Influence of Stone-Wales defects on the mechanical properties of graphene-like polyaniline (PANI) C3N nanosheets

Journal Pre-proof Influence of Stone-Wales defects on the mechanical properties of graphene-like polyaniline (PANI) C3N nanosheets

Sadegh Sadeghzadeh, Majid Ghojavand, Jafar Mahmoudi PII:

S0925-9635(19)30499-6

DOI:

https://doi.org/10.1016/j.diamond.2019.107555

Reference:

DIAMAT 107555

To appear in:

Diamond & Related Materials

Received date:

21 July 2019

Revised date:

5 September 2019

Accepted date:

23 September 2019

Please cite this article as: S. Sadeghzadeh, M. Ghojavand and J. Mahmoudi, Influence of Stone-Wales defects on the mechanical properties of graphene-like polyaniline (PANI) C3N nanosheets, Diamond & Related Materials (2019), https://doi.org/10.1016/ j.diamond.2019.107555

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© 2019 Published by Elsevier.

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Influence of Stone-Wales defects on the mechanical properties of graphene-like Polyaniline (PANI) C3N nanosheets Sadegh Sadeghzadeh1 a, Majid Ghojavand b, Jafar Mahmoudic Associate Professor, Smart Micro/Nano Electro-Mechanical Systems Lab (SMNEMS), Nanotechnology Department, School of Advanced Technologies, Iran University of Science and Technology, Tehran, Iran [email protected] School of Advanced Technologies, Iran University of Science and Technology, Tehran, Iran Professor. Email: [email protected]. Phone: +98-. Address: School of Chemistry, Damghan University, Damghan, Iran

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cAssistant

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Abstract

Today, Graphene-based 2D materials, i.e. those hexagonal arrangements that can be

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produced by making slight changes to graphene, are amongst the most interesting nanostructures for science and industry. Carbone nitrides (CNs) i.e. CxNy is one of the

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most important types of graphene-like materials, and C3N is the first two-dimensionally synthesized and most widely used of these materials. In this paper, the properties of C3N monolayer in the presence of Stone-Wales (7-5-7) defects, have been studied. After validating the model used for this purpose, by comparing the results with those of other works, the structures and arrangements that can be anticipated for Stone-Wales (SW) defects have been predicted. Then the effects of different SW defects, their position and density, and the effect of loading direction on elastic properties of pristine and defected sheets, have been simulated. The obtained results indicate that by increasing the number of SW defects, the failure stress and the failure strain of single1

Corresponding author. Tel: +98-21-73225812. E-mail: [email protected] (Sadegh Sadeghzadeh)

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layer C3N nanosheets are generally reduced, while its modulus of elasticity does not vary significantly. Generally, the defective armchair C3N nanosheet is more resistant than the zigzag configuration. Keywords: C3N nanosheets; Stone-Wales defect; Mechanical properties; Molecular dynamics; Polyaniline (PANI)

1 Introduction

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With the rise of graphene in 2004, it revived the legend of the production of exact 2D materials and the benefaction from their extraordinary properties as ultrahigh amounts

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of charge carrier’s mean free path and mobility, strength, specific surface, electrical and

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thermal conductivity. Since then, enormous efforts have been devoted to recognizing

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new 2D materials and nowadays more than 100 species are discovered. The main motive for searching alternative 2D materials has been achieving complementary

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characteristics of graphene, e.g. semiconducting characters. Hexagonal 2D boron nitride, silicene, germanene, stanene, and other new materials such as MoS2 and

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Borophene could be listed for example [1, 2].

