Atomic Structures of Pt Nanoclusters Supported on Graphene Grown

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Atomic Structures of Pt Nanoclusters Supported on Graphene Grown on Pt(111) Pei-Yang Cai, Yen-Wen Huang, Yi-Cheng Huang, Meng-Chin Cheng, Liang-Wei Lan, Chien-Cheng Kuo, Jeng-Han Wang, and Meng-Fan Luo J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b04119 • Publication Date (Web): 30 Jun 2018 Downloaded from http://pubs.acs.org on June 30, 2018

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The Journal of Physical Chemistry

Atomic Structures of Pt Nanoclusters Supported on Graphene Grown on Pt(111)

Pei-Yang Caia, Yen-Wen Huanga, Yi-Cheng Huanga, Meng-Chin Chengb, Liang-Wei Lanc, Chien-Cheng Kuoc, Jeng-Han Wangb,* and Meng-Fan Luoa,*

a

Department of Physics, National Central University, 300 Jhongda Road, Taoyuan 32001,

Taiwan b

Department of Chemistry, National Taiwan Normal University, Taipei, Taiwan

c

Department of Physics, National Sun Yat-sen University, 70 Lienhai Rd., Kaohsiung 80424,

Taiwan

1 Environment ACS Paragon Plus

The Journal of Physical Chemistry 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

ABSTRACT Atomic structures of Pt nanoclusters on graphene/Pt(111) were investigated with various techniques to probe the surface under ultrahigh-vacuum conditions and with calculations based on density-functional theory. Monolayer graphene was grown on thermal decomposition of ethylene on Pt(111) at 950 K and Pt clusters on deposition of Pt vapor onto graphene/Pt(111) at 300 K. The graphene had two predominant domains: one had a small angle of rotation between the graphene and underlying Pt lattice, structurally commensurate with the Pt(111) lattice (G0º), and the other was rotated about 30° with respect to the Pt lattice (G30º). G0º had a slightly corrugated structure, involving tetrahedral hybridization, and a stronger adsorption on Pt(111); in contrast, G30º was flat and weakly bound to Pt(111), via a van der Waals interaction. The grown Pt clusters were structurally ordered, having a fcc phase and growing in a (111) orientation, whereas they had correspondingly disparate nucleation modes and rotational configurations on the two major graphene domains. On G0º, the clusters were smaller, had a narrow size distribution and greater cluster density; they were structurally commensurate with the G0º lattice (with their [-110] (or [0-11]) axes along direction [1-100] of G0º). In contrast, on G30º, the clusters were larger, had an evidently broader size distribution and smaller cluster density; they preferred to rotate by 30° relative to the underlying G30º lattice. The former is attributed to a strong Pt-G0º interaction whereas the latter only partly to a weak Pt-G30º interaction; the preferential rotation of Pt clusters on G30º is governed not only by the graphene lattice, but largely by an indirect interaction between the Pt substrate and the clusters, likely through the charge transferred from the Pt substrate to graphene.


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1. INTRODUCTION Metal nanoclusters on a graphene monolayer grown on a metal surface are intensively studied because they can serve as a model for carbon-supported metal catalysts, and also as a prospective electrode in solar cells or fuel cells, because of graphene’s atypical electronic properties.1-7 The graphene monolayer has generally a superlattice at surfaces, exhibited as a Moiré pattern in STM images, largely because the lattices of graphene and the underlying metal surfaces are structurally mismatched.8-13 The supperlattice provides a novel template to form a two-dimensional cluster array.2,9,14 Many possible combinations of metal – graphene–metal have been investigated, including Ir,9 Rh,15 Pt,2, 6 W, Re, Fe and Au2 on graphene on Ir(111), Ni16 on Rh(111), Pt17-19, Ru20, Rh, Pd, Co, and Au18 on Ru(0001) and Pt21 on Pt(111). These investigations concentrated on the varied Moiré patterns, the nucleation of deposited metal atoms, the morphologies and possible arrays of grown clusters. The atomic structures of the nanoclusters on graphene are, however, less studied.22 Such structural information is crucial to understand both physical and chemical properties of the clusters, and certainly essential to explore their catalytic behavior. The present work has an aim to remedy this lack of knowledge and to acquire insight into the metal-graphene interaction. We have studied Pt nanoclusters on graphene grown on a Pt(111) single crystal. The graphene film was grown with thermal decomposition of ethylene on Pt(111) at 950 K and Pt clusters on depositing vapors of Pt onto graphene/Pt(111) at 300 K. The graphene on Pt(111) was previously shown to have varied Moiré domains and orientations;12-13, 21 the interaction between graphene and the underlying Pt(111) was considered to be weak, relative to that on other transition-metal surfaces, such as Rh(111) and Ru(0001),10, 12 such that the graphene monolayer was less structurally modified.10,

