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Electrochemical synthesis and study of poly(5-amino 1-naphthol) film in aqueous and organic media
Synthetic Metals, 63 (1994) 7-15
7
Electrochemical synthesis and study of poly(5-amino 1-naphthol) film in aqueous and organic media Minh-Chau Pham, Malik Mostefai, Monique Simon and Pierre-Camille Lacaze Institut de Topologie et de Dynamique des Syst~mes de l'Universitd Paris 7, Associd au CNRS, URA 34, 1 rue Guy de la Brosse, 75005 Paris (France)
(Received June 21, 1993; in revised form July 29, 1993; accepted October 15, 1993)
Abstract The electrooxidation of 5-amino 1-naphthol (5-NH2 1-NAP) in acidic aqueous and organic media leads to polymer film on glassy carbon and Pt electrodes. Poly(5-N'n 2 1-NAP) films show reversible, well-defined redox systems in both acidic organic and aqueous solutions. Films prepared in aqueous medium are conducting. The in situ infrared study during film growth and XPS analysis indicate that the electropolymerization occurs via the -NH2 groups while the -OH groups are not concerned. To confirm this finding, the study of the electrochemical oxidation of 5-methoxy 1-naphthylamine has been performed. The same conclusion can be formulated from the cyclic voltammograms of film growth and film electroactivity of the two monomers.
1. Introduction We are currently concerned with the synthesis of modified electrodes by electropolymerization of naphthol derivatives in organic solvent. Previously, we reported the synthesis and characterization of new conducting films, poly(NAP-1) and poly(NAP-2), respectively, from the electrochemical oxidation of 1naphthol and 2-naphthol in acetonitrile [1-3]. In continuation of our work, we have studied the electrochemical oxidation of amino naphthol derivatives (e.g. 5-amino 1-naphthol, 5-amino 2-naphthol, 8-amino 2-naphthol). Indeed, the amino naphthol derivatives which contain -NH2 and - O H functional groups, both electrooxidizable, will be interesting candidates for electropolymerization if the reaction could be performed selectively either via the -NH2 or the - O H group. In the literature, the synthesis of passivated films from the electrooxidation of 5-amino 1-naphthol in neutral acetonitrile solution was reported [4]. The electropolymerization seems to proceed via both -NH 2 and - O H groups and the conductivity of film in the dry state is 4.9x10 -7 S cm -1. In this paper, we report results concerning the electrooxidative polymerization of 5-amino 1-naphthol (5NH2 1-NAP) in acidic aqueous and organic media. Under our conditions, conducting polymer films can be formed in acidic aqueous solution. The film conductivity is c. 10 -3 S cm -1 and the film electroactivity is detected in both organic and aqueous media. The polymerization occurs selectively via the -NH2 groups.
0379-6779/94/$07.00 © 1994 Elsevier Sequoia. All rights reserved SSDI 0379-6779(93)02025-G
Experiments with 5-amino 2-naphthol and 8-amino 2-naphthol are underway and results will be published later.
NH 2
5-amino 1-naphthol
NH 2
5-methoxy 1-naphthylamine
2. Experimental 5-Amino 1-naphthol was of EP (extra pure) grade from TCI (Tokyo Kasei Organic Chemicals). The electrolytes (LiAsF6, NBu4C104) were all reagent grade (Fluka) and were used without further purification. Acetonitrile was spectrophotometric grade (Aldrich). Electrochemical measurements were performed with a PAR 173 potentiostat connected to a PAR 175 programmer. The working electrode was a Pt or carbon disk (Tokai) sealed in Teflon, or a germanium crystal coated with a thin layer of Pt deposited by sputtering (Balzers Model Sputron II) for in situ multiple reflection Fourier transform infrared spectroscopy (MIRFTIRS). 5-Methoxy 1-naphthylamine was prepared from 5amino 1-naphthol by a method described in the literature [5].
MIRFTIRS spectra were recorded on a Nicolet 60SX Fourier transform spectrometer. Details concerning the spectroelectrochemical cell have been published previously [6]. The reference electrode was a AgC1 covered silver wire dipping directly into the solution. In situ MIRFTIRS spectra at an indicated potential are transmittance difference spectra. For each spectrum, the transmittance spectrum of the system before polarization (the reference spectrum) is subtracted from that of the system at an indicated voltage. Scanning electron microscopy (SEM) was applied to analyse the polymer surface and to measure film thickness. A strip of polymer was scratched out from a polymer film deposited on a Pt substrate to reveal a cross section of the film. XPS spectra were recorded on an Escalab MKI vacuum generator with unmonochromatized Mg Ka Xrays and operated in the constant analyser energy (CAE) mode (200 W applied to Mg anode). The pressure is in the 10 -s mbar range. High resolution spectra were achieved using a 20 eV pass energy. XPS signals were fitted with a sum of Gaussian functions. XPS spectra were calibrated by assuming a 285.0 eV binding energy for aromatic and aliphatic carbons.
~ T2HA
0
0.5
i
I
VvsSCE D
Fig. 1. Cyclic v o l t a m m o g r a m s o f 10 -3 M 5 - a m i n o 1 - n a p h t h o l in 2 M H2SO4 solution at a scan rate of 50 m V s -1 with 0.3 c m d i a m e t e r glassy carbon disk electrode.
3. Results and discussion
3.1. Electrochemical synthesis of poly(5-NH2 1-NAP)
films Polymer films were deposited on Pt or glassy carbon disk electrodes at constant potential, constant current or by potential cycling in acidic aqueous or organic media.
3.1.1. Film formation in 2 M H2SO~ aqueous solution Typical cyclic voltammograms at a scan rate v of 50 mV s-1 taken during the oxidation of 10 -3 5-NH2 1NAP and poly(5-NHz 1-NAP) film growth at a glassy carbon electrode are shown in Fig. 1. The broad anodic peak on the first scan at 0.64 V corresponds to the oxidation of 5-NH2 1-NAP. As cycling continues, this peak height decreases and finally disappears. At more negative potentials, a well-defined system is formed at + 0.23 V. The current on the cathodic and anodic waves increases with time. The increasing current with each cycle reflects the growth of the polymer film. During the film growth, the potential limit of scanning progressively diminished from 0.9 to 0.7 V versus SCE and after about 10 cycles to 0.54 V, at a potential inferior to the oxidation potential of the monomer (0.64 V): poly(5-NH2 1-NAP) then grows without direct oxidation of the monomer. Film growth probably occurs by an autocatalytic mechanism as proposed by several groups for polyaniline growth [7, 8].
Fig. 2. SEM image of a poly(5-NH2 1-NAP) film prepared in 10-3 M 5-NH21-NAP+ 2 M HzSO4solutionat a constantpotential of 0.9 V vs. SCE for 10 min. The cross section of the film indicates a thickness of 0.43 /zm.
