Recovery of nano-Si from RHs. 3 g of RHs (Farmers’ Rice Co., USA) were first
refluxed with 45 mL of 10% HCl for 2 h to remove the metal ions inside. The leached
RHs were collected by filtration, washed with a large amount of deionized water, dried
at 100uC, and heated in air at 700uC for 2 h with a ramp rate of 5uC min21. A white
nano-SiO2 remnant formed with a yield about 0.5 g.
100 mg of the obtained nano-SiO2 was thoroughly ground along with 100 mg of
Mg powder (,325 mesh, 99.8%, Alfa Aesar) and sealed in a 5/8 in. Swagelok brass
union capped with two 5/8 in. Swagelok stainless steel plugs (Supplementary Fig. S3)
inside an Ar-filled glovebox. The powders were spread evenly at the bottom of the
horizontal reactor to avoid local heat accumulation during the exothermic reaction.
The reactor was heated inside a tube furnace under H2 or inert gas flow to 400uC over
10 min, then it was heated to 650uC with a ramp rate of 5uCmin21, and finally it was
held at 650uC for 2 h. After this process, the reactor was taken out of the furnace when
the temperature fell below 100uC. The obtained powders were soaked in 1 M HCl
solution (molar ratio of HCl 5 H2O 5 EtOH 5 0.66 5 4.72 5 8.88) for 6 h to remove
MgO and Mg2Si and then they were soaked in 5% hydrofluoric acid for 10 min to
ensure that any unreacted or newly formed SiO2 was removed. No difference were
noticed without HF etching. After drying under vacuum at room temperature, nano-
Si powders were obtained with a yield of about 30 mg.