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In situ Fourier transform infrared attenuated total reflection (FTIR-ATR) spectroscopic investigations on the base-acid transitions of leucoemeraldine
Electrochimica
mu
Pergamon PII: soo134fl6(96)0036l34
Acta, Vol. 42, No. II, pp. 16931700. 1997 0 1997 l%evier Science Ltd. All rights reserved. Printed in Great Britain 00134686/97 $17.00 + 0.00
In situ Fourier transform infrared attenuated
total reflection (FTIR-ATR) spectroscopic investigations on the base-acid transitions of leucoemeraldine Zhao Ping,* G. E. Nauer, H. Neugebauer, J. Theiner and A. Neckel Institute of Physical Chemistry, The University of Vienna, Waehringerstr.
42, A-1090, Vienna, Austria
(Received 4 June 1996; in revised form 4 September 1996) Abstract-Leucoemeraldine, the reduced form of the polyaniline, was obtained by electrochemical polymerization. Extensive infrared spectroscopic studies are discussed in this work. Ex situ FTIR spectra measured in NaReO,/HReO, electrolytes with different pH-values are presented and the absorption bands are assigned to different vibrational modes. Additionally, a distinct picture of the base-acid transition is evaluated using in sifu FTIR-ATR measurements in electrolytes containing the supporting salts mFb/HPFe, NaC104/HC104 and NaReOd/HReOd. The results demonstrate that in strong acidic media leucoemeraldine nitrogen atoms can be partly protonated. The protonation process starts in the pH-range 2.5 and 3 in the intercalation of anions from the electrolyte into the polymer structure is determined semi-quantitatively. 0 1997 Elsevier Science Ltd. All rights reserved. Key words: Polyaniline,
leucoemeraldine,
in situ
FTIR spectroscopy, conducting
INTRODUCTION
A large number of experimental works have been devoted in the last two decades to different forms of polyaniline. Depending on the average oxidation state, different compounds can be obtained [l]. Leucoemeraldine (LE), the fully reduced form of the polymer, is the simplest polymer since only phenyl rings in the benzenoid form are present. It was first synthesized in 1910 [2] and can be conveniently prepared as an analytically pure, white powder by the reduction of emeraldine [3]. The Raman spectral pattern of LE base form does not vary with the excitation wavelength and is interpreted as that of a p-disubstituted benzene [4, 51. Free-standing film can be obtained by electrochemical polymerization on an electrode surface or by casting from Nmethylpyrrolidone solution [6,7]. The surface of the particles in the powder or of the film are oxidized by air, and the film turns blue; however, “C and NMR studies show that the bulk of the blue material is still *Author to whom correspondence should be addressed. Current address: Department of Chemistry, The University of Georgia, Athens, GA-30602-2556, U.S.A.
polymer, electrochemistry.
in the LE reduced state [8]. In the LE base form, nitrogen atoms can be protonated and deprotonated depending on the pH of the medium. Potentiometric titration [9] of the LE salt form also demonstrated that in the region of pH = 0.5 the salt form is at least partly protonated. This feature of the base-acid transition is observed to be reversible. Many infrared spectroscopic studies [lO-141 have been made for the characterization of LE, however, experimental spectra differ from one another. We have made a series of research work on the PAN1 system [17-201. A number of vibrational spectroscopic investigations have been done on this system in order to determine structural changes and doping mechanisms during protonation and electrochemical redox processes. However, one problem arises from finding a suitable doping system because most of the anions used in the experiments have either no ir absorption bands (eg Cl-) or bands which overlap with polymer vibrational bands (eg ClO;). We have found that ReO; exhibits only one strong absorption band around 900 cm-’ in the infrared spectrum which is clearly separated from PANI vibrational bands. Although ClO; and PF; as doping anions can be used in the investigation of LE
1693
Zhao Ping et al. due to lower intensity changes of vibrational bands of the polymer in the base and acid forms, the NaReOd/HRe04 as a new doping system is more useful in the protonation doping investigations of the other forms of PANI [18] (emeraldine or pernigraniline) or in the electrochemical redox investigations of PANI [20]. In this paper, the attention is focused on the investigation of the base-acid transition process of LE to obtain information on the structure of the polymer via its vibrational properties and to measure in situ the change in the anion content of the polymer film during proton doping, which is beneficial eg for the improvement of PAN1 based pH-sensor.