Polymer films can also be formed at a constant potential of 0.9 V versus SCE. Under both conditions, the films are very adherent on the substrate and present a rather compact and uniform morphology as revealed by scanning electron microscopy (SEM) analysis (Fig. 2). A film obtained by polarizing the electrode at 0.9 V versus SCE for 10 min has a thickness of 0.43 /~m. The film presents electrochromic properties, being light yellow at the reduced state and dark green at the oxidized state.
3.1.2. Film formation in 0.1 M NBu4CI04+0.2 M HCI04 + acetonitrile solution In acetonitrile, 5×10 -2 M 5-NH2 1-NAP is electrooxidized at a more positive potential than in aqueous solution. Figure 3 presents cyclic voltammograms at a Pt electrode during film growth by scanning between
T 5~JA
(a) 0.1 I
1.o I
m.
Vvs SCE
Fig. 3. Cyclic voltammograms of 5 x 10 -2 M 5-NH2 1-NAP in 0.1 M NBu4CIO4 +0.2 M HC104+ acetonitrile solution at a scan rate of 50 mV s-1. Electrode: Pt disk (0.2 cm diameter).
0.1 and 1 V versus SCE at 50 mV s -~. T h e r e is no film catalytic effect in this case. If the potential limit is diminished to 0.7 V, for example, no film is formed. After 30 scans between 0.1 and 1 V versus SCE, a film 4.8 /zm thick is deposited (Fig. 4(a)). The morphology of films prepared in acetonitrile is very different from that of films formed in aqueous solution. The films are thicker but less adherent, more porous and present a globular structure, particularly for films prepared at constant potential (Fig. 4(b)). Soluble oligomers should be formed in this case as the solution darkens after electrolysis.
3.2. Film characterization 3.2.1. In situ MIRFTIRS study during poly(5-NH 2 INAP) film growth Figure 5 presents the six spectra recorded in situ during formation of poly(5-NH2 1-NAP)" at a constant potential of 0.9 V versus SCE. The medium was 0.1 M 5-NH2 1-NAP+0.1 M LiAsF6+0.I M HC104+ acetonitrile. The interval between two spectra is 130 s, the time duration to accumulate 500 interferograms scans. In the region 1800--630 cm -~, the band intensity increases from Sa to $6, indicating an increase in film thickness. To interpret the main bands observed in the film spectrum, we have recorded the spectrum of a KBr pellet of 5-NH2 1-NAP (Fig. 6). In the film spectrum (Fig. 5), two bands due to the electrolytic anions are observed, one band at 700 c m for AsF6- ions and a strong band at 1100 cm -1 for 0 0 4 - ions.
(b) Fig. 4. SEM images of a poly(5-NH2 1-NAP) film prepared in 5 × 10 -2 M 5-NH2 1-NAP + 10 -I M NBu4CIO4 + 0.2 M HCIO4 + CH3CN: (a) film formation by 30 scans between 0 and 1 V vs. SCE at 50 mV s -1 for a film thickness of 4.8 ~m; (b) film formation by polarizing the electrode at 0.9 V vs. SCE for 15 rain.
The bands at 1312 and 1259 cm -1 can be assigned to the C - N stretching vibrations of aromatic secondary amino groups [9, 10]. The C - N stretch for the primary amino groups in the monomer is observed at 1374 and 1297 cm -1 (Fig. 6(a)). A strong and broad band at 1581-1565 cm-1 could result from the overlap of two bands, a band at 1581 cm -1 due to C = C tic stretch and a band at 1565 cm -1 ascribable to the N - H deformation vibration of the secondary amine [9, 10] (Fig. 5). The N - H deformation vibration for primary amine is situated at higher frequency [9]. Indeed, a band is observed at 1596 c m - 1 in the monomer spectrum (Fig. 6). This band is also overlapped with that of the C = C . . . . . tic stretch which appears as a shoulder at 1630 cm -~ (Fig. 6). In the film spectrum (Fig. 5), a new band is visible at 1664 cm-1; this can be assigned to the stretching vibration of C = N - from the imino groups . . . . .
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Fig. 6. F T - I R spectrum o f a K B r pellet o f 5 - N H 2 1-NAP.
I
, 3;50
(C)
3000
2250 WAVENUMBERS
¢m -I
Fig. 5. In situ MIRFTIRS spectra recorded during poly(5-NH 2 1-NAP) film growth at E = 0 . 9 V vs. SCE. Medium: 0.1 M 5NH2 1 - N A P + 0 . 1 M LiAsF6+0.1 M HCIO4+CH3CN. Each spectrum was obtained using 500 interferometers (130 s). The spectra are referenced to that obtained with the system before polarization.
[9]. These IR data suggest that the -NH2 group takes part in the electropolymerization. Concerning the hydroxyl group, the C-O stretching vibrations are observed at 1046-1074 cm -1 and the O - H deformation vibrations at 1271 cm -1 for the monomer [9] (Fig. 6). In the film spectrum, they are, respectively, at 1051-1072 and 1181-1208 cm -1 (broad band). This fact could suggest that the - O H group
does not intervene in the electropolymerization. Another argument supporting this hypothesis is provided by results obtained from the electrochemical oxidation of 5-methoxy 1-naphthylamine that is presented in Section 3.4. In the region 2000-4000 cm -1, the N - H stretch of the aromatic primary amino groups in the monomer gives two sharp bands at 3374 and 3302 cm-1 and the O--H stretch gives a broad band at around 3030 cm-1 (Fig. 6(b)). In the film spectrum (Fig. 5(c)), a broad band is centred around 3078 c m - 1 which can be assigned to O - H stretch and a shoulder at 3289 cm-1 ascribable to N - H stretch of secondary amino and imino groups [11]. A negative band situated at 2252 cm -1 ( C = N stretch of CHsCN) indicates removal of the solvent from the electrode surface as a result of film formation.
11
The analysis of the 650-1000 cm-1 domain is helpful for information related to substitution on the aromatic nucleus. The two bands at 814 and 769 cm -~ in the film spectrum (Fig. 5(b)) are due to C - H out-of-plane vibrations of three adjacent aromatic hydrogen atoms. These bands are evidently detected in the case of monomer, because the three hydrogen atoms are present on the two nuclei: a very strong band is seen at 768 cm-1 with a shoulder at 796 cm -1. Besides the band at 814 cm -~ in the film, another new band of equal intensity is observed at 825 cm -~ and is assigned to the presence of 2H atoms [9]. From these data, it can be reasonably assumed that the remaining three hydrogen atoms are in the nucleus containing the - O H group and the coupling occurs on the nucleus containing the -NH2 function either at the ortho or para position leading to the presence of two adjacent H atoms remaining on this nucleus. The electropolymerization of 5-NH2 1-NAP then probably leads to the formation of C - N H - C , C = N - C via coupling of the naphthalene nuclei in the ortho or para positon with respect to the -NH2 group.
