Graphene and graphene nanomesh supported nickel clusters: Electronic, magnetic, and hydrogen storage properties

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Authors Fadlallah, M M; Abdelrahman, Ali; Schwingenschlögl, Udo; Maarouf, Ahmed A.

Citation Fadlallah MM, Abdelrahman A, Schwingenschlögl U, Maarouf AA (2018) Graphene and graphene nanomesh supported nickel clusters: Electronic, magnetic, and hydrogen storage properties. Nanotechnology. Available: http://dx.doi.org/10.1088/1361-6528/ aaee3c.

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DOI 10.1088/1361-6528/aaee3c

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Journal Nanotechnology

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ACCEPTED MANUSCRIPT Graphene and graphene nanomesh supported nickel clusters: Electronic, magnetic, and hydrogen storage properties

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1 2 3 4 5 6 7 8 Graphene and graphene nanomesh supported nickel 9 10 11 clusters: Electronic, magnetic, and hydrogen storage 12 13 14 15 properties 16 17 18 ∗,†,‡ ¶ § 19 Mohamed M. Fadlallah, Ali G. Abdelrahman, Udo Schwingenschl¨ogl, 20 21 and Ahmed A. Maarouf∗,k 22 23 24 †Department of Physics, Faculty of Science, Benha University, Benha, Egypt 25 26 ‡Center for Nanotechnology, Zewail City of Science and Technology, Giza 12588, Egypt 27 28 ¶Department of Basic Science, Faculty of Computer and Information Science, Ain Shams 29 30 University, Cairo, Egypt 31 32 §King Abdullah University of Science and Technology (KAUST), Physical Science and 33 34 Engineering Division (PSE), Thuwal 23955-6900, Saudi Arabia 35 36 kDepartment of Physics, Institute for Research and Medical Consultations, Imam 37 38 Abdulrahman Bin Faisal University, Dammam 31441, Saudi Arabia 39 40 41 E-mail: [email protected]; [email protected] 42 43 44 November 1, 2018 45 46 47 Abstract 48 49 Small-sized nanoparticles are widely used in applications such as catalysis, nano- 50 51 electronics, and hydrogen storage. However, the small size causes a common problem: 52 53 agglomeration on the support template. One solution is to use templates that limit 54 55 the mobility of the nanoparticles. Graphene nanomeshes (GNMs) are two dimensional 56 57 58 59 1 60 Accepted Manuscript AUTHOR SUBMITTED MANUSCRIPT - NANO-119391.R1 Page 2 of 17

1 2 3 porous structures with controllably passivated pores. In this work, we employ first prin- 4 5 ciples calculations to investigate the potential for using GNMs as support templates 6 7 for Ni clusters and, at the same time, study their magnetic and hydrogen storage 8 9 properties. We consider two Ni clusters (Ni6 and Ni13) and two GNMs (O-terminated 10 11 and N-terminated), comparing our results to those of isolated Ni clusters and those 12 13 of Ni clusters on graphene. High stability of the Ni clusters is found on the N-GNM 14 15 in contrast to the O-GNM. We quantify the hydrogen storage capacity by calculat- 16 ing the adsorption energy for multiple H molecules. The values on Nix/N-GNM are 17 2 18 significantly reduced as compared to the corresponding isolated Nix clusters, but a 19 20 high hydrogen storage capacity is maintained. The fact that Nix/N-GNM hosts spin 21 22 polarization is interesting for spintronic applications. 23 24 25 26 27 Keywords 28 29 30 Density functional theory, hydrogen storage, Ni cluster, graphene, nanomesh 31 32 33 34 1 Introduction 35 36 37 Carbon dioxide emissions from ordinary fuels and the consequent global warming are driving 38 39 the quest for clean, renewable, and cheap energy sources.1 Hydrogen is one potential can- 40 41 didate, as it is environmentally friendly and has the highest energy per mass of any fuel.2 42 43 Therefore, high-capacity hydrogen storage is a desirable technological goal and many exper- 44 45 imental3,4 and theoretical3,5 efforts have aimed to improve the process. In addition to the 46 47 gas/liquid storage of hydrogen in high-pressure vessels, a focus is developing on materials- 48 49 based storage.6,7 Due to their large surface area and moderate cost, C-based nanomaterials 50 51 are considered for hydrogen storage.8 For example, carbon nanotubes doped with Ca, Co, Fe, 52 53 Ni, and Pd have high storage capacities at standard temperature and pressure.9 Li trapped 54 10 11 55 in a graphene double vacancy and Ni-decorated graphene store up to 5 H2 molecules per 56 57 58 59 2 60 Accepted Manuscript Page 3 of 17 AUTHOR SUBMITTED MANUSCRIPT - NANO-119391.R1