12-13

The structural characteristics of

graphene/Pt(111) allow an investigation of atomic arrangements of clusters supported on a



free-standing-like graphene and how they alter with the varied domains and orientations. Pt clusters are studied because Pt has been a popular catalyst for the cathodic oxygen reduction23-28 and anodic alcohol oxidation.29-44 Our structural analysis of the graphenesupported Pt clusters could be further utilized to clarify the structure-reactivity correlation for these important reactions on Pt-based electrodes. The morphology and structures of the graphene/Pt(111) and clusters were characterized with scanning tunneling microscopy (STM) and reflection high-energy electron diffraction (RHEED). Furthermore, we used calculations based on density-functional theory (DFT) to investigate the structures of the Pt-graphene systems and the detailed mechanism for the metal-graphene interactions. The results show that graphene monolayers grew and had domains with varied rotations with respect to the underlying Pt (111) lattice, which agrees with earlier work.13, 21 Among them, two graphene domains are dominant: one is closely commensurate with the Pt(111) lattice whereas the other is rotated by 30° with respect to the Pt lattice. Analogously, Pt nucleation in two disparate manners was exhibited: one with larger clusters and smaller cluster density, and the other with smaller clusters and greater cluster density. The Pt clusters had a fcc phase and grew in orientation (111), but were exclusively commensurate with the underlying Pt(111) lattice, despite the two growth modes and two dominant graphene domains. This observation is atypical because, in the graphene domains, the Pt clusters prefer varied rotational angles relative to their underlying graphene lattices. The present report reveals how the graphene-Pt(111) interfacial interaction controls the growth and structures of the Pt clusters. In particular, we note that an indirect interaction between the Pt substrate and the clusters, through induced charge on the graphene, could account largely for the preferential rotation of the Pt clusters.

2. METHODS


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2.1 Experimental section Our experiments were performed in UHV chambers with a base pressure in a 10-10 Torr regime. A Pt(111) sample (MaTeck GmbH) was polished to a roughness less than 30 nm and an orientation accuracy better than 0.1°. To obtain a clean surface, the sample underwent alternative cycles of sputtering and subsequent annealing before each experiment. The cleanliness of the sample was monitored with Auger electron spectroscopy, low-energy electron diffraction and STM. A graphene film was formed on exposing a Pt(111) surface to 300-L (1.0 L = 10-6 Torr．s) ethylene at 950 K and then annealing the sample at 950 K for 10 mins.13 Ethylene gas was dosed by a doser pointing to the sample, with a background pressure 1 – 5 × 10-8 Torr. The grown graphene is mostly one layer thick according to our STM measurements. The sample was then quenched to 300 K for vapor deposition of Pt from ultra-pure Pt rod heated by electron bombardment in commercial evaporators (Omicron EFM 3). The rate of deposition of Pt was fixed about 0.05 ML/min, calculated according to the coverage prepared at 300 K. The coverage was estimated from the volume of the Pt clusters observed with STM; 1.0 ML corresponds to density 1.5 × 1015 atoms/cm2 of fcc Pt(111) surface atoms. STM images (recorded with a RHK UHV 300 unit), constant-current topographies, were obtained at 90 K with a sample bias voltage typically 2.4-2.8 V and a tunneling current 0.81.2 nA. The STM tip consisted of an electrochemically etched tungsten wire. RHEED was performed with an incident electron beam of energy 30 keV at a grazing angle 2-3° to the surface.

2.2 Computational method All the calculations were performed using Vienna Ab initio Simulation Package (VASP)4547

at the DFT level with a 3D periodic boundary condition. The computational method was at



GGA-PW91 level, in which the generalized gradient approximation48 with Perdew-Wang 1991 formulation49 was utilized for the exchange-correlation function, with the projectoraugmented wave method (PAW),50-51 in which the core electron-ion interaction was treated with cost-effective pseudopotential. The plane-wave basis for the valance electrons was cut off at 600 eV of its kinetic energy. The Brillouin-Zone (BZ) integration was computed in the reciprocal space and sampled by the Monkhorst-Pack scheme52 at 0.05 × 2 (1/Å) interval. The Pt(111) surface was constructed with five (3 × 4) metal layers containing a total of 60 atoms, in which the bottom two layers were fixed at the optimized Pt crystal lattice and the top three layers were free to relax. The graphene layers on Pt(111) in commensurate and 30ºrotation configurations (graphene/Pt(111)) were constructed with twelve (3 × 4) and sixteen (4 × 4) units, respectively, to examine the preferred adsorption of Pt single atoms and adlayers. For the calculations of 3D Pt clusters (Pt55, Pt37 Pt22, Pt10 and Pt4) on the substrates, a larger substrate was constructed, by enlarging the metallic slab to (6 × 8), to avoid the artificial interaction from the periodic images. The energetic convergence of 1 × 10-4 eV and a gradient convergence of 1 × 10-2 eV were applied for the structural optimization and energetic calculation, which includes the van der Waals correction by DFT-D3 method. The computed adsorption energies (Eads) of the graphene layer on Pt(111) surface and the Pt adspecies, including single atoms, layers and clusters, on graphene/Pt(111) were defined as E(graphene/Pt(111)) – E(graphene) – E(Pt(111)) and E(Pt adspecies/graphene/Pt(111)) – E(Pt adspecies) – E(graphene/Pt(111)), respectively, in which the E denotes the total energies of graphene/Pt(111), isolated graphene layer, Pt(111) surface, Pt adspecies on graphene/Pt(111) and isolated Pt adspecies. The induced charges upon adsorption of the optimized structures were analyzed by Bader charge.53-55 Some differences between the current and other charge analyses (e.g. the Mulliken charge) are expected; nevertheless, they provide qualitatively similar trends. The


