Insight on Thermally Activated Hydrocarbon Dehydrogenation on the

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Insight on Thermally Activated Hydrocarbon Dehydrogenation on the PtNi(111) Surface: From Adsorbed Hydrocarbons Up to Graphene Formation 3

Antonio Politano, Lin Wang, and Gennaro Chiarello J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b11102 • Publication Date (Web): 25 Jan 2018 Downloaded from http://pubs.acs.org on January 26, 2018

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The Journal of Physical Chemistry

Insight on Thermally Activated Hydrocarbon Dehydrogenation on the Pt3Ni(111) Surface: from Adsorbed Hydrocarbons up to Graphene Formation Antonio Politanoa,*, Lin Wangb,c and Gennaro Chiarellod a

Fondazione Istituto Italiano di Tecnologia, Graphene Labs, via Morego 30, 16163 Genova (Italy)

b

State Key Laboratory of Infrared Physics, Shanghai Institute of Technical Physics, Chinese Academy of Sciences, Shanghai, P. R. China c

Synergetic Innovation Center of Quantum Information & Quantum Physics, University of Science and Technology of China, Hefei, Anhui, P. R. China

d

Department of Physics, University of Calabria, via Ponte Bucci, cubo 31/C, 87036 Rende, Cosenza (Italy)

Corresponding Author *E-mail: [email protected]

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

The Pt3Ni(111) alloy is characterized by the presence of an outermost surface layer only composed by Pt atoms, i.e. the so-called Pt skin, which is at the basis of its outstanding performances in oxygen reduction reaction. The growth of graphene on this alloy is further motivated by the realization of a charge-neutral Gr/metal gate, which can be replaced by a pdoped Gr/oxide/metal configuration upon heating in an oxygen environment. Herein, we have followed the complete reaction pathway starting with adsorbed hydrocarbons and ending with the large-scale growth of a graphene overlayer. Experiments using temperature-programmed Xray photoelectron spectroscopy have been carried out for different conditions (C2H4 dose and sample temperature). Sub-monolayer graphene (0.08 ML) is reached upon heating at T=1000 K an ethene monolayer dosed at T=80 K. Conversely, monolayer graphene domains are achieved for C2H4 cracking at T=800 K. We also find that carbon atoms dissolved into the bulk segregate toward the substrate during the cooling.

Introduction The epitaxial growth of graphene (Gr) on metal substrates continues to attract a considerable interest 1-9, since it allows the obtainment of large-scale samples with high crystalline quality. An accurate choice of the underlying substrate can modify or introduce novel properties in the Gr overlayer 10. In particular, the direct synthesis of Gr on bimetallic alloys 11-16 has an intrinsic interest related to the possibility to control the physicochemical properties of the sample termination underlying the Gr sheet, with an extensive variety of achievable interfaces, ranging from Gr/metal gates up to Gr supported by high-κ dielectrics 17.


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Among the various alloys, Pt3Ni(111), widely studied in electrochemistry for its outstanding performances in oxygen reduction reaction 18-20, has unique features making it an ideal substrate for studying Gr growth. As a matter of fact, the matching of the work function of Pt3Ni(111) 21-22 with that of Gr

23

allows the realization of a charge-neutral Gr/metal contact

surface layer is only formed by Pt atoms (Pt-skin)

25-27

24

. The outermost

. Conversely, the second layer has

approximatively a Pt1Ni1 stoichiometry, whereas the third layer is Pt-enriched layers, the composition is the same as in the bulk, i.e. [Pt]:[Ni]=3:1

28

28

. For deeper

. When the surface is

exposed at high temperature to an oxygen atmosphere, Ni atoms segregate toward the surface with selective Ni oxidation, so as to form a NiOx skin 29-31. While the presence of a (111)-oriented Pt-skin makes Gr/Pt-skin-terminated/Pt3Ni(111) basically similar to Gr/Pt(111) concerning the morphological properties, with differently oriented domains 24 in the Gr overlayer, the occurrence of Ni atoms in the second layer in principle could lead to interface states arising from the hybridization with Ni 3d states, universally observed in any Gr/nickel interface 32-33. Herein, we have studied the epitaxial growth of Gr by thermally activated hydrocarbon dehydrogenation (TAHD) for various preparation procedures by means of temperatureprogrammed X-ray photoelectron spectroscopy (TP-XPS). This experimental tool, based on the use of synchrotron radiation, are particularly powerful for studying the growth and the electronic properties of Gr/metal alloys. In particular, TP-XPS enables monitoring core levels during the chemical reactions 34 and it has been recently used to find the optimal growth conditions of Gr on different substrates

