Anaerobic vs. aerobic preparation of silicon nanoparticles by stirred media milling. The effects of dioxygen, milling solvent, and milling time on particle size, surface area, crystallinity, surface/near-surface composition, and reactivity

A stirred media mill was hermetically sealed in a high-purity dinitrogen atmosphere glovebox and used to prepare silicon nanoparticles (SiNPs) by milling a slurry of 45-90 mu m pieces of metallurgical-grade silicon (MGS) suspended in anhydrous heptane (HEP), heptane containing a small amount of mesitylene (HEP/MES), or mesitylene containing 22 mM pyrene (MES/PYR) under anaerobic and anhydrous conditions (<= 5 ppm dioxygen (O-2) and water (H2O) vapor). Excess solvent and other volatiles were removed using a rotary evaporator also housed in the glovebox. The SiNPs were further dried at 22 +/- 1 degrees C under vacuum (<= 1 x 10(-5) Torr) for at least 15 h. The dark-brown or black pyrophoric nanoparticle powders were stored in a high-purity argon atmosphere glovebox. Samples were removed from the glovebox and analyzed by BET surface area (BET-SA) determination, tensimetric titrations with various pressures of O-2, PXRD, SEM, TEM, ATR-FTIR, and/or XPS. The effects of (i) anaerobic vs. aerobic milling, (ii) milling fluid, and (iii) milling time on BET-SA, degree of crystallinity, reactivity with O-2, and FTIR-or XPS-determined surface/near-surface composition of the SiNPs are discussed. The BET-SA of SiNPs milled anaerobically for 5 h in MES/PYR (258 m(2) g(-1)) was ca. twice as large as the BET-SA for SiNPs milled aerobically in the same milling fluid. In contrast, the BET-SA of SiNPs milled anaerobically in HEP (40 m(2) g(-1)) was four times smaller than for SiNPs milled aerobically in HEP. The PXRD-determined degree of crystallinity steadily decreased and the BET-SA steadily increased over time (1, 2, or 5 h) for SiNPs milled anaerobically in MES/PYR. The degree of crystallinity of SiNPs milled anaerobically in HEP also steadily decreased over time, but the BET-SAs for 1, 2, 3, 4, and 5 h milled samples were essentially the same. MES/PYR milled SiNPs reacted more slowly with O-2 and irreversibly absorbed less O-2 than HEP milled SiNPs.