Coaxial nanotubes

DOI: 10.1002/smll.200600055

Molecular-Dynamic Studies of Carbon–Water–CarbonComposite NanotubesJian Zou, Baohua Ji, Xi-Qiao Feng, and Huajian Gao*

We recently reported the discovery via molecular-dynamic simulationsthat single-walled carbon nanotubes (SWCNTs) with different diameters,lengths, and chiralities can coaxially self-assemble into multi-walledcarbon nanotubes (MWCNTs) in water via the spontaneous insertion ofsmaller tubes into larger ones.[1] Here, we extend that study to investigatethe various water structures formed between two selected SWCNTs aftersuch coaxial assembly. Depending on the tube geometry, typical waterstructures, besides the bulk phase, include a one-dimensional (1D)ordered water chain inside the smaller tube, a uniform or nonuniformwater shell between the two tubes, and a “boundary layer” of water nearthe exterior wall of the larger tube. It was found that a concentric watershell consisting of up to three layers of water molecules can form betweenthe two SWCNTs, which leads to a class of carbon–water–carboncomposite nanotubes. Analysis of the potential energy of the SWCNT–water system indicated that the composite nanotubes are stabilized byboth the tube–tube and tube–water van der Waals interactions. Geometri-cally confined between the two SWCNTs, water mono- and bilayers arefound to be stable, highly condensed, and ordered, although the averagenumber of hydrogen bonds per water molecule is reduced. In contrast, awater trilayer between the two CNTs can be easily disrupted by thermalfluctuations.

Keywords:· carbon nanotubes· molecular dynamics· self-assembly· simulations· water layers

1. Introduction

Carbon nanotubes (CNTs)[2] have great potential in nu-merous applications owing to their unique properties.[3,4] Forexample, the inner hollow cavity of a CNT may be used as aporous structure for filling or functionalization with varioustypes of nanoparticles and molecules,[5–13] as a fluid nano-

channel,[14–18] or as a low-dimensional confinement for phasetransitions.[19–25]

The interaction between water and CNTs has attractedconsiderable attention because of its significance in bothnanotechnology and biomedical applications.[15–17,21–27] It isknown that liquid around a solid surface tends to form lay-ered structures that mimick the structure of the solid,[28,29]

and that water confined in a nanometer-sized space may ex-hibit properties distinctly different from its macroscopicstate.[20–25] In a single-walled (SW) CNT with a sufficientlylarge radius, for example, (20,20), where (m,n) denotes theCNT indices, water tends to exhibit a three-dimensional(3D) bulklike structure.[21] When the CNT radius decreases,however, special water structures such as a one-dimensional(1D) water chain may form.[21–25] In addition, water mole-

[*] Dr. J. Zou, Prof. Dr. B. Ji, Prof. Dr. X.-Q. FengDepartment of Engineering Mechanics, Tsinghua UniversityBeijing 100084 (China)

Prof. Dr. H. GaoDivision of EngineeringBrown University182 Hope Street, Providence, RI 02912 (USA)Fax: (+1)401-863-9025E-mail: Huajian_Gao@brown.edu

1348 ? 2006 Wiley-VCH Verlag GmbH&Co. KGaA, Weinheim small 2006, 2, No. 11, 1348– 1355

full papersH. Gao et al.

SPECIALSE

CTION

cules close to the exterior[26] or interior[27] wall of a CNT arefound to exhibit characteristic layering similar to that at thegraphite–water interface.[30] Molecular dynamics (MD) hasbeen a reliable and powerful simulation tool in studyingsuch small systems.[1,8–11,20–27]

We have recently found via MD simulations thatSWCNTs of different size and chirality can self-assembleinto multi-walled (MW) CNTs in water via sequential co-axial insertion of smaller tubes into larger ones.[1] This co-axial assembly is driven by intertube van der Waals (vdW)forces and is found to be strongly tube-size dependent.Here, we investigate the various water structures formed be-tween two selected CNTs after such coaxial assembly. It isfound that water molecules confined in the annular gap be-tween the walls of two selected CNTs can form variousquasi-two-dimensional (Q2D) structures. Using special com-binations of CNTs, the annular Q2D water shell can be astable, highly condensed, and ordered monolayer or bilayerstructure that differs significantly from the water structuresconfined in an SWCNT.[21–25] The resulting concentriccarbon–water–carbon composite nanotubes, which resemblean MWCNT, are stabilized by both the tube–tube and tube–water vdW interactions. The water molecules confined inthe annular gap can also form a trilayer shell when thespace is large enough, but this trilayer structure is found tobe unstable due to relatively weak confinement.

