The photoelectrochemical behaviour of poly(o-methylaniline)

Synthetic Metals, 59 (1993) 243-247

243

Short Communication

The photoelectrochemical behaviour of poly(o-methylaniline)* Mu Shaolin and Xia Caifen Department of Chemistry, Yangzhou Teachers College, Yangzhou 225002 (China) (Accepted February 18, 1993)

Abstract The transitions of poly(o-methylaniline) semiconductor in NaC1 solutions from p-type into n-type or from n-type into p-type are caused by the change in potential and the photocurrent of a p-type semiconductor is very much less than that of an n-type one. At 0.20 V (versus SCE), the photocurrent of n-type poly(o-methylaniline) in solutions of concentration lower than 1 M increases with increasing C1- concentration and changes only a little for concentrations above 1 M. The effect of pH value on the photoelectrochemical properties of poly(o-methylaxdline) is very pronounced. The hydrolytic reaction of poly(omethylaniline) is a first-order reaction with a hydrolysis constant of 8.98× 10 -3 min -J.

Introduction Polyaniline has a very fast photoresponse property and a considerable p h o t o c u r r e n t ; it is a p r o m i s i n g p h o t o e l e c t r o c h e m i c a l m a t e r i a l [1, 2]. S h e n a n d T i a n h a v e d i s c o v e r e d t h a t t h e p r e s e n c e o f t h e r e d o x c o u p l e s in a c i d i c s o l u t i o n c a n h a v e a s u b s t a n t i a l i n f l u e n c e on b o t h m a g n i t u d e a n d s t a b i l i t y o f t h e p h o t o c u i T e n t o f p o l y a n i l i n e [3]. O u r r e s u l t s s h o w t h a t t h e r e a r e p h o t o c u r r e n t p e a k s o f n- a n d p - t y p e s e m i c o n d u c t o r s o n t h e p h o t o c u r r e n t - v o l t a g e c u r v e s o f p o l y a n i l i n e in t h e a b s e n c e o f a r e d o x c o u p l e [4]; t h i s is d i f f e r e n t f r o m an i n o r g a n i c s e m i c o n d u c t o r m a t e r i a l . In o r d e r t o s t u d y f u r t h e r t h e p r o p e r t i e s o f t h e p h o t o e f f e c t f o r c o n d u c t i n g p o l y m e r s in t h e a b s e n c e of a r e d o x c o u p l e , w e t o o k p o l y ( o - m e t h y l a n i l i n e ) as a r e s e a r c h s u b j e c t .

Experimental A s o l u t i o n c o n t a i n i n g 0.2 M o - m e t h y l a n f l i n e a n d 1.5 M HC1 w a s e l e c t r o l y s e d a t a p o t e n t i a l o f 0.5 V ( v e r s u s SCE). P o l y ( o - m e t h y l a n i l i n e ) (POMAN) film *Originally submitted to ICSM '92, GSteborg, Sweden, August 1992.

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© 1993 - Elsevier Sequoia. All rights reserved

244

was electrodeposited on a platinum foil (3 × 4 ram2), the other side of which was covered by adhesive tape material. The electrolytic cell for the photoelectrochemical experiment was made of three electrodes [4]. Saturated calomel electrode (SCE) was used as the reference electrode. The pH of NaC1 solution in the absence of a redox couple was adjusted with HCI. A 150 W xenon lamp focused by a pair of convex lens was c h o p p e d to an intermittent light source by a chopper (PAR 194A) with frequency 18 Hz. The light intensity was 5.4 roW, which was measured with a radiometer/ photometer (PAR 550). The photocurrent responses were measured under intermittent light pulses with a signal averager (PAR 4203) by summation averaging of 5000 times and recorded with an X-Y recorder (YEW 3036). All values of photocurrents in this paper were referred to the magnitude of p e a k - p e a k photocurrent.

Results and discussion

Cyclic voltammograms of POMAN The cyclic voltammograms of POMAN in the different pH values of 1 M NaC1 solutions are shown in Fig. 1. The peak current decreased quickly with increasing pH value; however, the peak potential hardly changed at the oxidation process. The change in the peak current with increasing pH at the reduction process was less than that at the oxidation process and the potential peak at the reduction shifted towards the negative direction of potential with increasing pH. In a solution of pH 3.68, although the oxidation peak disappeared, the reduction peak still remained (curve 2, Fig. 1). This indicates that POMAN easily lost its electrochemical activity with increasing pH during the oxidation process. At pH 4.75,,the current at the oxidation process reached nearly zero, namely, POMAN completely lost its electrochemical activity. This is caused by the hydrolysis of POMAN.

