymer coating, allows this method an environmental friendly process. An aqueous electrolyte in electrochemical polymerization of pyrrole makes it more environmentally friendly process as well. Oxalic acid is widely used as dopant to produce corrosion resistant coatings on metallic substrates [9]. Some researchers have used neutral salts derived from a strong acid and a strong base to produce polypyrrole freestanding films with high electrical conductivity [10,11]. It has been shown that performance of polymer coatings depends on the nature, characteristics, and morphology. These properties are controllable with various parameters including current density, substrate rotation speed, and pH and temperature of the solution [12]. In this respect, the morphology could play an important role. It has also been reported that properties, such as adhesion, corrosion resistance and conductivity are affected by the morphology. Commercial production of electropolymerized conductive coatings and films often dictates a continuous process. For this reason, several researchers have recommended rotating cylinder electrode (RCE) or rotating disc electrode (RDE) [13,14]. Winand has reported morphological aspects of metallic coatings electrochemically deposited under various current densities and rotating disc speeds using RDE
A. Ashrafi et al. / Synthetic Metals 156 (2006) 1280–1285
and/or RCE [15]. Diagrams regarding morphology against current density and disc rotating speed provide useful and important information, and applications of the coatings and films produced [16]. In this paper electrochemical polymerization of pyrrole on 316L stainless steel substrates was performed using RDE and galvanostatic technique. The morphology diagram of the resulting coatings under various current densities and rotating disc speeds is reported. 2. Experimental 2.1. Materials and equipments Deposition of polypyrrole was carried out according to electrochemical polymerization of pyrrole using an EG&G Potentiostat/Galvanostat model A273. The working electrode (anode) was made of 316L stainless steel. Disc specimens with 15 mm diameter were prepared from 3 mm thick sheet. The rotating speed of the anode was controlled with an EG&G electrode rotator. A stainless steel sheet was used as the counter electrode (cathode). The reference electrode was Ag/AgCl. Pyrrole (98 wt.%) and oxalic acid (98 wt.%) were purchased from Merck Company, Germany. Pyrrole was distilled at ambient pressure before each experiment and kept under nitrogen gas. A 100 ml solution of 0.2 M pyrrole and 0.1 M oxalic acid in double distilled water was used for each experiment. The solution was degassed by nitrogen purging before addition of pyrrole and kept under nitrogen gas atmosphere during the experiment. Morphology of coatings was studied using scanning electron microscope (SEM). The polypyrrole coatings were investigated by grazing-incidence small-angle X-ray scattering (GISAXS) using a Philips X-ray diffractometer.
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2.4. Grazing-incidence small-angle X-ray scattering Grazing-incidence small-angle X-ray scattering was used in order to determine the degree of crystallinity of thin polypyrrole coatings without interference of substrates. For this purpose ˚ was selected. A an X-ray energy of 8.048 keV (λ = 1.54076 A) 0.5 mm × 0.5 mm incident beam slit was used in order to limit and concentrate the beam on the specimen surface at small incident angles. The incident beam had a very small angle (1◦ ) with respect to the sample surface. Therefore, the path length of the X-ray beam through the film was increased and thus any diffraction from the stainless steel substrate was omitted. The scattering measurements were conducted using a Huber fourcircle diffractometer. The sample was arranged horizontally and rotated from 10 to 90◦ (2θ). A 0.5 mm × 4.0 mm receiving slit was used because of the low scattering power and small scattering volume. 3. Results and discussion 3.1. Potential–time (E–t) diagrams Polypyrrole coatings were achieved according to electrochemical deposition of pyrrole on 316L stainless steel substrates by a galvanostatic method. Fig. 1 shows electrochemical polymerization diagrams (E–t) for various conditions. As is shown in Fig. 1a and b, application of current to the specimen, increases the potential in all cases after their respective induction periods.
