Surface structure of polar Co(3)O(4)(111) films grown epitaxially on Ir(100)-(1 × 1)

Comparative study of Co3O4(111), CoFe2O4(111), and Fe3O4(111) thin film electrocatalysts for the oxygen evolution reaction

Water electrolysis to produce 'green H2' with renewable energy is a promising option for the upcoming green economy. However, the slow and complex oxygen evolution reaction at the anode limits the efficiency. Co3O4 with added iron is a capable catalyst for this reaction, but the role of iron is presently unclear. To investigate this topic, we compare epitaxial Co3O4(111), CoFe2O4(111), and Fe3O4(111) thin film model electrocatalysts, combining quasi in-situ preparation and characterization in ultra-high vacuum with electrochemistry experiments. The well-defined composition and structure of the thin epitaxial films permits the obtention of quantitatively comparable results. CoFe2O4(111) is found to be up to about four times more active than Co3O4(111) and about nine times more than Fe3O4(111), with the activity depending acutely on the Co/Fe concentration ratio. Under reaction conditions, all three oxides are covered by oxyhydroxide. For CoFe2O4(111), the oxyhydroxide's Fe/Co concentration ratio is stabilized by partial iron dissolution.

Active sites and deactivation of room temperature CO oxidation on Co3O4catalysts: combined experimental and computational investigations

Co₃O₄is a well-known low temperature CO oxidation catalyst, but it often suffers from deactivation. We have thus examined room temperature (RT) CO oxidation on Co₃O₄catalysts by operando DSC, TGA and MS measurements, as well as by pulsed chemisorption to differentiate the contributions of CO adsorption and reaction to CO₂. Catalysts pretreated in oxygen at 400 °C are most active, with the initial interaction of CO and Co₃O₄being strongly exothermic and with maximum amounts of CO adsorption and reaction. The initially high RT activity then levels-off, suggesting that the oxidative pretreatment creates an oxygen-rich reactive Co₃O₄surface that upon reaction onset loses its most active oxygen. This specific active oxygen is not reestablished by gas phase O₂during the RT reaction. When the reaction temperature is increased to 150 °C, full conversion can be maintained for 100 h, and even after cooling back to RT. Apparently, deactivating species are avoided this way, whereas exposing the active surface even briefly to pure CO leads to immediate deactivation. Computational modeling using DFT helped to identify the CO adsorption sites, determine oxygen vacancy formation energies and the origin of deactivation. A new species of CO bonded to oxygen vacancies at RT was identified, which may block a vacancy site from further reaction unless CO is removed at higher temperature. The interaction between oxygen vacancies was found to be small, so that in the active state several lattice oxygen species are available for reaction in parallel.

Operando Identification of the Reversible Skin Layer on Co3O4 as a Three-Dimensional Reaction Zone for Oxygen Evolution

Co oxides and oxyhydroxides have been studied extensively in the past as promising electrocatalysts for the oxygen evolution reaction (OER) in neutral to alkaline media. Earlier studies showed the formation of an ultrathin CoO_x(OH)_y skin layer on Co₃O₄ at potentials above 1.15 V vs reversible hydrogen electrode (RHE), but the precise influence of this skin layer on the OER reactivity is still under debate. We present here a systematic study of epitaxial spinel-type Co₃O₄ films with defined (111) orientation, prepared on different substrates by electrodeposition or physical vapor deposition. The OER overpotential of these samples may vary up to 120 mV, corresponding to two orders of magnitude differences in current density, which cannot be accounted for by differences in the electrochemically active surface area. We demonstrate by a careful analysis of operando surface X-ray diffraction measurements that these differences are clearly correlated with the average thickness of the skin layer. The OER reactivity increases with the amount of formed skin layer, indicating that the entire three-dimensional skin layer is an OER-active interphase. Furthermore, a scaling relationship between the reaction centers in the skin layer and the OER activity is established. It suggests that two lattice sites are involved in the OER mechanism.

