Investigation of the early stages of the electropolymerization of o-toluidine by UV-Vis reflectance spectroscopy

Synthetic Metals, 62 (1994) 9-15

9

Investigation of the early stages of the electropolymerization of o-toluidine by UV-Vis reflectance spectroscopy J . - M . L 6 g e r , B. B e d e n

a n d C. L a m y

Laboratoire de Chimie 1, URA CNRS 350, Universit~ de Poitiers, 40 Avenue du Recteur Pineau, 86022 Poitiers (France)

P. O c o n

a n d C. S i e i r o

Departamento de Quimica Fisica Aplicada, Facultad de Ciencas, Universidad Autonoma de Madrid, 28049 Madrid (Spain)

(Received February 9, 1993; in revised form June 22, 1993; accepted July 7, 1993)

Abstract The electrochemical synthesis of poly(ortho-toluidine) in acid medium was studied by 'in situ' UV-Vis differential reflectance spectroscopy. Thus, it was possible to follow the early stages of the formation of the conducting polymer at a gold electrode. With low concentrations of o-toluidine, an intermediate species was formed on the electrode surface during the first cycles. UV-Vis reflectance spectra of this species, recorded 'in situ', showed that it is different from that of the polymer itself, which can be observed after several potential cycles with low concentrations of monomer, or at the first cycle with concentrations of o-toluidine greater than 0.1 M. The likely role of this intermediate species as a precursor of the electropolymerization process is discussed.

Introduction The growing interest for practical applications of conducting polymers has led, in recent years, to numerous fundamental multi-disciplinary studies with a wide range of experimental techniques [1, 2]. Even if it is possible to synthesize conducting polymers by various chemical processes, electropolymerization is a very convenient way to form reproducible polymer layers. One of the main advantages of the electrochemical synthesis is that the electrode can act simultaneously as an initiator, a monitor and as a deposition substrate. Obviously, such an electrochemical preparation needs the use of a convenient solvent for the monomer molecule. For example, polyaniline, which is probably one of the most studied polymers, can be obtained by electropolymerization in aqueous or nonaqueous media. With ortho- or m e t a - t o l u i d i n e , it is also easy to obtain polymeric layers in acid aqueous media. Whatever the techniques used for their preparation, the conducting forms of polyaniline and polytoluidine are stable in air and water, and are insoluble in common organic solvents. These polymers have proton exchange and redox properties. Only a few studies have been devoted to the growth of such polymers [3-5] and the mechanisms are not yet well understood. According to some authors, the film growth under potential cycling conditions has been reported to be linear [6] or quadratic [7] with the number of cycles. This macrogrowth [8]

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occurs in three dimensions. Mechanistically, it can be achieved practically and described theoretically in two alternative ways: either three-dimensional nucleation and growth, or two-dimensional layer-by-layer nucleation and growth [9, 10]. However, one of the critical steps of the electrochemical polymerization is the formation of the first layer at the electrode surface. Further polymerization then occurs on the first layer and the growth process starts. The aim of the present work is to obtain information concerning the early stages of the electropolymerization ofo-toluidine at a gold electrode in an aqueous medium. For such a purpose, it is clear that only'in situ' techniques can give useful information. UV-Vis differential reflectance spectroscopy (UVDRS) was used in this work. This technique allows us to record a complete set of spectra in the range 250-800 nm during a voltammetric sweep. To our knowledge, only 'in situ' UV-Vis spectroscopic information taken after growth of the polymer layer is available in the literature. In the case of polyaniline, Stilwell and Park [11] recorded different UV-Vis spectra under different conditions. They were able to identify, thanks to their absorption bands, different structures of polyaniline corresponding either to conducting or to insulator forms. Such kinds of information are very valuable, but give no indication concerning the early stages of the electropolymerization process.

