Surface Formation Energy Research Articles

Ab Initio Prediction of Surface Stability of Fluorite Materials and Experimental Verification

Utilizing first-principle simulations [based on density functional theory (DFT) corrected for on-site Coulomb interactions (DFT+U)], we develop a model to explain the experimental stability in solution of materials having the fluorite structure, such as CaF2 and CeO2. It is shown that the stability of a surface is mainly dependent on its atomic structure and the presence of sites where atoms are deficiently bonded. Using as reference planes the surfaces with low surface formation energies, viz., (111), (100), and (110), our results reveal the relation between the surface energy of any Miller-indexed plane and the surface energy of those reference planes, being dependent on the fluorite surface structure only. Therefore, they follow the same trend for CaF2 and CeO2. Comparison with experimental results shows a correlation between the trends of dry surface energies and surface stabilities during dissolution of both CaF2 and CeO2, even though the chemical processes of dissolution of CeO2 and CaF2 are different. A deviation between ab initio predictions and experiments for some surfaces highlights the sensitivity of the developed model to the treatment of surface dipolar moments.

Density Functional Based Tight Binding Study on Wurzite ZnS Nanowires

We present a semi-abinitio - Density Functional Based Tight Binding (DFTB) study on the atomic and electronic structure of ZnS Nanowire (NW) and examine the dependence of surface stress on nanowire lateral size and shape. The ZnS wurtzite wire size ranges from 10 to 45Å and of various possible shapes - triangular, hexagonal and circular-like. We investigate the role of unsaturated dangling bond state in the region of band gap varying the wire's diameter. We have calculated the surface formation energy and find that it decreases with increasing the wire diameter and a greater stability (lower surface formation energy) comes with hexagonal and circular-like depending on wire's diameter. Other electronics and mechanical properties e.g. density of states (DOS), Young modulus... also are found to depend on the wire's size and shape. Futher the wire passivation by hydrogen has also investigated.

Investigating robustness of interatomic potentials with universal interface

We present a set of Python routines to perform basic tests of classical atomistic potentials and their example applications. These routines are implemented using universal Atomic Simulation Environment (ASE) and LAMMPS molecular dynamics code. ASE is utilized to create atomic configurations, to write input scripts for LAMMPS, and to read results from output files. Evaluated properties are formation energies and volumes of simple point defects (vacancies, substitutions, and interstitials), formation energies of basic surfaces, heats of formation of simple binary compounds, and elastic constants. The flexibility of LAMMPS allows easy switching between different semi-empirical potentials, while the universality of ASE allows to compare results with a variety of electronic structure codes.

Improved CO Adsorption Energies, Site Preferences, and Surface Formation Energies from a Meta-Generalized Gradient Approximation Exchange-Correlation Functional, M06-L.

A notorious failing of approximate exchange-correlation functionals when applied to problems involving catalysis has been the inability of most local functionals to predict the correct adsorption site for CO on metal surfaces or to simultaneously predict accurate surface formation energies and adsorption energies for transition metals. By adding the kinetic energy density τ to the density functional, the revTPSS density functional was shown recently to achieve a balanced description of surface energies and adsorption energies. Here, we show that the older M06-L density functional, also containing τ, provides improved surface formation energies and CO adsorption energies over revTPSS for five transition metals and correctly predicted the on-top/hollow site adsorption preferences for four of the five metals, which was not achieved by most other local functionals. Because M06-L was entirely designed on the basis of atomic and molecular energies, its very good performance is a confirmation of the reasonableness of its functional form. Two GGA functionals with an expansion in the reduced gradient that is correct through second order, namely, SOGGA and SOGGA11, were also tested and found to produce the best surface formation energies of all tested GGA functionals, although they significantly overestimate the adsorption energies.

