Ti Atomic Research Articles

Effect of vacancies and edges in promoting water chemisorption on titanium-based MXenes

The functionality of two-dimensional (2D) transition metal carbides and nitrides (MXenes) in technological applications greatly depends on their wettability. For instance, MXenes’ layer stability against degradative oxidation is notably reduced when stored in aqueous solutions, leading to the transformation into oxides. In this work, we study water adsorption on Ti-based MXenes by ab initio calculations. The energy gains for the molecular adsorption on Tin+1XnT2 is evaluated as a function of the termination (T = F, O, OH, mixture), the carbon/nitrogen ratio (X = C, N), the layer thickness (n) and water coverage. MXenes’ hydrophilicity tends to increase due to the presence of defects as vacancies and flake edges. We demonstrate that physical adsorption occurs through hydrogen bonding on both defect-free layers and layers containing C/N or Ti atomic vacancies, with –OH terminations providing the strongest interactions (0.40–0.65 eV). In contrast, strong water chemisorption is observed on surfaces with a single termination vacancy (0.60–1.20 eV), edges (0.75–0.85 eV), and clusters of defects (1.00–1.80 eV). We verified that the presence of undercoordinated Ti atoms on the surface is the key factor in promoting H2O chemisorption, i.e., the degradative oxidation.Graphical

Development of laser ablation absorption spectroscopy for nuclear fuel materials: plume expansion behavior for refractory metals observed by laser-induced fluorescence imaging spectroscopy

The dynamic behavior of an ablation plume in low pressure rare gas ambient was investigated with laser-induced fluorescence imaging spectroscopy for three refractory metals, i.e. titanium, zirconium and hafnium. A comparison of the plume expansion behaviors for the species of these elements revealed an atomic weight effect on the plume structure formation. A hemispherical thin layer and cavity structure reported previously for gadolinium were observed also for these elements. It was found that the plume size increases as well as the layer thickness decreases with increasing atomic weight. For ground state atoms of Ti, substantial amount of atoms were observed even at the center of the plume. Also, the persistence of the Ti atomic plume was as long as 300 μs, which was significantly longer than the other species studied. Furthermore, the mass-dependent elemental separation was observed in the ablation plume produced from a multielement sample. These results suggest that the observed plume structure arises from the ion-electron recombination process and the recoil of the ablated species during the multiple collisions with gas atoms.

Synthesis and characterization of Pt nanocatalyst on Ru0.7Ti0.3O2 support as a cathode for fuel cells application

Ruthenium oxide/titanium oxide, with a Ru:Ti atomic ratio of 7:3 was synthesized by modified sol–gel procedure and used as a support for platinum nanocatalyst for oxygen reduction reaction. The synthesized materials were characterized in terms of morphology, particle size distribution, chemical and phase composition by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), high angle annular dark filed scanning transmission electron microscopy (HAADF, STEM) and electron energy loss spectroscopy (EELS). XPS spectra revealed that Ru atoms were in mainly in Ru(4+) oxidation state, the Ti atoms in Ti(4+) oxidation state, whereas the Pt-atoms were in metallic state. TEM analysis proved that platinum nanoparticles nucleated at both oxide species and homogeneous distribution was observed. The average platinum nanoparticle size was 3.05nm.Electrochemically active surface area of platinum was 32m2g−1. Kinetics of the oxygen reduction was studied at rotating disc electrode in 0.5moldm−3 HClO4 solution, at 25°C. The catalytic activities expressed in terms of specific activity (per electrochemically active surface area of platinum) and mass activity (per mass of platinum) were determined and compared to Pt catalyst on carbon support. The high catalytic activity was proven by electrochemical characterization.

Doping mechanism of Vanadia/Titania nanoparticles in flame synthesis by a novel optical spectroscopy technique

Flame synthesis of V-doping TiO2 is studied by in situ diagnostic of phase selective laser-induced breakdown spectroscopy (LIBS). We apply this novel optical spectroscopy to tracing the gas-to-particle phase transition of V and Ti elements, as low-intensity laser only excites V and Ti atoms present in the particle phase but not in the gas phase. Both V and Ti atomic signals appear early at the burner exit and plateau downstream after a distance about 14mm. Compared with signals in pure TiO2 synthesis, the signal of Ti in the doping synthesis is significantly strengthened due to the lower band gap of V-doped TiO2. The doping mechanism is then inferred from the observations. It is deduced that the substantial collision and mixing of the nucleated V and Ti oxides occur even at the burner rim and persist through the entire process. The signal intensities of both V and Ti atoms increase with laser power and tend to plateau at about 20mJ/pulse. In the flatten region, the ratio of V and Ti signal intensities is almost proportional to the doping ratio of V and Ti elements in the particle phase, showing feasibility of utilizing the optical method in the doping ratio measurement.

