Sonochemical Effects on Sol-Gel Processes

Ultrafine nano-sized particles and spherical shaped particles, thin film coatings, fibers, porous and dense materials, as well extremely porous aerogels and xerogels are highly potential additives for the development and production of high performance materials. Advanced materials, including e.g. ceramics, highly porous, ultralight aerogels and organic-inorganic hybrids can be synthesized from colloidal suspensions or polymers in a liquid via the sol-gel method. The material shows unique characteristics, since the generated sol particles range in the nanometer size. Thereby, the sol-gel process is part of the nanochemistry.
In the following, the synthesis of nano-sized material via ultrasonically assisted sol-gel routes is reviewed.

Sol-Gel Process

Sol-gel and related processing includes the following steps:

1. making sol or precipitating powder, gelling the sol in a mold or on a substrate (in case of films), or making a second sol from the precipitated powder and its gelation, or shaping the powder into a body by non-gel routes;
2. drying;
3. firing and sintering. [Rabinovich 1994]

Table 1: Steps of Sol-Gel synthesis and the downstream processes

Sol-gel processes are a wet-chemical technique of synthesis for the fabrication of an integrated network (so-called gel) of metal oxides or hybrid polymers. As precursors, commonly inorganic metal salts such as metal chlorides and organic metal compounds such as metal alkoxides are used. The sol – consisting in a suspension of the precursors – transforms to a gel-like diphasic system, which consists in both a liquid and a solid phase. The chemical reactions that occur during a sol-gel process are hydrolysis, poly-condensation, and gelation.
During hydrolysis and poly-condensation, a colloid (sol), which consists in nanoparticles dispersed in a solvent, is formed. The existing sol phase transforms to the gel.
The resulting gel-phase is formed by particles which size and formation can vary greatly from discrete colloidal particles to continuous chain-like polymers. The form and size depends on the chemical conditions. From observations on SiO2 alcogels can be generally concluded that a base-catalyzed sol results in a discrete species formed by aggregation of monomer-clusters, which are more compact and highly branched. They are affected by sedimentation and forces of gravity.
Acid-catalyzed sols derive from the highly entangled polymer chains showing a very fine microstructure and very small pores that appear quite uniform throughout the material. The formation of a more open continuous network of low density polymers exhibits certain advantages with regard to physical properties in the formation of high performance glass and glass/ceramic components in 2 and 3 dimensions. [Sakka et al. 1982]
In further processing steps, by spin-coating or dip-coating it becomes possible to coat substrates with thin films or by casting the sol into a mold, to form a so-called wet gel. After additional drying and heating, a dense material will be obtained.
In further steps of the downstream process, the obtained gel can be further processed. Via precipitation, spray pyrolysis, or emulsion techniques, ultrafine and uniform powders can be formed. Or so-called aerogels, which are characterized by high porosity and an extremely low density, can be created by the extraction of the liquid phase of the wet gel. Therefore, normally supercritical conditions are required.

Table 2: Ultrasonic sol-gel synthesis of mesoporous TiO2 [Yu et al., Chem. Commun. 2003, 2078]

High Power Ultrasound and its Sonochemical Effects

High-power, low-frequency ultrasound offers high potential for chemical processes. When intense ultrasonic waves are introduced into a liquid medium, alternating high-pressure and low-pressure cycles with rates depending on the frequency occur. High pressure cycles mean compression, whilst low frequency cycles mean rarefaction of the medium. During the low-pressure (rarefaction) cycle, high power ultrasound creates small vacuum bubbles in the liquid. These vacuum bubbles grow over several cycles.
Accordingly to the ultrasound intensity, liquid compresses and stretches to varying degrees. This means the cavitation bubbles can behave in two ways. At low ultrasonic intensities of approximately 1-3 W/cm², the cavitation bubbles oscillate around an equilibrium size for many acoustic cycles. This phenomenon is termed stable cavitation. At higher ultrasonic intensities (up to 10 W/cm²), the cavitation bubbles form within a few acoustic cycles, reaching a radius at least twice their initial size before collapsing at a point of compression when the bubble can no longer absorb energy. This is termed transient or inertial cavitation. During bubble implosion, locally termed hot spots occur, featuring extreme conditions: very high temperatures (approximately 5,000 K) and pressures (approximately 2,000 atm) are reached. The implosion of the cavitation bubble also results in liquid jets with velocities of up to 280 m/s, which create very high shear forces. [Suslick 1998/ Santos et al. 2009]

Sono-Ormosil

Sonication is an efficient tool for the synthesis of polymers. During ultrasonic dispersing and deagglomeration, the caviational shear forces, which stretch out and break the molecular chains in a non-random process, result in a lowering of the molecular weight and poly-dispersity. Furthermore, multi-phase systems are very efficient dispersed and emulsified, so that very fine mixtures are provided. This means that ultrasound increases the rate of polymerisation over conventional stirring and results in higher molecular weights with lower polydispersities.
Ormosils (organically modified silicate) are obtained when silane is added to gel-derived silica during sol-gel process. The product is a molecular-scale composite with improved mechanical properties. Sono-Ormosils are characterized by a higher density than classic gels as well as an improved thermal stability. An explanation therefore might be the increased degree of polymerization. [Rosa-Fox et al. 2002]

Mesoporous TiO2 via Ultrasonic Sol-Gel Synthesis

Mesoporous TiO2 is widley used as photocatalyst as well as in electronics, sensor technology and environmental remediation. For optimized materials properties, it is aimed to produce TiO2 with high crystallinity and large surface area. The ultrasonic assisted sol-gel route has the advantage that the intrinsic and extrinsic properties of TiO2, such as the particle size, surface area, pore-volume, pore-diameter, crystallinity as well as anatase, rutile and brookite phase ratios can be influenced by controlling the parameters.
Milani et al. (2011) have demonstrated the synthesis of TiO2 anatase nanoparticles. Therefore, the sol-gel process was applied to the TiCl4 precursor and both ways, with and without ultrasonication, have been compared. The results show that ultrasonic irradiation have a monotonous effect on all components of the solution made by the sol-gel method and cause the breaking of loose links of large nanometric colloids in solution. Thus, smaller nanoparticles are created. The locally occurring high pressures and temperatures break the bondings in long polymer chains as well as the weak links binding smaller particles, by which larger colloidal masses are formed. The comparison of both TiO2 samples, in presence and in absence of ultrasonic irradiation, is shown in the SEM images below (see Pic. 2).
 

Pic. 2: SEM images of TiO2 pwder, calcinated at 400 degC for 1h and gelatinization time of 24h: (a) in the presence of and (b) in the absence of ultrasound. [Milani et al. 2011]

Furthermore, chemical reactions can profit from sonochemical effects, which include e.g. the breakage of chemical bonds, significant enhancement of chemical reactivity or molecular degradation.

Conclusion

The application of ultrasound to sol-gel processes leads to a better mixing and the particles’ deagglomeration. This results in smaller particles size, spherical, low-dimensional particle shape and enhanced morphology. So-called sono-gels are characterized by their density and fine, homogeneous structure. These features are created due to the avoidance of use of solvent during the sol formation, but also, and mainly, because of the initial cross-linked state of reticulation induced by ultrasound. After the drying process, the resulting sonogels present a particulate structure, unlike their counterparts obtained without applying ultrasound, which are filamentous. [Esquivias et al. 2004]
It has been shown that the use of intense ultrasound allows for the tailoring of unique materials from sol-gel processes. This makes high-power ultrasound a powerful tool for chemistry and materials’ research and development.