Synthesis and Characterization of a New Conducting Electropolymerized Film from 1-Naphthol

J. Electrochem. Soc., Vol. 138, No. 2, February 1991 �9 The Electrochemical Society, Inc. 449

Science," Barnes and Noble Book, New York (1972). 4. P. W. T. Lu and R. L. Ammon, This Journal, 127, 2610

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Dujka, Int. J. Hydrogen Energy, 7, 43 (1982). 7. P. W. T. Lu and R. L. Ammon, in "Hydrogen Energy

Process," T. N. Veziroglu, K. Fueki, and T. Ohta, Ed- itors, Vol. 1, p. 439, Pergamon Press, New York (1980).

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11. G. H. Fa rbman and G. H. Parker, in "Hydrogen: Pro- duct ion and Marketing," M. W. Smith and J. G. San- tagelo, Editors, p. 359, American Chemical Society, Washington, DC (1980).

12. E. Yeager and A. J. Salkind, "Techniques of Electro- chemistry," Vol. 1, p. 152, Wiley-Interscience, New York (1972).

13. A. Bewick and H. R. Thirsk, in "Modern Aspects of Electrochemistry," Vol. 5, p. 291, Plenum Press, New York (1969).

14. H. Kohler, D. L. Piron, and G. Belanger, This Journal, 134, 120 (1987).

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22. K. Wiesener, Electrochim. Acta, 18, 185 (1973).

Synthesis and Characterization of a New Conducting Electropolymerized Film from 1-Naphthol

Minh Chau Pham, Jamal Moslih, and Pierre-Camille Lacaze Insti tut de Topologie et de Dynamique des Syst~mes de l'Universit~ Paris 7, associ~ au C.N.R.S.-URA 34, 75005 Paris,

Cedex, France

ABSTRACT

A conduct ing and electroactive film, poly(NAP-1), has been electrochemically synthesized in acetonitri le solution. The polymer structure, the electropolymerizat ion mechanism, and the electrochemical propert ies were s tudied using in situ IR, XPS, and SEM spectroscopy.

In a pre l iminary s tudy (1), we have reported the prepara- t ion of a new conduct ing polymer film, poly (NAP-l), by electrochemical oxidat ion of 1-naphthol in acetonitrile.

We present in this paper details concerning the polymer structure, the electropolymerizat ion mechanism analyzed by in situ IR and XPS spectroscopy, and the electrochemical propert ies of this new type of polymer film.

Experimental Electrochemical measurements were performed with a

PAR 173 potent iostat connected to a PAR 175 pro- grammer.

The working electrode was a P t or glassy carbon disk sealed in Teflon, a P t plate for XPS experiments , or germanium crystal coated with a thin layer of P t deposi ted by sput ter ing (Balzers Model Sput ron II) for in situ IR analysis by the mul t ip le internal reflection Fourier t ransform infrared (MIRFTIRS) method.

Fi lms could be produced at constant current or constant potential (e.g., + 1.3V vs. Ag/AgC1) or by potential cycling between +0.2 and +l.3V. For example, a film formed by ten cycles has a thickness of 1.5 ~tm.

MIRFTIRS spectra were recorded on a Nicolet 60 SX Fourier t ransform spectrometer. Details concerning the spectroelectrochemical cell have been publ ished in a pre- vious paper (2).

In situ MIRFTIRS spectra at an indicated potential are t ransmit tance difference spectra. For each spectrum, the t ransmit tance spec t rum of the system before polarization (the reference spectrum) is subtracted from that of the system at an indicated voltage.

XPS spectra were recorded on a Vacuum Generators Es- calab MK1 Spectrometer , with an unmonochromated Mg K s x-ray source (power appl ied to the anode = 100W) under pressures in the 10 -8 mbar range. The analyzer was operated at constant pass energy (20 eV). The spectra were

digitized, summed, smoother, and reconstructed using Gaussian-shaped components . Binding energies are re- ferred to Cls 285 eV.

