3.4. Role of PVP on the Formation of the Hollow
Structure. The effect of the PVP concentration on the
formation of hollow nanomaterials was investigated. The
HSSNs were taken as an example. In the absence of PVP,
solid silica nanoparticles without a hollow interior were
obtained as shown in Figure 9A. In the presence of different
concentrations of PVP varying from 31.3, to 62.5, to 125 wt %,
the HSSNs were successfully synthesized. When the concentration
of PVP was 31.3 wt % (Figure 9B), which was lower
than the minimum PVP−H2O binding concentration (57 wt
%),33 there were a limited number of H2O molecules bound toPVP. Therefore, the majority of the synthesized nanomaterials
were solid silica nanoparticles (inset TEM image in Figure 9B)
with a few HSSNs. As the concentration of PVP increased to
62.5 wt %, well-dispersed HSSNs were obtained, as shown in
Figure 9C. All hollow nanoparticles appeared as a spherical
morphology with uniform size. When the concentration of PVP
was further increased to 125 wt % (Figure 9D), no significant
morphological change was observed except for a slight decrease
in size from 365 ± 49 to 359 ± 55 nm