Article pubs.acs.org/JPCC
Atomic Scale Insights into the Initial Oxidation of Ru(0001) Using Molecular Oxygen: A Scanning Tunneling Microscopy Study Benjamin Herd, Marcus Knapp, and Herbert Over* Department of Physical Chemistry, Justus-Liebig-University, Heinrich-Buff-Ring 58, D-35392 Gießen, Germany ABSTRACT: The initial oxidation of Ru(0001) has been studied by scanning tunneling microscopy (STM) using molecular oxygen as the oxidizing agent. The initial oxidation process reveals a complex behavior in which three-dimensional RuO2 domains (clusters) are exclusively formed at multiple steps, and only few of these clusters are able to initiate the growth of flat RuO2 domains with its (110) orientation along the (0001) direction of the Ru substrate. The oxide formation requires a minimum temperature of 500 K and a minimum pressure of about 1 × 10−5 mbar of oxygen. Below 1 × 10−5 mbar, oxide growth is very slow, although even at pressure of 10−6 mbar oxidation in step bunching regions is occasionally observed. The modified nucleation and growth mode of Ru(0001) oxidation is corroborated by growing the oxide in two separate steps: Starting with oxygen exposure of 1.5 × 10−5 mbar of O2 for 960 s at a sample temperature of 680 K, RuO2 nuclei are preferably formed. Subsequently, the further growth of flat oxide domains at 680 K is conducted by O2 pressures of 2.5 × 10−7 mbar, a pressure which is far below the threshold pressure for cluster formation. Chemical reduction experiments of ultrathin oxide layers by exposing CO at 600 K sample temperature allows to determine precisely the local thickness of the ultrathin RuO2(110) films. This reduction experiment reveals that these flat oxide layers penetrate into the topmost Ru layer starting from step edges.
1. INTRODUCTION Ruthenium is a transition metal with peculiar catalytic properties both in the oxidation and in the reduction of molecules in heterogeneous and homogeneous catalysis.1−4 For instance, ruthenium has shown to be an efficient oxidation catalyst for the CO oxidation at high pressures,5−7 while under ultrahigh vacuum (UHV) conditions in typical surface science experiments, single crystal Ru(0001) turned out to be practically inactive.8−10 This property was first considered as manifestation of a so-called pressure gap. Lately around the year 2000, this puzzle has been partly resolved and shown to be related to a materials gap rather than a pressure gap: Under oxidizing reaction conditions, the Ru(0001) surface transforms into a surface that is coated by an ultrathin RuO2(110) surface oxide. These ultrathin RuO2 films have shown to be the active component in the oxidation of CO.11 Not only is the CO oxidation catalyzed by RuO2(110), so is the industrially important HCl oxidation.12−14 During the past decade, the RuO2(110) surface has been thoroughly studied by a variety of typical surface science techniques, including low energy electron diffraction (LEED),15 density functional theory (DFT) calculations,16−18 scanning tunneling microscopy (STM),19−21 high-resolution core level shift spectroscopy (HRCLS),22 high-resolution energy electron loss spectroscopy HREELS,23 photoelectron emission microscope (PEEM), 24 scanning photoemission microscopy (SPEM),25,26 low energy electron microscopy (LEEM),27,28 and reflection absorption infrared spectroscopy (RAIRS).29 The interaction of Ru with oxygen has been intensively studied over the past 30 years as summarized in the next paragraphs. However, our understanding of the atomic processes in the © 2012 American Chemical Society
initial transformation of Ru(0001) into RuO2(110) is still largely elusive. When exposing the close-packed Ru(0001) surface to molecular oxygen under UHV conditions, oxygen molecules dissociate without activation forming chemisorbed oxygen in ordered (2 × 2)O30−32 or (2 × 1)O33−35 overlayers with coverages of 0.25 and 0.5 monolayer (ML), respectively.36 One ML corresponds to a number of surface atoms that equals that of Ru atoms in the topmost layer of Ru(0001). The dissociative sticking coefficient of molecular oxygen drops from almost one at low coverages to less than 10−3 at 0.5 ML. Therefore, under typical UHV conditions, the (2 × 1)O phase had been considered as the saturation surface O phase on Ru(0001).30 Dosing much more of molecular oxygen, say more than 1000 L (one Langmuir: 1 L corresponds to a dose of 1.3 × 10−6 mbar·s) or high exposures of NO2 (100 L) at surface temperatures of about 500 K, the Ru(0001) surface stabilizes two additional ordered surface phases of chemisorbed O, namely, the (2 × 2)-3O37−39 and the (1 × 1)O40 with coverages of 0.75 and 1.0 ML, respectively. The dissociative sticking coefficient of molecular oxygen over the Ru(0001)-(1 × 1)O surface is estimated to be less than 10−6.41 Therefore, oxygen uptake beyond a coverage of 1 ML is considered to be rate controlling for the initial oxidation of Ru(0001),42 requiring excessive exposures of molecular oxygen or NO2 at elevated temperatures (>500 K) in order to accommodate more than 1 ML of oxygen in the Ru(0001) system.43−50 Received: August 28, 2012 Revised: October 16, 2012 Published: October 23, 2012 24649