3.2.2. Ex situ IR film analysis Ex situ external reflection IR spectroscopy was used to analyse in the dry state films formed in both organic and aqueous media. Figure 7 presents the spectrum of a film synthesized in a n acetonitrile solution containing 0.1 M 5-NH2 1N A P + 0 . 1 M NBu4C104+0.2 M HC104 at a constant potential of 0.9 V versus SCE for 10 min. All the bands related to the polymer structure are confirmed (Fig. 7). The imino groups are detected by a C = N stretch at 1654 cm -1, secondary amino groups are observed by N - H deformation vibrations at 1560 cm -1, C-N stretch at 1258 cm -1 and N - H stretch at 3281 cm -1. The C = C . . . . . tic stretch is observed at 1578 cm-1. For the - O H group, O - H stretch is detected at 3076 cm -~, C - O stretch at 1047 cm -1 and O - H deformation vibration at 1179 cm -1. The presence of three adjacent H atoms is confirmed by the two bands at 809 and 751 cm-1 and that of two adjacent H atoms by the 821 cm-~ band. C 1 0 4 - ions are present in the film as indicated by a strong band at 1100 cm -1 (Fig.
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Fig. 7. Ex situ external reflection I R s p e c t r u m of a p o l y ( 5 - N H 2 1-NAP) film after its f o r m a t i o n at E = 0.9 V vs. S C E for 10 rain. M e d i u m : 0.1 M 5-NH2 1 - N A P + 0 . 1 M N B u 4 C I O 4 + 0 . 2 M ttCIO4 + CH3CN.
7). Figure 8 presents the spectrum of poly(5-NH2 1NAP) film prepared in an aqueous solution containing 10 -3 M 5-NH2 1-NAP and 2 M nClO4 at a constant potential of 0.9 V versus SCE for 10 min. Approximately the same bands are detected as for the film prepared in acetonitrile solution in Fig. 7. The imino groups are detected at 1658 cm-1; secondary amino groups at 1563 c m - ~ ( N - H deformation) and 1256 c m - 1 (C-N stretch); C=Ca~ stretch is visible at 1604 cm-1; C - O stretch at 1036 cm -1 and O - H deformation at 1182 cm -1. Two
l ~eo
t ~'~o
L ~DO
~6a WAVENUMBERS
6co ( cm-I
)
Fig. 8. E x situ external reflection I R s p e c t r u m o f a poly(5-NH2 I - N A P ) film in t h e dry state. T h e film w a s p r e p a r e d at 0.9 V vs. S C E for 10 min. M e d i u m : 10 -3 M 5-NH2 1 - N A P + 2 M HCIO4 a q u e o u s solution.
12 bands for three H are visible at 803 and 755 cm-~ and the 824 cm -~ band is relative to two H atoms. The 1102 cm -~ due to C104- ions is evidently present in the spectrum (Fig. 8).
3.2.3. Ex situ X-ray photoelectron spectroscopy film analysis XPS spectra display two C(ls) peaks: a first intense peak at 284.9 eV, due to aromatic or aliphatic carbon atoms coupled to carbon atoms of the same type, and a second less-intense peak at 286.40 eV, ascribed to carbon atoms coupled singly to oxygen atoms or to N atoms (C-N and C=N) [12, 13]. No peak is detected at c. 288 eV; this proves that no quinone group is present in the polymer chain (Fig. 9). The N(ls) spectrum (Fig. 10) displays three peaks. The peak at the lowest energy and centred at 399.5 eV can be attributed to the neutral amine nitrogen atom (-NH-) in the polymer [14]. The two peaks at higher energy centred at 400.8 and 402.6 eV can be assigned to protonated amine and imine nitrogen atoms [14]. A detailed XPS study is underway to evaluate the doping level and analyse the change occurring in the polymer structure during the film redox process. Data from size exclusion chromatography using Nmethyl-2-pyrrolidone (NMP) as eluent and polystyrene as reference indicate that the molecular weight is approximately 6200 (DP---39) for film prepared in acetonitrile solution. For film prepared in aqueous solution, DP is about 28.
790E Cls cps
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1BBe
1411
941
470
B
3~e 3~.
4~
4~1
4~
4;a3 4~4 4~s
Binding Energy eV
Fig. 10. N(ls) XPS spectrumof a poly(5-NH21-NAP)filmprepared at 0.9 V vs. SCE in acetonitrile.
3.3. Film electroactivity 3.3.1. Electroactivity of poly(5-NH2 1-NAP) films in 2 M H2S04 aqueous solution Poly(5-NH2 1-NAP) films synthesized either in acetonitrile or in aqueous solutions are both electroactive in aqueous acidic solution. Typical cyclic voltammetric responses of a 3/zm thick film-coated Pt electrode prepared in acetonitrile are seen in Fig. ll(a). A well reversible redox system is detected at 0.28 V (Epa) and 0.26 V (Epc) versus SCE. Peak currents increase linearly with increasing scan speed until 100 mV s -1 (Fig. ll(b)), indicating that the charge transport kinetics of the film are fast for this film. Cyclic voltammetric responses of a Pt electrode coated with a 0.7 ~m thick film prepared in aqueous medium (2 M H2SO4) are presented in Fig. 12(A). The system is observed at 0.22 V (Epa) and 0.1 V (E~) at scan rate 100 mV s -1. In this case, ditfusional kinetics are observed as peak currents increase linearly with (scan rate) u2.
3.3.2. Electroactivity of poly(5-NH2 I-NAP) films in acetonitrile solution
316i
1SB!
2~,3 2~4 2t35 2~6 2~7
2;3e
Binding Energy eV
Fig. 9. C(1 s) XPS spectrum of a poly(5-NH2 1-NAP) film prepared at 0.9 V vs. SCE in acetonitrile.
A film prepared in acetonitrile is electroactive in an acetonitrile solution containing 0.2 M HC104 +0.2 M NBu4CIO4 (Fig. 13). The redox system is observed at 0.52 V versus SCE; peaks are broader than in aqueous medium. Finally, a film prepared in aqueous medium presents the same redox system at 0.52 V but it is not stable. The film is partially soluble in the solvent after several scans.
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~c
(a)
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o
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Fig. 11. (a) Cyclic voltammetric r e s p o n s e s o f a poly(5-NH2 1N A P ) film-coated Pt electrode (film p r e p a r e d in acetonitrile) at different scan rates. M e d i u m : 2 M H2504; electrode surface: 0.03 cm 2. (b) Plot of peak intensity vs. scan rate.
3.4. Electrochemical oxidation of 5-methoxy 1naphthylamine Data from the IR study of poly(5-NH2 1-NAP) film growth (Section 3.2) suggest that the - O H group does not take part in the electropolymerization. In order to elucidate this finding, we have synthesized and studied the electrochemical oxidation of 5-methoxy 1-naphthylamine. 3.4.1. Film formation in 2 M H2S04 aqueous solution The electrooxidation of 5 x 1 0 -3 M 5-methoxy 1naphthylamine in 2 M H 2 8 0 4 results in film formation on Pt and glassy carbon electrodes.
