Stable Subnanometer Cobalt Oxide Clusters on ... - ACS Publications

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Stable Subnanometer Cobalt Oxide Clusters on Ultrananocrystalline Diamond and Alumina Supports: Oxidation State and the Origin of Sintering Resistance Glen A. Ferguson,†,⊥ Chunrong Yin,†,⊥ Gihan Kwon,† Eric C. Tyo,∥ Sungsik Lee,‡ Jeffrey P. Greeley,§ Peter Zapol,† Byeongdu Lee,‡ Sönke Seifert,‡ Randall E. Winans,‡ Stefan Vajda,*,†,§,∥ and Larry A. Curtiss*,†,§ †

Materials Science Division, ‡X-ray Science Division, and §Center for Nanoscale Materials, Argonne National Laboratory, Argonne, Illinois, United States ∥ Depatment of Chemical and Environmental Engineering, School of Engineering and Applied Science, Yale University, New Haven, Connecticut, United States ABSTRACT: The composition and stability of oxidized cobalt subnanometer clusters composed of four metal atoms supported on ultrananocrystalline diamond (UNCD) and alumina surfaces were studied using a combination of grazing-incidence X-ray absorption near-edge spectroscopy (GIXANES), grazing incidence small-angle X-ray scattering (GISAXS), and density functional calculations. GIXANES data revealed partially oxidized subnanometer cobalt clusters upon exposure to air, with similarity in the total degree of oxidation on both supports. The clusters were exposed to elevated temperatures of up to 300 °C under pure helium as well as oxygen and were found by GISAXS to be agglomeration resistant, whereas GIXANES showed the preservation of the composition of clusters during the heat treatment. Density functional calculations of cluster binding to model surfaces for UNCD and alumina were performed. The calculations indicate that the stability of the cobalt oxide clusters on UNCD is the result of electrostatic and dispersive interactions for the pristine hydrogen-terminated surfaces and covalent bonding between the cluster and defect sites on the surfaces. On alumina the origin of the stability is interactions between the cobalt and surface oxygens or the cluster oxygens with the surface aluminum atoms. These properties indicate that oxidized subnanometer cobalt clusters supported on UNCD and alumina are suitable candidate hybrid nanostructures for use as supported catalysts.

I. INTRODUCTION The production of next generation fuels and the improvement of energy storage capabilities are necessary for meeting future energy needs.1 One route to meeting these needs is the development of highly active, selective, low cost, and environmentally safe catalysts. This and other challenges, such as the identification of alternatives to precious metalbased catalysts, are strong driving forces for continuing catalyst research and development. The rational design of catalysts using subnanometer metal or metal oxide clusters is a promising area for future catalyst development that has yet to be fully exploited.2−27 Very small clusters can have properties surprisingly different from bulk materials or larger nanoparticles.6,28−63 In addition, the properties of the clusters may be altered by their interactions with supports. Several recent advancements in the use of surface-supported subnanometer clusters for catalysis demonstrate that such clusters can be highly active and selective for important chemical reactions.5,6,8,64−66 A major challenge for understanding and developing supported metal and metal oxide clusters for catalysis is achieving stability of the clusters at catalytically relevant conditions of temperature and pressure.2,8 The activity and selectivity of subnanometer clusters can also vary significantly © 2012 American Chemical Society

and nonmonotonically as a function of particle size, with only certain clusters having high activity or the desired selectivity.6,8 Under catalytically relevant conditions monodisperse clusters may sinter resulting in a broad distribution of sizes. This aggregation may lead to the deterioration of catalytic activity or a loss of selectivity. Such outcomes would eliminate the unique properties of the atomically precise cluster size and composition and with it the advantages of catalytic activity and selectivity. To most effectively utilize these systems, the clusters must be stationary on the surface, making particle stability a critical quantity for the rational design of clusterbased catalysts. Cobalt oxide is a promising possible candidate catalyst for a number of chemical transformation reactions such as oxidative dehydrogenation.67,68 The oxidation state of cobalt has also been of much interest for example in the Fischer−Tropsch reaction.69−72 In this study we have investigated both the degree of oxidation of as-prepared cobalt clusters and their sintering resistance. We have used grazing incidence small-angle X-ray scattering (GISAXS), grazing incidence X-ray-absorption Received: May 1, 2012 Revised: August 29, 2012 Published: October 19, 2012 24027

