Synthesis Methods For Colloids and Nano Particles

ColloidBulkMaterial Solution

Break down method

(Top down process)

Bottom up method

Two principle methods:

Break Down Method: Milling

Limitation of milling method:dry milling: approx. 3 µmwet milling: approx. 200 nm

Agglomeration preventsfurther disintegration intosmaller particles

Quartz flourd50 approx. 3,5µm

Break Down Methods

a) Frictional dispersion (low rotation speed in rotating mill)b) Hammering dispersion (medium rotation speed in ball mill)c) Centrifugal milling (high rotation speed)

Preparation of colloids/nano particles by milling:

Break Down Methods

„stirred ball mill“

Break Down Methods

Working principle of a disperserUltra Turrax colloidal mill (high speed disperser)approx. 4.000 – 30.000 rpm

Dispersion in a „colloidal mill“

Break Down Methods

Verticale colloidal mill(1.000 – 20.000 rpm)

Stator/Rotor gap adjustable/variablerequires water cooling !!!

Break Down Methods

Jet millmaterial is accelerated to 250-700 km/hMilling occurs when particles hit each other(autogeneous milling)

„single-stage homogenizer“Emulsion is squeezed through a narrowgap:

high shear forcesno mechanical partsused to homogenize milk

Shear milling

Break Down Methods

Ultrasound MillingIn the fluid, gas bubbles are formed by ultrasonic (cavitations)Gas bubbles collapse, fluid shoots through gas bubble at approx. 500 m/sPressures up to 1 GPa !Temperatures up to 10.000 K (products like from high temperature synthesis)Dissociation, formation of radicals, charge separation ( Ultrasonic chemistry)

Decay of a cavitation bubble Zn particle made by ultrasonic

Synthesis via Bottom up – Principle Methods

Synthesis of colloids/nanosfrom solution

Precipitation from solution by addition of a nonsolvent- precipitation of colloidal S from ethanol by addition of water- Colloidal carotenoid (food colorant) by addition of water to a solution of carotenoid in acetone

Ionic precipitation reactions- exceeding the solubility product- surface energy of particle increases with decreasingparticle size

Hydrolysis of organometal compounds- preparation of silica sol from Stöber process- preparation of TiO2 sol

Reduction of ions to the element followed by agglomeration- Preparation of gold ruby

Polymerisation and polycondensation of monomers intopolymers

Bottom Up Methods

Synthesis of colloidal sulfur sol (colloidal S particles are used as insecticides and fungicides)

a)Dilution of solution of S in ethanol with water: colloidal S precipitates

b)Comproportioning of S in water or air

SO2 + 2 H2S 3 S + 2 H2O

c)Acid degradation of sodium thiosulfate in water

Na2S2O3 1/8 S8 + Na2SO3

Bottom Up Methods: Seed Crystallisation

Change of free energy of a seed crystal with size;n= number of building units

A) Minor oversaturationB) High oversaturationnc= critical size from which seeds start to grow

Conditions necessary to achievemonodisperse particles:

A) Solution of particlesB) Concentration where seeds formC) Maximum oversaturation

I) No seed formationII) Range where seeds formIII) Growth regime for particles, seeds are no

longer formed(La Mer diagram)Opposite precipitatates (no seeds)

Bottom Up Methods: Nano Crystals by Seeding

- Principle: small particles possess higher solubility than large ones- large particles grow at the expense of small particles (Ostwald ripening)

- Method to achieve nano sized crystals:

a) initially high oversaturation spontaneous formation of many seed crystalsb) then lower concentration no more seeds are being formedc) maintain low concentration particles continue to grow

Bottom Up Reactions: Elemental SolsElemental sols of Au, Ag or Pt:Preparation by reduction from metal salt solutions with citrate, hydrazin, hydroxylamin, formaldehyde etc.

