- Email: [email protected]
The redox mechanism of polyaniline studied by simultaneous ESR–UV–vis spectroelectrochemistry
Synthetic Metals 107 Ž1999. 143–158 www.elsevier.comrlocatersynmet
The redox mechanism of polyaniline studied by simultaneous ESR–UV–vis spectroelectrochemistry Andreas Neudeck, Andreas Petr, Lothar Dunsch
)
Institut fur und Werkstofforschung, Institut fur Abteilung Elektrochemie und Leitfahige Polymere, Helmholtzstr. 20, ¨ Festkorper¨ ¨ Festkorperforschung, ¨ ¨ D-01069 Dresden, Germany Received 10 August 1998; received in revised form 17 March 1999; accepted 25 June 1999
Abstract The electrochemical redox process to charge and discharge polyaniline layers is studied by simultaneous Electron Spin Resonance ŽESR. and UV–vis spectroscopic measurements. For this purpose, optically transparent electrodes were applied under potential-controlled conditions. The use of a calibrated manganese ESR standard ensures the calculation of the absolute numbers of free spins in the polymer layer during electrochemical doping. A general procedure is given to analyse the simultaneously recorded electrochemical and spectroscopic data. The procedure proposed following the time dependence of charge, ESR absorption and the absorbance demonstrates that the electrochemical chargingrdischarging mechanism of polyaniline can be described on the base of three oxidation states. In this way, the separated UV–vis spectra and the timerpotential dependence of each redox state can be shown. Only little changes are observed in the shape of the separated spectra using the polaronrbipolaron model in comparison to the polaronrs-dimer model. A square ladder scheme of the redox mechanism is discussed, considering neutral, polaronic and bipolaronic states and the protonation equilibria. Furthermore, experimental evidence for the Faradaic nature of the ‘‘capacitive’’ current plateau is presented. q 1999 Elsevier Science S.A. All rights reserved. Keywords: Redox mechanism; Polyaniline; ESR–UV–vis spectroelectrochemistry
1. Introduction Since the increased interest in conducting polymers, both the polymerization and the redox mechanism of doping w1–35x of chemically and electrochemically synthesized conducting polymers w36–39x were studied. Big efforts were done in the chemical synthesis of doped polyacetylene and later in the electrochemical polymerization of aniline, pyrrole, thiophene etc. because of their conductivity, their ion exchange properties, the possibility of charge storage, and the switching of chemical and physical properties of such polymer layers by changing the oxidation state. The knowledge of the polymerization mechanism facilitate w40–43x the synthesis of conductive polymers with improved properties. The main processes of the electrochemical chargerdischarge mechanism in polymers are known. But there is no general model to describe the process and to get quantitative information on the distribution of the conjugation chain length w44x, w45x, the interactions of the generated polaronic and bipolaronic segments on the chains w46x related to a shift in the formal potential dependent on the level of doping, and the exchange of counter ions in connection with the change of acidrbase properties w47–51x of the conductive polymers during doping. The nature of the well known ‘‘capacitive behaviour’’ of charged polymers was not understood for a long time w44,46,52x. This behaviour is attributed to a current plateau behind the voltammetric redox peak during switching of the scan direction and by impedance measurements in this potential range. The high value of this capacity, which is 10,000 times larger than that of the bare electrode, cannot be explained by the extension of the electrochemical double layer due to the porosity of conductive polymer. A porosity below the nanometer scale is needed to get such high values. Recently, there are two models to explain the capacitive behaviour of conducting polymers by a redox current ŽFaradaic current. caused by the
)
Corresponding author. Fax: q49-351-4659-313; E-mail: [email protected]
0379-6779r99r$ - see front matter q 1999 Elsevier Science S.A. All rights reserved. PII: S 0 3 7 9 - 6 7 7 9 Ž 9 9 . 0 0 1 3 5 - 6
144
A. Neudeck et al.r Synthetic Metals 107 (1999) 143–158
distribution of the conjugated chain length w44,45x or by the influence of the doping level on the formal potential in a solid at high concentrations of polaronic and bipolaronic states w46x. Considering the conjugated chain length as statistically distributed w44x and the formal redox potential as indirectly proportional to the chain length as found for oligomers w53–55x, the Faradaic process shows a capacitive behavior that yields a current plateau in the cyclic voltammogram. The Faradaic nature can be described on the other hand by a model of Paasch et al. w46x. This model based on interactions of polaronic and bipolaronic states and on the description of soliton–soliton interactions derived by Brazovskii and Kirova w56x. It results in a modified Nernst equation with formal potentials of the polarons and bipolarons dependent on the oxidation level. Both models assume the ‘‘capacitive’’ behaviour to be mainly caused by the redox process. An extension of the electrochemical double layer into the porous polymer during the transfer of the polymer into the conducting state is not necessarily to be considered. Independent of the distribution of the conjugated chain length or the interactions of the increasing polaronic and bipolaronic states or both, the redox process is responsible for the constant current. As the total charge in the cyclic voltammogram represents a redox current, the ‘‘capacitive’’ current should not be subtracted to get the total amount of electrochemically created oxidation states inside the polymer layer. This is important to know in order to get additional kinetic and thermodynamic data by cyclovoltammetric measurements. It is the aim to distinguish between both effects Ždistribution of chain length and interactions at high doping level. that are superimposed by the heterogeneous charge transfer on the metalrpolymer interface connected with the propagation of the conductive state w17–35x. Therefore, in the diffusion of the counter ions w11–16x into the porous polymer layer and the acidrbase properties, cyclovoltammetric investigations have to be studied by spectroscopic methods w57–59x and with microgravimetic techniques like the electrochemical quartz micro balance technique. From quartz micro balance investigations and from the comparison of the charge for the electrochemical polymerization and the doping charge of the prepared polymer layer, it was found that approximately one positive charge can be stored per five monomer units in most conducting polymers w60,61x. Considering this result and using the polaronrbipolaron model Žwhich corresponds to a two electron transfer per ‘‘redox segment’’ of equal size., the extension of created polaronrbipolaron segments is in the range of 10 monomer units. In case of creating bonds in between the polaronic segments of different polymer chains that correspond to an electron transfer per redox segments, the extension of the polaronic state corresponds to five segments. Recently, a certain blue shift Žhypsochromic shift. was described during electrochemical doping by use of in situ UV–vis spectroelectrochemistry w57x. This result supports a distribution of the conjugated chain length over a large range what is in contrast to the sharpness of the voltammetric peak observed for the chargingrdischarging of polyaniline. That is why additional states are discussed by these authors. Based on a comparative study of the redox behaviour of the monomers and oligomers with a length up to the conjugation length and larger in electrochemically prepared polymer layers, the existence of chemical interactions between the polymer chains creating sigma bonds are discussed by Smie and Heinze w62x. It is necessary to get electrochemical and spectroscopic information on polymer charging at the same time and under the same conditions to distinguish between the effect of the phenomena discussed above and to describe the electrochemical chargerdischarge process of layers of conduction polymers dependent on the nature of the selected polymer. It will be shown that the use of in situ ESR and UV–Vis spectroelectrochemistry w63x at optically transparent laminated gold-m-meshes w64x permit the simultaneous study of layers of conductive polymers at a well controlled electrode potential that is demonstrated for electrochemically deposited polyaniline layers. A procedure is proposed to analyse the time dependence of the charge injected and the spectroscopic signals measured simultaneously to get the time dependence of each postulated oxidation state and the separation of the over modulated UV–vis spectra to get information of the oxidation states of the polymer layer and about the weight of the different phenomena discussed. 2. Experimental 2.1. Equipment The in situ ESR–UV–vis spectrocyclovoltammetric measurements were controlled by a home made program driving a potentiostat ŽPG285, HEKA-Elektronik, Lambrecht, Germany. by AD-DA plug-in boards ŽACAO and ACJr, Strawberrysoft, Sunnyvale, CA, USA. and triggering an ESR spectrometer ŽESP 300E, Bruker, Karlsruhe, Germany. as well as a 1024 diode array UV–visible spectrometer ŽINSTASPEC II, LOT Oriel, Darmstadt, Germany. as reported in a former paper w64x. The UV–vis spectrometer was used in the kinetic mode with external trigger using a integration time of 80 ms. The 75-W Xe lamp as a light source and the spectrometer were connected by optical wave guides to the modified optical ESR cavity OR280 ŽBruker, Karlsruhe, Germany. with its special adapters for this application w63x. As reference electrode, a silver chloride coated silver wire in the same electrolyte as used for the spectroelectrochemical measurement was fixed in a thin flexible Teflon tube Žouter diameter less than 300 mm.. One end of the tube was located
A. Neudeck et al.r Synthetic Metals 107 (1999) 143–158
145
close to the active electrode surface of the laminated gold-m-mesh inside the flat cell w64x like a Luggin capillary. The other end outside the cell was closed by a valve to be able to fill the tube with electrolyte from the flat cell by using the inert gas pressure inside the flat cell as already shown in Ref. w64x. Outside the electrochemical flat cell, a glass capillary filled with magnesium oxide containing traces of Mn2q was fixed. These Mn2q sample was calibrated with a Strong Pitch ŽVarian. in a double rectangular cavity TE104 ŽBruker, Karlsruhe, Germany. to determine the absolute numbers of the electrochemical generated spins in the polymer layer. A black mask with a rectangular hole smaller than the active electrode surface of the laminated Au-m-mesh was fixed on the flat cell. Thus, the replacement of the electrode and the re-positioning to the same position without changing the baseline of the UV–vis spectrum can be done. This was checked several times by recording the reference spectrum with the uncoated mesh inside the flat cell, removing and replacing the electrode to the same position and recording a spectrum using the former recorded reference spectrum. In each case, the derivation of the zero line had a maximum error of less than 0.05 absorbance units. Thus, the deposition of the polyaniline film is possible outside the cell under standard conditions used in former investigations w61,65,66x. In the same experiment, an ESR-spectrum was recorded as reference spectrum to separate the ESR signal of the polymer layer from the signal of the manganese sample. 2.2. Preparation of the laminated gold-m-mesh electrodes Commercially available thermal lamination foils Žlaminating pouches DOCUSEAL 100 mmr5 Mil, General Binding, Northbrook, IL, USA. which are resistant against solvents as acetonitrile ŽAN., dimethylsulfoxide ŽDMSO., dimethylformamide ŽDMF. and others were used to give gold-m-meshes a sufficient mechanical stability for stable use as optical transparent electrodes with insulated edges. Thermal lamination of the gold-m-mesh Žgold mesh LS148120 B G, Goodfellow, Cambridge, UK. are described elsewhere w64x. The electrode surface was polished in an ultrasonic bath in a solution of diamond suspension ŽMetadi 0.25 mm, Wirtz Buehler, Dusseldorf, Germany. and was found have a rectangular surface of 0.86 mm = 4.19 mm as determined ¨ by microscopy. 2.3. Chemicals, solution and preparation conditions of the PAni layer Acetonitrile ŽAN. and sulphuric acid were used as purchased of analytical grade ŽFluka.. Water was bidistilled before use. Solutions were prepared under inert conditions. The solvent reservoir bottle and test tubes Žall equipped with septa. were directly connected via stainless steel transfer needles. The inert gas Žnitrogen 5.0, over pressure p - 500 hPa. was used to deaerate and to move the solutions from the reservoir of the solvent over molecular sieve to 4 ml special test tubes to prepare the solution and transfer it via a flexible Teflon tube w67x into the spectroelectrochemical flat cell containing the laminated gold-m-mesh working electrode w63x. The electropolymerisation outside the spectroelectrochemical cell was carried out in a 70 mM solution of aniline in 1 M sulphuric acid in a mixture of acetonitrile and bidestilled water in the ratio 1:1 cycling the potential range from 0 to 850 mV vs. SCE and one initial scan between 0 and 1000 mV. The polymerisation was stopped after 15 cycles as used in former investigations of electrochemically prepared PAni layers w61x. The thickness of the PAni layer was estimated under the microscope by comparing the coated mesh with an uncoated mesh. A thickness of the carefully dried PAni layer was estimated to be 0.25 mm, whereas the images of the Au-m-mesh with a Pani layer after removing from the electrolyte clearly shows that the 11 mm = 11 mm meshes are completely filled with swelled polymer.
