Shape, Morphology, and Phase Transitions during Co Oxide Growth

Article pubs.acs.org/JPCC

Shape, Morphology, and Phase Transitions during Co Oxide Growth on Au(111) M. Li* and E. I. Altman Department of Chemical and Environmental Engineering, Yale University, New Haven, Connecticut 06520, United States ABSTRACT: The surface structure and morphology of cobalt oxide thin films grown on Au(111) were studied using scanning tunneling microscopy (STM) and ultraviolet and X-ray photoelectron spectroscopies. Initial growth in O2 led to 2-D CoO clusters exhibiting a superstructure in STM images characteristic of the lattice mismatch between the rocksalt (111) surface and the substrate. The superstructure governed the CoO cluster shape; as a result, the shapes of clusters containing up to thousands of Co atoms were modulated by the most efficient packing of spheres. Continued growth led to a transition to 3-D, fully oxidized, spinel phase Co3O4 clusters. These Co3O4 clusters were embedded with characteristic “Y” shaped grain boundaries that result from high reactivity toward oxygen at specific edges of the CoO clusters, and the doubling of the periodicity when CoO is oxidized to Co3O4. Exposure of the Co3O4 films to O atoms induced oxidation toward Co2O3, which roughened the surface. Reducing these overoxidized films produced CoO in a 3-D morphology. The results demonstrate how the shape and morphology of Co oxide nanostructures are intertwined with the support, the phase, and the history of the structures.

1. INTRODUCTION Recently, interest has emerged in the catalytic properties of cobalt and cobalt-containing oxides.1−4 Cobalt oxides have shown activity for a wide range of reactions, including oxygen reduction in fuel cells and solar cells,5,6 NO reduction,7 methane combustion,8 partial oxidation of hydrocarbons,9 low temperature, selective CO oxidation,10 ethanol steam reforming,2 and the water gas shift reaction.11 Many of these reactions typically require precious metals, making Co oxides attractive even if they may not be the optimal catalyst. Studies have revealed that the catalytic activity of Co oxides is sensitive to the full range of effects seen in heterogeneous catalysis, including particle size dependence,12,13 support effects,2,8 sensitivity to preparation methods,14,15 and dopants.16 The origins of these effects are poorly understood. As the systematic development of Co oxide catalysts relies on a deep understanding of the roles of oxidation state and structure on reactivity, we have been characterizing the reactivity of Co oxide surfaces on the atomic scale. In a recent paper, we showed how the combination of support interactions and structural differences between CoO and Co3O4 leads to an unexpected phase transition from CoO to Co3O4 as the cluster size increases.17 In this paper it will be shown that lattice mismatch between CoO clusters and the support can lead to size-dependent changes in cluster shape that persist up to clusters containing thousands of atoms; since specific island edges are more reactive toward oxygen, these shape changes can modulate reactivity. Cobalt oxide can crystallize in three structures: (1) rocksaltstructured CoO with Co2+ cations residing in octahedral (Oct) sites surrounded by O2− anions in an fcc structure with a lattice constant of 0.426 nm; (2) spinel-structured Co3O4 with 32 O2− © 2014 American Chemical Society

anions packed in a larger fcc unit cell with a lattice constant of 0.808 nm; and (3) corundum-structured Co2O3 with a and c lattice constants of 0.478 and 1.296 nm, respectively.18 The latter is only considered stable in the bulk form at high oxygen pressures (>104 atm),18,19 although evidence of its existence has been reported for thin films on α-Al2O3(0001) at low pressure20 and for nanoscale particles,21,22 though the particles can be amorphous. Each Co3O4 unit cell contains 16 Co3+ cations in octahedral sites and eight Co2+ cations situated in tetrahedral (Tet) sites.23 The lattice constant of Co3O4 is well matched to twice the lattice constant of Au (0.816 nm), hence, the choice of Au(111) for this work. Along [111] CoO is constructed of pairs of hexagonal O and Co planes, meanwhile the Co3O4 spinel structure is built up from hexagonal O planes alternating with two types of Co layers: one in which Co fills only Oct interstices, and a second in which Co fills Tet and Oct sites. Therefore, the thinnest [111]-oriented Co3O4 film is roughly twice as thick as the thinnest CoO film. Previous studies have shown that rocksalt- and spinelstructured transition metal oxides can be epitaxially grown on metal surfaces and imaged with scanning tunneling microscopy (STM).24,25 In particular, the growth of CoO(111) and Co3O4(111) multilayers on Ir(100) has been reported.24 phase transitions between the two oxides were observed depending on the growth temperature and film thickness. On the other hand, owing to its inertness and its unique long-range reconstruction, the Au(111) surface has been an ideal template for investigating the nucleation, growth, and properties of wellReceived: November 19, 2013 Revised: May 15, 2014 Published: May 23, 2014 12706

