Cobalt oxide nanolayers on Pd(100): The thickness-dependent structural evolution

The growth of interface-stabilized cobalt oxide (CoO_x) nanolayers on Pd(100) has been investigated and their structures are reported as a function of coverage. Several different phases have been observed by LEED and STM experiments, and they have been characterized spectroscopically by photoemission and X-ray absorption. The data indicate that in the low coverage regime (up to Θ_Co ≈ 2–3 ML) rock-salt CoO type phases are formed (defective in the single layer regime, and stoichiometric in multilayers) with (100) or (111) termination. At higher coverage (Θ_Co ≈ 10–20 ML) spinel Co₃O₄(111) and CoO(100) layers have been detected, in ratios dependent on the preparation conditions. The observed structures are discussed in relation to similar structures reported recently for CoO_x films on Ir(100) [W. Meyer et al., J. Phys.: Condens. Matter 20 (2008) 265011].     

Stoichiometry and surface reconstruction of epitaxial CuInSe2(112) films

CuInSe₂(112) films were grown on GaAs(111)A substrates by molecular beam epitaxy. The resulting surface stoichiometry was deduced by consideration of results from various surface analytic techniques. The obtainable Cu/In stoichiometry range in XPS was 0.4–1.2, where 1.2 marks the onset of Cu_2 ? xSe phase segregation at the surface and 0.4 corresponds to the copper-depleted surface with ordered defect compound (ODC) composition. For the stoichiometric CuInSe₂(112) surface, a c(4 × 2) reconstruction of the zinc blende surface periodicity is observed in the LEED pattern, with three rotational domains present on the flat GaAs(111) substrate. With the use of stepped (111) substrates, domain formation could be suppressed. By comparison of the LEED data and concentration depth profiles from angle-resolved XPS, two types of surface reconstructions could be distinguished. According to surface energy calculations in the literature, these correspond to surfaces stabilized by either Cu_In or 2V_Cu defects. The surface of copper-poor CuIn₃Se₅ shows no reconstruction of the zinc blende order.     

Infrared study of triruthenium dodecacarbonyl interactions with gold

We present here a study of the interaction of triruthenium dodecacarbonyl Ru₃(CO)₁₂ with gold surfaces using time-evolved and temperature-programmed infrared reflection absorption spectroscopy (IRAS) and STM. Ru₃(CO)₁₂ exhibits drastically different adsorption/desorption behavior on high-index surfaces of gold in comparison to the smooth Au(111) surface. On the smooth Au(111) surface, the adsorption of Ru₃(CO)₁₂ at 200 K is observed to be molecular and reversible with the molecule's Ru₃-plane oriented essentially perpendicular to the surface in the first and second layer. In the multilayer (> 3 ML), the molecule is oriented parallel (or moderately inclined) to the surface. On high-index gold surfaces, prepared by partial annealing of rough gold films, the molecules dissociate. Vibrational spectra reveal dissociation of carbonyl to Ru and CO at elevated temperature (> 250 K) with the formation of CO covered Ru-islands and the subsequent desorption of CO from Ru-islands. Increasing amounts of CO observed with increasing surface roughness demonstrate that the rate of Ru₃(CO)₁₂ dissociation is related directly to the surface roughness of the gold surface. STM images reveal at low coverage the formation of 2-D islands of carbonyl fragments with lateral sizes of 1 to 1.5 nm and at higher coverage the formation of larger 3-D islands of 1 to 3 layers and lateral sizes above 10 nm.     

Hot-pressed and die-upset Pr-Co magnets produced from mechanically activated alloys

Anisotropic die-upset PrCo₅-based magnets were produced by hot pressing and subsequent die-upsetting of alloy powders activated by high-energy ball milling. In addition to the PrCo₅ major phase the magnets contained Pr₂Co₁₇, even when prepared from a stoichiometric 1:5 alloy. Hard magnetic properties can be improved by partial Sm substitution for Pr and partial Cu substitution for Co. A higher cobalt/rare-earth ratio as well as the addition of Fe was found to be unfavorable for the deformation-induced anisotropy. The largest values of remanence, intrinsic coercivity and maximum energy product in hot-pressed magnets were obtained for Pr_0.7Sm_0.3Co_5.5 (7.8 kG, 14.1 kOe and of 13.1 MGOe) and in die-upset magnets for Pr_0.9Sm_0.1(Co_0.98Cu_0.02)₅ (9.2 kG, 10.5 kOe and 16.7 MGOe).     

