Interaction between Single-walled Carbon Nanotubes and Water Molecules

Transcription

1 Workshop on Molecular Thermal Engineering Univ. of Tokyo Interaction between Single-walled Carbon Nanotubes and Water Molecules Shohei Chiashi Dept. of Mech. Eng., The Univ. of Tokyo, Japan

2 Outline 1. Background 2. Objective 3. Methods (experiment and simulation) 4. Results & Discussion 4.1 Spectroscopy (PL & Raman) 4.2 MD Simulations 5. Conclusions

3 SWNT & Water Molecules a few molecules large aspect ratio large specific surface one atomic layer single-walled carbon nanotube (SWNT) Adsorption energy per molecules: 1-2 kj/mol (DFT cal.) [1] Interaction between a few water molecules and graphene. macro-scale hydrophobicity (wettablity) of SWNTs in nano-scale Water drop on CNT carpet. [2] high contact angle -> hydrophobic [1] O. Leenaerts, et al., Phys. Rev. B 79 (2009) [2] K. K. S. Lau, et al., Nano Lett., 3 (2003) 1701.

4 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 Excitation (nm) in vacuum (4.0 Pa) (9,7) Emission (nm) [1] Y. Homma, S. Chiashi, Y. Kobayashi, Rep. Prog. Phys, 72 (2009)

5 Methods (Experiments & Simulation) SWNT sample suspended SWNTs length: 7 µm, diameter: 1 nm SEM image of suspended SWNT. 5 µm MD simulation NTV emsemble (SCIGRESS ver. 2.3, Fujitsu Ltd.) H 2 O: SPC/E force field SWNT: Tersoff carbon-h 2 O: Universal force field [1] [1] A. K. Rappe, et al., JACS 114 (1992) PL excitation: nm emission: nm Raman excitation: 785 nm Conditions temperature: -15 to 80 C gas atmosphere: water temperature controller vacuum chamber sample vacuum pump & mass flow controller excitation laser Schematic illustration of the environmental PL & Raman scattering measurement system.

6 Ethanol Gas Pressure Dependence of PL Spectra Excitation Wavelength (nm) Excitation Wavelength (nm) (A) Emission Wavelength (nm) (B) Emission Wavelength (nm) PL mapping of (9,8) SWNT (A) in ethanol gas atmosphere and (B) in vacuum. ethanol (C) (10,5) d t =1.02 nm (9,7) d t =1.07 nm (10,6) d t =1.08 nm PL spectra measured with decreasing ethanol gas pressure. S. Chiashi, S. Watanabe, T. Hanashima, Y. Homma, Nano Letters 8(2008) 3097.

7 Adsorption and Desorption of Ethanol Molecules P t : transition pressure condensed ethanol molecule Emission Wavelength (nm) (a) (b) (c) 28.2 C 27.0 C 25.4 C 24.5 C ethanol layer coverage θ ethanol νin νout Ethanol Gas Pressure (Torr) Relationship between ethanol gas pressure and emission wavelength. dielectric constant ε env optical transition energy E ii

8 PL maps in Vacuum & Water Vapor Excitation Wavelength (nm) Excitation Wavelength (nm) (B) Emission Wavelength (nm) (A) E ex Emission Wavelength (nm) water vapor In vacuum E em PL maps from suspended SWNT measured in (A) vacuum and (B) water vapor. E em energy E ex k Coulomb interaction between electron and hole hν ex F hole = exciton ε : dielectric constant (electron polarization) environmental effect (ε env ) electron q 4π ε r 1 q2 1 rˆ 2 (binding energy of the exciton: ~100 mev) PL spectroscopy molecular adsorption effect on PL spectra [1] -> water molecules adsorb on SWNT surface [1] S. Chiashi, Homma, et al., Nano Lett. 8 (2008) 3097.

9 Temp. & Pressure Dependence Emission Wavelength λ (nm) P t : transition pressure 20 C 25 C P t 30 C 35 C desorption Water Vapor Pressure P (T) adsorption Temperature (T) and water vapor pressure (P) dependence of emission wavelength. (heat of evaporation of bulk water: 44 kj/mol at 25 C) ln(p t ) 0.5 ln(t ) exp(-e cond /k B T) : E cond ~80 kj/mol /T (1/K) Temperature dependence of the transition pressure (P t ). water molecules show a transition phenomenon on SWNT surface (extremely large condensation energy)

10 Raman Scattering Spectroscopy RBM: radial breathing mode Intensity (arb. units) RBM peak G band up-shift in water vapour (630 Pa) in vacuum (4.0 Pa) C-C bond Raman Frequency (cm 1 ) Raman scattering spectra from SWNT measured in vacuum and water vapor. water molecules mechanically affect the vibration property of SWNTs the up-shift of RBM peak indicates that water molecules uniformly adsorb on the SWNT surface

11 Adsorption Layer (MD Simulation) 2nd layer 1st layer y x 1 nm gas phase of H 2 O 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! (uniform and thermally stable adsorption layer)

12 Structure of the Adsorption Layer (MD Simulation) (A) Radial Density Distribution (g/cm 3 ) in bulk water 1st layer =1000 =800 =600 2nd layer 3rd layer Position r (nm) Normalized Density of Hydrogen Bonds (arb. units) (B) r =0.843 nm in bulk water =1000 =800 = Degree θ ( ) (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. hydrogen bond Water molecules form adsorption layer with lateral hydrogen bonding. the origin of the stability, large condensation energy?

13 Comparison between Exp. & Sim. RBM peak Experiment MD simulation (A) Intensity (arb. units) up-shift (630 Pa) in vacuum (4.0 Pa) Raman Frequency (cm 1 ) RBM Spectral Density (arb. units) water vapor In vacuum (B) in water vapour up-shift in vacuum =1000 =800 = Vibrational Frequency (cm 1 ) (A) Experimentally measured RBM peak and (B) calculated radial breathing mode frequency. quantitative agreement between experimental and simulation results

14 Conclusions 1. PL and Raman scattering spectroscopy indicate that water molecules adsorb on SWNT surface. [1] 2. MD simulation clearly elucidate the structure of the adsorption layer. [1] 3. Although the interaction between SWNT surface and water molecules is week, water molecule form stable and uniform adsorption layer on SWNT surface. [1] [1] Y. Homma, S. Chiashi, T. Yamamoto, et al., Phys. Rev. Lett., 110 (2013)