Ti-terminated Surface Research Articles

On-the-Fly Machine Learning Force Field Study of Liquid-Al/Solid-TiB2 Interfaces.

Using the on-the-fly machine learning force field, simulations were performed to study the atomic structure evolution of the liquid-Al/solid-TiB2 interface with two different terminations, aiming to deepen the understanding of the mechanism of TiB2 as nucleating particles in an aluminum alloy. We conducted simulations using MLFF for up to 100 ps, enabling us to observe the interfacial properties from a deeper and more comprehensive perspective. The nucleation potential of TiB2 particles is determined by the formation of various ordered structures at the interface, which is significantly influenced by the termination of the TiB2 (0001) surface. The evolution of the interface during heterogeneous nucleation processes with different terminations is described using structural information and dynamic characteristics. The Ti-terminated surface is more prone to forming quasi-solid regions compared to the B-termination. Analysis of mean square displacement and vibrational density of states indicates that the liquid layer at the Ti-terminated interface is closer in characteristics to a solid compared to the B-terminated interface. We also found that on the TiB2 (0001) surface different terminations give rise to distinct ordered structures at the interfaces, which is ascribed to their different diffusion abilities.

Theoretical insights into the behaviors of sodium and aluminum on the cathode titanium diboride surfaces

Density functional theory calculations were adopted to systematically investigate the adsorption and diffusion behaviors of sodium and aluminum over TiB2 surfaces or in TiB2 crystal to characterize the interaction mechanism between sodium and TiB2 cathode in aluminum reduction cells. Results suggest that Na and Al will stably adsorbe on the low-index TiB2 (0 0 0 1) surface, and the presence of vacant defects can significantly strengthen this adsorption. The migration of Na and Al over pristine TiB2 is anisotropic, with the largest energy barrier of 0.024/0.32 eV for Na and 0.28/1.57 eV for Al over Ti/B-terminated surfaces. The Ti vacancy in Ti-terminated surface is more effective to hinder Na and Al migration with the large diffusion barriers of 0.36 eV for Na and 2.07 eV for Al. Specially, for the B-terminated surface, B vacancy will promote the Na and Al diffusion with the lower barriers. Additionally, it is difficult for Na and Al to form interstitial defects and diffuse in covalent TiB2 crystal. Given these results, compared to graphite cathode, the sodium prefers to deposit on TiB2 surface, and the strong interaction between sodium and TiB2 promotes the early process of sodium penetration. On the other hand, the smoother landscape for Na diffusion on the TiB2 surface suggests the decreased stability of aluminum liquid, so that the current efficiency of aluminum reduction cell will decrease.

Atomically unraveling the dependence of surface microstructure on plasmon-induced hydrogen evolution on Au/SrTiO3

Strong light-matter interaction and coupled catalytic surface in plasmonic photocatalysts offer a unique opportunity for solar-to-chemical energy conversion. The interface/surface engineering is significant strategy to modulate the performance of plasmon-induced water splitting. This situation motivates the demand of a plasmonic heterostructure with well-defined atomic surface structures but identical bulk structure for plasmon-induced water splitting. In this work, using Au/SrTiO3 as a prototype, we found that altering the Ti-terminated and Sr-terminated surface of SrTiO3 gives rise to a remarkable difference in plasmon-induced hydrogen evolution activity. The efficiency of charge separation at the Sr-terminated surface is inferior compared with which at the Ti-terminated structure, while the reaction kinetics of Sr-terminated surfaces is faster than the counterpart, thus leading to a high plasmon-induced hydrogen evolution performance at Au/ SrTiO3 with surfaces of Sr-termination. Modulation of the interface/surface structure of Au/SrTiO3 changes not only charge separation but surface catalysis in plasmonic photocatalysts, where the catalysis process dominates the final photocatalytic performance. This work paves a way to design efficient plasmonic photocatalysts for solar-to-chemical energy conversion.

Electronic, magnetic and optical properties of various surfaces of TiHfIrZ (Z = Al and Ga) quaternary Heusler alloy

The electronic, magnetic and optical properties of (001), (110) and (111) surfaces of TiHfIrZ (Z = Al and Ga) quaternary Heusler alloy are studied by the first-principles method. All the seven surfaces investigated lose the bulk half-metallicity, and the TiHfIrZ-terminated (110) and Ti-terminated (111) surfaces of TiHfIrZ (Z = Al and Ga) with the high spin polarization are more suitable for the spintronics application using generalized gradient approximation (GGA) method. While the TiHfIrZ-terminated (110) surfaces of TiHfIrZ (Z = Al and Ga) present half-metallicity using the GGA + U method. The change of the bond lengths affects the d-d and p-d hybridization, and the atomic restrictions of the atoms, thus the electronic, magnetic and optical properties of the surfaces are different from those in the bulk alloy. The Ti-terminated (111) and TiHfIrZ(Z = Al and Ga)-terminated (110) surfaces are suitable for using in the infrared light transmission. The TiHfIrZ (Z = Al and Ga)-terminated (110) surface is suitable for using in both the infrared light transmission and the visual light absorption.

