C-terminated Surfaces Research Articles

Surface-termination dependence of propanoic acid deoxygenation on Mo2C

Although Mo2C is used as an alternative catalyst material for biomass conversion, the dependence of its reaction on the type of surface termination has not been studied. In the present study, we performed density functional theory calculations for the deoxygenation of propanoic acid on the two types of surface terminations possible for orthorhombic Mo2C (100), namely, on Mo- and C-terminated surfaces. The reaction energetics of the three possible deoxygenation pathways, namely, those for hydrodeoxygenation, decarbonylation, and decarboxylation, were compared by calculating the activation energies for their key surface reactions, such as hydrogenation and the scission of CC, CO, and OH bonds. The Mo-terminated surface was advantageous in the case of the hydrodeoxygenation pathway because of its low kinetic energy barrier for the hydrogenation of the oxygen species, while the C-terminated surface preferred the decarboxylation pathway because of its low kinetic energy barrier for OH bond scission.

Phase and Facet Control of Molybdenum Carbide Nanosheet Observed by In Situ TEM.

Transition metal carbides are of great potential for electrochemical applications. The phase and facet of molybdenum carbides greatly affect the electrochemical performance. Carburization of MoO3 inside a transmission electron microscope to monitor the growth process of molybdenum carbides is performed. Carbon sources with different activities are used and the controllable growth of molybdenum carbides is investigated. The results show that the relatively inert amorphous carbon film produces Mo2 C, where the interstitial sites formed by hexagonal closed packing molybdenum atoms are partially occupied by carbon atoms. In contrast, the carbon decomposed from the sucrose has a high portion of sp3 hybridized and crosslinked carbon atoms with high reactivity, leading to the formation of MoC with full occupation of interstitial sites by carbon atoms. In addition, the MoC growth experiences a (111) to (100) facets change with the increase of temperature. The (111) facet formed at low temperature has Mo-terminated or C-terminated surface with higher surface energy and higher reactivity, while the (100) facet with 1:1 C/Mo ratio on the surface exhibits enhanced stability. The phase and facet control by carbon source and temperature allow us to tune the crystal structures and surface atoms as well as their electrochemical properties.

Structural and electronic properties of bulk and low-index surfaces of zincblende PtC

Transition metal carbides have been extensively used in diverse applications over the past decade. Their versatility is in part thanks to their unique bonding, which displays a mixture of ionic, metallic and covalent character. While the bulk structure of zincblende (ZB) PtC has been investigated several times, a detailed understanding of the electronic and structural properties of its low-index surfaces is lacking. In this work, we present an ab initio investigation of the properties of five crystallographic ZB PtC surfaces (Pt/C-terminated PtC(1 0 0), PtC(1 1 0) and Pt/C-terminated PtC(1 1 1)). Upon geometry optimization, both polar and nonpolar surfaces undergo a mild interlayer relaxation, without extensive reconstructions. Calculated vacancy formation energies indicate facile C removal on the (1 1 1) surface while Pt-vacancy formation is endothermic. Finally, atomic O adsorption energies on all surfaces reveal a high affinity of the C-terminated surfaces towards this species.

Study of Ehrlich-Schwoebel Barrier in 4H-SiC Epitaxial Growths by Molecular Statics Method

A molecular statics method has been used to examine the Ehrlich-Schwoebel (ES) barrier for an adatom of 4H-SiC to diffuse from the {0001} to the {11-20} facet. As the calculated results shown, for the C-terminated surface, the inverse ES barrier exist for the silicon adatom, which could cause the step bunching; for the Si-terminated surface, ES barrier exist for the carbon adatom, which could cause the step meandering and result in the transition of step-flow growth to 2D-nucleation growth. Simultaneously, the C-terminated surface is more stable than the Si-terminated surface, which may be one reason that the quality of film grown on the carbon facet substrate is better.

