Carlo Ruberto

From Electronic Structure to Catalytic Activity: A Single Descriptor for Adsorption and Reactivity on Transition-Metal Carbides

Adsorption and catalytic properties of the polar (111) surface of transition-metal carbides (TMCs) are investigated by density-functional theory. Atomic and molecular adsorption are rationalized with the concerted-coupling model, in which two types of TMC surface resonances (SRs) play key roles. The transition-metal derived SR is found to be a single measurable descriptor for the adsorption processes, implying that the Brønsted-Evans-Polanyi relation and scaling relations apply. This gives a picture with implications for ligand and vacancy effects and which has a potential for a broad screening procedure for heterogeneous catalysts.

Trends in bulk electron-structural features of rocksalt early transition-metal carbides

A detailed and systematic density-functional theory (DFT) study of a series of early transition-metal carbides (TMCs) in the NaCl structure is presented. The focus is on the trends in the electronic structure and nature of bonding, which are essential for the understanding of the reactivity of TMCs. The employed approach is based on a thorough complementary analysis of the electron density differences, the density of states (DOS), the band structure and the real-space wavefunctions to gain an insight into the bonding of this class of materials and get a more detailed picture of it than previously achieved, as the trend study allows for a systematic identification of the bond character along the different bands. Our approach confirms the presence of both the well-known TM-C and TM-TM bonds and, more importantly, it shows the existence and significance of direct C-C bonds in all investigated TMCs, which are frequently neglected but have been identified in some cases (Zhang et al 2002 Solid State Commun. 121 411; Ruberto et al 2007 Phys. Rev. B 75 235438). New information on the spatial extent of the bonds, their k-space location within the band structure and their importance for the bulk cohesion is provided. Trends in covalency and ionicity are presented. The resulting electron-structural trends are analyzed and discussed within a two-level model.

Atomic and molecular adsorption on transition-metal carbide (111) surfaces from density-functional theory: a trend study of surface electronic factors

This study explores atomic and molecular adsorption on a number of early transition-metal carbides (TMCs) in NaCl structure by means of density-functional theory calculations. The investigated substrates are the TM-terminated TMC(111) surfaces, of interest because of the presence of different types of surface resonances (SRs) on them and because of their technological importance in growth processes. Also, TM compounds have shown potential in catalysis applications. Trend studies are conducted with respect to both period and group in the periodic table, choosing the substrates ScC, TiC, VC, ZrC, NbC, δ-MoC, TaC, and WC (in NaCl structure) and the adsorbates H, B, C, N, O, F, NH, NH(2), and NH(3). Trends in adsorption strength are explained in terms of surface electronic factors, by correlating the calculated adsorption-energy values with the calculated surface electronic structures. The results are rationalized by use of a concerted-coupling model (CCM), which has previously been applied successfully to the description of adsorption on TiC(111) and TiN(111) surfaces (Ruberto et al 2007 Solid State Commun. 141 48). First, the clean TMC(111) surfaces are characterized by calculating surface energies, surface relaxations, Bader charges, and surface-localized densities of states (DOSs). Detailed comparisons between surface and bulk DOSs reveal the existence of transition-metal localized SRs (TMSRs) in the pseudogap and of several C-localized SRs (CSRs) in the upper valence band on all considered TMC(111) surfaces. The spatial extent and the dangling bond nature of these SRs are supported by real-space analyses of the calculated Kohn-Sham wavefunctions. Then, atomic and molecular adsorption energies, geometries, and charge transfers are presented. An analysis of the adsorbate-induced changes in surface DOSs reveals a presence of both adsorbate-TMSR and adsorbate-CSRs interactions, of varying strengths depending on the surface and the adsorbate. These variations are correlated to the variations in adsorption energies. The results are used to generalize the content and applications of the previously proposed CCM to this larger class of substrates and adsorbates. Implications for other classes of materials, for catalysis, and for other surface processes are discussed.