dx.doi.org/10.1021/jp3041956 | J. Phys. Chem. C 2012, 116, 24027−24034

The Journal of Physical Chemistry C

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monitor possible sintering of clusters during the heat treatment under pure helium as well as in the presence of oxygen; GIXANES detected in fluorescence mode was employed to determine the oxidation state of cobalt. The GISAXS and GIXANES measurements were performed at the at the 12-ID-C beamline of the Advanced Photon Source in a reaction cell of own design.6,9,65,66,78 GISAXS with a geometry optimized for particle sizes starting above 1 nm was used to monitor possible sintering of clusters during the heat treatment; The GISAXS data were collected on a 1024 × 1024 pixel Platinum detector with X-rays of 7.68 keV as function of sample temperature. The two-dimensional GISAXS images were then processed by taking the average of three close cuts in the qy direction for horizontal information and in the qz direction for vertical information. The horizontal cuts were taken 35 pixels away from the center of the direct beam and the vertical cuts were taken 70 pixels away from vertical center. GIXANES data were collected using a four-element Vortex detector mounted perpendicular to the plane of incidence and photon energies between 7.397 and 8.197 keV. GIXANES data of the cobalt standards were collected under helium at room temperature. The GIXANES data were analyzed using the IFEFFIT interactive software package (with ATHENA and ARTEMIS graphical interfaces).79 We note that there has been a limited amount of GIXANES data reported in the scientific literature primarily due to the experimental difficulty of measurement.66,78,80,81 b. Theoretical. All calculations used Kohn−Sham density functional theory with a plane-wave basis set as implemented in the Vienna ab initio simulation package (VASP).82−85 The core electrons were described using the PAW potentials.86−88 The generalized gradient corrected PW91 functional89,90 and an energy cutoff of 400 eV were used for all calculations. Calculations were spin-polarized (a.k.a. spin unrestricted) due to the possible high magnetic moment (a.k.a. number of unpaired electrons) and the possible antiferromagnetic nature of the cobalt oxide clusters. Several supercells were used in this study. A 20 × 20 × 20 Å3 supercell was used for the gas phase species while a 20 × 15.1 × 10.1 Å3 supercell was used for the slab models of the diamond C(100)-2 × 1/H surface. An oblique supercell with the vectors (7.6, 3.8, 0.0), (0.0, 6.6, 0.0), and (0.0, 0.0, 30.0) Å was used for the C(111)/H surface slab. The results are converged for the differences in binding energies. Calculations of gas-phase clusters used a very small Gaussian smearing (σ = 0.01). The magnetic moment was scanned over a range of 0 to 14 (even magnetic moments only to maintain spin symmetry) to determine the ground state for the gas-phase cluster. A larger Gaussian smearing (σ = 0.2) was used for the surfaces. Brillouin zone sampling for gas phase clusters included only the Γ point while for surfaces a 3 × 3 × 1 Monkhorst-Pack grid was used. Gas-phase structures were relaxed until the forces were converged to at least 0.08 eV/Å, whereas the surface models were converged to 0.01 eV/Å. The binding energy is defined as the difference in energy at 0 K between the infinitely separated reactants and the surface with bound species. To determine the number of oxygens on the cluster and how the number of oxygens affect the cluster binding energy the Co4Ox (x = 0, 2, 4, and 6) cluster binding energies were calculated on the θ-alumina surface. The UNCD surface is complex containing grain boundaries and defects along with hydrogen terminated diamond surfaces. While the exact nature of the defects is unknown the hydrogen vacancy defects can be

near edge spectroscopy (GIXANES), and density functional theory (DFT) calculations to examine the stability and degree of oxidation of subnanometer cobalt oxide clusters on ultrananocrystalline diamond (UNCD) and alumina. UNCD is chosen because it can be deposited as a thin film on many materials with a tunable surface roughness at the nanoscale. UNCD also has excellent mechanical, chemical and electrical properties that can be tailored for the specific application.73,74 Alumina films75 are excellent at stabilizing subnanometer5,8,9,64,76 as well as nanometer sized clusters.6,75

