Phase Transitions of Cobalt Oxide Bilayers on Au(111) and Pt(111

Article pubs.acs.org/JPCB

Phase Transitions of Cobalt Oxide Bilayers on Au(111) and Pt(111): The Role of Edge Sites and Substrate Interactions Jakob Fester, Zhaozong Sun, Jonathan Rodríguez-Fernández, Alex Walton,† and Jeppe V. Lauritsen* Interdisciplinary Nanoscience Center (iNANO), Aarhus University, 8000 Aarhus C, Denmark S Supporting Information *

ABSTRACT: Well-characterized metal oxides supported on single crystal surfaces serve as valuable model systems to study fundamental chemical properties and reaction mechanisms in heterogeneous catalysis or as new thin film metal oxide catalysts in their own right. Here, we present scanning tunneling microscopy and X-ray photoelectron spectroscopy results for cobalt oxide nanoislands that reveal the detailed atomistic mechanisms leading to transitions between Co−O bilayer and O−Co−O trilayer, induced by oxidation in O2 and reductive vacuum annealing treatments, respectively. By comparing between two different noble metal substrates, Au(111) and Pt(111), we further address the influence of the substrate. Overall, nanoisland edges act to initiate both the oxidation and reduction processes on both substrates. However, important influences of the choice of substrate were found, as the progress of oxidation includes intermediate steps on Au(111) not observed on Pt(111), where the oxidation on the other hand takes place at a significantly higher rate. During reductive treatment of trilayer, the bilayer structure gradually reappears on Pt(111), but not on Au(111) where the reduction rather results in the appearance of a stacked cobalt oxide morphology. These observations point to strong differences in the catalytic behavior between Au and Pt supported cobalt oxides, despite the otherwise strong structural similarities.

1. INTRODUCTION It has generally been established that the interaction between metal and oxide support plays a key role for the catalyst performance for heterogeneous catalysts based on metal/oxide composites.1−3 Often the specific catalytically active centers are located at the interface between metal and oxide, as demonstrated in a number of model studies exploiting probing techniques with atomic precision detail, e.g., for the water−gas shift reaction and CO oxidation.4,5 The shape and composition of the catalyst nanostructure can be highly flexible and depend on specific reaction conditions such as surrounding reaction gas.6,7 The dynamical evolution during operation and correlation between structure, reactivity, and reaction environment can in principle be addressed by in situ characterization techniques,8−10 but phase transitions of materials near catalytic operating conditions is in general a fundamental issue in the understanding of catalysis yet to be explored. One particularly well studied supported transition metal (TM) oxide nanostructure is the inverse model catalyst system consisting of planar FeO films on Pt(111) and related noble metal surfaces. Upon oxidative treatment the iron oxide changes from an Fe−O bilayer structure to an O−Fe−O trilayer.7,11,12 Dynamic changes between FeO bilayer and FeO2 trilayer under real catalytic conditions have been reported by reductive H2 gas and oxygen pressure in the mbar range,13 determining the activity for CO oxidation due to the high activity for O2 dissociation at coordinatively unsaturated ferrous © XXXX American Chemical Society

(CUF) sites at the bilayer Fe−O/Pt(111) interface under the PROX (preferential oxidation of CO) reaction condition in excess of hydrogen.4 Under reaction conditions without H2, on the other hand, the trilayer FeO2‑x/Pt(111) interface was more recently shown to be most active.14 The stabilization of CUF metal sites originates from a strong interfacial bonding between Fe atoms and the underlying metal substrate (the so-called interface confinement effect) and varies with the choice of support metal.15 Support effects in CO oxidation catalysis comparing FeO(111) thin films on Au(111) and Pt(111) was recently found as well as a coverage dependency on the reaction rate on Au(111).16 In the same study, substantial reconstruction of the film morphology was observed on Au(111) under the reaction conditions, ascribed to the weak interaction of this substrate, whereas the film on Pt(111) was essentially still planar. Also, oxidation resistance of the Fe−O bilayer on Pt(111), as well as Co−O bilayer on both Pt(111) and Au(111), was recently reported via a dynamic size-effect for oxide islands with dimensions below 3.2 nm.17 However, little direct information is available on the actual transition processes between the TM-O and O-TM-O phases, although such knowledge is of utmost importance in the efforts toward an Special Issue: Miquel B. Salmeron Festschrift Received: May 22, 2017 Revised: August 11, 2017 Published: August 11, 2017 A

DOI: 10.1021/acs.jpcb.7b04944 J. Phys. Chem. B XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry B

Figure 1. Oxidation of Co−O bilayer on Au(111) and Pt(111) by stepwise exposure to 4 × 10−6 mbar O2 at 325 K. (a) and (b) STM images acquired through the dosing series (upper panel from left: Vt = −1.1 V, It = −0.240 nA/Vt = −1.2 V, It = −0.280 nA/Vt = −1.0 V, It = −0.240 nA/Vt = −1.3 V, It = −0.250 nA and lower panel: Vt = −1.4 V, It = −0.340 nA/Vt = 1.4 V, It = 0.130 nA/Vt = 1.4 V, It = 0.160 nA/Vt = 1.2 V, It = 0.210 nA). (c) STM images from exposure series of a low coverage sample (left: Vt = −1.2 V, It = −0.220 nA, right: Vt = 1.4 V, It = 0.290 nA). (d) Fraction of the cobalt oxide island area covered by the emerging trilayer morphology measured from STM images acquired from the samples shown in (a), (b), and (c). Note that the moiré pattern on bilayer in these images is hardly visible due to tuning of the color scale to include the trilayer apparent height, as well as because of disturbance by defect lines (in particular part (b) on Au(111)).