EXPERIMENTAL The in situ FTIR-ATR (attenuated total reflection) experiments were performed on a fully computerized Nicolet 60 SX spectrometer fitted with a liquid nitrogen cooled MCT-detector. The electrochemical equipment consisted of a potentiostat 1001 TNC (Fa.Jaissle, Neustadt/Waiblingen, Germany) and a three electrode spectroelectrochemical cell. A ZnSe reflection element (45”) coated with a thin Au/Pt minigrid layer, deposited by evaporation under high vacuum conditions, a sheet of Pt and a saturated calomel electrode (see) were used as working, counter and reference electrodes, respectively. All potentials refer to this see. The potential of the working electrode was controlled by a computer system, which allows also the data collection (SCADA, J. Theiner, Univ. Vienna). The polymerization was carried out in the ATR-cell [ 151 containing 0.1 M aniline and 1 M HCl solution by cycling the potential between -0.2 V and +0.8 V at a scan rate of 10 mV . s-l. After that, the cell was washed with tri-distilled water and then filled up with electrolytes of different pH-values, using KPFe/HPFs, NaC104/HC104 and NaReOd/HReOd solutions. The potential of the working electrode was kept at -200 mV in order to obtain the LE form. When equilibrium was established, FTIR-spectra were recorded consecutively in the following way: 512 interferograms were coadded for each single beam spectrum (Sss) with a resolution of 4 cm-‘. After the Fourier transformation, the spectra are presented as difference spectra Sn, ratioed to a reference spectrum Ss, Su = S&S,. The spectra were corrected for the Hz0 and CO2 content in the optical path and are base line corrected. In this presentation, upward-pointing features (increasing absorption) correspond to the gain of absorbing species or increasing intensity of vibrational modes, and downward-pointing features (decreasing absorption) correspond to the loss of absorbing species or decreasing intensity of vibrational modes. Difference spectra, if not otherwise -.__-._ *Intensity: s, strong; m, medium; w, weak; v, very.
indicated, were obtained by taking the ir-spectrum of the LE base form as a reference state. For the ex situ experiments, the polymerization of the polymer was performed in the same way as for the in situ experiments. The polymer was converted into different levels of protonation by equilibration in acidic electrolytes of various pH-values and dried in vacuum. FTIR spectra of the polymers on the ATR-crystal were recorded in transmission geometry, the reference state was the ZnSe reflection element without a polymer. RESULTS
AND DISCUSSION
When the acidity of the medium changes, LE can be transformed from the base form to the salt form. In this work, thirteen NaReOd/HReOG electrolytes with different pH-values were used for establishing definite forms of LE. Figure l(a) shows the influence of the pH on ex situ FTIR spectra of LE, the spectra at pH = 1.5 (curve a) and pH = 8 (curve b) are presented as examples. Doping the LE base form by protonation (changing the pH-value of the electrolyte) results in a significant enhancement of the infrared bands of the polymer due to the effect of the positive charge on the polymer chain, inducing a dipole moment [21]. At strong acidic conditions (pH = 1.5), the LE salt spectrum exhibits absorptions at 1606(m*), 1502(s), 1300(m), 1250(w), 1178(m), 814(vw) and 798(vw) cm-‘. When the pH of the medium is higher than 3, some additional bands at 1626(w), 1377(vw), 1336(m), 1151(sh) and 847(vw) cm-’ are detectable in combination with a shift of the bands at 1606, 1502 and 1178 cm-’ to lower wavenumbers and of the band at 1300 cm-’ to higher wavenumbers. The strong absorption band at 914 (910) cm-’ is attributed to the v3 ReOi anion vibration. The band at 814 cm-’ (LE salt form) is characteristic of a p-substitution of the aromatic ring and is assigned to a C-H out-of-plane deformation vibration of the benzenoid groups. The CH out-of-plane bending vibration of the monosubstituted benzene ring is not observed indicating the absence of oligomers, for example, in the phenyl capped octamer this vibrational mode is of appreciable intensity [22]. The rings are mostly coupled at C4 and N positions (“head to tail” coupling), the corresponding C-N stretching modes are observed in the LE base form at 1250 and 1219 cm-’ (in the LE salt form at 1250cm-I) [23]. Therefore, the LE base form structure is mainly para-(NH--CsH4-)n. The intense band around 1500 cm-’ in the infrared spectrum of LE results from a C-C ring-stretching mode and the band at about 1600 cm-’ with medium intensity is likely to arise from a disorder-induced C< ring stretching mode. The vibrational modes in the range of 1178-1165cm-’ and 1163-1150cm-’ are assigned as either C-H bending or ring amine bending vibrations. Theoretical studies of Raman spectra [23-261 have demonstrated that these modes
1695
Base-acid transitions of leucoemeraldine
1500 Wavenumbers
1000 (cm-
1)
l-
(b)
35’00 Wavenumbers
(cm-
30’00 1)
2:
IO
Fig. 1.(a) Ex situ FTIR spectra of leucoemeraldine, equilibrated at - 200 mV in NaReOd/HReOd electrolytes with different pH-values. Curve (a) pH = 1.5; (b) pH =8. (b) Ex siru FTIR spectra of leucoemeraldine in the wavenumber region 4000-2500 cm-‘, equilibrated at -200 mV in NaReOa/HReOd electrolytes with different pH-values. Curve (a) pH = I .5; (b) pH=3; (c) pH=5.
Zhao Ping et al.
