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Stable AA-Stacked Pt Nanoclusters Supported on Graphene/ Ru(0001) and the Selective Catalysis: A Theoretical Study Ding Yi, Wen Zhao, and Feng Ding ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.9b00359 • Publication Date (Web): 23 Apr 2019 Downloaded from http://pubs.acs.org on April 24, 2019

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ACS Applied Nano Materials

Stable AA-Stacked Pt Nanoclusters Supported on Graphene/Ru(0001) and the Selective Catalysis: A Theoretical Study Ding Yi,†,‡ Wen Zhao,† and Feng Ding†,§,*

†Center

for Multidimensional Carbon Materials (CMCM), Institute for Basic Science (IBS), Ulsan 44919, Republic of Korea

‡School

of Natural Science, Ulsan National Institute of Science and Technology (UNIST), Ulsan 44919, Republic of Korea

§School

of Materials Science and Engineering, Ulsan National Institute of Science and Technology (UNIST), Ulsan 44919, Republic of Korea

ABSTRACT: Most metals with close packed atomic arrangements have been found to adopt ABC- or AB-stacking, since this arrangement affords the largest possible

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coordination number of 12. Here, based on first principles calculations, we report for the first time, the predicted high stability of AA-stacked Pt nanoclusters when supported on a graphene/Ru(0001) surface. Our theoretical analysis reveals that this unusual AA stacking is favored due to chemical bonding between Pt atoms and graphene on Ru(0001) surface, where vertically aligned pz orbitals of C atoms tend to hybridize with the dz2 orbitals of the Pt atoms in the bottom most layer in the cluster. Consequently, dz2dz2 bonding between Pt layers is induced, leading to AA-stacking sequence in the Pt nanoclusters. Further analysis proves that these supported AA-stacked Pt clusters interact with adsorbed molecules or radicals in different strength and, can therefore, serve as high-selectivity catalysts for various applications.

KEYWORDS: nanocluster, templated growth, AA stacking, catalysis, DFT

INTRODUCTION

In the past decades, metal nanoclusters have attracted considerable attention because of their attractive and unique chemical and physical properties that are different from their bulk counterparts, and their diverse potential applications in catalysis, sensors and data


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storage, among others.1-4 To obtain a uniform response to external stimuli, metal nanoclusters with uniform size and shape are required. The controlled synthesis of such structures however remains a big challenge.4 One way to obtain well-ordered metal clusters is to deposit metal atoms onto a periodically structured template, such as a graphene moiré superstructure formed by growing a graphene layer on a transition metal surface.5,6 Moiré patterns can be clearly seen in as-grown graphene sheets on a transition metal substrate due to the lattice mismatch between graphene and the substrate.7 Among the many transition metals used to template graphene chemical vapor deposition (CVD) growth, graphene on Ru(0001) and Ir(111), denoted as Gr/Ru(0001) and Gr/Ir(111), respectively, have shown distinct periodic variations of the geometric and electronic properties8,9 and have been successfully used as templates to grow size-controlled nanoclusters of Pt, Rh, Ru and Pd on Gr/Ru(0001) surfaces9-14 and Ir, Pt, W, Re, and Rh nanoclusters on Gr/Ir(111) surfaces.15-19 Depending on the relative positions of the hexagons in graphene with respect to the top two layers of metal atoms of the substrate, a Gr/Ru(0001) or a Gr/Ir(111) surface has three typical high symmetry regions, namely FCC, HCP and ATOP. The three regions are


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named for whether an FCC- or an HCP-hollow, or a metal atom shows through the center of the carbon hexagons (see Figure 1a-c for details).8,10 Using scanning tunneling microscope (STM), Donner et al.11 and Pan et al.12 reported monodispersed Pt nanoclusters (PtNCs) arrays grown on Gr/Ru(0001) surfaces in their respective experiments, and they found that all PtNCs are confined in the FCC regions without coalescence. The mechanism of controlled growth of metal nanoclusters on Gr/Ru(0001) or Gr/Ir(111) is revealed by DFT calculations and it was found that the strong bonding between graphene and the metal substrate leads to the high activity of C atoms and, consequently, a strong bonding to metal nanoclusters.13,20 Experimentally, it is difficult to precisely determine the atomic configuration of metal nanoclusters grown on Gr/Ru(0001). It is well known that most transition metals have close packed structures, with some of them having a face centered cubic (FCC) structure with ABC stacking, and others with hexagonal close packed (HCP) atoms with ABAB stacking. To the best of our knowledge, all the models developed so far for metal clusters on graphene supported on transition metal substrates are based on ABC or ABAB stacking sequences.12-16,21,22 For example, Zhang et al. studied the growth mechanism of


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PtNCs on Gr/Ru(0001) and adopted AB stacking for two Pt atomic layers.13 In this paper, for the first time, we have systematically explored the different configurations of PtNCs adsorbed in the “FCC region” of a Gr/Ru(0001) surface, using first principles calculations. Our study shows that double layer PtNCs prefer to adopt AA stacking, which is triggered by the strong dz2-pz coupling between the bottom layer of the metal and the graphene layer, which in turn, is strongly influenced by its binding to the Ru substrate. Our finding paves the way to designing metal clusters to be used as efficient selective catalysts for various applications.