Model Studies on the Formation of the Solid Electrolyte Interphase: Reaction of Li with Ultrathin Adsorbed Ionic-Liquid Films and Co3 O4 (111) Thin Films

In this work we aim towards the molecular understanding of the solid electrolyte interphase (SEI) formation at the electrode electrolyte interface (EEI). Herein, we investigated the interaction between the battery‐relevant ionic liquid (IL) 1‐butyl‐1‐methylpyrrolidinium bis(trifluoromethylsulfonyl)imide (BMP‐TFSI), Li and a Co₃O₄(111) thin film model anode grown on Ir(100) as a model study of the SEI formation in Li‐ion batteries (LIBs). We employed mostly X‐ray photoelectron spectroscopy (XPS) in combination with dispersion‐corrected density functional theory calculations (DFT‐D3). If the surface is pre‐covered by BMP‐TFSI species (model electrolyte), post‐deposition of Li (Li⁺ ion shuttle) reveals thermodynamically favorable TFSI decomposition products such as LiCN, Li₂NSO₂CF₃, LiF, Li₂S, Li₂O₂, Li₂O, but also kinetic products like Li₂NCH₃C₄H₉ or LiNCH₃C₄H₉ of BMP. Simultaneously, Li adsorption and/or lithiation of Co₃O₄(111) to Li_nCo₃O₄ takes place due to insertion via step edges or defects; a partial transformation to CoO cannot be excluded. Formation of Co⁰ could not be observed in the experiment indicating that surface reaction products and inserted/adsorbed Li at the step edges may inhibit or slow down further Li diffusion into the bulk. This study provides detailed insights of the SEI formation at the EEI, which might be crucial for the improvement of future batteries.

Facet-Dependent Cobalt Ion Distribution on the Co3O4 Nanocatalyst Surface

Co₃O₄ is an important catalyst widely used for CO oxidation or electrochemical water oxidation near room temperature and was also recently used as support for single-atom catalysts (SACs). Co₃O₄ with a spinel structure hosts dual oxidation states of Co²⁺ and Co³⁺ in the lattice, leading to the complexity of its surface structure as the exposure of Co²⁺ and Co³⁺ has a significant impact on the performance of the catalysts. Although it is acknowledged that different facets exhibit varied catalytic activities and different abilities in hosting single atoms to provide active centers in SACs, the Co₃O₄ surface structure remains under-investigated. In this study, major facets of {111}, {110}, and {100} were studied down to subangstrom scale using advanced electron microscopy. We noticed that each facet has its own most stable surface configuration. The distribution of Co²⁺ and Co³⁺ on each facet was quantified, revealing a facet-dependent distribution of Co²⁺ and Co³⁺. Co³⁺ was found to be preferentially exposed on {100} and {110} as well as surface steps. Surface reconstruction was revealed, where a subangstrom scale shift of Co²⁺ was confirmed on facets of {111} and {100} due to polarity compensation and oxygen deficiency on the surface. This work not only improves our fundamental understanding of the Co₃O₄ surface structure but also may promote the design of Co₃O₄-based catalysts with tunable activity and stability.

Cobalt Oxide-Supported Pt Electrocatalysts: Intimate Correlation between Particle Size, Electronic Metal-Support Interaction and Stability

Oxide supports can modify and stabilize platinum nanoparticles (NPs) in electrocatalytic materials. We studied related phenomena on model systems consisting of Pt NPs on atomically defined Co₃O₄(111) thin films. Chemical states and dissolution behavior of model catalysts were investigated as a function of the particle size and the electrochemical potential by ex situ emersion synchrotron radiation photoelectron spectroscopy and by online inductively coupled plasma mass spectrometry. Electronic metal-support interaction (EMSI) yields partially oxidized Pt^δ+ species at the metal/support interface of metallic nanometer-sized Pt NPs. In contrast, subnanometer particles form Pt^δ+ aggregates that are exclusively accompanied by subsurface Pt⁴⁺ species. Dissolution of Co^x+ ions is strongly coupled to the presence of Pt^δ+ and the reduction of subsurface Pt⁴⁺ species. Our findings suggest that EMSI directly affects the integrity of oxide-based electrocatalysts and may be employed to stabilize Pt NPs against sintering and dissolution.