understanding of the flexibility and working principles of heterogeneous catalyst nanostructures exposed to various reactions gases and conditions. Well-ordered cobalt oxide islands can be synthesized on Au(111) and Pt(111) metal substrates and tuned by the synthesis conditions toward the related structures Co−O bilayer, O−Co−O trilayer, and O−Co−O−Co-O multilayer.18−21 Since cobalt oxide nanostructures have shown a great potential to play a role in next generation of earth abundant catalysts for e.g. hydrogen production via (photo)electrochemical water splitting22−26 and low temperature CO oxidation,27 this model system is an interesting and useful platform to study the nanoparticle properties in detail. Au(111) and Pt(111) are particularly interesting as substrates in this model system since synergistic effects have been reported between these metals and cobalt oxide. Enhancement effects were demonstrated in several studies using cobalt oxide and gold in CO oxidation,28−30 as well as the oxygen evolution reaction (half of the full water splitting reaction),25,31,32 and Pt in CO oxidation.33−35 Similar trends have been observed in the case of the related manganese oxide/Au system.36,37 However, the exact nature and mechanisms of the reported synergistic effects are not known. Structurally, the bilayer- and trilayer

morphologies, respectively, are identical on Au(111) and Pt(111), only differing by minor differences in interatomic spacing.18 This conveniently allows us here to make a direct comparison between the metal support influence on the conversion processes between bilayers and trilayers. As in the case of the Fe−O bilayer, the bilayer cobalt oxide exposes the (111) plane of a slightly distorted rocksalt structure when synthesized under low oxygen pressure conditions. The bilayer has a larger interatomic in-plane spacing than the underlying Au(111) and Pt(111) surfaces, resulting in a moiré pattern, whereas the trilayer structure appears to grow pseudomorphic with the substrate.21 In this work we shed light on the detailed transition mechanisms between bilayer structures and O-rich structures and the influence of noble metal supports on metal-oxide nanoparticles. We use scanning tunneling microscopy and Xray photoelectron spectroscopy to study both the oxidationand reduction processes leading to transitions between Co−O bilayer and O−Co−O trilayer islands supported on both Au(111) and Pt(111) under oxidizing and vacuum annealing conditions, respectively. The results point to the importance of island edges in the oxidation- as well as the reduction processes on both substrates. We observe pronounced substrate effects on B


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Figure 2. XPS Co 2p and O 1s spectra during series of bilayer oxidation and subsequent reduction in UHV. (a) Comparison between bilayer on Au(111) (upper) and Pt(111) (lower). SS indicates shakeup satellites. (b) Relative total O 1s areas from the data in (a) (filled circle) compared to expected values based on measurements from STM images (cross symbol) with 100% trilayer defined as a relative O 1s area of 1.8 as measured in XPS. (c) STM image (Vt = −0.7 V, It = −0.320 nA) of a bilayer sample exposed to 40 min O2 (4 × 10−6 mbar O2, 325 K) showing evidence of oxygen bound to the Pt(111) substrate.

STM was operated in constant current mode with an electrochemically etched W tip and the sample at room temperature. The sample biases stated were applied to the sample. XPS spectra were recorded in normal emission and energy calibrated toward the Au 4f 7/2 peak located at 84.0 eV. A polynomial background was subtracted from the O 1s region, whereas a linear background was used for the Co 2p region. Clean surfaces were obtained by repeated cycles of 1.5 kV Ar+ ion sputtering and annealing in UHV at 800 K for Au(111) and at 900 K in 5 × 10−7 mbar O2 with a subsequent flash anneal to 1100 K for Pt(111). Cleanliness was assessed by STM and XPS. The starting structures in our experiment are the wellcharacterized bilayer Co−O structures on both Au(111) and Pt(111) which were prepared following a recipe previously described.19 Briefly, this consists of reactive deposition of Co

the progression via intermediate states and the rate during oxidation. Our findings reveal surprising fundamental differences in the reaction pathways between bilayer and trilayers on the two structurally similar substrates, and in addition that the choice of substrate has a determining role for the reversibility of the transitions.

2. EXPERIMENTAL SECTION All experiments were performed in the same setup of UHV chambers with a base pressure below 1 × 10−10 mbar. The setup is divided in a preparation chamber with an Oxford Applied Research e-beam evaporator for metal deposition (model EGCO4) connected to an analysis chamber with a SPECS Phoibos 150 analyzer and Al Kα radiation (1486.6 eV) from a SPECS XR 50 source and an “Aarhus”-type STM.38 The C


