Hydrogen dissociation on CNT-Supported Pd-Clusters

Collaborators Group members   Chao Cao Luis Agapito Lan Li Lex Kemper Manoj Srivastava Yun-Wen Chen Yuning We Rashi Hadman Julio Palma Sabri Alkis

C. Zhang, Y. He, A. Malick J. Nicely, YX Wang P. Jiang, L.L. Wang A. Kolchin, M. Hedstrom R. Sedaghi, M. Du C. McKenney, S. Lauzat M. Wei, K. Morrison C. Schlubac

UF Nano-wire Jeffrey Krause (DOE)

High Tc Peter Hirschfeld

Multiscale simu. Rod Bartlett Jim Dufty Frank Harris Sam Trickey

Exptl. Art Hebard Andrew Rinzler Steve Hill

ORNL Xiao-Guang Zhang Predrag Krstic Paul Kent Thomas Maier

NW Univ. M. Ratner

Fudan Jing-Guang Che

Orsay (C.N.R.S) & Univ. of Paris Sud Catherine Brechignac Martin Schmidt Jerôme France

Sidney Yip (MIT) Pierre Deymier (UA)

Other Senior Collaborators

•  CNT- a natural choice for nano-molecular wire; stable, tunable Fermi energy by doping

•  Pd - a catalyst for H2 disociation; Pdn nano-catalyst

•  Pd+CNT - candidator for nano-chemical sensor/device: Pd for hydrogen detection

•  Understanding electron structure and transport properties of CNT doped by Pd4 and upon H2 adsorption

Recent development Experiments: High-Performance, Flexible Hydrogen Sensors That Use Carbon Nanotubes Decorated with Palladium

Nanoparticles, Yugang Sun* and H. Hau Wang Adv. Mater. 2007, 19, 2818–2823 Single-Walled Carbon Nanotubes Modified with Pd Nanoparticles: Unique Building Blocks for High-

Performance, Flexible Hydrogen Sensors, Yugang Sun,*,  H. Hau Wang,� and Minggang Xia, J. Phys. Chem. C 2008, 112, 1250-1259

Fabrication of interfaces between carbon nanotubes and catalytic palladium using dielectrophoresis and its applicationto hydrogen gas sensor, Junya Suehiro, Shin-Ichiro Hidaka, Shinji Yamane, Kiminobu Imasaka, Sensors and Actuators B 127 (2007) 505–511

Hydrogen sensors based on carbon nanotubes thin films, I. Sayagoa, E. Terradob,1, E. Lafuenteb,1, M.C. Horrilloa, W.K. Maserb,1, A.M. Benitob,1, R. Navarroc, E.P. Urriolabeitiac, M.T. Martinezb,1, J. Gutierreza, Synthetic Metals 148 (2005) 15–19

Sippe, Pd.D thesis with A. Rinzler (2005), J. Sippel-Oakley et al. Nanotechnology 16, 2218 (2006 J).

Theory Interactions of Hydrogen with Pd and Pd/Ni Alloy Chain-Functionalized Single Walled Carbon

Nanotubes from Density Functional Theory, Ling Miao, Venkat R. Bhethanabotla,* M. M. Ossowski, and Babu Joseph, J. Phys. Chem. B 2006, 110, 22415-22425

First-principle simuations of dissociated and molecular H2 adsorption on Pd4-cluester-functionalized carbon nanotubes, C.C Cao, Y. He, and H-P. Cheng, Phy. Rev. B77,045412 (2008)

Non-equilibrium Green’s function study of Pd4-cluester-functionalized carbon nanotubes ashydrongen sensor, Ca Chao, et al, submitted to PRB.

CNT film as hydrogen sensor

Expt. Of Prof. Rinzler At UF Sippel, thesis 2005

J. Sippel-Oakley et al. Nanotechnology 16, 2218 (2006).

Molecule-surface interaction

2) EF1)EF

EF

Orbital interaction Level shifting

emo es

o

Charge transfereo1 eo2

e1

e2

A typical semi-conductor CNT

Modification of the density of states of a (12,8) SWCNT- Blue-shaded regions are occupied by the electron after chemical doping. The position of V1 is the original Fermi energy level. (a) p-type; (b) n-type.

