Density Functional Investigation of Thioepoxidated and Thiolated Graphene

Employing first-principle calculations, we have investigated the interaction between graphene and the thioepoxide and thiol groups. The SH radical cannot be chemisorbed on perfect graphene, although it is physisorbed. The chemisorption energy can be increased to 0.4 eV if multiple SH groups are bonded to the sheet or if they are attached to Stones-Wales defects. However, when free-energy corrections are considered, the addition of SH groups to perfect graphene is not spontaneous. In the case of the SW defects, the addition is favorable if two SH groups are attached to the shortest CC bond and in opposite sites of the sheet. The single vacancy defect site has the highest affinity for the SH radical, which is dissociatively attached. Finally, employing nanoribbons, we have simulated the reactivity of bare and hydrogen-terminated edges of graphene. The SH group is dissociatively bonded to bare edges. However, hydrogen-terminated zigzag edges prefer to bind the SH group. Considering the different reactivities observed, the defect sites and edges of graphene can be labeled by employing SH radicals. The sites containing sulfur can be used to attach gold nanoparticles or create vertical arrays of graphene sheets on Au surfaces. Finally, for thioepoxidated graphene, we have determined that the binding energy per S atom is 0.49 eV, larger than that determined for the thiol group but very small to be achieved experimentally because the free-energy change is expected to be close to 0 for this process. These results confirm the experimental evidence, which indicated that the sulfur-containing groups present in sulfur-graphite nanocomposites are attached to the edges of graphite, although vacancy defect sites must be considered. The electronic properties of the functionalized and defective graphene sheets are discussed. As a byproduct, we have found that the free-energy term may turn the attachment of a single HO group to graphene to be not spontaneous. Thus, the OH groups observed in graphene oxide are present at defect sites or agglomerated, to have their binding energy increased due to cooperative effects, confirming earlier experimental results.