Graphene-Templated Growth of Pd Nanoclusters - The Journal of

Article pubs.acs.org/JPCC

Graphene-Templated Growth of Pd Nanoclusters Karin Gotterbarm,§,† Christian Steiner,§,‡ Carina Bronnbauer,† Udo Bauer,† Hans-Peter Steinrück,† Sabine Maier,*,‡ and Christian Papp*,† †

Lehrstuhl für Physikalische Chemie II, Universität Erlangen-Nürnberg, Egerlandstraße 3, 91058 Erlangen, Germany Department für Physik, Universität Erlangen-Nürnberg, Erwin-Rommel-Straße 1, 91058 Erlangen, Germany

‡

S Supporting Information *

ABSTRACT: Graphene grown on Rh(111) was used as a template for the growth of Pd nanoclusters. Using high-resolution synchrotron radiation-based X-ray photoelectron spectroscopy, we studied the deposition of Pd on corrugated graphene in situ. From the XP spectra, we deduce a cluster-by-cluster growth mode. The formation of clusters with 3 nm diameter was confirmed by low-temperature scanning tunneling microscopy measurements. The investigation of the thermal stability of the Pd particles showed three characteristic temperature regimes: Up to 550 K restructuring of the particles takes place, between 550 and 750 K the clusters coalesce into larger agglomerates, and finally between 750 and 900 K Pd intercalates between the graphene layer and the Rh surface.

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INTRODUCTION Graphene-palladium composite materials have recently been investigated1 with regard to numerous applications, for example, in catalytic C−C cross coupling2,3 and oxidation reactions4,5 as well as for hydrogen sensing.6 Graphene from exfoliated graphite is considered to be an excellent support material for the dispersion and stabilization of nanoparticles because of its large specific surface area and chemical inertness. High-quality graphene can be readily produced by chemical vapor deposition (CVD) on various transition-metal surfaces.7,8 On lattice-mismatched substrates, such as Pt(111), 9 Ir(111),10,11 Rh(111),12,13 and Ru(0001),13−15 Moiré superstructures are formed. The spatially varying interaction strengths between graphene and the substrate for different positions in the Moiré unit cell result in a periodic buckling of the graphene sheet.16 It has been shown that such corrugated graphene sheets are outstanding templates for the fabrication of nanocluster arrays, with a narrow size distribution of the clusters.10,17−19 Among the mentioned substrates, Rh(111) can be considered to be an intermediate case between strong and weak interaction of the substrate with the graphene layer, leading to a complex corrugation pattern.12,13,16 While the behavior of several metals evaporated onto graphene on Ir(111) and Ru(0001) has been studied,17−21 only a few studies exist for graphene on Rh(111): Sicot et al. investigated the deposition of Ni and Fe on graphene on Rh(111).22,23 Ni forms well-dispersed nanoclusters upon deposition at 150 K, while it grows in flat, triangular islands when deposited at room temperature.22 In contrast, Fe deposited at room temperature forms bulky, spherical clusters that are preferentially located at graphene domain boundaries.23 Upon annealing to 670 to 870 K, both metals intercalate underneath the graphene sheet, forming flat islands of monatomic height.23 We now present a study of Pd deposited on graphene on Rh(111). The preparation of well-defined noble metal clusters with a narrow © 2014 American Chemical Society

size distribution appears particularly interesting as model systems for various catalytic reactions.17 The growth and thermal stability of the Pd clusters were studied in detail by high-resolution synchrotron radiation-based X-ray photoelectron spectroscopy (HR-XPS), accompanied by scanning tunneling microscopy (STM) measurements for selected preparation steps.

