Quartz crystal microbalance characterization of electrochemical doping of polyaniline films

Synthetic Metals, 61 (1993) 291-296

291

Quartz crystal microbalance characterization of electrochemical doping of polyaniline films R.M. Torresi

a n d S.I. C o r d o b a

de Torresi

Departamento de Fisica Aplicada, Instituto de Fisica, Universidad Estadual de Campinas, Caixa Postal 6165, 13081 Campinas (SP) (Brazil)

C. G a b r i e l l i , M . K e d d a m

and H. Takenouti

UPR15 CNRS, Physique des Liquides et Electrochimie, Universit~ P. et M. Curie, Tour 22, 4 place Jussieu, 75252 Paris Cedex 05 (France)

(Received May 21, 1993; accepted July 1, 1993)

Abstract The electrochemical ionic exchange of polyaniline film-coated electrodes was studied in detail by electrogravimetry using an electrochemical quartz crystal microbalance. The influence of film thickness, electrolyte composition and potential sweep rate is discussed. The data obtained from these measurements establish the dependence of ion population inside the film on the degree of oxidation from -0.25 to 0.5 V (versus SCE). Analysis is made by calculating the relative contribution of anions and cations to the polymer electroneutrality from the electrochemical quartz crystal microbalance and electric charge data.

Introduction The electrochemical quartz crystal microbalance (EQCM) seems to be a very suitable method for analysing the role played by different ions and solvent molecules belonging to their solvation spheres in the redox processes occurring in conducting polymer films. After the pioneer work of Orata and Buttry [1] dealing with polyaniline (PANI) films, several papers focused on different aspects of the study of polymer films by quartz crystal electrogravimetry [2-5]. Polyaniline is, among conducting polymers, a particularly attractive material and it has been widely studied by the quartz crystal microbalance [2, 6-8]. In most of these papers, authors agree with the fact that oxidation/reduction processes produce the exchange of both anions and cations between the PANI film and the electrolyte in order to maintain polymer electroneutrality. Electrochemical impedance combined with EQCM in an a.c. regime allows the flux of cations and anions to be calculated individually [9]. Data obtained in this way demonstrated that neither the anion incorporation flux nor the cation expulsion flux becomes negligible in the potential range studied. Measurements performed under potentiodynamic conditions can be analysed quantitatively by calculating the molar fraction of total ions exchanged in the redox

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process from coulometric and gravimetric results. Recently, this analysis has been extended to polypyrrole films in aqueous media [10]. In the present work, PANI films are studied by the combination of E Q C M and potentiodynamic experiments. Quantitative analysis of the results is made for different experimental conditions in order to elucidate the influence of thickness and anion effect on PANI redox processes.

Experimental A platinum wire was used as the counter electrode and all potentials are referred to the saturated calomel electrode (SCE). Polyaniline films were grown on a gold electrode from a 1 M HC1 + 0.15 M C6HsNHz solution by applying triangular potential sweeps (v=0.05 V s -a) between - 0 . 2 and 0.7 V (versus SCE). The thickness of the film was determined by optical interferometry. After deposition, films were rinsed with purified water and placed in a conventional electrochemical cell with 1 M monomer-free HCI, HNO3 or HCIO4 electrolyte. Electrochemical measurements were performed under potentiodynamic conditions and the current density profiles were recorded simultaneously with the changes of resonance frequency of the quartz oscillator.

© 1993 - Elsevier Sequoia. All rights reserved

292

The working electrode was an 'AT-cut' 6 MHz quartz crystal (16 mm in diameter) with a gold layer deposited in key form on both sides (A = 0.2 cm2). This piezoelectric element was mounted in a Teflon holder [11] and one of the gold layers (in contact with the electrolytic solution) was attached to both the oscillator circuit and the potentiostat and thereby to ground. The resonance frequency shift was measured with a Schlumberger 2721 frequency counter. The Sauerbrey equation can be applied to relate the mass change per unit area, Am (in g cm-2), to the frequency shift h f (in Hz). The proportionality factor K value is 5.2×107 Hz g-1 cm 2, as determined by calibration of the EQCM by the silver deposition method [12]: Af= - K A m

(1)

