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Nano-materials and Nano-structures - Introduction
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Can nanotechnology cure cancer by 2015?
Economy of promises!
GETTYMost people learn about developments in science and medicine from the mass media.
• Nano-particles– Sol-gel; high-energy ball milling, hydrothermal
• Nano-tube, nano-wire, nano-rod and nano-belt– Evaporation (Science 291 (2001) p 1947)
– Laser ablation (Science 279 (1998) p 208)
– Anodization (J. Mater. Res. 16 (2001) p 3331)
– Electro-spinning (Nano lett. 3 (2003) p 555)
– Surface modification (Adv. Matls., 16[3] (2004) p 260, in print)
• Nano-porous structures– Emulsion templating (J. Mater. Res. 18 (2003) p 156)
– Photo-electrochemical etching (Electrochem. Solid-State. Lett. 1 (1998) p 175)
Fabrication Techniques
Sol-gel Technologies and Their products
MetalMetalAlkoxide Alkoxide SolutionSolution
HydrolysisPolymerization
Sol
Coating
Coating
Gelling
Precipitating
Heat
Uniform particles
Aerogel
Dense filmXerogel film
Wet gel Xerogel
Evaporation Heat
Dense ceramics
Extraction of solvent
Source: http://www.chemat.com/html/solgel.html
Sol-gel synthesis : OverviewProcess step Mechanistic step
organometallicsin solution viscous gel
SubstrateXerogel film
Crystalline film
heat
Spin/dip coat
Nano-particles
heat
Process Parameters Thin film & particle properties
gelation
crystallization
Poreelimination
pyrolysis dehydration
• Heterogeneous nucleation• Homogeneous nucleation• Growth• Phase transformation
Typical synthesis conditions Ti precursors
- Titanium isopropoxide
- Titanium butoxide
- Titanium methoxyethoxide
- Ti diisopropoxide bisacetylacetonate
- Titanium methoxypropoxide Solvent : Various alcohol
- 2-methoxy ethanol- Methanol- n-butanol
Complexing / chelating agents- Acetyl acetone- Acetic acid
Crystallization temperatures
- 100 ~ 1000 °C
Sn precursors - Tin Tetrachloride
- Tin ethoxide
- Tin ethylhexanoate
- di-n-butyl-tin-bis-acetylacetonate
Sol-gel synthesis of metal oxides
1. Hexavalent oxides, WO3 and MoO3
Tungsten ethoxide W(OEt)6 solutions stabilized by acetic acid have been used to form WO3 expensive and high sensitivity toward water
WOCl4 (Chloro-alkoxides) readily dissolves in all kinds of alcohols forming stable solutions of oxychloroalkoxides cheap and stable for several months
WOCl4 + xROH WOCl4-x(OR)x +xHCl
MoO3 can be deposited from alkoxides, chlorides, chloro-alkoxides or molybidic acid
2. Tetravalent oxides, TiO2 and SnO2
TiO2 can be easily deposited from alkoxide solutions. The best precursors seemsto be Ti(OBun)4
SnO2 can be obtained from the organic tin oxide precursor di-n-butyl-tin-bis-acetylacetonate ((C4H9)2Sn(acac)2)
Sol-gel silica spheres Sol-gel SnO2
Sol-gel sub-micron and nano-particles
J. Sol-gel Sci. Tech., 15, 263-270 (1999)
Sol-gel for BaTiO3 Ferroelectrics
TiO2 by Sol – gel for sensorsLa-modified TiO2 by a sol–gel
[La/Ti] atomic ratio of 5% and 10%
Sensors and Actuators B 111–112 (2005) 7–12
Addition of La stabilized the anatase phase and the grain sizeAt 600°C, 27 nm for 5% La/Ti atomic ratio and 11 nm for 10%
Response transients to 100 ppm ethanol at 500 °C
for the La–TiO2 films calcined at 900 °C
SnO2 by Sol - gel for sensors
Sensors and Actuators B 114 (2006) 646–655
SnO2, In2O3 and In2O3–SnO2 thin films by sol–gel technique
Fine and uniform SnO2 film
Application of nanoparticles as gas sensor: SnO2
Representative TEM micrograph of sample calcined at 550 C
Diameter 30 nmDiameter 18 nm
Time dependence reflectance at 620 nm during exposure at CO and O2 at 400 C for samples
Formation of oxygen vacancies in a reducing gas leads to a decrease in the reflectance of the SnO2 particles. In the smaller nanoparticles, oxygen vacancies can be formed to a higher extent leading to a higher sensitivity.
