Spontaneous graphenization of amorphous carbon on clean surfaces of nanometer-sized nickel particles at room temperature

We studied the structures of amorphous carbon (a-C) films deposited on nickel (Ni) nanoparticles with clean surfaces by transmission electron microscopy and Raman spectroscopy. The Ni nanoparticles were prepared on cleaved sodium chloride substrates by electron beam deposition at a base pressure of less than 3 × 10−7 Pa at 673 K, and subsequently the a-C film was deposited onto the nanoparticles at room temperature without exposure to air. The a-C around the Ni nanoparticles spontaneously formed a disordered graphitic structure without heat treatment. This contrasts markedly with the case in which Ni nanoparticles were exposed to air once, and no graphitic layer formed on the surfaces of the nanoparticles unless a heat treatment was performed. The present results suggest that clean surfaces of metal nanoparticles without exposure to air exhibit high catalytic activity and reduce the graphenization temperature of a-C.