Ti Clusters Research Articles

Synthesis, characterization, and rheological properties of hybrid titanium star‐shaped poly(n‐butyl acrylate)

AbstractThe synthesis of hybrid star‐shaped polymers was carried out by atom transfer radical polymerization of n‐butyl acrylate from a well‐defined multifunctional titanium‐oxo‐cluster initiator. Conditions were identified to prevent possible side reactions among monomer, polymer, and the titanium‐oxo‐cluster ligands. Polymerizations provided linear first‐order kinetics and the evolution of the experimental molecular weight is also linear with the conversion. 1H DOSY NMR and cleavage of the polymeric branches from the multifunctional initiator by hydrolysis were used to (i) prove the star‐shaped structure of the polymer, and (ii) demonstrate that the shoulder observed on size exclusion chromatograms is not due to a noncontrolled polymerization but to ungrafting of polymeric branches during analysis. Rheological properties of the hybrid star‐shaped poly(n‐butyl acrylate) were studied in the linear regime and show that the Ti‐oxo‐cluster not only increases significantly the viscosity of the polymer relative to its ungrafted arm but has a rheological signature which is qualitatively different from that of stars with organic cores suggesting that the Ti cluster reduces significantly the molecular mobility of the star. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011

Morphology and electronic properties of thermally stable TiOxnanoclusters on Au(111)

TiO${}_{x}$ clusters on atomically flat Au(111) films have been investigated by means of scanning tunneling microscopy and scanning tunneling spectroscopy. We present measurements of both the morphology and the electronic properties of these clusters. First, pure Ti clusters are produced in the gas phase and are deposited onto a clean Au(111) surface under controlled ultrahigh vacuum conditions. Next, the clusters are oxidized. Then, scanning tunneling microscopy measurements are performed before and after annealing up to high temperatures (970 K), indicating a high thermal stability of the deposited clusters. Scanning tunneling spectroscopy reveals an $n$-type semiconductorlike gap in the TiO${}_{x}$ cluster density of states. Local influence of the clusters on the electronic surface state of the Au(111) substrate is observed, which is relevant for catalytic cycles where certain steps of the cycles take place at the metal-oxide interface.

ChemInform Abstract: Gold Behaves as Hydrogen in the Intermolecular Self‐Interaction of Metal Aurides MAu4 (M: Ti, Zr, and Hf)

AbstractDFT calculations

Gold Behaves as Hydrogen in the Intermolecular Self‐Interaction of Metal Aurides MAu4 (M=Ti, Zr, and Hf)

We performed density functional calculations to examine the intermolecular self-interaction of metal tetraauride MAu(4) (M = Ti, Zr, and Hf) clusters. We found that the metal auride clusters have strong dimeric interactions (2.8-3.1 eV) and are similar to the metal hydride analogues with respect to structure and bonding nature. Similarly to (MH(4))(2), the (μ-Au)(3) C(s) structures with three three-center two-electron (3c-2e) bonds were found to be the most stable. Natural orbital analysis showed that greater than 96 % of the Au 6s orbital contributes to the 3c-2e bonds, and this predominant s orbital is responsible for the similarity between metal aurides and metal hydrides (>99 % H 1s). The favorable orbital interaction between occupied Au 6s and unoccupied metal d orbitals leads to a stronger dimeric interaction for MAu(4)-MAu(4) than the interaction for MH(4)-MH(4). There is a strong relationship between the dimeric interaction energy and the chemical hardness of its monomer for (MAu(4))(2) and (MH(4))(2).

Microstructure of Ti-Cr Nanoparticles Prepared by Electrical Wire Explosion

The reaction products of electrical wire explosion with 25 at% Cr-coated Ti wire were identified as � -TiCr2, � -Ti, � (Ti,Cr) and � -TiCr2 based on the selected area electron diffraction patterns. There was a bimodal size distribution where Ti-Cr particles were 40–50 nm in size, and fine particles less than 10 nm were determined to be pure Ti or Cr. The Ti-Cr nanoparticles are not only a mixture of Ti-Cr phases but they are also heterogeneous themselves at a single particle level, due to the non-equilibrium effects of the EWE process. The very rapid collision and subsequent coalescence of Ti and Cr atom clusters are believed to have contributed to the formation of chemically segregated particles. [doi:10.2320/matertrans.M2010277]

