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  1. 1. Chemistry of Nanoscale Materials Synthesis, Properties and Applications Potential Impacts of Nanoscale Materials Pharmacy Water purification Therapeutic drugs Catalysts Tagging DNA and DNA chips Sensors Information Storage Nanostructured Electrodes Chemical/Optical components Improved polymersEnvironmental/Green Chemistry Smart magnetic fluids Solar Cells Improved National Security Environmental remediation
  2. 2. DefinitionsNanoparticle: A solid particle in the 1-1000 nm range that could benoncrystalline, an aggregate of crystallites, or a single crystalliteNanocrystal: A solid particle that is a single crystal in the nanometersize range.Quantum dot. A particle that exhibits properties of quantumconfinement.Nanostructured/ Nanophase/ nanoscale material: Any solid material that has a nanometer dimension;Colloid: A stable liquid phase containing particles in the 1-1000nm range. A colloidal particle is one such 1-1000 nm sized particleCluster: A collection of units (atoms or reactive molecules) of upto about 50 units. Cluster compounds are such moietiessurrounded by a ligand shell that allows isolation of a molecularspecies (stable, isolable, soluble)
  3. 3. Size Relationships of Chemistry, Nanoparticles, and Condensed Matter Physics Nanoscale Condensed Atoms/Molecules Matter Particles 1 125 70,000 6 x 106 ∞ Diameter 1-10 nm Diameter 100-∞ nm Solid State Quantum Chemistry ? Physics In the nanoscale regime, neither quantum chemistry nor classical laws of physics hold
  4. 4. Factors Affected by Size Reduction: Bulk vs. Nano Melting Points Optical properties Colors Surface Reactivity Magnetic properties Conductivity Specific heats
  5. 5. About 2/3 of the Chemical Elements are Metals
  6. 6. Matter has Unusual Properties on the nm ScaleIf you take gold andmake particles about ruby-red10 nm in diameter, it stained glasslooks wine-red or from goldblue-gray, depending nanoparticleson how close theparticles are together
  7. 7. Preparation of Au Nanoparticles by a Chemical Route Reducing agent Capping agent Au nanoparticles Au3+ Salt
  8. 8. Transmission Electron Microscopy Images of Au Nanoparticles
  9. 9. Gold Nanospheres with Increasing Diameter SizeBulk Au 4 nm 12 nm 25 nm 37 nm• Optical properties of metal nanoparticles depend on their shape and size• Particle functionalization can be done on the surface• Visible optical changes occur
  10. 10. Origin of the Properties Bulk Metal Nanoscale metalUnoccupied Decreasing states the size… occupied states Separation between Close lying bands the valence and conduction bands Unbound electrons have Electron motion becomes motion that is not confined confined, and quantization sets in Particle size < mean free path of electrons
  11. 11. Band Structure in Metals EF (Fermi Level) EF depends on the density Density ρ = N/V (where N = Number of electrons, V = volume) Assuming all energy levels have the same number of electrons, δ = EF / N Since N ∝ V Therefore, δ ∝ 1/V V = L3 (where L = side length of the particle) Hence, δ ∝ EF/L3As the side length of the particle decreases the energy level spacing increase
  12. 12. Gold Nanospheres with Increasing Diameter SizeBulk Au 4 nm 12 nm 25 nm 37 nm• Optical properties of metal nanoparticles depend on their shape and size• Particle functionalization can be done on the surface• Visible optical changes occur
  13. 13. Surface Plasmon Absorption of Au Nanoparticles Surface plasmon absorption in metal nanoparticles arises from the collective oscillations of the free conduction band electrons that are induced by the incident electromagnetic radiation.
  14. 14. Factors that Affect the Surface Plasmon Absorbance of Metal Nanoparticles Plasmon absorption of metal nanoparticles is sensitive to the surrounding environment 1. Dielectric of the surrounding medium 2. Solvents (nature of the solvent)
  15. 15. Dielectric Constant and its Effect of the Surface Plasmon Absorption BandPosition of the plasmon absorption band can be discussedwithin the framework of the Drude model λ2 = λp2 (ε∝ = 2εm)Where λp is the bulk plasma wavelength, ε∝ is the high frequency dielectricconstants due to interband and core transitions, and εm is the medium dielectricconstant The refractive index is directly related to its dielectric constant n = (εm)1/2
  16. 16. Normalized surface plasmon absorption band of Au nanoparticles in cyclohexane and o-dichloromethane J. Phys. Chem. B. 2002, 106, 7729Inset shows the dependence of the square of the observed peak position of the surface plasmonband as a function of twice the medium dielectric function. ( m was determined from theexpression, m = n2)
  17. 17. How Does SP Band of Alkanethiolate-Au Clusters Vary with Refractive Index The optical dielectric of the ligand shell, and not that of the solvent, dominates the Au cluster dielectric environment Langmuir 1998, 14, 17Dodecanethiolate-stabilized Au cluster
