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Growth Mechanism of Graphene on Ru(0001) and O2 Adsorption on the Graphene/ Ru(0001) Surface Hui Zhang, Qiang Fu,* Yi Cui, Dali Tan, and Xinhe Bao* State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, People’s Republic of China ReceiVed: NoVember 30, 2008; ReVised Manuscript ReceiVed: March 22, 2009
The growth mechanism of monolayer (ML) graphene on Ru(0001) via pyrolysis of C2H4 was studied by scanning tunneling microscopy (STM), high-resolution electron energy loss spectroscopy (HREELS), and ultraviolet photoelectron spectroscopy (UPS). On the basis of the mechanistic understanding, graphene overlayers ranging from nanographene clusters to graphene film with 1 ML coverage were prepared in a well-controlled way. O2 adsorption on the graphene/Ru(0001) surface was investigated by STM, UPS, and X-ray photoelectron spectroscopy (XPS). It is revealed that the Ru(0001) surface fully covered by graphene becomes passivated to O2 adsorption at room temperature and only activated again at elevated temperatures (>500 K). The adsorbed oxygen intercalates between the topmost graphene overlayer and the Ru(0001) substrate surface. These intercalated oxygen atoms decouple the graphene layer from the Ru(0001) substrate, forming quasi-freestanding monolayer graphene atomic crystals floating on the O-Ru(0001) surface. 1. Introduction Nanostructured carbon materials, including carbon nanotubes (CNTs), carbon nanofilaments, as well as fullerene (C60) and related materials, have attracted extensive attention in past decades due to their potential applications in many fields, such as electronics, material science, and catalysis.1-7 These carbon materials have diverse morphologies and structures and, thus, present different properties and performance. Nevertheless, they all can be regarded as being made up of the same building block, graphene, which can be wrapped into zero-dimensional (0D) fullerenes, rolled into 1D nanotubes, or stacked into 3D graphite.8 Therefore, graphene is of great importance for a fundamental understanding of the relationship between structure and performance of the nanostructured carbon materials. The graphene has been long described only as an “academic” material until free-standing graphene was unexpectedly found by Geim and co-workers recently.9 These 2D carbon crystals have attracted a great deal of interest in physics in past years due to their unique electronic structure.10 Obviously, graphene also will be highly interesting in chemistry, in particular, catalysis.11 The well-defined 2D carbon structure provides a fertile ground to study the interactions of metals and/or gases with carbon, which will contribute to the basic knowledge of the role of various carbon forms in heterogeneous catalysis. For both basic research and applications of the new carbon material, it is highly demanding to prepare novel materials with wellcontrolled structure and size on a large scale. Epitaxial growth of graphene presents one of the most effective routes. Highquality and large area graphene overlayers have been obtained via pyrolysis of hydrocarbons on metal substrates, such as Ru(0001),12-14 Ir(111),15,16 and Pt(111)17,18 at elevated temperatures. Understanding the growth mechanism could promote the well-controlled and reproducible preparation of graphene and, thus, is worth exploring in detail. * To whom correspondence should be addressed. Phone: +86-41184686637. Fax: +86-411-84694447. E-mail:
[email protected] (X.B.) and
[email protected] (Q.F.).
