Iron Clusters Embedded in Graphene Nanocavities: Heat-Induced

Subscriber access provided by YORK UNIV

Article

Iron Clusters Embedded in Graphene Nanocavities: HeatInduced Structural Evolution and Catalytic C-C Bond Breaking Shuang Chen, Jie Bie, Wei Fa, Yucheng Zha, Yi Gao, and Xiao Cheng Zeng ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b02104 • Publication Date (Web): 21 Dec 2018 Downloaded from http://pubs.acs.org on December 25, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Nano Materials

Iron Clusters Embedded in Graphene Nanocavities: HeatInduced Structural Evolution and Catalytic C−C Bond Breaking Shuang Chen,*,† Jie Bie, †, ‡ Wei Fa, ‡ Yucheng Zha,† Yi Gao, § and Xiao Cheng Zeng*,⊥,# †

Kuang Yaming Honors School, Nanjing University, Nanjing, Jiangsu 210023, China

‡

National Laboratory of Solid State Microstructures and Department of Physics, Nanjing

University, Nanjing, Jiangsu 210093, China. §

Division of Interfacial Water and Key Laboratory of Interfacial Physics and Technology,

Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai 201800, China ⊥

Department of Chemistry, University of Nebraska–Lincoln, Lincoln, Nebraska 68588, United

States #

Department of Chemical & Biomolecular Engineering and Department of Mechanical &

Materials Engineering, University of Nebraska–Lincoln, Lincoln, Nebraska 68588, United States KEYWORDS: ultrafine Fe clusters, graphene edges, structural evolution, catalytic behavior, reaction mechanism, ab initial molecular dynamics simulations

ABSTRACT: Metal nanoclusters can be anchored at defective sites of graphene sheets to strengthen their thermal stability for potential device applications. A previous transmission electron microscopy (TEM) experimental study on the morphology change of an ultrafine iron

ACS Paragon Plus Environment

1

ACS Applied Nano Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 31

cluster embedded in a graphene nanocavity suggests that the underlying reaction mechanism is likely due to solid-solid transformation (Sci. Rep. 2012, 2, 995). The morphology change of Fe cluster may also assist the enlargement of graphene nanocavity. This TEM experiment reminds us that if the anchoring Fe nanocluster within the graphene nanocavity can efficiently catalyze graphene etching at certain operation temperature, the device application of graphene-metal nanocluster composite would be largely limited. As such, we have performed ab initio molecular dynamics (AIMD) simulations of a triangular hexagonal close-packed (HCP) Fe53 cluster in contact with either the edge of graphene nanocavity or graphene nanoribbon to investigate its structural evolution and catalytic behaviour at an elevated temperature (1173 K). Contrary to the previous TEM experiment, we suggest an alternative reaction mechanism, namely, the melting recrystallization for the structural transformation of Fe cluster. Moreover, we find that the molten iron cluster can etch and enlarge the graphene nanocavity. At the high temperature of 1173 K without H and O atoms, the Fe53 cluster undergoes a phase transition from the HCP structure to a liquid-like nanodroplet while in contact with the edge of either graphene nanocavity or graphene nanoribbon. Interestingly, the Fe53 cluster tends to saturate the graphene edges via forming Fe-C bonds but without breaking any C-C bonds. Meanwhile, the molten Fe53 cluster exhibits catalytic activity towards C-C bond dissociation at the graphene edge. Our reactive MD simulations show that the HCP Fe53 cluster can complete the reaction of carbide formation within 10 ps. Another independent climbing-image nudged elastic band calculations offer additional insight into Fecatalyzed reaction mechanism of C-C bond dissociation, C-C bond displacement or C-C bond rotation during graphene etching. We find that the Fe cluster can only efficiently catalyze the CC dissociation at the armchair edge, following the C-C displacement mechanism, due to the formation of strong bonds between Fe and dangling C atoms. We also find that the catalytic


2

Page 3 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


ability of Fe atoms seems less effective compared to that of Ni atoms, in part because Fe clusters tend to change their shapes during the reaction. Lastly, we perform AIMD simulations of the Fe53 cluster in contact with smaller-sized sp2-C flakes. We observe that the cluster can soak the carbon flake on its surface, followed by breaking the C-C bonds through C-C displacement or CC rotation. It appears that the catalytic ability of Fe53 cluster depends on size of carbon species being in contact with.

1. INTRODUCTION Due to its excellent electronic, thermal, and mechanical properties, coupled with its high chemical stability, graphene holds great potential in electronic and sensor applications. It can be also utilized in a variety of experimental settings, for example, as a coating layer in atomic force microscopy (AFM) characterization of structures of highly diffusive water1 or volatile organic molecules2, or as a support in transmission electron microscopy (TEM) characterization of structural evolution of ultrafine Fe clusters.3 In addition, when decorated with transition-metal nanoclusters or nanoparticles, the hybrid graphene-metal nanocluster (nanoparticle) composites may entail unconventional optical, electronic, magnetic, and mechanical properties for various applications, such as catalysis,4-5 gas sensors,6 molecular valves for gas separation,7 and nanoelectronics.8 Many research efforts have been devoted to enhance the performance of the graphene-transition-metal nanocomposites by controlling the growth of transition metals on pristine graphene, including changing the metal components,8-9 controlling the size of metal nanoclusters and nanoparticles,10 and realizing different microstructures of metals (i.e.


3


Page 4 of 31

superlattices),11 and even by creating defect sites to anchor metal clusters and particles on/in graphene.5-6, 8, 12 For many applications, the hybrid metal-graphene nanocomposites should keep subtle metalcarbon interaction to let the metal bond to the graphene strong enough to warrant the stability of metal nanoclusters or nanoparticles at the operation temperature but still weak enough not to destroy them as entities in themselves.11 Liu et al. systematically illustrated the growth morphology and thermal stability of various metals, including alkali metals, sp-simple metals, 3d and group-10 transition metals, noble metals, as well as rare-earth metals, on pristine graphene by theoretically analyzing subtle interactions within the metal-graphene nanocomposites.13-14 In addition, graphene should be inert enough to not deteriorate under the condition of use but active enough to bestow new functionalities to the metal nanoclusters or nanoparticles without destroying them.11 The graphene is generally a suitable support for transition metal nanoclusters and nanoparticles. However, the metal nanoclusters or nanoparticles may undergo migration, agglomeration, and sintering on the pristine graphene surface as the operation temperature increases. How to solve this problem becomes a significant topic of the device application of hybrid metal-graphene nanocomposites. One way to resolve the problem is to create nanocavities or nanopores within graphene basal plane to anchor metal nanoclusters and nanoparticles.5 Although thermal stability of pristine graphene is normally high enough without metal-catalyzed etching for device operation, the defective graphene (with nanocavities or nanopores) may be not so stable when in contacting with metal nanoclusters or nanoparticles. Indeed, Wang et al. found that the ultrafine Fe clusters embedded in graphene nanocavity tend to take regular planar shapes with morphology change by local atomic shuffling, a feature suggested in the hypothesis of solid-solid transformation in high-vacuum environment and under electron irradiation, which


