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Nanomaterials
- 1. What is Nanotechnology? •The study of the controlling of matter on an atomic and molecular scale. Generally nanotechnology deals with structures sized between 1 to 100 nanometer in at least one dimension, and involves developing or modifying materials or devices within that size.
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- 3. Disruptive Apps: Materials •Fiber that is stronger than spider web • Metal 100 x’s stronger than steel, 1/6 weight • Catalysts that respond more quickly and to more agents • Plastics that conduct electricity • Coatings that are nearly frictionless –(Shipping Industry) • Materials that change color and transparency on demand. • Materials that are self repairing, self cleaning, and never need repainting. • Nanoscale powders that are five times as light as plastic but provide the same radiation protection as metal.
- 4. • Fuel celltechnology becomes cost effective within 3 years. • Batteries that store more energy and are much more efficient • Plastics and paints that will store solar power and convert to energy for $1 per watt. Continue…..
- 5. • Nanostructured materialsare classified as Zero dimensional, one dimensional, two dimensional, three dimensional nanostructures. (a) 0 D spheres and clusters, (b) 1 D nanofibers, wires, and rods (c)2 D films, plates and networks, (d) 3D nanomaterials Classification of Nanomaterials
- 6. Examples of Nanomaterials Aunanoparticle Buckminsterfullerene FePt nanosphere Titanium nanoflower Silver nanocubes SnO2 nanoflower
- 7. Quantum dots • Quantumdots (QD) are nanoparticles/structures that exhibit 3 dimensional quantum confinement, which leads to many unique optical and transport properties. Lin-Wang Wang, National Energy Research Scientific Computing Center at Lawrence Berkeley National Laboratory. <http://www.nersc.gov> GaAs Quantum dot containing just 465 atoms.
- 8. Quantum dots properties •Quantum dots are usually regarded as semiconductors by definition. • Similar behavior is observed in some metals. Therefore, in some cases it may be acceptable to speak about metal quantum dots. • Typically, quantum dots are composed of groups II-VI, III-V, and IV-VI materials. • QDs are bandgap tunable by size which means their optical and electrical properties can be engineered to meet specific applications.
- 9. Quantum Confinement Definition: • QuantumConfinement is the spatial confinement of electron-hole pairs (excitons) in one or more dimensions within a material. – 1D confinement: Quantum Wells – 2D confinement: Quantum Wire – 3D confinement: Quantum Dot • Quantum confinement is more prominent in semiconductors because they have an energy gap in their electronic band structure. • Metals do not have a bandgap, so quantum size effects are less prevalent. Quantum confinement is only observed at dimensions below 2 nm.
- 10. Quantum Confinement • Recallthat when atoms are brought together in a bulk material the number of energy states increases substantially to form nearly continuous bands of states. N Energy Energy
- 11. Quantum Confinement • Thereduction in the number of atoms in a material results in the confinement of normally delocalized energy states. • Electron-hole pairs become spatially confined when the diameter of a particle approaches the de Broglie wavelength of electrons in the conduction band. • As a result the energy difference between energy bands is increased with decreasing particle size. Energy Eg Eg
- 12. Quantum Confinement • Thisis very similar to the famous particle-in-a-box scenario and can be understood by examining the Heisenberg Uncertainty Principle. • The Uncertainty Principle states that the more precisely one knows the position of a particle, the more uncertainty in its momentum (and vice versa). • Therefore, the more spatially confined and localized a particle becomes, the broader the range of its momentum/energy. • This is manifested as an increase in the average energy of electrons in the conduction band = increased energy level spacing = larger bandgap • The bandgap of a spherical quantum dot is increased from its bulk value by a factor of 1/R2, where R is the particle radius.* * Based upon single particle solutions of the schrodinger wave equation valid for R< the exciton bohr ra
- 13. Quantum Confinement • Whatdoes this mean? – Quantum dots are bandgap tunable by size. We can engineer their optical and electrical properties. – Smaller QDs have a large bandgap. – Absorbance and luminescence spectrums are blue shifted with decreasing particle size. Energy 650 nm555 nm
- 14. Quantum Dots (QD) •Nanocrystals (2-10 nm) of semiconductor compounds • Small size leads to confinement of excitons (electron-hole pairs) • Quantized energy levels and altered relaxation dynamics • Examples: CdSe, PbSe, PbTe, InP Eg
- 15. Quantum Dots Absorption andemission occur at specific wavelengths, which are related to QD size Eg
- 16. QD Synthesis: Colloidal Methods Journalof Chemcial Education. Vol. 82 No.11 Nov 2005 • Example: CdSe quantum dots • 30mg of Elemental Se and 5mL of octadecene are used to create a stock precursor Se solution. • 0.4mL of Trioctylphosphine oxide (TOPO) is added to the Se precursor solution to disassociate and cap the Se. • Separately, 13mg of CdO, 0.6mL of oleic acid and 10mL of octadecene were combined and heated to 225oC • Once the CdO solution reaches 225oC, room- temperature Se precursor solution was added. Varying the amount of Se solution added to the CdO solution will result in different sized QDs.