II. METHODS a. Experimental Section. Cluster Support Material. The UNCD-coated silicon wafers were purchased from Advanced Diamond Technologies. The wafers consisted of a 300 nm thick UNCD film deposited on a doped silicon wafer (UNCD, 25 Aqua DoSi).77 The use of doped silicon wafers for UNCD as well as alumina films as a base support facilitated conductivity for the measurements of the flux of the charged clusters landing on the support during cluster deposition and the determination of the amount of deposited metal. The approximately 3 ML thick alumina layer was prepared by atomic layer deposition on the top of naturally oxidized doped silicon wafer, yielding an amorphous surface with RMS roughness of about 0.69 nm.75 Cluster Deposition. A surface coverage equivalent to 0.1 atomic monolayer (ML) equivalent of Co was used for the samples. The size-selected cluster deposition method6,8,9,76 was used to produce and soft land clusters on UNCD and aluminum oxide supports. Briefly, a molecular beam of cobalt cluster cations was prepared by laser vaporization of a cobalt target rod using helium as carrier gas. Next, the cluster beam was guided through an assembly consisting of ion optics, quadrupole mass filter, and a quadrupole deflector, and the mass-selected positively charged clusters soft-landed on a biased support. During deposition, the flux of charged clusters was monitored using a picoampermeter, allowing online monitoring of the number of metallic cobalt clusters reaching the support. After deposition, the samples were exposed to air, which led to the subsequent oxidation of the cobalt clusters, as discussed below. Characterization of Clusters. The clusters were characterized using in situ GISAXS and cobalt K-edge GIXANES methods described elsewhere.6,9,65,66,78 The schematic of the experimental setup is shown in Figure 1. GISAXS was used to

Figure 1. Schematic of system setup for combined in situ GISAXS, GIXAS, and TPRx experiments. 24028



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reasonably assumed to be present on the surface.73,74,91 We have modeled surface adsorption by investigating several likely binding sites for the cobalt oxide cluster. To this end we have used the C(100)-2 × 1/H and C(111)/H surfaces with various hydrogen defects. While other surface orientations and reconstructions are possible, these surfaces give a good indication of the range of binding energies of the clusters to UNCD. The C(100)-2 × 1/H surface was modeled with a slab model. The model was constructed by optimizing the diamond crystal, cutting the crystal along the (100) plane followed by relaxation, which forms the surface dimers, and passivation with hydrogen. The C(100)-2 × 1/H surface supercell has a stoichiometry of C168H72 containing seven carbon layers. The dimer and the dangling bonds on the other side of the slab were hydrogen terminated. The top three layers of the slab were relaxed in all subsequent calculations. A varying number of hydrogen atoms are removed to create the defects studied. The C(111)/H slab was treated similarly. In this case, a cut along the (111) plane is not reconstructed. The C(111)/H unit cell has a stoichiometry of C90H18 containing ten carbon layers. The oxidized UNCD surface was modeled by taking the twodimer defect model of the (100) UNCD model, adding either two or three oxygens and optimizing the structure. The alumina surface was modeled as described elsewhere.6 In brief, the stoichiometry of the surface model is Al48O72, including six surface layers in the slab model. The nature of the surface is such that termination of dangling bonds is not required. A systematic search was carried out to find the most likely binding site on the θ-alumina surface for the Co4, Co4O2, Co4O4, and Co4O6 clusters.

Figure 3. GIXANES spectra of ALD-alumina supported cobalt tetramer clusters recorded as function of temperature under helium (left) and subsequently in helium seeded with 5% oxygen (right), using the temperature ramp shown in Figure 2. An offset has been applied to the spectra, for clarity.