4

Page 5 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


leads to preferential graphene carving.3 More interestingly, Zhao et al. observed that when under the electron irradiation in the TEM experiment, pure Fe can be found as nanocrystals forming on the surface of the graphene, but at the edge of graphene nanocavity, pure Fe tends to be as either isolated atoms or small clusters, or as two-dimensional (2D) crystalline membranes suspended across the perforations within the graphene.15 These fascinating phenomena of Fe atoms in graphene nanocavities associated with the formation of various iron-graphene hybrid structures and possible iron-assisted graphene etching due to subtle Fe-Fe, Fe-C, and C-C interactions require better understanding at the atomic level. Without H and O atoms and at an operation temperature, can Fe nanocluster catalyze graphene etching while gaining thermal energies? A complete understand would guide the design of metal-cluster-anchoring defective graphene systems for future device application. In previous TEM experiments, ultrafine Fe clusters appear to show some catalytic ability for graphene etching under electron irradiation in the high vacuum,3 which is somewhat unexpected. In general, if graphene can be efficiently etched by Fe nanoclusters, the hybrid Fe-C systems are expected to be immersed in H or O atmosphere at high temperature.16-19 In a recent TEM experiment without H and O, it is reported that a single Fe atom can diffuse along the freshly produced graphene edge and take catalytic action by removing and adding carbon atom at the graphene edge while under electron irradiation.20 This experiment motivates us to speculate that there may be a specific catalytic mechanism for Fe-catalyzed graphene etching due to certain synergistic effect between ultrafine Fe cluster and graphene nanocavity. We note that two mechanisms, namely, the solid-solid transformation and the melting recrystallization, are still under debate for explanation of the morphology change. It is possibly that the easy morphology change of ultrafine Fe cluster contributes to its catalytic activity for C-C bond breaking at the


5


Page 6 of 31

graphene edge. It will be insightful to see how the ultrafine Fe cluster changes its structure and how it can catalyze the graphene etching, and what the associated mechanism is in the absence of H and O atoms at an operation temperature. In this study, we use an experimentally characterized triangular Fe53 cluster with 3-layer hexagonal close-packed (HCP) structure3 as a model metal cluster to explore structural evolution of ultrafine Fe cluster embedded in graphene nanocavities and to investigate possible catalytic reactions between the Fe cluster and graphene edges, by using the ab initio molecular dynamics (AIMD) simulations and reactive molecular dynamics (MD) simulations. The implementation of AIMD or reactive MD simulations (without including H and O atoms) is to mimic the heating process associated with possible device operation condition, and to gain atomic insights into catalytic effect of the Fe cluster on the graphene edge. Furthermore, the climbing-image nudged elastic band (CI-NEB) calculations are employed to understand the associated reaction mechanism. Upon thermal activation, the model Fe53 cluster (in Figure 1) would become molten to reshape itself and Fe atoms may diffuse along the graphene edge when contacting with the graphene edge. With gaining enough thermal energy, the Fe cluster could catalyze the C-C dissociation at graphene edge through C-C displacement or C-C rotation to trigger further graphene etching. If so, the device application of metal-graphene nanocomposites would be limited by the operation condition.


6

Page 7 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Molten Fe Cluster Regular Shape

C-C Displacement Irregular Shape

C-C Rotation

Contacting with Graphene Edge

Reshaping Itself and Diffusing along Graphene Edge

Catalyzing C-C Dissociation of Graphene Edge

Figure 1. Sketch of structure evolution from regular shape to irregular shape and catalytic behaviour of ultrafine Fe cluster when it contacts with graphene edge through melting recrystallization by thermal annealing. For Fe-catalyzed C-C dissociation, two mechanisms, C-C displacement and C-C rotation, would trigger further graphene etching. Etching-related displacing and rotating C atoms are highlighted insets. 2. COMPUTATIONAL DETAILS 2.1 AIMD Simulations for Structural Evolution and Catalytic Behaviour of HCP Fe53 Cluster In order to simulate dynamics of the structural evolution and catalytic effect of the Fe53 cluster on C-C dissociation of graphene edge within graphene nanocavity in typical device operation condition (thermal activation plus high vacuum), the AIMD simulations are performed within the framework of the Kohn−Sham formulation of density functional theory (DFT) by using the Gaussian plane wave (GPW) method implemented in the QUICKSTEP program of the CP2K software package. The PBE-D3 functional is employed with the cut-off radius of 20 Å for all dispersion calculations. A polarized double-ξ quality Gaussian basis in conjunction with the norm-conserving Goedecker−Teter−Hutter pseudopotential is adopted. The auxiliary plane-wave


7


Page 8 of 31

basis set is defined by an energy cut-off of 330 Ry, accompanied by a relative cut-off of 33 Ry for Gaussian basis set collocation. The self-consistent-field (SCF) convergence is set to 10-6 a.u. In consideration of computational cost, the Brillouin zones for the supercells mentioned below are only sampled at the Γ point for our AIMD simulations. The time step in AIMD simulations is set as 1 fs. Note that a general rule of thumb for choosing the time step is that it is less than one tenth of the fastest molecular motions, e.g., the vibrational period of studied systems. Our system includes ultrafine Fe53 cluster and graphene, both are quite rigid even at high operation temperature considered in this study. Hence, the 1-fs time step for our AIMD simulations should be small enough for studying dynamic characteristics of the hybrid Fe-graphene nanocomposites. Here, the temperature is controlled by a chain of Nosé-Hoover chain thermostat. The spinpolarized computations are applied for the hybrid Fe-C system. The 3-layer Fe53 cluster, whose structure is taken from the TEM experiment,3 is placed in contact with either armchair (AC) edge or zigzag (ZZ) edge of a graphene nanocavity (GNCs), graphene nanoribbons (GNRs), or various small graphene flakes in Figures 2 and 3. The supercells with Fe53 cluster and GNC (including 240 C atoms) in Figures 2a and 2b have a volume of 2.98 nm × 2.95 nm × 3 nm. The GNR supercells have the same volume compared to the GNC supercells. Differently, the GNR supercells with armchair edge in Figure 2c or zigzag edge in Figure 2d own 112 or 96 graphene C atoms respectively. Especially, the models for small sp2-C flakes (Figure 3a) are built to dynamically observe the Fe-catalyzed C-C dissociation processes and further confirm the catalytic event in our AIMD simulations. Each supercell is set with the same volume (2.98 nm × 2.95 nm × 3 nm) as those of Fe53-GNC or Fe53-GNR models in Figure 2. The 5-ps or 10-ps AIMD simulations in constant-volume and constant-temperature (NVT) ensemble are performed at the temperature of 1173 K (900 °C) or 1800 K. As a


