Electrosynthesis and characterisation of polypyrrole doped with [Bi(dmit)2]−

Synthetic Metals 150 (2005) 21–26

Electrosynthesis and characterisation of polypyrrole doped with [Bi(dmit)2]− Robson Pacheco Pereira, James L. Wardell, Ana Maria Rocco ∗ Grupo de Materiais Condutores e Energia, Instituto de Qu´ımica, Universidade Federal do Rio de Janeiro, Cidade Universit´aria, Centro de Tecnologia, Bloco A, 21945-970 Rio de Janeiro, RJ, Brasil Received 5 April 2004; received in revised form 1 October 2004; accepted 19 December 2004 Available online 25 February 2005

Abstract The electrosynthesis and characterisation of polypyrrole doped with [Bi(dmit)2 ]− (Ppy/[Bi(dmit)2 ]) are reported. It was shown by means of IR spectroscopy that the anion [Bi(dmit)2 ]− is incorporated into the polymeric matrix. Thermogravimetric analysis showed that Ppy/[Bi(dmit)2 ] decomposes at 180 ◦ C, a higher temperature than [NBu4 ][Bi(dmit)2 ] or Ppy/DS. Cyclic voltammetry of Ppy/[Bi(dmit)2 ] exhibited one irreversible process with peak potentials independent of the scan rate. The superficial conductivity is about 10−4 S cm−1 and the material presented a double-layer capacitance of 70.5 ␮F cm−2 , approximately twice that of Ppy/DS. © 2005 Elsevier B.V. All rights reserved. Keywords: Polypyrrole; Electrosynthesis; Complexes; Dmit

1. Introduction Intrinsically conducting polymers (ICP) have attracted attention of researchers in the last 30 years due to its applications as electrochromic devices [1,2], high voltage solid state batteries [3], capacitors [4,5], sensors for oxygen and humidity [6], diodes [7], and artificial muscles [8,9]. The electrochemical syntheses of polypyrrole (Ppy) doped with complex anions of transition metals have been reported [10]. These polymer/metal complex materials are of fundamental interest for solid state science and they open up new perspectives on inorganic chemistry and in nanoscience and nanotechnology. Ppy incorporated with such counter-ions as BF4 − , ClO4 − , and PF6 − [11,12], as well as with some linear and planar metal complexes, such as [Au(CN)2 ]− [13] and [Ni(CN)4 ]2− [14] have also been studied. The doping anions cover a range of structural shapes, which enabled the effect of the anion structure on the physical properties of Ppy and the polymer arrangement to be evaluated [10,15,16]. As there is no strong evidence to the contrary, it is generally assumed ∗

Corresponding author. Tel.: +55 212 562 7250; fax: +55 212 562 7559. E-mail address: [email protected] (A.M. Rocco).

0379-6779/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.synthmet.2004.12.020

that the counter-ions maintain their physical integrity after incorporated in the polymer matrix. The conductivity values obtained for different ICP/metal complex systems exhibit considerable variations associated with the intrinsic properties of the counter-ion and the conditions used to prepare them [17]. Goward et al. [18] suggested the incorporation of large immobile inorganic anions, such as vanadate, in ICP. Such anions could participate in the redox processes of the resulting materials and could result in enhanced electrochemical stability and charge of the redox process, as well as effecting the charge transport in the ICP. The inter-chain (hopping) mechanism for the charge transport can be conductivity-limiting in either one or three dimensions [19] and the importance of inter-chain interactions is central to charge delocalisation, which leads to higher conductivity. Thus, the insertion of a complex anion and, consequently, its mobility in the solid, modify the charge transfer processes, which take place in the ICP. 1,2-Dithiolato complexes of metals, [Q]m [M(dmit)n ]m− (e.g., Q, univalent cation), have been extensively studied in the last two decades, especially in regard to non-linear optical applications, and use as molecular conductors, supercon-

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Fig. 1. Structural formula for [M(dmit)2 ]x− anions.