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The Journal of Physical Chemistry B metal in an oxygen atmosphere of 1 × 10−6 mbar O2 at an elevated sample temperature of 380 K. The deposition rate was ∼0.04 ML/min with 1 ML defined as the number of Au(111) or Pt(111) surface atoms. Following deposition, the samples were postannealed to 523 K for 10 min in 1 × 10−6 mbar O2 and 5 min in UHV. O−Co−O trilayer samples could be synthesized directly from bilayer samples by annealing 3 min at 400 K at a high local oxygen pressure (estimated to be 1 × 10−4 mbar O2 or above) at the sample by using a moveable doser tube placed within 1 mm from the sample surface. We then systematically followed the sequential formation of O−Co−O trilayers in an O2 gas exposure series (oxidation) and the gradual reduction during vacuum annealing as follows: In the oxidation series, the initial Co−O bilayer samples were exposed to 4 × 10−6 mbar O2 in the preparation UHV chamber with the sample temperature always kept at 325 K. The exposure was performed stepwise, interrupted by the sample being transferred to the analysis chamber for evaluation by means of STM and XPS. Starting from full oxidation, i.e., a sample consisting of entirely O−Co−O trilayer islands, the reduction was always carried out by heating to 423 K in UHV (unless otherwise specified) in time lapsed steps, while evaluating the sample by STM and XPS after each step. During the reduction series, the samples were cooled down to room temperature before any measurement.

predominantly taking place close to the edges of the cobalt oxide islands and then appears to progress inward toward the center of the islands. This phenomenon can be observed in the STM images in Figure 1b at 40 and 70 min exposure time (see also Figure S1 for further examples), and indicates that island edges are mediating the process. To further investigate this hypothesis, we synthesized a second sample with about half coverage of bilayer islands on Au(111) (0.19 ML Figure 1c), which results in smaller islands and a correspondingly higher density of edge sites.39 As calculated from STM images, the percentage of edge Co atoms out of the total number of Co atoms in the island basal plane increases from 12% to 16% as compared to the sample in Figure 1b), corresponding to the two different coverages of 0.19 and 0.43 ML, respectively. The progression of the oxidation process on the 0.19 ML sample is plotted in Figure 1d together with the previous results and appears to take place at a faster rate than in the case of the 0.43 ML sample, supporting the hypothesis of the edges mediating the conversion. In addition, the increased transformation rate on the high edge-density sample was backed up by the oxygen uptake of the samples for a fixed O2 dosage measured by XPS reflected by the normalized O 1s peak area, giving an average over the entire sample area to complement the STM image analysis. The O 1s area on the on high edge-density sample relative to the pristine bilayer (normalized to 1) displayed an increase of 51% (40 min exposure) and 15% (70 min exposure) compared to the high coverage sample in Figure 1a. The same percentages calculated from STM images differ from these values, yielding 139% and 18%, respectively, however they confirm the same trend as found based on the STM image analysis. On the Pt(111) substrate, we studied the onset of trilayer formation by means of a different strategy, as described in Section 3.4. 3.2. Transition between Bilayer and Trilayer Monitored by XPS. The same samples were also evaluated by means of XPS during the O2 exposure series. The Co 2p and O 1s core level regions were acquired for each exposure time and shown in Figure 2a for both Au(111) and Pt(111) substrates. The Co 2p spectra of the freshly synthesized samples display a broad main component and a shakeup satellite (SS) structure on both substrates which is characteristic of the Co2+ oxidation state in the pristine bilayer structure.19,40 Interestingly, despite these common features, the shape of the main Co 2p component differs between the two substrates, although the cobalt oxide morphology (bilayer) appears structurally identical in STM. The spectrum on Au(111) matches previously acquired synchrotron-19 as well as Al K-alpha X-ray source data,21 whereas the Co 2p spectrum for bilayers on Pt(111) was not reported in the literature before. As a function of O2 exposure in Figure 2a (greyscale spectra) the main component within the Co 2p peak changes toward a narrow shape and a binding energy of 779.3 eV on both substrates. The shakeup satellite (SS) is suppressed, but a weaker signature appears at a higher binding energy (indicated with a blue oval in Figure 2a similar to a peak also observed in reference spectra of Co3O4.41 These observations indicate a change to the Co3+ oxidation state40 in the trilayer structure, which is then stabilized in this oxidation state due to electron transfer from the substrate.21 Upon annealing the trilayer in UHV at 423 K (orange and red spectra), the shape of the original bilayer Co 2p spectrum readily returns on the Pt(111) sample after 10 min, but not in case of the Au(111), even after annealing for 100 min. This suggests that the transition

3. RESULTS 3.1. Oxidation Process of Bilayers Studied by STM. In Figure 1a,b a series of STM images reflecting the gradual transformation from bilayers supported on Pt(111) and Au(111) into trilayers are shown as a function of O2 dosage. Both samples were evaluated ex situ at selected dosages throughout the exposure series by means of STM and XPS (XPS results described in Section 3.2). Compared with the bilayer samples at zero O2 exposure, elevated but still atomically flat areas on the pristine islands appear with a larger apparent height (∼3 Å) than that of the bilayer (∼1.7 Å). This island height together with the appearance of a 3 × 3 R 30° superstructure on the basal plane relative to the underlying hexagonal cobalt lattice display the characteristics assigned to a cobalt oxide trilayer on Au(111) and Pt(111)18,21 (see also Figure 2c). The conversion process proceeds up to an almost complete transformation from bilayer to trilayer on both substrates, with an apparent trilayer to bilayer ratio of nearly 100% as evaluated from STM images. To compare the progression of the oxidation process from bilayers into trilayers on Au(111) and Pt(111), the percent of converted island area of the total island area was calculated at each stage in the exposure series. The conversion degree as a function of O2 dose is plotted in Figure 1d, and immediately shows that the oxidation is taking place at a significantly higher rate on Pt(111) compared to Au(111) at the same exposure conditions and coverage of the cobalt oxide (0.38 and 0.43 ML on the Pt(111) and Au(111) respectively, with 1 ML defined as the sample surface area fully covered by cobalt oxide islands). Particularly, 65% of the cobalt oxide on Pt(111) is already converted into trilayer after 10 min O2 exposure, whereas no trilayer can be observed at this point on the Au(111) sample. While the early onset of trilayer formation is not represented on the Pt(111) sample in the exposure series shown in Figure 1 due to the high conversion rate, it can be observed that the initial oxidation into the trilayer morphology on Au(111) is D


