Post on 22-Dec-2015
Top-down Approaches
• milling or attrition
• thermal cycles
• 10 ~ 1000 nm; broad size distribution
• varied particle shape or geometry
• impurities
• for nanocomposites and nanograined bulk materials (lower sintering temperature)
Bottom-up Approaches
• Two approaches– thermodynamic equilibrium approach
• generation of supersaturation
• nucleation
• subsequent growth
– kinetic approach• limiting the amount of precursors for the growth
• confining in a limited space
Homogeneous nucleation
• Liquid, vapor or solid
• supersaturation– temperature reduction– metal quantum dots in glass matrix by
annealing– in situ chemical reactions (converting highly
soluble chemicals into less soluble chemicals)
Homogeneous nucleation
• Energy barrier 2*
)3(
16
vGG
vGr
2*
Surface energy
Gibss free energy change
Nuclei
• formation favor:– high initial concentration or supersaturation
– low viscosity
– low critical energy barrier
• uniform nanoparticle size:– same time formation
– abruptly high supersaturation -> quickly brought below the minimum nucleation concentration
Nuclei growth
• Steps– growth species generation– diffusion from bulk to the growth surface– adsorption– surface growth
• size distribution– A diffusion-limited growth VS. a growth-
limited processes
Diffusion-limited growth
• monosized nanoparticles
• how?– Low/controlled supply growth species
concentration– increase the solution viscosity– introduction a diffusion barrier
Metallic nanoparticles
• Reduction of metal complexes in dilute solution– Diffusion-limited process maintaining– Example: nano-gold particles
• chlorauric acid (2.5 x 10-4 M) 20 ml boiling solution+ sodium citrate (0.5%) 1 ml
• 100°C till color change + water to maintain volume
• uniform and stable 20 nm particles
Other cases
HClHCHORhOHCHRhCl 32
3
2
333
ClNaCOHOHPdOHCONaPdCl 22)(2 3222322
OHPdHOHPd 222 2)(
ClClOHPtOHPtCl 3222
4 )( ClClOHPtOHClOHPt 222232 )()(
Reduction reagents
• Affect the size and size distribution– weak reduction reaction
• larger particles
• wider or narrower distribution (depends on “diffusion limited”)
• Affect the morphology– type, concentration, pH value
Polymer stabilizer
• To prevent agglomeration
• surface interaction:– surface chemistry of solid, the polymer, solvent and
temperature– Strong adsorbed stabilizers occupy the growth sites
and reduce the growth rate
• A. Henglein, Chem. Mater. 10, 444 (1998).– polyethyleneimine, sodium polyphosphate, sodium
polyacrylate and poly(vinylpyrrolidone)
Semiconductor nanoparticles
– Pyrolysis (熱裂解 ) of organometallic precursor(s) dissolved in anhydrate solvents at elevated temperatures in an airless environment in the presence of polymer stabilizer (i.e., capping material)
– Coordinating solvent• Solvent + capping material
• phosphine + phosphine oxide (good candidate)
• controlling growth process, stabilizing the colloidal dispersion, electronically passivating the surface
Process
– discrete nucleation by rapid increase in the reagent concentration -> Ostwald ripening (熟成 ) during aging at increased temperature (large particle grow)-> size selective precipitation
– Ostwald ripening• A dissolution-growth processes
• large particles grow at the expense of small particles
• produce highly monodispersed colloidal dispersions
Semiconductor nanocrystallites
• C.B. Murray (CdE, E=S, Se, Te), 1993– Dimethylcadmium (Me2Cd) + bis(trimethylsilyl)
sulfide ((TMS)2S) or trioctylphosphine selenide (TOPSe) or Trioctylphosphine telluride (TOPTe) + solvent (Tri-n-octylphosphine, TOP) + capping material (tri-n-octylphosphine oxide, TOPO)
– before aging (440 ~ 460nm), after aging at 230-260°C (1.5~11.5 nm)
– Size-selective precipitation
Oxide nanoparticles
• Several methods– principles: burst of homogeneous nucleation +
diffusion controlled growth– most commonly: sol-gel processing– most studied: silica colloids
Sol-gel process
• Synthesis– inorganic and organic-inorganic hybrid materials
colloidal dispersions– powders, fibers, thin film and monolith(整塊 )
– low temperature and molecular level homogeneity
• Ref– Sol-Gel Science by Brinker and Scherer; Introduction
to Sol-Gel Processing by Pierre; Sol-Gel Materials by Wright and Sommerdijk
Sol-gel process
• Hydrolysis– e.g.
• Condensation of precursors– e.g.