dx.doi.org/10.1021/jp411375w | J. Phys. Chem. C 2014, 118, 12706−12716

The Journal of Physical Chemistry C

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dispersed arrays of metal, sulfide, and oxide nanoparticles.26−29 In particular, the epitaxial growth of FeO and Fe3O4(111) thin films on Au(111) and the phase transition between the two oxides has been studied.26,30 The deposition of oxide clusters on metal supports, including Au, has also been the basis for “inverse model catalysts” that have been fruitful in unraveling interfacial interactions between the oxide and metal phase that lead to enhanced catalytic activity.31−33 These “inverse” systems can be more active than conventional oxide-supported metals due to the availability of low-coordinated sites on the oxide that would be covered by the metal in conventional catalysts.31 The focus of this paper is the oxide as an active phase not as a support; still, moving forward it will be important to consider possible contributions of Au−Co oxide edge sites to the reactivity of the system. Our recent study of the initial growth of cobalt oxide on Au(111) revealed only 2-D [111]-oriented CoO clusters up to coverages of 0.2 MLE (monolayer equivalent where 1 MLE is enough CoO to completely cover the surface) despite oxygen pressures orders of magnitude above the dissociation pressure of Co3O4.17 Under the same growth conditions, however, a transition to 3-D Co3O4 was observed well before the Au surface was completely covered. The stabilization of 2-D CoO clusters at low coverages was attributed to the interaction with the high surface energy Au support and the doubling of the thickness of the thinnest layer in going from CoO to Co3O4. As a result, oxidizing CoO clusters increases the exposed area of the high surface energy substrate and the population of high energy edge and corner sites, Co3O4, only forms when the energy gained by oxidation is sufficient to offset these energy costs. In this paper we show that Co oxide growth lifts the Au reconstruction, thereby liberating Au atoms that nucleate islands and surround and embed Co oxide clusters at step edges. The CoO islands display a moiré pattern due to lattice mismatch with the Au substrate; efficient packing of the moiré maxima within the CoO clusters modulates the cluster shape up to clusters containing thousands of atoms. The Co3O4 clusters display characteristic “Y” defects that could be associated with grain boundary formation due to high oxidation rates at specific CoO island edges. Exposure of Co oxide films in excess of 1 ML thick to atomic oxygen led to oxidation past Co3O4 toward Co2O3 and dramatic roughening; reducing these films produced 3-D CoO rather than the 2-D films formed during growth. Together, the results reveal how even weak interactions with an inert substrate can profoundly affect cluster shape, which in turn influences reactivity, and how the history of the clusters can determine their morphology.

Core-level XPS spectra were excited with nonmonochromatic 1253.6 eV Mg Kα1,α2 radiation; spectra were taken with 0.8 eV overall system resolution. The energy scale has been calibrated using Cu 2p3/2 (932.67 eV)37 and Au 4f7/2 (84.00 eV)37 peaks on a clean copper plate and a clean gold foil. The contribution of Mg Kα3,α4 satellite lines at 1262.0 and 1263.8 eV to the spectra were subtracted. The UPS spectra were obtained using He II (40.8 eV) radiation; spectra were referenced to the Fermi level, which was determined from a UPS spectrum of a clean gold foil, and corrected for satellite lines at 48.4 and 51.0 eV in the discharge lamp output. The XPS and UPS spectra were collected with the double-pass cylindrical mirror analyzer resolution set to 0.24 eV. In AES, the relative intensities of the Au NNV at 239 eV, Co LMM at 775 eV, and O KLL at 503 eV lines were used to characterize the surface. The Au(111) films on mica (typically 200 nm in thickness from PHASIS Sàrl) were initially outgassed in UHV by annealing slightly above 370 K for several hours to prevent bubbling of moisture trapped at Au/mica interface.38 The films were then cleaned by cycles of sputtering and annealing below 870 K in UHV until contaminants, typically C and S, were below the AES detection limit. Such treatments produced large flat terraces with the herringbone reconstruction. For the STM, AES, and LEED studies, cobalt was deposited by resistively heating a W basket filled with Co nuggets (99.99% in purity from Alfa Aesar) with typical growth rates ranging from 0.15 to 0.33 nm/min. To get rid of K impurities, the baskets were heated above the deposition temperature for several hours before Co was loaded. A quartz crystal microbalance was used to monitor the Co deposition rate. The film thickness is designated as monolayer equivalents (MLE) and calibrated by comparing the coverage of Co oxide islands determined from STM images, in the low coverage regime where only 2-D CoO is seen, with the coverage anticipated from the deposition time and the quartz crystal microbalance reading. The LEED characterization of the Au(111) films on mica revealed 12 broad spots spreading along an arc, indicating two primary rotational domains. Following Co oxide deposition, LEED only showed the Au(111) (1 × 1) pattern with increased background due to either small island sizes (for CoO) or small grain sizes and disorder for more oxidized films as discussed in 3.3.17 For the photoelectron spectroscopy measurements, Co was deposited using a high temperature effusion cell with the growth rate also measured using a quartz crystal microbalance. Here the microbalance was calibrated by translating it to the growth position. The ratios of peak-to-peak heights of Co/Au, O/Au, and O/Co in AES were cross-checked with those obtained for surfaces where the coverage is known from STM to ensure that the quartz crystal accurately measures the coverage. It is assumed that the mean free path is the same for Co and O and therefore that the AES peak ratio expected for CoO and Co3O4 does not depend on film thickness accordingly. Tungsten tips for STM were cleaned by electron beam bombardment prior to use. The tunneling current was 0.5 nA in all of the STM measurements. Throughout the paper, sample biases are reported so that negative biases probe occupied states and positive biases unoccupied states. The contrast in some of the STM images was enhanced by subtracting a plane fit to one of the terraces and cycling through the gray scale multiple times so that the structures on terraces at different heights could be visualized with the same vertical resolution.

