Stöber process

The Stöber process is a chemical process used to prepare silica (SiO2) 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 mechanism – a 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. The Stöber process is a chemical process used to prepare silica (SiO2) 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 mechanism – a 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 m2 g−1 and a density of 0.040 g cm−3 can be prepared. NASA has prepared silica aerogels with a Stöber-process approach for both the Mars Pathfinder and Stardust missions. The Stöber process is a sol-gel approach to preparing monodisperse (uniform) spherical silica (SiO2) materials that was developed by a team led by Werner Stöber and reported in 1968. The process, an evolution and extension of research described in Gerhard Kolbe's 1956 Ph.D. dissertation, was an innovative discovery that still has wide applications more than 50 years later. Silica precursor tetraethyl orthosilicate (Si(OEt)4, TEOS) is hydrolyzed in alcohol (typically methanol or ethanol) in the presence of ammonia as a catalyst: The reaction produces ethanol and a mixture of ethoxysilanols (such as Si(OEt)3OH, Si(OEt)2(OH)2, and even Si(OH)4), which can then condense with either TEOS or another silanol with loss of alcohol or water: Further hydrolysis of the ethoxy groups and subsequent condensation leads to crosslinking. It is a one-step process as the hydrolysis and condensation reactions occur together in a single reaction vessel. The process affords macroscopic particles of granular silica with diameters ranging from 50 to 2000 nm; particle sizes are fairly uniform with the distribution determined by the choice of conditions such as reactant concentrations, catalysts, and temperature. Larger particles are formed when the concentrations of water and ammonia are raised, but with a consequent broadening of the particle-size distribution. The initial concentration of TEOS is inversely proportional to the size of the resulting particles; thus, higher concentrations on average lead to smaller particles due to the greater number of nucleation sites, but with a greater spread of sizes. Particles with irregular shapes can result when the initial precursor concentration is too high. The process is temperature-dependent, with cooling (and hence slower reaction rates) leading to a monotonic increase in average particle size, but control over size distribution cannot be maintained at overly low temperatures. In 1999 Cédric Boissière and his team developed a two-step process whereby the hydrolysis at low pH (1 – 4) is completed before the condensation reaction is initiated by the addition of sodium fluoride (NaF). The two-step procedure includes the addition of a nonionic surfactant template to ultimately produce mesoporous silica particles. The main advantage of sequencing the hydrolysis and condensation reactions is the ability to ensure complete homogeneity of the surfactant and the precursor TEOS mixture. Consequently, the diameter and shape of the product particles as well as the pore size are determined solely by the reaction kinetics and the quantity of sodium fluoride introduced; higher relative fluoride levels produces a greater number of nucleation sites and hence smaller particles. Decoupling the hydrolysis and condensation process affords a level of product control that is substantially superior to that afforded by the one-step Stöber process, with particle size controlled nearly completely by the sodium fluoride-to-TEOS ratio. The two-step Stöber process begins with a mixture of TEOS, water, alcohol, and a nonionic surfactant, to which hydrochloric acid is added to produce a microemulsion. This solution is allowed to stand until hydrolysis is complete, much like in the one-step Stöber process but with the hydrochloric acid replacing the ammonia as catalyst. Sodium fluoride is added to the resulting homogeneous solution, initiating the condensation reaction by acting as nucleation seed. The silica particles are collected by filtration and calcined to remove the nonionic surfactant template by combustion, resulting in the mesoporous silica product.