The activation of FeTi for hydrogen absorption

FeTi is an interesting hydrogen storage material which has to be activated at ≈670 K for the absorption of hydrogen. We review critically the great number of previously published results and models on this activation process and emphasize the controversial points. To eliminate the controversy we analysed the variation of the surface composition of FeTi upon activation in the high-pressure cell of a photoelectron spectrometer. The initially passivating surface oxide is shown to be converted into a mixture of TiO₂ and Fe by surface segregation and chemical reduction. No evidence for the formation of Fe₂Ti₄O _x , FeTiO _x , and TiH _x is found. H₂/D₂ exchange reactions show that H₂ dissociates rapidly on Fe and FeTi, but not on TiO₂. The surface of FeTi is activated easily at 670 K. Difficulties encountered with the initial hydrogen absorption by virgin high purity FeTi are probably related to bulk (H diffusion, fracture toughness) rather than surface properties.     

Chemically shifted surface core-levels and surface segregation in EuAu and YbAu alloys

Chemically shifted surface core-level binding energies are observed for the rare-earth alloy component in Eu_1?xAu_x and Yb_1?xAu_x. Furthermore, these shifts are different from the chemical shifts of the bulk 4f levels. The surface core-levels are used to identify surface segregation of the lanthanide metal in the present alloys.     

Photon-induced chemical vapor deposition of molybdenum on Pt(1 1 1)

Temperature-programmed desorption (TPD) and X-ray photoelectron spectroscopy (XPS) have been employed to study the adsorption and photon-induced decomposition of Mo(CO)₆. Mo(CO)₆ adsorbs molecularly on a Pt(1 1 1) surface with weak interaction at 100 K and desorbs intact at 210 K without undergoing thermal decomposition. Adsorbed Mo(CO)₆ undergoes decarbonylation to form surface Mo(CO)_x (x ? 5) under irradiation of ultraviolet light. The Mo(CO)_x species can release further CO ligands to form Mo adatoms with CO desorption at 285 K. In addition, a fraction of the released CO ligands transfers onto the Pt surface and subsequently desorbs at 350-550 K. The resulting Mo layer deposited on the Pt surface is nearly free of contamination by C and O. The deposited Mo adatoms can diffuse into the bulk Pt at temperatures above 1070 K.     

Segregation at the surfaces of CuxPd1 − x alloys in the presence of adsorbed S

The influence of adsorbed S on surface segregation in Cu_xPd_1 ? x alloys (S/Cu_xPd_1 ? x) was characterized over a wide range of bulk alloy compositions (x = 0.05 to 0.95) using high-throughput Composition Spread Alloy Film (CSAF) sample libraries. Top-surface and near-surface compositions of the CSAFs were measured as functions of bulk Cu composition, x, and temperature using spatially resolved low energy ion scattering spectroscopy (LEISS) and X-ray photoemission spectroscopy (XPS). Preferential segregation of Cu to the top-surface of the S/Cu_xPd_1 ? x CSAF was observed at all bulk compositions, x, but the extent of Cu segregation to the S/Cu_xPd_1 ? x surface was lower than the Cu segregation to the surface of a clean Cu_xPd_1 ? x CSAF, clear evidence of an S-induced “segregation reversal.” The Langmuir–McLean formulation of the Gibbs isotherm was used to estimate the enthalpy and entropy of Cu segregation to the top-surface, ΔH_seg(x) and ΔS_seg(x), at saturation sulfur coverages. While Cu segregation to the top-surface of the clean Cu_xPd_1 ? x is exothermic (ΔH_seg < 0) for all bulk Cu compositions, it is endothermic (ΔH_seg > 0) for S/Cu_xPd_1 ? x. Segregation to the S/Cu_xPd_1 ? x surface is driven by entropy. Changes in segregation patterns that occur upon adsorption of S onto Cu_xPd_1 ? x appear to be related to formation of energetically favored PdS bonds at the surface, which counterbalance the enthalpic driving forces for Cu segregation to the clean surface.     

