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Preparation of polythiophene LB films and their gas sensitivities by the quartz crystal microbalance
ELSEVIER
Synthetic
Their
Metals 71 (1995)
2027-2028
Preparation of Polythiophene LB Films and Gas Sensitivities by the Quartz Crystal Microbalance
S.-R. Kim”, S.-A. Choi”, J.-D. Kim” and K. J. Kimb, C. Leeb, S. B. Rheeb
373-l
Qepartment of Chemical Engineering, KAIST, Kusung-dong, Yusong-gu, Taejon, 305-701, Korea
bFur:ctional Polymer Research Lab., KRICT, P. 0. BOX 9, Deadeog-Danji, Taejon, 305606, Korea
Abstract
The gas sensitivity of polythiophene LB film was investigated. As polythiophene has the conjugated double bond, the potential applications to the various devices using its conducting property have been proposed. The Langmuir-Blodgett(LB) technique has been used to manipulate conducting polymers to multilayer thin films with well defined structures and ordered molecular orientations. In this study, we have synthesized the polythiophene derivatives such as poly(octy1 thiophene)(POT), poly(propanoate thiophene)(PEPT) and poly(decanoate thiophene)(PEDT), and observed their behaviors at the air/water nnterface. For the preparation of polythiophene LB films with good quality, we have used the mixed monolayer with surface active materials like octadecylamine(ODA) and controlled the subphase conditions. Also, for the application to the sensing materials, we have tested the gas sensitivities of polythiophene derivatives LB films to NOz gas by the quartz crystal microbalance(QCM).
INTRODUCTION
The LangmuirBlodgett technique has recently been used to fabricate the multilayer of electroactive conjugated polymers with well defined composition, structure, thickness and ordered molecular organizations[l,21. In the hydrocarbon substituted thiophene, the heterocyclic ring have the weak dipole moment, so stable monolayers can be formed by mixing with stearic acid or other surface active materials[ll. The NOz sensitivity of polythiophenes has been reported and the mechanism of NO2 detection is known as the doping effect of NOz to the polythiophenel31. The QCM has been used in many other fields as mass sensitive device[41 and in our system, QCM gave the sensitivity for sensor.
EXPERIMENTAL
The mixture of polythiophene and ODA was spread from toluene solution on aqueous subphase containing 10e4 N NaOH and waiting about 20 minutes for toluene evaporated. Then, moving the barrier upto the surface pressure at 30 W/m and film deposition started. As the sensing device, 6 MHz gold coated QCM(purchased from Leybold Inficon) was used. It has a disk shape and its active area is about 0.29 &. QCM was connected to the oscillator circuit, driven at 15 V dc and the frequency signal output was measured by frequency counter on-lined to personal computer. To check the sensitivity of polythiophenes, standard gas made by the controlled mixing of 1.1 % NOz flow to pure NZ flow at room temperature and then, NOz gas of 550 ppn was continuously introduced to the sensor cell.
DETAILS
As the monolayer study and film deposition device, KSV-300 and a constant-perimeter barrier type of Langmuir film device were used. The ester substituted polythiophenes were prepared according to procedures described elsewhere[51. POT, PEPT and PEDT given in Figure 1 are spread over air-water interface, but none of them are transferred as itself. When they are mixed with surface active material like ODA or stearic acid and at high subphase pH condition, the stable LB film can be obtained. So in this study, we made LB film with good quality by mixing polythiophene with ODA(Aldrich co., 1:l mixture based on molar ratio of polythiophene repeating unit) and made subphase pH nearby 9 using NaOH.
0379-6779/95/$09.50 0 1995 Elsevier Science S.A. Ah rights reserved SSDI 0379-6779(94)03149-Z
Figure 1. The molecular
RESULTS
structures
of polythiophenes.
& DISCUSSION
The n-A isotherm of each polythiophene is shown in Figure 2. In the case of PEDT, there are two phase
S.-R. Kim et al. I Synthetic Metals 71 (1995) 2027-2028
2028
transitions while in the case of PEPT, only one observed. As shown in material structure in Figure 1, PEDT, PEPT have ester linkage and PEDT has a long side chain, so we infer that one transition is from ester linkage and the other is from side chain attached to the ester linkage. The limiting area of three polythiophenes are almost the same at high pressure, because the heterocyclic ring structure is same.
PEPT/DDA(I:I)
/ 0
““’ II
10
Surface
Figure 4. U-A
0
Figure
10
20
30
40
Surface
Area, ?/repeat
2. U-A
isotherm
50
i
‘1 20
Area,
f \.
30
2/mixed
,____I 10
50
60
n~olecule
isotherm of polythiophene./ODAf.l:l); Surface is based on the repeat unit of polythiophene.
60
unit
of polythiophene.