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electronic structures of density of state (DOS) were also examined to clarify the bonding characters of adsorptions.

3. RESULTS AND DISCUSSION 3.1 RHEED measurements of graphene grown on Pt(111) The monolayer graphene film grown on Pt(111), graphene/Pt(111), by chemical decomposition of ethylene has been previously studied, with STM and LEED.12-13, 21 The results showed varied rotational domains of monolayer graphene, in contrast with the single rotational ones grown on other transition metals, such as Ru(0001), Ir(111) and Ni(111).2, 9-10, 12

The structural features were attributed to a greater mismatch of the lattice parameters and a

weaker interaction between graphene and Pt(111).10, 12-13 The present RHEED measurements produced diffraction patterns for graphene and Pt(111) as a function of azimuthal angle, which allow readily quantitative analysis of diffraction intensities and thus measure the extents of the corresponding rotational domains. Figure 1 compares RHEED patterns from a clean Pt(111) single-crystal surface and graphene/Pt(111), obtained at azimuths [-110] and [-211] of Pt(111), referred to azimuthal angles 0˚ and 30˚ respectively. The graphene film was prepared on exposing Pt(111) to ethylene (300 L) at 950 K and then annealing the sample at the same temperature for 10 min; these conditions gave optimal diffraction patterns. The reflection streaks due to the Pt(111) surface became vague after the formation of graphene, such as the (10) beam in Figure 1(a),(c) and the (11) beam in Figure 1(b),(d), whereas the positions of the reflection streaks remained the same. The surface structure of the Pt substrate did not alter with the formation of graphene, confirming a weak interaction between grapheme and substrate (Pt).6-7, 10, 12-13 Two additional reflection streaks appeared at azimuth [-110] when graphene was grown, indicated with red notation (Figure 1(c)). As both graphene and Pt(111) have hexagonal surface lattices



and as similar patterns were observed at the same reciprocal lattice plane, the patterns are assigned to (10) and (20) reflection beams from the graphene film commensurate structurally with the Pt(111) surface. Consistent with that assignment, the (11) reflection beam from the graphene film appeared at azimuth [-211], shown in Figure 1(d) (red notation). Nevertheless, reflection beams (10) and (20) for graphene/Pt(111) are shown also at azimuth [-211] (blue notation in Figure 1(d)); at azimuth [-110], reflection beam (11) from graphene (likely mixed with the (20) signal from Pt(111)) appeared consistently. This observation indicates that a separate graphene domain was formed with rotation 30° with respect to the commensurate graphene or Pt lattice. Figure 1(e) illustrates schematically the observed diffraction streaks in reciprocal lattice space. The black and red circles denote the diffraction streaks (top view) from Pt(111) and the commensurate graphene domain, respectively; the same diffraction features appear at the same azimuthal angles. The blue circles indicate the diffraction streaks from the graphene domain rotated by 30° relative to the Pt lattice; the pattern differs from that of the commensurate grapheme by azimuthal angle 30°. In agreement with preceding studies,12-13, 21 the above results imply that graphene grew with varied rotations on Pt(111). To analyze quantitatively the extents of the rotationally varied graphene domains, we measured RHEED patterns from graphene/Pt(111) at varied azimuthal angles and plotted the angle-dependent intensities of characteristic patterns. Figure 1(f) shows the intensities of reflection rods (10) (squares) and (11) (triangles) as a function of azimuthal angle. Both (10) and (11) signals persist for all azimuthal angles (from 0° to 30°), implying that all rotations of the graphene domains are allowed, whereas they attained a maximum at 0° or 30°, implying two rotations are dominant. The red symbols represent signals dominantly from the graphene film closely commensurate with Pt(111), denoted as G0º (graphene(0001)[1-100]//Pt(111)[-110]), corresponding to small angles of rotation between the graphene and Pt lattices; in contrast, the blue symbols represent those from that