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Gas phase oxidation of Ru(0001) was first identified for high NO2 exposure at 800 K on the basis of Ru 3d core level shift spectra.48 The oxidation of Ru(0001) dosing molecular oxygen at elevated temperatures has been studied on the mesoscale, applying LEEM,27,28 SPEM,25 and PEEM.24 These studies indicated that the transition from oxygen adsorption to oxide formation on Ru(0001) is structurally intricate due to the coexistence of various oxygen phases on the surface and the roughening of the oxide surface,19 which leads to complex pattern formation in PEEM on the micrometer scale. Firm conclusions of the atomic scale processes during oxidation of Ru(0001) are difficult to draw from these mesoscopic experiments alone. Several researchers proposed the accumulation of oxygen in the subsurface region to precede the actual oxidation step of the surface.41,49,51−53 However, the experimental evidence whether subsurface O is required for this homogeneous oxidation process is not conclusive. On the basis of DFT calculations, a O−Ru−O trilayer has been postulated as the precursor for the surface oxidation of Ru(0001).54 Although there is no conclusive evidence for the precursor behavior of the O− Ru−O trilayers in the oxidation of Ru(0001), this trilayer motif has been unambiguously identified in the surface oxidation of other platinum group metal surfaces, such as Rh(111),55 Rh(110),56 Rh(100),57 and Ir(111).58 The first report of such an O−Me−O trilayer goes back to Mitchell and co-workers who studied the surface structures during initial oxidation of Zr(0001).59,60 The oxidation of Ru(0001) in a mixture of 115 ppm of O2 in Ar produced at 600−800 K a monolayer of ordered RuO2(110) in the form of rectangular stripes.61 The growth of these RuO2(110) stripes is unidirectional, starting from step edges. Combined STM and SPEM experiments on Ru(0001) have demonstrated that oxide films grow more promptly in step bunching regions than on flat terraces.62 This observation has been confirmed by in situ LEEM experiments during exposure of Ru(0001) to NO2.28 In situ surface X-ray diffraction (SXRD) measurements,45 where the X-ray intensity of a typical RuO2 related reflection is monitored as a function of the oxygen exposure time, are consistent with a nucleation and growth mode of the oxide layer on Ru(0001) as manifested in so-called Avrami curve. Alternatively, the oxidation process may also be considered as an autocatalytic oxidation process of ruthenium.42 the rationale behind the autocatalytic oxidation is that the sticking coefficient of oxygen is very low on the (1 × 1)O phase, but on the RuO2(110) island, the sticking coefficient is as high as 0.7.41 Therefore, a once formed oxide island is catalyzing the further oxidation assuming oxygen uptake, and the Ru supply is not rate limiting. The long induction period in the SXRD-derived Avrami curves45 together with the occurrence of a threshold O2 pressure of 1 × 10−5 mbar (at T = 650 K) for the oxidation of the Ru(0001) sample suggests that critical RuO2 nuclei are required to initiate the oxide growth. The formation of a RuO2 nucleus is a dynamical process of growth and decomposition. Only beyond a critical size is the RuO2 nucleus stable (critical nucleus), which affords a minimum oxygen pressure (critical/ threshold pressure) depending on the sample temperature. The grown RuO2(110) film on Ru(0001) is 1.6 nm thick over a growth temperature range from 550 to 650 K and pressure range of 10−4 to 10 mbar,45 manifesting a self-limited growth of RuO2(110) on Ru(0001). A similar thickness of the oxide was derived from a recent XPS study63 and STM study.64
From the l-scans in SXRD, which probes the structure normal to the surface, the RuO2(110) film surface is found to be very flat, i.e., both at the surface and at the interface Ru/RuO2 are atomically smooth. The oxide film grows epitaxially and incommensurately to the underlying Ru(0001) substrate,15 and the size of the RuO2(110) domains is several 10 nm across, when the oxide film is grown below 650 K.45 With SXRD, the in-plane lattice parameters of the RuO2(110) film were found to be 3.10 Å × 6.39 Å.45 These values correspond closely to values of the bulk-truncated unit cell, namely, 3.11 Å × 6.38 Å. Altogether, the RuO2(110) film can be considered to grow unstrained on the Ru(0001) surface. The layer distance between the Ru layers in RuO2(110) is found to be 3.1 Å,15,45 which is identical to the step heights of RuO2(110) as observed in STM.18,19 Since the RuO2(110) oxide film with its rectangular unit cell exhibits no 3-fold symmetry as the Ru(0001) substrate, three domains rotated by 120° coexist on the Ru(10001) surface. If the oxygen exposure is not too high, the (1 × 1)O phase coexists with the RuO2(110) oxide phase as observed in STM18 and also in LEED15 and SPEM.25 Although this brief review provides already plenty of information on the oxide formation on Ru(0001), it is fair to say that the atomic processes involved in the initial oxidation of Ru(0001) are still not settled. This unsatisfying situation was our main motivation to study the initial oxidation of Ru(0001) in detail with STM. The initial oxidation of Ru(0001) is shown to proceed via a modified nucleation and growth mode, starting at multiple step edges and step bunching areas. Chemical reduction experiments of the once prepared oxide films disclose that the ultrathin oxide layer grows and penetrates into the topmost Ru layer starting from the step edge.
