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Mechanism of underpotential deposition of metal on conducting polymers
Synthetic Metals 126 (2002) 337–345
Mechanism of underpotential deposition of metal on conducting polymers Yu-Chuan Liua,*, Kuang-Hsuan Yangb, Ming-Der Gerb a
Department of Chemical Engineering, Van Nung Institute of Technology, 1 Van Nung Road, Shuei-Wei Li, Chung-Li, Tao-Yuan 320, Taiwan, ROC b Department of Applied Chemistry, Chung Cheng Institute of Technology, University of National Defence 190, Sanyuan 1st St., Dashi Jen, Taoyuan, Taiwan, ROC Received 29 May 2001; received in revised form 9 October 2001; accepted 16 October 2001
Abstract From the idea of underpotential deposition (UPD), the electrochemically depositing and dissolving processes of copper (Cu) onto and from polypyrrole (PPy) were investigated. A mechanism was proposed to illustrate the whole metallization process. The results indicate that two forms of coppers, valence and elemental ones, can be formed on PPy at a constant cathodic potential. However, only the valence Cu can be left on PPy at the anodic stripping. From the X-ray photoelectron spectroscopy (XPS) and surface-enhanced Raman scattering (SERS) analyses, it is found that the valence Cu is interacted with the pyrrolylium nitrogen, resulting from an electron transfer from the Cu atom to the PPy nitrogen. The effect of the deposition methods of Cu onto PPy on its corresponding property was also discussed. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Depositing process; Valence copper; Polypyrrole; Underpotential deposition
1. Introduction Among the conjugating conducting polymers, polypyrrole (PPy) [1–4] is the most representative one for its easy polymerization and controllable property of switching it between conducting and insulating states by doping and undoping counterions into the polymer matrix, as well as its chemical and thermal stabilities. These characteristics make it widely used in microelectronic devices [5,6], batteries [7,8] and gas sensors [9,10]. It was also been found that the conductivity and the sensing behavior of PPy could be further improved by imbedding metal particles into the polymer matrix to form a metal–polymer complex [11–13]. Among a variety of methods for polymer metallization, electrodeposition via aqueous solutions is the most promising because of its convenience, good economy, and the wealth of current plating knowledge [11,14,15]. As is generally known, many underpotential deposition (UPD) elements can form a monolayer on the foreign metal substrate, they display catalytic effects for various electrochemical
*
Corresponding author. Tel.: þ886-3-451-5811x254; fax: þ886-2-8663-8557. E-mail address: [email protected] (Y.-C. Liu).
reactions [16–19]. Several mechanisms [20–22] have been proposed to explain the electrocatalytic characteristics, including modification of the electronic structure of the substrate, changes in the physical structure of the substrate and blocking of the adsorption of poisons. Meanwhile, the metal-modified PPy films, like other UPD species, also demonstrate enhanced effects on many electrochemical reactions. In 1986, Tourillon et al. [23] proposed a mechanism to illustrate the poly-3-methylthiophene–Cu interaction. In 1996, Rau et al. [24] also reported their studies on the interaction of Au (I)–PPy complex. In recent years, many researchers had focused their studies on the modification of conductive polymers with metals and the characteristics of the prepared complexes [25–28]. However, a detailed understanding of the electrodeposition process of metals onto the conductive polymers and the interaction of metalconducting polymer with the site of adsorbed metal was yet to be developed. Because copper is electrochemically active and readily undergoes oxidation/reduction from Cu (0) to Cu (I) or Cu (II), the UPD concept was employed to electrodeposit copper onto PPy and electrodissolve elemental copper from it in an aqueous solution in this work. The characteristics of copper and PPy on each step were examined. A mechanism was proposed to explicate these electrochemical processes.
0379-6779/02/$ – see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 9 - 6 7 7 9 ( 0 1 ) 0 0 5 8 1 - 1
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Also, two different methods for Cu electrodeposition were used to compare the specific properties of the prepared Cu-modified PPy.
2. Experimental 2.1. Formation of the valence Cu onto PPy The electropolymerization of PPy, the deposition of Cu onto PPy, and the dissolution of elemental Cu from PPy were performed in a three-compartment cell. In all experiments except for surface-enhanced Raman scattering (SERS) analysis, a sheet of platinum (a sheet of gold for the SERS experiment) foil with a working area of 0.238 cm2, a 2 cm 2 cm platinum foil and a silver–silver chloride (Ag/AgCl) electrode in a saturated KCl aqueous solution were employed as the working, counter and reference electrodes, respectively. All the potential used in the text was referred to Ag/AgCl. First, the electropolymerization of PPy was carried out at a constant anodic potential of 0.8 V in a deoxygenated aqueous solution containing 0.1 M pyrrole and 0.1 M LiClO4 (called oxidized PPy), followed by depositing Cu onto PPy at a constant cathodic potential of 0.05 V in a deoxygenated aqueous solution containing 0.02 M CuSO4 and 0.1 M H2SO4 (called elemental Cu and valence Cumodified PPy). In this Cu deposition, both elemental Cu (0) and valence Cu were deposited onto the PPy film. Then the deposited Cu was dissolved at 0.4 V in 0.1 M H2SO4 until all the elemental Cu had been completely dissolved, but the valence Cu onto PPy remained (called valence Cu-modified PPy). For comparison, except for stepping potential, pulse potential was also used to deposit Cu onto PPy. In depositing Cu with pulse potential, the potential was kept at 0.05 V for 1 s for Cu deposition, followed by turning off the cell for 30 s for Cu2þ ions equilibrium in each cycle. Besides stating in addition, the method of stepping potential was adopted in the text. In SERS experiments, the Au electrode was roughened by oxidation–reduction cycle (ORC) in a separate cell before the electropolymerization of PPy. The electrode was cycled in 0.1 M KCl from 0.28 V (holding 10 s) to 1.22 V (holding 2 s) at 500 mV s1 for 25 times. After every step the electrodes were rinsed throughout with deionized water, and finally dried in a vacuum dryer for 24 h at room temperature. Then the samples were placed in a desiccator with nitrogen before use. For different experimental requirements, the charges passed in depositing PPy for the conductivity, scanning electron microscopy (SEM), secondary ion mass spectroscopy (SIMS), rotating disk electrode (RDE), SERS and X-ray photoelectron spectroscopy (XPS) experiments were 1500, 250, 250, 100, 25 and 250 mC, respectively. All electrochemical experiments were controlled by a potentiostat (Model PGSTAT20, Eco Chemie), and were performed at room temperature 24 8C.
