Pattern Formation and Oscillatory Electropolymerization of Thiophene

Indian Journal of Chemistry Vol. 47A, December 2008, pp. 1798-1803

Pattern formation and oscillatory electropolymerization of thiophene Ishwar Das a, *, Namita Rani Agrawal b, Shoeb A Ansari a & Sanjeev Kumar Gupta a aChemistry Department, DDU Gorakhpur University, Gorakhpur 273 009, India Email: [email protected] bSt. Andrews College, Gorakhpur 273 009, India

Received 28 July 2008; revised 15 October 2008

Thiophene has been electrochemically polymerized using organic and inorganic oxidizing agents, 4-toluene sulphonic acid silver salt and perchloric acid, in the absence and presence of ZnSO 4. Electropolymerized aggregates have been characterized by morphological studies, growth kinetics, powder X-ray diffraction, differential scanning calorimetry, thermogravimetry and direct analysis in real time mass spectroscopy measurements. It is found that physical characteristics and morphology of polythiophene depend on the nature and concentration of the anion and field intensity. Morphologies of polymer aggregates using the silver salt of 4-toluene sulphonic acid and HClO 4 are fibrillar and compact respectively. Transitions in morphology have also been observed when doped with ZnSO 4. Growth kinetics during electropolymerization of thiophene has also been studied at different thiophene concentrations, field intensity, and concentrations of oxidizing agents as well as in the presence of ZnSO 4. Growth rate is found to be higher with HClO 4 than with 4-toluene sulphonic acid silver salt. Polymer obtained by using the silver salt is thermally more stable and of low molecular weight (416) than that obtained with HClO 4 (689). During electropolymerization, change in anode potential is observed and has been monitored as a function of time. Correlation between morphology and oscillatory behaviour has been studied.

Interest in conducting polymeric materials is growing Herein, we report results of studies on the rapidly 1,2 . Conducting polymers are at present electropolymerization of thiophene using different intensively studied in view of their multiple potential oxidizing agents. Growth morphologies, growth technological applications 3,4 . Conducting polymers kinetics at different experimental conditions and have recently acquired notable importance in influence of Zn SO 4 have been studied during the 5 nanoscale science and technology . Nanoscale process of electropolymerization. Oscillation in 6 conducting polymers can be used for biosensors , potential during electropolymerization of thiophene 7 electrochemical devices, single electron transistors , has also been reported. Polymer aggregates have been 8 nanotips in field emission displays and chemical characterized by XRD, thermal studies and DART 9,10 sensors etc. Among the numerous conducting mass spectrometry. polymers, polythiophene has become the subject of considerable interest. Although, polythiophenes are Materials and Methods prepared by means of two routes i.e., the chemical Thiophene (Spectrochem), perchloric acid 70% and electrochemical syntheses, the electrochemical (SD Fine Chem. Ltd., AR), 4-toluene sulphonic acid preparation of conjugated organic polymers has been silver salt (Dr. Theodor Schuchardt and Co.), zinc shown to be a useful method for obtaining high 11 sulphate (Qualigens LR), acetonitrile (SD Fine Chem. conducting good quality polymers . The Ltd., LR) were used as such. polymerization reaction is an electrophilic substitution reaction which retains the aromatic structure and Electrochemical synthesis and growth kinetics proceeds via a radical cation intermediate. A review The electrochemical synthesis of polythiophene on the synthesis, mechanism, structure and properties was carried out at room temperature using an of the polythiophene has also been published 9. experimental setup consisting of a petridish Although, the occurrence of oscillations and pattern containing a solution of monomer, acetonitrile and the formation in chemical 12-17 and electrochemical oxidizing agent, in a total of 10 ml of the solution. A systems 18-22 have been known for a long time, such cleaned platinum vertical cathode was immersed into phenomena during electropolymerization is rarely the solution while the other platinum vertical anode known. was put at air/liquid interface, 2.5 cm apart. These DAS et al. : PATTERN FORMATION AND OSCILLATORY ELECTROPOLYMERIZATION OF THIOPHENE 1799

