Nicola Seriani

Ab initio thermodynamics of lithium oxides: from bulk phases to nanoparticles

Lithium ion batteries are nowadays key devices for energy storage, and a great research effort is under way to develop and apply new materials. Recently, new approaches have been proposed that rely on the reversible formation of either Li2O or Li2O2 at the electrodes. The details of their formation and dissolution are, however, still unclear. As a first step towards the understanding of these processes, bulk lithium oxides, their surfaces and their nanoparticles have been here investigated by density functional theory and ab initio thermodynamics. At a pressure of 1 atmosphere of oxygen, Li2O2 is the stable bulk phase below 5 K, where a transition to Li2O takes place. Wulffs construction predicts an octahedral shape for Li2O nanoparticles and the form of a hexagonal prism for Li2O2. By taking into account the effect of the surfaces, a size-dependent phase diagram is calculated. At an oxygen pressure of 1 atmosphere and a temperature of 300 K, Li2O is the stable phase for particles with a diameter larger than approximately 2.5 nm. This size-dependent oxidation behavior of lithium should be taken into account in the design of nanostructured oxygen cathodes.

Platinum-group and noble metals under oxidizing conditions

Platinum-group metals and noble metals play an important role in catalysis, for total oxidation as well as for partial oxidation reactions. Only in recent years have advances in microscopic, spectroscopic and computer simulation techniques made it possible to investigate the interaction of oxygen with metallic substrates at an atomistic level. We present an overview on the formation of adsorption structures and surface oxides on Rh, Pd, Ag, Cu and Pt surfaces, with particular focus on the phase diagrams calculated from first-principles thermodynamics. The low-index (111), (100) and (110) surfaces as well as selected high-index surfaces have been considered. We predict the stability of novel structures such as the c(4 × 6) on Cu(100) and the α-PtO2 trilayer on Pt(100). The knowledge of the Gibbs free surface energies allows us to predict the adsorbate-induced changes in the thermodynamic equilibrium shape of metal nanoparticles. At low oxygen chemical potential, corresponding to clean surfaces, the (111) facets dominate the particle shape, with a significant contribution from (100) facets. But even under these conditions a small fraction of the overall surface corresponds to more open facets. As oxygen adsorption sets in, their contribution becomes larger. At high oxygen partial pressures, surface oxides form on the platinum-group metals. They do not only display different chemical properties than the metal, but also determine the exposed surface orientations of the particles. The latter effect might play an important role for the catalytic activity of transition metal nanoparticles.

Water adsorption and dissociation on α-Fe2O3(0001): PBE+U calculations.

Adsorption and dissociation of water on different oxygen- and iron-terminated hematite(0001) surfaces at monolayer coverage have been studied by density-functional theory calculations, including a Hubbard-like+U correction. We considered six possible surface terminations, including four oxygen- and two iron-terminations. Binding energy of water on these terminations can be as large as 1.0 eV. On these terminations the energy barrier for the dissociation of the molecularly adsorbed water is less than 0.3 eV, and in few cases the dissociation is even spontaneous, i.e., without any detectable barrier. Our results thus suggest that water can be adsorbed on the α-Fe2O3(0001) surface dissociatively at room temperature, as previously found by experiment. This study also presents a very first theoretical insight into the adsorption and dissociation of water on all known terminations of the hematite(0001) surface.

Carbon in palladium catalysts: A metastable carbide

The catalytic activity of palladium toward selective hydrogenation of hydrocarbons depends on the partial pressure of hydrogen. It has been suggested that the reaction proceeds selectively toward partial hydrogenation only when a carbon-rich film is present at the metal surface. On the basis of first-principles simulations, we show that carbon can dissolve into the metal because graphite formation is delayed by the large critical nucleus necessary for graphite nucleation. A bulk carbide Pd(6)C with a hexagonal six-layer fcc-like supercell forms. The structure is characterized by core level shifts of 0.66-0.70 eV in the core states of Pd, in agreement with experimental x-ray photoemission spectra. Moreover, this phase traps bulk-dissolved hydrogen, suppressing the total hydrogenation reaction channel and fostering partial hydrogenation.

Photo-driven oxidation of water on α-Fe2O3 surfaces: An ab initio study

Adopting the theoretical scheme developed by the Nørskov group [see, for example, Nørskov et al., J. Phys. Chem. B 108, 17886 (2004)], we conducted a density functional theory study of photo-driven oxidation processes of water on various terminations of the clean hematite (α-Fe2O3) (0001) surface, explicitly taking into account the strong correlation among the 3d states of iron through the Hubbard U parameter. Six best-known terminations, namely, Fe−Fe−O3− (we call S1), O−Fe−Fe−(S2), O2−Fe−Fe−(S3), O3−Fe−Fe− (S4), Fe−O3−Fe− (S5), and O−Fe−O3−(S6), are first exposed to water, the stability of resulting surfaces is investigated under photoelectrochemical conditions by considering different chemical reactions (and their reaction free energies) that lead to surfaces covered by O atoms or/and OH groups. Assuming that the water splitting reaction is driven by the redox potential for photogenerated holes with respect to the normal hydrogen electrode, UVB, at voltage larger than UVB, most 3-oxygen terminated substrates are stable. These results thus suggest that the surface, hydroxylated in the dark, should release protons under illumination. Considering the surface free energy of all the possible terminations shows that O3–S5 and O3–S1 are the most thermodynamically stable. While water oxidation process on the former requires an overpotential of 1.22 V, only 0.84 V is needed on the latter.

