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.     

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.     

Comprehensive visualization of detonation-diffraction structures and sizes in unstable and stable mixtures

The geometry and characteristic length of diffraction and re-initiation during a two-dimensional detonation propagation were revealed by visualization. C₂H₄ + 3O₂ (unstable), 2C₂H₂ + 5O₂ + 7Ar (stable) and 2C₂H₂ + 5O₂ + 21Ar (stable) were used as the test mixtures. Experiments were performed over the deviation angle range from 30° to 150° and the initial pressure range from 15.8 to 102.3 kPa. By self-emitting photography, we confirmed that the geometry and the characteristic length of diffraction are not different among test gases, with the exception of the fan-like structure of re-initiation that occurred regardless of whether the mixture was unstable or stable. We conducted a compensative experiment by changing the deviation angle and initial pressure, and summarized the detonation diffraction by shadowgraph. At deviation angles larger than 60°, we measured the distances from the vertex of the channel corner to the point where the transverse detonation wave reflected on the under wall (= wall reflection distance) and confirmed that wall reflection distances are approximately in the range of 10–15 times the cell width, whether the mixture is unstable or stable.     

Identification of iron oxide phases in thin films grown on Al2O3(0 0 0 1) by Raman spectroscopy and X-ray diffraction

We report on the identification of Fe₃O₄ (magnetite) and α-Fe₂O₃ (hematite) in iron oxide thin films grown on α-Al₂O₃(0 0 0 1) by evaporation of Fe in an O₂-atmosphere with a thickness of a few unit cells. The phases were observed by Raman spectroscopy and confirmed by X-ray diffraction (XRD). Magnetite appeared independently from the substrate temperature and could not be completely removed by post-annealing in an oxygen atmosphere as observed by X-ray diffraction. In the temperature range between 400 °C and 500 °C the X-ray diffraction shows that predominantly hematite is formed, the Raman spectrum shows a mixture of magnetite and hematite. At both lower and higher substrate temperatures (300 °C and 600 °C) only magnetite was observed. After post-annealing in an O₂-atmosphere of 5 × 10^?5 mbar only hematite was detectable in the Raman spectrum.     

Field-assisted sustainable O− ion emission from fluorine-substituted 12CaO·7Al2O3 with improved thermal stability

We examined the electric field-assisted thermionic emission of atomic oxygen radical anion (O^?) in a vacuum from fluorine-substituted derivatives of 12CaO·7Al₂O₃ (C12A7) with a composition of (12 ? x)CaO·7Al₂O₃·xCaF₂ (0 ≤ x ≤ 0.8). Unsubstituted C12A7 easily decomposed into 5CaO?3Al₂O₃ (C5A3) and 3CaO?Al₂O₃ (C3A) above 830 °C during the emission experiment in a vacuum. The decomposition temperature range became narrower as the amount of F^? ion substitution increased, e.g. the sample with x = 0.4 kept a single phase after the emission experiment at 900 °C. The emitted anionic species from the x = 0.4 sample were dominated by O^? ions (～ 92%) together with a small amount of O₂^? ions (～ 4%) and F^? ions (～ 4%). The absence of an O₂ gas supply to the opposite side of the emission surface led to a nearly steady co-emission of O^? ions and electrons with a ratio of < 1/1. The O₂ gas supply markedly enhanced the O^? ion emission, and suppressed the electron emission. A sustainable and high-purity O^? ion emission with a current density of 11 nA cm^? 2 was achieved at 830 °C with the supply of 40 Pa O₂ gas. The similarity in these emission features to the unsubstituted C12A7, together with the improved thermal stability demonstrates that the F^? ion-substituted C12A7 is a promising material for higher intensity O^? ion emission at higher temperatures.     

Adsorption of O2 and CO2 on the Si(111)-7 × 7 surfaces

The interaction of O₂ and CO₂ with the Si(111)-7 × 7 surface has been studied with X-ray photoelectron spectroscopy (XPS). It was found that both O₂ and CO₂ molecules can readily oxidize the Si(111)-7 × 7 surface to form thin oxide films. Two oxygen species were identified in the oxide film: oxygen atoms binding to on-top sites of adatom/rest atoms with an O 1s binding energy of ~ 533 eV as well as to bridge sites of adatom/rest atom backbonds at ~ 532 eV. These two oxygen species can be interconverted thermally during the annealing process. Due to the low oxidation capability, the silicon oxide film formed by CO₂ has a lower O/Si ratio than that of O₂.     

Desorption of O2 from a stepped platinum(113) surface during 308 nm laser irradiation

Angular and velocity distributions of desorbing O₂ during irradiation of 308 nm laser pulses were studied on a stepped Pt(1 1 3) surface. With increases in the coverage, three desorption components collimated at around 12°, 30° and 50° successively appeared when the desorption angle was changed in a plane along the step edge. The translational temperature also showed maxima at these collimation angles, and the values were slightly lower than previous results for 193 nm irradiation. Some possible desorption mechanisms are discussed.     

