Sulfur as an important co-factor in the formation of multilayer graphene in the thermolyzed asphalt reaction

Abstract

The results indicate a first ever hypothesized role of sulfur in the low temperature formation of graphene. Multilayer graphene was prepared with various hydrocarbons and sulfur mixtures by the University of Idaho Thermolyzed Asphalt Reaction (UITAR). Graphene films were synthesized with the UITAR process through the original flame-heated crucible, and in thermogravimetric analysis apparatuses. The latter was carried out under N₂ purge. The morphology of these films synthesized by cyclohexanol and sulfur was characterized by scanning electron, transmission electron, and atomic force microscopies, which indicate a flat (over several mm²) and layered structure consistent with graphitic structures. Elemental composition as determined by X-ray photoelectron spectroscopy indicated primarily sp² C with trace O and N impurities. Raman spectra of the UITAR graphene have D and G bands at 1350 cm⁻¹ and 1594 cm⁻¹. Based on the wavenumber positions of these bands, the Ferrari amorphization trajectory indicates that the UITAR graphene films are primarily sp² C with nano-crystalline characteristics. The I(D)/I(G) ratio is 0.97 with an average grain size of 5 nm as determined by the Tuinstra–Koenig relationship. A proposed scheme illustrates the role of sulfur in graphene growth based on thermogravimetric analyses. We hypothesize that elemental sulfur is involved with the dehydration/dehydrogenation and eventual crosslinking of cyclohexanol between 100 and 140 °C. In the range of 240–400 °C further dehydrogenation steps occur giving an unidentified intermediate with a sharp Raman peak at 1450 cm⁻¹. At 550 °C a mixture of graphene-like Raman D and G bands appear with the 1450 cm⁻¹ intermediate. At 600 °C the intermediate peak is lost with only bands characteristic of UITAR graphene. Therefore the minimum temperature of graphene formation with the UITAR reaction is 600 °C. The proposed mechanism is reinforced by results with other hydrocarbons. Other organics succeeded or failed in the UITAR reaction based on melting and boiling considerations.