2. Simulation Methods

The MD program GROMACS[31,32] was used to studythe structural characteristics of water around two uncappedand uncharged armchair SWCNTs dissolved in water con-tained in a 5<5<14-nm3 orthorhombic box at 300 K and anatmospheric pressure of 1 bar[33] with periodic boundaryconditions applied in three orthogonal directions. The elec-trostatic interactions were evaluated using the particle-meshEwald (PME) method with cubic spline interpolation and0.1-nm grid width.[34]

The carbon–carbon interaction of CNTs is described bya Morse potential for stretching, a harmonic cosine potentialfor bending, a twofold cosine potential for torsion, and aLennard-Jones (LJ) potential for nonbonding interac-tions:[26]

Uðrij;qijk; �ijklÞ ¼ KCr½expð�gðrij � rCÞÞ � 1�2

þ 12KCqðcosqijk � cosqCÞ2 þ

12KC�ð1þ cosð2�ijkl � �CÞÞ

þ4eCCsCC

rij

� �12

� sCC

rij

� �6� � ð1Þ

where rij denotes the distance between two bonded carbonatoms; qijk and �ijkl are the bending and torsional angles, re-spectively; rC, qC, and �C are the reference geometrical pa-rameters for graphene; KCr, KCq, and KCf are force constantsof stretching, bending, and torsion, respectively, and eCC andsCC are the carbon–carbon LJ parameters.

Accounting for intramolecular degrees of freedom, in-cluding the harmonic O�H bond stretching and H–O–H-

angle bending, the water solvent is described by the empiri-cal transferable intermolecular-potential 3-point (TIP3P)model as[35]

Uðrij;qijkÞ ¼12KWrðrij � rWÞ2 þ

12KWqðcos qijk � cos qWÞ2 ð2Þ

where rW and qW denote the reference O�H bond lengthand H–O–H angle, respectively, and KWr and KWq are thecorresponding force constants. The nonbonding interactionsbetween water molecules include an oxygen–oxygen LJ po-tential and an electrostatic potential between point chargeson oxygen and hydrogen atoms. Constraints using theSETTLE[36] algorithm are applied to the O�H-bond lengthfor water molecules.

The CNT–water interaction is modeled by a carbon–oxygen LJ potential with corresponding parameters eCO=

0.4787 kJmol�1 and sCO=0.3275 nm.[22] All interaction pa-rameters are summarized in Table 1 and Table 2.

In these simulations, two CNTs with the same length of4.79 nm but with different radii were initially coaxiallyaligned, with the nearest end-to-end distance of 0.2 nm, asshown in Figure 1. No position restraint was applied to theCNTs so that they were free to have both translational androtational motion. For all the systems studied in this paper,the radii of the small and the large CNTs, as well as the dif-ference DR, are listed in Table 3. Each simulation was car-ried out for at least 500 ps to ensure that the self-assemblyof CNTs had completed and that the whole system hadreached equilibrium, which was confirmed by monitoring

Table 1. Parameters for interaction potentials used in the simula-tions.

Parameters[a]

KCr=478.9 kJmol�1 rC=0.1418 nmg=21.867 nm�1

KCq=562.2 kJmol�1 qC=1208KCf=25.12 kJmol�1 fC=1808eCC=0.3601 kJmol�1 sCC=0.3400 nmeCO=0.4787 kJmol�1 sCO=0.3275 nm

[a] The bonding parameters for CNTs are taken from Ref. [26] (KCr, rCand g are the parameters for the Morse bond potential; KCq and qCare the parameters for the bending-angle potential; KCf and fC arethe torsion parameters), and the nonbonding parameters are takenfrom Ref. [22] (eCC and sCC are the Lennard-Jones parameters for car-bon–carbon interactions; eCO and sCO are the Lennard-Jones parame-ters for carbon–oxygen interactions). Carbon atoms are treated asuncharged particles, that is, qC=0.

