Ab Initio Atomistic Thermodynamics Research Articles

Tuning the crystal shape of materials by chemical potential: a combined theoretical and experimental study for NiSe2

Understanding and controlling the shape of materials has attracted considerable attention over the past few decades. Taking a typical transition metal chalcogenide NiSe2 as an example, we have figured out the role of chemical potential in the equilibrium shape of materials by a combined theoretical and experimental study. The theoretical equilibrium shape evolution of NiSe2 with chemical potential has been investigated using ab initio atomistic thermodynamics and Wulff construction. Moreover, in order to confirm our theoretical results, a series of NiSe2 crystals with different morphologies were also synthesized via a solvothermal route in mixed solvents of hydrazine hydrate, ethylenediamine and reactant. X-ray powder diffraction (XRD) and field emission scanning electron microscopy (FESEM) were used to analyze the products. Our results indicated that the theoretical equilibrium shapes of NiSe2 change from pyritohedron enclosed by (210) facet, truncated octahedron to perfect octahedron occupied by (111) facet, which agrees with the experimental shapes of NiSe2 synthesized with different volume ratios of mixed solvents. The present theory and experimental work indicates that controlling the chemical potential is a general and efficient way to tailor the shape of materials.

Adsorption and oxidation of sulfur dioxide on the yttria-stabilized zirconia surface: ab initio atomistic thermodynamics study

The interaction of SO2 with the yttria-stabilized zirconia (YSZ) (111) and oxygen-enriched YSZ (111) (YSZ+O) surface is investigated using the first-principles method based density functional theory (DFT). It is found that SO2 is adsorbed either as a molecule or forms SO2−3 species with new SO bonds to a surface oxygen on the YSZ (111) surface. In addition, there exist other species, e.g., SO3 and SO2−4 on the very active YSZ+O (111) surface. Using the ab initio atomistic thermodynamics method, we present a detailed analysis on the stability of the SO2YSZ/YSZ+O system as a function of the ambient conditions.

Reduction mechanisms of the CuO(111) surface through surface oxygen vacancy formation and hydrogen adsorption

We studied the reduction of CuO(111) surface using density functional theory (DFT) with the generalized gradient approximation corrected for on-site Coulomb interactions (GGA + U) and screened hybrid DFT (HSE06 functional). The surface reduction process by oxygen vacancy formation and H2 adsorption on the CuO(111) surface is investigated as two different reduction mechanisms. It is found that both GGA + U and HSE06 predict the same trend in the relative stability of oxygen vacancies. We found that loss of the subsurface oxygen is initially thermodynamically favourable. As the oxygen vacancy concentration increases, mixture of subsurface and surface vacancies is energetically preferred over full reduction of the surface or subsurface monolayer. The reduction of Cu(2+) to Cu(+) is found to be more favourable than that of Cu(+) to Cu(0) in the most stable vacancy structures at all concentrations. Consistent with the oxygen vacancy calculations, H2 adsorption occurs initially on under-coordinated surface oxygen. Water molecules are formed upon the adsorption of H2 and this gives a mechanism for H2 reduction of CuO to Cu. Ab initio atomistic thermodynamics shows that reducing CuO to metallic Cu at the surface is more energetically difficult than in the bulk so that the surface oxide protects the bulk from reduction. Using H2 as the reducing agent, it is found that the CuO surface is reduced to Cu2O at approximately 360 K and that complete reduction from Cu2O to metallic Cu occurs at 780 K.

Ab initio atomistic thermodynamics study on the sulfur tolerance mechanism of the oxygen-enriched yttria-stabilized zirconia surface

The first-principles method based on density functional theory (DFT) is used to investigate the reaction mechanism for the adsorption of H2S on the oxygen-enriched yttria-stabilized zirconia (YSZ+O) (111) surface. It is found that the H2S dissociation processes have low energy barriers (<0.5eV) and high exothermicities (2.5eV), and the dissociative S atoms may result in the poisoning of the YSZ+O surface by forming the SO and the hyposulfite (SO22−) species with very strong bonds to the surface. In addition, using the ab initio atomistic thermodynamics method, the surface regeneration or de-sulfurization process of a sulfur-poisoned (i.e. sulfur-covered) YSZ+O(111) surface is studied. According to the phase diagram, the adsorbed atomic sulfur can be oxidized to SO2 and removed from the YSZ+O surface by introducing oxidizing reagents, e.g. O2 and H2O.