2.2. Characteristics of the valence Cu onto PPy The conductivities of PPy-based films were determined by using a commercial instrument (Model RT-7, Napson) applying the four-probe technique with a direct current (DC) measurement at room temperature. The distribution of the deposited Cu in the PPy matrix was measured via SIMS (Model IMS-4f, Cameca) using oxygen atoms as the primary ions. For the XPS measurements a physical electronics PHI 1600 spectrometer with monochromatized Mg Ka radiation, 15 kV, 250 W, and an energy resolution of 0.1–0.8% DE/E was used. To compensate for surface charging effects, all XPS spectra are referred to the C 1s neutral carbon peak at 284.6 eV. The complex XPS peaks are deconvolved into component Gaussian peaks using the ‘‘automatic fitting’’ program provided with the XPS spectrometer. Raman spectra were obtained using a Renishaw 2000 Raman spectrometer employing a He–Ne laser of 25 mW and a charge couple device (CCD) detector with 1 cm1 resolution. The surface morphology of PPy-based films was obtained from SEM (Model 8360, Cambridge).
3. Results and discussion 3.1. Cu deposition and dissolution onto and from PPy From the viewpoint of UPD metal on a foreign metal (substrate), there are two different deposition processes [22,27], namely the two-dimensional (2D) metal adsorption deposition and the three-dimensional (3D) metal bulk deposition. 2D and 3D metal depositions are ascribed to underpotential and overpotential depositions (OPDs), respectively. Meanwhile, the electronic structure of the substrate strongly depends on the mass and charge transfer of UPD metal across the interface. Although the mechanism of the UPD metal onto the conductive polymer is not well defined, the phenomena for the deposition of Cu onto PPy observed in our experiments is similar with that of UPD metal on a metal substrate. Fig. 1 shows the I–E curve for Cu depositing and dissolving onto and from PPy and the Pt substrate. Inspecting the depositing and dissolving curves (a) and (b) in Fig. 1, it was found that metal copper was significantly deposited onto PPy at a potential more negative than 0.1 V vs. Ag/AgCl in the cathodic scan. Although the anodic dissolution peak of Cu from the PPy film is not clear enough, the deposited Cu can be markedly dissolved at a potential more positive than 0.3 V vs. Ag/AgCl in comparison with Cu dissolution from the Pt substrate. To adopt the idea from UPD, the potentials used for Cu deposition and dissolution were 0.05 and 0.4 vs. Ag/AgCl, respectively, in the text. As we known the undoping of PPy may compete with Cu depositing onto PPy at potential of 0.05 V vs. Ag/AgCl. The dominant reaction cannot be concluded from just observing the depositing process. However, these competitive
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passed, the relative charges applied to the Cu deposition (Cu2þ reducing to Cuþ, as shown later) and the undoping of PPy can be roughly estimated as follows:
Fig. 1. Cyclic voltammograms at 25 mV s1 showing the deposition– dissolution behaviors of Cu on the PPy film (curve a) and on the Pt substrate (curve b) in 0.02 M CuSO4 and 0.1 M H2SO4.
reactions can be illustrated from the Cu deposition in situ observing the mass change via an electrochemical quartz crystal microbalance (EQCM), as shown in Fig. 2. As we known, the undoping of PPy results in the repulsion of counterion ClO4 from it [15]. Thus, the mass decrease of PPy film will correspond to the frequency increase of EQCM. However, the Cu deposition onto PPy film results in mass increase corresponding to frequency decrease. Inspecting Fig. 2, the frequency decrease demonstrates two different processes in depositing Cu onto PPy. In the first deposition process, namely before ca. 4.2 mC was
Q1 þ Q2 ¼ 0:0042
(1)
Q1 63:5 Q2 99:5 ¼ 1:105 109 1530 96500 96500
(2)
where Q1 (C) and Q2 (C) are the charges applied to the Cu deposition and the PPy undoping, respectively. The Faraday’s constant is 96500 C mol1. The atomic weight of Cu and the molecular weight of ClO4 are 63.5 and 99.5, respectively. The mass sensitivity is 1:105109 g Hz1 [29] and the corresponding frequency decrease at applied charge of 42 mC is ca. 1530 Hz. Combining Eqs. (1) and (2), Q1 and Q2 are solved to be 0.00356 and 0.00063 C, respectively. Consequently, 85% quantity of charge is applied to Cu depositing onto PPy at 0.05 V vs. Ag/AgCl. However, the undoping of PPy more or less can occur at this cathodic potential. 3.2. Characteristics of Cu deposition and dissolution onto and from PPy At the UPD potential, valence Cu is expectantly deposited onto PPy and forms a more stable Cu–PPy complex than elemental Cu does. These phenomena show good agreement with the observations from Fig. 3. This figure shows the dissolution curves on the RDE at 1000 rpm after depositing 40, 20, 10 and 0 mC Cu onto 100 mC PPy. It is interesting that the anodic dissolution of deposited Cu disappears when the charge passed for depositing Cu is less than 10 mC. This indicates that a prior deposition of valence Cu onto PPy is formed with a stronger interaction, and this kind of Cu deposition cannot be removed in dissolution treatment.
Fig. 2. Coulometric curves of 16 mC Cu deposited onto 40 mC PPy (curve a) and corresponding resonance frequency (curve b) vs. time.
