Hydrogen plasma pretreatment is used to enforce the growth of vertically-aligned carbon nanotube forests on TiN substrates. The evolution of the substrate, catalyst, and nanotubes are studied by in situ and ex-situ photoemission and X-ray diffraction in order to understand the growth mechanism. We find that TiN retains its crystallographic structure and its conductivity during plasma pretreatment and nanotube growth, which is confirmed by electrical measurements. Plasma pretreatment is found to favor the growth of nanotube forests by root growth, as it binds the catalyst nanoparticles more strongly to the substrate than thermal pretreatment. We find that plasma pretreatment time should be limited, otherwise poor or no growth is found.

The rapid capacity decay of lithium-sulphur batteries has been a significant obstacle for practical application, which is generally considered to arise from dissolution of lithium polysulphide in the electrolyte and diffusion away from the cathode. As the lithium content in the polysuphide inceases with further discharge, capacity decay occurs also from the passivating effects by the formation of insoluble sulphides, further amplified by volume increase. More recently, weakening of sulphur adhesion to carbon with progress in discharge is also an important factor in the sulphur cathode degradation. In order to overcome capacity decay caused by all the above mechanisms, we have prepared a composite cathode made of sulphur and high density carbon nanotube (HD-CNT) forest scaffold that is able to interfacially adsorb and volumetrically confine the polysulphide species and accommodate the expansion of sulphur discharge products effectively. This cathode demonstrates very high electrochemical stability and high discharge capacity up to 200 full discharge/charge cycles even with the use of the basic organic ether electrolyte where polysulphide shows high solubility, thus providing evidence for confinement and interfacial contact. Retention and surface adsorption favoured by minimising the wall-to-wall distance between the aligned CNTs arising from a decrease in the reaction energy of the adsorption. Computational simulation of the interface between polysulphide species and carbon nanotube surface provides first-principles confirmation of improved binding between C and S in the polysulphides as wallto-wall distance is decreased. The HD-CNT scaffold is self-binding and highly-conducting thus the conventional additives of binder and carbon black are also fully eliminated. A high discharge capacity of 812 mAh g-1 of sulphur (corresponding to 503 mAh g-1 of the whole cathode material mass) is stably retained after 200 cycles at 400 mA g-1 with a small average capacity decay of only 0.054% per cycle on average These encouraging results provide novel approaches to designing and fabricating long cycle life cathode in a lithium sulphur battery.

The area density of vertically aligned carbon nanotubes forests is measured and analysed by the weight gain method. The mass density of a close packed array of single- and multi-walled nanotubes is analysed as a function of the average nanotube diameter and number of walls, and this is used to derive the area density, from which the filling factor can be extracted. Densities of order 1013 cm−2 tubes are grown from cyclic catalyst methods.

No abstract

We report both the growth of carbon nanotube forests and electrical conduction on W, Ti, and TiN substrates coated with an ultra-thin Al2O3 support layer. Varying the Al2O3 thickness, a good electrical contact and high nanotube density is possible for a 0.5 nm Al2O3 layer as such an ultra-thin film allows tunnelling. X-ray photoelectron spectroscopy shows that, when using these non-continuous Al2O3 films, Fe catalyst diffuses into the conducting substrates, eventually causing growth to stop. Forests grown on ultra-thin Al2O3 are potentially useful for applications as interconnects, supercapacitors, and heat spreaders.