Improvement of resistive switching in ZnO film by Ti doping

Pt/ZnO:Ti/n+–Si structures that showed reversible and steady resistive switching behaviors were fabricated by magnetron sputtering. The stability of the devices was improved by Ti doping and the switching mechanism of the resistive switching was theoretically studied under the guidance of the first principles. The influences of different Ti atomic doping concentrations on the crystal structure and resistive switching characteristics were also investigated. The results revealed that the oxygen vacancy appears easily around the Ti ions since the formation energies of oxygen vacancies had the minimum value when it located at the next nearest neighboring to Ti atoms.

Novel low-intensity phase-selective laser-induced breakdown spectroscopy of TiO2 nanoparticle aerosols during flame synthesis

Novel low-intensity laser-induced breakdown spectroscopy (LIBS) is employed to conduct an in situ study on swirl flame synthesis of TiO2 nanoparticles. Collected emissions agree well with Ti atomic spectra from the NIST database. In contrast to traditional application of LIBS on particles, the power used here is much lower (∼35mJ/pulse or 28J/cm2 at 532nm); and no macroscopic spark is visually observed. Nevertheless, the low-intensity LIBS shows interesting selectivity—only exciting Ti atoms in particle phase, with no breakdown emission occurring for gas molecules (e.g. titanium tetraisopropoxide precursor, air). The emission intensity increases as the nanoparticles grow in the synthesis flow field, plateauing as the particles become larger than 6nm, indicating the absorption efficiency to be size-dependent for small particles. The signals saturate at a fluence of ∼16J/cm2. When the precursor concentration is larger than 150ppm (corresponding to particle sizes of 6–8nm), the emission intensity increases linearly with precursor concentration. The selectivity of the low-intensity LIBS for such application could be advantageous for tracking nanoparticle formation and for measuring particle volume fraction during gas-phase synthesis or other systems. The size-dependent absorption efficiency could be used to measure nanoparticle size in situ.

Mechanical and tribological properties of Ti-DLC films with different Ti content by magnetron sputtering technique

Ti-doped diamond-like carbon (DLC) films were deposited on Si substrates at room temperature by magnetron sputtering Ti twin-target in methane and argon mixture atmosphere. The DLC films with different Ti concentrations were fabricated by varying the gas flow ratio of Ar/CH4. X-ray photoelectron spectroscopy (XPS), Raman spectra were used to analyze the composition and the microstructure of the films. The internal stress was calculated by using the Stoney equation, where the curvature of the film/substrate was measured by BGS 6341 type film stress tester. The mechanical and tribological properties of the films were systematically studied by the nano-indentor and reciprocating ball-on-disc tester, respectively. The Ti atomic concentration in the films increased from 0.41% to 8.2% as the Ar/CH4 flow ratio increased from 60/190 to 140/110. The Ti atoms exist mainly in the form of metallic-like Ti rather than TiC when Ti concentration is 0.41%, confirmed by XPS analysis. As the Ti concentration rose to 6.7%, the Ti-DLC films transformed to composite DLC films with carbide phase embedded in the DLC matrix because of the formation of TiC. As a result, the hardness is decreased, while the stress is dramatically increased. The Ti-DLC films with 0.41% Ti doping showed a relatively high hardness (13.75GPa), low stress (0.56GPa), extremely low wear rate (∼10−10mm3/Nm) and low friction coefficient (0.05).

