Article pubs.acs.org/JPCC
Ab Initio Molecular Dynamics Simulation of Ethylene Reaction on Nickel (111) Surface Rizal Arifin,*,†,§ Yasushi Shibuta,‡ Kohei Shimamura,† Fuyuki Shimojo,† and Shu Yamaguchi‡ †
Department of Physics, Kumamoto University, 2-39-1 Kurokami, Chuo-ku, Kumamoto 860-8555, Japan Department of Materials Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan § Faculty of Engineering, Universitas Muhammadiyah Ponorogo, Jl. Budi Utomo No. 10, Ponorogo 63471, Indonesia ‡
S Supporting Information *
ABSTRACT: We performed ab initio molecular dynamics (MD) simulations of ethylene molecules on the nickel (111) surface to understand the initial stage of graphene growth via a chemical vapor deposition process. Several hydrogen atoms are dissociated from ethylene molecules during the MD simulations in three different reaction mechanisms. It is seen that the ethylene molecules are easily chemisorbed on the nickel surface. This chemisorption contributes significantly to the dissociation reactions of ethylene molecules on the nickel (111) surface. Furthermore, it is found from additional MD simulations that carbon atoms diffuse more easily into the nickel subsurface than carbon dimers.
1. INTRODUCTION The number of investigations in the graphene field has increased significantly in recent years due to its desirable electronic, optical, and mechanical properties.1−3 To achieve mass production of graphene, efficient and economical growth methods are required. One of the successful methods to grow a large area graphene is chemical vapor deposition (CVD) in which the hydrocarbon molecules are decomposed into carbon atoms or molecules on the hot catalytic metal substrate such as nickel. By dissolving the metal substrate, the graphene can be transferred into another substrate or left as freestanding graphene.4−7 A number of experiments of graphene growth on the metal surface have been reported.8−15 For example, Reina et al. have successfully grown a large area of graphene on the nickel surface and transferred to another substrate such as silicon or silicon oxide.14 They have also demonstrated their ability to grow graphene directly on a prepatterned nickel substrate. Li and co-workers have investigated the evolution of carbon molecules during the graphene growth process on nickel and copper by using carbon isotope labeling.15 Nickel has been known for its strong catalytic properties for hydrocarbon molecules.16 Therefore, it is currently used in the graphene growth process via CVD as the catalyst and substrate as well.14 In the initial stage of the graphene growth process, the hydrocarbon molecules decompose into carbon atoms or molecules on the nickel substrate. It has been reported that single- and polycrystalline nickel substrates lead to the formation of monolayer and multilayer graphene, respectively.17 In recent CVD processes, ethylene molecules are widely used as carbon precursor molecules due to their high © 2015 American Chemical Society
reactivity on the nickel substrate. Therefore, the graphene can be grown efficiently at a few hundred kelvin.18 On the other side, a number of computational works on the graphene formation from carbon monomers or carbon dimers on the metal surface by Monte Carlo (MC)19,20 or molecular dynamics (MD) simulations21,22 have been reported. Several works using the classical MD simulations have demonstrated the adsorption and dehydrogenation of hydrocarbon molecules on the nickel cluster23 and the nickel surface.24 However, the discussion on dynamics of the dissociation process is beyond the precision of those simulations because the accuracy of empirical interatomic potentials is limited. It is desirable to use nonempirical methods, such as density functional theory (DFT). We have investigated the mechanism of hydrocarbon dissociation processes on metal clusters25,26 and metal surfaces27,28 by ab initio MD simulations, which enable us to describe the interactions between atoms precisely based on DFT. The ethylene and nickel are the most common combination for graphene growth via CVD techniques used in experiments. This paper extends our research to the study of ethylene dehydrogenation on the nickel (111) surface. So far, the adsorption energy of ethylene on the nickel (111) surface represented by Ni4 and Ni14 cluster has been calculated using DFT.29 Recently, other work has combined the experimental scanning tunneling microscopy and DFT to investigate the Received: December 5, 2014 Revised: January 21, 2015 Published: January 23, 2015 3210
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The Journal of Physical Chemistry C ethylene dissociation on flat and stepped Ni (111) surfaces.30 It was found that the stepped surface is more reactive than the flat surface. In these studies, however, small clusters or a small slab consisting of only six nickel atoms per layer were used. To carry out more reliable study, more realistic models are needed. In this paper, we investigate the reaction of ethylene molecules with the nickel (111) surface using moderately large size models. We are unaware of any theoretical studies of the behavior of ethylene molecules on the nickel surface based on dynamic simulations. The purpose of this paper is to reveal the mechanism of dynamic processes of ethylene dehydrogenation using ab initio MD simulations.
