OVERVIEW OF NANOTECHNOLOGY

Transcription

1 OVERVIEW OF NANOTECHNOLOGY Branko N. Popov Center of Electrochemical Engineering Department of Chemical Engineering University of South Carolina Columbia, SC 29208

2 The term nano corresponds to So one nanometer corresponds to one billionth of a meter. meter m m centimeter cm m millimeter mm m micrometer µm m nanometer nm m Nanotechnology as the name suggests is the study of materials of nanodimensions In general study of materials varying in size from 1 to 100 nm

3 Small solids act different Nanomaterials not only differ in size but also in most of the physical, chemical and biological properties. For example the bulk gold is well known as a yellowish solid, but gold of nanosize look more reddish. Nanogold is a very good catalyst too Bulk Gold Yellowish Nano Gold reddish

4 The Scale of Things -- Nanometers and More Things Natural 10-2 m 1 cm 10 mm Things Manmade Ant ~ 5 mm 10-3 m 1,000,000 nanometers = 1 millimeter (mm) Head of a pin 1-2 mm 21 st Century Challenge Dust mite 200 µm Human hair ~ µm wide Fly ash ~ µm The Microworld 10-4 m 10-5 m 0.1 mm 100 mm 0.01 mm 10 mm MicroElectroMechanical Devices µm wide O O O O O O O O O O O P O O O O Red blood cells with white cell ~ 2-5 µm 10-6 m 1,000 nanometers = 1 micrometer (mm) Visible spectrum Red blood cells Pollen grain O S O S O S O S O S O S O S O S ~10 nm diameter The Nanoworld 10-7 m 10-8 m 0.1 mm 100 nm 0.01 mm 10 nm Nanotube electrode Nanotube transistor Combine nanoscale building blocks to make functional devices, e.g., a photosynthetic reaction center with integral semiconductor storage ATP synthase 10-9 m 1 nanometer (nm) DNA ~2-1/2 nm diameter Atoms of silicon spacing ~tenths of nm m 0.1 nm Quantum corral of 48 iron atoms on copper surface positioned one at a time with an STM tip Corral diameter 14 nm Carbon nanotube ~2 nm diameter

5 Classification of nanostructured materials Based on the structure Nanostructured materials vary from zero dimensional atom clusters to three dimensional equiaxed grain structure. Each class has at least one dimension in the nanometer range

6 Atom clusters and filaments are defined as zero modulation dimensionality and can have any aspect ratio from 1 to Multilayered materials with layer thickness in the nanometer range are classified as onedimensionally modulated Layers in the nanometer thickness range consisting of ultrafine grains are twodimensionally modulated The last class is that consisting of three dimensionally modulated microstructure s or nanophase materials R.W. Siegel, Nanophase Materials, Encyclopedia of Applied Physics, vol. 11, VCH Publishers 1994, p 173

7 Types of Nanomaterials Nanotubes Nanocomposites Nanocatalysts Nanofilms Nanomachines

8 Properties of Carbon Nanotubes Inherent structural strength Presence of covalent enplane carbon bonds makes it one of the strongest. Extremely light high strength to density ration. Higher than steel

9 Carbon Nanotubes Properties High electrical and thermal conductivity Corrosion resistant Ideal for composite reinforcements Superconductivity at room temp is achieved Elastic

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11 Applications for carbon nanotubes Reinforcements for Composites Molds and Templates Field Emission Electrodes Quantum Wires Separation and Storage Technology Catalysis

12 Nanocomposites Optical Materials - nanocomposites do not scatter light, so they are currently being investigated for their optical applications, for use as optical thin films Electronic Materials - such as electronic ceramics, devices, and thin films SEM Photograph of Nanotube Epoxy Composite

13 Applications of Nanocomposites Abrasion and Wear Applications Erosion and Corrosion Applications Biotechnology - Nanocomposites have been used to encapsulate enzymes and biomolecules for their use in a drug delivery system This computer- generated simulation shows the building blocks of a layered nanocomposite. A copolymer is intercalated between micalike layers of crystalline silicate. The layers of such a structure are only a nanometer

14 Nanocatalysts Novel hybrid nano-structured materials will lead in development of high performance power sources High energy and power density supercapacitors 1 µm Lower capacity fade metal hydride alloys for electric vehicles High capacity Li-ion cells for consumer electronics Fuel cell assemblies for stationary power generation SEM Photograph of Nanostructured Catalyst

15 Thin Films Thin films are of great significance owing to their small size and less power consumption and numerous applications Properties of Nanocrystalline Diamond Films Drastic increase in surface smoothness. More defects and grain boundaries are present. Better electron emission than conventional diamond films. Less highly oriented grains. Enhanced optical transmission

16 Ref: Yasunori et al, Material Science engineering C 15(2001)

17 Synthesis of Nanomaterials Sol gel synthesis Vapor Condensation Mechanical alloying or high-energy ball milling, Sputtering - Plasma synthesis, and Electrodeposition

18 Sol-gel Technique for Nanoparticle Synthesis Hansung Kim, Branko N. Popov J. of Power Sources In Printing Objective Develop new technology for synthesizing RuO 2 /carbon nano-composite material. Increase specific capacitance and utilization of RuO 2 by decreasing particle size and dispersing evenly over carbon. Improve the power rate at high current discharge.

