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The interaction between ammonia and poly(pyrrole)
Synthetic Metals, 31 (1989) 163 - 179
163
THE INTERACTION BETWEEN AMMONIA AND POLY(PYRROLE) G. GUSTAFSSON, I. LUNDSTR(~M, B. LIEDBERG, C. R. WU and O. INGAN.~,S
Department of Physics and Measurement Technology, University of Link6ping, S-581 83 LinkSping (Sweden) O. WENNERSTROM
Department of Organic Chemistry, Chalmers University of Technology, S-412 96 GiSteborg (Sweden) (Received October 18, 1988; in revised form January 27, 1989; accepted February 1, 1989)
Abstract Reversible and irreversible interactions between ammonia and poly(pyrrole) have been studied using electrical conductivity measurements, optical spectroscopy, Fourier transform infrared spectroscopy (FT i.r.), X-ray photoelectron spectroscopy (XPS) and elemental analysis. The results show that in addition to the previously reported reversible change of the electronic properties of poly(pyrrole) upon treatment with ammonia, there is also a large irreversible change if the material is exposed to high concentrations of ammonia or ammonia plus water vapour for a long period of time. The measurements also indicate that amide-type carbonyl groups, amine groups and ammonium ions could be present in the irreversibly changed polymer. The results are used as a basis for a discussion on the interaction between conducting polymers and ammonia in general and between poly(pyrrole) and ammonia in particular.
Introduction Properties like the electrical conductivity and optical absorption are easily altered in conducting polymers by chemical or electrochemical methods. For example, electrical conductivity is controlled by doping with different electron
© Elsevier Sequoia/Printed in The Netherlands
164 work function changes, which can be detected by the device. Chague e t al. have shown that poly(p-phenylene azomethine) can be used as a sensor material for iodine [6]. We have already demonstrated how doped PPy can be used to detect ammonia gas [7]. Yoshino e t al. reported that undoped poly(p-phenylene) (PPP) and poly(p-phenylenevinylene) also showed reversible changes upon ammonia exposure [ 8 ] , but in this case the conductivity increased in contrast to PPy, where the conductivity decreases reversibly upon ammonia exposure. The conductivity of conducting polymers like doped poly(p-phenylene) and doped poly(acetylene) (PA) are, however, reported to decrease irreversibly by several orders of magnitude when they are exposed to ammonia [9, 10]. So far, the mechanisms of the interaction between conducting polymers and ammonia are considered to be simple doping or compensation effects. This is probably the case in some polymers. We think, however, that the general situation is more complex and that additional chemical processes occur. We have, for example, found that in PPy, besides the previously reported reversible decrease of the conductivity, there is also a large irreversible decrease if the material is exposed to ammonia or to ammonia plus water vapour for a long period of time [11]. It is important to gain an understanding of the interaction mechanisms when developing long-term stable and reversible chemical sensors. Therefore we have performed a more rigorous study of the interaction between PPy and ammonia using techniques like u.v./vis/n.i.r., FT i.r. spectroscopies and XPS together with conductivity measurements. The result of this study is used as a basis for a discussion of the interaction between conducting polymers and ammonia in general and between ammonia and PPy in particular.
Experimental PPy films were prepared by electropolymerization of 0.1 M pyrrole using 0.1 M Et4NC1Oa or 0.1 M Et4NCH3C6H4SO3 in CH3CN. The electrolyte was purged with Ar for about 30 minutes before addition of the pyrrole monomer. Evaporated Au or indium-tin-oxide (ITO) on glass was used as a working electrode. The counter electrode was a Pt coil or Au evaporated on glass (when making material for elemental analysis). All films were grown galvanostatically with 0.5-0.7 mA/cm 2. The film thickness was in the range 0 . 1 - 1 pm, as determined by surface profile measurements (Dektak 3030). Films used for elemental analysis were, however, thicker. Films for optical spectroscopy and XPS were used still attached to the ITO/glass electrode. Films for FT i.r. spectroscopy and electrical conductivity measurements were carefully peeled off the ITO/glass substrate in CH3CN and mounted in appropriate sample holders. Samples for elemental analysis were prepared by scraping off thick films of PPy from large-area Au electrodes.
165
All samples were exposed to laboratory atmosphere when they were transferred from the preparation to the measurement equipment. Optical spectroscopy was done mainly in air. Optical spectroscopy during ammonia exposure was performed by using an optical glass cell sealed with a Teflon lid and epoxy. The gases were supplied by Teflon tubes through the lid. When films were exposed to ammonia for a long period of time, they were put in a sealed container and ammonia gas (100%) was flushed through the container twice a day. Ammonia plus water {wet ammonia) exposures were done b y leading the ammonia gas through a saturated a m m o n i a - w a t e r solution before flushing it through the container. Pulses of low concentrations of ammonia during conductivity measurements were generated by allowing a stream of Ar gas, into which ammonia was injected, to pass over the film. The amount of ammonia gas was regulated b y a magnetic valve and a high-precision flow regulator. Exposure of the PPy film to low concentrations of ammonia plus water {wet ammonia) was done b y leading the carrier gas through water before injection of ammonia gas. Conductivity was measured using a four-probe technique. Ammonia exposure was done with the films still mounted in the probe. Optical spectra were recorded using a Perkin-Elmer Lambda 9 u.v./vis/n.i.r, dual-beam spectrophotometer. Infrared spectroscopy was carried out, at 2 cm -1 resolution, using a Bruker IFS l 1 3 v Fourier-transform spectrometer. XPS was done with a UHV instrument of local design and construction, at a pressure of 10 -1° Tort. The spectra were taken with unmonochromatized Mg K s radiation and an analyser resolution of 0.2 eV. Elemental analysis was performed b y Analytical Laboratories, Engelskirchen, F.R.G.
Results
Conductivity measurements The resistance of a film of PPy increases upon exposure to ammonia. As can be seen in Fig. 1, relatively small ammonia concentrations give rise to marked resistance changes within minutes. If the film is exposed to the same concentration of ammonia but with the carrier gas saturated with water vapour {wet ammonia), the change becomes even larger. The resistivity changes upon exposure to these relatively small ammonia concentrations are almost completely reversible. If higher concentrations of ammonia are used, and especially high concentrations of w e t ammonia, the resistivity changes become larger and partly irreversible. The resistivity of a PPy film increases b y a factor of 30 upon exposure to 1 atm of dry ammonia for 10 minutes. When ammonia is removed, the resistance returns to within a factor of two of its initial value. Further ammonia exposures also cause irreversible changes. The irreversible changes are significant if the film is exposed to high concentrations of dry or wet ammonia for a long period of time, as shown by Table 1. As expected, the reversible part of the resistivity change decreases with increasing total exposure time
166
DRY
WET
108
27 108
Ro=
/'~:"~
410G
"~
~
Ro= 550~
30rnin
Fig. 1. Resistance change of a film of poly(pyrrole) during 30 minute exposures to different concentrations of dry and wet ammonia in Ar. The labels indicate the concentration (in ppm) of NH3. Ar was flushed over the film between the ammonia pulses. TABLE 1 Resistivity changes of films of poly(pyrrole)(C104) exposed to 1 atm of dry and wet ammonia for various times. R0 is the resistance of the 'as prepared' film Treatment
R /R o
Dry NH3, 5 days Wet NH3, 4 days Wet NH3, 7 days
103 104 l0 s
under these e x t r e m e conditions. T he resistivity change measured after seven days o f w et a m m oni a exposure is c o m p l e t e l y irreversible. It was n o t even possible to increase the conductivity in such a film by electrochemical oxidation or b y immersing it into aqueous HC1 for 30 minutes. Optical spectra
The resistivity changes described above are accompanied b y changes in the optical spectra. Curve (a) in Fig. 2 shows the optical spectrum of a thin film o f d o p e d PPy. The spectrum contains two peaks, one at 1.0 eV and another, o f lower intensity, at 2.7 eV. During exposure to 1 atm of dry a m mo n ia (curve (b)), the low-energy peak decreases and shifts approximately 0.1 eV to higher energy. The high-energy peak increases and also shifts a b o u t 0.5 eV t o higher energy; this latter change can also be interpreted as a gradual shift in intensity between two peaks at 2.7 and 3.2 eV. When a m m o n i a is removed and Ar has been flowing through the cell f o r 50 minutes, a spectrum shown as curve (c) is obtained. The peaks shift back to lower energy, the intensity of the low-energy peak is increased and the intensity o f the high energy peak is lowered com pared to (b), b u t
167
a
f~
/ f-~,
°' ,' ,"
~t~ ~>,~
/
-,c---," / ,' --2"'--
<( 1.( O m ~
/ ~ J
/ J
/ / /
b¢
/ /
O.E
0.5
/ / / / c/
-
1'.o
~,'.o
:~.o
PHOTON ENERGY (eV}
4.0
1'.o
2'.o
3'.o
1o
PHOTON ENERGY (eV)
Fig. 2. Change of the optical spectrum of a thin film of doped poly(pyrrole) during a short pulse of ammonia exposure. Curve (a) shows the spectrum of the 'as prepared' film. Curve (b) shows the spectrum o f the film during exposure to 1 atm o f dry ammonia (after 10 minutes), and (c) the spectrum when ammonia is taken away and Ar has been flowing through the cell for 50 minutes. Fig. 3. Change of the optical spectrum o f doped poly(pyrrole) after long-term ammonia exposure. Curve (a) shows the spectrum of the 'as prepared' film and curve (b) the spectrum o f the film after five days' exposure to 1 atm o f dry ammonia. Curve (c) shows the spectrum of another film that was exposed to 1 atm o f wet ammonia for 14 days. Spectra were recorded with the films exposed to laboratory atmosphere.
