Surface Formation Energy Research Articles

A Bimetallic Strategy to Tailoring the D-Band Center and Surface Formation Energy of Ni-Based Catalyst by Cu for Efficient and Stable Catalytic Hydrogenation of Dioctyl Phthalate

A Bimetallic Strategy to Tailoring the D-Band Center and Surface Formation Energy of Ni-Based Catalyst by Cu for Efficient and Stable Catalytic Hydrogenation of Dioctyl Phthalate

Antiferromagnetic coupling in the initial stages of the MnN epitaxial growth on the CrN (0 0 1) surface

Spin polarized first principles calculations have been performed to study the structural and electronic properties, and spin coupling of the MnN deposit on the CrN (001) surface. Results indicate that Mn may transform from a paramagnetic to a ferromagnetic system, when adsorbed on a surface. Also, the first CrN layer switches from antiferromagnetic to ferromagnetic alignment. Surface formation energies show that epitaxial growth of θ-MnN on the CrN (001)-(1x1) surface is favorable for N-rich conditions and intermediate conditions. The double MnN layer exhibits an antiferromagnetic behavior, with the magnetic moments coupled parallel and alternating along c. Moreover, the exchange bias effect is manifested in the first CrN layer. The calculated projected densities of states spectra show a half metallic character in the first CrN layer. The Mn magnetic moments are in the range of 2.8–3.3 μB. This work identifies an antiferromagnetic system for possible applications in spintronics.

Modeling the half-metallicity of the CrN/GaN (1 1 1) heterostructure

Spin-polarized total-energy calculations are employed to investigate the structural, electronic and magnetic properties of the CrN epitaxial growth on the GaN (1 1 1) surface. Cr adsorption and incorporation were considered as initial stages. The adsorption takes place at high symmetry sites, and in the incorporation, Cr replaces Ga. The structures stability is studied with the surface formation energy (SFE) formalism. Results show that in the range from Ga-rich to Ga-poor conditions, and from Cr-rich to Cr-intermediate conditions, the ferromagnetic CrN monolayer is the most stable structure. The interface shows half-metal characteristics, where the minority spins displays a large band-gap. When depositing extra CrN layers, the structure displays the CrN rock-salt phase as the most stable configuration. The epitaxial growth of 2 and 3 CrN layers depicts antiferromagnetic (AFM) characteristics; moreover the interface remains with half-metal properties and Dirac-cone-like features. However, the band-gap of the minority spins is reduced, therefore, it is expected that a large number of CrN bilayers could drive the system to a half-metal to metallic transition. Calculations establish a strategy to generate 100% polarized carriers injection to GaN, which poses this CrN/GaN (1 1 1)||(1 1 1) heterostructure as a component to construct spintronic devices.

First-principles calculations of metal surfaces. I. Slab-consistent bulk reference for convergent surface properties

The first-principles computation of the surfaces of metals is typically accomplished through slab calculations of finite thickness. The extraction of a convergent surface formation energy from slab calculations is dependent upon defining an appropriate bulk reference energy. I describe a method for an independently computed, slab-consistent bulk reference that leads to convergent surface formation energies from slab calculations that also provides realistic uncertainties for the magnitude of unavoidable nonlinear divergence in the surface formation energy with slab thickness. The accuracy is demonstrated on relaxed, unreconstructed low-index aluminum surfaces with slabs with up to 35 layers.

Accurate Deep Potential model for the Al–Cu–Mg alloy in the full concentration space* *Project supported by the National Natural Science Foundation of China (Grant No. 11871110), the National Key Research and Development Program of China (Grant Nos. 2016YFB0201200 and 2016YFB0201203), and Beijing Academy of Artificial Intelligence (BAAI).

Combining first-principles accuracy and empirical-potential efficiency for the description of the potential energy surface (PES) is the philosopher’s stone for unraveling the nature of matter via atomistic simulation. This has been particularly challenging for multi-component alloy systems due to the complex and non-linear nature of the associated PES. In this work, we develop an accurate PES model for the Al–Cu–Mg system by employing deep potential (DP), a neural network based representation of the PES, and DP generator (DP-GEN), a concurrent-learning scheme that generates a compact set of ab initio data for training. The resulting DP model gives predictions consistent with first-principles calculations for various binary and ternary systems on their fundamental energetic and mechanical properties, including formation energy, equilibrium volume, equation of state, interstitial energy, vacancy and surface formation energy, as well as elastic moduli. Extensive benchmark shows that the DP model is ready and will be useful for atomistic modeling of the Al–Cu–Mg system within the full range of concentration.

