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CH4 dissociation in the early stage of graphene growth on Fe–Cu(100) surface: Theoretical insights
Accepted Manuscript Title: CH4 dissociation in the early stage of graphene growth on Fe–Cu(100) surface: theoretical insights Authors: Baoyang Tian, Tianhui Liu, YanYan Yang, Kai Li, Zhijian Wu, Ying Wang PII: DOI: Reference:
S0169-4332(17)32737-X http://dx.doi.org/10.1016/j.apsusc.2017.09.088 APSUSC 37167
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APSUSC
Received date: Accepted date:
5-7-2017 11-9-2017
Please cite this article as: Baoyang Tian, Tianhui Liu, YanYan Yang, Kai Li, Zhijian Wu, Ying Wang, CH4 dissociation in the early stage of graphene growth on Fe–Cu(100) surface: theoretical insights, Applied Surface Sciencehttp://dx.doi.org/10.1016/j.apsusc.2017.09.088 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
CH4 dissociation in the early stage of graphene growth on Fe–Cu(100) surface: theoretical insights Baoyang Tian a,b, Tianhui Liu,c YanYan Yang*,a, Kai Li *,b, Zhijian Wu*,b, and Ying Wang,b a
College of Chemistry and Pharmaceutical Engineering, Jilin Institute of Chemical Technology, Jilin, Jilin 132022
b
State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China. c
Electronic Information Products Supervision Inspection Institute of Jilin Province NO. 1381 Xinmin Avenue, Changchun, Jilin Province.
Highlights
Propose a theoretical catalyst to grow high quantity and larger area graphene.
Microkinetic model explains the effects of temperatures and H2 partial pressures.
The synergistic effect between Fe and Cu plays an important role in the CH4 dissociation process.
Abstract The mechanism of CH4 dissociation and carbon nucleation process on the Fe doped Cu(100) surface were investigated systematically by using the density functional theory (DFT) calculations and microkinetic model. The activity of the Cu(100) surface was improved by the doped Fe atom and the atomic Fe on the FeCu(100) surface was the reaction center due to the synergistic effect. In the dissociation process of CH4, CH3→CH2+H was regarded as the rate-determining step. The results obtained from the microkinetic model showed that the coverage of CHx(x=1-3) was gradually decreased with the temperature increasing and CH3 was always the major intermediate at the broad range of the temperature (from 1035 to 1080 ºC) and the ratio of H2/CH4 (from 0 to 5). It is also found that the reaction rates
*Corresponding author. E-mail: [email protected](Y. Yang); [email protected] (K. Li); [email protected] (Z. Wu); [email protected] (Y. Wang) 1
were increased with the temperature increasing. However, the reaction rates were reduced (or increased) at the range of H2/CH4=0-0.2 (or H2/CH4>0.2). It is noted that controlling the H2 partial pressure was an effective method to regulate the major intermediates and reaction rates of CH4 dissociation and further influence the growing process of graphene. Keywords: density functional theory; CH4 dissociation; microkinetic model; chemical vapor deposition; graphene growth 1. Introduction Graphene is considered to be the most attractive materials in this century due to its excellent electrical properties and optical properties [1, 2]. Owing to the excellent performance of graphene with high specific surface area [3], thermal conductivity [4], carrier mobility [5-9], transparency [10, 11], holes [5-8] and electrons half-integer quantum Hall effect [10, 11], the graphene-based materials have been widely employed in diverse fields, biomaterials, nanoelectronic devices, intercalation materials, catalysis, as well as drug delivery [12, 13]. The controllable synthesis of graphene is a necessary prerequisite for its broad applications. Currently, chemical vapor deposition (CVD) is regarded as one of the most promising methods for producing monolayer graphene on a commercial scale. Graphene CVD growth generally involves four steps: (1) dissociation of hydrocarbon molecules; (2) diffusion of C atoms on the catalyst surface; (3) formation of the nucleation graphene islands; and (4) growth of the graphene islands [14-16]. The first dissociation process determines the following steps of graphene growth, and the decomposition products directly affect the growth mechanism. It is well known that the dissociation of hydrocarbon molecules can not be occurred spontaneously and the appropriate catalysts are necessary. In general, transition metals were chosen for CVD graphene synthesis, such as Ru [17], Pd [18, 19], Co [20], Rh [21], Re [22], Pt [23, 24], Ir [25, 26], Au [27, 28], Fe [29] and Ag [30]. Among them, the 3d transition metal catalysts attracted more attention, especially Fe and Cu [31], due to the high activity and low price. A number of studies have been conducted to investigate the 2
effect factors and growth mechanism of graphene on 3d transition metal catalysts. It is found that the temperature affected the thickness and quality of graphene [32-34]. The optimum temperature was about ~600 ºC for graphene growth on nickel surface. If the temperature was higher than it, carbon will diffuse into nickel bulk and limit the surface growth rate. Conversely, a competing surface carbide phase could hinder the graphene formation [33]. Besides temperature, H2 partial pressure can also affect the quality of the graphene. It is found that the hexagonal graphene could be formed at high H2/CH4 ratios, whereas, the graphene was changed to a branched structure at a low H2/CH4 ratio [35]. In addition, the growth of high quality graphene also depends on the species and morphology of the catalysts [36]. The 3d transition metal Fe and Cu exhibited the distinct interactions with carbon atoms [37]. Fe possessed a high affinity to the C atom due to the partially filled Fe 3d-orbitals. Meanwhile, C atom had a finite solubility in Fe (0.022% carbon by weight in α-Fe at 727 ºC and 2.14% carbon by weight in γ-Fe at 1147 ºC) and formed metastable iron carbide (Fe3C, 6.67% carbon by weight) [38, 39]. C solubility in Cu was lower than in Fe (~0.008% carbon in Cu by weight at 1085 ºC) and no carbide was observed [40, 41]. Therefore, the metal-catalyzed CVD growth of graphene between Fe and Cu with the different M-C (M=Fe or Cu) interactions resulted in the formation of few-layer graphene on Fe foil [42] and monolayer graphene on Cu foil [43]. Although Cu was a better candidate catalyst for monolayer graphene growth, the activity of Cu was lower than that of Fe, which hindered the growth rates of graphene. Therefore, it is necessary to exploit an effective method to improve the Cu activity and remain the graphene growth process unchanged. Design an effective Cu-based catalyst, such as Fe doped Cu surface or Fe-Cu alloy, may be a good choice to balance the activity and the layered-control growth. Therefore, in this study, we firstly investigated the dissociation mechanism of methane on Fe-doped Cu surface to estimate the effect of Fe doping on the first step of graphene growth. Secondly, we investigated the carbon migration and polymerization process, which is related to the graphene nucleation. Thirdly, the flux effects of H2 and CH4, as well as the 3