  18. 18. Melting PointsProperty is a consequence of the averaged coordination numberof the participating atomsTypically, for bulk materials, surface atoms form a negligiblepart of the total number of atomsThe smaller a particle becomes, the more the proportion ofsurface atoms increasesFull shell clusters are constructed by successively packing layers – or shells – of metal atoms around a single metal atom The number of atoms per shell is (Sum of atoms + 10n2 + 2) where n = number of shell
  19. 19. The Relation Between the total number of atoms in Full shell (‘Magic Number’) clusters and the percentage of surface atoms Full-shell Total Surface Clusters Number of Atoms (%) Atoms 1 Shell 13 92 2 Shells 55 76 3 Shells 147 63 4 Shells 309 52 5 Shells 561 45 7 Shells 1415 35
  20. 20. Relation Between the Size of Gold Particles and Their Melting Point 1200 1000Melting Point (oC) 800 600 400 200 0 0 1 2 3 4 5 6 7 8 9 10 Particle Radius (nm)
  21. 21. Other Important Properties of Metal NanoparticlesGold nanoparticles have been shown to bephotoluminescentUnique electrochemical propertiesGold nanoparticles have shown electron – acceptorpropertiesEnhanced catalytic properties
  22. 22. Semiconductor Nanoparticles Group 14 (old group IV) Si, Ge III-V Materials: GaN, GaP, GaAs, InP, InAs II-VI Materials: ZnO, ZnS, ZnSe, CdS, CdSe, CdTeQuantum dots aresemiconductors particlesthat has all threedimensions confined to the1-100 nm length scale Colloidal CdSe quantum dots dispersed in hexane
  23. 23. Energy Diagrams Illustrating theSituation for a Nanoparticle, in Between a Molecule and a Bulk Semiconductor NANOPARTICLE MOLECULE LUMO BULK SOLID CB Energy ∆E Eg ∆E VB HOMO
  24. 24. Quantum Confinement in Semiconductor NanoparticlesEg (quantum dot) = Eg(bulk) + ( h2/8R2) (1/me + 1/mh) – 1.8e2/4πε 0εR Eg = bandgap energy of a quantum dot or bulk solid R = quantum dot radius mc = effective mass of the electron in the solid mh = effective mass of the hole in the solid ε = dielectric constant of the solid ε0 = permittivity of a vacuum
  25. 25. Room-Temperature Spectra of CdSe Quantum Dots(a) Absorption and photoluminescence spectra as a function of diameter(b) Quantum yield of photoluminescence as a function of size. Squares represents deep-trap emission, and circles represent band-edge emission Murray, C. B. Synthesis and characterization of II-VI quantum dots and their assembly into 3D quantum ot superlattices. Ph.D Thesis, MIT, Cambridge , MA 1995
  26. 26. Inorganic SemiconductorsTrap states are caused by defects, such as vacancies, locallattice mismatches, dangling bonds, or adsorbates at thesurface
  27. 27. Chemical Synthetic Routes for Metal and SemiconductorNanoparticles and Structures Additional Synthetic Approaches • Sonochemical • Electrochemical • Photochemical • Chemical Vapor Deposition
  28. 28. Aspects of Nanoparticle Growth in Solution Arrested precipitation Precipitation under starving conditions: a large number of nucleation centers are formed by vigorous mixing of the reactant solutions. If concentration growth is kept small, nuclei growth is stopped due to lack of material.Particles had to be protected from Oswald Ripening by stabilizers Oswald Ripening The growth mechanism where small particles dissolve, and are consumed by larger particles. As a result the average nanoparticle size increases with time and the particle concentration decreases. As particles increase in size, solubility decreases.
  29. 29. Synthetic Approaches for Metal and Semiconductor Nanoparticles via Chemical Routes 1. Metal Compound • Positively charge metal salt, or • Metal centers of complexes2. Solvents (depends on the nature of the salt) • Water • Polar organic solvents • Non-polar organic solvents3. Reducing agent (determined by the nature of the metal compound) • Gaseous hydrogen • Hydridic compounds • Reducing organics, e.g. alcohols Many others
  30. 30. Stabilization of Nanoclusters Against Aggregation1. Electrostatic stabilization Adsorption of ions to the + - - - - - + - - - + -- - + - surface. Creates an electrical - δ+ δ+ δ+ - δ+ δ δ+ -- -- -- double layer which results in a - δ+ δ+ - + δ+ δ+ - + - - --- - -- - - -+ Coulombic repulsion force + - between individual particles 2. Steric Stabilization Surrounding the metal center by layers of material that are sterically bulky, Examples: polymers, surfactants, etc
  31. 31. Synthetic Approaches for Metal and Semiconductor Nanoparticles via Chemical Routes 4. Stabilizers Role of stabilizers:Stabilizing agents/ligands/capping agents/passivating agents • prevent uncontrollable growth of particles • prevent particle aggregation • control growth rate • controls particle size • Allows particle solubility in various solvents
  32. 32. Other Common Stabilizers 1. Organic ligands • Thiols (thioethanol, thioglycerol, mercaptoethylamine, etc) • Amines • phosphates 2. Surfactants 3. Polymers 4. Solvents ether thioether5. Polyoxoanions
  33. 33. Schematic Procedure for Cluster Synthesis Add electron donor •• •••• •• •••••• + ++ •• ••• •••• • • ++ + •• • • • • •• •• •• ••••• ••• • Metal cations Metal atoms in Cluster in solution solution formation Ligand • •• • •••• • ••••• • • ••• Isolation in molecules (•) •••• •••• • • •• • • • solid form • •••• ••••• •• ••••••••• ••••• •••••• ••• •• •• • • •
