Faceting Transition at the Oxide–Metal Interface - American Chemical

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Faceting Transition at the Oxide−Metal Interface: (13 13 1) Facets on Cu(110) Induced by Carpet-Like Ceria Overlayer Marie Aulická, Tomás ̌ Duchoň, Filip Dvořaḱ , Vitalii Stetsovych, Jan Beran, Kateřina Veltruská,* Josef Mysliveček, Karel Mašek, and Vladimír Matolín Charles University in Prague, Faculty of Mathematics and Physics, Department of Surface and Plasma Science, V Holešovičkách 2, 18000 Praha 8, Czech Republic ABSTRACT: Structural transitions affect electronic structure of materials and consequently their catalytic properties. We report the observation of faceting of a low index metal surface at an oxide− metal interface in a catalytically relevant system of ceria on Cu(110). We observe formation of (13 13 1) facets on the Cu(110) surface covered by ceria upon annealing above 500 °C. The faceting transition occurs in spite of a weak adsorbate−substrate interaction, which manifests itself in ceria adopting a carpet-like growth mode. We rationalize the surface faceting under such conditions by oxide overlayer-induced modification of the roughening temperature of Cu(110). We describe the carpet-like ceria film in terms of elasticity theory and show that the specific structure of the ceria supported on Cu(13 13 1) can lead to a periodic modulation of the electronic structure of the ceria−copper interface. The reported structural transition indicates that surface faceting of metal can occur at the oxide−metal interface at relatively low temperatures with possible consequences for the catalytic properties of the interface. The oxide overlayer induced faceting transition can be expected to occur for other oxide−metal combinations and, as such, has perspective applications in preparation of functional oxide−metal nanostructures.

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INTRODUCTION Surface faceting is of importance for preparation of functional nanostructures, such as self-assembled quantum dots1 or tailored heterogeneous catalysts.2 Of particular interest is faceting of low index faces of metal crystals that are present in many technological applications.3 Typically, surface faceting of otherwise stable low index metal surfaces is caused by changes in surface free energy induced by chemisorption. Chemisorption-induced faceting of metals has been observed with various types of adsorbates: metal films,4 atomic species,5 molecular species,6 and organic adsorbates.7 The structural changes resulting from the faceting transitions affect catalytic and electronic properties of the faceted materials. Chemisorption induced faceting thus plays a crucial role during gas−solid catalytic reactions. One of the prominent materials used in many technologically relevant heterogeneous catalytic reactions is copper. The importance of understanding the faceting of copper has been highlighted by the finding that copper nanocrystals undergo reversible faceting transitions in different gas environments.8 Among the low index copper surfaces the Cu(110) has been found to facet especially easily. For example, adsorption of molecular chlorine on Cu(110) leads to formation of (210) facets,6 adsorption of larger molecules of formate and benzoate induces step bunching resulting in formation of (11 13 1) facets,9 and electrochemical annealing of bromide covered Cu(110) results in formation of (100) facets.10 In all the © 2015 American Chemical Society

mentioned cases the structural changes were linked to sufficient surface mobility of the substrate atoms (often achieved by thermal activation) and adsorbate−substrate interaction. In this contribution we explore the growth of thin ceria films on Cu(110) and extend the observations of a surface faceting of a low index metal surface to faceting induced by an oxide overlayer. The combination of ceria and copper has a prominent place in heterogeneous catalysis,11 mainly due to the specific properties of the ceria−copper interface.12 The electronic properties of the ceria−copper interface are dominated by a charge transfer from the copper metal to the ceria.13,14 The charge transfer results in occupation of the 4f level of cerium atoms, creating a population of Ce3+ sites at the interface, which are considered catalytically active.15−17 We show that the ceria overlayer induces faceting of the copper substrate and that such changes modify the electronic structure at the ceria−copper interface. Specifically, the ceria overlayer facilitates spontaneous creation of steps on the underlying substrate upon annealing above 500 °C. Equilibration of the ceria covered stepped copper surface leads to a formation of (13 13 1) facets, representing a local vicinal Cu(110) surface with a regular array of terraces with a periodicity of 23.5 Å. The faceted copper surface supports a Received: October 1, 2014 Revised: January 8, 2015 Published: January 8, 2015 1851

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performed with He+ ions with energy of 1500 eV and impact angle of 45°. STM experiments were performed with chemically etched tungsten tips thermally annealed in vacuum and the images were obtained by tunneling into the unoccupied states of the sample. RHEED patterns were taken using a CCD camera acquisition system at primary electron energy of 25 keV.

