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Oxygen interaction with Li-intercalated Single Wall Carbon Nanotube (SWCNT) bundles has been investigated by photoemission spectroscopy; we have compared the behaviour of the O 1s core-level and valence band spectra versus oxygen exposures and thermal treatments.
Our experiments were performed at an alkali concentration of 6 % corresponding to the lithium intercalation limit on our SWCNT sample at room temperature. Since oxygen does not adsorb on a perfectly pure SWCNT sample [1], the increased concentration of oxygen with respect to the clean sample is due to an alkali-induced enhanced reactivity of the sample. Oxygen concentration, after molecular gas exposure, grew up to 5%; after about ten hours lithium concentration grew up to 18% without having performed any further evaporation; also oxygen concentration slightly increased as expected from the above consideration on the reactivity of alkali-doped sample. The best-fit curve of the O 1s spectrum is achieved with two peaks located at 531eV and 533eV, which can be assigned, respectively, to oxygen belonging to lithium oxide (LiO2) and peroxide (Li2O2) [2,3]. So, we suppose that electronegative oxygen atoms attract lithium atoms from the bulk, leading to a dramatic increase of lithium concentration in the surface region, on which the formation of both LiO2 and Li2O2 occurs. The presence of both lithium oxide and peroxide was confirmed also by our valence band measurements carried out using a He discharge lamp.
A temperature-dependent study has shown that lithium peroxide is less stable than lithium oxide and starts desorbing at 400K; at temperatures higher than 1000K both oxygen and lithium desorb, and the electronic structure of the pristine sample is recovered.
[1] A. Goldoni, R. Larciprete, L. Petaccia, S. Lizzit, J. Am. Chem. Soc. 125, 11329 (2003)
[2] S.L. Qiu, C.L. Lin, J. Chen, and M. Strongin, Phys. Rev. B 39, 6194 (1989)
[3] G. Zhuang, Y. Chen, and P.N. Ross Jr., Surf. Sci. 418, 139 (1998)
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