Investigation on the interface characteristic between TiN and diamond by first-principles calculation

The electronic and elastic properties of TiN and diamond were calculated by first-principles method in this paper. Then, the work of adhesion (Wad) and charge density difference before and after relaxation of the TiN(111)/diamond(111) interface were analyzed. The calculated results show that TiN has a certain metallicity, which is mainly contributed by electrons in Ti-3d orbital. However, the diamond is an insulator with a bandgap of 4.77 eV. Furthermore, both diamond and TiN are with anisotropy, while the transverse deformation ability of the diamond is stronger than that of TiN. According to the two termination of TiN (111) surface, which are N-terminated and Ti-terminated surfaces, Ti/C (TiN (111) Ti-terminated/diamond (111)) and N/C (TiN (111) N-terminated/diamond (111)) interfaces were established. The bonding strength of Ti/C and N/C interfaces is decreased after relaxation, which is attributed to graphitization transformation of the diamond. The charge transfers at the interfaces of Ti/C and N/C are also changed before and after relaxation, which results in the change of the electrostatic interaction of atoms at the interface, therefore the interfacial bonding is reduced.

Insights into NinTim clusters adsorption and diffusion on B2-NiTi phase from atomistic simulations

In this work, we aim to study the adsorption and migration of NinTim (n,m=1-4) clusters on low-index B2-NiTi surfaces (100), (110) and (111). Ni and Ti diffusion was investigated using molecular static simulations with the conjunction of the MEAM formalism to describe atomic interactions. Ni-rich clusters are found to be more mobile than Ti-rich ones on Ni- and Ti-terminated (100) surfaces. On the other hand, the activation energy of Ni diffusion on the mixed (110) is found to be 0.51 eV, while that on Ni- and Ti-terminated (111) surfaces corresponds to barriers of 0.61 eV and 0.71 eV, respectively. These results demonstrate that on the (110) surface, a high thermal activity of clusters is expected due to the lower migration barriers. In addition, the diffusion mechanisms of Ti and Ni adatoms are quite similar and are characterized by a hopping mechanism between adsorption sites. While for the small cluster diffusion such as dimers, trimers and tetramers a variety of mechanisms is noted. Using classical molecular dynamics, we found that at 800 K, the Ni-terminated (100) and Ti- terminated (111) surfaces show an important segregation effect of Ni atoms on the surface. We have also found that the homogenous Ti clusters are stable on most surfaces with slight changes of their forms. While Ni clusters are found to show less stability with a higher mobility of their dissociated atoms.

Theoretical study of atomic relaxation, surface energy, electronic structure and properties of B2- and B19'-NiTi surfaces

NiTi shape memory alloy has been widely used in industrial and biological fields due to its excellent mechanical properties, unique shape memory effect and superelasticity. In this paper, the atomic relaxation, thermodynamic energy, structural stability, electronic structures and other properties of all low-index surfaces of B2- and B19'-NiTi alloys are systematically studied by using the first principles calculations based on density functional theory. The calculated results show that the atomic relaxations on all low-index surfaces of both B2- and B19'-NiTi alloys are mainly concentrated in 2−3 atomic layers on the surface, which means that the surface effect is mainly confined in two or three layers on the surface configuration. In addition, the atomic relaxation of Ti-terminated surface is most remarkable, and followed by Ni-terminated surface, while the atomic relaxation of Ni&Ti-terminated surface is insignificant. Furthermore, the valence charge density decays rapidly from the surface configuration to the vacuum layer. The calculation results of surface energy show that surface energy is inversely related to the coordinate number, and surface stability increases with the coordination number increasing. For B2- and B19'-NiTi, the surface energy of non-dense and non-stoichiometric surface depend on the chemical potential of Ti, and the surface energy is high. Therefore, the stabilities of these surfaces change with the chemical potential of Ti increasing. However, the surface energy values of dense surface configurations with stoichiometric ratio for B2-NiTi (101) and B19'-NiTi (010) are 1.81 J/m<sup>2</sup> and 1.93 J/m<sup>2</sup>, respectively, which are both lower than those for other non-dense surfaces in the most Ti chemical potentials range, showing excellent structural stability. Moreover, the electron density analysis indicates that the dominant bonding for B2-NiTi (101) surface is the chained Ni-Ti-Ni metallic bond, the distribution of electrons and the distance between Ni and Ti atoms on the B2-NiTi (101) surface are more uniform and smaller, respectively, than those for B19'-NiTi (010) surface. In summary, the B2-NiTi (101) surface shows the high stability.

