A Self-Templated Route to Hollow Silica Microspheres

A simple, mild, and effective self-templated approach has been developed to directly convert solid SiO 2 microspheres into hollow structures. The reaction involves initial partial dissolution of silica cores in a NaBH 4 solution and subsequent shell formation due to the redeposition of the silicate species back onto the colloid surfaces. The increasing concentration of NaBO 2 as the result of the slow decomposition of NaBH 4 in water is found to be responsible for the regrowth of the silica shell. This method allows the production of hollow silica spheres with sizes ranging from ∼70 nanometers to several micrometers, largely determined by the size of the starting silica colloids. The solid-to-hollow transformation mechanism is investigated in detail by transmission electron microscopy (TEM), scanning electron microscopy (SEM), Fourier Transform Infrared (FTIR) spectrometry, X-ray absorption spectroscopy (XAS), N 2 adsorption−desorption, and X-ray diffraction (XRD). We also study the reaction conditions that allow control over the wall thickness, surface morphology, and shell porosity.