Ultrafast Stratified Diffusion of Water Inside Carbon Nanotubes; Direct

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C: Physical Processes in Nanomaterials and Nanostructures

Ultrafast Stratified Diffusion of Water inside Carbon Nanotubes. Direct Experimental Evidence With 2D D-T NMR Spectroscopy 2

Jamal Hassan, Georgios Diamantopoulos, Lydia Gkoura, Marina Karagianni, Saeed M. Alhassan, Vijay Kumar Shankarayya Wadi, Marios S Katsiotis, Thomas Karagiannis, Michael Fardis, Nikolaos Panopoulos, Hae Jin Kim, Margarita Beazi-Katsioti, and Georgios Papavassiliou J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b01377 • Publication Date (Web): 17 Apr 2018 Downloaded from http://pubs.acs.org on April 17, 2018

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The Journal of Physical Chemistry

Ultrafast Stratied Diusion Of Water Inside Carbon Nanotubes. Direct Experimental Evidence With 2D D − T2 NMR Spectroscopy J. Hassan,

∗,†

¶

Kumar,

G. Diamantopoulos,

¶

M.S. Katsiotis,

Kim,

§

‡

L. Gkoura,

T. Karagiannis,

k

M. Beazi-Katsioti,

‡

¶

M. Karagianni,

‡

M. Fardis,

‡

¶

S. Alhassan,

‡

N. Panopoulos,

S.V.

H.J.

∗,‡

and G. Papavassiliou

†Department of Physics, Khalifa University of Science and Technology, UAE ‡Institute of Nanoscience and Nanotechnology, NCSR Demokritos, 15310 Aghia Paraskevi,

Attiki, Greece ¶Department of Chemical Engineering, Khalifa University of Science and Technology, UAE §Nano-Bio Electron Microscopy Research Group, Korea Basic Science Institute, 169-148,

Daejeon 305-806, Republic of Korea kSchool of Chemical Engineering, National Technical University of Athens, 9 Iroon

Polytechniou Street, 15780 Zografou, Athens, Greece E-mail: [email protected]; [email protected]

Abstract Water, when conned at the nanoscale acquires extraordinary transport properties. And yet there is no direct experimental evidence of these properties at nanoscale resolution.

Here, by using 2D NMR diusion-relaxation (D

spin-spin relaxation (T1

− T2 )

− T2 )

and spin-lattice -

spectroscopy, we succeeded to resolve at the nanoscale

water diusion in single and double-walled carbon nanotubes (SWCNT/DWCNT). In

1

ACS Paragon Plus Environment

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SWCNTs, spectra display the characteristic shape of uniform water diusion restricted in one dimension.

Remarkably, in DWCNTs water is shown to split into two axial

components with the inner one acquiring unusual ow properties: high fragility, ultrafast self-diusion coecient, and rigid molecular environment, revealing a stratied cooperative motion mechanism to underlie fast diusion in water saturated CNTs.

Introduction The behavior of water in the vicinity of nanoscaled surfaces and porous structures diverges signicantly from that of bulk water. For example, water in contact with graphene or carbon nanotubes (CNTs) shows astonishingly high transport properties,

1

with recent molecular

dynamics simulations (MDS) unveiling new exotic physics as the origin of this intriguing behavior.

Specically, the ultrafast diusion of water nanodroplets on graphene layers was

recently shown to be driven by propagating ripples in a motion resembling surng on sea waves.

2

Similarly, water molecules in CNTs were predicted to be coupled with the longitudi-

nal phonon modes of the nanotubes, leading to enhanced diusion of the conned water by more than 300%.

3

In the same consensus, membranes consisting of macroscopically aligned

CNTs with diameters as small as

2

nm are expected to exhibit orders of magnitude higher

water ow compared to predictions deriving from continuum hydrodynamics and Knudsen diusion models.

1

In sub-nanometer narrow CNT channels, MDS have shown that water forms an ordered one-dimensional chain, which spontaneously lls the hydrophobic CNT channels.

4

The es-

timated ow rates are comparable to those observed in biological pores such as aquaporin, gramicidin, and bacteriorhodosin.

57

Other MDS have shown that by increasing the CNT

diameter, water inside the CNTs forms ice nanotubes, comprised of a rolled sheet of cubic ice,

8

whilst by further increasing the CNT channel size, neutron scattering experiments

and MDS

1012

9

have shown that water molecules are organized in a stratied nanotubular

structure with a "chain"-like water component at the center of the CNTs. This intriguing

2


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molecular arrangement is central in explaining the fast water diusion in CNTs, is of critical importance in many aplications.

9,13

which

Today, it is generally considered that fast

dynamics in hydrophobic nanochannels such as CNTs takes place either in the form of a single le,

14

where water molecules can not surpass each other due to space limitation, or in

a ballistic way, i.e. very fast clusters of hydrogen bonded water molecules diuse in a highly coordinated way.

4,15

However, most of the facts regarding the nature of water diusion in

nanopores at molecular scale have been almost solely acquired by MDS studies; until now experimental work is very scarce. Here, on the basis of 2D

1

H NMR diusion-relaxation (D

− T2 )

and relaxation (T1

− T2 )

spectroscopy, we succeeded in resolving experimentally at the nanoscale the distribution of the self-diusion coecient that in SWCNTs, the (D

D

of water inside SWCNT and DWCNT. Our experiments show

− T2 )

NMR spectra exhibit the characteristic shape of a uniform

water diusion in randomly oriented one dimensional nanochannels with self-diusion coecient

D

values, similar to those of free water.

