Oxygen-Driven Porous Film Formation of Single-Crystalline Ru

May 13, 2016 - Oxygen-Driven Porous Film Formation of Single-Crystalline Ru Deposited on Au(111) ... This structural reorganization of Ru is driven by...
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Oxygen-Driven Porous Film Formation of Single-Crystalline Ru Deposited on Au(111) Benjamin Herd,† Daniel Langsdorf,† Christian Sack,† Yunbin He,†,‡ and Herbert Over*,† †

Department of Physical Chemistry, Justus-Liebig-University, Heinrich-Buff-Ring 58, D-35392 Gießen, Germany Hubei Collaborative Innovation Center for Advanced Organic Chemical Materials, Key Laboratory for Green Preparation and Application for Functional Materials, Ministry of Education, Faculty of Materials Science & Engineering, Hubei University, Wuhan 430062, China

‡

ABSTRACT: We examined the interaction of oxygen with ultrathin Ru layers deposited on a Au(111) substrate using scanning tunneling microscopy, X-ray photoelectron spectroscopy, and low-energy electron diffraction. The deposition of pure Ru below one monolayer (ML) at room temperature leads to the formation of clusters on the Au(111) surface, preferentially located at the elbow sites of the herringbone reconstruction. Subsequent exposure of molecular oxygen to such a Ru-covered Au(111) surface at 680 K results in the growth of two-layer-thick Ru islands that are embedded in the top Au(111) layer. This structural reorganization of Ru is driven by the minimization of surface energy and mediated by a mobile RuOx species. Deposition of an increasing amount of Ru at 620 K (0.5−10 ML, ML = monolayer) leads to a rough Ru film on Au(111). Subsequent oxygen treatment (10−5 mbar) at 680 K creates either a porous Ru film (6 ML) depending on the thickness of the Ru film.

1. INTRODUCTION

network of point dislocation sites at which deposited metal atoms can selectively nucleate.5,6 In the present study, we examined the deposition of Ru on Au(111) and how the Ru layers interact with oxygen, employing scanning tunneling microscopy (STM), low-energy electron diffraction (LEED), and X-ray photoemission spectroscopy (XPS). So far, only two STM studies have been reported on the deposition of Ru on Au(111). One study examined the electrochemical deposition of Ru on Au(111),7 and the other one, chemical vapor deposition using Ru3(CO)12 as a Ru precursor at elevated temperature.8 The oxidation of Ru deposits on Au(111) has not been studied previously and will be presented in this article.

Recent scanning tunneling microscopy (STM) studies indicated that the oxidation of a Ru(0001) surface proceeds via a modified nucleation and growth mechanism for temperatures higher than 550 K. In this process, first threedimensional oxide clusters are formed at the step edges of the Ru(0001) surface, which act as nucleation sites for the subsequent growth of flat RuO 2 domains with (110) orientation along the (0001) orientation of the ruthenium substrate.1 Flat RuO2(110) domains grow on Ru(0001) and also on TiO2(110)2 right from the start with a minimal thickness of three to four monolayers (ML) before lateral growth sets in. Therefore, we anticipate that a free-standing ultrathin crystalline Ru layer would need a minimum thickness of three monolayers (ML) to be oxidized by molecular oxygen at elevated temperatures above 550 K. To realize such a situation, we need to identify a substrate that is inert against oxygen adsorption and onto which Ru grows in a crystalline manner. The natural choice of such a substrate is Au(111), although the surface of Au(111) undergoes a so-called herringbone reconstruction.3 The reconstructed Au(111) surface consists of adjacent pairs of partial dislocation lines that separate fcc and hcp stacking regions in the topmost layer to relieve surface stress.3 A second and more isotropic stress relief is established by the formation of stress domains that alternate by 120° in a zigzag (so-called herringbone) pattern.4 The bending points (so-called elbows) of the herringbone reconstruction thereby offer a periodic © XXXX American Chemical Society

2. EXPERIMENTAL SECTION The STM and XPS experiments were conducted in a custombuilt three-chamber ultrahigh vacuum (UHV) system.1 The sample can be introduced into the load lock chamber and then transferred under UHV to the main chamber and from there to the STM chamber (VT-STM, Omicron). The main chamber is equipped with a mass spectrometer for residual gas analysis and a dual X-ray source together with a hemispherical analyzer (PSP Vacuum Technology) to perform XPS experiments. For the STM experiments, we used homemade tungsten tips. All Received: March 23, 2016 Revised: May 12, 2016

A

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Figure 1. STM images are taken at U = 1 V and I = 1 nA. (a, b) 150 nm × 150 nm; (c) 50 nm × 50 nm. (a) The Au(111) surface after the deposition of 0.3 ML Ru at room temperature. (b) The surface after subsequent annealing to 620 K for 5 min. (c) Close-up view of (b): Formation of flat Ru islands (triangular, A; elongated, B) with distinct orientation with respect to the Au(111) substrate.

