Monolayer Ti Research Articles

Dynamics of H2 dissociation on the 1/2 ML c(2 × 2)-Ti/Al(100) surface

The dissociation of H(2) on Ti-covered Al surfaces is relevant to the rehydrogenation and dehydrogenation of the NaAlH(4) hydrogen storage material. The energetically most stable structure for a 1/2 monolayer of Ti deposited on the Al(100) surface has the Ti atoms in the second layer with a c(2 × 2) structure, as has been confirmed by both low-energy electron diffraction and low-energy ion scattering experiments and density functional theory studies. In this work, we investigate the dynamics of H(2) dissociation on a slab model of this Ti/Al(100) surface. Two six-dimensional potential energy surfaces (PESs) have been built for this H(2) + Ti/Al(100) system, based on the density functional theory PW91 and RPBE exchange-correlation functionals. In the PW91 (RPBE) PES, the lowest H(2) dissociation barrier is found to be 0.65 (0.84) eV, with the minimum energy path occurring for H(2) dissociating above the bridge to top sites. Using both PESs, H(2) dissociation probabilities are calculated using the classical trajectory (CT), the quasi-classical trajectory (QCT), and the time-dependent wave-packet methods. We find that the QCT H(2) dissociation probabilities are in good agreement with the quantum dynamics results in the collision energy range studied up to 1.0 eV. We have also performed molecular beam simulations and present predictions for molecular beam experiments. Our molecular beam simulations show that H(2) dissociation on the 1/2 ML Ti/Al(100) surface is an activated process, and the reaction probability is found to be 6.9% for the PW91 functional and 1.8% for the RPBE at a nozzle temperature of 1700 K. Finally, we have also calculated H(2) dissociation rate constants by applying transition state theory and the QCT method, which could be relevant to modeling Ti-catalyzed rehydrogenation and dehydrogenation of NaAlH(4).

First-principles study of Ti intercalation between graphene and Au surface

We investigate the effects of Ti intercalation between graphene and Au surface on binding energy and charge doping by using the first-principles calculations. We show that the largest binding energy is realized by the intercalation of single mono-layer of Ti. We also show that electronic structure is very sensitive to the arrangement of metal atoms at the interface. If the composition of the interface layer is Ti0.33Au0.67 and the Ti is located at the top site, the Fermi level lies closely at the Dirac point, i.e., the Dirac cone of the ideal free-standing graphene is recovered.

Magnetic ordering in digital alloys of group-IV semiconductors with 3d-transition metals

The ab initio investigation of the magnetic ordering in digital alloys consisting of monolayers of 3dtransition metals Ti, V, Cr, Mn, Fe, Co, and Ni introduced into the Si, Ge, and Si0.5Ge0.5 semiconductor hosts is reported. The calculations of the parameters of the exchange interactions and totalenergy calcula� tions indicate that the ferromagnetic order appears only in the manganese monolayers, whereas the antifer� romagnetic order is more probable in V, Cr, and Fe monolayers, and Ti, Co, and Ni monolayers are nonmag� netic. The stability of the ferromagnetic phase in digital alloys containing manganese monolayers has been analyzed using the calculations of magnon spectra.

Confinement-induced changes in magnetic behavior of a Ti monolayer on Pt

Density functional theory is used to examine the magnetic properties of a Ti monolayer over Pt when is brought in contact with a Pt surface, at distances of 4–12 Å. The average magnetic moment of the Ti film decreases sharply as the surface–surface separation decreases, coincident with migration of surface electrons towards the gap, surface d-band broadening, and decrease of the density of states at the Fermi level. The phenomenon is analyzed using the Stoner criterion that correlates the ferromagnetic behavior with exchange splitting and density of states at the Fermi level obtained from a paramagnetic calculation.

Lattice Distortion Oriented Angular Self-Assembly of Monolayer Titania Sheets

Self-assembly of exfoliated monolayer titania sheets is investigated by detailed transmission electron microscopy and the force field calculations. It is demonstrated for the first time that slight but significant lattice distortions result in modified angular self-assembly of exfoliated monolayer Ti(0.87)O(2) sheets. These findings significantly broaden current knowledge of the self-assembly of exfoliated nanoscale layered sheets, which may render the potential manipulation of self-assembly of nanosheets.