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The Journal of Physical Chemistry B between bilayer and trilayer is only reversible on Pt(111). Both reduction processes were further investigated by STM, as will be discussed in Section 3.5. In the corresponding O 1s spectra, a high binding energy peak at 531 eV emerges during the O2 exposure, ∼ 1.9 eV above the main lattice oxygen component. This position and binding energy shift relative to the main peak is a signature typically assigned to hydroxyl species42 and agrees with the presence of a partial hydroxyl overlayer, previously assumed to originate from H2 or H2O in the rest gas dissociatively adsorbed on the formed trilayer as previously reported on both Au(111) and Pt(111).18 In agreement with this, we also observed the characteristic 3 × 3 R 30° superstructure on the basal plane in our STM images (as described in Section 3.1) that results from this overlayer. The increase in the total O 1s peak area normalized to the background further reflects the uptake of oxygen during the conversion. The areas relative to bilayer (normalized to 1) are plotted in Figure 2b together with the expected values according to the percentage of trilayer measured in STM images, bearing in mind that the lower layer oxygen toward the metal substrate signal is slightly attenuated through the upper layers of the cobalt oxide. An experimentally determined attenuation factor of 0.8 was used, based on a measurement of the oxygen signal arising from a bilayer sample before and after full conversion to trilayer. The obtained value is close to the prediction from the electron mean free path of 22.4 Å,43 yielding 0.9. Overall, the O 1s areas follow the STM data in case of Au(111), but goes significantly beyond the expected 1.8 for the complete trilayer on Pt(111). Whereas Au(111) is relatively chemically inert, the additional increase in O 1s signal can be explained by the ability of the Pt(111) surface to dissociate and bind oxygen, either as a thin oxide layer (in a previous study formed by means of atomic O44) or chemisorbed oxygen in a (2 × 2) pattern forming from dissociated O2 on steps with a desorption temperature of 375 K.45 In the STM images acquired during the conversion process from bilayer to trilayer on Pt(111), a (2 × 2) superstructure with periodicity of 5.7 ± 0.1 Å (twice of the Pt(111) in-plane interatomic distance of 5.6 Å) was indeed observed as shown in Figure 2c. It was reported44 that this oxygen layer gives rise to a peak at 530.8 eV in XPS, i.e., close to the hydroxyl position which is complicating the deconvolution of the spectrum into individual components. After 10 min of annealing at 423 K (Figure 2b), the O 1s area on the Pt(111) sample returns to ∼1, i.e., all the oxygen uptake on both the Pt(111) surface as well as in the trilayer is desorbed again in agreement with a reversible process. On Au(111), the trilayer is similarly seen to be reduced to the original oxygen content in the pristine sample, however at a much lower rate and not recovering the bilayer structure (see Section 3.5). O 1s spectra of the complete trilayer on Au(111) and Pt(111) are shown and compared in Figure 3. To minimize the presence of a contribution from oxygen on the Pt(111) surface, a sample with a high coverage of 0.81 ML trilayer was synthesized leaving only small areas of exposed Pt. Three individual peaks must be included to fit this spectrum at binding energies of 528.4, 529.8, and 531.2 eV reflecting the three distinct oxygen species expected: Upper layer oxygen, lower layer oxygen, and OH. Based on the size of the peaks compared to calculated values assuming a perfect 3 × 3 R 30° OH superstructure and the experimentally determined attenuation factor of 0.8 for lower layer oxygen (37,

Figure 3. XPS O 1s spectra of O−Co−O trilayer on Au(111) and Pt(111). Note the difference in the number of required individual components to peak fit spectra satisfactory.

44, and 19% of the total area for upper O, lower O, and OH, respectively), as well as comparison to previously reported XPS data of the similar FeO2 trilayer/Pt(111) system,46 we assign the low energy component at 528.4 eV to upper layer oxygen, although the relative area compared to the OH and lower layer peaks is smaller than expected. This discrepancy could be due to a contribution from oxygen adsorbed on the bare Pt(111) together with the phenomenon of a tail toward higher binding energy on the main component that is always present, even in the pristine bilayer spectrum (see Figure 2a). Surprisingly, the O 1s region of the trilayer on Au(111) can be approximated using only two components. Comparing the full width at halfmaximum (fwhm) of the main component of the bilayer and trilayer from the exposure series in Figure 2a, a slight increase from 1.75 to 1.91 eV is observed that might be associated with the appearance of two components at nearly the same position, corresponding to upper and lower oxygen species in the trilayer structure. The difference is likely a reflection of the more noble nature of the Au substrate compared with Pt in the interaction with the lower O layer in the O−Co−O trilayer. 3.3. Initial Stages of Oxidation on Au(111). Comparison of the O 1s areas measured in XPS and the expected values based on STM analysis reveals a slight mismatch, as the STM data shows a later apparent onset of the oxidation (Figure 2b). Since the STM analysis is solely based on the bilayer to trilayer area ratio determined from the brighter appearance of trilayer (Figure 1b), this suggests an initial oxygen uptake through an intermediate state which is not identified in the STM images as a full ∼3 Å high trilayer patch. Indeed, inspection of atomresolved STM images of the bilayer basal planes during the O2 exposure (Figure 4) reveals that oxygen adatom defect lines18,47 are incorporated into the bilayer prior to the onset of trilayer formation. The adatom lines are first organized in a high density, following an ordered zigzag pattern (Figure 4b), but even going beyond this stage and into an unordered and extremely densely packed abundance (Figure 4c). The O adatom line defects accommodate Co in a 4-fold coordination to O, and this contributes to the O uptake without full conversion into a trilayer. The very dense packing of line defects can therefore be considered an intermediate state before the trilayers are fully formed. In parallel with the formation of line defects, the edge structure of the bilayer islands change. By E