Gold sol: Gold ruby glas

Cassius´ gold ruby: reduction of gold salt solution with Sn2+ in acid solutionAbsorption is a function of colloidal/nano particle size

Absorption maximum of Au sol as a function of particle diameter

Preparation of gold sols possessing different particlesizesA) HAuCl4 solution 0,01 %B) Tri sodium citrate dihydrate solution 1,0 %

Bottom Up Synthesis: Sol – Gel ProcessPreparation of a silica sol

condensation of ortho silica acid (Si(OH)4)

Preparation of a silica sol by acidfrom aqueous sodium silicatesolution

Mono disperse silica

Bottom Up Synthesis: Sol – Gel Process

Preparation of monodisperse nano silica particles:

Stöber-Process:

hydrolysis of tetraalkylortho silicates (eg. tetraethylorthosilicate, TEOS)

Chemical reactionsinvolved in the Stöber process

Ammonia concentrationdetermines particle size(„morphological catalyst“)

Bottom Pp Synthesis of Nano Particles

colloidal ZnS colloidal CdCO3

colloidal TiO2 Fe2O3 barium ferrite

Star-shaped Anatase nano crystals

Bottom Up Synthesis of Nano Particles

Synthesis of Nano Particles – Carbon Black

Industrial manufacture of carbon black:

Incomplete combustion of aromatic compoundsGlobal production: approx. 6 - 7 mio. to/year

Completecombustion

Incompletecombustion

Water cooling

Air inlet

Industrial Carbon Black

Primary particles : diameter ~ 5 – 500 nm,

often aggregated to chains and intergrown

Chemical structure: layered, like graphite

• C6 rings, but irregular arrangement

• Extremely high surface area, ca. 10 – 1000 m2/g

• Ideal pigment: insoluble in all common solvents, resistent againstmost chemicals, UV stable, very intensive color

• As filler for strength enhancement of elastomers

30 nm

Carbon Black

Printing inks for newspapers(only 0,015 g of carbon black areneeded for one page!)

a point (.) on a newspaperpage contains 250.000.000.000 carbon black nano particles!!!

Examples for industrial use of carbon black

> 90 % as filler forelastomers, thereof 2/3 for tire industry, remainder for rubber

Coatings and paints

Synthesis of Pyrogenic Silica

Industrial manufacture byflame pyrolysis:

2 H2 + O2 2 H2OSiCl4 + 2 H2O SiO2 + 4 HCl

Combustion in O2/H2 flame

Manufacture of Pyrogenic Silica

Umsetzung in der Gasphase

Verwendung pyrogener Kieselsäure

In Siliconkautschuk als Verstärkerfüllstoff, aber auch in vielen anderen Anwendungen zur Kontrolle der Rheologie (z.B. Ketchup).

Zur Verdickung und Thixotropierung in Lacken und Farben, Druckfarben und ungesättigten Polyesterharzen sowie Epoxidharzen.

Als Zusatzstoff und Verarbeitungshilfsmittel in der Kosmetik und der pharmazeutischen Industrie sowie als Rieselhilfsmittel bei Lebensmitteln, Futtermitteln und in der chemischen Industrie eingesetzt.

Chemical Vapor Deposition (CVD)

Principle of „chemical vapor deposition“ : deposition from gas phase

Processes in use

Chemical Vapor Deposition (CVD)Preparation of a Cu nano film on a substrate

Schematic „hot-wall-reactor“ Schematic „cold-wall-reactor“

Chemical vapor deposition (CVD)

CVD von Al-Schichten aus Me3N-AlH3

At low T homogeneous Al surface

At high T Al layer contaminatedwith impurities of C

Deposition of Al layers from AliBu3

Chemical vapor deposition (CVD)Manufacture of diamond films for dental instruments etc.

Chemical vapor deposition (CVD)

Different morphologies achieved bydifferent gas pressures duringsynthesis

Manufacture of diamond films

Different morphologies achievedthrough different temperatures duringsynthesis

Sol-Gel Process

Condensation of two silica particles

Sol gel transformation is the result of a chemical reaction (condensation) between the sol particles

Sol-Gel Process

Difference between gel and precipitate:

Sol-Gel Process

4 5 6 7 8 9 10 11 12 13-25

-20

-15

-10

-5

0

5

10

15

20

25

30

35

40

45

50

0

10

20

30

40

50

60

70

80

90

100

Zetapotential

Zeta

pote

ntia

l (m

V)

pH-Wert

Viskosität

Vis

kosi

tät (

mPa

s)