Chemical etchant dependence of surface structure and morphology on GaN(0001) substrates

We studied the procedure of cleaning GaN(0001) substrate surfaces by wet etching and subsequent annealing in ultrahigh vacuum for two different types of freestanding GaN wafers: hydride vapor phase epitaxy (HVPE) crystal and Na flux liquid phase epitaxy (LPE) crystal wafers. A flat surface containing GaN(0001)2 × 2 reconstruction was successfully achieved on both HVPE and LPE surfaces by etching in HF and subsequent annealing at ～ 550 °C but was not achieved by etching in HCl, NaOH, and HNO₃.     

Preparation and characterization of iron–molybdate thin films

Mixed Fe–Mo oxides are used in industrial catalytic processes of selective oxidation of methanol to formaldehyde. For better understanding of the structure-reactivity relationships of these catalysts we aim to prepare well-ordered iron–molybdate thin films as model catalysts. Here we have studied Mo deposition onto Fe₃O₄ (111) thin films produced on Pt(111) as a function of Mo coverage and annealing temperature using LEED, AES, STM and IRAS. At low temperatures, the iron oxide film is covered by Mo = O terminated molybdena nanoparticles. Upon oxidation at elevated temperatures (T > 900 K), Mo species migrate into the film and form new bonds with oxygen in the film. The resulting films maintain the crystal structure of Fe₃O₄, and the surface undergoes a (√3 × √3)R30° reconstruction. The structure is rationalized in terms of Fe substitution by Mo in the surface layers.     

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.     

Conductivity of SnP2O7 and In-doped SnP2O7 prepared by an aqueous solution method

SnP₂O₇ and In-doped SnP₂O₇ have been prepared by an aqueous solution method using (NH₄)₂HPO₄ as phosphorous source. It was found that the solid solution limit in Sn_1 ? xIn_x(P₂O₇)_1 ? δ was at least x = 0.12. All pyrophosphates in the Sn_1 ? xIn_x(P₂O₇)_1 ? δ (x ≤ 0.12) series exhibit 3 × 3 × 3 superlattice structures. The conductivities of Sn_0.92In_0.08(P₂O₇)_1 ? δ in air are 6.5 × 10^? 6 and 8.0 × 10^? 9 S/cm at 900 and 400 °C, respectively, when prepared by an aqueous solution method and annealed at 1000 °C. The conductivity of undoped SnP₂O₇ is slightly lower. However, it was also found that the low-temperature conductivities of pyrophosphates annealed only at 650 °C are several orders of magnitude higher than those annealed at 1000 °C, which could be related to a trace amount of an amorphous secondary phase. The peak conductivity was in this case observed at around 250 °C, which is the same temperature as previously observed in In-doped SnP₂O₇ although the conductivity is still three orders of magnitude lower in the present study. These differences can be related to large differences in particle size and morphology, and all in all, the conductivities of SnP₂O₇-based materials are very sensitive to the synthetic history.     

Structural characterization of the V2O5/TiO2 system obtained by the sol–gel method

The influence of the vanadium load and calcination temperature on the structural characteristics of the V₂O₅/TiO₂ system was studied by X-ray diffraction and X-ray absorption spectroscopy (XAS) techniques. Samples of the V₂O₅/TiO₂ system were prepared by the sol–gel method under acid conditions and calcined at different temperatures. The rutile phase was found to predominate in pure TiO₂ calcined at 450 °C as a result of the reduction of phase transition temperature promoted by the sol–gel method under acid conditions. The anatase phase became predominant at 450 °C as the amount of vanadium increased from 6 to 9 wt%. A structural change in the TiO₂ phase from predominantly anatase to totally rutile with increased calcination temperature was observed in 6 wt% samples. An analysis of the vanadium X-ray Absorption Near Edge Structure (XANES) spectra showed that the oxidation state of vanadium atoms in the samples containing 6 and 9 wt% of vanadium and calcined at 450 °C was predominantly V⁴⁺. However, the presence of V⁵⁺ atoms cannot be ruled out. A qualitative analysis of extended X-ray absorption fine structure (EXAFS) spectra of the samples containing 6 and 9 wt% of vanadium calcined at 450 °C showed that the local structure around vanadium atoms is comparable to that of VO₂ crystalline phase, in which vanadium atoms are fourfold coordinated in a distorted structure. For the sample after calcination at 600 °C, the EXAFS and XANES results showed that a significant portion of vanadium atoms were incorporated in the rutile lattice with a V_xTi_(1−x)O₂ solid solution formation. The conditions of sample preparation used here to prepare V₂O₅/TiO₂ samples associated with different amounts of vanadium and calcination temperatures proved to be useful to modifying the structure of the V₂O₅/TiO₂ system.     