2. EXPERIMENTAL DETAILS The experiments were conducted in a home-built threechamber ultrahigh vacuum (UHV) system. The sample can be introduced via the load lock chamber, which contains a small sample manipulator, a magnetic rod for sample transfer, and a gas manifold to produce the oxide without deteriorating the pressure in the analysis chamber. The load lock chamber is pumped by a 70 L/s turbo pump and is separated from the analysis chamber by CF40 gate valve. From the load lock chamber, the sample can be transferred by a magnetic rod to the long-traveling sample manipulator of the analysis chamber. This chamber is furnished with a mass spectrometer and a dual X-ray source together with a semispherical analyzer (PSP Vacuum Technology) to perform X-ray photoelectron spectroscopy (XPS) experiments. In addition, the analysis chamber contains an atomic oxygen source (Oxford Applied Research: TC50, Universal Thermal Cracker). The analysis chamber is pumped by a magnetic turbo pump (Edwards: 300 l/s) together with an ion getter pump (400 l/s). The mechanical vibrations due to the turbopump are so small that the turbopump can run with full speed during the STM experiments without negative impact on the image quality. The scanning tunneling microscope (STM) chamber is separated from the analysis chamber by a CF150 gate valve and separately pumped by an ion getter pump (100 l/s). The sample can be transferred from the analysis chamber to the STM chamber just by translating the manipulator through the open CF150 gate valve and placing then the sample plate into the STM (VT-STM, Omicron) with a wobble stick. In general, we used homemade tungsten tips. For some of the experiments, we also used Pt tips, which were electrochemically etched by 24650
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sodium cyanide applying an AC voltage of 4 V.65 The advantage of Pt tips over W tips is their improved chemical resistance against weakly bound oxygen on the RuO2(110) surface. All STM images presented in this article are taken in the constant current mode. The sample temperature was not measured directly since the thermocouple cannot be transferred from one sample holder to another one. Rather, in a separate experiment, the stationary sample temperature for a specific heating power was determined by a fixed thermal couple of K type. In this way, the sample temperature is calibrated against the heating power. Above 600 K, the sample temperature was counter-checked by an IR-pyrometer. Accordingly, the error bars in the given sample temperatures are estimated to be at most ±30 K. The Ru(0001) sample was cleaned by Ar-ion sputtering for 10 min (p(Ar) = 10−5 mbar, 1.5 kV, and 15 mA) and roasted at 950 K in 10−7 mbar of oxygen in order to remove carbon contamination originating from carbon segregation from bulk Ru. Since the highest sample temperature achieved with the present sample holder is limited to 1100 K, we cannot remove all adsorbed oxygen from the Ru(0001) surface; still about 0.25 ML of adsorbed O remains on the surface after cleaning procedure of the Ru(0001) surface as determined by STM (mainly (2 × 2)O and rarely (2 × 1)O domains are observed). In addition, this temperature is not sufficient to anneal the bubbles induced by Ar-ion sputtering (cf. Figure 1).66 Oxidation of the sample was either performed in the load lock chamber or in the analysis chamber by exposing molecular oxygen as oxidizing agent. The STM images were taken at 300 K without dosing oxygen during the STM experiments. Figure 1. Oxygen uptake on Ru(0001) is shown as a function of the exposure time, while dosing 3 × 10−5 mbar of O2 at a sample temperature of 680 K. The oxygen up-take was measured by the integral intensity of O 1s in XPS. To each of the regions, 600 nm × 600 nm STM images are shown (U = 1.1 V; I = 1.0 nA). The oxygen uptake curve can be subdivided in four characteristic regions: (i) oxygen chemisorption on the terraces (a) and the formation of RuO2 clusters (b) and flakes (c) in the step-bunching region (0−600 s), (ii) slow formation of flat RuO2 domains (d,e) on the terraces of Ru(0001) (600−2000 s), (iii) oxide growth (e) (2000−4500 s), (iv) saturation (f) where the surface is fully covered by flat RuO2 domains. The flat oxide regions are emphasized with different colors for the three rotational domains.
3. RESULTS AND DISCUSSION 3.1. Oxygen Uptake during the Initial Oxidation of Ru(0001). Figure 1 summarizes the initial oxidation of Ru(0001) in terms of oxygen uptake as a function of O2 exposure time, keeping the partial oxygen pressure at 3 × 10−5 mbar and the sample temperature at 680 K. The oxygen concentration on the Ru(0001) surface is derived from the integral intensity of the O 1s spectrum (XPS). The integral intensity of the (2 × 1)O, which is produced after exposing 50 L of O2 at room temperature, is assigned to 0.5 ML, and all other oxygen coverages are calibrated against this value. We start the oxidation experiments from the Ru(0001)−(2 × 1)O phase. For thicker oxide layers, the oxygen content on the Ru(0001) surface is systematically underestimated due to attenuation of the O 1s signal originating from deeper layers. Also shown in Figure 1 are STM images of each of these characteristic regions. A STM image (a) taken at 0 s indicates a flat substrate, which is covered by (2 × 1)O phase, and many Ar-bubbles are visible. Already after 420 s (STM images (b)) oxide clusters are formed in step bunching regions. The averaged size of these clusters is 8−20 nm with a thickness of 1−2 nm. Oxygen uptake in the time interval of 400−700 s is quite fast. After 690 s, flat RuO2(110) layers are grown out of this step bunching regions (STM image (c)). During the exposure time (600−2400 s), oxygen take-up is slow, which is consistent with the slow formation of flat RuO2 domains (STM images (d) and (e)) on the terraces of Ru(0001). This oxidation stage may be envisioned as an induction regime for the growth of the flat oxide. After 2700 s (STM image (e)), flat RuO2(110) layers are covering the terraces although most of the terraces are still oxide-free. In the region (2400−4500 s) the oxygen up-take is accelerated. The oxide is now readily growing
across the surface. Finally, the region above 4000 s is characterized by a very slow growth of the oxide. In the STM image (f), the Ru(0001) surface is densely covered by an ultrathin RuO2(110) film. In this STM image, three rotational domains of RuO2(110) are discernible from the different textures. We should note that the oxygen uptake curve in Figure 1 deviates significantly from a typical S-shaped Avrami curve, which is reconciled with the more involved nucleation and growth process as indicated by STM. We have to keep in mind that the saturation part of the oxygen uptake curve is affected by the attenuation of O 1s photoelectrons coming from deeper layers. However, a quite similar behavior was also reported in a previous in situ SXRD study.45 In particular, also with SXRD, the oxide growth was found to saturate after one hour. The total amount of oxygen on the Ru(0001) surface after 4000 s of O2 exposure correspond to 2.5 ML (cf. Figure 1), which may actually be somewhat higher due to expected attenuation of the O 1s signal. From STM, an oxide film with 3−4 ML can be deduced, 24651
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Figure 2. (a) Clean Ru(0001) surface was exposed to 1 × 10−6 mbar of O2 at a sample temperature of 680 K for 1 h (U = 1.2 V; I = 1.0 nA; 1000 nm × 1000 nm). (b) Zoomed-in a region marked by the blue square: 200 nm × 200 nm. Two 3D-RuO2 domains (clusters) are located at multiple step edges of the substrate. (c) STM image (200 nm × 200 nm) of a step bunching region: clearly a few RuO2 clusters are recognizable. (d) Clean Ru(0001) surface was exposed to 1 × 10−6 mbar of O2 at a sample temperature of 680 K for 5 h, indicating scarcely occurring oxide islands in the step bunching regions (U = 0.8 V; I = 1.0 nA; 1000 × 1000 nm). (e) Scanning electron micrograph: Only very rarely are oxide domains in step bunching regions recognizable as dark small spots in the SEM image.
