Effect of Metal Impurities on the Tensile Strength of ... - ACS Publications

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Effect of Metal Impurities on the Tensile Strength of Carbon Nanotubes: A Theoretical Study Qinghong Yuan,†,‡ Li Li,*,‡ Qianshu Li,*,† and Feng Ding*,‡ †

MOE, Key Laboratory of Theoretical Chemistry of Environment, Center for Computational Quantum Chemistry and School of Chemistry and Environment, South China Normal University, Guangzhou, Guangdong 510006, People’s Republic of China ‡ Institute of Textiles and Clothing, Hong Kong Polytechnic University, Hong Kong, China ABSTRACT: Metal catalysts used in the synthesis of carbon nanotubes (CNTs) are one of the main impurities in CNT fibers. Using quantum mechanical computations, we show that these metal impurities have a large impact on the mechanical performance of CNTs and CNT fibers. One single Ni atom adsorption on the wall of a single-walled CNT (SWCNT) can decrease the tube’s critical strain about 40% by weakening the C−C bond on the wall of a tube. Further systematic calculations and theoretical analysis showed that the reduction of tube strength is nearly independent of the tube’s diameter. Thus, the tensile strength of the CNT fiber is expected to be greatly improved by washing away these metal impurities.

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INTRODUCTION

fundamental understanding of the mechanical properties and fiber strength of CNTs. We noticed that most CNT fibers are made by direct spinning of raw CNTs synthesized by various chemical vapor deposition (CVD) methods, in which transition-metal catalysts (e.g., Fe, Co, and Ni) play a crucial rule. In most CNT fibers, catalysts are not washed away, and thus, a high concentration of catalysts must exist in the CNT fibers. It is well-known that, chemically, these active catalysts may break the C−C bonds in a CNT wall easily22 and, thus, lower the mechanical performance of the CNTs in the fiber. As a consequence of CNT critical strain reduction, the tensile strength of the fibers must be lowered significantly. Motivated by the catalytic cutting of C−C bond by transition metal, in this paper, we systematically studied the effect of catalyst atoms (using Ni atom as an example) on the theoretical strength of CNTs. Our calculation shows that the theoretical critical strain of a CNT can be dramatically reduced by 40% by adding one Ni atom on the CNT surface, and the reduction is nearly independent of the CNT’s diameter and chirality. This shows that the existence of catalysts in CNT fiber is a negative factor for its mechanical performance, and the CNT fiber’s tensile strength is expected to be greatly enhancecd by washing out metal impurities properly.

Owning superior mechanical properties, such as a Young’s modulus of 1.0 TPa,1−4 a theoretical critical strain of 20% or higher, and ideal tensile strength of about 100 GPa,5 carbon nanotubes (CNTs) have been perceived as the strongest material in nature for about 2 decades. These superior mechanical properties have lately been proved by experimental observations.6−9 While, as CNTs are so tiny in diameter (diameter d ∼ 1−10 nm), scaling up is crucial for their macroscopic engineering applications. Technically, there are two most explored routes to use these CNTs in high-strength materials: (i) using CNTs as additive in composites10−12 and (ii) spinning CNTs into CNT fibers directly.13−15 In CNT composites, the concentration of CNTs is normally less than a few percent and thus is not enough for synthesizing super strong materials. In contrast, CNT fiber contains nearly 100% CNTs and therefore are expected to be the strongest macroscopic material with a tensile strength of 50 GPa or higher. Such a high tensile strength is about 5−10 times stronger than the recorded strength of today’s material, for example, ∼5−6 GPa for carbon fiber and ∼3−4 GPa for Dynemma.16,17 In the past decades, extensive efforts have been dedicated to the spinning of CNT fibers.14,18−20 Although the recorded strength of 9.0 GPa has been reported,13 the tensile strengths of most experimental CNT fibers are about 0.5−2.0 GPa, and, to date, the large gap between the experimental tensile strength and the theoretical prediction does not tend to be narrowed quickly. Although there have been extensive efforts dedicated to explore different means of improving CNT fiber’s strength,8,21 the progress is very slow because of the lacking of a © 2013 American Chemical Society