Table 2. Parameters for the TIP3P water model.[35]

Parameters[a]

KWr=502416 kJmol�1 rW=0.09572 nmKWq=628.02 kJmol�1 qW=104.528eOO=0.6367 kJmol�1 sOO=0.3151 nmqO= -0.834e qH=0.417e

[a] e is the electron charge.

small 2006, 2, No. 11, 1348– 1355 ? 2006 Wiley-VCH Verlag GmbH&Co. KGaA, Weinheim www.small-journal.com 1349

Coaxial Self-AssemblySPECIAL

SECTIO

N

the center-of-mass (COM) distance between the two CNTsand the total potential energy of the system. An additionalsimulation time of 500 ps was used to collect data to analyzethe equilibrated water structures in the composite-nanotubesystem.

3. Results and Discussion

To explore the CNT–water interactions, we performed aseries of MD simulations of the self-assembly of two select-ed CNTs with different radii, as shown in Figure 1. We fo-cused on the equilibrium structures of the CNT–water sys-tems formed through the coaxial insertion of a (5,5) CNTinto a larger armchair (m,m) nanotube, where m rangedfrom 10 to 25 (referred to as Cases A to P, as listed inTable 3). Only armchair CNTs are reported here becausethe simulations seemed to indicate that the CNT chiralitieshave little influence on the assembly processes and the equi-librated water structures.[12]

First, consider the self-assembly process of a (5,5) tubeinto a (10,10) tube in water (Case A). A simulation snap-shot after the assembly has completed is shown in Fig-ure 2A. The corresponding radial map of the time-averagedwater density at the equilibrium state is given in Figure 3A.It is observed that after the assembly has completed and the

whole system has reached equilibri-um, a stable 1D ordered waterchain inside the (5,5) tube and a“boundary layer” of water near theexterior wall of the (10,10) tubehave formed, but no water is foundinside the annular gap between thetwo CNTs because the difference inradii of the two CNTs, DR=

R(10,10)�R(5,5)=0.3385 nm, is aboutequal to the carbon–oxygen LJ pa-rameter sCO=0.3275 nm. Such anarrow gap does not allow watermolecules to enter. The waterradial-density profiles before andafter the assembly are compared inFigure 4A. Before the assemblyprocess starts (t=0 ps), water mole-cules have a continuous distributioninside the (10,10) tube. The systemreaches equilibrium shortly afterthe coaxial assembly has complet-ed, at which point the water densityinside the (10,10) tube becomesalmost zero. In other words, mostof the water molecules originallyinside the (10,10) tube are drivenout by the (5,5) tube.

For the assembly of a (5,5) tubeinto an (11,11) or a (12,12) tube(Cases B and C), it can be calculat-ed that DR=R(11,11)�R(5,5)=

0.4062 nm and DR=R(12,12)�R(5,5)=

Figure 1. The initial configuration of (5,5) and (10,10) CNTs in water. A) Side view. B) Top view. The two CNTshave the same length of 4.79 nm. The red and white spheres are oxygen and hydrogen atoms, respectively.

Figure 2. Snapshots of equilibrated CNT–water composites aftercoaxial self-assembly of a small tube into a large one. Subplots (A–P) correspond to the 16 cases listed in Table 3.

Table 3. The parameters for the different simulation systems of two SWCNTs in water.[a]

Case ACHTUNGTRENNUNG(m, n) for two CNTs Radius of small CNT [nm] Radius of large CNT [nm] DR [nm]

A ACHTUNGTRENNUNG(5,5), (10,10) 0.3385 0.6770 0.3385B ACHTUNGTRENNUNG(5,5), (11,11) 0.3385 0.7447 0.4062C ACHTUNGTRENNUNG(5,5), (12,12) 0.3385 0.8125 0.4739D ACHTUNGTRENNUNG(5,5), (13,13) 0.3385 0.8802 0.5416E ACHTUNGTRENNUNG(5,5), (14,14) 0.3385 0.9479 0.6093F ACHTUNGTRENNUNG(5,5), (15,15) 0.3385 1.0156 0.6770G ACHTUNGTRENNUNG(5,5), (16,16) 0.3385 1.0833 0.7447H ACHTUNGTRENNUNG(5,5), (17,17) 0.3385 1.1510 0.8125I ACHTUNGTRENNUNG(5,5), (18,18) 0.3385 1.2187 0.8802J ACHTUNGTRENNUNG(5,5), (19,19) 0.3385 1.2864 0.9479K ACHTUNGTRENNUNG(5,5), (20,20) 0.3385 1.3541 1.0156L ACHTUNGTRENNUNG(5,5), (21,21) 0.3385 1.4218 1.0833M ACHTUNGTRENNUNG(5,5), (22,22) 0.3385 1.4895 1.1510N ACHTUNGTRENNUNG(5,5), (23,23) 0.3385 1.5572 1.2187O ACHTUNGTRENNUNG(5,5), (24,24) 0.3385 1.6249 1.2864P ACHTUNGTRENNUNG(5,5), (25,25) 0.3385 1.6926 1.3541