STM images presented here are acquired in constant current mode at room temperature. For the low-energy electron diffraction (LEED) experiments, the Ru−Au(111) sample (Au(111) single crystal, Mateck) was prepared in the analysis chamber of the STM chamber and subsequently transferred to another UHV chamber equipped with a LEED setup (SPECS) where oxidation was also performed. The Au(111) sample was cleaned by three cycles of Ar ion sputtering for 20 min each (p(Ar) = 10−5 mbar, 1.5 kV, ion current: 24.1 μA) and subsequently annealed at 950 K in 10−7 mbar of oxygen for 30 min in order to remove contamination originating from the segregation of impurities coming from bulk Au(111). The sample temperature was measured with an infrared (IR) pyrometer that was precalibrated with a K-type thermocouple. In the LEED chamber, the temperature was measured with a K-type thermocouple directly connected to the Au(111) crystal. For the evaporation of Ru, a commercial one-pocket e-beam evaporator (Tectra) equipped with a Ru rod (Mateck) was employed. In all of our deposition experiments, Ru was deposited on Au(111) at 620 K. The deposition rate of Ru was previously determined by quantitative STM analysis and verified by XPS2 to be approximately 0.25 monolayer (ML) per minute. The Ru-covered Au(111) sample was oxidized in both chambers by backfilling the UHV chamber with molecular oxygen (p(O2) = 1 × 10−5 mbar) through a leak valve while keeping the sample at 680 K.

(Figure 1c) of a triangular island reveals that the bottom layer is 2.5 Å above the Au(111) substrate, whereas the higher region protrudes 1.5 Å from the bottom layer. For the oxidation of the Ru particles (cf. Figure 1a), the sample was annealed in an oxygen atmosphere (p(O2) = 1 × 10−5 mbar) at 680 K for 5 min. The STM images were taken after the sample was cooled to room temperature (cf. Figure 2a). Only a few randomly distributed clusters are discernible on

3. EXPERIMENTAL RESULTS 3.1. Oxidation of Ru Nanoclusters Deposited at Room Temperature. We focus first on the deposition of Ru on Au(111) at room temperature (RT). Figure 1a displays STM images of the obtained Au surface after the deposition of 0.3 ML Ru. The Au(111) surface is covered by Ru clusters that are aligned in evenly separated rows. A height of 2−4 Å and a mean lateral size of 6 nm were determined for these nanodisks. Annealing the surface up to 620 K for 5 min (cf. Figure 1b,c) causes a significant change in the morphology of the particles. Now, round but slightly larger particles (mean cluster size of 8−15 nm) are observed along with triangularly shaped islands as well as elongated islands (marked respectively with A and B in Figure 1c). The increase in the mean cluster size and the formation of flat islands indicate that the Ru clusters are starting to coalesce. The height of the particles is slightly increased to 6 Å. The observed triangular and elongated islands are aligned along a specific (likely high-symmetry) direction of the Au(111) surface and rotated by 120°. The height profile

Figure 2. Oxidation of 0.3 ML Ru at 680 K, deposited on Au(111) at room temperature (p(O2) = 1 × 10−5 mbar, 5 min). (a) Formation of flat and 1- to 2-ML-thick Ru islands (200 nm × 200 nm; U = 1.2 V, I = 4.5 nA). (b) Restored herringbone pattern with holes in the top Au(111) layer and a Ru island (100 nm × 100 nm; U = 1 V, I = 5 nA). (c) Close-up view of the Ru island in (b) (top layer (B)) (10 nm × 10 nm; U = 0.9 V, I = 0.8 nA). (d) Line scan across the triangular island.

the surface. Large triangular and hexagonal islands are formed, surrounded by the restored herringbone reconstruction pattern of Au(111). From this observation, we conclude that the mobility of deposited Ru strongly increased as a result of oxygen exposure at 680 K. As in the case of the gas-phase oxidation of Ru(0001), the high mobility of Ru is assigned to the formation of a mobile RuOx species.1 Also, one-monolayerdeep holes are discernible in the restored Au(111) surface (cf. Figure 2a,b). The newly formed triangular and hexagonal B