Oxygen Reduction and Evolution on Ni-modified Co3 O4 (1 1 1) Cathodes for Zn-Air Batteries: A Combined Surface Science and Electrochemical Model Study

The performance of structurally and chemically well-defined Ni-free and Ni-modified single-crystalline Co3 O4 (1 1 1) thin-film electrodes in the oxygen reduction and evolution reactions (ORR and OER) was investigated in a combined surface science and electrochemistry approach. Pure and Ni-modified Co3 O4 (1 1 1) film electrodes were prepared and characterized under ultrahigh-vacuum conditions by scanning tunneling microscopy and X-ray photoelectron spectroscopy. Both Ni decoration (by post-deposition of Ni) and Ni doping (by simultaneous vapor deposition of Ni, Co, and O2 ) induced distinct differences in the base cyclic voltammograms in 0.5 m KOH at potentials higher than 0.7 V compared with Co3 O4 (1 1 1) electrodes. Also, all oxide film electrodes showed a higher overpotential for the ORR but a lower one for the OER than polycrystalline Pt. Ni modification significantly improved the ORR current densities by increasing the electrical conductivity, whereas the OER onset of approximately 1.47 VRHE (RHE: reversible hydrogen electrode) at 0.1 mA cm-2 was almost unchanged.

Norbornadiene photoswitches anchored to well-defined oxide surfaces: From ultrahigh vacuum into the liquid and the electrochemical environment

Employing molecular photoswitches, we can combine solar energy conversion, storage, and release in an extremely simple single molecule system. In order to release the stored energy as electricity, the photoswitch has to interact with a semiconducting electrode surface. In this work, we explore a solar-energy-storing model system, consisting of a molecular photoswitch anchored to an atomically defined oxide surface in a liquid electrolyte and under potential control. Previously, this model system has been proven to be operational under ultrahigh vacuum (UHV) conditions. We used the tailor-made norbornadiene derivative 2-cyano-3-(4-carboxyphenyl)norbornadiene (CNBD) and characterized its photochemical and electrochemical properties in an organic electrolyte. Next, we assembled a monolayer of CNBD on a well-ordered Co₃O₄(111) surface by physical vapor deposition in UHV. This model interface was then transferred into the liquid electrolyte and investigated by photoelectrochemical infrared reflection absorption spectroscopy experiments. We demonstrate that the anchored monolayer of CNBD can be converted photochemically to its energy-rich counterpart 2-cyano-3-(4-carboxyphenyl)quadricyclane (CQC) under potential control. However, the reconversion potential of anchored CQC overlaps with the oxidation and decomposition potential of CNBD, which limits the electrochemically triggered reconversion.

Interfacial Reactions of Tetraphenylporphyrin with Cobalt-Oxide Thin Films

We have studied the adsorption and interfacial reactions of 2H-tetraphenylporphyrin (2HTPP) with cobalt-terminated Co₃ O₄ (111) and oxygen-terminated CoO(111) thin films using synchrotron-radiation X-ray photoelectron spectroscopy. Already at 275 K, we find evidence for the formation of a metalated species, most likely CoTPP, on both surfaces. The degree of self-metalation increases with temperature on both surfaces until 475 K, where the metalation is almost complete. At 575 K the porphyrin coverage decreases drastically on the reducible cobalt-terminated Co₃ O₄ (111) surface, while higher temperatures are needed on the non-reducible oxygen-terminated CoO(111). The low temperature self-metalation is similar to that observed on MgO(100) surfaces, but drastically different from that observed on TiO₂ (110), where no self-metalation is observed at room temperature.