2. EXPERIMENTAL SECTION The experiments were primarily conducted using an ultrahighvacuum (UHV) system equipped with an oxygen plasma source, an electron spectrometer for Auger electron spectroscopy (AES), low energy electron diffraction (LEED) optics and a custom designed variable temperature scanning tunneling microscope.34 The base pressure was maintained in the low 10−10 Torr range. Cobalt films were also grown using an oxygen plasma-assisted molecular beam epitaxy system with a base pressure kept in the low 10−9 Torr range and then characterized in an interconnected analysis chamber using X-ray and ultraviolet photoelectron spectroscopies (XPS and UPS).35 The flux of reactive oxygen species emitted from the plasma source is typically 1 × 1014/cm2·sec.36 12707



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3. RESULTS 3.1. Initial Growth and Structure of CoO. Figure 1 shows STM images of surfaces with 0.2 MLE Co oxide film

forming a symmetric hexagon; and (3) ten hexagonally arranged spots forming a larger elongated island. Triangularly shaped islands were also resolved near Au step edges and embedded into the step edges as shown in Figure 1b. The larger islands appear to adopt the asymmetric hexagonal shape typical of fcc (111) islands that is due to different energies for the alternating {110} and {100} step edges;39 however, it will be shown below that the shape depends strongly on size throughout this size regime. Interestingly, small islands composed of less than three bright spots without any internal structure can also be seen near the elbows of the bright herringbone dislocation ridges or soliton walls40 oriented in [112̅] direction as shown in Figure 1c; their size is consistent with an island containing a single moiré maximum. The Au(111) herringbone reconstruction41 was resolved surrounding the cobalt oxide islands on the flat terraces in Figure 1. In all cases, the bright soliton walls avoid the oxide islands. On the Au(111) reconstructed surface, the soliton walls separate fcc and hcp terminated regions; the fcc regions are the wider of the two.41 Following the soliton walls around the oxide clusters reveals that they sit on bulk-terminated fcc regions; thus, the Co oxide lifts the reconstruction. Gold islands and cobalt oxide clusters seemingly embedded within the substrate can also be seen in the images. Note, for example, the CoO clusters in the middle of Figure 1b that are partially surrounded by the step edge, and the CoO cluster in the middle of Figure 1c, which sits on top of an Au island. These features are due to two effects: (1) the lifting of the Au reconstruction and (2) impinging Co atoms substituting for Au in the surface region.42−44 The reconstructed Au(111) surface contains 23 Au atoms squeezed within 22 bulk lattice sites along the [110] direction; thus, lifting the reconstruction liberates Au atoms. Meanwhile, metals, including Co, can undergo place-exchange on Au surfaces;45,46 therefore, the cluster embedded in the Au terrace at the bottom left of Figure 1c can be explained by Co metal substituting into the Au surface prior to oxidation. In both cases, the liberated Au atoms either attach to step edges or nucleate Au islands. It is interesting to follow the evolution of the CoO cluster shapes as more Co atoms were incorporated into the clusters. As illustrated in Figure 2, the minimum number of bright spots (n) observed in moiré patterned clusters was three, which yielded triangular clusters. No clusters containing two, four, six, or eight bright spots were detected; meanwhile, the shape

Figure 1. STM images of 0.2 MLE cobalt oxides grown on Au(111). The film was prepared by depositing Co in O2 (1 × 10−6 Torr) at 380 K followed by annealing at 670 K for 10 min and cooled in O2 below 370 K. (a) CoO islands with different sizes on a flat Au substrate. Reproduced with permission from ref 17. Copyright 2014 Elsevier; (b) CoO island nucleation near Au steps and (c) CoO island nucleation on top of a Au island. The image biases are 2 V for (a) and (b) and 2.25 V for (c). The image contrast was enhanced by cycling through the gray scale multiple times to show the Au(111) substrate and island structures at the same gray level.

grown on Au(111) at 380 K in O2 (1 × 10−6 Torr) and annealed at 670 K for 10 min in O2. We previously showed using UPS in combination with STM that CoO(111) islands form on Au(111) under these conditions, and that the CoO islands were modulated by a moiré pattern with bright spots arranged in a hexagonal network.17 Three different sized islands were resolved in Figure 1a: (1) three spots arranged in an equilateral triangle to compose a small island; (2) seven spots

Figure 2. Evolution of the CoO cluster shapes as a function of the number of bright spots (n) in the moiré patterned clusters. The shaded balls represent the observed white spots on moiré patterned CoO clusters in STM. The white circles highlight kinks resolved in the cluster edges for n = 33 and 40. 12708



