Desorption of H2 from W(110) and Fe covered W(110) surfaces

The kinetics of H₂ desorption from H/W(110) and H/Fe₁/W(110) were studied by measuring work function changes Δø vs time at a number of temperatures. Combination with previously determined Δø vs coverage data and differentiation at various fixed coverages gave rate vs T data from which activation energies of desorption could be obtained. E vs coverage results agree well with previously determine ΔH_des results. In the case of H/Fe₁/W(110) this includes a rise from 20 to 30 kcal mol⁻¹ of H₂ at H/Fe = H/W > 0.3. Plots of rate −dθ/dt vs θ (θ being coverage in units of H/W) vary much more steeply than θ² at most coverages for both systems. The θ dependence can be explained almost quantitatively in terms of the variations of ΔH_des and surface entropy S_s with coverage, by assuming that rates of desorption are equal to the equilibrium rates of adsorption. The latter can be formulated thermodynamically, except for a sticking coefficient, s. Values for s(θ, T) can also be obtained and show relatively little temperature dependence.     

The application of ultrasound radiation to the synthesis of nanocrystalline metal oxide in a non-aqueous solvent

Highly crystalline metal oxide nanoparticles of TiO₂, WO₃, and V₂O₅ were synthesized in just a few minutes by reacting transition metal chloride with benzyl alcohol using ultrasonic irradiation under argon atmosphere in a non-aqueous solvent. The sonochemical process was conducted at a relatively low temperature, 363 K. A unique crystallization process of these nanoparticles has been observed and characterized by powder X-ray diffraction (PXRD), high resolution scanning electron microscopy (HRSEM), and BET. The particles’ size and shape measured from HRSEM reveal “quasi” zero-dimensional, spherical TiO₂ particles in the range of 3–7 nm. The V₂O₅ particles have a “quasi” one-dimensional ellipsoidal morphology, with lengths in the range of 150–200 nm and widths varying between 40 and 60 nm. The WO₃ particles were obtained as “quasi” two-dimensional platelets with square shapes having facets ranging from 30 to 50 nm. The thickness of these platelets was between 2 and 7 nm. The mechanism of the reactions leading to these three metal oxide nanoparticles in a non-aqueous system is substantiated by Nuclear Magnetic Resonance (NMR), and Electron Spin Resonance (ESR).     

Valence band and surface core-level shift studies of the oxygen-induced p(3 × 1) reconstruction of the W(1 0 0) surface

Using synchrotron radiation, W 4f_7/2 and valence-band (VB) photoelectron spectroscopy was used to study the oxidation states of the p(3 × 1) reconstructed surface of W(1 0 0) produced by oxygen adsorption at 1500 K. The W 4f_7/2 study showed two features at 0.40 and 1.50 eV higher binding energies relative to the bulk feature which represent shifts to lower binding energies compared to the W 4f_7/2 features of the as-grown oxygen films on W(1 0 0). Co-existence of these features is a characteristic of the formation of the p(3 × 1) structure. The VB studies of this system and as-grown oxygen films at room temperature (RT) showed an oxygen-induced feature for the p(3 × 1) structure for which the peak maximum had shifted by 0.9 eV to higher binding energies compared to the as-grown films. The direction of shifts in the W 4f_7/2 and VB spectra for the p(3 × 1) structure confirmed the reduction of the charge transfer from W to oxygen compared to the as-grown films. A study of the VB changes with photon energy shows that the main component of the oxygen-induced feature in the p(3 × 1) VB at 6.6 ± 0.2 eV is related to a hybridized state between W 5d and O 2p electrons.     

Sonochemical synthesis of nanocrystalline mercury sulfide, selenide and telluride in aqueous solutions

Nanocrystalline mercury chalcogenides HgE (E = S, Se, Te) were synthesized in a single step by a convenient, simple sonochemical method. Mercury nitrate, Hg(NO₃)₂, dissolved in 0.1 M ethylenediamine tetraacetic acid (EDTA), was used as the source of mercury and elemental chalcogenes, dissolved in a NaOH solution, as the sources of chalcogenide. The products were characterized by X-ray powder diffraction (XRD), transmission electron microscopy (TEM) and energy-dispersive X-ray spectroscopy (EDAX). The synthesis procedure is simple and uses less toxic reagents than the previously reported methods. The results showed that the complexing agent EDTA plays a crucial role in the process.     