In the present work, graphene was grown on Ru(0001) surface with ethylene (C2H4) as the carbon precursor. Decomposition of C2H4 and formation of graphene were explored in detail by using in situ surface techniques, including scanning tunneling microscopy (STM), ultraviolet photoelectron spectroscopy (UPS), and high-resolution electron energy loss spectroscopy (HREELS). On the basis of the understanding of the growth process, various well-defined graphene overlayers have been prepared on the Ru(0001) surface, which ranges from nanographene clusters to macroscopic graphene domains. O2 adsorption experiments indicate that it takes place via intercalation and the surface adsorbed oxygen atoms stay between the topmost graphene layer and the Ru(0001) substrate surface, which results in almost free-standing monolayer graphene floating on the O-Ru(0001) surface. 2. Experimental Section All experiments were performed in an Omicron multiprobe surface analysis system, which consists of a preparation chamber, a spectroscopic chamber, and a microscopic chamber.4,19 Ru(0001) substrate was cleaned by several cycles of Ar+ sputtering at 1 keV followed by annealing up to 1500 K in ultrahigh vacuum (UHV). In the case of surfaces covered by carbon, the initial annealing in O2 atmosphere (1.6 × 10-6 mbar) at 800 K is necessary. The cleanness of the surfaces was monitored by STM, which was conducted in the microscopic chamber at room temperature (RT) with a constant current mode using a homemade W-tip. The samples can be heated to 1100 K with electric resistance heaters. An electron-beam heater was applied for annealing from 1100 to 1500 K. The sample temperature was measured by chromel-alumel thermocouple in the region lower than 1000 K, while for temperatures higher than 1000 K it was measured by an infrared thermometer (Land Cyclops 100/100B). Graphene was prepared via exposure of the Ru surface to C2H4 (99.995% purity) at a certain temperature followed by annealing up to 1300 K in UHV. Introduction of the adsorbate
10.1021/jp810514u CCC: $40.75 2009 American Chemical Society Published on Web 04/16/2009
Growth Mechanism of Graphene on Ru(0001)
Figure 1. STM images of a clean Ru(0001) surface (a) and the Ru(0001) surface exposed to 24 L of C2H4 at RT (b) followed by annealing at temperatures of 700 (c), 1000 (d), and 1200 K (e), respectively. All the images are 200 nm × 200 nm. (f) A typical highresolution STM image of the graphene/Ru(0001) surface (15 nm × 15 nm).
onto the sample surfaces was carried out by backfilling the chamber through a leak valve. The pressure rise in the chamber and the exposure time were used to calculate the coverage of adsorbate in Langmuir (L). X-ray photoelectron spectroscopy (XPS) was carried out utilizing Mg KR (hν ) 1253.6 eV) radiation with a pass energy of 30 eV. UP spectra were acquired at normal emission with both He I (hν ) 21.2 eV) and He II (hν ) 40.8 eV) radiation. The HREELS measurements were operated with the primary beam energy at 8.77 eV. The angles of incidence and reflection electron beams are 60° with respect to the surface normal in the specular direction. 3. Results and Discussion Figure 1 displays STM images of a clean Ru(0001) surface and the surface dosed with 24 L of C2H4 at RT and annealed at different temperatures in UHV. It can be seen that C2H4 adsorption at RT has induced significant surface corrugations (Figure 1b). The Ru(0001) terraces were covered by a layer of ethylene-induced disordered structure, which may be from dissociated hydrocarbons on Ru as discussed below. When annealing the surface at a higher temperature, small protrusions are observed to distribute evenly on the terraces. For example, annealing at 700 K produces protrusions with sizes ranging from 1 to 2.5 nm and apparent height less than 0.3 nm (Figure 1c). With increasing temperature, the protrusions start to coalesce and form 2D islands on the terraces. Figure 1d shows an image of the surface annealed at 1000 K. There coexist large islands with typical Moire´ patterns at monolayer graphene structure on metals12-14 and nanographene clusters with an almost identical size at around 1.0 nm and monolayer thickness. These results indicate that the surface carbon species start to diffuse within single Ru(0001) terraces and form a well-ordered monolayer graphene structure with the annealing temperature higher than 700 K. At 1200 K, much stronger diffusion of the surface carbon species occurs, leading to further growth of islands and