electrodes were attached to a potentiostat (Scientific, presence of ZnSO 4 at different time of polymerization India) to supply constant potential. Polymers were were taken (Fig. 2). Thiophene concentration was also synthesized using 4-TSS and HClO 4 oxidizing agents varied in the range 0.5-2.0 M while concentration of in the absence and presence of ZnSO 4 (0.0008 M). 4-TSS or HClO 4 was in the range 0.025-0.125 M. Polymerization started at the anode as soon as the Field intensity was varied in the range 1.6-8.0 V/cm. potential was applied across the electrodes. The growth kinetics was studied by weighing the Microphotographs of aggregates were taken with a washed and dried aggregates as a function of time of Kodak digital camera (Fig. 1). Microphotographs of polymerization, field intensity and [Thiophene]. polythiophene using HClO 4 in the absence and Results are shown in Figs 2 and 3.

Table 1 — Intense lines in the XRD patterns of (a) polythiophene polymerized using 4-TSS (b) polythiophene polymerized in presence of 4-TSS and ZnSO 4 (c) polythiophene polymerized using HClO 4 and (d) polythiophene polymerized in presence of HClO 4 and ZnSO 4. [Conditions: [Thiophene] = 1.5 M,

[4-TSS] = [HClO 4] = 0.05 M, [ZnSO 4] = 0.0008 M, field intensity = 4.8 V/cm, separation between electrodes =2.5 cm] Polymer aggregates obtained from 4-TSS 4-TSS+ZnSO4 HClO4 HClO4+ZnSO4 2θ (°) Rel. int. 2θ (°) Rel. int. 2θ (°) Rel. int. 2θ (°) Rel. int. (%) (%) (%) (%) 3.412 100 19.058 100 22.732 100 24.666 100 4.616 91 4.770 99 25.907 94 25.752 86

Fig. 1 — Microphotographs of polythiophene aggregates obtained 24.361 86 4.374 94 26.424 83 19.341 85 in (a) 4-TSS and acetonitrile, (b) 4-TSS, ZnSO and acetonitrile, 4 27.430 84 23.991 80 13.917 70 22.644 84 (c) HClO 4 and acetonotrile and (d) HClO 4, ZnSO 4 and acetonitrile systems. Conditions: [Thiophene] = 1.5M, [4-TSS] = [HClO 4] = 23.254 84 0.05 M, [ZnSO 4] = 0.0008M, field intensity = 4.8 V/cm, separation between two vertical electrodes = 2.5 cm.

Fig. 2 — Plots showing amount of electropolymerized aggregates as a function of time in presence and absence of ZnSO 4 additive. Fig. 3 — Dependence of amount of electropolymerized [( ●) thiophene-4-TSS-acetonitrile, ( ▲) thiophene-4-TSS- ZnSO 4- aggregates on field intensity in (a) thiophene-4-TSS-acetonitrile acetonitrile , ( ○) thiophene-HClO 4-acetonitrile, ( ∆) thiophene- and (b) thiophene-HClO 4-acetonitrile systems. [Conditions: HClO 4-ZnSO 4-acetonitrile . Conditions: [Thiophene] = 1.5 M, [Thiophene] = 1.5 M, [4-TSS] = [HClO 4] = 0.05 M, field intensity [4-TSS] = [HClO 4] = 0.05 M, [ZnSO 4] = 0.0008 M, field intensity = 1.6, 3.2, 4.8, 6.4 and 8.0 V/cm, separation between two vertical = 4.8 V/cm, separation between two vertical electrodes = 2.5 cm]. electrodes =2.5 cm].