1696
are attributed to A, and BJ, Raman-active vibrations. When the symmetry of the LE chain is broken causing the Raman-active modes to become infrared active, these modes are thus observed also via infrared vibrations, connected with the change of the pH-value of the surrounding medium. The bands at about 1300 cm-’ have three contributions, coming from C-C ring deformation, C-H bending and C-N bending modes. The vibrational absorption at 1626 cm-‘, which is only detectable for the LE base form (pH = 8.5) should be assigned to a C=N vibrational mode. It seems that samples of undoped LE base form contain a few quinoid rings whose neighboring nitrogen atoms have no hydrogen atoms attached. This result is in agreement with an earlier report [27]. A remarkable difference between LE base and salt form is observed in the high frequency infrared region (Fig. l(b)). It is interesting to note that all the NH-groups (broad vibrational bands in the range of 3400 to 3350 cm-’ and 3300 to 3200 cm-‘, attributed to NH-stretching vibrations of -C6H4-NH-CbHagroups), are involved in H-bondings [28] (indicated by the disappearance of the absorption band at 3389 cm-’ and a strong shift to lower wavenumbers about 3240cm-‘). The difference in the intensities is caused by a difference in the H-bonding between the salt and the base form
-2OOmV
of LE. A large number of -NHgroups become lower H-bonded by the acid treatment, as indicated by a decreasing intensity of the absorbance at 3389 cm-’ for higher proton concentrations. An important implication of this is that each nitrogen in a polymer chain is located at a certain short distance from a nitrogen or a benzene ring in a neighboring polymer to form a H-bonded structure. Such close contacts of chains are considered to be important for charge transport in the conducting forms of the the absorption bands Moreover, polymer. ranging from 3000 to 2500 cm-’ are assigned to the vibrations associated with the NH: part in -C6H4NH:-C6H4groups. The intensity of vibrational bands at 2964 and 2877 increase with the decrease of the pH-value of the surrounding medium, which is also caused by the H-bonded structure change in the polymer chain. Because in ex situ FTIR spectroscopy only steadystate spectra are measurable and in order to get information on the dynamical changes of LE during the acid-base transition, in situ FTIR-ATR spectra were measured. This technique avoids the washing and drying processes and the handling of the polymer films in air. Establishing various experimental conditions, three different electrolytes (NaReOo/ HReO.+, NaC104/HClOd and KPFh/HPFb) were used. The results are presented in Fig. 2 and Fig. 3. As an
Leucoemeraldine
KPF6/HPFG
15’00 Wavenumbers
1 OS00 (cm-
1)
Fig. 2. Spectral changes measured in situ during the base-acid transition in KP&/HP& to 5.3 at -200 mV. The reference state is the ZnSe-crystal without a polymer film.
electrolyte in the pH-range 1.05
Base-acid transitions of leucoemeraldine
1697
.4
I
64
I 1500 Wavenumbers
-.1
I 1000 (cm-
1)
I
1500 Wavenumbers Fig. 3. (a and b).
1000 (cm-l)
1698
Zhao Ping et al. .ti
Acid
Form
J
2000
1000
1500 Wavenumbers
(cm-
1)
Fig. 3. Spectral changes during the base-acid transition process in different electrolytes measured by in situ FTIR-ATR spectroscopy. The reference state is the base form at -200mV. (a) KPFe/HPFa (in the pH-range l.OS5.3); (b) NaC104/HC104 (in the pH-range 065.3); (c) NaReO+‘HReOs (in the pH range 1.%X3).
example, the spectral changes in KPF6/HPFb electrolyte solutions are depicted in Fig. 2, using the spectrum of the bare ZnSe-crystal as the reference. Characteristic differences can be seen between the base and the acid form of LE, especially at wavenumbers 1604, 1500, 1178 and 845 cm-‘. For a better clarification of the spectral changes, difference spectra using the spectrum of the LE base form as the reference, are shown in Fig. 3. Note, that the absorbance scales in Fig. 3 are expanded and hence the sensitivity is higher in comparison to those displayed in Fig. 2. The patterns of the infrared spectra of the three systems in the wavenumber range below 2000 cm-’ are similar to one another, although there is a slight difference in the exact positions of the vibrational band maxima. An increase in the intensity was found for the absorption bands at 1604-1608, 1533-1539, 1317-1325, 1182-1186, 1128-1134, 930, 881, 798-800 and 768-769 cm-’ while a decrease in the intensity was observed for the absorption bands at 1500, 1280 and 1160 cm-‘. The absorption bands of the anions PF;, ReO;, ClO; are situated at 845, 904 and 1090 cm-‘, respectively. The infrared absorption data can be interpreted in terms of normal modes of para-disubstituted benzene. Several theoretical studies have shown that the ring-rotational conformation is important in
determining the ground-state electronic structure [22,24]. The LE base chain is built from unit cells containing one NH (bearing two electrons assumed to be perpendicular to the C-N-C plane) and one GHvring. In [25] an idealized polymer backbone structure is presented. In this model, spectral changes of LE during acid-base transition are considered. It is expected that ring-angle distortion and different
4
-
7.
-
0
I
2
3
4
s
‘
0’1
Fig. 4. Integrated intensities of the anion vibrational bands as a function of pH. The integration range is indicated in the brackets. (A) PF; (800-9OOcm-I); (+) CIO; (95&l 150 cm-‘); (m) ReO; (850-950 cm-‘).