Solar energy storage at an atomically defined organic-oxide hybrid interface

Molecular photoswitches provide an extremely simple solution for solar energy conversion and storage. To convert stored energy to electricity, however, the photoswitch has to be coupled to a semiconducting electrode. In this work, we report on the assembly of an operational solar-energy-storing organic-oxide hybrid interface, which consists of a tailor-made molecular photoswitch and an atomically-defined semiconducting oxide film. The synthesized norbornadiene derivative 2-cyano-3-(4-carboxyphenyl)norbornadiene (CNBD) was anchored to a well-ordered Co₃O₄(111) surface by physical vapor deposition in ultrahigh vacuum. Using a photochemical infrared reflection absorption spectroscopy experiment, we demonstrate that the anchored CNBD monolayer remains operational, i.e., can be photo-converted to its energy-rich counterpart 2-cyano-3-(4-carboxyphenyl)quadricyclane (CQC). We show that the activation barrier for energy release remains unaffected by the anchoring reaction and the anchored photoswitch can be charged and discharged with high reversibility. Our atomically-defined solar-energy-storing model interface enables detailed studies of energy conversion processes at organic/oxide hybrid interfaces.

Preparation of complex model electrocatalysts in ultra-high vacuum and transfer into the electrolyte for electrochemical IR spectroscopy and other techniques

Model studies at complex, yet well-defined electrodes can provide a better understanding of electrocatalytic reactions. New experimental devices are required to prepare such model electrocatalysts with atomic-level control. In this work, we discuss the design of a new setup, which enables the preparation of well-defined electrocatalysts in ultra-high vacuum (UHV) using the full portfolio of surface science techniques. The setup allows for direct transfer of samples from UHV and the immersion into the electrolyte without contact to air. As a special feature, the single crystal sample is transferred without any sample holder, which makes the system easily compatible with most electrochemical in situ methods, specifically with electrochemical infrared reflection absorption spectroscopy, but also with other characterization methods such as single-crystal cyclic voltammetry, differential electrochemical mass spectrometry, or electrochemical scanning tunneling microscopy. We demonstrate the preparation in UHV, the transfer in inert atmosphere, and the immersion into the electrolyte for a complex model catalyst that requires surface science methods for preparation. Specifically, we study Pt nanoparticles supported on well-ordered Co₃O₄(111) films which are grown on an Ir(100) single crystal. In comparison with reference experiments on Pt(111), the model catalyst shows a remarkably different adsorption and reaction behavior during CO electrooxidation in alkaline environments.

Electrifying model catalysts for understanding electrocatalytic reactions in liquid electrolytes

Electrocatalysis is at the heart of our future transition to a renewable energy system. Most energy storage and conversion technologies for renewables rely on electrocatalytic processes and, with increasing availability of cheap electrical energy from renewables, chemical production will witness electrification in the near future^1-3. However, our fundamental understanding of electrocatalysis lags behind the field of classical heterogeneous catalysis that has been the dominating chemical technology for a long time. Here, we describe a new strategy to advance fundamental studies on electrocatalytic materials. We propose to 'electrify' complex oxide-based model catalysts made by surface science methods to explore electrocatalytic reactions in liquid electrolytes. We demonstrate the feasibility of this concept by transferring an atomically defined platinum/cobalt oxide model catalyst into the electrochemical environment while preserving its atomic surface structure. Using this approach, we explore particle size effects and identify hitherto unknown metal-support interactions that stabilize oxidized platinum at the nanoparticle interface. The metal-support interactions open a new synergistic reaction pathway that involves both metallic and oxidized platinum. Our results illustrate the potential of the concept, which makes available a systematic approach to build atomically defined model electrodes for fundamental electrocatalytic studies.

Structure-Dependent Dissociation of Water on Cobalt Oxide

Understanding the correlation between structure and reactivity of oxide surfaces is vital for the rational design of catalytic materials. In this work, we demonstrate the exceptional degree of structure sensitivity of the water dissociation reaction for one of the most important materials in catalysis and electrocatalysis. We studied H₂O on two atomically defined cobalt oxide surfaces, CoO(100) and Co₃O₄(111). Both surfaces are terminated by O^2- and Co²⁺ in different coordination. By infrared reflection absorption spectroscopy and synchrotron radiation photoelectron spectroscopy we show that H₂O adsorbs molecularly on CoO(100), while it dissociates and forms very strongly bound OH and partially dissociated (H₂O) _n(OH) _m clusters on Co₃O₄(111). We rationalize this structure dependence by the coordination number of surface Co²⁺. Our results show that specific well-ordered cobalt oxide surfaces interact very strongly with H₂O whereas others do not. We propose that this structure dependence plays a key role in catalysis with cobalt oxide nanomaterials.