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COMPUTATIONAL DETAILS In this study, two different approaches are used in single-walled CNT (SWCNT) strength calculations. (i) Strength of pristine Received: January 7, 2013 Revised: February 15, 2013 Published: February 19, 2013 5470

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the Ni-activated C−C bond was broken first. Then, following it, the two neighboring C−C bonds broke. Finally, all the C−C bonds along the circumference direction broke in sequence under the critical tensile strain of 14%. We have demonstrated that a single Ni atom adsorption on a (8,0) SWCNT wall may reduce the tube’s critical strain about 40%. Such a dramatic reduction is quite surprising, as the Ni atom only bonds to one of the eight C−C bond along the (8,0) tube circumference. Figure 1a,c shows the failure procedure of the Ni-adsorbed C−C bond and the following brittle failure of the tube. In the first stage, the Ni atom adsorbed on a C−C bond weakens the bond strength by interacting with the π electrons of the two C atoms. As a consequence, the π−π interaction between the two C atoms was broken and the double CC bond was turned into a weakened single C−C bond. The weakened C−C bond was a bit longer than other C−C bonds around (see Figure 2a),

(8,0) SWCNT and Ni atom catalyzed (8,0) SWCNT were calculated with the density functional theory (DFT) method implemented in the Vienna ab initio simulation package (VASP).23,24 The ion−electron interactions were treated with the projected augmented wave (PAW) pseudopotentials,25 and the general gradient approximation (GGA) parametrized by Perdew, Burke, and Ernzerhof (PBE)26 was used as the exchange−correlation functional. For each structure with specified tensile strain, energies were optimized until the force component on every atom was less than 0.02 eV/Å. The kinetic energy cutoff in this study was set as 400 eV. (ii) A systematic calculation on a series of SWCNTs with different diameter and chirality was carried out via the density functional based tight binding (DFTB) approach,27 in which, a force of 1 × 10−6 au is used as the convergence criterion for geometry optimization and the DFT-based conjugate gradient method is employed to describe the electronic interaction.

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RESULTS AND DISCUSSIONS The failure procedure of a pristine (8,0) SWCNT was first examined. In our calculations, the tube was loaded by static tension under prescribed displacement until the C−C bond was broken. It is predicted that once a C−C bond was broken, the stress would drop abruptly under displacement controlled loading. The tensile strength is defined as the maximum stress the tube ever reaches during the strain loading process, and the critical strain is the strain that corresponds to this maximum stress. The calculated critical strain of pristine (8,0) SWCNT is about 22%, as shown in Figure 1a. Such a value is consistent

Figure 2. (a) The C−C bond lengths in a Ni-adsorbed (8,0) SWCNT without any strain. (b) The C−C bond lengths in the same SWCNT but with 10% tensile strain. (c) Bond dissociation energy curve for the CC bond. (d) Bond dissociation energy curve of the Ni-adsorbed CC bond. The molecule C2H4 is used as an example for this study.

and it can be more easily elongated than others during the stretching of the SWCNT (see Figure 2b). Beyond this, the active Ni atom can passivate the dangling C atoms and reduce both the energy compensation and barrier of the C−C bond cleavage. As shown in Figure 2c,d, the breakage of an isolated C−C bond (using C2H4 molecule as an example) clearly presents this effect. Without the Ni atom, the breakage of the CC double bond has a very high formation energy of 9.67 eV and the activation barrier is nearly the same (Figure 2c). When there is a Ni atom bridged on the C−C bond, the formation energy of the final state was dramatically reduced to −2.43 eV and the barrier of the bond breakage is reduced to 4.82 eV (Figure 2d), which is about one-half of the original one. The above analysis clearly shows that the Ni-bridged C−C bond was significantly weakened and thus it is not surprising that such a C−C bond can be broken at the critical strain of 14%. Now let us turn to the following breakage of other C−C bonds along the tube circumference. In material mechanics, the Griffith theory was normally applied to explain the fracture of materials on the macroscopic scale: the fracture of a brittle material is caused by the propagation of existing cracks or flaws. For a strained material with a crack, the stress will concentrate at the edges of the crack and thus lead to the further propagation of the crack. In Griffith theory, the critical stress σ of an object with a crack can be written as

