Sol-gel (my slave for LNMO)

You start with a solution, form a gel for heterogeneous dispersion of atomically mixed atoms (hopefully), then decompose gel and sinter -> dream of perfect atomically mixed compounds

• Formation of an oxide netowrk through polycondensation reactions of a molecular precursor in a liquid.
• Sol: stble dispersion of colloidal particles or polymers in a solvent.
• Gel: 3D-continous network
• Prevent problems w/coprecipitation
• Results in smaller particles
• Developed in 1960 due to nuclear industry
• Quick clay
• Shapes: Fibres, powders, xerogel films -> dense ceramic film
• Ex: PZT piezoelectric, stoichi hard to control via ceramic method (and needs 1100\(^\circ\)C); but w/sol-gel it is easier and only 570-700\(^\circ\)C needed.

Theory

Sol

• Colloidal particles not dissolved, but should not agglomerate or sediment.
• Agglomeration (by Van der Waals) is avoided by:
  • Electrostatic repulsion: Charges particle surface and makes them repel. Important for colloidal system.
  • Steric hindrance: Particles adsorb a thick layer of organic molecules(SURFACTANTS). Works best in concentrated dispersions. Branched adsorbates works best. Usual for nanomaterials.

Point of Zero Charge (PZC)

• e\(^-\)-static repulsion is because: Surface/particle is covered by ionic groups which determines surface potential. Counter ions in solution covers this layer, shielding the rest of the sol from the surface charges.
• For hydroxides, surface potential is determined by pH
• pH where particles is neutral is the PZC
• pH > PZC -> surface is - charged
• pH < PZC -> surface is + charged

• Size of surface potential \(\phi_0\) depends on difference between pH and PZC

Double layer

• Particle moves to electrode with opposite charge and carry adsorbed layer + counter ions
• “slip plane” divides what moves with particle and rest of sol.
  • Slip plane potential = Zeta Potential \(\phi_Z\)
  • Iso-electrical point (IEP) is which pH makes \(\phi_Z\)=0
  • Larger \(\phi_Z\) = greater stability colloid. (Should be >30-50mV)
• Given same surface potential, repulsive forces will increase with larger particles.

Coagulation / Flocculation

• Occur if:
  • \(\phi\) is lowered (by pH)
  • Increase # counter ions (decreases thickness of doublelayer)
  • Increase in
• In some cases, it can be re-dispersed:
  • Called Peptizing
  • Removing counter ions (e.g. by washing or adding charged ions(reestablishes double layer))

Netowork formation (gelation)

• Gel-time: The time it takes to gelate enough to put the container upside down.

Ostwald ripening: Can help with creating monodisperse particle sizes Ageing: The gel is still alive and will continue gelation Drying: Maintained structure -> aerogel, collapsed -> Xerogel

Applications

Silicates

• Forms gel easily: Not very electropositive and not susceptible to nucleophilic reactions.
• Sol-Gel in some steps:
  • Hydrolysis + condensation of molecules
  • Formation of Sol
  • Gelation(sol-gel transformation)
  • Ageing + Drying

Hydrolysis + condensation

• Alkoxides / silanols -> Siloxane groups (Si - O - Si)(with 3 dangling Si bonds)
• Hydrolysis: Si-O-R + H\(_2\)O -> Si-OH + ROH
• Condensation: Si-OH + Si-OR -> Si-o-Si + ROH

Slica-gel precursors

More for alkoxides:

• It is more complex, but might give more control
• 2 reaction paths:
  • pH< PZC -> species is positive
  • pH > PZC -> species is negative
• Acid or base must be used to catalyze reaction

Condensation parameters:

• Precursor type
• Water-alkoxide ratio (R\(_W\))
• Catalyst type
• Solvent type
• Temp , pH
• Relative + absolute consentration of reactants
• electron density on Si
  • Acid catalyzed demands high electron density
  • Base catalyzed demands low electron density
  • Acid catalyzed -> straight chains
  • Base catalyzed -> Branched networks
• R\(_W\) = 1 -> hydrolysis, no condensation

Metals

• Can create gels
• Often octahedrally coordinated with Oxygen
• Less free than Si tetrahedrons
• Hydrolysis of Ti(OR)\(_4\) 10\(^5\) faster than Si

Examples

• Pb(Zr\(_{0.48}\),Ti\(_{0.52}\))O\(_3\)
  • Start w/oxides which needs high temp to decompose
  • Need stoichi control + homogeneity
  • Pb is volatile, so it makes it stay.