"We think it is a novel and 'green' approach - from grass to carbon nanotubes," Enbo Wang told nanotechweb.org. "Using renewable natural products as the carbon source and the participation of oxygen, a benign oxidation reagent, offer numerous benefits ranging from environmental safety to further exploitation of existing farm and natural produce."

Wang and colleagues took grass from a field, before drying, crumbling and heating it at 250 °C for one hour. Then they heated the resulting material to 600 °C for around 20 minutes in a sealed container, together with about 15 ml of oxygen. Following cooling, the team reinjected oxygen and repeated the heat treatment. They carried out this cycle about 50 times.

The result was nanotubes about 1 µm long with outer diameters of 30-50 nm and inner diameters of 10-30 nm. The researchers estimate that the average yield for the process was roughly 15%.

"Recently it was shown that the participation of water can simplify the synthesis and purification of nanostructured carbon based on the complex chemistry in the C-H-O system," added Wang. "That inspired us to look for a new strategy for carbon nanotube fabrication directly from carbohydrates, based on the conversion from carbohydrate to pure carbon and water. This breaks through not only the limits of carbon source but also the traditional idea of obtaining active carbon atomic species and then assembling them into carbon nanotubes."

Many plants, including grass, contain vascular bundles that transport fluids throughout the organism. The bundles have a tubular structure and consist mainly of cellulose, hemicellulose and lignin. The researchers reckon that their pretreatment of the grass removed its protein and grease components, while the treatment at 600 °C dehydrated the cellulose and converted it into nanostructured carbon, as well as causing oxidative delignification.

The tubular structure of the carbon sources appeared to be crucial. Using the same heat treatment on carbohydrates that weren't in tubular form, such as glucose and saccharose, created only a very low number of nanotubes. But wood and hemp - both plant materials with a tubular structure - were good carbon sources for nanotube production.

"The obtained carbon nanotubes have some defects in the walls, which have potential applications in catalysis for use as catalyst supports," said Zhenhui Kang. "We intend to explore the effects of various reaction conditions on the yields of carbon nanotubes, and we will try to find an ideal juncture of high production yields and low cost. [This work] may also provide a new route for the further development of green carbon nanostructure synthesis."

The researchers reported their work in Nanotechnology.