First Principle Study on AlN Nano Wire

We present a first-principle study on the atomic and electronic structure of AlN Nanowire and examine the dependence of surface stress on nanowire lateral size and shape. The hexagonal wire size ranging as 10-16-22\AA\ and triangular wire ranging from 10-13-16-20\AA. We investigate the unsaturated dangling bond state in the region of bandgap with varying the wire�s diameter. We also calculated the surface formation energy and find that it decreases with increasing the wire diameter and a greater stability (lower surface formation energy) comes with hexagonal wires. We also study the dependence of theoretical prediction on various density functional theory (DFT) treatment using Dmol3 local orbital density functional method with an effective core potential.

Density Functional Theory Study on the Role of Ceria Addition in TixCe1–xO2 Adsorbents for Thiophene Adsorption

A TixCe1–xO2-based material has been reported as a potential adsorbent for ultradeep adsorptive desulfurization of liquid hydrocarbon fuels. To understand the roles of TiO2 and CeO2 in the adsorbent, density functional theory (DFT) calculations examined the energetics of adsorption site formation and corresponding thiophene adsorption energies on surfaces of TiO2 anatase, CeO2, and Ce-doped anatase. Compared with pure anatase surfaces, thiophene binds more strongly to Ce-doped anatase surfaces. O-rich sites (with activated O2) bind strongly to thiophene, but the formation of the O-rich sites is favored only on higher energy surfaces such as anatase (001) and ceria (100). The formation of a TixCe1–xO2 mixed surface decreases the relative surface formation energy of anatase (001), and thus a greater fraction of the (001) facet could be exposed on the adsorbent. Ceria may also act as an oxygen storage reservoir, providing oxygen for the formation of O-rich sites that strongly adsorb thiophenic species.

密着強度の評価における表面形成と塑性変形のエネルギー

密着強度の評価における表面形成と塑性変形のエネルギー

Determination of the electronic band structure of the rutile polymorph of TiO2: a quantum chemical approach

The aim of this work is the investigation of the relationship between the electronic band structure of the TiO2 rutile and the dimensionality of the system. For three dimensional system the bulk form of rutile was considered, while a slab model was chosen in order to represent the titanium (IV) dioxide (110) surface. The influence of changing the number of atomic layers on the bandgap value for the (110) surface was also examined. Density of states referring to the bands from the first valence band up to the bottom of the conduction band was projected on the whole set of atomic orbitals as well as on the significant shells of the titanium and oxygen atoms. Ab initio calculations with a B3LYP functional were carried out. Basis sets used were modified Ti_86-411(d31)G_darco_unpub and O 8_411_muscat_1999. The results are compared with experimental and computational data already available in the literature. Surface termination problem was discussed and the application of the obtained results as a starting point to obtain the first model of the rutile titania nanotube was proposed. The surface formation energies for rutile planes with a different surface terminations were compared and the modification to the equation needed for surface energy calculation was introduced.

Theoretical study of ZnO(1 0 [formula omitted] 0) and M/ZnO(1 0 [formula omitted] 0) (M = Cu, Ag and Au) surfaces with DFT approach

Density functional calculations with the plane-wave pseudopotential method have been carried out in order to determine the geometric and electronic structure of ZnO(101¯0) and M/ZnO(101¯0) (M=Cu, Ag and Au). Surface formation energies of the ZnO(101¯0) surfaces were calculated. Spilling effect and Friedel oscillation effect were observed for the ZnO(101¯0) surface. Cu and Ag are energetically favorable to connect with O atoms while Au is energetically favorable to connect with Zn atom on the surface at full coverage. Binding energies and vibrational frequencies were analyzed. The results were compared with previous theoretical studies and available experiments.

Thermodynamic analysis of SiC polytype growth by physical vapor transport method

Crystal growth of a certain polytype of SiC in a process of physical vapor transport was studied on the basis of classical thermodynamic nucleation theory in conjunction with numerical results obtained from a global model. Formation of a certain polytype in the nucleation stage is determined by the energy balance among surface energy, formation energy and supersaturation. The preferential growth condition of a certain polytype was estimated. The value of supersaturation was estimated using a numerical model obtained by a global model that includes species transport as well as heat transport in a furnace. The results of calculation showed that 4H polytype is more stable than 15R, 6H and 3C polytypes. Free energy difference between 4H and 6H polytypes decreased when total pressure in the furnace decreased.