17, 35-38

, as well as sticking coefficients for hydrocarbons and their

temperature-dependent saturation coverages

37, 39-41

. By means of TP-XPS, we have also



investigated the effect of the sample temperature on the adsorption process, by taking into account the evolution of the chemisorbed species at the surface during the TAHD process. We find that upon ethene dosage at T=80 K and successive annealing up to T=1000 K, Gr covers only the 8% of the Pt skin when starting from an adsorbed monolayer of ethene. By exposing ethene at T=300 K, ethylydine dominates the C-1s spectrum and after heating at T=800 K a wellordered Gr phase is formed. By directly exposing at T=800 K, monolayer Gr is formed.

2 Experimental Sample preparation The cleaning of the Pt3Ni(111) surface was achieved with reiterated series of Ar-ion sputtering (1.6 keV) and heating at 1100 K. The cleanliness was tested by X-ray photoelectron spectroscopy (XPS) and low-energy electron diffraction (LEED). LEED patterns were acquired also at University of Calabria, Department of Physics with an apparatus enabling higher resolution. The Pt-skin surface was attained before Gr growth by heating the crystal at 1100 K. Coverages of chemisorbed species were estimated by evaluating the behaviour of the photoemission intensity at a specific value of the binding energy (BE) as a function of the exposure to dosed gases. In this work, one monolayer is defined as the coverage corresponding to a fully covered layer. TP-XPS TP-XPS experiments were performed at the SuperESCA beamline of the Elettra synchrotron (Trieste, Italy). The C 1s spectra were collected with a photon energy of 400 eV by means of a Phoibos hemispherical electron analyser with 150 mm radius (SPECS GmbH, Germany). The


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experimental resolution was estimated to be about 40 meV. The acquisition of TP-XPS data was performed with the photon beam in grazing incidence (70°), while photoelectrons were acquired in normal emission. For TP-XPS measurements, the Pt3Ni(111) crystal was heated by electron bombardment and its temperature was measured with a K-type thermocouple spot-welded in a lateral position of the sample. A Shirley background was subtracted from raw XPS data. To fit C-1s core levels, Voigt lineshapes were used.

3 Results and discussion

3.1 TAHD for ethene dosed at T=80 K The adsorption of ethene on Pt-skin-terminated Pt3Ni(111) at T=80 K was followed by fast XPS (Figure 1, top panel) as a function of ethene doses, measured in L (1 L=1.33×10−6 mbar s). At low coverages, a single-component C 1s core level was measured at a BE of ~283.1 eV. The intensity of this component linearly increased with C2H4 exposure, up to an ethene dose of 0.45 L (bottom panel of Figure 1). In analogy with the cases of both Pt(111) 42-43 and Ni(111) 44, the saturated ethene overlayer on the (111)-oriented Pt skin of Pt3Ni(111) corresponds to a 2×2 overstructure and, correspondingly, to a coverage of 0.25 ML. The sticking coefficient is a key parameter to comprehend the reactivity of the Pt skin toward ethene. It defines the ratio between the quantity of chemical species which adsorb at the surface to the total number of particles impacting the same surface in the same time. The sticking



coefficient depends on various parameters, including coverage, sample temperature and kinetic energy of the impinging chemical species. The first derivative of the C 1s uptake data as a function of the total ethene dose was used in the estimation of the ethene sticking coefficient (σ) at T=80 K of ~0.55.

We also observe a shoulder at higher BE around 283.5 eV, whose origin will be discussed later. Moreover, for doses higher than 0.35 L, a second state at a BE of ~284.8 eV appears. This latter component has an increasing intensity up to 0.90 L and it should be evidently related to the formation of multilayer ethene. Beyond this value of the exposure, the intensity of the peak is nearly constant. Furthermore, a careful inspection of the C 1s region reveals that another component at a progressively higher BE (up to 284.0 eV) arises at higher doses. To understand the high BE value of the latter feature, one has to consider that the energy position of the core-level spectra is connected to the response of the electrons of the neighboring medium in screening the core hole generated by photoemission. For C atoms in the subsurface region of the ethene multilayer, the relaxation is less effective compared to the surface termination of the ethene multilayer, and, consequently, the photoelectron is emitted at a higher BE.


