The adsorption properties of CO molecules on single-layer graphene nanoribbons

The adsorption properties of CO molecules on graphene nanoribbons (GRNs) are studied through the molecular dynamics (MD) method. The AIREBO and LJ potentials are used to describe the C-C bonds in GNR and the interactions between the carbon atoms in GNR and CO molecules, respectively. The influences of the environmental pressure and charge density on the adsorption properties of CO molecules on GRNs are taken into account in this study. The effects of charges carried by GNRs on the adsorption properties are investigated in two aspects: atom distribution and energy evolution. Its observation from the results shows that the Coulomb force plays a more important role in the adsorption phenomenon than the van der Waals force, and the higher the charge density is, the larger the amount of the adsorbed CO molecules becomes. Low charge densities ( 4.937 ...


I. INTRODUCTION
Since graphene is one two-dimensional (2D) material made of carbon atoms in a hexagonal lattice, it has been subjected to a great upsurge in research since it was discovered in 2004 by Novoselov et al. 1 A perfect graphene is one of the most typical strictly two-dimensional crystalline materials, consisting of honeycomb carbon lattice structures.One carbon atom in a graphene film obtains four free electrons, and three electrons form covalent bonds with the other three adjacent carbon atoms in SP2 hybridization.
5][16][17][18] The adsorption of gas molecules on graphene obviously changes its properties, giving graphene the ability to be a kind of gas sensor.Compared with solid-state gas sensors, [19][20][21][22][23][24][25][26] gas sensors based on graphene or Carbon nanotubes [27][28][29][30][31][32] are more sensitive, even able to detect a single gas molecule.Regrettably, research into the adsorption properties of graphene is still in its starting phase, and the mechanism of the adsorption on graphene is still unknown.That mechanism and the properties of a gas adsorbed on a graphene surface need further study.Both their theoretical study and experimental research are hot points.
In this article, the adsorption properties of CO molecules on graphene nanoribbons (GNRs) are studied through the molecular dynamics (MD) method, hoping to lay the foundation for later theoretical and experimental studies and give a good understanding of the adsorption properties of a gas on a grapheme film.

II. SIMULATION PROCESS
Generally speaking, there are two typical adsorption mechanisms of gas molecules on a solid, including physical adsorption and chemical adsorption.In chemical adsorption, the gas molecule reacts with the adsorbent, producing a chemical bond between them.In physical adsorption, no chemical bond exists, and the intermolecular force plays an important role.What's more, monolayer adsorption and multilayer adsorption will occur in physical adsorption.In monolayer adsorption, some hypotheses are posed: (1) the unsaturated atomic force results in the adsorption of gas on the adsorbent; (2) as long as the kinetic energy of the adsorbed gas molecules on the adsorbent is able to overcome the adsorbent surface force field, the adsorbed molecules will return to the gas phase, which is called desorption.The adsorption rate is little affected by the neighboring adsorbed molecules and the adsorption position; (3) the adsorption and desorption achieve a dynamic balance.The adsorption of COs on a GNR investigated here is monolayer physical adsorption.
The large-scale atomic/molecular massively parallel simulator, i.e.LAMMPS, is a classical molecular dynamics code and was distributed as an open source code by Sandia National Laboratories.In this article, we utilized LAMMPS to perform the MD simulations and study the adsorption properties of CO molecules on single-layer graphene nanoribbons.
Four main steps should be carried out to perform an MD simulation.Firstly, the system models with one GNR lying in the middle of the system suffused with COs are built up as illustrated in Fig. 1(a).Due to the modeling limitations of LAMMPS, the GRN could not be established directly in LAMMPS.According to the perfect lattice structures of graphene, we deduced a set of physical model data and imported it into the system model in which the COs are uniformly distributed at the beginning of the simulation.Three groups of simulations are conducted in this article, corresponding  to three different environmental pressures, including 0.25 atm, 0.50 atm and 1.00 atm.In each group, there are three models in all, according to different charge densities of GNRs, which are achieved by applying different amount of charges on the GRN, for instance 0, 0.5, 0.75 and 1 charge on each carbon atom of GRN.Except for the contained charges in the GNRs, all the other settings are the same in each group.The size of the GNR is 120 nm × 60 nm, with an armchair edge in the length direction and a zigzag edge in the width direction.The carbon atoms and the oxygen atoms in the CO hold, respectively, 0.0644 charges and -0.0644 charges.Table I shows the CO ambient environments of different groups of simulations.According to the CO ambient pressure, the simulations are divided into three groups, which present different sizes of the simulation box and obtain different numbers of CO.The system pressures are not intentionally controlled, but evolve freely.The theoretical pressures are 0.25 atm, 0.50 atm and 1.00 atm, respectively.The corresponding actual pressures are 0.247 atm, 0.491 atm and 0.994 atm, respectively, which are much closed to the theoretical pressures.Secondly, the simulation conditions are set up.The canonical ensemble referred to as an NVT ensemble is applied to the system, and the time step is set at 1 fs.The environmental temperatures are all set at 300K, and a Nose-Hoover thermostat works during the MD simulations.The system pressures are not artificially regulated during the simulations.However, according to the ideal gas equation, the system pressure will remain unchanged only if the system's volume, temperature and particle number are all specific constants.Periodic boundaries are set for the global simulation system in each dimension.For MD simulations, potential functions are used to describe the interactions between the atoms in the system.Because they have a great effect on the accuracy of simulation results, choosing the appropriate potential functions is very crucial for MD simulations. 33Here, for the inner atoms of the GRNs, the energy functions are described by an adaptive intermolecular reactive empirical bond order (AIREBO) potential.The 12-6 Lennard-Jones (LJ) potential is used to describe the carbon-oxygen (C-O) bonds in the COs.In addition, the LJ potentials plus a Coulombic pairwise interaction is used to describe the C-C and C-O interatomic interaction, respectively, between the carbon atoms in GNRs and the carbon atoms in CO, and between the carbon atoms in GRNs and the oxygen atoms in COs.
Thirdly, energy minimization should be carried out to ensure the accuracy of simulation, which is one key step.Energy minimization helps reduce the error in the MD simulation and save on simulation time.After energy minimization, the initial configurations of the models are obtained.
Finally, a simulation time of at least 600 ps is needed for the systems to reach an equilibrium state, which is very important for MD simulations and at which state the simulation data should be obtained.In general, as the total energy of the simulated system becomes stable, it can be considered at a system equilibrium.

