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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 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.
Supplementary materials
Title
Supporting Information for The onset of CVD graphene formation on γ-Al2O3 is promoted by unsaturated CH2 end
Description
The supplementary material contains the following information: Energy diagram of the first two CH4 dehydrogenation steps on γ-Al2O3 (110) with different OH coverages (Figure S1); Structures in intermediates and translation states in the first two steps of the CH4 dehydrogenation on γ-Al2O3 surfaces (Figure S2), structures of γ-Al2O3 (110) with different OH coverage (Figure S3); NEB searching for C2H6 interacting with surface flexible carbon species (Figure S4) and NEB intermediates structures for C2H6 interacting with surface flexible carbon species (Figure S5); Energy diagram and structures of the C2H6 dehydrogenation (Figure S6); energy diagram and structures of C6H12 ring formation process (Figure S7); Energy diagram and structures of C6H12 ring desorption (Figure S8); Energy diagram and structures for H-transfer and H desorption after carbon ring (Figure S9).
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