Results and Discussion Electrochemical Synthesis . --Polymer films were

elaborated onto plat inum, graphite, or germanium/pla- t inum electrodes by electrochemical oxidation of 1 naphthol in acetonitr i le solution containing 0.1M of the electrolyte (NBu4C104, LiAsFs, LiC104, NBu4PF6). Satisfactory films could be produced at constant current or constant potential (e.g., + 1.3V vs. Ag/AgC1) or by potential cycling between +0.2 and +l .3V (Fig. 1).

Structure and Polymerization Mechanism Polymer structure.--The poly (NAP-l) film in its neutral

undoped state was demonst ra ted by in situ IR analysis (1) to be const i tuted by alternating naphthylene and furan rings.

0 fOH

Q ) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 128.151.244.46Downloaded on 2014-11-03 to IP

http://ecsdl.org/site/terms_use

450 J. Electrochem. Soc., Vol. 138, No. 2, February 1991 �9 The Electrochemical Society, Inc.

I .i001~A/cm

ol

10 th scan

I st scan

5 1,5V

s~ ss s~ +

0 T u,~ ~ ~,3

$2 V v$ Ag/AgCt S~ S s

Fig. 1. Cyclic voltammograms of poly(NAP-1) during film growth. Medium: l-naphthol 0.1M-NBu4CIO4 0.1M-acetonitrile. (a, top) The electrode is a glassy carbon disk with a surface of 0.07 cm2; the scan rate was 50 mV �9 s -~. (b, bottom) The electrode is a Pt-coated Ge prism; the scan rate was 20 mV �9 s -1.

In fact, the f i lm ob ta ined by ho ld ing the e lec t rode at a cons tan t po ten t ia l or by repe t i t ive cycl ing b e t w e e n 0.2 and 1.3V is d i rec t ly in the ox id ized conduc t ing state, poly (NAP-l ) +. Data f rom i n s i tu , e x si tu, IR, and X P S spectroscopy will conf i rm the p o l y m e r s t ruc tu re in its two states, the neut ra l (a) and the ox id ized fo rm (b).

b

I R s p e c t r o s c o p y . -

In Situ MIRFTIRS Spectra during Film Formation Figu re 2 p resen t s spect ra r ecorded in s i tu dur ing forma-

t ion of poly (NAP-l ) by repe t i t ive scans be tween 0.2 and 1.3V at 20 m V s-L The cycl ic v o l t a m m o g r a m s are those of Fig. lb . Three cycles were p e r f o r m e d b e t w e e n 0.2 and 1.3V and the four th scan was s topped at the h ighe r potent ia l l imi t o f 1.3V. Spec t ra S~, $3, $5, $7, w e r e r ecorded at the ends of the pos i t ive scans 0.2 --> 1.3V and $2, $4, $6 at the ends of t he reverse nega t ive scans 1.3 ~ 0.2V.

c~

o $ I

~o cm ~ $7

~iL ~Tg m.

~0t

b S 3

S~

ISo 1830 I~10 1~9o L~7o l iSo to3o w ~'~o ~ o mmVENUMBE~S

~o

L~

m -

=- :+

0~

WAVENUMBEfl5

w

~,-

~ -

N

o!

mmt WnVENUMBER5 Fig. 2. In situ MIRFTIRS spectra recorded during three repetitive

scans between 0.2 and 1.3V vs. Ag/AgCI. The cyclic voltammograms are those of Fig. 1 b. Each spectrum was obtained using 200 interferom- eter scan (50s). The spectra are referenced to that obtained with the system before polarization. The spectrum numbers are those noted in Fig. ! b.