19 Development of Novel Supercapacitors Based on Hybrid Metal-C Nanoparticles Capacitors deliver frequent pulses of energy in several electronic circuits Electrochemical capacitors Carbon based - Double layer Metal oxide (Ni, Ru, Co, Mn) - Faradaic reactions Need new devices which bridge gap between double layer and metal oxide capacitors

20 Electrode Preparation using the Colloidal Method Preparation of the colloidal solution using RuCl 3 xh 2 O (39.99 wt% Ru) and NaHCO 3 Adsorption of the colloidal particles using carbon black Filtration using a 0.45 µm filtering membrane Annealing in air Mixing with 5wt% PTFE Grounding to a pellet type electrode Cold pressing with two tantalum grids

21 TEM image of RuO 2 nh 2 O/carbon composite electrode (40 wt% Ru) 25 nm

22 SEM images of RuO 2.nH 2 O/carbon composite electrode 3 µm 3 µm (60 wt% Ru ) (80 wt% Ru)

23 SEM image of RuO 2 deposited on carbon particle by sol-gel method (10.6 wt% Ru) Y. Sato et al. Electrochem. Solid State Lett. 3 (2000) 113

24 Specific capacitance of RuO 2 nh 2 O as a function of Ru loading 900 Specific Capacitance (F/g of RuO 2 ) Weight Percent of Ru (%)

25 Comparison of Preparation Techniques for RuO 2 /carbon composite electrode Specific capacitance of RuO 2 Particle Size Ru loading limit Structure Annealing temperature Colloidal method 863 F/g 3 nm 40 wt% Amorphous 100 o C Conventional Sol-gel method 720 F/g 38 nm 10 wt% Amorphous 150 o C Heat decomposition 330 F/g 2 nm 50wt% Crystalline 320 o C

26 Synthesis of nanomaterials Vapor Condensation Gas Phase synthesis Atoms or molecules of the nano particle to be produced are converted into a vapor and nano sized clusters are condensed from the vapor Liquid Phase synthesis Nano sized materials can often be prepared by precipitant clusters of atoms in a liquid.

27 Gas phase synthesis

28 Synthesis of Nanophase Materials by Gas Condensation Processing Ref:Gas phase synthesis of nanocrystalline materials, Horst Hahn, NanoStruct. Mater., Vol. 9, pp.3-12, In this technique, the nano powder formed normally has the same composition as the starting material. The starting material, which may be a metallic or inorganic material is vaporized using some source of energy. The source of energy may be a Joule heating (Heat), laser or electron beam evaporation ect. These processes are normally carried out in an evacuated chamber with 1-50 mbar pressure of some inert gas like helium

29 Schematic of the Chemical Vapor Condensation Process for the Synthesis of Nanophase Materials n n n The precursor (metallic or inorganic) is vaporized with some source of energy. Precursor after passing through a hot walled reactor decomposes and nano particles nucleate in the gas phase. The size of the nano particles is determined by the particle residence time, temperature of the chamber, precursor composition and pressure.

30 Deep Reactive Ion Etching (DRIE) How it works Ions induced by ICP (Inductively Coupled Plasma) bombard the surface and etch away material. High density plasma is maintained at lower pressures less than 5 mtorr. Operating conditions Etching rate: ~4µm/min Side wall angle: 90 o ±2 o Aspect ratios: up to 30:1 Applications The basic technique of Micro-Electro-Mechanical Systems (MEMS) to fabricate micro mechanical elements, sensors and semiconductors 30 µm

31 Chemical Vapor Deposition Chemical vapor deposition, as its name implies, involves a gasphase chemical reaction occurring above a solid surface, which causes deposition onto that surface Precursors are activated This generally involves thermal (e.g. hot filament) or plasma (D.C., R.F., or microwave) activation, or use of a combustion flame (oxyacetylene or plasma torches CVD Diamond Group at the University of Bristol's School of Chemistry

32 Flow chart in thin film process Ref: Yasunori et al, Material Science engineering C 15(2001)

33 Common Low Pressure CVD Reactor May, Paul W., "CVD Diamond - a New Technology for the Future?", Endeavour Magazine, 19(3), pp