Oxidation of step edges on vicinal 4H-SiC(0001) surfaces

The oxidation processes of stepped SiC(0001) surfaces are studied within the ab initio atomistic thermodynamics approach. Our calculations show that a one-dimensional -Si-O- chain structure as a precursor for oxide growth on stepped SiC surfaces is formed along the step edge, promoting further oxidation of the step edges. Following the modified Deal-Grove oxidation model, we also find that the oxidation rate at steps is higher than that at terraces by three orders of magnitude. These findings give a reasonable explanation for the oxide thickness fluctuation between the step and the terrace observed in the previous experiments.

Tuning patterning conditions by co-adsorption of gases: Br2 and H2 on Si(001)

We have studied the co-adsorption of Br2 and H2 on Si(001), and obtained co-adsorption energies and the surface phase diagram as a function of the chemical potential and pressure of the two gases. To do this, we have used density functional theory calculations in combination with ab initio atomistic thermodynamics. Over large ranges of bromine and hydrogen chemical potentials, the favored configuration is found to be either one with only Br atoms adsorbed on the surface, at full coverage, in a (3 × 2) pattern, or a fully H-covered surface in a (2 × 1) structure. However, we also find regions of the phase diagram where there are configurations with either only Br atoms, or Br and H atoms, arranged in a two-atom-wide checkerboard pattern with a (4 × 2) surface unit cell. Most interestingly, we find that by co-adsorbing with H2, we bring this pattern into a region of the phase diagram corresponding to pressures that are significantly higher than those where it is observed with Br2 alone. We also find small regions of the phase diagram with several other interesting patterns.

First-Principles Predictions of the Structure, Stability, and Photocatalytic Potential of Cu2O Surfaces

For a photocatalytic reaction to be thermodynamically allowed, a semiconductor's band edges need to be placed appropriately relative to the reaction redox potentials. We apply a recently developed scheme for calculating band edges with density functional theory (DFT)-based methods to Cu2O, evaluating its available thermodynamic overpotential for redox reactions such as water splitting and conversion of CO2 to methanol. Because these calculations are surface dependent, we first study the low-index surfaces of Cu2O using periodic DFT+U theory to characterize and identify the most stable surface, which will be the most catalytically relevant. We employ various techniques to calculate the surface energy, including the method of "ab initio atomistic thermodynamics". The Cu2O(111) surface with (1 × 1) periodicity and surface copper vacancies is identified as the most stable at all oxygen partial pressures, although the ideal stoichiometric Cu2O(111) surface is relatively close in energy under oxygen-poor conditions. These surfaces are then used to calculate the band edges. Comparison of the band edges to redox potentials reveals that Cu2O is thermodynamically capable of photocatalytic reduction of CO2 to methanol and the reduction and oxidation of water.

Stability and Metastability of Clusters in a Reactive Atmosphere: Theoretical Evidence for Unexpected Stoichiometries ofMgMOx

By applying a genetic algorithm and ab initio atomistic thermodynamics, we identify the stable and metastable compositions and structures of MgMOx clusters at realistic temperatures and oxygen pressures. We find that small clusters (M≲5) are in thermodynamic equilibrium when x>M. The nonstoichiometric clusters exhibit peculiar magnetic behavior, suggesting the possibility of tuning magnetic properties by changing environmental pressure and temperature conditions. Furthermore, we show that density-functional theory with a hybrid exchange-correlation functional is needed for predicting accurate phase diagrams of metal-oxide clusters. Neither a (sophisticated) force field nor density-functional theory with (semi)local exchange-correlation functionals is sufficient for even a qualitative prediction.