In the d o m a i n 1750-700 cm -1, the bands in tens i ty increases f rom St to $7 wi th the film thickness . Different bands charac ter i s t ic o f the film are seen as in the case of film fo rmat ion af ter a s ingle potent ia l sweep at a 2 m V - s -1 scan rate (1). Fu ran r ings are ident i f ied by C~----C s t re tch at 1572 cm -1 (3), a symmet r i c v ibra t ion of C - - O - - C l inkage at

) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 128.151.244.46Downloaded on 2014-11-03 to IP



:-: , , , j , ' - :- i~ i l l !14

i ,

, oo ,o.o .... ,,o s ,.o NRVw NU~B[RS

Fig. 3. In situ MIRFTIRS spectra recorded during potential step ex- periment between | and 0.2V with u film-couted electrode. Medi,,m: 0.]M LiAsFa + CH3CN. The spectrawere obtained using 500 scans (130s) and referenced to that of the system before polarization.

1291 cm-l(3), and s y m m e t r i c v ib ra t ion of the same bond as a s t rong band at 1080 cm -t a c c o m p a n i e d by a shoulder at 1062 cm -~ (3). The C = C ~ s t re tch of naph thy l ene r ing is p resen t at 1589 cm -t (Fig. 2d).

A ve ry s t rong band at 768 cm -~ is re la ted to four ad jacent a romat ic h y d r o g e n a toms in po lynuc lea r a romat ic com- p o u n d s (4). This indica tes that in the poly (NAP-l) , the coupl ing occurs at the nuc leus conta in ing the - - O H group and does no t affect the second nuc leus of the ring. In fact, in the re fe rence s p e c t r u m of the sys tem before polariza- t ion (1), bands due to the m o n o m e r are visible, bes ides the band at 777 cm -1 wh ich is due to C - - H vibra t ions of the four ad jacen t a romat ic H a toms (4), two bands related to the C - - H out-of p lane v ibra t ions of three ad jacent H a toms are also obse rved at 750 and 798 cm -~ (4). These two lat ter bands are absen t in the p o l y m e r spec t rum (Fig. 2).

In situ MIRFTIRS Spectra of Films in the Oxidized and Reduced Forms In order to de tec t the d i f fe rence of the po lymer s t ruc ture

in the ox id ized and r educed form, a f i lm-coated e lec t rode

2

v v i p i I

~OCO

6 �84

N

D �84

I~O l~eO ~14~0 ~7 0 $ [50 ieo ~lo t~o ~ o ~VENOMBER$

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Fi 9. 4. Ex situ external reflection IR spectrum of the film after its formation by a potential cycle between 0 and 1.3V at 2 mV �9 s-L Medium: 1 -naphthol O. 1M, LiAsF60.1M-acetonitrile.

~TJ

'9.4J I

LsJ I I

~,6J I

I

A I

I

\

t \

BINI}IN6 ENER6Y EV

r I

i

A C~

10.5. i~ I

8,4J ] ' i

" ! / ' ! , t .; "x..

2.tJ '~

f J

BINDIN6 ENER6Y EV

/ )

Fig. 5, XPS spectra of a poly(NAP-1) film prepared by holding the electrode at 1.3V after three scans between 0 and 1.3V: (a, top) C l , , (b, bottom) O1,

was polar ized at 1V and fur ther s t epped to 0.2V in an acetoni t r i le solut ion con ta in ing only an electrolyte. The in s i t u spectra are p resen ted in Fig. 3. Sox cor responds to the ox id ized fo rm and Sr to the r educed form.

Sox presents a large band at 1223 cm -1 which can be ass igned to the C ~ - - C s t re tch in the furan rings; this band is pract ica l ly absen t in the spec t rum Sr of the reduced form. In the ox id ized form, the C~---C inter-r ing band at 1547 cm -1 is s t rong whi le in the r educed form, it is re- p laced by the C - - C band at 1124 cm -~ (Fig. 3).

Ex Situ IR Film Analysis A film in dry state has also been analyzed by e x s i t u ex-

te rnal ref lect ion spect roscopy. I t was synthes ized by a potent ia l cycle b e t w e e n 0 and 1.3 at 2 mV �9 s -~ wi th LiAsF8 as electrolyte.