Figure 1. (a) The failure processes of a pristine (8,0) SWNCT and a Ni-adsorbed (8,0) SWCNT and the corresponding structure before and after the critical strain. (b) Snapshots in the pristine (8,0) SWCNT failure process and (c) snapshots in the Ni-adsorbed (8,0) SWCNT failure process.

with previously published data.5 The SWCNT’s failure process is shown in Figure 1b. The loading of 22% strain made the C− C bond in the tube stretch to ∼1.85 Å while the tube is still stable. However, when a 23% strain was applied, the C−C bonds stretched, most broke abruptly, and a brittle fracture occurred. The energy greatly reduced from ∼110 to ∼30 eV because of the strain energy releasing and the edge reconstruction. Then let us consider the failure of the (8,0) SWCNT with one adsorbed Ni atom on the tube wall. As shown in Figure 1a, the adsorbed Ni atom radically reduced the critical strain to 13%, or decreased the critical strain by ∼40%. A brittle fracture was observed for the Ni-catalyzed tube, and the newly formed two edges have no edge reconstruction. The detailed failure process is shown in Figure 1c. Different from the failure of pristine (8,0) SWCNT, in which the breaking point is random, 5471

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(1)

in which σ0 is the applied stress, l is the half-length of the crack, and a is the distance between two neighboring atoms. Take the area around a broken C−C bond as a crack (Figure 1c), the critical stress of Ni-adsorbed SWCNT will be reduced to

σc = σc0√(a /l)

(2)

where σc and σc0 are the critical stresses of the cracked and flawless SWCNT, respectively. Intuitively, the two C−C bonds on both sides of the Ni atom will feel much higher stress than others after the breaking of the first C−C bond. After the cleavage of the two neighboring C−C bonds, the size of the crack becomes larger and the critical stress of the SWCNT will be further reduced. So we can see all C−C bonds break near simultaneously. Equation 2 indicates that the larger the crack is, the easier the tube breaks. This was verified by adding more Ni atoms on other C−C bonds, which should result in a larger crack after the bridged C−C bonds were broken. As shown in Figure 3, a Ni atom was placed on each C−C bond along the circumference. Eventually, the critical strain of the CNT was further reduced to 10%.

Figure 4. (a) Critical strain of various pristine SWCNTs and those with Ni impurities; the solid circles and squares represent the results calculated with the density functional based tight binding (DFTB) method, while the data shown with the hollow circle and hollow square for the (8,0) SWCNT are results calculated with the density functional theory (DFT). (b) The failure process of a (16,0) SWCNT. (c) The failure process of a (5,5) SWCNT.

Although the above discussions are mainly focused on SWCNT, we expect the MWCNT to have a similar cutting mechanism and reduction of mechanical strength based on the following reason. As illustrated in Figure 5, under a specific

Figure 3. The failure process of a (8,0) SWNCT with one Ni atom adsorbed on the wall and that of a (8,0) SWCNT with eight Ni atoms along the circumference.