Electronic and Magnetic Properties of Ti and Fe on Graphene

The structural, electronic and magnetic properties of Fe and Ti atomic wires and the complete covering when adsorbed on graphene are presented through ab initio calculations based on density functional theory. The most stable configurations are investigated for Fe and Ti in different concentrations adsorbed on the graphene surface, and the corresponding binding energies are calculated. The results show a tendency of the Ti atoms to cover uniformly the graphene surface, whereas the Fe atoms form clusters. The adsorption of the transition metal on the graphene surface changes significantly the electronic density of states near the graphene Fermi region. In all arrangements studied, a charge transfer is observed from the adsorbed species to the graphene surface due to the high hybridizations between the systems.

Atom Probe Tomographic Studies of Precipitation in Al-0.1Zr-0.1Ti (at.%) Alloys

Atom probe tomography was utilized to measure directly the chemical compositions of Al(3)(Zr(1)-(x)Ti(x)) precipitates with a metastable L1(2) structure formed in Al-0.1Zr-0.1Ti (at.%) alloys upon aging at 375 degrees C or 425 degrees C. The alloys exhibit an inhomogeneous distribution of Al(3)(Zr(1)-(x)Ti(x)) precipitates, as a result of a nonuniform dendritic distribution of solute atoms after casting. At these aging temperatures, the Zr:Ti atomic ratio in the precipitates is about 10 and 5, respectively, indicating that Ti remains mainly in solid solution rather than partitioning to the Al(3)(Zr(1)-(x)Ti(x)) precipitates. This is interpreted as being due to the very small diffusivity of Ti in alpha-Al, consistent with prior studies on Al-Sc-Ti and Al-Sc-Zr alloys, where the slower diffusing Zr and Ti atoms make up a small fraction of the Al(3)(Zr(1)-(x)Ti(x)) precipitates. Unlike those alloys, however, the present Al-Zr-Ti alloys exhibit no interfacial segregation of Ti at the matrix/precipitate heterophase interface, a result that may be affected by a significant disparity in the evaporation fields of the alpha-Al matrix and Al(3)(Zr(1)-(x)Ti(x)) precipitates and/or a lack of local thermodynamic equilibrium at the interface.

Ferromagnetic one dimensional Ti atomic chain

Using the full potential linearized augmented plane wave method, we have explored the magnetic properties of one dimensional (1D) Ti atomic chain. Interestingly, we have observed that the 1D Ti atomic chain has ferromagnetic ground state even on NiAl(110) surface. In addition, it has been found that the magnetic dipole interaction is a key factor in the study of magnetic anisotropy, not the magnetocrystalline anisotropy arising from spin-orbit interaction.

Influences of xTi/ xAl on atomic states, lattice constants and potential-energy planes of ordered FCC TiAl-type alloys

Based on the framework of systematic science of alloys, the basic information about states, volumes and potential energies of characteristic atoms in FCC Ti–Al system has been determined. For ordered FCC TiAl type alloys, the functions of the atomic states, atomic potential energies, cohesive energies, atomic volumes, the lattice constants and axis ratio c/ a′ varying with concentrations have also been established. The extreme points of curves v∼ x Al, a∼ x Al and c/ a′∼ x Al are all at the x Al=70%. When x Ti/ x Al=1, ψ (FCC TiAl) =ψ 8 Ti +ψ 4 Al = [Ar](3d n ) 0.125 (3d c ) 2.7125 (4s c ) 0.55 (4s f ) 0.6125+ [Ne](3s c ) 1.789 (3p c ) 0.75 (3s f +3p f ) 0.454 , calculated lattice constants a′=0.39954 nm, c=0.40777 nm, c/ a′=1.0206, which are in good agreement with experimental values. When x Ti/ x Al>1, ψ 8 Ti atoms decrease and ψ 7 Ti , ψ 6 Ti … atoms appear in succession with the increase of x Ti; ψ 4 Al atoms decrease and ψ 3 Al , ψ 2 Al … atoms appear one after another. Because the influence of the change of Ti atomic states exceeds that of the change of Al atomic states, rich Ti-FCC TiAl alloys have larger atomic volumes, larger lattice constants, c/ a′ much closer to unity and comparatively good ductility, though the stability of FCC lattice is reduced. When x Ti/ x Al<1, with the increase of x Al, the atomic states ψ 8 Ti and ψ 4 Al changes in the direction opposite to the case with x Ti/ x Al>1. Thus, ordered rich Al-FCC TiAl alloys have smaller atomic volumes, smaller lattice constants, larger c/ a′ which depart further from unity and poorer ductility, and there is an increase of stability for the FCC lattice.