All the bands related to the po lymer s t ructure are con- f irmed. For furan rings, C- -C s t re tch is seen at 1571 cm- ' , C - - O - - C l inkage at 1290 cm -~ and (1079-1057) cm-~; for naph thy l ene rings, C=Ca, s t re tch is obse rved at 1590 cm -I and C- -C s t re tch b e t w e e n m o n o m e r i c moie t ies at 1124 cm -~ (Fig. 4a).




The ex s i tu spectrum shows the presence of the naphthol groups at the ends of the polymer chains, as indicated by OH stretch at 3541 cm-' . The C--H vibrations of the four adjacent H atoms are situated at 765 cm -1 (Fig. 4).

An important part of the film is in the oxidized form as indicated by the C -~(~)~C large band at ca. 1215 cm associated with a strong band at 704 cm -~ related to the AsF6- counterions. The absorption increases in the 2000- 4000 cm -~ domain, indicating the presence of electronic conduct ing layers (5) (Fig. 4b).

X P S results .--A film obtained by holding the electrode at 1.3V for 5 min after three potential cyclings between 0 and 1.3V has been analyzed by XPS (Fig. 5).

The carbon spectrum displays two C~s signals; a first intense peak at 285 eV due to aromatic or aliphatic carbon atoms coupled to carbon atoms of the same type, a second less intense peak at 286.50 eV ascribed to carbon atoms coupled singly to oxygen atoms (6, 7) (Fig. 5a).

The oxygen spectrum exhibits two 0]8 signals: one at 532 eV ascribable to C--O--C bonds, another at 533.92 eV related to oxygen positively charged (Fig. 5b).

An approximate formula for the polymer can be calcu- lated from the peak intensities with experimental sensitiv- ity coefficients derived from a study of a series of organic compounds. We found (Ct0 O0.9~)n, which is in good agree- ment with the expected formula (C,0 O,)n. Peaks characteristic of the dopant are also seen, F]~ at 687.02 eV and AS3d at 48.5 eV (8).

Polymeriza t ion mechanism. - -To explain the poly(NAP-1) structure, we have proposed the following scheme for the mechanism: (i) oxidation of the monomer to radical- cation, (ii) dimerization of the radical cations, (iii) proton loss to yield a neutral dimer, (iv) oxidation of dimer to its radical cation, (v) reaction of dimer radical cation with another radical cation . . . . Concurrently with these reactions, the nucleophilic reactions of t he - -OH groups lead to cyclization and formation of furan functions in the chain.

In acetonitrile, the --OH is not dissociated. The anodic oxidation of 1-napththol gives principally two radical- cations, (I) and (II)

OH OH OH

( I ) (n) The coupling of two radical cations gives dimers A, B,

and C. OH OH

_ 2 H,,+ 2 II) "

Di~r A

OH

OH

named (p-o') and the following couplings will be always (p-o').

The coupling of radical cation (II) with dimer B will occur at the ortho position of- -OH. The coupling is (o-p') and the following couplings will be systematically (o-p').

The coupling of radical cation (II) with dimer C can be performed at the ortho or the para-position of- -OH. The (p-p') coupling gives the same trimer as that obtained with dimer B. The (o-p') coupling gives another trimer and the following couplings will be always (o-p').

The result is that for the three resulting polymers except for the two first monomeric units, the rest of the polymer chain will have the same structure.

In parallel with the radical cation couplings, nucIeo- philic reactions of t h e - - O H groups lead to cyclization and formation of furan rings in the chain.

OH

OH

o ~ O H

-le .2H +

Data from size exclusion chromatography using polysty- rene as reference indicate that the molecular weight is ap- proximately 376,000.

Another mechanism could be envisaged that is a nucleophilic attack o f - -OH from a monomer moiety at the radical cations derived from the monomer.

[1]

[2]

o r

OH

-2N +

The dimers obtained are again oxidized, and the reactions of dimer radical cations with another monomer moiety give trimers with two C- -O~C bonds. The resulting polymers then contain C--O--C as inter-ring bond.