Base-acid transitions
of leucoemeraldine
1699
pH > 4
PH
<1
A-
AA-:
ReOd’,
PFg’ , ClO4’
Fig. 5. Doping mechanism during acid-base transition processes of LE.
bond-lengths are responsible for the changes observed in the infrared spectrum of LE during protonation. Therefore, the decrease in the absorptions at 1500 and 1280 cm-‘, assigned to ring stretching and C-N bending vibrational modes, is caused by symmetry breaking and changing of the bond-angle. This effect cannot be seen ex situ because the handling of the film during the preparation procedures efface these small changes. Additionally, doping can cause an increase in the intensity of some vibrational bands. During the base-acid transition, the amount of anions in the polymer compensating the positive charges in the polymer chain changes. This demonstrates that the amount of protonated centers is increased on decreasing the pH-value of the electrolyte. Figure 4 shows, as a function of acidity, the anions content determined by integrating the anions vibrational absorption bands in an appropriate wavenumber range. The base-acid transition takes place at about pH = 3. In this pH-region the anion content shows an increase, verifying that the LE base form changes into the salt form. In principle, a similar picture is observable for all three systems used. In the non-oxidizing KPF6/HPF6 electrolyte,
the base-acid transition takes place in a very narrow pH-range (2.7-2.9) and the vibrational bands of the polymer indicate that the structure of the polymer is stable at pH-values higher than 3. It seems that the intercalation of anions must be associated with the oxidation of the polymer chain and with the base-acid transition process of the polymer based on the pH-value of the surrounding medium. According to in situ measurements of the anion vibrational intensities during base-acid transitions and structural changes of the polymer, the doping reactions are schematically presented in Fig. 5. At pH-values higher than 4, the LE base form is stable, converting in the pH-range l-3 to the partially protonated LE salt form. At pH-values lower than 1, the fully protonated LE salt form (50%) should be obtained. CONCLUSIONS The capability of in situ FTIR-ATR spectroscopy for the characterization of the base-acid transition of leucoemeraldine is demonstrated in this work. Depending on the pH-value of the electrolyte, different states of LE were found at a constant applied potential. At pH-values > 4, only the LE base
1700
Zhao Piing et al.
form is stable. The protonation process begins in the pH-range between 2.5 and 3, and in strong acidic media (pH-values < I), the LE nitrogen atoms are at least partly protonated. The vibrational features are indicative of H-bonded neighboring polymer chains, and of symmetry breaking and changing of bond-angles during protonation.
ACKNOWLEDGEMENTS One of the authors (Z.P.) acknowledges the financial support provided by the t)AAD (Office of Austrian Academic Exchange) and the Institute of Physical Chemistry (IPC), University of Vienna.
REFERENCES 1. A. Ray, A. F. Richter, A. G. MacDiarmid and A. J. Epstein, Synth. Met. 29, El51 (1989). 2. A. G. Green and A. E. Woodhead, J. Chem. Sot. Trans. 97, 2388 (1910). 3. A. G. Green and A. E. Woodhead, J. Chem. Sot. Trans. 101, 1117 (1912). 4. Y. Furukawa, T. Hara, Y. Hyodo and I. Harada, Synth. Met. 16, 189 (1986). 5. Y. Furukawa, F. Ueda, Y. Hyodo, I. Harada, T. Nakajima and T. Kawagoe, Macromolecules 21, 1297 (1987). 6. A. G. MacDiarmid, J. C. Chiang, M. Halpem, W. S. Huang, S. L. Mu, N. L. D. Somasiri, W. Wu and S. I. Yaniger, Mol. Cryst. Liq. Cryst. 121, 173 (1985). 7. P. M. McManus, S. C. Yang and R. J. Cushman, J. Chem. Sot., Chem. Commun. 1556 (1985). 8. S. Kaplan, E. M. Conwell, A. F. Richter and A. G. MacDiarmid, Synth. Met. 29, E235 (1989). 9. N. S. Sariciftci, H. Kuzmany, H. Neugebauer and A. Neckel, J. Chem. Phys. 92, 4530 (1990).
10. I. Harada. Y. Furukawa and F. Uecla. Synth. Met. 29, E303 (1989). 11. M. Ohria. T. Sakai. M. Takeuchi. Y. Kobavashi and M. Tsuji, Synth. Met.‘18, 347 (1987). * 12. R. P. McCall, J. M. Ginder, M. G. Roe, G. E. Asturias, E. M. Scherr, A. G. MacDiarmid and A. J. Epstein, Phys. Rev. I? 39, 10,174 (1989). 13. G. E. Asturias, A. G. MacDiarmid, R. P. McCall and A. J. Epstein, Synch. Met. 29, El57 (1989). 14. Y. Cao, P. Smith and A. J. Heeger, Synth. Met. 32,236
(1989). 15. Zhao Ping, H. Neugebauer and A. Neckel, Electrochim. Acta 41, 767 (1996). 16. Zhao Ping, H. Neugebauer and A. Neckel, Synth. Met. 69, 161 (1995). 17. Zhao Ping, H. Neugebauer and A. Ncckel, Electrochim. Acta 41, 767 (1996). 18. Zhao Ping, J. Chem. Sot., Faraday Trans. 92, 3063 (1996). 19. Zhao Ping, G. E. Nauer, H. Neugebauer and J. Theiner, J. Electroartal. Chem. in press. 20. Zhao Ping, G. E. Nauer, H. Neugebauer, J. Theiner and A. Neckel, J. Chem. Sot., Faraday Trans. 93 (1997). 21. J. T. Lopez Navarette and G. Zerbi, J. Chem. Phys. 94, 957, 965 (1991). 22. I. Harada and Y. Furukawa, Vib. Spectra. Struct. 19, 369 (1991). 23. S. Quillard, G. Louarn, J. P. Buisson, S. Lefrant, J. Masters and A. G. MacDiarmid, Springer Series in Solid-State Sciences 107. 271 (1992). 24. R. Kostic, D. Rakovic; I. E. Davidova and L. A. Gribov, Phys. Rev. B 45, 728 (1992). 25. S. Quillard, G. Louarn, J. P. Buisson, S. Lefrant, J. Masters and A. G. MacDiarmid, Synth. Met. 49-50, 525 (1992). 26. S. Quillard, G. Louarn, S. Lefrant and A. G. MacDiannid, Phys. Rev. R SO, 12,496 (1994). 27. A. J. Epstein, R. P. McCall, J. M. Grinder and A. G. MacDiarmid, in Spectroscopy of Advanced Materials (Edited by R. J. H. Clark and R. E. Hester), p. 355, John Wiley, New York (1991). 28. G. Socrates, Infrared Characteristic Group Frequencies, 2nd ed. J. Wiley, Chichester (1994).