- 17. QD color withsize Journal of Chemcial Education. Vol. 82 No.11 Nov 2005
- 18. Applications of QDs:Biological • Biological Tagging and Labeling – Biological assays and microarrays – Labeling of cells and intracellular structures – in vivo and in vitro imaging – Pathogen and Toxin detection
- 19. Applications of QDs:Biological • Biological Tagging – Organic fluorophores such as genetically encoded fluorescent protein, like GFP, or chemically synthesized fluorescent dyes have been the most common way of tagging biological entities. – Some limitations of organic fluorophores: • do not continuously fluoresce for extended periods of time • Degrade or photo-bleach • are not optimized for multicolor applications
- 20. Applications of QDs:Biological • The unique optical properties of quantum dots make them suitable for biological tagging and labeling applications. • QDs are excellent fluorophores. – Fluorescence is a type of luminescence in which the absorption of an incident photon triggers the emission of a lower energy or longer wavelength photon. – Quantum dots absorb over a broad spectrum and fluoresce over a narrow range of wavelengths. This is tunable by particle size. – So, a single excitation source can be used to excite QDs of different colors making them ideal for imaging multiple targets simultaneously.
- 21. Applications of QDs:Biological Absorption and emission Spectra of CdSe/ZnS QDs compared to Rhodamine, a common organic die. – The absorption spectrum (dashed lines) of the QD (green) is very broad, whereas that of the organic die (orange) is narrow. – Conversely, the emission spectrum (solid lines) of the QD is more narrow than that of the organic die Jyoti K. Jaiswal and Sanford M. Simon. Potentials and pitfalls of fluorescent quantum dots for biological imaging. TRENDS in Cell Biology Vol.14 No.9 September 2004
- 22. • A broadabsorption and narrow emission spectrum means a single excitation source can be used to excite QDs of different colors making them ideal for imaging multiple targets simultaneously. Gao, Xiaohu. "In vivo cancer targeting and imaging with." Nature Biotechnology 22(2004): 8. Applications of QDs: Biological CdSe/ZnS QDs used to image cancer cells in a live mouse.
- 23. Applications of QDs:Biological • Quantum dots are an attractive alternative to traditional organic dies because of their high quantum yield and photostability. • Quantum Yield = # emitted photons / # absorbed photons. – Quantum dots have a high quantum yield because they have a high density of energy states near the bandgap. – A higher quantum yield means a brighter emission. The quantum yield of some QDs is 20 times greater than traditional organic fluorophores. • Photostability is a fluorophore’s resistance to photobleaching or photochemical degradation due to prolonged exposure to the excitation source.
- 24. Applications of QDs:Biological X. Michalet, et al. Quantum Dots for Live Cells, in Vivo Imaging, and Diagnostics Science 307, 538 (2005) Common QD Materials, their size and emitted wavelengths
- 25. Applications of QDs:Biological Bioconjugated QDs: • The surface of QDs can be functionalized with affinity ligands: antibodies, peptides, or small-molecule drug inhibitors, to target specific types of cells for in vivo or in vitro imaging. • The affinity ligands are not bound directly to the surface of the quantum dots. They are usually connected to a linker molecule referred to as a capping ligand or coordinating ligand. • Polymers such as poly ethylene glycol (PEG) may be introduced to reduce nonspecific binding of the affinity ligands. Gao, Xiaohu. "In vivo cancer targeting and imaging with." Nature Biotechnology 22(2004): 8.