8

Page 9 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


benchmark, a 48-atom HCP Fe bulk is built, and the 10-ps AIMD simulation is performed at 1173 K to compare with results of the HCP Fe53 cluster, including the mean square displacement (MSD)-time curve, and Lindemann index (see Figure S1 of the Supporting Information) to estimate the phase behaviour of Fe cluster. 2.2 The First-Principles Calculations For the first-principles calculations, the PBE-D3 functional is employed, as implemented in the Vienna ab initio simulation package (VASP) 5.3.5. The electron-ion interaction is described by the projector augmented wave (PAW) potentials with an energy cut-off of 500 eV. The Methfessel-Paxton smearing with the order of 1 and the width of 0.02 eV are used for these calculations. For geometry optimization, the total energy change is set to be less than 10-5 eV and the magnitude of the largest force acting on the atoms is set to be less than 0.05 eV/Å for the CINEB calculations and less than 0.02 eV/Å for pure Fe53 cluster optimization, respectively. After geometry optimization, the more accurate spin-polarized single-point energy calculations are performed with the convergence criterion of the SCF computation being set to 10-6 eV. Moreover, the on-site Coulomb (U) and Exchange (J) interaction parameters for treating localized d electrons of Fe atoms are set to U = 4.0 eV and J = 1.0 eV, following the previous computational work.21-22 The previous first-principles study on the electronic and magnetic properties of substitutional Fe in graphene has shown that the magnetism of the Fe adatom on the defected graphene can be well accounted for when an appropriate value of U (> 2.4 eV) is used within the GGA + U framework.23

Previously, Liu et al. made a comparison between different computational methods to calculate the adsorption energies, diffusion barriers of various metal adatoms on graphene, and cohesive energies of the metals to understand the growth morphology and thermal stability of


9


Page 10 of 31

these metals on perfect graphene.13-14 They suggested the PBE/PAW method can give reasonable results for metal-graphene nanocomposites.13-14 Srivastava at al. also found that many physical properties, e.g., bond lengths, binding energies, magnetic moments, and charge transfer of hybrid Fe-C systems can be well described by the PBE functional, and the long-range vdW interactions may not be needed for describing these properties.24 In addition, their computational results agreed well with those based on other pseudopotentials with only small quantitative differences.24 Compared with the previous computational studies of Fe adatoms with pristine graphene, the spin-polarized PBE-D3/PAW calculations with suitable on-site Coulomb (U) and Exchange (J) interaction parameters are reasonable for studying the Fe cluster-contactinggraphene-nanocavity system. Reaction Pathways of C-C Dissociation of Graphene Edges. Two new slab models of graphene nanoribbons with armchair or zigzag edges are built to seek possible reaction pathways for C-C displacement and C-C rotation at the edges in Figure 4. In order to further demonstrate the catalytic effect of Fe clusters on C-C dissociation of these edges, more slab models (also in Figure 4) of graphene nanoribbons in contact with a smaller HCP Fe15 cluster, carved out from Fe bulk, are designed to investigate possible reaction pathways for comparison. The GNR model with the armchair edge (30 C atoms) in Figures 4a and 4b has a volume of 1.28 nm × 1.5 nm × 2 nm, and the one with zigzag edge (45 C atoms) in Figure 4c has a volume of 1.23 nm × 2.2 nm × 2 nm. The CI-NEB calculations are performed to seek any possible reaction pathway connecting the initial state (IS) as reactant to the final state (FS) as product via the transition state (TS). Generally, six images are used to conduct the CI-NEB calculations between two neighboring local minima along each reaction pathway. For the CI-NEB calculations, the sampling is at the Γ point to lower the computational cost. Then, the corresponding single-point energy of each


10

Page 11 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


located stationary point along the reaction pathway is calculated at the denser 5 × 1 × 1 k-point mesh within the Monkhorst−Pack scheme for F15 cluster-graphene nanoribbon composites. Geometry Optimization of Various Fe53 Clusters. To understand the easy structural change between different Fe53 clusters, several irregular Fe53 clusters (IC) taken from the final configuration of each AIMD simulation (see Figures 2 and 3) are further optimized. The regular Fe53 clusters directly carved out from the bulk crystals are also optimized. In sum, these regular Fe53 clusters include: 3-layer triangular HCP structure (RC1), equiaxial body-centered cubic (BCC) structure (RC2), triangular BCC structure (RC3), equiaxial face-centered cubic (FCC) structure (RC4), triangular FCC structure (RC5), and hexahedral HCP structure (RC6). All the Fe53 clusters mentioned above are finally summarized in Figure 5. With reference to the previous DFT calculations on magnetism of Fe clusters,25 we carefully set specific initial magnetic moment for each Fe atom in our spin-polarized calculations and found that almost all Fe 53 clusters are ferrimagnetic, except RC1 structure which is ferromagnetic. The supercells for Fe53 clusters in our

calculations have a volume of 3 nm × 3 nm × 3 nm. Surely, the Brillouin zone for each supercell is sampled at the Γ point here. 2.3 Reactive MD Simulations in Comparison with AIMD Simulations Because the simulation time in our AIMD simulations is too short, we did not observe Fecatalyzed C-C dissociation at edge of graphene nanocavities (see below in Figure 2). Thus, we performed the reactive MD simulations implemented in LAMMPS software to examine possible C-C dissociation at the edge of graphene nanocavities. The reactive force field (ReaxFF) parameters for Fe and C atoms, developed by Aryanpour et al.26 are adopted. To confirm these ReaxFF parameters in describing interatomic interactions of hybrid Fe-graphene nanocomposites, we compared the computed formation energies of Fe∙∙∙Fe, Fe∙∙∙C and C∙∙∙C, respectively, as a


11


Page 12 of 31

function of interatomic distance to those computed from either the spin-polarized calculations with the PBE-D3 functional and PAW method implemented in VASP software or the spinpolarized calculations with the PBE-D3 functional and GPW method implemented in CP2K software (see Figure S2). The agreement in bond lengths is quite good among different computational methods: the bond lengths of Fe∙∙∙Fe, Fe∙∙∙C, and C∙∙∙C are about 2.1 Å, 1.6 Å, and 1.3 Å, respectively. Learned from the formation energy curves for three interatomic interactions, when the interatomic distances are greater than their corresponding bond length, the ReaxFF formation energy curves exhibit very similar tendency as curves from DFT computation, particularly for the C∙∙∙C formation energy curve. However, the Fe∙∙∙Fe interaction values from ReaxFF are about 0.8 eV higher than those from DFT calculations, while the Fe∙∙∙C interaction values from ReaxFF are about 1.1 eV lower than those from DFT computations. Thus, the ReaxFF method would underestimate Fe∙∙∙Fe interaction and overestimate Fe∙∙∙C interaction. When the interatomic distances are less than the corresponding bond length, the ReaxFF generally yield lower formation energies. More specifically, the reactive MD simulations are performed in the canonical (NVT) ensemble with the temperature set to 1173 K. Compared to the AIMD simulations, a larger slab model with a volume of 6.82 nm × 6.89 nm × 5 nm is adopted in Figure 6. This 1500-atom graphene model has a square nanocavity with two armchair edges and two zigzag edges. The HCP Fe53 cluster is placed next to the graphene nanocavity, in contact with either one armchair edge or one zigzag edge. The 100-ps reaction MD simulation is performed for each model. The equations of motion are integrated by using the Verlet method with a time step of 0.1 fs. Dynamic trajectories are recorded every 1 fs for statistical analysis.


