Hcp Hollow Sites Research Articles

Atomic-level insights from density functional theory and ab initio molecular dynamics calculations for oxidation mechanism of transition metal doping Nb4AlC3 (0 0 0 1) surface

Poor oxidation resistance limits the further application of Nb4AlC3 MAX phase, a high-temperature structural candidate material. Transition metal doping can significantly address this problem. In this paper, we investigated the oxidation mechanism of the Nb4AlC3 (0 0 0 1) surface with Ti doping by combining density functional theory (DFT) and ab initio molecular dynamics (AIMD). DFT results indicate that the Nb surface has a lower surface energy, making it more easily exposed and reactive towards O2 molecules. O atoms are expected to preferentially occupy the HCP and FCC hollow sites on the surface. The Ti-doped surface has a stronger adsorption capacity for O atoms. O atoms gain electrons, while Nb/Ti atoms lose electrons. The formation of O–Nb/Ti bonds is facilitated by the hybridization of O-p and Nb/Ti-d orbitals. And O–Ti bonds are more stable than O–Nb bonds. AIMD simulation results show that O2 molecules on the clean and doped surfaces gradually dissociate into O atoms over time. These O atoms then combine with surface atoms to form Nb oxides, Ti oxides, and Nb–Ti oxides. With the passage of time, the concentration of Ti oxides increases while that of Nb oxides decreases. And the oxidation rate at 1073K is higher. The O–Nb/Ti bond population gradually increases over time, indicating that the bonding between O atoms and the surface is slowly strengthened. This study provides fundamental insights into the oxidation mechanism of transition metal-doped 413 MAX phase Nb4AlC3 surfaces.

Epitaxial growth of ultrathin gallium films on Cd(0001)

Growth and electronic properties of ultrathin Ga films on Cd(0001) are investigated by low-temperature scanning tunneling microscopy (STM) and density functional theory (DFT) calculations. It is found that Ga films exhibit the epitaxial growth with the pseudomorphic 1 × 1 lattice. The Ga islands deposited at 100 K show a ramified shape due to the suppressed edge diffusion and corner crossing. Furthermore, the majority of Ga islands reveal flat tops and a preferred height of three atomic layers, indicating the electronic growth at low temperature. Annealing to room temperature leads to not only the growth mode transition from electronic growth to conventional Stranski–Krastanov growth, but also the shape transition from ramified islands to smooth compact islands. Scanning tunneling spectroscopy (STS) measurements reveal that the Ga monolayer exhibits metallic behavior. DFT calculations indicate that all the interfacial Ga atoms occupy the energetically favorable hcp-hollow sites of the substrate. The charge density difference analysis demonstrates that the charge transfer from the Cd substrate to the Ga atoms is negligible, and there is weak interaction between Ga atoms and the Cd substrate. These results shall shed important light on fabrication of ultrathin Ga films on metal substrates with novel physical properties.

Density functional theory study of AlN/Hf interface

In this study, the performance of AlN (0001)/Hf (0001) interface, such as occupation characteristics and combined energy, were studied by using the first principles calculation package. In the AlN/Hf interface, the Al- and N-terminated interfaces have three stacking sites each (outermost top site, sub-outer top site, and hexagonal close-packed hollow site), comprising six different models. The most adhesion work occupies the N-terminal interface of the hcp-hollow site, and the interface energy was also the smallest and most stable, as well as a lower fracture work than the AlN/Ti interface. Moreover, the differential charge density were calculated, the partial density of states were plotted, and the bonding properties were discussed. Compared with other interfaces, the N-terminated interface with the hcp-hollow site exhibited more covalent characteristics; on the Al-terminated interface with the sub-outer top site, the Hf-Al metallic bond was formed. The most stable atomic occupation type in AlN/Hf interface is the hcp-hollow site of N atomic terminal.