5 (scan role )~/2 10 (inV. s -I )1/2
Fig. 12. (A) CV responses of a poly(5-NH2 1-NAP) film-coated Pt electrode (film p r e p a r e d in 2 M H2504 aqueous m e d i u m ) at various scan rates: a, 200; b, 100; c, 50; d, 20; e, 10; f, 5 m V s -~. M e d i u m : 2 M H2504; electrode surface = 1.4 cm 2. (B) Plot of peak intensity vs. (scan rate) ~ .
The cyclic voltammograms (CVs) during film growth at scan rate of 50 mV s-1 on a glassy carbon electrode are presented in Fig. 14. They are quite similar to the CVs recorded during poly(5-NH2 I-NAP) film growth (Fig. 1). On the first scan between 0 and 0.95 V versus SCE, an anodic peak is observed at 0.75 V. In subsequent scans, two redox systems are detected, system A (Epa=0.37 V; Em=0.33 V) and system B (Epa=0.32 V; E ~ = 0.2 V). During the film growth, system B seems to become preponderant compared to system A. A tentative explanation is that in the case of 5-methoxy 1-naphthylamine, the two couplings via ortho and para positions of the -NH2 group were performed during the first scans, and later only one kind of coupling was
14
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..... ,oo.,.-, ..... 50 ...... 20 •:.i:i 150
I
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o5
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--
Fig. 13. C V responses of a poly(5-NH2 1-NAP) film-coated Pt electrode (film p r e p a r e d in acetonitrile) at different scan rates. Medium: 0.2 M HCIO4+0.2 M NBu4CIOn+CH3CN; electrode surface: 0.03 cm 2.
V 0
I
0.5 I Vvs
SC E
Fig. 15. Cyclic voltammetric responses o f a poly(5-methoxy 1naphthylanfine)-coated carbon electrode in 2 M H2SO4 aqueous solution (electrode surface: 0.07 cm2).
3.4.2. Electroactivity of poly(5-methoxy 1naphthylamine) film in aqueous 2 M H2SO4 solution
0
Bl
0,95 Vv5SCE
Figure 15 presents the cyclic voltammetric responses of a film-coated carbon electrode. The film (= 200 nm thick) was prepared by scanning 30 times at 50 mV s -1 between 0 and 0.95 V versus SCE. It should be noted that the CVs present the same very reversible redox system (Epa= 0.28 V; Epc=0.23 V) as that recorded with poly(5-NHz 1-NAP) film in aqueous medium (Fig. ll(a)). The similarity of the CVs of f i l l growth and especially of film electroactivity during the study of 5-NH2 1-NAP and 5-methoxy 1-naphthylamine supports the argument that only -NH2 groups intervene in the electropolymerization of 5-NH2 1-NAP.
Fig. 14. Cyclic voltammograms of 5X 10 -3 M 5-methoxy l-naphthylamine in 2 M H2SO4 solution at a scan rate o f 50 m V s - l with a Pt disk electrode (surface: 0.03 cm2).
4. Conclusions
preferential, probably the para coupling due to steric hindrance. In the case of poly(5-NHz 1-NAP) film growth, only one redox system was observed at Epc = 0.23 V and Eva = 0.26 V (Fig. 1). If the potential limit is diminished as in the CVs of poly(5-NH2 1-NAP) f i l l growth (Section 3.1.1., Fig. 1), no f i l l is formed. The repetitive scans should be kept between 0 and 0.95 V to obtain f i l l deposit on the electrode surface. Films can also be formed at a constant potential of 0.9 V but they are thinner. The molecular weight for poly(5-methoxy 1-naphthylamine) is about 5000 (DP=29).
The electropolymerization of 5-amino 1-naphthol in acidic aqueous and acetonitrile solutions results in polymer films which are very adherent on glassy carbon and Pt electrodes. The fills prepared in aqueous medium are conducting, tr= 10 -3 S cm -1, while those prepared in acetonitrile are much less conducting, t r - 2 X l 0 -6 S cm -1. The infrared and XPS studies indicate that the polymerization occurs via the ortho or para position with respect to the -NH2 groups and leads to the formation of C-NH-C and C - N = C bonds, while the - O H groups are not concerned. The following structure could be proposed in the case ofpara coupling:
15
References
×
Y
These findings are confirmed by data from the study of the electropolymerization of 5-methoxy 1-naphthylamine. Besides the cyclic voltammograms during film growth, which are similar for the two monomers, the redox systems of the two films are practically the same in acidic aqueous medium and thus confirm the fact that coupling of nuclei occurs through the -NH2 group.
Acknowledgement The authors thank Professor L.H. Dao and Dr J.Y. Bergeron (Laboratoire de Recherche sur les Mat6riaux Avanc6s, INRS Energie, Varennes, Quebec, Canada) for help in size exclusion chromatography measurements.
1 M.-C. Pham, J. Moslih and P.-C. Lacaze,J. ElectroanaL Chem., 278 (1990) 415. 2 M.-C. Pham, J. Moslih and P.-C. Lacaze, J. Electrochem. Soc., 138 (1991) 449. 3 M.-C. Pham, J. Moslih and P.-C. Lacaze,J. Electroanal. Chem., 303 (1991) 297. 4 T. Ohsaka, M. Ohba, M. Sato, N. Oyama, S. Tanaka and S. Nakamura, J. Electroanal. Chem., 300 (1991) 51. 5 J. Lockett and W.F. Short, J. Chem. Soc., 1 (1939) 787. 6 M.-C. Pham, F. Adami, P.-C. Lacaze, J.P. Doucet and J.E. Dubois, J. Electroanal. Chem., 201 (1986) 413. 7 D.E. Stilwell and S.M. Park, J. Electroehem. Soc., 135 (1988) 2254 and 2491. 8 G. Zotti, S. Cattarin and N. Comisso, J. Electroanal. Chem., 239 (1988) 387. 9 G. Socrates, Infrared Characteristic Group Frequencies, Wiley, New York, 1980. 10 J.E. Stewart, J. Chem. Phys., 30 (1959) 1259. 11 A.G. Moritz, Spectroehim. Acta, 16 (1960) 1176. 12 A. Dilks, Electron Spectroscopy, Theory, Techniques and Applications, Vol. 4, Academic Press, London, 1981. 13 R.A. Dickie, J.S. Hammond, J.E. De Vries and J.W. Holubka, Anal Chem., 54 (1982) 2045. 14 P. Snauwaert, R. Lazzaroni, J. Riga and J.J. Verbist, J. Chem. Phys., 92 (1990) 2187.