3d 및 4d 전이금속과 Pd가 c(2×2) 합금을 이룬 단층의 자성에 대한 제일원리 연구

Pd과 3d(V, Ti) 및 4d(Mo, Ru, Rh) 전이금속이 c(<TEX>$2{\times}2$</TEX>) 합금을 이루고 있는 단층의 자성을 제일원리적 FLAPW 에너지띠 방법을 이용하여 연구하였다. 이들 합금과 비교를 위하여 Pd과 V, Ti 및 Mo, Ru, Rh 원자로만 이루어진 단층의 전자구조도 계산하였다. 3d 원소인 V와 Ti 만으로 이루어진 단층의 경우, V은 반강자성, Ti의 경우는 강자성상태가 안정적이었으나, 이들이 Pd와 c(<TEX>$2{\times}2$</TEX>) 합금을 이루었을 경우 모두 자기모멘트가 반대 방향을 가지는 준강자성 상태가 안정적이었다. 반면에 4d 원자인 Mo, Ru, Rh이 Pd와 c(<TEX>$2{\times}2$</TEX>) 합금을 이룰 경우에는 자기모멘트들이 같은 방향을 가졌다. 자기모멘트 값을 보면, Ru이나 Rh의 경우 순수단층이나 Pd과 합금을 이룬 경우 그리 큰 차이를 보이지 않았으나, Mo의 경우 <TEX>$0.02\;{\mu}_B$</TEX>에서 <TEX>$2.98\;{\mu}_B$</TEX>로 급격히 증가하였다. 합금을 이루는 두 원소사이의 전하이동은 전기음성도에 따르게 됨을 알았다. We investigated the electronic structure and magnetism of the (3d, 4d)-Pd alloyed c(<TEX>$2{\times}2$</TEX>) monolayer systems, by use of the FLAPW band method. For comparison, pure 3d- and 4d-transition metal monolayers are also considered. We found that the antiferromagnetic configuration of pure V monolayers is sustained in the V-Pd alloy system, while the Ti-Pd alloy system is changed to antiferromagnetic configuration from the ferromagnetic state in pure Ti monolayer. The 4d TM (Mo, Ru, Rh)-Pd monolayers are found to be stable in ferromagnetic configurations. The magnetic moments of Ru and Rh atoms in Ru-Pd and Rh-Pd systems are almost same with those of pure Ru and Rh monolayers, while the magnetic moment of Mo atom is increased to <TEX>$2.98\;{\mu}_B$</TEX> in Mo-Pd alloyed system from the value of Mo monolayer, <TEX>$0.02\;{\mu}_B$</TEX>.

A Theoretical Study of H2 Reacting on Ti/Al(100) Surfaces

Using density functional theory, we study H2 dissociation on Ti/Al(100) alloy surfaces, modeled by different Ti coverages varying from 1/4 to 1 monolayer. The minimum barrier height for H2 dissociation depends on the specific Ti coverage on the Al surface. In view of experimental information on Ti in hydrogenated and dehydrogenated NaAlH4, the most realistic model promoting H2 dissociation is found to be the (100) surface of Al covered by one monolayer of Ti in a c(2 × 2) structure, with Ti atoms in the first and third layers. This model has a late dissociation barrier with a height of only 0.23 eV, preceded by a deep molecular chemisorption well with a well depth of 0.45 eV. The projected density of states analysis provides a molecular orbital view in which the barrierless approach to the molecular chemisorption well is mainly explained by an occupied-virtual attraction between the H2 σg and Ti 3dz2 orbitals. The barrier separating the molecular chemisorption well and the dissociated state can be underst...