Study of the surface segregation of carbon vacancies in TiCx

Density functional theory (DFT) calculations have been performed to study the surface segregation of carbon vacancies in TiC_x in this work. The results confirm that the vacancies in the surface have the smaller formation energy and less effect on the electronic structure of TiC_x than those in the bulk, which indicates that the carbon vacancies in TiC_x prefer to segregate on the surface. It is also found that energy barriers for the migration of the vacancies are dependent on their locations, and the deeper the location of vacancies is in the bulk, the higher is the diffusion energy barrier needed.     

Density functional theory study into the adsorption of CO2, H and CHx (x = 0-3) as well as C2H4 on α-Mo2C(0 0 0 1)

The structures and energetics of the chemisorbed CO₂, CH_x species and H as well as C₂H₄ on the α-Mo₂C(0 0 0 1) surface have been computed at the GGA-RPBE level of density functional theory. It is found that CO₂ adsorbs dissociately into CO and O, in agreement with the experimental finding. The adsorbed O, CH_x and H species prefer the site of three surface molybdenum atoms over a second layer carbon atom (V_C site). On the basis of the calculated adsorption energies of CH_x and H, the sequential dehydrogenation of CH₄ and the C/C coupling reaction of CH_x have been discussed.     

Surface segregation in CuxNi1−x alloy

The tight-bonding Hartree Hamiltonian and a mixed Bethe-lattice approximation is used to study the surface segregation phenomenon of Cu_xNi_1−x alloy. It is found that Ni atoms do segregate to the surface layer in the Cu-rich range (0.75 < x < 1). This is in contrast with previous theories which predict the enrichment of Cu atoms at the surface for all bulk Cu concentrations. However, the present theory agrees reasonably well with the recent experimental observation of a crossover at x = 0.84 by Hashizume et al. [1].     

CO和O在无序二元合金NiCu表面上的化学吸附

本文用一维紧束缚模型和单电子化学吸附理论,在平均T矩阵近似下,研究了CO和O在无序二元合金Ni_xCu_1-x表面上的化学吸附特性,结果表明,CO和O在Ni_xCu_1-x表面上的化学吸附具有类似的性质,随合金中Ni浓度的增加,化学吸附能降低,化学吸附加强,Cu在NiCu合金表面的偏析在一定程度上减弱了CO和O在NiCu表面上的吸附。 关键词：     

Spin resonances in amorphous Y-Fe alloys

We present spin resonance measurements on amorphous Y_1?xFe_x alloys using both bulk (a-YFe₂) and thin film (x = 0.70, 0.77, 0.83) samples. At 300 K, in every sample, we have found one strong line (S₁) whose frequency and angular dependances imply that it is a uniform mode ferromagnetic resonance in a very thin (≈20–40 nm) surface layer. Previous static measurements indicate that these alloys have no long range order at 300 K. The effective magnetization of the surface layer is a non-monotonic function of the Fe content. Rutherford back scattering experiments on bulk samples are used to demonstrate that there is no significant variation in composition, with depth. We show that S₁ cannot be attributed to segregation, surface contamination or crystallinity. We propose that the surface layer is another amorphous phase of the alloy. A second resonance (S₃) has been ascribed to some of the superparamagnetic spin clusters known to be present in these systems. As reported earlier, we observe other resonances whose origin is not fully understood.     

Surface segregation of ZrC from a carbide solid solution

The surface segregation of zirconium carbide from carbide solid solutions is investigated. The spontaneous surface segregation of ZrC grains from solid solutions in the pseudobinary ZrC-NbC system is revealed for the first time. It is demonstrated that the ZrC precipitation is associated with the decomposition of the car-bide homogeneous solid solutions Zr_1?x Nb_xC ≡ (ZrC)_1?x (NbC)_x. The boundaries of the latent solid-phase decomposition region formed at T<1200 K are determined for the solid solutions formed by ZrC_y and NbC _y′ carbides with different nonstoichiometry. The experimental and theoretical estimates obtained for the segregation energy of ZrC are equal to ?50 and ?31 kJ mol^?1, respectively.     