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changed from a trapezoid for a five-spot cluster to a regular hexagon for a seven-spot cluster and back to a trapezoid for the nine-spot cluster. No 11-spot clusters were seen, but the shape changed from an elongated hexagon for the 10-spot cluster to the asymmetric hexagon expected for fcc-structured (111) islands for n = 12 and then back to a regular hexagon for n = 19. Above 30 bright spots, the expected asymmetric hexagonal shape was consistently observed. Interestingly, the kinks on the island edges occur on the length scale of the moiré pattern as highlighted by the circles in Figure 3 for n = 33 and 40. Such kinks have also been observed on FeO islands grown on Au(111) under similar conditions.26

size remained the same. Figure 3b shows a higher resolution image of a small area in Figure 3a where dark features could be resolved atop the cobalt oxide islands. A straight groove can be seen to run across the 6 nm wide hexagonal island labeled “A”. Coalesced to island A at its upper edge is a larger 8 nm wide island labeled “B” on which a “Y” shaped groove could be resolved. Similar “Y” grooves could be seen in all the remaining cobalt oxide islands in Figure 3b. As in Figure 1, the Au reconstruction soliton walls could be resolved surrounding the Co oxide islands. The oxidation states of the Co oxide clusters grown under the conditions in Figure 3 were characterized using XPS; the results are compared in Figure 4 with reference spectra for

Figure 4. XPS spectra of Co oxide surfaces: (a), (b), and (c) are reference spectra for a LaCoO3(110) single crystal, a Co3O4(111) film grown on α-Al2O3(0001), and a CoO(100) film grown on Fe3O4/ MgO(100), respectively; (d) 0.5 MLE as-grown cobalt oxide film on Au(111) at similar conditions, as in Figure 3; (e) reducing the film in (d) by UHV anneal at 770 K for 5 min, and (f) reoxidizing in oxygen plasma plume at 670 K for 15 min.

CoO (Figure 4a),47 a 12 nm Co3O4 film grown on αAl2O3(0001) (Figure 4b)20 and LaCoO3 (Figure 4c).20 All of the spectra exhibit strong peaks in the Co 2p core level region at 780 and 796 eV and a pair of weaker satellite peaks whose intensity varies depending on the material. The two intense peaks have been assigned to emission from the Co 2p1/2 and 2p3/2 levels with simultaneous transfer of an electron from an O 2p valence level to the partially filled d band of the ionized Co atom.48,49 Meanwhile, the satellite peaks have been attributed to ionization of the Co 2p core levels without any of the remaining electrons formally changing state.48,49 The probability of the electron transfer from O to Co when Co is ionized is enhanced the more covalent the Co−O bond.50 The reference spectra show very little difference between the two more intense peaks for CoO, where all the Co is 2+, and those for LaCoO3, where all the Co is 3+. Much more obvious differences between the Co oxides can be seen in the intensities of the satellite peaks. Focusing on the region between the 2p3/2 and 2p1/2 peaks, a strong peak can be seen for CoO that is completely absent in LaCoO3; the spectrum for Co3O4 shows curvature between the two main peaks, indicative of a weak satellite peak. These changes can be associated with more covalent Co−O bonds when Co is in the 3+ oxidation state. Comparing the spectrum in Figure 4d for the as-grown film with the reference spectra reveals the curvature between the two 2p peaks consistent with Co3O4. Reducing the film by

Figure 3. (a) STM image of 0.5 MLE cobalt oxide on Au(111) prepared under similar conditions in Figure 1; (b) small scale image of the same surface resolving “Y” shaped domain boundaries. The image biases are 2 V for (a) and (b). The image in (b) was processed to show the Au(111) substrate and island structures with the same contrast.

3.2. Formation and Reduction of Co3O4. Increasing the coverage to 0.5 MLE under similar growth conditions, as in Figure 1, resulted in a greater variety of shapes, including hexagonal and rhombic clusters, as well as more irregularly shaped clusters that appear to be due to island coalescence, as shown in Figure 3a. Compared to Figure 1, the average island 12709