carpet-like ceria overlayer, suggesting a weak interaction of the ceria layer with the substrate. The equilibrated CeOx/Cu(110) system thus represents a model of a weakly interacting adlayer on a vicinal surface. The carpet-like ceria overlayer has modified electronic structure at the copper steps due to a loss of binding energy to the substrate. Moreover, the local vicinal Cu(110) surfaces have modified surface states as a consequence of the regular 1D step array, possibly leading to electron confinement and superlattice effects.18 The modification of the electronic properties of the ceria−copper interface as a result of the specific structure of the CeOx/Cu(110) system can be exploited in preparation of functional nanostructures.19 The reported structural transition can be expected to occur for other oxide−metal combinations, providing that the intralayer interaction in the oxide is stronger than binding to the metal substrate and that sufficient surface mobility of substrate metal atoms can be reached.

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EXPERIMENTAL RESULTS Deposition of 1−10 ML of ceria onto Cu(110) at 250 or 300 °C using the preparation procedures described in the Experimental Section produces weakly ordered layers. The weak ordering is apparent from diffuse hexagonal LEED spots of ceria accompanied by low intensity linear streaks in [001] direction of the copper substrate. An example of a LEED pattern taken immediately after deposition of 2 ML of ceria is shown in Figure 1a. Both preparation procedures described in

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EXPERIMENTAL SECTION The experiments were performed in three ultrahigh vacuum systems. All the systems have a base pressure of 1 × 10−8 Pa and are equipped, along with all the necessary sample cleaning and preparation facilities (ion gun, sample heating through current and electron bombardment, chromel−alumel thermocouple, O2 and Ar gas lines), for X-ray photoelectron spectroscopy (XPS) and electron diffraction, either low energy electron diffraction (LEED) or reflection high energy electron diffraction (RHEED), experiments. Furthermore, one system features scanning tunneling microscopy (STM) and another one ion scattering spectroscopy (ISS). Cu(110) single crystal (MaTecK) with a purity of 99.999%, diameter of 1 cm, and thickness of 2 mm was used as a substrate. Cleaning of the sample was realized by several cycles of Ar+ bombardment (1 keV) and annealing in vacuum at 500 °C and checked by XPS and electron diffraction. Cerium oxide layers were prepared either by evaporation of metallic cerium (GoodFellow, 99.9%) onto preoxidized copper surface at elevated substrate temperature of 300 °C or by reactive evaporation of metallic cerium in oxygen background at 250 °C. The thickness of the resulting ceria films was determined from the attenuation of Cu 2p3/2 signal in XPS (using Al Kα Xray radiation, 1486.6 eV) using inelastic mean free path from TPP-2M formula20 and neglecting elastic scattering of photoelectrons. The following definition of one monolayer is used throughout this paper: One monolayer of ceria corresponds to a vertical stack of O−Ce−O layers with a thickness of 3.1 Å and Ce atom density of 7.9 × 1014 cm−2. In the discussion of our results, 1−2 ML thick ceria films are referred to as thin films, while ≥3 ML thick ceria films are referred to as thick films. Both preparation methods allow for the control of the stoichiometry of the resulting layers by varying the oxygen exposure of the copper crystal and the pressure of oxygen during evaporation (with 5 × 10 −5 Pa resulting in stoichiometric CeO2), respectively. However, the first method is limited by the amount of oxygen that can be stored at the copper surface, the limit being surface copper oxide phase with a c(6 × 2) reconstruction with respect to Cu(110)21 and, as such, can be used only for preparation of ultrathin oxide layers. In some cases the stoichiometry of the prepared layers was further adjusted by Ce−ceria interfacial reaction and O2 exposure, as described in ref 22. LEED patterns were taken with primary electron energy ranging from 15 to 100 eV. Ion scattering spectroscopy was

Figure 1. LEED images of CeOx/Cu(110) taken at an electron energy of 33 eV. (a) 2 ML ceria film on Cu(110) after preparation at 300 °C, (b) 8 ML ceria film annealed at 600 °C, (c) 2 ML ceria film annealed at 600 °C, (d) splitted (3 × 3) diffraction pattern of Ce6O10/Cu(110) (6 ML). High symmetry directions of the copper substrate are marked in (a) along with the position of the first order spots of Cu(110) (green circles). The spot splitting distance highlighted by identical markers in (b) and (c) corresponds to angular distance of 6.6 ± 0.5°. Two domains of the splitted (3 × 3) pattern of Ce6O10 are indicated by solid red and dotted green lines in (d).

