Anomalous Enhancement of Mechanical Properties in the Ammonia Adsorbed Defective Graphene

Pure graphene is known as the strongest material ever discovered. However, the unavoidable defect formation in the fabrication process renders the strength of defective graphene much lower (~14%) than that of its perfect counterpart. By means of density functional theory computations, we systematically explored the effect of gas molecules (H2, N2, NH3, CO, CO2 and O2) adsorption on the mechanical strength of perfect/defective graphene. The NH3 molecule is found to play a dominant role in enhancing the strength of defective graphene by up to ~15.6%, while other gas molecules decrease the strength of graphene with varying degrees. The remarkable strength enhancement can be interpreted by the decomposition of NH3, which saturates the dangling bond and leads to charge redistribution at the defect site. The present work provides basic information for the mechanical failure of gas-adsorbed graphene and guidance for manufacturing graphene-based electromechanical devices.

Scientific RepoRts | 6:33810 | DOI: 10.1038/srep33810 the effect of gas molecule adsorption on the mechanical properties of the graphene layer would help us understand this issue, which is important for its applications in flexible electronic nanodevices and other related fields.
In this paper, we performed a systematic study of gas adsorption on the perfect/defective graphene layer to evaluate the mechanical strength change. Many common gases, namely H 2 , N 2 , NH 3 , CO, CO 2 and O 2 , have been considered due to their importance to environmental and industrial applications. We first investigate the structural and mechanical properties of the perfect graphene (P-graphene) with adsorbed gas molecules. Then, the change of ideal strain for the D-graphene upon gas molecule adsorption is investigated. Finally, the mechanisms behind the strength change are analysed to gain more insights into the effects of adsorbed molecules on the mechanical properties of D-graphene.

Computational Method
Our spin polarized DFT computations were performed by utilizing Viena ab intio simulation package (VASP) code 30,31 with the implemented projector augment wave (PAW) method 32,33 . The generalized gradient approximation in the Perdew-Burke-Ernzerhof form (GGA-PBE) 34 was used to describe the exchange-correlation for electrons. A dispersion correction to the total energy (DFT-D3 method) 35 was employed to simulate the long-range van der Waals interaction. The plane-wave energy cutoff was set to 400 eV for geometry optimization and to 500 eV for static electronic structure calculations. To study two-dimensional (2D) systems under the periodic boundary conditions, a vacuum layer with a thickness of at least 20 Å was used to avoid the interaction between periodic images. All the geometry structures were fully relaxed until energy and force were converged to 1.0 −5 eV and 0.005 eV/Å, respectively. A 5 × 5 × 1 and 17 × 17 × 1 Monkhorst-Pack k-point sampling was used for geometry optimizations and static electronic structure calculations, respectively. The adsorption energy (E ad ) of a gas molecule on graphene layer was obtained by the following equation: E ad = E (gas+G) − (E G + E gas ), where E (gas+G) , E G and E gas are the total energies of the gas adsorbed graphene system, graphene and the adsorbed gas molecule, respectively. According to this definition, a more negative E ad value indicates a more favourable adsorption.
The strain 36,37 was added by changing the lattice parameters a, which is determined by the expression a = a 0 (1 + ε ). Here, a 0 is the equilibrium lattice constants of graphene at 0% strain. The values of ε increased in steps of 0.4% until the graphene sheet fractured and the strain-stress relation was obtained. To eliminate the artificial effect of the out-of-plane thickness of the simulation box on the stress, we employed the second Piola-Kirchhoff stress 37,38 to express the 2D forces with units of N/m. In order to lift the constraints imposed by periodic boundary conditions 39 , a 3 × 3 supercell was used for all the calculations. Note that only a single vacancy in each supercell was considered in order to reduce the complexity.

Results and Discussion
Structural and mechanical properties of the perfect graphene with adsorbed gas molecules.
First, we search for the lowest-energy configuration when a gas molecule is adsorbed on P-graphene. To find the most favourable position for gas adsorption, we place the single gas molecule above the graphene layer at different distances and orientations. After full optimization, the obtained configurations are compared, and the energetically most favourable states are selected for further discussions.
As a representative, the case for NH 3 is presented in Fig. 1. The NH 3 molecule prefers to locate in the hexagon center (Fig. 1a), and the calculated distance (see Table 1) between the NH 3 molecules and graphene is 2.84 Å. The    computed adsorbent-graphene distances, adsorption energies, and magnetic moments for other gas molecules (H 2 , N 2 , CO, CO 2 and O 2 ) are given in the Table 1. In the equilibrium, all these gas molecules are physically adsorbed on graphene with a distance between 2.68-3.32 Å. However, only the adsorption of H 2 , NH 3 and O 2 is exothermic while others are not favoured energetically, and only O 2 adsorbed system is magnetic. Before studying the strain effect on the gas-adsorbed graphene, we investigated the strain-stress relationship in the P-graphene as shown in the Fig. 2a. In an attempt to evaluate the "minimum" ideal strain for graphene, both armchair and zigzag directions seem possible to be chosen. However, a previous report 40 revealed that the armchair direction possesses lower ideal strain than the zigzag direction, since it is parallel to the carbon-carbon bond which dominates the mechanics of graphene 41 . Therefore, only the armchair direction is considered in our calculations. Our computations show that the P-graphene sheet can sustain a maximum stress up to ~32 N/m and its structure does not fracture even when the strain exceeds 30% (Fig. 2b), which is in consistent with previous theoretical work 40 .
However, the adsorption of a single gas molecule on the top of graphene sheet can significantly decrease its mechanical strength (Fig. 2b). The largest reduction derives from the adsorption of NH 3 , where the fracture point drops to only 15.6% relative to that of P-graphene (> 30% of the fracture point). These results well explain why the synthesized graphene layers exhibit relatively low intrinsic strength (~15% of the fracture point) under realistic experimental conditions, which include gas exposure.