Effects of doping with nanoscale Co3O4 particles on the superconducting properties of powder-in-tube processed MgB2 tapes

Nanoscale Co₃O₄ particles were doped into MgB₂ tapes with the aim of developing superconducting wires with high-current-carrying capacity. Fe-sheathed MgB₂ tapes with a mono-core were prepared using the in situ powder-in-tube (PIT) process with the addition of 0.2–1.0 mol% Co₃O₄. The critical temperature decreased monotonically with an increasing amount of doped Co₃O₄ particles for all heat-treatment temperatures from 600 to 900 °C. However, the transport critical current density (J_c) at 4.2 K varied with the heat-treatment temperatures. The J_c values in magnetic fields ranging from 7 to 12 T decreased monotonically with increasing Co₃O₄ doping level for a heat-treatment temperature of 600 °C. In contrast, some improvements on the J_c values of the Co₃O₄ doped tapes were observed in the magnetic fields below 10 T for 700 and 800 °C. Furthermore, J_c values in all the fields measured increased as the Co₃O₄ doping level increase from 0 to 1 mol% for 900 °C. This heat-treatment temperature dependence of the J_c values could be explained in terms of the heat-treatment temperature dependence of the irreversibility field with Co₃O₄ doping.     

Effect of reaction temperature on the size and morphology of scorodite synthesized using ultrasound irradiation

Synthesis of scorodite (FeAsO₄·2H₂O) using dynamic action agglomeration and the oxidation effect from ultrasound irradiation was investigated. The effect of different reaction temperatures (90, 70, 50, and 30 °C) on the size and morphology of scorodite particles synthesized under O₂ gas flow and ultrasound irradiation was explored because the generation of fine bubbles depends on the solution temperature. At 90 °C, the size of scorodite particles was non-homogeneous (from fine particles (<1 μm) to large particles (>10 μm)). The oxidation–reduction potential (ORP) and yield at 90 °C showed lower values than those at 70 °C. The scorodite particles, including fine and non-homogeneous particles, were generated by a decrease in the oxidation of Fe(II) to Fe(III) and promotion of dissolution caused by the generation of radicals and jet flow from ultrasound irradiation. Using ultrasound irradiation in the synthesis of scorodite at low temperature (30 °C) resulted in the appearance of scorodite peaks in the X-ray diffraction (XRD) pattern after a reaction time of 3 h. The peaks became more intense with a reaction temperature of 50 °C and crystalline scorodite was obtained. Therefore, ultrasound irradiation can enable the synthesis of scorodite at 30 °C as well as the synthesis of large particles (>10 μm) at higher temperature. Oxide radicals and jet flow generated by ultrasound irradiation contributed significantly to the synthesis and crystal growth of scorodite.     

Synthesis and electrical transport properties of Lu2+xTi2−xO7−x/2 oxide-ion conductors

The Lu_2+xTi_2−xO_7−x/2 (x = 0; 0.052; 0.096; 0.286; 0.44; 0.63; 33.3–49 mol% Lu₂O₃) nanoceramics with partly disordered pyrochlore-type structure are prepared by sintering freeze-dried powders obtained by a co-precipitation technique with 1600 °C annealing. Similar to pyrochlore-like compositions in the zirconate system, some of the new titanates are good oxide-ion conductors in air. The new solid-state electrolytes have oxide-ion conductivity in the interval of 1.0 × 10^− 3 – 2.5 × 10^− 3 S/cm at 740 °C in air. This value of conductivity is comparable with that of ZrO₂/Y₂O₃ ceramics. The conductivity of Lu_2+xTi_2−xO_7−x/2 depends on the chemical composition. The highest ionic conductivity is exhibited by nearly stoichiometric Lu_2+xTi_2−xO_7−x/2 (x = 0.096; 35.5 mol% Lu₂O₃) material containing ∼ 4.8 at.% Lu_Ti anti-site defects.     

Effect of substrate temperature on few-layer graphene grown on Al2O3 (0 0 0 1) via direct carbon atoms deposition

Few-layer graphene (FLG) was grown on Al₂O₃ (0 0 0 1) substrates at different temperatures via direct carbon atoms deposition by using solid source molecular beam epitaxy (SSMBE) method. The structural properties were characterized by reflection high energy electron diffraction (RHEED), Raman spectroscopy and near-edge X-ray absorption fine-structure (NEXAFS). The results showed that the FLG started to form at the substrate temperature of 700 °C. When the substrate temperature increased to 1300 °C, the quality of the FLG was the best and the layer number was estimated to be less than 5. At higher substrate temperature (1400 °C or above), the crystalline quality of the FLG would be deteriorated. Our experiment results demonstrated that the substrate temperature played an important role on the FLG layer formation on Al₂O₃ (0 0 0 1) substrates and the related growth mechanism was briefly discussed.     