[a] All the CNTs have the same length of 4.79 nm. (m,n) denote the CNT indices. DR denotes the differ-ence in the two CNT radii.

1350 www.small-journal.com ? 2006 Wiley-VCH Verlag GmbH&Co. KGaA, Weinheim small 2006, 2, No. 11, 1348– 1355

full papersH. Gao et al.

SPECIALSE

CTION

0.4739 nm, respectively, values which are smaller than2sCO=0.6500 nm but larger than sCO=0.3400 nm. In thesecases, the spacing between the two assembled CNTs is toosmall to accommodate a uniform annular water layer. How-ever, the (5,5) tube was observed to shift away from thetube axis toward the interior wall of the large tube due tothe tube–tube vdW attraction, and the initially circular crosssection of the large tube deformed into an elliptical shapebecause of the pressure from surrounding water moleculesand the asymmetry of the CNT–water system. After thisshift, the maximum spacing of the gap between the (5,5)and (12,12) tubes is estimated to be more than DR<2�sCC=0.6078 nm, nearly twice that of sCO. Consequently, asmall number of water molecules are observed to “swim”into this gap (see Figures 2C and 3C). Similarly, when a(13,13) or (14,14) CNT is used as the large tube (Cases Dand E), it undergoes an even larger deformation and morewater molecules are accommodated between the two CNTs(Figures 2E and 3E for Case E).

The assembly of a (5,5) tube into a (15,15) tube (CaseF) is an especially interesting case. The (5,5) and (15,15)CNTs are observed to assemble into a concentric structurewith a uniform water monolayer confined between the twoCNTs (Figures 2F and 3F). In this case, DR=0.6770 nm isabout twice that of sCO so that the annular gap between thetwo CNTs is large enough to accommodate a complete cy-

Figure 3. Radial maps of time-averaged water density for equilibratedCNT–water composites after coaxial assembly of two CNTs. Subplots(A–P) correspond to the 16 cases listed in Table 3.

Figure 4. The radial-density profiles of water at t=0 ps and at equilibrium states, normalized by the bulk-water density r0 and averaged overthe cylindrical shells centered at the axis of the CNTs with radius r. A) (5,5) and (10,10), B) (5,5) and (15,15), C) (5,5) and (19,19), andD) (5,5) and (24,24) CNTs. The positions of the walls of the two CNTs are indicated by the dashed lines.

small 2006, 2, No. 11, 1348– 1355 ? 2006 Wiley-VCH Verlag GmbH&Co. KGaA, Weinheim www.small-journal.com 1351

Coaxial Self-AssemblySPECIAL

SECTIO

N

lindrical water monolayer, or a “water nanotube”. The re-sulting (5,5)–water–(15,15) composite nanotubes exhibit anearly perfect concentric ring structure resembling a triple-walled CNT.

For a (5,5) CNT entering a (16,16), (17,17), or (18,18)CNT (Cases G, H, and I), the size of the annular gap be-tween the two CNTs increases but cannot accommodate auniform water bilayer. Driven by the internanotube vdW in-teractions, the (5,5) tube shifts away from the tube axistoward the interior wall of the large tube and the latter de-forms into an elliptical shape. Consequently, the water mol-ecules confined in the gap form a nonuniform ring consist-ing of a partial monolayer on one side and a partial bilayeron the other (Figures 2I and 3I for Case I).

For the case of a (5,5) tube assembled into a (19,19)tube (Case J), the difference in radii DR=0.9479 nm isclose to 2sCO+sOO=0.9701 nm, which is large enough to ac-commodate a uniform water bilayer, as shown in Figures 2Jand 3J, which leads to a tetralayered structure of (5,5)–bi-layer–(19,19).