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T=80 K

Intensity [Arb. Units]

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


monolayer C2H4 multilayer C2H4

0

1

2

3

4

C2H4 dose [L]

Figure 1. (top panel) Behavior of the C-1s core level as a function of the C2H4 dose for ethene adsorbed at T=80 K on Pt3Ni(111). On the right, we report the colour scale related to the intensity (measured in counts) of the photoemission intensity. (bottom panel) Behavior of the photoemission intensity in correspondence of the BE of monolayer (283.1 eV, red curve) and multilayer (284.8 eV, black curve) ethene on Pt3Ni(111).



On the basis of results in Figure 1, we have studied TAHD for monolayer (Figure 2) and bilayer (Figure 3) ethene dosed at T=80 K. Figure 2 shows the behaviour of the intensity of the C 1s core level when heating monolayer ethene, obtained with exposure at T=80 K. In the uptake at T=80 K, the most intense component in the C 1s has a BE of 283.05 eV, which is accompanied by features at 283.43 and 283.93 eV. A fit procedure also reveal another component at 283.80 eV (Supporting Information, SI, Figure S1). The comparison with the case of C2H4/Pt(111)37 (red circles in the bottom panel of Figure 2) clarifies the origin of the additional components at BE 283.43 and 283.80 eV: they arise from transitions to the first and second vibrationally excited states in the final-state ions, respectively. The existence of these features is thus related to a fine structure associated to the excitation of the C–H stretching vibration the photoemission process. The energy separation between the adiabatic and the first excited mode is 370 ± 10 meV, which coincides (within the experimental inaccuracy) with the energy of C-H stretching observed by vibrational spectroscopies 45-46. The amplitude of the composite vibronic transition is evaluated by the product of Poisson distributions over the amplitudes of the symmetry allowed vibronic transitions. For each of them the resulting Franck-Condon factor is 47:

with ν the vibrational quantum number and with S the “S-factor”, that is the ratio of the area of the first excited peak to the adiabatic peak, i.e. I(1–0)/I(0–0). In our case, the S-factor is estimated to be 0.50. For the sake of comparison, we report that its value for ethene/Pt(111) is 0.36

37

. The presence of only one adiabatic C 1s peak indicates that the two C atoms are

equivalent. Consequently, ethene adsorb in a symmetric configuration with the C–C axis parallel


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to the surface, as for C2H4/Pt(111)41. We can conclude that ethene molecularly adsorbs into a in a di-σ configuration. The feature at a BE 283.93 eV arises from ethylidyne (CCH3)37. Upon heating at 240 K, the component of monolayer ethene suddenly disappears. Around 400 K, a broadening is observed. It can be explained by considering that ethylidyne dehydrogenates to CH2−C39, even if the back-hydrogenation to CH3−CH is in principle feasible for its similar activation energy

39

. For higher temperatures, components related to CH2−C disappear, thus

evidencing the occurrence of C-C bond breaking. In the temperature range between 400 and 1000 K only a weak component at 283.99 eV arising from isolated C atoms is observed. The formation of an ordered Gr phase (BE of 284.03 eV) is evident around T=1000 K. By considering the area of the C 1s core level, we can estimate that Gr coverage is only 0.08 ML.

1000

800

Temperature [K]

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


600

400

200

284.5

284.0

283.5

283.0

282.5

282.0

Binding Energy [eV]



Figure 2. (top panel) Behavior of the C 1s core level for monolayer ethene/Pt3Ni(111), obtained by dosage at T=80 K, and successively annealed up to ~1100 K. (bottom panel) comparison of the C 1s line-shape acquired immediately after ethene uptake at T=80 K up to the saturation of monolayer ethene on Pt3Ni(111) (black curve) and after heating monolayer ethene up to T=1100 K (yellow curve). A reference spectrum for C2H4/Pt(111)37 in similar experimental conditions (albeit uptake temperature in Ref. 37 is slightly higher, i.e. T=120 K) is also reported.

Figure 3 shows the evolution of the C 1s core level during the heating of a Pt3Ni(111) surface covered with a bilayer of ethene at 80 K. The desorption of the multilayer ethene is complete only for temperatures higher than T=500 K, as indicated by the behaviour of the intensity at a BE of 284.8 eV reported in the SI, Section S5. At a temperature of T=600 K, ethylydine is formed. The breaking of C-C bonds occurs only at T=700 K. However, it is not possible to attain a Gr overlayer in these conditions. Even after heating at T=900 K, only disordered C atoms are present at the surface with a corresponding broad C 1s line-shape.


