Electrolyte effects on various properties of ...

Thin Solid Films 519 (2010) 1016–1019

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Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / t s f

Electrolyte effects on various properties of polycarbazole Bhavana Gupta, Arun Kumar Singh, Rajiv Prakash ⁎ School of Materials Science and Technology, Banaras Hindu University, Varanasi-221005, India

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Available online 25 August 2010 Keywords: Conducting polymer Polycarbazole Electrochemical polymerization Thermal analysis Polymer crystallinity

a b s t r a c t Effect of electrolyte during electrochemical synthesis of polycarbazole (PCz) is reported in this paper. Two different supporting electrolytes tetrabutylammonium perchlorate (TBAClO4) and tetrabutylammonium tetrafluoroborate (TBABF4) dissolved in acetonitrile are used for the synthesis of PCz. The electrolyte effect with regard to yield, structure, electroactivity, crystallinity and thermal stability of the polymer is studied through various techniques including XRD, cyclic voltammetry, differential scanning calorimetry and electrochemical impedance spectroscopy. High doping and facilitation of dopant ion movement in the case of PCz/TBAClO4 in comparison to PCz/TBABF4 film is revealed in conductivity measurements and electrochemical investigations. XRD shows a significant difference in the crystallinity of the polymers synthesized using in two different electrolytes. Melting point of the conducting polymer is also observed for the first time during thermal investigations (in case of PCz/TBABF4). The amorphous to crystalline transformation of PCz is also observed on heating. A comparative analysis of both the electrolytes also shows that TBAClO4 is a better supporting electrolyte than TBABF4 in various aspects for synthesis of PCz. © 2010 Elsevier B.V. All rights reserved.

1. Introduction Conducting polymers such as polypyrrole, polyaniline, polythiophene, and poly (N-vinyl carbazole) have been extensively studied for their synthesis, characterization and applications [1–7]. However, lately other classes of polymers like polycarbazole are gaining considerable attention due potential applications in light emitting diodes (OLED); as green and blue light emission was achieved by using carbazole (Cz) derivatives [8–15], sensors [16] and rechargeable batteries [17]. Anodic oxidation of carbazole and its N-substituted derivatives were extensively studied by Ambrose et al. [18]. They investigated the reactivity of cation radicals formed from 76 ring substituted carbazole using electrochemical and spectroscopic techniques. It was reported that the 3, and 9 positions on carbozole were extremely reactive and anodic oxidation of carbazole produced very unstable cations, such as 9,9′- and 3,3′-bicarbazyls. Though conducting polymers can be polymerized by two methods viz chemical and electrochemical techniques, electrochemical method offers a better control over properties as they can be tailored by changing experimental conditions [19–21] such as electrolyte, oxidation potential, solvent, etc. To date, polycarbazole is extensively synthesized using electrochemical techniques [22,23] and this polymer is mainly studied for its optical and electrochemical properties [24]. In this article, we report the electro-oxidation of carbazole (Cz) and formation of PCz, using optimum conditions, which have been

⁎ Corresponding author. E-mail address: [email protected] (R. Prakash). 0040-6090/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2010.08.034

deduced from the cyclic voltammogram of carbazole in two different electrolytes and further characterized for structural, thermal and electrochemical properties. 2. Experiment 2.1. Material Carbazole monomer and tetrabutylammonium perchlorate (TBAClO4) and tetrabutylammonium tetrafluoroborate (TBABF4) purchased from Aldrich Chemicals, USA. Acetonitrile (HPLC grade solvent) were obtained from Merck, India. All chemicals used were of analytical grade. Double distilled deionized water was used in all the experiments. 2.2. Instrument Mini four probes assembly (Raman Scientifics, Roorkee, India) connected with Keithley digital multimeter and current source measurement instrument (model 2000), USA is used for conductivity measurements. Electrochemical workstation (mode CHl7041C), CHInstrument Inc., USA was used for the electrochemical characterization using three electrodes assembly. XRD measurements were carried out using 18 kW rotating anode (Cu Kα wavelength 1.543 Å) based on Rigaku, Japan powder diffractometer operating in the Bragg–Brentano geometry and fitted with a graphite monochromator in the diffracted beam with 3°/min scan rate. Differential scanning calorimetry was carried out using (METTLER TOLEDO, model

B. Gupta et al. / Thin Solid Films 519 (2010) 1016–1019

FP900 Thermosystem) at a heating rate of 10 °C/min under N2 atmosphere.