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Fig. 3. Anodic dissolution at polarization potential of 0.4 V vs. Ag/AgCl on RDE at 1000 rpm of deposited Cu with different charges passed from 100 mC PPy: (a) 40 mC; (b) 20 mC; (c) 10 mC; (d) 0 mC.
In this work, two different methods were used in depositing Cu onto PPy. They were expected to demonstrate different properties of the prepared Cu-modified PPy. Fig. 4 shows the depth profiles of deposited Cu onto 250 mC PPy with different methods. As shown in curve (d) of Fig. 4, the thickness of the PPy film is about 2.3 mm, which is measured from the point of inflection on the intensity/depth profile of
Fig. 4. Depth profiles of deposited Cu onto 250 mC PPy matrix in depositing Cu with different charges passed by different methods: (a) 50 mC passed by stepping potential; (b) 25 mC passed by stepping potential; (c) 25 mC passed by pulse potential; (d) depth profile of the Pt substrate used in the sample of curve (a).
the Pt substrate. This film thickness is consistent with 500 mC cm2 corresponding to 1 mm estimated by a correlation between thickness and charge passed, as reported in the literature [30]. As shown in curve (b), the charge passed in depositing Cu by stepping potential being one-tenth of that passed in depositing PPy, most of the deposited Cu is concentrated on the surface of PPy. As for the deposition process beginning at the PPy surface or at the Pt substrate, it can be revealed from the conductivity of PPy at 0.05 V in Cu deposition. As discussed before, the undoping of PPy is slight at this cathodic potential. And it still maintains the conducting property, as confirmed later. According to the similar result reported by Jovic et al. [28], metals start to be deposited on the surface of conducting polymers. Therefore, the Cu deposition undoubtedly begins at the surface of PPy. It may also result from the diffusion-controlled reaction in depositing Cu via stepping potential. Then it is deposited at the open pores of PPy and the deposition is toward the Pt substrate. When the charge passed in depositing Cu is double, as shown in curve (a), the spatial distribution of deposited Cu onto PPy matrix can be uniform, namely PPy can be evenly modified throughout the film. From the economic viewpoint, the PPy can be also evenly modified by depositing Cu via pulse potential with only one-tenth charge of that passed in depositing PPy, as shown in curve (c) of Fig. 4. As we know, the advantage in depositing Cu with the pulse potential method is that less problem about ions diffusing into the polymer matrix exist. Thus, Cu can be evenly deposited onto the whole PPy matrix at every pulse deposition potential. The similar reports were shown in the literature [31,32]. Room temperature conductivity measured in air is listed in Table 1. As expected, the conductivity of the PPy modified with Cu deposition significantly increases by elemental Cu. The valence Cu forming a complex with PPy also exhibits an excellent conductivity. It suggested that the valence Cu can intrinsically improve the PPy conductivity. During Cu deposition, the current passed at a cathodic potential corresponds two different processes, the Cu deposition and the undoping of the oxidized PPy. For a precise comparison, the conductivity of the reduced PPy is measured under the same conditions, i.e. treated at the potential of Cu deposition in 0.1 M H2SO4 without Cu2þ ions for 70 s, which is the same period of time as that used in Cu deposition. The small decrease in conductivity reveals that the undoping process is unimportant and the Cu deposition onto PPy dominates the whole reaction under these conditions. Also, the PPy maintains its conducting property in Cu deposition. Fig. 5 shows the SEM photographs of the distribution of the Cu deposited on the surface of PPy. The right parts of these figures represent the backscattering electron images (BEIs) of the left parts of the identical figures, which reflect brighter images for elements with higher atomic numbers. It is helpful to distinguish the Cu particle from the PPy matrix. Comparing Fig. 5(a) and (b), it is obvious that the Cu
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Table 1 Conductivities of various 1500 mC PPy-based films PPy-based films
Conductivity (S cm1)
Oxidized PPya
Partially reduced PPyb
Elemental Cu and valence Cu-modified PPyc
Elemental Cu and valence Cu-modified Ppyd
Valence Cu-modified PPye
30.2
27.3
102
118
62.3
a
Oxidized PPy prepared at 0.8 V in a solution containing 0.1 M pyrrole and 0.1 M LiClO4. Oxidized PPy treated after (a) by reducing at 0.05 V in 0.1 M H2SO4 without Cu2þ ions for 70 s. c Cu deposited onto the oxidized PPy after (a) at 0.05 V by stepping potential in a solution containing 0.02 M CuSO4 and 0.1 M H2SO4 for 70 s. d Cu deposited onto the oxidized PPy after (a) at 0.05 V by pulse potential in a solution containing 0.02 M CuSO4 and 0.1 M H2SO4. The charge passed for Cu deposition is the same as that in sample (c). e Valence Cu obtained after (c) by dissolving the deposited elemental Cu at 0.4 V in 0.1 M H2SO4. b
particles on PPy are denser in depositing Cu with stepping potential method. This phenomenon is consistent with that shown in the SIMS analysis. Fig. 6 shows the SEM photographs of different types of Cu deposited on the surface of PPy. At cathodic potential of 0.05 V, both elemental Cu and valence Cu were deposited onto PPy with a dendritic structure, which can be confirmed via energy dispersive
X-ray (EDX). On the contrary, the SEM micrograph shows a typical morphology of PPy with a nodule shape in character [33,34] after elemental Cu was completely dissolved from PPy. None of the left valence Cu was detected via EDX. It may be ascribed to the limitation of the instrumental sensitivity. However, the valence Cu interacted with the pyrrolylium nitrogen can be further confirmed via XPS analysis.
Fig. 5. SEM micrographs of 25 mC Cu deposited onto 250 mC PPy via different deposition methods: (a) stepping potential method; (b) pulse potential method.
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Fig. 6. SEM micrographs of PPy modified with different types of Cu: (a) elemental Cu and valence Cu; (b) valence Cu.
