Nano-materials and Nano-structures - Introduction

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• 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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