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EXPERIMENTAL METHODS All XP spectra were measured at beamline U49/2-PGM1 of the synchrotron radiation facility BESSY II at the HelmholtzZentrum Berlin. The experiments were performed in a transportable UHV apparatus described in detail elsewhere.24 The setup consist of two chambers: The preparation chamber is equipped with low-energy electron diffraction (LEED) optics, a quartz crystal microbalance (QCM), and an electron beam evaporator installed opposite to the QCM. The hemispherical electron energy analyzer, a quadrupole mass spectrometer (QMS), and the supersonic molecular beam are connected to the analyzer chamber. Additionally, a second electron beam evaporator for Pd evaporation during XPS measurements is mounted in the analyzer chamber. The sample was a cylindrical Rh(111) crystal with 10 mm diameter, 3.0 mm thickness, and an orientation accuracy of 0.1°. The sample is cooled with liquid nitrogen and heated resistively. With this setup, sample temperatures between 100 and 1400 K are achievable. The sample temperature is measured with two independent type-K thermocouples spotwelded to the Rh crystal. Spectra were recorded either at normal emission (0°) or with an angle of 69° to the analyzer to allow for evaporation of the metal while performing XPS measurements. Pd 3d5/2 spectra were recorded Received: May 28, 2014 Revised: July 3, 2014 Published: July 5, 2014 15934

dx.doi.org/10.1021/jp5052563 | J. Phys. Chem. C 2014, 118, 15934−15939

The Journal of Physical Chemistry C

Article

with a photon energy of 450 eV and a total energy resolution of 180 meV. C 1s and Rh 3d5/2 spectra were recorded with 380 eV photon energy and a total energy resolution of 170 meV. The acquisition time was 10 s for Pd 3d5/2 and 9 s for C 1s and Rh 3d5/2 core-level spectra. All spectra are referenced to the Fermi level and corrected with a linear background. Pd 3d5/2 and C 1s signals were fitted with an asymmetric Doniach−Sunjic function convoluted with a Gaussian function.25 Graphene was grown by CVD using 2 × 10−8 mbar propene at 920 K.26 Pd was deposited on the sample by electron beam evaporation at 150 K or even lower temperatures. Pd coverages were calibrated with the QCM. One ML of Pd thereby corresponds to one Pd atom per Rh surface atom. STM measurements were performed with an Omicron lowtemperature STM operated at a base pressure lower than 10−10 mbar. The same Rh(111) crystal as in the XPS experiments was used, and similar cleaning procedures and CVD conditions for the graphene growth were applied. The graphene quality was monitored with LEED and STM. Pd was evaporated from an electron beam evaporator with the sample held at room temperature. All STM images presented in this paper were acquired in the constant current mode at a sample temperature of ∼78 K. The STM images were analyzed using the WSxM software.27

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RESULTS AND DISCUSSION After producing graphene on the Rh(111) surface by CVD (for details, see ref 26), we subsequently deposited Pd by electron beam evaporation. Figure 1a shows selected Pd 3d5/2 spectra recorded at an emission angle of 69° (with respect to the surface normal) during the Pd evaporation onto graphene on Rh(111) at 150 K. The evaporation process is illustrated in Figure 1c. With increasing coverage, a broad asymmetric peak evolves at 335.1 eV, with a shoulder at 335.5 eV. At 0.1 ML Pd (blue spectrum), the shoulder is clearly visible and the full width at half-maximum (fwhm) of the entire Pd 3d5/2 signal is 1.1 eV. For higher coverages, the broad peak becomes sharper and the shoulder less distinct. At 0.3 ML (green spectrum), the fwhm is reduced to 1.0 eV, and at 0.7 ML (red spectrum) it is reduced to 0.9 eV. The spectra are described very well with three Pd contributions, in agreement with previous work by Gladys et al.28 The fit of the red spectrum with the highest coverage (0.7 ML Pd) is shown in Figure 1b. The green contribution at 335.1 eV is assigned to Pd atoms in the bulk of the particles; the gray signals at 334.8 and 335.5 eV denote the surface contributions S1 and S2, respectively. For coverages smaller than 0.1 ML Pd (blue spectrum in Figure 1a), the bulk signal is small in comparison with the surface signals. With increasing coverage, the bulk signal rises more than the surface signals. This development is clearly evident from the inset in Figure 1b, where the ratio of the bulk to the surface signal areas is plotted against the Pd coverage: Up to 0.2 ML, the ratio increases steeply; thereafter, the slope decreases and finally saturates at a bulk-to-surface ratio of 0.8; 90% of this saturation value is reached for a Pd coverage of 0.3 ML. (Note that at normal emission (0°) a saturation value of 0.9 is obtained.) Similar Pd bulk-to-surface ratios were obtained for several independent Pd depositions onto graphene on Rh(111), with Pd coverages from 0.4 to 1.2 ML (data not shown). Because the bulk-to-surface ratio is a measure for the average cluster size, its saturation suggests that further deposition of Pd leads to an increase in cluster number rather than average cluster size. These findings indicate a template-assisted cluster-by-cluster