reversible integrator40 with a time step of 0.242 fs. The MD simulation has been performed for 15 000 MD steps. Several dissociation reactions of ethylene molecules on the catalytic nickel (111) surface are observed during MD simulation. These chemical reactions occur by the presence of nickel substrate as the catalyst. The activation energies of reactions without the nickel catalyst are very high.41 The C CH fragment is the lowest hydrogen coordinated fragment obtained from the MD simulation. Figure 2 shows the time
2. COMPUTATIONAL METHODS The electronic states are calculated by the projectoraugmented-wave (PAW)31,32 method within the framework of DFT. The Perdew−Burke−Ernzerhov (PBE)33 generalized gradient approximation (GGA) is employed for the exchangecorrelation energy. In the MD simulation, the empirical correction of the van der Waals interaction is described by the DFT-D approach.34 The plane wave cutoff energies are 30 and 250 Ry for the electronic pseudo-wave function and pseudo-charge density, respectively. The energy functional is minimized iteratively using a preconditioned conjugate-gradient method.35 The Brillouin zone is sampled using Γ point. Projector functions are generated for the 3d, 4s, and 4p states of nickel, the 2s and 2p states of carbon, and the 1s state of hydrogen. We have obtained the adsorption energy of a hydrogen atom on the nickel (111) surface by using the PBE functional, which is in agreement with the experimental value36 and previous theoretical calculation.37 Therefore, we employed the PBE functional in this study.
Figure 2. Time evolution of the number of dissociated hydrogen atoms. The specific reactions are labeled by [a], [b], ..., [r] which are discussed in the text for details.
evolution of the number of dissociated hydrogen atoms. A hydrogen atom is considered to be dissociated from carbon atoms if the distance between them is larger than 1.6 Å for more than 50 MD steps (12.1 fs). It is shown that a high rate of dissociation reaction occurs in the early stage of the MD simulation. In total, 10 hydrogen atoms are dissociated (noted with [a]−[h] and [j]−[k] in Figure 2), and one hydrogen atom makes a bond with one of the ethylene fragments (noted with [i] in Figure 2) during 1250 fs. At each [a], [b], [c], and [f] in Figure 2, a hydrogen atom is dissociated from a H2CCH2 molecule. Three CCH2 fragments are formed at [d], [e], and [g] in Figure 2 by the dissociation of three hydrogen atoms from three HCCH2 fragments. Two hydrogen atoms are dissociated from CCH2 fragments to form two CCH fragments ([h] and [k] in Figure 2). At [j] in Figure 2, one HCCH molecule is formed from a HCCH2 fragment by releasing one hydrogen atom into the nickel surface. After 1250 fs, only one hydrogen atom dissociates from an ethylene fragment permanently. However, some C−H bonds are temporarily dissociated and formed. At [l] in Figure 2, a hydrogen atom is temporarily dissociated from a HCCH molecule for 60 fs before the bonding is restored. At [m] in Figure 2, a hydrogen atom dissociates from a HCCH molecule, and soon it makes a bonding with a H2CCH2 molecule to form a H2C−CH3 molecule. Another H2C−CH3 molecule is formed after few femtoseconds later (noted with [n] in Figure 2). This molecule is however preserved only for 36 fs. The remaining H2C−CH3 molecule (noted with [o] in Figure 2) makes a bond with a hydrogen atom to form a H3C− CH3 molecule ([p] in Figure 2) for 306 fs. At 3197 fs, a hydrogen atom is dissociated from H3C−CH3 to form a H2C− CH3 molecule ([q] in Figure 2). Later, another hydrogen atom is also dissociated from H2C−CH3 at 3281 fs to form a H2C CH2 molecule ([r] in Figure 2). The hydrogen atoms dissociated from ethylene molecules move on the nickel surface while some of them diffuse into subsurface and recombine with ethylene fragments. In the end
3. RESULTS AND DISCUSSION 3.1. Ethylene Reaction on Nickel (111) Surface. The system includes three layers of the nickel (111) plane containing 30 nickel atoms in each layer and 24 ethylene molecules arranged on the upper space as shown in Figure 1.
Figure 1. Initial configuration of the MD simulation.
