The idea behind synthesis of nanoparticles by precipitation, is that one chemically reacts a soluble precursor in solution to produce an insoluble intermediate. As the intermediate is insoluble, it precipitates allowing us to remove the water [how] and harvest the precipitate. It doesn't stop there, however. The precipitate is normally a hydroxide-salt complex, which we would like to remove. This is done by heating the precipitate (normally between 600 and 1000 deg C) in order to decompose the complex, and which leaves metal-oxide nanoparticles.
General direction:
React soluble precursor in solution to form insoluble precipitate. Remove liquid and heat precipitate between 600 and 1000 deg C in order to decompose hydroxide-salt complex. Produces metal-oxide nanoparticles.
Pros: Relatively cheap and simple method for producing nanoparticles. Doesn't require high pressures.
Cons: Can only produce oxides, not pure metals. Must start with soluble precursor.
Example 1, gold nanoparticles:
Materials: AuCl4-, NaBH4 and long chain thiol molecules.
Chemical reduction of gold salt, AuCl4-, using NaBH4 and then using long chain thiol molecules as steric separation agents. The NaBH4 reduces the gold nanoparticles from Au3+ to Au, and then the added thiol molecules stick on to the surface of the gold due to their R-S-H group, which has an affinity to the gold nanoparticle. The nanoparticles can be precipitated [explanation] and then heated to remove the thiol compound.
Example 2, BaTiO3 nanopowders for ferroelectric ceramics:
Materials: BaCl2, TiCl4, H2O and H2C2O4 or Ba(CH3COO)2, Ti(C3H7O)4 and H2C2O4 in alcohol.
React one of the two groups above to reduce the barium and titanium ions and produce insoluble Barium Titanate (BaTiO3). Isolate the precipitate, wash it, and thermally decompose the organics (800 C) to produce nanosized BaTiO3.
Hydrothermal synthesis
General directions
Mix soluble precursors in a "high-pressure cooker", raising the temperature up to 350 C and pressure up to 35MPa (≈350 atmospheres). This directly precipitates metal oxide crystals, without agglomerates.
Can be sped up by feeding in supercritical fluid heated from somewhere else.
Pros: Directly produces crystals with no need for calcination (heating), no agglomerates, sizes below 100 nm and good control of stoichiometry.
Cons: Expensive: £70,000 for a 1.8 litre reactor. Slow: takes hours, up to days for reaction. Production of less than 1 g/h.
Example, synthesis of Hydroxyapatite:
Materials: Ca(NO3)2*4H2O, (NH4)2HPO4, acids/bases.
Mix ingredients at a ratio of 5:3. Adjust pH using acids/bases to needed pH. Heat at 350 degrees at 35MPa to precipitate nanoparticles. Wash them. Ca10(PO4)6(OH)2 produced.
Sol-gel synthesis
General direction:
Gels formed from metal alkoxides (e.g. Ti(OC3H7)4, general R-O-) or citric acid and ethylene glycol to form organic gel with trapped dissolved metal ions.
Definitions:
Sol: Semi-rigid solid containing trapped liquid
.
The solid 3D framework may be composed of
(a) polymers OR (b) linked particles.
Gel: Solution precusor to a gel (a) solution of molecules which subsequently react to form polymers (gel) OR- (b) suspension of nanoparticles which subsequently condense to form 3D chains of particles (gel)