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In 1989 covalent solids of carbon nitrides (CNs) accentuated for their probable

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extraordinary high bulk modules, based on an empirical model hypothetically [3]. Afterward, the researchers exceedingly attracted to further develop such structures [4]. As a longtime preexisting case, graphitic C3N4 (g-C3N4) have been amongst the most matured covalent solids of CNs, owing to their facile synthesis, appealing electronic band structure, high physicochemical stability, and earth-plentiful nature [5]. Regarding their lamellar structure and semiconducting nature, such graphitic structures of CNs triggered inquiries to synthesize exact 2D CNs, complementing the graphene role in graphene-based nanoelectronics as an ideal conductor. While the beginning efforts to realize exact 2D structures of CNs failed due to intolerable amounts of defects, recently exact 2D C3N has been synthesized successfully [6, 7]. Studies show that these

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materials will have massive applications in the near future, such as hydrogen storage [8]. Revealing the capability of CNs to be prepared in exact 2D forms, scientists have been prompted to explore this field more precisely through numerical procedures. In fact, due to the expensive preparation and testing, especially in the structures with one layer of atoms, simulations are always very helpful to obtain an optimal design before further developments. For instance, in [9], the mechanical properties of some hypothetical

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graphene-based graphitic CNs monolayers have been studied by means of molecular

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simulations, including g-CN, triazine-based g-C3N4 and heptazine-based g-C3N4. However, as a realized case, C3N monolayer has attracted more attention and has been

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simulated by using density functional theory (DFT) and MD methods, in order to

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identify its mechanical, electronic, optical and thermal properties [10, 11]. In [10] it is revealed that C3N monolayers are indirect semiconductors with low 1.04 eV bandgap,

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indicating their potential use as a complementary element with graphene as an ideal

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conductor in nanoelectronics. The stability over 4000 k, a very closed elastic modulus (341 GPa.nm) and tensile strength (35.2 GPa.nm) to graphene, and a comparable

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thermal conductivity (800 W/mK) to the ultrahigh amount of graphene’s counterpart are the findings of C3N monolayers in [11]. Some numerical studies were performed

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recently [12]. In order to study more realistic cases, the presence of some type of defects have been considered in C3N monolayers including single vacancy [13], divacancies [14] and crack and notch [13, 15]. In fact, in the synthesis process, various intrinsic defects, including vacancies, adatoms, and reconstruction of the covalent bonds, randomly occur in the crystalline network. Moreover, sometimes, by using proper techniques such as the focal electron or ion beam irradiation, selective artificial defects introduced into these 2D crystals locally, in order to control the band structure for electronics applications as well as chemical properties [16-18].

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SW defects are amongst the most important defects dealt with in graphene-based systems, due to their abundance, stability and precious effect on the local properties. More precisely, regarding their lowest formation energy amongst the other defects, they are generated more readily during the heat treatment, resulting in their myriad [19]. At the same time, their migration energy is the largest amongst the other defects resulting in their location stability [19]. This is important in local engineering of the sheets, in particular as we are aware of the prosperous effect of such defect on the local electronic

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band structure and chemical properties [20].

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Inspired by their important role in graphene systems, several studies have been conducted to consider SW defect in other 2D systems. The geometries, formation

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energies, and reactivities of SW defects in a series of graphene-like boron nitride-

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carbon heterostructures were studied using DFT [21].

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In this paper, the effect of 5-7-5 Stone-Wales (SW) defect on the mechanical properties of C3N monolayer has been evaluated. As the simplest reconstructive defects, The 5-7-5

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SW defect (also called 7-5 ring defect) is created in 2D hexagonal crystals, when two neighboring atoms that define the edges of two hexagons rotate 90º about an axis that

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passes through the midpoint between these atoms and is perpendicular to the 2D sheet

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(fig 1). Contrary to graphene in which only two distinct cases of Stone-Wales defects are possible due to the uniformity of all its constituent atoms, in C3N, because of the presence of carbon and nitrogen atoms, four types of rotatable bonds can form distinctive defects. These four types of bonds have been depicted in figure 1 along with their counterparts. According to this figure, by rotating each of the bonds (1, 2, 3 and 4), a new type of defective structure will be obtained.

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Four different types of bonds whose rotation causes distinct Stone-Wales defects in C3N

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nanosheets

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Figure 1.