ductors and molecular ferromagnets [20–23]. The interest in dmit (1,3-dithiole-2-thione-4,5-dithiolate) based complexes (e.g., see Fig. 1) has been particularly centred on molecular electronics [24], with metallic conductivity and superconductivity being the most active researched properties. These studies gain much from being multidisciplinary with inputs from synthetic and material chemists, photochemists, crystallographers, applied, and theoretical physicists. Up to now, insertion of metal-dmit complexes in ICP has not been reported in the literature to our knowledge. The dmit unit exhibits considerable electron delocalisation and can be considered as a pseudo-aromatic system. Consequently, ␲–␲ interactions can occur between [M(dmit)n ]x− units as well as between these complex anions and other components present in the materials. In addition, the various S atoms – thione and thiolato – in the dmit moiety can take part in interanionic interactions, e.g., M–S and S–S [25]. Insertion of [M(dmit)n ]x− anions in ICP can provide a high ␲ electron density and enhance the number of accessible electronic states for the electronic conduction process. This paper describes the electrochemical polymerisation and spectroscopic, thermal and electrochemical characterisation of polypyrrole doped with [Bi(dmit)2 ]− .

2. Experimental [NBu4 ][Bi(dmit)2 ] was prepared as previously reported [10]. Galvanostatic polymerisation was carried out on a three electrodes cell, using platinum sheets (with 1 cm2 area) as working and counter electrodes and SCE as reference. Samples of polypyrrole doped with [NBu4 ][Bi(dmit)2 ] (Ppy/[Bi(dmit)2 ]) were synthesised using a constant current density of 1 mA cm−2 for 10 min (600 mC) at a temperature of 15 ◦ C from degassed solutions of 0.10 M in pyrrole and 0.5 mM in [NBu4 ][Bi(dmit)2 ] in acetonitrile. All the films were washed and dried under vacuum before analysis. For comparison, samples of polypyrrole doped with dodecyl sulfate [26,2] were obtained using the same conditions. IR spectra were recorded in KBr discs on a Nicolet 760 FTIR, with resolution of 2 cm−1 and 128 scans. UV–vis spectra were obtained using a Varian Cary 1E spectrometer. Cyclic voltammetry was carried out on a Voltalab 40 equipment, in a three electrode cell, similar to that used in the synthesis, with a platinum sheet covered with the ICP as the working electrode and 0.5 M aqueous KCl as support electrolyte, with scan rates of 25, 60, 80, and 90 mV s−1 . Electrochem-

ical impedance spectra (EIS), in the range of 10–100 kHz, were obtained on the same equipment and experimental arrangement as the cyclic voltammetry, with values of the polarisation of +400 and −500 mV. Thermogravimetric analysis (TGA) was carried out on a TGA7 Perkin-Elmer, with a heating rate of 10 ◦ C min−1 and nitrogen atmosphere, from 30 to 450 ◦ C in an alumina pan. Superficial conductivity was obtained as mean values often measurements from dry samples at room temperature with an electrometer connected to a four-probe system.

3. Results and discussion 3.1. Electrochemical characterisation of [NBu4 ][Bi(dmit)2 ] The cyclic voltamogram of [NBu4 ][Bi(dmit)2 ] is shown in Fig. 2. Three anodic and three cathodic irreversible peaks are observed with a scan rate of 50 mV s−1 . The second anodic peak exhibits a fast increase in the current, indicating the deposition of conductive species on the surface of the anode. Consistent with this fact, a film deposited on the electrode surface was observed. Thus, the process is not purely diffusion controlled. A post peak, indicating a strong adsorption of reactant, is also observed [27] and the current showed a v1 dependence, further confirming it as an adsorption peak [28]. The first and second anodic peaks, in analogy to dmit complexes of nickel and palladium [29], studied by Faulmann et al., could correspond to electronic transfer processes, which, in the case of [Bi(dmit)2 ]− , involves monoanionic and neutral species. The formation of a dark red deposit on the electrode and the voltammetric behaviour are similar to the observed in [29]. The generation of that neutral species could lead to a non-integer oxidation state radical anion {[Bi(dmit)2 ]x− }n+m , formed by a reaction between [Bi(dmit)2 ]− and [Bi(dmit)2 ]0 in the diffusion layer, in analogy to the {[Ni(dmit)2 ]x− }n+m compound.

