Mat Res Innovat (2001) 4:126–130

© Springer-Verlag 2001

ORIGINAL ARTICLE

Arzu Ersöz · Meral Arca · Kadir Pekmez Attila Yildiz

Electrochemical polymerization of benzene in the presence of Ag+, Pb2+ and Cu+ ions

Received: 18 September 2000 / Reviewed and accepted: 20 September 2000

Abstract Poly(p-phenylene) (PPP) films were synthesized by using benzene and fluorosulphonic acid (FSO3H) as a strong acid containing Ag+, Pb2+ and Cu+ ions in methylene chloride (CH2Cl2) solution. Addition of Ag+ or Pb2+ ions into the polymerization medium improved the PPP films formation, but Cu+ ion did not have an effect on polymerization. PPP films were characterized by cyclic voltammetry, IR and TGA. Dry conductivities were measured by using four probe technique. Keywords Conducting polymers · Poly(p-phenylene) · Electrooxidation of benzene

Introduction and previous work Poly(p-phenylene) (PPP) continues to receive considerable attention due to its outstanding physical and chemical properties. In particular, it is known for its exceptional thermal stability in the neutral state, its resistance to environmental oxidation and irradiation, its very wide conductivity range, the possibilty of carrying out chemical or electrochemical n- or p- doping of the polymer similar to that of polyacetylenes [1]. This n- and p-doping property of PPP has gained a lot of interest in applications. Recently, conducting polymers have attracted attention in applications to batteries, supercapacitors, electrochromic devices, semiconductors and various sensors [2] and lastly, recently discovered, applications of PPP material for electroluminescent devices [1].

A. Ersöz (✉) · K. Pekmez · A. Yildiz Department of Chemistry, Hacettepe University, 06532 Beytepe, Ankara, Turkey e-mail: [email protected] A. Ersöz Department of Chemistry, University of Kentucky, Lexington, 40506 Kentucky, USA M. Arca Department of Chemistry, Kocaeli University, Kocaeli, Turkey

Because of the more complicated conditions required, anodic synthesis of PPP is considerably smaller. Up to 1984 [3] nothing was known about benzene electropolymerization in organic solvents and only since 1986 have some suitable organic electrolytes been proposed for this reaction [4]. The influence of different parameters such as electrolyte, solvent and presence of residual water on the final film has been studied both in organic media [5] and in an acidic aqueous solution [6]. Also the influence of temperature on both the film formation and on the redox response has been determined [7]. It has been shown that polymer formation is facilitated by the addition of strong Lewis and Brönsted acids which lower the benzene oxidation potential [8]. The electrolytic media used for benzene electropolymerization have to be strongly anhydrous (H2O<10–3 M) and it was shown that PPP formation occurs only in solvents with a donor number DN<15 or with a pKBH+<-10 [9]. PPP can be polymerized by using both chemical and electrochemical methods. The most common chemical syntheses are the Kovacic method [10] and the Yamamoto method [11]. Early studies on the preparation of PPP by electrochemical polymerization of benzene consisted of observations of unidentified layers on the electrode [12] or of insoluble polyphenylenes which precipitated into the solution [13]. PPP films have been obtained by polymerization of benzene at a rather low potential in different acidic media such as CF3SO3H [14], SbF5 [15] and AlCl3 [16]. PPP films are also obtained by polymerization the monomer in aqueous solution of an acidic microemulsion of benzene/sulfuric acid and sodium dodecyl sulphate [17]. Electroreductive polymerization of 1,4-dibromobenzene has also been reported [18, 19]. Good quality PPP films have been obtained by electrochemical polymerization of biphenyl in different organic media [5, 20, 21]. PPP film has been electrochemically deposited onto a stainless steel electrode surface [22] and Ag/Ag+ electrode [23] was used by direct oxidation of benzene in BF3-diethylether solution. PPP film can also be obtained by the electrochemical polymerization of benzene and diphenyl us-

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ing composite electrolytes of CuCl2+LiAsF6 [24]. Athouel et al. [25] have characterized PPP oligomers such as parasexiphenyl and paraoctiphenyl by applying UV-visible and IR absorption spectroscopies, Raman scattering spectroscopy and photoluminescence at 77 K. The aim of the present report was to study the electrochemical oxidation of benzene in the presence of Ag+, Pb2+ and Cu+ ions as a catalyst in the production of polyphenylene films and investigate its properties applying IR spectroscopy, TG analysis and dry conductivity measurements.

