Work function of the oxygen functionalized graphenic surfaces - Integral experimental and theoretical approach

The effect of oxygen plasma functionalization of graphenic surface was thoroughly investigated experimentally (SEM, XPS, Raman spectroscopy, work function measurements, TG) and corroborated by DFT molecular modeling using periodic slabs (work function, surface dipoles, EF/DOS). It was found that the introduction of oxygen substantially modified the electrodonor properties of the graphenic surface and the work function changed in a non-monotonous way upon the plasma treatment time. At the short contact time (5-30 s) the work function significantly increased from 4.2 eV (parent graphenic surface) to 5.6 eV and after passing the maximum value it reached plateau at the level of 5.2 eV after 100-150 s. The XPS revealed that the plasma treatment leads to an increase in the oxygen surface concentration (up to ~10 at.%). A comparison of SEM images and Raman spectra of unmodified and plasma-modified graphenic surfaces indicated that the induced changes are limited to the outermost surface. Indeed, the TGA stability profiles did not indicate any bulk structural changes. Based on the experimental and DFT results, the molecular models of the surface modification process was proposed taking into account the various location of surface oxygen functionalities. At the early stage of the plasma treatment, a generation of strongly polar surface functional groups (surf-OH with the local dipole moment of 2.7 D) leads to the formation of an electrostatic potential barrier for electron transfer from the surface (observed increase in work function). Prolonging modification results in the insertion of oxygen heteroatoms into the carbon surface, more uniform electron density distribution, and hence work function decrease. Since the electronic properties of graphenic materials play a key role in their various applications, the obtained results provide rational guidelines for their design and optimization.