10

Experimental Electrochemical measurements were carried out with a PAR Model 362 potentiostat-galvanostat. The spectroelectrochemical cell used was a standard three-electrode system with quartz windows in order to obtain reflectance spectra in the UV-Vis range. A gold disk (area = 0.50 cm2) was used as a working electrode. Prior to each measurement, the gold disk electrode was polished to mirror-finish with alumina powders (down to 0.1 Izm). The reference electrode was a Hg/Hg2SO4/ saturated KzSO4 electrode (MSE) and the counter electrode was a gold wire. The supporting electrolyte (sulfuric acid solution) was prepared from Merck 'suprapur' product and ultrapure water (Milli Q from Millipore). ortho-Toluidine was provided by Fluka and was used after distilling it to eliminate oxidized impurities. The distilled o-toluidine was stored under nitrogen atmosphere in the dark. All experiments were performed at room temperature. During the experiments, a nitrogen atmosphere (U quality from L'Air Liquide) was maintained above the electrolytic solution. The electrochemical polymerization of o-toluidine was carried out in solutions containing o-toluidine at a concentration varying from 0.01 to 0.5 M in 1 M sulfuric acid. Typically, the electropolymerization on the gold surface was obtained by continuous potential cycling between -0.5 and 0.55 V versus MSE at 5 mV s -1. This sweep rate was chosen in order to be compatible with the spectroscopic measurements. UV-Vis differential reflectance measurements were carried out using a Harrick RSS-C rapid scan spectrometer. Spectra were accumulated and averaged with a Nicolet 370 data acquisition system connected to a PC-AT microcomputer. Specific software for data acquisition, data transfer and graphic treatment (in twoor three-dimensional diagrams) was used [12]. One of the main advantages of this equipment is to give the possibility to record UV-Vis spectra with a high acquisition rate, typically 10 ms per individual spectrum in this work.

-1,0

os

0 -O'Z.

(a)

'

-02 '

0

0.2 . .

-02

0

0.2

o

o'2

.

04 . E/V(HSE)

q'6.~ ~10

05

0

-05 -0A-

(b) ~'8 ,~

0¢

E/V[HSE)

s

0

'-o'2

(c)

'

' EIVIMSE)

Fig. 1. Voltammograms of a gold electrode in 1 M sulfuric acid + x M o-toluidine: (a) x = 0 . 0 2 ; (b) x = 0.07; (c) x=0.5. Scan rate 5 mV s - l ; room temperature.

Results and discussion Voltammetric response Figure 1 presents different voltammograms recorded during the electropolymerization of o-toluidine on a gold surface. All the curves were obtained at a low sweep rate (5 mV s-1) and for different organic concentrations (0.02, 0.07 and 0.5 M) in 1 M sulfuric acid. For rather high o-toluidine concentrations (Figs. l(b) and (c)) the voltammograms obtained are similar to those already published [13-16]. Starting from the lower potential limit, polymerization begins only when a po-

tential greater than 0.3 V versus MSE is applied. The huge peak current, which appears then, is generally attributed to the oxidation of the monomer. It is important to mention that the intensity of this peak is similar irrespective of monomer concentration. This peak can be clearly related to the oxidation of the monomer previously adsorbed at the electrode surface. The electropolymerization itself starts only during the following potential cycles. When the electropolymerization is initiated, different peaks, typical of the polymer, appear both during the

11

positive and negative sweeps. The rate of growth of the polymer layer depends greatly on toluidine concentration. Even under very dilute conditions, the polymer is formed, as visible in the voltammograms (Fig. l(a)) by the continuous increase of the current peaks in the region - 0 . 3 to 0.3 V versus MSE, due to the oxido-reduction of the polymer. However, even after several tens of potential cycles, no visible polymer layers appear on the gold surface, but the shape of the voltammograms and the UVRDS measurements (see below) confirm the presence of such a polymer layer with a very small thickness at the electrode surface. Conversely, with more concentrated solutions (greater than 0.1 M), a dark-green layer appears rapidly; the greater the concentration, the faster the polymerization, so that the layer formed darkens only after a few cycles.

2.00

aso

a6o

47o

~ao

89o

coo

Wave length /rim

Fig. 3. Same as in Fig. 2, but in a two-dimension diagram (absorbance vs. wavelength) for different electrode potentials.

