Effects of Mechanical Deformation on Outer Surface Re Activity of Carbon Nanotubes

8/7/2019 Effects of Mechanical Deformation on Outer Surface Re Activity of Carbon Nanotubes

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Effects of Mechanical Deformation on Outer Surface Reactivity of Carbon Nanotubes

Xiaohui Song1, Sheng Liu1,2*, Han Yan2, Zhiyin Gan2 1Research Institute of Micro/Nano Science and Technology, Shanghai Jiao Tong University, Shanghai, P. R. China, 200302 Wuhan National Lab for Optoelectronics, Huazhong University of Science & Technology, Wuhan, P. R. China, 430074

*Corresponding author: Sheng Liu, Telephone: 86-13871251668, Fax number: 86-27-87557074, Email:[email protected]

Abstract

An ab initio approach of Car-Parrinello molecular dynamics is used to study the chemisorption of a singleoxygen atom on outer surface of zigzag single-wall carbonnanotubes under various uniaxial strains and bendingdeformation. The effect of mechanical deformation onadsorption of oxygen atom on CNT is demonstrated bylinking the chemical reactivity and structural deformation.The adsorption energy E b and pyramidalization angle θ P areobtained by structural relaxation calculations, and ground-state electronic structures are described according to densityfunctional theory (DFT) within a plane-wave pseudopotential

framework. Our results show that the surface reactivity of CNT is mostly determined by its pyramidalization angle of carbon atom. For bending SWCNT, both E b and θ P vary withadsorption sites, the E b is higher at sites with larger

pyramidalization angle. An approximate linear relation of strain and adsorption energy can be obtained. It is indicatedthat the structure of CNT is crucial for its surface reactivity,and the mechanical deformation can be a method for controlling the surface reactivity of CNT and offeringadsorption site selectivity as the adsorption is facilitated onthe sites with higher pyramidalization angle.

Introduction

Carbon nanotubes (CNTs), discovered in 1991 [1], have

been identified to be one class of the novel candidatematerials to continue the miniaturization of microelectronicsto a new level [2]. It is expected to produce a variety of novelnanodevices by using foreign atoms and molecules for functionalization of CNTs [3]. In this context, the adsorptionof oxygen on CNTs is of particular interest. Both simulation[4] and experimental data [5] have shown that the electrontransport properties of singe-wall carbon nanotubes (SWNT)change dramatically upon exposure to gas molecules of oxygen at ambient temperature. Such process offers theopportunity for the chemical modification of nanotubes to

produce distinct structural and electronic properties.Therefore, the CNTs can be used as sensitive material of gas

sensors. On the other hand, the change of electron transport properties should be avoided when the CNTs to be used asinterconnect materials, where the reactivity should be reducedto guarantee the reliability of interconnects.

So far most theoretical studies of the interaction of molecular or atomic oxygen with carbon nanotube focused on

physisorption [6-9] and chemisorption [9, 10] mechanism.They suggest that oxygen is predominantly physisorbed onCNTs. Most of these studies have concluded that thereactivity of the CNTs is correlated with strains produced bythe curved surface which induces transformation of carbon

atoms from sp2 to sp3 hybridization, and the physisorbedmolecular oxygen get chemisorbed at some special sites withhigh local strain such as various defects. For perfect CNTs,the side-wall of a carbon nanotube is considerably lessreactive when the tube diameter is over 1 nm, furthermore, thereactivity decreases as the tube diameter increases due to thereduction in curvature. Therefore, in order to produce avariety of novel nanodevices by using foreign atoms andmolecules for functionalization of CNTs, the surfacereactivity of CNTs should be tuned. Some studies havereported that tuning the atom adsorption on CNTs by radialdeformation [11-13], which can be a method to controlsurface reactivity of the CNTs via bringing out local strainsthat induce change of the hybridization of carbon atoms inCNTs. It indicated that the mechanical deformation not onlyradical deformation, but also other types of deformation suchas tension and bending which can produce local strain couldtune the surface reactivity of CNTs.

The mechanical deformation can be a very useful tool tocontrol the interaction of atomic oxygen with CNTs. Theadvantage of this method is the extent of the deformation can

be easily controlled by macroscopic variable of force and pressure, and such modification is more extensive than thelocalized groups at the end caps and offers a natural way tochange the properties of the carbon nanotube. Furthermore,the adsorption sites can be controlled by mechanical

deformation that induces local structural change of anindividual SWCNT.