The atoms in the bottom-most layer of nickel substrate are fixed in order to mimic the infinite thickness of substrate. The dimensions of supercell are 12.94 × 12.95 × 20.0 Å3 in the x-, y-, and z-directions, respectively. In order to mimic the experimental process done in the constant temperature condition, the Nosé−Hoover thermostat38,39 is employed in the canonical-ensemble MD simulation at 1500 K. The equations of motion are numerically solved via an explicit 3211
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The Journal of Physical Chemistry C of our MD simulation, among 11 hydrogen atoms, five hydrogen atoms exist in the subsurface, one of them recombines with the CCH2 fragment to form a standing CCH3 fragment, and the rest stay on the surface. In our investigation, three reaction mechanisms are observed, except the temporary reactions. We refer to these reactions as reactions 1, 2, and 3 described in detail in Figure 3. Reaction 1
In order to give a clear insight into the detail of the ethylene reaction on the nickel (111) surface, we choose two reactions as examples: the dissociation reaction of H2CCH2 into HCCH2 and a hydrogen atom and the formation reaction of a standing CCH3 fragment from the standing CCH2 and a hydrogen atom (the details of the other reactions are provided in the Supporting Information). The first example is a hydrogen dissociation from an ethylene molecule in reaction 1. Figure 4a shows the time evolution of
Figure 3. Diagrams of three mechanisms of ethylene reactions on the nickel (111) surface for (a) reaction 1, (b) reaction 2, and (c) reaction 3. The order of the reactions is denoted by the arrows.
occurs twice while the two other reactions occur once during the MD simulation. These three reaction mechanisms are initiated by the adsorption of ethylene molecules on the nickel (111) surface. Figure 3 shows that the final products of reactions 1 and 3 are a CCH fragment and three hydrogen atoms, and however, the intermediate products are different. In reaction 1, a standing CCH2 fragment is produced in the intermediate process while a symmetric HCCH molecule is found in reaction 3. Reaction 2 produces a CCH3 fragment. The dissociation mechanism of the ethylene molecules into C CH2 and two hydrogen atoms is identical with the dissociation in the reaction 1. However, in reaction 2, a hydrogen atom (from reaction 1) makes a bonding with the standing CCH2 to form a standing CCH3 fragment. In reaction 3, we have a symmetric HCCH molecule. It was proposed by computational studies that the HCCH molecule is produced also during the graphene growth on the copper (111) surface by using methane molecules as a carbon precursor.28,42 The HCCH molecules on the copper surface come from the recombination of two CH fragments before the methane molecule is completely dehydrogenated. Note that the situation of methane on the nickel (111) surface is different; i.e., the methane molecules are decomposed into monomer carbon atoms without forming the HCCH molecule in the intermediate process.27
Figure 4. (a) Time evolution of bond-overlap population and (b) snapshots of atomic configuration of the hydrogen dissociation process of an ethylene molecule. The other molecules are not shown for clarity.
bond-overlap populations (OVP) between atoms associated with this reaction. The OVP analysis43 generalized to the PAW method44 is used to quantify the change of the bonding properties of atoms involved in the reaction. The value of OVP describes the strength of the covalent bonding between two atoms; i.e., a higher value of OVP represents the stronger bonding between the corresponding atoms. The bonding between H1 and Ni1 atoms is formed at about 40 fs as shown clearly in Figure 4a. The snapshots of atomic configuration are shown in Figure 4b. About 20 fs later, C1 and Ni1 atoms make a bonding. These bonds become stronger over the time in contrast with the bonding between C1 and H1 atoms, which is weakened continuously. Finally, the bonding between C1 and H1 atoms is broken at about 85 fs after the bonding between H1 and Ni2 is formed. This dissociation process influences the bonding between two carbon atoms of the ethylene molecule only weakly even after dissociation. It is clearly shown from Figure 4a that the bond between C1 and C2 atoms remains strong. Even though the atomic configurations 3212
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complete dehydrogenation of the ethylene molecule on the nickel surface. The carbon dimer can segregate to form a carbon chain, or it can dissociate into two carbon atoms. An additional MD simulation is carried out in order to know the behavior of the carbon dimer on the nickel surface. We performed a MD simulation of carbon dimers on the nickel (111) substrate. The system is prepared first by eliminating undissociated ethylene molecules from the final configuration of MD simulation explained in subsection 3.1 remaining four ethylene fragments on the nickel surface. Second, the hydrogen atoms are removed from the ethylene fragments to obtain four carbon dimers on the nickel surface. Then, the MD simulation is performed at 1500 K for 5000 steps. In total, two (from four) carbon dimers dissociate into four carbon atoms. Figure 6a shows that the carbon dimers, which maintain the chemical bonds, stay on the nickel surface as shown in Figure 6c. Once the dimer dissociates into two carbon atoms, carbon atoms diffuse in the nickel subsurface as shown in Figures 6b and 6d for the carbon z coordinates with respect to the nickel surface and the snapshot of atomic configuration,
are different in respective hydrogen dissociation reactions of ethylene molecules, the mechanisms are similar to each other. The second example is the C−CH3 formation observed in reaction 2. From Figure 5a, we clearly see that the reaction is
Figure 5. (a) Time evolution of bond-overlap population and (b) snapshots of atomic configuration of the formation reaction of a C− CH3 fragment from a CCH2 fragment and a free hydrogen atom. The other molecules are not shown for clarity.