III. RESULTS AND DISCUSSION The GIXANES data of bulk cobalt standards obtained under helium at room temperature are shown in Figure 2a, and the Figure 4. GIXANES spectra of UNCD supported cobalt tetramer clusters recorded as function of temperature under helium (left) and subsequently in helium seeded by 5% oxygen (right), using the temperature ramp shown in Figure 2. An offset has been applied on the spectra for clarity.

implying an oxidized cobalt oxide cluster with a composition of Co4O4. The broadened features in the spectra of the subnanometer clusters can be caused by quantum size effects as well as by the presence of various structural isomers and possible inhomogeneities in the nature of the binding sites of the support. The slight changes in the spectral features during sample heating may be a reflection of structural change and temperature-dependent cluster-support interactions that can include charge transfer. We note that similar subtle changes in the oxidation state under reaction conditions were recently reported for somewhat larger 27-atom cobalt clusters on the same support.66,81 The in situ GIXANES data demonstrate excellent stability of the composition of the clusters during the approximately 6 h long heat treatment (∼3 h under He plus ∼3 h under oxygen), with temperatures reaching 300 °C. It is important to note that due to the lack of subnanometer CoxOy standards it is not possible to determine if the modest differences observed in the spectral features hint at slight differences in the support-dependent degree of clusters oxidation (i.e., oxygen content), differences in structure, charge state or a combination thereof. Typical vertical and horizontal cuts of the GISAXS images recorded in the temperature range of 25 to 300 °C under

Figure 2. Left: GIXANES spectra of cobalt oxide standards collected under helium at room temperature. Right: The temperature ramp applied for the size-selected clusters samples. An offset has been applied on the spectra, for clarity.

temperature profile used for clusters samples under helium and under 5% oxygen in helium is plotted in Figure 2b. The temperature and environment dependent GIXANES data obtained on the alumina and the UNCD supported clusters are shown in Figures 3 and 4, respectively. The cluster samples were first heated under helium and then subsequently in helium seeded with 5% oxygen. The spectra reveal similarity in the oxidation state of the alumina and UNCD supported cobalt tetramers, closely resembling the composition of bulk CoO, as shown in Figure 2a, (i.e., a cobalt to oxygen ratio of 1:1), thus 24029



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Figure 5. Horizontal (a) and vertical (b) cuts of the GISAXS images of alumina-supported Co4±1 clusters as function of temperature under helium. Horizontal (c) and vertical (d) cuts of UNCD-supported Co4±1 recorded between temperatures 25 and 300 °C exposed to pure helium. An offset is applied on the curves for clarity. The unchanged patterns confirm stable, sintering-resistant clusters.

Table 1. Binding Energies, Antiferromagnetic State and Magnetic Moment of the Four-Atom Cobalt Clusters on Diamond Surfaces surface

defect type

C(100)-2 × 1/hydrogen passivated hydrogen vacancy

number none one two

four C(111)-1 × 1/hydrogen passivated hydrogen vacancy

C(100)-2 × 1/hydrogen passivated with oxygen defects

hydrogen replaced by oxygen

none one two three two three

location single carbon (point) one-dimer onerow across dimer row two-dimer onerow

adjacent sites adjacent sites single dimer along two dimers

Co4O4 cluster, binding energy, eV

antiferromagnetic ground state found

cluster magnetic moment, μB

1.47 2.82

yes yes

0.4 1.4

2.36

no

7.8

3.37

yes

2.0

4.21

no

5.0

0.46 0.97 1.82 4.78 1.20 1.97

no no yes yes yes yes

7.7 8.4 5.6 2.7 5.6 7.1

Figure 6. Geometries of the Co4O4 clusters interacting with different defect sites on a C(100)- 2 × 1/H surface along with the pristine C(100)- 2 × 1/H surface: (a) The complete hydrogen termination, (b) point defect, (c) across dimer row defect, (d) two-dimer one-row defect, (e) one-dimer one-row defect, (f) two oxygen inclusion defect, and (g) three oxygen inclusion defect. The Co4O4 cluster on the complete hydrogen terminated UNCD surface is representative of the gas-phase cluster.

clusters, without signs of sintering under the applied heat treatment conditions. GISAXS data (not shown) collected during the heat treatment of the clusters under oxygen yielded

helium are illustrated for Co4±1 clusters on alumina and UNCD in Figure 5. The changes in the GISAXS patterns of all samples were of very minor nature; indicative of highly stable supported 24030