1 2 3 Li/Ni atom. High hydrogen storage capacities also are predicted for Ca-decorated zigzag 4 5 graphene nanoribbons,12 porous graphene oxide,13 and porous graphene doped with various 6 7 elements.14,15 8 9 A graphene nanomesh (GNM, also called porous graphene) is a two dimensional material 10 11 formed by creating a lattice of pores in graphene. The sizes and geometries of the pores con- 12 13 trol the electronic properties of the GNM, which can be semiconducting or semimetallic.16–19 14 15 Pore passivation by species such as H, O, and N can be used to dope GNMs18,20,21 and con- 16 17 trol their magnetic properties.22 The pores also offer a possible route to boost the chemical 18 19 reactivity, allowing GNMs to be utilized as templates in various applications, for example, as 20 21 electrostatic and chemical traps.23 GNMs have been fabricated with pore sizes between 5 nm 22 23 and 200 nm,24,25 making them suitable templates for catalytic nanoparticles,11,23 molecular 24 25 sensors,26,27 and nanoparticles for hydrogen storage.15 26 27 In this work, we consider Ni6 and Ni13 clusters, isolated, on graphene, and on O- and 28 29 N-passivated GNMs (O-GNM and N-GNM, respectively). We study their magnetic and 30 31 hydrogen storage properties both with and without templates. 32 33 34 35 36 2 Computational methods 37 38 39 All calculations are performed using the QUANTUM ESPRESSO plane wave density func- 40 28 41 tional theory package. Spin-polarized calculations are employed to determine the electronic 42 43 and magnetic properties. For the exchange correlation functional we use the generalized gra- 44 29 45 dient approximation in the scheme of Perdew-Burke-Ernzerhof. A 9×9×1 Monkhorst-Pack 46 47 k-mesh and a 40 Ry energy cut-off are used. A 6 × 6 graphene supercell is built, with a 15 48 49 A˚ thick vacuum separation to prevent interaction along the c-direction. GNMs with hexag- 50 51 onal pores are obtained by removing 12 C atoms and saturating the pore edge with 6 O 52 53 or 6 N atoms. All systems are structurally relaxed to reduce the atomic forces below 0.002 54 55 Ry/Bohr. At the start of the structural relaxation the bottom of the Ni clusters is located 56 57 58 59 3 60 Accepted Manuscript AUTHOR SUBMITTED MANUSCRIPT - NANO-119391.R1 Page 4 of 17

1 2 3 about 3 A˚ above the graphene/GNM plane. 4 5 6 7 8 3 Results and discussion 9 10 11 3.1 Ni and Ni clusters 12 6 13 13 14 We begin by studying the magnetic and hydrogen storage properties of Ni6 and Ni13 clusters, 15 16 which are found experimentally30 and theoretically31 to be the most stable Ni clusters. Their 17 18 optimized structures (Fig. 1a,b) show average Ni-Ni bond lengths of 2.31 A˚ and 2.37 A,˚ 19 20 respectively, and magnetic moments per Ni atom of 1.4 µB and 0.9 µB, in agreement with 21 22 previous calculations.31,32 The density of states (DOS) (Fig. 1c,d) demonstrates that the 23 24 Fermi energy is located in the spin-down Ni 3d states for Ni6 with a spin-up gap of about 2 25 26 eV, while for Ni13 both spin channels are metallic. We note that all atoms of Ni6, but only 27 28 12 atoms of Ni13, constitute the surface of the cluster. 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 4 60 Accepted Manuscript Page 5 of 17 AUTHOR SUBMITTED MANUSCRIPT - NANO-119391.R1

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 Figure 1: Optimized structures of (a) Ni and (b) Ni , and (c,d) corresponding DOS. 34 6 13 35 Optimized structures of (e) Ni6 and (f) Ni13 with H2 molecules adsorbed, and corresponding 36 (g) Ead and (h) µ per Ni atom as functions of n. 37 38 39 40 We calculate the storage capacities of the Nix clusters (x = 6, 13) by adding H molecules, 41 2 42 one at a time,allowing the system to structurally relax after each addition. The adsorption 43 44 energy of the nth molecule is 45 46 47 n 48 ( ) Ead(H2 ) = E(Nix + (n − 1) H2) + E(H2) − E(Nix + n H2), (1) 49 50 51 52 where E(Nix + n H2) is the total energy of the cluster with n adsorbed H2 molecules and 53 54 EH2 is the total energy of an isolated H2 molecule. The isolated Ni6 and Ni13 clusters are 55 56 capable of adsorbing up to 13 H2 molecules with Ead larger than 0.1 eV (Fig. 1e,f). For both 57 58 59 5 60 Accepted Manuscript AUTHOR SUBMITTED MANUSCRIPT - NANO-119391.R1 Page 6 of 17

