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Synthesis of polyacetylene ultra-thin film
ELSEVIER
Synthetic Metals 101 (1999) 67-68
Synthesis of polyacetylene ultra-thin
film
T. S. Liang,K. Akagi, H. Shirakawa Instituteof MaterialsScience,Universityof Tsukuba,Tsukuba,Ibaraki 30.54573,Japan Abstract We have synthesizedpolyacetyleneultra-thin films with thicknessof 500- 1000A order. The films were synthesizedusinghighly concentrated catalystof Ti(O-n-Bu),-EtsAlwith thin layer. Activation energy(&) of acetylenepolymerizationunderlow initial pressure wasestimatedabout7.7 kJ/mole. Diametersof tibrils weresubstantiallyfiner than thoseof standardpolyacetylenes. This situation resultedin unexpectedreductionof in-planealignmentof tibrils. The electricalconductivity (0% IO3S/cm) after iodinedopingwas lowerthanthe predictedvalue( u > 10”S/cm). Keywords: Polyacet)tiene,Ultra-thin film,In-planealignment,Fibrii morphology I.
Introduction
It has been recently predicted that when the thicknessof polyacetylenefilm is lessthan 1.0 pm, the in-planealignmentof fibril becomeslarge, leadingto an enhancementof electrical conductivity [1,2]. Here,we synthesizedpolyacetyleneultra-thin filmsto examinethe abovepredictionandalsoto achievethe high conductivity. 2. Experimental Acetylene polymerizationswere carried out with the solvent evacuationmethod[3]. Cumenewas usedas a solvent. The catalyst solution was prepared under conditions that the concentration of tetra-n-butoxytitanium [Ti(O-n-Bu)d] was 0.1 mole/l, and the mole ratio between triethylaluminium [EtsAl] andthe titanium,[Al] / [Ti], was4. The catalystsolution was agedat room temperaturefor 1 hour and subsequentlyat 150“C for 1 hour. Since the catalystsolution after the aging was highly viscous, it was diluted with cumene until the concentrationof [Ti] was below 0.005 mole/l. The catalyst solutionthusdilutedwashomogeneously coatedon the innerwall of Schlenkflask. Then,the cumenewasevacuatedwith vacuum pumpingto give the highly concentratedcatalystwith thin layer. The thicknessof the catalystlayer wasestimatedfrom the quantity of the catalystandthe coatingareaof the innerwall of the flask. Acetylenepolymerizationswere performedundertwo kindsof conditions:(1) the thicknessof catalystlayer (d,,) andthe initial acetylene pressure(PO) were about 1.5 pm and 700 Torr, respectively,and(2) dUt andPOwereabout5.5 w and 12Torr, respectively. Even after the polymerization,the Schlenkflask was kept cool at -77 “C, usinga dry ice - ethanol. The films werewashedseveraltimeswith toluenethat hadbeenpre-cooled at -77 “C, to avoid a cis-trans thermal isomerization. The characterizations of the films were carried out through measurements of FT-IR and UV-Vis absorptionspectra. The film thicknesswasevaluatedfrom the calibrationcurve derived Tomthe relationshipbetweenIR absorbance andfilm thickness.
3. Resultsand Discussion 3.1. Propertiesof ultra-thinfilms The ultra-thin films with 500 - 1000A ( 0.05 - 0.1 pm) in thickness were synthesized under the both polymerization conditions. UV-Vis absorptionspectraof the films areshownin Fig. 1. The film synthesizedunderthe high acetylenepressure (POw 700 Torr) showeda narrowerabsorptionbandcompared with the caseof the low acetylenepressure(POm 12 Tot-r), indicating that the former has a narrower distribution of the conjugationlengththanthe latter [4-61.
