Ab Initio Atomistic Thermodynamics Research Articles

First-principles investigations of the structure and stability of oxygen adsorption and surface oxide formation at Au(111)

We perform density-functional theory calculations to investigate the adsorption of oxygen at the Au(111) surface, including on-surface, subsurface, and surface oxide formation. We find that atomic oxygen adsorbs weakly on the surface and is barely stable with respect to molecular oxygen, while pure subsurface adsorption is only metastable. Interestingly, however, we find that the most favorable structure investigated involves a thin surface-oxide-like configuration, where the oxygen atoms are quasithreefold-coordinated to gold atoms, and the gold atoms of the surface layer are twofold, linearly coordinated to oxygen atoms. By including the effect of temperature and oxygen pressure through the description of ab initio atomistic thermodynamics, we find that this configuration is the most stable for realistic catalytic temperatures and pressures, e.g., for low-temperature oxidation reactions, and is predicted to be stable up to temperatures of around $420\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ at atmospheric pressure. This gives support to the notion that oxidized Au, or surface-oxide-like regions, could play a role in the behavior of oxide-supported nanogold catalysts.

Nitrogen adsorption and thin surface nitrides on Cu(1 1 1) from first-principles

Experimental studies of nitrogen adsorbed on a Cu(1 1 1) surface show that the surface layer undergoes a reconstruction to form a pseudo-(1 0 0) structure. We use ab initio techniques to demonstrate the theoretical stability of this reconstructed surface phase over a range of conditions. We systematically investigate the chemisorption of N on the Cu(1 1 1) surface, from 0.06 to 1 ML coverage. A peculiar atomic relaxation of N atoms for 0.75 ML is identified, which results in the formation of a (metastable) “N-trimer cluster” on the surface. We have also investigated surface nitride formation, as suggested from experiments. A surface nitride-like structure similar to the reported pseudo-(1 0 0) reconstruction is found to be highly energetically favored. Using concepts from “ ab initio atomistic thermodynamics”, we predict that this surface nitride exists for a narrow range of nitrogen chemical potential before the formation of bulk Cu 3N.

Surface oxides of the oxygen–copper system: Precursors to the bulk oxide phase?

To gain an initial understanding of the copper-based catalysts in commercially important chemical reactions such as the oxygen-assisted water–gas shift reaction, we performed density-functional theory calculations, investigating the interaction of oxygen and copper, focusing on the relative stability of surface oxides and oxide surfaces of the O/Cu system. By employing the technique of “ ab initio atomistic thermodynamics”, we show that surface oxides are only metastable at relevant pressures and temperatures of technical catalysis, with no stable chemisorption phase observed even at very low coverage. Although exhibiting only metastability, these surface oxides resemble the bulk oxide material both geometrically and electronically, and may serve as a precursor phase before onset of the bulk oxide phase. Having identified the bulk oxide as the most stable phase under realistic catalytic conditions, we show that a Cu 2O(1 1 1) surface with Cu vacancies has a lower free energy than the stoichiometric surface for the considered range of oxygen chemical potential and could be catalytically relevant.

Theoretical investigations on the potential-induced formation of Pt-oxide surfaces

Using the extended ab initio atomistic thermodynamics approach together with density functional theory calculations the interfacial structure and composition of Pt-electrodes in electrochemical environments at elevated electrode potentials was studied. Focusing on the electrode potential region, at which the oxide-formation occurs, the bulk systems and all low-index surfaces of a-PtO2, b-PtO2, and PtO bulk-oxides were calculated. On the basis of the bulk-oxide formation energies we first deduced the stability ranges at which the bulkoxides are the thermodynamically stable phases. In agreement with experimental observations, we find that at experimental temperature and pressure conditions a-PtO2 and b-PtO2 bulk-oxides are stable above 1.2 V, while PtO requires D/ > 1.3 V. Afterwards the corresponding (p, T,/)-phase diagrams of surface structures were obtained, showing a preference for a-PtO2(0 0 1), b-PtO2(1 1 0), and PtO(1 0 0), respectively, having bulk-like compositions even on their surfaces. However, in case of thin oxide layers a PtO composition might also be present. 2007 Elsevier B.V. All rights reserved.