12

Page 13 of 31

Final 10

Mean Square Displacement Linear Fitting

8 6

Self-diffusion coefficients D = 2.72×10-5 cm2/s

4 2 0

0

1

2

3

4

0.12

Lindemann Index

2

Mean Square Displacement (Å )

Initial

0.09 0.06 0.03 0.00

5

0

2

4

6

8

10

8

10

4

5

4

5

Time (ps)

Time (ps)

10


8 6

D = 2.88×10-5 cm2/s

4 2 0

0

1

2

3

4

0.12

Lindemann Index

2


(a) Fe53@GNC_AC 0.09 0.06 0.03 0.00

5

0

2

4

6

Time (ps)

Time (ps)

6


4 2 0 0.0

D = 3.26×10-5 cm2/s 0.5

1.0

1.5

2.0

0.12

Lindemann Index

2


(b) Fe53@GNC_ZZ 0.09 0.06 0.03 0.00

2.5

0

1

2

3

Time (ps)

Time (ps)

(c) Fe53@GNR_AC 6


4 2 0 0.0

D= 0.5

2.45×10-5 1.0

1.5

2.0

cm2/s 2.5

Time (ps)

0.12

Lindemann Index

2


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.09 0.06 0.03 0.00

0

1

2

3

Time (ps)

(d) Fe53@GNR_ZZ

Figure 2. Initial and final (after 10-ps AIMD simulations for graphene nanocavity and after 5-ps AIMD simulations for graphene nanoribbon) structures of Fe53 cluster in contact with (a) armchair (AC) edge and (b) zigzag (ZZ) edge of the GNCs; or in contact with (c) AC edge and (d) ZZ edge of the GNRs. The temperature in AIMD simulations is controlled at 1800 K for GNCs and 1173 K for the GNRs. The corresponding MSD-time curves (black solid line) with linear fitting (red solid line) of the diffusion coefficients and the obtained curves of Lindemann indexes (blue solid line) are also present to characterize the phase behaviour of Fe53 cluster. 3. RESULTS AND DISCUSSION


13


Page 14 of 31

In our AIMD simulations, the ultrafine HCP Fe53 cluster is in contact with either armchair (AC) or zigzag (ZZ) edges of a square-like graphene nanocavity (see Figure 2a and 2b), or in contact with either an armchair or zigzag edge of a graphene nanoribbon (se Figure 2c and 2d). The system temperature is controlled at 1173 K, with reference to the experimental temperature at which graphene etching by Fe nanoparticle was observed in H2 atmosphere.16 5-ps AIMD simulation is performed for every system. As shown in Figure 2, even with the temperature controlled at 1173 K, Fe-catalyzed C-C dissociation is not observed. Next, the simulation temperature is elevated to 1800 K and the simulation time is extended to 10 ps for each graphene nanocavity system. Again, Fe-catalyzed C-C dissociation is not observed, due to the relatively short simulation time. However, the Fe53 cluster, either in graphene nanocavity or in contact with graphene nanoribbon, can easily rearrange itself to accommodate the shape of graphene edge. Meanwhile, the cluster has the tendency to form as many Fe-C bonds as possible. This behavior also resembles the experimentally observed behavior of Fe cluster3 or Pt cluster27 in the graphene nanocavity. Notably, near the graphene edge, Fe atoms tend to form an Fe chain, similar to the previously observed stable metal chain-graphene edge structures;28-29 far away from graphene edge, Fe atoms tend to form a cluster. This is because the metal-C bond is stronger than the metal-metal bond. For Pt cluster embedded in a graphene nanocavity under electron irradiation, the Pt cluster tends to be flattened and form as many peripheral Pt-C bonds as possible.27 Dong et al. also used the thermal-activated MD simulations to obtain a Pt cluster with the regular shape of a truncated octahedron. This cluster was also observed in their TEM experiment under electron irradiation.27 They pointed out the similarity to the thermal activation process supports early hypothesis30 in simulating the electron irradiation effect by a high ambient temperature. 27 Hence the thermal-activated AIMD simulations appear to be able to mimic certain aspect of heating process by electrons irradiation in qualitative fashion, despite not all aspects of the heating process. As shown in


14

Page 15 of 31

Figure 2a, the Fe53 cluster diffuses along the armchair edge from the right to the left to form new Fe-C bonds with the left zigzag edge. Similarly, the Fe53 cluster diffuses along the zigzag edge from the top to the bottom to form more Fe-C bonds with the bottom armchair edge (Figure 2b). Here, the bond formation at the graphene edge strongly prevents the diffusion of Fe53 cluster on top of the graphene surface. For GNR systems (Figures 2c and 2d), the diffusion of Fe cluster along the edge is not so obvious due to the less number and location of dangling C atoms, compared to graphene nanocavity.

(a) Initial Contacting of Fe53 Cluster with Different sp2-C 3-layer HCP Fe53 Cluster Flakes Side View

ZZ Edge

AC Edge Top View

AC32

ZZ32

AC12

ZZ12

ZZ1

AC1 ZZ3

AC3 Interact with AC5

ZZ5

(b) Possible Dissociation of C-C Bonds (Final Structures)

AC1

AC3

AC5

AC12

AC32

ZZ1

ZZ3

ZZ5

ZZ12 (10 ps)

ZZ32

(c) Evidence of Molten Nanodroplet for Fe53 Cluster 3.0


0.07 0.06

2.5 2.0 1.5 1.0

Self-diffusion coefficients D = 1.95×10-5 cm2/s

0.5 0.0 0.0

0.5

1.0

1.5

Time (ps)

2.0

2.5

Lindemann Index

2


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.05 0.04 0.03 0.02 0.01 0.00 0

1

2

3

4

5

Time (ps)


15


Page 16 of 31

Figure 3. Fe-catalyzed C-C dissociation based on our 5-ps AIMD simulations at 1173 K: (a) initial configurations of Fe53 cluster with 3-layer HCP structure contacting with different graphene flakes (named according to contacting edges and number of 6-membered rings), (b) final configurations for easy recognition of possible dissociation of C-C bonds, and (c) evidence that the ultrafine Fe cluster turns into a molten nanodroplet, based on MSD-time curve (black solid line) and Lindemann index (blue solid line). Further learned from Figure 2, we find this cluster, either in contact with the graphene nanocavity or in contact with GNR, behaves like a molten droplet at 1800 K or 1173 K, since the computed diffusion coefficient of the Fe53 cluster is on the order of 10-5 cm2/s and the Lindemann index approaches to 0.1. The latter is a commonly used value for the Lindemann index to characterize the solid/liquid phase transition.31 When the Lindemann index increases beyond 0.1 as the simulation time increases, we view that the Fe clusters become liquid-like. For comparison, an

independent AIMD simulation of bulk Fe with temperature being controlled at 1173 K indicates that the bulk Fe still behaves like a solid. This is because 1173 K is still much below the measured melting point of bulk Fe. As shown in Figure S1, the MSD–time curve levels off and the Lindemann index is less than 0.01 during the 10-ps AIMD simulation of bulk Fe. In contrast, the Fe53 cluster at 1173 K behaves like a molten droplet and can gradually saturate the edge of the graphene nanocavity, but it cannot soak the edge of large-sized graphene and yield C-C dissociation. In the previous TEM experiment, the ultrafine Fe cluster tends to exhibit a non-zero displacement rate along the graphene edge, followed by gradually etching the graphene edge under the electron irradiation, while losing its original shape due to its close contact with the continuously enlarged graphene nanocavity within a few seconds.3 In addition, the previous