temperature effects were investigated by the microkinetic models. Our current results showed that the synergistic effect of Fe and Cu was very important to improve the activity of Cu surface and the Fe-Cu(100) surface could be deemed as a potentially catalyst to achieve the high quality and larger scale graphene growth. 2. Computational details 2.1.Method The geometric structures and the energies were calculated by using Vienna abinitio simulation package (VASP) [44-47]. The interaction between the valence electron and the ion cores was described by Blöchl's all-electron-like projector augmented wave (PAW) method [48, 49]. The Perdew-Burke-Ernzerhof (PBE) was used as the exchange correlation function [50]. The cut off kinetic energy was set to 300 eV. The wave functions were expanded with a plane wave basis set at each kpoints. Brillouin zone integration was approximated by a sum over special selected kpoints using the Monkhorst-Pack method [51] and they were set to 3×3×1. An energy smearing by the electron occupancies were determined according to Fermi Scheme of 0.1 eV. Geometries were optimized until the energy was converged to 1.0×10-6 eV/atom and the force was converged to 0.01 eV/Å. Spin polarization was considered in all calculations due to the presence of magnetic atoms Cu and Fe. The previous study [52] showed that CH4 decomposition was dominated by van der Waals (vdW) interaction. Therefore, in the current calculations, a semi-empirical decentralized corrected density functional theory (DFT-D2) force field [53, 54] method was used to describe the van der Waals (vdW) interaction between the reactants and the substrates. The structures of transition states (TSs) and the reaction pathways were obtained by using the climbing image micro-elastic band (CI-NEB) method [55]. To characterize the properties of all the stationary points and to obtain zero energy (ZPE) correction, harmonic vibration frequency calculations were carried out for all the transition states and the most stable configurations of CHx(x=0-4) on Fe-Cu(100) surface. 4
2.2. Model The previous experimental studies [56] found that the Cu(100) surface showed a remarkable performance in CVD graphene growth. Therefore, the (100) surface of Cu was chosen in our study. The (100) surface was obtained by cutting the optimized bulk face centered cubic Cu crystal along (100) direction and three layer slabs were selected. This model has been used in the previous calculations of CH4 dissociation on metal surface and was proved to be effective [57]. A (3×3) supercell was used to model the coverage of a 1/9 monolayer. Then, we replaced one of the surface Cu atoms by a Fe atom to simulate 1:8 Fe–Cu(100) surface. The atoms in the bottom were fixed in their bulk positions and the other two layers were allowed to relax. To avoid periodic interactions, the vacuum layer as large as 12 Å between repeated slabs was used along the z direction. The chemisorption energies of CHx(x=0–4) on Fe-Cu(100) surface, Eads, was defined as follows Eads =Eeadsorbatest/slabe-(Eslab + Eadsorbates)
(1)
Where Eadsorbates/slab is the total energy of adsorbate and the Fe-Cu(100) surface, Eslab is the total energy of the Fe-Cu(100) surface and Eadsorbates is the total energy of free adsorbates. The first two terms were calculated with the same parameters. The third term was estimated by setting the isolated adsorbate in a box of 12×12×12 Å. The negative Eads indicates an exothermic chemisorption, and the positive value suggests an endothermic chemisorption. 3. Results and discussion 3.1.Adsorption of CHx(x=0-4) on Fe-Cu(100) Five typical adsorption sites for CHx(x=0-4) on Fe-Cu(100) surface were investigated, i.e. Top site (TFe and TCu), Bridge site (BFe-Cu and BCu-Cu) and Hollow site, as shown in Fig.1. Followed structure optimization, the possible adsorption configurations of CHx(x=0-4) and the adsorption energies on the Fe-Cu(100) surface were shown in the Supporting information, Fig. S1. 5
3.1.1 Methane adsorption Pervious researches showed that methane adsorption was a physisorption process with small adsorption energy (-0.50 ~ -0.05 eV) on the pure metal surfaces [58-60]. In Fig. S1 a-c, we showed the structure of CH4 adsorbed on the Fe-Cu(100) surface and the adsorption energies. It is seen that CH4 on the Fe-Cu bridge site was the most stable structure. The distance between Fe and C atoms in CH4 was 3.10 Å and the adsorption energy was -0.46 eV. Compared with pure Cu(100) (-0.21 eV) [61], the adsorption energy was enhanced by 0.25 eV, indicating the doped Fe atom slightly strengthen the interaction between Cu(100) and CH4. 3.1.2 CH3 and CH2 adsorption on Fe-Cu(100) The optimized structures of CH3 and CH2 on the Fe-Cu(100), together with the adsorption energies, were shown in Fig.S1 d-j (CH3 and CH2 corresponding to d to g and h to j). It is seen that the CH3 can be stacked stably on the top site and bridge sites, respectively, due to the negative adsorption energies. Among the four optimized structures, the most stable adsorption site was Fe-Cu bridge site with the adsorption energy of -2.42 eV and the distance between Fe and C atom of CH3 was 2.04 Å, which is shorter than that of pure Cu(100) (2.15 Å) [61]. This suggested that the interaction between the Fe and C of CH3 was stronger than that of pure Cu(100). Thus, the doped Fe enhanced the adsorption of CH3 on Cu(100) (Eads=-2.42 and -1.83 eV for Fe-Cu(100) and Cu(100), respectively)[61]. For CH2, the most stable structure was CH2 adsorbed on a hollow site with the adsorption energy of -4.28 eV. It is seen that one H atom of CH2 bonds to Fe atom (Fe-H bond of 1.84 Å), which was favorite to the further dehydrogenation. For the least stable adsorption configuration of CH2 (Fig. S1 h), it is seen that the surface was deformed, which further reduced the adsorption energy of CH2. Compared with pure Cu(100) (-3.58 eV) [61], the adsorption energy was reduced by 0.70 eV. Therefore, the doped Fe improved the CH2 adsorption on Fe-Cu(100) surface. 3.1.3 CH and C adsorption on Fe-Cu(100) 6
For CH and C chemisorption on Fe-Cu(100), five CH and C possible adsorption sites were considered. After structure optimization, only the stable site of the nearest hollow site to Fe atom was found. The stable absorption structures and energies were shown in Fig. S1 (k for CH and l for C). For CH, the adsorption energies were -6.71 eV, which was lower than that of the pure Cu(100) surface (-5.88 eV) [61]. The distances of Fe–C was 1.89 Å, which was shortened compared to that on the pure Cu(100) (1.98 Å) [61]. This suggests that the doped Fe had stronger interaction with the C of CH than that of Cu, and this was consistent with the improved adsorption of CH on Fe-Cu(100). All the C adsorption structures on Fe-Cu(100) were converged to the most stable four fold site, i.e. C (Fig. S1l) at the nearest hollow site to the Fe atom and the adsorption energy was -7.61 eV. Similar to the configuration of CH adsorption, the doped Fe was more affinity to the adsorbed C atom (1.76 Å) than that of pure Cu(100) (1.91 Å) [61]. Furthermore, it is seen that the adsorption energies of CHx was increased as the number of the H atom in CHx reduced [52, 61-63]. This trend was consistent with the charge transition of CHx(x=0-3) on Fe-Cu(100) surface, as shown in Table 1. The more charge transferred to CHx, the stronger adsorption was obtained. 3.2.Methane dissociation on Fe-Cu(100) To investigate the mechanism of methane dissociation on the Fe-Cu(100) surface, the minimum energy paths of CHx → CHx-1+H were estimated. In these calculations, the most stable geometric structures of CHx were regarded as the ISs (initial states). And the most stable CHx-1+H co-adsorption configurations were set as the FSs (final states). The geometric structures of ISs, FSs and TSs (transition states) for CH4 dissociation were shown in Fig.2. 3.2.1 CH4→CH3+H The first step of CH4 dissociation, i.e. CH4→CH3+H, was shown in Fig. 2a. The most stable geometric structure of CH4 on Fe-Cu bridge site was set as the IS. CH3 at BFe-Cu with H at hollow site was set as the FS. In TS1, the CH3 was adsorbed at the 7