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2 Materials and Methods

Due to the successful experience in simulating single-layer C3N sheets [14, 15], this

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paper uses Large-scale Atomic/Molecular Massively Parallel Simulator (LAMMPS)

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[22] to study the effect of Stone-Wales defects on the mechanical properties of this material. Using this software, a standard uniaxial mechanical test was designed and

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applied to samples with different types and percentages of Stone-Wales defects. The Tersoff potential is used to simulate the dynamics of carbon and nitrogen atoms. The

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most important feature of this potential is its ability to model the elements of the fourth

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group of the periodic table, including germanium, carbon, and silicon. A recent version of this potential has also been developed for the modeling of the interaction of boron, nitrogen, and carbon that is used in this paper. In this potential, the interactions between two atoms are modeled with two general attractive and repulsive terms, multiplied by a function dependent on the distance that plays the role of the cut-off radius. All pair coefficients were used from [23]. This potential, which is appropriate for Ge-Si, Ge-C, Si-Ge and Si-C bonds, is written as 𝑈𝑇 =

1 ∑ ∑ 𝑈𝑖𝑗 2 𝑖

)1 (

𝑗≠𝑖

where ?????? is the interaction energy between the ith and jth atoms. 5

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𝑈𝑖𝑗 = 𝑓𝑐 (𝑟𝑖𝑗 )[𝑓𝑅 (𝑟𝑖𝑗 ) + 𝑏𝑖𝑗 𝑓𝐴 (𝑟𝑖𝑗 )]

The coefficients in (2) can be expressed as 1

𝑟𝑖𝑗 < 𝑅𝑖𝑗

𝜋(𝑟𝑖𝑗 − 𝑅𝑖𝑗 ) 1 1 + cos [ ] 2 2 𝑆𝑖𝑗 − 𝑅𝑖𝑗 { 0 𝑓𝐴 (𝑟𝑖𝑗 ) = −𝐵𝑖𝑗 𝑒 − 𝜇𝑖𝑗𝑟𝑖𝑗 𝑓𝑅 (𝑟𝑖𝑗 ) = 𝐴𝑖𝑗 𝑒 − 𝜆𝑖𝑗𝑟𝑖𝑗 𝑓𝑐 (𝑟𝑖𝑗 ) =

𝐵𝑖𝑗 = 𝑋𝑖𝑗 (1 +

𝑅𝑖𝑗 ≤ 𝑟𝑖𝑗 ≤ 𝑆𝑖𝑗 𝑟𝑖𝑗 < 𝑆𝑖𝑗

}

1 𝑛𝑖 𝑛𝑖 −2𝑛𝑖 𝛽𝑖 𝜉𝑖𝑗 )

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𝜉𝑖𝑗 = ∑ 𝑓𝐶 (𝑟𝑖𝑘 ) 𝑔(𝜃𝑖𝑗𝑘 ) 𝑘≠𝑖,𝑗

𝐶𝑖 2

2−

𝑑𝑖 2 + (−ℎ𝑖 + 𝐶𝑜𝑠𝜃𝑖𝑗𝑘 ) (𝜆𝑖 + 𝜆𝑗 ) (𝜇𝑖 + 𝜇𝑗 ) 𝜆𝑖𝑗 = , 𝜇𝑖𝑗 = 2 2 𝐴𝑖𝑗 = √𝐴𝑖 𝐴𝑗 , 𝐵𝑖𝑗 = √𝐵𝑖 𝐵𝑗 𝑅𝑖𝑗 = √𝑅𝑖 𝑅𝑗 , 𝑆𝑖𝑗 = √𝑆𝑖 𝑆𝑗

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𝑑𝑖

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𝐶𝑖 2

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𝑔(𝜃𝑖𝑗𝑘 ) = 1 +

)3 (

In this equation, subscripts i, j and k mark the atoms where the bond between atoms i

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and j is modified by a third atom k. The potential parameters and their corresponding values are chosen from [23]. Similar to what mentioned in [11], the carbon's cutoff of

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the optimized Tersoff potential is modified from 0.18 nm to 0.20 nm here to simulate the mechanical response of pristine and also defected graphene.

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The method used in this paper is that, after verifying the model used, the effect of different parameters on the mechanical properties of the sheets in the presence of various Stone-Wales defects is studied. Periodic boundary conditions are applied to the system so that physically it can be ensured that the dimensions of the sheet are large enough to guarantee real conditions. The time step is considered to be 0.25 fs, and the number of atoms, except for the validation (that is equal to 14,000), is considered to be 5000 atoms. This number of particles is equivalent to a 40×60 Angstroms sheet. After constructing a pristine C3N sheet without defect, using a handwritten algorithm in MATLAB software, different Stone-Wales defects are constructed as shown in figure 1.