16

Page 17 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


experiment proposed that the characteristic time scale for C-C bond breakage at graphene edge is on the order of 10 s.27 In our AIMD simulations, however, the simulation time is considerably shorter compared to the time scale of TEM measurement. Here, within the much shorter simulation time and in the absence of H and O atoms at 1173 K, the Fe53 cluster in contact with graphene edge becomes molten. After that, it dynamically reshapes itself, diffuses along the graphene edge, and forms more and more Fe-C bonds to stabilize the dangling C atoms, but without showing the Fe-catalyzed C-C dissociation. This behavior maintains, even with doubly increased temperature and reaction time in our AIMD simulations. Additionally, the hybrid Fe-C systems can be still trapped into a low-energy state to impede further C-C dissociation in our AIMD simulations. Alternatively, the reactive MD is performed with the Fe53-cluster-contacting graphene nanocavity for 100-ps (see Figure 6). Once the HCP Fe53 cluster in contact with either armchair edge or zigzag edge of the square nanocavity, the Fe atoms start to react with edge C atoms to form 2D Fe-C carbide. As shown by snapshots in Figure 6, the Fe atoms behave like in liquid state within the first 10-ps simulations for both Fe53@GNC_AC and Fe53@GNC_ZZ models, due to the linearly increasing MSD curve with time (Figures 6a and 6b) and rapidly increasing of Lindemann index (Figure 6a and 6b) with a maximum of 0.18. After 10-ps reactive MD simulations, almost all Fe atoms form carbide with the edge C atoms without further diffusion, as indicated by level-off MSD curve after reaching 49 Å2 for Fe53@GNC_AC (Figure 6a) or 40 Å2 for Fe53@GNC_ZZ (Figure 6b) and the quickly decreasing Lindemann indexes. The selfdiffusion coefficients are estimated to be ~10−4 cm2/s, based on the first 7-ps MSD results, three or four times larger than those estimated based on the AIMD simulations. These results indicate more rapid diffusion of Fe atoms to induce reaction with graphene C atoms. The metal-carbide


17


Page 18 of 31

formation was also observed in graphene-Ti-electrode contact.32 It is believed there the sputter Ti atoms would “penetrate” through the vacancies of the top graphene layers to be in contact with the bottom layer directly.32 Although the formation of iron carbides is within 10 ps, quite fast, the qualitative picture is believed to be rational. For AIMD simulations, we expect the C-C dissociation or carbide formation processes could be observed for our studied Fe-C system be being given much longer simulation time. Without the success in reproducing Fe-catalyzed C-C dissociation of graphene edges, we invoke the static CI-NEB calculations to seek possible reaction mechanism. Limited by the computational resource, a smaller HCP Fe15 cluster is employed to investigate its reaction with armchair and zigzag graphene edges of GNRs with less C atoms compared to the slab models in Figures 2c and 2d (Figure 4). Note that in Ni-nanoparticle-catalyzed graphene etching in atmosphere of H2, Qiu et al. proposed a “Pac-Man” mechanism in which C-C bond breaking is catalyzed by Ni atoms without any involvement of H atoms at the initial stage. 33 The Ni-C interaction can weaken and break edge C-C bond. With more C-C bonds broken by Ni atoms, the dangling C atoms are increasingly surrounded by Ni atoms.33 The driving force was attributed to the strong Ni-C interaction, as proposed by Qiu et al.33 Eventually, the enclosed C atoms are taken place by Ni nanoparticle.33 For our studied system in the absence of H atoms, Fe atoms can catalyze the C-C dissociation of graphene edge and at the same time they may behave like Ni atoms, since the Fe-C interaction24 is comparable to Ni-C interaction. Generally, the C-C dissociation of

graphene edges stems from two mechanisms, C-C displacement and C-C rotation. The reaction pathways for C-C displacement and C-C rotation at graphene edges are derived, with and without the Fe atoms as a comparison in Figure 4.


18

Page 19 of 31

2.583 Å

(a) C-C Displacement at AC Edge 8

FS 5.95

6

Energy (eV)

1.231 Å

12

4 2

IS (ISFe) 0.0

0

no Fe Fe-catalyzed

TSFe 0.05

Im1Fe −0.75

Im3Fe Reaction Coordinate −0.89

FSFe −0.32

-2 1.231 Å

1.424 Å

1.811 Å

2.583 Å

2.004 Å

Side View

(b) C-C Rotation at AC Edge

93.1° 125.6°

4

Energy (eV)

2

0

89.0°

12 3

125.8°

IS (ISFe) 0.0

TS1 1.24

Reaction Coordinate

TS2 2.34

81.1°

Im 0.97

TSFe 0.87

ImFe 0.24

FS 1.30 FSFe −1.54

-2 123.8°

-4

98.7°

no Fe Fe-catalyzed

90.0°

124.5°

(c) C-C Displacement along ZZ Edge 86.4° 57.4°

4

no Fe Fe-catalyzed

124.3°

3 12 2

Energy (eV)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0

-2

-4

TS 2.44 IS (ISFe) 0.0 119.9°

Reaction Coordinate

TSFe −1.93

FS 1.61 FSFe −1.13

96.3° 65.5°

Figure 4. Reaction energies and stationary points along the reaction pathways of (a) C-C displacement and (b) C-C rotation at armchair graphene edge and (c) C-C displacement at zigzag graphene edge without and with Fe atoms. IS, Im, TS, and FS indicate the initial state (reactant), reaction intermediate, transition state, and final state (product), respectively. In the absence of Fe atoms, the most favorable pathway for C-C displacement at armchair graphene edge would be first related to the C1-C2 bond breakage (see Figure 4a). This pathway


19


Page 20 of 31

has been confirmed by the previous metadynamics simulations, from which Qiu et al. predicted similar C-C breakage with a free-energy barrier of 1.11 eV.33 However, our CI-NEB calculations suggest that the energy increases all the time, with an energy change of about 5.95 eV as the C1C2 bond length is elongated from 1.231 Å to 2.583 Å, different from the results of the previous metadynamics simulations.33 When the C-C displacement is catalyzed by the HCP Fe15 cluster, the reaction energy barrier is largely reduced (Figure 4a). During the C1-C2 bond breakage, the Fe15 cluster appears to be rigid, contributing importantly to the C-C displacement. This behavior is consistent with the observation that during metal-catalyzed cutting, the shape of the nanoparticle does not change much.33 Notably, the reaction energy barrier is much lowered to be about 0.70 eV, comparable to the energy barrier of ~0.50 eV for Ni-catalyzed C-C displacement of graphene armchair edge.33 This markedly lowered energy barrier can be attributed to the strong bonds between Fe atoms and the dangling C atoms of the graphene edge, especially for dangling C1 and C2 atoms. Overall, Fe atoms tend to be located in imperfect regions of graphene, such as edges, with a high affinity of a few electron volts. This behavior is also similar to that in the Cr-graphene system.34 Although the energy barrier of Fe-catalyzed C-C displacement at the armchair graphene edge is relatively low (0.70 eV), the thermal condition is still insufficient in our AIMD simulations to activate the C-C bond dissociation largely due to the very short simulation time. As shown in Figure 4b, without Fe atoms, the armchair edge gives a more favorable reaction pathway of C-C rotation, for C-C dissociation, compared to the C-C displacement in Figure 4a. For the C-C rotation, the C2 atom first moves away as the C2-C3 bond length increases from 1.409 Å to 1.532 Å. Next, the C1-C2 bond rotates to be almost perpendicular to the initial orientation of C1-C2 bond. This reaction pathway undergoes two main steps, the elongation of


