From Hollow Nanosphere to Hollow Microsphere: Mild Buffer Provides Easy Access to Tunable Silica Structure

A facile synthesis method for producing silica hollow spheres from nano- to microsize level is realized by adjusting the hydrolysis and condensation kinetics of silane precursors in a mild buffer solution (NaH2PO4-Na2HPO4, pH approximate to 7.0) in the presence of F127 (EO106PO70EO106) surfactant. Characterization using transmission electron microscopy, field-emission scanning electron microscopy, and nitrogen sorption techniques reveals that the silica hollow nanospheres with outer diameter of about 12 nm can be obtained with tetramethoxysilane as silane precursor. Silica hollow nanospheres (similar to 20 nm) can also be prepared using tetraethoxysilane (TEOS) as silane precursor with the addition of hydrolysis and condensation catalyst NH4F. Using TEOS as the silane precursor without F-, the formation of silica hollow microspheres (0.5 similar to 8 mu m) was observed. Time-resolved in situ UV-Raman results show that TEOS does not hydrolyze at room temperature, and the hydrolysis of TEOS occurs immediately at room temperature when NH4F was added to the buffer solution. The fast hydrolysis and condensation rates of silane precursor favor the formation of hollow nanospheres through the condensation of silicate around a single micelle of F127 in NaH2PO4-Na2HPO4 buffer solution. The construction of hollow microspheres is attributed to the formation of O/W emulsion by the hydrophobic TEOS with the aid of F127 surfactant due to the existence of unhydrolyzed TEOS at room temperature under current synthetic conditions.