the initial spectrum is not recovered. Thus, the optical changes obtained after an initial exposure to ammonia are not reversible. The change in the optical spectrum after exposure to 1 atm of dry and wet ammonia for a long time can be seen in Fig. 3. The spectrum of the film after exposure to dry ammonia for five days still resembles the initial spectrum to some extent. The two original peaks are still observed, even though their intensities have decreased and their positions shifted towards higher energy. The ultraviolet absorption has also become very intense. When a film has been exposed to wet ammonia for 14 days, the optical spectrum has changed completely, as can be seen in curve (c). This spectrum shows very little structure and is just a continuous increase of the absorption with the energy. However, there are two very weak shoulders at 0.7 eV and 1.4 eV. Treatment of the films in HC1 or attempts to oxidize the films electrochemically did not change the spectra of the long-term ammonia-exposed films appreciably.
Infrared spectra The FT i.r. spectrum of a PPy film during ammonia exposure is not very useful, since the vibrational spectrum of the ammonia molecules obscures all the interesting features of the PPy spectrum. This means that
168
the FT i.r. technique cannot be used for investigating the reversible mechanism. The irreversible mechanism, however, can be studied. To test the generality of the observations, the measurements were performed with two different anions, perchlorate and toluene sulfonate. The spectra of PPy films with perchlorate and toluene sulfonate anions, exposed to different ammonia atmospheres for varying times, are shown in Figs. 4 and 5 respectively. A c o m m o n feature of these spectra was an overall decrease in absorption upon ammonia exposure. This was also observed in the optical spectra (decrease of the low-energy peak). Changes induced b y the treatments were qualitatively the same in dry and wet ammonia and all changes that were observed in the dry case were also observed in the wet case, b u t were enhanced in the latter. After exposure to 1 atm of w e t ammonia for 14 days, a new, broad absorption band develops at 2 7 0 0 - 3 5 0 0 cm -1. The shape of this band is not the same in the two different materials. In the spectrum of PPy cio;,
a
a
c.3-@-so~
i !AA,,'v,,
b
5 d
d (J Z < m
O Z < m er
n-
o
O
(n m ,<
\.
\ \
a
4000
,
I
t
3000
i
2000
WAVENUMBERS, cm -1
=
1000
I
4000
3000
2 0 0=0
1 0 J0 0
WAVENUMBERS, cm -I
Fig. 4. F T i.r. s p e c t r a o f t h i n , f r e e s t a n d i n g films of p o l y ( p y r r o l e ) ( C 1 0 4 ) t r e a t e d in diff e r e n t a m m o n i a a n d w a t e r e n v i r o n m e n t s for varying times. Curve (a) is t h e s p e c t r u m o f t h e 'as p r e p a r e d ' film, (b) t h e s p e c t r u m a f t e r five days in 1 a t m o f d r y a m m o n i a a n d (c) t h e s p e c t r u m o f a film e x p o s e d t o 1 a t m o f w e t a m m o n i a for 14 days. Curve (d) shows t h e same film as in (c) b u t a f t e r rinsing in w a t e r for 12 hours. The asterisks indicate a b s o r p t i o n b a n d s t h a t are caused b y t h e a n i o n . Fig. 5. F T i.r. s p e c t r a o f t h i n , f r e e s t a n d i n g films o f p o l y ( p y r r o l e ) ( C H a C 6 H a S O a ) t r e a t e d in the same way as t h e films in Fig. 4.
169
toluene sulfonate the band is asymmetric and shows a peak at 3178 cm -1, while it is symmetric and shows almost no structure in the spectrum of PPy perchlorate. A shoulder occurs at 1650 cm -1 in all samples treated with dry or wet ammonia. It is, however, most pronounced in the spectra of films exposed to wet ammonia for 14 days. The occurrence of new absorption bands between 1580 and 1600 cm -1 is also evident in those spectra. The shoulder at 1082 cm -1 and the sharp peak at 617 cm -~ in the spectrum of 'as prepared' PPy perchlomte and the absorptions at 562, 614, 679, 811, 1010, 1035 and 1122 cm -1 in the spectrum of 'as prepared' PPy toluene sulfonate may in part be assigned to the anion. In the spectra of the most exposed samples these peaks appear more clearly. After rinsing such films in water for 12 hours these peaks disappear, as can be seen in spectra (d) in Figs. 4 and 5; in addition the shape of the band at 30003500 cm -1 changes. The spectra of the two films {prepared with different anions) are very similar after ammonia treatment and rinsing in water. Treatment of the films with aqueous HC1 (37%) after the long-term ammonia exposure did not change the spectra of the films significantly. XPS
Figure 6 shows the change of the N(ls) and the C(ls) peaks of PPy perchlorate upon exposure to wet ammonia for 14 days. The N(ls) peak in the spectrum of the 'as prepared' sample is asymmetric and shows an increased intensity on the high binding-energy side compared to the lowenergy side. The lineshape for the ammonia-treated sample is completely different. The spectrum shows an increased intensity on the low-energy side of the main peak, which is located at 399.8 eV, and a decreased intensity on the high,energy side compared to the original spectrum.
N (ls)
C Ils} ,'.,
i,/
...'.
.-
•
BefO¢lt exp. """:"
'
.. "'"'....."
.
.
.
.
.
"-'"'"""'"
..'..
iV Wet ammonia
. ........
....
.'"
"'.•....,
....
-"""
....
BINDING ENERGY (eV)
Fig. 6. N(ls) and C(ls) photoelectron spectra of an 'as prepared' poly(pyrrole)(C104) film, (i) and a film exposed to 1 atm of wet ammonia for 14 days, (ii).
170 The C(ls) peak in the spectrum of the 'as prepared' sample is asymmetric and shows an increased intensity on the low binding-energ~¢ side compared to the high-energy side. In the spectrum of the sample exposed to wet ammonia for 14 days, a shoulder appears around 288 eV. The peak is also more symmetric compared to the peak in the original spectrum.
Elemental analysis The elemental compositions of an 'as prepared' film, a film exposed to wet ammonia for 12 days and a film exposed to wet ammonia for 12 days and then rinsed in water for 24 hours are listed in Table 2. All films showed good material balance and the drying loss was 2 - 6 % when dried at 50 °C and 0.1 mBar. The 'as prepared' film shows a good agreement with the expected elemental composition. If we assume that the chlorine exists in the form of perchlorate ions, then there is approximately one anion per three pyrrole rings. There is no oxygen excess in the 'as prepared' film. The sample exposed to wet ammonia has a large nitrogen and hydrogen excess compared to the 'as prepared' film and the oxygen c o n t e n t has also increased in this sample. With the assumption that chlorine exists in the form of perchlorate ions, the excess of oxygen is 0.71 per pyrrole ring. The excess of hydrogen and nitrogen in the ammonia-exposed film decreases considerably when the film is rinsed in water. Also the oxygen and chlorine contents decrease. The chlorine c o n t e n t is almost zero after rinsing and the decrease of oxygen is nearly four times the decrease of chlorine. The resulting film, after ammonia exposure and water treatment, does, however, still show a large excess of both oxygen and nitrogen. There are approximately 0.8 extra nitrogens and 0.8 extra oxygens per pyrrole ring. TABLE 2 Elemental analysis of 'as prepared', ammonia-treated (1 atm) and water-rinsed poly(pyrrole)(C104) Sample
C
H
N
O
C1
As prepared Wet ammonia, 12 days Wet ammonia, 12 days followed by water rinsing, 24 h
4.00 a 4.00 a 4.00 a
3.12 4.56 3.33
1.01 2.24 1.81
1.41 1.79 0.91
0.35 0.27 0.02
aThe carbon content was assumed to be four carbons per pyrrole monomer. Discussion
Reversible effects The results of the conductivity measurements presented above show that ammonia, at low concentrations and short exposure times, mainly interacts reversibly with PPy.