Fig. 2. Cyclic voltammogram of [NBu4 ][Bi(dmit)2 ] (10−4 M in acetonitrile) at 50 mV s−1 .

R.P. Pereira et al. / Synthetic Metals 150 (2005) 21–26

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Fig. 3. UV–vis spectra for: (a) [NBu4 ][Bi(dmit)2 ] in CH3 CN; (b) pyrrole and [NBu4 ][Bi(dmit)2 ] in CH3 CN before synthesis; and (c) after the synthesis. Fig. 4. IR spectra for [NBu4 ][Bi(dmit)2 ], Ppy/DS, and Ppy/[Bi(dmit)2 ].

3.2. Ppy/[Bi(dmit)2 ] During the electrosynthesis, a black thin film was deposited on the electrode. The UV–vis spectra of the remaining reaction solution indicated that [NBu4 ][Bi(dmit)2 ] had been consumed, see Fig. 3. After 10 min, the concentration of [NBu4 ][Bi(dmit)2 ] remaining had fallen to approximately half the original value. Further reaction time indicated no consumption of the anion. IR spectra of [NBu4 ][Bi(dmit)2 ], Ppy/[Bi(dmit)2 ] and the spectra of Ppy doped with the linear counter ion, dodecylsulphate (DS) [2], are shown in Fig. 4; attributions of selected bands are listed in Table 1. The IR spectra of Ppy/DS exhibits characteristic bands for δ(C N) at 1637 cm−1 , for sulphate at 1227 cm−1 and ν(C H) at 2920 and 2821 cm−1 , characterising the pyrrol ring and the counter ion. In the IR spectra

of Ppy/[Bi(dmit)2 ], bands, ν(C C), δ(C N)Ppy , δ(S C S), and ν(C S) can be observed, as well as a band at 885 cm−1 , attributed to dmit ring out of plane vibration. From the main bands in the IR spectra it is clear that the anion, [Bi(dmit)2 ]− , has been incorporated into the Ppy matrix. The only changes in the spectra of [Bi(dmit)2 ]− on incorporation involve only delocalised internal modes [30] of the dmit ring, as accounted for by the change in the chemical environment from a pure crystalline material to the one in Ppy/[Bi(dmit)2 ]. 3.3. Thermogravimetric analyses The TGA curves for the obtained materials are shown in Fig. 5. The curve of [NBu4 ][Bi(dmit)2 ] exhibits a weight loss

Table 1 IR band attributions for the different studied systems System

Wavenumber (cm−1 )

Theorya (cm−1 )

Attribution

[NBu4 ][Bi(dmit)2 ]

1414 1055 1030 1012 879 519 460

1454 1050 1046 997 898 510 471

C C stretch S C S + C S stretch C C displacement out of plane Thione out of plane Ring dmit out of plane Ring dmit internal bending Ring bend + ring-Bi-ring bend

Ppy/DS

2920, 28510 1637 1402 1226

C H stretch (DS) C N, CH2 C C (py ring) Sulphate

Ppy/[Bi(dmit)2 ]

1637 1402 1051 1010 885

C N (py) C C stretch (both py and dmit) S C S + C S stretch (dmit) Thione out of plane (dmit) Ring dmit out of plane

a

From [30].

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Fig. 5. TGA curves for [NBu4 ][Bi(dmit)2 ], Ppy/DS, and Ppy/[Bi(dmit)2 ].

of approximately 35% from 130 to 450 ◦ C. TGA of Ppy/DS showed a weight loss of approximately 45% in the range of 133–300 ◦ C. The Ppy/[Bi(dmit)2 ] exhibited a higher thermal stability than Ppy/DS or [NBu4 ][Bi(dmit)2 ], with the onset temperature of thermal decomposition being 180 ◦ C. It was assumed that the conductive polymers obtained were free from low molar mass components since there was no weight loss at temperatures below 100 ◦ C. 3.4. Cyclic voltammetry The cyclic voltammograms for Ppy/[Bi(dmit)2 ] in KCl aqueous media at different scan rates are shown in Fig. 6.

