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Growth of polyaniline films: Evidence for fractal surface
0038-1098/87 $3.00 + .00 Pergamon Journals Ltd.
~Solid State Cormunicat ions, Vol.64,No.4, pp.435-437, 1987o Printed in Great Britain.
GROWTH OF POLYANILINE FILMS : EVIDENCE FOR FRACTAL SURFACE B. VILLERET, M. NECHTSCHEIN Centre d'Etudes Nucl~aires de Grenoble D~partement de Recherche Fondamentale/Service de Physique Groupe Dynamique de Spin et Propri~t~s Electroniques 85X 38041 Grenoble cedex France Received 3 July 1987 by E.F. Bertaut
During electropolymerization of aniline using cyclic potential scanning the surface area of the polyaniline films grows like a power law of their volume, with the exponent e z 0.4. Implications of this result are discussed. Au cours de la polym~risatiod ~lectrochimique de l'aniline par balayages cycliques du potentiel, la surface des films croit comme une loi de puissance du volume, avec un exposant e z 0,4. Des implications ~e ce r~sultat sont discut~es.
Electrodeposition is known to give rise to fractal structures in certain experimental conditions. Recently, the electrochemical polymerization of the conducting polymer polypyrrole, in the diffusion-limited regime, has been shown to induce a fractal growth of the polymer*. We, here, report a study of the growth of polyaniline films during electrochemical polymerization in normal conditions. Using only Cyclic Voltammetry data we have determined geometrical features of the films. We have obtained a power law for the surface area to volume ratio, which is evidence that the surface exhibits a fractal character. Electropolymerization of polyaniline, similarly to other conducting polymers, can be achieved : (i) galvanostatically (constant current), ( i i ) potentiostatically (constant applied potential), or (iii) by cyclic potential scanning. It proceeds by removal of two electrons per added aniline monomer and subsequent condensation with loss of two hydrogen atoms z. At the potential needed for the polymerization the polymer is in an oxidized state. Using cyclic potential scanning the two processes of (a) polymer oxidation with about one hole for every 2.5 monomer units, and (b) polymerization with removal of 2 electrons per monomer units, can be well separated and visualized on the cyclic voltammogramm as shown in Fig.l. Process (a) gives rise to a peak at the polymer redox potential (Vox), which occurs at a potential value significantly less than that of the monomer oxidation. The potential is rised above this peak up to the monomer oxidation potential, at which process (b) can proceed. The maximum potential (Vs) is conveniently limited to the starting part of the monomer oxidation peak in order to control the polymerization and because application of a too high potential may
°x
11t
S
V (volts)VsCu a
i. Cyclic voltammogram during polyaniline electropolimerization. The OX peak corresponds to polymer oxidation and the S peak to the polymer synthesis
435
436
GROWTH OF POLYANILINE FILMS : EVIDENCE FOR FRACTAL SURFACE
disturb the regularity of the chains. Upon cycling the potential between a minimum value (Va), at which the polymer is neutral, and V s, the film grows layer by layer. We have used cyclic potential scanning for polymerizing aniline and we have followed the film growth at every cycle. Electrochemical syntheses have been performed on a platinum wire (0.5 mm diameter) in a 1 M solution of aniline in the super-acidic medium N H 4 F - 2.35 HF, which is known to favour good
1o
Vol. 64, No. 4
_L ~M
/
f
~2
O I
os - j
o.6~ 0"4
quality polyaniline films 3. The potential was scanned at the rate of 20 mV/s between V = -0.2 V and V s = 0.7 V versus a copper ram terence electrode with a PAR 273 potentiostat. The growth of the film was followed cycle by cycle up to total charges for the synthesis of 30 - 100 mC, corresponding to film thicknesses in the range of 0.2 - 0.6 ~m. About 15 to 40 cycles were used for a given synthesis. Since the film thickness is always much smaller than the diameter of the cylindrical electrode, it can be considered that the film is growing on a plane electrode. At every cycle the charge corresponding to the oxidation of the polymer, Qox' was measured by integration of the current as the potential was rised from V to V (see a b Fig.l). Since the oxidation concerns the whole chains, Qox scales with the mass, M, of the film already synthesized : e Qox = n e - ~ (I) m where n is the number of electrons removed per e monomer unit, e is the electron charge, and m is the mass of a monomer unit. The charge needed for the polymerization process, Qs' has also been determined at every cycle. Attention has been paid to the fact that, in the potential region where the polymerization takes place, there are two contributions to the current : one corresponding to the polymerization itself, and another one due to the so-called "capacitive oxidation" (which gives the non zero plateau right side of the oxidation peak) 4. By integrating the current during upward (from V b to Vs), and downward (from V to V = V ) potentials, the charges S