1800 INDIAN J CHEM, SEC A, DECEMBER 2008

Fig. 4 — Anode potential changes with time during electropolymerization of thiophene and their growth morphologies (a, b) in thiophene-4-TSS-acetonitrile system and (c, d) thiophene- HClO 4-acetonitrile systems. [Conditions: [Thiophene] = 1.5 M, [4-TSS] = [HClO 4] = 0.05 M, [thiophene] was varied in the range Fig. 5 — Anode potential changes with time during electro- 0.5-2.0 M. Field intensities: 4.8V/cm in each case]. polymerization of thiophene and growth morphologies in the presence of ZnSO 4 (a, b) thiophene-4-TSS- ZnSO 4-acetonitrile Oscillation in anode potential with time during system and (c, d) thiophene-HClO 4-ZnSO 4-acetonitrile system. electropolymerization [Conditions: [Thiophene] = 1.5 M, [4-TSS] = [HClO 4] = 0.05 M, The same experimental setup as employed earlier Field intensity = 4.8V/cm]. was used to monitor the potential changes at the anode during the electropolymerization of thiophene (Fig. 1c) morphologies were obtained for organic using oxidizing agents 4-TSS (0.05 M) or HClO 4 sulphonate and inorganic perchlorate ions (0.05 M) at an air/liquid interface. Calomel reference respectively. The anion has a substantial effect on the electrode was used. Influence of ZnSO 4 (0.0008 M) structure, growth rate and the degree of anisotropy of on oscillatory behaviour was also studied. Results are the resulting polymer. Influence of Zn SO 4 on the shown in Figs 4 (a, d) and 5 (a, c). morphology of polymer aggregates was also studied and results are shown in Fig. 1 (b, d). In the case of Characterisation 4-TSS, addition of ZnSO showed a morphological Powder X-ray diffraction patterns of polythiophene 4 transition from fibrillar to compact while in case of aggregates obtained using 4-TSS and HClO as 4 HClO , a transition from compact to branched was oxidizing agents in the absence and presence of Zinc 4 observed. In the latter case, the dependence of sulphate were taken in the 2 θ range 0-100 o. Results morphology on time of polymerization was also are recorded in Table 1. studied in the absence and in presence of ZnSO . Thermal characteristics of polymer aggregates 4 were studied by DSC and TG up to 500°C using Growth kinetics of polymerization was studied by Mettler Toledo differential scanning calorimeter measuring the weight of polymerized aggregates as a (model DSC 821). function of time in (a) thiophene – 4-TSS – aceto- Direct analysis in real time (DART) mass spectra nitrile, (b) thiophene – 4-TSS – ZnSO 4 – acetonitrile, of polymer samples were taken using Jeol-Accu TOF (c) thiophene – HClO 4 – acetonitrile and (d) thiophene JMS-T100LC mass spectrometer. – HClO 4 – ZnSO 4 – acetonitrile systems (Fig. 2). These results show that weight of the polymerized Results and Discussion aggregate depends on the time of polymerization. In Polythiophene aggregates were synthesized systems (a) and (c) the system follows an empirical electrochemically using (i) 4-toluene sulphonic acid equation, w2 = mt + c , where w is the weight of silver salt and (ii) perchloric acid oxidizing agents in polymer aggregate at time t, m and c are slope and the absence and presence of ZnSO 4. Acetonitrile was intercept respectively. It is also observed that the used as solvent. Corresponding morphologies are growth rate of polymerization was higher when shown in Fig. 1. Fibrillar (Fig. 1a) and compact HClO 4 was used than that observed with 4-TSS. This

