As discussed earlier, the pore size of mesoporous SiO2 (7 nm) is
much bigger than that of N719 dye molecules (1 nm) [27] which
facilitates the diffusion of dye molecules through proper channels
and therefore does not affect dye loading when incorporated in
mesoporous TiO2 [28]. Fig. 4(b) shows SEM images of mesoporous
TiO2 incorporated with SiO2 revealing inter-connecting
nanocrystallites having porous structure. The figure clearly shows
that morphology of the mesoporous TiO2 (Fig. 4(a)) did not change
when mesoporous SiO2 was incorporated in TiO2 and the rod like
structure of mesoporous SiO2 is entirely covered by TiO2.