Hydrothermal synthesis of mesoporous Co3O4 nanobelts by means of a compound precursor

In the present work, high surface area mesoporous cobalt oxide (Co₃O₄) nanobelts have been synthesized by thermal treatment of cobalt hydroxide carbonate (CHC) precursors. CHC nanobelts were prepared by a facile hydrothermal method. Control experiments with variations in reaction time, solvent and different cobalt source revealed that temperature and sulfates are key factors in determining the formation of CHC nanobelts. Scanning electron microscopy and transmission electron microscopy images showed that the Co₃O₄ nanobelts consisted of mesoporous nanobelts with the average width of 40 nm. Brunauer–Emmett–Teller (BET) gas adsorption measurement further indicated that the products presented a rather large surface area (172.09 m² g^?1).     

Fe3O4 surface electronic structures and stability from GGA + U

The electronic structures, stabilities and magnetic properties of the Fe₃O₄(111), (110) and (001) surfaces have been computed at the level of density functional theory by including the Hubbard parameter (U) for describing the on-site Coulomb interaction of iron 3d electrons. Among the six Fe₃O₄(111) terminations, the Fe_tet1 (exposing tetrahedral coordinated iron) and Fe_oct2 (exposing octahedral coordinated iron) terminations are more stable and have metallic character. For the Fe₃O₄(110) surface, strong surface distortion has been found; the A-layer termination (exposing tetrahedral coordinated iron) has metallic character, while the B-layer termination (exposing tetrahedral and octahedral coordinated iron) has half-metal character. For the Fe₃O₄(001) surface, both A-layer (exposing tetrahedral coordinated iron) and B-layer (exposing octahedral coordinated iron) terminations have half-metal character. The surface stability of (111) > (001) > (110) on the basis of the computed surface energies agrees well with the experimental findings, and explains reasonably the observed diversity and complexity of the experiments.     

Effect of oxygen gas pressure on orientations of Cu2O nuclei during the initial oxidation of Cu(100), (110) and (111)

The orientations of oxide nuclei during the oxidation of Cu(100), (110) and (111) surfaces have been examined by in situ transmission electron microscopy. Our results indicate that the epitaxial nucleation of oxide islands on these surfaces cannot be maintained for a whole range of oxygen gas pressure varying from 10^? 5 Torr to 750 Torr. The critical oxygen gas pressure, pO₂, leading to the transition from nucleating epitaxial to non-epitaxial oxide nuclei shows a dependence on the crystallographic orientations of the Cu substrates with pO₂⁽¹⁰⁰⁾ > pO₂⁽¹¹¹⁾ > pO₂⁽¹¹⁰⁾. By fitting the experimentally determined critical oxygen pressures to a kinetic model, we find that such dependence can be attributed to the effect of surface orientations of the Cu substrates on the oxygen surface adsorption and diffusion, which dominate the kinetic processes of oxide nucleation.     

Type B epitaxy of Ge on CaF2(111) surface

It was known experimentally that type B orientation, which is rotated 180° about the [111] axis, dominated the heteroepitaxial growth of Ge(111) on a CaF₂(111) substrate at an elevated temperature. We performed first principles calculations using density functional theory to determine the energetics of the Ge(111)/CaF₂(111) interface and found that the type B orientation of the Ge film is most likely a result of a direct bonding between Ge atoms and Ca²⁺ at the CaF₂ surface with the top F^? layer depleted. Our theoretical prediction is supported by our X-ray diffraction experiments on {111} < 121> biaxially textured Ge/CaF₂ samples.     