In this paper, the effects of unaxial tension and bendingdeformation on the reactivity of the CNT are examined byfirst principles calculations. Various unaxial tensions and a

bending angle are applied to a zigzag single wall carbonnanotube and oxygen atom adsorptions on different sites of CNT are calculated respectively. In order to illustrate thiscoupling effect, the relation between chemical reactivity andstructure deformation will be studied from point of view of

pyramidalization angle.

Method

In this work, the Car-Parrinello molecular dynamics [14]simulation package was used for structural relaxationcalculations. Ground-state electronic structures weredescribed according to density functional theory (DFT) withina plane-wave pseudopotential framework [15].

We used a periodically repeated supercell approach andsample the Brillonin Zone with a single k-point, and our supercell was large enough to avoid spurious interaction

between periodic replicas of an oxygen atom. For atoms insimulation, the electron-ion interaction was described byTronullier-Martins nonlocal norm-conserving

978-1-4244-2231-9/08/$25.00 ©2008 IEEE 2091 2008 Electronic Components and Technology Conference



pseudopotentials. The generalized gradient correctedexchange functional Becke-Lee-Yang-Par (BLYP) wasadopted, and the plane wave basis set had a kinetic energycutoff of 60 Ry.

In order to illustrate the interaction of atomic oxygen withdeformed SWCNT, a semiconducting (10, 0) single wallcarbon nanotube (SWCNT) with 140 atoms was modeled. Aoxygen atom is placed at a distance of 1.5A above the center

of a hexagon on each adsorption sites, as shown in Fig.1.

(a)

(b)Fig. 1. Schematic for computational model: (a) axial tension; (b) bendingdeformation.

. The uniaxial strains was obtained by axial tension [16,17] which was applied by shifting the end atoms along theaxis by small steps, and then relaxing the model via theconjugate-gradient method, while keeping each endconstrained as shown in Fig.1(a). The strains of 2.5%, 5%,7.5%, 10%, 12.5%, and 15% were applied and computedrespectively with the same pseudopotentials and generalizedgradient corrected exchange function.

The bending angle of 30° is applied and six differentadsorption sites that denote different strains were considered.To obtain relax structures under bending and calculate atomicadsorption properties, the atoms of two rings at each endswere fixed, as shown in Fig.1(b), and then relaxing the modelvia the conjugate-gradient method. The binding energy of atomic oxygen on different system was calculated as:

b E =E (oxygen) + E (CNT) - E (oxygen + CNT) （1）

where E (oxygen) was the energy of oxygen atom, E (CNT) was the energy of isolated CNT system and E (oxygen +

CNT) was the energy of the whole system including theadsorbed oxygen atom, all calculation of total energy atcertain strain used the same supercell and k-points. A positive

E b corresponded to an adsorption process.

Results and Discussion

First we study the adsorption of atomic oxygen on a pristine SWCNT, the results show that the atomic oxygen isvery reactive and adsorption takes place with no barrier toform epoxide-like structure, as shown in Fig.2, the adsorptionenergy is 2.12eV, the bond length of C-C and C-O are 1.573Å and 1.465 Å respectively, and the oxygen atom

preferentially attacks “twisted C-C bonds” shown in Fig.2, because they are of a larger curvature and more reactive. Theresults are in agreement with previous study [10].

Fig. 2. The structure of epoxide-like product after adsorption of atomicoxygen.

Next, the structure is considered under various uniaxialstrains, with the adsorption energy E b and epoxide-likestructure parameters for the optimized structures listed in

Table 1. It is shown that the adsorption energy increases withincreasing uniaxial strain, the C-C bonds elongate and are aptto break with the adsorption of oxygen atom, and the changeof C-O distance has a general trend towards more stableequilibrium under larger strains. All these changes of adsorption energy and CNT structure parameters indicate thatthe uniaxial strains enhance the reactivity of CNT surface toadsorb oxygen atom, an approximate linear relation betweenstrain and adsorption energy can be obtained form thecalculation, as plotted in Fig. 3.

Fig. 3. The variation of E b and θ P against strain.

