Synthesis Methods For Colloids and Nano Particles

Transcription

1 Synthesis Methods For Colloids and Nano Particles Two principle methods: Bulk Material Colloid Solution Break down method (Top down process) Bottom up method

2 Break Down Method: Milling Limitation of milling method: dry milling: approx. 3 µm wet milling: approx. 200 nm Agglomeration prevents further disintegration into smaller particles Quartz flour d 50 approx. 3,5µm

3 Break Down Methods Preparation of colloids/nano particles by milling: a) Frictional dispersion (low rotation speed in rotating mill) b) Hammering dispersion (medium rotation speed in ball mill) c) Centrifugal milling (high rotation speed)

4 Break Down Methods stirred ball mill

5 Break Down Methods Dispersion in a colloidal mill Ultra Turrax colloidal mill (high speed disperser) approx rpm Working principle of a disperser

6 Break Down Methods Verticale colloidal mill ( rpm) Stator/Rotor gap adjustable/variable requires water cooling!!!

7 Break Down Methods Shear milling Jet mill material is accelerated to km/h Milling occurs when particles hit each other (autogeneous milling) single-stage homogenizer Emulsion is squeezed through a narrow gap: high shear forces no mechanical parts used to homogenize milk

8 Break Down Methods Ultrasound Milling In the fluid, gas bubbles are formed by ultrasonic (cavitations) Gas bubbles collapse, fluid shoots through gas bubble at approx. 500 m/s Pressures up to 1 GPa! Temperatures up to 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

9 Synthesis via Bottom up Principle Methods Synthesis of colloids/nanos from 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 decreasing particle size Hydrolysis of organometal compounds - preparation of silica sol from Stöber process - preparation of TiO 2 sol Reduction of ions to the element followed by agglomeration - Preparation of gold ruby Polymerisation and polycondensation of monomers into polymers

10 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 SO H 2 S 3 S + 2 H 2 O c)acid degradation of sodium thiosulfate in water Na 2 S 2 O 3 1/8 S 8 + Na 2 SO 3

11 Bottom Up Methods: Seed Crystallisation Change of free energy of a seed crystal with size; n= number of building units A) Minor oversaturation B) High oversaturation n c = critical size from which seeds start to grow Conditions necessary to achieve monodisperse particles: A) Solution of particles B) Concentration where seeds form C) Maximum oversaturation I) No seed formation II) Range where seeds form III) Growth regime for particles, seeds are no (La Mer diagram) longer formed Opposite precipitatates (no seeds)

12 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 crystals b) then lower concentration no more seeds are being formed c) maintain low concentration particles continue to grow

13 Bottom Up Reactions: Elemental Sols Elemental 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 Sn 2+ in acid solution Absorption 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 particle sizes A) HAuCl 4 solution 0,01 % B) Tri sodium citrate dihydrate solution 1,0 %

14 Bottom Up Synthesis: Sol Gel Process Preparation of a silica sol condensation of ortho silica acid (Si(OH) 4 ) Preparation of a silica sol by acid from aqueous sodium silicate solution Mono disperse silica

15 Bottom Up Synthesis: Sol Gel Process Preparation of monodisperse nano silica particles: Stöber-Process: hydrolysis of tetraalkylortho silicates (eg. tetraethylorthosilicate, TEOS) Chemical reactions involved in the Stöber process Ammonia concentration determines particle size ( morphological catalyst )

16 Bottom Pp Synthesis of Nano Particles colloidal ZnS colloidal CdCO 3 colloidal TiO 2 Fe2 O 3 barium ferrite

17 Bottom Up Synthesis of Nano Particles Star-shaped Anatase nano crystals

18 Synthesis of Nano Particles Carbon Black Industrial manufacture of carbon black: Incomplete combustion of aromatic compounds Global production: approx. 6-7 mio. to/year Air inlet Complete combustion Incomplete combustion Water cooling

19 Industrial Carbon Black Primary particles : diameter ~ nm, 30 nm often aggregated to chains and intergrown Chemical structure: layered, like graphite C 6 rings, but irregular arrangement Extremely high surface area, ca m 2 /g Ideal pigment: insoluble in all common solvents, resistent against most chemicals, UV stable, very intensive color As filler for strength enhancement of elastomers

20 Carbon Black Examples for industrial use of carbon black > 90 % as filler for elastomers, thereof 2/3 for tire industry, remainder for rubber Printing inks for newspapers (only 0,015 g of carbon black are needed for one page!) Coatings and paints a point (.) on a newspaper page contains carbon black nano particles!!!

21 Synthesis of Pyrogenic Silica Industrial manufacture by flame pyrolysis: 2 H 2 + O 2 2 H 2 O SiCl H 2 O SiO HCl Combustion in O 2 /H 2 flame

22 Manufacture of Pyrogenic Silica

23 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.

