Nano materials Optoelectronics Laboratory

IMS

Carbon Nanotubes: synthesis, acidic oxidation and

application as ultra sharp and high aspect ratio

CNT AFM probes

Vaneet Kumar Sharma

May 2nd, 2008

Department of Chemistry,

University of Connecticut

OUTLINE

• Introduction to single wall carbon nanotubes (SWNTs)

• Synthesis of single walled carbon nanotubes

• Acidic oxidation of single walled carbon nanotubes

• Application as ultra sharp and high aspect ratio carbon nanotube AFM probes

•Atomic Force

Microscope

(AFM) tips

•Nanoelectronics

Molecular Electronics

Nanosized Conductors

•Field emission displays

•Electromagnetic

Shielding

•Specialty Sensors

•Advanced Composites

•Actuators

•Hydrogen storage

•Nanometric test tubes

•Cancer therapy

Physico-Chemical

Large Surface Area (~1600 m2/g)

Amenable to electrochemical doping

Hollow, molecule storage/nanoreactors

Thermal conductivity twice as good as

diamond (2000 W/m/K)

Good thermal stability (750°C in air,)

Electrical

Metallic or Semiconducting (1-D)

met-SWNTs are ballistic conductors (109 A/cm2)

Mechanical

Strongest known fiber (Young’s modulus, ~1 TPa)

Highly flexible, Buckle-prone

Large aspect ratio (~103)

SWNT Unique Properties

Number of different (n,m) SWNTs in HiPco

Metallic Semiconducting

SWNTs DOS and Eii

SWNTs Density of States (DOS) and Eii

Ch=na1 + ma2

n = m : Metallic (zero band gap)

n - m = 3k : Semi-metallic

(0.04 eV band gap)

n-m ≠ 3k : Semiconducting

(0.6~1.2 eV band gap)

where k is integer

It was not until 1991, when Sumio Iijima of the NEC

Laboratory, Tsukuba used High Resolution Transmission

electron microscope to observe Carbon nano tubes,

In his own words it was "Serendipity“, discovery by chance

In his own words it was "Serendipity“, discovery by chance

Methods of Synthesis

Arc discharge

Laser ablation

Chemical Vapor Deposition (CVD)

Chemical Vapor Deposition Apparatus Diagram

C2H2 / CH4 /CO

H2

Ar / He

Quartz boat contains

catalyst, Fe/ Co/ Ni

nanoparticles or Fe-Mo supported

Catalyst

•Pressure: 1atm

•Temperature: 800 ° - 900°C

A typical CVD set-up consists of catalyst held in a

quartz tube placed inside of a furnace and have the

following advantages over other synthesis procedures.

Potential for a large-scale synthesis of high-quality SWNTs

Increased Control (in terms of narrow range of diameter)

Lower Temperatures (as compared to the arc discharge or

laser ablation where the temperature is as high as 1400°C)

Catalyst (composition, the nature of metal and type

of support material )

1. Metal nanoparticle catalyst (eg. Fe, Co, Ni)

2. Supported metal nanopaticle catalyst (Fe/MgO,

Fe/Al2O3, Fe-Mo/MgO, Co-SBA-15, Co-MCM41 etc)

To aqueous iron salt (Fe(NO3)3.9H2O)

solution (NaOH, Na2CO3, NH3, NaHCO3) is

added under vigorous stirring at room

temperature, then heating at 100°C, baked

at 150°C and finally calcination at ~ 500°C,

The quality and yield of SWNTs are very sensitive to catalyst supports,

To induce uniformity in size for these metal nanoparticles these are well

dispersed on these support,

Supported metal nanoparticle catalyst

(Fe/MgO, Fe/Al2O3, Co-SBA-15, Co-MCM41)

Support like MgO, Al2O3 are prepared by adding base to metal salt aqueous

solution under vigorous stirring, and then heating it to 100°C, baked at 150°C and

finally calcination at 500°C

MCM-41

(Mobile Crystalline Material)

Mesoporous materials are those

with pores in the range 20-500Å

in diameter. They have huge

surface areas, providing a vast

number of sites where sorption

processes can occur.

For supported catalyst like MgO or Al2O3, we load 1% catalyst on the support in

methanol or butanol as solvent, under vigorous stirring, and then heating at 100°C,

baked at 150°C, finally calcination at ~ 500°C

Metal loading

Fe-MCM-41, Co-MCM-41 is prepared by isomorphously substituting metal ions

for Si ions in the silica matrix of the MCM-41, the metal loaded in this case is also

~1%

The MgO support offered a high nanotube yield due to

the strong metal-support interaction. The MgO support

has another advantage that it can be removed by the

relatively mild acidic treatment, while many support

materials, such as alumina, silica, and zeolite, require the

highly toxic HF treatment

1. Small size of metal nanoparticle. (α diameter of the nanotube formed)

2. Small size and high surface area of the support.

3. Highly uniform, well dispersed catalyst sample, no aggregation or

stacking of particles.

