Ti Clusters Research Articles

Ab initio calculations of elastic properties of titanium nanoclusters

Density functional theory calculations have been used to study the bulk modulus of a nanosized Ti cluster. The applicability of various equations of states to nanosized and bulk titanium has been analyzed.

Optical Properties and Quasiparticle Band Gaps of Transition-Metal Atoms Encapsulated by Silicon Cages

Semiconductors assembled upon nanotemplates consisting of metal-encapsulating Si cage clusters (M@Sin) have been proposed as prospective materials for nanodevices. To make an accurate and systematic prediction of the optical properties of such M@Sin clusters, which represent a new type of metal–silicon hybrid material for components in nanoelectronics, we have performed first-principles calculations of the electronic properties and quasiparticle band gaps for a variety of M@Si12 (M = Ti, Cr, Zr, Mo, Ru, Pd, Hf, and Os) and M@Si16 (M = Ti, Zr, and Hf) clusters. At first stage, the electronic structure calculations have been performed within plane-wave density functional theory in order to predict equilibrium geometries, polarizabilities, and optical absorption spectra of these endohedral cagelike clusters. The quasiparticle calculations were performed within the GW approximation, which predict that all of these systems are semiconductors exhibiting large band gaps. The present results have demonstrated that the independent-particle absorption spectra of M@Sin, calculated within the local density or generalized gradient approximations to density functional theory, are dramatically influenced by many-body effects. On average, the quasiparticle band gaps were significantly increased, in comparison with the independent-particle gaps, giving values in the 2.45–5.64 eV range. Consequently, the inclusion of many-body effects in the electron–electron interaction, and going beyond the mean-field approximation of independent particles, might be essential to realistically describe the optical spectra of isolated M@Sin clusters, as well as their cluster-assembled materials.

The Ti4 cluster activates water dissociation on defective graphene

Activation of the Ti4 cluster on defective graphene for water adsorption and dissociation was investigated via density functional theory. Both the vacancy and the Ti cluster can promote the water dissociation reaction. The vacancy can efficiently enhance the adsorption of Ti atoms to stabilize the cluster. However, compared to the role of the vacancy, the cluster plays a more important role in activating water dissociation. A single water molecule and a second one can almost freely dissociate with (or without) a low barrier on the Ti4 cluster, the barriers are substantially lower than that on a single Ti atom. The Ti4 cluster decorated graphene is a promising candidate for activation of the dissociation of water molecules.

Ab initio study on the effects of dopant–defect cluster on the electronic properties of TiO2-based photocatalysts

We present a first-principles study on the electronic properties of TiO2 containing both dopants and defects on the basis of density-functional theory. We show that the formation energies of defects can be reduced by up to 80% in N-doped TiO2 (N substitutes O), as compared to those in perfect TiO2, but the H-doping (H substitutes O) has less effect on the defect formation. We predict that dopant-interstitial Ti cluster plays an important role in the narrowing of band gap of N-doped anatase and brookite TiO2, and H-doped rutile TiO2. Importantly, the defect bands within the band gaps can enhance the visible-light absorption and improve the photocatalytic performance of the systems due to the reduced gap between the bands. The dopant–defect cluster in the doped TiO2 may be responsible for the observed visible-light absorption in experiments. The present study provides a new route to enhance the performance of photocatalyst.

Al12X (X = Ni, Pd, Pt, Ti, and Zr) Clusters: Promising Low-Cost and High-Activity Catalysts for CO Oxidation

CO oxidation on the surface of various Al12X (X = Ni, Pd, Pt, Ti, and Zr) clusters is investigated by density functional theory (DFT) calculations. The molecular structures and surface–adsorbate interaction energies of CO and O2 on the Al12X (X = Ni, Pd, Pt, Ti, and Zr) cluster surfaces are predicted. The frontier orbital picture (FOP) successfully predicted the cluster–CO or cluster–O2 ground-state configurations for all Al12X clusters. The calculated results indicate that the CO oxidation reaction is sensitive to the properties of different transition metals, and the order of activation barrier for CO reaction with molecular O2 and CO reaction with the adsorbed oxygen atom is different for Al12X (X = Ni, Pd, Pt) clusters. A trimolecular Langmuir–Hinshelwood (LH) mechanism with two coadsorbed CO molecules and one coadsorbed O2 molecule is proposed, which will lead to the spontaneous formation (due to extremely low energy barrier) of two CO2 molecules as product. The calculated results show that the order...

