Work functions and surface functional groups of multiwall carbon nanotubes

We have studied the work function and density of states (DOS) of multiwall carbon nanotubes (MWNTs) using ultraviolet photoelectron spectroscopy (UPS) . Raw MWNTs were purified by successive sonication, centrifugation, sedimen tation, and filtration processes with the aid of a nonionic surfactant. The purified MWNTs showed a slightly lower work function (4.3 eV) than that of highly oriented pyrolytic graphite (4.4 eV). Effects of three different ox idative treatments, air-, oxygen plasma-, and acid-oxidation, have also bee n studied. It was found that oxidative treatments affect the DOS of valence bands and increase the work function. X-ray photoelectron spectroscopy (XP S) measurements have suggested that gas-phase treatment preferentially form s hydroxyl and carbonyl groups, while liquid-phase treatment forms carboxyl ic acid groups on the surface of MWNTs. These surface chemical groups disru pt the pi-conjugation and introduce surface dipole moments, leading to high er work functions up to 5.1 eV. We expect the information on the work funct ion of the MWNTs to be of importance to the development of electronic or op toelectronic applications.