4. In case of mesoporous materials (MCM 41, SBA 15), the catalyst should

be isomorphously substituted, that is it should be in the framework of

the material and not distributed on the surface.

5. Metal loading 1% in methanol, butanol solvent.

6. Calcination temperature has a very important role to play,

sintering should be avoided.

what is good catalyst for SWNT’s synthesis ??

There are 2 distinct theories and there followers

1 Those who believe that with metal nanoparticles the best results

come with methane and then recently they have realized or it is now

more often reported methane/hydrogen

2 Those who work with supported metal catalyst and for them

acetylene is the ultimate carbon feed

Carbon source, amount and time of carbon feed

The argument which differs the two is that

Acetylene have advantage that at the reaction temp ~ 800°C it is

more reactive than methane, and more carbon is available for the

reaction

And disadvantage lies in the fact that more carbon availability leads

to impurities like amorphous carbon, graphite, MWNTs

Now more or less CH4/H2 is preferred but then C2H2 has its

admirers.

H2 is passed along with the methane gas at ~800°C as hydrogen

prevents excess carbon deposition from poisoning the catalyst and

sustains nanotube growth over an extended period of time, hence

minimize the impurities.

But the window of use of CH4/ H2 is very narrow,

large quantities will Suppress the SWNT’s yield, and

less quantities will lead to pyrolytical growth (but no SWNT’s)

Sometimes H2 is also passed around 450-600°C before the reaction so

as to preactivate the catalyst, so as to provide a pre reduction

treatment

SWNT’s synthesis is a very vigorous reaction, So vigorous

that 1 micron is formed in 1 ms, so much carbon is available,

and all carbon wants to rush into the catalyst zone so as to

grow

what is the amount of carbon feed in the Reaction ?????

There are no exact values or say rules for it, nobody knows exactly

what is the best amount of carbon feed ??

The general rule is that u pass more carbon in less time or u pass

less carbon in more time

generally the carbon feed is 20-40 times lower than the argon or

inert gas which is being passed in the reaction,

If the flow of argon gas is 2000 SCCM (cubic centimeter under

standard conditions of temp. and pressure) then 40-70 SCCM of

acetylene or methane is passed at the reaction temperature of ~800°C

What are ideal conditions for the synthesis of SWNT’s by CVD ??

1. In previous slide I have already told what is good catalyst for

SWNT’s synthesis ?? (small size, uniform, 1%loading etc)

2. CH4/H2 is preferred but then C2H2 has its admirers.

3. Generally the carbon feed is 20-40 times lower than the argon

or inert gas which is being passed in the reaction, preferably

there is 30- 45 minute feed for ~40-50 SCCM (CH4 or C2H2)

4. Reaction temperature is ~800-850 °C, below 800°C MWNT’s

are formed and at temperature higher than 850-900 ° C

defects are formed such that Amorphous carbon and graphite

reduce the yield of SWNT’s

Oxidation of SWNTs

Sonication assisted

HNO3 / H2SO4,

HNO3 +H2SO4

Liu, J. et al. Science 280, 1253 (1998).

• Hydrophobic side-wall and hydrophilic

end. •Driving factor for the physical interaction with

hydrophilic substrate and other SWNTs

Name Position (cm-1) Origination (mode)

G’ ~ 2700 Overtone of D-band

G 1550-1605 Graphite related mode (A,

E1, and E2)

D ~ 1350 Defect-induced (non-sp2)

RBM 400~150 In phase radial displace-

ments (A)

Band Characteristic

G Lorenzian at wG+ and BWF (Breit-

Wigner-Fano) at wG-: Metallic

Lorenzian at wG+ and Lorenzian at wG

-:

Semiconducting

RBM Diameter dependent

Kukovecz et al, Eur. Phy. J. B, 28, 223, (2002).