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in ] M NaC] soluUons: 1, pH 4.75; 2, pH 3.68; 3,

245

Effect of applied potential on photocurrent response Figure 2 shows the changes in the photocurrents of POMAN in 1 M NaC1 solution of pH 1.67 with potential. The change in photocurrent from 0.0 V to the positive directiol~ of potential is shown in curve 1 of Fig. 2. The photocurrent increased with increasing potential in the region of 0.0 to 0.20 V. This indicates that POMAN is an n-type semiconductor. At potentials higher than 0.20 V, the photocurrent decreased with increasing potential and a peak formed at 0.20 V. This is because the conductivity of POMAN decreased when it was far from the equilibrium potential of 0.20 V, which offset an increase of photocurrent. This is similar to the photocurrent response of polyaniline [4]. In general, conducting polymer films, such as polyaniline in 1 M NaC1 solution of pH 2.0 in the region of 0.0 to 0.3 V, accept anions from the solution and become p-type semiconductors during the oxidation process. However, POMAN does not become a p-type semiconductor in this potential region. This difference between polyaniline and POMAN was caused by the effect of pH value on their own properties; for example, there were two oxidation and two reduction peaks on the cyclic voltammogram of polyaniline at pH 4.1, then the pH value of POMAN with two oxidation and two reduction peaks on the cyclic voltammogram shifted to pH 1.5. In fact, the potential region of the p-type semiconductor of polyaniline shifted towards the negative direction of potential with increasing pH. Finally, the p-type semiconductor behaviour of polyaniline in the solution pH 5.1 disappeared [4]. When the applied potential changed from 0.70 V towards the negative direction of potential, the photocurrent response is shown in curve 2 of Fig. 2, where the peak potential shifted to - 0 . 1 0 V. When the potential decreased to - 0.18 V, the photocurrent became zero and then the photocurrent increased a little with decreasing potential. This fact indicates that the semiconductor transition occurred from n- to p-type; however, the photocurrent of the ptype semiconductor is very small. The results described above are similar to the photoelectrochemical behaviour of polyaniline, but the photocurrent of the p-type semiconductor nor polyaniline is very large.

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0 0.2 0.4 0.6 -0.4 -0.2 O 0.2 0.4 E / V(VS.SCE) E I V(,,S.SCE) Fig. 2. Relationship between the photocurrent of POMAN and applied potential in 1 M NaC1 solution of pH 1.67: 1, oxidation process; 2, reduction process. Fig. 3. Relationship between the photocurrent of POMAN and applied potential in 1 M NaC1 solution of pH 3.68: 1, oxidation process; 2, reduction process.

246

Effect o f the solution p H on photocurrent Experiments were carried out in 1 M NaC1 solutions of pH 1.67, 2.27, 2.98, 3.68 and 4.75. One of the changes in the p h o t o c u r r e n t of the POMAN in the solution of pH 3.68 with potential is shown in Fig. 3. The peak potential o c c u r r e d at 0.19 and 0.05 V when the potential was stepped from 0.0 V towards the positive direction and from 0.5 V to the negative direction, respectively. The s e m i c o n d u c t o r transition from n- to p-type o c c u r r e d at - 0 . 2 0 V. The p h o t o c u r r e n t of the p-type sem i conduct or was very small. Finally, the p-type s e m i c o n d u c t o r behaviour of POMAN disappeared in a solution of pH 4.75. W h en the oxidation of POMAN was in the region of pH 1.67 to 3.68, all the peak potentials of n-type semiconductors occurred at about 0.20 V. F r o m Fig. 1, we can also see that the peak potential during the oxidation p r o c e s s of POMAN was almost independent of solution pH. This is due to the fact that H + ent er e d t he solution from the pol ym er film during the oxidation process; t h e r e f o r e the effect of solution pH on the peak potential was small. However, the p h o t o c u r r e n t de c reased markedly with increasing pH value; this was caused by decreasing conductivity. When POMAN was reduced, it a c c e p t e d H + from the solution and the effect of pH value on the peak potential was m or e pr onounc e d , but the effect of pH on the p h o t o c u r r e n t during the reduction pr oces s was smaller than that during the oxidation process. Effect o f the solution concentration on photocurrent In this section, all the pH values were 2.27 and the electrode potential of POMAN was set at 0.20 V. The change in p h o t o c u r r e n t of an n-type s e m i c o n d u c t o r with solution concent r a t i on is shown in Fig. 4 during the oxidation process. The p h o t o c u r r e n t initially increased with increasing concentration of the solution and t hen did not change in solutions of concentration higher than 1 M. During the oxidation process, the POMAN film a c c e p t e d C1- ions from the solution and thus the rates of diffusion and migration of