J. Phys. Chem. C, Vol. 113, No. 19, 2009 8297 attachment of the most islands to the step edges, as is seen in Figure 1e. At the same time, the Ru(0001) steps become irregular and faceted, in contrast to the straight steps on the clean Ru(0001) surface. Above 1300 K, large and continuous graphene layers with the size of micrometers which span several steps appear on the surface. An atomic resolution STM image of the graphene surface was displayed in Figure 1f. The periodicity of the Moire´ patterns at the monolayer graphene structure on Ru(0001) was derived to be C(12×12)-Ru(11×11).14,20 Nevertheless, it should be noted that the superstructure of graphene on Ru(0001) is still in dispute. C(11×11)-Ru(10×10)13 and C(25×25)-Ru(23× 23)21 have also been suggested. The formation of the Moire´ patterns is due to the mismatch between graphitic layer and Ru(0001) lattice, which induces the structural corrugation and charge inhomogeneity in the graphene sheet.13,14,18,22 The Moire´ patterns of graphene on Pt(111) and Ir(111) have been attributed mainly to the electronic effects, and the resulting geometric corrugation at graphene surfaces is quite small, less than 1 Å.15,18 In comparison, on the graphene/Ru(0001) surface the geometric buckling of the graphene layer has been recently calculated to be as large as 1.5 Å.22 Thus, the Moire´ patterns on the graphene/ Ru(0001) surface may originate from the geometric corrugation in addition to the charge density fluctuation in graphene sheet, both of which result from strong interactions between graphene and Ru. HREELS shows that upon C2H4 adsorption at RT, the main features are at 736, 959, 1131, 1267, 1332, and 2968 cm-1 with a shoulder at 2908 cm-1. The peak at 736 cm-1 can be assigned to the C-H bending mode in the CCH species.23 The losses at 959, 1131, 1267, 1332, and 2968 cm-1 should be assigned to F(CH3), C-C stretch in CCH3, C-C stretch in CCH, CH3 bends, and CH3 symmetric stretch.23-25 It has been reported that C2H4 adsorbs molecularly on noble metal surfaces at low temperatures, such as 120 K, with CdC bonds parallel to the surface.24,26,27 Close to RT, C2H4 dissociates to ethylidyne species (CH3-Ca) and part of the ethylidyne may further dehydrogenate to form acetylide (-CtCH), with C-C and CtC bonds in the hydrocarbon intermediates perpendicular to the surface. The dissociative adsorption of C2H4 has been observed on Ru(0001),23-25,27,28 Pt(111),29,30 Pd(111),31,32 and Rh(111).33 Above RT, strong dissociation of the ethylidyne species happens, producing more acetylide. Thus, it is expected that the intensity of the loss features from the acetylide species increases, which explains why the peak at 736 cm-1 shifts to 776 cm-1 and its intensity increases when annealing at 500 K (Figure 2b). Between 500 and 700 K, complete decomposition of the surface adsorbed hydrocarbon intermediates takes place, and eventually only carbon remains on the surface. As shown in Figure 2c, the C-H peaks at 776 and 2968 cm-1 and the peak at 1131 cm-1 from C-C bond stretch disappear upon annealing at 700 K. At 1250 K, well-defined graphene domains form on the surface (Figure 1e), which is accompanied by two new peaks at 700 and 1470 cm-1. The peak at 700 cm-1 is due to the vertical optical phonon and the 1470 cm-1 loss to the longitudinal optical phonon in graphene.20 It should be mentioned that the losses at ∼420 and ∼1970 cm-1 are always observed in most HREELS spectra, and they are due to the surface adsorbed CO from the background. Figure 3 shows the selected UP spectra of the Ru(0001) surface adsorbed with 24 L of C2H4 at RT and annealed at the indicated temperatures. On the as-adsorbed surface, two peaks at ∼6-8 and ∼11.2 eV are observed, and both of them are from the surface dissociated ethylidyne species.26,27,34 At 400
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Figure 2. HREELS spectra from a Ru(0001) surface exposed to 24 L of C2H4 at RT (a) followed by annealing at 500 (b), 700 (c), and 1250 K (d).