Phosphonic Acids on an Atomically Defined Oxide Surface: The Binding Motif Changes with Surface Coverage

We have studied the anchoring mechanism of a phosphonic acid on an atomically defined oxide surface. Using time-resolved infrared reflection absorption spectroscopy, we investigated the reaction of deuterated phenylphosphonic acid (DPPA, C₆H₅PO₃D₂) with an atomically defined Co₃O₄(111) surface in situ during film growth by physical vapor deposition. We show that the binding motif of the phosphonate anchor group changes as a function of coverage. At low coverage, DPPA binds in the form of a chelating tridentate phosphonate, while a transition to a chelating bidentate occurs close to monolayer saturation coverage. However, the coverage-dependent change in the binding motif is not associated with a major change of the molecular orientation, suggesting that the rigid phosphonate linker always maintains the DPPA in a strongly tilted orientation irrespective of the surface coverage.

Gluing Ionic Liquids to Oxide Surfaces: Chemical Anchoring of Functionalized Ionic Liquids by Vapor Deposition onto Cobalt(II) Oxide

Ionic liquids (IL) hold a great potential as novel electrolytes for applications in electronic materials and energy technology. The functionality of ILs in these applications relies on their interface to semiconducting nanomaterials. Therefore, methods to control the chemistry and structure of this interface are the key to assemble new IL-based electronic and electrochemical materials. Here, we present a new method to prepare a chemically well-defined interface between an oxide and an IL film. An imidazolium-based IL, which is carrying an ester group, is deposited onto cobalt oxide surface by evaporation. The IL binds covalently to the surface by thermally activated cleavage of the ester group and formation of a bridging carboxylate. The anchoring reaction shows high structure sensitivity, which implies that the IL film can be adhered selectively to specific oxide surfaces.

Nucleation of Graphene Layers on Magnetic Oxides: Co3O4(111) and Cr2O3(0001) from Theory and Experiment

We report directly grown strongly adherent graphene on Co₃O₄(111) by carbon molecular beam epitaxy (C MBE) at 850 K and density functional theory (DFT) findings that the first graphene layer is reconstructed to fit the Co₃O₄ surface, while subsequent layers retain normal graphene structure. This adherence to the Co₃O₄ structure results from partial bonding of half the carbons to top oxygens of the substrate. This structure is validated by X-ray photoelectron spectroscopy and low-energy electron diffraction studies, showing layer-by-layer graphene growth with ∼0.08 electrons/carbon atom transferred to the oxide from the first graphene layer, in agreement with DFT. In contrast, for Cr₂O₃ DFT finds no strong bonding to the surface and C MBE on Cr₂O₃(0001) yields only graphite formation at 700 K, with C desorption above 800 K. Thus strong graphene-to-oxide charge transfer aids nucleation of graphene on incommensurate oxide substrates and may have implications for spintronics.

CO Adsorption on Reconstructed Ir(100) Surfaces from UHV to mbar Pressure: A LEED, TPD, and PM-IRAS Study