Cation diffusion along basal dislocations in sapphire

The diffusion behavior of impurity cations was examined in deformed α-Al 2O 3 single crystals (sapphire) that had a high density of unidirectional basal dislocations. This behavior was examined in the temperature range of 1150–1400 °C by secondary ion mass spectrometry depth profiling techniques. In order to investigate different pipe diffusion behaviors with different diffusing elements, diffusion behaviors of Cr and Ti were studied. Cr was expected to exhibit behavior similar to that of Al because both are isovalent and because of the complete mutual solubility of Cr 2O 3 and Al 2O 3 at high temperatures. Ti was selected because it is representative of an element that is hardly soluble and thus is expected to segregate at dislocations. The lattice and pipe diffusion kinetics were best described by the equations D l ,Ti = ( 8.2 × 10 - 4 – 3.2 × 10 - 1 ) exp - 5.3 - 0.5 + 0.3 ( eV ) kT (m 2 s –1) and a 2 D p ,Ti eff = ( 6.5 × 10 - 30 – 2.9 × 10 - 20 ) exp - 1.9 - 1.5 + 1.4 ( eV ) kT (m 4 s –1) for Ti, respectively and by equations D l ,Cr = 2.1 - 1.6 + 6.9 × 10 - 10 exp - 3.1 - 0.2 + 0.4 ( eV ) kT (m 2 s –1) and a 2 D p ,Cr eff = ( 1.4 × 10 - 26 – 1.3 × 10 - 20 ) exp - 3.2 - 0.9 + 0.9 ( eV ) kT (m 4 s –1) for Cr, respectively. A drastic decrease in the activation energy for Ti penetration was observed; however, such behavior was not observed for Cr penetration. This fact well explains that the electrical conductivity of deformed sapphire, which results from diffused Ti nanowires along the dislocations of the crystal. The behavior of Cr was similar to that of 18O, examined previously, in that their activation energies for lattice and pipe diffusion were similar. Because Cr diffusion was expected to mimic cation self-diffusion, it was assumed that the activation energy for pipe diffusion was not very low compared to that for bulk diffusion. This assumption is consistent with the fact that the activation energy for grain boundary diffusion in alumina and other ceramics is equal to or higher than that of bulk diffusion. Thus, the absolutely low temperature dependence of Ti pipe diffusion in sapphire may not be because of the enhanced migration of Ti along dislocations but because of other effects, such as the segregation of Ti or Ti clustering at dislocations.

Large Polyoxotitanate Clusters: Well-Defined Models for Pure-Phase TiO2 Structures and Surfaces

Careful control of the temperature and duration of the reaction between titanium tert-butoxide and acetic acid leads to the formation of new polyoxotitanate clusters with 14, 18, and 28 Ti atoms. They are considered intermediates in the growth of Ti/O nanoclusters of increasing size. The Ti(28) cluster is the largest crystallized to date. UV/vis spectroscopy on this cluster reveals that the optical band gap is blue-shifted with respect to a cluster containing only 17 titanium atoms indicating that the optical properties are more heavily influenced by the internal structure of the particle than its size.

A Chemical Modification Strategy for Hydrogen Storage in Covalent Organic Frameworks

We developed a two-step doping strategy for chemical modification of covalent organic frameworks (COFs) as hydrogen storage materials. The first step was the boron (B) doping of organic building units, suppressing the clustering of subsequent metal dopants on frameworks. The second step was the doping of metal atoms, forming trapping centers for H2 molecules on B-doped COFs. Using ab initio calculations, we explored the doping processes and studied the dependence of the structural stability and electronic properties of doped building units on the B concentration. In organic building units of COF-1, two-B para substitution was energetically more favorable than other B substitutions. The clustering of Sc, Ti, and Ca was suppressed on such B-doped COF-1. Sc and Ti preferred the double-sided adsorption, while Ca only favored the single-sided adsorption, due to their different interactions with the substrates. Each Sc and Ti atom bound four H2 molecules through the Kubas interaction, while each Ca atom could adsorb six H2 molecules via the polarization of H2 molecules under the electric field surrounding the Ca. A hydrogen storage gravimetric capacity larger than 6.5 wt % was achieved in these modified frameworks. Our work indicates that COFs modified by the two-step doping are promising for hydrogen storage applications in fuel cell-powered vehicles.

