GasGas--phase Synthesis of Nanoparticles, phase Synthesis of Nanoparticles, NanotubesNanotubes, and , and NanowiresNanowires

Stephen D. Tse and Bernard KearMechanical & Aerospace Engineering

Materials Science and EngineeringRutgers, the State University of New Jersey

Piscataway, NJ 08854

28 February 2008

Areas of DiscussionAreas of DiscussionFabricating nanoenergetic structures strategically so that the fundamental thermo-chemical-physical combustion mechanisms can be extracted and optimized– Directly compare nanomaterials with conventional materials

Overcoming processing difficulties to obtain desired particle size, surface characteristics, physical & chemical propertiesFundamental understanding of growth mechanisms– Reproducible products

Building in long-term passivation without losing faster reaction rate Eye towards mechanical propertiesScalability

Synthesis RoutesSynthesis RoutesGas-phase– Nanometric fuels (e.g. n-Al < 50nm)

Sol-Gel– Synthesizing/incorporating nanodimensional organic

and inorganic materials in a supporting gel (e.g. high degree of mixing and uniformity of Fe2O3/Al Nanocomposite)

Vacuum– Atomic layer deposition

Fundamental understanding and tailoring heat releaseConnecting with realistic reacting materials performance

Al-M

oO3

Al-C

uO

Al-W

O3

Al-F

e2O

3

Al-B

i2O

3

Al-M

nO2

Al-N

i

B-T

i

B-Z

r

Al-T

eflo

n

0

5

10

15

20

25

30

Hea

t Rel

ease

, ∆H

[kJ/

g]

Full OxidationThermite/Intermetallic Reaction

Some Reactive SystemsSome Reactive Systems

Why from GasWhy from Gas--Phase?Phase?Provides a cheap alternative to expensive vacuum synthesis techniques in thin or thick filmsMuch higher deposition rate as compared to vacuum techniques, enabling mass productionUsually a continuous process versus a batch process (e.g. liquid-based synthesis processes or milling processes)– Characteristics variations from batch to batch

Key AdvantageVery short process times and nanoscaled powders of high purity with a narrow particle size distribution

ConsiderationsConsiderationsParameters influencing the time-temperature history/profile during the synthesis process– Reaction temperature– Reaction pressure– Mass flows of reactants and carrier gas– Precursor material (e.g. of different

decomposition kinetics and reaction enthalpies)

– Method of precursor delivery– Type of carrier gas– Reactor geometry

Precursor

Plasma gas

Auxiliary gas

RF CoilEvaporationDissociationReactionCondensationNucleationQuenching

Cold Substrate

Synthesis Parameters• RF power• Gas (Ar, H2, N2, CH4)• Ambient pressure & gas• Mass flow rates

• Precursor loading• Substrate temperature• Substrate voltage bias• Substrate distance

In-Situ CharacterizationGas-Phase diagnostics• Temperature (SRS, FRS, OES. LIF)• Chemical species (SRS, LIF, OES)• Flow field velocity (PIV)

Particle diagnostics• Composition (LIBS)• Crystallinity (Raman)• Size/Distribution (LII)

Ex-Situ Powder Characterization• Phase & crystallinity (XRD)• Morphologies & primary particle size (TEM)• Aggregate particle size (DLS)

Computational ModelingGas-Phase phase• Temperature• Chemical species• Flow field velocity

Particle growth & transport• Sectional Model (precursor

decomposition, nucleation, surface growth, coagulation, coalescence)

• Aerosol dynamics

• Powder surface area (BET)• Particle size distribution (nano-SMPS)• Precursor conversion (TGA)

Correlation Correlation

Optimize Process

Predict & Guide Experiments

ValidationPrecursor

Plasma gas

Auxiliary gas

RF CoilEvaporationDissociationReactionCondensationNucleationQuenching

Cold Substrate

Synthesis Parameters• RF power• Gas (Ar, H2, N2, CH4)• Ambient pressure & gas• Mass flow rates