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rotated

by

about

30°

relative

to

the

commensurate

one,

denoted

as

G30º

(graphene(0001)[1000]//Pt(111)[-110]), corresponding to large angles of rotation between the graphene and Pt lattices. G0º was indicated earlier to be more corrugated and had a greater Moiré periodicity, whereas G30º was flatter and had a smaller Moiré periodicity.13, 21 The domination of the two graphene domains was also reflected in the preceding and present (Figure S1, Supporting Information) LEED measurements for samples prepared under similar conditions. The lattice parameter of the grown graphene was estimated, according to the RHEED patterns, to be 2.45 ± 0.03 Å, essentially the same as that (2.46 Å)56-57 for a freestanding graphene. The comparable lattice parameter adds weight to the preceding argument that the graphene-Pt(111) interaction is weak and that the electronic structure of the grown graphene is comparable to that of a free-standing graphene.10, 12-13, 21

3.2 Pt nanoclusters on graphene/Pt(111) Pt nanoclusters were grown with deposition of Pt vapor onto graphene/Pt(111) at 300 K. We characterized the morphologies and structures of the supported Pt clusters with STM and RHEED. The STM measurements showed that the grown Pt clusters had a mean diameter from 2.3 to 3.5 nm and height from 0.68 to 0.94 nm evolving with the coverage (0.1 – 2.0 ML) before the coalescence of the clusters. More notably, two distinct nucleation features were identified: one had larger clusters but smaller cluster density, and the other smaller clusters and greater cluster density. Figure 2 exemplifies the STM image for Pt clusters (1.0 ML) on graphene/Pt(111). The cluster density in the upper part of Figure 2(a) is about one third that in the lower part (3.31 × 1012 cm-1 vs 1.03 × 1013); in contrast, the clusters in the upper part have a broader distribution of both diameter and height (Figure 2(b) vs Figure 2(d) and Figure 2(c) vs Figure 2(e)), and significantly a greater mean size: diameter 3.2 nm vs 2.7 nm and height 0.94 nm vs 0.61 nm. The area for either larger clusters (denoted as regime A)



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or smaller ones (denoted as regime B) shares about half the surface. The distinct nucleation features were persistently observed for varied Pt coverages on the graphene surface. This result indicates that regimes A and B correspond to graphene surfaces of two structurally distinct kinds and that in regime B clusters form more readily. They correspond to the above G0º and G30º. Regime B is likely referred to more corrugated G0º (than regime A to less corrugated G30º) as a corrugated surface typically provides more preferential nucleation sites or stronger bondings (to adspecies),21 leading to facile formation of new clusters. Further evidence for this assignment comes from DFT modeling below. The clusters in regime A show a bimodal distribution of height and a broad distribution of diameter (Figure 2(b),(c)), as two-dimensional (2D) Pt clusters with height 0.2 – 0.3 nm and smaller diameter 1.2 - 1.8 nm persistently appeared for Pt coverage ≤ 1.0 ML. They are evident in STM images (Figure 2(f)) and share a fraction 10 – 20 % of total cluster number in regime A. The 2D clusters imply a distinct cluster-graphene interaction and thus another nucleation mode in regime A. Similar 2D clusters were observed for Au on graphene/Ru(0001).18

In

comparison

with

previous

work

on

Pt

clusters

on

graphene/Pt(111),21 the present clusters have a smaller diameter (for instance, 1.0 ~ 3.0 nm vs 3.0 ~ 5.0 nm at 0.2 ML) and a comparable height but a greater cluster density. The greater cluster density implies more preferential nucleation sites or easier formation of clusters on the present surface. As our IRAS spectra of CO adsorbed as a probe showed no CO on the graphene surface even at 100 K, the nucleation sites do not correspond to reactive defects, such as point defects, on the surface. The preferential growth of the clusters along the domain boundaries, mentioned in a previous report,21 was observed also in the present work  the cluster chains near the middle bottom and right top corner in Figure 2(f). Nevertheless, the diffusion of Pt atoms on the graphene surface must be reasonably rapid because the present mean diameter attained 2.2 nm even at a small coverage such as 0.1 - 0.2 ML. No multilayer


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graphene was indicated; all steps observed, such as at the right side of Figure 2(f), have a height about 2.2 Å, corresponding to the steps at the Pt(111) surface. Figure 3 exemplifies the RHEED patterns for Pt clusters on graphene/Pt(111) obtained at azimuths [-110] and [-211] of the Pt(111) substrate. At 0.5-ML Pt, the reflection streaks due to the graphene remained visible, as indicated in Figure 3(a),(b); the additional diffraction spots are ascribed to structurally ordered Pt clusters. With increased Pt coverage, the diffraction signals from the graphene vanished whereas those from the clusters became enhanced, as shown in Figure 3(c),(d). These patterns indicate that the Pt clusters had an fcc phase, and grew with their facets (111) parallel to the graphene surface and their axes [-110] or [0-11] along direction [-110] of the Pt(111) substrate. The corresponding points of the reciprocal lattice at the two azimuths are plotted schematically in Figure 3(e),(f). As Pt(111) facets have six-fold symmetry, the Pt clusters rotated by 60° about direction (111), such as those with axes [-110] or [0-11] along direction [-110] of the Pt(111) substrate, are structurally the same with respect to the substrate. We note that the graphene surface consisted of both G0 and G30º (Figure 3(a),(b)) but the diffraction patterns do not indicate Pt clusters of two kinds rotated relative to each other by 30°. To investigate whether the rotation of the clusters altered with the graphene, we measured the RHEED patterns for Pt clusters/graphene/Pt(111) at varied azimuthal angle. Figure 4(a),(b) plot the intensities of diffraction spots 311/220 (squares) and 240/13-1 (triangles) from Pt clusters (0.5 and 4.0 ML), as a function of azimuthal angle. The spots 311/220 are characteristic diffraction spots in reciprocal lattice planes [0-11]/[-110], and 240 and 13-1 are those in reciprocal lattice plane [-211] (Figure 3). The trend is notably the same for the clusters of varied size. The intensities of spots 311 and 220 attained a maximum about 0° (azimuth [-110]), but decreased near the background level above 15°; likewise, those of 240 and 13-1 reached a maximum about 30° (azimuth [-211]), but decreased near the