III. SIMULATION RESULTS AND ANALYSIS
As the simulation results show, the adsorption of COs on a GNR is a monolayer physical adsorption, and the adsorption rate is the same at any position on the GNR.The systems of GNRs containing different charges under different pressures are simulated firstly.As shown in Fig. 1(b) and 1(d), it can be found that after 600 ps of simulations, the CO molecules are basically distributed uniformly in the simulated domain and some CO molecules are adsorbed on the GRN surface.In the following, let's study the processes of the COs being adsorbed on the GNRs and the influence of the charge on the adsorption.As defined from the macroscopic point of view, adding 0 charges, 0.5 charges, 0.75 charges and 1 charge on each carbon atom of GRN equals electrifying the graphene It is observed that COs can be adsorbed on GNRs by the van der Waals force or the Coulomb force.Fig. 2 illustrates the adsorption of COs on GRN without charges under a pressure of 1.00 atm.The simulation results indicate that there is no Coulomb force between the GNRs and the CO when the GNRs are not charged, and the interactions are realized by the van der Waals force only, leading to a relatively weaker interaction.The intensity of interaction between the carbon atoms and the GNR is basically the same as the interaction between the oxygen atoms and the GNR.As shown in Fig. 2(a), few COs are adsorbed on the GRN and the adsorbed CO molecules are almost paralleled with the GNR surface.Also, this adsorption of COs on GRN is not yet steady.During the simulation process, we found that the adsorbed COs moved quickly on the GNR surface and flew away after a short period of time.The adsorption phenomena under other environmental pressures are similar to that under 1.00 atm.
The adsorption status of COs on a charged GNR with a charge density of 6.582 C/m 2 under the environmental pressure of 1.00 atm is illustrated in Fig. 3.Besides the van der Waals force between the COs and the GNR, the Coulomb force also plays an important role.Many more COs than for the one without charge are adsorbed on the GNR, which indicates that the interaction between the COs and the GNR is much more powerful.Moreover, this adsorption is more stable, though the adsorbed COs will fly away as well.The Coulomb force between the oxygen atoms in the COs and the GNR plays an attraction role, and a repulsion affect between the carbon atoms in the COs and the GNR.Consequently, as described in Fig. 3, those O atoms of COs are adsorbed on the GNR.greater probability of finding COs at a distance of 5.2 Å from the GNR.It can be concluded that the interaction between 6.582 C/m 2 GNRs and CO is far more intense than that between GNRs charged with lower charge densities (0 C/m 2 , 3.291 C/m 2 and 4.937 C/m 2 in this article) and CO.In the other words, a lower charge density on a GNR has little effect on the adsorption.Fig. 4(b) gives the G(r) curves with charge density 6.582C/m 2 at different pressures.The G(r) curves under 0.50 atm and 1.00 atm almost coincide, which indicates that the COs distribution regularity is similar to each other.But because the CO molecules densities are definitely different at different environmental pressures, the higher the environmental pressure is, the larger the amount of CO molecules adsorbed on the GRN becomes.
Fig. 5 gives the curves of total energy evolution process under 1.00 atm with different charge densities.The curves under 0.25 atm and 0.50 atm are basically the same.As can be seen in Fig. 5(a), at time zero, the different systems under the same pressure contain the same total energy, and the total energies have all experienced increasing, decreasing and stable processes.The 0 C/m 2 curve and 3.291 C/m 2 curve differ little throughout almost the whole simulation process.In the first two stages, the energy of the 4.937 C/m 2 system is higher than those with 0 C/m 2 and 3.291 C/m 2 .As the simulation continues, the differences between the three curves get smaller and smaller, basically coinciding.The 6.582 C/m 2 curve obviously differs from the other three curves.The total energy of the 6.582 C/m 2 system is far greater.The increasing and decreasing processes are both over a large range.The total energy curve of the 6.582 C/m 2 system shakes intensively.This indicates that a lower charge density (<4.937C/m 2 ) affects the system little, while a relatively greater current affects the system obviously.Fig. 5(b) gives the total energy of GRNs with different charge density.According to the total energy curves of the system in Fig. 5(a), the total energies of the GRNs in Fig. 5(b) are a little lower, with corresponding charge density, which shows that the total energy of the COs in the system accounts for a small part of the total energy of the whole system.Fig. 6 illustrates the evolution process of the total energy of GNRs under 0.50 atm and 1.00 atm (the curves under 0.25 atm are basically the same).At time zero, the four curves basically hold the same energy.The energy under 1.00 atm is a little higher than that under 0.5 atm.That is because of the fact that the higher the system pressure is, the greater the CO density is, and a GNR under higher pressure will interact with COs more intensely, resulting in a higher potential energy.With the same kinetic energy added at time 0, the higher the pressure is, the greater the total energy is.The same as with the curves in Fig. 5, the GNR energy experience increasing, decreasing and stable processes.The 0 C/m 2 curve, 3.291 C/m 2 curve and 4.937 C/m 2 curve coincide in the stable stage.However, the 6.582 C/m 2 curve is a little greater in the first two phases.The energy of the 6.582 C/m 2 GNR is larger than those of the other three.Again, this indicates that the relatively higher charge density (6.582 C/m 2 ) markedly affects the GNR, and a low charge density (<4.937C/m 2 ) basically does no work on the system.Fig. 7 shows the evolution process of the total energies of GNRs only with different charge densities.The two curves with 0 C/m 2 and 6.582 C/m 2 charge densities are very similar with each other, vibrating at a statistical average value of -21316 eV.What's more, the 3.291 C/m 2 GNR and 4.937 C/m 2 GNR also hold the same energies, indicating that charges have little effect on the total energies of the GNRs.According to other sets of simulation results of COs only under different environmental pressures, the total energies of COs are listed in Table II.Different systems have different total energies.
The adsorption energy is given by where E ads denotes the adsorption energy, E CO is the total energy of the COs only, E GRN is the total energy of the GRN only, and E CO-GRN is the total energy of the whole system including COs and GRN.The adsorption energies are listed in Table III.Relatively low charge densities (<3.291C/m 2 ) have little effect on the adsorption, and the adsorption energies are the same.With increasing charge density, the adsorption energy increases.The environmental pressure also affects the adsorption, i.e. the higher the pressure is, the greater the adsorption energy becomes.Higher pressure means more CO molecules contained per unit volume, and the CO molecules interact with the GNR more frequently, resulting in higher adsorption energies.