Wavelength ( lo3 nm ) 2
1
0.5
1
2
Wavenumber
0.2
3
4
5
( lo4 cm-’ )
Fig. 1 Absorption spectraof ultra-thin films ( d = 500 10008, ) synthesized underPO= 700Torr andPO= 12 Torr. 3.2. Relationshipbetweenthe cis-contentandinitial acetylene pressure. The polymerizationtemperaturewas varied from -77 “C to room temperature. The &s-contentof the film dependson the
0379-6779/99/$ - see front matter 0 1999 Elsevier Science S.A. All rights reserved. PII: SO379-6779(98)01 144-8
68
T.S.Liang et al. I Synthetic Metals IO1 (1999) 6748
polymerizationtemperatureas expected(Fig. 2), but depends slightly on the initial acetylenepressure. 3.3. Activation energyfor acetylenepolymerization Assuming that the polymerization under the low initial acetylenepressure(Pa= 12 Torr) can be expressedby the tirstorderkinetics,the rateconstant(k) in the equationof In [P/PO] = - k t was evaluatedby monitoringthe changeof acetylene pressure(P) as a function of the polymerizationtime (t). The activation energy (ELI)was then calculatedby plotting the rate constantyersusthe polymerizationtemperature(T), asshownin Fig. 3, accordingto an Arrheniusequation; Ink=
1nA-&/ET,
?e 605 5
40 -
P 5
20-
3.4. Fibril morphologyandelectricalconductivity Fig. 4 showsa photographof scanningelectronmicroscope (SEM) of the ultra-thin film synthesizedunder PO= 700 Torr. The highly condensed fibrillar morphology is observed, irrespectiveof the initial acetylenepressure.This isquitedifferent from the porousfibrillar morphologyof the standardfilm [l], and rathercloseto the globularmorphologyof the highly stretchable film [3]. In fact, the presentfilm wassynthesizedwith the hightemperatureagedcatalyst. It is importantthat the fibrils of the film are lessthan 1OOAin diameterandthey are evidently finer thanthoseof the standardfilm [4]. Thisallowedthe fibrils to be randomlyorientedin termsof sideof film thickness,as well as sidesof film lengthandwidth, evenin the ultra-thinfilm. Thus the formationof the in-planealignmentof flbril is suppressed in the presentfilm. This resulted in an unexpectedelectrical conductivity of lo3 S/cmafter iodinedoping,which waslower thanthepredictedvalueof lo4 S/cm.
Polyacetyleneultra-thin films with thicknessof 500- 1000A order were synthesizedby controllingthe thicknessof catalyst layer and initial acetylene polymerization. The electrical conductivity of the films was lower by one order than the predictedvalue. This is dueto the randomorientationof the fine tibrils, which gives an unexpectedsuppression of the in-plane alignmentof libril evenin the ultra-thinfilm.
I
-50
!
0
Polymerization temperature
I
50
( “C )
Fig. 2 Cis-contentsof ultra-thin films synthesizedunder different polymerization temperaturewith high and low acetylenepressures.
Polymerization temperature T ( “C ) -50 -100 -5 50 : p -! -6 1
* -7 A -8
.104 Conclusion
I
0 1 -100
whereA andR arefrequencyfactor andgasconstant,respectively. The resultantactivationenergywasabout7.7 kJ/mol.
8
3
4 103/T
5 (K-l)
6
Fig. 3 Plot of Ink vs. 1/T underPO* 12Torr
5 References [l] H. Shirakawa,Yu-Xi Zang, K. Mochizuki, K. Akagi, H. Kyotani, Y. Tanabe,,Sj&r. Met., 41 (1991)13. [2] K. Akagi andH. Shlrakawa,Synth.Met., 60 (1993)85. [3] K. Akagi, M. Suezaki, H. Shirakawa,H. Kyotani, M. Shlmomura, Y. Tanabe,S’ynth.Met., 28 (1989)Dl. [4] H. E&hard& J. Chem.P&s., 79 (1983)208.5. [5] T. Kubo, T. Watanabe,T. Nishioka,H. Takezoe,A. Fukuda, Jpn.J. Appl. P&s., 31 (1992)37. [6] T. Nishioka,K. Suruga,K. Ishikawa,H. Takezoe,A. Fukuda, Jpn. J. Appl. Phys.,33 (1994)L953.