Clean and stable surface modifications of an iridium (100) single crystal, i.e., the (1 × 1) phase, the (5 × 1) reconstruction, and the oxygen-terminated (2 × 1)-O surface, were prepared and characterized by low energy electron diffraction (LEED), temperature-programmed desorption (TPD), infrared reflection absorption spectroscopy (IRAS) and polarization modulation IRAS (PM-IRAS). The adsorption of CO in UHV and at elevated (mbar) pressure/temperature was followed both ex situ and in situ on all three surface modifications, with a focus on mbar pressures of CO. The Ir(1 × 1) surface exhibited c(4 × 2)/c(2 × 2) and c(6 × 2) CO structures under low pressure conditions, and remained stable up to 100 mbar and 700 K. For the (2 × 1)-O reconstruction CO adsorption induced a structural change from (2 × 1)-O to (1 × 1), as confirmed by LEED, TPD, and IR. For Ir (2 × 1)-O TPD indicated that CO reacted with surface oxygen forming CO₂. The (5 × 1) reconstruction featured a reversible and dynamic behavior upon CO adsorption, with a local lifting of the reconstruction to (1 × 1). After CO desorption, the (5 × 1) structure was restored. All three reconstructions exhibited CO adsorption with on-top geometry, as evidenced by IR. With increasing CO exposure the resonances shifted to higher wavenumber, due to adsorbate-adsorbate and adsorbate-substrate interactions. The largest wavenumber shift (from 2057 to 2100 cm^-1) was observed for Ir(5 × 1) upon CO dosing from 1 L to 100 mbar.

Structure-Dependent Anchoring of Organic Molecules to Atomically Defined Oxide Surfaces: Phthalic Acid on Co3O4(111), CoO(100), and CoO(111)

We have performed a model study to explore the influence of surface structure on the anchoring of organic molecules on oxide materials. Specifically, we have investigated the adsorption of phthalic acid (PA) on three different, well-ordered, and atomically defined cobalt oxide surfaces, namely 1) Co3O4(111), 2) CoO(111), and 3) CoO(100) on Ir(100). PA was deposited by physical vapor deposition (PVD). The formation of the PA films and interfacial reactions were monitored in situ during growth by isothermal time-resolved IR reflection absorption spectroscopy (TR-IRAS) under ultrahigh vacuum (UHV) conditions. We observed a pronounced structure dependence on the three surfaces with three distinctively different binding geometries and characteristic differences depending on the temperature and coverage. 1) PA initially binds to Co3O4(111) through the formation of a chelating bis-carboxylate with the molecular plane oriented perpendicularly to the surface. Similar species were observed both at low temperature (130 K) and at room temperature (300 K). With increasing exposure, chelating mono-carboxylates became more abundant and partially replaced the bis-carboxylate. 2) PA binds to CoO(100) in the form of a bridging bis-carboxylate for low coverage. Upon prolonged deposition of PA at low temperature, the bis-carboxylates were converted into mono-carboxylate species. In contrast, the bis-carboxylate layer was very stable at 300 K. 3) For CoO(111) we observed a temperature-dependent change in the adsorption mechanism. Although PA binds as a mono-carboxylate in a bridging bidentate fashion at low temperature (130 K), a strongly distorted bis-carboxylate was formed at 300 K, possibly as a result of temperature-dependent restructuring of the surface. The results show that the adsorption geometry of PA depends on the atomic structure of the oxide surface. The structure dependence can be rationalized by the different arrangements of cobalt ions at the three surfaces.

Functionalized Porphyrins on an Atomically Defined Oxide Surface: Anchoring and Coverage-Dependent Reorientation of MCTPP on Co3O4(111)

We have studied the adsorption of tetraphenylporphyrin (2HTPP) and its carboxylated counterpart mono-para-carboxyphenyltriphenylporphyrin (MCTPP) on an atomically defined Co3O4(111) film under ultrahigh vacuum (UHV) conditions. Using time-resolved infrared reflection absorption spectroscopy (TR-IRAS), we show that 2HTPP adsorbs molecularly in a flat-lying orientation, whereas MCTPP binds to the surface via formation of a chelating bidentate carboxylate upon deposition at 400 K. Combining TR-IRAS and density-functional theory (DFT), we determine the molecular tilting angle as a function of coverage. We show that the MCTPP adsorption geometry changes from a nearly flat-lying orientation (tilting angle <30°) at low coverage to a nearly perfectly upright-standing orientation (tilting angle of approximately 80°) in the full monolayer.

