Nano materials

Nano materials Module2 Nanoscale Nanoscale Nano - Dwarf Nano size: nm = 10 -6 millimeter (mm) = 10 -9 meter (m) nm Combination of atoms or molecules to form objects of nanometer scale Cross section of human hair Scale •Nano-materials: Used by humans for 100 of years, the beautiful ruby red color of some glass is due to gold Nano particles trapped in the glass (ceramic) matrix • The decorative glaze known as luster Ruby Red glass pot (entrapped with gold nanoparticles) What’s special with Nano? The properties of nanomaterials deviate from those of single crystals or polycrystals (bulk) For example, the fundamental properties like electronic, magnetic, optical, chemical and biological Surface properties: energy levels, electronic structure, and reactivity are different for nano materials Exhibit size dependent properties, such as lower melting points, higher energy gaps etc On the Surfaces and interfaces basics: Bulk In bulk materials, only a relatively small percentage of atoms will be at or near a surface or interface (like a Nano In nanomaterials, large no of atomic features near the interface crystal grain boundary) Nanostructured materials • • • • • Nanoparticles Nanowires Nanotubes Nanorods Nanoporous materials Bulk and Nanoscale Density of states for 3D, 2D, 1D, 0D showing discretization of energy and discontinuity of DOS Size variation Various size of CdSe nanoparticles and their solution The bulk CdSe is black Effects of Nano size • • • • • • • • • • Properties depends on size, composition and structure Nano size increases the surface area Change in surface energy (higher) Change in the electronic properties Change in optical band gap Change in electrical conductivity Higher and specific catalytic activity Change thermal and mechanical stabilities Different melting and phase transition temperatures Change in catalytic and chemical reactivities Bulk and Nanoscale Bulk (eg Gold) Nano (eg Gold) o High melting point (1080 C) 8 Tough with high tensile strength 25 nm — Red reflected Inert-unaffected by air and most reagents 50 nm — Green reflected Lustrous–Shiny surface when polished Malleable–Can be hammered, bent or rolled→any desired shape Ductile–Can be drawn out into wires Yellow colour when in a mass Heat & electricity conductor High densities Vary in appearance depending on size & shape of cluster Are never gold in colour! Are found in a range of colours Are very good catalysts Are not “metals” but are semiconductors Melts at relatively low temperature (~940º C) Size & Shape of the nanoparticles determines the color For example; Gold particles in glass: (Unexpected visible properties & they are small enough to scatter visible light rather than absorb) CNT • Tubular structure MWNT & SWNT High resolution scanning tunneling micrograph of two single walled nanotubes High resolution Transmission electron micrograph of two multi walled nanotubes SEM of spun CNT fibers §4.3.2 Chemical bonding Structure Fig.8 Two or more nested tubes of CNTs Comparative sheet live structures are also given sp boned carbon sp boned carbon Fig Chemical structure of graphite (A) and CNT (B) Both having sp bonded carbons 50 CNT Synthesis •Preparation by Arc Method (as like C60, but experimental conditions are different) by graphite electrodes •Conditions: • Larger amount of He gas (0.7 atm pressure/500 torr) • Distance between the graphite electrode ~1mm • Arc evaporation of graphite with He or Ar or CH4 or H2 (effective) • Maintaining Plasma condition •Carbon fibre like deposit on the –ve graphite electrode 51 CNT synthesis – Electric arc method CNT synthesis Use of double furnace – catalytic chemical vapour deposition (CCVD) Organometallic/hydrocarbon copyrolysis a – sublimation of precursor b – decompostion of precursor and growth on the substrate at high temperature furnace c – densly packed and aligned MWNT grown by CCVD • Highly crytalline Multiwalled carbon nanotube (MWNT) by the arc method with liquid N (Arc submerged