171 The reversible changes in the conductivity are accompanied by changes in the electronic structure, as observed in the optical spectra. The peaks at 1.0 eV and 2.7 eV in the optical spectrum of an 'as prepared' PPy film have been assigned to transitions from the valence band to two bipolaron bands in the band gap [12]. The bipolaron bands are induced by the dopant. The oscillator strength of the u-~* transition is very low in doped PPy and the absorption band of this transition is believed to be a part of the high-energy tail of the 2.7 eV bipolaron peak [12, 13]. The change of the peak at 2.7 eV upon ammonia exposure could thus either be interpreted as an increase of the absorption and a 0.5 eV shift towards higher energy of this peak, or as mainly a gradual change of the relative intensities of a peak, positioned at around 3.2 eV (Tr-Tr* transition) and the bipolaron peak at 2.7 eV. The former interpretation is not very probable, since the low-energy bipolaron peak is shifted by only 0.1 eV and has decreased upon ammonia exposure. The changes of the optical properties could thus be interpreted as a lowering of the bipolaron peaks and an increase of the ~-u* peak. This indicates that the polymer becomes less doped during ammonia exposure. The a m m o n i a - p o l y m e r interaction in p-doped conducting polymers is generally considered to be a compensation effect. Thus, ammonia molecules, which are electron donating, act as n-type dopants that could decrease the doping level of the polymer chain by compensating the effect of the original dopant. This nucleophilic interaction may be written as: Polymer+A - + NH3 -
~ Polymer ° .... NH3 +, A -
(1)
It should be noted that, theoretically, it is possible to compensate a material even though it is not n-dopable since different energy levels are involved in the two processes. The ease with which such a compensation occurs, and consequently the reversibility of the reaction, should be related to the ionization potentials of the polymer and of ammonia [8]. It should, however, be noted that the energy levels could change considerably due to the interaction, so a direct comparison of the gas-phase ionization potentials of ammonia and the polymer does not give the relevant information. If we assume, however, that the changes of the energy levels due to the p o l y m e r - a m m o n i a interaction are approximately the same for the different polymers, we could get an indication of the reversibility of the reactions by studying the order of the ionization potentials of the polymers. As an example, we could compare the ionization potentials (as reflected in the redox potentials) of PPy, PANi, PA, PPP and poly(thiophene) (PT) and see how the order is related to their interaction with ammonia. PPy and PANi have the lowest redox potentials (--0.2 V and 0.2 V v e r s u s SSCE respectively) [14]. These polymers interact reversibly with ammonia. The other polymers PT (0.7 V), PA (0.7 V) and PPP (1.6 V) [14] all have higher redox potentials and the interaction is reported to be irreversible in these polymers [ 8 - 101.
172
An alternative mechanism for the compensation reaction, which involves proton transfer between the polymer and ammonia, could be written as
Polymer+A - + NH3 ~
Polymer(--H) ° + NH4+A-
(2)
Ammonia is a rather strong base and should attack protons that are acidic. This deprotonation should leave a negative charge on the chain, which could combine with the hole and u n d o p e the polymer. This reaction, which could be reversible, can only occur in polymers with protons that are acidic, like PPy and PANi, and thus could n o t explain the interaction between PPP, PA or PT and ammonia. In PPy and PANi the most acidic protons are those bonded to the nitrogen. These could easily be removed b y ammonia, forming ammonium ions that electrostatically bind to the negative anion. The reaction for PPy is: NH3 t
A®
H H ~ ..~N/ ~~N / L N® ~ ~ /IN,,~." A® H
H
H
"
N\ ~.../N //--~ , -.~ / ~ N / ~ ~ IN,J,L- +NH~A® (3) A® H
H
Deprotonation in PPy has been studied earlier [1, 2]. It was shown that in aqueous bases the conductivity decreases irreversibly b y three to four orders of magnitude. The conductivity could be restored b y treatment in acid. In the case of PPy exposed to ammonia gas, the reversible change in conductivity was between one and t w o orders of magnitude. The difference could be explained b y the fact that the former experiment was performed in solution where a m m o n i u m - a n i o n complexes can be transported o u t of the polymer, while in the latter case these inorganic species remain in the polymer. The latter reaction can therefore be considered as an equilibrium reaction, whereas the first one goes to completion. To determine which of the two types of compensation reactions causes the reversible interaction b y PPy is difficult, since most of the analytical tools available to us cannot be used while the sample is still exposed to ammonia. One indication could, however, be given b y studying the relation between the change of resistance and the partial pressure of ammonia. Earlier work done by us has shown that the response (AR/Ro) appears to follow approximately (PNrI~)i/2 at small ammonia concentrations, where PNH3is the partial pressure of NH3 in the Ar carrier gas [15]. An acid-base reaction with a subsequent formation of a mobile NH4+A - complex leads to a bimolecular reaction. The assumption that the change in conductivity is linearly related to the number of imine nitrogens n o w gives an expression that fits rather well with a square r o o t dependence [15]. However, the above argument is very weak, since there is not a simple relationship between the conductivity and the number of charge carriers in the polymer chains.
173 We have also tried to distinguish between the t w o models b y exposing a film of PPy to triethylamine, a non-nucleophilic base with almost the same PKa value as ammonia. The exposure resulted in a very slow increase of the resistance. This indicates that proton transfer is a possible mechanism of the a m m o n i a - P P y interaction. The very slow response of the polymer to the triethylamine exposure could be due to a very slow diffusion of the gas into and out of the polymer, since triethylamine is a bulky molecule.
Irreversible effects A reversible compensation reaction based on proton transfer should to a certain extent 'protect' the polymer from further nucleophilic attacks, since the chains are less positively charged in the compensated polymer. However, since the reversible reaction is based on an equilibrium reaction, there are always segments that axe n o t compensated and where irreversible nucleophilic attacks could occur. This should lead to a slow degradation of the polymer. Furthermore, it is likely that the a m m o n i u m - a n i o n complex formed in the reversible reaction diffuses freely in the polymer matrix. In a short time perspective such a diffusion probably causes no changes in the reversibility of the reaction. In a longer time perspective the diffusion could, however, result in salt crystal formation at the surface of the polymer, which would change the reversibility appreciably. In our long-term exposure experiments, we have observed salt crystals at the surface of the material, b u t so far have not been able to analyse them. Diffusion of the a m m o n i u m - a n i o n complex and subsequent crystal formation should result in a polymer with the nitrogens irreversibly deprotonated. These imine nitrogens should be observed b y XPS and FT i.r. spectroscopy. In an FT i.r. spectrum of base-treated PPy, a shoulder appears at 1605 cm -1 . This has been assigned to imine-type nitrogen [16]. Thus, the peak appearing at 1600 cm -1 in the long-term ammonia-exposed films could in part come from irreversibly deprotonated nitrogen. In XPS spectra imine-type nitrogens axe reported to give rise to a pronounced shoulder on the low binding-energy side of the N(ls} peak [ 1 , 2 , 1 6 ] . Long-term ammonia-exposed samples show no such sharp shoulder, b u t there is an overall increased intensity at the lowenergy side of the main peak. Part of this increase could be due to imine nitrogens. The changes in the FT i.r. spectrum and the excess of nitrogen and oxygen in the elemental analysis show that the polymer undergoes chemical changes u p o n long-term ammonia exposure. The broad band at 2700 - 3500 cm -1 in the FT i.r. spectrum, after ammonia exposure, is a mixture of different absorptions. N--H stretch modes are usually found in that region, which suggests that structures containing amines (primary and secondary) and amides could be present in the polymer. It is reported that a structure like [A] gives a doublet in the region 3360 - 3460 cm -1 [17].
174 CH= CH(OEt) 2
I
NC
The N--H stretch of PPy in the neutral form is observed at 34O0 cm -1 [18]. The absorption band at 3200 cm -1 , which is clearly resolved in the spectrum of PPy toluene sulfonate after ammonia treatment, is probably caused by ammonium ions. O--H bonds also give rise to absorptions in the region 2 7 0 0 - 3 5 0 0 cm -1, and we cannot exclude the possibility that hydroxyl groups are incorporated in the polymer exposed to ammonia. However, hydroxypyrroles exist predominantly in the oxo form [19, 20] and it is probable that the hydroxyl groups are converted to carbonyl groups. At 1600 cm -1 a new peak appears in the FT i.r. spectrum of PPy exposed to ammonia. This could be assigned to primary amines, since such groups usually show strong absorptions in this region. However, there is also a possibility that this peak, or a part of it, could come from iminetype nitrogen, as mentioned above. The peak at 1650 cm -~, observed in the spectra of long-term ammoniaexposed samples, is most probably caused by a carbonyl group. Carbonyl groups in poly(N-methylpyrrole) have been reported to give an absorption peak around 1700 cm -1 [21]. The difference in peak position in the two materials should be due to hydrogen bonding in the former polymer [22]. The XPS results support the assignments of the new peaks in the FT i.r. spectrum. The N(ls) core level spectrum indicates that amine groups have been introduced into the chain upon ammonia treatment, since the intensity in the region around 398.5 eV, the binding energy where primary amines can be found [23], has increased. This assignment is, however, somewhat uncertain since there is almost no structure of the lineshape in this region and since other groups, like imines, are expected to give rise to a peak in the binding~nergy range in question. The formation of carbonyl groups on exposure of PPy to ammonia is also supported by XPS spectra where the shoulder at 288.0 eV in the C(ls) spectrum can be assigned to carbonyl carbons. A similar assignment has been done in aged PA by Munstedt [24]. There are two species that are likely to attack the polymer chain during ammonia exposure. The first is ammonia itself, which is a good nucleophile, and the other is the hydroxide ion, which is an even better one. Hydroxide ions are generated by the following reaction: NH3 + H20 ~
NH4 + + OH-
(4)
In experiments with dry ammonia exposure, the water needed for the reaction must be supplied by the polymer matrix itself. PPy films are reported to contain a large amount of water, which is bound to the polymer and cannot be desorbed by mild heating [25]. Treatment of PPy films with wet ammonia will of course increase the hydroxide ion concentration.
