Nanomaterials 1 Nanoscopic Materials Nanomaterials 2 "Prey", the latest novel by Michael Crichton, author of "Jurassic Park". The horrible beasties threatening humanity in this new thriller are not giant dinosaurs, but swarms of minute "nanobots" that can invade and take control of human bodies. Last summer, a report issued by a Canadian environmental body called the action group on erosion, technology and concentration took a swipe at nanotechnology. It urged a ban on the manufacture of new nanomaterials until their environmental impact had been assessed. The group is better known for successfully campaigning against biotechnology, and especially against genetically modified crops. The research, led by a group at the National Aeronautics and Space Administration's Johnson Space Centre in Houston, has found in preliminary studies that inhaling vast amounts of nanotubes is dangerous. Since they are, in essence, a form of soot, this is not surprising. But as most applications embed nanotubes in other materials, they pose little risk in reality. Nanostructural Materials Nanomaterials 3 Room at the Bottom Nanomaterials 4 Nanoscale Writing Nanomaterials 5 Nanoscopic Materials Size is another variable to change physical and chemical properties Nanomaterials 6 Nanoscopic Materials Small Negligible light scattering New optics Quantum size effects Information technology, Storage media High surface area Catalysts, Adsorbents Large Interfacial area New composites Surface modifications Targeted Drug Delivery Nanomaterials 7 Nanoscale regime Size 1 100 nm (traditional materials > 1 m) Physical and chemical properties depend on the size !! Natural examples: Human teeth, 1-2 nm fibrils of hydroxyapatite Ca5(PO4)3(OH) + collagen Asbestos, opals, calcedon Primitive meteorites, 5 nm C or SiC, early age of the Solar system Nanoscopic Materials Nanomaterials 8 Nanoparticles 1 100 nm Traditional materials > 1 m Nanoscopic Materials Nanomaterials 9 Nanomaterials 10 The nano-Family At least one dimension is between 1 - 100 nm 0-D structures (3-D confinement): ˇ Quantum dots ˇ Nanoparticles 1-D structures (2-D confinement): ˇ Nanowires ˇ Nanorods ˇ Nanotubes 2-D structures (1-D confinement): ˇ Thin films ˇ Planar quantum wells ˇ Superlattices Nanomaterials 11 Nanomaterials 12 Coherence Length XRD patterns of iron oxide nanocrystals of 4, 6, 8, 9, 10, 11, 12, 13, and 15 nm Nanomaterials 13 Decreasing grain size = Increasing volume fraction of grain boundaries (50% for 3 nm particles) Surface Effects Nanomaterials 14 Surface Effects Dispersion F = the fraction of atoms at the surface F is proportional to surface area divided by volume N = total number of atoms V ~ r3 ~ N n = number of atoms at the cube edge33 2 11 Nrr r F F Nanomaterials 15 Surface Effects Atoms at surfaces have fewer neighbours than atoms in the bulk lower coordination and unsatisfied bonds surface atoms are less stabilized than bulk atoms The smaller a particle the larger the fraction of atoms at the surface, and the higher the average binding energy per atom. the melting and other phase transition temperatures scale with surface- to-volume ratio and with the inverse size Example: the melting point depression in nanocrystals 2.5 nm Au particles 930 K bulk Au 1336 K Nanomaterials 16 Surface Effects Calculated mean coordination number as a function of inverse radius, represented by N-1/3 for Mg clusters (triangles = icosahedra, squares = decahedra, diamonds = hcp coordination number Nanomaterials 17 Surface Effects Atom binding (vaporization) energies lower in nanoparticles, fewer