Figure 4. STM images of graphene layers grown on Ru(0001) prepared via exposure of 24 L of C2H4 to Ru(0001) at 700 K followed by annealing at 1300 K in UHV (200 nm × 200 nm) (a) and exposure of 3 L of C2H4 to Ru(0001) at RT followed by annealing at 900 K in UHV (100 nm × 100 nm) (b).
Figure 3. He II UP spectra taken from a Ru(0001) surface (a) and the Ru(0001) surface exposed to 24 L of C2H4 at RT (b) followed by annealing at 400 (c), 700 (d), 1000 (e), and 1200 K (f).
K, the main peaks are located at 9.1 and 12 eV, which can be attributed to the 3σ and 2ss* orbitals of the CCH(ads) species.27 The complete decomposition of ethylidyne and other hydrocarbon intermediates at 700 K leaves out the characteristic features from C-C and C-H bonds, and only a very weak peak at 9.2 eV can be seen, which has been reported to be from C(ads).27,35 On the surfaces annealed at 1000 and 1200 K, new peaks at 7.5 and 9.5 eV appear and become stronger at higher annealing temperatures. Since well-ordered graphene has already formed under such conditions (Figure 1d,e), these peaks should be characteristic of graphene structure and originate from the graphite-derived π and σ states in the valence band. Compared to the features from bulk graphite surfaces, these peaks shift to higher binding energies, which may be caused by hybridization of the electronic states of graphene with the d states of Ru.36,37 Accordingly, the growth mechanism of graphene on Ru(0001) through pyrolysis of C2H4 can be derived based on the above results. First, C2H4 dissociatively adsorbs on Ru(0001) at RT, producing a layer of surface adsorbed ethylidyne and possibly other hydrocarbon intermediates. Above RT, further dehydrogenation and decomposition of these intermediates take place, resulting in formation of pure carbon clusters on the surface at 700 K. With the annealing temperature higher than 700 K, the carbon clusters may coalesce due to surface diffusion forming
monolayer graphene islands on the Ru(0001) terraces. Further increasing temperature above 1000 K, carbon tends to aggregate to steps resulting in large monolayer graphene islands. The strong interaction of carbon with Ru(0001) steps causes preferential growth of graphene along the steps and induces the reconstruction of Ru(0001) steps. The decomposition of C2H4 has been investigated on other metal surfaces.18,31-33,38,39 For example, Land et al. has presented nice STM studies in the chemistry of C2H4 on Pt(111) and they reported a similar surface process of ethylene decomposition at elevated annealing temperatures.18,38,39 The above growth process involves the saturated adsorption of C2H4 at RT and, thus, the coverage of the resulting graphene is limited by the amount of C2H4 adsorbed on metal surfaces at RT. As shown by the STM data (Figure 1e), The coverage of graphene obtained by this process is only about 0.25 ML. It has been shown that complete dehydrogenation of C2H4 takes place at 700 K and the surface deposited carbon can be controlled at a high coverage if exposing Ru(0001) to C2H4 at this temperature. To improve the graphene coverage, a new process has been attempted here by exposing Ru(0001) to 24 L of C2H4 at 700 K followed by UHV annealing at 1300 K. This modified deposition process produces graphene with a coverage above 0.9 ML (Figure 4a). Alternatively, graphene growth has been tried at other conditions. For example, C2H4 was dosed on Ru(0001) at RT with 3 L exposure followed by annealing up to 900 K, and this preparation process results in the formation of monolayer graphene clusters with a size around 1.0 nm (Figure 4b). The results (Figure 4a,b) suggest that the coverage and morphology of graphene can be simply controlled by manipulating the growth conditions.
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Figure 6. O 1s XPS spectra from a 0.9 ML graphene/Ru(0001) surface (a) and the surface exposed to 360 L of O2 at RT (b), 500 K (c), and 600 K (d). O 1s XPS spectra from the graphene/Ru(0001) surface adsorbed with oxygen at 600 K and subsequently annealed at 800 (e) and 1000 K (f) in UHV.