Figure 1. Deposition of Pd on graphene/Rh(111) at 150 K. (a) Selected Pd 3d5/2 spectra measured during electron beam evaporation of Pd; spectra were measured with an emission angle of 69° with respect to the surface normal. (b) Fit of the spectrum with highest coverage shown in panel a. Inset: bulk-to-surface ratio as a function of Pd coverage; data points are calculated from the fit areas of the experiment shown in panel a. (c) Illustration of the evaporation process.

growth mode. Similar behavior has been found for comparable systems such as Ir, Pt, Au, and Fe clusters on graphene/ Ir(111)17 and Pt clusters on graphene/Ru(0001).19 To confirm the formation of clusters and to gain more detailed information about their size and geometry, we performed STM measurements. Figure 2a shows an STM image of Pd deposited on graphene/Rh(111) at room temperature. The image shows clusters of similar size covering around one-third of the graphene surface. Neighboring clusters do not agglomerate. The clusters exhibit apparent heights of 6.6 ± 1.0 Å independent of the applied bias voltage, which corresponds to roughly 3 fcc (111) planes of Pd. The height and diameter distributions of the clusters are shown in Figure 2b,c. The clusters are 3.0 ± 0.3 nm in diameter. (Note that the actual size could be somewhat smaller due to tip effects.) A white rhombus in the inset indicates the supercell of the Moiré superstructure. The diameter of the clusters resembles the dimensions of the Moiré unit cell, which has a periodicity of 2.9 ± 0.05 nm.22 Thus, each cluster occupies one unit cell, as has been reported for clusters on graphene on Rh(111)22 and on 15935



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Figure 2. (a) STM measurement of Pd on graphene/Rh(111) deposited at room temperature. Pd adsorbs as clusters, which do not agglomerate. In the inset, the supercell of the Moiré superstructure is indicated by a white rhombus. (b,c) Height and diameter distribution of 76 clusters, respectively. The apparent height of the clusters is 6.6 ± 1.0 Å. The diameter of the clusters along the fast scan direction is 3.0 ± 0.3 nm. Tunneling parameters: U = 5 mV, I = 9 pA.