neighbors to keep atoms from escaping Plasticity of nanocrystalline ceramics Nanomaterials 18 Properties of grain boundaries Lower coordination number of atoms Reduced atomic density (by 10 30 %) Broad spectrum of interatomic distances Si . Experimental evidence HREM EXAFS, reduced number of nearest and next-nearest neighbors Raman spectroscopy Mössbauer spectroscopy, quadrupole splitting distribution broadened Diffusivity enhanced by up to 20 orders of magnitude !! Solute solubility in the boundary region Ag (fcc) and Fe (bcc) immiscible in (s) or (l), but do form solid solution as nanocrystalline alloy EPR, nano-Si gives a sharp signal Surface Effects Nanomaterials 19 GibbsThomson Equation rH V T TT m sl l mol b m b mm -= - 2 Tm = mp of the cluster with radius r Tm b = mp of the bulk Vmol l = the molar volume of the liquid sl = the interfacial tension between the s and l surface Hm = the bulk latent heat of melting DSC In nanoparticles confined in pores bulk Nanomaterials 20 Phase Transitions Phase transitions are collective phenomena With a lower number of atoms in a cluster a phase transition is less well defined, it is therefore broadened Small clusters behave more like molecules than as bulk matter bulk bulk Nanomaterials 21 Surface Effects Correlation between the unit- cell volume (cubic) and the XRD particle size in -Fe2O3 nanoparticles Nanomaterials 22 Surface Effects The inter-ionic bonding in nanoparticles has a directional character ions in the outermost layer of unit cells possess unpaired electronic orbitals associated electric dipole moments, aligned roughly parallel to each other point outwards from the surface the repulsive dipolar interactions increase in smaller particles reduced by allowing unit cell volume to increase Nanomaterials 23 Finite-size effects MO to Band transition Quantum Confinement Effects Physical and chemical properties depend on the size !! Nanomaterials 24 Metal-to-Insulator Transition Nanomaterials 25 Nanomaterials 26 Band gap increases with decreasing size Metal-to-Insulator Transition Metallic behavior Single atom cannot behave as a metal nonmetal to metal transition 100-1000 atoms Magnetic behavior Single domain particles large coercive field Nanomaterials 27 Metal-to-Insulator Transition Variation of the shift, E, in the core-level binding energy (relative to the bulk metal value) of Pd with the nanoparticle diameter The increase in the core-level binding energy in small particles poor screening of the core charge the size-induced metal-nonmetal transition in nanocrystals Nanomaterials 28 Electrical Conductivity Particle size Bulk value Relativeconductivity Nanomaterials 29 Photoelectron spectra of Hg clusters of nuclearity n The 6p peak moves gradually towards the Fermi level the band gap shrinks with increase in cluster size 6s HOMO 6p LUMO Nanomaterials 30 a) Absorption spectra of CdSe nanocrystals (at 10 K) of various diameters b) Wavelength of the absorption threshold and band gap as a function of the particle diameter for various semiconductors. The energy gap in the bulk state in parenthesis Nanomaterials 31 Quantum Confinement Effects Fluorescence of CdSeCdS coreshell nanoparticles with a diameter of 1.7 nm (blue) up to 6 nm (red), smaller particles have a wider band gap Nanomaterials 32 Optical properties nc-TiO2 is transparent Blue shift in optical spectra of nanoparticles Quantum Confinement Effects Nanomaterials 33 NANO -particles, crystals, powders -films, patterned films -wires, rods, tubes -dots Nanostructured materials = nonequilibrium character good