UV-Vis reflectance spectra With the UVDRS equipment used in this work, it is possible to record complete UV-Vis reflectance spectra in a rather short time. As a first example, Fig. 2 presents a three-dimensional absorbancewavelength-potential diagram obtained with spectra recorded typically each 50 mV during both positive and negative sweeps, at 5 mV s -~. Each spectrum is the average of 50 individual spectra, which takes only 0.5 s, and is subtracted from the first one, taken as a reference. In order to have a more precise idea of the position of the absorption peaks, it is more convenient to present the spectra in two dimensions, absorbance versus wavelength for each electrode potential (Fig. 3). As can be seen in Fig. 2, during the first potential positive sweep, the differential reflectance spectra display no significant bands until 0.3 V versus MSE, which corresponds to the beginning of the oxidation peak visible in the voltammogram (Fig. l(c)). For potentials

,

/

7 2.00

PO

ent

0.00 1a 1

250

525

800

Wave leng~.h /nm

Fig. 2. Three-dimensional diagrams (absorbance-wavelengthpotential) recorded during the growth of a poly(o-toluidine) layer. All spectra are presented after subtraction from the first one, taken as a reference. Each experimental spectrum, taken every 50 mV, is obtained by accumulation and averaging of 50 spectra (accumulation time: 0.5 s); gold electrode in 1 M sulfuric acid + 0.5 M o-toluidine; 5 m V s-~; first potential cycle; room temperature.

g

tint.

I eI

2~0

~a5

1

sOO o. oo

Wevelangth /nm

Fig. 4. Same as in Fig. 2, b u t f o r t h e 2nd p o t e n t i a l cycle.

greater than 0.3 V, two clear bands are visible in the spectra and their intensities grow rapidly with potential. When they appear, these bands are situated around 460 and 700 nm, but their positions vary with potential and shift progressively towards 490 and 680 nm, mainly during the reverse sweep. Two shoulders are also seen at around 360 and 750 nm. It should be noted that, even after one cycle, the polymer layer formed on the electrode surface is clearly seen in the UV-Vis reflectance spectra. The evolution of the spectra during continuous cycling is illustrated in Figs. 4 and 5 for the second and the fourth potential cycles. The more significant change observed in the spectra is the growth with potential of a strong absorption band around 615 nm at the end of the second cycle. Thus, Fig. 5 corresponds to a rather thick layer of polymer formed on the gold electrode surface. In this case, the shape of the spectra obtained becomes independent of the potential range and, in a two-dimensional representation (Fig. 6), the two main bands are visible at 510 and 610 nm, with a clear shoulder at around 360 nm. Under these conditions, the electrode is totally black. This kind of

12

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~

-o. so.

t.',o o. 5

V/14SE

5

v/me

~

o. s~

i . oo

~

f Po -~0"5

P°tlnt

250 ~

525

le 1 Nevelength

800

ten t

0.00

250

300

la 1

•evelength

/rim

0.00

/nm

Fig. 7. Three-dimensional diagram (absorbance-wavelengthpotential) recorded during the growth of a poly(o-toluidine) layer. Each spectrum, taken every 50 mV, is obtained by accumulation and averaging of 50 spectra (accumulation time: 0.5 s); gold electrode in 1 M sulfuric acid+0.01 M o-toluidine; 5 m V s - l ; first potential cycle; room temperature.

Fig. 5. Same as in Fig. 2, but for the 4th potential cycle.

300

2.40

1

2.50

2.00

8 t.2o

0.60

0.00

i

230

i

i

i

i

360 Were l e n g t h

470

i

i

580

i

i

690

i

i

1.00

300

/nm

Fig. 6. Same as in Fig. 5, but in a two-dimensional diagram

0.50

(absorbance vs. wavelength) for different electrode potentials: (a) - 0.50, - 0.45 and - 0.40 V vs. M S E ; (b) - 0 . 3 5 V; (c) - 0.30 V; (d) - 0 . 2 5 , - 0 . 2 0 and - 0 . 1 5 V; (e) from - 0 . 1 0 to 0 (negative sweep) and from 0 to 0.50 V (positive sweep), a spectrum each 50 mV.

0.00 230

360 dsvelength

470

5fl0

sgo

800

/rim

Fig. 8. Same as in Fig. 7, but in a two-dimensional diagram

(absorbance vs. wavelength) for different electrode potentials.