BFe-Cu and the breaking H atom was located at the nearest hollow site with the C-H of 1.61 Å. The reaction energy barrier was calculated as 0.64 eV with slightly endothermic of 0.07 eV. For Cu(100), Cu(111), Ni(111), Fe(111) and Fe(100) surfaces [61, 64-66], the reaction energy barrier of CH4→CH3+H was calculated to be 0.83, 1.31, 0.65, 1.02 and 0.71 eV, respectively. And the reaction energies were 0.58, -0.63 and -0.13 eV for the Cu(100), Fe(111) and Fe(100), respectively [61, 64, 65]. Therefore, the dissociation process of CH4→CH3+H was improved on the Fe-Cu(100) both kinetically and thermo-dynamically. 3.2.2 CH3→CH2+H For CH3 →CH2 +H, the most stable geometric structure of CH3 on Fe-Cu bridge site was set as the IS, and CH2 with H at the hollow site was set as the FS. For TS2 in Fig. 2b, the CH2 was adsorbed at the Fe-Cu bridge site and the breaking H was located at the nearest Fe-Cu bridge site with the C-H of 1.59 Å. The reaction energy barrier was calculated to be 1.00 eV with endothermic of 0.46 eV. For Cu(100), Cu(111), Ni(111), Fe(111) and Fe(100) surface [61, 64-66], CH3→CH2+H, the reaction energy barrier was calculated to be 0.98, 1.31, 0.65, 1.02 and 0.71 eV. The reaction energies on Cu(100), Fe(111) and Fe(100) were 0.71, -0.32 and 0.37 eV, respectively [61, 64, 65]. It indicated that CH3 dehydrogenation reaction was improved on Fe-Cu(100) compared to pure Cu(100). However, its reaction activity was still lower than those on Fe(111), Ni(111) and Fe(100) [64-66]. 3.2.3 CH2→CH+H The dissociation process of CH2→CH+H, was shown in Fig. 2c. The most stable geometric structure of CH2 on hollow site was set as the IS. The CH and H adsorbed at hollow site was set as the FS. For TS3, (Fig. 2a), the CH3 was adsorbed at the hollow site and the breaking H atom was located at the nearest hollow site with the CH of 1.58 Å. The reaction energy barrier was calculated as 0.47 eV with exothermic of -0.20 eV. For Cu(100), Cu(111), Ni(111), Fe(111) and Fe(100) surface [61, 64-66], the barriers of CH2→CH+H were 0.63, 0.93, 0.31, 0.13 and 0.75 eV. The reaction 8
energies of Cu(100), Fe(111) and Fe(100) were 0.05, -0.69 and 0.64 eV [61, 64, 65]. Compared with pure Cu(100) [61], it is seen that CH2 dehydrogenation reaction became the only exothermic process in the whole CH4 dissociation process on the FeCu(100). Meanwhile, the C–H dissociation barrier (0.47 eV) was very low and comparable with those of Ni(111) (0.31 eV) and Fe(111) (0.13 eV) [63, 64]. This promises that CH can be formed automatically once CH2 is produced. 3.2.4 CH→C+H CH→C+H, was shown in Fig. 2d. CH adsorbed at hollow site was set as IS. C at hollow site with H at BFe-Cu was set as FS. Regarding to TS4, as shown in Fig.2(d), atomic C was adsorbed at the hollow site and the breaking H atom was located at the nearest Fe top site (TFe) with C-H of 1.86 Å. The calculated reaction energy barrier was 0.74 eV with an endothermic of 0.13 eV. For Cu(100), Cu(111), Fe(111) and Fe(100) surfaces [61, 64-66], the reaction energy barriers of CH→C+H were 1.39, 1.97, 1.04 and 0.76 eV, respectively. The reaction energies were 0.74, 0.04 and 0.41 eV for Cu(100), Fe(111) and Fe(100), respectively [61, 64, 65]. It is clearly seen that the doped Fe enhanced the CH dissociation process owing to the lower reaction barrier and less endothermic compared to the pure Cu and Fe. 3.3.The effects of the doped Fe on Cu(100) surface As discussed above, for the Fe-Cu(100), Fe showed a higher catalytic activity than Cu and the major catalytic center was observed as the doped Fe atom. In order to illustrate the catalytic activity of the doped Fe on Cu(100) surface, the CH4 dissociation potential energy on Fe-Cu(100) was shown in Fig. 4. For comparison, the results of the pure Cu(100) surface were also displayed in the same figure [61]. For pure Cu(100), it is seen that all four dehydrogenation steps were endothermic with the higher reaction barriers. Therefore, the continuous CH4 dissociation process on the Cu(100) surface was unfavorable both kinetically and thermodynamically. The rate-determining step was CH→C+H on Cu(100) surface [61]. On the contrary, for the Fe-Cu(100) surface, CH2→CH+H was found to be an exothermic process and the rate-determining step was CH3→CH2+H. Compared with Cu(100) surface [61], the reaction barriers of the most 9
elementary were reduced significantly (such as, CH4→CH3+H, by 0.19 eV; CH2→CH+H, by 016 eV; CH→C+H, by 0.65 eV). However, for the reaction CH3→CH2+H, the reaction barrier was increased slightly by ~0.02 eV. Moreover, the total barrier of CH4 dissociation was reduced from 3.83 to 2.85 eV after the introduction of the Fe atom. Therefore, the synergistic effect between Fe and Cu improved the catalytic activity for CH4 dissociation. To further explain the synergistic effect, the electronic structure of the FeCu(100) had been investigated. It is seen that the charge of Cu(100) was redistributed by the doped Fe atom. On the Fe-Cu(100), ~0.22 e was transferred from Fe to Cu atoms, suggesting that the Fe-Cu bond was polarized. Since the radicals of CHx(x=1-3) have redundant electrons, they preferred to be adsorbed at the Fe atom. In order to explain the high activity of Fe-Cu(100), the d-band center was examined [67-69]. It is found that the average d-band centers of Fe was -1.87 eV, which was closer to Fermi level than that of surface Cu(100) (-2.41 eV) [61], suggesting that Fe was more active than that of Cu. On the other hand, it is seen that the average d-band center of Cu (-2.41 eV) in Fe-Cu(100) was far away from Fermi level compared with that of pure Cu(100) (2.30 eV) [61]. This result indicated that the electron transferred from Fe led to the shift of d-orbital of Cu atoms. 3.4.The migration and polymerization of carbon atoms on Fe-Cu(100) For the growth of graphene, except for the dissociation of hydrocarbon molecules, the migration and polymerization of carbon atoms on the catalytic metal surface were also the important reactions in the graphene nucleation process. Therefore, we also investigated the behavior of C atoms in this study. The possible migration path of C atom on Fe-Cu(100) surface were shown in Fig. 3a. C atoms moved around the surface of Cu atoms, i.e. C atoms moving between the two hollow sites in the counter clock wise direction (Fig. 3a). The reaction energy barrier was 1.42 eV and endothermic was 0.30 eV, indicated that C atoms was migrate difficultly on the Fe-Cu(100) surface.