background level below 15°. The angle-dependent intensities of diffraction spots from the clusters match, exclusively, those of the deflection streaks from G0º (Figure 1(f)). This result indicates that the structurally ordered Pt clusters are mostly commensurate with G0º or the Pt(111) surface: their facets (111) are parallel to the graphene surface and their axes [-110] ([0-11]) are along direction [1-100] of G0º or along direction [-110] of the Pt(111) substrate, as illustrated schematically in Figure 4(c) (denoted as Pt0º/G0º; 0°° in Pt0º indicates the angle relative to the Pt(111) lattice). Alternatively or simultaneously, the Pt clusters on G30º could have a rotation 30° with respect to the underlying G30º lattice, illustrated schematically in Figure 4(d) (denoted as Pt0º/G30º), which also have the Pt clusters structurally commensurate with the Pt(111) surface and produce angle-dependent diffraction intensities as observed in Figure 4(a),(b). The results hence indicate that Pt30º/G0º and Pt30º/G30º, for which the Pt clusters have rotation 30° with respect to the Pt(111) lattice, do not exist; G0º and G30º are structurally different, so favor rotationally different Pt clusters. DFT calculations presented below explain in detail the atypical behaviors resulting from the Pt-graphene interfacial interaction and even an indirect interaction between the Pt clusters and Pt(111) substrate.

3.3 DFT calculations for Pt nanoclusters on graphene/Pt(111) We examine first the adsorption structures for monolayer graphene on a Pt(111) surface. The stable structures for graphene commensurately (G0°°) and 30º-rotated (G30°°) adsorbed on the Pt(111) surface are shown in Figure 5, with the corresponding Eads, charge and DOS results. The graphene layers with other rotations failed to adsorb or retain their structures upon adsorption, attributable to a lattice mismatch between graphene and the Pt(111) surface. Shown in Figure 5(a), the adsorption of G0°° has a much stronger Eads (-0.63 eV/unit),58 a smaller distance between the graphene layer and Pt(111) surface (2.18 Å) and denser induced charges than those for G30°° (-0.19 eV/unit and 3.63 Å). The shapes of the induced charges


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imply also that G0°° corrugates slightly and features an electronic tetrahedral (sp3) hybridization; the corrugation is more evident from the side view in direction [-110] of the Pt(111) surface (Figure S2(a)). The thinning-induced charge in G30°° such that the positive and negative ones were separately distributed on the Pt(111) and graphene layer sides, respectively, implies a rather weak van der Waals interaction; G30°° thus remains intact and flat. Additionally, the top views of the charge analysis (the lower part of Figure 5(a)) found that the charge distributions are highly ordered on both G0°° and G30°°, implying that the preferred adsorption orientations of graphene (0o and 30o) are involved with not only the Pt (111) lattice but also their charge distributions. The DOS analysis shown in Figure 5(b) confirms the above results. The strong Eads of G0°° corresponds to a lower energetic d-band center (-2.99 eV) for the bonding band whereas the weak van der Waals interaction of G30°° is related to a higher energetic d-band center (-2.36 eV) and has a d-band shape almost identical to that of a clean Pt(111) surface; the DOS of the graphene p band in the antibonding regime agrees with the Eads results (Figure S2(b)). The computational results agree with earlier STM observations that the graphene with a small rotational angle relative to the underlying Pt(111) lattice is more corrugated than that with a large rotational angle.13, 21 To examine Pt nanoclusters on graphene/Pt(111), we first placed a Pt atom on G0°° and G30°° to find their preferential adsorption sites for the formation of clusters. On G0°°, the Pt atom adsorbs stably on both top and hollow sites, with strong Eads, -1.26 and -1.59 eV, respectively; likewise, these two sites are preferred on G30°°, but with rather weak Eads, -0.26 eV for either site, shown in Figure 6(a). The strong Eads on G0°°, arising largely from the corrugated graphene surface, implies for deposited Pt atoms have a smaller critical size for nucleation, a smaller diffusion length, and therefore facile formation of Pt clusters, which corresponds to a greater cluster density and smaller clusters, as observed in regime B. Based on the preferential adsorption sites, we further added a Pt layer on G0°° and G30°°. Two possible