Structural, energetic and magnetic properties of small Tin (n = 2–13) clusters: a density functional study

The structural, energetic, and magnetic properties of Tin clusters (n = 2 to 13 atoms) have been studied using Density Functional Theory (DFT), applying the pseudopotential LCAO method and the generalized gradient approximation for the exchange-correlation functional. The binding energy and the dissociation energy were calculated using the PBE and BLYP approximations for the exchange-correlation potential, in order to evaluate the performance of this functionals in predicting the energetic properties of small Ti clusters. The experimentally observed trend in the dissociation energy of Tin as a function of the cluster size is reproduced by both PBE and BLYP calculations. The effects of structural distortion on the magnetism of clusters were studied comparing the ground state structure against non-distorted clusters. It was found that the structural distortion has no effect on the total magnetic moment. For all studied clusters using the BLYP functional, with exception of Ti6 and Ti7, magnetism is predicted.

All-electron relativistic calculation on Au5X (X = Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu and Zn) clusters

All-electron scalar relativistic calculations on Au5X (X = Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu and Zn) clusters have been performed by using density functional theory with the generalized gradient approximation. Our calculation results indicate that all the lowest energy geometries of Au5X clusters have planar structures; the doped X atoms prefer to occupy the fourfold coordination site. Except Au5Fe, Au5Co and Au5Zn, for other clusters including pure Au6 cluster, the HOMO are delocalized obviously with a contribution from all atoms in the cluster. On the contrary, the electron localization in Au5Zn is very strong resulting in the least stability of this cluster. Au5Cu cluster with six delocalized electrons being defined as magic number for two-dimensional system has the largest VIP and deepest HOMO energy level. With the substitution Au for X atoms, the metallicity of all Au5X clusters is reinforced.

Electronic theory study on high temperature oxidation mechanism of Nb-Ti-Al alloy

The electronic structure parameters of density of electronic states, atoms embedding energy, affinity energy and cluster energy of Nb alloy have been calculated using the recursive method. The high-temperature oxidation mechanism of Nb alloy was investigated. Studies show that the oxygen adsorption on Nb alloy surface can lower the adsorption energy, so oxygen is easily adsorbed on the alloy surface, and gradually diffuses into the Nb alloy matrix. Oxygen has a high solubility in the Nb alloy matrix because of the negative atom embedding energy, and it has similar density of states to Nb. Because the atom embedding energies of Ti and Al in the alloy matrix are higher than that on the alloy surface, Ti and Al atoms diffuse from the alloy matrix to the alloy surface and segregate on the alloy surface, ultimately making the Nb surface rich in Ti and Al. The clustering energy calculation shows that Ti and Al atom tend to gather on their own area, forming Ti and Al atom clusters respectively. Oxygen can interact with Nb, Ti and Al because of the negative affinity energy with Nb, Ti and Al on Nb alloy surface, to generate the Nb2O5, TiO2 and Al2O3 mixed oxide film which has protective effect on Nb alloy.

Surface and interstitial transition barriers in rutile (110) surface growth

We present calculated surface and interstitial transition barriers for Ti, O, O-2, TiO, and TiO2 atoms and clusters at the rutile (110) surface. Defect structures involving these small clusters, including adcluster and interstitial binding sites, were calculated by energy minimization using density-functional theory (DFT). Transition energies between these defect sites were calculated using the NEB method. Additionally, a modified SMB-Q charge equilibration empirical potential and a fixed-charge empirical potential were used for a comparison of the transition energy barriers. Barriers of 1.2-3.5 eV were found for all studied small cluster transitions upon the surface except for transitions involving O-2. By contrast, the O-2 diffusion barriers along the [001] direction upon the surface are only 0.13 eV. The QEq charge equilibration model gave mixed agreement with the DFT calculations, with the barriers ranging between 0.8 and 5.8 eV.

Hydrogen sorption properties of a Mg–Y–Ti alloy

The catalytic effect of titanium on the hydrogen sorption properties of a Mg–Y–Ti alloy has been investigated. The alloy is formed by a majority phase Mg 24+ x Y 5, a minor phase of solid solution of Y in Mg and Ti clusters randomly dispersed in the sample. During the first hydrogen absorption cycle 5.6 wt.% hydrogen was absorbed at temperatures above 613 K. The alloy decomposed almost completely to MgH 2 and YH 3. After hydrogen desorption pure Mg and YH 2 were formed. For further absorption/desorption cycles the material had a reversible hydrogen capacity of 4.8 wt.%. The MgH 2 decomposition enthalpy was determined to −68 kJ/mol H 2, and the calculated activation energy of hydrogen desorption of MgH 2 was 150(±10) kJ/mol.