175
Due to the disordered nature of the polymer, with sites of different sensitivities to nucleophilic attack, various reactions are possible. The bipolarons are the most electrophilic sites due to their positive charge and are thus the prime target for nucleophilic attack. In the schemes below we have selected a few examples of reactions that seem reasonable and are consistent with the experimental results.
(a) Hydroxide ion attack at the (J-carbon
I NH3
ibH®
H
N
H A®
(,
~
.N®
/ ~ N ~ N
''~
A® H
+ NH® 4
H OH
H
H
+ 14
NH A ®
14
l H OHH
Tautomerism
H
+ NHOA®
l
4
A® H
H
HO~.~
H
+ 2 NH®A® 4
H
H
Similar mechanisms have been used to explain the degradation of PPy during over-oxidation in aqueous electrolytes [26], and during base treatment followed by electrochemical oxidation [27].
(b) Attack at the s-carbon followed by ring opening IOH® H ... N ~
H
+NH?
AQ H
H
I H
AO
Ao H
H
"~/O~H N H ~ N
N ~'"
" N~A G
~"~O
~
H
Ao
N° N ~
N~" H
+ NH?A®
176
This mechanism has been proposed b y Otero et al. as an explanation for the degradation of PPy at anodic potentials in aqueous electrolytes [28]. The carbonyl groups introduced b y both these reactions are of the amide t y p e (vinylic amides). (c) and (d) Attach o f ammonia on the chain in a similar way to that o f the hydroxide ion NH3 H -~. N
AG
® H~ A N'~ ". H 1
®
H NH 3
NH 3 H -/
AQ
H
j NH3 A® N
A®
H
H NH2 N H
H
~
1
A® H
N H ~
NH3 H H
H
NH 3
1
IAO H
N A®
A® .
H
N
_
+ NH?A O H
N
.
/:~ NH2 H
N
~
NH4QA® H
The initial step of reactions (c) and (d) with ammonia could eventually be reversible and thus constitute the reversible nucleophilic interaction (eqn. (1)) in the case of PPy. Eventually all the initial steps of all the reactions (a)- (d) could be reversible, with different time constants. As in the deprotonation reaction, these reactions (a) - (d) produce NH4÷A - complexes, which could diffuse and form ammonium salt crystals. This would be a truly irreversible step that would degrade the polymer. The result of all the reactions described above would be that charges are taken from the polymer chain and that new groups are introduced into the chain. Some of those groups give rise to sp 3 defects, which effectively break the conjugation. The resulting polymer would therefore consist of a variety of conjugation lengths. In addition, a lot of ammonium salts would be incorporated in the film during ammonia treatment. This could give rise to a very disordered material. This is in agreement with the optical observations, which show that the gap states induced by the dopant almost vanish. Furthermore, the featureless, continuous increase of the absorption with energy in the spectrum of the long-term (14 days) ammonia-exposed film could indicate a
177
disordered material with an increased amount of segments or species with short conjugation length. FT i.r. spectra and elemental analysis of an ammonia-treated film, before and after rinsing in water, support a model that includes ammonium ion formation and subsequent formation of salts, since the results show that N--Hx derived species are formed, which, together with the anion, can easily be washed away with water. The elemental analysis shows that during ammonia exposure we introduce nitrogen and oxygen into the film. Part of this nitrogen and oxygen excess can be washed away by rinsing in water (the oxygen that leaves the film is probably the anion oxygen, as indicated by FT i.r. spectroscopy). However, after rinsing we are still left with a large excess of nitrogen and oxygen (the analysis gives 0.8 oxygen and 0.8 extra nitrogen per pyrrole ring). This cannot be explained by the reactions described above, since they are based upon the principle that only the charged parts of the chain are attacked by nucleophiles. This would then give around one or two extra nitrogens or oxygens for every bipolaron. At a doping level of 30%, this only gives around 0.15 oxygen and 0.15 extra nitrogen per pyrrole ring. However, the introduction of carbonyl groups does increase the electrophilicity of the chain and make it sensitive to nucleophilic attacks, even though there are no charges left. Two examples of such reactions are the following:
H O~
H
NH3 N
/-~
N '~
A® ~
H
I H20 H 0%
.
I H20 HNH
A@
H
Reactions like these increase the number of nitrogens introduced into the chain. They cannot, however, quite explain the result of the elemental analysis.
Conclusions The interaction between ammonia gas and a p-doped conducting polymer is generally thought of as a compensation effect. Compensation of a polymer with ammonia can be explained by two different models,
178 based on electron and proton transfer respectively. The first involves electron transfer from the ammonia molecule to the polymer. This makes the polymer more neutral and less conducting. The ionization potential of the polymer is important for the reversibility of such an adsorption. This interaction may in some polymers constitute the first step in a multistep reaction, leading to an irreversible degradation of the polymer. The other model involves reversible proton transfer from acidic parts of the polymer to ammonia. This proton transfer would leave a negative charge, which is likely to recombine with the hole in the polymer; the resulting material will be more neutral and less conducting. We propose that different polymers interact differently with ammonia and that the mechanism for the interaction in a certain polymer can be described by either of the models, or perhaps by a combination of both. In PPy we have not been able to conclude definitely which of the models is more plausible. However, the experiments point to a mechanism that involves proton transfer reactions. In addition to the interaction described above, there is another interaction in PPy. This interaction has a very long time constant compared to the above described interaction and the changes of conductivity, which are irreversible, can become quite large under certain conditions. FT i.r. spectra and elemental analysis show that the irreversibly changed film contains N--H~ derived species, probably ammonium ions, which can easily be washed away with water together with the anion. Furthermore, FT i.r. and XPS measurements show that amide-type carbonyl groups are introduced into the chain during ammonia exposure. Some indications of the presence of amine groups have also been obtained. The results reported earlier on the interaction between conducting polymers and gases and the fact that there is a variety of combinations of gases and polymers indicate that conducting polymers have a good potential as gas sensors. However, the combination of a p-doped polymer, with several positive centres, with a nucleophile like ammonia does lead to some kind of irreversible attack on the chain. Such attacks could eventually be reduced by preparing samples with less defects and longer chains, since this technique has been shown to improve the environmental stability of other polymers such as polyacetylene [29].
Acknowledgements We thank Dr. R. Erlandsson, Dr. R. BjSrklund and Prof. W. R. Salaneck for stimulating discussions. Our work is supported by grants from the Swedish Natural Science Research Council (NFR), the National Swedish Board for Technical Development (STU) and the Engineering Research Council of STU (STUF).
