Adsorption on Carbon Nanotubes Studied Using Polarization-Modulated Infrared Reflection−Absorption Spectroscopy

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Abstract
Single-wall carbon nanotubes (SWNTs), deposited onto an Al substrate from a liquid suspension, have been cleaned by annealing in ultrahigh vacuum. The effects of exposing the sample in situ to atomic H (or D) and/or to dimethyl methylphosphonate [DMMP, (CH3O)2(CH3)PO] were then studied using polarization-modulated infrared reflection−absorption spectroscopy. Atomic H reacts preferentially near strained or defective regions in the nanotube wall to produce a spectrum consistent with alkane-like species (>CH2 and CH3). Only a small fraction of the >CC< sites in the nanotube wall react with H, and there is no clear evidence for monohydride >C(H)C(H)< species. For DMMP, data were obtained under steady-state conditions in reagent pressures in excess of half the room-temperature vapor pressure. Adsorption occurs via the PO group with a coverage that depends on the ambient pressure. Varying the DMMP coverage by changing the pressure causes changes in the spectrum that can be related to the strength of the DMMP/SWNT interaction. Preadsorbed H is seen to have little or no effect on the subsequent adsorption of DMMP. For DMMP, the molecular features are superimposed on a broad, smoothly varying background that can be related to adsorption-induced changes in the Drude parameters characterizing the SWNT free-carrier density and scattering lifetime.