1 2 3 clusters we find that the average Ni-Ni bond length does not change after H adsorption, 4 2 5 while the average Ni-H bond length is 1.61 A.˚ The average E is 0.36 eV for Ni and 0.41 6 ad 6 7 eV for Ni . 8 13 9 The magnetic moments of both clusters tend to decrease with increasing number of 10 11 adsorbed H molecules (Fig. 1g,h). To explain this observation, we analyze the L¨owdin 12 2 13 charges and find for Ni that adsorption of an H molecule shifts Ni 4s spin-up states above 14 6 2 6 15 the Fermi energy and Ni 3d spin-down states below the Fermi energy, thereby decreasing the 16 17 magnetic moment of the cluster. Moreover, a partial charge of 0.04 electrons is transferred 18 19 to the H2 molecule, which is therefore elongated from 0.75 A˚ to 0.89 A.˚ This dissociative H2 20 21 33–36 adsorption has been reported for Ni and other metal particles. Additional H2 molecules 22 23 lead to similar effects. L¨owdin charge analysis certifies that the magnetic moments of the Ni 24 25 atoms with H2 molecules adsorbed are reduced (typically by 15 %) as compared to the other 26 27 Ni atoms. H2 adsorption on Ni13 also shifts spin-up and spin-down states across the Fermi 28 29 energy in opposite directions and the magnetic moment of the cluster decreases. The fact 30 31 that this decrease is smaller than for Ni6 can be attributed to the fact that only the surface 32 33 atoms participate in the Ni-H bonding. 34 35 36 37 3.2 Ni clusters on pristine graphene 38 39 40 We next discuss results for the Ni clusters adsorbed on graphene (Nix/G), with a binding 41 42 energy of 43 1 Eb = E(Y) + E(Nix) − E(Nix + Y)
, (2) 44 x 45 46 47 where Y is the graphene or GNM template. Ni6 shows slight deformations (Fig. 2a) with an 48 ˚ ˚ 49 average Ni-Ni distance of 2.35 A. The average Ni-C distance is 2.09 A, the binding energy 50 51 per Ni atom is 0.16 eV, and the magnetic moment per Ni atom is 1.0 µB. For Ni13 (Fig. 52 ˚ ˚ 53 2b) we obtain an average Ni-Ni bond length of 2.41 A, an average Ni-C distance of 2.07 A, 54 55 a binding energy per Ni atom of 0.20 eV, and a magnetic moment per Ni atom of 0.9 µB. 56 57 58 59 6 60 Accepted Manuscript Page 7 of 17 AUTHOR SUBMITTED MANUSCRIPT - NANO-119391.R1

1 2 3 The deformation of the Ni cluster is not significantly affected by how it faces graphene; 4 13 5 it ends up having triangular faces only. L¨owdin charge analysis indicates that the average 6 7 charge transfer from a Ni atom to graphene is 0.4 electrons for Ni and 0.6 electrons for Ni , 8 6 13 9 which explains the difference in the binding energy per Ni atom between the two clusters. 10 11 The (partial, P) DOS (Fig. 2c,d) shows mainly Ni 3d spin-down states at the Fermi energy 12 13 and spin-up gaps of 0.25 eV for Ni /G and 0.70 eV for Ni /G. 14 6 13 15 The relaxed structures after H2 adsorption (Fig. 2e,f) demonstrate that the maximum 16 17 number of adsorbed H2 molecules is lower than in the case of the isolated clusters, which is 18 19 due to the binding between some Ni atoms and graphene. The adsorption has no significant 20 21 effect on the average Ni-C distance. Both systems can adsorb 8 H2 molecules (Fig. 2g,h) 22 23 with an average H-H distance of 0.91 A˚ and 0.84 A˚ and an average Ead of 0.38 eV and 24 25 0.50 eV for Ni6/G and Ni13/G, respectively. Because of the interaction with graphene, the 26 27 magnetic moments are lower than those of the isolated clusters, with Ni6/G being completely 28 29 depolarized at maximum H2 load. When more H2 molecules are adsorbed, again Ni spin- 30 31 up/down states are shifted above/below the Fermi energy, which decreases the magnetic 32 33 moment. 34 35 36 37 3.3 Ni clusters on GNMs 38 39 40 We find that the Ni clusters bind weakly (binding energy per Ni atom: 0.08 eV for Ni6 and 41 42 0.07 eV for Ni13) to the O-GNM, which is reflected by average Ni-O distances of 2.10 A˚ for 43 44 Ni6 and 2.13 A˚ for Ni13 (Fig. 3a,b). The average Ni-Ni distance is 2.31 A˚ for Ni6 and 2.40 A˚ 45 46 for Ni13. We find at the Fermi energy mainly Ni 3d spin-down states (Fig. 3c,d). Moreover, 47 48 6 H2 molecules can be adsorbed on Ni6/O-GNM and 8 H2 molecules on Ni13/O-GNM (Fig. 49 50 3e,f), with an average H-H distance of 0.90 A˚ and 0.85 A˚ and an average Ead of 0.52 eV and 51 52 0.49 eV, respectively. Again, the adsorption of H2 molecules reduces the magnetic moment 53 54 (Fig. 3g,h). 55 56 57 58 59 7 60 Accepted Manuscript AUTHOR SUBMITTED MANUSCRIPT - NANO-119391.R1 Page 8 of 17