c
b
from the reversible (capacitive) oxidation and reduction processes cancel each other, and only Qs remains. For a given cycle, Qs scales with the mass increase ~ , of the film : e Qs = (2 + n e) - ~M (2) m By summing the Qs over all the cycles one obtains a total polymerization charge which is proportional to the film mass. e Qs = (2 + ne ) - M (3) m In Fig.2 we give a log-log plot of Qox' and Qs as a function of Qs" It appears clearly that the mass increment obeys a power law a function of the total mass. Over three orders of magnitude, at least, one has : ~a ~ M = (4) with ~ = 0.40 ± 0.03 (the uncertainty being estimated from the spread of the values obtained in about i0 experiments). From the distance
O+R.i
_M .~
o!2 I o~ Io.~lc Oox
I~
~°~ ~.d ~ Q s
l&I~11~ 2'0 I 4'oP4o1~oo ~'~
(mC)
2. Log-log plot of the charge during the synthesis peak, QsO~, as a function of the oxidation charge Qo×(O), and of [Qs(e). Qo× and ~Qs are proportional to the mass M of the film (see text).
between
the straightlines corresponding
to the
two sets of data, we obtain (2 + n e )/n e = 6 ± 0.2 hence n = 0.40 + 0.015. This value, e determined at a potential of 0.3 V above the oxidation peak, is in very good agreement with previous measurements of the doping level of polyaniline in its first oxidized state : one electron per 2.5 monomer units ~. The increase of AM with M shows that, as the film grows up, there are more and more chains involved in the polymerization process. Since the polymerization proceeds from the surface, it follows that ~ is proportional to the surface area, A. With the reasonable assumption that the density is constant, the film volume, V, is proportional to the mass, and we also obtain a power law for the surface area as a function of volume : A ~ V~ (5) Thus, the surface area increases with volume, in contrast to the behaviour of a regular growth on a plane electrode which would yields = 0. This result evidences the fractal structure of the film surface. For the idealized structure of a Bathe lattice one would have = I. It is tempting to compare our results to the behaviour of the Eden model. The Eden model, which has been proposed to describe cluster growth and aggregation phenomena, can also be considered as a simplified model for polymerization. It consists of the following : particles are added one after another to a growing cluster with the prescription that each new particle sticks on any point of the surface of the cluster with equal probability. It has been shown that this model leads to objects with compact bulk and fractal surface. The height of the surface points, h, is statistically distributed with a standard deviation, ~ : the "surface thickness". In the case of thin clusters the relation ~ ~ h ~, with ~ ~ 0.3 has been obtained by computation ~. For a given surface pattern it is reasonable to assume that the surface thickness scales with the surface
Vol. 64, No. 4
GROWTH OF POLYANILINE FILMS : EVIDENCE FOR FRACTAL SURFACE
area : A ~ ~, which suggests to compare the value of ~ to our finding for a. The two values are rather close to each other. However, it 3hould be noted that the computations yielding ~ 0.3 have been performed in the case of 2 dimensional clusters. In d = 3 the ~ value is not definitely established, hut seems to be smaller than in the case of d = 2 5 In conclusion we have given evidence that, during polyaniline film growth, the surface
area increases like a power law of the volume. This result implies an indented structure for the surface. It should also have important consequences for the doping/dedoping process which proceeds through the surface. Acknowledgement - We acknowledge M. Lapkowski for help in the sample preparation and F. Devreux and J.-P. Travers for critical reading of the manuscript.
References I. J.H. Kaufman, C.K. Baker, A.I. Nazzal, M. Flickner and O.R. Melroy, Phys. Rev. Lett. 56, 1932 (1986) 2. See for instance A.F. Diaz and J. Bargon in "Handbook of Conducting Polymers" Vol.l, edited by A. Skotheim, M. Dekker, Inc. New York
3. E.M. Geni#s and C. Tsintavis, J. Electroanal. Chem. 195, 109 (1985) 4. J. Tanguy, N. Mermillod and M. Hoclet, Synth. Met. 18, 7 (1987) 5. R. Jullien and R. Botet, J. Phys. A : Math. Gen. 18, 2279 (1985)
437
~Solid State Cormunicat ions, Vol.64,No.4, pp.435-437, 1987o Printed in Great Britain.
