Carbon Nanotubes: Development of Nanomaterials for

Hydrogen Storage

Hongjie Dai

Department of Chemistry & Laboratory for Advanced Materials

Stanford University

GCEP, September 19, 2006

Outline• Can carbon nanotubes (CNTs) be used for

efficient hydrogen storage?- Physisorption (non-covalent)?- Chemisorption (covalent)?

• Synthesis of CNTs for hydrogen storage

• Interaction of CNTs with reactive H-species

Funding: GCEP; Collaborators: A. Nilson; K. Cho; B. Clemens.

Single-Walled Carbon Nanotubes (SWNT)

•diameter ~1-2 nm, ‘molecular wire’• length: 10 nm to 10mm

• Sp2 C-C bonding: one of the strongest bond in nature

• Multi-walled CNTs: ~ 10-20 nm in diameter

• Radius of curvature is important to physical/chemical properties

Chiral angleθ

~600 ºCplasma

Synthesis by Chemical Vapor Deposition

1-2 nm seed

CxHy catalyst

Furnace500 - 1000°C

CH4 → C+ H2

600 °C

plasma

Base growth

CVD PECVD

Yuegang Zhang, Yiming Li, J. Phys. Chem, 1999;

CxHy

Jing Kong, Hyongsok Soh, Calvin Quate, H. DaiNature, 1998.

Previous: Patterned Growth of SWNTs by CVD

CVD

pattern individual “seeds” grow 1 tube per “seed”

SWNTs Synthesis From Individual Nanoparticles

100 nm

2nm Fe seed

250 nm

CVD

300 nm

Ali Javey, H. Dai, JACS, 2005

CVD Growth of Vertically Aligned Multi-Walled CNTs

Shoushan Fan, Nathan Franklin, H. Dai., Science, 1999.

600 ºC5 nm

plasmaCH4

SWNT

A quasi-remote plasma enhanced CVD approach

Synthesis of Single-Walled Nanotubes by PECVD

CH4 → C+ H2

600-800 °C

1% O2

2 nm Fe particle as growth seed

High Yield Vertical-SWNTs by PECVD

G

100 200 300 1400 1600

Raman shift (cm-1)

RBM

20 nm

Exclusive single-walled, no multi-walled; Highly repeatable

Guangyu Zhang, et al., PNAS, 2005; GCEP patent

Grown byoxygenAssistedPECVD

Monolayer of ~2 nm Fe particles

(a) (b)

(e)

(c) (d)

(f)

1 inch

SiO2

V-SWNT film

Patterned Growth of Vertical SWNTs

Reproducible growth of vertical SWNTs

(a) (b) (c)

20 µm20 µm

(d) (e)

Vertical SWNTs (V-SWNT) Patterns

O2 Addition (~1%) Drastically Enhances PECVD of SWNTs From CH4 Plasma

5 µm

A B

With O2 Addition O2-free CH4 growth

Same catalyst, drastically different yield with and without oxygen!In plasma CVD, H species are detrimental to SWNT formationOxygen scavenging of H: O + H→ OH favors SWNT formation

O Scavenging of H: O + H→ OH Favors SWNT Growth

250 300 350

Wavelength (nm)

OH

Opt

ical

Em

issi

on

Plasma of CxHy: CxHy → C + H (H is inevitable)C is needed for growth of SWNT. H does not favor SWNT formation, especially for small tubes

Solution to the dilemma of PECVD: add oxygen (<4%)

H OH

×

C-feedingto growth

H – attacking sp2 C

Fe-C

O

C

Vertical SWNTs Transferred onto Various Substrates (Cu, polymer…)

A

B

H2O bath

V-SWNT film floating on H2O

Si substrate

Cu substrate

Cu

V-SWNT

Polymer binding layer

H2O bath

Vertical SWNTs on Various Substrates (Cu, polymer…)

Cu

Plastic

Glass

An Artistic Presentation of V-SWNT Films

2.5 nm

Hydrogenation @300K of SWNTs(Guangyu Zhang, et al., JACS, 2006)

200nm

SWNTs ‘swell’ upon hydrogenationSwelling uniform along tube length

• Atomic structure change due to hydrogenation

K.Park, et al., J. Phys. Chem B 109, 8967 (2005).

Theoretical Prediction

G

D

200

100

1800 1600 1400 1200Raman Shift (cm -1)

As-grownPlasmaAnnealed

Raman Spectroscopy of Hydrogenation/dehydrogenation of SWNTs

Hydrogenation decreases ‘G’/‘D’ band ratio.Complete reversal upon 500 ºC annealing (starts at ~200ºC).

IR

Tran

smitt

ance

1.00

0.99

3000 2900 2800 2700wavelength (cm-1)

RT Plasma 500 annealed

Infrared Spectroscopy of Hydrogenated/dehydrogenated SWNTs

CHx species observed on SWNTs, x=1, 22920cm-1: sp3 CH stretch/asymmetric stretch of sp3 CH2 groups.2850cm-1: symmetric stretch of sp3 CH2

A

CH/CH2

CH2

(c)

10-10

10-9

10-8

10-7

I ds (A

)

-20 -10 0 10 20Vgs (V)

S

D

Electrical Transport in a SWNT Through Hydrogenated/Dehydrogenation

Hydrogenation decreases conductance by orders of magnitudeReversible upon 500ºC annealing.

@300K, ‘high’Plasma power, 3min

10 min

Hydrogenation/Hydro-carbonation (Etching) Under ‘Harsh’ Plasma

200 300 4001.0

1.5

2.0

Dia

met

er (n

m)

T (οC)

200ºCplasma

300 ºCplasma

400 ºC plasma

10min

Hydrogenation/Hydro-carbonation (Etching) Under 200-400ºC

• Smaller diameter SWNTs are more easily etched• Diameter dependent chemical reactivity !

• Reversible hydrogenation can be achieved with SWNTs under specific hydrogenation conditions

• SWNTs can retain structural integrity through hydrogenation/dehydrogenation processes

• A viable hydrogen storage approach

• Optimum diameter of may SWNTs exists for repeated reversible hydrogenation/dehydrogenation at low temperatures

Conclusions Based on Microscopy, Spectroscopy & Electrical Data

Summary• Role of hydrogen in PECVD growth identified.• Highly efficient growth of SWNTs achieved.

• Reversible hydrogenation of SWNTs can be achieved with retained structures and properties.

• Hydrocarbonation/etching can occur.• Diameter dependent reactivity elucidated.

• Next: Identify optimum diameter of SWNTs that allow for hydrogenation and reversal at low temperatures.

AcknowledgementsGuangyu ZhangDavid MannPengfei QiYuerui LuXinran WangAli Javey

James GibbonsYoshio NishiAnders NilsonK. J. ChoBruce Clemens

GCEP