sinterability high catalytic activity difficult handling adsorption of gases and impurities poor compressibility PREPARATION METHODS Top-down: from bulk to nanoparticles Bottom-up: from atoms to nanoparticles Nanoscopic Materials Nanomaterials 34 a) Variation of the nonmetallic band gap with nanocrystal size b) in CdS nanocrystals Nanomaterials 35 NANO -particles, crystals, powders -films, patterned films -wires, rods, tubes -dots Nanostructured materials = nonequilibrium character good sinterability high catalytic activity difficult handling adsorption of gases and impurities poor compressibility PREPARATION METHODS Top-down: from bulk to nanoparticles Bottom-up: from atoms to nanoparticles Nanoscopic Materials Nanomaterials 36 Bottom-up Synthesis: Atom Up Nanomaterials 37 NANOSTRUCTURAL MATERIALS Atom Aggregation Method GEM gas evaporation method evaporation by heating resistive, laser, plasma, electron beam, arc discharge the vapor nucleates homogeneously owing to collisions with the cold gas atoms condensation in an inert gas (He, Ar, 1kPa) on a cold finger, walls - metals, intermetallics, alloys, SiC, C60 in a reactive gas O2 TiO2, MgO, Al2O3, Cu2O N2, NH3 nitrides in an organic solvent matrix Bottom-up Synthesis Nanomaterials 38 NANOSTRUCTURAL MATERIALS SMAD the solvated metal atom dispersion 1 2 g of a metal, 100 g of solvent, cooled with liquid N2 more polar solvent (more strongly ligating) gives smaller particles Ni powder: THF < toluene < pentane = hexane Carbide formation Ni(g) + pentane NixCyHz Ni3C 77 to 300 K 180 °C, octane Bottom-up Synthesis Nanomaterials 39 NANOSTRUCTURAL MATERIALS Thermal or Sonocative Decomposition of Precursors Fe(CO)5 nc-Fe + 5 CO sono [Co(en)3]WO4 nc-WC 23% Co PhSi(OEt)3 + Si(OEt)4 + H2O gel -SiC (CH3SiHNH)n (l) Si3N4 + SiC laser M(BH4)4 (g) borides MB2+x (M = Ti, Zr, Hf) Si(OEt)4 + Ag+ or Cu2+ + H2O SiO2/Ag+ /Cu2+ SiO2/Ag/Cu Ar, 1500 °C 300-400°C H2, 550 °C Bottom-up Synthesis Nanomaterials 40 NANOSTRUCTURAL MATERIALS Reduction of Metal Ions Borohydride Reduction - Manhattan Project Aqueous, under Ar 2 Co2+ + 4 BH4 - + 9 H2O Co2B + 12.5 H2 + 3 B(OH)3 Under air 4 Co2B + 3 O2 8 Co + 2 B2O3 Nonaqueous Co2+ + BH4 - + diglyme Co + H2 + B2H6 TiCl4 + 2 NaBH4 TiB2 + 2 NaCl + 2 HCl + H2 MXn + n NR4[BEt3H] M + NR4X + n BEt3 + n/2 H2 M = group 6 to 11; n = 2,3; X = Cl, Br mixed-metal particles Bottom-up Synthesis Nanomaterials 41 NANOSTRUCTURAL MATERIALS Au colloidal particles HAuCl4 + NaBH4 in toluene/H2O system, TOABr as a phase transfer agent, Au particles in the toluene layer, their surface covered with Br, addition of RSH gives stable Au colloid Au fcc S S S S S S S S S S S S S Bottom-up Synthesis Nanomaterials 42 Bottom-up Synthesis SH Nanomaterials 43 Two-dimensional array of thiol-derivatised Au particles (mean diam 4.2 nm) Nanomaterials 44 TEM micrograph of hexagonal arrays of thiolized Pd nanocrystals: a) 2.5 nm, octane thiol b) 3.2 nm, octane thiol Nanomaterials 45 The d-l phase diagram for Pd nanocrystals thiolized with different alkane thiols. The mean diameter, d, obtained by TEM. The length of the thiol, l, estimated by assuming an all-trans conformation of the alkane chain. The thiol is indicated by the number of carbon atoms, Cn. The bright area in the middle encompasses systems which form close-paced organizations of nanocrystals. The surrounding darker area includes disordered or low-order arrangements of nanocrystals. The area enclosed by the dashed line is derived from calculations from the soft sphere model Nanomaterials 46 NANOSTRUCTURAL MATERIALS K + K+ Mg Mg in dry anaerobic