179
References 1 0 . Ingan//s, R. Erlandsson, C. Nylander and I. Lundstr6m, J. Phys. Chem., 45 (1984) 427. 2 R. Erlandsson and I. Lundstr6m, J. Phys. (Paris) Colloq., 44 (1983) C3-713. 3 J.-C. Chiang and A. G. MacDiarmid, Synth. Met., 13 (1986) 193. 4 M. Josowicz and J. Janata, Proc. 2nd Int. Meet. Chem. Sensors, Bordeaux, France, July 7 - 10, 1986, p. 457. 5 M. Josowicz and J. Janata, Anal. Chem., 58 (1986) 514. 6 B. Chague, J. P. Germain, C. Maleysson and H. Robert, Sensors and Actuators, 7 (1985) 199. 7 C. Nylander, M. Armgarth and I. Lundstr6m, Proc. Int. Meet. Chem. Sensors, Fukuoka, Japan, 1983, p. 203. 8 K. Yoshino and H. B. Gu, Jpn. J. Appl. Phys., 25 (1986) 1064. 9 L. W. Shacklette, R. R. Chance, D. M. Ivory, G. G. Miller and R. H. Baughman, Synth. Met., 1 (1979) 307. 10 C. K. Chiang, C. R. Fincher, Jr., Y. W. Park, A. J. Heeger, H. Shirakawa, E. J. Louis, S. C. Gau and A. G. MacDiarmid, Phys. Rev. Lett., 39 (1977) 1098. 11 G. Gustafsson and I. Lundstr6m, Synth. Met., 21 (1987) 203. 12 J. L. Bredas, J. C. Scott, K. Yakushi and G. B. Street, Phys. Rev. B, 30 (1984) 1023. 13 J. Fink, B. Scheerer, W. Wernet, M. Monkenbusch, G. Wegner, H.-J. Freund and H. Gonska, Phys. Rev. B, 34 (1986) 1101. 14 A. F. Diaz, J. Crowley, J. Bargon, G. P. Gardini and J. B. Torrance, J. Electroanal. Chem., 121 (1981)355. 15 G. Gustafsson and I. Lundstr6m, to be published. 16 O. Inganh's and B. Liedberg, unpublished work. 17 A. Albert, Heterocyclic Chemistry, Athlone Press, London, 2nd edn., 1968, p. 205. 18 G. B. Street, in T. A. Skotheim (ed.), Handbook o f Conducting Polymers, Voh 1, Marcel Dekker, New York, 1986, p. 281. 19 A. Albert, Heterocyclic Chemistry, Athlone Press, London, 2nd edn., 1968, p. 208. 20 A. R. Katritzky and A. P. Ambler, in A. R. Katritzky (ed.), Physical Methods in Heterocyclic Chemistry, Vol. 2, Academic Press, New York, 1963, p. 318. 21 K. Hyodo and A. G. MacDiarmid, Synth. Met., 11 (1985) 167. 22 J. Manzur and G. Gonzalez, Z. Naturforsch., 36B (1981) 763. 23 R. Nordberg, R. G. Albridge, T. Bergmark, U. Ericsson, J. Hedman, C. Nordling, K. Siegbahn and B. J. Lindberg, Ark. Kemi, 28 (1968) 257. 24 H. Munstedt, Polymer, 29 (1988) 296. 25 M. Salmon, A. F. Diaz, A. J. Logan, M. Krounbi and J. Bargon, Mol. Cryst. Liq. Crist., 83 (1982) 265. 26 F. Beck, P. Braun and M. Oberst, Bet. Bunsenges. Phys. Chem., 91 (1987)967. 27 G. Wegner, W. Werned, D. T. Glatshofer, J. Lrlanski, Ch. Kr6hnke and M. Mohammadi, Synth. Met., 18 (1987) 1. 28 T. F. Otero, R. Tejada and A. S. Elola, Polymer, 28 (1987) 651. 29 H. Naarman and N. Theophilou, Synth. Met., 22 (1987) 1.
163
THE INTERACTION BETWEEN AMMONIA AND POLY(PYRROLE) G. GUSTAFSSON, I. LUNDSTR(~M, B. LIEDBERG, C. R. WU and O. INGAN.~,S
Department of Physics and Measurement Technology, University of Link6ping, S-581 83 LinkSping (Sweden) O. WENNERSTROM
Department of Organic Chemistry, Chalmers University of Technology, S-412 96 GiSteborg (Sweden) (Received October 18, 1988; in revised form January 27, 1989; accepted February 1, 1989)
Abstract Reversible and irreversible interactions between ammonia and poly(pyrrole) have been studied using electrical conductivity measurements, optical spectroscopy, Fourier transform infrared spectroscopy (FT i.r.), X-ray photoelectron spectroscopy (XPS) and elemental analysis. The results show that in addition to the previously reported reversible change of the electronic properties of poly(pyrrole) upon treatment with ammonia, there is also a large irreversible change if the material is exposed to high concentrations of ammonia or ammonia plus water vapour for a long period of time. The measurements also indicate that amide-type carbonyl groups, amine groups and ammonium ions could be present in the irreversibly changed polymer. The results are used as a basis for a discussion on the interaction between conducting polymers and ammonia in general and between poly(pyrrole) and ammonia in particular.
Introduction Properties like the electrical conductivity and optical absorption are easily altered in conducting polymers by chemical or electrochemical methods. For example, electrical conductivity is controlled by doping with different electron
© Elsevier Sequoia/Printed in The Netherlands
164 work function changes, which can be detected by the device. Chague e t al. have shown that poly(p-phenylene azomethine) can be used as a sensor material for iodine [6]. We have already demonstrated how doped PPy can be used to detect ammonia gas [7]. Yoshino e t al. reported that undoped poly(p-phenylene) (PPP) and poly(p-phenylenevinylene) also showed reversible changes upon ammonia exposure [ 8 ] , but in this case the conductivity increased in contrast to PPy, where the conductivity decreases reversibly upon ammonia exposure. The conductivity of conducting polymers like doped poly(p-phenylene) and doped poly(acetylene) (PA) are, however, reported to decrease irreversibly by several orders of magnitude when they are exposed to ammonia [9, 10]. So far, the mechanisms of the interaction between conducting polymers and ammonia are considered to be simple doping or compensation effects. This is probably the case in some polymers. We think, however, that the general situation is more complex and that additional chemical processes occur. We have, for example, found that in PPy, besides the previously reported reversible decrease of the conductivity, there is also a large irreversible decrease if the material is exposed to ammonia or to ammonia plus water vapour for a long period of time [11]. It is important to gain an understanding of the interaction mechanisms when developing long-term stable and reversible chemical sensors. Therefore we have performed a more rigorous study of the interaction between PPy and ammonia using techniques like u.v./vis/n.i.r., FT i.r. spectroscopies and XPS together with conductivity measurements. The result of this study is used as a basis for a discussion of the interaction between conducting polymers and ammonia in general and between ammonia and PPy in particular.
Experimental PPy films were prepared by electropolymerization of 0.1 M pyrrole using 0.1 M Et4NC1Oa or 0.1 M Et4NCH3C6H4SO3 in CH3CN. The electrolyte was purged with Ar for about 30 minutes before addition of the pyrrole monomer. Evaporated Au or indium-tin-oxide (ITO) on glass was used as a working electrode. The counter electrode was a Pt coil or Au evaporated on glass (when making material for elemental analysis). All films were grown galvanostatically with 0.5-0.7 mA/cm 2. The film thickness was in the range 0 . 1 - 1 pm, as determined by surface profile measurements (Dektak 3030). Films used for elemental analysis were, however, thicker. Films for optical spectroscopy and XPS were used still attached to the ITO/glass electrode. Films for FT i.r. spectroscopy and electrical conductivity measurements were carefully peeled off the ITO/glass substrate in CH3CN and mounted in appropriate sample holders. Samples for elemental analysis were prepared by scraping off thick films of PPy from large-area Au electrodes.
165
All samples were exposed to laboratory atmosphere when they were transferred from the preparation to the measurement equipment. Optical spectroscopy was done mainly in air. Optical spectroscopy during ammonia exposure was performed by using an optical glass cell sealed with a Teflon lid and epoxy. The gases were supplied by Teflon tubes through the lid. When films were exposed to ammonia for a long period of time, they were put in a sealed container and ammonia gas (100%) was flushed through the container twice a day. Ammonia plus water {wet ammonia) exposures were done b y leading the ammonia gas through a saturated a m m o n i a - w a t e r solution before flushing it through the container. Pulses of low concentrations of ammonia during conductivity measurements were generated by allowing a stream of Ar gas, into which ammonia was injected, to pass over the film. The amount of ammonia gas was regulated b y a magnetic valve and a high-precision flow regulator. Exposure of the PPy film to low concentrations of ammonia plus water {wet ammonia) was done b y leading the carrier gas through water before injection of ammonia gas. Conductivity was measured using a four-probe technique. Ammonia exposure was done with the films still mounted in the probe. Optical spectra were recorded using a Perkin-Elmer Lambda 9 u.v./vis/n.i.r, dual-beam spectrophotometer. Infrared spectroscopy was carried out, at 2 cm -1 resolution, using a Bruker IFS l 1 3 v Fourier-transform spectrometer. XPS was done with a UHV instrument of local design and construction, at a pressure of 10 -1° Tort. The spectra were taken with unmonochromatized Mg K s radiation and an analyser resolution of 0.2 eV. Elemental analysis was performed b y Analytical Laboratories, Engelskirchen, F.R.G.
