Interaction between
Single-walled Carbon Nanotubes and
Water Molecules
Shohei Chiashi
Dept. of Mech. Eng., The Univ. of Tokyo, Japan
Workshop on Molecular Thermal Engineering Univ. of Tokyo 2013. 07. 05
Outline
1. Background 2. Objective
3. Methods (experiment and simulation) 4. Results & Discussion
4.1 Spectroscopy (PL & Raman) 4.2 MD Simulations
5. Conclusions
SWNT & Water Molecules
single-walled carbon nanotube (SWNT)
Water drop on CNT carpet. [2]
high contact angle -> “hydrophobic”
Adsorption energy per molecules:
1 - 2 kJ/mol (DFT cal.)[1]
Interaction between a few water molecules and graphene.
a few molecules
macro-scale
hydrophobicity (wettablity) of SWNTs in nano-scale
• large aspect ratio
• large specific surface
• one atomic layer
[1] O. Leenaerts, et al., Phys. Rev. B 79 (2009) 235440.
[2] K. K. S. Lau, et al., Nano Lett., 3 (2003) 1701.
Objective
Investigation of the interaction
between an SWNT and water molecules
by using
suspended SWNTs
[1],
photoluminescence (PL) spectroscopy, Raman scattering spectroscopy,
and molecular dynamics (MD) simulation
[1] Y. Homma, S. Chiashi, Y. Kobayashi, Rep. Prog. Phys, 72 (2009) 066502.
1250 1300 750
800
Emission (nm) Excitation (nm) in vacuum
(4.0 Pa)
(9,7)
Methods (Experiments & Simulation)
Schematic illustration of the environmental PL
& Raman scattering measurement system.
5 µm
SEM image of suspended SWNT.
• suspended SWNTs length: 7 µm,
diameter: 1 nm SWNT sample
• NTV emsemble
(SCIGRESS ver. 2.3, Fujitsu Ltd.)
• H2O: SPC/E force field
• SWNT: Tersoff
• carbon-H2O: Universal force field [1]
[1]A. K. Rappe, et al., JACS 114 (1992)10024.
excitation laser vacuum pump &
mass flow
controller sample
temperature controller
vacuum chamber
• temperature: -15 to 80 °C
• gas atmosphere: water PL
Raman
Conditions
• excitation: 690-830 nm
• emission: 1000-1600 nm
• excitation: 785 nm
MD simulation
Ethanol Gas Pressure Dependence of PL Spectra
PL spectra measured with decreasing ethanol gas pressure.
(9,7)
dt=1.07 nm (10,6)
dt=1.08 nm (10,5)
dt=1.02 nm
S. Chiashi, S. Watanabe, T. Hanashima, Y.
Homma, Nano Letters 8(2008) 3097.
PL mapping of (9,8) SWNT (A) in ethanol gas atmosphere
and (B) in vacuum.
(B)
1300 1350 1400
700 750 800
Emission Wavelength (nm)
Excitation Wavelength (nm)
1300 1350 1400
700 750 800
Emission Wavelength (nm)
Excitation Wavelength (nm) (A)
ethanol
(C)
10–1 100 101 102 1340
1360 1380
28.2 °C 27.0 °C 25.4 °C 24.5 °C
Ethanol Gas Pressure (Torr)
Emission Wavelength (nm)
Adsorption and Desorption of Ethanol Molecules
ethanol layer condensed ethanol
molecule
νin νout
ethanol
Relationship between ethanol gas pressure and emission wavelength.
(a) (b)
(c)
coverage θ dielectric constant εenv optical transition
energy Eii Pt : transition pressure
PL maps in Vacuum & Water Vapor
1100 1200 1300
700 750 800
Emission Wavelength (nm) Excitation Wavelength (nm)
1100 1200 1300
700 750 800
Emission Wavelength (nm) Excitation Wavelength (nm)
k Eem Eex
energy
Eem Eex
In vacuum water vapor
PL maps from suspended SWNT measured in (A) vacuum and
(B) water vapor.
hνex
electron hole
exciton
Coulomb interaction between electron and hole
ε : dielectric constant (electron polarization)
”environmental effect” (εenv)
2 2
1
1 ˆ
4 r
r q
q
ε
= π F F F F
(binding energy of the exciton: ~100 meV)
[1] S. Chiashi, Homma, et al., Nano Lett. 8 (2008) 3097.
PL spectroscopy molecular adsorption effect on PL spectra [1]
-> water molecules adsorb on SWNT surface (B)
(A)
0.01 0.1 1 10 1180
1200 1220 1240
20°C
25°C 30°C 35°C
Water Vapor Pressure P (T)
Emission Wavelength λ (nm)
Temp. & Pressure Dependence
Temperature (T) and water vapor pressure (P) dependence of emission wavelength.
0.0032 0.0033 0.0034 0
1 2 3
1/T (1/K) ln ( P )– 0 .5 l n ( T )
tPt: transition pressure
Pt
Temperature dependence of the transition pressure (Pt).
exp(-Econd/kBT) : Econd ~80 kJ/mol (heat of evaporation of bulk water:
44 kJ/mol at 25 °C)
adsorption
desorption
water molecules show a transition phenomenon on SWNT surface (extremely large condensation energy)
Raman Scattering Spectroscopy
1550 1600
200 250
Raman Frequency (cm
–1)
In te n s it y ( a rb . u n it s )
in water vapour (630 Pa) in vacuum (4.0 Pa)
G–band RBM peak
RBM: radial breathing mode
water molecules mechanically affect the vibration property of SWNTs up-shift
the up-shift of RBM peak indicates that water molecules uniformly adsorb on the SWNT surface
C-C bond
Raman scattering spectra from SWNT measured in vacuum and water vapor.
Adsorption Layer (MD Simulation)
x 1 nm y
2nd layer
1st layer
Snapshot of MD simulation of water molecules on an SWNT (13,0) at 25 °C.
water molecules form adsorption layer on SWNT, which is hydrophobic!
gas phase of H2O
(uniform and thermally stable adsorption layer)
Structure of the Adsorption Layer (MD Simulation)
–90 –60 –30 0 30 60 90
Degree θ (°)
Normalized Density of
N =600 N =800 N =1000
Hydrogen Bonds
r =0.843 nm
in bulk water H O
H O
(arb. units) H O
2 2 2
0.5 1 1.5 2
0 2 4
Position r (nm)
Radial Density N =600 N =800 N =1000 in bulk water 1st layer
2nd layer 3rd layer
H O H O H O
Distribution (g/cm )3 2
2 2
Water molecules form adsorption layer with lateral hydrogen bonding.
the origin of the stability, large condensation energy?
(A) Radial distribution functions of water
molecules around SWNT in a water vapor. (B) Angle distribution between hydrogen bonds in the
1st layer and the tangential plane of the tube surface.
(A) (B)
hydrogen bond
200 250
Raman Frequency (cm
–1) In te n s it y ( a rb . u n it s )
(630 Pa)in vacuum (4.0 Pa)
Comparison between Exp. & Sim.
200 250
Vibrational Frequency (cm
–1) R B M S p e c tr a l D e n s it y ( a rb . u n it s )
in
vacuum in water
vapour
N =600 N =800 N =1000H O
H O H O
2 2 2 Experiment
In vacuum water vapor
MD simulation
quantitative agreement between experimental and simulation results up-shift
up-shift RBM peak
(A) (B)
(A) Experimentally measured RBM peak and (B) calculated radial breathing mode frequency.
Conclusions
[1]Y. Homma, S. Chiashi, T. Yamamoto, et al., Phys. Rev. Lett., 110 (2013) 157402.