Epitaxial cobalt oxide films on Ir(100)-the importance of crystallographic analyses

Epitaxial cobalt oxide films on Ir(100) exhibit a rich scenario of different structural phases which are reviewed in this paper. The great majority of phases could be, as a rare case, crystallographically described by the joint application of atomically resolved STM and quantitative LEED, whereby structural surprises were more the rule than the exception. So, the oxide grows in the polar (111) orientation for both the Co3O4 and CoO stoichiometry on the bare Ir substrate in spite of the latter's square symmetry. Moreover, the film orientation can be tuned to non-polar (100) growth when one or several pseudomorphic Co layers are introduced as an interface between oxide and Ir substrate. By using the nanostructured Ir(100)-(5 × 1)-H phase as a template a nanostructured Co film can be formed whose oxidation leads to a nanostructured oxide. The nominally polar films circumvent the polarity problem by appropriate surface terminations. That of CoO(111) is, again as a surprise, realized by a switch from rocksalt-type to wurtzite-type stacking near the surface, by which the latter becomes metallic. The stepwise oxidation of a pseudomorphic Co layer on the bare Ir substrate leads to the sequential formation of rocksalt-type tetrahedral Co-O building blocks (with intermediate BN-type blocks) whereby the Co species more and more assume positions determined by the inner-oxidic binding.

Noncontact atomic force microscopy study of the spinel MgAl(2)O(4)(111) surface

Based on high-resolution noncontact atomic force microscopy (NC-AFM) experiments we reveal a detailed structural model of the polar (111) surface of the insulating ternary metal oxide, MgAl(2)O(4) (spinel). NC-AFM images reveal a 6√3×6√3R30° superstructure on the surface consisting of patches with the original oxygen-terminated MgAl(2)O(4)(111) surface interrupted by oxygen-deficient areas. These observations are in accordance with previous theoretical studies, which predict that the polarity of the surface can be compensated by removal of a certain fraction of oxygen atoms. However, instead of isolated O vacancies, it is observed that O is removed in a distinct pattern of line vacancies reflected by the underlying lattice structure. Consequently, by the creation of triangular patches in a 6√3×6√3R30° superstructure, the polar-stabilization requirements are met.

Direct graphene growth on Co3O4(111) by molecular beam epitaxy

Direct growth of graphene on Co(3)O(4)(111) at 1000 K was achieved by molecular beam epitaxy from a graphite source. Auger spectroscopy shows a characteristic sp(2) carbon lineshape, at average carbon coverages from 0.4 to 3 ML. Low energy electron diffraction (LEED) indicates (111) ordering of the sp(2) carbon film with a lattice constant of 2.5(±0.1) Å characteristic of graphene. Sixfold symmetry of the graphene diffraction spots is observed at 0.4, 1 and 3 ML. The LEED data also indicate an average domain size of ~1800 Å, and show an incommensurate interface with the Co(3)O(4)(111) substrate, where the latter exhibits a lattice constant of 2.8(±0.1) Å. Core level photoemission shows a characteristically asymmetric C(1s) feature, with the expected π to π* satellite feature, but with a binding energy for the 3 ML film of 284.9(±0.1) eV, indicative of substantial graphene-to-oxide charge transfer. Spectroscopic ellipsometry data demonstrate broad similarity with graphene samples physically transferred to SiO(2) or grown on SiC substrates, but with the π to π* absorption blue-shifted, consistent with charge transfer to the substrate. The ability to grow graphene directly on magnetically and electrically polarizable substrates opens new opportunities for industrial scale development of charge- and spin-based devices.

Tuning the growth orientation of epitaxial films by interface chemistry

The support of epitaxial films frequently determines their crystallographic orientation, which is of crucial importance for their properties. We report a novel way to alter the film orientation without changing the substrate. We show for the growth of CoO on the Ir(100) surface that, while the oxide grows in (111) orientation on the bare substrate, the orientation switches to (100) by introducing a single (or a few) monolayer(s) of Co between the oxide and substrate. This tunability of the orientation of epitaxial films by the appropriate choice of interface chemistry most likely is a general feature.