UV-Vis reflectance spectra is typical of a thick layer of conducting polymer. Similar spectra were obtained with polyaniline films [17]. However, it should be noted that, with such thick layers, the reflectivity of the electrode surface is very poor and its change during a potential cycle is weak. This first approach showed the fast modification of the spectra with potential for thin layers, then the dependence of the spectra on the thickness of the polymer layer. However, under these experimental conditions, it was still difficult to obtain clear information on the early stages of the polymerization process. This information is important because the whole electropolymerization process is supposed to depend on the formation of this first layer. One of the possibilities to decrease the rate of electropolymerization, as pointed out above, is to use more dilute solutions. As described in Fig. l(a), electropolymerization with low concentrations of o-toluidine is very slow. The reflectance spectra obtained with 0.01 M of monomer are shown in Figs. 7 to 11. The three-dimensional

1.50 -0.50.

o.7~ g VI~

t t• 1

525

250 Navelength

800

0.00

/nm

Fig. 9. Same as in Fig. 7, but for the 2nd potential cycle.

diagram for the first sweep (Fig. 7) is rather different from that recorded in more concentrated solutions (Fig. 2). If there are again no absorption bands before 0.3 V versus MSE, a strong and sharp band appears then and increases continuously until the upper potential limit (0.55 V versus MSE). During the first reverse

13

1,00 -0.50

o.so

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0.55

Po

tJal

250

525 Wavelength

600 0.00

/nm

Fig. 10. Same as in Fig. 7, but for the 15th cycle.

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0.40

0.20

0.00

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L

i 360

i

i 470

i

~ 560

i

i 690

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i

BOO

W~velength /nm

Fig. 11. Same as in Fig. 10, but in a two-dimensional diagram (absorbance vs. wavelength) for different electrode potentials.

sweep, this band decreases progressively and disappears completely for potentials below 0.3 V versus MSE. The wavelength position of this band is very stable and can be pointed at 504 nm, as seen in the two-dimensional diagram of Fig. 8. However, the spectra recorded at the lower potential limit, at the end of the first sweep, present important differences with those recorded at the beginning of the first sweep. This spectrum is represented in Fig. 9 as the first spectrum obtained during the second sweep. As in the case of the first sweep, the spectra observed during the second positive sweep present a sharp absorption band of increasing intensity for potentials greater than 0.3 V versus MSE. This band is situated exactly at the same position as that obtained during the first sweep and disappears also during the reverse sweep. During subsequent sweeps, the same phenomena are reproduced, but the band at 504 nm becomes progressively smaller and the spectra obtained at the end of each cycle are modified slowly, mainly in the region between 300 and 450 nm. Figures 10 and 11 represent the three- and two-dimensional diagrams for the 15th cycle, with only a small band at 504 nm for a potential corresponding to the upper limit and an

increase of absorbance in the region around 700 nm. This type of spectra is close to that observed in more concentrated conditions at the beginning of the polymerization process (see Fig. 2). It can be noted that the band at 504 nm exists only in the potential range corresponding to the current peak generally attributed to the electrochemical oxidation of the monomer. Another important remark is that this band does not correspond to any absorption band observed by UV-Vis transmission spectroscopy of an o-toluidine solution in acid medium. To summarize, the spectra observed with the two ranges of concentrations of the monomer present significant differences. In dilute solutions, the UV-Vis spectra exhibit a new absorption band, probably corresponding to the formation of a precursor of the electropolymerization process which appears later. UV-Vis spectra, characteristic of the polymer layer, appear only after several cycles (15 in a 0.01 M otoluidine solution). To confirm the modification observed on the UV-Vis diagrams observed with different concentrations, experiments were carried out for various intermediate concentrations of o-toluidine monomer. The reflectance spectra observed show a continuous modification from the two limiting cases described in detail above. As an example, the spectra recorded during the electropolymerization of 0.02 M o-toluidine display again the band situated at 510 nm, which is formed during the positive sweep, but this band appears less sharp and the spectra corresponding to the polymer layer, after one potential cycle, show bands around 350 and 700 nm, slightly more intense than with 0.01 M o-toluidine. With 0.03 M o-toluidine (Figs. 12 and 13), the gradual evolution of the diagrams is clearly visible, but the absorption band around 505 nm appears wider and wider, with a clear shoulder at 420 nm. Finally, with 0.07 M o-toluidine in acid solution, the three-dimensional diagram (Fig. 14) recorded during the first positive sweep for potentials greater than 0.3 V versus MSE

Novl

length

/nm

Fig. 12. Same as in Fig. 7, but with 0.03 M o-toluidine.

