The Stöber process is a chemical process used to prepare silica () particles of controllable and uniform size for applications in materials science. It was pioneering when it was reported by Werner Stöber and his team in 1968, and remains today the most widely used wet chemistry synthetic approach to silica nanoparticles. It is an example of a sol-gel process wherein a molecular precursor (typically tetraethylorthosilicate) is first reacted with water in an alcoholic solution, the resulting molecules then joining together to build larger structures. The reaction produces silica particles with diameters ranging from 50 to 2000 nm, depending on conditions. The process has been actively researched since its discovery, including efforts to understand its kinetics and mechanisma particle aggregation model was found to be a better fit for the experimental data than the initially hypothesized LaMer model. The newly acquired understanding has enabled researchers to exert a high degree of control over particle size and distribution and to fine-tune the physical properties of the resulting material in order to suit intended applications.
In 1999 a two-stage modification was reported that allowed the controlled formation of silica particles with small holes. The process is undertaken at low pH in the presence of a surface-active molecule. The hydrolysis step is completed with the formation of a microemulsion before adding sodium fluoride to start the condensation process. The non-ionic surfactant is burned away to produce empty pores, increasing the surface area and altering the surface characteristics of the resulting particles, allowing for much greater control over the physical properties of the material. Development work has also been undertaken for larger pore structures such as macroporous monoliths, shell-core particles based on polystyrene, cyclen, or polyamines, and carbon spheres.
Silica produced using the Stöber process is an ideal material to serve as a model for studying colloid phenomena because of the monodispersity (uniformity) of its particle sizes. Nanoparticles prepared using the Stöber process have found applications including in the delivery of medications to within cellular structures and in the preparation of biosensors. Porous silica Stöber materials have applications in catalysis and liquid chromatography due to their high surface area and their uniform, tunable, and highly ordered pore structures. Highly effective thermal insulators known as aerogels can also be prepared using Stöber methods, and Stöber techniques have been applied to prepare non-silica aerogel systems. Applying supercritical drying techniques, a Stöber silica aerogel with a specific surface area of 700 m g and a density of 0.040 g cm can be prepared. NASA has prepared silica aerogels with a Stöber-process approach for both the Mars Pathfinder and Stardust missions.