The surface passivation mechanism of nanocrystalline silicon powder was studied. The liquid nitrogen/argon was used as the medium to prepare the nanocrystalline silicon powder, using a cryomilling technology. The X-ray diffraction, transmission electron microscopy, plasma emission spectroscopy and infrared spectrum were used to analyze the prepared samples, and density functional theory was used to investigate the cryomilling process. For nanocrystalline silicon powder cryomilled with liquid N2, the amorphous outer layer with N element is formed on the surface, and chemisorption caused by the formation of Si-N-Si bond leads to the surface passivation, although physisorption also be confirmed, the Si-N bond is steady after exploded in air for 30 days and no new bond is observed. For nanocrystalline silicon powder cryomilled with liquid Ar, no new chemical bond is observed, Ar element absorbs on the surface of the prepared powder only through physisorption, and after exploded in air for 30 days, a Si-O bond can be observed obviously.
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