Sonochemical Effects on Sol-Gel Processes
by Kathrin Hielscher, Hielscher Ultrasonics GmbH, www.hielscher.com
Introduction
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 counted to 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]
Precursor
Suspension
Gelation
Drying
Sintering
Sol-Gel
Table 1: Steps of Sol-Gel synthesis
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.
High Power Ultrasound
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 lowpressure (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
~1-3Wcm-2, the cavitation bubbles oscillate about some equilibrium size for many acoustic
cycles. This phenomenon is termed stable cavitation. At high ultrasonic intensities (≤10Wcm 2
) the cavitational bubbles are formed within a few acoustic cycles to a radius of at least
twice their initial size and collapse at a point of compression when the bubble cannot absorb
more energy. This is termed transient or inertial cavitation. During bubble implosion, locally
so-called hot spots occur, which feature extreme conditions: During the implosion, locally
very high temperatures (approx. 5,000K) and pressures (approx. 2,000atm) are reached. The
implosion of the cavitation bubble also results in liquid jets of up to 280m/s velocity, which
act as very high shear forces. [Suslick 1998/ Santos et al. 2009]
The application of power ultrasound is a well-known tool to create extremely fine
dispersions and emulsions. In chemistry such extremely fine-size dispersions are used to
improve chemical reactions by enhanced mass transfer. This means that the interfacial
contact area between two or more immiscible liquids becomes dramatically enlarged and
provides thereby a better, more complete and/or faster course of the reaction.
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.
Sono-Gels
In sono-catalytically assisted sol-gel reactions, ultrasound is applied to the precursors. The
resulting materials with new characteristics are known as sonogels. Due to the absence of
additional solvent in combination with the ultrasonic cavitation, a unique environment for
sol–gel reactions is created, which allows for the formation of particular features in the
resulting gels: high density, fine texture, homogeneous structure etc. These properties
determine the evolution of sonogels on further processing and the final material structure.
[Blanco et al. 1999]
Suslick and Price (1999) show that the ultrasonic irradiation of Si(OC2H5)4 in water with an
acid catalyst produces a silica “sonogel”. In conventional preparation of silica gels from
Si(OC2H5)4, ethanol is a commonly used co-solvent due to the non-solubility of Si(OC2H5)4 in
water. The use of such solvents is often problematic as they can cause cracking during the
drying step. Ultrasonication provides a highly efficient mixing so that volatile co-solvents
such as ethanol can be avoided. This results in a silica sono-gel characterized by a higher
density than conventionally produced gels. [Suslick et al. 1999, 319f.]
Conventional aerogels consist of a low-density matrix with large empty pores. The sonogels,
in contrast, have finer porosity and the pores are quite sphere-shaped, with a smooth
surface. Slopes greater than 4 in the high angle region reveal important electronic density
fluctuations on the pore-matrix boundaries [Rosa-Fox et al. 1990].
Pic. 1: Ultrasonic glass flow cell for continuous sonication (©www.hielscher.com)
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 polydispersity. 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]
The images of the surface of the powder samples show clearly that using ultrasonic waves
resulted in greater homogeneity in the average size of the particles and resulted in smaller
particles. Due to sonication, the average particle size decreases by approx. 3 nm. [Milani et
al. 2011]
The positive effects of ultrasound are proven in various resear studies. E.g., report Neppolian
et al. in their work the importance and advantages of ultrasonication in the modification and
improvement of the photocatalytic properties of mesoporous nano-size TiO2 particles.
[Neppolian et al. 2008]
Table 2: Ultrasonic sol-gel synthesis of mesoporous TiO2 [Yu et al., Chem. Commun. 2003, 2078]
Nanocoating via ultrasonic sol-gel reaction
Nanocoating means covering material with a nano-scaled layer or the coverage of a nanosized entity. Thereby encapsulated or core-shell structures are obtained. Such nano
composites feature physical and chemical high performance properties due to combined
specific characteristics and/or structuring effects of the components.
Exemplarily, the coating procedure of indium tin oxide (ITO) particles will be demonstrated.
ITO particles are coated with silica in a two-step process, as shown in a study of Chen (2009).
In the first chemical step, the indium tin oxide powder undergoes a aminosilane suface
treatment. The second step is the silica coating under ultrasonication. To give a specific
example of sonication and its effects, the process step presented in Chen’s study, is
summarized below:
A typical process for this step is as follows: 10 g GPTS was mixed slowly with 20 g of water
acidified by hydrochloric acid (HCl) (pH = 1.5). 4 g of aforesaid aminosilane treated powder
was then added to the mixture, contained in a 100 ml glass bottle. The bottle was then
placed under the probe of the sonicator for continuous ultrasound irradiation with output
power of 60 W or above. Sol-gel reaction was initiated after approximately 2-3 min
ultrasound irradiation, upon which white foam was generated, due to the release of alcohol
upon extensive hydrolysis of GLYMO. Sonication was applied for 20 min, after which the
solution was stirred for several more hours. Once the process was finished, particles were
gathered by centrifuging and were washed repeatedly with water then either dried for
characterisation or kept dispersed in water or organic solvents. [Chen 2009, p.217]
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.
Pic. 3: High power ultrasound (1kW) setup for sonochemical applications (© www.hielscher.com)
For more information, please contact Hielscher Ultrasonics!
Phone: +49 3328-437-3
Email: [email protected]
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