The POT/ODA(l:l) LB film has a great NOz sensitivity as shown in Figure 3. For the same number of layers, the frequency change is larger in case of POT than the other two. In Figure 2, at 30 mN/m, the differences in area per repeat unit of each polythiophene are very large, but after mixing with ODA, the differences are not so considerable (Figure 41, therefore the frequency change difference is not due to the mass loaded on the QCM. The head group of each polythiophene is same, so the difference of sensitivity might be because of the ester linkage in side chain. In the Figure 5, as the LB film layer increases the frequency change i.e. sensitivity increase linearly. So we can control the sensitivity by the film thickness. After exposed to NOz, the frequency recovery is only 5-10% of the total change, even after treating in vacuum oven at 40 G during 1 tiv, the recovery to initial frequency is not achieved.
Layers ok’POT LB Film Figure 5. Layer effect of POT/ODAIl:l) for NOz (550 ppn).
LB film
CONCLUSION Polythiophene LB films with good quality have been made by mixing with surface active materials and by controlling the subphase conditions. Transfer ratio was nearly one up to considerable multilayers. The NOz sensitivity characteristics of POT, PEPT and PEDT were tested, and POT showed the best response to NOz of the three materials used in this study. And the sensitivity increase linearly with film thickness enough to control the sensitivity by layer numbers.
REFERENCES 1.
T. A. Skotheim, X. Q. Yang
and T. lnagaki,
Thin Solid
Films, 178, (1989) 233. 2.
NO, ON -120
~~~~‘~~~~,.~ 0 5 10
NOp OFF .~.~~.I~~ I5 20
1 .,,-.’ 25
30
Time. min Figure 3. Frequency change pattern of 14 layers polythiophene LB film(l:l mixture with ODA) for NOz (550 pm).
M.F. Rubner and T.A. Skotheim, in Conjugated Polymers: Controlled Molecular Assemblies of Electrically Conductive Polymers, eds. by J. L. Bredas and R. Silbey, Kluwer Academic Publishers, pp 3633403. 3. T.Hanawa, S. Kuwabata, H. Hashimoto and H. Yoneyama, Synthetic Metals, 30, (1989) 173. 4. J. F. Alder and J. J. McCallum, Analyst, 108, (1983) 1169. 5. C. Lee, K. J. Kim and S. B. Rhee, “The Effect of Ester Substitution and Alkyl Chain Length on the Properties of Poly(thiophene)s”, to be published in proc., ICSM, 1994.
Synthetic
Their
Metals 71 (1995)
2027-2028
Preparation of Polythiophene LB Films and Gas Sensitivities by the Quartz Crystal Microbalance
S.-R. Kim”, S.-A. Choi”, J.-D. Kim” and K. J. Kimb, C. Leeb, S. B. Rheeb
373-l
Qepartment of Chemical Engineering, KAIST, Kusung-dong, Yusong-gu, Taejon, 305-701, Korea
bFur:ctional Polymer Research Lab., KRICT, P. 0. BOX 9, Deadeog-Danji, Taejon, 305606, Korea
Abstract
The gas sensitivity of polythiophene LB film was investigated. As polythiophene has the conjugated double bond, the potential applications to the various devices using its conducting property have been proposed. The Langmuir-Blodgett(LB) technique has been used to manipulate conducting polymers to multilayer thin films with well defined structures and ordered molecular orientations. In this study, we have synthesized the polythiophene derivatives such as poly(octy1 thiophene)(POT), poly(propanoate thiophene)(PEPT) and poly(decanoate thiophene)(PEDT), and observed their behaviors at the air/water nnterface. For the preparation of polythiophene LB films with good quality, we have used the mixed monolayer with surface active materials like octadecylamine(ODA) and controlled the subphase conditions. Also, for the application to the sensing materials, we have tested the gas sensitivities of polythiophene derivatives LB films to NOz gas by the quartz crystal microbalance(QCM).
INTRODUCTION
The LangmuirBlodgett technique has recently been used to fabricate the multilayer of electroactive conjugated polymers with well defined composition, structure, thickness and ordered molecular organizations[l,21. In the hydrocarbon substituted thiophene, the heterocyclic ring have the weak dipole moment, so stable monolayers can be formed by mixing with stearic acid or other surface active materials[ll. The NOz sensitivity of polythiophenes has been reported and the mechanism of NO2 detection is known as the doping effect of NOz to the polythiophenel31. The QCM has been used in many other fields as mass sensitive device[41 and in our system, QCM gave the sensitivity for sensor.