the Experimental Section result in qualitatively the same LEED patterns. However, the thickness of the prepared films influences the intensity distribution between the spots and the linear streaks. The intensity of the linear streaks is greatly diminished in the thicker films, while the thin films predominantly feature the linear streaks. Furthermore, substrate spots are visible on the LEED patterns of the thin films, which indicates that the thin films are not continuous after preparation. Annealing the prepared films above 500 °C greatly enhances the ordering. This is indicated by increased intensity and sharpness of the hexagonal ceria spots. Furthermore, a splitting of the ceria spots is observed in LEED after the annealing. 1852

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The Journal of Physical Chemistry C LEED patterns of thick and thin ceria films annealed at 600 °C for 10 min are shown in Figure 1b,c, respectively. The spots are split in Cu[001] direction, which is the direction of the linear streaks on the as-prepared films. Correcting for instrumental aberrations the splitting distance remains the same across the probed energy range (20−80 eV). Using a calibration of 0.1 deg precision rotary manipulator the splitting distance corresponds to angular distance of 6.6 ± 0.5° on the LEED screen. Again, a difference between the thin and thick films is observed. The thin films exhibit a central spot in between the split ones, which is not the case for the thicker films. Moreover, the annealing enhances the intensity of the spots of the copper substrate on the thin films and might lead to appearance of low intensity substrate spots in the thick films depending on their thickness. The appearance of the substrate spots after the annealing indicates restructuring of the ceria film that results in uncovering of the copper substrate. The presence of the substrate spots in the LEED patterns taken on the annealed thin films allows for straightforward determination of the ceria lattice parameter (the copper spots are suppressed at primary electron energy of 33 eV due to being in a minimum of the I−V curve and thus not visible in Figure 1a,c). The obtained value of the surface lattice parameter is 3.91 ± 0.05 Å, which corresponds nominally to the surface lattice parameter of reduced ceria.23 The respective crystallographic orientation between the ceria epitaxial film and the copper single crystal can be represented by the following epitaxial relationships: CeOx(111)∥Cu(110); CeOx[11̅0]∥Cu[11̅0]. The observed spot splitting is independent of the stoichiometry of the ceria film. In fact, ceria films with stoichiometries ranging from CeO2 to Ce2O3 exhibit exactly the same spot splitting. This is best illustrated by utilizing the Ce− ceria interfacial interaction and O2 exposure24 to continuously explore the cerium−oxygen phase diagram in the respective range. Distinct surface reconstructions arising from specific ordering of oxygen vacancies have been previously observed at certain points during the reversible transition from CeO2 to Ce2O3.22 Accordingly, on Cu(110) these surface reconstructions are splitted in the same manner as is the hexagonal LEED pattern of ceria. As an example a characteristic (3 × 3) LEED pattern of CeO1.67 splitted on Cu(110) is shown in Figure 1d. Complementing the crystallographic information obtained by LEED, the morphology of the ceria films was studied by STM. Immediately after preparation, a disordered ceria film with a typical average terrace width of 8 nm covers the substrate uniformly, with copper substrate being visible in the case of the thin films. Many of the ceria islands are found to be elongated in the [110̅ ] high symmetry direction of the copper substrate, but no trend in the length is apparent. An example of an STM image taken on a 3 ML ceria film after preparation is shown in Figure 2a. The enhancement of ordering of the films observed in LEED after annealing is clearly visible in STM. A considerable mass transport at the elevated temperatures results in anisotropic lateral growth of the elongated ceria islands. After annealing at 500 °C, the surface is covered with linear finger-like structures, hundreds of nm long (150−300 nm) in the Cu[11̅0] direction and tens of nm wide (10−30 nm) in the Cu[001] direction. The enhancement of ordering is illustrated by a stepwise annealing of the surface displayed in Figure 2a at 400 and 500 °C for 10 min each, the obtained STM images are shown in Figure 2b and c, respectively.