(Keynote) Solution-Phase Growth of Cu Microplates: A Multi-Scale Theoretical Analysis from First Principles

We use first-principles density functional theory (DFT) to quantify the role of iodide in the solution-phase growth of Cu microplates. Our calculations show that a Cu adatom binds more strongly to hcp hollow sites than fcc hollow sites on iodine-covered Cu(111) – the basal facet of two-dimensional (2D) Cu plates. This feature promotes the formation of stacking faults during seed and plate growth which, in turn, promotes 2D growth. We also found that iodine adsorption leads to strong Cu atom binding and prohibitively slow diffusion of Cu atoms on Cu(100) – a feature that promotes Cu atom accumulation on the {100} facets of a growing 2D plate. Incorporating these DFT insights into analog experiments, in which we initiated the growth of Cu plates from small seeds consisting of magnetic spheres, we confirmed that two or more stacking faults are required for lateral plate growth, consistent with prior studies. Moreover, plates can take on a variety of shapes during growth: from triangular and truncated triangular to round and hexagonal – consistent with experiment. Using absorbing Markov chain calculations, we assessed the propensity for 2D vs. 3D kinetic growth of the plates. At experimental temperatures, we predict plates can grow to achieve lateral dimensions in the 1–5-micron range, as observed in experiments.

Structural and electronic properties of Sn sheets grown on Cd(0001)

We investigate the growth and electronic properties of the Sn sheets on Cd(0001) with a low-temperature scanning tunneling microscopy (STM) and density functional theory (DFT) calculations. It is found that both the first and second layer of Sn reveal the epitaxial growth with a 1 × 1 commensurate lattice. Scanning tunneling microscopy (STS) measurements indicate the Sn monolayer exhibits a metallic behavior. DFT calculations indicate that all the Sn atoms in the first Sn layer occupy the energetically preferable hcp-hollow sites. Very small amount of charge is transferred from Cd(0001) to the Sn monolayer, indicating the interface of Sn/Cd(0001) is governed by the weak van der Waals interaction.

Atomic level insights into the Ti2AlC oxidation mechanism by the combination of density functional theory and ab initio molecular dynamics calculations

Ti2AlC oxidation mechanism was investigated by density functional theory (DFT) and ab initio molecular dynamics (AIMD). DFT results showed that the most stable adsorption site was HCP hollow sites, followed by FCC. O atom gained electrons, while Al atom lost electrons through the hybridisation of O-p and Al-s/p orbitals. AIMD results show that O2 molecule gradually dissociated into O atoms at 1273 K and formed Al-Ti mixed oxide film. The hybridisation between O-p and Al-s/p orbitals formed Al-O bond, while the hybridisation of O-p and Ti-d orbitals formed Ti-O bond.

Atomistic processes of boron and nitrogen near the Pt(111) surface

Two-dimensional h-BN is grown by CVD using precursor molecules containing B and N, which then chemically decompose into constituent atoms on catalytic substrates. Using first-principles calculations, we study adsorption, diffusion, and penetration of B and N atoms separated from the precursors on the Pt(111) surface. These are the essential steps to understand the h-BN growth mechanism on the catalytic substrate. Both B and N atoms are stably adsorbed at the fcc and hcp hollow sites, to form both covalent and ionic bonds with Pt. The B atom becomes more stable in the subsurface, while does not the N atom. The penetration barrier of B is only 0.36 eV. The diffusion barriers of the B atom is 0.49 and 0.61 eV on the surface and in the subsurface, respectively. Whereas a single B-N dimer, which can act as seed for h-BN islands, is not found stable on Pt(111), the linear chains of the B-N dimers become more stable with more B and N atoms. For three B-N dimers, the circular shape less stable than the corresponding linear chain, but the energetics reverses for hydrogenated ones. Based on our results, we discuss the h-BN growth and argue that understanding of the precursor molecules is required further.

Atomic and electronic structure of an epitaxial Nb2O3 honeycomb monolayer on Au(111)

The experimental discovery and theoretical analysis of an epitaxial (2 \ifmmode\times\else\texttimes\fi{} 2) honeycomb $\mathrm{N}{\mathrm{b}}_{2}{\mathrm{O}}_{3}$ monolayer on a Au(111) surface is reported. The oxide monolayer is grown by Nb deposition and subsequent annealing in an oxidizing atmosphere. Scanning tunneling microscopy (STM) images show that the films form a well-ordered honeycomb lattice, and low energy electron diffraction patterns confirm that the films adopt a (2 \ifmmode\times\else\texttimes\fi{} 2) periodicity with respect to the Au(111) substrate. Density functional theory (DFT) modeling shows that the Nb atoms are located in Au(111) threefold hollow sites and the O atoms are located in on-top positions. DFT also demonstrates the existence of a strong interfacial interaction characterized by a large electron transfer towards the Au substrate, an increase of the Nb oxidation state, and substantial film rumpling. High-resolution STM images, supported by simulations, are able to discriminate between Nb atoms adsorbed in fcc or hcp hollow sites on the Au(111) substrate.