Ultrathin Y2O3(111) films on Pt(111) substrates

The growth of ultrathin films of Y₂O₃(111) on Pt(111) has been studied using scanning tunneling microscopy (STM), X-ray photoemission spectroscopy (XPS), and low energy electron diffraction (LEED). The films were grown by physical vapor deposition of yttrium in a 10^? 6 Torr oxygen atmosphere. Continuous Y₂O₃(111) films were obtained by post-growth annealing at 700 °C. LEED and STM indicate an ordered film with a bulk-truncated Y₂O₃(111)–1 × 1 structure exposed. Furthermore, despite the lattices of the substrate and the oxide film being incommensurate, the two lattices exhibit a strict in-plane orientation relationship with the [11?0] directions of the two cubic lattices aligning parallel to each other. XPS measurements suggest hydroxyls to be easily formed at the Y₂O₃ surface at room temperature even under ultra high vacuum conditions. The hydrogen desorbs from the yttria surface above ~ 200 °C.     

The studies on structural and thermal properties of delithiated LixNi1/3Co1/3Mn1/3O2 (0 < x ≤ 1) as a cathode material in lithium ion batteries

The structural and thermal properties of the delithiated Li_xNi_1/3Co_1/3Mn_1/3O₂ (0 < x ≤ 1) material have been investigated by using diffraction and thermoanalytical techniques such as XRD and TG-DSC methods. XRD result shows that the delithiated materials maintain the O3-type structure with defined stoichiometric number at the range of 0.24 < x ≤ 1, exhibiting good crystal structural stability. The cobalt and nickel ions in the delithiated materials change their valence state (i.e. Co³⁺ to Co⁴⁺ and Ni³⁺ to Ni⁴⁺) when x < 0.49; the irreversible changes of the transformation may affect the first cycle of charge–discharge efficiency of the materials. A comparison of the results of TG-DSC with TPD-MS shows that the irreversible change of oxygen species during the delithiation process of Li_xNi_1/3Co_1/3Mn_1/3O₂ have great influence on the structural and thermal stability and reversibility of the materials.     

Structure sensitivity of propene oxidation over Co-Mn spinels

Single-phase cobalt–manganese spinel oxides (Co_3?nMn_nO₄, CMO) were studied for the catalytic oxidation of propene in a systematic optimization strategy. CMO films were synthesized by pulsed-spray evaporation chemical vapor deposition (PSE–CVD) and characterized by X-ray diffraction (XRD), Fourier transform infrared (FTIR), Raman and Ultraviolet–Visible (UV–Vis) spectroscopy. The effect of Co/Mn ratio in the mixed oxide systems on their catalytic activity was investigated in a fixed-bed reactor at T = 100–800 °C, with a space velocity of 90,000 mL/g_cat h and a feed of 2% C₃H₆/20% O₂/78% Ar. XRD patterns, FTIR and Raman spectroscopy reveal that a cubic single-phase spinel structure is obtained for n ? 1.23, while a tetragonal spinel structure is observed for n > 1.23. With increasing of the manganese content, the temperature–programmed analysis demonstrates a lower reducibility, a general decrease of the temperature required for the reduced samples to be re-oxidized and increasing thermal stability. The catalytic tests show that the involvement of cobalt–manganese oxides in propene oxidation suppresses the formation of reaction intermediates, favoring the selectivity toward CO₂ at low temperatures. Co_2.35Mn_0.65O₄ exhibits the best catalytic performance, which follows in line with its better reducibility compared with the other compositions in the series of CMO oxides. These results show the great potential of CMO for future industrial application as a low-temperature catalytic system which does not rely on precious metals.     

Surface oxides of Ir(111) prepared by gas-phase oxygen atoms

The Ir(111) surface is oxidized with gas-phase oxygen atoms under vacuum condition to achieve an oxidation level beyond its saturation coverage for chemisorption. Two surface oxides, rutile IrO₂ of (100) domain and corundum Ir₂O₃ of (001) domain, have been grown at 550 K with different oxygen exposure of 3.6 × 10⁵ L and 7.2 × 10⁵ L respectively. The temperature programmed desorption (TPD) experiment of rutile IrO₂(100) shows its desorption curve (at 4 K s^? 1) peaks at 750 K, followed by a long tail of less pronounced desorption features. On the other hand, TPD of corundum Ir₂O₃(001) displays a symmetric trace, peaking at 880 K. Carbon monoxide titration experiments show that adsorbed CO reduces corundum Ir₂O₃(001) at 400 K, but CO does not adsorb on rutile IrO₂(100) and no reduction reaction occurs. Evidently, among the two surface oxides, corundum Ir₂O₃(001) involves in catalysis of carbon monoxide oxidation, while rutile IrO₂(100) does not. The formation of two surface oxides is also compared, we conclude that the atom arrangement favors Ir₂O₃(001) at the oxide/metal interface.     