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UHV annealing at 770 K for 5 min led to a broadening of the two major Co 2p peaks, as shown in Figure 4e, and now clearly discernible satellite peaks at higher binding energies than the main 2p features. The increase in the intensity of the satellite peaks can be attributed reduction to CoO leading to the exclusive population of octahedral sites by Co2+.51 Under similar reduction conditions, STM showed that the surface was populated by islands exhibiting the characteristic CoO(111) moiré pattern due to the lattice mismatch with Au;17 thus, the Y defects are characteristic of Co3O4. Annealing the resulting CoO film in reactive oxygen species emitted by the plasma source at 670 K for 15 min led to the considerable weakening of the satellite peaks, as shown in Figure 4f, indicating reoxidation. 3.3. Impact of High Oxygen Activities. At 0.5 MLE Co oxide and above, the structure, morphology, and phase of the Co oxide was strongly influenced by the preparation conditions. As shown in Figure 5a when Co was deposited in vacuum then exposed to O2 (1 × 10−6 Torr) at 300 K before a final UHV anneal at 690 K for 10 min, the surface was covered by hexagonal islands with sizes ranging from 5 to 15 nm; the step heights and orientations were similar to those seen for CoO in Figures 1 and 2. In addition, AES indicated the same O/Co peak-to-peak ratio as in Figure 1; therefore, the islands in Figure 5a can be assigned as CoO. Exposing the surface in Figure 5a to reactive oxygen species emanating from the oxygen plasma did not simply oxidize the CoO to Co3O4. As illustrated in Figure 5b, these reactive oxygen species dramatically roughened the surface, creating bright spots on the terraces along with locally ordered small patches and dark pits. In addition, scattered white features were observed on the surface that could not be attributed to impurities as only Au, Co, and O were detected by AES. The AES spectra also revealed a 1.9 times higher O/Co peak-to-peak ratio than the Co3O4 surface in Figure 3, suggesting extra oxygen incorporated into the surface. As shown in Figure 5c, similar results were obtained for a 1 MLE Co oxide film also grown at 300 K in O2 (6 × 10−6 Torr) and then annealed to 670 K in the plume of reactive oxygen species from the plasma source and cooled to 370 K in O2. Figure 5c reveals rough clusters terminated by bright spots similar to 5b), and a linear alignment of the clusters leaving dark trenches running diagonally from the upper left to the lower right of the image. Auger electron spectroscopy detected an O/Co peak ratio comparable to that for the image in Figure 5b. It has been reported that strong oxidants, including atomic oxygen and ozone, can effectively oxidize Au surfaces, dramatically roughening them in the process.52 However, STM measurements on a bare Au substrate exposed to the oxygen plasma under conditions similar to those in Figure 5b,c only resolved the Au herringbone reconstruction on large flat terraces. As atomic oxygen was reported to desorb from Au(111) at 550 K53 and the reactive oxygen exposure from plasma in Figure 5c was above 550 K, it is not surprising in this case that the active species from the oxygen plasma had no effect on the Au. The degree of surface roughening is also temperature-dependent, with more dramatic roughening at 200 K than 400 K,52,54 thus, we would expect less severe roughening at the higher temperatures where the surface was exposed to the reactive species from the plasma starting at 670 K. Still, oxygen-induced reconstructions on the Au may be expected but were not observed. This may be due to the time required to cool and transfer the sample to the microscope and the high

Figure 5. STM images of cobalt oxides grown on Au(111) under different preparation conditions. (a) 0.5 MLE prepared by depositing Co in UHV and exposing it to O2 (1 × 10−6 Torr) at room temperature followed by annealing at 690 K for 10 min; (b) the surface in (a) annealed in the oxygen plasma at 670 K for 20 min and cooled in the plasma below 370 K; and (c) 1 MLE with Co deposited in O2 (6 × 10−6 Torr) at room temperature, annealed in the oxygen plasma at 670 K for 40 min and cooled in O2 below 370 K. The image biases are 1.75 V for (a), 1 V for (b), and 1.5 V for (c).

reactivity of oxygen on Au toward CO,54 the main component in the background gas in the chamber. Thus, we can conclude that the surface roughening and the extra oxygen detected by AES compared to Co3O4 are due to the uptake of atomic oxygen to form Co3+ rich structures past Co3O4. The surface morphology changed drastically after reactive oxygen-treated films were annealed in UHV. As shown in Figure 6, annealing a reactive oxygen-treated film in UHV at 820 K for 5 min removed much of the small scale roughness seen in Figure 5b,c. Instead, the surface was terminated by multilayers of small triangle shaped terraces with a typical size of 10 nm. Auger spectroscopy indicated the same O/Co peakto-peak ratio, as in Figure 1, suggesting the formation of CoO(111) terraces after reduction. Such triangular multilayers have been observed for rock-salt structured FeO(111) 12710



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the treatment conditions. Starting with the as-grown film in Figure 7a, there is some evidence of weak emission at ≈789.5 eV. In comparison to the reference spectra in Figure 4, the region between the two intense peaks is not as flat as that seen for LaCoO3 while on the other hand the satellite emission is weaker and appears at a higher energy than that expected for Co3O4. The data then suggest that the film is oxidized past Co3O4, though probably not all the way to Co2O3. After annealing at 520 K in UHV (Figure 7b), the satellite peaks became more obvious and shifted to lower binding energy, consistent with the formation of Co3O4. Increasing the temperature above 670 K (Figure 7c,d) led to strong satellite peaks both between the now broader main peaks at 786 eV and below the main 2p1/2 peak at 802.5 eV, indicative of a surface dominated by Co2+. Reoxidizing the film in the plume of the oxygen plasma (Figure 7e) essentially restored the spectrum to its initial state. Auger spectra for the surface in Figure 7e revealed a 1.3 times higher O/Co peak-to-peak ratio than the Co3O4 surface pictured in Figure 3. Though not as large as the value seen for the rough surfaces in Figure 5b,c, the XPS data indicate that the reactive oxygen species emanating from the plasma source oxidize Co beyond Co3O4, which can account for the morphological changes seen in Figure 5. It should be noted that, although O2 does not adsorb on Au, atomic oxygen can oxidize Au surfaces.56 Analysis of the Au 4f core level region, however, revealed no evidence of oxidation when the Au-supported Co oxide films were exposed to the oxygen plasma. In Figure 8, the Co 2p3/2 satellite peak area between the two intense Co 2p peaks is shown for the Co oxide films under the

Figure 6. STM images of 2 MLE Co oxide film with Co deposited in UHV at room temperature, annealed in plasma plume at 670 K for 20 min, followed by UHV annealing at 820 K for 5 min. The image bias was 1.5 V.

