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.
Download ReportTranscript 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.
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 Gv Fig 3.1 Homogeneous nucleation • Energy barrier G * Surface energy 16 (3 G v ) 2 r 2 * Gv 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 growthlimited 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 RhCl PdCl 3 3 2 Na 2 CO 3 2 H 2 O Pd ( OH ) 2 H 2 CO 3 2 Na 2 CH 3 OH Rh 3 HCHO 3 HCl 2 Pd ( OH ) 2 H 2 Pd 2 H 2 O PtCl 2 4 H 2 O Pt ( H 2 O ) Cl 3 Cl Pt ( H 2 O ) Cl 3 H 2 O Pt ( H 2 O ) 2 Cl 2 Cl 2 Cl 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 230260°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. M ( OEt ) 4 xH 2 O M ( OEt ) 4 x ( OH ) x xEtOH • Condensation of precursors – e.g. M ( OEt ) 4 x ( OH ) x M ( OEt ) 4 x ( OH ) x ( OEt ) 4 x ( OH ) x 1 MOM ( OEt ) 4 x ( OH ) x 1 H 2 O • typical precursors: metal alkoxides or inorganic and organic salts 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 diffusioncontrolled 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 Vigorous stirring • catalyst: – ammonia • solvent: – various alcohols water 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