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Langmuir islands are oriented along specific directions of the gold substrate. Similar results were reported for the growth of Ni on Au(111) without oxidation.6 The line scan in Figure 2d indicates that the islands consist of two layers. The lower layer (marked with A in Figure 2b) is approximately 0.7 Å higher than the surrounding Au surface. Most likely this difference in height is not due solely to a topographic effect but rather to an electronic effect. We need to recall that the density of states (DOS) values of Au and Ru capped with oxygen are different, thus affecting the apparent height seen in STM. A chemisorbed oxygen phase on Au(111) can be excluded because molecular oxygen cannot dissociate on the inert (macroscopic) gold surface. The topmost (brighter) layer (marked with B in Figure 2b) of the triangular island forms a step edge that is approximately 1.7 Å high that is significantly contracted with respect to a single step height of Ru(0001) (2.1 Å). A close-up view of the topmost layer of the triangular island reveals a row distance of 5.4 Å (cf. Figure 2c) that corresponds to the row distance found for a chemisorbed (2 × 1)-O adsorbate layer on Ru(0001).9 It is known that Ru(0001) layers with chemisorbed oxygen are imaged more deeply than simple topography would presume.10 With no visible chemisorbed oxygen phase on the bottom Ru layer, the reduced step height of 1.7 Å is tentatively ascribed to an electronic effect by the chemisorbed oxygen on the top Ru layer. Alternatively, the reduced step height may be due to the electronic modification of the bottom Ru layer by the underlying Au layer, which may contribute to the electronic effect of STM imaging. Additionally, the depths of the holes in the Au(111) substrate were quantified with a line scan analysis. For smaller holes, a depth of 1 ML is ascertained, whereas larger holes are 2 to 3 ML deep. This finding may be the first hint that Ru atoms have been intermixed into the top Au layer after deposition and upon O2 exposure the Ru atoms are extracted to form islands. Altogether, we infer that these islands on Au(111) consist of a Ru double layer that is embedded in the topmost Au(111) layer. 3.2. Deposition of Ru on Au(111) at High Temperature. We focus now on the physical vapor deposition of Ru on Au(111) at elevated substrate temperature, i.e., 620 K. Figure 3 shows a series of STM images of Ru/Au(111) with an increasing amount of deposited Ru ranging from 0.5 to 10 ML. After the deposition of 0.5 ML Ru at 620 K, STM (Figure 3a) reveals the formation of small particles randomly distributed across the surface. Well-ordered rows of clusters, as observed for Ru deposition at room temperature (cf. Figure 1), are not formed. A magnification of the surface structure (cf. Figure 3b) discloses a strongly disturbed herringbone pattern that may be related to surface alloying as discussed in the literature.11 The step edges of the Au(111) substrate are not straight as on the clean Au(111) surface, but rather these are highly irregular with various orientations. This observation has been discussed in terms of surface alloying for the Mo−Au(111) system.9 A closer look reveals a depletion zone of Ru particles along the step edges of the upper terraces (marked by the white dashed line). Figure 3c shows the surface after the deposition of 1 ML Ru at 620 K. In comparison to 0.5 ML, a significant increase of the number of clusters and the formation of two-dimensional islands (Figure 3d) are discernible. The appearance of these islands indicates that with progressing Ru deposition the observed clusters start to merge, forming a metal adlayer on

Figure 3. Deposition of 0.5−10 ML Ru on Au(111) at 620 K. (a, b) 0.5 ML Ru, (c, d) 1 ML Ru, (e, f) 1.5 ML Ru, (g) 4 ML Ru, and (h) 10 ML. (b, d) Close-ups of images (a) and (c), respectively. (f) Closeup of (e), including a line scan. STM images (a, c, e, g, h): 200 nm × 200 nm, (b, d) 100 nm × 35 nm, and (f) 60 nm × 60 nm. All STM images were recorded with U = 1 V, I = 1 nA.

gold. Still, a cluster-free depletion zone at the step edges is apparent in STM. Figure 3e,f display the surface after the deposition of 1.5 ML Ru. On this flat surface area, large islands are visible that are partially covered by hexagonal islands (close-up in Figure 3f), indicating an hcp(0001) orientation for the growing Ru islands. A height profile across the islands reveals step heights of 2.1 and 4.2 Å corresponding to 1 and 2 layers of Ru. If the islands are pseudomorphically grown on Au(111), then the lateral lattice is expanded by 5% so that the layer distance should shrink to about 1.9 Å according to Liouville’s theorem (conservation of the unit cell volume under stress). Therefore, we presume that these Ru islands have already the native lattice parameters of Ru(0001). After the deposition of 4 ML of Ru (cf. Figure 3g), a rough surface is visible, where the former step arrangement of the Au(111) substrate is masked. In Figure 3h, an STM image of a 10-ML-thick Ru film is shown. The surface is substantially rougher than for 4 ML Ru. With continuing Ru deposition, the C

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Langmuir growth of Ru on Au(111) proceeds via the formation of flat three-dimensional islands and their coalescence to a closed but rough layer. This behavior is actually not reconciled with the higher surface free energy of Ru(0001) in comparison to that of Au(111), which would rather favor the three-dimensional growth of ruthenium on Au(111). 3.3. Oxygen Interaction with Layers of Ru Grown on Au(111) at 680 K. To study the influence of oxygen on the Ru/Au(111) system, 10−5 mbar of oxygen was exposed at 680 K for 30 min to previously prepared Ru samples on Au(111) with Ru coverages of 1.5 and 4 ML. Figure 4a displays the resulting surface morphology after oxygen exposure to a Au(111) surface covered with 1.5 ML of Ru (cf. Figure 3e). Again, as observed with the oxygen exposure to 0.3 ML of Ru (cf. Figure 2), the morphology of the surface changed dramatically. Instead of hexagonal compact islands (cf. Figure 3e), a film with large terraces and randomly distributed 2- to 10-nm-wide holes covers the entire surface. The magnification of a large hole is depicted in Figure 4b, showing additional holes in the second film layer, thus manifesting a porous film structure. A height profile across this large hole (cf. Figure 4c) reveals layer distances of 2.1 and 4.2 Å, in accord with the step height of Ru(0001). These porous Ru films are stabilized by adsorbed oxygen, and their formation is driven by the lower surface energy of the O/Ru layer in comparison to that of the pure Ru layer on Au. Figure 4d displays a 4 ML Ru-covered Au(111) surface after oxygen treatment at 680 K. A closed film with less randomly distributed holes is formed. Line scans across the surface (cf. Figure 4f) reveal a step height of about 2.1−2.2 Å. The corrugation of the terrace is, however, about 1 Å, so the step height is difficult to measure precisely. In addition, small clusters (7−12 nm wide and 1−2 nm high) located at single steps are visible in STM (cf. Figure 4 d,f), which may be attributed to the initial oxidation of the deposited Ru layer. With progressing oxygen treatment (90 min) (cf. Figure 4g), the number and the size of clusters increase (10−25 nm with a height of 3−5.5 nm) as shown in Figure 4i. A few of these clusters are elongated and rotated by 120° with respect to each other (A−C in Figure 4g). This may be considered to be the first indication of the formation of RuO2(110) domains. Similar rotated clusters have been reported for the oxidation of Ru(0001), although these clusters are exclusively formed at double steps.1 To investigate the chemical nature of the newly formed porous Ru film, the oxygen treatment of a 4 ML Ru precovered Au(111) surface was investigated with XPS. Figure 5 displays the evolution of the Au 4p3/2, O 1s, Ru 3p3/2, and Ru 3d5/2 emissions with oxygen exposure time. With the first oxygen treatment (cf. Figure 5, spectrum c; p(O2)= 10−5 mbar, t = 30 min (∼18 000 L O2)), an O 1s peak arises at 530.1 eV that grows slightly in intensity with increasing oxygen exposure. Because of the weak interaction of Au with molecular oxygen, the adsorbed oxygen is assigned to O bound to metallic Ru. The observed O 1s binding energy of 530.1 eV agrees well with the binding energy reported for a chemisorbed oxygen adlayer phase on Ru(0001), namely, 530.07 eV.12 During oxygen treatment, the Ru 3d5/2 signal shifted from 279.9 to 280.4 eV, pointing to a change in the chemical environment of Ru. This peak shift of 0.5 eV to higher binding energies can be attributed to the formation of a chemisorbed oxygen adlayer on Ru. High-resolution X-ray photoelectron spectroscopy studies in combination with DFT calculations by Lizzit et al.13 were