Comparison of H 2 production from ethanol and ethylene glycol on M/Pt(1 1 1) (M = Ni, Fe, Ti) bimetallic surfaces

The reactions of ethylene glycol and ethanol have been studied on Fe/Pt(1 1 1) and Ti/Pt(1 1 1) bimetallic surfaces utilizing temperature programmed desorption (TPD). These results are compared to our previous studies on Ni/Pt(1 1 1) to illustrate the trend in the reforming activity on 3d-Pt bimetallic surfaces. The oxygenates decomposed on these surfaces to produce mainly H 2 and CO. The bimetallic surfaces were prepared by thermal evaporation of Fe or Ti onto Pt(1 1 1), using Auger electron spectroscopy (AES) to monitor surface compositions. Surfaces prepared by deposition of a monolayer of Fe or Ti on Pt(1 1 1), designated Fe–Pt–Pt(1 1 1) or Ti–Pt–Pt(1 1 1), displayed higher reforming activity for both ethylene glycol and ethanol than the corresponding subsurface monolayer Pt–Fe–Pt(1 1 1) and Pt–Ti–Pt(1 1 1) structures or clean Pt(1 1 1). The reforming yield increased as the surface d-band center, calculated from density functional theory (DFT), shifted closer to the Fermi level. The reforming selectivity of oxygenates, especially ethanol, began to decrease as the d-band center shifted closer to the Fermi level. Combining results in the current work with previous studies on Ni/Pt(1 1 1), a general criterion can be formulated for selecting 3d-Pt bimetallic surfaces with desirable reforming activity and selectivity.

Growth of atomically smooth MgO films on graphene by molecular beam epitaxy

We investigate the growth of MgO films on graphene by molecular beam epitaxy and find that surface diffusion promotes a rough morphology. To reduce the mobility of surface atoms, the graphene surface is dressed by Ti atoms prior to MgO deposition. With as little as 0.5 ML (monolayer) of Ti, the MgO overlayer becomes atomically smooth. Furthermore, no aggregation of MgO is observed at the edges of the graphene sheet. These results are important for the fabrication of nanoscale electronic and spintronic devices.

Electronic structure of point defects in controlled self-doping of theTiO2(110) surface: Combined photoemission spectroscopy and density functional theory study

Point defects in metal oxides such as ${\text{TiO}}_{2}$ are key to their applications in numerous technologies. The investigation of thermally induced nonstoichiometry in ${\text{TiO}}_{2}$ is complicated by the difficulties in preparing and determining a desired degree of nonstoichiometry. We study controlled self-doping of ${\text{TiO}}_{2}$ by adsorption of 1/8 and 1/16 monolayer Ti at the (110) surface using a combination of experimental and computational approaches to unravel the details of the adsorption process and the oxidation state of Ti. Upon adsorption of Ti, x-ray and ultraviolet photoemission spectroscopy (XPS and UPS) show formation of reduced Ti. Comparison of pure density functional theory (DFT) with experiment shows that pure DFT provides an inconsistent description of the electronic structure. To surmount this difficulty, we apply DFT corrected for on-site Coulomb interaction $(\text{DFT}+U)$ to describe reduced Ti ions. The optimal value of $U$ is 3 eV, determined from comparison of the computed $\text{Ti}\text{ }3d$ electronic density of states with the UPS data. $\text{DFT}+U$ and UPS show the appearance of a $\text{Ti}\text{ }3d$ adsorbate-induced state at 1.3 eV above the valence band and 1.0 eV below the conduction band. The computations show that the adsorbed Ti atom is oxidized to ${\text{Ti}}^{2+}$ and a fivefold coordinated surface Ti atom is reduced to ${\text{Ti}}^{3+}$, while the remaining electron is distributed among other surface Ti atoms. The UPS data are best fitted with reduced ${\text{Ti}}^{2+}$ and ${\text{Ti}}^{3+}$ ions. These results demonstrate that the complexity of doped metal oxides is best understood with a combination of experiment and appropriate computations.

