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Preparation and properties of fullerene doped polyaniline
S ImTlUI TIIf ELSEVIER
Synthetic Metals 70 (1995) 1463-1464
Preparation and properties of fullerene doped polyaniline H.Y. Lim, a S.K. Jeong,a J.S. Suh, a E.J. Oh, a Y.W. Park, b K.S. Ryuc and C.H. Yoc aDepartment of Chemistry, Myong Ji University, Yong-In, Kyong-Gi Do, 449-728, Korea bDepartment of Physics, Seoul National University, Seoul, 151-742, Korea CDepartment of Chemistry, Yonsei University, Seoul, 120-749, Korea Abstract C60 (0.5-15mo1%) doped polyaniline(emeraldine base:EB) was prepared from NMP(N-methyl-2-pyrrolidinone)/toluene solution. Free standing films cast from this solution showed the conductivities up to 6.2xl05S/cm. UV/Vis. spectra of C60 doped EB solution in NMP/toluene showed the decrease in absorption intensity as the content of C60 increased. The first and the second redox processes in the cyclic voltammogram were observed to shift to higher potential. It is believed that the former is attributed to the steric effect which is due to bulky C60 and the latter is attributed to the electronic effect which is due to the electron withdrawing nature of C60. From the above results, the formation of charge transfer complex between the polymer as electron donor and C60 as electron acceptor was suggested. Also thermal stability of C60 doped polyaniline was compared to that of parent polyaniline. 1. INTRODUCTION Recently, much attention has been focused to the one of the pure carbon compound, fullerene[1], because it showed many interesting chemical and physical properties[2] and can be produced in large scale[3]. For example, upon doping with alkali metals, C60 is reduced and becomes a superconductor at low temperatures[4]. C60 undergoes nucleophilic addition[5] to satisfy Huckel rule and to be stabilized. In this case fullerene behaves as a good acceptor. Recently, C60 has been used as a dopant for p-type conducting polymers, polythio phene etc. Polyaniline (EB) behaves as a good electron donor because of its non-pair electron of nitrogen. If we consider the nature of emeraldine base as an electron donor and that of C60 as an electron acceptor, it can be expected that C60 affect the electronic structure of polyaniline. In this paper we report the doping effect of fullerene on the properties of polyaniline with the results of UV/Vis. spectra, cyclic voltammetry, thermal stability and conductivity measurements.
the cyclic voltammograms, the resulting solution of the reaction products was deposited on the Pt electrode by solution casting in NMP/toluene. After the solvent was evaporated, cyclic voltammogram was recorded over the potential range from -0.2 to 1V in 1N HC1 at a scan rate of 50mV/sec. Conductivities were measured by 4-probe technique with as prepared films. Thermogravimetric analysis was carried out on a Rigaku THERMOFLEX TG 8110 system. 3. RESULT AND DISCUSSION UV/Vis. spectra for C60 (0.5-15mo1%) doped polyaniline (emeraldine base) in NMP/toluene solution are shown in Fig. 1. These spectra show that as the content of C60 increases, (1) the n-n* transition located at 328nm decreases in absorbance intensity, (2) the exciton transition centered at 637 nm decreases in absorbance intensity, which is similar with the case of HC1 doped emeraldine base, (3) new (weak) peak appears around 820nm region. It is believed to be due to the formation of polaron upon C60 doping and (4) another new
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2. EXPERIMENTAL
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Polyaniline was synthesized by oxidative polymerization of aniline monomer with ammonium peroxydisulfate as an oxidant and then converted to emeraldine base by deprotonation with 0.1N NH4OH[6, 7]. C60 (>99%) was purchased from Texas Fullerenes Corporation. To prepare the C60 doped emeraldine base free standing films, emeraldine base was weighed by 3wt% and dissolved in NMP. Also C60 was weighed (0.5, 1, 3mo1% per nitrogen ring unit) and dissolved in toluene respectively. EB-NMP solution was poured into the C60-toluene solution and the resulting solution was stirred for lhr and soniccated for 20 hrs. This homogeneous solution was cast into a clean glass plate and the solvents were allowed to evaporate at 45°C. UV/Vis. spectra were measured in the range of 260-1600nm with these homogeneous solutions. To obtain 0379-6779/95/$09.50 © 1995 Elsevier Science S.A. All rights reserved S S D ! 0379-6779(94)02919-P
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.