First-principles calculations on Mg/TiB2 interfaces

In order to explore the interfacial structure of Mg (0001)/TiB2 (0001) interface and understand the heterogeneous nucleation potential of α-Mg grains on TiB2 particles, the electronic structure, adhesion work, bonding properties and interface energy of Mg(0001)/TiB2(0001) interface were investigated by the first-principles calculations method based on density functional theory (DFT). The Mg (0001) slabs with five atomic layers and TiB2 (0001) slabs with 9 atomic layers were adopted for interfacial models. Mg (0001)/TiB2 (0001) models with different stacking positions (“OT” stacking sequence, “HCP” stacking sequence and “MT” stacking sequence) of different terminations (Ti- and B-) were investigated. Metallic/covalent mixed bonds and ion bonds were formed respectively when Mg atoms combined with Ti-terminated TiB2 surface and B-terminated TiB2 surface. The interface energy of Mg/TiB2 interface is much larger than that of α-Mg/Mg melt interface, indicating that TiB2 particles possess poor nucleation potency for α-Mg grains from thermodynamical considerations.

Topological Dirac nodal-net fermions in AlB2 -type TiB2 and ZrB2

Based on first-principles calculations and effective model analysis, a Dirac nodal-net semimetal state is recognized in AlB$_2$-type TiB$_2$ and ZrB$_2$ when spin-orbit coupling (SOC) is ignored. Taking TiB$_2$ as an example, there are several topological excitations in this nodal-net structure including triple point, nexus, and nodal link, which are protected by coexistence of spatial-inversion symmetry and time reversal symmetry. This nodal-net state is remarkably different from that of IrF$_4$, which requires sublattice chiral symmetry. In addition, linearly and quadratically dispersed two-dimensional surface Dirac points are identified as having emerged on the B-terminated and Ti-terminated (001) surfaces of TiB$_2$ respectively, which are analogous to those of monolayer and bilayer graphene.

Investigation on adhesion strength, stability and electronic properties of Ti/TiB<sub>2</sub> interface by first-principles

The adhesion strength between Ti and TiB2, commonly employed in wear-resistant and oxidation-resistant coatings, depends on their interfacial properties. However, it is difficult to clearly understand the interaction of the Ti/TiB2 interface at the atomic or even electronic scale through experimental methods. Therefore, the first-principles calculation has been employed to study the Ti/TiB2 interface in this article. First of all, properties of the Ti(0001) surface, TiB2(0001) surface, and Ti(00011)/TiB2(0001) interfaces were investigated by first-principles calculations based on density functional theory (DFT). Considering two termination and three possible sequences, six interface models were studied in this work. Additionally, the work of adhesion ( W ad), interface energy ( γ int), and electronic structure of these interfaces were calculated. The results show that the calculated bulk properties of Ti and TiB2 are in good agreement with the experimental data and the values of other studies, indicating that the parameters used in our calculations are reliable. For the TiB2(0001), the B-terminated surface is less stable than the C-terminated one at low Δ μ Ti The surface energy of the B-terminated surface decreases with increasing Δ μ Ti, whereas the energy of the Ti-terminated surface increases. Consequently, the B-terminated surface is more stable when Δ μ Ti≥ - 1.27 eV/unit cell. The calculated W ad results indicate that both of W ad and interfacial separation ( d 0) are affected by the termination and stacking sequences. The W ad of the Ti/TiB2 interfaces with B termination is larger than that of the ones with Ti termination, which suggests that the stability and adhesion strength of the former are better than that of the latter. For the Ti/TiB2 interfaces with the same termination, the B-terminated hollow-stacked (BTH) interface and Ti-terminated hollow-stacked (TTC) interface are considered as the optimal ones. For the B-terminated interfaces, the W ad of the interfaces with same terminations follows the order: hollow site (HS)>center site (CS)>top site (TS), while the interfacial separation exhibits the opposite sequence. The reason could be that each interfacial B atom of TiB2(0001) side interact with three nearest-neighbor Ti atoms of Ti(0001) side in HS interface, which results in the strong interaction. While one interfacial B atom of TiB2(0001) side interact with only one nearest-neighbor Ti atom of Ti(0001) side in the CS and TS interface, leading to the relatively weak interaction. Moreover, the γ int of the interfaces with B-termination follows the sequence: HS W ad. In addition, for the entire range of Ti chemical potential, the BTH interface exhibits the smallest γ int among the six different interfaces. Therefore, the BTH interface has the best stability and adhesion strength. The electron density, charge density difference and partial density of states (PDOS) indicate that the BTH interface primarily consists of Ti–B covalent bonds and Ti–Ti metallic bonds, and the Ti–B covalent bonds is mainly contributed from the hybridization between interfacial B 2p and Ti 3d orbitals.