mu
Pergamon PII: soo134fl6(96)0036l34
Acta, Vol. 42, No. II, pp. 16931700. 1997 0 1997 l%evier Science Ltd. All rights reserved. Printed in Great Britain 00134686/97 $17.00 + 0.00
In situ Fourier transform infrared attenuated
total reflection (FTIR-ATR) spectroscopic investigations on the base-acid transitions of leucoemeraldine Zhao Ping,* G. E. Nauer, H. Neugebauer, J. Theiner and A. Neckel Institute of Physical Chemistry, The University of Vienna, Waehringerstr.
42, A-1090, Vienna, Austria
(Received 4 June 1996; in revised form 4 September 1996) Abstract-Leucoemeraldine, the reduced form of the polyaniline, was obtained by electrochemical polymerization. Extensive infrared spectroscopic studies are discussed in this work. Ex situ FTIR spectra measured in NaReO,/HReO, electrolytes with different pH-values are presented and the absorption bands are assigned to different vibrational modes. Additionally, a distinct picture of the base-acid transition is evaluated using in sifu FTIR-ATR measurements in electrolytes containing the supporting salts mFb/HPFe, NaC104/HC104 and NaReOd/HReOd. The results demonstrate that in strong acidic media leucoemeraldine nitrogen atoms can be partly protonated. The protonation process starts in the pH-range 2.5 and 3 in the intercalation of anions from the electrolyte into the polymer structure is determined semi-quantitatively. 0 1997 Elsevier Science Ltd. All rights reserved. Key words: Polyaniline,
leucoemeraldine,
in situ
FTIR spectroscopy, conducting
INTRODUCTION
A large number of experimental works have been devoted in the last two decades to different forms of polyaniline. Depending on the average oxidation state, different compounds can be obtained [l]. Leucoemeraldine (LE), the fully reduced form of the polymer, is the simplest polymer since only phenyl rings in the benzenoid form are present. It was first synthesized in 1910 [2] and can be conveniently prepared as an analytically pure, white powder by the reduction of emeraldine [3]. The Raman spectral pattern of LE base form does not vary with the excitation wavelength and is interpreted as that of a p-disubstituted benzene [4, 51. Free-standing film can be obtained by electrochemical polymerization on an electrode surface or by casting from Nmethylpyrrolidone solution [6,7]. The surface of the particles in the powder or of the film are oxidized by air, and the film turns blue; however, “C and NMR studies show that the bulk of the blue material is still *Author to whom correspondence should be addressed. Current address: Department of Chemistry, The University of Georgia, Athens, GA-30602-2556, U.S.A.
polymer, electrochemistry.
in the LE reduced state [8]. In the LE base form, nitrogen atoms can be protonated and deprotonated depending on the pH of the medium. Potentiometric titration [9] of the LE salt form also demonstrated that in the region of pH = 0.5 the salt form is at least partly protonated. This feature of the base-acid transition is observed to be reversible. Many infrared spectroscopic studies [lO-141 have been made for the characterization of LE, however, experimental spectra differ from one another. We have made a series of research work on the PAN1 system [17-201. A number of vibrational spectroscopic investigations have been done on this system in order to determine structural changes and doping mechanisms during protonation and electrochemical redox processes. However, one problem arises from finding a suitable doping system because most of the anions used in the experiments have either no ir absorption bands (eg Cl-) or bands which overlap with polymer vibrational bands (eg ClO;). We have found that ReO; exhibits only one strong absorption band around 900 cm-’ in the infrared spectrum which is clearly separated from PANI vibrational bands. Although ClO; and PF; as doping anions can be used in the investigation of LE
1693
Zhao Ping et al. due to lower intensity changes of vibrational bands of the polymer in the base and acid forms, the NaReOd/HRe04 as a new doping system is more useful in the protonation doping investigations of the other forms of PANI [18] (emeraldine or pernigraniline) or in the electrochemical redox investigations of PANI [20]. In this paper, the attention is focused on the investigation of the base-acid transition process of LE to obtain information on the structure of the polymer via its vibrational properties and to measure in situ the change in the anion content of the polymer film during proton doping, which is beneficial eg for the improvement of PAN1 based pH-sensor.
