Mesoporous silica with tunable structural properties is of great interest for a wide range of applications. Various parameters, such as surfactant, water amount and temperature, can be tuned to control the shape, size and pore structure of mesoporous silica particles.
The influence of cetyltrimethylammonium bromide (CTAB) as template on the morphology and pore structure of mesoporous silicas has been investigated. Various morphologies can be generated, including spherical, aggregate, bean-like and faceted shaped particles.
Sol-Gel Reactions
The synthesis of mesoporous silica by sol-gel reactions is very common and can be used to produce mesoporous materials with tunable porosity, size and structure. Many of these are used in applications such as catalysis, adsorption, optical devices, drug delivery and bio-imaging. Achieving good control of the morphology, dispersity and uniformity of these materials is essential to their successful application. Methods to achieve this are described here. They include fast self-assembly, soft and hard templating, a modified Stober process, dissolving-reconstruction and modified aerogel methods.
In most sol-gel reactions, the starting material is a "sol" that is a stable dispersion of colloidal silica particles (amorphous or crystalline) or polymer chains in a solvent. In gel formation, van der Waals forces or hydrogen bonds cause the particles to interact and aggregate into a three-dimensional network. The gel may then undergo a number of reactions, including hydrolysis and polycondensation, which lead to the formation of metal-oxo-metal or metal-hydroxy-metal bonds in the resulting mesoporous materials. The resulting gel may then be dried to form a dense amorphous material, an aerogel or, if required, a porous molecular sieve with tunable pores.
Sol-gel chemistry is very pH sensitive, due to the fact that acids (H+) and bases (OH-) accelerate reactions by different mechanisms. It is therefore important to control the acidity of a solution and to ensure that it is at an appropriate pH when reacting. A solution that is too acidic will be unable to form a gel and will decompose into its constituent parts; a solution that is too alkaline will not have sufficient viscosity for effective gelation.
Sol-gel chemistry can also be limited by the availability of suitable metal alkoxides and water-soluble structure directing agents or templates. In order to overcome these limitations, alternative metal-alkoxide reactions have been developed which employ aqueous metal salts instead of alkoxides. In the case of aqueous metal salts, the adsorption behaviour can be modified through the use of a co-catalyst or by altering the synthesis conditions. A typical example of these reactions would be the synthesis of mesoporous cobalt-based SBA-15 molecular sieves by mixing a hydrate of Co(NO)3 with a sodium phosphate (SB), a surfactant, and tetraethyl orthosilicate (TEOS). This allows the pore size to be tuned by varying the concentration of the aqueous metal salt and the amount of TEOS added.
Hydrothermal Reactions
For the hydrothermal reactions, various silica precursors may be used, but tetraethyl orthosilicate (TEOS) is preferably used because of its superior catalytic properties. The reaction is carried out by heating the silicate solution to high temperatures under a high pressure, and then cooling it down to lower temperatures. The mixture is subsequently subjected to hydrolysis, during which the water molecules cause the crystals to dissolve and form mesoporous silica with controlled pore size distribution.
Recently, a new method was developed that allows the formation of mesoporous silica by introducing transition metal salts during the hydrothermal process. These salts react with the silica to generate metal oxide nanoparticles with a specific structure. In addition to being useful as a mesoporous carrier, these particles have many other applications such as catalysts, adsorbents, and photocatalytic devices.
To make this reaction work, tetraethyl orthosilicate is mixed with an organic compound that can act as a modifier, such as a carboxylic acid, amine, or phosphonate. The modification is conducted in the supercritical condition of the hydrothermal reaction, which is a much more efficient way to modify mesoporous silica than conventional methods, such as adsorption capping or silane capping.
Another advantage of this method is that the surface of the mesoporous silica can be chemically modified with a high degree of control. This feature makes it possible to synthesize mesoporous materials with a large variety of structures.
In one study, a mesoporous carbon composite material with a thickness-controllable carbon layer was prepared using this method. Amino-modified silica carriers were first prepared, and then the carbon layer was coated on them. The result was that the ordered mesoporous silica could be covered with a carbon layer of controlled morphology and particle size, and it was also possible to prepare disordered mesoporous silica that had an amino-modified carbon coating.
The preparation of mesoporous silica using soft template strategy is also possible, but it is difficult to control the reaction conditions and material structures because of the limitations of available soft templates. In contrast, when hard template strategy is employed, a mesoporous silica gel with controlled pore structure can be prepared first and then the carbon precursors can be filled into the template pores through wet impregnation or chemical vapor deposition.