O2 O2CO
General Conclusion of Sol - gelVersatile
powders, films, fibers, monoliths, aerogels, xerogels
High purity
controlled by purity of starting solution
Homogeneity
controlled by hydrolysis, condensation and polymerization
reactions
Limited mass-scale production
particles, coatings, thin-films
High-energy ball milling
xSnO2-(1-x)αFe2O3 are fabricated by mechanical alloying
Sens. Actuators B 65 (2000) 361
VS (Vapor-Solid)
1. No metal catalysts used
2. Vapor phase chemical species adsorbs on the surface of the substrate due to T leading to 1-D nucleation and growth
SiH4(g) Si (g) + 2H2 Continuation of adsorption of Si
Mechanism is not well established
Screw dislocation (Sears), Defect (twins or stacking fault) for nucleation sites
Low temperature zone
substrate
TEM image showing the silicon nanowires prepared via VS2
2P.P. Yu, Synthesis of nano-scale silicon wires by excimer laser ablation at high temperature, Solid state communication, 105(1998) 403-407.
• Experimental setup
FurnaceAlumina tube
SnO2 powder
Ar
Aluminasubstrate
Ar Arevaporate deposition
Coolingwater
Powders are placed at the center of the tube Alumina substrate is placed downstream inside the tube Evacuate tube to around 210-3 torr Evaporation is conducted at 1350 °C in Ar atmosphere
+SnO2
Nanobelts (Evaporation Technique)
Source: Science, 291 (2001) 1947
Typical widths of nano-belts are in the range of 50-200 nm
The length is several hundred micrometer or more.
It has rectangular cross-section Each nano-belt is a single crystal without dislocations Nano-belts of ZnO, SnO2, In2O3, CdO, Ga2O3, PbO2 can be easily
produced by evaporation
Nano-belts (Evaporation Technique)
VLS (Vapor-Liquid-Solid)
Liquid metal droplet forms Droplet captures X from gas leading to saturation and nucleation
X nanowire forms by diffusion of X
FeSix metal liquid droplet
Substrate (Si)
X: SiH4 Si + 2H2
substrate substrate
Typical Si nanowires1
Metal catalyst cap
1S.Q. Feng, The growth mechanism of silicon nanowires and their quantum confinement effect, Journal of crystal growth, 209(2000) 513-517.
Nanowire fabrication by electrospinning
Schematic view of the setup for electrospinning
Nanowire source materials 1. A polymer solution or melt is injected from a small nozzle under the influence of an electric field.
2. The build up of electrostatic charges on the surface of a liquid droplet induces the formation of a continuous ultrathin fiber.
3. Various engineering plastics, biopolymers, electrically conductive polymers, and oxide nanowires have been produced by the technique.
Controlling variables: Electric field strength, polymer molecular weight and deposition distance
TiO2 nanowires by electrospinning
TEM image of the same sample after it was calcined in air at 500 C for 3 hrs
TEM image of TiO2/PVP composite nanowires fabricated by electrospinning
For the inorganic nanowires like TiO2, inorganic precursors are required to be added in liquid polymer solutions. For the above results, titanium tetraisopropoxide (Ti(OiPr)4), poly vinyl pyrrolidone (PVP, a liquid polymer) and ethanol were used. Added acetic acid to stabilize the solution and control the hydrolysis. As-spun nanowires in air transformed to TiO2 by the hydrolysis of Ti(OiPr)4. Finally, PVP is removed by calcining at 500 C in air.