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results imply the formation of strong binding to the surface for most of the defects. The strong surface-cluster bond energies explain why the clusters are resistant toward sintering. The C(111) surface differs significantly from the C(100)-2 × 1 surface, Figure 7. The geometries of the clusters adsorbed to

the same results i.e., no indication of sintering. Thus in both environments the particles are highly stable. To understand these experimental observations demonstrating the stability of the oxide clusters against sintering, we calculated the binding energies of Co4O4 clusters on UNCD with hydrogen and oxygen surface defects and on alumina supports. Table 1 lists the binding energies of the Co4O4 cluster on each type of surface defect and whether the ground state is antiferromagnetic along with the calculated magnetic moments. The gas-phase Co4O4 cluster is shown in Figures 6. The structure of this cluster is almost completely flat with an antiferromagnetic ground state (two electrons spin up and two electrons spin down). The structures of Co4O4 clusters on the pristine hydrogenpassivated diamond (100) surfaces and the diamond surfaces with hydrogen or oxygen defects are shown in Figure 6. In the case of the diamond surfaces the surface hydrogen defects can have a large impact on the cluster geometry, with the overall shape of the supported cluster significantly changed compared to the gas-phase structure in many cases. The cluster adsorbed on the defect-free diamond surface, C(100)-2 × 1, has bonds that are only slightly changed as shown in Figure 6a. Absorption at the point defect in Figure 6b has several significant changes. For the cobalt−cobalt bonds that include the cobalt bound to the carbon surface the bonds are lengthened by 0.4 Å (from 3.8 Å to 4.2 Å). The cobalt−cobalt bonds that do not include the cobalt bound to the surface are shortened by 0.2 Å (from 2.6 Å to 2.4 Å). The remaining clusters are differentiated from the gas-phase cluster by having a geometry that is in-between the tetrahedron and the rhombus. The change in dihedral angle of the four cobalt atoms for the across dimer defect is from 0° to 43° shown in Figure 6c. The most dramatic change in dihedral angle is for the two-dimer one-row defects in Figure 6d with a change of 0° to 52° and least dramatic for the one-row one-dimer defect in Figure 6e with a change from 0° to 25°. The clusters binding to the pristine surface, Figure 6a, and the point defects, Figure 6b, are flat. The bonds for the two-dimer one-row and the one-row one-dimer defects are only slightly changed while for the across dimer-row defect the cobalt−cobalt bonds for the cobalt bound to the carbon dimer are increased from 2.3 to 2.5 Å. The cobalt−oxygen bonds are ∼1.7 Å for all clusters and the cobalt−carbon bonds are ∼2.0 Å. Binding to the oxygen defects results in the clusters becoming more tetrahedral with respect to the geometry of the Co4 atoms. The dihedral is 30° for the two oxygen defect in Figure 6f compared to 60° for the three oxygen defect in Figure 6g. Otherwise, the bonds remain close to these found in the gas-phase cluster. Bonds between the cobalt and the surface oxygen are 2.2 Å for the two-oxygen defect. The cluster bound to the three-oxygen defect has a distance of 1.8 Å for the two oxygens to which the cluster is bound and a distance length of 2.7 Å between cluster and the remaining oxygen. The difference in binding energy between the two-oxygen defect and the three-oxygen defect is primarily due to the reduced strain in the cluster upon binding. The binding energy for the different hydrogen defect types on the C(100)-2 × 1/H surface ranges from 2.36 to 4.21 eV. On the C(111)/H surface, the range is 0.97 to 4.78 eV.74 The results for the UNCD surface with oxygen defects also indicate a range of binding energies. The cluster is bound to the structure with two oxygen defects by 1.20 eV and to the structure with three oxygen defects by 1.97 eV (Table 1). These

Figure 7. Co4O4 cluster interactions on the pristine C(111)/H surface (a) and single hydrogen defect (b), double hydrogen defect (c), and triple hydrogen defect (d) sites.

