Journal of Advanced Research (2012) 3, 195–223 Cairo University Journal of Advanced Research REVIEW ARTICLE The synthesis, properties and uses of carbon materials with helical morphology Ahmed Shaikjee a,b, Neil J. Coville a,b,* a DST/NRF Centre of Excellence in Strong Materials, University of the Witwatersrand, Johannesburg 2050, South Africa b Molecular Sciences Institute, School of Chemistry, University of the Witwatersrand, Johannesburg 2050, South Africa Received 6 April 2011; revised 21 May 2011; accepted 23 May 2011 Available online 3 August 2011 KEYWORDS Abstract Carbon nanostructures have been widely studied due to their unique properties and Coiled carbon nanotubes; potential use in various applications. Of interest has been the study of carbonaceous material with Coiled carbon nanofibers; helical morphologies, due to their unique chemical, mechanical, electrical and field emission prop- Carbon coil; erties. As such it is envisaged that these materials could be excellent candidates for incorporation in Carbon helix; numerous nanotechnology applications. However in order to achieve these aspirations, an under- Synthesis; standing of the growth mechanisms and synthetic strategies is necessary. Herein we consider histor- Properties ical and current investigations as reported in the literature, and provide a comprehensive outline of growth mechanisms, synthetic strategies and applications related to helical carbon nanomaterials. ª 2011 Cairo University. Production and hosting by Elsevier B.V. All rights reserved. Introduction synthetic processes, carbon can be tailored into a myriad of structures, particularly those in the nanometre range [2–4]. Carbon is an amazing element, not just because it is the element In 1991, Ijima published his landmark paper which described required for all life processes, but also due to the fact that it can the appearance of carbon filaments with diameters in the range exist in numerous allotropic forms [1]. Additionally, by means of of nanometres [5,6]. These carbon materials would come to be known as carbon nanotubes (CNTs), and play a fundamental role in leading scientific and industrial research endeavours in nanotechnology. Indeed within a matter of years CNTs have ta- * Corresponding author. Tel.: +27 11 7176738; fax: +27 11 7176749. ken centre stage in the nano-science arena. It is no exaggeration E-mail address: [email protected] (N.J. Coville). to say that one of the most active fields of research in the area of nanotechnology currently is the synthesis, characterization and 2090-1232 ª 2011 Cairo University. Production and hosting by application of CNTs [5,7,8]. This has naturally led to a renewed Elsevier B.V. All rights reserved. interest in the synthesis of other forms of carbon nanomaterials: Peer review under responsibility of Cairo University. graphene, fibers, horns, buds, onions, helices etc. [8–11]. It is this doi:10.1016/j.jare.2011.05.007 diversity in the morphology of carbon materials that provides the flexibility to modify the properties of carbon. Thus, the de- sign and production of carbon materials with unusual morphol- Production and hosting by Elsevier ogies is a promising way to exploit the morphology-property correlation of carbon nano-materials. 196 A. Shaikjee and N.J. Coville Of particular interest to scientists has been the study of car- [16–18]. It is expected that the same should also apply to springs bon nanomaterials with a helical or non-linear morphology (helices) made from nanomaterials. shown in Fig. 1. Helical carbon nano-materials have a long While mechanically useful, springs or coils have also been history, having first been reported by Davis et al. [12] in used in electro-magnets, solenoids, inducers etc. This is due 1953. However these fibrous materials were initially considered to the ability of coiled materials to exhibit interesting electro- a curiosity and efforts were focused on their prevention rather magnet properties since a current flowing through a wire than on their synthesis [13,14]. It was not until the 1990s, stim- wound into a coil produces both electric and magnetic fields ulated by the discovery of CNTs, that there was a renewed [16,18]. This property of electromagnetism has created a revo- interest in carbon fibers and tubes, especially those with unu- lution in many fields from the development of plasma televi- sual (e.g., helical/spring-like) morphology [2,3]. sions to memory storage devices. It is envisaged that carbon The helical shape is a common form seen in the universe nano-materials with helical morphology could also be used (from spiralling galaxies to DNA) and it is thus not unexpected as components in future nano-technology devices [13,19,20]. that this should also be a common motif found in carbon nano- Macro sized coils and springs are manufactured by a top structures [15]. Indeed innumerable macro-devices have been down process. While this approach could also be used to form