Aleksandra Vojvodic

Solar hydrogen production with semiconductor metal oxides: new directions in experiment and theory

An overview of a collaborative experimental and theoretical effort toward efficient hydrogen production via photoelectrochemical splitting of water into di-hydrogen and di-oxygen is presented here. We present state-of-the-art experimental studies using hematite and TiO(2) functionalized with gold nanoparticles as photoanode materials, and theoretical studies on electro and photo-catalysis of water on a range of metal oxide semiconductor materials, including recently developed implementation of self-interaction corrected energy functionals.

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.

Binding of polycyclic aromatic hydrocarbons and graphene dimers in density functional theory

An early van der Waals density functional (vdW-DF) described layered systems (such as graphite and graphene dimers) using a layer-averaged electron density in the evaluation of nonlocal correlations. This early vdW-DF version was also adapted to approximate the binding of polycyclic aromatic hydrocarbons (PAHs) (Chakarova S D and Schroder E 2005 J. Chem. Phys. 122 054102). In parallel to that PAH study, a new vdW-DF version (Dion M, Rydberg H, Schroder E, Langreth D C and Lundqvist B I 2004 Phys. Rev. Lett. 92 246401) was developed that provides accounts of nonlocal correlations for systems of general geometry. We apply here the latter vdW-DF version to aromatic dimers of benzene, naphthalene, anthracene and pyrene, stacked in sandwich (AA) structure, and the slipped-parallel (AB) naphthalene dimer. We further compare the results of the two methods as well as other theoretical results obtained by quantum-chemistry methods. We also compare calculations for two interacting graphene sheets in the AA and the AB structures and provide the corresponding graphene-from-graphite exfoliation energies. Finally, we present an overview of the scaling of the molecular–dimer interaction with the number of carbon atoms and with the number of carbon rings.

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.

Steam Reforming on Transition-metal Carbides from Density-functional Theory

A screening study of the steam reforming reaction on clean and oxygen covered early transition-metal carbides surfaces is performed by means of density-functional theory calculations. It is found that carbides provide a wide spectrum of reactivities, from too reactive via suitable to too inert. Several molybdenum-based systems are identified as possible steam reforming catalysts. The findings suggest that carbides provide a playground for reactivity tuning, comparable to the one for pure metals.Graphical AbstractCalculated potential energy diagram for the steam reforming reaction on transition metal carbide TMC(111) and TM2C(100) surfaces

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.