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Computer simulations explain how nanotubes detect toxic gases in extremely low concentrations

ALEXANDRE ROCHA/UFABCVolatile connections: ammonia molecules (blue and white spheres) interact with nitrogen atoms (blue spheres) in the wall of nanotubes, altering the transmission of electricityALEXANDRE ROCHA/UFABC

Some years ago the team of Mexican researchers Maurício and Humberto Terrones showed that films coated with carbon nanotubes, microscopic cylinders formed from thousands of carbon atoms arranged in hexagons, can function as toxic gas detectors. In less than half a second these films detected the presence of very low concentrations of ammonia, nitric oxide and ethanol, irritating gases that may cause lung damage and in extreme cases kill. In their experiments the group from Mexico also saw that the most efficient sensors were not those that had nanotubes formed exclusively of carbon. The most sensitive and capable of detecting a few molecules of toxic gas in billions of other molecules, contained nanotubes with atoms of the chemical element nitrogen in their composition – when the nanotubes have elements other than carbon, the physicists say they contain impurities.

In the article in Chemical Physics Letters in which they described the results in 2004 the Terrones explained the good performance of the sensors by the interaction of the toxic gas molecules with the atoms of nitrogen in the walls of the nanotubes. Analyzing these results physicists from the University of São Paulo (USP) and Federal University of the ABC (UFABC) showed in two recent articles that the Mexicans were only partially right. The molecules of ammonia, nitric oxide and ethanol really do connect with the nitrogen atoms of the nanotube but apparently not in the way the Terrones had suggested.

Using computer simulations Mariana Rossi Carvalho, Antonio José Roque da Silva and Adalberto Fazzio observed that the impurities formed by four nitrogen atoms that occupy positions around the space left by two carbon atoms are more stable than when three nitrogen atoms are arranged around the void left by a carbon. Unlike the Brazilians the Mexicans believed that the ammonia atoms linked to the impurities consisted of three nitrogen atoms. In her Masters thesis Mariana noted that in addition to being more stable the removal of two carbon atoms and the substitution of another two by four nitrogen atoms also consumes less energy, the reason why they are likely to occur in greater quantity in nanotubes.

It was necessary, however, to check whether the atom substitutions forecast in the theoretical model would explain the consequences seen in practice – the substitution of carbon atoms by nitrogen atoms alters the transport of electrical charges by the nanotubes. With Fazzio and Da Silva, physicist Alexandre Rocha developed a computer program capable of representing situations close to those of reality in which thousands of impurities are randomly distributed over the nanotubes formed by up to 100,000 carbon atoms. “This is the first time that rigorous calculations have been used to deal with the transport of the charge in nanotubes formed by such a large number of atoms, of up to 1,000 nanometers in length [a nanometer corresponds to a millionth of a millimeter and is some 100,000 times smaller than the thickness of a hair]”, says Fazzio, a professor from USP and dean of the UFABC.

Rocha simulated the interaction of ammonia molecules, comprising one nitrogen atom and three hydrogen atoms (NH3), with two types of impurity in the nanotubes: one in which three nitrogen atoms substitute one carbon atom and one in which four nitrogen atoms occupy the space left by the two carbon atoms. He saw that both in the first case as well as the second, the ammonia molecule breaks down in the same way: one hydrogen atom connects to the two nitrogen atoms of the impurity, while the nitrogen atoms and the other two hydrogen atoms remaining in the ammonia bond with a third nitrogen atom.

The difference is in the energy consumed by these interactions. Ten times more energy is necessary for the ammonia molecule to break down and its by-products bond to the impurity formed by three nitrogen atoms than is needed by that of four atoms, according to an article published in the Physical Review Letters of May 2 this year. This result indicates that it is very probable that the ammonia bonds to the impurities formed by four nitrogen atoms. As in this case the bonding energy is lower it becomes easier to reuse the gas sensor formed by nanotubes. “It’s possible to remove the ammonia from the nanotube with a jet of air or by increasing the temperature”, comments Rocha.

What was needed was to analyze what happened after the ammonia bonded with the two types of impurity in the nanotubes. In running the program  hundreds of times using different levels of this gas, the physicist observed that electrical charges flowed more easily through the nanotubes as the ammonia molecules bonded to the defects in the three nitrogen atoms. In the case of the defects formed by the four nitrogen atoms the opposite occurred: the more the ammonia molecules adhered to the nanotube the greater was the resistance to the charge transport, which was similar to that registered by the Mexicans in their experiments with toxic gases. This was proof that the structure proposed by the Brazilians explained the experimental results of the Terrones brothers. “This result is important for manufacturing gas nano-sensors, because it indicates that it’s feasible to produce very sensitive devices”, comments Da Silva. “We observed significant changes in the capacity to transmit electrical charges in low concentrations of ammonia.”

The project
Simulation and modeling of nanostructures and complex materials (nº 05/59581-6); Modality Thematic project; Coordinator Adalberto Fazzio – IF-USP; Investment R$ 692,178.08 (FAPESP)

Scientific articles
VILLALPANDO-PÁEZ, F. et al. Fabrication of vapor and gas sensor using films of aligned CNx nanotubes. Chemical Physics Letters. v. 386, p. 137-142. 6 fev. 2004.
ROCHA, A.R. et al. Designing real nanotube-based gas sensors. Physical Review Letters. v. 100. 2 May. 2008.

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