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Microstructured conducting polymers
ELSEVIER
Synthetic
Metals85 (1997) 1401-1402
Microstructured
conducting
polymers
L. Dunsch, P. Rapta*, A. Neudeck, A. Bar& R.-M. Peters**, D. Reinecke**, I. Apfelstedt** IFW Dresden e.V., Institut i%r Festkorperforschung, Helmholtzstr. 20, D-01069 Dresden, Germany *Permanent address: Slovak Technical University, Radlinskeho 9, SK-812 37 Bratislava, Slovak Republic **microParts Gesellschaft t?ir Mikrostrukhutechnik mbH, Hauert 7, D-44227 Dortmund; Germany
ABSTRACT LroA-technique allows the fabrication of microstructured electrodes used for spectroscopic applications in a new spectroelectrochemical cell. Thus the electrochemical and spectroscopic properties of redox systems in aqueous and non aqueous solvents can be studied. In this work the deposition of polypyrrole in molded Lroa-forms is studied on the basis of former work on electrochemical polypyrrole deposition at high current densities. The stable polypyrrole-LIGA structures can be formed with high aspect ratios by electropolymerization of monomeric pyrrole in aqueous solutions. With this electrochemical preparation precise ~O~~TX~-LIGA structures can be prepared with a stick width of lo-50 pm and a structure height of 110 pm. The new structurized polymer films were characterized by scanning electron microscopy, electrical conductivity, ESR spectroscopy and voltammetric measurements. The application of the polypyrrole-LrGA structures in the spectrcelectrochtical LIGA~.~ developed earlier by the group is presented. Keywords: polypyrrole, electrochemical polymerization, in situ electrochemical spectroscopy, electron spin resonance, sensor
1. Introduction
2. Experimental
The lithographic structuring and galvanic deposition (LIGA) technique [I] has recently been used for the fabrication of electrochemical and microstructured electrodes for spectroelectrcchemical investigations [2]. Up till now such electrodes are made of metals like gold, palladium or nickel using the electroforming step of the LIGA process [l]. LIGAstructured metals have been preferably used for spectroscopic applications in a new spectroelectrochemical cell described elsewhere [2a] and are well characterized with respect to their electrochemical and spectroscopic properties using several redox systems in aqueous and organic solvents [2b]. The preparation of microstructured conductive polymers such as polypyrrole (PPy), polyaniline and polythiophene with high electrical conductivity, good environmental stability and interesting redox properties is important due to their possible applications in electrocatalysis, electrochemical synthesis, energy conversion, sensors, electrochromic applications, micr* electronics, battery electrodes etc. [see e.g. 31. In this report the possibility to prepare honeycombmicrostructured electrodes using the new family of conducting materials - the electroactive polymers and the special polymethylmethacrylate (PMMA>LIGA masks is presented. It is shown that stable PpY-LIGA structures can be formed using masks prepared on the LIGA-technique of any desired structure. The LIGA-structured polymer layers were characterized by scanning electron microscopy (SEM), electrical conductivity, electron spin resonance spectroscopy (ESR) and voltammetric measurements [4]. The application of the polypyrrole-LIGA structures in a new kind of spectroelectrochemical LIGA-cell is demonstrated. o37g6779/97/$17.00 0 1997 Else&r PII s0379-6779(96)04410-4
Science S.A All rights reserved
The new electrochemical preparation cell used for the microscopic control of the PPy-LIGA structure formation is shown in Fig.1. The preparation cell consists of a teflon ring cell, two metal blccks and the special transparent electrodes. The PMMA-LIGA mask with a thin gold layer of approximately
Fig. 1: The scheme of the electrochemical preparation cell for the observation of the polymerization process under the microscope. (A) metal blocks, (B) Silicon rubber seals, (C) PPycoated gold array electrode, (Dl) Teflon ring cell, (D2) Silicon rubber ring seal, (E) gold-PMMA-LIGA mask.