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Then, an initial velocity was induced to all atoms proportional to the initial temperature of the system. Then, using the Nose-Hoover barostat and thermostat (NPT) ensemble, the total pressure of the system was imposed to be zero in all directions, so that the system was released without pre-tension. Then, the simulation box in the larger direction (assumed to be in line with x) was elongated in such a way that its elongation rate is equal to strain rate. The strain at any instant is calculated from relation ϵ×dt; where ϵ is the strain rate (in 1/s) and dt is the time step (in sec). During this stretching,

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the pressure along with the pulled-out direction is not fixed and the rest of the directions are still under the previous ensemble. At any moment, the Virial stress of all atoms is

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calculated and printed. Obtaining the position and stress of all atoms at any moment, the

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tension contours are drawn for different types. Also, by summing the stresses of the atoms and dividing it by the total volume of the system, the overall tolerated stress is

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calculated and, with the determination of the strain, at each time, the stress-strain

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diagram is drawn.

To validate the model and algorithm used, the mechanical properties of the C3N sheets

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without defects are compared with previous works. The results showed that the stressstrain diagram for a 60×100 Å sheet with the same conditions as in reference [11] was

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only 0.4% and the elasticity modulus was estimated to be 311 to 314 Gpa.nm that

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corresponds to what presented in previous works (340 Gpa.nm in [24] and [25]). This value is lower than that of graphene only between 7% [26] and 11% [27], which shows that the mechanical properties of C3N are very close to graphene. To further examine and assure more of the model used, the energy level of various Stone-Wale defects has also been studied. The energy level of each of the shown cases in figure 1 can specify the distinction between these different arrangements. By changing the bond numbers from one to twelve, only two types of bonds as C-C and C-N have to be rotated in order to create the Stone-Wales defects. Therefore, according to figure 2, only two energy levels can be observed for various types of defects. However, due to the major differences in 7

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overall sheet configurations, various types of defects will lead to different mechanical properties. This figure shows the energy levels for a 5 × 5 nm2 C3N sheet in its initial arrangement and after 125 ps, when it has reached a stable equilibrium under NVE (constant energy) ensemble. -7.965

-7.97

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-7.98

Potential Energy/ atom at time =0 Potential Energy/ atom at time =125 ps Total Energy/ atom at time =0 Total Energy/ atom at time =125 ps

-7.985

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Energy per atom (eV)

-7.975

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-7.99

Figure 2.

1

2

3

4

5

6 7 Bond number

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10

11

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Energy levels per atom for a 5 × 5 nm2 C3N sheet in its initial configuration and also

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after 125 ps, when it has reached stable equilibrium under NVE (constant energy) ensemble

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Furthermore, the formation energy of various SW types may be useful. As energy

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needed to form an SW defect, differences between the potential energy of pristine sheet and defected sheet with various SW types are plotted in figure 3. Since there is no need to remove any atom to form the SW defects, 𝐸𝑓∗ maybe defined as 𝐸𝑓∗ = 𝐸𝑆𝑊 − 𝐸𝑃𝑟𝑖𝑠𝑡𝑖𝑛𝑒

)4 (

Where, 𝐸𝑆𝑊 is potential energy of defected sheet with an SW defect and 𝐸𝑃𝑟𝑖𝑠𝑡𝑖𝑛𝑒 is the potential energy of a pristine C3N sheet with dimensions exactly equal to the defected one. Figure 3 demonstrates that formation energy for types 1, 2, 4, 7, 8 and 11 is similar and larger than the formation energy of types 3, 5, 6, 9, 10 and 12. 8

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E* (eV)

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26

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23

2

3

5

6 7 8 SW Types

9

10

11

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The formation energy of various SW types

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Figure 3.