surface. Exceedingly, most of the Ru(0001) surface is still oxidefree. With scanning electron microscopy (SEM) (cf. Figure 2e) we studied the oxide growth on the mesoscopic scale (15 μm × 15 μm). As indicated in Figure 2e, the growth of flat oxide domains is indeed a rare event. Therefore, these experiments are consistent with previous SXRD and XPS experiments that infer a critical pressure of 1 × 10−5 mbar of O2 for the (macroscopic) oxide growth. 3.2.2. Oxidation at O2 Pressure of 10−5 mbar. In order to study the initial oxidation of Ru(0001) more effectively, the next oxidation experiment was conducted with the critical pressure of 1 × 10−5 mbar of O2 for 960 s, keeping the sample temperature at 680 K. From the STM image in Figure 3a, it is obvious that most of the terraces are oxide-free. This means that this oxidation experiment is still within the induction regime of the oxidation. The step edges are straight as observed with the clean Ru(0001) surface (cf. Figure 3) and there is no indication of corrosion of the step edges. Only rarely can one recognize oxide islands as shown in Figure 3a. This ultrathin oxide island is located at a multiple step (cf. Figure 3b) and grows alongside the step edge on the upper terrace. A single cluster is protruding the oxide islands with a height of 1.5 nm. Whether this cluster represents the nucleus from which the oxide island has grown cannot be judged from this STM image. However, this cluster is top-flatted with an apparent height of 1.5 nm above the top layer of the oxide island (cf. Figure 3c, blue linescan) corresponding to a 5-fold step of RuO2(110). Therefore, this RuO2 cluster may have the proper (110) orientation of RuO2 to induce the growth of a flat RuO2 (110) domain. From the general shape of these oxide layers in comparison with earlier studies,20 one can already infer that this flat oxide domain is RuO2 in (110) orientation (cf. Figure 3b). The oxide island exposes several well-resolved layers with a layer distance of 3.1 Å, which corresponds to the expected layer spacing of RuO2(110) and therefore the step height of RuO2(110) (cf. green linescan in Figure 3c). The apparent thickness of the oxide island is 0.7 nm above the upper Ru(0001)−(1 × 1)O terrace. This thickness can be associated with real thickness of 2−3 RuO2 units in (110) orientation (i.e., 0.6−0.9 nm). An atomically resolved STM image (inset of Figure 3b) reveals a row-like structure on two terraces, which is characteristic for the RuO2(110) orientation and has been
which is in reasonable agreement with the integrated O 1s intensity and with SXRD.45 3.2. Initial Oxidation of Ru(0001) by Molecular Oxygen. 3.2.1. Oxidation at O2 Pressure of 10−6 mbar. We start the oxidation experiments from a clean Ru(0001) surface with about 0.25 ML of adsorbed oxygen (cf. Figure 2a). From SXRD45 and XPS experiments,67 it is known that the oxidation of Ru(0001) needs a threshold pressure of 1 × 10−5 mbar of O2 and temperature above 500 K in order to form an oxides layer that can macroscopically be studied. In the following experiment, we check whether this conclusion is also reconciled with STM on the microscopic level. When exposing the clean Ru(0001) surface to 1 × 10−6 mbar of O2 for 1 h (cf. Figure 2) one can see isolated clusters at step bunches with STM, which can be assigned to RuO2. In the flat region of the Ru(0001) surface, only occasionally are RuO2 clusters observed at multiple step edges of the substrate (cf. Figure 2b). It is remarkable that, at such low O2 pressures, RuO2 clusters are formed at all with a typical size of 10 nm across and 1 nm high (cf. Figure 2b). Rather, one would have expected only to see the (2 × 1)O chemisorption phase as the saturation surface structure. Much smaller clusters than 6 nm have not been observed with STM. These big clusters are already supercritical in that these clusters do not disappear after switching off the O2 exposure. The typical critical cluster size in the oxidation of Ru(0001) is expected to be several RuO2 units, say below 10 units. Since we see with STM only RuO2 clusters with lateral dimensions exceeding 10 nm, we may argue that the growth of the initially formed critical nuclei proceeds autocatalytically, and as a cluster size of about 10 nm is reached, the growth slows down so that only this size of the cluster is encountered on the surface. The next question is whether a flat oxide layer can be formed from these oxide nuclei by prolonged O2 exposure. For this reason, we exposed the Ru(0001) surface to 1 × 10−6 mbar of O2 at a sample temperature of 680 K for 5 h. The growth of a flat oxide domain has been only witnessed in the step bunching region with a lateral dimensions of several 100 nm (cf. Figure 2d), and the observed clusters increased in size; the clusters are now typically 15−20 nm across and 2−3 nm high. However, we have to emphasize that one has to search the surface for two days with STM to find such an oxide layer on the Ru(0001) 24652
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Figure 4. (a) Clean Ru(0001) surface was exposed to 1 × 10−5 mbar of O2 at a sample temperature of 680 K for 960 s (U = 1.1 V; I = 1.0 nA; 300 nm × 300 nm). (b) Zoomed-in region with a round oxide cluster (U = 0.9 V; I = 1.0 nA; (d) 60 nm × 60 nm) together with a line scan through the cluster. (c) Zoomed-in region with an elongated oxide cluster (U = 0.9 V; I = 1.0 nA; 60 × 60 nm) together with a line scan through the cluster.
terraces. A more careful inspection of this STM images (cf. Figure 4) reveals that clusters are placed exclusively along double and multiple step edges, while single steps are clusterfree. The clusters reveal various shapes with lateral size of 10−20 nm and an averaged height of 1−2 nm. Figure 4b shows a magnified STM image of a round-shaped cluster. We may presume that these clusters consist of RuO2 since without extensive oxygen exposure such clusters are not formed. The height of the specific cluster in Figure 4b is 2 nm, while the base size is 10 nm × 15 nm. This cluster sits on a depression, which is by 0.2 nm deeper than the surrounding terrace. It seems therefore that this cluster has grown into the topmost layer of Ru(0001). A simple estimation reveals that the Ru atoms from the corroded area of the upper Ru(0001) terrace around the cluster accounts for about 20% of the observed cluster size. The rest of the required Ru atoms for the growth of the RuO2 cluster are presumably taken from step edges of Ru(0001).
