TGA and DTA were employed to further investigate this reaction.
The mass of the mixture did not change during the reaction with a
slow ramp rate (Fig. 3g). With a higher ramp rate (20uC min21),
however, the powders were ejected out of the TGA crucible at
433uC, indicating a violent reaction. DTA directly shows that the
reaction occurs around 400–450uC, and that the higher ramp rate
results in more severe heat accumulation (Fig. 3h), which ultimately
leads to the violent reaction and ejection of reactants. Therefore, in
order to obtain nano-Si, a slower heating rate of about 5uC min21 is
necessary. Further decreasing the heating rate from 5 to 0.5uC min21
does not decrease the particle size or influence the crystallinity. The
obtained Si nanoparticles (Si-RH-5) have sizes around 22 nm
(Fig. 2f, Fig. 3c–d). Theoretically, spherical 22 nm single crystalline
Si particles have a specific surface area of 117 m2 g21, which is only
half of the measured value (Supplementary Table S1). Hence, the Si
nanoparticles obtained here are porous, which is consistent with the
pore diameter analysis (Fig. 3f, inset). This naturally-inherited porous
nanoparticle morphology meets the requirement of a stable Si
anode to accommodate the volume change induced by lithium insertion/
extraction (discussed below).