- 26. Applications of QDs:Biological Bioconjugated QDs: • The surface of QDs can be functionalized with affinity ligands: antibodies, peptides, or small-molecule drug inhibitors, to target specific types of cells for in vivo or in vitro imaging. • The affinity ligands are not bound directly to the surface of the quantum dots. They are usually connected to a linker molecule referred to as a capping ligand or coordinating ligand. • Polymers such as poly ethylene glycol (PEG) may be introduced to reduce nonspecific binding of the affinity ligands. Gao, Xiaohu. "In vivo cancer targeting and imaging with." Nature Biotechnology 22(2004): 8.
- 27. Applications of QDs:Biological Bioconjugated QDs: • The coordinating ligands serve a dual purpose: 1. To bind the affinity ligands to the surface of the QD. 2. To encapsulate the quantum dot in a protective layer that prevents enzymatic degradation and aggregation. • The coordinating ligands dictate the hydrodynamic behavior of the QD and are chosen according to the desired biocompatibility. • Common coordinating ligands: • Avidin-biotin complex • Protein A or protein G • Simple polymers and amphiphilic lipids
- 28. Applications of QDs JyotiK. Jaiswal and Sanford M. Simon. Potentials and pitfalls of fluorescent quantum dots for biological imaging. TRENDS in Cell Biology Vol.14 No.9 September 2004 QDs conjugated with antibody molecules (blue) by using avidin (purple) or protein A (green) as linkers. Between 10 and 15 linker molecules can be attached covalently or electrostatically to a single QD, which facilitates the binding of many or a few antibody molecules on each QD.
- 29. Applications of QDs:Biological DNA assays and microarrays Each pixel contains a different DNA sequence Fluorescence observed if sample binds QD-functionalized DNA Image source: Wikipedia: Gene Expression Profiling BioMems Applications Overview SCME: www.scme-nm.org
- 30. Applications of QDs:Light Emitters • The discovery of quantum dots has led to the development of an entirely new gamut of materials for the active regions in LEDs and laser diodes. • Indirect gap semiconductors that don’t luminesce in their bulk form such as Si become efficient light emitters at the nanoscale due quantum confinement effects. • The study of QDs has advanced our understanding of the emission mechanisms in conventional LED materials such as InGaN, the active region of blue LEDs. • The high radiative-recombination efficiency of epitaxial InGaN is due to self-assembled, localized, In rich clusters that behave like QDs.
- 31. Additional Applications ofQDs • New applications for QDs are continuously being discovered. • For example: Solar cells that incorporate QDs may lead to more efficient light harvesting and energy conversion.
- 32. Quantum Dot SolarCells Possible benefits of using quantum dots (QD): • “Hot carrier” collection: increased voltage due to reduced thermalization • Multiple exciton generation: more than one electron-hole pair per photon absorbed • Intermediate bands: QDs allow for absorption of light below the band gap, without sacrificing voltage MRS Bulletin 2007, 32(3), 236.
- 33. QDs: Collect HotCarriers Eg Conduction Band Valence Band 1. Tune QD absorption (band gap) to match incident light. 2. Extract carriers without loss of voltage due to thermalization. Band structure of bulk semi- conductors absorbs light having energy > Eg. However, photo- generated carriers thermalize to band edges.
- 34. QDs: Multiple ExcitonGeneration In bulk semiconductors: 1 photon = 1 exciton Eg In QDs: 1 photon = multiple excitons Impact ionization The thermalization of the original electron-hole pair creates another pair. Absorption of one photon of light creates one electron-hole pair, which then relaxes to the band edges.
- 35. QDs: Multiple ExcitonGeneration 1 2 3 4 5 Photon Energy (Ehv/Eg) QuantumEff(%) 100 150 200 250 300 Quantum efficiency for exciton generation: The ratio of excitons produced to photons absorbed >100% means multiple exciton generation Occurs at photon energies (Ehv) much greater than the band gap (Eg)
- 36. QDs: Intermediate Bands Eg Intermediate bandformed by an array of QDs Conventional band structure does not absorb light with energy < Eg Intermediate bands in the band gap allow for absorption of low energy light
- 37. P3HT:CdSe Solar Cells J.Am. Chem. Soc., 2004, 126 (21), 6550.
- 38. CdSe Sensitizers/Nano TiO2 “RainbowCell” 2.3 2.6 3.0 3.7 nm J. Am. Chem. Soc., 2008, 130 (12), 4007.
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- 40. from informaworld.com Carbon nanotubes Rolledup sheet of sp2 bonded carbon atoms
- 41. 1-D Electronic Structures:Carbon Nanotubes Wrapping vector: 2211 anann Diameter: 0.0783nm21 2 2 2 1 nnnnd The folding of the sheet controls the electronic properties of the nanotubes.