3. Spectroelectrochemical characterisation of the Pani layer on the Au-m-mesh electrode Fig. 1 shows a schematic drawing of the Au-m-mesh. If a polymer thickness of approximately a quarter of a micrometer is considered the amount of monomer units in the film can be estimated by using an approximated density of the Pani layer of 1 grcm3. The estimation that one redox segment corresponds to 10 monomer units results on electrochemical quartz crystal micro balance investigations and the comparison of the polymerization charge with the redox charge of 1:10 w60x based on a two electron transfer per segment and using the Faradaic law which can be written as m deposited s
Q polym . 2F
Mmonomer units s
with Q polym .s Qdeposition y Qredox
Q redox 2F
Msegment f
Qredox 2F
Ž 10 Mmonomer units . s
Qredox 0.2 F
Mmonomer units
146
A. Neudeck et al.r Synthetic Metals 107 (1999) 143–158
Fig. 1. Schematic drawing of the Au-m-mesh coated with a PANI layer for the definition of volumes and areas at the electrode.
If the extension of a radical cation Žor polaronic state. on the polymer chain has the same extension the maximal numbers of redox segments of the film can be estimated to be n8 s 2 nmol s 1.2 10 15 segments. After preparation of the Pani layer, the electrode was fixed in the spectroelectrochemical cell and several cyclovoltammograms with scan rates in the 2–20 mVrs range and simultaneously 21 UV–vis and ESR spectra were recorded Žcf. Fig. 2.. The Mn2q ESR lines permit the determination of Q-factor changes and therefore the determination of the absolute spin numbers generated in the PAni film during electrochemical oxidation by comparison with the Strong Pitch sample. The shape of the ESR spectra and the line width do not change during electrochemical oxidationrreduction of the PAni film at low modulation amplitudes. For the determination of the time dependence of the free spins in the PAni layer, the ESR intensity of the overmodulated ESR signal at constant magnetic field conditions was followed to get a continuously recorded ESR intensity–timerpotential-dependence. For calibration, the calculation of the double integral of the ESR-signal under same conditions including modulation amplitude was done. Spectrocyclovoltammetric experiments were carried out by following the over-modulated ESR intensity continuously and recording of 81 UV–vis spectra simultaneously.
-100
0
100
200
300
400
-100
0
Potential I m V
0
100
200
200
300
400
300
Potential I m V
-0.0105
400
-100
0
100
200
300
400
Potential I m V
10
3
Magnetic Field 1Gauss
Magnetic Field 1 Gauss
-0.0105
10
3
Magnetic Field 1 Gauss
Trigger
Trigger 3560
A. Neudeck et al.r Synthetic Metals 107 (1999) 143–158
-100
100
Potential I m V
Trigger
Trigger Magnetic Field 1Gauss
3560
Fig. 2. ESR–UV–vis spectrocyclovoltammogram of a Pani layer Ž df 25 mm. on a laminated Au-m-mesh Ž0.86 mm=4.19 mm. in a quartz flat cell with a slit of 0.5 mm. Ža. Cyclovoltammetric curve with trigger markers for the ESR and the UV–vis measurement. The background curve was recorded without a polymer layer under the same conditions in the flat cell with a scan rate of 4.5 mVrs. Žb. Integrated cyclovoltammetric curve vs. time with and without correction of the experimental determined background current with trigger markers. Žc. UV–vis spectra recorded during the cyclovoltammetric scan. Žd. ESR spectra recorded during the cyclovoltammetric scan Žsuperimposed by two lines of the Mn standard sample for calibration.. Že. ESR spectra of the PAni layer from the Mn signal separated by subtraction of an ESR spectrum recorded under the same condition with the uncoated Au-m-mesh. Žf. ESR–intensity–potential plot at a field of 3461G ŽFig. 3e. for the change of the spin concentration in the polymer layer during recording the cyclovoltammetric curve. 147
A. Neudeck et al.r Synthetic Metals 107 (1999) 143–158
148
4. Results and discussion Our model for the analysis is as follows: the reduced initial state of the polymer chain should consist of segment A on the polymer chain, which can be oxidized by subtraction of one electron to be transferred to a radical or polaronic segment B and by transfer of two electrons into the segment C. The protonation equilibria are assumed in a first approximation to be concerned with the electron transfer. A homogeneous redox process, the comproportionation equilibrium inside the polymer film, is taken into account additionally and results in the following model. The proposed model for the analysis of the spectrocyclovoltammetric data is based only on thermodynamic conditions, but no kinetic restrictions are taken into account. Therefore, it cannot describe the cyclovoltammetric current potential curve. The given model without any charge transfer kinetics, neglecting also the kinetic of the protonationrdeprotonation, yields completely symmetric current peaks without the capacitive current step at the switching potential and the hysteresis in the charge potential curve. As the protonationrdeprotonation equilibria and the charge transfer kinetics are not introduced to the model, the hysteresis of the cyclovoltammetric data cannot be described. But for the analysis of the spectroelectrochemical data, it is, first of all, only important to know that the ‘‘capacitive’’ current at low scan rates in the range of a few millivolts per second is mainly caused by a Faradaic process. By application of this simplified model Žcf. Scheme 1., the normalized charge passing the polymer layer can be expressed as the sum of the generated amount of segments of the first oxidation state B and those of the second oxidation state C, multiplied by two according to the two-electron transfer QŽ t . F
s n B Ž t . q 2 nC Ž t .
Ž 1.
The timerpotential dependencies of the amount of generated states B and C are given by the timerpotential dependence of the calibrated ESR signal and the charge passed according to n B Ž t . s ´ ESR SESR , Ž t . s
nC Ž t . s
1 QŽ t . 2
ž
F
SESR , Ž t .
Ž 2.
FESR
y ´ ESR SESR Ž t .
/
Ž 3.
where ´ ESR s Ž1rFESR . is the calibration factor of the ESR signal and has to be determined versus a known sample. To get the timerpotential dependence of the initial state A it is necessary to know the initial amount n8 of segments A in the film. The microscopic estimation of the thickness of PAni layer gives rise of a larger error and can only used to control the
Scheme 1. Simplified chargerdischarge process based on three oxidation states of the polymer chains from the thermodynamical point of view. Possible protonationrdeprotonation reaction are assumed to be concerned with the charge transfer.
A. Neudeck et al.r Synthetic Metals 107 (1999) 143–158
149
determined values in the orders of magnitudes. Especially, the determination of the numbers of redox active segments is connected with a large experimental error Žsee Fig. 3.. Considering the comproportionation equilibrium K comp s
nB Ž t .
2
Ž 4.
nA Ž t . n C Ž t .
the total amount of segments and the equilibrium constant are available by neglecting interactions of the oxidized segments that have a density of more than 10 20 segmentsrcm3 or in concentration more than 1 M. These interactions are responsible for changing the driving force Žthe free molar enthalpy and therefore the standard potential. of the oxidation process and can be described by the model of Paasch et al. w46x. The interactions of the positive charged segments show a similar influence on the formal potential of the polaronic and of the bipolaronic state which keeps the difference of the formal potential in a first approximation constant, and therefore the equilibrium constant of the comproportionation equilibrium. The sum of the amounts of all redox segments of the polymer layer are constant during the electrochemical chargerdischarge process n8 s nA Ž t . q n B Ž t . q n C Ž t .
Ž 5.
Combining of Eqs. Ž4. and Ž5. yields 2
n B Ž t . s K comp n8n C Ž t . y K comp Ž n B Ž t . n c Ž t . q n c Ž t .
2
Ž 6.
.
This can be generally written as: z Ž t . s k1 x Ž t . q k 2 y Ž t . with 2
2
z Ž t . s n B Ž t . , x Ž t . s n c Ž t . and y Ž t . s n B Ž t . n c Ž t . q n c Ž t . . Thus, the determination of the constants k 1 and k 2 by a linear fit w67–69x is possible. By these constants, the equilibrium constant of the comproportionation equilibrium and the total amount of segments n8 can be calculated to be K com s 0.232
Fig. 3. Potential dependence of the three postulated redox states A, B and C of PAni during the spectrocyclovoltammetric experiment Žcf. Fig. 2. as determined from the time dependence of the charge and the ESR intensity normalized to the total amount of spins.
A. Neudeck et al.r Synthetic Metals 107 (1999) 143–158
150
X X Žwhich corresponds to a formal potential difference E8B,C y E8A,B s y38 mV. and n8 s 1.79 nmol. The value of the equilibrium constant corresponds to a formal potential difference of D E s y38 mV of the first and the second electron transfer. Therefore, the second step Žreaction from B to C. is energetically favoured. This value should be treated critically because of the large experimental error and a protonation equilibria assumed to be concerned with the charge transfer. Then the deprotonated product of the first electron transfer should easier to be oxidized. The results correspond to a scheme of an EC eq E process with a fast chemical equilibrium. The determined number of segments Žca. 1.8 nmol. is in the same order as the estimated number of segments from the microscopic estimated thickness Ž0.25 mm. of the deposited layer under dried conditions Ž2 nmol.. The determined value of n8 permits the calculation of the time dependence of the total amount of states A, B and C inside the PAni film during recording a cyclic voltammogram. If it is taken into account further that the resulting absorbance is a linear combination of three components or oxidation states, the Lambert–Beer law can be written:
Abs Ž l ,t . s ´˜A Ž l . nA Ž t . q ´˜ B Ž l . n B Ž t . q ´˜ C Ž l . n C Ž t .
Ž 7.