diglyme, THF, ethers, xylene NiCl2 + 2 K Ni + 2 KCl AlCl3 + 3 K Al + 3 KCl Reduction by Glycols or Hydrazine "Organically solvated metals" Alkali Metal Reduction Nanomaterials 47 Alkalide Reduction 13 K+(15-crown-5)2Na- + 6 FeCl3 + 2CBr4 THF -30 °C 2 Fe3C (nano) + 13 K(15-crown-5)2Cl0.43Br0.57 + 13 NaCl Anealed at 950 °C / 4 h Fe3C: 2 15 nm Nanomaterials 48 NANOSTRUCTURAL MATERIALS Reactions in Porous Solids Zeolites, Mesoporous materials Ion exchange in solution, reaction with a gaseous reagent inside the cavities M2+ + H2E ME M = Cd, Pb; E = S, Se Ship-in-the-Bottle Synthesis Ru3+ + Na-Y Ru(III)-Y Ru(III)-Y + 3 bpy Ru(bpy)3 2+ reduction of Ru(III) Conducting carbon wires Acrylonitrile introduced into MCM-41 (3 nm diam. channels) Radical polymerization Pyrolysis gives carbon filaments Bottom-up Synthesis Nanomaterials 49 NANOSTRUCTURAL MATERIALS Gel or Polymer Matrices Sol-Gel Method Aerogels, supercritical drying Aerosol Spray Pyrolysis Aqueous solution, nebulization, droplet flow, solvent evaporation, chemical reaction, particle consolidation, up to 800 °C 3Gd(NO3)3 + 5 Fe(NO3)3 Ga3Fe5O12 + 6 O2 + 24 NO2 MnCl2 + 2 FeCl3 + 4 H2O MnFe2O4 + 8 HCl Mn(NO3)2 + Fe(NO3)3 no go, why? Bottom-up Synthesis Nanomaterials 50 NANOSTRUCTURAL MATERIALS Inverse Micelles H2O + octane H2O H2O Cd2+ Se2- CdSe CdSe PhSeSiMe3 Bottom-up Synthesis Nanomaterials 51 Nanomaterials 52 NANOSTRUCTURAL MATERIALS Properties on Nanostructured Materials Metallic behavior Single atom cannot behave as a metal nonmetal to metal transition : 100-1000 atoms Magnetic behavior Single domain particles, large coercive field Depression of melting points in nanocrystals bulk Au mp 1064 °C 10 nm Au 550 °C Nanomaterials 53 LaMer mechanism Supersaturated solution Burst of nucleation Slow growth of particles without additional nucleation Separation of nucleation and growth Nanomaterials 54 Watzky-Finke mechanism Slow continuous nucleation Fast autocatalytic surface growth Nanomaterials 55 Seed-mediated mechanism Au nanoclusters as seeds Bi, Sn, In, Au, Fe, Fe3O4 Nanomaterials 56 Other mechanisms Digestive rippening Surfactant exchange Nanomaterials 57 Thermal Decomposition of Precursors Fe(CO)5 oleic acid trioctylamine 350 o C, 1 h 350 o C, 1 h Fe Me3NO Fe2O3 6 nm Separation of nucleation and growth Fe(CO)5 thermal decomposition at 100 oC contributes to nucleation Fe(oleate) thermal decomposition at 350 oC contributes to growth OH O Nanomaterials 58 Top-down Synthesis: Bulk Down Nanomaterials 59 NANOSTRUCTURAL MATERIALS Introduction of Crystal Defects (Dislocations, Grain Boundaries) High-Energy Ball Milling final size only down to 100 nm, contamination Extrusion, Shear, Wear High-Energy Irradiation Detonative Treatment Crystallization from Unstable States of Condensed Matter Crystallization from Glasses Precipitation from Supersaturated Solid or Liquid Solutions Top-down Synthesis: Bulk Down Nanomaterials 60 XRD patterns of iron oxide nanocrystals of 4, 6, 8, 9, 10, 11, 12, 13, and 15 nm Nanomaterials 61 Nanomaterials 62 NANOSTRUCTURAL MATERIALS Optical properties nc-TiO2 is transparent Blue shift in optical spectra of nanoparticles Nanomaterials 63 NANOSTRUCTURAL MATERIALS Atom binding (vaporization) energies lower in nanoparticles, fewer neighbors to keep atoms from escaping Plasticity of nanocrystalline ceramics Nanomaterials 64 Electrical conductivity Nanomaterials 65 Applications Destruction of dangerous organic compounds (organophosphates - VX, chlorinated - PCB) Nanomaterials 66 Asymmetric heterogeneous catalysis on nanoparticles Nanomaterials 67 CNT growth