Si boride-coated Si nanoparticles with improved thermal oxidation resistance

• Si boride nanoshell (~15 nm) is formed with a molten salt of sodium borohydride.
• Si boride-coated Si NPs have remarkably improved thermal oxidation resistance.
• The core–shell NPs retain optical stability after annealing at 850 °C in air.

A new fabrication technique is devised to synthesize conformal Si core–shell nanoparticles (NPs) with Si boride nanoshell by reacting the surface atoms of Si NPs (50–200 nm diameter) with a molten salt of sodium borohydride. The shell layer, about 10–20 nm thick, consists of a mixture of crystalline phase (SiBx) and other amorphous phases as identified by TEM and EELS analysis. New absorbance peaks for Si–Si boride core–shell NPs appear at the wavenumber of 940 and 777–677 cm−1 in FT-IR analysis. TGA analysis reveals that the core–shell structured Si–Si boride NPs exhibit a remarkably improved resistance to thermal oxidation by a factor of 4.6 at 750 °C and at by a factor of 3.5 at 850 °C compared to bare Si. Optical measurements show that spectrally selective coating (SSC) layers made of Si–Si boride NPs have a superior optical stability to that of the bare Si NPs after annealing at high temperature, and desirably exhibit a lower reflectance in the visible spectrum range than the bare Si NPs. These surface-protected, oxidation-resistant semiconductor materials and their novel fabrication methods exhibit further broad applicability of boride nanolayers which can be used for high temperature applications such as solar thermal power generation.

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