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3 Results and discussion

Effects of different Stone-Wales defects on mechanical properties

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3.1

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Figure 4 illustrates the effects of different types of defects on the stress-strain diagrams

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for the armchair configuration of C3N nanosheets. One can see that despite the elasticity modulus being the same for all types of Stone-Wales defects, fracture stress and

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fracture strain diminish with similar rates for defect types 1 through 4. Sheets with

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types 1 defect and 4 seem to be weaker than the others, because of the rotation of a horizontal bond (which was along with the tension before the rotation and is no longer in the same direction after the rotation) and thus the reduction in the overall resistance and strength of sheet bonds. With the change in tension direction (chirality), sheets with types 2 and 3 defects are expected to become weaker. Nevertheless, for the armchair arrangement simulated here, sheets with types 1 and 4 defects are weaker than the others.

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Stress-strain curves and the variations of fracture stress, fracture strain and elasticity

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Figure 4.

defects

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modulus for sheets of armchair configuration in pristine (defect-free) and with various Stone-Wales

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Relatively different results can be observed for sheets with a zigzag configuration (see

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figure 5). With the rotation of the C-C bond in the sheet with defect type 1, the failure

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of the whole sheet occurs more quickly. Conversely, in the sheet with type 4 arrangement, despite the fact that the bond is in a horizontal direction before rotation

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(like in type 1 configuration), since the bond is of C-N type, its rotation leads to a

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different situation relatively higher fracture stress and strain values.

Figure 5.

Stress-strain diagrams and the variations of fracture stress, fracture strain, and elasticity

modulus for sheets of zigzag configuration in pristine (defect-free) state and with various Stone-Wales defects

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A similar study for graphene sheets was performed [28]. As we can see, because the number of atoms in all states is the same, the overall stiffness of the sheets is almost equal in all cases. But, what is clearly different are the failure strain and stress. In fact, at the moment of failure, the dangling bonds indicate which structure is earlier and which structure will later fail. Figure 6 shows the first bond rupturing moment in the elongation test plotted on the initial state of that defected armchair sheet. Clearly, as shown in figure 7, the defected sheet with type 4 behaves differently. In that case, the

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carbon-nitrogen bond and in the other types the carbon-carbon bond initially ruptures. In figure 6, the black bonds and atoms belong to the initial configuration and the green

(2)

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(1)

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bonds and atoms belong to the moment that the first bond failed.

(3) Figure 6.

(4)

The first bond rupturing moment in the elongation test plotted on the initial state of that

defected armchair sheet. Larger particles are nitrogen atoms and the smaller ones are carbon. In every case, the bond placed at the depicted oval has broken and a crack along the arrows is outbreak.

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The first bond in these structures approximately ruptured at about 90% of the time of the total failure of the sheet. This time is roughly equivalent to the time when, according to strain-stress diagrams, the C3N sheet has a self-hardening since then. This phenomenon is clearly observable in the stress-strain diagrams. Small fluctuations show that rupturing the C-N bond leads to less strain energy loss than rupturing the C-C bond, this is the type 4 that fails faster than the others. In figure 6, the crack formation and propagation, which is approximately perpendicular to the firstly ruptured bond, is

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demonstrated with an arrow. A similar analysis can be made for zigzag sheets. It was observed that the crack formation and propagation, is approximately perpendicular to

Stress distribution

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the firstly ruptured bond in zigzag sheets.

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Figure 7 shows the stress distribution levels of defect-free sheets and the armchair

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sheets with various Stone-Wales defects at the fracture moment in the midsections of sheets. The depicted times for the fracture points indicates that some configurations fail

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sooner and some later. Chronologically, the order of the failure of the sheets was 12, 10,

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8, 6, 11, 4, 1, 2, 3, 7, 5, and 9, respectively.

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SW Type 1

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Pristine

SW Type 2

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SW Type 3

SW Type 4

SW Type 6

SW Type 8

SW Type 9

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SW Type 7

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SW Type 5

SW Type 10

Figure 7.

SW Type 11

SW Type 12

Stress distribution in the midsection of sheets with the armchair arrangement in pristine

(defect-free) and defected with various Stone-Wales defects at the moment of fracture

Figure 8 illustrates the stress distribution levels of pristine and defected zigzag sheets at the moment of failure. In this figure, the Stone-Wales defect has been considered at 13

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different SW defects. As depicted in figure 8, the crack starts from the defect location and the direction of crack propagation is along the length direction of the SW defect.