Figure 3. (a) Clean Ru(0001) surface was exposed to 1 × 10−5 mbar of O2 at a sample temperature of 680 K for 960 s (U = 1.1 V; I = 1.0 nA; 500 nm × 500 nm). (b) Zoomed-in region with an oxide island (U = 0.9 V; I = 1.0 nA; 160 nm × 160 nm). In the inset, an atomically resolved STM image is shown from the oxide layer (16 nm × 16 nm2) The large protrusions are caused by Ar-bubbles. (c) Line scans through the 2D oxide island with step heights of the oxide of 0.3 nm and an apparent height of the first oxide layer of 0.7 nm. A second line scan goes through the top-flatted cluster with a height of 1.5 nm above the top terrace.
assigned to bridging O rows.20 The distance between the rows is, however, 7.2 Å instead of 6.4 Å. A similar elongation of the unit cell of RuO2 was reported by He et al.68 (7.6 Å) and observed by Rösler et al. (7.2 Å).69 From Figure 4a, it becomes evident that all clusters decorate exclusively the step edge regions; no cluster is found on the flat 24653
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Figure 5. STM image of a bat like oxide consisting of a high cluster and two flat RuO2 layers, which are rotated by 120°. STM parameters: U = 1.1 V; I = 1.0 nA; 250 nm × 250 nm. Various line scans (a−d) through the bat-shaped oxide domain are indicated.
Figure 6. (a) Clean Ru(0001) surface: The holes are due to argon ion sputtering and insufficient annealing to temperatures not higher than 1100 K. (b) Clean Ru(0001) surface was exposed to 5 × 10−5 mbar of O2 at a sample temperature of 680 K for 960 s. The resulting RuO2 film is dense but quite rough. From the texture of the oxide flakes, three rotational domains are discernible. (c) These rotational domains are color coded (blue, red, and yellow). STM parameters: (a) U = 1.1 V; I = 1.0 nA; 500 nm × 500 nm; (b) U = 1.2 V; I = 1.0 nA; 500 nm × 500 nm.
round shaped and cannot be assigned readily to exposed oxide facets with specific orientations. The base size of this cluster is 50 nm long and 20 nm wide. The flat oxide regions at the wings of the bat are only a few Ru−O layers thick since the Arbubbles from the substrate are still visible through the oxide islands. The lateral size of the lower wing is approximately 180 nm × 60 nm, while that of the upper wing is 90 nm × 60 nm. Again, around this flat oxide region, the topmost Ru layer of Ru(0001) is corroded (see left side of the red line scan (c)), indicating that the flat oxide has penetrated the topmost Ru layer of Ru(0001). A line scan across the flat oxide region (red line scan (c) in Figure 5) reveals an apparent height of 1.2 nm, which is consistent with about four RuO2(110) layers. Summarizing, Figure 5 provides clear evidence that flat RuO2(110) islands grow out of a RuO2 cluster. 3.2.3. Oxidation at O2 Pressure of 5 × 10−5 mbar. At higher O2 pressures, the oxidation of Ru(0001) should be accelerated as confirmed in Figure 6. Exposing a clean Ru(0001) surface (cf. Figure 6a) to 5 × 10−5 mbar of O2 at 680 K for 960 s leads to a surface that is completely covered by an oxide layer (cf. Figure 6b) with RuO2 flakes displaying lateral dimensions of 20 nm × 10 nm across as also observed previously.19,45,69 From the textures of the oxides domains, one can recognize that this surface area of the oxide film exposes three rotational domains (cf. Figure 6c). The 2−4 nm deep holes seen on the clean Ru(0001) surface (caused by frequent Ar-ion sputtering and insufficient annealing) are still recognizable in the oxide film, although the oxide film has overgrown the whole surface
An elongated cluster is shown in more detail in Figure 4c. Also, this cluster, at least the flat part of it, grows into the topmost layer of the Ru(0001) substrate. Along the long side of this cluster a line scan reveals a step height of 0.3 nm, which is indicative of a step of RuO2(110). Therefore, one may consider this elongated cluster as a RuO2 cluster whose surface normal is oriented along the (110) direction. From the line scan, the step edge of the Ru(0001) substrate is identified with a triple step edge with a total height of 0.65 nm. From Figure 4a, one can recognize 5−6 elongated clusters, which are either directed with the long side along the same direction or rotated by 120° as expected for the growth of rotational domains of RuO2(110) on Ru(0001). In another region of the oxidized Ru(0001) surface, we can see how two flat oxide domains rotated by 120° grow out from a single cluster in the center, resembling a bat (cf. Figure 5). This type of oxide domain is not a singular event; similar aggregations are seen for instance in the STM image of Figure 7b. The cluster is located at a multiple step edge of the Ru(0001) substrate: As can be seen from the STM image, the extrapolation of the step edges in the upper left part and that of bottom right part of the STM image meet at the cluster position. The left part of the big cluster consists of two steep facets (cf. purple line scan in image (d)) forming a wedge with an angle of about 140°. Therefore, the two facets may be assigned to (101) and (100), assuming that the flat region of the oxide cluster is oriented along the (110) direction. The height of this cluster is 4 nm on the left side and 7 nm on the right side (line scan (a)). The right side of the cluster is 24654