with ´˜ i s ´ i Ž d pathrVlayer . and ´ i , d path , V layer stands for the absorbance coefficient of the species i and d for the path length of the beam through the polymer layer and its volume. Eq. Ž7. holds only if the meshes of the electrode are completely covered with the PAni layer and the time scale of the experiment is long enough to neglect a concentration gradient inside a single mesh filled with polymer perpendicular to the light beam. If the film is thinner than half of the mesh size as observed from the microscopic images for a dried layer, the absorbance observed has to be corrected to get that absorbance of the layer to fulfill the Lambert–Beer law:
Abs layer s ylog 10
ž
10yAbs detected y 1 y
A layer A mesh_opening
A layer A mesh_opening
/
0
2 . for a single mesh opening of with A layer s A mesh_opening s 11 mm = 11 mm s 121 mm2 and A layer s 4Ž11 mm d layer y d layer 11 mm = 11 mm. For the proposed mechanism, the time dependence of the absorbance at each wavelength should be described by a linear combination of the time dependence of the three redox states A, B and C determined from ESR signal and the charge passed. In this way, the relative absorbance coefficients ´˜ i Ž l. at each wavelength for each species are available. Each ´˜ i Ž l. dependence represents the separated spectra of one of the three redox states of the polymer Žcf. Fig. 4.. The separation of the UV–vis spectra by using the time dependence of the charge and the ESR signal were done at first for the uncorrected absorbance and for different ratio of A layerrA mesh_opening beginning at the lowest possible value corresponding to the maximum value of the observed absorbance up to 1. The square least sum was calculated for each analysis and should have a minimum at the real thickness of the film. In contrast, we found the lowest square least sum for a ratio A layerrA mesh_opening of 1 Žcf. Fig. 5.. A film thickness of more than 3 mm can be estimated from the maximum value of the observed absorbance values in the range of 0.2. It is concluded that the PAni layer is swelled in contact with the solvents Žwater:acetonitriles 1:1.. Therefore, the meshes must be completely filled with polymer under these conditions, and a linear relation of the amount of substance and the absorbance observed is valid. This results are in agreement with the microscopic image of a freshly prepared PAni layer on the gold mesh one can get in the time scale of 1 h after preparation. As a result of the numerical separation of the UV–vis spectra using the ESR signal and the charge passed, the spectrum of the state B shows negative values in the 550–700 nm range. It seems to be obvious that intermediate redox states are responsible for this behaviour. However, if the absorbance is calculated from ´˜ B Ž l. at the time of the highest concentration of B, the negative value corresponds to y0.012 absorbance units. Therefore, the absolute value is smaller than the shift in the baseline observed by a replacement of the electrode. With the determined time dependence ŽFig. 4. and the separated spectra of each oxidation state A, B and C of the polymer layer, the portion on the total change of the absorbance can be calculated Žcf. Fig. 6.. The sum of the portions of the separated spectra subtracted from the experimental spectra yields an absolute error of less than 0.01 absorbance units over the whole potential range ŽFig. 6d.. The same result was found by following the ESR intensity and using the calibration factor to get the total amount of spins and by increasing the number of recorded UV–vis spectra for scan rates from 5 mVrs up to 20 mVrs as it is shown in Fig. 7. The 2d plot ŽFig. 7b. of the experimental UV–vis spectra recorded during a cyclic voltammetric scan shows a behaviour like a ‘‘hypsochromic shift’’ in the 600–800 nm range at the laminated Au-m-mesh electrode. This behaviour was also
A. Neudeck et al.r Synthetic Metals 107 (1999) 143–158
151
Fig. 4. Separated UV–vis spectra of the redox states A, B and C in PAni using the calculated amount of substance vs. potential dependencies of Fig. 3 and the measured UV–vis spectra ŽFig. 2c.. Ža. absorption ´ U -wavelength plot n ´ ´ Abs s ´ dc s ´ d s n s ´ U n, ´ U s . Ad A A
observed on ITO electrodes w57x and by the oxidation of reduced PAni layers under air w70x. As the simultaneous use of in situ ESR and UV–vis spectroscopy permits the separation of the superimposed spectra, their comparison ŽFig. 7a and c. clearly shows that the so called ‘‘hypsochromic shift’’ results from the superimposed spectra of the redox states of A, B and C. There is no need to discuss further intermediate oxidation states than the three mentioned. The separation of the UV–vis spectra based on the charge passed and the ESR signal to separate the polaronic state which was done using the polaronrbipolaron model w71x can also be applied for the polaronrs-dimer-dication w62x model, which describes the interactions of polaronic states between the polymer chains discussed in former papers w72–77x as a reversible dimerisation. Considering the redox segments of equal length for all redox states on the chains and only for the s-dimer-dication between two chains, the double extensions it was found by using the comproportionation equilibrium that the amount of the starting reduced state on the chains tends to zero at the switching potential as expected. Therefore, the analysis of the data can simplified by determination of n8 at the switching potential based on the boundary condition nAŽ Esw . s 0.
Fig. 5. Square least sum of the fit of the absorbance–time curves by the charge and the ESR signal for corrected absorbance values corresponding to the thickness of the polymer layer inside the meshes. Ž d layer s 5.5 corresponds to the uncorrected absorbance values..
152 A. Neudeck et al.r Synthetic Metals 107 (1999) 143–158 Fig. 6. Dependencies of the separated spectra of the three redox states B and C in PAni during recording the spectrocyclovoltammogram ŽFig. 2.. Ža. Portion of the educt A Žthe reduced film.. Žb. Portion of the radical cation or polaronic state B. Žc. Portion of the product of the dication or bipolaronic state C. Žd. Difference of the sum of the portions Fig. 6a–c to the measured spectra.
A. Neudeck et al.r Synthetic Metals 107 (1999) 143–158 Fig. 7. ESR–UV–vis spectrocyclovoltammogram of a Pani layer on a laminated Au-m-mesh Ž0.86 mm=4.19 mm. in a quartz flat cell in 0.1 M H 2 SO4 ŽH 2 O:ACNs1:1. with a slit of 0.5 mm with 81 equidistant triggered UV–vis spectra and simultaneously followed ESR intensity at constant field Ž3461G.. Ža. Separated UV–vis spectra of the redox states A, B and C calculated from the potential dependence of the redox states and the UV–vis spectra using Eq. Ž7.. Žb. 2d plot of the experimental UV–vis spectra. Žc. 2d plot of the calculated UV–vis spectra from the time dependence of the three separated redox species.
153
A. Neudeck et al.r Synthetic Metals 107 (1999) 143–158
154
Under this condition the comparison of both models can be written as:
|| |
||
Ža. PolaronrBipolaron y A ye qe y B ye y B qe y C A q C 2B A . . . segment of the reduced film B . . . polaronic segment C . . . bipolaronic segment
Žb. PolaronrDimer y A ye qe y B 2B D
a. PolaronrBipolaron Ž1. Total amount of segments n8:
b. PolaronrDimer
A . . . segment of the reduced film B . . . polaronic segment C . . . dimer of two polaronic segments
n8 s nAŽ t . q n B Ž t . q n C Ž t .
Ž8a.
n8 s nAŽ t . q n B Ž t . q 2 n D Ž t .
Ž8b.
Ž9a.
n8 s n B Ž Esw . q 2 n D Ž Esw .
Ž9b.
Concerning nAŽ Esw . s 0 it yields: n8 s n B Ž Esw . q n C Ž Esw .
Ž2. Time dependence of the amount of polaronic states: nBŽ t . s
S ESR Ž t . FESR
Ž10a.
nBŽ t . s
Ž11a.
nC Ž t . s
S ESR Ž t .
Ž10b.
FESR
Ž3. Time dependence of the third redox state: nC Ž t . s
1 2
ž
QŽ t .
y
S ESR Ž t .
F
FESR
/
1 2
ž
QŽ t .
y
F
S ESR Ž t . FESR
/
Ž11b.
Ž4. Time dependence of the reduced segment A resulting from Eqs.Ž8a., Ž8b., Ž9a., Ž9b., Ž10a., Ž10b., Ž11a. and Ž11b.: nAŽ t . s
1 2
ž
Q Ž Esw .
1 y 2
q
F
ž
S ESR Ž Esw t . FESR
QŽ t .
q
F
S ESR Ž t . FESR
/
nAŽ t . s
/
Q Ž Esw . F
y
QŽ t . F
Ž12b.
Ž12a.
In both cases, the time dependence of the absorbance at each wavelength based on Lambert–Beer law Žcf. Eq. Ž7.. can be written as a linear combination of the amount of substances of three states A, B and C for the polaronrbipolaron model
Ž a . Abs Ž l ,t . s ´˜A Ž l . nA Ž t . q ´˜ B Ž l . n B Ž t . q ´˜ C Ž l . n C Ž t .
Ž 13a.
as well as of A, B and D for the polaronrdimer model
Ž b . Abs Ž l ,t . s ´˜A Ž l . nA Ž t . q ´˜ B Ž l . n B Ž t . q ´˜ D Ž l . n D Ž t . .
Ž 13b.
The separation of the superimposed spectra of the three states A, B and C as well as A, B and D can be done using the same linear fit procedure as described above. The resulting spectra of the three states for each model differs only a little and the square sum of errors is identical. The identical square least sum can be explained by replacing the amount of substances in Eqs. Ž13a. and Ž13b. by the expressions of Eq. Eqs. Ž9a., Ž9b., Ž10a., Ž10b., Ž11a., Ž11b., Ž12a., Ž11b., Ž12a. and Ž12b. which results in the polaronrbipolaron mode in Abs Ž l ,t . s
´˜A Ž l . 2 q
ž
Q Ž Esw .
q
F
S ESR Ž Esw . FESR
y´˜A Ž l . q 2 ´˜ B Ž l . y ´˜ C Ž l . 2 FESR
/
q
Ž y´˜A Ž l . q ´˜ C Ž l . . 2F
SESR Ž t .
QŽ t .
Ž 14a.
A. Neudeck et al.r Synthetic Metals 107 (1999) 143–158
155
Scheme 2. Chargerdischarge mechanism of polyaniline including the acidrbase equilibria of each oxidation state.
and in the case of the polaronrdimer model in Abs Ž l ,t . s
´˜A Ž l . Q Ž Esw . 2F
q
Ž y2 ´˜A Ž l . q ´˜ D Ž l . . 2F
QŽ t . q
y2 ´˜A Ž l . q 2 ´˜ B Ž l . q ´˜ D Ž l . 2 FESR
SESR Ž t .
Ž 14b.
A. Neudeck et al.r Synthetic Metals 107 (1999) 143–158
156
Eqs. Ž14a. and Ž14b. show in both cases that the time dependence of the absorbance at each wavelength is a linear combination of the charge passed and the ESR signal vs. time which can be written as Abs Ž l ,t . s k 0 q k 1 Ž l . Q Ž t . q k 2 Ž l . s ESR Ž t . and the linear fit yields independent of the used model the constants k 0 , k 1 and k 2 . The two models differ only in the interpretation of the constants determined: Ža. PolaronrBipolaron k0 s
k1 s
k2 s
´˜A Ž l . 2
ž
Q Ž Esw .
Žb. PolaronrDimer q
F
S ESR Ž Esw . FESR
Ž y´˜A Ž l . q ´˜ C Ž l . . 2F y´˜A Ž l . q 2 ´˜ B Ž l . y ´˜ C Ž l . 2 FESR
/
k0 s
k1 s
k2 s
´˜A Ž l . Q Ž Esw . 2F
Ž y2 ´˜A Ž l . q ´˜ D Ž l . . 2F y2 ´˜A Ž l . q 2 ´˜ B Ž l . q ´˜ D Ž l . 2 FESR
.
Comparing both models, the fit of the data shows the same square least sum and yields at the same time to small differences in the separated spectra for the three redox states by using the different models. The decision which model fits better with the experimental results cannot be done on the basis of the accuracy of fit. The discussion of the shape of the separated spectra using the different models may allow the preference of one of the model. For both models the separated spectra of the polaronic state shows negative values of the absorbance coefficient in the range 460–620 nm range. The range and the absolute values become larger by using the polaronrdimer model, but not significantly.
5. Conclusions Simultaneous spectroelectrochemical studies result in mechanism of charging and discharging of conducting polymers where three oxidation states of the PAni layer A, B and C are involved. The initial state A is the protonated PAni in 0.1 M sulphuric acid. The oxidation state B is characterized by a free spin and a positive charge if the electron transfer is connected with a deprotonation step Žcf. Scheme 2.. The estimation of the number of monomer units in the deposited layer corresponds to a delocalization of this state over ca. 10 monomer units. The third state is connected with a two electron transfer product of the polymer segment on the chain. The determined equilibrium constant K comp for the comproportionation reaction AqC
| 2B
of 0.232 corresponds to a difference of the standard potentials on D E s y38 mV and shows the second electron transfer energetically to be favoured. As the first electron transfer is connected with a deprotonation of the polymer segment Žcf. Scheme 2. that is the reason of the ‘‘inverted’’ formal potentials of E8A,B and E8B,C Ž E8B,C y E8A,B s y38 mV.. The spectra of each oxidation state in their time dependence determined from the charge flown and the calibrated ESR-signal results in a ‘‘hypsochromic shift’’ of the UV–vis spectra as observed earlier w57x. This is shown in the in situ UV–vis spectra of Fig. 7. It is not necessary to discuss more than three different redox states in PAni during charging and discharging. The full mechanism of charging and discharging must be written as follows. Considering the protonation equilibria differences, the structure of the three separated oxidation states A, B and C of polyaniline are hard to be differentiate. In Scheme 2, the initial polymer chain redox state A is protonated w47x and forms a type of protonated leuco-emeraldine salt. State B can be described as a radical cation state or polaronic state Žspin s 1r2, charge s q1, delocalized over 10 monomer units. and C corresponds to the protonated chinoid structure which is a bipolaronic state. A deprotonated form which is not bipolaronic is less probable. The p K a value of the protonation equilibria and the protonation kinetics may explain the hysteresis in the charge–potential curve as well as the different shapes of the forward and backward scan. The change of the pH value inside of the polyaniline layer has an influence on the follow-up equilibrium of the heterogeneous charge transfer. If we include the protonation equilibrium into the Nernst equation, the resulting formal potential shifts with the level of doping. The same effect was introduced by Paasch et al. w46x by application of the soliton model on polaronic and bipolaronic states resulting in a shift of the formal potentials with the doping level. Thus, it is proved in this work by the in situ ESR–UV–Vis spectroelectrochemistry that the capacitive current
A. Neudeck et al.r Synthetic Metals 107 (1999) 143–158
157
plateau is caused by a Faradaic process as in the potential region of this plateau further quinoid polymer structures are formed. There are three phenomena giving a shift of the driving force during the charging process: Ž1. a distribution of different conjugation length in the electrochemical prepared conductive polymers due to different structural units differing from the linear model; Ž2. the interaction of polaronic and bipolaronic segments on the chain; and Ž3. the acidrbase equilibria connected with a change of the pH value inside the polymer layer during charging. The third effect should be depressed by use of low pH values but the current plateau does not disappear by use of 0.1 M sulphuric acid. But the two processes cannot describe the large peak separation in the cyclic voltammograms and the hysteresis in the spectroscopic signals. That means they are superimposed by kinetic processes where the propagation of the conductivity area in to the polymer layer w17–35x and the exchange of the counter ions w11–16x should play an important role and shows the chargingrdischarging of PAni as a very complex process.