HiPco SWNTs, Elaser = 2.41 eV

500 1000 1500 2000 2500

Wavenumber (cm-1)

Ram

an In

ten

sity

G (wG+)

D RBM

G’ G (wG

-)

1-

2

1-

1

21

cm 5.8 :

,cm 239 : RBM,at peak :

C

C

Cd

C

RBM

t

RBM

w

w

Resonance Raman spectroscopy of SWNTs

1425 1500 1575 1650

sem-SWNTs

1450 1500 1550 1600 1650

met-SWNTs

Resonance Raman studies of effect of acid treatment on SWNTs

Effect of

1) Nitric acid

2) Sulfuric acid

3) 1:2(Nitric:Sulfuric acid)

4) 1:3(Nitric:Sulfuric acid)

5) 1:4(Nitric: Sulfuric acid)

514 nm or 2.4 ev

Resonance Raman characterization

632 nm or 1.96 ev Resonance Raman characterization

785 nm or 1.56 ev

Micro-electronics / semiconductors

CNTs AFM probe

Controlled Drug Delivery/release

Solar storage

Biosensors

Field Effect transistors

Nano lithography

Single electron transistors

Batteries

Field emission flat panel displays

Nano electronics

Nano balance

Nano tweezers Data storage

Magnetic nanotube

Nanogear

Nanotube actuator

Molecular Quantum wires

Hydrogen Storage

Noble radioactive gas storage

Artificial muscles

Waste recycling

Electromagnetic shielding

Dialysis Filters

Thermal protection

Nanotube reinforced composites

Reinforcement of armour and other

materials

Reinforcement of polymer

Avionics

Collision-protection materials

Fly wheels

Future Applications

Single walled Carbon nanotubes AFM nanoprobes by

dielectrophoresis (DEP)

When a dielectric particle is subjected to an electric field, a dipole moment is induced in the particle. If the

electric field is spatially nonuniform, the polarized particle experiences a force imbalance. The direction of

this force depends on the polarizability of the particle relative to the polarizability of the medium.

When an electric field is applied to a particle in a medium, the resulting torque aligns the particle parallel to

the electric field.

Positive DEP corresponds to movement of the particle towards the high electric field,

negative DEP corresponds to movement of the particle toward the low electric field.

Positive DEP means that the particles (carbon nanotubes) have higher dielectric constant than the medium

hence movement towards the AFM probe.

Conversely the negative DEP has lower dielectric constant than the medium hence move in opposite

direction

Proposed mechanism (0.001~0.01 mg/ml)

In AFM probe the tip of the nanotube solution have the highest

electric field area,

SWNTs solution used is a mixture of metallic (met-) and

semiconducting (sem-) nanotubes,

Sem- SWNTs have finite dielectric constant with εsem < 5 while

met- SWNTs are expected to have a very large εmet- owing to the

mobile carriers.

met- SWNTs are expected to migrate towards the high field region

(AFM tip ends) under the electric field gradients,=

The deionized water and DMF are chosen as the nanotube

dispersion medium whose dielectric constants are 80 and 39

respectively.

AC240-HiPCO-6

2.1 MHz, 10V, 20 sec immersion time

AFM CNT Tips

AC240-AFM-CNT probes (HiPCO NTs in naturalized FMN solution)

10V, 2.1 MHz, 15 sec immersion time

The dimensions (diameters and length) and morphology (straightness

and orientation) of the fabricated CNT tips depends on the several parameters

1) External electric field,

2) Concentrations of the nanotube dispersion,

3) Immersion time,

4) Pulling rate,

5) Humidity,

6) AFM tip wetting properties and its alignment

AC field with 2.1 MHz and 6 or 8 V or 10 V????

Concentration ranges

between 0.001~0.005 mg/ml

Thicker solution requires less time, however thinner

solution was preferred, since there are lesser impurites

and better dispersion, normal immersion time is 10 -15

seconds

Pulling rate should be slower than the nanotube deposition

rate

It was control by sealing the cell which was saturated with water

vapour to minimize the solution evaporation

Less the wetting, better results, as

smaller capillary force, thus minimize

the disturbance to the pulling process

Common defects

Conclusions

• Resonance Raman characterization indicated that the separation efficiency of

octadecylamine mediated process was 89% for sem-SWNTs.

• Metallicity and diameter dependent dedoping characteristics of p-dopants from HiPco

SWNTs were revealed by resonance Raman spectroscopy study.

• Charge-stabilization provided a SWNT separation where dedoped SWNTs preferentially

precipitated leaving doped SWNTs in highly dielectric DMF media

• The modeled SWNTs reduction Gibbs free energy towards dedoping exhibited matching

trends with the observed nanotubes dedoping and separation behavior.

• Variation in de-doping characteristics of various (n,m) SWNT has been identified as the

primary reason for metallicity and diameter enrichment.

• Starting from a narrow diameter distribution SWNT sample and performing the “right”

redox jump is essential to attain selective diameter and type (metallic vs. semiconducting)

enrichment.

Acknowledgements