Figure 4. STM images of the oxidation of 1.5 ML of Ru (a) and 4 ML of Ru (d, g) (p(O2) = 1 × 10−5 mbar for 30 min (a, d) and 90 min (g) at 680 K). (a, d, g) 250 nm × 250 nm, (a, d) U = 1 V, I = 2 nA, (g) (U = 1 V, I = 1 nA, (b, e) 70 nm × 70 nm. (b) Close-up of (a); (e) closeup of (d). (c, f) Line profiles showing a step height of 2 Å; (h) line profile of a cluster in (d); and (i) line profile of three clusters in (g). The three rotational domains of the clusters (i) are marked with A, B, and C and red arrows.

reported for the Ru 3d5/2 core level at a chemical shift of up to 1 eV to higher binding energy, depending on the formed oxygen adlayer phase. Similar to Ru 3d5/2, the Ru 3p3/2 peak also shifts by 0.7 eV to higher binding energies upon oxygen exposure (cf. Figure 5, spectra d and e). For Au 4p3/2, only a slight decrease in the peak intensity is observed with Ru deposition as a result of the attenuation of the Au signal by the covering Ru layers. The main result of the XPS experiment is that a 4 ML Ru film does not form flat domains of RuO2(110) under oxidation conditions that are typical for the oxide D

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Figure 5. XPS: Evolution of Au 4p3/2, O 1s, Ru 3p3/2, and Ru 3d5/2 during oxygen exposure (p(O2) = 1 × 10−5 mbar at 680 K). (a, Gray) Clean surface; (b, blue) deposition of 4 ML of Ru on Au(111) at 620 K; (c) 30 min oxygen treatment; (d) 60 min oxygen treatment; and (e) 90 min oxygen treatment.

Figure 6. LEED pattern taken at 62 eV: (a) clean Au(111) surface, (b) after deposition of 4 ML of Ru at 620 K, and (c) after further oxygen treatment (9000 L; p(O2) = 10−5 mbar) at 680 K.

Figure 7. Starting with a surface that was obtained after the oxidation of 4 ML of Ru on Au(111) at 680 K (cf. Figure 6c): (a) LEED pattern after an annealing step to 850 K for 3 min; (b) STM image of a similarly oxidized surface after annealing to 750 K for 120 min (the line scan displays step heights of 4 and 8 Å) (200 nm × 200 nm, U = 1.1 V, I = 2.4 nA); and (c) LEED pattern after an additional oxygen treatment (36 000 L of O2) at 680 K.

formation on Ru(0001).1 However, the STM image taken after 90 min of O2 treatment (cf. Figure 4g) reveals the formation of larger particles, with several of them elongated along specific directions. Because the elongated particles are mutually rotated by 120° with respect to each other, we may presume the formation of RuO2(110) domains that are not discernible in our XPS experiments. To gain structural information about the 4-ML-thick Ru film on Au(111) and its transformation in an

oxygen atmosphere at 680 K, we employed low electron energy diffraction (LEED). LEED is complementary to STM because LEED is an integrating technique, whereas STM is a local technique. Figure 6a displays the diffraction pattern of a clean Au(111) surface. Upon deposition of 4 ML of ruthenium at 620 K (cf. Figure 6b), the LEED pattern vanishes as a result of a covering and obviously rough Ru film. This is consistent with STM (cf. Figure 3g), which indeed reveals a rough surface. E