The surface chemistry of methyl and methylene radicals adsorbed on Cu(111)

The interactions of methyl and methylene radicals on Cu(111) were investigated with XPS, AES and HREELS under various exposure conditions. The CH₂ and CH₃ radicals are generated through a hot nozzle source with ketene and azomethane gases. It is shown that with substrate at 300 K, the impinging CH₃ radicals are trapped mainly as CH₃(ads), while a part of the adsorbate decomposes to form CH₂(ads) and H(ads). H atoms are found to desorb at about 380 K, while the chemisorbed hydrocarbon adspecies desorb at about 420 K. In drastic contrast, exposing the clean Cu surface to methylene radicals results not only in the trapping of CH₂(ads), but also in the formation of complex aromatic species. The adlayer is sensitive to annealing at elevated temperatures. Desorption and partial conversion to methylidyne take place at around 420 K. The CH(ads) species can survive up to 700 K and then decomposes to form residual carbon above 800 K. In both radical-Cu(111) systems, surface coverage appears to saturate near one monolayer. The relative concentrations of different surface species in the adlayer, however, depend on the amount of radical exposure. The reaction properties of the two systems are compared and discussed.     

CO adsorption on clean and oxidized Pt3Ti(111)

CO adsorption on clean and oxidized Pt₃Ti(111) surfaces has been investigated by means of Auger Electron Spectroscopy (AES), Thermal Desorption Spectroscopy (TDS), Low Energy Electron Diffraction (LEED) and High Resolution Electron Energy Loss Spectroscopy (HREELS). On clean Pt₃Ti(111) the LEED patterns after CO adsorption exhibit either a diffuse or a sharp c(4 × 2) structure (stable up to 300 K) depending on the adsorption temperature. Remarkably, the adsorption/desorption behavior of CO on clean Pt₃Ti(111) is similar to that on Pt(111) except that partial CO decomposition on Ti sites and partial CO oxidation have also been evidenced. Therefore, the clean surface cannot be terminated by a pure Pt plane. Partially oxidized Pt₃Ti(111) surfaces (< 135 L O₂ exposure at 1000 K) exhibit a CO adsorption/desorption behavior rather similar to that of the clean surface, showing again a c(4 × 2) structure (stable up to 250 K). Only the oxidation of CO is not detectable any more. These results indicate that some areas of the substrate remain non-oxidized upon low oxygen exposures. Heavily oxidized Pt₃Ti(111) surfaces (> 220 L O₂ exposure at 1000 K) allow no CO adsorption indicating that the titanium oxide film prepared under these conditions is completely closed.     

Theoretical studies on chemisorption of oxygen on β-Mo2C catalyst and its surface oxidation

Oxygen chemisorption on β-Mo₂C surface and its oxidation have been investigated by using the density functional theory with the periodic models. Two surfaces of (011) and (101) were chosen to perform the calculations and the most stable surface structures together with the energetics of oxygen stepwise adsorption were identified. Thermodynamic effect of temperature and reactant pressure on the chemisorption and surface oxidation was investigated. The results suggest that the (101) surface is more active than the (011) surface towards the oxygen adsorption. The (101) surface can be fully oxidized by O₂ at P_O2/P⁰ of 10^? 21–10⁴ and temperature of 100–700 K. For the (011) surface with O₂ as the oxidant, the most stable structure is that with 1/2 ML or 7/8 ML oxygen coverage, depending on the temperature and P_O2/P⁰ value. The increase of gaseous oxidant pressure or decrease of temperature can enhance the oxidation of β-Mo₂C surface and lead a more negative reaction Gibbs free energy. High temperature and low oxidant pressure may hinder the surface oxidation process.     