epitaxially grown on Pt(111) surface;55 curiously, we did not observe such structures when CoO was grown directly. To determine how reactive oxygen species affect the chemical state of the CoOx/Au(111) surface, Co oxide films grown in the plume of the oxygen plasma source were characterized using photoelectron spectroscopy in a separate UHV system. In this case, the films were grown in the plasma plume at 670 K and then cooled either in the plasma plume or O2 to below 370 K. Figure 7 shows the Co 2p XPS results as a 2

Figure 8. Fraction of Co 2p3/2 satellite peak area in the total peak area including both satellite and main Co 2p3/2 peaks for Co oxide films under the different treatment conditions in Figure 7. The red and blue dashed lines mark the fractions calculated from the CoO and Co3O4 reference spectra in Figure 4. The fraction for LaCoO3 reference spectra was presumed to be zero, indicating the complete oxidation of Co to 3+.

Figure 7. Co 2p XPS spectra of 2 MLE Co oxide films grown in oxygen plasma and progressively annealed in UHV at different temperatures. (a) As-grown film with Co deposited in plasma plume at 670 K and cooled in the oxygen plasma plume below 370 K; UHV annealing for 5 min at (b) 520, (c) 670, and (d) 770 K; (e) surface reoxidized in the plasma plume at 670 K for 40 min and cooled in the plasma plume below 370 K. The dashed line is the Gaussian fit of the Co p3/2 peak.

different treatments in Figure 7. A Shirley background57 was first subtracted from each spectrum by choosing the energy end points of 776 and 792 eV that encompasses both Co p3/2 and its satellite peak. The Co 2p3/2 satellite peak area was calculated by subtracting the Co 2p3/2 peak area from the total area obtained by integrating the spectra between 775 and 792 eV. The area of the main Co 2p3/2 peak was obtained by integrating a Gaussian fit of the peak. As illustrated by the dashed line in Figure 7c, the peak fitting started below the peak on the low binding energy side to just past the peak on the high binding

MLE thick Co oxide film was annealed to progressively higher temperatures in UHV before being re-exposed to the reactive oxygen species emitted by the plasma. Similar to 0.5 MLE Co oxide film in Figure 4, the spectra are dominated by peaks at 780 and 796 eV that vary little with treatment, while the intensities and positions of the satellite peaks are sensitive to 12711



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energy side where the contribution of the satellite to the total intensity was still small. As shown in Figure 8, the calculated satellite peak areas for the as-grown and reoxidized films are below the red dashed line calculated from the Co3O4 reference spectra but far above the LaCoO3 reference line at zero, which indicates that both films were slightly overoxidized past Co3O4. The satellite peak areas increased past the Co3O4 red reference line toward the CoO blue reference line as the film was annealed from 520 to 770 K, indicating that Co2+ dominated surfaces annealed at the higher temperatures. Figure 9 shows the O 1s XPS spectra for the same film in Figure 7. It is noted that a shoulder appeared at the higher

Figure 10. UPS spectra of the same surfaces in Figure 7. (a) Au(111) substrate; (b) as-grown film in Figure 7a; progressive UHV annealing for 5 min at (c) 520, (d) 670, and (e) 770 K; (f) reoxidized in oxygen plasma plume at 670 K.

peak at 0.7 eV in Figure 10b is far more intense relative to the broad emission between 2−8 eV than is seen for Co3O4, where the peak is typically roughly half as intense as the broad feature.49,50 Thus, the spectrum is consistent with oxidation past Co3O4 toward Co2O3. When the film was annealed in UHV at 520 K for 5 min, the distinct peak diminished in intensity and shifted to 1 eV as shown in Figure 10c, consistent with reduction to Co3O4. Continued annealing at 670 K led to the disappearance of the well-defined peak entirely in Figure 10d, indicating reduction to CoO.50 At 770 K (Figure 10e), features associated with the Au substrate became more apparent, including the step in intensity at the Fermi level, the peak at 6.5 eV and the d band edge at 2.5 eV, suggesting that the CoO has clustered on the surface consistent with the STM observation in Figure 6. Finally, after re-exposure to reactive species from the plasma source, Figure 10f shows that the peak associated with Co3+ returned and the Au features became obscured again, consistent with a redispersion of the Co oxide. The intensity of the distinct peak is not as large as following growth, suggesting the reactive oxygen species do not restore the film quite to its initial state, consistent with the calculated Co 2p3/2 satellite peak area in Figure 8. Figure 10c−f also shows a feature in the 9−11 eV range that does not appear correlated with oxidation and reduction, or at least the intensity of the peak at 0.7 eV. Such a feature is typically associated with a satellite of the Co 3d emission with an origin similar to that of the satellites in the Co 2p region.50 This suggests that its intensity should increase as the intensity of the peak at 0.7 eV decreases; however, the distinctness and position of the satellite peak is also sensitive to the structural order of the film, small variations in stoichiometry, and the inelastic background.50,60 Considering the roughness of the Co oxide film following plasma treatment, it is not surprising that heating to 520 K would result in an oxide containing Co3+ and Co2+ but not in the well-defined spinel structure.