2.3. Process of polymerization Electrochemical syntheses and examinations were performed in a three electrode and one-compartment cell with working, counter and reference electrodes. Cyclic voltammetric (potentiodynamic synthesis of PCz) experiments was performed using platinum disc (diameter of 2 mm) and platinum gauze as a working and counter electrode by applying potential range from 0 to 1.4 V. To obtain a sufficient amount of polymer for thermal and structural characterizations, gold plate with a surface area of 10 cm2 each were employed as the working as well as counter electrodes vs. Ag/AgCl reference electrode and polymer formed at 1.3 V at constant potential. Electrodes mentioned earlier were carefully polished with abrasive paper (1500 mesh), and cleaned with water and acetone successively before each examination. All potentials were referred to an Ag/AgCl electrode. The amount of the carbazole, TBAClO 4 and TBABF 4 were 60 mM, 0.1 M respectively. 3. Results and discussion 3.1. Electropolymerization of carbazole on Pt disc electrode by cyclic voltammetry Cyclic voltammograms of PCz thin films electrochemically deposited on Pt disc electrode recorded in 0.1 M TBAClO4 and TBABF4 as supporting electrolytes in acetonitrile as shown in Fig. 1. The onset potentials of carbazole oxidation with TBAClO4/ACN and TBABF4/ACN were obtained at 0.96 and 1.05 V respectively as shown in Fig. 1. After the first cycle, the first peak intensity increased and potentials shifted toward higher values. Gradual increase in the intensity of the cathodic wave with repeated scans indicate that the product is gradually deposited on the surface of the Pt disc electrode (cf. Fig. 1). In TBABF4/ ACN solution, the anodic peaks of PCz shifted to higher potential from 0.86 to 0.91 V and cathodic peaks from 0.70 to 0.86 V to that of TBAClO4/ACN (cf. Fig. 1) for the second cycle, probably due to difficult diffusion of ion in and out the polymer film. The cyclic voltammogram of carbazole in TBABF4/ACN exhibited oxidation of the monomer to relatively higher potential, which affected thickness of the polymer film. The current density that was obtained for TBAClO4/ACN which

Fig. 1. Voltammogram (multisweep) of PCz in 0.1 M TBABF4/ACN (black) and TBAClO4/ACN (red) on Pt electrode. Scan rate: 100 mV/s. Cycles: 10 (between 0.0 and 1.4 V) [Cz] = 0.06 M.

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was higher than that obtained for TBABF4/ACN (cf. Fig. 1) supports thin and less doped state of the polymer in case of TBABF4/ACN.

3.2. Effect of scan rate in monomer-free solution The coated film was washed with monomer-free electrolyte solution and redox behavior was studied. Oxidation and reduction peaks were observed by increasing the applied potential for TBAClO4/ACN and TBABF4/ACN (cf. Figs. 2 and 3). These anodic processes are occurring at 0.86 V and 1.15 V for TBAClO4/ACN and 0.94 V and 1.3 V for TBABF4/ACN respectively. Such behavior may correspond to the formation of the radical cations of carbazolic units during the first oxidation step followed by their oxidation into dications through the second step [25]. Redox process is more reversible according to the Ipa/Ipc ratio in case of TBAClO4/ACN than TBABF4/ACN. The redox peaks are much broader and at higher potential in the case of PCz/TBABF4 than PCz/TBAClO4 probably because of slower ion exchange process, which collaborate to less BF4 ions in the synthesized film. Apart from that the ion exchange also affected by texture of the film. Porosity of the film facilitates the diffusion of the ion in and out of the film [26] which supports higher porosity of the PCz film synthesized using TBAClO4. The scan rate dependence of the electro-active film peak current was investigated on the second peak as shown in Figs. 2 and 3 (inset). The peak current (ip) for a reversible voltammogram at 25 °C is given by the following equation: ip = (2.69 × 105)· A · D1/2 · C0 · m1/2 where m is the scan rate, A is electrode area, D is the diffusion coefficient of electro-active species and C0 is the concentration of electro-active species in the solution. Peak current was proportional to m1/2 in the range of scan rates which showed diffusion control process [27]. Diffusion coefficients for PCz/TBABF4 than PCz/ TBAClO4 are 1.80 × 10− 7 and 1.9010− 7 respectively. 3.3. Impedance analysis Impedance spectra of PCz films (TBAClO4/ACN and TBABF4/ACN) recorded in monomer-free solution at open circuit potential in the frequency range of 0.01 to 104 hertz using pt disc electrode. The potential used for potentiostatic polymerization was 1.3 V for 300 s. Electrochemical impedance spectroscopy is a powerful tool for the analysis of surface treatment [28]. Apart from that ac impedance spectroscopy can be used to obtain a measure of the resistance associated with the potential specific power of the electro chemical capacitor [29]. The most important point in the impedance spectra was the resistances associated with the transport process within

Fig. 2. Voltammogram (multisweep) of PCz in 0.1 M TBABF4/ACN at various scan rates (from 20 to 500 mV/s) on Pt electrode. Current vs root scan rate (inset).