On the dual deuterium/deuteron nature of D charge distribution in the Ti host matrix: A DFT analysis

In the present work we have investigated the charge distribution on deuterium atom inside a titanium host matrix and their interaction energy. This analysis is motivated by the role that Ti-D alloys play in nuclear processes, i.e. the reported neutron generation as a consequence of the pressure exerted on such hybrid systems. A two-fold DFT procedure has been employed in order to carry out our analysis, namely a periodic and a cluster approach. Both show that the D atom, at variance with the prediction by T. Asami et al. (T. Asami, Journal of Condensed Matter Nuclear Science 2011; 5:7–16; T Asami, N. Sano, Journal of Condensed Matter Nuclear Science 2012; 9:1–9), tends to avoid the deuteron form when entering the Ti tetrahedral site. In particular, according to the cluster approach results, both the Voronoi Deformation Density atomic charge analysis and the interaction energy decomposition analysis indicate that the deuterium enters the tetrahedron cage retaining its electron, provided that a high energy barrier could be overcome. The largest contribution to the interaction energy is the charge-transfer orbital interaction term but the sizeable energy barrier is mainly due to the electrostatic repulsive interaction between Ti cluster and D+. We thus conclude that the experimental conditions (i.e., the combination of pressure and temperature) could be responsible for the hypothesized “switching on” of the deuterium-to-deuteron charge oxidation.

Oxygen adsorption on small Ti clusters: A first-principles study

We have used density functional theory calculations to investigate the interaction of titanium (Ti) nanoparticles with oxygen. We found the energy-favorable site for oxygen atoms on a Ti13 cluster surface and investigated the atomic structure of the oxidized cluster. Our results showed that during oxidation, the oxygen atoms advantageously occupied positions on the titanium clusters that are similar to “bulk” interstitial sites.

The Microstructure and Tribological Behavior of Ti/a-C and Ti/a-C:H Films Prepared by Magnetron Sputtering

This paper reports two films, Ti/a-C and Ti/a-C:H, prepared on the Ti-6Al-4V alloys by magnetron sputtering in PVD and CVD process, respectively. The morphology and microstructure were characterized by Raman spectroscopy, scanning electron microscopy and transmission electron microscopy. Both films show nanosized Ti clusters incorporated into the amorphous carbon matrix. Mechanical properties of the films were investigated by nanoindentation and ball-on-disk tribo-test. Ti/a-C film shows a hardness as high as 40.9 GPa, while that of Ti/a-C:H is 12.2 GPa. Both films show reduced friction and high wear resistance.

Identification of Ti Clusters during Nucleation and Growth of Sol—Gel-Derived TiO2 Nanoparticles

In the present work, electrospray ionization mass spectrometry (ESI-MS) was used to investigate the titanium clusters present during nucleation and growth of sol-gel derived TiO2 nanoparticles. Depending on the alkoxide precursor used, Ti clusters of different sizes were identified [titanium isopropoxide (TTIP)-11-12 Ti atoms, titanium butoxide (TTB)-10-11 Ti atoms and titanium tetraethoxide (TTE)-5-7 Ti atoms]. The Ti-O-Ti backbone/core of the titanium clusters were found to be quite stable after formation and do not easily break up into smaller clusters. The results of this investigation suggest that these clusters are used as building blocks in the growth during the induction period. The h-ratio (n H2O/n alcoxide) and the pH were not found to influence the identity of the Ti clusters present during nucleation of the growth of sol-gel derived TiO2 nano-particles, although the induction period was greatly influenced. The reactivity of the alkoxy group are OEt > OPri > OBu. The difference in reactivity of the alkoxides is generally believed to be due to the increasing size of the alkoxy group which causes sterical hindrance during the nucleophilic substitution reaction.

Interaction of Bacterial Cells with Cluster-Assembled Nanostructured Titania Surfaces: An Atomic Force Microscopy Study

The nanoscale interaction of bacterial cells with solid surfaces is a key issue in biomedicine because it constitutes the first pathogenic event in the complex series of biofilm development on prosthetic devices. We report on an Atomic Force Microscopy study of the interaction of Escherichia coli and Pseudomonas aeruginosa bacterial cells with nanostructured titania thin films with controlled and reproducible nanometer-scale morphology, produced by assembling Ti clusters from the gas phase in a Supersonic Cluster Beam Deposition apparatus. The results demonstrate that bacterial adhesion and biofilm formation are significantly influenced by a pure physical stimulus, that is, the nanoscale variation of surface topography. The increase of nanoscale film roughness promotes bacterial adhesion with respect to flat substrates; remarkably, Pseudomonas aeruginosa cells lose their flagella on nanostructured TiO2 thin films upon adhesion, as opposed to same bacteria onto reference smooth glass substrates. Further, we have observed increased cell biovolume and other biofilm properties on nanostructured substrates in comparison with smooth glasses. These findings suggest that the design of innovative biomaterials with a suitable patterning of biomaterials surfaces can be an effective approach to control the adhesion of microorganisms to in vivo implant surfaces with active biological functionalities.