Investigation of LEPS potential energy surface for the interaction of a Pt(111) surface with a hydrogen atom

In this work, the identification of a potential energy surface between H atom and Pt(111) surface has been studied through the use of London-Eyring-Polanyi-Sato potential energy function (PEF).  The energy values for the H–Pt(111) interaction calculated using density functional theory were used to determine the parameters of this PEF by using a nonlinear least-squares method.  For this study, four symmetric sites on the surface were considered as a top site, bridge site, fcc-hollow site and hcp hollow site.  It can be determined which sites on the Pt surface are penetration region, adsorption site or scattering site by defining the potential energy surface.  It is found that both of the hollow sites of the surface are regions where H atom can penetrate directly to subsurface and it can be held easily on the surface.

Adsorption mechanism of CO molecule on Al(111) surface: periodic DFT investigation

The adsorption mechanism of the CO molecule on Al(111) surface has been investigated systematically at the atom-molecule level by the method of periodic density functional theory. The adsorption energies, adsorption structures, charge transfer, and density of states have been calculated in a wide range of coverage. It is found that the hcp-hollow site is the energetically favorable site. A significant positive correlation has been found between the adsorption energy (Eads) and coverage. The adsorbed CO molecules are almost perpendicular on the surface with the C atom facing the surface. There is an obvious charge transfer from Al atoms to the C atom; the Al atoms that have interaction with the C atom offer the most charge. The 4σ, 1π, and 5σ molecular orbitals of CO are found to contribute to bonding with the Al. The charges filling in the 2π molecular orbital contribute to C–O bond activation. In conclusion, the passivation of aluminum surface and the activation of CO molecule occur simultaneously in the adsorption of CO on Al surface.

Catalytic Hydrogenation of p-Chloronitrobenzene to p-Chloroaniline Mediated by γ-Mo2N.

Promoting the production of industrially important aromatic chloroamines over transition-metal nitrides catalysts has emerged as a prominent theme in catalysis. This contribution provides an insight into the reduction mechanism of p-chloronitrobenzene (p-CNB) to p-chloroaniline (p-CAN) over the γ-Mo2N(111) surface by means of density functional theory calculations. The adsorption energies of various molecularly adsorbed modes of p-CNB were computed. Our findings display that, p-CNB prefers to be adsorbed over two distinct adsorption sites, namely, Mo-hollow face-centered cubic (fcc) and N-hollow hexagonal close-packed (hcp) sites with adsorption energies of −32.1 and −38.5 kcal/mol, respectively. We establish that the activation of nitro group proceeds through direct pathway along with formation of several reaction intermediates. Most of these intermediaries reside in a significant well-depth in reference to the entrance channel. Central to the constructed mechanism is H-transfer steps from fcc and hcp hollow sites to the NO/–NH groups through modest reaction barriers. Our computed rate constant for the conversion of p-CNB correlates very well with the experimental finding (0.018 versus 0.033 s–1 at ∼500 K). Plotted species profiles via a simplified kinetics model confirms the experimentally reported high selectivity toward the formation of p-CAN at relatively low temperatures. It is hoped that thermokinetics parameters and mechanistic pathways provided herein will afford a molecular level understanding for γ-Mo2N-mediated conversion of halogenated nitrobenzenes into their corresponding nitroanilines; a process that entails significant industrial applications.