Si/C and H coadsorption at 4H-SiC{0001} surfaces

Density functional theory (DFT) study of adsorption of 0.25 monolayer of either Si or C on 4H-SiC{0001} surfaces is presented. The adsorption in high-symmetry sites on both Si- and C-terminated surfaces was examined and the influence of the preadsorbed 0.25ML of hydrogen on the Si/C adsorption was considered. It was found out that for Si on C-terminated surface and C on Si-terminated the most favourable is threefolded adsorption site on both clean and H-precovered surface. This is contrary to the bulk crystal stacking order which would require adsorption on top of the topmost surface atom. In those cases, the presence of hydrogen weakens the bonding of the adsorbate. Carbon on the C-terminated surface, only binds on-top of the surface atom. The CC bond-length is almost the same for the clean surface and for one with H and equals to ∼1.33Å which is shorter by ∼0.2 than in diamond. The analysis of the electronic structure changes under adsorption is also presented.

First-principles study of Mn adsorption on Al4C3(0 0 0 1) surface

First-principle calculation based on the density functional theory was adopted to investigate the adsorption energy, stability, electronic structure and bonding of Mn atom adsorption on Al-terminated and C-terminated Al4C3(0001) surface under 0.25ML and 0.5ML. Results show that the structure of Mn adsorption on C-terminated Al4C3(0001) surface is more stable than that on Al-terminated surface according to the formation energy calculation. For Mn adsorption on Al-terminated surface, Mn is more favorable to reside at the site H1 comparing with other sites. As well, for Mn adsorption on C-terminated surface, the structure of Mn adsorption at site H′1 is the most stable one. By analyzing the electronic structure and bonding, it is found that the mixed metallic/covalent bonds are formed between Mn atoms and Al-terminated surface, while the covalent bonds are formed between Mn atoms and C-terminated surface. According to the interlayer spacing calculation, Al4C3(0001) surfaces are reconstructed after Mn adsorption, which in turn affect the following stacking of Mg atoms on Al4C3(0001) surface. The above analysis provided effective theoretical support to the experimental phenomenon that high Mn content has negative influence on the heterogeneous nucleation of Al4C3 particles for α-Mg grains.

Influence of SiC surface polarity on the wettability and reactivity in an Al/SiC system

The wetting of (0001) 6H-SiC single crystals by molten Al was investigated using a dispensed sessile drop method in a high vacuum at 973–1173K. The wettability and reactivity in this system are sensitive to the surface polarity of SiC. The interfacial reaction on the Si-terminated surface is rapid. The formation of a continuous Al4C3 product layer at the interface leads to an equilibrium contact angle of 56±1° at 1173K. In comparison, the interfacial reaction on the C-terminated surface is sluggish. The interface is only partially covered by discrete Al4C3 platelets even after dwelling at 1173K for 2h. The final wettability, however, is much better (θF=41±1°) than that of the Si-terminated surface which was covered by a dense Al4C3 layer, suggesting that the formation of Al4C3 should not always contribute to the wetting in the Al/SiC system. A plausible explanation is that the clean (i.e., deoxidized) C-terminated surface should be well wetted by molten Al in nature, owing to the strong chemical interactions between liquid Al and the surface atoms of the C-terminated SiC. It is likely that the presence of the oxide film at the surface of the molten Al drop or the SiC substrate and the rapid formation of Al4C3, which prevent the establishment of a real Al/SiC interface, conceal the intrinsic wettability of this system.

New Insights into the Structure of the C-Terminated β-Mo2C (001) Surface from First-Principles Calculations

Periodic density functional theory (DFT) calculations have been used to study the stability of five different C-terminations of the Mo2C(001) surface in the most stable orthorhombic (β) phase of this important material. The different terminations all have similar values of the work function or atomic charges, indicating a similar electronic structure, although the analysis of the cleavage energy suggests that the non polar C-terminated surfaces will be the extended terminations seen under thermodynamic equilibrium. Nevertheless, the calculated DFT cleavage energy values together with statistical arguments indicate that different C-terminated motifs are likely to coexist even at temperatures just below annealing conditions.