  34. 34. Schematic Procedure for Cluster Synthesis Add Add Ligand electron molecules (•) donor ++ + + + + •••• •••• •••• •• •• •• •••• • •••• • •••• ++ + • +• • • + • • ••• • ••• •• •• •••• • •••• + ++ • • •• • • + +• •• •• •• ••••••• ••••• •••• ••••••••• •• • • • • • ••••• • • ••• ••••• •• • •• • • • • • • •• •• ••• •••Metal cations in solution Stabilized Metal particles in solution
  35. 35. Synthesis of Metal Nanoparticles in Organic MediaBiphasic reduction procedure Add phase transfer reagent Extract e.g. tetraoctyl ammonium Aqueous bromide (TOAB)solution of metal salt Add Reducing agent
  36. 36. TEM Image of Au Nanoparticles Prepared in the Presence of a Surfactant (CTAB) CTAB = cetyltrimethylammonium bromide J. Phys. Chem. B. 2001, 105, 4065
  37. 37. Nucleation and Growth Homogeneous nucleation occurs via a stepwise sequence ofbimolecular additions until a nucleus of critical size is obtained. a. Nucleation from supersaturated solution nS Sn b. Diffusion-Controlled Growth Sn + S Sn+1 LaMer et al. J. Am. Chem. Soc. 1950, 72, 4847Highly monodisperse nanoparticles are formed if the processes of nucleation and growth can be successfully separated • Nucleation process must be fast • Growth process must be slow
  38. 38. NucleationNucleus Radius is calculated as follows: ∆G = 4πσ(r2 – [2r3 / 3r*]) Where r = nucleus radius r* = critical nucleus radius σ = surface tension ∆G(nucleus) = n(∆G formation, bulk – ∆G formation, free atom) + σA Where A = particle surface area
  39. 39. Dendrimer-Templated Nanocluster SynthesisStructure of poly(amidoamine) dendrimer (PAMAM) Generation 2 PAMAM Dendrimer
  40. 40. Dendrimer-Templated Nanocluster Synthesis Pioneered in 1998, by Donald A. Tomalia (Michigan Molecular Institute Richard M. Crooks (TAMU) Hydrazine PAMAM + CuAc2 1 x 10-4 mol 1 x 10-6 mol Cu Nanoclsuters 1 x 10-5 mol Formation of Cu nanoclusters can be monitored by UV-vis spectrophotometry Reaction is pH dependent: Presumably H+ ions compete with Cu2+ ions for the tertiary amine sites J. Am. Chem. Soc. 1998, 120, 4877 J. Am. Chem. Soc 1998, 120, 7355
  41. 41. Reverse Micelles Water-in-oil droplets [H2O] Water pool w = [surfactant] Particle size is controlled by the size of the water droplets in which synthesis takes placeConsider that: V = volume , R = radius, R = 3V/A A = surface area 3Vaq[H2O] σ = head polar group area Rw = Vaq = volume of water σ[s]
  42. 42. Parameters Affecting Particle Growth/ Shape/ Structure• Type of capping agent/stabilizers• Concentration of the reactants• pH value of the solution• Duration of heat treatment
  43. 43. Sonochemical Approaches for Nanoscale Particle Synthesis Rectified Diffusion ‘area’ effect ‘shell’ effect + BubbleAcousticpressure Liquid shell - Bubble
  44. 44. Sonochemical Approaches for Nanoscale Particle Synthesis Step 1: • Bubble expands when surrounding medium experiences –ve pressure • Bubble collapses when surrounding medium experiences +ve pressure• Bubble collapse leads to extreme temperatures (5,000 – 50, 000 K), and pressure (100 atm) within the bubbleStep 2: Solvent or solute molecules present within the bubbles aredecomposed under these extreme conditions and generate highly reactive radicals
  45. 45. Formation of Highly Reactive Radicals Depending on the liquid medium, sonication leads to the generation of oxidizing and reducing radicals In aqueous solution H2O • H + •OH M+ + H • M + H+ In solutes like alcohols, sonication leads to secondary radicals RHOH + H (OH• ) • ROH• + H2(H2O) ROH • + M+ M + RO + H+
  46. 46. Examples of Metal Nanoparticles Prepared by Sonication Ag nanoparticles prepared in aqueous solution at 1 MHz H2O • • H + OH Ag+ + H • Ag + H+ J. Phys. Chem. 1987, 91, 6687Au nanoparticles prepared by sonication ROH • AuCl4- Au + Products Langmuir 2002, 18, 7831-7836
  47. 47. Synthesis of CdS NanoparticlesGeneral: Anionic orCd(II) salt + Lewis basic + Sulfide 1-10 nm polymers source CdS SodiumCd(NO3)2.4H2O + polyphosphate + Na2S CdS 2 x 10-4 M 2 x 10-4 M 2 x 10-4 M Chem. Mater. 1999, 11, 3595
  48. 48. Synthesis of CdSe Nanoparticles General: High Phosphine oxideCd(CH3)2 + Se reagent + temperature surfactant CdSe HAD-TOPO-TOP Cd(CH3)2 + (C8H17)3PSe CdSeHAD-TOP-TOP = hexadecylamine-trioctylphosphine oxide-trioctylphosphine J. Am. Chem. Soc. 1993, 115, 8706
  49. 49. Synthesis of ZnSe NanoparticlesZn(CH3CH2)2 + (C8H17)3PSe Zn/Se TOP SolutionZn/Se TOP + Hexadecylamine 270 C o ZnSe Solution J. Phys. Chem. B. 1998, 102, 3655 TOPO binds too strongly to Zn TOP binds too weaklyAmines, however have intermediate strength
  50. 50. Synthesis of III-V Semiconductor NanoparticlesSynthesis of III-V semiconductor nanoparticles is quite complex Requires high temperature 370 – 400 oCGaCl3 + tris-(trimethylsilyl)phosphine + TOPO-TOP GaP GaP particles prepared in this manner lacked monodispersity GaCl3 + As(SiMe3)3 ~ 700 oC GaAs Chem. Mater. 1989, 1, 4 J. Am. Chem. Soc. 1990, 112, 9438