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Pristine

SW2, time=727 ps

SW4, time=717 ps

SW5, time=767 ps

SW3, time=742 ps

SW6, time=705 ps

SW8, time=704 ps

SW9, time=803 ps

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SW1, time=727 ps

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SW7, time=736 ps

SW11, time=711 ps SW12, time=607 ps

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SW10, time=664 ps

Figure 8.

Stress distribution level of pristine and defected zigzag C3N sheets at the moment of

failure in the tensile test. The Stone-Wales defect has been considered at different locations. The crack starts from the defect location and the direction of crack propagation is along the length direction of the SW defect.

As it is observed, the fracture is initiated at the nanosheet corners in pristine C3N nanosheets and all defected sheets have disrupted from the point of defect. The performed simulations don’t show any signature about the effect of defect location in the fracture process; which is relevant to the applied periodic boundary condition. By 14

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other words, as we use such a boundary condition, we cannot be assured about the obtained local results.

3.3

The effect of the density of Stone-Wales defects

Figure 9 displays the stress-strain graphs (left) and the variations of fracture stress, fracture strain and elasticity modulus (right) versus the number of Stone-Wales defects in a single layer armchair C3N nanosheet. In this figure, for bringing the diagrams to the same level, the values of fracture strain and elasticity modulus have been multiplied by

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100 and 0.1, respectively. As it is observed, fracture stress and fracture strain generally

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diminish with the increase in the number of Stone-Wales defects. Although the modulus

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of elasticity follows a similar decreasing trend, its changes are not as evident.

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As the density of the SW defects increases, after the sheet is elongated more than 0.1% (e = 0.1), some local variations observed and strain-stress curves are nonmonotic. These

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variations are shown in figure 9 with a separate box. These nonmonotics are due to the fact that as the number of SW defects increases, some bonds break locally, thus

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experiencing a sudden decrease in the total stress. Immediately, because the level of

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stress has not yet reached the yield stress, all atoms rearrange and the diagram proceeds

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with a slope close to its behavior prior to the local failure.

Nonmonotics after ε=0.1

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Journal Pre-proof Figure 9.

The stress-strain curves (left) and the fracture stress variations, fracture strain and

elasticity modulus (right) versus the number of Stone-Wales defects in a single-layer armchair C3N nanosheet. For bringing the diagrams to the same level, the values of fracture strain and elasticity modulus have been multiplied by 100 and 0.1, respectively.

Figure 10 shows the stress-strain graphs (left) and the variations of fracture stress, fracture strain and elasticity modulus (right) versus the number of Stone-Wales defects in a single layer zigzag C3N nanosheet. Here also, with the increase of Stone-Wales defects, fracture stress and fracture strain diminish; and although the modulus of

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elasticity also experiences a decline, its variations are not so pronounced. By comparing

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the graphs presented in figures 9 and 10, it is realized that the increase of defects in the

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armchair C3N sheets causes less change in fracture stress and strain values than the increase of defects in the zigzag sheets. The stress-strain curves of all defected zigzag

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C3N sheets (with different numbers of defects) are significantly different from those of

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the pristine (defect-free) sheet. So, in general, C3N nanosheets with armchair configuration enjoy a higher failure strength. This can be better demonstrated by

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figure 10 (25.96 GPa.nm).

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comparing the average modulus of elasticity in figure 9 (26.87 GPa.nm) with that in

Figure 10.

The stress-strain diagrams (left) and the variations of fracture stress, fracture strain and

elasticity modulus (right) versus the number of Stone-Wales defects in a single-layer zigzag C3N nanosheet. For bringing the diagrams to the same level, the values of fracture strain and elasticity modulus have been multiplied by 100 and 0.1, respectively.

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3.4

The effects of loading direction in C3N nanosheets with/without StoneWales defects Figure 11 shows the stress-strain diagrams (left) and the variations of fracture stress, fracture strain and elasticity modulus (right) versus the direction of tension in C3N sheets with or without Stone-Wales defects. As it is observed, two major types of chirality influence the mechanical behavior of C3N sheets. These two chirality types include chirality type 0 (comprising tension directions of 0, 30, 60 and 90º) and

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chirality type 15 (comprising tension directions of 15, 45 and 75º). Therefore, as expected, the mechanical behavior of C3N nanosheets will recur by changing the

Figure 11.

