Preparation of boehmite—silica colloids: Rods, spheres, needles and gels

A method is presented for the synthesis of amorphous silica rods with a boehmite core. The addition of boehmite needles to a mixture of tetraethoxysilane (TES), ethanol and aqueous tetramethylammonium hydroxide solution produces gels or aggregates. However, when boehmite needles are previously coated with silica in an aqueous sodium silicate solution, they serve in the TES solution as nuclei for the polymerisation of part of the hydrolysed TES into discrete, amorphous silica rods. The rods tend to stick together probably because of the increase in ionic strength caused by the TES hydrolysis. In the absence of boehmite nuclei, silica only forms discrete, non-aggregated spheres, which demonstrates that the Stöber silica sphere synthesis can be performed with a base other than ammonia.