.
I
C6o (mol%) a.O
b. 0.5 c. 3.0 d. 15 v
260 400
800 ~. (rim)
1200
1600
Figure 1. UV/Vis. spectra of polyaniline (EB) with various concentrations of C60 •
H.Y. Lim et al. / Synthetic Metals 70 (1995) 1463-1464
1464
absorption peak at higher energy (-280nm) than that of ~-n* transition of EB is observed only at high concentration (15mo1%) of C60. It is believed to be originated from the absorption of C60 itself. The electrical conductivity of emeraldine base is -109S/cm. However, it was observed that as the content of C60 increases, the electrical conductivity of C60 (0.5-3mo1%) doped polyaniline increases up to 6.2xl05S/cm. Conductivity data are listed in Table 1. Actually fine solution could be obtained up to 3mo1% of fullerene doped emeraldine base. But above this concentration of solution, it was difficult to make fine solution due to the limited solubility of the reaction product. It appears that conductivities reach the constant value (-10" 5S/cm) at -1 mol% doping concentration of C60. But the compressed pellet of 15mo1% C60 doped EB which was prepared by grinding above two solutions in mortar (the resulting mixture did not form fine solution) and then evaporating solvent in air showed the value lower than 107S/cm. Although its doping level in the mixture is not clear, there should be significant effect of bulky C60. Morita et al. explained that the decrease in conductivity of polyalkylthiophene upon doping above some doping level is due to the large size of C60 [8]. In our case C60 makes polyaniline chain to be distorted, so disturbs polarons formed upon doping sense each other which leads lower conductivity. Table 1. Conductivities of C60 doped polyaniline (EB) films mol% of
C60
in the film
0 0.5 1
3 15 (compressed pellet) -
Conductivity (S/cm) 10 -9 3.5 x 10-6 4 . 3 x 10-5 6.2 x 10-5 <10 -7 5
C6o(mol%) -[~"-'xc/...'"a ........
a.O
/
/~
.......
t
i
i
r
i
I
i
I
-0.2
0.0
0.2
0.4
0.6
0.8
1.0
Potential, Volts vs. SCE Figure 2. Cyclic voltammograms of emeraldine base and C60 doped emeraldine base measured in 1N HC1.
b. 3mo1% C60-EB
\' \
c. EB
\ c\
EB
100
200
300
400
500
600
Temperature (°C) Figure 3. TGA profile of C60, emeraldine base (powder) and C60 doped (3mo1%) emeraldine base film.
EB.HC1
The cyclic voltammogram of the C60 doped polyaniline and parent polyaniline is shown in Fig. 2. It exhibits two distinct reversible redox process of El/2 = 0.3V (vs. SCE), El/2' = 0.85 V (vs. SCE) for 3mo1% C60 doped polyaniline. Thus it can be clearly seen in Fig. 2 that, starting with pure polyaniline, two sets of reversible peaks located at El/2 = 0.2V (vs. SCE) and E1/2'= 0.59V (vs. SCE) move to higher potentials. It is believed that the first redox process is attributed to the steric effect due to bulky C60. This steric effect makes polymer chain distorted and electron movement not easy. Therefore high potential is required electron to move. The second redox process is attributed to the electronic effect which is due to the electron withdrawing nature of C60. Electron withdrawing behavior of C60 reduces electron density and increases basicity of nitrogen atoms on emeraldine base. So higher potential is needed to remove proton in second redox process. The TGA profile obtained from the thermal analysis perfor med in air is shown in Fig. 3. As synthesized emeraldine base powder shows two principal weight loss up to 600°C. The first weight loss is completed at -100°C, which is due to the removal of water and the second weight loss process occurs over the temperature range between 350 and 600°C due to the decomposition of the polymer chain. C60 shows the weight loss of -38% between room temperature and 600°C which accounts for the oxidation of C60 and is consistent with Ajie's result[9]. 3mo1% doped polyaniline shows 3 steps weight loss