COMPUTATIONAL DETAILS

To calculate the formation energies of Pt nanoclusters on Gr/Ru(0001) surfaces and their electronic properties, first-principles calculations were performed using density functional theory (DFT) method implemented in the Vienna Ab-initio Simulation Package (VASP) code.23,24 Local density approximation (LDA) method25,26 for exchangecorrelation functional and projector augmented wave (PAW) potentials27 were adopted. The plane-wave energy cutoff was fixed at 300 eV for all the structural optimization to


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save computational cost, due to the very large size of the system. The convergence of energy in self-consistent field procedure and force in ionic relaxation were set as 10−4 eV and 0.02 eV/Å, respectively. For the calculation of electronic properties based on optimized structures, parameters were chosen to obtain higher accuracy, including 400 eV energy cutoff, 2×2×1 k-point sampling, and 10-5 eV energy convergence. The STM images were simulated using the Tersoff−Hamann approach.28 To validate our results, the generalized gradient approximation (GGA) with the PerdewBurke-Ernzerhof type exchange-correlation functional29 as well as a correction for van der Waals (vdW) interactions using DFT-D3 method of Grimme,30 were also adopted for some configurations.

RESULTS AND DISCUSSION


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Figure 1. (a)-(b) Top and side views of a graphene layer on Ru(0001) surface. The three typical regions are marked by different colors; red for FCC, purple for HCP and yellow for ATOP. (c) Zoom-in views of the three typical regions. (d)-(g) Top and side views of Gr/Ru(0001)-supported single layer PtNCs, Pt19 and Pt27.

We first explore single layer PtNCs on a Gr/Ru(0001) surface. PtNCs of different geometry including regular hexagons (Pt19 and Pt37) and truncated triangles (Pt12 and Pt27) were selected to be placed in the “FCC regions” of a Gr/Ru(0001) surface (see Figure 1 and Figure S1). In the FCC region, half of the carbon atoms bond strongly to underlying Ru atoms and the graphene is partially sp3 hybridized. The superposition of Pt atoms leads to further sp3 hybridization of the other half of the carbon atoms in this region. In the top view of this structure, it is clearly seen that there is a carbon atom beneath each of the Pt atoms in PtNC. The side views (Figure 1e & 1g) reveal that only the edge Pt atoms are strongly bonded with the C atoms beneath them, as characterized by the small bond length, dPt-C ~ 2.1 Å, whereas those away from the edges are lifted from the


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substrate, and the distance of a Pt atom to its nearest carbon neighbor is much larger,

dPt-C > 3 Å. The large distance implies a weak van der Waals interaction.

Figure 2. (a)-(b) Top and side views of Gr/Ru(0001) surfaces-supported 2-layer PtNCs with AB stacking and AA stacking, Pt27+19 and Pt27+27 are shown as two examples. (c) Formation energies of 2-layer PtNCs of different stacking sequences, as well as 1-layer PtNCs, as a function of the number of Pt atoms. Brown, gray and cyan balls represent Pt, C and Ru atoms, and the second layers of Ru substrates and PtNCs are distinguished by lighter colors.

Figure 2a-b shows the top and side views of Gr/Ru(0001) surface-supported 2-layer PtNCs. 2-layer PtNCs are characterized by the general formula PtN1+N2, where N1 and


