Electrically Conductive Low Dimensional Nanostructures: Synthesis, Characterisation and Application

Multi-walled carbon nanotubes modified by covalent grafting of poly(2-vinylpyridine) (CNT-g-P2VP) form stable dispersions in good solvents for P2VP, including water at low pH. The positively charged P2VP shell allows the immobilization of the CNTs onto oppositely charged surface. Deposition of CNT-g-P2VP from aqueous dispersions at low pH is an effective method to prepare homogeneous ultra-thin films with a tunable CNTs density. A percolation threshold of 0.2 μg/cm and critical exponent α=1.24 are found from dc conductivity measurements. The sheet resistance value agrees with the percolation theory for 2D-films. According to AFM and electrical measurements, even when only 5% of the surface is covered by CNT-g-P2VPs, the sheet resistance is of the order of 1MΩ/sq, which indicates that conductivity is imparted by a network of an ultra-low density. When the film transmittance decreases down to ~70% at 550nm, the occupied surface area is ~15% and sheet resistance falls down to ~90kΩ/sq. These data show that undesired in-plane clustering does not occur upon the dispersion casting of the films and that high quality networks of CNT-g-P2VPs are built up. The electrosteric stabilization of the CNT-g-P2VP dispersions in water at low pH is at the origin of this desired behavior. Although the MWNT films prepared in this work are less conductive and less transparent than the SWNTs films, they could find applications, e.g., in touch screens, reflective displays, EMI shielding and static charge dissipation. Moreover, the polymer shell of CNT-g-P2VP serves as a scaffold for immobilization of various materials that have an affinity to P2VP, including nanoclusters of Prussian Blue. These nanoparticle-based nanostructures might be useful materials for manufacture of electrooptical devices, biosensors or mechanically robust ion-sieving membranes. 3.1. Synthesis and solubilization of carbon nanotubes: state of the art Carbon nanotubes are molecular-scale tubes of graphitic carbon with outstanding properties. They are well-known by the stiffest and strongest fibres and have remarkable electronic properties and many other unique characteristics. Chapter 3 Nanotubes modified by covalent grafting of poly(2-vinylpyridine) For these reasons, they have attracted huge academic and industrial interest. The current interest in nanotubes is a direct consequence of the series of experiments on the vaporisation of graphite making by Harry Kroto and Richard Smalley who obtained fullerene, C60 , in 1985. The discovery that carbon could form stable, ordered structures other than graphite and diamond stimulated researchers worldwide to search for other new forms of carbon. The search was given new impetus when it was shown in 1990 that C60 could be produced in a simple arc-evaporation apparatus readily available in all laboratories and as an additional products carbon nanotubes were discovered by the electron microscopist Summio Iijima, of the Nec laboratories in Japan, in 1991. Carbon nanotubes are recognized as the ultimate carbon fiber with the highest strength of any material and the highest thermal conductivity, and, as been shown, to possess outstanding field emission properties. Transport in metallic carbon nanotubes has had ballistic nature, than means, that electron moves in a medium with negligible electrical resistance. They often have been used as the active semiconductors in nanoscale devices, due to their unique topologically controlled electronic properties. The tubes containing one-two layers of graphite are called single-walled nanotubes (SWNTs). The tubes which consist of many layers are designated as multi-walled nanonotubes (MWNTs). Nanotube diameters range from 0.4nm to more than 3nm for SWNTs and from 1.4nm to at least 100nm for MWNTs. Useful composite research requires a bulk supply of nanotubes of high purity and in an usable (i.e., easily dispersible) form. Existing technologies for the production of single-wall nanotubes (SWNTs) do not yield sufficient quantities and lack the required purity. Purification of these materials is often tedious, low in yield, and damaging to the tube’s structure through oxidative shortening. 22 For some applications such as conductive fillers or as reinforcing fibers, MWNTs are likely to be preferred over SWNTs on a cost basis. In the next paragraph, devoted to nanotubes I focus on the synthesis of carbon nanotubes in general with preferential attention to MWNT. Synthesis. The hybridisation of carbon’s orbital in nanotubes is sp2, with each atom joined to three neighbours, as in graphite. The tubes can therefore be considered as rolled-up graphene sheets (graphene is an individual graphite layer). There are three distinct ways in which a 13 Iijima, S. Nature 1991, 354, 56-58 14 Yu, M. F.; Files, B. S.; Arepalli, S.; Ruoff, R. S. Phys. Rev. Lett. 2000, 84, 5552-5555. 