DAS et al. : PATTERN FORMATION AND OSCILLATORY ELECTROPOLYMERIZATION OF THIOPHENE 1801

may be due to the difference in anion-polymer Polymer aggregates were characterized by XRD, interaction. Perchloric acid is more rapidly ionized in DSC, TG and DART mass spectroscopy. XRD acetonitrile than 4-TSS and thus forces the polymer patterns of the polymer aggregates, obtained by using backbone to release more and more electrons to the 4-TSS and HClO 4 were compared (Table 1). Results cathode. In the case of oxidation with 4-TSS, indicate that the aggregates are different. On incorporation of ZnSO 4 in the medium reduced the incorporation of Zn SO 4, the product is changed in growth while in the case of HClO 4, the growth was both the cases (Table 1). The broadening is related increased. In all these cases the empirical equation with the particle size using Scherrer formula for m w = ct was followed as evident by linear plots spherical particles 24 , between log w vs log t . Growth also depends on the field intensity and [Thiophene] (Fig. 3). 4 9.0 λ D = × Results show that the growth of polymerization 3 3Bcos θ depends on the concentration of oxidizing agent Non- 2 where D the diameter of the spherical particles, is linear plots obeying the empirical equation w = related to the full width at half maximum B of the m[oxidizing agent]+ c for systems (a) and (b) were peak centered at θ recorded with X-rays of obtained. The growth of polymerization also depends wavelength λ. This relation explains that with change on the field intensity and [Thiophene]. The sigmoidal in B, the particle size will also change. shaped curve was obtained when 4-TSS was used Simultaneous DSC and TG thermograms of while with HClO 4 growth depends on field intensity polymer aggregates (systems a and c) were taken upto and [thiophene] with no induction time obeying the 500°C. Polymer aggregates obtained by using 4-TSS equations w2 = mV + c and w = c[Thiophene] m and HClO 4 show different thermal properties. When respectively. Figure 4 shows anode potential changes 4-TSS was used, the aggregate shows two endotherms with time during electropolymerization and the at 83.4° and 300°C with corresponding weight losses corresponding morphologies when 4-TSS and HClO 4 of 3.19% and 29.12% respectively, while with HClO 4 oxidizing agents were used. Oscillations were these endotherms were obtained at 100.1° and observed in each case. Small portion of traces after 5 280.5 °C with the corresponding weight losses of minutes of polymerization are shown in Fig. 4 (a, c). 14.94% and 30.4% respectively. Thus, polythiophene Amplitude of oscillation was more in the case of with 4-TSS is found to be thermally more stable than thiophene-4-TSS-acetonitrile system than in the that with HClO 4. Polythiophenes synthesized with thiophene-HClO 4 – acetonitrile system. It is generally 4-TSS and HClO 4 have different molecular weights believed that the morphology of crystals strongly and thus length of conjugation as evident by their depend on the distance from the thermodynamic DART mass spectra. equilibrium. Under these conditions, pattern formation is accompanied by oscillations in potential. The mechanism of electropolymerization of Weight of polymer aggregates ( w) was plotted as a thiophene starting from the monomer can be function of [Thiophene]. In the case of oxidation with schematically represented as shown in steps (i)-(v): 4-TSS, w versus [Thiophene] plot is sigmoidal with (i) In presence of electric field, the oxidation of an induction period and the morphology is fibrillar thiophene monomer to the radical cation is the (Fig. 4b) while in the case of HClO 4, there is no first step (Scheme 1). induction period and the morphology is compact. (ii) The electron thus released is associated with the + + Sigmoids and morphology (fibrillar) both support Ag from 4-TSS or H from HClO 4 (Scheme 2). instability. When ZnSO was doped, the amplitude of 4 (iii) Formation of a dimer: There are two possibilities oscillations was increased in both the systems (Fig. 5) for the formation of a dimer and a transition from compact to branch was observed. Such a transition in morphology may be (a) Radical cation-monomer coupling explained in terms of the surface instability 23 . (Scheme 3). Diffusion limited aggregation is a process where in (b) Radical cation-radical cation coupling successive particles moving in space undergo a (Scheme 4). random walk until they interact with a growing (iv) Chain propagation via oxidation of the fractal. extending polymer (Scheme 5).

1802 INDIAN J CHEM, SEC A, DECEMBER 2008

Scheme 1

Ag + + e - Ag (deposited at cathode)

+ - H + e ½ H 2 (g) Scheme 2

Scheme 3

Scheme 4

Scheme 5

DAS et al. : PATTERN FORMATION AND OSCILLATORY ELECTROPOLYMERIZATION OF THIOPHENE 1803

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