Figure 2. STM images of CeOx/Cu(110) illustrating the improvement of ordering during annealing and the resulting structure. Subsequent annealing steps of 3 ML ceria film on Cu(110) are shown in (a) after deposition at 300 °C, (b) after annealing at 400 °C and (c) after annealing at 500 °C. (d) Line profiles of distinct planes observed on the linear structures after annealing. The planes are inclined with respect to the copper surface by α = 18.6 ± 0.4° and β = 3.3 ± 0.4°. The line profiles are marked in (c) and offset in z for clarity. The size of the STM images is 100 × 133 nm.

Statistical analysis of the linear structures reveals a distinct plane inclined with respect to the copper substrate by 3.3 ± 0.4°. The planes are found in two domains, rotated by 180°, with equal population (see blue line in Figure 2c,d). In the case of large structures, the domains are accompanied by a steeper plane with an inclination angle of 18.6 ± 0.4° (see red line in Figure 2c,d) that can be assigned to a (210) facet of copper.6 STM images of 1 ML high ceria islands present on the thin ceria films reveal a regular step-like height modulation in the Cu[001] direction (see Figure 3). The height modulation can be fitted by a succession of step functions f(x) = a/(1 + e(b−x)/t), where a is the step height, b is the center of the step, and t is the slope of the step edge. The average step height a resulting from the fitting procedure is 1.3 ± 0.1 Å. The lateral periodicity of the observed modulation has been determined to be 23.5 ± 0.7 Å by averaging the distance between centers of neighboring steps on more than 100 of 1 ML high ceria islands. Thicker islands (≥2 ML) do not exhibit the step-like height modulation. Instead, their height profile corresponds to an inclined plane in the Cu[001] direction, with the inclination angle of 3.3 ± 0.4°. STM image of thin ceria film consisting of 1 and 2 ML high islands is shown in Figure 3a along with respective height profiles in Figure 3c. The above-mentioned step-like height modulation corresponds to ceria islands growing over 1 ML high steps of the copper substrate in a carpet-like growth mode (see Figure 3b,c).25 The height of the modulation observed on 1 ML high ceria islands matches the height of the steps on Cu(110) [1.3 Å, in contrast to the 3.1 Å high steps on CeO2(111)] and on STM images no lateral change of positions of oxygen atoms is detected on the ceria islands in the proximity of the underlying copper steps (see Figure 3b). The copper steps below ceria islands are ceria induced and do not extend to the bare copper surface (see Figure 3b). The inclined planes of ceria observed on 2 ML or thicker ceria islands in the STM explain the splitting of ceria spots in LEED as the inclination transfers to the reciprocal space. We 1853

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Figure 4. RHEED diffraction pattern of 8 ML ceria film on Cu(110) after annealing at 600 °C along with its interpretation. The pattern was taken in [11̅0] direction of the copper substrate. The red and green dashed lines correspond to two ceria domains inclined with respect to each other. The angle formed by the reciprocal lattice rods is 6.4 ± 0.4°.

inclined with respect to the copper (110) plane. Fitting the line profiles in Figure 4 yields the value of the inclination angle 3.2 ± 0.2°, which is in agreement with the STM measurements and the spot splitting observed in LEED (angular distance from surface normal is half of the observed 6.6° spot splitting). In the Cu[001] direction, only one set of the reciprocal rods perpendicular to the surface is observed. The symmetrical intensity modulation of the two sets of reciprocal rods shown in Figure 4 reveals that each domain of ceria planes exists in two populations with different stacking of the fcc (111) planes: ABCABC and ACBACB.26 Such two population would not be detected in the 60° symmetrical LEED patterns without I−V LEED analysis as they are rotated by 180°. The observed structural changes are kinetically limited as a considerable mass transport is needed for them to occur. Annealing above 500 °C is necessary to give rise to the inclined ceria planes. ISS was used to further study the effect of annealing. The specific property of ISS is its sensitivity to the topmost atomic layers, which makes it especially useful for determining whether the substrate is covered continuously by an adlayer. In our case a 3 ML ceria layer completely covers the copper substrate after preparation. Stepwise annealing reveals a gradual discontinuation of the layer, as copper substrate is uncovered. This is illustrated in Figure 5. The signal of copper starts to be visible after annealing at 400 °C, with the biggest increase of its intensity occurring after annealing at 600 °C. The increase of Cu intensity after annealing happens at the expense of the Ce signal, which points at a decrease of the ceria coverage. This suggests a vertical growth of the large linear structures observed in STM.