The Atomic Structure of the Se-Passivated CuInSe2

Through the first-principles calculations, we studied the atomic structure of Se-passivated CuInSe2. It is revealed that, the CuInSe2 can be passivated by selenium, under Se-rich condition, the structures with a full Se layer plus dimerized Se adatoms are favored. The formation energy of Se-terminated surface and the surface with Se overlayer are different, the formation energy of the surface with Se overlayer is more lower than the pure Se-terminated surface, that is to say, the surface with Se overlayer is more stable. On the other hand, the atomic structure of the Se-passivated CuInSe2 are also different from the atomic structure of the pure Se-terminated surface. Studying the structure of the Se-passivated CuInSe2 is very helpful for the high-efficiency solar cells.

Selenium Passivation of the CuInSe2 for High-Efficiency Solar Cells

In this paper, selenium passivation of the CuInSe2 for high-efficiency solar cells is studied through first-principles calculations. It is revealed that, the formation energy of Se-terminated surface and the surface with Se overlayer are different. The surface with Se overlayer are more stable, it guarantees an ideal growth that can efficiently limit the formation of point defects. It is further proved that the unique surfaces of CuInSe2 play the critical roles on the high efficiency of the based thin film solar cell. It is the significant foundation for improving the growth of related solar cells and it is very important for the formation and control of related defects in the growth of CIS. Due to the different surface energy and property, the study of the growth of Selenium passivation of CIS is very important.

Factors determining surface oxygen vacancy formation energy in ternary spinel structure oxides with zinc.

Spinel oxides are an important class of materials for heterogeneous catalysis including photocatalysis and electrocatalysis. The surface O vacancy formation energy (EOvac) is a critical quantity for catalyst performance because the surface of metal oxide catalysts often acts as a reaction site, for example, in the Mars-van Krevelen mechanism. However, experimental evaluation of EOvac is very challenging. We obtained the EOvac for (100), (110), and (111) surfaces of normal zinc-based spinel oxides ZnAl2O4, ZnGa2O4, ZnIn2O4, ZnV2O4, ZnCr2O4, ZnMn2O4, ZnFe2O4, and ZnCo2O4. The most stable surface is (100) for all compounds. The smallest EOvac for a surface is the largest in the (100) surface except for ZnCo2O4. For (100) and (110) surfaces, there is a good correlation, over all spinels, between the smallest EOvac for the surface and bulk formation energy, while the ionization potential correlates well in (111) surfaces. Machine learning over EOvac of all surface sites in all orientations and for all compounds to find the important factors, or descriptors, that decide the EOvac revealed that bulk and surface-dependent descriptors are the most important, namely the bulk formation energy, a Boolean descriptor of whether the surface is (111) or not, and the ionization potential, followed by geometrical descriptors that are different in each O site.

Unveiling Temperature-Induced Structural Domains and Movement of Oxygen Vacancies in SrTiO3 with Graphene.

Heterointerfaces coupling complex oxides exhibit coexisting functional properties such as magnetism, superconductivity, and ferroelectricity, often absent in their individual constituent. SrTiO3 (STO), a canonical band insulator, is an active constituent of such heterointerfaces. Temperature-, strain-, or mechanical stress-induced ferroelastic transition leads to the formation of narrow domains and domain walls in STO. Such ferroelastic domain walls have been studied using imaging or transport techniques and, often, the findings are influenced by the choice and interaction of the electrodes with STO. In this work, we use graphene as a unique platform to unveil the movement of oxygen vacancies and ferroelastic domain walls near the STO surface by studying the temperature and gate bias dependence of charge transport in graphene. By sweeping the back gate voltage, we observe antihysteresis in graphene typically observed in conventional ferroelectric oxides. Interestingly, we find features in antihysteresis that are related to the movement of domain walls and of oxygen vacancies in STO. We ascertain this by analyzing the time dependence of the graphene square resistance at different temperatures and gate bias. Density functional calculations estimate the surface polarization and formation energies of layer-dependent oxygen vacancies in STO. This corroborates quantitatively with the activation energies determined from the temperature dependence of the graphene square resistance. Introduction of a hexagonal boron nitride (hBN) layer, of varying thicknesses, between graphene and STO leads to a gradual disappearance of the observed features, implying the influence of the domain walls onto the potential landscape in graphene.