made based upon a helical design and used by humankind from nano sized springs, the bottom up process starting from atoms ancient times (e.g., the Archimedes water screw) to the present and molecules is expected to be the preferred procedure to make (e.g., support springs for cellular keypads) [16]. It is expected the components needed to form helical nano-materials. The that nano materials with helical morphology should possess growth of helical carbonaceous materials from carbon precur- both similar and unique physical and chemical properties to sors via a bottom up approach in the presence of a catalyst is their macro components. Nano helices should thus behave in expected to proceed by equivalent methods used to synthesize a comparable manner to macro materials with similar morphol- straight fibers and tubes [5,7]. The mechanism commonly pro- ogy. The ability of a macro scale spring to change shape in re- posed for carbon fiber growth involves adsorption and dissoci- sponse to an external force (compression, extension, torsion ation of a carbon precursor on the surface of a catalyst particle etc.), and return to its original shape when the force is removed and dissolution of carbon into the catalyst particle. Once the has made springs an important component in cellular technol- catalyst particle has been saturated with carbon, the carbon ogy, time keeping, medical as well as shock absorbing devices crystallizes out of the metal particle and is extruded to form a Fig. 1 Various types of helical carbon nanomaterials with non-linear morphology. Helical carbon nanomaterials 197 CNT or CNF [5,20]. Typically CNTs exist as cylinder/s of rolled controlling the synthesis of helical carbon materials and hence up graphene sheets [7], giving rise to single walled, double walled the manufacture of sophisticated and economically viable and multi-walled entities, Fig. 2. CNFs by contrast are com- nano-devices containing carbon nano helices. posed of graphene sheets that stack upon each other, to produce both hollow and solid carbon structures, Fig. 2. These structures Structural origin and growth aspects of carbon helices do not need to be straight; they can take on a helical morphol- ogy. As such, two categories of helical materials exist; (i) coiled After the discovery of CNTs, researchers began to study fibers, Fig. 3a, where the fiber is a dense structure with no inner other forms of carbon in greater detail; in particular those hollow and (ii) coiled tubes, Fig. 3b, where an inner hollow exists that exhibited non-linear geometry. The use of a graphene throughout the length of the coil. sheet or honeycomb network rolled into a cylinder (used Helical carbon fibers and tubes can be divided into different to model CNTs) could not be used to explain the geometry categories based upon the helical nature of the material: single observed in non-linear carbon structures. In early studies it helix, double helix, triple helix, braid, spiral, coil, spring etc. was realised that fullerenes achieved their curvature by the [3,15,19]. The diversity of helical materials provides a myriad introduction of pentagonal rings into graphene (positive cur- of shaped carbons, Fig. 1. The use of helical carbons in tech- vature) while the insertion of heptagonal and/or octagonal nological applications will be dependent on our ability to con- rings led to ‘negative’ curvature [21,22]. Before long it was trol the coil morphology and coil geometry of these materials. appreciated that a judicious insertion of a series of pentago- This includes control of the coil diameter, pitch and fiber/tube nal and heptagonal rings within a hexagonal matrix would thickness, Fig. 3c. The growth of carbon nano-materials can be yield helically coiled carbon nano-materials. As such the is- controlled by varying temperature, gas environment and the sue of helical growth is then to achieve the correct combina- type of catalyst. The alteration of any of these variables will tion of polygonal rings (5, 6 and 7) that would generate a result in a significant change in the type and amount of helical helix [22–25]. carbon nano-materials formed [3]. To achieve this control, an understanding of the growth mechanism and the role played Structural origin of helices in CNTs by the various parameters is needed. To date control over the synthesis of a specific type of helical carbon nano-material has been met with only limited success. In order to develop a model that can describe the helical nature In this review we attempt to provide a summary of the var- of coiled CNTs, carbon in the form of a fullerene or torus must ious synthetic procedures employed, the relevant mechanistic first be considered. Dunlap [21,26] showed that the insertion of explanations that have been given to explain helical growth pentagon and heptagon rings at the junction of two CNTs can patterns and the current technological applications associated yield what he called a ‘knee structure’.