Theoretical study on the encapsulation of Pd3-based transition metal clusters inside boron nitride nanotubes

Chemical functionalization of the boron nitride nanotube (BNNT) allows a wider flexibility in engineering its electronic and magnetic properties as well as chemical reactivity, thus making it have potential applications in many fields. In the present work, the encapsulation of 13 different Pd(3)M (M = Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Pd, Pt, and Au) clusters inside the (10, 0) BNNT has been studied by performing comprehensive density functional theory (DFT) calculations. Particular attention is paid to searching for the stable configurations, calculating the corresponding binding energies, and evaluating the effects of the encapsulation of Pd(3)M cluster on the electronic and magnetic properties of BNNT. The results indicate that all the studied Pd(3)M clusters can be stably encapsulated inside the (10, 0) BNNT, with binding energies ranging from -0.96 (for Pd(3)Sc) to -5.31 eV (for Pd(3)V). Moreover, due to a certain amount of charge transfer from Pd(3)M clusters to BNNT, certain impurity states are induced within the band gap of pristine BNNT, leading to the reduction of the band gap in various ways. Most Pd(3)M@BNNT nanocomposites exhibit nonzero magnetic moments, which mainly originate from the contribution of the Pd(3)M clusters. In particular, the adsorption of O(2) molecule on BNNT is greatly enhanced due to Pd(3)M encapsulation. The elongation of O-O bonds of the adsorbed O(2) molecules indicates that Pd(3)M@BNNT could be used to fabricate the oxidative catalysis.

Scandium-Decorated MOF-5 as Potential Candidates for Room-Temperature Hydrogen Storage: A Solution for the Clustering Problem in MOFs

Transition-metal-based systems show promising binding energy for hydrogen storage but suffer from clustering problem. The effect of light transition metal (M = Sc, Ti) decoration, boron substitution on the hydrogen storage properties of MOF-5, and clustering problem of metals has been investigated using ab initio density functional theory. Our results of solid-tate calculations reveal that whereas Ti clusters strongly Sc atoms do not suffer from this problem when decorating MOF-5. Boron substitution on metal-decorated MOF-5 enhances the interaction energy of both the metals with MOF-5. Sc-decorated MOF-5 shows a hydrogen storage capacity of 5.81 wt % with calculated binding energies of 20–40 kJ/mol, which ensures the room-temperature applicability of this hydrogen storage material.

Ti-doped LiAlH4 for hydrogen storage: Rehydrogenation process, reaction conditions and microstructure evolution during cycling

One-step rehydrogenation of Ti-doped LiAlH4 from its dehydrogenated precursor (Ti-doped LiH/Al), proceeds in Me2O solution under mild conditions (25 °C; 50–100 bar H2). The formation process, the effects of reaction conditions (temperature and H2 pressure) on the yield of LiAlH4, and the microstructure evolution during hydrogenation–dehydrogenation cycles were investigated comprehensively. No evidence was obtained for the intermediate Li3AlH6 in the product. Higher temperatures adversely affect the yield and stability of the Ti-doped LiAlH4 product. High hydrogen pressure favors the formation of LiAlH4 but the influence of pressure in the range 50–100 bar is slight if other variables are maintained constant. The cycling performance depends on the microstructure of the material: larger particle size; lower surface area; and the formation of less active Ti clusters and Ti–Al alloys upon cycling correspond to a decrease in cycling performance. Incorporation of an extra ball milling stage before each rehydrogenation significantly improves the cycling performance.

First Principles Study of Hydrogen Desorption from the NaAlH4 Surface Doped by Ti Clusters

On the basis of density functional calculations, we show that a Ti4 cluster adsorbed at the (101) surface of NaAlH4 is able to catalyze both the release of H2 and the formation of AlH52– groups which have been previously proposed to be the mobile species leading to the formation of the Na3AlH6 product during the dehydrogenation of Na alanate.

Tuning the magnetic and transport property of graphene with Ti atom and cluster

Atomic structure of Ti and Ti4 cluster on the surface of graphene are studied by the density functional theory and nonequilibrium Green’s function formalism. The most stable configurations for Ti and Ti4 adsorbed on the graphene surface are investigated, and the binding energies corresponding to the configurations are calculated. We found that the Ti4 clusters can strongly interact with pristine graphene surface, and the interaction can be further enhanced when structural vacancies are introduced onto the graphene surface. Electronic properties of metal/graphene composites are also presented to show their highly spin polarized behaviors. The transport properties of systems are dependent on the applied bias voltage. Our results also indicate that the Ti4/graphene composite is a simple and typical model to study the interactive effects of metal clusters absorbed on graphene. The results of this work could help to design novel devices such as nanoelectrodes and magnetic sensors based on graphene.