Theoretical investigation of the AlN (0 0 0 1)-(2 × 2) surface doped with nickel: Structural, electronic and magnetic properties

Structural, electronic and magnetic properties of the nickel doped AlN (0 0 0 1)-(2 × 2) surfaces are investigated by spin-polarized first-principle total-energy calculations. The exchange–correlation potential energies are treated within the generalized gradient approximation (GGA) with the Perdew, Burke and Enzernof (PBE) functional. Electron-ion interactions are dealt with the pseudopotential approach. According to results the H3 site is the most favorable structure in the adsorption of Ni at different coverage from ¼ to 1 monolayer (ML). When coverage is in the range from ½ to 1 ML, we found ferromagnetic (FM) behavior as a consequence of the Ni presence with magnetic moment (>0.7 μB/atom). Also, we found the formation of dimers, trimers and atomic chains in the ½, ¾ and 1ML coverage, respectively. The minimum energy pathways in the Ni adsorption and incorporation show activation energies of 1.53 and 1.26 eV, respectively. The deposit of Ni into substitutional sites suggests the possibility of growing NiN epitaxially. We employed the surface formation energy formalism to investigate the different structures stability. Results indicate that the Ni doping is favorable under Ni-rich conditions in all the range of chemical potential ΔμAlN, finding four AlN surfaces with different magnetic properties: the 1Ni ads and 4Ni subs models are non-magnetic, however the 1Ni subs model is FM and the 2Ni subs model is antiferromagnetic (AFM) with magnetic moments of the order of 0.8 μB/atom. Therefore, we can conclude that high concentrations of Ni induce a non-magnetic behavior, and low concentrations favor the magnetic properties, provided that the Ni atoms occupy substitutional sites. The density of states shows that only the 1st ML of surfaces is metallic, in contrast layers beneath the first layer are semiconductor, where the valence and conductions bands are mainly formed by the N-p and Al-p orbitals, respectively.

Shallow Iodine Defects Accelerate the Degradation of α-Phase Formamidinium Perovskite

Summary Shallow defects are mostly benign in covalent semiconductors, such as silicon, given that they do not constitute non-radiative recombination sites. In contrast, the existence of shallow defects in ionic perovskite crystals might have significant repercussions on the long-term stability of perovskite solar cells (PSCs) because of the metastability of the ubiquitous formamidinium lead triiodide (FAPbI3) perovskite and the migration of charged point defects. Here, we show that shallow iodine interstitial defects ( I i ) can be generated unintentionally during commonly used post-fabrication treatments, which can lower the cubic-to-hexagonal transformation barrier of FAPbI3-based perovskites to accelerate its phase degradation. We demonstrate that concurrently avoiding the generation of I i and the more effective passivation of iodine vacancies ( V I ) can improve the thermodynamic stability of the films and operational stability of the PSCs. Our most stable PSC retained 92.1 % of its initial performance after nearly 1,000 h of continuous illumination operational stability testing.

Density Functional Theory Study of NiFeCo Trinary Oxy-Hydroxides for an Efficient and Stable Oxygen Evolution Reaction Catalyst.

NiOOH and its doped species are widely used as electrocatalysts for the oxygen evolution reaction (OER) in alkaline media. In this work, we carried out comprehensive density functional theory (DFT) simulations of Ni-based electrocatalysts for the OER by applying suitable dopants in β-NiOOH. A range of Fe and Co atoms (%) are employed as doping agents to increase the overall catalytic ability, stability, and feasibility of NiOOH. Our simulations indicate that Ni88%Fe6%Co6%OOH is efficient, stable, and provides more catalytic sites at the surface of resulting catalysts for water adsorption and dissociation, which facilitate the OER. The lower overpotential for the OER is estimated from the higher adsorption energy of water molecule over the surface of Ni88%Fe6%Co6%OOH, followed by other electronic properties such as band structure, electrostatic potential, the density of states, and surface formation energy.

A brief review of formation energies calculation of surfaces and edges in semiconductors

To have a high quality experimental growth of crystals, understanding the equilibrium crystal shape (ECS) in different thermodynamic growth conditions is important. The factor governing the ECS is usually the absolute surface formation energies for surfaces (or edges in 2D) in different orientations. Therefore, it is necessary to obtain an accurate value of these energies in order to give a good explanation for the observation in growth experiment. Historically, there have been different approaches proposed to solve this problem. This paper is going to review these representative literatures and discuss the pitfalls and advantages of different methods.