•  Density Functional Theory (PWSCF Package) Plane wave basis up to 32 Ryd; PBE exchange correlation functional; RRKJ ultra-soft pseudopotential; Density cut-off 400 Ryd; 1x1x32 Gamma centered K-points; Quasi Newtonian BFGS for ionic minimization; Gaussian smearing with width=0.007 Ryd; Löwdin’s charge analysis

•  Non-equilibrium Green function (smeagol) Geometries: DFT/PBE with double-Zeta numerical atomic basis; Troullier-Martin pseudo-potentials; Energy cut-off: 200 Ryd Conductance: single-zeta numerical atomic basis, LDA exchange-correlation functional, Device region Hatree potential is matched with leads.

+

(This picture is not to scale)

In x- and y- direction, a super-cell of 40 Angstrom is introduced to eliminate interactions between neighboring images.

x

y

y

x z

(5,5) CNT

Atop-Site: Physisorption Edge-site: Chemisorption

From Ernst D. German, et al, J. Phys. Chem. A, 2001, 105, 11312-11326

Pd-Pd (1): 2.67 Å (within 1 pyramid) Pd-Pd (2): 2.86 Å (across 2 pyramids)

Eb=1.06eV

Eb=0.37eV

Pd-H: 1.85 Å Pd-Pd (3): 2.64 Å (x-y plane) Pd-Pd (4): 2.90 Å (z)

H-H: 0.81 Å (0.75 Å) Pd-H: 1.86 Å Pd-Pd (1): 2.69 Å Pd-Pd (2): 2.83 Å

Cao, He, & Cheng, PRB 77, 045412(2008)

Two (5,5) CNT units per Pd4 cluster

No H2

Dissociated H2

Conductor

Semi-conductor Eg=128meV

Semi-conductor Eg=52meV

Ef is aligned

to 0.0 eV

Molecular H2

No Pd

Pd4- doped

Dissociated H2

† C(1) refers to C atoms (34) not bonded with Pd4 Ef is aligned to 0.0 eV

No H2

Molecular H2

Type Pure CNT CNT+Pd Molecular H2

Dis- H2

All C 160,00 160.09 160.12 160.08

C (1)† 136.00 135.81 135.85 135.89

All Pd - 39.91 39.90 39.83

All H - - 1.98 2.062

total 160.00 200.00 202.00 200.00

Close to the Fermi energy level: the Pd clusters dominate the highest occupied band, Pd clusters and CNT contribute roughly the same to the lowest unoccupied band.

C atom in contact with Pd4 looses electrons, but a (5,5) CNT gains electron H2 looses; 2H gain (slightly)

Localization of electrons: ρ = ρCNT+Pd - ρCNT- ρPd Red: electron rich; Blue: hole rich This leads to a semiconductor

Pd4 recovers its original configuration once H2 Is removed, indicating a robust nano-catalyst

For chemisorption,a flat band across Fermi level ⇒  lower vF hence conductance vF= 3.3x104 m/s vs. 1.1x105 m/s (no H2, and H2)

No H2

H2

B.E = 0.45 eV

2H B.E. = 1.30 eV

No H2

H2

B.E = 0.45 eV

2H B.E. = 1.30 eV

A systematic change in the band-gap width versus the coverage percentage is observed, which suggests the possibility of manipulating the CNT band structure by appropriate palladium doping. Especially with 100% palladium coverage, the CNT system turns out to be semiconducting due to the electron localization effect. All the investigated systems showed an increase in conductance upon hydrogen chemisorption but the mechanisms vary: at high coverage, the adsorption of hydrogen induced a substantial structural change and the clusters formed an atomic wire; at medium and low coverage, the dissociation of hydrogen greatly reduced the localization of the binding electrons between CNT and Pd4 clusters. The conductance change in this system suggests that the Pd-cluster-doped CNT can potentially serve as a hydrogen sensor.