Advances on surface structural determination by LEED

In the last 40 years, low energy electron diffraction (LEED) has proved to be the most reliable quantitative technique for surface structural determination. In this review, recent developments related to the theory that gives support to LEED structural determination are discussed under a critical analysis of the main theoretical approximation-the muffin-tin calculation. The search methodologies aimed at identifying the best matches between theoretical and experimental intensity versus voltage curves are also considered, with the most recent procedures being reviewed in detail.

Direct subangstrom measurement of surfaces of oxide particles

Using aberration-corrected transmission electron microscopy combined with first-principles calculations, we show that the surface structure of Co3O4, a typical complex oxide, can be directly imaged and quantitatively analyzed at the subangstrom scale. The atomic positions of both light oxygen and heavier cobalt within the surface layers have been measured to an accuracy of several picometers. The surface electronic structure analysis suggests a polarity compensation model based on the electronic polarizability of surface ions.

Substoichiometric cobalt oxide monolayer on Ir(100)-(1 × 1)

A substoichiometric monolayer of cobalt oxide has been prepared by deposition and oxidation of slightly less than one monolayer of cobalt on the unreconstructed surface of Ir(100). The ultrathin film was investigated by scanning tunnelling microscopy (STM) and quantitative low-energy electron diffraction (LEED). The cobalt species of the film reside in or near hollow positions of the substrate with, however, unoccupied sites (vacancies) in a 3 × 3 arrangement. In the so-formed 3 × 3 supercell the oxide's oxygen species are both threefold and fourfold coordinated to cobalt, forming pyramids with a triangular and square cobalt basis, respectively. These pyramids are the building blocks of the oxide. Due to the reduced coordination as compared to the sixfold one in the bulk of rock-salt-type CoO, the Co-O bond lengths are smaller than in the latter. For the threefold coordination they compare very well with the bond length in oxygen terminated CoO(111) films investigated recently. The substoichiometric 3 × 3 oxide monolayer phase transforms to a stoichiometric c(10 × 2)-periodic oxide monolayer under oxygen exposure, in which, however, cobalt and oxygen species are in (111) orientation and so form a CoO(111) layer.

Phases and phase transitions of hexagonal cobalt oxide films on Ir(100)-(1 × 1)

Cobalt oxides on the unreconstructed Ir(100) surface were prepared by reactive deposition of Co established by simultaneous oxygen flux at about 50 °C and subsequent annealing. The films were investigated by low-energy electron diffraction (LEED), scanning tunnelling microscopy (STM) and thermal desorption spectroscopy (TDS). We show that in spite of the quadratic unit mesh of the substrate, oxide films of (111) orientation develop. As long as oxygen-rich conditions are maintained they are of spinel-type Co(3)O(4)(111). They are non-pseudomorphic and transform to rocksalt-type CoO(111) when oxygen loss is induced by annealing at elevated temperatures. Thin films of CoO(111) are commensurate, and so, in order to realize that, they exhibit a slightly distorted unit cell when below a thickness equivalent to about seven cobalt monolayers. With increasing film thickness the uniaxial strain accompanied by the commensurability is gradually relieved by the insertion of dislocations so that eventually the film assumes ideal hexagonality. All CoO(111)-type surfaces are reconstructed at low sample temperatures equivalent to a [Formula: see text] superstructure. They reversibly transform into a (1 × 1) phase at about 50 °C.

Coexistence of rocksalt and wurtzite structure in nanosized CoO films

Cobalt oxide (CoO) films epitaxially grown on Ir(100) in (111) orientation were investigated by means of quantitative low-energy electron diffraction and scanning tunneling microscopy. We find with high crystallographic precision that in the bulk of the films the rocksalt structure prevails while near the surface there is a switch towards the wurtzite structure. As a consequence, nanosized CoO cannot be considered as a single structural phase. The film surfaces prove to be metallic, apparently connected with polarity compensation.