GROWTH OF POLYANILINE FILMS : EVIDENCE FOR FRACTAL SURFACE B. VILLERET, M. NECHTSCHEIN Centre d'Etudes Nucl~aires de Grenoble D~partement de Recherche Fondamentale/Service de Physique Groupe Dynamique de Spin et Propri~t~s Electroniques 85X 38041 Grenoble cedex France Received 3 July 1987 by E.F. Bertaut
During electropolymerization of aniline using cyclic potential scanning the surface area of the polyaniline films grows like a power law of their volume, with the exponent e z 0.4. Implications of this result are discussed. Au cours de la polym~risatiod ~lectrochimique de l'aniline par balayages cycliques du potentiel, la surface des films croit comme une loi de puissance du volume, avec un exposant e z 0,4. Des implications ~e ce r~sultat sont discut~es.
Electrodeposition is known to give rise to fractal structures in certain experimental conditions. Recently, the electrochemical polymerization of the conducting polymer polypyrrole, in the diffusion-limited regime, has been shown to induce a fractal growth of the polymer*. We, here, report a study of the growth of polyaniline films during electrochemical polymerization in normal conditions. Using only Cyclic Voltammetry data we have determined geometrical features of the films. We have obtained a power law for the surface area to volume ratio, which is evidence that the surface exhibits a fractal character. Electropolymerization of polyaniline, similarly to other conducting polymers, can be achieved : (i) galvanostatically (constant current), ( i i ) potentiostatically (constant applied potential), or (iii) by cyclic potential scanning. It proceeds by removal of two electrons per added aniline monomer and subsequent condensation with loss of two hydrogen atoms z. At the potential needed for the polymerization the polymer is in an oxidized state. Using cyclic potential scanning the two processes of (a) polymer oxidation with about one hole for every 2.5 monomer units, and (b) polymerization with removal of 2 electrons per monomer units, can be well separated and visualized on the cyclic voltammogramm as shown in Fig.l. Process (a) gives rise to a peak at the polymer redox potential (Vox), which occurs at a potential value significantly less than that of the monomer oxidation. The potential is rised above this peak up to the monomer oxidation potential, at which process (b) can proceed. The maximum potential (Vs) is conveniently limited to the starting part of the monomer oxidation peak in order to control the polymerization and because application of a too high potential may
°x
11t
S
V (volts)VsCu a
i. Cyclic voltammogram during polyaniline electropolimerization. The OX peak corresponds to polymer oxidation and the S peak to the polymer synthesis
435
436
GROWTH OF POLYANILINE FILMS : EVIDENCE FOR FRACTAL SURFACE
disturb the regularity of the chains. Upon cycling the potential between a minimum value (Va), at which the polymer is neutral, and V s, the film grows layer by layer. We have used cyclic potential scanning for polymerizing aniline and we have followed the film growth at every cycle. Electrochemical syntheses have been performed on a platinum wire (0.5 mm diameter) in a 1 M solution of aniline in the super-acidic medium N H 4 F - 2.35 HF, which is known to favour good
1o
Vol. 64, No. 4
_L ~M
/
f
~2
O I
os - j
o.6~ 0"4
quality polyaniline films 3. The potential was scanned at the rate of 20 mV/s between V = -0.2 V and V s = 0.7 V versus a copper ram terence electrode with a PAR 273 potentiostat. The growth of the film was followed cycle by cycle up to total charges for the synthesis of 30 - 100 mC, corresponding to film thicknesses in the range of 0.2 - 0.6 ~m. About 15 to 40 cycles were used for a given synthesis. Since the film thickness is always much smaller than the diameter of the cylindrical electrode, it can be considered that the film is growing on a plane electrode. At every cycle the charge corresponding to the oxidation of the polymer, Qox' was measured by integration of the current as the potential was rised from V to V (see a b Fig.l). Since the oxidation concerns the whole chains, Qox scales with the mass, M, of the film already synthesized : e Qox = n e - ~ (I) m where n is the number of electrons removed per e monomer unit, e is the electron charge, and m is the mass of a monomer unit. The charge needed for the polymerization process, Qs' has also been determined at every cycle. Attention has been paid to the fact that, in the potential region where the polymerization takes place, there are two contributions to the current : one corresponding to the polymerization itself, and another one due to the so-called "capacitive oxidation" (which gives the non zero plateau right side of the oxidation peak) 4. By integrating the current during upward (from V b to Vs), and downward (from V to V = V ) potentials, the charges S
