Chemical Reactivity of Lithium Doped Monolayer and Bilayer Graphene

Herein, we perform a comparative investigation on the reactivity of pristine and lithium doped monolayer and bilayer graphene. To study the addition of free radicals and 1,3 dipolar cycloadditions, we employed periodic DFT calculations. In all cases lithium increases the reactivity, even though the effect is weaker for the 1,3 dipolar cyclo addition as compared with that exhibited by free radicals. The effect of lithium doping is local and decreases as the functional group is attached far from the lithium atom. The increased reactivity can be explained by analyzing the band structure at the Fermi level. Lithium doped graphene is metallic thus showing an increased reactivity toward free radicals as observed for metallic nanotubes. However, the metallic character of lithium doped graphene is lost when free radicals like OH, SH, H, F, and CH3 are attached; the Fermi level is raised and a gap is opened. The reactivity of bilayer graphene was almost unaffected by the lithium doping. Therefore, to enhance the reactivity of graphene it is crucial to use the monolayer form. Also, it is important to eliminate defects to decrease lithium diffusion to the other side of the sheet because Li atoms can remove the functional groups attached to graphene, forming compounds like LiOH, LiF, etc. Finally, for comparative purposes, we performed M06-2X and MP2 calculations for the addition of free radicals to lithium doped benzene. The results showed similar trends to those observed for graphene. This work demonstrates that lithium doping can dramatically increase the reactivity of graphene to such an extent that chemical groups that do not react with graphene become bonded when lithium is underneath. Thus, lithium doping can open new avenues for investigating the chemistry of the rather unreactive sp(2) framework of graphene.