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Fig. 4. R e l a t i o n s h i p b e t w e e n t h e p h o t o c u r r e n t o f POMAN a n d c o n c e n t r a t i o n o f t h e NaC1 s o l u t i o n o f p H 2 . 2 7 at 0 . 2 0 V. Fig. 5. D e t e r m i n a t i o n o f t h e h y d r o l y t i c r e a c t i o n k i n e t i c s o f POMAN in 1 M NaC1 s o l u t i o n o f p H 4 . 7 5 at 0 . 2 0 V.

247 C1- to the e l e c t r o d e surface w e r e p r o p o r t i o n a l to the c o n c e n t r a t i o n of C l - , w h i c h led to an i n c r e a s e in p h o t o c u r r e n t . As t h e p h o t o c u r r e n t c o n t i n u e d increasing in the NaCl solution, the p h o t o c u r r e n t r e s p o n s e c u r v e (Fig. 4) flattened; the p h o t o r e s p o n s e c u r r e n t was i n d e p e n d e n t of solution c o n c e n tration. Determination of hydrolysis constant

The c h a n g e in p h o t o c u r r e n t with time was o b s e r v e d during d e t e r m i n a t i o n o f the p h o t o r e s p o n s e of POMAN in 1 M NaCl solution o f p H 4.75 and 0 . 2 0 V. H o w e v e r , this p h e n o m e n o n did n o t o c c u r in the solution o f p H 3.60. This indicates that a hydrolytic r e a c t i o n t o o k place in the solution o f p H 4.75, w h i c h led to a d e c r e a s e of p h o t o e l e c t r o c h e m i c a l activity. Curve 1 on the cyclic v o l t a m m o g r a m (Fig. 1) also s h o w e d t h a t the e l e c t r o c h e m i c a l activity o f POMAN was lost c o m p l e t e l y in the solution o f p H 4.75. W e d e t e r m i n e d the c h a n g e in p h o t o c u r r e n t o f POMAN with time; the plot o f log i against time s h o w e d a straight line (Fig. 5). This is a first-order r e a c t i o n . B a s e d o n the slopes o f straight lines 1 and 2, w e c a l c u l a t e d the v a l u e s o f hydrolysis constant: 9 . 1 5 × 10 -3 and 8.81 × 10 -3 rain -~, respectively. T h e m e a n value is 8 . 9 8 × 1 0 -3 min -~.

Conclusions C o m p a r e d with polyaniline, the p h o t o c u r r e n t o f the p-type s e m i c o n d u c t o r of POMAN u n d e r irradiation with light is m u c h smaller t h a n t h a t o f the nt y p e one, and the p-type s e m i c o n d u c t o r o c c u r s at l o w e r potentials. This is due to the f a c t t h a t the c o n d u c t i v i t y o f POMAN is t h r e e o r d e r s o f m a g n i t u d e less t h a n that o f polyaniline and the hydrolysis o f POMAN o c c u r s at a lower pH value. This m e a n s t h a t t h e r e is an i n h e r e n t relationship c o n c e r n i n g conductivity, p r o p e r t i e s o f the p - t y p e s e m i c o n d u c t o r and p r o t o n a t i o n of the conducting polymers.

References 1 2 3 4

E. M. Geni~s and M. Lapkowski, Synth. Met., 24 (1988) 69. E. M. Geni~s, P. Hany and M. ~pkowski, Synth. Met., 25 (1988) 29. P. K. Shen and Z. Q. Tian, Electrochim. Acta, 34 (1989) 1611. Dong Yaohua and Mu Shaolin, Electrochim. Acta, 36 (1991) 2015.