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Table 1. The calculated adsorption energy E b and optimized epoxide-likestructure parameters

Strain E b (eV) C-C (Å) C-O (Å) θP (°) Parallel C-C (Å) Twisted C-C(Å)

0.000 2.12 1.573 1.465 5.11 1.434 1.422

0.025 2.25 1.666 1.442 5.36 1.456 1.4350.050 2.55 2.101 1.398 5.59 1.485 1.440

0.075 2.76 2.112 1.399 5.75 1.518 1.4420.100 3.03 2.174 1.403 5.91 1.566 1.443

0.125 3.29 2.179 1.405 6.11 1.642 1.4440.150 3.57 2.185 1.408 6.29 1.676 1.445

From our simulation results we can see that there is closerelation between surface reactivity and CNT structure, anyeffects that alter the structure may induce change of surfacereactivity. So we can explain the effect of uniaxial strain onsurface reactivity of CNTs from point of view of structuredeformation. It is known that a carbon nanotube is made byrolling up a graphene sheet into a cylinder [1], and the strains

produced by the curved surface inducesσ-л hybridization.An addition reaction can take place with carbon atomstransformed from sp2 to sp3 hybridization. The reactivity iscorrelated with the extent of this hybridization transformationdenoted by pyramidalization angle θ P [11], as shown in Fig. 4.For θ P =0°, there is the sp2 hybridization as a flat graphenewhich exhibits lower surface reactivity [5], and for θ P =19.5°,there is the sp3 hybridization as CH4. The θ P value for carbonatom on a carbon nanotube falls between these two values,and both the curvature and the reactivity increase with the θ P value [18]. For the (n, 0) tubes in our calculation, the

pyramidalization angle θ P can be calculated by the formula:

1

1

1

x 1

y 0

z 0

−⎡ ⎤ ⎡ ⎤⎢ ⎥ ⎢ ⎥=⎢ ⎥ ⎢ ⎥⎢ ⎥ ⎢ ⎥⎣ ⎦ ⎣ ⎦

(2)

2 12

2 12

2

x cos

y sin

z 0

θ

θ

−⎡ ⎤ ⎡ ⎤⎢ ⎥ ⎢ ⎥=⎢ ⎥ ⎢ ⎥⎢ ⎥ ⎢ ⎥⎣ ⎦ ⎣ ⎦

(3)

313

23 12 313

12

32

2 23 12 31

31

12

cos x

cos cos cos y

sin z

cos cos cos1 cos

sin

θ

θ θ θ

θ

θ θ θ θ

θ

⎡ ⎤⎢ ⎥⎢ ⎥−⎢ ⎥⎡ ⎤

−⎢ ⎥⎢ ⎥ = ⎢ ⎥⎢ ⎥⎢ ⎥⎢ ⎥⎣ ⎦ ⎢ ⎥

⎡ ⎤−⎢ ⎥− − ⎢ ⎥⎢ ⎥⎣ ⎦⎣ ⎦

(4)

and

( )

( ) ( ) [ ]

1 2 3

2 22

2 3 3 2 1 3 2 1 2 3 1

90 pcos

x y z

y z z x x y ( x x ) y ( x x )

θ + =

+ − + − − −⎡ ⎤⎣ ⎦

o

(5)

as reported by Haddon [19]. The results of pyramidalization angle θ P at different adsorption sites areshown in Table 1.

Fig. 4. Schematics for the θ P angle.

In previous report [20], an approximate linear relation between adsorption energy and pyramidalization angle has been fitted by calculating various pristine carbon nanotubes of different diameter to illustrate the crucial effect of

pyramidalization angle on surface reactivity of pristine CNT.However, in our study, an approximate linear relation

between adsorption energy and pyramidalization angle canonly be obtained in strains that are below 10%, as the strain

increases, the pyramidalization angle alters little but theadsorption energy alters a lot, as plotted in Fig. 5. It is clear that the pyramidalization angle is not the only factor affectsthe surface reactivity of CNT.

Fig. 5. The E b plotted versus θ P .