Origin of the unique activity of Pt/TiO2 catalysts for the water–gas shift reaction

Periodic density functional theory calculations and microkinetic modeling are used to illustrate the specific role of the three-phase boundary (TPB) in determining the activity and selectivity of TiO2-supported Pt catalysts for the water–gas shift (WGS) reaction. The Pt8/TiO2(110) catalyst model identified from a systematic ab initio atomistic thermodynamics study is used to investigate the redox mechanism and associative pathway with redox regeneration of the WGS reaction. Analysis of a microkinetic model determined exclusively from first principles suggests that a CO-promoted redox pathway dominates in the low-temperature range of 473–623K and the classical redox pathway becomes dominant at temperatures above 673K. The improved activity of the TPB compared to the Pt(111) surface can be explained by a reduced CO adsorption strength on Pt sites at the TPB, an increased number of oxygen vacancy at the TPB, and a significantly facilitated water activation and dissociation.

Reaction cycles and poisoning in catalysis by gold clusters: A thermodynamics approach

In heterogeneous catalysis, a catalytic process takes place at finite temperature and at finite pressure of the atmosphere of the reactant gases. By applying ab initio atomistic thermodynamics to the model case of free Au2 and clusters in an atmosphere of O2 and CO, we derive all the thermodynamically possible reaction paths for the oxidation of CO to CO2. This analysis lets us explain how gold clusters enable oxidation reactions without breaking the spin-conservation rule. Furthermore, we identify special cluster + ligands compositions such as reaction intermediates and poisoned species. In particular, a thermodynamically driven poisoning is identified for the catalytic system containing free Au2, and the experimental (p, T) conditions that avoid its formation are suggested. This implies that for some systems a catalytic cycle can be established, on thermodynamics grounds, only in a defined range of temperatures and pressures. In addition, our predictions for provide the so far most complete interpretation of the available experimental data (Socaciu et al, J. Am. Chem. Soc. 2003). © 2013 Wiley Periodicals, Inc.

An atomistic thermodynamics study of the structural evolution of the Pt3Ni(111) surface in an oxygen environment

The structural evolution of the Pt 3 Ni(111) surface under oxidizing conditions was studied by ab initio atomistic thermodynamics. The thermodynamic phase diagram from Ni-rich to Pt-rich conditions with oxygen coverages up to one monolayer was constructed from their 560 possible surface structures. With an increase in the oxygen chemical potential, there were only two types of thermodynamically stable structures, which were a clean Pt-skin surface and a Ni-skin surface with chemisorbed oxygen, regardless of the underlying Pt-rich or Ni-rich conditions. Bimetallic surfaces with chemisorbed oxygen were only metastable. The detail analysis revealed that the structural evolution is determined by the factors of segregation cost, difference between oxygen-metal (Pt and Ni) bonding strength, and oxygen chemical potential. With a gradual increase of oxygen chemical potential, the clean Pt-skin surfaces of Pt 3 Ni(111) transit to the oxygen-chemisorbed Ni-skin surfaces without stable oxygen-chemisorbed PtNi surface alloys formed.

First-principles prediction of half-metallicity at the low index surfaces of rocksalt KS

The structural, electronic and magnetic properties of the low index (111), (110) and (001) surfaces of the rocksalt Potassium Sulfide (rs-KS) are investigated by using pseudo-potential calculations in the framework of spin density functional theory. The results show that the bulk half-metallicity of this compound is well preserved on its low index surfaces. The spin flip gap at the studied surfaces is close to the bulk value except for the potassium terminated (111) surface which exhibits about half of the bulk spin flip gap. The calculated surface energies in the framework of ab-initio atomistic thermodynamics indicate that the potassium terminated (111) surface of KS has the lowest energy over entire allowed range of the chemical potentials, while the surface energy of the (001) and (110) surfaces approaches the surface energy of the stable surface at the sulfur rich environment. In the framework of the Heisenberg model, it is argued that the surface effects have small effects on the interlayer exchange interaction in the stable K-terminated (111) surface, compared with the bulk KS.

