Letter
Elastic properties of graphyne-based nanotubes

https://doi.org/10.1016/j.commatsci.2019.109153Get rights and content

Abstract

Graphyne nanotubes (GNTs) are nanostructures obtained from rolled up graphyne sheets, in the same way carbon nanotubes (CNTs) are obtained from graphene ones. Graphynes are 2D carbon-allotropes composed of atoms in sp and sp2 hybridized states. Similarly to conventional CNTs, GNTs can present different chiralities and electronic properties. Because of the acetylenic groups (triple bonds), GNTs exhibit large sidewall pores that influence their mechanical properties. In this work, we studied the mechanical response of GNTs under tensile stress using fully atomistic molecular dynamics simulations and density functional theory (DFT) calculations. Our results show that GNTs mechanical failure (fracture) occurs at larger strain values in comparison to corresponding CNTs, but paradoxically with smaller ultimate strength and Young’s modulus values. This is a consequence of the combined effects of the existence of triple bonds and increased porosity/flexibility due to the presence of acetylenic groups.

Introduction

Graphene became one of the most studied structures in materials science since its first experimental realization in 2004 [1]. The advent of graphene created a renewed interest in the investigation of other 2D carbon-based nanostructures such as carbon nitride [2], pentagraphene [3], phagraphene [4] and the so-called graphynes [5], among others. Proposed by Baughman, Eckhardt and Kertesz in 1987, graphyne is a generic name for a family of 2D carbon-allotropes formed by carbon atoms in sp and sp2 hybridized states connecting benzenoid-like rings [5]. The possibility of creating different graphyne structures with different porosities, electronic and/or mechanical properties can be exploited in several technological applications, such as energy storage [6], [7] and water purification [8], [9]. The recent advances in synthetic routes to some graphyne-like structures [10] have attracted much attention to graphyne, since theoretical calculations have revealed interesting mechanical properties of single-layer [11], [12] and multi-layer graphyne [13], as well the presence of Dirac cones [14].

Similarly to 2D, quasi-1D carbon structures have also received special attention in the last decades. For example, CNTs have been used as field-emission electron sources [15], tissue scaffolds [16], actuators [17], and artificial muscles [18]. Because CNTs can be conceptually seen as graphene sheets rolled up into cylindrical form [19], [20], [21], the same concept has been used to propose graphyne-based nanotubes (GNTs). Preserving the same CNT (n,m) nomenclature to describe nanotubes of different chiralities, different GNT families were theoretically predicted by Coluci et. al. [22], [23], [24] (Fig. 1). GNTs exhibit different electronic properties in comparison to CNTs, for instance, γ-GNTs are predicted to have the same band gap for any diameter [25]. There is a renewed interest in their electronic properties [26]. Likewise the electronic behavior and mechanical properties of GNTs also show interesting features [27], [28]. For example, molecular dynamics (MD) simulations have shown that, under twisting deformations, GNTs would be superplastic and more flexible than CNTs, with fracture occurring at angles three times larger than those of CNTs [29]. In another recent MD work [25] carried out with AIREBO potential, the mechanical properties of graphynes-based nanotubes of γ type (γ-GNTs) were predicted to not be very sensitive to their length and to the strain rate, while the Young’s modulus (Y) values increase with larger diameters.

Although there is a great interest in the properties of GNTs, a fully comprehensive investigation of their mechanical properties has not been yet fully carried out and it is one the objectives of the present work. In this work we have investigated the behavior of GNTs under mechanical tensile stress using fully atomistic reactive molecular dynamics (MD) and density functional theory (DFT) calculations.

Section snippets

Molecular dynamics simulations

Fully atomistic reactive MD simulations were carried out to predict the tensile stress/strain behavior of CNTs, α-GNTs, and γ-GNTs (Fig. 2). These simulations were performed using the LAMMPS (Large-scale Atomic/Molecular Massively Parallel Simulator) [30] code with the reactive Force Field (ReaxFF) [31]. ReaxFF is a classical reactive potential suitable for studying fracture mechanics and breaking/formation. There are many ReaxFF parameter sets, in the present work we used the parametrization

Results

The obtained critical strain (εc) and the ultimate strength (US) MD values for the studied nanotubes are presented in Table 2. The complete structural failure (fracture) of both zigzag-alligned CNTs and γ-GNTs occurred around similar εc. On the other hand, different εc were observed for armchair CNTs and γ-GNTs. Especially, α-GNTs showed the highest εc values for both zigzag and armchair nanotubes. We attributed these differences to the large pore size (see Fig. 1), notably for α-GNTs, and the

Conclusions

We investigated the structural and mechanical properties of graphyne tubes (GNTs) of different diameters and chiralities, through fully atomistic reactive molecular dynamics and DFT calculations. We also considered conventional carbon nanotubes (CNTs), for comparison purposes. Our results show that the complete structural failure (fracture) of both zigzag-alligned CNTs and γ-GNTs occurred around similar critical strain values ((εc)), but quite distinctly for armchair CNTs and γ-GNTs. In

Data availability

The data required to reproduce the work reported in the manuscript can be found in José Moreira de Sousa – email:[email protected].

CRediT authorship contribution statement

J.M. De Sousa: Conceptualization, Methodology, Software, Validation, Data_curation, Writing_%E2%80%93_original_draft, Writing_%E2%80%93_review_%26_editing. R.A. Bizao: Methodology, Software, Validation, Data_curation, Writing_%E2%80%93_review_%26_editing. V.P. Sousa Filho: Validation, Data_curation, Writing_%E2%80%93_review_%26_editing. A.L. Aguiar: Methodology, Software, Validation, Data_curation, Writing_%E2%80%93_review_%26_editing. V.R. Coluci: Validation, Data_curation,

Acknowledgements

This work was supported by CAPES, CNPq, FAPESP, ERC and the Graphene FET Flagship. J.M.S., R.A.B. and D.S.G. thank the Center for Computational Engineering and Sciences at Unicamp for financial support through the FAPESP/CEPID Grant 2013/08293-7. J.M.S., A.G.S.F, A.L.A. and E.C.G. acknowledge support from PROCAD 2013/CAPES program. E.C.G. acknowledges support from CNPq (Process No. 307927/2017-2, and Process No. 429785/2018-6). J.M.S., A.L.A. and E.C.G. thank the Laboratório de Simulação

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