Ir(111).17 From high-resolution images we conclude that the clusters preferably adsorb near steps and defect sites in the graphene, as seen from the underlying Moiré pattern. Because it is possible to manipulate the clusters by decreasing the tip− sample distance (data not shown), we conclude that they are loosely bound to the substrate. Furthermore, the Pd clusters do not intercalate, when evaporated at room temperature, which is in agreement with the results found for Ni and Fe on graphene/Rh(111).23 To investigate the thermal stability of the Pd clusters, we annealed a sample with a nominal coverage of 0.3 ML deposited at 150 K in 100 K steps up to 850 K. After each annealing step, all relevant core levels were measured. Selected Pd 3d5/2 and C 1s spectra are shown in Figure 3a,b, respectively. Additional O 1s/Pd 3p and Rh 3d5/2 spectra are shown in the Supporting Information, Figure S2. The total signal intensities of the C 1s, Rh 3d5/2, and Pd 3d5/2 core levels during the entire experiment are displayed in Figure 4a−c, respectively. In a complementary experiment, a sample with 0.7 ML Pd deposited at 150 K was first annealed to 500 K and subsequently to 900 K in 50 K steps. Again, all relevant core levels were measured after each annealing step. Selected C 1s spectra of this experiment are shown in Figure 3c. The C 1s, Rh 3d5/2, and Pd 3d5/2 signal intensities are shown in Figure 4d−f, respectively. For better comparison with Figure 4f, the Pd 3d5/2 intensity displayed in Figure 4c was multiplied by two. Selected Pd 3d5/2 spectra during the annealing of the 0.7 ML sample are shown in the Supporting Information, Figure S3. In the following, the Pd 3d5/2 and C 1s spectra as well as their signal intensities will be discussed together for three temperature ranges, as indicated by vertical lines in Figure 4. For clarification, a simplified scheme of the processes occurring in the three temperature ranges is shown in Figure 5.

The blue Pd 3d5/2 spectrum in Figure 3a was measured at 150 K. It shows a broad signal at 335.2 eV with an fwhm of 1.2 eV and a pronounced shoulder at 335.7 eV. The total peak width and also intensity of the shoulder at 335.7 eV are significantly higher than observed in the experiment shown in Figure 1, indicating an additional signal at similar binding energies as the S2 contribution. After heating to 550 K (green spectrum), the peak maximum is shifted by 0.1 eV to lower binding energies, and the shoulder at 335.7 eV decreased and shifted closer to the main peak; consequently, the fwhm of the signal was reduced to 1.0 eV, that is, identical to the value found in Figure 1. The blue C 1s spectrum in Figure 3b at 150 K exhibits two characteristic graphene signals at 285.2 and 284.6 eV, corresponding to strongly and weakly bound graphene regions, respectively.16,26 The small shoulder at 283.8 eV indicates a minor amount of carbidic carbon. The green spectrum measured after heating to 550 K shows small changes: The signal at 285.2 eV increases while the graphene signal at 284.6 eV and the carbidic carbon decrease. The intensities of the strongly and weakly bound carbon species are shown in Figure 4a,d (open and full blue symbols) along with the total C 1s intensity (black). From 150 to 550 K, the C 1s intensity increases by 8% for the 0.3 ML sample and by 24% for the 0.7 ML case. The Rh 3d5/2 intensity (Figure 4b,e) shows a simultaneous increase of 17 and 23%, respectively. In this temperature range, the Pd 3d5/2 intensity continuously decreases by 23% for 0.3 ML and 10% for 0.7 ML Pd. These observations are explained as follows: After deposition at 150 K, the Pd particles are rather flat. Upon heating to 550 K, they are restructured toward a more 3-D shape, which is energetically more favorable due to a smaller surface area. This process explains the loss of intensity in the Pd 3d5/2 core level as well as the increase in C 1s and Rh 3d5/2 intensity. When the clusters become more 3-D (i.e., thicker), the topmost Pd layers 15936



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Figure 4. Total signal intensities from fitting of C 1s, Rh 3d5/2, and Pd 3d5/2 core-level spectra measured after annealing of 0.3 and 0.7 ML Pd on graphene/Rh(111).

Figure 3. Thermal stability of Pd deposited on graphene/Rh(111). Selected (a) Pd 3d5/2 spectra of 0.3 ML Pd, (b) C 1s spectra of 0.3 ML Pd, and (c) C 1s spectra of 0.7 ML Pd measured after annealing to different temperatures. All spectra were recorded at normal emission.