1402
L. Dmsch
et al. /SyntheticMetals
2 urn on the bottom of the structure was used as working electrode (WE). A gold wire is fixed by conductive silver glue at the PMMA-LIGA mask to connect working electrode with the potentiostat. A polypyrrole coated gold array electrode serves as a counter electrode (CE) and is located on top of the ring cell. A silver wire coated with silver chloride (Ag/AgCl) serves as a reference electrode (RE). PMMA honeycomb structures with holes of a spanner width of 10-50 urn, a wall thickness 10 p and 50 v and a structure height of 110 urn were used as microstructured nonconducting material. For polymerization an aqueous solution of 0.01 M pyrrole in 0.1 M p-toluene-sulfonic acid (tosylate) was rigorously deaerated before use, filled in the polymerization cell and polymerized at an applied potential in the range +500 mV to +lOOO mV vs. Ag/AgCl electrode. The polymer growth process was simultaneously observed under the microscope. ESR spectra were recorded on an ERS 221 X-band spectrometer (ZWG Berlin) with 100 kH.z modulation at a microwave power of 1 mW. The electrochemical experiments were done with the computer driven PAR 173 potentiostatgalvanostat (EG&G) for preparative work. Spectroelectrochemical measurements with up to 60 UV-Vis measurements during on single scan in cyclovoltammetry were done. For WVIS spectra an InstaSpec II diode array spectrometer (LOT Oriel) was used driven by a computerand linked to that of the HEKA potentiostatPG 285.
85 (1997)
1401-1402
materialse.g. epoxy resin, catalyst, enzymesetc. In prolonged polymerizationthe wholestructureis overggown. iv) After removingthe PPy film or epoxyresin from the top of the PMMA structuresand dissolvingof PMMA the final PPyLIGA structuresareobtained(Fig.Zd). The electrical conductivity of 5uch PPy structuresin the oxidized state is in the range of some 10Scm-’ which is sufficient for the application in electrochemicalstudies as workingelectrodematerials. The ESR spectroscopicstudy of LIGA structured PPy indicatesthat the structureshave the samepammagnetic centres asin two-dimensional PPystructures[4,5]. II-I W-Vis spectroelectrochemiclexperimentsa diazepin oxidation was studied.Absorptionbandsat about 300 nm and 398run are typical for dissolveddiazepin. Further absorption bandsappearat 326mn, 390run and410nm when increasing
3. Results and Discussion
The processof the polymergrowth on the PMMA-LIGA mask consistsof the followingsteps(Fig. 2): i) At first, the thin gold layer on the bottom of the LIGAmaskis coveredby a thin polymerlayer(Fig.2a). ii) During tiu-ther polymerization PPy grows at PMMA walls until “chimney” structuresare formed (Fig.Zb). PMMAfacing surfaceof polymeris smoothand homogenous, i. e. the regularhoneycombstructuredchannelswith holesin the range of 10to 50 urn canbe preparedandusedfor electrochemical and spectroelectrcchemical studiesasdescribedbelow.
(a)~~~~ @)~ * ................................. .-.................................
tc)~ Cd) .................. 4 PPy
Fig. 3: UV-Vis sprectraobservedin cyclovoltammetryof 5.10q3 M diazepin in 0.1 M LiClO, MeCN solution at a PPy-LIGA structuredworking electrode;scan +200mV + +900 mV + +200 mV, scanrate20 mV s-r the potential fYom200mV to 900 mV. In the reversescanthe new bandsdisappearandthe original spectrumof the substance wasobservedagain.The spectraof the spectrocyclovoItammetric experiment are causedby the product of the one electron oxidation of diazepin. The experiment demonstratesthe suitability of PFy-LIGA structure in spectroelectrochemical LIGA cells. References
q
Epoxy resin
Fig.2.: The schematicdiagramof the preparationprocessof PPyLIGA structure iii) “Square-wave”structureswith large surfacearea are obtainedby a total coverageof PMMA with PPy (Fig.2c). SEM micrographof the polymer surfacefacing the polymer solution showsmoreheterogeneous polymersurfacesat its outer face [4]. Needlesformed in the growing processcan be suppressed by optimizing experimental conditions. At this degree of polymerizationthe obtainedstructurescanbe filled with various
[l] (a) W. Ehrfeld andE. W. Becker,KfK-Nachrichten19 (1987)380,(b) W. Ehrfeld and D. Munchmeyer,Nucl. Instr. Meth. Phys.Res.A 303(1991) 523. [2] (a) A. NeudeckandL. Dunsch,Ber. Bunsenges. Phys. Chem.97 (1993)407,(b) A. NeudeckandL. Dunsch,J. Electroanal.Chem.370(1994) 17, (c) A. NeudeckandL. Dunsch,J. Electroanal.Chem.386 (1995) 135 [3] Ch. R. Martin, Adv. Mat. 7 (1995) 487. [4] L. Dunsch,P. Rapta,A. Neudeck,A. Bar& R.-M. Peters,D. Reinecke,I. Apfelstedt,in MicrosystemTechnologyfor ChemicalandBiologicalMicroreactors,DECHEMA Monographs132(1996)p. 205. L. Dunsch [5] G. Paasch,D. Schmei.Ser, A Bar-U,H. Naarmasm, andW. Gopel, Synth.Met. 66 (1994) 135.