Surface structure of γ-Fe2O3(111)

The surface structure of γ-Fe₂O₃(111) has been investigated with a range of surface techniques. Two different surface structures were discovered depending upon surface preparation techniques. Sputtering followed by annealing in vacuum produced a reduced surface characterised by a (2 × 2) LEED pattern, whereas sputtering followed by annealing in 1 × 10^? 6 mbar oxygen produced a surface characterised by a (√3 × √3)-R30° LEED pattern. The latter appears to be a very low conductivity surface, whereas the former has the band gap expected for maghemite (~ 2.0 eV). We propose that the reduced surface is a magnetite-like layer, whereas the oxidised surface is an Fe₂O₃-like layer.     

In-situ surface-sensitive X-ray diffraction study on the influence of iodide over the selective electrochemical etching of Cu3Au (111)

During selective etching (dealloying) surface-sensitive X-ray diffraction employing Synchrotron light has been used to in-situ monitor the potential-controlled formation of Au-rich films on the surface of Cu₃Au (111) in iodide-containing electrolytes. Similar to the case in pure sulfuric acid we observed a sequence of structural transformations starting from a well-prepared pristine surface to a porous film consisting of substrate-oriented Au ligaments. Also stacking-reversed ultrathin Au-rich films and Au islands form as intermediate steps but no passive-like behavior was observed in iodide-containing electrolytes, i.e. the surface quickly developed Au ligaments after reaching the Cu dissolution potential. At low overpotentials comparatively coarse Au islands point to a higher mobility of Au/electrolyte interfaces in iodide-containing solutions. At higher overpotentials and also with higher iodide concentrations an epitaxial Cu-iodide precipitate film showed an orientation relation of CuI (111) || CuAu (111) and two azimuthal domains of < ? 2, 2, 0 > || < ? 2, 2, 0 > and < ? 2, 2, 0 > || < 2, ? 2, 0>. This partially dissolution-inhibiting bulk CuI layer is observed to produce a bimodal pore size instead of usually obtained homogeneous porosity. The X-ray data and supporting ex-situ AFM and SEM images show marked differences in the morphology and connectivity of the forming nanoporous Au layer. Precipitation layers are thus suggested to provide means for controlling the nanoporosity for applications of dealloyed films and surfaces.     

Thermal stability of the Co/β-Si3N4/Si(111) interface: A photoemission study

The thermal stability of the Co/β-Si₃N₄/Si(111) interface has been studied by high-resolution photoemission spectroscopy in a temperature range extending from room temperature to 650 °C. It is demonstrated the ability of a very thin crystalline buffer layer of silicon nitride to prevent the interfacial reaction between cobalt and silicon at room temperature. The behaviour of the interface at higher temperature shows the formation of cobalt silicides already at 300 °C. Moreover, the presence of new components in the decomposition of the photoemission spectra is discussed in the light of the existing literature.     

Oxygen nonstoichiometry,mixed valency and mixed conduction in (La,Sr)(Co,Fe)O3−δ

Defect chemistry for a mixed conductor, La_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ was studied. Samples were treated under controlled oxygen partial pressure, P(O₂), conditions at 1273 K [10^− 11.1 ≤ P(O₂)/atm ≤ 1], and cooled to room temperature. Oxygen nonstoichiometry and valences of transition metal ions for the treated samples were evaluated by iodometry and X-ray absorption spectroscopy, respectively. With decreasing P(O₂), preferential reduction of Co³⁺ to Co²⁺ was observed, while iron preserved its higher valence above 3 under conditions studied. A dependency of its electrical conductivity on P(O₂) was discussed along with a change in concentration of oxygen vacancies and mixed valences.     

Crystal structure and the structural disorder of ceria from 40 to 1497 °C

Nuclear-density distribution of ceria (CeO₂, space group: Fm3¯m) has been investigated between 40 and 1497 °C by the maximum-entropy method (MEM) and MEM-based pattern fitting combined with the Rietveld method using neutron powder diffraction data. The refined unit-cell and atomic displacement parameters increased with an increase of temperature. The results of the MEM analysis reveal that the oxide ions have a positional disorder spreading over a wide area and shift to the <111> directions from the ideal position 8c 1 / 4,1 / 4,1 / 4 of the fluorite-type structure. The disorder of the oxide ions is more significant at higher temperatures and suggests the diffusion paths in the <111> and <100> directions.