Structural and electronic properties of phosphorus-doped titanium clusters: A DFT study

Based on the density-functional theory, geometric, energetic, electronic, and magnetic properties of phosphorus-doped small titanium clusters Ti n P ( n = 1–12) have been investigated. For the ground state structures of Ti 10P, Ti 11P, and Ti 12P clusters, the P atom completely fell into the center of the clusters forming the P-encapsulating Ti cage clusters. Doping of the P atom enhanced the stability of the Ti clusters. The binding energies, second-order energy differences, and fragmentation energies with the size of clusters showed Ti 6P and Ti 12P to be endowed with special stability. The total magnetic moments of Ti n P clusters exhibited odd–even staggering behavior. The quenching of the magnetic moment has not been found in Ti n P clusters. The density of states of clusters was discussed to understand the origin of peculiar magnetic properties.

First-principles calculations of 17O nuclear magnetic resonance chemical shielding in Pb(Zr1/2Ti1/2)O3 and Pb(Mg1/3Nb2/3)O3: Linear dependence on transition-metal/oxygen bond lengths

First-principles density functional theory oxygen chemical shift tensors were calculated for A(B,B′)O3 perovskite alloys Pb(Zr1/2Ti1/2)O3 (PZT) and Pb(Mg1/3Nb2/3)O3 (PMN). Quantum chemistry methods for embedded clusters and the gauge including projector augmented waves (GIPAW) method [C. J. Pickard and F. Mauri, Phys. Rev. B 63, 245101 (2001)]10.1103/PhysRevB.63.245101 for periodic boundary conditions were used. Results from both methods are in good agreement for PZT and prototypical perovskites. PMN results were obtained using only GIPAW. Both isotropic δiso and axial δax chemical shifts were found to vary approximately linearly as a function of the nearest-distance transition-metal/oxygen bond length, rs. Using these results, we argue against Ti clustering in PZT, as conjectured from recent 17O NMR magic-angle-spinning measurements. Our findings indicate that 17O NMR measurements, coupled with first-principles calculations, can be an important probe of local structure in complex perovskite solid solutions.

Magnesium nanoparticles with transition metal decoration for hydrogen storage

We report on the hydrogen storage behaviour of Mg nanoparticles (NPs) (size range 100 nm–1 μm) with metal-oxide core–shell morphology synthesized by inert gas condensation and decorated by transition metal (TM) (Pd or Ti) clusters via in situ vacuum deposition. The structure and morphology of the as-prepared and hydrogenated NPs is studied by electron microscopy, X-ray diffraction including in situ experiments and X-ray absorption spectroscopy, in order to investigate the relationships with the hydrogen storage kinetics measured by the volumetric Sieverts method. With both Pd and Ti, the decoration deeply improves the hydrogen sorption properties: previously inert NPs exhibit complete hydrogenation with fast transformation kinetics, good stability and reversible gravimetric capacity that can attain 6 wt%. In the case of Pd-decoration, the occurrence of Mg–Pd alloying is observed at high temperatures and in dependence of the hydrogen pressure conditions. These structural transformations modify both the kinetics and thermodynamics of hydride formation, while Ti-decoration has an effect only on the kinetics. The experimental results are discussed in relation with key issues such as the amount of decoration, the heat of mixing between TM and Mg and the binding energy between TM and hydrogen.

Aberration-Corrected STEM Investigation of Metallic Ti Cluster Formation in Mg-Ti Hydrides

Extended abstract of a paper presented at Microscopy and Microanalysis 2011 in Nashville, Tennessee, USA, August 7–August 11, 2011.

Ti-doped nano-porous graphene: A material for hydrogen storage and sensor

Clustering of Ti on carbon nanostructures has proved to be an obstacle in their use as hydrogen storagematerials. Using density functional theory we show that Ti atoms will not cluster at moderate concentrations when doped into nanoporous graphene. Since each Ti atom can bind up to three hydrogen molecules with an average binding energy of 0.54 eV/H2, this material can be ideal for storing hydrogen under ambient thermodynamic conditions. In addition, nanoporous graphene is magnetic with or without Ti doping, but when it is fully saturated with hydrogen, the magnetism disappears. This novel feature suggests that nanoporous graphene cannot only be used for storing hydrogen, but also as a hydrogen sensor.