Effect of adsorbed hydrogen on the stability of titanium atoms on aluminum surfaces

The thermal stability of Ti atoms evaporated onto polycrystalline Al and Al(100) surfaces was studied using ion scattering spectroscopy. Measurements were made in vacuum and in the presence of deuterium. When heated in vacuum, Ti begins to disappear from the surface at substrate temperatures below $100\text{ }\ifmmode^\circ\else\textdegree\fi{}\text{C}$ and is not detected on the surface at temperatures above $250\text{ }\ifmmode^\circ\else\textdegree\fi{}\text{C}$. When measurements are made while the sample is being exposed to atomic D, Ti persists on the surface to noticeably higher temperatures. Diffusion coefficients for surface Ti moving into the Al substrate were obtained at temperatures in the range $25--150\text{ }\ifmmode^\circ\else\textdegree\fi{}\text{C}$. The experimental results are compared to density-functional theory calculations, which indicate that direct and Al mediated H-Ti interactions stabilize surface Ti. At H coverages around 1 ML (monolayer), the most stable structure has 1/2 ML Ti in the second layer and 1 ML H on specific bridge sites on the top Al layer. Ti directly exposed to the surface is less stable and thus likely to be less active for Ti catalysis of H sorption in alanate H storage materials.

Electronic and magnetic properties of basic nanosystems of early 3d transition metals (Sc, Ti, V, Cr, and Mn)

This detailed and systematic theoretical study on the behavior of basic low dimensional (one- and two-dimensional) systems of early 3d transition metals should serve as a guideline to experimentalists as well as to theoreticians. We find that, lowering of dimensionality is favorable for emergence of magnetic ordering in all the systems studied, except Ti monolayers (MLs). For Ti MLs, both nonmagnetic and ferromagnetic states are degenerate within the numerical limits. For such a case, the interactions with substrate would play a decisive role in the magnetic ordering of the atoms in the ML. The total energy calculations show that the nonmagnetic and ferromagnetic states are almost degenerate for Cr and V MLs too; however, anti-ferromagnetic ordering is favored in these. The ferromagnetic ordering in Sc linear chains and anti-ferromagnetic ordering in MLs of Mn and Cr are found to be favored by a relatively larger margin showing good stability. Some low dimensional systems, showing electrons with only one kind of spin available at Fermi energy, may be suitable for spintronics related applications. The linear chains of Cr and Mn, and MLs of Sc are likely to form stable magnetic nanosystems as these exhibit almost saturated magnetic moment per atom around the equilibrium separation. The magnetic moment strengthens considerably as one goes from two- to one-dimension. Our results are supported qualitatively by available experimental results and offer a good insight into these nanosystems.

Alternate stacking of transition metal ions and terephthalic acid molecules for the fabrication of self-assembled multilayers

Self-assembled multilayers consisting of transition metal ions and biscarboxyl acid molecules have been fabricated by a layer-by-layer chemisorption technique. As transition metal ion, zirconium (Zr(IV)) or titanium (Ti(IV)) was employed, while terephthalic acid (TPA) was used as biscarboxyl molecule. In the multilayers, two TPA monolayers were bridged by one monolayer of Zr(IV) or Ti(IV) most likely through coordination bonds between the metal ions and the carboxyl groups in the TPA molecules. Although the both transition metal ions were successfully applied to construct multilayers, the multilayer structure of the Ti–TPA system was more disordered than that of the Zr–TPA system as revealed by grazing incidence X-ray reflectivity.

Wear and corrosion behavior of W/WC bilayers deposited by magnetron sputtering

AbstractWC/W coatings were deposited by reactive magnetron sputtering using 40%, 60% and 80% methane CH4 in the gas mixture. The bilayers were grown on to AISI 420 stainless‐steel substrates in order to study the wear and corrosion behavior. Before growing the bilayers, one Ti monolayer was grown to improve the adherence of the coatings to the substrate. The wear resistance and the friction coefficient of the coatings were determined using a pin‐on‐disk tribometer. All coatings had a friction coefficient of about 0.5. The measured weight lost of the bilayers from each probe allowed the qualitative analysis of wear behavior all coatings. The bilayers grown with 80% methane showed the best abrasive wear resistance and adhesion without failure through the coating in the wear track for dry pin‐on‐disk sliding. Electrochemical corrosion test showed that the bilayers grown with 80% methane were more resistant to corrosion than the ones uncoated. (© 2007 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Structural and mechanical properties of titanium and titanium diboride monolayers and Ti/TiB 2 multilayers