Development of a segregation model beyond McLean based on atomistic simulations

In the present atomistic study, layer-specific segregation to the surface is investigated for an exemplary (100) surface in iron–chromium alloys using an embedded-atom potential. Through a continuous variation of the chemical potential difference in the semi-grandcanonical ensemble, the full composition range is explored at temperatures between 600 K and 1400 K. The obtained layer-specific segregation curves demonstrate the well-known limitations of the widely used McLean model for interface segregation, i.e. a monolayer model imposing ideal behavior. However, keeping the original idea of McLean, the segregation model is extended in two ways in order to provide a complete analytical description of segregation: Firstly, the entire surface is hypothetically replaced by an equivalent single layer with identical segregation properties, which accounts for all subsurface layer effects, but also enables the application of an effective monolayer model with only one single energy of segregation. Secondly, directly following from the general treatment assuming non-ideal solution behavior, the energy of segregation becomes composition-dependent. The validity and accuracy of the proposed analytical model is confirmed by fitting the composition-dependent energy of segregation to the thermodynamically unambiguous solute excess. The change in surface formation energy according to the interfacial adsorption equation can be described excellently over the entire composition range for all investigated temperatures. The model can be applied to experimental data and directly transferred to grain boundaries.

Design of Ceria Catalysts for Low‐Temperature CO Oxidation

AbstractA catalyst active to carbon monoxide (CO) oxidation at low temperature is essential for environmental conservation, saving fuel and improvement of the quality of human life. Rational design of CO oxidation catalyst on the basis of comprehensive understanding of physicochemical properties of catalytic materials, rather than simply searching for the catalyst based on trial‐and‐error, is a promising approach to meet the increasingly stringent regulations. This review covers metal‐doped and ‐loaded system based on CeO2 catalysts as strategies to significantly improve CO oxidation activity at low temperature. When incorporated into CeO2 lattice, active metals significantly lower the oxygen vacancy formation energy (Evf) of the catalyst surface, resulting in high catalytic activities at low temperature. When the active metals are loaded on the CeO2 surface, many active sites could be acquired by increasing the dispersion, and the catalytic activity can be dramatically improved by newly introducing the interfacial sites between the metals and the CeO2 support. Doping the support could further improve this loaded system in terms of specific surface area, oxygen vacancy formation, and spillover effects. In this review, based on this knowledge, we propose a rational design approach to a robust low‐temperature CO oxidation catalysts. The desirable CO oxidation catalysts identified from the interplay between theoretical and experimental approaches would ultimately improve the quality of human life, and create potential economic benefits by alleviating air pollution.

Identifying the Origin of the Limiting Process in a Double Perovskite PrBa0.5Sr0.5Co1.5Fe0.5O5+δ Thin-Film Electrode for Solid Oxide Fuel Cells.

Oxygen reduction reaction in a double perovskite material, PrBa0.5Sr0.5Co1.5Fe0.5O5+δ (PBSCF), was studied for application as a cathode in a solid oxide fuel cell (SOFC). Electrochemical measurements were performed on a geometrically well-defined dense thin-film (0.8-2 μm thickness) electrode, fabricated as a symmetric cell. In combination with density functional theory (DFT) and molecular dynamics (MD) simulations, experiments provided an insight into the operating mechanism of the SOFC material tested at an open-circuit voltage. The dense thin-film electrode of PBSCF showed a thickness-dependent electrochemical performance, suggesting bulk diffusion limitation. To understand the origin of this diffusion-limiting electrochemical performance, DFT calculations were utilized to calculate the surface (γ) and oxygen vacancy formation (EOV) energies. For example, EOV in the Pr plane (190 kJ/mol) of PBSCF was measured to be lower than that of the BaSr plane (EOV = 297 kJ/mol). In addition, oxygen vacancies were difficult to be created in the BaSr/CoFe terminal surface (EOV = 341.6 kJ/mol) as compared to other terminal surfaces. MD simulations further elaborated on the nature of cation disordering in the surface and subsurface regions, consequently leading to the preferential segregation of the Ba cations to the surface, which is a known phenomenon in such double perovskite materials. Because of cation disordering and segregation of Ba species, the oxygen anion diffusivity (∼10-12 cm2 s-1), calculated from MD, in the near-surface region was observed to be 2 orders of magnitude lesser than that of the bulk (D = 2.98 × 10-10 cm2 s-1) of the material at 973 K. Surface characterization of the thin-film electrode using X-ray photoelectron spectroscopy was indicative of a nonperovskite Ba2+ phase on the electrode surface. The segregation of Ba cations was linked with the transport of oxygen anions, which was limiting the electrochemical performance of the electrode.