Acknowledgements The authors gratefully acknowledge grants from the Deutsche Forschungsgemeinschaft ŽSFB 287.. We also thank Dr. G. Wendrock ŽIFW-Dresden, Germany. for the microscopic studies of the coated mesh electrodes and Prof. G. Paasch ŽIFW-Dresden, Germany. for discussion.
References w1x w2x w3x w4x w5x w6x w7x w8x w9x w10x w11x w12x w13x w14x w15x w16x w17x w18x w19x w20x w21x w22x w23x w24x w25x w26x w27x w28x w29x w30x w31x w32x w33x w34x w35x w36x w37x w38x
A.T. Hubbard, F.C. Anson, J. Electroanal. Chem. 54 Ž1970. 129. R.F. Lane, A.T. Hubbard, J. Phys. Chem., 77 Ž1978. 1401. E. Laviron, J. Electroanal. Chem., 100 Ž1979. 263. H. Angerstein-Kozlowska, J. Klinger, B.E. Conway, J. Electroanal. Chem., 75 Ž1977. 41. H. Angerstein-Kozlowska, J. Klinger, B.E. Conway, J. Electroanal. Chem. 75 Ž1977. 61. E. Laviron, J. Electroanal. Chem. 52 Ž1974. 355. E. Laviron, J. Electroanal. Chem. 52 Ž1974. 395. A.T. Hubbard, J. Electroanal. Chem. 22 Ž1968. 165. H. Angerstein-Kozlowska, B.E. Conway, J. Klinger, J. Electroanal. 87 Ž1978. 301. H. Angerstein-Kozlowska, B.E. Conway, J. Klinger, J. Electroanal. 87 Ž1978. 321. F.B. Kaufmann, A.H. Schroder, E.M. Engler, S.R. Kramer, J.Q. Chambers, J. Am. Chem. Soc. 102 Ž1980. 483. ¨ P. Daum, J.R. Lenhard, D.R. Rolison, R.W. Murray, J. Am. Chem. Soc. 102 Ž1980. 4649. P. Daum, R.W. Murray, J. Phys. Chem. 85 Ž1981. 389. K. Doblhofer, K. Braun, R. Lange, J. Electroanal. Chem. 206 Ž1986. 93. J. Heinze, R. Bilger, Ber. Bunsenges. Phys. Chem. 97 Ž1993. 502. M.A. Vorotyntsev, E. Vieil, J. Heinze, Russ. J. Electrochem. 31 Ž1995. 1027. K. Aoki, T. Aramoto, Y. Hoshino, J. Electroanal. Chem. 340 Ž1992. 127. K. Aoki, Y. Tesuka, J. Electroanal. Chem. 267 Ž1989. 35. K. Aoki, Y. Tesuka, K. Shinozaki, H. Sato, D. Kagatu, J. Electrochem. Soc. Jpn. 57 Ž1989. 397. K. Aoki, J. Cao, Y. Hoshino, Electrochim. Acta 38 Ž1993. 1711. K. Aoki, J. Electroanal. Chem. 373 Ž1994. 67. Y. Tesuka, T. Ishii, K. Aoki, J. Electroanal. Chem. 402 Ž1996. 161. K. Aoki, M. Kawase, J. Electroanal. Chem. 377 Ž1994. 125. K. Aoki, J. Cao, Y. Hoshino, Electrochim. Acta 39 Ž1994. 2291. M. Vuki, M. Kalaji, L. Nyholm, L.M. Peter, J. Electroanal. Chem. 332 Ž1992. 315. M. Vuki, M. Kalaji, L. Nyholm, L.M. Peter, Synth. Met. 55–57 Ž1993. 1515. J.C. Lacroix, K. Kanazawa, A.F. Diaz, J. Electrochem. Soc. 5 Ž1989. 136. J.C. Lacroix, Dissertation, Universitat ¨ Paris 11, Paris, 1990. O. Genz, Dissertation, Universitat 1995. ¨ Dusseldorf ¨ O. Genz, M.M. Lohrengel, J.W. Schultze, Fast Charge–Discharge Kinetics in Intrinsically Conducting Polymers — Intercalation and Film Relaxation, in: Proceedings NATO-ARW XX, Kluwer Academic Publishers, 1996, 321. O. Genz, M.M. Lohrengel, J.W. Schultze, Electrochim. Acta 39 Ž1994. 179. O. Genz, M.M. Lohrengel, Mater. Sci. Forum 191 Ž1995. 207. M.M. Lohrengel, O. Genz, Mechanism of redox process of conducting polymers, Ionics 1 Ž1995. 304. S. Winkels, M.M. Lohrengel, Electrochim. Acta 42 Ž1997. 3117. K. Meeholz, J. Heinze, Angew. Chem. 102 Ž1990. 695. L. Dunsch, J. Prakt. Chem. 317 Ž1975. 409. A.G. MacDiarmid, The polyanilines: a novel class of conducting polymers, in: W.R. Salaneck, I. Lundstrom, ¨ B. Ranby ŽEds.., Conjugated Polymers and Related Material — The Interconnection of Chemical and Electronic Structure, Oxford Univ. Press, Oxford, 1993, p. 73. A.P. Monkman, D. Bloor, G.C. Stevens, J.C.H. Stevens, J. Phys. Appl. Phys. 20 Ž1987. E227.
158
A. Neudeck et al.r Synthetic Metals 107 (1999) 143–158
w39x M. Angelopoulos, G.E. Asturias, S.P. Ermer, E. Ray, E. Schorr, A.G. MacDiarmid, M. Akhtar, Z. Kiss, A.J. Epstein, Mol. Cryst. Liq. Cryst. 160 Ž1988. 151. w40x C.P. Andrieux, P. Hapiot, J.-M. Saveant, J. Am. Chem. Soc. 112 Ž1990. 243. ´ w41x P. Garcia, J.-M. Pernaut, P. Hapiot, V. Wintgens, P. Valat, F. Garnier, D. Delaboulise, J. Phys. Chem. 97 Ž1993. 513. w42x H. Yang, A.J. Bard, J. Electroanal. Chem. 339 Ž1992. 423. w43x E.M. Genies, J.F. Penneau, M. Lapkowski, A. Boyle, J. Electroanal. Chem. 269 Ž1989. 63. w44x K. Meerholz, J. Heinze, Synth. Met. 57 Ž1993. 5040. w45x K. Yakushi, L.J. Lauchlan, T.C. Clarke, G.B. Street, J. Chem. Phys. 79 Ž1983. 4774. w46x G. Paasch, P.H. Nguyen, A. Fischer, Chem. Phys. 227 Ž1998. 219. w47x Z. Ping, G.E. Nauer, H. Neugebauer, J. Theimer, A. Neckel, Electrochim. Acta 42 Ž1997. 1693. w48x P. Zhao, G.E. Nauer, H. Neugebauer, J. Theiner, J. Electroanal. Chem. 420 Ž1997. 301. w49x Z. Ping, G.E. Nauer, H. Neugebauer, J. Theiner, A. Neckel, J. Chem. Soc. Faraday Trans. 93 Ž1997. 121. w50x T. Moll, J. Heinze, Synth. Met. 55 Ž1993. 1521. w51x L. Dunsch, K.-H. Lubert, Electrochim. Acta Ž1997.. w52x K. Roßberg, G. Paasch, L. Dunsch, S. Ludwig, J. Electroanal. Chem. Ž1997.. w53x F. Garnier, A. Yassar, R. Hajlaoui, G. Horowitz, F. Deloffre, B. Sevet, S. Ries, P. Alnot, J. Am. Chem. Soc. 115 Ž1993. 8716. w54x J. Heinze, Electrochemically conducting polymers, Top. Curr. Chem. 152 Ž1990. 1. w55x P. Bauerle, U. Segelbacher, A. Maier, M. Mehring, J. Am. Chem. Soc. 115 Ž1993. 10217. ¨ w56x S.A. Brazovskii, N.N. Kirova, JETP Lett. 33 Ž1982. 4. w57x V. Brandl, R. Holze, Ber. Bunsenges. Phys. Chem. 101 Ž1997. 251. w58x G. Zotti, G. Schiaron, Synth. Met. 30 Ž1989. 151. w59x C. Visy, J. Lukkari, J. Kankare, J. Electroanal. Chem. 319 Ž1991. 85. w60x A. Thyssen, A. Borgerding, J.W. Schultze, Makromol. Chem., Macromol. Symp. 8 Ž1987. 143. w61x A. Zimmermann, U. Kunzelmann, L. Dunsch, Synth. Met. 93 Ž1998. 17–25. ¨ w62x A. Smie, J. Heinze, Angew. Chem. 109 Ž1997. 375. w63x A. Petr, L. Dunsch, A. Neudeck, J. Electroanal. Chem. 412 Ž1996. 153. w64x A. Neudeck, L. Kress, J. Electroanal. Chem. 437 Ž1997. 141. w65x L. Dunsch, Habilitationsschrift, TU-Dresden, 1995. w66x R. Kessel, Dissertation, Universitat 1990. ¨ Dusseldorf, ¨ w67x A. Neudeck, Habilitationsschrift, TU-Dresden, 1998. w68x A. Neudeck, L. Dunsch, J. Electroanal. Chem. 386 Ž1995. 135. w69x A. Neudeck, L. Dunsch, Electrochim. Acta 40 Ž1995. 1427. w70x J. Yano, K. Terayama, S. Yamasaki, K. Aoki, Electrochim. Acta 44 Ž1998. 337. w71x J.L. Bredas, J.C. Scott, K. Yakushi, G.B. Street, Phys. Rev. B 30 Ž1984. 1023. ´ w72x M.G. Hill, R.K. Mann, L.L. Miller, J.P. Penneau, J. Am. Chem. Soc. 114 Ž1992. 2728. w73x M.G. Hill, J.F. Penneau, B. Zinger, K.R. Mann, L.L. Miller, Chem. Mater. 4 Ž1992. 1106. w74x P. Bauerle, U. Segelbacher, K.-U. Gaudl, D. Huttenlocher, M. Mehring, Angew. Chem. 105 Ž1993. 125. ¨ w75x U. Segelbacher, N.S. Sariciftci, A. Grupp, P. Bauerle, M. Mehring, Synth. Met. 55–57 Ž1993. 4728. ¨ w76x P. Hapiot, P. Audebert, K. Monnier, J.-M. Pernaut, G. Garcia, Chem. Mater. 6 Ž1994. 1549. w77x P. Audebert, P. Hapiot, J.-M. Pernaut, P. Garcia, J. Electroanal. Chem. 361 Ž1993. 283.