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Fig. 3. Voltammogram (multisweep) of PCz in 0.1 M TBAClO4/ACN at various scan rates (from 20 to 500 mV/s) on Pt electrode. Current vs root scan rate (inset).

electrode. These resistances were evaluated from the difference in the real part (Z′) of the impedance between the low and high frequencies [30]. The results of electrochemical impedance are a Nyquist plot of half a semicircle: the high frequency part (intercept) gives solution resistance (Rs) as 340 Ω and width of the semicircle gives the real impedance of the studied layer, i.e. ohmic resistance or ionic charge transfer resistance (Rct) of a layer on the electrode. Fig. 4 gives the Nyquist plot of the electrode coated by PCz/TBAClO4 and PCz/TBABF4. Distorted semicircle in the higher frequency region is because of asymmetric configuration of the electrode [31]. Nyquist plot of PCz/ TBAClO4 and TBABF4/ACN contain a linear part at lower and middle frequency regions attributed to the diffusion controlled Warburg behavior. Deviation is in vertical line from 45° is probably because of irregular surface and fast diffusion. More deviation from the vertical line was observed for PCz/TBAClO4 than TBABF4/ACN which supports fast diffusion of ion through PCz/TBAClO4 film probably because of more porosity of the film in the case of PCz/TBAClO4. The width of semicircle of PCz/TBAClO4 is smaller than PCz/TBABF4 (12 and 90 Ω/ cm2 respectively) probably because of high doping of polymer in TBAClO4/ACN electrolyte and also the fast diffusion of dopant ions at

Fig. 4. AC impedance response of the polymer films at open circuit potential synthesized in TBAClO4/ACN (black) and TBABF4/ACN (red).

Fig. 5. XRD patterns of PCz synthesized in TBAClO4/CAN (black) and TBABF4/ACN (red).

working potential (at open circuit potential) in comparison to TBABF4/ACN as shown in Fig. 4. The results of quantitative analysis are in well agreement with qualitative analysis, further approving the conclusion of slower ion exchange that corresponds to less doping in the case of PCz/TBABF4 drawn from cyclic voltammetry. 3.4. X-ray diffraction pattern PCz formed by TBABF4/ACN was more crystalline than the TBAClO4/ACN system probably because of better alignment of the polymer chain as shown in Fig. 5. Regular arrangement of the polymer film probably affects the morphology of the polymer film. Compact and dense polymer film obtained in TBABF4/ACN with high crystallinity, which reduces the diffusion of the ion in and out of the polymer chain, thus affects the electroactivity and conductivity. The crystal structure of crystalline polycarbazole was analyzed by considering the triclinic crystal system based on matching 2θ values. The cell parameters obtained by using this data are a = 11.2368, b = 9.2374 and c = 6.9157 (these were determined by using Unit Cell program, Method of TJB Holland & SAT Redfern 1995) [32]. The miller indices corresponding to reflections from different planes are also shown in same Fig. 5. The value of the percentage crystallinity is 39.92 based on

Fig. 6. Best fit XRD pattern of PCz synthesized in TBABF4/ACN.


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in Fig. 8. A first order phase transition was observed in both the cases. When the polymer was synthesized in TBABF4/ACN a clear melting point observed showing an endothermic transition at 203 °C, which was also supported by high crystalline nature of the PCz formed in the same electrolyte. However, a clear crystallization temperature was observed in amorphous PCz formed in TBAClO4/ACN. The crystallization temperature was nearly the same as the melting temperature. 4. Conclusion Electropolymerization of carbazole was carried out in two different supporting electrolytes using acetonitrile as a solvent. The effects of supporting electrolytes on electropolymerization, redox property, impedance and crystallinity were studied. The two different electrolytes resulted into two different natures of the polymers i.e. amorphous and crystalline. Further, thermal studies also indicated the crystalline nature by showing clear melting point. The amorphous to crystalline transformation of PCz was also observed on heating, which was shown by DSC. Acknowledgment Fig. 7. I–V plot of PCz-TBAClO4/ACN (♦) and TBABF4/ACN (◄).

Authors are thankful to Prof. D. Pandey for his support in structural characterization of the polymers. the Lorentzian best fitting method and corresponding best fit curve is shown in Fig. 6. 3.5. Conductivity measurement Electrical properties of the composites were studied using mini four probes assembly connected with a 6-1/2 digit digital multimeter. Conductivity of the film was measured using four probe technique at room temperature. I–V study was also carried out by current voltage plot for PCz films, which showed an ohmic current with a symmetric current plot at both sides in the positive and negative voltage ranges as shown in Fig. 7. The conductivity of PCz-TBAClO4 and PCz-TBABF4 was 9.544596 × 10− 2 and 20.44 × 10− 3 respectively. 3.6. Thermal property Thermal properties of the conducting polymers are crucial for application purpose. The differential scanning coulometry (DSC) studies of the polycarbazole films performed in the heating range of 50–350 °C and were conducted at a heating rate of 10°/min as shown

Fig. 8. DSC of PCz synthesized in TBAClO4/ACN (black) and TBABF4/ACN (red).

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