Fig. 7 shows the XPS survey spectra of oxidized PPy and valence Cu-modified PPy. The oxidized PPy was prepared with the electrolyte of LiClO4. To maintain the electronic neutrality, ClO4 was doped into the PPy film. However, the valence Cu-modified PPy was prepared with the electrolyte containing SO4 2 . The dopant ClO4 is easily replaced by the dopant SO4 2 [35], as shown in Fig. 7. Inspecting Fig. 8, the Cu 2p3/2 main peak of the valence Cu is located at 932.9 eV with a 0.5 eV positive shift with respect to that of the elemental Cu and valence Cu. Hammond and Winograd [36] have also reported a 0.95 eV shift of the UPD Cu vs. elemental Cu binding energy on Pt substrate. As discussed above, a valence Cu is indeed deposited onto PPy. The next step is to confirm unequivocally that the source of attraction for cations, whether as a result of a strong binding between the Cu ion and either the pyrrolylium nitrogen or the pyrrolylium backbone, or simply as a loose attachment. This can be revealed from the N 1s and C 1s XPS spectra. In Fig. 9, the doping level is defined as the oxidation level of PPy, the ratio of the area of Nþ ðBE > 401 eVÞ to that of
Fig. 7. XPS survey spectra: (a) oxidized PPy; (b) valence Cu-modified PPy.
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the total N 1s gives the fraction of positively charged N atoms designed the Nþ/N ratio. It is notable that the doping level of oxidized PPy is 0.36. However, this doping level decreases significantly to 0.24 or 0.21 after it is modified with elemental Cu and valence Cu or just valence Cu, respectively. When a valence Cu is deposited onto the PPy, electrons transfer from the Cu atom to the imine group,
and a complex is formed between the imine group and the Cu atom. This is a similar phenomenon to that observed in vacuum deposition of Cu onto polyvinylimidazole, as reported by Inagaki et al. [37]. As a result, the decrease of the oxidized nitrogen atom with a higher binding energy occurs. In this situation, the original oxidized nitrogen atom of PPy can be reduced to a neutral state. However, the influence of the interaction of Cu and pyrrolylium ring on the electrons binding energy of carbon atom cannot be observed from XPS analysis. As we known, PPy is a conjugated polymer with an aromatic-like structure and the delocalized p electrons, which come both from the C and N atoms. An ambiguous influence on the electrons binding energy of carbon atoms may indicate that the charge transfer from the deposited Cu atom mainly to the N atoms with a lone pair of electrons in this situation. The interaction of valence Cu with pyrrolylium nitrogen would demonstrate an effect on the vibration modes of Cumodified PPy. As shown in Fig. 10, the most interesting phenomenon is the change of the broader Raman spectra of N–H in plane deformation [38,39], appearing in the range of 1000–1150 cm1. This broader Raman peak can be further deconvolved into distinguishable double peaks at about 1053 and 1083 cm1. The reason of the decrease in intensity of the band at 1083 cm1 for the spectra of the elemental Cu and the valence Cu-modified PPy films is that the copper inclines to locate at nitrogen atoms with a lone pair of electrons or at the PPy ring with p electrons. Thus, an electron transfer from the Cu to the nitrogen atom of PPy, results in a Cu–PPy complex. Therefore, the N–H in-plane deformation would be hindered by the Cu–PPy interaction.
Fig. 9. N 1s XPS core-level spectra: (a) oxidized PPy; (b) elemental Cu and valence Cu-modified PPy; (c) valence Cu-modified PPy.
Fig. 10. SERS survey spectra: (a) oxidized PPy; (b) elemental Cu and valence Cu-modified PPy; (c) valence Cu-modified PPy.
Fig. 8. XPS Cu 2p3/2 spectrum: (a) elemental Cu and valence Cu on PPy; (b) valence Cu on PPy.
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Fig. 11. Electrochemical depositing and dissolving processes of copper onto and from PPy films.
3.3. Mechanism of Cu deposition and dissolution onto and from PPy From the detailed discussion above, the mechanism of Cu deposited onto and dissolved from PPy can be proposed, as illustrated in Fig. 11. At the 0.05 V UPD potential, oxidized PPy was partially reduced (process (1)) and UPD valence Cu was first simultaneously deposited onto PPy (process (2)) with an interaction on pyrrolylium nitrogen. Since elemental Cu was found on PPy, it was realized that OPD Cu clusters were second deposited on valence Cu and elemental Cu (process (3)) at 0.05 V. In anodic stripping at the 0.4 V dissolution potential (process (4)), the valence Cu obtained in process (2) cannot be dissolved resulting from its strong interaction with pyrrolylium nitrogen. On contrast, most of the Cu clusters were readily dissolved and less of them were left and also bound with pyrrolylium nitrogen. That is why only the valence Cu can be existed in the Cu-modified PPy complex after the final anodic dissolution process.
4. Conclusion The electrochemically depositing and dissolving processes of Cu onto and from PPy were investigated in detailed. In depositing Cu, two deposition methods of stepping and pulse potentials were used. The result indicates that the spatial distribution of deposited Cu is even with pulse potential method. And two forms of coppers, valence and elemental ones, can be formed on PPy at the constant cathodic potential. However, only the elemental Cu can be dissolved from PPy and less valence Cu was left on PPy at the anodic dissolution. From the XPS and SERS analyses, valence Cu is found to be interacted with the pyrrolylium nitrogen, resulting from an electron transfer from the Cu atom to the PPy nitrogen. Excluding the improvement in conductivity, the elemental Cu demonstrates less influence on the characteristics of the Cu-modified PPy. From the economic viewpoint, excess of elemental Cu is unnecessary to modify the property of PPy. The charge used in depositing Cu is estimated to be only 10% of that used in depositing PPy, it can improve the property of PPy. The proposed
mechanism can successfully explain the deposition and dissolution processes of Cu onto and from PPy. Acknowledgements We thank the National Science Council of the Republic of China (NSC-89-2214-E-238-001), Van Nung Institute of Technology, and Chung Cheng Institute of Technology for their financial supports.