Most important, diusion in DWCNTs

becomes non-uniform; a second water component is observed, which is assigned to an axial water component with

D

values four times that of bulk water at

T = 285K.

To the best of

our knowledge, this is the rst direct experimental evidence of stratied water diusion in CNTs. It is furthermore noticed that the time window of the NMR diusion measurements is

2−3

orders of magnitude longer than in MDS. This allows assessment even of ultra-slow

motion, such as of macromolecules through biological membranes, which makes the presented methodology applicable in important biological processes.

Experimental Section Puried and open ended single and double walled carbon nanotubes, SWCNT and DWCNT, were purchased from SES research, USA. The inner diameter of the CNTs used in this work were

≈ 1.2

nm for the SWCNT and

≈ 3.5

nm for the DWCNT. Both types of CNTs had

3



average length

≈ 1 µm.

Page 4 of 20

The samples were characterized using a TEM-FEI Tecnai G20

instrument with a 0.11 nm point to point resolution. More information on the samples and their TEM images are available in the Supporting Information. For the NMR experiments, CNT powders were used with no further treatment and double distilled water was used in all the measurements. 2D

D − T2

H NMR two-dimensional diusion-relaxation

measurements were performed in the stray eld of a

34.7

magnet providing a

101.324MHz. 5000

1

4.7T Bruker superconductive

T/m constant magnetic eld gradient at

1

H NMR frequency of

The experiments were carried out by using a pulse sequence with more than

pulses (more detail can be found in Supporting Information).

The temperature was

controlled by an ITC5 temperature controller in a ow type Oxford cryostat. The accuracy of the temperature was

0.1 K. A thirty-minute time window was allowed at each temper-

ature before collecting data. NMR data were analyzed using a 2D non-negative Tikhonov regularization inversion (discussed in the Supporting Information) algorithm code, developed

1

by the authors. The

H Magic Angle Spinning (MAS) NMR experiments were performed at

room temperature on a BRUKER (AVANCE 400) NMR spectrometer, in 4 mm rotors, and at spinning frequency of

12kHz.

Results and discussion Fig. 1a, shows the

1

shows the relevant

1

rate of

12

H NMR

T1

distribution analysis of water in DWCNTs. The right inset

H magic angle spinning (MAS) NMR spectrum, obtained at spinning

kHz. Two peaks are observed, in agreement with the literature;

16

a broad one at

2.73 ppm, which is attributed to water inside the CNTs (nanotubular water) and possibly to water in the space between the CNT bundles (interstitial water) and a relatively narrow peak at

4.9 ppm, very close to the frequency of bulk water, 4.8 ppm.

The frequency shift of the rst

peak from the frequency of the bulk water may be assigned either to less number of hydrogen bonds per water molecules inside CNTs, or to shielding from ring currents induced on the

4


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1 Figure 1: (a) H MAS NMR

T1

right inset shows the relevant Peak

1

analysis of water in DWCNTs, at room temperature. The 1 H MAS NMR spectrum at spinning-frequency of 12kHz.

corresponds to bulk water and the broad peak 2 to nanotube/interstitial water. The 1 H MAS NMR relaxation data from the shaded areas of the spectrum

main panel shows

(saturation recovery curves).

The solid lines are theoretical ts by using an 1D inversion

algorithm. The left inset shows the relevant T1 distributions of bulk and conned water, 1 respectively, obtained by the inversion. (b) Contour plot of the static (no MAS) H NMR

T1 − T2

spectrum of water in DWCNTs, at room temperature. Based on the

T2 /T1

ratios,

bulk, interstitial, and nanotube water can be resolved.

CNT walls.

16

The main panel shows the experimental relaxation data acquired separately

on each of the two peaks, by using a saturation recovery technique.

17

The

T1

analysis was

then performed by implementing an 1D inversion algorithm on the relaxation data (left inset of Fig.

1a).

18

Details on the inversion can be found in the Supporting Information.

It is observed that bulk water acquires

T1

corresponding to the shaded area around holds

T1

distribution with a peak at

distribution with peak at

4.9ppm),

≈ 0.15

≈ 1.2

s (violet line

while the nanotube/interstitial water

s (cyan line corresponding to the shaded area

around

2.73

T1 ≈ 2

s, due to the unavoidable presence of paramagnetic impurities on the CNT walls.

ppm).

The weak violet

T1

Both values are suciently shorter than that of distilled bulk water

peak at

≈ 0.15

s belongs to the tail of the nanotubular water NMR

signal, which overlaps with the NMR signal of the bulk water. In addition, a second weak

T1

peak at

≈1

s is observed in the cyan line, which shows that a small component of the

conned water acquires bulk water dynamics.

5



To further examine dierent water groups in CNTs,

1

Page 6 of 20

H NMR

T1 − T2

correlation spec-

troscopy was performed under static conditions (no MAS), as shown in Fig. 1b. 2D

T1 − T2

provides information about the liquidity of the local molecular environment:

in gen-

T2 ≈ T1 ,

whereas

eral, molecules in unconstraint liquid environment are characterized by molecules in a rigid (solid) environment show

T2 ≈ T1 , T2

Ultrafast Stratified Diffusion of Water Inside Carbon Nanotubes; Direct

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