Transition Metal Carbide Surfaces Research Articles

Electronic structure of monolayer graphite on some transition metal carbide surfaces

It has been revealed that monolayer graphite on the (111) surfaces of transition-metal carbides differs substantially from bulk graphite in the lattice constant and the phonon dispersion, while those of monolayer graphite on the (100) surfaces are hardly modified. In this work, the electronic structure of monolayer graphite formed on TiC(111), (100), and TaC(100) surfaces has been investigated by XPS, UPS, and EELS. From the results of XPS and UPS, we have concluded that these large changes are caused not by charge transfer but mainly by orbital hybridization between the graphite overlayer and the (111) surface. This is in contrast to the case of graphite intercalation compounds. By angle-resolved EELS, in addition, we have clearly detected a two-dimensional character of the plasmons in monolayer graphite.

Synthesis, characterization, and catalytic properties of clean and oxygen-modified tungsten carbides

High surface area WC and β-W 2C powders (30–100 m 2g −1) were prepared by direct isothermal carburization of WO 3 and W 2N in CH 4-H 2 mixtures. After surface cleaning with H 2, their surfaces are equilibrated with bulk stoichiometric carbides and free of polymeric carbon; they chemisorb 0.2–0.4 monolayers of CO and H. These carbides catalyze neopentane hydrogenolysis with high selectivity. Chemisorbed oxygen also inhibits hydrogenolysis rates and introduces surface sites for neopentane isomerization, a reaction that occurs only on Pt, Ir, and Au metals. Chemisorbed oxygen also inhibits hydrogenolysis of n-hexane and n-heptane on tungsten carbides and introduces surface sites that lead to high isomerization selectivity (70–99%). Kinetic and isotopic tracer studies of n-heptane, 3,3 dimethylpentane, methylcyclohexane, propylene, and methanol reactions show that dehydrogenation reactions and methyl-shifts of unsaturated intermediates occur on oxygen-modified WC powders. Carbidic sites (WC x) catalyze C-H activation reactions; chemisorbed oxygen titrates such WC x sites and introduces Bronsted acid surface sites (WO x). Thus, these materials catalyze both dehydrogenation and carbenium-ion reactions, reflecting the bifunctional nature of oxygen-modified transition metal carbide surfaces.

Monolayer Graphite on Transition-metal Carbides and Application to Field Emitter

Monolayer graphite can be grown epitaxially on transition metal carbide surfaces with very easy procedure. Surface periodicity, phonon dispersions, electron energy dispersions, STM image and so on were measured. Monolayer graphite has similar properties to pristine graphite, although the interaction between the graphite and the substrate is so large that it affects also intra-layer π bond. The surface becomes inert and stable once monolayer graphite is formed. In case transition metal carbides are applied to field emitter, the formation of monolayer graphite dramatically improves the current stability.

Erratum: Ab initio pseudopotential study of structural and high-pressure properties of SiC

Erratum: Ab initio pseudopotential study of structural and high-pressure properties of SiC

SURFACE PHONONS OF THE (100) SURFACE OF SOME COMPOUNDS WITH A ROCK-SALT CRYSTAL STRUCTURE

The energy dispersion curves of surface phonons of the (100) surface of some compounds with a rock-salt crystal structure (TiC, ZrC, HfC, NbC, TaC, MgO, and NiO) have been investigated using electron energy loss spectroscopy. Drastic changes in force constants on all the carbide surfaces are correlated with a rippled structure. On the other hand, the surface phonons of insulating compounds, NiO and MgO (100), have showed dispersion curves agreeing with the predicted ones calculated on the basis of a shell model with bulk dynamical parameters. No changes in the parameters near the NiO and MgO (100) surfaces have been detected. The shielding effect of free electrons on the surface phonon for the transition metal carbide surfaces is discussed.

Bond softening in monolayer graphite formed on transition-metal carbide surfaces.

Full phonon-dispersion curves of the graphitic layer on some transition-metal carbides were measured by using high-resolution electron-energy-loss spectroscopy. The graphitic layer on (111) surfaces of TaC, HfC, and TiC showed anomalous softening within the layer, whereas the graphite layer on TaC(001) is similar to bulk graphite. The measured phonon dispersion has been analyzed with a force-constant model, and it has been revealed that the force constants for vertical angle bending and for bond twisting are much weaker in the former case. This is ascribed to the charge transfer into the overlayer from the substrate, resulting in the weakening of the \ensuremath{\pi} bond in graphite. The clear contrast in variations of phonons between the graphitic layer on TaC(111) and that on TaC(001) indicates that the microscopic structure at the interface determines the charge transfer.

Phonon dispersion in monolayer graphite formed on Ni(111) and Ni(001)

The phonon dispersion relations of monolayer graphite on Ni(111) and on Ni(001) were measured by using electron energy loss spectroscopy. Both samples gave almost the same results. The data were analysed with a force constant model in a slab geometry, and it was revealed that the vertical bond bending force constant and the bond twisting force constant were greatly softened. These force-constant changes are comparable with the previously reported case of monolayer graphite on (111) surfaces of transition-metal carbides. However, the interaction between the Ni substrate and the graphite overlayer is not so strong as that on the carbide (111) surface. The similar phonon structures between graphitic layers on Ni(111) and on Ni(001) suggest that both substrates have similar charge transfer capability.

Effects of chemical bonding on the electronic transition in low energy He + scattering

Scattering of low energy (10−300 eV) He + ions from surfaces of transition metal carbides is studied. It will be demonstrated that the electronic transition in He + scattering from TaC is strongly dependent on the chemical state of the target atoms: the He + spectra from Ta at TaC(111) exhibit inelastic scattering due to excitation of the C 2p electron above a threshold energy as small as 30 eV where neutralization induced by the C 2p electron occurs simultaneously. The neutralization probability increases with increasing the number of the coordinating carbon atoms. These phenomena are not attributable to the electronic transition from the delocalized broad band but caused by the local electronic transition mediated by a molecular state of a ( He + Ta) + complex formed during collision. The electronic transition is such that the C 2p electron is excited due to interaction with the promoted ( He + Ta)+ state or that the electron is resonantly transferred to the promoted molecular state.

(1×1) rippled relaxation of (100) transition-metal carbide surfaces

An investigation of (100) surfaces of TaC and HfC was performed using low-energy electron diffraction (LEED) and angle-resolved ultraviolet photoemission spectroscopy with synchrotron radiation. Detailed LEED I–V analyses show that, on both surfaces, carbon atoms displace outward and metal atoms inward, giving rise to (1×1) rippled relaxations, with the magnitude of the ripple being larger on TaC(100). Photoemission spectra of metal 4f levels from these surfaces show no obvious surface core-level shifts. Using x-ray photoemission spectroscopy (XPS) and Auger electron spectroscopy (AES), changes in (bulk) core-level binding energies, valence-band structures, and local carbon environments were determined for TaCx (0.5<x<1.0) and HfCx (0.6<x<1.0). From these bulk studies, it was inferred that, as x decreases, added negative charge accumulates in the vicinity of metal atoms while some charge transfers away from remaining carbon atoms. Insight obtained from the XPS and AES investigations is used in this paper to discuss the charge distribution at the (100) surfaces of TaC and HfC. It is concluded that carbon atoms in the outermost layer move outward to allow their compressed s,p electrons to expand to a state of lower kinetic energy, and metal atoms move inward because of added charge accumulating in their vicinity. We propose that the larger ripple on TaC(100) results form a larger accumulation of charge near the metal atoms on that surface.