We report 3C-SiC nano-crystals synthesis by thermal annealing of SiO2/Si wafers in CO2 gas. The nano-crystals have been characterized using scanning electron microscopy and atomic force microscopy. These results are correlated with selective area electron diffraction paterns, and transmission electron microscopy observations that evidence the formation of cubic SiC nano-crystals epitaxied on Si. In our experimental conditions, the crystals size is in the range 10-60 nm, increasing with the treatment time, as the crystals density. Using isotopic labelled SiO2 associated with Nuclear Reaction Analysis (NRA) and Nuclear Narrow Resonance Profiling (NRP), oxygen exchanges between CO2 and SiO2 could be evidenced.