Synthetic
Metals85 (1997) 1401-1402
Microstructured
conducting
polymers
L. Dunsch, P. Rapta*, A. Neudeck, A. Bar& R.-M. Peters**, D. Reinecke**, I. Apfelstedt** IFW Dresden e.V., Institut i%r Festkorperforschung, Helmholtzstr. 20, D-01069 Dresden, Germany *Permanent address: Slovak Technical University, Radlinskeho 9, SK-812 37 Bratislava, Slovak Republic **microParts Gesellschaft t?ir Mikrostrukhutechnik mbH, Hauert 7, D-44227 Dortmund; Germany
ABSTRACT LroA-technique allows the fabrication of microstructured electrodes used for spectroscopic applications in a new spectroelectrochemical cell. Thus the electrochemical and spectroscopic properties of redox systems in aqueous and non aqueous solvents can be studied. In this work the deposition of polypyrrole in molded Lroa-forms is studied on the basis of former work on electrochemical polypyrrole deposition at high current densities. The stable polypyrrole-LIGA structures can be formed with high aspect ratios by electropolymerization of monomeric pyrrole in aqueous solutions. With this electrochemical preparation precise ~O~~TX~-LIGA structures can be prepared with a stick width of lo-50 pm and a structure height of 110 pm. The new structurized polymer films were characterized by scanning electron microscopy, electrical conductivity, ESR spectroscopy and voltammetric measurements. The application of the polypyrrole-LrGA structures in the spectrcelectrochtical LIGA~.~ developed earlier by the group is presented. Keywords: polypyrrole, electrochemical polymerization, in situ electrochemical spectroscopy, electron spin resonance, sensor
1. Introduction
2. Experimental
The lithographic structuring and galvanic deposition (LIGA) technique [I] has recently been used for the fabrication of electrochemical and microstructured electrodes for spectroelectrcchemical investigations [2]. Up till now such electrodes are made of metals like gold, palladium or nickel using the electroforming step of the LIGA process [l]. LIGAstructured metals have been preferably used for spectroscopic applications in a new spectroelectrochemical cell described elsewhere [2a] and are well characterized with respect to their electrochemical and spectroscopic properties using several redox systems in aqueous and organic solvents [2b]. The preparation of microstructured conductive polymers such as polypyrrole (PPy), polyaniline and polythiophene with high electrical conductivity, good environmental stability and interesting redox properties is important due to their possible applications in electrocatalysis, electrochemical synthesis, energy conversion, sensors, electrochromic applications, micr* electronics, battery electrodes etc. [see e.g. 31. In this report the possibility to prepare honeycombmicrostructured electrodes using the new family of conducting materials - the electroactive polymers and the special polymethylmethacrylate (PMMA>LIGA masks is presented. It is shown that stable PpY-LIGA structures can be formed using masks prepared on the LIGA-technique of any desired structure. The LIGA-structured polymer layers were characterized by scanning electron microscopy (SEM), electrical conductivity, electron spin resonance spectroscopy (ESR) and voltammetric measurements [4]. The application of the polypyrrole-LIGA structures in a new kind of spectroelectrochemical LIGA-cell is demonstrated. o37g6779/97/$17.00 0 1997 Else&r PII s0379-6779(96)04410-4
Science S.A All rights reserved
The new electrochemical preparation cell used for the microscopic control of the PPy-LIGA structure formation is shown in Fig.1. The preparation cell consists of a teflon ring cell, two metal blccks and the special transparent electrodes. The PMMA-LIGA mask with a thin gold layer of approximately
Fig. 1: The scheme of the electrochemical preparation cell for the observation of the polymerization process under the microscope. (A) metal blocks, (B) Silicon rubber seals, (C) PPycoated gold array electrode, (Dl) Teflon ring cell, (D2) Silicon rubber ring seal, (E) gold-PMMA-LIGA mask.