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Figure 4. STM images representing different intermediate stages in oxidation of Co−O bilayer on Au(111). (a) Co−O bilayer reduced by annealing in UHV 5 min at 523 K (Vt = −1.1 V, It = −0.340 nA). (b) Bilayer as synthesized after cooling down to room temperature in 1 × 10−6 mbar O2 (Vt = −0.5 V, It = −0.340 nA). (c) Bilayer exposed to 20 min O2 at 4 × 10−6 mbar O2 (Vt = −1.1 V, It = −0.340 nA). (d) Sketches of reduced (left) and oxidized (right) bilayer. Arrows indicate observed distortions (not included in the structural model). (e) Coexistence of a high density of line defects and the onset of trilayer formation. The O2 exposure was 25 min at 4 × 10−6 mbar O2, 325 K (Vt = −1.3 V, It = −0.210 nA). Dotted lines show assignments of island corner shapes.

Figure 5. Onset of trilayer formation on Co−O bilayer on Pt(111). (a) STM image (Vt = −0.9 V, It = −0.310 nA) of coexisting bilayer and trilayer. Note that only few line defects are incorparated in the bilayer structure. i and ii indicate two different types of line defects. O2 exposure: 10 min at 4 × 10−6 mbar O2, 325 K. (b) STM image (Vt = 1.1 V, It = 0.230 nA) of a Co−O bilayer film with holes of uncovered Pt(111) (black spots) exposed to 0.5 min O2 at 4 × 10−6 mbar O2, 325 K. Note the onset of trilayer formation located at the edges of the pits.

due to oxygen deficient line defects on CoO films supported on Pt(111).20 In this study, the observed wiggling of long defect lines, that would at first hand be expected to be perfectly straight, was associated with the tendency of the CoO film to adjust relative to the substrate toward a favorable fcc-like stacking sequence. Finally, the image in Figure 4e shows the onset of trilayer formation together with the basal plane in a stage with a high density of oxygen adatom line defects. Interestingly, the observed initial incorporation of line defects and change in edge structure is reversible upon annealing the sample (see Figure S2), whereas the conversion into trilayer on Au(111) is not.

structural analysis of the oxygen adatom defect lines in the island basal planes, as described in a previous publication,18 the edge types can be assigned. From the situation of pristine (reduced) bilayer structure before oxidation that displays both of the two possible cobalt and oxygen terminations (see sketch in Figure 4d, left), all edges after oxidation appear to be oxygen terminated (Figure 4d, right). The particle corners simultaneously change from a sharp 120 degree angle to a rounded shape (see the STM images in Figure 4b, c, and e). This observed distortion of the beforehand perfect hexagonal lattice is indicated by arrows in Figure 4d, but note that the ball sketch itself does not show this stretch. A similar phenomenon has been reported previously in the case of lattice displacements F


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Figure 6. O−Co−O trilayer reduction experiment on Au(111). (a) STM image (Vt = −1.2 V, It = −0.220 nA) of bilayer exposed to 40 min O2 at 4 × 10−6 mbar O2, 325 K. (b) Image of the same sample as in (a) (Vt = 1.5 V, It = 0.110 nA) after full conversion to trilayer and annealed in UHV for 20 min, 423 K. Θ is the measured fraction of Au(111) surface area covered by cobalt oxide islands.

3.4. Initial Stages of Oxidation on Pt(111). In contrast to the case of the oxidation process of bilayers on Au(111), we do not observe the early stages of oxygen uptake by defect line incorporation and change in edge termination on the Pt(111) sample. The STM image in Figure 5a is acquired after 20 min of O2 exposure at 4 × 10−6 mbar, i.e., well beyond the onset of trilayer formation (see Figure 1). However, the bilayer in the image is still in its pristine state without a high density of defect lines on neither the basal plane or along the edges (compare to Figure 4e). It is therefore unlikely that formation of adatom line defects proceed before trilayer formation. Only two small defect lines (marked as i and ii) are observed, indicating that this structure is indeed visible in the STM tip mode during the recording of this image, whereas the contrast of defect lines in other tip modes can change and make them not clearly recognizable due to the complicating role of this phenomenon in STM imaging of oxide surfaces.48 Whereas the defect line denoted ii appears similar to the oxygen adatom defect lines described in Figure 4, the defect line i is most likely an oxygen deficient structure as further discussed in Section 3.5. Inspired by the results on Au(111), indicating that edges play a role in the onset of trilayer formation, a sample with a high coverage of bilayer on Pt(111) was synthesized, leaving only local holes in the film exposing edges at the interface between cobalt oxide and the Pt(111) substrate. By dosing a small amount of O2 (0.5 min at 4 × 10−6 mbar, 325 K), it was possible to capture the onset of conversion from bilayer into trilayer, as shown in Figure 5b. With a few exceptions, the patches of trilayer grow preferentially from edges of the holes in the bilayer film. To quantify this observation, a large number of STM images were acquired and the trilayer patches in these images assigned as either located at an edge or on the basal plane. On average, 88% of 159 counted trilayer patches were situated at locations that could be clearly identified as Pt(111)/ CoO edges, indicating that these edges host sites that facilitate the onset of the oxidation process. 3.5. Reduction of Trilayer on Au(111) and Pt(111). At this point we turn to the reverse process of trilayer formation, investigating the reduction of the trilayer by UHV annealing the samples at a temperature of 423 K. This temperature is enough to reduce the structure significantly, and the tendencies for the two structures to reversibly give off O are important catalytic parameters explaining low temperature CO oxidation. Figure 6 shows STM images of the same synthesis on Au(111) during

