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 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 nucleate 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.
One-step process
The Stöber process is a sol-gel approach to preparing monodisperse (uniform) spherical silica () 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 PhD dissertation, was an innovative discovery that still has wide applications more than 50 years later. Silica precursor tetraethyl orthosilicate (, TEOS) is hydrolyzed in alcohol (typically methanol or ethanol) in the presence of ammonia as a catalyst: Si(OEt)4 + H2O -> Si(OEt)3OH + EtOH Si(OEt)4 + 2H2O -> Si(OEt)2(OH)2 + 2EtOH The reaction produces ethanol and a mixture of ethoxysilanols (such as , , and even ), which can then condense with either TEOS or another silanol with loss of alcohol or water: 2Si(OEt)3OH -> (EtO)3Si-O-Si(OEt)3 + H2O Si(OEt)3OH + Si(OEt)4 -> (EtO)3Si-O-Si(OEt)3 + EtOH Si(OEt)3OH + Si(OEt)2(OH)2 -> (EtO)3Si-O-Si(OEt)2OH + H2O
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 microscopic particles of colloidal 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 distribution cannot be maintained at overly low temperatures.
Two-step process 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.
The selection of conditions for the process allows for control of pore sizes, particle diameter, and their distributions, as in the case of the one-step approach. Porosity in the modified process is controllable through the introduction of a swelling agent, the choice of temperature, and the quantity of sodium fluoride catalyst added. A swelling agent (such as mesitylene) causes increases in volume and hence in pore size, often by solvent absorption, but is limited by the solubility of the agent in the system. Pore size varies directly with temperature, bound by the lower out of the surfactant cloud point and the boiling point of water. Sodium fluoride concentration produces direct but non-linear changes in porosity, with the effect decreasing as the added fluoride concentration tends to an upper limit.
Kinetics The LaMer model for the kinetics of the formation of hydrosols is widely applicable for production of monodisperse systems, and it was originally hypothesized that the Stöber process followed this monomer addition model. This model includes a rapid burst of nucleation forming all of the particle growth sites, then proceeds with hydrolysis as the rate-limiting step for condensation of triethylsilanol monomers to the nucleation sites. The production of monodisperse particle sizes is attributed to monomer addition happening at a slower rate on larger particles as a consequence of diffusion-limited mass transfer of TEOS. However, experimental evidence demonstrates that the concentration of hydrolyzed TEOS stays above that required for nucleation until late into the reaction, and the introduction of seeded growth nuclei does not match the kinetics of a monomer addition process. Consequently, the LaMer model has been rejected in favour of a kinetic model based around growth via particle aggregation.
Under an aggregation-based model, nucleation sites are continually being generated and absorbed where the merging leads to particle growth. The generation of the nucleation sites and the interaction energy between merging particles dictates the overall kinetics of the reaction. The generation of the nucleation sites follows the equation below:
where J is the nucleation rate, k1 and k2 are rate constants based on the concentrations of H2O and NH3 and gs is the normalization factor based on the amount of silica precursor. Adjusting the concentration ratios of these compounds directly influences the rate at which nucleation sites are produced.
Merging of nucleation sites between particles is influenced by their interaction energies. The total interaction energy is dependent on three forces: electrostatic repulsion of like charges, vanderWaals attraction between particles, and the effects of solvation. These interaction energies (equations below) describe the particle aggregation process and demonstrate why the Stöber process produces particles that are uniform in size.
The van der Waals attraction forces are governed by the following equation:
where AH is the Hamaker constant, R is the distance between the centers of the two particles and a1, a2 are the radii of the two particles. For electrostatic repulsion force the equation is as follows: where Where ε is the dielectric constant of the medium, kB is the Boltzmann constant, e is the elementary charge, T is the absolute temperature, κ is the inverse Debye length for a 1:1 electrolyte, x is the (variable) distance between the particles, and φ0 is the surface potential. The final component of the total interaction energy is the solvation repulsion which is as follows:
where As is the pre-exponential factor (1.5 × 10−3 J⋅m−2) and L is the decay length (1 × 10−9 m).
This model for controlled growth aggregation fits with experimental observations from small-angle X-ray scattering techniques and accurately predicts particle sizing based on initial conditions. In addition, experimental data from techniques including microgravity analysis and variable pH analysis agree with predictions from the aggregate growth model.
Morphological variations Several different structural and compositional motifs can be prepared using the Stöber process by the addition of chemical compounds to the reaction mixture. These additives can interact with the silica through chemical and/or physical means either during or after the reaction, leading to substantial changes in morphology of the silica particles.
Mesoporous silica
The one-step Stöber process may be modified to manufacture porous silica by adding a surfactant template to the reaction mixture and calcining the resulting particles. Surfactants that have been used include cetrimonium bromide, cetyltrimethylammonium chloride, and glycerol. The surfactant forms micelles, small near-spherical balls with a hydrophobic interior and a hydrophilic surface, around which the silica network grows, producing particles with surfactant- and solvent-filled channels. Calcining the solid leads to removal of the surfactant and solvent molecules by combustion and/or evaporation, leaving mesopore voids throughout the structure, as seen in the illustration at right.