Lecture - 3
Sol – Gel
Outline
Principle/Outline of the Process
Methodology
Process Controlling Parameters
Advantages of the process
Disadvantages of the Process
Typical example
Concept
Important Definitions
What is Hydrolysis?
What is Sol?
What is Aerosol?
What is Gel?
What is Aerogel?
What is Xerogel?
• 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.
The sol-gel process: (a) sol; (b) gel.
Solid materials synthesis procedure, performed in a liquid and normally at low temperatures
Particles in a dispersed state in the solvent – independent colloidal suspension –Sol
The colloidal particles are linked together to form a 3-dimensional open grid – Gel
Overview of Sol-gel Process
Sol
Metal Alkoxide (M-O-R)
Hydrolysis and Condensation
Polymerisation
Solution
Coating
Wet gel
Xerogel film
Extraction of solvent
Evaporation
substrate
Aero gel Xero gel
Heating
Heating
Fibers
substrate
Dense Uniform
ceramics particles
Dense
Film
ADVANTAGES
Uniformity DISADVANTAGES
High purity Sensitivity for Atmosphere Condition
Easy Operation Expensive Raw Materials
Cost Effective
Low temperature
Controlled structure
Coating on irregular shapes
Selective doping
FACTORS EFFECTING
pH (acid / base)
Temperature
Water and Solvent
Reagent concentration
Catalyst
Dipping (spinning) speed
Effect of Concentration and pH on sol gel process
TEOS – Tetra Ethyl Ortho Silicate
Effect of Time and Temperature
The problem of Drying
Nanocrystalline Metal Oxide Preparation
Sol-Gel Process
Inorganic gels
SiO2, TiO2, ZrO2, Mg(OH)2 etc.
Organic gels
-(C-C)- covalent bonds
resorcinol-formaldehyde (RF), melamine
formaldehyde
Carbon gels
Organic gels heat at 500 oC in N2 → C gel
Nanocrystalline Metal Oxide Preparation
Sol-Gel Process
The usual molecular precursors are metallo-organic compounds such as
alkoxides M(OR)n, where M is a metal like Si, Ti, etc. R is an alkyl group (R = CH3, C2H5,
etc.).
For example, tetraethylorthosilicate (TEOS), Si(OC2H5)4, is commonly used in the sol–
gel synthesis of silica and glasses.
Similarly Ti(iOC3H7)4 is used for the preparation of TiO2.
Ti(iOC3H7)4 + 4 H2O TiO2 + 4 C3H7OH
Ethanol/Methanol, Catalyst
Ti(iOC3H7)4 + x H2O Ti(iOC3H7)4 –x(OH)x+ x C3H7OH
“Titanol”
Ti(iOC3H7)4 –x(OH)x + Ti(iOC3H7)4 –x(OH)x Ti(iOC3H7)4 –x(OH)x -1OTi(OH)x-1(OC3H7)4-x + H2O
Metal Salt (MXn) as precursor
Hydrolysis reaction
Mz+ + H2O → M(OH2)z+
H2O groups are replaced by OH
(loss of proton)
Metal Salt (MXn) as precursor
Condensation reaction
H2O groups are eliminated
Hydrolysis Mechanism
Nanocrystalline Metal Oxide Preparation
Sol-Gel Process
Ti(iOC3H7)4 + 4 H2O TiO2 + 4 C3H7OH
Hydrolysis (Initiation)
Ethanol/Methanol, Catalyst
Ti(iOC3H7)4 + x H2O Ti(iOC3H7)4 –x(OH)x+ x C3H7OH
“Titanol”
Condensation (Propagation)
Ti(iOC3H7)4 –x(OH)x + Ti(iOC3H7)4 –x(OH)x Ti(iOC3H7)4 –x(OH)x -1OTi(OH)x-1(OC3H7)4-x + H2O
Sol-gel example: silica
• Precursors:
– silicone alkoxides with different alkyl ligand
sizes Vigorous stirring
• catalyst:
– ammonia
• solvent:
water
– various alcohols
Monodisperse SiO2 Particles
Monodisperse SiO2 Particles from MTMS
Monodisperse SiO2 Particles from TEOS
Photos of a Cu2+ loaded gel (left)
and a blank gel (right)
21
Synthesis of Silver NanoWires
Application