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distribution of 2.4 ± 0.3 nm. It is worth noting that most of these clusters are not able to initiate the formation of flat RuO2(110) domains. With continuing O2 exposure at pressure of 2.5 × 10−7 mbar, both the 2D-RuO2(110) domains and the 3D-domains grow simultaneously in size, although the growth rate for the 2DRuO2(110) domains is substantially higher (cf. Figure 7c,d). The size of the oxide island has increased from about 50 nm × 10 nm to more than 300 nm × 100 nm upon exposing the surface with nuclei to 2.5 × 10−7 mbar of O2. The thickness of the oxide islands has not changed and amounts to 1−3 RuO2(110) units so that this growth is purely lateral or twodimensional. The flat oxide domains are significantly thinner than the typical height of the RuO2 clusters of about 2 nm. The number of flat RuO2 domains has increased too, indicating that further RuO2 nuclei were able to initiate the growth of flat oxide islands. Half of the Ru(0001) surface is now covered by an ultrathin and flat oxide layer, exposing all three rotational domains (cf. Figure 7d). The seed and growth experiment demonstrates nicely that we were able to decouple the nucleation from the actual growth process of flat oxide films. Both oxide clusters and the flat RuO2 domains grow continuously in size by exposing 2.5 × 10−7 mbar of O2. Figure 7 displays that most of the oxide islands are attached to a RuO2 cluster so that we can presume that these clusters have also initiated the growth of the flat oxide islands. The growth of RuO2 islands directly from a cluster has been visualized by STM in Figure 5. The seed and growth experiments provide compelling evidence that at least two different reaction pathways are operative in the oxidation of Ru(0001): first, the formation of RuO2 clusters and, subsequently, the growth of these clusters as well as the growth of flat oxide islands starting from some of the RuO2 clusters. The observed RuO2 clusters are critical in the sense that they continue to grow after pressure reduction of O2 to 2.5 × 10−7 mbar. The formation of RuO2 clusters highly depends on the applied O2 pressure. Only above 10−5 mbar of O2 is a sufficient number of RuO2 clusters formed and stable against decomposition at 680 K, while 2.5 × 10−7 mbar of O2 is too low in pressure to form additional stable RuO2 clusters within one hour. 3.4. Complete Chemical Reduction of RuO2(110)/ Ru(0001) by CO: What Kind of Morphology Is Left on the Surface? The seed and growth produced RuO2 film (cf. section 3.3) was subsequently chemically reduced by exposure of 50 L of CO at 600 K. The original oxide islands are decomposed in that oxygen from the oxide transforms CO into CO2, thereby leaving an aggregation of round-shaped Ru metal clusters (cf. Figure 8a,b) or round-shaped monoatomic Ru metal islands (cf. Figure 8b) on the Ru(0001) surface. These aggregates serve as fingerprints for the thickness distribution of the previously reduced oxide film. The surface temperature was chosen to be high enough to allow for efficient chemical reduction70 but low enough to suppress annealing of the Ru(0001) surface and the formation of smooth terraces. The oxide-free terraces remain flat after reduction. The oxide clusters do apparently not alter too much upon chemical reduction. However, a careful statistical analysis of the cluster size (see Figure 11) reveals that the cluster size reduces. This is also expected as the transformation of RuO2 to metallic ruthenium is accompanied by a 30% shrinkage in volume. The aggregation of round-shaped metallic Ru clusters in Figure 8 with a height of more than 1 nm is assigned to reduced
area of Ru(0001). The oxide film is only 3−4 RuO2 layers thick as Ar bubbles are still discernible in STM. 3.3. Seed and Growth Experiments. If the nucleation and growth mechanism is an appropriate description for the oxide formation on Ru(0001), then one could envision an experiment where the nucleation process is purposely separated from the subsequent growth process. As shown in Figure 7, O2
Figure 7. Seed and growth experiment of the initial oxidation of Ru(0001). (a) First, in a seed experiment, RuO2 clusters are synthesized by exposing 10−5 mbar of O2 at 680 K for 960 s. (b) Subsequently, the oxide was further grown by exposing 2.5 × 10−7 mbar of O2 at 680 K for 1 h (growth). The various rotational domains of the oxide islands in images a and b are shown in different colors (blue, red, and yellow) in images c and d, respectively. STM imaging parameters: U = 1.1 V; I = 1.0 nA; 600 nm × 600 nm.
exposure of 1 × 10−5 mbar at 680 K for 960 s results in a Ru(0001) surface that is covered by many RuO2 clusters decorating exclusively multiple step edges of the Ru(0001) substrate, and only very few small flat oxide islands are formed (cf. Figure 7a,c). This preparation step may be considered as a seed experiment in which stable oxide nuclei for the continuing oxidation of the Ru(0001) are prepared. In Figure 7c, the rare events of oxide islands are color coded for different rotational oxide domains. This situation is well within the induction regime of the oxidation process so that one should be able to advance the oxide growth with much smaller oxygen pressure, say 2.5 × 10−7 mbar. Without the seed step, this pressure is far from being sufficient to initiate the oxidation on the Ru(0001) surface within one hour. In Figure 7b, the cluster-precovered Ru(0001) is shown after 60 min of oxide growth at 2.5 × 10−7 mbar of O2 and 680 K. Obviously the clusters are still sitting exclusively at the double and multiple step edges. The number of clusters has not changed as expected for a pure growth mode. However, the apparent size of the clusters has increased significantly (cf. Figure 11), consistent with a continuing cluster growth upon low-pressure O2 exposure. The averaged cluster size after the seed process is 11 ± 3 nm with a height of 1.8 ± 0.3 nm. Upon growing at 2.5 × 10−7 mbar of O2 for 60 min, the RuO2 clusters increase in size (18 ± 3 nm) with a height 24655
dx.doi.org/10.1021/jp3085155 | J. Phys. Chem. C 2012, 116, 24649−24660
The Journal of Physical Chemistry C
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Figure 9. STM image of a solitary cluster after chemical reduction by CO exposure (200 L of CO, 600 K). The cluster is hexagonal shaped and the linescan of the left side reveal that the cluster height is clearly related to the step height of Ru(0001), namely, two atomic Ru from the top terrace and four atomic layer from the bottom terrace.