Surface Stability of NiTi Alloy: A Density-Functional Theory Study

The low-index (001), (111) and (110) surfaces of austenitic NiTi were investigated by the use of first principles calculations. The calculated results showed that the non-polar NiTi (110) surface was the most stable under most of the Ti chemical potential. The polar Ni terminated NiTi (001) surface was the most stable under Ni-rich conditions. The Ni-terminated surfaces were more stable than their corresponding Ti-terminated surfaces in the entire range of Ti chemical potential. The surface interlayer relaxations of the Ni-terminated surfaces were much larger than those of the corresponding Ti-terminated surfaces. The Ti atoms in the surface layer of the non-polar NiTi (110) surface were more outward than Ni atoms.

Grain Refinement Mechanism of Al-5Ti-1B Master Alloy by <i>Ab Initio</i> Calculations

To exactly understand the grain refining mechanism of α-Al by the Al-5Ti-1B master alloy, the structural properties of α-Al/solid-TiB2 (S/S) and liquid-Al/solid-TiB2 (L/S) interfaces were studied using the first-principles method. Different ordered structures were formed on the interfaces with different terminations of TiB2 (0001) surface, which determines the nucleant potency of TiB2. The heterogeneous nucleation of α-Al on the B-terminated surface is frustrated by an AlB2-like structure formed at the interface. In contrast, a five-layer quasi-solid region with stacking sequence of fcc-Al (111) planes forms on the Ti-terminated TiB2 (0001) surface, which is the basis of successful heterogeneous nucleation of α-Al. Moreover, when redundant Ti solute being added into the liquid Al region of Ti-terminated liquid-Al/TiB2 interface, the quasi-solid Al region further extends until entire solidification. The reason for using the Al-5Ti-1B master alloy rather than TiB2 powders as the commercial refiner in Al industry lies in two aspects: the excessive Ti atoms in the master alloy could guarantee sufficient Ti chemical potential to form Ti-terminated surface of TiB2, and the redundant Ti solute in inoculated melts could facilitate the growth of quasi-solid Al region at the solid/liquid interface.

The influence of boron on the adsorption of Ti and C on TiC surfaces

The first-principles calculations have been performed to study the influence of boron on the adsorption of Ti and C on different TiC surfaces. It is found that boron can be adsorbed on both TiC (001) and (111) surfaces. When boron is present and carbon supply is high during the preparation of TiC, boron can bond with carbon atoms to form B-C clusters on (001) surface, but the formation of them is less favorable than that of Ti-C clusters. However, under the low carbon-supply condition, both B-B and Ti-Ti clusters can be formed, and, once being formed, B-B clusters are more stable than the Ti-Ti ones. On Ti-terminated (111) surfaces, boron adatoms are more likely to be moved to form B-B clusters. The study of the diffusion of the adatoms on the surfaces demonstrates that boron adatoms can be more easily migrated on (111) surfaces, which further confirms the above results.

The half-metallic characteristics of the (001) surface of zinc-blende TiTe

In this paper, we employ the first-principles full-potential linearized augmented plane-wave method, which is based on the density functional theory to investigate the structural, electronic, and magnetic properties of the bulk and (001) surface of zinc-blende (ZB) TiTe. The bulk ZB TiTe is found to be a half-metallic ferromagnet at an equilibrium lattice constant of (6.41Ǻ). At the similar equilibrium lattice constant, the half-metallicity verified in the bulk TiTe, it is maintained at both Ti(Te)-terminated (001) surfaces and subsurfaces. Furthermore, by calculating the atomic magnetic moment, we find that the magnetic moment increases at the surface, but decreases at the subsurface. In addition, we evaluate the surface stability and found that the Te-terminated (001) surface is energetically more stable than the Ti-terminated (001) surface.

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.