20

Page 21 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


C2-C3 bond with an energy barrier of about 1.24 eV and the rotation of C1-C2 bond with a barrier of about 1.37 eV. The rate-limiting step is the latter one. Figure 4b also shows that when the Fe atoms are involved in the reaction process, the shape of the HCP Fe15 cluster changes a lot from the initial state (ISFe) to final state (FSFe). At the first step towards the breakage of C2-C3 bond, the Fe atoms markedly lower the energy of the system due to the formation of Fe-C bonds to stabilize the dangling C2 and C3 atoms. The energy is lowered by 2.5 eV. Next, with further rotation of the C1-C2 bond, the energy continuously increases, giving a high barrier of 4.8 eV, much higher than the energy barrier without Fe catalyst. Overall, the Fe atoms cannot efficiently catalyze C-C dissociation at armchair edge via the C-C rotation mechanism. For the zigzag graphene edge, the most probable C-C dissociation is through the displacement of C1 (Figure 4c). Without Fe atoms, C1 atom slowly moves away from C2 atom with relative distance from 1.391 Å to 3.084 Å, and eventually forms 3-membered ring with C3 atom and its neighboring atom. The reaction energy barrier for this path is about 2.44 eV. However, when this reaction is catalyzed by Fe atoms, the energy barrier is even higher, with a value of about 3.2 eV, compared to that without Fe catalyst. As the reaction proceeds, the shape of Fe15 cluster changes a lot as well. Once the dangling C atoms emerge during the breakage of C1-C2 bond, Fe cluster can stabilize them to reach a low-energy intermediate state along the reaction pathway. Nevertheless, we deposit the HCP Fe15 cluster on the reactive sites to seek more possible reaction pathways for both C-C rotation and C-C displacement. After optimization, we find that the Fe cluster tends to leave the graphene surface and move to the graphene edge to form more bonds with dangling C atoms while reshaping itself. This behavior of Fe cluster can be ascribed to its relatively low affinity to the pristine graphene.34 Hereafter, the Fe15 cluster is only


21


deposited to be in direct contact with the graphene edge in Figure 4, as it would lead to the most probable reaction pathway. To summarize, in the absence of Fe atoms, the most favorable reaction pathway for C-C dissociation of graphene edges is C-C rotation at the armchair edge, in line with the experimental observation.35 The C-C dissociation of graphene edge exhibits edge dependence. Although the Fe cluster is in presence to form strong bonds with original or newly generated (from C-C dissociation) dangling C atoms at graphene edges to stabilize the reaction system, the Fe cluster cannot efficiently catalyze all types of the C-C dissociation, except through the C-C displacement mechanism at the armchair edge. However, the energy barriers are still not low enough to let the C-C dissociation be observed within the short time scale of AIMD simulations. The catalytic behavior of Fe atoms is also different from that of Ni atoms. Ni clusters appear to be more rigid during etching to easily take place the position of dissociated C atoms due in part to strong cohesive energy of Ni atoms.33 In contrast, Fe clusters appear to be “soft” to move along the graphene edge while reshaping themselves. As a result, the catalytic ability of Fe clusters may be less effective than that of Ni clusters for C-C dissociation of graphene edges.

RC2 RC1 (triangular 3-layer HCP) (equiaxial BCC) Relative Energy per Fe Atom (eV/atom)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 31

RC3 (triangular BCC)

RC4 (equiaxial FCC)

RC5 (triangular FCC)

RC6 (Hexahedral HCP)

0.20

IC2

Irregular Clusters

RC3

0.15 0.10

RC2

Regular Clusters

0.05 0.00

RC1

RC4

IC14

IC1

RC5 RC6

IC13

IC4 IC9 IC10

IC3

IC7 IC5

IC6

IC11

IC12

IC8

-0.05

Fe53 Clusters


22

Page 23 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 5. Comparison of relative energies of various Fe clusters with 53 atoms. The regular clusters are carved out from the bulk crystals and further optimized, including 3-layer triangular HCP structure (RC1), equiaxial BCC structure (RC2), triangular BCC structure (RC3), equiaxial FCC structure (RC4), triangular FCC structure (RC5), and hexahedral HCP structure (RC6). The irregular clusters (IC) are taken from the final structures of corresponding AIMD simulations in Figures 2 and 3 and further optimized. As mentioned above, the lack of direct evidence of C-C dissociation in our AIMD simulations is largely due to short simulation time. In order to accelerate our AIMD simulations and on-thefly reproduce the Fe-catalyzed C-C dissociation of graphene edges, a new series of AIMD simulations are performed, in which the Fe53 cluster is in direct contact with various graphene flakes along their armchair or zigzag edges (see Figure 3a). Note that the length scale of flakes is much smaller than that of GNC or GNR edges shown in Figure 2. Interestingly, although this Fe53 cluster cannot catalyze the C-C dissociation of these graphene edges within 10-ps simulation time, it can almost catalyze C-C dissociation of smaller graphene flakes without H and O atoms in 5 ps. Except the ZZ12 flake (even with 10 ps simulation time), other nine graphene flakes exhibit breakage of C-C bonds or changed aromatic C structures during 5-ps AIMD simulations with temperature controlled at 1173 K (Figure 3b). Again, the MSD-time curve and Lindemann index in Figure 3c indicate that the Fe53 cluster behaves like a molten droplet at 1173 K, and it undergoes the phase transition from triangular shape to spherical shape as shown in Figure 3b. The C-C bonds in the graphene flakes dissociate still through low-energy path of C-C rotation35 or high-energy path of C-C displacement.35 As shown in Figure 3b, the ZZ32 flake undergoes C-C rotation to achieve the structural transformation from a 6-membered ring to a 5-membered ring after 5-ps simulation. In another ZZ1 flake (Figure 3b), the 6-


23


Page 24 of 31

membered ring breaks into two C wires, with 4 atoms and 2 atoms, respectively. Because the small-sized graphene flakes are soaked on the molten Fe53 cluster, C atoms diffuse fast in molten Fe to promote the breakage of C-C bonds.36 Here, the Fe53 cluster does not show preferential dissociation of C-C bonds along AC edges or ZZ edges, since all of these small graphene flakes immediately occupy the surface of molten Fe53 cluster and quickly diffuse on the cluster surface to promote the C-C dissociation through C-C rotation or C-C displacement. The newly-formed dangling C atoms would be stabilized by Fe atoms. Nearly all small graphene flakes can be etched by the HCP Fe53 cluster within a few ps. This time scale is comparable to that for the C-C dissociation or iron carbide formation observed in the reactive MD simulations, thus also validating the reactive MD simulations. Hence, we expect to see that the Fe53 cluster can still efficiently catalyze the C-C dissociation at edge of graphene nanocavity or nanoribbon in a long enough AIMD simulation time. As shown in Figure 5, the relative energies of various ultrafine Fe53 clusters at 0 K, including clusters carved out from Fe bulks and irregular clusters taken from the final configuration of each AIMD simulation are within 0.2 eV/atom. Thus, the structural evolution between regular clusters and irregular clusters is expected to be easy at 1173 K. Regardless of the initial structures of the Fe clusters, at 1173 K the ultrafine Fe clusters easily turn into a molten droplet. As such, the ultrafine Fe cluster in contact with graphene edge would undergo the structural evolution, diffuse along the graphene edge, and exhibits catalytic ability for C-C dissociation of graphene edge, as summarized in Figure 1.