also the shape has changed. The reduced RuO2 particle is flat and of hexagonal shape with a height of 2 atomic Ru layers (0.42 nm) from the top terrace or 4 atomic Ru layers from the bottom terrace. This Ru cluster is still located at a multiple step of Ru(0001) as observed with the original oxide clusters. A magnified STM image of a pretty flat surface region is depicted in Figure 10b where the ultrathin oxide film had been Figure 8. Reduction of an initially grown oxide film prepared by the seed and growth method shown in Figure 6. The chemical reduction of this oxide surface was conducted by exposure of 50 L of CO at 600 K. The reduced oxide leaves a characteristic mark on the Ru(0001) surface, which provides unprecedented details about the growth and the thickness of the oxide layer. STM imaging parameters: U = 1.2 V; I = 1.0 nA; (a) 400 nm × 400 nm and (b) 100 nm × 100 nm. (c) Line scan and (d) reducing the oxidized surface by exposing 200 L of CO at 600 K (U = 1.1 V; I = 1.0 nA; 400 nm × 400 nm).
thick RuO2(110) domains as also previously seen in STM.19−21 The thicker the original RuO2(110) film was in this region of the Ru(0001) substrate, the higher the aggregates of Ru clusters after chemical reduction (cf. Figure 8b, line scan). Ultrathin RuO2(110) islands reduced in a way that only single atomic high Ru(0001) islands are formed (cf. Figure 8b, line scan) with a step height of 2.2 Å on an otherwise flat terrace of Ru(0001). These Ru islands are covering about 70% of the terrace where the oxide had been grown consistent with 30% volume shrinkage of RuO2 upon chemical reduction. The underlying Ru(0001) substrate in Figure 8b is atomically flat, clearly indicating that the interface between Ru(0001) and the former RuO2(110) film is atomically smooth, which is consistent with conclusions drawn from previous SXRD measurements.45 Therefore, from the line scan of the reduced oxide region, one can unambiguously derive the actual thickness of the former oxide film to be 1 ML of RuO2. The higher regions 1.2 nm above the flat Ru(0001) terrace are compatible with six layers of Ru (cf. linescan in Figure 8c). Reducing the oxidized Ru(0001) surface by 200 L of CO at 600 K results in a surface that is flatter than the reduced surface by 50 L of CO exposure (cf. Figure 8d). Obviously the rough surface is annealed by prolonged CO exposure. At the moment we do not know whether this is a pure temperature effect or CO forms mobile Ru−CO surface complexes, which may facilitate the annealing process. Adsorbate-driven morphological changes have recently been reported in literature.71,72 The reduced RuO2 clusters are still visible in STM, clearly indicating that, in each former flat RuO2 domain, at least one cluster was located. In Figure 9, a high-resolution STM image of such a reduced RuO2 cluster is shown. Clearly not only the size but
Figure 10. Detailed STM images for the chemical reduction of an initially grown oxide film prepared by the seed and growth method shown in Figure 6. The chemical reduction of this oxide surface was conducted by exposure of 100 L of CO at 600 K. The reduced oxide leaves a characteristic mark of Ru islands on the Ru(0001) surface, which provides unprecedented details about the growth and the thickness of the reduced oxide layer. STM imaging parameters: U = 1.2 V; I = 1.0 nA; (a) 400 nm × 400 nm and (b) 100 nm × 100 nm. (c) Regions of Ru islands with the same heights after chemical reduction are color coded. (d) Reconstructed height distribution of the RuO2(110) domains.
chemically reduced by 100 L of CO at 600 K. The height analysis of the reduced oxide film on Ru(0001) is summarized in Figure 10c,d. While most of the surface was covered by 1 ML of RuO2, part of the surface was also covered by 2−4 ML of RuO2 (lower part of Figure 10b). The regions of Ru islands with the same heights after chemical reduction are color coded in Figure 10c. The assignment of the thickness of the 24656
dx.doi.org/10.1021/jp3085155 | J. Phys. Chem. C 2012, 116, 24649−24660
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the cluster reduces from 17 ± 3 nm to 11 ± 2 nm, fully consistent with the volume change during chemical reduction of RuO2 to metallic ruthenium by about 30%. A similar shrinking is observed for the height of the clusters, namely, from 2.4 to 1.6 nm. The size of the reduced RuO2 clusters is (on the average) similar to that after the seeding experiment. It is worthwhile noting that no RuO2 cluster is observed that is smaller than 6 nm.