7
Electrochemical synthesis and study of poly(5-amino 1-naphthol) film in aqueous and organic media Minh-Chau Pham, Malik Mostefai, Monique Simon and Pierre-Camille Lacaze Institut de Topologie et de Dynamique des Syst~mes de l'Universitd Paris 7, Associd au CNRS, URA 34, 1 rue Guy de la Brosse, 75005 Paris (France)
(Received June 21, 1993; in revised form July 29, 1993; accepted October 15, 1993)
Abstract The electrooxidation of 5-amino 1-naphthol (5-NH2 1-NAP) in acidic aqueous and organic media leads to polymer film on glassy carbon and Pt electrodes. Poly(5-N'n 2 1-NAP) films show reversible, well-defined redox systems in both acidic organic and aqueous solutions. Films prepared in aqueous medium are conducting. The in situ infrared study during film growth and XPS analysis indicate that the electropolymerization occurs via the -NH2 groups while the -OH groups are not concerned. To confirm this finding, the study of the electrochemical oxidation of 5-methoxy 1-naphthylamine has been performed. The same conclusion can be formulated from the cyclic voltammograms of film growth and film electroactivity of the two monomers.
1. Introduction We are currently concerned with the synthesis of modified electrodes by electropolymerization of naphthol derivatives in organic solvent. Previously, we reported the synthesis and characterization of new conducting films, poly(NAP-1) and poly(NAP-2), respectively, from the electrochemical oxidation of 1naphthol and 2-naphthol in acetonitrile [1-3]. In continuation of our work, we have studied the electrochemical oxidation of amino naphthol derivatives (e.g. 5-amino 1-naphthol, 5-amino 2-naphthol, 8-amino 2-naphthol). Indeed, the amino naphthol derivatives which contain -NH2 and - O H functional groups, both electrooxidizable, will be interesting candidates for electropolymerization if the reaction could be performed selectively either via the -NH2 or the - O H group. In the literature, the synthesis of passivated films from the electrooxidation of 5-amino 1-naphthol in neutral acetonitrile solution was reported [4]. The electropolymerization seems to proceed via both -NH 2 and - O H groups and the conductivity of film in the dry state is 4.9x10 -7 S cm -1. In this paper, we report results concerning the electrooxidative polymerization of 5-amino 1-naphthol (5NH2 1-NAP) in acidic aqueous and organic media. Under our conditions, conducting polymer films can be formed in acidic aqueous solution. The film conductivity is c. 10 -3 S cm -1 and the film electroactivity is detected in both organic and aqueous media. The polymerization occurs selectively via the -NH2 groups.
0379-6779/94/$07.00 © 1994 Elsevier Sequoia. All rights reserved SSDI 0379-6779(93)02025-G
Experiments with 5-amino 2-naphthol and 8-amino 2-naphthol are underway and results will be published later.
NH 2
5-amino 1-naphthol
NH 2
5-methoxy 1-naphthylamine
2. Experimental 5-Amino 1-naphthol was of EP (extra pure) grade from TCI (Tokyo Kasei Organic Chemicals). The electrolytes (LiAsF6, NBu4C104) were all reagent grade (Fluka) and were used without further purification. Acetonitrile was spectrophotometric grade (Aldrich). Electrochemical measurements were performed with a PAR 173 potentiostat connected to a PAR 175 programmer. The working electrode was a Pt or carbon disk (Tokai) sealed in Teflon, or a germanium crystal coated with a thin layer of Pt deposited by sputtering (Balzers Model Sputron II) for in situ multiple reflection Fourier transform infrared spectroscopy (MIRFTIRS). 5-Methoxy 1-naphthylamine was prepared from 5amino 1-naphthol by a method described in the literature [5].
MIRFTIRS spectra were recorded on a Nicolet 60SX Fourier transform spectrometer. Details concerning the spectroelectrochemical cell have been published previously [6]. The reference electrode was a AgC1 covered silver wire dipping directly into the solution. In situ MIRFTIRS spectra at an indicated potential are transmittance difference spectra. For each spectrum, the transmittance spectrum of the system before polarization (the reference spectrum) is subtracted from that of the system at an indicated voltage. Scanning electron microscopy (SEM) was applied to analyse the polymer surface and to measure film thickness. A strip of polymer was scratched out from a polymer film deposited on a Pt substrate to reveal a cross section of the film. XPS spectra were recorded on an Escalab MKI vacuum generator with unmonochromatized Mg Ka Xrays and operated in the constant analyser energy (CAE) mode (200 W applied to Mg anode). The pressure is in the 10 -s mbar range. High resolution spectra were achieved using a 20 eV pass energy. XPS signals were fitted with a sum of Gaussian functions. XPS spectra were calibrated by assuming a 285.0 eV binding energy for aromatic and aliphatic carbons.
~ T2HA
0
0.5
i
I
VvsSCE D
Fig. 1. Cyclic v o l t a m m o g r a m s o f 10 -3 M 5 - a m i n o 1 - n a p h t h o l in 2 M H2SO4 solution at a scan rate of 50 m V s -1 with 0.3 c m d i a m e t e r glassy carbon disk electrode.
3. Results and discussion
3.1. Electrochemical synthesis of poly(5-NH2 1-NAP)
films Polymer films were deposited on Pt or glassy carbon disk electrodes at constant potential, constant current or by potential cycling in acidic aqueous or organic media.
3.1.1. Film formation in 2 M H2SO~ aqueous solution Typical cyclic voltammograms at a scan rate v of 50 mV s-1 taken during the oxidation of 10 -3 5-NH2 1NAP and poly(5-NHz 1-NAP) film growth at a glassy carbon electrode are shown in Fig. 1. The broad anodic peak on the first scan at 0.64 V corresponds to the oxidation of 5-NH2 1-NAP. As cycling continues, this peak height decreases and finally disappears. At more negative potentials, a well-defined system is formed at + 0.23 V. The current on the cathodic and anodic waves increases with time. The increasing current with each cycle reflects the growth of the polymer film. During the film growth, the potential limit of scanning progressively diminished from 0.9 to 0.7 V versus SCE and after about 10 cycles to 0.54 V, at a potential inferior to the oxidation potential of the monomer (0.64 V): poly(5-NH2 1-NAP) then grows without direct oxidation of the monomer. Film growth probably occurs by an autocatalytic mechanism as proposed by several groups for polyaniline growth [7, 8].
Fig. 2. SEM image of a poly(5-NH2 1-NAP) film prepared in 10-3 M 5-NH21-NAP+ 2 M HzSO4solutionat a constantpotential of 0.9 V vs. SCE for 10 min. The cross section of the film indicates a thickness of 0.43 /zm.
Polymer films can also be formed at a constant potential of 0.9 V versus SCE. Under both conditions, the films are very adherent on the substrate and present a rather compact and uniform morphology as revealed by scanning electron microscopy (SEM) analysis (Fig. 2). A film obtained by polarizing the electrode at 0.9 V versus SCE for 10 min has a thickness of 0.43 /~m. The film presents electrochromic properties, being light yellow at the reduced state and dark green at the oxidized state.