Fig. 7. Dependence of jp with v1/2 for Ppy/[Bi(dmit)2 ].

An irreversible redox pair is exhibited at all the studied scan rates. It can also be observed that the oxidation and reduction potentials do not change with the scan rate and consequently, Ep is not a function of v, indicating that the redox process is rapid enough to maintain the oxidised and reduced forms at equilibrium on the electrode [31]. The dependence of peak current (jp ) with scan rate (v) for Ppy/[Bi(dmit)2 ] is presented in Fig. 7, from which, one can note that both the cathodic and anodic processes exhibit a linear dependence of jp with v1/2 for all the studied scan rates. This behaviour is associated with a diffusion controlled process on the surface of the electrode [32], from which it can be concluded that the diffusion of the anion from the polymer bulk to the solution and vice versa is rate limiting of the redox process for this ICP. Values obtained from the area of the anodic and cathodic peaks related to the associated charge of the oxidation (QA ) and reduction (QC ) processes for Ppy/[Bi(dmit)2 ] and PpyDS are represented in Table 2. For Ppy/[Bi(dmit)2 ], QA /QC tend to 1 for scan rates lower than 25 mV s−1 . For lower scan rates, the polymer should exhibit charge compensation in the associated oxidation/reduction processes. However, for v greater than 60 mV s−1 , the charge associated with the oxidation is higher than that for the reduction, as found similarly for Ppy/DS [2]. A decrease of the charge values for the oxidation and reduction processes for Ppy/[Bi(dmit)2 ], compared to Ppy/DS, was observed. Table 2 Values of QA , QC and QA /QC from CV of Ppy/[Bi(dmit)2 ]

Fig. 6. Cyclic voltamograms for Ppy/[Bi(dmit)2 ] at different scan rates.

Scan rate (mV s−1 )

QA

QC

QA /Qc

25 60 80 90

0.11815 0.20952 0.28870 0.20570

0.13579 0.43446 0.66009 0.74011

0.870 0.482 0.437 0.278

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Fig. 8. EIS of Ppy/[Bi(dmit)2 ] (polarisation +400 mV).

3.5. Electrochemical impedance spectroscopy Electrochemical impedance spectroscopy (EIS) from 10 to 100 kHz of Ppy/[Bi(dmit)2 ] is shown in Figs. 8 and 9 (polarisation +400 and −500 mV, respectively). In Table 2, the charge transfer resistance (Rct ), characteristic frequency (ωo ) and double-layer capacitance (Cd ) values obtained from EIS are listed. The electrochemical behaviour and the different regions of frequency response in impedance spectra reflect the different mechanisms and characteristic time-dependencies for electrochemical processes. Considering a model where anions in solution migrate through the polymer matrix to the electrode surface and then a redox process takes place (which applies for the majority of ICP), the high frequency region is associated with the electrolyte resistance (Re ). The mid-frequency region is associated with the charge transfer resistance (Rct )