an O2 exposure series (a) and after full conversion to trilayer plus subsequent annealing (b), respectively. The image in (b) was taken after 100 min annealing, at which point the oxygen content as measured by XPS decreased to 0.99 relative to the original bilayer, i.e., the sample was fully reduced. However, images of samples exposed to this treatment still show approximately the same density of protruding areas, generally located near the island edges, together with the reappearance of bilayer. At first sight, the protruding areas look similar to trilayer patches, but are clearly reduced according to the XPS data and exhibit several differences. First, the apparent height varies between STM tip imaging modes, but is in general larger here (3.5 Å on average) than for the trilayer (∼2.9 Å).21 Second, the protruding areas are not flat, but corrugated and no ordered surface structure could be resolved in STM. Also, the island shape seems to have changed from containing sharp 120 degree corners (Figure 6, image a) toward a less regularly hexagonal shape of the edges. Finally, the coverage in terms of Au(111) surface area covered by cobalt oxide islands decrease by 26% from 0.20 to 0.16 ML. Together with the XPS measurement, implying a Co to O stoichiometry equal to that in the bilayer, i.e., 1:1, these observations might suggest that the protruding areas result from a growth to a 3D like phase very similar to a double bilayer,19 facilitated by the energy provided through heating the sample. On this basis, the transition between bilayer and trilayer on Au(111) does not seem to be reversible under the conditions of the reduction experiment. We further investigated whether a lower annealing temperature could prevent the amorphous 3D growth during reduction. However, by heating to only 390 K, the formation of higher 3D like patches appear while trilayer still persists within the islands, and domains with three different apparent heights corresponding to bilayer, trilayer and 3D growth coexist together. Finally, we investigated the reduction of trilayers on Pt(111) by STM. The sample corresponding to the XPS data series in Figure 2 on Pt(111) after conversion to trilayer and subsequent 10 min annealing in UHV at 423 K was also evaluated by STM. At this stage, the XPS O 1s area decreased to ∼1.1 relative to the pristine bilayer (Figure 2b). The STM images reveal that, in contrast to the case of Au(111), the bilayer morphology reappears in an almost perfect condition with only a few percent trilayer persisting on the surface. Interestingly, the basal planes were found to contain a type of line defect different from G


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STM images, e.g., Figure 7b,c, the bilayer is observed to initially emerge in the vicinity of island edges (examples highlighted with dotted ovals), in agreement with the coverage series experiment. On Pt(111) we therefore conclude that the oxidation and annealing induced reduction processes between bilayer and trilayer are reversible and both appear to be mediated by the presence of island edges.

the oxygen adatom line defects described in Figure 4, characteristic of their dark appearance (see inset in Figure 7a

4. DISCUSSION We have studied the processes of oxidation, as well as reduction by vacuum annealing, leading to transformations between cobalt oxide bilayer- and trilayer morphologies on the two substrates Au(111) and Pt(111). Common to the oxidation processes on Au(111) and Pt(111) is the role of island edges that facilitate trilayer formation. This is in line with the special reactivity of cobalt terminated island edges reported in our previous study of H2O dissociation at room temperature in the same model system.39 In comparison to our experiments, to generate the trilayer on bilayer films of FeO supported on Pt(111), previous studies utilized an atomic oxygen source,11 elevated O2 pressure in the mbar regime from 0.5 mbar to 50 mbar at elevated temperature of 450−500 K7,11−13,46,51 or NO2.46 However, it was found that nanoislands, as opposed to a closed monolayer film, oxidized more readily at 1.3 × 10−6 mbar O2 and 475 K.11 Our present results on the comparable CoO system are in good agreement with this behavior, related to the important role of nanoparticle edges in the fundamental oxidation mechanism. 4.1. Oxidation Mechanism. Overall, the results suggest a trilayer formation process initiated by dissociation of O2 at the island edges. This leaves the possibilities of (1) dissociation on the edge of CoO bilayer islands and, most likely, (2) at the Au/ Pt−Co ensemble at the interface between metal oxide and the support, similar to the activation of molecular oxygen at coordinatively unsaturated ferrous (CUF) sites at the interface between FeO and Pt(111).4,52 Alternatively, the trilayer edge, once formed, could compete in the O2 dissociation, as could in principle the appearing bilayer/trilayer boundary, bearing in mind that there has been some evidence that this interface, in case of the Fe−O bilayer/O−Fe−O trilayer system on Pt(111), is capable of dissociating H2O,46 and therefore might be expected to exhibit a different reactivity than the basal plane in these systems. However, which dissociation site that is most favorable can not be concluded from our data. The further progress of the oxidation process is most likely to proceed via O diffusion at the island/substrate interface, with the exception of O2 dissociation being preferred at the bilayer/ trilayer boundary, in which case no diffusion is in principle needed. The observed apparent deintercalation of interface O in the vicinity of edges in our vacuum reduction experiments seems to support the picture of mobile lower layer oxygen. On Pt(111), an alternative oxidation mechanism could be considered, since it is known that O2 can dissociate on the bare Pt(111) substrate.45 This feature of the system was supported by our XPS results that reveal an increase in total O 1s area due to oxygen stored on the Pt. In relation to trilayer formation, the surface oxygen on Pt(111) provides a potential source of O species available for oxidation of the cobalt oxide, and hence suggests a possible oxidation mechanism by migration of oxygen from the platinum underneath the cobalt oxide bilayer. On the bare Au(111) substrate, on the other hand, dissociation of O2 is not expected or evidenced in our experiments, leaving