24 Chemical Vapor Deposition (CVD) Principle of chemical vapor deposition : deposition from gas phase Processes in use

25 Chemical Vapor Deposition (CVD) Preparation of a Cu nano film on a substrate Schematic hot-wall-reactor Schematic cold-wall-reactor

26 Chemical vapor deposition (CVD) Deposition of Al layers from Al i Bu 3 At low T homogeneous Al surface At high T Al layer contaminated with impurities of C CVD von Al-Schichten aus Me 3 N-AlH 3

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

28 Chemical vapor deposition (CVD) Manufacture of diamond films Different morphologies achieved by different gas pressures during synthesis Different morphologies achieved through different temperatures during synthesis

29 Sol-Gel Process Sol gel transformation is the result of a chemical reaction (condensation) between the sol particles Condensation of two silica particles

30 Sol-Gel Process Difference between gel and precipitate:

31 Sol-Gel Process Sol-gel transformation Zetapotential (mv) Zetapotential Viskosität ph-wert Viskosität (mpas) Gel formation only occurs at specific ph values where particles can get in such proximity that a condensation reaction can take place Example: Zeta potential and Brookfield viscosity as a function of ph value of a Al 2 O 3 sol at a concentration of 0,95 mol/l Gel formation occurs best at ph 9.5

32 Sol-Gel Process Sol-gel transformation 10 Zetapotential [mv] Gelation times of silica sols as a function of the SiO 2 concentration of the sols ph-wert ph dependent zeta potential of silica

33 Sol-Gel Process Condensation of silica particles possessing different sizes Bond between smaller particles is stronger finer particles produce more stable gels Network of particles in a gel Characteristics of gels - low mechanical stability - at least two phases - 3D network of solid phase - interstitial space filled with: a) Water Hydrogel b) Alcohol Alcogel c) Gas Aerogel/Xerogel

34 Sol-Gel Process Site Percolation in a square lattice, shown for three different particle concentrations p. A gel is formed only when p = 0,75 (when at least 75 % of all site are occupied by particles)

35 Sol-Gel Process Formation of a 3D network during gelation Aggregates, agglomerates and networks Aggregates: irreversibel Agglomerates: reversibel Network: labile

36 Sol-Gel Process: Stabilization of Sols Surface modification of sol particles to prevent condensation/gelation Surface inertisation and electrostatic stabilization allows higher solids content in a sol enhances shelf life (stability) of a sol

37 Sol-Gel Process: Xerogel and Aerogel Xerogel / Aerogel Xerogel: evaporation of solvent causes Shrinkage or collapse of 3D network in sol Relationship between particle geometry and gel structure: a) Spherical network structure b) Platelet structure c) Fibrous structure of gel Aerogel: careful removal of solvent under sustainment of 3D network (supercritical CO 2 )

38 Sol-Gel Process: Preparation of CaCO 3 Nano Particles, Xerogel and Aerogel CaCO 3 aerogel, CaCO 3 xerogel and CaCO 3 nano particles from sol-gel transformation of calciumdi(methylcarbonate) Reaction scheme: + 2CO 2 + H 2 O CaO + 2CH3OH Ca(OCH 3) 2+ H2O Ca(OCOOCH 3 ) 2 CaCO 3 + CH3OH Sol/Alkogel CaO in MeOH suspendiert ph Reaktion Filtration Rückstand CO 2 L Filtration Sol Gelierung Gel überkritische Trocknung mit CO 2 CaCO 3 Aerogel CaCO 3 Nanopartikel Trocknung an Luft CaCO 3 Xerogel

39 Sol-Gel Process: CaCO 3 Nano Particles 100 nm Primary CaCO 3 nano particles (Ø 1-5 nm!) TEM image of CaCO 3 nano particles SEM image of 500 nm CaCO 3 nano particles

40 Sol-Gel Process: CaCO 3 Alcogel and Aerogel CaCO 3 Alkogel ESEM image of a CaCO 3 Aerogel CaCO 3 Aerogel obtained from supercritical drying

41 Sol-Gel Process: Nanos, Xerogels, Aerogels, Ceramics Ceramic Coatings, Bodies and Fibers: Application of a xerogel, followed by solvent evaporation and calcination

42 Sol-Gel Process: Preparation of Cement Clinker Phases Wasser kolloidales Teilchen Netzwerk Wasser+ gelöste Nitrate + Ca(NO ) ph-wert 3 2 Sol Hydrogel Netzwerk mit Nitraten Luft Klinkerphase T - H2O Xerogel T Zersetzung Ca(NO ) 3 2 reine Klinkerphase

43 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 homogeneity and small particle sizes