Elasticity, band structure, and piezoelectricity of BexZn1−xO alloys

Lattice constants, elasticity, band structure and piezoelectricity of hexagonal wide band gap Be_xZn_1−xO ternary alloys are calculated using first-principles methods. The alloys' lattice constants obey Vegard's law well. As Be concentration increases, the bulk modulus and Young's modulus of the alloys increase, whereas the piezoelectricity decreases. We predict that Be_xZn_1−xO/GaN/substrate (x=0.022) multilayer structure can be suitable for high-frequency surface acoustic wave device applications. Our calculated results are in good agreement with experimental data and other theoretical calculations.     

Electronic band structure and optical absorption of nanotubular zinc oxide doped with Iron, Cobalt, or Copper

The absorption spectra of the precursor-derived solid solutions Zn_{1 ? x} M _x O (M = Fe, Co, Cu) with a tubular morphology of aggregates have been investigated in the ultraviolet and visible regions. The maximum metal concentration x in the Zn_{1 ? x} M _x O solid solutions is 0.075 for iron, 0.2 for cobalt, and 0.1 for copper. It has been found that the optical absorption and the band gap of the Zn_{1 ? x} M _x O compounds depend on the type of dopant. The obtained experimental data have been interpreted using the results of the performed ab initio calculations of the electronic band structure and optical absorption.     

Optical and structural properties of Zn1−x Mg x O ceramic materials

This paper reports structural, optical and cathodoluminescence characterizations of sintered Zn_1?x Mg _x O composite materials. The effects of MgO composition on these film properties have been analyzed. X-ray diffraction (XRD) confirms that all composites are polycrystalline with prominent hexagonal wurtzite structure along two preferred orientations (002) and (101) for the crystallite growth. Above doping content x = 10 %, the formation of the hexagonal ZnMgO alloy phase and the segregation of the cubic MgO phase start. From reflectance and absorption measurements, we determined the band gap energy which tends to increase from 3.287 to 3.827 eV as the doping content increases. This widening of the optical band gap is explained by the Burstein–Moss effect which causes a significant increase of electron concentration (2.89 × 10¹⁸?5.19 × 10²⁰ cm^?3). The luminescent properties of the Zn_1?x Mg _x O pellets are studied by cathodoluminescence (CL) at room and liquid nitrogen temperatures under different electron beam excitations. At room temperature, the CL spectra of the Zn_1?x Mg _x O composites exhibit a dominant broad yellow-green light band at 2.38 eV and two ultraviolet emission peaks at 3.24 and 3.45 eV corresponding to the luminescence of the hexagonal ZnO and ZnMgO structures, respectively. For the doped ZnO samples, it reveals also new red peaks at 1.72 and 1.77 eV assigned to impurities’ emissions. However, the CL spectra recorded at 77 K show the presence of excitonic emission peaks related to recombination of free exciton (X _A), neutral donor-bound excitons (D⁰X) and their phonon replicas. The CL intensity and energy position of the green, red and ultraviolet emission peaks are found to depend strongly on the MgO doping content. The CL intensity of the UV and red emissions is more enhanced than the green light when the MgO content increases. CL imaging analysis shows that the repartition of the emitting centers in Zn_1?x Mg _x O composites is intimately connected to the film composition and surface morphology.     