Adhesion, stability and electronic properties of Ti2AlN(0001)/TiAl(111) coherent interface from first-principles calculation

This research work aims at investigating the adhesion, stability and electronic properties of Ti2AlN(0001)/TiAl(111) coherent interface by first-principles calculation based on density functional theory. Twelve kinds of Ti2AlN(0001)/TiAl(111) interfacial models were investigated in consideration of two different Ti2AlN(0001) surface terminations, three different interfacial atom stacking sites, and two different atomic layer stacking sequences of TiAl(111) surface slab. The adhesive work, interfacial energy, electronic structure of the interface models were calculated. The results show that the adhesive work of Ti(Al)-terminated Ti2AlN(0001)/TiAl(111) interface is larger than that of Al-terminated Ti2AlN(0001)/TiAl(111) interface. For both the Ti(Al)- and Al-terminated Ti2AlN(0001)/TiAl(111) interface, the adhesive work increases, and the interfacial energy decreases as the order of on-top, hcp-hollow and fcc-hollow sites. For the interfaces with same interfacial termination and stacking site, the atomic layer stacking sequence of TiAl(111) surface slab has a weak influence on the adhesive work and interfacial energy. The Ti(Al)-terminated Ti2AlN(0001)/TiAl(111) interface with the stacking site of fcc-hollow and stacking sequence of “(Ti2AlN)ABABA-C′B′A′C′B′(TiAl)” shows the largest adhesive work (2.99 J/m2) and lowest interfacial energy (0.57 J/m2). The calculated electronic properties reveal that the interfacial bonding of Ti(Al)-terminated Ti2AlN(0001)/TiAl(111) interface is mainly contributed from Ti-Al covalent and Ti-Ti metallic interactions, and the dominant interfacial bonding is Al-Al metallic and Ti-Al covalent bonds for Al-terminated Ti2AlN(0001)/TiAl(111) interface. The interfacial bond strength of Ti(Al)-terminated interface is stronger than that of Al-terminated interface.

A theoretical study on the mechanism of hydrogen evolution on non-precious partially oxidized nickel-based heterostructures for fuel cells.

It is desirable, yet challenging, to utilize non-precious metals instead of noble-metals as efficient catalysts in the renewable energy manufacturing industry. Using first principles calculations, we study the structural characteristics of partially oxidized nickel-based nanoheterostructures (NiO/Ni NHSs), and the interfacial effects on hydrogen evolution. The origin of the enhanced hydrogen evolution performance is discussed at the microscopic level. This study identifies two types of active sites of the exposed Ni surface available for the hydrogen evolution reaction (HER). One is the hcp-hollow sites near the perimeter boundary that exhibit a more excellent HER performance than platinum (Pt), and the other the second nearest neighbor fcc-hollow sites away from the boundary that exhibit a similar performance to Pt. The interfacial effects result from the competitive charge transfer between NiO and Ni surfaces in NHSs, and enhance the reactivity of NiO/Ni NHSs by shifting the d-states of surface atoms down in energy. The illumination of the mechanism would be helpful for the design of more efficient and cheap transition metal-based catalysts.

Adsorption Behaviors of Cobalt on the Graphite and SiC Surface: A First-Principles Study

Graphite and silicon carbide (SiC) are important materials of fuel elements in High Temperature Reactor-Pebble-bed Modules (HTR-PM) and it is essential to analyze the source term about the radioactive products adsorbed on graphite and SiC surface in HTR-PM. In this article, the adsorption behaviors of activation product Cobalt (Co) on graphite and SiC surface have been studied with the first-principle calculation, including the adsorption energy, charge density difference, density of states, and adsorption ratios. It shows that the adsorption behaviors of Co on graphite and SiC both belong to chemisorption, with an adsorption energy 2.971 eV located at the Hollow site and 6.677 eV located at the hcp-Hollow site, respectively. Combining the charge density difference and density of states, it indicates that the interaction of Co-SiC system is stronger than Co-graphite system. Furthermore, the variation of adsorption ratios of Co on different substrate is obtained by a model of grand canonical ensemble, and it is found that when the temperature is close to 650 K and 1700 K for graphite surface and SiC surface, respectively, the Co adatom on the substrate will desorb dramatically. These results show that SiC layer in fuel element could obstruct the diffusion of Co effectively in normal and accidental operation conditions, but the graphite may become a carrier of Co radioactivity nuclide in the primary circuit of HTR-PM.