structures, Pt0°°/G0°° and Pt0°°/G30°°, were indicated: these Pt layers adsorb, without structural distortion, on the graphene surfaces stably and uniformly. For Pt0°°/G0°° (Figure 6(b)), its Pt adlayer is commensurate with both the graphene sheet and Pt(111), and has a strengthened Eads (-2.15 eV/unit)58 and dense induced charge; its graphene layer becomes even more corrugated as the tetrahedral hybridization is further induced by the Pt adlayer. For Pt0°°/G30°°, its Pt adlayer is rotated by 30° with respect to the graphene sheet but commensurate with the Pt(111) surface; it has a rather weak Eads (-0.14 eV/unit)58 via a van der Waal interaction and sparse induced charge. The Eads results agree with the above single-atom ones and are confirmed by the DOS analysis (Figure S3) in that the d band center of the Pt adlayer in Pt0°°/G0°° has a lower energy (-2.91 eV) whereas that in Pt0°°/G30°° has a higher one (-2.33 eV). For Pt30°°/G0°° and Pt30°°/G30°°, in which the Pt adlayers are rotated by 30º relative to the Pt(111) lattice, the stacking of Pt atoms is unstable; their structures become disordered (Figure S4(a)). Furthermore, we placed a series of 3D Pt clusters with a radius ranging from 1.6 (Pt4) to 7.5 Å (Pt55) on both G0°° and G30°°; the Pt atoms in the 3D clusters were stacked based on the stable 2D structures above (Pt0°°/G0°° and Pt0°°/G30°°). The largest ones, Pt55, on both surfaces are shown in Figure 6(c), for example; the others are shown in Figure S5. The optimized structures reveal that all these Pt clusters adsorb stably and with alike structures on both surfaces － they have a fcc phase and grow in (111) orientation; on either G0°° or G30°°, they are structurally commensurate with the underlying Pt(111), like the above Pt adlayers. The Pt clusters on G0°° have much stronger Eads and corrugate further the graphene surfaces, whereas the weakly adsorbed Pt clusters on G30°° have greater cluster-graphene distances and rather flat graphene surfaces (Figure 6(c)). These computed results have corroborated the experimental observation on the structures and orientations of the Pt clusters.


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The above results indicate that Pt clusters adsorb commensurately or 30º-rotated on the graphene sheet, like graphene layers on Pt(111) (G0°° and G30°°). Nevertheless, in either case, the Pt clusters are commensurate with the Pt(111) surface beneath. That no Pt30°°/G0°° exists is rational as the Pt cluster-G0°° interaction is strong and a commensurate Pt cluster on G0°° is strongly favored, but that condition does not apply to Pt30°°/G30°°. The Pt(111) surface beneath controls the rotation of the Pt adspecies not only through the varied structures of the grown graphene layers (corrugated G0°° and flat G30°°) but also through another indirect interaction. Such an indirect interaction is confirmed on examining the adsorption of a Pt layer on freestanding monolayer graphene, with no Pt(111) substrate. The Pt layer is allowed to adsorb on the free-standing graphene sheet in an ordered structure either commensurate with (Pt0°°/G; 0°° in Pt0º indicates the angle relative to the graphene lattice) or rotated by 30º with respect to the graphene lattice (Pt30°°/G) (Figure S4(b)). Without the Pt(111) surface beneath, the adsorption becomes weak (-0.19 and -0.10 eV/unit),58 indicating a van der Waals interaction. Such a Pt adlayer-graphene interaction (Pt0°°/G or Pt30°°/G) resembles that of Pt30°°/G30°° or Pt0°°/G30°°, whereas, on G30°° (having Pt(111) surface as a support), only the adsorption of 30º-rotation with respect to the graphene lattice, Pt0°°/G30°°, is allowed. The comparison indicates that the Pt(111) substrate plays a role in determining the rotation of the Pt clusters, despite the lack of direct contact. The orderly distributed charges of Pt0°°/G30°° (Figure 6(b)) suggests that its Pt adlayer has matched the graphene lattice － both 0o and 30o rotations are allowed, and also the charge distribution combining those from the graphene and beneath Pt(111) (Figure 5(a)) － the 0o rotation is exclusively allowed. Accordingly, the charge induced on the graphene sheet by the Pt(111) substrate governs the rotational angle of the Pt clusters. Likewise, the significantly induced charge in Pt0°°/G0°° (Figure 6(b)) could contribute to the preferential rotation of the clusters.