  51. 51. Synthesis of InP and InCl3 260 oC, TOP InP nanocrystals InCl3 + [(CH3)3Si]3PSynthesis of InAs via Dehalosilylation Me3SiCl Me3SiCl Me3SiCl evolved evolved evolved InCl3 + (Me3Si)3As InAs 3 days, rt 70-75 C o 150 C o 4 days 4 days Chem. Mater. 1989, 1, 4
  52. 52. Factors Affecting the Nature of the Nanoparticle• Particle size and shape• Surface properties• Particle-solvent interactions• Particle-particle interactions
  53. 53. Common Methods for Nanoparticle Characterization Surface stateParticle Surface Surface Surface Size Area composition structure; Topography Surface Complexes Electron Microscopy X-ray diffraction LEEDMagnetic Measurements AES, SEM XPS, TEM SIMS, EXAFS EPMA, EXAFS IR, UV-Vis, ESR, NMR, Raman
  54. 54. UV-Visible Spectroscopy • Particularly effective in characterizing semiconductor and metal particles• Useful for metal nanoparticle characterization whose surface plasmon absorbance lies in the visible range, e.g. Cu, Au, Ag • Can be used to determine particle size:(For semiconductor nanoparticles: as the radius decreases, the band gap increases and λmax shifts to lower energy. • Particle aggregation • Information about the surface, e.g. presence of adsorbates
  55. 55. Infrared Spectroscopy IR has been used as a surface probe for nanostructures Example illustrated by Bardley:Adsorbing CO onto the metal nanoparticle surface resulted in IR depending on particle size More face: More edge: bridged CO linear CO is is stable stable 2.5 nm 4 nm 6 nmAs particle size increased, the ratio of terminal CO to bridging CO decreased et al. Chem. Mater. 1992, 4, 1234 Bradley,
  56. 56. Nuclear Magnetic Resonance (NMR) Two uses: 1. Probing the ligands that surround metal core2. Probing the intra-core metallic atoms (difficult) Probing the intra-core metallic atomsNuclear spin relaxation time, and nuclear resonance frequency,are sensitive to any metallic property the particle may exhibitChange in frequency (known as ‘Knight shift’) is a consequenceof the interaction of the metal nucleus with the conduction band electrons If particles are very small, in favorable cases, the Knight shift allows resonances for surface and interior metal particles to be identified
  57. 57. Microscopy• TEM: High voltage beam passes through a very thin sample. The sample areas that do not allow passage of electrons allow image to be presented• STM: Involves dragging a sharp needlelike probe across a sample very close to the surface. The tunneling current between the sample and probe tip can be monitored . As probe approaches an elevated portion, the probe moves up and over, and produces a surface map.• AFM: The probe tip is essentially touching the surface, and the surface can be mapped by the weak interaction between the tip and the sample.
  58. 58. Transmission Electron Microscopy • Provides direct visual information of size, shape, dispersity, structure and morphology • Routine magnifications > 40,000 to 0.2 nm Drawbacks • Samples are dried and examined under high vacuum conditions• Therefore, no direct information is gained on how particles exist in solution • Only a finite number of particles can be examined and counted, which may not be a representative of the sample as a whole • Requires electron beam in which case, some nanoparticles may undergo structural rearrangement, aggregation or decomposition.
  59. 59. Scanning Tunneling Microscopy (STM) Makes possible the determination of the total diameter of the nanoparticle, including the stabilizing ligand shell Effective probe of the electronic properties of nanoparticles Reetz et al. Science 1995, 267, 367-369A combined STM/TEM study of Pd nanoparticles stabilized by R4N+Br-. Determined thickness of stabilizing ligand shell by subtracting the STM determined diameter from the TEM determined diameter
  60. 60. Shortcomings to STM• Nanoparticles may not stick well to the substrate surface, preventing good images from being obtained• Geometry of the tip shape may lead to inaccurate measurements or artifacts in the image• Tunneling mechanism is not well understood• Samples have to be dried• Specific techniques applied to imaging are not mature, i.e. standard literature protocols have not been established
  61. 61. Atomic Force Microscopy• Technique is purely mechanical – A cantilevered tip attached to a spring is dragged across a sample – Increase or decrease in tip height is measured yielding a surface height profile as a function of distance – Can be carried out on non-conducting samples Shortcomings • Can reliably determine particle height but not diameter • Cannot distinguish between subtle shape differences, or image particles that are not spatially close to each other
  62. 62. High Aspect Ratio Nanoparticles • What is a high aspect ratio nanoparticle?• Aspect ratio refers to the ratio of a particles length to its width Aspect ratio = length width• High aspect ratio nanoparticles have elongated structure Examples: nanotubes, nanowires, nanorods• Often have distinctive properties as opposed to the bulk materials or even spherical particles e.g. Chemical, electrical, magnetic, optical, etc.
