Band structure, work function and interfacial diagrams of oxygen-functionalized carbon nano-onions

Abstract

Carbon nano-onions (CNOs) are an emerging class of carbon nanomaterials with a wide range of applications. Few is known about their electronic structure and the factors that affect it. Here, we study for the first time the effect of two oxidation treatments on the electronic structure of CNOs using ultraviolet photoelectron spectroscopy (UPS) and X-ray photoelectron spectroscopy (XPS). CNOs were oxidized by physical (Ar/O2 radio frequency plasma) and chemical (HNO3/H2SO4) methods. It was found that the physical oxidative treatment introduced preferentially hydroxyl and carbonyl groups on the outer shells of CNOs and changed the electronic state from metallic to semiconductor with a work function of 4.3 eV and a valence band located 0.2 eV below the Fermi level. In contrast, the chemical oxidation produced CNOs with hydroxyl and carboxylic acid moieties and increased the work function from 4.3 to 4.8 eV with preservation of the metallic structure of CNO. The variations on the electronic structure were explained in terms of the elimination/generation of structural defects on the outer shells of CNO and the electronic effects that resulted from the introduction of different oxygen-containing functional groups on the graphitic layers. The interfacial energy diagrams for pristine and oxidized CNOs were constructed from the UPS data. These diagrams can be used to explain electronic interfacial properties of these materials and the design of novel electrochemical or photovoltaic devices.