We continued to increase DR by using (20,20), (21,21),(22,22), (23,23), and (24,24) CNTs as the large tube (CasesK–O). For Cases K–M, the water confined between the twoCNTs has an asymmetrically ring-shaped structure consist-ing of a partial bilayer and a partial trilayer. The corre-sponding snapshots and radial-density maps are shown inFigures 2K–O and 3K–O, respectively. It was observed thata water trilayer can form between the (5,5) and (24,24)CNTs, as shown in Figure 2O and 3O. For the combinationof a (5,5) tube and a (25,25) tube, the water trilayer betweenthe two CNTs becomes rather disrupted by thermal fluctua-tions.

We also examined the effects of DR of the two CNTs onthe insertion speed of the self-assembly process for Cases Ato K. It is shown in Figure 5A that the insertion speeddrops almost exponentially with the increase in DR, that is,ACHTUNGTRENNUNGV=V0expACHTUNGTRENNUNG(�DR/R0). The two parameters, V0 and R0, arefitted as V0=1837 nmns�1 and R0=0.203 nm in the range of0.34<DR<0.61 nm (before a full water monolayer isformed between the two CNTs) or V0=364 nmns�1 andR0=0.260 nm in the range of 0.68<DR<1.02 nm (after acomplete cylindrical water monolayer has formed). Howev-er, the number of water molecules in the gap increases onlyproportionally with respect to DR in the range 0.41<DR<

1.02 nm (Figure 5B). Therefore, the insertion speed of asmall CNT into a large one depends strongly on the layeredwater structures between the two CNTs but is less sensitiveto the number of water molecules in the gap.

The reduction in the CNT–CNT vdW energy during theself-assembly process for Cases A–K is found to decrease agreat deal after a complete cylindrical monolayer of waterhas formed between the two CNTs (Figure 5C). For a smallDR (Cases A–E), the space between the two CNTs is notlarge enough to accommodate a complete water monolayer,and the observed reduction in insertion speed is causedmainly by the gradually weakened CNT–CNT vdW interac-tion as DR increases. After a complete cylindrical monolay-er of water has formed (Cases F–K), the two CNTs are sep-arated from each other, which leads to a sharp drop in inter-

tube vdW interactions and insertion speed, as shown in Fig-ure 5A and C, as the main driving force for the self-assem-bly of CNTs is the CNT–CNT vdW attraction.[12] Thisobservation also indicates that water molecules betweentwo CNTs do not lubricate intertube sliding, which is in con-trast to the conventional wisdom that, in the macroscopicworld, water usually reduces friction between two parallelsurfaces. This is because CNTs already have atomicallysmooth surfaces, and water molecules between two CNTslead to increasing energy dissipation via thermal fluctua-tions.

Figure 5. Effects of water on the coaxial assembly of different combi-nations of CNTs (Cases A–K as listed in Table 3). A) The insertionspeed as a function of radius difference DR. B) The number of watermolecules confined between two CNTs as a function of DR. C) Thereduction of intertube vdW energy as a function of DR.

1352 www.small-journal.com ? 2006 Wiley-VCH Verlag GmbH&Co. KGaA, Weinheim small 2006, 2, No. 11, 1348– 1355

full papersH. Gao et al.

SPECIALSE

CTION

The observed water monolayer in the (5,5)–water–(15,15) system is very stable within the simulation timeperiod of up to 1 ns (Case F), as is consistent with the simu-lations done by Kalra et al.[37] To explore the physical mech-anism for the stabilization of the water monolayer shown inFigures 2F and 3F, we can calculate the potential energychange DEpot during the coaxial assembly process of the(5,5) and (15,15) CNTs, which contains three parts