15 Hone, J.; Batlogg, B.; Benes, Z.; Johnson, A. T.; Fischer, J. E. Science 2000, 289, 1730-1733 16 de Heer, W. A.; Chatelain, A.; Ugaarte, D. A. Science 1995, 270, 1179-1180 17 Frank, S.; Poncharal, P.; Wang, Z. L.; de Heer, W. A. Science 1998, 280, 1744-1746 18 Collins, P. G.; Arnold, M. S.; Avouris, P. Science 2001, 292, 706-708 19 Hu, J.; Odom, T. W.; Lieber, C. M. Acc. Chem. Res. 1999, 32, 435-445 20 Baughman, R. H.; Zakhidov, A. A.; de Heer, W.A. Science 2002, 297, 787. 21 Sinnott, S. B.; Andrews, R. Solid State Mater. Sci. 2001, 26, 145-249. 22 Ebbesen, T. W. CRC Press: Boca Raton 1997, 226-248. 21 Chapter 3 Nanotubes modified by covalent grafting of poly(2-vinylpyridine) graphene sheet can be rolled into a tube. The first two, known as “armchair” and “zig-zag” have a high degree of symmetry. The terms "armchair" and "zig-zag" refer to the arrangement of hexagons around the circumference. The third class of tubes, which in practice is the most common, is known as chiral, meaning that it can exist in two mirror-related forms. Carbon nanotubes can be produced using two techniques: non-catalytic and catalytic. The “classical” method of non-catalytic technique is arc-evaporation synthesis. The original method was represented by Iijima that was slightly modified by Kraetschmer-Huffman. The electrodes are two graphite rods, usually of high purity, although there is no evidence that exceptionally pure graphite is necessary. Typically, the anode is a long rod approximately 6mm in diameter and cathode is a much shorter rod 9mm in diameter. Efficient water-cooling of the cathode has been shown to be essential in producing good quality nanotubes. The position of the anode should be adjustable from outside the chamber, so that a constant gap can be maintained during arc-evaporation. A voltage-stabilized DC power supply is normally used, and discharge is typically carried out at a voltage of 20V. The current depends on the diameter of the rods, their separation, the gas pressure, etc. But it is usually in the range of 50100A. When the pressure is stabilized, the voltage should be turned on. At the start of the experiment the electrodes should not be touching, so no current will initially flow. The moveable anode is not gradually moved closer to the cathode until arcing occurs. When a stable arc achieved, the gap between the rods should be maintained at approximately 1mm or less; the rod is normally consumed at a rate a few millimeters per minute. When the rod is consumed, the power should be turned off and the chamber left to cool before opening. A number of factors have been shown to be important in producing a good yield of high quality nanotubes. The pressure of the helium in the evaporation chamber (500Torr appears to be the optimum) and the current are the most important parameters. Non-catalytic methods are characterized with high quality tubes possess different structure. These methods are simple and useful for SWNTs and MWNTs, but the yield of the pure tubes is not so high. In the best sample, the ratio of the nanotubes to nanoparticles is of the order of 2:1 and, as a result, an additional purification of seed is necessary. This leads to high prices, especially for SWNT. Catalytic methods of syntheses were proposed to produce high-quality aligned nanotubes in bulk at low cost. It was well known for over the century that filamentous carbon can be formed by catalytic decomposition of a carbon containing gas on a hot surface. The phenomenon was lijima, S. Nature 1991, 354, 56 . 24 Ebbesen, T.W.; Ajayan, P.M. Nature 1992, 358, 220. 22 Chapter 3 Nanotubes modified by covalent grafting of poly(2-vinylpyridine) firstly observed by Kroto and C. Schultzenberger in 1890. Work, established in the 1950s, was shown that filaments could be produced by the interaction of wide range of hydrocarbons and other gases with metals, the most effective of which were iron, cobalt and nickel. The system for synthesis of CNTs is relatively simple and consisting of a quartz tube reactor within a multizone furnace (Figure 3.1). The system configuration involves entraining a mixture of xylene and ferrocene (as suggested by Rao et al.) into an inert gas stream. During the decomposition of the ferrocene-xylene mixtures at temperatures in the range 625775°C and at atmospheric pressure, iron nanoparticles are nucleated and begin to deposit carbon (from xylene and ferrocene) as well-aligned pure nanotubes arrays on the quartz surfaces (reactor walls and substrates). From short-duration experiments (2-10min), it appears that once growth initiates, the MWNTs rapidly grow to a maximum length (typically 50μm). The most effective catalyst have been shown to be iron, nickel, and cobalt . Figure 3.1 Schematic of floating catalytic CVD reactor system to produce MWNTs. Nanotube synthesis by chemical vapour deposition (CVD) has many advantages over other production routes, including the high purity of the product, which requires no further processing to yield a usable product. However, the significantly lower temperatures used in the synthesis, compared to those of arc or laser production methods, tend to produce nanotubes with less well-defined graphene structure. This can be rectified by a posttreatment in which the nanotubes are heated in an inert atmosphere to a high temperature in the range 1800-2600°C. This graphitisation process removes many of the structural defects, 25 Schultzenberger, C. R. Acad. Sci., 1890, 111, 774-778. 26 Heath, J. R.; O’Brien, S. C.; Kroto H. W.; Smalley, R..E. Comments Cond. Mat. Phys. 1987, 13, 19. 27 Sen, R.; Govindaraj A.; Rao, C. N. R. Chem. Phys. Lett. 1997, 267, 276-280. 28 Rao, C. N. R.; Sen, R.; Satishkumar, B. C.; Govindaraj A. Chem. Commun. 1998, 1525-1526. 29 Derbyshire, F.; Presland, A. E. B.; Trimm, D. L. Carbon 1975, 13, 111-113. 30 Ebbesen, T. W. CRC Press: Boca Raton 1997, 226-248. 23 Chapter 3 Nanotubes modified by covalent grafting of poly(2-vinylpyridine) but it has been also shown to be a successful method for removing all of the residual iron catalyst. Solubilization. It is very difficult to explore the full application potential for pure carbon nanotubes. The major barrier is their poor solubility and low dispersibility in aqueous and organic liquids, leading to difficulties in their manipulation. CNTs typically exist as ropes or bundles. They are a few micrometers long; the CNTs ropes are entangled together in the solid state to form a highly dense, complex network structure. The low dispersibility stems from the strong tendency to aggregation due to the van der Waals attraction rather than to disperse in common solvent. A recent calculation for SWNTs suggests that the typical intertube attraction is 36×kB×T (where kB is Boltzmann constant, T is absolute temperature) for every nanometer of overlap between adjacent tubes, leading to cohesive energy of a few thousands of kB×T pro micrometer-long tubes. Unlike the case of classical colloids, the attraction is short-ranged and decay to a negligible value over a distance of a few nanometers. Most of the methods for dispersion and modification are designed to reduce the short-range attraction between the nanotubes via the introduction of a repulsive interaction of similar strength. A number of groups have reported successful modification and solubilisation of carbon nanotubes. These reactions may be roughly divided into covalent and non-covalent attachment of functional groups or polymers direct to the graphite surfaces. It is possible to wet the SWNT raw soot in refluxing nitric acid, whereby the end caps of the tubes are oxidized to carboxylic acid and other weakly acidic functionalities. These “acidpurified” SWNTs can be dispersed in various amide-type organic solvents under the influence of an ultrasonic force field. The nitric acid purifies the carbon nanotubes by removal of some of the metal catalysts used in the synthesis of the tubes and some of the amorphous carbon that is a by-product of most synthetic methods. However, the nitric acid treatment introduces defects on the nanotube surface, oxidizes (hole dopes) the carbon nanotubes and produces impurity states at the Fermi level of the nanotubes. This latter effect may be viewed as an intercalation of the nanotube lattice by oxidizing agents, with concomitant effects on the electronic properties of the nanotubes, a process which is quite familiar from the intercalation of graphite and fullerene lattices by various redox reagents. The defect sites 31 Andrews, R.; Jacques, D.; Qian, D.; Dickey, E. C. Carbon 2001, 39, 1681-1687. 32 Shvartzman-Coher, R.; Levi-Kalisman, Y.; Nativ-Roth, E.; Yerushalmi-Rozen, R. Langmuir (Letter) 2004, 15, 6085-6088. 33 G. Viswanathan, N. Chakrapani, H. Yang, B. Wei, H. Chung, K. Cho, C. Y. Ryu, P. M. Ajayan, J. Am. Chem. Soc. 2003, 125, 9258. 34 Y-P. Sun, K. Fu, Y.Lin , W.Huang Acc Chem Res,. 2002, 35, 1096. 35 Garg, A., Sinnott, S.B. Chem Phys Lett 1998, 295, 273. 24 Chapter 3 Nanotubes modified by covalent grafting of poly(2-vinylpyridine) that are introduced into the carbon nanotubes can lead to shorten and eventually destroy the carbon nanotubes under similar oxidizing conditions. Direct reaction of the acid-purified SWNTs with long-chain amines led to soluble materials by the formation of zwitterions (Scheme 3.1). A variety of oligomeric and polymeric compounds have been also used in the functionalization of carbon nanotubes.