1402
L. Dmsch
et al. /SyntheticMetals
2 urn on the bottom of the structure was used as working electrode (WE). A gold wire is fixed by conductive silver glue at the PMMA-LIGA mask to connect working electrode with the potentiostat. A polypyrrole coated gold array electrode serves as a counter electrode (CE) and is located on top of the ring cell. A silver wire coated with silver chloride (Ag/AgCl) serves as a reference electrode (RE). PMMA honeycomb structures with holes of a spanner width of 10-50 urn, a wall thickness 10 p and 50 v and a structure height of 110 urn were used as microstructured nonconducting material. For polymerization an aqueous solution of 0.01 M pyrrole in 0.1 M p-toluene-sulfonic acid (tosylate) was rigorously deaerated before use, filled in the polymerization cell and polymerized at an applied potential in the range +500 mV to +lOOO mV vs. Ag/AgCl electrode. The polymer growth process was simultaneously observed under the microscope. ESR spectra were recorded on an ERS 221 X-band spectrometer (ZWG Berlin) with 100 kH.z modulation at a microwave power of 1 mW. The electrochemical experiments were done with the computer driven PAR 173 potentiostatgalvanostat (EG&G) for preparative work. Spectroelectrochemical measurements with up to 60 UV-Vis measurements during on single scan in cyclovoltammetry were done. For WVIS spectra an InstaSpec II diode array spectrometer (LOT Oriel) was used driven by a computerand linked to that of the HEKA potentiostatPG 285.
85 (1997)
1401-1402
materialse.g. epoxy resin, catalyst, enzymesetc. In prolonged polymerizationthe wholestructureis overggown. iv) After removingthe PPy film or epoxyresin from the top of the PMMA structuresand dissolvingof PMMA the final PPyLIGA structuresareobtained(Fig.Zd). The electrical conductivity of 5uch PPy structuresin the oxidized state is in the range of some 10Scm-’ which is sufficient for the application in electrochemicalstudies as workingelectrodematerials. The ESR spectroscopicstudy of LIGA structured PPy indicatesthat the structureshave the samepammagnetic centres asin two-dimensional PPystructures[4,5]. II-I W-Vis spectroelectrochemiclexperimentsa diazepin oxidation was studied.Absorptionbandsat about 300 nm and 398run are typical for dissolveddiazepin. Further absorption bandsappearat 326mn, 390run and410nm when increasing
3. Results and Discussion
The processof the polymergrowth on the PMMA-LIGA mask consistsof the followingsteps(Fig. 2): i) At first, the thin gold layer on the bottom of the LIGAmaskis coveredby a thin polymerlayer(Fig.2a). ii) During tiu-ther polymerization PPy grows at PMMA walls until “chimney” structuresare formed (Fig.Zb). PMMAfacing surfaceof polymeris smoothand homogenous, i. e. the regularhoneycombstructuredchannelswith holesin the range of 10to 50 urn canbe preparedandusedfor electrochemical and spectroelectrcchemical studiesasdescribedbelow.
(a)~~~~ @)~ * ................................. .-.................................
tc)~ Cd) .................. 4 PPy
Fig. 3: UV-Vis sprectraobservedin cyclovoltammetryof 5.10q3 M diazepin in 0.1 M LiClO, MeCN solution at a PPy-LIGA structuredworking electrode;scan +200mV + +900 mV + +200 mV, scanrate20 mV s-r the potential fYom200mV to 900 mV. In the reversescanthe new bandsdisappearandthe original spectrumof the substance wasobservedagain.The spectraof the spectrocyclovoItammetric experiment are causedby the product of the one electron oxidation of diazepin. The experiment demonstratesthe suitability of PFy-LIGA structure in spectroelectrochemical LIGA cells. References
q
Epoxy resin
Fig.2.: The schematicdiagramof the preparationprocessof PPyLIGA structure iii) “Square-wave”structureswith large surfacearea are obtainedby a total coverageof PMMA with PPy (Fig.2c). SEM micrographof the polymer surfacefacing the polymer solution showsmoreheterogeneous polymersurfacesat its outer face [4]. Needlesformed in the growing processcan be suppressed by optimizing experimental conditions. At this degree of polymerizationthe obtainedstructurescanbe filled with various
[l] (a) W. Ehrfeld andE. W. Becker,KfK-Nachrichten19 (1987)380,(b) W. Ehrfeld and D. Munchmeyer,Nucl. Instr. Meth. Phys.Res.A 303(1991) 523. [2] (a) A. NeudeckandL. Dunsch,Ber. Bunsenges. Phys. Chem.97 (1993)407,(b) A. NeudeckandL. Dunsch,J. Electroanal.Chem.370(1994) 17, (c) A. NeudeckandL. Dunsch,J. Electroanal.Chem.386 (1995) 135 [3] Ch. R. Martin, Adv. Mat. 7 (1995) 487. [4] L. Dunsch,P. Rapta,A. Neudeck,A. Bar& R.-M. Peters,D. Reinecke,I. Apfelstedt,in MicrosystemTechnologyfor ChemicalandBiologicalMicroreactors,DECHEMA Monographs132(1996)p. 205. L. Dunsch [5] G. Paasch,D. Schmei.Ser, A Bar-U,H. Naarmasm, andW. Gopel, Synth.Met. 66 (1994) 135.