Effect of film thickness The stabilized j-E and Am-E potentiodynamic profiles recorded for three different film thicknesses are shown in Fig. 2. Both the total anodic charge and the gain of mass increase with film thickness but they behave in two different ways: while the charge increases linearly in the thickness range studied here, the gain of mass follows a nonlinear behaviour as the film swelling is more important for thicker films. These fact, shown in Fig. 3, indicate that the redox process takes place in the bulk of the polymer matrix and not only at the electrode surface. In order to elucidate the role played by ion exchange in the redox processes, the representation of Am versus Q/F was chosen. From the slope of this kind of plot, it is possible to calculate the relationship between the molecular weight of the species and the number of electrons involved in the reaction according to

Q =nF£izixi

Results and discussion

and

Film formation Figure 1 shows the mass gain as a function of time during the electropolymerization of polyaniline when the electrode is polarized by the potential-time programme shown in the inset. It must be noted that appreciable change in mass was only observed after one hour of cycling (100 cycles). From this point, mass changes produced by redox processes in the film began to be observed and they became greater as the electropolymerization time increased. It must be pointed out that the ionic exchange during oxidation/reduction became more relevant when the thickness of the film was larger. A very important feature to be observed in Fig. 1 is the fact that the slope of the curve Am/t keeps increasing with time. This shows that the electrodeposition efficiency increases as the cycling continues. This phenomenon is opposite to that observed by Gottesfeld and co-workers [8] for films grown under galvanostatic or potentiostatic conditions.

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,

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Fig. 2. j-E (a) a n d A m - E (b) p o t e n t i o d y n a m i c profiles r e c o r d e d with different t h i c k n e s s e s o f P A N I films: ( - - ) d = 270; ( - - - ) d = 340; ( - - - ) d = 500 n m . 1 M HC1 electrolytic solution; v = 0.1 Vs-l.

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Fig. 1. G a i n o f m a s s as a function o f time r e c o r d e d d u r i n g electropolymerization o f PANI film.

0

I 0

250

0 500

d/nm

Fig. 3. T o t a l m a s s c h a n g e (©) a n d total anodic charge (O) as a f u n c t i o n of film thickness.

293

•n = n ~ixiMWi

I

Thus

075 x

Am = Q ~,x, MW, F Zizixi

where x, is the molar fractional contribution of the ith species to the mass changes, so ~]~xi= 1, and MW~ is the molecular weight of these species. The charge density was calculated by integration of the E--j potentiodynamic profiles recorded between -0.25 and 0.5 V. In Fig. 4, the Am versus QS/F diagrams for the three different film thicknesses are shown. According to eqn. (2), the slope of these plots should be independent of film thickness. However, experimental data show that, for a given Q/F value, the thicker the polymer film the greater is the gain of mass. This fact indicates that the relationship between the amount of incorporated anions and ejected protons (oxidation process) depends on the amount of electroactive polymer. In order to consider the fractional contribution of protons and anions in both the oxidation and reduction processes, it is possible to calculate thexi values involved in the charge compensation as a function of potential from the changes of mass and electric charge referred to the initial value of the potential sweep. Taking into account that ZA- =ZH~o+ = 1

(3)

eqn. (2) can be rewritten as Am(E)=

Q(E) F X {xA-(E)MWA- - (1 - x ^ - (E))MWH.~O+}

(4)

25 d=500 nrn

.o

15 I0

0 15 d,340nm

z <

5 0 I0

d-270nm

0

0

I

2

0.5

(2)

t

i

3

4

[O/F] x I0t/mol cm 2

Fig. 4. M a s s c h a n g e as a function o f Q/F for different t h i c k n e s s e s of P A N I films in 1 M HCI electrolytic solution. D a t a t a k e n from Fig. 2.

l

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0 75

x

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025

0 -025

J

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0

025

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05

EIV

Fig. 5. M o l a r fractional c o n t r i b u t i o n o f p r o t o n s a n d chloride ions as a f u n c t i o n o f potential: ( A ) d = 2 7 0 ; ( t t ) d = 3 4 0 ; (O) d = 5 0 0 n m . D a t a t a k e n f r o m Fig. 4.

