The carbon chain growth during the onset of CVD graphene formation on γ-Al2O3 is promoted by unsaturated CH2 ends

Abstract

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 nano porous 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 theory calculations provide insights into the initial stages of graphene growth. We have modelled the mechanism of CH₄ dissociation on the (111), (110), (100), and (001) γ-Al₂O₃ surfaces. Subsequently, we have considered the reaction pathway leading to the formation of a C6 ring. The γ-Al₂O₃(110) and γ-Al₂O₃(100) are both active for CH₄ dissociation, but the (100) surface has 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 C_nH_2n* (n = 2–6) intermediates with unsaturated CH₂ ends. The formation of these species, which are not bound to the surface-active sites, promotes the sustained carbon growth in a nearly barrierless process. Also, the short distance between terminal carbon atoms leads to strong interactions, which might lead to the high activity between unsaturated CH₂* of the hydrocarbon chain. Analysis of the electron localization and geometries of the carbon chains reveals the formation of C–Al–σ bonds with the chain growing towards the vacuum rather than C–Al–π bonds covering the γ-Al₂O₃(100) surface. This growth behaviour prevents catalyst poisoning during the initial stage of graphene nucleation.