Figure 7. O−Co−O trilayer reduction experiment on Pt(111) showing pairs of images from three samples of full trilayer (left) and again after reductive vacuum annealing treatment for different coverages. (a) Low coverage (Vt = 0.9 V, It = 0.520 nA/Vt = −1.1 V, It = −0.270 nA). Insets show the surface atomic structure of trilayer (left, Vt = 0.9 V, It = 0.360 nA) and reduced bilayer with defect lines appearing as depressions (rigth, Vt = −1.0 V, It = −0.280 nA). (b) Intermediate coverage (Vt = 1.3 V, It = 0.200 nA/Vt = −0.9 V, It = −0.250 nA). (c) High coverage (Vt = −1.1 V, It = −0.240 nA/Vt = −1.0 V, It = −0.310 nA). Dotted ellipses indicate the first emerging bilayer areas along island edges.

after annealing). These features are very similar to the oxygen vacancy dislocation lines described in the FeO/Pt system,49,50 and indicate that the bilayer in our experiment is in fact reduced beyond its pristine state, that contained a number of oxygen adatom defect lines (see Figure 1b). To investigate whether oxygen deintercalation could be facilitated by the ability of oxygen to migrate under the Co−O sheet and be released in the vicinity of edges, we synthesized trilayer samples with different coverages to vary the relative amounts of edge length and basal plane area. Figure 7 shows three as prepared trilayer samples of 0.28, 0.45, and 0.80 ML that were exposed to the same treatment of 35 min UHV annealing at 423 K. The reduction process is seen to happen at a much lower rate on the sample with increased coverage. The average trilayer percent, as measured from STM images, yield 7, 41, and 71%, respectively, following a trend suggesting that a high abundance of edges facilitate the reduction. In individual H


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Second, since our study highlights an influence of edge abundance, we note that the distribution of bilayer on Pt(111) is organized differently than on Au(111) toward a triangular island shape18 and elongated islands (see Figure 1a), opposed to the hexagonal island shape on Au(111). Calculation of the ratio between edge atoms and basal plane atoms on the two samples in our experiment (Figure 1), of approximately the same coverage (0.38 and 0.43 ML, respectively), yields a higher edge atom density on Pt(111) of 14% of the total cobalt atoms compared to 12% on the Au(111) sample. In addition to this, the cobalt terminated edge type (see Figure 4d) is dominating, i.e., favored over the oxygen termination, on Pt(111).18 This has a potential influence on the oxidation process since O2 adsorption and dissociation is likely to occur on exposed metal sites.4 Finally, there might be a substrate effect on the involved O2 dissociation barrier itself at the island/substrate interface that we propose to be the most likely first step in the oxidation process. The role of the pronounced defect line incorporation and change in edge structure as an intermediate step in the oxidation process on Au(111) should also be considered. Interestingly, the change of edge type to oxygen terminated edges (a change that is resulting from the appearance and organization of defect lines) is equivalent to the recently reported structural dynamics of small FeO nanoislands on Pt(111) in O2, also leading to reconstruction of edges to the oxygen termination, albeit the mechanism there is a collective shift of all oxygen atoms in the basal plane.17 This behavior is on the basis of DFT and observations of small nanoislands in O2 concluded to cause resistance toward oxidation (i.e., trilayer formation) due to a change in the barrier for oxygen penetration into the FeO-Pt(111) interface. This picture agrees well with our observation of slow trilayer formation on the Au(111) substrate, and we speculate that the intermediate step of line defect incorporation as such causes resistance toward oxidation. We also note that while the restructuring of FeO nanoislands is a size effect leading to strongest oxidation resistance of small islands, the reconstruction of edges by defect line incorporation in our system does not seem to be dependent on island size. The further oxidation from the intermediate structure might progress via a slow channel from oxygen edges or alternatively from imperfections in the edge structure (see STM images in Figure 4b,c). Previously, the stability of bilayer and trilayer in terms of surface energy was calculated on both Pt(111) and Au(111) by means of density functional theory (DFT).18 This study showed that the thermodynamic transition point from bilayer to trilayer is located at a lower oxygen chemical potential on Pt(111) compared to Au(111), i.e., the relative stability of trilayer versus bilayer is highest on Pt(111). This might be an additional driving force behind the fast rate of conversion observed in the present study on Pt(111). Also, that study predicted a larger charge transfer to the substrate and higher stability of bilayer on Pt(111) than Au(111) which, in turn, may explain why the bilayer structure is fully recovered on Pt(111) after the reduction treatment, but not on Au(111). Particularly, it was found in a the same previous work18 that the stacked double bilayer morphology, that coexists with (single) bilayers on Au(111), was never observed on Pt(111). We speculate, that the ability of the system to form this stacked island morphology on Au(111) may be the reason behind the nonreversible conversion on the Au(111) substrate, where