Nano-multilayers represent a new class of engineering materials that are made up of alternating nanometer scale layers of two different components. In the present work a titanium (Ti) monolayer was combined with titanium diboride (TiB 2) to form a Ti/TiB 2 nano-multilayer. Designed experimental parameters enabled an evaluation of the effects of direct current bias voltage ( U b) and bilayer thickness ( Λ) during multilayer deposition on the mechanical properties of reactively sputtered Ti/TiB 2 multilayer films. Their nanostructures and mechanical properties were characterized and analyzed using X-ray photoelectron spectroscopy (XPS), low-angle and high-angle X-ray diffraction (XRD), plan-view and cross-sectional high-resolution transmission electron microscopy (HRTEM), and microindentation measurements. Under the optimal bias voltage of U b = − 60 V, it was found that Λ (varied from 1.1 to 9.8 nm) was the most important factor which dominated the nanostructure and hardness. The hardness values obtained varied from 12 GPa for Ti and 15 GPa for TiB 2 monolayers, up to 33 GPa for the hardest Ti/TiB 2 multilayer at Λ = 1.9 nm. The observed hardness enhancement correlated to the layer thickness, followed a relation similar to the Hall-Petch strengthening dependence, with a generalized power of ∼ 0.6. In addition, the structural barriers between two materials (hcp Ti/amorphous TiB 2) and stress relaxation at interfaces within multilayer films resulted in a reduction of crack propagation and high-hardness.

Structural investigation of the c(√2 × 5√2)R45° surface alloy formed by Ti deposition on Cu(0 0 1)

The alloying process of Ti deposited on Cu(0 0 1) was studied by means of XPS, LEIS, XPD and LEED intensity analysis. With the sample held at 570 K, a linear decrease of the Cu LEIS signal as a function of the amount of Ti deposited is observed in the early stages of deposition until a constant value is reached. At the onset of the plateau a c(√2 × 5√2)R45° LEED pattern starts to be visible. XPD and LEED intensity measurements were performed for the c(√2 × 5√2)R45° phase prepared depositing ca. 1.5 monolayer of Ti. The angle-scanned XPD curves measured for the phase c(√2 × 5√2)R45° reveal that Ti atoms substitute Cu atoms in the fcc lattice of the substrate. The polar XPD curves show that at least the first four layers of the substrate are involved in the alloying process. We found that the (3 1 0) plane of the Cu 4Ti alloy (D1 a type-structure) fits, without significant contraction or expansion of the lattice parameters, the c(√2 × 5√2)R45° structure. The intensity versus energy curves of the diffracted beams were calculated on the basis of this structural model using the tensor LEED method. The results of the LEED intensity analysis provide a further evidence of the formation of a slab of Cu 4Ti(3 1 0) layers.

Theoretical study of Ti and Fe surface alloys on Al(001) substrate

Abstract Accurate density-functional calculations are performed to investigate the formation of Ti and Fe ultrathin alloys on Al(0 0 1) surface. It is demonstrated that a deposition of Ti monolayer on Al(0 0 1) substrate leads to the formation of Al3Ti surface alloy with Ti atoms arranged according to the L12 stacking, distinct from the D022 structure characteristic of a bulk Al3Ti compound. A quest for the reason of this distinct atomic arrangement led us to the study of the surface structure of Al3Ti(0 0 1) compound. It is concluded that even the Al3Ti(0 0 1) surface is terminated with three layers assuming a L12 stacking and hence this stacking fault can be classified as a surface-induced stacking fault. Several possibilities of Fe atoms distributed in the surface region of Al(0 0 1) have been examined. The most stable configuration is the one with the compact Fe monolayer on Al(0 0 1) and covered by one Al monolayer. Lastly, our calculations show that there is no barrier for the penetration of Fe adatoms below the Al(0 0 1) surface; however, such a barrier is present for a Ti-alloyed Al(0 0 1) surface.