24

Final

Self-diffusion coefficients 0.30 = 1.10×10-4 cm2/s

Mean Square Displacement D Linear Fitting

60

0.25 50

Lindemann Index

2

Initial

40 30

7.8 ps

20

10 ps

0.20 0.15 0.10 0.05

10 0

0.00 0

10

20

30

40

0

50

20

Time (ps)

40

60

80

100

80

100

Time (ps)

(a) Fe53@GNC_AC 60


50

D = 9.35×10-5 cm2/s

0.30 0.25

Lindemann Index

2


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60



Page 25 of 31

40 30

8 ps

20

10 ps

0.20 0.15 0.10 0.05

10

0.00

0 0

10

20

30

40

50

0

20

Time (ps)

40

60

Time (ps)

(b) Fe53@GNC_ZZ

Figure 6. 100-ps reactive MD simulations of Fe53 cluster in contact with (a) armchair (AC) edge and (b) zigzag (ZZ) edge of a square nanocavity of graphene with 1500 C atoms. The temperature is controlled at 1173 K. The initial and final structures and selected snapshots are highlighted in insets. The corresponding MSD-time curves (black solid line) with the linear fitting (red solid line) of the diffusion coefficients and Lindemann index curves (blue solid line) are also present to characterize the phase change of Fe cluster. Here, the linear fitting of diffusion coefficient is estimated based on the first 7-ps MSD results for both models. 4. CONCLUSIONS

Based on systematic AIMD simulations, we show that the ultrafine HCP Fe53 cluster in contact with graphene edges behaves like a molten droplet at 1173 K and in the high vacuum (without H and O atoms). This molten Fe cluster can further reshape itself, diffuse along the graphene edge, and catalyze graphene-etching-related C-C dissociation. In our AIMD simulations, this Fe53


25


Page 26 of 31

cluster, when contacting with edges of the graphene nanocavity and graphene nanoribbons, tends to saturate the graphene edge via forming Fe-C bonds while without breaking any C-C bonds. Our reactive MD simulations show that within 10 ps the ultrafine Fe53 cluster can complete its carbide formation with C atoms at graphene nanocavity. Furthermore, by means of CI-NEB calculations, we show that the catalytic ability of this ultrafine Fe cluster is ascribed to form strong bonds between Fe atoms and dangling C atoms of graphene edges. However, the Fe cluster can only efficiently catalyze the C-C dissociation at armchair edge, following the C-C displacement mechanism. The catalytic ability of Fe atoms is less efficient than that of Ni atoms, because Fe clusters can easily reshape during reaction. The catalytic behavior of the Fe53 cluster is strongly dependent on the length scale of graphene. When in contact with smaller-sized graphene flakes, the molten Fe cluster can dissociate the C-C bonds through C-C rotation or C-C displacement by soaking the flakes on its surface, even without the contribution of H or O atoms. Our comprehensive study suggested that the device application of graphene nanocavity-Fe nanocluster composites would be limited by its operation temperature since the thermal activation can cause Fe-catalyzed etching of graphene edge.

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website. MSD-time curve of Fe bulk and interatomic energies versus interatomic distance. (PDF) AUTHOR INFORMATION


26

Page 27 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Corresponding Author *E-mail: [email protected]. *E-mail: [email protected]. ACKNOWLEDGMENT Shuang Chen received funding from the National Natural Science Foundation of China (Grant No. 21603097), the Natural Science Foundation of Jiangsu Province (Grant No. BK20160613), and the Fundamental Research Funds for the Central Universities (Grant No. 0215-14380011). The computations were performed in both the High Performance Computing Center (HPCC) of Nanjing University and the University of Nebraska Holland Computing Center. Wei Fa and Jie Bie received funding from the National Natural Science Foundation of China (Grant No. 11474150). REFERENCES (1)

Xu, K.; Cao, P.; Heath, J. R. Graphene Visualizes the First Water Adlayers on Mica at Ambient

Conditions. Science 2010, 329, 1188−1191. (2)

Cao, P.; Xu, K.; Varghese, J. O.; Heath, J. R. Atomic Force Microscopy Characterization of

Room-Temperature Adlayers of Small Organic Molecules through Graphene Templating. J. Am. Chem. Soc. 2011, 133, 2334−2337. (3)

Wang, H.; Li, K.; Yao, Y.; Wang, Q.; Cheng, Y.; Schwingenschlögl, U.; Zhang, X. X.; Yang, W.

Unraveling the Atomic Structure of Ultrafine Iron Clusters. Sci. Rep. 2012, 2, 995 (1−6). (4)

Liu, J.; Ma, Q.; Huang, Z.; Liu, G.; Zhang, H. Recent Progress in Graphene-Based Noble-Metal

Nanocomposites for Electrocatalytic Applications. Adv. Mater. 2018, 1800696 (1−20). (5)

Chen, Z.; Wang, W.; Zhang, Y.; Liang, Y.; Cui, Z.; Wang, X. Pd Nanoparticles Confined in the

Porous Graphene-like Carbon Nanosheets for Olefin Hydrogenation. Langmuir 2018, 34, 12809−12814. (6)

Vedala, H.; Sorescu, D. C.; Kotchey, G. P.; Star, A. Chemical Sensitivity of Graphene Edges

Decorated with Metal Nanoparticles. Nano Lett. 2011, 11, 2342−2347.


27


(7)

Page 28 of 31

Wang, L.; Drahushuk, L. W.; Cantley, L.; Koenig, S. P.; Liu, X.; Pellegrino, J.; Strano, M. S.;

Scott Bunch, J. Molecular Valves for Controlling Gas Phase Transport Made from Discrete ÅngströmSized Pores in Graphene. Nature Nanotech. 2015, 10, 785−790. (8)

Ruffino, F.; Giannazzo, F. A Review on Metal Nanoparticles Nucleation and Growth on/in

Graphene. Crystals 2017, 7, 219 (1−40). (9)

Zhou, Z.; Gao, F.; Goodman, D. W. Deposition of Metal Clusters on Single-Layer

Graphene/Ru(0001): Factors that Govern Cluster Growth. Surf. Sci. 2010, 604, L31−L38. (10)

Wang, B.; Yoon, B.; König, M.; Fukamori, Y.; Esch, F.; Heiz, U.; Landman, U. Size-Selected

Monodisperse Nanoclusters on Supported Graphene: Bonding, Isomerism, and Mobility. Nano Lett. 2012, 12, 5907−5912. (11)

N’Diaye, A. T.; Gerber, T.; Busse, C.; Mysliveček, J.; Coraux, J.; Michely, T. A Versatile