c
b
from the reversible (capacitive) oxidation and reduction processes cancel each other, and only Qs remains. For a given cycle, Qs scales with the mass increase ~ , of the film : e Qs = (2 + n e) - ~M (2) m By summing the Qs over all the cycles one obtains a total polymerization charge which is proportional to the film mass. e Qs = (2 + ne ) - M (3) m In Fig.2 we give a log-log plot of Qox' and Qs as a function of Qs" It appears clearly that the mass increment obeys a power law a function of the total mass. Over three orders of magnitude, at least, one has : ~a ~ M = (4) with ~ = 0.40 ± 0.03 (the uncertainty being estimated from the spread of the values obtained in about i0 experiments). From the distance
O+R.i
_M .~
o!2 I o~ Io.~lc Oox
I~
~°~ ~.d ~ Q s
l&I~11~ 2'0 I 4'oP4o1~oo ~'~
(mC)
2. Log-log plot of the charge during the synthesis peak, QsO~, as a function of the oxidation charge Qo×(O), and of [Qs(e). Qo× and ~Qs are proportional to the mass M of the film (see text).
between
the straightlines corresponding
to the
two sets of data, we obtain (2 + n e )/n e = 6 ± 0.2 hence n = 0.40 + 0.015. This value, e determined at a potential of 0.3 V above the oxidation peak, is in very good agreement with previous measurements of the doping level of polyaniline in its first oxidized state : one electron per 2.5 monomer units ~. The increase of AM with M shows that, as the film grows up, there are more and more chains involved in the polymerization process. Since the polymerization proceeds from the surface, it follows that ~ is proportional to the surface area, A. With the reasonable assumption that the density is constant, the film volume, V, is proportional to the mass, and we also obtain a power law for the surface area as a function of volume : A ~ V~ (5) Thus, the surface area increases with volume, in contrast to the behaviour of a regular growth on a plane electrode which would yields = 0. This result evidences the fractal structure of the film surface. For the idealized structure of a Bathe lattice one would have = I. It is tempting to compare our results to the behaviour of the Eden model. The Eden model, which has been proposed to describe cluster growth and aggregation phenomena, can also be considered as a simplified model for polymerization. It consists of the following : particles are added one after another to a growing cluster with the prescription that each new particle sticks on any point of the surface of the cluster with equal probability. It has been shown that this model leads to objects with compact bulk and fractal surface. The height of the surface points, h, is statistically distributed with a standard deviation, ~ : the "surface thickness". In the case of thin clusters the relation ~ ~ h ~, with ~ ~ 0.3 has been obtained by computation ~. For a given surface pattern it is reasonable to assume that the surface thickness scales with the surface
Vol. 64, No. 4
GROWTH OF POLYANILINE FILMS : EVIDENCE FOR FRACTAL SURFACE
area : A ~ ~, which suggests to compare the value of ~ to our finding for a. The two values are rather close to each other. However, it 3hould be noted that the computations yielding ~ 0.3 have been performed in the case of 2 dimensional clusters. In d = 3 the ~ value is not definitely established, hut seems to be smaller than in the case of d = 2 5 In conclusion we have given evidence that, during polyaniline film growth, the surface
area increases like a power law of the volume. This result implies an indented structure for the surface. It should also have important consequences for the doping/dedoping process which proceeds through the surface. Acknowledgement - We acknowledge M. Lapkowski for help in the sample preparation and F. Devreux and J.-P. Travers for critical reading of the manuscript.
References I. J.H. Kaufman, C.K. Baker, A.I. Nazzal, M. Flickner and O.R. Melroy, Phys. Rev. Lett. 56, 1932 (1986) 2. See for instance A.F. Diaz and J. Bargon in "Handbook of Conducting Polymers" Vol.l, edited by A. Skotheim, M. Dekker, Inc. New York
3. E.M. Geni#s and C. Tsintavis, J. Electroanal. Chem. 195, 109 (1985) 4. J. Tanguy, N. Mermillod and M. Hoclet, Synth. Met. 18, 7 (1987) 5. R. Jullien and R. Botet, J. Phys. A : Math. Gen. 18, 2279 (1985)
437