- 42. D = diameterof the nanotube The component of the wave vector perpendicular to the CNT long axis is quantized D n k 2 Metallic behavior: the allowed values of intersect the k points at which the conduction and valence bands meet. CNTs can behave either like metal or semiconductors depending on their chirality. k
- 43. Development of electronicproperties as a function of cluster size Each band has a width that reflects the interaction between atoms, with a bandgap between the conduction and the valence bands that reflects the original separation of the bonding ad antibonding states.
- 44. pz electrons hybridizeto form π e π* valence and conduction bands that are separated by an energy gap of about 1V (semiconductor). For certain high simmetry directions (the K points in the reciprocal lattice) the material behaves like a metal. Conduction in CNTs Apex: at this point CB meets VB for graphene sheets (metal-like behavior) Allowed statesK k wave vector perpendicular to the CNT long axis
- 45. Field effect transistormade from a single semiconducting CNT connecting source and drain connectors.
- 46. Semiconductor Nanowires • Ga-P/Ga-Asp/n nanojunctions Copyright Stuart Lindsay 2009 (IOP) TEM images Line profiles of the composition through the junction region
- 47. from rice.edu Single walled carbonnanotube (single sheet of carbon atoms) Multiwalled carbon nanotube (several sheets of carbon atoms) Carbon nanotubes can be formed from a single sheet of C atoms or several sheets
- 48. Carbon sheets canalso be rolled up in different directions to give different types of nanotubes.
- 49. from phycomp.technion.ac.il Electrical conductor Electricalinsulator The properties of nanotubes depend on how they are rolled up
- 50. Nanotube sizes alsodepend on how they are rolled up
- 51. Carbon Nanotubes Applications Carbonnanotubes are strong and light at the same time. It makes them a perfect choice for using as reinforcing additive in composite fibers, to make them extremely strong and electrically conductive simultaneously. There is enormous potential for the nanotubes to used in the design and development of space elevators, bulletproof wear, clothing and others. Also, the sufficient electrical and thermal conductivity of carbon nanotubes makes them potentially useful in revolutionizing the industry with use in solar cells, as sensors, in batteries, and in transistors.
- 52. Carbon Nanotubes • 4nm width (smaller diameter than DNA) • 100x’s stronger than steel 1/6 weight • Thermal/electrically conductive • Metallic and Semi-Conductive
- 53. Graphene Graphene is classifiedas a semi-metal and is an allotrope of carbon. It is a 2-dimensional, single layer of graphite, made of 6 carbon molecules join with others to form a perfect hexagonal lattice-shaped structure. Incidentally, graphene is the fundamental structural element of other carbon allotropes, including carbon nanotubes. Graphene is extremely malleable and elastic. Since it is only one- atom-thick, graphene is nearly transparent however it is also an excellent electrical and thermal conductor. Graphene is very thin and flexible yet is highly conductive. Hence, it finds potential application as a transparent conductor for use in photovoltaic cells and other types of flexible electronic devices. Also, its larger ratio of surface area to mass makes it excellent potential for use in energy storage or chemical sensing.
- 54. BuckyBalls – C60 •Roundest and most symmetrical molecule known to man • Compressed – becomes stronger than diamond • Third major form of pure carbon • Heat resistance and electrical conductivity
- 55. Top - Down method Highenergy Ball Milling Lithography Gas condensation Severe plastic deformation Synthesis of Nanomaterials
- 56. Synthesis of Nanomaterials Top -down method (Destruction) Bottom-up method (Construction) Synthesis of Nanomaterials To synthesize nanomaterials, either to assemble atoms together or to dis-assemble (Break or dissociate) bulk solids into finer pieces until they are constituted of only few atoms.
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- 59. Mechanical Grinding (PhysicalMethod) Mechanical milling is achieved using high energy shaker, planetary ball, or tumbler mills Energy transferred to the powder from refractory depends on the • Rotational speed, size and number of the balls • Ratio of the ball to powder mass • The time of milling • Milling atmosphere Nanoparticle are produced by the shear action during grinding.
