polypyrrole composites: Formation of polypyrrole wires

ELSEVIER

Electrosynthesis

Synthetic Metals 101 (1999) 3-4

of polyacrylidpolypyrrole

composites: formation

of polypyrrole

wires

C. Jerome*,D. Labaye,I. Bodart,R. Jerome Centre for Education and Research on Macromolecules (GERM) University of Likge, B6, Sart-Tilman, B-4000 Lk?ge, Belgium

Abstract A newpreparationmethodis reportedfor the synthesis of conductingpolypyrrolenanowires.This two-stepall-electrochemical methodconsistsin the cathodicgrafting of a thin polyacrylatefilm on a carbonelectrode,which is then usedas an anodefor the electropolymerization of pyrrole. Scanningelectronmicroscopyshowsthe formationof polypyrrolewires(diameter-600 nm lenght-300 pm) growing through holesin the polyacxylateprecoating.The effect of the experimentalconditions,suchas natureof the grafted polymer,solvent,andelectrode,on the wire formationis discussed. Keywords: Electropolymerization, Polypyrrole,Nanowires

1. Introduction

3. Resultsand discussion

Researchon metal and semiconductor nanowiresis a rapidly expandingfield stimulatedby the need for molecular conductorsof nanometricdimensions.In this respect,several researchgroupsarepayingattentionto the confinementof doped conductingpolymers,suchaspolypyrrole (PPy), in appropriate porousstructure[l]. Quite recently, we have reportedon a new electrochemical techniquefor the synthesisof PPy nanowirest-21. A carbonelectrodeis modifiedby the cathodicelectrograftingof a polyethylacrylate(PEA) film, which is then usedasa template for the electrosynthesisof PPy. This paper deals with the dependenceof the nanowire formation on experimental parameters,such as nature of the solvent, the electrografted polymer,andthe electrodesubstrate.

3.1 Effect of the sobent

Pyrrole has beenelectropolymerizedin DMF by the controlled polarization (250 mC) of a glassy carbon anode previouslycoatedby anelectrograftedPEA film whichis actually swollenby DMF. After careful washingby acetonitrile(ACN). the anodehasbeenanalyzedby SEM, and PPy wireshasbeen observedto emergefrom the grafted layer (thicknessof the grafted film -500 nm) (Fig. 1). These wires are randomly dispersed on the electrodesurface,with diameterin the rangeof 400nmto 1 pm andlengthof ca. 300pm.

2. Experimental technique The first electropolymerizationreactionconsistsin the electrograftingof a polyacrylateprecoatby scanningthe potential down to -1.8V/Pt. Accordingto previousworks[3], acrylonitrile (AN) and ethylacrylate(EA) can be electrograftedonto various substratessuch as metals (Pt, Fe, Ni) and carbon, in dimethylfotmamide(DMF) added with tetraethyl ammonium perchlorate (lEAP:O.OSM) as conducting salt. These thin polyacrylate films have the uniqueproperty of being attached onto the electrodesurfacealthoughthey are formed in a good ‘solvent for the polymer. The swellingof thesefilms by DMF allows the coated electrodeto be used as the anodefor the electropolymerization of pyrrole. PPy is synthesized by chronopotentiometry undera constantcurrentof +OSmA/cm*for ,a constanttime of 500sin a DMF solutiondecimolarin Py and 10.05Min TEAP. *CJ is grateful to the “Fonds de la Recherche pour I’Industrie et I’Agriculkre” for a fellowship. The authors are indebted to the “Services FedCraux des Affaires Scientifiques, Techniques et CultureIles” for support in the frame of the “Poles d’Attractions Interuniversitaires: Supramolecular Catalysis and Supramolecular Chemistry”

10pm Fig. 1 Scanningelectronmicrographof a PPy/CIOswire grown from a PEA graftedcarbonelectrode It mustbe notedthat PPy hasa low conductivity when electiopolymerizedin DMF, i.e. a solvent of a high donor number(S -5x10’ S/cm [4]), comparedto the conductivity

- see front matter 0 1999 Elsevier Science S.A. All rights reserved. II: SO379-6779(98)00524-4