attenuate the electrons emitted from the underlying Pd atoms. Consequently the Pd 3d5/2 signal loses intensity. This effect is smaller for higher Pd amounts because the particles are already more 3-D to begin with. At the same time, the deposited Pd covers a smaller part of the graphene/Rh surface. Therefore, the carbon and rhodium signals are less attenuated and the C 1s and Rh 3d5/2 intensities increase. (Note that this increase is more pronounced for the Rh 3d5/2 than for the C 1s signal due to the larger inelastic mean free path at lower kinetic energy of the former (73 vs 95 eV, at a photon energy of 380 eV).) The restructuring of the Pd clusters to 3-D islands is supported by the STM experiments in Figure 2, which show clusters of around three layer thickness after deposition at room temperature. At this point we come back to the large width of the Pd 3d5/2 peak at 150 K and the decrease in the shoulder at 335.7 eV upon heating. (See previous.) On the one hand, this is in line with the proposed structural change that leads to a lower number of surface atoms. On the other hand, at 150 K water from the residual gas of the chamber absorbs on the Pd particles. From O 1s spectra (see Figure S2 in the Supporting Information), we know that these water impurities desorb from the Pd clusters until 200 K. The additional intensity at 335.7 eV

Figure 5. Model of the structural changes of the Pd clusters during the thermal stability experiment shown in Figures 3 and 4.

at 150 K therefore partially originates from the surface core level shift of water adsorbed on Pd at low temperatures.28 Upon heating to 750 K (orange spectra in Figure 3a,b) the XP spectra show further changes: the Pd 3d5/2 signal decreases significantly and shifts to lower binding energies by an additional 0.1 eV, while its fwhm stays constant at 1.0 eV. The decrease in the Pd 3d5/2 intensity (Figure 4c,f) between 550 and 750 K is steeper than that for lower temperatures, and at 750 K, the Pd 3d5/2 intensity reaches its minimum for both 0.3 and 0.7 ML Pd coverage. In the same temperature regime, for 0.3 ML Pd, the C 1s and Rh 3d5/2 intensities (Figure 4a,b) stay nearly constant. For 0.7 ML Pd, the findings are more complicated: Between 550 and 650 K both C 1s and Rh 3d5/2 signals show a further increase. Starting around 650 K, an enhanced increase in the C 1s core level is observed, while the Rh 3d5/2 intensity starts to decrease. This behavior marks the onset of intercalation, which will be discussed later. In the temperature regime between 550 and 750 K, the Pd particles 15937



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CONCLUSIONS We investigated the growth and thermal stability of Pd clusters deposited onto graphene on Rh(111) by synchrotron radiationbased X-ray photoelectron spectroscopy. From the XP spectra, we deduce a constant bulk-to-surface ratio for Pd coverages higher than 0.3 ML; this behavior indicates that above this coverage the amount of deposited Pd increases mainly the number of clusters but not the average cluster size. Additional STM measurements of Pd on graphene/Rh(111) confirmed that Pd adsorbs as clusters on the graphene/Rh(111) Moiré structure. When elucidating the thermal stability of the Pd clusters by XPS, three temperature regimes can be distinguished: Up to 550 K, the Pd particles do not coalesce but undergo restructuring toward more 3-D clusters. Between 550 and 750 K, the particles become more mobile and agglomerate into larger clusters. Finally, up to 900 K the Pd intercalates completely between the graphene sheet and the Rh surface.