Figure 3. STM images illustrating the carpet-like growth of ceria on Cu(110). (a) Thin ceria film (2 ML) after annealing at 600 °C with a population of 1 and 2 ML thick ceria islands. (b) 1 ML thick ceria islands imaged with high resolution. (c) Line profiles corresponding to the marked areas in (a) and (b): 1 ML (blue and green line) and 2 ML (red line) thick ceria islands. The line profiles have been offset in z for clarity, the black horizontal lines in the graph illustrate the height of steps on the copper substrate. Size of the STM images is as follows: (a) 62 × 62 nm; (b) 18.5 × 18.5 nm.

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have imaged the reciprocal rods of the inclined ceria planes using RHEED. A RHEED pattern taken in Cu[110̅ ] direction on an annealed 8 ML ceria film is shown in Figure 4 along with its interpretation. Two sets of reciprocal rods of ceria are observed. These correspond to two domains of ceria planes

DISCUSSION We divide the discussion of our results into three parts. In the first, we propose a microscopic model of the annealed CeOx/ 1854

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respectively. The model value of the lattice parameter of ceria agrees well with the one determined by LEED (3.91 ± 0.05 Å). A top and side view of the proposed model is shown in Figure 6.

Figure 5. ISS spectra of 3 ML thick ceria layer on Cu(110) taken after preparation and subsequent annealing steps. The spectra have been offset in intensity for clarity.

Cu(110) system that explains our experimental observations. In the second, we rationalize the formation and the stability of the proposed system. In the last part, we describe the ceria film in the framework of a carpet-like growth mode and discuss several specific properties of the CeOx/Cu(110) model system. Combining the results of our LEED, STM, and RHEED experiments, we have enough information to conclude that the (111) planes of ceria on the annealed thick ceria films are inclined with respect to the Cu(110) surface by 3.2 ± 0.2° in the Cu[001] direction. Furthermore, the diffraction experiments show that the inclined ceria planes are essentialy unperturbed. Specifically, no sign of faceting of the ceria film is observed. Faceting (essentially a formation of a periodic array of steps) would lead to splitting of the reciprocal rods27 and, consequently, to an energy dependent splitting of ceria spots in LEED. However, the reciprocal rods do not show such splitting (see Figure 4) and the spot splitting in LEED is constant during an energy scan. The constant splitting behavior can be explained by the LEED patterns of the two domains of inclined ceria planes (rotated by 180°) not converging to the same spot on the LEED screen with lowering of the energy [the two (0,0) spots do not overlap]. The observed inclination is thus not a result of a faceting of the ceria film. Accordingly, in creating our model we assume a reorganization of the copper substrate into regular array of terraces that support the ceria film and give rise to the observed inclination. Cutting a bulk crystal of copper with a plane inclined by 3.1° with respect to the (110) plane produces a (13 13 1) facet made up of an array of terraces consisting of seven copper atoms, each. The periodicity of the array is 23.5 Å, which corresponds very well to the periodicity of the height modulation observed on 1 ML thick ceria islands by STM (23.5 ± 0.7 Å; Figure 3b,c). This arrangement also provides a favorable coincidence between the lattice parameters of ceria and copper. Taking into account the epitaxial relations determined by diffraction techniques and considering a bulk lattice parameter of copper (3.61 Å) and a slightly perturbed inplane lattice constant of ceria (3.87 Å), the coincidence parameters are 1.5× and 6.5× the distance between the copper atoms in the Cu[11̅0] and Cu[001] direction,

Figure 6. Microscopic model of CeOx/Cu(13 13 1). The copper atoms are shown in red, ceria atoms in yellow and oxygen atoms in blue. (a) Top view with indicated high symmetry direction of the copper substrate and the ceria overlayer. The ceria overlayer is not shown in the top part in order to reveal the substrate. (b) Side view of a 3 ML thick ceria layer, (c) side view of a 1 ML thick ceria layer. Black rectangle in (a) highlights the lattice parameter coincidence between ceria and copper. The black vertical lines show the position of the steps on the copper surface.