14 1.75

i .50

i

.25

t .20

0.75

0.90 \

i

0.25

O. 30

-0.25

-0.75

I 250

I 360

I

Wavelength

I 470

I

I 580

I

I 690

I

I" 800

2.00 -0.50.

1.00 ~

P°te n ~

, 250

I 250

I 380

0.00

525 Wavslengt~

800 /nm

Fig. 14. Same as in Fig. 7, but with 0.07 M o-toluidine.

exhibits an absorption band with a maximum situated at 509 nm and a shoulder at 370 nm. A third absorption band appears at 666 nm when the potential reaches the upper limit. Then it increases regularly with number of sweeps with a shift towards higher wavelengths, as seen in the two-dimensional diagram corresponding to the third sweep (Fig. 15). After growth of the polymer for 25 cycles, UV-Vis reflectance spectra are very similar to those observed after only four cycles in 0.5 M otoluidine (Fig. 6).

Discussion

All the UV-Vis reflectance diagrams presented in this paper show clearly that 'in situ' spectroscopic techniques allow us to gain information, not only on the conducting polymeric layer, but also on the early steps of the formation of this layer. By using different increasing concentrations of o-toluidine, it is possible to follow the modifications of the absorbance spectra. Table 1 summarizes the main features observed during continuous cycling of a gold electrode in an o-toluidine acid solution. By modifying the concentration of the

*

i

Wavelength

/rim

Fig. 13. Same as in Fig. 12, but in a two-dimensional diagram (absorbance vs. wavelength) for different electrode potentials.

t l II 1

0.00

470

I

I 580

I

I 690

I

I 800

/nm

Fig. 15. Same as in Fig. 14, but in a two-dimensional diagram (absorbance vs. wavelength) for the 3rd potential cycle and for different electrode potentials.

monomer, it is possible to visualize the formation of the first layer, which is the crucial step for further growth of the polymer layer. The polymer itself is characterized by UV-Vis bands situated around 510 and 610 nm, when the layer is sufficiently thick (see Fig. 6). However, it is better to assign the absorption bands of the polymer when the layer is not too thick, i.e. when the reflectivity is rather important. Under such conditions, the spectrum of the polymer displays clearly two bands: one at = 450-470 nm and another at 680-690 nm (see Figs. 13 and 15). When the thickness of the polymer layer is progressively increased, the positions of these bands shift, respectively, to around 510 and 610 nm. However, the most interesting information obtained during this work is that during the first positive sweep, a different intermediate species is produced by the electro-oxidation of the o-toluidine molecule. The UV-Vis reflectance spectrum of this species is characterized by a sharp band at 505 nm. This band is very stable in wavelength position irrespective of potential and when the concentration of the monomer is increased. During continuous cycling, this band exists only close to the upper limit of potential (between 0.3 and 0.55 V versus MSE under these experimental conditions during both positive and negative sweeps). During the polymerization process, the intensity of this band decreases progressively and the UV-Vis reflectance spectra of the conducting polymer layer appear progressively. Different authors have presented spectra in the UV-Vis range always after the growth of the polymer [11, 13] but, to our knowledge, never during the growth process. Thus, we have demonstrated that 'in situ' UV-Vis differential reflectance techniques allow us to follow the first stages of the formation of the polymer layer, leading to the formation of an intermediate