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800

Temperature [K]

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


600

400

200

288

287

286

285

284

283

282

281

Binding Energy [eV]

Figure 3. Behavior of the C 1s core level for bilayer ethene/Pt3Ni(111), obtained by dosage at T=80 K, and successively annealed up to ~900 K.

3.2 TAHD for ethene dosed at T=300 K

The top panel of Figure 4 displays the C 1s core level measured at T=300 K in an ethene environment. The C 1s core level measured at room temperature in these conditions is dominated by the ethylydine component at a BE of 283.88 eV. Even after fit procedure (Figure S2 in the SI) no trace of C-H groups, whose BE is ~283.6 eV 37, is found. Contrariwise, C-H groups are stably adsorbed at room temperature on Pt(111) 37. It should be mentioned that the conversion of ethene in ethylidyne on Pt(111) starts at T=255 K 48

and it is completed at T=320 K

37

. In the bottom panel, it is reported that the surface is

saturated with ethylydine only for an ethene dose higher than ~30 L.


T=80 K 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


T=300 K

Figure 4. (top panel) Behavior of the C 1s core level for Pt3Ni(111) exposed to ethene at T=300 K. (bottom panel) Behavior of the ethylydine coverage, expressed in ML, as a function of the ethene dose, measured in L for the case of Pt3Ni(111) exposed to ethene at T=300 K.

TP-XPS experiments have been also carried out for TAHD in a Pt skin saturated with ethene at T=300 K (Figure 5 and Figure S3 in the SI). Ethylydine is transformed into –CH groups upon heating the sample in the 435-715 K range (Figure S3,d in the SI). The spectral component coming from –CH groups vanishes at T=740 K. The C 1s core level in the temperature range


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775-825 K displays a unique sharp Voigt line-shape at 284.08 eV (Figure S3,e in the SI), arising from the formation of an ordered Gr phase. 1000 900

Temperature [K]

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


800 700 600 500 400

285.0 284.0 283.0 282.0

Binding Energy [eV]

Figure 5. (top panel) Behavior of the C 1s core level for Pt3Ni(111) saturated with ethene at T=300 K, and successively annealed up to ~1000 K. (bottom panel) comparison of the C 1s lineshape acquired immediately after exposure to ethene at T=300 K up to saturation (black curve) and after heating to T=1100 K and successive cooling at T=300 K (yellow curve). A reference



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spectrum for the Pt(111) surface exposed to C2H4 at T=300 K up to saturation

37

is also

displayed (red open circles).

The bottom panel of Figure 5 reports a comparison of the C 1s line-shape for a saturated ethene layer on Pt3Ni(111) (black curve) and Pt(111) (red empty circles) at T=300 K. We note striking changes between the two cases. In the case of ethene-exposed Pt(111), the predominant feature at 283.96 eV comes from ethylidyne CCH3, while that at 283.48 eV, originated from ethylidene species (CHCH3) 37, is a minoritary, albeit well-resolved, component. On ethene-exposed Pt-skin-terminated Pt3Ni(111) the intense peak associated to ethylidyne at 283.88 eV is accompanied by a vibrational fine structure (S=0.19). However, a fit procedure (Figure S2 in the SI) also reveals ethylidene at 283.42 eV (only 7.5% of the signal). To understand the surface termination of the Pt3Ni crystal underneath the Gr cover, one has to consider that no Ni segregation occurs during heating, as demonstrated by TP-XPS data in Figure S4 of the SI. By considering the BE of the C 1s BE of the Gr phase (284.1 eV) and the LEED pattern (Figures S5 and S6,a in the SI) strongly resembling that of Gr/Pt(111) (Figure S7 in the SI) it is evident that Gr grows on the Pt skin of the Pt3Ni(111) surface. In particular, the BE of the C-1s core level coincides with the case of Gr/Pt(111) features at 284.4 and 284.8 eV are observed in Gr/Ni(111)

50-52

49

, whereas two well-distinct

, as a consequence of the co-

occurrence of bridge-fcc and top-fcc configurations 53.