The suggested microscopic model explains our experimental observations very well. However, a considerable mass transport is required for the proposed reorganization of the copper substrate. As a consequence, the necessary structural transition is kinetically limited and the system has to be annealed to achieve sufficient surface atom mobility. This is apparent from our results, as the transition occurs after annealing at 500 °C or above. Nevertheless, the annealing temperature is still too low to explain the transition only on the basis of surface roughening, as spontaneous creation of steps on (110) surface of copper has been observed only at temperatures above 1000 K.28−30 The roughening temperature can be lowered due to a presence of adsorbates, either through adsorbate−substrate or adsorbate−adsorbate interactions.31 In our case, the ceria adlayer may affect the roughening temperature. The roughening temperature of copper surface partially covered with ceria would be inhomogeneous, with differences between the regions below the ceria adlayer and the free copper surface. In fact, the inhomogeneity of the roughening temperature explains our observation that the reorganization of the copper substrate occurs only below the ceria adlayer, while the uncovered copper substrate remains intact (Figure 3b). Furthermore, the linearity of the finger like ceria islands can be explained by anisotropy of the roughening temperature on Cu(110) as the probability of a step creation in Cu[11̅0] direction is less than half of a step creation in Cu[001] direction at the respective annealing temperature.32 Elongation of islands along the Cu[11̅0] direction is a direct consequence of this anisotropy. While the modification of the roughening temperature is most 1855

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distorted areas increases with increasing film thickness. In the case of CeOx/Cu(110), 1 ML thick ceria islands show height modulation in STM, with the distorted regions being around 11 Å long. This value has been determined as the distance between the points where the step function used to fit the height modulation reaches 5% and 95% of its amplitude, which is expected to be a reasonable upper bound in the precision limited by other parameters of our model. Thicker ceria islands show continuous height changes in STM, which implies that starting from 2 ML the ceria forms inclined planes on the vicinal copper surface (Figure 7b). This means that the length of the distorted areas is larger than the width of the copper terraces. Knowing the length of the distorted areas in 1 ML thick ceria islands we can use the equation to estimate the binding energy loss of the ceria overlayer to the copper substrate in the distorted region.37 Inputting elastic modulus in [112̅] direction of the fluorite lattice of ceria E[112̅] = 98−114 GPa (calculated from known values of elastic constants of CeO238) we arrive at a value of 0.04−0.05 eV. It has to be noted that the value of the elastic modulus depends on the stoichiometry. While there are no reported values of the elastic constants for the cubic Ce2O3 phase, the elastic modulus in the relevant direction is twice as large in the case of the hexagonal phase of Ce2O3.39 Our value of eb might thus be underestimated by up to a factor of 2. The carpet-like growth mode helps us in interpreting finer features in our LEED patterns, too. The central spot (unsplitted) visible in the LEED patterns of the 1−2 ML thick ceria films originates from the unperturbed parts of 1 ML thick islands. The distorted regions contribute intensity into a line in Cu[001] direction going through the central spot. This explains why the central spot disappears with increasing thickness of the ceria film, when the population of 1 ML thick ceria islands decreases. With the resolution of our LEED instrument we do not see any additional substrate spots due to the regular array of terraces under the ceria film. The linear streaks present in the films after preparation (before annealing) are attributed to random distribution of the registry of small ceria islands with respect to the substrate in Cu[001] direction, which makes them as likely to be in phase as out of phase with each other.40 The size of the ceria islands increases with the thickness of the prepared ceria film and subsequently the in phase condition for the diffraction is fulfilled on larger areas of the sample. This effect is in agreement with our observation of the inhomogeneous distribution of intensity between the linear streaks and ceria spots with amount of deposited ceria. The specific structure of the CeOx/Cu(110) systems leads to modifications of the electronic structure of the copper−ceria interface. First, the binding energy loss of ceria in the distorted areas creates specific sites of ceria in the vicinity of the copper steps. This is especially interesting in the case of the 1 ML high ceria islands, where such regions alternate periodically with unperturbed ones. The second electronic effect of the specific structure of the CeOx/Cu(110) system arises due to the regularity of the copper terraces. Vicinal surfaces have modified electronic structure due to electron confinement and superlattice effects.18 As such, the surface states of the Cu(110) located at the interface with ceria are perturbed. We expect the copper surface states to survive at the copper−ceria interface due to the weak interaction between the substrate and the overlayer, albeit the copper will be slightly hole doped by the ceria overlayer.13 Further angle resolved photoemission experiments and theoretical studies will have to be carried out in