34 Below, you can see the differences between a conventional CVD film (left) and a nanocrystalline film (right). These pictures are from the CVD Diamond Group at the University of Bristol's School of Chemistry Notice the difference in crystallinity between the two films. The nanocrystalline film is not well-faceted or regular in appearance. Ref:from Bristol

35 Sputtering Sputtering is a technique by which atoms and ions of Argon or other gases from a plasma bombard a target thereby knocking atoms off of the target. These material atoms travel to a substrate where they are deposited and form a thin film. DC sputtering, RF sputtering, ion beam sputtering, Bias sputtering, Reactive sputtering Schematic diagram of ion beam sputtering

36 Sputtering Sputter deposition is commonly used in the formation of superconducting alloys. Nonconductive and oxide materials also can be deposited using RF sputtering Alloy and composite can be deposited with keeping the ratio of elements

37 Synthesis of Nanostructured Materials by Electrochemical Processes Underpotential Deposition (UPD) of monolayers of Zn, Ni, Bi onto hard alloys Novel autocatalytic reduction process for deposition of amorphous alloys (Ni-P, Ni-Co-P) Galvanostatic pulse treatments for deposition of ternary and quarternary composites based on Zn, Ni, Cd, P Superior corrosion and catalytic properties Superior electrocatalytic properties (long cycle life, low self discharge, high rate capabilities) Superior mechanical properties (low rates of hydrogen permeation and corrosion)

38 Underpotential Deposition of Nanostructured Monatomic Layers of Zn, Pb and Bi UPD occurs with a formation of monatomic layers at potentials more noble than the reversible Nernst potential. UPD has been optimized for Zn, Pb and Bi by using the work functions of these metals and the work function of the substrate. The Underpotential shift (E) when the monatomic layers are formed is determined by the difference in work functions in electron volts of both metals. UPD formed monatomic layers of Pb, Zn and Bi on steel surface inhibit corrosion due to lowering of the binding energy of the hydrogen adatoms on Zn, Pb and Bi adsorbates.

39 Autocatalytic Reduction Process for Deposition of Nanostructured Composites One step process No external current is used for deposition. High temperature and large concentration of reducing agent (hypophosphite) during encapsulation leads to hydrogen evolution. Evolved hydrogen penetrates the hydride particles in the bath and results in lowering the particle size. EPMA of cobalt encapsulated LaNi 4.27 Sn 0.24 alloy Nanosized amorphous layers of Co-P, Ni-P are deposited by controlling the substrate particle size, the concentration of Co ++ or Ni ++ in the electrolyte and by controlling the deposition rate (ph, temperature and presence of leveling agents). 1 µm

40 Comparison of Bare and Nanostructured Co Coated Hydride Alloy 400 Capacity (mah/g) Co Mixed Bare Co encapsulated, Thickness = mm Co encapsulated, Thickness = mm Number of cycles

41 DC and Pulse Deposition of Nanostructured Multilayers The particle nucleation rate and the grain size is controlled by the peak cathodic potential, the pulse period and the relaxation period and the duty cycle. Thin films and nanostructured deposits have been deposited by optimizing the duty cycle and the concentration of leveling agents. The film grain size is proportional to the crystal growth rate and inversely proportional to the nucleation rate. Pulse deposition increases the nucleation rate, decreases the crystal growth rate. Nanostructured Zn-Ni-Cd 1 µm 1 µm Multiple Layers of Zn-Ni

42 Comparison of Microhardness for Nanocrystalline Mild steel and Microcrystalline mild steel Microhardness kg per square mm Microcrystalline Nanocrystalline Nanocrystaline Microcrystalline Structure Substrate Copper Brass Mild Steel Mild steel Hardness Range kg/square mm Average VHN A.M. El-Sherik and U.Erb, Plating and Surface Finishing, pg,85-89,september 1995,

43 Variation of Microhardness with Grain size Ref: C.Cheung, G.Palumbo and U.Erb. Scripta Metallurgica et Materialia, Vol31, N06, pp ,1994

44 Improved Performance of Nanocrystalline metals over Microcystalline metals Properties Hardness Wear resistance Friction Corrosion Resistance Strength Magnetic Hydrogen diffusion Electrocatalytic Descriptions 5 times harder 170 times increase Cut friction in half Reduce or stop localized corrosion 3 to 10 times stronger Lower coercivity,resistivity 3 times increased,ms reduced by 5 % Higher hydrogen diffusion Improved electrocatalytic activities for hydrogen evolution and hydrogen oxidation reactions

45 Development of Pt-Ru Nanoparticles as Electrocatalysts for Polymer Fuel Cells Present polymer electrolyte fuel cells have low utilization of active material Electrode catalysts also suffer from CO poisoning