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N2 are the numbers of Pt atoms in the first and second layers, respectively. By adsorbing Pt atoms at different sites on the second layer, both AB (Pt atoms are adsorbed on the hollow sites) and AA (Pt atoms are adsorbed on the top sites) stacking are realized. Top views of the different models of 2-layer PtNCs are given in Figure S1. Different from 1layer PtNCs, after adding the second layer of Pt atoms, it is seen that all the Pt atoms of the bottom layer are strongly bonded to the C atoms below them. As shown in the side view of Figure 2a and b, 50% of the C atoms of the region bond to Pt atoms and the other 50% bond to the top layer of Ru atoms. The formation energy of a PtNC is calculated as according to: 𝐸𝑓 = 𝐸𝑡𝑜𝑡 ― 𝐸𝑠𝑢𝑏 ―𝑁 × 𝜇𝑃𝑡, where 𝐸𝑡𝑜𝑡 and 𝐸𝑠𝑢𝑏 are the energies of the system with and without Pt clusters, and 𝜇𝑃𝑡 is the energy of one Pt atom in the bulk and 𝑁 is the number of Pt atoms in the clusters. The calculated formation energies as a function of the number of Pt atoms are shown in Figure 2c (formation energies per Pt atom are also shown in Figure S2). Comparing the formation energy of 1- and 2-layer PtNCs, we find that the critical size for a transition from single layer to a bilayer is ~27 (more data points around Pt27 can be found in Figure S3).


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The critical size is larger than that reported previously (~10)13, and the difference might be due to the limited configurations considered in their study. Surprisingly, when comparing the formation energies of 2-layer PtNCs with different stacking arrangements, it is observed that AB-stacked PtNCs are less stable than AA stacked ones, which is in contrast with free standing ones as shown in Figure S4. Depending on the size of the PtNCs, the difference in formation energy ranges from 1.0 to 3.0 eV in the range of N from ~Pt30 to Pt60, which is significant. To understand the high stability of AA stacked PtNCs, we first calculated the binding energies of PtNCs of different configurations, to the substrate; two of them are shown in Figure 3a. The binding energy is defined as: 𝐸𝑏 = 𝐸𝑡𝑜𝑡 ― 𝐸𝑠𝑢𝑏 ― 𝐸𝑃𝑡, where 𝐸𝑡𝑜𝑡 and 𝐸𝑠𝑢𝑏 are the energies of the system with and without Pt clusters, and the third term 𝐸𝑃𝑡 is the energy of the Pt cluster. The binding energy of AA-stacked Pt19+19 is 30.07 eV, which is ~ 1.6 eV per Pt atom in the bottom layer. Such a high binding energy implies a strong chemical bond instead of a weak van der Waals interaction between graphene and metal surface, for which, the binding energies are mostly in the range of 0.1-0.2 eV


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per atom. The strong chemical bond can be characterized by charge density differences (CDD) as shown in Figure 3c. The binding energy of AB stacked Pt19+12 is 24.79 eV or 1.3 eV/atom, which also implies a chemical bond between the Pt atoms of the bottom and the graphene; as characterized by the CDD (Figure 3b). The binding energy difference between the two clusters is ~ 5.3 eV, which is larger than the energy difference between AA and AB stacked clusters. This result indicates that the stronger binding to the Gr/Ru substrate is responsible for the high stability of AA stacked PtNCs on it, although the freestanding AA clusters are less stable than AB stacked ones.


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Figure 3. (a) Binding energies of Pt19+12 (AB stacking) and Pt19+19 (AA stacking) with the Gr/Ru substrate. (b)-(c) Charge density differences of Pt19+12 and Pt19+19 on Gr/Ru(0001) surfaces before and after adding the second layer of PtNCs, where yellow and blue represent, respectively, the charge density increase and decrease with an isosurface level of 0.005 e/Bohr3. (d)-(h) Projected density of states (PDOS) of C pz orbitals (red curve) of Gr/Ru (d), Pt dz2 orbitals (black curve) of free standing Pt19+12 (AB-Pt) (e) and Pt19+19 (AA-Pt) (f) and those in Gr/Ru supported Pt19+12 (AB-Pt/Gr/Ru) (g) and Pt19+19 (AAPt/Gr/Ru) (h). The 7 central Pt atoms of the first layer and the 7 C atoms below have been chosen for the PDOS plot. More details can be found in Figure S5

The CDD of AA stacked PtNCs clearly shows characteristics of dz2-dz2 bonding between the Pt atoms in the two layers and is drastically different from that observed in AB stacked clusters. The polarized dz2 orbitals of the bottom layer Pt atoms have the same symmetry as the pz orbitals of the carbon atoms, which ensures a large orbital overlapping between