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Mechanism of underpotential deposition of metal on conducting polymers Yu-Chuan Liua,*, Kuang-Hsuan Yangb, Ming-Der Gerb a
Department of Chemical Engineering, Van Nung Institute of Technology, 1 Van Nung Road, Shuei-Wei Li, Chung-Li, Tao-Yuan 320, Taiwan, ROC b Department of Applied Chemistry, Chung Cheng Institute of Technology, University of National Defence 190, Sanyuan 1st St., Dashi Jen, Taoyuan, Taiwan, ROC Received 29 May 2001; received in revised form 9 October 2001; accepted 16 October 2001
Abstract From the idea of underpotential deposition (UPD), the electrochemically depositing and dissolving processes of copper (Cu) onto and from polypyrrole (PPy) were investigated. A mechanism was proposed to illustrate the whole metallization process. The results indicate that two forms of coppers, valence and elemental ones, can be formed on PPy at a constant cathodic potential. However, only the valence Cu can be left on PPy at the anodic stripping. From the X-ray photoelectron spectroscopy (XPS) and surface-enhanced Raman scattering (SERS) analyses, it is found that the valence Cu is interacted with the pyrrolylium nitrogen, resulting from an electron transfer from the Cu atom to the PPy nitrogen. The effect of the deposition methods of Cu onto PPy on its corresponding property was also discussed. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Depositing process; Valence copper; Polypyrrole; Underpotential deposition
1. Introduction Among the conjugating conducting polymers, polypyrrole (PPy) [1–4] is the most representative one for its easy polymerization and controllable property of switching it between conducting and insulating states by doping and undoping counterions into the polymer matrix, as well as its chemical and thermal stabilities. These characteristics make it widely used in microelectronic devices [5,6], batteries [7,8] and gas sensors [9,10]. It was also been found that the conductivity and the sensing behavior of PPy could be further improved by imbedding metal particles into the polymer matrix to form a metal–polymer complex [11–13]. Among a variety of methods for polymer metallization, electrodeposition via aqueous solutions is the most promising because of its convenience, good economy, and the wealth of current plating knowledge [11,14,15]. As is generally known, many underpotential deposition (UPD) elements can form a monolayer on the foreign metal substrate, they display catalytic effects for various electrochemical
*
Corresponding author. Tel.: þ886-3-451-5811x254; fax: þ886-2-8663-8557. E-mail address: [email protected] (Y.-C. Liu).
reactions [16–19]. Several mechanisms [20–22] have been proposed to explain the electrocatalytic characteristics, including modification of the electronic structure of the substrate, changes in the physical structure of the substrate and blocking of the adsorption of poisons. Meanwhile, the metal-modified PPy films, like other UPD species, also demonstrate enhanced effects on many electrochemical reactions. In 1986, Tourillon et al. [23] proposed a mechanism to illustrate the poly-3-methylthiophene–Cu interaction. In 1996, Rau et al. [24] also reported their studies on the interaction of Au (I)–PPy complex. In recent years, many researchers had focused their studies on the modification of conductive polymers with metals and the characteristics of the prepared complexes [25–28]. However, a detailed understanding of the electrodeposition process of metals onto the conductive polymers and the interaction of metalconducting polymer with the site of adsorbed metal was yet to be developed. Because copper is electrochemically active and readily undergoes oxidation/reduction from Cu (0) to Cu (I) or Cu (II), the UPD concept was employed to electrodeposit copper onto PPy and electrodissolve elemental copper from it in an aqueous solution in this work. The characteristics of copper and PPy on each step were examined. A mechanism was proposed to explicate these electrochemical processes.
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Also, two different methods for Cu electrodeposition were used to compare the specific properties of the prepared Cu-modified PPy.
2. Experimental 2.1. Formation of the valence Cu onto PPy The electropolymerization of PPy, the deposition of Cu onto PPy, and the dissolution of elemental Cu from PPy were performed in a three-compartment cell. In all experiments except for surface-enhanced Raman scattering (SERS) analysis, a sheet of platinum (a sheet of gold for the SERS experiment) foil with a working area of 0.238 cm2, a 2 cm 2 cm platinum foil and a silver–silver chloride (Ag/AgCl) electrode in a saturated KCl aqueous solution were employed as the working, counter and reference electrodes, respectively. All the potential used in the text was referred to Ag/AgCl. First, the electropolymerization of PPy was carried out at a constant anodic potential of 0.8 V in a deoxygenated aqueous solution containing 0.1 M pyrrole and 0.1 M LiClO4 (called oxidized PPy), followed by depositing Cu onto PPy at a constant cathodic potential of 0.05 V in a deoxygenated aqueous solution containing 0.02 M CuSO4 and 0.1 M H2SO4 (called elemental Cu and valence Cumodified PPy). In this Cu deposition, both elemental Cu (0) and valence Cu were deposited onto the PPy film. Then the deposited Cu was dissolved at 0.4 V in 0.1 M H2SO4 until all the elemental Cu had been completely dissolved, but the valence Cu onto PPy remained (called valence Cu-modified PPy). For comparison, except for stepping potential, pulse potential was also used to deposit Cu onto PPy. In depositing Cu with pulse potential, the potential was kept at 0.05 V for 1 s for Cu deposition, followed by turning off the cell for 30 s for Cu2þ ions equilibrium in each cycle. Besides stating in addition, the method of stepping potential was adopted in the text. In SERS experiments, the Au electrode was roughened by oxidation–reduction cycle (ORC) in a separate cell before the electropolymerization of PPy. The electrode was cycled in 0.1 M KCl from 0.28 V (holding 10 s) to 1.22 V (holding 2 s) at 500 mV s1 for 25 times. After every step the electrodes were rinsed throughout with deionized water, and finally dried in a vacuum dryer for 24 h at room temperature. Then the samples were placed in a desiccator with nitrogen before use. For different experimental requirements, the charges passed in depositing PPy for the conductivity, scanning electron microscopy (SEM), secondary ion mass spectroscopy (SIMS), rotating disk electrode (RDE), SERS and X-ray photoelectron spectroscopy (XPS) experiments were 1500, 250, 250, 100, 25 and 250 mC, respectively. All electrochemical experiments were controlled by a potentiostat (Model PGSTAT20, Eco Chemie), and were performed at room temperature 24 8C.