10
The possible polymerization of two carbon atoms on Fe-Cu(100) surface was shown in Fig. 3b. The reaction energy barrier was calculated as 0.85 eV and exothermic of -0.24 eV, which suggested that the carbon polymerization was occurred more easily due to the low reaction barrier and the exothermic process. Therefore, the doped Fe atom was not only as an activity center for CH4 dissociation but also as a carbon polymerization center for graphene nucleation. Furthermore, compared with the pure Cu(100) surface, the migration barrier on Fe-Cu(100) surface was decreased by 0.43 eV (Ea = 1.85 eV on Cu(100)) and the polymerization barrier was increased by 0.10 eV (Ea = 0.75 eV on Cu(100)). Therefore, the introduction of Fe accelerated the migration of the C and hindered the nucleation process kinetically compared to the reactions on Cu(100) surface. This promised that the formed atomic carbon having sufficient time to migrate to the most stable position or to the edges of the formed graphene islands during the nucleation and growth processes. Ultimately, it led to a formation of a perfect graphene pattern. We proposed that Fe-Cu(100) surface is a potentially catalyst to achieve the high quality and larger scale graphene growth. 3.5 Microkinetic model As mentioned before, the growth process of graphene was affected by the factors of temperature and H2 pressure. Therefore, in this study the microkinetic model was also performed to explain the effect of above two factors. Based on the DFT calculated results and microkinetic model [67, 68] (Supporting information for a brief description of the model), we calculated the possible coverage of each intermediate (CHx) at different experimental temperatures and H2 partial pressures, as shown in Table 2, Table 3, Fig. 5, and Table. S1, respectively. To simulate the experimental conditions, the temperature was ranged from 1035 to 1080 ºC, and H2 partial pressure was varied from 0 to 0.12 Torr. It is noted that in the microkinetic calculations the total pressure of CH4 and H2 was fixed at a constant of 0.12 Torr according to the experimental conditions [70]. Furthermore, free C atom was more active than the graphene, therefore, we assumed that the formed C by dehydrogenation process could 11
combine immediately to form the graphene islands. In other words, the coverage of free C atom was set as zero. At 1035 ºC and H2/CH4=0.2, the coverage of CH3 was ~134.2 and 8.0 times higher than that of CH2 and CH on Fe-Cu(100) surface (Table 2). For 0
12
high quality growth of graphene through adjusting H2 partial pressure, consistent to the experimental observations. 4. Conclusions The mechanism of CH4 dissociation and carbon nucleation process on FeCu(100) surface were investigated by using DFT calculation and microkinetic model. The synergistic effect between Fe and Cu improved the catalytic activity on CH4 dissociation due to the polarization and the shift d-band center. The CH4 dissociation can be improved on Cu(100) once Fe atom was introduced. For CHx→CHx-1+H (x=14), the doped Fe strengthened the adsorption of CHx, meanwhile, all the elementary reactions was improved both kinetically and thermodynamically, except for CH3→CH2+H. For CH4 dissociation, CH3→CH2+H was the rate-determining step on the Fe-Cu(100). On the other hand, Fe atom was also regarded as the C polymerization center for graphene nucleation. Based on the microkinetic model, we found that both the temperature and H2/CH4 have an influence on the reaction rates. The reaction rate was kept increasing with the temperature increasing from 900 to 1100 ºC. On the other hand, the H2 partial pressure reduced (or increased) the reaction rate at the range of H2/CH4=0-0.2 (or H2/CH4>0.2). The major intermediate was observed as CH3 species and the maximum coverage of CH3 can be obtained at 1035 ºC and H2/CH4=0. In addition, it is also found that the doped Fe reduced the temperature sensitivity of CH4 dissociation on Fe-Cu(100) and. Over all, doping Fe in the Cu(100) can not only improve the CH4 dissociation but also the polymerization of C atom, which were beneficial for improving the growth rate of graphene. Moreover, the appropriate H2 partial pressure can control the CH4 dissociation rate on FeCu(100) to further control the growth of graphene. Acknowledgements This work is supported by the National Natural Science Foundation of China (21503210, 21521092, 21673220), Jilin Province Natural Science Foundation (20150101012JC), Special Program for Applied Research on Super Computation of the NSFC-Guangdong Joint Fund (the second phase), Educational Commission of 13
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20
Fig 1. Top view of the possible adsorption sites on the Fe-Cu (100) surface.
21
Fig 2. The structures of stationary points (including IS, TS, and FS), imaginary frequency and C-H bond length in each TS (Imag(TS) and C-H(TS), the units are cm1
and Angstrom), as well as the reaction barrier (Ea) and reaction energy (H) for
four CH4 dissociation pathways on Fe–Cu(100) surface: (a) CH4→CH3+H; (b) CH3 22
→CH2+H; (c) CH2→CH+H; (d) CH→C+H.
Fig 3. The behavior of carbon atom on Fe–Cu(100) (a) C-migration, (b) Cpolymerization.
Fig 4. Potential energy curves of CH4 dissociation on Fe-Cu(100) and Cu(100) surfaces, respectively. 23
Fig 5. The coverage of CH3 (red), CH2 (green) and CH (black) at the different temperatures and H2/CH4 ratios.
24
Fig 6. The branching reaction rate for CH4 dissociation on surface Fe-Cu(100) (black), Ni-Cu(111) (red) and Cu(100) (blue) as a function of (a) temperature; and (b) H2/CH4.
25
Table 1. The Bader charge of Fe atom, Cu atom and CHx(x=3–0) on Fe-Cu(100) surface.