Structural and thermal properties of calcium using an MEAM potential

Structural and thermal properties of Ca are examined using a modified embedded-atom method (MEAM) interatomic potential. We developed an MEAM interatomic potential for calcium (Ca) using a first-principles method based on density functional theory (DFT). The material parameters, such as the cohesive energy, equilibrium atomic volume, and bulk modulus, are used to determine the MEAM potential parameters. The elastic constants and various point defect energies, such as monovacancy and interstitial defects, are also examined while developing the potential. Several structural properties of Ca, such as different surface formation energies, stacking fault energies, and thermal properties, such as coefficient of thermal expansion, specific heat and melting temperature, are investigated using the potential. We found that the present MEAM potential gives a good overall agreement with DFT calculations and experiments.

Properties of rutile TiO 2 surfaces from a Tight-Binding Variable-Charge model. Comparison with ab initio calculations

Up until now, variable charge models appeared to be inadequate for surface simulations of TiO 2. Particularly, the relative stability of the low-index surfaces was not well predicted, and the formation energy of the most stable (110) surface was far from recent ab initio predictions (Surf. Sci. 504, (2002) 115). In the present paper, low index (110), (100) and (001) surfaces of rutile TiO 2 have been successfully described at the atomic scale by means of a new variable-charge model. In this model, the iono-covalent metal–oxygen bond energy is calculated thanks to an analytical expression derived from a tight-binding description of the electronic structure of the oxide. This expression, which is a function of charge, is included in the charge equilibration procedure, which modifies the equilibrium state of the crystal with respect to previous models. Thus, the model becomes very stable with respect to charge transfers and leads to the stabilization of the (110), (100) and (001) surfaces of rutile TiO 2 with the right order in energy. Surface formation energies, atomic relaxations and charge transfers have been calculated and compared with density functional theory (DFT) calculations using the generalized gradient approximation (GGA) and the B3LYP hybrid functional with the CRYSTAL06 code. The surface energies calculated with the atomic model (0.42, 0.49 and 1.26 J m −2 respectively) are in very good agreement with our DFT calculations (0.48, 0.68 and 1.36 J m −2). Moreover, the atomic relaxations and the charge transfers at surfaces obtained with the model compare very reasonably both with the DFT results and the available experimental measurements. Our new model, whose transferability has been previously tested on different polymorphs of TiO 2, appears to be very useful in studying chemical properties of metal oxide surfaces.

Trends in Metal Oxide Stability for Nanorods, Nanotubes, and Surfaces

The formation energies of nanostructures play an important role in determining their properties, including their catalytic activity. For the case of 15 different rutile and 8 different perovskite metal oxides, we used density functional theory (DFT) to calculate the formation energies of (2,2) nanorods, (3,3) nanotubes, and the (110) and (100) surfaces. These formation energies can be described semiquantitatively (mean absolute error ≈ 0.12 eV) by the fraction of metal−oxygen bonds broken and the metal d-band and p-band centers in the bulk metal oxide.

Study of Superhydrophobic Electrospun Nanocomposite Fibers for Energy Systems

Polystyrene (PS) and polyvinyl chloride (PVC) fibers incorporated into TiO(2) nanoparticles and graphene nanoflakes were fabricated by an electrospinning technique, and then the surface morphology and superhydrophobicity of these electrospun nanocomposite fibers were investigated. Results indicated that the water contact angle of the nanocomposite fiber surfaces increases to 178° on the basis of the fiber diameter, material type, nanoscale inclusion, heat treatment, and surface porosity/roughness. This is a result of the formation of the Cassie-Baxter state in the fibers via the nanoparticle decoration, bead formation, and surface energy of the nanofiber surface. Consequently, these superhydrophobic nanocomposite fibers can be utilized in designing photoelectrodes of dye-sensitized solar cells (DSSCs) as self-cleaning and anti-icing materials for the long-term efficiency of the cells.