The redox mechanism of polyaniline studied by simultaneous ESR–UV–vis spectroelectrochemistry Andreas Neudeck, Andreas Petr, Lothar Dunsch
)
Institut fur und Werkstofforschung, Institut fur Abteilung Elektrochemie und Leitfahige Polymere, Helmholtzstr. 20, ¨ Festkorper¨ ¨ Festkorperforschung, ¨ ¨ D-01069 Dresden, Germany Received 10 August 1998; received in revised form 17 March 1999; accepted 25 June 1999
Abstract The electrochemical redox process to charge and discharge polyaniline layers is studied by simultaneous Electron Spin Resonance ŽESR. and UV–vis spectroscopic measurements. For this purpose, optically transparent electrodes were applied under potential-controlled conditions. The use of a calibrated manganese ESR standard ensures the calculation of the absolute numbers of free spins in the polymer layer during electrochemical doping. A general procedure is given to analyse the simultaneously recorded electrochemical and spectroscopic data. The procedure proposed following the time dependence of charge, ESR absorption and the absorbance demonstrates that the electrochemical chargingrdischarging mechanism of polyaniline can be described on the base of three oxidation states. In this way, the separated UV–vis spectra and the timerpotential dependence of each redox state can be shown. Only little changes are observed in the shape of the separated spectra using the polaronrbipolaron model in comparison to the polaronrs-dimer model. A square ladder scheme of the redox mechanism is discussed, considering neutral, polaronic and bipolaronic states and the protonation equilibria. Furthermore, experimental evidence for the Faradaic nature of the ‘‘capacitive’’ current plateau is presented. q 1999 Elsevier Science S.A. All rights reserved. Keywords: Redox mechanism; Polyaniline; ESR–UV–vis spectroelectrochemistry
1. Introduction Since the increased interest in conducting polymers, both the polymerization and the redox mechanism of doping w1–35x of chemically and electrochemically synthesized conducting polymers w36–39x were studied. Big efforts were done in the chemical synthesis of doped polyacetylene and later in the electrochemical polymerization of aniline, pyrrole, thiophene etc. because of their conductivity, their ion exchange properties, the possibility of charge storage, and the switching of chemical and physical properties of such polymer layers by changing the oxidation state. The knowledge of the polymerization mechanism facilitate w40–43x the synthesis of conductive polymers with improved properties. The main processes of the electrochemical chargerdischarge mechanism in polymers are known. But there is no general model to describe the process and to get quantitative information on the distribution of the conjugation chain length w44x, w45x, the interactions of the generated polaronic and bipolaronic segments on the chains w46x related to a shift in the formal potential dependent on the level of doping, and the exchange of counter ions in connection with the change of acidrbase properties w47–51x of the conductive polymers during doping. The nature of the well known ‘‘capacitive behaviour’’ of charged polymers was not understood for a long time w44,46,52x. This behaviour is attributed to a current plateau behind the voltammetric redox peak during switching of the scan direction and by impedance measurements in this potential range. The high value of this capacity, which is 10,000 times larger than that of the bare electrode, cannot be explained by the extension of the electrochemical double layer due to the porosity of conductive polymer. A porosity below the nanometer scale is needed to get such high values. Recently, there are two models to explain the capacitive behaviour of conducting polymers by a redox current ŽFaradaic current. caused by the
)
Corresponding author. Fax: q49-351-4659-313; E-mail: [email protected]
0379-6779r99r$ - see front matter q 1999 Elsevier Science S.A. All rights reserved. PII: S 0 3 7 9 - 6 7 7 9 Ž 9 9 . 0 0 1 3 5 - 6
144
A. Neudeck et al.r Synthetic Metals 107 (1999) 143–158
distribution of the conjugated chain length w44,45x or by the influence of the doping level on the formal potential in a solid at high concentrations of polaronic and bipolaronic states w46x. Considering the conjugated chain length as statistically distributed w44x and the formal redox potential as indirectly proportional to the chain length as found for oligomers w53–55x, the Faradaic process shows a capacitive behavior that yields a current plateau in the cyclic voltammogram. The Faradaic nature can be described on the other hand by a model of Paasch et al. w46x. This model based on interactions of polaronic and bipolaronic states and on the description of soliton–soliton interactions derived by Brazovskii and Kirova w56x. It results in a modified Nernst equation with formal potentials of the polarons and bipolarons dependent on the oxidation level. Both models assume the ‘‘capacitive’’ behaviour to be mainly caused by the redox process. An extension of the electrochemical double layer into the porous polymer during the transfer of the polymer into the conducting state is not necessarily to be considered. Independent of the distribution of the conjugated chain length or the interactions of the increasing polaronic and bipolaronic states or both, the redox process is responsible for the constant current. As the total charge in the cyclic voltammogram represents a redox current, the ‘‘capacitive’’ current should not be subtracted to get the total amount of electrochemically created oxidation states inside the polymer layer. This is important to know in order to get additional kinetic and thermodynamic data by cyclovoltammetric measurements. It is the aim to distinguish between both effects Ždistribution of chain length and interactions at high doping level. that are superimposed by the heterogeneous charge transfer on the metalrpolymer interface connected with the propagation of the conductive state w17–35x. Therefore, in the diffusion of the counter ions w11–16x into the porous polymer layer and the acidrbase properties, cyclovoltammetric investigations have to be studied by spectroscopic methods w57–59x and with microgravimetic techniques like the electrochemical quartz micro balance technique. From quartz micro balance investigations and from the comparison of the charge for the electrochemical polymerization and the doping charge of the prepared polymer layer, it was found that approximately one positive charge can be stored per five monomer units in most conducting polymers w60,61x. Considering this result and using the polaronrbipolaron model Žwhich corresponds to a two electron transfer per ‘‘redox segment’’ of equal size., the extension of created polaronrbipolaron segments is in the range of 10 monomer units. In case of creating bonds in between the polaronic segments of different polymer chains that correspond to an electron transfer per redox segments, the extension of the polaronic state corresponds to five segments. Recently, a certain blue shift Žhypsochromic shift. was described during electrochemical doping by use of in situ UV–vis spectroelectrochemistry w57x. This result supports a distribution of the conjugated chain length over a large range what is in contrast to the sharpness of the voltammetric peak observed for the chargingrdischarging of polyaniline. That is why additional states are discussed by these authors. Based on a comparative study of the redox behaviour of the monomers and oligomers with a length up to the conjugation length and larger in electrochemically prepared polymer layers, the existence of chemical interactions between the polymer chains creating sigma bonds are discussed by Smie and Heinze w62x. It is necessary to get electrochemical and spectroscopic information on polymer charging at the same time and under the same conditions to distinguish between the effect of the phenomena discussed above and to describe the electrochemical chargerdischarge process of layers of conduction polymers dependent on the nature of the selected polymer. It will be shown that the use of in situ ESR and UV–Vis spectroelectrochemistry w63x at optically transparent laminated gold-m-meshes w64x permit the simultaneous study of layers of conductive polymers at a well controlled electrode potential that is demonstrated for electrochemically deposited polyaniline layers. A procedure is proposed to analyse the time dependence of the charge injected and the spectroscopic signals measured simultaneously to get the time dependence of each postulated oxidation state and the separation of the over modulated UV–vis spectra to get information of the oxidation states of the polymer layer and about the weight of the different phenomena discussed. 2. Experimental 2.1. Equipment The in situ ESR–UV–vis spectrocyclovoltammetric measurements were controlled by a home made program driving a potentiostat ŽPG285, HEKA-Elektronik, Lambrecht, Germany. by AD-DA plug-in boards ŽACAO and ACJr, Strawberrysoft, Sunnyvale, CA, USA. and triggering an ESR spectrometer ŽESP 300E, Bruker, Karlsruhe, Germany. as well as a 1024 diode array UV–visible spectrometer ŽINSTASPEC II, LOT Oriel, Darmstadt, Germany. as reported in a former paper w64x. The UV–vis spectrometer was used in the kinetic mode with external trigger using a integration time of 80 ms. The 75-W Xe lamp as a light source and the spectrometer were connected by optical wave guides to the modified optical ESR cavity OR280 ŽBruker, Karlsruhe, Germany. with its special adapters for this application w63x. As reference electrode, a silver chloride coated silver wire in the same electrolyte as used for the spectroelectrochemical measurement was fixed in a thin flexible Teflon tube Žouter diameter less than 300 mm.. One end of the tube was located
A. Neudeck et al.r Synthetic Metals 107 (1999) 143–158
145
close to the active electrode surface of the laminated gold-m-mesh inside the flat cell w64x like a Luggin capillary. The other end outside the cell was closed by a valve to be able to fill the tube with electrolyte from the flat cell by using the inert gas pressure inside the flat cell as already shown in Ref. w64x. Outside the electrochemical flat cell, a glass capillary filled with magnesium oxide containing traces of Mn2q was fixed. These Mn2q sample was calibrated with a Strong Pitch ŽVarian. in a double rectangular cavity TE104 ŽBruker, Karlsruhe, Germany. to determine the absolute numbers of the electrochemical generated spins in the polymer layer. A black mask with a rectangular hole smaller than the active electrode surface of the laminated Au-m-mesh was fixed on the flat cell. Thus, the replacement of the electrode and the re-positioning to the same position without changing the baseline of the UV–vis spectrum can be done. This was checked several times by recording the reference spectrum with the uncoated mesh inside the flat cell, removing and replacing the electrode to the same position and recording a spectrum using the former recorded reference spectrum. In each case, the derivation of the zero line had a maximum error of less than 0.05 absorbance units. Thus, the deposition of the polyaniline film is possible outside the cell under standard conditions used in former investigations w61,65,66x. In the same experiment, an ESR-spectrum was recorded as reference spectrum to separate the ESR signal of the polymer layer from the signal of the manganese sample. 2.2. Preparation of the laminated gold-m-mesh electrodes Commercially available thermal lamination foils Žlaminating pouches DOCUSEAL 100 mmr5 Mil, General Binding, Northbrook, IL, USA. which are resistant against solvents as acetonitrile ŽAN., dimethylsulfoxide ŽDMSO., dimethylformamide ŽDMF. and others were used to give gold-m-meshes a sufficient mechanical stability for stable use as optical transparent electrodes with insulated edges. Thermal lamination of the gold-m-mesh Žgold mesh LS148120 B G, Goodfellow, Cambridge, UK. are described elsewhere w64x. The electrode surface was polished in an ultrasonic bath in a solution of diamond suspension ŽMetadi 0.25 mm, Wirtz Buehler, Dusseldorf, Germany. and was found have a rectangular surface of 0.86 mm = 4.19 mm as determined ¨ by microscopy. 2.3. Chemicals, solution and preparation conditions of the PAni layer Acetonitrile ŽAN. and sulphuric acid were used as purchased of analytical grade ŽFluka.. Water was bidistilled before use. Solutions were prepared under inert conditions. The solvent reservoir bottle and test tubes Žall equipped with septa. were directly connected via stainless steel transfer needles. The inert gas Žnitrogen 5.0, over pressure p - 500 hPa. was used to deaerate and to move the solutions from the reservoir of the solvent over molecular sieve to 4 ml special test tubes to prepare the solution and transfer it via a flexible Teflon tube w67x into the spectroelectrochemical flat cell containing the laminated gold-m-mesh working electrode w63x. The electropolymerisation outside the spectroelectrochemical cell was carried out in a 70 mM solution of aniline in 1 M sulphuric acid in a mixture of acetonitrile and bidestilled water in the ratio 1:1 cycling the potential range from 0 to 850 mV vs. SCE and one initial scan between 0 and 1000 mV. The polymerisation was stopped after 15 cycles as used in former investigations of electrochemically prepared PAni layers w61x. The thickness of the PAni layer was estimated under the microscope by comparing the coated mesh with an uncoated mesh. A thickness of the carefully dried PAni layer was estimated to be 0.25 mm, whereas the images of the Au-m-mesh with a Pani layer after removing from the electrolyte clearly shows that the 11 mm = 11 mm meshes are completely filled with swelled polymer.