Bader Charge (|e|)
Fe
Cu
CH3
CH2
CH
C
-0.22
0.01
0.31
0.48
0.72
0.88
Table 2. The coverage of CHx(x=3–1) (/mol) at the different experimental temperatures (ºC) with fixed H2/CH4 ratio of 0.2 on Fe-Cu(100) surface. 0.2
1035
1050
1060
1080
CH3
1.38×1013
1.27×1013
1.22×1013
1.11×1013
CH2
1.78×1010
1.80×1010
1.81×1010
1.83×1010
CH
1.70×1011
1.65×1011
1.62×1011
1.56×1011
Table 3. The coverage of CHx(x=3–1) (/mol) at the different H2/CH4 ratios with experimental temperature of 1050 ºC on Fe-Cu(100) surface. 1050
0
0.2
0.5
1.0
2.0
5.0
CH3
3.66×1014
1.27×1013
2.79×1013
4.70×1013
7.30×1013
1.16×1014
CH2
5.12×1011
1.80×1010
3.94×1010
6.63×1010
1.03×1011
1.63×1011
CH
4.77×1012
1.65×1011
3.63×1011
6.11×1011
9.51×1011
1.51×1012
26
S0169-4332(17)32737-X http://dx.doi.org/10.1016/j.apsusc.2017.09.088 APSUSC 37167
To appear in:
APSUSC
Received date: Accepted date:
5-7-2017 11-9-2017
Please cite this article as: Baoyang Tian, Tianhui Liu, YanYan Yang, Kai Li, Zhijian Wu, Ying Wang, CH4 dissociation in the early stage of graphene growth on Fe–Cu(100) surface: theoretical insights, Applied Surface Sciencehttp://dx.doi.org/10.1016/j.apsusc.2017.09.088 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
CH4 dissociation in the early stage of graphene growth on Fe–Cu(100) surface: theoretical insights Baoyang Tian a,b, Tianhui Liu,c YanYan Yang*,a, Kai Li *,b, Zhijian Wu*,b, and Ying Wang,b a
College of Chemistry and Pharmaceutical Engineering, Jilin Institute of Chemical Technology, Jilin, Jilin 132022
b
State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China. c
Electronic Information Products Supervision Inspection Institute of Jilin Province NO. 1381 Xinmin Avenue, Changchun, Jilin Province.
Highlights
Propose a theoretical catalyst to grow high quantity and larger area graphene.
Microkinetic model explains the effects of temperatures and H2 partial pressures.
The synergistic effect between Fe and Cu plays an important role in the CH4 dissociation process.
Abstract The mechanism of CH4 dissociation and carbon nucleation process on the Fe doped Cu(100) surface were investigated systematically by using the density functional theory (DFT) calculations and microkinetic model. The activity of the Cu(100) surface was improved by the doped Fe atom and the atomic Fe on the FeCu(100) surface was the reaction center due to the synergistic effect. In the dissociation process of CH4, CH3→CH2+H was regarded as the rate-determining step. The results obtained from the microkinetic model showed that the coverage of CHx(x=1-3) was gradually decreased with the temperature increasing and CH3 was always the major intermediate at the broad range of the temperature (from 1035 to 1080 ºC) and the ratio of H2/CH4 (from 0 to 5). It is also found that the reaction rates
*Corresponding author. E-mail: [email protected](Y. Yang); [email protected] (K. Li); [email protected] (Z. Wu); [email protected] (Y. Wang) 1
were increased with the temperature increasing. However, the reaction rates were reduced (or increased) at the range of H2/CH4=0-0.2 (or H2/CH4>0.2). It is noted that controlling the H2 partial pressure was an effective method to regulate the major intermediates and reaction rates of CH4 dissociation and further influence the growing process of graphene. Keywords: density functional theory; CH4 dissociation; microkinetic model; chemical vapor deposition; graphene growth 1. Introduction Graphene is considered to be the most attractive materials in this century due to its excellent electrical properties and optical properties [1, 2]. Owing to the excellent performance of graphene with high specific surface area [3], thermal conductivity [4], carrier mobility [5-9], transparency [10, 11], holes [5-8] and electrons half-integer quantum Hall effect [10, 11], the graphene-based materials have been widely employed in diverse fields, biomaterials, nanoelectronic devices, intercalation materials, catalysis, as well as drug delivery [12, 13]. The controllable synthesis of graphene is a necessary prerequisite for its broad applications. Currently, chemical vapor deposition (CVD) is regarded as one of the most promising methods for producing monolayer graphene on a commercial scale. Graphene CVD growth generally involves four steps: (1) dissociation of hydrocarbon molecules; (2) diffusion of C atoms on the catalyst surface; (3) formation of the nucleation graphene islands; and (4) growth of the graphene islands [14-16]. The first dissociation process determines the following steps of graphene growth, and the decomposition products directly affect the growth mechanism. It is well known that the dissociation of hydrocarbon molecules can not be occurred spontaneously and the appropriate catalysts are necessary. In general, transition metals were chosen for CVD graphene synthesis, such as Ru [17], Pd [18, 19], Co [20], Rh [21], Re [22], Pt [23, 24], Ir [25, 26], Au [27, 28], Fe [29] and Ag [30]. Among them, the 3d transition metal catalysts attracted more attention, especially Fe and Cu [31], due to the high activity and low price. A number of studies have been conducted to investigate the 2
effect factors and growth mechanism of graphene on 3d transition metal catalysts. It is found that the temperature affected the thickness and quality of graphene [32-34]. The optimum temperature was about ~600 ºC for graphene growth on nickel surface. If the temperature was higher than it, carbon will diffuse into nickel bulk and limit the surface growth rate. Conversely, a competing surface carbide phase could hinder the graphene formation [33]. Besides temperature, H2 partial pressure can also affect the quality of the graphene. It is found that the hexagonal graphene could be formed at high H2/CH4 ratios, whereas, the graphene was changed to a branched structure at a low H2/CH4 ratio [35]. In addition, the growth of high quality graphene also depends on the species and morphology of the catalysts [36]. The 3d transition metal Fe and Cu exhibited the distinct interactions with carbon atoms [37]. Fe possessed a high affinity to the C atom due to the partially filled Fe 3d-orbitals. Meanwhile, C atom had a finite solubility in Fe (0.022% carbon by weight in α-Fe at 727 ºC and 2.14% carbon by weight in γ-Fe at 1147 ºC) and formed metastable iron carbide (Fe3C, 6.67% carbon by weight) [38, 39]. C solubility in Cu was lower than in Fe (~0.008% carbon in Cu by weight at 1085 ºC) and no carbide was observed [40, 41]. Therefore, the metal-catalyzed CVD growth of graphene between Fe and Cu with the different M-C (M=Fe or Cu) interactions resulted in the formation of few-layer graphene on Fe foil [42] and monolayer graphene on Cu foil [43]. Although Cu was a better candidate catalyst for monolayer graphene growth, the activity of Cu was lower than that of Fe, which hindered the growth rates of graphene. Therefore, it is necessary to exploit an effective method to improve the Cu activity and remain the graphene growth process unchanged. Design an effective Cu-based catalyst, such as Fe doped Cu surface or Fe-Cu alloy, may be a good choice to balance the activity and the layered-control growth. Therefore, in this study, we firstly investigated the dissociation mechanism of methane on Fe-doped Cu surface to estimate the effect of Fe doping on the first step of graphene growth. Secondly, we investigated the carbon migration and polymerization process, which is related to the graphene nucleation. Thirdly, the flux effects of H2 and CH4, as well as the 3
