Transcript Wai-Leung Yim, XG Gong, and Zhi-Feng LiuJ. Phys. Chem. B 2003

Nanotube as a gas Sensor
Outline
• What are Carbon nanotubes?
– Types
– Properties
– Applications
• Motivation
• Approach
• Results
– Preliminary results
• Symmetry and
• Basis set Effect
– NO2 +CNT
• Future Work
• Conclusion
• Acknowledgements
• References
What are carbon nanotube
•Carbon nanotubes, long, thin cylinders of carbon, were
discovered in 1991 by S. Iijima
• They can be considered as rolled up graphene tubes of carbon
• There are two types: SWNT and MWNT.
•It has very strong C-C chemical bonding.
Types of SWNT
A chiral vector Ch characterizes the
nanotubes Ch=na1+na2, where a1 and a2
are lattice vectors of the 2D hexagonal
lattice, and n and m are integers
Properties of nanotubes
• They can be either metals or semiconductors with different
size energy gaps, depending diameter and helicity of the
tubes, on the indices (n,m)
• A SWNT is considered metallic if the value n - m is
divisible by three. Otherwise, the nanotube is semi
conducting.
• Ultra-small SWNTs (diameter 4Å) exhibit
Superconductivity below 20K.
• Nanotubes are very strong with very high Youngs Modulus
and extremely flexible
• High thermal conductivity.
• High sensitivity to gas adsorption
Applications
• Micro-electronics / semiconductors
• Controlled Drug Delivery/release
• Field Effect transistors and Single electron transistors
• Nano electronics
• Nanogear
• Hydrogen Storage
Motivation
• Use of carbon nanotubes as chemical sensors for gases
like NH3 and NO2 was first demonstrated by Kong et al.
• Electrical conductance of an SWNTs increases by three
orders of magnitude when exposed to NO2 and to decrease
by 2 orders of magnitude in the presence of Ammonia.
• In general all the papers have predicted physisorption
between NO2, followed by charge transfer from tube to
molecule
• There is very little or no interaction between NH3 and
SWNT . The interaction between NH3 and SWNT was
studied by photoemission spectroscopy and it was found
out that the tube is sensitive to NH3 even though the
sensitivity is lesser as cmpd to NO2
Approach
• The motivation for out study was to study the nanotube gas
interaction by approximating the nanotube as a molecule of
specific length and using semi empirical method (PM3) to
predict the change in properties.
• A 10, 0 semiconducting nanotube was used for our study.
• To study the change in the electronic structure of the tube
– when NO2 physisorbs
– NO2 chemisorbs
– PES with varying C-N bond length
– Rotational PES for NO2 in chemisorbed well and
physisorbed well
– Effect of adsorption of 2 NO2 molecules
Effect of symmetry
D10d symmetry
D10h symmetry
•The symmetry of the nanotube depends on the number of hexagons
along the tube axis and along the circumference.
•They to, 2 different types of point groups, Dnh (with horizontal
mirror planes) and Dnd (with dihedral mirror planes)
Effect of symmetry on the LUMO and
HOMO
Single point calculations using DFT(B3PW91 ) and HF
Effect of Symmetry
LUMO and HOMO of D10h tube
Band gap and dipole moment
Basis set effect
•The DFT and HF calculations done using the STO3G basis set concentrate the frontier orbitals of the
carbon nanotube on the edge carbon atoms While the
Hf / 3-21G split valence basis set distributes the
orbitals along the axis of the nanotubes resulting in
delocalized HOMO- LUMO orbitals.
Length effect
HOMO of (10,0 )N=5 and N= 7
energy band gap of N= 5 tube = 0.255
LUMO 10,0 N=5 ,and 7
HOMO and LUMO ,CNT +NO2
HOMO and LUMO orbital for NO2 at a dist of 1.8 and 2.61A
PES wrt C-N
PES of Rotation
PES of rotation in Chemisorbed well
Pes for rotation in physisorbwell
1.646
1.6382
1.638
1.6378
Series1
1.6376
1.6374
Ene rgy ( ha rtre e s)
dist(1.8)
energy at (2.61 A)
1.6384
1.645
1.644
1.643
Series1
1.642
1.641
1.64
1.6372
0
50
100
150
Angle between No2 and CNT z axis(deg)
C-N = 2.61A
200
0
50
100
Angle (deg)
C-N = 1.8A
150
200
NO2 in
Type
Energy
band gap
plain CNT
2.2320
0.045
Physisorbed
1.6374
0.255
Chemisorbed
1.6481
0.261
No2in
1.6105
0.272
Model Chemistry
Type
Band gap
CNT
0.045
CNT+NO2(PM3)
0.255
CNT+NO2(ROHF/3-21g
0.016
CNT +NO2 in (PM3)
0.272
CNT +NO2 in (ROHF/3-21g)
0.011
2NO2(PM3)
0.06
Binding energy
BE = (Emolecule+CNT – ECNT –ENO2 )
Type
binding energy
Physisorbed
-0.5916
Chemisorbed
-0.5787
NO2 in
-0.618
2NO2
0.0326
Future Work
• To evaluate the binding energies of the structures
using ROHF/3-21G level of theory.
• To determine the amt and type of charge transfer
in the system
• To study the method for regeneration process
– Either by the route of Chemical reaction or
– By investigating the Energy difference for 2 NO2
molecules on the surface
• To study the behavior of the tube in the presence
of the electric field
• To compare the more favourable position for NO2
inside or outside the nanotube
Conclusion
• Symmetry of the nanotube fragment affects the
nature of the frontier orbitals
• As the length of the nanotube increases , the
orbitals are less delocalized
• PM3 introduces a spin contamination which can
be potentially solved by doing a single point at
higher level of theory
• From the current calculation NO2 prefers to be
inside the Nanotube
• Between chemisorbed and Physisorbed region,
physisorption seems to be the preferred state.
Acknowledgment
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Dr Schlegel
Dr Goldfield
Dr Hratchian
Dr Anand
Dr Knox
Jie Lie
Stan Smith
Barbara Munk
References
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http://www.pa.msu.edu/cmp/csc/ntproperties/
http://dagotto.phys.utk.edu/condensed/noppi.carbon.2.pdf
http://physicsweb.org/articles/world/11/1/9
http://www.e-nanoscience.com/application.html
Teri Wang Odom; Jin-Lin Huang; Philip Kim; Charles M. Lieber, J.
Phys. Chem. B, 2000, 104, 2794-2809.
L.G. Bulusheva; A.V. Okotrub; D.A. Romanov; D. Tomanek, J. Phys.
Chem.A, 1998, 102, 975-981.
M.J.Frisch et al., GAUSSIAN 03, Revision B.05, Gaussian Inc.,
Pittsburgh PA, 2003.
Shu Peng a,, Kyeongjae Cho a, Pengfei Qi , Hongjie Dai, Chemical
Physics Letters 387 (2004) 271–276
Wai-Leung Yim, X. G. Gong, and Zhi-Feng LiuJ. Phys. Chem. B
2003, 107, 9363-9369