An ab initio molecular dynamics study: liquid-Al/solid-TiB2 interfacial structure during heterogeneous nucleation

The structural properties of the liquid/solid interface between a TiB2 substrate and Al melts during a nucleation process are investigated using ab initio molecular dynamics simulation at 2 K undercoolings. Different ordered structures are formed on the interfaces with different terminations of TiB2 (0 0 0 1) surface, which determines the nucleant potency of TiB2 particles. It is found that five Al layers stacking like fcc-Al (1 1 1) planes on the Ti-terminated surface naturally extend into the liquid region, which is helpful in effective heterogeneous nucleation. In contrast, the heterogeneous nucleation of α-Al on the B-terminated surface is frustrated by both the AlB2-like structure formed within the first Al layer and the area with sparse Al atoms between the localized Al layer and liquid regions. An evaluation factor based on the Z-directional projection is proposed to quantitatively characterize the in-plane ordering, which could draw a clear boundary between the quasi-solid region and the liquid phase in the liquid/solid interface system.

Bulk and surface half-metallicity: Metastable zinc-blende TiSb

Motivated by the recent experimental fabrication of half-metallic MnSb and CrSb thin films with metastable zinc-blende structure [Aldous et al. Phys. Rev. B 85, 060403(R) (2012); Deng et al. J. Appl. Phys. 99, 093902 (2006)], we investigate the structural, electronic, and magnetic properties of TiSb in both ground-state NiAs and metastable zinc-blende phases by using the first-principles calculations. It is shown that the ground-state NiAs phase is metallic and nonmagnetic, but the metastable zinc-blende phase exhibits half-metallic ferromagnetism with a magnetic moment of 1.00 μB per formula unit. We also reveal that the half-metallicity in bulk ZB TiSb is lost at the Sb-terminated (001) surface due to the appearance of surface states within the gap of the minority-spin channel, but the Ti-terminated (001) surface retains the bulk half-metallicity, which makes ZB TiSb a promising candidate for the possible epitaxial growth of half-metallic thin films or multilayers on semiconductor substrates for spintronic applications.

Ilmenite (0001) surface investigated using hybrid density functional theory

The ilmenite (0001) surface is examined using the hybrid density functional B3LYP. A number of surface configurations are studied, with the Fe-O(3)-Ti-terminated surface computed to have the lowest cleavage energy, and also computed to be the most stable surface at high vacuum. Under oxygen-rich conditions, the O(3)-Ti-Ti-terminated surface is computed to be most stable. The calculated geometries of the surfaces, when compared to experimental measurement, suggest that experimentally cleaved ilmenite at low oxygen partial pressures can form a metastable O=Fe-O(3)- terminated surface.

Confined high-mobility electron gas at the Ruddlesden–Popper type heterointerfaces

We study the transport properties of Ruddlesden–Popper type interfaces grown on SrTiO3 substrates, showing that in contrast to perovskite-type interfaces, no mobile carriers are observed in the substrate even when La ions of the rocksalt layer are in direct contact with the Ti-terminated surface of SrTiO3. Annealing at 800 °C and above can, however, be used to perform local chemical doping of the interface by La-ion diffusion into the SrTiO3 substrate, forming a metallic interface with confined high-mobility carriers. The technique can be used to reliably fabricate δ-doped perovskite quantum wells with sheet carrier densities below 1013 cm−2.

Towards understanding the effects of carbon and nitrogen-doped carbon coating on the electrochemical performance of Li4Ti5O12 in lithium ion batteries: a combined experimental and theoretical study

We investigate the effects of carbon coating, with and without nitrogen-dopants, on the electrochemical performance of a promising anode material Li(4)Ti(5)O(12) (LTO) in lithium ion battery applications. The comparative experimental results show that LTO samples coated with nitrogen-doped carbon derived from pyridine and an ionic liquid exhibit significant improvements in rate capability and cycling performance compared with a LTO sample coated by carbon derived from toluene and the pristine LTO sample. For the first time, we construct an atomistic model for the interface between the lithium transition metal oxide and carbon coating layers. Our first-principles calculations based on density functional theory reveal that at this interface there is strong binding between the graphene coating layer and the Ti-terminated LTO surface, which significantly reduces the chemical activity of LTO surfaces and stabilizes the electrode/electrolyte interface, providing a clue to solve the swelling problem for LTO-based batteries. More importantly, electron transfer from the LTO surface to graphene greatly improves the electric conductivity of the interface. Nitrogen-dopants in graphene coatings further increase the interfacial stability and electric conductivity, which is beneficial to the electrochemical performance in energy storage applications.