  63. 63. Types of High Aspect Ratio NanoparticlesNanowires Aspect ratio Synthetic MethodNi, Au, Pt, Ag, Co, Cu, ZnO Templated electrodeposition Up to 250 TiO2, ZnO, SiO2 Sol-gel 250 Silicon Nanocluster-mediated vapor-liquid- > 100 solid growth > 20MnO2, Fe2O3, Cu2O, Pd, Electrodeposition on graphite Cu, Au, Ag surfaceNanotubes Gold Templated electroless deposition 250 Silica Sol-gel 250Carbon High temperature: laser > 100 ablation, arc discharge, othersNanorodsGold Surfactant/seed mediated synthesis ~20CdSe Surfactant/seed mediated synthesis 2-10 Cu Micellar growth 1.7-3.7 Se Crystal growth > 100
  64. 64. UV-Visible Absorption Spectrum ofAu Nanorods with Aspect Ratio 3.3 J. Phys. Chem. B. 1999, 103, 3073
  65. 65. Synthesis of Au Nanorods1. Formation of 4 nm “seed” by reduction of HAuCl4 HAuCl4 NaBH4 solution + Sodium citrate2. Seed-mediated growth in the presence of cetyltrimethylammonium bromide (CTAB) produces rod-like Au spheroids and nanorods HAuCl4 Solution + ascorbic acid in CTAB 3. Seed-mediated growth in the presence of cetyltrimethyammonium bromide (CTAB) of rod-like Au nanoparticles leads to Au nanorods HAuCl4 Solution + ascorbic acid in CTAB CTAB = cetyltrimethylammonium bromide (a surfactant)
  66. 66. Increase in Intensity of the LongitudinalPlasmon Band with Increase in Nanorod Concentration Chem. Mater. 2003, 15, 1957
  67. 67. TEM Image of Au Nanorods Prepared by a Seed-Mediated Growth MethodAspect Ratio = 13 J. Phys. Chem. B. 2001, 105, 4065
  68. 68. Gold Nanoparticles with Increasing Aspect Ratio Increasing aspect ratio 1 18 Obare, S. O.; Jana, N. R.; Murphy, C. J. Unpublished results
  69. 69. Increase in Aspect Ratio of AuNanoparticles Shifts the Longitudinal Plasmon Band to the NIR Chem. Mater. 2003, 15, 1957
  70. 70. Calculated absorption spectra of Au nanoparticles with Varying Medium Dielectric Constant Link, S.; Mohamed, M. B.; El-Sayed, M. A. J. Phys. Chem. B. 1999, 103 (16), 3073-3077
  71. 71. Synthesis of Ag Nanorods and Nanowires Formation of 4 nm “seed” by reduction of AgNO 3 AgNO3 NaBH4 solution + Sodium citrate Ag seed Formation of Ag nanorods; aspect ratio was varied by changing the seed concentration; pH is higher ~ 11.8 AgNO3 solution + ascorbic acid NaOH Formation of Ag nanowires; pH is low AgNO3 + ascorbic acid solution NaOH CTAB = cetyltrimethylammonium bromide (a surfactant)
  72. 72. TEM Images of Silver Nanorods and Nanowires Chem. Commun. 2001, 617
  73. 73. Silver Nanoparticles with Increasing Aspect Ratio Increasing aspect ratio1 10 Murphy, C. J.; Jana, N. R. Adv. Mater. 2002, 14, 80
  74. 74. Electrochemical Synthesis of Nanoparticles Synthesis of High Aspect Ratio Nanoparticles Nanoporous Membrane Templated Fabrication SEM image of typical alumina membrane.Typically membranes consists of 1. Anodized alumina 2. Track etch polycarbonate www. J. Vac. Sci. Technol. B 2003, 35, 1097
  75. 75. Nanowire Synthesis by Electrodeposition Sputter Place membrane in copper on aqueous solution of bottom metal salt Nanoporous alumina membrane M+ Apply potential e- - e M+(aq) + e- M(s) Remove copper Dissolve alumina with CuCl/HCl in warm .5 M solution KOH
  76. 76. Current–Time Transient for the Deposition of 60 nmNickel Nanowires into a 6 µm Polycarbonate Template Science 1993, 261, 1316
  77. 77. The Sol-Gel ProcessM-O-R + H2O → MOH + R-OH (Hydrolysis)M-OH + HO-M → M-OH + H2O (Condensation) Sol Gel Metal Hydrolysis Gelling Alkoxide Solution
  78. 78. Sol-Gel Synthesis for High Aspect Ratio Fabrication Materials synthesized by sol-gel method: TiO2: Chem. Mater. 1997, 9, 2544, Adv. Mater. 1994, 8, 857. CdS: Adv. Mater. 2001, 13, 1393.SiO2: Chem. Mater. 1997, 9, 2544. J. Am. Chem. Soc. 1995, 117, 2651 In2O3: J. Mater. Chem. 2001, 11, 2901. Ga2O3: J. Mater. Chem. 2001, 11, 2901. V2O5: Chem. Mater. 1997, 9, 2544 MnO2: Chem. Mater. 1997, 9, 2544 WO3: Chem. Mater. 1997, 9, 2544
  79. 79. SEM Image of TiO2 Nanostructures Prepared by the Sol-Gel ProcessSEM image of TiO2 nanostructures obtained by immersing the template membrane in the sol for (A) 5, (B) 25, and (C) 60 s.
  80. 80. SEM Image of MnO2 Nanostructures Prepared by the Sol-Gel ProcessSEM image of MnO2 fibers prepared in the 200-nm-pore-diameter alumina template membrane
  81. 81. SEM Image of V2O5 Nanostructures Prepared by the Sol-Gel Process SEM images of the template-synthesized V2O5 microstructures
  82. 82. SEM Image of Co3O4 Nanostructures Prepared by the Sol-Gel Process SEM image of Co3O4 fibers prepared in the 200-nm-pore-diameter alumina template membrane
  83. 83. Synthesis of Carbon Nanotubes Synthesis of carbon 1993, Nature 1993, 363, 603 Iijima an coworkers first synthesized carbon nanotubes involves high nanotubes via the thermal decomposition of temperature approaches hydrocarbons. High temperature decomposition of vapors such as benzene or acetylene, in the presence of Co, Fe, or Ni catalysts, formed single walled carbon nanotubes. 1996, Science 1996, 273, 483 Laser Ablation method (Smalley and coworkers) Method produced nanotubes formed into ropes of 100-500 carbon nanotubes, at yields of more than 70%.A single walled carbon nanotube 2002, Nano Lett. 2002, 2, 1043Carbon nanotubes are cylindrical Catalyst-free synthesis (Avouris and coworkers)structures consisting of rolled-up Method developed for carbon nanotube synthesis graphene sheets with fullerene on a silicon surface. Advantage is that catalyst caps. removal is not necessary for purification.
  84. 84. Ligands and the Surfaces they are Reported to FunctionalizeLigand Name Surface Proposed modified linkage R–S–H Thiols Au, Ag, Cu, Hg, FeR–S–S–R Disulfides R–C N Isocyanides Pt, Pd Carboxylic acids Metal oxides Phosphonates Metal oxides Siloxanes Metal oxides Hydroxamic acids Metal oxides
  85. 85. Selective Functionalization 1-butaneisocyanide 2-mercaptoethylamine Adv. Mater. 1999, 11, 1021
  86. 86. Applications of High Aspect Ratio NanoparticlesSeveral applications of high aspect ratio nanoparticles have been shown, and many others continue to be unfolded Applications is areas such as: •Biology •Gene therapy •Bioseparations •Separations •Catalysis •Sensing •Electronics •Optical applications Etc.