Structure of NO dimer multilayer on Rh(111)

Molecular self-assembly is the spontaneous organization of molecules under thermodynamic equilibrium conditions into well-defined arrangements via cooperative effects between chemical bonds and weak noncovalent interactions. Molecules undergo self-association without external instruction to form hierarchical structures. Molecular self-assembly is ubiquitous in nature and has recently emerged as a new strategy in chemical biosynthesis, polymer science and engineering. NO monomer is apt to be absorbed on the surfaces of some metals such as Ir(111), Ni(111), Pd(111), Pt(111), Rh(111) and Au(111), and the interactions of NO monomer with the metal surfaces have been extensively studied. When NO monomer is weakly adsorbed on the noble-metal surface, it cannot be reduced completely but forms a stable structure, which is named NO dimer. The first-principle technique is employed to determine the structures of NO dimer ((NO)2) molecular chains and monolayers on virtual Rh(111), as well as (NO)2 monolayer and multilayer on Rh(111). First, (NO)2 monomers are assembled into two stable molecular chains on the virtual Rh(111) surface, whose bind energies are 0.309 and 0.266 eV, respectively. The molecular chains are self-assembly systems, in which (NO)2 monomers are parallel and ordered, and the O atoms and N atoms are shown to be of (100) and (111) structures, respectively. Then, the two molecular chains are assembled into two stable monolayers (denoted as M1 and M2) on the virtual Rh(111)-(13), and the coverage is 1.00 ML. In the M1 monolayer, the angle between the NN bond of (NO)2 monomer and the substrate is in a range of 70-90, and in the M2 monolayer, the NN bond is parallel to the substrate.In the adsorption system of M2/Rh(111), (NO)2 molecules can be adsorbed on the top as well as the hcp and fcc hollow sites. When (NO)2 molecules are adsorbed on the top site, the adsorption system is best described by the electron structure Rh+0.14N0=O-0.14, and when (NO)2 molecules are absorbed on the two hollow sites, the adsorption system is described by the electron structure Rh+0.34N-0.18=O-0.16. Therefore, (NO)2 molecules are more apt to be adsorbed on the two hollow sites than on the top site. In the adsorption systems of M1+M2/Rh(111) and M1+(M1+M2)/Rh(111), (NO)2 molecules are adsorbed vertically on the two hollow sites, the NN bond is parallel to the substrate in the first monolayer, and the angle between the NN bond and the substrate is in a range of 70-90 in the second and third monolayers. The interaction between the neighbor monolayers is about 0.01 eV, and the thickness of the vacuum layer is 0.31 nm0.02 nm.

Structure of BP3S monolayer on Au(111)

The first-principle technique is employed to determine the structure of the BP3S monomer, the structures of the molecular chains and monolayers on virtual Au（111), and the atomic structure of BP3S/Au（111) adsorption system. The results show that the BP3S monomer presents a symmetric structure, and the angle between two benzene rings is 3510. At first, many BP3S monomers are assembled into one stable molecular chain in the virtual Au（111), the distance between the neighbor monmers is 0.516 nm, and the bind energy between the monmer and the molecular chain is 0.071 eV. It is a self-assembly system. Then many molecular chains are assembled into two stable monolayers in the virtual Au（111)-(37) and Au（111)-(313), and their coverages are 0.20 ML and 0.14 ML, respectively. In the virtual Au（111)-(37) and Au（111)-(313), the angles between the molecular chains and the virtual surface are 60 and 30, respectively, and the binding energies between the monmer and two monolayers are 0.101 eV and 0.125 eV, respectively. They are both the self-assembly systems. Finally, two monolayers are adsorbed on the Au（111)-(37) and Au（111)-(313) at four adsorption sites. The S atom is easy to obtain two electrons and turn into S2- ion, and the Au atom is easy to lose one electron and become Au+ ion, so the bridge site（two Au+ ions) is more stable than the top site（one Au+ ion), while the hcp and fcc hollow sites（three Au+ ions) are both unstable. In the Au（111)-(37), the chemisorption energy of the bridge site（-1.879 eV) is lower than that of the top site（-1.511 eV). And in the Au（111)-(313), the chemisorption energy of the bridge site（-1.691 eV) is lower than that of the top site（-1.492 eV). The results are confirmed in the other S-Au adsorption systems, such as the C6H13S/Au（111). A comparison between the structures of the BP3S monolayer before and after being adsorbed on Au（111) clearly shows that the structural parameters of the adsorption system depend mainly on the interaction in the monolayer, and that the contribution of Au（111) to the structure of the monolayer is weak. These results are confirmed in the other self-assembly adsorption systems.