Figure 7. O 1s XPS spectra from a clean Ru(0001) surface (a) and the surface exposed to 360 L of O2 at RT (b), 500 K (c), and 600 K (d). O 1s XPS spectra from the Ru(0001) surface adsorbed with oxygen at 600 K and subsequently annealed at 800 (e), 1000 (f), and 1300 K (g) in UHV.
Figure 5. STM images of the 0.9 ML graphene/Ru(0001) surface exposed to 360 L of O2 at 600 K: 200 nm × 200 nm (a) and 10 nm × 10 nm (b). The surface was annealed to 800 K in UHV, and STM images showing the coexistence of the graphene domains with Moire´ patterns and the smooth surface domains induced by oxygen adsorption: 200 nm × 200 nm (c) and 15 nm × 15 nm (d).
In the following part, the surface chemistry of graphene was studied by using O2 as a probe gas. O2 was introduced to the 0.9 ML graphene/Ru(0001) surface, and the adsorption was studied by STM, XPS, and UPS. Figure 5a shows a typical STM image from the graphene/Ru(0001) surface exposed to 360 L (1.6 × 10-6 mbar, 5 min) of O2 at 600 K. The monolayer surface
Figure 8. He II UP spectra from a clean Ru(0001) surface (a), a clean 0.9 ML graphene/Ru(0001) surface (b), the surface in part b adsorbed with 360 L of O2 at 600 K (c), and the surface in part c annealed at 800 K in UHV (d).
structure is still present on the Ru(0001) terraces but the characteristic Moire´ patterns could no longer be observed. The atomic image of the surface in Figure 5b shows that the graphitic structure remains intact but the surface corrugation becomes much less. Before O2 exposure the surface height modulation is 1 Å, while upon O2 adsorption the height modulation reduces to 0.2 Å. When annealing the oxygen-exposed surface at 800 K, the graphene structure with the typical Moire´ patterns recovers on most regions of the surface. This transition can be seen more clearly in Figure 5c, which shows an STM image
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Figure 9. Schematic illustrating a graphene/Ru(0001) surface (a) and the surface with oxygen atoms intercalated between graphene and Ru substrate (b).
recorded from a region consisting of the graphene domains with Moire´ patterns and the smooth surface domains induced by oxygen adsorption. An atomic image of a boundary between the two domains is given in Figure 5d, showing continuous transition across the domain boundary. The STM data clearly demonstrate that both surfaces have a similar atomic structure and the difference in the surface corrugation comes from the different surface superstructure in the two domains. Figure 6 displays XPS O 1s spectra from the 0.9 ML graphene/Ru(0001) surface upon O2 adsorption. Exposure to 360 L O2 at RT leads to adsorption of a small amount of oxygen on the surface, but a strong oxygen signal is observed at the adsorption temperature of 600 K. The results are different from O2 adsorption on the clean Ru(0001), where strong oxygen signals are observed upon RT adsorption of the same amount of O2 (Figure 7). Thus, the Ru(0001) surface covered by a layer of graphene becomes passivated against oxygen adsorption near room temperature.40 Annealing the oxygen-adsorbed graphene/ Ru(0001) surface in UHV at 800 K could remove most of the surface oxygen. Oxygen signal completely disappears after annealing at 1000 K (Figure 6). Again, it is quite different from the situation on the clean Ru(0001) surface, where the surface adsorbed oxygen remains unchanged at 800 K and a strong decrease of XPS O 1s signals takes place only at temperatures higher than 1300 K (Figure 7). The oxygen adsorption also results in significant changes in the surface electronic structure, as evidenced by UPS (Figure 8). The characteristic UPS features of graphene/Ru(0001) at ∼7.5 and ∼9.5 eV remain unchanged upon O2 adsorption at temperatures below 600 K. Exposure of the surface to O2 at 600 K causes the shift of the peaks to 5.3 and 6.9 eV, respectively, similar to those of the bulk graphite surface. After annealing the oxygen-exposed graphene/Ru(0001) surface at 800 K in UHV, the UPS spectrum almost resembles the line shape and line position of the clean graphene/Ru surface again. These UPS data