At the same time, Si
coats and fuses around the MgO that forms, and after dissolving
MgO with HCl, macroporous Si remains (Fig. 3a). In sample Si-
RH-40, some MgO might be completely coated with Si so that HCl
cannot access it during etching, which is why relatively intense MgO
peaks appear in the XRD pattern of Si-RH-40 (Fig. 3e). Another result of this violent inhomogeneous reaction was the formation of
Mg2Si in Si-RH-40 (Supplementary Fig. S6).