In summary, the computational results show that regime B, having a greater cluster density and smaller Pt clusters commensurate structurally with the graphene sheet, corresponds to the more corrugated G0°° surface and its stronger Eads for Pt clusters (Pt0°°/G0°°). In contrast, the flat G30°° surface and its weaker Eads for Pt clusters (Pt0°°/G30°°) explain a smaller cluster density and larger Pt clusters rotated by 30° relative to the graphene sheet in regime A. Moreover, the induced charge, from the Pt(111) surface to the graphene sheet, could control the preferential rotation of the Pt clusters; only the clusters commensurate with the underlying Pt(111) surface (Pt0°°/G0°° and Pt0°°/G30°°) adsorbed in an orderly and uniform manner on the graphene sheet, as indicated by the RHEED patterns.

4. CONCLUSION With STM, RHEED and DFT calculations, we investigated the atomic structures of Pt nanoclusters on graphene/Pt(111). The monolayer graphene was prepared on decomposing ethylene on Pt(111) at 950 K and Pt clusters on depositing vapor of Pt onto graphene/Pt(111) at 300 K. The graphene had two predominant domains: one was structurally commensurate with the Pt(111) lattice (G0°°), having a small angle of rotation between the graphene and the Pt lattice, and the other was rotated by 30° with respect to the Pt lattice (G30°°). G0°° involved electronic tetrahedral (sp3) hybridization and had a greater Eads on Pt(111), so corrugated a little; G30°° had a rather weak Eads, through a van der Waals interaction, so was flat. The Pt clusters grown on the graphene had a mean diameter 2.3 – 3.5 nm and height 0.68 – 0.94 nm evolving with the coverage (before the coalescence of the clusters). They had a fcc phase, grew with facets (111) parallel to the graphene surface, but showed disparate nucleation modes and rotational configurations in the two graphene domains. On corrugated G0°°, the clusters were smaller, had a narrow size distribution and greater cluster density, like the growth on a surface with abundant preferential nucleation sites; they were preferentially


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commensurate with the G0°° lattice (with their axes [-110] (or [0-11]) along direction [1-100] of G0º). In contrast, on flat G30°°, the clusters were larger, had a broad size distribution and smaller cluster density, like that on a defect-free surface; they preferred to rotate by 30° with respect to the G30°° lattice (with their axes [-110] (or [0-11]) along direction [1000] of G30º). Such growth modes and a rotational preference result primarily from the characteristic interactions of Pt clusters with corrugated G0°° and flat G30°°. Notably, the indirect interaction between the Pt substrate and the clusters on G30°°, likely through the charge transferred from the Pt substrate to the graphene sheet, controls the preferential rotation of the Pt clusters.

ASSOCIATED CONTENT Supporting Information Additional LEED patterns for graphene/Pt(111) and DFT simulations. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Authors *E-mail for M.-F. Luo: [email protected] *E-mail for J.-H. Wang: [email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS National Science Council of Taiwan provided support (NSC-103-2112-M-008-014-MY2) for the work. CPU time at Taiwan’s National Center for High-performance Computing (NCHC)



and Department of Applied Chemistry in Private Chinese Culture University (PCCU) was greatly appreciated. We also thank Yu-Cheng Wu for technical support.

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Captions of figures Figure 1. (a)-(d) RHEED patterns for clean Pt(111) and graphene/Pt(111) surfaces; (a) and (c) show patterns obtained at azimuth [-110], and (b) and (d) show those at azimuth [-211]. (e) schematic for observed diffraction streaks in reciprocal lattice space from the top view and (f) plot of intensities of reflection streaks (10) (squares) and (11) (triangles) as a function of azimuthal angle. In (f) red and blue notations indicate signals predominantly from the graphene structurally commensurate with and rotated by 30° relative to the Pt lattice, respectively. The graphene was grown on exposing Pt(111) to ethylene (300 L) at 950 K and then annealing the sample at the same temperature for 10 min.

Figure 2. (a) STM image of Pt (1.0 ML) clusters on graphene/Pt(111); (b),(c) histograms of diameters and heights for larger clusters at the upper part of the STM image and (d),(e) those for smaller clusters at the lower part. (f) STM image of Pt (0.2 ML) clusters on graphene/Pt(111). The boundary of regimes A and B in (a) is indicated by a dash line. The Pt was deposited on graphene/Pt(111) at 300 K.


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Figure 3. (a)-(d) RHEED patterns for Pt (0.5 ML and 4.0 ML) clusters on graphene/Pt(111) surfaces; (a) and (c) show patterns obtained at azimuth [-110], and (b) and (d) show those at azimuth [-211]. (e) and (f) show schematic reciprocal-lattice points at incident directions [110] and [-211], respectively. The Pt was deposited on graphene/Pt(111) at 300 K.

Figure 4. Plots of intensities of diffraction spots 311 and 220 (squares) and 240 and 13-1 (triangles) from Pt clusters at (a) 0.5 and (b) 4.0 ML, as a function of azimuthal angle. Schematic diagrams illustrating (c) Pt clusters structurally commensurate only with G0°°, Pt(111) [-110] clusters//graphene(0001)[1-100]//Pt(111)[-110], and (d) those rotated by 30° with respect to the G30°° lattice, Pt(111) [-110] clusters//graphene(0001)[1000]//Pt(111)[-110]. The Pt was deposited on graphene/Pt(111) at 300 K.