Models for transport calculations

Electrode II Electrode I

: extended molecule, scattering region

Left lead Right lead

Right reservoir

µL µR

µIµII

What is measured when you measure a resistance - TheLandauer formula revisited, A. Douglas Stone A. Szafer, IBM Res. Develop. 32, No.3 384 (1988).

Molecular nanoelectronics Edited by Mark Reed and Takhee Lee, American Scientific

Publishers, 2003

First Experiment: M. Reed etal. Science, 278, 252(1997) Molecule as conductor: Takao Ishida etal., J. Phys. Chem. B 106, 5886 (2002);

David J. Wold etal. J. Phys. Chem. B, 106, 2813, (2002) Many more…..

First theoretical work: Aviram & Ratner, Chem. Phys. Lett. 29, p277 (1974) Planewave DFT plus scattering matrix: Wan, Mozos, H.G. et. al. APL, 71,

419(1997); Choi and Ihm, PRB, 59, 2267(1999) Planewave DFT plus Lippmann-Schwinger Eqn: Lang, PRB 52, 5335(1995); Di

Ventra & Lang, PRB 65, 045402 (2001); Tsukada, PRB 55, 4991(1997) Real space DFT plus Green’s functions: P. S. Krstic, X.-G. Zhang and W. H.

Buttler, Phys. Rev. B 66, 205319 (2002) Nardelli & Bernholc, PRB 64, 245423(2001); Palacios et.al, PRB 64, 115411(2001); Kobayashi etal (2003).

Real space DFT plus Keldysh non-equilibrium Green’s functions: Taylor, Guo, Wang, PRB 63, 245407(2001); Brandbyge, Mozos, Ordejon, Taylor & Stokbro, PRB 65, 165401(2002); Taylor, Stokbro & Guo, (2003); Xue, Datt, and Ratner, Chem. Phys. 281, 151 (2002).

Also: Yang et al

Linear Response Method Landauer formula ∑==

'

2'

22 22nn

nntheT

he

σ

Caroli formula ][ −+ΓΓ= GGTrT RL

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Γ{L,R } = i[Σ(L,R )+ − Σ(L ,R )

− ]

channel channel

( )±±±

Σ+Σ−−=

RLmolHEG 1

A tight-binding framework: i ja b

h

'h

Surface Green’s functions of two semi-infinite leads gaa and gbb can be evaluated in standard method, for details see P. S. Krstic, X.-G. Zhang and W. H. Buttler, Phys. Rev. B 66, 205319 (2002)

'nn

RΣ

Gij =Gijmol +Gii

molh+gaahGij +Gijmolh'gbbh

' +Gjj

C. Caroli et al,, J. Phys. C: Solid State. Phys.4, 916 (1971)

€

Σ

€

ΣL R

Rigorous when using DFT

Non-equilibrium Green’s function in conjunction density function theory

G < ( r

→

, r'→

,E) = G ij< (E)φi

*

ij∑ ( r

→

)φ j( r'→

)

G R, A(E) =

ψ iψ i*

E ± iε + − Eii∑

G < E( ) = G R E( )∑< G A E( )

ΓL,R = i[∑(L,R)

R −(∑L,RA )]

∑< E( ) = ∑L

< E( ) + ∑R< E( ) = iΓ L E( ) f E − µL( ) + iΓR E( ) f E − µR( )

T (E,V ) = Tr[Γ L(E,V )G R (E,V )ΓR (E,V )G A(E,V )]

GRG< : Counting electrons

: Electronic structure

G MM

R = E+SMM − HMM − ∑LR −∑R

R( )−1,∑L,R

R = E+SM(L,R) − HM(L,R)( )G LL,RRR E+S(L,R)M − H(L,R)M( )

Introduce G<

S :overlap matrix;  H :Hamiltonian matrix with DFT + bias

density matrix : ρij =

12�i

dE∫ G ij< (E)

Conductance : G =

2e2

hT (E f )

S.Datta, Electronic Transport in Mesoscopic Sysstem; Xue, Datt, and Ratner, Chem. Phys. 281, 151 (2002)