the pristine hydrogen passivated C(111) surface and the surface with a point defect have only slight changes from the results for the C(100)-2 × 1 surface. The C(100)/H surface with two and three defects have several important geometric differences. For the two-defect surface the single cobalt bound to the surface binds to two carbon atoms. This arrangement makes the geometry around this particular cobalt closer to a tetrahedron. The cobalt−cobalt bonds at this site are lengthened by 0.2 Å (from 2.3 to 2.5 Å). The cobalt−carbon distances are 2.0 and 2.3 Å showing one stronger bond. The three-defect surface includes a carbon−oxygen bond between the cluster oxygen and a surface carbon. This bonding lengthens cobalt−oxygen bonds for the oxygen bound to the surface from 1.8 to 1.9 Å. The carbon−oxygen bond length is 1.5 Å, slightly longer than typical. On the C(111)/H surface the range of binding energies is from 0.97 to 4.78 eV, Table 1. The large interaction energies indicate the clusters are stable on the surface and form strong metal−carbon bonds. The pristine surfaces have binding energies of 1.47 and 0.47 eV for the C(100)-2 × 1/H and C(111)/H surfaces, respectively. This lower binding energy indicates that it might be possible to have some sintering of the clusters on the pristine surfaces. To explore the possibility that the clusters are covalently bound to the surface we need to consider the bond lengths of the carbon−carbon dimer bond of the surface. On the C(100)-2 × 1/H surface the unpassivated carbon dimers form structures with significant double bond character.92 The bond length of these carbon−carbon dimers is 1.4 Å from our calculations. This value is close to a carbon− carbon double bond (sp2 hybridized) distance in molecular systems of 1.3 Å. When the bonds are hydrogen-terminated, the carbon dimer bond distance is 1.6 Å, which could be interpreted as a strained carbon−carbon single bond (sp3 24031



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Figure 8. Co4 (a), Co4O2 (b), Co4O4 (c), and Co4O6 (d) clusters bound to alumina. Bonds from the cobalt to the surface oxygens and the cluster oxygens to the surface aluminum atoms are shown. Structures shown are the site with the highest binding energy.

The alumina results in Table 2 show the binding energy for the cluster with successive oxygens added to it. The binding

hybridized) in molecular systems, 1.5 Å. In cases when the clusters are bound to a carbon dimer and both carbons are not passivated, the carbon−carbon dimer bond length is between 1.6 and 1.7 Å. This indicates the possibility that the bond is a carbon−carbon single bond between the cobalt atoms and the dimer carbon atoms. The structures of the Co4, Co4O2, Co4O4, and Co4O6 clusters on the θ-alumina surface are shown in Figure 8. This series of calculations was used to determine the most probable oxidation state of the cluster. The first and second O2 dissociations on the Co4 on θ-alumina are calculated to occur without barrier. GIXANES indicates the cluster oxidation is similar for both supports so the results on the alumina support will also be valid for the UNCD support. The large number and variety of defect sites on the UNCD surface makes this type of calculation more challenging to perform and less clear to interpret. For this reason calculation of oxidation states was performed only for alumina. The addition of molecular oxygen to the cluster was calculated until the binding energy of the oxidized cluster was lower than the separated reactants, i.e., Co4O6. In this case the Co4O4 + O2 is thermoneutral compared to Co4O6 implying that even at moderate temperatures the additional oxygen is not likely to be bound. If one adds the translational entropy lose of the O2 molecule upon adsorption to the energy (∼0.47 eV) the likelihood of any significant amount of Co4O4 oxidizing to Co4O6 is unlikely. As can be seen in Figure 8, the O2 molecule is only loosely bound to the cluster. This theoretical evidence also implies that the oxidation state of the cluster is CoO. The degree of oxidation is likely to be limited by interactions with the surface. In this study the originally deposited clusters were metallic. Upon exposure to oxygen the clusters will oxidize to Co4O4. This structure contains cobalt atoms that are undercoordinated likely resulting in a structure that is more reactive then those synthesized as cobalt oxide in the gas phase. In this case the surface acts as a reagent by binding to the cluster and preventing complete oxidation of the cobalt. The cluster oxidation state is therefore a direct result of the fabrication method of depositing metallic particles on the surface and then allowing them to oxidize. The metallic clusters bind to the surface reducing tendency to oxidize. We can speculate that it may be possible to achieve other undercoordinated metal oxides that are stable at room temperature, using this method. The geometry of the most probable oxidized cluster, Co4O4, is distorted from the gas-phase by folding slightly. The most significant change is the cobalt−cobalt bonds for the cobalt atoms nearest to the surface, which are increased by ∼0.1 Å compared to the gas-phase cluster. The oxygen atoms close to the surface have bond lengths of 1.9 Å for the cobalt−oxygen and aluminum−oxygen bonds.

