Nanoparticles

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: • 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 • 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 230-260°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 • 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 • e.g. • Condensation of precursors • e.g. • 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 substrate @ low temperature • example: 2~3 nm silver particles • may migrate and agglomerate

30. 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

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 • 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

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 • 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 NH4HCO3 @ 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