Francesc Viñes

A systematic density functional theory study of the electronic structure of bulk and (001) surface of transition-metals carbides

A systematic study of the bulk and surface geometrical and electronic properties of a series of transition-metal carbides (TMC with TM = Ti, V, Zr, Nb, Mo, Hf, Ta, and W) by first-principles methods is presented. It is shown that in these materials the chemical bonding is strongly covalent, the cohesive energies being directly related to the bonding-antibonding gap although the shift of the center of the C(2s) band related peak in the density of states with respect to diamond indicates that some metal to carbon charge transfer does also take place. The (001) face of these metal carbides exhibits a noticeable surface rumpling which grows along the series. It is shown that neglecting surface relaxation results in very large errors on the surface energy and work function. The surface formation induces a significant shift of electronic energy levels with respect to the corresponding values in the bulk. The extent and nature of the shift can be understood from simple bonding-antibonding arguments and is enhanced by the structural rippling of this surface.

Methane Activation by Platinum: Critical Role of Edge and Corner Sites of Metal Nanoparticles

Complete dehydrogenation of methane is studied on model Pt catalysts by means of state-of-the-art DFT methods and by a combination of supersonic molecular beams with high-resolution photoelectron spectroscopy. The DFT results predict that intermediate species like CH(3) and CH(2) are specially stabilized at sites located at particles edges and corners by an amount of 50-80 kJ mol(-1). This stabilization is caused by an enhanced activity of low-coordinated sites accompanied by their special flexibility to accommodate adsorbates. The kinetics of the complete dehydrogenation of methane is substantially modified according to the reaction energy profiles when switching from Pt(111) extended surfaces to Pt nanoparticles. The CH(3) and CH(2) formation steps are endothermic on Pt(111) but markedly exothermic on Pt(79). An important decrease of the reaction barriers is observed in the latter case with values of approximately 60 kJ mol(-1) for first C-H bond scission and 40 kJ mol(-1) for methyl decomposition. DFT predictions are experimentally confirmed by methane decomposition on Pt nanoparticles supported on an ordered CeO(2) film on Cu(111). It is shown that CH(3) generated on the Pt nanoparticles undergoes spontaneous dehydrogenation at 100 K. This is in sharp contrast to previous results on Pt single-crystal surfaces in which CH(3) was stable up to much higher temperatures. This result underlines the critical role of particle edge sites in methane activation and dehydrogenation.

Establishing the Accuracy of Broadly Used Density Functionals in Describing Bulk Properties of Transition Metals.

The performance of various commonly used density functionals is established by comparing calculated values of atomic structure data, cohesive energies, and bulk moduli of all transition metals to available experimental data. The functionals explored are the Ceperley-Alder (CA), Vosko-Wilk-Nussair (VWN) implementation of the Local Density Approximation (LDA); the Perdew-Wang (PW91) and Perdew-Burke-Ernzerhof (PBE) forms of the Generalized Gradient Approximation (GGA), and the RPBE and PBEsol modifications of PBE, aimed at better describing adsorption energies and bulk solid lattice properties, respectively. The present systematic study shows that PW91 and PBE consistently provide the smallest differences between the calculated and experimental values. Additional calculations of the (111) surface energy of several face centered cubic (fcc) transition metals reveal that LDA produces the most accurate results, while all other functionals significantly underestimate the experimental values. RPBE severely underestimates surface energy, which may be the origin for the reduced surface chemical activity and the better performance of RPBE describing adsorption energies.

Bulk Properties of Transition Metals: A Challenge for the Design of Universal Density Functionals

Systematic evaluation of the accuracy of exchange-correlation functionals is essential to guide scientists in their choice of an optimal method for a given problem when using density functional theory. In this work, accuracy of one Generalized Gradient Approximation (GGA) functional, three meta-GGA functionals, one Nonseparable Gradient Approximation (NGA) functional, one meta-NGA, and three hybrid GGA functionals was evaluated for calculations of the closest interatomic distances, cohesive energies, and bulk moduli of all 3d, 4d, and 5d bulk transition metals that have face centered cubic (fcc), hexagonal closed packed (hcp), or body centered cubic (bcc) structures (a total of 27 cases). Our results show that including the extra elements of kinetic energy density and Hartree-Fock exchange energy density into gradient approximation density functionals does not usually improve them. Nevertheless, the accuracies of the Tao-Perdew-Staroverov-Scuseria (TPSS) and M06-L meta-GGAs and the MN12-L meta-NGA approach the accuracy of the Perdew-Burke-Ernzerhof (PBE) GGA, so usage of these functionals may be advisable for systems containing both solid-state transition metals and molecular species. The N12 NGA functional is also shown to be almost as accurate as PBE for bulk transition metals, and thus it could be a good choice for studies of catalysis given its proven good performance for molecular species.

Bandgap Engineering of Graphene by Physisorbed Adsorbates

Graphene, a material consisting of a single graphite layer, has recently attracted considerable scientifi c interest due to its unique electronic properties, which makes it a promising candidate for carbon-based electronics. A prerequisite for its usage in electronic devices is the ability to generate bandgaps in graphene, which can be considered as a gap-less semiconductor, or more generally the capability to tune its band structure at will in a fl exible and technically feasible fashion. Here, we show by means of density-functional calculations that the physisorption of molecules with specifi c electronic structure, characterized by a lowest unoccupied molecular orbital energetically located in the vicinity of the Dirac point, opens a bandgap and makes an engineering of band structures possible. The suggested approach is highly versatile since bandgaps and the form of the graphene bands can be tuned by modifying the adsorption geometry, the coverage of active adsorbate molecules, or by chemical modifi cations of the adsorbate. Graphene is a novel material [ 1 ] consisting of a single graphite layer. It has recently attracted considerable interest, among others things, due to its extraordinary electronic properties, making it highly interesting from a technical point of view as a promising candidate for carbon-based electronics. [ 2 − 10 ]

Desulfurization of thiophene on Au/TiC(001): Au-C interactions and charge polarization.