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 Figure 2: Optimized structures of (a) Ni6/G and (b) Ni13/G, and (c,d) corresponding 45 DOS/PDOS. Optimized structures of (e) Ni6/G and (f) Ni13/G with H2 molecules adsorbed, 46 and corresponding (g) Ead and (h) µ per Ni atom as functions of n. 47 48 49 50 51 52 53 54 55 56 57 58 59 8 60 Accepted Manuscript Page 9 of 17 AUTHOR SUBMITTED MANUSCRIPT - NANO-119391.R1

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 Figure 3: Optimized structures of (a) Ni6/O-GNM and (b) Ni13/O-GNM, and (c,d) corre- 33 sponding DOS/PDOS. Optimized structures of (e) Ni6/O-GNM and (f) Ni13/O-GNM with 34 H molecules adsorbed, and corresponding (g) E and (h) µ per Ni atom as functions of n. 35 2 ad 36 37 38 39 Turning to the N-GNM, we first notice that both clusters form short bonds (1.94 A˚ for 40 41 Ni6 and 1.97 A˚ for Ni13) to the N atoms at the pore edge (Fig. 4a,b), because N has one 42 43 valence electron more available than O. The average Ni-Ni distance is 2.31 A˚ for Ni6 and 44 45 2.39 A˚ for Ni13. We find at the Fermi energy that the C pz states dominate in the spin-up 46 47 channel and the Ni 3d states in the spin-down channel (Fig. 4c,d). The Ni-N bonds at the 48 49 pore edge are much stronger than the Ni-O bonds in the case of the O-GNM (binding energy 50 51 per Ni atom: 1.42 eV for Ni6 and 0.7 eV for Ni13). Both starting configurations of the Ni13 52 53 cluster in the GNM pore give the same equilibrium structure. The cluster becomes slightly 54 55 deformed having only triangular faces, different from the severe deformations occurring at 56 57 58 59 9 60 Accepted Manuscript AUTHOR SUBMITTED MANUSCRIPT - NANO-119391.R1 Page 10 of 17

1 2 3 unpassivated graphene vacancies.37 We obtain for Ni a hydrogen storage capacity of 6 H 4 6 2 5 molecules (Fig. 4e), with an average H-H distance of 0.87 A˚ and an average E of 0.36 6 ad 7 eV. As for Ni a smaller fraction of the cluster atoms is involved in the Ni-N binding at 8 13 9 the pore edge, a higher hydrogen storage capacity of 8 H molecules is achieved (Fig. 4f), 10 2 11 with an average H-H distance of 0.85 A˚ and an average E of 0.45 eV. Similar to the other 12 ad 13 studied cases, the adsorption of 6 H molecules reduces the magnetic moment (Fig. 4g,h). 14 2 15 It turns out that the N-GNM offers the lowest mobility for the anchored Ni clusters (Fig. 16 17 5). However, the H2 storage capacity is low due to the fact that charge is consumed by 18 19 the covalent bonds with the pore edge. This can be remedied by using large pore N-GNMs 20 21 to host bigger Ni clusters and achieve H2 storage properties close to those of the isolated 22 23 clusters. 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 10 60 Accepted Manuscript Page 11 of 17 AUTHOR SUBMITTED MANUSCRIPT - NANO-119391.R1