become more mobile and thus agglomerate into larger clusters. Because of the increased self-damping in the larger and thicker clusters, the intensity of the Pd 3d5/2 signal drops significantly. Simultaneously, the damping of the underlying substrate is reduced. Thus, the intensity of the C 1s and (initially also the) Rh 3d5/2 signals increase. For the 0.3 ML Pd deposition, this effect on the substrate is so small (note that with a cluster thickness of three layers only 10% of the surface is covered) that we do not observe significant intensity changes in this temperature range. Upon heating of 0.3 ML Pd to 850 K (red spectra in Figure 3a,b), the Pd 3d5/2 intensity increases again and the binding energy remains constant at 335.0 eV. The peak shows a reduced fwhm of 0.9 eV and its final intensity is 64% of the original value at 150 K. For 0.7 ML, the increase in the Pd signal is much more pronounced, yielding a final intensity of 151% of the original value at 150 K. At the same time, the spectral shape of the C 1s signals shows pronounced changes. For 0.3 ML Pd, at 850 K the signal of the strongly bound graphene regions is shifted 0.2 eV toward lower binding energies, from 285.2 to 285.0 eV, and the signal of weakly bound graphene regions at 284.6 eV increases by 25% and is broadened (Figure 3b). For 0.7 ML, these effects are much more pronounced (red spectrum in Figure 3c). Between 750 and 850 K, for both 0.3 and 0.7 ML Pd, the total C 1s intensity increases, while the Rh 3d5/2 intensity decreases. This behavior is a strong indication for the intercalation of Pd between the graphene sheet and the Rh surface at these temperatures: If intercalated, the Pd atoms are arranged in a flat, monatomic layer, which leads to a more efficient attenuation of the Rh 3d5/2 signal, as compared with the arrangement in clusters. In the flat configuration Pd atoms are no longer damped by other Pd atoms on top; therefore, the Pd 3d5/2 intensity increases. Simultaneously, graphene becomes the topmost layer, which explains the increase in the C 1s intensity. This characteristic behavior has also been used to prove intercalation in the past.29,30 The shape of the C 1s spectra at 850 or 900 K (red spectra in Figure 3b for 0.3 ML Pd and in Figure 3c for 0.7 ML, respectively) provides further evidence of intercalation. Graphene on Pd(111) exhibits a C 1s signal at 284.3 eV.31 Therefore, upon intercalation an additional signal at 284.3 eV is expected, which explains the increase and broadening of the signal at 284.5 eV. This effect also leads to the increase in the weakly bound carbon signal and the decrease in the strongly bound carbon signal in Figure 4a,d (full and open blue symbols, respectively). Overall, the observed effects due to intercalation are more pronounced for the sample with 0.7 ML Pd coverage. In a separate experiment (data not shown), a 0.7 ML Pd layer prepared at 150 K was annealed to 500 K for 1 min, allowed to cool down to 200 K, and heated to 500 K again. The Pd 3d5/2, C 1s, and Rh 3d5/2 spectra recorded after cooling down to 200 K for the first and the second time are identical. This experiment proves that after the first annealing step the particles are stable up to the annealing temperature. Additional STM measurements (see Supporting Information, Figure S1) confirm that Pd clusters prepared at room temperature are stable after annealing the sample to 370 and 420 K for 10 min. In these experiments, both the size and the number of clusters remain virtually constant. Neither intercalation nor agglomeration of the clusters is observed under these conditions, in agreement with the XPS results.

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ASSOCIATED CONTENT

S Supporting Information *

STM images upon annealing, additional O 1s/Pd 3p3/2 and Rh 3d5/2 spectra to the data shown in Figure 3 for 0.3 ML Pd, the Pd 3d5/2 spectra for 0.7 ML Pd (used for the analysis in Figure 4), and Pd 3d5/2, C 1s and valence band spectra of 0.4 ML Pd/ graphene/Rh(111). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*S.M. Phone: +49 9131 85 27268. E-mail: sabine.maier@ physik.uni-erlangen.de. *C.P.: Phone: +49 9131 85 27326. E-mail: christian.papp@fau. de. Notes

The authors declare no competing financial interest. § K.G. and C.S. share first authorship.

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ACKNOWLEDGMENTS We thank HZB for the allocation of synchrotron radiation beamtime and BESSY staff for support during beamtime. Funding from SFB 953 “Synthetic Carbon Allotropes”, the Cluster of Excellence “Engineering of Advanced Materials” at the University of Erlangen-Nuremberg, and the Bavarian Academy of Science is gratefully acknowledged. K.G. thanks the “Fonds der chemischen Industrie” for providing her a Ph.D. grant.