Dimer B

(I) + (11) - - . - - ~ ~ ~[ H

Dimer C

The dimer is again oxidized to radical cation; coupling with a radical cation derived from the monomer leads to a trimer.

The coupling of radical cation (I) with dimer A can occur only at the para position of t he - -OH group; the coupling is

Fig. 6. Scanning electron micrographs of poly(NAP-1 ) film on Pt. The film was formed by six scans between 0 and 1.3V; the electrolyte is LiAsF6.




Fig. 7. SEM of poly(NAP-] ) film on Pt. The film was formed at o constant potential of 1.3V for 10 min; the electrolyte is NBu4CIO4.

If the radical cat ion-substrate couplings mechanism does exist, the electrochemical oxidat ion of 2-methyl 1- naphthol should lead to the selective para-coupling between radical cation (II) and the substrate (e.g., Eq. [2]). The o-coupling between radical-cation (I) and the substrate

C . c r ~ z

"1o

g 1.2 U

O.B

0./.

/ / / /.f

1 0 5 10 thichness/~Jm

Fig. 8. Plot of polymerization charge density vs. film thickness for poly(NAP-1 ) films.

0.99 t

0,93

T

I 0,90

50 mV/sec

20

]0

200 m V / s e c

I00

50

I

0,/, 1 V vs Ag/A~Ct

i

0.t, 1 V vs Ag/AgCL

tpo

20

10.

'HA .I

10 50 100 200 vl~V#o�9 }

"Pc

20

o// / |, i i i

0 10 50 100 200 v (~w/~)

Fig. 9. (a) Cyclic voltammograms of a poly(NAP-1)-coated Pt electrode in 0.1M LiAsF6 + acetonitrile. Film thickness = 500 nm. (b) Plot of peak intensity vs. scan speeds for anodic peak (i~) and cathodic peak (ip~).




cannot occur because the o-position is blocked by a --CH3 group (e.g., Eq. [1]). In fact, we have found that no film was obtained by electrochemical oxidation of 2-methyl 1~ naphthol in the same conditions as for 1-naphthol. Proba- bly, the kinetics of couplings between two radical cations is much more rapid than that of couplings between a radical cation and a substrate. It can be deduced that the latter mechanism should be eliminated.

Another feature indicating the exclusion of the radical cation substrate coupling mechanism is that by this mechanism, the polymer structure will be the same as that obtained by electro-oxidation of 1-naphthol in a basic methanolic medium (9); the film contains poly (oxide) groups (C--O--C) as inter-ring bonds. We have thoroughly studied these polymer structures by XPS (9a) and IR spectroscopy (9b); poly (NAP-l) obtained in acetonitrile solution gives completely different results with XPS and IR analysis.

Coming back to the first mechanism that we adopt, IR spectra show that the - -OH band is negligible while C- -O~C bonds of furan rings are present (Fig. 2, 4). The existence of a furan function proves that the --OH groups are oxidized and the cyclization reaction leads to furan rings in the chain.

Physical Properties of Poly(NAP-I ) Film Film thickness and morphology.--The polymer surface is

analyzed by scanning electron microscopy (SEM). The film appears to be a dense homogeneous material (Fig. 6). SEM was also used to measure film thickness. On a polymer film prepared on a plat inum plate, a strip of polymer was scratched out to reveal a cross section of the film. For example, for a film prepared by holding the electrode at 1.3V for 10 min, the thickness is ca. 7.8 ~m (Fig. 7).

A plot of thickness vs. polymerization charge density for a series of similar films is shown in Fig. 8. These data ex- hibit a linear relationship with a slope of 0.18 C - cm -~ ~m -1.

