A closer look at how symmetry constraints and the spin-orbit coupling shape the electronic structure of Bi(111).

Fully relativistic density-functional-theory calculations of Bi(111) thin films are analyzed to revisit their two metallic surface-states branches. We first contrast these metallic branches with surface states arising at gaps in the valence band opened by the spin-orbit coupling (SOC). We find that the two metallic branches alongΓM‾do not overlap with the bulk band at the zone boundary,M. We show that the spin texture observed in such states cannot be traced to the lifting of Kramers' degeneracy. Instead, we track them to themj=±1/2-mj=±3/2SOC splitting, the potential anisotropy for in-plane and out-of-plane states, and the coupling between the opposite surfaces of a slab occurring nearM, which is driven by a spatial redistribution of the four metallic states composing the two metallic branches. Each of these branches appears to be non-degenerate at the tested surface, yet each is degenerate with another state of opposite spin at the other surface. Nevertheless, the four metallic states bear some contribution on both surfaces of the film because of their spatial redistribution nearM. The overlapping among these states nearM, afforded by their spatial redistribution on both surfaces, causes a hybridization that perpetuates the splitting between the two branches, makes the film's electronic structure thickness dependent nearM, extinguishes the magnetic moment of the metallic states avoiding the magnetic-moment discontinuity atM, and denies the need or expectancy of the metallic branches becoming degenerate atM. We propose that theoppositespin polarization observed for the two metallic branches occurs because the surface atoms retain their covalent bonds and thus cannot afford magnetic polarization. We show that the Rashba-splitting of the metallic states for inversion-asymmetric films does not have a fixed magnitude but can be tuned by changing the perturbation breaking inversion symmetry.

The perturbation energy: A missing key to understand the "nobleness" of bulk gold.

The nobleness of gold surfaces has been appreciated since long before the beginning of recorded history. Yet, the origin of this phenomenon remains open because the so far existing explanations either incorrectly imply that silver should be the noblest metal or would fail to predict the dissolution of Au in aqua regia. Here, based on our analyses of oxygen adsorption, we advance that bulk gold's unique resistance to oxidation is traced to the large energy cost associated with the perturbation its surfaces undergo upon adsorption of highly electronegative species. This fact is related to the almost totally filled d-band of Au and relativistic effects, but does not imply that the strength of the adsorbate-Au bond is weak. The magnitude of the structural and charge-density perturbation energy upon adsorption of atomic oxygen­which is largest for Au­is assessed from first-principles calculations and confirmed via a multiple regression analysis of the binding energy of oxygen on metal surfaces.

Gold-doped graphene: A highly stable and active electrocatalysts for the oxygen reduction reaction.

In addressing the growing need of renewable and sustainable energy resources, hydrogen-fuel-cells stand as one of the most promising routes to transform the current energy paradigm into one that integrally fulfills environmental sustainability. Nevertheless, accomplishing this technology at a large scale demands to surpass the efficiency and enhance the cost-effectiveness of platinum-based cathodes, which catalyze the oxygen reduction reaction (ORR). In this work, our first-principles calculations show that Au atoms incorporated into graphene di-vacancies form a highly stable and cost-effective electrocatalyst that is, at the same time, as or more (dependently of the dopant concentration) active toward ORR than the best-known Pt-based electrocatalysts. We reveal that partial passivation of defected-graphene by gold atoms reduces the reactivity of C dangling bonds and increases that of Au, thus optimizing them for catalyzing the ORR and yielding a system of high thermodynamic and electrochemical stabilities. We also demonstrate that the linear relation among the binding energies of the reaction intermediates assumed in computational high-throughput material screening does not hold, at least for this non-purely transition-metal material. We expect Au-doped graphene to finally overcome the cathode-related challenge hindering the realization of hydrogen-fuel cells as the leading means of powering transportation and portable devices.

Rational Design of Competitive Electrocatalysts for Hydrogen Fuel Cells.

The large-scale application of one of the most promising clean and renewable sources of energy, hydrogen fuel cells, still awaits efficient and cost-effective electrocatalysts for the oxygen reduction reaction (ORR) occurring on the cathode. We demonstrate that truly rational design renders electrocatalysts possessing both qualities. By unifying the knowledge on surface morphology, composition, electronic structure, and reactivity, we solve that trimetallic sandwich-like structures are an excellent choice for optimization. Their constituting species are expected to couple synergistically yielding reaction-environment stability, cost-effectiveness, and tunable reactivity. This cooperative-action concept enabled us to predict two advantageous ORR electrocatalysts: Pd/Fe/W(110) and Au/Ru/W(110). Density functional theory calculations of the reaction free-energy diagrams suggest that these materials are more active toward ORR than the so-far best Pt-based catalysts. Our designing concept advances also a general approach for engineering advanced materials.

Nature of the binding of a c(2×2)-CO overlayer on Ag(001) and surface mediated intermolecular coupling.

We present a first-principles study of the nature of the binding of a c(2×2)-CO overlayer on Ag(001) and of the origin of CO-CO interactions upon adsorption. Electronic structural changes induced by molecular adsorption provide an interpretation for earlier X-ray photoemission valence band spectra of CO/Ag(001). Our results establish that CO chemisorbs on clean Ag(001) and follows the Blyholder model of donation and back-donation between CO and metal orbitals. We analyze the origin of the dispersion of the C-O stretch mode and attest that it is caused by the metal-CO coupling. Specifically, the coupling of CO to Ag, although the weakest of those between it and transition and other noble metals, greatly enhances the intermolecular force constants. We also find that the response of the charge density around CO is much stronger and of longer range when the molecule stretches than when it rigidly vibrates against the surface. This difference explains why the C-O stretch mode disperses while the Ag-CO stretch mode does not.

Application of density functional theory to CO tolerance in fuel cells: a brief review.

The large scale practical application of fuel cells in the hydrogen economy is possible only with a dramatic reduction of the cost and significant improvement of the electrocatalytic properties of the electrodes. This can be achieved through rational design of new materials, which requires an understanding of the microscopic mechanisms underlying electrocatalysis. We review briefly some applications of density functional theory (DFT) to this problem, with particular reference to the observed high CO tolerance of Pt-Ru-based anodes. These DFT-based calculations trace the changes in the surface electronic structure and the energy landscape induced by formation of Pt islets on facets of Ru nanoparticles which lead to the preferred diffusion of CO from Pt sites to Ru, where it exhibits a high rate of reaction with hydroxyls, which are generally present. We also consider the energetics of stabilization of the Pt islets on the Ru nanoparticles.