the cobalt edge atoms as the most likely sites for oxygen dissociation. In relation to the oxidation mechanism, we also note one special feature of the layered cobalt oxide system on Au(111) and Pt(111) which is the apparent pseudomorphic growth of the trilayer,18,21 opposite the often encountered tendency of bilayer thin oxide films to expand the in-plane lattice relative to their bulk lattices (e.g., FeO53 and CoO20), in the case of ZnO even adopting a one-plane graphene-like structure.54−56 Due to this feature, the conversion from bilayer to trilayer implies a lateral contraction of the film, decreasing the in-plane Co−Co interatomic distance (e.g., from 3.1 ± 0.1 Å to 2.7 ± 0.2 Å on Pt(111)18). The “island nature”, i.e., presence of edges, might facilitate this compression and provide an additional favoring of trilayer formation in the vicinity of island edges. This phenomenon, however, is in contrast to the FeO system where trilayer forms exclusively on a certain domain type in the moiré structure and does not change the in-plane lattice spacing of the oxide.12,57 One complication in the picture of oxidation mechanism is the inclusion of how the partial hydroxyl overlayer on the resulting trilayer structure on both substrates forms. We emphasize that the O−Co−O trilayer structure does not appear to exist without the 3 × 3 R 30° superstructure of H which was previously speculated to play a stabilizing role.21 Most likely, H2 or H2O dissociates on island edges during the transformation, an idea that is supported by the known tendency of bilayer on Au(111) to hydroxylate by water dissociation at edge sites already at H2O pressures below 1 × 10−7 mbar.39 Whether the availability of H2O or H2 in the vacuum rest gas is a determining parameter in the oxidation process is still not evident and cannot be addressed by our present data. 4.2. Substrate effects. Our experiments point out several noteworthy influences of the choice of substrate on the oxidation and reduction processes studied, and can be summarized as follows: • The rate of the oxidation, as well as reduction, is significantly higher on Pt(111) compared to Au(111). • The bilayer structure is only stabilized again on Pt(111) after reduction of the trilayer, i.e., the oxidation/ reduction reactions are reversible only on this substrate. • On Au(111) an intermediate state of the cobalt oxide exists in the oxidation process, characterized by closely packed adatom oxygen line defects and a change in edge structure toward exclusively the oxygen terminated edge type. Several factors could possibly contribute to the fast rate of the conversion processes on Pt(111). First, since the Pt(111) surface is capable to dissociating O2 (see section 4.1), the resulting additional source of O atoms available for oxidation might facilitate the generation of trilayer. This O source could be thought to increase the rate of the process, either contributing to the likely O2 dissociation at cobalt oxide edge atoms or eventually as the primary source of oxygen. In both cases the oxidation mechanism would be different from the Au(111) supported system, where O2 dissociation is unlikely on the bare substrate, and therefore O2 dissociation has to occur exclusively on edge sites. Likewise, the reverse O association process needed for the release of O2 during reduction of the islands may also be influenced, as the energy barriers in this process could also be different on Pt and Au. I


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areas of stacked cobalt oxide appear along island edges during UHV annealing (see Section 3.5).

5. CONCLUSIONS By exposing Co−O bilayer and O−Co−O trilayer on Au(111) and Pt(111) supports to relatively mild oxidizing and reducing conditions, the transition processes between the two morphologies could be followed, revealing interesting details about the mechanisms and substrate effects involved. Overall, our observations are governed by the role of island edges as mediator of both oxidation and reduction processes driven by O2 exposure and vacuum annealing, respectively. Prior to the actual trilayer formation from the Co−O bilayer on Au(111), we found initial transformations of the island peripheries toward oxygen terminated edges accompanied by a change in the island shape and an oxygen uptake by incorporation of line defects to an extraordinary high density. These intermediate stages were not observed on the Pt(111) substrate. Besides this difference, we found a significantly higher oxidation rate on Pt(111) and that the reduction process recovering Co−O bilayer from the O−Co−O trilayer could only be accomplished on this substrate. The direct comparison of equivalent structures exposed to the same controlled conditions on different support metals emphasizes the importance of the interplay between nanoparticles and support material in metaloxide heterogeneous catalyst systems.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcb.7b04944. STM images of early stages of trilayer formation on Au(111) and synthesized bilayer on Au(111) with a high density of oxygen adatom defect lines treated by annealing in vacuum to increasing temperatures (PDF)

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AUTHOR INFORMATION

Corresponding Author

*[email protected]. ORCID

Jeppe V. Lauritsen: 0000-0003-4953-652X Present Address †

School of Chemistry, University of Manchester, Manchester M13 9PL, UK Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The Lundbeck Foundation and Villum Fonden is acknowledged for financial support. Zhaozong Sun would like to acknowledge financial support from the Chinese Scholarship Council (CSC).

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REFERENCES

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Phase Transitions of Cobalt Oxide Bilayers on Au(111) and Pt(111

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