First-principles investigation on the interaction of Boron atom with Nickel part I: From surface adsorption to bulk diffusion

In this work, Boron atom adsorption, absorption and diffusion on the surface of and in the bulk of the Nickel are studied with a first-principles density functional theory approach and the climbing image nudged elastic band method. The calculated results indicate that, on the Ni (111) surface, there are three stable sites for B atom adsorption, which are Top site, Hcp-hollow site and Fcc-hollow site, and Hcp-hollow site is the most preferred one. While below the surface, the Boron atom prefers to reside at the Oct site with the absorption energy of −1.38 eV, and the energy barrier for Boron atom diffusing from Hcp-hollow site to Oct site is 1.22 eV. In bulk Ni, the Tet and Oct sites are both stable ones for solution of the B atoms, the energy barrier for B atom diffusing from Tet site to its nearest neighboring Oct site is 0.29 eV, while that for B atom diffusing between two neighboring Oct sites are 1.65 eV, which indicate that the B atom diffuses between two neighboring Oct sites is harder than that from Tet site to neighboring Oct site.

Orientation-Dependent Interaction between CO2 Molecules Adsorbed on Ru(0001).

Determining the molecular structure of CO2 adsorbed on metal surfaces and its mutual interactions is important to understand its catalytic conversion reactions. Here, we study CO2 adsorption on Ru(0001) using scanning tunneling microscopy (STM) and density functional theory (DFT) calculations. Adsorbed at 77 K, the CO2 molecules form mainly disordered structures at submonolayer coverage, except for small (2 × 2) domains. The adsorbed molecules are no longer linear as in the gas phase, but instead, they adopt a "V"-shape geometry with the carbon atom occupying three-fold hcp hollow sites and possess three symmetry-equivalent orientations. Annealing to 250 K causes partial desorption of the molecules, while the remaining molecules form trimers of three different configurations with different interaction energies determined by their relative orientations. The "strong"-interacting trimer shows a cyclic structure, about 40 meV more stable than the "weak"-interacting trimer that is composed of three parallel molecules.

Ab initio study on the adsorption of oxygen on Co(111) and its subsurface incorporation

The adsorption of oxygen on Co(111) and its subsurface incorporation is studied by density-functional theory calculations. Calculated adsorption energies, geometries and electronic structures are discussed in comparison with that of O/Co(0001). The results indicate that oxygen adsorption in hcp-hollow sites on Co(111) are the most preferred in a coverage range of 0.11–1.0 ML, and the surface electronic structure modifications for O/Co(111) points to a consistent picture of Hammer-Norskov model. In addition, present calculations show that oxygen atoms incorporate into the first subsurface layer forming an O-Co-O trilayer on top Co(111), which possibly undergoes conversion to cobalt oxides. The behaviors of oxygen on Co(111) are similar to that of O/Co(0001).

Scanning tunneling microscopy study of Fe, Co and Cr growth on Re(0001)

Atomically flat terraces of the Re(0001) surface with a contaminant density below 0.5% have been obtained by oxygen annealing followed by a flash to higher temperature. This Re(0001) single crystal has been used as a substrate for the deposition of Fe, Co and Cr atoms. Scanning Tunneling Microscopy experiments characterize the growth mode for the submonolayer coverage regime. Co, Cr and Fe atoms self-assemble to form monolayer high islands. Despite a large lattice mismatch between film and substrate, Co and Fe grow pseudomorphically up to half a monolayer. Cr islands are pseudomorphic only for a size below 10nm. Higher coverage leads to reconstructed islands with an element-dependent reconstruction pattern. Scanning Tunneling Spectroscopy measurements at 8K reveal the electronic properties of Fe and Re. Differential conductance measurements on the Re(0001) show the presence of standing waves, possibly due to a rhenium surface state. Atomic resolution images of Fe attached to a Re step edge lead to the conclusion that the Fe atoms occupy hcp hollow sites. A Néel magnetic ordered state of the Fe hcp monolayer is revealed with Spin-Polarized Scanning Tunneling Microscopy and Magnetic Atom Manipulation Imaging.