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REFERENCES

(1) Blanita, G.; Lazar, M. D. Review of Graphene-Supported Metal Nanoparticles as New and Efficient Heterogeneous Catalysts. Micro Nanosyst. 2013, 5, 138−146. (2) Siamaki, A. R.; Khder, A. E. R. S.; Abdelsayed, V.; El-Shall, M. S.; Gupton, B. F. Microwave-Assisted Synthesis of Palladium Nanoparticles Supported on Graphene: A Highly Active and Recyclable Catalyst for Carbon-Carbon Cross-Coupling Reactions. J. Catal. 2011, 279, 1−11. (3) Li, Y.; Fan, X.; Qi, J.; Ji, J.; Wang, S.; Zhang, G.; Zhang, F. Palladium Nanoparticle-Graphene Hybrids as Active Catalysts for the Suzuki Reaction. Nano Res. 2010, 3, 429−437. (4) Singh, R. N.; Awasthi, R. Graphene Support for Enhanced Electrocatalytic Activity of Pd for Alcohol Oxidation. Catal. Sci. Technol. 2011, 1, 778−783.

15938



Article

(25) Doniach, S.; Šunjić, M. Many-Electron Singularity in X-Ray Photoemission and X-Ray Line Spectra from Metals. J. Phys. C Solid State 1970, 3, 285−291. (26) Gotterbarm, K.; Zhao, W.; Höfert, O.; Gleichweit, C.; Papp, C.; Steinrück, H.-P. Growth and Oxidation of Graphene on Rh(111). Phys. Chem. Chem. Phys. 2013, 15, 19625−19631. (27) Horcas, I.; Fernandez, R.; Gomez-Rodriguez, J. M.; Colchero, J.; Gomez-Herrero, J.; Baro, A. M. WSXM: A Software for Scanning Probe Microscopy and a Tool for Nanotechnology. Rev. Sci. Instrum. 2007, 78, 0137051−0137058. (28) Gladys, M. J.; El Zein, A. A.; Mikkelsen, A.; Andersen, J. N.; Held, G. Chemical Composition and Reactivity of Water on Clean and Oxygen-Covered Pd(111). Surf. Sci. 2008, 602, 3540−3549. (29) Koch, R. J.; Weser, M.; Zhao, W.; Viñes, F.; Gotterbarm, K.; Kozlov, S. M.; Höfert, O.; Ostler, M.; Papp, C.; Gebhardt, J.; et al. Growth and Electronic Structure of Nitrogen-Doped Graphene on Ni(111). Phys. Rev. B 2012, 86, 0754011−0754016. (30) Dedkov, Y. S.; Fonin, M.; Rüdiger, U.; Laubschat, C. GrapheneProtected Iron Layer on Ni(111). Appl. Phys. Lett. 2008, 93, 0225091−0225093. (31) Gao, J.-H.; Ishida, N.; Scott, I.; Fujita, D. Controllable Growth of Single-Layer Graphene on a Pd(111) Substrate. Carbon 2012, 50, 1674−1680.