15 TABLE 1. Absorption bands of the spectra obtained for different concentrations of o-toluidine Concentration C (M)

0.01 0.02

Cycle no. 1

2

3

504

504

504

507 754 a

507 754 a

507 754a

507 754 a

504 720 ~

430 479 720 ~

433 482--,503 726a

415 494 730~

420 503 666 ~

415 470 680

420 479 684a~690

515---,479

430 485 683

494

494

683 a

425 464 680

680~663

690a ~ 632

458~488 690~678

482--,494 708---616

494 617

509 611

0.03

0.07

0.20 0.50

Indicates changes of the band position during the potential sweep, species, which is p r e s u m a b l y the p r e c u r s o r s t a t e o f t h e e l e c t r o p o l y m e r i z a t i o n itself. This s t e p is crucial in t h e p o l y m e r i z a t i o n p r o c e s s b e c a u s e it c o r r e s p o n d s to t h e f o r m a t i o n o f t h e first layer, t h e o n e with c h e m i c a l b o n d s b e t w e e n the o r g a n i c c o m p o u n d a n d t h e metallic surface. W h e n such b o n d s a r e i m p o s s i b l e o r very difficult to c r e a t e , the e l e c t r o c h e m i c a l g r o w t h of the c o n d u c t i n g p o l y m e r r e m a i n s impossible. O t h e r e x p e r i m e n t a l w o r k s h o u l d b e d o n e in o r d e r to identify t h e i n t e r m e d i a t e s p e c i e s r e s p o n s i b l e for t h e p r e p o l y r n e r i z a t i o n stage. P r e l i m i n a r y 'in situ' E S R m e a surements, carried out under the same experimental c o n d i t i o n s , d i d n o t allow t h e d e t e c t i o n of radicals, p r o b a b l y d u e to t h e i r s h o r t lifetime. A t the m o m e n t it is difficult to p r o p o s e s o m e s t r u c t u r e for this species. It is n o t a d s o r b e d o - t o l u i d i n e , which has no a b s o r p t i o n b a n d s in t h e c o r r e s p o n d i n g w a v e l e n g t h region a n d no d e t e c t a b l e U V - V i s b a n d s a r e seen for p o t e n t i a l s b e l o w 0.3 V versus M S E , a r e g i o n w h e r e the m o n o m e r is a d s o r b e d . But it is c l e a r t h a t it is f o r m e d directly from t h e e l e c t r o - o x i d a t i o n o f the m o n o m e r previously ads o r b e d at low p o t e n t i a l . T h i s p r e c u r s o r state could be a d i m e r o r a t r i m e r f o r m o f o - t o l u i d i n e , as p r o p o s e d in t h e l i t e r a t u r e for t h e f o r m a t i o n o f aniline [3] but, until now, n o s p e c t r o s c o p i c e v i d e n c e has b e e n given.

Acknowledgement This w o r k was d o n e u n d e r t h e f r a m e w o r k o f t h e ' A c t i o n Int6gr6e F r a n c o - E s p a g n o l e ' , No. H F 119.

6

15 504 703

494 635 ~ 650

aShoulder.

References 1 J.-L. Br6das and G. Street, Acc. Chem. Res., 18 (1985) 309. 2 T.A. Skoteim (ed.), Handbook of Conducting Polymers, Marcel Dekker, New York, 1986. 3 G. Mengoli, M.T. Munari, P. Biacco and M.M. Musioni, J. Appl. Polym. Sci., 26 (1981) 4287. 4 A. Volkov, G. Tourillon, P.C. Lacaze and J.E. Dubois, J. Electroanal. Chem., 115 (1980) 279. 5 C. Carlin, C.J. Kepley and A.J. Bard, J. Electrochem. Soc., 132 (1985) 353. 6 A. Kitami, J. lzumi, J. Yano, X. Hiromoto and K. Sasaki, Bull. Chem. Soc. Jpn., 57 (1984) 2254. 7 S.H. Glarom and J.H. Marshall, J. Electrochem. Soc., 134 (1987) 142. 8 E. Bosco and S. Rangarajan, J. Electroanal. Chem., 134 (1982) 213. 9 R.D. Armstrong and A.A. Metcalfe, J. Electroanal. Chem., 63 (1975) 19. 10 R.D. Armstrong and J.A. Harrisson, J. Electrochem. Soc., 116 (1969) 328. 11 D.E. Stilwell and S.M. Park, J. Electrochem. Soc., 136 (1989) 427. 12 A. Rakotondrainibe, A. Spinelli, C. Lamy and B. Beden, Spectroscopy, (1993) in press. 13 S. Cattarin, L. Doubova, G. Mengoli and G. Zotti, Electrochim. Acta, 33 (1988) 1077. 14 E.M. Geni6s, J.-F. Penneau and M. Lapkowski, NewJ. Chem., 12 (1988) 765. 15 P. Ocon and P. Herrasti, New Z Chem., 16 (1992) 501. 16 M.L. Bafion, V. Lopez, P. Ocon and P. Herrasti, Synth. Met., 48 (1992) 355. 17 H. Laborde, Thesis, University of Poitiers, France, 1992.