temperature effects were investigated by the microkinetic models. Our current results showed that the synergistic effect of Fe and Cu was very important to improve the activity of Cu surface and the Fe-Cu(100) surface could be deemed as a potentially catalyst to achieve the high quality and larger scale graphene growth. 2. Computational details 2.1.Method The geometric structures and the energies were calculated by using Vienna abinitio simulation package (VASP) [44-47]. The interaction between the valence electron and the ion cores was described by Blöchl's all-electron-like projector augmented wave (PAW) method [48, 49]. The Perdew-Burke-Ernzerhof (PBE) was used as the exchange correlation function [50]. The cut off kinetic energy was set to 300 eV. The wave functions were expanded with a plane wave basis set at each kpoints. Brillouin zone integration was approximated by a sum over special selected kpoints using the Monkhorst-Pack method [51] and they were set to 3×3×1. An energy smearing by the electron occupancies were determined according to Fermi Scheme of 0.1 eV. Geometries were optimized until the energy was converged to 1.0×10-6 eV/atom and the force was converged to 0.01 eV/Å. Spin polarization was considered in all calculations due to the presence of magnetic atoms Cu and Fe. The previous study [52] showed that CH4 decomposition was dominated by van der Waals (vdW) interaction. Therefore, in the current calculations, a semi-empirical decentralized corrected density functional theory (DFT-D2) force field [53, 54] method was used to describe the van der Waals (vdW) interaction between the reactants and the substrates. The structures of transition states (TSs) and the reaction pathways were obtained by using the climbing image micro-elastic band (CI-NEB) method [55]. To characterize the properties of all the stationary points and to obtain zero energy (ZPE) correction, harmonic vibration frequency calculations were carried out for all the transition states and the most stable configurations of CHx(x=0-4) on Fe-Cu(100) surface. 4
2.2. Model The previous experimental studies [56] found that the Cu(100) surface showed a remarkable performance in CVD graphene growth. Therefore, the (100) surface of Cu was chosen in our study. The (100) surface was obtained by cutting the optimized bulk face centered cubic Cu crystal along (100) direction and three layer slabs were selected. This model has been used in the previous calculations of CH4 dissociation on metal surface and was proved to be effective [57]. A (3×3) supercell was used to model the coverage of a 1/9 monolayer. Then, we replaced one of the surface Cu atoms by a Fe atom to simulate 1:8 Fe–Cu(100) surface. The atoms in the bottom were fixed in their bulk positions and the other two layers were allowed to relax. To avoid periodic interactions, the vacuum layer as large as 12 Å between repeated slabs was used along the z direction. The chemisorption energies of CHx(x=0–4) on Fe-Cu(100) surface, Eads, was defined as follows Eads =Eeadsorbatest/slabe-(Eslab + Eadsorbates)
(1)
Where Eadsorbates/slab is the total energy of adsorbate and the Fe-Cu(100) surface, Eslab is the total energy of the Fe-Cu(100) surface and Eadsorbates is the total energy of free adsorbates. The first two terms were calculated with the same parameters. The third term was estimated by setting the isolated adsorbate in a box of 12×12×12 Å. The negative Eads indicates an exothermic chemisorption, and the positive value suggests an endothermic chemisorption. 3. Results and discussion 3.1.Adsorption of CHx(x=0-4) on Fe-Cu(100) Five typical adsorption sites for CHx(x=0-4) on Fe-Cu(100) surface were investigated, i.e. Top site (TFe and TCu), Bridge site (BFe-Cu and BCu-Cu) and Hollow site, as shown in Fig.1. Followed structure optimization, the possible adsorption configurations of CHx(x=0-4) and the adsorption energies on the Fe-Cu(100) surface were shown in the Supporting information, Fig. S1. 5
3.1.1 Methane adsorption Pervious researches showed that methane adsorption was a physisorption process with small adsorption energy (-0.50 ~ -0.05 eV) on the pure metal surfaces [58-60]. In Fig. S1 a-c, we showed the structure of CH4 adsorbed on the Fe-Cu(100) surface and the adsorption energies. It is seen that CH4 on the Fe-Cu bridge site was the most stable structure. The distance between Fe and C atoms in CH4 was 3.10 Å and the adsorption energy was -0.46 eV. Compared with pure Cu(100) (-0.21 eV) [61], the adsorption energy was enhanced by 0.25 eV, indicating the doped Fe atom slightly strengthen the interaction between Cu(100) and CH4. 3.1.2 CH3 and CH2 adsorption on Fe-Cu(100) The optimized structures of CH3 and CH2 on the Fe-Cu(100), together with the adsorption energies, were shown in Fig.S1 d-j (CH3 and CH2 corresponding to d to g and h to j). It is seen that the CH3 can be stacked stably on the top site and bridge sites, respectively, due to the negative adsorption energies. Among the four optimized structures, the most stable adsorption site was Fe-Cu bridge site with the adsorption energy of -2.42 eV and the distance between Fe and C atom of CH3 was 2.04 Å, which is shorter than that of pure Cu(100) (2.15 Å) [61]. This suggested that the interaction between the Fe and C of CH3 was stronger than that of pure Cu(100). Thus, the doped Fe enhanced the adsorption of CH3 on Cu(100) (Eads=-2.42 and -1.83 eV for Fe-Cu(100) and Cu(100), respectively)[61]. For CH2, the most stable structure was CH2 adsorbed on a hollow site with the adsorption energy of -4.28 eV. It is seen that one H atom of CH2 bonds to Fe atom (Fe-H bond of 1.84 Å), which was favorite to the further dehydrogenation. For the least stable adsorption configuration of CH2 (Fig. S1 h), it is seen that the surface was deformed, which further reduced the adsorption energy of CH2. Compared with pure Cu(100) (-3.58 eV) [61], the adsorption energy was reduced by 0.70 eV. Therefore, the doped Fe improved the CH2 adsorption on Fe-Cu(100) surface. 3.1.3 CH and C adsorption on Fe-Cu(100) 6
For CH and C chemisorption on Fe-Cu(100), five CH and C possible adsorption sites were considered. After structure optimization, only the stable site of the nearest hollow site to Fe atom was found. The stable absorption structures and energies were shown in Fig. S1 (k for CH and l for C). For CH, the adsorption energies were -6.71 eV, which was lower than that of the pure Cu(100) surface (-5.88 eV) [61]. The distances of Fe–C was 1.89 Å, which was shortened compared to that on the pure Cu(100) (1.98 Å) [61]. This suggests that the doped Fe had stronger interaction with the C of CH than that of Cu, and this was consistent with the improved adsorption of CH on Fe-Cu(100). All the C adsorption structures on Fe-Cu(100) were converged to the most stable four fold site, i.e. C (Fig. S1l) at the nearest hollow site to the Fe atom and the adsorption energy was -7.61 eV. Similar to the configuration of CH adsorption, the doped Fe was more affinity to the adsorbed C atom (1.76 Å) than that of pure Cu(100) (1.91 Å) [61]. Furthermore, it is seen that the adsorption energies of CHx was increased as the number of the H atom in CHx reduced [52, 61-63]. This trend was consistent with the charge transition of CHx(x=0-3) on Fe-Cu(100) surface, as shown in Table 1. The more charge transferred to CHx, the stronger adsorption was obtained. 3.2.Methane dissociation on Fe-Cu(100) To investigate the mechanism of methane dissociation on the Fe-Cu(100) surface, the minimum energy paths of CHx → CHx-1+H were estimated. In these calculations, the most stable geometric structures of CHx were regarded as the ISs (initial states). And the most stable CHx-1+H co-adsorption configurations were set as the FSs (final states). The geometric structures of ISs, FSs and TSs (transition states) for CH4 dissociation were shown in Fig.2. 3.2.1 CH4→CH3+H The first step of CH4 dissociation, i.e. CH4→CH3+H, was shown in Fig. 2a. The most stable geometric structure of CH4 on Fe-Cu bridge site was set as the IS. CH3 at BFe-Cu with H at hollow site was set as the FS. In TS1, the CH3 was adsorbed at the 7