Activation mechanisms of silver toward CO oxidation by tungsten carbide substrate: A density functional theory study

The development of efficient catalysts for low temperature CO oxidation is important to the application of fuel cells. In this work, we report that the Ag monolayer on WC (0001) surface (Ag_ML/WC) could effectively catalyze CO oxidation through the L-H mechanism (CO + O₂ → OOCO → CO₂ + O). The most sluggish reaction step is suggested to be CO + O → CO₂ with a barrier of 0.48 eV, which is 1.21 eV lower than the barrier of O₂ dissociation. The electronic structure and d-band center analyses demonstrate that the promoted activity may originate from the synergistic effect between Ag monolayer and WC. The present study is conducive to design new efficient and cost-effective catalysts for CO oxidation without using of the noble platinum.     

Surface diffusion of dysprosium on the W(1 1 1) facet

Diffusion of dysprosium on the (1 1 1) facet of a tungsten micromonocrystal was investigated by means of spectral analysis of field emission current fluctuations. The experimental spectral density functions of the current fluctuations were analysed by using Gesley and Swanson’s theoretical spectral density function, which enables to determine the surface diffusion coefficient D for dysprosium. Derived from the temperature dependence of D, the diffusion activation energy E is presented for some Dy coverages θ_(1 1 1). In the temperature range 400–600 K, the E first drops from 1.25 eV per atom at θ₍₁₁₁₎≈0.25 ML to 0.48 eV per atom at θ₍₁₁₁₎≈1 ML (corresponding to the minimum of the work function of the system), then increases to 1.03 eV per atom at θ₍₁₁₁₎≈1.3 ML. The results are discussed from the aspects of the substrate structure and interaction in the adsorbed layer.     

Dynamic study of W atoms and clusters on W (1 1 1) surfaces

Using a field ion microscope, the diffusion behaviors and atomic processes of W atoms and clusters on W (1 1 1) surfaces were observed directly. The activation energy of W clusters diffusion on W (1 1 1) as a function of cluster size has an oscillatory and increasing behavior. But, the activation energy of a single W atom is especially high. The compact geometric structures are more stable and have higher activation energies of surface diffusion than structures with extra atoms at the periphery. Besides the terrace diffusion, other atomic processes such as the ascending, descending, detachment motion on W (1 1 1) surfaces were also observed. Unlike the general systems, their occurrence temperatures are quite near. These experimental results were used to discuss the formation mechanism of single atom W tips.     

Surface vibrations of CO on W(100)

High resolution electron-energy-loss spectroscopy has been used to study the surface vibrations of CO on a W(100) surface at 300 K. For small exposures (β-CO) two losses at ～68 meV and ～78 meV are observed. This vibrational spectrum of β-CO is a clear indication of dissociative adsorption with the carbon and oxygen atoms in fourfold coordination sites each. With further exposure to CO two additional losses at 45 meV and 258 meV are observed, which represent the vibration of undissociated α-CO in upright position on top of a W atom. Furtheron results of coadsorption of H₂/CO and O₂/CO on W(100) are reported.     