Bulk and surface energetics of crystalline lithium hydride: Benchmarks from quantum Monte Carlo and quantum chemistry

We show how accurate benchmark values of the surface formation energy of crystalline lithium hydride can be computed by the complementary techniques of quantum Monte Carlo (QMC) and wavefunction-based molecular quantum chemistry. To demonstrate the high accuracy of the QMC techniques, we present a detailed study of the energetics of the bulk LiH crystal, using both pseudopotential and all-electron approaches. We show that the equilibrium lattice parameter agrees with experiment to within 0.03 %, which is around the experimental uncertainty, and the cohesive energy agrees to within around 10 meV per formula unit. QMC in periodic slab geometry is used to compute the formation energy of the LiH (001) surface, and we show that the value can be accurately converged with respect to slab thickness and other technical parameters. The quantum chemistry calculations build on the recently developed hierarchical scheme for computing the correlation energy of a crystal to high precision. We show that the hierarchical scheme allows the accurate calculation of the surface formation energy, and we present results that are well converged with respect to basis set and with respect to the level of correlation treatment. The QMC and hierarchical results for the surface formation energy agree to within about 1 %.

Catalytic effect and nucleation stability of Au on GaAs(111)B surface

We perform comprehensive first-principles calculations to study the catalytic effect and nucleation stability of Au on GaAs(111)B surface during the initial growth of Au-assisted III-V nanowires (NWs). The atomic structures and energetics of GaAs(111)B surface with the deposited Au atoms are systematically discussed for various possible surface configurations. With the deposition of Au, the As trimer on GaAs(111)B-(2×2) surface undergoes a desorption process. The surface phase diagrams as functions of temperature and As2 pressure reveal that the As-trimer desorption on the (2×2) surface with Au adatom occurs beyond 540–650 K, while the desorption without Au adatom does beyond 800–970 K. The result shows the catalytic effect of Au on the desorption of As trimer. The calculated surface formation energies indicate that the on-surface adsorption of Au on GaAs(111)B surface is unstable, and the Au adatoms prefer to be incorporated on GaAs(111)B surface with a (3×3) phase during the initial deposition of Au. The simulated scanning tunneling microscopy image for the (3×3) surface with the incorporating Au atom is in good agreement with the experimental observation. Nevertheless, the added Au adatoms tend to form Au droplets on the surface when the Au coverage is further increased. It is expected that our calculations can contribute to the understanding of Au wetting experiments and the initial growth of Au-assisted III-V NWs.

A reactive force-field (ReaxFF) Monte Carlo study of surface enrichment and step structure on yttria-stabilized zirconia

To investigate surface segregation in yttria-stabilized zirconia (YSZ), DFT energies describing surface energy as a function of yttrium lattice position were used to parameterize a reactive-force field (ReaxFF). We used ReaxFF to perform Monte Carlo (MC) simulated annealing to sample structural configurations of flat YSZ (111) and vicinal YSZ (111) stepped surfaces. We evaluated yttrium surface segregation, oxygen vacancy position, and surface step composition for flat and stepped YSZ surfaces. It is thermodynamically favorable for yttrium atoms to segregate to the surface of YSZ, and specifically to step edge sites. Surface saturation of yttrium occurs at approximately 40% (40:60 Y:Zr ratio) while yttrium concentration at the step edge does not approach a saturation value, suggesting that steps on the YSZ surface are mainly yttria-terminated. We found that it is thermodynamically favorable for oxygen vacancies to occupy positions in the subsurface layer of YSZ, and a higher fraction of vacancies occupy positions NN to Y than NN to Zr. Yttrium segregation to step edges on the YSZ surface does not lower the surface formation energy of the stepped surface below that of the flat (111) termination, suggesting that the stability of YSZ surface steps observed experimentally is due to kinetic barriers for surface re-ordering.