• typical precursors: metal alkoxides or inorganic and organic salts
xEtOHOHOEtMOxHOEtM xx )()()( 424
OHOHOEtMOMOHOEt
OHOEtMOHOEtM
xxxx
xxxx
21414
44
)()()()(
)()()()(
Multicomponents materials
• Sol-gel route– ensure hetero-condensation reactions between
different constituent precursors• reactivity, electronegativity, coordination number,
ionic radius
• precursor modification: attaching different organic ligands (e.g. reactivity: Si(OC2H5)4 < Si(OCH3)4) )
• chemically modify the coordination state of the alkoxides
• multiple step sol-gel
Organic-inorganic hybrids
• Incorporating organic components into an oxide system by sol-gel processing– co-polymerization– co-condense– trap the desired organic (or bio) components
inside the network– biocomponents-organic-inorganic hybrids
Sol-gel products
• Monodispersed nanoparticles– temporal nucleation followed by diffusion-
controlled growth– complex oxides, organic-inorganic hybrids,
biomaterials– size = f(concentration, aging time)– colloid stabilization: not by polymer steric
barrier, by electrostatic double layer
Sol-gel example: silica
• Precursors:– silicone alkoxides with different alkyl ligand
sizes
• catalyst:– ammonia
• solvent:– various alcohols
water
Vigorous stirring
Vapor phase reactions
• Same mechanism as liquid phase reaction
• Elevated temperatures + vacuum (low concentration of growth)
• Collection on a down stream non-sticking substrate @ low temperature
• example: 2~3 nm silver particles
• may migrate and agglomerate
Vapor phase reactions
• Agglomerates:– large size spherical particles– needle-like particle
• Au on (100) NaCl and (111) CaF substrate• Ag on (100) NaCl substrate
– change in temperature and precursor concentration did not affect the morphology
• size affections– reaction and nucleation temperature
Solid state phase segregation
• applications– metals and semiconductor particles in glass matrix
• homogeneous nucleation in solids state– metal or semiconductor precursors introduced to and
homogeneously distributed in the liquid glass melt at high temperature
– glass quenching to room temperature
– glass anneal above the Tg
– solid-state diffusion and nanoparticles formed
Solid state phase segregation
• Glass matrix (or via sol-gel, polymerization):– metallic ions
• Reheating (or UV, X-ray, gamma-ray):– metallic atoms
• Nuclei growth by solid-state diffusion (slow!)
Heterogeneous nucleation
• A new phase forms on a surface of another material– thermal oxidation, sputtering and thermal oxidation, Ar
plasma and ulterior thermal oxidation
– associate with surface defects (or edges)
Kinetically confined synthesis
• Spatially confine the growth– limited amount of source materials or available
space is filled up
• groups– liquid droplets in gas phase (aerosol & spray)– liquid droplets in liquid (micelle & microemulsion)– template-based– self-terminating
Micelles or microemulsion
• micelles– surfactants or block polymers– two parts: one hydrophilic and one hydrophobic– self-assemble at air/aqueous solution or
hydrocarbon/aqueous solution interfaces
• microemulsion– dispersion of fine organic liquid droplets in an
aqueous solution
Micelle
• CdSe nanoparticles by Steigerwald et al.– surfactant AOT (33.3g) + heptane (1300ml)+ water
(4.3ml)
– stirred -> microemulsion
– 1.0M Cd2+ (1.12 ml) + microemulsion
– Se(TMS)2 (210μl) + heptane (50ml) + microemulsion (syringe, 注射 )
– formation of CdSe crystallites
Polymer nanoparticles
• Water-soluble initiator + surfactant + water + monomer– monomer (large droplets, 0.5 ~ 10μm )– initiator – polymerization– nanoparticles (50 ~ 200nm)
Aerosol synthesis
• Characteristics– Regarded as top-down (maybe?)– can be polycrystalline– needs collection and redispersion
• process– liquid precursor -> mistify -> liquid aerosol ->
evaporation or reaction -> nanoparticles– polymer particle 1~20 μm (from monomer droplets)
Spray pyrolysis
• Solution process– metal (Cu, Ni …) and metal oxide powders– converting microsized liquid droplets of precursor
or precursor mixture into solid particles through heating
– droplets -> evaporation -> solute condensation -> decomposition & reaction -> sintering
– e.g. silver particle: Ag2CO3, Ag2O and AgNO3 with NH4HCO3 @ 400°C
Template-based synthesis
• Templates– cation exchange resins with micropores– zeolites– silicate glasses
• ion exchange
• gas deposition on shadow mask (template)
Core-shell nanoparticles
• The growth condition control– no homogeneous nucleation occur and only
grow on the surface– concentration control: not high enough for
nucleation but high enough for growth• drop wise addition
• temperature control