Nanocarbons, especially carbon nanotubes and nanofibres, are key materials in nanotechnology but producing them on an industrial scale remains expensive. Current processes for producing these nanomaterials include wet chemistry techniques that involve preparing a support, normally silica or alumina, and impregnating these with catalytically active metals.
Now, Dang Sheng Su and colleagues of the Fritz-Haber Institute in Berlin may have found an inexpensive new way to produce nanocarbons directly from the gas phase by using volcano rock from Mount Etna as the catalyst. The researchers explain that the naturally occurring iron oxide particles in lava make it an effective catalyst and that their technique could possibly pave the way to a more efficient production method for the nanomaterials.
Su and co-workers begin by pulverizing the volcano rock and heating it to temperatures of 700 °C under a hydrogen atmosphere. This reduces the iron oxide particles to elemental iron. Next, they pass a mixture of the hydrogen and ethylene gas over the powder. The iron particles catalyse the decomposition of ethylene to elemental carbon, which is deposited on the lava rock in the form of nanotubes and fibres.
The new technique has many advantages says Su. First, the catalyst is produced in large quantities and is thus cheap – Mount Etna is Europe's largest and most active volcano and is thought to have produced as much as 10 million cubic metres of lava when it erupted in 2002. Second, researchers do not need to deposit the catalytic iron on any kind of substrate since the lava acts as both catalyst and substrate. Finally, the process works without any wet chemical steps, therefore simplifying the production process.
Apart from the important commercial aspect, the results also raise questions from a geological point of view. The researchers have shown that if a carbon source is available, then nanocarbons can grow on minerals at relatively moderate temperatures. And since volcanoes produce reducing gases, such as methane and hydrogen, does this mean that the Earth could have already produced nanotubes and fibres by itself millions of years ago? Furthermore, atomic hydrogen, carbon oxides and metallic iron are present in interstellar space, so could nanocarbons also be formed in the cosmos?
Natural fullerenes have already been identified in fulgurite, a glassy rock that forms where lightning hits the ground, and shungite, a highly metamorphosed carbon-rich rock within Precambrian sediments, write the researchers. "The remaining question is how to find carbon nanotubes and nanofibres in nature," they say.
The work was published in Angewandte Chemie International Edition.