Figure 9. O 1s XPS spectra of 2 MLE Co oxide films grown in the oxygen plasma plume (a), reduced in UHV at 770 K (b), and reoxidized in the plasma plume at 670 K for 40 min and cooled in the plasma plume below 370 K (c).

binding energy side of the dominant peak at 531 eV on the asgrown film in Figure 9a, and this feature disappeared after the film was reduced by annealing in UHV at 770 K (Figure 9b). When the surface was reoxidized, the O 1s peak at 531 eV appeared slightly broader on its higher binding energy side, as shown in Figure 9c. A similar feature has been reported for spinel oxides, including Co3O4 and Fe3O4, which may not be simply attributed to surface defects or the surface hydroxyl population, but rather to the cation coordination change from tetrahedral in the monoxide to octahedral in the more oxidized spinels.49,58 The impact of reactive oxygen species on the valence band structure of the Co oxide films was characterized using UPS; results are provided in Figure 10 for the same surfaces examined in Figure 7. As shown in Figure 10a, the Au(111) substrate is characterized by peaks in the Au 5d band centered at 3 and 6.5 eV and a weak feature extending to the Fermi level due to the Au sp band.59 For the as-grown Co oxide film in Figure 10b, the spectrum shows a broad band between 2−8 eV and an intense, well-defined peak 0.7 eV below the Fermi level. The general features in the spectrum are similar to those previously reported for Co oxides where broad emission between 2−8 eV is due to a combination of O 2p derived states above 5 eV and Co 3d derived states closer to the Fermi level; the distinct peak at ≈0.7 eV has been attributed to Co3+.50 The

4. DISCUSSION The results indicate that Co oxide growth on Au(111) proceeds as follows. Initially, only monolayer-high [111]-oriented CoO islands form on the surface even when the oxygen pressure is high enough to form Co3O4 by several orders of magnitude. The CoO surface exhibits a moiré pattern due to the lattice mismatch with Au. As the Co oxide coverage increases, 3-D 12712



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Co3O4 clusters form with characteristic Y-shaped defects. Although only CoO and Co3O4 are thermodynamically stable at normal pressures, atomic oxygen supplied by a plasma source could oxidize the Co oxide films past Co3O4 toward Co2O3; the overoxidized films were rough on the atomic scale. Photoelectron spectroscopy results indicate that once the coverage enters the regime where Co3O4 is stable on the surface, the system can be cycled between CoO, Co3O4, and the overoxidized state. Moiré patterns form when two lattices with the same symmetry but different periodicities overlay each other. The periodicity of the moiré pattern for CoO on Au(111) indicates little deviation of the CoO lattice constant from its bulk value,17 suggesting that the interaction of the oxide with the Au surface is weak, or at least insufficient to strain the CoO the 4.3% required to match Au lattice constant. Still, the CoO−Au(111) interaction is sufficiently strong to influence both the Au substrate and the Co oxide islands in several ways. First, as highlighted in Figure 1, the Co oxide lifts the Au(111) reconstruction. In addition, we previously showed that the attractive CoO/Au(111) interaction in conjunction with the higher surface energy of the Au substrate and the intrinsic bilayer structure of Co3O4 plays a key role in stabilizing CoO islands against oxidation.17 It is this effect that is responsible for only seeing CoO during the initial growth even though the oxygen pressure was easily high enough to form Co3O4. Although the moiré pattern creates only a modest rumpling of the CoO layer, 0.015 nm assuming a hard sphere model, it has a profound effect on the shapes of the CoO islands. Typically, the equilibrium shape of islands on surfaces is determined by the edge energies. For fcc and rock salt (111) islands, an asymmetric hexagonal shape is expected because of the different energies of the alternating {100} and {111} edges. This assumes of course that the islands are large enough that a continuum description of the edge energies is appropriate. For clusters containing only a handful of atoms, the shape is governed by the packing that maximizes the coordination of the atoms within the cluster. As a result, clusters with certain numbers of atoms are much more stable than clusters with just one more or one less atom; for example, an fcc (111) island containing seven atoms arranged in a regular hexagon is much more stable than clusters with six or eight atoms, even though it does not adopt the expected asymmetric hexagonal shape.61,62 Interestingly, Figure 2 shows that this same pattern occurs for CoO on Au(111) except with the maxima of the moiré pattern replacing individual atoms. Each bright ball in the moiré pattern contains 110 Co atoms, and so the smallest stable CoO clusters contain more than 300 Co atoms. More interestingly, up to clusters containing at least 12 bright spots, or over 1300 Co atoms, the variation in shape expected based on the efficient packing of spheres is seen and cluster sizes that do not yield efficient packings are generally not seen. For the larger islands with over 30 bright spots, the asymmetric hexagonal shape is finally consistently seen. Still, even for these large islands the kinks on the island edges take the form of entire moiré maxima rather than individual atoms. This effect can be understood in terms of a strong driving force to terminate the island edge at specific locations within the moiré pattern. Tip convolution effects at step edges make it difficult to determine this exact location but the images suggest the edges prefer to terminate at the valleys between maxima. Nonetheless, the end result is that the moiré maxima with 110 Co atoms each replace individual atoms in governing the shape of the CoO islands.