RuO2(110) film is particularly easy for the 1 ML oxide domains since only single atomic Ru layer islands are observed on the Ru(0001) surface after reduction, and these islands cover 70% of this part of the terrace. The single atom high Ru islands on the terraces are assigned to come from a chemically reduced single RuO2(110) layer which has been grown (penetrated) into the topmost Ru layer of the terrace (cf. Figure 10b). The growth of a single oxide layer into the topmost Ru layer is well recognized by the unaffected terrace region on the left and right side of the region with single atomic Ru islands. This means that the oxide does not simply overgrow the Ru(0001) substrate, but rather the Ru atoms from the topmost ruthenium layer are consumed to build up the 1 ML oxide film. Only at few places in the STM image is deeper growth of the oxide into the terrace evident (small round-shaped pits). Altogether, this 1 ML RuO2(110) can be envisioned as a wetting layer adhering/anchoring the oxide to the Ru(0001) surface. The surplus 30% of ruthenium atoms of the topmost Ru(0001) layer are not used to start with the growth of a second layer on top of the first RuO2 layer. Otherwise, the reduction of such RuO2(110) films would have led to Ru islands, which are partly two layer thick but experimentally not observed. Therefore, the 30% surplus Ru atoms from the topmost Ru(0001) layer has to be shuffled out of this region. A 1 ML RuO2 grows on the lower terrace (purple region) and the upper terrace (yellow region) (cf. Figure 10d), but at the lateral interface, the further growth of the RuO2(110) layer is suppressed. Neither has the 1 ML RuO2 of the upper terrace overgrown the 1 ML RuO2 on the lower terraces, nor has the 1 ML RuO2 of the lower terrace undergrown the upper 1 ML RuO2 to form a 2 ML thick RuO2. In the lower part of the STM image in Figure 10b, also multilayers of RuO2(110) have been chemically reduced. The thickness of this part of the oxide film is indicated in Figure 10d. One 3 ML thick RuO2 island was growing into the lower terrace, stopping at the next step edge of the substrate. The 3D clusters in Figure 10a also reduce in size upon chemical reduction as revealed by the size distribution of the cluster size before and after chemical reduction by an exposure of 200 L of CO at 600 K (cf. Figure 11). The averaged size of
4. CONCLUSIONS With a combination of core level spectroscopy of O 1s (XPS) and microscopy (STM), the oxidation process of Ru(0001) has been studied on the atomic scale. First, chemisorbed O layers are formed on Ru(0001) followed by the growth of RuO2 clusters and small oxide islands solely located in step bunching regions. Finally, the flat oxide layers grow across the surface until most of the Ru(0001) is covered by an ultrathin oxide film RuO2 in (110) orientation. From STM experiments, we conclude that the oxidation of Ru(0001) proceeds via 3D cluster formation exclusively located at multiple steps of the substrate (nucleation phase). For the generation of such RuO2 clusters, a critical pressure of 1 × 10−5 mbar is required. O2 exposure below this threshold pressure does barely produce clusters. For instance, for an O2 pressure of 1 × 10−6 mbar and exposure times of five hours, only very few clusters are identified with STM, preferentially in the step bunching regions. The observed 3D domains consist of RuO2 since chemical reduction by CO exposure at 600 K let the cluster size shrink by about 30% consistent with a volume reduction of 30% during the transformation of RuO2 to metallic Ru. An open question is where the Ru atoms are coming from for the growth of these RuO2 clusters. In STM, we observe only little corrosion on the terraces and at the step edges during the growth of the clusters. Maybe the applied sample temperature is high enough to heal the corroded step edges, thus leaving the Ru(0001) steps straight. Another point is how the RuO2 clusters are formed. On the clean Ru(0001) surface, Ru atoms from the step edge become mobile enough only above 850 K to anneal a sputtered surface. However, exposing oxygen to the Ru(0001) surface at 600 K suffices to form RuO2 clusters at multiple step edges. This may hint to a mobile Ru−O species, which run along the step edges even at 600 K and finally aggregating into RuO2 clusters when several of these Ru−O species meet. The mobility of such Ru−O species is restricted to the step region since all the RuO2 cluster observed in STM are found at double and multiple steps. Future ab initio (e.g., DFT) calculations together with molecular dynamics simulations may help to understand the process of RuO2 cluster formation. Particular attention in these simulations should be devoted to explain why RuO2 clusters are exclusively found at multiple step edges. After the nucleation phase, the growth of flat RuO2(110) domains starts from the 3D-domains into the topmost Ru(0001) layer. The orientation of the flat RuO2 domains was determined based on both the measured step heights of RuO2(110) of 3.1 Å and the observation of row-like structure in atomically resolved STM images. The thickness of these RuO2(110) islands depends on the applied O2 pressure. The higher the O2 pressure, the thicker the oxide film has grown. Obviously, there are two different kinds of RuO2 clusters present on the Ru(0001) surface, one minority species, which is able to initiate 2D growth of the oxide, and the other majority
Figure 11. Statistical distribution of the lateral size (left) and the height (right) of 3D-RuO2 clusters after seeding (red), after growing (blue), and after chemical reduction (purple) by exposing 200 L of CO at 600 K. 24657
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that RuO2 clusters grow preferentially at meandering single atom high step edges of Ru(0001) with an averaged size of 5 nm in diameter and 2−4 nm tall; multiple steps do not play a particular role in the electro-oxidation process. We presume that these clusters are oriented along the (100) direction as observed in a previous RHEED study.74 Meandering step edges are indicative of corrosion of the Ru(0001) substrate during electrochemical oxidation. This corrosion process may supply the growing oxide clusters with ruthenium. In the same STM study of Adzic et al.,61 the gas phase oxidation of Ru(0001) was studied. STM reveals the growth of flat ultrathin RuO2(110) layers starting from the single atomic step edge and growing onto the lower terrace of Ru(0001). Quite in contrast, we observe in our STM study that ultrathin RuO2(110) layers start to grow exclusively along the multiple step edge of Ru(0001) and then penetrating into the upper terrace. It may be that the prior electro-oxidation of Ru(0001)61 has changed the step edge morphology in a way that the gas phase oxidation proceeds differently. We may also compare the gas phase oxidation of Ru(0001) with that of Ru(1010̅ ). On Ru(1010̅ ), the RuO2 film grows preferentially along the (100) orientation as a flat oxide film.75−78 This proves that the interfacial energy between the oxide and the Ru substrate is an important ingredient to the delicate energy balance determining the final orientation of the oxide layer. Other orientations of RuO2 have been observed as minor species on Ru(101̅0).78 In an elegant experiment, the thickness of the oxide films was determined by chemical reduction via CO exposure of CO at a sample temperature of 600 K. The reduced oxide leaves a characteristic patchwork of Ru islands on the Ru(0001) surface, which provides unprecedented details about the growth and the thickness of the oxide layer (cf. Figure 12). The oxide films
species, which do not initiate the growth of 2D-RuO2(110) domains. In the many STM images we took over the past 2 years, there has been no indication of O−Ru−O trilayer formation as suggested by Reuter et al.54 Concerning the typical nucleation and growth mechanism, we have to bear in mind that the observed RuO2 clusters in STM are typically larger than 10 nm across and therefore well beyond the size for a critical nucleus (critical size typically