Results
Conductivity measurements The resistance of a film of PPy increases upon exposure to ammonia. As can be seen in Fig. 1, relatively small ammonia concentrations give rise to marked resistance changes within minutes. If the film is exposed to the same concentration of ammonia but with the carrier gas saturated with water vapour {wet ammonia), the change becomes even larger. The resistivity changes upon exposure to these relatively small ammonia concentrations are almost completely reversible. If higher concentrations of ammonia are used, and especially high concentrations of w e t ammonia, the resistivity changes become larger and partly irreversible. The resistivity of a PPy film increases b y a factor of 30 upon exposure to 1 atm of dry ammonia for 10 minutes. When ammonia is removed, the resistance returns to within a factor of two of its initial value. Further ammonia exposures also cause irreversible changes. The irreversible changes are significant if the film is exposed to high concentrations of dry or wet ammonia for a long period of time, as shown by Table 1. As expected, the reversible part of the resistivity change decreases with increasing total exposure time
166
DRY
WET
108
27 108
Ro=
/'~:"~
410G
"~
~
Ro= 550~
30rnin
Fig. 1. Resistance change of a film of poly(pyrrole) during 30 minute exposures to different concentrations of dry and wet ammonia in Ar. The labels indicate the concentration (in ppm) of NH3. Ar was flushed over the film between the ammonia pulses. TABLE 1 Resistivity changes of films of poly(pyrrole)(C104) exposed to 1 atm of dry and wet ammonia for various times. R0 is the resistance of the 'as prepared' film Treatment
R /R o
Dry NH3, 5 days Wet NH3, 4 days Wet NH3, 7 days
103 104 l0 s
under these e x t r e m e conditions. T he resistivity change measured after seven days o f w et a m m oni a exposure is c o m p l e t e l y irreversible. It was n o t even possible to increase the conductivity in such a film by electrochemical oxidation or b y immersing it into aqueous HC1 for 30 minutes. Optical spectra
The resistivity changes described above are accompanied b y changes in the optical spectra. Curve (a) in Fig. 2 shows the optical spectrum of a thin film o f d o p e d PPy. The spectrum contains two peaks, one at 1.0 eV and another, o f lower intensity, at 2.7 eV. During exposure to 1 atm of dry a m mo n ia (curve (b)), the low-energy peak decreases and shifts approximately 0.1 eV to higher energy. The high-energy peak increases and also shifts a b o u t 0.5 eV t o higher energy; this latter change can also be interpreted as a gradual shift in intensity between two peaks at 2.7 and 3.2 eV. When a m m o n i a is removed and Ar has been flowing through the cell f o r 50 minutes, a spectrum shown as curve (c) is obtained. The peaks shift back to lower energy, the intensity of the low-energy peak is increased and the intensity o f the high energy peak is lowered com pared to (b), b u t
167
a
f~
/ f-~,
°' ,' ,"
~t~ ~>,~
/
-,c---," / ,' --2"'--
<( 1.( O m ~
/ ~ J
/ J
/ / /
b¢
/ /
O.E
0.5
/ / / / c/
-
1'.o
~,'.o
:~.o
PHOTON ENERGY (eV}
4.0
1'.o
2'.o
3'.o
1o
PHOTON ENERGY (eV)
Fig. 2. Change of the optical spectrum of a thin film of doped poly(pyrrole) during a short pulse of ammonia exposure. Curve (a) shows the spectrum of the 'as prepared' film. Curve (b) shows the spectrum o f the film during exposure to 1 atm o f dry ammonia (after 10 minutes), and (c) the spectrum when ammonia is taken away and Ar has been flowing through the cell for 50 minutes. Fig. 3. Change of the optical spectrum o f doped poly(pyrrole) after long-term ammonia exposure. Curve (a) shows the spectrum of the 'as prepared' film and curve (b) the spectrum o f the film after five days' exposure to 1 atm o f dry ammonia. Curve (c) shows the spectrum of another film that was exposed to 1 atm o f wet ammonia for 14 days. Spectra were recorded with the films exposed to laboratory atmosphere.
the initial spectrum is not recovered. Thus, the optical changes obtained after an initial exposure to ammonia are not reversible. The change in the optical spectrum after exposure to 1 atm of dry and wet ammonia for a long time can be seen in Fig. 3. The spectrum of the film after exposure to dry ammonia for five days still resembles the initial spectrum to some extent. The two original peaks are still observed, even though their intensities have decreased and their positions shifted towards higher energy. The ultraviolet absorption has also become very intense. When a film has been exposed to wet ammonia for 14 days, the optical spectrum has changed completely, as can be seen in curve (c). This spectrum shows very little structure and is just a continuous increase of the absorption with the energy. However, there are two very weak shoulders at 0.7 eV and 1.4 eV. Treatment of the films in HC1 or attempts to oxidize the films electrochemically did not change the spectra of the long-term ammonia-exposed films appreciably.
Infrared spectra The FT i.r. spectrum of a PPy film during ammonia exposure is not very useful, since the vibrational spectrum of the ammonia molecules obscures all the interesting features of the PPy spectrum. This means that
168
the FT i.r. technique cannot be used for investigating the reversible mechanism. The irreversible mechanism, however, can be studied. To test the generality of the observations, the measurements were performed with two different anions, perchlorate and toluene sulfonate. The spectra of PPy films with perchlorate and toluene sulfonate anions, exposed to different ammonia atmospheres for varying times, are shown in Figs. 4 and 5 respectively. A c o m m o n feature of these spectra was an overall decrease in absorption upon ammonia exposure. This was also observed in the optical spectra (decrease of the low-energy peak). Changes induced b y the treatments were qualitatively the same in dry and wet ammonia and all changes that were observed in the dry case were also observed in the wet case, b u t were enhanced in the latter. After exposure to 1 atm of w e t ammonia for 14 days, a new, broad absorption band develops at 2 7 0 0 - 3 5 0 0 cm -1. The shape of this band is not the same in the two different materials. In the spectrum of PPy cio;,
a
a
c.3-@-so~
i !AA,,'v,,
b
5 d
d (J Z < m
O Z < m er
n-
o
O
(n m ,<
\.
\ \
a
4000
,
I
t
3000
i
2000
WAVENUMBERS, cm -1
=
1000
I
4000
3000
2 0 0=0
1 0 J0 0
WAVENUMBERS, cm -I
Fig. 4. F T i.r. s p e c t r a o f t h i n , f r e e s t a n d i n g films of p o l y ( p y r r o l e ) ( C 1 0 4 ) t r e a t e d in diff e r e n t a m m o n i a a n d w a t e r e n v i r o n m e n t s for varying times. Curve (a) is t h e s p e c t r u m o f t h e 'as p r e p a r e d ' film, (b) t h e s p e c t r u m a f t e r five days in 1 a t m o f d r y a m m o n i a a n d (c) t h e s p e c t r u m o f a film e x p o s e d t o 1 a t m o f w e t a m m o n i a for 14 days. Curve (d) shows t h e same film as in (c) b u t a f t e r rinsing in w a t e r for 12 hours. The asterisks indicate a b s o r p t i o n b a n d s t h a t are caused b y t h e a n i o n . Fig. 5. F T i.r. s p e c t r a o f t h i n , f r e e s t a n d i n g films o f p o l y ( p y r r o l e ) ( C H a C 6 H a S O a ) t r e a t e d in the same way as t h e films in Fig. 4.
169
toluene sulfonate the band is asymmetric and shows a peak at 3178 cm -1, while it is symmetric and shows almost no structure in the spectrum of PPy perchlorate. A shoulder occurs at 1650 cm -1 in all samples treated with dry or wet ammonia. It is, however, most pronounced in the spectra of films exposed to wet ammonia for 14 days. The occurrence of new absorption bands between 1580 and 1600 cm -1 is also evident in those spectra. The shoulder at 1082 cm -1 and the sharp peak at 617 cm -~ in the spectrum of 'as prepared' PPy perchlomte and the absorptions at 562, 614, 679, 811, 1010, 1035 and 1122 cm -1 in the spectrum of 'as prepared' PPy toluene sulfonate may in part be assigned to the anion. In the spectra of the most exposed samples these peaks appear more clearly. After rinsing such films in water for 12 hours these peaks disappear, as can be seen in spectra (d) in Figs. 4 and 5; in addition the shape of the band at 30003500 cm -1 changes. The spectra of the two films {prepared with different anions) are very similar after ammonia treatment and rinsing in water. Treatment of the films with aqueous HC1 (37%) after the long-term ammonia exposure did not change the spectra of the films significantly. XPS
Figure 6 shows the change of the N(ls) and the C(ls) peaks of PPy perchlorate upon exposure to wet ammonia for 14 days. The N(ls) peak in the spectrum of the 'as prepared' sample is asymmetric and shows an increased intensity on the high binding-energy side compared to the lowenergy side. The lineshape for the ammonia-treated sample is completely different. The spectrum shows an increased intensity on the low-energy side of the main peak, which is located at 399.8 eV, and a decreased intensity on the high,energy side compared to the original spectrum.
N (ls)
C Ils} ,'.,
i,/
...'.
.-
•
BefO¢lt exp. """:"
'
.. "'"'....."
.
.
.
.
.
"-'"'"""'"
..'..
iV Wet ammonia
. ........
....
.'"
"'.•....,
....
-"""
....
BINDING ENERGY (eV)
Fig. 6. N(ls) and C(ls) photoelectron spectra of an 'as prepared' poly(pyrrole)(C104) film, (i) and a film exposed to 1 atm of wet ammonia for 14 days, (ii).
170 The C(ls) peak in the spectrum of the 'as prepared' sample is asymmetric and shows an increased intensity on the low binding-energ~¢ side compared to the high-energy side. In the spectrum of the sample exposed to wet ammonia for 14 days, a shoulder appears around 288 eV. The peak is also more symmetric compared to the peak in the original spectrum.
