Early stages of surface graphitization on nanodiamond probed by x-ray photoelectron spectroscopy

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We have investigated the early stages of graphitization on detonation nanodiamond during sequential annealing treatments under vacuum using x-ray photoelectron spectroscopy. Two different temperature-dependent regimes were observed. Below 900 \ifmmode^\circ\else\textdegree\fi{}C, the nanodiamond surface reconstructs into graphitic domain but does not alter the diamond core. Above 900 \ifmmode^\circ\else\textdegree\fi{}C, graphitization, i.e., carbon hybridization changes from $s{p}^{3}$ to $s{p}^{2}$, occurs from the nanodiamond surface toward the diamond core. Graphitization is observed at much lower temperatures on nanodiamonds than on bulk diamond due to the high concentration of structural defects on their surface. These results indicate that low-temperature annealing under vacuum is an efficient method to uncouple surface and bulk graphitization. Hybrid nanocarbons formed in these conditions, constituted of a diamond core with a thin graphitic outer shell, may have interesting catalytic and chemical properties.