For the first time, researchers have been successful in bringing about spontaneous chemical reactions between two different varieties of carbon nanotubes without using any chemicals. In fact, a simple mechanical grinding of the carbon nanotubes with a mortar and pestle was all that was required to induce chemical reactions. The end result was the production of valuable graphene nanoribbons.
Graphene nanoribbons are being increasingly used in composite materials.
To achieve this, carbon nanotubes containing two different chemical additives — carboxyl groups and hydroxyl groups — were chosen for the study. When ground for about 20 minutes, the additives reacted with one another and unzipped the nanotubes to form one atomic layer thin graphene nanoribbons. The reaction of the two different chemical additives is exothermic in nature and the heat released ultimately unzips the nanotubes.
To be certain, the researchers repeated the experiment using various ratios of the two varieties of carbon nanotubes and in many conditions — standard lab conditions, vacuum, in open air and at variable humidity, temperatures, times and seasons.
“Water is formed in this reaction, and its detection during the process of grinding proves the chemical reaction. Mechano-chemistry was proven this way,” said Prof. T. Pradeep of the Department of Chemistry, IIT Madras who along with Prof. Pulickel M. Ajayan of the Department of Material Sciences and Nano Engineering, Rice University, Houston undertook this novel study. Prof. Ajayan is also a distinguished visiting professor at IIT Madras. The results of the study were published on June 16 in the journal Nature Communications.
Till date, there has not been any reported instance of graphene nanoribbons being formed by grinding the carbon nanotubes and in the complete absence of other chemicals. “This opens up the possibility of producing novel nanostructured products with specific properties by mechanical agitation,” Prof. Pradeep said.
The next step is to generalise this in all kinds of nanosystems. The teams are looking at such chemistry with other functionalised carbon nanotubes. Applications of such chemically synthesised nanoribbons remain to be explored.
“Identification of the process as mechanochemistry was our contribution,” he said. “To prove this, we detected the release of water by mass spectrometry. A combination of novel chemistry and modified instrumentation allowed us to observe this.”
Prof. Ajayan had observed the disappearance of carbon nanotubes upon grinding and shared this information with Prof. Pradeep.
“I suggested that mechano-chemistry might be the reason. I had just come back after a class which dealt with triboluminescence, the emergence of luminescence by grinding. I showed him that and told him that chemical reactions can happen similarly. Maybe we could detect water to prove this mechano-chemistry. That is how this started,” Prof. Pradeep recalled.