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on the interface; for ICP, this process involves migration of ions from solution to the solid matrix. The relaxation effect related to this process is shown in Nyquist plot as a semicircle, whose time constant is given by the product of Rct and Cd (double-layer capacitance). These parameters can be obtained from semicircle diameter (in real axis) and characteristic frequency of relaxation in semicircle maximum [33,34]. At low frequencies, impedance is dominated predominantly by diffusional processes. Usually, two regions can be identified, both linear (in a Nyquist plot), one with phase angle ␲/4, corresponding to a linear semi-infinite diffusion (kinetics depending on diffusion of species), represented by Warburg impedance (ZW ) and a second region at lower frequencies with phase angle ␲/2. Since the diffusion of ions in solution is faster than in polymeric matrix, the mass transport is dominated by ion transport in the solid and the linear region with phase angle of ␲/2 is associated with a purely capacitive response. Below this frequency, the diffusion process is limited by the charge storage on polymeric matrix, resulting in a limit capacitance [33,35]. From Table 3 it is seen that Rct and Cd for Ppy/[Bi(dmit)2 ] are in different orders of magnitude than that for Ppy/DS. Rct for the oxidised form (+400 mV) of Ppy/[Bi(dmit)2 ] is one order of magnitude smaller than that for Ppy/DS. It could indicate a minor charge transfer resistance or could be due to a superposition of semicircles for different processes, if these present time constants of similar values [36]. The charge transfer resistance obtained for polarisation at −500 mV (reduced form of Ppy/[Bi(dmit)2 ]) is about twice that for Ppy/DS. For polarisation at −500 mV, Cd is in the range of processes involving double-layer polarisation (2.97 ␮F cm−2 ), however, for the oxidised form, this value is one order of magnitude higher (70.5 ␮F cm−2 ). This high value of Cd reflects a high charge density on the surface of Ppy/[Bi(dmit)2 ]. This capacitive behaviour can be useful to the application of this material as a polymeric capacitor. 3.6. Superficial conductivity Superficial conductivity of Ppy/[Bi(dmit)2 ] exhibited a mean value of 1.71 × 10−4 S cm−1 , while for Ppy/DS the conductivity is 31.8 S cm−1 . In Table 4 lists conductivity values for other Ppy/complex anion systems. From these data, it can be seen that the value obtained for Ppy/[Bi(dmit)2 ] is within the range of conductivity normally found for such Table 3 Parameters obtained from EIS for Ppy/[Bi(dmit)2 ] and Ppy/DS Polarisation (mV)

Fig. 9. EIS of Ppy/[Bi(dmit)2 ] (polarisation −500 mV).

ωo (kHz)

Rct ()

Cd (␮F cm−1 )

Ppy/DS

+400 −500

2.5 2.2

5.0 5.0

45.1 40.0

Ppy/[Bi(dmit)2 ]

+400 −500

8.9 14.0

0.8 12.0

70.5 2.97

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Table 4 Conductivity values for some Ppy/complex anion systems System

Conductivity (S cm−1 )

[NBu4 ][Bi(dmit)2 ]a Ppy/DSa Ppy/[Bi(dmit)2 ]a

1.63 × 10−3 31.8 1.71 × 10−4

Ppy/[Cu(dpp)2 ]+ Ppy/[MoS4 ]2− Ppy/[CuPcTs]4− Ppy/[Re2 Cl8 ]2− Ppy/[Pt(CN)4 ]2− Ppy/[Fe(CN)4 ]3−

2.7 × 10−5 2.6 × 10−2 5.0 × 10−2 5.0 17.0 20

Values obtained from H.S. Nalwa (Ed.), Handbook of Conductive Polymers: Spectroscopy and Physical Properties. a This work.

materials: as noted these conductivity values vary with the nature of the incorporated anion. The crystal structure of [NBu4 ][Bi(dmit)2 ] [25] and other [Q][Bi(dmit)2 ] complexes consists of well separated cations but interacting anions, linked by interanionic Bi–S interactions, which results in six-coordinate Bi centers. Even if the anions are not interacting in the Ppy/[Bi(dmit)] material, due to the dilute concentration of the anions within the polypyrrole matrix, they will not have a planar structure. As a consequence, the Ppy chains will be less ordered which will result in the lower value of conductivity observed.

4. Conclusion A new polymeric material was obtained by galvanostatic polymerisation of pyrrole in the presence of [NBu4 ][Bi(dmit)2 ], as confirmed by spectroscopical analysis. The material exhibited an onset decomposition temperature of 180 ◦ C. The superficial conductivity is about 10−4 S cm−1 and the material presented a Cd value of 70.5 ␮F cm−2 , approximately twice the value for Ppy/DS. This Cd value enables the application of Ppy/[Bi(dmit)2 ] as capacitive material and the electrochemical impedance spectra are different from Ppy/DS, indicating that the new material presents different electronic properties, induced by the presence of the anion [Bi(dmit)2 ]− .

Acknowledgments Authors would like to thank CNPq for fellowships and Prof N.M. Comerlato for supplying the [NBu4 ][Bi(dmit)2 ] samples.

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