Experimental The experiments were performed in a three-electrode one compartment cell using “Model 175 Universal Programmer, Model 173 and 273 Potentiostat-Galvonastat, Model 179 Digital Coulometer”. The voltammetric curves were recorded on a B.B.C. Matrawatt Goertz X-Y recorder. TBAClO4 (TBA:Tetrabutylammonium) was prepared by reacting concentrated perchloric acid solution (BDH) with the solution of tetrabutylammonium hydroxide (Aldrich) and recrystallized from ethanol several times. TBABF4 was prepared by reacting tetrafluoroboric acid (Aldrich) with the solution of tetrabutylammonium hydroxide (Aldrich) and recrystallized from ethanol:water mixture several times. They were kept under nitrogen atmosphere after vacuum drying for 24 h at 120°C. Acetonitrile (MeCN) (Aldrich) was purified with five-steped fractional distillation. CH2Cl2 (Aldrich) and benzene (Aldrich) were washed with H2SO4, H2O, Na2CO3 and distilled. Benzene was stored in the presence of metallic sodium. Biphenyl (Aldrich) and FSO3H (Aldrich) were used as received. All experiments were taken under nitrogen atmosphere. The reference electrode was either Ag/AgCl or Ag wire and the counter electrode was a Pt spiral. The working electrode for the cyclic voltammetric studies was a Pt disc (area: 0.0132 cm2) and for the conductivity, IR and TG analysis was Pt macroelectrode (area: 1.0 cm2). The working microelectrode was cleaned by polishing with Al2O3 slurry and the working macroelectrode was cleaned by holding it in a flame for a few minutes before polymerization. Benzene samples were obtained either as a film or powdery material. Powdery material was pressed into pellets. Dry conductivity values were measured by using four-probe measuring technique at room temperature. The ohmic contact to the films was made with Au-plated four-probe tips. The PPP films were prepared by potential scanned electrolysis of benzene solutions between 0.0 and +1.5 V vs. Ag wire electrode. Anhydrous Ag+, Pb2+ and Cu+ solutions were prepared by potentialcontrolled electrolysis using Ag, Pb and Cu wires as

Fig. 1a–c Cyclic voltammogram of the oxidation of benzene in CH2Cl2+0.1 M TBABF4 vs Ag/Ag(sat.); C(Benzene): 0.1 M, (a) C(FSO3H): 0.2 M, (b) C(FSO3H): 0.3 M, (c). C(FSO3H): 0.4 M, sweep rate: 100 mV/cm

working electrode. Concentrations of these ions in CH2Cl2 solvent were determined coulometrically.