Theoretical and experimental studies of hydrogen adsorption and desorption on Ir surfaces

We report adsorption and desorption of hydrogen on planar Ir(210) and faceted Ir(210), consisting of nanoscale {311} and (110) facets, by means of temperature programmed desorption (TPD) and density functional theory (DFT) in combination with the ab initio atomistic thermodynamics approach. TPD spectra show that only one H2 peak is seen from planar Ir(210) at all coverages whereas a single H2 peak is observed at around 440 K (F1) at fractional monolayer (ML) coverage and an additional H2 peak appears at around 360 K (F2) at 1 ML coverage on faceted Ir(210), implying structure sensitivity in recombination and desorption of hydrogen on faceted Ir(210) versus planar Ir(210), but no evidence is found for size effects in recombination and desorption of hydrogen on faceted Ir(210) for average facet sizes of 5-14 nm. Calculations indicate that H prefers to bind at the two-fold short-bridge sites of the Ir surfaces. In addition, we studied the stability of the Ir surfaces in the presence of hydrogen at different H coverages through surface free energy plots as a function of the chemical potential, which is also converted to a temperature scale. Moreover, the calculations revealed the origin of the two TPD peaks of H2 from faceted Ir(210): F1 from desorption of H2 on {311} facets while F2 from desorption of H2 on (110) facets.

Ab initioatomistic thermodynamics study of the early stages of Cu(100) oxidation

Using an ab initio atomistic thermodynamics framework, we identify the stable surface structures during the early stages of Cu(100) oxidation at finite temperature and pressure conditions. We predict the clean surface, the 0.25 monolayer oxygen-covered surface, and the missing-row reconstruction as thermodynamically stable structures in range of 100--1000 K and 10${}^{\ensuremath{-}15}$--10${}^{5}$ atm, consistent with previous experimental and theoretical results. We also investigate the thermodynamic stabilities of possible precursors to Cu${}_{2}$O formation including missing-row reconstruction structures that include extra on- or subsurface oxygen atoms as well as boundary phases formed from two missing-row nanodomains. While these structures are not predicted to be thermodynamically stable for oxygen chemical potentials below the nucleation limit of Cu${}_{2}$O, they are likely to exist due to kinetic hindrance.

Surface morphology of Hägg iron carbide (χ-Fe5C2) from ab initio atomistic thermodynamics

Ab initio atomistic thermodynamics is utilized to achieve the understanding of the surface structure and stability of χ-Fe5C2 under CO and synthesis gas (H2/CO) pretreatment conditions in catalyst activation for Fischer–Tropsch synthesis (FTS). On the basis of the computed surface free energy (γ) as a function of the carbon chemical potential (μC) by considering temperature, pressure, and H2/CO ratios, it is found that CO pretreatment favors stable carbon-rich facets, while small amount of H2 added into CO leads to a large decrease in μC and thus stabilizing carbon-poor facets. The high activity of surface carbon toward hydrogenation might explain the enhanced initial activity of the FTS catalysts activated from CO pretreatment. Moreover, under both CO and H2/CO pretreatments, either low temperature or low pressure can lead to stable carbon-rich facets.

Cu/ZnO(0001) under oxidating and reducing conditions: A first-principles survey of surface structures

We investigate the stability of Cu-exposed ZnO(0001) surface structures in an oxygen environment using density functional theory and the method of ab initio atomistic thermodynamics. A two-dimensional phase diagram is constructed which identifies stable surface structures as a function of the copper and oxygen chemical potentials. Two structures with a $(\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3})$R30${}^{\ensuremath{\circ}}$ unit cell are found to be prominently stable at intermediate oxygen and copper chemical potentials. These phases are characterized by a single adlayer of Cu${}_{4}$O${}_{3}$ and Cu${}_{12}$O${}_{13}$ stoichiometry on ZnO(0001). We rationalize recent experimental observations in the literature in terms of our results.