2.2. Electrochemical polymerization Stainless steel specimens were mechanically polished with emery papers from #80 to #1200. After polishing, the specimens were degreased ultrasonically with isopropanol for 10 min. Electrochemical deposition was carried out using a galvanostatic method. Various current densities (0.05, 0.10, 0.25, 0.50, 0.75, and 1.0 mA/cm2 ) and disc rotation speeds (0, 50, 500, 1000, and 1500 rpm) were used. The specimen in a rotating disc electrode cell was set perpendicularly to the direction of rotation [17]. In all cases the charge (Q = It) was kept constant (0.5Q) at the polymerization potential. This means that for instance, the deposition time was 2000 s for coating of the specimen using current density of 0.1 A/cm2 . The polymerization experiments were conducted at 25 ◦ C under nitrogen atmosphere. 2.3. Elemental analysis Elemental analysis of C, H, and N was performed using an elemental analyzer Vario III (Elementar). Before the measurement, the specimens were rinsed with ethanol and kept at 50 ◦ C for 24 h to remove any absorbed water.
Fig. 1. Potential–time diagrams for various rotation speeds and current densities (a) without rotation (0 rpm) and various current densities, (b) with constant current density (0.25 mA/cm2 ) and various rotation speeds: (a) 0.05 mA/cm2 (—); 0.1 mA/cm2 (- - -); 0.25 mA/cm2 (· · ·); 0.5 mA/cm2 (- · -); 0.75 mA/cm2 ) and (b) 0 rpm (- · · -); 50 rpm (- · -); 500 rpm (· · ·); (- · · -); 1 mA/cm2 ( 1000 rpm (- - -); 1500 rpm (—).
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Fig. 2. Variation of peak potential against (a) current density and (b) disc rotation speed.
Thereafter the potential increases beyond the polymerization potential, i.e. beyond the horizontal line of potential. At this time, nucleation begins and then the potential decreases. This decrease is assumed to be due to change of surface potential accompanied with formation of the first polypyrrole nuclei. Following this stage, a very slight and gradual increase in potential is seen. This would be due to decreasing concentration of the monomer and oligomers near the surface of the specimen. Thus, the polymerization needs higher potential to proceed. This indicates that the electrochemical polymerization is based on a diffusion controlled process. By increasing the current density, the peak due to nucleation potential increases (Fig. 2a). It is believed that as the current density is increased, more monomer in the solution converts to oligomers and the chain length of the oligomers in the solution increases as well. Therefore, the potential needed to initiate coating is assumed to increase. It is also seen that the peak potential increases with increasing the disc rotation speed (Fig. 2b). It is also observed that increasing current density decreases the time needed to reach the polymerization potential (induction period) (Fig. 3a). It is believed that increasing the current density increases the possibility of the substrate to reach the polymerization potential. Therefore, the induction period would decrease. As mentioned before, rotating the specimen and/or increasing the rotation speed decrease the polymerization potential. It should be mentioned that during polymerization, hydrogen gas evolution occurred on the substrate surface. By rotating the specimen, removal of hydrogen gas from the surface is assumed to be promoted. Therefore, polymerization is assumed to take place at potentials lower than those needed in the case with no rotation. It is also observed that increasing the disc rotation speed gives rise to substantially no measurable change in induction period (Fig. 3b).
Fig. 3. Induction period as a function of (a) current density and (b) disc rotation speed.