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direction of tension by 30º.

The directions of tension simulated for a pristine (defect-free) C3N nanosheet (left) and

the variations of fracture stress, fracture strain and elasticity modulus (right) versus the direction of tension in C3N sheets with/without Stone-Wales defects

To study the effect of SW defects on the elastic properties of C3N sheets stretched along with different directions, the diagrams in figure 11 have also been plotted for sheets with one Stone-Wales defects. It may be important to note that in general, the presence of a Stone-Wales defect has no significant effect on fracture stress and strain and modulus, but it considerably affects the modulus of elasticity at the directions of tension associated with the second type of chirality. 17

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4 Conclusion In this paper, the effect of 5-7-5 Stone-Wales defect on the mechanical properties of single-layer C3N nanosheets was investigated. The elasticity modulus of a single-layer C3N sheet at room temperature was estimated in the range of 311-314 GPa.nm, and the influences of various parameters on the mechanical properties of these sheets were subsequently evaluated. In studying the effects of different Stone-Wales defects on mechanical properties, only two energy levels were observed at various types of

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defects. Despite having the same elasticity modulus in the presence of all types of

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Stone-Wales defects, fracture stress and fracture strain diminish with similar rates for defect types 1 through 4. By plotting the stress contours of C3N nanosheets it was

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observed that the pristine (defect-free) C3N sheet has ruptured from points far away

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from the center, while all defected sheets have failed from the point of the defect.

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In studying the effect of Stone-Wales defect position on the mechanical characteristics of C3N sheets, it was observed that by displacing these defects, the resulting stress-

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strain diagrams are not significantly affected; therefore, considering the periodicity of

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the problem’s boundary conditions, the obtained results are sufficiently credible. In exploring the effect of Stone-Wales defect density, it was observed that the fracture

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stress and fracture strain of C3N nanosheets generally diminish with the increase in the number of Stone-Wales defects. Although the modulus of elasticity of these sheets also decreases, their variations are not tangible. In general, C3N nanosheets with armchair configuration have greater resistance against failure. This can be better demonstrated by comparing the average modulus of elasticity of armchair sheets (26.87 GPa.nm) with that of zigzag sheets (25.96 GPa.nm). By analyzing the effect of loading direction on C3N nanosheets with/without StoneWales defects, it was observed that two major types of chirality influence the mechanical behavior of C3N sheets. These two chirality types include chirality type 0 (comprising tension directions of 0, 30, 60 and 90º) and chirality type 15 (comprising 18

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tension directions of 15, 45 and 75º). The mechanical behavior of C3N nanosheets recurs by changing the direction of tension by 30º. In general, compared to a defect-free C3N sheet, the presence of Stone-Wales defects has no significant effect on the fracture stress and strain of sheets subjected to the tensile test in different directions, but it considerably affects the modulus of elasticity at the directions of tension associated with the second type of chirality.

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Highlights for: Influence of Stone-Wales defects on the mechanical properties of graphene-like Polyaniline (PANI) C3N nanosheets Sadegh Sadeghzadeh a, Majid Ghojavand b, Jafar Mahmoudic 1

a

Associate Professor, Smart Micro/Nano Electro-Mechanical Systems Lab (SMNEMS), Nanotechnology Department, School of Advanced Technologies, Iran University of Science and Technology, Tehran, Iran [email protected] School of Advanced Technologies, Iran University of Science and Technology, Tehran, Iran Professor. Email: [email protected]. Phone: +98-. Address: School of Chemistry, Damghan University, Damghan, Iran b

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 The properties of defected PANI with SW defects are studied.

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 Different types, position, density and chirality of SWs were discussed.  Increasing the number of SWs, the failure stress and strain reduced.

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 Defect density does not affect the modulus of elasticity significantly.

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 Defective armchair C3N sheet is more resistant than the zigzag.

1

Corresponding author. Tel: +98-21-73225812. E-mail: [email protected] (Sadegh Sadeghzadeh)

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