DEpot ¼ DECC þ DECW þ DEWW ð3Þ

where DECC represents the change in carbon–carbon inter-action energy, mainly due to the reduction of the vdW forcebetween the (5,5) and (15,15) CNTs; DECW is the change inCNT–water vdW energy, and DEWW arises due to the water–water interaction energy. For the coaxial assembly of the(5,5) and (15,15) CNTs (Figure 2F), the energy changes ofthese three terms are calculated to beDECC ¼ �382:18 kJmol�1, DECW ¼ �434:19 kJmol�1 andDEWW ¼ �8:44 kJmol�1, which leads to a change in thetotal potential energy of DEpot ¼ �824:81 kJmol�1. It can beseen that the CNT–CNT and CNT–water vdW energiesdominate the assembly process. We can also calculate theenergy changes for a noncoaxial assembly process as in Fig-ure 2E and 3E, by initially placing the (5,5) tube close toone side of the (15,15) tube wall. It is calculated thatDECC ¼ �1787:00 kJmol�1, DECW ¼ �1159:37 kJmol�1,DEWW ¼ �8:54 kJmol�1, and DEpot ¼ �636:17 kJmol�1. Theequilibrium state of the coaxial (5,5)–water–(15,15) systemis therefore more energetically favorable, and the formationof the concentric CNT–water–CNT trilayer lowers both theCNT–CNT and the CNT–water vdW energies. In compari-son, the noncoaxial structures are seen to lower the CNT–CNT vdW energy but increase the CNT–water vdW energy.

The coaxial (5,5)–water–(15,15) composite nanotube wasfurther studied by examining the water radial-density pro-file, as shown in Figure 4B. There are two plateaus in thewater density: 1=0 in the ranges of r=0.18–0.50 nm andr=0.85–1.20 nm, which correspond to the walls of the (5,5)and (15,15) CNTs, respectively. Between the two CNTwalls,there exists a narrow water-density peak at r=0.68 nm, withthe maximum 1 around four times the bulk water density10. This indicates that water confined between the twoCNTs forms a highly condensed monolayer.

Besides the water monolayer, three other typical struc-tures can be recognized in Figure 4B, namely, the 1D or-dered water chain,[22] the boundary layer at the CNT–waterinterface,[26,27] and the bulk water. The sharp peak at r=0with a maximum relative density 1/10=4.2 corresponds tothe 1D ordered water chain in the (5,5) tube. Another peakvalue 1/10=2.4 at r=1.35 nm shows the boundary layer ofwater about 0.33 nm away from the wall of the (15,15) tube.At r>1.73 nm, the water density 1 approaches 10, which in-dicates that the water boundary layer has a thickness ofabout 0.7 nm.

Although the water monolayer between the (5,5) and(15,15) CNTs exhibits a highly condensed structure with adensity close to that of the 1D water chain, the configura-tion of oxygen atoms lacks long-range order, as can be seen

from the oxygen–oxygen radial-distribution function (RDF)in Figure 6A for the (5,5)–water–(15,15) composite. In con-trast, the RDF plot of the 1D water chain exhibits multipleequidistant peaks, which shows that water molecules in the1D chain are periodically ordered as a 1D crystal. For all

four water structures, the first peak of the oxygen–oxygenRDF plot is at r=0.28 nm, which is mainly caused by thestrong hydrogen-bonding network in water.

The changes in the water–hydrogen-bonding networkare attributed to the different degrees of confinement in thefour typical water structures, namely, the 1D water chain,the 2D water monolayer, the water boundary layer at the(15,15)–water interface, and bulk water. The averagenumber of hydrogen bonds per water molecule for thesefour structures are calculated as 1.42, 2.97, 2.91, and 3.34, re-spectively, through a geometrical criterion with a maximumdonor–acceptor (DA) distance of 0.35 nm and a hydrogen–donor–acceptor (HDA) angle of 308.[30,38] In comparisonwith bulk water, the 1D water chain loses on average twoout of four hydrogen bonds per water molecule because ofits very strong confinement due to the (5,5) tube, while boththe water monolayer and the water boundary layer lose ap-proximately one hydrogen bond due to their Q2D confine-ment. Note that the average number of hydrogen bonds per

Figure 6. Radial distribution function (RDF) of oxygen atoms in fourdifferent water structures. A) CNTs (5,5) and (15,15) (Case F inTable 3). B) CNTs (5,5) and (19,19) (Case J in Table 3).

small 2006, 2, No. 11, 1348– 1355 ? 2006 Wiley-VCH Verlag GmbH&Co. KGaA, Weinheim www.small-journal.com 1353

Coaxial Self-AssemblySPECIAL

SECTIO

N

water molecule in bulk water is lower than four, partiallydue to thermal fluctuations.