The room temperature growth of Ti on sputter-cleaned Si(1 0 0): Composition and nanostructure of the interface

The formation of the interface during the deposition of titanium on sputter-cleaned Si(1 0 0) substrates has been studied at room temperature using X-ray photoelectron spectroscopy (XPS), angle resolved XPS (ARXPS), ultraviolet photoelectron spectroscopy (UPS) and ion scattering spectroscopy (ISS). The experimental results are consistent with a two-stage mechanism for Ti growth: a first stage characterized by the formation of a uniform layer ∼4 monolayer (ML) thick of TiSi x ( x ≈ 0.78), followed by a second stage in which metallic Ti grows over the titanium silicide formed previously. During the second stage, metallic Ti grows according to a Stranski–Krastanov mechanism, with the formation of a Ti monolayer followed by the growth of metallic titanium islands with an average thickness of 7 ML. The formation of a titanium silicide during the first stage of growth, involves charge transfer from Ti to Si.

The Characteristics Improvement of Ozone Water Production by Multi-layer Electrodes at Water Electrolysis Cell by Solid Polymer Electrolyte

Ozone is recently used for many purposes as an environmental friendly oxidant, so the ozone production device with high ozone concentration and low production energy is desired. One of the candidates for such device is the ozone water production by water electrolysis cell using the solid polymer electrolyte with PbO2 anode catalyst, which has merits to be compact and to produce high-concentration ozone water directly by the deionized water electrolysis. In this study, we have tested ozone water production by changing electrodes and electrolytes constitution in order to improve the ozone production performance. Tested two electrolytes are Nafion117 and a membrane-electrode assembly (MEA) with Pt catalyst on cathode side of Nafion117. Tested two electrodes are mono-layer of Ti expanded metal and four different mesh layers of Ti expanded metal. Ozone water production tests are performed under long-term operations changing temperature and flow rate to optimize experimental conditions. The voltage-current characteristic for electrolysis cell have been improved significantly, when the electrode is four layers of Ti metal and the electrolyte is the MEA with Pt catalyst. The stable ozone water concentration has been obtained by operating the cell for about 8 hours. The optimum temperature and water flow rate for ozone water production are 25˜30° and 33L/h, respectively. Furthermore, the optimum overpotential was measured by a reference electrode at the cathode-side MEA, and the anode catalyst suitable for ozone water production was identified to be βPbO2 by the X ray diffraction pattern.

Thermal stability of ultrathin titanium films on a Pt(111) substrate

We report annealing studies using He +-ion scattering spectroscopy (He +-ISS) and X-ray photoelectron spectroscopy (XPS) to evaluate the thermal stability of ultrathin Ti films deposited on a Pt(111) single-crystal surface at 300 K. These results establish that strong Pt–Ti intermetallic bonding provides sufficient driving force for thermal interdiffusion of Ti and Pt at a (111)-oriented interface at 700 K (427 °C), even in the absence of oxygen and a minimal influence of grain boundaries. Identification of this critical temperature provides additional fundamental information for Ti/Pt bilayer film processing considerations. This data was obtained by using He +-ISS to monitor the concentration of Ti in the topmost surface layer as a function of temperature. In these studies, we define 1 dose unit (DU) to be the amount of deposited Ti that is required to eliminate the Pt signal in He +-ISS. This amount should be close to that required to produce a one-monolayer Ti film, but it could exceed one monolayer of Ti if alloying or clustering occurs on the substrate at 300 K under these deposition conditions. For Ti films less than 1 DU, heating to 700 K slightly decreased the Ti surface concentration. Higher annealing temperatures of up to 950 K caused extensive loss of surface Ti, and this is attributed to Ti diffusion into the Pt crystal sub-surface region. For a 1-DU Ti film, temperatures of 750–800 K were required to cause sufficient interdiffusion so that Pt atoms were detected in the topmost layer, strongly suggesting that the 1-DU Ti film was more thermally stable than those of lower Ti coverage. Thicker Ti films of greater than 1 DU required temperatures of 800–1050 K before any change was detected in the Ti surface concentration as probed by the He +-ISS signal intensity. After annealing to temperatures higher than 1000 K, XPS revealed a chemical shift for the Ti (2p) peaks of 1.5 eV to higher binding energy, indicating formation of a Ti/Pt alloy.