Structural and magnetic studies on Zr doped ZnO diluted magnetic semiconductor

In this study, Zirconium doped Zn_1−xZr_xO (with x = 0.00–0.10) samples have been prepared by formal solid-state reaction technique. The Zr doped ZnO samples annealed at 1100 °C and characterized by different characterization techniques, such as X-ray diffraction (XRD), Scanning electron microscope (SEM), Vibrating sample magnetometer (VSM) and Fourier transform infrared spectroscopy (FTIR). The X-ray diffraction (XRD) used to study the structural properties. XRD pattern showed that lattice parameters, “a”, “c”, unit cell volume and Zn–O bond length increase with doping content (x ≤ 0.04) where as these decrease with x > 0.04. On the other hand, reverse trend observed with lattice distortion. The crystallite size decreases with increasing doping content of Zr. FTIR employed to investigate functional chemical bonding properties of different elements and compounds present in materials. The low, medium and high frequency absorption bands observed at 630, 1500 and 3435 cm⁻¹, which were the common features of Zn–O, H–O–H and O–H bond respectively. SEM used to study surface morphology and measured grain size of specimen. The surface becomes dense and grain size decreases with increasing degree of Zr contents. The SEM micrograph also shows the presence of spherical micro size particles and formation of pores in samples. Magnetic properties were obtained using VSM. The samples exhibit room temperature ferromagnetism. The magnetic hysteresis loops show variation in the value of magnetic parameter. The saturation magnetization (M_s) and coercivity (H_c) decrease, while remanence magnetization (M_r) shows gradually increasing trend with Zr content. VSM measurement reveals that sample Zn_0.96Zr_0.4O show better result as compared to x = 0.06–0.10.     

Effect of surface segregation on the sharpness of heteroboundaries in multilayered Si(Ge)/Si1−x Gex structures grown from atomic beams in vacuum

The main reasons for composition intermixing in the vicinity of heteroboundaries in an Si(Ge)/Si_1?x Ge_x heterosystem grown by the molecular beam epitaxy method are considered. The proposed model explains all the experimentally observed peculiarities, such as the clearly manifested asymmetry of the solid solution profile in layers, and demonstrates the noticeable erosion of the composition profile at the boundaries even in the absence of the surface segregation effect. It is shown that the surface segregation in the Ge/Si_1?x Ge_x system may play a positive role, leading to an increase in the profile steepness of the Si_1?x Ge_x layer near the boundaries.     

Temperature-programmed surface reaction (TPSR) of CH4 synthesis by PdxNi100−x nanoparticles

A series of Pd_xNi_100−x nanoparticles were prepared by the co-precipitation method and analyzed using a temperature-programmed surface reaction (TPSR) of their methanation reactions. ESCA measurement suggested that the as-prepared Pd-Ni alloys had Pd-core/Ni-shell structure. Surface Pd segregation occurred during H₂ reduction and resulted in a surface composition close to the nominal value. The TPSR experiments were performed by pre-adsorption of CO with H₂ to form methane. The peak temperature of methanation increased as Pd content increased, indicating that a methanation reaction is favored on Ni and Ni-rich alloy nanoparticles. For physical mixtures of Pd and Ni nanoparticles, methanation behaviors is similar to those of alloy nanoparticles; but the methanation temperatures of physical mixtures are always higher than those of alloy nanoparticles. This may be due to the formation of a Pd-enriched alloy surface layer during reduction in H₂ at 400 °C, or because the CO molecules adsorbed on the Pd sites spill over onto the Ni sites for methanation. Using TPSR technique and measuring methanation temperature, the top-most surface of such bimetallic nanoparticles can be probed.     

Ion-plasma treatment of Cd1 ? x Zn x Te (x ～ 0.04) single crystals and application of antireflection diamond-like carbon films

The influence of ion-plasma treatment of Cd_{1 ? x} Zn _x Te (x ?? 0.04) single crystals, which precedes the deposition of an antireflection coating (a diamond-like a-C: H: N film) on the optical properties of the resulting structure, is investigated. Treatment is carried out in an argon, nitrogen, and hydrogen plasma. As follows from the optical properties of the a-C: H: N/Cd_{1 ? x} Zn _x Te, the structure treated in an argon plasma offers the highest transmission. A model of the optical system is proposed that takes into account the formation of transition layers after ion-plasma treatment and the effect of the type of ion on their characteristics. The absorption spectra of the a-C: H: N/Cd_{1 ? x} Zn _x Te system are analyzed, and bands assigned to the stretching and bending vibrations of C-H bonds, as well as to C-C and C=O bonds, are revealed. The presence of these bonds should be taken into account in designing and synthesizing these optical coatings.     