3. Spectroelectrochemical characterisation of the Pani layer on the Au-m-mesh electrode Fig. 1 shows a schematic drawing of the Au-m-mesh. If a polymer thickness of approximately a quarter of a micrometer is considered the amount of monomer units in the film can be estimated by using an approximated density of the Pani layer of 1 grcm3. The estimation that one redox segment corresponds to 10 monomer units results on electrochemical quartz crystal micro balance investigations and the comparison of the polymerization charge with the redox charge of 1:10 w60x based on a two electron transfer per segment and using the Faradaic law which can be written as m deposited s
Q polym . 2F
Mmonomer units s
with Q polym .s Qdeposition y Qredox
Q redox 2F
Msegment f
Qredox 2F
Ž 10 Mmonomer units . s
Qredox 0.2 F
Mmonomer units
146
A. Neudeck et al.r Synthetic Metals 107 (1999) 143–158
Fig. 1. Schematic drawing of the Au-m-mesh coated with a PANI layer for the definition of volumes and areas at the electrode.
If the extension of a radical cation Žor polaronic state. on the polymer chain has the same extension the maximal numbers of redox segments of the film can be estimated to be n8 s 2 nmol s 1.2 10 15 segments. After preparation of the Pani layer, the electrode was fixed in the spectroelectrochemical cell and several cyclovoltammograms with scan rates in the 2–20 mVrs range and simultaneously 21 UV–vis and ESR spectra were recorded Žcf. Fig. 2.. The Mn2q ESR lines permit the determination of Q-factor changes and therefore the determination of the absolute spin numbers generated in the PAni film during electrochemical oxidation by comparison with the Strong Pitch sample. The shape of the ESR spectra and the line width do not change during electrochemical oxidationrreduction of the PAni film at low modulation amplitudes. For the determination of the time dependence of the free spins in the PAni layer, the ESR intensity of the overmodulated ESR signal at constant magnetic field conditions was followed to get a continuously recorded ESR intensity–timerpotential-dependence. For calibration, the calculation of the double integral of the ESR-signal under same conditions including modulation amplitude was done. Spectrocyclovoltammetric experiments were carried out by following the over-modulated ESR intensity continuously and recording of 81 UV–vis spectra simultaneously.
-100
0
100
200
300
400
-100
0
Potential I m V
0
100
200
200
300
400
300
Potential I m V
-0.0105
400
-100
0
100
200
300
400
Potential I m V
10
3
Magnetic Field 1Gauss
Magnetic Field 1 Gauss
-0.0105
10
3
Magnetic Field 1 Gauss
Trigger
Trigger 3560
A. Neudeck et al.r Synthetic Metals 107 (1999) 143–158
-100
100
Potential I m V
Trigger
Trigger Magnetic Field 1Gauss
3560
Fig. 2. ESR–UV–vis spectrocyclovoltammogram of a Pani layer Ž df 25 mm. on a laminated Au-m-mesh Ž0.86 mm=4.19 mm. in a quartz flat cell with a slit of 0.5 mm. Ža. Cyclovoltammetric curve with trigger markers for the ESR and the UV–vis measurement. The background curve was recorded without a polymer layer under the same conditions in the flat cell with a scan rate of 4.5 mVrs. Žb. Integrated cyclovoltammetric curve vs. time with and without correction of the experimental determined background current with trigger markers. Žc. UV–vis spectra recorded during the cyclovoltammetric scan. Žd. ESR spectra recorded during the cyclovoltammetric scan Žsuperimposed by two lines of the Mn standard sample for calibration.. Že. ESR spectra of the PAni layer from the Mn signal separated by subtraction of an ESR spectrum recorded under the same condition with the uncoated Au-m-mesh. Žf. ESR–intensity–potential plot at a field of 3461G ŽFig. 3e. for the change of the spin concentration in the polymer layer during recording the cyclovoltammetric curve. 147
A. Neudeck et al.r Synthetic Metals 107 (1999) 143–158
148
4. Results and discussion Our model for the analysis is as follows: the reduced initial state of the polymer chain should consist of segment A on the polymer chain, which can be oxidized by subtraction of one electron to be transferred to a radical or polaronic segment B and by transfer of two electrons into the segment C. The protonation equilibria are assumed in a first approximation to be concerned with the electron transfer. A homogeneous redox process, the comproportionation equilibrium inside the polymer film, is taken into account additionally and results in the following model. The proposed model for the analysis of the spectrocyclovoltammetric data is based only on thermodynamic conditions, but no kinetic restrictions are taken into account. Therefore, it cannot describe the cyclovoltammetric current potential curve. The given model without any charge transfer kinetics, neglecting also the kinetic of the protonationrdeprotonation, yields completely symmetric current peaks without the capacitive current step at the switching potential and the hysteresis in the charge potential curve. As the protonationrdeprotonation equilibria and the charge transfer kinetics are not introduced to the model, the hysteresis of the cyclovoltammetric data cannot be described. But for the analysis of the spectroelectrochemical data, it is, first of all, only important to know that the ‘‘capacitive’’ current at low scan rates in the range of a few millivolts per second is mainly caused by a Faradaic process. By application of this simplified model Žcf. Scheme 1., the normalized charge passing the polymer layer can be expressed as the sum of the generated amount of segments of the first oxidation state B and those of the second oxidation state C, multiplied by two according to the two-electron transfer QŽ t . F
s n B Ž t . q 2 nC Ž t .
Ž 1.
The timerpotential dependencies of the amount of generated states B and C are given by the timerpotential dependence of the calibrated ESR signal and the charge passed according to n B Ž t . s ´ ESR SESR , Ž t . s
nC Ž t . s
1 QŽ t . 2
ž
F
SESR , Ž t .
Ž 2.
FESR
y ´ ESR SESR Ž t .
/
Ž 3.
where ´ ESR s Ž1rFESR . is the calibration factor of the ESR signal and has to be determined versus a known sample. To get the timerpotential dependence of the initial state A it is necessary to know the initial amount n8 of segments A in the film. The microscopic estimation of the thickness of PAni layer gives rise of a larger error and can only used to control the
Scheme 1. Simplified chargerdischarge process based on three oxidation states of the polymer chains from the thermodynamical point of view. Possible protonationrdeprotonation reaction are assumed to be concerned with the charge transfer.
A. Neudeck et al.r Synthetic Metals 107 (1999) 143–158
149
determined values in the orders of magnitudes. Especially, the determination of the numbers of redox active segments is connected with a large experimental error Žsee Fig. 3.. Considering the comproportionation equilibrium K comp s
nB Ž t .
2
Ž 4.
nA Ž t . n C Ž t .
the total amount of segments and the equilibrium constant are available by neglecting interactions of the oxidized segments that have a density of more than 10 20 segmentsrcm3 or in concentration more than 1 M. These interactions are responsible for changing the driving force Žthe free molar enthalpy and therefore the standard potential. of the oxidation process and can be described by the model of Paasch et al. w46x. The interactions of the positive charged segments show a similar influence on the formal potential of the polaronic and of the bipolaronic state which keeps the difference of the formal potential in a first approximation constant, and therefore the equilibrium constant of the comproportionation equilibrium. The sum of the amounts of all redox segments of the polymer layer are constant during the electrochemical chargerdischarge process n8 s nA Ž t . q n B Ž t . q n C Ž t .
Ž 5.
Combining of Eqs. Ž4. and Ž5. yields 2
n B Ž t . s K comp n8n C Ž t . y K comp Ž n B Ž t . n c Ž t . q n c Ž t .
2
Ž 6.
.
This can be generally written as: z Ž t . s k1 x Ž t . q k 2 y Ž t . with 2
2
z Ž t . s n B Ž t . , x Ž t . s n c Ž t . and y Ž t . s n B Ž t . n c Ž t . q n c Ž t . . Thus, the determination of the constants k 1 and k 2 by a linear fit w67–69x is possible. By these constants, the equilibrium constant of the comproportionation equilibrium and the total amount of segments n8 can be calculated to be K com s 0.232
Fig. 3. Potential dependence of the three postulated redox states A, B and C of PAni during the spectrocyclovoltammetric experiment Žcf. Fig. 2. as determined from the time dependence of the charge and the ESR intensity normalized to the total amount of spins.
A. Neudeck et al.r Synthetic Metals 107 (1999) 143–158
150
X X Žwhich corresponds to a formal potential difference E8B,C y E8A,B s y38 mV. and n8 s 1.79 nmol. The value of the equilibrium constant corresponds to a formal potential difference of D E s y38 mV of the first and the second electron transfer. Therefore, the second step Žreaction from B to C. is energetically favoured. This value should be treated critically because of the large experimental error and a protonation equilibria assumed to be concerned with the charge transfer. Then the deprotonated product of the first electron transfer should easier to be oxidized. The results correspond to a scheme of an EC eq E process with a fast chemical equilibrium. The determined number of segments Žca. 1.8 nmol. is in the same order as the estimated number of segments from the microscopic estimated thickness Ž0.25 mm. of the deposited layer under dried conditions Ž2 nmol.. The determined value of n8 permits the calculation of the time dependence of the total amount of states A, B and C inside the PAni film during recording a cyclic voltammogram. If it is taken into account further that the resulting absorbance is a linear combination of three components or oxidation states, the Lambert–Beer law can be written:
Abs Ž l ,t . s ´˜A Ž l . nA Ž t . q ´˜ B Ž l . n B Ž t . q ´˜ C Ž l . n C Ž t .
Ž 7.