Rearranging eqn. (4), XA-(E) can be written as MWn,~o+ +MWA-

XA-(E) = MWH3o+

Am(E)F + Q(E)(MWH3o+ +MWA-)

(5)

Using eqn. (5), the dependence of x,- on potential was calculated for the three film thicknesses studied. It is necessary to indicate that in the anodic sweep xi-i3o+ is the expelled proton fractional contribution and XA- is the incorporated anion fractional contribution. We consider that solvent molecules take part in the anionic exchange process as belonging to the solvation sphere of protons. As in the cathodic sweep protons are incorporated and anions are expelled, the sense of fractional contributions is opposite to that given for the anodic sweep. These plots are shown in Fig. 5. During the anodic sweep, for the 500 nm thick film, the incorporation of chloride ions is predominant and the proton expulsion is only appreciable at low potentials (from 0.0 to 0.15 V). Conversely, for thinner films, the potential range is larger where proton ejection is observed. During the cathodic process, the ionic exchange is less dependent on film thickness. It can be observed from Fig. 5 that proton transport is more important when oxidation or reduction begins. This fact agrees with results already reported using a quartz crystal microbalance in a.c. regime [9]. The morphology details of PANI films of different thicknesses are compared by SEM micrographs (Fig. 6). In Fig. 6(a), the surface of a 270 nm thick polymer film is shown. A homogeneous and compact polymer layer can be observed which is in contrast with the

294 125

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0.25 -025

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Fig. 8. Molar fractional contribution of protons and different anions as a function of potential: ( 0 ) anodic sweep; (O) cathodic sweep. Data taken from Fig. 7.

(b) Fig. 6. Scanning electron micrographs of P A N I films of different thicknesses: (a) 270; (b) 500 nm. Marks inside the Figure are in p.m.

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film grown to 500 nm. The micrograph of the latter film is shown in Fig. 6(b) and it can be seen that thicker PANI films present a spongy globular structure which leads to a higher surface/volume ratio. It has already been pointed out [8, 13, 14] by means of optical and ellispometric observations that film density changes from a more compact structure near the electrode surface to a less dense structure with increasing distance from the metal substrate. This density variation is also predicted by the theory of de Gennes [15]. According to the ionic fractional contributions calculated from eqn. (5), the lower the film density the higher is the participation of anions in the conservation of electroneutrality. These features could explain the nonlinear behaviour of mass as a function of thickness shown in Fig. 3 because anion incorporation is more important for thicker films.

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05

E/V

Fig. 7. j-E (a) and A m - E (b) potentiodynamic profiles recorded with different 1 M electrolytic solutions for d = 270 nm and v = 0.1 V s - l : ( - - ) HCI; ( - - - ) HNO3; ( - - - ) HCIO4.

Effect of electrolyte composition The shape of the voltammograms of polyaniline films (d = 270 nm) was practically unaffected by the nature of the electrolyte used (HCI, HNO3 or HCIO4), as shown in Fig. 7(a). This could indicate that all anions studied here move freely inside the film. However, the change of mass strongly depends on the mass of the anion present in the electrolyte solution (Fig. 7(b)). Considering the same approach as discussed above, it is possible to calculate xi(E) values for the different electrolytes used. These results are shown in Fig. 8. During the anodic scan, it is not possible to establish marked differences due to the change of the electrolytic solution and the anion incorporation process is more

295 important at higher potentials. On the contrary, during the cathodic scan (reduction process), there are striking differences between the curve recorded with C 1 0 4 - containing solutions and curves obtained for NO3- or CI- solutions. In the case of H C l O 4 solution, in almost the entire potential range studied, the charge balance is made principally by anion expulsion. The participation of protons in the charge balance is only appreciable at the beginning of the reduction process. This behaviour could be explained by taking into account the fact that the electrostatic interactions between the anion and the positive centres of the polyaniline matrix are weaker when the charge/size ratio of the anion is lower. This situation can be summarized considering that, during the oxidation process, the charge/size ratio of the anion does not play a relevant role for maintaining the polymer electroneutrality. However, during the reduction process, the amount of expelled anions increases for a larger monovalent anion.

