Reference:

• C. J. Brinker and G. W. Scherer, Sol-Gel Science - The Physics and Chemistry of Sol-Gel Processing, New York, Academic Press, 1990.

• Sol-gel chemistry http://www.psrc.usm.edu/mauritz/solgel.html

• The Sol-gel gatewayhttp://www.solgel.com

• The Sol-Gel process allows to synthesize ceramic materials of high purity and homogeneity by a process occurs in liquid solution of organometallic precursors (TMOS, TEOS, Zr(IV)-Propoxide, Ti(IV)-Butoxide, etc. ), which, by means of hydrolysis and condensation reactions, lead to the formation of a new phase (SOL). M-O-R + H2O M-OH + R-OH (hydrolysis) M-OH + HO-MM-O-M + H2O (water condensation) M-O-R + HO-M M-O-M + R-OH (alcohol condensation) (M = Si, Zr, Ti)

• The SOL is made of solid particles of a diameter of few hundred of nm suspended in a liquid phase.

• Then the particles condense in a new phase (GEL) in which a solid macromolecule is immersed in a liquid phase (solvent). Drying the GEL by means of low temperature treatments (25-100 C), it is possible to obtain porous solid matrices (XEROGELs).

• The fundamental property of the solgel process is that it is possible to generate ceramic material at a temperature close to room temperature.

• Therefore such a procedure opened the possibility of incorporating in these glasses soft dopants, such as fluorescent dye molecules and organic chromophores.

Precusor=> Oligomer=> Polymer=> Colloids=> Sol =>

gel

Xerogel, Aerogel Organic/Inorganic

Ceramics, Glass Porous Materials

Traditional Sol-Gel Methodology

Hydrolysis

Condensation

Random Network of SiO2

Hydrolysis

• (8) Olation and (9) Oxolation

Factors affecting Reactivity

• pH, 

• Water content, 

• Concentration, 

• Temperature,

• Drying conditions 

Condensation rate of silica

Sili

ca c

ond

ensa

tion

rat

e

2 1210864

pH

Zero Charge

PositiveCharge

Negative ChargeSilica

Dissolves

How to control hydrolysis rates?

• By contrast to silicon alkoxides whose hydrolysis requires catalysts for efficient gelation rates, hydrolysis of most metal alkoxides is rapid and can lead to uncontrolled precipitation. The electronegative alkoxide groups make the metal highly prone to nucleophilic attack by water. The more electrophilic metal centres –as compared to silicon- as well as a larger and thus more stereolabile coordination sphere result in a higher hydrolytic susceptibility. The following sequence of reactivity is usually found

Si(OR)4 << Sn(OR)4 ~ Ti(OR)4 < Zr(OR)4 ~ Ce(OR)4.7

Strategy for slowing down hydrolysis• Changing the nature of the organic group R:

alkoxides with primary organic groups such as n-butoxides are less sensitive to hydrolysis than secondary ones such as isopropoxides; 

• Increase of the metal coordination number thus hindering attack of water and formation of the metal hydroxyl bond, M-OH, necessary for the development of the network; 

• Decreasing the functionality of the precursor by partial substitution of the OR ligands by anionic ligands such as carboxylates or b-diketonates leading to M-Z bonds less susceptible to hydrolysis

Typical Starting Materials

Sol, Gel and Flocculate

The sol-gel process: (a) sol; (b) gel.

Gel Process

The problem of Drying

• Freshly prepared silica gels contain an appreciable amount of solvent (usually 70 to 90 wt %) which must be eliminated. The solid silica network formed by hydrolysis and polycondensation of Si alkoxides is made up by Silica species of a few tens of nanometer size.Capillary stress appears when the liquid move inside the pores during drying and form a liquid-gas curve interface

The problem of Drying

Supercritical Drying

Monolithic Aerogels

Hydrophobic Surface

Contact Angle of Hydrophobic surface

Applications of Aerogel

• Thermal Insulation• Acoustic Insulation• Catalyst Support• Optical applications• Nuclear Waste Storage • Filler for paints or others• low dielectric constant materials• Batteries

• Hydrolysis: – The reaction of a metal alkoxide (M-OR) with water, forming a metal hydroxide (M-OH).

• Condensation: – A condensation reaction occurs when two metal hydroxides (M-OH + HO-M) combine to

give a metal oxide species (M-O-M). The reaction forms one water molecule. • Sol:

– A solution of various reactants that are undergoing hydrolysis and condensation reactions. The molecular weight of the oxide species produced continuously increases. As these species grow, they may begin to link together in a three-dimensional network.

• Gel Point: – The point in time at which the network of linked oxide particles spans the container holding

the Sol. At the gel point the Sol becomes an Alcogel. • Alcogel (wet gel):

– At the gel point, the mixture forms a rigid substance called an alcogel. The alcogel can be removed from its original container and can stand on its own. An alcogel consists of two parts, a solid part and a liquid part. The solid part is formed by the three-dimensional network of linked oxide particles. The liquid part (the original solvent of the Sol) fills the free space surrounding the solid part. The liquid and solid parts of an alcogel occupy the same apparent volume.

• Supercritical fluid: – A substance that is above its critical pressure and critical temperature. A supercritical fluid

possesses some properties in common with a liquids (density, thermal conductivity) and some in common with gases. (fills its container, does not have surface tension).

• Aerogel: – What remains when the liquid part of an alcogel is removed without damaging the solid part

(most often achieved by supercritical extraction). If made correctly, the aerogel retains the original shape of the alcogel and at least 50% (typically >85%) of the alcogel's volume.

• Xerogel: – What remains when the liquid part of an alcogel is removed by evaporation, or similar

methods. Xerogels may retain their original shape, but often crack. The shrinkage during drying is often extreme (~90%) for xerogels.

Hybride Synthesis Strategy

• Although they initially have been worked out by chemists from the sol-gel scientific community today hybrids are elaborated by researchers coming from a variety of disciplines, polymers chemists, solid state chemists, catalysis, materials researchers etc. Each of these communities elaborate hybrids using their own tools, specific disciplinary methods and more important their own raw materials. It is not seldom to see that a polymerist will work out an hybrid system having an emphasis on the polymer side of the hybrid, using even pre-formed polymers, capped oligomers etc. Sol-gel and inorganic chemists will preferably use as precursors, silicon or metal alkoxides or even inorganic building units such as clusters or nanoparticles. They can also use lamellar inorganic compounds as host for organic components. Many names have been given to these materials: Ceramers, Polycerams, Ormosils or Ormocers.

• However it is now commonly accepted that a molecular approach for the synthesis of hybrids reflects better the wide opportunities offered by this compounded chemistry.

Schematic illustration of the organic template approach

Shape of different products available through processing by sol-gel

• Organo(alkoxy)silanes and metal alkoxides serving as precursors for sol-gel-derived

materials.

Cross-linking of inorganic clusters or nanoparticles via photochemically induced polymerization of

(meth)acrylic functional groups.

Selected Organic Dye for Sol-gel

One-pot synthesis of chromophoric sol_gel hybrid materials.

Angew. Chem. Int. Ed. 2002, 41, No. 10

xerogels prepared from fluoroaromatics 1 to 5(from left to right) and APS or MAPS in TEOS

Industrial ApplicationsFunctional Coating for Glass

Commercial magnifying lens(PMMA) euipped with an abrasion resistant, UV cured hybrid coating