In this work, we reveal the dual roles of polymeric capping ligands in the hollowing of silica nanospheres during their surface-protected etching. We first show that polymeric capping ligands, if they have a stronger interaction with the surface Si-OH groups than water, can reduce the condensation of the silica network, allowing the diffusion of OH- ions through the shell to dissolve the inner silica. Also, the polymeric ligands can passivate the surface silica, making it less likely to be dissolved by OH- ions. The combination of these two roles ensures highly selective etching of the interior of the colloidal silica spheres, making the surface-protected etching a robust process for the synthesis of hollow silica nanoshells. Our insight into the specific roles of the ligands is expected to elucidate the impact of polymeric ligands on the colloidal chemistry of silica, particularly in its condensation and etching behaviors, and offer new opportunities in the design of silica and other oxide-based nanostructures.
Keywords: condensation; hollow silica nanospheres; poly(vinylpyrrolidone); silicomolybdate method; surface-protected etching.