Electrical conductivity and thermal expansion of the oxy-cuspidine Gd4Al2O9 substituted with Ca and Sr

The series of Gd_4 ? xM_xAl₂O_9 ? x/2 (M = Ca, Sr) with x = 0, 0.01, 0.05, 0.10 and 0.25 was prepared by the citrate complexation method. Both Gd_4 ? xCa_xAl₂O_9 ? x/2 and Gd_4 ? xSr_xAl₂O_9 ? x/2 show the monoclinic cuspidine structure with space group of P2₁/c up to 0.05–0.1 and 0.01–0.05 mol for Ca and Sr, respectively. Beyond the substitution limit of Gd₄Al₂O₉, GdAlO₃ and SrGd₂Al₂O₇ appear as additional phases. The highest electrical conductivity obtained at 900 °C yielded σ = 1.49 × 10^? 4 S/cm for Gd_3.95Ca_0.05Al₂O_8.98. In comparison, the conductivity of pure Gd₄Al₂O₉ was σ = 1.73 × 10^? 5 S/cm. The conductivities determined are in a similar range as those of other cuspidine materials investigated previously. The thermal expansion coefficient of Gd₄Al₂O₉ at 1000 °C was 7.4 × 10^? 6 K^? 1. The phase transition between 1100 and 1200 °C reported earlier changes with increasing substitution of Ca and Sr.     

Oxygen permeation characteristics of zirconia with surface modification

Yttria-stabilized zirconia (YSZ) can be used as an oxygen-permeating membrane at elevated temperature (> 1400 °C) due to its chemical and mechanical stability. It was previously shown that the oxygen transport through YSZ membrane in reducing oxygen partial pressure (P_O2) was highly influenced by the surface-exchange kinetics that can be improved by porous surface coating layers such as YSZ, GDC (Gd-doped ceria) or YSZ–GDC mixture [H.J. Park, G.M. Choi, J. Eur. Ceram. Soc. 25 (2005) 2577]. However, the increased oxygen flux was still lower than that estimated assuming bulk-diffusion limit and rapidly decreased with time due to the sintering of coating layers and the reaction between bulk YSZ and coating layers. In this study, the oxygen fluxes through YSZ with LaCrO₃, GDC + LaCrO₃ (bilayer), LaCrO₃ + 5 wt.% GDC (mixture), or LaCr_0.7Co_0.3O₃ coatings were measured under controlled P_O2 gradient (permeate-side P_O2: ∼ 3 × 10^− 12 atm, feed-side P_O2: 2 × 10^− 10–2 × 10^− 8 atm) at 1600 °C. The oxygen flux drastically increased with these coatings. The highest increase in oxygen flux was shown with GDC + LaCrO₃ (bilayer) coating and was maintained for a long time. The presence of highly catalytic Ce ions while maintaining porous structure in the coating layer may explain the observation. The prevention of formation of resistive layer due to ceria coating may also be partly responsible for the observation.     

Synthesis and crystallographic studies of garnet-related lithium-ion conductors Li6CaLa2Ta2O12 and Li6BaLa2Ta2O12

High-purity specimens of Li₆CaLa₂Ta₂O₁₂ and Li₆BaLa₂Ta₂O₁₂ have been successfully synthesized by solid-state reactions. The analytical chemical compositions of these samples were in good agreement with the nominal compositions of Li₆CaLa₂Ta₂O₁₂ and Li₆BaLa₂Ta₂O₁₂. The Rietveld refinements verified that these compounds have the garnet-type framework structure with the lattice constants of a = 12.725(2) Å for Li₆CaLa₂Ta₂O₁₂ and a = 13.001(4) Å for Li₆BaLa₂Ta₂O₁₂. All of the diffraction peaks of X-ray powder diffraction patterns were well indexed on the basis of cubic symmetry with space group Ia-3d. To make a search for Li sites, the electron density distributions were precisely examined by using the maximum entropy method. Li⁺ ions occupy partially two types of crystallographic site in these compounds: (i) tetrahedral 24d sites, and (ii) distorted octahedral 96h sites, the latter of which are the vacant sites of the ideal garnet-type structure. The present Li₆CaLa₂Ta₂O₁₂ and Li₆BaLa₂Ta₂O₁₂ samples exhibit the conductivity σ = 2.2 × 10^? 6 S cm^? 1 at 27 °C (E_a = 0.50 eV) and σ = 1.3 × 10^? 5 S cm^? 1 at 25 °C (E_a = 0.44 eV), respectively.