We report on the transformation of dichloromethane to low-chlorine carbon nanoparticles by the exposure of aqueous dichloromethane solution and/or dichloromethane/water biphasic solution to femtosecond laser pulses. The pH of either solution immediately decreased due to the reduction of dichloromethane by hydrated electrons. The time evolution of transmittance revealed that particle formation was suppressed when the initial pH was low because protons scavenge hydrated electrons. The size and morphology of nanoparticles was independent of the initial pH and the reaction solution used. The analysis of the elemental composition of carbon particles showed that the Cl/C atomic ratio was, independent on the initial pH values, greatly reduced to 0.06. We propose that the violent destruction of dichloromethane and elimination of chlorine atoms not only as Cl − but also Cl2 excludes chlorine atoms from the carbon network construction processes toward the formation of carbon nanoparticles.

SUMMARY The decomposition of aqueous chlorohydrocarbons regarded as environmental pollutants has been widely studied. However, it is usually difficult to separate liquid and/or volatile products originated from chlorohydrocarbons from reactant solution. We succeeded to precipitate hydrophobic carbon nanoparticles and their agglomerates from aqueous dichloromethane by femtosecond laser (0.8 μm, 40 fs) irradiation. In the case of water/dichloromethane bilayer solution, the precipitates were produced in abundance when the water layer was exposed to femtosecond laser pulses. The time evolution of pH and transmittance revealed that the precipitation of carbon particles was preceded by dechlorination. Focusing intense femtosecond laser pulses onto water creates a high density of reactive species in a well‐confined volume; that is, plasma filament. The graphitization hence solidification by using laser pulses could be a useful way to remove chlorinated compounds from their aqueous solution.

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