Photodesorption of oxygen from carbon nanotubes

Abstract

We propose to use photodesorption as a noninvasive tool to clean carbon nanotubes from oxygen, offering a clear advantage over thermal and chemical treatments. The detachment of chemisorbed oxygen from atomic vacancies is triggered by a resonant Auger process, initiated by an O 1s → O 2p transition, which leaves two holes in the O 2s level. In the electronically excited state, oxygen desorbs spontaneously, with no damage to the carbon network or the cylindrical nanotube shape. Subsequent reaction of oxygen atoms with H 2 molecules is shown to prevent reoxidation of the nanotube.Oxidation is commonly used to purify carbon nanotubes (CNTs) from amorphous carbon soot. 1 The efficiency of this process relies on the fact that carbon soot converts easily to CO and CO 2 , which subsequently evaporate, in contrast to structurally perfect nanotubes and graphite, which are rather unreactive. Ab initio density functional theory (DFT) calculations 2,3 indeed suggest that the interaction of molecular oxygen with carbon nanotubes should be weak, in stark contrast to the dissociative adsorption of oxygen atoms at the edges 4 and at defect sites 5 of carbon nanotubes. There, chemisorbed oxygen forms chemically strong C u O u C complexes, which have been observed by near-edge x-ray absorption fine-structure spectroscopy. 6 Whereas physisorption of O 2 causes hole doping and increases the electronic conductivity of semiconductor nanotubes, 2 formation of C u O u C complexes destroys the perfect -bonding network and thus decreases the conductivity of nanotubes. Finding an effective way to remove the chemisorbed oxygen is a necessary prerequisite for a practical use of carbon nanotubes in electronic devices.Removal of the chemisorbed oxygen atoms by heat treatment comes with the inevitable side effect of damaging the carbon network surrounding the O atom, due to the higher stability of the C u O bond in comparison to C u C bonds. 4 Such structural damage is evidenced in temperature programed mass spectra of oxidized carbon nanohorns, 7 with a graphitic network similar to nanotubes, which show a preferential desorption of CO and CO 2 molecules rather than oxygen at high temperatures. The stability of the C u O bond also makes alternative chemical processes, designed for selectively breaking the C u O bond, very unlikely. The inertness of the C u O u C complex in a defective nanotube was confirmed by our DFT calculations, which suggest that H atoms preferentially attach to carbon rather than oxygen sites. Inspired by the observed disintegration of H 2 O molecules by a resonant Auger process, 8 we decided to study photochemical processes in oxidized nanotubes as an alternative to chemical and thermal purification methods.Here we propose to use photodesorption as an efficient, noninvasive way to clean carbon nanotubes from chemisorbed oxygen. Results of our first-principles simulations for electron ion dynamics (FPSEID) 9 suggest that the strong C u O bonds can be efficiently ruptured by optically promoting the bonding electrons into an...

Real-time propagation time-dependent density functional theory study on the ring-opening transformation of the photoexcited crystalline benzene

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Nonthermal Current-Stimulated Desorption of Gases from Carbon Nanotubes

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Decomposition of nitrous oxide on carbon nanotubes

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