The sticking and dissociation of NH3 on W(110): a three-state model

The kinetics of the adsorption of NH₃ on W(110) and its subsequent dissociation have been investigated using molecular beam techniques and temperature programmed desorption (TPD) for surface temperatures ranging from 140 to 700 K. NH₃ shows a wide desorption peak around 270 K and a smaller peak at 170 K while H₂ and N₂, produced by dissociation, desorbed at 550 and 1350 K, respectively, with kinetic parameters similar to those reported for H and N generated by adsorption of H₂ and N₂. At normal incidence and for a surface temperature of 140 K, the NH₃ sticking coefficient was found to decrease from unity at a beam energy of 0.8 kcal/mol to 0.5 for a beam energy of 5.4 kcal/mol. The sticking coefficient generally decreases with surface temperature to a value of 0.05 at 700 K, but, for a 5.4 kcal/mol beam, it exhibits a relative minimum near 300 K. The reflection coefficient of NH₃, for an angle of incidence of 49°, increases with temperature and incident beam energy in agreement with the sticking measurements. The TPD peak positions, sticking and reflection data are all well reproduced by a three-state model based on simple kinetics. The model assumes that NH₃ initially traps in a molecular state and that dissociation occurs by thermal activation into an intermediate state. At no temperature is the sticking probability enhanced by increasing the kinetic energy of the incident molecules and there is no evidence for a direct dissociation channel which has a translational energy barrier less than 5.4 kcal/mol.     

Atomic structures on faceted W(111) surfaces induced by ultrathin films of Pd

The faceting of Pd/W(111) surfaces has been studied using a Scanning tunneling microscope (STM). Three-sided pyramidal facets having {211} faces with dimensions ranging from 3 to 15 nm can be induced by ultrathin Pd films (≥ 1 monolayer), upon annealing to 700 K or higher. From atomic-resolution STM-images of these surfaces, we obtain direct confirmation of the {211} structure on individual facets of the 3-sided pyramids. In addition, the atomic structure of the facet edges indicates that edge energy may play a role in faceting. When the as-deposited coverage of Pd is greater than the critical value ( 1 monolayer) for inducing faceting, the extra Pd atoms diffuse to form 3-dimensional clusters, some with discernible crystalline structures, upon annealing.     

Quantum-well states in bilayers of Ag and Au on W(1 1 0)

sp-Like quantum-well states (QWS) in thin monocrystalline bilayer films of Ag and Au on W(1 1 0) and of single Ag films were studied by angle-resolved photoemission. We find that the propagation of the electronic states in the bilayer films along [1 1 1] depends on the energy relative to the band edge of Au metal at the L point of the Brillouin zone. In particular, QWS with binding energies less than this band-edge energy (1.1 eV) are strongly confined to the Ag layer, while for higher binding energies the QWS extend across the whole bilayer film. This clearly demonstrates the weakness of the potential barrier at the Ag/Au interface in the context of QWS formation at energies where electronic states exist in both metals.     

Sonochemical water splitting in the presence of powdered metal oxides

Kinetics of hydrogen formation was explored as a new chemical dosimeter allowing probing the sonochemical activity of argon-saturated water in the presence of micro- and nano-sized metal oxide particles exhibiting catalytic properties (ThO₂, ZrO₂, and TiO₂). It was shown that the conventional sonochemical dosimeter based on H₂O₂ formation is hardly applicable in such systems due to catalytic degradation of H₂O₂ at oxide surface. The study of H₂ generation revealed that at low-frequency ultrasound (20 kHz) the sonochemical water splitting is greatly improved for all studied metal oxides. The highest efficiency is observed for relatively large micrometric particles of ThO₂ which is assigned to ultrasonically-driven particle fragmentation accompanied by mechanochemical water molecule splitting. The nanosized metal oxides do not exhibit particle size reduction under ultrasonic treatment but nevertheless yield higher quantities of H₂. The enhancement of sonochemical water splitting in this case is most probably resulting from better bubble nucleation in heterogeneous systems. At high-frequency ultrasound (362 kHz), the effect of metal oxide particles results in a combination of nucleation and ultrasound attenuation. In contrast to 20 kHz, micrometric particles slowdown the sonolysis of water at 362 kHz due to stronger attenuation of ultrasonic waves while smaller particles show a relatively weak and various directional effects.     

Photoemission from the valence bands of Ce(1 1 1) on W(1 1 0)

We report a photoelectron spectroscopic study of the valence bands of epitaxial Ce(1 1 1) films grown on W(1 1 0) at room temperature. The evolution of γ → α → γ like phase transition of Ce is observed with increasing Ce coverage and the valence-band structures of γ-like Ce film are determined. The 4f and 5d photoemission cross sections in the photon energy region from 20 eV to 130 eV are presented and discussed. A 5d-like surface state and a 6s band bottom are identified.     