Bonding properties and structures of titanium clusters on (10, 0) single wall carbon nano capsule

We optimize whole structures of Ti clusters, Tin (n=1, 3 and 7), on a zigzag-type (10, 0) single wall carbon nano capsule, C160, by using the first principles molecular dynamics within the spin density functional approximation, and discuss the bonding properties of Ti clusters on the C160. For all the clusters, the most stable configuration includes a site above a center of C-hexagonal or pentagonal ring. The shape of Ti7 cluster is very different by the located site. The cluster in which Ti atoms are located around centers of hexagon has a flat bottom and flatten the curvature of C wall below it.

Synthesis, Characterization, Cytotoxicity, and Hydrolytic Behavior of C2‐ and C1‐Symmetrical TiIV Complexes of Tetradentate Diamine Bis(Phenolato) Ligands: A New Class of Antitumor Agents

We recently introduced a new class of bis(isopropoxo)-Ti(IV) complexes with diamine bis(phenolato) ligands that possess antitumor activity against colon HT-29 and ovarian OVCAR-1 cells that is higher than that of the known Ti(IV) compounds titanocene dichloride and budotitane as well as that of cisplatin. Herein, we elaborate on this family of compounds; we discuss the effect of structural parameters on the cytotoxic activity and hydrolytic behavior of these complexes, seeking a relationship between the two. Whereas complexes with small steric groups around the metal center possess high activity and lead mostly to formation of O-bridged polynuclear complexes with bound bis(phenolato) ligand upon water addition, bulky complexes hydrolyze to release all free ligands and are inactive. Slightly increasing the size of the N-donor substituents probably weakens the ligand binding in solution, and, thus, rapid hydrolysis is observed, leading to a lack of cytotoxicity, supporting the requirement for ligand inertness. Replacing the two isopropoxo ligands with a single catecholato unit gives a complex with a different geometry that exhibits slower hydrolysis and reduced cytotoxicity, suggesting some participation of labile ligand hydrolysis in the cytotoxicity mechanism. A crystallographically characterized O-bridged polynuclear species obtained from a biologically active bis(isopropoxo) complex upon water addition is inactive, which rules out its participation as the active species, yet suggests some role of the particular steric and electronic requirements allowing its formation in the activity mechanism. Additional measurements support rapid formation of the active species in the presence of cells prior to O-bridged Ti(IV) cluster formation.

The magnetism of M13-doped cagelike structure of gold clusters (M = Fe, Ti)：density functional calculations

The geometric and magnetic properties of M13 and M13-doped Au20 （M = Fe, Ti） clusters have been studied using the generalized gradient approximation based on the density functional theory. The optimized geometries of the clusters are close to the Ih structure within 0.006—0.05 nm tolerance. The lowest-energy spin states of the Fe13 and Fe13-doped Au20 clusters are 44 μB and 38 μB, respectively, while there is week ferromagnetic interaction between the Fe and Au atoms for the Fe13-doped Au20 cluster. On the other hand, the lowest energy spin states of the Ti13 and Ti13-doped Au20 clusters are 6 μB and 4 μB, respectively. The magnetic ordering is in a week ferromagnetic arrangement between the 12 surface Ti atoms and Au atoms, while in a week antiferromagnetic arrangement between the 12 surface Ti atoms and the core Ti atom. Comparing with the bare Fe13 and Ti13 clusters, the magnetic moments of Fe13 and Ti13 in Fe13-doped Au20 and Ti13-doped Au20 clusters are reduced by 6.81 μB and 2.88 μB, respectively.

Core/Shell Morphology in Ti Clusters Prepared by Plasma Gas Condensation and Post Annealing

Ministry of Education, Science, Culture and Sports, Japan; Aichi Prefecture, Nagoya City and Aichi Science and Technology Foundation

Interaction of D2 with Ti-Adsorbed Polyaniline and Implication for Hydrogen Storage