For the water bilayer between the (5,5) and (19,19)CNTs (Case J), shown in Figures 2J and 3J, the maximumrelative densities 1/10 (Figure 4C) are 2.6 and 2.7, respec-tively, much lower than that of a monolayer but comparableto that of the boundary layer at the CNT–water interface.The RDF plot in Figure 6B reveals that the structure of awater bilayer is less ordered than that of a monolayer. Cor-respondingly, the average number of hydrogen bonds perwater molecule increases from 2.97 for a monolayer to 3.15for a bilayer. Therefore, as a result of the relatively weakconfinement, the water bilayer is less condensed and less or-dered in comparison with the monolayer.

For the water trilayer formed between the (5,5) and(24,24) CNTs (Case O), the radial-density profile of watershows three peak values for the maximum relative densitiesof 2.4, 1.7, and 2.5 (Figure 4D). Such a trilayer structure isnot as stable as the monolayer or bilayer discussed abovebecause of weaker confinement, which is also consistentwith the findings of Kalra et al.[37] Consequently, the (5,5)tube may move toward the interior surface of the (24,24)tube under large thermal fluctuations, in which case thewater trilayer is broken into a structure consisting of a parti-al bilayer and a parACHTUNGTRENNUNGtial tetralayer similar to the structureshown in Figure 3N.

4. Conclusions

Molecular-dynamic simulations have been used to inves-tigate water structures formed during the coaxial self-assem-bly of two selected CNTs in water. The water moleculesconfined between the walls of two CNTs were found tohave structural characteristics that are strongly tube-size de-pendent, and were found to exhibit stable, highly con-densed, and ordered monolayer or bilayer structures forspecial CNT combinations. The resulting carbon–water–carbon composite nanotubes were energetically stabilizedby both the CNT–CNT and the CNT–water vdW interac-tions.

Four different water structures were observed, namely, a1D ordered water chain, a 2D water monolayer or bilayer, aboundary layer of water at the CNT–water interface, andbulk water, in different regions in the equilibrated CNT–water system, depending on the degree of confinement. Thewater monolayer formed between CNTs is highly condensedwith a more ordered structure than the boundary layer butit lacks the long-range order seen for a 1D water chain.Compared with the monolayer, the water bilayer is less con-densed and less ordered owing to weaker confinement. Atrilayer water structure was also observed in our simulationsbut seemed to be easily disrupted by thermal fluctuations.

Acknowledgements

Financial support of this work was provided by NSFC (GrantNo. 10525210, 10121202, 10442002 and 10502031), 973

Project (Grant No. 2004CB619304) and Fok Ying Tong Educa-tion Foundation. In addition, J.Z. acknowledges a VisitingPh.D. Student Fellowship at the Max Planck Institute forMetals Research during 2002–2003 and many helpful dis-cussions with Dr. Y. Kong, Dr. Z.P. Xu, and Dr. L.F. Wang. H.G.acknowledges support from the Max Planck Society, an Over-seas Young Investigator Award from NSFC, and a Chang JiangScholarship from Tsinghua University.

[1] J. Zou, B. Ji, X. Q. Feng, H. Gao, Nano Lett. 2006, 6, 430–434.[2] S. Iijima, Nature 1991, 354, 56–58.[3] R. H. Baughman, A. A. Zakhidov, W. A. de Heer, Science 2002,

297, 787–792.[4] C. R. Martin, P. Kohli, Nat. Rev. Drug Discovery 2003, 2, 29–37.[5] P. M. Ajayan, S. Iijima, Nature 1993, 361, 333–334.[6] E. Dujardin, T. W. Ebbesen, H. Hiura, K. Tanigaki, Science 1994,

265, 1850–1852.[7] B. W. Smith, M. Monthioux, D. E. Luzzi, Nature 1998, 396, 323–

324.[8] K. Hirahara, K. Suenaga, S. Bandow, H. Kato, T. Okazaki, H. Shi-

nohara, S. Iijima, Phys. Rev. Lett. 2000, 85, 5384–5387.[9] H. Gao, Y. Kong, D. Cui, C. S. Ozkan, Nano Lett. 2003, 3, 471–

473.[10] H. Gao, Y. Kong, Annu. Rev. Mater. Res. 2004, 34, 123–150.[11] I. C. Yeh, G. Hummer, Proc. Natl. Acad. Sci. USA 2004, 101,

12177–12182.[12] S. Arcidiacono, J. H. Walther, D. Poulikakos, D. Passerone, P.