BFe-Cu and the breaking H atom was located at the nearest hollow site with the C-H of 1.61 Å. The reaction energy barrier was calculated as 0.64 eV with slightly endothermic of 0.07 eV. For Cu(100), Cu(111), Ni(111), Fe(111) and Fe(100) surfaces [61, 64-66], the reaction energy barrier of CH4→CH3+H was calculated to be 0.83, 1.31, 0.65, 1.02 and 0.71 eV, respectively. And the reaction energies were 0.58, -0.63 and -0.13 eV for the Cu(100), Fe(111) and Fe(100), respectively [61, 64, 65]. Therefore, the dissociation process of CH4→CH3+H was improved on the Fe-Cu(100) both kinetically and thermo-dynamically. 3.2.2 CH3→CH2+H For CH3 →CH2 +H, the most stable geometric structure of CH3 on Fe-Cu bridge site was set as the IS, and CH2 with H at the hollow site was set as the FS. For TS2 in Fig. 2b, the CH2 was adsorbed at the Fe-Cu bridge site and the breaking H was located at the nearest Fe-Cu bridge site with the C-H of 1.59 Å. The reaction energy barrier was calculated to be 1.00 eV with endothermic of 0.46 eV. For Cu(100), Cu(111), Ni(111), Fe(111) and Fe(100) surface [61, 64-66], CH3→CH2+H, the reaction energy barrier was calculated to be 0.98, 1.31, 0.65, 1.02 and 0.71 eV. The reaction energies on Cu(100), Fe(111) and Fe(100) were 0.71, -0.32 and 0.37 eV, respectively [61, 64, 65]. It indicated that CH3 dehydrogenation reaction was improved on Fe-Cu(100) compared to pure Cu(100). However, its reaction activity was still lower than those on Fe(111), Ni(111) and Fe(100) [64-66]. 3.2.3 CH2→CH+H The dissociation process of CH2→CH+H, was shown in Fig. 2c. The most stable geometric structure of CH2 on hollow site was set as the IS. The CH and H adsorbed at hollow site was set as the FS. For TS3, (Fig. 2a), the CH3 was adsorbed at the hollow site and the breaking H atom was located at the nearest hollow site with the CH of 1.58 Å. The reaction energy barrier was calculated as 0.47 eV with exothermic of -0.20 eV. For Cu(100), Cu(111), Ni(111), Fe(111) and Fe(100) surface [61, 64-66], the barriers of CH2→CH+H were 0.63, 0.93, 0.31, 0.13 and 0.75 eV. The reaction 8
energies of Cu(100), Fe(111) and Fe(100) were 0.05, -0.69 and 0.64 eV [61, 64, 65]. Compared with pure Cu(100) [61], it is seen that CH2 dehydrogenation reaction became the only exothermic process in the whole CH4 dissociation process on the FeCu(100). Meanwhile, the C–H dissociation barrier (0.47 eV) was very low and comparable with those of Ni(111) (0.31 eV) and Fe(111) (0.13 eV) [63, 64]. This promises that CH can be formed automatically once CH2 is produced. 3.2.4 CH→C+H CH→C+H, was shown in Fig. 2d. CH adsorbed at hollow site was set as IS. C at hollow site with H at BFe-Cu was set as FS. Regarding to TS4, as shown in Fig.2(d), atomic C was adsorbed at the hollow site and the breaking H atom was located at the nearest Fe top site (TFe) with C-H of 1.86 Å. The calculated reaction energy barrier was 0.74 eV with an endothermic of 0.13 eV. For Cu(100), Cu(111), Fe(111) and Fe(100) surfaces [61, 64-66], the reaction energy barriers of CH→C+H were 1.39, 1.97, 1.04 and 0.76 eV, respectively. The reaction energies were 0.74, 0.04 and 0.41 eV for Cu(100), Fe(111) and Fe(100), respectively [61, 64, 65]. It is clearly seen that the doped Fe enhanced the CH dissociation process owing to the lower reaction barrier and less endothermic compared to the pure Cu and Fe. 3.3.The effects of the doped Fe on Cu(100) surface As discussed above, for the Fe-Cu(100), Fe showed a higher catalytic activity than Cu and the major catalytic center was observed as the doped Fe atom. In order to illustrate the catalytic activity of the doped Fe on Cu(100) surface, the CH4 dissociation potential energy on Fe-Cu(100) was shown in Fig. 4. For comparison, the results of the pure Cu(100) surface were also displayed in the same figure [61]. For pure Cu(100), it is seen that all four dehydrogenation steps were endothermic with the higher reaction barriers. Therefore, the continuous CH4 dissociation process on the Cu(100) surface was unfavorable both kinetically and thermodynamically. The rate-determining step was CH→C+H on Cu(100) surface [61]. On the contrary, for the Fe-Cu(100) surface, CH2→CH+H was found to be an exothermic process and the rate-determining step was CH3→CH2+H. Compared with Cu(100) surface [61], the reaction barriers of the most 9
elementary were reduced significantly (such as, CH4→CH3+H, by 0.19 eV; CH2→CH+H, by 016 eV; CH→C+H, by 0.65 eV). However, for the reaction CH3→CH2+H, the reaction barrier was increased slightly by ~0.02 eV. Moreover, the total barrier of CH4 dissociation was reduced from 3.83 to 2.85 eV after the introduction of the Fe atom. Therefore, the synergistic effect between Fe and Cu improved the catalytic activity for CH4 dissociation. To further explain the synergistic effect, the electronic structure of the FeCu(100) had been investigated. It is seen that the charge of Cu(100) was redistributed by the doped Fe atom. On the Fe-Cu(100), ~0.22 e was transferred from Fe to Cu atoms, suggesting that the Fe-Cu bond was polarized. Since the radicals of CHx(x=1-3) have redundant electrons, they preferred to be adsorbed at the Fe atom. In order to explain the high activity of Fe-Cu(100), the d-band center was examined [67-69]. It is found that the average d-band centers of Fe was -1.87 eV, which was closer to Fermi level than that of surface Cu(100) (-2.41 eV) [61], suggesting that Fe was more active than that of Cu. On the other hand, it is seen that the average d-band center of Cu (-2.41 eV) in Fe-Cu(100) was far away from Fermi level compared with that of pure Cu(100) (2.30 eV) [61]. This result indicated that the electron transferred from Fe led to the shift of d-orbital of Cu atoms. 3.4.The migration and polymerization of carbon atoms on Fe-Cu(100) For the growth of graphene, except for the dissociation of hydrocarbon molecules, the migration and polymerization of carbon atoms on the catalytic metal surface were also the important reactions in the graphene nucleation process. Therefore, we also investigated the behavior of C atoms in this study. The possible migration path of C atom on Fe-Cu(100) surface were shown in Fig. 3a. C atoms moved around the surface of Cu atoms, i.e. C atoms moving between the two hollow sites in the counter clock wise direction (Fig. 3a). The reaction energy barrier was 1.42 eV and endothermic was 0.30 eV, indicated that C atoms was migrate difficultly on the Fe-Cu(100) surface.