Electroactivity of the poly(NAP-1) f i lm.--Cyclic voltammograms of the poly(NAP-1) film coated Pt electrode in 0.1M LiAsF6-acetonitrile solution are shown in Fig. 9a. An anodic current peak appears at a potential of ca. 0.93V while the cathodic current peak appears as a broad one at a potential of ca. 0.9V at 50 mV - s -1. Peak currents increase linearly with increasing scan speed, indicating that the charge transport kinetics of the film are fast (Fig. 9b). This behavior is nevertheless true only for thin films. The peak current intensities vary linearly with film thickness until ca. 500 nm. For thicker films, the anodic peak is not well defined.

Electronic conduct ivi ty . --The electronic resistances of a series of poly(NAP-1) films on Pt were measured using a mercury contact to the surface of the film. The conductiv- ity varies between ca. 10 -2 and 10 -1 S �9 cm -z depending on the conditions of film formation.

Conclusion A new type of conduct ing polymer film is synthesized by

electrochemical oxidation of 1-naphthol in acetonitrile so-

lution. Instead of insulator poly (naphthalene oxide) film obtained when 1-naphthol is electrochemically oxidized in alkaline methanolic solution (ga), conducting film formed in acetonitrile is constituted by alternating naphthylene and furan rings. The mechanism and the film structure have been elucidated by in situ IR analysis and XPS. Dur- ing the doping process, oxygen of furan rings are positively charged binded by counterions. The process is quite reversible as shown by an in situ IR study which will be presented later.

Acknowledgments We wish to thank Mrs. Monique Simon and Mr. Michel

Leclerc for technical assistance in MIRFTIRS experiments and XPS study, respectively. Size exclusion chromatography was performed at Universit6 de Qu6bac, INRS-Energie. We are grateful for this help from Pr. Le H. Dao and Dr. M. T. Nguyen.

Manuscript submitted Feb. 28, 1990; revised manuscript received Sept. 14, 1990. This was, in part, Paper 724 presented at the Montreal, Quebec, Canada, Meeting of the Society, May 6-11, 1990.

Inst i tut de Topologie et de Dynamique des Syst~mes de L'Universit~ Paris 7 assisted in meeting the publication costs o f this article.

REFERENCES 1. M. C. Pham, J. Moslih, and P. C. Lacaze, J. Electroanal.

Chem., 278, 415 (1990). 2. (a) M. C. Pham, F. Adami, P. C. Lacaze, J. P. Doucet,

and J .E. Dubois, ibid., 201, 413 (1986); (b) M.C. Pham, F. Adami, and J. E. Dubois, This Journal, 134, 2166 (1987).

3. (a) A. H. J. Cross, S. G. E. Stevens, and T. H. E. Watts, J. Appl. Chem., 7, 562 (1957); (b) A. R. Katritzky and J.M. Lajowski, J. Chem. Soc., 657 (1959); (c) H.W. Thompson and R. B. Temple, Trans Faraday Soc., 41, 27 (1945).

4. G. Socrates, in "Infrared Characteristic Group Fre- quencies," Edited by John Wiley and Sons, Chi- chester, New York, Brisbane (1980).

5. G. B. Street, in "Handbook of Conducting Polymers," Vol. 1, T.A. Skotheim, Editor, p. 279, Marcel Dek- ker, New York (1986).

6. A. Dilks, "Electron Spectroscopy, Theory, Techniques and Applications," Vol. 4, p. 277, Academic Press, London (1981).

7. R. A. Dickie, J. S. Hammond, J. E. De Vries, and J. W. Holubka, Anal. Chem., 54, 2045 (1982).

8. "Handbook of X-Ray Photoelectron Spectroscopy," C.D. Wagner, W.M. Riggs, L.E. Davis, J .F . Moulder, and G.E. Muilenberg, Editors, Perkin Elmer, Physical Electronics Division, Eden Prairie, MN (1978).

9. a) M. C. Pham, A. Hachemi, and M. Delamar, J. Electro- anal. Chem., 134, 197 (1985); (b) A. Hachemi, Th~se de Docteur-Ing6nieur, Universit6 Paris 7 (1985).



Synthesis and Characterization of a New Conducting Electropolymerized Film from 1-Naphthol

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