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Langmuir LEED is known to be very sensitive to surface roughness because of multiple scattering of the outgoing electrons (in contrast to surface XRD: single scattering and therefore much less sensitivity to surface roughness). After oxygen exposure is carried out at 680 K (9000 L of O2) (cf. Figure 6c), a sharp hexagonal LEED pattern appears with a slightly increased reciprocal unit cell; the LEED pattern of the Au(111) substrate remains invisible. Assuming an in-plane lattice constant of Au(111) of aAu(111) = 2.88 Å, the lattice constant of the covering Ru film is inferred to be 2.71 Å. This value corresponds nicely to the bulk truncated surface unit cell of Ru(0001), namely, aRu= 2.72 Å. To learn more about the thermal stability of the porous Ru− O layer on Au(111), the film was annealed to 850 K for 3 min (cf. Figure 7a). New diffraction spots become visible and can be assigned to a (2 × 2)O overlayer on Ru(0001), while weak diffraction spots of the Au(111) substrate reappear. Figure 7b shows an STM image of a similarly prepared surface after thermal treatment at 750 K for 120 min. Only large hexagonal islands are observed on the surface. The porous structure of the film has disappeared. The inserted height profile reveals island heights of 4.0 and 8.0 Å, matching 2- and 4-ML-high Ru islands. The LEED pattern implies that these Ru layers do not fully cover the Au(111) surface, which is consistent with the STM image and the expected dewetting behavior of Ru on Au(111) when oxygen has desorbed. Figure 7c shows a diffraction pattern after an additional oxygen treatment (36 000 L of O2) at 680 K with the evolution of (2 × 2) spots but no sign of oxide formation. The porous structure does not reappear, thus indicating that the porous structure is metastable. 3.4. Oxidation of 10 ML of Ru on Au(111) at 680 K. In the following section, we will present experiments on the interaction of oxygen with a 10-ML-thick Ru film deposited on Au(111), with particular attention drawn to the formation of RuO2. Figure 8a displays an STM image taken after 80 min of oxygen treatment at 680 K (p(O2) = 10−5 mbar). Clearly, two large rotational domains of stoichiometric flat RuO2(110) are visible. In the magnification of the flat oxide terrace, rows of bridging oxygen that are typical of RuO2(110) become visible. This is quite surprising when compared to the starting surface of a very rough 10 ML Ru film (cf. Figure 3h). Obviously, substantial transport occurs even at such a low temperature of 680 K. A line scan across the surface (cf. Figure 8b) reveals the thickness of the oxide domains of up to 2.1 nm, which corresponds to seven layers of RuO2(110). Besides flat RuO2(110) domains, many clusters are formed with a typical height of up to 7 nm. Figure 8c shows a line scan across one of these elongated clusters. The height profile exhibits a triangular shape, which can be attributed to the formation of microfacets of RuO2, e.g., RuO2(100) as reported previously.14 An STM image (cf. Figure 8d) of an area where no stoichiometric RuO2(110) domains and few clusters are present reveals that the underlying surface is flat and not an oxide, presumably a flat Ru layer. The LEED pattern (cf. Figure 8e) taken after 90 min of oxygen treatment shows a rotational smearing of the RuO2(110)-related LEED spots that is indicative of a continuous rotation of the RuO2(110) domains. A similar smearing of diffraction intensity was previously observed for the growth of RuO2(110) on Ru(0001) when prepared at higher temperature (740 K) but for similar O2 pressure conditions.15 No Au-related reflections are observed in the LEED pattern, indicating that Ru completely covers the Au(111) surface. The

Figure 8. Oxidation of 10 ML of Ru deposited on Au(111) at 620 K (cf. Figure 3h): (a) STM image after oxygen treatment (p(O2) = 10−5 mbar) at 680 K for 80 min (700 nm × 700 nm, U = 1.5 V, I = 0.8 nA); close-up: 18 nm × 18 nm, U = 1.2 V, I = 0.8 nA). (b) Line scan across the oxide domain revealing a thickness of 2.1 nm (seven layers). (c) Line scan across the particle indicates microfacets with a height of 7 nm). (d) STM image of an area where no flat RuO2(110) domains are present (130 nm × 130 nm) U = 1.5 V, I = 0.6 nA). (e) Global LEED pattern taken after 90 min under similar oxidation.

LEED pattern exhibits reflections coming from a relaxed Ru film, which confirms the interpretation that the clusters observed by STM (cf. Figure 8e) are grown on a Ru layer and not on Au(111). Figure 9 summarizes the evolution of the O 1s and Ru 3p3/2 XPS signals upon oxygen treatment after 30 (b), 50 (c), and 80

Figure 9. Corresponding XPS measurements of samples shown in Figure 8. Development of the Au 4p3/2, O 1s, and Ru 3p3/2 signals during oxygen exposure (p(O2) = 1 × 10−5 mbar) at 680 K: (a, blue) deposition of 10 ML of Ru at 620 K, (b) after 30 min oxygen treatment, (c) after 50 min oxygen treatment, and (d) after 80 min oxygen treatment. F