Gel formation only occurs at specific pH values where particlescan get in such proximity that a condensation reaction can takeplace

Example:

Zeta potential and Brookfieldviscosity as a function of pH valueof a Al2O3 sol at a concentration of 0,95 mol/l

Gel formation occurs best at pH 9.5

Sol-gel transformation

Sol-Gel Process

Gelation times of silica sols as a function of the SiO2 concentration of the sols

-60

-50

-40

-30

-20

-10

0

10

0 1 2 3 4 5 6 7 8 9 10 11 12

pH-Wert

Zeta

pote

ntia

l [m

V]

pH dependent zeta potential of silica

Sol-gel transformation

Sol-Gel Process

Network of particlesin a gel

Condensation of silicaparticles possessingdifferent sizes

Bond between smallerparticles is stronger

finer particles producemore stable gels

Characteristics of gels

- low mechanical stability- at least two phases- 3D network of solid phase- interstitial space filled with:

a) Water Hydrogelb) Alcohol Alcogelc) Gas Aerogel/Xerogel

Sol-Gel Process„Site Percolation“ in a square lattice, shown for threedifferent particle concentrations p.

A gel is formed only when p = 0,75 (when at least 75 % of all site are occupied by particles)

Sol-Gel Process

Formation of a 3D network during gelation

Aggregates, agglomerates and networks

Aggregates: irreversibelAgglomerates: reversibelNetwork: labile

Sol-Gel Process: Stabilization of Sols

Surface modification of sol particles to prevent condensation/gelation

Surface inertisation and electrostaticstabilization

allows higher solids content in a solenhances shelf life (stability) of a sol

Sol-Gel Process: Xerogel and Aerogel

Xerogel / Aerogel

Xerogel: evaporation of solvent causesShrinkage or collapse of 3D network in sol

Aerogel: careful removal of solvent under sustainment of 3D network(supercritical CO2)

Relationship between particle geometry and gelstructure:

a) Spherical network structureb) Platelet structurec) Fibrous structure of gel

Sol-Gel Process: Preparation of CaCO3 Nano Particles, Xerogeland Aerogel

CaO in MeOHsuspendiert

Rückstand

Filtration

Trocknung an Luft

CaCO3 Aerogel

CO2

pH

L

Reaktion

Sol

Filtration

GelierungGel

CaCO3 Xerogel

überkritische Trocknung mit CO2

CaO + 2CH OH3 Ca(OCH ) + H O3 2 2 Ca(OCOOCH )3 2

+ 2CO2 CaCO + CH OHSol/Alkogel

3 3

+ H2O

CaCO3 aerogel, CaCO3 xerogel and CaCO3 nano particles from sol-gel transformationof calciumdi(methylcarbonate)

Reaction scheme:

CaCO3Nanopartikel

Sol-Gel Process: CaCO3 Nano Particles

SEM image of 500 nm CaCO3 nano particles

100 nm

TEM image of CaCO3

nano particles

Primary CaCO3 nanoparticles (Ø 1-5 nm!)

Sol-Gel Process: CaCO3 Alcogel and Aerogel

CaCO3 Aerogel obtainedfrom supercritical drying

ESEM image of a CaCO3 Aerogel

CaCO3 Alkogel

Sol-Gel Process: Nanos, Xerogels, Aerogels, Ceramics

Ceramic Coatings, Bodies and Fibers:Application of a xerogel, followed by solvent evaporation and calcination

Sol-Gel Process: Preparation of Cement Clinker Phases

Sol

Wasser

kolloidalesTeilchen

Hydrogel

+ Ca(NO )pH-Wert

3 2

Netzwerk

Wasser+gelöste Nitrate

- H O2

Xerogel

T

Luft

Netzwerkmit Nitraten

reine Klinkerphase

T ZersetzungCa(NO )3 2

Klinkerphase

Sol-Gel Process: Industrial Applications

Inorganic coatings

- reactive oxides (e.g. manufacture of pure cement clinker phases)

- ceramic materials

- ceramic coatings on temperature sensitive surfaces

- glasses

- manufacture of silica gels (adsorbens, chromatography, substrate for catalysts etc.)

In solid state reactions, the sol-gel process ensures high homogeneityand small particle sizes