2.2. Characteristics of the valence Cu onto PPy The conductivities of PPy-based films were determined by using a commercial instrument (Model RT-7, Napson) applying the four-probe technique with a direct current (DC) measurement at room temperature. The distribution of the deposited Cu in the PPy matrix was measured via SIMS (Model IMS-4f, Cameca) using oxygen atoms as the primary ions. For the XPS measurements a physical electronics PHI 1600 spectrometer with monochromatized Mg Ka radiation, 15 kV, 250 W, and an energy resolution of 0.1–0.8% DE/E was used. To compensate for surface charging effects, all XPS spectra are referred to the C 1s neutral carbon peak at 284.6 eV. The complex XPS peaks are deconvolved into component Gaussian peaks using the ‘‘automatic fitting’’ program provided with the XPS spectrometer. Raman spectra were obtained using a Renishaw 2000 Raman spectrometer employing a He–Ne laser of 25 mW and a charge couple device (CCD) detector with 1 cm1 resolution. The surface morphology of PPy-based films was obtained from SEM (Model 8360, Cambridge).
3. Results and discussion 3.1. Cu deposition and dissolution onto and from PPy From the viewpoint of UPD metal on a foreign metal (substrate), there are two different deposition processes [22,27], namely the two-dimensional (2D) metal adsorption deposition and the three-dimensional (3D) metal bulk deposition. 2D and 3D metal depositions are ascribed to underpotential and overpotential depositions (OPDs), respectively. Meanwhile, the electronic structure of the substrate strongly depends on the mass and charge transfer of UPD metal across the interface. Although the mechanism of the UPD metal onto the conductive polymer is not well defined, the phenomena for the deposition of Cu onto PPy observed in our experiments is similar with that of UPD metal on a metal substrate. Fig. 1 shows the I–E curve for Cu depositing and dissolving onto and from PPy and the Pt substrate. Inspecting the depositing and dissolving curves (a) and (b) in Fig. 1, it was found that metal copper was significantly deposited onto PPy at a potential more negative than 0.1 V vs. Ag/AgCl in the cathodic scan. Although the anodic dissolution peak of Cu from the PPy film is not clear enough, the deposited Cu can be markedly dissolved at a potential more positive than 0.3 V vs. Ag/AgCl in comparison with Cu dissolution from the Pt substrate. To adopt the idea from UPD, the potentials used for Cu deposition and dissolution were 0.05 and 0.4 vs. Ag/AgCl, respectively, in the text. As we known the undoping of PPy may compete with Cu depositing onto PPy at potential of 0.05 V vs. Ag/AgCl. The dominant reaction cannot be concluded from just observing the depositing process. However, these competitive
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passed, the relative charges applied to the Cu deposition (Cu2þ reducing to Cuþ, as shown later) and the undoping of PPy can be roughly estimated as follows:
Fig. 1. Cyclic voltammograms at 25 mV s1 showing the deposition– dissolution behaviors of Cu on the PPy film (curve a) and on the Pt substrate (curve b) in 0.02 M CuSO4 and 0.1 M H2SO4.
reactions can be illustrated from the Cu deposition in situ observing the mass change via an electrochemical quartz crystal microbalance (EQCM), as shown in Fig. 2. As we known, the undoping of PPy results in the repulsion of counterion ClO4 from it [15]. Thus, the mass decrease of PPy film will correspond to the frequency increase of EQCM. However, the Cu deposition onto PPy film results in mass increase corresponding to frequency decrease. Inspecting Fig. 2, the frequency decrease demonstrates two different processes in depositing Cu onto PPy. In the first deposition process, namely before ca. 4.2 mC was
Q1 þ Q2 ¼ 0:0042
(1)
Q1 63:5 Q2 99:5 ¼ 1:105 109 1530 96500 96500
(2)
where Q1 (C) and Q2 (C) are the charges applied to the Cu deposition and the PPy undoping, respectively. The Faraday’s constant is 96500 C mol1. The atomic weight of Cu and the molecular weight of ClO4 are 63.5 and 99.5, respectively. The mass sensitivity is 1:105109 g Hz1 [29] and the corresponding frequency decrease at applied charge of 42 mC is ca. 1530 Hz. Combining Eqs. (1) and (2), Q1 and Q2 are solved to be 0.00356 and 0.00063 C, respectively. Consequently, 85% quantity of charge is applied to Cu depositing onto PPy at 0.05 V vs. Ag/AgCl. However, the undoping of PPy more or less can occur at this cathodic potential. 3.2. Characteristics of Cu deposition and dissolution onto and from PPy At the UPD potential, valence Cu is expectantly deposited onto PPy and forms a more stable Cu–PPy complex than elemental Cu does. These phenomena show good agreement with the observations from Fig. 3. This figure shows the dissolution curves on the RDE at 1000 rpm after depositing 40, 20, 10 and 0 mC Cu onto 100 mC PPy. It is interesting that the anodic dissolution of deposited Cu disappears when the charge passed for depositing Cu is less than 10 mC. This indicates that a prior deposition of valence Cu onto PPy is formed with a stronger interaction, and this kind of Cu deposition cannot be removed in dissolution treatment.
Fig. 2. Coulometric curves of 16 mC Cu deposited onto 40 mC PPy (curve a) and corresponding resonance frequency (curve b) vs. time.