Density-functional bridge between surfaces and interfaces

Interfaces are brought into focus by many materials phenomena, e.g., contacting, materials strength, and wetting. The class of interfaces includes ultra-high-vacuum surfaces, which provide a meeting place for numerous accurate experimental techniques and advanced theory. Such meetings stimulate detailed comparisons on the quantum level between experiment and theory, which develop our theoretical tools and understanding. This creates good positions for broadened applications, e.g., other interfaces, which typically lack adequate experimental tools. Density-functional theory is one key bridge between surfaces and other interfaces. The paper presents some recent typical applications from our group, including brief reports on interface structures (VN/Fe, TiC/Co, TiC/Al 2 O 3 ), dynamic processes at surfaces and interfaces (O 2 /Al(111), scanning-tunneling microscopic spectroscopy and manipulation), adsorption and desorption (CO, N 2 , NO, and O 2 on Al(111)), electronic and magnetic properties at surfaces and interfaces (magnetic effects on TiC/Co, surface state on κ-Al 2 O 3 (001)), and epitaxial growth on surfaces (Al(111) and alike). Similar progress in many worldwide materials groups and networks gives a basis for the ongoing paradigm shift in materials science.

Bridging between micro- and macroscales of materials by mesoscopic models

Abstract The importance of bridging length scales for materials is illustrated by three examples, nematic liquid crystals, strength of materials, and epitaxial growth. Emphasis is on the microscopic scale, with first-principles calculations of molecule–surface interaction, stacking-fault energies, interlayer interactions, diffusion barriers, and adsorbate–adsorbate interactions. Some pilot examples of using such information on the meso- and macroscales with models using director fields, misfit densities of dislocation, and monomer and island densities are presented. The area is predicted to have a great future.

Ab initio structure modelling of complex thin-film oxides: thermodynamical stability of TiC/thin-film alumina

We present a strategy to identify energetically favourable oxide structures in thin-film geometries. Thin-film candidate configurations are constructed from a pool of sublattices of stable and metastable oxide bulk phases. Favourable stoichiometric compositions and atomic geometries are identified by comparing total and Gibbs free energies of the relaxed configurations. This strategy is illustrated for thin-film alumina on TiC, materials which are commonly fabricated by chemical vapour deposition (CVD) and used as wear-resistant multilayer coatings. Based on the standard implementation of ab initio thermodynamics, with an assumption of equilibrium between molecular O(2) and the oxide, we predict a stability preference of TiC/alumina configurations that show no binding across the interface. This result is seemingly in conflict with the wear-resistant character of the material and points towards a need for extending standard ab initio thermodynamics to account for relevant growth environments.

Coarse-grained model for growth of α- and k-Al2O3 on TiC and TiN(111): thin alumina films from density-functional calculations

We propose a coarse-grained model for growth of α- and k-Al2O3 on TiX(111) (X = C or N) surfaces, generalisable to similar systems. We perform the first step in the model and investigate the structure and energetics of a thin alumina film on TiX(111) with density-functional theory calculations. Results show that the stable alumina structure consists of two oxygen layers with a full aluminum layer in octahedral coordination in between and that the stability is higher on the TiC surface than on TiN.

Hard-materials-surface prediction of one-dimensional electron gas

A new and robust one-dimensional electron gas (1DEG) is predicted on the (001) surface of κ-Al 2 O 3 by means of a density-functional-theory (DFT) study. Our first-principles calculations show that this surface is Al terminated, with the Al atoms lying in [100] zigzag lines, and has an excess of electrons due to the charge asymmetry in the bulk κ-Al 2 O 3 . The κ-Al 2 O 3 phase itself is metastable up to 1000 K. To test the robustness of our materials-theory prediction of the 1DEG along the zigzag Al lines, the critical temperature due to a Peierls metal-insulator instability has been estimated to be in the temperature range [0,1] K, using a tight-binding model, where the model parameters are determined with additional DFT calculations. The virtues of both the materials-theory design and the possible 1DEG realization are also discussed.