Table 2. Binding Energies, Antiferromagnetic State, and Magnetic Moment of the Four-Atom Cobalt Clusters on θAlumina surface θalumina

cluster

BE, eV

antiferromagnetic ground state found

cluster magnetic moment, μB

Co4

3.18

no

8.0

Co4O2 Co4O4

3.59 4.59

yes yes

3.9 7.5

energy of the Co4 cluster to the surface of 4.59 eV indicates that there is a slightly stronger binding per cobalt-surface bond than the oxidized clusters due to the greater possible number of binding sites. From this result, it is possible to understand the effect of oxidation on the bonding of the cobalt cluster. The difference between the Co4, Co4O2, and Co4O4 is large with the bonding increasing with the number of oxygens. The oxygens do not decrease the binding energy but increase it by two different mechanisms. For the first oxidation, Co4 + O2 → Co4O2, the increased bond strength comes from the decrease in strain caused by the longer cobalt−cobalt bond distances. The Co4O4 has increased opportunities for bonding through cluster oxygen to the surface in comparison to Co4O2. The implication of these results is that other similarly oxidized clusters larger cobalt clusters are also likely to be stable on the surface. The alumina results show a high binding energy of 4.59 eV similar to the UNCD surfaces. The surface oxygen−cobalt bond lengths are longer than the cobalt−oxygen bonds within the cluster. The main difference between the oxygen is that the surface oxygen are likely to be more ionic than those in the cluster. Using the difference in binding between the Co4O2 and the Co4O4 clusters we can estimate the interaction between the surface aluminum and cluster oxygen to be ∼0.5 eV. While individually the cluster surface interactions are relatively weak their sum indicates a strongly bound cluster that is unlikely to sinter at low-temperatures.

IV. CONCLUSIONS This investigation of cobalt oxide clusters Co4Ox on UNCD and alumina supports using in situ X-ray techniques, combined with DFT calculations, has revealed the nature of the Co4Ox cluster and its sintering resistance. As indicated by GIXANES the oxidation state of the Co4 cluster on UNCD and alumina is implicated to be CoO when compared to bulk standards. DFT calculations of oxidation after deposition on alumina support this oxidation state assignment and indicate that some cobalt atoms of the cluster will be undercoordinated. In addition the 24032



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clusters do not sinter or desorb up to 300 °C as evidenced by GISAXS during the several hour-long heat treatments under helium and oxygen. These observations are supported by DFT calculations that indicate strong interactions between the clusters atoms and the surface resulting in stable clusters at defect sites on UNCD and on the alumina surface. The results indicate that the method of sample fabrication plays a significant role in producing undercoordinated stable metal oxide clusters. Specifically, the reactive character of the metallic clusters landing particles enables the strong binding to the support, and reduces the tendency of the cluster to completely oxidize. Such strong binding to the support would not be expected for clusters that land after oxidation. Due to the undercoordination and sintering resistance it is likely possible to use these clusters as subnanometer catalysts.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; [email protected]. Author Contributions ⊥

Equally contributing first authors.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Drs. J. W. Elam and J. A. Libera for providing the alumina-coated silicon chips. The authors would like to thank Dieter Gruen and Michael Sternberg for insightful discussions concerning UNCD. The U.S. Department of Energy, BES-Materials Sciences, and BES-Scientific User Facilities under Contract DE-AC-02-06CH11357 supported the work performed at Argonne National Laboratory with the UChicago Argonne LLC, the operator of Argonne National Laboratory. E.T. gratefully acknowledges the support by the U.S. Air Force Office of Scientific Research under AFOSR MURI grant FA9550-08-0309.

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