In situ XPS and MS study of methanol decomposition and oxidation on Pd(111) under millibar pressure range

The methanol decomposition and oxidation on a Pd(111) single crystal have been investigated in situ using ambient-pressure X-ray photoelectron spectroscopy (XPS) and mass-spectrometry (MS) in the temperature range of 300–600 K. It was found that even in the oxygen presence the methanol decomposition on palladium proceeds through two competitive routes: fast dehydrogenation to CO and H₂, and slow decomposition of methanol via the C–O bond scission. The rate of the second route is significant even in the millibar pressure range, which leads to a blocking of the palladium surface by carbon and to a prevention of the further methanol conversion. As a result, no gas phase products of methanol decomposition were detected by mass-spectrometry at 0.1 mbar CH₃OH in the whole temperature range. The methanol C–O bond scission produces CH_x species, which fast dehydrogenate to atomic carbon even at room temperature and further partially dissolve in the palladium bulk at 400 K with the formation of the PdC_x phase. According to in situ XPS data, the PdC_x phase forms even in the oxygen excess. The application of an in situ XPS–MS technique unambiguously shows a good correlation between a decrease in the surface concentration of all carbon-containing species and the rate of methanol conversion. Since these carbon species have a high reactivity towards oxygen, heating of Pd(111) above 450 K in a methanol–oxygen mixture yields CO, CO₂, and water. The product distribution indicates that the main route of methanol conversion is the dehydrogenation of methanol to CO and hydrogen. However, under the experimental conditions used, hydrogen is completely oxidized to water, while CO is partially oxidized to CO₂. No palladium oxide was detected by XPS in these conditions.     

Structural,electronic, optical and thermodynamic properties of NaxRb1−xH and NaxK1−xH alloys

A theoretical study of the structural, electronic, optical and thermodynamic properties of Na_xRb_1?xH and Na_xK_1?xH ternary alloys in NaCl phase has been carried out using the first-principles method. We modeled the alloys at some selected compositions with ordered structures described in terms of periodically repeated supercells. The dependences on the composition of the lattice constant, band gap, dielectric constant, refractive index, Debye temperature, mixing entropy and heat capacities were analyzed for x=0, 0.25, 0.50, 0.75 and 1. The lattice constants of Na_xRb_1?xH and Na_xK_1?xH exhibit a marginal deviation from Vegard's law. A strong deviation of the bulk modulus from linear concentration dependence was observed for both alloys. We found that the composition dependence of the energy band gap is highly non linear and the large bowing coefficient for Na_xRb_1?xH is sensitive to the composition. Using the approach of Zunger and co-workers, the microscopic origins of the gap bowing were detailed and explained. The thermodynamic stability of these alloys was investigated by calculating the phase diagram. The thermal effect on some macroscopic properties was investigated using the quasi-harmonic Debye model. There is a good agreement between our results and the available experimental data for the binary compounds, which is a support for those of the ternary alloys that we report for the first time.     

Oxygen flux influence on the morphological, structural and optical properties of Zn1−xMgxO thin films grown by plasma-assisted molecular beam epitaxy

The Zn_1−xMg_xO thin films were grown on Al₂O₃ substrate with various O₂ flow rates by plasma-assisted molecular beam epitaxy (P-MBE). The growth conditions were optimized by the characterizations of morphology, structural and optical properties. The Mg content of the Zn_1−xMg_xO thin film increases monotonously with decreasing the oxygen flux. X-ray diffractometer (XRD) measurements show that all the thin films are preferred (0 0 2) orientated. By transmittance and absorption measurements, it was found that the band gap of the film decreases gradually with increasing oxygen flow rate. The surface morphology dependent on the oxygen flow rate was also studied by field emission scanning electron microscopy (FE-SEM). The surface roughness became significant with increasing oxygen flow rate, and the nanostructures were formed at the larger flow rate. The relationship between the morphology and the oxygen flow rate of Zn_1−xMg_xO films was discussed.