started by bonding formation between H1 atom and both Ni1 and Ni2 atoms at about 860 fs. A bond between H1 and Ni3 is also formed 10 fs later. Then, the position of H1 atom becomes very close to the standing CCH2 fragment. We must note that during the MD simulation most of the dissociated hydrogen atoms continuously move between different sites on the nickel surface, while a few of them diffuse into the substrate. When the standing CCH2 fragment tilts to the H1 side due to thermal vibration, a C−CH3 fragment is formed. We guess that if there are more hydrogen atoms on the surface, the formation of C−CH3 fragments is easier than the hydrogen dissociation of the CCH2 fragment. Since the hydrogen atoms easily move on the surface, there is a high chance for hydrogen atoms to make a bonding with the standing CCH2 fragments. The rebonding of the dissociated hydrogen atoms to the hydrocarbon fragments disturbs the dehydrogenation process. Then, we can say that excess hydrogen atoms on the surface make the graphene growth process ineffective. From Figure 5a, we can also see that the formation of the C− CH3 fragment changes the typical double bond between carbon atoms in an ethylene molecule to the single bond. The OVP between C1 and C2 atoms is decreasing by almost half. 3.2. Carbon Dimers on the Nickel (111) Surface. The carbon dimer is expected to be the final product of the
Figure 6. Time evolution of z-coordinates of carbons with respect to the nickel surface. The z-coordinate of average nickel surface is set to be zero. (a) The lines labeled “C dimer (1)” and “C dimer (2)” represent the z-coordinates of the first- and second-carbon dimers, respectively. (b). The lines labeled “C atom (1)” and “C atom (2)” represent the z-coordinates of the first and second pairs of carbon atoms, respectively, dissociated from carbon dimers. Snapshots of the atomic configuration of (c) carbon dimers and (d) carbon atoms on the nickel surface after 5000 MD steps. 3213
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The Journal of Physical Chemistry C respectively. This fact is consistent with the experimental observation45,46 and our previous work.27 We have reported that carbon atoms easily diffuse into the nickel surface after they are dissociated from a methane molecule.27 3.3. Methane and Ethylene Adsorption on Nickel (111) Surface. As it is explained in subsection 3.1, the ethylene molecules are adsorbed on the nickel surface before the dissociation reactions are observed. The close distance between the molecules and the nickel surface is needed to initiate the reaction. The information on the molecule adsorption on the catalytic surface is absolutely needed to understand the mechanism of the reaction. Additional DFT calculations to obtain the adsorption energy of ethylene and methane molecules on the nickel (111) surface are also performed. Figure 7 shows the system used, in which
Figure 8. Adsorption sites of (a) an ethylene molecule and (b) a methane molecule on the nickel (111) surface. The dashed △ triangle and ▽ triangle correspond to the fcc and hcp hollow sites, respectively.