Role of SiC substrate polarity on the growth and properties of bulk AlN single crystals

Two symmetrically nonequivalent silicon carbide (SiC) substrate orientations, (0001) Si-terminated and \((000\overline{1} )\) C-terminated, were used in the physical vapour transport growth of bulk aluminium nitride (AlN) single crystals. The crystals grown on Si-faces always exhibit an Al-polar growth surface. AlN growth on \((000\overline{1} )\) C-terminated surfaces of the SiC substrates was performed to obtain N-polar growth surfaces. An abrupt interface was observed between the AlN crystal and the C-face substrate which is in contrast to the growth on Si-faces where hexagonally shaped SiC hillocks are formed. The growth on C-faces is usually dominated by multi-site nucleation. Applying similar supersaturation conditions that led to step-flow growth on Si-faces to the C-faces resulted in a spiral growth mode, even on highly off-oriented substrates. The obtained broad X-ray diffraction rocking curves of such samples (full-width at half-maximum ≈380 arcsec) indicate the presence of more misfit dislocations and significant misfit stress. In addition, polarity inversion is observed in C-face grown crystals. Though the structural properties of the crystals grown on C-face are inferior to that of the crystals grown on Si-face, the incorporation of unintentional Si impurity was found to be lower (<2 wt%).

Density functional study of H2O adsorption and dissociation on WC(0 0 0 1)

Adsorption and dissociation of water on the W- and C-terminated WC(0001) surfaces have been studied with periodical slab model by PW91 approach of GGA within the framework of density functional theory (DFT). Adsorption energies, preferred adsorption sites, and configurations for H2O and its dissociation products (OH, H, and O) are determined, as are the minimum energy paths of H2O decomposition. Our results suggest H2O, OH and O bind preferentially on the metal surface, while H prefers C termination over W termination. For the W-terminated surface, the barriers of H2O dehydrogenation are 0.74eV for the removal of hydrogen from the H2O to produce adsorbed OH and 0.75eV for the subsequent decomposition of OH to hydrogen and oxygen. For the C-terminated surface, the corresponding barriers of two dehydrogenation steps are 0.09eV and 0.44eV respectively. Both dissociation steps are found to be highly exothermic on the W- and C-terminated surfaces. The H2 desorption from the C-terminated surface is unlikely due to the formation of stable C–H bond, but it is easy to happen on the metal surface. Compared to Pt(111), the results here indicate that the activity of WC(0001) toward the decomposition of water is significantly higher.

Half-metallicity and stability of the rock salt BaC and SrC (111) surfaces: A density functional study

The electronic structure and magnetic properties of relaxed (111) surfaces of the alkaline-earth monocarbides BaC and SrC in the stable rock salt structure, are calculated on the basis of first principle density functional theory within the framework of self-consistent field plane wave pseudo-potential method, using the generalized gradient approximation for the exchange-correlation functional. The results of this study reveal that the C-terminated (111) surfaces retain the bulk half-metallic property in both BaC and SrC. The half-metallicity of the C-terminated BaC surface is found to be more robust compared to the bulk BaC due to the larger half-metallic energy gap. In contrast, the half-metallic energy gap of the C-terminated SrC surface is found to be smaller than that of the bulk. The Ba-terminated surface of BaC and the Sr-terminated surface of SrC, however, lose their bulk half-metallicity due to the formation of surface states in the majority spin band gap. The calculations also show that the atomic magnetic moments at the half-metallic C-terminated surfaces in both BaC and SrC increase considerably with respect to the corresponding bulk values, which is explained in terms of an increase in the number of unpaired 2p electrons of the carbon atom at the surface. We also discuss the stability of the surfaces via the calculated bulk formation energies. The bulk formation energies for both BaC and SrC in the rock salt structure are found to be positive, which indicate that the surfaces are not stable at normal pressure and temperature conditions, and non-equilibrium growth techniques may be required for the realization of BaC and SrC thin films.