EXPERIMENTAL
The mixture of polythiophene and ODA was spread from toluene solution on aqueous subphase containing 10e4 N NaOH and waiting about 20 minutes for toluene evaporated. Then, moving the barrier upto the surface pressure at 30 W/m and film deposition started. As the sensing device, 6 MHz gold coated QCM(purchased from Leybold Inficon) was used. It has a disk shape and its active area is about 0.29 &. QCM was connected to the oscillator circuit, driven at 15 V dc and the frequency signal output was measured by frequency counter on-lined to personal computer. To check the sensitivity of polythiophenes, standard gas made by the controlled mixing of 1.1 % NOz flow to pure NZ flow at room temperature and then, NOz gas of 550 ppn was continuously introduced to the sensor cell.
DETAILS
As the monolayer study and film deposition device, KSV-300 and a constant-perimeter barrier type of Langmuir film device were used. The ester substituted polythiophenes were prepared according to procedures described elsewhere[51. POT, PEPT and PEDT given in Figure 1 are spread over air-water interface, but none of them are transferred as itself. When they are mixed with surface active material like ODA or stearic acid and at high subphase pH condition, the stable LB film can be obtained. So in this study, we made LB film with good quality by mixing polythiophene with ODA(Aldrich co., 1:l mixture based on molar ratio of polythiophene repeating unit) and made subphase pH nearby 9 using NaOH.
0379-6779/95/$09.50 0 1995 Elsevier Science S.A. Ah rights reserved SSDI 0379-6779(94)03149-Z
Figure 1. The molecular
RESULTS
structures
of polythiophenes.
& DISCUSSION
The n-A isotherm of each polythiophene is shown in Figure 2. In the case of PEDT, there are two phase
S.-R. Kim et al. I Synthetic Metals 71 (1995) 2027-2028
2028
transitions while in the case of PEPT, only one observed. As shown in material structure in Figure 1, PEDT, PEPT have ester linkage and PEDT has a long side chain, so we infer that one transition is from ester linkage and the other is from side chain attached to the ester linkage. The limiting area of three polythiophenes are almost the same at high pressure, because the heterocyclic ring structure is same.
PEPT/DDA(I:I)
/ 0
““’ II
10
Surface
Figure 4. U-A
0
Figure
10
20
30
40
Surface
Area, ?/repeat
2. U-A
isotherm
50
i
‘1 20
Area,
f \.
30
2/mixed
,____I 10
50
60
n~olecule
isotherm of polythiophene./ODAf.l:l); Surface is based on the repeat unit of polythiophene.
60
unit
of polythiophene.
The POT/ODA(l:l) LB film has a great NOz sensitivity as shown in Figure 3. For the same number of layers, the frequency change is larger in case of POT than the other two. In Figure 2, at 30 mN/m, the differences in area per repeat unit of each polythiophene are very large, but after mixing with ODA, the differences are not so considerable (Figure 41, therefore the frequency change difference is not due to the mass loaded on the QCM. The head group of each polythiophene is same, so the difference of sensitivity might be because of the ester linkage in side chain. In the Figure 5, as the LB film layer increases the frequency change i.e. sensitivity increase linearly. So we can control the sensitivity by the film thickness. After exposed to NOz, the frequency recovery is only 5-10% of the total change, even after treating in vacuum oven at 40 G during 1 tiv, the recovery to initial frequency is not achieved.
Layers ok’POT LB Film Figure 5. Layer effect of POT/ODAIl:l) for NOz (550 ppn).
LB film
CONCLUSION Polythiophene LB films with good quality have been made by mixing with surface active materials and by controlling the subphase conditions. Transfer ratio was nearly one up to considerable multilayers. The NOz sensitivity characteristics of POT, PEPT and PEDT were tested, and POT showed the best response to NOz of the three materials used in this study. And the sensitivity increase linearly with film thickness enough to control the sensitivity by layer numbers.
REFERENCES 1.
T. A. Skotheim, X. Q. Yang
and T. lnagaki,
Thin Solid
Films, 178, (1989) 233. 2.
NO, ON -120
~~~~‘~~~~,.~ 0 5 10
NOp OFF .~.~~.I~~ I5 20
1 .,,-.’ 25
30
Time. min Figure 3. Frequency change pattern of 14 layers polythiophene LB film(l:l mixture with ODA) for NOz (550 pm).
M.F. Rubner and T.A. Skotheim, in Conjugated Polymers: Controlled Molecular Assemblies of Electrically Conductive Polymers, eds. by J. L. Bredas and R. Silbey, Kluwer Academic Publishers, pp 3633403. 3. T.Hanawa, S. Kuwabata, H. Hashimoto and H. Yoneyama, Synthetic Metals, 30, (1989) 173. 4. J. F. Alder and J. J. McCallum, Analyst, 108, (1983) 1169. 5. C. Lee, K. J. Kim and S. B. Rhee, “The Effect of Ester Substitution and Alkyl Chain Length on the Properties of Poly(thiophene)s”, to be published in proc., ICSM, 1994.