probably responsible for the lower kinetic limit, we have to further rationalize the regularity of the terraces. First, we argue that the proposed model represents a thermodynamic equilibrium. There are several observations supporting this statement: further annealing (up to 800 °C) has no effect on the structure, there is no dependence on the preparation procedure with respect to oxygen supply (reactive evaporation, oxygen preexposition) or stoichiometry of the supported ceria film, and increasing the thickness of the thin films by further ceria deposition and annealing leads to qualitatively the same results as obtained on the thick films. Consequently, in thermodynamic equilibrium the regularity of the terraces arises from the interplay between the repulsive interaction of the steps33 and the favorable coincidence of the in-plane lattice constant of CeOx(111) and Cu(110). The formation of the local vicinal copper surface might lead to a reduction of surface stress and consequently to the stability of the reorganized surface.34 When the (13 13 1) facet representing a local vicinal surface of copper is stabilized, the ceria adlayer on top of it can be described in the framework of the so-called carpet-like growth mode.25 The carpet-like growth mode is a consequence of the large difference between the step height of the vicinal copper surface (1.3 Å) and of the ceria film (3.1 Å). Due to the weak adsorbate−substrate interaction,35 the ceria can grow over the copper steps with only a small distortion in its lattice. Similar behavior of ceria has also been observed on Ru(0001).36 The distortion in the ceria overlayer can be described using elasticity theory. Following from a model of Schwennicke et al.,37 the length of the distorted area in the vicinity of the substrate step can be estimated as Λ = (3Ed3h2a20/2eb)1/4, where E is the elastic modulus, d is the thickness of the film, h is the bending distance, a0 is the lattice parameter, and eb is an average binding energy loss per primitive lattice unit in the distorted region. The parameters are illustrated in Figure 7a. The term representing the lateral stretch in the equation of Schwennicke et al. has been neglected as we do not expect distortion in the Cu[11̅0] direction due to the coincidence of in-plane lattice constants. From the equation it is evident that the length of the

Figure 7. (a) Illustration of the parameters used in determination of the average loss of the binding energy eb. The bending distance h is equal to the height of a copper step. Thickness of the film d equals the thickness of the ceria film. The length of the distorted and unperturbed regions is marked as Λ and Γ, respectively. (b) Illustration of the boundary condition arising from our experimental data in the case of 2 ML or thicker ceria films. 1856

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The Journal of Physical Chemistry C

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order to gauge the magnitude of the effect of the modification of the electronic structure at the interface between copper and ceria on chemical properties of the system.

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CONCLUSIONS We have performed a detailed structural study of surface faceting of Cu(110) induced by a ceria overlayer. The CeOx/ Cu(110) system can be equilibrated by annealing above 500 °C. The annealing improves the ordering noticeably and introduces copper steps at the interface between copper and ceria through modification of the roughening temperature of Cu(110). The distance between the steps is constant in the equilibrium, creating (13 13 1) facets on the copper surface. The ceria film on top can be described by the so-called carpetlike growth mode; the ceria grows over the copper steps, accommodating the steps by a small distortion in the ceria layer. In more than 1 ML thick ceria islands, the distortion spreads over the whole terrace, creating an inclined plane of ceria, with inclination angle of 3.3° with respect to the Cu(110) plane. Practically, the system consists of a local vicinal Cu(110) surface covered with rigid ceria film. The specific structure can give rise to modifications of electronic structure at the copper−ceria interface. The elasticity theory predicts a loss of binding energy in the ceria film supported on the Cu(13 13 1) facets. On 1 ML thick ceria islands, the binding energy loss at the copper steps may lead to a creation of an array of alternating regions of ceria with different binding energy to the substrate. Furthermore, the regular 1D step array on the local vicinal surface of Cu(110) modifies the surface states, possibly leading to electron confinement and superlattice effects. The CeOx/Cu(110) system represents an observation of surface faceting of a low index metal surface induced by an oxide overlayer and can be employed as a model system for the study of the effects of structural modifications of electronic structure on the chemical properties of metal supported oxide overlayers. The reported structural transition can be expected to occur for other oxide−metal combinations and, as such, opens a pathway for the preparation of oxide−metal functional nanostructures.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: katerina.veltruska@mff.cuni.cz. Phone: +420 221 912 243. Fax: +420 284 685 095. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Czech Science Foundation (GAČ R 204/11/1183) and by the Ministry of Education of the Czech Republic (LD13054). M.A., T.D., F.D., and V.S. acknowledge the support of the Granty Agency of the Charles University (GAUK 320313, GAUK 794313, GAUK 610112, GAUK 339311).

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DOI: 10.1021/jp5099359 J. Phys. Chem. C 2015, 119, 1851−1858

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DOI: 10.1021/jp5099359 J. Phys. Chem. C 2015, 119, 1851−1858