- 60. Wet Chemical Synthesis(Chemical Method) Wet Chemical Synthesis Top down method (Electrochemical etching) Bottom up method (Sol-gel) • In Top down method single crystals are etched in an aqueous solution for producing nanomaterials, e.g., synthesis of porous silicon by electrochemical etching. • In Bottom up method (Sol-gel, precipitation etc.) materials containing the desired precursors are mixed in a controlled fashion to form a colloidal solution.
- 61. Sol-gel process The sol-gelprocess may be described as: ”Formation of network (oxide) through poly- condensation reactions of a molecular precursor in a liquid.” One of the highly attractive features of the sol-gel process is the possibility to shape the material into any desired form such as monoliths, films, fibers, and monosized powders, and subsequently to convert it into a ceramic material by heat treatment.
- 62. What is aSol? A sol is a stable dispersion of colloidal particles or polymers in a solvent. A sol consists of a liquid with solid particles which are not dissolved, but do not agglomerate or sediment. The particles may be amorphous or crystalline. (An aerosol is particles in a gas phase, while a sol is particles in a liquid). Agglomeration of small particles are due to van der Waals forces and a tendency to decrease the total surface energy. Van der Waals forces are weak, and extend only for a few nanometers. In order to counter the van der Waals interactions, repulsive forces must be established. May be accomplished by: Electrostatic repulsion. By adsorption of charged species onto the surface of the particles, repulsion between the particles will increase and agglomeration will be prevented. Most important for colloidal systems. Steric hindrance. By adsorbing a thick layer of organic molecules, the particles are prevented from approaching each other reducing the role of the van der Waals forces. Works best in concentrated dispersions. Branched adsorbates works best. Usual for nanomaterials.
- 63. What is agel? A gel consists of a three dimensional continuous network, which encloses a liquid phase. In a colloidal gel, the network is built from agglomeration of colloidal particles. In a polymer gel the particles have a polymeric sub-structure made by aggregates of sub- colloidal particles. Generally, the sol particles may interact by van der Waals forces or hydrogen bonds. A gel may also be formed from linking polymer chains. In most gel systems used for materials synthesis, the interactions are of a covalent nature and the gel process is irreversible. The gelation process may be reversible if other interactions are involved.
- 64. Why sol-gel synthesis? Sol-gelsynthesis may be used to prepare materials with a variety of shapes, such as porous structures, thin fibers, dense powders and thin films.
- 65. Sol gel synthesis •Theidea behind sol-gel synthesis is to “dissolve” the compound in a liquid in order to bring it back as a solid in a controlled manner. •Multi component compounds may be prepared with a controlled stoichiometry by mixing sols of different compounds. •The sol-gel method prevents the problems with co- precipitation, which may be inhomogeneous, be a gelation reaction. •Enables mixing at an atomic level. •Results in small particles, which are easily sinterable.
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- 67. Depending on thenature of the precursors sol-gel process can be of two type: a) the precursor is an aqueous solution of an inorganic salt or, b) a metal organic compound
- 68. Aqueous sol-gel Chemistry Conversionof a precursor solution into an inorganic solid via inorganic polymerization reactions induced by water. In general, precursor (or starting compound) is an inorganic metal salt (chloride, nitrate, sulfate etc) or a metal organic compound such as an alkoxide. (Alkoxides of Si, Ti, Al, Zr, (or, Fe, Ni, Co, Mn, Cu, Y, Nb, Ta… ) The sol-gel process consists of the following steps: (i) Preparation of a homogeneous solution either by dissolution of metal organic precursors in an organic solvent that is miscible with water, or by dissolution of inorganic salts in water; ii) conversion of the homogeneous solution into a sol by treatment with a suitable reagent (generally water with or without any acid/base); iii) aging; iv) shaping; and v) thermal treatment/sintering.
- 69. Aqueous sol-gel Chemistryp.2 The first step in a sol-gel reaction is the formation of an inorganic polymer by hydrolysis and condensation reactions, i.e., the transformation of the molecular precursor into a highly crosslinked solid. Hydrolysis leads to a sol, a dispersion of colloidal particles in a liquid, and further condensation results in a gel, an interconnected, rigid and porous inorganic network enclosing a continuous liquid phase. This transformation is called the sol-gel transition. There are two possibilities to dry the gels. Upon removal of the pore liquid under hypercritical conditions, the network does not collapse and aerogels are produced. When the gel is dried under ambient conditions, shrinkage of the pores occurs, yielding a xerogel.