1 Pm Fig. 4 SEM photographof ultra-thinPA film with thickness of 1000A
Synthetic Metals 101 (1999) 67-68
Synthesis of polyacetylene ultra-thin
film
T. S. Liang,K. Akagi, H. Shirakawa Instituteof MaterialsScience,Universityof Tsukuba,Tsukuba,Ibaraki 30.54573,Japan Abstract We have synthesizedpolyacetyleneultra-thin films with thicknessof 500- 1000A order. The films were synthesizedusinghighly concentrated catalystof Ti(O-n-Bu),-EtsAlwith thin layer. Activation energy(&) of acetylenepolymerizationunderlow initial pressure wasestimatedabout7.7 kJ/mole. Diametersof tibrils weresubstantiallyfiner than thoseof standardpolyacetylenes. This situation resultedin unexpectedreductionof in-planealignmentof tibrils. The electricalconductivity (0% IO3S/cm) after iodinedopingwas lowerthanthe predictedvalue( u > 10”S/cm). Keywords: Polyacet)tiene,Ultra-thin film,In-planealignment,Fibrii morphology I.
Introduction
It has been recently predicted that when the thicknessof polyacetylenefilm is lessthan 1.0 pm, the in-planealignmentof fibril becomeslarge, leadingto an enhancementof electrical conductivity [1,2]. Here,we synthesizedpolyacetyleneultra-thin filmsto examinethe abovepredictionandalsoto achievethe high conductivity. 2. Experimental Acetylene polymerizationswere carried out with the solvent evacuationmethod[3]. Cumenewas usedas a solvent. The catalyst solution was prepared under conditions that the concentration of tetra-n-butoxytitanium [Ti(O-n-Bu)d] was 0.1 mole/l, and the mole ratio between triethylaluminium [EtsAl] andthe titanium,[Al] / [Ti], was4. The catalystsolution was agedat room temperaturefor 1 hour and subsequentlyat 150“C for 1 hour. Since the catalystsolution after the aging was highly viscous, it was diluted with cumene until the concentrationof [Ti] was below 0.005 mole/l. The catalyst solutionthusdilutedwashomogeneously coatedon the innerwall of Schlenkflask. Then,the cumenewasevacuatedwith vacuum pumpingto give the highly concentratedcatalystwith thin layer. The thicknessof the catalystlayer wasestimatedfrom the quantity of the catalystandthe coatingareaof the innerwall of the flask. Acetylenepolymerizationswere performedundertwo kindsof conditions:(1) the thicknessof catalystlayer (d,,) andthe initial acetylene pressure(PO) were about 1.5 pm and 700 Torr, respectively,and(2) dUt andPOwereabout5.5 w and 12Torr, respectively. Even after the polymerization,the Schlenkflask was kept cool at -77 “C, usinga dry ice - ethanol. The films werewashedseveraltimeswith toluenethat hadbeenpre-cooled at -77 “C, to avoid a cis-trans thermal isomerization. The characterizations of the films were carried out through measurements of FT-IR and UV-Vis absorptionspectra. The film thicknesswasevaluatedfrom the calibrationcurve derived Tomthe relationshipbetweenIR absorbance andfilm thickness.
3. Resultsand Discussion 3.1. Propertiesof ultra-thinfilms The ultra-thin films with 500 - 1000A ( 0.05 - 0.1 pm) in thickness were synthesized under the both polymerization conditions. UV-Vis absorptionspectraof the films areshownin Fig. 1. The film synthesizedunderthe high acetylenepressure (POw 700 Torr) showeda narrowerabsorptionbandcompared with the caseof the low acetylenepressure(POm 12 Tot-r), indicating that the former has a narrower distribution of the conjugationlengththanthe latter [4-61.
