Carbon-Isotope Labeling Rules Out Covalent Sulfur Addition to Graphene

Covalent functionalization of graphene is commonly inferred from Raman spectroscopy, photoelectron spectroscopy, microscopy, and first-principles calculations. However, these methods may not distinguish bonding to the graphene lattice from the adsorption of carbon-containing reaction products. Here, we investigate this ambiguity using the gas-phase reaction of sulfur radicals with graphene as a case study. Sulfur treatment produces new Raman bands, substantial changes in the photoemission spectra, and surface compositions consistent with sulfur addition. Density functional theory calculations further show that these observations can be reproduced by a stable graphene episulfide structure in which sulfur bridges two lattice carbon atoms. A decisive control experiment using isotopically pure 13C graphene nevertheless rules out this assignment. Although the intrinsic graphene Raman bands undergo the expected isotope shifts, the characteristic reaction–product bands near 1260 and 1450 cm–1 remain unchanged. The carbon atoms contributing to these modes therefore originate from extrinsic species rather than from the graphene lattice. Detection of similar products on hexagonal boron nitride further supports the formation of organosulfur species in the gas phase, followed by their selective deposition on hydrophobic surfaces. These results show that an extensive agreement among conventional characterization methods and theoretical predictions can still lead to an incorrect structural assignment. Isotopically labeled graphene provides a direct and broadly applicable control for establishing whether carbon atoms in proposed functional groups originate from the graphitic lattice.