3.1.2. Film formation in 0.1 M NBu4CI04+0.2 M HCI04 + acetonitrile solution In acetonitrile, 5×10 -2 M 5-NH2 1-NAP is electrooxidized at a more positive potential than in aqueous solution. Figure 3 presents cyclic voltammograms at a Pt electrode during film growth by scanning between
T 5~JA
(a) 0.1 I
1.o I
m.
Vvs SCE
Fig. 3. Cyclic voltammograms of 5 x 10 -2 M 5-NH2 1-NAP in 0.1 M NBu4CIO4 +0.2 M HC104+ acetonitrile solution at a scan rate of 50 mV s-1. Electrode: Pt disk (0.2 cm diameter).
0.1 and 1 V versus SCE at 50 mV s -~. T h e r e is no film catalytic effect in this case. If the potential limit is diminished to 0.7 V, for example, no film is formed. After 30 scans between 0.1 and 1 V versus SCE, a film 4.8 /zm thick is deposited (Fig. 4(a)). The morphology of films prepared in acetonitrile is very different from that of films formed in aqueous solution. The films are thicker but less adherent, more porous and present a globular structure, particularly for films prepared at constant potential (Fig. 4(b)). Soluble oligomers should be formed in this case as the solution darkens after electrolysis.
3.2. Film characterization 3.2.1. In situ MIRFTIRS study during poly(5-NH 2 INAP) film growth Figure 5 presents the six spectra recorded in situ during formation of poly(5-NH2 1-NAP)" at a constant potential of 0.9 V versus SCE. The medium was 0.1 M 5-NH2 1-NAP+0.1 M LiAsF6+0.I M HC104+ acetonitrile. The interval between two spectra is 130 s, the time duration to accumulate 500 interferograms scans. In the region 1800--630 cm -~, the band intensity increases from Sa to $6, indicating an increase in film thickness. To interpret the main bands observed in the film spectrum, we have recorded the spectrum of a KBr pellet of 5-NH2 1-NAP (Fig. 6). In the film spectrum (Fig. 5), two bands due to the electrolytic anions are observed, one band at 700 c m for AsF6- ions and a strong band at 1100 cm -1 for 0 0 4 - ions.
(b) Fig. 4. SEM images of a poly(5-NH2 1-NAP) film prepared in 5 × 10 -2 M 5-NH2 1-NAP + 10 -I M NBu4CIO4 + 0.2 M HCIO4 + CH3CN: (a) film formation by 30 scans between 0 and 1 V vs. SCE at 50 mV s -1 for a film thickness of 4.8 ~m; (b) film formation by polarizing the electrode at 0.9 V vs. SCE for 15 rain.
The bands at 1312 and 1259 cm -1 can be assigned to the C - N stretching vibrations of aromatic secondary amino groups [9, 10]. The C - N stretch for the primary amino groups in the monomer is observed at 1374 and 1297 cm -1 (Fig. 6(a)). A strong and broad band at 1581-1565 cm-1 could result from the overlap of two bands, a band at 1581 cm -1 due to C = C tic stretch and a band at 1565 cm -1 ascribable to the N - H deformation vibration of the secondary amine [9, 10] (Fig. 5). The N - H deformation vibration for primary amine is situated at higher frequency [9]. Indeed, a band is observed at 1596 c m - 1 in the monomer spectrum (Fig. 6). This band is also overlapped with that of the C = C . . . . . tic stretch which appears as a shoulder at 1630 cm -~ (Fig. 6). In the film spectrum (Fig. 5), a new band is visible at 1664 cm-1; this can be assigned to the stretching vibration of C = N - from the imino groups . . . . .
I0
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S!
Ixl U Z I--
~r (/I Z
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_.1
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i
i
1670
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i
1150
630
SS I-
(a)
t~o
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~eo
S6 tll U Z I--
~r tn z
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(b)
i
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tlJ > I-..J I/J
rr
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4A%
s2 s3
~
st,
z
•
55
,,
o
3310
(b)
WAVEN U M B E R S
crn-1
Fig. 6. F T - I R spectrum o f a K B r pellet o f 5 - N H 2 1-NAP.
I
, 3;50
(C)
3000
2250 WAVENUMBERS
¢m -I
Fig. 5. In situ MIRFTIRS spectra recorded during poly(5-NH 2 1-NAP) film growth at E = 0 . 9 V vs. SCE. Medium: 0.1 M 5NH2 1 - N A P + 0 . 1 M LiAsF6+0.1 M HCIO4+CH3CN. Each spectrum was obtained using 500 interferometers (130 s). The spectra are referenced to that obtained with the system before polarization.
[9]. These IR data suggest that the -NH2 group takes part in the electropolymerization. Concerning the hydroxyl group, the C-O stretching vibrations are observed at 1046-1074 cm -1 and the O - H deformation vibrations at 1271 cm -1 for the monomer [9] (Fig. 6). In the film spectrum, they are, respectively, at 1051-1072 and 1181-1208 cm -1 (broad band). This fact could suggest that the - O H group
does not intervene in the electropolymerization. Another argument supporting this hypothesis is provided by results obtained from the electrochemical oxidation of 5-methoxy 1-naphthylamine that is presented in Section 3.4. In the region 2000-4000 cm -1, the N - H stretch of the aromatic primary amino groups in the monomer gives two sharp bands at 3374 and 3302 cm-1 and the O--H stretch gives a broad band at around 3030 cm-1 (Fig. 6(b)). In the film spectrum (Fig. 5(c)), a broad band is centred around 3078 c m - 1 which can be assigned to O - H stretch and a shoulder at 3289 cm-1 ascribable to N - H stretch of secondary amino and imino groups [11]. A negative band situated at 2252 cm -1 ( C = N stretch of CHsCN) indicates removal of the solvent from the electrode surface as a result of film formation.
11
The analysis of the 650-1000 cm-1 domain is helpful for information related to substitution on the aromatic nucleus. The two bands at 814 and 769 cm -~ in the film spectrum (Fig. 5(b)) are due to C - H out-of-plane vibrations of three adjacent aromatic hydrogen atoms. These bands are evidently detected in the case of monomer, because the three hydrogen atoms are present on the two nuclei: a very strong band is seen at 768 cm-1 with a shoulder at 796 cm -1. Besides the band at 814 cm -~ in the film, another new band of equal intensity is observed at 825 cm -~ and is assigned to the presence of 2H atoms [9]. From these data, it can be reasonably assumed that the remaining three hydrogen atoms are in the nucleus containing the - O H group and the coupling occurs on the nucleus containing the -NH2 function either at the ortho or para position leading to the presence of two adjacent H atoms remaining on this nucleus. The electropolymerization of 5-NH2 1-NAP then probably leads to the formation of C - N H - C , C = N - C via coupling of the naphthalene nuclei in the ortho or para positon with respect to the -NH2 group.
