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Nanoparticles
Lecture 2郭修伯
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
• Driving force
Fig 3.1
vG
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
Table 3.1
Other cases
HClHCHORhOHCHRhCl 323
23
33
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
Fig 3.10
Fig 3.12
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)
stabilizer concentration
temperature
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
OHOHOEtMOMOHOEtOHOEtMOHOEtM
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 alcoholswater
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!)
Solid state phase segregation
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)
Heterogeneous nucleation
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)
Size control by termination
• Termination by organic components or alien ion occupation
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
Semiconductor industry
Semiconductor industry