D. Li, Y. Xia, Fabrication of titania nanofibers by electrospinning, Nano Letters 3 (2003) 555-560.
Titania nano-tube fabricated by electrospinning.
Co-axial process helps fabricate hollow tubes. Mineral oil is extracted (etched) in octane and calcined in air at 500 C to remove PVP.
D. Li, Y. Xia, Direct Fabrication of Composite and Ceramic Hollow Nanofibers by Electrospinning, Nano Lett. 4 (2004) 933-938.
MoO3 nanowires by electrospinning
TEM image of MoO3/PVP composite nanowires before calcination
HRTEM image of MoO3 nanowires after calcination at 500 C in air
0.5 M molybdenum isopropoxide sol was prepared and mixed with 0.1mM polyvinylpyrrolidone. The solution was electrospun in air using a DC voltage power supply at 20 kV.
• Experimental setup
Electrolyte
TiO2
Ti
I
Ti4+
OH-, O2-
TiO2
Ti
Electrolyte
oxidation
dissolution
Nanotubes (Anodization)
TiO2 nano-tubes fabricated by anodization Diameter of nano-tubes: 10 – 80 nm Nano-tubes are oriented and perpendicular to the surface Nano-tube length increases with anodization time (400 nm in 20 min) As-received nano-tubes are amorphous and oxygen deficient Annealing in oxygen atmosphere is required
Source: Adv. Mater. 15 (2003) 624
Nano-tubes (Anodization)
Photo-electrochemical Etching
UV supply
power
supply
mirror
Current
H2SO4TiO2
UV light
Counterelectrode
TiO2 + SO42- + 2h+ → TiO·SO4 + 1/2O2
Ev
EF
Ec
TiO2 H2SO4
hν
Eg
h+
e-
Nano-honeycomb structure
Sintering
1300 °C for 6hrs 700°C for 4hrs in 10% H2/N2
H2 heat treatment PEC etching
- Nano-fibers are parallel, oriented in the same direction - Diameter of nano-fibers: 15 – 50 nm Length of nano-fibers: up to 5 μm
heat-treatment
1 μm 1 μm
Process
Sintering
1200 °C for 6hrs 700°C for 8hrs in 5% H2/N2
H2 heat treatment
Adv. Matls., 16[3], 260 (2004) Patent appl. 2003-678772
Invention of Titania Nano-fibers
Wide variety of possibilities by gas-phase reaction - CISM
Nano-channels(different sintering)
Nano-whiskers(doped with Fe2O3)
Nano-lamellar(two-phase)
Nano-fibers
Ceramic Nano-machining?
Patterned TiO2 Nanowires
Planar view Tilted view
<001>
<110>
Gas-Phase Growth in SnO2
• Experimental Conditions:– Sintered 1300 C for 24 hours
– Heat treat in a humid 5% H2 – 95% N2 atmosphere at 700 C for 4 hours
– 1 L/min gas flow
– Au-coated SnO2
2 m
TiO2 nano-wire on Ti64 alloy in Ar
Magnified image for the circled area
Nano-wire diameter : ~ 50-100 nmLength of nano-wire : ~ 3-5 m
200 nm
(Gd,Ce)O2 thin film on YSZ substrate breaks up into a psuedo-periodic array of single crystal islands with average size of 200 nm upon annealing at 1150oC.
Self-assembled Nano-islands
Mass-scale synthesis and fabrication Materials R&D, manufacturing
Assembling into devices and structuresNano-packaging, bonding, adhesion
Stability in hostile environments Surface and interface chemistry and physics
Fundamental understanding of propertiesMechanism and modeling
Potential applicationsSensing, catalysis, bio-medical, nano-electronicsnano-composites, TBC, corrosion
Challenges and Opportunities
Success Code: RTDAD
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