Cathodic needle growth from Mo(CO)6 and Cr(CO)6 vapors at lower electric fields

A cathodic needle growth which is possibly associated with the temperature-field (T-F) electron emission is described. Experimental data disclosed that densely populated needle crystals exhibiting a dendritic configuration growth at the tip area of a pointed cathode when it is operated in an Mo(CO)₆ or Cr(CO)₆ atmosphere at a field strength just insufficient to draw field electrons. The needle growth occurs always at elevated temperatures of ∼600–∼700 K, indicating that it is triggered by T-F electrons emitted from the cathode tip. Needle crystals produced are not single crystals but composed of linearly packed micro-crystals, and those crystals obtainable from Mo(CO)₆ are a face-centered cubic phase of Mo₂C. Needles arising from Cr(CO)₆ have not been identified, but they are believed to be an unknown phase of chromium carbide.     

On the preparation and electronic properties of clean W(1 1 0) surfaces

We have studied the influence of oxygen pressure during the cyclic annealing used for the cleaning of W(1 1 0) surfaces. For this purpose the surface morphology and electronic properties are measured by means of scanning tunneling microscopy (STM) and spectroscopy (STS), respectively. It is found that the surfaces with impurity atom densities as low as 2 × 10⁻³ can be obtained by gradually reducing the oxygen pressure between subsequent annealing cycles down to about 2 × 10⁻⁸ mbar in the final cycle. Only on the clean surface a bias-dependent spatial modulation of the local density of states (LDOS) is observed at step edges and around impurity sites by STS. In addition, we find a pronounced peak in the occupied states. In combination with density functional theory calculations these features can be traced back to a dispersive p_z-d_xz-type surface resonance band and the lower band edge of a surface state, respectively.     

Self-organized Fe nanostructures on W(1 1 0)

The magnetic domain structure of Fe wires, ribbons and islands with different shapes that have been prepared under ultra high vacuum conditions on W(1 1 0) are studied with spin polarized low energy electron microscopy. The dimensions of the nanostructures, which are of the order of tens and hundreds of nm, can be controlled by the temperature at which they are produced, by the average Fe coverage and by the substrate morphology. The domain structure of the nanostructures depends on their shape and is determined by the competition between magnetoelastic, shape and magnetocrystalline anisotropies.     

Oxygen-induced p(3 × 1) reconstruction of the W(1 0 0) surface

The oxidation of the W(1 0 0) surface at elevated temperatures has been studied using room temperature STM and LEED. High exposure of the clean surface to O₂ at 1500 K followed by flash-annealing to 2300 K in UHV results in the formation of a novel p(3 × 1) reconstruction, which is imaged by STM as a missing-row structure on the surface. Upon further annealing in UHV, this surface develops a floreted LEED pattern characteristic of twinned microdomains of monoclinic WO_x, while maintaining the p(3 × 1) missing-row structure. Atomically resolved STM images of this surface show a complex domain structure with single and double W〈0 1 0〉 rows coexisting on the surface in different domains.     

在Si(111)上用有机溶胶凝胶甩膜热解法制备(0001)定向的6H-SiC薄膜

在Si(111)衬底上用聚苯乙烯溶胶凝胶甩膜并经950℃真空(10^-3Pa)热解处理法,制备出晶态SiC薄膜.用FTIR,XRD,TEM,RamanXPS等方法研究了SiC薄膜的晶体结构、微结构、组成以及各元素的化学态等性质.结果表明制得的是沿(0001)高度择优取向的晶态6H-SiC薄膜.膜中SiC晶粒沿c轴柱状生长,其最大尺寸约150nm,膜厚约为0.3μm,SiC中的Si/C比约为1.表层有少许污染C(CH和CO)和少量O(Si₂O₃,CO态氧和吸附氧).从对比实验可知,在热解时将甩膜的Si片与另一空白Si片面面相贴可明显增加SiC的生成量. 关键词： 碳化硅 薄膜 溶胶凝胶