Production of Subsurface Carbon by Collision Induced Absorption and Its Vibrational Spectroscopic Identification in Au–Ni(111)

Carbon buried beneath the surface of a metal catalyst has long been suspected to play a key role in graphene and nanotube synthesis, carbon gasification, carbon fuel cells, and Fischer-Tropsch reactions, but the lack of a technique to detect this carbon unambiguously has stymied its definitive identification in that chemistry. This study reports the first vibrational spectroscopic identification of carbon occupying sites beneath a surface, along with its distinction from surface-adsorbed carbon, by the incident energy dependence of its high-resolution electron energy loss intensity. Its assignment is corroborated by its purposeful synthesis via collision-induced absorption, a technique designed to cleanly synthesize bulk carbon by bombardment of surface-bound carbon with 6 eV Xe atoms that pound it beneath the surface. On a Au-Ni(111) surface alloy, subsurface carbon is found to occupy multiple interstitial sites within the dislocation loop below the alloy surface and in Ni octahedral sites, yielding frequencies of 368, 442 and 509, and 694 cm^-1, respectively, consistent with previous density functional calculations. Discovery of the spectroscopic signature of bulk carbon provides a handle for its role and/or its reactivity to be explored and ultimately controlled and optimized over that of surface bound carbon in heterogeneous catalytic reactions and materials properties.