First-principles calculations on Mg/Al4C3 interfaces

In order to explore the interfacial structure of Mg/Al4C3 interface and clarify the heterogeneous nucleation potential of Al4C3 particles in Mg melt. The atomic structure, bonding, and interfacial energy of Mg/Al4C3 interfaces were studied by first-principles calculations to analyze the sequence of Mg atoms onto the surface of Al4C3 (0001) slab. Surface energy calculations show that the outmost layer of Al4C3 free surface having a preference of C atom termination. And polar covalent/ionic mixed bonds are formed across interface during the combination of Mg atoms with C-terminated Al4C3 surface. The calculated interfacial energy of Mg/Al4C3 interface is much smaller than that between α-Mg and magnesium melts, proving the excellent nucleation potency of Al4C3 particles for α-Mg grains from interfacial atomic structure and atomic bonding energy considerations.

Atomic and electronic structure of molybdenum carbide phases: bulk and low Miller-index surfaces

The geometric and electronic structure of catalytically relevant molybdenum carbide phases (cubic δ-MoC, hexagonal α-MoC, and orthorhombic β-Mo2C) and their low Miller-index surfaces have been investigated by means of periodic density functional theory (DFT) based calculations with the Perdew-Burke-Ernzerhof (PBE) exchange-correlation functional. Comparison to available experimental data indicates that this functional is particularly well suited to study these materials. The calculations reveal that β-Mo2C has a stronger metallic character than the other two polymorphs, both β-Mo2C and δ-MoC have a large ionic contribution, and δ- and α-MoC exhibit the strongest covalent character. Among the various surfaces explored, the calculations reveal the high stability of the δ-MoC(001) nonpolar surface, Mo- and C-terminated (001) polar surfaces of α-MoC, and the nonpolar (011) surface of β-Mo2C. A substantially low work function of only 3.4 eV is predicted for β-Mo2C(011), suggesting that this system is particularly well suited for (electro)catalytic processes where surface → adsorbate electron transfer is essential. The overall implications for heterogeneously catalysed reactions by these molybdenum carbide nanoparticles are also discussed.

First-principles calculations of zinc-blende GeC(0 0 1) surfaces

We report the structural parameters, electronic and chemical bonding properties of zinc-blende GeC(001) surfaces investigated using the plane-wave ultrasoft pseudopotential technique based on density-functional theory (DFT). Both Ge- and C-terminated relaxed structures of two slabs have been analyzed and it can be concluded that the C atoms relax inward while Ge atoms relax outward. The electronic band structures and density of states of two slabs have been discussed, showing a metallic surface properties accompanied with overlaps of valence and conduction bands for Ge-terminated surface and a P-type semiconductor property with a direct band gap of 0.6eV for C-terminated surface. The surface energy and stability have been calculated, which indicates Ge-terminated slab surface is more stable than C-terminated surface.

Ab Initio Studies of Al and Ga Adsorption on 4H-SiC{0001} Surfaces

Changes in the atomic and electronic structure of Siand C-terminated 4H-SiC{0001} surfaces resulting from aluminium and gallium adsorption have been studied within density functional theory framework. Al and Ga coverages ranging from a submonolayer to one monolayer were considered. Our results show that Al binds more strongly to both surfaces than Ga. The binding is stronger to the C-terminated surface for both metals. The sites occupied by Al and Ga atoms at 1 monolayer are di erent and it is due to a di erent charge transfer from metal to the substrate.