with ´˜ i s ´ i Ž d pathrVlayer . and ´ i , d path , V layer stands for the absorbance coefficient of the species i and d for the path length of the beam through the polymer layer and its volume. Eq. Ž7. holds only if the meshes of the electrode are completely covered with the PAni layer and the time scale of the experiment is long enough to neglect a concentration gradient inside a single mesh filled with polymer perpendicular to the light beam. If the film is thinner than half of the mesh size as observed from the microscopic images for a dried layer, the absorbance observed has to be corrected to get that absorbance of the layer to fulfill the Lambert–Beer law:
Abs layer s ylog 10
ž
10yAbs detected y 1 y
A layer A mesh_opening
A layer A mesh_opening
/
0
2 . for a single mesh opening of with A layer s A mesh_opening s 11 mm = 11 mm s 121 mm2 and A layer s 4Ž11 mm d layer y d layer 11 mm = 11 mm. For the proposed mechanism, the time dependence of the absorbance at each wavelength should be described by a linear combination of the time dependence of the three redox states A, B and C determined from ESR signal and the charge passed. In this way, the relative absorbance coefficients ´˜ i Ž l. at each wavelength for each species are available. Each ´˜ i Ž l. dependence represents the separated spectra of one of the three redox states of the polymer Žcf. Fig. 4.. The separation of the UV–vis spectra by using the time dependence of the charge and the ESR signal were done at first for the uncorrected absorbance and for different ratio of A layerrA mesh_opening beginning at the lowest possible value corresponding to the maximum value of the observed absorbance up to 1. The square least sum was calculated for each analysis and should have a minimum at the real thickness of the film. In contrast, we found the lowest square least sum for a ratio A layerrA mesh_opening of 1 Žcf. Fig. 5.. A film thickness of more than 3 mm can be estimated from the maximum value of the observed absorbance values in the range of 0.2. It is concluded that the PAni layer is swelled in contact with the solvents Žwater:acetonitriles 1:1.. Therefore, the meshes must be completely filled with polymer under these conditions, and a linear relation of the amount of substance and the absorbance observed is valid. This results are in agreement with the microscopic image of a freshly prepared PAni layer on the gold mesh one can get in the time scale of 1 h after preparation. As a result of the numerical separation of the UV–vis spectra using the ESR signal and the charge passed, the spectrum of the state B shows negative values in the 550–700 nm range. It seems to be obvious that intermediate redox states are responsible for this behaviour. However, if the absorbance is calculated from ´˜ B Ž l. at the time of the highest concentration of B, the negative value corresponds to y0.012 absorbance units. Therefore, the absolute value is smaller than the shift in the baseline observed by a replacement of the electrode. With the determined time dependence ŽFig. 4. and the separated spectra of each oxidation state A, B and C of the polymer layer, the portion on the total change of the absorbance can be calculated Žcf. Fig. 6.. The sum of the portions of the separated spectra subtracted from the experimental spectra yields an absolute error of less than 0.01 absorbance units over the whole potential range ŽFig. 6d.. The same result was found by following the ESR intensity and using the calibration factor to get the total amount of spins and by increasing the number of recorded UV–vis spectra for scan rates from 5 mVrs up to 20 mVrs as it is shown in Fig. 7. The 2d plot ŽFig. 7b. of the experimental UV–vis spectra recorded during a cyclic voltammetric scan shows a behaviour like a ‘‘hypsochromic shift’’ in the 600–800 nm range at the laminated Au-m-mesh electrode. This behaviour was also
A. Neudeck et al.r Synthetic Metals 107 (1999) 143–158
151
Fig. 4. Separated UV–vis spectra of the redox states A, B and C in PAni using the calculated amount of substance vs. potential dependencies of Fig. 3 and the measured UV–vis spectra ŽFig. 2c.. Ža. absorption ´ U -wavelength plot n ´ ´ Abs s ´ dc s ´ d s n s ´ U n, ´ U s . Ad A A
observed on ITO electrodes w57x and by the oxidation of reduced PAni layers under air w70x. As the simultaneous use of in situ ESR and UV–vis spectroscopy permits the separation of the superimposed spectra, their comparison ŽFig. 7a and c. clearly shows that the so called ‘‘hypsochromic shift’’ results from the superimposed spectra of the redox states of A, B and C. There is no need to discuss further intermediate oxidation states than the three mentioned. The separation of the UV–vis spectra based on the charge passed and the ESR signal to separate the polaronic state which was done using the polaronrbipolaron model w71x can also be applied for the polaronrs-dimer-dication w62x model, which describes the interactions of polaronic states between the polymer chains discussed in former papers w72–77x as a reversible dimerisation. Considering the redox segments of equal length for all redox states on the chains and only for the s-dimer-dication between two chains, the double extensions it was found by using the comproportionation equilibrium that the amount of the starting reduced state on the chains tends to zero at the switching potential as expected. Therefore, the analysis of the data can simplified by determination of n8 at the switching potential based on the boundary condition nAŽ Esw . s 0.
Fig. 5. Square least sum of the fit of the absorbance–time curves by the charge and the ESR signal for corrected absorbance values corresponding to the thickness of the polymer layer inside the meshes. Ž d layer s 5.5 corresponds to the uncorrected absorbance values..
152 A. Neudeck et al.r Synthetic Metals 107 (1999) 143–158 Fig. 6. Dependencies of the separated spectra of the three redox states B and C in PAni during recording the spectrocyclovoltammogram ŽFig. 2.. Ža. Portion of the educt A Žthe reduced film.. Žb. Portion of the radical cation or polaronic state B. Žc. Portion of the product of the dication or bipolaronic state C. Žd. Difference of the sum of the portions Fig. 6a–c to the measured spectra.
A. Neudeck et al.r Synthetic Metals 107 (1999) 143–158 Fig. 7. ESR–UV–vis spectrocyclovoltammogram of a Pani layer on a laminated Au-m-mesh Ž0.86 mm=4.19 mm. in a quartz flat cell in 0.1 M H 2 SO4 ŽH 2 O:ACNs1:1. with a slit of 0.5 mm with 81 equidistant triggered UV–vis spectra and simultaneously followed ESR intensity at constant field Ž3461G.. Ža. Separated UV–vis spectra of the redox states A, B and C calculated from the potential dependence of the redox states and the UV–vis spectra using Eq. Ž7.. Žb. 2d plot of the experimental UV–vis spectra. Žc. 2d plot of the calculated UV–vis spectra from the time dependence of the three separated redox species.
153
A. Neudeck et al.r Synthetic Metals 107 (1999) 143–158
154
Under this condition the comparison of both models can be written as:
|| |
||
Ža. PolaronrBipolaron y A ye qe y B ye y B qe y C A q C 2B A . . . segment of the reduced film B . . . polaronic segment C . . . bipolaronic segment
Žb. PolaronrDimer y A ye qe y B 2B D
a. PolaronrBipolaron Ž1. Total amount of segments n8:
b. PolaronrDimer
A . . . segment of the reduced film B . . . polaronic segment C . . . dimer of two polaronic segments
n8 s nAŽ t . q n B Ž t . q n C Ž t .
Ž8a.
n8 s nAŽ t . q n B Ž t . q 2 n D Ž t .
Ž8b.
Ž9a.
n8 s n B Ž Esw . q 2 n D Ž Esw .
Ž9b.
Concerning nAŽ Esw . s 0 it yields: n8 s n B Ž Esw . q n C Ž Esw .
Ž2. Time dependence of the amount of polaronic states: nBŽ t . s
S ESR Ž t . FESR
Ž10a.
nBŽ t . s
Ž11a.
nC Ž t . s
S ESR Ž t .
Ž10b.
FESR
Ž3. Time dependence of the third redox state: nC Ž t . s
1 2
ž
QŽ t .
y
S ESR Ž t .
F
FESR
/
1 2
ž
QŽ t .
y
F
S ESR Ž t . FESR
/
Ž11b.
Ž4. Time dependence of the reduced segment A resulting from Eqs.Ž8a., Ž8b., Ž9a., Ž9b., Ž10a., Ž10b., Ž11a. and Ž11b.: nAŽ t . s
1 2
ž
Q Ž Esw .
1 y 2
q
F
ž
S ESR Ž Esw t . FESR
QŽ t .
q
F
S ESR Ž t . FESR
/
nAŽ t . s
/
Q Ž Esw . F
y
QŽ t . F
Ž12b.
Ž12a.
In both cases, the time dependence of the absorbance at each wavelength based on Lambert–Beer law Žcf. Eq. Ž7.. can be written as a linear combination of the amount of substances of three states A, B and C for the polaronrbipolaron model
Ž a . Abs Ž l ,t . s ´˜A Ž l . nA Ž t . q ´˜ B Ž l . n B Ž t . q ´˜ C Ž l . n C Ž t .
Ž 13a.
as well as of A, B and D for the polaronrdimer model
Ž b . Abs Ž l ,t . s ´˜A Ž l . nA Ž t . q ´˜ B Ž l . n B Ž t . q ´˜ D Ž l . n D Ž t . .
Ž 13b.
The separation of the superimposed spectra of the three states A, B and C as well as A, B and D can be done using the same linear fit procedure as described above. The resulting spectra of the three states for each model differs only a little and the square sum of errors is identical. The identical square least sum can be explained by replacing the amount of substances in Eqs. Ž13a. and Ž13b. by the expressions of Eq. Eqs. Ž9a., Ž9b., Ž10a., Ž10b., Ž11a., Ž11b., Ž12a., Ž11b., Ž12a. and Ž12b. which results in the polaronrbipolaron mode in Abs Ž l ,t . s
´˜A Ž l . 2 q
ž
Q Ž Esw .
q
F
S ESR Ž Esw . FESR
y´˜A Ž l . q 2 ´˜ B Ž l . y ´˜ C Ž l . 2 FESR
/
q
Ž y´˜A Ž l . q ´˜ C Ž l . . 2F
SESR Ž t .
QŽ t .
Ž 14a.
A. Neudeck et al.r Synthetic Metals 107 (1999) 143–158
155
Scheme 2. Chargerdischarge mechanism of polyaniline including the acidrbase equilibria of each oxidation state.
and in the case of the polaronrdimer model in Abs Ž l ,t . s
´˜A Ž l . Q Ž Esw . 2F
q
Ž y2 ´˜A Ž l . q ´˜ D Ž l . . 2F
QŽ t . q
y2 ´˜A Ž l . q 2 ´˜ B Ž l . q ´˜ D Ž l . 2 FESR
SESR Ž t .
Ž 14b.
A. Neudeck et al.r Synthetic Metals 107 (1999) 143–158
156
Eqs. Ž14a. and Ž14b. show in both cases that the time dependence of the absorbance at each wavelength is a linear combination of the charge passed and the ESR signal vs. time which can be written as Abs Ž l ,t . s k 0 q k 1 Ž l . Q Ž t . q k 2 Ž l . s ESR Ž t . and the linear fit yields independent of the used model the constants k 0 , k 1 and k 2 . The two models differ only in the interpretation of the constants determined: Ža. PolaronrBipolaron k0 s
k1 s
k2 s
´˜A Ž l . 2
ž
Q Ž Esw .
Žb. PolaronrDimer q
F
S ESR Ž Esw . FESR
Ž y´˜A Ž l . q ´˜ C Ž l . . 2F y´˜A Ž l . q 2 ´˜ B Ž l . y ´˜ C Ž l . 2 FESR
/
k0 s
k1 s
k2 s
´˜A Ž l . Q Ž Esw . 2F
Ž y2 ´˜A Ž l . q ´˜ D Ž l . . 2F y2 ´˜A Ž l . q 2 ´˜ B Ž l . q ´˜ D Ž l . 2 FESR
.
Comparing both models, the fit of the data shows the same square least sum and yields at the same time to small differences in the separated spectra for the three redox states by using the different models. The decision which model fits better with the experimental results cannot be done on the basis of the accuracy of fit. The discussion of the shape of the separated spectra using the different models may allow the preference of one of the model. For both models the separated spectra of the polaronic state shows negative values of the absorbance coefficient in the range 460–620 nm range. The range and the absolute values become larger by using the polaronrdimer model, but not significantly.
5. Conclusions Simultaneous spectroelectrochemical studies result in mechanism of charging and discharging of conducting polymers where three oxidation states of the PAni layer A, B and C are involved. The initial state A is the protonated PAni in 0.1 M sulphuric acid. The oxidation state B is characterized by a free spin and a positive charge if the electron transfer is connected with a deprotonation step Žcf. Scheme 2.. The estimation of the number of monomer units in the deposited layer corresponds to a delocalization of this state over ca. 10 monomer units. The third state is connected with a two electron transfer product of the polymer segment on the chain. The determined equilibrium constant K comp for the comproportionation reaction AqC
| 2B
of 0.232 corresponds to a difference of the standard potentials on D E s y38 mV and shows the second electron transfer energetically to be favoured. As the first electron transfer is connected with a deprotonation of the polymer segment Žcf. Scheme 2. that is the reason of the ‘‘inverted’’ formal potentials of E8A,B and E8B,C Ž E8B,C y E8A,B s y38 mV.. The spectra of each oxidation state in their time dependence determined from the charge flown and the calibrated ESR-signal results in a ‘‘hypsochromic shift’’ of the UV–vis spectra as observed earlier w57x. This is shown in the in situ UV–vis spectra of Fig. 7. It is not necessary to discuss more than three different redox states in PAni during charging and discharging. The full mechanism of charging and discharging must be written as follows. Considering the protonation equilibria differences, the structure of the three separated oxidation states A, B and C of polyaniline are hard to be differentiate. In Scheme 2, the initial polymer chain redox state A is protonated w47x and forms a type of protonated leuco-emeraldine salt. State B can be described as a radical cation state or polaronic state Žspin s 1r2, charge s q1, delocalized over 10 monomer units. and C corresponds to the protonated chinoid structure which is a bipolaronic state. A deprotonated form which is not bipolaronic is less probable. The p K a value of the protonation equilibria and the protonation kinetics may explain the hysteresis in the charge–potential curve as well as the different shapes of the forward and backward scan. The change of the pH value inside of the polyaniline layer has an influence on the follow-up equilibrium of the heterogeneous charge transfer. If we include the protonation equilibrium into the Nernst equation, the resulting formal potential shifts with the level of doping. The same effect was introduced by Paasch et al. w46x by application of the soliton model on polaronic and bipolaronic states resulting in a shift of the formal potentials with the doping level. Thus, it is proved in this work by the in situ ESR–UV–Vis spectroelectrochemistry that the capacitive current
A. Neudeck et al.r Synthetic Metals 107 (1999) 143–158
157
plateau is caused by a Faradaic process as in the potential region of this plateau further quinoid polymer structures are formed. There are three phenomena giving a shift of the driving force during the charging process: Ž1. a distribution of different conjugation length in the electrochemical prepared conductive polymers due to different structural units differing from the linear model; Ž2. the interaction of polaronic and bipolaronic segments on the chain; and Ž3. the acidrbase equilibria connected with a change of the pH value inside the polymer layer during charging. The third effect should be depressed by use of low pH values but the current plateau does not disappear by use of 0.1 M sulphuric acid. But the two processes cannot describe the large peak separation in the cyclic voltammograms and the hysteresis in the spectroscopic signals. That means they are superimposed by kinetic processes where the propagation of the conductivity area in to the polymer layer w17–35x and the exchange of the counter ions w11–16x should play an important role and shows the chargingrdischarging of PAni as a very complex process.