ethylene molecule stay on the hcp hollow and top sites. The position of the carbon atom on the nickel hollow site is nearer to the surface (1.64 Å) than that of the carbon atom staying on the nickel top site (2.02 Å), as seen in Table 1. The hydrogens in the adsorbed ethylene molecule are in higher positions than the bonded carbon. H1 and H2 atoms, which is bonded with C1 atom (see Figure 8a) on the hollow site, are in 1.94 and 1.90 Å away from the surface, respectively. The hydrogen atoms H3 and H4 bonded with C2 atom on the top site are in 2.46 and 2.41 Å away from the nickel surface, respectively. From the calculated adsorption energy, we conclude that the energy released from the adsorption process contributes significantly to the dissociation reaction of ethylene on the nickel surface. Figure 8b shows that the methane molecule prefers to stay on the top of nickel atom with two hydrogen atoms on the lower positions and other two hydrogens on the upper positions with respect to the carbon atom. From the adsorption energy data in Table 1, the small value of adsorption energy (−0.01 eV) indicates that the methane molecule interacts weakly with the nickel surface and the carbon atom stays 3.47 Å away from the nickel surface. In the methane case, therefore, the dissociation is mainly driven from external energy, such as kinetic energy from collision or thermal vibration. These results are consistent with the experimental fact that, while the high temperature at about 1000 K or more is needed to grow the graphene using methane as the carbon precursor molecule,14 only a few hundred kelvin is sufficient if ethylene molecules are used.18,47
Figure 7. (a) Top view and (b) side view of the system used to obtain the adsorption energy of ethylene on the nickel (111) surface.
four layers of the nickel (111) plane (16 atoms per layer) are used for the substrate and one ethylene or methane molecule is located on the nickel surface. The atoms in two bottom-most nickel layers are fixed. The Brillouin zone is sampled using 2 × 2 k-points. The ferromagnetic spin polarization is taken into account for the nickel atoms. The system is relaxed using a quasi-Newton optimization method. We examine the adsorption of an ethylene molecule on the nickel surface by taking the configuration from MD simulation before the dissociation reaction. For comparison, the adsorption of a methane molecule on the nickel surface from our previous MD simulation27 is also investigated. The adsorption energy, Eads, is calculated by using a formula Eads = E(sub+mol) − (Esub + Emol), where E(sub+mol), Esub, and Emol are the total energies of the system consisting of the substrate and the molecule, of an isolated substrate, and of an isolated molecule, respectively. Table 1 lists the z-coordinates (the distance from the surface) of H and C atoms in the optimized structure as well as the adsorption energy. The ethylene molecule is easier to adsorb than the methane molecule on the nickel surface, indicated by the lower adsorption energy (−0.76 eV) and the smaller distance from the ethylene molecule to the surface. In Figure 8a, it is clearly seen that the carbon atoms of the
4. CONCLUSION We performed ab initio MD simulations of the ethylene dehydrogenation on the nickel (111) surface. Three different reaction mechanisms are found consisting of the hydrogen dissociation from ethylene molecules and the formation of a C−CH3 fragment from a CCH2 fragment and a hydrogen atom. We also investigated the behavior of carbon dimers on the nickel (111) surface. During MD simulation for 1.21 ps, carbon dimers from the ethylene dehydrogenation on the nickel surface keep staying on the surface, while carbon atoms prefer to diffuse into the nickel subsurface. We observed that all the reactions are started by the chemisorption of ethylene molecules on the nickel (111) surface. Therefore, we calculated the adsorption energy of an ethylene molecule on the nickel surface to clarify the contribution of the adsorption process on the dissociation reaction. We found that the ethylene molecule has rather low adsorption energy, much lower than the methane molecule, on the nickel (111) surface, which indicates the significant contribution of the adsorption energy on the dissociation process.
Table 1. Adsorption Energy Eads, z-Coordinates of C Atom ZC, and z-Coordinates of H Atom ZH with Respect to the Nickel (111) Surface for Methane and Ethylenea molecule
Eads [eV]
ZC [Å]
ethylene
−0.76
C1: 1.64 C2: 2.02
methane
−0.01
ZH [Å] H1: H2: H3: H4:
1.94 1.90 2.46 2.41
3.47
a The average of z-coordinates of the nickel atoms on the topmost layer is set to be zero. The atomic configuration of C1, C2, H1, H2, H3, and H4 can be seen in Figure 8.
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ASSOCIATED CONTENT
S Supporting Information *
Time evolution of OVPs and snapshots of atomic configurations for HCCH2 → CCH2 + H; HCCH2 → C CH2 + H; HCCH2 → HCCH + H; and HCCH → CCH + H reactions on the nickel (111) surface. This material is available free of charge via the Internet at http:// pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*(R.A.) E-mail Rizal.Arifi
[email protected]; Ph +81-90-6632-8197. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors acknowledge the Supercomputer Center, Institute for Solid State Physics, The University of Tokyo, and the Research Institute for Information Technology, Kyushu University, for the use of their facilities.
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REFERENCES
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DOI: 10.1021/jp512148b J. Phys. Chem. C 2015, 119, 3210−3216
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DOI: 10.1021/jp512148b J. Phys. Chem. C 2015, 119, 3210−3216