Adding Co atoms to CoO during continued growth in oxidizing condition leads to spinel-structured Co3O4 islands on Au(111), which display “Y”-shaped depressions (STM Figure 3b) and no superstructure. The lack of a superstructure is easy to explain: oxidation to Co3O4 nearly completely eliminates the lattice mismatch with Au. The formation of the “Y”-shaped depressions can be explained by domain boundaries forming due to Co3O4 nucleation at alternating CoO step edges. In contrast to CoO, along the [111] direction, the Co3O4 repeat unit requires at least two alternating O and Co layers. The hexagonal O layers are the same as those in CoO except with the spacing between O atoms reduced to 0.286 nm. In bulk Co3O4 (111), the Co arrangement alternates between an octahedral (Oct) layer in which Co fills 1/4 of the octahedral interstices (Figure 11a) and one in which Co fills upper and lower tetrahedral (Tet) sites in addition to octahedral sites creating the Tet-Oct-Tet layer in Figure 11b. Therefore, the schematic of the complete Co3O4 cobalt stack can be illustrated as the Co Oct layer on top of the Co Tet-Oct-Tet layer as shown in Figure 11c. The result is a Co sublattice with a (2 × 2) periodicity with respect to the O sublattice, as opposed to CoO where the Co and O sublattices are equivalent. Therefore, if oxidation of CoO to Co3O4 initiates at island step edges, domain boundaries may form as the growth fronts merge. Preferential oxidation at either {100} or {111} steps will lead to “Y”-shaped domain boundaries as the growth fronts approach the island center as illustrated in Figure 11c. The appearance of three domain boundaries in a “Y” shape and two distinct step orientations in [111]-oriented CoO islands, therefore, implicates one of the step edges as the specific site for initiating CoO oxidation to Co3O4. In contrast, oxidation initiated on the top (111) terraces would yield either single or random domain structures, while if the steps were uniformly reactive, islands with six domains would be expected. As illustrated above, the lattice mismatch with the Au(111) causes the CoO shape and thus relative abundance of {100} and {111} steps to vary with size; since only one of the step orientations is active for oxidation, the Au substrate will cause the reactivity toward oxygen to depend on size. Although bulk Co2O3 in the corundum structure is only considered thermodynamically stable at oxygen pressures in the thousands of atmospheres,18,19 our results indicate that atomic oxygen at low pressures can oxidize Co3O4 toward Co2O3. Recently, we also identified a Co2O3 corundum phase as an interfacial layer during the epitaxial growth of Co3O4 on αAl2O3(0001) in an oxygen plasma.20 In this case, the structural match to the substrate was considered an important factor that allowed Co2O3 to form. On Au(111), the situation is somewhat different as the highly oxidized Co oxide is poorly ordered. Still, the results indicate that under oxidizing conditions thin Co layers can contain far more Co3+ than Co3O4, and thus oxidized phases that are not stable in the bulk must be considered in understanding catalytic reactions on nanoparticle surfaces carried out under such conditions.

5. SUMMARY In conclusion, small CoO clusters are stabilized on Au due to the combination of the high surface energy of the support, the intrinsic bilayer structure of Co3O4, and the attractive interaction between Co oxide and Au which lifts the Au herringbone reconstruction. The lattice mismatch between CoO and Au(111) creates a moiré pattern that profoundly affects the evolution of the CoO cluster shapes. The moiré 12713



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change in the growth conditions. The resulting Co3O4 clusters exhibited “Y”-shaped domain boundaries that could be associated with the oxidation of CoO proceeding preferentially at alternating island edges. Since the Au substrate induces the moiré pattern which governs the island shape as a function of size which in turn determines the population of the reactive edges, the substrate plays an additional role in modulating the reactivity of CoO. Finally, it was shown that although only CoO and Co3O4 are thermodynamically favored in the bulk at normal oxygen pressures, films with stoichiometries approaching Co2O3 could be produced.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: +1 203-432-4332. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

The authors thank Georg Simon, Xiaodong Zhu, Matthew Herdiech, and Victor Henrich. Discussions with Dr. Jan Götzen in the Mechanical Engineering and Materials Science Department of Yale University are also acknowledged. This project is supported by the Department of Energy through Basic Energy Sciences Grant Number DE-FG02-98ER14882. The authors also acknowledge use of facilities supported by the National Science Foundation through the Yale Materials Research Science and Engineering Center (Grant No. MRSEC DMR1119826).

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Figure 11. Schematics illustrating the formation of the “Y” domain boundary (db) on Co3O4. (a) The top Co octahedral layer of Co3O4; (b) the Co3O4 Tet-Oct-Tet Co layer with cyan, blue and red dots representing the middle octahedral (Oct) Co layer, upper tetrahedral Co layer (Tet 1) and lower tetrahedral Co layer (Tet 2) respectively; and (c) domain boundary formation as Co3O4 grows from the island step edges. In (a) the parallelogram represents the unit cell, which is (2 × 2) with respect to the O sublattice and CoO. The parallelograms in (c) illustrate the unit cells and the lines of the dbs that form as the (2 × 2) unit cells are constructed starting from alternating edges of a (1 × 1) CoO island.

maxima replace the individual CoO units as the building blocks for the islands causing the shapes and stabilities of the islands to be governed by the most efficient packing of spheres up to islands containing thousands of atoms. A transition from CoO to Co3O4 occurred as the Co coverage was increased despite no 12714



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