Chemical Reactivity of Electron-Doped and Hole-Doped Graphene

The chemical reactivity of electron-doped and hole-doped graphene was studied by means of first principles calculations, on the basis of dispersion corrected density functional theory. To model hole-doped graphene, the widely known electron acceptor 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4-TCNQ) was utilized, while the electron donor tetrathiafulvalene (TTF) was selected for the electron-doped case. The results demonstrate that the reactivity of graphene can be modified by the adsorption of electron donating/withdrawing molecules. The reactions considered were the addition of fluorine atoms and hydroxyl radicals. In both cases, it was observed that the adsorption of F4-TCNQ and TTF increased the reactivity of graphene. This outcome was expected for electron-doped graphene because we have recently shown that lithium increases the reactivity of graphene. Yet, for F4-TCNQ the finding is surprising given that this molecule accepts 0.4 e(-) from graphene. The gas phase free energies of association are calculated to be negative for F4-TCNQ and TTF, but for the latter is only -2.5 kcal/mol. The results obtained employing infinite models and using the VDW-DF and M06-L functionals are supported by cluster model calculations performed with the M06-2X method. When TTF was adsorbed onto graphene, a charge transfer from TTF to graphene was not observed. However, when TTF and F4-TCNQ are simultaneously adsorbed on opposite sides of the graphene sheet, the amount of charge accepted by F4-TCNQ and donated by TTF is increased. This work clearly suggests that dual doping is a useful tool to expand, even to a greater extent, the possibilities to tune the properties of graphene. Further work must be devoted to synthesize better electron donors and acceptors and thus allow for larger charge transfer.