Effect of sweep rate Potentiodynamic experiments with a 270 nm polyaniline film in 1 M HC1 solution at different sweep rates were performed. From the charge of the voltammogram and the change in mass, xi(E) values were calculated according to eqns. (4) and (5). The results are shown versus the potential in Fig. 9. The variation of sweep rate does not modify appreciably the relationship x a - ( E ) / W ~ o ÷ (E) during the oxidation process. When potentials were scanned towards more negative values at 10 or 100 mV s-1 sweep rates, both anions and protons moved inside the polymer to maintain electroneutrality. On the contrary, at 1 mV s-1 sweep rate, protons entered the polymer matrix until a potential of 0.25 V was reached; after that, charge balance was achieved only by ejecting anions. 1.25

i

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t

z~

0.25

'~ 0 7 5

05

0.5

025

0.75 0 o25

'5 075 I x O5 025 -0 25

+o I x

x

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05 i 0 25

0

075 0.5

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Fig. 9. Molar fractional contribution of protons and chloride ions as a function of potential for PANI films (d=270 nm) in 1 M HCI electrolytic solutions cycled at different potential sweep rates: (A) 0.001; (0) 0.01; (0) 0.1 V s-'.

TABLE 1. Current peak-potential sweep rate relationship, anodic charge (Qa) and anodic charge-cathodic charge relationship for polyaniline film (d=270 nm) in 1 M HCI

v (my s -I) 1 10 100

• ,/3-1 Jp.a (mF cm -2)

100 90 120

Q~ (mC cm -2)

Qa/Qc

24 23 27

1.10 0.95 1.05

Different parameters as a function of potential sweep rate are shown in Table 1. The electric charge involved in the electro-oxidation process is independent of sweep rate in the range studied here. This fact proves that the amount of active material is always the same. However, mainly in the cathodic sweep, the way in which electroneutrality is achieved depends on the sweep rate: the slower the potential sweep the higher is the participation of the anion in the ionic exchange process• Furthermore, when the system is perturbed in a fast way, film electroneutrality is mainly reached by the movement of protons; when the system is slowly perturbed, the species responsible for maintaining the electroneutrality are principally the anions.

Conclusions

The results described above reveal that in the PANI electropolymerization by cycling, the rate of film growth increases with time. This phenomenon is opposite to that observed by other authors for PANI films grown by potential or current steps. With respect to the polymer redox behaviour in monomer-free solutions, EQCM experiments show that proton transport is predominant at the beginning of the oxidation and reduction processes. For different thicknesses of films, the anion transport becomes more important as film thickness increases probably due to the increase of surface area. Experiments performed with different electrolytes show that the chemical nature of the anion only affects the reduction process where the amount of expelled anions increases whilst the charge/size ratio of the anion diminishes. Data obtained for different potential sweep rates show a faster kinetics for proton incorporation than for anion expulsion during the reduction process. The scan rate has not a marked influence on the relative contribution of protons and anions during the oxidation process.

296

Acknowledgement The authors thank Dr S. Pereira Nunes (IQ-UNICAMP) for SEM photomicrographs.

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6 H. Daifuku, T. Kawagoe, N. Yamamoto, T. Ohsata and N. Oyama, J. Electroanal. Chem., 274 (1989) 313. 7 H. Daifuku, T. Kawagoe, T. Matsunaga, N. Yamamoto, T. Ohsata and N. Oyama, Synth. Met., 41-43 (1991) 2897. 8 J. Rishpon, A. Redondo and S. Gottesfeld, Z Electroanal. Chem., 294 (1990) 72. 9 S. Cordoba de Torresi, C. Gabrielli, M. Keddam, H. Takenouti and R. Torresi, J. Electroanal. Chem., 290 (1990) 269. 10 R.C.D. Peres, M.A. de Paoli and R.M. Torresi, Synth. Met., 48 (1992) 259. 11 C. Gabrielli, M. Keddam, H. Takenouti and R. Torresi, in T.A. Ramanarayaram and H.L. Tuller (eds.), Proc. Syrup. Ionic and Mixed Conducting Ceramics, Vol. 91-12, The Electrochemical Society, Pennington, NJ, 1991, p. 172. 12 C. Gabrielli, M. Keddam and R. Torresi, J. Electrochem. Soc., 138 (1991) 2657. 13 C.M. Carlin, L.J. Kelper and A.J. Bard, J. Electrochem. Soc., 132 (1985) 353. 14 J. Bacskai, V. Kertesz and G. Inzelt, Electrochim. Acta, 38 (1993) 393. 15 P.G. de Gennes, Macromolecules, 14 (1981) 1637.