in): – – – • Vacumm is replaced with liq N in the chamber After the arc discharge, carbon deposits near the –ve electrodes→not sticked Reaction product ~70% MWNT Chemical vapour deposition (CVD): – – – – Carbon source in the gas phase and plasma with resistively heated coil → transfer to carbon molecule Common carbon sources: CO, Methane and Acetylene Energy source cracks the molecule into atomic carbon → diffuse towards heated coil (Ni, Fe or Co) and bind on it Two steps involved: • • • (1) Catalyst Preparation (Fe, Ni, Co or alloys) (2) Actual CVD (Yeild ~30%) Other CVD methods: Plasma-enhanced CVD, alcohol catalytic CVD, Aero-gel supported CVD and Laser-assisted CVD 54 CNT synthesis • Template technique - Catalyst free formation of CNT SWNT Purification • Arc method synthesized SWNT always have impurity of the metal catalyst particles – Other impurities are; soot, amorphous carbon & smaller fullerenes • Strong oxidation & Acid Refluxing techniques are commonly used in the Industry for the cleaning • Methods: Structure selective • Specific Techniques: • Oxidation: Eg treatment with H2O2 and H2SO4 Size selective Good Way to remove carbonaceous impurities or to clear the metal on the surface 56 • Limitations: – Oxidation of both impurities & SWNTs – Experimental conditions must be controlled • Damage to SWNT is relatively less than the damage to the impurities • The process depend upon: – Metal impurity content – Oxidation timings – Environment – Oxidizing agent and Temperature • Example: The H2O2 and H2SO4 → can clean the metal surface • If O2 present in the medium → rupture the CNT Metal (M) CNT MOx + MOx oxidized 57 • Acid Treatment: – – – • High temperature vacuum treatment (1873 K) → the metal will be melted and removed Metal & Impurities are separated due to the strong vibrations Solvent & surfactant having critical role in the process Time also a controlling factor Magnetic Purification: – • Egs HNO3 or 4M HCl Ultrasonication: – – – • + No effect to CNT; H only effect to metal Annealing (heating): – • Will remove the metal catalyst (impure) Ferromagnetic (catalytic) particles are mechanically removed Micro-filtration: – Using CS2 as solvent (fullerenes soluble) → filtered 58 CNT applications • • • • • CNT composites used for electrostatically applying paints to the car components They can form transparent, conducting and flexible polymer composite Used in field emission device Used in tips of atomic force microscopy CNT with higher charge carrier mobility (20000 cm2 / V s) finds application in transistors, replacing the metal oxides CNT applications • • • • • Precise mass and charge measurements ( Addition of single atom or charge to CNT is detected by measuring the change in resonant frequency) Large non linear absorption of Light( NLO material) Used in optical switches In Lithium ion batteries: N-doped CNT show good Li+ ion storage capacity than normal graphite Sensors: N doped MWNT shows fast response and organic solvents (milliseconds) to toxic gases CNT • In field effect transistors Infineons vertical CNT-FET conncept Metal oxide give high power dissipation, with CNT this is avoided CNT can replace Si, Ge, InSb CNT • SWNT attached to the tip of the Si cantilever for atomic force microscopy ( nanoprobes) [...]... change the optical properties of nano sized gold §4.1.1.1 Gold nanoparticles Red Yellow Size increase Size increase Green Blue Fig 1 Size and shape dependent colors of Au & Ag Orange Brown nanoparticles 12 nanotubes Au - Nanocrystals Au - Nanorods Au - Nanotubes Different shapes of gold nanostructures were prepared using different methods Silver : Bulk - Nano • In nano size not only the surface area... conduction band SP band provides some information regarding NPs band structure Silicon Si - SiO2 - Nanotubes SiO2 - Nanospheres SiO2 - Nanotubes Nanomaterials synthesis approach 1.Top down approach: Breaking of bulk material 2.Bottom approach: Build up of material Atom→molecule→cluster Preparation Nanomaterials