3.2.2. Ex situ IR film analysis Ex situ external reflection IR spectroscopy was used to analyse in the dry state films formed in both organic and aqueous media. Figure 7 presents the spectrum of a film synthesized in a n acetonitrile solution containing 0.1 M 5-NH2 1N A P + 0 . 1 M NBu4C104+0.2 M HC104 at a constant potential of 0.9 V versus SCE for 10 min. All the bands related to the polymer structure are confirmed (Fig. 7). The imino groups are detected by a C = N stretch at 1654 cm -1, secondary amino groups are observed by N - H deformation vibrations at 1560 cm -1, C-N stretch at 1258 cm -1 and N - H stretch at 3281 cm -1. The C = C . . . . . tic stretch is observed at 1578 cm-1. For the - O H group, O - H stretch is detected at 3076 cm -~, C - O stretch at 1047 cm -1 and O - H deformation vibration at 1179 cm -1. The presence of three adjacent H atoms is confirmed by the two bands at 809 and 751 cm-1 and that of two adjacent H atoms by the 821 cm-~ band. C 1 0 4 - ions are present in the film as indicated by a strong band at 1100 cm -1 (Fig.
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810
I---
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Fig. 7. Ex situ external reflection I R s p e c t r u m of a p o l y ( 5 - N H 2 1-NAP) film after its f o r m a t i o n at E = 0.9 V vs. S C E for 10 rain. M e d i u m : 0.1 M 5-NH2 1 - N A P + 0 . 1 M N B u 4 C I O 4 + 0 . 2 M ttCIO4 + CH3CN.
7). Figure 8 presents the spectrum of poly(5-NH2 1NAP) film prepared in an aqueous solution containing 10 -3 M 5-NH2 1-NAP and 2 M nClO4 at a constant potential of 0.9 V versus SCE for 10 min. Approximately the same bands are detected as for the film prepared in acetonitrile solution in Fig. 7. The imino groups are detected at 1658 cm-1; secondary amino groups at 1563 c m - ~ ( N - H deformation) and 1256 c m - 1 (C-N stretch); C=Ca~ stretch is visible at 1604 cm-1; C - O stretch at 1036 cm -1 and O - H deformation at 1182 cm -1. Two
l ~eo
t ~'~o
L ~DO
~6a WAVENUMBERS
6co ( cm-I
)
Fig. 8. E x situ external reflection I R s p e c t r u m o f a poly(5-NH2 I - N A P ) film in t h e dry state. T h e film w a s p r e p a r e d at 0.9 V vs. S C E for 10 min. M e d i u m : 10 -3 M 5-NH2 1 - N A P + 2 M HCIO4 a q u e o u s solution.
12 bands for three H are visible at 803 and 755 cm-~ and the 824 cm -~ band is relative to two H atoms. The 1102 cm -~ due to C104- ions is evidently present in the spectrum (Fig. 8).
3.2.3. Ex situ X-ray photoelectron spectroscopy film analysis XPS spectra display two C(ls) peaks: a first intense peak at 284.9 eV, due to aromatic or aliphatic carbon atoms coupled to carbon atoms of the same type, and a second less-intense peak at 286.40 eV, ascribed to carbon atoms coupled singly to oxygen atoms or to N atoms (C-N and C=N) [12, 13]. No peak is detected at c. 288 eV; this proves that no quinone group is present in the polymer chain (Fig. 9). The N(ls) spectrum (Fig. 10) displays three peaks. The peak at the lowest energy and centred at 399.5 eV can be attributed to the neutral amine nitrogen atom (-NH-) in the polymer [14]. The two peaks at higher energy centred at 400.8 and 402.6 eV can be assigned to protonated amine and imine nitrogen atoms [14]. A detailed XPS study is underway to evaluate the doping level and analyse the change occurring in the polymer structure during the film redox process. Data from size exclusion chromatography using Nmethyl-2-pyrrolidone (NMP) as eluent and polystyrene as reference indicate that the molecular weight is approximately 6200 (DP---39) for film prepared in acetonitrile solution. For film prepared in aqueous solution, DP is about 28.
790E Cls cps
632i
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1BBe
1411
941
470
B
3~e 3~.
4~
4~1
4~
4;a3 4~4 4~s
Binding Energy eV
Fig. 10. N(ls) XPS spectrumof a poly(5-NH21-NAP)filmprepared at 0.9 V vs. SCE in acetonitrile.
3.3. Film electroactivity 3.3.1. Electroactivity of poly(5-NH2 1-NAP) films in 2 M H2S04 aqueous solution Poly(5-NH2 1-NAP) films synthesized either in acetonitrile or in aqueous solutions are both electroactive in aqueous acidic solution. Typical cyclic voltammetric responses of a 3/zm thick film-coated Pt electrode prepared in acetonitrile are seen in Fig. ll(a). A well reversible redox system is detected at 0.28 V (Epa) and 0.26 V (Epc) versus SCE. Peak currents increase linearly with increasing scan speed until 100 mV s -1 (Fig. ll(b)), indicating that the charge transport kinetics of the film are fast for this film. Cyclic voltammetric responses of a Pt electrode coated with a 0.7 ~m thick film prepared in aqueous medium (2 M H2SO4) are presented in Fig. 12(A). The system is observed at 0.22 V (Epa) and 0.1 V (E~) at scan rate 100 mV s -1. In this case, ditfusional kinetics are observed as peak currents increase linearly with (scan rate) u2.
3.3.2. Electroactivity of poly(5-NH2 I-NAP) films in acetonitrile solution
316i
1SB!
2~,3 2~4 2t35 2~6 2~7
2;3e
Binding Energy eV
Fig. 9. C(1 s) XPS spectrum of a poly(5-NH2 1-NAP) film prepared at 0.9 V vs. SCE in acetonitrile.
A film prepared in acetonitrile is electroactive in an acetonitrile solution containing 0.2 M HC104 +0.2 M NBu4CIO4 (Fig. 13). The redox system is observed at 0.52 V versus SCE; peaks are broader than in aqueous medium. Finally, a film prepared in aqueous medium presents the same redox system at 0.52 V but it is not stable. The film is partially soluble in the solvent after several scans.
13
cl
....
~
20
o2
o
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(A)
~c
(a)
pA
~pA aoo
lOO 200
5o
(B)
o
(b)
lo
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Fig. 11. (a) Cyclic voltammetric r e s p o n s e s o f a poly(5-NH2 1N A P ) film-coated Pt electrode (film p r e p a r e d in acetonitrile) at different scan rates. M e d i u m : 2 M H2504; electrode surface: 0.03 cm 2. (b) Plot of peak intensity vs. scan rate.
3.4. Electrochemical oxidation of 5-methoxy 1naphthylamine Data from the IR study of poly(5-NH2 1-NAP) film growth (Section 3.2) suggest that the - O H group does not take part in the electropolymerization. In order to elucidate this finding, we have synthesized and studied the electrochemical oxidation of 5-methoxy 1-naphthylamine. 3.4.1. Film formation in 2 M H2S04 aqueous solution The electrooxidation of 5 x 1 0 -3 M 5-methoxy 1naphthylamine in 2 M H 2 8 0 4 results in film formation on Pt and glassy carbon electrodes.