• Precursor loading• Substrate temperature• Substrate voltage bias• Substrate distance

In-Situ CharacterizationGas-Phase diagnostics• Temperature (SRS, FRS, OES. LIF)• Chemical species (SRS, LIF, OES)• Flow field velocity (PIV)

Particle diagnostics• Composition (LIBS)• Crystallinity (Raman)• Size/Distribution (LII)

Ex-Situ Powder Characterization• Phase & crystallinity (XRD)• Morphologies & primary particle size (TEM)• Aggregate particle size (DLS)

Computational ModelingGas-Phase phase• Temperature• Chemical species• Flow field velocity

Particle growth & transport• Sectional Model (precursor

decomposition, nucleation, surface growth, coagulation, coalescence)

• Aerosol dynamics

• Powder surface area (BET)• Particle size distribution (nano-SMPS)• Precursor conversion (TGA)

Correlation Correlation

Optimize Process

Predict & Guide Experiments

Validation

Research StrategyResearch Strategy

B

C N

BN

B4+δC

“C3N4”

B2CN2

BC2NBC4N BC3N

B4N

mol

-% B

mol-%

Bmol-% C

“C3N4” Cdiamond

c-BN

B4C

TiCSiC

Al2O3 TiNSi3N4

superhard

hard

hardness

B

C N

BN

B4+δC

“C3N4”

B2CN2

BC2NBC4N BC3N

B4N

mol

-% B

mol-%

Bmol-% C

B

C N

BN

B4+δC

“C3N4”

B2CN2

BC2NBC4N BC3N

B4N

mol

-% B

mol-%

Bmol-% C

“C3N4” Cdiamond

c-BN

B4C

TiCSiC

Al2O3 TiNSi3N4

superhard

hard

hardness

“C3N4” Cdiamond

c-BN

B4C

TiCSiC

Al2O3 TiNSi3N4

superhard

hard

hardness

Materials InvestigatedMaterials Investigated

cc--BN NanopowderBN Nanopowder

GaNGaN NanopowderNanopowder

10 20 30 40 50 60 70 80

2 Theta

GaN Ga (metal) Ga2O3

GaN: 72% Ga (metal): <15% Ga2O3: <10%

Morphologies of AluminumMorphologies of AluminumNot only particles but also wires/ribbons

Quasi 1Quasi 1--D Flame D Flame

Stagnation premixed flame

Flame

Flat Flame Burner

Cooling Water

Substrate

Plate ElectrodeDC High Voltage

(0 to +/- 10kV)

Premixed H2 + O2

&Precursor vapor + Carrier gas

Temperature

Atomic Structure of Single CrystalsAtomic Structure of Single Crystals

HRTEM image of TiO2 powder (-300V)

• Circles highlight individual primary particle fringes• Primary particle size ~ 5nm

5 nm

Nanoparticle FormationNanoparticle Formation

Characteristic CVD curves: film growth rate w as a function of inverse process temperature and precursor partial pressure

Dotted lines mark the cross over from film growth to particle formation

Diagnostic Facilities & SetupDiagnostic Facilities & Setup

Optics for Raman

Optics for LIF

RemovableMirror

Dye Laser

PMT

Balanced Photo Receiver for TDLAS

Monochrometerfor LIF

Nd:YAG Laser

External Cavity Diode Laser

CatalyticProbe

Spectrometerfor Raman

ICCD

MFCs

MFCs

Fuel + Inert

O2 + Inert

Detailed GasDetailed Gas--Phase ChemistryPhase Chemistry

Mueller, M.A., Kim, T.J., Yetter, R.A., and Dryer, F.L., Int. J. Chem. Kinet. 31:113-125 (1999)