Chlorination of the Cu(110) Surface and Copper Nanoparticles: A Density Functional Theory Study

The interaction of atomic chlorine with the Cu(110) surface is studied using density functional theory and ab initio atomistic thermodynamics. The calculated surface free energies of different Cl/Cu(110) structures as a function of chlorine chemical potential show that under ultrahigh-vacuum conditions, the c(2 × 2)-Cl structure is the most stable for coverages up to and including 1/2 ML, whereas the (2 × 3)-Cl and (1 × 4)-Cl configurations are the most stable at 2/3 and 3/4 ML coverages, respectively. It is also shown that although there are thermodynamically stable geometries for high (1 ML) coverage of Cl, these structures are only kinetically accessible. The morphology of a copper nanostructure terminated by low-index Cu surfaces in a chlorine environment has been predicted using a Wulff construction. It is found that the (111) facets dominate at low chlorine concentration, but as the chlorine concentration is increased, the (100) planes become predominant, resulting in a cubical crystal shape.

Half-metallic properties of the Co 2Ti 1− xFe xGa Heusler alloys and Co 2Ti 0.5Fe 0.5Ga (0 0 1) surface

Electronic and magnetic properties of the bulk Co 2Ti 1− x Fe x Ga Heusler alloys and Co 2Ti 0.5Fe 0.5Ga (0 0 1) surfaces are studied within the framework of density functional theory using the augmented plane wave plus local orbital (APW+lo) approach. It will be shown that all alloys have the spin polarization of the ideal 100% value except the Co 2FeGa alloy with spin polarization about 98%. Co 2Ti 0.5Fe 0.5Ga is an example that is stable against the effects destroying the half-metallicity due to the position of the Fermi energy ( E F) in the middle of the minority band gap. The phase diagram obtained by ab-initio atomistic thermodynamics shows that in the higher limit of μ Ga three surfaces of FeGa, TiGa and TiFeGa are accessible in the Co 2Ti 0.5Fe 0.5Ga alloy but on decreasing μ Ga, the accessible region gradually moves towards FeGa termination. It is discussed that, at the ideal surfaces, half-metallicity of the alloy is lost, although the TiGa surface keeps high spin polarization (about 95%).

Free gold clusters: beyond the static, monostructure description

The thermodynamical stability of free, pristine gold clusters at finite temperature, and of cluster+ligands complexes at finite temperature and in the presence of an atmosphere composed of O2 and CO, is studied employing parallel tempering and ab initio atomistic thermodynamics. We focus on Au13, which displays a significant fluxional behavior: Even at low temperature (100 K) this cluster exhibits a multitude of structures that dynamically transform into each other. At finite temperature, the preference of this cluster for three-dimensional versus planar structures is found to result from entropic effects. For gold clusters containing one to four gold atoms in an O2 + CO atmosphere, we apply ab initio atomistic thermodynamics. On the basis of these considerations, we single out a likely reaction path for CO oxidation catalyzed by gold clusters.

An equilibrium ab initio atomistic thermodynamics study of chlorine adsorption on the Cu(001) surface

The effect of chlorine (Cl) chemisorption on the energetics and atomic structure of the Cu(001) surface over a wide range of chlorine pressures and temperatures has been studied using equilibrium ab initio atomistic thermodynamics to elucidate the formation of cuprous chloride (CuCl) as part of the Deacon reaction on copper metal. The calculated surface free energies show that the 1/2 monolayer (ML) c(2 × 2)-Cl phase with chlorine atoms adsorbed at the hollow sites is the most stable structure for a wide range of Cl chemical potential, in agreement with experimental observations. It is also found that at very low pressure and exposure, but elevated temperature, the 1/9 ML and 1/4 ML phases become the most stable. By contrast, a high coverage of Cl does not lead to thermodynamically stable geometries. The subsurface adsorption of Cl atoms, however, dramatically increases the stability of the 1 ML and 2 ML adsorption configurations providing a possible pathway for the formation of the bulk-chloride surface phases in the kinetic regime.