(5) Yang, J.; Tian, C.; Wang, L.; Fu, H. An Effective Strategy for Small-Sized and Highly-Dispersed Palladium Nanoparticles Supported on Graphene with Excellent Performance for Formic Acid Oxidation. J. Mater. Chem. 2011, 21, 3384−3390. (6) Johnson, J. L.; Behnam, A.; Pearton, S. J.; Ural, A. Hydrogen Sensing Using Pd-Functionalized Multi-Layer Graphene Nanoribbon Networks. Adv. Mater. 2010, 22, 4877−4880. (7) Wintterlin, J.; Bocquet, M.-L. Graphene on Metal Surfaces. Surf. Sci. 2009, 603, 1841−1852. (8) Voloshina, E.; Dedkov, Y. Graphene on Metallic Surfaces: Problems and Perspectives. Phys. Chem. Chem. Phys. 2012, 14, 13502− 13514. (9) Sutter, P.; Sadowski, J. T.; Sutter, E. Graphene on Pt(111): Growth and Substrate Interaction. Phys. Rev. B 2009, 80, 2454111− 24541110. (10) N’Diaye, A. T.; Bleikamp, S.; Feibelman, P. J.; Michely, T. TwoDimensional Ir Cluster Lattice on a Graphene Moiré on Ir(111). Phys. Rev. Lett. 2006, 97, 2155011−2155014. (11) Coraux, J.; N’Diaye, A. T.; Engler, M.; Busse, C.; Wall, D.; Buckanie, N.; Meyer zu Heringdorf, F.; van Gastel, R.; Poelsema, B.; Michely, T. Growth of Graphene on Ir(111). New J. Phys. 2009, 11, 0230061−02300622. (12) Voloshina, E. N.; Dedkov, Y. S.; Torbrügge, S.; Thissen, A.; Fonin, M. Graphene on Rh(111): Scanning Tunneling and Atomic Force Microscopies Studies. Appl. Phys. Lett. 2012, 100, 2416061− 2416064. (13) Iannuzzi, M.; Hutter, J. Comparative Study of the Nature of Chemical Bonding of corrugated Graphene on Ru(0001) and Rh(111) by Electronic Structure Calculations. Surf. Sci. 2011, 605, 1360−1368. (14) Marchini, S.; Günther, S.; Wintterlin, J. Scanning Tunneling Microscopy of Graphene on Ru(0001). Phys. Rev. B 2007, 76, 0754291−0754299. (15) Martoccia, D.; Willmott, P. R.; Brugger, T.; Björck, M.; Günther, S.; Schlepütz, C. M.; Cervellino, A.; Pauli, S. A.; Patterson, B. D.; Marchini, S.; et al. Graphene on Ru(0001): A 25 × 25 Supercell. Phys. Rev. Lett. 2008, 101, 1261021−1261024. (16) Preobrajenski, A. B.; Ng, M. L.; Vinogradov, A. S.; Martensson, N. Controlling Graphene Corrugation on Lattice-Mismatched Substrates. Phys. Rev. B 2008, 78 (073401), 073401−073404. (17) N’Diaye, A. T.; Gerber, T.; Busse, C.; Myslivecek, J.; Coraux, J.; Michely, T. A Versatile Fabrication Method for Cluster Superlattices. New J. Phys. 2009, 11, 1030451−10304519. (18) Donner, K.; Jakob, P. Structural Properties and Site Specific Interactions of Pt with the Graphene/Ru(0001) Moiré Overlayer. J. Chem. Phys. 2009, 131, 1647011−16470110. (19) Pan, Y.; Gao, M.; Huang, L.; Liu, F.; Gao, H.-J. Directed SelfAssembly of Monodispersed Platinum Nanoclusters on Graphene Moiré Template. Appl. Phys. Lett. 2009, 95, 0931061−0931063. (20) 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. (21) 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. (22) Sicot, M.; Bouvron, S.; Zander, O.; Rüdiger, U.; Dedkov, Y. S.; Fonin, M. Nucleation and Growth of Nickel Nanoclusters on Graphene Moiré on Rh(111). Appl. Phys. Lett. 2010, 96, 0931151− 0931153. (23) Sicot, M.; Leicht, P.; Zusan, A.; Bouvron, S.; Zander, O.; Weser, M.; Dedkov, Y. S.; Horn, K.; Fonin, M. Size-Selected Epitaxial Nanoislands Underneath Graphene Moiré on Rh(111). ACS Nano 2012, 6, 151−158. (24) Denecke, R.; Kinne, M.; Whelan, C. M.; Steinrück, H.-P. In-situ Core-Level Photoelectron Spectroscopy of Adsorbates on Surfaces Involving a Molecular Beam  General Setup and First Experiments. Surf. Rev. Lett. 2002, 9, 797−801. 15939


Graphene-Templated Growth of Pd Nanoclusters - The Journal of

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