Flame ModelFlame ModelSandia SPIN code (Kee)Chemical mechanism from Mueller, M.A., Kim, T.J., Yetter, R.A., and Dryer, F.L., Int. J. Chem. Kinet. 31:113-125 (1999)Transport properties computed with CHEMKIN subroutinesSurface reactions set such that H, O, OH, and HO2recombine with unit sticking probability at substrateBoundary conditions at burner specify inlet mass flux and temperatureBoundary conditions at substrate incorporate no-slip condition and constant surface temperature

Laser Induced FluorescenceLaser Induced Fluorescence

OH OH VibrationalVibrational TransitionsTransitions

OH LaserOH Laser--Induced Fluorescence Induced Fluorescence Tunable Dye LaserM

M

BS

Photo-diode

Monochromator

L

BeamDump

x

yz

3-D translationCVD Chamber

with optical access

Pumpingbeam

Excitingbeam

ND2x Nd:YAG Laser

PinholeFilter

ComputerDigitalOscilloscope

PMT

• Relative OH profile: Q1(7)• Two-line temperature: P2(7) and P2(9) of the (1-0) band excited

0

500

1000

1500

2000

2500

0 0.5 1 1.5 2 2.5 3 3.5 4Substrate-Burner gap (cm)

Tem

pera

ture

(K)

pressure 20 torrpressure 40 torr

Axial Temperature Profile Axial Temperature Profile

• Constant mass flow rate

burnersubstrate

0

0.005

0.01

0.015

0.02

0.025

0.03

0 0.5 1 1.5 2 2.5 3 3.5 4Substrate-Burner gap (cm)

OH

mol

e fra

ctio

n

20torr40torr

Axial OH Profile Axial OH Profile

• Constant mass flow rate

burnersubstrate

1st stage N2 Dilution

PressureGauge

Excess Exhaust

3rd stage N2 Dilution

2nd stage N2 Dilution

Aerosol Storage Chamber

DMACPC

Low-Pressure Synthesis Chamber

Vacuum Pump

Premixed H2 + O2& Precursor Vapor + Carrier Gas

Cooling WaterCooling Water

Flat Flame Burner

Substrate

Computer

Valve 1

Valve 3

Val

ve 2

MFC

MFC

MFC

Bypass

Aerosol Sampling into Aerosol Sampling into nanonano--SMPS SMPS

Particle Growth Modeling Particle Growth Modeling Sectional Model: particle size distribution discretized into different volume sections which are evenly spaced on a lognormal size scale. All particles belonging to a section are supposed to be of the same size (zeroth order) as opposed to higher order sectional methods.

GDE:

surf

k

coal

k

coag

k

nucl

k

decomp

kk

dtdN

dtdN

dtdN

dtdN

dtdN

dtdN

++++=

Quasi 1Quasi 1--D Flame D Flame

Counterflow diffusion flame

Raman ScatteringRaman Scattering

VibrationalVibrational Raman SpectraRaman Spectra

NN22 VibrationalVibrational QQ--Branch Raman Branch Raman SpectrumSpectrum

Spontaneous Raman Spectroscopy Spontaneous Raman Spectroscopy 2x Nd:YAG LaserPulse-Stretcher

M

M

BSPowerMeter

Image Rotator

ICCD Camera

Prism

ImagingSpectrometer

L

L

Raman Notch FilterAchromats

BeamDump

Retroreflector

x

yz

3-D translation

CVD Chamber with optical access

Delay/GateGenerator

Computer

beam

Flame Structure Validation (Raman)Flame Structure Validation (Raman)

Temperature & major species With correction from interference from C-containing radicals

0. 00%

10. 00%

20. 00%

30. 00%

40. 00%

50. 00%

60. 00%

-0. 6 -0. 3 0 0. 3 0. 6 0. 9posi t i on/mm

0

500

1000

1500

2000

2500O2

CH4

H20

H2

Sum/4

T

0. 00%

2. 00%

4. 00%

6. 00%

8. 00%

10. 00%

12. 00%

-0. 6 - 0. 3 0 0. 3 0. 6 0. 9posi t i on/mm

CO2

CO

C2H2

Tungsten Oxide Tungsten Oxide

Our Synthesis MethodOur Synthesis MethodFlame synthesis is employed to grow well-aligned, single-crystal nanowires with:– diameters ranging from 20-50nm– coverage density of 109-1010cm-2