Equation 2 also implies that the critical stress of a Niadsorbed SWCNT is a function of the crack size and thus is independent of the tube’s diameter, which is somehow against our intuition. To test this predication, we have further studied the failure of a series of SWCNTs, including (n,0) (n = 4, 5, 6, 7, 8, 12, 16), (6,3), and (5,5) SWCNTs. Calculations on these tubes were carried out by using the DFTB method, considering that DFT calculations are very time-consuming and there is a small difference between DFT and DFTB results. For example, the critical strain of the (8,0) SWCNT and the Ni-adsorbed (8,0) SWCNT calculated by DFT/DFTB methods are 22%/ 20% and 13%/10%, respectively, as shown in Figure 4a. DFTB calculations showed that the critical strain of these SWCNTs is around 18%, in spite of the tube diameter and chirality, which is consistent with previous reports.28−31 As predicated by eq 2, the critical strain reduction of the Ni-adsorbed SWCNTs has no strong correlation with tube diameter but depends on the chiral angle slightly. For all Ni-catalyzed SWCNTs investigated in this paper, (n,0) tubes have a critical strain of ∼11%, while both (6,3) and (5,5) tubes have a critical strain of ∼13%.

Figure 5. Illustration of the failure of a multiwalled carbon nanotube (MWCNT) induced by a metal impurity. (a, b) The outer wall of the MWCNT was broken by the catalytic cutting of the metal first under a tensile strain. (c, d) Then the metal is able to directly interact with the second wall of the SWCNT and cut it in a similar manner.

tensile loading, the outer wall of a MWCNT will be broken first due to the metal impurity’s catalystic cutting effect, as discussed above. During the breakage of the outer wall, the inner walls only have a minor effect because of the very weak van der Waals (VDW) interaction between them and the specific layerby-layer structure of the MWCNT. After the outer wall was broken, the metal is able to interact with the second outer wall directly and serves as a catalyst for its cutting. Then, the third outer and deeper inner walls break in sequence. Overall, the breakage of all the walls follows the mechanism of catalytic 5472

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cutting, and thus, we can expect a similar reduction of the mechanical strength of MWCNT as that in the SWCNT. On the other hand, metal atoms encapsulated inside the nanotube are also possible, although we mainly discussed the case of physically absorbed Ni atom. For the encapsulated case, metal atoms substitute for the carbon atoms, and defective sites with weak carbon−metal interactions are formed. Compared with the Ni atom adsorbed outside, the encapsulated metal atoms have more profound effect on the tensile strength of CNTs, and the critical strain of CNT is expected to be further decreased. Experimentally, a possible way to remove the metal catalysts is to purify the CNT using acids or bases. Meanwhile, the purification introduces a lot of oxygen-containing carboxyl and hydroxyl groups and defect sites on the surfaces of the CNTs.32 Those chemical functionalization groups will greatly decrease the tensile strength of CNT because the broken double CC bonds produce cracks on the wall of the CNT, which is similar to the case we discussed above. Hence, washing away the physically adsorbed metal catalysts and simultaneously keeping the crystallinity of the CNTs is a good way to improve the CNTs’ mechanical strength. It should be noted that, in addition to metal catalysts and chemical functionalization, dopant such as nitrogen or boron could also influence the tensile strength of CNTs.33,34 It has been reported that the nitrogen-doped MWNTs exhibited a certain degree of plastic behavior before failure because of the kink formation and motion caused by doped nitrogen atom,34 which is different from the brittle bondbreaking mechanism we mentioned for the nickle-adsorbed MWNTs.

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CONCLUSIONS In conclusion, the effect of metal catalyst Ni on the mechanical strength of SWCNTs has been explored. It is found that single Ni atom adsorption on the wall of a SWCNT can decrease the tube’s critical tensile stain by about 40%. The critical strain reduction is independent of the tube diameter and weakly depends on the tube’s chiral angle. By removing the metal catalysts during the spinning of CNT fiber, the mechanical strength of CNT yarns is expected to be greatly improved.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (F.D.); [email protected]. cn (Q.L.); [email protected] (L.L.). Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The work in Hong Kong Polytechnic University was supported by Hong Kong GRF grant (B-Q26K) and PolyU internal grant (A-PM35; A-PK89; A-PJ50; G-YX4Q).

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REFERENCES

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