First-Principles Study of TiC(111) Surface

The structural and electronic properties of TiC(111) surfaces are calculated using the first-principles total-energy plane-wave pseudopotential method based on density functional theory. As a polar surface, (111) surface shows large charge depletion in the upper part of the atoms, while charge accumulation happens in the inferior part of the atoms, interlayer Ti-C chemical bonds are reinforced and the outermost interlayer distances are largely reduced. Meanwhile, the charge accumulation and depletion for Ti-terminated surface is more than that for C-terminated surface on the same position of the two slabs after full relaxation. The surface energy of C-terminated surface is in the range from 7.61 to 9.83 J/m2, much larger than that of Ti-terminated surface from 3.13 to 1.35 J/m2, and the Ti-terminated surface is thermodynamically more favorable over all of the range of (chemical potential of TiC slab). This present work makes a beneficial attempt at exploring TiC surface as an ab initio method for studying possible nucleation mechanism of Aluminum on it.

Theoretical Model for CO Adsorption and Dissociation on Clean and K-Doped β-Mo2C Surfaces

We studied the effect of K on the adsorption and dissociation of CO on the β-Mo2C (001) surface by density functional theory calculations. Molecular CO adsorbs more strongly on Mo-terminated surfaces than on C-terminated ones. Adsorption is energetically more favorable in the presence of preadsorbed potassium. The CO molecule withdraws electron density from the surface, being more extended on the K-doped surface. The CO dissociation was also evaluated, and reaction pathways were modeled, revealing that the C-terminated surface is energetically less favorable than the Mo-terminated one. For both surfaces, the activation energy barrier for dissociation increases with the K content. C–O vibrational frequencies were also computed on K-modified surfaces.

Structure and energetics changes during hydrogenation of 4H-SiC{0001} surfaces: a DFT study

The changes in the atomic and electronic structure of Si- and C-terminated hexagonal SiC{0001} surfaces resulting from on-surface and subsurface hydrogen adsorption have been studied within the density functional theory framework. Hydrogen coverages ranging from a submonolayer to one monolayer were considered. Our results show that a monolayer of adsorbed H almost completely suppresses the relaxation of the SiC surface atomic layers. On both terminations H binds strongly to the surface and the binding is about 2 eV stronger in on-surface sites than subsurface. Hydrogen binding to the C-terminated surface varies very little with coverage and is distinctly stronger than to the Si-terminated surface.

Molecular dynamics study of temperature-dependent ripples in monolayer and bilayer graphene on 6H—SiC surfaces

Using classical molecular dynamics and a simulated annealing technique, we show that microscopic corrugations occur in monolayer and bilayer graphene on 6H—SiC substrates. From an analysis of the atomic configurations, two types of microscopic corrugations are identified, namely periodic ripples at room temperature and random ripples at high temperature. Two different kinds of ripple morphologies, each with a periodic structure, occur in the monolayer graphene due to the existence of a coincidence lattice between graphene and the SiC terminated surface (Si- or C-terminated surface). The effect of temperature on microscopic ripple morphology is shown through analysing the roughness of the graphene. A temperature-dependent multiple bonding conjugation is also shown by the broad distribution of the carbon-carbon bond length and the bond angle in the rippled graphene on the SiC surface. These results provide atomic-level information about the rippled graphene layers on the two polar faces of the 6H—SiC substrate, which is useful not only for a better understanding of the stability and structural properties of graphene, but also for the study of the electronic properties of graphene-based devices.

Reactivity of H2O and the Si-terminated surface of silicon carbide studied with ONIOM method

The reactivity of H2O and the Si-terminated silicon carbide surface (001) was investigated on the triplet potential energy surface with the combined first principle and molecular mechanics ONIOM(CASSCF:AM1:UFF) method for the (SiC)192·H2O model. It was found that the H2O molecule and the surface can form three physisorption complexes and follow five reaction paths to produce eight products, in which there are five main products having necessary energy barriers less than 300 kJ mol−1. Compared with that on the C-terminated surface, the interaction with the Si-terminated surface has stronger physisorption energy, smaller lowest necessary energy barrier, more main and more stable products.