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(cf. Figure 1a). In the presence of oxygen, the Ru/Au system is able to minimize the surface free energy by forming a wetting layer of oxygen-covered Ru islands. Surprisingly, the Ru islands are additionally embedded in the Au surface (cf. Figure 2). Evidently, the surface energy of Ru is substantially reduced by oxygen adsorption, thus allowing the formation of large Ru islands. The embedding of the Ru double-layer islands could be explained by the fact that Au atoms are now also able to wet the step edges of the double-layer Ru islands. A similar stabilization effect has been reported previously for Cu on Pb(111).25 When Ru is deposited on Au(111) at an elevated temperature of 620 K, then surface alloying is suggested by the formation of serrated step edges. At the step edges, Ru replaces Au atoms, and these Au atoms are incorporated into the next step edge, leading to the observed step flow. The formation of serrated step edges during metal deposition has previously been reported for other metal−metal systems including Mo and Ni on Au(111),5,11,26 Pd on Cu or Ag, and Au on Ag27−29 and is commonly explained by a place exchange process where substrate metal atoms are ejected from the surface and replaced by atoms of the deposited metal. Because of the high temperature, the ejected substrate atoms diffuse to the nearby step. This results in step flow with serrated step edges and a cluster-depleted zone as observed in STM (cf. Figure 3). Larger amounts of Ru (2−10 ML) deposited at 620 K lead to the growth of Ru islands and finally to the formation of a rough, completely covering Ru layer, presumably with the same lattice constant as for Ru(0001). These Ru layers transform into a flat film when exposed to oxygen at 680 K for 30 min. For coverages below 4 ML, no oxidation of the Ru layer into flat RuO2(110) domains is observed in STM, LEED, or XPS. Instead, the Ru films forms a porous or perforated structure upon O2 exposure with randomly distributed 2- to 10-nm-wide holes covering the entire surface. The perforated Ru film reveals a sharp LEED pattern of Ru(0001) that is indicative of a singlecrystalline film. From XPS, we know that these perforated films are covered by adsorbed oxygen. In addition, small particles are observed whose density increases with oxygen exposure. These particles may be oxides such as observed in the initial oxidation of Ru(0001) single crystals by molecular oxygen.1 The reported oxide particles1 are formed by a mobile RuOx species and are located at step edges of Ru(0001) as also observed for the porous Ru layer in Figure 4. The formation of nanoholes in the porous layer is likely due to kinetics, i.e., the transport process involved in the transformation of a rough Ru layer to a flat but porous layer. Also, this process is mediated by a mobile RuOx species. For thicker Ru films (e.g., 10 ML), the same oxygen treatment leads to the formation of flat and 2.1-nm-thick RuO2(110) domains. A very similar thickness of the flat RuO2(110) layer was also reported for the oxidation of Ru(0001) when O2 was exposed at 680 K.30 These flat RuO2(110) domains on the 10-ML-thick Ru film are in coexistence with a high density of 7-nm-high and 30-nm-wide RuO2 clusters with varying rotational orientations at the surface (cf. Figure 8). STM experiments indicate that these clusters are growing on flat Ru layer rather than on RuO2. For the oxidation of Ru(0001),1 is has been shown that the oxidation process starts with the formation of a mobile RuOx species by a corrosion of the step edges. In a second step, stable RuO2 nuclei are formed. This process depends critically on the applied oxygen pressure and should be >10−5 mbar. Some of

min (d). After 30 min of oxygen exposure, an O 1s peak at 530.2 eV appears that is assigned to chemisorbed oxygen on Ru. With progressing oxygen exposure (50 and 80 min), the O 1s peak shifts to 529.7 eV, which is assigned to the formation of RuO2.12 Compared to the 4 ML Ru/Au(111) sample, a 2 times higher peak intensity of the O 1s signal is observed for the oxidized 10 ML Ru/Au(111) surface. The significantly higher O 1s intensity and the shift of the Ru 3p3/2 feature by 1 eV to higher binding energies corroborate that RuO2 is formed during this oxygen treatment. The intensity of Ru 3p3/2 decreases upon oxygen exposure, which is reconciled with a 30% lower Ru density in the RuO2 clusters than in metallic Ru.

4. CONCLUDING DISCUSSION Although Au and Ru do not form bulk alloys,16,17 we have experimental evidence that Ru and Au form a surface alloy in the topmost Au layer at higher deposition temperature. At room temperature, the Ru-induced exchange of Au is not observed. Rather, at room temperature Ru nucleates at the elbow of the herringbone reconstruction,6,11,18,19 forming clusters in a well-ordered row structure. This has been observed with STM when 0.3 ML of ruthenium is deposited on Au(111) at room temperature. The formation of well-ordered cluster rows was previously reported in the literature for the electrochemical deposition of Ru on Au(111)7 and chemical vapor deposition using Ru3(CO)12 as a Ru precursor at elevated temperature.8 In both of these studies, STM images revealed that Ru particles nucleate preferentially at the elbow sites of the reconstructed Au(111) surface, which are also expected to be the most reactive nucleation sites for deposited Ru, consistent with previous DFT calculations that reveal that Ru nanoparticles on the Au(111) surface are energetically preferred to embed in the Au(111) surface.19 The preferential nucleation of metals at the elbow sites of the reconstructed Au(111) surface is an established phenomenon and was reported for the deposition of various other metals including Fe,20 Co,21 Ni,5,6,18 and Mo.11 In the case of the electrochemical deposition of Ni on Au(111), it was proposed that the initial nucleation step proceeds via an atom-exchange process, where deposited metal atoms substitute for Au atoms in the topmost gold layer. This exchange process was evidenced by the formation of small holes at the elbow sites with an apparent depth of 0.5 Å followed by the growth of Ni nanoclusters due to the trapping of additional Ni atoms.18 The observed hole depth of 0.5 Å was explained by the smaller size of the embedded Ni atoms. Oxidation of the Ru nanoparticles in an oxygen atmosphere (p(O2) = 1 × 10−5 mbar) at 680 K for 5 min induces the rearrangement of these clusters into large double-layer islands that are embedded in the topmost layer of Au(111). Obviously oxygen exposure creates a mobile RuOx species, as recently reported for the oxidation of Ru(0001),1 that is responsible for the large mass transport for the transformation of particles into double-layer islands. The driving force for the observed growth behavior can be broadly explained by the variation of the surface energy of Ru during oxygen exposure. On the basis of the surface free energy of gold σAu, ruthenium σRu, and oxygen -covered ruthenium σRu/O, the following relationship is derived from our STM experiments: σRu > σAu > σRu/O.22 The higher surface energy of Ru(0001) (3.05 J/m2) compared to that of Au(111) (1.50 J/m2)23,24 and the fact that Ru is immiscible in bulk Au suggest that Ru will form only 3D particles on the Au(111) surface, which has indeed been observed with STM for the room-temperature deposition of Ru G