3.2. Morphology Using SEM, morphology of various polypyrrole coatings produced on 316L stainless steel specimens was investigated. Examples are shown in Fig. 4. As shown in this figure, various morphologies, such as semicrystalline, planar, semiplanar, island-type, semispheroid, spheroid (cauli-flower), needle-like, and trunk shape were obtained. At very low current densities and disc rotation speeds, such as 0.05 mA/cm2 and 50 rpm, respectively, a semicrystalline morphology is formed. This morphology looks like a hexagonal type microstructure. The nucleation and growth of these hexagonal crystals are assumed to continue until the entire substrate surface is covered with polypyrrole coating. It must be mentioned that these crystals are different from ferrous oxalate crystals which are formed during initial stages of polymerization. This difference is shown in their GISAXS diffraction peaks in Fig. 6. The ferrous oxalate crystals have been reported to consist of monoclinic and/or orthorhombic structure [18]. The semicrystalline polypyrrole coating shows peaks at 2θ = 13, 22, 27, 29, 47, and 48◦ . The ferrous oxalate dihydrate, on the other hand, shows peaks at 2θ = 18.5, 29.1, 44.6, and 44.7◦ and the amorphous polypyrrole only a broad around 2θ = 25◦ . These observations clearly ruled out the possibility. It is also worth mentioning that the polypyrrole coating is black while the ferrous oxalate dihydrate deposit is grey. By increasing the current density or disc rotation speed, the morphology of coatings is changes from semicrystalline to needle-like through planar and spheroid. At a very high current density, such as more than 1 mA/cm2 , the rate of polymerization would be higher than the rate of deposition. Therefore,
A. Ashrafi et al. / Synthetic Metals 156 (2006) 1280–1285
some inhomogeneities, such as trunk-shape morphologies are assumed to form. These inhomogeneities are assumed to be due to the depolarization of substrate as well as hydrogen gas evolution on the surface of the specimen. As mentioned previously, the hydrogen gas is a by-product of electropolymerization of pyrrole. Fig. 5 shows a morphology diagram as a function of current density as well as disc rotation speed. With increasing current density at a constant rotation disc speed, the mor-
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phology changes in the order of planar, semiplanar, semispheroid, needle-like, and finally trunk-shape. The SEM observations of the coatings show that the smoothness decreases in the order of, semicrystalline, planar, semiplanar, island type, semispheroid, spheroid, needle-like, and trunk-shape. Thus, it could be concluded that smoothness of coatings decreases with increasing current density. It is worth mentioning that the tendency is almost the same in the metallic electrochemically deposited coatings. In the metal electrochem-
Fig. 4. Morphology of polypyrrole coatings on 316L stainless steel substrates (a) semicrystalline, (b) planar, (c) semiplanar, (d) island type, (e) semispheroid, (f) spheroid, (g) needle-like, (h) trunk-shape.
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Fig. 4. (Continued ).
ical deposition, however, the morphologies are compact crystalline, sharp crystalline, crystalline/nodular, and open nodular [11,12]. It is also observed that the area of smooth morphology increases with increasing the disc rotation speed (Fig. 5). At low current densities (such as 0.05 mA/cm2 ), the area of planar morphology increases as the disc rotation speed is increased and finally extend to other morphology zones, such as semiplanar, spheroid type, and so on.
3.3. Grazing-incidence small-angle X-ray scattering The GISAXS pattern of amorphous polypyrrole shows a single broad peak, which indicates that the elecropolymerized polypyrrole is amorphous. This amorphous peak has been reported to be observed at 2θ = 25◦ [19]. In the case of semicrystalline polypyrrole, on the other hand, three sharp peaks are observed at 2θ = 15.9, 22.5, 27.2, 29.1, 35.7, 39.3, 42.9, 47.1, and 48.3◦ . The peaks strongly indicate that the electropolymer-
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nique using rotating disc electrode in oxalic acid aqueous solution. The following conclusions were derived from the results obtained:
Fig. 5. Morphology diagram of polypyrrole coatings on 316L stainless steel substrates (Scr: semicrystalline; Pr: planar; Spr: semiplanar; I: island; Ssph: semispheroid; Sph: spheroid; N: needle-like; T: trunk-shape).