Computation of Large Systems with Economic Basis Set: Simulation of Diamond Metallization Using Titanium

Economic basis sets for ab initio calculations of systems involving titanium (Ti) atom have been studied. The basis set in which the polarization and diffuse functions are applied only to Ti and its nearest neighboring atoms is shown to give reliable binding energies and geometries for various carbon-based compounds. Such a basis set is used as the economic basis set for calculations of medium-sized clusters simulating Ti and diamond interactions. For large clusters composed of Ti and C atoms, the more economic basis set 6-31G is considered for obtaining reliable energetics with the correction using high-level single-point energy calculations with 6-31+G*. The approach to determine the economic basis set is expected to be useful for a general system containing transition metal element. With the economic basis sets, simulations of diamond metallization have revealed reasonable behavior of the Ti atomic interaction with diamond film. On the basis of the determined energetics of Ti deposition, the high Ti concentration in CVD diamond after metallization is conjectured to occupy the grain boundaries rather than the bulk of diamond grain. For a Ti atom with high kinetic energy, the Ti atom may penetrate through a diamond (001) surface more easily than through a diamond (111) surface. In the diamond bulk, the Ti atom may energetically favor to substitute a C site more than diffusion.

Characterization of microstructure and mechanical behavior of sputter deposited Ti-containing amorphous carbon coatings

We report on the characterization of microstructure and mechanical properties of sputter deposited Ti-containing amorphous carbon (Ti-aC) coatings as a function of Ti composition. Ti-aC coatings have been deposited by unbalanced magnetron sputter deposition, in an industrial-scale four-target coating deposition system. The composition and microstructure of the Ti-aC coatings have been characterized in detail by combining the techniques of Rutherford backscattering spectrometry (RBS) and hydrogen elastic recoil detection (ERD), transmission electron microscopy (TEM), X-ray absorption near edge structure (XANES) spectroscopy and extended X-ray absorption fine structure (EXAFS) spectroscopy. At Ti compositions <4at.%, Ti atoms dissolve in an amorphous carbon (a-C) matrix. The dissolution limit of Ti atoms in an a-C matrix is determined to be between 4 and 8 at.%. At Ti compositions >8 at.%, XANES and EXAFS data indicate that the average Ti atomic bonding environment in Ti-aC coatings resembles that in cubic B1-TiC, consistent with TEM observation of precipitation of TiC nanocrystallites in the a-C matrix. Beyond the Ti dissolution limit, the Ti-aC coatings are nanocomposites with nanocrystalline TiC clusters embedded in an a-C matrix. A large scale, quasi one-dimensional composition modulation in the Ti-aC coatings was observed due to the particular coating deposition geometry. Elastic stiffness and hardness of the Ti-aC coatings were measured by instrumented nanoindentation and found to vary systematically as a function of Ti composition. Unlubricated friction coefficient of Ti-aC coatings against WC–Co balls was found to increase as the Ti composition increases. As Ti composition increases, the overall mechanical behavior of the Ti-aC coatings becomes more TiC-like.

Ti atomic bonding environment in Ti-containing hydrocarbon coatings

We report characterization of the average Ti atomic bonding environment in Ti-containing hydrocarbon (Ti–C:H) coatings by x-ray absorption near edge structure (XANES) spectroscopy, extended x-ray absorption fine structure (EXAFS) spectroscopy, and high-resolution transmission electron microscopy (TEM). Ti–C:H coatings have been synthesized in a hybrid chemical vapor deposition/physical vapor deposition deposition system, which combines inductively coupled plasma and sputter deposition. Combining x-ray absorption spectroscopy with high resolution TEM imaging, we have determined that the dissolution limit of Ti atoms in an amorphous hydrocarbon (a-C:H) matrix is between 0.9 and 2.5 atomic percent. At Ti compositions &amp;gt;2.5 at. %, XANES and EXAFS data indicate that the average Ti atomic bonding environment in Ti–C:H resembles that in cubic B1–TiC, consistent with direct TEM observation of the precipitation of TiC nanocrystallites in an a-C:H matrix. Beyond the Ti dissolution limit, Ti–C:H coatings are in fact TiC/a-C:H nanocomposites, in which the TiC nanoprecipitates are very much bulk like.