- 70. Xerogel and Aerogel •If the gel is dried by evaporation, then the capillary forces will result in shrinkage, the gel network will collapse, and a xerogel is formed. • If drying is performed under supercritical conditions, the network structure may be retained and a gel with large pores may be formed. This is called an aerogel, and the density will be very low. A record is < 0.005 g/cm3.
- 71. Aqueous sol-gel Chemistryp.3 The sol-gel conversion of metal alkoxides involves two main reaction types: hydrolysis and condensation. During hydrolysis, the alkoxide groups (-OR) are replaced via the nucleophilic attack of the oxygen atom of a water molecule under release of alcohol and the formation of a metal hydroxide. Condensation reactions between two hydroxylated metal species leads to M-O-M bonds under release of water (oxolation), whereas the reaction between a hydroxide and an alkoxide leads to M-O-M bonds under release of an alcohol (alkoxolation).
- 72. Aqueous sol-gel Chemistryp.4 • The chemical reactivity of metal alkoxides towards hydrolysis and condensation depends mainly on the electronegativity of the metal atom, its ability to increase the coordination number, the steric hindrance of the alkoxy group, and on the molecular structure of the metal alkoxides (monomeric or oligomeric). The amount of added water in the hydrolysis step and how the water is added, determines, whether the alkoxides are completely hydrolyzed or not and which oligomeric intermediate species are formed. Additional parameters are the polarity, the dipole moment, and the acidity of the solvent.
- 73. Non –aqueous sol-gelChemistry Nonaqueous processes can be divided into two general methodologies: Surfactant directed and Solvent controlled. • Surfactant-Directed vs. Solvent-Controlled Surfactant-controlled synthesis routes involve the transformation of the precursor species into the oxidic compound in the presence of stabilizing ligands in a typical temperature range of 250 to 350 ◦C. Alternative to surfactants is the use of common organic solvents, which act as reactant as well as control agent for particle growth, enabling the synthesis of high-purity nanomaterials in surfactant-free medium. In comparison to the synthesis of metal oxides in the presence of surfactants, the solvent-controlled approaches are simpler, because the initial reaction mixture just consists of two components, the metal oxide precursor(s) and a common organic solvent.
- 74. Non –aqueous sol-gelChemistry p.2
- 75. Non –aqueous sol-gelChemistry p.3 Equation 1 displays the condensation between metal halides and metal alkoxides (formed upon the reaction of metal halides with alcohols) under release of an alkyl halide. One of the early examples was the preparation of anatase nanocrystals from titanium isopropoxide and titanium chloride. Eq 2 (Ether elimination) leads to the formation of a M–O–M bond upon condensation of two metal alkoxides under elimination of an organic ether. This mechanism was reported for the formation of hafnium oxide nanoparticles. The ester elimination process involves the reaction between metal carboxylates and metal alkoxides (eq 3), as reported for zinc oxide, titania,and ndium oxide. Analogous to ester eliminations are amide eliminations. Reacting metal oleates with amines, for example, enabled the controlled growth of titania nanorods. In the case of ketones as solvents, the release of xygen usually involves aldol condensation, where two carbonyl compounds react with each other with elimination of water. The water molecules act as the oxygen-supplying agent for metal oxide formation (eq 4). Literature examples include the synthesis of ZnO and TiO2 in acetone.
- 76. Liquid solid reactions(precipitation) • In this method, ultrafine particles of the desired materials are produced by precipitation from a solution. The presence of desired nuclei in the solution is a necessary condition to initiate the process. TiO2 powders with particle sizes in the range 70-300 nm have been prepared from titanium tetraisopropoxide by this method. Similarly ZnS powders may be produced by reaction of aqueous zinc salt solutions with thioacetamide. Hydrothermal synthesis • It is based on the ability of water an aqueous solutions to dilute the substances at high temperatures and pressures, which are practically insoluble under normal condition. These include oxides, silicates and sulphides. Autoclaves are used for this purposes because they can withstand high temperature and pressure for long period of time. The possible advantages of the process over the other are the ability to create crystalline phase that are not stable at the melting point. This method is used for the synthesis of materials which have high vapor pressure near their melting point.
- 77. Gas phase synthesisof nanomaterials • This method allow elegant way to control process parameter in order to be able to produce size, shape, and chemical composition controlled nanostructures. • In conventional chemical vapor deposition (CVD) synthesis, gaseous products either are allowed to react homogeneously or heterogeneously depending on a particular application. 1. In homogeneous CVD, particles form in the gas phase and diffuse towards a cold surface due to thermophoretic forces, and can either be scrapped of from the cold surface to give nano powders or deposited on to a substrate to yield what is called ‘particulate films’. 2. In heterogeneous CVD, the solid is formed on the solid surface, which catalyses the reaction and a dense film is formed.
