Characterizing the Dopant Behavior of Functionalized Carbon Nanotubes in Conducting Polymers
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Characterizing the Dopant Behavior of Functionalized Carbon Nanotubes in Conducting Polymers Mark Hughes1,*, Graeme A. Snook1, George Z. Chen2, Milo S. P. Shaffer3, Derek J. Fray1 and Alan H. Windle1 1 Department of Materials Science and Metallurgy, University of Cambridge, Pembroke Street, Cambridge, CB2 3QZ, UK. 2 School of Chemical, Environmental and Mining Engineering, University of Nottingham, Nottingham, NG7 2RD, UK. 3 Department of Chemistry, Imperial College, London, SW7 2AZ, UK. ABSTRACT The electrochemical polymerization of conducting polymers, such as polypyrrole, generally requires the incorporation of an anionic dopant to balance the positive charge on the oxidized conducting polymer chains. The susceptibility of multiwalled carbon nanotube (MWNT) surfaces to functionalization makes them exciting candidates for a new class of dopant for conducting polymers. In this work, the doping of polypyrrole with functionalized MWNTs is investigated using a combination of electrochemical impedance spectroscopy, scanning electron microscopy, and quartz crystal microbalance work. The findings described here are particularly relevant in light of recent reports indicating that carbon nanotube-conducting polymer composites hold great promise for use in electrochemical capacitors, also known as supercapacitors [1,2]. INTRODUCTION Electrically conducting polymers, such as polypyrrole (PPy), are frequently deposited as films via electrochemical oxidation of the relevant monomer. During deposition, an anionic dopant is typically incorporated into the ECP to balance the positive charge on the oxidized conducting polymer chains. The susceptibility of multiwalled carbon nanotube (MWNT) surfaces to functionalization makes them exciting candidates for a new class of dopant for conducting polymers. In this work, hydroxyl, carbonyl and carboxylic groups are attached to the surface of MWNTs via an acid-treatment process involving nitric and sulfuric acid. These functional groups give the acid-treated MWNTs a net negative charge, enabling them to provide charge balance for the oxidized conducting polymers, as do more conventional anionic dopants. Once deposited, the functionalized MWNTs are permanently embedded within the polymer, forming a continuous nanocomposite film. The work described here relates to MWNT-PPy nanocomposite films in which the MWNTs are the only dopant used during film deposition. Electrochemical impedance spectroscopy, scanning electron microscopy, and electrochemical quartz crystal microbalance work are used to characterize the doping behavior of the functionalized MWNTs. A comparison is also made between MWNT-PPy composite films made using functionalized MWNTs and MWNT-PPy composite films made using pristine MWNT (not functionalized) arrays. These findings are particularly relevant in light of recent reports indicating that carbon nanotube-conducting polymer composites hold great promise for use in electrochemical capacitors, frequently referred to as supercapacitors [1,2].
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