Elemental analysis The elemental compositions of an 'as prepared' film, a film exposed to wet ammonia for 12 days and a film exposed to wet ammonia for 12 days and then rinsed in water for 24 hours are listed in Table 2. All films showed good material balance and the drying loss was 2 - 6 % when dried at 50 °C and 0.1 mBar. The 'as prepared' film shows a good agreement with the expected elemental composition. If we assume that the chlorine exists in the form of perchlorate ions, then there is approximately one anion per three pyrrole rings. There is no oxygen excess in the 'as prepared' film. The sample exposed to wet ammonia has a large nitrogen and hydrogen excess compared to the 'as prepared' film and the oxygen c o n t e n t has also increased in this sample. With the assumption that chlorine exists in the form of perchlorate ions, the excess of oxygen is 0.71 per pyrrole ring. The excess of hydrogen and nitrogen in the ammonia-exposed film decreases considerably when the film is rinsed in water. Also the oxygen and chlorine contents decrease. The chlorine c o n t e n t is almost zero after rinsing and the decrease of oxygen is nearly four times the decrease of chlorine. The resulting film, after ammonia exposure and water treatment, does, however, still show a large excess of both oxygen and nitrogen. There are approximately 0.8 extra nitrogens and 0.8 extra oxygens per pyrrole ring. TABLE 2 Elemental analysis of 'as prepared', ammonia-treated (1 atm) and water-rinsed poly(pyrrole)(C104) Sample
C
H
N
O
C1
As prepared Wet ammonia, 12 days Wet ammonia, 12 days followed by water rinsing, 24 h
4.00 a 4.00 a 4.00 a
3.12 4.56 3.33
1.01 2.24 1.81
1.41 1.79 0.91
0.35 0.27 0.02
aThe carbon content was assumed to be four carbons per pyrrole monomer. Discussion
Reversible effects The results of the conductivity measurements presented above show that ammonia, at low concentrations and short exposure times, mainly interacts reversibly with PPy.
171 The reversible changes in the conductivity are accompanied by changes in the electronic structure, as observed in the optical spectra. The peaks at 1.0 eV and 2.7 eV in the optical spectrum of an 'as prepared' PPy film have been assigned to transitions from the valence band to two bipolaron bands in the band gap [12]. The bipolaron bands are induced by the dopant. The oscillator strength of the u-~* transition is very low in doped PPy and the absorption band of this transition is believed to be a part of the high-energy tail of the 2.7 eV bipolaron peak [12, 13]. The change of the peak at 2.7 eV upon ammonia exposure could thus either be interpreted as an increase of the absorption and a 0.5 eV shift towards higher energy of this peak, or as mainly a gradual change of the relative intensities of a peak, positioned at around 3.2 eV (Tr-Tr* transition) and the bipolaron peak at 2.7 eV. The former interpretation is not very probable, since the low-energy bipolaron peak is shifted by only 0.1 eV and has decreased upon ammonia exposure. The changes of the optical properties could thus be interpreted as a lowering of the bipolaron peaks and an increase of the ~-u* peak. This indicates that the polymer becomes less doped during ammonia exposure. The a m m o n i a - p o l y m e r interaction in p-doped conducting polymers is generally considered to be a compensation effect. Thus, ammonia molecules, which are electron donating, act as n-type dopants that could decrease the doping level of the polymer chain by compensating the effect of the original dopant. This nucleophilic interaction may be written as: Polymer+A - + NH3 -
~ Polymer ° .... NH3 +, A -
(1)
It should be noted that, theoretically, it is possible to compensate a material even though it is not n-dopable since different energy levels are involved in the two processes. The ease with which such a compensation occurs, and consequently the reversibility of the reaction, should be related to the ionization potentials of the polymer and of ammonia [8]. It should, however, be noted that the energy levels could change considerably due to the interaction, so a direct comparison of the gas-phase ionization potentials of ammonia and the polymer does not give the relevant information. If we assume, however, that the changes of the energy levels due to the p o l y m e r - a m m o n i a interaction are approximately the same for the different polymers, we could get an indication of the reversibility of the reactions by studying the order of the ionization potentials of the polymers. As an example, we could compare the ionization potentials (as reflected in the redox potentials) of PPy, PANi, PA, PPP and poly(thiophene) (PT) and see how the order is related to their interaction with ammonia. PPy and PANi have the lowest redox potentials (--0.2 V and 0.2 V v e r s u s SSCE respectively) [14]. These polymers interact reversibly with ammonia. The other polymers PT (0.7 V), PA (0.7 V) and PPP (1.6 V) [14] all have higher redox potentials and the interaction is reported to be irreversible in these polymers [ 8 - 101.
172
An alternative mechanism for the compensation reaction, which involves proton transfer between the polymer and ammonia, could be written as
Polymer+A - + NH3 ~
Polymer(--H) ° + NH4+A-
(2)
Ammonia is a rather strong base and should attack protons that are acidic. This deprotonation should leave a negative charge on the chain, which could combine with the hole and u n d o p e the polymer. This reaction, which could be reversible, can only occur in polymers with protons that are acidic, like PPy and PANi, and thus could n o t explain the interaction between PPP, PA or PT and ammonia. In PPy and PANi the most acidic protons are those bonded to the nitrogen. These could easily be removed b y ammonia, forming ammonium ions that electrostatically bind to the negative anion. The reaction for PPy is: NH3 t
A®
H H ~ ..~N/ ~~N / L N® ~ ~ /IN,,~." A® H
H
H
"
N\ ~.../N //--~ , -.~ / ~ N / ~ ~ IN,J,L- +NH~A® (3) A® H
H
Deprotonation in PPy has been studied earlier [1, 2]. It was shown that in aqueous bases the conductivity decreases irreversibly b y three to four orders of magnitude. The conductivity could be restored b y treatment in acid. In the case of PPy exposed to ammonia gas, the reversible change in conductivity was between one and t w o orders of magnitude. The difference could be explained b y the fact that the former experiment was performed in solution where a m m o n i u m - a n i o n complexes can be transported o u t of the polymer, while in the latter case these inorganic species remain in the polymer. The latter reaction can therefore be considered as an equilibrium reaction, whereas the first one goes to completion. To determine which of the two types of compensation reactions causes the reversible interaction b y PPy is difficult, since most of the analytical tools available to us cannot be used while the sample is still exposed to ammonia. One indication could, however, be given b y studying the relation between the change of resistance and the partial pressure of ammonia. Earlier work done by us has shown that the response (AR/Ro) appears to follow approximately (PNrI~)i/2 at small ammonia concentrations, where PNH3is the partial pressure of NH3 in the Ar carrier gas [15]. An acid-base reaction with a subsequent formation of a mobile NH4+A - complex leads to a bimolecular reaction. The assumption that the change in conductivity is linearly related to the number of imine nitrogens n o w gives an expression that fits rather well with a square r o o t dependence [15]. However, the above argument is very weak, since there is not a simple relationship between the conductivity and the number of charge carriers in the polymer chains.
173 We have also tried to distinguish between the t w o models b y exposing a film of PPy to triethylamine, a non-nucleophilic base with almost the same PKa value as ammonia. The exposure resulted in a very slow increase of the resistance. This indicates that proton transfer is a possible mechanism of the a m m o n i a - P P y interaction. The very slow response of the polymer to the triethylamine exposure could be due to a very slow diffusion of the gas into and out of the polymer, since triethylamine is a bulky molecule.
Irreversible effects A reversible compensation reaction based on proton transfer should to a certain extent 'protect' the polymer from further nucleophilic attacks, since the chains are less positively charged in the compensated polymer. However, since the reversible reaction is based on an equilibrium reaction, there are always segments that axe n o t compensated and where irreversible nucleophilic attacks could occur. This should lead to a slow degradation of the polymer. Furthermore, it is likely that the a m m o n i u m - a n i o n complex formed in the reversible reaction diffuses freely in the polymer matrix. In a short time perspective such a diffusion probably causes no changes in the reversibility of the reaction. In a longer time perspective the diffusion could, however, result in salt crystal formation at the surface of the polymer, which would change the reversibility appreciably. In our long-term exposure experiments, we have observed salt crystals at the surface of the material, b u t so far have not been able to analyse them. Diffusion of the a m m o n i u m - a n i o n complex and subsequent crystal formation should result in a polymer with the nitrogens irreversibly deprotonated. These imine nitrogens should be observed b y XPS and FT i.r. spectroscopy. In an FT i.r. spectrum of base-treated PPy, a shoulder appears at 1605 cm -1 . This has been assigned to imine-type nitrogen [16]. Thus, the peak appearing at 1600 cm -1 in the long-term ammonia-exposed films could in part come from irreversibly deprotonated nitrogen. In XPS spectra imine-type nitrogens axe reported to give rise to a pronounced shoulder on the low binding-energy side of the N(ls} peak [ 1 , 2 , 1 6 ] . Long-term ammonia-exposed samples show no such sharp shoulder, b u t there is an overall increased intensity at the lowenergy side of the main peak. Part of this increase could be due to imine nitrogens. The changes in the FT i.r. spectrum and the excess of nitrogen and oxygen in the elemental analysis show that the polymer undergoes chemical changes u p o n long-term ammonia exposure. The broad band at 2700 - 3500 cm -1 in the FT i.r. spectrum, after ammonia exposure, is a mixture of different absorptions. N--H stretch modes are usually found in that region, which suggests that structures containing amines (primary and secondary) and amides could be present in the polymer. It is reported that a structure like [A] gives a doublet in the region 3360 - 3460 cm -1 [17].