3.3 TAHD for ethene dosed at T=800 K The top panel of Figure 6 shows the behaviour of the C 1s core level upon exposing to ethene with the sample kept at T=800 K. The core-level spectra are characterized by a single component


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at 284.1 eV, due to the Gr phase. In the bottom panel of the same Figure, we report the behaviour of the coverage as a function of the ethene exposure. Interestingly, for doses less than 10 L, the photoemission intensity at a BE of 284.1 eV (peculiar of the Gr phase) is vanishing. Correspondingly, carbon atoms derived from TAHD are dissolved into the bulk of the crystal (see also Section 3.4). For higher doses, the intensity of the C 1s line-shape at the BE of the Gr phase increases up to saturate for exposures around 60 L.

T=800 K

Figure 6. (top panel) Behavior of the C 1s core level for Pt3Ni(111) exposed to ethene at T=800 K as a function of the dose. (bottom panel) Behavior of the Gr coverage, expressed in ML, as a function of the ethene dose, measured in L for the case of Pt3Ni(111) exposed to ethene at T=800 K. The Gr coverage has been estimated by considering the photoemission intensity at its typical BE of 284.1 eV.



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Near-edge X-ray absorption fine structure (NEXAFS) measurements (see Section S4 in the SI for technical details) for the C K-edge acquired for the Gr phase formed upon ethene dosage at T=800 K are reported in Figure 7 for two geometries, with photon incidence grazing (red curve) and nearly-normal (black curve) with respect to the sample in order to probe the 1s → π* (~285 eV) and 1s → σ * (291-292 eV) transitions, respectively54. The highly dichroic C K-edge signal, due to the inherent nature of Gr band structure, with in-plane σ* and out-of-plane π* states, confirms the excellent quality of the Gr layer on Pt3Ni(111). The pre-edge shoulder observed at ~284 eV has been revealed in Gr/Pt(111)55, with a weak feature at 288.1 eV

56

. Features in the

region between 1s → π∗ and 1s → σ∗ transitions are due to interface states, as observed for Gr/Ni(111)57. These excitations correspond to electronic transitions toward the interface state above the Fermi level (around the M-point in the Brillouin zone) arising from the hybridization of C-pz orbitals with Ni d bands. The intensity of interface states is reduced in the Gr/Pt3Ni(111) system, because of the presence of Ni atoms only in the second layer of the Pt3Ni surface58. Moreover, their energy is dissimilar with respect to Gr/Ni(111), for which interface states are observed at 286.3 eV

57

. The occurrence of interface states in Gr/Pt3Ni(111) indicates orbital

mixing, which is instead absent in Gr/Pt(111)55. The 1s → σ∗ transition is composed of two distinct features at 291.4 eV and 292.4 eV. The sharp peak at 291.4 eV corresponds to the creation of an exciton correlation effects of electron–hole pairs within the Gr sheet

60

59

. Its intensity reflects strong

. The broad peak at 292.6 eV is

related to the transition from the C-1s level to the relatively nondispersing σ* states at the Γ point of the Brillouin zone

60-61

. NEXAFS features related to 1s → σ∗ transitions in graphite and


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Gr/Pt(111) are quite similar to those of Gr/Pt3Ni(111), while the two features in strongly

x10

12

interacting Gr/metal interfaces, as Gr/Ni(111), overlap resulting into a broad peak57.

Intensity (Arb. Units)

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


280

290

300

310

320

Photon Energy (eV) Figure 7. X-ray absorption spectra at the C K-edge measured for Gr/Pt3Ni(111) at grazing (20°, red curve) and normal incidence (90°, black curve).

The LEED pattern observed Gr grown on Pt3Ni(111) at a fixed temperature of 800 K (Figure 8) exhibits spots from the hexagonal lattice of Pt3Ni(111) (inner spots) superimposed with semiarcs arising from diffraction from the Gr overstructure (outer spots). The presence of semi-arcs indicates that differently rotated domains exist in the Gr superlattice. Such a LEED pattern is similar but not identical to that of Gr on Pt(111) (Figure S6). In Gr on Pt(111), three domains are predominant: one aligned with the substrate, other ones rotated by ±19° and another one rotated by 30° 62. The quantitative analysis of the intensity of diffraction spots as a function of the angle I(θ) reveals that, compared to Gr/Pt(111) (Figure S5, middle panel), diffraction peaks in Gr/Pt3Ni(111) are very broad (Figure S5, bottom panel), as a result of higher disorder in the Gr



overlayer. Nevertheless, the analysis of I(θ) indicates that in Gr/Pt3Ni(111) the same domains observed in Gr/Pt(111) exist, due to the presence of a Pt skin in Pt3Ni(111) 25-27. Therefore, as for Gr/Pt(111)

63-64

, the Gr overlayer grown on the Pt skin of Pt3Ni(111) is

characterized by a moiré superlattice, originated from the minimization of the absolute value of the strain between the Gr and the Pt skin for the different orientations between both atomic lattices.