- 78. Advantages of gasphase processes • An excellent control of size, shape, crystallinity and chemical composition . • Highly pure materials can be obtained • Multicomponent systems are relatively easy to form • Easy control of the reaction mechanisms Most of the synthesis routes are based on the production of small clusters that can aggregate to form nano particles.
- 79. Schematic representation ofgas phase process of single phase nanomaterials from a heated crucible
- 80. Flame assisted ultrasonicspray pyrolysis • In this process precursors are nebulized and then unwanted components are burnt in a flame to get required materials. ZrO2 has been obtained by this method from a precursor of Zr(CH3 CH2 CH2O)4. • This process is mainly used for synthesis of fumed silica. • Silicon tetrachloride is heated in an oxy-hydrogen flame to give highly dispersed silica. The resulting amorphous powder consists of spherical particles with sizes in the range 7-40 nm. • In combustion flame synthesis mixture of acetylene and oxygen or hydrogen and oxygen supplies the energy to initiate the pyrolysis of precursor. This process is widely used for industrial production of powders in large quantities, such as carbon black, fumed silica, and titanium dioxide. • At high pressure, highly agglomerated powders are produced which is disadvantageous for subsequent processing. The basic ides for low pressure flame synthesis is to reduce agglomeration. Aerosol scientists widely use this process. • Exact control of the flame produces a flat flame front. Due to oxidative atmosphere of the flame, this synthesis process is limited to the formation of the oxides.
- 81. Schematic diagram offlame assisted ultrasonic spray pyrolysis
- 82. Chemical vapour condensation(CVC) • This technique involves pyrolysis of vapors of metal organic precursors in a reduced pressure atmosphere. • Nanoparticles of ZrO2, Y2O3, have been produced by this method. • The evaporative source used in GPC is replaced by a hot reactor in the Chemical vapor condensation method. • Nanosized particles metals and ceramics can be obtained in good amounts by adjusting the residence time of the precursor molecules, changing the gas flow rate, pressure difference between the precursor delivery system and the main chamber and the temperature of the reactor. • The production capacities in CVC process are much higher than GPC system.
- 83. • In additionto the formation of nanoparticles of a single precursor The CVC reactor can also be used for the synthesis of mixture of nanoparticles of two phases by supplying two precursors at the front end of the reactor, and • Coated nanoparticles like n-ZrO2 coated with n-Al2O3 or vice versa, by supplying a second precursor at the second stage of the reactor.
- 84. Schematic diagram ofset up for GPC system followed by consolidation in a mechnical press or collection in an appropriate solvent media
- 85. Laser ablation • Thismethod is extensively used for preparation of nanoparticles and particulate films. • In this process a laser beam is used as the primary excitation source of ablation for generating clusters directly from solid sample. • The small dimension of the particles and the possibility to form thick films make this process an efficient tool for the production of ceramic particles and coatings. • Synthesis of ZrO2, SnO2 nanoparticls thick films are synthesized using this process. • Synthesis of other materials such as lithium manganate, silicon and carbon has also been carried out by this technique.
- 86. Thermolysis • This methodinvolves solvent free pyrolysis of a suitable precursor material. One of the oldest nanomaterials, activated charcoal is prepared using this method. Precursor materials are pyrolyzed at 600-900 °C in the absence of air to obtain charcoal of specific surface area. • In the process the materials are carbonized in oxidizing atmosphere of CO2,O2 or steam at 600-1200 °C and activation is carried using chemicals such as KOH, NaOH, ZnCl2 or phosphoric acid.
- 87. Solvothermal synthesis • Thisis a versatile method for synthesizing nanomaterials at temperatures below 200 C. This technique enables synthesis of crystalline products at low temperature and also helps in control the size and morphology of the resultant products. Electrodeposition • This technique involves creation of solid materials from electrochemical reactions in liquid. A conducting substance is placed in liquid containing electrolytes. When a potential is applied, redox reaction takes place and the material is deposited as thin film at the cathode. Semiconductor ZnO nanotube arrays are synthesized by direct electrochemical deposition from aqueous solutions into the pore channels of anodically-formed alumina .