. The weight loss appeared up to 100°C due to the removal of water and toluene. The weight loss observed between 100°C and 210°C accounts for the removal of NMP and the third weight loss process begins at - 3 5 0 ° C is due to the decomposition of polymer chain. Compared with emeraldine base, C60 doped emeraldine base shows slower polymer decomposition. 4. REFERENCES 1. H.W. Kroto, A.W. Heath, S.C. O'Brien, R.F. Curl and R.E. Smally, Nature, 318 (1985) 162. 2. W. Kratschmer, L.D. Lamb, K. Fostiropoulos and D.R. Huffman, Nature, 347 (1990) 354. 3. H.K. Kroto, A.W. Allafand and S.P. Balm, Chem. Rev., 91 (1991) 1213 ; P.J. Fagan, J.C. Calabrese and B.Malore, Acc . Chem. Res., 25 (1992) 134. 4. A.F. Hebarb, M.J. Rosseinsky, R.C. Haddon, D.W. Murphy , S.H. Glarum, T.T. Palstra, A.P. Ramirez and A.R. Kortan, Nature, 350 (1990) 600. 5. Bent, H.A. Chem. Ref., 61 (1961) 275. 6. Y. Wei, W.W. Focke, G.E. Wnek, A. Ray and A.G. MacDiarmid, J. Phys. Chem., 93 (1989) 495. 7. Y. Wei, G.W. Jang, K.F. Hssueh, E.M. Scherr, A.G. MacDiarmid and A.J. Epstein, Polymer, 33 (1992) 314. 8. S. Morita, A.A. Zakhidov, T. Kawai, H. Araki and K. Yoshino, Jpn. J. Appl. Phys., 31 (1992) L890. 9. H. Ajie, M.M. Alvarez, S.J. Anz, R.D. Beck, K.E. Schriver , W. Kratschmer, J. Phys. Chem., 94 (1990) 8630.
Synthetic Metals 70 (1995) 1463-1464
Preparation and properties of fullerene doped polyaniline H.Y. Lim, a S.K. Jeong,a J.S. Suh, a E.J. Oh, a Y.W. Park, b K.S. Ryuc and C.H. Yoc aDepartment of Chemistry, Myong Ji University, Yong-In, Kyong-Gi Do, 449-728, Korea bDepartment of Physics, Seoul National University, Seoul, 151-742, Korea CDepartment of Chemistry, Yonsei University, Seoul, 120-749, Korea Abstract C60 (0.5-15mo1%) doped polyaniline(emeraldine base:EB) was prepared from NMP(N-methyl-2-pyrrolidinone)/toluene solution. Free standing films cast from this solution showed the conductivities up to 6.2xl05S/cm. UV/Vis. spectra of C60 doped EB solution in NMP/toluene showed the decrease in absorption intensity as the content of C60 increased. The first and the second redox processes in the cyclic voltammogram were observed to shift to higher potential. It is believed that the former is attributed to the steric effect which is due to bulky C60 and the latter is attributed to the electronic effect which is due to the electron withdrawing nature of C60. From the above results, the formation of charge transfer complex between the polymer as electron donor and C60 as electron acceptor was suggested. Also thermal stability of C60 doped polyaniline was compared to that of parent polyaniline. 1. INTRODUCTION Recently, much attention has been focused to the one of the pure carbon compound, fullerene[1], because it showed many interesting chemical and physical properties[2] and can be produced in large scale[3]. For example, upon doping with alkali metals, C60 is reduced and becomes a superconductor at low temperatures[4]. C60 undergoes nucleophilic addition[5] to satisfy Huckel rule and to be stabilized. In this case fullerene behaves as a good acceptor. Recently, C60 has been used as a dopant for p-type conducting polymers, polythio phene etc. Polyaniline (EB) behaves as a good electron donor because of its non-pair electron of nitrogen. If we consider the nature of emeraldine base as an electron donor and that of C60 as an electron acceptor, it can be expected that C60 affect the electronic structure of polyaniline. In this paper we report the doping effect of fullerene on the properties of polyaniline with the results of UV/Vis. spectra, cyclic voltammetry, thermal stability and conductivity measurements.