IV. CONCLUSIONS
System models of graphene nanoribbons (GNRs) with different charge densities, including 0 C/m 2 , 3.291 C/m 2 , 4.937 C/m 2 and 6.582 C/m 2 , lying in CO ambient environment under different pressures, such as 0.25 atm, 0.50 atm and 1.00 atm, are built up and simulated through Molecular Dynamics (MD) methods at room temperature (300K).The AIREBO potential is used to describe the interactions of the C-C bonds in the GNR and the Lennard-Jones (LJ) potential for the interactions between the CO and the GNR.The adsorption morphology and the radial pair distribution functions between CO and GNR are drawn, and the adsorption energies are calculated.The study indicates that both the charge density and the environmental pressure have a great influence on the adsorption properties of CO molecules on GRNs: with increasing charge density, the amount of the adsorbed CO molecules will become larger and larger; and the higher the pressure is, the greater the adsorption energy becomes.Observating the simulation results, it can be seen that most of the adsorbed CO molecules lie at a distance of about 5.2 angstroms (Å) from the GRN, which is close to the zero potential distance of the LJ potential between the oxygen atoms of the COs and the carbon atoms in the GRN.
FIG. 1. System configurations under 1.00 atm with charge density of 6.582 C/m 2 (a) Initial configuration of the system; (b) After 600 ps simulations; (c) One CO molecule in the system; (d) CO adsorbed on the GNR.

FIG. 5 .
FIG. 5. Evolution curves of total energy under 1.00 atm pressure with different charge densities; (a) total energy of the whole systems; (b) total energy of GRNs only.

FIG. 6 .
FIG. 6.Total energy of GNRs evolution curves under different pressures with different charge densities.

TABLE I .
CO ambient environments of different groups of simulation.

TABLE II .
Total energy of COs.

TABLE III .
Adsorption energies (eV) between COs and different charge density GNRs under different pressures.