Predicting surface phase transitions from ab initio based statistical mechanics and thermodynamics

The atomic structure and stoichiometry of a solid surface dictates its physical and chemical properties, and hence plays a crucial role in the performance of a material in technological applications. Indeed, in the field of surface science, a significant proportion of studies address the determination of surface atomic geometries and associated phase transitions. From the theoretical side, the desire is to obtain a first-principles based description that can accurately predict arbitrary surface structures and phase transitions under various conditions, such as temperature, partial pressure, and adparticle coverage. In the present paper, we discuss recent theoretical approaches through various examples, which address this goal. In particular, ab initio atomistic thermodynamics, which affords determination of the surface free energy as a function of relevant atom chemical potentials, and varying pressure and temperature conditions, as well as more powerful schemes that combine first-principles quantum mechanical calculations with statistical mechanics, namely, the lattice-gas or cluster-expansion approach in combination with Monte Carlo simulations.

Thermodynamic stability and structure of copper oxide surfaces: A first-principles investigation

To obtain insight into the structure and surface stoichiometry of copper-based catalysts in commercially important chemical reactions such as the oxygen-assisted water-gas shift reaction, we perform density-functional theory calculations to investigate the relative stability of low-index copper oxide surfaces. By employing the technique of ``ab initio atomistic thermodynamics,'' we identify low-energy surface structures that are most stable under realistic catalytic conditions are found to exhibit a metallic character. Three surfaces are shown to have notably lower surface free energies compared to the others considered and could be catalytically relevant; in particular, under oxygen-rich conditions, they are the ${\mathrm{Cu}}_{2}\mathrm{O}(110):\mathrm{Cu}\mathrm{O}$ surface, which is terminated with both Cu and O surface atoms, and the ${\mathrm{Cu}}_{2}\mathrm{O}(111)\text{\ensuremath{-}}{\mathrm{Cu}}_{\mathrm{CUS}}$ surface, which contains a surface (coordinatively unsaturated) Cu vacancy, while for the oxygen-lean conditions, the ${\mathrm{Cu}}_{2}\mathrm{O}(111)$ surface with a surface interstitial Cu atom is found to be energetically most favorable, highlighting the importance of defects at the surface.

Surface processes and phase transitions from ab initio atomistic thermodynamics and statistical mechanics

Knowledge of the surface composition and atomic geometry is a prerequisite for understanding the physical and chemical properties of modern materials such as those used in, for example, heterogeneous catalysis, corrosion resistance, sensors, and fuel cells. To understand the function of surfaces under realistic conditions, it is crucial to take into account the influence of environmental gas at finite (possibly high) temperatures and pressures. Recent experimental and theoretical studies have shown that when transition metal surfaces are exposed to high oxygen pressures, thin oxide-like structures can form which may have little similarity to the bulk oxides, and thus possess unique chemical and physical properties. Given that technological oxidation catalysis typically involves oxygen-rich conditions, such structures may be present, and in fact be the active material for the reaction, as opposed to the traditionally believed pure metal. Using the approach of ab initio atomistic thermodynamics, free energy phase-diagrams for oxygen/transition metal systems in (T, p)-space ranging from ultra-high vacuum to technically relevant pressures, p, and temperatures, T, are discussed. In addition, results of ab initio statistical mechanical schemes, namely, the Lattice-gas Hamiltonian plus Monte Carlo (MC) simulations, are presented for oxygen/transition metal and alkali-atom/metal systems, where for the latter, the recently introduced “Wang–Landau” algorithm is employed, which affords an accurate estimation of the density of (configurational) states, therefore allowing a direct determination of all major thermodynamic functions.

Preserving the Half-Metallicity at the Heusler AlloyCo2MnSi(001)Surface: A Density Functional Theory Study

We have studied the stability, the electronic, and the magnetic properties of Co2MnSi(001) thin films for 15 different terminations using density functional theory calculations. The phase diagram obtained by ab initio atomistic thermodynamics shows that in practice the MnSi, pure Mn, or pure Si terminated surfaces can be stabilized under suitable conditions. Analyzing the surface band structure, we find that the pure Mn termination, due to its strong surface-subsurface coupling, preserves the half-metallicity of the system, while surface states appear for the other terminations.

Atomic Structure of theGaAs(001)−c(4×4)Surface: First-Principles Evidence For Diversity of Heterodimer Motifs

The GaAs(001)-c(4x4) surface was studied using ab initio atomistic thermodynamics based on density-functional theory calculations. We demonstrate that in a range of stoichiometries, between those of the conventional three As-dimer and the new three Ga-As-dimer models, there exists a diversity of atomic structures featuring Ga-As heterodimers. These results fully explain the experimental scanning tunneling microscopy images and are likely to be relevant also to the c(4x4)-reconstructed (001) surfaces of other III-V semiconductors.