Koumoutsakos, Phys. Rev. Lett. 2005, 94, 105502.[13] T. Ito, L. Sun, R. M. Crooks, Chem. Commun. 2003, 1482–1483.[14] R. E. Tuzun, D.W. Noid, B. G. Sumpter, R. C. Merkle, Nanotech-

nology 1996, 7, 241–246.[15] Y. Gogotsi, J. A. Libera, A. Guvenc-Yazicioglu, C. M. Megaridis,

Appl. Phys. Lett. 2001, 79, 1021–1023.[16] S. Niyogi, R. C. Haddon, Proc. Natl. Acad. Sci. USA 2004, 101,

6331–6332.[17] M. Majumder, N. Chopra, R. Andrews, B. J. Hinds, Nature 2005,

438, 44–44.[18] S. P. Adiga, D.W. Brenner, Nano Lett. 2005, 5, 2509–2514.[19] L. D. Gelb, K. E. Gubbins, R. Radhakrishnan, M. Sliwinska-Bart-

kowiak, Rep. Prog. Phys. 1999, 62, 1573–1659.[20] K. Koga, H. Tanaka, X. C. Zeng, Nature 2000, 408, 564–567.[21] M. C. Gordillo, J. Marti, Chem. Phys. Lett. 2000, 329, 341–345.[22] G. Hummer, J. C. Rasaiah, J. P. Noworyta, Nature 2001, 414,

188–190.[23] K. Koga, G. T. Gao, H. Tanaka, X. C. Zeng, Nature 2001, 412,

802–805.[24] W. H. Noon, K. D. Ausman, R. E. Smalley, J. P. Ma, Chem. Phys.

Lett. 2002, 355, 445–448.[25] Y. Maniwa, H. Kataura, M. Abe, S. Suzuki, Y. Achiba, H. Kira, K.

Matsuda, J. Phys. Soc. Jpn. 2002, 71, 2863–2866.[26] J. H. Walther, R. Jaffe, T. Halicioglu, P. Koumoutsakos, J. Phys.

Chem. B 2001, 105, 9980–9987.[27] T. Werder, J. H. Walther, R. L. Jaffe, T. Halicioglu, F. Noca, P. Kou-

moutsakos, Nano Lett. 2001, 1, 697–702.[28] M. A. Henderson, Surf. Sci. Rep. 2002, 46, 5–308.[29] P. A. Thiel, T. E. Madey, Surf. Sci. Rep. 1987, 7, 211–385.[30] J. Marti, J. Chem. Phys. 1999, 110, 6876–6886.[31] H. J. C. Berendsen, D. van der Spoel, R. van Drunen, Comput.

Phys. Commun. 1995, 91, 43–56.[32] E. Lindahl, B. Hess, D. van der Spoel, J. Mol. Model. 2001, 7,

306–317.[33] H. J. C. Berendsen, J. P. M. Postma, W. F. van Gunsteren, A.

DiNola, J. R. Haak, J. Chem. Phys. 1984, 81, 3684–3690.[34] U. Essmann, L. Perera, M. L. Berkowitz, T. Darden, H. Lee, L. G.

Pedersen, J. Chem. Phys. 1995, 103, 8577–8593.

1354 www.small-journal.com ? 2006 Wiley-VCH Verlag GmbH&Co. KGaA, Weinheim small 2006, 2, No. 11, 1348– 1355

full papersH. Gao et al.

SPECIALSE

CTION

[35] W. L. Jorgensen, J. Chandrasekhar, J. D. Madura, R. W. Impey,M. L. Klein, J. Chem. Phys. 1983, 79, 926–935.

[36] S. Miyamoto, P. A. Kollman, J. Comput. Chem. 1992, 13, 952–962.

[37] A. Kalra, S. Garde, G. Hummer, Proc. Natl. Acad. Sci. USA 2003,100, 10175–10180.

[38] D. van der Spoel, P. J. van Maaren, P. Larsson, N. Timneanu, J.Phys. Chem. B 2006, 110, 4393–4398.

Received: February 2, 2006

small 2006, 2, No. 11, 1348– 1355 ? 2006 Wiley-VCH Verlag GmbH&Co. KGaA, Weinheim www.small-journal.com 1355

Coaxial Self-AssemblySPECIAL

SECTIO

N