Direct Growth of Flexible Carbon Nanotube Electrodes

The abundance of carbon, together with its accessibility, processability, chemical stability, and wide electrochemical potential window are reasons why carbon, in all its allotropes, represents a very attractive material for use in energy conversion and storage. It has found widespread use as an electrode material in capacitors or batteries, and even as component of fuel cells. Common energy-storage devices such as batteries and capacitors rely on large-surface-area electrodes to function. Hence, having a high electroactive surface area combined with high conductivity and useful mechanical properties makes carbon nanotubes (CNTs) attractive candidates for electrodes in these devices. Following the successful synthesis of aligned CNT forests, carbon nanotube electrochemical devices came into sight, and several chemical vapor deposition (CVD) methods have since been developed to grow aligned multiwalled and single-walled carbon nanotube forests. However, while impressive results were reported, these approaches suffer from the inherent problem that growth must be established on nonconducting substrates. Once synthesized, the aligned forests need to be transferred to a conducting substrate, or require a post-processing step where metal contacts are deposited on top of the forest, before they are suitable as electrode materials for integration into devices. More recently, direct growth of CNTs on Ni-containing alloys was reported, and the obtained materials were subsequently effectively used as capacitors or field-emission devices. Additionally, the growth of CNTs on metals using plasma-enhanced CVD was reported, but this technique has still issues of scalability. CNTs have commonly been grown from metallocenes of Fe, Co, Ni, Ru, and/or metal oxides, nitrates, or pentacarbonyls Much less common in the literature is the use of organic iron salts as catalysts, such as iron(III)-tosylate (Fe(III)TS), iron(III)-dodecylbenzenesulfonate (Fe(III)DBS) and iron(III)-pyridinesulfonate (Fe(III)PS). Here, we report that by modifying the generally employed route to CVD synthesis it is possible to grow carbon nanotube networks integrated into a carbon layer (CL) on insulating graphitic carbon (glassy carbon) and even on metallic substrates (e.g. aluminum or copper foil). Although networks derived through this new process contain unaligned multiwalled nanotubes only, they are vastly superior in all electrochemical aspects when compared to vertically aligned forests grown in the same furnace. This, combined with the robust nature and flexibility of the structures produced, provides an exciting advance. In a typical experiment (see Experimental Section), a thin film of iron(III) p-toluenesulfonate (Fe(III)pTS) catalyst was spin-coated onto quartz plates from organic solutions. After annealing, which facilitates solvent removal, the CVD process was initially carried out at 600 °C under Ar/H2 gas flow to reduce the Fe catalyst to iron nanoparticles. A CNT growth phase followed, carried out at 800 °C with C2H2 as the carbon source. The resultant CNT films (Fig. 1a) were observed to be very much unlike those grown using conventional catalysts. During CNT growth a reflective layer was formed beneath the carbon film (Fig. 1b). The formed carbon nanotube/carbon layer (CNT/CL) paper appears as a matt-black layer on top of the quartz plate (Fig. 1a), while a flexible, shiny carbon layer forms the lower layer of the CNT/CL paper (Fig. 1b). The CNT/CL paper was easily removed from the substrate and the resulting freestanding film could be rolled around a glass rod without visible signs of degradation (Fig. 1c). The size of the CNT/CL 3D networks is easily scaled up to 100 cm by using a larger CVD quartz tube, given that scale up is not limited by the CVD process itself. The CNT/CL paper was characterized using a range of methods. Scanning electron microscopy (SEM) (Fig.1d) and transmission electron microscopy (TEM) confirmed the top layer indeed consisted of CNTs, whilst SEM of the cross-sectional area (Fig. 1e) revealed a highly porous 3D structured CNT network grown on top of a dense CL with less than 1 lm thickness. The long nanotubes obtained in the network are multiwalled carbon nanotubes (MWCNTs) with an external diameter of 20–40 nm. An SEM image of the CL (Fig. 1f) displays a uniformly dense continuous film. Raman spectroscopy (Supporting Information, Fig. S1) of the nanotube layer produced D(1347 cm) and G-bands (1598 cm) within the accepted literature range for MWCNT samples. XRD spectra of the carbon layer revealed peaks in the 2h degree regions of 25° and 42°, identical to those obtained for a commercial Carbon Black sample (Fig. S2). The electrical resistance was measured using a standard four-probe system, with the CNT/CL C O M M U N IC A TI O N