The specific surface area and pore size of the mesoporous silica
were determined by employing nitrogen-sorption analysis at 77 K
(Figure 3). The adsorption-desorption isotherm exhibits a typical
type IV isotherm for the N2-sorption branch with an pronounced
adsorption at ca. 0.4 P/P0, indicating that uniform cylindrical mesopores
are present. The pore size distribution is calculated using the adsorption branch of the capillary condensation region utilizing the
Barrett-Joyner-Halenda (BJH) algorithm14. The results show that a
narrow distribution of mesopore diameters centered at 3 nm are
present (inset of Figure 3), which is consistent with the pore size
measured from the TEM image (Figure 2a). Also, the adsorption
and desorption branches display very small hysteresis, demonstrating
that the mesopores are uniform and open without any poreblocking
effects occurring during desorption15. Consequently, the
specific surface area of the vertically aligned mesoporous silica,calculated by using the Brunauer-Emmett-Teller method16 is
1030 m2/g, is considerably higher than that of previously reported
mesoporous silica13,17,18. The adsorption-desorption isotherm of the
silicon nanoparticles resulting from the magnesiothermic reduction
reaction displays small hysteresis of type H3 at 0.4-0.8 P/P0 (see
Supplementary Fig. S3 online), indicating plate-like particles that
do not display pore-blocking effects during desorption15. The specific
surface area of the completely reduced silicon nanoparticles is
363 m2/g, a value that is large as compared to that obtained in previously
generated silicon nano-structures produced by using magnesiothermic
reduction19–21. This outcome is the consequence of the
fact that the starting silicon particles with ca. 10 nm size are well
distributed over the rGO sheets without significant agglomeration or
a restacking of rGO sheets.