Acknowledgements The authors gratefully acknowledge grants from the Deutsche Forschungsgemeinschaft ŽSFB 287.. We also thank Dr. G. Wendrock ŽIFW-Dresden, Germany. for the microscopic studies of the coated mesh electrodes and Prof. G. Paasch ŽIFW-Dresden, Germany. for discussion.
References w1x w2x w3x w4x w5x w6x w7x w8x w9x w10x w11x w12x w13x w14x w15x w16x w17x w18x w19x w20x w21x w22x w23x w24x w25x w26x w27x w28x w29x w30x w31x w32x w33x w34x w35x w36x w37x w38x
A.T. Hubbard, F.C. Anson, J. Electroanal. Chem. 54 Ž1970. 129. R.F. Lane, A.T. Hubbard, J. Phys. Chem., 77 Ž1978. 1401. E. Laviron, J. Electroanal. Chem., 100 Ž1979. 263. H. Angerstein-Kozlowska, J. Klinger, B.E. Conway, J. Electroanal. Chem., 75 Ž1977. 41. H. Angerstein-Kozlowska, J. Klinger, B.E. Conway, J. Electroanal. Chem. 75 Ž1977. 61. E. Laviron, J. Electroanal. Chem. 52 Ž1974. 355. E. Laviron, J. Electroanal. Chem. 52 Ž1974. 395. A.T. Hubbard, J. Electroanal. Chem. 22 Ž1968. 165. H. Angerstein-Kozlowska, B.E. Conway, J. Klinger, J. Electroanal. 87 Ž1978. 301. H. Angerstein-Kozlowska, B.E. Conway, J. Klinger, J. Electroanal. 87 Ž1978. 321. F.B. Kaufmann, A.H. Schroder, E.M. Engler, S.R. Kramer, J.Q. Chambers, J. Am. Chem. Soc. 102 Ž1980. 483. ¨ P. Daum, J.R. Lenhard, D.R. Rolison, R.W. Murray, J. Am. Chem. Soc. 102 Ž1980. 4649. P. Daum, R.W. Murray, J. Phys. Chem. 85 Ž1981. 389. K. Doblhofer, K. Braun, R. Lange, J. Electroanal. Chem. 206 Ž1986. 93. J. Heinze, R. Bilger, Ber. Bunsenges. Phys. Chem. 97 Ž1993. 502. M.A. Vorotyntsev, E. Vieil, J. Heinze, Russ. J. Electrochem. 31 Ž1995. 1027. K. Aoki, T. Aramoto, Y. Hoshino, J. Electroanal. Chem. 340 Ž1992. 127. K. Aoki, Y. Tesuka, J. Electroanal. Chem. 267 Ž1989. 35. K. Aoki, Y. Tesuka, K. Shinozaki, H. Sato, D. Kagatu, J. Electrochem. Soc. Jpn. 57 Ž1989. 397. K. Aoki, J. Cao, Y. Hoshino, Electrochim. Acta 38 Ž1993. 1711. K. Aoki, J. Electroanal. Chem. 373 Ž1994. 67. Y. Tesuka, T. Ishii, K. Aoki, J. Electroanal. Chem. 402 Ž1996. 161. K. Aoki, M. Kawase, J. Electroanal. Chem. 377 Ž1994. 125. K. Aoki, J. Cao, Y. Hoshino, Electrochim. Acta 39 Ž1994. 2291. M. Vuki, M. Kalaji, L. Nyholm, L.M. Peter, J. Electroanal. Chem. 332 Ž1992. 315. M. Vuki, M. Kalaji, L. Nyholm, L.M. Peter, Synth. Met. 55–57 Ž1993. 1515. J.C. Lacroix, K. Kanazawa, A.F. Diaz, J. Electrochem. Soc. 5 Ž1989. 136. J.C. Lacroix, Dissertation, Universitat ¨ Paris 11, Paris, 1990. O. Genz, Dissertation, Universitat 1995. ¨ Dusseldorf ¨ O. Genz, M.M. Lohrengel, J.W. Schultze, Fast Charge–Discharge Kinetics in Intrinsically Conducting Polymers — Intercalation and Film Relaxation, in: Proceedings NATO-ARW XX, Kluwer Academic Publishers, 1996, 321. O. Genz, M.M. Lohrengel, J.W. Schultze, Electrochim. Acta 39 Ž1994. 179. O. Genz, M.M. Lohrengel, Mater. Sci. Forum 191 Ž1995. 207. M.M. Lohrengel, O. Genz, Mechanism of redox process of conducting polymers, Ionics 1 Ž1995. 304. S. Winkels, M.M. Lohrengel, Electrochim. Acta 42 Ž1997. 3117. K. Meeholz, J. Heinze, Angew. Chem. 102 Ž1990. 695. L. Dunsch, J. Prakt. Chem. 317 Ž1975. 409. A.G. MacDiarmid, The polyanilines: a novel class of conducting polymers, in: W.R. Salaneck, I. Lundstrom, ¨ B. Ranby ŽEds.., Conjugated Polymers and Related Material — The Interconnection of Chemical and Electronic Structure, Oxford Univ. Press, Oxford, 1993, p. 73. A.P. Monkman, D. Bloor, G.C. Stevens, J.C.H. Stevens, J. Phys. Appl. Phys. 20 Ž1987. E227.
158
A. Neudeck et al.r Synthetic Metals 107 (1999) 143–158
w39x M. Angelopoulos, G.E. Asturias, S.P. Ermer, E. Ray, E. Schorr, A.G. MacDiarmid, M. Akhtar, Z. Kiss, A.J. Epstein, Mol. Cryst. Liq. Cryst. 160 Ž1988. 151. w40x C.P. Andrieux, P. Hapiot, J.-M. Saveant, J. Am. Chem. Soc. 112 Ž1990. 243. ´ w41x P. Garcia, J.-M. Pernaut, P. Hapiot, V. Wintgens, P. Valat, F. Garnier, D. Delaboulise, J. Phys. Chem. 97 Ž1993. 513. w42x H. Yang, A.J. Bard, J. Electroanal. Chem. 339 Ž1992. 423. w43x E.M. Genies, J.F. Penneau, M. Lapkowski, A. Boyle, J. Electroanal. Chem. 269 Ž1989. 63. w44x K. Meerholz, J. Heinze, Synth. Met. 57 Ž1993. 5040. w45x K. Yakushi, L.J. Lauchlan, T.C. Clarke, G.B. Street, J. Chem. Phys. 79 Ž1983. 4774. w46x G. Paasch, P.H. Nguyen, A. Fischer, Chem. Phys. 227 Ž1998. 219. w47x Z. Ping, G.E. Nauer, H. Neugebauer, J. Theimer, A. Neckel, Electrochim. Acta 42 Ž1997. 1693. w48x P. Zhao, G.E. Nauer, H. Neugebauer, J. Theiner, J. Electroanal. Chem. 420 Ž1997. 301. w49x Z. Ping, G.E. Nauer, H. Neugebauer, J. Theiner, A. Neckel, J. Chem. Soc. Faraday Trans. 93 Ž1997. 121. w50x T. Moll, J. Heinze, Synth. Met. 55 Ž1993. 1521. w51x L. Dunsch, K.-H. Lubert, Electrochim. Acta Ž1997.. w52x K. Roßberg, G. Paasch, L. Dunsch, S. Ludwig, J. Electroanal. Chem. Ž1997.. w53x F. Garnier, A. Yassar, R. Hajlaoui, G. Horowitz, F. Deloffre, B. Sevet, S. Ries, P. Alnot, J. Am. Chem. Soc. 115 Ž1993. 8716. w54x J. Heinze, Electrochemically conducting polymers, Top. Curr. Chem. 152 Ž1990. 1. w55x P. Bauerle, U. Segelbacher, A. Maier, M. Mehring, J. Am. Chem. Soc. 115 Ž1993. 10217. ¨ w56x S.A. Brazovskii, N.N. Kirova, JETP Lett. 33 Ž1982. 4. w57x V. Brandl, R. Holze, Ber. Bunsenges. Phys. Chem. 101 Ž1997. 251. w58x G. Zotti, G. Schiaron, Synth. Met. 30 Ž1989. 151. w59x C. Visy, J. Lukkari, J. Kankare, J. Electroanal. Chem. 319 Ž1991. 85. w60x A. Thyssen, A. Borgerding, J.W. Schultze, Makromol. Chem., Macromol. Symp. 8 Ž1987. 143. w61x A. Zimmermann, U. Kunzelmann, L. Dunsch, Synth. Met. 93 Ž1998. 17–25. ¨ w62x A. Smie, J. Heinze, Angew. Chem. 109 Ž1997. 375. w63x A. Petr, L. Dunsch, A. Neudeck, J. Electroanal. Chem. 412 Ž1996. 153. w64x A. Neudeck, L. Kress, J. Electroanal. Chem. 437 Ž1997. 141. w65x L. Dunsch, Habilitationsschrift, TU-Dresden, 1995. w66x R. Kessel, Dissertation, Universitat 1990. ¨ Dusseldorf, ¨ w67x A. Neudeck, Habilitationsschrift, TU-Dresden, 1998. w68x A. Neudeck, L. Dunsch, J. Electroanal. Chem. 386 Ž1995. 135. w69x A. Neudeck, L. Dunsch, Electrochim. Acta 40 Ž1995. 1427. w70x J. Yano, K. Terayama, S. Yamasaki, K. Aoki, Electrochim. Acta 44 Ž1998. 337. w71x J.L. Bredas, J.C. Scott, K. Yakushi, G.B. Street, Phys. Rev. B 30 Ž1984. 1023. ´ w72x M.G. Hill, R.K. Mann, L.L. Miller, J.P. Penneau, J. Am. Chem. Soc. 114 Ž1992. 2728. w73x M.G. Hill, J.F. Penneau, B. Zinger, K.R. Mann, L.L. Miller, Chem. Mater. 4 Ž1992. 1106. w74x P. Bauerle, U. Segelbacher, K.-U. Gaudl, D. Huttenlocher, M. Mehring, Angew. Chem. 105 Ž1993. 125. ¨ w75x U. Segelbacher, N.S. Sariciftci, A. Grupp, P. Bauerle, M. Mehring, Synth. Met. 55–57 Ž1993. 4728. ¨ w76x P. Hapiot, P. Audebert, K. Monnier, J.-M. Pernaut, G. Garcia, Chem. Mater. 6 Ž1994. 1549. w77x P. Audebert, P. Hapiot, J.-M. Pernaut, P. Garcia, J. Electroanal. Chem. 361 Ž1993. 283.