Insight into the modification of electrodonor properties of multiwalled carbon nanotubes via oxygen plasma: Surface functionalization versus amorphization

Abstract

The effect of oxygen plasma treatment on multi-walled carbon nanotubes (MWCNTs) surfaces towards controlled tuning of their electrodonor properties (gauged by the work function) was investigated experimentally (X-ray diffraction, Raman spectroscopy, thermogravimetry, transmission electron microscopy, X-ray photoelectron spectroscopy) and by molecular modeling (Density Functional Theory). The nanotubes were treated with oxygen plasma (0.2 mbar) at varied generator power (20–60 W) and exposure time (0.1–30 min). It was found that the work function changes nonmonotonously upon plasma treatment: after significant increase (from 4.5 eV to 5.9 eV) and passing through maximum the work function decreases and finally reaches a plateau. The experimental results supported by DFT calculations allowed to propose a molecular model explaining the changes in carbon nanotube surface induced by oxygen plasma. Two different oxygen adatom locations: out-of-plane (Csurf–Oadatom) and in-plane (Csurf–Osurf–Csurf) were identified, leading to formation of surface dipoles (formation of negative potential barrier, work function increase) and incorporation of oxygen into the carbon structure (cancellation of the potential barrier, work function decrease), respectively. The critical regions of plasma parameters for oxygen surface decoration and amorphization were identified providing guidelines for rational designing and tuning of MWCNT electrodonor properties.