3.5. Possible Mechanism. A possible mechanism of the
formation of hollow nanomaterials with tunable morphologies
was proposed based on the above results (Scheme 1). The
formation of PVP−H2O droplets and the intrinsic properties of
applied solvents play critical roles in the synthesis process. PVP
is an amphiphilic, nonionic polymer that is soluble in H2O and
many nonaqueous solvents because of the presence of a highly
polar amide group within the pyrrolidone ring. Therefore, PVP
was commonly viewed as a surfactant and a stabilizer in
material synthesis. One important property of PVP is that, at a
high concentration of PVP (≥57 wt %) in aqueous solution,
H2O molecules preferred binding with PVP instead of acting as
the solvent, which was referred to as bound H2O.33 When
TEOS was added, its hydrolysis and condensation took place at
the interface of the hydrophobic phase and the PVP−H2O
droplets. The hydrolyzed Si(OH)x(OCH2CH3)4−x molecules
were polar and tended to accumulate at the oil−H2O interface
of the emulsion. As the hydrolysis and condensation
continuously proceeded, the resulting silica assembled on the
surface of PVP−H2O droplets, eventually resulting in a hollow
interior after removal of the PVP−H2O droplets