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.