Initial stages of the epitaxial growth of MnN on the GaAs (001)-(2 × 2) surface: First-principle study

First-principle total-energy calculations have been employed to investigate the epitaxial growth of manganese nitride (MnN) on the non-reconstructed GaAs (001)-(2 × 2) surface. Also, we have studied the surface nitridation process proposed by Gwo and Tokumoto. According to results, the surface nitridation generates a (3 × 3) flat surface that is composed by an array of missing N atomic rows along both [−110] and [110] directions. The initial stages of the epitaxial growth of MnN have been investigated by considering the adsorption and incorporation of Mn atoms. Calculations show that for ¼ ML coverage the most favorable adsorption site of Mn induces an fcc configuration and for the Mn incorporation an hcp-2 arrangement. The surface formation energy (SFE) yields the adsorption of Ga adatoms at the hcp-2 site as a stable configuration; in addition, a surface with 3 Mn atoms replacing 3 Ga atoms of the 1st ML is stable for intermediate and As-rich conditions. When only one MnN ML is considered, three interfaces may be formed with 25%, 75% and 100% of Mn, which yield stable configurations showing the FM/AFM interface. A second MnN ML is considered; in this case, only one structure is stable, and an Mn/MnN interface is formed. Calculations show that the interface exhibits an FM/AFM coupling structure.

Precious‐Metal‐Free Electrocatalysts for Activation of Hydrogen Evolution with Nonmetallic Electron Donor: Chemical Composition Controllable Phosphorous Doped Vanadium Carbide MXene

AbstractThe insufficient strategies to improve electronic transport, the poor intrinsic chemical activities, and limited active site densities are all factors inhibiting MXenes from their electrocatalytic applications in terms of hydrogen production. Herein, these limitations are overcome by tunable interfacial chemical doping with a nonmetallic electron donor, i.e., phosphorization through simple heat‐treatment with triphenyl phosphine (TPP) as a phosphorous source in 2D vanadium carbide MXene. Through this process, substitution, and/or doping of phosphorous occurs at the basal plane with controllable chemical compositions (3.83–4.84 at%). Density functional theory (DFT) calculations demonstrate that the PC bonding shows the lowest surface formation energy (ΔGSurf) of 0.027 eV Å−2 and Gibbs free energy (ΔGH) of –0.02 eV, whereas others such as P‐oxide and PV (phosphide) show highly positive ΔGH. The P3–V2CTx treated at 500 °C shows the highest concentration of PC bonds, and exhibits the lowest onset overpotential of –28 mV, Tafel slope of 74 mV dec−1, and the smallest overpotential of ‐163 mV at 10 mA cm−2 in 0.5 m H2SO4. The first strategy for electrocatalytically accelerating hydrogen evolution activity of V2CTx MXene by simple interfacial doping will open the possibility of manipulating the catalytic performance of various MXenes.

Manganese‑germanium nanostructure formation on the GaAs(111)–(1 × 1)A surface: Stability and magnetic properties

Spin-polarized first-principles total energy calculations have been developed to explore the Mn and Ge monolayer adsorption and incorporation into the unreconstructed GaAs(111)–(1 × 1)A surface. Surface formation energy results show that a germanium terminated surface is stable in a wide range of chemical potential; in this structure, the first Ga layer is replaced by a Ge layer. Upon increasing the As coverage, it is possible to find a stable structure with a manganese bilayer under an As monolayer. The density of states shows that a Ge terminated structure displays a semiconductor behavior whereas the Mn bilayer is metallic, with evident antiferromagnetic characteristics induced by the Mn atoms. Mn magnetic moments are of the order of ~4 μ B . The magnetic arrangement was confirmed by spin density distribution. Such antiferromagnetic structure may be suggested as a substrate to form magnetic junctions with other magnetic materials. • Ge deposition drives to formation of Ge layers on the GaAs surface. • Mn incorporates to form antiferromagnetic bilayers. • As atoms present magnetic moments induced by proximity to Mn.

Critical Role of Mullite-type Oxides’ Surface Chemistry on Catalytic NO Oxidation Performance

By combining low energy ion scattering spectroscopy and density functional theory calculation, we study the surface composition and surface formation energy of AMn2O5 (A = Sm, Bi) mullite-type oxides synthesized by different methods and their effects on NO catalytic performance. It is well-known that hydrothermal (HT) synthesis at low temperatures produces materials with higher specific surface areas (SSAs) compared with those synthesized by coprecipitation (CP) and high-temperature calcination; however, it is less clear how synthesis methods affect surface chemistry. We find that the NO oxidation performance of SmMn2O5–HT does not match the SSA increase when compared to the lower SSA SmMn2O5–CP. Combined experimental and theoretical investigation reveals that SmMn2O5–HT includes a higher fraction of inactive Sm-terminated surfaces, which explains its lower than expected activity. However, the surface chemistry change depends strongly on the A-site element. The exposed surfaces of BiMn2O5–CP are predomina...