Interaction of D2 with Ti-adsorbed polyaniline (PANI) and TiO2 at low Ti coverages has been investigated by temperature programmed desorption, Auger electron spectroscopy, and H2-D2 exchange in a high pressure cell. In contrast to recent DFT calculations (Lee et al. Phys. Rev. Lett. 2006, 97, 56104-56107) that have shown a multiple number of molecular hydrogen chemisorption on Ti-decorated PANI, only 0.52 D2 molecules per Ti atomically adsorb on Ti-deposited PANI at 87 K at a nominal Ti coverage of 2 ML. Broad TPD spectra of D2, HD, and H2 were observed, which showed a common peak at 250 K and another isotope-dependent peak at higher temperature. Ti deposited on TiO2 forms clusters with a size distribution at 87 K, on which D2 atomically adsorbs. As the Ti coverage increases, the D2 desorption peak gradually shifts from 200 to 260 K and the number of D2 molecules adsorbed per Ti also increases from 0.26 at 0.1 ML to 0.87 at 2 ML. We attribute this to the fact that D2 adsorbs on larger clusters with a greater adsorption energy. TPD spectra of D2 desorbing from Ti (2 ML)/PANI and Ti (1 ML)/TiO2 look very similar to each other. This led us to conclude that Ti also adsorbs on PANI in clusters. We suggest that no molecular D2 adsorption on Ti/PANI at 87 K is due to a reduced electron backdonation ability of Ti upon clustering. The small number of adsorbed D2 per Ti on both substrates was ascribed to irreversible D2 adsorption only on relatively large Ti clusters, while D2 adsorbs reversibly on small clusters. This was indirectly confirmed by the observation that continuous H2-D2 isotope exchange occurs to produce HD on Ti/PANI as well as on Ti/TiO2 at 106 K in a high pressure cell at 5.7 mTorr. Desorption mechanisms involving H atom abstraction from PANI by Ti clusters are proposed to account for the complex isotope-dependent TPD spectra. Implication of the present results for hydrogen storage based on Ti-dispersed polymers is briefly addressed.

Energetics and kinetics of Ti clustering on neutral and charged C60 surfaces

Using ab initio spin density functional theory, we investigate the energetics and kinetics of Ti clustering on both neutral and charged C(60) surfaces. We compare the formation energy of sparsely dispersed zero-dimensional (0D), compact single-layered two-dimensional (2D), and clustered three-dimensional (3D) Ti(N) configurations as a function of cluster size (N < or = 12) and further study the transformation kinetics between them. We find that 0D configuration is always less stable than that of 2D and 3D configurations and 0D to 2D transformation involves in a single Ti diffusion process with kinetic barrier of < or = 0.7 eV. On the other hand, there exists a critical cluster size (N(C)) of N(C) = 5, below which 2D layers are preferred to 3D clusters. Hole- or B-doping greatly enhance the Ti-fullerene interaction and lead to stronger dispersion of Ti atoms. Even so, for moderate charge doping (less than seven holes) the critical size of Ti atoms on neutral C(60) surprisingly remains unchanged or only slightly increases to N(C) = 6 by B-doping. However, we find that the formation of 3D clusters may be hindered by a high kinetic barrier related to the process of single Ti atoms climbing up a single Ti layer. This barrier is approximately 1 eV or even 1.47 eV for B-doped C(60) surfaces which is high enough to stabilize larger 2D structures (N > or = N(C)) at low temperatures. These findings may prove to be instrumental in stabilizing transition metal coated nanostructures and especially homogeneously Ti-coated fullerenes, which are believed to be a very promising material for hydrogen storage.

Analysis of hydrogen adsorption in the bulk and on the surface of magnesium nanoparticles

The stability of magnesium hydride (MgH x ) nanoparticles (x = 0.5, …, 2) is investigated using ab initio calculations. It is shown that for a nanoparticle diameter of D ∼ 5 nm, the internal pressure becomes lower than 3 kbar; for this reason, the structure of hydride nanoparticles coincides with the structure of this hydride in crystalline form. It is found that the phase of partly saturated MgH x hydrides (x < 2) must decompose into the phase of pure hcp magnesium and the α phase of MgH2. The frequencies of jumps of hydrogen atoms within the hcp phase of magnesium and in the α phase of MgH2 are calculated; it is shown that slow diffusion of hydrogen in magnesium is due to the large height of potential barriers for motion of hydrogen within MgH2. To attain high diffusion rates, the structures of Mg53Sc and Mg53Ti crystals and their hydrides are calculated. It is found that the frequency of jumps of H atoms in Mg53ScH108 near the Sc atoms does not noticeably change as compared to the frequency of jumps in the α phase of MgH2, while the frequency of jumps in Mg53TiH108 near Ti atoms is higher by approximately a factor of 2.5 × 106. This means that diffusion in manganese hydride with small admixtures of titanium atoms must be considerably eased. Chemical dissociation of hydrogen molecules on the (0001) surface of hcp magnesium, on the given surface with adjoined individual Ti atoms, and on the surface of a one-layer titanium cluster on the given surface of magnesium is investigated. It is found that dissociation of hydrogen at solitary titanium atoms, as well as on the surface of a Ti cluster, is facilitated to a considerable extent as compared to pure magnesium. This should also sharply increase the hydrogen adsorption rate in magnesium nanoparticles.