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the pz orbitals of the C atoms and the dz2 orbitals of the Pt atoms, resulting in a very strong binding between the bottom Pt atoms and the Gr/Ru substrate. To further illustrate the bonding of AA stacked PtNCs and AB stacked PtNCs on the Gr/Ru substrate, the projected density of states (PDOS) of Gr/Ru, freestanding AA and AB stacked Pt clusters, and Pt clusters on Gr/Ru-support, are shown in Figure 3d-h. Among the PDOSs of all the orbitals, the pz orbital of Gr/Ru and the dz2 orbital of AA stacked PtNCs have an obvious peak near the Fermi level (see Figure S5 for the PDOS of all orbitals). After the AA stacked PtNC is placed on the Gr/Ru surface, both peaks near the Fermi level disappear and two peaks, one below the Fermi level and one above, representing bonding and the antibonding states, respectively, are clearly seen. In comparison, the freestanding AB stacked PtNC has a much smaller peak near the Fermi level and the PDOS of both bonding and antibonding states of the supported cluster have smaller peaks than those of AA stacked clusters. As a result of the stronger binding of AA stacked PtNC to Gr/Ru as compared to that for the AB stacked cluster, the gap between the bonding and antibonding states of the former AA is ~ 0.3 eV larger.


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We have used the LDA method in the above calculations. Although this method affords an appropriate description of binding energies and growth modes of metal clusters on Gr/Ru or Gr/Ir surfaces,13,20,21,31 it is normally considered to be less accurate than the GGA method with vdW correction. To further validate our results, some typical configurations were checked using PBE+D3 (GGA of Perdew-Burke-Ernzerhof with DFTD3 correction of Grimme) method and the results are shown in Figure S6. Although the formation energies obtained by the PBE+D3 method are higher than those calculated by the LDA method, the overall preference for AA stacking is maintained, with the energy difference between AA and AB stacking becoming even larger.

Figure 4. Simulated STM images of (a) Pt19+12, (b) Pt27+19, (c) Pt19+19 and (d) Pt27+27 on Gr/Ru(0001) surfaces with 2×2 supercells at a bias of ±0.1 V. The lower left corners are


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overlaid with atomic structures and the enlarged STM images of PtNCs are shown in the circles.

In order to gain further insights into the surface morphology of the clusters with different stacking sequences and facilitate the experimental identification of them, we simulated STM images of four typical cluster structures, Pt19+12 and Pt27+19 with AB stacking sequence, and Pt19+19 and Pt27+27 with AA stacking sequence, at a bias of ±0.1 V and the results are shown in Figure 4. From it, we can clearly see that the clusters with AB stacking sequence have higher electron densities (brighter) near the edges, while the images of clusters with AA stacking sequence look more uniform and dim.


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Figure 5. (a)-(b) Side views of Pt/Gr/Ru (AB stacking and AA stacking) with adsorbates on them, taking CO molecules as example. (c) Binding energies of different adsorbates to Gr/Ru(0001) surfaces-supported AB-stacked Pt slab, AA-stacked Pt slab, and Pt(111) surfaces. (d) Calculated free energy diagram for hydrogen evolution on the three kinds of substrates.

Our analysis thus confirms the high stability of AA stacked PtNCs on Gr/Ru substrates and we attribute this high stability to dz2-dz2 coupling between the Pt atoms and the resultant strong bonding between PtNC and the Gr/Ru substrate. The new configuration


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of PtNCs undoubtedly affects their electronic properties and therefore opens a way to modulate the structure of transition metal clusters in various applications, which are of crucial importance for energy management and catalysis.32,33 To further demonstrate the difference between AA and AB stacked PtNCs, we calculated the adsorption of some molecules, atoms and radicals (e.g., CO, H, O, OH, CH, CH2 and CH3) on their surfaces. The optimized structures are shown in Figure 5a-b and Figure S7, and the calculated binding energies of different adsorbates on both AB and AA stacked 2-layer Pt slabs on Gr/Ru and Pt(111) surfaces are shown in Figure 5c with the definition of 𝐸𝑏’ = 𝐸𝑡𝑜𝑡’ ― 𝐸𝑠𝑢𝑏’ ― 𝐸𝑎𝑑, where 𝐸𝑡𝑜𝑡’ and 𝐸𝑠𝑢𝑏’ are the energies of the system with and without the adsorbates, and 𝐸𝑎𝑑 is the energy of the adsorbate. As compared to Pt(111) surfaces, binding of all the adsorbates are weakened on AA-Pt/Gr/Ru but enhanced on AB-Pt/Gr/Ru surface. This difference in adsorption energies implies that specific catalytic effects can be achieved by tuning the structure of Pt clusters and more efficient catalysts might be designed, for example, for hydrogen evolution reaction (HER), oxygen evolution reaction (OER), oxygen reduction reaction (ORR) or methane decomposition, among others.34-38


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In order to demonstrate the selectivity-activity of AA stacked Pt, the HER performances of AA-, AB-stacked Pt structure on Gr/Ru (0001) and Pt(111) surface were evaluated by computing the reaction free energy defined as: ∆𝐺 = ∆𝐸 + ∆𝐸𝑍𝑃𝐸 ―𝑇∆𝑆, where ∆𝐸 is the adsorption energy of a hydrogen atom (setting the energy of 1/2 H2 as the reference) on different substrates, and ∆𝐸𝑍𝑃𝐸 and ∆𝑆 are the differences in the zeropoint energy and the entropy, respectively, between the adsorbed and the gas phase. The calculated free energies are shown in Figure 5d and we can find that AA stacked Pt slab has higher activity for HER than common Pt catalyst since the red line is closer to zero.