the cyclic voltammograms, the resulting solution of the reaction products was deposited on the Pt electrode by solution casting in NMP/toluene. After the solvent was evaporated, cyclic voltammogram was recorded over the potential range from -0.2 to 1V in 1N HC1 at a scan rate of 50mV/sec. Conductivities were measured by 4-probe technique with as prepared films. Thermogravimetric analysis was carried out on a Rigaku THERMOFLEX TG 8110 system. 3. RESULT AND DISCUSSION UV/Vis. spectra for C60 (0.5-15mo1%) doped polyaniline (emeraldine base) in NMP/toluene solution are shown in Fig. 1. These spectra show that as the content of C60 increases, (1) the n-n* transition located at 328nm decreases in absorbance intensity, (2) the exciton transition centered at 637 nm decreases in absorbance intensity, which is similar with the case of HC1 doped emeraldine base, (3) new (weak) peak appears around 820nm region. It is believed to be due to the formation of polaron upon C60 doping and (4) another new
" ¢ • i ,
2. EXPERIMENTAL
•
i
.
.
.
.
.
.
.
i
.
.
.
.
.
a
Polyaniline was synthesized by oxidative polymerization of aniline monomer with ammonium peroxydisulfate as an oxidant and then converted to emeraldine base by deprotonation with 0.1N NH4OH[6, 7]. C60 (>99%) was purchased from Texas Fullerenes Corporation. To prepare the C60 doped emeraldine base free standing films, emeraldine base was weighed by 3wt% and dissolved in NMP. Also C60 was weighed (0.5, 1, 3mo1% per nitrogen ring unit) and dissolved in toluene respectively. EB-NMP solution was poured into the C60-toluene solution and the resulting solution was stirred for lhr and soniccated for 20 hrs. This homogeneous solution was cast into a clean glass plate and the solvents were allowed to evaporate at 45°C. UV/Vis. spectra were measured in the range of 260-1600nm with these homogeneous solutions. To obtain 0379-6779/95/$09.50 © 1995 Elsevier Science S.A. All rights reserved S S D ! 0379-6779(94)02919-P
.
.
,
.
.
.
.
.
.
.
I
C6o (mol%) a.O
b. 0.5 c. 3.0 d. 15 v
260 400
800 ~. (rim)
1200
1600
Figure 1. UV/Vis. spectra of polyaniline (EB) with various concentrations of C60 •
H.Y. Lim et al. / Synthetic Metals 70 (1995) 1463-1464
1464
absorption peak at higher energy (-280nm) than that of ~-n* transition of EB is observed only at high concentration (15mo1%) of C60. It is believed to be originated from the absorption of C60 itself. The electrical conductivity of emeraldine base is -109S/cm. However, it was observed that as the content of C60 increases, the electrical conductivity of C60 (0.5-3mo1%) doped polyaniline increases up to 6.2xl05S/cm. Conductivity data are listed in Table 1. Actually fine solution could be obtained up to 3mo1% of fullerene doped emeraldine base. But above this concentration of solution, it was difficult to make fine solution due to the limited solubility of the reaction product. It appears that conductivities reach the constant value (-10" 5S/cm) at -1 mol% doping concentration of C60. But the compressed pellet of 15mo1% C60 doped EB which was prepared by grinding above two solutions in mortar (the resulting mixture did not form fine solution) and then evaporating solvent in air showed the value lower than 107S/cm. Although its doping level in the mixture is not clear, there should be significant effect of bulky C60. Morita et al. explained that the decrease in conductivity of polyalkylthiophene upon doping above some doping level is due to the large size of C60 [8]. In our case C60 makes polyaniline chain to be distorted, so disturbs polarons formed upon doping sense each other which leads lower conductivity. Table 1. Conductivities of C60 doped polyaniline (EB) films mol% of
C60
in the film
0 0.5 1
3 15 (compressed pellet) -
Conductivity (S/cm) 10 -9 3.5 x 10-6 4 . 3 x 10-5 6.2 x 10-5 <10 -7 5
C6o(mol%) -[~"-'xc/...'"a ........