  87. 87. Nanowire Synthesis by Electrodeposition Sputter Place membrane in copper on aqueous solution of bottom metal salt Nanoporous alumina membrane M+ Apply potential e- - e M+(aq) + e- M(s) Remove copper Dissolve alumina with CuCl/HCl in warm .5 M solution KOH
  88. 88. Au-Ni-Au Nanorods Bound and Unbound to Fluorescein-tagged Poly-His Confocal fluorescence image of Au-Ni-Au after modification of Ni portion with Fluorescein-tagged poly-His SEM image of Au-Ni-Au NanowiresAngew. Chemie. Int. Ed. 2004, 43, 3048
  89. 89. Exposure of Fluorescein-tagged Poly-His Proteins to Au-Ni-Au Nanorods Left: Fluorescein-tagged poly-His solutionRight: Fluorescein-tagged poly-HisSolution after exposure to Au-Ni-Au Nanorods Angew. Chemie. Int. Ed. 2004, 43, 3048
  90. 90. Separation of His-tagged Proteinsfrom a Non-His-tagged Structures a. Solution of IgG protein (no a b c His)-Green Alexa dye mixed with His-tagged protein-Red Alexa Dye b. After exposure to Au-Ni- Au; (non-His protein remains in solution) c. Separated Au-Ni-Au from a in solution Angew. Chemie. Int. Ed. 2004, 43, 3048
  91. 91. Smart Nanotubes for Bioseparations and Biocatalysis J. Am. Chem. Soc. 2002, 124, 11864
  92. 92. Smart Nanotubes for Bioseparations and Biocatalysis Nanotubes preferentially reside in cyclohexane due to Vial containing cyclohexane the outer hydrophobic (upper) and water (lower) surface Add nanotubes Acyclohexane H2 O Nanotubes A. Dansylamide on inner void and C 18 on outer surfaces Danyslamide dye fluoresces green J. Am. Chem. Soc. 2002, 124, 11864
  93. 93. Smart Nanotubes for Bioseparations and Biocatalysis Nanotubes preferentially Vial containing cyclohexane reside in Aqueous phase due (upper) and water (lower) to the outer hydrophilic surface Add nanotubes Bcyclohexane H2 O Nanotubes B. Quinineurethan on inner and no silane on outer surfaces Quinineurethan dye fluoresces blue J. Am. Chem. Soc. 2002, 124, 11864
  94. 94. Smart Nanotubes for Bioseparations and Biocatalysis Nanotubes A preferentially reside in Organic phase while Vial containing cyclohexane nanotubes B reside in the (upper) and water (lower) aqueous phase Add nanotubes Acyclohexane Add nanotubes B H2 O Nanotubes A. Dansylamide on inner void and C 18 on outer surfaces Nanotubes B. Quinineurethan on inner and no silane on outer surfaces J. Am. Chem. Soc. 2002, 124, 11864
  95. 95. Multifunctional Nanorods for Gene Delivery Nature Materials 2003, 2, 668 Goal of gene therapy: to introduce foreign cells into somatic cells to supplement defective genes, or provide additional biological functions H2N H2N S-S S-S DNA* HOOC OOC Au Ni HN-DNA* HN-DNA* S-S S-S OOC S-T OOC HS-Transferrin Observations 1. Nanorods were internalized into the cell but did not enter the nucleus 2. GFP was observed in the nucleus indicating delivery of the reporter gene to the nucleus Human kidney cell 3. Au-S-T served to promote cellular uptake 4. Disulfide linkage acts as a cleavable point to promote release of DNA within the cellDNA* = DNA plasmids which encodes Green fluorescent Protein GFP
  96. 96. What is Nanoparticle Engineering/Surface ModificationTailored synthesis of core-shell nanoparticles with defined morphologies and properties shell core
  97. 97. Why Surface Modification? 1. The shell can alter the charge, functionality, and reactivity of the surface2. The shell can enhance the stability and dispersibility of the colloidal core 3. Magnetic, optical, or catalytic functions may be readily imparted to the dispersed colloidal core4. Encasing colloids in a shell of different composition may also protect the core from extraneous chemical and physical changes
  98. 98. Effects of Surface ModificationChemical and Colloidal Stability • Nanoparticle degradation through chemical etching • Agglomeration caused by strong van der Waals attractive forcesTuning of Physical Properties For example, the optical properties of metal nanoparticles are influenced by their environments. Controlled surface modification can alter these propertiesControl of Interparticle Interactions Within Assemblies • Collective properties of nanoparticle assemblies are influenced to a large extent by the separation between the particles. • Coating the particles with a uniform shell of inert material could control the distance between the particles
  99. 99. Types of Core-Shell Nanoparticles • Metal-Polymer • Metal-Metal • Semiconductor- Semiconductor • Semiconductor-Metal • Metal - Semiconductor
  100. 100. Polymers on Metals• Main reason is for nanoparticle stabilization• Could also be used to assemble nanoparticles• Examples: – Chem. Mater. 1998, 10, 1214 – J. Am. Chem. Soc. 1999, 121, 8518 – Adv. Mater. 1999, 11, 34 – Adv. Mater. 1998, 10, 132 – Chem. Commun. 1998, 351 – Adv. Mater. 1999, 11, 131 – J. Am. Chem. Soc. 1999, 121, 10642 – Nano Lett. 2002, 2, 3