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Langmuir the oxide clusters can further grow to form flat RuO2 islands in the (110) orientation. This growth process can be accomplished at much lower oxygen pressures, i.e., nucleation and growth can be decoupled to optimize the morphology of the oxide film.1 XPS experiments (cf. Figure 9) suggest that cluster formation on the 10-ML-thick Ru film may be reduced by decreasing the O2 exposure. This may open the door to preparing well-defined and flat RuO2(110) layers on Au(111). First, we may apply a high O2 pressure of 10−5 mbar to form a few critical RuO2 nuclei; subsequently, these nuclei grow slowly in a low-O2pressure environment while suppressing the formation of further nuclei. This technology may allow for the preparation of flat model electrodes for studying for instance the stability of ultrathin RuO2 layers on Au(111) in the electrochemical oxygen evolution reaction (OER).

(4) Narasimhan, S.; Vanderbilt, D. Elastic Stress Domains and the Herringbone Reconstruction on Au(111). Phys. Rev. Lett. 1992, 69, 1564−1567. (5) Chambliss, D. D.; Wilson, R. J.; Chiang, S. Nucleation of Ordered Ni Islands Arrays on Au(111) by Surface Lattice Dislocations. Phys. Rev. Lett. 1991, 66, 1721−1724. (6) Cullen, W. G.; First, P. N. Island Shapes and Intermixing for Submonolayer Nickel on Au(111). Surf. Sci. 1999, 420, 53−64. (7) Strbac, S.; Magnussen, O. M.; Behm, R. J. Nanoscale Pattern Formation During Electrodeposition: Ru on Reconstructed Au(111). Phys. Rev. Lett. 1999, 83, 3246−3249. (8) Cai, T.; Song, Z.; Chang, Z.; Liu, G.; Rodriguez, J. A.; Hrbek, J. Ru Nanoclusters Prepared by Ru3(CO)12 Deposition on Au(111). Surf. Sci. 2003, 538, 76−88. (9) Pfnur, H.; Held, G.; Lindroos, M.; Menzel, D. Oxygen induced Reconstruction of a Close-Packed Surface − a LEED-IV Study on Ru(0001)-p(2 × 1)O. Surf. Sci. 1989, 220, 43−58. (10) Wintterlin, J.; Trost, J.; Renisch, S.; Schuster, R.; Zambelli, T.; Ertl, G. Real-time STM Observations of Atomic Equilibrium Fluctuations in an Adsorbate System: O/Ru(0001). Surf. Sci. 1997, 394, 159−169. (11) Biener, M. A.; Biener, J.; Schalek, R.; Friend, C. M. Surface Alloying of Immiscible Metals: Moon Au(111) Studied by STM. Surf. Sci. 2005, 594, 221−230. (12) Over, H.; Seitsonen, A. P.; Lundgren, E.; Wiklund, M.; Andersen, J. N. Spectroscopic Characterization of Catalytically Active Surface Sites of a Metallic Oxide. Chem. Phys. Lett. 2001, 342, 467− 472. (13) Lizzit, S.; Baraldi, A.; Groso, A.; Reuter, K.; Ganduglia-Pirovano, M. V.; Stampfl, C.; Scheffler, M.; Stichler, M.; Keller, C.; Wurth, W.; Menzel, D. Surface Core Level Shifts of Clean and Oxygen-Covered Ru(0001). Phys. Rev. B: Condens. Matter Mater. Phys. 2001, 63, 205419. (14) Assmann, J.; Crihan, D.; Knapp, M.; Lundgren, E.; Löffler, E.; Muhler, M.; Narkhede, V.; Over, H.; Schmid, M.; Varga, P. Understanding the Structural Deactivation of Ruthenium Catalysts on an Atomic Scale Under Both Oxidizing and Reducing Conditions. Angew. Chem., Int. Ed. 2005, 44, 917−920. (15) Goritzka, J. C.; Herd, H.; Krause, P. P. T.; Falta, J.; Flege, J. I.; Over, H. Insights into the Gas Phase Oxidation ̀of Ru(0001) on the Mesoscopic Scale Using Molecular Oxygen. Phys. Chem. Chem. Phys. 2015, 17, 13895−13903. (16) Curtarolo, A.; Morgan, D.; Ceder, G. Accuracy of Ab Initio Methods in Predicting the Crystal Structures of Metals: A review of 80 Binary Alloys. CALPHAD: Comput. Coupling Phase Diagrams Thermochem. 2005, 29, 163−211. (17) Schwank, J. Gold in Bimetallic Catalysts. Gold Bull. 1985, 18, 2− 10. (18) Möller, F. A.; Kintrup, J.; Lachenwitzer, A.; Magnussen, O. M.; Behm, R. J. In situ STM study of the ElectroDeposition and Anodic Dissolution of Ultrathin Epitaxial Ni Films on Au(111). Phys. Rev. B: Condens. Matter Mater. Phys. 1997, 56, 12506−12518. (19) Liu, P.; Rodriguez, J. A.; Muckerman, J. T.; Hrbek, J. Interaction of CO, O, and S with Metal Nanoparticles on Au(111): A Theoretical Study. Phys. Rev. B: Condens. Matter Mater. Phys. 2003, 67, 155416. (20) Voigtländer, B.; Meyer, G.; Amer, N. M. Epitaxial-growth of Fe on Au(111) - a Scanning Tunneling Microscopy Investigation. Surf. Sci. 1991, 255, L529−L535. (21) Voigtländer, B.; Meyer, G.; Amer, N. M. Epitaxial-Growth of Thin Magnetic Cobalt Films on Au(111) Studied by Scanning Tunneling Microscopy. Phys. Rev. B: Condens. Matter Mater. Phys. 1991, 44, 10354−10357. (22) Langsdorf, D.; Herd, B.; He, Y.; Over, H. Oxidation-Induced Dispersion of Gold on Ru(0001): A Scanning Tunneling Microscopy Study. J. Phys. Chem. C 2015, 119, 16046−16057. (23) Vitos, L.; Ruban, A. V.; Skriver, H. L.; Kollár, J. The Surface Energy of Metals. Surf. Sci. 1998, 411, 186−202. (24) Aqra, F.; Ayyad, A. Surface Energies of Metals in Both Liquid and Solid States. Appl. Surf. Sci. 2011, 257, 6372−6379.