Structural and mechanical properties of the defective graphene with adsorbed gas molecules.
Normally, a material's ideal (intrinsic) strength can be traced down to its bond strength 42 . The bond strength, in principle, can be greatly affected by the amount of charge in the bond. Removing one atom from the nanosheet will have a significant impact on the charge distribution of the layer, resulting in weakening the bond strength. As a result, the ideal strength of the material would be reduced. For example, the P-graphene is able to sustain an ideal strain above 30% (Fig. 2a), while the unavoidable defects in fabrication process (e.g. carbon atom vacancies) can significantly affect its strength, i.e. decreasing to only 14.8% (Fig. 3a). The introduction of foreign atoms in the defect sites of graphene brings new electron to interact with the dangling bonds, which therefore affect the bond strength and thus change the strain of the host materials. Motivated by this rationale, we explore the gas  Our computations show that the adsorption of H 2 , N 2 , CO, CO 2 or O 2 molecules makes the critical strain for D-graphene decrease by 0.4~2.0% (Fig. 3b). The most significant reduction comes from the CO or CO 2 adsorbed D-graphene, where its breaking strain drops to only 12.8%. However, there is a shining exception: the NH 3 dissociative adsorption enhances the ideal strain of D-graphene by approximately 1%, i.e. to 15.6%. Compared with the gas adsorption on the P-graphene, all the gas adsorptions turn to be energetically favourable on the D-graphene sheet (Table 2), and the remarkable high E ad (9.083 eV) for NH 3 also indicates that it dissociates on the D-graphene sheet. Furthermore, all of the six gas-adsorbed systems become magnetic.
With regard to the NH 3 adsorption on defective graphene, experimentally it has been demonstrated that the NH 3 molecule can easily dissociate on this host layer 43 . To validate the possibility of gas decomposition, we performed the nudged elastic band (NEB) 44 DFT calculations for the NH 3 + D-graphene system at different strains, as shown in Fig. 4. By computing the transition states, we find that the NH 3 decomposition needs to overcome an energy barrier (E a ) of 0.5 eV for the strain-free D-graphene, while the E a value decreases as the strain increases. As a 0.5 eV energy barrier is not sufficient to hinder the NH 3 decomposition, it is reasonable to expect the NH 3 decomposition to occur without stain, while the finite strains can accelerate this process.
The high possibility of NH 3 decomposition on defective graphene, as revealed by our NEB calculations, is further verified by the optimized structural configuration of NH 3 + D-graphene system (Fig. 5). The NH 3 decomposition happens above the D-graphene sheet. When the strain is less than 12.8%, the NH 3 molecule is dissociated into NH 2 and H, NH 2 is physisorbed at the carbon atom with a dangling bond, while H bonds to a neighbouring carbon atom (-c-H) (Fig. 5a-c). This dissociative NH 3 adsorption induces obvious corrugation on the D-graphene layer (Fig. 5b) due to the strong electron coupling between the adsorbed species and the D-graphene sheet.
Interestingly, when the D-graphene is further stretched with a strain greater than 12.8%, the NH 2 is further dissolved into NH and -c-H (Fig. 5d). Note that the H fragment plays a dominant role in saturating the dangling bond at the defect site, which would affect the charge distribution and strengthen the carbon bond (the neighbouring bond length reduces to 1.40 Å). As a result, the ideal strain for D-graphene is enhanced by the dissociative adsorption of NH 3 . Furthermore, we have also checked the structure for H 2 /N 2 /CO/CO 2 /O 2 + D-graphene system, but no gas decomposition is found. Thus, the experimentally observed difference on the mechanical strength can be attributed to the differing impact of different gas adsorption at the defected site of graphene.
Finally, we examine the relationship between the stress and the strain for the NH 3 + D-graphene system (Fig. 5e). The yield point of NH 3 adsorbed graphene is at 12% strain. Before this point, it is in the elastic range where the deformation is reversible and the stretched layer can return to its original geometry when the tension is released. Further extension after the yield point would induce an irreversible plastic deformation, and the 2D structure will eventually rupture (see the structural change in Fig. 5d)

Conclusion
We systematically investigated the ideal strength for P-/D-graphene adsorbed by several common gas molecules including H 2 , N 2 , NH 3 , CO, CO 2 and O 2 by means of DFT computations. All the gas adsorption on P-graphene can significantly reduce its breaking strain, and the similar trend is also revealed for D-graphene that adsorbed by H 2 , N 2 , CO, CO 2 and O 2 . Most importantly, the NH 3 adsorption on D-graphene can significantly enhance its ideal strain to 15.6%, and the fundamental mechanism has been analysed in terms of NH 3 dissociation. These results supply useful information for the ideal strength of gas adsorbed graphene system, and provide guidance to the fabrication of graphene-based electromechanical devices.