A first-principles study of bulk oxide formation on Pd(100)

The catalytic oxidation activity of palladium is influenced by the oxidation state of the metal. Under technologically relevant conditions, bulk and surface oxides may form and decompose. By employing first-principles calculations based on density functional theory, we have investigated the transition from the surface oxide to the bulk oxide on Pd(100). We show that the most stable orientation of the oxide film is PdO(101)@Pd(100) at any film thickness. The monolayer has unique electronic, chemical, and thermodynamic properties in comparison to thicker oxide films. In particular, carbon monoxide adsorbs by approximately 0.3 eV more strongly on thicker oxides than on the surface oxide, a fact that should influence the catalytical activity. Finally, we show that a simple model employing density functional theory energies predicts a Stranski-Krastanov growth mode for the oxide film, with a critical thickness of 1 ML. Our results give a framework for the interpretation of experiments of Pd oxide growth.

Effects of Al-doping on the properties of Li–Mn–Ni–O cathode materials for Li-ion batteries: an ab initio study

The key properties of a successful cathode material, such as the structural stability during delithiation, the battery voltage, and the Li mobility, were investigated for Al-doped Li–Mn–Ni oxide structures, using density-functional theory and the nudged-elastic band method. The rhombohedral layered structure of LiMn0.5Ni0.5O2 with zigzag and flower arrangements of transition metal atoms as well as the monoclinic structure of Li(Li1/6Ni1/6Mn2/3)O2 were used as base structures. A stabilizing effect of Al-doping was found for all partially lithiated systems considered. The derived battery voltages at zero temperature are generally enhanced by Al-doping. The calculated activation energies for Li jumps suggest slower Li mobility. The Al-doped Li-rich monoclinic structure seems to be most promising as a cathode material because of a comparatively high battery voltage.

Ab initio study of element segregation and oxygen adsorption on PtPd and CoCr binary alloy surfaces

The segregation behavior of the bimetallic alloys PtPd and CoCr in the case of bare surfaces and in the presence of an oxygen ad-layer has been studied by means of first-principles modeling based on density-functional theory (DFT). For both systems, change of the d-band filling due to charge transfer between the alloy components, resulting in a shift of the d-band center of surface atoms compared to the pure components, drives the surface segregation and governs the chemical reactivity of the bimetals. In contrast to previous findings but consistent with analogous PtNi alloy systems, enrichment of Pt atoms in the surface layer and of Pd atoms in the first subsurface layer has been found in Pt-rich PtPd alloy models, despite the lower surface energy of pure Pd compared to pure Pt. Similarly, Co surface and Cr subsurface segregation occurs in Co-rich CoCr alloys. However, in the presence of adsorbed oxygen, Pd and Cr occupy preferentially surface sites due to their lower electronegativity and thus stronger oxygen affinity compared to Pt and Co, respectively. In either cases, the calculated oxygen adsorption energies on the alloy surfaces are larger than on the pure components when the more noble components are present in the subsurface layers.

Ab initio parameterization of an all-atom polarizable and dissociable force field for water

A novel all-atom, dissociative, and polarizable force field for water is presented. The force field is parameterized based on forces, stresses, and energies obtained form ab initio calculations of liquid water at ambient conditions. The accuracy of the force field is tested by calculating structural and dynamical properties of liquid water and the energetics of small water clusters. The transferability of the force field to dissociated states is studied by considering the solvation of a proton and the ionization of water at extreme conditions of pressure and temperature. In the case of the solvated proton, the force field properly describes the presence of both Eigen and Zundel configurations. In the case of the pressure-induced ice VIII/ice X transition and the temperature-induced transition to a superionic phase, the force field is found to describe accurately the proton symmetrization and the melting of the proton sublattice, respectively.

Defective α‐Fe2O3(0001): An ab Initio Study

By using density functional theory calculations at the PBE+U level, we investigated the properties of hematite (0001) surfaces decorated with adatoms/vacancies/substituents. For the most stable surface termination over a large range of oxygen chemical potentials (muO), the vacancy formation and adsorption energies were determined as a function of muO. Under oxygen-rich conditions, all defects are metastable with respect to the ideal surface. Under oxygen-poor conditions, O vacancies and Fe adatoms become stable. Under ambient conditions, all defects are metastable; in the bulk, O vacancies form more easily than Fe vacancies, whereas at the surface the opposite is true. All defects, that is, O and Fe vacancies, Fe and Al adatoms, and Al substituents, induce important modifications to the geometry of the surface in their vicinity. Dissociative adsorption of molecular oxygen is likely to be exothermic on surfaces with Fe/Al adatoms or O vacancies.