Nanomaterials Module2

Nanoscale

Cross section of human hair Nano - Dwarf Nano size: 1 nm = millimeter (mm) = meter (m) nm

Scale

5 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) Bulk. In bulk materials, only a relatively small percentage of atoms will be at or near a surface or interface (like a crystal grain boundary). crystal grain boundary Nano. In nanomaterials, large no. of atomic features near the interface. 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:

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

Effect of Nano size Nano size increase the surface Change in surface energy Change in the electronic properties Change in optical band gap Change in electrical conductivity Higher and specific catalytic activity

Bulk and Nanoscale Bulk (eg. Gold)Nano (eg. Gold) 1.Lustrous–Shiny surface when polished. 2.Malleable–Can be hammered, bent or rolled  any desired shape. 3.Ductile–Can be drawn out into wires 4.Yellow colour when in a mass 5.Heat & electricity conductor 6.High densities 7. High melting point (1080 o C) 8. Tough with high tensile strength 9. Inert-unaffected by air and most reagents 1.Vary in appearance depending on size & shape of cluster. 2.Are never gold in colour!. 3.Are found in a range of colours. 4.Are very good catalysts. 5.Are not “metals” but are semiconductors. 6.Melts at relatively low temperature (~940º C). 7.Size & Shape of the nanoparticles determines the color. 8.For example; Gold particles in glass: 25 nm—Red reflected 50 nm—Green reflected (Unexpected visible properties & they are small enough to scatter visible light rather than absorb)

Optical properties of Gold NPs Size, shape, change the optical properties of nano sized gold

12 § Gold nanoparticles Size increase Red Green Orange Yellow Blue Brown Fig 1. Size and shape dependent colors of Au & Ag nanoparticles

nanotubes Au - Nanotubes

Silver : Bulk - Nano In nano size not only the surface area increased the electronic properties are modified. This influence the optical, electrical, catalytic properties It improve the selectivity in catalysus Silver nanocubes

Silicon SiO 2 - 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 Ball milling Laser ablation Gas condensation processing (GPC) Chemical Methods Sol-gel synthesis Solution phase ( stabilizing Ligands) Precipitation method Chemical vapour condensation Catalytic chemical vapour deposition Template assisted CVD Electrochemical method

Preparation Any Preparation technique should provide: 1.Identical size of all particles (mono sized or uniform size distribution). 2.Identical shape or morphology. 3 Identical chemical composition and crystal structure. 4 Individually dispersed or mono dispersed i.e., no agglomeration.

19 Schematic representation of the principle of mechanical milling. Violent or agitation, ~50  m  nm High-Energy ball milling (Top down approach) : *Interest in the mineral, ceramic processing, and powder metallurgy industry. * Involves milling process include particle size reduction. * Restricted to relatively hard, brittle materials which fracture and/or deform during the milling operation. * Different purposes including; tumbler mills, attrition mills, shaker mills, vibratory mills, planetary mills, etc. *Hardened steel or tungsten carbide (WC) coated balls  the basic process of mechanical attrition (rubbing away). Preparation – Physical method

Preparation – physical method Limitation of Ball milling: (Even though high production rates) 1.Severe plastic deformation associated with mechanical attrition due to generation of high temperature in the interphase, 100 to 200º C. 2.Difficulty in broken down to the required particle size. 3.Contamination by the milling tools (Fe) and atmosphere (trace elements of O2, N2, in rare gases) can be a problem. (inert condition necessary)

21 (B) Gas Condensation Processing (GPC)-Bottom-up approach: Nanoparticles deposits (2-50nm) Metal in crucible Thermal or electric or e- beam evaporation (like PVD) Metal cluster (gaseous state) Homogenous nucleation in gas phase Cooling (Rotating cylinder) Liquid N 2 (-80 o C) Collection of the nanoparticles scrapping General Scheme for GPC for the nanoparticle synthesis Preparation – Physical method

22 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. Major advantage over conventional gas flow is the improved control of the particle sizes. 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, i.e. a dynamic vacuum is generated by means of both continuous pumping and gas inlet via mass flow controller. Limitation:- 1.Control of the composition of the elements has been difficult and reproducibility is poor. 2.Oxide impurities are often formed. The method is extremely slow.

23 Fig. 5 A schematic of a typical CVC reactor Involves pyrolysis (heat treatment) of vapors of metal organic precursors (starting materials) like Hexamethyldisilazane (CH 3 ) 3 Si-NH- Si-(CH 3 ) 3 to produce SiC x N y O z. Evaporate source in the GPC is replaced by a hot wall reactor in the CVC process (Fig.5). Precursor residence time is the key parameter to control the size of nanoparticle here (gas flow rate, pressure, heating temperature can be controlled). Other procedure similar to GPC. Production capabilities are much larger than in the GPC processing. (C) Chemical Vapour Condensation (CVC) (Bottom-up approach): Preparation

Tubular furnace for synthesis of nanomaterials, nanowires by Chemical vapour deposition

25 Preparation - Chemical Methods (Bottom-up approachs): 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. 3. Metal or metalloid element surrounded by various reactive ligands (Si(OCH 3 ) 4, tetramethoxy silane, TMOS, alkoxide) is the reactant. 4. The starting material is processed to form a sol in contact with water or dilute acid. Removal of the liquid from the sol yields the gel, and the sol/gel transition controls the particle size and shape. Calcination of the gel produces the product (eg. Oxide).