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Fig. 3. Anodic dissolution at polarization potential of 0.4 V vs. Ag/AgCl on RDE at 1000 rpm of deposited Cu with different charges passed from 100 mC PPy: (a) 40 mC; (b) 20 mC; (c) 10 mC; (d) 0 mC.
In this work, two different methods were used in depositing Cu onto PPy. They were expected to demonstrate different properties of the prepared Cu-modified PPy. Fig. 4 shows the depth profiles of deposited Cu onto 250 mC PPy with different methods. As shown in curve (d) of Fig. 4, the thickness of the PPy film is about 2.3 mm, which is measured from the point of inflection on the intensity/depth profile of
Fig. 4. Depth profiles of deposited Cu onto 250 mC PPy matrix in depositing Cu with different charges passed by different methods: (a) 50 mC passed by stepping potential; (b) 25 mC passed by stepping potential; (c) 25 mC passed by pulse potential; (d) depth profile of the Pt substrate used in the sample of curve (a).
the Pt substrate. This film thickness is consistent with 500 mC cm2 corresponding to 1 mm estimated by a correlation between thickness and charge passed, as reported in the literature [30]. As shown in curve (b), the charge passed in depositing Cu by stepping potential being one-tenth of that passed in depositing PPy, most of the deposited Cu is concentrated on the surface of PPy. As for the deposition process beginning at the PPy surface or at the Pt substrate, it can be revealed from the conductivity of PPy at 0.05 V in Cu deposition. As discussed before, the undoping of PPy is slight at this cathodic potential. And it still maintains the conducting property, as confirmed later. According to the similar result reported by Jovic et al. [28], metals start to be deposited on the surface of conducting polymers. Therefore, the Cu deposition undoubtedly begins at the surface of PPy. It may also result from the diffusion-controlled reaction in depositing Cu via stepping potential. Then it is deposited at the open pores of PPy and the deposition is toward the Pt substrate. When the charge passed in depositing Cu is double, as shown in curve (a), the spatial distribution of deposited Cu onto PPy matrix can be uniform, namely PPy can be evenly modified throughout the film. From the economic viewpoint, the PPy can be also evenly modified by depositing Cu via pulse potential with only one-tenth charge of that passed in depositing PPy, as shown in curve (c) of Fig. 4. As we know, the advantage in depositing Cu with the pulse potential method is that less problem about ions diffusing into the polymer matrix exist. Thus, Cu can be evenly deposited onto the whole PPy matrix at every pulse deposition potential. The similar reports were shown in the literature [31,32]. Room temperature conductivity measured in air is listed in Table 1. As expected, the conductivity of the PPy modified with Cu deposition significantly increases by elemental Cu. The valence Cu forming a complex with PPy also exhibits an excellent conductivity. It suggested that the valence Cu can intrinsically improve the PPy conductivity. During Cu deposition, the current passed at a cathodic potential corresponds two different processes, the Cu deposition and the undoping of the oxidized PPy. For a precise comparison, the conductivity of the reduced PPy is measured under the same conditions, i.e. treated at the potential of Cu deposition in 0.1 M H2SO4 without Cu2þ ions for 70 s, which is the same period of time as that used in Cu deposition. The small decrease in conductivity reveals that the undoping process is unimportant and the Cu deposition onto PPy dominates the whole reaction under these conditions. Also, the PPy maintains its conducting property in Cu deposition. Fig. 5 shows the SEM photographs of the distribution of the Cu deposited on the surface of PPy. The right parts of these figures represent the backscattering electron images (BEIs) of the left parts of the identical figures, which reflect brighter images for elements with higher atomic numbers. It is helpful to distinguish the Cu particle from the PPy matrix. Comparing Fig. 5(a) and (b), it is obvious that the Cu
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Table 1 Conductivities of various 1500 mC PPy-based films PPy-based films
Conductivity (S cm1)
Oxidized PPya
Partially reduced PPyb
Elemental Cu and valence Cu-modified PPyc
Elemental Cu and valence Cu-modified Ppyd
Valence Cu-modified PPye
30.2
27.3
102
118
62.3
a
Oxidized PPy prepared at 0.8 V in a solution containing 0.1 M pyrrole and 0.1 M LiClO4. Oxidized PPy treated after (a) by reducing at 0.05 V in 0.1 M H2SO4 without Cu2þ ions for 70 s. c Cu deposited onto the oxidized PPy after (a) at 0.05 V by stepping potential in a solution containing 0.02 M CuSO4 and 0.1 M H2SO4 for 70 s. d Cu deposited onto the oxidized PPy after (a) at 0.05 V by pulse potential in a solution containing 0.02 M CuSO4 and 0.1 M H2SO4. The charge passed for Cu deposition is the same as that in sample (c). e Valence Cu obtained after (c) by dissolving the deposited elemental Cu at 0.4 V in 0.1 M H2SO4. b
particles on PPy are denser in depositing Cu with stepping potential method. This phenomenon is consistent with that shown in the SIMS analysis. Fig. 6 shows the SEM photographs of different types of Cu deposited on the surface of PPy. At cathodic potential of 0.05 V, both elemental Cu and valence Cu were deposited onto PPy with a dendritic structure, which can be confirmed via energy dispersive
X-ray (EDX). On the contrary, the SEM micrograph shows a typical morphology of PPy with a nodule shape in character [33,34] after elemental Cu was completely dissolved from PPy. None of the left valence Cu was detected via EDX. It may be ascribed to the limitation of the instrumental sensitivity. However, the valence Cu interacted with the pyrrolylium nitrogen can be further confirmed via XPS analysis.
Fig. 5. SEM micrographs of 25 mC Cu deposited onto 250 mC PPy via different deposition methods: (a) stepping potential method; (b) pulse potential method.
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Fig. 6. SEM micrographs of PPy modified with different types of Cu: (a) elemental Cu and valence Cu; (b) valence Cu.
