J. Phys. Chem. B 2005, 109, 13205-13209
13205
Microscopy and Spectroscopy of Interactions between Metallopolymers and Carbon Nanotubes Fiona Frehill,† Marc in het Panhuis,*,‡ Nigel A. Young,‡ William Henry,† Johan Hjelm,† and Johannes G. Vos† National Centre for Sensor Research, School of Chemical Sciences, Dublin City UniVersity, Dublin 9, Ireland, and Department of Chemistry, The UniVersity of Hull, Hull, HU6 7RX, United Kingdom ReceiVed: March 21, 2005; In Final Form: May 24, 2005
The interaction between redox polymers, based on Ru- or Os-bis(2,2′-bipyridyl)-poly(4-vinylpyridine), and carbon nanotubes was investigated by spectroscopic and microscopic techniques. These metallopolymers were found to be excellent dispersants for nanotubes, as a result of a good wetting interaction between polymer and nanotubes. The results obtained show that well-coated individual nanotubes can be obtained. In addition, interactions between nanotubes and polymers did not significantly affect the electronic and electrochemical properties of the metallopolymers. On the basis of the electrochemical properties of the polymers this opens the possibility of adding functionality through interaction with nanotubes, either as redox active materials with enhanced mechanical properties or by using these modified nanotubes as nanosized electrochemical sensors.
1. Introduction Since their discovery in 19911 carbon nanotubes have attracted considerable attention, mainly because their unique combination of mechanical, thermal, optical, and electrical properties may form the basis of many potential applications.2-4 However, processing and manipulation of carbon nanotubes is limited by their lack of solubility in most solvents. Therefore, successful incorporation of nanotubes into materials requires the development of methods for their effective dispersion. Decreasing nanotube aggregation in solution and improving solubility are areas of intensive research. Compounds such as saccharides,5,6 surfactants,7-10 ceramics,11 biological entities,12,13 synthetic polymers,14-25 and chemically modified soy oil26 have been used to disperse single-walled (SWNT) and multiwalled (MWNT) carbon nanotubes through noncovalent functionalization. Polymers are attractive materials since they may be easily processed and fabricated into solid-state forms such as thin films required in most applications.27 In addition to acting as efficient nanotube dispersing agents, employing polymers in composites is of interest because they allow for a novel combination of nanotube and polymer properties. Previous work established that the formation of stable polymer-nanotube composites depends on a good wetting interaction between polymers and nanotubes.14,16 This in turn has been found to be dependent in particular on the polymer chain conformation.16,19 In addition it was determined that certain polymers form crystalline layers around nanotubes thereby increasing polymer backbone-nanotubes interactions.23,24 Composite materials from nanotubes and conducting polymers have been investigated.19,21,22,25 The electrochemical activity in these polymers is based on the electronic and electrochemical properties of the polymer backbone. On the other hand, for redox * To whom correspondence
[email protected]. † Dublin City University. ‡ The University of Hull.
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Figure 1. Structure of metallopolymers [M(bpy)2PVP10Cl]Cl, with M ) Ru or Os.
polymers such as materials of the type [M(bpy)2(PVP)10Cl]Cl (see Figure 1), where M ) Ru or Os, bpy is 2,2′-bipyridine, and PVP is poly(4-vinyl)pyridine, considered in this paper, the electrochemically active moieties are not related to the backbone but are based on pendent groups of the type M(bpy)2-, which are covalently linked to the polymer backbone.28-30 The polymers are of interest since their electrochemistry is extremely reversible and the materials are very stable in several oxidation states. They have therefore been used extensively as redox catalysts for the sensing of analytes such as Fe(II)/Fe(III), nitrite, and ascorbic acid and have been applied in biosensor assemblies.31-35 In addition, there is a well-developed synthetic chemistry for these redox metallopolymers containing poly(pyridyl) complexes of ruthenium(II) and osmium(II) and their redox potential can be varied systematically.36 In this contribution, (noncovalent) interactions between two redox metallopolymers and single-walled and multiwalled carbon nanotubes are investigated, using a range of microscopic and spectroscopic techniques. 2. Experimental Details SWNT37 were prepared by high-pressure decomposition of carbon monoxide (HiPco) obtained from Carbon Nanotechnologies, Inc. (Houston, USA, batch no. P0-185). Catalytic chemical vapor deposition (CVD) produced MWNT were
10.1021/jp051459o CCC: $30.25 © 2005 American Chemical Society Published on Web 06/18/2005
13206 J. Phys. Chem. B, Vol. 109, No. 27, 2005 obtained from Nanocyl S.A. (Belgium, batch no. NFL60). The metallopolymers [Ru(bpy)2PVP10Cl]Cl and [Os(bpy)2PVP10Cl]Cl where bpy is 2,2′-bipyridine and PVP is poly(4-vinyl)pyridine were prepared as described previously.36 Nanotube dispersions were prepared by sonicating SWNT or MWNT in a 0.5% (weight per volume) methanolic solution of the polymer until a dense black dispersion had formed. The resulting composites are the following: [Ru(bpy)2PVP10Cl]ClMWNT 1, [Ru(bpy)2PVP10Cl]Cl-SWNT 2, [Os(bpy)2PVP10Cl]Cl-MWNT 3, and [Os(bpy)2PVP10Cl]Cl-SWNT 4. UV-vis absorption spectra (accuracy (2 nm) were recorded in 10 mm quartz cuvettes on a Shimadzu UV-vis-NIR 3100 spectrophotometer interfaced with an Elonex PC466, using a UV-vis data manager. Raman spectra were collected in 180° scattering geometry with a Bruker FRA 106/S Raman module mounted on a Bruker Equinox55 FTIR bench with a cw Nd: YAG 1064 nm laser at ca. 100 mW, a CaF2 beam splitter, and a liquid nitrogen cooled Ge detector. Transmission electron microscopy (TEM) images were obtained on a JEOL 2011 TEM. Diluted composite solutions were evaporated onto Cu 300 mesh grids. Tapping mode atomic force microscopy (TM-AFM) was carried out in an air environment with use of a Digital Instruments Nanoscope IIIa. Laser alignment was performed manually, and cantilever tuning and drive frequency selection was automatically performed by the DI software. The cantilevers used are made of silicon and have a triangular pyramid shape, a tip radius