are consistent with the STM and XPS results. They indicate that the oxygen-adsorbed graphene/Ru(0001) surface presents a unique morphology and electronic structure in comparison to the clean graphene/Ru(0001) surface. First, the graphene layer on the oxygen-adsorbed surface is atomically smooth, and no additional adsorbate atom, e.g., O species, could be observed atop even though a large amount of oxygen has been detected on the oxygen-adsorbed surface (Figure 6). Second, the Moire´ patterns formed by the interaction between graphene and the Ru(0001) substrate disappear, suggesting that the interaction strength between graphene and the substrate decreases after O2 adsorption. Finally, the UPS features from the oxygen-adsorbed graphene on Ru(0001) become more similar to those of bulk graphite surfaces. On the basis of the above results, we suggest that oxygen adsorption on the graphene/Ru(0001) surface should take place via intercalation of oxygen atoms between the graphene and the Ru(0001) substrate surface (Figure 9). The introduction of oxygen at the graphene/Ru(0001) interface could weaken the interaction of the graphene layer with the Ru substrate and further decouple
them. The intercalation effect at the oxygen-adsorbed graphene/ Ru(0001) surface leads to less height modulation in STM images (Figure 5) and a shift of electronic π states of the graphitic sheet to lower binding energies in the UPS spectra (Figure 8).41,42 Since oxygen interacts weakly with graphite at RT,43-45 the decoupled graphene layer should behave more like a freestanding graphitic monolayer floating on the oxygen-adsorbed Ru(0001) surface. The intercalation of substances in bulk graphite has been known for a long time,46,47 and the insert of adsorbates underneath graphene layers grown on metal substrates was also confirmed recently. For example, the intercalation of Au atoms at the interface between graphene and Ni substrate produces quasi-freestanding graphene on Ni.41,42 The deposited Au layers were also found to intercalate between graphitic BN monolayer (h-BN) and Ru(0001) upon annealing at 1050 K.48 These intercalation processes can be attributed to the stronger interaction between the adsorbate and the metal substrate than that between the adsorbate and the topmost graphitic layer. Therefore, we propose that oxygen intercalation at the graphene/ Ru(0001) interface might also result from stronger bonding of oxygen with the Ru surface than with the carbon surface. 4. Conclusion The growth mechanism of graphene on Ru(0001) via pyrolysis of C2H4 at elevated temperatures has been elucidated by STM, HREELS, and UPS. C2H4 was found to dissociatively adsorb on Ru(0001) at RT and dehydrogenate completely at 700 K. Above 700 K, carbon species start to diffuse on Ru terraces and form well-ordered graphene structures. The C2H4 adsorption amount, adsorption temperature, and the subsequent UHV annealing temperature are the most critical growth parameters. Depending on these conditions, graphene overlayers ranging from nanographene clusters to micrometric graphene domains can be obtained. The covering of the Ru(0001) surface by graphene layers passivates the adsorption of oxygen. It could occur only at elevated temperatures, such as 600 K. The oxygen adsorption takes place via intercalation at the graphene/Ru(0001) interface, which lifts the graphene layer from the Ru substrate. The weak interaction between the intercalated oxygen and the graphene leads to formation of the quasi-freestanding graphene layer. Such a “floating” monolayer graphene should represent the unique atomic and electronic structures of free 2D carbon atomic crystal. Acknowledgment. This work was financially supported by the National Natural Science Foundation of China (Nos. 20603037, 20733008, and 20873143), Ministry of Science and Technology of China, and Chinese Academy of Sciences (“Bairen” program). The authors thank Dr. Xiulian Pan for reading the manuscript carefully. References and Notes (1) Serp, P.; Corrias, M.; Kalck, P. Appl. Catal., A 2003, 253, 337.
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