Hydrolysis-Polycondensation Reactions
The hydrolysis-polycondensation reaction of TEOS with functional silanes can be used to synthesize mesoporous silica. This reaction involves the simultaneous hydrolysis of TEOS to form silanol groups, and the condensation of these groups with ethoxy groups to produce silicon-oxygen bridges in the mesoporous structure. The synthesis is controlled by the temperature, pH, and the concentration of a solvent, such as methanol or water. The kinetics of the reaction is monitored by spectroscopy (nuclear magnetic resonance, FTIR, Raman and XRD), chromatography (liquid chromatography, gas permeation chromatography, and mass spectrometry), and microscopy (scanning electron microscopy, transmission electron microscopy, and atomic force microscopy).
It was found that a high concentration of methanol accelerates the rate of hydrolysis of TEOS, while at a lower concentration the reaction proceeds to the condensation stage much more rapidly. This is because the methanol concentration affects the relative position of the hydrolysis and condensation reactions, which is related to their molar absorptivities. The point at which the hydrolysis reactions become overtaken by the condensation reactions is determined by the observation of a shift in the FTIR Si-OH stretching vibration peak to a less intense value (i.e. the emergence of the Si-O-Si bond).
In order to further improve the mesoporous silica synthesis process, the hydrolysis and condensation kinetics can be controlled by the addition of inorganic salts (IS). For example, adding nitrate salts accelerates the densification and condensation of TEOS and also allows for the formation of mesoporous silica with a hexagonal particle shape and cylindrical pores.
Other research on mesoporous silica has focused on the synthesis of anionic surfactant-templated mesoporous silica through the hydrolysis-polycondensation of tetramethyl orthosilicate with vinyltriethoxysilane, allyltriethoxysilane, and (3-mercaptopropyl)trimethoxysilane. This approach has allowed the generation of mesoporous silica monoliths containing different pore wall chemistries by varying the molar ratio between TMOS and each of the three functional silanes.
It was also reported that the hydrolysis-polycondensation reactions of TMOS and functional silanes can be modulated by pH, temperature, and the presence of a catalyst such as magnesium nitrate. They found that, in an alkaline solution, the dimerization reactions predominated over the hydrolysis reactions because the energy barriers for the SN2 mechanism are significantly lower than those of the SN1 mechanism. However, in the presence of a fluoride ion (F-), the reaction follows the SN1 mechanism, which is much more energetically favorable, and the hydrolysis reaction predominated.
Electrochemical Reactions
Ordered mesoporous silica has attracted great interest for its fascinating applications in redox probes, separation, catalysis, drug delivery and fuel cells due to its unique combination of properties such as high porosities, narrow pore size distributions and excellent activity-structure relationships2,3,4,5. In order to realize these remarkable applications, it is critical to fully understand the filling and diffusion of metal species inside mesoporous silica.
To address this issue, several methodologies have been developed including TEM characterizations, powder X-ray diffraction patterns, N2 adsorption/desorption isotherms and voltammetric methods6–8. A variety of methods has also been employed to functionalize the surface of mesoporous silica in order to enable effective mass transport of electroactive ions or molecules. This is achieved by using surfactants, organosilane coupling agents and post-synthesis grafting approaches.
In addition to its unique structure, mesoporous silica also has a strong affinity towards redox metals. This is why a variety of techniques has been developed in order to encapsulate metal species in mesoporous silica, and more specifically in ordered mesoporous silica templates. In particular, the soft-enveloping synthetic strategy (Fig. 1a) enables the controlled growth of monodispersed metal nanocrystals within the mesoporous channels of a mesoporous silica template. A wide range of heterogeneous structures such as Au nanowires, 3D mesoporous gold, and AuAg alloy networks have been successfully synthesized. All these samples exhibit enhanced catalytic performances in methanol oxidation reactions.
A further important aspect of mesoporous silica is its ability to be modified by incorporating organic moieties into the pores of the material, which can be useful for a broad variety of applications. This can be achieved by either the physical mixing of as-synthesized mesoporous silica with conductive particles such as graphite or tin oxide, or by chemically modifying the mesoporous material with various organosilane reactants before insertion of the organic compounds.
In this context, the Stober’s method is an interesting and easy-to-use approach for obtaining functionalized mesoporous silica for electrodes. The spherical mesoporous silica resulting from this process can be used as a matrix in electrically-modified carbon electrodes. A number of promising applications have been demonstrated in this regard, including in-situ redox probing and electrocatalysis.
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