It has been reported that it costs less energy to change bond angle than bond length of CNT under smalldeformation, as evidenced from MD simulation [21] andfinite element method [22]. Therefore, the CNT structure isapt to deform by changing bond angle shown in Fig. 4 rather than bond length under low strains, and the uniaxial strainmakes the diameter that determines pyramidalization angle of the CNT structure decrease with increasing of uniaxial strain,

nevertheless hexagon structure of CNT under strains is closeto that of pristine CNT for little change of bond length. So theapproximate linear relation between adsorption energy and

pyramidalization angle is obtained in low uniaxial strains as plotted in Fig. 5, and pyramidalization angle plays a crucialrole in surface reactivity of CNT in which the hexagonstructure is close to that of pristine CNT. Furthermore, thereis an approximate linear relation between strain and

pyramidalization angle in low strains as plotted in Fig. 3.




On the other hand, with the increase of strain, elongationof “parallel C-C bond” will be more obvious，as shown inTable 1, and the CNT structure is not approximate hexagonstructure as pristine CNT, the results are in agreement withthe theoretical study on structural mechanics of zigzag carbonnanotubes [22].. It is clear that elongation of “parallel C-C

bonds” shown in Table 1 will change the electron structuregreatly under larger strains, and the electron structure has

been reported as an important factor affects surface reactivity[20]. In this case, the hybridization is complex and can not bedenoted by pyramidalization angle, and the elongation of C-C

bonds induces high surface potential energy that increase thesurface reactivity of CNT. That is why we can also obtainapproximate linear relation between strain and adsorptionenergy in high strains.

At last, the structure was considered under bending angleof 30°, with the adsorption energy E b at each site for theoptimized structures listed in Table 2. The structure of anepoxide-like product after adsorption of atomic oxygen isshown in Fig. 6. It is shown that the application of bendingangle to a carbon nanotube produces bond elongation or

shortening at different location, and the adsorption energy ateach site is different. The adsorption energy levels at site_1,site_2 and site_3 which are under tension strains are larger than those on a pristine SWCNT, while the adsorption energylevels at site_4, site_5 and site_6 which are under compression strain is smaller than those on a pristineSWCNT.

Table 2. The calculated adsorption energy E b and pyramddalization angle θ P

Fig. 6. The structure of epoxide-like product after adsorption of atomicoxygen.

The bending deformation of SWCNT which can beillustrated as classic elastic beam by generalized Hooke’s lawand elastic models theory produces different strains on thesurface of SWCNT, for sites 1-3, the tension strains induce

larger pyramidalization angles than pristine SWCNT, the pyramidalization angle increases with the strain increasing, asa result, the site 1 has the largest pyramidalization angle. Incontrast, for sites 4-6, the compression strain induces smaller

pyramidalization angles, and the pyramidalization angledecreases with the compression strain increasing, so that thesite_6 has the smallest pyramidalization angle. Since the

pyramidalization angle is directly related to the reactivity, it

indicates that the six sites in our study has different surfacereactivity, and the oxygen atom will be tend to be adsorbed onsite_1 which has the highest surface reactivity. Therefore, the

bending deformation of CNT offers a method to control the pattern of adsorption.

Conclusions

In our study, the effort of mechanical deformation onadsorption of oxygen atom on CNTs is demonstrated bylinking the chemical reactivity and structural deformation.The results indicate that CNT surface is very reactive andoxygen atom adsorption takes place without barrier to formepoxide-like structures, for a certain type of CNT, itsreactivity is mostly determined by its pyramidalization angle

of carbon atom. Although we have considered only a limitedset of structures with axial tension and bending angle, our calculations show that the surface reactivity of CNT can betuned by structural deformation which produce differentstrains on the surface of CNTs that induce different

pyramidalization angle at various adsorption sites, and anapproximate linear relation between adsorption energy and

pyramidalization angle has been obtained form our calculation.

Therefore, the effect should be fully considered in theapplication of CNTs. We can tune the surface reactivity of CNT by mechanical method, which can be determined bymacroscopic variables of force or pressure; for another thing,

the method offers site selectivity, as the adsorption isfacilitated on the sites with higher pyramidalization angle. Onthe other hand, when the CNTs are used as interconnectionsmaterial, the effect should be avoided to reduce the reactivitythat may induce molecules adsorption. It suggests that theapplication of the mechanical deformation will be a promisingmethod to control the functionalization of CNTs by usingforeign atoms and molecules.

Acknowledgments

This work is supported by National High TechnologyResearch Program (863) of China (No. 2007AA04Z348).

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Effects of Mechanical Deformation on Outer Surface Re Activity of Carbon Nanotubes

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