Figure 5. (a) Optimized structures, induced charges and related Eads of graphene commensurately and 30º-rotated adsorbed on a Pt(111) surface (G0°° and G30°°, respectively). The rotations of adsorbed graphene and the Pt(111) surface are noted with blue and black arrows and numbers, respectively; C and Pt atoms are represented with red and white balls, respectively. Induced charges upon graphene adsorption inside the supercell are plotted with yellow/blue spheres for +/– 0.05 |e| of densely charged G0°° and +/– 0.005 |e| of sparsely charged G30°°. (b) DOS analysis for the d bands of surface Pt atoms in G0°° and G30°° are shown as red and blue lines, respectively, referred to that of a clean Pt(111) surface (black line). Their d-band centers are marked with short vertical lines and indicated with numbers in the legend. The corresponding DOS analysis of their p bands is shown in Supporting Information in Figure S2(b).



Figure 6. (a) Top and side views of optimized structures and related Eads of a single Pt atom adsorbed on two preferential sites, top and hollow sites, on G0°° (left) and G30°° (right) surfaces. (b) Optimized structures, induced charges and related Eads of Pt monolayers flatly and orderly adsorbed on graphene/Pt(111) surfaces, Pt0°°/G0°° (left) and Pt0°°/G30°° (right); the Pt adlayer on G0°° is commensurate with the G0°° lattice (Pt0°°/G0°°) whereas that on G30°° is rotated by 30o relative to the G30°° lattice (Pt0°°/G30°°; 0°° in Pt0º indicates the angle relative to the Pt(111) lattice). (c) Top and side views of optimized structures and related Eads of the 3D Pt clusters, Pt55, on G0°° (left) and G30°° (right). The adsorption for the other smaller 3D Pt clusters, Pt37 Pt22, Pt10 and Pt4, is shown in Figure S5. The rotations of the Pt adlayer (or cluster) and underlying Pt(111) (grey balls) are noted with black arrows and numbers; those of graphene (red balls) domains are noted in blue. In (b), induced charges are plotted with yellow/blue spheres for +/– 0.05 |e| of densely charged G0°° and +/– 0.005 |e| of sparsely charged G30°°.


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Figure 1

(a) 0˚[]

(b) 30˚ [11]

Pt(111)

Pt(111)

-20

-10

10

20

-1-1

000

000

(c) 0˚

(d) 30˚

Graphene/Pt(111)

Graphene/Pt(111)

000

(e)

11

1010 112020 Pt G GPt G

000

Pt(111) substrate Graphene Graphene R30˚

30˚ 15˚ 0˚ 10 10 Pt G

1120 20 G Pt G


10 11 1120 G Pt G G


(f) Graphene/Pt(111) 8 Graphene 11

Intensity/a.u.

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

Graphene 10 reflection rod Graphene 11 reflection rod R0˚ R30˚ Graphene 10

6 4 2 0 10

Graphene 11 R30˚

Graphene 10 R30˚

20

30

20

10

0

-10

-20

Azimuthal angle/degree


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Figure 2 (a) (b)

25 0 0.0

1.2

Regime A

300

3.6 2.4 Diameter/nm

200 100

10 nm

25

400

(d)

0 0.0

(c)

0 0.0

4.8

Counts

Regime B

50

Counts

Counts

50

Counts

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


1.2

2.4 3.6 4.8 Diameter/nm

0.4

0.8 1.2 Height/nm

1.6

(e)

200 0 0.0

(f)

10 nm


0.4

0.8 1.2 Height/nm

1.6


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Figure 3

(a) 0˚ []

(b)

0.5 ML Pt

0.5 ML Pt

11] 30˚ [

240

311 220

13 G 10

000

(c)

GG 11 20

000

(d)

0˚

G 10

G 11

30˚

4.0 ML Pt

4.0 ML Pt

240

311 220

131

000

(e)

000

] ] [ [

000

(f)

11] [ 204

240

000


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Figure 4

(a)

Intensity/a.u.

311 and 220 diffraction spots diffraction spots 240 and 13

0.5 ML Pt on graphene/Pt(111)

8 6 4 2 0 10

20

30

20

10

0

-10

-20


(b) 311 and 220 diffraction spots diffraction spots 240 and 13

4.0 ML Pt on 8 graphene/Pt(111)

Intensity/a.u.

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


6 4 2 0 10

20

30

20

10

0

-10



-20


(c) PtCluster

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(d) (111)

PtCluster

(111)

Graphene PtSubstrate

(111)

Graphene PtSubstrate


(111)

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Figure 5

Figure 6




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TOC Graphic A weak Pt-G interaction, larger clusters

Regime A

Regime B

A strong Pt-G interaction, smaller clusters Counts

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10 nm


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Atomic Structures of Pt Nanoclusters Supported on Graphene Grown

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