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 Figure 4: Optimized structures of (a) Ni6/N-GNM and (b) Ni13/N-GNM, and (c,d) corre- 35 sponding DOS/PDOS. Optimized structures of (e) Ni6/N-GNM and (f) Ni13/N-GNM with 36 H molecules adsorbed, and corresponding (g) E and (h) µ per Ni atom as functions of n. 37 2 ad 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 Figure 5: Binding energy per Ni atom of the Ni clusters on the different templates. 53 54 55 56 For Nix/G the spin-up DOS is always gapped and the spin-down DOS always finite at the 57 58 59 11 60 Accepted Manuscript AUTHOR SUBMITTED MANUSCRIPT - NANO-119391.R1 Page 12 of 17

1 2 3 Fermi energy, which indicates that this system can be used as spin filter (because transport 4 5 is permitted only for one spin component). As an analysis of the wavefunction demonstrates 6 7 for the Ni /N-GNM that the Ni 3d states (dominate at the Fermi energy the spin-down 8 13 9 channel) are localized on the cluster while the C pz states (dominate at the Fermi energy 10 11 the spin-up channel) are delocalized, also this system can act as spin filter. When the pore 12 13 density is decreased the spin-down states will slowly recover their graphene character and 14 15 the spin filtering properties will be lost. 16 17 18 19 20 4 Conclusion 21 22 23 GNMs are porous graphene structures that are currently considered for applications in na- 24 25 noelectronics, chemical separation, and molecular sensing. During or post fabrication, their 26 27 pore edges can be selectively passivated to match the target application. Here, we use den- 28 29 sity functional theory to explore possible utilization for hydrogen storage as templates for 30 31 anchoring Ni clusters on that H2 molecules can be adsorbed. We also study the magnetic 32 33 properties. We find that the N-GNM is capable of firmly anchoring the Ni clusters through 34 35 strong binding to the pore N atoms, while the O-GNM binds clusters only weakly. On the 36 37 N-GNM the Ni6 and Ni13 clusters can host up to 6 and 8 H2 molecules, respectively. The 38 39 H2 storage properties of larger clusters are expected to approach those of Ni clusters on 40 41 graphene, as the fraction of Ni atoms involved in the binding to the GNM decreases. We 42 43 also find that for small numbers of adsorbed H2 molecules (n = 2 to 4 or 5) there is some 44 45 variation in the adsorption energy per molecule (∼ 0.1 eV) due to the low density of H2 46 47 molecules. This variation decreases as more H2 molecules are added to the Ni clusters. For 48 49 larger clusters less variation in the adsorption energies is expected, as the molecules are more 50 51 homogeneously distributed over the cluster. 52 53 We observe half-metallic properties for Nix/G, opening potential for spin filtering appli- 54 55 cations. On the other hand, Ni13/N-GNM shows at the Fermi energy a finite DOS for both 56 57 58 59 12 60 Accepted Manuscript Page 13 of 17 AUTHOR SUBMITTED MANUSCRIPT - NANO-119391.R1

1 2 3 spin channels. However, spin filtering is also possible in this case, as one spin channel is due 4 5 to delocalized graphene-like states, and the other due to localized Ni states. 6 7 8 9 10 Acknowledgment 11 12 The authors would like to acknowledge the use of the resources of the Supercomputing 13 14 Laboratory at KAUST, and the resources and technical services provided by the Scientific 15 16 and High Performance Computing Center at Imam Abdulrahman Bin Faisal University, 17 18 Dammam, Saudi Arabia. M. Fadlallah would also like to thank Ulrich Eckern for fruitful 19 20 discussions. 21 22 23 24 25 References 26 27 28 (1) Solomon, S.; Plattner, G.-K.; Knutti, R.; Friedlingstein, P. Irreversible climate change 29 30 due to carbon dioxide emissions. Proceedings of the National Academy of Sciences 2009, 31 32 1704–1709. 33 34 (2) Jain, I. Hydrogen the fuel for 21st century. International Journal of Hydrogen Energy 35 36 2009, 34, 7368–7378. 37 38 39 (3) Chen, P.; Zhu, M. Recent progress in hydrogen storage. Materials Today 2008, 11, 40 41 36–43. 42 43 44 (4) Durbin, D.; Malardier-Jugroot, C. Review of hydrogen storage techniques for on board 45 46 vehicle applications. International Journal of Hydrogen Energy 2013, 38, 14595–14617. 47 48 49 (5) Han, S. S.; Mendoza-Cortes, J. L.; Goddard III, W. A. Recent advances on simula- 50 51 tion and theory of hydrogen storage in metal-organic frameworks and covalent organic 52 53 frameworks. Chemical Society Reviews 2009, 38, 1460–1476. 54 55 56 57 58 59 13 60 Accepted Manuscript AUTHOR SUBMITTED MANUSCRIPT - NANO-119391.R1 Page 14 of 17

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