10
The possible polymerization of two carbon atoms on Fe-Cu(100) surface was shown in Fig. 3b. The reaction energy barrier was calculated as 0.85 eV and exothermic of -0.24 eV, which suggested that the carbon polymerization was occurred more easily due to the low reaction barrier and the exothermic process. Therefore, the doped Fe atom was not only as an activity center for CH4 dissociation but also as a carbon polymerization center for graphene nucleation. Furthermore, compared with the pure Cu(100) surface, the migration barrier on Fe-Cu(100) surface was decreased by 0.43 eV (Ea = 1.85 eV on Cu(100)) and the polymerization barrier was increased by 0.10 eV (Ea = 0.75 eV on Cu(100)). Therefore, the introduction of Fe accelerated the migration of the C and hindered the nucleation process kinetically compared to the reactions on Cu(100) surface. This promised that the formed atomic carbon having sufficient time to migrate to the most stable position or to the edges of the formed graphene islands during the nucleation and growth processes. Ultimately, it led to a formation of a perfect graphene pattern. We proposed that Fe-Cu(100) surface is a potentially catalyst to achieve the high quality and larger scale graphene growth. 3.5 Microkinetic model As mentioned before, the growth process of graphene was affected by the factors of temperature and H2 pressure. Therefore, in this study the microkinetic model was also performed to explain the effect of above two factors. Based on the DFT calculated results and microkinetic model [67, 68] (Supporting information for a brief description of the model), we calculated the possible coverage of each intermediate (CHx) at different experimental temperatures and H2 partial pressures, as shown in Table 2, Table 3, Fig. 5, and Table. S1, respectively. To simulate the experimental conditions, the temperature was ranged from 1035 to 1080 ºC, and H2 partial pressure was varied from 0 to 0.12 Torr. It is noted that in the microkinetic calculations the total pressure of CH4 and H2 was fixed at a constant of 0.12 Torr according to the experimental conditions [70]. Furthermore, free C atom was more active than the graphene, therefore, we assumed that the formed C by dehydrogenation process could 11
combine immediately to form the graphene islands. In other words, the coverage of free C atom was set as zero. At 1035 ºC and H2/CH4=0.2, the coverage of CH3 was ~134.2 and 8.0 times higher than that of CH2 and CH on Fe-Cu(100) surface (Table 2). For 0
12
high quality growth of graphene through adjusting H2 partial pressure, consistent to the experimental observations. 4. Conclusions The mechanism of CH4 dissociation and carbon nucleation process on FeCu(100) surface were investigated by using DFT calculation and microkinetic model. The synergistic effect between Fe and Cu improved the catalytic activity on CH4 dissociation due to the polarization and the shift d-band center. The CH4 dissociation can be improved on Cu(100) once Fe atom was introduced. For CHx→CHx-1+H (x=14), the doped Fe strengthened the adsorption of CHx, meanwhile, all the elementary reactions was improved both kinetically and thermodynamically, except for CH3→CH2+H. For CH4 dissociation, CH3→CH2+H was the rate-determining step on the Fe-Cu(100). On the other hand, Fe atom was also regarded as the C polymerization center for graphene nucleation. Based on the microkinetic model, we found that both the temperature and H2/CH4 have an influence on the reaction rates. The reaction rate was kept increasing with the temperature increasing from 900 to 1100 ºC. On the other hand, the H2 partial pressure reduced (or increased) the reaction rate at the range of H2/CH4=0-0.2 (or H2/CH4>0.2). The major intermediate was observed as CH3 species and the maximum coverage of CH3 can be obtained at 1035 ºC and H2/CH4=0. In addition, it is also found that the doped Fe reduced the temperature sensitivity of CH4 dissociation on Fe-Cu(100) and. Over all, doping Fe in the Cu(100) can not only improve the CH4 dissociation but also the polymerization of C atom, which were beneficial for improving the growth rate of graphene. Moreover, the appropriate H2 partial pressure can control the CH4 dissociation rate on FeCu(100) to further control the growth of graphene. Acknowledgements This work is supported by the National Natural Science Foundation of China (21503210, 21521092, 21673220), Jilin Province Natural Science Foundation (20150101012JC), Special Program for Applied Research on Super Computation of the NSFC-Guangdong Joint Fund (the second phase), Educational Commission of 13
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20
Fig 1. Top view of the possible adsorption sites on the Fe-Cu (100) surface.
21
Fig 2. The structures of stationary points (including IS, TS, and FS), imaginary frequency and C-H bond length in each TS (Imag(TS) and C-H(TS), the units are cm1
and Angstrom), as well as the reaction barrier (Ea) and reaction energy (H) for
four CH4 dissociation pathways on Fe–Cu(100) surface: (a) CH4→CH3+H; (b) CH3 22
→CH2+H; (c) CH2→CH+H; (d) CH→C+H.
Fig 3. The behavior of carbon atom on Fe–Cu(100) (a) C-migration, (b) Cpolymerization.
Fig 4. Potential energy curves of CH4 dissociation on Fe-Cu(100) and Cu(100) surfaces, respectively. 23
Fig 5. The coverage of CH3 (red), CH2 (green) and CH (black) at the different temperatures and H2/CH4 ratios.
24
Fig 6. The branching reaction rate for CH4 dissociation on surface Fe-Cu(100) (black), Ni-Cu(111) (red) and Cu(100) (blue) as a function of (a) temperature; and (b) H2/CH4.
25
Table 1. The Bader charge of Fe atom, Cu atom and CHx(x=3–0) on Fe-Cu(100) surface.
Bader Charge (|e|)
Fe
Cu
CH3
CH2
CH
C
-0.22
0.01
0.31
0.48
0.72
0.88
Table 2. The coverage of CHx(x=3–1) (/mol) at the different experimental temperatures (ºC) with fixed H2/CH4 ratio of 0.2 on Fe-Cu(100) surface. 0.2
1035
1050
1060
1080
CH3
1.38×1013
1.27×1013
1.22×1013
1.11×1013
CH2
1.78×1010
1.80×1010
1.81×1010
1.83×1010
CH
1.70×1011
1.65×1011
1.62×1011
1.56×1011
Table 3. The coverage of CHx(x=3–1) (/mol) at the different H2/CH4 ratios with experimental temperature of 1050 ºC on Fe-Cu(100) surface. 1050
0
0.2
0.5
1.0
2.0
5.0
CH3
3.66×1014
1.27×1013
2.79×1013
4.70×1013
7.30×1013
1.16×1014
CH2
5.12×1011
1.80×1010
3.94×1010
6.63×1010
1.03×1011
1.63×1011
CH
4.77×1012
1.65×1011
3.63×1011
6.11×1011
9.51×1011
1.51×1012
26