1. Nanoparticles
Lecture 2
郭修伯

2. 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)

3. 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

4. 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)

5. Homogeneous nucleation
Driving force
Fig 3.1

6. Homogeneous nucleation
Surface energy
Energy barrier
Gibss free energy change

7. Nuclei formation favor: uniform nanoparticle size:
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

8. Nuclei growth Steps size distribution growth species generation
diffusion from bulk to the growth surface
adsorption
surface growth
size distribution
A diffusion-limited growth VS. a growth-limited processes

9. Diffusion-limited growth
monosized nanoparticles
how?
Low/controlled supply growth species concentration
increase the solution viscosity
introduction a diffusion barrier

10. 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

11. Table 3.1

12. Other cases

13. 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

14. Fig 3.10

15. Fig 3.12

16. 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)

17. stabilizer concentration

18. temperature

19. 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

20. 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

21. 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 °C (1.5~11.5 nm)
Size-selective precipitation

22. Oxide nanoparticles Several methods
principles: burst of homogeneous nucleation + diffusion controlled growth
most commonly: sol-gel processing
most studied: silica colloids

23. Sol-gel process Synthesis Ref
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

24. Sol-gel process Hydrolysis Condensation of precursors
e.g.
Condensation of precursors
typical precursors: metal alkoxides or inorganic and organic salts

25. 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

26. 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

27. 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

28. Sol-gel example: silica
Precursors:
silicone alkoxides with different alkyl ligand sizes
catalyst:
ammonia
solvent:
various alcohols
Vigorous stirring
water

29. Vapor phase reactions Same mechanism as liquid phase reaction
Elevated temperatures + vacuum (low concentration of growth)
Collection on a down stream non-sticking low temperature
example: 2~3 nm silver particles
may migrate and agglomerate

30. Vapor phase reactions Agglomerates: size affections
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

31. 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

32. 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!)

33. Solid state phase segregation

34. 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)

35. Heterogeneous nucleation

36. 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

37. Micelles or microemulsion
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

38. 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

39. Polymer nanoparticles
Water-soluble initiator + surfactant + water + monomer
monomer (large droplets, 0.5 ~ 10μm )
initiator
polymerization
nanoparticles (50 ~ 200nm)

40. Aerosol synthesis Characteristics process
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)

41. Size control by termination
Termination by organic components or alien ion occupation

42. 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 400°C

43. Template-based synthesis
Templates
cation exchange resins with micropores
zeolites
silicate glasses
ion exchange
gas deposition on shadow mask (template)

44. 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

45. Semiconductor industry

46. Semiconductor industry