  101. 101. Sketch of the surface reactions involved inthe formation of a thin silica shell on citrate- stabilized gold particles Langmuir 1996, 12, 4329
  102. 102. UV-visible spectra of sodium citrate-stabilized, 15 nm diameter gold colloids 1day after addition of different amounts of APS APS: polymerization initiator 3-aminopropyltrimethoxysilane Langmuir 1996, 12, 4329
  103. 103. 15 nm gold particles coated with thin silica layers 18 hours after addition 42 h after addition 5 days after addition of active silicaThe silica shell keeps on growing, but eventually small silica particles also nucleate out of the solution. Langmuir 1996, 12, 4329
  104. 104. Silica-Coated Au Nanoparticles Langmuir 1996, 12, 4329Transmission electron micrographs of silica-coated gold particles produced during the extensivegrowth of the silica shell around 15 nm Au particles with TES in 4:1 ethanol/water mixtures. Theshell thickness are (a, top left) 10 nm, (b, top right) 23 nm, (c, bottom left) 58 nm, and (d, bottom right) 83 nm
  105. 105. Influence of thin silica shells on the UV- visible spectra of aqueous gold colloids Experimental Calculated Langmuir 1996, 12, 4329
  106. 106. Influence of thick silica shells on the UV- visible spectra of ethanolic gold colloids Experimental Calculated Langmuir 1996, 12, 4329
  107. 107. Effect of Solvent Refractive Index on theColor of Dispersions of 15 nm Gold Particles with a 60 nm Silica Shell The solvent refractive indices (left to right) are 1.45, 1.42, 1.39, and 1.36 Langmuir 1996, 12, 4329
  108. 108. Silica Coating of Silver Colloids NaBH4 AgClO4 + sodium citrate 10 nm Ag nanoparticles Ag + 3-aminopropyltrimethoxysilane + sodium silicate Ag@SiO2nanoparticlesSilicate ion concentration 0.02 % 0.01 % 0.005 % Langmuir 1998, 14, 3740
  109. 109. Emulsion PolymerizationEmulsion polymerization is a type of polymerization that takes place in anemulsion typically incorporating water, monomer, and surfactant. The most common type of emulsion polymerization is an oil-in-water emulsion, in which droplets of monomer (the oil) are emulsified (with surfactants) in a continuous phase of water. In aqueous solution surfactant Monomer + free radical initiator → Polymer
  110. 110. Polymer-Coated Silver Nanoparticles TEM images of silver particles: (A) uncoated particle, (B) polystyrene/methacrylate coated particles, (C) polystyrene/methacrylate coated particles with a covalently boundBSA layer, and (D) the same as panel C after exposure to gold colloids. Negative staining by phosphotungstic acid used for all images J. Am. Chem. Soc. 1999, 121, 10642
  111. 111. Preparation of Polymer-Coated Functionalized Silver Nanoparticles Extinction spectra of silver particles: (A) uncoated particles and (B) polystyrene coatedparticles. Solid line: suspension in water. Dotted line: suspension in water, after 1 h in 1.8 M NaCl J. Am. Chem. Soc. 1999, 121, 10642
  112. 112. Synthetic Protocols for the Synthesis of Coupled 1D Nanoparticle Arrays Procedures for (A) Ppy- linked Au ColloidsAlkyldithiolate-Linked Au Colloids Ppy = poly(pyrrole) Chem. Mater. 1998, 10, 1214
  113. 113. Au Colloids Linked by PPyTransmission electron microscope images of 1D and near- 1D arrays of Au colloids linked by Ppy Chem. Mater. 1998, 10, 1214
  114. 114. Semiconductor on SemiconductorTailoring optical propertiesEnhancing the luminescence Shell Energy Core core shell shell core
  115. 115. Energies of Various Semiconductors TiO2 GaP Energy (eV) GaAs CdS CdSe ZnO TiO2 WO3 1.4 3.0 2.25 1.7 2.5 3.2 3.2 3.2 Values at pH = 1
  116. 116. Inorganic SemiconductorsTrap states are caused by defects, such as vacancies, locallattice mismatches, dangling bonds, or adsorbates at the surface
  117. 117. Examples for Semiconductor-Semiconductor Core-Shell Nanoparticles Examples include: ZnS on CdSe CdS on CdSe CdSe on CdS, etc J. Phys. Chem. B. 1997, 101, 9463 J. Phys. Chem. B. 1998, 102, 1884 J. Phys. Chem. 1993, 97, 5333 J. Phys. Chem. 1996, 100, 6381 J. Phys. Chem. 1996, 100, 8927 J. Phys. Chem. 1996, 100, 13226 J. Phys. Chem. 1996, 100, 20021
  118. 118. CdSe Coated with ZnS NanoparticlesMe2Cd + TOPSe CdSe 300 oC (TMS)2/Me2Zn/TOP ∆T ZnS CdSe J. Phys. Chem. 1996, 100, 468 TEM picture of (CdSe)ZnS nanocrystals
  119. 119. CdSe Coated with ZnS Nanoparticles J. Phys. Chem. 1996, 100, 468 Absorption spectrum of the (CdSe)TOPO (dotted line) and the (CdSe)ZnS Normalized fluorescence spectra of CdSe-TOPOnanocrystals (solid line). The fluorescence (dotted line) and CdSe@ZnS (solid line) withof the (CdSe)ZnS is also shown (solid line) 470 nm excitation
