Chemical vapor deposition of methane onto a template of alumina (Al2O3) nanoparticles is a prominent synthetic strategy of graphene meso-sponge, a new class of nanoporous carbon materials consisting of single-layer graphene walls. However, the elementary steps controlling the early stages of graphene growth on Al2O3 surfaces are still not well understood. In this study, density functional calculations provide insights into the initial stages of graphene growth. We have modelled the mechanism of CH4 dissociation on (111), (110), (100), and (001) γ-Al2O3 surfaces. Subsequently, we have considered the reaction pathway leading to the formation of a C6 ring. We found the γ-Al2O3(110) and γ-Al2O3(100) are both active for CH4 dissociation, but the (100) surface has a higher catalytic activity towards the carbon growth reaction. The overall mechanism involves the formation of the reactive intermediate CH2* that then can couple to form CnH2n* (n = 2-6) species. The unsaturated CH2 end promotes the sustained carbon growth in a nearly barrierless process. Also, the short length between terminal carbon atoms leads to strong interactions, which might lead to the high activity among unsaturated CH2* of hydrocarbon chain. Analysis of the electron localization and geometries of the carbon chains reveal the formation of C-Al-σ bonds with the chain growing towards the gas rather than C-Al-π bonds covering the γ-Al2O3(100) surface. This growth behaviour prevents catalysis poison during the initial stage of graphene nucleation.
Supporting Information for The onset of CVD graphene formation on γ-Al2O3 is promoted by unsaturated CH2 end