Figure 8. LEED pattern of the as-grown Gr sheet on Pt3Ni(111) at T=800 K and successively cooled at room temperature for LEED experiments. The primary electron beam energy is 70 eV. 3.4 TP-XPS data acquired during cooling To prove that carbon atoms are dissolved into the bulk whenever ethene is dosed at temperature higher than ~820 K, we have followed in real-time the evolution of the C-1s core level during the cooling of the sample (Figure 9a). A Pt3Ni(111) crystal, exposed to ethene at a temperature of 970 K, has been cooled at a rate of -16.8 K/min. Remarkably, the signal of the C-1s core level, initially null, appears at a temperature around 820 K (Figure 9b). Interestingly, we note that the BE is 284.4 eV and the corresponding line-shape is too broad to be attributed to a Gr phase. Only 18 Environment ACS Paragon Plus

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at T=650 K the BE returns to be that of the Gr phase, i.e. 284.1 eV and, congruently, the lineshape becomes sharp. Further cooling down to T=300 K does not produce noticeable changes in the C-1s core-level spectra (not shown). Thus, one can conclude that, during the cooling, carbon atoms dissolved into the bulk diffuse toward the surface.

Figure 9. (a) Behavior of the signal of the C-1s core-level acquired by TP-XPS during the cooling of a sample of Pt3Ni(111) previously exposed to ethene at 970 K. The cooling has been carried out at a rate of -16.8 K/min. Panel (b) shows the behaviour of the photoemission intensity at the BE of 284.1 eV, peculiar of the Gr phase, as a function of temperature (reported in the bottom axis) and of the time (reported in the top axis). During the TP-XPS experiments, the pressure remained in the 10-10 mbar range in order to avoid surface contamination. 19 Environment ACS Paragon Plus


Conclusions

In this work, we have investigated the whole reaction pathway beginning with adsorbed hydrocarbons and terminating with the large-scale growth of a Gr overlayer. Experiments have been repeated for different conditions by changing (i) ethene dose and (ii) sample temperature. When heating monolayer ethene dosed at T=80 K, only submonolayer islands of Gr are achieved. No significant improvement is achieved upon TAHD with bilayer ethene. Monolayer graphene domains fully covering the Pt skin of the Pt3Ni alloy are attained for ethene cracking at T=800 K.

We also demonstrate that carbon atoms are dissolved into the bulk whenever the Pt3Ni(111) sample is exposed at a temperature higher than 825 K. Upon cooling, carbon atoms dissolved into the bulk diffuse toward the surface.

SUPPORTING INFORMATION Fit procedure of the C-1s line-shape; demonstration of the absence of any change in sample composition upon heating; comparative evaluation of LEED patterns of graphene on Pt-skinterminated Pt3Ni(111) and on Pt(111); technical details of NEXAFS experiments; dependence of the C-1s signal of bilayer ethene on sample temperature.

AUTHORS’ CONTRIBUTIONS


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GC and AP devised the project; AP did the experiments at the Elettra synchrotron; LEED experiments were performed by GC; Data analysis was carried out by AP and LW; GC, LW and AP wrote the manuscript. ACKNOWLEDGMENTS AP is grateful to Paolo Lacovig and Silvano Lizzit for support during the beamtimes at the SuperESCA beamline and for helpful discussions. Moreover, AP thanks Sincrotrone Trieste S.C.p.A. for economic funding. LW acknowledges support from the Youth Innovation Promotion Association (CAS), the State Key Program for Basic Research of China (2017YFA0305500), and National Natural Science Foundation of China (61405230, 61675222).

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TOC graphic

1000 900

Temperature [K]

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

800 700 600 500 400

285.0 284.0 283.0 282.0

Binding Energy [eV]


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Insight on Thermally Activated Hydrocarbon Dehydrogenation on the

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