Photoemission and first-principles DF calculations were used to study the interaction of thiophene with TiC(001) and Au/TiC(001) surfaces. The adsorption strength of thiophene on TiC(001) is weak, and the molecule desorbs at temperatures below 200 K. The molecule binds to Ti centers of TiC(001) through its sulfur atom with negligible structural perturbations. In spite of the very poor desulfurization performance of TiC(001) or Au(111), a Au/TiC(001) system displays a hydrodesulfurization activity higher than that of conventional Ni/MoS(x) catalysts. The Au<-->TiC(001) interactions induce a polarization of electron density around Au which substantially increases the chemical reactivity of this metal. Au nanoparticles drastically increase the hydrodesulfurization activity of TiC(001) by enhancing the bonding energy of thiophene and by helping in the dissociation of H(2) to produce the hydrogen necessary for the hydrogenolysis of C-S bonds and the removal of sulfur. H(2) spontaneously dissociates on small two-dimensional clusters of gold in contact with TiC(001). On these systems, the adsorption energy of thiophene is 0.45-0.65 eV larger than that on TiC(001) or Au(111). Thiophene binds in a eta(5) configuration with a large elongation (approximately 0.2 A) of the C-S bonds.

Formation of one-dimensional electronic states along the step edges of CeO2(111)

Scanning tunneling microscopy (STM) combined with density functional theory (DFT) are used to analyze the structural and electronic properties of step edges on the surface of CeO(2)(111) films grown on Ru(0001). Depending on the preparation conditions, 211 or 110-oriented steps develop on the surface, which results in the formation of ceria ad-islands with hexagonal or triangular shapes. STM conductance spectroscopy reveals pronounced differences in the electronic properties of the step edges, as reflected in different onset positions of the ceria conduction band. The band shifts are related to the development of distinct edge electronic states that split-off from the ceria conduction band, as shown with DFT calculations. The separation of the edge states from the main band is governed by the atom-coordination and local charge-distribution along the edge, the latter giving rise to the development of electrostatic dipoles. We expect that the observed edge morphologies determine not only the electronic properties but also the adsorption behavior of step edges on the CeO(2)(111) surface.

Carbon on platinum substrates: from carbidic to graphitic phases on the (111) surface and on nanoparticles.

The formation of carbonaceous deposits on Pt(111) surfaces and Pt nanoparticles has been studied using suitable models and density-functional calculations. The study addresses a broad range of processes, from the very first stage of carbon deposition up to a final building of graphene monolayers (ML) defined as a 1:1 ratio of the number of C atoms to surface Pt atoms. A carbidic phase is formed below a coverage of approximately 0.3 ML, when negatively charged carbon atoms are strongly adsorbed preferentially on fcc hollow sites. On Pt nanoparticles, the adsorption of carbon atoms seems to be enhanced near particle edges due to the special flexibility of defect sites. Above a coverage of approximately 0.3 ML, the formation of small C(n) aggregates becomes possible. Interestingly, thermodynamics favors the formation of C(3) trimers at a coverage of 0.33 ML, whereas the formation of C(2) dimers requires a higher coverage of 0.5 ML. The covalently bonded C(2) species is supposed to be the key fragment for the formation of benzene-like rings at coverages above 0.6 ML. These rings are expected to be the building blocks for the graphene monolayer. However, the typical electronic structure of graphene is not observed until a coverage above approximately 1.8 ML is reached. We corroborated the experimentally suggested carbon double-layer to be stable. It is proposed to consist of a monolayer of carbidic atoms C adsorbed on Pt with a graphene layer adsorbed on the carbidic layer. Some of the carbidic atoms serve as anchors for the graphene layer, with noticeably strong covalent bonds formed. This double-layer model would imply a much higher adhesion of the graphene layer than in the single-layer model.

Adsorption of gold on TiC(001): Au–C interactions and charge polarization

High-resolution photoemission and first-principles density-functional slab calculations were used to study the adsorption of gold on a TiC(001) surface. A positive shift in the binding energy of the C 1s core level is observed after the deposition of Au on the metal carbide surface. The results of the density-functional calculations corroborate the formation of Au-C bonds. In general, the bond between Au and the TiC(001) surface exhibits very little ionic character, but there is a substantial polarization of electrons around Au that affects its chemical properties.

Theoretical assessment of graphene-metal contacts

Graphene-metal contacts have emerged as systems of paramount importance in the synthesis of high-quality and large-size patches of graphene and as vital components of nanotechnological devices. Herein, we study the accuracy of several density functional theory methods using van der Waals functionals or dispersive forces corrections when describing the attachment of graphene on Ni(111). Two different experimentally observed chemisorption states, top-fcc and bridge-top, were put under examination, together with the hcp-fcc physisorption state. Calculated geometric, energetic, and electronic properties were compared to experimental data. From the calculations, one finds that (i) predictions made by different methodologies differ significantly and (ii) optB86b-vdW functional and Grimme dispersion correction seem to provide the best balanced description of stability of physisorption and chemisorption states, the attachment strength of the latter on Ni(111) surface, the graphene-Ni(111) separation, and the bandstructure of chemisorbed graphene. The collation suggests that accurate and affordable theoretical studies on technologies based on graphene-metal contacts are already at hand.