174 CH= CH(OEt) 2
I
NC
The N--H stretch of PPy in the neutral form is observed at 34O0 cm -1 [18]. The absorption band at 3200 cm -1 , which is clearly resolved in the spectrum of PPy toluene sulfonate after ammonia treatment, is probably caused by ammonium ions. O--H bonds also give rise to absorptions in the region 2 7 0 0 - 3 5 0 0 cm -1, and we cannot exclude the possibility that hydroxyl groups are incorporated in the polymer exposed to ammonia. However, hydroxypyrroles exist predominantly in the oxo form [19, 20] and it is probable that the hydroxyl groups are converted to carbonyl groups. At 1600 cm -1 a new peak appears in the FT i.r. spectrum of PPy exposed to ammonia. This could be assigned to primary amines, since such groups usually show strong absorptions in this region. However, there is also a possibility that this peak, or a part of it, could come from iminetype nitrogen, as mentioned above. The peak at 1650 cm -~, observed in the spectra of long-term ammoniaexposed samples, is most probably caused by a carbonyl group. Carbonyl groups in poly(N-methylpyrrole) have been reported to give an absorption peak around 1700 cm -1 [21]. The difference in peak position in the two materials should be due to hydrogen bonding in the former polymer [22]. The XPS results support the assignments of the new peaks in the FT i.r. spectrum. The N(ls) core level spectrum indicates that amine groups have been introduced into the chain upon ammonia treatment, since the intensity in the region around 398.5 eV, the binding energy where primary amines can be found [23], has increased. This assignment is, however, somewhat uncertain since there is almost no structure of the lineshape in this region and since other groups, like imines, are expected to give rise to a peak in the binding~nergy range in question. The formation of carbonyl groups on exposure of PPy to ammonia is also supported by XPS spectra where the shoulder at 288.0 eV in the C(ls) spectrum can be assigned to carbonyl carbons. A similar assignment has been done in aged PA by Munstedt [24]. There are two species that are likely to attack the polymer chain during ammonia exposure. The first is ammonia itself, which is a good nucleophile, and the other is the hydroxide ion, which is an even better one. Hydroxide ions are generated by the following reaction: NH3 + H20 ~
NH4 + + OH-
(4)
In experiments with dry ammonia exposure, the water needed for the reaction must be supplied by the polymer matrix itself. PPy films are reported to contain a large amount of water, which is bound to the polymer and cannot be desorbed by mild heating [25]. Treatment of PPy films with wet ammonia will of course increase the hydroxide ion concentration.
175
Due to the disordered nature of the polymer, with sites of different sensitivities to nucleophilic attack, various reactions are possible. The bipolarons are the most electrophilic sites due to their positive charge and are thus the prime target for nucleophilic attack. In the schemes below we have selected a few examples of reactions that seem reasonable and are consistent with the experimental results.
(a) Hydroxide ion attack at the (J-carbon
I NH3
ibH®
H
N
H A®
(,
~
.N®
/ ~ N ~ N
''~
A® H
+ NH® 4
H OH
H
H
+ 14
NH A ®
14
l H OHH
Tautomerism
H
+ NHOA®
l
4
A® H
H
HO~.~
H
+ 2 NH®A® 4
H
H
Similar mechanisms have been used to explain the degradation of PPy during over-oxidation in aqueous electrolytes [26], and during base treatment followed by electrochemical oxidation [27].
(b) Attack at the s-carbon followed by ring opening IOH® H ... N ~
H
+NH?
AQ H
H
I H
AO
Ao H
H
"~/O~H N H ~ N
N ~'"
" N~A G
~"~O
~
H
Ao
N° N ~
N~" H
+ NH?A®
176
This mechanism has been proposed b y Otero et al. as an explanation for the degradation of PPy at anodic potentials in aqueous electrolytes [28]. The carbonyl groups introduced b y both these reactions are of the amide t y p e (vinylic amides). (c) and (d) Attach o f ammonia on the chain in a similar way to that o f the hydroxide ion NH3 H -~. N
AG
® H~ A N'~ ". H 1
®
H NH 3
NH 3 H -/
AQ
H
j NH3 A® N
A®
H
H NH2 N H
H
~
1
A® H
N H ~
NH3 H H
H
NH 3
1
IAO H
N A®
A® .
H
N
_
+ NH?A O H
N
.
/:~ NH2 H
N
~
NH4QA® H
The initial step of reactions (c) and (d) with ammonia could eventually be reversible and thus constitute the reversible nucleophilic interaction (eqn. (1)) in the case of PPy. Eventually all the initial steps of all the reactions (a)- (d) could be reversible, with different time constants. As in the deprotonation reaction, these reactions (a) - (d) produce NH4÷A - complexes, which could diffuse and form ammonium salt crystals. This would be a truly irreversible step that would degrade the polymer. The result of all the reactions described above would be that charges are taken from the polymer chain and that new groups are introduced into the chain. Some of those groups give rise to sp 3 defects, which effectively break the conjugation. The resulting polymer would therefore consist of a variety of conjugation lengths. In addition, a lot of ammonium salts would be incorporated in the film during ammonia treatment. This could give rise to a very disordered material. This is in agreement with the optical observations, which show that the gap states induced by the dopant almost vanish. Furthermore, the featureless, continuous increase of the absorption with energy in the spectrum of the long-term (14 days) ammonia-exposed film could indicate a
177
disordered material with an increased amount of segments or species with short conjugation length. FT i.r. spectra and elemental analysis of an ammonia-treated film, before and after rinsing in water, support a model that includes ammonium ion formation and subsequent formation of salts, since the results show that N--Hx derived species are formed, which, together with the anion, can easily be washed away with water. The elemental analysis shows that during ammonia exposure we introduce nitrogen and oxygen into the film. Part of this nitrogen and oxygen excess can be washed away by rinsing in water (the oxygen that leaves the film is probably the anion oxygen, as indicated by FT i.r. spectroscopy). However, after rinsing we are still left with a large excess of nitrogen and oxygen (the analysis gives 0.8 oxygen and 0.8 extra nitrogen per pyrrole ring). This cannot be explained by the reactions described above, since they are based upon the principle that only the charged parts of the chain are attacked by nucleophiles. This would then give around one or two extra nitrogens or oxygens for every bipolaron. At a doping level of 30%, this only gives around 0.15 oxygen and 0.15 extra nitrogen per pyrrole ring. However, the introduction of carbonyl groups does increase the electrophilicity of the chain and make it sensitive to nucleophilic attacks, even though there are no charges left. Two examples of such reactions are the following:
H O~
H
NH3 N
/-~
N '~
A® ~
H
I H20 H 0%
.
I H20 HNH
A@
H
Reactions like these increase the number of nitrogens introduced into the chain. They cannot, however, quite explain the result of the elemental analysis.
Conclusions The interaction between ammonia gas and a p-doped conducting polymer is generally thought of as a compensation effect. Compensation of a polymer with ammonia can be explained by two different models,
178 based on electron and proton transfer respectively. The first involves electron transfer from the ammonia molecule to the polymer. This makes the polymer more neutral and less conducting. The ionization potential of the polymer is important for the reversibility of such an adsorption. This interaction may in some polymers constitute the first step in a multistep reaction, leading to an irreversible degradation of the polymer. The other model involves reversible proton transfer from acidic parts of the polymer to ammonia. This proton transfer would leave a negative charge, which is likely to recombine with the hole in the polymer; the resulting material will be more neutral and less conducting. We propose that different polymers interact differently with ammonia and that the mechanism for the interaction in a certain polymer can be described by either of the models, or perhaps by a combination of both. In PPy we have not been able to conclude definitely which of the models is more plausible. However, the experiments point to a mechanism that involves proton transfer reactions. In addition to the interaction described above, there is another interaction in PPy. This interaction has a very long time constant compared to the above described interaction and the changes of conductivity, which are irreversible, can become quite large under certain conditions. FT i.r. spectra and elemental analysis show that the irreversibly changed film contains N--H~ derived species, probably ammonium ions, which can easily be washed away with water together with the anion. Furthermore, FT i.r. and XPS measurements show that amide-type carbonyl groups are introduced into the chain during ammonia exposure. Some indications of the presence of amine groups have also been obtained. The results reported earlier on the interaction between conducting polymers and gases and the fact that there is a variety of combinations of gases and polymers indicate that conducting polymers have a good potential as gas sensors. However, the combination of a p-doped polymer, with several positive centres, with a nucleophile like ammonia does lead to some kind of irreversible attack on the chain. Such attacks could eventually be reduced by preparing samples with less defects and longer chains, since this technique has been shown to improve the environmental stability of other polymers such as polyacetylene [29].
Acknowledgements We thank Dr. R. Erlandsson, Dr. R. BjSrklund and Prof. W. R. Salaneck for stimulating discussions. Our work is supported by grants from the Swedish Natural Science Research Council (NFR), the National Swedish Board for Technical Development (STU) and the Engineering Research Council of STU (STUF).
179
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