  120. 120. Observations on the Optical Characteristics of CdSe/ZnS Nanoparticles Fluorescence of CdSe-TOPO shows the broad tail, due to surface traps. CdSe/ZnS fluorescence spectrum has a flat baseline; thisindicates that the ZnS reduces the traps present on the CdSe (TOPO) surface Fluorescence of CdSe (CdSe/ZnS) was stable for months compared to uncapped CdSeNo reduction in the CdSe quantum yield was observed for months with the CdSe/ZnS nanoparticles J. Phys. Chem. 1996, 100, 468
  121. 121. Synthesis of HgS/CdS Core-Shell Nanostructures J. Phys. Chem. 1993, 97, 5333 HgScore CdSshell HgCl2 + H2S + sodium polyphosphate → HgS HgS + Cd(ClO4)2 + H2S → HgS/CdSCdScore HgSshell Cd(ClO4)2 + H2S + sodium polyphosphate → CdS CdS + HgCl2 + H2S → CdS/HgS Note: Due to the much lower solubility of HgS compared with CdS particles result in an exchange of Cd2+ by Hg2+ (CdS)n + mHgCl2 → (CdS)n-m(HgS)m + mCdCl2
  122. 122. CdS/HgS Mixed Colloids J. Phys. Chem. 1993, 97, 5333HgS nanoparticles HgS coated with CdS
  123. 123. Absorption Spectra of Core-Shell CdS on HgS Nanoparticles J. Phys. Chem. 1993, 97, 5333 A = HgS
  124. 124. Fluorescence Spectra of Core-Shell CdS on HgS Nanoparticles HgS nanoparticles do not fluoresce CdS coated HgS nanoparticles fluoresce: Possibly due to removal of traps for nonradiative recombinations or Fluorescence could arise from band to band recombination in HgS core J. Phys. Chem. 1993, 97, 5333
  125. 125. CdS/HgS Mixed Colloids J. Phys. Chem. 1993, 97, 5333
  126. 126. Metal – Metal Core-Shell NanostructuresExamples reported in the literature Au/Ag : J. Chem. Phys. 1964, 41, 3357-3363 Au/Cd : Ber. Bunsenges. Phys. Chem. 1994, 98, 180-189 Au/Pb : Ber. Bunsenges. Phys. Chem. 1994, 98, 180-189 Au/Sn : J. Phys. Chem. 1994, 98, 6931-6935 Au/Tl : Ber. Bunsenges. Phys. Chem. 1994, 98, 180-189 Ag/Pb : Ber. Bunsenges. Phys. Chem. 1992, 96, 754-759 Ag/Cd : J. Phys. Chem. 1994, 98, 6931-6935 Ag/In : Ber. Bunsenges. Phys. Chem. 1992, 96, 2411-2414 Au/Pt : J. Phys. Chem. B. 2000, 104, 2201-2203
  127. 127. Pt/Au Core-Shell Nanoparticles Synthesis of Aucore Ptshell Nanoparticles High TNaAuCl4 + sodium citrate Au nanoparticles (~ 20 nm) H2 PtCl 4 2- + Au nanoparticles Au/PtSynthesis of Ptcore Aushell Nanoparticles H2 PtCl4 + sodium polyacrylate 2- Pt nanoparticles (~12 nm) γ-rays Pt nanoparticles + K2Au(CN)2 MeOH Pt/Au J. Phys. Chem. B. 2000, 104, 2201-2203
  128. 128. PtcoreAushell NanoparticlesPt nanoparticles 1:1 Pt/Au nanoparticles 1:2 Pt/Au nanoparticles J. Phys. Chem. B. 2000, 104, 2201-2203
  129. 129. Pt/Au Core-Shell Nanoparticles J. Phys. Chem. B. 2000, 104, 2201-2203Absorption spectra of Pt nanoparticlesbefore and after deposition of various amounts of gold. Overall Pt concentration is 1 x 10-4 MConcentration of Pt:Au is given on the Absorption spectra of Au nanoparticles curves before and after deposition of various amounts of Pt. Overall Au concentration: 3 x 10-4 M Molar of Au:Pt is given on the curves
  130. 130. AucorePtshell NanoparticlesElectron micrograph of Au core particles before (left) and after (right) deposition of Pt in the ratio 1:2 J. Phys. Chem. B. 2000, 104, 2201-2203
  131. 131. Metal-Semiconductor Core-Shell Nanoparticles Metals can be used as templates to make hollow semiconductor nanostructures Fabrication of composite nanoparticles with a large electroniccapacitance, i.e. a large difference in the Fermi level of the core relativeto the conduction band edge of the shell will enable electrons to diffuse through the shell and be trapped in the core for a long time Examples: Au/CdSe: J. Mater. Res. 1998, 13, 905-908 Au/CdS: J. Phys. Chem. B. 1997, 101, 7675 Ag/TiO2: Langmuir 2000, 16, 2731-2735 Au/TiO2: J. Phys. Chem. B. 2000, 104, 10851 TiO2/Ag: Langmuir 1999, 15, 7084-7087 ZnO/Au: J. Phys. Chem. B. 2003, 107, 7479-7485
  132. 132. Au/CdS Composite Nanoparticles Au nanoparticles Au/CdS composite nanoparticles J. Phys. Chem. B. 1997, 101, 7675
  133. 133. Synthesis of CdS-Capped Au Nanoparticles High TNaAuCl4 + sodium citrate Au nanoparticles (~ 20 nm) MNA = 2-mercaptonicotinic acid J. Phys. Chem. B. 1997, 101, 7675
  134. 134. Absorption Spectra of Au/CdS Nanocomposites Absorption properties of Au/CdS are not the result of a simple addition of the spectra of two nanoclusters, but rather an influence of the CdS on the Au. Au Au/CdSAu/MNA CdS J. Phys. Chem. B. 1997, 101, 7675
  135. 135. Emission Spectra of Au/CdS Nanocomposites Emission quenching of CdS is indicative of the occurrence ofelectron transfer from excited CdS into the Au core.Conduction band energy for CdS = - 1.0 V vs. NHE Fermi level of Au = + 0.5 V vs. J. Phys. Chem. B. 1997, 101, 7675 NHE