5. SUMMARY We provide evidence that Ru and Au(111) may form a surface alloy at 620 K, but there is no experimental signature for surface alloying at room temperature. The surface alloy becomes dealloyed upon oxygen exposure at 680 K, resulting in embedded Ru double-layer islands. When the Au(111) surface is covered by 1.5 to 4 ML of Ru, rough Ru films pseudomorphically grow on Au(111). Upon oxidation at 680 K, these Ru films transform to a perforated single-crystalline Ru layer that presents promising model catalysts with new functionalities (holes). Oxidation of the Ru layer does not form flat RuO2(110) domains as long as the Ru coverage is below 4 ML. However, the oxidation of 2- to 4-ML-thick Ru layers leads to the formation of small particles that can be assigned to RuO2. Flat RuO2(110) domains do form after the oxidation of 10-ML-thick Ru layers grown on Au(111). These flat RuO2(110) domains coexist with large RuO2 particles. The produced Ru films on Au(111) can serve as promising model electrodes in electrocatalysis. For instance, one can study the dissolution behavior of RuO2(110) and/or metallic Ru in the electrochemical oxygen evolution reaction.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful for financial support from the German Science Foundation (Ov21-9/1). REFERENCES

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Langmuir (25) Nagl, C.; Platzgummer, E.; Schmid, M.; Varga, P.; Speller, S.; Heiland, W. Subsurface Islands and Superstructures of Cu on Pb(111). Surf. Sci. 1996, 352-354, 540−545. (26) Meyer, J. A.; Baikie, I. D.; Kopatzki, E.; Behm, R. J. Preferential Island Nucleation at the Elbows of the Au(111) Herringbone Reconstruction Through Place Exchange. Surf. Sci. 1996, 365, L647−L651. (27) Murray, P. W.; Throshaug, S.; Stensgaard, I.; Besenbacher, F.; Laegsgaard, E.; Ruban, A. V.; Jacobsen, K. W.; Kopidakis, G.; Skriver, H. L. Heteroepitaxial Subsurface Growth Mode Resulting in Interlayer Mixing. Phys. Rev. B: Condens. Matter Mater. Phys. 1997, 55, 1380− 1383. (28) Hirschorn, E. S.; Lin, D. S.; Hansen, E. D.; Chiang, T. C. Atomic burrowing and hole formation for Au growth on Ag(110). Surf. Sci. 1995, 323, L299. (29) Murray, P. W.; S; Stensgaard, I.; Laegsgaard, E.; Besenbacher, F. Mechanisms of Initial Alloy Formation for Pd on Cu(100) Studied by STM. Phys. Rev. B: Condens. Matter Mater. Phys. 1995, 52, 14404− 14407. (30) He, Y. B.; Knapp, M.; Lundgren, E.; Over, H. Ru(0001) Model Catalyst Under Oxidizing and Reducing Reaction Conditions: In-situ High-Pressure Surface X-Ray Diffraction Study. J. Phys. Chem. B 2005, 109, 21825−21830.

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DOI: 10.1021/acs.langmuir.6b01139 Langmuir XXXX, XXX, XXX−XXX