CONCLUSIONS

In summary, through systematic density functional calculations and electronic structure analysis, we have shown that the AA stacked bilayer Pt clusters are highly stable on Gr/Ru(0001) surface and the stability is induced by the strong coupling of pz orbitals of the C atoms in graphene and the dz2 orbitals of the Pt atoms directly above. The finding


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of highly stable AA stacked transition metal clusters provides a great opportunity for various applications, particularly in catalysis.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: Models of PtNCs on Gr/Ru(0001), formation energies per Pt atom of PtNCs, formation energies of PtNCs with the same number of Pt atoms and corresponding models, formation energies of free-standing PtNCs, PDOS of all C p-orbitals and Pt d-orbitals, formation energies of PtNCs obtained by PBE+D3 method, models of adsorbates on Pt/Gr/Ru(0001).

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]

ACKNOWLEDGMENT


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The authors acknowledge support from the Institute for Basic Science (IBS-R019-D1) of South Korea and the usage of IBS-CMCM high performance computing system, Cimulator. REFERENCES (1) de Heer, W. A. The physics of simple metal clusters: experimental aspects and simple models. Rev. Mod. Phys. 1993, 65, 611-676. (2) Haruta, M. Size- and support-dependency in the catalysis of gold. Catal. Today 1997, 36, 153-166. (3) Tyo, E. C.; Vajda, S. Catalysis by clusters with precise numbers of atoms. Nature Nanotechnol. 2015, 10, 577-588. (4) Lu, Y.; Chen, W. Sub-nanometre sized metal clusters: from synthetic challenges to the unique property discoveries. Chem. Soc. Rev. 2012, 41, 3594-3623. (5) N’Diaye, A. T.; Bleikamp, S.; Feibelman, P. J.; Michely, T. Two-Dimensional Ir Cluster Lattice on a Graphene Moiré on Ir(111). Phys. Rev. Lett. 2006, 97, 215501. (6) Liu, X.; Han, Y.; Evans, J. W.; Engstfeld, A. K.; Behm, R. J.; Tringides, M. C.; Hupalo, M.; Lin, H.-Q.; Huang, L.; Ho, K.-M. et al. Growth Morphology and Properties of Metals on Graphene. Prog. Surf. Sci. 2015, 90, 397-443. (7) Seah, C.-M.; Chai, S.-P.; Mohamed, A. R. Mechanisms of graphene growth by chemical vapour deposition on transition metals. Carbon 2014, 70, 1-21. (8) Voloshina, E. N.; Fertitta, E.; Garhofer, A.; Mittendorfer, F.; Fonin, M.; Thissen, A.; Dedkov, Y. S. Electronic structure and imaging contrast of graphene moiré on metals. Sci. Rep. 2013, 3, 1072. (9) Sutter, E.; Albrecht, P.; Wang, B.; Bocquet, M.-L.; Wu, L.; Zhu, Y.; Sutter, P. Arrays of Ru nanoclusters with narrow size distribution templated by monolayer graphene on Ru. Surf. Sci. 2011, 605, 1676-1684. (10) 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. (11) Donner, K.; Jakob, P. Structural properties and site specific interactions of Pt with the graphene/Ru(0001) moiré overlayer. J. Chem. Phys. 2009, 131, 164701. (12) Pan, Y.; Gao, M.; Huang, L.; Liu, F.; Gao, H.-J. Directed self-assembly of monodispersed platinum nanoclusters on graphene Moiré template. Appl. Phys. Lett. 2009, 95, 093106. (13) Zhang, L. Z.; Du, S. X.; Sun, J. T.; Huang, L.; Meng, L.; Xu, W. Y.; Pan, L. D.; Pan, Y.; Wang, Y. L.; Hofer, W. A. et al. Growth Mechanism of Metal Clusters on a Graphene/Ru(0001) Template. Adv. Mater. Interfaces 2014, 1, 1300104.


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Stable AA-Stacked Pt Nanoclusters Supported on ... - ACS Publications

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