Wavelength ( lo3 nm ) 2
1
0.5
1
2
Wavenumber
0.2
3
4
5
( lo4 cm-’ )
Fig. 1 Absorption spectraof ultra-thin films ( d = 500 10008, ) synthesized underPO= 700Torr andPO= 12 Torr. 3.2. Relationshipbetweenthe cis-contentandinitial acetylene pressure. The polymerizationtemperaturewas varied from -77 “C to room temperature. The &s-contentof the film dependson the
0379-6779/99/$ - see front matter 0 1999 Elsevier Science S.A. All rights reserved. PII: SO379-6779(98)01 144-8
68
T.S.Liang et al. I Synthetic Metals IO1 (1999) 6748
polymerizationtemperatureas expected(Fig. 2), but depends slightly on the initial acetylenepressure. 3.3. Activation energyfor acetylenepolymerization Assuming that the polymerization under the low initial acetylenepressure(Pa= 12 Torr) can be expressedby the tirstorderkinetics,the rateconstant(k) in the equationof In [P/PO] = - k t was evaluatedby monitoringthe changeof acetylene pressure(P) as a function of the polymerizationtime (t). The activation energy (ELI)was then calculatedby plotting the rate constantyersusthe polymerizationtemperature(T), asshownin Fig. 3, accordingto an Arrheniusequation; Ink=
1nA-&/ET,
?e 605 5
40 -
P 5
20-
3.4. Fibril morphologyandelectricalconductivity Fig. 4 showsa photographof scanningelectronmicroscope (SEM) of the ultra-thin film synthesizedunder PO= 700 Torr. The highly condensed fibrillar morphology is observed, irrespectiveof the initial acetylenepressure.This isquitedifferent from the porousfibrillar morphologyof the standardfilm [l], and rathercloseto the globularmorphologyof the highly stretchable film [3]. In fact, the presentfilm wassynthesizedwith the hightemperatureagedcatalyst. It is importantthat the fibrils of the film are lessthan 1OOAin diameterandthey are evidently finer thanthoseof the standardfilm [4]. Thisallowedthe fibrils to be randomlyorientedin termsof sideof film thickness,as well as sidesof film lengthandwidth, evenin the ultra-thinfilm. Thus the formationof the in-planealignmentof flbril is suppressed in the presentfilm. This resulted in an unexpectedelectrical conductivity of lo3 S/cmafter iodinedoping,which waslower thanthepredictedvalueof lo4 S/cm.
Polyacetyleneultra-thin films with thicknessof 500- 1000A order were synthesizedby controllingthe thicknessof catalyst layer and initial acetylene polymerization. The electrical conductivity of the films was lower by one order than the predictedvalue. This is dueto the randomorientationof the fine tibrils, which gives an unexpectedsuppression of the in-plane alignmentof libril evenin the ultra-thinfilm.
I
-50
!
0
Polymerization temperature
I
50
( “C )
Fig. 2 Cis-contentsof ultra-thin films synthesizedunder different polymerization temperaturewith high and low acetylenepressures.
Polymerization temperature T ( “C ) -50 -100 -5 50 : p -! -6 1
* -7 A -8
.104 Conclusion
I
0 1 -100
whereA andR arefrequencyfactor andgasconstant,respectively. The resultantactivationenergywasabout7.7 kJ/mol.
8
3
4 103/T
5 (K-l)
6
Fig. 3 Plot of Ink vs. 1/T underPO* 12Torr
5 References [l] H. Shirakawa,Yu-Xi Zang, K. Mochizuki, K. Akagi, H. Kyotani, Y. Tanabe,,Sj&r. Met., 41 (1991)13. [2] K. Akagi andH. Shlrakawa,Synth.Met., 60 (1993)85. [3] K. Akagi, M. Suezaki, H. Shirakawa,H. Kyotani, M. Shlmomura, Y. Tanabe,S’ynth.Met., 28 (1989)Dl. [4] H. E&hard& J. Chem.P&s., 79 (1983)208.5. [5] T. Kubo, T. Watanabe,T. Nishioka,H. Takezoe,A. Fukuda, Jpn.J. Appl. P&s., 31 (1992)37. [6] T. Nishioka,K. Suruga,K. Ishikawa,H. Takezoe,A. Fukuda, Jpn. J. Appl. Phys.,33 (1994)L953.
1 Pm Fig. 4 SEM photographof ultra-thinPA film with thickness of 1000A