Fig. 7 shows the XPS survey spectra of oxidized PPy and valence Cu-modified PPy. The oxidized PPy was prepared with the electrolyte of LiClO4. To maintain the electronic neutrality, ClO4 was doped into the PPy film. However, the valence Cu-modified PPy was prepared with the electrolyte containing SO4 2 . The dopant ClO4 is easily replaced by the dopant SO4 2 [35], as shown in Fig. 7. Inspecting Fig. 8, the Cu 2p3/2 main peak of the valence Cu is located at 932.9 eV with a 0.5 eV positive shift with respect to that of the elemental Cu and valence Cu. Hammond and Winograd [36] have also reported a 0.95 eV shift of the UPD Cu vs. elemental Cu binding energy on Pt substrate. As discussed above, a valence Cu is indeed deposited onto PPy. The next step is to confirm unequivocally that the source of attraction for cations, whether as a result of a strong binding between the Cu ion and either the pyrrolylium nitrogen or the pyrrolylium backbone, or simply as a loose attachment. This can be revealed from the N 1s and C 1s XPS spectra. In Fig. 9, the doping level is defined as the oxidation level of PPy, the ratio of the area of Nþ ðBE > 401 eVÞ to that of
Fig. 7. XPS survey spectra: (a) oxidized PPy; (b) valence Cu-modified PPy.
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the total N 1s gives the fraction of positively charged N atoms designed the Nþ/N ratio. It is notable that the doping level of oxidized PPy is 0.36. However, this doping level decreases significantly to 0.24 or 0.21 after it is modified with elemental Cu and valence Cu or just valence Cu, respectively. When a valence Cu is deposited onto the PPy, electrons transfer from the Cu atom to the imine group,
and a complex is formed between the imine group and the Cu atom. This is a similar phenomenon to that observed in vacuum deposition of Cu onto polyvinylimidazole, as reported by Inagaki et al. [37]. As a result, the decrease of the oxidized nitrogen atom with a higher binding energy occurs. In this situation, the original oxidized nitrogen atom of PPy can be reduced to a neutral state. However, the influence of the interaction of Cu and pyrrolylium ring on the electrons binding energy of carbon atom cannot be observed from XPS analysis. As we known, PPy is a conjugated polymer with an aromatic-like structure and the delocalized p electrons, which come both from the C and N atoms. An ambiguous influence on the electrons binding energy of carbon atoms may indicate that the charge transfer from the deposited Cu atom mainly to the N atoms with a lone pair of electrons in this situation. The interaction of valence Cu with pyrrolylium nitrogen would demonstrate an effect on the vibration modes of Cumodified PPy. As shown in Fig. 10, the most interesting phenomenon is the change of the broader Raman spectra of N–H in plane deformation [38,39], appearing in the range of 1000–1150 cm1. This broader Raman peak can be further deconvolved into distinguishable double peaks at about 1053 and 1083 cm1. The reason of the decrease in intensity of the band at 1083 cm1 for the spectra of the elemental Cu and the valence Cu-modified PPy films is that the copper inclines to locate at nitrogen atoms with a lone pair of electrons or at the PPy ring with p electrons. Thus, an electron transfer from the Cu to the nitrogen atom of PPy, results in a Cu–PPy complex. Therefore, the N–H in-plane deformation would be hindered by the Cu–PPy interaction.
Fig. 9. N 1s XPS core-level spectra: (a) oxidized PPy; (b) elemental Cu and valence Cu-modified PPy; (c) valence Cu-modified PPy.
Fig. 10. SERS survey spectra: (a) oxidized PPy; (b) elemental Cu and valence Cu-modified PPy; (c) valence Cu-modified PPy.
Fig. 8. XPS Cu 2p3/2 spectrum: (a) elemental Cu and valence Cu on PPy; (b) valence Cu on PPy.
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Fig. 11. Electrochemical depositing and dissolving processes of copper onto and from PPy films.
3.3. Mechanism of Cu deposition and dissolution onto and from PPy From the detailed discussion above, the mechanism of Cu deposited onto and dissolved from PPy can be proposed, as illustrated in Fig. 11. At the 0.05 V UPD potential, oxidized PPy was partially reduced (process (1)) and UPD valence Cu was first simultaneously deposited onto PPy (process (2)) with an interaction on pyrrolylium nitrogen. Since elemental Cu was found on PPy, it was realized that OPD Cu clusters were second deposited on valence Cu and elemental Cu (process (3)) at 0.05 V. In anodic stripping at the 0.4 V dissolution potential (process (4)), the valence Cu obtained in process (2) cannot be dissolved resulting from its strong interaction with pyrrolylium nitrogen. On contrast, most of the Cu clusters were readily dissolved and less of them were left and also bound with pyrrolylium nitrogen. That is why only the valence Cu can be existed in the Cu-modified PPy complex after the final anodic dissolution process.
4. Conclusion The electrochemically depositing and dissolving processes of Cu onto and from PPy were investigated in detailed. In depositing Cu, two deposition methods of stepping and pulse potentials were used. The result indicates that the spatial distribution of deposited Cu is even with pulse potential method. And two forms of coppers, valence and elemental ones, can be formed on PPy at the constant cathodic potential. However, only the elemental Cu can be dissolved from PPy and less valence Cu was left on PPy at the anodic dissolution. From the XPS and SERS analyses, valence Cu is found to be interacted with the pyrrolylium nitrogen, resulting from an electron transfer from the Cu atom to the PPy nitrogen. Excluding the improvement in conductivity, the elemental Cu demonstrates less influence on the characteristics of the Cu-modified PPy. From the economic viewpoint, excess of elemental Cu is unnecessary to modify the property of PPy. The charge used in depositing Cu is estimated to be only 10% of that used in depositing PPy, it can improve the property of PPy. The proposed
mechanism can successfully explain the deposition and dissolution processes of Cu onto and from PPy. Acknowledgements We thank the National Science Council of the Republic of China (NSC-89-2214-E-238-001), Van Nung Institute of Technology, and Chung Cheng Institute of Technology for their financial supports.
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