First-Principles Study of Initial Growth of InP Nanowires: Self-Catalytic Effect and Nucleation Mechanism of In Adatoms

We perform systematic density-functional calculations on the self-catalytic effect and the nucleation mechanism of In adatoms on InP(111)B substrate during the initial growth of self-catalyzed InP nanowires. We first determine the equilibrium structure and stability of pure InP(111)B surface. The calculated surface formation energies show that the (2 x 2) surface with P trimer is a stable phase for a wide range of P chemical potential. Based on the P trimer structure, the adsorption of In atoms can induce the desorption of P trimer on InP(111)B surface. The adsorption-desorption phase diagrams as functions of temperature and P 2 pressure indicate that the desorption of P trimer on the (2 x 2) surface without In adatom occurs beyond 970—1190 K, while the desorption with the In adatom does beyond 570-700 K. The P trimer desorption promoted by In adatom suggests the self-catalytic role of In adatom. It can be interpreted by the electron-counting rule. The structural relaxations and the calculated Gibbs free energies of In adatoms for a wide range of coverage indicate that the In adatoms can be stabilized on InP(111)B surface only at a low coverage (Θ In ≤ 1 ML). In contrast, the In adatoms prefer to form the In droplets on the surface when the In coverage is continuously increased. The In adatoms of InP(111)B surface can be nucleated because the bonding interaction between the In adatoms and surface P atoms becomes weak with the increase of In coverage. Once the substrate is heated under In-rich condition, the surface In atoms easily form the In droplets. Our results offer a valuable information to insight into the initial growth of self-catalyzed InP nanowires by the vapor—liquid—solid mechanism on InP(111)B substrate.

First-principles investigation of the structure and electronic properties of CdS/CdSe/CdS and CdS/CdTe/CdS quantum wells using a slab approximation

The structure and electronic properties of CdS, CdSe, and CdTe thin films and multilayer CdS/CdSe/CdS and CdS/CdTe/CdS heterostructures simulating quantum wells of these compositions are performed using density functional theory and a slab approximation. Various crystal modifications and various crystallographic surfaces of the specified compounds are considered. In the calculations, the GGA approximation for the exchange-correlation functional (PW91) and ultrasoft pseudopotentials are used. For the systems in study, the work functions are calculated for various surfaces. The calculated structural data are in good agreement with experiments. The calculated surface reconstruction can be qualitatively described as an inward displacement of cadmium atoms into the surface crystal layer and an outward displacement of chalcogen atoms. The calculated surface energies indicate that the (110) surface formation energy is lower than the (100) surface formation energy. The calculated work functions decrease in the sequence CdS > CdSe > CdTe, while the calculated work functions for multilayer structures are between the corresponding values for the pure components of the structure.

Mechanical properties, electronic structure and bonding of α- and β-tricalcium phosphates with surface characterization

The mechanical properties and electronic structure of alpha- and beta-tricalcium phosphate (TCP) crystals are studied by using two ab initio density functional methods, the Vienna Ab initio Simulation Package (VASP) and the orthogonalized linear combination of atomic orbitals method. Based on the VASP optimized crystal structures, the elastic constants of alpha- and beta-TCP are obtained using an effective stress-strain computational scheme. From the calculated elastic constants, the bulk modulus, shear modulus, Young's modulus and Poisson's ratios are obtained. The results show that the mechanical properties of the two crystals are comparable and that alpha-TCP is somewhat softer than beta-TCP. Comparison with experimental extrapolations of the elastic constants shows significant differences, which attest to the difficulty of obtaining single crystal samples. The calculated electronic structure results show that both crystals are large gap insulators with a direct band gap of 4.89 eV for alpha-TCP and 5.25 eV for beta-TCP. Effective charge calculations show that, on average, beta-TCP has slightly less charge transfer per Ca than alpha-TCP. The (010) ((001)) surface model for alpha-TCP (beta-TCP) is studied using a supercell slab geometry and fully relaxed to obtain the optimized structures. The estimated surface formation energies are 0.777 and 0.842 J m(-2) for alpha-TCP and beta-TCP, respectively. The electronic structures of the two surface models are compared with the bulk models. Charge density analysis shows that the surfaces of both TCP crystals are positively charged overall owing to the presence of Ca ions near the surfaces.