Self-consistent Procedure Obtain GLL

0  & GRR0Seperate lead  

calculation

Initial  guess      HMM

in Solve GMM  or GMM<

   Calculate density matrix

ρij =12πImag dZ  Gij Z( )

c∫

Mix HMMin  and  HMM

out

predict  new HMMin

Calculate HMMout

Compare HMMout  and  HMM

in

converged

not converged

Analyze data

ρij =12πi

dZ  Gij< Z( )

c∫

or

Left/Right leads: gold; device region contains 2 unit cells of leads on each side and 2 unit cells of Pd4 doped CNT (Medium coverage)

(8,0) CNT (5,5) CNT

(a) Clean Pd4

(b) H2 physisorption

(c) H2 chemisorption

Conductance

Conductance G of pure (5,5) and (8,0) CNT without Pd doping are 0.62 G0 and 0.23 G0, G0 = 2e2/h. Resistance comes from the contact of CNT and Au leads for (5,5) CNT. After Pd doping, G of the metallic CNTs decreases to 0.28 G0, the semiconducting CNTs increases to 0.54 G0; Upon H2 adsorption, G of (8,0) decreases to 0.27, (0.24) G0, for phys. (chem), G of (5,5) increases to 0.36 (0.53) Go for phys. (chem). Trend is reversed.

Sensitivity S=max(Gf/Gi,Gi/Gf) i and f are system before and after H2 adsorption

S~2 for (8,0) CNT, since 1/3 metal, 2/3 semiconductor, overall S~1.35 Agree with experimental data J. Kong et al Adv. Mater. 13, 1384 (2001)

Projected Density of States

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ρ(E ) = −1/πTr[ImG +(E )]

“Noise” in near EF in DOS of (5,5) CNT => Localized electron =>decreased G

New states in the gap, gap reduces G increases

Consistent with band calculations!

€

ρk(E) = −1/πTr[ImGk+(E)]

DOS Projected on CNT

Charge Transfer

Charge transfer causes different effects in the two types of systems: Semiconducting system, a significant shift in the conductance peaks and brings one of the peaks very close to EF, leading to a large increase in the conductance. Metallic system, the shift in the conductance peak near EF is much smaller, less than 0.01 eV. The large change in the conductance is mainly due to the sharpness of the peak. When integrated over a typical bias voltage window the effect of the charge transfer on the conductance in the metallic system is greatly reduced.

Hartree Potential

Hydrogen physisorption or chemisorption substantially modifies the electrostatic potential within the device region. No big differences are seen at the Au lead/CNT interfaces (around ±12˚A).

Local Density of State

(5,5) (8,0)

For metallic CNT based systems, little difference is observed on CNT backbone. However the states at the CNT/Pd-cluster interface are greatly reduced by hydrogen chemisorption. These states are strong chemical bonding between CNT/Pd-cluster, which serves as a scattering center in device region. Thus, the hydrogen chemisorption enhancs the electron transport.

Summary II •  Upon Pd doping, the conductance of metallic CNTs decreases

due to electron localization effects, and the conductance of the semiconducting CNTs increases since it creates new states that reduces the band gap.

•  For metallic (5,5) system, the conductance increases by ~90% with hydrogen chemisorption

•  For semiconducting (8,0) system, the adsorption suppresses the conductance by ~60%. These behavior is dominated by electron localization effect upon hydrogen adsorption, and is related to charge transfer.

•  Therefore both metallic and semiconducting CNTs are much better hydrogen sensing materials individually than mixed ensemble CNTs.

Current and future work •  GW approach to correlation at finite •  LDA+U for comparison

Complex physical models for CNT film, Pd doping

Projected DOS

€

ρk(E) = −1/πTr[ImGk+(E)]

Device Region

SMEAGOL (based on SIESTA) Single-zeta numerical basis LDA Zero Bias

Over 40% increase in Conductance

Ef is aligned to 0.0 eV

•  Localized states form when CNT is doped with Pd -> Semi-conductor

•  Molecular H2 adsorption has little effect on electronic properties of Pd doped CNT

•  Substantial structural change when H2 is dissociated -> Conductor

Thank you