preparation Physical Methods Chemical Methods Ball milling Sol-gel synthesis Gas condensation... improve the selectivity in catalysis Silver nanowires Silver nanocubes Silver nanoparticles Change in shape and size shows the difference in the visible spectrum of Ag nanoparticles The scattering or absorption is due to the localized surface plasmon resonance SPR • • Surface plasmon resonance SPR: The collective oscillations of the electron gas at the surface of nanoparticles ( eg 6s electrons of the conduction... parameter to control the size of nanoparticle here (gas flow rate, pressure, heating temperature can be controlled) Fig 5 A schematic of a typical CVC reactor •Other procedure similar to GPC Production capabilities are much larger than in the GPC processing 26 Preparation Tubular furnace for synthesis of nanomaterials, nanowires by Chemical vapour deposition The precursor materials were converted to desired... on starting materials and reaction conditions Wet chemical synthesis • Preparation nanoparticles HAuCl4 + Stabilizing agent + NaBH4 Au nanoparticles AgNO3 + Stabilizing agent + NaBH4 Ag nanoparticles HAuCl4 + AgNO3 + Stabilizing agent + NaBH4 AuAg alloy NPs Stabilizing agents – Sodium citrate, Alkanethiols, alkylammonium salts, R- amines, R-COOH, surfactants etc • Citrate method of Au nanoparticle... Preparation of different types of nanostructures Stabilizing agents - eg., Citrate, Thiols, Amines, Carboxylates, Phosphine, Phosphine oxide, Surfactants, coordinating polymer Coordination of stabilizing agents with the nanostructures Stability of the nanostructures depends on the chemical nature of the stabilizing agents too Control on the composition, size, shape of the nanostructures Larger control... When catalyst is used it is known as catalytic chemical vapour deposition (CCVD) Preparation - Chemical Methods (Bottom-up approaches): Wet Chemical Synthesis of nanomaterials (Sol-gel Process) 1 Very popular & widely employed to prepare oxide materials (SiOx) 2 The sol-gel process: formation of a colloidal suspension (sol)→ gelation of the sol to form a network in a continuous liquid phase (gel) →solid... vaporization cluster beam is used→ preparing nanoparticle web-like structure 3 2 High energy pulsed laser (107 W/cm , under After Ablation plasma) focused on the analyst substrate→ can generate substrate vapor 2 -8 (1014-1015 atoms/0.01cm /10 pulse) and T = 104K→ liquefaction process → nanoparticles After Ablation Picture of metal – Ablation 3 ZrO2 and SnO2 nanoparticulates thick films were synthesized... Fig 4 Schematic representation of typical set-up for gas condensation synthesis of nanomaterials followed by consolidation in a mechanical press or collection in an appropriate solvent media •These methods allow for the continuous operation of the collection device and are better suited for larger scale synthesis of nanopowders •However, these methods can only be used in a system designed for gas flow,... Nanoparticles Metal in Metal cluster crucible (gaseous state) deposits (2-50nm) Homogenous nucleation in gas phase scrapping Collection of the nanoparticles Advantages of Gas Phase synthesis * An excellent control of size, shape, crystallinity and chemical composition * Highly pure materials can be obtained * Multicomonent systems are relatively easy to form * Easy control of the reaction mechanisms •Major advantage ... (like a Nano In nanomaterials, large no of atomic features near the interface crystal grain boundary) Nanostructured materials • • • • • Nanoparticles Nanowires Nanotubes Nanorods Nanoporous materials. .. materials Nanomachining Nanodevices Nanolithography Magnetic storage disk materials Thermoelectric materials Piezoelectric materials Nanoelectrodes Carbon nanotubes (CNT) Carbon nanotubes Carbon Nanotubes... of nano sized gold §4.1.1.1 Gold nanoparticles Red Yellow Size increase Size increase Green Blue Fig Size and shape dependent colors of Au & Ag Orange Brown nanoparticles 12 nanotubes Au - Nanocrystals