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C. Jfir8me et al. i Synthetic Metals

observedwhenACN is substitutedfor DMF (S - 80 S/cm[S]). So,the electropolymerization of Py hasbeenrepeatedin ACN all the other conditions being kept unchanged(PEA grafted C electrode,TEAP O.OSM,Py 0.lM, I = 0.5 mA/cm’, t = 500 s). Figure2 comparesthe chronopotentiograms observedin the two solventsfor the samecurrent density.The potentialobservedin ACN (Fig. 2B) is lower thanin DMF (Fig. 2A), whichindicatesa betterconductivity. However,no wire is observedby SEM for the samplepreparedin ACN, but rather a PPy depositionthat exhibitsthe cauliflower aspectusually observedin caseof non previouslycoatedanodes. 2400 ,

101 (1999)

3-4

of the graftedPEA film comparedto a plateauat 1.33V when PAN is graftedontothe anode. SEM observationof the PANIpPy composite-shows a nodularmorphologyfor PPy (Fig. 3), which supportsthat the structural characteristicsof the conducting polymer may be deeply affectedby the insulatingpolymer which is pregrafted onto the electrode a@ acts as a template for the Py polymerization.

I

2100-1800-1500--

600 -300 -0 --300 I -50

2.5pm 100

250 t (4

400

550

Fig. 2 Chronopotentiogram of PPy synthesis(I = 0.5 mA): A. in DMF onto PEA graftedC; B. in ACN onto PEA graftedC; C, in DMF ontoPAN graftedC. SinceACN and DMF are good solventsfor PEA, the swellingof this graftedPEA film is expectedto be comparable in the two solvents.Themainsolventeffect shouldconcernthe PPy conductivity. According to the proposed mechanismfor the formationandgrowth of the PPy wire [2], we haveto admitthat in DMF, the PPy is not growing on itself but that the polymerizationonly occursfrom the foot of the wire near the electrodesurface. Wire formation actually indicatesthat Py cannotbe oxidizedon the surfaceof PPy that hasgrownthrough someholes in the electrograftedPEA film. This conclusionis consistentwith the low conductivity of PPy formedin DMF [4]. In contrast,the conductingpropertiesof PPy areimprovedwhen formedin ACN, so that Py can be oxidized on the PPy surface leadingaccordinglyto a threedimensionalgrowthratherthan to an unidimensional one.This tentativeexplanationof the solvent effect will be substantiated by extendingtheanalysisto additional solvents(DMSO, PC,...). 3.2 Effect of the grafled polymer The role of the graftedfilm on the formationof PPy wireshasbeenstudiedby substitutinga thermoplasticPAN film for the soft PEA film used until now. Fig. 2C showsthe chronopotentiogram observedwhen PPy is synthesizedin DMF with a C anodepreviouslygraftedby PAN. The potentialremains lower in DMF whenthe electrodeis graftedby PAN ratherthan by PEA (Fig. 2A and2C). Thebettercoatingpropertiesof PEA is expectedto limit the Py diffusionto the electrodesurfacein line with the overpotentialobservedat 1.8V for the first 200sin case

Fig. 3 Scanningelectronmicrographof PPy/c10~synthesized ontoa PAN graftedC electrode. 3.3 Effect of the electrode substrate

Finally, thenatureof the conductingsubstrate-hasbeen modified.PEA hasbeengraftedonto Pt insteadof glassycarbon. Sincethe Py polymerizationrequiresan anodicpotential,the use of metalsproneto oxidation, suchas Fe or Ni, is much less convenient.The chronopotentiogram (Fig. 2D) showsan initial overpotentialat 2.3V that rapidly decreases down to a plateau valueat l&V. The morphologyof the PPy formed on the Pt/PEA electrodeis alsonodular,and no wire is observed.The actual effect of the substrateon the morpholo,gof PPy is uri?learyet, and requiresadditional experimentsbefore drawing definite conclusions. 4. Conclusion PPy wirescangrow from a PEA graftedcarbonanode when Py is electropolymerizedin DMF added with TEAP. Modification of experimentalparameters, suchassolvent(ACN insteadof DMF), grafled polymer template(PAN rather than PEA), andelectrodesubstrate(Pt insteadof C) greatly affectsthe morphologyof PPywhichturnsout to looseits uniquecapability of formingwires.A deeperanalysisis requiredto understandthe basicmechanism for thePPy wire formation. 5. References [l] G.A. Ozin, Adv. Mater.,4 (1992)612 [2] C. Jerome,R. Jerome,Angew.Chem.,acceptedMay (1398) [3] R, JCrBme,M. Mertens,L. Martinot, Adv. Mater. 7 (1995) 807 [4] J. Jianyang,Y. Li, Polymer38 (1997)197-i ~_ [5] M. Omastova,S.Kosina,M. Lazar,S. Baluch,Eur. Polym. J. 30 (1994)1319