46 Development of Pt-Ru Nanoparticles as Electrocatalysts for Polymer Fuel Cells Develop superior fuel cell electrodes with better utilization of electrocatalyst Construct membrane electrodes with nanoparticles of carbon and catalyst particles Catalysts - Pt/Ru/Pd/Ni Synthesize mixed alloy catalysts with Pt- Ru-C and Pd-Ru-C Nanostructures will lead to better utilization of noble metal and hence low electrode cost inhibiting CO poisoning

47 Pulse electrodeposition for preparing PEM fuel cell electrode K.H. Choi, H. Kim, T. H. Lee J. of Power Sources 75 (1998) 230 Objective Reducing cost of electrode by decreasing Pt loading Presenting Pt on the surface contacting with polymer electrolyte. Decreasing particle size and increasing activity of Pt

48 Process of pulse electrodeposition The rate of nuclei formation, v: = 2 k exp k v 1 η Overpotential of DC deposition η DC = η ln 0 i i DC Overpotential of PC deposition 0 η i a η PC = η = i PC DC = η 0 i ln i DC a 0 + η + η 0 t ln t 0 off on t ln t off on i p : peak current density i a : average current density

49 Effect of pulse and DC electrodeposition at current density of 50mA/cm 2 Pulse 20 nm DC 200 nm

50 Effect of particle size of Pt on the performance of PEMFC A A B B

51 Nanostructure Synthesis by Chemical Reduction Supporting Structure Supporting Structure 2-3 nm Mechanism of Nanostructure Formation Reducing agent Membrane Salt of metal to be reduced Nanostructure Deposition on Membranes Deposition of gold: HAuCl 4 salt of metal and NaBH 4 reducing agent

52 Synthesis of Next-generation Catalysts by Control of Structure at the Nanoscale Catalytic nanoparticles are grown inside dendrimers, which allow for precise control of sizes and shapes. Nanoparticles are then a) supported on high surface area and porous materials, b) self-assembled into ordered nanoarrays on flat surfaces, and c) attached to the walls of nanotubular reactors within a membrane catalyst.

53 Template Synthesis Technique Underlying principle is akin to that of producing components through the use of replication e.g, die casting or mold casting Materials are deposited in to the pores of this membranes by either electrochemical or chemical(electroless) reduction of metal ion. Types of Membranes Track Etch membranes in the form of nuclear track filters (NTFs)are widely used. Pores are generated by ion track etch technique. Porous alumina membranes are produced electrochemically from aluminium.

54 Alumina anodisc membrane approach Anodization of aluminum in an acid solution results in the growth of an oxide with a high level of porosity These pores can then be filled with metal by electrochemical reduction of a metal salt The alumina matrix in which the metal particles are held can then be removed by a chemical wash

55 Gold Nanowires Formed in an Alumina Nanoporous Membrane by Electrochemical Reduction of KAuCl 4 Ref: Charles R.Martin, Science Vol 266, pg , December 1994

56 Summary Nanostructures-Applications: In catalysis they provide templates and pores In material science new advanced metals alloys and composites with high strength and corrosion resistances will be synthesized In electronics, nanostructures will provide new processor architectures In molecular biology nanostructures are the fundamental machines that drive the cell Giant magnetoresistant (GMR) materials have been introduced into commercial use Inorganic nanostructures will be used in medicine and biology as markers

57 Summary High density information storage (nano-cds) Materials showing giant and tunneling magnetoresistive effects New types of components for information processors based on quantum mechanical principles (single electron transistors) Aerogels-sponge like materials show promises in catalysis and energy applications Addition of aluminum oxides nanoparticles that convert aluminum metal into a material with wear resistance equal to that of the best bearing steel

58 Objectives for the next decade Development of new catalysts for chemical automotive, pharmaceutical and food industries Synthesis and processing of organic, inorganic and biological nanostructures with controlled size, structure, morphology, shape Assemblies of nanoscale building blocks including (i) construction of functional assembles, (ii) develop techniques which will be used as methods for fabrication of nanostructures and nanosystems such as scanning probe writing, combination of electron beam and lithography, (iii)synthesis of oorrosion resistant materials, magnetic structures, zeolites, aeorgels

59 Future studies Studies should be carried out in order to: To understand the size-dependent properties of materials such as (i) electrical, (ii) magnetic, (iii) chemical, (iv) thermal, (v) mechanical (vi) biological and (vii) stability, hardness ductility and wear resistance To understand the properties of isolated nanstructures such as (i) building blocks with different sizes and shapes, (ii) building identical nanostructures (iii) the nucleation and growth rate as a function of process variables To design and make structures with atomic level precision To define the fundamentals of molecular electronics