EXPERIMENTAL The in situ FTIR-ATR (attenuated total reflection) experiments were performed on a fully computerized Nicolet 60 SX spectrometer fitted with a liquid nitrogen cooled MCT-detector. The electrochemical equipment consisted of a potentiostat 1001 TNC (Fa.Jaissle, Neustadt/Waiblingen, Germany) and a three electrode spectroelectrochemical cell. A ZnSe reflection element (45”) coated with a thin Au/Pt minigrid layer, deposited by evaporation under high vacuum conditions, a sheet of Pt and a saturated calomel electrode (see) were used as working, counter and reference electrodes, respectively. All potentials refer to this see. The potential of the working electrode was controlled by a computer system, which allows also the data collection (SCADA, J. Theiner, Univ. Vienna). The polymerization was carried out in the ATR-cell [ 151 containing 0.1 M aniline and 1 M HCl solution by cycling the potential between -0.2 V and +0.8 V at a scan rate of 10 mV . s-l. After that, the cell was washed with tri-distilled water and then filled up with electrolytes of different pH-values, using KPFe/HPFs, NaC104/HC104 and NaReOd/HReOd solutions. The potential of the working electrode was kept at -200 mV in order to obtain the LE form. When equilibrium was established, FTIR-spectra were recorded consecutively in the following way: 512 interferograms were coadded for each single beam spectrum (Sss) with a resolution of 4 cm-‘. After the Fourier transformation, the spectra are presented as difference spectra Sn, ratioed to a reference spectrum Ss, Su = S&S,. The spectra were corrected for the Hz0 and CO2 content in the optical path and are base line corrected. In this presentation, upward-pointing features (increasing absorption) correspond to the gain of absorbing species or increasing intensity of vibrational modes, and downward-pointing features (decreasing absorption) correspond to the loss of absorbing species or decreasing intensity of vibrational modes. Difference spectra, if not otherwise -.__-._ *Intensity: s, strong; m, medium; w, weak; v, very.
indicated, were obtained by taking the ir-spectrum of the LE base form as a reference state. For the ex situ experiments, the polymerization of the polymer was performed in the same way as for the in situ experiments. The polymer was converted into different levels of protonation by equilibration in acidic electrolytes of various pH-values and dried in vacuum. FTIR spectra of the polymers on the ATR-crystal were recorded in transmission geometry, the reference state was the ZnSe reflection element without a polymer. RESULTS
AND DISCUSSION
When the acidity of the medium changes, LE can be transformed from the base form to the salt form. In this work, thirteen NaReOd/HReOG electrolytes with different pH-values were used for establishing definite forms of LE. Figure l(a) shows the influence of the pH on ex situ FTIR spectra of LE, the spectra at pH = 1.5 (curve a) and pH = 8 (curve b) are presented as examples. Doping the LE base form by protonation (changing the pH-value of the electrolyte) results in a significant enhancement of the infrared bands of the polymer due to the effect of the positive charge on the polymer chain, inducing a dipole moment [21]. At strong acidic conditions (pH = 1.5), the LE salt spectrum exhibits absorptions at 1606(m*), 1502(s), 1300(m), 1250(w), 1178(m), 814(vw) and 798(vw) cm-‘. When the pH of the medium is higher than 3, some additional bands at 1626(w), 1377(vw), 1336(m), 1151(sh) and 847(vw) cm-’ are detectable in combination with a shift of the bands at 1606, 1502 and 1178 cm-’ to lower wavenumbers and of the band at 1300 cm-’ to higher wavenumbers. The strong absorption band at 914 (910) cm-’ is attributed to the v3 ReOi anion vibration. The band at 814 cm-’ (LE salt form) is characteristic of a p-substitution of the aromatic ring and is assigned to a C-H out-of-plane deformation vibration of the benzenoid groups. The CH out-of-plane bending vibration of the monosubstituted benzene ring is not observed indicating the absence of oligomers, for example, in the phenyl capped octamer this vibrational mode is of appreciable intensity [22]. The rings are mostly coupled at C4 and N positions (“head to tail” coupling), the corresponding C-N stretching modes are observed in the LE base form at 1250 and 1219 cm-’ (in the LE salt form at 1250cm-I) [23]. Therefore, the LE base form structure is mainly para-(NH--CsH4-)n. The intense band around 1500 cm-’ in the infrared spectrum of LE results from a C-C ring-stretching mode and the band at about 1600 cm-’ with medium intensity is likely to arise from a disorder-induced C< ring stretching mode. The vibrational modes in the range of 1178-1165cm-’ and 1163-1150cm-’ are assigned as either C-H bending or ring amine bending vibrations. Theoretical studies of Raman spectra [23-261 have demonstrated that these modes
1695
Base-acid transitions of leucoemeraldine
1500 Wavenumbers
1000 (cm-
1)
l-
(b)
35’00 Wavenumbers
(cm-
30’00 1)
2:
IO
Fig. 1.(a) Ex situ FTIR spectra of leucoemeraldine, equilibrated at - 200 mV in NaReOd/HReOd electrolytes with different pH-values. Curve (a) pH = 1.5; (b) pH =8. (b) Ex siru FTIR spectra of leucoemeraldine in the wavenumber region 4000-2500 cm-‘, equilibrated at -200 mV in NaReOa/HReOd electrolytes with different pH-values. Curve (a) pH = I .5; (b) pH=3; (c) pH=5.
Zhao Ping et al.