5 (scan role )~/2 10 (inV. s -I )1/2
Fig. 12. (A) CV responses of a poly(5-NH2 1-NAP) film-coated Pt electrode (film p r e p a r e d in 2 M H2504 aqueous m e d i u m ) at various scan rates: a, 200; b, 100; c, 50; d, 20; e, 10; f, 5 m V s -~. M e d i u m : 2 M H2504; electrode surface = 1.4 cm 2. (B) Plot of peak intensity vs. (scan rate) ~ .
The cyclic voltammograms (CVs) during film growth at scan rate of 50 mV s-1 on a glassy carbon electrode are presented in Fig. 14. They are quite similar to the CVs recorded during poly(5-NH2 I-NAP) film growth (Fig. 1). On the first scan between 0 and 0.95 V versus SCE, an anodic peak is observed at 0.75 V. In subsequent scans, two redox systems are detected, system A (Epa=0.37 V; Em=0.33 V) and system B (Epa=0.32 V; E ~ = 0.2 V). During the film growth, system B seems to become preponderant compared to system A. A tentative explanation is that in the case of 5-methoxy 1-naphthylamine, the two couplings via ortho and para positions of the -NH2 group were performed during the first scans, and later only one kind of coupling was
14
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~
v=200mVsec-I
//~
"'-"--~..::i :i: ,~o
...... 20
//
..... ,oo.,.-, ..... 50 ...... 20 •:.i:i 150
I
0'0
o5
VvsSCE ,0
--
Fig. 13. C V responses of a poly(5-NH2 1-NAP) film-coated Pt electrode (film p r e p a r e d in acetonitrile) at different scan rates. Medium: 0.2 M HCIO4+0.2 M NBu4CIOn+CH3CN; electrode surface: 0.03 cm 2.
V 0
I
0.5 I Vvs
SC E
Fig. 15. Cyclic voltammetric responses o f a poly(5-methoxy 1naphthylanfine)-coated carbon electrode in 2 M H2SO4 aqueous solution (electrode surface: 0.07 cm2).
3.4.2. Electroactivity of poly(5-methoxy 1naphthylamine) film in aqueous 2 M H2SO4 solution
0
Bl
0,95 Vv5SCE
Figure 15 presents the cyclic voltammetric responses of a film-coated carbon electrode. The film (= 200 nm thick) was prepared by scanning 30 times at 50 mV s -1 between 0 and 0.95 V versus SCE. It should be noted that the CVs present the same very reversible redox system (Epa= 0.28 V; Epc=0.23 V) as that recorded with poly(5-NHz 1-NAP) film in aqueous medium (Fig. ll(a)). The similarity of the CVs of f i l l growth and especially of film electroactivity during the study of 5-NH2 1-NAP and 5-methoxy 1-naphthylamine supports the argument that only -NH2 groups intervene in the electropolymerization of 5-NH2 1-NAP.
Fig. 14. Cyclic voltammograms of 5X 10 -3 M 5-methoxy l-naphthylamine in 2 M H2SO4 solution at a scan rate o f 50 m V s - l with a Pt disk electrode (surface: 0.03 cm2).
4. Conclusions
preferential, probably the para coupling due to steric hindrance. In the case of poly(5-NHz 1-NAP) film growth, only one redox system was observed at Epc = 0.23 V and Eva = 0.26 V (Fig. 1). If the potential limit is diminished as in the CVs of poly(5-NH2 1-NAP) f i l l growth (Section 3.1.1., Fig. 1), no f i l l is formed. The repetitive scans should be kept between 0 and 0.95 V to obtain f i l l deposit on the electrode surface. Films can also be formed at a constant potential of 0.9 V but they are thinner. The molecular weight for poly(5-methoxy 1-naphthylamine) is about 5000 (DP=29).
The electropolymerization of 5-amino 1-naphthol in acidic aqueous and acetonitrile solutions results in polymer films which are very adherent on glassy carbon and Pt electrodes. The fills prepared in aqueous medium are conducting, tr= 10 -3 S cm -1, while those prepared in acetonitrile are much less conducting, t r - 2 X l 0 -6 S cm -1. The infrared and XPS studies indicate that the polymerization occurs via the ortho or para position with respect to the -NH2 groups and leads to the formation of C-NH-C and C - N = C bonds, while the - O H groups are not concerned. The following structure could be proposed in the case ofpara coupling:
15
References
×
Y
These findings are confirmed by data from the study of the electropolymerization of 5-methoxy 1-naphthylamine. Besides the cyclic voltammograms during film growth, which are similar for the two monomers, the redox systems of the two films are practically the same in acidic aqueous medium and thus confirm the fact that coupling of nuclei occurs through the -NH2 group.
Acknowledgement The authors thank Professor L.H. Dao and Dr J.Y. Bergeron (Laboratoire de Recherche sur les Mat6riaux Avanc6s, INRS Energie, Varennes, Quebec, Canada) for help in size exclusion chromatography measurements.
1 M.-C. Pham, J. Moslih and P.-C. Lacaze,J. ElectroanaL Chem., 278 (1990) 415. 2 M.-C. Pham, J. Moslih and P.-C. Lacaze, J. Electrochem. Soc., 138 (1991) 449. 3 M.-C. Pham, J. Moslih and P.-C. Lacaze,J. Electroanal. Chem., 303 (1991) 297. 4 T. Ohsaka, M. Ohba, M. Sato, N. Oyama, S. Tanaka and S. Nakamura, J. Electroanal. Chem., 300 (1991) 51. 5 J. Lockett and W.F. Short, J. Chem. Soc., 1 (1939) 787. 6 M.-C. Pham, F. Adami, P.-C. Lacaze, J.P. Doucet and J.E. Dubois, J. Electroanal. Chem., 201 (1986) 413. 7 D.E. Stilwell and S.M. Park, J. Electroehem. Soc., 135 (1988) 2254 and 2491. 8 G. Zotti, S. Cattarin and N. Comisso, J. Electroanal. Chem., 239 (1988) 387. 9 G. Socrates, Infrared Characteristic Group Frequencies, Wiley, New York, 1980. 10 J.E. Stewart, J. Chem. Phys., 30 (1959) 1259. 11 A.G. Moritz, Spectroehim. Acta, 16 (1960) 1176. 12 A. Dilks, Electron Spectroscopy, Theory, Techniques and Applications, Vol. 4, Academic Press, London, 1981. 13 R.A. Dickie, J.S. Hammond, J.E. De Vries and J.W. Holubka, Anal Chem., 54 (1982) 2045. 14 P. Snauwaert, R. Lazzaroni, J. Riga and J.J. Verbist, J. Chem. Phys., 92 (1990) 2187.