– growth rates of microns per minute– no pretreatment or catalysts– in open environments

Method is also robust in that the combustion process inherently provides for:– an elevated enthalpy source to evaporate the metal substrate

atoms,– the gas-phase chemical species (e.g. oxidizer, water vapor,

hydrogen, carbon dioxide) to produce the requisite oxide,– a favorable temperature gradient for growth of the nanowires

ScalabilitySelf-gettering

Aligned Aligned NanowiresNanowires

Dense yield of nanomaterials grown directly on a tungsten substrateDiameters of 20-50nm, lengths > 10µm, 10min sampling durationEDX tungsten oxide

TEM of Tungsten Oxide (WOTEM of Tungsten Oxide (WO2.92.9) )

20 nm

Different crystal structures include: cubic and monoclinic WO3, tetragonal WO2.9, and monoclinic W18O49Indexed SAED pattern with the first three highest intensities of 3.779Å, 3.126Å, and 2.67Å match very well with the tetragonal phase of WO2.9 with lattice constants of a = 5.3Å, b = 5.3Å, and c = 3.83Å (PDF card #18-1417)d-spacings correspond to {110}, {101}, and {200}, respectively

10 nm

0.378 nm

[110]

CrystallinityCrystallinity

Dislocation-free, single-crystalline2-D Fourier transform pattern gives average spacing for lattice planes of 3.78Å, which corresponds to the reflections from d-spacings of (110) planes of the tetragonal WO2.9 phase Preferable growth along the [110] direction

Growth Mechanism Growth Mechanism

Growth appears to be by vapor-solid (VS) mechanism– no metal nanoparticle at its tip (VLS)– thickening by a ledge-growth mechanism

20 nm

(b) (a)

–10 V applied

Voltage Bias on Substrate Voltage Bias on Substrate

–100 V applied

Minimizing electrostatic energy coming from ionic charges on the polar surfaces?

Other Metal OxidesOther Metal OxidesCuO nano- wires and ribbons

Fe2O3 nanowires Fe2O3 nano- wires and ribbons.

(a) (b) (c) (d) (e) (f)

(g) (h) (i) (j) (k) (l)

(m) (n) (o) (p) (q) (r)

Various MorphologiesVarious Morphologies

Encapsulating Reactive Nanoparticles in Encapsulating Reactive Nanoparticles in Carbon Carbon NanotubesNanotubes

Inert Co-flow

Premixed Flame

{Spinel Substrate}

Fuel + Oxygen + InertΦ > 1

Aero-dynamic

Jet

[Alloy Substrate]

CNTsCNTs

Inert Co-flow

Premixed Flame

{Spinel Substrate}

Fuel + Oxygen + InertΦ > 1

Aero-dynamic

Jet

[Alloy Substrate]

CNTsCNTs

Nan

opar

ticle

s en

caps

ulat

ed in

gro

win

g ca

rbon

nan

otub

e

Very WellVery Well--Aligned Aligned CNTsCNTs from Ni/Tifrom Ni/Ti

Online Feedback Control Online Feedback Control Laser Induced Breakdown Spectroscopy (LIBS)Raman of nanoparticles during synthesis

0. 00E+001. 00E+042. 00E+043. 00E+044. 00E+045. 00E+046. 00E+047. 00E+048. 00E+04

100 200 300 400 500 600 700

InIn--situ Raman of situ Raman of NanopowdersNanopowders

0. 00E+001. 00E+042. 00E+043. 00E+044. 00E+045. 00E+046. 00E+047. 00E+048. 00E+04

100 200 300 400 500 600 700