Results and discussion In order to establish optimum electropolymerization conditions, different solvents such as MeCN and CH2Cl2, electrolytes such as TBABF4 and TBAClO4, strong acids such as HBF4 and FSO3H were used. Results of the experiments showed that the use of slightly acidic solvent such as CH2Cl2 instead of MeCN made film formation easier and the use of BF4– ion as anion instead of ClO4– ion gave an adherent film. Because of the easier oxidation of benzene in acidic media the strong acid, FSO3H, was used. The oxidation potential slightly shifted toward less anodic potential (about 0.8 V) in the presence of strong acid. After choosing the suitable solvent, electrolyte and acid, the concentrations of benzene, TBABF4 and FSO3H were determined. The effect of monomer and acid concentration on the growth of the PPP film was investigated by growing films in solutions of varying benzene and acid concentrations in the range of 50– 400 mM. Concentrations of 0.1 M benzene and 0.4 M FSO3H were found to be the optimum. In these studies, Ag/AgCl and Ag wire were used as reference electrodes, respectively. The effect of acid concentration on polymerization are presented in Figs. 1 and 2. The cyclic voltammogram of 0.1 M benzene in CH2Cl2+0.1 M TBABF4 containing different concentrations of FSO3H solution by using Ag/AgCl electrode recorded between 0.0 and +2.0 V, is shown in Fig. 1. Figure 2 shows the cyclic voltammogram of benzene under the above conditions by using Ag wire electrode recording between 0.0 and +1.5 V. As shown in the Figs. 1 and 2, when Ag wire was used as the reference electrode, increase of the current in voltammograms was easily observed than that of Ag/AgCl electrode. A black PPP film was easily obtained also in the case of Ag wire electrode. The above results also indicated that Ag+ ion affected growth and formation of PPP film. In order to determine the effect of Ag+ ion on the growth of the PPP film, the films were grown in solutions of varying Ag+ concentra-

128 Fig. 2a–c Cyclic voltammogram of the oxidation of benzene in CH2Cl2+0.1 M TBABF4 vs Ag wire; C(Benzene): 0.1 M, (a) C(FSO3H): 0.2 M, (b) C(FSO3H): 0.3 M, (c) C(FSO3H): 0.4 M, sweep rate: 100 mV/cm

concentration of Ag+ ion was increased up to 6.0 mM the film formation also increased, beyond this concentration precipitation of AgBF4 was observed. The catalyst effect of Pb2+ and Cu+ ions were also examined with the same method. Solutions with varying concentrations of Pb2+ and Cu+ ions were prepared as explained above. Using the same procedure for Ag+, the effect of Pb2+ and Cu+ ions on the growth of polymer film was studied. Figure 4 shows the cyclic voltammogram of benzene solutions containing 5 mM of (a)Ag+, (b) Pb2+ and (c) Cu+ ions separately. When the voltammograms in these figures are compared with each other, the catalytic effect of Pb2+ ion on the formation and growth of the PPP film could easily be seen (Fig. 4b). This is similar to the effect of Ag+ ion. Although in a previous study [26] Cu+ ion was claimed to be very effective in increasing the amount and improving the quality of the polyaniline film in the present study solutions of Cu+ ion did not have any effect in the polymerization of PPP. The above observations could be interpreted as follows. Ag+, Pb2+ and Cu+ ions which were formed in the electrolytic medium prior to polymerization produced the catalytically active species in organic solvents according to the following reactions: Ag+→Ag2++e–

+1.8 V (vs. Ag/AgCl)

Pb2+→Pb4++2e– +1.7 V (vs. Ag/AgCl) Cu+→Cu2++e– Ag+,

Fig. 3 Cyclic voltammogram of benzene using (a) 0.5 mM, (b) 1.5 mM, (c) 3.5 mM and (d) 5.0 mM Ag+ ion concentrations vs. Ag/AgCl electrode; C(Benzene): 0.1 M, C(FSO3H): 0.4 M; sweep rate: 100 mV/cm

tions (0.5–7.5 mM) by cycling the potential between 0.0–2.0 V for the same period of time using Ag/AgCl electrode. Figure 3 shows the cyclic voltammograms of benzene of these solutions which contain increasing amounts of Ag+ ion. As seen from the Fig. 3, when the

Pb2+

+1.2 V (vs. Ag/AgCl) Cu+

and ions which were produced anodically in CH2Cl2+0.1 M TBABF4 electrolyte were firstly oxidized to Ag2+, Pb4+ and Cu2+ ions, which in fact acted as catalysts, after the addition of benzene and FSO3H acid at the potential of electropolymerization. Then these ions were reduced to Ag+, Pb2+ and Cu+ ions chemically during polymerization and after that, Ag+, Pb2+ and Cu+ ions were reoxidized to Ag2+, Pb4+ and Cu2+ ions electrochemically. Ag2+, Pb4+ and Cu2+ ions increased the speed of electropolymerization by oxidizing the monomer or polymer according to the following reaction:

129 Fig. 4 Cyclic voltammograms of benzene in 5.0 mM (a) Ag+, (b) Pb2+ and (c) Cu+ ions vs. Ag/AgCl electrode, C(Benzene): 0.1 M, C(FSO3H): 0.4 M; sweep rate: 100 mV/cm

In the electropolymerization of benzene, radical formation was very difficult, so only thin film could be obtained. Because of the low conductivity of thin PPP film growing on the Pt electrode at the beginning of electropolymerization, oxidation of the film was difficult and when catalysts were not used the rate of electropolymerization was slower. The catalysts were increased the radical formation and an adherent PPP film was obtained even Ag/AgCl electrode was used. As mentioned above Ag+, Pb2+ and Cu+ ions were produced at the end of the chemical reaction of these ions with the monomer or PPP and again they were oxidized to Ag2+, Pb4+ and Cu2+ ions. This cycle continued if the potential was kept constant at a higher value than those of the oxidation potentials of these ions. When the oxidation potential of these ions were considered, it could be seen that the activity of metal ions based on the oxidation potentials of Ag2+, Pb4+ and Cu2+ ions. So, Ag+ and Pb2+ ions which had +1.7 and +1.8 V oxidation potential, respectively, had catalyst effect on the electropolymerization, while Cu2+ ion having an oxidation potential of +1.2 V had no catalytic effect on electropolymerization of benzene. As a result the potential of Cu2+ ion was not enough to oxidize benzene or polymer. The PPP product was identified by IR spectra, TGA analysis and conductivity measurements. This product was peeled as a film for conductivity measurements and pressed into pellets for IR spectra. The IR spectra of PPP film which was anodically generated during polymerization of benzene in the presence of Ag+ ion is shown in

Fig. 5 IR spectrum of PPP film obtained by electrooxidation of benzene in the presence of Ag+ ion

Fig. 5. The strong band at 806 cm–1 in the IR spectrum of PPP clearly showed that the benzene rings in the product were linked at para positions. Two out-of plane vibration bands due to the terminal phenyl group at 740–760 and 662 cm–1[10] were also seen in IR spectrum of PPP. An absorption band was also seen at 880 cm–1 which indicated presence of trisubstituted benzene ring. Peaks at 907 and 938 cm–1 were regarded as vibrations from mono substituted phenyl rings and were interpreted to originate from the adsorption of dimer at the electrode surface [28]. Thermal stability of PPP films was also investigated under nitrogen atmosphere. The film lost approximately 10% of its weight up to 100°C. This loss was because of the removing of BF4– ion which is found as a counter an-

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ion in the film structure by changing into BF3. The PPP film was thermally stable up to a temperature of approximately 400°C. In addition to thermal stability, activation energy of PPP film was also calculated as 400 kJ/mol by using Freeman and Caroll method [28]. The conductivity measurements were made by using the four-probe technique. When Ag/AgCl electrode was used, a good quality film could not be obtained. But using Ag+ ion solution PPP film was obtained and the conductivity of PPP was measured as a film or pellet. The conductivity of cathodically reduced PPP film which was obtained by the electrooxidation of benzene was found as 1.3×10–5 S/cm. After doping process, the conductivity reached to 10–2 S/cm.

Conclusion It can be concluded that PPP can be electrochemically generated from benzene in CH2Cl2. A strictly anhydrous medium was necessary for the electrochemical synthesis of PPP by oxidation of benzene. In the absence of acid, the electropolymerization did not proceed. Due to high oxidation potential of benzene, polymerization without a catalyst was very difficult. Ag2+ and Pb4+ ions formed during polymerization had catalytical effect on the polymerization. Cu2+ ion did not have any catalytic effect on the film formation due to its low oxidation. The film thus formed was a conducting film in the oxidized state and an insulator when neutral. The conductivity of BF4– doped film was 10–2 S/cm and PPP film was thermally stable up to 400°C.

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