Sol – Gel synthesis

Sol-Gel Classic sol-gel reaction scheme Sol-gel processing refers to the hydrolysis and condensation of alkoxide-based precursors such as Si(OEt) 4 (tetraethyl orthosilicate, or TEOS). The reactions involved in the sol-gel chemistry based on the hydrolysis and condensation of metal alkoxides can be described as follows:

28 Over all Steps: Step 1: Formation of different stable solutions of the alkoxide (the sol). Step 2: Gelation resulting from the formation of an oxide- or alcohol- bridged network (the gel) by a polycondensation or polyesterification reaction Step 3: Aging of the gel, during which the polycondensation reactions continue until the gel transforms into a solid mass, accompanied by contraction of the gel network and expulsion of solvent from gel pores. Step 4: Drying of the gel, when water and other volatile liquids are removed from the gel network. – If isolated by thermal evaporation, the resulting monolith is termed a xerogel. – If the solvent (such as water) is extracted under supercritical or near super critical conditions, the product is an aerogel. Step 5: Dehydration, during which surface- bound M-OH groups are removed, there by stabilizing the gel against rehydration. This is normally achieved by calcining the monolith at temperatures up to C. Step 6: Densification and decomposition of the gels at high temperatures (T>800 0 C). The pores of the gel network are collapsed, and remaining organic species are volatilized. The typical steps that are involved in sol-gel processing are shown in the schematic diagram below.

Wet chemical synthesis Use of chemical stabilizing agents Preparation of different types of nanostructures Stabilizing agents - eg., Citrate, Thiols, Surfactants, coordinating polymer Use of different reducing agents 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 on the reaction rates during preparation

Wet chemical synthesis Preparation nanoparticles HAuCl 4 + Stabilizing agent + NaBH 4 Au nanoparticles AgNO 3 + Stabilizing agent + NaBH 4 Ag nanoparticles HAuCl 4 + AgNO 3 + Stabilizing agent + NaBH 4 AuAg alloy NPs Stabilizing agents – Sodium citrate, Alkanethiols, alkylammonium salts, R- amines, R-COOH, surfactants etc

31 Nanomaterials are produced by precipitation from a solution. The method involves high degree of homogenization and low processing temperature. ZnS powders were produced by reaction of aqueous zinc salt solutions with thioacetamide (TAA). Precursor zinc salts were chloride, nitric acid solutions, or zinc salts with ligands (i.e., acetylacetonate, trifluorocarbonsulfonate, and dithiocarbamate). Eg.1: The 0.05 M cation solution was heated in a thermal bath maintained at 70° or 80 °C in batches of 100 or 250 ml. Acid was added dropwise to bring it to a pH of 2. The reaction was started by adding the TAA to the zinc salt solution, with the molar ratio of TAA and zinc ions being set to an initial value of either 4 or 8. Precipitation method

Applications of nanomaterials In major view nanomaterials has found their application in many major areas Electronics Medicine Industries Environment Sensing

Applications of nanomaterials Catalysis Nanotransitors, Field effect transistors Field emission Scanning probes in STM MEMS devices Hydrogen storage Energy conversion devices Nanomedicine Chemical, bio, pressure, thermal sensors Magnetic materials

Applications of nanomaterials Gaint magnetic resistance materials Nanomachining Nanodevices Nanolithography Magnetic storage disk materials Thermoelectric materials Piezoelectric materials Nanoelectrodes

35 Applications of nanomaterials Carbon Nanotubes (CNTs)/Basics Fig. 6 Prof. Iijuma (Japan) with a CNT model. ~1 mm nm Fig.7 Shape and structure of Carbon nanotube (SWNT). * Discovered accidently during bulk preparation of C 60 by the arc method. *Graphite carbon needles grew on the -ve side carbon electrode (arc method) *CNT also member of Fullerene structural family.

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

38 §4.3.2 Chemical bonding Structure sp 2 boned carbon Fig. 9. Chemical structure of graphite (A) and CNT (B). Both having sp 2 bonded carbons. Fig.8 Two or more nested tubes of CNTs. Comparative sheet live structures are also given.

39 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 CH 4 or H 2 (effective) Maintaining Plasma condition Carbon fibre like deposit on the –ve graphite electrode.

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

42 Highly crytalline Multiwalled carbon nanotube (MWNT) by the arc method with liquid N 2 (Arc submerged in): – Vacumm is replaced with liq. N 2 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.

CNT synthesis Template technique - Catalyst free formation of CNT

44 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: Specific Techniques: 1. Oxidation: Eg. treatment with H 2 O 2 and H 2 SO 4 Good Way to remove carbonaceous impurities or to clear the metal on the surface. 1. Structure selective 2. Size selective

45 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 H 2 O 2 and H 2 SO 4  can clean the metal surface If O 2 present in the medium  rupture the CNT CNT Metal (M)MOx + MOx oxidized

46 2. Acid Treatment: – Will remove the metal catalyst (impure) – No effect to CNT; H + only effect to metal – Egs. HNO 3 or 4M HCl 3. Annealing (heating): – High temperature vacuum treatment (1873 K)  the metal will be melted and removed 4. Ultrasonication: – Metal & Impurities are separated due to the strong vibrations – Solvent & surfactant having critical role in the process – Time also a controlling factor 5. Magnetic Purification: – Ferromagnetic (catalytic) particles are mechanically removed 6. Micro-filtration: – Using CS 2 as solvent (fullerenes soluble)  filtered