a.O
/
/~
.......
t
i
i
r
i
I
i
I
-0.2
0.0
0.2
0.4
0.6
0.8
1.0
Potential, Volts vs. SCE Figure 2. Cyclic voltammograms of emeraldine base and C60 doped emeraldine base measured in 1N HC1.
b. 3mo1% C60-EB
\' \
c. EB
\ c\
EB
100
200
300
400
500
600
Temperature (°C) Figure 3. TGA profile of C60, emeraldine base (powder) and C60 doped (3mo1%) emeraldine base film.
EB.HC1
The cyclic voltammogram of the C60 doped polyaniline and parent polyaniline is shown in Fig. 2. It exhibits two distinct reversible redox process of El/2 = 0.3V (vs. SCE), El/2' = 0.85 V (vs. SCE) for 3mo1% C60 doped polyaniline. Thus it can be clearly seen in Fig. 2 that, starting with pure polyaniline, two sets of reversible peaks located at El/2 = 0.2V (vs. SCE) and E1/2'= 0.59V (vs. SCE) move to higher potentials. It is believed that the first redox process is attributed to the steric effect due to bulky C60. This steric effect makes polymer chain distorted and electron movement not easy. Therefore high potential is required electron to move. The second redox process is attributed to the electronic effect which is due to the electron withdrawing nature of C60. Electron withdrawing behavior of C60 reduces electron density and increases basicity of nitrogen atoms on emeraldine base. So higher potential is needed to remove proton in second redox process. The TGA profile obtained from the thermal analysis perfor med in air is shown in Fig. 3. As synthesized emeraldine base powder shows two principal weight loss up to 600°C. The first weight loss is completed at -100°C, which is due to the removal of water and the second weight loss process occurs over the temperature range between 350 and 600°C due to the decomposition of the polymer chain. C60 shows the weight loss of -38% between room temperature and 600°C which accounts for the oxidation of C60 and is consistent with Ajie's result[9]. 3mo1% doped polyaniline shows 3 steps weight loss
. The weight loss appeared up to 100°C due to the removal of water and toluene. The weight loss observed between 100°C and 210°C accounts for the removal of NMP and the third weight loss process begins at - 3 5 0 ° C is due to the decomposition of polymer chain. Compared with emeraldine base, C60 doped emeraldine base shows slower polymer decomposition. 4. REFERENCES 1. H.W. Kroto, A.W. Heath, S.C. O'Brien, R.F. Curl and R.E. Smally, Nature, 318 (1985) 162. 2. W. Kratschmer, L.D. Lamb, K. Fostiropoulos and D.R. Huffman, Nature, 347 (1990) 354. 3. H.K. Kroto, A.W. Allafand and S.P. Balm, Chem. Rev., 91 (1991) 1213 ; P.J. Fagan, J.C. Calabrese and B.Malore, Acc . Chem. Res., 25 (1992) 134. 4. A.F. Hebarb, M.J. Rosseinsky, R.C. Haddon, D.W. Murphy , S.H. Glarum, T.T. Palstra, A.P. Ramirez and A.R. Kortan, Nature, 350 (1990) 600. 5. Bent, H.A. Chem. Ref., 61 (1961) 275. 6. Y. Wei, W.W. Focke, G.E. Wnek, A. Ray and A.G. MacDiarmid, J. Phys. Chem., 93 (1989) 495. 7. Y. Wei, G.W. Jang, K.F. Hssueh, E.M. Scherr, A.G. MacDiarmid and A.J. Epstein, Polymer, 33 (1992) 314. 8. S. Morita, A.A. Zakhidov, T. Kawai, H. Araki and K. Yoshino, Jpn. J. Appl. Phys., 31 (1992) L890. 9. H. Ajie, M.M. Alvarez, S.J. Anz, R.D. Beck, K.E. Schriver , W. Kratschmer, J. Phys. Chem., 94 (1990) 8630.