Review Understanding the Physicoelectrochemical Properties of Carbon Nanotubes: Current State of the Art Xiaobo Ji, a Rashid O. Kadara, b Jaanus Krussma, c Qiyuan Chen, a Craig E. Banks b * a College of Chemistry and Chemical Engineering, Central South University, Changsha 410083, P.R. China b Faculty of Science and Engineering, School of Biology, Chemistry and Health Science, Division of Chemistry and Materials, Manchester Metropolitan University, Chester Street, Manchester M15GD, Lancs, UK *e-mail: c.banks@mmu.ac.uk c Institute of Physical Chemistry, University of Tartu, Jakobi Street 2, Tartu, 51014, Estonia Received: October 6, 2009 Accepted: November 10, 2009 Abstract Carbon nanotubes receive considerable attention in the area of electrochemistry not only due to their reported structural, mechanical or electronic properties but because they represent the worlds smallest electrodes allowing electrochemistry to be performed where other electrode materials cannot penetrate. In this review, we overview recent developments in this area summarizing the fundamental advances in understanding the various factors and parameters that can significantly affect the electrochemical reactivity of carbon nanotubes, which is essential for their continual use and successful implementation in a plethora of areas and applications. Keywords: Nanoelectrochemistry, Electrocatalysis, Carbon nanotubes, Edge plane pyrolytic graphite electrodes, Electrochemical reactivity, Nanotubes, Catalysis DOI: 10.1002/elan.200900493 1. Introduction Carbon is an ideal electrode material owing to its attractive features, including good corrosion resistance, high electrical conductivity, low cost and a wide potential window in aqueous solutions [1 – 3]. According to the degree of graphitization, carbon is morphologically diverse and a variety of bulk carbon structures exist which have different forms, such as carbon black to carbon fibers, through to pyrolytic graphite. Graphite has a hexagonal structure with sp 2 -carbon atoms where the C  C bond length is around 1.42  arranged in hexagonal layered planes[4 – 6]. A large variety of graphite products have been used as working electrodes, such as amorphous carbon, glassy carbon, carbon black, carbon fibers, powdered graphite, pyrolytic graphite (PG) and highly ordered pyrolytic graphite (HOPG), each having different chemical and physical properties. A key structural factor which leads to an assortment of different materials is the average graphite microcrystallite size, viz lateral grain size, the average size of the hexagonal lattices that make up the macro structure. The smallest lateral grain sizes are found in amorphous carbon, glassy carbon and carbon black and can be as low as 10 . In the intermediate range are carbon fibers and pyrolytic graphite with lateral grain sizes around 100  and 1000  respectively. In contrast, the highest grade of HOPG (ZYA and SPI 1 grade) can have lateral grain sizes of 1 – 10 mm. From a slab of HOPG, edge plane pyrolytic graphite (EPPG) and basal plane pyrolytic graphite (BPPG) electro- des can be readily fabricated. Figure 1 shows a schematic representation of HOPG. An electrode is made up of many of these graphite sections. If the graphite crystal is cut, as shown, perpendicular to these sheets an edge plane electrode is formed whereas if the crystal is cut parallel to the sheets then a basal plane electrode is formed. In the latter case the surface should not be thought of as perfect but rather as illustrated (see Fig. 1), there will be surface defects, where graphite crystals meet, in the form of steps exposing the edges of the graphite layer. Such steps are typically 1 – 6 sheets of graphite in height; Figure 1 also shows an actual atomic force microscopy (AFM) image detailing the edge steps observed at an HOPG surface. In carefully prepared samples of basal plane HOPG, the distance between the edge steps can be as much as 1 – 10 mm. Due to the nature of the chemical bonding, the two phases, edge and basal exhibit completely different electrochemical properties. For vol- tammetry, the edge plane very often exhibits considerably faster electrode kinetics in comparison with basal plane. This means that an electrode consisting entirely of edge plane, an edge plane pyrolytic graphite electrode may show a nearly reversible voltammogram whilst an electrode consisting mainly of basal plane can show irreversible behavior. For this reason, as discussed below, edge plane electrodes are often preferable to basal plane or other electrodes for electrochemistry, particularly electroanalyt- ical purposes. Another fascinating form of carbon, carbon nanotubes (CNTs) have attracted attention around the world since Review Electroanalysis 2010, 22, No. 1, 7 – 19  2010 Wiley-VCH Verlag GmbH&Co. KGaA, Weinheim 7