Fabrication Method for Cluster Superlattices. New J. Phys. 2009, 11, 103045 (1−19). (12)

Kamat, P. V. Graphene-Based Nanoarchitectures. Anchoring Semiconductor and Metal

Nanoparticles on a Two-Dimensional Carbon Support. J. Phys. Chem. Lett. 2010, 1, 520−527. (13)

Liu, X.; Wang, C. Z.; Hupalo, M.; Lu, W. C.; Tringides, M. C.; Yao, Y. X.; Ho, K. M. Metals on

Graphene: Correlation between Adatom Adsorption Behavior and Growth Morphology. Phys. Chem. Chem. Phys. 2012, 14, 9157−9166. (14)

Liu, X.; Wang, C.-Z.; Hupalo, M.; Lin, H.-Q.; Ho, K.-M.; Tringides, M. C. Metals on Graphene:

Interactions, Growth Morphology, and Thermal Stability. Crystals 2013, 3, 79−111. (15)

Zhao, J.; Deng, Q.; Bachmatiuk, A.; Sandeep, G.; Popov, A.; Eckert, J.; Rümmeli, M. H. Free-

Standing Single-Atom-Thick Iron Membranes Suspended in Graphene Pores. Science 2014, 343, 1228−1232. (16)

Datta, S. S.; Strachan, D. R.; Khamis, S. M.; Johnson, A. T. C. Crystallographic Etching of Few-

Layer Graphene. Nano Lett. 2008, 8, 1912−1915. (17)

Wang, J.; Ma, L.; Yuan, Q.; Zhu, L.; Ding, F. Transition-Metal-Catalyzed Unzipping of Single-

Walled Carbon Nanotubes into Narrow Graphene Nanoribbons at Low Temperature. Angew. Chem. Int. Ed. 2011, 50, 8041−8045. (18)

Ma, L.; Wang, J.; Yip, J.; Ding, F. Mechanism of Transition-Metal Nanoparticle Catalytic

Graphene Cutting. J. Phys. Chem. Lett. 2014, 5, 1192−1197. (19)

Chen, Q.; Ma, L.; Wang, J. Making Graphene Nanoribbons: a Theoretical Exploration. WIREs

Comput. Mol. Sci. 2016, 6, 243−254. (20)

Zhao, J.; Deng, Q.; Avdoshenko, S. M.; Fu, L.; Eckert, J.; Rümmeli, M. H. Direct in situ

Observations of Single Fe Atom Catalytic Processes and Anomalous Diffusion at Graphene Edges. Proc. Natl. Acad. Sci. 2014, 111, 15641−15646.


28

Page 29 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(21)

Morari, C.; Allmaier, H.; Beiuşeanu, F.; Jurcuţ, T.; Chioncel, L. Electronic Structure and

Magnetic Properties of Metallocene Multiple-Decker Sandwich Nanowires. Phys. Rev. B 2012, 85, 085413 (1−9). (22)

Chen, S.; Zeng, X. C. Interaction between Iron and Graphene Nanocavity: Formation of Iron

Membranes, Iron Clusters, or Iron Carbides. ACS Appl. Mater. Interfaces 2017, 9, 12100−12108. (23)

Santos, E. J. G.; Ayuela, A.; Sánchez-Portal, D. First-principles Study of Substitutional Metal

Impurities in Graphene: Structural, Electronic and Magnetic Properties. New J. Phys 2010, 12, 053012 (1−32). (24)

Srivastava, M. K.; Wang, Y.; Kemper, A. F.; Cheng, H.-P. Density Functional Study of Gold and

Iron Clusters on Perfect and Defected Graphene. Phys. Rev. B 2012, 85, 165444 (1−15). (25)

Wu, M.; Kandalam, A. K.; Gutsev, G. L.; Jena, P. Origin of the Anomalous Magnetic Behavior of

the Fe13+ Cluster. Phys. Rev. B 2012, 86, 174410 (1−5). (26)

Aryanpour, M.; van Duin, A. C. T.; Kubicki, J. D. Development of a Reactive Force Field for

Iron−Oxyhydroxide Systems. J Phys. Chem. A 2010, 114, 6298−6307. (27)

Dong, C.; Zhu, W.; Zhao, S.; Wang, P.; Wang, H.; Yang, W. Evolution of Pt Clusters on

Graphene Induced by Electron Irradiation. J. Appl. Mech. 2013, 80, 040904 (1−8). (28)

Wu, M.; Pei, Y.; Dai, J.; Li, H.; Zeng, X. C. Tri-Wing Graphene Nano-Paddle-Wheel with a

Single-File Metal Joint: Formation of Multi-Planar Tetracoordinated-Carbon (ptC) Strips. J. Phys. Chem. C 2012, 116, 11378−11385. (29)

Wu, M.; Pei, Y.; Zeng, X. C. Planar Tetracoordinate Carbon Strips in Edge Decorated Graphene

Nanoribbon. J. Am. Chem. Soc. 2010, 132, 5554−5555. (30)

Ding, F.; Larsson, P.; Larsson, J. A.; Ahuja, R.; Duan, H.; Rosén, A.; Bolton, K. The Importance

of Strong Carbon-Metal Adhesion for Catalytic Nucleation of Single-Walled Carbon Nanotubes. Nano Lett. 2008, 8, 463−468. (31)

Li, H.-B.; Page, A. J.; Hettich, C.; Aradi, B. a.; Köhler, C.; Frauenheim, T.; Irle, S.; Morokuma,

K. Graphene Nucleation on a Surface-Molten Copper Catalyst: Quantum Chemical Molecular Dynamics Simulations. Chem. Sci. 2014, 5, 3493−3500. (32)

Liu, W.; Wei, J.; Sun, X.; Yu, H. A Study on Graphene—Metal Contact. Crystals 2013, 3,

257−274. (33)

Qiu, Z.; Song, L.; Zhao, J.; Li, Z.; Yang, J. The Nanoparticle Size Effect in Graphene Cutting: A

"Pac-Man" Mechanism. Angew. Chem. Int. Ed. 2016, 55, 1-5. (34)

Wang, H.; Feng, Q.; Cheng, Y.; Yao, Y.; Wang, Q.; Li, K.; Schwingenschlögl, U.; Zhang, X. X.;

Yang, W. Atomic Bonding between Metal and Graphene. J. Phys. Chem. C 2013, 117, 4632-4638.


29


(35)

Page 30 of 31

Sun, L.; Banhart, F.; Warner, J. Two-Dimensional Materials under Electron Irradiation. MRS

Bulletin 2015, 40, 29-37. (36)

Rao, R.; Pierce, N.; Liptak, D.; Hooper, D.; Sargent, G.; Semiatin, S. L.; Curtarolo, S.;

Harutyunyan, A. R.; Maruyama, B. Revealing the Impact of Catalyst Phase Transition on Carbon Nanotube Growth by in Situ Raman Spectroscopy. ACS Nano 2013, 7, 1100-1107.


30

Page 31 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table of Contents

Molten Fe Droplet

Contacting with Graphene Edge

Reshaping Itself and Diffusing along Graphene Edge

Catalyzing C-C Dissociation of Graphene Edge


31

Iron Clusters Embedded in Graphene Nanocavities: Heat-Induced

Recommend Documents