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are attributed to A, and BJ, Raman-active vibrations. When the symmetry of the LE chain is broken causing the Raman-active modes to become infrared active, these modes are thus observed also via infrared vibrations, connected with the change of the pH-value of the surrounding medium. The bands at about 1300 cm-’ have three contributions, coming from C-C ring deformation, C-H bending and C-N bending modes. The vibrational absorption at 1626 cm-‘, which is only detectable for the LE base form (pH = 8.5) should be assigned to a C=N vibrational mode. It seems that samples of undoped LE base form contain a few quinoid rings whose neighboring nitrogen atoms have no hydrogen atoms attached. This result is in agreement with an earlier report [27]. A remarkable difference between LE base and salt form is observed in the high frequency infrared region (Fig. l(b)). It is interesting to note that all the NH-groups (broad vibrational bands in the range of 3400 to 3350 cm-’ and 3300 to 3200 cm-‘, attributed to NH-stretching vibrations of -C6H4-NH-CbHagroups), are involved in H-bondings [28] (indicated by the disappearance of the absorption band at 3389 cm-’ and a strong shift to lower wavenumbers about 3240cm-‘). The difference in the intensities is caused by a difference in the H-bonding between the salt and the base form
-2OOmV
of LE. A large number of -NHgroups become lower H-bonded by the acid treatment, as indicated by a decreasing intensity of the absorbance at 3389 cm-’ for higher proton concentrations. An important implication of this is that each nitrogen in a polymer chain is located at a certain short distance from a nitrogen or a benzene ring in a neighboring polymer to form a H-bonded structure. Such close contacts of chains are considered to be important for charge transport in the conducting forms of the the absorption bands Moreover, polymer. ranging from 3000 to 2500 cm-’ are assigned to the vibrations associated with the NH: part in -C6H4NH:-C6H4groups. The intensity of vibrational bands at 2964 and 2877 increase with the decrease of the pH-value of the surrounding medium, which is also caused by the H-bonded structure change in the polymer chain. Because in ex situ FTIR spectroscopy only steadystate spectra are measurable and in order to get information on the dynamical changes of LE during the acid-base transition, in situ FTIR-ATR spectra were measured. This technique avoids the washing and drying processes and the handling of the polymer films in air. Establishing various experimental conditions, three different electrolytes (NaReOo/ HReO.+, NaC104/HClOd and KPFh/HPFb) were used. The results are presented in Fig. 2 and Fig. 3. As an
Leucoemeraldine
KPF6/HPFG
15’00 Wavenumbers
1 OS00 (cm-
1)
Fig. 2. Spectral changes measured in situ during the base-acid transition in KP&/HP& to 5.3 at -200 mV. The reference state is the ZnSe-crystal without a polymer film.
electrolyte in the pH-range 1.05
Base-acid transitions of leucoemeraldine
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.4
I
64
I 1500 Wavenumbers
-.1
I 1000 (cm-
1)
I
1500 Wavenumbers Fig. 3. (a and b).
1000 (cm-l)
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Zhao Ping et al. .ti
Acid
Form
J
2000
1000
1500 Wavenumbers
(cm-
1)
Fig. 3. Spectral changes during the base-acid transition process in different electrolytes measured by in situ FTIR-ATR spectroscopy. The reference state is the base form at -200mV. (a) KPFe/HPFa (in the pH-range l.OS5.3); (b) NaC104/HC104 (in the pH-range 065.3); (c) NaReO+‘HReOs (in the pH range 1.%X3).
example, the spectral changes in KPF6/HPFb electrolyte solutions are depicted in Fig. 2, using the spectrum of the bare ZnSe-crystal as the reference. Characteristic differences can be seen between the base and the acid form of LE, especially at wavenumbers 1604, 1500, 1178 and 845 cm-‘. For a better clarification of the spectral changes, difference spectra using the spectrum of the LE base form as the reference, are shown in Fig. 3. Note, that the absorbance scales in Fig. 3 are expanded and hence the sensitivity is higher in comparison to those displayed in Fig. 2. The patterns of the infrared spectra of the three systems in the wavenumber range below 2000 cm-’ are similar to one another, although there is a slight difference in the exact positions of the vibrational band maxima. An increase in the intensity was found for the absorption bands at 1604-1608, 1533-1539, 1317-1325, 1182-1186, 1128-1134, 930, 881, 798-800 and 768-769 cm-’ while a decrease in the intensity was observed for the absorption bands at 1500, 1280 and 1160 cm-‘. The absorption bands of the anions PF;, ReO;, ClO; are situated at 845, 904 and 1090 cm-‘, respectively. The infrared absorption data can be interpreted in terms of normal modes of para-disubstituted benzene. Several theoretical studies have shown that the ring-rotational conformation is important in
determining the ground-state electronic structure [22,24]. The LE base chain is built from unit cells containing one NH (bearing two electrons assumed to be perpendicular to the C-N-C plane) and one GHvring. In [25] an idealized polymer backbone structure is presented. In this model, spectral changes of LE during acid-base transition are considered. It is expected that ring-angle distortion and different
4
-
7.
-
0
I
2
3
4
s
‘
0’1
Fig. 4. Integrated intensities of the anion vibrational bands as a function of pH. The integration range is indicated in the brackets. (A) PF; (800-9OOcm-I); (+) CIO; (95&l 150 cm-‘); (m) ReO; (850-950 cm-‘).