- Increasing the current density at constant rotation disc speed increases the potential peak and decreases induction period. - Increasing the rotation disc speed at constant current density decreases both the potential peak and induction period. - Various morphologies, such as semicrystalline, planar, semiplanar, island-type, semispheroid, spheroid, needle-like, and trunk-shapes were observed depending upon the conditions of the deposition. - Apart from morphologies reported previously, a new semicrystalline morphology was observed in the coating prepared at a very low current density (0.05 mA/cm2 ) and rotation disc speed lower than 50 rpm. Its crystallinity was 68%. - Increasing the rotation disc speed increases the area of smoother morphology zone. - At low rotation disc speed, increasing the current density promoted rougher morphologies, whereas at high speed, smoother morphologies. Acknowledgements The authors would like to thank the Vice Chancellor for Research and Center of Graduate Studies, Isfahan University of Technology for the financial support received. References
Fig. 6. GISAXS pattern of amorphous polypyrrole, semicrystalline polypyrrole, and iron oxalate hydrate.
ization conditions induce crystals. The degree of crystallinity of this coating is defined as follows [20]: Total area under crystalline peaks × 100% Total area under crystalline and amorphous peaks Using the fitted results obtained from GISAXS patterns (Fig. 6), the degree of crystallinity of semicrystalline polypyrrole coatings produced was estimated to be 68%. 3.4. Elemental analysis The elemental analysis result of semicrystalline sample is as follows: C = 52.35%, H = 3.79% and N = 16.33%. These results are in good agreement with calculated percentage for carbon (61.35%), hydrogen (3.79%), and nitrogen (15.90%) content in the polymer repeating unit. The results show that the ratio of pyrrole to dopant (oxalate) molecules is 4:1. 4. Summary and conclusion Electrochemical deposition of pyrrole on 316L stainless steel substrates was successfully carried out by galvanostatic tech-
[1] B. Lakard, G. Herlem, S. Lakard, R. Guyetant, B. Fahys, Polymer 46 (2005) 12233. [2] A.A. Hermas, M. Nakayama, K. Ogura, Electrochim. Acta 50 (2005) 3640. [3] H.K. Hans, A.D. Child, W.C. Kimbrell, Synth. Met. 71 (1995) 2139. [4] S. Kuwabata, S. Masui, H. Tomiyori, H. Yoneyama, Electrochim. Acta 46 (2000) 91. [5] M.A. Malik, R. Wlodarczyk, P.J. Kulesza, H. Bala, K. Miecznikowski, Corros. Sci. 47 (2005) 771. [6] J.O. Iroh, W. Su, Electrochim. Acta 46 (2000) 15. [7] A. Ashrafi, M.A. Golozar, S. Mallakpour, Iran Polym. J. 12 (2003) 485. [8] Z. Weiss, D. Mandler, G. Shustak, A.J. Domb, J. Polym. Sci. A 42 (2004) 1658. [9] J.O. Iroh, W. Su, Electrochim. Acta 46 (2000) 15. [10] M. Ogasawara, K. Funahashi, T. Demura, T. Hagiwara, K. Iwata, Synth. Met. 14 (1986) 61. [11] M. Yamaura, K. Sato, T. Hagiwara, K. Iwata, Synth. Met. 48 (1992) 337. [12] W. Su, J.O. Iroh, Synth. Met. 95 (1998) 159. [13] P. Kathirgamanathan, M.M.B. Qayyum, J. Electrochem. Soc. 141 (1994) 147. [14] P. Kathirgamanathan, A.M. Souter, D. Baluch, J. Appl. Electrochem. 24 (1994) 283. [15] R. Winand, Electrochim. Acta 39 (1994) 1091. [16] D.R. Gabe, G.D. Wilcox, J.C. Garcia, F.C. Walsh, J. Appl. Electrochem. 28 (1998) 759. [17] V.S. Kolosnitsyn, O.A. Yapryntseva, Russ. J. Appl. Chem. 77 (2004) 222. [18] W. Su, J.O. Iroh, Electrochim. Acta 44 (1999) 4655. [19] Y. Nogami, J.P. Pouget, T. Ishiguro, Synth. Met. 62 (1994) 257. [20] L. Mandelkern, Crystallization of Polymers, second ed., Cambridge University Press, 2002.