Base-acid transitions
of leucoemeraldine
1699
pH > 4
PH
<1
A-
AA-:
ReOd’,
PFg’ , ClO4’
Fig. 5. Doping mechanism during acid-base transition processes of LE.
bond-lengths are responsible for the changes observed in the infrared spectrum of LE during protonation. Therefore, the decrease in the absorptions at 1500 and 1280 cm-‘, assigned to ring stretching and C-N bending vibrational modes, is caused by symmetry breaking and changing of the bond-angle. This effect cannot be seen ex situ because the handling of the film during the preparation procedures efface these small changes. Additionally, doping can cause an increase in the intensity of some vibrational bands. During the base-acid transition, the amount of anions in the polymer compensating the positive charges in the polymer chain changes. This demonstrates that the amount of protonated centers is increased on decreasing the pH-value of the electrolyte. Figure 4 shows, as a function of acidity, the anions content determined by integrating the anions vibrational absorption bands in an appropriate wavenumber range. The base-acid transition takes place at about pH = 3. In this pH-region the anion content shows an increase, verifying that the LE base form changes into the salt form. In principle, a similar picture is observable for all three systems used. In the non-oxidizing KPF6/HPF6 electrolyte,
the base-acid transition takes place in a very narrow pH-range (2.7-2.9) and the vibrational bands of the polymer indicate that the structure of the polymer is stable at pH-values higher than 3. It seems that the intercalation of anions must be associated with the oxidation of the polymer chain and with the base-acid transition process of the polymer based on the pH-value of the surrounding medium. According to in situ measurements of the anion vibrational intensities during base-acid transitions and structural changes of the polymer, the doping reactions are schematically presented in Fig. 5. At pH-values higher than 4, the LE base form is stable, converting in the pH-range l-3 to the partially protonated LE salt form. At pH-values lower than 1, the fully protonated LE salt form (50%) should be obtained. CONCLUSIONS The capability of in situ FTIR-ATR spectroscopy for the characterization of the base-acid transition of leucoemeraldine is demonstrated in this work. Depending on the pH-value of the electrolyte, different states of LE were found at a constant applied potential. At pH-values > 4, only the LE base
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Zhao Piing et al.
form is stable. The protonation process begins in the pH-range between 2.5 and 3, and in strong acidic media (pH-values < I), the LE nitrogen atoms are at least partly protonated. The vibrational features are indicative of H-bonded neighboring polymer chains, and of symmetry breaking and changing of bond-angles during protonation.
ACKNOWLEDGEMENTS One of the authors (Z.P.) acknowledges the financial support provided by the t)AAD (Office of Austrian Academic Exchange) and the Institute of Physical Chemistry (IPC), University of Vienna.
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10. I. Harada. Y. Furukawa and F. Uecla. Synth. Met. 29, E303 (1989). 11. M. Ohria. T. Sakai. M. Takeuchi. Y. Kobavashi and M. Tsuji, Synth. Met.‘18, 347 (1987). * 12. R. P. McCall, J. M. Ginder, M. G. Roe, G. E. Asturias, E. M. Scherr, A. G. MacDiarmid and A. J. Epstein, Phys. Rev. I? 39, 10,174 (1989). 13. G. E. Asturias, A. G. MacDiarmid, R. P. McCall and A. J. Epstein, Synch. Met. 29, El57 (1989). 14. Y. Cao, P. Smith and A. J. Heeger, Synth. Met. 32,236
(1989). 15. Zhao Ping, H. Neugebauer and A. Neckel, Electrochim. Acta 41, 767 (1996). 16. Zhao Ping, H. Neugebauer and A. Neckel, Synth. Met. 69, 161 (1995). 17. Zhao Ping, H. Neugebauer and A. Ncckel, Electrochim. Acta 41, 767 (1996). 18. Zhao Ping, J. Chem. Sot., Faraday Trans. 92, 3063 (1996). 19. Zhao Ping, G. E. Nauer, H. Neugebauer and J. Theiner, J. Electroartal. Chem. in press. 20. Zhao Ping, G. E. Nauer, H. Neugebauer, J. Theiner and A. Neckel, J. Chem. Sot., Faraday Trans. 93 (1997). 21. J. T. Lopez Navarette and G. Zerbi, J. Chem. Phys. 94, 957, 965 (1991). 22. I. Harada and Y. Furukawa, Vib. Spectra. Struct. 19, 369 (1991). 23. S. Quillard, G. Louarn, J. P. Buisson, S. Lefrant, J. Masters and A. G. MacDiarmid, Springer Series in Solid-State Sciences 107. 271 (1992). 24. R. Kostic, D. Rakovic; I. E. Davidova and L. A. Gribov, Phys. Rev. B 45, 728 (1992). 25. S. Quillard, G. Louarn, J. P. Buisson, S. Lefrant, J. Masters and A. G. MacDiarmid, Synth. Met. 49-50, 525 (1992). 26. S. Quillard, G. Louarn, S. Lefrant and A. G. MacDiannid, Phys. Rev. R SO, 12,496 (1994). 27. A. J. Epstein, R. P. McCall, J. M. Grinder and A. G. MacDiarmid, in Spectroscopy of Advanced Materials (Edited by R. J. H. Clark and R. E. Hester), p. 355, John Wiley, New York (1991). 28. G. Socrates, Infrared Characteristic Group Frequencies, 2nd ed. J. Wiley, Chichester (1994).